- Email: [email protected]
High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates
International Biodeterioration & Biodegradation xxx (2015) 1e5
Contents lists available at ScienceDirect
International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates Maliha Jabeen, Zeshan*, Sohail Yousaf, Muhammad Rizwan Haider, Riffat Naseem Malik Department of Environmental Sciences, Quaid-i-Azam University, Islamabad 45320, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2015 Received in revised form 16 March 2015 Accepted 17 March 2015 Available online xxx
Low C/N ratio and high biodegradability are major limitations of anaerobic digestion of food waste which results in process inhibition due to rapid accumulation of volatile fatty acids. In the present study, C/N ratio of food waste was adjusted by mixing with rice husk which has low biodegradability. Co-digestion of the two wastes was performed in continuous pilot scale anaerobic reactor operated at organic loading rates (OLR) of 5, 6 and 9 kg VS/m3/d and mesophilic (37 C) temperature under plug flow mixing mode. At organic loading rate of 5 and 6 kg VS/m3/d, the volatile fatty acids/alkalinity ratio ranged from 0.15 to 0.24 which indicated higher buffering capacity of digester. While at OLR of 9 kg VS/m3/d, volatile fatty acids/alkalinity ratio of 0.94 was recorded. Daily biogas production and gas production rate were 196 L/ d and 2.36 L/L/d respectively at OLR of 6 kg VS/m3/d. However at loading rate of 9 kg VS/m3/d, daily biogas production drastically decreased from 196 L/d to 136 L/d. Highest volatile solids removal of 82% was achieved at 5 kg VS/m3/d. Biogas production, reactor stability and volatile solids removal efficiency decreased with increase in organic loading rate and decrease in hydraulic retention time. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Co-digestion Rice husk Food waste Dry anaerobic digestion Organic loading rate
Introduction Food and agriculture waste management is one of the biggest challenges faced by many countries. According to Environmental Protection Agency of Pakistan, average municipal solid waste generation rate is 0.613 kg/capita/d and 90% of total household waste is biodegradable (PEPA, 2005). Similarly, huge quantities of agricultural waste are also produced in Pakistan, of which rice husk production is about 1.78 million tons annually (Mirani et al., 2013). Improper management of food and agricultural waste is a major source of environmental pollution all over the world. Food waste is easily biodegradable due to high content of water and this is major reason for its unstable combustion in incinerator, production of bad odor and abundant leachate (Cheng and Hu, 2010), and emission of greenhouse gases (i.e. methane) in landfills (Liu et al., 2012). Methane traps 25 times more heat and its warming effect is 72 times higher as compared to carbon dioxide (IPCC, 2007). Therefore, there is a dire need for safe management of organic waste
* Corresponding author. Tel.: þ92 51 9064 4140. E-mail addresses: [email protected] (M. Jabeen), [email protected] (Zeshan), [email protected] (S. Yousaf), [email protected] (M.R. Haider), [email protected] (R.N. Malik).
including food waste and agricultural waste. In this regard, anaerobic digestion has proven to be an environment friendly option (Liu et al., 2012). However the main hindrance in anaerobic digestion of agriculture waste is slow biodegradation rate because of its chemical composition and structure of ligno-cellulosic materials. On the other hand, anaerobic digestion of food waste alone is not feasible owing to its low C/N ratio and high biodegradation rate which results in digester failure in a very short period of time due to rapid production and accumulation of volatile fatty acids (Shen et al., 2013). Another problem associated with anaerobic digestion of food waste alone is the operational instability at high organic loading rates which inhibits methanogenesis (Zhang et al., 2012). Anaerobic digester works efficiently when provided with the proper composition of feedstock. Optimum carbon to nitrogen ratio for anaerobic digestion is in the range of 25e30 (Okeh et al., 2014). The C/N ratio of food waste can be adjusted to optimum range by mixing it with another suitable organic waste prior to anaerobic digestion. This process of simultaneous digestion of multiple substrates is known as co-digestion. Apart from adjustment of C/N ratio, co-digestion also leads to dilution of toxic compounds. Moreover, it allows increased load of organic matter in digestion and missing nutrients also become available to microorganisms that results in high biogas yield (Zhang et al., 2012, 2013). Several
http://dx.doi.org/10.1016/j.ibiod.2015.03.023 0964-8305/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
2
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5
studies have reported the co-digestion of food waste with other substrates. Kim and Oh (2011) reported that co-digestion of food and paper waste resulted in 79.8% volatile solids removal and a methane yield of 250 L CH4/kg COD. Similarly, co-digestion of food, fruit and vegetable waste and dewatered sewage sludge resulted in gas production rate of 4.25 L/L/d at OLR of 6 kg VS/m3/d (Liu et al., 2012). In the light of previous literature, it is suggested that if food waste is co-digested with some suitable organic waste, digester stability and higher biogas yield can be achieved (Liu et al., 2012; Zhang et al., 2012, 2013; Shen et al., 2013; Agyeman and Tao, 2014). However little information is available on co-digestion of food waste using rice husk as co-substrate. In the present study, high solids co-digestion of food waste and rice husk was performed under mesophilic (37 ) conditions ndez et al., 2008), because it requires smaller energy expense (Ferna and higher solubilization of food waste is also related to this temperature (Komemoto et al., 2009). The specific objective of this research work was to investigate effectiveness of high solids anaerobic co-digestion of food waste and rice husk for biogas production without volatile fatty acids inhibition and to determine the effect of different organic loading rates on digester stability and performance. Materials and methods This research work was carried out in two phases, startup phase and continuous phase. For startup, reactor was operated in batch mode as well as at low organic loading rates (1e4 kg VS/m3/d). After successful and stable startup, continuous phase of reactor was started in which it was operated at higher organic loading rates (5, 6 and 9 kg VS/m3/d). Experimental setup Pilot scale single stage anaerobic digester was constructed and operated under plug flow mode. A horizontally placed stainless steel container having 79 cm length, 18 cm radius making 80 L total volume served as anaerobic digester. The digester was equipped with inlet and outlet valves for feeding and digestate withdrawal. There were two biogas valves on the top of digester, one of which was connected to the biogas meter through a pipe while other was kept spare. A schematic diagram of reactor design is shown in Fig. 1.
