The treatment performance of different subsoils in Ireland receiving on-site wastewater effluent.

Current Irish guidelines require a comprehensive site assessment of a percolation area for wastewater disposal before planning permission is granted for dwellings in rural areas. For a site to be deemed suitable, the subsoil must have a percolation value equivalent to a field saturated hydraulic conductivity in the range 0.08 to 4.2 m d(-1) using a falling head percolation test. A minimum of 1.2 m of unsaturated subsoil must also exist below the invert of the percolation area receiving effluent from a septic tank (or 0.6 m for secondary treated effluent). During a 2-yr period, the three-dimensional performance of four percolation areas treating domestic wastewater was monitored. At each site samples were taken at 0, 10, and 20 m along each of the four percolation trenches at depths of 0.3, 0.6, and 1.0 m below each trench to ascertain the attenuation effects of the unsaturated subsoil. The two sites with septic tanks installed performed at least as well as the other two sites with secondary treatment systems installed and appeared to discharge a better quality effluent in terms of nutrient load. An average of 2.1 and 6.8 g total N d(-1) remained after passing through 1-m depth of subsoil beneath the trenches receiving septic tank effluent compared with 12.7 and 16.7 g total N d(-1) on the sites receiving secondary effluent. The research also indicates that the septic tank effluent was of an equivalent quality to the secondary treated effluent in terms of indicator bacteria (E. coli) after percolating through 0.6-m depth of unsaturated subsoil.

Effective distribution of domestic wastewater effluent between percolation trenches in on-site treatment systems.

On-site treatment systems discharging to groundwater rely on the effective distribution of effluent across a percolation area to provide an appropriate loading rate for the subsoil. In Ireland, this is achieved in a distribution box which splits the effluent evenly between the requisite number of percolation pipes. The flow regime experienced at four different distribution boxes was monitored continuously over twelve month periods which established that the most common flow rates at the distribution unit were in the range 1-4 litres/minute for a four to five person dwelling. In addition, the average flow rate from the four sites was only 100 litres per person per day, compared to recommended design value of 180 litres per person per day. Two distribution boxes were also tested in the laboratory to assess their distribution efficiency over a range of loading rates. The most commonly installed unit was found to significantly favour two out of the four trenches and both units were shown to perform particularly poorly at a range of different off-level installation angles. Modifications to the boxes were also tested, involving plastic V-notch inserts which were shown to greatly improve the hydraulic distribution and make the unit much less sensitive to off-level installation or subsidence.