You could think of it as a vast water bank account.

For the past eight years, the San Antonio Water System (SAWS) has used the Twin Oaks Aquifer Storage and Recovery (ASR) facility to ensure a reliable water supply to the city's 1.4 million residents, even in the face of drought.

The $250 million ASR system allows SAWS to store water deep underground during wet periods, then withdraw it during dry spells — such as the severe drought of 2011. The system, about 10 miles south of the city limits, includes a well field with 32 deep wells. When the system is in withdrawal mode, a high-service pump station on the site sends water through a 30-mile concrete pipeline to pumping stations on the city's north side for distribution to customers.

The 3,200-acre site also includes the 30 mgd Twin Oaks water plant, which treats water from the Carrizo-Wilcox Aquifer below the property, providing a supplement to the city's main water source in the Edwards Aquifer.

Caring for the facility around the clock are plant manager Roberto Macias and 10 crew members who monitor and operate the system and perform planned maintenance on the treatment plant, pumps and other critical equipment.

Use it or lose it

SAWS distributes on average about 200 mgd to San Antonio residents. The ASR facility was a key part of the system's 50-year Water Supply Plan addressing the need to develop and manage water resources for a growing population.

"Ten or 15 years ago, all our eggs were in one basket," says Macias. "The Edwards Aquifer was the sole source of water for San Antonio. We were basically mandated to start trying to find other sources."

Edwards Aquifer, one of the world's most prolific artesian aquifers, serves nearly 2 million water users in semi-arid south-central Texas. Water withdrawals from the aquifer are regulated by the Edwards Aquifer Authority (EAA), established by the State of Texas in 1996 and overseen by an elected board of directors.

"Every year, we purchase a certain allotment of water from the EAA," Macias says. "We have to predict how much we're going to use. If we don't use our full allotment, whatever is left just stays on the table and goes away. We can't roll it over."

The Twin Oaks ASR allows the city to claim its full allotment of Edwards Aquifer water every year. In wet years when the allotment is high, SAWS can store water in excess of its needs in the ASR. In dry years, when the aquifer level and water allotments go down, "We can take that water back," Macias says.

The water is injected into the Carrizo-Wilcox Aquifer, whose water is higher in iron and manganese and has lower hardness and pH than Edwards Aquifer water. "They are two very different waters," says Macias. "When we put the Edwards water in, it kind of pushes away the local water — it forms a 'bubble,' and there is very little mixing. We put it in at a controlled rate so that there is limited turbulence. It just pushes the other water away, and when we draw it back, the 'bubble' collapses."

Making it real

Turning the ASR from concept to reality took some innovative engineering. The planning process started in the mid-1990s, and work was completed in 2004. SAWS acquired 3,200 acres of farm and ranch land and ultimately drilled 32 wells 400 to 600 feet deep for water injection and withdrawal.

Edwards Aquifer water travels to the ASR site through the 60-inch pipeline from the Artesia, Seale and Randolph pumping stations. Because the ASR's elevation is 200 feet lower, the water arrives with significant force. Therefore, it first goes through a pressure-reducing station (Singer Valve) that consists of a manifold with throttling valves that can be opened and closed to control the pressure going out to the well field.

"Getting the water in is easy," Macias says. "It's like a big ball rolling downhill. It's just the opposite when we're sending water into town — we have to overcome 200 feet of head. So we have four huge high-service pumps, each one with a motor rated at 1,750 hp." (Motors from US Motors/Nidec Motor Corp., pumps from Goulds Water Technology). Each well pump in the ASR field has a 300 hp motor (US Motors).

The biggest operating challenge is keeping track of the water. "The state requires us to monitor every gallon of water we use or store," Macias says. "It's like an air traffic control system, keeping track of all the water going in or out. We basically share the wealth, so to speak, and evenly balance the water, so that we don't put too much in one well and not enough in another."

To that end, the operations team relies on a SCADA system supplied by Transdyn. The system is accessible from the well field and at the main plant, and the data is also transmitted to SAWS headquarters in the city.