Reactor was equipped with thermostatically controlled water jacket to maintain mesophilic temperature (37 C). Feedstock preparation Food waste used in this study was collected from cafeteria of Quaid-i-Azam University, Islamabad, Pakistan and rice husk was obtained from a local rice huller. Food waste was collected in polythene bags every day for a period of one week and stored in refrigerator at 4 C. Food waste consisted of food residues such as cooked rice, potato, vegetables, chicken and meat. The indigestible materials such as polythene bags, bones, plastics and egg shells were manually removed. It was then manually chopped to a size of about 10 mm and mixed to get a representative sample. The prepared food waste was then mixed with rice husk to get a C/N ratio of 28. Characteristics of individual substrates and feedstock mixture have been presented in Table 1. Operating conditions For startup, reactor was loaded with fresh cow dung to the working volume (80% of total volume) and was operated in batch mode for a period of 44 days. Once reactor stabilized with cow dung, it was then operated at low OLRs (starting from 1 kg VS/m3/ d through gradual increase to 4 kg VS/m3/d in 20 days) to achieve stabilized conditions for digestion and biogas production. After startup and stabilization period, continuous loading phase of the reactor was started in which it was fed with the feedstock mixture at OLRs of 5, 6 and 9 kg VS/m3/d one after another. The hydraulic retention time at OLR 5, 6 and 9 kg VS/m3/d was 26, 25 and 14 days respectively. Operation or run time at each organic loading rate was almost double the retention time of its respective OLR (i.e. 27, 52 and 30 days), which is also equal to the number of replications for each OLR. During continuous loading phase, feeding with feedstock mixture and digestate withdrawal from reactor was performed on daily basis. Mesophilic temperature of 37 C was maintained in the reactor throughout the study period. The total solids content of feedstock mixture was adjusted to 20% by addition of water. Analytical methods Digestate characteristics such as total solids (TS) and volatile solids (VS) were determined as per standard methods (APHA,
Fig. 1. Schematic diagram of anaerobic digester.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5 Table 1 Characteristics of inoculum, substrates and feedstock mixture. Parameters
Inoculum
Food waste
Rice husk
Mixture
Total solids (%) Volatile solidsa (%) Total organic carbona (%) Total nitrogena (%) C/N ratio
13.79 87.70 37.60 2.80 13.42
27.45 91.99 51.10 3.04 16.81
90.00 81.09 45.03 1.17 38.48
54.40 84.80 48.72 1.74 28.00
a
% of total solids.
2005). Digestate was centrifuged at 5000 rpm for 20 min to obtain supernatant. The supernatant was then filtered and used for determination of volatile fatty acids (VFA) and alkalinity. Volatile fatty acids were measured by titrating the samples with 0.1 N NaOH (USEPA, 1983) and alkalinity was determined according to standard methods (APHA, 1998). Operational parameters for anaerobic digestion such as volatile fatty acids, alkalinity, total solids and volatile solids were performed twice a week. The pH of digestate was measured daily according to method described by Rhoades (1982), with the help of a pH meter, Crison model pH 25þ. Daily biogas produced was measured with the help of wet tip gas meter. Kruskal Wallis One Way ANOVA with duns procedure (two tailed test) as post hoc test was used to find out significance of the results using XLSTAT add-in for Microsoft Excel. Results and discussion Monitoring of reactor stability during startup and continuous phase pH is a very basic parameter that describes the stability of the anaerobic digestion system. During startup phase, initially pH of reactor was low due to high concentration of volatile fatty acids (7871 mg/L) because of rapid hydrolysis rate of cow dung used for startup. At this stage, pH was adjusted artificially to achieve stable anaerobic digestion conditions. During continuous loading at low OLRs, pH of the reactor remained above 7.0 and maintained naturally without addition of any chemicals. The pH decreased with the increase in OLR but did not drop below 7.0 till OLR of 6 kg VS/m3/ d (Fig. 2, Table 2). This is because of high buffering capacity of the system due to high alkalinity, which helped to maintain a balance between acidogenesis and methanogenesis. As the OLR was increased to 9 kg VS/m3/d, the pH ranged from 7.1 to 7.6 initially, but after day 158 it decreased rapidly to an average of 6.54 due to high acid accumulation (Fig. 2). Optimum pH range suggested for successful anaerobic digestion is 6.8e7.8 (Lahav and Morgan, 2004). This range is relatively wide in the plant scale and the finest value of pH varies with substrate and digestion technique. pH of 7.1 is required for mono-digestion of food waste as reported by
3
Somashekar et al., 2014. However, enhanced enzymatic activity of hydrolytic enzyme leads to acidic pH during hydrolysis of food waste digestion (Lim et al., 2008). Highest concentration of volatile fatty acids and volatile fatty acids/alkalinity ratio of 10,445 mg/L and 1.11 respectively was observed on 165th day during reactor operation at OLR of 9 kg VS/m3/d as shown in Fig. 3. This indicated that at higher OLRs, volatile fatty acids were not effectively consumed by methanogens. These results are in agreement with previous studies (Lim et al., 2008; Jiang et al., 2013; Shen et al., 2013). In present study, high volatile fatty acids/alkalinity ratio (0.94) at OLR of 9 kg VS/m3/d is one of reasons of digester instability. The optimum range of volatile fatty acids/alkalinity ratio is 0.3e0.4 (Liu et al., 2012) or 0.1e0.25 (Di Maria et al., 2014). Shen et al. (2013) also reported that, co-digestion of food and fruit & vegetable waste was less stable at higher organic loading rates because of accumulation of propionate (1576 mg/L), which accounted for 68.5% of volatile fatty acids and could not be easily utilized by methane producing bacteria. Volatile fatty acids concentration of 6000e8000 mg/L, and beyond, has inhibitory effect on biogas production (Polprasert, 2007). Solids reduction by anaerobic reactor During startup phase, total solids content of digestate was 9.61%. However, it began to increase with the increase in OLR. At OLR of 5 kg VS/m3/d, total solids content of digestate was 10.43% which increased to 11.09% at OLR of 6 kg VS/m3/d. During OLR of 6 and 9 kg VS/m3/d, total solids content of digestate was in the range of 10e13% with an average of 11.57%. Similar observations were made by Somashekar et al. (2014) who reported that continuous feeding for digestion process is very efficient in removing the total solids. However, high total solids content of digestate indicates acidic conditions in the reactor which results in no further increase in biogas volume (Somashekar et al., 2014). The average and standard deviation values of total solids and volatile solids of digestate at each OLR are shown in Table 2. Highest volatile solids removal of 82% was achieved at OLR of 5 kg VS/m3/d which decreased with increase in OLR. Lowest volatile solids removal was observed at OLR of 9 kg VS/m3/d (Table 3 and Fig. 4). This can be attributed to increase in organic load of digester at higher OLRs. It could also be due to the overall increase in rice husk's ligno-cellulosic content in the digester used in feedstock preparation which has comparatively low degradability. Increase in total solids content with increase in organic loading rate could also be a reason of decrease in volatile solids removal efficiency of the reactor. Similar results have also been reported in other studies (Duan et al., 2012; Somashekar et al., 2014), in which volatile solids removal reduced with increase in total solids content of digestate. Specific biogas yield also showed similar trends where it decreased with increase in OLR from 5 kg VS/m3/d. It suggested a relationship between specific biogas yield and volatile solids removal, therefore biogas yield can be increased by increasing the volatile solids removal efficiency of the anaerobic reactor and vice versa. The statistical analysis indicated that volatile solids removal at OLR of 9 kg VS/m3/d differed significantly (p 0.004) from volatile solids removal at OLR of 5 and 6 kg VS/m3/d. Kim and Oh (2011) reported volatile solid reduction of 79.8% and 71.5% from separate co-digestion of food waste with paper waste and livestock waste, respectively. Biogas production of anaerobic reactor
Fig. 2. pH of digestate at various stages of digester operation.
