The importance of testing and removing phosphates

by Dave Flaherty | October 12, 2020 12:25 pm

By Alicia Stephens and Anjelica Swanson

People purchase hot tubs for many different reasons, including health benefits, therapeutic massage, and overall relaxation.

Regardless of the motivation to purchase, hot tub users want their ownership experience to be as easy and trouble-free as possible. However, due to the hot water and aeration that are key elements to hot tubs, their maintenance can be challenging. Therefore, establishing an effective maintenance routine that includes testing the water regularly is key to keeping them clean, clear, and ready for use. Many hot tub owners are unaware of the challenges phosphates can cause. Consistently testing for and removing phosphates is important to ensuring an effective hot tub water maintenance program.

What is a phosphate?

There are many different types of phosphates and phosphoric acids; however, the form tested in pools and hot tubs is orthophosphate. These nuisance phosphate-based compounds are formed when a phosphorous-containing compound, such as polyphosphate or a phosphonate, react with oxygen in an oxidation-reduction reaction. The usual cause of this reaction is from the addition of chlorine into a pool when a phosphorous-containing substance is already present in pool water. Not all phosphorous-based compounds are problematic. In fact, there are many different phosphorous-based products used in water treatment applications due to their efficacy and ease of use. It is the end product, orthophosphate, which provides no benefit to hot tub water but, instead, reacts with normally soluble minerals to form insoluble compounds that may lead to poor water quality or create other problems in hot tubs.

Where do phosphates come from?

The goal for hot tub maintenance is to sustain a phosphate level of less than 125 parts per billion (ppb) in the water. Maintaining this level can be a challenge due to the almost constant introduction of phosphate-containing contaminants into the hot tub environment. Phosphates enter hot tubs from many different sources. Outdoors, for example, fertilizers and other lawn-treatment products may contain phosphates that can inadvertently be carried into the water by wind or rain runoff from the yard. Phosphates can also be unintentionally added into hot tubs from the soaps, cosmetics, and detergents used by bathers, as well as through their urine and sweat. Environmental factors such as dust, plant debris, some source water (including wells), and soil are additional sources of phosphates.

Certain specialty products, such as sequestering or chelating agents, contain phosphonic or phosphoric acid, which eventually break down into orthophosphate in the water. Stain and scale-control products often contain phosphorous-based compounds due to their best-in-class control of scale, which is prevalent in the warm water environment in hot tubs. These products also help prevent iron and copper staining or water discolouration due to metals, and some will help prevent corrosion. Many municipalities add phosphorous-based scale products at water-treatment plants to help reduce buildup and prevent corrosion in the distribution lines. In fact, it is not uncommon for some areas to have readings between 500 and 1000 ppb straight from the tap.

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.

Preventing issues related to phosphates

Phosphate testing and removal should be included as part of the hot tub maintenance routine. Phosphates are still not tested as widely as standard water parameters such as pH, total alkalinity, and calcium hardness. To prevent issues related to high phosphate concentrations, levels must be monitored diligently to ensure they remain low. Orthophosphates are the only form of phosphates that will show up on water test kits and the only form that can be affected by current phosphate remover products.

Maintaining a phosphate residual of less than 125 ppb is the key to preventing phosphate-related issues in the hot tub environment. Near-zero levels of phosphates lead to less accumulation of calcium phosphate scale, as well as an increase in water clarity and overall water quality. Fewer phosphates in the water lead to less contaminants, which means less problems to deal with in the hot tub. Phosphate removers are specialty chemicals used to reduce orthophosphates from pool and hot tub water. These use minerals that react quickly with orthophosphate to produce an insoluble phosphate compound that can be removed by the filter.

In a hot tub, the white precipitation formed when adding a phosphate remover will cause the water to appear hazy for a short period of time, but the rapid turnover rates in hot tubs will see the water become clean in a few hours. The filter will physically remove phosphates from the water as it filters out the insoluble compound. Per application directions, phosphate removing products outline an initial dose to be followed with a weekly maintenance dose to keep the phosphate level as low as possible.

It is imperative to clean the hot tub filter before and after the addition of the initial dose of hot tub phosphate remover. A clean filter upon product addition will allow for the efficient removal of phosphates, and cleaning the filter after the treatment will ensure proper filtration as regular maintenance resumes.

Conclusion

Removing phosphate is not just a passing trend in water care. With more hot tubs using newer technologies, maintaining water quality is more important than ever. A complete hot tub maintenance routine should include phosphate testing and removal in addition to the traditional components of sanitizing, oxidizing, and balancing hot tub water chemistry.

Water balance and equipment protection, as well as water clarity, is easier to maintain when phosphates are less than 125 ppb. In saltwater hot tubs, removing phosphates is a key component in limiting the formation of calcium phosphate and ensuring the efficient operation of the chlorine generator. Hot tub manufacturers that offer saltwater systems often require the control of phosphate levels to validate warranties. By adding a phosphate remover to traditional hot tub maintenance programs will decrease water and equipment problems and increase customer satisfaction with the look and feel of their water.

Alicia Stephens is the education and training manager for Biolab Inc. In her 19 years with the company, she has focused primarily on education, training, and development, as well as technical support and new product research and integration. Currently, Stephens supports all education and training initiatives for the Biolab Pro Dealer division. She can be reached via email at alicia.stephens@biolabinc.com.

Anjelica Swanson is a technical services analyst for Biolab Inc. She is a Certified Pool Operator (CPO) and has more than eight years of experience working in research and development. She received her bachelor’s degree in biology from the University of Tennessee at Chattanooga. Swanson can be reached via email at anjelica.swanson@biolabinc.com.

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