Technical, economic, and carbon dioxide emission analyses of managing anaerobically digested sewage sludge through hydrothermal carbonization

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

By

Luis Armando Huezo Sanchez

Graduate Program in Food, Agricultural & Biological Engineering

The Ohio State University

2020

Dissertation Committee:

Dr. Ajay Shah

Dr. Katrina Cornish

Dr. Steve Culman

Dr. Jay Martin

Copyrighted by

Luis Armando Huezo Sanchez

2020

Abstract

Sewage sludge is the solid byproduct from wastewater treatment plants, and some of it is treated by anaerobic digestion (AD), which is a biological method that produces biogas and an AD effluent (ADE). Biogas is typically used for energy and heat. Anaerobic digestion effluent has a high concentration of microbes, nutrients, carbon, and water. It is typically dewatered, and its fates include incineration, landfilling, composting, or application to agricultural fields; in all these options, ADE needs to be transported. The storage and transportation of ADE have environmental impacts on water, soil, and air.

Dewatering ADE is energy and cost intensive. A viable alternative to process ADE could be thermochemical methods, such as hydrothermal carbonization (HTC), that can treat

ADE at high temperatures and pressures without the need to remove the water. HTC produces a carbonized char-like material called hydrochar with potential uses as solid fuel and soil amendment. Hydrochar as soil amendment has the potential to improve the properties of the soil and crop yield. Therefore, the objective of this study was to assess the technical, economic, and environmental feasibility of producing hydrochar through HTC of ADE from sewage sludge and analyze its use as soil amendment.

Hydrothermal carbonization of ADE from sewage sludge was conducted between

180 and 260°C for a residence time between 30 and 70 minutes following a central composite design. The process parameters evaluated were temperature, time, and feedstock

ii pH; the response variables included hydrochar and liquor yields and properties. The produced hydrochar was used as soil amendment at 1, 3, 5, 10, and 15 g per kg of soil.

Seedling flats were filled with the char-soil mixtures, and lettuce seeds were planted and placed in a greenhouse. Soil properties and plant responses, such as nutrient retention, seed germination, and biomass production were analyzed based on char sources and rates. To scale-up the combined AD-HTC system to a sewage input flow of 15 ton hr-1, the process

was modeled and a techno-economic analysis was performed with data from wastewater

treatment plants, equipment and process conditions; properties of feedstock, intermediates,

and final products; and cost of feedstock, materials, equipment, and utilities. The direct

carbon dioxide emissions of the combined AD-HTC system were estimated.

Temperature was the most influential parameter in producing hydrochar. Higher temperatures resulted in lower hydrochar yields, higher ash contents, and a more

carbonized material. Soil amended with hydrochar had higher pH, phosphorus content, and

cation exchange capacity compared to soil with no amendment. Lettuce emergence rates

in soils amended with hydrochar were similar and higher compared to pyrochar and no-

char. All dry weights from roots, leaves, and whole plants for amended soils were greater

than those for no-char. When the combined AD-HTC system was scaled-up, the capital investment was calculated to be ~US$36 million, with a payback time of less than six years, internal rate of return of ~12%, and an operating cost of ~US$1,300 ton-1 of hydrochar.

The direct carbon dioxide emissions of the combined AD-HTC system decreased compared to scenarios without AD or HTC to manage sewage sludge. In conclusion, the production of hydrochar from sewage sludge through a combined AD-HTC system has the potential

iii to be technically, economically, and environmentally feasible and to be implemented in the current wastewater treatment plants.

iv

Dedicated to my sister, parents, and grandparents.

v

Acknowledgments

I thank my advisor Dr. Ajay Shah for his guidance, support, dedication, and trust during my doctoral studies. I also thank Dr. Katrina Cornish, Dr. Steve Culman, and Dr.

Jay Martin for serving on my committee and for their support and encouragement during the process. Thanks to Dr. Juliana Vasco-Correa for her technical support and coaching during my studies, experiments, and writing. Thanks to Dr. Ashish Manandhar, Asmita

Khanal, Seyed Mousavi, Junqi Wang, Yangyang Li, and Zhifang Cui from the Biosystems

Analysis Laboratory for working alongside and their assistance during this journey. I thank

Quasar Energy Group members Josh Andre and Xumeng Ge for their help while running the experiments. Special thanks to Peggy Christman, Mary Wicks, Scott Wolfe, Mike

Klingman, and Candy McBride, for their invaluable constant help and patience. I thank Dr.

Luis Cañas, the Zamorano community, and Jhony Mera for opening the door to this opportunity. I thank my friends Uchit Nair, Aditya Raj, Dr. Ramon Salcedo, Andrea

Landaverde, Dr. Anirudh Akula, Shyam Sivaprasad, Parisa Nazemi, Asmita Khanal,

Champ Zhang, Dr. David Ramirez, Hugo Pantigoso, Juan Quijia, Andres Sanabria, and

Julio Ramirez for their friendship and encouragement. Finally, I thank Kaylee South for her unconditional love and support in all aspects of my professional and personal life.

vi

Vita

December 2011 ...... B.Sc. Environmental Science and

Socioeconomic Development, Zamorano

University

May 2017 ...... M.Sc. Food, Agricultural, and Biological

Engineering, The Ohio State University

May 2015 to present ...... Graduate Research Associate, Department

of Food, Agricultural, and Biological

Engineering, The Ohio State University

Fields of Study

Major Field: Food, Agricultural & Biological Engineering

vii

Table of Contents

Abstract ...... ii

Acknowledgments...... vi

Vita ...... vii

Table of Contents ...... viii

List of Tables ...... xiii

List of Figures ...... xv

Chapter 1: Introduction ...... 1

1.1. Background ...... 1

1.2. Problem statement ...... 2

1.3. Dissertation objectives ...... 4

1.4. Dissertation organization ...... 5

Chapter 2. Literature Review ...... 7

2.1. Introduction ...... 7

2.2. Types of organic waste ...... 8

2.2.1. Manure ...... 9

2.2.2. Sewage sludge ...... 9 viii 2.3. Waste biomass conversion methods and bioproducts ...... 10

2.3.1. Biochemical conversion methods ...... 11

2.3.2. Thermochemical conversion methods ...... 12

2.4. Hydrothermal carbonization of biomass ...... 15

2.5. Biochar, pyrochar, and hydrochar ...... 18

2.6. Soil properties ...... 20

2.6.1. Soil pH ...... 20

2.6.2. Cation exchange capacity ...... 21

2.7. Techno-economic and life cycle analyses of anaerobic digestion and hydrothermal

carbonization systems ...... 22

2.7.1. Techno-economic analysis of anaerobic digestion and hydrothermal

carbonization systems ...... 24

2.7.2. Life-cycle assessment of anaerobic digestion and hydrothermal carbonization

systems...... 29

2.8. Conclusion ...... 32

Chapter 3. Hydrothermal carbonization of anaerobic digestion effluent from sewage sludge for hydrochar production ...... 34

3.1. Abstract ...... 34

3.2. Introduction ...... 35

3.3. Methodology ...... 39

ix 3.3.1. Feedstock, source, pH, and storage ...... 39

3.3.2. Experimental design and statistical analysis ...... 40

3.3.3. Hydrothermal carbonization and downstream process ...... 41

3.3.4. Hydrochar and liquor characterization ...... 42

3.4. Results and Discussion ...... 44

3.4.1. Mass balance...... 44

3.4.2. Yield of hydrothermal carbonization products ...... 45

3.4.3. Hydrochar properties ...... 49

3.4.4. Liquor composition and properties ...... 58

3.5. Conclusions ...... 66

Chapter 4. Effect of hydrochar from anaerobically digestated sewage sludge and manure as soil amendment on soil properties and plant responses ...... 68

4.1. Abstract ...... 68

4.2. Introduction ...... 69

4.3. Methodology ...... 73

4.3.1. Experimental design and data analysis ...... 73

4.3.2. Soil, chars, and amended soils ...... 74

4.3.3. Greenhouse study ...... 75

4.4. Results and Discussion ...... 77

4.4.1 Properties of amended soils ...... 77 x 4.4.2. Effect of chars on lettuce emergence ...... 82

4.4.3. Effect of chars on plant development ...... 86

4.4.4. Effects of chars on nutrient content of lettuce ...... 90

4.5. Conclusions ...... 93

Chapter 5. Techno-economic and direct carbon dioxide emissions analyses of a combined anaerobic digestion and hydrothermal carbonization system ...... 94

5.1. Abstract ...... 94

5.2. Introduction ...... 95

5.3. Methodology ...... 99

5.3.1. System boundary of the combined anaerobic digestion and hydrothermal

carbonization system ...... 99

5.3.2. Scenarios ...... 102

5.3.3. Techno-economic and direct carbon dioxide emission estimation ...... 104

5.3.4. Sensitivity analysis ...... 107

5.4. Results and Discussion ...... 108

5.4.1. Capital investment for upgrading base cases to the combined AD-HTC system

...... 108

5.4.2. Mass balance of the combined anaerobic digestion and hydrothermal

carbonization system ...... 110

xi 5.4.3. Operating cost for the combined anaerobic digestion and hydrothermal

carbonization system ...... 111

5.4.4. Net revenue for base cases and the combined anaerobic digestion and

hydrothermal carbonization system ...... 113

5.4.5. Direct carbon dioxide emissions...... 114

5.4.6. Sensitivity analysis ...... 116

5.5. Conclusions ...... 117

Chapter 6. Conclusions and recommendations ...... 119

6.1. Conclusions ...... 119

6.2. Recommendations for future research ...... 120

References ...... 122

Appendix A: Hydrothermal carbonization of anaerobic digestion effluent from sewage sludge for hydrochar production ...... 147

Appendix B: Effect of hydrochar from anaerobically digestated sewage sludge and manure on soil properties and plant responses ...... 152

xii

List of Tables

Table 1. Reaction conditions for hydrothermal processes...... 15

Table 2. Techno-economic analyses of hydrothermal carbonization systems...... 25

Table 3. Life-cycle assessments performed on hydrothermal carbonization systems...... 30

Table 4. Properties of anaerobic digestion effluent at original and modified pH...... 40

Table 5. Mass balance for all the reaction temperatures and times studied for both pH .. 45

Table 6. Volatile fatty acids of anaerobic digestion effluent and liquor at different

hydrothermal carbonization conditions...... 61

Table 7. Sugars, acids, and alcohols identified in the liquor at different hydrothermal

carbonization conditions...... 63

Table 8. Elements measured in lettuce roots and leaves from the highest to lowest

concentration...... 92

Table 9. Equipment present and required to be installed for each scenario...... 104

Table 10. Equipment capacities and purchase cost of the combined AD-HTC system. 106

Table 11. Assumptions for the technical, economic, and environmental analyses...... 107

Table 12. Hydrothermal carbonization product yields and properties...... 147

Table 13. Composition of the anaerobic digestion effluent, hydrochar, and liquor...... 148

Table 14. Mean comparison of soil properties by amendment rate within char types. .. 152

Table 15. Mean comparison of soil properties by char type within amendment rates. .. 153

xiii Table 16. Elements measured in lettuce roots and leaves...... 154

xiv

List of Figures

Figure 1. Wastewater treatment plant (WWTP), anaerobic digestion (AD), and hydrothermal carbonization (HTC)...... 2

Figure 2. Central composite design for the HTC runs...... 41

Figure 3. Hydrothermal carbonization reaction and downstream processes...... 42

Figure 4. Effect of reaction temperature and time on hydrochar, liquor, and gas yields .. 48

Figure 5. Content of N, C, H, S, O, and ash in hydrochar at different HTC conditions compared to the ADE...... 51

Figure 6. Van Krevelen diagram...... 53

Figure 7. Calorific value...... 54

Figure 8. Scanning electron microscopy of hydrochar...... 56

Figure 9. Color of hydrochar and liquor at different hydrothermal carbonization conditions...... 57

Figure 10. Content of N, C, H, S, O, and ash in liquor at different HTC conditions compared to the ADE...... 59

Figure 11. Layout of the experimental designs for the emergence and lettuce growth assay...... 76

Figure 12. Effects of char types and amendment rates on soil properties...... 79

xv Figure 13. Soil properties for the control treatment compared with the maximum values.

...... 81

Figure 14. Effect of char rates per char type on lettuce emergence from day 4 to 10...... 84

Figure 15. Effect of char types and amendment rates on dry weights of the whole plants,

roots, and leaves, and growth index...... 87

Figure 16. Lettuce grown in soil amended with different char types 60 days after sowing.

...... 88

Figure 17. Schematic of the combined AD-HTC system for the treatment of sewage sludge and the production of hydrochar...... 99

Figure 18. Process flow diagram of the combined AD-HTC system...... 101

Figure 19. Scenarios for the combined anaerobic digestion and hydrothermal

carbonization (AD-HTC) system plus transportation to the agricultural field...... 103

Figure 20. Framework for techno-economic and environmental analyses...... 105

Figure 21. Capital investment for upgrading scenarios 1-5 to the combined anaerobic

digestion and hydrothermal carbonization system...... 109

Figure 22. Mass balance for the treatment of 1,000 kg of sewage sludge...... 110

Figure 23. Operating cost of the combined anaerobic digestion and hydrothermal

carbonization system...... 112

Figure 24. Net revenue per year for the base case scenarios and the combined anaerobic

digestion and hydrothermal carbonization system...... 114

Figure 25. Carbon dioxide emissions for all the base scenarios and the combined

anaerobic digestion and hydrothermal carbonization system...... 116

xvi Figure 26. Sensitivity analysis of hydrochar production cost, payback time, and internal rate of return...... 117

xvii

Chapter 1: Introduction

1.1. Background

Wastewater treatment plants (WWTP) receive and treat wastewater from

households, industries, and institutions to produce clean water to be recycled into society

and generate sewage sludge as a byproduct (Figure 1). Sewage sludge is the collective term used for the solids from wastewater treatment; even though this is the solid fraction, it just contains about 8% of total solids (TS). WWTP worldwide produce ~150 million tons of sewage sludge annually (Lundqvist et al., 2017). In the United States (US), ~8 million tons yr-1 of dry sewage sludge are produced (Center for Sustainable Systems, 2019; U.S.

Environmental Protection Agency, 2006) from ~16,000 WWTP, from which ~10% of them treat the sewage sludge through anaerobic digestion (AD) (Cybersecurity and

Infrastructure Security Agency, n.d.; Water Environment Federation, 2015), the remaining

90% dewater the sewage sludge and deal with it through incineration, landfill, or land application (Figure 1). Anaerobic digestion is a biological conversion method that can process wet biomass in the absence of oxygen into biogas and anaerobic digestion effluent

(ADE). Biogas is often used to generate heat and electricity to run equipment within the

1 WWTP or outside the plant. Anaerobic digestion effluent is microbe-, nutrient-, carbon-,

and water-rich (≥ 80% water content).

Figure 1. Wastewater treatment plant (WWTP), anaerobic digestion (AD), and hydrothermal carbonization (HTC).

1.2. Problem statement

Anaerobic digestion effluent is typically dewatered, and its fates include landfilling, composting (Sheets et al., 2015), or application to agricultural fields (Nkoa, 2014) (Figure

1). Field application of ADE is challenging because the application window is small due to regulations that determine when and where ADE can be applied to the field (Beecher et al., 2007). While waiting for the appropriate time to apply the ADE, it needs to be stored in lagoons for months (Lukicheva et al., 2014). Storage of ADE in lagoons also serve as

2 sedimentation basins for the ADE where the solid fraction settles and undergo anaerobic

digestion, with a difference that in this case, biogas is not collected, but released to the

atmosphere. Storage of ADE in lagoons poses environmental risks for water and soil

contamination, such as nutrient runoff (Nkoa, 2014), ammonia volatilization (Génermont

& Cellier, 1997), and odor issues. Transportation of ADE from where it is generated to the agricultural field is also challenging and costly due to the high-water content, and

dewatering ADE is energy and cost intensive (Delzeit & Kellner, 2013).

The current obstacles to the management of ADE have pushed the exploration of

alternative technologies to cope with its generation. Some of the alternatives are

thermochemical conversion methods, which use heat to process biomass into different

products. Hydrothermal carbonization (HTC) is a thermochemical method that can process

wet biomass, such as ADE, without prior dewatering (Figure 1). During HTC the

temperature of the feedstocks is elevated under high pressures to keep the water in the

feedstock in its liquid state (Toor et al., 2011). Hydrothermal carbonization opens

possibilities for alternative treatments of ADE, with the potential to produce higher value

products. Different feedstock and process conditions during HTC yield different char

characteristics. The main product of HTC is hydrochar (Figure 1), which is a carbonized

material with potential uses as solid fuel (Lin et al., 2015) and soil amendment (Eibisch et

al., 2015; McLaughlin et al., 2009). Hydrochar as soil amendment could improve soil

properties (Kammann et al., 2012) and crop yields (Upadhyay et al., 2014).

This dissertation conducted a technical, economic, and direct carbon dioxide

emission analyses of using HTC for treating ADE of sewage sludge generated from

WWTP. Most of the previous research have focused in AD or HTC independently, and not 3 in a combined AD and HTC system (Figure 1). Hydrothermal carbonization experiments were conducted to produce hydrochar and use it as soil amendment source. Technical, economic, and environmental analyses of the combined AD-HTC system (Figure 1) were performed through techno-economic analysis (TEA) and direct carbon dioxide emission analysis. The TEA was used as a tool to estimate the technical and economic requirements for the system, facilitate comparison of scenarios, analyze units’ operations, and aid in the decision making for improving processes. A direct carbon dioxide emission analysis was used to quantify and evaluate the emissions of the system.

1.3. Dissertation objectives

The main objective of this dissertation was to assess the technical, economic, and environmental feasibility of producing hydrochar through HTC of ADE from sewage sludge and analyze its use for soil amendment. Three specific objectives were set:

Objective 1. Assess the effects of HTC temperature, time, and initial ADE pH on the

yields and properties of hydrochar and its potential uses.

Objective 2. Analyze the effect of hydrochar from anaerobic digestion effluent from

sewage sludge and manure, and pyrochar as soil amendments, at different

rates, and their effects on the soil properties and plant responses including

seed emergence, below and above-ground biomass production, and plant

nutrient content.

4 Objective 3. Conduct the techno-economic and direct carbon dioxide emissions

analyses of the combined AD-HTC system for the treatment of sewage

sludge, production of hydrochar, and its transportation to an agricultural

field.

1.4. Dissertation organization

The dissertation includes six chapters. Chapter 1 is the introduction and contains background information, objectives of the dissertation, and organization of the dissertation.

Chapter 2 is a literature review and discusses biomass conversion into bioproducts and system analysis of conversion methods. Biomass conversion includes biochemical and thermochemical methods with special emphasis on AD and HTC. The bioproducts review focusses on hydrochar and pyrochar and their uses as soil amendments. Systems analysis includes technical, economic, and environmental analysis of AD and HTC. Chapters 3, 4, and 5 are projects that have been conducted, written, and prepared for publication in peer- review scientific journals. In Chapter 3, the effects of HTC temperature, time, and initial pH on the yields, and properties of hydrochar and liquor were assessed. The data generated during this study were used for performing technical, economic, and environmental modelling, and the hydrochar produced was used as a soil amendment. In Chapter 4 the soil properties and plant responses to the use of hydrochar as soil amendment were analyzed. Responses included soil properties, emergence rate, and biomass production and nutrient content. Chapter 5 evaluated the technical, economic, and environmental 5 feasibility of a combined AD-HTC system. Several scenarios for the treatment of sewage sludge and production of hydrochar were evaluated. The combined AD-HTC system included sewage sludge as feedstock, AD, HTC, and the transportation of hydrochar to an agricultural field. Chapter 6 summarizes the conclusions from the chapters 3, 4, and 5 and recommends research directions for future studies.

6

Chapter 2. Literature Review

2.1. Introduction

Sewage sludge is the solid byproduct (~8% total solids) of wastewater treatment

plants (WWTP) and ~150 million tons of sewage sludge are generated annually worldwide

(Lundqvist et al., 2017). In the United States ~90% of all the WWTP dispose of their sewage sludge by landfill or trough incineration. The remaining ~10% treat their sewage sludge through anaerobic digestion (AD).

The purpose of AD in a WWTP is to manage the sewage sludge, produce biogas for energy and heat, and reduce the quantity of sewage sludge waste. Anaerobic digestion generates an effluent (ADE), which is microbe-, nutrient-, carbon-, and water-rich. The

ADE from sewage sludge is typically dewatered, and its fates include incineration or landfilling (Sheets et al., 2015), or application to agricultural fields. ADE dewatering is energy intensive and translates into higher costs (Delzeit & Kellner, 2013). Field application of ADE is challenging because of the short application window and long-term storage needed (Lukicheva et al., 2014). Transportation of ADE from where it is generated to the agricultural field is inefficient and costly due to the high-water content (Delzeit &

Kellner, 2013).

7 An alternative treatment for ADE is hydrothermal carbonization (HTC), which is a

thermochemical conversion method than can process wet biomass, without prior

dewatering. Hydrothermal carbonization works at high temperatures and pressures and

produces hydrochar, which has potential use as a soil amendment.

Previous research on AD and HTC have considered HTC modeling, hydrochar properties (Ahmed et al., 2016) and applications (Rom et al., 2018), hydrothermal

processes (Kumar et al., 2018; Tekin et al., 2014), techno-economic analysis (TEA) and

life-cycle assessment (LCA) of thermochemical conversion methods (Brown, 2015; Patel et al., 2016), AD (Vasco-correa et al., 2018), and conversion of other feedstocks (Quinn &

Davis, 2015). The TEA and LCA for AD systems, to date, have focused on the generation of energy, but few have focused on the ADE and its uses. Studies on TEA of HTC mostly have focused on lignocellulosic waste, rather than ADE. Life-cycle assessment for HTC have paid more attention to the use of hydrochar as energy and few as soil amendment. The objective of this review was to summarize the knowledge in organic waste management and previous research on the techno-economics and life cycle of AD and HTC for the production of hydrochar from ADE of sewage sludge.

2.2. Types of organic waste

The properties of organic waste generated by different processes may become a

feedstock for another process or be disposed as waste, depending on its composition and

consistency. Organic waste can be in solid or liquid form. Solid organic wastes include the 8 organic fraction of municipal solid waste, agricultural waste, and industrial crop waste from processing. The organic fraction of municipal solid waste is defined as the organic waste collected by municipalities from household, commercial, and institutional facilities

(Nizami et al., 2017). The organic fraction of municipal solid waste is ideally treated separately to reduce the amount of organic waste sent to the landfill. Wet organic waste

(>70% water) includes wastes from household, industries, or animal farms operations.

Industries and households generate wastewater and sewage sludge. Animal farm operations generate manure.

2.2.1. Manure

Manure is composed of liquids (water, urine) and solids (inorganic and organic).

Inorganic solids in manure include soil, sand, and grit; organic solids include bedding material, feed waste, undigested plant material, feces, and bacteria. The quantity and properties of manure depend on the animal species, age, animal productivity (quantities of milk, meat, etc.), feed, digestibility, nutrient levels, bedding, feed waste, soil, surface water, wash water, environment, ventilation, and handling method (James et al., 2006).

2.2.2. Sewage sludge

In the US, ~8 million dry tons of sewage sludge, also called biosolids, are generated per year, from which ~60% is applied to the agricultural fields and forestry, and the remaining is either incinerated or sent to the landfill (Center for Sustainable Systems, 2019;

U.S. Environmental Protection Agency, 2006). Sewage sludge contains heavy metals such as zinc, chromium, lead, polycycle aromatic hydrocarbons, and polychlorinated biphenyl. 9 All these components make its disposal challenging (Feng et al., 2015). The US classifies sewage biosolids in two categories: class A, biosolids treated to reduce bacteria before land application; and class B, untreated biosolids (Havlin et al., 2014). When the sewage sludge

is treated by AD, some of its water is removed, usually by a belt thickener, and the

remaining partially dewatered sludge is sent to an equalization tank where the flow is

normalized before being sent to AD (Cies̈ lik et al., 2015).