In order to evaluate the performance of anaerobic digester, daily biogas yield, specific biogas yield, gas production rate and volatile solids removal were determined and the results have been
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
4
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5
Table 2 Characteristics of digestate at different stages of digester operation. Parameters
Units
Organic loading rate (kg VS/m3/d)
Startup phase
5 pH Volatile fatty acids Alkalinity VFA/ALKa Total solids Volatile solidsb a b
7.45 3672 13,714 0.27 9.61 62.35
mg/L mg/L % %
± ± ± ± ± ±
0.36 2391 968 0.18 1.30 4.12
6
7.75 1935 12,905 0.15 10.43 74.18
± ± ± ± ± ±
0.07 342 1135 0.02 0.44 3.30
7.68 2413 11,400 0.22 11.09 73.92
9 ± ± ± ± ± ±
0.08 494 1301 0.06 0.97 2.77
6.95 8344 8879 0.94 12.55 88.20
± ± ± ± ± ±
0.43 1511 668 0.13 0.52 0.81
Volatile fatty acids/Alkalinity ratio. % of total solids.
Fig. 3. Variation in volatile fatty acids, alkalinity and volatile fatty acids/alkalinity ratio at different stages of digester operation.
presented in Table 3. Among all OLRs studied, highest daily biogas production (196 L/d) was attained at OLR of 6 kg VS/m3/d. Non parametric Kruskal Wallis One Way ANOVA (a ¼ 0.05) with Dunn's procedure for pair wise comparisons of daily biogas yield at various organic loading rates were performed to find significant differences among all possible pairs. The results showed that daily biogas yield at OLR of 6 kg VS/m3/d differed significantly (p 0.0001) from biogas yield at OLR of 5 and 9 kg VS/m3/d. Low daily biogas yield at OLR of 9 kg VS/m3/d was attributed to high volatile fatty acids and volatile fatty acids/alkalinity ratio owing to high organic load. Specific biogas yield (SBY) was the highest at OLR of 5 kg VS/m3/ d. However specific biogas yield decreased with increase in OLR to 6 and 9 kg VS/m3/d. Average specific biogas yield of 446 and 399 L/ kg VS was recorded at organic loading rates of 5 and 6 kg VS/m3/ d respectively. While specific biogas yield of 215 L/kg VS was obtained at OLR of 9 kg VS/m3/d. The possible reasons for higher specific biogas yield and volatile solids removal at 5 kg VS/m3/ d could be higher solid retention time and lower load while at higher OLR, the specific biogas yield and volatile solids removal decreased because of overloading of reactor and lower solid retention time. Similar results have also been reported by Zeshan et al. (2012). To define a relationship between specific biogas yield and volatile solids removal at OLR of 5, 6 and 9 kg VS/m3/d,
Fig. 4. Specific biogas yield and VS removal at different organic loading rates.
regression analysis was performed. High value of regression coefficient (0.98) showed a strong relationship between specific biogas yield and volatile solids removal. Decline in specific biogas yield along with volatile solids removal at high OLR was also observed by Zeshan et al. (2012). Gas production rate (GPR) is another performance parameter of dry anaerobic digestion process. It is measurement of biogas generation rate per unit volume of reactor. Initially the biogas production rate was low, but with the stabilization of reactor and increase in OLR, gas production rate also increased. Organic loading rate directly affects the rate of gas production under normal digestion conditions. Fig. 5 showed an increase in gas production rate at 5, 6 and 9 kg VS/m3/d. However, decline in gas production rate was observed at day 152 during OLR of 9 kg VS/m3/d. Through co-digestion of food waste and rice husk, maximum average gas production rate of 2.36 L/L/d was obtained at OLR of 6 kg VS/m3/d. However, a decrease in average gas production rate to 1.89 L/L/ d was observed at OLR of 9 kg VS/m3/d. The gas production rate
Table 3 Performance of anaerobic digester at different organic loading rates. Parameters
Units
Daily biogas yield Specific biogas yield Volatile solids removal Gas production rate
L/d L/kg VS % L/L/d
Organic loading rate (kg VS/m3/d) 5 179 446 82.41 2.23
6 ± ± ± ±
40.8 101 0.55 0.51
196 399 73.12 2.36
9 ± ± ± ±
18.2 38 3.85 0.26
136 215 35.42 1.89
± ± ± ±
73.2 121 2.79 1.02
Fig. 5. Gas production rate at different stages of digester operation.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5
started to decline during OLR of 9 kg VS/m3/d because of increase in volatile fatty acids/alkalinity ratio and/or drop in pH and alkalinity. A temporary decline in gas production rate was observed for first few days of each run, when the loading rate was changed, but later the gas production rate used to get stable with the passage of time. The reason for this change in gas production rate might be the sudden increase in organic load which disturbed the microbial activity. Agyeman and Tao (2014) worked on co-digestion of food waste and dairy manure and stated that more than ten days are required for stable operation of anaerobic digester at higher OLRs. The type of digester is also very important in biogas production. In single phase digestion system, the adaptation and regeneration period for microorganisms is 15 days which increases up to 30 days in two stage anaerobic system after change in organic loading rate (Shen et al., 2013). The gas production rate in present study was in accordance with values reported in literature related to digestion of food waste with different co-substrate. The gas production rate of 1.88 and 1.75 L/L/d at OLR of 4e5 kg VS/m3/d from co-digestion of food, fruit and vegetable waste in single and two stage anaerobic digester respectively were achieved in a study where higher risks of volatile fatty acids accumulation and low biogas yield at high OLR have also been reported (Shen et al., 2013). Conclusions Stable performance of the reactor was observed at OLR of 5 and 6 kg VS/m3/d where volatile fatty acids/alkalinity ratio ranged between 0.15 and 0.24. Therefore, rice husk can be used to overcome rapid acid accumulation in anaerobic digestion of food waste without external chemical addition. Daily biogas production rate was higher at OLR of 6 kg VS/m3/d while specific biogas yield was higher at OLR of 5 kg VS/m3/d. Similarly, volatile solids removal of 82% was achieved at OLR of 5 kg VS/m3/d. Biogas production, volatile solids removal and stability of digester decreased with increase in OLR to 9 kg VS/m3/d because of excessive volatile fatty acids production due to overfeeding of the reactor. References Agyeman, F.O., Tao, W., 2014. Anaerobic co-digestion of food waste and dairy manure: effects of food waste particle size and organic loading rate. J. Environ. Manag. 133, 268e274. APHA, AWWA, WEF, 2005. Standard Methods for the Examination of Water and Waste Water, 21st ed. American Public Health Association, Washington D.C., USA. APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Waste Water, 20th ed. American Public Health Association, Washington D.C., USA. Cheng, H., Hu, Y., 2010. Municipal solid waste (MSW) as a renewable source of energy: current and future practices in China. Bioresour. Technol. 101, 3816e3824.
