New plant increases city’s water options

San Antonio, Texas, residents used to rely on groundwater for their water supply, but development and ongoing droughts were jeopardizing the Edwards Aquifer that provided it. The city knew it would need an alternative water source that did not tax the aquifer – or the city’s coffers. Its answer was the country’s largest drinking water ultrafiltration system.

Developed in France, ultrafiltration is a mechanical process that treats surface water by forcing it through a thin plastic membrane. The membrane acts as a strainer, preventing unwanted and potentially unsafe particles from passing through, but allowing water, minerals and small molecules to pass. It filters everything smaller than .01 micrometers, including bacteria, viruses, algae and fungi.

Ultrafiltrated water requires less chlorine than is necessary with traditional treatment methods. Additionally, because of the sophisticated technology, the ultrafiltration facilities occupy less space – typically less than a quarter of that required by traditional plants.

The BexarMet Water District, the city’s local water utility, determined that long-term contracting would be the least expensive way to get its new plant. However, because Texas law proscribes long-term contracts, the district was forced to create a non-profit association – the BexarMet Development Corp. (BMDC) – which could do so. BMDC ultimately partnered with Harrington Park, N.J.-based United Water for a Design/Build/Operate (DBO) project, issuing tax-exempt revenue bonds to finance the plant’s $25 million construction tab. (The arrangement was the first use of tax-exempt financing of a DBO project in Texas.)

Once the plant began operating (January 2000), the BexarMet Water District could lease or buy back water from BMDC, which uses the proceeds to pay the debt service on the bonds. The plant processes 9 mgd, but its capacity can be expanded up to 27 mgd.

Finally, BMDC is working with United Water, the National Park Service, the Bexar Land Trust, the Southwest Independent School District, Alamo Community College and the Audubon Society to create a nature center on the plant grounds. Recreational hiking and biking trails have been constructed throughout the property.