In 1999, the Ralph Brennan Water Treatment Plant in Daytona Beach, Fla., embarked on a program to improve or replace its 25-year old gaseous chlorination system with a disinfection technology that would put the plant in better compliance with Risk Management Plant guidelines.
Three disinfection alternatives were evaluated: rehabilitation or replacement of the existing gaseous chlorination system, bulk purchase of commercially available 12% sodium hypochlorite, and on-site generation of 0.8% sodium hypochlorite.
Generating sodium hypochlorite onsite is a simple process that uses salt, water and electricity to generate the disinfection solution. Because eliminating the use of any hazardous chemical at the plant was a priority to the city, they favored the 0.8 % solution strength produced through the use of an on-site generation system. When the city evaluated the cost of ownership for the on-site generation system over a 20-year life cycle, the analysis showed a potential for payback in the fifth year of operation.
Process design considerations
With an average flow rate of 13.5 million gallons per day (mgd), Daytona Beach services a daily population of more than 100,000. Therefore, a 2,000-lb per day (lb/day) ClorTec on-site generation system would meet its daily disinfection needs.
Daytona Beach is on a peak load usage agreement with the Florida Power & Light Company that provides a favorable off-peak kilowatt-hour rate of $0.03/kWh, half that of the peak rate. This agreement includes financial penalties for excessive power consumption during peak hours. It was determined that the 24-hour disinfection demand could be cost-effectively met by operating the on-site generation system during the 14-hour off-peak power period.
Therefore, the city increased the system design capacity from 2,000 lb/day to 4,000 lb/day to benefit from the operational savings. The total design-build cost for the 4,000 lb/day (with designed-in expansion capability to 6,000 lb/day) on-site generation facility was $2,227,267.
Start-up and operations
Two 2,000-lb/day ClorTec on-site sodium hypochlorite generators went online at Daytona Beach in January 2001 and were producing 0.8% sodium hypochlorite solution within two hours of start up.
The system has a storage tank capacity of 108,000 gal, or 7,200 lb of equivalent gaseous chlorine, allowing for up to three days of disinfection storage.
Due to preexisting concerns related to raw water TOC and subsequent formation of trihalomethanes (THMs), chloraminated water, a combined chlorine and ammonia compound, was used for the residual disinfectant. When the on-site generation system came online, the chlorine injection point was located at the top of the filters and the ammonia injection point was located at the bottom of the clearwell.
In June 2001, a strategic decision was made to place the chlorine and ammonia injection points within two feet of one another, leading to dramatic results. Within one month the chlorine bleach demand fell from 784,665 gal to 445,770 gal. By simply moving the injection points, the two compounds could be more efficiently mixed.
In December 2005, however, the city of Daytona was required to measure free chlorine contact time to meet CT requirements and as a result, the chlorine demand increased.
Low cost, low maintenance
Since installation, the ClorTec system has proven to be effective and reliable, producing more than 2,011,887 lb of free available chlorine at an average cost of $0.17 per pound (see Figure 1 ). Labor and maintenance for the on-site system includes mostly manufacturer-recommended actions. The on-site system maintenance has consisted of the following:
- Acid washing of the electrolytic cells once each year – 3 to 4 days labor;
- Cleaning the brine dissolver tanks once every four years – 1 to 2 days labor;
- Cleaning/changing filters once each month;
- Replacing diodes as needed; and
- Cleaning the chlorine product tanks once every four years – 2 to 3 days labor.
The ClorTec system on-site sodium hypochlorite generating system has exceeded the expectations of the city of Daytona Beach by cost-effectively eliminating the use of hazardous chemicals. The system’s flexibility and reliability make for a robust disinfection system while ongoing operational efficiencies with low maintenance have provided excellent cost savings.
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