Potential and optimisation of agriculture-based anaerobic digestion for environmental mitigation of agriculture-associated pollution
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The current work has taken a holistic approach to understanding the potential for mitigation of pollution from agriculture using anaerobic digestion (AD), with a particular focus on reduction of pathogen load to the environment. AD is a natural process whereby multi-species microbial communities operate synergistically to break down complex organic matter. This process produces biogas which can be used to generate electricity and/or heat, or upgraded to biomethane and injected into the gas grid or used as transport fuel. The residue from AD is called ‘digestate’ and can be used as an organic fertiliser/soil improver. Materials that fall under the scope of the EU Animal By-product (ABP) Regulations (EU Regulation 1069/2009 and EU Regulation 142/2011) are subject to rules aimed at protecting public and animal health. These Regulations require pasteurisation of AD raw materials or digestate at 70 °C for a minimum of 60 min with a maximum particle size of 12mm. The EU legislation allows for derogation from the requirement for a pasteurisation treatment in AD plants transforming manure and non-ABP materials such as fats, oils and grease, “provided the competent authority does not consider it to present a risk for the spread of any serious transmissible diseases.” The Irish Department of Agriculture, Food and the Marine, the competent authority responsible for adherence to EU ABP legislation, established an alternative pasteurisation standard for digestate, known as the “National Transformation Parameter”, 60 °C for 96 hours. The overall aim of this work was to determine the microbial sanitisation efficacy of the National Transformation Parameter when compared with the EU standard. Within this aim, the possibility for optimising the efficacy of AD as a tool for mitigation of the environmental impacts of agriculture was examined. Finally, a holistic analysis was undertaken of the potential for microbial, nutrient and metal transmission to watercourses, soil and grass, as well as gaseous emissions from landspreading of unprocessed slurry compared with slurry co-digested in AD. Initial storage experiments demonstrated the efficacy of slurry co-digestion with fats, oils and grease as a means of reducing faecal indicator bacteria. Miniature-scale trials were validated as proxies for investigation of faecal indicator bacteria (FIB) survival and biogas production where necessary for simultaneous examination of multiple variables. On that basis, 50 mL CSTR trials were established at different ratios of co-digestion feedstock, temperatures, retention times and loading rates. Response surface analysis was applied to model and optimise process parameters for different operational conditions. The model developed identified that with a combination of low organic loading and longer retention time, digestate sanitisation sufficient to satisfy EU standards is possible in AD at temperatures of 20 or 25°C, whilst also maintaining satisfactory methane production. The Irish AD industry predominantly utilises mesophilic CSTR of slurry co-digested with food production waste. Hence, the aim of optimisation of sanitisation and biogas production under those conditions was addressed. By changing the feeding regime from daily to a three-day system, biogas yield per gram VS fed was increased by greater than 50% and coliform and E.coli numbers were reduced below the EU pasteurisation standard. An initial examination of the metagenomic datasets demonstrated the changing community dynamics, with increased abundance and diversity of key hydrolysers and methanogens, as well as some interesting shifts in bacteriophage concentrations. Landspreading of unprocessed slurry presents risks of mobilisation during rainfall events thereby contributing to pathogen, nutrient and metal incidental losses. Field trials carried out as part of this work demonstrated the reduced microbial load from application of digestate from slurry co-digestion to grassland and consequent reduced runoff compared with unprocessed slurry. Pasteurisation at two conditions further reduced microbial contamination. These results have been used by project partners in a risk analysis to demonstrate reduced risk to human and animal health from landspreading of pasteurised and unpasteurised digestate, compared with slurry. Metal and nutrient analysis of soil, grass and runoff also demonstrated reduced pollution potential from digestate compared with slurry. Finally, a comparative examination of greenhouse gas and ammonia emissions following landspreading found 72% and 50% lower methane and N2O emissions respectively from plots treated with digestate compared with slurry. NH3 emissions were not significantly different between treatments but were higher than untreated controls, while CO2 emissions were not significantly different between treatments and controls. Taken holistically, this work highlights the efficacy of AD with or without pasteurisation as a means of reducing agricultural pollution. Where the requirement for pasteurisation is a prohibiting factor for development of agriculture-based AD, this work demonstrates the potential for optimisation of sanitisation through adjustment of operational parameters. In that scenario, processing of slurry with food production waste is a multi-beneficial solution to reducing the environmental impacts of unmitigated landspreading of animal manure slurries.