Removal Efficiency Of Sudden Cadmium-Contaminated Water By Coagulation Adsorption-complexation Ultrafiltration Combined Process

With the rapid economic development,the application of cadmium and cadmium governance can not be synchronized,heavy metal cadmium pollution incident frequentlly breaks out of water.Although the complexation ultrafiltration process can deal with sudden heavy metal cadmium polluted water,the problem of membrane pollution and limited water treatment capacity for high-concentration sudden heavy metal cadmium polluted water have been restricting its wider application,so it is necessary to conduct process combination to seek efficient methods to remove Cd²⁺.Coagulation and powder activated carbon adsorption are identified as two key emergency water purification technologies.Therefore,in this experiment,the coagulation adsorption-complexation ultrafiltration combined process was adopted to treat raw water with simulated excessive cadmium pollution of 100 times,to explore the best process conditions that could make the effluent cadmium reach the standard and slow down the pollution of ultrafiltration membrane,and to provide basis for the engineering practice of water treatment with sudden heavy metal cadmium pollution.Firstly,the experiment changed the conditions of coagulant type,coagulant dosage,adsorbent dosage,adsorption time,initial concentration of Cd²⁺,etc.Based on the UV₂₅₄value of samples and the retention rate of heavy metal cadmium,the optimal conditions of coagulation adsorption pretreatment were studied.The experimental results showed that the coagulant with the strongest load resistance was PACl,and the optimal dosage was 20mg/L.The optimal adsorption dosage of PAC was 50mg/L and the optimal adsorption time was 30min.The removal rate of organic matter and the interception rate of heavy metal cadmium were the largest when adsorption followed by coagulation.Therefore,the optimal reaction sequence of the coagulation adsorption process is the first adsorption and coagulation.Sodium polyacrylate was used as complexing agent.The effects of adsorption-coagulation process,complexation ultrafiltration process and combination process on the retention rate of heavy metal cadmium,effluent water quality and flocculant morphological characteristics of simulated raw water polluted by heavy metal cadmium were investigated when the reaction system pH,humic acid concentration,inlet turbidity,initial concentration of Cd²⁺and coexisting cation concentration were changed.The results showed that the retention rate of Cd²⁺increased with the increase of pH value.When pH=7,the combined process makes the effluent cadmium discharge up to standard,and at this time,the removal rate of UV₂₅₄54 and TOC is the largest,the turbidity of effluent is the smallest,the average particle size of floc is the largest,and the coagulation effect is the best.Therefore,pH=7 is the best reaction condition;With the increase of humic acid concentration,the Cd²⁺retention rate of the water polluted by heavy metal cadmium treated by the three methods gradually decreases;Kaolin has a certain adsorption property,and it can remove cadmium metal when no agent is added.With the increase of water turbidity,the retention rate of Cd²⁺increased;The change of initial Cd²⁺concentration has great influence on the retention rate of metal Cd²⁺,but the removal rate of UV₂₅₄54 and TOC has little change;In the three ways,the concentration of Na⁺and Ca²⁺increases,and the retention rate of metal Cd²⁺decreases.The influence degree of Na⁺is less than that of Ca²⁺.When the ion strength is strong,the combined process can still make the effluent cadmium discharge up to the standard.However,for raw water with increased Ca²⁺concentration,the combined process could not make the effluent cadmium reach the standard.At last,the changes of membrane permeability ratio flux and membrane reversible pollution degree produced by ultrafiltration under various technological conditions were investigated experimentally,and the morphological characteristics of the cake layer on the membrane surface were observed by SEM.At the same time,PES ultrafiltration membrane with weak hydrophobicity and PVDF ultrafiltration membrane with strong hydrophobicity were found to have little difference in turbidity,basically no difference in removal rate of UV₂₅₄,and no influence on the retention rate of heavy metal cadmium.However,the TOC removal rate of PES ultrafiltration membrane was higher than that of PVDF ultrafiltration membrane.PES membrane surface filter cake layer is more compact than PVDF membrane surface filter cake layer,with more serious membrane hole blockage and more irreversible pollution.The PVDF membrane surface filter cake layer is relatively thick,but the structure of the filter cake layer is relatively loose,with high porosity and strong permeability,so the total membrane pollution is relatively large,and most of them are reversible membrane pollution caused by concentration polarization and coagulation layer resistance,which can be eliminated by hydraulic backwashing.