5
Di Maria, F., Sordi, A., Cirulli, G., Gigliotti, G., Massaccesi, L., Cucina, M., 2014. Cotreatment of fruit and vegetable waste in sludge digesters. An analysis of the relationship among bio-methane generation, process stability and digestate phytotoxicity. Waste Manag. 34, 1603e1608. Duan, N., Dong, B., Wu, B., Dai, X., 2012. High-solid anaerobic digestion of sewage sludge under mesophilic conditions: feasibility study. Bioresour. Technol. 104, 150e156. ndez, J., Perez, M., Romero, L., 2008. Effect of substrate concentration on dry Ferna mesophilic anaerobic digestion of organic fraction of municipal solid waste (OFMSW). Bioresour. Technol. 99, 6075e6080. IPCC, 2007. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, New York, USA. Jiang, J., Zhang, Y., Li, K., Wang, Q., Gong, C., Li, M., 2013. Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate. Bioresour. Technol. 143, 525e530. Kim, D.-H., Oh, S.-E., 2011. Continuous high-solids anaerobic co-digestion of organic solid wastes under mesophilic conditions. Waste Manag. 31, 1943e1948. Komemoto, K., Lim, Y., Nagao, N., Onoue, Y., Niwa, C., Toda, T., 2009. Effect of temperature on VFA's and biogas production in anaerobic solubilization of food waste. Waste Manag. 29, 2950e2955. Lahav, O., Morgan, B., 2004. Titration methodologies for monitoring of anaerobic digestion in developing countriesda review. J. Chem. Technol. Biotechnol. 79, 1331e1341. Lim, S.-J., Kim, B.J., Jeong, C.-M., Ahn, Y.H., Chang, H.N., 2008. Anaerobic organic acid production of food waste in once-a-day feeding and drawing-off bioreactor. Bioresour. Technol. 99, 7866e7874. Liu, X., Gao, X., Wang, W., Zheng, L., Zhou, Y., Sun, Y., 2012. Pilot-scale anaerobic codigestion of municipal biomass waste: focusing on biogas production and GHG reduction. Renew. Energy 44, 463e468. Mirani, A.A., Ahmad, M., Kalwar, S.A., Ahmad, T., 2013. A rice husk gasifier for paddy drying. Sci. Tech. Dev. 32, 120e125. Okeh, O.C., Onwosi, C.O., Odibo, F.J.C., 2014. Biogas production from rice husks generated from various rice mills in Ebonyi State, Nigeria. Renew. Energy 62, 204e208. PEPA (Pakistan Environmental Protection Agency), 2005. Guideline for Solid Waste Management. Pakistan Environmental Protection Agency, Islamabad, Pakistan. Available online at: http://environment.gov.pk/EA-GLines/SWMGLinesDraft. pdf. Polprasert, C., 2007. Organic Waste Recycling Technology and Management, third ed. IWA Publishing, London, UK. ISBN: 184339121X. Rhoades, J., 1982. Cation exchange capacity. Methods of soil analysis. Part 2. Chem. Microbiol. Prop. 149e157. Shen, F., Yuan, H., Pang, Y., Chen, S., Zhu, B., Zou, D., Liu, Y., Ma, J., Yu, L., Li, X., 2013. Performances of anaerobic co-digestion of fruit & vegetable waste (FVW) and food waste (FW): single-phase vs. two-phase. Bioresour. Technol. 144, 80e85. Somashekar, R., Verma, R., Naik, M.A., 2014. Potential of biogas production from food waste in a uniquely designed reactor under lab conditions. Int. J. Geol. Agric. Environ. Sci. 2. USEPA, 1983. Methods for Chemical Analysis of Water and Wastes, Method 305.1 (Titrimetric). Office of Research and Development, U.S Environmental Protection Agency, Washington, DC. Available online at: http://www.state.in.us/dnr/ fishwild/files/Methods_Analysis_Water_Wastes_USEPA_March1983.pdf. Zeshan, Karthikeyan, O.P., Visvanathan, C., 2012. Effect of C/N ratio and ammonia-N accumulation in a pilot-scale thermophilic dry anaerobic digester. Bioresour. Technol. 113, 294e302. Zhang, C., Li, J., Liu, C., Liu, X., Wang, J., Li, S., Fan, G., Zhang, L., 2013. Alkaline pretreatment for enhancement of biogas production from banana stem and swine manure by anaerobic codigestion. Bioresour. Technol. 149, 353e358. Zhang, Y., Banks, C.J., Heaven, S., 2012. Co-digestion of source segregated domestic food waste to improve process stability. Bioresour. Technol. 114, 168e178.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
Contents lists available at ScienceDirect
International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates Maliha Jabeen, Zeshan*, Sohail Yousaf, Muhammad Rizwan Haider, Riffat Naseem Malik Department of Environmental Sciences, Quaid-i-Azam University, Islamabad 45320, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2015 Received in revised form 16 March 2015 Accepted 17 March 2015 Available online xxx
Low C/N ratio and high biodegradability are major limitations of anaerobic digestion of food waste which results in process inhibition due to rapid accumulation of volatile fatty acids. In the present study, C/N ratio of food waste was adjusted by mixing with rice husk which has low biodegradability. Co-digestion of the two wastes was performed in continuous pilot scale anaerobic reactor operated at organic loading rates (OLR) of 5, 6 and 9 kg VS/m3/d and mesophilic (37 C) temperature under plug flow mixing mode. At organic loading rate of 5 and 6 kg VS/m3/d, the volatile fatty acids/alkalinity ratio ranged from 0.15 to 0.24 which indicated higher buffering capacity of digester. While at OLR of 9 kg VS/m3/d, volatile fatty acids/alkalinity ratio of 0.94 was recorded. Daily biogas production and gas production rate were 196 L/ d and 2.36 L/L/d respectively at OLR of 6 kg VS/m3/d. However at loading rate of 9 kg VS/m3/d, daily biogas production drastically decreased from 196 L/d to 136 L/d. Highest volatile solids removal of 82% was achieved at 5 kg VS/m3/d. Biogas production, reactor stability and volatile solids removal efficiency decreased with increase in organic loading rate and decrease in hydraulic retention time. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Co-digestion Rice husk Food waste Dry anaerobic digestion Organic loading rate
