The City of Phoenix conducted the Lake Pleasant Water Quality and Testing Study Project to assess the viability of various advanced treatment processes to meet anticipated drinking water regulations. Lake Pleasant is a man-made storage facility that stores Colorado River Water delivered via the Central Arizona Project (CAP) Canal. The water from the lake is released during the summer season to meet the peak water demands of the Phoenix Metropolitan area. Many water treatment facilities are being served by the CAP Canal and Lake Pleasant, including the City of Phoenix= existing Union Hills Water Treatment Plant (UHWTP) and the proposed Lake Pleasant WTP. The City conducted pilot and demonstration scale testing of various processes such as conventional treatment, microfiltration (MF), ultrafiltration (UF), granular activated carbon, nanofiltration, ballasted flocculation, and several preoxidants. These unit processes were configured in alternative treatment trains to optimize removal of turbidity, pathogens, disinfection by-product (DBP) precursors, and taste and odor compounds. The project will include potential stress testing of the treatment processes with simulated high algae and/or high turbidity raw water, during periods of lake releases. A significant component of this testing program was the assessment of residuals handling systems to treat waste streams from these various water treatment processes. Membrane treatment options were evaluated as a possible alternative to commonly used residual treatment processes like clarification. The membrane treatment residual treatment options evaluated include ultrafiltration of the decant stream obtained after settling of the microfiltration backwash residual stream. The recovered water from this process can be recycled without risking microbial contamination. Another innovative process that was evaluated was the treatment of microfiltration backwash using tubular UF membranes operated in hybrid mode. It was found that the hybrid mode operation of the UF system allows the backwash to be directly sent to the membrane without any settling, resulting in cost savings. This is compared to treatment of MF backwash by settling and subsequent treatment of the decant through a hollow fiber UF system. During each test cycle, the feed and permeate were monitored for TOC, turbidity, TSS, UV254, pH, and particle counts. Once per day, the feed, permeate, and backwash streams were also monitored for Crypto, Giardia, and Total coliforms. A chemical cleaning procedure was performed on the membrane modules at the end of each test to evaluate the effect of cleaning on flux recovery. The results provide an indication of the viability of the membrane treatment processes for the treatment of residuals from microfiltration backwash. The hollow fiber membrane was tested at an initial permeate flux of 60 gfd with different hydraulic cleaning frequencies to evaluate the effect of cleaning frequency on the evolution of transmembrane pressure. Permeate flux dropped to an average of 50.7 gfd after 50 minutes of filtration for the settled MF backwash. For the MF backwash that was sent directly to the membrane without settling, the permeate flux dropped to an average of 46.8 gfd after 50 minutes of filtration. The hollow fiber membrane was run at 80 minute cycles between hydraulic cleanings. Following 80 minutes of filtration, the flux consistently dropped to an average of 30 gfd for direct MF backwash with no settling. The tubular hybrid ultrafiltration membrane was tested at initial fluxes of 80 and 120 gfd with cycles of 50 minutes between hydraulic cleanings for the settled MF backwash. After 50 minutes of filtration the permeate flux dropped to an average of 47.8 gfd and 64.5 gfd for the respective fluxes. An initial flux of 53 gfd was tested with the direct MF backwash with no settling. After 50 minutes of filtration the permeate flux dropped to an average of 40.1 gfd. Operational parameters like flux, transmembrane pressure and temperature were taken every 15 minutes during the runs.
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