Temperature-phased anaerobic digestion of the hydromechanically separated organic fraction of municipal solid waste with sewage sludge

• Water separated municipal organic waste was co-digested with sewage sludge.
• The TPAD system with a thermophilic stage SRT of 1 day was the most effective.
• The maximum methane yield obtained in the TPAD system was 333 dm3/kgVS.
• An inhibition of methanogenesis by free ammonia and fatty acids was observed.

This paper examines the temperature-phased anaerobic digestion (TPAD) process treating the mixture of municipal sewage sludge with the hydromechanically separated organic fraction of municipal solid waste (HS-OFMSW). The experiments showed that the performance of the TPAD process strongly depends on the conditions applied in the thermophilic stage. The TPAD system operated at a solids retention time (SRT) of 1 and 14 days in the first and second steps, respectively, achieved the overall methane yield of 333 l CH4 kgVS−1 and the volatile solids reduction of 52.1%, whereas the corresponding values reported for a control single-stage mesophilic process were 230 l CH4 kgVS−1 and 37.23%, respectively. However, when an SRT of the thermophilic reactor was extended to 2 days, the methane production in the subsequent mesophilic stage significantly decreased. It was therefore concluded that the prolonged exposure of feedstock to the thermophilic temperatures can lead to greater intensity of protein degradation. Consequently, higher amounts of ammonia are liberated to the liquid phase, which results in the inhibition of methanogenesis in the subsequent mesophilic step. Moreover, ammonia affects the release and distribution of volatile fatty acids, which also influences the performance of the whole TPAD system.