Introduction Food and agriculture waste management is one of the biggest challenges faced by many countries. According to Environmental Protection Agency of Pakistan, average municipal solid waste generation rate is 0.613 kg/capita/d and 90% of total household waste is biodegradable (PEPA, 2005). Similarly, huge quantities of agricultural waste are also produced in Pakistan, of which rice husk production is about 1.78 million tons annually (Mirani et al., 2013). Improper management of food and agricultural waste is a major source of environmental pollution all over the world. Food waste is easily biodegradable due to high content of water and this is major reason for its unstable combustion in incinerator, production of bad odor and abundant leachate (Cheng and Hu, 2010), and emission of greenhouse gases (i.e. methane) in landfills (Liu et al., 2012). Methane traps 25 times more heat and its warming effect is 72 times higher as compared to carbon dioxide (IPCC, 2007). Therefore, there is a dire need for safe management of organic waste
* Corresponding author. Tel.: þ92 51 9064 4140. E-mail addresses: [email protected] (M. Jabeen), [email protected] (Zeshan), [email protected] (S. Yousaf), [email protected] (M.R. Haider), [email protected] (R.N. Malik).
including food waste and agricultural waste. In this regard, anaerobic digestion has proven to be an environment friendly option (Liu et al., 2012). However the main hindrance in anaerobic digestion of agriculture waste is slow biodegradation rate because of its chemical composition and structure of ligno-cellulosic materials. On the other hand, anaerobic digestion of food waste alone is not feasible owing to its low C/N ratio and high biodegradation rate which results in digester failure in a very short period of time due to rapid production and accumulation of volatile fatty acids (Shen et al., 2013). Another problem associated with anaerobic digestion of food waste alone is the operational instability at high organic loading rates which inhibits methanogenesis (Zhang et al., 2012). Anaerobic digester works efficiently when provided with the proper composition of feedstock. Optimum carbon to nitrogen ratio for anaerobic digestion is in the range of 25e30 (Okeh et al., 2014). The C/N ratio of food waste can be adjusted to optimum range by mixing it with another suitable organic waste prior to anaerobic digestion. This process of simultaneous digestion of multiple substrates is known as co-digestion. Apart from adjustment of C/N ratio, co-digestion also leads to dilution of toxic compounds. Moreover, it allows increased load of organic matter in digestion and missing nutrients also become available to microorganisms that results in high biogas yield (Zhang et al., 2012, 2013). Several
http://dx.doi.org/10.1016/j.ibiod.2015.03.023 0964-8305/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
2
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5
studies have reported the co-digestion of food waste with other substrates. Kim and Oh (2011) reported that co-digestion of food and paper waste resulted in 79.8% volatile solids removal and a methane yield of 250 L CH4/kg COD. Similarly, co-digestion of food, fruit and vegetable waste and dewatered sewage sludge resulted in gas production rate of 4.25 L/L/d at OLR of 6 kg VS/m3/d (Liu et al., 2012). In the light of previous literature, it is suggested that if food waste is co-digested with some suitable organic waste, digester stability and higher biogas yield can be achieved (Liu et al., 2012; Zhang et al., 2012, 2013; Shen et al., 2013; Agyeman and Tao, 2014). However little information is available on co-digestion of food waste using rice husk as co-substrate. In the present study, high solids co-digestion of food waste and rice husk was performed under mesophilic (37 ) conditions ndez et al., 2008), because it requires smaller energy expense (Ferna and higher solubilization of food waste is also related to this temperature (Komemoto et al., 2009). The specific objective of this research work was to investigate effectiveness of high solids anaerobic co-digestion of food waste and rice husk for biogas production without volatile fatty acids inhibition and to determine the effect of different organic loading rates on digester stability and performance. Materials and methods This research work was carried out in two phases, startup phase and continuous phase. For startup, reactor was operated in batch mode as well as at low organic loading rates (1e4 kg VS/m3/d). After successful and stable startup, continuous phase of reactor was started in which it was operated at higher organic loading rates (5, 6 and 9 kg VS/m3/d). Experimental setup Pilot scale single stage anaerobic digester was constructed and operated under plug flow mode. A horizontally placed stainless steel container having 79 cm length, 18 cm radius making 80 L total volume served as anaerobic digester. The digester was equipped with inlet and outlet valves for feeding and digestate withdrawal. There were two biogas valves on the top of digester, one of which was connected to the biogas meter through a pipe while other was kept spare. A schematic diagram of reactor design is shown in Fig. 1.
