Meeting Increasing Water Demand

May 20, 2008

Many communities in the water-stressed desert areas of the Southwest are turning to surface water sources for potable water supplies instead of overburdened aquifers. In 1969, the completion of the DeCordova Bend Dam on the Brazos River formed Lake Granbury, which now provides a surface water alternative.

However, the lake is brackish because of a subterranean salt bed outcropping, which underlies and contaminates the Brazos River. In response, the Brazos River Authority (BRA) established the Lake Granbury Surface Water and Treatment System (SWATS), a reverse osmosis (RO) desalination plant in Granbury, Texas. The resulting potable water is transmitted to the towns of Granbury and Acton through the Acton MUD distribution system.

Because of increasing demand, BRA needed to upgrade and expand the SWATS plant. The project focused on enhancements to the ultrafiltration (UF) system used to provide high-quality, prefiltered surface water as feed to an existing RO system. Once completed, the UF upgrade would increase the overall plant capacity.

BRA selected Layne Christensen Company’s Membrane Technology Group to perform the SWATS upgrade. The supervising engineer came from NRS Engineers.

During treatment, raw water is pumped from Lake Granbury to a clarifier, followed by recarbonation with carbon dioxide. It is then directed to either a 7.5-million-gal-per-day (mgd) sand filter or the membrane building, where it is treated by the UF system. The UF product water is then processed by a 7.5-mgd RO system, and any excess is delivered to finished water storage for use in the distribution network.

The project began in August 2006, and was substantially complete by October 2007. The company fulfilled the upgrade by building and installing a 1.8-mgd UF skid that uses vertically mounted Norit X-Flow membranes. The additional skid increased the overall system capacity from 7.3 to 9.1 mgd. A total of 80 membrane modules are used in each of five UF trains. The new UF train was used to take the production load off the original trains and to allow easy retrofit without interruption to the Brazos River Authority’s customers during the transition.

The retrofit of the existing UF trains was accomplished onsite as a construction retrofit project. Operational changes included replacing valves to reduce water hammer issues, engineering changes to the backwash system and control programming changes to the existing control system to optimize system performance.

Among the control upgrades were installation of a remote data collection and monitoring service, which allowed the observation of system operation and performance throughout the retrofit and subsequent testing activities. Additionally, Layne upgraded the PLC network to an Ethernet format and installed a VPN to allow remote access to the SCADA system and all PLCs in the UF system. This feature allowed for customer support and system adjustments on a virtually instantaneous basis.

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