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

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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 3.9g CO2-eq to 1.3g CO2-eq, but increased emissions of nitrous oxide (N2O) from 0.6g CO2-eq to 2.0g 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.