Pollution Characteristics Of Chemical Pulp Effluent And Its Tertiary Treatment With Catalytic Ozonation

The southern hardwood chemical pulp effluent contains high concentration of organiccompounds and presents deep color after physicochemical treatment combined withbiological treatment, and the biologically treated effluent could not meet emission standard.Furthermore, the effluent would contaminate receiving water and endanger ecologicalenvironment if tertiary treatment is ignored. In this research, pollution characteristics ofsouthern hardwood chemical pulp effluent and its treatment efficiency and mechanism ofozonation and heterogeneous catalytic ozonation were studied.Pollution feature, characteristic pollutants of southern hardwood chemical pulp effluentand its modification during biological treatment process were investigated throughconventional analysis, infrared （IR） and ultraviolet （UV） spectra, gas chromatography （GC）and mass spectrometry. It was found that in the effluent concentration of dissolved organiccompounds is high, with the dissolved chemical oxygen demand (COD_cr) and dissolvedbiochemical oxygen demand （BOD₅） of1.5kg·m^-3and0.4kg·m^-3respectively. In addition,the value of BOD₅/COD_cris0.27, indicating bad biodegradability of the effluent. An analysisfor the data from GC-MS shows that the effluent contains a large number of organiccompounds, mainly including lipoid substances, phenols, alkanes and organic acids, and theralative content of aromatic compounds is up to58.16%. Simultaneously, based on thephotobacterium EC50of the major organic compounds measured, the main pollutants to becontrolled in the effluent are phenols, chlorophenols, organic chlorides and phthalates.After biological treatment of hydrolytic acidification combined with active sluge process,COD_crand BOD₅removal rates of the effluent were67%and88%respectively, and BOD₅/COD_crof the effluent decreased to0.14, indicating great decreasement of pollution conditionof the effluent. Furthermore, organic pollutant species of the effluent obviously reduced andchromatographic peak area and strength greatly decreased after biological treatment. However,some recalcitrant organic compounds still exist in the biologically treated effluent which aremainly Di-n-octyl phthalate,6,6’-di-tert-butyl-2,2’-methylenedi-p-cresol and other lignindegradation products. These recalcitrant organic compounds contribute greatly to the residualCOD_crof the biologically treated effluent. Simultaneously, color of the effluent deepens after biological treatment, which may be ascribed to the generation of carbonyl and hydroxylgroups in the structure of organic compounds in the biological treatment process.In order to search for the feasible method of tertiary treatment and removal of recalcitrantorganic compounds of the effluent, reactor and flow of ozonation and heterogeneous catalyticozonation of the effluent were designed, and catalysts of alumina loaded with transition metaloxides were prepared through impregnation method. Then, treatment efficiency of the effluentwith ozonation and catalytic ozonation was researched. It was found that catalysts with stablephysical properties could be prepared under proper condition, and MnO_x, NiO_xand ZrO_xcould be stably loaded both on exterior and interior surface of alumina particles in the processof catalysts preparation other than TiO_x, which could only be loaded on the exterior surface ofcatalysts. Simultaneously, there are numerous micropores, microparticles or tiny slices on thesurface of the prepared catalysts, which accounts for the large specific surface area of thecatalysts. These properties of the prepared catalysts indicate that in the process of catalyticozonation, organic compounds and ozone in the effluent could closely contact with MnO_x,NiO_x, ZrO_xor TiO_xloaded on the surface of the prepared catalysts to initiate the reactions ofcatalytic ozonation.In the process of single ozonation of the effluent, COD_crremoval rate and reaction rateconstant both increased with the pH ascending, but organic compounds in the effluent couldnot be thoroughly mineralized, with COD_crremoval rates of41%and49.1%for pH of3.38and8.17respectively. Simultaneously, the effluent color was effectively removed throughsingle ozonation, with color removal rate of more than90%after45min ozonation.When the effluent was treated with ozonation catalyzed by the prepared catalysts, COD_crand color removal efficiency obviously enhanced and biodegradability of the effluentimproved evidently, compared with single ozonation. While catalytic performance of theprepared catalysts is not only dependent on the metal oxides loaded on surface of catalyst, butalso the preparation condition. It was found that COD_crremoval rates of the effluent afterozonation catalyzed by MnO_x/Al₂O₃-600and MnO_x-TiO_x/Al₂O₃-600for120min are72.6%and79.6%which are higher than single ozonation by23.5%and30.5%, and color removalrates are98%and99%respectively. Simultaneously, BOD₅/COD_crof the effluent increasesfrom0.14to0.59after ozonation for20min catalyzed by MnO_x/Al₂O₃-600, indicating obvious enhancement of biodegradability of the effluent.For the catalysts loaded with composite metal oxides, catalytic performance is dependenton the combined metal oxides and preparation condition of the catalysts. The preparedMnO_x-TiO_x/Al₂O₃and MnO_x-TiO_x-ZrO_x/Al₂O₃presented high catalytic ability and effectivelyenhanced COD_crremoval efficiency and biodegradability of the effluent during ozonation.An analysis for the data from IR spectra, UV spectra and GC-MS shows that recalcitrantorganic compounds of Di-n-octyl phthalate and6,6’-di-tert-butyl-2,2’-methylenedi-p-cresoland other organic compounds of the effluent could be effectively degraded and removed byheterogeneous catalytic ozonation, and chromophores of the organic compounds were greatlydestroyed, which obviously decreased the pollution condition and toxicity of the effluent.In order to search for the mechanism of heterogeneous catalytic ozonation, modificationof ozone concentration dissolved in effluent and in reactor outlet gas was investigated andeffect of radical scavenger on treatment efficiency was studied both for single ozonation andcatalytic ozonation. It was found that ozone concentrations dissolved in effluent and in reactoroutlet gas during catalytic ozonation was both lower than single ozonation, and that althoughthe effect of radical scavenger on treatment efficiency of single ozonation was unobvious, theexistence of radical scavenger in reaction system greatly reduced treatment efficiency ofcatalytic ozonation. Based on these results, it is suggested that during the process ofheterogeneous catalytic ozonation, ozone rapidly decomposes on surface of catalyst togenerate hydroxyl radical and ozone concentration dissolved in effluent accordingly decreases,which accelerates the diffusion of ozone from gas to effluent and reduces the ozoneconcentration in reactor outlet gas. Simultaneously, hydroxyl radicals generated react withorganic compounds in effluent, which results in rapid decrease of COD_crof the effluent. As aresult, ozone consumption increases, and ozone utilization rate and efficiency obviouslyenhance for catalytic ozonation. Consequently, it is reasonable to conclude that enhancementof removal efficiency of organic compounds during heterogeneous catalytic ozonation lies onthe continuous generation of hydroxyl radicals. The result is helpful to understand themechanism of heterogeneous catalytic ozonation.