Making a case for Phosphate Control in Swimming Pools and Hot Tubs

Calcium phosphate scale occurs when orthophosphate combines with calcium in pool/hot tub water, particularly in the presence of heated water.

Calcium phosphate scale is a relatively new problem creeping into the swimming pool and hot tub industry. However, the problem has long been recognized in the boiler industry. Calcium phosphate scale became prevalent when reverse osmosis (RO) technology was widely applied to municipal waste water. Due to recent water shortages, municipal waste water recycling and reuse has become a major area where this water purification technology has become prevalent. As such, preventative actions have become commonplace for commercial boiler water and now, it seems, it may become standard for treating swimming pool and hot tub water, as well.

One of the main reasons for the increased appearance of calcium phosphate scale in swimming pools and hot tubs could be the more frequent use of metal remover products, which contain polyphosphonates, 1-Hydroxethylidene-1, 1-Diphosphonic Acid (HEDP). When these metal removal products are used in pools and/or hot tubs with high calcium levels, there is a chance it may lead to calcium phosphate scale formation—especially at the heat source. Not only can scale cause cloudy water, but it can also damage equipment, particularly heater exchangers.

Another possible factor could be a high level of orthophosphates in the source water, which can cause calcium phosphate scale formation on heat exchangers as calcium precipitates more readily in higher water temperatures.

What is calcium phosphate scale?

Calcium phosphate scale occurs when orthophosphate combines with calcium in pool/hot tub water, particularly in the presence of heated water. The result is an insoluble white scale, which is difficult to remove. Further, if left untreated, it can lead to the destruction of heat exchangers.

Calcium scale buildup is typically more problematic in hot tubs due to the higher water temperature. Scale forms when calcium attaches to carbonate in the water; calcium carbonate becomes more insoluble at higher water temperatures. Further, due to increased temperatures and aeration, the hot tub’s evaporation rate is rapid. This leads to an increased buildup of total hardness and total dissolved solids (TDS).

When water evaporates, pure water leaves, while all other contaminants are left behind. That said, scaling in a hot tub can be especially difficult to treat once it occurs and, if not caught in time, can damage the surface finish. This is not the only concern as calcium scale also causes discomfort to bathers and can even cause skin abrasions and rashes. Further, with the advent of salt systems, high phosphate levels and the formation of calcium phosphate scale on generator cells can cause the system to fail and not keep up with sanitizer demand.

It is much easier and more cost-effective to prevent water problems from the start than it is to treat them after they have occurred. In the case of scale buildup in a hot tub, for example, once it has formed, the only thing left to do is grab a mild scale remover and start scrubbing. (Phosphate scaled cells or surfaces are best cleaned with a mild acid solution.)

Unfortunately, in today’s fast-paced world, many key elements of proper water management are simply overlooked. For instance, checking calcium hardness and phosphate levels is a step many operators tend to skip. This is a mistake for two important reasons:

  1. Low calcium hardness can result in a corrosive environment that is harmful to bathers, equipment, and pool/hot tub surfaces.
  2. High calcium hardness can result in cloudy water, stains, and scale. High phosphate content in source water, or buildup in the hot tub, can lead to further scale issues.

Another case for phosphate testing

This detrimental type of scale provides another reason for aquatic professionals to conduct regular phosphate testing and to keep phosphate levels down to prevent scale formation. It is particularly important if phosphate-based metal removal or sequestering products are being used regularly in heated pools or hot tubs where calcium is present. This includes pools in areas with high water hardness levels or where calcium hypochlorite (cal-hypo) (Ca[ClO]2) is regularly used.

In hot tubs, calcium hardness testing (also referred to as total hardness) is important in preventing scale buildup. Calcium hardness is a measurement of the mineral salts present in hot tub water, which include: calcium (Ca), magnesium (Mg), aluminum (Al), iron (Fe), and manganese (Mn). When it comes to hot tub water, 70 to 75 percent of the total hardness is made up of calcium.

