Inland Empire Utilities Agency has three parallel anaerobic digestion systems each using threephase thermophilic digestion. The first phase is an acid digester operating at a temperature of approximately 35°C. The second phase is an anaerobic thermophilic gas digester operating
in the range of 50°C to 56°C. No heat is added to the third phase. The operating temperature of the third phase depends on ambient weather conditions. In one of the three process trains the thermophilic second phase is fed in a semi-batch basis (feed/hold/draw) and is operated
at 56°C. The feed period is 4 hours, the hold period is 17 hours and the draw time is 3 hours. As such, the biosolids produced from this train meets the Class A requirements using Alternative 1 of 40 CFR Part 503 (U.S. EPA, 1993). The second and third process trains are being fed continuously
and the thermophilic second phase is being operated at 50°C. Biosolids from these trains meet the Class A requirements of 40 CFR 503 using Alternative 3. No significant difference has been observed between the semi-batch process operated at 56°C and the continuously fed train operated
at 50°C. To date, all the pathogen samples for both processes have been non-detect. In fact, over 85% of the fecal coliforms samples have been nondetect
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