Chemical Treatment of Private Swimming Pools

Scale Formation in Swimming Pools

The formation of scale in swimming pools is due to the precipitation of calcium or magnesium in the water as carbonates.

There are several factors that influence this as follows;

1. Amount of calcium or magnesium in the water
2. pH
3. Total Alkalinity
4. Dissolved carbon dioxide
5. Total dissolved solids

Langelier developed an index for determining when a water would become scale forming or corrosive. In practice to control this entails the adjustment of one or more of the following, value, alkalinity and calcium hardness.

Increase of any of these values beyond a certain point will cause scale formation.

As mentioned before, sodium hypochlorite will raise the pH but does not add calcium salts. Calcium hypochlorite adds calcium as well as raising the pH. However, sodium dichloroisocyanurate does not add any scale forming impurities and does not affect the pH.

Salts Concentration in a Pool

However, a factor that is often overlooked, is the concentration of salts in a pool due to solar evaporation.

In tropical and sub-tropical climates the evaporation rate from a pool could be as high as 60" in a season. This means that if there is no loss due to backwashing, the solid contents in the water could be doubled in a season. This is because the salts are left behind as the water evaporates and their additional salts are added in the make up water. This is more noticeable in diatomaceous and Hi Rate sand filters where the back wash rate is small.

The obvious method to reduce this is to empty portion of the pool water periodically and make up with fresh water. however, this will lower the cyanuric acid level required for stabilisation.

The pH can be reduced by adding hydrochloric acid or sodium bisulphate 2.4 lb of hydrochloric acid (33 %) or 2 lbs. sodium bisulphate will neutralise 1 lb. of calcium carbonate.

(NOTE: 5 ppm of calcium carbonate is equivalent to 1 lb. in 20,000 gallons)

Another method is to precipitate the calcium as Tricalcium phosphate by the addition of trisodium phosphate. 1 lb. of anhydrous trisodium phosphate will precipitate 1 lb. of calcium carbonate. This is readily removed from the bottom of the pool by vacuuming.

In very hard waters it is possible to use a sequestering agent such as "Calgon" (sodium hexa-metaphosphate) at the rate of 1/3 lb. per 5000 gallons, which is repeated at intervals. This compound ties the calcium up in a complex compound that prevents it precipitating out.

Scale Solution

When scale has built up, the pH should be reduced by adding not more than 3 pints of hydrochloric of hydrochloric acid to 10,000 gallons at one time. At least an hour should elapse before checking the pH and repeating the procedure if required. The dropping of the pH will increase the solubility of the water for the calcium deposits that will slowly redissolve with subsequent rise in pH. This is called "pH bounce" and will continue until such time as all scale has redissolved. Similarly with new concrete work, the pH will continually rise without the addition of chemicals, due to the leaching out of calcium deposits in the fresh cement. This may take several months before the pH can be stabilised.

Sudden growths of algae in a pool will also raise the pH due to the removal of carbon dioxide from the water by the algae for food.