Why Co-Digestion for Food Waste and Sewage Sludge?

Drainage Service Department Research & Development Forum

Smart City • Innovative Wastewater Management Co-digestion of Food Waste with Sewage Sludge

HKU team： Chunxiao WANG, Yubo WANG, Yulin WANG, Tong ZHANG

DSD team: Sussana LAI, KK CHEUNG

Prof. Tong Zhang Environmental Biotechnology Laboratory The University of Hong Kong 5th December, 2018  Introduction  Materials and Method  Results and Discussion  Conclusion

民以食為天《漢書》

2 https://newint.org/features/2008/12/01/food-crisis-facts; https://www.youtube.com/watch?v=ExH6kSwoFBw  Introduction  Materials and Method  Results and Discussion  Conclusion  What is the current situation of food waste around the world?

( Peek , 2014)

 Nearly about 1.4 billion tonnes food waste (one third of total food production) generated from farmland to meal table annually till 2011 revealed by the United Nations (FAO, 2014).  3584 tonnes food waste generated daily accounting 36% of the municipal solid waste landfilled in Hong Kong (EPD, 2017).  Conventionally, food waste was disposed in composting, incineration, landfill and anaerobic digestion (Astals et al., 2014; Wang et al., 2014; Chiu & Lo, 2016; ).  AD of food waste became more attractive due to high moisture and organic content in the FW and higher energy recovery potential (Zhang et al., 2007; Astals et al., 2014; Ingrid et al., 2014; Chiu& Lo, 2016).

3  Introduction  Materials and Method  Results and Discussion  Conclusion  Why co-digestion for food waste and sewage sludge?

From engineering perspective:  There are excess capacities in the anaerobic digesters. Food waste could be added and co-digested with sewage sludge to recovery more energy (EPA, 2013; Wang et al., 2015).

From technical perspective: Table 1. Biogas production performance of co-fermentation system

FW/FSS ratio (basis) Methane yield (mL/g VS) Improvement (%) Reference

10:90 (Volume) 293 18.1 Cabbai et al., 2013

20:80 (Volume) 600 54 Zupancic et al., 2008

50:50 (Volume) 365 47.2 Cabbai et al., 2013  More energy recovered from the bio-wastes (FSS and FW):  Increase in methane (biogas) production rate (Zhang et al., 2007; Sonsnowski et al., 2008; Cabbai et al., 2013).

 Increasing the stability of anaerobic-digesters:  Nutrition balance (C/N) ratio: 5< C/N <30 is regarded as optimal C/N ratio for a stable operated AD system (Dai et al., 2013).

4  Introduction  Materials and Method  Results and Discussion  Conclusion Co-digestion at a US wastewater treatment plant If there is excess capacity in the anaerobic digesters, food waste can be added to generate more energy. In California alone there are almost 140 wastewater treatment facilities that utilize anaerobic digesters, with an estimated excess capacity of 15-30%. Turning Food Waste into Energy at the East Bay Municipal Utility District (EBMUD)

http://www3.epa.gov/region9/waste/features/foodtoenergy/index.html 5  Introduction  Materials and Method  Results and Discussion  Conclusion

6 EBMUD Process If 50% of the food waste generated each year in the U.S. was anaerobically digested, enough electricity would be generated to power over 2.5 million homes for a year.

To digest food waste in anaerobic digesters, food waste must be 1) pre-treated into a slurry in the slurry tank 2) grinded into small pieces of 2 inches 3) to remove heavy debris. 4) added to the anaerobic digester as pulp after going through the paddle finisher. http://www3.epa.gov/region9/waste/features/foodtoenergy/index.html 6  Introduction  Materials and Method  Results and Discussion  Conclusion Co-digestion at a Germany wastewater treatment plant Braunschweig wastewater treatment plant

This plant currently achieves 100% electricity self-supply (energy neutral). Plant with biological sewage treatment and thermophilic digestion of sludge. • Capacity (Sewage flow): 52,000 m3/day. • Co-digestion of sludge with biowaste (grease and oil). • Recycling of biogas from landfill. • Recycling of methane from fermentation of green waste nearby.

7  Introduction  Materials and Method  Results and Discussion  Conclusion  Materials: Feeding Sewage Sludge (FSS)  PS and TSAS from TaiPo sewage treatment work Air Disinfection

Raw Grit Primary Secondary screening Aeration Efflunet wastewater chamber settling setting

Screenings Grits

PS Primary sludge TSAS Thickened secondary activated slduge

Anaerobic Sludge digester dewatering

Sewage Sludge (SS): TP-STW: PS /TSAS = 4.5/1(v/v)

 Anaerobic seed sludge: Digested sludge, sampled from anaerobic digester

8  Introduction  Materials and Method  Results and Discussion  Conclusion  Materials: Food Waste (FW) Table 2 The reported compositions of food waste from different sources on dry weight (%) basis

