Water Treatment: Thinking Small

May 6, 2003
Editorial Emphasis

Environmental issues in the 21st Century, particularly those concerning meeting water demand and requirements, revolve around the problem of dealing with growth. As populations grow, as urban, suburban and ex-urban areas expand, demand for water increases and safety standards rise. An effective and creative way to deal with problems of growth and resource management might just be to think small.

A case in point is the Olivenhain Municipal Water District's (OMWD) treatment facility in San Diego County, California. The Olivenhain facility is the largest ultrafiltration water treatment plant in North America.

Yes, it's the largest, but read on. We are still thinking small.

Much has already been written about the Olivenhain facility and its use of one of the newer approaches to water treatment. The plant features ultrafiltration rather than traditional water treatment methods. Canada-based Zenon Environmental provided the membrane filters.

Use of ultrafiltration alone could be considered a step toward the future. Membrane technologies such as Zenon's ZeeWeed® have been seen to remove cryptosporidium, giardia and other bacteria with less chemical treatment. But a closer look reveals some other eye-openers in the details of the plant's design.

Tom Kennedy, the Olivenhain plant's operations manager, provides a look at some of those details. "There is no clear well and no forebay in this plant. Water comes in, it's in the plant for seven minutes, and it's gone."

Two considerations, both squarely in the realm of managing limited resources, dictated a plant design that had no forebay. One was real estate. Another was energy recovery. A relatively compressed site, combined with a need to conserve energy, resulted in a plant with a small footprint that maximizes limited land resources. It also resulted in a plant that generates nearly two-thirds of its own energy as the plant's pumps double as dam intake and turbine generators. Power generated is stored on site and used by the plant. A forebay design would have increased overall size while losing energy generation.

We're still not thinking small enough, though. One element that might seem insignificant on the face of it, but that makes the overall design work, is the use of static mixers. Static mixers are in general use, but the Olivenhain plant presents special think-small requirements.

Chris Martin of Boyle Engineering, the Olivenhain plant's designers, says, "Some chemical treatment is still required. Static mixers were necessary to ensure good mixing of the chemicals. And the relatively compressed site required shorter pipe length."

These requirements led Boyle to specify the Model 2800 Static Mixers by Westfall Manufacturing of Bristol, Rhode Island.

Martin continues, "We used the Westfall mixers because in cases of large piping, it is difficult to find mixers with a shorter lay length. The normally configured static mixer can be several pipe diameters long. With a 42-inch pipe, that can be very long.

"The membrane filtration used in Olivenhain meets the space requirements, and allows better treatment in a smaller footprint. This kind of design lends itself to situations where there is no on site storage."

The Westfall Model 2800 is a motionless static mixer in which fluids are injected and rapidly mixed by a combination of alternate vortex shedding and intense shear zone turbulence. Westfall's smaller design is a perfect fit for both the space and regulatory requirements of the Olivenhain plant.

Two 42-inch mixers and one 8-inch mixer are installed on the plant side, feeding all of the filter galleries. Water flows through the Zenon process to the outfall through two more 42-inch mixers.

"We want to measure the chemicals and maintain levels across distance (to the reservoirs, 4 1/2 and 6 miles distant.)," Martin goes on to say. "We have to monitor both upstream and downstream, to maintain levels for safe water for customers."

With a short detention time and an "on the fly" operation, mixing must be fast and it must be efficient, as well as space saving. Olivenhain plant manager Tom Kennedy puts it this way, "We can't vary the flow rate. That's why we had to have the static mixers. They mix well within about 8 feet, better than the manufacturer's specifications."

The space saving design of the Olivenhain plant maximizes limited real estate resources, while at the same time conserving energy. Ultrafiltration technology addresses the growing concern over water quality and safety. And the smaller, efficient static mixer makes it possible.

Innovative treatment technologies, right down to the smallest components, are the future of water treatment plants. Dealing with growing problems sometimes requires thinking small.

About the Author

Richard Merwin

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