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Percolation and Tertiary Treatment

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Trench Systems

A trench percolation system uses a series of specially designed 4" perforated pipes in stone lined trenches to distribute treated effluent to the subsoil, where it undergoes further treatment. Each pipe is connected to a single distribution box which evenly distributes the wastewater between each trench. A permeable geo-textile membrane covers the stone trench, preventing clay from the top soil from washing down and sealing drainage paths.

Distribution of water along the trench relies on gravity, therefore accurate levels are critical. As this can be difficult to achieve in long trenches, Molloy Precast recommend that a gravity system and hence a trench system is not installed for trench lengths greater than 100m.

Trench Systems

    Trench kits supplied by Molloy Precast include:

  • Distribution box and accessories
  • Specially designed 4” distribution pipes
  • Geo-textile membrane cover
  • Installation instruction and best practice manual



The design of a percolation area will vary with both the site characteristics and method of wastewater treatment. The minimum design requirements are stated in the following tables, as per EPA Code of Practice for Wastewater Treatment and Disposal Systems Serving Single Houses, 2009. For this reason, each design should be site specific.

Minimum Percolation Trench Length Required with Secondary Treatment Input

P-T Values Percolation Trench Discharge Trench Length Required per no. of People (m)
    4 5 6 7 8 10 12
3-21 ≤50 24 30 36 42 48 60 72
21-40 ≤25 48 60 72 84 96 120 144
41-50 ≤25 48 60 72 84 96 120 144
51-75 ≤16 75 94 113 131 150 188 225
10m Maximum Trench Length (EPA, CoP, 2009)

Further Information

For further information do not hesitate to contact our design team. We would also strongly recommend a visit to our interactive training and display facility where you can see exactly how a trench system is constructed and operates.