Before water can be passed into the public supply, it is necessary to remove all potentially pathogenic micro-organisms. Since these micro-organisms are extremely small, it is not possible to guarantee their complete removal by sedimentation and filtration, so the water must be disinfected to ensure its quality. Disinfection is the inactivation of pathogenic organisms and is not to be confused with sterilisation, which is the destruction of all organisms.
Worldwide, chlorine is the most popular disinfecting agent for drinking water, although the use of ozone has recently become more widespread. The use of chlorine in water treatment, while not being acceptable to all, does save lives. In Peru, the reduction of the chlorine dose led to a cholera outbreak in which thousands died. Chlorine acts as a strong oxidizing agent which can penetrate microbial cells, killing the micro-organisms. It kills most bacteria but not all viruses. It is relatively cheap and extremely soluble in water (up to 7000 g m−3). It has some disadvantages. If organics are present in the water being disinfected, it can lead to the formation of potentially carcinogenic disinfection by-products (e.g. trihalomethanes; see below). The World Health Organization has given health-based guidelines for a variety of disinfection by-products, such as chloroform. If the water has been previously treated by coagulation and flocculation, the chances of organic pollutants being present to form trihalomethanes are remote. Slow sand filters are effective in removing trace organics.
Chlorine is a dangerous chemical and so requires careful handling. It can also give rise to taste and odour problems: for example, in the presence of phenols it forms chlorophenols which have a strong medicinal odour and taste.
HOCl, hypochlorous acid, is the disinfecting agent and is referred to as free available chlorine. Since chlorine is an oxidizing agent, it reacts with all compounds in water which can be oxidized, e.g. converting nitrites to nitrates, and sulphides to sulphates. As mentioned above, it also reacts with any organics present and can form trihalomethanes (THMs). These are single carbon compounds with the general formula CHX3 where X may be any halogen atom (e.g. chlorine, bromine, fluorine, iodine, or a combination of these). Some THMs are known to be carcinogenic. There is evidence to link long-term low-level exposure and rectal, intestinal and bladder cancers. There is therefore a limit of 100 μg l−1 for total THMs in water supplied for potable use. Chlorine also reacts with ammonia to form chloramines. Thus when chlorine is added to water there is an immediate chlorine demand which must be satisfied before a residual of chlorine exists for disinfection.
The formation of chloramines is as shown below:
The chloramines are disinfectants but not nearly as effective as free chlorine (they may have to be 25 times more concentrated to have the same effect).
Chlorine in compounds such as chloramines is referred to as combined residual chlorine. Although not as effective as free chlorine in disinfection, combined chlorine is less likely to produce objectionable tastes and smells. One reason for this is that combined chlorine does not react with phenols, which may be present, to form chlorophenols. In fact, ammonia is sometimes added to water for this reason. Combined residuals also last longer than free chlorine.
For disinfection with chlorine, the World Health Organization (WHO) guidelines recommend a minimum free chlorine concentration of 0.5 mg l−1 after a contact time of 30 minutes at a pH less than 8, provided that the turbidity is less than 1 NTU. The water leaving the chlorine contact tank is usually discharged with a chlorine concentration of 0.5–1.0 g m−3 to ensure that the water is kept safe throughout the supply and distribution system.
Concern with hazards of chlorine storage has led to the adoption of electrolytic generation of chlorine on large water treatment plants. In this process, sodium hypochlorite solution with a chlorine content of 6–9% is generated through the electrolysis of a solution of sodium chloride.
Recently, ozone (O3, a blue gas and a very strong oxidizing agent) has become popular as a disinfectant, particularly as it is effective against viruses and spores. In the UK, it is often used to oxidize any pesticide residuals present. Also, ozonation does not produce toxic by-products such as trihalomethanes which can occur with chlorine. It can, however, form toxic bromates if bromine is present in the water. In France, there are about 600 water treatment plants using ozone as a disinfectant. The drawback with ozone, however, is that it is not possible to have a residual level, as there is for chlorine, to confer protection in the supply and distribution system (O3 rapidly breaks down to oxygen when any particles are present). In ultra-clean water, however, it will remain as O3). Hence, after ozonation, the water is chlorinated before it goes into the supply system. The ozone used in water treatment plants is usually generated by passing dry air or oxygen between plates, across which a high voltage is imposed. It is expensive to produce, and the necessary equipment is complex.
Ultraviolet radiation can also be used to disinfect water, but care must be taken to ensure that no suspended solids are present which could shield the micro-organisms and prevent them from being destroyed. UV systems are generally only used in small-scale water treatment units. They do not give a residual for protection in the distribution system.