Reactor was equipped with thermostatically controlled water jacket to maintain mesophilic temperature (37 C). Feedstock preparation Food waste used in this study was collected from cafeteria of Quaid-i-Azam University, Islamabad, Pakistan and rice husk was obtained from a local rice huller. Food waste was collected in polythene bags every day for a period of one week and stored in refrigerator at 4 C. Food waste consisted of food residues such as cooked rice, potato, vegetables, chicken and meat. The indigestible materials such as polythene bags, bones, plastics and egg shells were manually removed. It was then manually chopped to a size of about 10 mm and mixed to get a representative sample. The prepared food waste was then mixed with rice husk to get a C/N ratio of 28. Characteristics of individual substrates and feedstock mixture have been presented in Table 1. Operating conditions For startup, reactor was loaded with fresh cow dung to the working volume (80% of total volume) and was operated in batch mode for a period of 44 days. Once reactor stabilized with cow dung, it was then operated at low OLRs (starting from 1 kg VS/m3/ d through gradual increase to 4 kg VS/m3/d in 20 days) to achieve stabilized conditions for digestion and biogas production. After startup and stabilization period, continuous loading phase of the reactor was started in which it was fed with the feedstock mixture at OLRs of 5, 6 and 9 kg VS/m3/d one after another. The hydraulic retention time at OLR 5, 6 and 9 kg VS/m3/d was 26, 25 and 14 days respectively. Operation or run time at each organic loading rate was almost double the retention time of its respective OLR (i.e. 27, 52 and 30 days), which is also equal to the number of replications for each OLR. During continuous loading phase, feeding with feedstock mixture and digestate withdrawal from reactor was performed on daily basis. Mesophilic temperature of 37 C was maintained in the reactor throughout the study period. The total solids content of feedstock mixture was adjusted to 20% by addition of water. Analytical methods Digestate characteristics such as total solids (TS) and volatile solids (VS) were determined as per standard methods (APHA,
Fig. 1. Schematic diagram of anaerobic digester.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5 Table 1 Characteristics of inoculum, substrates and feedstock mixture. Parameters
Inoculum
Food waste
Rice husk
Mixture
Total solids (%) Volatile solidsa (%) Total organic carbona (%) Total nitrogena (%) C/N ratio
13.79 87.70 37.60 2.80 13.42
27.45 91.99 51.10 3.04 16.81
90.00 81.09 45.03 1.17 38.48
54.40 84.80 48.72 1.74 28.00
a
% of total solids.
2005). Digestate was centrifuged at 5000 rpm for 20 min to obtain supernatant. The supernatant was then filtered and used for determination of volatile fatty acids (VFA) and alkalinity. Volatile fatty acids were measured by titrating the samples with 0.1 N NaOH (USEPA, 1983) and alkalinity was determined according to standard methods (APHA, 1998). Operational parameters for anaerobic digestion such as volatile fatty acids, alkalinity, total solids and volatile solids were performed twice a week. The pH of digestate was measured daily according to method described by Rhoades (1982), with the help of a pH meter, Crison model pH 25þ. Daily biogas produced was measured with the help of wet tip gas meter. Kruskal Wallis One Way ANOVA with duns procedure (two tailed test) as post hoc test was used to find out significance of the results using XLSTAT add-in for Microsoft Excel. Results and discussion Monitoring of reactor stability during startup and continuous phase pH is a very basic parameter that describes the stability of the anaerobic digestion system. During startup phase, initially pH of reactor was low due to high concentration of volatile fatty acids (7871 mg/L) because of rapid hydrolysis rate of cow dung used for startup. At this stage, pH was adjusted artificially to achieve stable anaerobic digestion conditions. During continuous loading at low OLRs, pH of the reactor remained above 7.0 and maintained naturally without addition of any chemicals. The pH decreased with the increase in OLR but did not drop below 7.0 till OLR of 6 kg VS/m3/ d (Fig. 2, Table 2). This is because of high buffering capacity of the system due to high alkalinity, which helped to maintain a balance between acidogenesis and methanogenesis. As the OLR was increased to 9 kg VS/m3/d, the pH ranged from 7.1 to 7.6 initially, but after day 158 it decreased rapidly to an average of 6.54 due to high acid accumulation (Fig. 2). Optimum pH range suggested for successful anaerobic digestion is 6.8e7.8 (Lahav and Morgan, 2004). This range is relatively wide in the plant scale and the finest value of pH varies with substrate and digestion technique. pH of 7.1 is required for mono-digestion of food waste as reported by
3
Somashekar et al., 2014. However, enhanced enzymatic activity of hydrolytic enzyme leads to acidic pH during hydrolysis of food waste digestion (Lim et al., 2008). Highest concentration of volatile fatty acids and volatile fatty acids/alkalinity ratio of 10,445 mg/L and 1.11 respectively was observed on 165th day during reactor operation at OLR of 9 kg VS/m3/d as shown in Fig. 3. This indicated that at higher OLRs, volatile fatty acids were not effectively consumed by methanogens. These results are in agreement with previous studies (Lim et al., 2008; Jiang et al., 2013; Shen et al., 2013). In present study, high volatile fatty acids/alkalinity ratio (0.94) at OLR of 9 kg VS/m3/d is one of reasons of digester instability. The optimum range of volatile fatty acids/alkalinity ratio is 0.3e0.4 (Liu et al., 2012) or 0.1e0.25 (Di Maria et al., 2014). Shen et al. (2013) also reported that, co-digestion of food and fruit & vegetable waste was less stable at higher organic loading rates because of accumulation of propionate (1576 mg/L), which accounted for 68.5% of volatile fatty acids and could not be easily utilized by methane producing bacteria. Volatile fatty acids concentration of 6000e8000 mg/L, and beyond, has inhibitory effect on biogas production (Polprasert, 2007). Solids reduction by anaerobic reactor During startup phase, total solids content of digestate was 9.61%. However, it began to increase with the increase in OLR. At OLR of 5 kg VS/m3/d, total solids content of digestate was 10.43% which increased to 11.09% at OLR of 6 kg VS/m3/d. During OLR of 6 and 9 kg VS/m3/d, total solids content of digestate was in the range of 10e13% with an average of 11.57%. Similar observations were made by Somashekar et al. (2014) who reported that continuous feeding for digestion process is very efficient in removing the total solids. However, high total solids content of digestate indicates acidic conditions in the reactor which results in no further increase in biogas volume (Somashekar et al., 2014). The average and standard deviation values of total solids and volatile solids of digestate at each OLR are shown in Table 2. Highest volatile solids removal of 82% was achieved at OLR of 5 kg VS/m3/d which decreased with increase in OLR. Lowest volatile solids removal was observed at OLR of 9 kg VS/m3/d (Table 3 and Fig. 4). This can be attributed to increase in organic load of digester at higher OLRs. It could also be due to the overall increase in rice husk's ligno-cellulosic content in the digester used in feedstock preparation which has comparatively low degradability. Increase in total solids content with increase in organic loading rate could also be a reason of decrease in volatile solids removal efficiency of the reactor. Similar results have also been reported in other studies (Duan et al., 2012; Somashekar et al., 2014), in which volatile solids removal reduced with increase in total solids content of digestate. Specific biogas yield also showed similar trends where it decreased with increase in OLR from 5 kg VS/m3/d. It suggested a relationship between specific biogas yield and volatile solids removal, therefore biogas yield can be increased by increasing the volatile solids removal efficiency of the anaerobic reactor and vice versa. The statistical analysis indicated that volatile solids removal at OLR of 9 kg VS/m3/d differed significantly (p 0.004) from volatile solids removal at OLR of 5 and 6 kg VS/m3/d. Kim and Oh (2011) reported volatile solid reduction of 79.8% and 71.5% from separate co-digestion of food waste with paper waste and livestock waste, respectively. Biogas production of anaerobic reactor
Fig. 2. pH of digestate at various stages of digester operation.
