Treatment of dairy wastewater with two-stage anaerobic sequencing batch reactor systems--thermophilic versus mesophilic operations

A two-stage anaerobic sequencing batch reactor (ASBR) system was studied in the laboratory to evaluate a temperature-phased anaerobic digestion process for dairy wastewater treatment. The temperature-phased anaerobic digestion system incorporates thermophilic digestion and mesophilic digestion processes into one system which has the potential to offer the benefits of high digestion rate, fecal bacteria destruction, process stability, and favorable economics for animal wastewater treatment. Two thermophilic (55 degrees C)-mesophilic (35 degrees C) systems (II and III) were evaluated against one mesophilic (35 degrees C)-mesophilic (35 degrees C) system (I) at two system hydraulic retention times (HRTs) (3 and 6 days) and five volatile solids (VS) loading rates (2, 3, 4, 6 and 8 g/l/day). The three systems were designed with different first- and second-stage volume ratios: 1:4 for systems I and II and 1:2 for system III. The effects of temperature configurations and volume ratios between the two stages, HRTs, and VS loading rates on the performance of the two-stage ASBR system were studied. A relatively high solid retention time (SRT) of 13-18 days was achieved at both 3- and 6-day HRTs for all three systems. The performance of each system depends on the HRT and the VS loading rate. All three systems at the 3-day HRT deteriorated when the VS loading rate reached 6 g/l/day. The total solids (TS) and volatile solids (VS) removals were similar for 3- and 6-day HRTs at the VS loading rates of 2, 3 and 4 g/l/d. For the same VS loading rate in the range of 2-4 g/l/day and the same first- to second-stage volume ratio (1:4), the VS removal and specific methane production rate of the thermophilic-mesophilic system (II) were 43.8-44.1% and 0.41-0.82 l/l/day for 3-day HRT, respectively, and 37.1-38.9% and 0.34-0.72 l/l/day for 6-day HRT, respectively, which are higher than the VS removal and specific methane production rate of the mesophilic-mesophilic system (I) (29.3-30.2% and 0.27-0.56 l/l/day for 3-day HRT, respectively; 26.1-29.1% and 0.27-0.48 l/l/day for 6-day HRT, respectively). With the thermophilic-mesophilic system, the 1:4 ratio for the first- and second-stage reactors yielded better solids removals and methane production rate than the 1:2 ratio. Both thermophilic-mesophilic systems were found to be more effective in destroying the total coliforms present in the wastewater than the mesophilic-mesophilic system. As far as solids removal, biogas production, and coliform bacteria destruction are concerned, thermophilic-mesophilic systems are recommended over the mesophilic-mesophilic systems. However, the increased heating energy requirement of the former system needs to be considered when comparing the economics of these two treatment systems.