What Causes the Septic Tank Soakaway to Fail?

The soakaway starts filling up from day one and the FOG used in cooking is not broken down by anaerobic bacteria. Soakaways start to block from day one.

Gradually all the soil gets blocked so that water can no longer escape. If the water backs up in the Septic Tank so that the level of water rises above the Tee on the outlet pipe then the FOG, the main part of the floating solides instead of being retained, escapes!

The FOG soon smothers the soil and the good aerobic bacteria is prevented from breaking down the the organic matter. The soakaway is very soon not fit for anything.

Septic Tank Emptying

The water between the floating solids and the sinking solids is called the settlement zone. This should be about a metre deep, giving time for the fine solids to settle onto the bottom. If the floating solids accumulate at the top, and the sinking solids accumulate at the bottom, then clearly the zone between them gets smaller - meaning that the water travels faster through the tank.

By speeding up the rate of flow, the solids don’t have time to settle out. This means that raw sewage ends up in the soakaway! This is why the Septic Tank should be emptied once a year.

WPL septic tank installation

About Septic Tanks

A Septic Tank is a tank in the ground which partially treats raw sewage before the dirty water goes into a soakaway.

For many generations Septic Tanks were the standard way of treating sewage. Although the effluent should go into a soakaway, sometimes it goes illegally into the river system (including ditches). Environmental guidelines have changed recently, so it is important to ensure your Septic Tank is up to date.  A Septic Tank works by:

  1. The raw sewage is directed downwards by a T shaped inlet pipe.

The idea is that the sinking solids settle at the bottom, and the floating solids float on the surface. -Floating solids are composed of fats, oils and greases (FOG).

  1. To prevent the floating solids escaping, we have a similar T shaped outlet pipe.

If the water in the soakaway gets above the top of this T pipe, the floating solids can escape.

Sewage treatment plant installation


If the man doing the desludging happens to break the Tee on the Outlet pipe then clearly all the floating solids, including FOG, simply flow out into the soakaway once the Septic Tank is full, usually after a few days. No one notices but the soakaway now has to cope with the worst part of the organic matter.

The Tee on the lnlet pipe is about 600mm long, so as to direct all solids down into the part of the tank furthest from the Outlet pipe.


Sewage Pipes



Sewage Treatment Plant or Septic Tank?

You’ve got a failed septic tank system, what are your options?

  1. Replace the existing soakaway.
  2. Install a pump station, and the pump the effluent to a shallower soakaway.
  3. Convert the Septic Tank to a sewage treatment plant.
  4. Replace the Septic Tank with a sewage treatment plant and a soakaway.
  5. Replace it with Sewage Treatment Plant and pump to a shallow soakaway.
  6. Replace it with Sewage Treatment Plant and pump to the ditch.
  7. Replace it with Sewage Treatment Plant and simply gravitate to a ditch.

Having a product that works is a worthwhile investment for the long term. There are a variety of factors to consider such as:

  • the number of people on the system (Population Equivalent)
  • do you need to employ a pump
  • how much space and access is available for the installation
  • the work should be carried out by qualified British Water engineers

Septic Tank

If you will be discharging into a river, keep in mind that the Environmental Agency requires effluent to be discharged at 20 BOD (Biological Oxygen Demand). Read more about this process here.  A Septic Tank can only produce 200 BOD with the best conditions, so in this situation we recommend installing an Allerton ConSept inside the Septic Tank.

Sewage Treatment Plant

A sewage treatment plant is a great option for treating the effluent from your property. We have products for large and small buildings, plus everything in between. 

Septic Tanks vs Sewage Treatment Plant