In order to evaluate the performance of anaerobic digester, daily biogas yield, specific biogas yield, gas production rate and volatile solids removal were determined and the results have been
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
4
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5
Table 2 Characteristics of digestate at different stages of digester operation. Parameters
Units
Organic loading rate (kg VS/m3/d)
Startup phase
5 pH Volatile fatty acids Alkalinity VFA/ALKa Total solids Volatile solidsb a b
7.45 3672 13,714 0.27 9.61 62.35
mg/L mg/L % %
± ± ± ± ± ±
0.36 2391 968 0.18 1.30 4.12
6
7.75 1935 12,905 0.15 10.43 74.18
± ± ± ± ± ±
0.07 342 1135 0.02 0.44 3.30
7.68 2413 11,400 0.22 11.09 73.92
9 ± ± ± ± ± ±
0.08 494 1301 0.06 0.97 2.77
6.95 8344 8879 0.94 12.55 88.20
± ± ± ± ± ±
0.43 1511 668 0.13 0.52 0.81
Volatile fatty acids/Alkalinity ratio. % of total solids.
Fig. 3. Variation in volatile fatty acids, alkalinity and volatile fatty acids/alkalinity ratio at different stages of digester operation.
presented in Table 3. Among all OLRs studied, highest daily biogas production (196 L/d) was attained at OLR of 6 kg VS/m3/d. Non parametric Kruskal Wallis One Way ANOVA (a ¼ 0.05) with Dunn's procedure for pair wise comparisons of daily biogas yield at various organic loading rates were performed to find significant differences among all possible pairs. The results showed that daily biogas yield at OLR of 6 kg VS/m3/d differed significantly (p 0.0001) from biogas yield at OLR of 5 and 9 kg VS/m3/d. Low daily biogas yield at OLR of 9 kg VS/m3/d was attributed to high volatile fatty acids and volatile fatty acids/alkalinity ratio owing to high organic load. Specific biogas yield (SBY) was the highest at OLR of 5 kg VS/m3/ d. However specific biogas yield decreased with increase in OLR to 6 and 9 kg VS/m3/d. Average specific biogas yield of 446 and 399 L/ kg VS was recorded at organic loading rates of 5 and 6 kg VS/m3/ d respectively. While specific biogas yield of 215 L/kg VS was obtained at OLR of 9 kg VS/m3/d. The possible reasons for higher specific biogas yield and volatile solids removal at 5 kg VS/m3/ d could be higher solid retention time and lower load while at higher OLR, the specific biogas yield and volatile solids removal decreased because of overloading of reactor and lower solid retention time. Similar results have also been reported by Zeshan et al. (2012). To define a relationship between specific biogas yield and volatile solids removal at OLR of 5, 6 and 9 kg VS/m3/d,
Fig. 4. Specific biogas yield and VS removal at different organic loading rates.
regression analysis was performed. High value of regression coefficient (0.98) showed a strong relationship between specific biogas yield and volatile solids removal. Decline in specific biogas yield along with volatile solids removal at high OLR was also observed by Zeshan et al. (2012). Gas production rate (GPR) is another performance parameter of dry anaerobic digestion process. It is measurement of biogas generation rate per unit volume of reactor. Initially the biogas production rate was low, but with the stabilization of reactor and increase in OLR, gas production rate also increased. Organic loading rate directly affects the rate of gas production under normal digestion conditions. Fig. 5 showed an increase in gas production rate at 5, 6 and 9 kg VS/m3/d. However, decline in gas production rate was observed at day 152 during OLR of 9 kg VS/m3/d. Through co-digestion of food waste and rice husk, maximum average gas production rate of 2.36 L/L/d was obtained at OLR of 6 kg VS/m3/d. However, a decrease in average gas production rate to 1.89 L/L/ d was observed at OLR of 9 kg VS/m3/d. The gas production rate
Table 3 Performance of anaerobic digester at different organic loading rates. Parameters
Units
Daily biogas yield Specific biogas yield Volatile solids removal Gas production rate
L/d L/kg VS % L/L/d
Organic loading rate (kg VS/m3/d) 5 179 446 82.41 2.23
6 ± ± ± ±
40.8 101 0.55 0.51
196 399 73.12 2.36
9 ± ± ± ±
18.2 38 3.85 0.26
136 215 35.42 1.89
± ± ± ±
73.2 121 2.79 1.02
Fig. 5. Gas production rate at different stages of digester operation.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023
M. Jabeen et al. / International Biodeterioration & Biodegradation xxx (2015) 1e5
started to decline during OLR of 9 kg VS/m3/d because of increase in volatile fatty acids/alkalinity ratio and/or drop in pH and alkalinity. A temporary decline in gas production rate was observed for first few days of each run, when the loading rate was changed, but later the gas production rate used to get stable with the passage of time. The reason for this change in gas production rate might be the sudden increase in organic load which disturbed the microbial activity. Agyeman and Tao (2014) worked on co-digestion of food waste and dairy manure and stated that more than ten days are required for stable operation of anaerobic digester at higher OLRs. The type of digester is also very important in biogas production. In single phase digestion system, the adaptation and regeneration period for microorganisms is 15 days which increases up to 30 days in two stage anaerobic system after change in organic loading rate (Shen et al., 2013). The gas production rate in present study was in accordance with values reported in literature related to digestion of food waste with different co-substrate. The gas production rate of 1.88 and 1.75 L/L/d at OLR of 4e5 kg VS/m3/d from co-digestion of food, fruit and vegetable waste in single and two stage anaerobic digester respectively were achieved in a study where higher risks of volatile fatty acids accumulation and low biogas yield at high OLR have also been reported (Shen et al., 2013). Conclusions Stable performance of the reactor was observed at OLR of 5 and 6 kg VS/m3/d where volatile fatty acids/alkalinity ratio ranged between 0.15 and 0.24. Therefore, rice husk can be used to overcome rapid acid accumulation in anaerobic digestion of food waste without external chemical addition. Daily biogas production rate was higher at OLR of 6 kg VS/m3/d while specific biogas yield was higher at OLR of 5 kg VS/m3/d. Similarly, volatile solids removal of 82% was achieved at OLR of 5 kg VS/m3/d. Biogas production, volatile solids removal and stability of digester decreased with increase in OLR to 9 kg VS/m3/d because of excessive volatile fatty acids production due to overfeeding of the reactor. References Agyeman, F.O., Tao, W., 2014. Anaerobic co-digestion of food waste and dairy manure: effects of food waste particle size and organic loading rate. J. Environ. Manag. 133, 268e274. APHA, AWWA, WEF, 2005. Standard Methods for the Examination of Water and Waste Water, 21st ed. American Public Health Association, Washington D.C., USA. APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Waste Water, 20th ed. American Public Health Association, Washington D.C., USA. Cheng, H., Hu, Y., 2010. Municipal solid waste (MSW) as a renewable source of energy: current and future practices in China. Bioresour. Technol. 101, 3816e3824.
