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

Quantifying in-house emissions of methane (CH4) from liquid manure (slurry) is difficult due to high background emissions from enteric processes, yet of great importance for correct estimation of CH4 emissions from manure management and effects of treatment technologies such as anaerobic digestion. In this study CH4 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 CH4 kg-1 VS (volatile solids). Current estimates of CH4 emissions from pig and cattle manure management correspond to 0.032 and 0.015 kg CH4 kg-1, respectively, indicating that slurry pits under animal confinements are a significant source. Fractions of degradable volatile solids (VSd, kg kg-1 VS) were estimated using an aerobic biodegradability assay and total organic C analyses. The VSd 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 VSd in fresh excreta from pigs and cattle, respectively, had been lost at the time of sampling. An empirical model of CH4 emissions from slurry was reparameterised based on experimental results. A sensitivity analysis indicated that predicted CH4 emissions were highly sensitive to uncertainties in the value of lnA of the Arrhenius equation, but much less sensitive to uncertainties in VSd or slurry temperature. A model application indicated that losses of carbon in VS as CO2 may be much greater than losses as CH4. Implications of these results for the correct estimation of CH4 emissions from manure management, and for the mitigation potential of treatments such as anaerobic digestion, are discussed.由于肠道发酵过程会产生大量本底甲烷（CH4）排放，因此量化液态粪浆（slurry）的场内甲烷排放颇具难度，但这对于准确估算粪肥管理环节的甲烷排放以及厌氧消化等处理技术的减排效果至关重要。本研究从6家代表性养殖场的漏缝地板下方采集了20份猪粪浆与11份牛粪浆样品，并在采样后24小时内、于接近采样时粪污池内温度的条件下测定了甲烷产率。猪粪浆与牛粪浆的甲烷产率分别为0.030 kg CH4·kg⁻¹挥发性固体（volatile solids, VS）与0.011 kg CH4·kg⁻¹ VS，二者差异显著。当前针对猪、牛粪肥管理环节的甲烷排放估算值分别为0.032 kg CH4·kg⁻¹与0.015 kg CH4·kg⁻¹，这表明圈养栏舍下方的粪污池是重要的甲烷排放源。研究采用好氧生物降解测试与总有机碳分析方法，估算了可降解挥发性固体（degradable volatile solids, VSd，单位为kg·kg⁻¹ VS）的占比。猪粪浆与牛粪浆中的可降解挥发性固体平均占比分别为0.51 kg·kg⁻¹ VS与0.33 kg·kg⁻¹ VS；经估算，采样时猪、牛新鲜排泄物中分别有平均43%与28%的可降解挥发性固体已发生流失。基于本实验结果，对粪浆甲烷排放的经验模型进行了重新参数化。敏感性分析结果显示，预测的甲烷排放对阿伦尼乌斯方程（Arrhenius equation）中lnA值的不确定性极为敏感，但对可降解挥发性固体占比或粪浆温度的不确定性敏感度较低。模型应用结果表明，挥发性固体中的碳以二氧化碳形式流失的量可能远高于以甲烷形式流失的量。本文还讨论了本研究结果对准确估算粪肥管理环节甲烷排放，以及厌氧消化等处理技术的减排潜力所具有的意义。