Effects Of Different Treatment-returning Modes Of Pig Farm Wastewater On Greenhouse Gas Emission

The sixth assessment report of the Intergovernmental Panel on Climate change（IPCC）emphasizes that agriculture is an important source of global greenhouse gas emissions,in which manure from aquaculture is one of the main sources of agricultural greenhouse gases,and China’s pig industry as one of the largest producers of manure,how to reduce greenhouse gas emissions from pig farms is of great significance for our country to implement the"double carbon"Strategy.There are four main resource utilization modes of pig farm manure in actual agricultural production:"Sewage direct return to field"model,"Sewage complete anaerobic digestion-field return"model,"Sewage incomplete anaerobic digestion-field return"mode and"Sewage complete anaerobic digestion-biogas slurry storage for 60 days-field return"mode.And there are few studies on the characteristics of greenhouse gas emissions under the above models.In this study,indoor simulation was used to study the greenhouse gas emission characteristics of pig farm sewage during fermentation,storage and returning to the field,and the relationship between the change characteristics of related environmental factors and greenhouse gas emission.The sewage treatment-returning model which is beneficial to the reduction of greenhouse gas emission was selected.The main conclusions are as follows:（1）In the"Direct return of sewage"mode,the total greenhouse gas emission equivalent is2557.19±62.56 mg CO₂e,in which the greenhouse gas emission equivalents of soil CH₄,CO₂ and N₂O are 0.39±0.07 mg CO₂e,2483.91±56.39g CO₂e and 72.89±6.10mg CO₂e;,respectively.Under this mode,the greenhouse gas equivalent produced by 1kg sewage returning to the field is18265.64±446.86 mg CO₂e.（2）Under the mode of"complete anaerobic digestion of sewage and returning to the field",the total greenhouse gas emission equivalent was 12314.41±251.88 mg CO₂e,in which the greenhouse gas equivalent produced by the soil in the anaerobic digestion stage was 10424.22±195.25 mg CO₂e;and the total greenhouse gas equivalent produced by the soil in the returning stage was 1890.19±56.63 mg CO₂e;1 kg and 41458.56±561.10 mg CO₂e.（3）Under the mode of"complete anaerobic digestion of unpolluted water and returning to the field",the total greenhouse gas emission equivalent was 34099.72±688.11 mg CO₂e,in which the greenhouse gas equivalent produced by the soil in the anaerobic digestion stage was 32002.38±631.48 mg CO₂e;and the total greenhouse gas emission equivalent produced by biogas slurry was 2097.34±56.63 mg CO₂e;1 kg in the returning stage.The total greenhouse gas emission equivalent was 95009.31±803.93mg CO₂e.（4）Under the mode of"sewage complete anaerobic digestion-biogas slurry storage for 60days-returning to the field",the total greenhouse gas emission equivalent was 4821.07±481.51mg CO₂e,in which the greenhouse gas emission characteristics in the anaerobic digestion stage was the same as that in the"sewage complete anaerobic digestion-returning"model,the greenhouse gas emission equivalent was 127.39±19.35mg CO₂e in the biogas slurry storage stage and 930.11±10.67mg CO₂e in the soil greenhouse gas emission equivalent in the returning stage.In this mode,the total greenhouse gas equivalent produced by 1kg biogas slurry is 28980.90±426.39mg CO₂e.（5）In the process of anaerobic digestion,the number of archaea in biogas slurry of complete anaerobic digestion is much larger than that of incomplete anaerobic digestion,and the stage of incomplete anaerobic digestion is the most active period of acid production by bacteria.the stage of complete anaerobic digestion is the most active period of methane production by archaea.In the process of returning different treated sewage to the field,the soil was in a flooded and anaerobic environment in a short time,and the abundance of archaea was relatively high,but with the absorption of soil and the passage of time,the abundance of archaea decreased gradually in the aerobic environment.The rest are some Digestive/Denitrifying bacteria related to soil digestion/denitrification.In a word,this study quantifies the greenhouse gases produced under different modes of sewage treatment-returning to the field.according to the greenhouse gas equivalent,the order of total greenhouse gas emission under each mode is as follows:"Incomplete anaerobic digestion biogas slurry-returning field"model>"Complete anaerobic digestion biogas slurry-returning field"mode>"Complete anaerobic digestion biogas slurry-biogas slurry storage for 60 days-returning to field"mode.It is suggested that large-scale farms/sewage treatment plants should first carry out complete anaerobic digestion and then put the biogas slurry for 30～60 days and then return it to the field.