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Improved biogas production from food waste by co-digestion with de-oiled grease trap waste
Accepted Manuscript Improved biogas production from food waste by co-digestion with de-oiled grease trap waste Li-Jie Wu, Takuro Kobayashi, Hidetoshi Kuramochi, Yu-You Li, Kai-Qin Xu PII: DOI: Reference:
S0960-8524(15)01594-1 http://dx.doi.org/10.1016/j.biortech.2015.11.061 BITE 15794
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
15 October 2015 18 November 2015 22 November 2015
Please cite this article as: Wu, L-J., Kobayashi, T., Kuramochi, H., Li, Y-Y., Xu, K-Q., Improved biogas production from food waste by co-digestion with de-oiled grease trap waste, Bioresource Technology (2015), doi: http:// dx.doi.org/10.1016/j.biortech.2015.11.061
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Improved biogas production from food waste by co-digestion with de-oiled grease trap waste Li-Jie Wua, Takuro Kobayashib, Hidetoshi Kuramochib, Yu-You Lia,c and Kai-Qin Xub,d, a Department of Civil and Environmental Engineering, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan b Center of Material Cycles and Waste Management Research, National Institute for Environmental Studies, Tsukuba 305-8506, Japan c Department of Frontier Science for Advanced Environment, Graduate School of Environmental Studies, Tohoku University, Sendai, 980-8579, Japan d School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ------------------------------------------------------------------------------------------------------------Corresponding author: Takuro Kobayashi National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, Ibaraki 305-8506, Japan Phone: +81-29-850-2400. Fax: +81-22-850-2560. E-mail: [email protected]
Abstract The objective of this study was to assess the feasibility of co-digesting food waste (FW) and de-oiled grease trap waste (GTW) to improve the biogas production. A lab-scale mesophilic digester (MD), a temperature-phased anaerobic digester (TPAD) and a TPAD with recycling (TPAD-R) were synchronously operated under mono-digestion (FW) and co-digestion (FW+ de-oiled GTW). Co-digestion increased the biogas yield by 19% in the MD and TPAD-R, with a biogas yield of 0.60 L/g VS added. Specific methanogenic activity in the TPAD-R was much higher than that in the MD. In addition to methane, hydrogen at a yield of approximately 1 mol/mol hexose was produced in the TPAD-R. Alkalinity was consumed more in the co-digestion than in mono-digestion. Co-digestion resulted in more lipid accumulation in each digester. The MD favored the degradation of lipid and conversion of long-chain fatty acids more than the TPAD and TPAD-R.
Keywords:
Food
waste
(FW);
Grease
trap
waste
(GTW);
Anaerobic
digestion;
Temperature-phased anaerobic digestion (TPAD); Two-stage; Long-chain fatty acid (LCFA)
1. Introduction Food waste (FW) is the largest fraction in increasing municipal solid waste. The generation of FW is predicted to increase with 44% by 2025 (Ariunbaatar et al., 2014). The uncontrolled discharge of FW causes severe environmental pollution, such as decay and odor. Anaerobic digestion has proved to be an effective technology to treat FW, such an easily biodegradable organic matter with high moisture, carbohydrate, lipid, and protein contents, due to the capabilities in energy recovery in the form of methane, waste reduction and stabilization, low carbon emission and limited pollution (Zhang et al., 2014). Nonetheless, in many cases, anaerobic digestion of municipal solid waste in smaller facilities often confronts the issues that biogas utilization is less economically or operationally appealing than flaring excess biogas. As a consequence, smaller waste treatment facilities do not commonly use anaerobic digesters for solids management, let alone implement biogas utilization (Wang et al., 2013). One feasible way to improve the economics of biogas utilization is to substantially increase biogas production by anaerobic co-digestion with substrates of high biogas production potential. The co-digestion, the simultaneous digestion of two or more substrates, can improve methane yield because of the supply of additional nutrients from the co-substrates, and more efficiently use the equipment and share cost by processing multiple waste streams in a single facility, as compared to mono-digestion (Alatriste-Mondragón et al., 2006). In addition to FW, another major stream of organic waste from restaurant is the grease trap waste (GTW). The GTW is from a small passive or mechanized oil/water separator installed inside the food service establishments in case of the blockage in the wastewater pipe. Conversion of oily content in GTW to biodiesel is widely studied because of concerns relating to the quality of
the feedstock, specifically the presence of high moisture content and free fatty acids (Montefrio et al., 2010; Toba et al., 2011). The de-oiled GTW turns out to be appropriate as a substrate for anaerobic digestion (Kobayashi et al., 2014). Furthermore, due to still high lipid content in the de-oiled GTW, more methane can be produced owing to theoretically higher methane production potential of lipid than other organic matters (Angelidaki and Sanders, 2004). Theoretically, 1 g of glycerol trioleate (C57H104O6), a common lipid in nature, is equivalent to 1.08 L of methane at standard temperature and pressure, while 1 g of glucose (C6H12O6) is equivalent to only 0.37 L (Kim and Shin, 2010). A number of studies have shown that such a lipid-rich organic waste could become potential co-substrate (Li et al., 2013; Silvestre et al., 2014). Thus, it is possible to take advantage of de-oiled GTW as the co-substrate of FW to increase the economics of biogas utilization. Moreover, the proximity of GTW to FW favors the collection of the waste for the following treatment. However, previous studies focus their attention on the co-digestion of lipid-rich waste to enhance the biogas production of sewage sludge, and the investigation on the co-digestion of de-oiled GTW with FW has almost not been found. In anaerobic digestion of lipid-containing waste, lipid loading is a critical parameter. When the de-oiled GTW is overdosed, accumulation of long-chain fatty acids (LCFAs), propionate and acetate can result in a pH drop and even process failure, due to limited substrate and product transport, damaged cells and reduced activity of stressed microbial communities (Wang et al., 2013). Thermophilic condition (55℃) for lipid-containing waste is more advantageous than mesophilic condition (35℃), due to increased ability to degrade LCFAs and a smaller scum layer. It is because that under thermophilic condition, lipid becomes more accessible to microorganisms and their lipolytic enzymes caused by increased diffusion coefficients and lipid solubility in
aqueous media with increasing temperature (Chipasa and Medrzycka, 2006). However, thermophilic bacteria may be more sensitive to LCFA inhibition than mesophilic bacteria (Hwu et al., 1997). Thus, it is possible to apply temperature-phased anaerobic digestion (TPAD), which integrates the advantages of thermophilic and mesophilic digestion, to the degradation of mixed substrate of FW and de-oiled GTW to improve the operation performance. In fact, the relevant studies have never been found yet. Hwu et al. found that the wash-out biomass exhibited a greater LCFA degradation capability than the biomass remaining inside the reactor (Hwu and Lettinga, 1997). Therefore, the biomass recycling was recommended to reduce the biomass washout and the toxic effect on the acetogenic and methanogenic populations (Pereira et al., 2001). On the basis of TPAD, the introduction of a biomass recycling from the end stage to the front stage, namely TPAD-R, is likely to further optimize the performance of TPAD in degrading lipid-containing waste. The feasibility of TPAD-R in digesting FW has been demonstrated by many previous studies (Kobayashi et al., 2012; Lee et al., 2010), but little attention was paid on the application of TPAD-R to the digestion of de-oiled GTW, let alone the co-digestion of FW and de-oiled GTW. The goal of this study was to investigate the feasibility to improve the biogas production by co-digesting FW and de-oiled GTW in single-stage mesophilic digestion (MD), TPAD and TPAD-R. The specific objectives were to (a) determine the extent to which the biogas production is improved by co-digestion, (b) evaluate the operation differences among MD, TPAD and TPAD-R in digesting FW and mixed substrates, and (c) study the effects of biomass recycling on the TPAD.
2. Materials and methods 2.1 Experimental setup The anaerobic experiments were conducted using three systems, the single-stage MD and the two-stage TPAD and TPAD-R systems as shown in Fig.1. The MD is comprised of a reactor system, while both the TPAD and TPAD-R included two reactor systems. Each reactor system consists of a continuous stirred tank reactor equipped with a temperature controller, a feeding and decanting system, and a biogas collection system (WNK-0.5, Shinagawa Corporation, Japan). The effective volumes for the MD, TPAD and TPAD-R were set as 6 L, 10L and 7.5 L, respectively. The TPAD and TPAD-R were operated with the ratios of the front stage (stageⅠ) to the end stage (stageⅡ) 1:4, and under this condition better operation performance was achieved than other ratios (Coelho et al., 2011; Riau et al., 2012). A recycle system was introduced from the stageⅡof the TPAD-R to stageⅠ, with the same flow-rate as influent of the system. The temperature controllers were used to make sure the required temperature of each reactor by heaters and water jackets. The feeding and decanting systems worked semi-continuously using peristaltic pumps. Particularly, the feedstock of each system was fed manually once a day.
2.2 Substrates and inoculums The substrates used in this study contained prepared FW and de-oiled GTW. Raw FW was collected from a dining hall at the National Institute for Environmental Studies, Tsukuba, Japan. It was mixed with tap water as 1:1.4 (raw FW: tap water) in a tank (4 ℃) equipped with the cooling system before it was shredded with a shear pump to the particle size less than 5 mm, in order to meet the requirement of wet anaerobic digestion. The prepared FW was stirred in the tank to
ensure the homogeneity. The GTW relevant to this study was collected from grease traps of 11 different types of restaurants in Japan, mainly supplying meat products and serving for the banquet. Subsequently, the mixture of different restaurant sources was heated to 60℃ for at least 6 hours, which caused layer separation. After the upper oil layer was pumped away for biodiesel recovery, de-oiled GTW was obtained. The characteristics of single substrate FW and mixed substrate FW+ de-oiled GTW used in this study are shown in Table 1. Due to deficient trace elements in restaurant waste for anaerobic digestion, necessary trace elements, such as Fe, Co and Ni, were supplemented to the feedstock in case of biogas production limitation caused by deficient trace elements. The prepared stock metal solution contained Fe (FeCl2·4H2O) 10000 mg/L, Co (CoCl2·6H2O) 1000 mg/L and Ni (NiCl2·6H2O) 1000 mg/L. The stock solution was added to the substrate as the ratio of 1 mL stock solution: 100 mL substrate (Kobayashi et al., 2012). The mesophilic digesting sludge fetched from a wastewater treatment plant in Ibaraki Prefecture for sewage sludge treatment was used as mesophilic inoculum, while the thermophilic digesting sludge acclimated from mesophilic sludge at least one month as the reported method was used as thermophilic inoculums (Bou V S Kov A et al., 2005).
