It is estimated that there are 400,000 septic tanks in Ireland. These treat the effluent from 1.2m people and deal with 230m litres per day.

While septic tanks can work quite effectively the key aspect of wastewater treatment performance is maintenance. The septic tank will only provide partial treatment of domestic effluent. It is necessary to have a percolation area to complete the treatment process.

The septic tank is most commonly a concrete tank with one or two chambers. The incoming domestic effluent enters below the surface so as to cause minimal disruption. The solids settle to the bottom and a scum layer forms on the surface. The solids are broken down aerobically by bacteria and the outflow is discharged from beneath the surface to the percolation area.

The treatment process is completed by bacteria which attach themselves to the stones and treat the effluent as it trickles through the percolation area. The septic tank requires emptying when either the scum layer or the solids at the bottom approach the outlet level. The solids or scum would cause blockages of the percolation area, prevent the septic tank discharging and lead to poor final effluent treatment.