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Fast start-up of semi-pilot scale anaerobic digestion of food waste acid hydrolysate for biogas production Chao Huang, Cheng Zhao, Hai-Jun Guo, Can Wang, Mu-Tan Luo, Lian Xiong, Hai-Long Li, Xue-Fang Chen, and Xin-De Chen J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b04005 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Journal of Agricultural and Food Chemistry


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Fast start-up of semi-pilot scale anaerobic digestion of food waste acid hydrolysate

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for biogas production

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Chao Huang a,b,c,e, Cheng Zhao b,d, Hai-Jun Guo a,b,c,e, Can Wang a,b,c,e, Mu-Tan Luo b,d,

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Lian Xiong a,b,c,e, Hai-Long Li a,b,c,e, Xue-Fang Chen a,b,c,e, Xin-De Chena,b,c,e,*

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b

CAS Key Laboratory of Renewable Energy, Guangzhou 510640, PR China;

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, PR China;

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c

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, PR China;

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15 16 17

e

University of Chinese Academy of Sciences, Beijing 100049, PR China;

R&D Center of Xuyi Attapulgite Applied Technology, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Xuyi 211700, PR China

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*Corresponding author.

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Address: No.2 Nengyuan Road, Tianhe District, Guangzhou 510640, PR China.

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Tel. /fax: +86 20 37213916

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E-mail address: [email protected] (X. D. Chen) 1

ACS Paragon Plus Environment


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Abstract

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In this study, fast start-up of semi-pilot scale anaerobic digestion of food waste acid

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hydrolysate for biogas production was carried out for the first time. During the period of

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fast start-up, more than 85% of COD can be degraded and even more than 90% of COD

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can be degraded during the later stage of anaerobic digestion. During this anaerobic

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digestion process, the biogas yield, the methane yield, and the CH4 content in biogas

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were 0.542±0.056 m3/kg COD consumption, 0.442±0.053 m3/kg COD consumption, and

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81.52±3.05%, respectively, and these values were high and stable. Besides, the

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fermentation pH was very stable that no acidification was observed during the anaerobic

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digestion process (outlet pH was 7.26±0.05 for the whole anaerobic digestion). Overall,

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start-up of this anaerobic digestion can be completed in a short period (the system can

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be stable two days after substrate was pumped into the bioreactor) and anaerobic

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digestion of food waste acid hydrolysate is feasible and attractive for industrial

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treatment of food waste and biogas production.

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Keywords: Anaerobic digestion; food waste acid hydrolysate; fast start-up; biogas

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production; semi-pilot scale

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Introduction Food waste is usually one semi-solid organic waste (total solid ranged from 12.9 to

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38.7 wt% on the basis of dry weight) discharged from various fields such as kitchen,

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restaurant, hotel, canteen, etc. 1 and it has become one worldwide serious environmental

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issue nowadays as the global growth of population and economic development 2.

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Traditionally, chemical method such as hydrothermal reactions was applied widely to

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treat food processing wastes 3. More recently, food waste was treated more by the

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biological method to generate some bio-products including ethanol 4, lactic acid 5, etc.

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Nowadays, anaerobic digestion is considered as one attractive method to treat food

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waste due to its mild condition, easy for operation, and great potential for clean energy

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(biogas) generation 6.

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Generally, hydrolysis, acidogenesis, acetogenesis, and methanogens are four main

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biological steps for anaerobic digestion. Usually, the efficiency of anaerobic digestion is

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not high especially at its beginning because above four steps cannot be completed

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simultaneously that hydrolysis is much longer than other processes and thus considered

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as the step of rate-limiting 6, 7. To make the anaerobic digestion of food waste to be more

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efficient, pretreatment of food waste including mechanical (ultrasound, high pressure

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and lysis), thermal, chemical (ozonation, alkali) and biological pretreatments is one

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method applied commonly today 8, 9, but these pretreatments will inevitably increase the

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operation cost and cannot solve the issue of anaerobic digestion completely because the

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four biological steps of anaerobic digestion are still carried out in one bioreactor

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(one-stage bioprocess). Changing anaerobic digestion from one-stage bioprocess to

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two-stage bioprocess can increase the efficiency of biogas production 10, 11, but this

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unfortunately also increases the total operation cost and requires longer period.