Ensuring reliability

Macias and his team are charged with making sure the pumps operate when needed. The team members, drawn from the ranks of veteran SAWS employees, received extensive vendor-sponsored training, and most have dual state certification in water and wastewater operations. "We wanted them to have well-rounded backgrounds so that we would have the cream of the crop working here," says Macias who holds A-level certifications in water and wastewater and has 29 years of experience in water and wastewater treatment. Team members and their certifications are:

Andrew McBride, technician III, A water, B wastewater, 10 years with SAWSAnthony Woolfork, technician IV, C groundwater, A wastewater, 26 yearsArturo B. Valdez, technician III, C groundwater, B wastewater, 32 yearsClark Hebert, technician IV, C groundwater B wastewater, 20 yearsDavid Stoker, technician III, C groundwater, 10 yearsKevin Shoemake, technician III, C groundwater, C wastewater, 16 yearsManuel Huron, technician III, C groundwater, B wastewater, 23 yearsMarvin Gorzell, technician IV, B groundwater, B wastewater, 10 yearsRogelio Valero, trainee, D water, 8 yearsSharon Surra, technician III, A water, A wastewater, 2.5 years

The ASR facility operates around the clock regardless of the mode of operation. Four two- or three-member crews work 12-hour shifts. "They are constantly checking on the pumps and the pipes and taking water samples to make sure the water is within the standards required by the state and the U.S. EPA," Macias says.

Computerized maintenance management software (Infor Public Sector) tracks planned maintenance tasks and issues work orders. The main concern for plant operations is power outages, usually related to storms or lightning strikes. A 100 kW diesel-fueled backup generator (Generac) keeps plant control systems, lighting and other basic equipment operating through outages. An emergency system sized to operate the pumps would be too costly, Macias says.

Treating water

The other item on the operators' agenda is the Twin Oaks treatment plant. SAWS is authorized to withdraw 6,400 acre-feet per year from the Carrizo-Wilcox Aquifer, and this water requires treatment.

"We're lucky in San Antonio in that the Edwards Aquifer water doesn't require a lot of treatment — just chlorination," Macias says. "We have to treat the Carrizo-Wilcox Aquifer water to make it compatible with Edwards water, so that there are no problems in our distribution system."

In the treatment process, carbon dioxide and lime are added to increase pH, hardness and alkalinity. A step-feed aeration process removes remaining carbon dioxide, helps oxidize iron, and increases dissolved oxygen.

Polymer is added to the aerated water to assist coagulation of suspended solids into settleable particles. Potassium permanganate, hydrogen sulfide and chlorine are then introduced to oxidize manganese into an insoluble form that can be removed by sedimentation and filtration. Solids-contact clarifiers remove the settleable particles. Finally, dual-media filters remove remaining solids.

Successful performance

With an assist from the weather, the ASR facility has performed beyond expectations. Available storage in the aquifer is estimated at 200,000 to 400,000 acre-feet. "At our peak, the most we have ever stored is 95,000 acre-feet before we had to start tapping into it," says Macias.

"During six or seven of the years since the facility was built, it was wet here in San Antonio, so we were just storing water. But almost all of last year we were in a recovery mode because of the drought. We average about 24 inches of rain per year, but in 2011, we were about 14 inches below the average. The ASR really saved our bacon last year. Our studies show that we have enough water stored to survive the five-year drought of record."

The ASR site and aggressive water conservation measures help SAWS prevent water shortages. "It's cheaper to conserve water than to buy new sources," notes Sarah Gatewood, communications specialist with SAWS.

"We have water rules that mainly focus on outdoor irrigation. Year-round, watering is not permitted between 10 a.m. and 8 p.m. When the Edwards Aquifer drops below certain levels and the EAA limits how much of our permit we can use, we limit customers to watering during those same hours, once per week, on an assigned day."

While SAWS does not restrict indoor water use or water for business purposes, it does offer free faucet aerators, showerheads and low-flow toilets. The utility also works with businesses to customize water-efficient retrofits.

Water efficiency measures apply even on the ASR site. A cistern collects runoff from structure roofs, and that water is used for site irrigation, along with the treatment plant's filter backwash water. Site landscaping uses native plants that need little water. Most of the property is rented to cattle ranchers and to farmers for growing peanuts, strawberries and other crops.

Looking ahead

For the years ahead, SAWS is exploring renewable energy at the ASR site. One attractive option, Macias says, is a hydroturbine in the incoming pipeline from San Antonio to capture energy from the water flowing downhill. Another possibility is photovoltaic energy, as the site provides abundant vacant land that could host solar panels. The facility already saves energy by using high-efficiency motors with soft starts and variable-speed drives.

"Another interesting thing coming to the ASR area is a brackish desalination plant," notes Gatewood. The plant will draw water from 2,000 feet deep in the Wilcox Aquifer and treat it with reverse osmosis. "We are drilling the production wells right now, and we expect the plant to be online by 2016."

When that plant is complete, San Antonio will have yet another source of high-quality water for a growing and thirsty community.