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Role of Microbial Hydrolysis in Anaerobic Digestion

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Role of Microbial Hydrolysis in Anaerobic Digestion energies Review Role of Microbial Hydrolysis in Anaerobic Digestion Theresa Menzel, Peter Neubauer and Stefan Junne * Bioprocess Engineering, Institute of Biotechnology, Technische Universität Berlin, Ackerstr. 76, ACK 24, D-13355 Berlin, Germany; [email protected] (T.M.); [email protected] (P.N.) * Correspondence: [email protected] Received: 26 August 2020; Accepted: 20 October 2020; Published: 23 October 2020 Abstract: There is a growing need of substrate flexibility for biobased production of energy and value-added products that allows the application of variable biodegradable residues within a circular economy. It can be used to balance fluctuating energy provision of other renewable sources. Hydrolysis presents one of the biggest limitations during anaerobic digestion. Methods to improve it will result in broader process applicability and improved integration into regional material cycles. Recently, one focus of anaerobic digestion research has been directed to systems with a separate hydrolysis–acidogenesis stage as it might be promised to improve process performance. Conditions can be adjusted to each class of microorganisms individually without harming methanogenic microorganisms. Extensive research of separate biomass pretreatment via biological, chemical, physical or mixed methods has been conducted. Nevertheless, several methods lack economic efficiency, have a high environmental impact or focus on specific substrates. Pretreatment via a separate hydrolysis stage as cell-driven biotransformation in a suspension might be an alternative that enables high yields, flexible feeding and production, and a better process control. In this review, we summarize existing technologies for microbial hydrolytic biotransformation in a separate reactor stage and the impacts of substrate, operational parameters, combined methods and process design as well as remaining challenges. Keywords: anaerobic digestion; biogas; biomass pretreatment; hydrolysis stage; multi-stage digestion; biological pretreatment; biodegradable waste; biorefinery 1. Introduction In 2019, the European Commission presented their plan of action to reach zero net emissions of greenhouse gases in the EU by 2050 and decouple economic growth from resource use in a clean, circular economy [1]. Including this concept in the bioeconomy sector requires processes that add value to waste and side products from agriculture, industry and the society [2]. Anaerobic digestion (AD) can produce bioenergy and value-added products from biodegradable residues; it represents an approach to close the loop in a circular bioeconomy. Examples are summarized in [3,4], among others. AD is a well-established and mature technology in Europe with more than 18,000 production plants in 2018 that are already providing about 14% of renewable energy. Moreover, as calculated by the World Biogas Association, the biogas and biomethane sector of AD can potentially reduce greenhouse gas emissions by 10–13%, and can thus play a key role in achieving the plan of action [5]. Energies 2020, 13, 5555; doi:10.3390/en13215555 www.mdpi.com/journal/energies Energies 2020, 13, 5555 2 of 29 Energies 2020, 13, x FORIn PEER brief, AD REVIEW of biodegradable residues consists of four metabolic stages, as shown in Figure1. 2 of 28 During hydrolysis, bulk biomass is degraded to soluble carbohydrates, proteins and lipids followed by include mechanical,acidogenesis, thermal, where these chemical, are converted biological mainly to and short-chain mixed carboxylic pretreatments. acids (SCCA) andThe alcohols. most promising of In acetogenesis and methanogenesis, acetic acid is consumed or assimilated and converted to these methodsmethane concerning and carbon hydrolysis dioxide. For efficiency the vast majority need of biodegradablehigh-energy waste, inputs hydrolysis (e.g., represents thermal/microwave treatment) or hugethe process amounts bottleneck of due chemicals to slow rates (acidic and incomplete pretreatment), degradation which [6,7]. Many makes reviews the have process costly in industrial scalebeen [6,7]. published The onintroduction pretreatment methods of a ofsepa variousrate biogenic hydrolysis residues tostage improve in hydrolysisAD has which already shown to include mechanical, thermal, chemical, biological and mixed pretreatments. The most promising of increase the netthese energy methods concerningoutput hydrolysisin pilot e ffiscaleciency needfermentations, high-energy inputs making (e.g., thermal it promising/microwave as potential biological pretreatment.treatment) or hugeWhile amounts hydrolytic of chemicals and (acidic acidogenic pretreatment), microorganisms which makes the process favor costly a