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Recently, it is reported that changing solid lignocellulosic biomass into liquid

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lignocellulosic hydrolysate can improve the performance of biogas production by

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solving the issue of inefficient mass transfer in traditional solid-state anaerobic

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digestion 12. Similarly, if the semi-solid food waste can be turned into liquid materials

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with fermentability, the “water soluble food waste constituents” can be utilized as

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substrate for anaerobic sludge more efficiently, and therefore this liquid-state anaerobic

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digestion has higher efficiency than that of traditional solid-state anaerobic digestion

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(detail scheme can be seen in Scheme 1). Many researches have proved that food waste

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can be converted to liquid substrate simply by acid hydrolysis, and the food waste acid

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hydrolysate can be used for fermentation to produce many bio-products such as ethanol,

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single cell protein, etc. 13, 14. However, little study use food waste acid hydrolysate for

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anaerobic digestion to generate biogas and whether this technology is suitable for

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application or not is still unknown.

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Anaerobic digestion of food waste is one low efficient bioprocess especially at its

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beginning, thus its start-up usually requires long period (even for several weeks) and the

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CH4 content in biogas is usually low (even less than 50%) 15, which is unsuitable for

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industrial application. Therefore, the performance of start-up is focused by many

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researches on food waste anaerobic digestion 16. Undoubtedly, if the start-up is able to

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be completed quickly, the competiveness of this anaerobic digestion can be increased

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greatly for actual application.

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In this study, food waste acid hydrolysate was used as substrate for anaerobic

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digestion to evaluate the performance of its start-up in a 100 L bioreactor for the first

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time. During this period, the performance of COD removal, biogas generation, and pH

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stability was evaluated systematically to learn the bioprocess. By this research, the

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possibility and prospect of using food waste acid hydrolysate for anaerobic digestion

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can be proven and one efficient method for shortening the start-up period of food waste

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anaerobic digestion can be obtained.

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Methods and materials

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Feedstock

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Granular anaerobic sludge used for anaerobic digestion was offered by local chemical

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enterprise. Before anaerobic digestion, the granular anaerobic sludge was inoculated

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into the bioreactor and the granular anaerobic sludge occupied about 50% of the volume

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of bioreactor (reaction part). Food waste acid hydrolysate used as substrate for

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anaerobic digestion was obtained from ZHONGKE New Energy Technological

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Development Limited Company (ZNETD Co., Ltd), Huai-An, China. As described by

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ZNETD Co., Ltd, the food waste was hydrolyzed at 170 oC by 2% (w/v) H2SO4 for 60

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min in a 50 L hydrolysis equipment, and after that the food waste hydrolysate was

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separated from the solid hydrolysis residue by vacuum filtration. The original COD of

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food waste acid hydrolysate was 92479±381 mg/L. After hydrolysis, lime was added

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into the food waste acid hydrolysate and its pH was neutralized to about 6.0. Then, the

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food waste hydrolysate was separated from solid residue after lime treatment by vacuum

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filtration. Before anaerobic digestion, the pH of food waste hydrolysate was further

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adjusted to certain pH (around 7.5-9.0) by Na2CO3 and the hydrolysate was recovered

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again by vacuum filtration. After that, tap water was added into the food waste

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hydrolysate to dilute its COD to certain value. Except the beginning of anaerobic

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digestion (during the first two days) that urea (its concentration was 1/40 of COD) and

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KH2PO4 (its concentration was 1/200 of COD) were added into the food waste

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hydrolysate for rejuvenation of granular anaerobic sludge, no extra nutrient was added

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into the hydrolysate throughout the process of anaerobic digestion.

