Bioresource Technology 154 (2014) 215–221

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Comparison of high-solids to liquid anaerobic co-digestion of food waste and green waste Xiang Chen a, Wei Yan a, Kuichuan Sheng a,⇑, Mehri Sanati b a b

College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, China Faculty of Engineering, Department of Design Sciences, Lund University, P.O. Box 118, SE-22100 Lund, Sweden

h i g h l i g h t s  High-solids and liquid co-digestion of food waste (FW) and green waste (GW).  Optimal biogas production was achieved at FW:GW mixing ratio of 40:60.  Methane yields at 15–20% total solids (TS) were higher than that at 5–10% TS.  Organic overloading at high TS content (25%) caused inhibition of methanogenesis.  Volumetric productivity at 15–25% TS was 3.8- to 4.6-fold higher than that at 5% TS.

a r t i c l e

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Article history: Received 17 October 2013 Received in revised form 10 December 2013 Accepted 12 December 2013 Available online 22 December 2013 Keywords: High-solids anaerobic digestion Co-digestion Food waste Green waste Biogas

a b s t r a c t Co-digestion of food waste and green waste was conducted with six feedstock mixing ratios to evaluate biogas production. Increasing the food waste percentage in the feedstock resulted in an increased methane yield, while shorter retention time was achieved by increasing the green waste percentage. Food waste/green waste ratio of 40:60 was determined as preferred ratio for optimal biogas production. About 90% of methane yield was obtained after 24.5 days of digestion, with total methane yield of 272.1 mL/g VS. Based the preferred ratio, effect of total solids (TS) content on co-digestion of food waste and green waste was evaluated over a TS range of 5–25%. Results showed that methane yields from high-solids anaerobic digestion (15–20% TS) were higher than the output of liquid anaerobic digestion (5–10% TS), while methanogenesis was inhibited by further increasing the TS content to 25%. The inhibition may be caused by organic overloading and excess ammonia. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Bioenergy recovery and pollution control through anaerobic digestion (AD) of organic wastes is a promising greenhouse gas mitigation option and considered to be a sustainable waste treatment practice (Pantaleo et al., 2013; Rajagopal et al., 2013). Since methane rich biogas is the main end product of AD, methane production must be improved to maximize revenues from energy generation and hence, to make digestion facilities more profitable (Fdez-Güelfo et al., 2012). Driven by a complex and diverse community of microbial organisms, the performance of AD is affected by a variety of operational factors, such as temperature, pre-treatment of substrates, and digester mixing. The total solids (TS) content in association with the organic loading rate is also one of the key factors that affect the performance, cost and stability of AD systems (Alvarez and Liden, 2008; Wu et al., 2009). It has been ⇑ Corresponding author. Tel.: +86 571 8898 2179; fax: +86 571 8898 2191. E-mail address: [email protected] (K. Sheng). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.12.054

reported that the TS content affects the following parameters: rheology and viscosity of the digester contents, fluid dynamics, clogging, and solid sedimentation that can directly influence the overall mass transfer rates within the digesters (Karthikeyan and Visvanathan, 2013). Since the TS content is an important parameter, two main types of AD processes have been developed: liquid and high-solids AD. Liquid AD (L-AD) systems typically operate with 0.5–15% TS, while high-solids AD (HS-AD) refers to a process that generally operates at 15–40% TS (Shi et al., 2013). It has been claimed that HS-AD is advantageous over L-AD for a number of reasons including higher volumetric loading capacity, reduced energy input for heating and mixing, and greater ease in handling the compost-like digestate (Li et al., 2011). However, both HS-AD and L-AD have their own advantages and disadvantages with respect to methane production maximization and process optimization. Even though the HS-AD process is reported to tolerate high organic loadings, low operational stability still hinders wide application of HS-AD technology (Schievano et al., 2010). HS-AD may be particularly sensitive to