2.3 Experimental design The stageⅠof TPAD and TPAD-R was controlled at thermophilic temperature (55 ℃), and MD and the stage Ⅱ of TPAD and TPAD-R was set at mesophilic temperature (35 ℃ ). Thermophilic reactors and mesophilic reactors in MD, TPAD and TPAD-R were initially inoculated with thermophilic inoculum and mesophilic inoculum, respectively, until the working volume of each reactor was achieved. FW was firstly pumped into each system as hydraulic
retention time (HRT) 100 days. Start-up process was accomplished by shortening the HRT from 100 days, to 50 days and then to 30 days. The performance at HRT 30 days was selected as the final operation results. Under the condition of total HRT 30 days, the HRTs for the MD, stage Ⅰand stageⅡ Ⅱof the TPAD, and stageⅠ Ⅰand stageⅡ Ⅱof the TPAD-R are 30 days, 6 days, 24 days, 3 days, and 12 days, respectively. The period in HRT 100 days and 50 days took about 30 days before the HRT was shortened to 30 days. Through over 100 days’ operation, the feedstock of all systems was changed to mixed substrate of FW and de-oiled GTW, with a lipid/total solid (TS) 40%, until the experiment ended. It is worth pointing out that the value of lipid/TS at 40% has proved appropriate for the prevention of inhibition caused by lipid (Li et al., 2002). After the steady state of the co-digestion was reached, the residual biogas production and specific methanogenic activity (SMA) test (with acetate as substrate) for the effluents of each system were carried out.
2.4 Analytical methods Biogas production was noted on a daily basis and normalized to standard conditions according to the daily local meteorological data. Nearly every day biogas samples were taken to analyze the content of nitrogen, methane and carbon dioxide with a gas chromatograph (GC-8A, Shimadzu), equipped with a thermal conductivity detector and a stainless steel column packed with Shincarbon ST (Shimadzu GLC). Sludge samples were taken from the sampling ports of each reactor twice a week, together with substrate, for the subsequent analysis. The pH, TS, volatile solid (VS), and alkalinity were measured according to the Standard Method (APHA, 2012). Chemical oxygen demand (COD) was determined by COD Digest Vials (HACH). Volatile fatty
acids (VFAs) and LCFAs were analyzed referring to the previous report (Kobayashi et al., 2014). Samples for soluble items analysis, such as VFA, soluble COD (SCOD), alkalinity and NH4+-N were prepared by centrifuging samples at 13000 rpm for 5 min and filtering them with 0.45 µm pore-size filters. The organic matter carbohydrate and protein was analyzed using spectrophotography. A methanol-chloroform extraction and weight method was taken to measure lipid. The methods to measure and analyze the residual biogas production and SMA test referred to the previous reports (Kobayashi et al., 2015). Each sample was set in duplicate.
3. Results and discussion 3.1 Feedstock characterization The characteristics of the prepared FW and the mixed substrate of FW and de-oiled GTW used in this study are shown in Table 1. Both of them were adjusted to the similar solid concentration, lower than 10%, to meet the requirement of the wet anaerobic digestion. The percentages of VS/TS in the single substrate FW and the mixed substrate were similar as well, with a percentage of 95%. Such a high VS concentrations in the substrates revealed a pretty high organic matter content, indicating a desirable methane production potential. After the FW was mixed with the de-oiled GTW, the concentrations of carbohydrate and protein decreased to some extent, while the lipid concentration increased from 13.9 g/L (lipid/TS=20%) to 32.9 g/L (lipid/TS=40%). Since the same mass concentration for lipid is representative of more COD than for carbohydrate and protein, the higher lipid concentration in the mixed substrate led to increased COD 1.86 g/g TS.
3.2 Biogas production The biogas production characteristics over time in the MD, TPAD and TPAD-R are presented in Fig.2. At the preliminary stage of the operation, the production rates in each system increased gradually. After more than two turnovers at HRT of 30 days for mono-digestion, all the systems achieved a steady biogas production. The stable biogas production was achieved earlier in both the MD and TPAD-R than in the TPAD. Due to the successful start-up in the period of mono-digestion, the time to achieve stable biogas production for co-digestion was shortened, especially for the MD and TPAD. The operation performance on biogas production in the steady state is summarized in Table 2. Co-digestion increased the biogas yields of the mono-digestion in the MD, TPAD and TPAD-R by 19%, 8% and 19%, respectively. In addition, the biogas production rates for both the mono-digestion and co-digestion were slightly higher in the TPAD-R than in the MD. Similar biogas production rates were observed in stageⅠ(1.82 L/L/d) and stageⅡ (1.91 L/L/d) of the TPAD-R for mono-digestion. However, after the co-digestion started, the biogas production rate in stageⅠof the TPAD-R decreased to less than 1 L/L/d, while improved biogas production rate 2.86 L/L/d occurred in the following stageⅡ. Hydrogen was the main effective biogas component in stageⅠof the TPAD-R, and preferred substrate for hydrogen production is carbohydrate (Kapdan and Kargi, 2006; Kobayashi et al., 2012; Okamoto et al., 2000). Therefore, the reduced biogas production in stageⅠof the TPAD-R possibly resulted from the less carbohydrate in the FW than in the mixture of FW and de-oiled GTW. As a result of almost no biogas produced in stageⅠof the TPAD, despite higher biogas production rates in stage Ⅱof the TPAD than in the MD, the capability of the whole TPAD to produce biogas decreased. In the initial start-up period, hydrogen was produced in stageⅠof the TPAD. However,
hydrogen disappeared in stageⅠof the TPAD immediately with the shortened HRT. Methane production was never observed in this stage. A different biogas composition from that in stageⅠ of the TPAD existed in stageⅠof the TPAD-R. In addition to a little methane, hydrogen accounted for a significant proportion in stageⅠof the TPAD-R, with a percentage of 31.7% for the mono-digestion and 26.7% for the co-digestion. Hydrogen production was not observed in the MD. The percentages of methane and carbon dioxide in stageⅡof the TPAD and TPAD-R were similar with that in the MD, with a methane percentage of more than 60%. In particular, methane as the most significant biogas fraction in the MD, TPAD and TPAD-R was further elaborated. Almost the same methane yields 0.45 L/g VS added were achieved in the three systems in the period of mono-digestion. The extent to which methane yields were improved was similar in the MD and TPAD-R by co-digestion, with an improved percentage of approximately 33%. Moreover, co-digestion improved the methane yield in the TPAD by 16%, which was lower than the improved extent in the other systems. In the steady state, hydrogen was only produced in stageⅠof the TPAD-R. The hydrogen yields, calculated according to the hydrogen productivity and carbohydrate reduction, were similar in the mono-digestion (1.12 mol/mol hexose) and co-digestion (0.85 mol/mol hexose). They are consistent with the previous investigation into anaerobic digestion of FW (Shin et al., 2004; Shin and Youn, 2005). Such a hydrogen yield is caused by the more dominant butyrate fermentation than acetate fermentation (Kapdan and Kargi, 2006).
3.3 Removal The solid concentrations in each reactor of the MD, TPAD and TPAD-R over time are shown in
Fig.3. Due to the incomplete degradation initially, solids accumulated in the MD, with the TS of over 4% on the 20th day. After an operation time of over 30 days, TS concentration in the MD decreased to a stable level, with a TS removal rate of 66.3% in the steady state of mono-digestion (Table 3). When the feedstock was changed into the mixed substrate, the similar trend for the changes of solid concentrations was observed in the MD. Around the 150th day, the steady state of the co-digestion was reached, with a TS removal rate of 72.9% and VS removal rate of 77.4%. Since the difference of solid contents existed between the initial inoculums in stageⅠof the TPAD and the feedstock, the insufficient degradation of the feedstock in this stage resulted in a gradually increasing solid concentrations. Ultimately, solids were almost not removed in stageⅠof the TPAD for both the mono-digestion and co-digestion. Nevertheless, the buffering action of stage Ⅰof the TPAD, the solid contents in the following stage fluctuated in a narrow range, with a similar solid removal in the mono-digestion and co-digestion. Unlike the changes of solid contents in stageⅠof the TPAD, stageⅠof the TPAD-R played a role in solid removal for mono-digestion, with a TS removal rate of 25.7% and a VS removal rate of 29.1%. However, when the ratio of lipid in the feedstock increased to lipid/TS 40%, in spite of the lower solid contents in stageⅠof the TPAD-R than in the mixed substrate resulting from the dilution of the feedstock by recycling, solids were almost not removed in this stage. The residual carbohydrate and protein in the effluent of the TPAD-R were always lower than in the effluent of the MD and TPAD, indicating the enhanced degradation ability of the TPAD-R for carbohydrate and protein. The increased removal rates of protein in the TPAD-R were 5% in the mono-digestion and 11% in the co-digestion, as compared to those in the MD. Furthermore, the recycle system in the two-stage system favored the degradation and conversion of carbohydrate in stageⅠ, with a removal rate of 76.1% in
mono-digestion and 67.0% in co-digestion. It is consistent with the hydrogen production in stageⅠof the TPAD-R, for carbohydrate makes more contribution to the hydrogen production in an anaerobic digester than the other organic matter (Kobayashi et al., 2012). Solubilization of particulate organic matter is indispensable during the anaerobic digestion of solid waste, and the rate in this step often limits the whole process (Eastman and Ferguson, 1981). As shown in Table 3, all the SCOD in the effluents of the MD, TPAD and TPAD-R were reduced to less than 5 g/L, indicating a sufficient conversion of the solubilized COD. Relatively high SCOD concentrations in stageⅠof the two-stage systems revealed a certain solubilization in the thermophilic stages. In addition, possibly the dilution caused by recycling in the TPAD-R resulted in a lower SCOD concentration in stageⅠof the TPAD-R than in stageⅠof the TPAD. Similar characteristics were observed in soluble carbohydrate, protein and lipid.
3.4 pH and VFA The pH and VFA concentrations in the MD, TPAD and TPAD-R over time are shown in Fig.4. It is reported that most methanogenic bacteria function in a pH range of 6.5-8.0 (Van Lier et al., 2001). All the pH values in the effluents of the MD, TPAD and TPAD-R were maintained in a neutral range throughout the process, indicating an appropriate pH condition for the growth of methanogenic bacteria. It is worth pointing out that the pH in the effluents of each system was lower in co-digestion than in mono-digestion, which was corresponding to the lower alkalinity in co-digestion than in mono-digestion. That is to say, more alkalinity was consumed in the co-digestion than in the mono-digestion, which resulted in the decreased pH in the co-digestion. In addition, except for a certain amount of VFA detected in the start-up period, VFA accumulation
was almost not observed in the effluents of the MD, TPAD and TPAD-R. Therefore, the operation in all systems was stable in the experimental period. VFA accumulation in stageⅠof the TPAD led to the pH drop to less than 4.0. In particular, VFA concentration in stageⅠof the TPAD attained over 10000 mg HAc/L in the co-digestion, much higher than in the mono-digestion. Similar performance on pH and VFA was observed in stageⅠof the TPAD-R as well. The difference between the TPAD and TPAD-R was that the improved alkalinity caused by recycling resulted in the neutralization of partial VFA and pH increase in stageⅠof the TPAD-R. The appropriate pH environment made hydrogen production possible in stageⅠof the TPAD-R (Shin et al., 2004).
