Microbial Load, Physical–Chemical Characteristics, Ammonia, and GHG Emissions from Fresh Dairy Manure and Digestates According to Different Environmental Temperatures

This study evaluated chemical and physical parameters, volatile fatty acids (VFAs), pathogens indicators, ammonia, and greenhouse gas (GHG: CO2, CH4, N2O) emissions from fresh and digested dairy manure under controlled laboratory conditions, simulating storage at 18 °C and 28 °C. Manure and digestate samples were collected during summer 2023 from three dairy farms in Northern Italy, all operating similar mono-substrate, mesophilic anaerobic digesters at 42 °C with short hydraulic retention times (HRT) of ~30 days, instead of the longer HRTs commonly used (up to 90 days). Gas emissions were measured using a static chamber method over 40 min sessions, and cumulative GHG losses were converted to CO2 equivalents. Anaerobic digestion significantly increased ammonia emissions (p < 0.001), in comparison with fresh manure samples. Anaerobic digestion affected pH variations, while reducing CH4 and N2O emissions by up to 67% and 50%, respectively. Storage at 28 °C increased total GHG fluxes by 74% for fresh manure and 66% for digestate. Residual methane emissions suggest incomplete digestion, likely due to short HRT and low digestion temperatures. Among pathogens, only clostridia showed significant reduction post-digestion. Overall, anaerobic digestion effectively lowers the global warming potential (GWP) of dairy manure, but higher environmental temperatures exacerbate ammonia and GHG emissions during storage, highlighting the need for optimized post-digestion handling in warm climates.