According to the National Swimming Pool Foundation (NSPF), the ideal range for calcium hardness in a hot tub is 150 to 250 parts per million (ppm). Calcium hardness is raised by adding calcium chloride (CaCl2). This chemical predominantly comes in a powder form and must be diluted before it is added to the hot tub water. A liquid version of this chemical is also available for hot tubs, which is much more convenient to use and gets into solution much quicker. However, before adding calcium chloride, the source water should be tested for phosphates first.

High levels of phosphates in fill water, combined with adding calcium chloride, can lead to the formation of calcium scale—especially when the hot tub is being heated. Many products for controlling metals and scale in hot tubs contain phosphoric (H3PO4) or phosphonic acid (C-PO[OR]2), which break down to orthophosphate and can potentially combine with the calcium in the water to create calcium scale. Therefore, non-phosphate metal treatment and scale products should be used to prevent this occurrence. Lowering calcium hardness can only be accomplished by draining and replacing the water. Once the water is properly adjusted, the calcium hardness level will increase over time naturally due to the high evaporation rate in hot tubs. Calcium hardness should be monitored at least monthly. Most manufacturers recommend hot tubs be drained every three to four months.

Hot tub water should also be regularly tested for phosphates, especially if they are tied into a salt system to generate either chlorine (Cl) or bromine (Br). Phosphate levels should be maintained no higher than 250 parts per billion (ppb). Keep in mind, phosphates are a major pollutant to waterways; therefore, hot tubs should always be tested and treated for phosphate removal before each draining.

Phosphate testing and using phosphate removers are recommended for swimming pools to keep resistant algae strains from flourishing in the absence of a chlorine sanitizer or an Environmental Protection Agency (EPA)-registered algaecide. Phosphate removal has become a standard in the swimming pool and hot tub industry as it is a prime nutrient, which algae can use to resist typical chlorine and algaecide treatments. In fact, it has been proven quite successful in these circumstances.

Hot Tub Scaling
Oil, bather waste and scum buildup above the waterline can attract calcium as part of the scaling process. To prevent this from happening, adding a natural-based clarifier on a weekly basis is recommended. This will trap and filter out contaminants, which add to the unsightly scaling, thus reducing any buildup above the waterline.

Another reason for keeping phosphate levels down—especially in areas with high water hardness or where calcium hypochlorite and calcium chloride are used in pools and/or hot tubs—is it appears 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.

As stated earlier, most metal removal and prevention products (e.g. phosphoric or phosphonic acid) are effective in dealing with certain metal stains; however, they also eventually break down to orthophosphate, which accumulates over time. When high levels of orthophosphate are combined with calcium and high temperatures, e.g. at the point of the heat exchanger, the potential for calcium phosphate scale to precipitate and damage the exchanger is extremely high.

Covered in scale

In one particular case, for example, a commercial aquatic facility was using a phosphate-based chemical on a weekly basis. The swimming pool was heated and also had high levels of calcium. As a result, the facility went through several heat exchangers in a short timeframe. When inspected, the heat exchangers were covered with a white ‘scale-like’ material. After laboratory testing, it was determined to be calcium phosphate scale. Having this knowledge, the aquatic facility conducted a phosphate removal treatment procedure and the phosphate-based chemical was switched to a non-phosphate product. Since switching, the facility has operated for several years without any further heat exchanger problems.

Research continues

Presently, there is ongoing research into calcium phosphate scale and fieldwork confirmations and different problems and resolutions are being accumulated.

Due to the complexity of phosphate chemistry, it is not easy to predict a threshold level of phosphate scaling. To minimize risk, it is important to reduce not only orthophosphate, but also the calcium concentration. Regular draining and dilution can also help. Keeping the pH at the lower end of the acceptable range (7.2 to 7.4) is another method which can help reduce scale formation.

Recommended Water Parameter Levels
• Phosphates 200 parts per billion (ppb) or below
• Calcium hardness 150 to 300 parts per million (ppm)

So, while research continues, it does appear for now that regular testing of phosphate levels and calcium hardness, along with phosphate removal treatments, can help extend the life of heat exchangers in high calcium swimming pools and hot tubs.

This article was written by Terry Arko and originally appeared on Pool & Spa Marketing [link].