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the inhibition caused by overproduction of volatile fatty acids (VFAs) and ammonia, due to organic overloading. However, so far, information is lacking concerning the quantitative threshold of the TS content below which methane production from HS-AD is higher or comparable to the output of L-AD. There are some studies related to the effect of the TS content on the performance of AD process. Forster-Carneiro et al. (2008) analyzed the AD process of food waste with three different TS levels. The results showed that reactors at 20% TS achieved a higher methane production compared to 25% and 30% TS. In a study conducted by Wu et al. (2009), no significant differences were observed in the methane production ranging from 351 to 381 mL/g VSfeedstock, applied to four TS contents of 1%, 2%, 5% and 10%. Recently, Brown et al. (2012) evaluated several lignocellulosic feedstocks (switchgrass, corn stover, wheat straw, yard waste, leaves, and maple) for biogas production under L-AD (5% TS) and HS-AD (18–19% TS). The study found no significant difference in methane yield between L-AD and HS-AD. These studies investigated the influence of TS control levels on AD, but the TS contents studied were within a narrow range; studies on a wider range of TS contents affecting performance of anaerobic reactors under both L-AD and HS-AD are limited. Food waste and green waste are available year round and account for a significant portion of municipal solid waste (MSW) (Brown and Li, 2013). The use of food waste and green waste may improve the overall economic benefits of AD process due to the low or zero cost associated with collecting these two feedstocks (Brown et al., 2012; Brown and Li, 2013). However, due to the high biodegradability and relatively low carbon to nitrogen (C/N) ratio, mono-digestion of food waste may encounter various potential inhibitors, including fast VFAs production from starch and free ammonia from protein (Brown and Li, 2013; Xu and Li, 2012). Mono-digestion of lignocellulosic green waste also faces challenges, including its poor nutrient content, slow start-up and long retention time (Pohl et al., 2013). Better methane production performance is expected in co-digestion systems. Co-digestion is a well-accepted process that enhances organic matter degradation and biogas production by synergistic and complementary effects, which improve the balance of nutrients and dilute inhibitory compounds (Kim and Oh, 2011; Wan et al., 2011). Consequently, anaerobic co-digestion may be a promising solution for centralized treatment of food waste and green waste. The objective of this study was to investigate the effect of TS control levels on anaerobic co-digestion of food waste and green waste. Anaerobic batch tests were conducted under L-AD and HS-AD, with TS contents ranging from 5% to 25%. A preferred food waste to green waste (FW/GW) mixing ratio was also needed to optimize biogas production for co-digestion. Hence, the effect of FW/GW mixing ratio on the performance of co-digestion was assessed first. Then, a comparison of high-solids to liquid anaerobic co-digestion of food waste and green waste was evaluated based on the pre-determined preferred FW/GW ratio.

2. Methods 2.1. Feedstock and inoculum The food waste was collected from one student canteen in Zhejiang University, Hangzhou, China. Impurities contained in the food waste, such as bones, eggshell, wastepaper and plastics were removed manually after sampling. Then the food waste was ground up using a blender (CPEL-23, Shanghai Guosheng, China). The ground food waste slurry was sealed in plastic bags and stored in a freezer at 20 °C. The food waste was thawed overnight under ambient conditions before usage.

Green waste was collected on the campus of Zhejiang University and mainly contained grass clippings and fallen leaves. The green waste was air-dried at room temperature for 48 h, and then ground with a grinder (DYQ-188, Ruian Huanqiu, China). Then the ground green waste was screened through a 5-mm sieve, and stored at 4 °C until used. The anaerobic sludge taken from the bottom settlement of a mesophilic anaerobic digester in Hangzhou, China was used as inoculum. The digester was a 300 m3 tank fed with livestock manure. Before sampling, the digester stirring was stopped for 1 day. The sludge was kept in air-tight buckets under ambient conditions (about 25 °C) after sampling.

2.2. Batch anaerobic digestion system Each batch AD system consisted of a 500-mL digestion glass bottle, a 2-L gas collection glass bottle and a 500-mL liquid collection beaker. The digestion bottle was loaded with feedstock and inoculum. Once biogas was produced in the digestion bottle, it was automatically distributed into the gas collection bottle which was filled with diluted hydrochloric acid solution (pH < 3), and then an equivalent volume of acid solution to the produced biogas was displaced to the liquid collection beaker. Thus, the biogas production volume could be measured periodically by means of the water displacement method.

