Carbon/Nitrogen Ratio and Anaerobic Digestion of Swine Waste

ABSTRACT THE greatest volume of methane production was observed in digesters operating on a sub-strate of high carbon/nitrogen (C/N) ratio. How-ever, these digesters tended to be unstable ecologically and operated on the verge of failure. Digesters operated on a low C/N ratio (high nitrogen) substrate, although exhibiting lower gas production, were seen to become highly buffered and more stable. The low C/N digesters, because of their stability, were better able to adjust to increases in organic loading. The C/N range of 15.5/1 to 19/1 was found to be the optimum range in terms of maximum methane production based on variation in loading rates. Maximum gas production (690 mL CH4/g organic carbon loaded) occurred at a loading rate of 2.24 g volatile solids (VS)/L/day and a C/N ratio of 19/1. Due to unstable operating conditions observed at high C/N ratios, it is suggested that a C/N ratio of 16/1 may provide more operational stability with minimum loss in gas production. This corresponds to a 55 to 60 percent reduction in total organic carbon (TOC).