According to the World Health Organisation (WHO) over 2000 chemical contaminants have been found to be generally occurring in waters. Of these, 750 have been found in drinking water, of which 600 are organic and the WHO suggests that these may only represent a small fraction of the total organic matter present in drinking water.
Organic compounds merit a whole branch of chemistry in their own right. They are all based on carbon. Carbon is a very significant element because of its ability to form chains with itself and other atoms to form a wide range of different molecules. Indeed, carbon is the basic element of life itself. For this reason, organic contaminants in very small quantities can interfere with the body’s biochemistry with far-reaching effects.
Organic compounds found in drinking water fall into several categories as follows:
- CHLORINATED ALKANES AND ALKENES – solvents widely used in industry for cleaning and degreasing.
- CHLOROBENZENES – solvents.
- PESTICIDES – including fungicides which control plant diseases, herbicides which kill weeds, and insecticides. Three hundred different pesticides are used in the UK.
- POLY AROMATIC HYDROCARBONS (PAH's) – found in tar pitch used to line the inside of water mains.
- PHENOLS – strong antiseptics.
- NATURAL ORGANIC SUBSTANCES – formed by the decomposition of plant material e.g. humus.
- OTHERS – including pharmaceuticals, water treatment chemicals, non-chlorinated solvents, detergents and organometallics.
The main health concern in relation to organic contamination is carcinogenicity. This is the ability of a substance to trigger cancers. Many organic substances found in water have been proven to have carcinogenic properties when present in higher concentrations. However, it must be emphasised that there is little evidence to date to indicate that the concentrations of organic substances found in drinking water have any of these effects.
A survey carried out in London did not show any statistically significant relationships between cancer and water re-use, although a survey of this type is difficult to interpret due to the number of variables involved.
Some organic substances found in drinking water are present as a result of the water treatment process itself. Chlorination of water containing natural organic substances give rise to a group of contaminants known as Trihalomethanes (THM's) which have shown to be carcinogens. Although it is difficult to show any relationship between THM's in drinking water and health problems, there is currently concern because it has been shown that the formation of THM's is time-dependent and continues following chlorination in the water distribution system.
Another group of compounds formed during chlorination are chlorophenols. Phenols are found in raw waters as manmade and natural substances and they have a slight taste of their own in low concentrations. When chlorinated, they form chlorophenols, which have a very strong antiseptic taste and are responsible for a large number of taste complaints.
It can be seen from the above that it is very difficult to ascribe health effects to organic substances in drinking water. It is, however, well known that organic substances are responsible for aesthetic and organoleptic (affecting the sense organs) complaints.
In recognition of this problem some water treatment processes use activated carbon to remove most tastes and odours.