Greenhouse gas emission from covered wastewater storage tank on dairy farm in summer

Abstract: Global warming is mainly caused by emission of greenhouse gases such as CO2, CH4 and N2O to the atmosphere. Agriculture plays an important role in the increasing of global greenhouse gases emission with large numbers of livestock and poultry industries. This study focuses on investigating the emission dynamics of CO2, CH4 and N2O in covered storage process of manure slurry. The experiment was carried out during summer, in a dairy farm in Beijing, with a population of 3 400 cows. On this farm, dairy manure is cleaned by scrapers, and later solid-liquid separated. After separation, the liquid part is directly fed into the covered storage lagoon for anaerobic fermentation, while the solid part is used to produce organic fertilizer. A storage tank was built in the dairy processing area of ??the dairy farm with a floating film and a base membrane to prevent leaching and pollution of groundwater. The cover film kept floating on the surface and served to reduce on the smell nuisance and ammonia loss. After a certain storage period and stabilization in the covered storage tank, the digested sludge could be used onto the farm as fertilizer. The storage tank was 200 m in length, 25 m in width and 3 m in depth. There were 14 ventilation holes (5 cm inner diameter) on both sides along the length and width of the tank. Based on the enclosed condition, the amount of exhausted gas from this storage tank could be calculated. The test was conducted for 7 days from July 28th to August 3rd . The emitted gases were collected every 3 hours for the further composition analysis. Similarly, at every 3 h, slurry samples were also collected and analyzed for pH value, dissolved oxygen (DO), chemical oxygen demand（COD）and ammonium. The environmental condition was also recorded by parameters of humidity, pressure and air temperature. Results showed that, under this covered storage tank, the emitted concentration of CO2, CH4 and N2O were (634.01 ± 81.54) g/m3, (215.33 ± 18.59) g/m3 and (0.19 ± 0.07) mg/m3, respectively. The emission were (4.18 ± 0.53) g/(m2?h), (1.42 ± 0.12) g/(m2?h) and (0.0013±0.0005) mg/(m2·h), respectively. The CO2 equivalent of emitted CH4 and N2O were 35.50 and 0.0004 g/(m2·h), respectively. The contribution rates of emitted CO2, CH4 and N2O were 10.53%, 89.46% and 0, respectively. During this experiment, the emission of the CO2 and CH4 showed clearly positive correlation tendency with temperature, however, the relationships were not significant (P>0.05). It may due to the test period of the experiment was only during the summer, the seasonal change with a bigger temperature range may cause the more clear positive correlations, which need to be studied in future. With a low emission of N2O under anaerobic condition, the emission of CH4 plays the main role and should be well controlled. Thus, the appropriate method to effectively utilize the CH4 produced from the covered storage tank in the dairy factory has become the key issue for the environmental protection and renewable energy utilization.