Ozone pretreatment upstream of microfiltration membranes; A pilot scale evaluation

Ozonation prior to microfiltration membranes was operated at the pilot scale to evaluate the benefits of enhanced organic carbon removal and advanced oxidation of Tuolumne River water in California. The pilot study was conducted to support the design and construction of a new treatment plant for the Turlock Irrigation District, an irrigation and power company in California. It was anticipated that pre-ozone in conjunction with alum would increase iron, manganese, color, turbidity, and organic carbon removal and improve membrane performance compared to coagulant addition alone. As a result of improved organic carbon removal, it was anticipated that a decrease in disinfection by-product (DBP) formation would be observed. Results from the pilot evaluation showed that although iron, manganese, and turbidity levels were significantly decreased through membrane filtration, the reduction was consistent with and without pre-ozone. Increased reduction in the specific ultraviolet absorption (SUVA), ultraviolet absorption at 254 nm (UV254), and total organic carbon (TOC) levels was observed in conjunction with a decrease in the total trihalomethane (THM) and haloacetic acid (HAA) formation for pre-ozone and alum addition compared to membrane treatment alone or with post-ozone treatment. Oxidation with ozone resulted in a reduction in organic carbon, a DBP precursor, and the chlorine dose necessary to provide adequate disinfection residual. Combined, these two factors reduced the total THM and HAA formed during 7-day simulated distribution system (SDS) evaluations. Impact of pre-ozone on membrane system performance was membrane system dependent. Pretreatment with ozone alone or combined with alum was found to significantly decrease the rate of membrane fouling for one system, but to increase the fouling rate of a second system. Fouling experienced by the third membrane system during ozonation was similar to the fouling observed without chemical addition. Solids carryover due to poor sludge drainage was likely the cause of some of the membrane fouling for the second membrane system. However, the first membrane system did not appear to be impacted by the solids carryover and continued to show improved performance during this period. Based on these piloting results, the recommended treatment train for the new water treatment plant for the Turlock Irrigation District includes pre-ozonation, alum coagulation, flocculation, plate settlers, microfiltration membranes, and post-chlorination.