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 degrees 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 CH4 yield of 201mLg(-1) initial VS was observed in the 5 TS category with VSWS being 50. The highest specific biogas volume produced (318mLg(-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 253.9mgL(-1) was found, and a total loss of methane production occurred when the FAN concentration reached 1000mgL(-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.
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