Bioenergy conversion studies of organic fraction of MSW: kinetic studies and gas yield organic loading relationships for process optimisation

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1 Bioresource Technology 95 (2004) Bioenergy conversion studies of organic fraction of MSW: kinetic studies and gas yield organic loading relationships for process optimisation M.S. Rao, S.P. Singh * Biomass and Waste Management Laboratory, School of Energy and Environmental Studies, Faculty of Engineering Sciences, Devi Ahilya University, Khandwa Road Campus, Indore, MP PIN , India Received 24 July 2003 Abstract Batch digestion of municipal garbage was carried out for 100 days at room temperature (26 ± 4 C; average temperature 25 C) and at ambient temperature (32 ± 10 C; average temperature 29 C) conditions for total solids concentrations varying between 45 and 135 g/l. A first order model based on the availability of substrate as the limiting factor was used to perform the kinetic studies of batch anaerobic digestion system. Effect of organic solids concentration and digestion time on biogas yield was studied and mass and energy balance analysis was conducted for batch digestion. The net bioenergy yield from municipal garbage and corresponding bioprocess conversion efficiency over the length of the digestion time were observed to be 12,528 kj/kg volatile solids and 84.51% respectively. The methane content of the biogas generated from the reactors was in the range of 62 72% with the overall average methane content of the biogas, computed over the total digestion period was 65 vol%. Ó 2004 Published by Elsevier Ltd. Keywords: Anaerobic digestion; Bioenergy; Municipal garbage; Kinetics; Mass and energy balance 1. Introduction Municipal solid waste generation is rapidly increasing in Indian urban areas and started creating enormous waste disposal problems in the recent past. Anaerobic digestion of source sorted and shredded garbage can be an attractive option for both energy generation as well as waste disposal. Though anaerobic digestion of cattle dung has been the commercial biogas energy production technique for many years, application of anaerobic digestion to other wastes such as municipal garbage is yet to obtain commercial acceptance in India. Batch and semi-continuous anaerobic digestion systems are two widely used techniques for bioenergy conversion of organic fraction of wastes in developing countries like India. Batch digestion systems are the simplest ones to use due to their ease of application, operation, low investment and associated maintenance costs. * Corresponding author. Tel.: ; fax: address: sp_singh@excite.com (S.P. Singh). Several studies have been made on the bioconversion of biomass by different researchers, for example Mata- Alvarez et al. (1992) carried out experiments on Barcelona s central food market organic wastes, Lane (1984) and Prema Viswanath et al. (1992) on fruit and vegetable wastes, Krishna Nand et al. (1991) on canteen wastes, Ranade et al. (1987) on market waste and Cho et al. (1995) on Korean food wastes. Webb and Hawkes (1985) have studied the gas yield organic loading relationships for anaerobic digestion of poultry litter. Rao et al. (2000) have studied the ultimate bioenergy production potential of municipal garbage in batch reactors. There is a large number of factors which affect biogas production efficiency such as environmental conditions like ph, temperature, type and quality of substrate, mixing (Molnar and Bartha, 1989), process inhibitory parameters like high organic loading, formation of high volatile fatty acids, inadequate alkalinity etc. Therefore, the amenability of substrate for biogasification, gas yield organic loading relationships, bioprocess conversion efficiency and process inhibitory parameters vary from substrate to substrate, /$ - see front matter Ó 2004 Published by Elsevier Ltd. doi: /j.biortech