3.5 Co-digestion vs. mono-digestion Due to the higher methane production potential of the de-oiled GTW than the FW, co-digestion of the FW with de-oiled GTW improved the biogas production, with an increased methane yield of over 30% in the MD and TPAD-R. The removal rates of solids and organic matter were similar between the mono-digestion and co-digestion, with a COD removal rate of approximately 80%. It suggested that the effects of the increased lipid loading in the co-digestion on the degradation and conversion of organic matter were slight. Nevertheless, analyses about protein and lipid showed that more residues were found in the effluents of the co-digestion, indicating that the increased lipid loading decreased the capability of the anaerobic systems to degrade protein and lipid. According to the analysis about LCFA concentration, as presented in Fig.5, co-digestion made more LCFAs not degraded than mono-digestion. Particularly, the residual LCFA concentration in the TPAD remained up to 4.73 mmol/ L in the co-digestion, as compared to 1.28 mmol/ L in the
mono-digestion, with palmitic acid dominant in the LCFAs. Amounts of unsaturated LCFAs, such as oleic acid and linoleic acid, produced in stageⅠof the two-stage systems were one of the possible reasons. Thus, more LCFA accumulation in the digesters was responsible for the reduced capability to remove lipid in the co-digestion. In addition, co-digestion aggravated the consumption of alkalinity in each reactor, resulting in a pH decrease. It could be attributed to the more VFA accumulation in the co-digestion than in the mono-digestion.
3.6 Two-stage processes vs. single-stage process The biogas production during the mono-digestion was similar in the MD, TPAD and TPAD-R, with a methane yield of 0.45 L/g VS added. Co-digestion improved the methane yields in the MD and TPAD-R to a similar extent. Accordingly, as shown in Fig.6, the residual biogas production in the effluents of each system was also similar, with a final biogas volume of around 120 mL/g VS. However, analyses about SMA indicated that the TPAD-R achieved much higher capability to degrade acetate than the MD and TPAD, with a maximum methane production rate of 1225 mL/g VS/d. Biogas was almost not produced in stageⅠof the TPAD, which was corresponding to a nearly impossible degradation of the organic matter. That is to say, stageⅡtook all the responsibility in the TPAD to degrade organic matter and to produce biogas. Thus, under the condition of the identical organic matter loading for the MD and TPAD, stageⅡof the TPAD bore more loading rate than the MD. Consequently, the increased lipid loading in the stageⅡof the TPAD resulted in the reduced methane yield 0.52 L/g VS added in the co-digestion, as compared to 0.60 L/g VS added in the MD and TPAD-R. Correspondingly, the removal rate of VS and COD was a little lower in the TPAD than in the MD and TPAD-R. The removal rates of protein
decreased in the systems as the order of the TPAD-R, MD and TPAD, which suggested an improved capability to degrade protein in the TPAD-R than in the MD and TPAD. Lipid was remained less in the MD than in the TPAD and TPAD-R. The actual higher lipid loading in the TPAD than in the MD and LCFA accumulation like oleic acid and linoleic acid in stageⅠof the TPAD led to the reduced lipid removal in the TPAD. In the case of the TPAD-R, the less lipid degradation than that in the MD could be attributed to the improved accumulation of refractory lipid in the system caused by recycling. Moreover, the recycle system in the two-stage system, which replenished a certain amount of alkalinity to stageⅠ, resulted in the improved alkalinity and pH in stageⅠ. It provided the possibility for hydrogen production in this stage.
4. Conclusions Co-digestion of FW and de-oiled GTW could lead to an increase of up to 19% in biogas yield, with a biogas yield of 0.60 L/g VS added. The two-stage system with recycling achieved the highest specific methane production activity 1225 mL/g VS/d and a hydrogen yield about 1 mol/ mol hexose. Lipid was converted more complete in the single-stage system than in the two-stage systems, which was attributed to more accumulation of long-chain fatty acids in the two-stage systems to some extent. Palmitic acid accumulated most in the effluents of co-digestion, responsible for the reduced capability to degrade lipid.
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recirculation of digester sludge. Bioresource Technol. 101, S42-S47. 18. Li, C.X., Champagne, P., Anderson, B.C., 2013. Effects of ultrasonic and thermo-chemical pre-treatments on methane production from fat, oil and grease (FOG) and synthetic kitchen waste (KW) in anaerobic co-digestion. Bioresource Technol. 130, 187-197. 19. Li, Y.Y., Sasaki, H., Yamashita, K., Seki, K., Kamigochi, I., 2002. High-rate methane fermentation of lipid-rich food wastes by a high-solids co-digestion process. Water Sci. Technol. 45, 143-150. 20. Montefrio, M.J., Xinwen, T., Obbard, J.P., 2010. Recovery and pre-treatment of fats, oil and grease from grease interceptors for biodiesel production. Appl. Energ. 87, 3155-3161. 21. Okamoto, M., Miyahara, T., Mizuno, O., Noike, T., 2000. Biological hydrogen potential of materials characteristic of the organic fraction of municipal solid waste. Water Sci. Technol. 41, 25-32. 22. Pereira, A., Mota, M., Alves, M., 2001. Degradation of oleic acid in anaerobic filters: The effect of inoculum acclimatization and biomass recirculation. Water Environ. Res. 73, 612-621. 23. Riau, V., De la Rubia, M.A., Pérez, M., 2012. Assessment of solid retention time of a temperature phased anaerobic digestion system on performance and final sludge characteristics. J. Chem. Technol. Biot. 87, 1074-1082. 24. Shin, H., Youn, J., Kim, S., 2004. Hydrogen production from food waste in anaerobic mesophilic and thermophilic acidogenesis. Int. J. Hydrogen Energ. 29, 1355-1363. 25. Shin, H.S., Youn, J.H., 2005. Conversion of food waste into hydrogen by thermophilic acidogenesis. Biodegradation 16, 33-44.
26. Silvestre, G., Illa, J., Fern A Ndez, B., Bonmat I, A., 2014. Thermophilic anaerobic co-digestion of sewage sludge with grease waste: Effect of long chain fatty acids in the methane yield and its dewatering properties. Appl. Energ. 117, 87-94. 27. Toba, M., Abe, Y., Kuramochi, H., 2011. Hydrodeoxygenation of waste vegetable oil over sulfide catalysts. Catal. Today 164, 533-537. 28. Van Lier, J.B., Tilche, A., Ahring, B.K., Macarie, H., Moletta, R., Dohanyos, M., Pol, L.W.H., Lens, P., Verstraete, W., 2001. New perspectives in anaerobic digestion. Water Sci. Technol. 43, 1-18. 29. Wang, L., Aziz, T.N., de Los Reyes III, F.L., 2013. Determining the limits of anaerobic co-digestion of thickened waste activated sludge with grease interceptor waste. Water Res. 47, 3835-3844. 30. Zhang, C., Su, H., Baeyens, J., Tan, T., 2014. Reviewing the anaerobic digestion of food waste for biogas production. Renewable and Sustainable Energy Reviews 38, 383-392.
MD
TPAD
Mesophilic digestion
Temperature-phased anaerobic digestion M
Temperature-phased anaerob Biogas
Gas meter Feedstock
Biogas
M
Gas meter
Feedstock Feedstock
Biogas
M
M
Gas meter
Gas meter
Mesophilic reactor (35℃)
Fig.1
Stage Ⅰ(55℃)
Stage Ⅱ(35℃)
Scheme of experimental setup for the MD, TPAD and TPAD-R
The figure has been adjusted, and the acronyms in the figure have been expanded.
Stage Ⅰ(55℃
Recycle system
Lipid/TS=40%
Lipid/TS=20%
Lipid/TS=20%
5
5
0 100
3
2
1
0 100 CH4
80
60
40
20
0
0 20 40 60 80 100 120 140 160 180
Stage I: Stage II:
Operation time (days)
H2 H2
CH4 CH4
CO2 CO2
2
1
Stage I: Stage II:
H2 H2
CH4 CH4
CO2 CO2
80
80
60
60
40
40
20
20
0
3
0 100
CO2
Gas composition in the TPAD (%)
H2
Gas composition in the MD (%)
Gas production rate (L/L/d)
1
4
Gas composition in the TPAD-R (%)
Gas production rate (L/L/d)
Gas production rate (L/L/d)
2
Fig.2
TPAD-R (stage I) TPAD-R (stage II)
4
3
Lipid/TS=40%
5
TPAD (stage I) TPAD (stage II)
MD
4
Lipid/TS=20%
Lipid/TS=40%
0 0
20 40 60 80 100 120 140 160 180
Operation time (days)
Biogas production characteristics over time
The font size in the figure has been increased.
