Evaluation of pathogen concentration in anaerobic digestate using a predictive modelling approach (ADRISK)

Farmyard manure and slurry (FYM&S) is a valuable feedstock for anaerobic digestion (AD) plants. However, FYM&S may contain high concentrations of pathogens, and complete inactivation through the AD process is unlikely. Thus, following land application of digestate, pathogens may contaminate a range of environmental media posing a potential threat to public health. The present study aimed to combine primary laboratory data with literature-based secondary data to develop an Excel-based exposure assessment model (ADRISK) using a gamma generalised linear model to predict the final microorganism count in the digestate. This research examines the behaviour of a suite of pathogens (Cryptosporidium parvum, norovirus, Mycobacterium spp., Salmonella spp., Listeria monocytogenes, Clostridium spp., and pathogenic Escherichia coli) and indicators (total coliforms, E. coli, and enterococci) during mesophilic anaerobic digestion (M-AD) at 37 °C, pre/post-AD pasteurisation, and after a period of storage (with/without lime) for different feedstock proportions (slurry:food waste: 0:1, 1:3, 2:1, and 3:1). ADRISK tool simulations of faecal indicator bacteria levels across all scenarios show that the digestate can meet the EU standard without pasteurisation if the AD runs at 37 °C or a higher temperature with a higher C:N ratio (recipe 3) and a hydraulic retention time ≥ 7 days. The storage of digestate also reduced levels of microorganisms in the digestate. The Irish pasteurisation process (60 °C for 4 days), although more energy-intensive, is more effective than the EU pasteurisation (70 °C for 1 h) specification. Pre-AD pasteurisation was more effective for C. parvum, norovirus, Mycobacterium thermoresistibile. However, post-AD literature-based pasteurisation is most likely to assure the safety of the digestate. The information generated from this model can inform policy-makers regarding the optimal M-AD process parameters necessary to maximise the inactivation of microorganisms, ensuring adverse environmental impact is minimised, and public health is protected.