Water chlorination is the process of adding chlorine (Cl₂) to water. It is used to inactivate bacteria and other microbes, and prevent the spread of waterborne diseases such as cholera, dysentery and typhoid. (Many of these diseases are still prevalent in parts of the world where clean water and chlorination is not available/used.)

Chlorination provides an extra layer of protection against waterborne disease by leaving a chlorine residual in the water. This residual makes its way through our network of pipes and reservoirs to the very last house at the end of the line. This ‘residual’ of chlorine in the network is just as important as the chlorine at the treatment plant. It protects customers from the risk of re-contamination which may occur in the network.

Chlorination offers residual protection against a wide range of bacteria including eColi and Campylobacter (which was the bacteria that caused the problem in Havelock North). It ensures that any bacteria that enters the system after the initial treatment is inactivated.

Yes, chlorine is used in all of TDC's drinking water supplies. The Downlands, Orari, Peel Forest, Pleasant Point, Seadown, Te Moana, Temuka, Timaru, Upper Pareora and Winchester water supplies are all treated with chlorine.

There may be some minor taste implications, but you do have choices.

• Fill a jug of water and leave it on the bench or in your fridge overnight. The chlorine will dissipate naturally over a few hours.
• Install a carbon filter

We will use as little chlorine as possible to keep your water safe. This is expected to be roughly 0.8mg of chlorine for every litre of water. As water travels through our reticulation, some of the chlorine gets used up. So the chlorine residual will lower to about 0.5mg per litre at the end of the network as it enters your household plumbing. For reference the safe limit for chlorine is 4mg per litre.

On-site generation of sodium hypochlorite, which is a liquid form of chlorine.

Chlorine has been used safely around the world for about 120 years. It keeps millions of people all over the globe – and in New Zealand – safe from waterborne illness. (Note: over 80% of the country is supplied with chlorinated water).

Under the Drinking Water Standards and based on current knowledge, you can only use treatment that will constitute no significant risk to the health of a person who consumes 2 litres of water a day over their lifetime. Chlorine meets this safety threshold.

Not in the concentration that we will be using for disinfection.

The International Agency for Cancer Research (IARC) does not believe chlorinated water is either a probable, or even possible, cause of cancer.

The Journal of the National Cancer Institute (United Kingdom) states that “water chlorination is one of the major disease prevention achievements of the 20th century”, and that it “has become the principal means of effectively reducing waterborne enteric diseases”, which the World Health Organisation has stated account for a significant number of deaths every year, even in developed countries. So chlorine actually saves lives.

There are no known health impacts from drinking water effectively treated with chlorine. Carbon filters are installed on dialysis units and these mitigate the risk.

No, chlorine is safe for pets, including cats, dogs, birds etc.

However, water for a fish tank may need to be neutralised (see question about pet fish below).

If you have fish in outside ponds you will need to either turn down in-coming water to an absolute trickle (this dilutes the chlorine level to a safe amount for your fish), or fill up drums of water and let them sit for at least 24 hours before using (the UV light from the sun evaporates chlorine).

For fish tanks or bowls inside, fill up a container of water, let it sit for at least 24 hours and replace 1/3 of this water at a time with what is in the tank already.

De-chlorinating kits (sodium thiosulfate) can be purchased at pet supply stores.

You could also install a carbon filter at your tap to remove chlorine from the water you use for your fish.

Chlorine can be an irritant for existing skin conditions such as eczema. If you feel your skin getting dry or itchy, use a moisturiser after having a shower or bath. If you notice increased skin irritation, or eczema symptoms talk to your GP. To minimise exposure to chlorine, try bathing at times of low water demand – in the middle of day on weekdays, early in the morning (before 7.30am), or late in the evening (after 9.30pm). You can also install a carbon filter to remove chlorine from your household water.

There is a risk that a contamination event could go undetected. Backflow is one of the major causes of contamination. We are aware of an incident when a water main was shut off to repair a pipe and a consumer had left a hose running on the compost bin. This led to an internal backflow event where grass was discharged from the kitchen tap. Things like this are a risk because we may not be advised and the property may be unaware that backflow is occurring. Since 2017, 1% of our 700 tests within the Geraldine water supply reticulation network have detected bacteria.

We have a legal and moral obligation to the community to do all that we can to protect the quality of their drinking water. TDC is also obligated to learn from the Havelock North experience, where a serious contamination event occurred in 2016.

In 2016, Havelock North experienced a major outbreak of campylobacter that resulted in over 5,000 people getting sick, and four deaths. Several reasons were identified for why this occurred including contamination from the source, no treatment processes and ineffective barriers to contamination.

Following the subsequent independent inquiry, commissioners made a recommendation to government and all water suppliers that there should be residual disinfection within the pipe network. This led to the recent legislative changes under the Water Services Act 2021.

Chlorine provides a residual disinfection, and effectively a safeguard against potential contamination.

Yes. However, no other options are, at this time, as effective or guaranteed to meet our obligations.

This statement is not correct. Council has chlorinated the Geraldine Water Supply after contamination events in the past.

For example, in 2017, there were two samples taken within the Geraldine reticulation that detected the presence of bacteria. Chlorine was added to the water at the source to eliminate the contamination.

The status quo is no longer a realistic option in light of legislative changes under the Water Services Act 2021.

Yes, this is intended as a permanent solution.

• Water  NZ  - Why is chlorine important in drinking water?
• Water NZ - How can we make water safe to drink?

As mentioned above, chlorine and any associated by-products can be removed by using a household granular activated carbon (GAC) filter. These come in a range of different sizes and models starting from small units which can be purchased locally from hardware stores starting at approximately $100. There may be other options available - talk to plumbers or other suppliers for options that meet your budget and your requirements.