2.3. Waste biomass conversion methods and bioproducts

The conversion methods for treating waste biomass can be classified as biochemical

and thermochemical. Biochemical conversion methods include AD, fermentation, and

composting (aerobic digestion). Thermochemical conversion methods include combustion,

torrefaction, gasification, pyrolysis, and hydrothermal processes such as HTC,

hydrothermal liquefaction, and hydrothermal gasification. The use of waste biomass as

feedstock fulfills two goals, to manage waste in a more sustainable way and to produce a

higher value bioproduct.

Organic waste is rich in carbon and nutrients and can be used in future processes to

produce energy or chemicals. The worst type of organic waste management is to send it to

the landfill, where it is degraded into methane and released to the environment.

10 2.3.1. Biochemical conversion methods

Biochemical conversion methods treat biomass with the help of a biological agent, usually anaerobic or aerobic microbes with the objective to produce a higher value product or generate a less toxic waste (Chen & Wang, 2017). Three of the key biochemical conversion methods that have been used to treat organic waste are fermentation, composting, and anaerobic digestion. Fermentation microbially converts the sugar, starch, or cellulose fractions of the wet biomass slurry using bacteria and fungi to produce metabolites (butanol, citric acid, lactic acid, and others). Composting is an aerobic process that converts wastes, including manure, sludge, food waste, municipal solid organic waste, and industrial waste into decomposed material used as compost soil amendment. Anaerobic digestion converts waste biomass into biogas and an ADE with the use of anaerobic microorganisms.

Anaerobic digestion occurs in the absence of oxygen at temperatures between 37°C to and 55°C for 19 to 45 days. Wet biomass typically treated by AD includes sewage sludge, food waste, and manure. Biogas is mainly made of methane (CH4) (60 to 70%) and

carbon dioxide (CO2) and is typically used for energy and heat. Anaerobic digestion includes four steps: hydrolysis, which breaks down organic molecules into monomers;

acidogenesis, which converts the monomers into volatile fatty acids; acetogenesis, which

occurs when the volatile fatty acids are converted into acetic acid, CO2, and hydrogen gas

(H2); and methanogenesis, which transforms acetic acid and some H2 into CH4 and CO2

(Mao et al., 2015). Each step involves a different community of microbes. Anaerobic

digestion reduces the quantity of solids that otherwise would go to the landfill, minimizing

emissions and effluents. 11 2.3.2. Thermochemical conversion methods

Thermochemical conversion methods treat dry and wet biomass with heat to produce char, oils, and gases. Thermochemical conversion methods of dry organic wastes include combustion, torrefaction, gasification, and pyrolysis, while hydrothermal processes are used for wet waste feedstocks. Combustion produces heat and gases (CO2, water vapor,

nitrogen gas [N2], carbon monoxide [CO], sulfur oxide [SOx], and nitrogen oxides [NOx])

from dry matter in an oxygen-rich environment at temperatures between 700°C and

1,000°C. Torrefaction occurs between 200°C and 300°C from minutes to hours, with heating rates <50°C min-1 (Boersma et al., 2005). It is used as a pretreatment to dry

biomass, to upgrade lignocellulosic biomass, reduce microbial activity (Trifonova et al.,

2009), and increase biomass energy content (Bridgeman & Jones, 2008). Gasification produces synthesis gas (syngas) made up of H2 and CO. It occurs at temperatures between

750 and 1,100°C, from seconds to minutes, with moderate and very fast heating rates, up

to 10,000°C s-1, and limited oxygen input (Brown, 2011; Schurtz & Fletcher, 2009; Zhu et

al., 2016).

Pyrolysis occurs in the absence of oxygen (Bridgwater & Peacocke, 2000; Brown

& Brown, 2014), under atmospheric pressure (Venderbosch & Prins, 2011) and produces

condensable gases, non-condensable gases, and char. The distribution of products depends

on the type of pyrolysis, slow or fast (Al Arni, 2018). Slow pyrolysis (conventional

pyrolysis or carbonization) has long residence times (minutes to hours), slow heating rates

(~10°C min-1), and temperatures between 300°C to 500°C, and its main product is pyrochar

(Al Arni, 2018; Nachenius et al., 2013; Venderbosch & Prins, 2011). Fast pyrolysis

produces biooil as the main product (Nachenius et al., 2013). It has high heating rates (≥ 12 100°C s-1) (Nachenius et al., 2013), to achieve temperatures between 400°C to 600°C, for

short reaction times (<2 s) (Venderbosch & Prins, 2011). One of the issues with pyrolysis,

combustion, torrefaction, and gasification is that they require dry biomass, and are not ideal

for wet biomass. Drying of wet biomass is energy intensive and translates into more

expensive processes.

2.2.2.1. Hydrothermal processes

Hydrothermal processes are thermochemical conversion methods that can treat wet biomass at elevated water temperatures and high pressures, to maintain the water in its liquid phase above 100°C. Water at subcritical temperatures behaves like a polar solvent, while at supercritical temperatures it behaves as an organic solvent (Elliott, 2011). These

properties of water facilitate mass transfer (Siskin & Katritzky, 1991) and accelerate

chemical reactions. However, this behavior of water and the conversion of wet biomass

into oxygenated fragments cause the formation of organic acids, which lowers pH and leads

to metal corrosion (Elliott, 2011).

Hydrothermal processes for wet biomass present some advantages over

thermochemical processes for dry biomass, as well as some challenges. Advantages of

hydrothermal processes include the use of lower temperatures than in thermochemical

methods for dry biomass, no need of extensive pretreatment of the feedstock, the ability to

combine with other technologies for recycling and improvement, and the recovery of heat

from hot streams. Some of the challenges to overcome are that hydrothermal processes

require feedstock with small particle size and complex and expensive reactors for handling

water at high temperatures and pressures (Elliott, 2011; Zhang et al., 2010). Hydrothermal 13 processes produce char, oil, and gases. The distribution of the products depends on the process conditions and feedstock characteristics.

Hydrothermal processes include hydrothermal liquefaction, hydrothermal gasification, and HTC (Table 1). In hydrothermal liquefaction, the solid biopolymeric structures are broken down into liquid components that are later purified into liquid fuels and chemicals (Elliott, 2011). During hydrothermal gasification, the liquid components from the solid polymeric structures are gasified into H2-rich gas, and some CH4 and CO2

(Schmieder et al., 2000). Hydrothermal carbonization is also known as wet torrefaction

(Acharya et al., 2015; Chen et al., 2015) or wet pyrolysis (Libra et al., 2011). It produces

hydrochar through dehydration and decarboxylation of biomass. After HTC, the hydrochar

sludge is separated in its solid and liquid phases. The main product of hydrothermal

carbonization is a carbonized char-like product called hydrochar.

14 Table 1. Reaction conditions for hydrothermal processes.

Conversion method HTCa HTLb HTGc 180-300 100-490 175-767 Temperature (°C) (Peng et al., 2016) (Kumar et al., (Kumar et al., 2018) 2018) 4 3 Heating rate (°C s-1) (Peng et al., 2016) (Kumar et al., 2018) 1-6 5 to 40 8.8-105 Pressure (MPad) (Ramke et al., 2009; (Kumar et al., (Kumar et al., Uddin et al., 2013) 2018) 2018) 20-480 0.2 to 120 15-45 Time (min) (Peng et al., 2016) (Kumar et al., (Kumar et al., 2018) 2018) Target product Char Oil Gas

a HTC: hydrothermal carbonization; b HTL: hydrothermal liquefaction; c HTG: hydrothermal gasification; d MPa: Megapascal

2.4. Hydrothermal carbonization of biomass

Hydrothermal carbonization opens the possibilities for alternative treatments of

ADE, which currently has technical, economic, and environmental challenges that limit its

wider use. Dewatering of ADE is challenging because the water that it contains is intrinsic

(García-Bernet et al., 2011), which means is bound to the solids and not easily removed by traditional physical methods. Application of ADE to agricultural fields is also challenging because it can only be applied at certain times of the year and then it needs to be stored for months usually in lagoons (Lukicheva et al., 2014). Lagoons serve as sedimentation basins for the ADE where the solid fraction settles in the bottom and undergo anaerobic digestion, with a difference that in this case, biogas is not collected, but released to the atmosphere.

15 Lagoon storage poses environmental risks for water and soil contamination (Lukicheva et

al., 2014). The transportation of ADE from where it is generated to the agricultural field is inefficient because ADE is mostly water (Delzeit & Kellner, 2013). By processing the ADE

through HTC, dewatering and disposing of ADE to the landfill or incineration are avoided

and nutrients are recycled.

Previous research in HTC has focused on different feedstocks (Bach et al., 2013;

Berge et al., 2011; Elaigwu & Greenway, 2016; Fernandez et al., 2015; Kalderis et al.,

2014; Kim et al., 2017; Seames et al., 2010; Titirici et al., 2007; Wang et al., 2019; Wang

et al., 2018), HTC temperatures(Bach et al., 2014; Danso-Boateng et al., 2015; Kim et al.,

2014; Peng et al., 2016; Saha et al., 2019; vom Eyser et al., 2015; Yan et al., 2010; Zhai

et al., 2017; Zhang et al., 2014) and times (Hussain et al., 2015; Mäkelä et al., 2016;

Parshetti et al., 2013; Saha et al., 2019; Zhai et al., 2017). Feedstock that have been used

include lignocellulosic biomass (Bach et al., 2013; Bach et al., 2014; Lynam et al., 2011;

Titirici et al., 2007; Yan et al., 2010), Propopis africana shell (Elaigwu & Greenway,

2016), coconut rice husk (Kalderis et al., 2014), orange peels (Fernandez et al., 2015),

ADE from corn silage (Mumme et al., 2011), food waste (Wang et al., 2018; Tengfei Wang

et al., 2018; Zhai et al., 2018), municipal waste streams (Berge et al., 2011), manure (Dai

et al., 2015; Heilmann et al., 2014; Wu et al., 2018), pulp and paper mill sludge (Mäkelä

et al., 2016), human feces (Danso-Boateng et al., 2015), sewage sludge (He et al., 2013;

Koottatep et al., 2016; Parshetti et al., 2013; Peng et al., 2016; vom Eyser et al., 2015;

Wang et al., 2019; Zhai et al., 2017; Zhao et al., 2014), sewage sludge mixed with other

solid wastes (Zhai et al., 2017), and ADE from sewage sludge (Kim et al., 2014).

Temperatures and times previously studied have ranged from 175°C (Bach et al., 2013; 16 Bach et al., 2014) to 300°C (Peng et al., 2016) and 10 min (Bach et al., 2014) to 12 hr (He

et al., 2013). Hydrothermal carbonization pressures reported have been between 1.5 to 26

MPa (Bach et al., 2014; Parshetti et al., 2013; vom Eyser et al., 2015). The most common

uses of hydrochar have been as solid fuel (He et al., 2013; Kim et al., 2014; Parshetti et

al., 2013; Zhao et al., 2014) and soil amendment (Eibisch et al., 2015; Hitzl et al., 2015;

McLaughlin et al., 2009) for crop improvement or carbon sequestration (Titirici et al.,

2007).

Other researchers have also evaluated feedstock particle size and additives during

HTC (Bach et al., 2013; Lynam et al., 2011; Wang et al., 2018; Zhai et al., 2018). Smaller particle sizes have shown to decrease hydrochar yield, but have enhanced the calorific value and carbon content (Bach et al., 2013). The use of acetic acid and lithium chloride as additives for the pretreatment of lignocellulosic biomass has resulted in a 30% increase in the calorific value of hydrochar, but decreased the yield, compared to the hydrochar with no pretreatment (Lynam et al., 2011). Food waste combined with other materials, such as woody biomass and molasses, have been used as feedstock to produce hydrochar pellets for solid fuel (Wang et al., 2018; Zhai et al., 2018). High ratios of food waste mixed with lignocellulosic biomass as feedstock for HTC have enhanced fuel properties by decreasing the O/C and H/C ratios (Wang et al., 2018). The increase of the lignocellulosic fraction compared to the food waste, has shown to increase the tensile strength of the pellet (Wang et al., 2018). Hydrochar from food waste and molasses/lime as binders has resulted in stronger pellets due to the solid bridge from molasses and agglomeration by the lime, and increased calorific values at higher HTC conditions (Zhai et al., 2018). Hydrothermal carbonization has also been used to reduce pharmaceutical loads (diclofenac, phenazone, 17 carbamazepine, among others) from sewage sludge (vom Eyser et al., 2015). The reduction

has been attributed to high HTC temperatures and the limited thermal stability of some

pharmaceutical components (vom Eyser et al., 2015).

2.5. Biochar, pyrochar, and hydrochar

Hydrochar is the main product of HTC, which is similar to pyrochar (commercially

known as biochar). Biochar is used to denote a carbonized biomass typically used in the soil; however there is not a prescribed use for it (IBI, 2015). Pyrochar is the main product of pyrolysis (Eibisch et al., 2015), one of the most commonly used thermochemical processes. Pyrochar and hydrochar feedstocks include dry biomass, such as agricultural residues, energy crops, or forest residues (Budai et al., 2014; Liu et al., 2010). Hydrochar feedstocks include wet biomass, such as manure (Dai et al., 2015; Lang et al., 2019) and sewage sludge (Cao et al., 2011; Wiedner et al., 2013).

Hydrochar can be used right after production (Bargmann et al., 2014; Wagner &

Kaupenjohann, 2014) or be further processed to maximize or minimize specific properties.

Post-production processes include activation with chemicals to enhance hydrochar properties (Puccini et al., 2017) and washing (Fei et al., 2019) to eliminate potential toxic chemicals that may harm plants or inhibit germination (Busch et al., 2013). However, washing the hydrochar may not be practical at commercial scales (Kalderis et al., 2018) and would produce more wastewater (Hitzl et al., 2018).

18 Pyrochar and hydrochar properties are very dependent on process conditions

(feedstock particle size, reactor type, heating rate, reaction temperature, and time) and feedstock characteristics (i.e., type, nature, and origin) (Lehmann et al., 2011; Zhang et al.,

2008). Higher reaction temperatures result in higher carbon content, porosity, cation exchange capacity (CEC) (Bird et al., 2008; Dieguez-Alonso et al., 2018), surface area

(Kloss et al., 2012; Sun et al., 2014), and lower O/C and H/C ratios (Ronsse, 2016), which means higher stability in the soil (Brewer et al., 2012; Kammann et al., 2012; Spokas,

2010). Pyrochar in the soil is more stable than hydrochar, in terms of resistance against physical, chemical, and microbial degradation (Soja et al., 2016).Volatile solids and H/C and O/C ratios are good indicators of the level of carbonization, but not for agronomic properties. According to the International Biochar Initiative Guidelines (2015), for a product to be considered as pyrochar, its H/C ratio has to be ≤ 0.7.

Both hydrochar and pyrochar have the potential to be used as soil amendments

(Busch & Glaser, 2015; Eibisch et al., 2015; McLaughlin et al., 2009) to improve soil properties (Kammann et al., 2012) such as pH (Fellet et al., 2011), structure, CEC (Fellet et al., 2011; Kalderis et al., 2018), bulk density, pore volume (Soja et al., 2016), organic matter (Kalderis et al., 2018), water retention (Abel et al., 2013; Soja et al., 2016), and carbon storage. One of the most important benefits of pyrochar is carbon sequestration.

Only soil has a big enough scale to accommodate biochar and make a relevant long-term mitigation of climate change (Sohi et al., 2010). Other potential uses of hydrochar are as solid fuel (Lin et al., 2015) and adsorbent (Fang et al., 2018). Uses of pyrochar include reclamation and mitigation of mining sites, seed coating, potting media, storm water

19 filtration, and soil restoration (Delaney, n.d.; Dumroese et al., 2011; Eibisch et al., 2015;

Fellet et al., 2011).

Pyrochar, depending on its initial feedstock (Dieguez-Alonso et al., 2018), conversion method (Dieguez-Alonso et al., 2018), processing conditions, and soil characteristics has the potential to improve agricultural production (Sohi et al., 2010), offset agricultural gas emissions such as CH4 and nitrous oxide (Bergman et al., 2015;

Renner, 2007; Soja et al., 2016; Yanai et al., 2007), and reduce leakage of nitrate into groundwater.

2.6. Soil properties

Some of the most important soil characteristics for crop production are soil pH,

CEC, and organic matter.

2.6.1. Soil pH

The pH is a measurement of acidity or alkalinity; normal soil pH is between 4 and

8. Soil pH is stable due to its buffering capacity, which increases with higher CEC and higher organic matter. Soil pH is influenced by its parental material, climate, living organisms, topography, age, season, agricultural practices, soil horizon, and water content

(Troeh & Thompson, 2005). The pH influences some of the charges on minerals and organic matter and, therefore, the availability of nutrients in the soil solution, which are

20 critically important to plants. Nutrient requirements and optimal pH differ for each plant species.

2.6.2. Cation exchange capacity

Cation exchange capacity represents the capacity of the negatively charged surfaces

of clays, minerals, organic matter, and roots to exchange/resupply cations/nutrients into the

soil solution (Havlin et al., 2014), and is measured in centimoles of charge per kilogram of

-1 soil (cmolc kg ). Dissolved nutrients can be taken up by plant roots. CEC is a reversible

process and the most important buffering mechanism in the soil. It is essential for nutrient

availability and retention (Lee et al., 2013). Pure soil organic matter has a CEC between

-1 100 and 300 cmolc kg soil (Havlin et al., 2014), roots have a CEC between 10 to 100

-1 cmolc kg .

Cations have a positive charge and are adsorbed by negatively charged surfaces.

These charges are distributed throughout the surface and are not dependent on the soil

solution pH. However, the negative charges on the edges of the minerals are dependent on

the soil solution pH. At lower pH, there is more hydrogen ions (H+) and the edges are

positively charged. As the pH increases, the H+ are removed, the charge becomes negative, and CEC increases.

2.6.3. Soil organic matter (SOM)

Soil organic matter is made up of plant residues and living microorganisms, active

SOM, and stable SOM (humus) (Fenton et al., 2008). The SOM increases the CEC and

buffering capacity of the soil, improves the decomposition of minerals, provides structure 21 to the soil, and improves aeration and water holding capacity. Soil organic matter content in the topsoil is commonly between 1 and 6% by weight, but can be up to 90% in low- elevation wet areas, and as low as 1% in deserts.

Soil organic matter mostly contains soil organic carbon, ~57% (Chatskikh et al.,

2013), which is important for long term stability, soil fertility, and nutrient availability

(Blanco-canqui & Lal, 2009; Tiad, 1997). The quality and quantity of soil organic carbon is very important for the sustainability of energy crops, as it improves their physical, chemical, and microbiological properties (Bessou et al., 2011).

2.7. Techno-economic and life cycle analyses of anaerobic digestion and hydrothermal carbonization systems

Systems analyses provide indicators for decision making to improve the system efficiency and sustainability. The production of bioproducts from biomass does not make them sustainable just because biomass is used instead of fossil feedstocks. There are other aspects to consider, such as production or collection of the biomass, transportation to the processing plant, conversion of the biomass into a range of bioproducts, and use and recyclability of the byproducts. Each of these steps uses resources different than biomass, such as electricity, water, chemicals, fuels, labor, which need to be considered when analyzing the sustainability of a biosystem.

Techno-economic analysis and life-cycle assessment are some of the commonly used tools for conducting systems analyses. These methods evaluate the performance of a 22 system, as well as identify hotspots that may need improvement. Data availability is an

issue while performing biosystems analysis. Whenever data is not available, clear

estimations and assumptions needs to be used. Sensitivity and uncertainty analyses are

important to deal with uncertainty.

Techno-economic analysis is a tool to evaluate the technical and economic feasibility of a product, process, or project. It is performed during the early stages of development when it serves to choose the best alternative and set operation targets for process improvement. To perform a TEA of a processing plant, the basic data required includes plant capacity, feedstock and materials, processes, flow diagram, products,

byproducts, waste, and mass and energy flows, equipment and plant life, depreciation,

maintenance and labor requirements, and raw materials and chemical quantities. The

results from the TEA include technical and economic details. The results also include a set

of indicators, which can be used to evaluate and compare production systems, including

capital investment, payback time, operating cost, net present value, and internal rate of

return. Results from a TEA help in the decision making of production systems, identify

bottlenecks to improve operation efficiency, and identify research and development gaps.

Life-cycle assessment evaluates the environmental impacts involved in the life-

cycle of a product from its extraction, production or collection, and conversion into a

product, to the use of the product and disposal (ISO, 2006; Shah et al., 2016). It accounts

for all the emissions generated throughout its life cycle. The most studied environmental

sub-impact categories in an ecosystem are global warming potential, acidification, and

eutrophication (Patel et al., 2016).

23 The LCA methodology considers four steps: goal and scope definition, inventory

analysis, impact assessment, and interpretation (International Organization for

Standardization, 2006). Goal and scope definition outline the methodology, assumptions,

limitations, functional unit, system boundary, geographic location, allocation method, and

selected impact categories. In the inventory analysis the consumption of resources, energy,

and the generation of emissions, effluents, and waste through the life cycle are estimated.

It identifies process steps, inventory required, and inventory flows. The impact assessment

consists of the evaluation and understanding of environmental impacts from the system.

During this step, the impact categories, category indicators, and characterization models

are selected and analyzed. The interpretation phase interacts with the other three phases,

and the results are evaluated in relation to the goal and scope. Results from LCA consider

the impacts of the system on the environment and human health.

2.7.1. Techno-economic analysis of anaerobic digestion and hydrothermal carbonization

systems

There are several studies focusing separately on TEA of AD (Astill & Shumway,

2016; Gebrezgabher et al., 2010; Kabir et al., 2015; Khan et al., 2014; Klavon et al., 2013;

Lantz, 2012; Li et al., 2018; Lin et al., 2019; Shafiei et al., 2011; Theodorou et al., 2017)

and HTC systems (Kempegowda et al., 2017; Lucian & Fiori, 2017; Mahmood et al., 2016;

Saba et al., 2019; Unrean et al., 2018; Wirth et al., 2011; Zeymer et al., 2017). There are a few TEAs of HTC systems in the literature (Kempegowda et al., 2017; Lucian & Fiori,

2017; Mahmood et al., 2016; Saba et al., 2019; Unrean et al., 2018; Wirth et al., 2011;

Zeymer et al., 2017) (Table 2) and thus there is an acknowledged need for more of these 24 studies (Kumar et al., 2018). To date, there is no TEA of HTC of ADE from sewage sludge.

Studies have focused on the use of hydrochar as fuel or byproduct, but not as soil amendment. Some of the TEAs of AD and HTC are described below.

Table 2. Techno-economic analyses of hydrothermal carbonization systems.

Feedstock Hydrochar Capacity (ton yr-1) Capital cost MSPa Biomass Hydrochar (million (US$ US$) ton-1) Miscanthus and coal fuel 378,000 12.3 114-117 (Saba et al., 2019) Forest and agricultural fuel 60 25.2 residues (Kempegowda et al., 2017) Green waste fuel 5,000 2,211 170-262 (Zeymer et al., 2017) Forest residues fuel 28,000 5,317 (Wirth et al., 2011) Compost and grapefruit fuel 14,000 7,000 226 (Lucian & Fiori, 2017) Food waste byproduct 660,000 30-60 (Mahmood et al., 2016)

a MSP: minimum selling price

A comparison of small AD systems for different farm sizes and determination the

economic viability was performed by Klavon et al. (2013). They pointed out the

importance of the use of the ADE, biogas, and CO2 credits. They found out that, for the

AD system to be feasible, the minimum farm size is 250 cows. Rajendran et al. (2013)

25 investigated a 2-m3 digester for households in developing countries and estimated that it could provide sufficient fuel for cooking for a family of four to six.

The economic performance of a 70,000 ton yr-1 AD plant was analyzed by

Gebrezgabher et al. (2010). This plant treated pig and poultry manure, corn, food waste,

and flower bulbs for 40 days at 40°C. They proposed several options to treat ADE including

fractionation, ultrafiltration, and reverse osmosis. The use of reverse osmosis-treated-ADE

as a fertilizer resulted in lower transportation costs and was more profitable than the other

options.

A Swedish TEA comparison of a large AD plant with a farm-scale AD plant, both

based on combined biogas-based heat and power production from cow manure found that

neither size was profitable and proposed that ADE utilization should be studied as an

additional profit (Lantz, 2012).