0
20 40 60 80 100 120 140 160 180
Operation time (days)
VS
8
6
4
2
0
0
20
40
60
80 100 120 140 160 180
Stage I: Stage II:
TS TS
VS VS
6
4
2
0 0
20
TS and VS concentrations in the MD, TPAD and TPAD-R
40
60
80
Lipid/TS=40%
Lipid/TS=20%
10
8
Operation time (days)
Fig.3
Lipid/TS=40%
Lipid/TS=20%
10
Solid content in the TPAD (%)
Solid content in the MD (%)
TS
Lipid/TS=40%
Solid content in the TPAD-R (%)
Lipid/TS=20%
10
100 120 140 160 180
Operation time (days)
Stage I: Stage II:
TS TS
VS VS
8
6
4
2
0 0
20
40
60
80
100 120 140 160 180
Operation time (days)
Lipid/TS=20%
Lipid/TS=40%
Lipid/TS=20%
9
9
Lipid/TS=20%
9
8
8
7
7
7
6
6
6
pH
8
pH
pH
Lipid/TS=40%
TPAD (stage I) TPAD (stage II)
MD
5
5
5
4
4
4
3
3
12000
3
12000
12000
TPAD (stage I) TPAD (stage II)
MD
TPAD-R (stage I) TPAD-R (stage II)
8000
8000
8000
4000
2000
0
0
6000
4000
2000
20 40 60 80 100 120 140 160 180
Operation time (days)
Fig.4
VFA (mg HAc/L)
10000
VFA (mg HAc/L)
10000
VFA (mg HAc/L)
10000
6000
0
Lipid/TS=40%
TPAD-R (stage I) TPAD-R (stage II)
6000
4000
2000
0
20 40 60 80 100 120 140 160 180
0
Operation time (days)
pH and VFA concentrations in the MD, TPAD and TPAD-R
0
20 40 60 80 100 120 140 160 180
Operation time (days)
LCFA concentration (mmol/L)
10
8
6
4
2
0
MD
TPAD (stage I)
TPAD (stage II)
TPAD-R (stage I) TPAD-R (stage II)
Tridecanoic acid (C13:0) Caprylic acid (C8:0) Capric acid (C10:0) Lauric acid (C12:0) Myristic acid (C14:0) Myristoleic acid (C14:1) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Heptadecanoic acid (C17:0) Steatic acid (C18:0) Elaidic acid (C18:1) Oleic acid (C18:1 (n-9)) Linoleic acid (C18:2) Arachidic acid (C20:0) cis-11-Eicosenoic acid (C20:1(w-9)) Linoleic acid (C18:3) Total LCFAs Behenic acid (C22:0) Erucic acid (C22:1)
Fig.5 LCFA concentrations in the MD, TPAD and TPAD-R (The columns on the left are the LCFA concentration in mono-digestion, and the columns on the right are LCFA concentration in co-digestion)
700
120
SMA test (mL/g VS)
500 90
400
300
60
200 30 100
0 0
5
10
15
20
25
30
Residual biogas production (mL/g VS)
600
150
Residual-MD Residual-TPAD Residual-TPAD-R
SMA-MD SMA-TPAD SMA-TPAD-R
0 35
Time (days) Fig.6 Residual biogas production and SMA test in the steady state of the co-digestion in the MD, TPAD and TPAD-R
Nomenclature FW
food waste
GTW
grease trap waste
MD
mesophilic digestion
TPAD
temperature-phased anaerobic digestion
TPAD-R
temperature-phased anaerobic digestion with a recycle system
StageⅠ Ⅰ
the front stage in the two-stage systems
StageⅡ Ⅱ
the end stage in the two-stage systems
HRT
hydraulic retention time
TS
total solid
VS
volatile solid
COD
chemical oxygen demand
SCOD
soluble chemical oxygen demand
VFA
volatile fatty acid
LCFA
long-chain fatty acid
SMA
specific methanogenic activity
Table 1
Feedstock characterization Single substrate: FW
Mixed substrate: FW+de-oiled GTW
pH
3.65±0.06
3.99±0.04
TS (%)
7.62±0.29
8.18±0.14
VS (%)
7.21±0.29
7.82±0.22
VS/TS (%)
94.6
95.6
COD (g/L)
101.2±6.1
152.2±13.4
Carbohydrate (g/L)
31.0±5.4
15.9±0.9
Protein (g/L)
14.1±1.9
10.0±0.7
Lipid (g/L)
13.9±2.2
32.9±3.0
Lipid/TS (%)
20
40
Table 2
Biogas production for the MD, TPAD and TPAD-R in the steady state Mono-digestion
Co-digestion
TPAD MD
TPAD-R
TPAD
Ⅰ
Ⅱ
Ⅰ+Ⅱ
Ⅰ
Ⅱ
Ⅰ+Ⅱ
1.79±0.07
0.05±0.01
2.20±0.04
1.77
1.82±0.08
1.91±0.08
1.89
0.74
0.00
0.70
0.74
0.13
0.75
1.00
0.00±0.00
0.96
1.02
0.44
H2
0.0±0.0
0.0±0.0
0.0±0.0
0.0
CH4
60.5±1.9
0.0±0.0
61.0±0.6
CO2
39.5±3.9
25.7±2.1
38.9±0.7
MD
TPAD-R
Ⅰ
Ⅱ
Ⅰ+Ⅱ
Ⅰ
Ⅱ
Ⅰ+Ⅱ
2.29±0.05
0.03±0.00
2.59±0.13
2.08
0.78±0.12
2.86±0.05
2.44
0.79
0.88
0.00
0.79
0.80
0.05
0.68
0.94
1.40
0.98
1.14
0.00
0.07
0.09
0.00
1.02
1.19
31.7±1.1
0.0±0.0
6.1
0.0±0.0
0.0±0.0
0.0±0.0
0.0
26.7±1.0
0.0±0.0
1.7
60.6
0.5±0.0
70.4±1.0
57.0
70.4±0.9
0.0±0.0
65.4±0.2
65.2
3.6±0.2
68.7±0.4
64.5
38.8
69.9±2.6
30.7±1.0
38.2
30.4±0.6
4.0±0.8
34.6±0.2
34.5
66.4±1.1
31.2±0.3
33.4
Biogas production Production rate (L/L/d) Yield (L/g VS added) Recovery rate (L/g VS destroyed) Composition (%) Methane production Production rate (L/L/d)
1.08
0.0
1.34
1.07
0.00
1.34
1.07
1.61
0.00
1.69
1.36
0.03
1.96
1.58
Yield (L/g VS added)
0.45
0.00
0.43
0.45
0.00
0.53
0.45
0.62
0.00
0.52
0.52
0.00
0.47
0.60
0.60
0.00
0.58
0.62
0.00
0.98
0.56
0.80
0.00
0.70
0.71
0.00
0.70
0.77
Recovery rate (L/g VS destroyed)
Table 3
Characteristics of digested sludge and material removal Mono-digestion
Co-digestion
TPAD
TPAD-R Ⅰ+
MD Ⅰ
Ⅱ
TPAD Ⅰ+
Ⅰ Ⅱ
Ⅰ+
MD
Ⅱ
TPAD-R
Ⅰ
Ⅱ
Ⅱ
Ⅰ+ Ⅰ
Ⅱ
Ⅱ
Ⅱ
2.57±0
7.91±
2.47±
2.47±
3.57±
1.99±
1.99±
2.22±
8.17±0
2.50±
2.50±
5.46±
2.08±
2.08±
.08
0.02
0.09
0.09
0.19
0.06
0.06
0.13
.49
0.06
0.06
0.15
0.08
0.08
66.3
0.0
68.8
67.6
25.7
44.2
73.9
72.9
0.1
69.4
69.4
0.0
61.9
74.6
1.84±0
7.53±
2.03±
2.03±
3.06±
1.42±
1.42±
1.77±
7.88±0
2.09±
2.09±
5.03±
1.66±
1.66±
.05
0.04
0.05
0.05
0.06
0.05
0.05
0.09
.52
0.04
0.04
0.14
0.01
0.01
74.5
0.0
73.0
71.8
29.1
53.6
80.3
77.4
0.0
73.5
73.3
0.0
70.0
78.8
17.3±0
101.3
16.6±
16.6±
54.2±
12.1±
12.1±
27.9±
147.9±
32.7±
32.7±
95.8±
27.6±
27.6±
.9
±3.2
1.0
1.0
1.2
1.0
1.0
1.0
19.4
1.3
1.3
16.9
1.1
1.1
82.9
0.0
83.6
83.6
4.3
77.7
88.0
81.7
2.8
77.9
78.5
0.0
71.2
81.9
2.6±0.
36.4±
1.8±0
1.8±0
22.7±
1.7±0
1.7±0
2.2±0
28.7±1
3.3±0
3.3±0
17.6±
1.7±0
1.7±0
2
1.2
.3
.3
0.9
.2
.2
.1
.0
.3
.3
1.3
.1
.1
2.3±0.
29.3±
2.8±0
2.8±0
3.9±0
1.7±0
1.7±0
2.6±0
13.8±2
2.1±0
2.1±0
2.9±0
1.7±0
1.7±0
2
1.0
.3
.3
.2
.1
.1
.4
.1
.3
.3
.2
.1
.1
92.6
5.5
90.4
91.0
76.1
56.4
94.5
83.6
13.2
84.8
86.8
67.0
41.4
89.3
0.4±0.
11.0±
0.3±0
0.3±0
1.4±0
0.2±0
0.2±0
0.3±0
3.0±0.
0.3±0
0.3±0
0.9±0
0.2±0
0.2±0
1
1.3
.1
.1
.0
.0
.0
.0
3
.1
.1
.2
.0
.0
5.2±0.
12.0±
6.8±0
6.8±0
6.8±0
4.5±0
4.5±0
8.1±0
8.5±0.
9.0±1
9.0±1
7.5±0
7.0±0
7.0±0
1
1.2
.4
.4
.9
.2
.2
.7
7
.1
.1
.8
.7
.7
63.1
14.9
43.3
51.8
26.9
33.8
68.1
19.0
15.0
0.0
10.0
11.8
6.7
30.0
0.7±0.
3.0±0
0.6±0
0.6±0
0.9±0
0.3±0
0.3±0
0.4±0
1.5±0.
0.5±0
0.5±0
1.4±0
0.4±0
0.4±0
1
.2
.1
.1
.3
.0
.0
.0
2
.0
.0
.2
.0
.0
1.1±0.
14.4±
2.4±0
2.4±0
6.9±1
2.4±0
2.4±0
3.6±0
29.1±2
5.5±0
5.5±0
8.9±0
4.3±0
4.3±0
4
2.1
.2
.2
.2
.2
.2
.6
.4
.3
.3
.5
.2
.2
92.1
0.0
83.3
82.7
15.3
65.2
82.7
89.1
11.6
81.1
83.3
52.2
51.7
86.9
TS (%)
TS removal (%)
VS (%)
VS removal (%)
COD (g/L)
COD removal (%) SCOD (g/L) Carbohydr ate (g/L) Carbohydr ate removal (%) Sa-Carboh ydrate (g/L) Protein (g/L) Protein removal (%) S-Protein (g/L)
Lipid (g/L)
Lipid removal (%)
- 30 -
S-Lipid (g/L)
0.0±0.
0.8±0
0.0±0
0.0±0
0.3±0
0.0±0
0.0±0
0.2±0
0.4±0.
0.2±0
0.2±0
0.4±0
0.2±0
0.2±0
0
.2
.0
.0
.0
.0
.0
.2
1
.1
.1
.1
.1
.1
7.77±0
3.61±
7.32±
7.32±
5.36±
7.59±
7.59±
7.39±
3.99±0
6.89±
6.89±
5.28±
7.17±
7.17±
.06
0.04
0.04
0.04
0.10
0.05
0.05
0.04
.11
0.02
0.02
0.04
0.02
0.02
0±0
0±0
0±0
0±0
0±0
0±0
0±0
0±0
0±0
pH
VFA (mg
3034
4334
0±0 HAcb/L)
±280
10120
±950
8086
±584
±863
Alkalinity 5455± (mg
3840
3840
1963
5131
5131
2917
±228
±228
±131
±38
±38
±66
107.0
107.2
98.0
107.0
105.2
109.0
0±0
1800
1800
1281
2971
2971
±163
±163
±92
±112
±112
100.4
97.6
107.3
99.0
107.4
0±0
253 CaCO3/L) COD mass balance
108.6
100.3
(%)
Note: a‘S’ is ‘Soluble’.
b
All of the individual VFA was calculated as acetic acid.
