Overcoming the pathogen hurdle
Climate change, the energy crisis and the drive for agricultural sustainability have moved anaerobic digestion centre stage. Teagasc researchers are investigating and eliminating bacterial, viral and parasitic pathogens in feed stock and digestate.
Anaerobic digestion will sanitise organic waste while providing cheap, sustainable energy and fertiliser
Wastewater treatment sludge (WWTS) is the solid, semi-solid or slurry by-product of treatment processes in sewage plants. European legislation prohibiting disposal at sea or in landfill is encouraging the use of WWTS in agriculture, where other organic wastes, such as animal manure and slurries, are already used as natural fertilisers. However, the full potential of these waste materials – an essential part of the future circular economies mandated in the EC Green Deal – can only be realised if their use does not present a public or veterinary health risk.
Anaerobic digestion (AD), followed by pasteurisation, offers a means of ‘sanitising’ animal wastes and WWTS, while generating biogas and digestate, a nutrient-rich organic fertiliser. AD therefore offers future farmers and society clean and cheap fertiliser and energy, while solving the issue of WWTS disposal without compromising the health of the environment, animals or humans.
Optimising pathogen destruction
Unlocking the potential of this technology relies on pathogen destruction in the bioreactor and during subsequent processes. Teagasc research, in collaboration with University College Dublin, the University of Galway and the Technological University of Dublin, will investigate the fate of bacteria, viruses and parasites under different bioreactor conditions to inform the design of AD processes that optimise both pathogen destruction and biogas production.
This is not as simple as it might first appear, as biogas production is reliant on maintaining a healthy population of methanogenic bacteria in the bioreactor. The challenge is to selectively target one group of bacteria to be reduced or eliminated without adversely affecting another. Potential strategies include manipulating bioreactor conditions to maximise ammonia and volatile fatty acid concentrations, but research is required to ensure biogas production is not reduced. Increasing solid retention time would also enhance pathogen destruction, but the times required to completely sanitise the waste are currently too long.
Research findings
Our research has already shown that under the right conditions, mesophilic (approximately 30-40°C) digestion will reduce bacteria such as Escherichia coli, Salmonella spp., Yersinia spp. and Enterococcus spp., but other pathogens will survive. Some will even survive thermophilic (approximately 50-60°C) digestion, especially spore-formers. Post bioreactor stages in the AD process are therefore critical for pathogen control.
Current EU legislation requires a pasteurisation step (70°C for one hour or equivalent) to eliminate any E. coli to below the limit of detection and Enterococci numbers to below 1,000 colony-forming units per gram. Our data suggests dangerous bacteria such as Salmonella spp. and L. monocytogenes may be reduced by pasteurisation, but research on time-temperature combinations is required to optimise pathogen destruction.
Drying is another thermal process, undertaken to reduce the weight and volume of digestate during transport and storage. It also offers an opportunity to sanitise the digestate, but is not commonly practised in Ireland as it requires energy input and produces greenhouse gas emissions. Moreover, the digestate has a lower fertilising value, because nitrogen is lost through ammonia volatilisation.
The final stage in the process is storage; digestate is only applied to land at specific times throughout the year. It may be possible to manipulate the storage conditions and/or apply disinfectants, such as calcium oxide, to ensure the digestate is pathogen-free before land spreading.
To date, our collaborative studies have established the potential for pathogen destruction during AD and subsequent processes. The planned research for 2023–24 will quantify the pathogen load in animal wastes and WWTS and establish the bioreactor conditions for optimal pathogen destruction.
FUNDING
The Safe Waste project is funded by the Department of Agriculture, Food and the Marine (grant number 2021R453).
Contributor
Declan Bolton, Principal Research Officer, Teagasc Food Research Centre
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