Another TEA examined biogas-based heat and power generation from cow manure

and sheep dung (Akbulut et al. 2012) treated by AD for 33 days in a 22,713 m3 digester.

Results indicated profits and a payback time between 3.7 and 5.3 years. A slightly shorted

payback period of 2.6 to 4 years was calculated for a small AD system used to generate

power and purify water for 30 households in Bangladesh (Khan et al., 2014). This study

was very dependent on the socio-economic context (Khan et al., 2014). Rajendran et al.

(2014) performed a TEA on the AD of the organic fraction of municipal solid waste. They

built six scenarios, the basic scenario considered a pretreatment, AD, and biogas upgrading;

the other scenarios built on it and varied the upgrading method, biogas amount, and AD

reactors. They found that producing and selling biogas as a vehicle fuel was profitable for

26 most scenarios, but there were still some uncertainties to address such as collection of the feedstock, fees, and capacities.

Other TEAs of AD systems have focused on power generation (Akbulut, 2012;

Lantz, 2012), biomass pretreatment methods (Dhar et al., 2012; Shafiei et al., 2013;

Teghammar et al., 2014), combination of treatment methods (Barta et al., 2010; Shafiei et al., 2011), nutrient management systems (Astill & Shumway, 2016), and dealt with real operating plants (Fichtner, 2012). Different feedstocks have also been studied, such as, seaweed and microalgae (Dave et al., 2013; Zamalloa et al., 2011), food waste (Theodorou et al., 2017), yard trimmings (Lin et al., 2019), co-digestion of different feedstocks (Li et al., 2018). Few studies have focused on the TEA of AD with special attention to the ADE

(Gebrezgabher et al., 2010).

A TEA and LCA of AD of the organic fraction of municipal solid waste for electricity production compared to landfill was performed by Sanscartier et al. (2012). This study evaluated a program implemented in Canada called Feed-to-Tariff that promoted the generation of electricity from renewable energy. They found that the electricity produced from the organic fraction of municipal solid waste was not enough to displace the amount of coal used for energy; however, the new system reduced greenhouse gas emissions, was cheaper, had the potential to improve at larger scales, and helped the waste management system. Collet et al. (2017) studied CH4 production from AD and power-to-gas technology through TEA and LCA and found that the system was not completely feasible at the time.

The techno-economics of HTC of miscanthus and coal to produce 346,000 ton hydrochar yr-1 as solid fuel for a 110 MWeq coal-fired power plant were estimated by Saba et al. (2019). They estimated a minimum selling price of US$106-117 ton-1 of hydrochar 27 and found that the production cost was very sensitive to plant size, feedstock cost, and hydrochar yield. The total capital investment was estimated at US$12.3 million, while the

operational cost was US$38.9 million yr-1. The cost of a gigajoule (GJ) of hydrochar was estimated to be US$4.49, considerably more expensive that bituminous coal (US$1.95 GJ-

1) or natural gas ($3.42 GJ-1).

A TEA of a two-stage HTC of forest and agricultural residues to produce hydrochar

slurry and activated carbon was performed by Kempegowda et al. (2017). This slurry was

wet-milled and combusted for heat and power. They estimated a capital investment of

US$29 million for the HTC. Wirth et al. (2011) investigated the impacts of plant capacity,

feedstock type, and transportation distances on HTC of forest residues The HTC reactor

was the most expensive piece of equipment, around 16% of the investment. Biomass supply

was the most influential factor on profitability. Lucian and Fiori (2017) used off-

specification compost and grape marc as feedstock in a continuous HTC reactor at 220°C

for 1 hr. The investment was ~US$2 million, production cost was ~US$0.8 million yr-1,

and the minimum selling price was ~US$226 ton-1 for a break-even point. Mahmood et al.

(2016) produced hydrochar and biooil through hydrothermal oxidation and found a

minimum selling price for hydrochar to be US$30-60 ton-1, however, biooil was the target

product with a minimum selling price between US$100-2,100 ton-1.

Hydrothermal carbonization of green waste considering different development

stages and heat supply options have also been studied (Zeymer et al., 2017). In Scenario 1

the hydrochar was pelletized, the green waste input was 5,000 ton yr-1 with a 4 hr residence

time. Scenario 2 did not pelletize the hydrochar. Scenario 3 was the same as Scenario 1

plus it assumed that the processing plant was located next to another biorefinery that 28 provided free feedstock and heat. Scenario 4 was the same as Scenario 2 multiplied 5 times.

Scenario 5 was the same as scenario 1 plus half the residence time and double the capacity.

Its functional unit was 1 megajoule (MJ) of heat generated by each of the scenarios. The

impact category assessed by the LCA was greenhouse gas emissions. The minimum selling

prices for scenarios 1 and 2 were US$215 and US$262 ton-1, respectively; they established

that it would become profitable below US$170 ton-1. The scenario with the lowest

production cost was Scenario 4 with US$163 ton-1. The LCA determined that all scenarios

had the potential to mitigate greenhouse gas emissions, compared to natural gas or heating

oil if the heat produced in the process was recirculated and used. The greenhouse gas

-1 emissions for the scenarios were between 61 gCO2-eq kWh (scenario 3) and 112 gCO2-

-1 -1 eq kWh (Scenario 1) generated, compared to 284 and 375 gCO2-eq kWh for natural gas

and heating oil, respectively.

2.7.2. Life-cycle assessment of anaerobic digestion and hydrothermal carbonization

systems

Many studies have focused on LCA of AD (Lijó et al., 2014; Mezzullo et al., 2013;

Nayal et al., 2016; Smith et al., 2014; Timonen et al., 2019), but only few on HTC systems

(Benavente et al., 2017; Berge et al., 2015a; Liu et al., 2017; Owsianiak et al., 2016;

Owsianiak et al., 2018) (Table 3). Most of the LCAs of HTC have focused on hydrochar for energy (Liu et al., 2017; Owsianiak et al., 2016) and just a few for soil amendment

(Gievers et al., 2015; Owsianiak et al., 2018). Most of these studies used some sort of waste as feedstock, and just one used ADE (Owsianiak et al., 2016).

29 Table 3. Life-cycle assessments performed on hydrothermal carbonization systems.

Feedstock Functional unit Impact category Green waste 1 MJa GHGb (Zeymer et al., 2017) Biowaste 1 kg in soil GWP, GTPc, & CTPd (Owsianiak et al., 2018) (others) Loblolly pine & coal 1 kWh GHG (Liu et al., 2017) Olive waste 1 kg of treated waste 9 categories (Benavente et al., 2017) Four wastes 1 MJ 15 categories (Owsianiak et al., 2016) Food waste 1 kg of treated food waste 9 categories (Berge et al., 2015)

a Megajoules; b Greenhouse gas; c Global temperature change potential; d Comparative toxicity potentials

Four AD reactor systems were investigated by Smith et al. (2014): AD membrane

reactor, high rate activated sludge with AD, conventional activated sludge with AD, and

aerobic membrane reactor with AD. The functional unit was 5 million treated gallon of

wastewater per day. They found that the AD membrane reactor recovered most of the

energy, but at the same time had higher energy demands and environmental emissions.

Life-cycle assessment of HTC of biowaste into hydrochar for soil amendment and

six different crops (barley, wheat, sugar beet, fava bean, onion, and lucerne) in Spain and

Germany was performed by Owsianiak et al. (2018). The functional unit was the average

application and storage of 1 kg of hydrochar to a temperate agricultural soil. Thirty-six

scenarios were built varying crop, and country, and assessing hydrochar CO2 and CH4

emissions. Both, replacement of inefficient waste management systems and temporary storage of carbon in the soil, resulted in global warming potential benefits that outweighed 30 the emissions produced from the hydrochar production and transportation to the field. The

sustainability of hydrochar used as soil amendment is dependent on the impact category analyzed, location, specific crop, and whether gas emissions are taken in account.

A cradle-to-grave LCA of HTC on greenhouse gas emissions and life cycle energy

was performed by Liu et al. (2017) for the conversion of coal and loblolly pine into

briquettes of coal fines with 0 - 100% hydrochar. The functional unit was 1 kWh of

electricity from the co-formed briquettes of hydrochar and coal fines. They found that the

electricity generated from these blended briquettes had lower greenhouse gas emissions

and higher life cycle energy used than just using coal. Most of the greenhouse gases were

emitted by the HTC reactor, which contributed 58% of the total.

The life cycle of the HTC of olive mill waste compared to composting, AD, and

incineration was conducted by Benavente et al. (2017). The functional unit was 1 kg of

treated olive waste. There were environmental benefits from most of the indicators, except

for freshwater ecotoxicity and freshwater eutrophication, probably because the liquor was

not treated. Hydrothermal carbonization plus hydrochar combustion for energy was more sustainable than the other options, except for incineration with energy recovery. The LCA of HTC of food waste, organic fraction of municipal solid waste, and ADE was performed by Owsianiak et al. (2016). The functional unit was 1 MJ of heat output. All the hydrochars had less impact that fossil fuels. In order of sustainability, hydrochar produced from green waste had the least environmental impacts and hydrochar produced from ADE the largest impact. Life-cycle assessment of HTC of 18 food waste scenarios based on liquid treatment, electricity needs, fate of metals, fate of nutrients, and HTC conditions regarding packing material, and reaction time and temperature was conducted by Berge et al. (2015a). 31 The functional unit was 1 kg of treated food waste. Liquor management and hydrochar

combustion for energy recovery were the most influential on lowering the environmental

impacts.

An LCA of potential uses of hydrochar from sewage sludge was conducted by

Gievers et al. (2015). The intended uses were for agriculture, horticulture, and incineration in a municipal solid waste power plant, and in a lignite power plant. The functional unit was the treatment of 1 kg of sewage sludge. They found that substituting peat and lignite with hydrochar as fuel led to the greatest reductions in global warming potential and depletion of fossil fuel reserves.

2.8. Conclusion

Previous researchers have performed experimental studies and system analyses of

AD and HTC. Anaerobic digestion is a well-studied topic; however, it is not commonly used in the WWTP in the US. Hydrothermal carbonization is a topic that has a lot of potential for treating biomass and producing a high value product; however, its potential is not yet known. Studies on HTC have focused on lignocellulosic biomass and its resulting hydrochar for energy use. Little research has been done on HTC from ADE of sewage sludge, therefore, there is little understanding on how it performs. In order to be able to improve the waste management system, especially in WWTP, alternative methods such as

HTC could be implemented. For this reason, more studies on HTC using ADE from sewage

32 sludge as feedstock need to be conducted. This will help to understand what type of

hydrochar can be produced from ADE.

Hydrochar research has focused on its use as solid fuel and not much attention has been put to other uses. Sewage sludge, manure, and ADE from both have all been used as

soil amendment at different rates on different crops, but not hydrochar from ADE of

sewage sludge or manure. In order to better understand the potential benefits of hydrochar

as soil amendment, it needs to be produced and tested in the soil.

System analyses using TEA and LCA of AD and HTC have been performed. Most

of these analyses have been for independent processes, AD or HTC, not as a combined

system. These studies have used different feedstock, capacities, processing conditions, and

focused mainly on hydrochar for energy. None, to the best of our knowledge, have

considered a combined AD-HTC system using sewage sludge as feedstock with the

purpose of producing hydrochar that can be applied to the field. There is an acknowledged

need for more of this type of research (Kumar et al., 2018).

Many studies have been performed on HTC using different feedstocks, its techno-

economics, and life-cycle environmental impacts. However, most of them have focused on

lignocellulosic biomass as feedstock and use of hydrochar for energy, very few of them

have focused on HTC from ADE of sewage sludge and its potential as soil amendment.

These studies have created a roadmap where different aspects of HTC and hydrochar have

been investigated, but there are unexplored spaces of this map that this study aims to fill.

33

Chapter 3. Hydrothermal carbonization of anaerobic digestion effluent from sewage sludge for hydrochar production

3.1. Abstract

Hydrothermal carbonization is a thermochemical conversion method that can

process wet biomass such as anaerobic digestion effluent (ADE) from sewage sludge into

hydrochar and liquor. Conventional treatment of ADE typically begins with dewatering, followed with options of incineration, landfilling, composting, or application to agricultural fields when not dewatered. All these methods to manage ADE have their own

challenges, especially when drying is required. Hydrothermal carbonization (HTC) was investigated as an alternative treatment method. The objective of this study was to assess the effects of HTC temperature, time, and initial ADE pH on the yields and properties of hydrochar and liquor. Anaerobic digestion effluent from sewage sludge (pH ~7.9) was collected from a wastewater treatment plant in Ohio and the pH of a subsample was acidified with sulfuric acid to pH 6.6. Hydrothermal carbonization of the ADE from sewage sludge at two different pH levels was conducted from 180 to 260°C, and for 30 to 70 min

following a central composite design. Hydrochar yields from ADE at the original pH and modified pH ranged from 70-80% and 62-73%, respectively. Higher temperatures resulted

in lower hydrochar and liquor yields. The calorific values for hydrochar ranged from 9.4 34 to 12 MJ kg-1 and were similar to the calorific value of the ADE, possibly due to the

hydrochar high ash and low carbon contents. The carbon content of hydrochar did not

increase compared to the carbon content of the ADE. As low calorific values, high ash

content, and high O/C and H/C ratios were found, hydrochar from ADE from sewage

sludge has more potential as a soil amendment than as a solid fuel.

3.2. Introduction

Sewage sludge is the nutrient-rich, solid byproduct from wastewater treatment plants (WWTP). Most of it is currently being sent to the landfill or incinerated where it generates greenhouse gas emissions and its nutrients are not utilized. Just a small fraction of the sewage sludge is processed through anaerobic digestion, which is a biochemical conversion method that can convert wet biomass, such as sewage sludge, into biogas and an anaerobic digestion effluent (ADE).

The ADE is a microbe-, nutrient-, carbon-, and water-rich (≥70%) sludge that is

typically applied to agricultural fields (Nkoa, 2014), or dewatered and sent for incineration,

landfilling, or composting (Sheets et al., 2015). The application of ADE to agricultural fields is challenging because its application window is short, therefore it needs to be stored in lagoons for several months until it is the right time for application (Lukicheva et al.,

2014), and requires transportation from these lagoons to the agricultural field (Delzeit &

Kellner, 2013). Storage and transportation of ADE poses environmental contamination risks for water and soil (Lukicheva et al., 2014), such as nutrient runoff (Nkoa, 2014), 35 ammonia volatilization (Génermont & Cellier, 1997), and odor issues. Dewatering of ADE

is energy intensive because it contains intrinsic water (Chen & Yang, 2012), which means

it is bound to the solid particles, and is not easily removed by traditional physical methods,

such as press filtering or centrifugation, resulting in high drying costs (Delzeit & Kellner,

2013). These obstacles have incentivized researchers to analyze alternative technologies to

manage ADE. Some of these are thermochemical conversion methods, which use heat to

process biomass into different products.

Hydrothermal carbonization (HTC) is a thermochemical method that treats wet

biomass, such as ADE, at high temperatures and pressures to produce hydrochar; it is also

known as wet torrefaction (Acharya et al., 2015) or wet pyrolysis (Libra et al., 2011). The

HTC process is robust due to its ability to treat heterogeneous feedstock without extensive

pretreatment (Berge et al., 2011; Hoekman et al., 2011). It works by elevating the

temperature of the feedstock between 180 and 260°C (Mäkelä et al., 2015). Temperatures

below 180ºC are not able to achieve carbonization (Basso & Castello, 2013; Yu et al.,

2011) and temperatures above 260ºC belong to a different process called hydrothermal

liquefaction (Gollakota et al., 2018). Due to autogenous pressure in the reactor, the water

remains liquid and acts as a polar solvent (Elliott, 2011) and catalyst, facilitating mass

transfer (Siskin & Katritzky, 1991) and accelerating the reactions (Toor et al., 2011).

Pressures reported have ranged from 1.5 to 26 MPa (Bach et al., 2014; Parshetti et al.,

2013; vom Eyser et al., 2015). The products of HTC are a gas fraction composed of carbon dioxide (CO2, 95%) and methane (CH4, 5%) (Kempegowda et al., 2017; Libra et al., 2011),

wastewater or liquor, and a carbonized char-like material called hydrochar. Different process conditions and feedstocks result in different hydrochar and liquor characteristics. 36 The potential uses of hydrochar include solid fuel (He et al., 2013; Kim et al., 2014; Lin et

al., 2015; Parshetti et al., 2013; Zhao et al., 2014), adsorbent (Fang et al., 2018), and soil amendments (Eibisch et al., 2015; Hitzl et al., 2015; McLaughlin et al., 2009).

Previous research in HTC has focused on different feedstocks (Bach et al., 2013;

Berge et al., 2011; Elaigwu & Greenway, 2016; Fernandez et al., 2015; Kalderis et al.,

2014; Kim et al., 2017; Seames et al., 2010; Titirici et al., 2007; Wang et al., 2019; Wang

et al., 2018), HTC temperatures(Bach et al., 2014; Danso-Boateng et al., 2015; Kim et al.,

2014; Peng et al., 2016; Saha et al., 2019; vom Eyser et al., 2015; Yan et al., 2010; Zhai

et al., 2017; Zhang et al., 2014) and times (Hussain et al., 2015; Mäkelä et al., 2016;

Parshetti et al., 2013; Saha et al., 2019; Zhai et al., 2017). Temperatures and times

previously studied have ranged from 175 (Bach et al., 2013; Q. V. Bach et al., 2014) to

300°C (Peng et al., 2016) for 10 min (Bach et al., 2014) to 12 hr (He et al., 2013).

Feedstocks that have been used for HTC include lignocellulosic biomass (Bach et

al., 2013; Bach et al., 2014; Lynam et al., 2011; Titirici et al., 2007; Yan et al., 2010),

Propopis africana shell (Elaigwu & Greenway, 2016), coconut rice husk (Kalderis et al.,

2014), orange peels (Fernandez et al., 2015), ADE from corn silage (Mumme et al., 2011), food waste (Wang et al., 2018; Wang et al., 2018; Zhai et al., 2018), municipal waste streams (Berge et al., 2011), manure (Dai et al., 2015; Heilmann et al., 2014; Wu et al.,

2018), pulp and paper mill sludge (Mäkelä et al., 2016), human feces (Danso-Boateng et al., 2015), sewage sludge (He et al., 2013; Koottatep et al., 2016; Parshetti et al., 2013;

Peng et al., 2016; vom Eyser et al., 2015; Wang et al., 2019; Zhai et al., 2017; Zhao et al.,

2014), and ADE from sewage sludge (Kim et al., 2014). Hydrothermal carbonization of

ADE of sewage sludge at temperatures between 180 to 250°C has been shown to improve 37 dewaterability of the hydrochar sludge and decreased H/C and O/C ratios of the ADE from

1.99 and 0.90 to 1.01 and 0.65, respectively (Kim et al., 2014).

Hydrothermal carbonization of sewage sludge at temperatures between 180 and

300°C for 15 min to 8 hr have been conducted (He et al., 2013; Koottatep et al., 2016;

Parshetti et al., 2013; Peng et al., 2016; Zhai et al., 2017; Zhao et al., 2014). Atomic H/C

and O/C ratios were different under different HTC conditions; at 200°C, the resulting H/C

and O/C ratios were 1.53 and 0.39 (He et al., 2013); at 300°C, 0.92 and 0.04 (Zhai et al.,

2017); and at 300°C for 0.5 hr, 1.84 and 0.07 (Peng et al., 2016). Atomic H/C and O/C ratios are important because they denote the degree of aromaticity of a carbon compound.

The lower the atomic ratios the more stable the carbon compound and the higher potential it has to be used as a solid fuel. Calorific values of hydrochar from sewage sludge produced at 250°C for 15 min have resulted in 15.8 MJ kg-1 (Parshetti et al., 2013) and at 300°C for

1 hr in 21.3 MJ kg-1 (Zhai et al., 2017). Sewage sludge has also been mixed with catalyst

(acetic acid, lithium chloride, borax, and zeolite) and other biomass types (cassava pulp,

dried leaves, pig manure, and rice husk) resulting in more optimal combinations, such as

sewage sludge mixed with acetic acid and cassava pulp for a calorific value of 28 MJ kg-1

(Koottatep et al., 2016). Sewage sludge as feedstock for HTC mixed with sawdust,

corncob, and rape straw at temperatures >260°C resulted in a 50% enhancement in

dewaterability (Zhai et al., 2017). Reaction temperature has been the most important

parameter regarding biofuel properties as it increases the energy density of biomass

(Lynam et al., 2011; Yan et al., 2010).

Hydrothermal carbonization has the potential to improve the waste management

system to cope with the generation of ADE and produce a higher value product. Most of 38 the studies previously performed on HTC have focused on other feedstocks not on ADE

from sewage sludge. More research on this feedstock is required to better understand its

potential and the type of hydrochar that can be produced from it. This will help us

understand the best use of hydrochar from ADE of sewage sludge Therefore, the objective

of this study was to assess the effects of HTC temperature, time, and initial pH of the ADE

from sewage sludge on the yields and properties of hydrochar and liquor, and identify the

potential uses of hydrochar from ADE of sewage sludge. The ADE was treated by HTC at

temperatures ranging from 180 to 260°C for 30 to 70 min at two different pH levels.

Physical and chemical properties of hydrochar and liquor were measured and analyzed.

3.3. Methodology

3.3.1. Feedstock, source, pH, and storage

Anaerobic digestion effluent from sewage sludge was obtained from a WWTP in

Akron, Ohio. The ADE collected had a total solids (TS) content of 8.73% and a pH of 7.90

±0.46 (3 replicates). Half of the ADE was separated, and its pH was lowered with 98% sulfuric acid (H2SO4) to around pH 6.60 ±0.15. The ADE with the original pH (pH-O) and

modified pH (pH-M) were stored at -20ºC. Properties of the ADE are shown in Table 4.

39 Table 4. Properties of anaerobic digestion effluent at original and modified pH.

ADEa Ash (%) O/Cb ratio H/Cc ratio HHVd (MJ kg-1) pH-O 42 ±4 0.57 ±0.03 1.29 ±0.05 11.64 ±0.76 pH-M 23 ±3 1.14 ±0.09 1.29 ±0.06 10.95 ±0.30 e f -1 g -1 pH TS (%) VFA (mg kg ) COD (g L ) pH-O 8.2 ±0.02 0.7% ±0.00 3,194 ±330.70 10.23 ±0.38 pH-M 7.2 ±0.02 2.2% ±0.00 2,346 ±134.80 9.31 ±0.26 a Anaerobic Digestion Effluent; b Oxygen/Carbon; c Hydrogen/Carbon; d Higher heating value in Megajoules per kilogram; e Total Solids; f Volatile Fatty Acids; g Chemical Oxygen Demand;

3.3.2. Experimental design and statistical analysis

The HTC of ADE followed a central composite design with a center point (three replicates) and four axial points (Figure 2). The factors evaluated were reaction temperature and time, for each initial ADE pH. Temperature was set between 180 and 260°C, retention time between 30 and 70 min, and the initial ADE pH was considered a categorical factor,

ADE pH-O and ADE pH-M. The center point was at 220°C and 50 min. The axial points extended from 163 to 277°C and from 22 to 78 min. The combination of reaction temperature and time was defined as a severity condition, higher temperatures and longer times denoted higher severity conditions, and lower temperatures and shorter times denoted lower severity conditions. Response variables included yields of hydrochar, liquor, and gas, and hydrochar and liquor properties. Gas yield was calculated by difference when hydrochar and liquor were subtracted from 100%. Alpha values <0.05 were considered significant. The data was analyzed using JMP Pro® 14.

40

Figure 2. Central composite design for the HTC runs. Temperature is the ‘x’ axis and time is the ‘y’ axis. Temperature ranged from 163 to 277°C and time from 22 to 78 min. The center point was 220°C and 50 min.

3.3.3. Hydrothermal carbonization and downstream process

Hydrothermal carbonization was performed in a 1-L stirred reactor (Parr

Instruments Company, Moline, IL, USA) with a heater jacket and autonomous pressure.

