Pool / Aquatics
Failing Salt Generators
The Phosphate Connection
By Terry Arko
Salt generators have gained in popularity recently, but the technology goes back to 1800, when scientists discovered the technology of splitting molecules in a solution using what is called electrolysis. Salts like sodium chloride or sodium bromide are used in a solution that is subject to low-voltage electrical current and a pair of cells with opposing charges. One cell, the anode, contains positive charges, and the cathode contains negative charges. Electrical ions flow back and forth between the two cells. When sodium chloride passes across these cells, molecules are split and chlorine gas is produced at the anode. Hydrogen gas is produced at the cathode. Salt generators produce hydrogen gas, chlorine gas and a solution of sodium hydroxide. When salt generators are working properly, they are continuously producing free available chlorine to proper set levels.
When a unit begins to fail, it will not produce sufficient free chlorine to keep up with demand. There are numerous reasons for failure, including dirty or calcified cells that need to be cleaned, no power to cells, or insufficient levels of salt in the water. But, there is one main factor to consider first when a salt chlorine generator fails: the presence of phosphates in the pool water.
When levels of phosphate exceed 500 ppb, the unit can cease to produce enough free available chlorine to keep up with demand. Most manufacturers of salt chlorine generators will confirm that when there is a problem with production of free available chlorine a phosphate test is recommended. If the phosphate levels are near or over 500 ppb, a phosphate removal treatment is advised.
A typical example of salt generator failure is what happened recently to Steve White, owner of Underwater Pool Masters in Massachusetts. He installed a new salt chlorine generator at an outdoor commercial swimming pool, but could not get a decent chlorine read for over two weeks after the installation. "The situation was such that we ended up having to continue to shock the water to sanitize the pool until we could make sure the new salt generator was producing the optimum chlorine levels," White said.
So, what is going on when phosphate levels climb in a salt pool? We need to look to the industrial uses of orthophosphate in water treatment. It is important to understand that orthophosphate is the detrimental form of phosphate that exists in water. Orthophosphate will not only interfere with salt generators, it can also cause excessive algae blooms to occur in both traditional and salt pools as well. A product known as zinc orthophosphate is used in drinking water systems because it adheres to metal pipes and acts as an anti-corrosion agent. So, from this we know that orthophosphate likes to cling to metals. The real interference of phosphates in chlorine generators is still somewhat theoretical. It appears that since orthophosphates attach to metals and they are anodic or negative ions, they could attach to the cathode and cause an interference with the flow of electrons between the anode and the cathode of the salt chlorine generator. We do know that higher levels of orthophosphate seem to cause a definite interference with the normal operation of the salt chlorine generators.
Phosphates can be introduced into swimming water from a multitude of sources, including fertilizers, organic debris and soil, detergent cleaners, tile cleaners and metal sequestering chemicals. Also, phosphate can come from human perspiration and urea.
There are several different forms of phosphate depending on the source, all, however, eventually end up in the form of orthophosphate. Metal products can be one of the main culprits that cause failure in salt chlorine generator pools. This is because staining from metals is more prevalent in salt chlorine pools due to the potential of galvanic corrosion from the high TDS of the salt and dissimilar metals present in the water. Most manufacturers of salt chlorine generators will recommend a metal sequestering product be used. Most metal sequestering products available for pools are phosphonic or phosphoric acid based formulas. When these are added to a salt pool the result will be a breakdown of the phosphate to free orthophosphate, which is what will cause the problems with cells in the chlorine generator. For this reason it is best to use a phosphate-free metal sequestering product.
The situation experienced at the pool in Massachusetts turned out to be the result of high phosphates introduced by the city's water supply. The swimming pool in question used approximately 15,000 gallons of water, which was replaced at the time the salt generator was installed. About two-thirds of the new water added was delivered by truck while the remaining one-third came from the local town water supply. This community is known to have high levels of metals in its water—especially iron. As a result, the community regularly adds metal sequestering, which are high in phosphates to avoid having the metals from pipes cause unsightly metal stains in sinks and toilets.
In doing his detective work to figure out why he wasn't getting a good chlorine read, White checked the water in the bathroom of the pool facility and found that the phosphate levels were at 1,500 ppb. "I learned from the manufacturer of the salt generator that phosphates might be the problem, so I immediately started treating the pool with phosphate remover. Once I got the phosphate levels back down to 100 ppb the salt generator started producing chlorine and we were able to get the chlorine read levels where they needed to be."
It is important to understand that orthophosphate in pool water exists in a soluble form. The most effective phosphate removers work by making the soluble orthophosphate precipitate out as a solid. This will cause some cloudiness in the water, which can be filtered out readily with the use of a clarifier. However, keep in mind that the more phosphate, the more cloudiness there will be. In extreme cases it can take up to three days for the cloudiness to clear completely. It is important during this cloudy period to have the salt chlorine generator turned off until the water clears. During this period liquid chlorine can be used to keep the residual up. The other option for extreme levels of phosphate when levels are near or over 5,000 ppb would be to drain and dilute some or all of the water. The drain and dilution method may be better used for commercial pools where shut-down time is limited and cloudiness of the water is regulated. In these cases it is advisable to dilute out as much phosphate as possible and then use maintenance doses of phosphate remover to keep levels managed.
It is important in a salt chlorine pool to be diligent about keeping phosphate levels down. The following are some guidelines that can help:
- Test water for phosphates on a weekly basis.
- Treat with a phosphate remover weekly to keep levels down.
- Use only non-phosphate metal products and cleaners.
- Clean and remove grass, leaves and any organic debris from the pool as quickly as possible.
- Test tap water to see what levels of phosphate are in the source.
- Test for and treat for phosphates after any periods of extreme weather or heavy swimming.
For commercial pools avoid using any phosphate-based cleaners for the decking or tile. Many seasonal outdoor pools are near lakes. Operators should discourage patrons from visiting the pool after swimming in lakes. Thorough rinsing in showers should be done. Source water should be tested for phosphates as well. Many water municipalities treat with straight orthophosphates at different times. If this is the case then phosphate treatment is recommended at filling and whenever topping off.
The method for winterizing a salt generator pool is not that different from a regular pool. In areas where winter is extreme and a hard shut-down is used, the pool should be drained down and winterizing chemicals added as usual. The salt generator should be inactivated and in some cases disassembled and stored for the winter. Check with the generator manufacturer for specific winterizing instructions. In some areas a complete shut-down may not be necessary. In these areas you would simply turn down the hours of filtration operation and reset the generator to run in accordance. In winter water temperatures there should be sufficient chlorine produced from the unit in two hours a day.
Knowing the underlying water chemistry in pools is a key component to making sure that salt generators work properly. In a salt chlorine pool, keeping the phosphates low is the secret to letting the free chlorine flow!
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