Recently, I presented at the Beca Water Technical Conference on chlorination (or not) of drinking water supplies. It’s a topical issue and one worthy of wider dissemination. First, to give you some background…

Since August 2016, Havelock North has been infamous as the place with the water supply that made people sick. Through a number of systemic failures in water security practices, storm runoff from a sheep farm, contaminated with Campylobacter, entered the town’s water supply at a borehole. It led to widespread gastro-intestinal illness. Of 13,000 residents, 5,500 became ill; 45 of whom were hospitalised and, tragically, four died from associated causes. It has the stigma of being New Zealand's largest recorded outbreak of waterborne disease. At the time, the water supply wasn’t chlorinated, nor had any other form of protection. 

Since the Havelock North outbreak, many councils have become more aware of their own vulnerabilities. Some have taken the decision to add chlorination to reduce risks, particularly those with untreated groundwater-sourced drinking water; often sparking negative feedback from residents because of the unpleasant taste and other issues. 

My interest in the chlorination debate goes back to the fundamentals of what we’re trying to achieve with the treatment of community water supplies. With polarising views being publicised, I’d like to try to present a balanced view of the pros and cons in the argument. 

On the pro side, what are the benefits of maintaining an ongoing, low level of chlorine in water supplies – otherwise known as a chlorine residual?

  • Maintenance of microbiological quality through the distribution system to the tap. Water is by no means a sterile product – biological activity in the distribution system is significant. This quote neatly summarises the context: “Water can be considered a perishable product with a shelf life (detention time in system), packaging, (pipes and storage facilities) and preservatives (free chlorine or total chlorine) (Kirmeyer et al., 1999). 
  • Disinfection from distribution contamination event. A free chlorine residual has been shown to provide a significant reduction in contamination risk - with sewage mixing with chlorinated potable water at up to around 1% by volume.
  • Barrier in the event of a water treatment failure or similarly a raw water contamination. Using the “Swiss cheese” model of public health risk management, even if not essential to achieve compliance with drinking water standards, chlorine provides another barrier to reduce the risk of “the holes aligning”.
  • Tracer to monitor for contamination. Monitoring bug levels in a water supply is complicated and time-consuming. Conversely, measuring free chlorine residual is simple and quick. By seeing a chlorine residual in a water supply, we can be confident that bug levels are being suppressed to a safe drinking level. 

And on the con side, why not maintain a chlorine residual?

  • Consumer preference / taste. This is widely given as a reason, particularly where communities are not accustomed to chlorine. Consumer confidence shouldn’t be underrated – lack of confidence in a water supply can lead to people not drinking enough water or making less healthy choices as to what they drink. In most cases, what is identified as a “chlorine taste” is actually due to a reaction with ammonia or organic matter, forming compounds with a taste identified as chlorine-like. This can include systems which have historically operated chlorine-free, so the addition of chlorine to well-developed biofilm can cause taste issues – although complaints can be minimised where the addition of chlorine has been gradual. 
  • Cultural values. This is a significant issue, particularly in some of our smaller, iwi-managed, water supplies. For example, the relationship of Māori with fresh water has been summarised by the Iwi Chairs Forum as - “Our wai (water) is an inseparable part of our whakapapa and our identity, and is a fundamental part of what drives our very existence. The future health and wellbeing of our waters are a matter of utmost importance to all iwi, as well as all New Zealanders.” As explained to me, Māori feel a strong connection to their water supply, and by choice, would prefer to collect water directly from the source. This is balanced against the convenience and value of reticulation to the home, and the health benefits of removing pathogens from the community water supply. Nonetheless, chlorination is seen as removing the life force of the water. I know of two communities, each supplying around 100 households, where they’ve said they would discontinue operating their supply if forced to chlorinate.
  • Chlorinated by-products. The formation of by-products from the reaction of chlorine with organics in the water supply, many of which are known carcinogens, is an issue. Yet it’s generally considered to be a much lower risk in comparison to the benefits of eliminating pathogens. It is, however, the primary reason for a number of European supplies going chlorine-free.
  • Cost. Yes, chlorination will add cost to operation of a water supply but, in comparison to other disinfection options, offers excellent value for money. In terms of large water supplies, it is generally those supplies that are prioritising quality well ahead of cost that have been going chlorine-free. Hence cost is generally not a significant issue in the decision to go chlorine-free.


