AWWA MTC64546

The projected potable water demands for the City of Scottsdale indicate that the City will need to increase the supply of potable water by 2009. As such, the City is planning to expand the net treatment capacity at the Central Arizona Project Water Treatment Plant (WTP) from 50 mgd to 80 mgd. The existing facility uses a conventional treatment process (flocculation, sedimentation, and dual-media filtration). Because of space constraints at the site, it was not possible to expand the facility using a conventional treatment process. A process train consisting of high-rate clarification followed by membrane filtration (MF) was selected for the expansion because of its reliability with respect to water quality and production. A pilot-testing program was developed to confirm the design criteria for the high-rate clarification and MF facilities and to pre-quality vendors to participate in the bid process. As part of the pilot testing program, bench-scale testing was completed to evaluate membrane permeability of clarified water produced by dissolved-air flotation (DAF) and sand-ballasted flocculation (SBF) processes. Membranes are sensitive to certain water treatment chemicals that may irreversibly foul the membrane. When this happens, the membrane flux cannot be recovered to its original condition even after chemical cleaning. There is a concern that some polymers can cause irreversible membrane fouling. Polymers have molecular weights ranging from 1,000,000 Daltons to 100,000,000 Daltons and are virtually 100 percent rejected in most microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes. As such, it is very important to regulate the addition of polymer during the pretreatment phase by only adding polymer that is compatible with membranes at doses that minimize carryover to the downstream processes. During the bench-scale testing, commercially available ultrafiltration cells were used to test the performance of flat-sheet membranes when variously pretreated waters are applied to the membrane surface. Samples were collected from the effluent of each of the three pretreatment processes and filtered through the flat-sheet membranes in these cells to determine maintenance of filterability. Testing in this manner is not a standard practice. This approach was used to reduce the cost and duration of pilot testing and to determine a preliminary result on the effect of membrane permeability as a function of pretreatment. Although the bench-scale membrane tests cannot be directly scaled up to predict membrane performance on a full-scale level, the testing provided some insight as to the compatibility of the pre-treated waters with membrane filtration. The paper describes the bench-scale testing materials (including polymers), testing schedule, and procedures used to evaluate the membrane permeability of the pretreatment alternatives. Includes table, figures.