Comparison of Air Emissions from Raw Liquid Pig Manure and Biogas Digester Effluent Storages

Biogas digesters are commonly used in livestock farming in China. The digestion process converts large amounts of raw liquid manure (RLM) to biogas digester effluent (BDE). The BDE is then stored on the farm for some time before land application as crop or orchard nutrients. Storage of RLM or BDE is associated with gas emissions, although little information is available concerning comparison of air emissions between the two handling practices. This study was conducted to compare methane (CH4), carbon dioxide (CO2), nitrous oxide (N2O), and ammonia (NH3) emissions from RLM and BDE storages using dynamic emission vessels (DEVs). Both media were stored in closed vessels (50 L) at a 40 cm storage depth, a constant storage temperature of 30°C, and a headspace air exchange rate of 15 to 17 air changes per hour (ACH) for 22 days. The results showed that the average daily gas emission rates for RLM and BDE, in mg L-1 d-1, were, respectively, 102.9 and 125.3 CO2 (p < 0.05), 0.72 and 3.33 N2O (p < 0.05), 29.2 and 0.32 CH4 (p < 0.05), and 1.21 and 0.66 NH3 (p = 0.08). The total greenhouse gas (GHG = CH4 + CO2 + N2O) emissions were similar for RLM and BDE, 1.05 and 1.12 g CO2-eq L-1 d-1, respectively (p = 0.26). Nitrous oxide (N2O) emissions accounted for 88.2% of the CO2-eq GHG emissions for BDE, whereas CH4 emissions accounted for 69.7% of the CO2-eq GHG emissions for RLM. The high N2O emissions from BDE likely resulted from the lower COD/N ratio in BDE than RLM under the storage conditions. Differences in gaseous emission characteristics between RLM and BDE were attributed to the differences in methanogen species and the population of ammonia-oxidizing bacteria (AOB).