Many pool operators consider using saline generators for sanitation instead of traditional chlorine or bromine disinfection methods. Many feel this is an easy way to have a chlorine-free pool; however, saline pools use the same chlorine as any other pool, they just use salt from the pool or a brine tank to produce the chlorine on site as opposed to having someone deliver it.
The Basics
Saline systems use non-ionized, coarse, sun-dried or pelletized salt (normally in 40 lb bags). One treatment method involves adding the salt to the pool water to develop a concentration of 0.3% to 0.7% (3,000 ppm to 7,000 ppm). Twenty seven pounds of salt per 1,000 gal will produce a 3,000 ppm salinity concentration. This concentration is similar to that of human tears, so the saline solution is very gentle on swimmers.
The other method is to use a brine tank to hold the saline solution, mixing it with the pool water as needed to achieve the required salt concentration.
As the saline solution passes through the electrolytic chamber, hypochlorous acid is produced when an electrical DC current is passed between the positively charged anode plates and the negatively charged cathode plates in the chlorinator cell. While the filtration system is operating, the system will introduce electrolytically produced hypochlorous acid into the swimming pool either on a continuous basis or as requested by a chemical controller. The system does not operate unless the filtration system is operating.
This introduction of hypochlorous acid ensures the continuous sanitization of the pool water and will provide the necessary chlorine residual when the equipment is operated properly. When the hypochlorous acid has destroyed the bacteria in the pool, it reverts to salt (sodium chloride). This means that salt is continuously recycled during this process. Because the chlorine ultimately is converted back into salt, it gets recycled and extra salt will need to be added infrequently, such as when salt is lost due to splash-out or backwashing.
Salt generator sizing is based on several factors including gallonage, surface areas, bather load per 24 hours, hours of operation, additional water features and location. Salt systems should be sized for peak bather load periods. Backup feed systems can be utilized where bather loads cannot be anticipated, such as in new pools, or pools with excessive load in a short period of time. Systems ranging in size from 1.5 lb per day to 25 lb per day in both vertical and horizontal configurations allow optimization of the system to the body of water and equipment room. Optimal sizing yields greater operating efficiencies and costs.
The following are general guidelines for sizing:
- Outdoor sizing recommendations are 0.65 lb per 10,000 gal for low use; 0.80 lb per 10,000 gal for medium use; and 1.0 lb per 10,000 gal for heavy outdoor use.
- Indoor sizing recommendations are 0.4 lb per 10,000 gal for low use; 0.5 lb per 10,000 gal for medium use; and 0.6 lb per 10,000 for heavy indoor use.
Maintaining 5 ppm as the target residual with an appropriately sized system will allow up to two times the normal demand to be accommodated. A traditional erosion feeder is recommended at a redundant system in all commercial applications where saline generation is being designed.
An example of the plumbing layout in a chlorine generator field installation.
Maintenance Matters
Salt systems require regular preventive maintenance just like any other piece of equipment. The main area of concern for saline systems is the chlorination cell, where the electrolysis happens. Some cells have a self-cleaning feature that reverses the polarity of the cathode plates, similar to rotating the tires on your car. This minimizes the amount of buildup that will occur. However, over time these plates will need to be manually cleaned. This process involves soaking the cells in a mild acid solution (five parts water to one part acid).
Over time, the cells will need to be replaced. Smaller residential type cells generally last only 3,000 hours, where commercial cells can last approximately 30,000 hours, with warranty periods lasting around three years or 15,000 hours.
Reap the Rewards
Salt systems generate pure sodium hypochlorite at a near-neutral pH and therefore have less effect on pH than most other pool chlorines. This means that while the pool still uses chlorine, it is using a pure form of the chemical with no additional binders of inhibitors. Saline generators have the least impact on pH of any commonly used sanitizer, reported between 7.7 and 8.8 pH. Manufacturers report the following advantages of salt systems:
- Save money;
- Offer ease of fixed versus variable chemical costs;
- Improve the water quality and enhances bather comfort;
- Stabilize pH and maintain easier water balance;
- Enhance indoor air quality;
- Reduce pool maintenance; and
- Offer ease of operation.
While saline generators have a higher upfront cost when compared to a traditional chlorine feed method, they typically have a two- to four-year return on investment. Payback periods will depend on the local cost of electricity and traditional chlorine. Areas that have a higher cost for chlorine delivery and a lower cost for energy will see a rapid payback.
The Omni Nashville Hotel uses a saline generator in its pools.
Overcoming Obstacles
Saline systems that spike the pool with salt increase the total dissolved solids (TDS) levels to between 3,000 and 6,000 ppm. Additionally, commercial pools have historically operated below 1,500 ppm of TDS. While TDS levels between 3,000 and 6,000 ppm are not considered extremely high, they are above what typically is considered normal. Manufacturers state that salinity levels up to 6,000 ppm of sodium chloride do not show an appreciable increase in corrosion. However, where improper grounding is present, increased salinity can increase the rate of galvanic corrosion. Many owners will select the brine tank method, which allows them to control the TDS levels.
High TDS can have an impact on water quality and an increase in TDS has been used as an indicator of increased organic concentrations because it can be measured. TDS has no impact on water clarity, as everything has been dissolved in the water, but many manufacturers in the aquatic industry are uncomfortable guaranteeing their products when TDS levels are continuously more than 1,600 ppm.
Another area owners often are concerned with is the taste of the water. Salt systems usually require a minimum of 3,000 ppm of salt. The normal taste threshold for salt is generally considered to be 3,500 ppm. At 5,000 ppm, the salt taste is detectable, but not offensive. The human eye is approximately 7,000 ppm of salt. None of these are even close to sea water, which is more than 30,000 ppm of salt. With this relatively low level of salt, most swimmers hardly taste the salt in the water. As an example, chicken soup has approximately 4,400 ppm of sodium chloride.
Saline generators have been used successfully in commercial applications across the country. Saline generators may not make sense in all applications, but by producing chlorine on-site, they have proven to reduce operating costs and minimizing exposure to hazardous chemicals. It is best to evaluate the pros and cons of these systems compared to traditional methods.
