Distribution Change And Oxygen Transfer Law Of Obstructed Gas Region During Sludge Aerobic Compost

The emission of harmful exhaust gas is one of the important problems restricting the development of the urban sludge aerobic composting industry.The generation and release of these harmful gases are related to the existence of the anaerobic zone in the compost,and the obstructed gas region is the main factor affecting the distribution of the particle anaerobic zone.Changes and the law of oxygen transfer are not clear.Therefore,in response to this problem,this study takes the sludge aerobic composting process as the research object,and carries out the composting process under different composting stages and process operating conditions（turning,adding bulking agent）,and the obstructed gas region of the composting material is carried out.The inversion and O₂ consumption first-order reaction rate constants were fitted and verified,and the spatial distribution of dissolved oxygen in particles with different surface oxygen concentrations during the composting process was studied.The following main conclusions are obtained:（1）With the progress of the composting,the ratio coefficient（β）of the material flow area of the compost increases gradually and then decreases,while the gas exchange coefficient（ω）of the two-region is opposite to it.Turnover treatment can increase the porosity of materials at each phase,increasing the mesophilic and thermophilic phases,but reduce theβof materials in the raw-material and curing phases.In the four stages,the gas exchange between the two-region slowed down,and theβof the materials were 0.62/0.60,0.58/0.60,0.52/0.54 and 0.59/0.55 in each stage（raw-material,mesophilic,thermophilic,and curing phases）before and after turning over,ωis 0.26/0.22,0.17/0.05,0.66/0.08,0.94/0.17,respectively.（2）There is a difference in the effect of adding a bulk agent on the balance of materialβandωat each phase.Compared with the non-added treatment（SR）,the addition of wheat straw（SRW）and peanut shells（SRP）slightly decreased and increased the obstructed gas region in the matrix,and both reduced the gas exchange between the two regions.Theβof the four-phases material of the SR/SRP/SRW is 0.58/0.58/0.61,0.58/0.55/0.58,0.56/0.52/0.57 and0.58/0.56/0.59,and theωis 0.04/0.01/10-6,0.19/10-6/10-6,0.21/0.34/0.09 and 10-6/10-6/10-6.In addition to the curing phase,the two-region flow model can better simulate the temporal change curve of O₂ in the small-scale test tail gas of the above-mentioned treatment.（3）With the increase of the surface O₂ concentration,the diffusion depth to the inside of the compost material gradually increases.When the surface O₂ concentration is2%/5%/10%/20%/40%,the depth of the aerobic layer of the three phases（raw-material,mesophilic,and thermophilic phases）compost material（from the outer surface to the inner surface）is 0/0/25/425/1225,0/0/325/450/2000,0/0/2000/2000/2000μm,corresponding to the thickness of the anaerobic layer（from the core to the outside）is 1975/1750/1825/1475/525,1875/1550/625/1175/0,and 1575/0/0/0/0μm,respectively.When the surface O₂ concentration in the raw-material and mesophilic phases is 48.7%and 40.2%,it can basically ensure that there is no anaerobic zone in the reactor,and the particle dissolved oxygen（DO）diffusion depth function(f _A(CO_2,m),f _AN(CO_2,im))combined with the two-region flow model of O₂ mass transfer-consumption in composting matrix,the mathematical model of the spatial distribution of the aerobic-anaerobic zone of the reactor material particles based on the two-region flow model can be obtained,which can be used to quantitatively simulate the particle anaerobic zone distribution in the matrix.