EFFECT OF SOLIDS RETENTION TIME ON THE PERFORMANCE OF THERMOPHILIC AND MESOPHILIC DIGESTION

This paper compares the steady-state performance of thermophilic and mesophilic digestion as a function of SRT. Laboratory thermophilic (55°C) and mesophilic (35°C) digesters were operated in parallel at SRTs ranging from 4 to 15 days, treating prethickened combined primary and secondary waste sludge, to compare the performance of thermophilic and mesophilic digestion under equal solids loading conditions. Results from pilot plant operation of thermophilic and mesophilic digesters at 20 days SRT (King County, WA) are also included.The pH, alkalinity, and average ammonia concentrations were higher for the thermophilic digester than the mesophilic digester at each SRT. Thermophilic digestion had higher VFA and SCOD concentrations at all SRTs, with a VFA composition similar to mesophilic digestion at the 15 day SRT. However, at lower SRTs, the thermophilic digester showed much higher propionate and slightly more butyrate.Gas production rates and methane content of the digester gases were similar at long SRTs (10 days and greater), while the thermophilic digester produced more gas with a higher methane content at lower SRTs (6 days and less). Carbohydrates were readily degraded by both digesters, protein was the major component in the sludge at the long SRTs, and lipid and protein content was in the same range at the 6-day SRT.The TS and VS concentrations of the thermophilic and mesophilic digesters increased with decreasing SRT, and there was no statistically significant difference (student's t test, 95 percent confidence level) between the VS destruction efficiencies of the thermophilic and mesophilic digesters at SRTs of 6, 10, and 20 days.At the 4-day SRT, methanogenic activity was maintained in the thermophilic digester but not in the mesophilic digester, indicating a greater maximum specific growth rate for thermophilic microorganisms.A first order model was used to fit the VS destruction efficiency with time for both the thermophilic and mesophilic digesters. The results show that staged digestion could result in significant improvement in VS destruction efficiency, and suggest that staged mesophilic digestion would provide similar VS destruction efficiencies as a TPAD system.