Onsite Sodium Hypochlorite Generating System
In 1999, the city of Daytona Beach, Fla. had 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.
Staff discovered that approximately 40 hours were spent on site-specific RMP paperwork, maintenance and operational procedures every month. Therefore, the city decided to replace the gaseous chlorine system at the Brennan Plant with a treatment technology that would better comply with RMP guidelines.
The city staff conducted evaluations of three treatment alternatives: rehabilitation/replacement of the existing system, bulk purchase of commercially available sodium hypochlorite solution, and onsite generation of sodium hypochlorite.
The city staff decided to emphasize eliminating hazardous disinfection chemicals at the plant—a decision that accelerated the evaluation of the last two treatment alternatives, bulk and onsite sodium hypochlorite.
Readily available potable grade bulk sodium hypochlorite solution ranges from 12 to 15% concentration, a solution strength that is considered hazardous and subject to containment requirements.
Therefore, the city favored the 0.8% solution strength produced through the use of onsite generating systems. Before making the final decision to install an onsite generation treatment system at the Brennan Plant, however, the city operations and engineering staff visited plants in Vero Beach, Fla., West Palm Beach, Fla., and Ocoee, Fla. that were using onsite generation systems.
Because representatives of the facilities in Palm Beach and Ocoee detailed the simplicity and the reliability of the operation, Daytona Beach chose onsite 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). This facility services a daily population of more than 100,000, but its transient population can increase to almost 500,000 during special city events.
To effectively meet its daily disinfection needs, a 2,000-lb per day (lb/day) onsite generating system would be required. Other requirements of the system included redundancy, system layout and power demands.
The Brennan Plant is on a peak load usage agreement with Florida Power and Light. This agreement provides for a favorable off-peak rate of $0.03/KWh compared to the peak rate of $0.06/KWh.
Because of this the city looked more closely into determining if the onsite 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 onsite generation system during off-peak hours, the city was 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 11⁄2 mgd.
Meeting city expectations
Generating sodium hypochlorite on site is a simple and straightforward process that uses three common consumables: salt, water and electricity. The ClorTec onsite 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. When it reaches the low-level set point, the system automatically restarts to replenish its supply. Three peristaltic hose pumps inject the hypochlorite as needed.
On Jan. 2, 2001, two ClorTec MC 2,000-lb/day onsite sodium hypochlorite generation systems went online 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. The system is fully automated and has manual operation features if needed.
During Jan. to June of 2001, the monthly production of chlorine bleach was more than 700,000 gal, or 46,667 equivalent pounds of chlorine.
Because of concerns related to raw water TOC and subsequent formation of trihalomethanes, chloraminated water (a combined chlorine and ammonia compound) was used for the residual disinfectant at the Brennan Plant.
When the onsite generation system came online, 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.
From 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.
However, by December 2005, Daytona was required to measure free chlorine contact time to meet CT requirements and as a result, their chlorine demand decreased. Over the operational period from Jan. to Oct. 2006, the total bleach production in gallons was 30,178,316.
In January 2006, new regulatory requirements were met for free chlorine contact time (C/T0 resulting in a daily average consumption of 1,125 lb per day.) For one month, the demand increased up to 514,688 gal. Since installation, the system has proven to be an effective and reliable system producing more than 2,011,887 lb of free available chlorine at an average cost of $0.17 per pound.
Labor and maintenance for the onsite 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 onsite system. The onsite 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);
- Replacing diodes; and
- Cleaning the chlorine product tanks.
When switching over to an emergency generator power source, the resulting power surges can cause damage to diodes within the onsite sodium hypochlorite generating system, requiring replacement. Despite this, 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 because emergency power alleviates their need to brave the conditions to work on faulty or downed equipment. In addition, the use of emergency power sources ensures uninterrupted water treatment.
By almost any measure, the ClorTec onsite sodium hypochlorite generating system at the Brennan Plant has exceeded the expectations of the city of Daytona Beach. Onsite generation eliminates the use of the hazardous disinfection chemicals used in gaseous chlorine disinfection. The system’s flexibility makes for a robust disinfection system and quick system installation. Ongoing operational efficiencies and low, predictable maintenance combine to provide excellent cost savings.