2.3. Experimental design and set-up Two sets of experiments were carried out in the batch AD system. The first set of experiments studied the effect of FW/GW mixing ratios on biogas production via anaerobic co-digestion. Six feedstock mixing ratios (FW/GW: 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100, based on VS) were studied. Based on the initial TS contents of the food waste, green waste and inoculum, a sufficient amount of deionized water was added in each condition to adjust the TS content of the mixture inside the batch system to 15%. After completing the first set of experiments, a preferred FW/ GW mixing ratio for optimal biogas production was determined: 40:60. Based on this ratio, the second set of experiments investigated the effect of TS content on co-digestion of food waste and green waste. Food waste and green waste (40% food waste and 60% green waste, based on VS) were digested at five TS levels: 5%, 10%, 15%, 20% and 25%. Based on the initial TS contents of the feedstock and inoculum, a sufficient amount of deionized water was added in each digestion test to adjust the corresponding TS content. For the digestion tests with higher TS contents (i.e. 20% and 25%), the inoculum sludge was centrifuged (Centrifuge 5810R, Eppendorf, Germany) at 3000 rpm for 30 min. After removing the decanted liquid from the solid, the solid portion was collected, and then its TS and volatile solids (VS) contents were measured again for the digestion tests. In all the digestion tests, the feedstock and inoculum were loaded into the batch system at a feedstock/inoculum ratio of 1.0 (5.0 g VS of feedstock and 5.0 g VS of inoculum were added). The feedstock/inoculum ratio was calculated based on the amount of feedstock to the amount of inoculum on a VS basis. Blank trials containing inoculum only were performed to correct for the biogas produced from the inoculum. All the tests were carried out in duplicate. After adding the feedstock and inoculum, the anaerobic reactor was tightly closed with a rubber stopper and a screw cap, and then flashed with argon gas for 5 min. Thereafter, the AD systems were incubated at 37 ± 1 °C.

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3. Results and discussion 3.1. Characteristics of feedstocks and inoculum Characteristics of the food waste, green waste and inoculum sludge are shown in Table 1. TS contents of the food waste and green waste were 26.9% and 86.8%, respectively on a wet weight basis, both of which were above 25%. This indicates that these two materials with high solids contents were suitable for HS-AD. The anaerobic sludge had a TS content of 13.7%. Centrifugation of the sludge was adopted in this study to ensure that the TS of feedstock and sludge mixture in the reactors were around 20% or above. It has been reported that up to 50% of inoculum is required in HS-AD systems for a rapid start-up (Brown and Li, 2013; Martin et al., 2003). Consequently, a highly active and concentrated

Table 1 Characteristics of feedstocks and anaerobic sludge. Parameters

Food waste

Green waste

Anaerobic sludge

TS (%, w.b.) VS (%, w.b.) VS/TS (%) Total carbon (%, d.b.) Total nitrogen (%, d.b.) C/N pH Cellulose (%, d.b.) Hemicellulose (%, d.b.) Lignin (%, d.b.)

26.9 ± 0.3 25.2 ± 0.3 93.6 ± 0.5 46.3 ± 0.7 2.1 ± 0.2 22.0 ± 1.1 4.51 ± 0.01 ND ND ND

86.8 ± 0.3 74.3 ± 0.9 85.7 ± 1.2 45.3 ± 0.3 1.1 ± 0.1 41.2 ± 1.3 ND 32.1 ± 0.9 23.7 ± 0.7 14.1 ± 0.7

13.6 ± 0.2 6.4 ± 0.1 47.1 ± 0.2 29.4 ± 0.3 2.6 ± 0.5 11.3 ± 0.9 7.46 ± 0.01 ND ND ND

Note: w.b., wet base; d.b., dry base; ND, not determined.

3.2. Effect of food waste to green waste mixing ratios on biogas production The daily and accumulative biogas yields during the co-digestion of food waste and green waste at different mixing ratios are shown in Fig. 1. The biogas production processes ran for about 50 days until no more biogas production was observed. For all the digestion tests, biogas production started immediately from the first day, and peak daily biogas production rates were observed after 1.5 days of digestion. The highest biogas production rate was obtained at an FW/GW mixing ratio of 100:0, with a peak daily biogas production rate of 80.6 mL/g VS/d, which was 1.5-fold (p < 0.05) higher than that of the digestion system with an FW/ GW mixing ratio of 0:100. However, biogas production rate dropped immediately after the peak for the digestion system with an FW/GW mixing ratio of 100:0, and no biogas was produced from day 6.5 to 12.5, indicating that an apparent severe inhibition occurred. The inhibition was probably caused by the higher digestibility of food waste compared to the lignocellulosic green waste, leading to overproduction of VFAs that inhibited the

