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.

The highest quality of biosolids is called exceptional quality. To qualify for this classification, biosolids must comply with three criteria: (1) metal concentrations, (2) vector‐attraction reduction, and (3) the Class A pathogen‐density requirements. The City of Los Angeles Bureau of Sanitation Hyperion Treatment Plant (HTP) (Playa del Rey, California) meets the first two requirements. Thus, the objective of this study was to ensure that HTP's biosolids production would meet the Class A pathogen‐reduction requirements following the time–temperature regimen for batch processing (U.S. EPA, 1993; Subsection 32, Alternative 1). Because regulations require the pathogen limits to be met at the last point of plant control, biosolids sampling was not limited to immediately after the digesters, i.e., the digester outflows. The sampling extended to several locations in HTP's postdigestion train, in particular, the last points of plant control, i.e., the truck loading facility and the farm for land application. 
 
 A two‐stage, thermophilic‐continuous‐batch process, consisting of a battery of six egg‐shaped digesters, was established in late 2001 for phase I of this study and modified in early 2002 for phase II. As the biosolids were discharged from the second‐stage digesters, the Salmonella sp. (pathogen) and fecal‐coliform (indicator) densities were well below the limits for Class A biosolids, even though the second‐stage‐digester temperatures were a few degrees below the temperature required by Alternative 1. Salmonella sp. densities remained below the Class A limit at all postdigestion sampling locations. Fecal‐coliform densities were also below the Class A limit at postdigestion‐sampling locations, except the truck‐loading facility (phases I and II) and the farm for final use of the biosolids (phase II). Although federal regulations require one of the limits for either fecal coliforms or Salmonella sp. to be met, local regulations in Kern County, California, where the biosolids are land‐applied, require compliance with both bacterial limits. Additional work identified dewatering, cooling of biosolids after the dewatering centrifuges, and contamination as possible factors in the rise in density of fecal coliforms. These results provided the basis for the full conversion of HTP to the Los Angeles continuous‐batch, thermophilic‐anaerobic‐digestion process. During later phases of testing, this process was demonstrated to produce fully disinfected biosolids at the farm for land application.

This study compares the effect of a rapid increase of the digester temperature (from 54 degrees C to 58 degrees C in 2 weeks) with a slow increase (from 53.9 degrees C to 57.2 degrees C at a rate of 0.55 degrees C per month) on full-scale thermophilic anaerobic digestion at Hyperion Treatment Plant. The short-term test demonstrated that rapidly increasing the digester temperature caused elevated production of volatile sulfur compounds, most notably methyl mercaptan, but volatile solids destruction and methane production were not significantly affected. The increase of the volatile fatty acid to alkalinity ratio from 0.1 to over 0.3 indicated a transient change in digester biochemistry, which was reversed by lowering the temperature. In the long term-test, a slow increase of digester temperature, the production of hydrogen sulfide increased above temperatures of 56.1 degrees C, but was controlled by increased injection of ferrous chloride. Methyl mercaptan was detected in trace amounts at the highest temperature tested (57.2 degrees C). This test showed insignificant effects on other digestion parameters, although some temperature-independent changes were observed that could have been seasonal effects over the year that the long-term test lasted. Thus a slow temperature increase was preferable. This observation contrasts with previous results showing the desirability of a rapid temperature rise to first establish a thermophilic culture when converting from mesophilic operation. Further research is warranted on temperature limits and process changes to optimize thermophilic anaerobic digestion.

Several wastewater treatment plants that were investigated in this study produce biosolids that meet the Class A pathogen reduction requirements for both fecal coliforms (indicator) and Salmonella sp. (pathogen). The U.S. EPA Part 503 Biosolids Rule specifies that the Class A limits for these bacteria shall be met at the last point of plant control, which is usually interpreted as the truck loading facility when the biosolids are prepared for transport or at the farm for land application. However, a recurrence of fecal coliforms was observed in post-digestion biosolids from the City of Los Angeles Hyperion and Terminal Island Treatment Plants and in the biosolids at the farm for land application. Biosolids sampled from dewatering centrifuges, silos and the farm sometimes contained fecal densities in densities up to 10 7 MPN/g dry wt, causing exceedance of the Class A limit for fecal coliforms. In contrast, recurrence of Salmonella sp. was never observed, which indicates that fecal coliforms are a poor indicator of the pathogen content in biosolids after digestion. Regrowth tests with fecal coliforms in biosolids from several locations in the post-digestion train indicated the presence of a self-disinfecting effect that caused killing of fecal coliforms spiked to biosolids from the digester. Such an effect was not present in centrifuge and silo biosolids. Hence, the dewatering centrifuge was identified as the most likely location where the recurrence of fecal coliforms originated. Laboratory simulations of centrifuge dewatering achieving only 10% total solids could not confirm the field observations and the possible role of centrifuge dewatering in the recurrence of fecal coliforms remained unknown. Comparison of observations at three wastewater treatment plants indicated that the recurrence of fecal coliforms can possibly be related to the following combination of factors: i) contamination of Class A biosolids with fecal coliforms from external sources; ii) a large drop of the post-digestion biosolids temperature to below the maximum for fecal coliform growth; iii) the loss of a self-disinfecting effect that causes killing of fecal coliforms in digested sludge. Recurrence of fecal coliforms in biosolids during postdigestion and at the farm was prevented by insulation of the post-digestion train of the Hyperion Treatment Plant, which maintained a biosolids temperature above the maximum for growth of fecal coliforms. When silo and farm biosolids were subsequently stored at 21 0 C, no recurrence of fecal coliforms was observed for at least 6 days. KEYWORDSBiosolids regulations, Class A, last point of plant control, indicator and pathogenic bacteria, anaerobic digestion, regrowth and inactivation, laboratory simulations, selfdisinfecting