When dealing with pool water the term ‘hardness’ can be somewhat confusing! Obviously water cannot be hard in a conventional sense unless it is frozen, so what does it mean and how does it affect the running of a swimming pool? Put simply hardness is the concentration of calcium and magnesium salts in the water. For the purpose of swimming pools we shall only deal with calcium in this post.
A rough guide to water supplies in the UK.
The water supply to our homes is often described as hard or soft.
Hard water occurs due to the water supply filtering through soluble rock such as limestone and chalk, resulting in mineral salts like calcium dissolving into it. Water supplies in the south east of England are normally hard and this can lead to scale appearing on kettles and pool equipment. Hard water also stops your soap foaming as well. The main chemical which makes water hard is calcium bicarbonate. The calcium part is the hardness whilst the bicarbonate is the alkalinity. As a result hard water has a very high pH level.
Soft water has very little minerals in it and as a result is very hungry to dissolve metals such as lead as well as concrete or grout in pools. To stop this from happening you will need to add calcium to the water which is discussed in more detail later on in this post.
Calcium hardness is a measure of the amount of calcium dissolved in water.
Why is calcium hardness important in swimming pools?
Most swimming pools are made of concrete and are often tiled with grout between the tiles. Pool equipment, particularly pumps and heaters are made of metal. As mentioned earlier soft water is hungry to dissolve anything soluble it comes into contact with so it will dissolve the concrete, grout and metals until it is satisfied. Therefore it is necessary to add calcium to the water to prevent this from happening.
What should the calcium hardness level be?
The ideal water for swimming pools is medium to hard water. The PWTAG publication “Swimming Pool Water – Treatment and Quality Standards for Pools and Spas” recommends the following concentrations of calcium in pool water:
- Minimum 80 ppm
- Maximum 150 ppm
Pools in hard water areas have over 200 ppm of calcium in the water supply so they will never have to add calcium. Pools that chlorinate using calcium hypochlorite will usually operate with calcium concentrations well over 150 ppm. This will not do any harm in a swimming pool unless it is very high (over 500 ppm).
PLEASE NOTE – These levels apply to swimming pools they do not apply to domestic hot tubs. Water hardness in hot tubs is discussed in a separate blog post.
The loss of grout on swimming pools is often blamed on low concentrations of calcium hardness in the water. Pool builders often recommend boosting the calcium hardness level to as high as 400 ppm to prevent grout loss. PWTAG carried out research on this and could find no evidence to support using a higher concentration of calcium hardness.
How to adjust calcium hardness
To increase the calcium level you need to add calcium builder (calcium chloride) or use calcium hypochlorite as your source of chlorine.
1.5kg of calcium chloride will increase the calcium hardness level by approximately 20ppm in a 10,000 gallon (45m3) pool.
In a soft water area the calcium level can be decreased by diluting with mains water (backwashing). In hard water areas the only way of decreasing the calcium level is to use a water softener which is not recommended or necessary in swimming pools.
How to test for calcium hardness
Calcium hardness can be tested using test strips, a comparator, a photometer or the traditional tablet counting method, which in my opinion is more accurate than either of the aforementioned methods.
Please note once you have established that your water supply is hard, you rarely need to check the calcium hardness level.
Tablet Count Testing Method:
- Fill a dilution tube with 100ml of pool water.
- Add calcium hardness tablets one at a time and shake the sample to ensure they are dissolved.
- When the water changes colour from pink to bright purple the test is complete.
- The calcium level is obtained by the following formula; (the number of tablets used x 20 ) – 10
For example if 7 tablets are used the calcium level is (7 x 20) – 10 = 130 ppm