| Municipal sludge and other solid wastes can be effectively disposed through aerobic composting.However,aerobic composting is accompanied by the greenhouse gas emissions,which may worsen the greenhouse effect.The anaerobic region of particles is affected by the air-immobile region.The presence of the air-immobile region hinders oxygen mass transfer and may expand the range of anaerobic region of particles,thereby exacerbating the greenhouse gas emissions.Therefore,observing the changes in the characteristics of the air-immobile region contributes to analyze the emission laws of greenhouse gases in composting.This study conducted sludge composting experiments under different oxygen supply strategies,mainly exploring the effects of ventilation rate and oxygen supply concentration on the characteristics of the air-immobile region and greenhouse gas（N2O and CH4）emissions,and establishing mathematical models for greenhouse gas emissions.The parameters of two-region model were obtained by gas tracer-parameter inversion to describe the characteristics of the air-immobile region and calculate the volume ratio of anaerobic region in particles.Then the impact mechanism of oxygen supply strategy on the emissions of N2O and CH4 were analyzed,and the quantitative relationship between the anaerobic region and the emissions of N2O and CH4were established.The conclusions were as follows:（1）As ventilation rate increased,the proportional coefficient of the air-immobile region（φ）,gas exchange coefficient inside and outside the air-immobile region（α）,and the average gas flow rate in the free air space（v）increased,while the volume ratio of anaerobic region in particles decreased,and the emissions of N2O and CH4 increased.The increase inαand v was the main reason for promoting N2O and CH4 emissions.For the pile with the ventilation rate of 0.2/0.4 L·kg-1 DM·min-1（DM represents dry mass）during temperature-increasing,thermophilic and curing phases,φvalues were0.43±0.08/0.47±0.04,0.46±0.03/0.48±0.05 and 0.42±0.04/0.50±0.02,respectively,αvalues were<0.001/0.006±0.003,<0.001/0.010±0.007 and<0.001/0.019±0.004 min-1,respectively.Finally,the cumulative emissions of N2O and CH4 from the treatments of0.2/0.4 L·kg-1 DM·min-1 were 5.07/8.93 and 157.91/223.58 mg·kg-1 DM,respectively.（2）As oxygen supply concentration increased,the values ofφincreased in the temperature-increasing and thermophilic phases while decreased in the curing phase,the values ofαshowed little change,and the volume ratio of anaerobic region in particles,N2O emission and CH4 emission decreased.The reduction of anaerobic region distribution was the main reason for limiting N2O and CH4 emissions.For the pile with oxygen supply concentration of 20.9%/40.0%,φvalues of temperature-increasing,thermophilic and curing phases were 0.40±0.10/0.44±0.10,0.47±0.05/0.52±0.11 and0.42±0.10/0.41±0.11,respectively,φvalues were both<0.001/<0.001 min-1 in the temperature-increasing and curing phases while were 0.003±0.003/0.002±0.002 min-1in the thermophilic phase,and the cumulative emissions of N2O and CH4 were4.11/3.33 and 167.99/111.26 mg·kg-1 DM,respectively.（3）Based on sludge composting experiments under different ventilation rates and oxygen supply concentrations,the mathematical model for N2O emissions was established using ventilation rates and the volume ratios of anaerobic,anaerobic and aerobic regions as independent variables,and N2O emission rates as dependent variable;the mathematical model for CH4 emissions was established using ventilation rates and the volume ratios of anaerobic regions as independent variables,and CH4 emission rates as dependent variable.The mathematical models of greenhouse gas emissions can be used to predict emission rates of greenhouse gases under different process conditions. |
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