Thermophilic‐Anaerobic Digestion to Produce Class A Biosolids: Initial Full‐Scale Studies at Hyperion Treatment Plant

Abstract

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.

Factors Affecting Disinfection and Stabilization of Sewage Sludge

“…They include ammonia, amines, organic acids, aldehydes, and ketones . Metabolites like ammonia, volatile fatty acids, and hydrogen-sulfide are known to have toxic effects on microorganisms (Iranpour et al, 2006). Volatile fatty acids can be a potent bactericide when in their protonated form which occurs at low pHs (Aitken et al, 2005).…”

DISINFECTION OF FECAL COLIFORMS AND SALMONELLA IN THERMOPHILICALLY DIGESTED BIOSOLIDS

“…Since 2000, several tests were conducted to convert HTP to thermophilic anaerobic digestion. During Phase I (late 2001) and Phase II (early 2002), a two-stage continuous-batch process was investigated in a designated battery of six thermophilic digesters and a designated thermophilic postdigestion train that was isolated from mesophilic plant operations to prevent contamination of the thermophilically digested biosolids (Iranpour et al, 2002b(Iranpour et al, , 2004 (Figure 1a). It was originally intended that Phase III at HTP (mid 2002) would be a test of a plant-wide thermophilic process that was guaranteed to meet the batch time/temperature requirement of Alternative 1 of 40 CFR 503.…”

“…The indicator/pathogen density requirements for Class A biosolids usually need to be met at the last point of control by the plant, which usually is at the truck loading facility or the farm for land application. In response to local bans and ordinances on biosolids land application and growing public concern on the safety of biosolids, an increasing number of plants are implementing thermophilic anaerobic digestion to produce a higher quality of biosolids with indicator/pathogen densities below the limits for Class A qualification (Drury et al, 2002;Iranpour et al, 2002bIranpour et al, , 2004Mittsdörffer et al, 1990;Wilson and Dichtl, 1998). Recent investigations have indicated the recurrence of fecal coliforms in post-digestion biosolids at some wastewater treatment plants that produce Class A biosolids by thermophilic anaerobic digestion (Iranpour et al, 2002a).…”

The Effect of Digester Temperature on the Production of Volatile Organic Sulfur Compounds Associated with Thermophilic Anaerobic Biosolids

“…In this case, the specific microbial community of interest is a group of enteric human pathogens accustomed to life at standard body temperature (37°C), or mesophilic conditions. Previous research has shown that the viability of pathogenic organisms can be significantly affected by high temperature incubation and/or treatment (Berg et al 1980;Watanabe et al 1997;Ghosh 1998;Iranpour et al 2006). The aforementioned processes each use increased temperatures between 50°C-65°C to deactivate non-thermostable components of the pathogenic cells and viruses.…”