Water Treatment: Thinking Small

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.

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