PERCOLATION AREA DESIGN NOTES

INTRODUCTON
These recommendations deal with the BC 8 Wastewater Treatment plants used to treat and dispose of domestic sewage from Single Dwelling Houses. All procedures and recommendations outlined below are as outlined in THE NATIONAL STANDARDS AUTHORITY OF IRELAND'S publication SR6 1991 SEPTIC TANK SYSTEMS, RECOMMENDATIONS FOR DOMESTIC EFFLUENT TREATMENT AND DISPOSAL FOR A SINGLE DWELLING. It should be the responsibility of the builder that all construction work on the percolation area should be in strict accordance with the recommendations contained in the publication and any subsequent recommendations.

The uPVC pipe for the BC 8-treatment plant to the percolation area should be watertight and be 100mm nominal diameter laid to 1 in 60 fall. This pipe should terminate in a 600mm by 600mm distribution box from which the distribution pipes for the percolation area will begin. The distribution box should evenly distribute the effluent the distribution pipes in the percolation area.

Location of Percolation area

Sites should preferably be level well-drained areas, or, on convex slopes. Sites that are in depressions, at the bottom of slopes or on concave slopes should be avoided.
Percolation areas should be located at furthest point away from groundwater sources of drinking water.

Refer to table below for suggested distance between a well and a percolation area.

(Table 1)

Type of Soil Minimum dept of subsoil above
rock (m)		 Minimum depth of water table
(m)		 Minimum Distance from percolation
area (m)
Clayey till, silty clay, clay where
(low percolation rate)
'T' more than 30
 1.0
 1.0
 30.0
Sandy clay, sandy till
(medium percolation rate)
'T between 15 and 30
 1
2 to 5
5
 1
1
1
 45
30 to45
30
Sand and Gravel ( high percolation rate)
'T' less than 15
 1
2 to 5
5
 1
1
5
 60
40 to 60
30

Also the following conditions should be followed when siting a percolation area.
1. Preferably they should be located to the rear of a housing development furthest away from road.
2. No part of the percolation area should be within 20 m of the nearest dwelling house
3. No part of the percolation area should be within 3 m of the boundary of the adjoining site.
4. No water mains or service pipes should be located within the percolation area
5. No access roads, driveways, or paved areas should be located within the percolation area.
6. No part of the percolation area should be within 10 m of the nearest road boundary, stream or ditch

Size of Percolation Area

The size or area of the percolation area is dependent on the results of the percolation tests carried out on the site of the percolation area. Refer to the standard test sheet for determining the 'T' value for the site. The 'T' value is the time in minutes for a 1" or 25mm drop in the water level in the test hole(s)

The relationship between the value of 'T' and the length of distribution piping required for the percolation area is as given in Table 2.

(Table 2)

Value of 'T' Length of piping (m)
From 5 up to 10
Above 10 and up to 15
Above 15 and up to 30
Above 30 and up to 60
 30
36
48
96

Note 'T' values less than 5 may indicate that the percolation rate is too fast possibly polluting the groundwater
'T' values greater than 30 may indicate that the percolation rate may be too slow and leading to ponding of the effluent at the site of the percolation area. Where this occurs a raised percolation area may be necessary. Recommendations for its construction are in appendix A attached.

Percolation Area Distribution pipes

o Pipe should preferably be 110mm diameter perforated smoothwall plastic drainage pipes with perforations of 8mm diameter at about 75mm centers along the pipe at about 4, 6 and 8 o'clock positions.
Note: Wavin Plastics supply these pipes

Percolation area Construction

- The pipe trenches shall be set out at 2 m centers
- The percolation pipes should be laid in trenches at least 450mm wide and bedded on 250mm depth of crushed stone (20 - 30 mm in size)
- Pipes should be laid to fall of 1 in 200 approx.
- Pipes should be interconnected at end of pipe runs
- Once the drainage pipes are in place they should be surrounded and covered to a depth of 150mm of crushed stone which should extend the full width of the trench
- Before the trenches are backfilled with the topsoil the crushed stone should be covered with peat fiber, geotextiles or similar permeable or durable materials. This is to prevent the stone being silted up with topsoil.

Maintenance of Percolation area

The percolation area should be inspected periodically and any signs of malfunctioning noted. This will show itself by obvious sings of blockage at distribution box, ponding, smell or pollution in the surrounding area. In this event contact our sales office. A reserve percolation area may have to be constructed.

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