By Larry Grubb
Water district officials in the south supply area of Phoenix, Ariz., concerned over the risks in using chlorine gas to chlorinate their water supply, chose an alternative method - calcium hypochlorite tablets and erosion feeders. This decision resulted from a long-term water evaluation study the district commissioned in 1990 to examine the issue of risks.
The Phoenix Water District serves approximately 2 million people and is divided into the north and south supply areas. Combined, the two areas cover 240 square miles. Both the north and south areas used chlorine gas to chlorinate their water. Somewhat independent of each other, however, the south supply area began to explore alternatives. The south supply area chlorinates nearly 10 million gallons of water per day from 48 well sites.
Because the district serves such a large area, water district authorities were concerned about the amount of time it took to respond to a gas leak at a remote site (often several hours). Added to this was the city's requirement that four staff professionals respond to any leak.
As a replacement to the existing system, Phoenix officials selected a system that uses 3-inch calcium hypochlorite (cal hypo) tablets and erosion feeders. They installed the system at two well sites, two elevated storage tanks and one main station. The city chose to use a scale-inhibiting form of the cal hypo tablet because of concerns about hard water. The south supply area also upgraded its system within the past two years to include new technology that allows for improved automation, monitoring, chlorine analysis and paperless reports from various sites.
The technology behind erosion feeders - or chlorinators, as they are known - is based on the premise that a side stream of circulating water is directed into the chlorinator to erode the calcium hypochlorite tablet as it passes over a sieve plate and discharges through an outlet. Chlorination is controlled by the inlet water flow. A properly installed and operated erosion feeder will not feed any chlorine when the water flow is turned off, as the tablets do not soak in water. Since the chlorine solution produced is only about 200-400 PPM chlorine, severe over-chlorination is prevented.
With the calcium hypochlorite erosion feeders, a higher water flow through the chlorinator yields a higher chlorine delivery. The feeders are NSF 61-rated and come in different sizes, depending upon the amount of chlorine needed to sanitize and disinfect a water source. Furthermore, the feeders require little maintenance and have the capacity to operate for days without supervision.
Since switching to calcium hypochlorite and an erosion feeder system, the district's south supply area has reported good results. To date, the new system has reduced maintenance and manpower requirements, and eliminated worries about responding to gas leaks. The south supply area recently completed a three-month study to determine if the system outperformed its former water chlorination methods. The report found that the erosion feeder system provided accurate and dependable chlorine delivery.
According to Todd Milsap, maintenance mechanic II and acting treatment facility supervisor for the Phoenix Water District, 20 new tablet chlorination systems have been ordered for installation in the south supply area, bringing the total number of units to nearly 40.
"By far the greatest benefit we've realized is that our safety concerns have been virtually eliminated with the tablet chlorination system," said Milsap. "Chemical usage has gone way down, and that's good.
Key endorsements from the NSF 61, US Department of Agriculture (USDA), American Water Works Association (AWWA) and EPA assure the public that components used to manufacture the system will have no adverse health effects. Experts, however, remind users that standard storage procedures should be followed when using solid calcium hypochlorite. Product containers should be kept closed and stored in a cool, dry place. Solid calcium hypochlorite should not be disposed of in the trash, since it could react with organic materials such as greases and oils.
