Experimental Study On Treatment Of Silica Gel Wastewater By Coagulation-Nanofiltration

China is a country with water shortage, insufficient per capita water resources and serious water pollution. With population growth and economic development, the conflict between water resources supply and demand is increasingly obvious. Silica gel production is high water consumption industry and silica gel wastewater has always been a key source of pollution in the field of industrial production. In order to treat the silica gel wastewater and recycle the water, in this paper the key parameters of the process of treating silica gel wastewater with membrane was studied, which have reference to actual project.According to the characters of silica gel wastewater, coagulation was used as the pretreatment of nanofiltration. The effects of coagulant type, coagulant dosage and pH on silica removal were inspected in the coagulation experiments. With a series of tests, AlCl3 was screened out to be the best coagulant, and the operating conditions influencing coagulation effect were optimized. The optimal conditions of pre-coagulation were as follows: the dosage of AlCl3 was 120mg/L, pH value was 7.5-9.5, stirring time was 4min, standing time after being stirred was 60min. Under optimum conditions, the removal rate of silica was over 93%, Silica was lower than 20mg/L in the treated effluent.Based on the coagulation experiments, the influence of NF operation parameters(operation pressure, operation period and feed water temperature)on running status and membrane separation performance was explored in the experiments. Solution-diffusion model was used to explain the influence of operation pressure on membrane flux and separation performance: with the operating pressure increasing, the permeation flux increased and the removal rate of ions increased first and then decreased. Along with the operation period extension, the membrane permeation flux decreased and the removal rate of ions decreased, which revealed the effect of operation period to membrane. Analysis was made for the effects of feed water temperatures on membrane performance using solution-diffusion model and Arrhenius equation. In the test range, Na2SO4 rejection of NFⅠwas higher than 99%, permeate conductivity was lower than 170μs·cm-1; Na2SO4 rejection of NFⅡwas higher than 98%, permeate conductivity was lower than 370μs·cm-1, so the permeate quality can meet different requirement.According to the characteristics of the membrane process and fouling of the membrane, several kinds of cleaning strategies were applied during the experiments, and the SEM and the EDS analyses were carried out. The cleaning efficiency and the fouling mechanism were thus investigated. It was found that the reason of the nanofiltration membrane fouling was concentration polarization, and the flux was almost recovered after cleaning with 0.1%NaOH+1%Na2-EDTA and 0.2% HCl.It was shown that the Coagulation-Nanofiltration system was technically feasible, and the system can effectively remove silica, Na2SO4 and hardness. The permeate quality was close to municipal tap water, which can meet the reuse requirement of common enterprises. The Coagulation-Nanofiltration system is efficient way to treat the silica gel wastewater.