November 2007, Vol. 19, No.11

Plant Profile

Elmendorf

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Twin Oaks Aquifer Storage and Recovery Facility

Location: Elmendorf, Texas
Startup date: November 2004
Number of employees: 12
Design capacity: 30 mgd (114,000 m3/d)
Annual operating cost: $1.5 million


The 30-mgd (114,000-m3/d) Twin Oaks Aquifer Storage and Recovery Facility (Elmendorf, Texas) helps ensure sustainable water resources near San Antonio. The facility does this by capturing excess flows pumped from the Edwards Aquifer,
the region’s main drinking water supply, for storage and later use, as well as treating water from the Carrizo sand aquifer to make it compatible with Edwards Aquifer water. Carrizo water is typically higher in iron and manganese and has lower hardness and pH than Edwards Aquifer water.

The San Antonio Water System (SAWS), which operates the Twin Oaks facility, can withdraw up to 6400 ac-ft (7.9 million m3) per year from the Carrizo sand aquifer. Carbon dioxide and lime are added to the raw Carrizo water to increase pH, hardness, and alkalinity. A step-feed aeration process over three 10.7-mgd (40,500-m3/d) cascade aeration units removes any remaining carbon dioxide, oxidizes iron, and increases the dissolved oxygen concentration.

Next, polymer is added to the aera ted water to assist coagulation of suspended solids into large, settleable particles. Potassium permanganate and chlorine are then added to the water to oxidize manganese into an insoluble form that can be removed by sedimentation and filtration. Three 10.7-mgd (40,500-m3/d) solids contact clarifiers remove the settleable

Six dual-media (sand–anthracite) filters remove any remaining solid particles. Two backwash pumps and a filter backwash waste clarifier accommodate the backwashing of the filters.
The Twin Oaks Aquifer Storage and Recovery Facility (Elmendorf, Texas) can withdraw up to 6400 ac-ft (7.9 million m3) per year from the Carrizo sand aquifer. The water is treated first by cascade aeration to remove carbon dioxide, oxidize iron, and increase dissolved oxygen concentration. Photo by Ana Julia Pena-Tijerina.

Finally the water is chlorinated and stored in a 3-million-gal (11-million-L) clear well tank until it is needed.particles.

Storage Options
Even though the water supply demand in San Antonio fluctuates greatly between the hot, dry summers and the relatively cool, wet winters, what doesn’t change is the amount of water SAWS is legally allowed to withdraw from the Edwards Aquifer. Allocations from Edwards Aquifer are limited by legislation. There is no carryover or credit for pumping rights not used in a particular year, so to avoid losing the rights to that water, SAWS uses Twin Oaks.

The facility collects excess flows from the Edwards Aquifer so that extra water can be injected into the Carrizo sand aquifer for storage and later use. Such storage and recovery allows for flexibility in storage and management of water resources throughout the year. Using an aquifer instead of a surface reservoir also has some advantages. Underground storage allows less evaporation. Stored water is less vulnerable to contamination. Most land directly above the storage aquifer can be used regularly.

In the second stage of treatment at the Twin Oaks Aquifer Storage and Recovery Facility (Elmendorf, Texas), polymer is added and flocculation, coagulation, and settling all occur in a solids contact clarifier. Photo by Ana Julia Pena-Tijerina. During wet years, up to 30,000 ac-ft (37 million m3) of excess chlorinated Edwards Aquifer water is diverted to storage in the Carrizo sand aquifer (injection mode). When stored water is withdrawn from the Carrizo sand aquifer during dry periods (recovery mode), it is again chlorinated and then used to help maintain spring flows in the area and ensure protection of endangered species habitats.

By design, the 25-mi (40-km) pipeline that connects Twin Oaks to the SAWS distribution system always has flow moving
through it. The movement ensures that the water remains fresh.

Growing Supply and Demand
Phase 1 of the project, completed in 2004, includes the 30-mgd (114,000-m3/d) treatment facility, 16 wells, a high service pump station, and 30 mi (48 km) of main to convey water to local pumping stations.

Phase 2 of the project includes construction of 12 additional wells, interconnecting pipeline, a second clear well, and other improvements, which will bring the facility’s overall capacity to 60 mgd (227,000 m3/d). When Phase 2 is complete — which is scheduled for January 2008 — Twin Oaks will be the second largest aquifer storage and recovery facility in the United States.