90 Daily biogas yield (mL/(g VS·d))

The TS and VS contents of food waste, green waste, inoculum sludge and digestate were determined according to the Standard Methods (APHA, 1998). Total carbon and nitrogen contents were measured by an elemental analyzer (EA 1112, CarloErba, Italy). To determine pH, NH4–N, total VFAs, and alkalinity (total inorganic carbon) before and after each test, samples were prepared by mixing a 5-g sample with 50 mL of deionized water, and subsequently the dilution was centrifuged at 10000 rpm for 15 min. The supernatant was then filtered through qualitative filter paper, and the filtrate was analyzed. The pH was determined by a pH meter (PHS-3D, Shanghai Jinghong, China). The NH4–N concentration was measured by spectrophotometry according to the Standard Methods (APHA, 1998). Total VFAs and alkalinity were determined through a two-step titration method (Voß et al., 2009). Cellulose, hemicellulose, and lignin contents in the green waste were analyzed according to the procedure described by Van Soest et al. (1991). The biogas composition (CH4, CO2, H2, and N2) from the headspace of the AD system was analyzed using a gas chromatograph (GC 2014, Shimadzu, Japan) equipped with a thermal conductivity detector. The temperatures of the column oven, injector port and detector were 100, 120, and 120 °C, respectively. Argon at a flow rate of 30 mL/min was used as a carrier gas. The biogas and methane yields at the end of each test were calculated by dividing the cumulative gas yields by the mass of VS in the feedstock loaded into the reactors at start-up. The volumetric methane productivity expressed in Vmethane/Vwork was calculated as the volume of methane production (Vmethane) per unit working volume of the reactor (Vwork). Analysis of variance (ANOVA) was performed using Microsoft Excel 2007 software to determine statistical significance with a threshold p-value of 0.05.

inoculum source was critical to speed up the HS-AD process (Brown and Li, 2013; Forster-Carneiro et al., 2008). The effluent of L-AD after dewatering and drying, or the digestate of HS-AD can be utilized in industrial HS-AD reactors. The cellulose, hemicellulose, and lignin contents of green waste were 32.1%, 23.7%, and 14.1%, respectively.

(a)

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100% FW + 0% GW 80% FW + 20% GW

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60% FW + 40% GW 40% FW + 60% GW

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20% FW + 80% GW

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0% FW + 100% GW

40 30 20 10 0 0

450 Cumulative biogas yield (mL/g VS)

2.4. Analytical methods and data analysis

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40% FW + 60% GW 20% FW + 80% GW

50

0% FW + 100% GW

0 0

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Fig. 1. (a) Daily and (b) accumulative biogas yields during co-digestion of food waste and green waste at different mixing ratios (note: FW = food waste, GW = green waste).

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methanogenesis process (Brown and Li, 2013). Then after about 10 days of self-recovery, the system started again to produce biogas, and a higher daily biogas production occurred from day 30 to 45. Digestion systems with FW/GW mixing ratios of 80:20 and 60:40 also had similar biogas production processes. For the digestion systems with FW/GW mixing ratios of 40:60, 20:80 and 0:100, the biogas production rates could be separated into two phases: an initial rapid production for the first 12 days followed by a slower rate over the rest of the digestion test. With increasing the amount of green waste from 60% to 100% in the feedstock, biogas production increased until day 32.5, 24.5, and 16.5, respectively, and subsequently biogas was produced at a negligible level until the end of experiments (day 50.5). It was found that higher daily biogas production occurred later in the digestion process when the food waste was the major component in the feedstock (60–100% food waste). The retention time for monodigestion of food waste (50.5 days) was two times longer than that for mono-digestion of green waste (16.5 days). It seemed the addition of green waste can result in shorter retention time. From Fig. 1b, it can also be seen that at the end of the digestion process, the total biogas yields were 409.8, 389.4, 388.8, 390.2, 324.7, and 270.9 mL/g VS for the digestion system with FW/GW mixing ratios of 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100, respectively. Approximately 44.0%, 43.7%, 66.5%, 90.3%, 96.7%, and 96.9% of the total biogas yields were obtained after the first 24.5 days of digestion, respectively, for FW/GW mixing ratios of 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100. During the period of 20–32.5 days, the highest accumulative biogas yield was observed for the FW/GW mixing ratio of 40:60. Consequently, increasing the amount of food waste in the feedstock led to an increase in the biogas yield, while higher biogas production efficiency was achieved by increasing the amount of green waste. The average methane contents of biogas produced from codigestion of food waste and green waste at different mixing ratios are shown in Fig. 2. Statistical analysis shows that the mixing ratios had significant effects (p < 0.05) on methane contents. The highest methane content of 79.7% was observed in the digestion system with 100% food waste, which was comparable to a study of the HS-AD (with 20% TS) of the organic fraction of MSW where the methane content remained practically constant at 80% in the period 15–40 days (Fernández et al., 2010). With the addition of green waste, methane content of the biogas started to decrease. The higher the composition ratio of green waste, the lower was the methane content in the digestion system. The system with 100% green waste had the lowest methane content of 60.7%. The higher methane content at higher composition ratios of food waste was

400 Methane content Methane yield

90 Methane content (%)

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70 60

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30 100 20

Methane yield (mL/g VS)