The reaction was performed at temperatures and times following the central composite design described above, at 120 rpm. The pressures autogenerated in the reactor for 163,

180, 220, 260, and 277°C were 0.83, 1.10, 2.24, 4.96, and 6.07 MPa, respectively. After the HTC was completed (Figure 3), the hydrochar sludge was cooled to 20°C, centrifuged at 5,000 rpm for 10 min, and dewatered through a vacuum-filtered (Whatman quantitative filter paper ashless, Grade 40) to separate the solids (hydrochar cake) and the liquid

41 (liquor). The hydrochar cake was dried in a convection oven at 105ºC for 24 hr, ground,

and stored in sealed containers. Liquor was stored at -20°C.

Figure 3. Hydrothermal carbonization reaction and downstream processes.

3.3.4. Hydrochar and liquor characterization

Hydrochar characterization included visual, proximate (moisture, total solids, and

ash content), and ultimate (composition) analyses. The hydrochar was analyzed visually to assess its color. Moisture, total solids, and ash content (%) were measured using a convection oven (105°C), and a muffle furnace (550°C). Carbon (C), hydrogen (H), nitrogen (N), and sulfur (S) contents were measured using an Elementar Analyzer

(Elementar Vario Max NCHS, Elementar Americas, Mt. Laurel, NJ, USA). Oxygen (O) was estimated by difference when ash, C, N, H, and S were subtracted from 100%. Other elements include aluminum (Al), arsenic (As), boron (B), barium (Ba), beryllium (Be), calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe),

potassium (K), lithium (Li), magnesium (Mg), manganese (Mn), molybdenum (Mo),

42 sodium (Na), nickel (Ni), phosphorus (P), lead (Pb), sulfur (S), antimony (Sb), selenium

(Se), silicon (Si), strontium (Sr), thallium (Tl), vanadium (V), and zinc (Zn) that were measured by Inductive Coupled Plasma-Optical Emission Spectrometry (ICP-OES,

Agilent 5110, Service Testing Analysis Laboratory, The Ohio State University, Wooster,

Ohio). The calorific value was measured by a bomb calorimeter (C2000 basic, IKA) and a decomposition unit (model: C 5010). Surface morphology for hydrochar pH-O was observed by scanning electron microscopy (Hitachi Schottky field emission SU5000,

Molecular and Cellular Imaging Center, The Ohio State University, Wooster, Ohio).

Hydrochar yield was determined as the ratio of the final dried hydrochar after HTC to the initial dried ADE feedstock.

Liquor characterization included visual, proximate and ultimate analyses. The same elements quantified in the hydrochar described above were measured in the liquor. The liquor was filtered and analyzed through gas and liquid chromatography. Volatile fatty acids (VFA) were measured by gas chromatography (GC-2014 Shimadzu Gas

Chromatograph) with a Stabilwax-DA column (30 m×0.32 mm×0.5µm). Sugars, furans, and phenolic compounds were measured by high-performance liquid chromatography

(HPLC, Agilent Technologies 1200 series) with a Rezex ROA-Organic acid H+

(Phenomenex®, Torrance, CA, USA). Chemical oxygen demand (COD) was conducted by a COD reactor block (HACH, DRB 200) and read using a COD photometer (Chemetrics).

43 3.4. Results and Discussion

3.4.1. Mass balance

At higher HTC severity conditions, similar trends were found for hydrochar during the downstream process regardless of the pH (Table 5). Higher severity conditions during

HTC decreased hydrochar sludge and increased gas yield. At higher HTC severity conditions compared to lower conditions, the dewatering of hydrocar sludge slightly decreased the hydrochar cake yield and increased the liquor yield, relative to the hydrochar sludge. After drying, based on the hydrochar cake, higher HTC severity conditions resulted in higher hydrochar recovered and lower vapor. Based on these results, it was observed that the dewaterability of hydrochar sludge tended to increase at higher HTC severity conditions.

44 Table 5. Mass balance for all the reaction temperatures and times studied for both pH

HTC condition after HTC after dewatering after dryinge Ta tb pHc HCd Gas HC Liquor HC vapor sludge cake °C min g g g % g % g % g % 163 50 O 98.1 1.9 18.6 18.9 79.6 81.1 6.4 34.3 12.2 65.7 180 30 O 94.2 5.8 19.8 21.0 74.4 79.0 6.2 31.6 13.5 68.4 180 70 O 97.1 2.9 19.2 19.7 77.9 80.3 6.4 33.3 12.8 66.7 220 22 O 89.3 10.7 20.3 22.8 69.0 77.2 6.1 30.2 14.2 69.8 220 50 O 85.3 14.7 18.1 21.2 67.2 78.8 5.9 32.4 12.2 67.6 220 78 O 88.6 11.4 17.5 19.8 71.1 80.2 6.1 35.0 11.4 65.0 260 30 O 83.3 16.7 16.4 19.7 66.9 80.3 6.0 36.4 10.4 63.6 260 70 O 79.3 20.7 13.9 17.6 65.3 82.4 5.8 41.5 8.2 58.5 277 50 O 77.7 22.3 13.2 17.0 64.5 83.0 5.7 43.6 7.4 56.4 163 50 M 99.1 0.9 18.8 18.9 80.3 81.1 5.9 31.3 12.9 68.7 180 30 M 94.6 5.4 16.7 17.6 77.9 82.4 5.6 33.5 11.1 66.5 180 70 M 99.3 0.7 19.2 19.3 80.1 80.7 6.0 31.3 13.2 68.7 220 22 M 92.6 7.4 15.6 16.9 77.0 83.1 5.5 35.4 10.1 64.6 220 50 M 88.7 11.3 17.6 19.9 71.0 80.1 5.5 31.0 12.2 69.0 220 78 M 89.6 10.4 15.4 17.2 74.2 82.8 5.6 36.3 9.8 63.7 260 30 M 88.8 11.2 14.1 15.9 74.6 84.1 5.5 38.9 8.6 61.1 260 70 M 79.8 20.2 12.2 15.2 67.6 84.8 5.1 42.2 7.0 57.8 277 50 M 81.1 18.9 12.1 14.9 69.0 85.1 5.2 42.8 6.9 57.2

Note: Starting material is 100 g of anaerobic digestion effluent. a Temperature; b time, c pH refers to original or modified pH of the anaerobic digestion effluent; and d hydrochar; e drying was carried out to a final hydrochar with 80% total solids.

3.4.2. Yield of hydrothermal carbonization products

Hydrothermal carbonization converted the ADE into hydrochar, liquor, and gas;

different process conditions yielded different percentage of products (Figure 4). Hydrochar

yield was significantly higher from ADE pH-O than ADE pH-M (p 0.0002) and ranged from 70% to 78% for pH-O and 62% to 73% for pH-M (Figure 4a and d). The effect of

45 initial pH on hydrochar yield is contradictory, some studies have found higher yields at

lower initial pH (Ghanim et al., 2017, 2018; Wikberg et al., 2015), while others have found

a decrease in hydrochar yield (Lu et al., 2014; Lynam et al., 2011; Reza et al., 2015). The

decrease in hydrochar yield at lower pH may be due to more dissolution and degradation

of components from the solids to the liquid fraction because of the presence of the H2SO4, as different compounds solubilize at different pH (Heilmann et al., 2014). More dissolution and degradation resulted in lower final solids, similar to the effect of higher temperatures, which also promote dehydration and depolymerization. Other researchers also found a decrease in hydrochar yield at lower pH (Ghanim et al., 2017; Lu et al., 2014; Lynam et al., 2011; Reza et al., 2015). The effect of pH from 2 to 12 on hydrochar yield was studied by Reza et al. (2015), and they found a decrease when the pH of the feedwater (water added to the feedstock prior HTC) was low. Lynam et al. (2011) used acetic acid (CH3COOH)

and found lower yields at lower pH because cellulose was more reactive at lower pH. Lu

et al. ( 2014) used hydrochloric acid (HCl) or H2SO4 to decrease feedwater pH to 2.2 and

found cellulose dissolution. Ghanim et al. (2017) concluded that the type of acid used to

decrease the pH also had an effect on hydrochar yield. Ghanim et al. (2017) used H2SO4

and CH3COOH and found that hydrochar yield increased with H2SO4 and decreased with

CH3COOH. Another potential explanation for not having observed an increase in hydrochar yield may have been the close distance between pH-O (7.8) and pH-M (6.6), compare to larger ranges of pH that others have studied (Lu et al., 2014; Reza et al., 2015).

Temperature during HTC had a significant effect on hydrochar yield for both pHs

(p 0.0052 and 0.0003 for pH-O and pH-M, respectively). As temperature increased, hydrochar yield decreased, this is because at higher temperatures the organic matter 46 dissolves and decomposes into the gas as found by others (Chen et al., 2017; Guo et al.,

2016; Peng et al., 2016; Xu & Jiang, 2017; Zhao et al., 2018). The highest hydrochar yields were found at 180°C and 70 min at both pHs and were 78% and 73% for pH-O and pH-M, respectively.

Liquor yields ranged from 9% to 20% and 17% to 27% for pH-O and pH-M, respectively (Figure 4b and e). They were significantly higher for ADE pH-M than ADE pH-O (p <0.0001) and decreased at higher HTC temperatures (p <0.0001 and 0.0003 for pH-O and pH-M, respectively). This is because at lower initial pH and higher temperatures more dissolution and degradation of components from the solids to the liquid fraction occurred, which increased hydrochar condensation and therefore improved its dewaterability, allowing more free water to be filtered. Gas yields ranged from 3% to 20% and 1% to 22% for pH-O and pH-M, respectively. Gas yield increased at higher temperatures. (Figure 4c and f).

47 a) d)

b) e)

c) f)

Figure 4. Effect of reaction temperature and time on hydrochar, liquor, and gas yields Hydrochar (a and d), liquor (b and e), and gas yields (c and f) for pH-O (a, b, and c) and pH-M (d, e, and f). Yield data is found in appendix A Table 12.

48 3.4.3. Hydrochar properties

3.4.3.1. Composition of hydrochar

Content of C, H, N, S, O, and ash in hydrochar were influenced by HTC conditions

(Figure 5). The C content of hydrochar ranged from 21.2% to 28.8%, for both pH-O and pH-M, which were slightly lower than the C content of ADE, 28.0%. The low C content in hydrochar is because of the low C content of the ADE, which is the result of the C being consumed by microorganisms during anaerobic digestion and its conversion to CH4 and

CO2. The C content of hydrochar from ADE of sewage sludge has been reported to be as low as 26% (Escala et al., 2013) and 28% (Berge et al., 2011), similar to the values found in this study. As expected, low C feedstocks produce low C hydrochar (Berge et al., 2011;

Escala et al., 2013; Wang et al., 2017). Hydrochar made of other feedstocks generally has higher C content (Berge et al., 2011; Wang et al., 2018). Hydrochar from food waste and woody biomass have C contents between 51% and 72% (Wang et al., 2018); and hydrochar made from coconut fibers (initial C content 48%) and eucalyptus leaves (initial C content

47%) have C contents of 67 to 78% and 61 to 72%, respectively (Liu et al., 2013).

Hydrochar with high C contents are produced from feedstocks with high initial C content.

According to the International Biochar Initiative (IBI, 2015) biochar with carbon content above 60% is categorized as class 1, between 30% and 60% is class 2, and from 10% to

30% is class 3. Our hydrochar is class 3. When HTC was performed at 220°C and 50 min for ADE pH-M, the distribution of carbon among the hydrochar, liquor, and gas fractions was 62%, 22%, and 16%, respectively, similar to an earlier report (Berge et al., 2011).

Thus, most of the C was retained by the solid fraction. 49 Hydrogen content for hydrochar was significantly higher in the hydrochar pH-M,

2.77 ±0.35%, than pH-O, 2.52 ±0.33% (p 0.0055). The difference in H content may be due to the extra H in the H2SO4 added to lower the pH. The content of H was not affected by duration of HTC (Figure 5), but as temperature increased, H content significantly decreased

(p 0.0014 and <0.0001 for pH-O and pH-M respectively), as previously reported (Liu et al., 2013). Nitrogen content decreased with increasing HTC severity. Sulfur, similar to H, was significantly higher in hydrochar pH-M due to the extra S added in the H2SO4 (p

<0.0001). Sulfur content remained almost unchanged from the S content of the ADE for both, pH-O and pH-M. Oxygen content decreased with increasing HTC severity, likely due to temperature enhancement of decarboxylation and dehydration reactions during HTC

(Chen et al., 2017).

Hydrochar ash content increased as temperature increased (Figure 5), perhaps because the minerals in the ADE became less soluble and precipitated, as previously suggested (Liu et al., 2013). Precipitated minerals are solids that contribute to the ash content. Ash content in the hydrochar ranged from 52.3% to 67.8% (Appendix A Table

12), similar to an earlier report where ash content was between 48% to 67% (Mäkelä et al.,

2015). Feedstocks with higher C content such as lignocellulosic biomass, generate less ash after HTC (Liu et al., 2013) because on a percent basis, C occupies a larger percentage of the biomass. Hydrochars from lignocellulosic biomass have ash contents from 6% to 14%, the higher values produced at the higher HTC temperatures (Liu et al., 2013). Elements contained in the ashes include Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn,

Mo, Na, Ni, P, Pb, Sb, Se, Si, Sr, Tl, V, and Zn (Appendix A Table 13).

50

Figure 5. Content of N, C, H, S, O, and ash in hydrochar at different HTC conditions compared to the ADE.

51 3.4.3.2. Fuel properties

The van Krevelen diagram displays O/C and H/C atomic ratios where lower ratios

indicate a higher aromaticity and higher solid fuel value (Mahmood et al., 2016; Sun et al.,

2017). Hydrochar from ADE of sewage sludge resulted in a more carbonized material than

the initial ADE (Figure 6). Anaerobic digestion effluent pH-O and pH-M started with an average O/C and H/C ratio of 0.86 and 1.29, respectively, and changed to O/C ratios between 0.01 and 0.44, and H/C ratios between 1.03 and 1.42 after HTC. The O/C and H/C ratios of commercial pyrochar are ≤ 0.6 (Spokas, 2010) and ≤ 0.7 (IBI, 2015), respectively.

Hydrothermal processes promote low O/C and H/C ratios through depolymerization, demethanation, dehydration, and decarboxylation (Tekin et al., 2014). There was a reduction in the O/C ratio, but not in the H/C ratio, attributed to C remaining mostly unchanged under all the different processing conditions, due to readily transformable C being consumed and converted into CH4 and CO2 during anaerobic digestion leaving more stable carbon in the ADE. The lower O/C ratios in hydrochar than ADE suggests the occurrence of decarboxylation and dehydration reactions during HTC. Lower O/C and H/C ratios also indicate higher degrees of carbonization and therefore more potential for it be used as solid fuel and higher stability in the soil when hydrochar is used as a soil amendment.

52

Figure 6. Van Krevelen diagram. Van Krevelen diagram of anaerobic digestion effluent of sewage sludge, hydrochar produced under different hydrothermal carbonization severity conditions, compared to lignocellulosic biomass, peat, lignite, coal, and anthracite (Ahmad & Subawi, 2013). Hydrochar O/C and H/C ratios shown are categorized in pH-O (blue dots) and pH-M (green dots) for temperatures below, equal to, and above 220°C.

The calorific values of ADE and its hydrochar were lower than other carbon-rich feedstocks (Figure 7). The maximum calorific value of hydrochar was 12.26 MJ kg-1 from

220°C and 22 min, which is similar to other values found for hydrochar from sewage sludge

(Peng et al., 2016). Most of the energy in the produced hydrochar from ADE has already

been converted to biogas during anaerobic digestion. No clear trend was observed in the

calorific value of hydrochar based on pH, reaction time, or temperature. There was no

53 significant difference in HHV between hydrochar pH-O and pH-M (p-value 0.1168). Thus, hydrochar from ADE of sewage sludge is not fit for use as a solid fuel.

Figure 7. Calorific value. Calorific value of anaerobic digestion effluent and hydrochar from this experiment compared to other carbon-rich feedstocks (Ahmad & Subawi, 2013). The bars for each compound represent the range of higher heating value measured (anaerobic digestion effluent and hydrochar) or reported. sb coal: sub-bituminous coal.

3.4.3.3. Potential use of hydrochar

Because of the low carbon content, high ash content, no reduction in the H/C ratio, and low calorific value, hydrochar from ADE of sewage sludge has more potential to be used as a soil amendment than as solid fuel. The low C content of the hydrochar was the reason for the lack of reduction in H/C ratio and the low calorific value. The high ash content is the most important factor preventing hydrochar from ADE of sewage sludge from being used as fuel (Wang et al., 2019). The ash content of coal has to be lower than

50% to be used as fuel (Radovic, 1997; Schweinfurth, 2009). The hydrochar produced from 54 ADE of sewage sludge had ash contents between 52.3% to 67.8%. However, the presence

of N, P, and K indicates that hydrochar can add nutrients to the soil when used as an

amendment.

3.4.3.3. Hydrochar morphology

The surface morphology and particle size of hydrochar produced under different

HTC conditions showed visual differences (Figure 8). Hydrochar produced at 163°C and

50 min (Figure 8b) have smoother sections, and larger particles than hydrochar produced at 277°C and 50 min (Figure 8d) which had a fine granular texture. Hydrochar produced at

220°C and 78 min (Figure 8a) seemed to have smaller particles compared to hydrochar at

220°C and 22 (Figure 8e), which are produced at the same temperature, but different reaction time. Based on the SEM images, reaction temperature seemed qualitatively to have a higher impact on morphology than reaction time, although no quantitative measurements were attempted.

55 a) b)

c) d)

e)

Figure 8. Scanning electron microscopy of hydrochar. a) 220°C, 78 min; b) 163°C, 50 min; c) 220°C, 50 min; d) 277°C, 50 min; e) 220°C, 22 min

56 3.4.3.4. Color for hydrochar and liquor

Hydrochar color darkened while liquor color lightened (Figure 9a and b) with increasing severity of the HTC conditions, similar to earlier reports (Hoekman et al., 2017;

H. Li et al., 2018; Xu & Jiang, 2017). Hydrochar darkening may be caused by the more carbonized material and Maillard and caramelization reactions, which is the heat-induced reduction of sugars and amino acids (Lam et al., 2012; Li et al., 2018). Also, higher ash contents are found in darker hydrochar (Li et al., 2018). In contrast, liquor lightening may be due to less total solids, and more dissolved and decomposed material than at lower HTC severity conditions.

a) b)

Figure 9. Color of hydrochar and liquor at different hydrothermal carbonization conditions. The arrangement in the figure follows the same arrangement as the central composite design (Figure 2).

57 3.4.4. Liquor composition and properties

3.4.4.1. Composition of liquor

The liquor contained organic and inorganic compounds, including sugars, volatile

fatty acids, furans, phenolic compounds, and polycyclic aromatic hydrocarbons dissolved

or suspended in water (Figure 10, and Table 6 and 7). Contents of C, H, N, S, O, and ash

were influenced by the initial pH of the feedstock (Figure 10). Contents of N, S, and O

were significantly higher in liquor pH-M (p <0.0001, <0.0001, and <0.0001, respectively);

while C was significantly higher in liquor pH-O (p <0.0001). No significant differences were found based on pH in H and ash contents. Sulfur content was higher for liquor pH-M due to the H2SO4 added to decrease the initial pH. Nitrogen contents ranged from 7% to

14%, C from 14% to 51%, H from 3% to 6%, S from 2% to 16%, and O from 15% to 43%

(Table 13). Temperature had the largest effect of all parameters on the content of C, H, N,

S, O, and ash. Contents of N, C, and H for both pH-O and pH-M decreased with

temperatures, and C and H also decreased with longer HTC times. Ash and S increased

with higher temperatures. However, O content in liquor pH-O, decreased with increasing

temperature and longer time, while in pH-M, liquor increased with temperature.

58

Figure 10. Content of N, C, H, S, O, and ash in liquor at different HTC conditions compared to the ADE.

59 Volatile fatty acids are formed when ADE is used as an HTC substrate (Berge et

al., 2011). In our study, VFA measured in the liquor included acetic, propionic, isobutyric, butyric, isovaleric, and valeric acids (Table 6). The concentration of total VFA appeared to be higher in liquor pH-M than pH-O (p 0.011). The total VFA and their concentration, significantly increased (p <0.0001 for pH-O, and <0.0001 for pH-M) as temperature increased, similar to earlier reports (Nilsson, 2017; Wang et al., 2019). Acetic acid, the predominant VFA produced during HTC by the decomposition of hydrolysis products

(Becker et al., 2014; Berge et al., 2011; Goto et al., 2004; Nilsson, 2017; Yan et al., 2010), was significantly higher in liquor pH-M than pH-O (p 0.0281) and increased with temperature.

60 Table 6. Volatile fatty acids of anaerobic digestion effluent and liquor at different hydrothermal carbonization conditions.

Temp. Time Acetic Propionate Isobutyric C min mg kg-1 mg kg-1 mg kg-1 ADE-O 1,932 ±261 384 ±15 230 ±24 163 50 1,135 ±51 247 ±4 103 ±3 180 30 1,295 ±18 254 ±3 107 ±0 180 70 1,136 ±52 242 ±2 100 ±1 220 22 1,556 ±29 272 ±1 110 ±2 220 50 1,571 ±37 288 ±4 114 ±3 220 78 1,622 ±30 265 ±1 112 ±2 260 30 2,222 ±39 328 ±5 118 ±1 260 70 1,945 ±32 309 ±2 110 ±2 277 50 1,991 ±84 346 ±5 119 ±3 ADE-M 1,436 ±114 311 ±5 135 ±12 163 50 1,442 ±50 285 ±13 139 ±1 180 30 1,205 ±10 249 ±2 ±102 ±1 180 70 1,771 ±26 279 ±2 ±114 ±7 220 22 1,634 ±5 275 ±4 110 ±3 220 50 1,897 ±32 308 ±11 120 ±3 220 78 1,906 ±26 303 ±12 114 ±1 260 30 2,006 ±178 304 ±14 109 ±2 260 70 2,351 ±24 350 ±3 120 ±6 277 50 2,033 ±36 318 ±5 117 ±4

ADE: anaerobic digestion effluent; pH-O: original pH; pH-M: modified pH.

61 Table 6 Continued

Temp. Time Butyric Isovaleric Valeric COD C min mg kg-1 mg kg-1 mg kg-1 g L-1 ADE-O 172 ±8 348 ±47 128 ±2 10.2 ±0.4 163 50 106 ±2 152 ±6 125 ±1 17.8 ±0.8 180 30 111 ±2 166 ±2 125 ±0 18.4 ±0.7 180 70 104 ±2 142 ±5 125 ±0 15.1 ±0.1 220 22 108 ±2 160 ±3 126 ±1 18.6 ±0.5 220 50 112 ±3 153 ±2 127 ±0 15.1 ±1.0 220 78 112 ±5 145 ±1 125 ±0 16.0 ±0.2 260 30 110 ±1 168 ±3 129 ±2 14.3 ±0.5 260 70 111 ±0 146 ±1 128 ±1 15.5 ±0.7 277 50 111 ±4 145 ±3 131 ±1 15.8 ±0.3 ADE-M 143 ±7 192 ±25 129 ±1 9.3 ±0.3 163 50 142 ±4 206 ±4 127 ±3 12.0 ±0.0 180 30 107 ±1 157 ±1 126 ±1 13.1 ±0.6 180 70 110 ±3 161 ±10 133 ±3 14.3 ±0.2 220 22 112 ±2 151 ±0 126 ±0 15.4 ±0.1 220 50 116 ±2 161 ±2 129 ±1 15.3 ±0.8 220 78 114 ±1 155 ±1 128 ±2 16.2 ±0.5 260 30 110 ±3 158 ±8 126 ±1 13.6 ±0.8 260 70 116 ±2 171 ±1 129 ±0 15.2 ±0.6 277 50 113 ±3 166 ±1 127 ±1 13.0 ±0.2

COD: chemical oxygen demand

Sugars, acids, and alcohol were identified in the liquor (Table 7). Sugars measured

included cellobiose, glucose, mannose, galactose, xylose, and arabinose. Acids included

succinic and lactic. Alcohols measured included xylitol, glycerol, and ethanol. Cellobiose,

glucose, and xylitol concentrations decreased in liquor from ADE pH-M, while mannose, galactose, and xylose concentrations increased. For liquor pH-O, the concentrations of

62 cellobiose, xylose, and succinic acid decreased with higher HTC temperatures, as sugars

degrade more at higher temperatures (Hoekman et al., 2011; Peterson et al., 2008).

