Environ. Sci. Technol. 2005, 39, 5804-5809
Inactivation of Ascaris suum and Poliovirus in Biosolids under Thermophilic Anaerobic Digestion Conditions MICHAEL D. AITKEN,* MARK D. SOBSEY, KIMBERLY E. BLAUTH, MINA SHEHEE,† PHILLIP L. CRUNK,‡ AND GLENN W. WALTERS Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina, Chapel Hill, North Carolina 27599-7431
There is considerable interest in the United States in production of Class A (low pathogen content) biosolids from the treatment of municipal wastewater sludge. Current requirements imposed by the U.S. Environmental Protection Agency make it difficult for thermophilic anaerobic digestion, in its simplest process configurations, to achieve Class A status. In particular, the time-temperature requirements necessitate long batch treatment times at temperatures associated with thermophilic anaerobic digestion. The time-temperature requirements are meant to ensure extensive inactivation of helminth eggs and enteric viruses, considered to be the most heat-resistant of the relevant pathogen classes. However, data on inactivation kinetics of these pathogens at precisely controlled and well-characterized temperatures are scarce. We measured inactivation of vaccine-strain poliovirus and eggs from the helminth Ascaris suum at temperatures from 49 to 55 °C in a lab-scale batch reactor containing biosolids from a continuous-flow thermophilic anaerobic digester. Both microbes were inactivated rapidly, with Ascaris more resistant to inactivation than poliovirus, and the relationships between inactivation rate and temperature were steep. The Arrhenius correlation between inactivation rate and temperature over the range 49-53 °C is consistent with protein denaturation as the inactivation mechanism for both microbes. The least stringent of the EPA timetemperature equations for thermal processes requires batch treatment times more than 2 orders of magnitude greater than would be required for three-log reduction of Ascaris at the rates we measured, suggesting an overly conservative regulatory approach. Such a grossly conservative approach can hinder full-scale implementation of thermophilic anaerobic digestion.
Introduction More than half of the approximately 6 million tons (dry weight) of biosolids generated annually from municipal wastewater treatment in the United States is applied to * Corresponding author phone: 919-966-1481; fax: 919-966-7911; e-mail:
[email protected]. † Present address: North Carolina Department of Public Health, Raleigh, NC. ‡ Present address: Limno-Tech, Inc., Washington, DC. 5804
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 15, 2005
agricultural lands (1). Land application and other forms of biosolids reuse or disposal are regulated by the U.S. Environmental Protection Agency (EPA) under 40 CFR Part 503. Biosolids designated for land application are classified as Class A or Class B, depending on the presumed pathogen content resulting from the process used to treat the source sludge (2). Class A biosolids are intended to have undetectable levels of three classes of pathogens: bacteria, enteric viruses, and helminths (parasitic intestinal worms). Correspondingly, the restrictions on management of and access to the land application site that apply to Class B biosolids do not apply to Class A biosolids. Such favorable conditions for the site owner and the presumed lower risk to public health (1) have led to a growing national interest in the production of Class A biosolids at municipal wastewater treatment plants. The Part 503 regulations identify six methods by which a sludge treatment process can be assumed to produce Class A biosolids (2). Some treatment processes, such as pasteurization, are designated as “processes to further reduce pathogens” (PFRPs) and are assumed to produce biosolids that meet the Class A pathogen criteria as long as specified operating conditions are met. Thermophilic anaerobic digestion, a high-temperature (g50 °C) modification of conventional (and widely practiced) anaerobic digestion, is not identified as a PFRP. Another method by which a treatment process can be assumed to meet the Class A criteria is to operate the process at a specified combination of operating temperature and treatment time; different timetemperature equations are used depending on the solids content of the source sludge and the magnitude of the treatment time. It is difficult to comply with the timetemperature approach in most types of continuous-flow reactors (2), disqualifying the simplest means of implementing thermophilic anaerobic digestion for sludge treatment. While it is possible to treat sludge in batch reaction systems, the holding times required by the time-temperature equations may not be implemented easily at full scale. At 50 °C the required treatment time for sludge with 39.3d < 3.5 0.17 0.5 38.9 3.5 0.5 0.5 NDe ND
