Chlorine

Is my water supply chlorinated?

All the water that we supply to Wellington, Porirua, Lower Hutt and Upper Hutt is chlorinated.

Why is water chlorinated?

Chlorine is a disinfectant that kills water-borne bacteria and viruses that could cause diseases. The water treatment processes that we use are designed to remove these germs as well as protozoa (such as giardia and crypyosporidium) from the water supply, but disinfection of treated water with chlorine provides another level of protection for the public. In the unlikely event that some germs have not been removed from water during treatment, chlorine will kill them. It also has a residual effect, so will kill bacteria and viruses if they get into the supply system between our water treatment plants and consumers' taps, for instance through a cracked pipe.

Water-borne diseases such as typhoid and cholera were a common cause of death in New Zealand during the 19th and early 20th centuries, but have been controlled, largely due to improved water supplies and sanitation.

How much chlorine is added to water?

For supplies from unsecure water sources, the Ministry of Health's Drinking Water Standards for New Zealand require that there be a minimum of 0.2 milligrams of chlorine per litre of water (at pH 8.0) available everywhere within a distribution system, to provide effective disinfection. One milligram per litre is one part per million parts of water. Chlorine breaks down when it comes into contact with the organic matter that it neutralises, so the level of free available chlorine at any point within a distribution system isn't constant over time. Our dosing levels are based on maintaining a minimum chlorine residual (free available chlorine) of around 0.4 mg/L.

The Drinking Water Standards give a maximum allowable value for free avaiilable chlorine of 5.0 mg/L. Chlorine is also listed as an "aesthetic determinand", because it can adversely affect the taste and odour of water. The aesthetic guideline value for chlorine in consumers' water is 0.6 mg/L. We typically add 0.6 to 0.8 mg/L of chlorine at our treatment plants to achieve the necessary chlorine residual in the distribution system.

Who should I contact about chlorine taste and odour problems?

Taste and odour may occur if chlorine acts on organic material within water distribution pipes. Please ring your city council to log the issue - this allows the issue to be tracked.