By Terry Arko
As salt water pools become more popular in both residential and commercial pool installations, issues with salt chlorine generators have also been garnering more attention.
In a pool setting, salt generators produce hydrogen (H) and chlorine (Cl2) gas, and a solution of sodium hydroxide (NaOH). When salt generators are working properly, they continuously produce free available chlorine (FAC) to proper set levels. However, when they fail, there will not be enough free chlorine to keep up with demand. While failure can be caused by several factors, this article will focus on one: high phosphate levels.
How salt chlorine generators work
The technology behind salt chlorine generators goes back to the 1800s, when scientists first developed electrolysis, a process that involves electric current being passed through a substance to elicit a chemical change.
In the case of salt chlorine generators, salts (e.g. sodium chloride [NaCl] or sodium bromide [NaBr]), are used in a solution that is subjected to a low-voltage electrical current. The current flows between a pair of electrodes with opposing charges—the anode, which is positively charged, and the cathode, which is negatively charged. Electrical ions flow back and forth between the two electrodes; when the salt passes across them, molecules are split. Chlorine gas is produced at the anode, while hydrogen gas is produced at the cathode.
There are numerous reasons for salt chlorine generator failure, including dirty or calcified electrodes, lack of power to electrodes and/or insufficient salt levels in the water. There is, however, one primary factor to consider when a salt chlorine generator fails—the presence of phosphates in the pool water.
The phosphate connection
When phosphate levels exceed 500 parts per billion (ppb), the salt chlorine generator may cease to produce enough FAC to keep up with demand. In fact, most manufacturers recommend a phosphate test when there is a problem with production of FAC. If phosphate levels are near or over 500 ppb, a phosphate removal treatment is advised. This can help salt chlorine generators function as they intended.
High phosphate levels are directly linked to industrial uses of orthophosphate in water treatment. Orthophosphates are detrimental to salt chlorine generators, as they serve as nutrients for algae; they can contribute to scale, in both traditional and salt water pools. Zinc (Zn) orthophosphate is used in drinking water systems because it adheres to metal pipes and acts as an anti-corrosion agent.
The real interference of phosphates in chlorine generators is still somewhat theoretical. However, it appears that since orthophosphates attach to metals and are anodic or negative ions they could attach to the anode and cause an interference with the flow of electrons between the anode and the cathode of the salt chlorine generator. Another theory is that since algae spores feed on nutrients such as phosphate and begin to bloom, chlorine is consumed at higher levels. More work research needs to be performed to define the exact cause of interference from orthophosphates. Regardless of the mechanisms involved, it is clear that higher orthophosphate levels seem to cause interference with the normal operation of the salt chlorine generators.