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The number of registered domestic waste water treatment systems, such as septic tanks used by homes not connected to a public sewer, increased by 1.2% to 498,283 in 2024 (See Table 1).
In 2024, Cork had the highest number of registered domestic waste water treatment systems at 57,009 (11.4%) (See Table 1).
The seven counties of Cork, Galway, Kerry, Donegal, Mayo, Tipperary, and Wexford accounted for more than half of all domestic water treatment systems in 2024 for the fifth year in a row (See Table 1).
There were 5,926 new registrations in 2024, which was 469 more than in 2023, and an increase of 8.6% (See Table 2).
In 2024, household owners accounted for 97% of all registered wastewater treatment systems, 483,481 out of a total of 498,283 systems (See Table 3), and 95.3% of new registrations, 5,649 out of 5,926 (See Table 4).
New registrations may include an element of late registrations.
Map 1 illustrates the domestic wastewater treatment system registrations by county in 2024, including both new and existing systems. Cork recorded the highest number of total registrations, with 57,009 systems, representing 11.4% of the national total. Of these, 728 were new registrations, accounting for 12.3% of all new systems registered in 2024. Galway followed with 44,073 total registrations (8.8%) and 571 new systems (9.6%). The map highlights the concentration of registrations in a small number of counties.
Figure 1 shows total, existing, and new registrations of domestic waste water treatment systems for 2020-2024. The bar chart shows the existing and new registrations. Total registrations are shown through the line graph. The amount of total registrations has steadily risen from 475,742 in 2020 to 498,283 in 2024. The amount of new registrations went from 4,157 in 2020 to 5,926 new registrations in 2024. There was an increase of 8.6% from the new registrations in 2024 compared with 2023.
Table 1 presents total registrations of domestic waste water treatment systems by county for 2020-2024. Cork recorded the largest share of registered domestic wastewater treatment systems, accounting for 11.4% of the total, with 57,009 systems. This was the fifth consecutive year where the seven counties of Cork (57,009 or 11.4%), Galway (44,073 or 8.8%), Kerry (34,672 or 7%), Donegal (32,041 or 6.4%), Mayo (30,406 or 6.1%), Tipperary (27,164 or 5.5%), and Wexford (26,863 or 5.4%) accounted for more than 50% of the registered systems. Dublin had the lowest share of registered domestic waste water treatment systems with 6,601 registrations (1.3%) in 2024.
Figure 2 presents a bar chart of the seven counties with the highest number of domestic wastewater treatment system registrations in 2024. These counties collectively account for more than half of all registrations with 252,228 systems highlighting a continued regional concentration in terms of registered systems.
Table 2 presents new registrations of domestic waste water treatment systems by county for 2020-2024. In 2024, there were 5,926 new systems registered, up from the 5,457 new systems registered in 2023, representing an increase of 8.6%. Of the new systems registered in 2024, Cork accounted for the largest proportion of new registrations with 12.3% (728). Galway had the next highest share with 9.6% (571). Meanwhile, Carlow had the fewest number of new systems registered with 1.2% (73).
To further examine the trends within the counties with the highest shares, Figure 3 displays the top seven counties by new registrations in 2024, which together account for 2,976 systems, or more than 50% of the total 5,926 registered. Notably, Cork (728 or 12.3%), Galway (571 or 9.6%), Donegal (430 or 7.3%), Wexford (351 or 5.9%), and Mayo (320 or 5.4%) feature in both the top seven for total and new registrations, reflecting consistent trends. Roscommon (296 or 5%) and Meath (280 or 4.7%), however, emerge specifically in the top seven for new registrations, highlighting the increase in registrations in those counties.
Table 3 presents total registrations of domestic waste water treatment systems by type of owner in 2024. Of the 498,283 systems registered in 2024, households owned 483,481 or 97% of total systems. Public Authority and Other Non-Domestic systems each account for 1.5% of the total. Longford had the highest percentage of Public Authority systems at 4.2% (338), while Dublin recorded the highest percentage of Other Non-Domestic systems at 3.6% (238).
Table 4 presents the new registrations by owner type in 2024. Domestic owners accounted for the majority of new registrations in 2024, representing 95.3% (5,649). Other Non-Domestic registrations comprised 4.4% (260), while Public Authorities made up the remaining 0.3% (17). Galway stands out due to it making up the majority of the new Public Authority registrations with 12 systems. Figure 4 reinforces the distribution of owner types, visually highlighting the predominance of Domestic ownership.
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Statistician's Comment
The Central Statistics Office (CSO) has today (26 September 2025) published Domestic Waste Water Treatment Systems 2024.
Josh O'Driscoll, Statistician in the Environment Division of the CSO, noted that: "Domestic waste water treatment systems, such as septic tanks, are used by homes not connected to public sewers. In 2024, the registrations of these systems rose by 1.2% to 498,283 (See Figure 1 and Table 1). There were increases in every county with Cork recording the highest proportion of domestic waste water treatment systems with 57,009 (11.4% of total). This was followed by Galway (8.8%), Kerry (7%), Donegal (6.4%), Mayo (6.1%), Tipperary (5.5%), and Wexford (5.4%), with these seven counties accounting for more than half of all registered systems (See Table 1 and Figure 2).
In 2024, 5,926 new systems were registered, an 8.6% increase compared with 2023. Cork had the largest registration of new systems at 12.3%, followed by Galway (9.6%), and Donegal (7.3%) (See Table 2). Almost all systems (97%) were registered by households (Domestic), with only a very small share belonging to Public Authorities or Other Non-Domestic (See Table 3). Households also accounted for the majority of new registrations in 2024, with 5,649 (95.3%) waste water treatment systems. Other Non-Domestic registrations totalled 260 (4.4%), while Public Authorities registered 17 (0.3%) systems (See Table 4 and Figure 4)."