Treatment of Low Strength Swine Waste Using Anaerobic Sequencing Batch Reactors (ASBRs)

Abstract

Use of anaerobic digestion for the treatment and recovery of biogas from animal waste has a long history but its adoption has been hindered by to poor economics. The economic challenges are even worse when considering anaerobic digestion of low strength animal wastewaters because of large volumes of reactors and the corresponding high energy input requirements. A relatively new reactor-technology known as anaerobic sequencing batch reactor (ASBR), has shown promise of improving the economics of the treatment of such dilute wastewaters. This paper reports preliminary optimization studies of an ASBR treating dilute swine slurries from pitrecharge manure management systems. Based on the results, optimum biogas yields from anaerobic digestion of low strength swine waste (0.3 - 0.4% TS) were approximately 0.14 mL/mg COD and 0.16 mL/mg COD at a HRT of 5.25, and 6 days at digestion temperatures of 20oC and 35oC, respectively. Higher operational temperatures improved the specific biogas yield but the qualities of biogas produced at the two temperatures (65 – 70% CH4 and 17 – 20% CO2) were not significantly different. On the other hand, higher cycle-frequency for the same HRT significantly decreased the biogas yield but not the quality of the biogas produced. Maximum COD reductions of approximately 90 and 84% were achieved at HRTs of 7.2 and 9.1 days, at digestion temperatures of 20oC and 35oC, respectively. Higher COD reduction (implying more bio-stabilized effluents) in the lower-temperature digester was attributed to less biomass wash-out most probably due to more efficient solids settling. Results of suspended solids in the effluents showed that effectiveness of settling was degraded by higher cycle-frequency. The volatile fatty acids reduction at both reactortemperatures and the two cycle-frequencies (1 and 3 cycles/day) were significantly high from a mean of 639 mg/L in the influent to means less than 120 mg/L in the effluents; significantly mitigating the potential of odor generation from the effluents. Additionally, it was observed that the nutrients (both N and P) levels in the effluents remained about the same as in the influents during the entire testperiod.