Table 7. Sugars, acids, and alcohols identified in the liquor at different hydrothermal carbonization conditions.

Temp. Time Cellobiose Glucose Man-Gal-Xyl Arabinose C min ADE-O 41.1 0.0 8.5 ±5.0 163 50 43.7 ±0.7 41.2 ±1.9 180 30 51.4 ±2.4 57.5 ±1.8 53.3 ±0.6 180 70 45.9 ±2.4 20.3 0.0 84.6 ±1.0 73.7 ±1.0 220 22 47.4 ±0.4 61.0 ±2.1 39.3 ±0.7 220 50 43.7 ±0.0 49.0 ±0.0 25.4 ±0.0 220 78 35.2 ±1.8 64.7 ±27.8 27.0 ±18.8 260 30 0.0 50.4 ±2.9 14.9 260 70 0.0 52.9 ±2.6 0.0 277 50 0.0 69.1 ±1.7 38.1 ADE-M 6.5 ±3.1 8.7 ±1.9 163 50 53.6 ±1.3 62.1 ±1.5 180 30 37.7 ±3.4 92.5 ±3.8 69.4 ±1.3 180 70 34.6 ±2.4 36.4 0.0 97.2 ±1.8 91.0 ±1.2 220 22 35.5 ±0.8 69.2 ±2.3 33.5 ±0.3 220 50 18.5 29.9 ±0.0 62.3 ±0.0 24.0 ±0.0 220 78 15.8 25.8 ±0.2 62.5 ±0.7 16.2 ±0.5 260 30 61.9 ±1.1 12.0 ±4.3 260 70 77.8 ±1.0 14.2 ±0.7 277 50 67.8 ±3.0 33.8 ±2.3

63 Table 7. Continued

Temp. Time Xylitol-Succ. Ac. Lactic Acid Glycerol C min ADE-O 63.8 ±4.8 41.0 ±16.2 21.2 ±2.7 163 50 210.4 ±2.5 145.8 ±10.4 171.0 ±1.3 180 30 246.9 ±2.8 162.4 ±48.3 191.7 ±2.0 180 70 357.1 ±28.8 262.1 ±15.6 270.8 ±3.7 220 22 314.6 ±114.4 283.7 381.9 ±4.0 220 50 279.1 ±0.1 266.3 ±0.0 332.1 ±0.0 220 78 309.7 ±8.6 303.3 ±51.8 289.9 ±1.5 260 30 188.1 ±4.2 274.0 ±4.5 183.2 ±5.1 260 70 174.5 ±4.1 284.7 ±5.9 173.7 ±2.8 277 50 163.0 ±1.5 315.8 ±2.7 209.6 ±2.4 ADE-M 60.4 ±7.2 23.8 ±5.6 163 50 178.7 ±56.8 122.9 ±4.9 181.6 ±1.2 180 30 211.5 ±73.5 232.9 ±52.5 232.8 ±6.2 180 70 326.2 ±3.0 315.3 ±80.2 345.0 ±1.3 220 22 120.5 ±2.8 293.3 ±49.8 310.3 ±4.9 220 50 146.2 293.9 ±0.0 288.8 ±0.0 220 78 170.7 ±2.0 273.0 ±1.4 276.7 ±2.3 260 30 148.7 ±3.3 216.5 ±3.3 188.5 ±3.8 260 70 151.2 ±2.7 232.4 ±1.0 179.7 ±3.1 277 50 115.6 ±2.9 208.9 ±2.3 167.0 ±0.7

64 Table 7. Continued

Temp. Time Ethanol HMF Furfural C min ADE-O 35.0 ±3.4 2.5 ±0.1 3.8 163 50 83.5 ±6.9 3.0 ±0.6 180 30 113.9 ±5.8 4.1 ±0.2 180 70 183.3 ±10.4 8.8 ±1.4 220 22 329.4 ±9.0 9.6 ±4.8 220 50 282.5 ±0.1 10.3 ±0.0 220 78 212.3 ±4.1 11.8 ±0.7 27.5 260 30 158.5 ±5.5 7.5 ±3.8 36.3 ±4.4 260 70 147.8 ±5.1 9.8 ±3.6 24.9 277 50 182.1 ±3.1 16.8 ±1.0 44.0 ADE-M 34.0 ±3.2 3.0 ±0.7 3.8 ±0.2 163 50 101.4 ±5.3 4.2 ±0.2 3.5 180 30 138.7 ±15.3 3.8 ±0.9 3.6 ±0.0 180 70 202.7 ±12.2 16.3 ±1.1 7.0 ±1.5 220 22 303.8 ±19.3 16.4 220 50 280.5 ±0.0 14.5 ±0.1 23.6 ±0.0 220 78 203.3 ±3.3 8.0 ±5.9 0.0 260 30 181.2 ±4.9 20.6 ±4.3 0.0 260 70 179.1 ±5.0 21.1 ±1.3 0.0 277 50 201.1 ±28.0 31.0 ±8.8 14.2

Note: T: reaction temperature; t: time; COD: chemical oxygen demand; ADE: anaerobic digestion effluent; pH-O: original pH; pH-MP: modified pH; Glu: glucose; Man: mannose; Gal: galactose; Xyl: xylose; Ara: arabinose; Succ: succinic; Gly: glycerol; EtOH: ethanol; and Fur: Furfural.

3.4.4.2. Chemical Oxygen Demand

Chemical oxygen demand of the liquor at all HTC conditions for both pH

treatments was higher than that of the COD of ADE (Table 6). This is because many water- insoluble components become soluble after HTC (Appels et al., 2011). Chemical oxygen

demand values of the liquors from ADE pH-O and pH-M were 10.23 and 9.31 g L-1, 65 respectively, and ranged from 13.8 and 19.4, and 11.9 and 16.9 g L-1, for liquors pH-O and

pH-M, respectively. Typical COD values for liquor are between 10 and 40 g L-1 (Nilsson,

2017), and indicate a potential for biogas production if recirculated into anaerobic digestion

processes. The COD was significantly higher in the liquor pH-O than pH-M (p <0.0001).

Both temperature and time had significant effects on the COD for liquor pH-O (p 0.0007 temperature and 0.0022 time) and pH-M (p 0.0235 temperature and 0.0008 time). As

temperature and time increased, COD decreased in the liquor pH-O, while in liquor pH-M,

as time increased COD increased.

3.5. Conclusions

Hydrochar produced from ADE of sewage sludge was most impacted by reaction

temperature. Based on its properties, it has the potential to be used as a soil amendment.

At higher temperatures, hydrochar achieved higher degrees of carbonization and lower

yields. This is because higher temperatures promoted more dehydration and

decarboxylation of the ADE. Lower pH promoted more dissolution and degradation of

components from the solids to the liquid fraction, which resulted in lower hydrochar yields.

Hydrochar from ADE of sewage sludge resulted in high ash content and low C and H

contents. This is because of the high ash content and low C and H contents originally

present in the ADE. The H/C ratio did not show a decrease after HTC and the calorific

values of hydrochar were low compared to other carbon materials typically used as solid

fuels. Due to the hydrochar’s high ash content, low carbon content, and low calorific value, 66 hydrochar from ADE of sewage sludge has higher potential as soil amendment than as solid fuel.

For future studies, more parameters can be explored to better understand HTC. In the present study, ADE from a WWTP was used as feedstock. The ADE used was representative of one WWTP, but even for the same plant, the properties of the ADE change daily. Anaerobic digestion effluent of multiples plants and at different times of the year are encouraged to be analyzed. It is recommended for future studies to increase the range of pH using different acids and alkaline compounds to better understand the effect of pH and pH modifiers in the properties of hydrochar. It is also recommended to expand the range of reaction time to better analyze its effect on hydrochar properties.

67

Chapter 4. Effect of hydrochar from anaerobically digestated sewage sludge and manure as soil amendment on soil properties and plant responses

4.1. Abstract

Soil amendments have the potential to enhance the properties of the soil. Pyrochar

and hydrochar are soil amendments made of biomass and have been suggested to improve

nutrient availability, soil organic matter, and increase crop yield. Hydrochar is produced

through hydrothermal carbonization of wet biomass; pyrochar is produced through

pyrolysis of dry biomass. The objective of this study was to investigate the effects of

hydrochar from anaerobic digestion effluent (ADE) of sewage sludge, ADE of manure,

and commercial pyrochar as soil amendments on the soil properties and plant responses.

Hydrochar for this study was produced from the ADE of sewage sludge and manure. The

chars were amended to soil at 1, 3, 5, 10, and 15 g per kg of soil. Seedling flats were filled

with the char-soil mixtures, and lettuce (Lactuca sativa) was planted and set in a

greenhouse. Soil properties and plant responses including nutrient content, seed emergence, and above-ground biomass production were analyzed based on the char type and amendments rates, and were compared to no-char. Soil amended with hydrochar from

ADE of sewage sludge and manure had higher pH, phosphorus content, and cation exchange capacity compared to pyrochar and no-char. All char types and the no-char 68 treatment resulted in emergence rates higher than 80%. Roots grown in soil amended with

10 and 15 g kg-1 for all chars had more dry mass than the roots grown with no-char. No negative effects were observed in emergence and dry biomass, which indicated that hydrochar from sewage sludge and manure at rates from 1 to 15 g kg-1 could be used and applied to the soil without negative impact in the crop yield.

4.2. Introduction

Soil amendments are different than fertilizers, and are added to the soil to improve its physical characteristics in order to provide a better environment for the roots (Davis &

Whiting, 2013). Amendments satisfy plants’ needs indirectly by modifying the properties of the soil, such as pH and structure; fertilizers do it directly, by providing nutrients to the soil and plant. Soil amendments have the potential to sequester carbon (Sohi et al., 2010); reduce greenhouse gas emissions (Karhu et al., 2011); and improve soil properties

(Kammann et al., 2012) by increasing pH (Fellet et al., 2011), cation exchange capacity

(CEC) (Fellet et al., 2011; Kalderis et al., 2018), soluble organic matter (Kalderis et al.,

2018), carbon storage (Raimundo et al., 2019; Zwieten et al., 2010), and water holding capacity (Fellet et al., 2011; Raimundo et al., 2019; Soja et al., 2016); retain and supply nutrients (Eibisch et al., 2015; Nelson et al., 2011); remove pollutants from soil-water

(Eibisch et al., 2015); and provide higher stability of soil structure and pore volume. These further enhances crop production.

69 The effects of amendments on soil and plants depend on several factors, such as the

nature of the feedstock for the amendment, method of application to the soil, properties of

the soil (Dugan et al., 2010), and type of crop being grown (Bargmann et al., 2014). The

nature of the amendment refers to the feedstock used (Dieguez-Alonso et al., 2018),

feedstock conversion method (Dieguez-Alonso et al., 2018), and process conditions

(Lehmann et al., 2011; Zhang et al., 2008). Methods of application refers to its application

on the surface or any depth below the surface, in solid or liquid form, rate of application,

frequency, and history of application. The properties of the soil include type of soil, texture,

structure, water content, and weather.

Biochar is a carbonized material derived from biomass, including plant matter, and

organic waste from agriculture, food, and municipal systems, which has the potential to be used as soil amendment. Biochar can be produced through pyrolysis of dry biomass and hydrothermal carbonization of wet biomass. Biochar produced through pyrolysis is called pyrochar (Jian et al., 2018), while biochar produced through hydrothermal carbonization is called hydrochar. The most common feedstock for pyrochar is lignocellulosic biomass,

for hydrochar it can be any wet biomass such as sewage sludge or manure. Higher pyrolysis

temperatures make the pyrochar more stable in the soil (Brewer et al., 2012; Kammann et al., 2012).

Pyrochar and hydrochar have the potential to improve soil properties and therefore crop quality; however, they sequester carbon differently. The carbon content in pyrochar is more stable and less available in the soil, therefore a better method for carbon

sequestration compared to hydrochar (Breulmann et al., 2017; Busch & Glaser, 2015).

Until 2010, carbon sequestration was the most important reason to apply pyrochar to the 70 soil; however, more recently it is also applied for agricultural purposes to improve soil

properties and crop yields (Sohi et al., 2010). Pyrochar has the potential to reduce nitrous

oxide generation in the soil by increasing immobilization of individual nitrogen species

(Bergman et al., 2015; Renner, 2007; Soja et al., 2016; Yanai et al., 2007). The mineral

contents of pyrochar and hydrochar increase soil productivity (Nelson et al., 2011) and

improve its fertility (Raimundo et al., 2019). Pyrochar and hydrochar of corn digestate,

miscanthus, and woodchips have also proven to reduce the availability of contaminants in soils, such as isoproturon, by the sorption capacity of chars (Eibisch et al., 2015).

Previous research on pyrochar and hydrochar as soil amendment have considered variations in feedstock, application rates, particle sizes, combination with other amendments, toxicity, crop production, and purpose of application (Breulmann et al., 2017;

Busch et al., 2013; Castellini et al., 2015; Dugan et al., 2010; Karhu et al., 2011; Paneque et al., 2019; Puccini et al., 2018; Roig et al., 2012; Wagner & Kaupenjohann, 2014; Yuan et al., 2016; Yue et al., 2017). Feedstock used in the production of pyrochar has included lignocellulosic biomass (Anderson et al., 2011; Castellini et al., 2015; Wagner &

Kaupenjohann, 2014) and sewage sludge (Yuan et al., 2016; Yue et al., 2017); for hydrochar, lignocellulosic biomass (Puccini et al., 2018; Wagner & Kaupenjohann, 2014), sewage sludge (Yue et al., 2017), food waste (Kalderis et al., 2018), and agricultural waste

(Raimundo et al., 2019). Lignocellulosic biomass as feedstock for both pyrochar and hydrochar has been used for remediation while the response of plant development has been assessed (Eibisch et al., 2015; Wagner & Kaupenjohann, 2014). Anaerobic digestion effluent (ADE) of sewage sludge has been applied directly to the soil (Roig et al., 2012), but also used as feedstock for both pyrochar and hydrochar (Breulmann et al., 2017; Yue 71 et al., 2017). Pyrochar and hydrochar from sewage sludge has been studied regarding their

carbon sequestration potential and nutrient content (Breulmann et al., 2017).

Amendment rates for pyrochar have ranged from 2.5 to 100 g of char kg-1 of soil

(Anderson et al., 2011; Castellini et al., 2015; Dugan et al., 2010; Karhu et al., 2011);

however, no conclusions have been achieved regarding amendment rates, since different

studies have used different types of char, feedstocks, and plants. Pyrochar and hydrochar

amendment to soil have been tested with barley (Owsianiak et al., 2018), lettuce (Kalderis

et al., 2018; Puccini et al., 2018; Trupiano et al., 2017; Upadhyay et al., 2014), lucerne

(Owsianiak et al., 2018), potato (Upadhyay et al., 2014), ryegrass (Paneque et al., 2019),

sugar beet (Owsianiak et al., 2018), switchgrass (Ashworth et al., 2014), turf grass (Yue et

al., 2017), and wheat (Castellini et al., 2015; Owsianiak et al., 2018).

Hydrochar from ADE of sewage sludge and manure has the potential to enhance

soil properties and crop production. However, to date, most research has focused on using

sewage sludge, manure, and ADE from both, as soil amendments, but not hydrochar from

ADE of sewage sludge or manure. To better understand the effects of hydrochar on soil and plants and promote its use, its properties and impacts need to be analyzed. Therefore, the objective of this study was to investigate the effects of hydrochar from anaerobic digestion effluent (ADE) of sewage sludge, ADE of manure, and commercial pyrochar at

different rates as soil amendments on soil properties and plant responses.

72 4.3. Methodology

4.3.1. Experimental design and data analysis

Three different types of chars were amended to the soil at five different rates and were compared against soil with no char (control) to evaluate their effect on soil properties and plant development. The three types of chars included hydrochar from ADE of sewage

sludge (HC-ADE-SS), hydrochar form ADE of manure (HC-ADE-M), and commercial

pyrochar (PC) from lignocellulosic biomass. The five amendment rates for the three char

types were based on rates previously used in the other studies (Ahmad et al., 2014;

Castellini et al., 2015; Dugan et al., 2010; Karhu et al., 2011), and were 1, 3, 5, 10, and 15 g char kg-1 of soil (i.e., 0.1, 0.3, 0.5, 1.0, and 1.5 wt. % of soil or 2.0, 5.9, 9.8, 19.6, and

29.4 ton ha-1). These in addition to the control, which was soil with no char, resulted in a

total of 16 different soil/char mixtures. All treatments were replicated five times in a full

factorial completely randomized design. Response variables included soil properties,

emergence rates, and biomass production properties such as growth index, dry weight, and

composition including aluminum (Al), arsenic (As), boron (B), barium (Ba), beryllium

(Be), calcium (Ca), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe),

potassium (K), lithium (Li), magnesium (Mg), manganese (Mn), molybdenum (Mo),

sodium (Na), nickel (Ni), phosphorus (P), lead (Pb), sulfur (S), antimony (Sb), selenium

(Se), silicon (Si), strontium (Sr), thallium (Tl), vanadium (V), and zinc (Zn). The effects of

char type and amendment rate were analyzed. Analysis of variance (ANOVA) was performed to test for significant effects, alpha values <0.05 were considered significant.

Tukey’s post-hoc test was used for mean comparison when ANOVA was found to be 73 significant. Differences between means in graphs are shown by different letters. The data

were analyzed using JMP Pro® 14.

4.3.2. Soil, chars, and amended soils

The soil used for the experiment was a Wooster-Riddle silt loam and was collected from a previously grass-covered area from the top 20 to 30 cm in Wooster, Ohio. The soil

was sieved at 5 mm and amended with different chars at the specified rates. The soil/char

mixtures analysis were conducted at an external laboratory (Spectrum Analytic,

Washington Court House, Ohio) following standard procedures (Soil and Plant Analysis

Council Inc., 1999), and included pH, organic matter (SOM), P, K, Mg, Ca, CEC, K

saturation, Mg saturation, Ca saturation, K/Mg ratio, and Ca/Mg ratio. Element saturation

refers to the percentage of the CEC that is occupied by a particular element (Culman et al.,

2019; Spectrum Analytic, n.d.). Potassium, Mg, and Ca are the three major cations that

have an alkaline reaction in the soil, the sum of their saturation is called base saturation

(Culman et al., 2019; Spectrum Analytic, n.d.). Hydrochar was produced from ADE of

sewage sludge and manure through hydrothermal carbonization at 220°C for 50 min, which

was considered as an optimal condition from a previous study (Chapter 3). Pyrochar was

produced from lignocellulosic biomass and was purchased from WakefieldTM BioChar.

74 4.3.3. Greenhouse study

4.2.3.1. Emergence assay

The assay was conducted to test the effects of char types and amendment rates on seed emergence. Rouxai red Lettuce (Lactuca sativa, Johnny’s Selected Seeds) was used as a bioindicator crop because it is sensitive to soil properties (IBI, 2015; Zwieten et al.,

2010). The emergence assay followed a full factorial completely randomized design with

16 treatments, 5 replicates per treatment, and 10 plants per replicate (Figure 11). Lettuce seeds were planted in 200-cell seedling flats and emergence rates were recorded from day

4 up to day 25 after sowing. Emergence rate was calculated as the percentage of plants emerged above-ground per replicate from day 4 to day 10 after sowing.

75 Rep 1 Char rate Char type Plants/rep g/kg 1 3 PC 5 1 10 10 15 1 3 Rep 2 Rep 3 Rep 4 Rep 5 HC-ADE-M 5 1 10 10 15 1 3 HC-ADE-SS 5 1 10 10 15 No-char 0 1 10

Figure 11. Layout of the experimental designs for the emergence and lettuce growth assay. There were 16 different treatments, 5 for pyrochar (PC), 5 for hydrochar from anaerobic digestion effluent from manure (HC-ADE-M), 5 for hydrochar from anaerobic digestion effluent from sewage sludge (HC-ADE-SS), and 1 for no-char, and five replicates per treatment. For the emergence assay there were 10 plants per replicate. For the biomass assay, there was one plant per replicate selected randomly from among the 10 plants used in the germination assay.

4.2.3.2. Lettuce growth assay

The lettuce growth assay was conducted to study the effects of char types and

amendment rates on growth index, dry biomass production, and composition of lettuce’s

roots and leaves. After the emergence assay, five plants per treatment (one plant per

replicate) from the emergence assay were randomly selected and transplanted into pots

(355 mL) in the same char/soil mixture that they were in the seedling flats. The lettuce

growth assay followed a full factorial completely randomized design with 16 treatments 76 and five replicate per treatment (Figure 11). Pictures and growth index measures were

taken for all the plants before harvest (60 days after sowing). The growth index ([widest

width +perpendicular width +height]/3) provided an indication of the size of the plant

(Shreckhise et al., 2018). All the lettuce plants were harvested at the soil level. The fresh

and dry weights of the harvested lettuce’s roots and leaves were recorded. Roots and leaves were dried at 60°C for 72 hr. Elements composition of one replicate per treatment was analyzed by Inductive Coupled Plasma-Optical Emission Spectrometry (ICP-OES, Agilent

5110, Service Testing Analysis Laboratory, The Ohio State University, Wooster, Ohio) for determining the content in lettuce roots and leaves when grown in different amendments and rates. Not all the replicates were analyzed due to resources constraints.

4.4.Results and Discussion

4.4.1 Properties of amended soils

Char types and amendment rates had a significant effect in most of the soil properties analyzed (Figure 12 and Appendix B Tables 14 and 15). Effects of amendment rate within char types were significant in PC for pH and Mg content; in HC-ADE-SS for P content; and in HC-ADE-M for pH, contents of P, K, Mg, Ca, and saturations of K, Mg, and Ca (p-values for significant effects based on char types are shown in Appendix B Table

14).

Differences in soil properties were more apparent at the higher amendment rates when compared to the lower rates (Figure 12). Significant effects were found for P content 77 at 1 and 3 g kg-1; and for P, pH, and saturation of Mg and Ca at 5 g kg-1. Significant effects

at 10 g kg-1 were found for all the properties described above and for content of K, Mg, and K saturation. In addition to the parameters with significant effect at 10 g kg-1, the

effects of SOM, Ca content, and CEC were also significant at 15 g kg-1. Ratios of K/Mg

and Ca/Mg did not show any clear trend by char type or amendment rate (p-values for

significant effects based on amendment rates are shown in Appendix B Table 15).

78

Figure 12. Effects of char types and amendment rates on soil properties.

79 Soil properties shown include pH, CEC, P, SOM, K, Mg, Ca, K saturation, Mg saturation, Ca saturation, K/Mg ratio, and Ca/Mg ratio. Points refer to the mean of 3 replicates, error bars represent the standard deviation, and horizontal or incline lines represent the trend of the measurements with respect to the amendment rate. Comparisons between rates within each char type are performed comparing the data points of the same char type (same color); comparisons between chars within the same rate are performed vertically comparing the three data points in the same rate (same column). P-values and letter report for mean comparison are included in the Appendix B Tables 14 and 15.

For most of the properties, the highest average values were for the soil amended with HC-ADE-M (Figure 13). It had significantly higher values for pH, P, K, Mg, K

saturation, Mg saturation, and Ca saturation (p-values in Appendix B Table 14). The

prevalence of higher values for most of the soil properties belonging to soils amended with

HC-ADE-M may be because of the nature of the manure, which still contains undigested

agricultural residues and these residues containing residual agricultural inputs such as

nutrients from fertilization. Soil amended with HC-ADE-SS reported the highest values for

Ca/Mg ratio (Appendix B Table 14).

80

Figure 13. Soil properties for the control treatment compared with the maximum values. Maximum average values (blue bars) for all the soil properties measured versus the control (orange bars). The numbers next to the orange bars refers to the value of the control; the number next to the blue bars refers to the maximum value measured, corresponding to a char type and amendment rate.

At higher char amendment rates, pH, and content of P and Mg increased, except for pH in HC-ADE-SS. The addition of PC and HC-ADE-M increased the pH value of the soil from 7.00 to 7.06 and 7.19, respectively. Similar trends have been observed elsewhere by using pyrochar from lignocellulosic material (Carter et al., 2013; Nelson et al., 2011;

Trupiano et al., 2017), where the pH of soil have increased from 6.9 to 8.0. The increase 81 in pH by the addition of char may be due to the feedstock from which the char was made.

