Using spades and shovels, workers are hand mining a 450-ft-long tunnel under Lake Houston Parkway in northeast Houston for massive pipes that will carry raw water from the lake to what will be one of the largest drinking-water treatment plants in the nation. The low-tech work is a sharp contrast to the design of the state-of-the-art drinking-water plant, its complex construction and how difficult it will be to treat the lake’s muddy water.

The water plant, the largest public works project in city history, is “enormously complicated,” says Paul Vranesic, project director for Houston Waterworks, a joint venture of Jacobs and CDM Smith. The JV is building the 320-million-gallon-per-day (mgd) expansion to the Northeast Water Purification plant under a progressive design-build contract with the city of Houston and its four partners.

“Bringing all of the players together has been key to the success of the project so far,” says Ravi Kaleyatodi, Houston water project director for the Northeast Water Purification Plant Expansion.

He admits there are ups and downs when working on such a huge project. “It’s a consensus-driven process and it has to be transparent.”

The $1.76-billion plant entails more than 1 million cu yd of earthwork, and 215,000 cu yd of structural concrete. It is one component of an overall $3.5-billion program that will also include a $350-million project to transfer water 23 miles from the Trinity River into Lake Houston and two sets of water transmission lines, costing $1.25 billion, to move the water throughout the region.

With the design 90% complete and construction and procurement at about 10%, more than 10 major subcontractors are on site, advancing some $300 million of early work packages, including Balfour Beatty, McCarthy, Fisk Electric, Barnard and Lazer. After the Houston City Council approves the final amended contract with the joint venture this summer, the remaining contracts will be awarded.

The project is necessary to meet the region’s growth—the metropolitan area, with about 6 million people today, is adding another million people every decade.

But just as importantly, the plant is designed to meet a goal of reducing groundwater use 80% by 2035 to stop the region from sinking.

From 1906 to 2016, all areas of Houston experienced some subsidence, and some neighborhoods sank as much as 10 ft. Since 2013, areas northeast of downtown have dropped almost 1 ft, according to the Harris-Galveston Subsidence District, which regulates groundwater withdrawals and has mandated the reduction of groundwater use.

The trend cannot be reversed, but it can be stopped.

Right now, the area gets 70% of its water from the region’s aquifers. When the water is removed, the soils compact and sink. The region’s clay soils mean that aquifers recharge very slowly—½ in. to 1 in. per year. Manual recharge through aquifer storage and recovery is difficult because of the layers of aquifers and the soils, says Mike Turco, general manager of the subsidence district.

The progressive design-build plant is the largest contract of its kind in the world, according to the team, and will allow Houston to begin treating raw Lake Houston water more quickly when it receives the first 80-mgd treatment train by the end of 2022. The completed project—all four trains with the potential to treat up to 448 mgd—will be finished in 2025, when the region must get at least 60% of its water supply from surface water.

The design-build approach has allowed the team to best utilize a tight 90 acres and to issue $300 million in early work packages after 60% of the design was completed.

The progressive design-build model was chosen because Houston wanted to be heavily involved in the design of the plant, says Kaleyatodi. The city was not involved with the design of the existing 80-mgd Northeast Water Purification Plant, which began operations in 2005. That plant has had difficulty treating the Lake Houston water.

The Houston Waterworks team held 240 meetings with owners and the owner advisor, Carollo. The design went through five or six iterations, says Randy Rogers, engineering manager for the team. “The plant we have bears little resemblance to the one we started with.”

The difficulties arise because the plant must treat huge quantities of “flashy” Lake Houston water at the end of the watershed. After the lake, the water goes into the Gulf of Mexico.

“This is the most challenging water I’ve ever treated,” says Rogers. During heavy rains, the lake can be completely flushed up to 12 times a day, changing the turbidity, pH, alkalinity and organic levels of the water each time.

To treat the water, the group had to think big and unconventionally. The water will be pumped from a 30,000-sq-ft pump station on Lake Houston through two 108-in. steel pipes to the plant. From there, a coagulation, flocculation and sedimentation process will remove the bulk of the inert solids and about 50% of the total organic carbon in the raw water. The plant will use plate settlers to minimize the sedimentation basin footprint, Rogers says. The settled water will be treated with ozone for primary disinfection and taste- and odor-removal purposes—the first use of ozone in Houston. The ozone will also break the residual organic matter down further, allowing it to be more readily assimilated by the downstream biologically active filters. The filters are 2,300 sq ft each and have a backwash rate of 75 mgd.

Filtered water will then flow to the post-filtration chemical addition basins where chlorine, fluoride and alkalinity-stabilizing chemicals will be added to produce potable water. Two 224-mgd transfer pump stations will then move the water to four 12-million-gallon above-ground concrete storage tanks from which the finished water will be pumped to two transmission mains, 84 in. and 120 in. All of this will take place over about four hours.

The plant’s residuals treatment will be even more complex, with the sludge from the sedimentation basins moved into gravity thickeners—the largest such thickeners used for  drinking-water treatment in the U.S., Rogers says—where the sludge will be concentrated. The thickened sludge will then be conveyed to four 44-in.-dia centrifuges, which can be used for either thickening or dewatering, and are the largest in the world to be used in a drinking-water plant, Rogers says. The centrifuges will further dewater the solids before they are hauled off to a landfill.

Houston Waterworks self-performed about half of the design, and, under a requirement from the city, trained local minority- and women-owned small businesses to design the other 50%, “None of whom has ever done anything like this before,” says Vranesic. “We’re leaving a legacy that these firms will be able to tackle complex endeavors like this in the future.”

The plant was developed in 3D using Microstation. Slices of those models were used as the design drawings, and drone photos are routinely superimposed on the model to make sure work is going according to plan.

“We find problems and we fix them for the cost of the fraction of what reworks would be,” Vranesic says.

Engineers revised some of those designs after Hurricane Harvey hit Houston in 2017. Floodwaters threatened to swamp the backwash pond at the existing facility, which would have knocked the plant out of operation and put the city’s entire drinking-water system at risk. The quick thinking of then-plant manager Drew Molly and his team, which stayed on site throughout the storm, saved the plant.

After the storm, the plant was redesigned to be at least 2 ft higher than the highest level Hurricane Harvey’s floodwaters reached. During a future such storm, the roads will act as drainage, and the processes will be islands standing above the water.

“From my perspective … the most important aspect of the new plant were modifications made to the elevations of critical infrastructure as a direct result of the impacts from Hurricane Harvey, says Molly, now the city’s director of  drinking-water operations.

Because the site is surrounded by neighborhoods, most of the equipment and materials will have to be assembled off site, including 35-ft valves that have to be assembled by cranes and moved to the site. Labor, which will reach a peak of 1,500, must be bused in.

The project is being funded by Houston and four partner agencies that will share portions of the cost, including the North Harris County Regional Water Authority, Central Harris County Regional Water Authority, West Harris County Regional Water Authority and North Fort Bend Water Authority. The group secured low-interest loans for the projects from the Texas Water Development Board, which will be repaid by customers.

More than 500 municipal utilities, including in Katy, about 50 miles west, will receive drinking water from the plant.

The Northeast plant is “a necessary project for us,” says Turco. “It’s a great example of the regulated and regulatory community working together to ensure an ample water supply.”