Microfiltration (MF) is a unit process commonly used in treatment trains to treat municipal wastewater effluent to indirect potable reuse standards. Organic fouling of MF membranes affects flux, cleaning frequencies, and module replacement intervals. West Basin Municipal Water District's (West Basin's) Edward C. Little Water Recycling Facility treats non-nitrified secondary effluent for indirect potable reuse using polypropylene MF membranes. West Basin is installing new polyvinylidene fluoride (PVDF) MF membranes to increase filtration capacity. Ozonation is also being installed to improve MF reliability and membrane longevity by reducing organic fouling. Earlier pilot testing demonstrated ozone pretreatment of polypropylene MF membranes improved performance, but ozonation had not been tested with PVDF MF membranes. This project filled that gap by studying pretreatment of PVDF MF membranes by ozonation. Other aspects topics investigated were the effects of preozonation on reverse osmosis (RO) performance, ozone dose control using ozone effluent ultraviolet transmittance at 254 nm (UVT), and N-nitrosodimethylamine (NDMA) formation by ozonation. Testing included a control treatment train of MF with ferric chloride addition and special cleanings with chlorine and citric acid followed by RO. MF membranes in both treatment trains were operated at 27 gfd with a goal of 21 days of continuous operation, and the RO units were operated at 12 gfd and 80% water recovery. Initial tests included manual ozone dose control with average transferred doses of 8 to 10 mg/L. Unexpected inorganic fouling of MF membranes in the ozone train was likely deep pore fouling caused by subcolloidal manganese. However, this inorganic fouling was mitigated by two approaches: (1) adding 10 mg/L ferric chloride into the ozone MF feed and (2) controlling ozone dose based on ozone effluent UVT. These findings indicated ozonation could be used with PVDF MF membranes. While RO performance was initially better for the ozone train, by the end of testing both RO units were performing similarly. These data indicated there was no detrimental effect of MF pretreatment by ozonation on downstream RO. The ozone dose control strategy was generally successful at maintaining ozone effluent UVT within its target range, although there is room for optimization to minimize ozone generation costs while still maintaining acceptable MF performance. NDMA formation by ozonation was shown to be significant and highly variable (30 to 241 ng/L) and should be studied further to determine its potential impact on the design of advanced oxidation processes used to remove NDMA. Additional research is recommended to optimize ozone dose control in front of the polypropylene and PVDF MF membranes operating in parallel. Studying RO performance over a longer period of time would help determine if biofouling, which might not be apparent during the first several months of operation with new membranes, could manifest later.
展开▼