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Anaerobic digestion

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A 100 L internal circulation (IC) reactor CH003 (ZNETD Co., Ltd) whose ratio of

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height to diameter (h/d) was 8:1 was used for continuous anaerobic digestion of food

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waste acid hydrolysate and the anaerobic digestion was carried out at 37 oC (mesophilic

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temperature). To evaluate the performance of this technology for fast start-up, a

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relatively high inlet COD (around 18000 mg/L) was applied at the beginning of

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anaerobic digestion. To prevent possible acidification, the inlet pH was adjusted to high

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level (around 9.5) and the inlet flow rate was set at low level (about 0.6 kg/h) at the

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beginning of anaerobic digestion. As the continuous anaerobic digestion went on, the

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Organic Loading Rates (OLRs) could be higher while the inlet pH could be lower if the

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performance of COD removal, biogas generation, and pH stability was normal, or the

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inlet COD, flow rate, and pH would be adjusted to reduce the OLRs and prevent

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acidification. Overall, the Hydraulic Retention Time (HRT) was about 5 d during this

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start-up process. The performance of COD removal, biogas generation, and pH stability

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was evaluated systematically throughout the whole start-up process.

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Analytical methods

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OLRs, HRT, and COD removal were defined as described by previous study 12. COD

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of inlet and outlet substrate after filtration by 0.45 µm filter membrane was measured

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once a day by a COD Water Quality Meter CM-05B (Shuanghui Jingcheng Electronic

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Product Limited Company, Beijing, China). Inlet flow rate of food waste hydrolysate

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was determined by measuring the reducing weight of inlet substrate in one hour. Biogas

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production was measured in duplicate once a day by water displacement. pH value was

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measured by pH meter (Hangzhou Leichi Ltd., China) once a day. Biogas was collected

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once a day by a gas sampling bag for measurement of the main composition of biogas

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(CH4 and CO2) by gas chromatography (GC). The GC analysis was carried out by

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GC-9800 (Shanghai Kechuang Instrument Limited Company, China) equipped with a

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TDX-01 column and a thermal conductivity detector according to previous method 12.

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Results and discussion

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Performance of COD removal

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In this study, food waste acid hydrolysate was applied as substrate for biogas

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production, and this completed liquid-state anaerobic digestion has great potential to

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overcome the disadvantages of traditional anaerobic digestion of food waste with

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semi-solid state. Usually, COD removal is one key value reflecting the efficiency of

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anaerobic digestion of liquid substrate and for a typical stable anaerobic digestion

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system, the value of COD removal is generally above 80% 17. In this research, the

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situation of COD removal was evaluated for the whole anaerobic digestion process (Fig.

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1).

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At the beginning of anaerobic digestion, a relatively high inlet COD (about 18000

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mg/L) was tried and during the first two days of anaerobic digestion, the outlet COD

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increased a little approximately from 1000 mg/L to 1500 mg/L. For safety (to prevent

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fast and great increase of outlet COD), the inlet COD was reduced to about 15000 mg/L.

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From the 1st day to the 4th day, the value of COD removal decreased a little from 93.9%

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to 87.5%, indicating that the granular anaerobic sludge did not completed acclimated to

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the environment of food waste hydrolysate. Because the COD removal was more stable

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from the 4th day to the 5th day (about 87.5%), the inlet COD was set higher again to

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about 20000 mg/L from the 4th day to the 6th day. From then on, the inlet COD

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maintained relatively stable. Interestingly, although the inlet COD increased again, the

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COD removal increased simultaneously to about 90%, and maintained at this value

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stably from the 6th day to the 10th day (90.0±0.9%), showing that this anaerobic

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digestion system has excellent and stable performance for COD removal and thus has

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great potential for the biological treatment of food waste. However, although the COD

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removal was suitable for application, the value of outlet COD was still high (>1000

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mg/L) thus this bio-resources (food waste hydrolysate) was wasted a little and the outlet

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hydrolysate required to be treated by some biological method such as aerobic digestion

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before drainage 18. Overall, during these ten days of anaerobic digestion, the value of COD removal was

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higher than 85% even when the inlet COD value was high (around 20000 mg/L),

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indicating that the granular anaerobic sludge has great adaptability to the environment

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of food waste acid hydrolysate and therefore the start-up is able to be completed in a

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short time.