- 31 -
97.2
Highlights •
A single-stage and two two-stage anaerobic systems were synchronously operated.
•
Co-digestion with de-oiled grease trap waste increased the biogas yield up to 19%.
•
Two-stage process with recycling achieved a hydrogen yield of 1 mol/mol hexose.
•
More alkalinity was consumed in the co-digestion than the mono-digestion.
•
Single-stage process favored the conversion of lipid and long-chain fatty acids.
- 32 -
S0960-8524(15)01594-1 http://dx.doi.org/10.1016/j.biortech.2015.11.061 BITE 15794
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
15 October 2015 18 November 2015 22 November 2015
Please cite this article as: Wu, L-J., Kobayashi, T., Kuramochi, H., Li, Y-Y., Xu, K-Q., Improved biogas production from food waste by co-digestion with de-oiled grease trap waste, Bioresource Technology (2015), doi: http:// dx.doi.org/10.1016/j.biortech.2015.11.061
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Improved biogas production from food waste by co-digestion with de-oiled grease trap waste Li-Jie Wua, Takuro Kobayashib, Hidetoshi Kuramochib, Yu-You Lia,c and Kai-Qin Xub,d, a Department of Civil and Environmental Engineering, Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan b Center of Material Cycles and Waste Management Research, National Institute for Environmental Studies, Tsukuba 305-8506, Japan c Department of Frontier Science for Advanced Environment, Graduate School of Environmental Studies, Tohoku University, Sendai, 980-8579, Japan d School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ------------------------------------------------------------------------------------------------------------Corresponding author: Takuro Kobayashi National Institute for Environmental Studies, Onogawa 16-2, Tsukuba, Ibaraki 305-8506, Japan Phone: +81-29-850-2400. Fax: +81-22-850-2560. E-mail: [email protected]
Abstract The objective of this study was to assess the feasibility of co-digesting food waste (FW) and de-oiled grease trap waste (GTW) to improve the biogas production. A lab-scale mesophilic digester (MD), a temperature-phased anaerobic digester (TPAD) and a TPAD with recycling (TPAD-R) were synchronously operated under mono-digestion (FW) and co-digestion (FW+ de-oiled GTW). Co-digestion increased the biogas yield by 19% in the MD and TPAD-R, with a biogas yield of 0.60 L/g VS added. Specific methanogenic activity in the TPAD-R was much higher than that in the MD. In addition to methane, hydrogen at a yield of approximately 1 mol/mol hexose was produced in the TPAD-R. Alkalinity was consumed more in the co-digestion than in mono-digestion. Co-digestion resulted in more lipid accumulation in each digester. The MD favored the degradation of lipid and conversion of long-chain fatty acids more than the TPAD and TPAD-R.
Keywords:
Food
waste
(FW);
Grease
trap
waste
(GTW);
Anaerobic
digestion;
Temperature-phased anaerobic digestion (TPAD); Two-stage; Long-chain fatty acid (LCFA)
1. Introduction Food waste (FW) is the largest fraction in increasing municipal solid waste. The generation of FW is predicted to increase with 44% by 2025 (Ariunbaatar et al., 2014). The uncontrolled discharge of FW causes severe environmental pollution, such as decay and odor. Anaerobic digestion has proved to be an effective technology to treat FW, such an easily biodegradable organic matter with high moisture, carbohydrate, lipid, and protein contents, due to the capabilities in energy recovery in the form of methane, waste reduction and stabilization, low carbon emission and limited pollution (Zhang et al., 2014). Nonetheless, in many cases, anaerobic digestion of municipal solid waste in smaller facilities often confronts the issues that biogas utilization is less economically or operationally appealing than flaring excess biogas. As a consequence, smaller waste treatment facilities do not commonly use anaerobic digesters for solids management, let alone implement biogas utilization (Wang et al., 2013). One feasible way to improve the economics of biogas utilization is to substantially increase biogas production by anaerobic co-digestion with substrates of high biogas production potential. The co-digestion, the simultaneous digestion of two or more substrates, can improve methane yield because of the supply of additional nutrients from the co-substrates, and more efficiently use the equipment and share cost by processing multiple waste streams in a single facility, as compared to mono-digestion (Alatriste-Mondragón et al., 2006). In addition to FW, another major stream of organic waste from restaurant is the grease trap waste (GTW). The GTW is from a small passive or mechanized oil/water separator installed inside the food service establishments in case of the blockage in the wastewater pipe. Conversion of oily content in GTW to biodiesel is widely studied because of concerns relating to the quality of
the feedstock, specifically the presence of high moisture content and free fatty acids (Montefrio et al., 2010; Toba et al., 2011). The de-oiled GTW turns out to be appropriate as a substrate for anaerobic digestion (Kobayashi et al., 2014). Furthermore, due to still high lipid content in the de-oiled GTW, more methane can be produced owing to theoretically higher methane production potential of lipid than other organic matters (Angelidaki and Sanders, 2004). Theoretically, 1 g of glycerol trioleate (C57H104O6), a common lipid in nature, is equivalent to 1.08 L of methane at standard temperature and pressure, while 1 g of glucose (C6H12O6) is equivalent to only 0.37 L (Kim and Shin, 2010). A number of studies have shown that such a lipid-rich organic waste could become potential co-substrate (Li et al., 2013; Silvestre et al., 2014). Thus, it is possible to take advantage of de-oiled GTW as the co-substrate of FW to increase the economics of biogas utilization. Moreover, the proximity of GTW to FW favors the collection of the waste for the following treatment. However, previous studies focus their attention on the co-digestion of lipid-rich waste to enhance the biogas production of sewage sludge, and the investigation on the co-digestion of de-oiled GTW with FW has almost not been found. In anaerobic digestion of lipid-containing waste, lipid loading is a critical parameter. When the de-oiled GTW is overdosed, accumulation of long-chain fatty acids (LCFAs), propionate and acetate can result in a pH drop and even process failure, due to limited substrate and product transport, damaged cells and reduced activity of stressed microbial communities (Wang et al., 2013). Thermophilic condition (55℃) for lipid-containing waste is more advantageous than mesophilic condition (35℃), due to increased ability to degrade LCFAs and a smaller scum layer. It is because that under thermophilic condition, lipid becomes more accessible to microorganisms and their lipolytic enzymes caused by increased diffusion coefficients and lipid solubility in
aqueous media with increasing temperature (Chipasa and Medrzycka, 2006). However, thermophilic bacteria may be more sensitive to LCFA inhibition than mesophilic bacteria (Hwu et al., 1997). Thus, it is possible to apply temperature-phased anaerobic digestion (TPAD), which integrates the advantages of thermophilic and mesophilic digestion, to the degradation of mixed substrate of FW and de-oiled GTW to improve the operation performance. In fact, the relevant studies have never been found yet. Hwu et al. found that the wash-out biomass exhibited a greater LCFA degradation capability than the biomass remaining inside the reactor (Hwu and Lettinga, 1997). Therefore, the biomass recycling was recommended to reduce the biomass washout and the toxic effect on the acetogenic and methanogenic populations (Pereira et al., 2001). On the basis of TPAD, the introduction of a biomass recycling from the end stage to the front stage, namely TPAD-R, is likely to further optimize the performance of TPAD in degrading lipid-containing waste. The feasibility of TPAD-R in digesting FW has been demonstrated by many previous studies (Kobayashi et al., 2012; Lee et al., 2010), but little attention was paid on the application of TPAD-R to the digestion of de-oiled GTW, let alone the co-digestion of FW and de-oiled GTW. The goal of this study was to investigate the feasibility to improve the biogas production by co-digesting FW and de-oiled GTW in single-stage mesophilic digestion (MD), TPAD and TPAD-R. The specific objectives were to (a) determine the extent to which the biogas production is improved by co-digestion, (b) evaluate the operation differences among MD, TPAD and TPAD-R in digesting FW and mixed substrates, and (c) study the effects of biomass recycling on the TPAD.
2. Materials and methods 2.1 Experimental setup The anaerobic experiments were conducted using three systems, the single-stage MD and the two-stage TPAD and TPAD-R systems as shown in Fig.1. The MD is comprised of a reactor system, while both the TPAD and TPAD-R included two reactor systems. Each reactor system consists of a continuous stirred tank reactor equipped with a temperature controller, a feeding and decanting system, and a biogas collection system (WNK-0.5, Shinagawa Corporation, Japan). The effective volumes for the MD, TPAD and TPAD-R were set as 6 L, 10L and 7.5 L, respectively. The TPAD and TPAD-R were operated with the ratios of the front stage (stageⅠ) to the end stage (stageⅡ) 1:4, and under this condition better operation performance was achieved than other ratios (Coelho et al., 2011; Riau et al., 2012). A recycle system was introduced from the stageⅡof the TPAD-R to stageⅠ, with the same flow-rate as influent of the system. The temperature controllers were used to make sure the required temperature of each reactor by heaters and water jackets. The feeding and decanting systems worked semi-continuously using peristaltic pumps. Particularly, the feedstock of each system was fed manually once a day.
2.2 Substrates and inoculums The substrates used in this study contained prepared FW and de-oiled GTW. Raw FW was collected from a dining hall at the National Institute for Environmental Studies, Tsukuba, Japan. It was mixed with tap water as 1:1.4 (raw FW: tap water) in a tank (4 ℃) equipped with the cooling system before it was shredded with a shear pump to the particle size less than 5 mm, in order to meet the requirement of wet anaerobic digestion. The prepared FW was stirred in the tank to
ensure the homogeneity. The GTW relevant to this study was collected from grease traps of 11 different types of restaurants in Japan, mainly supplying meat products and serving for the banquet. Subsequently, the mixture of different restaurant sources was heated to 60℃ for at least 6 hours, which caused layer separation. After the upper oil layer was pumped away for biodiesel recovery, de-oiled GTW was obtained. The characteristics of single substrate FW and mixed substrate FW+ de-oiled GTW used in this study are shown in Table 1. Due to deficient trace elements in restaurant waste for anaerobic digestion, necessary trace elements, such as Fe, Co and Ni, were supplemented to the feedstock in case of biogas production limitation caused by deficient trace elements. The prepared stock metal solution contained Fe (FeCl2·4H2O) 10000 mg/L, Co (CoCl2·6H2O) 1000 mg/L and Ni (NiCl2·6H2O) 1000 mg/L. The stock solution was added to the substrate as the ratio of 1 mL stock solution: 100 mL substrate (Kobayashi et al., 2012). The mesophilic digesting sludge fetched from a wastewater treatment plant in Ibaraki Prefecture for sewage sludge treatment was used as mesophilic inoculum, while the thermophilic digesting sludge acclimated from mesophilic sludge at least one month as the reported method was used as thermophilic inoculums (Bou V S Kov A et al., 2005).
