There are almost half a million registered domestic wastewater treatment systems in Ireland, according to new figures released by the CSO.

There were 5,481 new systems set up last year, a one per cent increase compared to 2020. Overall, there are 481,694 systems in the country.

These systems are usually septic tanks which collect, treat and discharge wastewater from households that are not connected to public sewer systems.

Six coastal counties accounted for 45% of registrations last year.

Mayo accounted for 6% as the county has 28,458 domestic water treatment systems, with 289 installed in 2021.

Cork has 55,034 domestic wastewater treatment systems, the highest proportion in the whole country. It accounted for 11% of all systems, and 12% of new registrations from last year.

Galway had 42,577 and Kerry had 33,890 systems, accounting for 9% and 7% respectively.

Next were Donegal, Mayo and Tipperary, who each accounted for 6% of all systems and Wexford at 5%. These seven counties accounted for roughly half of all individual wastewater treatment systems.

Household owners accounted for 97% of all systems registered last year.

The registration and inspection system for domestic wastewater treatment systems was introduced under the Water Services (Amendment) Act 2012.

The Act was in response to a European Court of Justice ruling against Ireland in October 2009. The purpose of the Act is to protect ground and surface water from pollution from malfunctioning systems.

Each water services authority is required to establish and maintain a register of domestic waste water treatment systems situated within its functional area.