Chlorination disinfection and contact time
Chlorine is used worldwide to make water safe to drink. In order to work, enough chlorine needs to be added for a sufficient length of time. This is called Contact Time.
One of the most important steps in the water treatment process is disinfection. The most widely used method of disinfecting treated water is through the addition of chlorine.
Chlorine is used throughout the world for this purpose and is highly effective in killing harmful bacteria that could cause illness to you or your family.
In order for chlorine to kill bacteria in drinking water, there needs to be enough chlorine added, but it also needs sufficient time. This is called Contact Time and refers to how much chlorine has been added to the water and how long that chlorine has been allowed to mix and react with the water.
Contact time notices
Where Irish Water has confirmed a contact time issue at a particular property, we hand deliver a specific advisory notice to the occupants of that property. This advisory notice is prepared and issued in consultation with the Health Services Executive (HSE).
Click on the counties below to see all water supplies where there is a current Contact Time issue. It details the type of notice that's in effect for each places which are either:
- Dual: A Dual notice offers the consumer two choices to ensure the water is safe to drink i.e. leave stand for a minimum of 30 minutes or boil and cool the water.
- Boil Water Notices (BWN): A BWN requires that the consumer boils and cools the water only.
It also shows the number of properties and population affected in comparison to the whole supply. Only the affected properties were issued with a specific advisory notice.
If you are on any of these water supplies and have not received a hand delivered notice, then you are not affected by this issue.
Contact Time is a minimum standard set down by the World Health Organisation (WHO) which all drinking water supplies must meet. Irish Water is applying this standard as part of its National Disinfection Programme, which has the primary objective of ensuring all supplies receive full and proper disinfection.
Contact Time is expressed in a compound unit called milligram minutes per Litre (mg.mins/Litre). According to the WHO, water with a Ct of 15mg.min/L is the minimum standard for disinfection, for example:
- If your water has 0.5mg/L chlorine present and it has been allowed to fully mix for 30 minutes, then the Ct is 0.5mg/L x 30 minutes = 15mg.min/Litre (which is the minimum WHO standard for disinfection)
- If your water has 0.5mg/L chlorine present and it has been allowed to fully mix for only 20 minutes, then the Ct is 0.5mg/L x 20 minutes = 10mg.min/Litre (which does not meet the minimum WHO standard for disinfection)
- If too much chlorine is added to your drinking water to compensate for the short contact time, then your water may develop an unacceptable chlorine taste.
National Disinfection Programme
Through the roll out of the National Disinfection Programme, Irish Water is undertaking a more rigorous technical assessment of the existing chlorination processes at all our plants, including the calculation of Contact Time levels for the properties being supplied, particularly those that are closest to the treatment plant.
This programme is looking at over 800 sites across the country. Arising from this programme we have discovered a small number of sites with some properties in close proximity to the treatment plant. The water supply to these properties has not had sufficient time to react with the added chlorine. There is sufficient chlorine in the water however it needs a minimum of 30 minutes contact with the water to achieve the minimum standard for disinfection.
Properties located in close proximity to the treatment plant with insufficient contact time are receiving drinking water that has not been fully disinfected to WHO standards.
We have engaged with the HSE and the EPA on this issue and to ensure the public health of our customers is fully protected, we have issued an appropriate Advisory Notice to any affected properties. Because of the relatively small numbers of properties affected to date, these notices have been hand delivered to each property.
Advice to properties
To ensure the health of the consumer, but equally to reduce as much as possible any inconvenience, we have issued notices that offer two options to guarantee the water is safe to drink:
- Fill a clean container with cold water from the kitchen sink tap and leave it stand for a minimum of 30 minutes prior to consuming, as this will allow the water enough time to ensure it is adequately disinfected; or
- Boil and cool the water before consuming.
Prior to issuing these notices we measured chlorine levels at the affected properties. We did this to ensure the advice contained within these notices posed the least inconvenience as possible to householders.
For those properties where we are assured that adequate disinfection can be achieved through giving the water enough time to react with the chlorine, we issued a “two-option advisory notice”. For a small number of properties, we issued a Boil Water Notice without the option of leaving the water stand for a minimum of 30 minutes.