2 174 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) and for different environmental and operating conditions. The objectives of the present study are to investigate the way in which biogas yield, volume of biogas produced per unit weight of organic material fed, varies with digestion time and organic solids concentration; to evaluate the effect of organic loading on kinetics and bioprocess conversion efficiency of batch anaerobic digestion system and to optimise the organic loading that can be fed to the reactor with digestion time; and to assess the quality of the digested solids and liquid effluent for their further use Reactor set-up Aspirator bottles of capacity 3.25 I made of glass with bottom sampling outlets were used as bioreactors. The bottles were closed by rubber stoppers equipped with glass tubes for gas removal and for adjusting the ph. The glass tube was dipped inside the slurry to avoid gas loss during the ph adjustments. A thermometer of range 0 60 C having an accuracy ±0.5 C was used to measure the slurry temperature in the reactors maintained at room temperature where as a multi-channel digital temperature indicator equipped with copper constantan thermocouples (type t ) having an accuracy ±0.1 C were used to measure the slurry temperature in the reactors maintained at ambient temperature Reactor start-up 2. Methods 2.1. Raw material preparation and analysis The raw material constituted of food wastes emanating from fruit and vegetable markets, households, hotels and juice centres. The wastes, collected and combined in approximately equal proportion, were then mixed several times in the laboratory, shredded and ground into a size of approximately cm prior to analysis for chemical composition. Initially, 25 samples were analysed for moisture content, total solids, total volatile solids, ash content, alkalinity, volatile fatty acids as acetic acid, protein, Kjeldahl nitrogen, fat, cellulose, hemicellulose and lignin using analytical methods given by Jain et al. (1989). Chemical oxygen demand as biodegradable chemical oxygen demand was also determined using standard techniques (APHA, 1989). The ph of the slurry was measured using a digital ph meter having an accuracy of ±0.01 ph unit. The mean chemical composition of the municipal garbage is given in Table 1. Table 1 Chemical composition of municipal garbage Parameter Weight fraction or ratio Moisture Total solids Total volatile solids a Ash a Total organic carbon a Kjeldahl nitrogen a 1.10 C/N ratio Fat a 8.50 Protein a 6.87 Cellulose a Hemicellulose a 9.50 Lignin a 8.50 a Indicates wt.% in total solids. Seed-inoculum amounting to 15% of the bioreactor working volume was collected from a nearby Khadi and Village Industries Commission (KVIC) cattle dungbased biogas plant and added to each reactor to initiate digestion Gas measurement Gas production was measured at a fixed time each day by the water displacement method, with water prepared as specified in standard methods (APHA, 1989) Gas analysis Gas samples were collected by gas sampling injectors and a sample of 100 ll was used for each run. The biogas composition (CH 4 +CO 2 ) was determined using a Gas Chromatograph (NUCON 5700) equipped with a thermal conductivity detector and stainless steel column of length 6 ft, OD 1/4 in., ID 2 mm, Porapak Q 100 having mesh range The carrier gas used was H 2 and the analysis was carried out at a carrier gas flow rate of 30 ml/min with the injector, column and detector temperatures maintained at 120, 90 and 120 C, respectively. The gas quality was checked once a week Experimental procedure The substrate was prepared in a way similar to that used to prepare the substrate from the Barcelona central food market waste (Mata-Alvarez et al., 1992). The substrate was mixed with tap water along with the inoculum to make slurry having a total volume of 2 l. The substrate concentration was expressed as weight of solids/total volume of solids plus water, assuming that the density of the solids is approximately equal to the density of water. Five bioreactors each were run under

