Upgrade foul septic tank to package treatment plant:
Onn Point were appointed to assist a private client as they wanted to upgrade an existing septic tank to a treatment plant. The outdated septic tank was successfully replaced with a modern treatment plant that met the latest environmental standards. This upgrade, using the help of soakaway design calculations, not only improved effluent quality but also ensured compliance with regulations.
Full scale infiltration tests were carried out in line with BS6297 confirming ground conditions were suitable for the use of infiltration. A sustainable drainage strategy diverting the existing system to a treatment plant was produced, allowing treated effluent to discharge to a foul field drain.
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Send us the site address and the condition wording, or the planning reference, and we will tell you exactly what is needed. If you do not need us, we will say so.
The engineering behind it
A five-bedroom property coming off an ageing septic tank, where the whole scheme turned on one number from the percolation test.
The number that sized everything
Percolation testing returned an average time to drain of 2,700 seconds, giving a Vp of 18. Applied to a five-bedroom property that works out at 7 × 18 × 0.25 = 31.5m² of drainage field, which at a 0.9m trench width is 35 linear metres. A slower percolation rate would have meant a longer field than the plot could take, and a different solution entirely.
Replacing the septic tank
The existing septic tank is decommissioned and replaced with a package treatment plant, with a downstream sample chamber and the field laid at a constant 1:200 gradient. The field also had to clear the statutory separations: 50m from any water abstraction, 10m from a watercourse and 15m from any dwelling — which on a rural plot is often what decides where it can physically go.
The numbers
| Percolation result | Vp 18 (2,700 seconds average) |
| Drainage field area | 31.5m² |
| Drainage field length | 35 linear metres at 0.9m wide |
| Property size | Five bedrooms |
| Field gradient | Constant 1:200 |
| Separation distances | 50m abstraction, 10m watercourse, 15m dwelling |
| Foul | Septic tank decommissioned, package treatment plant installed |
| Testing | PT (West Midlands) Ltd |
Taken from the drawing set for this job. The scheme was approved at planning. Site named by town only; the client’s name and the full address are not published.