2.3 Experimental design The stageⅠof TPAD and TPAD-R was controlled at thermophilic temperature (55 ℃), and MD and the stage Ⅱ of TPAD and TPAD-R was set at mesophilic temperature (35 ℃ ). Thermophilic reactors and mesophilic reactors in MD, TPAD and TPAD-R were initially inoculated with thermophilic inoculum and mesophilic inoculum, respectively, until the working volume of each reactor was achieved. FW was firstly pumped into each system as hydraulic
retention time (HRT) 100 days. Start-up process was accomplished by shortening the HRT from 100 days, to 50 days and then to 30 days. The performance at HRT 30 days was selected as the final operation results. Under the condition of total HRT 30 days, the HRTs for the MD, stage Ⅰand stageⅡ Ⅱof the TPAD, and stageⅠ Ⅰand stageⅡ Ⅱof the TPAD-R are 30 days, 6 days, 24 days, 3 days, and 12 days, respectively. The period in HRT 100 days and 50 days took about 30 days before the HRT was shortened to 30 days. Through over 100 days’ operation, the feedstock of all systems was changed to mixed substrate of FW and de-oiled GTW, with a lipid/total solid (TS) 40%, until the experiment ended. It is worth pointing out that the value of lipid/TS at 40% has proved appropriate for the prevention of inhibition caused by lipid (Li et al., 2002). After the steady state of the co-digestion was reached, the residual biogas production and specific methanogenic activity (SMA) test (with acetate as substrate) for the effluents of each system were carried out.
2.4 Analytical methods Biogas production was noted on a daily basis and normalized to standard conditions according to the daily local meteorological data. Nearly every day biogas samples were taken to analyze the content of nitrogen, methane and carbon dioxide with a gas chromatograph (GC-8A, Shimadzu), equipped with a thermal conductivity detector and a stainless steel column packed with Shincarbon ST (Shimadzu GLC). Sludge samples were taken from the sampling ports of each reactor twice a week, together with substrate, for the subsequent analysis. The pH, TS, volatile solid (VS), and alkalinity were measured according to the Standard Method (APHA, 2012). Chemical oxygen demand (COD) was determined by COD Digest Vials (HACH). Volatile fatty
acids (VFAs) and LCFAs were analyzed referring to the previous report (Kobayashi et al., 2014). Samples for soluble items analysis, such as VFA, soluble COD (SCOD), alkalinity and NH4+-N were prepared by centrifuging samples at 13000 rpm for 5 min and filtering them with 0.45 µm pore-size filters. The organic matter carbohydrate and protein was analyzed using spectrophotography. A methanol-chloroform extraction and weight method was taken to measure lipid. The methods to measure and analyze the residual biogas production and SMA test referred to the previous reports (Kobayashi et al., 2015). Each sample was set in duplicate.
3. Results and discussion 3.1 Feedstock characterization The characteristics of the prepared FW and the mixed substrate of FW and de-oiled GTW used in this study are shown in Table 1. Both of them were adjusted to the similar solid concentration, lower than 10%, to meet the requirement of the wet anaerobic digestion. The percentages of VS/TS in the single substrate FW and the mixed substrate were similar as well, with a percentage of 95%. Such a high VS concentrations in the substrates revealed a pretty high organic matter content, indicating a desirable methane production potential. After the FW was mixed with the de-oiled GTW, the concentrations of carbohydrate and protein decreased to some extent, while the lipid concentration increased from 13.9 g/L (lipid/TS=20%) to 32.9 g/L (lipid/TS=40%). Since the same mass concentration for lipid is representative of more COD than for carbohydrate and protein, the higher lipid concentration in the mixed substrate led to increased COD 1.86 g/g TS.
3.2 Biogas production The biogas production characteristics over time in the MD, TPAD and TPAD-R are presented in Fig.2. At the preliminary stage of the operation, the production rates in each system increased gradually. After more than two turnovers at HRT of 30 days for mono-digestion, all the systems achieved a steady biogas production. The stable biogas production was achieved earlier in both the MD and TPAD-R than in the TPAD. Due to the successful start-up in the period of mono-digestion, the time to achieve stable biogas production for co-digestion was shortened, especially for the MD and TPAD. The operation performance on biogas production in the steady state is summarized in Table 2. Co-digestion increased the biogas yields of the mono-digestion in the MD, TPAD and TPAD-R by 19%, 8% and 19%, respectively. In addition, the biogas production rates for both the mono-digestion and co-digestion were slightly higher in the TPAD-R than in the MD. Similar biogas production rates were observed in stageⅠ(1.82 L/L/d) and stageⅡ (1.91 L/L/d) of the TPAD-R for mono-digestion. However, after the co-digestion started, the biogas production rate in stageⅠof the TPAD-R decreased to less than 1 L/L/d, while improved biogas production rate 2.86 L/L/d occurred in the following stageⅡ. Hydrogen was the main effective biogas component in stageⅠof the TPAD-R, and preferred substrate for hydrogen production is carbohydrate (Kapdan and Kargi, 2006; Kobayashi et al., 2012; Okamoto et al., 2000). Therefore, the reduced biogas production in stageⅠof the TPAD-R possibly resulted from the less carbohydrate in the FW than in the mixture of FW and de-oiled GTW. As a result of almost no biogas produced in stageⅠof the TPAD, despite higher biogas production rates in stage Ⅱof the TPAD than in the MD, the capability of the whole TPAD to produce biogas decreased. In the initial start-up period, hydrogen was produced in stageⅠof the TPAD. However,
hydrogen disappeared in stageⅠof the TPAD immediately with the shortened HRT. Methane production was never observed in this stage. A different biogas composition from that in stageⅠ of the TPAD existed in stageⅠof the TPAD-R. In addition to a little methane, hydrogen accounted for a significant proportion in stageⅠof the TPAD-R, with a percentage of 31.7% for the mono-digestion and 26.7% for the co-digestion. Hydrogen production was not observed in the MD. The percentages of methane and carbon dioxide in stageⅡof the TPAD and TPAD-R were similar with that in the MD, with a methane percentage of more than 60%. In particular, methane as the most significant biogas fraction in the MD, TPAD and TPAD-R was further elaborated. Almost the same methane yields 0.45 L/g VS added were achieved in the three systems in the period of mono-digestion. The extent to which methane yields were improved was similar in the MD and TPAD-R by co-digestion, with an improved percentage of approximately 33%. Moreover, co-digestion improved the methane yield in the TPAD by 16%, which was lower than the improved extent in the other systems. In the steady state, hydrogen was only produced in stageⅠof the TPAD-R. The hydrogen yields, calculated according to the hydrogen productivity and carbohydrate reduction, were similar in the mono-digestion (1.12 mol/mol hexose) and co-digestion (0.85 mol/mol hexose). They are consistent with the previous investigation into anaerobic digestion of FW (Shin et al., 2004; Shin and Youn, 2005). Such a hydrogen yield is caused by the more dominant butyrate fermentation than acetate fermentation (Kapdan and Kargi, 2006).
3.3 Removal The solid concentrations in each reactor of the MD, TPAD and TPAD-R over time are shown in
Fig.3. Due to the incomplete degradation initially, solids accumulated in the MD, with the TS of over 4% on the 20th day. After an operation time of over 30 days, TS concentration in the MD decreased to a stable level, with a TS removal rate of 66.3% in the steady state of mono-digestion (Table 3). When the feedstock was changed into the mixed substrate, the similar trend for the changes of solid concentrations was observed in the MD. Around the 150th day, the steady state of the co-digestion was reached, with a TS removal rate of 72.9% and VS removal rate of 77.4%. Since the difference of solid contents existed between the initial inoculums in stageⅠof the TPAD and the feedstock, the insufficient degradation of the feedstock in this stage resulted in a gradually increasing solid concentrations. Ultimately, solids were almost not removed in stageⅠof the TPAD for both the mono-digestion and co-digestion. Nevertheless, the buffering action of stage Ⅰof the TPAD, the solid contents in the following stage fluctuated in a narrow range, with a similar solid removal in the mono-digestion and co-digestion. Unlike the changes of solid contents in stageⅠof the TPAD, stageⅠof the TPAD-R played a role in solid removal for mono-digestion, with a TS removal rate of 25.7% and a VS removal rate of 29.1%. However, when the ratio of lipid in the feedstock increased to lipid/TS 40%, in spite of the lower solid contents in stageⅠof the TPAD-R than in the mixed substrate resulting from the dilution of the feedstock by recycling, solids were almost not removed in this stage. The residual carbohydrate and protein in the effluent of the TPAD-R were always lower than in the effluent of the MD and TPAD, indicating the enhanced degradation ability of the TPAD-R for carbohydrate and protein. The increased removal rates of protein in the TPAD-R were 5% in the mono-digestion and 11% in the co-digestion, as compared to those in the MD. Furthermore, the recycle system in the two-stage system favored the degradation and conversion of carbohydrate in stageⅠ, with a removal rate of 76.1% in
mono-digestion and 67.0% in co-digestion. It is consistent with the hydrogen production in stageⅠof the TPAD-R, for carbohydrate makes more contribution to the hydrogen production in an anaerobic digester than the other organic matter (Kobayashi et al., 2012). Solubilization of particulate organic matter is indispensable during the anaerobic digestion of solid waste, and the rate in this step often limits the whole process (Eastman and Ferguson, 1981). As shown in Table 3, all the SCOD in the effluents of the MD, TPAD and TPAD-R were reduced to less than 5 g/L, indicating a sufficient conversion of the solubilized COD. Relatively high SCOD concentrations in stageⅠof the two-stage systems revealed a certain solubilization in the thermophilic stages. In addition, possibly the dilution caused by recycling in the TPAD-R resulted in a lower SCOD concentration in stageⅠof the TPAD-R than in stageⅠof the TPAD. Similar characteristics were observed in soluble carbohydrate, protein and lipid.