100

50

10

0

0 100:0

80:20 60:40 40:60 20:80 Food waste to green waste mixing ratio

0:100

Fig. 2. Average methane contents and total methane yields produced from codigestion of food waste and green waste at different mixing ratios.

probably caused by the high protein content contained in the food waste. Compared with carbohydrate-rich feedstocks, such as lignocellulosic green waste, methane content in the biogas produced during the degradation of protein-rich materials was higher (Weiland, 2010). A study by Liu et al. (2009) also found higher methane contents in the biogas were achieved from digestion of food waste than from green waste and the mixture (50% food waste and 50% green waste, based on VS). The total methane yield of the digestion systems can also be seen in Fig. 2. Methane yield comparisons closely resemble those of the methane content comparisons at the same FW/GW mixing ratios. By decreasing the food waste percentages in the feedstock from 100% to 0%, the methane yield decreased by 49.6% (from 326.4 to 164.6 mL/g VS). The methane yield from digestion of green waste was lower than the yield obtained from food waste. Since food waste and green waste were co-digested at different mixing ratios, the synergistic effect of co-digestion could be estimated as an additional methane yield for co-substrates over the weighted average of the individual substrate’s experimental methane yield (EMY) (Labatut et al., 2011). If the differential (EMY – Weighted EMY) was positive and greater than the standard deviation (SD) of EMY, the synergistic effect could be confirmed. The weighted EMY of co-substrates was calculated according to the following formula:

Weighted EMY ¼ EMYFW  PFW þ EMYGW  PGW

ð1Þ

where, Weighted EMY is the weighted average of experimental methane yield for co-substrates (mL/g VS); EMYFW and EMYGW are the experimental methane yields for food waste and green waste, respectively (mL/g VS); PFW and PGW are the percentage of food waste and green waste in the co-substrates, respectively on a VS basis. As seen in Table 2, synergistic effects were found in all the codigestion systems, since the positive differentials in methane yields were all greater than their SD. In the digestion system with an FW/GW mixing ratio of 40:60 in particular, the EMY was 18.7% higher than the weighted EMY. The synergism observed in the codigestion may arise from the adjustment of the C/N ratios in the reactors from 14.4 (food waste alone) and 16.9 (green waste alone) to 14.9–16.4, since the C/N ratios generally fell in the optimum range of 15–30 (Weiland, 2010). Co-digestion can actually improve the methane production of the AD process (Ward et al., 2008). In summary, the first set of experiments showed that the FW/ GW mixing ratios had significant effects (p < 0.05) on anaerobic co-digestion of food waste and green waste. In AD systems operating at a TS content of 15%, increasing the percentage of food waste in the feedstock led to an increase in the methane yield, while shorter retention time was achieved by increasing the amount of green waste. Considering balanced methane production capacity and efficiency, an FW/GW mixing ratio of 40:60 was regarded as a preferable mixing ratio. With this ratio, 90% of the methane yield was obtained after 24.5 days of digestion, and the total methane yield was determined to be 272.1 mL/g VS. 3.3. Comparison of liquid to high-solids anaerobic co-digestion of food waste and green waste 3.3.1. Biogas production A preferable FW/GW mixing ratio of 40:60 was determined in the first set of experiments to further study the effect of the TS control level on the co-digestion of food waste and green waste. Co-substrates were digested with five different TS contents: 5%, 10%, 15%, 20% and 25%, and the corresponding initial VS loadings were 15.4, 30.7, 46.1, 61.5, and 76.9 g VS/L, respectively. The daily and accumulative biogas yields during the co-digestion of food waste and green waste at different TS contents are shown in Fig. 3.

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X. Chen et al. / Bioresource Technology 154 (2014) 215–221 Table 2 Synergistic effect evaluation of co-digestion of food waste and green waste. FW/GW mixing ratioa

C/N

EMY

SD of EMY

Weighted EMY

Differential (EMY – Weighted EMY)

Increasing rate of methane yield (%)b

Synergistic effect

100:0 80:20 60:40 40:60 20:80 0:100

14.4 14.9 15.3 15.8 16.4 16.9

326.4 305.9 284.6 272.1 211.2 164.6

30.7 7.2 10.0 26.6 9.1 4.8

326.4 294.0 261.7 229.3 196.9 164.6

– 11.9 22.9 42.8 14.3 –

– 4.0 8.8 18.7 7.2 –

– Synergistic Synergistic Synergistic Synergistic –

FW: food waste; GW: green waste; C/N: C/N ratio in the mixture of food waste, green waste and anaerobic sludge; EMY: experimental methane yield (mL/g VS); SD: standard deviation. a Based on volatile solids (VS). b The increasing rate of methane yield was calculated based on the comparison between EMY and weight EMY.

