Impact of substrate to inoculum ratio in anaerobic digestion of swine slurry

Batch tests using swine slurry as substrate were conducted to elucidate the impact on anaerobic digestion of different ratio of substrate (COD)/inoculum (VS). Soluble COD, volatile fatty acids (VFAs) concentration, and pH were periodically analyzed in order to completely understand possible inhibitions. In addition, ammonia concentration was also calculated at the start up and final stage of the test.The results showed that the same methane yield was achieved with the three experimental ratios (COD/VS of 1, 2 and 3), however methane production rates were clearly different. This later parameter decreased with increasing ratios. This behaviour was explained by the accumulation of VFAs (predominantly acetic (HAc) and propionic acid (HPr)), reflected on the soluble COD profile as well. The possible inhibition by free ammonia was null since the values calculated were far below the threshold concentration reported in literature.