DISINFECTION OF FECAL COLIFORMS AND SALMONELLA IN THERMOPHILICALLY DIGESTED BIOSOLIDS

Abstract

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

Recurrence of Fecal Coliforms and Salmonella Species in Biosolids Following Thermophilic Anaerobic Digestion

“…Anaerobic digestion is one of the processes most widely used by wastewater treatment plants for the treatment of wastewater solids, with most digesters currently being operated at mesophilic temperatures for the production of Class B biosolids ( Goldstein, 2000 ; U.S. EPA, 1999 ). However, 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 for the production of Class A biosolids according to 40 CFR Part 503 requirements ( Drury et al, 2002 ; Iranpour, Cox, Starr, Fan, Mundine, Abkian, and Haug, 2004 ; Iranpour et al, 2006 ; Mittsdörffer et al, 1990 ; Oh et al, 2005 ; Wilson and Dichtl, 1998 ).…”

Full‐Scale Class A Biosolids Production by Two‐Stage Continuous‐Batch Thermophilic Anaerobic Digestion at the Hyperion Treatment Plant, Los Angeles, California

“…Following the recommendations of Phases I and II, it was decided to proceed with the complete conversion to thermophilic operation, which would eliminate the possibility of contamination by mesophilically digested biosolids. In addition, the postdigestion train was insulated and electrically heat‐traced to maintain a high biosolids temperature throughout postdigestion. The Phase III process was a two‐stage continuous process, with 90% of the plant's feed sludge being digested at a thermophilic temperature ( Iranpour et al, 2004a ). This was a temporary process configuration, necessitated for completing the conversion of HTP to thermophilic operation.…”

“…The HTP has been entirely thermophilic since October 2002, using a two‐stage, continuous‐batch process ( Iranpour et al, 2004a ). The first stage of this process contains 16 digesters that are operated in a continuous mode.…”

“…In addition, the postdigestion train was insulated and electrically heattraced to maintain a high biosolids temperature throughout postdigestion. The Phase III process was a two-stage continuous process, with 90% of the plant's feed sludge being digested at a thermophilic temperature (Iranpour et al, 2004a). This was a temporary process configuration, necessitated for completing the conversion of HTP to thermophilic operation.…”

Disinfection and Antimicrobial Processes

“…the final fecal coliform concentration exceeded 10 4 /g in the undigested biosolids sample and remained below 10 3 /g in the thermophilic biosolids sample (Jolis et al, 2004). A study conducted by Iranpour et al (2004) also suggested that fecal coliforms are a poor indicator of pathogen content in biosolids after digestion due to their recurrence rate, whereas recurrence of Salmonella sp. was not observed.…”