Estimation of Methane Emissions from Slurry Pits below Pig and Cattle Confinements

Quantifying in-house emissions of methane (CH(4)) from liquid manure (slurry) is difficult due to high background emissions from enteric processes, yet of great importance for correct estimation of CH(4) emissions from manure management and effects of treatment technologies such as anaerobic digestion. In this study CH(4) production rates were determined in 20 pig slurry and 11 cattle slurry samples collected beneath slatted floors on six representative farms; rates were determined within 24 h at temperatures close to the temperature in slurry pits at the time of collection. Methane production rates in pig and cattle slurry differed significantly at 0.030 and 0.011 kg CH(4) kg(-1) VS (volatile solids). Current estimates of CH(4) emissions from pig and cattle manure management correspond to 0.032 and 0.015 kg CH(4) kg(-1), respectively, indicating that slurry pits under animal confinements are a significant source. Fractions of degradable volatile solids (VS(d), kg kg(-1) VS) were estimated using an aerobic biodegradability assay and total organic C analyses. The VS(d) in pig and cattle slurry averaged 0.51 and 0.33 kg kg(-1) VS, and it was estimated that on average 43 and 28% of VS(d) in fresh excreta from pigs and cattle, respectively, had been lost at the time of sampling. An empirical model of CH(4) emissions from slurry was reparameterised based on experimental results. A sensitivity analysis indicated that predicted CH(4) emissions were highly sensitive to uncertainties in the value of lnA of the Arrhenius equation, but much less sensitive to uncertainties in VS(d) or slurry temperature. A model application indicated that losses of carbon in VS as CO(2) may be much greater than losses as CH(4). Implications of these results for the correct estimation of CH(4) emissions from manure management, and for the mitigation potential of treatments such as anaerobic digestion, are discussed.