Keep an Eye on Your Pool
The Case for Phosphate Control in Swimming Pools
By Terry Arko
Calcium phosphate scale is a new problem that has been creeping into the swimming pool industry. However, the problem has long been recognized in the boiler industry. Calcium phosphate scale became prevalent when reverse osmosis technology was widely applied to municipal wastewater. Because of recent water shortages, municipal wastewater recycling and reuse has become a major application area of reverse osmosis. Because of the use of this technology, preventive actions for calcium phosphate scaling have become commonplace for commercial boiler water and now, it seems, might become commonplace in swimming pool water, as well.
One of the main reasons for the increased appearance of calcium phosphate scale in swimming pools is the increase in the use of metal remover products that contain polyphosphonates HEDP. When these metal removal products are used in pools with high calcium levels, this may lead to the formation of calcium phosphate scale. Not only can the scale cause cloudiness in swimming pool water, but it can also damage equipment. Another factor causing an increase in calcium phosphate scale could be the higher levels of orthophosphate from source water added to high calcium water. The higher levels of orthphosphates from source water could be causing calcium phosphate scale formation, particularly on heat exchangers.
Calcium phosphate scale occurs when orthophosphate combines with calcium in pool water, particularly in the presence of heated water. The result is an insoluble white scale that is difficult to remove. Not only is the scale difficult to remove, but if left untreated, can lead to the destruction of heat exchangers.
This detrimental scale provides another reason for pool professionals to conduct regular phosphate testing and to keep phosphate levels down in order to prevent this scale formation from occurring. It is particularly important if phosphate-based metal removal or sequestering products are being used regularly in heated pools where calcium is present. This would include pools in high hardness areas such as Arizona or pools that regularly use calcium hypo-chlorite.
Typically, testing for phosphate and using phosphate removers are recommended to keep resistant algae strains from flourishing in the absence of a chlorine sanitizer or an EPA-registered algaecide. Phosphate removal has become a standard in the swimming pool industry to remove this prime nutrient, which algae can use to resist typical chlorine and algaecide treatments as well. It has proven very successful in these circumstances.
Now it seems there is another very important reason for keeping phosphate levels down, especially in high-water-hardness areas or where calcium hypochlorite and calcium chloride are used in pools. It appears that in the orthophosphate calcium scenario, tri-calcium phosphate is formed from the reaction of orthophosphate with calcium at the heat exchanger. The formation precipitates and is insoluble in water and even more so at higher temperatures. The calcium phosphate scale will adhere to heat exchangers, causing damage and failure.
Most metal removal and prevention products are phosphoric or phosphonic acid based. While these are effective at dealing with certain metal stains, they also eventually break down to orthophosphate, which will accumulate over time. When there are high levels of orthophosphate combined with calcium and you have a high temperature, such as at the point of the heat exchanger, the potential for calcium phosphate scale to precipitate and damage the exchanger is very high.
In one particular case recently, a commercial aquatic facility in Arizona was using a phosphate-based chemical on a weekly basis. The pool was heated and also had high levels of calcium. The facility went through several heat exchangers in a short timeframe. Each of the heat exchangers were all covered with a white "scale-like" material. After laboratory testing, the white material was determined to be calcium phosphate scale. A phosphate removal treatment was then conducted and the phosphate-based chemical was switched to a non-phosphate-based product. As a result, the facility has been running over a year now with no problem to the heat exchanger.
Presently, there is ongoing research into this reaction and confirmations from the field of problems and solutions to the problem of calcium phosphate being accumulated. "We are looking at the history of this in the boiler treatment industry as we have been receiving numerous reports of this occurrence from the pool industry," said Everett J. Nichols, Ph.D., MSPH and scientific director of Water Treatment Technologies for HaloSource Inc. Because of the complexity of phosphate chemistry, it is not easy to predict a threshold level of phosphate scaling. To minimize the risk of phosphate scaling, it is important to reduce not only orthophosphate, but also calcium, fluoride and aluminum concentration. A low feed pH helps to control phosphate scaling. There are also commercially available anti-scalants that are good for phosphate scale removal. Keep in mind that phosphate scaled membranes are best cleaned at low pH.
So, while continued research is being sought on the phosphate calcium scale problem, it does appear for now that regular testing of phosphate levels and phosphate removal treatments can help extend the life of heat exchangers in high calcium pools.