Study On Optimization Of PAC-MBR Operational Efficiency And Carbon Footprint Accounting

In this paper,the membrane bioreactor（MBR）was used as the research object.Aiming at the problem that the MBR did not take into account the problem of reducing pollution and reducing carbon,six operating conditions were constructed under the conditions of gas-water ratio of 50:1,65:1,80:1,PAC dosage of 1.0 g/L and 2.0 g/L,and each operating condition was operated for 15 days.The relationship between the changes of pollutant removal efficiency and the related parameters of mixed liquid characteristics under different gas-water ratios and powdered activated carbon（PAC）was analyzed.The mechanism of the coupling effect of aeration and PAC to enhance the operation efficiency of the process and the mechanism of alleviating membrane fouling were revealed.The carbon footprint model of PAC-MBR combined process was constructed according to the microbial metabolic behavior in the reaction tank.With the help of IPCC（Intergovernmental Panel on Climate Change,IPCC）emission factor method and chemical measurement method,the PAC-MBR carbon emission accounting algorithm was constructed,and the carbon emission accounting was preliminarily explored.Based on the above research,FAHP（Fuzzy Analytic Hierarchy Process,FAHP）was used to comprehensively evaluate the operating conditions of PAC-MBR process,and the operating parameters that can take into account the treatment effect,operating cost and environmental benefits were obtained.The main conclusions are as follows:（1）The effect of gas-water ratio on the operation efficiency of PAC-MBR combined process was studied.The effects of different gas-water ratios and PAC dosage on the removal of organic matter,nitrogen and phosphorus in MBR were investigated,and the coupling effect of aeration and PAC on the membrane filtration cycle was analyzed.The results showed that:（1）The effects of gas-water ratio and PAC dosage on the removal of different pollutants were different.Under different gas-water ratios,PAC dosage had no significant effect on COD removal rate,which was maintained above 90%.Compared with the low dosage of PAC,the higher dosage of PAC can alleviate the effect of high gas water ratio on nitrogen removal.Under all gas-water ratio conditions,the phosphorus removal rate increased with the increase of PAC dosage.（2）The overall treatment effect of D₂-B（gas-water ratio 65:1,PAC dosage 2.0g/L）was the best,and the removal rates of COD,NH₄⁺-N,TN and TP were 92.73%,93.23%,69.34%and 74%,respectively.（3）Compared with high PAC dosage,low PAC dosage can slow down the process of membrane fouling.（2）The effects of gas-water ratio on the characteristics of PAC-MBR sludge mixture were tested and analyzed.The results showed that:（1）The coupling of aeration and PAC could improve the characteristics of the mixture to a certain extent.Under the conditions of 2.0 g/L PAC dosage and all gas-water ratios,the specific oxygen consumption rate and MLVSS/MLSS of microorganisms were higher,and the microbial activity was stronger.Under the condition of 2.0 g/L PAC dosage and low air-water ratio（50:1）,the stability of sludge flocs is strong and the sedimentation is good,and the total amount of EPS is low.At this time,the molecular weight distribution of various types of EPS is also at a low level.High PAC dosage mainly reduces EPS by adsorbing protein（Protein,PN）in EPS.（2）Under the condition of 1.0g/L PAC dosage and high gas-water ratio（80:1）,it is beneficial to the formation of large-size sludge flocs,which is helpful to alleviate membrane fouling.High gas-water ratio will promote the increase of extracellular polymeric substances（EPS）,which has a great influence on the content of polysaccharides（PS）in various types of EPS.（3）The carbon emission accounting algorithm and carbon emission accounting are established.The results showed that:（1）different gas-water ratio and PAC dosage had great influence on N₂O emission.When the dosage of PAC is 1.0 g/L,the gas-water ratio is proportional to the CO₂ emission of N₂O and relative aerobic area.（2）When the dosage of PAC was 2.0 g/L,the N₂O emission decreased first and then increased,but it was higher as a whole.（3）The increase of gas-water ratio and PAC dosage will promote CO₂ emission in the relatively aerobic area.The emission of CH₄decreased first and then increased,and it was strongly inhibited when the gas-water ratio was 65:1.Under the condition of low air-water ratio,the change of PAC dosage has a greater impact on CO₂ emission.In the relative anaerobic zone,when the gas-water ratio is 50:1 or 65:1,the endogenous metabolism of microorganisms can be weakened by increasing the amount of PAC,and the emission of CO₂ can be reduced.When the gas-water ratio is 80:1,this process shows the opposite trend.High gas-water ratio and low PAC dosage can inhibit CH₄ emission.（4）According to the operation of each working condition,indirect carbon emissions can be sorted from large to small as D₂-C>D₁-C>D₂-B>D₁-B>D₂-A=D₁-A.In general,PAC dosage has a stronger impact on total carbon emissions,ranking from large to small as D₂-C>D₁-C>D₁-B>D₂-B>D₁-A>D₂-A.（4）Comprehensive evaluation of PAC-MBR combined process operating conditions based on FAHP.The results showed that the scores of each working condition were ranked as follows:D₂-B>D₁-B>D₁-C>D₂-C>D₁-A>D₂-A.In other words,when other conditions were constant in this study,when the gas-water ratio was65:1 and the PAC dosage was 2.0 g/L,the process could take into account the treatment effect,environmental benefits and economic benefits.This study provides a reference method for the theoretical research of MBR treatment process in the field of pollution reduction and carbon reduction.