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Percolation and Tertiary Treatment

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Pressurised Systems

A pressurised system distributes treated wastewater by discharging it through a system of small diameter pipes. The distribution network is made up of a supply manifold and specially designed and drilled distribution pipes. The wastewater is pumped at pressure from the treatment tank, ensuring it flows to all points of the network. This pipe network is laid out on a bed of stone and subsoil, covered with a geo-textile membrane and covered with topsoil.

Pressurised systems are particularly useful in raised percolation beds. Raised beds are constructed by creating a mound on top of the normal ground level using imported soil of suitable percolation characteristics. They are installed where difficult site conditions such as high water table, insufficient subsoil depth or poor percolation characteristics are encountered. 

Pressurised Systems
    Pressurised System kits supplied by Molloy Precast include:

  • Supply Manifold
  • Specially drilled 32mm distribution pipes
  • Geo-textile membrane cover
  • Installation and best practice manual
  • Delivery and/or installation of pressurised kits is available throughout the country.

These kits are normally used in conjunction with secondary treatment systems. Usually the pump included in our AswaFlow treatment system can be used to feed the pipe network, however, distance from the system, height above the system and size of the pipe network may prevent this. In these circumstances we recommend the installation of a larger pump fitted inside a separate pump chamber which can also be supplied by Molloy Precast.


Each system is designed to be site specific. Size and layouts are dependent on the occupancy of the dwelling and the permeability of the soil. The following table outlines the minimum required bed sizes (EPA CoP, 2009).

P-T Values Loading rate on plan area (l/sq. m/day) Area required per PE (sq. m)
    4 5 6 7 8 10 12
3-20 ≤20 30 37.5 45 52.5 60 75 90
21-40 ≤10 60 75 90 105 120 150 180
41-50 ≤5 120 150 180 210 240 300 360
51-75 ≤3 200 250 300 350 400 500 600
(EPA, CoP, 2009)

Further Information

Generic drawings are available. Site specific drawings can also be prepared on request. 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 pressurised system is constructed and operates.