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Some concerns about chlorination.

Water chlorination is of increasing environmental concern because of the large amount of chlorine used and its ability to produce toxic byproducts

Chlorine is used as a disinfectant because its high oxidising capacity makes it biocidal. It is not a selective agent; it does not target one group of organisms over another. It kills bacteria before it kills higher organisms because bacteria are smaller and have a proportionally greater surface area exposed to its effects; but all are susceptible (for marine organisms lethal concentrations of residual chlorine range from 0.01 to 0.1 mg/l).

Current large-scale uses of chlorine for this purpose include:

* disinfection of domestic water supplies. Attempts to arrest bacterial proliferation in water distribution systems often require the addition of large amounts of chlorine. In Europe, a daily use of one gram of chlorine per person has been calculated (200 l water per inhabitant, chlorinated at a level of 5 mg/l). |1~

In North America, equivalent water use per inhabitant is often higher than in Europe, and increases with affluence. In the case of our home town, at certain times of the year the tap water reeks, and we have personally proven that this water will kill tropical fish and inhibit initiation of fermentation in batches of home-brew. Chlorine in water distribution systems also increases the rate of corrosion of metal water pipes.

* anti-fouling treatment of cooling waters. One 1,000-MW thermal or nuclear power plant chlorinating its incoming cooling water can consume as much chlorine as a city of 2.6 million inhabitants. The outgoing water contains residual chlorine. Lost in the concern about potential hazards from the effluent of nuclear power plants, little attention is paid to this outflow.

* disinfection of urban wastewater. It is traditional (in fact mandatory in many jurisdictions) for urban wastewaters, after primary or higher treatment, to be chlorinated before discharge. This can create problems in the receiving waters, whether fresh or salt.

Seawater differs from freshwater in its reaction with chlorine because of the presence of bromide ion in all seawater and its absence from most freshwater. Free residual chlorine cannot exist for long in seawater; it reacts rapidly with bromide ion to give bromine and chloride ion. The bromine then reacts rapidly to give, among other products, brominated derivatives of the organic matter in seawater. So the effectiveness of chlorine is sharply reduced once the effluent is dispersed to seawater and it does not have the persistence of action that is noticeable in potable water supplies.

As an example, chlorine usage for the proposed Halifax-Dartmouth Metropolitan Wastewater Management System would be 4 g per day per person served by the system (ultimate capacity 550,000 people). |2~ It would be used to disinfect the effluent from the (primary) treatment plant before this is discharged to the sea. Primary treatment alone may achieve bacterial removal of 50 to 90% (but is inefficient in the removal of viruses; median 10%). We must consider whether we gain by a chlorination that does little to reduce pathogen numbers while in the discharge pipe (contact time 15 to 20 min),and nothing to destroy pathogens once the waste stream reaches seawater, but does create potentially troublesome byproducts during the process and discharges these to inshore waters.

* pulp and paper-mill effluent. In the diminishing number of paper mills in which chlorine is used as the bleaching agent, the total effluent has been defined to be a hazardous substance under the Environmental Protection Act. The need to close fisheries because of contamination by chlorinated compounds (such as polychlorodibenzofurans), which has occurred on Canada's west coast, would disappear if chlorinated substances were absent from effluents before discharge. In the case of other mills, where chlorination is used to disinfect wastewater before discharge, the same considerations apply as for urban wastewater.

Waters that have been chlorinated contain, in addition to free chlorine, a whole suite of chlorinated compounds (see L. McCarty's paper, p. 22) created by the process.

Though opinions may differ on the degree to which these chlorinated byproducts are immediately harmful to humans and other organisms at the concentrations at which they occur (though this can be up to 0.3 mg/l in the case of chloroform in the drinking water of various cities), there is no dispute that they include many persistent and potentially hazardous substances, some of which are bioaccumulative. Among these compounds are potent mutagens (e.g., chloro-derivatives of purine and pyrimidine bases), and some compounds that have been proven to have carcinogenic properties (including trihalomethanes, the production of which is closely correlated with free chlorine residuals).

