Why chlorination ?

Probably one of the most widely recognized uses of chlorine in everyday life is disinfection of drinking water.  Chlorine was first used to make water safe in the late 19th century.  At that time, it proved invaluable to control the spread of water-borne diseases such as typhoid, cholera, dysentery and gastro-enteritis.

Fighting such diseases, which have killed more people than all the wars in history, remains vital today.  For example, according to the World Health Organization (WHO) "3.4 million people in developing countries, most of them children, die every year from diseases associated with lack of safe drinking water, inadequate sanitation and poor hygiene[1]".

A powerful disinfection agent, elemental chlorine and its derivatives, sodium hypochlorite and chorine dioxide, quickly kill bacteria and other micro-organisms when added to water in minute quantities.  Chlorine has the major advantage of ensuring clean water right to the tap , whereas the action of other disinfection techniques - such as ozone, ultraviolet light and ultrafiltration - is much more temporary.  In addition to purifying water, chlorine not only helps to remove unpleasant tastes or odors, but also helps eliminate unwanted nitrogenous contaminants, and control the growth of slime and algae in distribution pipes and storage tanks.

Today, most of world's drinking water is made safe with the help of chlorine.  It is an essential part of the purification processes used by water companies to supply our homes, industry and ensure that discharges of wastewater to rivers and seas are safe.

What makes chlorination more interesting than other techniques ?

There are other processes designed to purify drinking water, such as ozone, ultraviolet irradiation, and ultrafiltration.  However, the principal advantage over alternative disinfection processes is that chlorination is the only method that persists long enough to keep water germ-free right up to the tap "the remanence effect".

Pathogens can easily enter the distribution system downstream of a water treatment plant especially in older or poorly-maintained pipeline networks.  Chlorine also prevents the growth of biofilm (for example: algae in pipes and slime formation in storage tanks).  Another distinct advantage is that it can be used with relatively simple techniques and at low costs, which makes chlorination a particularly appropriate disinfection technique for lesser-developed countries.

What about chlorination byproducts ?

During the water treatment process, chlorine reacts with organic matter such as decomposing leaves or soil to create what are called disinfection by-products (DBPs).  The most common class of DBPs is trihalomethanes (THMs).

Toxicological studies suggested that some THMs are carcinogenic to laboratory animals, but only at levels many thousands of times greater than those found in drinking water.  Recent studies on chloroform (the main type of THMs) show that it is unlikely to cause cancer at the extremely low levels found in drinking water.

The World Health Organization's (WHO) International Programme on Chemical Safety recently concluded that "There is insufficient epidemiological evidence to support a causal relationship between bladder cancers and exposures to chlorinated drinking water."[2] EPA and Health Canada have also reached similar conclusions.[3]

Based on these findings, the WHO maintains that the "Risks to health from DBPs are extremely small in comparison with inadequate disinfection."[4]

In addition to concerns about carcinogenicity attention has also focused on possible relationships between DBPs and adverse reproductive and developmental effects.  Overall however, evidence for such effects are sparse and the findings inconsistent.

Based on available research, WHO concluded that the "risks to health from DBPs are extremely small in comparison with inadequate disinfection" and urged that disinfection not be compromised in attempts to control DBPs (N.B. In 1993, WHO issued Guidelines for drinking water quality[5]).

This sets values for various DBPs on the basis of a single excess case of cancer in a given large population of people each drinking two liters of water per day for 70 years).

It is essential to remember that the primary purpose of chlorination is the protection of public health and that any minimal or hypothetical adverse effects have to be balanced against the benefits gained from chlorination.  This is well illustrated by the tragic consequences of a misinterpretation of US Regulations in 1991 by the Peruvian Government that led to suspension of the chlorination of that country's water supplies.  The resulting cholera epidemic, which also spread to neighboring countries in Latin America, caused about 800,000 cases of cholera and more than 6,000 deaths.[6]