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.
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.