3 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) ambient and room temperatures respectively with equal working volumes (2 l) but different total solids concentrations varying between 45 and 135 g/l. All the reactors were fed with municipal garbage, tap water and cattle dung slurry (inoculum), used as the starter in the reactors. Liquid samples were drawn from each reactor periodically and analysed for ph, volatile fatty acids as acetic acid, chemical oxygen demand and alkalinity. The ph was measured daily for the first 15 days of the experiment and measured for every 10 days thereafter. Volatile fatty acids as acetic acid and alkalinity were measured daily for the first 10 days of the experiment and measured every 10 days thereafter. Chemical oxygen demand was measured every 10 days throughout the experiment. The characteristics of inoculum and digested slurry are given in Table 2. The experiment was carried out for 100 days at room temperature (26 ± 4 C; average temperature 25 C) for the first five reactors and at ambient temperature (32 ± 10 C; average temperature 29 C) for the other five reactors, maintaining similar total solids concentrations in reactors in both conditions. One reactor each was fed with inoculum, used as starter in the other 10 reactors and run parallel at ambient and room temperature conditions. All the gas productions were evaluated after deducting the gas production from the control, by first multiplying the volume of gas produced by the control by 0.15 (the ratio of the volume of inoculum to bioreactor working volume). All the gas volumes were measured at an average temperature of 25 C and corrected to 0 C standard temperature and 1 atm pressure. The substrate was mixed once each day, at the time of the gas measurement, to maintain intimate contact between the microorganisms and the substrate. The C:N weight ratio of the organic matter was adjusted to 25:1 using urea, as this is the optimum ratio for maximum microbial activity (Mata-Alvarez et al., 1992, 1993; Molnar and Bartha, 1989). 3. Data analysis and calculations 3.1. Kinetic study Kinetic studies of anaerobic digestion process are useful to predict the performance of digesters and design appropriate digesters. Kinetic studies are also helpful in understanding inhibitory mechanisms of biodegradation. First-order kinetic models are the simplest models applied to the anaerobic digestion of complex substrates as they provide a simple basis for comparing stable process performance under practical conditions. Therefore, a first order model based on the availability of substrate as the limiting factor was used (Llabres- Luengo and Mata-Alvarez, 1987; Hahimoto, 1989; Turick et al., 1991; Mata-Alvarez et al., 1993; Chen and Hahimoto, 1996; Sanchez et al., 1996) to perform the present study. The basic equation is db=dt ¼ kb ð1þ where k is the first-order substrate utilisation rate constant (time 1 ) and B (mg/l) represents the biodegradable substrate concentration. On integration, Eq. (1) becomes B=B 0 ¼ expð ktþ ð2þ where B 0 (mg/l) represents initial substrate concentration. Substrate concentration can be correlated with biogas production (G), as mentioned below. ðg 1 GÞ=G 1 ¼ B=B 0 ð3þ where G 1 (l) is the ultimate biogas production. From Eqs. (2) and (3), the integrated equation for the firstorder model which gives an analytical relation between the volume of biogas produced and digestion time was obtained and used to quantify the extent of process inhibition is as follows: G ¼ G 1 ½1 expð ktþš ð4þ Table 2 Inoculum and digested slurry (after 100 days) characteristics Parameter reactor ph Total VFA as acetic acid (mg/l) Total alkalinity as CaCO 3 (mg/l) Inoculum ,400 Ambient conditions R R ,890 R ,610 R ,810 R ,800 Room conditions R ,120 R ,910 R ,640 R ,110 Total chemical oxygen demand (mg/l)