In this study, an increase of pH was found when PC and HC-ADE-M were amended.

Pyrochar was made from lignocellulosic biomass and HC-ADE-M contain bedding

material, and undigested plant material, which still contained lignocellulosic biomass, and

that may have been a factor for the increase in pH. Another reason why an increase in pH

has been observed with pyrochar is because it has liming potential, however its effect is

dependent on the type of char and soil (Nelson et al., 2011). The alkalization of the soil by

the addition of chars could also be beneficial for earth worms and other soil

microorganisms (Beesley & Dickinson, 2011; Trupiano et al., 2017). Soils amended with

HC-ADE-SS had lower pH compared to the control, which is similar to the findings of

George et al. (2012), who found lower pH when they amended hydrochar from spent

brewer’s yeast; however, no reason was provided for the decrease in pH. The reduction in

pH in this study may be associated to the nature of the HC-ADE-SS, since sewage sludge

is highly heterogeneous.

Amendment rate had a significant effect on P content. Higher P contents were found at higher amendment rates. The effect of rates on P content was higher in HC-ADE-M and

HC-ADE-SS than pyrochar. This may be due to the presence of higher P in organic waste chars (manure and sewage sludge) compared to lignocellulosic biomass chars (Schneider

& Haderlein, 2016; Zhang et al., 2016; Zhao et al., 2015).

4.4.2. Effect of chars on lettuce emergence

All the treatments resulted in emergence rates higher than 80% (Figure 14). Lettuce started to emerge on day 4 after sowing. After day 10, the number of plants remained 82 unchanged in all the treatments. The highest and the lowest emergence percentages based

on char type were for the soils amended with pyrochar and HC-ADE-SS, respectively.

All the amendment rates in all char types, except 1 and 5 g kg-1 of HC-ADE-SS,

resulted in higher rates than the control at 10 days after sowing; however, not all were significantly different. The highest emergence rate per char type at day 10 were at1 g kg-1

for PC, 5 g kg-1 for HC-ADE-M, and 3, 10, and 15 g kg-1 for HC-ADE-SS, from which

HC-ADE-M at 5 g kg-1 was the only significantly higher emergence rate compared to the

control (p 0.0361). The highest emergence rate for day 4 after sowing was pyrochar at 3 g

kg-1 and from day 5 to 10 HC-ADE-M 5 g kg-1. When comparing char types within the

same amendment rate for 1, 3, 5, 10 and 15 g kg-1 at day 10 after sowing, the highest

emergence per char type within specific rates were for PC (1 g kg-1), HC-ADE-M (3 g kg-

1), HC-ADE-M (5 g kg-1), HC-ADE-M (10 g kg-1), and both PC (15 g kg-1) and HC-ADE-

M (15 g kg-1), respectively. Significant differences between char types at specific

amendment rates at 10 days after sowing were found just at 1 g kg-1 where the emergence

rate of lettuce was significantly higher in soil amended with pyrochar compared to soil

amended with HC-ADE-SS (p 0.0398).

83

Figure 14. Effect of char rates per char type on lettuce emergence from day 4 to 10. From top to bottom, emergence of lettuce grown in (a) soils amended with in pyrochar (PC), (b) hydrochar from anaerobic digestion effluent from manure (HC-ADE-M), and (c) hydrochar from anaerobic digestion effluent from sewage sludge (HC-ADE-SS). ANOVA and mean comparisons were performed just for emergence at day 10 after sowing. Mean comparison between amendment rates within char type was performed in hydrochar from anaerobic digestion effluent from manure (HC-ADE-M) (b), which was the only char type that had a significant effect of amendment rate. Mean comparison for amendment rates within char types is shown in the lower-case letters. Mean comparisons between char types at the same amendment rates were performed using upper-case capital letters for 1, 5, and 15 g kg-1, which were the rates that showed significant effects of char type.

84 An increase in emergence rate was observed compared to the control at all

amendment rates (except for HC-ADE-SS 1 g kg-1), which differs to what others have

found (Busch et al., 2012; Kalderis et al., 2018; Libra et al., 2011). Others have attributed

the decrease in emergence to the phytotoxicity of the chars (Busch et al., 2012; Kalderis et al., 2018). Some studies have explored the effect of the post production treatments for pyrochar and hydrochar to reduce potential toxicity to the soil and plants (Busch et al.,

2013; Kalderis et al., 2018; Yue et al., 2017). They have found that washing the hydrochar

(Busch et al., 2013; Puccini et al., 2018) or letting it mature (Kalderis et al., 2018) allow the hydrochar to lose some of the phytotoxic compounds that damage the plants. Kalderis et al. (2018) used fresh and processed hydrochar from orange peels (mature 4 weeks and washed) on barley and found a negative effect on germination with fresh hydrochar, but less phytotoxicity when the hydrochar was washed or matured. The phytotoxicity of the hydrochar has been attributed to the presence of phenols, organic acids, and furfurals that are typically found in hydrochar, regardless of the feedstock (Kalderis et al., 2018; Libra et al., 2011).

Germination studies have been performed using pyrochar and hydrochar from the same feedstock and have found that pyrochar have not negatively affected the germination while hydrochar have decreased it (Bargmann et al., 2013; Busch et al., 2012). These results have been observed on barley when pyrochar and hydrochar from wet beet-root chip at 50, 100, and 250 g kg-1 (5, 10, and 25% vol) (Busch et al., 2012), and hydrochar and

pyrochar from the same feedstock at 20, 40, and 100 g kg-1 (2, 4, and 10% w/w) (Bargmann et al., 2013) have been used. The inhibitions have been attributed to the substances produced during HTC (e.g. phenols), which are different than the compounds produced 85 during pyrolysis (e.g. PHAs). No negative effects for hydrochar amendment were found in

the emergence assay.

4.4.3. Effect of chars on plant development

Dry weights of the roots, leaves, and whole plant from all char types at all amendments rates were heavier than the control (Figure 15). There were significant effects in the dry weight of roots between amendment rates within plants grown in soils amended with HC-ADE-M (p 0.0004) and HC-ADE-SS (p 0.0027), but not within plants with PC.

Plants in soil amended with hydrochar were heavier than the ones in soils with PC and the control. The heaviest dry weight for plants, leaves, and roots were 0.49 g (HC-ADE-M at

10 g kg-1), 0.34 g (HC-ADE-SS at 3 g kg-1), and 0.14 g (HC-ADE-M at 15 g kg-1), compared to 0.30 g, 0.23 g, and 0.07 g for the control, respectively. The dry weights of roots were significantly higher in soils amended with HC-ADE-M at 10 and 15 g kg-1 than

in the control (p 0.0094 and 0.0026, respectively). Higher amendment rates resulted in

heavier dry roots, but this trend was not the same for leaves, whole plant, and growth index.

No significant effects were observed for growth index based just on char type or

amendment rate; however, visual differences were observed in the plants (Figure 16). The

highest value for growth index was 8.7 (PC 15 g kg-1) and the lowest 6.8 (HC-ADE-M 5 g

kg-1).

86

Figure 15. Effect of char types and amendment rates on dry weights of the whole plants, roots, and leaves, and growth index. From top to bottom, dry weights of the whole plant (green), roots (blue), and leaves (orange), and growth index (GI) (purple dots) for lettuce grown in soils amended with (a) pyrochar (PC), (b) hydrochar from anaerobic digestion effluent from manure (HC-ADE- M), and (c) hydrochar from anaerobic digestion effluent from sewage sludge (HC-ADE- SS). GI ‘y’ axis is in the right, from 0 to 10. Mean comparison between amendment rates within char type was performed in HC-ADE-M, which was the only char type that had a significant effect of amendment rate for roots dry weight and is shown in lower-case letters. Mean comparisons between char types at the same amendment rates were performed using upper-case capital letters for 1, 5, and 15 g kg-1, which were the rates that showed significant effects of char type for roots dry weight. Significant effects in GI are shown at 3 g kg-1 between char types. 87 a) b)

c) d)

e) f)

g) h)

Figure 16. Lettuce grown in soil amended with different char types 60 days after sowing. A profile (a, c, e, and g) and top (b, d, f, and h) pictures are shown for lettuce grown in the control (a and b), soils amended with pyrochar (c and d), hydrochar from anaerobic digestion effluent from manure (HC-ADE-M) (e and f), and from sewage sludge (HC- ADE-SS) (g and h).

88 At all amendment rates of PC, HC-ADE-M and HC-ADE-SS, the weights of the whole plants, leaves and roots were higher compared to the control. These findings support and contradict what others have found (Bargmann et al., 2014; Kalderis et al., 2018; Rillig et al., 2010; Roß et al., 2015; Schimmelpfennig et al., 2014; Upadhyay et al., 2014; Wagner

& Kaupenjohann, 2014; Yue et al., 2017). An increase in dry biomass or yields have been found in lettuce leaves with soils amended with pyrochar from green waste at rates from 5 to 50 g kg-1 (10 to 100 ton ha-1) (Upadhyay et al., 2014); with pyrochar on Avena sativa

compared to the control (Wagner & Kaupenjohann, 2014); on fermented hydrochar from

corn silage and digestate on spring wheat shoots (Roß et al., 2015); with pyrochar and

hydrochar from Miscanthus × giganteus on Lolium perenne (Schimmelpfennig et al.,

2014); with pyrochar from peanut hull, on Lolium perenne (Kammann et al., 2012); with

pyrochar from sewage sludge, on grass (Yue et al., 2017); with hydrochar from spent

brewers grain at 60 g kg-1 (60 ton ha-1 at 0.1 m depth) on barley (Bargmann et al., 2014),

with hydrochar from beet-root chips at 4% wt. (60 ton ha-1 at 0.1 m depth) (Bargmann et

al., 2014), with hydrochar on Phaseolus beans; and with hydrochar from beet-root chips

and spent brewers’ grain on beans (Bargmann et al., 2014). Other researchers have found

reduction on plant dry matter or yields of Avena sativa when plants were grown in soils

amended with hydrochar from poplar wood (Wagner & Kaupenjohann, 2014); with

hydrochar from orange peels on corn yield due to phytotoxicity (Kalderis et al., 2018); and

with hydrochar from beet root chips at > 10% vol on taraxacum (Rillig et al., 2010).

The decrease in dry biomass and yield in the literature have been attributed to

phytotoxic compounds in the hydrochar; increase in dry biomass has been attributed to

improved soil properties a more nutrient availability. In this study, char increased pH and 89 P content, and consequently nutrient availability, which may be a reason why more biomass

was produced than with the control. Negative effects on dry matter have been attributed to

phytotoxicity of the hydrochar; however, in this experiment HC-ADE-M and HC-ADE-SS were used without any additional process and no negative effects were observed.

4.4.4. Effects of chars on nutrient content of lettuce

Elements in lettuce were more concentrated in the roots than in the leaves, and few elements showed significant differences with respect to char type (Table 8). Roots grown in soil with HC-ADE-SS had higher concentration in 19 out of 26 elements. In the case of leaves, most of them had lower concentrations compared to the leaves in the control.

Macro- and micro-nutrients contents showed some trends in the lettuce tissue.

(Table 8) Macronutrients present in the lettuce included K, Ca, P, Mg, and S. Higher

concentration of Ca in the char treatments was found in the roots compared to the control.

Roots in char treatments generally showed higher concentrations of macronutrients

compared to the control, contrary to leaves that showed less concentration of

macronutrients. Micronutrients found in lettuce included Fe, Mn, B, Cu, Zn, and Mo. Roots

grown in HC-ADE-SS showed higher concentrations in all the micronutrients compared to

the control. The content of Mo was higher for both, roots and leaves, in all char treatments

compared to the control. Boron was higher in all roots of char treatments compare to the

control. The purpose of hydrochar as soil amendment is not as a fertilizer; however, its

ashes contain nutrients that benefits the plants (Fei et al., 2019). The higher emergence rate

and dry weights of lettuce grown in soils amended with chars may be because chars

increased the content of some of the required nutrients for plant development. 90 There are some concerns about the use of hydrochar as soil amendment because of the presence of heavy metals such as As, Pb, and Cd, for risks of toxicity (Kambo & Dutta,

2015). Arsenic was found at lower concentrations in lettuce grown in PC, HC-ADE-M, and HC-ADE-SS compared to the control, except for the lettuce leaves in PC. Lead was found in lower concentrations than the control in roots of PC and HC-ADE-M, and leaves of HC-ADE-SS, but higher concentration in roots of HC-ADE-SS and in leaves of HC-

ADE-M. Cd was not detected in leaves. Cadmium in roots was lower in all char treatments compared to the control. No increased risk of contamination by As, Pb, Cd was proven.

91 Table 8. Elements measured in lettuce roots and leaves from the highest to lowest concentration.

µg g- NC PC HC-ADE-M HC-ADE-SS 1 Roots Leaves Roots Leaves Roots Leaves Roots Leaves K ab17,300 58,037 ab18,900 53,970 a21,332 56,892 b14,196 54,690 Na 8,477 8,431 9,999 8,183 8,353 8,848 9,936 7,701 Ca 7,954 x10,632 8,503 x10,673 8,996 y9,033 9,096 x10,785 Mg 5,435 3,364 5,291 3,298 6,371 2,764 6,040 3,292 Al 5,227 544 4,882 669 4,774 722 6,233 587 Fe 4,744 496 4,611 629 4,601 688 6,695 550 S 2,689 x2,643 2,828 x2,440 2,492 y1,897 2,916 xy2,263 P 2,458 x2,918 2,788 x2,907 2,369 y2,361 3,128 x2,972 Si b339 292 a531 326 b390 322 b374 295 Mn 116 69 118 56 133 71 249 58 Cu ab85.24 x11.67 a87.53 y8.50 b52.18 y8.07 a88.57 y7.07 Ba 72.68 14.76 64.55 14.23 81.86 12.07 93.03 13.06 Zn 46.70 x63.24 45.08 y38.48 43.77 y39.70 92.29 y37.79 Sr 33.69 x22.77 36.82 x23.21 38.32 y18.56 36.47 x21.76 B 19.16 29.17 23.79 27.52 21.70 27.99 19.85 29.42 V 11.33 1.27 10.89 1.47 11.19 1.59 13.98 1.32 Cr 6.729 5.021 0.951 5.137 1.084 7.805 1.127 Ni 6.623 0.692 5.412 0.840 5.500 0.798 7.602 0.667 Pb 5.188 0.617 4.389 0.521 4.925 0.780 7.105 0.690 As 4.088 4.616 1.869 2.787 3.703 Co 2.484 2.297 2.697 3.932 Se 1.378 1.745 2.309 1.957 4.397 1.452 2.006 2.828 Li 1.2740 0.3103 0.2112 0.2390 0.1971 0.2468 0.9337 0.2439 Cd 0.9779 0.9091 0.9491 1.1009 Mo 0.5929 1.2279 0.3185 1.4563 0.2519 1.1876 0.2815 Sb 2.9610 0.5238 1.7608 0.7473 1.3141 0.9386 1.5490

Each element in a row represents the average of each char type for roots or leaves. The colors in the char type (PC, HC-ADE-M, and HC-ADE-SS) indicate if the element content of the roots or leaves is higher (blue) or lower (orange) compared to the control. White cells mean that no data was available. Values preceded with letters showed statistical significance in roots (a,b) and leaves (x,y). Elements are arranged from highest to lowest concentration. Full data can be found in Appendix B Table 16.

92 4.5. Conclusions

Hydrochars from ADE of sewage sludge and manure at rates from 1 to 15 g kg-1

have the potential to be used as soil amendment for lettuce production. The properties of

soil and plants from soils amended with hydrochar responded similar to or better than the

properties of soil and plants amended with commercial pyrochar or the control (i.e., no

char). The presence of beneficial elements in the ashes of hydrochar may have caused the

increase in emergence and plant development compared to the control. No negative effects

were observed in emergence or dry biomass, which indicates that hydrochars from ADE of

sewage sludge and manure can be applied to the soil.

The results from this study are specific to the chars and rates used on a Wooster-

Riddle silt loam soil on a specific variety of lettuce. The effects of char on soil properties and plant responses depend on the feedstock used, conversion method, process conditions, application method of the char, rate of application, frequency, history of application, properties of the soil, and weather. Therefore, for future studies, it is recommended to consider several of these parameters to better understand the effects of hydrochar on soils and plants.

93

Chapter 5. Techno-economic and direct carbon dioxide emissions analyses of a combined anaerobic digestion and hydrothermal carbonization system

5.1. Abstract

In the United States, about 10% of the wastewater treatment plants use anaerobic digestion (AD) to treat sewage sludge, and the other 90% dewater the sewage sludge and manage it through landfilling, incineration, or land application. A potential technology to treat anaerobic digestion effluent is hydrothermal carbonization (HTC), which is a thermochemical conversion method that works at high temperatures and pressures. A combined AD-HTC system was proposed in this study to treat sewage sludge and produce hydrochar, which has the potential to be used as soil amendment. The objective of this study was to analyze the techno-economics and direct carbon dioxide emissions of the combined AD-HTC system and transportation of hydrochar to the field. Process modeling of the combined AD-HTC system was performed in SuperPro Designer to conduct a techno-economic analysis and quantification of the direct carbon dioxide emissions from transportation of the final product to the field, not accounting the carbon dioxide emissions generated during the HTC. Information required for the analyses included data from wastewater treatment plants, equipment and process conditions; properties of feedstock, intermediates, and final products; cost of feedstock, materials, equipment, and utilities; and 94 carbon dioxide emission factors. Results from the analyses indicated that the capital

investment required to install the necessary equipment to a combined AD-HTC system was

about US$36 million, which included AD, HTC, filtering, and drying, with a payback time

of less than six years, an internal rate of return of ~13%, and an operating cost for producing

1 ton of hydrochar of US$1,298, and cost of treating 1 ton of sewage sludge of US$80. The

direct carbon dioxide emission for the combined AD-HTC system added up to 23 ton yr-1

The production of hydrochar from sewage sludge through a combined AD-HTC system

has the potential to be technically and economically feasible when implemented in current

wastewater treatment plants.

5.2. Introduction

Wastewater treatment plants (WWTP) worldwide produce ~150 million tons of

sewage sludge annually (Lundqvist et al., 2017). Sewage sludge are the solids nutrient-rich

organic matter resulted as byproduct from the wastewater treatment (Peccia & Westerho,

2015; U.S. Environmental Protection Agency, 2006). In the United States, ~8 million tons

yr-1 of dry sewage sludge are produced (Center for Sustainable Systems, 2019; U.S.

Environmental Protection Agency, 2006) from ~16,000 WWTP, from which ~10% of them treat sewage sludge through anaerobic digestion (AD) (Cybersecurity and Infrastructure

Security Agency, n.d.; Water Environment Federation, 2015), while the remaining 90% dewaters the sewage sludge and deals with it through landfill, incineration, or land application. 95 The purpose of AD in a WWTP is to manage the sewage sludge and produce biogas

for energy and heat generation. Anaerobic digestion reduces the organic matter in the

effluent by ~50% (Fricke et al., 2005), decreases the disposal costs, reduces the microbial

load of the material, and lowers the risks of contamination to the environment such as

nutrient runoff (Nkoa, 2014), ammonia volatilization (Génermont & Cellier, 1997), and

odor issues. For WWTP without AD reactors, sewage sludge is the final product, while the

plants with AD reactors end up with anaerobic digestion effluent (ADE) as byproduct, in

addition to biogas as the main product. In both cases, WWTP have to manage the sewage

sludge or ADE, which is expensive due to their high water content (Delzeit & Kellner,

2013). Transportation of sewage sludge or ADE becomes more expensive as more water

needs to be managed instead of managing solids, as more trips to dispose the same quantity

of solids need to be made. Current methods for the treatment of ADE include dewatering

and sending to the landfill, composting, (Sheets et al., 2015), incineration, or land

application (Nkoa, 2014).

Anaerobic digestion effluent can also be managed through thermochemical processes, such as hydrothermal carbonization (HTC), which can be used to treat wet waste. Hydrothermal carbonization works at temperatures in the range 180-260°C, under

autogenous pressures, and produces hydrochar, an effluent liquid called liquor, and non-

condensable gases. Hydrochar is a carbonized material and one of its potential uses is as

soil amendment.

Previous researchers have performed techno-economic analyses (TEA) and life cycle assessment (LCA) of AD and HTC, mostly as independent processes, not as a combined system. There are several studies focusing on TEA of AD (Astill & Shumway, 96 2016; Dhar et al., 2012; Gebrezgabher et al., 2010; Kabir et al., 2015; Khan et al., 2014;

Klavon et al., 2013; Li et al., 2018; Lin et al., 2019; Mel et al., 2015; Shafiei et al., 2011;

Theodorou et al., 2017) and LCA of AD systems (Collet et al., 2017; De Vries et al., 2012;

Franchetti, 2013; Lijó et al., 2014; Mezzullo et al., 2013; Nayal et al., 2016; Riaño &

García-González, 2015; Sanscartier et al., 2012; Smith et al., 2014). Fewer studies have been conducted on TEA of HTC (Kempegowda et al., 2017; Lucian & Fiori, 2017;

Mahmood et al., 2016; Saba et al., 2019; Unrean et al., 2018; Wirth et al., 2011; Zeymer

et al., 2017) and LCA of HTC systems (Benavente et al., 2017; Berge et al., 2015a; Liu et

al., 2017; Owsianiak et al., 2016; Owsianiak et al., 2018; Unrean et al., 2018; Zeymer,

2017).

A TEA and LCA for AD and for HTC have been performed separately by

Sanscartier et al. (2012) and Zeymer et al. (2017), respectively. Sanscartier et al. (2012)

used the organic fraction of municipal solid waste for energy generation, and found that it could just replace a small fraction of the amount of coal used for energy; however, it reduced greenhouse gas emissions, was cheaper, had the potential to improve at larger scales, and was a more sustainable waste management method. Zeymer et al. (2017)

performed a TEA and LCA of HTC of plant material in Germany and found that the

minimum selling price of hydrochar ranges from €150 to €230 ton-1 (US$212 to US$257

ton-1) at two different production levels (2,211 and 11,056 ton yr-1), the low cost of hydrochar may be because the system did not include steam or cooling water as part of utilities, and the low operating cost did not consider specific percentages for labor and facility dependent costs, and less and lower cost of their equipment. The cost of feedstock in Zeymer et al. (2017) represented the 22% and 31% of the total operating cost for the two 97 production levels. Owsianiak et al. (2016) analyzed the HTC of plant matter, food waste, organic fraction of municipal solid waste, and ADE for the production of energy and found that hydrochar produced from ADE had the highest negative environmental impacts on global warming potential compared to the other feedstocks. These results were found in part because the system already in place for ADE was an incinerator that generated heat and energy for other processes (Owsianiak et al., 2016), therefore the ADE was already being utilized as energy source. This was the reason why hydrochar from ADE performed poorly compared to hydrochar produced from other feedstocks that did not have an efficient waste management system already in place (Owsianiak et al., 2016).

Most of the TEA and LCA studies performed on AD and HTC have focused on individual processes. These studies have used different feedstock, capacities, processing conditions, and focused mainly on hydrochar for energy. None, to the best of our knowledge, have considered a combined AD-HTC system using sewage sludge as feedstock with the purpose of producing hydrochar that can be applied to the field. There is an acknowledged need for more of this type of research (Kumar et al., 2018)

The objective of this study was to conduct the techno-economic, and direct carbon dioxide emissions analyses of a combined AD-HTC system for the treatment of sewage sludge, production of hydrochar, and its transportation to an agricultural field. This study fills the gap in systems analysis for a combined AD-HTC system using sewage sludge as feedstock. This analysis considered technical, economic, and environmental inputs to the system in order to quantify equipment and materials required for the production, associated costs, and direct carbon dioxide emissions to the environment from the production and transportation of the hydrochar. 98 5.3. Methodology

5.3.1. System boundary of the combined anaerobic digestion and hydrothermal

carbonization system

The system boundary for the combined AD-HTC system (Figure 17) for this study included all the operations, from feedstock (sewage sludge), to the transportation of the hydrochar to the agricultural fields. Sewage sludge entered AD to produce biogas and

ADE. Then, ADE flowed to the HTC reactor and produced hydrochar still mixed in a sludge, it was filtered where the liquor was separated and the hydrochar cake passed through the dryer, and the final dry hydrochar was transported to an agricultural field.

