Chlorine and trihalomethanes

The addition of chlorine is an essential step to ensure harmful bacteria are eliminated from your drinking water. The formation of Trihalomethanes is a consequence of this process.

Trihalomethanes (THMs) occur when the chlorine that is used for disinfection reacts with organic matter in the water.

This reaction primarily depends on the purity of the treated water (i.e. the amount of organic matter present), the amount of chlorine added, and the length of time before the drinking water reaches your tap.

How THMs are formed

Most drinking water in Ireland comes from surface water sources e.g. rivers, lakes and streams. These water sources often contain tiny remaining particles of dirt, twigs, leaves, etc. which are referred to as organic matter. 

In order to make the water safe to drink, chlorine is added during the disinfection process. If you drink water that has not been properly disinfected, there is a risk of serious and possibly life-threatening illness. 

A consequence of adding chlorine to the treated water is the formation of THMs which occurs when the chlorine that is used for disinfection purposes reacts with organic matter in the water. This reaction depends on the purity of the treated water (i.e. the amount of organic matter present), the amount of chlorine added, and the length of time before the drinking water reaches your tap.

In some of our water supplies we have detected levels of THMs which are higher than the allowable limit. 

Removing Trihalomethanes

Factors for THMs forming

Amount of organic matter in the source water

Higher concentrations of organic matter may impact the ability of the treatment plant to remove it

Amount of chlorine added to the treated water

This is because the higher the concentration of chlorine, the more likely THMs are to form.

Time chlorine has to react with any organic matter in the treated water

This depends on how far the water travels from the treatment plant to your tap. Most water supplies have at least one storage reservoir and several kilometres of watermains. The longer it takes, the more likely that THMs will form.

The time of year as THM formation can be seasonal

THM formation is more likely to occur during the autumn where there is a much higher presence of dead and decaying leaves. The temperature of the water is also higher at this time, which can also influence THM formation.

Health and THMs

There are some concerns about possible health risks from drinking water with elevated THM levels. Issues associated with exposure to THMs from drinking water in the long and short term are uncertain and unproven. According to the Health Service Executive (HSE), several scientific studies have been undertaken in this area but none have found any conclusive evidence of a link between THMs and serious illness such as cancer

“...benefits of using chlorine to treat our drinking water are much greater than any possible health risk from THMs.“

Health Service Executive (HSE)

What we're doing

We are taking action to remove the risk of elevated levels of THMs forming in all public water supplies. Key to achieving this is to ensure the treatment process removes as much organic matter as possible, thereby, removing this primary precursor of THM formation.

In order to achieve this, we have initiated a prioritised programme of investment to address all treatment deficiencies, including those for THMs. With this programme we are committed to improving water quality through investment in water services infrastructure and are targeting specific Water Supply Zones where the risk of elevated THMs are highest. 

As well as prioritising investment in removing the risk of elevated levels of THMs in public water schemes, we are upgrading and optimising the treatment processes at all water treatment plants across the country. This includes controlling the concentration of chlorine in the water, which is one of the key measures in controlling THM formation. 

Additional measures we are taking to minimise THM formation, includes the reorganisation of networks and regular flushing of storage reservoirs and pipelines.

Further information

To learn more about THMs and your drinking water visit the Environmental Protection Agency website. If you would like to know more about THMs in drinking water and your health, visit the Health Service Executive website.