4 176 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) where k (time 1 ) is the first-order biogas production rate constant. The values of G 1 and k were obtained from an exponential regression analysis using Curve Expert, version Mass balance analysis Mass balance analysis by the determination of volatile solid mass removal in the present study was done based on the volume and composition of biogas produced. Biogas as produced contains methane, carbon dioxide, water vapour and trace amounts of other gases. From a volumetric point of view, the trace gases are neglected and considered only the dry biogas for determining mass removals. Dry biogas is assumed to behave as an ideal gas and consist entirely of carbon dioxide and methane. The biogas volumes are measured at one atmosphere so that no pressure correction is required. Conversion of as-measured biogas volumes to standard conditions at 0 C and one atmosphere of pressure was done in a way similar to that used by Richards et al. (1991) excluding volume occupied by water vapour and thermal expansion effects. The biogas mass is calculated using the molecular weights of methane and carbon dioxide (16 and 44 g/mol, respectively), the molar volume of an ideal gas at STP ( l/mol) and normalised individual gas contents (vol%) as follows: G m ¼ V d ½ð16 CH 4 =100Þþð44 CO 2 =100ÞŠ=22:413 ð5þ where G m is the biogas mass (g); V d, the dry biogas volume at STP (l); CH 4, the biogas normalised methane content (vol%); CO 2, the biogas normalised carbon dioxide content (vol%) Energy balance analysis Net energy generation (NEG) from anaerobic digestion process is determined by carrying out energy balance analysis using stoichiometric energy potential of the substrate (SEP), bioprocess conversion factor (BCF); ratio of substrate utilised (mg/l) at time t and total substrate available for bioconversion (mg/l) and energy consumed in the process (EC) at time t as follows: NEG ðkj=kg VSÞ ¼½fSEP ðkj=kg VSÞBCFg fec ðkj=kg VSÞgŠ ð6þ The stoichiometric gas energy yield (CO 2 +CH 4 ) that can be obtained from anaerobic decomposition of organic matter has been estimated from the total organic carbon (TOC), as 1 kg of carbon in the substrate will yield 1/12 kmol of gas. Thus, per kg of carbon degraded, the gas yield should be m 3 gas measured at 0 C standard temperature and 1 atm pressure. The stoichiometric gas energy yield (m 3 /kg volatile solids) was determined using the TOC:VS:COD ratio 1:2.21:2.84 obtained for municipal garbage by Rao et al. (2000). It is evident from the fact that a certain amount of substrate (energy) is consumed by the micro-organisms during their reproduction and growth in the process. The energy consumed in the process was estimated indirectly using the volumetric difference between the stoichiometric biogas energy potential of the substrate consumed (m 3 /kg TOG) and the actual biogas produced. Bioenergy yield was estimated using volumetric higher heating value of biogas in energy per unit mass. The biogas composition was assumed to contain CH 4 and CO 2, neglecting the trace gases. The higher heating value of the biogas was determined at pressure 1 atm and temperature 0 C based on the higher heating value of CH 4 at pressure 1 atm and temperature 20 C (Culp, 1991) assuming that the mixture behaves as an ideal gas. 4. Results and discussion Degradation of substrate started almost immediately and proceeded without problems in the reactors maintained at ambient temperature. However, for the reactors maintained at room temperature, it took about 6 8 days for initiation of biogas production. The cumulative biogas production at different total solids concentration maintained at room and ambient temperature along with the predicted values using the first-order kinetic model described by Eq. (4) are shown in Figs It has been observed that the cumulative biogas production was fit well with the first-order kinetic model as is evident from the standard error and correlation coefficient (r) between the experimental and predicted values given along with the parameter estimates in Table 3. A decrease in ph was observed during the first few days of digestion due to the high volatile fatty acids formation, hence the ph was adjusted to 7.0 ± 0.2 using NaHCO 3 solution, when the ph before adjustment and immediately after mixing the substrate varied between 5.5 and 6.5. Thereafter, the ph was maintained between 6.8 and 7.4, which is the optimum ph range for methane production. The profile of ph and volatile fatty acids as acetic acid (mg/l) over the length of the digestion period at different organic loading concentrations under ambient and room temperatures are shown in Figs. 6 and 7 respectively. The maximum specific biogas production rates were observed to be 2 and 1.9 vol. gas/vol. slurry/day, respectively, on the 4th and 6th days of the digestion period observed for reactors fed at g VS/l at ambient and room temperatures respectively. The initial ph drop and high volatile fatty acid concentration and high specific biogas production rates show that the substrate contains some easily biodegradable constituents.

5 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Fig. 1. Cumulative biogas production (TS concentration 45 g/l). Fig. 2. Cumulative biogas production (TS concentration 67.5 g/l). Fig. 3. Cumulative biogas production (TS concentration 90 g/l).