There are effective alternative disinfectants available and in use, including ozone and UV. However, these disinfectants do not provide a persistent residual within the network. The only viable alternatives that can provide a persistent residual are chloramination (or combined chlorine) and chlorine dioxide.  

Chloramination involves dosing ammonia to form a more stable, but less effective, form of chlorine. It’s not used currently in New Zealand but is used in many larger centres internationally, including Brisbane, Sydney and Melbourne. Chloramines are an effective “preservative” but have limited disinfectant strength.

Chlorine dioxide has niche applications, but has limitations including the maximum dose being limited due to the formation of chlorite and chlorate, relatively low persistence, along with dosing complexity. 

Epidemic Versus Endemic Risk

The risks associated with ‘chlorine-free’ can be considered under two categories: epidemic risk and endemic risk.  

Epidemic Risk 

These risks are reasonably well known, with the existence of the more significant events – such as Havelock North – being undeniable. Some 35% of water supply contamination events are known to be due to reticulation contamination. In a significant proportion of these, a chlorine residual could have significantly reduced the consequences. Similarly, for source (including untreated groundwater) or treatment failures, chlorine can reduce the consequences.  

Endemic Risk

The endemic risk is somewhat harder to quantify: what is the background level of illness caused by pathogens in the water supply?

To understand this, we need to consider the level of bacteria in our water supplies. An HPC (heterotrophic plate count) of around 10/ml is considered acceptable for a treated potable water, yet this concentration can increase 10-fold through the distribution system. Note that HPCs are the culturable bacteria, which may be only 1% of the total bacteria present. Hence the compliant, chlorinated water at your tap could contain 10,000/ml, or 2 million bugs in a glass! 

Fortunately, the vast majority of these bacteria are non-pathogenic to healthy individuals. However, there are exceptions – Aeromonas can grow in the distribution and, although not pathogenic for most healthy individuals, can be pathogenic for susceptible individuals. Legionella can also multiply in distribution systems; a significant problem at elevated temperatures but they can also grow at ambient temperatures in a distribution system.

Studies have compared the outcomes of people supplied with compliant, chlorinated tap water, against groups drinking point-of-use high-purity reverse osmosis-treated or bottled water. In one case, the higher-quality water reduced gastro-intestinal illness by 35%, and another by 14 - 40%.  Yet two other studies found no difference, illustrating the difficulty in proving the background endemic impact water supplies may be having.


So, what’s my personal view? I have to say I’ve moved a step closer to the compulsory chlorination camp, but still consider chlorination shouldn’t be compulsory. I think the current Drinking Water Standards and Water Safety Plan requirements are close to the mark; compliance is easier and chlorination is encouraged, but not compulsory. 

Some points that should be considered if going chlorine-free include:

  • The size of the population, and the proportion of the most vulnerable immune-compromised people within this population. If chlorine-free, do the vulnerable need additional treatment of their water supply?
  • Level of risk in the source and treatment. Is the treated water a high-quality, biologically-stable product?
  • Distribution management – this is about minimising contamination risks, providing good turnover and reducing opportunities for biological growth within the distribution. The best European examples of chlorine-free systems include, among the risk management measures implemented, maintaining distribution minimum velocities – something that may be contrary to our fire-fighting codes.
  • Monitoring – without being able to use residual chlorine as an indicator for bug-free water, increased monitoring should be applied, including monitoring for a greater range of pathogens. The assumptions around E. coli being a sound surrogate for all pathogens may not be valid in a chlorine-free system.  

A question was asked at the end of my talk: would I drink chlorine-free water? The answer is ‘yes - I believe I have a healthy immune system, and the risk we are talking about is low for me as an individual, but still significant risk if we are dealing with large populations.’

So, if you were given a choice, would you be prepared for a chlorine-free water supply?

About the Author
Philip la Roche

Technical Director - Water Engineering

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