Anaerobic digestion of untreated manure: Environmental risk assessment of resultant digestates

Anaerobic digestion (AD) of poultry and goat manure was performed to estimate the biochemical methane potential (BMP) and biosafety of the digestates using standard analytical and microbiological methods. The effects of residence time (RT) on BMP, process performance indicators, potential pathogens and indicator bacteria reduction as well as heavy metals concentrations were determined. The experiment was performed in a semi-batch mode at mesophilic temperature of 30 ± 0.2°C using 20 L prototype biodigesters over 45 days RT. The cumulative biogas yield from goat manure (31,703 ml/gVS) was > yield from poultry manure (30,275 ml/gVS). The process performance indicators after digestion revealed a minimal variation in pH (6.0 to 7.5) with notable reduction in total solids (55.0% in goat manure >50.6% in poultry manure) and volatile solid (56.1% in goat manure > 44.2% in poultry manure). Besides methanogens (Methanothrix, Methanobacterium and Methanosarcina species), Bacillus (100%) and Clostridium (87.5%) species were the most predominant bacterial genera. Sanitary assessment revealed a significant reduction (p < 0.05) of indicator and potentially pathogenic bacteria at residence time ≥ 30 days. At 45 days RT, faecal coliform, Staphylococcus and Vibrio species were undetected in both poultry and goat manure digestates, while total coliforms (3.6 log CFU/ml) and Salmonella count (3.2 log CFU/ml) in poultry manure digestate were above tolerable limit. A negligible amount (p < 0.05) of heavy metals was observed with higher zinc and copper concentrations in poultry and goat manure respectively. Extension of residence time and/or further treatment is critical to ensure digestate meets the United States EPA/EU permissible limit of 3.0 log CFU/ml before farmland application