6 178 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Fig. 4. Cumulative biogas production (TS concentration g/l). Fig. 5. Cumulative biogas production (TS concentration 135 g/l). Table 3 Values of fitting functions and statistical measures for the kinetic model TS concentration (g/l) Parameters Statistical measures Ultimate biogas production (l) Biogas production rate constant (per day) Standard error Ambient temperature Room temperature Correlation coefficient

7 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Fig. 6. Profile of ph and volatile fatty acids (VFA) concentration (ambient temperature). Fig. 7. Profile of ph and VFA concentration (room temperature). The reactor fed with 135 g TS/l maintained at room temperature was failed to continue with biogas production where high volatile fatty acids (VFA) formation (18,500 mg acetic acid/l) lowered ph of slurry to 5.1 and ultimately lead to formation of thick scum layer on the top of the substrate. Though addition of NaHCO 3 solution increased the ph of slurry to 7.3, it could not revive gas production in this reactor. Failure of anaerobic reactors appears to have been due to inadequate levels of alkalinity to balance the levels of VFA in the digestion liquors. It was found that careful attention to alkalinity levels during digestion of municipal garbage is crucial to the success of the digestion. For stable digestions, it is imperative that a satisfactory ratio be maintained between VFA and alkalinity levels. This ratio is given by the empirical relationship that for balanced digestion, alkalinity (mg/l) 0.7 VFA (mg/l) should not be less than 1500 (Lane, 1984). On the basis, an alkalinity of 12,000 mg/l in the presence of 18,500 mg VFA/l is grossly inadequate and digestion failure would be anticipated. Stenstrom et al. (1983) have conducted experiments on anaerobic digestion of municipal solid waste and found that at high organic loading it was very difficult to avoid forming a thick scum layer of undigested solids on the surface of the digesters. According to Niekerk et al. (1987) foaming is a result of inadequate mixing, high alkalinity, ammonia and volatile fatty acid levels. Therefore in the present study too, inadequate mixing of substrate and presence of higher concentrations of alkalinity and volatile fatty acids in the reactor might have caused formation of thick scum layer of undigested solids on the surface of the reactor. However, this foaming problem was not encountered with the reactor fed with same volatile solids concentration but run at ambient temperature conditions. The biogas yield, biogas produced per kg organic solids (volatile solids) for different concentrations of organic loading over a 100-day digestion time at room

8 180 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Fig. 8. Biogas yield vs organic loading at ambient temperature. Fig. 9. Biogas yield vs organic loading at room temperature. and ambient temperatures are shown in Figs. 8 and 9 respectively. The rates of biogas production differed significantly according to the organic loading. It can be observed from Figs. 8 and 9 that bulk of substrate degradation takes place up to a period of 60 days suggesting that the digesters should preferably be run at a digestion time close to 60 days for optimum energy yield. The methane content of the biogas generated from the reactors was in the range of 52 56% during the first 2 4 days of the digestion and remained in the range of 62 72% for the remaining period. The average methane content of the biogas generated from the reactors fed at 35 and 67.5 g TS/l maintained under room temperature was observed to be 70 vol%. The average methane content of the biogas generated from the rest of the reactors maintained at both room and ambient temperatures was observed to be 65 vol%. The relative methane yield as defined by Brummeler and Koster (1990), is the sum of the methane produced at any time divided by the potential methane yield of the organic fraction, as determined in a batch assay at low solids. The potential methane yield estimated in the earlier study by Rao et al. (2000) was used for determining relative methane yields at different concentrations of organic loading. The relative methane yields obtained for different organic loadings over the length of the digestion period, 100 days are shown in Figs. 10 and 11. The effect of organic loading on methane production rates at ambient and room temperatures are shown in Figs. 12 and 13. It was observed that methane production rate was increased with the increase of organic loading up to a concentration of g VS/l and decreased thereafter. This means that the anaerobic digestion is inhibited significantly owing to the build-up

9 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Fig. 10. Relative methane yield curves at different organic loading concentrations under ambient temperature. Fig. 11. Relative methane yield curves at different organic loading concentrations under room temperature. Fig. 12. Effect of organic loading on methane production rate (ambient temperature).

10 182 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Fig. 13. Effect of organic loading on methane production rate (room temperature). Fig. 14. Bioprocess conversion efficiency profile over time at different total solids (TS) concentration (ambient temperature). Fig. 15. Bioprocess conversion efficiency profile over time at different total solids (TS) concentration (room temperature).

