The impact of orthophosphate
Phosphorous-based products are broken down into orthophosphates (the simplest form) via oxidation, hydrolysis, or enzymatic digestion. It is this form, orthophosphate, which acts as one of many nutrient sources for algae while no longer adding value as a sequestering agent or corrosion inhibitor.
Orthophosphates contribute directly to water quality issues such as cloudy water and scale formation. Calcium phosphate scale formation requires increased levels of orthophosphate and the presence of calcium ions. If these conditions are met, the phosphate ion combines with the calcium, resulting in a gritty precipitate that forms in the water. Calcium phosphate tends to collect in dead spots where circulation is low, such as the footwell or recessed seats; however, if other scales are present on surfaces it can attach and form bulky scale deposits. Calcium phosphate can negatively impact the hot tub heater or chlorine generator cell (if one is present).
One method that can be used to identify whether the material is calcium phosphate scale is to pour distilled/bottled water (no phosphate) into a phosphate test vial and put a pinch of the material in the vial. To avoid receiving a false positive test, tap water or hot tub water should not be used. This test is performed by inverting the vial until the scale is no longer visible and has dissolved into the distilled water. Complete dissolution is not necessary and not all the scale may dissolve, but if it is calcium phosphate scale, the test results will show a phosphate reading in the vial.
Since hot tubs require a specific amount of calcium hardness to protect surfaces and equipment, controlling phosphate levels with the use of a phosphate remover can be an effective method to prevent the formation of scale. In areas with hard water, application of a sequestering agent as part of routine maintenance may also be necessary to prevent scale formation.
Impact of phosphates on chlorine generators
There are many different options for hot tub sanitization and maintenance. Chlorine, bromine, and biguanide products are all effective at killing bacteria and helping keep hot tub water clear. Saltwater hot tubs are quickly gaining in popularity and provide an alternative method to adding chlorine to the water. Overall, saltwater hot tubs offer reduced maintenance, but the presence of phosphate in these hot tubs can lead to trouble, specifically with scaling in the chlorine generator cell.
To create active sanitizer, the saltwater contained in the hot tub must pass through the chlorine generator cell. As it passes through the cell, an electric charge is passed over the ruthenium-coated plates found inside the cell. The charge breaks apart the existing salt molecule and ultimately creates hypochlorous acid, the active form of chlorine that kills bacteria in the water. The byproducts of this process contribute to elevated pH levels, which creates an environment where calcium carbonate scale is more likely to occur. Scale buildup on the electrolytic cell plates of a chlorine generator in a hot tub can cause stress on the system’s ability to perform.
A hot tub will normally have a high pH because of the hot water environment and the loss of carbon dioxide from the water. The pH rises even faster when a salt chlorinator is present. This presents an ideal environment for the formation of calcium carbonate scale, which builds up on the ruthenium plates inside the chlorine generator cell, as well as on the heating element of the hot tub. Once calcium carbonate scale has formed, calcium phosphate can easily adhere to the gritty texture of the existing scale. The combination can lead to decreased efficiency with a hot tub heater or chlorine generator. As scale builds up on cell plates, the electric charge needed to create active chlorine passes through the plates less effectively. This leads to decreased efficiency in breaking apart the salt molecule and a decrease in the amount of active sanitizer available in the hot tub, leading to an inability to maintain a proper sanitizer residual.
As scale builds up, the chlorine generator output setting must be increased to produce enough sanitizer to establish the desired residual. Increasing the output setting will decrease the life of the chlorine generator cell. The prevention of scale buildup, including calcium phosphate scale, is imperative to protect the chlorine generator cell in saltwater hot tubs.
