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Anaerobic digestion of animal manure to produce soil amendments

Madison, WI(WCM Newsletter) -

As more anaerobic manure digestors go on-line there has been increased interest in modifying the by-product solids to create marketable soil amendments for non-farm use. It has long been recognized that manure has the potential to contain infectious pathogens. With improved medical diagnostics, the significance of zoonotic (animal to human) transmission of infectious diseases has gained renewed attention. Recently, researchers and practitioners are re-considering the operation of manure digesters to achieve pathogen reduction goals. Lessons can be borrowed from domestic wastewater treatment practices to determine if this is possible.

The objective of sludge digestion in domestic wastewater treatment processes is to produce an organically stable and pathogen-free biosolids product that may be land applied with little or no risk to public health and the environment. Key factors that can produce stress on microorganisms that lead to inactivation in digesters include temperature, treatment time, pH, other chemicals in the reactor environment, and microbial competition. The U.S. Environmental Protection Agency (EPA) has set out pathogen and indicator limits to meet Class A standards, essentially unrestricted use, for processed domestic wastewater solids (i.e. biosolids, see Table 1).

Table 1. Class A Pathogen and Indicator Limits and Pathogen Destruction Requirements

 The EPA has also funded and conducted research into process conditions that results in acceptable pathogen destruction and removal efficiencies. Under the Federal Part 503 regulations, in order for a biosolids product to meet Class A, it must meet one of six pathogen-reduction criteria and/or meet maximum pathogen (indicator) concentration levels (McFarland, 2001). To meet Alternative 1 for thermally treated biosolids of less than 7 percent solids, the temperature of biosolids during treatment must be 50¢ªC (122¢ªF) or higher for at least 30 minutes. Alternative 2 applies to high pH/high temperature processes. If Alternative 3 is chosen, the biosolids must meet pathogen concentration limits for enteric viruses and viable helminth ova each required monitoring period. To meet Alternative 4, unknown process, the biosolids must be monitored for all four indicator and pathogens each required monitoring period and meet concentration limits for all four microbials (see Table 1). Alternatives 5 and 6 are met by applying processes to further reduce pathogens (PFRP). PFRP criteria have been defined by EPA for composting, heat drying, heat treatment, thermophilic aerobic digestion, β-ray irradiation, γ-ray irradiation, and pasteurization. There currently is no criteria specific for anaerobic digestion.  Depending on the operational conditions of the individual anaerobic digestion process, fecal coliform and Salmonella densities above Class A levels may remain in digested biosolids. By inference, the potential for transmission of pathogens then also remains.

For anaerobically digested manure, pathogen safety can be evaluated by testing for target microorganisms. The allowed analytical methods for biosolids are summarized in Table 2. The EPA recommends Methods 1680 and 1681 for measuring fecal coliforms in sewage sludge/biosolids, and Method 1682 for measuring Salmonella (72 Fed. Reg. 57 (26 March 2007)).

Table 2. Microbial Methods Biosolids Required Under Part 503