3.4 pH and VFA The pH and VFA concentrations in the MD, TPAD and TPAD-R over time are shown in Fig.4. It is reported that most methanogenic bacteria function in a pH range of 6.5-8.0 (Van Lier et al., 2001). All the pH values in the effluents of the MD, TPAD and TPAD-R were maintained in a neutral range throughout the process, indicating an appropriate pH condition for the growth of methanogenic bacteria. It is worth pointing out that the pH in the effluents of each system was lower in co-digestion than in mono-digestion, which was corresponding to the lower alkalinity in co-digestion than in mono-digestion. That is to say, more alkalinity was consumed in the co-digestion than in the mono-digestion, which resulted in the decreased pH in the co-digestion. In addition, except for a certain amount of VFA detected in the start-up period, VFA accumulation
was almost not observed in the effluents of the MD, TPAD and TPAD-R. Therefore, the operation in all systems was stable in the experimental period. VFA accumulation in stageⅠof the TPAD led to the pH drop to less than 4.0. In particular, VFA concentration in stageⅠof the TPAD attained over 10000 mg HAc/L in the co-digestion, much higher than in the mono-digestion. Similar performance on pH and VFA was observed in stageⅠof the TPAD-R as well. The difference between the TPAD and TPAD-R was that the improved alkalinity caused by recycling resulted in the neutralization of partial VFA and pH increase in stageⅠof the TPAD-R. The appropriate pH environment made hydrogen production possible in stageⅠof the TPAD-R (Shin et al., 2004).
3.5 Co-digestion vs. mono-digestion Due to the higher methane production potential of the de-oiled GTW than the FW, co-digestion of the FW with de-oiled GTW improved the biogas production, with an increased methane yield of over 30% in the MD and TPAD-R. The removal rates of solids and organic matter were similar between the mono-digestion and co-digestion, with a COD removal rate of approximately 80%. It suggested that the effects of the increased lipid loading in the co-digestion on the degradation and conversion of organic matter were slight. Nevertheless, analyses about protein and lipid showed that more residues were found in the effluents of the co-digestion, indicating that the increased lipid loading decreased the capability of the anaerobic systems to degrade protein and lipid. According to the analysis about LCFA concentration, as presented in Fig.5, co-digestion made more LCFAs not degraded than mono-digestion. Particularly, the residual LCFA concentration in the TPAD remained up to 4.73 mmol/ L in the co-digestion, as compared to 1.28 mmol/ L in the
mono-digestion, with palmitic acid dominant in the LCFAs. Amounts of unsaturated LCFAs, such as oleic acid and linoleic acid, produced in stageⅠof the two-stage systems were one of the possible reasons. Thus, more LCFA accumulation in the digesters was responsible for the reduced capability to remove lipid in the co-digestion. In addition, co-digestion aggravated the consumption of alkalinity in each reactor, resulting in a pH decrease. It could be attributed to the more VFA accumulation in the co-digestion than in the mono-digestion.
3.6 Two-stage processes vs. single-stage process The biogas production during the mono-digestion was similar in the MD, TPAD and TPAD-R, with a methane yield of 0.45 L/g VS added. Co-digestion improved the methane yields in the MD and TPAD-R to a similar extent. Accordingly, as shown in Fig.6, the residual biogas production in the effluents of each system was also similar, with a final biogas volume of around 120 mL/g VS. However, analyses about SMA indicated that the TPAD-R achieved much higher capability to degrade acetate than the MD and TPAD, with a maximum methane production rate of 1225 mL/g VS/d. Biogas was almost not produced in stageⅠof the TPAD, which was corresponding to a nearly impossible degradation of the organic matter. That is to say, stageⅡtook all the responsibility in the TPAD to degrade organic matter and to produce biogas. Thus, under the condition of the identical organic matter loading for the MD and TPAD, stageⅡof the TPAD bore more loading rate than the MD. Consequently, the increased lipid loading in the stageⅡof the TPAD resulted in the reduced methane yield 0.52 L/g VS added in the co-digestion, as compared to 0.60 L/g VS added in the MD and TPAD-R. Correspondingly, the removal rate of VS and COD was a little lower in the TPAD than in the MD and TPAD-R. The removal rates of protein
decreased in the systems as the order of the TPAD-R, MD and TPAD, which suggested an improved capability to degrade protein in the TPAD-R than in the MD and TPAD. Lipid was remained less in the MD than in the TPAD and TPAD-R. The actual higher lipid loading in the TPAD than in the MD and LCFA accumulation like oleic acid and linoleic acid in stageⅠof the TPAD led to the reduced lipid removal in the TPAD. In the case of the TPAD-R, the less lipid degradation than that in the MD could be attributed to the improved accumulation of refractory lipid in the system caused by recycling. Moreover, the recycle system in the two-stage system, which replenished a certain amount of alkalinity to stageⅠ, resulted in the improved alkalinity and pH in stageⅠ. It provided the possibility for hydrogen production in this stage.
4. Conclusions Co-digestion of FW and de-oiled GTW could lead to an increase of up to 19% in biogas yield, with a biogas yield of 0.60 L/g VS added. The two-stage system with recycling achieved the highest specific methane production activity 1225 mL/g VS/d and a hydrogen yield about 1 mol/ mol hexose. Lipid was converted more complete in the single-stage system than in the two-stage systems, which was attributed to more accumulation of long-chain fatty acids in the two-stage systems to some extent. Palmitic acid accumulated most in the effluents of co-digestion, responsible for the reduced capability to degrade lipid.
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26. Silvestre, G., Illa, J., Fern A Ndez, B., Bonmat I, A., 2014. Thermophilic anaerobic co-digestion of sewage sludge with grease waste: Effect of long chain fatty acids in the methane yield and its dewatering properties. Appl. Energ. 117, 87-94. 27. Toba, M., Abe, Y., Kuramochi, H., 2011. Hydrodeoxygenation of waste vegetable oil over sulfide catalysts. Catal. Today 164, 533-537. 28. Van Lier, J.B., Tilche, A., Ahring, B.K., Macarie, H., Moletta, R., Dohanyos, M., Pol, L.W.H., Lens, P., Verstraete, W., 2001. New perspectives in anaerobic digestion. Water Sci. Technol. 43, 1-18. 29. Wang, L., Aziz, T.N., de Los Reyes III, F.L., 2013. Determining the limits of anaerobic co-digestion of thickened waste activated sludge with grease interceptor waste. Water Res. 47, 3835-3844. 30. Zhang, C., Su, H., Baeyens, J., Tan, T., 2014. Reviewing the anaerobic digestion of food waste for biogas production. Renewable and Sustainable Energy Reviews 38, 383-392.
MD
TPAD
Mesophilic digestion
Temperature-phased anaerobic digestion M
Temperature-phased anaerob Biogas
Gas meter Feedstock
Biogas
M
Gas meter
Feedstock Feedstock
Biogas
M
M
Gas meter
Gas meter
Mesophilic reactor (35℃)
Fig.1
Stage Ⅰ(55℃)
Stage Ⅱ(35℃)
Scheme of experimental setup for the MD, TPAD and TPAD-R
The figure has been adjusted, and the acronyms in the figure have been expanded.
Stage Ⅰ(55℃
Recycle system
Lipid/TS=40%
Lipid/TS=20%
Lipid/TS=20%
5
5
0 100
3
2
1
0 100 CH4
80
60
40
20
0
0 20 40 60 80 100 120 140 160 180
Stage I: Stage II:
Operation time (days)
H2 H2
CH4 CH4
CO2 CO2
2
1
Stage I: Stage II:
H2 H2
CH4 CH4
CO2 CO2
80
80
60
60
40
40
20
20
0
3
0 100
CO2
Gas composition in the TPAD (%)
H2
Gas composition in the MD (%)
Gas production rate (L/L/d)
1
4
Gas composition in the TPAD-R (%)
Gas production rate (L/L/d)
Gas production rate (L/L/d)
2
Fig.2
TPAD-R (stage I) TPAD-R (stage II)
4
3
Lipid/TS=40%
5
TPAD (stage I) TPAD (stage II)
MD
4
Lipid/TS=20%
Lipid/TS=40%
0 0
20 40 60 80 100 120 140 160 180
Operation time (days)
Biogas production characteristics over time
The font size in the figure has been increased.
