OCCURRENCE OF FECAL COLIFORM AND SALMONELLA SP. FOLLOWING THERMOPHILIC DIGESTION AND POST-DIGESTION PROCESSING AT THE HYPERION TREATMENT PLANT

The City of Los Angeles Hyperion Treatment Plant (HTP) has conducted a multiphase study of thermophilic digestion to meet a newly stringent pathogen standard in the counties where it sends its biosolids for land application. The original plan was that Phase III of the studies would be a test of a batch process that was guaranteed to meet the time/temperature requirement of Alternative 1 of 40 CFR 503, the Federal sludge regulation. However, operational problems delayed the startup of this process. Consequently, Phase III became a full-scale test of a continuous two-stage thermophilic anaerobic digestion process. These tests were conducted in August and September, 2002. Thermophilic digestion was at average digester temperatures in the first and second stage of 128.1 – 131.7 °F (53.4 – 55.4 °C) and 122.0 – 127.8 °F (50 – 53.2 °C), respectively, and at an overall mean HRT of about 12 days. Almost full compliance with the Class A pathogen requirements was demonstrated in biosolids at the Truck Loading Facility and the farm for biosolids land application. Salmonella sp. in raw sludge were completely removed and these pathogens were non-detect in all samples taken at the farm. Kern County requires testing for both Salmonella sp. and fecal coliforms. Reactivation and/orgrowth of fecal coliforms in biosolids after the digesters, as was previously observed during Phase I (late 2001) and Phase II (early 2002) tests, did not occur during Phase III. This can possibly be attributed to two modifications implemented in the Phase III process. First, insulation of the post-digestion train prevented cooling of biosolids during post-digestion processing, i.e., biosolids temperatures from the digester outflow to the Truck Loading Facility decreased only by approximately 1 °F. Second, the full conversion of HTP to thermophilic treatment prevented the risk of contamination bymesophilic plant operations. Further studies in the laboratory with biosolids from the Truck Loading Facility and the farm demonstrated that no reactivation and/or growth offecal coliform occurred when the biosolids were stored for up to six days at 75 °F. The Phase IV Class A Biosolids program at HTP (October 2002) will test the timetemperature relationship for batch treatment (Alternative 1 of 40 CFR 503). Experimentswere conducted during this write-up and the results will also be presented at the conference.