A COMPARATIVE STUDY OF TEMPERATURE-PHASED DIGESTION TO SINGLE STAGE THERMOPHILIC AND MESOPHILIC DIGESTION

Three bench scale anaerobic digestion systems, operating at the same time, treated a blend of primary and secondary residuals (at 4% total solids) from Blue Plains wastewater treatment plant (WWTP), Washington, D.C. The digestion systems were temperature-phased anaerobic digestion (TPAD), single stage thermophilic (SST) and single stage mesophilic (SSM) anaerobic digestion. Solids retention time (SRT) and organic loading rates (OLR) were changed between each of three study phases. TPAD demonstrated the best performance by destroying more volatile solids and having lower residual biological activity than the other digesters during each phase of the study. Maximum volatile solids reduction (VSR) was 67% by the TPAD system with a 15 day SRT (each stage at 7.5 days). The a SSM digester had 9% higher VSR than the SST digester when operated at 20 days SRT, indicating that a fraction of the feed sludge was only degraded under mesophilic conditions. The results of this study, combined with additional studies on odor production and dewatering, have demonstrated an advantage to TPAD digestion as a process for Blue Plains.