Septic Tank | Design and Information

 Cesspit Septic Tank - How it works - Design

A Septic Tank, commonly called a Cesspit, is simply a big concrete, brick, fibreglass or polyethylene tank, buried in the ground that takes all the wastewater from the house. 

The septic tank was invented in the 1860's and is the original piece of sewage equipment used for the treatment of sewage in rural areas.  The Septic Tank has now been superseded by the sewage treatment plant.

Septic tanks are a minimum of 2700 litres (650 gallons) for a 2 bedroom house.  Wastewater flows into the tank at one end and leaves it at the other.  A brick or concrete septic tank looks something like this in cross-section:

septic tank cross section  .
In the top picture, you can see that the tank is 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 of the tank and aerobic bacteria colonise it, digesting the organic material and preventing the effluent from becoming too septic.  This layer known as the scum layer or crust. Anything heavier than water sinks to the bottom to form the sludge layer. In the middle is a relatively clear effluent layer. This body of effluent contains anaerobic bacteria and chemicals like nitrogen and phosphorous, plus 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.

Types of Septic Tanks - traditional and fibreglass

The 'onion' type fibreglass tank does not work as well as the old fashioned brick one in our experience.  They tend to produce an effluent which is far more 'septic' (oxygen-stripping) than the traditional design due to the fact that the only air/sewage contact interface is the small area in the neck of the tank.  There is also only a very short distance for horizontal settlement between the inlet and outlet pipe.

Septic tank inlets and outlets

The inlet and outlet pipes are 'T' pipes which direct the inflow to under the water line. This slows down the passage of sewage and helps prevent the contents of the Primary Chamber from disturbances, allowing easier settlement. The 'T' pipes need to protrude at least 300mm. above the water line to prevent the scum crust from flowing into the top of the pipe.

The outlet 'T' takes water from below the water line so that the scum crust cannot exit the tank and block up the soakaway. If this 'T' pipe is missing, it can knock years off the life of your septic tank drainfield.

The central baffle is an 'H' pipe, which again prevents scum crust from transferring from the primary to the secondary chambers and also prevents disturbance of the secondary chamber settled solids.

Depth for Settlement.

There needs to be a minimum of 1 metre from the bottom of the outlet pipe to the bottom of the tank to allow for adequate settlement.

There is usually a 50mm. drop between the primary and secondary chambers of the septic tank.

We can also provide plans to enable you to build your own septic tank using a concrete base and blocks or bricks.
Septic Tank Access Points

Both the primary and the secondary chambers need good sized lids to allow access to all parts of the septic tank chambers for de-sludging.

Septic Tank Gases - Bad for the Environment

A septic tank sewage system naturally produces methane and hydrogen sulphide gases (caused by bacteria breaking down the organic material in the wastewater).  These gases smell, are dangerous, and are 'greenhouse gases' which are 21 times more polluting than carbon dioxide. Sinks and baths, therefore, have loops of pipe called P-traps that hold water in the lower loop and block the septic tanks gases from flowing back into the house. The gases flow up a ventilation or soil vent pipe instead -- if you look at any house, you will see one or more vent pipes going up the side of the house or poking through the roof.  There should also be a vent pipe fitted on the outlet side of the tank.

Soakaway Drainfields

As new wastewater enters the septic tank, it displaces the water that's already there. This water flows out of the tank and into soakaway drainfield. A drain field is made of solid perforated pipes (NOT 'Flexicoil' land drainage pipe!), a minimum of 2 metres apart and a maximum of 700mm. below ground, buried in trenches filled with 20mm to 50mm size washed gravel to a depth of 300mm under and 50mm. over the pipe. This is to enable the soakaway to be colonised by AEROBIC bacteria instead of ANAEROBIC bacteria which produce a clogging black slime that blocks the soakaway.  As most 'onion' septic tanks have an outlet which is over 1 metre below ground (why?) these need to be cut down to enable the above depth!.  Unfortunately, some soakaways installed by ignorant contractors are either installed lower than 700mm. below ground, or are nothing but pits filled with stones and both have a very limited lifespan!  The top of the gravel in the soakaway trench is covered with geotextile membrane to prevent soil trickling through.

The following diagram shows an overhead view of a septic tank distribution box and drain field:

Septic Tank soakaway design drawing 

The effluent 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 soakaway drainage pipe you will need for this type of sewage equipment.  In places where the ground is made up of primarily clay soil that absorbs water very slowly, a drain field will NOT work.  Even in good soils, soakaways have a limited lifespan as septic effluent contains a lot of suspended solids which block the soil porosity over time.  If the soakaway has been installed deeper than 700mm. below ground level, then anaerobic bacteria will grow in the soakaway and gradually, over a period of years, block it with slime.  Please click on Septic Tank Problems for your solution.

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

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

Minimum Distances for Wastewater Systems

The minimum distances that septic tanks should be from other sites are:-

10 metres from a dwelling

5 metres from a boundary

50 metres from a well or borehole or spring

10 metres from a watercourse

In addition, the soakaway should be 5 metres from a hedge or tree root zone as the roots will find and block the drain.

Other Septic Tank Information

Sewage effluent discharges - What you can discharge and where

Septic tank regulations - New rules for septic tank owners

Septic tank surveys - How to Do It Yourself

Can a farmer empty my septic tank?

Septic Tank Dangers


 Septic Tank Specialists