Aerobic thermophilic and anaerobic mesophilic treatment of swine waste

Laboratory experiments were conducted to investigate two-stage aerobic thermophilic and anaerobic mesophilic treatment of swine waste. The two-stage system included a 1-day sludge retention time (SRT) aerobic thermophilic reactor operating at 62°C and with 1.0 mg dissolved oxygen/l, followed by a 5, 9, and 14-day SRT anaerobic mesophilic digester operating at 37°C. A single stage anaerobic mesophilic digester operating at 6, 10, and 15-day SRT and 37°C was used as the control. Feed swine waste slurry average composition included total solids (TS)=4.3%; volatile solids (VS)=67% of TS; supernatant chemical oxygen demand (COD)=14,330 mg/l; fecal coliform density=7.2×108 MPN/g TS; pH=7.1; and alkalinity=2700 mg CaCO3/l. Two-stage system operating at 6, 10, and 15-day system SRT reduced VS by 46, 54, and 61%, respectively, and was significantly better than the control at each SRT. Supernatant COD reduction by the two-stage system (56–67%) was significantly better than that obtained in the control (44–60%). Fecal coliform density was reduced to <103 MPN/g TS at all SRT by the two-stage system, whereas, the control did not reduce the fecal coliform density below 105 MPN/g TS at all SRT. The two-stage system anaerobic digester produced 0.56–0.64 m3 CH4/kg VS destroyed compared to lower levels of 0.47–0.51 m3 CH4/kg VS destroyed by the control, both operating at 6, 10, and 15-day system SRT. The methane gas production by the two-stage system of 0.26, 0.32, and 0.39 m3/kg VS fed at 6, 10, and 15-day system SRT, respectively, was significantly higher than that by the control system (0.17, 0.22, and 0.25 m3/kg VS fed at 6, 10, and 15-day SRT, respectively). The biogas produced by the two-stage system anaerobic digester contained 353–387 ppm (v/v) H2S content compared to 569–609 ppm (v/v) H2S content in the biogas from the control anaerobic digester. The time-to-filter values of the product sludge from the two-stage system (245, 197, and 158 s at 6, 10, and 15-day system SRT, respectively) were about 50% lower than those of the product sludge from the control (355, 295, and 250 s at 6, 10, and 15-day system SRT, respectively), indicating better dewaterability of the two-stage system product sludge.