The answer most people are given is "every year". That came from the 2009 code. The EPA replaced it in 2021 with a table based on how many people live in the house and how big the tank is — and for most households the real interval is longer.
Source: Table 12.2, EPA Code of Practice for Domestic Waste Water Treatment Systems (Population Equivalent ≤ 10), 2021. This is the EPA's guidance on frequency, not a booking and not a legal minimum — a neglected tank or unusual water use can need attention sooner.
| Tank volume | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|
| 3 m³ | 5 | 3 | 2 | 2 | 1 | 1 | 1 |
| 4 m³ | 5 | 5 | 3 | 3 | 2 | 2 | 1 |
| 5 m³ | 5 | 5 | 5 | 3 | 3 | 2 | 2 |
| 6 m³ | 5 | 5 | 5 | 5 | 3 | 3 | 2 |
| 2.5 m³ | 5 | 3 | 2 | 1 | 1 | 1 | 1 |
| 3.5 m³ | 5 | 4 | 3 | 2 | 2 | 1 | 1 |
| 4.5 m³ | 5 | 5 | 4 | 3 | 2 | 2 | 2 |
| 5.5 m³ | 5 | 5 | 5 | 4 | 3 | 2 | 2 |
Columns are the number of people living in the house. Cells are years between de-sludging.
Table 12.2 of the EPA's 2021 Code of Practice for Domestic Waste Water Treatment Systems. They are reproduced exactly, with no adjustment. The underlying research is EPA Research Report 253 on desludging rates and mechanisms for domestic wastewater treatment system sludges in Ireland.
Use the 'I do not know' option. The EPA Code says that where the tank volume is not known, a volume of 2.5 cubic metres should be assumed and the frequency determined on house occupancy alone. That is exactly what the calculator does. If you want to measure it instead, the useable volume of a rectangular tank is its length multiplied by its width multiplied by the liquid depth — the depth of the liquid, not the depth down to the lid.
Neither. It is the EPA's guidance on frequency, and it is a starting point rather than a rule that fits every household. A house with unusual water use, a system that has been neglected for years, or a tank that has never been emptied at all may need attention sooner. Your own de-sludging records and an operator who has actually seen the tank beat any table.
Because most of that advice is from the 2009 code, which recommended de-sludging every year regardless of household size or tank volume. The 2021 Code replaced that blanket figure with this table. A great many Irish pages have not been updated.