The Cobb County - Marietta Water Authority employees in Marietta, Ga., know the value of consistency. At the public utility’s James E. Quarles Water Treatment Plant, many of the more than 30 operations and maintenance staff members have been with the company for 20 to 25 years. The plant’s division manager has been with the authority for 37 years. The utility’s consistent performance helped it to earn recognition as the best operated water system in the southeast U.S. by the U.S. Environmental Protection Agency’s District IV office.
When the authority decided to switch disinfection systems at the 59-year-old Quarles Plant, its goal was to select a consistently reliable disinfection system. The technology it selected has delivered exactly that.
Formed in 1951, the authority provides treated drinking water on a wholesale basis to 14 retail water customers, including the Cobb County Water System. Cobb County, located just north of Atlanta, is one of the fastest growing counties in the country, with population growth five times the national average from 1970 to 2005. As a result, the authority expanded the Quarles Plant from 64 million gal per day (mgd) to 86 mgd in 2003. Now serving a population of around 400,000, the plant utilizes a conventional disinfection/coagulation/flocculation/sedimentation/filtration process. It draws its water from the 430-mile-long Chattahoochee River, the water source for about half of Georgia’s population and for portions of Alabama and Florida.
In 2008, the authority began investigating alternatives to its use of chlorine gas for disinfection at its two plants: Quarles and the Hugh A. Wyckoff Water Treatment Plant in Acworth. When the Quarles Plant was first built in 1952, its location about 5 miles east of Marietta was rural. But with the region’s rapid growth over the past four decades, Marietta is now densely populated and the plant is surrounded by new homes, schools, a Cobb County government center and businesses. Like many other water utilities in quickly growing areas, the authority determined that the storage of chlorine gas cylinders at the plant was a risk that needed to be addressed.
Choosing an Alternative
After undertaking a whole life costs analysis between bulk and onsite-generated hypochlorite, the authority focused on onsite generation as its alternative disinfection of choice. Onsite systems from three leading manufacturers were compared.
In the end, according to the utility’s process engineer, Patrick Pherson, “we looked at what systems other utilities were using for onsite generation, including a facility in nearby Clayton County. Reliability and reputation were important to us, and so was after-sales service. With all that in mind and with the guidance of our design engineer, CH2M Hill, we chose the ClorTec system  from Severn Trent Services  .” With more than 3,000 installations worldwide, the ClorTec onsite sodium hypochlorite system is a proven, cost-effective, reliable and safe alternative treatment to chlorine gas and bulk sodium hypochlorite disinfection.
Three 1,500-lb-per-day ClorTec systems were provided by Carter/Verplanck in August 2009. To accommodate future expansion needs, space also was reserved for a fourth 1,500-lb-per-day generator. Without the ability to increase the plant’s footprint to accommodate the new equipment, the onsite system and the ancillary equipment, including water heaters, softeners, storage tanks and metering pumps, were installed where the gaseous chlorine storage and feed system had been housed. In addition, a temporary bulk sodium hypochlorite feed system was installed and used to provide uninterrupted operation of the Quarles Plant while the ClorTec systems were being installed.
Seven storage tanks are used with the system: two 72-ton brine tanks containing salt, four 18,400-gal 0.8% sodium hypochlorite solution tanks and one 18,400-gal dual-service tank capable of storing 0.8% sodium hypochlorite solution or 12% bulk sodium hypochlorite.
The efficiency of the onsite generation process is maximized when inlet water temperatures are between 65°F and 80°F, so three water heaters were installed. At a flow rate of 45 gal per minute (gpm), each water heater can raise the minimum temperature of supply water from 35°F to 65°F.
To eliminate calcium buildup caused by hard water, four water softeners with a combined maximum capacity of 188 gpm also were installed.
According to Pherson, the ClorTec generators have been operating well in the year and a half since their installation. Any operational issues that have popped up since the system’s installation have been handled with the assistance of Carter/Verplanck and Severn Trent Services representatives.
“The Cobb County - Marietta Water Authority works hard to be a responsible community citizen,” Pherson said. “That means investing in our system and our people so that our operations are running safely and efficiently. Replacing our gaseous chlorine system with onsite sodium hypochlorite generation was the right thing to do.”