5
Di Maria, F., Sordi, A., Cirulli, G., Gigliotti, G., Massaccesi, L., Cucina, M., 2014. Cotreatment of fruit and vegetable waste in sludge digesters. An analysis of the relationship among bio-methane generation, process stability and digestate phytotoxicity. Waste Manag. 34, 1603e1608. Duan, N., Dong, B., Wu, B., Dai, X., 2012. High-solid anaerobic digestion of sewage sludge under mesophilic conditions: feasibility study. Bioresour. Technol. 104, 150e156. ndez, J., Perez, M., Romero, L., 2008. Effect of substrate concentration on dry Ferna mesophilic anaerobic digestion of organic fraction of municipal solid waste (OFMSW). Bioresour. Technol. 99, 6075e6080. IPCC, 2007. In: Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., Miller, H.L. (Eds.), Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, New York, USA. Jiang, J., Zhang, Y., Li, K., Wang, Q., Gong, C., Li, M., 2013. Volatile fatty acids production from food waste: effects of pH, temperature, and organic loading rate. Bioresour. Technol. 143, 525e530. Kim, D.-H., Oh, S.-E., 2011. Continuous high-solids anaerobic co-digestion of organic solid wastes under mesophilic conditions. Waste Manag. 31, 1943e1948. Komemoto, K., Lim, Y., Nagao, N., Onoue, Y., Niwa, C., Toda, T., 2009. Effect of temperature on VFA's and biogas production in anaerobic solubilization of food waste. Waste Manag. 29, 2950e2955. Lahav, O., Morgan, B., 2004. Titration methodologies for monitoring of anaerobic digestion in developing countriesda review. J. Chem. Technol. Biotechnol. 79, 1331e1341. Lim, S.-J., Kim, B.J., Jeong, C.-M., Ahn, Y.H., Chang, H.N., 2008. Anaerobic organic acid production of food waste in once-a-day feeding and drawing-off bioreactor. Bioresour. Technol. 99, 7866e7874. Liu, X., Gao, X., Wang, W., Zheng, L., Zhou, Y., Sun, Y., 2012. Pilot-scale anaerobic codigestion of municipal biomass waste: focusing on biogas production and GHG reduction. Renew. Energy 44, 463e468. Mirani, A.A., Ahmad, M., Kalwar, S.A., Ahmad, T., 2013. A rice husk gasifier for paddy drying. Sci. Tech. Dev. 32, 120e125. Okeh, O.C., Onwosi, C.O., Odibo, F.J.C., 2014. Biogas production from rice husks generated from various rice mills in Ebonyi State, Nigeria. Renew. Energy 62, 204e208. PEPA (Pakistan Environmental Protection Agency), 2005. Guideline for Solid Waste Management. Pakistan Environmental Protection Agency, Islamabad, Pakistan. Available online at: http://environment.gov.pk/EA-GLines/SWMGLinesDraft. pdf. Polprasert, C., 2007. Organic Waste Recycling Technology and Management, third ed. IWA Publishing, London, UK. ISBN: 184339121X. Rhoades, J., 1982. Cation exchange capacity. Methods of soil analysis. Part 2. Chem. Microbiol. Prop. 149e157. Shen, F., Yuan, H., Pang, Y., Chen, S., Zhu, B., Zou, D., Liu, Y., Ma, J., Yu, L., Li, X., 2013. Performances of anaerobic co-digestion of fruit & vegetable waste (FVW) and food waste (FW): single-phase vs. two-phase. Bioresour. Technol. 144, 80e85. Somashekar, R., Verma, R., Naik, M.A., 2014. Potential of biogas production from food waste in a uniquely designed reactor under lab conditions. Int. J. Geol. Agric. Environ. Sci. 2. USEPA, 1983. Methods for Chemical Analysis of Water and Wastes, Method 305.1 (Titrimetric). Office of Research and Development, U.S Environmental Protection Agency, Washington, DC. Available online at: http://www.state.in.us/dnr/ fishwild/files/Methods_Analysis_Water_Wastes_USEPA_March1983.pdf. Zeshan, Karthikeyan, O.P., Visvanathan, C., 2012. Effect of C/N ratio and ammonia-N accumulation in a pilot-scale thermophilic dry anaerobic digester. Bioresour. Technol. 113, 294e302. Zhang, C., Li, J., Liu, C., Liu, X., Wang, J., Li, S., Fan, G., Zhang, L., 2013. Alkaline pretreatment for enhancement of biogas production from banana stem and swine manure by anaerobic codigestion. Bioresour. Technol. 149, 353e358. Zhang, Y., Banks, C.J., Heaven, S., 2012. Co-digestion of source segregated domestic food waste to improve process stability. Bioresour. Technol. 114, 168e178.
Please cite this article in press as: Jabeen, M., et al., High-solids anaerobic co-digestion of food waste and rice husk at different organic loading rates, International Biodeterioration & Biodegradation (2015), http://dx.doi.org/10.1016/j.ibiod.2015.03.023