Septic Tanks

How a Septic Tank works

A septic tank is simply a big concrete, brick, fibreglass or polyethelene tank, buried in the ground that takes all the wastewater from the house. Septic tanks are a minimum of 2800 litres (650 gallons) for a 2 bedroom house.  Wastewater flows into the septic tank at one end and leaves the tank at the other. Brick or concrete septic tanks look something like this in cross-section:

In this picture, you can see that septic tanks are divided into two sections.  The first section is the primary settlement tank (PST) and the second section is the secondary settlement tank (SST)  The sewage enters the PST  from the sewer pipes in the house.. Anything that floats rises to the top and forms a layer known as the scum layer. Anything heavier than water sinks to form the sludge layer. In the middle is a relatively clear water layer. This body of water contains bacteria and chemicals like nitrogen and phosphorous that act as fertilizers, and a fairly large proportion of suspended solids – tiny bits that float around in the water.  The effluent then transfers via a baffle, pipe or weir to the SST where the process is repeated.

A septic tank naturally produces gases (caused by bacteria breaking down the organic material in the wastewater), and these gases smell and are dangerous.. Sinks therefore have loops of pipe called P-traps that hold water in the lower loop and block the gases from flowing back into the house. The gases flow up a ventilation or soil vent pipe instead -- if you look any house, you will see one or more vent pipes going up the side of the house or poking through the roof.

As new wastewater enters the septic tank, it displaces the water that's already there. This water flows out of the septic tank and into a drain field or soakaway. A drain field is made of perforated pipes buried in trenches filled with gravel. The following diagram shows an overhead view of a distribution box and drain field:  Unfortunately, some soakaways, installed by contractors, are nothing but pits filled with stones and have a very limited lifespan!

The water is slowly absorbed and filtered by the ground in the drain field. The size of the drain field is determined by how well the ground absorbs water. A percolation test determines how many metres of saokaway drinage pipe you will need.  In places where the ground is hard clay that absorbs water very slowly, the drain field will not work.

It is illegal to discharge septic tank effluent into a watercourse as it is very polluting. 

A septic tank system is normally powered by nothing but gravity. Water flows down from the house to the septic tank, and down from the tank to the drain field. It is a completely passive system.

You may have heard the expression, "The grass is always greener over the septic tank." Actually, it's the drain field, and the grass really is greener -- it takes advantage of the moisture and nutrients in the drain field.

Please click on the following:-

Septic Tank Problems  Typical problems occuring with septic tanks and their soakaways

Septic Tank Care           How to look after your septic tank

Waste Tech Environmental Ltd.    

UK Tel 01757 288 

Ireland Tel 063 81321

Septic tank and sewage treatment specialists