Figure 17. Schematic of the combined AD-HTC system for the treatment of sewage sludge and the production of hydrochar. The system boundary considered all the elements withing the thick-line rectangle.

99 The analysis was based on a sewage sludge treatment plant with an input of 15 ton hr-1 (Figure 18) (Quasar Energy Group, 2015). Sewage sludge with 7.6% total solids (TS) entered the equalization tank that homogenized the inflow and acted as a buffer prior entering the AD reactors. The equalization tank was set as a continuous process with a retention time of 3 days. The AD was set as a continuous process with a retention time of

20 days at 37°C. The ADE was pressurized by pumps and heated by a counterflow heat exchanger. It is necessary to increase the pressure of the flow prior to increasing the temperature above the boiling point of water, to avoid the vaporization of the water (Saba et al., 2019). The pressurized and heated ADE flowed into the HTC reactor, which was considered a semi continuous process at 220°C and retention time of 50 min. After HTC the hydrochar sludge passed through a flash tank where it was depressurized and cooled to

150°C. After the flash tank, the hydrochar sludge was further cooled, from 150 to 42°C, in the previously described heat exchanger. Gas generated in the flash tank, composed of carbon dioxide (CO2, 95%) and methane (CH4, 5%) (Kempegowda et al., 2017; Libra et al., 2011) because of decarboxylation (Ramke et al., 2009), was sent to a steam generator to combust CH4 and release CO2. The steam generated was directed to a steam turbine to generate electricity, which was used to offset part of the electricity consumed by the plant.

The cooled hydrochar sludge was dewatered in a belt filter where the liquor was separated and recirculated to the WWTP. The hydrochar cake (32% TS) passed through the drier to get the final hydrochar (80% TS). Hydrochar was transported to the agricultural field 25 miles away from the WWTP, this is a conservative distance given the fact that some

WWTP in the US have disposed sewage sludge more than 1,000 miles away (States

100 Environmental Protection Agency - Office of Water, 2000). Properties of sewage sludge,

ADE, hydrochar, liquor, and operating conditions can be found elsewhere (Chapter 3).

Figure 18. Process flow diagram of the combined AD-HTC system. AD: anaerobic digestion; ADE: anaerobic digestion effluent; HTC: hydrothermal carbonization; HC: hydrochar

101 5.3.2. Scenarios

Based on the processes described above, five scenarios were modeled (Figure 19);

considering the equipment present and to be installed to upgrade to a combined AD-HTC

system (Table 9). Scenario 1 did not include AD and its final product was sewage sludge

(~7.6% TS, based on experimental measurements). Scenario 2 did not include AD, but

included the filtration step resulting in sewage cake as final product (~32.4% TS, based on

experimental measurements). Scenario 3 included AD and its final product was ADE

(~8.2% TS, based on experimental measurements). Scenario 4 included AD followed by

the filtration step; and its final product was ADE cake (~32.4% TS, based on experimental

measurements). Scenario 5 included AD, followed by filtration and drying, and its final

product was dried ADE (~80.0% TS, based on experimental measurements). The last one,

is the upgraded scenario, and included AD, HTC, filtration, and drying; its final product was dry hydrochar (~80.0% TS, based on experimental measurements). All the base case scenarios were considered to be upgraded to scenario 6. The final product in all the scenarios was transported to be disposed. Scenarios 1 and 2 did not include AD reactors and are representative of ~90% of all the WWTP in the US (Cybersecurity and

Infrastructure Security Agency, n.d.; Water Environment Federation, 2015). Scenarios 3,

4, and 5 included AD and represent ~10% of all the WWTP in the US (Cybersecurity and

Infrastructure Security Agency, n.d.; Water Environment Federation, 2015). Scenarios 1 through 4 paid US$50 ton-1 (Neo Water Treatment, 2019; U.S. Environmental

ProtectionAgency, 2014) as waste disposal fee for sewage sludge, ADE, sewage cake, or

ADE cake. The price of hydrochar was based on the TS% of the final product. At 100%

TS, the price of hydrochar was set at US$2,000 ton-1, considering that the price of pyrochar 102 ranged between US$2,538 to US$7,150 ton-1 (Biochar Supreme, n.d.; Hydrobuilder, n.d.;

Porter & Laird, 2019), and that there were no commercially available hydrochar to refer.

The hydrochar produced by the combined AD-HTC system had 80% TS and its price was

set at US$1,600 ton-1. The price of dried ADE for Scenario 5 was calculated. The operating cost and emissions for the first five scenarios were estimated, as well as the cost of upgrading them to scenario 6.

Figure 19. Scenarios for the combined anaerobic digestion and hydrothermal carbonization (AD-HTC) system plus transportation to the agricultural field. Scenarios in yellow (scenarios 1 and 2) represent 90% of all the plants in the United States where there is no AD installed. Scenarios in orange (scenarios 3, 4, and 5) represent the 10% that do have AD reactors as part of their operation. Scenario in green, scenario 6 is the combined AD-HTC system, which includes HTC as part of the operation, and is proposed in this study.

103 Table 9. Equipment present and required to be installed for each scenario.

Scenarios AD HTCa Filter Drier S1 to install to install to install to install S2 to install to install present to install S3 present to install to install to install S4 present to install present to install S5 present to install present present AD-HTC present present present present

a Installation of HTC comes along with the pumps, heat exchanger, flash tank, boiler, and generator.

5.3.3. Techno-economic and direct carbon dioxide emission estimation

All the scenarios along with technical information about the processes were inputs

to the technical model, and their results were used as the input data to the economic and

environmental models (Figure 20). Technical information includes equipment and process

conditions, and chemical and physical properties of feedstock, intermediate, and final

products. The outputs of the technical analysis were the number and size of equipment, as

well as mass and energy balances. These data were inputs to the economic model along

with the cost associated with them. The outputs of the economic analysis were the capital

investment (US$), operating cost (US$ yr-1), net present value (NPV, US$), net revenue

(US$), internal rate of return (IRR, %), payback time (yr), sewage sludge treatment cost

(US$ ton-1), and hydrochar production cost (US$ ton-1). Net revenue was calculated as the

difference between the total revenue (US$ yr-1) minus the operating cost (US$ yr-1).

Technical outputs were also used as inputs to the environmental model along with

environmental factors to calculate the amount of direct CO2 emissions from the system.

104

Figure 20. Framework for techno-economic and environmental analyses.

Data considered for the system modeling included equipment capacities and purchase costs (Table 10), feedstock, products, plant parameters, and direct CO2 emission factors (Table 11). Process modeling and simulation were conducted using SuperPro

Designer (Intelligen Inc. v10, n.d.). No material losses were assumed.

105 Table 10. Equipment capacities and purchase cost of the combined AD-HTC system.

Equipment Description Unit cost Size/capacitya (US$) Flat Bottom Tankb 962 m3 US$ 25,000 AD reactorb (3) 3,000 m3 US$ 3,456,000 Pumpb 5.18 kW US$ 87,000 Heat Exchangerb 45.67 m2 US$ 28,000 Pumpc 5.18 kW US$ 87,000 HTC reactordef(Lucian & Fiori, 2017) 20,101 kg hr-1 US$ 196,000 Flash Drumd 0.48 m3 US$ 150,000 Boilerc 1,331 kg hr-1 US$ 61,000 Generatorc 94 kW US$ 46,000 Belt Filterc 3 m US$ 149,000 Sludge Dryerb 1,354 kg hr-1 US$ 63,000 Truckg 32 m3 US$ 100,000 Unlisted Equipmenth US$ 1,087,000 Total equipment purchase cost US$ 5,455,000 a Capacity refers to the capacity used for this study; b (Lin et al., 2019); c (Intelligen Inc. v10, n.d.); d (Lucian & Fiori, 2017); e (Saba et al., 2019); f (Zeymer et al., 2017); g (Commercial Truck Trader, n.d.); h Unlisted equipment included skids, storage units, and lighting.

106 Table 11. Assumptions for the technical, economic, and environmental analyses.

Parameters Values Units Days of operationa 330 days Operating hoursa 7,920 hr yr-1 Sewage sludge inputb 15 ton hr-1 Laborc 22 US$ hr-1 Electricity costa 0.10 US$ kWh-1 Cooling watera 0.05 US$ ton-1 Steama 12.00 US$ ton-1 Steam high pressurea 20.00 US$ ton-1 Steam for driera 0.28 US$ ton-1 Price of biogasd 0.18 US$ m3-1 Hydrochar yield 7,306 ton yr-1 Waste disposal fee – liquoref 0.74 US$ ton-1 g -5 -1 GHG emission factor for tanker 6.72×10 ton CO2eq yr g -5 -1 GHG emission factor for full truckload 8.69×10 ton CO2eq yr a (Intelligen Inc. v10, n.d.); b (Quasar Energy Group, 2015); c (Occupational Outlook handbook, 2019); d (Klavon et al., 2013); e (Prieto et al., 2016); f (Saba et al., 2019); g (U.S. Environmental Protection Agency, 2018)

5.3.4. Sensitivity analysis

Four important factors were subjected to sensitivity analysis by varying their values by ±20% from the base values, and their impacts were analyzed on selected responses.

Variability was included for HTC reactor cost (US$), hydrochar selling price (US$ ton-1), hydrochar yield (ton hr-1), and AD reactor cost (US$). The effects of their variability were estimated on hydrochar production cost (US$ ton-1), payback time (yr), and IRR (%).

107 5.4. Results and Discussion

5.4.1. Capital investment for upgrading base cases to the combined AD-HTC system

The capital investment to upgrade the base scenarios to the combined AD-HTC system ranged from US$5.6 (Scenario 5) to US$35.8 million (Scenario 1) (Figure 21). The capital investment to upgrade the scenarios included the direct fixed capital, working capital, and startup cost (Figure 21a) for the installation of AD reactors, HTC reactor (plus pumps, heat exchanger, flash tank, boiler, and generator), filter, and dryer (Figure 21b).

The capital investments for scenarios 3 to 5 were lower than scenarios 1 and 2 because there was no need to install AD reactors, which are the main cost of the upgrade. Most of the WWTPs in the US are represented by scenarios 1 and 2, which are the scenarios that need more investment to upgrade to a full combined AD-HTC system.

108 a)

b)

Figure 21. Capital investment for upgrading scenarios 1-5 to the combined anaerobic digestion and hydrothermal carbonization system. a) Breakdown of capital investment based on direct fixed cost, working capital, and start- up cost; b) Breakdown of capital investment based on different processes.

109 5.4.2. Mass balance of the combined anaerobic digestion and hydrothermal carbonization

system

The fate of the solids and water in the combined AD-HTC system is shown in

Figure 22. These results are based on 1,000 kg of sewage sludge entering the combined

AD-HTC system. In 1,000 kg of sewage sludge are 76 kg of solids, from which 49 kg

become hydrochar (Figure 22). The hydrochar yield based on the TS of ADE, as typically

compared in the literature (Nizamuddin et al., 2017), was of 71%. At full scale, with an

input of 15 ton hr-1 of sewage sludge, the potential hydrochar yield is 0.92 ton hr-1 at 80%

TS. Different process conditions (i.e., temperature and time) and properties of the initial feedstock, as explored in Chapter 3, result in changes in hydrochar yield.

Figure 22. Mass balance for the treatment of 1,000 kg of sewage sludge. Black box indicates the total mass; brown text indicates total solids; and blue text indicates water for each inflow and outflow through the combined anaerobic digestion and hydrothermal carbonization system.

110 5.4.3. Operating cost for the combined anaerobic digestion and hydrothermal

carbonization system

The operating cost of the combined AD-HTC system is based on the hydrochar produced (ton yr-1) and on the sewage sludge treated (ton yr-1). The operating cost to produce 1 ton of hydrochar was US$1,298 and to treat 1 ton of sewage sludge was US$80

(Figure 23). Once the base cases have been upgraded to the combined AD-HTC system, the operating cost is the same for all the scenarios. The labor cost for hydrochar production was US$50 ton-1 including the operators in the plant and the driver that transport the

hydrochar to the field. Operating cost related to the facility accounted for US$871 ton-1 and

considered the costs related to AD (64%), HTC reactors (12%), filter and dryer (4%), and

skids, storage units, lightning, and other equipment (20%). Laboratory dependent cost was

US$7 ton-1, which considered the daily sampling and analysis of sewage sludge, ADE,

liquor, and hydrochar. The waste fee for the combined AD-HTC system to recirculate the

wastewater back to the WWTP was US$7 ton-1 of hydrochar produced. Utilities costs were

US$361 ton-1 and included electricity, steam, and cooling water. Fuel cost for

transportation of the hydrochar to the field was US$1 ton-1 of hydrochar produced. The total transportation cost for 1 ton of hydrochar including depreciation of the truck, salary for driver, and fuel was ~US$3.1. Operating cost of hydrochar based on the combined AD-

HTC system was US$1,298 ton-1, but when just the HTC reactor and side equipment

(pumps, heat exchanger, flask tank, boiler, and generator) were considered, the production

cost decreased to US$462 ton-1. Others have found hydrochar selling prices between

US$106 to 205 ton-1 (Lucian & Fiori, 2017; Saba et al., 2019; Zeymer et al., 2017), while

111 the market prices for pyrochar ranged from US$2,538 to US$7,150 ton-1 (Biochar Supreme, n.d.; Hydrobuilder, n.d.; Porter & Laird, 2019).

As scenarios upgraded to the combined AD-HTC, payback time decreased from 5.3

yr in Scenario 1 to 0.8 yr in Scenario 5 and IRR increased from 13% in Scenario 1 to 63%

in scenario 5. By having a combined AD-HTC system there is a constant supply of free

feedstock (ADE) and the potential to treat HTC liquor in AD or WWTP. This type of

systems where AD is combined with HTC enhances the economic feasibility of the HTC

as found by Suwelack et al. (2016).

a) b)

Figure 23. Operating cost of the combined anaerobic digestion and hydrothermal carbonization system. Classification of elements considered in the operating cost (a), from which the facility costs represented the greatest percentage (b).

112 5.4.4. Net revenue for base cases and the combined anaerobic digestion and hydrothermal

carbonization system

Net revenues for scenarios 1 through 5 were negative because the operating costs were higher than the total revenues, while for the combined AD-HTC system the net

revenue was positive because the system sold hydrochar (Figure 24). Scenarios 1 and 2 did

not produce any final product that could be sold, sewage sludge and sewage cake,

respectively. In addition, their final product had to be disposed at a waste disposal fee of

US$50 ton-1. Scenarios 3 and 4 produced biogas that was sold, and ADE and ADE cake

that was also disposed with the waste disposal fee. The combined AD-HTC system had a

positive net revenue because it sold biogas and hydrochar. The operating cost varied in

different scenarios because they dealt with different equipment and products. Scenario 2

had the lowest operating cost because it had minimal equipment use, a filter, and low

transportation cost due to higher TS of the sewage cake (32.4%). Scenario 1 had less

equipment than Scenario 2, just the truck for transporting the sewage sludge and no other

equipment, but the TS of the sewage sludge was low (7.6%), therefore, more trips needed

to be made to dispose the sewage sludge, compare with the fewer trips of Scenario 2.

Scenarios 4 and 5 also had lower operating cost than Scenario 3, even though they had

more equipment in the system. The lower costs of scenarios 4 and 5 were due to the less volume of waste generated and therefore lower waste management cost. Scenario 3 dealt

with the disposal of ADE with high water content, while scenario 4 had a filter which

makes the transportation of ADE cake cheaper. Scenario 5 had a dryer to produce dried

hydrochar, it was transported to the field, and no waste disposal fee was paid. In Scenario

5, the production cost of 1 ton of dry ADE was US$680, however its selling price was 113 considered to be zero. In order to avoid a negative net revenue, the dried hydrochar has to

be sold at US$263 ton-1, which resulted in a total revenue equal to the operating cost achieving a neutral net revenue. Scenario 6 sold hydrochar at US$1,600 ton-1 as previously

described.

Figure 24. Net revenue per year for the base case scenarios and the combined anaerobic digestion and hydrothermal carbonization system. A-H: combined AD-HTC system.

5.4.5. Direct carbon dioxide emissions

Total direct CO2 emissions calculated for the scenarios ranged between 23 and 284

-1 ton CO2 yr (Figure 25). The only source of direct CO2 emissions for scenarios 1 through

5 was transportation of the final product to the field because no other process released gases

in those scenarios. Total direct CO2 emissions tended to decrease in scenarios 1 through 5

114 as less final product was generated with higher TS, which translated into fewer miles driven

to transport that material to the field.

In the combined AD-HTC system, HTC produced CO2 and CH4, which were passed

through a boiler to combust CH4, release CO2, and generate steam, following the approach

for HTC gas management similar to Owsianiak et al. (2016). The amount of CO2 generated

-1 -1 in the boiler was 15,927 ton yr that resulted in 2.18 ton CO2 ton hydrochar, when all the

-1 CO2 was attributed to hydrochar, or 1.39 ton CO2 ton hydrochar when the generated CO2

was also attributed to the energy produced through the combustion of HTC gases. Figure

25 compares the direct emissions, not considering the CO2 released by the boiler.

Other researchers have managed HTC gases in different ways (Benavente et al.,

2017; Lucian & Fiori, 2017; Mahmood et al., 2016; Saba et al., 2019). Some have sent the

hydrochar sludge along with the CO2 generated to a heat exchanger (Saba et al., 2019), released the gases (Lucian & Fiori, 2017), considered them negligible and not accounted for them (Mahmood et al., 2016), or stated that these gases being mostly CO2 did not represent a big environmental impact (Benavente et al., 2017). There is a need to study better potential practices to manage gases generated during HTC. A way to manage it could include CO2 capture, compression, and pipeline transport for energy production and

geologic storage (Fry et al., 2017).

115

Figure 25. Carbon dioxide emissions for all the base scenarios and the combined anaerobic digestion and hydrothermal carbonization system. A-H: combined AD-HTC system.

5.4.6. Sensitivity analysis

The impacts of ±20% variations in the hydrochar yield, selling price, and costs of

AD and HTC reactors on the hydrochar production cost, payback time, and IRR for the combined AD-HTC system are summarized in Figure 26. The two most influential

parameters were hydrochar yield followed by hydrochar selling price. When hydrochar

yield decreased by 20%, hydrochar production cost and payback time increased, while IRR

decreased. The cost of the HTC reactor had a minimal influence on hydrochar production

cost, payback time, and IRR, compared to the large influence of the cost of AD reactors,

due to the cost of the high cost AD reactors. At higher production levels and cheaper

equipment higher revenues will be expected.

116

Figure 26. Sensitivity analysis of hydrochar production cost, payback time, and internal rate of return. Blue (higher value) and orange (lower value) bars indicate the impacts of ±20% in the variable.

5.5.Conclusions

Upgrading current WWTP in the US to a combined AD-HTC system for the treatment of sewage sludge and production of hydrochar has the potential to be technically, economically, and environmentally feasible. The capital investment to upgrade the base scenarios to the combined AD-HTC is dependent on the equipment that needs to be installed. As less equipment is required, the capital investment decreased. However, in the

117 US, 90% of the WWTP are represented in Scenario 1 and 2, which require the highest

investment because they currently do not have AD. As WWTP are upgraded to the combined AD-HTC system, the plants will be able to utilize or sell biogas and hydrochar, which will decrease operating cost and increase revenue. The direct CO2 emissions related

to the transportation of the final product after upgrading to the combined AD-HTC system will decrease as the final product has less water, fewer miles will be driven and therefore less CO2 will be emitted. Options to manage the CO2 emissions from the HTC process are

capture, compression, and pipeline transport for energy production and geologic storage.

It is recommended to perform an extended environmental assessment of the combined AD-

HTC system.

118

Chapter 6. Conclusions and recommendations

6.1. Conclusions

Hydrochar produced through the combined anaerobic digestion and hydrothermal carbonization (AD-HTC) system from sewage sludge has the potential to be technically and economically feasible, reduce direct carbon dioxide emissions, and to be used as soil

amendment for crop production. The combined AD-HTC system considers AD reactors,

HTC reactor, filter, and dryer. The capital investment to upgrade the base scenarios to the combined AD-HTC is dependent on the equipment that needs to be installed. As less

equipment is required, the capital investment decreased; however, in the US, most of the

WWTP will required the highest investment. Once upgraded, they will be able to utilize or

sell biogas and hydrochar, which will decrease operating cost and increase revenue.

Reaction temperature during HTC was the parameter with the most impact on the

production of hydrochar from sewage sludge. At higher temperatures, hydrochar achieved

higher degrees of carbonization and lower yields. The hydrochar produced from anaerobic

digestion effluent (ADE) from sewage sludge showed high ash content, low carbon

content, and low calorific value. Because of these properties, hydrochar would be better used as a soil amendment than as solid fuel.

119 Soil was amended with hydrochar from ADE from sewage sludge and lettuce was

grown on the amended soil. The emergence rate of lettuce on soils amended with hydrochar

was similar or higher than the emergence rate of the control. The dry weights of roots,

leaves, and whole plants were heavier than the control. The enhancement of soil properties

and presence of nutrients may have been a reason for the increase in emergence and

development of plants grown in soils amended with hydrochar when compared to the

control. The effects of char on soil properties and plant responses depend on the feedstock used, conversion method, process conditions, application method of the char, rate of application, frequency, history of application, properties of the soil, and weather. No negative effects were observed in emergence or dry biomass, which indicate that hydrochar from ADE of sewage sludge can be used and applied to the soil.

This analysis showed that by upgrading the current WWTP in the US to a combined

AD-HTC system there is a potential for a more efficient waste management system, higher

economic return, reduction in direct CO2 emissions, and improvement in crop yield.

6.2. Recommendations for future research

Hydrothermal carbonization of ADE from sewage sludge was conducted at temperatures ranging from 180 to 260°C for 30 to 70 min. This was done for both pH levels, original (~7.9) and modified (~6.6). Further studies could increase the range of reaction time to better understand its effect on hydrochar and liquor yields and properties.

Also, the pH could be decreased and increased with different chemicals and at a larger range to better understand their effects and potential. 120 The use of char as soil amendment was evaluated for hydrochar from ADE of

sewage sludge and manure, and pyrochar from lignocellulosic biomass. The current

research on the effect of hydrochar in agriculture is very limited and isolated. More

research could be done to understand the broad effects of hydrochar in the soil and how it

affects plants in general. For future studies, it is recommended to evaluate hydrochar

produced from different feedstock at different processing conditions on different types of

soils, with more amendment rates, and with different crops. The use of the liquor as

irrigation water should also be evaluated.

In the system analysis of the combined AD-HTC system, the direct carbon dioxide

emissions were accounted, considered the transportation of the hydrochar to the field, but did not include the gases generated from the HTC process. Currently, there is not an established procedure to manage the emissions from HTC, for which it is recommended to create different scenarios, analyze, and optimize the best way to deal with these emissions.

A full LCA is suggested to account for more impact categories and better understand the negative and positive effects of HTC on the current system.

121

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146

Appendix A: Hydrothermal carbonization of anaerobic digestion effluent from sewage sludge for hydrochar production

Table 12. Hydrothermal carbonization product yields and properties.

