FULL-SCALE THERMOPHILIC ANAEROBIC DIGESTION AT THE HYPERION TREATMENT PLANT: Experiments for The Production of EQ Biosolids

The highest quality of biosolids is called Exceptional Quality (EQ). In order to qualify for this classification, biosolids must comply with three criteria: metal concentrations, vector attraction reduction, and the Class A pathogen density requirement. The City of Los Angeles Bureau of Sanitation Hyperion Treatment Plant (HTP) meets the first two requirements. Thus, the objective of this study is to ensure that HTP's biosolids processing meets the pathogen density requirement by satisfying time-temperature regimen for Class A (U. S. EPA 40 CFR Part 503, Subsection 32, Alternative 1).A two-stage thermophilic batch process, consisting of a battery of 6 egg-shaped digesters, was established at HTP. As the biosolids were discharged from the second stage digester, the Salmonella sp. (pathogen) and fecal coliform (indicator) densities were well below the maxima for Class A biosolids. Analysis of the cycle of filling, holding, and withdrawal from the digesters in the second stage indicated that the process satisfies the time-temperature relation to guarantee killing nonbacterial pathogens even when the temperature in these digesters is a few degrees below the target temperature, providing reliability of results despite operational fluctuations. Salmonella sp. densities remained below the Class A limit at all post-digestion sampling locations. Fecal coliform densities were also below the Class A limit at all sampling locations except at the truck loading facility. Since the Bureau seeks the greatest feasible biosolids safety, additional work has been done, such as temperature profiling and regrowth testing, to understand this apparent recontamination or regrowth of fecal coliforms before the truck is loaded, and control measures are available for the possible explanations that have been identified. Nevertheless, since the Salmonella sp. results show that the federal Class A standard is met by the present process, these biosolids are indeed EQ.