How do sewage systems work?

Find out how septic tanks work.

Septic tank, sewage treatment plant and cesspit emptying, how they work.

There are basically two types of septic tanks: traditional, masonry-built pits or more modern GRP (glass fibre) units, often referred to as “onions”. Whatever the type, their function is the same – a settlement tank in which the sewage sludge is retained for sufficient time for the organic matter to undergo anaerobic decomposition. The final effluent from the septic tank should then be drained into an underground soakaway system. The soakaway must be of adequate size to deal efficiently with the volume of the final effluent. Whichever type of septic tank you have, the most common problem is the failure of the soakaway.
Above is a picture of an ‘onion’ type septic tank which is split in to three segments. It shows the waste coming in on the right with two baffles to separate the solids and on the left the effluent exits to the soakaway.
Here is a picture of brick type septic tank. Here is how it works, You have the flow being forced down by the inlet T-Piece to allow the solids (sludge) to settle. You have the outlet T-Piece just below the layer of floating scum to allow the effluent to drain down the soakaway.

As you can see, if you allowed the solids to build up in the tank it could easily block the outlet and cause a blockage or worse an overflowing septic tank! So yearly emptying will help keep you tank healthy and work correctly.

How sewage treatment plantswork.

The Sewage Treatment Plant process is similar to the way that a Septic Tank works but mechanical components provide a process to help break down solids to produce a cleaner and more environmentally friendly effluent.

Wastewater and sewage are fed into the primary settlement zone where solids and liquids separate and the liquid flows into the secondary zone chamber. In the chamber, a pump draws the liquid up the “air lift” and spreads the liquid over the bale and encourages good bacteria to digest the organic matter, breaking it down and purifying it.

As it leaves the final chamber known as the humus chamber the effluent is typically 95% clean and ready to discharge into land drainage systems, or into a local ditch or watercourse (subject to consent by the Environment Agency).

On the right is a picture of an aerated sewage treatment plant and below is a quick guide as to how the process works.

  • 1.Inlet from the property.
  • 2.Settlement zone.
  • 3.Secondary zone.
  • 4.Airlift draws effluent up from the final chamber with the blower (Fig 6) and spreads it over the bale where it filters through and the bacteria on the bale purify it.
  • 5.Bale. Where the bacteria live.
  • 6.Air blower and housing.
  • 7.Outlet from final chamber to a soakaway, watercourse or ditch.

    • How often should you have your tank emptied ?

    Regular emptying of your septic tank or sewage treatment plant is essential otherwise there maybe an excessive build up of sludge which could block the soakaway or pollute the land around the soakaway. Ideally you should have your tank emptied at least once a year but it also depends on the size of your tank and the number of people living in the property. Please contact us for free advice.

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