Mr Fixit Property Maintenance and Renovation Services
Property Renovation & Maintenance
Contents Technical Advice Services
About Mr Fixit Contact us Energy Efficient Buildings Building Insulation MEP Design Chimneys & Flues Rabies

Septic Tank - How Large?

Standard sizes for Septic Tanks and why they overfill.

How large should a septic tank be? It is very important that septic tanks are large enough for the number of people they serve. Why is my septic tank full? There are several reasons why a septic tank can become full and overflow.

Here we look at why a septic tank can become full, how a septic tank works and how to calculate how large a septic tank you will need.

At the bottom of the page you will find British and American standard septic tank sizes.

See also:

Why is my Septic tank full?

Last week I received an interesting question:

"I have a faulty septic tank. The tank is quite large, approx. 5 x 2 x 1.5, but fills rapidly - in the wet it needs to be pumped out often, sometimes every two weeks, the dry considerably slower. Obviously, ground water must be entering, but what to do about it is the question?"

When operating normally a septic tank should not fill up and, as a guideline, should only need pumping out probably once every 5 years if that. This is a very large septic tank (15 cubic metres, most tanks are only a quarter of this size) and should take a while to fill up.

The first question is “has the tank been working properly in the past or has it always needed pumping out regularly”. If it has always needed pumping out then it is likely that there was a fault with the original design or installation.

In the opening question the fact that the tank fills faster in the wet season is a bit of a give away and supports the writer's suspicion of ground water entering the tank. We don't go to the toilet more or have more showers in the wet season do we?

If excess water is getting into the tank it may well be that drainage water has been connected to the tank or somehow surface water is running into it. Septic tanks should be isolated from drainage and should only take waste from toilets, showers, washbasins, washing machines and sinks.

Drainage water entering the tank is a health issue in addition to the problem of the tank filling up when it rains. We need to check if the pipework is connected correctly or if there is a pool of water over the tank after heavy rain?

It may be that the tank has been built in an area of clay (rice fields are lined with clay) and the whole area the tank is in fills up with water that can't drain away. It is important that the area around the tank can drain properly.

It may possibly be that the tank is cracked and ground water is getting in from the surrounding ground. This is highly unlikely unless the tank has been incorrectly situated in clay as above. It is very seldom that the water table is higher than the top of a septic tank and so water would more likely drain out from a cracked tank rather than into it.

If the tank is filling up then it may be that the outlet pipe from the tank is blocked. In other countries there are strict regulations regarding what is known as the “soak field” into which septic tanks drain. Here in Bali most people do not give much consideration to where the semi processed effluent from the septic tank goes to, most people have never heard of a soak field even though most people use septic tanks. This “blind eye” attitude to the discharge pipes of septic tanks is probably the main reason why there are so many bowel problems associated with Bali.

A final possibility is that the flow from the primary chamber to the secondary chamber inside the septic tank is blocked. It could be that the level of sediment in the primary chamber of the tanks has risen above the level of the pipe and blocked it. If the tank has been pumped out this is unlikely but possible.

Septic tank design

The design Of a 2 chamber septic tank

Septic tanks should have at least 2 chambers (sometimes 3) in them. The first chamber provides settling of solids to the bottom as a sludge. Bacterial action starts to break down the sludge and waste water. Scum resulting from the bacterial action is trapped on the surface.

An internal pipe allows semi processed waste to pass from the first chamber to the second chamber where continuing bacterial action breaks down the waste water further. The processed water drains from this secondary chamber out of the tank to the soak field.

If the pipe between the first and second chambers becomes blocked then the first chamber will overflow. This is not likely but could happen particularly if the tank has filled up with silt washed into the tank from drainage water.

How to find the cause of the problem

So to summarise.

1. Start by checking that the septic tank is in fact full. If the pipe to the tank becomes blocked then waste will backup to showers and toilets and the septic tank will not be full.

2. Next check to see if drainage or surface water is entering the tank and take steps to stop it. You may have to check to make sure that some highly intelligent person has not connected drainage pipes into the septic system. Also make sure that during wet weather water is not pooling over the top of the tank.

3. Next check the drain from the tank. Get someone to dig out the discharge pipe from the tank and check that it is clear and that the pipe is not backing up. Also check to make sure the outlet pipe from the tank is lower than the inlet pipe.

4. If these steps fail then it will be necessary to dig out and gain access to the tank. It should be checked to see if it has silted up inside (this is not likely), to make sure it is not cracked or leaking and to make sure all the pipework is operating properly.

If the tank is cracked it will need to be repaired properly and you should seek technical advice on this. These days plastic septic tanks are available and are far more able to withstand ground movement than the majority of existing tanks that are usually built from bricks or concrete blocks.

Calculating Septic Tank Size - How Big?

This whole discussion raises the question of how large a septic tank should be. This is of course dependent on how much waste is being produced and is directly related to our water consumption for “internal” purposes such as showers, toilets, baths, dishwashing, clothes washing and face, hair and teeth cleaning.

Obviously when considering water usage we have to consider lifestyle differences. Some people tend to use a lot of water, some like to bath instead of shower, in hot climates people tend to shower more often. It has also been found that the more affluent we are the more water we waste. To help determine your water usage you can go to the water usage calculator on the website at

American Calculation (compiled from several sources)

0.95 cubic metres plus 0.95 cubic metres per bedroom. So:

No. Bedrooms 1 2 3 4 5
Tank Size in Cubic Metres 1.9 2.85 3.8 4.75 5.7

British Calculation (from the British Standard BS 6297)

2,000 litres (2 cu metres) plus 180 litres per person. So:

No people 1 2 3 4 5 6 7 8 9 10
Tank Size in Cubic Metres 2.18 2.36 2.54 2.72 2.9 3.08 3.26 3.44 3.62 3.8

See also:

Copyright © Phil Wilson August 2011
This article, or any part of it, cannot be copied or reproduced without permission from the copyright owner.