An Operational Profile of a On-site Sodium Hypochlorite Generating System
In 1999, the city of Daytona Beach, Fla., has one water treatment and two wastewater treatment facilities within its service area that exceeded the 2,500-lb threshold limit for gaseous chlorine.
As a result, a Risk Management Plan (RMP) would have to be prepared and submitted to the Environmental Protection Agency Risk Management Planning-Accidental Release Prevention Program. While developing their RMP, city staff further evaluated the chlorination facilities at one particular location, the Ralph Brennan Water Treatment Plant, and determined that the 25-year old system was reaching the end of its useful life.
Further, staff discovered that approximately 40 hours were spent on site-specific RMP paperwork, maintenance and operational procedures every month. Therefore, the city decided to embark on a program to replace the gaseous chlorine system at the Brennan Plant with a treatment technology that would put them in better compliance with or eliminate RMP guidelines at that location.
The city staff conducted evaluations of three treatment alternatives: rehabilitation/replacement of the existing gaseous chlorine system, bulk purchase of commercially available sodium hypochlorite solution, and on-site generation of sodium hypochlorite.
Rehabilitating/replacing the gaseous chlorine system was not a desirable option because it would entail a substantial cost to install a containment system to address an accidental release, and it still exposed the treatment staff and community to a hazardous disinfection chemical. The city staff decided to place a greater emphasis on trying to eliminate the use of any hazardous disinfection chemical at the plant, a decision that accelerated the evaluation of the last two treatment alternatives; bulk and on-site sodium hypochlorite.
Readily available potable grade bulk sodium hypochlorite solution ranges between a 12-15% concentration, a solution strength that is still considered a hazardous material and is subject to containment requirements.
Therefore, the 0.8 % solution strength produced through the use of on-site generating systems was favored by the city. Before making the final decision to install an on-site generation treatment system at the Brennan Plant, however, the city operations and engineering staff visited several facilities, including plants in Vero Beach, Fla., West Palm Beach, Fla., and city of Ocoee, Fla., that were using on-site generation systems.
The city of Daytona Beach staffers discussed, in depth, the operational and maintenance requirements of the technology. Representatives of the facilities in Palm Beach County and Ocoee detailed the simplicity of the process and the reliability of the operation. As a result, the city of Daytona Beach chose on-site generation as its preferred disinfection process.
Process design considerations
The Brennan Plant operates at an average flow rate of 13.5 million gallons per day (MGD), maximum 16.7 MGD and minimum 6.0 MGD during some winter months. This facility services a daily population of more than 100,000, but its transient population can increase to almost 500,000 around special city of Daytona events.
To effectively meet its daily disinfection needs, a 2,000-lb per day (lb/day) on-site generating system would be required. Other requirements of the system included redundancy, system layout and power demands. The treatment plant has nine full-time operators on staff.
The Brennan Plant is on a peak load usage agreement with Florida Power and Light. This agreement provides for a favorable off-peak kilowatt-hour rate of $0.03/KWh compared to the peak rate of $0.06/KWh, and includes stiff financial penalties for excessive power consumption during peak hours.
Because of the substantial difference in price and cost savings that could be realized, the city looked more closely into determining if the on-site generation system could produce a 24-hour disinfection demand under the 14-hour off-peak power period. After determining that their disinfection demand could be met by operating the on-site generation system during off-peak hours, the city was even able to increase the system design capacity from 2,000 lb/day to 4,000 lb/day because of the cost savings. This provided system redundancy and enabled greater flexibility during required maintenance.
Moreover, the increase to the disinfection system capacity would enable to city to meet expanding service demands.
By 2007, the city of Daytona expects their service demand to increase by 1 to 1.5 MGD.
The city?s need to get the process installed and operating in a timely manner, coupled with the simplicity of the on-site process, was factored into its decision to consider proceeding with the project through the design-build method.
The project featured the development of a main building and secondary building housing in addition to miscellaneous site work. The total 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.
Meeting city expectations
Generating sodium hypochlorite onsite is a simple and straightforward process that uses three common consumables: salt, water and electricity. The ClorTec on-site generation system 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 the 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 a day tank and, when it reaches the low-level set point, the system automatically restarts to replenish its supply. Three peristaltic hose pump injects the hypochlorite as needed.
On January 2, 2001, two ClorTec MC 2,000-lb/day on-site sodium hypochlorite generation systems went on line and were producing 0.8% hypochlorite solution within two hours of start up. The system has a storage capacity of 108,000 gal. or 7,200 lb of product; allowing for up to an eight-day supply of equivalent chlorine disinfection. Morton Solar Salt is used in the process. The system is fully automated and has manual operation features if needed.
During the first half of 2001 (January -June), the monthly production of chlorine bleach was more than 700,000 gal.
Because of 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 at the Brennan Plant.
When the on-site generation system came on line, the chlorine feed equipment was located at the top of the filters and the ammonia feed 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, May to June, the chlorine bleach production fell from 784,665 gal. to 445,770 gal., just from the adjustment to injection points and the more efficient mixing of the two compounds that resulted. Over the four-year operational period (2001 to 2004), the lowest monthly chlorine bleach production was recorded at 278,048 gal.
However, by December 2005, the city of Daytona will be required to measure free chlorine contact time to meet CT requirements and as a result, their chlorine demand may increase up to ten percent.
After four years of operation, the ClorTec system has proven reliable while producing 1,404,085.0 lb of sodium hypochlorite at an average cost of $0.014 per pound. Labor and maintenance for the on-site system has been fairly straightforward, including mostly manufacturer-recommended actions. The labor and maintenance hours and costs are inclusive of all plant/building-related items such as daily housekeeping activities and larger projects like the installation of a sprinkler system for the building that houses the on-site system. The on-site system maintenance has consisted of the following:
- Acid washing of the electrolytic cells (1x per year x 3-4 days);
- Cleaning the salt tanks (1 x per 4 years x 1-2 days);
- Cleaning/changing filters (1 x per month);
- Replace diodes; and
- Cleaning the chlorine product tanks.
ClorTec On-site Sodium Hypochlorite Generating System Maintenance & Material Costs (2000-2005)
Total Bleach Production (gal) 21,061,271.30
Total Hours of Generation 7947.6
Total Labor/Maintenance Hours 435.4
Total Labor Costs $4,226.18
Total Material Costs $13,952.45
Total Maintenance/Material Cost per Pound of Chlorine $0.0129
Total Salt Cost per Pound of Chlorine $0.0735
Total Power Cost per Pound of Chlorine $0.058
Production cost per Pound of Chlorine (4 year) $0.14
When switching over to an emergency generator power source, the resulting power surges can cause damage to diodes within the on-site sodium hypochlorite generating system, requiring replacement. Despite the occurrence of diode damage, using emergency generated power is a must in Florida, where lightning strikes are common. During severe weather, employees are protected from the harsh external elements since emergency power alleviates their need to brave the conditions to work on faulty or down equipment. In addition, the use of emergency power sources ensures uninterrupted water treatment.
By almost any measure, the ClorTec on-site sodium hypochlorite generating system at the Brennan Plant has exceeded the expectations of the city of Daytona Beach, confirming the favorable reviews from plants in Vero Beach, West Palm Beach and the city of Ocoee. On-site generation eliminates the use of the hazardous disinfection chemicals used in gaseous chlorine disinfection. The system?s flexibility and reliability make for a robust disinfection system and quick system installation, ongoing operational efficiencies with low, predictable maintenance combine to provide excellent cost savings.
Marvin D. Owens is the plant superintendent, Utilities Department Plant Operations, for the city of Daytona Beach, Fla. He can be reached at firstname.lastname@example.org .
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