Food waste origin Carbohydrates Proteins Lipids References

Household 55 17 13 (la Cour et al.,2004) Household 61 14 14 (Hansen et al.,2007) Urban (Households, markets, 78 17 5 (Redonals et al., 2012) restaurants)

University dining hall 64 15 17 (Ferris et al.,1995) Military facilities 57 18 22 (Ferris et al.,1995) Institution restaurant 64 21 12 (Yan et al.,2011) This test 78 16 6 -

Table 3 Preparation of synthesized food waste

Category Wet Weight (g)

Meat 95 Vegetable (lettuce) 300 Fruit (apple) 140 Steamed Rice 400

Bread 350 9  Introduction  Materials and Method  Results and Discussion  Conclusion  FW:SS ratios: FW:FSS (TS:TS) ratios Volatile solid reduction (%)  Higher FW:FSS ratios correspond with higher biodegradation efficiency in term of VSR (from 36% to 57%).

Methane yield (mL/g VSR) Methane production rate (mL/L/d)

Table 5 Bioconversion efficiency of the co-digestion process

Methane Methane 605 mL/g VSR 564 mL/g VSR yield production 526 mL/g VSR (mL/g VSR) rate (mL/L/d) R1 504 279 R2 515 402 R3 526 804

10  Introduction  Materials and Method  Results and Discussion  Conclusion  Solid Retention Time (SRTs) Volatile solid reduction (%) 70 Methane content (%) 60 50 SRT_5d 40 SRT_10d SRT_15d 30 SRT_25d 20 10

Methane content (%) 0 6 12 18 24 30 36 42 48 54 60 66 Digestion day (d)

900 Methane yield (mL/g VSR)  Longer SRT corresponded with higher VSR (from 32% to 47%). 800  Methane content decreased significantly at short 700 SRT conditions (5 d, 10 d and 15 d) and kept at Theoretical methane yield=571 mL/g VSR 600 a stable level during the whole fermentation process at SRT of 25 days. 500  Methane yield decreased more quickly along 400 with shorter SRT conditions (SRT of 5, 10 and 300 15 days).  At SRT of 25 days, the methane yield kept 200 stable during the whole digestion process. 100  The highest methane yield (547 mL/g VSR) was

methane yield (mL/gVSR) obtained in R4 with SRT of 25 days. 0 6 12 18 24 30 36 42 48 54 60 66 Digestion day (d) 11  Introduction  Materials and Method  Results and Discussion  Conclusion  Solid Retention Time (SRTs) pH 700 Methane production rate (mL/L/d) 7.5

600 SRT_5d 7.0 SRT_10d 500 SRT_15d 6.5 SRT_25d 400 6.0 300 pH 5.5 200 5.0 100 4.5 0 Methane production rate (mL/L/d) 6 12 18 24 30 36 42 48 54 60 66 6 12 18 24 30 36 42 48 54 60 66 Digestion day (d) Digestion day (d) Total organic carbon (mg/L) 8000

7000  The methane production rate (MPR) kept stable in R4 (289 mL/L/d) with SRT of 25 days and shorter 6000 SRT conditions led sharper MPR decrease. 5000 4000  Along with the pH drop, methane production 3000 decreased significantly and kept in low level (less than 100 mL/L/d). 2000

1000

Total organic carbon (mg/L) 6 12 18 24 30 36 42 48 54 60 66 12 Digestion day (d)  Introduction  Materials and Method  Results and Discussion  Conclusion  FW composition

R1 _ boiled meat R5 _ soup slag

R2 _ herbal tea R4 _ fresh meat

R3 _ canteen leftover

Table 8 Food waste composition in collected real food waste

Reactor ID FW description Carbohydrates Proteins Lipids

R1 R1_boiled meat 27% 39% 34%

R2 R2_herbal Tea 75% 14% 11%

R3 R3_cateen leftover 58% 26% 16%

R4 R4_fresh meat 3% 73% 24%

R5 R5_soup slag 1% 62% 36%

R6 R6_Equal TS of all five types of FW 35% 41% 24%

 The FW composition fed in R2 was similar with the artificial FW synthesized based on

food consumption pattern in Hong Kong. 13  Introduction  Materials and Method  Results and Discussion  Conclusion

R1 _ boiled meat R5 _ soup slag Carbohydrate protein lipid 100%

80% R2 _ herbal tea R4 _ fresh meat 60%

40%

20%

R3 _ canteen leftover 0% R1 R2 R3 R4 R5 R6

Ammonium nitrogen concentration (mg/L)

+  The highest protein content were detected in the FW fed to R4 and R5 and the NH4 -N after digestion was corresponded with the protein content in the feed (FW). 14  Introduction  Materials and Method  Results and Discussion  Conclusion

Volatile solid reduction (%) Methane yield (mL/g VSR)

633 620 615 589 579 561

 The VSR is not only related with hydrolyzed microbes, but also the characteristics of the substrates, e.g. whether the substrate is easy to be destroyed and pretreated (R1).