Daily biogas yield (mL/(g VS·d))

5% TS 10% TS 15% TS 20% TS 25% TS

50 40 30 20 10 0 0

5

10

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40

45

50

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150

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100

15% TS 20% TS

50

25% TS

0 0

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10

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20 25 30 35 Digestion time (Days)

40

45

50

production process for the HS-AD systems (20% and 25% TS). After the biogas production peak on day 1.5, the rate declined rapidly, and the digesters had zero biogas production from day 4.5 to 8.5. Then, after about 6 days of self-recovery, biogas production resumed in the reactors, and a higher daily biogas production was observed from day 24.5 to 45. Fig. 3b shows that the total biogas yields from the digestion systems operating at TS of 5%, 10%, 15%, 20% and 25% were 312.8, 335.7, 390.2, 348.4, and 269.0 mL/g VS, respectively. After 16.5 days of digestion, about 82.8%, 82.7%, 73.0%, 46.1%, and 50.2% of the total biogas yield was achieved, and after 24.5 days of digestion, about 95.8%, 96.1%, 90.3%, 64.3%, and 63.1% of the total biogas yield was achieved, respectively, for TS of 5%, 10%, 15%, 20% and 25%. It can be seen that the retention time of L-AD systems with 5% and 10% TS was about 25 days for complete digestion, while a 2.0-fold increased retention time was found for the HSAD systems with 20% and 25% TS. This could be explained that the mass transportation in HS-AD was much slower than that in L-AD (Li et al., 2011). The average methane contents of biogas produced from co-digestion of food waste and green waste at different TS control levels are shown in Fig. 4. The statistical analysis shows that the TS control level had significant effects (p < 0.05) on methane contents. Methane contents varied from 65.5% to 70.9% at the TS level range of 5–25%. The total methane yields were therefore calculated to be 215.9, 224.0, 272.1, 246.9 and 176.3 mL/g VS, respectively at TS levels of 5%, 10%, 15%, 20% and 25% (shown in Fig. 4). The higher methane yields at TS levels of 15% and 20% indicated that methane production from HS-AD could be higher or comparable to the output of L-AD (Li et al., 2011; Luning et al., 2003). However, the continual increase of TS content from 20% to 25% caused a 35.2% decrease in the methane yield, with respect to the yield at 15%

Fig. 3. (a) Daily and (b) accumulative biogas yields during co-digestion of food waste and green waste at different TS control levels.

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Similar to the first set of experiments, biogas production started immediately from the first day for all the digestion tests, indicating fast acclimation of the microorganisms to the co-substrates. The daily biogas production rates in the digestion systems at TS levels of 5%, 10%, 15%, 20% and 25% reached their peak values of 42.5, 40.2, 55.3, 60.5 and 39.0 mL/g VS/d, respectively, after 1.5 days of digestion. For the digestion systems at TS contents of 5%, 10% and 15%, the biogas production rate dropped immediately after the first 4.5 days of digestion, and then resumed to reach another peak at day 10.5. This was followed by a slower rate over the rest of the digestion test. For the L-AD systems (5% and 10% TS), biogas production increased until day 24.5, and then remained almost constant until the end of experiments (day 50.5). Similar to digestion systems with higher percentages of food waste in the first set of experiments, there was a suspension phase during the biogas

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Methane yield (mL/g VS)

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Fig. 4. Average methane contents and total methane yields produced from codigestion of food waste and green waste at different TS control levels.

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TS. Additionally, the methane yield at 25% TS was 18.3% lower than that at 5% TS. These results were not completely consistent with a previous study conducted by Brown et al. (2012). They evaluated the methane production from yard waste under L-AD (5% TS) and HS-AD (18–19% TS), and found no significant difference in methane yield between L-AD and HS-AD. However, the HS-AD of yard waste at higher TS contents (above 20% TS) was not investigated by Brown et al. (2012). In this study, methane yields obtained from HS-AD at TS levels of 15–20% were comparable to the yields of L-AD, while the methane yield started to decrease with an increasing TS level to 25%. The volumetric methane productivities of the digestion systems in both L-AD and HS-AD are presented in Fig. 5. It can be seen that volumetric productivity comparisons generally resemble those of the methane yield comparisons at the same TS levels. L-AD at 5% TS showed the lowest volumetric productivity (0.9 Lmethane/Lwork). Increasing the TS level from 5% to 10% increased the volumetric productivity 2.2-fold (p < 0.05). HS-AD systems at TS levels of 15%, 20%, and 25% had similar volumetric productivity (3.6–4.3 Lmethane/Lwork), which showed increases of 278–357% compared to that of L-AD at 5% TS. The higher volumetric productivity of HS-AD confirmed a main advantage over L-AD due to higher volumetric loading capacity and smaller reactor volume (Brown et al., 2012; Guendouz et al., 2008). 3.3.2. VS reduction The VS reduction of the co-substrates is also shown in Fig. 5. It can be seen that the VS reduction values were highly correlated