Yield Hydrochar T t Hydrochar Liquor Gas Calorific value Ash °C min % % % J kg-1 % ADE pH-O 10,880 163 50 79.5% 19.7% 0.8% 11,089 56% 180 30 75.5% 17.6% 6.9% 9,428 59% 180 70 77.8% 20.0% 2.1% 11,816 57% 220 22 76.3% 15.0% 8.8% 10,914 61% 220 50 72.2% 13.5% 14.4% 9,589 65% 220 78 75.0% 12.8% 12.2% 10,222 62% 260 30 73.4% 9.4% 17.2% 9,607 63% 260 70 71.9% 8.7% 19.4% 11,434 67% 277 50 69.9% 7.9% 22.2% 10,278 68% ADE pH-M 10,656 163 50 72.1% 26.8% 1.1% 10,894 52% 180 30 72.4% 27.5% 0.1% 11,240 57% 180 70 73.2% 26.8% 0.0% 10,904 54% 220 22 68.4% 24.8% 6.8% 12,001 62% 220 50 65.5% 20.9% 13.6% 10,119 59% 220 50 67.3% 22.7% 10.0% 10,441 57% 220 50 67.4% 22.8% 9.8% 10,465 62% 220 78 67.6% 22.3% 10.1% 10,558 63% 260 30 66.7% 21.6% 11.7% 11,192 61% 260 70 61.9% 16.7% 21.3% 9,366 67% 277 50 63.2% 19.0% 17.8% 10,607 67%

Note: T: reaction temperature; t: time; ADE: anaerobic digestion effluent; pH-O: original pH; pH-M: modified pH. 147 Table 13. Composition of the anaerobic digestion effluent, hydrochar, and liquor.

pH T t Fe Ca P Al Mg Mn K O ADE 30,441 29,468 18,082 10,425 6,881 3,118 2,701 M ADE 30,278 25,945 17,364 10,410 6,598 3,101 2,688 163 50 33,370 33,230 20,130 10,660 7,790 3,601 1,934 180 30 34,210 35,280 20,890 10,380 8,121 3,728 1,882 180 70 34,150 34,590 21,340 10,590 7,747 3,768 1,809 220 22 34,350 35,330 21,700 10,260 8,236 3,685 1,795 O - 220 50 36,630 37,530 23,440 14,780 8,994 3,907 3,115 pH 220 78 33,810 35,660 21,860 10,720 8,087 3,656 2,011 260 30 36,360 37,010 23,050 11,510 8,907 3,847 2,051

260 70 36,360 37,000 23,490 11,920 8,715 3,846 2,209 277 50 37,550 37,880 24,760 11,780 8,748 4,083 1,958 163 50 30,580 25,870 17,370 11,090 6,241 3,091 1,917

Hydrochar 180 30 32,010 27,960 18,730 9,336 6,754 3,326 1,484 180 70 32,580 28,860 19,810 10,080 6,882 3,486 1,614 220 22 32,890 28,900 19,370 12,360 7,518 3,413 2,284 M - 220 50 33,920 29,810 20,270 11,400 8,024 3,566 2,119 pH 220 78 33,990 30,730 20,790 9,428 7,716 3,570 1,364 260 30 35,920 33,960 22,420 11,500 8,385 3,731 1,782 260 70 35,310 34,470 23,400 10,700 8,720 3,786 1,793 277 50 35,060 33,980 22,870 10,600 8,456 3,685 1,506 163 50 38.63 26.80 164.90 3.77 10.60 1.95 169 180 30 36.71 17.11 159.90 1.77 3.97 1.07 168 180 70 29.54 15.61 119.70 1.02 3.72 0.54 151 220 22 26.33 14.53 104.60 0.43 0.08 0.15 192 O - 220 50 25.46 13.66 79.95 0.43 3.66 0.38 176 pH 220 78 21.32 14.60 80.24 0.79 6.42 0.60 167 260 30 4.10 16.39 92.66 0.23 0.08 0.10 168

260 70 3.18 15.10 73.52 0.30 0.08 0.11 172 277 50 2.32 13.27 83.50 0.37 0.08 0.08 191 163 50 34.27 232.2 161.4 1.59 65.7 6.99 163 Liquor 180 30 28.91 222.5 152.7 0.94 52.7 5.83 162 180 70 40.89 229.0 143.1 0.49 66.0 8.56 175 220 22 21.16 173.9 148.0 0.38 32.8 4.06 173 M - 220 50 22.67 150.7 113.9 0.87 16.0 1.88 180 pH 220 78 15.10 143.3 110.4 0.20 11.3 1.10 192 260 30 2.25 90.9 79.9 0.33 5.5 0.51 172 260 70 5.76 51.6 39.4 0.30 0.1 0.05 202 277 50 1.05 89.8 51.7 0.77 6.1 0.58 173

Note: T: reaction temperature; t: time; ADE: anaerobic digestion effluent; pH-O: original pH; pH-M: modified pH. 148 Table 13 Continued

pH T t Na Zn Si Ba Cu Sr Pb Cr O ADE 1,337 1,401 644 515 338 123 72.1 59.7 M ADE 1,327 1,383 620 495 329 108 74.9 62.2 163 50 924 1,581 635 580 377 141 81.8 66.1 180 30 1,017 1,624 552 606 399 148 82.9 66.2 180 70 928 1,669 648 627 421 150 90.9 67.1 220 22 974 1,624 533 591 385 144 81.0 63.1 O - 220 50 1,001 1,724 557 642 423 160 86.1 77.7 pH 220 78 951 1,603 478 591 381 147 81.1 65.6 260 30 918 1,709 527 627 410 150 87.8 76.6

260 70 946 1,716 407 632 413 153 85.3 74.9 277 50 900 1,841 396 679 438 161 90.4 76.0 163 50 715 1,505 667 539 359 116 73.4 67.3

Hydrochar 180 30 817 1,546 648 552 372 119 78.4 62.9 180 70 848 1,633 671 590 389 126 80.8 65.7 220 22 738 1,569 607 569 386 120 79.0 65.7 M - 220 50 822 1,577 593 566 374 122 87.2 67.3 pH 220 78 729 1,637 514 577 390 125 83.9 62.5 260 30 750 1,723 536 622 428 136 85.4 74.2 260 70 816 1,650 494 601 393 137 85.1 72.7 277 50 633 1,699 385 611 408 134 83.6 69.4 163 50 119 1.43 33.5 <0.9 0.25 0.09 <0.5 <0.9 180 30 119 0.98 33.1 <0.9 0.16 0.06 <0.5 <0.9 180 70 105 0.35 34.4 <0.9 <0.1 0.05 <0.5 <0.9 220 22 136 <0.3 34.3 <0.9 <0.1 0.06 <0.5 <0.9 O - 220 50 126 <0.3 48.4 <0.9 <0.1 0.07 <0.5 <0.9 pH 220 78 133 <0.3 59.5 <0.9 <0.1 0.08 <0.5 <0.9 260 30 118 <0.3 35.9 <0.9 <0.1 0.13 <0.5 <0.9

260 70 122 <0.3 35.8 <0.9 <0.1 0.14 <0.5 <0.9 277 50 136 <0.3 38.7 <0.9 <0.1 0.13 <0.5 <0.9 163 50 118 <0.3 60.4 <0.9 <0.1 0.58 <0.5 <0.9 Liquor 180 30 121 <0.3 36.0 <0.9 <0.1 0.62 <0.5 <0.9 180 70 131 <0.3 47.1 <0.9 <0.1 0.64 <0.5 <0.9 220 22 127 <0.3 47.3 <0.9 <0.1 0.70 <0.5 <0.9 M - 220 50 132 <0.3 51.8 <0.9 <0.1 0.69 <0.5 <0.9 pH 220 78 141 <0.3 42.6 <0.9 <0.1 0.68 <0.5 <0.9 260 30 123 <0.3 34.9 <0.9 <0.1 0.53 <0.5 <0.9 260 70 149 <0.3 35.0 <0.9 <0.1 0.34 <0.5 <0.9 277 50 125 <0.3 44.9 <0.9 <0.1 0.57 <0.5 <0.9

Note: T: reaction temperature; t: time; ADE: anaerobic digestion effluent; pH-O: original pH; pH-M: modified pH. 149 Table 13. Continued

pH T t Ni V Tl B Co As Mo Se O ADE 39.7 21.9 25.2 18.3 21.5 14.6 11.0 6.8 M ADE 40.3 21.8 30.0 17.6 21.2 15.4 11.6 6.6 163 50 41.7 21.1 34.3 8.2 21.5 9.1 4.3 6.1 180 30 44.1 20.3 32.6 6.7 22.4 8.6 4.7 10.1 180 70 43.5 21.1 32.0 5.3 22.3 8.6 5.6 3.3 220 22 43.5 21.0 31.3 4.9 22.5 7.7 8.8 7.7 O - 220 50 51.9 30.3 32.5 10.7 24.0 7.1 9.9 6.4 pH 220 78 44.8 22.4 31.1 6.3 22.6 8.6 11.1 8.7 260 30 50.2 24.3 31.8 6.3 24.7 10.0 13.2 4.7

260 70 50.8 24.8 32.9 6.9 24.9 11.5 13.9 5.9 277 50 52.3 24.3 30.2 5.6 26.9 8.3 14.2 4.2 163 50 42.7 23.0 27.9 8.5 20.4 12.8 11.5 1.8

Hydrochar 180 30 42.3 19.6 26.9 4.2 21.6 11.5 11.8 4.1 180 70 43.5 21.3 29.5 6.6 22.1 10.0 12.0 3.2 220 22 46.1 26.0 27.7 8.1 22.0 9.2 11.1 4.7 M - 220 50 47.1 24.2 27.2 9.1 21.9 10.8 11.7 5.1 pH 220 78 45.4 20.1 27.2 3.4 22.7 10.4 12.6 4.7 260 30 49.1 23.6 27.0 4.8 23.8 13.0 12.9 5.0 260 70 49.1 22.2 34.4 6.0 23.5 5.9 10.0 4.9 277 50 48.8 21.6 26.3 4.2 23.8 10.6 13.2 4.3 163 50 0.36 <0.2 <1.1 0.66 <0.9 <0.9 0.49 <1.0 180 30 0.29 <0.2 <1.1 0.64 <0.9 <0.9 0.47 <1.0 180 70 0.21 <0.2 <1.1 0.62 <0.9 <0.9 0.36 <1.0 220 22 <0.3 <0.2 <1.1 0.76 <0.9 <0.9 <0.2 <1.0 O - 220 50 <0.3 <0.2 <1.1 0.81 <0.9 <0.9 0.32 <1.0 pH 220 78 <0.3 <0.2 <1.1 0.72 <0.9 <0.9 <0.2 <1.0 260 30 <0.3 <0.2 <1.1 0.70 <0.9 <0.9 <0.2 <1.0

260 70 <0.3 <0.2 <1.1 0.78 <0.9 <0.9 <0.2 <1.0 277 50 <0.3 <0.2 <1.1 0.84 <0.9 <0.9 <0.2 <1.0 163 50 <0.3 <0.2 <1.1 0.67 <0.9 <0.9 <0.2 <1.0 Liquor 180 30 <0.3 <0.2 <1.1 0.68 <0.9 <0.9 <0.2 <1.0 180 70 <0.3 <0.2 <1.1 0.73 <0.9 <0.9 <0.2 <1.0 220 22 <0.3 <0.2 <1.1 0.74 <0.9 <0.9 <0.2 <1.0 M - 220 50 <0.3 <0.2 <1.1 0.72 <0.9 <0.9 <0.2 <1.0 pH 220 78 <0.3 <0.2 <1.1 0.76 <0.9 <0.9 <0.2 <1.0 260 30 <0.3 <0.2 <1.1 0.69 <0.9 <0.9 <0.2 <1.0 260 70 <0.3 <0.2 <1.1 0.81 <0.9 <0.9 <0.2 <1.0 277 50 <0.3 <0.2 <1.1 0.65 <0.9 <0.9 <0.2 <1.0

Note: T: reaction temperature; t: time; ADE: anaerobic digestion effluent; pH-O: original pH; pH-M: modified pH. 150 Table 13. Continued

pH T t Li Sb Cd Be O ADE 4.22 5.13 5.24 1.70 M ADE 4.24 4.11 5.17 1.70 163 50 4.15 4.79 4.98 <0.9 180 30 0.68 4.74 5.19 <0.9 180 70 1.70 6.03 5.16 <0.9 220 22 3.33 5.09 5.10 <0.9 O - 220 50 1.65 4.64 5.49 <0.9 pH 220 78 3.06 4.85 5.04 <0.9 260 30 0.17 5.03 5.40 <0.9

260 70 3.41 4.49 5.42 <0.9 277 50 3.90 5.67 5.83 <0.9 163 50 4.56 3.55 4.65 <0.9

Hydrochar 180 30 2.70 5.45 4.78 <0.9 180 70 3.90 5.48 4.98 <0.9 220 22 4.63 5.05 4.88 <0.9 M - 220 50 3.86 5.08 4.98 <0.9 pH 220 78 2.17 6.30 5.27 <0.9 260 30 2.75 5.72 5.44 <0.9 260 70 2.54 5.53 5.31 <0.9 277 50 2.26 6.71 5.36 <0.9 163 50 <0.2 <0.4 <0.9 <0.9 180 30 <0.2 <0.4 <0.9 <0.9 180 70 <0.2 <0.4 <0.9 <0.9 220 22 <0.2 <0.4 <0.9 <0.9 O - 220 50 <0.2 <0.4 <0.9 <0.9 pH 220 78 <0.2 <0.4 <0.9 <0.9 260 30 <0.2 <0.4 <0.9 <0.9

260 70 <0.2 <0.4 <0.9 <0.9 277 50 <0.2 <0.4 <0.9 <0.9 163 50 <0.2 <0.4 <0.9 <0.9 Liquor 180 30 <0.2 <0.4 <0.9 <0.9 180 70 <0.2 <0.4 <0.9 <0.9 220 22 <0.2 <0.4 <0.9 <0.9 M - 220 50 <0.2 <0.4 <0.9 <0.9 pH 220 78 <0.2 <0.4 <0.9 <0.9 260 30 <0.2 <0.4 <0.9 <0.9 260 70 <0.2 <0.4 <0.9 <0.9 277 50 <0.2 <0.4 <0.9 <0.9

Note: T: reaction temperature; t: time; ADE: anaerobic digestion effluent; pH-O: original pH; pH-M: modified pH. 151

Appendix B: Effect of hydrochar from anaerobically digestated sewage sludge and manure on soil properties and plant responses

Table 14. Mean comparison of soil properties by amendment rate within char types.

PC HC-ADE-M HC-ADE-SS 0 1 3 5 10 15 0 1 3 5 10 15 0 1 3 5 10 15 0.0166 <0.0001 pH b b ab ab ab a c c c b ab a

SOM

CEC

<0.0001 <0.0001 P e de d c b a e de cd bc b a ppm)

- <0.0001 K b b b b a a 0.0274 <0.0001 Mg ab b ab ab a a c c bc b a a 0.0006 Content (m3 Ca b b b b ab a <0.0001 K c bc bc ab a ab <0.0001 Mg c c c b a a <0.0001 Saturation (%) Saturation Ca c c c b ab a K/

Mg

Ratio Ca /Mg

SOM: soil organic matter; CEC: cation exchange capacity

152 Table 15. Mean comparison of soil properties by char type within amendment rates.

-1 -1 -1 -1 -1 1 g kg 3 g kg 5 g kg 10 g kg 15 g kg M SS PC M SS PC M SS PC M SS PC M SS PC 0.0044 0.0038 0.0014 pH a b ab a b b a c b 0.0372 SOM a ab b 0.0459 CEC a ab b

0.0061 0.0009 <0.0001 0.0003 <0.0001 P a b b a b b a b b a b b a b c ppm)

- 0.003 0.0006 K a b b a b b 0.0003 <0.0001 Mg a b b a b b 0.0070 Content (m3 Ca a b b 0.0053 0.0365 K a b b a b ab 0.0062 0.0003 0.0008 Mg a b b a b b a b b 0.0356 0.0017 0.0009

Saturation (%) Saturation Ca a b ab a c b a c b

K/Mg

Ratio Ca/Mg

153 Table 16. Elements measured in lettuce roots and leaves.

Treatment (ug g-1) Al As B Ba Ca Cd Co Cr 0 4.60 14.2 8,267 0.98 9.37 0 3,841 24.4 61.9 7,985 1.99 3.96

Roots 0 6,613 3.58 18.9 83.5 7,611 2.98 6.86

NC 0 657 30.2 15.3 11,340 0 481 25.6 16.1 9,727

Leaves 0 494 31.7 12.9 10,830 1 4,850 6.51 25.1 72.8 9,007 0.91 2.72 4.98 3 4,392 24.4 59.6 7,970 2.08 4.46 5 4,887 3.65 24.1 63.4 8,862 2.09 5.08 Roots 10 5,482 21.4 62.6 7,736 2.42 5.76 15 4,797 3.69 23.9 64.3 8,940 2.18 4.82 PC 1 638 28.3 14.3 10,500

3 665 26.4 12.9 9,734 5 915 1.29 28.9 15.8 10,440 0.95 Leaves 10 623 2.45 24.7 14.3 11,690 15 507 29.4 13.8 11,000 1 5,461 24.3 88.2 8,898 0.98 2.75 6.06 3 4,104 2.32 19.2 67.0 7,376 2.18 4.108

5 5,365 4.60 23.6 95.1 10,350 0.93 3.10 5.98 Roots M - 10 5,097 3.34 21.6 90.2 9,910 3.36 5.38 15 3,841 0.88 19.9 68.9 8,447 2.10 4.158

ADE 1 449 25.2 10.7 9,292 3 848 27.1 13.0 9,385 HC - 5 658 30.7 12.29 9,460 Leaves 10 1,021 28.0 13.2 8,572 1.08 15 634 29.0 11.3 8,458 1 7,813 3.68 19.3 88.6 8,308 0.89 3.79 8.30 3 5,000 3.14 20.1 64.6 8,001 0.81 2.99 5.16

5 3,964 2.53 17.3 65.2 2.21 4.38 Roots SS

- 10 4,573 3.43 21.1 81.5 8,413 0.86 2.72 5.07 15 9,815 5.74 21.6 165.2 11,660 1.86 7.96 16.12

ADE 1 726 28.5 14.9 11,260 3 489 29.1 12.4 10,050

HC - 5 1,081 25.2 16.5 10,670 1.13 Leaves 10 506 29.9 12.8 11,090 15 273 33.5 10.6 11,330

154 Table 16. Continued

Treatment (ug g-1) Cu Fe K Li Mg Mn Mo 0 53.5 17,220 5,994 0.83 0 114.8 3,487 20,000 5,247 88.8 0.21

Roots 0 87.4 6,001 14,680 1.27 5,063 143 0.74

NC 0 13.7 57,840 3,549 92.3 0 9.0 480 57,350 0.32 3,170 40.7

Leaves 0 12.2 512 58,920 0.30 3,373 74.9 1 106.3 4,482 21,790 0.18 6,585 122.3 1.42 3 81.4 4,203 20,460 5,152 105.3 1.65 5 84.2 4,840 13,700 0.22 4,584 115.1 0.82 Roots 10 75.5 5,236 17,980 0.22 4,743 133.5 0.88 15 90.2 4,292 20,570 0.22 5,389 112.8 1.38 PC 1 8.1 604 54,670 0.25 3,171 51.7 0.41

3 8.6 651 49,380 0.23 3,290 67.7 5 8.0 797 53,270 0.18 3,157 57.9 0.17

Leaves 10 623 57,430 0.27 3,664 49.2 0.20 15 9.4 470 55,100 0.26 3,206 54.5 0.49 1 77.1 5,041 17,790 0.20 6,329 144.0 1.28 3 53.0 3,662 17,950 6,161 79.3 0.65

5 42.5 5,687 20,370 7,122 142.8 1.14 Roots M - 10 49.0 5,038 22,140 6,696 191.9 1.82 15 39.4 3,578 28,410 5,549 104.6 2.39

ADE 1 9.193 409 51,960 0.27 2,794 67.1 3 8.2 752 55,370 0.29 3,066 73.6 HC - 5 7.6 662 57,680 0.23 2,709 68.5 0.30

Leaves 10 7.1 907 58,470 0.21 2,664 70.7 0.25 15 8.2 712 60,980 0.25 2,589 73.3 0.20 1 105.2 7,601 12,490 1.24 6,089 166.5 1.12 3 80.7 4,999 15,150 0.62 5,871 118.0 0.84

5 92.0 3,932 16,120 0.26 5,003 110.4 0.57 Roots SS

- 10 72.0 5,022 15,430 0.24 6,098 133.0 0.87 15 93.0 11,920 11,790 2.31 7,141 716.5 2.53

ADE 1 9.0 719 57,060 0.28 3,446 71.5 3 7.4 477 54,840 0.28 3,363 52.5

HC - 5 7.8 1,004 56,070 0.20 3,132 53.1 0.34

Leaves 10 6.4 458 0.23 3,218 67.7 15 6.7 261 53,160 0.27 3,453 58.09 0.22

155 Table 16. Continued

Treatment (ug g-1) Na Ni P Pb S Sb Se Si 0 5,503 8.59 2,184 6.87 2,312 405 0 10,520 4.91 2,842 2.71 2,735 2.52

Roots 0 9,409 6.37 2,349 5.98 3,019 0.24 274

NC 0 9,876 2,456 0.74 2,730 1.84 346 0 7,088 0.48 3,188 0.50 2,442 1.65 269

Leaves 0 8,329 0.90 3,111 2,757 261 1 11,510 5.78 2,998 4.33 3,048 0.34 546 3 8,552 5.04 2,527 4.14 2,433 2.67 590 5 7,872 5.25 2,582 4.36 2,468 2.52 516 Roots 10 8,982 6.07 2,613 5.32 2,751 3.64 435 15 13,080 4.92 3,221 3.80 3,440 0.52 2.38 569 PC 1 8,040 0.96 2,847 0.46 2,279 0.70 302

3 8,177 0.78 2,880 2,283 1.76 303 5 7,396 1.13 2,941 0.44 2,329 2.46 2.20 424

Leaves 10 9,539 0.66 2,984 0.67 2,806 2.27 322 15 7,763 0.68 2,885 2,502 2.13 1.41 280 1 9,310 6.34 2,483 5.60 2,659 0.66 5.34 3 9,429 4.35 2,091 3.76 2,733 1.35 342

5 8,664 6.88 2,016 6.05 2,362 5.84 397 Roots M - 10 7,226 5.51 2,343 5.24 2,245 8.27 454 15 7,135 4.42 2,913 3.98 2,460 0.83 1.19 368

ADE 1 7,915 0.59 2,378 2,229 1.70 1.00 254 3 8,974 0.88 1,836 0.73 2,057 402 HC - 5 8,411 0.72 2,312 0.71 1,783 0.39 1.43 292

Leaves 10 8,462 1.03 2,140 0.94 1,671 0.91 399 15 10,480 0.77 3,138 0.73 1,745 1.86 2.47 263 1 9,864 8.39 2,333 6.88 2,732 0.94 0.95 339 3 11,020 5.86 2,655 4.69 2,908 1.44 396

5 10,140 4.49 2,617 4.64 3,173 1.41 354 Roots SS

- 10 10,150 6.47 2,909 5.43 2,895 2.52 392 15 8,507 12.8 5,125 13.89 2,873 0.93 3.72 388

ADE 1 8,352 0.87 2,801 0.79 2,320 0.50 353 3 7,226 0.75 3,359 0.57 2,370 1.76 279

HC - 5 7,067 0.98 2,794 0.86 2,278 3.89 422 Leaves 10 8,246 0.59 2,839 0.63 1,975 1.50 279 15 8,263 0.35 2,896 2,429 1.60 201

156 Table 16. Continued

Treatment (ug g-1) Sr V Zn 0 51.3 0 34.8 8.54 44.6

Roots 0 32.6 14.11 44.2

NC 0 23.5 1.50 61.3 0 20.6 1.16

Leaves 0 24.3 1.15 65.1 1 38.6 10.74 48.6 3 34.9 10.00 41.7 5 35.6 11.14 46.4 Roots 10 35.1 12.04 41.9 15 40.0 10.55 46.8 PC 1 22.0 1.40 34.9 3 20.6 1.51 28.4 5 22.3 1.93 37.2

Leaves 10 26.5 1.40 50.4 15 24.7 1.12 41.4 1 37.9 12.22 56.4 3 33.3 9.32 31.6 5 41.5 13.00 49.2 Roots

10 41.9 12.57 38.9 15 37.0 8.82 42.8 1 18.0 0.99 60.0 HC M 3 19.4 1.89 38.2 5 19.0 1.51 29.3

Leaves 10 18.1 2.14 31.5 15 18.4 1.42 39.6 1 34.2 17.45 52.6 3 33.7 11.50 48.3 5 28.9 8.82 54.6 Roots

10 34.3 10.92 64.7 15 51.2 21.22 241.2 1 22.9 1.66 50.0 HC SS 3 20.4 1.09 39.5 5 22.6 2.39 32.0

Leaves 10 22.20 1.144 38.21 15 21.83 0.669 41.44

157