Research On Treatment Of Organic Matter In Reverse Osmosis Concentrated Water

In recent years,the discharge of industrial wastewater has continuously increased,environmental regulation has become increasingly strict,and freshwater resources are scarce.The "zero discharge" process has become a popular choice for enterprises.The zero discharge project of coking phase IV wastewater carried out by a company in Shanghai used the technology of activated carbon adsorption,ultrafiltration,reverse osmosis,nanofiltration salt separation,freezing crystallization,ion-exchange membrane electrodialysis and evaporative crystallization technologies to complete the classified recovery and reuse of salt resources of industrial sodium chloride and anhydrous sodium sulfate.Among them,the concentrated water produced in the second round of reverse osmosis process contains 200～260 mg/L COD,which can adversely affect the electrodialysis membrane(ED membrane),shorten the life of electrodialysis membrane,and affect the continuous and stable operation of the subsequent process.Therefore,in order to reduce the content of organic matter in the RO concentrated water and mitigate its impact on the electrodialysis membrane,this work uses activated carbon adsorption,thermal activated sodium persulfate oxidation,activated carbon activated sodium persulfate oxidation,ozone oxidation,ultraviolet ozone combined oxidation and electrochemical oxidation processes to treat the RO concentrated water,and the aim is to reach the effluent COD of<150 mg/L by the company standard.Moreover,this work will also evaluate the influence of different parameters on the treatment process,and establish the optimum treatment plan through the comparison of treatment efficiency and economic cost.The main obtained results are as follows:(1)In the activated carbon adsorption experiment,granular activated carbon is applied.The effects of activated carbon dosage,initial pH and reaction time on RO concentrated water were investigated,and the molecular weight classification was also conducted on the organic matter in the concentrated water before and after adsorption.The research shows that the adsorption capacity of the studied activated carbon is unsatisfactory,and the required dosage is large.The optimal process parameter is at initial pH of 7.4,the reaction time is 1 h,the dosage of GAC is 40.0 g/L.The COD in the effluent is 120 mg/L,and the removal rates of COD is 33.3%.Molecular weight analysis shows that the distribution of different molecular weight organics in the concentrated reverse osmosis water before and after the reaction is relatively consistent,mainly less than 1 KDa.The adsorption scale of organic matter on this granular activated carbon mainly ranges from 1 to 3.5 kDa molecular weight,which is not suitable for the treatment.(2)In the sodium persulfate oxidation experiment,the investiaged activation methods are thermal activation and granular activated carbon activation.It was found that the initial reaction rate of activated carbon activation process was higher than that of thermal activation process due to the synergistic effect in the early stage of reaction,but the overall treatment efficiency by thermal activation process was better than that of activated carbon activation process due to the limitation of catalysis.The thermal activated chemical process is highly correlated with pH than the activated carbon activation process,and the oxidation efficiency is slightly better under acidic conditions.The optimal conditions of thermal activation process are at initial pH of 7.6～7.7,80℃,reaction time of 5 h,sodium persulfate dosage of 12.0 g/L.The effluent COD is 130 mg/L,and the removal efficiencies of COD and TOC are 48.0%and 48.1%respectively.The optimal conditions of activated carbon activation process are at initial pH of 7.7,reaction time of 4 h,sodium persulfate dosage of 8.00 g/L,and activated carbon dosage of 8.00 g/L.At this condition,the effluent COD is 140 mg/L,and the removal rates of COD and TOC are 40.4%and 47.1%,respectively.(3)The ozone oxidation and ultraviolet ozone combined oxidation experiments found that the ultraviolet ozone combined oxidation experiment had a synergistic effect of oxidation when compared with the simple ozone oxidation,and the oxidation efficiency was significantly higher.Compared to other pH conditions,ozone oxidation and ultraviolet ozone combined oxidation processes have better treatment effects under acidic conditions.The optimal parameters of ozonation process are:3 L/min gas flow,initial pH of 7.7,reaction time of 1 h and 30 mg/L ozone concentration.Under optimal condition,the effluent COD is 135 mg/L,the removal rates of COD and TOC are 48.1%and 11.3%,respectively.The optimal parameters of the UV ozone combined oxidation process are:initial pH of 7.7,45 min reaction,3 L/min gas flow,15 W UV lamp power,and 30 mg/L ozone concentration.The effluent COD is 145 mg/L,and the removal rates of COD and TOC are 44.2%and 15.1%,respectively.(4)In the electrochemical experiment,graphite+graphite,graphite+Ti/PbO2 electrochemical system were used.The experiments showed that the Ti/PbO2 anode was better than that of graphite anode,but the required time for the two systems to meet the required discharge standard,and the cost of Ti/PbO2 electrode plate was much higher than that of graphite electrode plate.Graphite anode was selected for subsequent research.Increasing the current density and decreasing the initial pH can improve the removal rate.The best experimental parameters are graphite+graphite electrode plate,the plate spacing is 1.0 cm,the stirring speed is 500 rpm,the current density is 30 mA/cm2,the pH of raw water,and the reaction time is 45 min.Under the above-mentioned condition,the effluent COD is 135 mg/L,and the removal rates of COD and TOC are 43.8%and 17.5%,respectively.(5)Three dimensional fluorescence spectrum analysis shows that the organics in RO concentrated water are mainly humic acid like organics,soluble microbial metabolites and certain fulvic acid like substances.Except for electrochemical oxidation,other treatment processes basically remove fluorescent substances,and some fulvic acid like substances still remain after electrochemical oxidation.(6)After comparison,sodium persulfate oxidation process,ozone oxidation,ultraviolet ozone combined oxidation and electrochemical oxidation process have relatively stable treatment effect.Among them,ozone oxidation process and electrochemical oxidation process are more economical.However,the electrochemical oxidation process is prone to produce toxic by-products,and the consumption and maintenance of the electrode plate need higher later costs.The final economic option is the ozone oxidation process.