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Volumetric productivity (L methane/Lwork )

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25

Fig. 5. Volumetric methane productivity and VS reduction at different TS control levels.

with the methane production yields at the same TS control levels (Fig. 4). Higher VS reduction values were obtained in the digestion systems with higher methane yields. The highest methane yield and VS reduction were observed at the TS level of 15%. By increasing the TS level from 5% to 15%, a 1.2-fold increase in VS reduction was achieved. The continual increase of TS content from 15% to 25% caused a 17.7% decrease in the VS reduction. Although the methane yield at 25% TS was 18.3% lower than that at 5% TS, the VS reduction of 39.6% at 25% TS was comparable to the VS reduction of 40.4% at 5% TS. The lower methane yield but comparable VS reduction at 25% TS may be due to the conversion of VS into intermediate products such as high concentrations of VFAs (Brown and Li, 2013). 3.3.3. Digestion system characteristics It is in the interest of AD plant owners to run the plant at its operational optimum to maximize methane production, while most biogas plants run at a less-than-optimum loading rate to prevent instability in the anaerobic digesters resulting from poor monitoring systems (Ward et al., 2008). Monitoring of the key chemical and physical parameters is necessary to achieve optimal control of the AD process. The instability in a disturbed AD process is often caused by excess accumulation of VFAs, resulting in a dramatic decrease in pH if the buffering capacity of the system is not sufficient (Brown and Li, 2013; Ward et al., 2008). The ratio of total VFAs to the total inorganic carbonate (VFA/alkalinity) has been recognized as a guide value for assessing process disturbances at an early stage (Lossie and Pütz, 2010). The VFA/alkalinity ratio is a more reliable parameter for monitoring digester imbalance than simple measurements of pH, since an accumulation of VFAs will lead to a significant decrease of the buffering capacity before the pH decreases. Therefore, VFA/alkalinity ratios and pH were measured in this study to monitor the AD process (Table 3). The initial pH in the digestion systems at different TS levels ranged from 7.3 to 8.0, which were within or close to the optimum pH interval of 7.0–8.0 recommended by Weiland (2010). After the reaction, a slight increase was observed for the final pH in all the digestion systems with respect to the initial pH. The digestion systems had final pH values ranging from 7.4 to 8.7. As shown in Table 3, the initial and final VFA/alkalinity ratios were below 0.3 for the digestion systems at TS levels in the range of 5–20%, except for the system at 25% TS, where the initial and final VFA/alkalinity ratios were 0.38 and 0.64, respectively. According to the observations and recommendations provided by Lossie and Pütz (2010), VFA/alkalinity ratios between 0.3 and 0.4 are generally regarded as optimal for biogas production at a maximum, ratios below 0.3 are regarded as having deficient feedstock input, and ratios exceeding 0.6 are regarded as having excessive

Table 3 Variation of pH, VFA/alkalinity ratio, and TAN concentration during co-digestion of food waste and green waste. Food/Green mixing ratioa

100:0 80:20 60:40 40:60 20:80 0:100 40:60 40:60 40:60 40:60 40:60 a b

Based on volatile solids (VS). mg HAceq/mg CaCO3.

TS level (%)

15 15 15 15 15 15 5 10 15 20 25

VFA/alkalinity ratiob

pH

TAN concentration (mg/L)

Initial

Final

Initial

Final

Initial

Final

7.66 ± 0.05 7.76 ± 0.00 7.81 ± 0.03 7.81 ± 0.03 7.88 ± 0.01 7.94 ± 0.04 7.30 ± 0.01 8.03 ± 0.01 7.81 ± 0.03 7.89 ± 0.04 7.95 ± 0.04

8.36 ± 0.06 8.39 ± 0.06 8.34 ± 0.04 8.37 ± 0.08 8.36 ± 0.00 8.37 ± 0.00 7.39 ± 0.08 8.25 ± 0.01 8.37 ± 0.08 8.70 ± 0.11 8.71 ± 0.10

