Septic Tank sizing
This article lists all the steps you need to go through to size a septic tank for your home. It is a simple calculation with 3 steps, however you can jump to the answer in the useful table if you wish.
If you need to know how big your septic tank needs to be, you will find the answer here. These three steps to calculating your septic tank size make it very easy to understand. This guide is for septic tank sizes for a home, you may be looking to size a septic tank for an office of commercial building, in which case it is a different calculation.
You must follow british water flows and loads rules for establishing the maximum number of people in your house. There is a simple rule, 1-3 bedrooms must be classed as 5 people, anything above that and you take the number of bedrooms and add 2. For your convenience, here is a table to help:
| Number of Bedrooms | Number of people |
|---|---|
| 1-3 Bedrooms | 5 People |
| 4 Bedrooms | 6 People |
| 5 Bedrooms | 7 People |
| 6 Bedrooms | 8 People |
| 7 Bedrooms | 9 People |
| 8 Bedrooms | 10 People |
You can very easily calculate the maximum flow per day by multiplying maximum number or people in the property by 150 Litres. 150 Litres is the British water standard for the average volume of water produced per person in a normal household. To make this simpler for you, i have included a table below:
| Number of People | Maximum Flow Per Day |
|---|---|
| 5 | 750 |
| 6 | 900 |
| 7 | 1050 |
| 8 | 1200 |
| 9 | 1350 |
| 10 | 1500 |
Once you know your maximum flow per day, you will add 2000 Litres to this figure. The reason for this is to ensure the septic tank is large enough to allow solid matter to separate and fall down to the bottom of the tank. If a Septic Tank was too small, then there would be too much disturbance in the water, resulting in sludge build up quickly exciting the chamber in to your drainage field.
| Number of Bedrooms | Maximum number or people | Minimum size of septic tank |
|---|---|---|
| 1-3 | 5 | 2750 Litres |
| 4 | 6 | 2900 Litres |
| 5 | 7 | 3050 Litres |
| 6 | 8 | 3200 Litres |
| 7 | 9 | 3350 Litres |
| 8 | 10 | 3500 Litres |
At least 2750 Litres
If your septic tank is too small, sludge build up will happen fast and can exit the tank into the drainage field. You should increase the frequency of desludging to ensure you protect your drainage field.
No, it is not advisable to use a larger tank than needed for the application. This is because waste must move through the tank and to the drainage field quickly after settlement. If you size the tank much larger than needed, there is a risk of septicity. If the water stays too long in your tank, it will start to smell.
Pumping (or de-sludging) a septic tank is a task that should be performed once a year by a "tankering company" with a waste carriers license.
A correctly sized and operated septic tank should not smell. However, there are some things you should not put down the toilets to ensure your tank operates efficiently.
The size of drainage field is determined by a number of factors. Ground porosity and number of bedrooms make up the key variables.