FIRST STEP IN WATER QUALITY
The water we use every day for cleaning, drinking or washing is termed potable, that is, it is safe for human consumption, but is not completely pure. It contains many dissolved materials including mineral salts and gasses.
For water to be comfortable to swim in, it is essential that it be non-corrosive, non-scaling and correctly balanced. The various factors involved are described below. Water that is balanced, and healthy for the swimmer will also benefit the water proofing membrane of the pool surfaces and prevent corrosion and erosion of equipment.
The recommended parameters and factors relating to water balance are :-
|pH||7.2 - 7.8|
|Total Alkalinity :||80 - 120 ppm as CaCO3|
|Calcium Hardness||250+ ppm as CaCO3|
|Total Dissolved Solids||less than 1500 ppm|
ppm is parts per million; it is the same numerically as milligrams per litre - mg/l
pH is a logarithmic scale indicating whether the. pool water is either acid or alkaline (basic). The scale runs from 0 -14. Neutral water (neither acid or alkaline) is approx pH 7. At pH values of less than 7, the water is acidic and may cause eye irritation, metal components may corrode and pool liners may be damaged; at pH values above 7 the water is alkaline, chlorine becomes progressively less effective in destroying bacteria and controlling algae growth, the water may become cloudy and scale formation may occur.
Swimming pool water must be maintained within the range 7.2 - 7.8. Ideal range is 7.3 - 7.5.
Alkalinity describes the amount of alkaline compounds (carbonates, bicarbonates and hydroxides) present in pool water. Many cheap pool test sets do not have any means of measuring alkalinity and its importance is often underestimated. If the alkalinity is low (below 60 ppm) there can be rapid changes in ph, corrosion of exposed metals, eye irritation and even coloured water (usually green. High alkalinity (above 160 ppm) is not so serious but can increase resistance to change in pH, making ph adjustment difficult and may cause cloudy water.
The ideal range is 80 - 120 ppm as CaCO3.
Total hardness is made up of dissolved calcium salts and smaller quantities of other mineral salts such as magnesium. In swimming pool chemistry it is usual to measure only calcium carbonate or calcium hardness. Low calcium hardness (below 50 ppm) can cause itching, corrosion of metal surfaces and/or grout loss.
When water evaporates from a swimming pool, the dissolved minerals, such as dissolved calcium carbonate, remain in the pool water. Thus the concentration of calcium carbonate tends to increase. The ideal levels should be:
250 ppm to prevent grout loss
500 ppm to inhibit corrosion
TOTAL DISSOLVED SOLIDS
The total dissolved solids (TDS) is the sum of the weight of the soluble compounds in the pool water. Normal drinking water contains a few hundred parts per million TDS. Brackish water contains 3,000 - 5,000 ppm. Sea water typically contains 35,000 TDS. Chemicals added to pool water to maintain sanitation, pH, alkalinity calcium hardness and to combat algae etc. all add total dissolved solids to the pool water. Dirt, dust and other debris introduced into the pool by bathers also contribute to dissolved solids.
High levels of TDS and the accompanying turbidity will impair the performance of the disinfectant, algicide, and other treatment chemicals added to pool water. As TDS levels rise, more "active disinfectant" is required to eliminate organisms in the pool water. Because organisms embedded in the solid particles, causing cloudy water, are protected from the disinfectant this is not generally a problem to TDS levels of 1500 ppm, at normal pool disinfectant levels. In addition high levels of TDS can enhance galvanic corrosion of metals, reducing the life of heaters, pipes and pumps.
BALANCING THE WATER
Balancing the water means selecting and achieving a set of conditions which will not cause corrosion but will maintain the water in a slightly scale forming condition.
Experience has shown that the Langelier Saturation Index gives the Pool Manager a reliable indication as to the water's condition. This simple formula uses the pH, alkalinity,calcium hardness, temperature and TDS readings. The alkalinity, calcium hardness, temperature and TDS are converted to factors as in the tables below:
|TEMPERATURE||BICARBONATE ALKALINITY||CALCIUM HARDNESS||TOTAL DISSOLVED SOLIDS|
LANGELIER SATURATION INDEX = (pH+AF+CF+TF)-TDSF
pH = actual pool water pH
AF = alkalinity factor
CF = calcium hardness factor
TF = temperature factor
TDSF = total dissolved solids factor
It should be noted that each of these components has its own set of parameters which should be adhered to. These parameters are:
|pH||7.3 - 7.5|
|Alkalinity||80 - 120 ppm|
|Calcium Hardness||500 ppm|
|Temperature||84 - 96°F|
|Total Dissolved Solids||below 1500 ppm|
Although this example shows a positive index and therefore should indicate a scale forming condition, the problem here is the high TDS. Experience has shown that under these conditions corrosion would still take place.
Action: Reduce the TDS below 1500 ppm by the introduction of fresh water.
|pH = 7.2||Factor 7.2|
|Alkalinity = 15||Factor 1.1|
|Calcium Hardness = 15||Factor 0.7|
|Temperature = 84°F||Factor 0.7|
|TDS = 2000||Factor12.2|
This example from a soft water area shows the typical problems. The pool would not only show signs of corrosion but also grout loss from the pool tank. Indicated by the negative index and the very low hardness level.
Action: Reduce alkalinity by the addition of sodium bicarbonate. Raise the calcium hardness by the addition of calcium chloride or, preferably a change of sanitiser to calcium hypochlorite. Reduce the TDS level by dilution with fresh water.
|pH = 7.3||Factor 7.3|
|Alkalinity = 100||Factor 2.0|
|Calcium Hardness = 500||Factor 2.3|
|Temperature = 84°F||Factor 0.7|
|TDS = 1500||Factor12.15|
This is an example of a well managed pool.. Action has been taken to obtain the correct results and maintain the saturation index just on the scale forming side.
It is essential when operating a pool to test the source water. It is very difficult to balance the pool water if the starting point is unknown. For example a pool in a soft water area using a sanitiser which significantly increases the TDS level has a dilemma.
The TDS level can only be reduced by dilution with fresh water. This introduces a large quantity of fresh soft water. This reduces the calcium hardness level. To raise the calcium hardness level we need to add calcium chloride. This also adds to the TDS level requiring the introduction of more fresh water again lowering the calcium level.
An overall look at the chemicals being used and their effects should be closely examined and a suitable combination selected. In the above example a change to a calcium based sanitiser would not only reduce TDS level but also automatically increase the calcium level.