Laboratory Evaluation of Thermophilic‐Anaerobic Digestion to Produce Class A Biosolids. 2. Inactivation of Pathogens and Indicator Organisms in a Continuous‐Flow Reactor Followed by Batch Treatment

Abstract

Thermophilic‐anaerobic digestion in a single‐stage, mixed, continuous‐flow reactor is not approved in the United States as a process capable of producing Class A biosolids for land application. This study was designed to evaluate the inactivation of pathogens and indicator organisms in such a reactor followed by batch treatment in a smaller reactor. The combined process was evaluated at 53°C with sludges from three different sources and at 51 and 55°C with sludge from one of the sources. Feed sludge to the continuous‐flow reactor was spiked with the pathogen surrogates Ascaris suum and vaccine‐strain poliovirus. Feed and effluent were analyzed for these organisms and for indigenous Salmonella spp., fecal coliforms, Clostridium perfringens spores, and somatic and male‐specific coliphages. No viable Ascaris eggs were observed in the effluent from the continuous reactor at 53 or 55°C, with greater than 2‐log removals across the digester in all cases. Approximately 2‐log removal was observed at 51°C, but all samples of effluent biosolids contained at least one viable Ascaris egg at 51°C. No viable poliovirus was found in the digester effluent at any of the operating conditions, and viable Salmonella spp. were measured in the digester effluent in only one sample throughout the study. The ability of the continuous reactor to remove fecal coliforms to below the Class A monitoring limit depended on the concentration in the feed sludge. There was no significant removal of Clostridium perfringens across the continuous reactor under any condition, and there also was limited removal of somatic coliphages. The removal of male‐specific coliphages across the continuous reactor appeared to be related to temperature. Overall, at least one of the Class A pathogen criteria or the fecal coliform limit was exceeded in at least one sample in the continuous‐reactor effluent at each temperature. Over the range of temperatures evaluated, the maximum time required to meet the Class A criteria by batch treatment of the continuous‐reactor effluent was 1 hour for Ascaris suum and Salmonella spp. and 2 hours for fecal coliforms.

Male-Specific Coliphages as Indicators of Thermal Inactivation of Pathogens in Biosolids

“…The construction and operation of the continuous and batch laboratory reactors are described elsewhere (2,3). The concentrations of F ϩ coliphages in the untreated sludges ranged from about 3 log 10 /g total solids (TS) to about 5 log 10 /g TS (2).…”

“…The concentrations of F ϩ coliphages in the untreated sludges ranged from about 3 log 10 /g total solids (TS) to about 5 log 10 /g TS (2). Concentrations in the treated biosolids were 0.4 to 3.1 log 10 /g TS, depending on the source sludge and operating temperature (2).…”

“…The source of the coliphages was an ultrafiltered concentrate of membrane-filtered (0.45-m pore size) raw wastewater from the municipal wastewater treatment plant in Beaufort, North Carolina. Total F ϩ coliphages were quantified by a modification of EPA method 1601 (28), in which the presence-absence test format for 1-liter volumes was replaced with a most-probable-number format (2).…”

“…A significant fraction of the F ϩ coliphages survived treatment at temperatures as high as 55°C, and in subsequent batch treatments of the effluent from the continuous digester, there was limited further reduction at any temperature studied (2). In the present study, the rates of inactivation of total F ϩ coliphages in biosolids were quantified at 51 and 53°C.…”

Survey of Wastewater Indicators and Human Pathogen Genomes in Biosolids Produced by Class A and Class B Stabilization Treatments

“…The majority of U.S. class A operations choose to meet class A status with time and temperature treatmentbased alternatives. Based on these regulatory monitoring targets, most culture-or PCR-based biosolid studies have focused on these microorganisms and similar enteric pathogens (i.e., Listeria monocytogenes, enterovirus, and Clostridium perfringens) but have not diversified to other relevant airborne pathogens, like Legionella pneumophila, that may proliferate during stabilization where enteric pathogens cannot (1,9,22,28,34,40,41,53). Only recently, and in limited sample sizes, have researchers begun to directly compare the diverse sludge stabilization methods available, including MAD, liming, and composting, to understand how selected enteric pathogens and indicators are removed (18,22,32,34).…”