Methane production from low solid concentration liquid swine waste using conventional anaerobic fermentation

A study was conducted to determine the methane production characteristics of low concentration liquid swine waste using conventional dispersed growth anaerobic fermentation at 35°C. Raw waste was obtained from a conventional under floor flushing system and screened using an 18 mesh vibrating liquid–solid separator. The volatile solid (VS) concentration of the influent waste used in the study was 15 g/l. Hydraulic retention times (HRTs) of 5, 3, 2, and 1 days were replicated three times with one 300 l bench scale fermenter. Results from the study show that conversion to methane is practical for the 5 and 3 day HRT but that considerable stress occurred at the 2 day HRT. One replicate failed during the 2 day HRT and all three replicates failed at the 1 day HRT. Failure of digestion was determined based on steady-state gas production. Methane productivity (LCH4/g VS added) ranged from 0.36 to 0.22 for the 5 and 2 day HRTs, respectively. VS reduction showed a high of 51.6% for the 5 day HRT and a low of 34.5% for the 2 day. Steady-state operating levels of ammonia and total volatile fatty acids (TVFA) suggest no inhibition from either at any of the HRTs. Alkalinity levels were low (1200–2000 mg/l as CaCO3) when compared to digestion studies using longer HRTs. Although foaming occurred at the 2 and 1 day HRT, TVFA levels remained low (<700 mg/l as acetic) and pH was >7. This suggests the failure mechanism was bacterial washout, not organic overloading.