Owls Hall, Farm Blackmore End, Braintree, Essex CM7 4DF
CONTACT OWLS HALL
01844 877 030
EMAIL OWLS HALL

How to size a septic tank

HOW TO SIZE A SEPTIC TANK

Owls Hall Environmental How To Guide

A septic tank is used where no mains sewage connection is available, and the water can be discharged through the ground via a soakaway or drainage field. If you want to know how to size a septic tank, look no further.

Septic tanks are not allowed to discharge to a ditch. Upgrading from a septic tank to a sewage treatment plant will allow discharging to a ditch and provide cleaner water increasing the life of a soakaway should one be used. Learning how to size a Septic Tank is important to ensuring it conforms with building regulations and provides long term performance. 

The below is a guide to sizing a septic tank, with worked examples for single and multiple properties.

Step 1 – Find out how many people the property can accommodate.

British water flows and loads 4 document states that:

A single property of up to 3 bedrooms must be sized at 5 people.
A single property of 4 bedrooms must be sized at 6 people.
A single property of 5 bedrooms must be sized at 7 people
A single property of 6 bedrooms must be sized at 8 people
Multiple properties are sized individually and then added together.

Example A:

4 bedrooms = 6 people

A reduction factor is used where populations exceed 12 people This is to account for the fact that it is highly unlikely each property will be full. From 13-25 population, you can reduce the figure by multiplying the population by 0.9. From 26-50 population, you can reduce the figure by multiplying by 0.8.

Example B:

4 bedroom + 5 bedroom property = 6 people + 7 people = 13 people.
Then multiply by 0.9 and round up: 13 x 0.9 = 11.7 (12 rounded up)

Step 2: Calculate the number of litres produced from the property or properties

British water flows and loads 4 document states each person creates 150L a day. Therefore, take the number of people in step one and multiply by 150.

Example A:

4 bedrooms = 6 people. 6 x 150 = 900L per day

Example B:

4 bedroom + 5 Bedroom properties = 12 population: 12 x 150 = 1800L Per day

Step 3: Add 2000L to the answer in step 2. This will be the minimum size you must have to comply with building regulations and the environment agency general binding rules.

Example A:

4 bedrooms = 6 people. 6 x 150 = 900L + 2000L = 2900L

Example B:

4 Bedroom + 5 Bedroom = 12 people: 12 x 150 = 1800L = 2000L = 3800L

Step 4: Round the number up to the nearest available septic tank size. Under-sizing your septic tank will result in the water produced being high in suspended solids and risks blocking your soakaway.

For ease, we have created a simple table:

Bedrooms Minimum size of septic tank
1-3 bedrooms 2750L
4 bedrooms 2900L
5 bedrooms 3050L
2 x 1-3 bed properties 3500L
2 x 4 bedrooms 3800L
2 x 5 bedrooms 3950L

For sizing of a septic tank for a commercial building, more detailed calculations must be made to ensure the daily flow is accurate. This is done using the British water flows and loads 4 Document. For assistance with this, our technical team can help on 01844 202121

CALL US NOW

Need help and would like to speak to us?
Calls will be charged at the local rate

01371 850 537

EMAIL US NOW

Email our customer care team at Owls Hall
and we will get back to you.

info@owlshall.co.uk