Study On Removing Organic Matters From Backwash Water By Coagulation-PAC Adsorption-Microfiltration Process

Luan River was treated by two pilot-scale membrane systems (microfiltration and ultrafiltration) combined with coagulation process in Yangliuqing Water Treatment Plant, Tianjin. Operation data showed that water productivity of the microfiltration system was 90.9%, and that of the ultrafiltration system was 85.7%. To improve the water productivity of both pilot systems, a bench-scale coagulation-PAC adsorption-microfiltration system was adopted for treating the above-mentioned membrane backwash water (MBW). The distributions of organic compounds in different molecular weight (MW) ranges of Luan River, MBW, and treated water were tested. Correlation and linear regression analysis were applied to analyze the relationship of UV254 with DOC and CODMn, and the model of trihalomethanes formation potential (THMFP) was studied as well.The DOC distribution in MBW from the microfiltration system mainly distributed in MW fractions larger than 30 ku and lower than 1 ku, with proportions of 38.8% and 37.3%, respectively. A majority of UV254 and THMFP centralized in MW less than 1 ku. There is a positive correlation between SUVA and THMFP/DOC. Although small organics were the major THMs precursors, organics with MW between 3 ku~1 ku had the greatest reactivity to form THMs. When the MBW was treated by coagulation-PAC adsorption-microfiltration process, organic matters in treated water concentrated in MW less than 1 ku. Coagulation is effective to eliminate large molecular organics, PAC adsorption can get rid of organics with MW lower than 1 ku, and membrane separation plays an important role to remove THMs precursors with MW larger than 30 ku.The organics MW distribution in MBW from the ultrafiltration system was similar to that from the microfiltration system. The values of CODMn, UV254 and THMFP increased as the filtration time was prolonged. Obvious removal rate of UV254 was observed after adding PAC,which means PAC is useful to adsorb UV-absorbing constituents. Treated by coagulation-PAC adsorption-microfiltration process, DOC distribution in treated water is relatively even.In prechlorination period, the THMs concentration in finished water from coagulation-ultrafiltration system was far below the limits, which was regulated by Standards for Drinking Water Quality (GB5749-2006). The THMs concentration in MBW was about 100μg/L, but the THMs concentration in treated water maintained at 20~40μg/L. Chlorination disinfection was simulated in the laboratory, and the result revealed that after chlorination, the THMs concentration was linearly correlated with sodium hypochlorite dosages.The results of correlation analysis and linear regression made it possible to consider the UV254 valve as an index of organic matters in the MBW and treated water. The THMFP predictive model was also applicable for the MBW.