The steady-state performance of thermophilic (55 °C) and mesophilic (35 °C) anaerobic digestion as a function of solids retention time (SRT) was evaluated in laboratory digesters at SRTs ranging from 4 to 15 days, and in pilot-plant digesters at a 20-day SRT. All of the digesters
were fed the same source of municipal combined primary and secondary waste sludge. The destruction efficiency of volatile solids increased from 53% to 66% as the SRT was increased from 6 days to 20 days. The average destruction efficiency of volatile solids was 3 percentage points higher
for the thermophilic digester at the 6-day SRT and approximately 1 percentage point higher for the higher SRTs, but the difference was only statistically significant at the 15-day SRT. Based on volatile suspended solids measurements, the thermophilic solids destruction efficiency was approximately
4 percentage points higher at the 10- and 15-day SRTs. At a 4-day SRT, methanogenic activity could only be maintained in the thermophilic digester. The pH, alkalinity, ammonia, volatile fatty acid, and soluble chemical oxygen demand concentrations were higher for the thermophilic digester
at each SRT. At SRTs of 10 days and less, the thermophilic digester had a much higher propionate and slightly higher butyrate concentration. Carbohydrates were readily degraded by both digesters, protein was the major component in the sludge at the long SRTs, and lipid degradation increased
with increasing SRT.
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