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Performance of biogas generation For a long time, food waste was utilized as substrate for biogas (methane) production

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from 0.2 to 0.4 m3/kg VS (volatile solids) 19. In this research, the performance of biogas

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generation was evaluated during the whole anaerobic digestion process (Fig. 2 and 3).

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. Generally, the methane yield from food waste such as fruit and vegetable waste ranges

For traditional solid-state anaerobic digestion of organic waste, the biogas generation

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requires a long period 20. In this study, after the food waste hydrolysate was pumped

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into the bioreactor, the biogas can be immediately produced. Even a little lower than

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that of rest of fermentation period, the biogas production for the 1st day of fermentation

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was as high as 4539 mL/h, suggesting that the environment of food waste acid

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hydrolysate affected little on the biogas production capacity of granular anaerobic

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sludge. Obviously, this completed liquid-state anaerobic digestion was beneficial for the

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conversion of substrate to biogas by anaerobic sludge and this is one main reason for the

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phenomenon of fast biogas generation and the achievement of fast start-up. And after

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adaption of granular anaerobic sludge in the 1st day of anaerobic digestion, the biogas

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production was relatively stable (7641±1421 mL/h) from the 2nd day to the 10th day of

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anaerobic digestion. The biogas production was partly associated with the OLRs (Fig. 2),

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and thus the biogas production can be controlled by adjusting OLRs in actual

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application.

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Biogas (methane) yield is another important value to evaluate the performance of

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biogas generation 21, 22. According to this value, the biogas (methane) production per

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substrate utilization (COD degradation in food waste hydrolysate) can be evaluated. The

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biogas (methane) yield during the whole anaerobic digestion process is shown in Fig. 2.

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Throughout the anaerobic digestion process, the biogas yield and methane yield were

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0.542±0.056 and 0.442±0.053 m3/kg COD consumption, respectively. No matter the OLRs

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was high or low, the biogas (methane) yield was stable relatively, indicating that this

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technology is suitable and stable for biogas generation. Specially, during the later stage

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of the anaerobic digestion process (from the 6th day to the 10th day), the biogas and

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methane yield increased to higher level (0.567±0.053 and 0.457±0.060 m3/kg COD

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consumption,

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better as the anaerobic digestion went on. Considering that the biogas (methane) yield

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was measured for this short and early period of anaerobic digestion (ten days after

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pumping the food waste hydrolysate into the bioreactor), the biogas (methane) yield of

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this anaerobic digestion is attractive for industrial application.

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respectively), suggesting that the performance of biogas generation would be

For biogas produced by anaerobic digestion of organic wastes, CH4 and CO2 are its

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main components (their total content in biogas is usually higher than 98% and even

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close to 100%) and CH4 content (CH4/CO2 ratio) is an important parameter for

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industrial application of biogas. If the CH4 content (CH4/CO2 ratio) is higher, the energy

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potential of biogas is greater 23, 24. For common anaerobic digestion, the CH4 content in

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biogas is ranged from 53% to 70% 25. In this research, the main composition of biogas

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(CH4 and CO2) was analyzed during the whole anaerobic digestion process and the

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CH4/CO2 ratio was calculated accordingly (Fig. 3).

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Generally for one typical anaerobic digestion, the CH4 content (CH4/CO2 ratio) in

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biogas is low at the beginning of start-up and then the CH4 content will increase

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gradually as the anaerobic digestion goes on because of the adaptation of methanogenic

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bacteria in anaerobic sludge 26. In some special situation when the phenomenon of

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acidification exists during the start-up process, it is possible that the CO2 content in

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biogas is even higher than that of CH4 content, and this biogas is undoubtedly not

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suitable for actual application. In this research, the CH4 content in biogas was

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surprisingly high (>80%) even immediately after pumping the food waste hydrolysate

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into the bioreactor. During this fast and short period of anaerobic digestion, the CH4

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content was highly stable (81.52±3.05%), and this value is attractive for industrial

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application especially to produce biogas with high calorific value such as “Bio-natural

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Gas” (biogas generated by anaerobic digestion and following purification, whose

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composition and calorific value are almost the same as the conventional natural gas)

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because the cost of biogas upgrading (removal of CO2) can be saved greatly 7, 27.

