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Located 20 miles south of Tucson, Ariz., the unincorporated active living community of Green Valley is home to more than 22,000 residents. With few surface water alternatives, the community is entirely dependent on groundwater and like many Southwestern communities, the available groundwater contains naturally occurring arsenic above the current regulated levels. Untreated, Community Water Co.’s fresh water withdrawals would be non-compliant with the U.S. Environmental Protection Agency’s (EPA) maximum contaminant level (MCL) of 10 ppb for arsenic in drinking water.
To assure compliant drinking water supplies, Community Water Co. awarded a contract to Layne Christensen Co. in September 2005 for the design and installation of arsenic treatment plants at four well sites, each utilizing co-precipitation, which is recognized as the best available technology for arsenic removal. The systems were designed to use LayneOx , a highly efficient granular catalytic filter media with a naturally high manganese dioxide content. The LayneOx process provides the ability to remove iron, manganese, hydrogen sulfide and arsenic, all in one cost-effective backwash process.
An important consideration with water treatment systems is disposal of the resulting concentrated contaminant. In this case, arsenic disposal is easily accomplished. The co–precipitation process combines arsenic with iron, resulting in ferric arsenate, which can be disposed of as non-hazardous.
The two largest of the four treatment systems (like the one shown pictured) are comprised of six 8.5-ft-diameter pressure filters, each containing 9 cu yards of LayneOx manganese dioxide filter media. The plant’s six pressure filters have a combined treatment capacity of 2,400 gal per minute.
The arsenic treatment plant shown here is comprised of a 3,000-gal sodium hypochlorite pre-oxidant storage tank and injection system, a 550-gal ferric chloride injection system, six 8.5-ft-diameter LayneOx filters, a 62,000-gal settling tank, a decant and sludge pump skid and a 300,000-gal storage tank for final treated water.
One unique aspect of this project is that Community Water Co.’s staff had assisted in the onsite construction of the Layne Christensen Co.-designed treatment plants and has gained an intimate knowledge of system operation, which assures optimal system performance and long-term system maintainability.
The system has been on-line since 2007, and the treated water contains arsenic levels below the 10 ppb MCL.