Optimization of anaerobic sequencing batch reactors treating dilute swine slurries

Use of anaerobic digestion for the treatment and recovery of biogas from concentrated animal waste effluents is a technically viable approach, but widespread acceptance has been limited due to poor economics. This challenge is magnified several fold when considering anaerobic digestion of low-strength or dilute animal slurries because of the larger digester volumes and the corresponding high energy input requirements. These constraints could be mitigated by using an anaerobic sequencing batch reactor (ASBR). This technology has shown tremendous potential to improve the economics for the treatment of dilute animal waste effluents. This article reports preliminary optimization (biogas production and organic strength reduction) of an ASBR treating dilute swine slurries from pit-recharge manure management systems commonly found in confined swine housing. Based on the results, optimum biogas yields from anaerobic digestion of low-strength swine waste (0.3% to 0.4% TS) were approximately 0.14 mL/mg COD and 0.16 mL/mg COD at 5.25 and 6 days HRT at digestion temperatures of 20 degrees C and 35 degrees C, respectively. Higher operational temperature improved the specific biogas yield, but the qualities of biogas produced at the two temperatures, although high (65% to 70% CH4 and 17% to 20% CO2), were not significantly different. Maximum COD reductions of approximately 90% and 84% would be achieved at 7.2 and 9.1 days HRT at digestion temperatures of 20 degrees C and 35 degrees C, respectively. Higher COD reduction (implying more bio-stabilization of slurry) in the lower-temperature digester was attributed to less biomass washout, which is likely due to more efficient solids settling. The volatile fatty acids at both reactor temperatures were reduced from a mean of 639 75 mg/L in the influent to mean values of 74 +/- 12 and 85 +/- 17 mg/L in the effluents at 20 degrees C and 35 degrees C, respectively, which significantly mitigated the potential of odor generation from the effluents. Additionally, it was observed that the nutrient (both N and P) levels in the effluents remained about the same as in the influent.