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Obviously, the high CH4 content in biogas showed that the start-up of anaerobic

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digestion was efficient because the CH4 content in biogas is usually low in uncomplete

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start-up process 15.

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Performance of pH stability In actual application, pH value is one important parameter for anaerobic digestion.

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Generally, pH ranged from 6.8 to 7.2 is suitable for methanogenic bacteria while for

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fermentative bacteria, wider pH ranged from 4.0 to 8.5 is acceptable 6,17. Therefore, too

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low or too high pH is negative for methane formation and should be avoided during

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anaerobic digestion. Generally, low pH is more fatal to methanogenic bacteria than high

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pH: the activity of methanogenic bacteria is difficult to be recovered if it stay at the

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environment with low pH for long time; In contrast, the activity of methanogenic

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bacteria can be recovered simply by adjusting pH from high value to suitable value. For

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a fast start-up process of anaerobic digestion, pH stability is very important because

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acidification of fermentation system is easy and normal to happen especially at the

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beginning of anaerobic digestion. When the outlet pH decreases quickly, the inlet pH

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must be adjusted to higher value by adding of alkaline materials to prevent acidification

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of anaerobic digestion system 28.

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In this study, the value of inlet pH and outlet pH was determined during the whole

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anaerobic digestion process (Fig. 4). To prevent the possible acidification at the

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beginning of anaerobic digestion, the inlet pH was adjusted to a high level (>9.0) for the

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initial days of anaerobic digestion (the 1st and the 2nd day). During the operation, the

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outlet pH value was monitored carefully and when the outlet pH value was suitable for

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biogas production, the inlet pH could be decreased gradually. As shown in Fig. 4, the

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outlet pH value was surprisingly very stable (7.26±0.05), manifesting that this system is

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excellent for fast start-up of anaerobic digestion. Finally after 10 days of anaerobic

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digestion, the inlet pH can be decreased to the value close to outlet pH, and thus the

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alkaline materials used for neutralization of substrate (food waste hydrolysate) can be

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saved.

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Comparison with other anaerobic digestion of food waste

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For better evaluation on the effect of continuous anaerobic digestion of food waste

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using its acid hydrolysate as substrate on the performance of start-up, this anaerobic

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digestion mode was compared with other anaerobic digestion (Table 1). For anaerobic

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digestion of food waste, solid-state mode is usually applied (see in Table 1) and the high

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solid content of substrate will reduce the efficiency of anaerobic digestion and therefore

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requires longer start-up period 20, 25, 29. The start-up period can be shortened by some

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special technologies such as co-digestion and leachate recirculation 30, 31, but the start-up

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period is still long for actual application. In this research, almost immediately after

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pumping food waste hydrolysate into the bioreactor, the performance including COD

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removal and biogas generation could be very stable and efficient even with relatively

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high OLRs. Namely, the start-up can be completed quickly after the beginning of

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anaerobic digestion, and this technology does not need a long period to reach steady

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phase which make it suitable for actual application.

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As shown in Table 1, most food waste anaerobic digestion was carried out with batch

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mode. For batch anaerobic digestion, the bioprocess is usually completed in certain

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period thus the anaerobic digestion cannot last for a long time and the biogas generation

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will weaken at the later period of anaerobic digestion 26, 30, 32. Obviously, batch

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anaerobic digestion is unsuitable for actual application particularly for the situation

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requiring continuous biogas production. In contrast, continuous mode used in this study

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can treat food waste and produce biogas uninterruptedly, thus it is more promising for

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actual application. Besides, most food waste anaerobic digestion is still carried out with

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lab or small scale (see in Table 1), and the scale of anaerobic digestion (100 L semi-pilot

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scale) in this study is much larger, therefore it can reflect whether this technology is

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promising for industrial application or not more clearly.