slightly in acidic pH industrial scale [6,7]. The introduction of a separate hydrolysis stage in AD has already shown around 5.0–6.0,to the increase methanogenic the net energy species output in are pilot rather scale fermentations, sensitive and making thrive it promising at neutral as potential pH and mesophilic conditions. Separationbiological pretreatment. of the hydrolysis–acidogen While hydrolytic and acidogenicesis and microorganisms acetogenesis–methanogenesis favor a slightly acidic pH enables the optimization ofaround process 5.0–6.0, parameters the methanogenic to speciesthe differe are rathernt sensitive conditions and thrive of atthe neutral corresponding pH and mesophilic microorganism, conditions. Separation of the hydrolysis–acidogenesis and acetogenesis–methanogenesis enables the enabling higheroptimization efficiency of process in hydrolysis parameters toand the dimethfferentanogenesis conditions of the[8–11]. corresponding While microorganism, there are some examples of a separate hydrolysisenabling higher and efficiency acidogenesis in hydrolysis and in methanogenesis a three stage [8–11 AD,]. While the there strong are some syntrophic examples of relationship a separate hydrolysis and acidogenesis in a three stage AD, the strong syntrophic relationship between between acetogensacetogens and and methanogens methanogens makes makes the separation the sepa of theration last stages of the adverse last [3 ,stages4]. adverse [3,4]. Figure 1. MetabolicFigure 1. pathwaysMetabolic pathways of anaerobic of anaerobic digestiondigestio andn involvedand involved microorganisms. microorganisms. LCFA—long-chain LCFA—long- fatty acids, SCCA—short chain carboxylic acid. chain fatty acids, SCCA—short chain carboxylic acid. While several reviews have been published on these two-stage AD systems for biogas production, only a few focus on the operation and application of the hydrolysis stage itself [4,12,13]. Since currently more complex and unsteady sources of organic residues are used as substrates for AD, flexibilization of AD is of high importance. Microbial communities in a hydrolysis stage are able to adapt to changing substrates and loading rates and are able to digest the biogenic residues under optimal process conditions [14]. Depending on the process operation, either pure hydrolysis or combined hydrolysis and acidogenesis can be performed, leading to SCCA as products. Sometimes, hydrolysis and acidogenesis cannot be separated from each other, thus, the methods which are described in this review might include results from both processes. The review, thus, aims to summarize existing technologies for microbial hydrolytic biotransformation in a separate reactor stage and the impacts of substrate, operational parameters, combined methods and process design as well as remaining challenges. Alternating process conditions, e.g., caused by flexible feedstock utilization, can make the hydrolysis stage suitable as first reaction step for improving operational robustness. It might provide a substantial contribution for a better integration of biomass-derived energy into renewable energy supply systems. Energies 2020, 13, 5555 3 of 29 While several reviews have been published on these two-stage AD systems for biogas production, only a few focus on the operation and application of the hydrolysis stage itself [4,12,13]. Since currently more complex and unsteady sources of organic residues are used as substrates for AD, flexibilization of AD is of high importance. Microbial communities in a hydrolysis stage are able to adapt to changing substrates and loading rates and are able to digest the biogenic residues under optimal process conditions [14]. Depending on the process operation, either pure hydrolysis or combined hydrolysis and acidogenesis can be performed, leading to SCCA as products. Sometimes, hydrolysis and acidogenesis cannot be separated from each other, thus, the methods which are described in this review might include results from both processes. The review, thus, aims to summarize existing technologies for microbial hydrolytic biotransformation in a separate reactor stage and the impacts of substrate, operational parameters, combined methods and process design as well as remaining challenges. Alternating process conditions, e.g., caused by flexible feedstock utilization, can make the hydrolysis stage suitable as first reaction step for improving operational robustness. It might provide a substantial contribution for a better integration of biomass-derived energy into renewable energy supply systems. 2. Biodegradable Waste for Anaerobic Digestion The application of renewable crops as substrate is seen critical, as anaerobic digestion should not be in competition with the agricultural food industry and rather use sidestreams in a circular economy [2,15]. There are various types
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