Identifying saltwater problems related to salt crystal chemistry

by Sally Bouorm | August 1, 2012 8:42 am

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By Bob Harper

It goes without saying salt is a critical component of saltwater pools. Without it, electrolysis within the electrolytic chlorine generator (ECG) could not take place, and chlorine could not be produced to sanitize the water. Yet, ironically, poor quality salt can produce several problems in a saltwater pool, including staining, scale, and increased chlorine demand.

The chemistry of salt crystals (i.e. sodium chloride [NaCl]) is the same for all pool salts. However, pool salt contains more than just NaCl. Where the salt originates and how it was produced—whether mined, mechanically evaporated, or via solar evaporation of salt water ponds—affects the levels and types of contaminants that are embedded within the salt crystals themselves.

What cannot be seen can harm a pool

Some pool salts contain organic contaminants, which are primary culprits for cloudy water and high chlorine demand. Others contain varying amounts of inorganic contaminants such as calcium (Ca), nitrates (NO₃⁻), silicates (SiO₄^4-), sulfates (SO²⁻₄) and metals (manganese [Mn], copper [Cu], iron [Fe]), many of which can affect water clarity, dissolution rate, stain and scale potential, and other important water balance factors.

Sometimes the presence of contaminants can be identified by the shape of the salt crystal, even before it is added to the pool. Generally speaking, the more irregular the salt crystal, the more impurities are embedded within. This is especially true of solar and mined (rock) salt since these salts undergo little, if any, processing to remove naturally occurring contaminants.

Mechanically evaporated pool salt, which tends to have a more uniform cubic shape, is purer, making it a better choice for pools. Mechanical evaporation involves solution mining (i.e. in-situ leaching [ISL] mining process), along with high heat, to produce salt from underground deposits. The high heat used to evaporate the salt eliminates many of the contaminants found in solar or mined salt.

After salt is added to the pool, other immediate indicators of contaminants may become apparent, such as:

• Slow dissolution, evidenced by the amount of time and brushing needed to completely dissolve the salt. Allowing salt to remain on pool surfaces while it dissolves may weaken the plaster surface and can cause staining and etching;
• Staining, seen immediately or gradually over time; and
• Cloudy water and chlorine demand, which commonly occur when solar salt is added to pool water. High levels of organic contaminants can cause visibly cloudy water almost immediately.

Pool professionals should always use salt specifically labelled as pool salt. Other variations can contain additives, which can harm a saltwater pool. Water conditioning salts, for example, may have added phosphates, which create a thin layer of scale to protect home plumbing systems. On pools, however, this scale can reduce the effectiveness of the ECG and eventually shut it down. These salts may also contain an anti-caking agent called yellow prussiate of soda (YPS), which contains iron that can stain pool surfaces.

The following tips can help pool professionals identify and solve some of the most common water quality issues associated with the use of improper or poor quality pool salt:

Scale

Of the issues caused by poor quality pool salt, scale is perhaps the most damaging as it hampers the ECG’s effectiveness and lifespan. Small white flakes coming out of the returns or accumulating on the pool floor is usually evidence of scale formation on the ECG plates. Further, unusual reduction or difficulty in maintaining chlorine levels, without other factors such as heavy rain or high bather load, can also indicate scale formation. The cleaner the cell plates, the more efficiently they will produce chlorine.

Most common scales, such as calcium carbonate (CaCO₃), can be confirmed by visually inspecting the cell and removed using diluted muriatic acid (hydrochloric acid). This typically involves soaking the cell plates for 10 to 15 minutes in a solution of five parts water to one part muriatic acid. Once it is removed, regular use of preventatives and maintaining proper water balance will help stop scale from reforming as well as protect the life of the cell.

Staining

Another problem, which can be caused by inorganic contaminants in the salt itself, is staining. Early warning signs for staining are purely visual and include mild streaking down the sides of the pool or discoloured pool surfaces and/or water.

Once the type of stain is identified, it can usually be treated with products containing citric (C₆H₈O₇), ascorbic (C₆H₈O₆), or sulfamic (H₃NSO₃) acid. Removing dissolved stain-causing metals can usually be accomplished with sequestering agents and filter aids. Maintaining good water balance as well as using high quality pool salt and stain prevention products can ward off future stains.

Cloudy water/chlorine demand

Water quality is another area of concern. Chlorine demand in saltwater pools (i.e. the condition in which the amount of chlorine generated by the ECG is not enough to overcome the level of organic contaminants in the pool) is often caused by poor salt quality.

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Organic contaminants from bather load, yard debris, or other sources will cause pool water to become cloudy if the ECG cannot produce enough chlorine. Since the ECG produces chlorine at a fixed rate, it can be difficult to overcome the chlorine demand caused by a large amount of contaminants. Depending on the magnitude of demand, multiple chlorine shocks may be required to solve the problem.

Inorganic contaminants, as mentioned previously, can also affect water clarity—whether dissolved or suspended in the pool water. This problem can be identified by cloudy water, or, if metal particles are present, the pool water will be discoloured. For example, copper typically causes a green tint, and manganese and iron cause varying brown, blackish and/or purple discolourations.[3]

Removing metal particles is fairly straightforward. It involves using a sequestering agent and filter aid to collect and remove particles. After this is performed, the filter should be cleaned. Removing minerals, e.g. nitrates, calcium, and sulfates, can be accomplished by partially draining the pool and adding fresh water to dilute.

Prevention is the best cure

First, it is important to check with the supplier to learn exactly where and how the salt is manufactured. The supplier should be able to guarantee the quality and consistency of the salt source and production process.

Similar to selecting the appropriate salt, it is important to choose the right maintenance and treatment products as well, as these can either help or create further issues when maintaining a saltwater pool. Therefore, it is important to use products specifically formulated for saltwater pools, without phosphates or sulfates. Phosphates can be found in some anti-scale products and sulfates are found in bisulfate (dry acid), which is often used to lower pH.

Due to the ECG’s high pH, temperature, and chlorine levels, products that are not formulated for saltwater pools can break down in this harsh environment and any phosphates or sulfates present can form additional scale. Further, phosphorous-based compounds also result in the formation of orthophosphates, which are nutrients for algae.

Although phosphate levels in certain treatment products may be minute, it only takes a small amount to cause large-scale problems. For instance, they can combine with calcium to form calcium phosphate, which is a particularly tricky form of scale in that it is not noticeable upon visual inspection, yet can hamper chlorine production significantly.

Some of the more advanced pool salts on the market contain anti-scale and anti-stain agents and are guaranteed to work in saltwater pools. Some even use all three types of scale control mechanisms—dispersion (typically a polymer that adds a positive or negative charge to pool water so mineral ions repel rather than attract); crystal modification (which rounds the edges of mineral molecules so they cannot stack into a cubic scale structure); and chelation (which keeps minerals suspended in water)—for the ultimate protection against scale.

Vendors and distributors can also educate pool professionals about the ingredients in the products they use and why some may be preferred over others. By avoiding harmful contaminates and using the purest pool salt available, pool professionals can guarantee their customers will get the most enjoyment out of their pool.

 

 

Bob Harper is vice-president of sales for Compass Minerals’ consumer and industrial business and also serves as general manager of the company’s Pristiva subsidiary. He has 25 years of experience in the pool and spa industry and is an expert in saltwater pool systems. He can be reached via e-mail at harperb@pristiva.net[4].

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