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The City of Los Angeles Hyperion Treatment Plant (HTP) has conducted a multiphase study of thermophilic anaerobic digestion to meet federal (Part 503 Biosolids Rule) and local (Kern County ordinance) requirements for Exceptional Quality Class A biosolids. The study consisted of five Phases of full-scale testing.

A designated battery of six thermophilic digesters for digestion of 20% of HTP/s sludge was used in Phases I and II (October 2001 – March 2002). The tests demonstrated compliance with the Class A limits for fecal coliforms and Salmonella in a two-stage batch process. However, fecal coliform densities in biosolids increased during postdigestion, causing non-compliance with local regulations. Potential causes for fecal coliform recurrence in thermophilic biosolids were: a) contamination by mesophilic biosolids; b) a large drop of the biosolids temperature which would allow reactivation/growth of fecal coliforms.

The Phase III process (August – September 2002) was a continuous process with 90% of the plant in thermophilic operation and the post-digestion train insulated between the digesters and silos to maintain a high biosolids temperature. This process almost fully complied with the Class A pathogen requirements for biosolids at the silos and the farm. Recurrence of fecal coliforms during post-digestion did not occur.

The Phase IV process (October 2002) was a two-stage batch process that complied with the time-temperature relation in Alternative 1 of the Part 503 Biosolids Rule. Complete disinfection of fecal coliforms and Salmonella was achieved and both groups of bacteria were non-detect in farm biosolids. Relatively high digestion temperatures, however, caused unacceptable emissions of odors.

To reduce odor emissions from thermophilic operations, the digester temperatures in the Phase IV process were lowered, which then became Phase V testing. The Phase V process (November 2002) produced farm biosolids with non-detect levels of fecal coliforms, Salmonella, helminth ova and enteric viruses. This process, therefore, complied with Alternative 3 of the Part 503 Biosolids Rule.

The City of Los Angeles received the permit to land apply HTP/s Exceptional Quality biosolids at its farm in Kern County on December 27, 2002, four days before the ban on Class B biosolids became effective. Monthly testing in 2003 confirms that the standards for Exceptional Quality biosolids are consistently being met.

Document Type: Research Article


Publication date: January 1, 2003

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