Chemical pollution refers to all chemicals that are dissolved or suspended in the swimming pool water. There are a large number of different chemicals that are introduced into the water, either deliberately as part of the treatment regime, or incidentally due to the nature of the source water or via transfer from bathers bodies. Some examples:
- Disinfectant (e.g. chlorine)
- Disinfectant by-products
- pH correctant (e.g. Acid)
- pH buffer (e.g. Sodium bicarbonate)
- Calcium harness increaser (e.g. Calcium chloride)
- Calcium minerals (i.e. hard water)
- Cosmetics (e.g. de-odorants etc.)
It is worth noting that much of the chemical pollution is caused by the addition of treatment chemicals (chlorine, pH correctant etc.) and that the rate of addition of these chemicals is often directly proportional to the addition of chemical (and other) pollution introduced by bathers. The key message here is that the importance of embedding a culture of pre-swim showering cannot be under-estimated.
The levels of chemical pollution present in the pool water can be measured using an instrument called a ‘Total Dissolved Solids’ meter (TDS meter). This measures the electrical conductivity of the pool water. As pure water is not a conductor of electricity, the more conductive the swimming pool water is, the more it must contain by way of elements other than water, i.e. it is the chemicals dissolved within the swimming pool water that is conducting the electricity, not the water itself.
The levels of chemical pollution need to be kept under control, otherwise the pool will look cloudy and unclean and will also cause a bad ‘chlorine smell’ and cause bathers discomfort through eye irritation and rashes etc. The TDS level of the swimming pool should be kept well below 1000mg/m3 above the TDS level of the mains water supply. The best way of controlling chemical pollution is via prevention. Minimising the amount of chemical pollution being introduced into the pool via bathers is the first step. This will then lead to the pool operator not having to add as much disinfectant to the pool, which leads to not having to add as much pH correctant onto the pool either. Bather pre-showering is the most effective way of minimising the amount of chemical pollution that bathers introduce into the swimming pool water.
Chemical pollution can also be controlled re-actively (after the pollution has entered the water) by a process of dilution, which is where an adequate supply of clean, fresh water (usually from the mains supply) is introduced into the pool on a regular basis. The recommended rate of dilution is 30 litres of fresh water to be added per bather, per day.
There can be several different sources of water to fill swimming pools, some examples:
Water can be supplied to a location in two main ways:
- Supplied by water companies
- Abstracted locally under a license from the Environment Agency
The chemical content of source water can vary according to where it comes from and how it’s treated. Records are kept by water suppliers and pool operators should ask for copies and use the information as a key input into the pool water treatment regime.
Alkalinity is a measure of the alkaline salts dissolved in it and has an influence on how resistant the water is to changes in the pH value. The higher the alkalinity level, the more resistant the water is to changes in pH. There is more detail on this on chapter 10. pH Control.
Total hardness is a measure of the calcium and magnesium salts dissolved in it. Calcium hardness is the part of total hardness that consists of calcium salts and has an influence on how corrosive or scale-forming the water is.
Sulphates are salts of sulphuric acid that occur widely in everyday life and will therefore be present in pool water. High sulphate levels in combination with high TDS levels can attack cement and grout, so they need to be controlled to be lower than 360mg/l. If this is not possible (due to high usage of sodium bisulphate and aluminium sulphate), sulphate-resistant Portland cement and epoxy grout may be required to resist sulphate attack.
Sweat and urine from bathers are nitrogenous products that will form ammonia through decomposition. When chlorine reacts with ammonia, chloramines are formed. There are potentially three stages:
- Monochloramine (Chlorine + Ammonia)
- Dichloramine (Chlorine + Ammonia + Monochloramine)
- Trichloramine (Chlorine + Ammonia + Monochloramine + Dichloramine)
Chlorine will also react with and oxidise nitrogenous compounds (the precursors to ammonia; mainly urine and sweat). These reactions form irritant chemical by-products and are also a waste of chlorine (which is needed for disinfection of biological pollution, such as harmful bacteria).
Trihalomethanes (THM’s) are organic compounds that occur in swimming pools as a by-product of the reaction between chlorine and bather-added pollution such as sweat and urine. They can also arise in source water due to the reaction between humic acid and disinfection compounds. THM’s can be found both in the swimming pool water and in the air above it, giving rise to several routes of entry into the body (skin contact, ingestion and inhalation). THM’s have been associated with an increased risk of bladder cancer and studies have suggested that it is reasonable to assume that exposure to high levels of THM’s for long periods may present some poorly defined risks to pregnant women.
Swimming pool should aim to operate with THM’s at less than the UK drinking water standard of 100 micrograms per litre. However, monitoring for THM’s is difficult and expensive, so does not form part of the routine monitoring regime for swimming pools. Pools managers should be mindful of THM’s though and seek to control levels by adopting procedures to encourage pre-swim showering, control of bather loading and responsible use of disinfectants (as little as will get the job done and an avoidance of shock-dosing).