0
20 40 60 80 100 120 140 160 180
Operation time (days)
VS
8
6
4
2
0
0
20
40
60
80 100 120 140 160 180
Stage I: Stage II:
TS TS
VS VS
6
4
2
0 0
20
TS and VS concentrations in the MD, TPAD and TPAD-R
40
60
80
Lipid/TS=40%
Lipid/TS=20%
10
8
Operation time (days)
Fig.3
Lipid/TS=40%
Lipid/TS=20%
10
Solid content in the TPAD (%)
Solid content in the MD (%)
TS
Lipid/TS=40%
Solid content in the TPAD-R (%)
Lipid/TS=20%
10
100 120 140 160 180
Operation time (days)
Stage I: Stage II:
TS TS
VS VS
8
6
4
2
0 0
20
40
60
80
100 120 140 160 180
Operation time (days)
Lipid/TS=20%
Lipid/TS=40%
Lipid/TS=20%
9
9
Lipid/TS=20%
9
8
8
7
7
7
6
6
6
pH
8
pH
pH
Lipid/TS=40%
TPAD (stage I) TPAD (stage II)
MD
5
5
5
4
4
4
3
3
12000
3
12000
12000
TPAD (stage I) TPAD (stage II)
MD
TPAD-R (stage I) TPAD-R (stage II)
8000
8000
8000
4000
2000
0
0
6000
4000
2000
20 40 60 80 100 120 140 160 180
Operation time (days)
Fig.4
VFA (mg HAc/L)
10000
VFA (mg HAc/L)
10000
VFA (mg HAc/L)
10000
6000
0
Lipid/TS=40%
TPAD-R (stage I) TPAD-R (stage II)
6000
4000
2000
0
20 40 60 80 100 120 140 160 180
0
Operation time (days)
pH and VFA concentrations in the MD, TPAD and TPAD-R
0
20 40 60 80 100 120 140 160 180
Operation time (days)
LCFA concentration (mmol/L)
10
8
6
4
2
0
MD
TPAD (stage I)
TPAD (stage II)
TPAD-R (stage I) TPAD-R (stage II)
Tridecanoic acid (C13:0) Caprylic acid (C8:0) Capric acid (C10:0) Lauric acid (C12:0) Myristic acid (C14:0) Myristoleic acid (C14:1) Palmitic acid (C16:0) Palmitoleic acid (C16:1) Heptadecanoic acid (C17:0) Steatic acid (C18:0) Elaidic acid (C18:1) Oleic acid (C18:1 (n-9)) Linoleic acid (C18:2) Arachidic acid (C20:0) cis-11-Eicosenoic acid (C20:1(w-9)) Linoleic acid (C18:3) Total LCFAs Behenic acid (C22:0) Erucic acid (C22:1)
Fig.5 LCFA concentrations in the MD, TPAD and TPAD-R (The columns on the left are the LCFA concentration in mono-digestion, and the columns on the right are LCFA concentration in co-digestion)
700
120
SMA test (mL/g VS)
500 90
400
300
60
200 30 100
0 0
5
10
15
20
25
30
Residual biogas production (mL/g VS)
600
150
Residual-MD Residual-TPAD Residual-TPAD-R
SMA-MD SMA-TPAD SMA-TPAD-R
0 35
Time (days) Fig.6 Residual biogas production and SMA test in the steady state of the co-digestion in the MD, TPAD and TPAD-R
Nomenclature FW
food waste
GTW
grease trap waste
MD
mesophilic digestion
TPAD
temperature-phased anaerobic digestion
TPAD-R
temperature-phased anaerobic digestion with a recycle system
StageⅠ Ⅰ
the front stage in the two-stage systems
StageⅡ Ⅱ
the end stage in the two-stage systems
HRT
hydraulic retention time
TS
total solid
VS
volatile solid
COD
chemical oxygen demand
SCOD
soluble chemical oxygen demand
VFA
volatile fatty acid
LCFA
long-chain fatty acid
SMA
specific methanogenic activity
Table 1
Feedstock characterization Single substrate: FW
Mixed substrate: FW+de-oiled GTW
pH
3.65±0.06
3.99±0.04
TS (%)
7.62±0.29
8.18±0.14
VS (%)
7.21±0.29
7.82±0.22
VS/TS (%)
94.6
95.6
COD (g/L)
101.2±6.1
152.2±13.4
Carbohydrate (g/L)
31.0±5.4
15.9±0.9
Protein (g/L)
14.1±1.9
10.0±0.7
Lipid (g/L)
13.9±2.2
32.9±3.0
Lipid/TS (%)
20
40
Table 2
Biogas production for the MD, TPAD and TPAD-R in the steady state Mono-digestion
Co-digestion
TPAD MD
TPAD-R
TPAD
Ⅰ
Ⅱ
Ⅰ+Ⅱ
Ⅰ
Ⅱ
Ⅰ+Ⅱ
1.79±0.07
0.05±0.01
2.20±0.04
1.77
1.82±0.08
1.91±0.08
1.89
0.74
0.00
0.70
0.74
0.13
0.75
1.00
0.00±0.00
0.96
1.02
0.44
H2
0.0±0.0
0.0±0.0
0.0±0.0
0.0
CH4
60.5±1.9
0.0±0.0
61.0±0.6
CO2
39.5±3.9
25.7±2.1
38.9±0.7
MD
TPAD-R
Ⅰ
Ⅱ
Ⅰ+Ⅱ
Ⅰ
Ⅱ
Ⅰ+Ⅱ
2.29±0.05
0.03±0.00
2.59±0.13
2.08
0.78±0.12
2.86±0.05
2.44
0.79
0.88
0.00
0.79
0.80
0.05
0.68
0.94
1.40
0.98
1.14
0.00
0.07
0.09
0.00
1.02
1.19
31.7±1.1
0.0±0.0
6.1
0.0±0.0
0.0±0.0
0.0±0.0
0.0
26.7±1.0
0.0±0.0
1.7
60.6
0.5±0.0
70.4±1.0
57.0
70.4±0.9
0.0±0.0
65.4±0.2
65.2
3.6±0.2
68.7±0.4
64.5
38.8
69.9±2.6
30.7±1.0
38.2
30.4±0.6
4.0±0.8
34.6±0.2
34.5
66.4±1.1
31.2±0.3
33.4
Biogas production Production rate (L/L/d) Yield (L/g VS added) Recovery rate (L/g VS destroyed) Composition (%) Methane production Production rate (L/L/d)
1.08
0.0
1.34
1.07
0.00
1.34
1.07
1.61
0.00
1.69
1.36
0.03
1.96
1.58
Yield (L/g VS added)
0.45
0.00
0.43
0.45
0.00
0.53
0.45
0.62
0.00
0.52
0.52
0.00
0.47
0.60
0.60
0.00
0.58
0.62
0.00
0.98
0.56
0.80
0.00
0.70
0.71
0.00
0.70
0.77
Recovery rate (L/g VS destroyed)
Table 3
Characteristics of digested sludge and material removal Mono-digestion
Co-digestion
TPAD
TPAD-R Ⅰ+
MD Ⅰ
Ⅱ
TPAD Ⅰ+
Ⅰ Ⅱ
Ⅰ+
MD
Ⅱ
TPAD-R
Ⅰ
Ⅱ
Ⅱ
Ⅰ+ Ⅰ
Ⅱ
Ⅱ
Ⅱ
2.57±0
7.91±
2.47±
2.47±
3.57±
1.99±
1.99±
2.22±
8.17±0
2.50±
2.50±
5.46±
2.08±
2.08±
.08
0.02
0.09
0.09
0.19
0.06
0.06
0.13
.49
0.06
0.06
0.15
0.08
0.08
66.3
0.0
68.8
67.6
25.7
44.2
73.9
72.9
0.1
69.4
69.4
0.0
61.9
74.6
1.84±0
7.53±
2.03±
2.03±
3.06±
1.42±
1.42±
1.77±
7.88±0
2.09±
2.09±
5.03±
1.66±
1.66±
.05
0.04
0.05
0.05
0.06
0.05
0.05
0.09
.52
0.04
0.04
0.14
0.01
0.01
74.5
0.0
73.0
71.8
29.1
53.6
80.3
77.4
0.0
73.5
73.3
0.0
70.0
78.8
17.3±0
101.3
16.6±
16.6±
54.2±
12.1±
12.1±
27.9±
147.9±
32.7±
32.7±
95.8±
27.6±
27.6±
.9
±3.2
1.0
1.0
1.2
1.0
1.0
1.0
19.4
1.3
1.3
16.9
1.1
1.1
82.9
0.0
83.6
83.6
4.3
77.7
88.0
81.7
2.8
77.9
78.5
0.0
71.2
81.9
2.6±0.
36.4±
1.8±0
1.8±0
22.7±
1.7±0
1.7±0
2.2±0
28.7±1
3.3±0
3.3±0
17.6±
1.7±0
1.7±0
2
1.2
.3
.3
0.9
.2
.2
.1
.0
.3
.3
1.3
.1
.1
2.3±0.
29.3±
2.8±0
2.8±0
3.9±0
1.7±0
1.7±0
2.6±0
13.8±2
2.1±0
2.1±0
2.9±0
1.7±0
1.7±0
2
1.0
.3
.3
.2
.1
.1
.4
.1
.3
.3
.2
.1
.1
92.6
5.5
90.4
91.0
76.1
56.4
94.5
83.6
13.2
84.8
86.8
67.0
41.4
89.3
0.4±0.
11.0±
0.3±0
0.3±0
1.4±0
0.2±0
0.2±0
0.3±0
3.0±0.
0.3±0
0.3±0
0.9±0
0.2±0
0.2±0
1
1.3
.1
.1
.0
.0
.0
.0
3
.1
.1
.2
.0
.0
5.2±0.
12.0±
6.8±0
6.8±0
6.8±0
4.5±0
4.5±0
8.1±0
8.5±0.
9.0±1
9.0±1
7.5±0
7.0±0
7.0±0
1
1.2
.4
.4
.9
.2
.2
.7
7
.1
.1
.8
.7
.7
63.1
14.9
43.3
51.8
26.9
33.8
68.1
19.0
15.0
0.0
10.0
11.8
6.7
30.0
0.7±0.
3.0±0
0.6±0
0.6±0
0.9±0
0.3±0
0.3±0
0.4±0
1.5±0.
0.5±0
0.5±0
1.4±0
0.4±0
0.4±0
1
.2
.1
.1
.3
.0
.0
.0
2
.0
.0
.2
.0
.0
1.1±0.
14.4±
2.4±0
2.4±0
6.9±1
2.4±0
2.4±0
3.6±0
29.1±2
5.5±0
5.5±0
8.9±0
4.3±0
4.3±0
4
2.1
.2
.2
.2
.2
.2
.6
.4
.3
.3
.5
.2
.2
92.1
0.0
83.3
82.7
15.3
65.2
82.7
89.1
11.6
81.1
83.3
52.2
51.7
86.9
TS (%)
TS removal (%)
VS (%)
VS removal (%)
COD (g/L)
COD removal (%) SCOD (g/L) Carbohydr ate (g/L) Carbohydr ate removal (%) Sa-Carboh ydrate (g/L) Protein (g/L) Protein removal (%) S-Protein (g/L)
Lipid (g/L)
Lipid removal (%)
- 30 -
S-Lipid (g/L)
0.0±0.
0.8±0
0.0±0
0.0±0
0.3±0
0.0±0
0.0±0
0.2±0
0.4±0.
0.2±0
0.2±0
0.4±0
0.2±0
0.2±0
0
.2
.0
.0
.0
.0
.0
.2
1
.1
.1
.1
.1
.1
7.77±0
3.61±
7.32±
7.32±
5.36±
7.59±
7.59±
7.39±
3.99±0
6.89±
6.89±
5.28±
7.17±
7.17±
.06
0.04
0.04
0.04
0.10
0.05
0.05
0.04
.11
0.02
0.02
0.04
0.02
0.02
0±0
0±0
0±0
0±0
0±0
0±0
0±0
0±0
0±0
pH
VFA (mg
3034
4334
0±0 HAcb/L)
±280
10120
±950
8086
±584
±863
Alkalinity 5455± (mg
3840
3840
1963
5131
5131
2917
±228
±228
±131
±38
±38
±66
107.0
107.2
98.0
107.0
105.2
109.0
0±0
1800
1800
1281
2971
2971
±163
±163
±92
±112
±112
100.4
97.6
107.3
99.0
107.4
0±0
253 CaCO3/L) COD mass balance
108.6
100.3
(%)
Note: a‘S’ is ‘Soluble’.
b
All of the individual VFA was calculated as acetic acid.
- 31 -
97.2
Highlights •
A single-stage and two two-stage anaerobic systems were synchronously operated.
•
Co-digestion with de-oiled grease trap waste increased the biogas yield up to 19%.
•
Two-stage process with recycling achieved a hydrogen yield of 1 mol/mol hexose.
•
More alkalinity was consumed in the co-digestion than the mono-digestion.
•
Single-stage process favored the conversion of lipid and long-chain fatty acids.
- 32 -