Percolation | Polishing Filters
Raised Bed Polishing Filter/Percolation Area
Raised Beds are installed where difficult site conditions are encounterd including (EPA, CoP, 2009):
- High water tables
- Insufficient subsoil depth
- Insufficient percolation characteristics of native soil
In these cases secondary treatment systems are normally installed.
Construction of a Raised Bed
- Imported soil with suitable characteristics
- Clean pebble or broken stone
- Distribtution pipes
- Layer of clean stone to secure pipes
- Geotextile
- Soil overlay
A raised bed normally consists of the following layers from bottom up:
It is essential in order to prevent bleeding that 1m of clay is placed between the edges of the stone pad and the side of the bed.
Types of Raised Beds
There are two pipe systems availible from Molloy Precast for raised beds:
Pressurised System:
We supply pressurised kits which can be delivered 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 nework 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.
The Molloy Precast pressured kit includes:
- Manifold
- Specially drilled distribtion pipes
- Delivery pipe
- Geotextile cover
- Installation manual
We have standard systems for different sizes, however, our team would be happy to design and manufacture kits to suit your site with regard to size and layout. We also offer an installation service.
Gravity System:
These systems use 4" perforated pipes to distribute wastewater over the infiltration area. Wastewater is pumped from the treatemnt tank via a stilling chamber into a distribution box where flow is divided evenly into each outlet pipe. Gravity fed Kits are available and include:
- 4" distribution pipes
- Distribution box with accessories
- Geotextile cover
- Installation manual
We have standard systems for different sizes, however, our team would be happy to design and manufacture kits to suit your site with regard to size and layout.
Sizing
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 required bed sizes (EPA, CoP, 2009).
| Minimum Bed Area Required | ||||||||
| 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 availible, also site specific drawings can 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 faciltiy where you can see exactly how a raised system is constructed and operates.
