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 (SiO44-), 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.
