Membrane Fouling Mechanisms Of Typical Organic Matters In Secondary Effluent By The Combined Process Of Coagulation-precipitation And Nanofiltration

At present,recycled water reuse is an important means to solve the problem of water shortage,and secondary effluent from urban wastewater plants is a common source of high-quality recycled water.The common regeneration depth treatment processes for secondary effluent from wastewater plants include coagulation and sedimentation,adsorption,oxidation and membrane technology.Among them,membrane technology is widely used in the secondary effluent reuse of urban wastewater plants with the advantages of high efficiency,small area and clean process,etc.The problem of membrane fouling is the focus of research and difficulty in the application of membrane separation technology at this stage.In this paper,the effect of direct nanofiltration,coagulation-precipitation-nanofiltration combined process on the treatment of typical organic matter and other pollutants in the secondary effluent under different water quality is compared and analyzed by using sodium alginate to simulate polysaccharide organic matter,kaolin to simulate turbidity,and Ca Cl₂ to change the ion concentration.The effects of three factors,namely,concentration of organic matter,turbidity and Ca²⁺,on the membrane flux and membrane fouling index were investigated,and the membrane fouling was characterized.The results showed that under different water quality conditions,the combined coagulation-precipitation-NF process could ensure that the removal rates of organic matter,turbidity,conductivity and Ca²⁺were maintained above 90%,96%,26%and 70%,respectively,and the removal effect of conductivity was most significantly improved compared with direct NF,which was also more easily affected by water quality conditions.Coagulation and precipitation pretreatment can significantly increase the specific flux of NF membrane,and the increase of organic concentration and turbidity will reduce the specific flux of direct NF and coagulation and precipitation-NF;while the increase of Ca²⁺concentration in water will lead to the trend of decreasing and then increasing the specific flux of membrane in both treatment processes;the concentration of organic matter and Ca²⁺concentration have more influence on the specific flux of membrane than turbidity.Under direct NF conditions,both reversible and irreversible resistance increased due to the increase of organic concentration;the increase of turbidity mainly caused the increase of reversible resistance,while the increase of Ca²⁺concentration mainly affected the irreversible resistance,so that the membrane fouling resistance showed a trend of first increasing and then decreasing.Coagulation and precipitation-NF mainly reduces the irreversible resistance to keep the fouling resistance of the membrane at about 5×10¹¹m^-1.Organic matter and Ca²⁺affect membrane fouling mainly by changing the compactness of the filter cake layer,and turbidity affects membrane fouling mainly by changing the thickness of the filter cake layer;coagulation precipitation-NF produces lighter membrane fouling,and can be basically removed by hydraulic flushing.Hermia membrane fouling model and x DLVO theory were applied to discuss the membrane fouling mechanism of the combined coagulation-precipitation-NF process.The results showed that in the direct NF process,the membrane fouling was mainly due to the intermediate blockage of organic matter at the beginning of the filtration stage and the cake layer jointly generated by contaminants at the later stage;while in the coagulation precipitation-NF process,the membrane fouling was mainly caused by the cake layer;the membrane fouling index was lower and the membrane fouling trend was smaller in coagulation precipitation-NF than in direct NF under different water quality conditions.Compared with direct NF,the energy barrier between contaminants in membrane feed water and NF membrane and between contaminants in membrane feed water and membrane surface contaminants both increased in the combined coagulation and precipitation-NF process,among which the increase of energy barrier between contaminants in membrane feed water and NF membrane was more significant,which effectively alleviated irreversible membrane fouling.