Abstract
Manure and slurry may contain a range of bacterial, viral and parasitic pathogens and land application of these organic fertilisers typically occurs without prior treatment. In-situ treatment through farm-based anaerobic digestion (AD) of such organic fertilisers co-digested with food-production wastes is multi-beneficial due to energy recovery, increased farm incomes and noxious gas reduction. Before risk assessment can be carried out at field scale an investigation of the fate of relevant target pathogens during the actual AD process must be undertaken, requiring the development of practical test systems for evaluation of pathogen survival. The present study examines miniature (50 mL) and laboratory (10 L) scale AD systems. Treatments included slurry co-digested with fats, oils and grease (FOG) under typical operating and pasteurisation conditions used in farm-based AD, in batch-fed miniature and laboratory mesophilic (37°C) continuously stirred tank reactors. Biogas production, pH, chemical oxygen demand, volatile solids and ammonia concentration were measured throughout the trial, as were faecal indicator bacteria (FIB) i.e. total coliforms, Escherichia coli and Enterococcus species. The miniature and laboratory bioreactors performed similarly in terms of physicochemical parameters and FIB die-off. In the absence of pasteurisation, after 28 days, enterococci numbers were below the <1000 cfu g-1 threshold required for land application, while E. coli was no longer detectable in the digestate. For comparison, FIB survival in slurry was examined and after 60 days of storage, none of the FIB tested was <1000 cfu g-1, suggesting that slurry would not be considered safe for land application if FIB thresholds required for AD digestate were to be applied. Taken together we demonstrate that i) miniature-scale bioreactors are valid proxies of farm-based AD to carry out targeted pathogen survival studies and ii) in situ AD treatment of slurry prior to land application reduces the level of FIB, independently of pasteurisation, which in turn might be indicative of a decreased potential pathogen load to the environment and associated public health risks.
Highlights
1.4 billion tons of manure are produced in Europe each year, 80% of which is in the form of slurry, predominantly from cattle (Crowe et al, 2000; Foged et al, 2011)
The slurry collected from the three farms was tested prior to anaerobic digestion (AD), for initial FIB levels as well as total solids and volatile solids (Table 1)
Volatile solids (VS) degradation was comparable for the 50 mL and 10 L bioreactors with 64 and 61% VS removal, respectively within the first 7 days (Figure 1A)
Summary
1.4 billion tons of manure are produced in Europe each year, 80% of which is in the form of slurry, predominantly from cattle (Crowe et al, 2000; Foged et al, 2011). A number of methods for limiting the impact of manure storage and landspreading, both in terms of GHG capture or mitigation and pathogen reduction have been examined, including aeration, and acidification during storage, animal diet manipulation, or alternative landspreading techniques (Nicholson et al, 2004; Franz et al, 2005; Webb et al, 2010). These proposed solutions, consider either pathogens or GHG in isolation. In this context, the recommended direct incorporation of slurry into soil might lead to increased pathogen survival, as it inevitably reduces UV exposure (Avery et al, 2004; Hutchison et al, 2004)
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