In good health: wastewater


Waste treatment is the poor relation on every build. The last thing to get anyone’s attention – until the planners start asking questions! In all cases, having a well-functioning sewage treatment system may make the difference between safe, clean ground and surface waters, and contamination of drinking water and habitats. 

This article is a guide to checking your system, making repairs and improvements and knowing what is involved in a site assessment to enable you to choose the right system.

Site and system
Older houses usually have a septic tank and percolation area whilst newer ones may have a mechanical treatment system, but the initial check will be much the same. Before you even leave the house you may already know two things. One, does your wc flush freely? Two, are you aware of any contamination of your own or a neighbour’s well? A poor flush may simply be a blocked pipe in need of rodding, or it may be that the tank or percolation area have become congested and in need of an overhaul. Well water contamination can be difficult to link to a particular pollution source, but may indicate the need for better treatment prior to percolation.

Moving outside, check the percolation area. If you don’t know where it is, then have a look for trees or shrubs that are thriving, particularly down-gradient of your septic tank. This often indicates a ready supply of nutrients. Some older properties may only have a soak pit, which basically bypasses the treatment element of percolation and introduces water closer to the groundwater level. If you have well contamination, this may be a possible cause.

A ‘percolation area’ (ROI) or ‘soakaway’ (NI) is the name given to a dispersed infiltration area which filters the effluent through the subsoil before it reaches the groundwater. In ROI a ‘soak-pit’ is a rubble-filled pit used as a disposal method for storm water from roof surfaces etc. Soak-pits do not filter water and are unsuitable for septic tank effluent or grey water disposal.

The ground around the percolation area should be free of surface ponding. Adjacent streams or drains should be free of black sludge, sewage fungus or other obvious signs of enrichment. If you spot any of these it’s a clear indication of inadequate infiltration. The recent septic tank inspection process in ROI found that the largest cause of failure was chronic lack of tank maintenance. Desludging the tank every year or two (depending upon the number of people in the house and the tank size), should therefore solve most problems.

If blockages or pollution persist then ‘secondary treatment’ in the form of mechanical aeration or a constructed wetland, for example, will help to reduce the pollution levels and lower the potential for clogging up the percolation area. Where greater treatment is needed, using ‘tertiary polishing’ via, say, a soil or sand polishing filter or reed bed, may be necessary. This applies to sites where the groundwater is vulnerable or with no infiltration in the soil so that the effluent may need to be routed to a drain or stream. A discharge licence is required for discharges to surface waters, and this won’t typically be given in ROI, (In NI it’s easier but not easy) so this option is really only suitable for existing problem sites rather than new-build projects.

Technically there should be a distribution box to split the septic tank effluent evenly between approximately four percolation pipes. Check that all the pipes have an even flow leaving the box. These are a newer addition and not usually found on properties more than fifteen years old.


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Feidhlim Harty

About Feidhlim Harty

Féidhlim is the director of FH Wetland Systems, an environmental consultancy business specialising in the design and planting of constructed wetlands, gravel reed beds and zero discharge willow facilities. FH Wetland Systems Ltd., 30 Woodlawn, Lahinch Rd, Ennis, Co Clare, tel. 065 6797355,

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