Below Ground Percolation - Trenches

Size

The size of the percolation area depends on the hydraulic loading rate and on the treatment that precedes the percolation area. It must be constructed to facilitate effective filtration of the treated effluent into the subsoil. The following table gives the minimum length of percolation trench for secondary treated wastewater for the different percolation values. (Ref. p. 47 ref sec. 4.11.4.3 and Table 15, p.48 of EPA Waste Water Treatment Manuals: Treatment Systems for Single Houses 2000).

Construction

Construct the distribution trenches using a series of either 4 or 7 trenches, each 18 m long and arranged 2.4 m apart from centre to centre. Excavate 450 mm wide trenches and fill the trenches with clean broken stone (sized 15mm to 30mm i.e.1” chippings) ensuring that the surface is level. Place pipes on the stones. Secure the pipes with more stones until the distribution pipes are just covered. The distribution pipes must be rigid, 100mm in diameter, with perforations positioned at 5 o’clock and 7 o’clock. Cover the stones with geotextile membrane or impervious plastic sheet to prevent ingress of clay/silt etc. In turn, cover the impervious membrane or sheet with 300 mm of soil. No deep rooting plants should be allowed to interfere with the distribution pipes of the percolation area. All rows must be ventilated at both ends i.e. the distribution manhole end and the opposite end to the manhole. Join all trench ends vents together, if necessary and locate the vent in a secure vandal proof area, preferably using a u- shaped down turned vent pipe.

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Trenches - 4 Rows	Trenches - 7 Rows	Trenches - Photo