A COMPARATIVE STUDY OF TWO-STAGE THERMOPHILIC, SINGLE STAGE MESOPHILIC, AND TEMPERATURE PHASED ANAEROBIC DIGESTION

Blue Plains WWTP is planning to construct an egg shaped digester facility scheduled for completion in 2011. Designers are currently assessing process options for the digesters. To aid in the design, Virginia Tech is conducting a study to compare the performance of lab scale digesters operated as temperature phased anaerobic digestion (TPAD) systems, single stage thermophilic, and single stage mesophilic anaerobic digestion systems. Temperature phased anaerobic digestion is the desired process option for the facility due to its higher performance and ability to control product odor.The digester set up for this study was selected to compare a TPAD system, a two-stage thermophilic system, and two mesophilic conventional digesters using the same feedstock. The digesters were fed a blend of thickened primary and waste activated sludges from Blue Plains, mixed at a 50/50 ratio by weight. The total solids (TS) concentration of the feed was around 4% until each digester achieved stable operating conditions. The study included a short-term increase in loading and a decrease in the hydraulic detention time to simulate peak loading conditions.Results from the study demonstrated an advantage in favor of the TPAD system. The volatile solids reduction (VSR) was significantly higher (67%) and there was less fluctuation in VSR and volatile fatty acids (VFA) under the peak loading condition. Furthermore, problems associated with ammonia toxicity may be lessened in a second stage mesophilic versus a second stage thermophilic digester. Though not presented in this manuscript, total headspace sulfur odors were also lower in the TPAD digestion system versus the others systems.Based on the results of this study and other studies in this laboratory, it is recommended that DCWASA continue to pursue TPAD as a process option for Blue Plains' new anaerobic digestion facility.