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In 2006, the W.R. Wise Water Treatment Plant in Greenwood, S.C., converted its disinfection system from gaseous chlorine to onsite generation of sodium hypochlorite (0.8%); from gaseous anhydrous ammonia to commercial aqua ammonia (19%); and from gaseous chlorine and solid sodium chlorite to liquid sodium chlorate and 78% sulfuric acid for onsite generation of chlorine dioxide.
The use of gaseous chlorine at water treatment plants has, for many years, been an effective method of disinfection. It remains by far the most common method used globally to disinfect water and wastewater and is the most commonly used disinfectant in treatment plants throughout the U.S.
The safety of chlorine gas has, however, come under increased scrutiny in recent years. At many plants, minor chlorine gas leaks occasionally occur with faulty valves and poor connections, and the risk for major leaks is always present. At low levels, chlorine gas can cause eye, skin and respiratory irritation, while exposure in high enough doses can be fatal.
In addition to the side effects related to chlorine gas exposure, the nature of the gas allows migration to distances well beyond the point of release. In addition, storing, moving, handling and changing the one-ton chlorine gas cylinders are cumbersome and potentially hazardous.
The regulatory compliance issues related to chlorine also are significant. The OSHA Process Safety Management Plan, which the Wise Plant was required to maintain, was voluminous. Another regulatory challenge was the U.S. Environmental Protection Agency’s (EPA) Risk Management Plan, which the plant was required to develop and maintain.
Decision to Convert
The decision to undertake the conversion project at the Wise Plant was driven by the result of vulnerability assessment recommendations in 2002. The utility was looking to develop a flexible alternative disinfection strategy throughout the plant that would enable the facility to further ensure operator and community safety, reduce Hazmat and PPE training and meet EPA regulations concerning disinfection byproducts (DBPs)—regulations that no longer allowed the use of free Cl2 as the primary disinfectant.
The Wise Plant evaluated several alternative disinfection options including free chlorine, chlorine dioxide, chloramines and ozone. Ultimately, the Wise Plant decided to install a 1,500-lb/day ClorTec onsite sodium hypochlorite generation system from Severn Trent Services. The ClorTec system offers significant benefits over competitive onsite systems—particularly its ability to improve water quality through the reduction of total trihalomethanes and haloacetic acids levels and improvements in chloramine levels.
The flexibility of the selection paid off immediately as the plant was able to utilize existing space to save costs. The chlorine gas cylinder storage area was used for the sodium hypochlorite storage area and brine tanks. The gas cylinders’ handling area was used to house the new liquid feeders for sodium hypochlorite.
Generating sodium hypochlorite on site is a simple and straightforward process that uses three common consumables: salt, water and electricity. The onsite generation system selected by the Wise Plant 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. The salt solution is then passed through electrolytic cell(s), which apply a low-voltage DC current to the brine to produce the sodium hypochlorite. The sodium hypochlorite is then safely stored in three bulk storage tanks. When it reaches the low-level set point, the system automatically restarts to replenish its supply. Three liquid feeders inject the hypochlorite as needed.
In addition to significant improvements in water quality, the use of onsite sodium hypochlorite generation offers several advantages over the use of gaseous chlorine for disinfection. First, the disinfectant is produced and stored in liquid form. The 0.8% solution generated by an onsite system is nonhazardous, eliminating the need for system users to develop and maintain an EPA Risk Management Plan. Hazmat training is not required for handling the disinfectant, and there is no need for the use of self-contained breathing apparatuses with onsite sodium hypochlorite disinfection systems. In addition, onsite 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 onsite generation process also is a cost-effective disinfection alternative. A pound of chlorine equivalent can be generated on site using salt, water and electricity for as low as 20 cents/lb of chlorine equivalent (2006 pricing). Systems typically provide a return on initial investment within three to five years.
Dramatic Reductions, Improvements
Once the new disinfection system was online at the Wise Plant, there was a noticeable improvement in water quality, including a dramatic reduction in total trihalomethanes and haloacetic acids levels, improvement of chloramine levels in the distribution system, a reduction in flushing required at dead-ends and savings through the reduced need for post-pH adjustment chemicals.
Annual flushing has been reduced by 76% and the flushing process takes less time, equating to a cost savings of $12,688 per year. Post-pH adjustment with lime or caustic has resulted in a reduction of 30 lb-per-mgd, a savings of $11,436 per year.
The Wise Plant used chlorine dioxide as the primary disinfectant, which was key to reducing DBPs prior to coagulation. The 0.8% sodium hypochlorite and chloramines used before and after filtration, respectively, provided more stability in maintaining the chloramine residuals in the distribution system with no additional production of DBPs. The 0.8% hypochlorite is more stable and much easier to feed compared to gaseous chlorine, and less chlorine is lost to the atmosphere because of better mixing and retention in the treated water as compared to gaseous chlorine.
The conversion from gas to liquid disinfection at the Wise Plant has proven to be extremely cost-effective. The onsite sodium hypochlorite generation system provides water that is free of pathogens, reducing acute risks; eliminates the production of DBPs, dramatically reducing chronic risks; and maintains a residual disinfectant in the distribution system for continued pathogen deactivation. In addition to the aforementioned cost savings, the liquid disinfectants are safer and less hazardous to handle. The onsite system is more stable and consistent than the gaseous chlorine system it replaced, all the while reducing the risk for exposure to the staff and community.
The 2006 conversion project reflects the Greenwood Commissioners of Public Works’ commitment to delivering superior water quality for its customers. This commitment was acknowledged in 2006 when the utility became only the fourth U.S. water utility ever to receive Phase IV “Excellence in Water Treatment” recognition from the Partnership for Safe Water. The partnership is sponsored by the American Water Works Association, Association of Metropolitan Water Agencies, Association of State Drinking Water Administrators, EPA, National Association of Water Companies and the American Water Works Association Research Foundation.