Study On Preteatments Prior To Ultrafiltration Membrane And Membrane Pollution Control Mechanism Of Coal Gasification Wastewater Advanced Treatment

Coal gasification industry is one of important fields of coal chemical.However,a large amount of non-biodegradable wastewater is produced in the process of coal to natural gas,and the concentration of refractory organic pollutants in the effluent is still high after the long process anaerobic-anoxic-aerobic process is used,which is bad for zero discharge of coal gasification wastewater.Membrane technology has the advantages of compact equipment and simple operation,which can be used as an effective way for wastewater reuse,however,the membrane pollution problem has always been the main obstacle restricting the membrane reuse of wastewater.In this paper,aiming at the ultrafiltration membrane(UF)pollution arising from refractory organic pollutants in the coal gasification biochemical effluent reuse,the removal effect and the mechanism of refractory organic pollutants in the membrane pretreatment and ultrafiltration membrane treatment were studied.The following conclusions are drawn:1)The refractory organic pollutants in the biochemical effluent from coal gasification were mainly phenols,alcohols,esters and alkanes,the typical pollutants were hydrophilic organic compounds 2,4-di-tert-butylphenol,2-palmitoyl-rac-glycerin and hydrophobic organic compounds 2-Stearoyl-rac-glycerol and n-octadecane.The water quality showed the characteristic of cake fouling to ultrafiltration membrane.Hydrophobic organic compounds and hydrophilic organic compounds were the main reasons for the ultrafiltration membrane cake fouling,and the hydrophilic organic substances were the main reasons for the fouling in the ultrafiltration membrane pore.2)The combined process of polysilicate aluminium ferric sulphate(PSAFS)coagulation and H₂O₂ Fenton-like was used to treat biochemical effluent from coal gasification,and the degradation effect of hydrophilic organics in biochemical effluent of coal gasification was better than that of hydrophobic organics.The total COD removal rate reached 73.5%?77.6%,which was 25.2%?29.3%higher than that of traditional Fenton process.The residual iron form was changed with the p H,when p H was 6.5,the main forms of residual iron in effluent were iron polynuclear hydroxyl complexes such as Fe₂(OH)₂⁴⁺?Fe₃(OH)₄⁵⁺;when p H was 7.5,the residual iron in the effluent mainly existed in the form of iron polymer.The experimental results show that the effect of iron polymer as catalyst for Fenton-like reaction is better than that of iron polynuclear hydroxyl complex.3)The porosity formed by mesoporous and macroporous of the the semi coke prepared from low rank lignite accounted for 81.9%of the total porosity,and the surface functional groups were mainly benzene ring and halide.The combination of semi coke and PSAFS was used to treat biochemical effluent from coal gasification,and the removal effect of hydrophobic organic compounds were better than that of hydrophilic organics.Under the optimal process conditions,semi coke and PSAFS played a synergistic role in the removal of COD and TSS,the highest removal rates of COD and TSS reached 69.0%and 79.0%.At neutral p H,the coexisting system of semi coke and PSAFS reduced the energy barrier of four typical organic compounds,the results showed physical adsorption performance for hydrophobic organics and chemical adsorption characteristics for hydrophilic organics.Electrostatic neutralization and hydrogen bonding occurred between hydrophilic organics and PSAFS,by which dense flocs with larger granularity were formed in semi coke and PSAFS reaction system that was good for pollutants removal.4)The nano manganese titanate(Mn Ti O₃)semi coke hydrogels with semi coke as raw material was developed,in this material the pore volume increased significantly,forming a relatively complete mesopores and macroporous systems,and hydrogen bonds existed on the surface.Experiments showed that the photocatalytic assisted semi coke hydrogel loading of nano Mn Ti O₃-H₂O₂ had good removal effect on hydrophobic organics and hydrophilic organics,the removal rate of TOC was over 90%.In this process,the adsorption effect on hydrophobic organics was much higher than that of hydrophilic organics.About 90%of the hydrophobic organics were removed in the inner or surface of nano Mn Ti O₃ semi coke hydrogel;only about 40%of hydrophilic organics were removed due to the adsorption on the surface of nano Mn Ti O₃ semi coke hydrogel,about50%hydrophilic organic compounds were removed depended on hydroxyl radicals in the liquid phase.5)The mechanism of removal of hydrophilic/hydrophobic organic compounds by photocatalytic assisted semi coke hydrogel loading of nano Mn Ti O₃-H₂O₂ revealed:the photoelectrons and photogenerated holes were generated by Mn elements on the hydrogel in the role of visible light,and the photoelectrons were transfered on Mn-O-Ti,then active species were generated and organics were oxidized;H₂O₂ acted as an acceptor of photogenerated electrons so as to reduce the recombination of photo generated carriers and further to bring about more active species such as hydroxyl radical.At neutral p H,hydrophobic organics were adsorbed by hydrogels,and catalyzed by hydroxyl radical active species generated by hydrogel surface and oxidized by·HO₂ generated by Mn³⁺catalyzing H₂O₂;only one part of hydrophilic organics were adsorbed by hydrogel and catalyzed by hydroxyl radical active species generated by hydrogel surface,while the unadsorbed hydrophilic organics were catalytic oxidized by the active species in the liquid phase generated on the hydrogel surface.6)Compared with hydrophobic organics,hydrophilic organics were liable to lead membrane specific flux decline and irreversible pollution.Both weaken the cake fouling and control the irreversible and reversible pollution were important factors when the control effect of UF membrane fouling was evaluated aiming at pretreatments.Among the three pretreatments,PSAFS coagulation-H₂O₂ Fenton-like process had obvious effect on the reduction of internal resistance of membrane pore,and had better control effect on the irreversible pollution than reversible pollution;and semi coke-PSAFS process was effective to reduce the filter cake resistance,and with better removal effect on reversible pollution;the photocatalytic assisted semi coke hydrogel loading of nano Mn Ti O₃-H₂O₂process could effectively reduce the above two membrane resistances,and greatly reduce the irreversible and reversible pollution of the ultrafiltration membrane at the same time,and the average porosity of the cake formed on the ultrafiltration membrane was the lowest,the cake quality was the lightest,the cake layer resistance was the smallest,was therefore favorable for the UF pretreatment.