Comparative Studies of Alternative Anaerobic Digestion Technologies

Abstract

Washington D.C. Water and Sewage Authority is planning to construct a new anaerobic digestion facility at its Blue Plains WWTP by 2008. The research conducted in this study is to aid the designers of this facility by evaluating alternative digestion technologies. Alternative anaerobic digestion technologies include thermophilic, acid/gas phased, and temperature phased digestion. In order to evaluate the relative merits of each, a year long study evaluated the performance of bench scale digestion systems at varying solids retention times (SRT) and organic loading rates (OLR). The digesters were fed a blend of primary and secondary residuals from the Blue Plains wastewater treatment facility.

In each study phase, temperature phased anaerobic digestion was compared to single stage mesophilic digestion (the industry standard) at the same SRT. Single stage thermophilic digestion was evaluated by sampling the first thermophilic stage of the temperature phased digestion systems throughout the study. Additionally, the first phase study compared acid/gas phased digestion to temperature phased and single stage mesophilic digestion.

Results of the study demonstrated that the temperature phased digestion system consistently performed better than the other systems during each study phase by having higher volatile solids reduction (VSR), higher methane production, and lower residual biological activity. The highest observed VSR during the study (67%) occurred in a temperature phased digestion system operated at 7.5 days in each stage. Based on these results, it seems a suitable candidate for the Blue Plains digestion facility. Additionally, odor studies performed in conjunction with the research presented in this paper have shown distinct advantages for the temperature phased process.