0.16 ± 0.03 0.12 ± 0.09 0.14 ± 0.00 0.06 ± 0.00 0.19 ± 0.06 0.14 ± 0.12 0.27 ± 0.02 0.08 ± 0.00 0.06 ± 0.00 0.12 ± 0.01 0.38 ± 0.03

0.35 ± 0.03 0.22 ± 0.21 0.17 ± 0.00 0.22 ± 0.06 0.13 ± 0.07 0.14 ± 0.07 0.25 ± 0.02 0.12 ± 0.01 0.22 ± 0.06 0.15 ± 0.08 0.64 ± 0.28

1865.6 ± 11.1 1865.3 ± 66.7 1963.5 ± 155.6 1946.6 ± 44.4 2078.3 ± 55.6 2360.2 ± 33.3 607.8 ± 32.3 1488.1 ± 11.1 1946.6 ± 44.4 2214.7 ± 54.3 2954.6 ± 36.8

3456.5 ± 22.2 3228.1 ± 11.1 3117.4 ± 300.0 3423.6 ± 44.4 3504.8 ± 77.8 3254.9 ± 44.4 769.2 ± 49.5 1853.6 ± 33.3 3423.6 ± 44.4 2764.9 ± 144.5 4243.4 ± 22.2

X. Chen et al. / Bioresource Technology 154 (2014) 215–221

feedstock input. As the feedstock/inoculum ratio in this study was fixed at 1.0, more biogas production may be achieved by increasing the feedstock/inoculum ratio for the digestion systems at TS levels in the range of 5–20%, while for the digestion system at 25% TS, increasing the amount of inoculum may improve the gas production performance. For the HS-AD system at 25% TS, although the final pH was high (pH = 8.7), a high final VFA/alkalinity ratio of 0.64 was found, which was also reflected by the low methane yield (Fig. 4). This was likely caused by the accumulation of VFAs due to overfeeding. Ammonia accumulation is potentially encountered during AD of N-rich feedstock due to the degradation of proteinaceous materials (Sung and Liu, 2003). An optimal ammonia concentration ensures sufficient buffering capacity in the AD system thus increasing the stability of the AD process, while high ammonia is reported as a strong inhibitor of biogas production (Chen et al., 2008; Rajagopal et al., 2013). Chen et al. (2008) reported that the inhibiting total ammonia nitrogen (TAN) concentration that caused a 50% decrease in the methane yield varied much from 1.7 to 14 g/L. Most of the studies on ammonia inhibition have been focused on traditional L-AD over the past few decades. As seen in Table 3, the initial and final TAN concentrations in the digestion systems generally increased with the increase of TS levels from 5% to 25%. The digestion system at 25% TS had a 5.5-fold higher final TAN concentration than that of the system at 5% TS. It seems that ammonia inhibition is more likely to be encountered in HS-AD, since lower water content affects dilution (Wang et al., 2013). Considering that the methane yields in HS-AD at 15–20% TS were higher or comparable to the yields in L-AD, and that the methane yield started to decrease with further increasing TS level to 25% (Fig. 4), it can be concluded that a higher TAN concentration (4.2 g/L) in the digestion system at 25% TS may initiate inhibition of methanogenesis, leading to lower methane yields. Consequently, careful consideration should be taken to avoid ammonia inhibition of the HS-AD process. 4. Conclusion The optimal performance for co-digestion of food waste and green waste was achieved at their mixing ratio of 40:60. Under this preferred ratio, the effect of TS content (5–25%) on anaerobic codigestion was investigated in batch systems. The results indicate that methane yields from HS-AD (15% and 20% TS) were higher or comparable to the output of L-AD (5% and 10% TS), while 25% TS content corresponded to a threshold at which methane production was inhibited. Considering the volumetric productivity, HS-AD systems (15–25% TS) showed increases of 278–357% compared to that of L-AD at 5% TS. Acknowledgements This work was financially supported by National Science & Technology Pillar Program of China (No. 2012BAC17B02). The authors also would like to thank Mrs. Eileen Deaner (Department of Design Sciences, Lund University) for proofreading and language correction. References Alvarez, R., Liden, G., 2008. Semi-continuous co-digestion of solid slaughterhouse waste, manure, and fruit and vegetable waste. Renew. Energy 33, 726–734.

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Comparison of high-solids to liquid anaerobic co-digestion of food waste and green waste.

Co-digestion of food waste and green waste was conducted with six feedstock mixing ratios to evaluate biogas production. Increasing the food waste per...
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