Ultrafiltration processes for removing humic substances: effect of molecular weight fractions and PAC treatment

The role of humic substances in general and different molecular weight fractions of humic substances in particular on the membrane performance in terms of quality (chlorination byproducts) and quantity (permeate flux) is unclear. One of the goals of the present study is to evaluate the effect of fractionated humic substances on the ultrafiltration (UF) performance. A commercial humic solution was subject to gel filtration chromatography, and the resultant four fractions representing the apparent molecular weight (AMW) from 180 to 22 600 Daltons (Da) were filtered through a single module UF system, with a molecular weight cutoff 100 kDa. The fraction with the largest AMW (6.5–22.6 kDa) exhibits the worst flux decline with the best permeate quality, whereas the smallest AMW fraction (160–650 Da) exerts little effect on flux decline. Although the UF system is able to remove a significant portion of THMFP (trihalomethanes formation potential) in larger AMW fractions, the permeate THM in terms of μg THMs/mg carbon is still high. Thus, UF systems are not effective in removing disinfection-byproducts. The use of powdered activated carbon (PAC) for pretreatment of humic substances, or in combination with different UF operating systems, to reduce membrane fouling actually yields adverse effects. The PAC was ineffective in removing those AMW fractions less than 300 or greater than 17 000 Da; the higher the PAC dosage, the more removal in the middle range of thee AMW factions. The flux decline for PAC-treated streams is worse than the original humic substances. Similarly, the combined UF-PAC system significantly decreases the flux.