The city of Cape Coral (Fla.) Reverse Osmosis Water Treatment Plant has switched to sodium hypochlorite as its main disinfectant after a full-scale test indicated that it was safer and slightly less expensive to use than gas chlorine in the specific application. The cost savings was primarily due to a reduction in the use of caustic soda needed to maintain a proper pH level.
Cape Coral had been using about 450 pounds of chlorine a day as its main disinfectant but began to look for alternatives with the advent of the EPA Risk Management Program.
Chlorine reduced the pH of the treated water, so the city was required to add caustic soda (sodium hydroxide) to raise the finished water pH to 8.8 from about 6.0 after primary treatment. Since sodium hypochlorite naturally tends to raise the pH, the city was able to noticeably reduce the amount of caustic soda it used.
The sodium hypochlorite, when compared pound for pound with gas, was 2.6 times more expensive for meeting comparable chlorine demand. However, the use of sodium hypochlorite resulted in a 55 percent drop in caustic soda usage. The net effect was a 0.2 cent reduction in chemical costs per 1,000 gallons treated, when compared to using chlorine gas and caustic soda together.
The Cape Coral RO Plant staff also determined that a storage and delivery system for sodium hypochlorite could be installed at a lower capital cost than installing an emergency scrubber and enclosing the existing chlorine gas storage area.
Since the finished water target pH has always been above 8.5, free chlorine in the citys potable water has always existed predominately as a hypochlorite ion. This is an important point, because it meant that switching to sodium hypochlorite resulted in no noticeable difference in the free chlorine water chemistry.
Testing Program
Before switching disinfectants, the staff at Cape Coral conducted an extensive full-scale test to prove the system would work. Sodium hypochlorite has long been known as an effective disinfectant for potable water, but has typically been used in systems with an equivalent gas chlorine demand of less than 10 pounds per day.
Testing Program
After examining the costs and other issues involved, the staff decided to purchase 11 percent strength commercial grade sodium hypochlorite. Plant staff estimated that a fully operational storage and delivery system would cost about $70,000. This would include a new building, three 4,000-gallon storage tanks with self containment and eight chemical metering pumps. Staff estimated the plant would need approximately 410 gallons per day of solution for disinfection.
Testing Program
Allied Universal Corp, which supplies sodium hypochlorite for canal pumping stations that supply non-potable water for the citys irrigation system, agreed to lease and install three temporary tanks with self containment for the test, and would deliver the 11 percent sodium hypochlorite twice per week. Bill Clover of Blankenship & Associates agreed to loan the plant three Prominent chemical metering pumps for the full-scale test.
Testing Program
After startup of adding sodium hypochlorite, free chlorine residuals were taken on both clearwells every 15 minutes for the first 24 hours. Once proper feed rates were established, residuals were taken hourly on both clearwells and the plant finished water entering the distribution system. After several days, residuals were taken every two hours as per normal operating procedure. There were no problems with establishing feed rates or maintaining proper free chlorine residuals.
Testing Program
Free chlorine residuals were taken daily at three remote sites in the distribution system. Proper residuals were maintained at these sites throughout the test period.
Testing Program
The Lee County Health Department collected 106 samples (about 90 samples normally are collected) throughout the distribution system during the first month that sodium hypochlorite was being used. Each sample was tested for free chlorine residual, pH and coliform bacteria analysis. All samples showed adequate chlorine residuals and no confirmed coliform bacteria.
Testing Program
Samples on both clearwells and plant finished water entering the distribution system were analyzed for sodium, chloride and TDS, both before and after adding sodium hypochlorite. Although sodium levels went up and chloride levels went down, the changes were not significant.
Testing Program
The testing also involved monitoring chlorate and chlorite levels and testing for corrosion control verification. Lead and copper analyses were also completed. No problems were uncovered in those areas.
Testing Program
Shelf life is a concern with sodium hypochlorite, since it loses stability and strength over time if it is exposed to direct sunlight or high temperatures. The test plant had minimal storage and usage patterns indicated that the plant would be at the mercy of the supplier for ensuring uninterrupted plant operation. Staff suggested that keeping the sodium hypochlorite solution cool and out of direct sunlight would dramatically increase shelf life and allow for larger quantities to be stored on site to help ensure uninterrupted plant operation.