 The R2 with the lowest protein content (14%) in the feed realizes the highest methane yield (534 mL/g VSR) and production rate (408 mL/L/d) and less TOC accumulation (2622 mg/L) comparedMethanewith productionother digesters rate (mL/L/d). Total organic carbon (mg/L)

 The low methane yield and production rate are detected in the R4 (315 mL/g VSR and 211 mL/L/d), R5 (309 mL/g VSR and 226 mL/L/d).

15  Introduction  Materials and Method  Results and Discussion  Conclusion

Volatile solid reduction (%) Total organic carbon (mg/L)

 The highest methane yield (581 mL/g VSR) and production rate (1051 mL/L/d) were detected in L2 with mixing condition of 80 rpm.

 Along with the increasing mixing intensity, less methane yield and production were observed in L3 and L4.

Methane yield (mL/gVSR) Methane production rate (mL/L/d)

16  Introduction  Materials and Method  Results and Discussion  Conclusion  Microorganisms in reactors under microscope

We may observe the shapes microorganisms under microscope. But it is difficult to know their names and functions.

17  Introduction  Materials and Method  Results and Discussion  Conclusion

Moore’s Law

“in the long view of history, the impact of DNA sequencing will Carlsonbe on a par Curve with that of the microscope.”

https://en.wikipedia.org/wiki/Whole_genome_sequencing 18  Introduction  Materials and Method  Results and Discussion  Conclusion  Bioinformatics Environmental Bioinformatics A “new frontier” in Environmental Engineering

Bioinformatics : translation (from the different combinations of A, T, G and C to some biological terms, such as names of bacteria species and names of genes/enzymes) of big data, based on databases (like “dictionaries”).

Bioinformatics : another kind of the “microscope” to study microorganisms in wastewater reactors. It tells us the names and functions of different microbial populations.

19  Introduction  Materials and Method  Results and Discussion  Conclusion  Classification of microorganisms at phylum level

1.5% 1.5% 1.6% 7.7% 2.3% Firmicutes (hydrolyser) Bacteroidetes (hydrolyser) 31.9% 4.5% Proteobacteria (fermenter) Chloroflexi 5.4% Actinobacteria (fermenter) 6.7% Thermotogae (hydrolyser) Euryarchaeota (methanogen) Spirochaetae Cloacimonetes 14.9% Planctomycetes 21.9% Others

 Bacteria accounted for 97.4%, Archaea 2.3%, plus a minor part of unknown sequences.

 The top six phyla were Firmicutes, Bacteroidetes, Proteobacteria, Chloroflexi, Actinobacteria, and Thermotogae were general hydrolysers and/or fermenters (acidogens).

20  Introduction  Materials and Method  Results and Discussion  Conclusion  Diversity of methanogens in the co-digestion reactor

Methanosaetaceae Methanosarcinaceae 19% 4%

Methanocorpusculaceae Methanospirillaceae 64% 1%

Methanoregulaceae 1%

Methanomicrobiales Incertae Sedis 2%

Methanomicrobiaceae 9%

 The most abundant family was Methanocorpusculaceae. This family was reported to

grow on H2/CO2 or formate with the optimal pH of 7 under mesophilic conditions (Zellner et al., 1989).  The family of Methanosaetaceae was well-reported to use acetate for the production of methane (Cavalier-Smith.,2002; Zielińska et al., 2013).

21  Introduction  Materials and Method  Results and Discussion  Conclusion

Co-digestion is one of the promising solutions to treat food waste.

Reactor operation and performance :

 Waste stabilization (VSR): • Higher VSR result was obtained from the reactors with higher FW:FSS ratio & longer SRT conditions. • Aggressive mixing conditions assisted creating homogenous distribution of substrates, heat and mass transfer, and facilitate particle size reduction. • Positive effect was also observed if the FW was pretreated before the co-digestion (e.g. R1_boiled meat).

 Energy recovery (Methane/Biogas production): : • FW:FSS ratio of 40:60 with SRT ≥17 days were the most appropriate operating parameters for the co- 3 digestion system, considering the volatile solid reduction (57%) and methane yield (0.53 m CH4/kg-VSR). • A sufficient SRT is highly need to ensure the complete methanogenic co-digestion process (no less than 17 days). • Suitable mixing conditions (80 rpm) would ensure the integrity of microbial consortia proximity but not impeded the mass and heat transfer among the functional groups to guarantee the efficiency of energy recovery (referred as methane/biogas production).

Microbial analysis :  Major hydrolysers : Firmicutes, Bacteroidetes, and Thermotogae  Major fermenters: Proteobacteria and Actinobacteria  Major methanogens: Methanocorpusculaceae

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