Author: David Tuck, Water Plant Superintendent, Greenwood Commissioners of Public Works
In 2006, the W.R. Wise Water Treatment Plant in Greenwood, South Carolina converted its disinfection system from gaseous chlorine to on-site 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 on-site generation of chlorine dioxide.
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 Personal Protective Equipment training and meet USEPA 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 plant decided upon and installed a 1,500 lb/day ClorTec® on-site sodium hypochlorite generation system from Severn Trent Services. The ClorTec system offers significant benefits over competitive on-site 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 our 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. And the gas cylinders handling area was used to house the new liquid feeders for sodium hypochlorite.
The on-site generation system selected operates by feeding softened water into a brine dissolver. A brine solution is formed, 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 on-site sodium hypochlorite generation offers several advantages over gaseous chlorine. First, the disinfectant is produced and stored in liquid form. The 0.8% solution is non-hazardous, eliminating the need to develop and maintain a USEPA 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. In addition, on-site systems do not suppress finished water pH to the extent that gaseous chlorine disinfection does, so the amount of post-pH adjustment chemical (i.e., lime or caustic) necessary before distribution of finished water is reduced.
The on-site generation process also is a cost-effective disinfection alternative. A pound of chlorine equivalent can be generated on site for as low as 20 cents/lb (2006 pricing). Systems typically provide a return on initial investment within three to five years.
Dramatic reductions in Total Trihalomethanes and Haloacetic Acids; Improved Chloramine Levels
Once the new disinfection system was online at the Wise Plant, there was a dramatic reduction in Total Trihalomethanes and Haloacetic Acids levels, improvement of Chloramine levels, 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/year. Post-pH adjustment with lime or caustic has resulted in a savings of $11,436/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.
The conversion from gas to liquid disinfection at the Wise Plant has proven to be extremely cost-effective. The on-site 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, reducing the risk for exposure to the staff and community.
The conversion project reflects the Greenwood Commissioners of Public Works’ commitment to delivering superior water quality for our 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, which is sponsored by a number of water and environmental agencies and associations.
For more information, e-mail info@severntrentservices.com.
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