By Jack Flanders

After 25 years of frenetic growth, Houston has worked up a big thirst.

Finding a drink of water, though, isn't as easy as it used to be.

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To bring surface water into the water distribution system in broader fashion and fairly quickly, the city last year began construction of one of the state's largest water purification systems, a $97.6 million water treatment plant on the western shore of Lake Houston.

Owned by the city, the northeast Houston plant will provide treated water to the city of Houston, North Harris County Regional Water Authority, Humble and numerous other utility districts.

The treatment plant is a design/build/operate partnership between the city's Houston Area Water Corp. and MWH Constructors Inc. of Broomfield, Colo. Called the Northeast Water Purification System, the project is one of the largest design/build/operate partnerships in the nation, and one of the first to be used in a Texas public works project, Houston officials said.

"Choosing design/build/operate over the traditional design-bid process consolidates decision-making and oversight and reduces bidding, allowing a much faster construction schedule," said Todd Larson, PE, vice president of MWH and program manager for the Lake Houston project. "This process will produce a savings of about $80 million."

Once construction is completed in mid-June, MWH will manage the facility for 10 years with an option to continue managing for an additional 10 years.

Jeff Taylor, vice president of HAWC and deputy director of public utilities for the city, said officials chose the design/build/operate process because time is of the essence.

"Having one firm provide both design and construction saves a lot time, and time is money,'' Taylor added. "We anticipated that this process would save us between $50 million and $80 million, and based on what we've seen so far, we'll easily save that.''

Sitework began in spring 2002 on the 110-acre plant. When completed, the plant will be able to treat 40 million gallons of water per day, with the ability to expand to 400 mgd by 2030.

Larson said construction has moved quickly, due in no small part to the CAD 3-D designs MWH created early in the process. Easement acquisition and Corps of Engineers permitting took about nine months and were done concurrently with the detail design.

Only two setbacks have occurred, Larson said. The first came during excavation work in July 2002.

During that period it rained for nearly 70 days, making the select fill too wet to ensure dependability. To solve the problem, MWH poured a nearly 4-in. concrete seal slab on top of the soil to bolster the regular foundations.

The second problem came during construction of the raw water intake at Lake Houston. Original plans called for construction of a steel-sheet cofferdam, which would provide a dry space from which workers could tunnel the intake pipeline.
However, because the soil proved too unstable to support the tunneling operation, MWH had to abandon that plan and turn to surface barges with dredge lines.

Even with the setbacks, the project will be completed early, Larson said.

Work on the Lake Houston treatment plant began with construction of the single-level intake, which employs four adjacent screens about 20 ft. deep. The intake construction was the most technically challenging aspect of the project, said MWH construction manager Bob Lewis.

When the cofferdam process proved unworkable, rectangular FlexiFlow barge components were transported to the site by flatbed truck, then assembled. Three barges were used for the dredging work - two to support 110-ton American 999 cranes and the other to accommodate miscellaneous equipment.

The 84-in. steel pipe was assembled in sections, then floated on airlift bags - with crane support - to the proper location in the lake. It then was lowered into the underwater trench and secured with precast concrete weights and clamps.

Once the pipe connects with the raw water intake beneath the shoreline, it extends on shore another 190 ft. at a depth of 24 ft. It connects to the raw water pump station, where the lake water goes into steel pump barrels 30 ft. below ground level. The raw water pump station consists of two fairly small, windowless concrete block buildings - one housing vertical turbine pumps and the other power.

From the raw water pump station, water is pumped through 8,600 lin. ft. of 66-in. steel pipe to the main plant about 2 mi. miles away. The land was purchased by the city nearly 30 years ago.

At the treatment plant, the water enters a pumped flash system where chemicals are mixed with the raw water. It then flows into a distribution box, where the flow is split into five flocculation/sedimentation basins. The concrete basins, 11 to 13 ft. deep, are open and rectangular in shape.

"The water flows in serpentine fashion down a chute configuration that drops from 13 ft. to 11 ft.," Larson said. "This causes the dirt particles to attach to each other, making them larger and easier to settle out.''

At this point, the treatment process begins to utilize several innovative technologies, he added. From the flocculation basin, the water flows into a sedimentation basin, where floating sludge collectors are used.

"Normally in a sedimentation basin you would see a rake assembly, but to minimize moving parts we decided to use sludge suckers that are attached to floating pontoons,'' Larson said. "The sludge suckers move back and forth and siphon sludge off the bottom of the tank, much like a vacuum cleaner. It's a fairly uncommon process but one that requires less maintenance than a conventional rake system."

After sludge is removed, water enters a complex of eight enclosed, concrete filter cells, each with a 5.71 mgd capacity. The cell complex is where most of the project's sophisticated mechanical and electrical functions are centered. At this point, the system employs 15 in. of sand and 15 in. of anthracite for filtration and incorporates 50 electrically activated valves, three transfer pumps, two filter-to-waste pumps and two blowers. The blowers supply air for backwash.

Once water leaves the cells, it goes into the disinfection building that houses four ultraviolet in-line reactors, each with a 13.33 mgd rating. The reactors look like long steel pipes, each with seven ultraviolet bulbs at 45-degree angles. They are enclosed in a masonry block structure about 60 ft. long and 40 ft. wide.

Taylor said that to his knowledge, the Lake Houston ultraviolet system is the largest in America used in an industrial application. "When it comes to technology, this plant is way ahead of the curve,'' he added.

Once the water is treated in the ultraviolet cells, it is pumped to a 10 million gallon, prestressed concrete storage tank that rises about 30 ft. above ground. The tank's walls are 4 in. thick, Larson said.

A high-service pump station moves water from the tank to the five locations where it enters the city's distribution system. To reach the distribution points, MWH constructed about 12 mi. of 84-in. and 42-in. steel transmission main.

The plant complex includes a 23,000-sq.-ft. administrative building that will house computerized monitoring equipment, an auditorium, workshop and laboratory. The building has architectural masonry exterior, a metal roof and glaze-block windows.

Also housed inside the building is a bottling facility that will allow the plant to produce Houston's own version of private-label bottled water.

"Initially we'll bottle the water in 5-gallon containers and provide it to city offices around town," Larson said. "We'll also make it available at special events. We haven't figured out what to call it - maybe something like Bayou City Blue."

Consultants and subcontractors providing services on the project have included PBS&J, Binkley & Barfield Inc., Nathelyne Kennedy, Aviles Engineering Corp. and Geotest Engineering.

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