The city of Gastonia, N.C., water treatment plant was constructed in 1922 near what is now the center of the city. Gastonia, a city of 70,000 residents, is located 20 miles west of Charlotte in the state’s southern Piedmont region. Its 25.2-mgd water treatment 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.
Using gaseous chlorine for disinfection at the water treatment facility, the city had developed an efficient evacuation plan in the event of a large-scale gas leak as required by its EPA risk management plan. However, in the 1990s a three-story courthouse, a jail, and a social services building were built near the plant, with the jail and courthouse adjacent to the chlorine storage building. With an evacuation plan now affecting thousands of residents — including inmates — rather than just hundreds, the city knew it was time to either move the plant or change disinfection methods.
Considering the switch from gaseous chlorine
Even before the construction of the courthouse, jail, and social services building, the city’s public works and utilities department had been considering a switch to an alternative means of disinfection. And when the 9/11 terrorist attacks took place in 2001, increasing awareness of the potential hazard of transportation and storage of high-pressure chlorine cylinders, 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, [Charlotte, N.C.-based CDM], we performed a fairly sophisticated analysis using different price points for the cost of bulk sodium hypochlorite,” said Ed 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, CDM’s principal design engineer on the project, recommended the selection of an on-site sodium hypochlorite generating system from Severn Trent Services. (Click on process diagram above to enlarge.) Two 750-lb generating units were installed in January 2010, along with four 12,500-gal storage tanks. The equipment was provided by Premier Water in Charlotte. Max Foster, the company’s sales representative who worked with the city, also provided timely technical information and support.
The technology came highly recommended by another local water utility. The City of Hickory, N.C., had installed the state’s first on-site sodium hypochlorite system in 2003, and the system has provided ongoing operational efficiencies with low, predictable maintenance; increased safety; and demonstrable cost savings. “When we checked with other water utilities using various on-site systems, the system had the best reputation and history of long-term performance,” Cross said.
Advantages of on-site generation
The use of on-site sodium hypochlorite generation offers several advantages over gaseous chlorine for disinfection. The disinfectant is produced and stored in liquid form. Therefore, there is no danger of leaks from chlorine gas cylinders. It is also not necessary for facilities using on-site sodium hypochlorite generating systems to develop and maintain a risk management plan. HAZMAT training is not required for handling the disinfectant; nor is there any need for the use of self-contained breathing apparatuses. In addition, on-site sodium hypochlorite disinfection systems do not suppress finished water pH to the extent that gaseous chlorine disinfection does. Therefore, the amount of pH adjustment chemical (i.e., lime or caustic) necessary before distribution of finished water is reduced.
The on-site generation process is simple, as three common consumables are used in sodium hypochlorite generation: salt, water, and electricity. The system operates by feeding softened water into a brine dissolver. The salt dissolves to form a brine solution, which is further diluted to the desired salt solution and then passed through electrolytic cells. The cells apply a low-voltage DC current to the brine to produce the sodium hypochlorite. The solution is then safely stored in two of the 12,500-gal storage tanks. When it reaches the low-level set point, the system automatically restarts to replenish its supply. The 0.8 percent sodium hypochlorite solution is non-hazardous; the only by-product is hydrogen gas, which is safely vented to the atmosphere.
Safety of on-site generation
“From the beginning, safety was the driving force behind eliminating the use of gaseous chlorine at our facility” said Cross. “When the chlorine gas cylinders were being hauled away, we were jumping for joy. Choosing on-site generation for our disinfection needs was in line with our safety requirements, and the switch provided us with cost savings, too. When we applied for deregistration from our risk management plan and no longer had to perform HAZMAT training, we realized how significant the savings were. We haven’t completed a full cost analysis yet, but we are certain the system is providing long-term savings. We have also been impressed with the intuitiveness of the system’s software package, too. When there is an operational issue, the system tells you what’s wrong, and our staff has been able to handle every maintenance issue. The system has performed exactly as advertised, and that’s a testament to the technology and to the Severn Trent Services sales and technical team.”