11 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) of volatile fatty acids at a higher loading (a lower inoculum ratio) as well as due to the fact that increasing the density of the solid waste tends to decrease the gas production, possible because of a decrease in the effective surface area exposed to hydrolysis. It was also observed that methane production rates were more for the first 20 days in all the reactors and decreased thereafter. These results indicated that methane gas is produced first at a high rate and then at a reduced rate which is a factor of less than the initial rate over the length of the digestion time of 100 days. The high initial rates, which were sustained for up to 20 days, indicate that a major amount of the gas-producing solid waste fraction was readily and rapidly biodegradable. The actual amount of energy recovery from the solid waste will depend on the efficiency of the conversion process used. The bioprocess conversion efficiency, as defined by Rao et al. (2000); ratio of biodegradable substrate utilised and ultimate biodegradable substrate available for bioprocess conversion multiplied by 100, over the length of the digestion period at different concentrations of organic loading are shown in Figs. 14 and 15. It is evident from Figs that biogas yields obtained under ambient conditions were better than those obtained under room temperature. It indicates that temperature has shown to be the environmental variable having considerable effect on gas production especially at higher organic loading. It was observed that at organic solids concentration g/l and at the end of 60-day digestion time, the relative methane yield achieved was highest, about 60% with highest bioprocess conversion process efficiency of over 74%. From Fig. 12, it can be observed that methane production rates were observed to be highest for the reactor fed at g VS/l run under ambient temperature. The summary of performance of batch reactors mentioning the characteristics of initial and digested substrate, along with their degradation percentages, under different conditions are given in Tables 4 and 5. It was observed that % of the total volatile matter in the substrate was converted and the total biogas yield was m 3 /kg volatile solids for organic loading concentrations g VS/l. The methane content in the biogas was observed to be in the range vol%. The overall average methane content of the biogas, computed over the total digestion period was 65 vol% which is well comparable with values reported in the literature vol% for fruit and vegetable wastes by Lane (1984), 61 vol% for water hyacinth by Radwan et al. (1991) and 60 vol% for mixture of agricultural wastes by Molnar and Bartha (1989). The bioenergy yield from municipal garbage was observed to be in the range of 24,000 27,450 kj/m 3 biogas. C:N ratio is most often used to indicate both the stability of organic matter and the quality of the digested substrate for its further use. C:N ratio of digested Table 4 Summary of performance of batch reactors under ambient temperature R1 R2 R3 R4 R5 Parameter TS (g) VS (g) TOC (g) KN (g) C/N COD (g)

12 184 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Table 5 Summary of performance of batch reactors under room temperature Parameter R1 R2 R3 R4 TS (g) VS (g) TOC (g) KN (g) C/N COD (g) substrate in the range 15:1 17:1 is considered to be stable and high quality compost (Molnar and Bartha, 1988). From Tables 4 and 5, it is evident that the composition of the digested substrate respectively at total solids concentration between 35 and 90 g/l under ambient temperature and total solids concentration between 35 and 67.5 g/l under room temperature can be used as compost or biofertilizer. 5. Conclusions From the results obtained, it can be concluded that digesters should preferably be run at 67.5 g TS/l under ambient temperature with a digestion time close to 60 days for optimum energy yield. The net bioenergy yield from municipal garbage and corresponding bioprocess conversion efficiency over the length of the digestion time were observed to be 12,528 kj/kg volatile solids and 84.51% respectively. About 88% of the net energy yield was recovered at 60-day digestion time. The low C/N weight ratio (15:1) in the digested substrate indicates that it can be utilised as biofertilizer or soil conditioner. However, the effluent chemical oxygen demand concentration indicates that it should be treated before using it for other applications. Further study is required to determine the effect of winter season temperatures on biogas production and enhancement of gas production rates during winter at higher temperatures provided by a non-conventional energy input such as a solar greenhouse. Acknowledgements The authors are thankful to Madhya Pradesh Council of Science and Technology, Bhopal for providing financial assistance. References APHA, Standard Methods for the Examination of Water and Wastewater, 17th ed. Washington, DC. Brummeler, E., Koster, I.W., Enhancement of dry anaerobic digestion of the organic fraction of municipal solid waste by an aerobic pretreatment step. Biological wastes 31, Chen, T.H., Hahimoto, A.G., Effects of ph and substrate:inoculum ratio on batch methane fermentation. Bioresource Technology 56, Cho, J.-K., Park, S.-C., Chang, H.-N., Biochemical methane potential and solid state anaerobic digestion of Korean food wastes. Bioresource Technology 52, Culp Jr., A.W., Principles of Energy Conversion. McGraw-Hill Series in Mechanical Engineering. McGraw-Hill, Singapore. p Hahimoto, A.G., Effect of inoculum/substrate ratio on methane yield and production rate. Biological wastes 28, Jain, M.C., Chhonkar, P.K., Kumar, S., Analytical Methods in Biogas Technology. Indian Agricultural Research Institute Publications, New Delhi.

