Effects of temperature and mixing modes on the performance of municipal solid waste anaerobic slurry digester

Purpose Anaerobic digestion is a promising technology for simultaneous treatment of biodegradable organic matter of municipal solid waste (MSW) and production of renewable energy. Mixing modes and temperature have influences on biogas production in anaerobic digesters treating MSW. Therefore, in this study, digester was operated at different modes of mixing and temperatures to obtain design criteria. Methods The experiments were carried out in a semi-continuous digester. In the first part of the investigation, temperature was set at 25, 28, 31 and 34 degrees C. During this step, digester content was mixed in an intermittent mode by mechanical mixers. In the second part of the study, mixing condition of the digester was set at various modes: continuous, intermittent (15 min on and 30 min off) and minimal (twice in a batch). Results Digestion with a temperature in this range resulted in biogas yield of 0.23-0.33 m(3) biogas/kg VS, with a methane content of 60.2-71.8% in biogas. The methane content and yield decreased with reduction of digestion temperature. However, this reduction was almost negligible from 34 to 31 degrees C. In addition, in comparison to intermittent mixing, continuous and minimal mixing modes reduced the biogas production by 40% and 50%, respectively. Therefore, in this digester greatest biogas yield of 0.33 +/- 0.02 m(3) biogas/kg VS were obtained at 34 degrees C and intermittent mixing mode. Conclusions Based on the data obtained from this study, temperature in the range of 31-34 degrees C and intermittent mixing is suggested as a base for design purposes.