Thermophilic-anaerobic digestion to produce class A biosolids: initial full-scale studies at Hyperion Treatment Plant.

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.     

Full-scale class A biosolids production by two-stage continuous-batch thermophilic anaerobic digestion at the hyperion treatment plant, Los Angeles, California.

The City of Los Angeles Hyperion Treatment Plant (HTP) (California) converted its anaerobic digesters to thermophilic operation to produce Class A biosolids. Phase IV tests demonstrated compliance of a two-stage, continuous-batch process with Alternative 1 of U.S. Environmental Protection Agency 40 CFR Part 503 (U.S. EPA, 1993), which defines the time-temperature requirement for batch treatment (T > or = 56.3 degrees C at 16-h holding). Fecal coliforms, Salmonella sp., viable helminth ova, and enteric viruses were not detected in biosolids in the postdigestion train, including the truck-loading facility and the farm for land application as the last points of plant control where compliance is to be demonstrated. The same results were achieved during Phase V tests, after lowering the second-stage holding temperature to 52.6 degrees C to reduce the elevated methyl mercaptan production that was observed during Phase IV. Hence, the Phase V process complied with Alternative 3 of 40 CFR Part 503. Currently, HTP operates its digesters under the same conditions as tested in Phase V. In 2003, monthly monitoring of the biosolids at the truck-loading facility and the farm for land application demonstrated consistent compliance with Alternative 3.