Recent statistical analyses of epidemiological investigations into the association between chlorination byproducts in drinking water and cancer suggest a positive association between consumption of chlorination byproducts in drinking water and bladder and rectal cancer in humans. |3~ Even if this accounts for only a small proportion of the total occurrence of these types of cancer, the overall incidence of cancer in our society is far higher than that of waterborne infectious diseases, so we should balance the risks posed by aquatic pathogens against those attributable to chemicals in chlorinated water.

Effect upon fisheries

Although chlorinated and brominated compounds (e.g., bromoform) are produced naturally by marine algae, discharge of chlorinated residues to the sea contributes significantly more of them to the coastal zone. Field observations show decreased biomass and lower species diversity in receiving waters affected by chlorinated wastes. |4~

A less dramatic effect of chlorination is "taint": the acquisition by a fish product of a pronounced taste unacceptable to the customer. Though this is not a public health problem, it can be devastating in its economic effects.

Molluscan shellfish (e.g., oysters) can filter 10 to 24 litres of water an hour and are thereby highly susceptible to picking up trace components of chlorinated effluents that taint their flesh. Producers have sustained commercial losses when their product has become unsaleable for this reason.

Alternatives to chlorination include ozonation and UV-irradiation technology. These are as immediately effective against bacterial pathogens as is chlorine, and are additionally effective against hepatitis A, Giardia lamblia, and Cryptosporidium (the disease-causing organisms of greatest current concern in most developed countries) that are resistant to chlorine. Both techniques are becoming increasingly economical as compared to chlorination, ozone disinfection in particular is being widely-used in Europe.


The potential negative effects of chlorination may outweigh any potential benefits. There needs to be a serious evaluation to determine, in each individual situation, the need for chlorination, the risks/benefits, the long-term effects of chlorine use, and the feasibility of alternative treatments.

For example, in the case of the Halifax Harbour Cleanup Project, the highly-oxygenated cold salt water to which the effluent is to be discharged is an effective natural disinfectant for many land-derived pathogens. The health risk (from contact -- even Bluenosers do not voluntarily drink seawater) is in any case primarily at swimming beaches that are open (for reason of low air and water temperature) only a few months of the year.

Under the conditions for release for the Halifax Harbour project (N.S. Dept. Environ., Environ. Review Branch, Sept. 3, 1993), alternative methods of effluent disinfection (including UV irradiation technology) must be evaluated by the proponent on an ongoing basis in relation to both primary and advanced primary treatment. It is encouraging that a Canadian company (Trojan Technologies Inc., London, ON) is a major manufacturer, if not the manufacturer, of this equipment worldwide.


1. Abarnou, R. and L. Miossec, 1992. Chlorinated waters discharged to the marine environment: chemistry and environmental impact. An overview. The Science of the Total Environment, 126, 173.

2. Anon., 1993. Federal-Provincial Environmental Assessment Review Panel for the Halifax-Dartmouth Wastewater Management System. Halifax, Nova Scotia, April 1, 1993 (Evening Session). international Rose Reporting, Inc., 255 pp.

3. Morris, R.D., Audet, A.-M., Angelillo, I.F., Chalmers, T.C., and F. Mosteller, 1992. Chlorination, chlorination by-products, and cancer: a meta-analysis. American Journal of Public Health, 82, 955.

4. Osborne, L.L., 1985. Response of Sheep River, Alberta, macroinvertebrate communities to discharge of chlorinated municipal sewage effluent. In: Jolley et al. (eds.), Water Chlorination, 5, 481.

5. Smith, R.A., 1994. Water quality and health: a global perspective. Geotimes, 39, 19.

Roger Pocklington, FCIC, is a research scientist at the Bedford Institute of Oceanography in Dartmouth, NS; David Wimberly is a Dartmouth citizen actively involved in environmental issues.
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Author:Pocklington, Roger; Wimberly, David
Publication:Canadian Chemical News
Date:Mar 1, 1994
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