Greenhouse gas and ammonia emissions from digested and separated dairy manure during storage and after land application

Manure management at dairy production facilities, including anaerobic digestion (AD) and solid-liquid separation (SLS), has shown strong potential for the abatement of greenhouse gas (GHG) and ammonia (NH3) emissions. However, previous study results are inconsistent and the combined effect of AD + SLS remains to be quantified. This study evaluated the effects of AD, SLS, and AD + SLS on GHG and NH3 emissions during manure storage through land application over nine months. AD and SLS alone significantly (P < 0.05) reduced total GHG emissions for storage and land application compared to untreated manure slurries by 25% and 31%, respectively. The majority of that reduction was from methane during storage. SLS had a greater potential for methane reduction in storage than AD, but the variability in digester performance likely impacts the reduction potential. Digestion with subsequent separation further decreased CH4 emissions from 3.9 g CO2-eq to 1.3 g CO2-eq, but increased emissions of nitrous oxide (N2O) from 0.6 g CO2-eq to 2.0 g CO2-eq during storage eliminating a further reduction of GHG emissions as compared to AD alone. AD resulted in a gas emission tradeoff as it increased NH3 emissions by 81% during storage, which could be mitigated by subsequent SLS, manure storage covers, or other beneficial management practices.