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Overall, the methane yield and CH4 content in biogas in this study can be compared

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with and even higher than many other anaerobic digestion of food waste (Table 1).

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However, it is worth noting that many studies required a long time to reach the highest

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level of methane yield and CH4 content in biogas. For example, in previous study, the

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CH4 content in biogas can be close to 80% for the food waste anaerobic digestion, but

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this value was obtained after a long period (almost 50 days of anaerobic digestion) and

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during the beginning of anaerobic digestion, CO2 content in biogas was high (even

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higher than 80%) 26. Obviously, this long start-up period is negative for actual

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application . In this research, the CH4 content in biogas was larger than 80% almost

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immediately after pumping food waste hydrolysate into the bioreactor, again showing

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that this technology has excellent performance for start-up and biogas production.

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The potential of industrial application

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For improvement of biogas production from food waste, pretreatment is the most

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common procedure before anaerobic digestion. Generally, the pretreatment methods

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mainly include chemical, physical, and biological ones 8, 9. For industrial application,

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the procedures of pretreatment will undoubtedly increase the total cost of anaerobic

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digestion by using extra materials (e.g. acid, base, oxidant, etc.), energy consumption,

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operation cost especially for long period. In this study, for acid hydrolysis and later

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neutralization, merely small concentration of sulphuric acid and lime was used, and the

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cost is acceptable for industrial application (the market prices of sulphuric acid and lime

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are about 400 RMB/ton and 300 RMB/ton respectively in China 12). Besides, except that

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a small of urea and KH2PO4 were added into the inlet substrate, no extra nitrogen or

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phosphorus source was added into the fermentation system. Therefore, except the cost

308

of operation, almost no cost is need for this technology.

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In this study, acid hydrolysis of food waste firstly and then generating biogas from

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food waste hydrolysate is one novel technology for biogas production from food waste.

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In experiment, the COD of food waste acid hydrolysate was close to 100000 mg/L (0.1

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kg/L), and the average biogas yield was higher than 0.5 m3/kg COD consumption, namely 20

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L of food waste can generate about 1 m3 of biogas. For the treatment of 20 L food waste,

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the cost of raw materials used for acid hydrolysis and neutralization is very small (less

315

than 0.5 RMB). Besides, the cost for biogas production from food waste acid

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hydrolysate is mainly for the transport of food waste, electricity and steam used for

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operation, wastewater treatment, and human labor. Usually, the cost for above parts can

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be controlled in a suitable level (less than 1 RMB for production of 1 m3 of biogas).

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Nowadays, the general natural gas market prices in China is approximately 3.5 RMB/m3

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12

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the cost for its upgrading (obtaining biogas with higher CH4 content like natural gas) is

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relatively low. Considering this point, the potential market price of the biogas produced

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in this work can be estimated as 2.5 RMB/m3. Thus, the profit of this technology is

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attractive for industrialization.

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. In this research, the biogas produced had high CH4 content (approximately 80%) and

For larger scale application of food waste hydrolysate anaerobic digestion in much

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greater level, some issues should be taken into account carefully. For instance,

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hydrolysis of food waste and following lime treatment of food waste hydrolysate should

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also be scaled up to suitable level to satisfy the supply of fermentation substrate.

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Therefore, the equipment and operation of these technologies should be further

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optimized. Moreover, the integration of hydrolysis, lime treatment, and anaerobic

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digestion should be well designed. Last but not least, the treatment of waste generated

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by the technology in this study should be paid attention: the wastewater after anaerobic

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digestion can be treated by traditional biological method 18, and the solid residue with

334

low fermentability after hydrolysis of food waste can be further treated by traditional

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method of food waste treatment such as landfill, incineration, composting 33. Above

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points should be focused in further research. In conclusion, food waste acid hydrolysate was shown as one promising substrate for

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anaerobic digestion that it is possible to complete the start-up quickly with relatively

339

high OLRs. Overall, the performance of COD removal, biogas generation, and pH

340

stability shows that the technology in this study is promising for industrialization.

