Study On The Mechanism And Control Technology Of The Fouling Of Ultrafiltration Membrane In Filtration Of Dyeing Wastewater

Dyeing wastewater,as the main source of water pollution,has the characterics of complex composition,high loading of organic matter,high colority,great change of water quality,and the low rate of recycling.With the increasing shortage of water resources,it is necessary to reuse the dyeing wastewater.Hybrid membrane technology of ultrafiltration and reverse osmosis is one of the effective technologies for the depth treatment and reuse of dyeing wastewater,but the ultrafiltration as pretreatment of the reverse osmosis process is very easy to be fouled,leading to the decrease of separation efficiency and the shortening of service life.Therefore,it is great importance to mitigate membrane fouling on the premise of the quality of the filtrated water.In this study,the fouling behavior of the ultrafiltration membrane in filtration of dyeing wastewater was first investigated by adopting the model of resistance in series.Both the fouling mechanism and main foulants were analyzed through calcaulating the resistances to permeation.Then,the effects of the pretreatments of coagulation and ozonation on the mitigation of membrane fouling and the efficiencies of different cleaning methods on the flux recovery of the fouled membrane were investigated to determine the optimum pretreatment method and cleaning procedure to motigate and control the fouling of ultrafiltration membrane in filtration of dyeing wastewater.The following conclusions were obtained:(1)The resistances to permeation of the UF membrane in filtration of dyeing wastewater could be determined through the model of resistance in series.The results demonstrated that the fouling of UF membrane in filtration of deying wastewater was manily due to surface fouling,which was attributed to the formation of cake layer on the membrane surface.The resistance of the surface fouling accounted for 75.3 % of the total fouling resistance,while the resistance of pore-blocking accounted for 24.7 % of the total fouling resistance.The results of single-factor experiments also indicated that the macromolecular organic materials of the slurry and additives such as PAM,PVA and CMC were the main foulants to the UF membrane in filtration of dyeing wastewater.(2)The results of coagulation experiments showed that the combination of ferric chloride and polyacrylamide exhibited the better coagulation efficiency when compared with other coagulants.The coagulation efficiency improved with increasing dosage of ferric chloride.Under the dosages of 300 mg/L ferric chloride and 2 mg/L polyacrylamide,the removals of COD and UV254 of the dyeing wastewater were 26.8 % and 13.1 % respectively.The optimum parameters for coagulation pretreatment were as follows: pH of wastewater was alkaline,temperature=20~30 ℃ and the settling time=100 min.After the coagulation treatment with 300mg/L of ferric chloride,the increasing rate of transmembrane pressure of ultrafiltration membrane declined from 55 kPa/h in filtration of the raw wastewater to about 6 kPa/h under the constant permeate flow of 75 L/(m2h).(3)The ozone oxidation experiments illustrated that the removal of organic material by ozone oxidation was largely affected by the pH of the wastewater,and increased with the increase of pH and changed slightly when the pH of the wastewater was higher than 11.The temperature of ozonation was set as at the room temperature since the effect of temperature on the ozonation was not obvious.After oxidation using 80 mg/L ozone,the removals of COD and UV254 were 21.8 % and 44.8 % respectively,and the increasing rate of transmembrane pressure of ultrafiltration membrane declined from 55 kPa/h in filtration of the raw wastewater to about32 kPa/h under the constant permeate flow of 75 L/(m2h).The effect of ozone oxidation on the mitigation of the fouling of ultrafiltration membrane was worse than that of the treatment of coagulation.(4)The supernatant fraction of the dyeing wastewater after coagulation treatment was subjected to ultrafiltration under the model of constant operating pressure of 0.03 MPa to investigate the efficiency of different cleaning methods.The optimum cleaning procedure for the ultrafiltration membrane was as follows: physical cleaning of backwashing using the produced water under 0.1 MPa for 2 min was performed with membrane after each filtration of one hour;chemical cleaning with 2000 mg/L of sodium hypochlorite aqueous solution of pH 12 adjusted with sodium hydroxide was performed after each filtration of 10.0 hours,through which,long-term stable operation could be attained for the ultrafiltration system.