By Ali Giti
The City of Gastonia, NC, with 70,000 residents, is located 20 miles west of Charlotte in the state’s southern Piedmont region. Its 25.2-mgd water treatment facility was constructed in 1922 near what is now the center of the city. The facility uses a traditional disinfection / coagulation / flocculation / sedimentation / filtration process to treat surface water drawn from Mountain Island Lake. The man-made lake is fed by the Catawba River and is the primary source of drinking water for residents of Gaston County.
With gaseous chlorine as the primary disinfection method at the water treatment facility, the city was required to develop an evacuation plan in the event of a large scale gas leak under its USEPA risk management plan. However, in the 1990s a three-story courthouse, a prison and a social services building were built near the plant. In fact, the prison and courthouse were positioned adjacent to the chlorine storage building. The evacuation plan that once affected hundreds of residents now affected thousands – including inmates. As a result, the city knew it was time to either move the plant or change disinfection methods.
Alternative Disinfection
Even before the construction of the courthouse, prison and social services building, the city’s public works and utilities department had considered a switch to an alternative means of disinfection. When the 9/11 terrorist attacks took place in 2001, awareness of the potential hazard of transportation and storage of high-pressure chlorine cylinders increased, and the city redoubled its efforts to secure funding for the disinfection switch over. With funding finally assured in 2007, the city began investigating two alternatives: bulk sodium hypochlorite and on-site sodium hypochlorite generation.
“Under the direction of our design engineer, CDM (Charlotte), we performed a fairly sophisticated analysis using different price points for the cost of bulk sodium hypochlorite,” said Edward Cross, division manager, water supply and treatment for the city of Gastonia. “At the time of the analysis, the cost of bulk was relatively low – but now costs have risen again. The operational considerations were significant, too. With on-site generation, a shipment of salt would be delivered every six weeks, affording uninterrupted service in between. However, with bulk, shipments would be received every few days. The frequency of shipments would have required a lot of extra labor to coordinate and physically handle the incoming material.”
An additional benefit of on-site generation over the use of bulk sodium hypochlorite is that, because the unit produces sodium hypochlorite on demand, the technology alleviates the problem of chlorate by-product generation that typically results from the storage of bulk material.
After analyzing the two disinfection methods, Laurin Kennedy, PE, CDM’s principal design engineer on the project, recommended the selection of the ClorTec® on-site sodium hypochlorite generating system from Severn Trent Services. Two 750-lb generating units were installed in January 2010, along with four 12,500-gallon storage tanks. The equipment was provided by Premier Water in Charlotte, with technical information and support offered through the company’s sales representative, Max Foster.
