PROTOCOLS AND PLANS FOR CLASS A CERTIFICATION OF THERMOPHILIC ANAEROBIC DIGESTION BY DEMONSTRATING EQUIVALENCY TO PROCESSES TO FURTHER REDUCE PATHOGENS

The City of Los Angeles Hyperion Treatment Plant (HTP) completed the conversion of its anaerobic digesters to thermophilic operation in August 2002. Extensive monitoring of the digesters in 2003 indicated stable thermophilic digestion performance: pH of 7.5; 300 mg/L volatile fatty acids (VFA) as acetic acid; VFA to alkalinity ratio of 0.074; 61% volatile solids destruction. Since December 2002, the two-stage continuous-batch process has been certified as Class A under Alternative 3 of the U.S. EPA Part 503 Biosolids Rule. Salmonella sp., viable helminth and enteric viruses were not detected during monthly monitoring of the biosolids at the Truck Loading Facility and the farm for land application. Fecal coliforms were always at least ten times below the Class A limit, and often not detected. Hence, near complete disinfection of the biosolids was achieved, even though the holding time and temperature in the second stage (16 hours at 129.2°F) was well below the time-temperature requirement of Alternative 1. The City of Los Angeles Bureau of Sanitation aims to demonstrate that the process at HTP is equivalent to Processes to Further Pathogens (PFRP equivalency: Alternative 6). Detailed protocols have been developed to demonstrate a 2-log reduction of viable helminth ova and a 3-log reduction of enteric viruses, as required by U.S. EPA for Alternative 6. The experiments entail a laboratory setup with six 20-Liter digesters that will simulate the operation of HTP's second-stage batch digesters. The U.S. EPA Pathogen equivalency Committee gave final approval of the protocols in June 2004, and experiments are expected to start in 2005.