13 M.S. Rao, S.P. Singh / Bioresource Technology 95 (2004) Krishna Nand, Sumithra Devi, S., Prema Viswanath, S., Somayaji Deepak, Sarada, R., Anaerobic digestion of canteen wastes for biogas production: process optimisation. Process Biochemistry 26, 1 5. Lane, G., Laboratory scale anaerobic digestion of fruit and vegetable solid waste. Biomass 5, Llabres-Luengo, P., Mata-Alvarez, J., Kinetic study of the anaerobic digestion of straw-pig manure mixtures. Biomass 14, Mata-Alvarez, J., Llabres, P., Cecchi, F., Pavan, P., Anaerobic digestion of the Barcelona central food market organic wastes: experimental study. Bioresource Technology 39, Mata-Alvarez, J., Mtz-Viturtia, A., Llabres-Luengo, P., Cecchi, F., Kinetic and performance study of a batch two-phase anaerobic digestion of fruit and vegetable wastes. Biomass and Bioenergy 5 (6), Molnar, L., Bartha, I., High solids anaerobic fermentation for biogas and compost production. Biomass 16, Molnar, L., Bartha, I., Factors influencing solid-state anaerobic digestion. Biological wastes 28, Niekerk, A., Kawahigashi, J., Reichlin, D., Malea, A., Jenkins, D., Foaming in anaerobic digesters a survey and laboratory investigation. Journal WPCF 59 (5), Prema Viswanath, S., Sumithra Devi, S., Krishna Nand, Anaerobic digestion of fruit and vegetable processing wastes for biogas production. Bioresource Technology 40, Radwan, A.M., Sebak, H.A., Mitry, N.R., El-Zanati, E.A., Hamad, M.A., Dry anaerobic fermentation of agricultural residues. Biomass and Bioenergy 5 (6), Ranade, D.R., Yeole, T.Y., Godbole, S.H., Production of biogas from market waste. Biomass 13, Rao, M.S., Singh, S.P., Singh, A.K., Sodha, M.S., Bioenergy conversion studies of the organic fraction of MSW: assessment of ultimate bioenergy production potential of municipal garbage. Applied Energy 66, Richards, B.K., Cummings, R.J., White, T.E., Jewell, W.J., Methods for kinetic analysis of methane fermentation in high solids biomass digesters. Biomass and Bioenergy 1 (2), Sanchez, E., Borja, R., Lopez, M., Determination of the kinetic constants of anaerobic digestion of sugar-mill-mud waste (SMMW). Bioresource Technology 56, Stenstrom, M.K., Adam, S., Bhunia, P.K., Abramson, S.D., Anaerobic digestion of municipal solid waste. Journal of environmental engineering 109 (5), Turick, C.E., Peck, M.W., Chynoweth, D.P., Jerger, D.E., White, E.H., Zsuffa, L., Kenney, A.W., Methane fermentation of woody biomass. Bioresource Technology 37, Webb, A.R., Hawkes, F.R., Laboratory scale anaerobic digestion of poultry litter: gas yield loading rate relationships. Agricultural wastes 13,