Methane production from anaerobic co-digestion of sludge with fruit and vegetable wastes: effect of mixing ratio and inoculum type

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Methane production from wastes, like sewage sludge and fruit and vegetable wastes, has double benefits; first is minimizing these wastes and second is energy recovery. The goal of this research is to enhance methane yield from anaerobic co-digestion of primary sludge (PS) with fruit and vegetable wastes (FVW) in Egypt using different mixture and various inoculum types. Bio-chemical methane potential (BMP) tests were conducted in a 500-mL glass reactor under mesophilic conditions (35–37 °C). In the first BMP tests, six mixtures with PS to FVW ratios of 100:0, 70:30, 50:50, 30:70, 20:80, and 0:100 (based on volatile solids) were performed to obtain the best mixture for an optimal methane production. In the second BMP tests, three types of inoculum (fresh cow manure, activated sludge, and excess sludge) were used to identify the optimal inoculum for the greatest methane production. The highest methane yield was observed at PS to FVW ratio of 50:50 (141 mL/g VS), which was higher than the individual digestion of the other used feedstock. However, the minimum methane yield was recorded at PS to FVW ratio of 20:80. On the other hand, using the activated sludge as inoculum improved the methane yields from anaerobic co-digestion of PS with FVW compared with the other types of inoculum. Statistical analysis of the results was conducted using ANOVA test. The results conducted that the production of methane was improved by anaerobic co-digestion of PS with FVW and using the activated sludge as inoculum.