The order the decisions happen in
A replacement is not a shopping decision followed by a site visit. It runs the other way. The site characterisation — desk study, trial hole, percolation tests — establishes what the ground will accept, and only then is a system selected. The site assessor records the result on a site characterisation form and recommends the appropriate system and discharge route.
Percolation value is the headline number, but it is not the only gate. The EPA also requires a suitable slope, all the minimum separation distances, and the required depth of unsaturated soil or subsoil above bedrock and the water table beneath the infiltration area. Other things can rule a site out entirely — underground utilities, certain sites within SACs, SPAs or NHAs, and floodplains.
Permitted percolation values
Straight from Table 6.4 of the EPA's 2021 Code of Practice:
- Septic tank and percolation area: 3 to 50
- Secondary treatment and soil polishing filter — pumped or underlying gravity discharge: 3 to 75
- — gravity discharge, 500mm wide trenches: 3 to 75
- — low-pressure pipe, 300mm wide trenches: 3 to 90
- — drip dispersal: 3 to 120
- Tertiary treatment and infiltration area: 3 to 75
Where a system is installed at the surface, the subsurface percolation value must be between 3 and 90.
Why this matters more here than elsewhere
Low-permeability subsoils are common across the west, and heavy clay is the defining ground condition in much of Leitrim and Roscommon. Under the 2009 code a great many of those sites were simply unsuitable for anything discharging to ground. The 2021 Code did not lower the environmental bar — it added engineered ways of meeting it on slow ground. If a site was assessed and written off before June 2021, it is worth knowing that the rules it was judged against have since changed.
One caveat worth stating plainly: a willow bed evapotranspiration system can be used for secondary treatment, but monitoring of full-scale systems found that full evapotranspiration was not achieved, so discharging final effluent to ground still requires an offset tertiary soil polishing filter.
Frequently Asked Questions
01 What decides which system I can install?
The percolation value of the ground, established by a site characterisation. The EPA's Table 6.4 sets the permitted ranges: a conventional septic tank with a percolation area needs a value between 3 and 50. A secondary treatment system with a soil polishing filter covers 3 to 75 for pumped or gravity discharge, 3 to 90 for low-pressure pipe in 300mm trenches, and 3 to 120 for a drip dispersal system. A tertiary treatment system with an infiltration area is 3 to 75. Where a system is installed at the surface, the subsurface value must be 3 to 90.
02 My ground drains too slowly for a normal system. Is that the end of it?
Not since 2021. Under the old 2009 code the maximum percolation value for discharge to ground was 75, and anything slower was simply unsuitable. The 2021 Code added two options for low-permeability soils: low-pressure pipe distribution, which works up to a value of 90, and drip dispersal, which works up to 120. The EPA describes drip dispersal as specialised, requiring expert design and installation and increased maintenance — it is not a like-for-like swap for a percolation area.
03 What if the percolation value is above 120?
Then the site is unsuitable for a system discharging to ground. Discharge to surface water may be an alternative, but that requires a Water Pollution Act licence from the local authority. Site improvement works are also ruled out where the slope exceeds 1:8, where the average value from the three tests exceeds 120, or where the required separation distances cannot be met.
04 What if the value is below 3?
The ground drains too fast. Below a value of 3 the retention time in the soil and subsoil is too short to treat the effluent properly, so importing soil or subsoil with a slower percolation rate and installing a suitable system may be considered. As with slow ground, discharge to surface water is an alternative that needs a licence.
05 What does a replacement cost?
We do not publish a figure, and we would rather say so than invent one. The concrete tank manufacturers serving Ireland — O'Reilly Oakstown, Molloy Precast, Carlow Concrete Tanks — all quote rather than publish list prices, and no installer in the five counties publishes a rate either. Figures circulating in forum threads and on aggregator sites are homeowner anecdote, not price data. Get a written quote from a site visit, and remember the grant covers 85% up to €12,000 if you qualify.
06 Is a treatment system better than a septic tank?
Not automatically — it is a different answer to a different ground condition. A conventional septic tank and percolation area is simpler, passive, and has no running cost, and where the percolation value allows it, it is usually the sensible choice. A secondary treatment system exists because the ground will not take the effluent from a plain septic tank. The EPA also lists factors worth weighing beyond the ground: simplicity, longevity, capital cost, running cost, maintenance cost, required de-sludging frequency, power requirements and carbon footprint.
Where We Do This Work
Septic Tank Replacement & New Systems is available across Galway, Mayo, Roscommon, Sligo and Leitrim — Galway, Mayo, Roscommon, Sligo and Leitrim. Each county page covers what is specific to it, from published prices in Galway to slow-draining ground in Leitrim.