AWWA MTC64632

The Metropolitan Water District of Southern California is evaluating conventional treatment with and without pre-ozonation followed by biofiltration (O₃/BF) as possible pretreatment options for reverse osmosis (RO) salinity control of mildly brackish Colorado River water. The use of existing conventional treatment facilities instead of microfiltration (MF) would reduce the cost of implementing RO desalination, but this economic benefit may be offset by lower operating RO membrane flux and increased fouling. This project evaluated the following pretreatment processes to isolate the effect of different fouling mechanisms on membrane performance: MF - baseline condition; MF with pre-ozonation - established the effect of organic fouling with minimal colloidal and biological interferences; MF with pre-ozonation (MF-O₃) followed by biologically active dual-media filtration (biofiltration) - established the effect of organic and biological fouling with minimal colloidal interferences; coagulation/flocculation/sedimentation/filtration (conventional treatment) - established the effect of colloidal fouling; conventional treatment with pre-ozonation - established the effect of organics on colloidal fouling with minimal biological interferences; and, conventional treatment with pre-ozonation followed by biofiltration - established the effect of organics on colloidal and biological fouling. Silt density index and turbidity data showed MF and MF-O₃ to produce higher quality RO feed than the other pretreatment processes. However, the conventional treatment processes removed higher levels of total organic carbon and ultraviolet absorbing material, indicating organic fouling may be less problematic. RO performance data revealed potential interaction between aluminum chlorohydrate, the coagulant used during the conventional treatment processes, and Flocon 260 antiscalant that caused a rapid decline in specific flux. Additional RO data showed the MF processes generally performed better than the conventional processes. The addition of pre-ozonation and biofiltration appeared to decrease the efficacy of conventional pretreatment. Analysis of autopsied membranes supported the findings observations from the RO performance data. Includes 6 references, tables, figures.