4.1 Water supply contaminantsThere are four main types of contaminants that can be found in water that is taken from bores, rivers, billabongs and lakes. These are germs, suspended solids, dissolved salts and chemicals.
GermsThese deserve the greatest attention because of the health risk they present to everyone in the community.
Nearly all the water collected from bores, rivers, lakes and billabongs has to be checked regularly and if necessary treated to make sure it is free of germs. Rainwater collected with equipment known to be free of germs is probably the only type of water supply that does not normally have to be treated.
When searching for the source of germ contamination of a water supply, it may be necessary to check the whole supply system to try and find the point at which the germs are entering the water. This may be at the water supply source, the tank, anywhere in the pipelines or a breakdown in the water treatment system.
Suspended solidsSuspended solids include small particles of clay, iron oxide or plant matter which hang in the water and give it a murky (dirty and cloudy) appearance.
These solids can be removed by letting the water stand to allow solids to settle. Suspended solids can also be removed from water by filtration. This means running the water through very fine material which will catch the solids.
Fig. 6.26: Water filtration through coarse and fine sand and gravel.
Dissolved saltsAs rainwater runs over the ground and down into the soil it sometimes comes into contact with limestone and similar rocks. Small amounts of minerals from these rocks dissolve in the water, in much the same way as sugar is dissolved in cups of tea. These minerals are the salts of sodium, calcium and magnesium. For example, sodium chloride (common salt), calcium carbonate (limestone) and magnesium sulphate.
The dissolved salts make the water hard. Hard water is what causes the white crust to form on the elements of electric kettles and on the inside parts of toilet cisterns. Soap will not lather easily in hard water. People may get an upset stomach from drinking hard water.
Hard water can be made soft by treatment with chemicals. However, this is not often done. Provided the mineral content is not too great and a danger to health, most people can put up with hard water.
Top of Page
4.2 DisinfectionTreating a water supply to kill germs is called disinfection. Communities get their water from sources such as bores, rivers, lakes and dams. The water from these sources is often contaminated; sometimes only slightly, sometimes badly. This is why the water supplier makes provision for water treatment (usually chlorination) between the water source and the storage tank or in the tank. This treatment should keep the water free of live germs and parasites.
These are some methods of disinfection:
ChlorinationChlorination uses chlorine chemicals to kill the germs and should leave sufficient
free residual chlorinein the water. This is a little extra chemical in the water which acts as a safety buffer against further contamination. That is, if all the germs in the water at the storage point are killed, there is still some chlorine left to attack any other germs which might get into the water system in the tank or the pipes which take the water to the community, for example, via a cracked or leaking pipe or tank.
The recommended level of free residual chlorine in drinking water is between 0.2 and 0.6 ppm (parts per million) or mg/L (milligrams per litre).
This means that there is between 0.2 and 0.6 parts of chlorine per million parts of water, or 0.2 and 0.6 milligrams of chlorine per litre of water. These units of measure are basically the same and either can be used in detailing the measured level of chlorine.
Swimming pool free residual chlorine levels are much higher than the level in drinking water.
The length of time which the chlorine needs to kill the germs depends upon the level of water contamination. It is important to note that at times the water supply, especially at the source, may be so badly contaminated that normal levels of chlorination will not be enough.
- a rotting carcass of an animal such as a cow or dog may have contaminated the water source
- a sewage leak or sewage dumped near the water source
- rubbish dumped near the water source
- water has high levels of iron
There are three main chemicals used to chlorinate water:
Chlorine gasMany communities have a gas chlorination system for their water supply. Cylinders of chlorine gas are connected to the water supply line. The gas is automatically fed into the water at the correct dosage to make sure that all germs are killed.
Fig 6.27: Gas chlorination of a small community water supply.
Chlorine gas is yellow-green in colour and has an irritating, sharp smell. It is an extremely poisonous gas and breathing even small quantities can be fatal.
If the gas chlorination system breaks down and causes chlorine gas to leak into the air, the EHP should make sure that no-one goes anywhere near the area and that the water supplier, is notified (told) immediately. People who enter areas into which chlorine gas has leaked must wear full breathing equipment (air tanks).
Top of Page
Sodium hypochloriteThe chlorine can also be combined with other substances. These can be in solid form or as a solution (liquid).
Sodium hypochlorite is one of these substances. This comes in a liquid form. Sodium hypochlorite is used where the chemical has to be added to the water on a regular basis. For example, in swimming pools or water tanks where the chlorine level needs to be checked every few days and sodium hypochlorite added as necessary.
Particular steps need to be taken in checking the chlorine level in drinking water and in adding more chlorine to the water. These are dealt with in Section 6.1.
Calcium hypochloriteThis is another chemical in which chlorine is combined with other substances.
Calcium hypochlorite comes as a white powder. It is often referred to as ‘A chlorine'. It is used for the same purpose and in the same way as sodium hypochlorite. It is also discussed in Section 6.1.
Calcium hypochlorite is not as strong as sodium hypochlorite in its germ killing action. However, it is cheaper to buy and is used more often.
Ultraviolet (UV) lightUltraviolet light cannot be seen by the human eye. However, when it is produced in a lamp (tube) other types of light are also produced which can be seen.
When the ultraviolet light is strong enough it is able to kill germs. The water flows through a container in which ultraviolet light producing tubes are set. The water pipes are placed between the ultraviolet light tubes. These pipes are made of Teflon which allows the UV light to pass through into the water and kill any germs present.
Fig 6.28: Ultraviolet light unit for water disinfection.
One disadvantage of ultraviolet light disinfection of water is that there is no residual effect. Germs are killed only at the point of contact with the ultraviolet light. Germs will not be killed if contamination occurs after the water has left the disinfection plant. Chlorine may still have to be added to provide the residual effect.
FiltrationWhen water is run through fine sand, the filtration process removes suspended solids.
Water supplies for large towns often have their disinfection systems assisted by filtering the water through large sand beds before chlorination. This will reduce the chemicals required for disinfection. However, this is rarely used in smaller water supplies.