Soakaway, Advice, Design and Installation

Septic Tank Soakaway Systems, Problems and Drainage Fields - How they work

A septic tank soakaway system performs two functions:-
  • It uses naturally occurring aerobic soil bacteria to further clean the pollutants in the effluent
  • It disperses septic effluent into the soil,

Over 60% of all sites in the UK FAIL the three mandatory tests required to install a septic tank soakaway. If you can't have a soakaway, you can't have a septic tank and must consider using a sewage treatment plant instead.

The 3 tests are:

  1. The site must not be in an Environment Agency Groundwater Source Protection Zone. This is to protect abstracted drinking water
  2. The site must pass the deep Trial Site Assessment Hole Test and never have either groundwater (the water table) or rock within 1 metre of the soakaway drain pipe
  3. The site must consist of soil that passes the Percolation Tests - not too sandy or heavy.

The Environment Agency has confirmed to us that DEEP BOREHOLE soakaway drilling as a method of solving drainfield issues is NOT generally acceptable to them, neither are SOAKAWAY TUNNELS, CRATES, ETC. as these are for rainwater soakaways only.  BEWARE of such adverts on the Internet and report them to Trading Standards!

If you pass the above tests, 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 Environment Agency only allow septic tanks for small scale works up to around 15 persons.

Proper modern septic tank system soakaways are constructed with trenches filled with a clean washed stone, fed by solid perforated drainage pipe. These trenches are interconnected and have a design similar to an oven grid tray.

However, older septic tank drainfields sometimes did not have to conform to any Building Regulations and may be a simple A failed septic tank soakaway pitlong trench, a herringbone drainage layout or a large stone filled pit.  All these latter types are often not fit for the purpose as percolation tests were not carried out and they are often inadequate.

All soakaways 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

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 1.2 metres of the drainfield pipework.

If you have a question about your septic tank soakaway give us a call on 01757 288022 and we can normally arrange a free survey and diagnosis of the extent of the soakaway system problem.

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Installers Wanted
Crystal is currently seeking installers for sewage plant and septic tanks which DO have the EN12566-3 throughout many parts of the UK.
OASIS Rainwater Harvesters
British Water Members

Please telephone us on 01759 369915 as sewage treatment enquiries are too complex for email correspondence.
 
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