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A septic tank drainfield has two functions:-
- It uses naturally occurring aerobic soil bacteria to destroy the pathogens in the effluent
- It disperses septic effluent into the soil and soaks it away into the lower ground levels
With over 60% of land in the UK failing the three tests which must be passed in order to install a septic tank soakaway drainfield, it is difficult to obtain permission for a septic tank. If you can't have a soakaway, you can't have a septic tank and must consider using a sewage treatment plant which discharges to a ditch instead.
The 3 mandatory tests in the UK are:
1. The drainfield area must not be in an Environment Agency Groundwater Source Protection Zone. This is to protect abstracted public drinking water.
SOAKAWAY TUNNELS, CRATES, ETC. must not be used for sewage effluent drainfields as these are for rainwater soakaways only.
If the tests above are satisfactory, then the size of the soakaway system or drainage field will be dependant on the following factors:
1. The porosity of the soil.
2. The size of the property (based on the number of bedrooms) and the number of residents it serves. The UK Environment Agency only allows septic tanks for small sewage systems up to around 15 persons.
Modern septic tank soakaway drainfields are constructed using trenches filled with a clean washed stone, fed by solid slotted drainage pipe. These trenches are interconnected and have a design similar to an oven grid shelf.
Older septic tank drainfields did not have to conform to any Building Regulations and may be a simple long trench, a herringbone drainage system or a large pit filled with rubble. All these systems are often not fit for the purpose and they usually fail.
All drainfields have a limited lifespan which is made shorter if the correct emptying and maintenance of the septic tank is ignored. If you do not empty your tank on a regular basis the suspended solids from the tank will be forced down the drains and into the soakaway, clogging the pipes and contaminating the soil with a black, slimy biomatt, making the drainfield no longer effective.
When this happens, the septic tank soakaway system comes to the end of its life and a new drainfield must be installed. Tell-tale signs that this is happening are when your tank fills to a level above that of the outlet pipe, especially after heavy rainfall. If the water in the septic tank rises then it is probable the drainage in the drainfield is damaged.
Rules and Regulations for Septic and Foul Drainage soakaways in the UK
Minimum distances that the drainfield should be from:
- Buildings - 15 metres
- Boundaries - 2 metres
- Water abstraction point (well, spring, bore hole) - 50 metres
- Watercourse (ditch, stream, river) - 10 metres
No access roads, driveways, paths, buildings, etc. should be within the drainfield area. If it is in a field, then tractors are not allowed to drive over it.
No water or electric cable or pipes should be within the drainfield area.
The bedrock or water table should not, at any time, be within 1 metre of the drainfield pipework.
Never use normal drainage pipe, it needs to be specific to sewage.