AWWA MTC64546

The projected potable water demands for the City of Scottsdale indicate that the City will needto increase the supply of potable water by 2009. As such, the City is planning to expand the nettreatment capacity at the Central Arizona Project Water Treatment Plant (WTP) from 50 mgd to 80 mgd. The existingfacility uses a conventional treatment process (flocculation, sedimentation, and dual-mediafiltration). Because of space constraints at the site, it was not possible to expand the facilityusing a conventional treatment process.A process train consisting of high-rate clarification followed by membrane filtration (MF) wasselected 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 clarificationand MF facilities and to pre-quality vendors to participate in the bid process. As part of the pilot testingprogram, bench-scale testing was completed to evaluate membrane permeability ofclarified 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 themembrane. When this happens, the membrane flux cannot be recovered to its originalcondition even after chemical cleaning. There is a concern that some polymers can causeirreversible membrane fouling. Polymers have molecular weights ranging from 1,000,000Daltons 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 toregulate the addition of polymer during the pretreatment phase by only adding polymer that iscompatible with membranes at doses that minimize carryover to the downstream processes.During the bench-scale testing, commercially available ultrafiltration cells were used to test theperformance of flat-sheet membranes when variously pretreated waters are applied to themembrane surface. Samples were collected from the effluent of each of the three pretreatmentprocesses and filtered through the flat-sheet membranes in these cells to determinemaintenance of filterability.Testing in this manner is not a standard practice. This approach was used to reduce the costand duration of pilot testing and to determine a preliminary result on the effect of membranepermeability as a function of pretreatment. Although the bench-scale membrane tests cannotbe directly scaled up to predict membrane performance on a full-scale level, the testing providedsome 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 membranepermeability of the pretreatment alternatives. Includes table, figures.

Product Details

Edition:

Vol. - No.

Published:

03/01/2007

Number of Pages:

6

File Size:

1 file , 820 KB