Pilot Testing Microfilters and Cloth-Media Filters for Lake Augmentation

Microfiltration and cloth-media filtration were pilot tested for plant expansion and water supply augmentation purposes. The performance of the pilot units was assessed by evaluating key influent and effluent data, including suspended solids, turbidity, total phosphorous, hydraulic loading rate, flux, and chemical addition. Results showed that one of the cloth-media filters, and both of the microfiltration membranes tested could meet all forecasted requirements for augmentation, achieving lower than 0.5 mg/L total phosphorous on the cloth-media filter, and 0.3 mg/L on both microfilters. The microfiltration membranes were tested on both secondary effluent and filtered effluent. Sustainable flux rates on filtered effluent were approximately 40% higher than secondary effluent.

A recent long-range water-planning study concluded that Dallas Water Utilities (DWU) will need to reuse 120 mgd of treated wastewater effluent. It is envisioned that 60 mgd of this will come from the Dallas Southside Wastewater Treatment Plant (Southside), and this treated wastewater will be used to augment Lake Ray Hubbard, a source of raw drinking water for DWU.

The Phase IV Expansion of Southside will increase the plant’s average daily flow capacity by 30 million gallons per day (mgd) – from approximately 110 to 140 mgd. Water used for lake augmentation must meet stringent nitrogen and phosphorus limits, and this expansion provides a timely opportunity to produce effluent quality suitable for this reuse option. Phases I through III at Southside provide advanced treatment with nitrogen removal and effluent filtration. To achieve the necessary quality for lake augmentation, cloth-media filtration is being considered for Phase IV, and microfiltration is being considered for use throughout all four Phases of the plant to provide polishing for phosphorus and pathogen removal.

Pilot testing occurred over an eight-month (approximately) period between November 2005 and June 2006. This period was selected because it included the time of year with the coldest wastewater and highest viscosities, and thereby established worst-case scenarios for hydraulic loading rates on the filters and membranes.

The goals of the pilot study were to determine the basic design criteria for the wastewater treatment facility, to provide performance data, and to allow the Southside staff to become familiar with the technologies being considered. In particular, the study was to focus on the design flux rate for membrane filtration of both filtered effluent and secondary effluent, and on removal efficiencies of all technologies tested. Prior to testing, it was expected that sustainable flux rates of filtered effluent would be much higher than secondary effluent flux rates; but it was also possible that flux rates on secondary effluent would be high enough to warrant full-scale installation without filters.

Four technologies were piloted as part of the project: two cloth-media filters, a pile-cloth type, and the other a thin polyester type (hereinafter referred to as CMF1 and CMF2, respectively); and two microfiltration membrane systems, a pressure type, and the other a submerged vacuumtype (hereinafter referred to as MF1 and MF2, respectively). These treatment technologies were selected for testing based on their solids removal and polishing capabilities, and for the fundamentally different approaches designed by two manufacturers of the same technology.

The testing facility, shown schematically in Figure 1, allowed for multiple influent combinations for the microfilters. Under this arrangement, the microfilters could treat existing plant filter effluent (anthracite mono-media), secondary effluent, and pilot-scale cloth-media-filter effluent. Chemical addition was used upstream of the pilot units to precipitate phosphorous and enhance removal.

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