Anaerobic co-digestion of poultry litter and wheat straw affected by solids composition, free ammonia and carbon/nitrogen ratio

Abstract

In this study, the effects of three total solids levels (2%, 5% and 10% TS) of the mixtures of poultry litter and wheat straw at different percent volatile solids from wheat straw (0%, 25% and 50% VSWS), free ammonia nitrogen (FAN) and C/N ratio on co-digesting poultry litter with wheat straw were studied in batch experiments operated at 37 °C. The results showed that adjusting the substrate C/N ratio alone could not always result in high methane yields and biogas volumes. The maximum CH 4 yield of 201 mL g –1 initial VS was observed in the 5% TS category with VSWS being 50%. The highest specific biogas volume produced (318 mL g –1 initial VS) was also obtained in the same solid category. A polynomial regression between FAN and the methanogenic activity was obtained with the correlation coefficient being 0.9472. A FAN inhibitory threshold concentration of 253.9 mg L –1 was found, and a total loss of methane production occurred when the FAN concentration reached 1000 mg L –1 . An optimal TS content for anaerobic digestion of poultry litter with wheat straw was around 4.15% to achieve the best biogas yield. Similarly, for digesting poultry litter without addition of wheat straw, the TS content should be kept at around 6.8%.