Impact of thermal pre-treatment on anaerobic co-digestion of sewage sludge and landfill leachate

Abstract

The complex microbial flocs with rigid cell walls and undigested organic matter in the domestic sewage sludge defy anaerobic digestion, especially hydrolysis, the rate-limiting step of anaerobic digestion. The anaerobic digestion of municipal solid waste landfill leachate is challenging because of its high refractory organics and ammoniacal nitrogen. However, if sewage sludge and landfill leachate are mixed in an optimal ratio and pre-treated, they can be anaerobically co-digested and clean bioenergy can be harnessed. Among the pre-treatment methods, thermal pre-treatment is preferred because it is simple and economical yet effective. In this investigation, sewage sludge from a domestic sewage treatment plant and landfill leachate from an active municipal solid waste landfill were mixed in the ratio of 75:25 and thermally pre-treated at 70 °C for 60 min. The pre-treated substrates were anaerobically digested at 37 ± 1 °C using fresh cow dung slurry as the inocula. The results of this experiment were compared with the control experiment in which substrates without pre-treatment were used. Though biogas production from the fresh substrates was higher during the initial days, it decreased after the seventh day. For the thermal pre-treated substrates, the biogas production was low during the first week, which started to increase from the tenth day until the end of the experiment. The cumulative biogas yields from untreated and thermal pre-treated reactors were 723.5 and 418 mL, respectively.