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Funding The authors acknowledge the financial support of the Key Research and

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Development Plan of Jiangsu Province, China (BE2016706), project of Huai-An

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Science and Technology (HAS2015035), National Natural Science Foundation of China

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(51378486, 21606229), Pearl River S&T Nova Program of Guangzhou (201610010014),

346

Youth Innovation Promotion Association CAS (2015290), Special Support Project of

347

Guangdong Province (2016TQ03N881), the Science and Technology Project of

348

Guangdong Province (2016A010105016), and Foundation of Key Laboratory of

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Renewable Energy, Chinese Academy of Sciences (Y707j41001).

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References

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1.

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energy: A review. Fuel 2014, 134, 389-399.

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442

Figure captions

443

Scheme 1 Pathway of biogas production from food waste acid hydrolysate.

444 445

Fig. 1 Performance of COD removal throughout the anaerobic digestion: (●) COD

446

removal; (△) inlet COD; (□) OLRs; (■) inlet flow rate; (▼) outlet COD.

447 448

Fig. 2 Performance of biogas generation throughout the anaerobic digestion: (○) Biogas

449

production; (■) Biogas yield; (△) Methane yield.

450 451

Fig. 3 CH4 and CO2 content in biogas, CH4 / CO2 ratio throughout the anaerobic

452

digestion: (■) CH4 content; (○) CO2 content; (△) CH4 / CO2 ratio.

453 454

Fig. 4 Inlet and outlet pH value throughout the anaerobic digestion: (■) Inlet pH; (○)

455

outlet pH.

456

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457

Scheme 1

458 459

Huang et al., (2017)

460

21



Figure 1

6

22500

COD removal (%)

80

Inlet and outlet COD (mg/L)

20000 5

17500 4

15000

60

12500 3

10000

40

20

2

7500 5000

1 2500

0

0

0 1

2

3

4

5

6

7

8

Fermentation time (d) 462 463


22


9

10

Inlet velocity (kg/h)

100

3 Organic Loading Rate kg COD/(m .d) /

461

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Figure 2

1.0

14000 0.8 12000

3

Biogas production (mL/h)

16000

10000

0.6

8000 0.4

6000 4000

0.2 2000 0

0.0 1

2

3

4

5

6

7

8


Biogas yield / methane yield (m /kg COD consumption)

464


467 468

23


9

10


Figure 3

CH4 and CO2 content (%)

100

10

8

80

6

60

4

40 2

20 0

1

2

3

4

5

6

7

8



472 473

24


9

10

CH4 / CO2 ratio

469

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474

Figure 4

10

pH value

9

8

7

6 1

2

3

4

5

6

7

8

Fermentation time (d)

475 476


25


9

10


Table 1 Comparison of anaerobic digestion of food waste with different modes and scales Anaerobic Start-up Substrate Anaerobic digestion mode Methane yield digestion scale period Food waste and waste Batch solid-state anaerobic 120 mL 5-7 days 0.326 m3/kg VSa activated sludge co-digestion Hay and soybean Batch solid-state anaerobic 2L 8-12 days 0.258 m3/kg VSa processing waste co-digestion Dog food 4.5 L Batch dry anaerobic digestion 3-4 weeks NAb

1

2 3

Page 26 of 27

a b

Food waste

5L

Food waste

10 L

Food waste

60 L

Food waste acid hydrolysate

100 L

Batch dry anaerobic digestion Semi-continuous dry anaerobic digestion Sequence batch dry anaerobic digestion Continuous liquid-state anaerobic digestion

20 days 10-20 days

VS refers to volatile solids. NA refers to not available.

26


0.22 m3/kg VSa 0.10 m3/kg food

CH4 content

Reference

NAb

31

Around 70% NAb Close to 80%

30 16 26

61.89%

29

66%

15

81.52%

This study

waste

2 weeks 2 days

NAb 0.442 m3/kg COD consumption

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