HOW TO SIZE A SEPTIC TANK
Owls Hall Environmental How To Guide
If you want to know how to calculate septic tank size, look no further. 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.Important septic tank information
Learning how to size a septic tank is essential for ensuring it conforms with building regulations and provides long term performance. Remember, 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.
4 steps to size a septic tank
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.
Septic tank size guide
| Bedrooms | Minimum size of septic tank | Recommended product |
| 1-3 bedrooms | 2750L | 3000L Septic Tank |
| 4 bedrooms | 2900L | 3000L Septic Tank |
| 5 bedrooms | 3050L | 4000L septic tank |
| 2 x 1-3 bed properties | 3500L | 4000L septic tank |
| 2 x 4 bedrooms | 3800L | 4000L septic tank |
| 2 x 5 bedrooms | 3950L | 4000L septic tank |
| 3 x 2 bedrooms | 3800L | 4000L septic tank |
| 3 x 3 bedrooms | 4250L | 5000L septic tank |
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.
Further Reading:
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3000L Millennium Septic Tank
£ 650.00 excl VAT£ 780.00 incl VATView Details -
4000L Millennium Septic Tank
£ 800.00 excl VAT£ 960.00 incl VATView Details -
5000L Millennium Septic Tank
£ 930.00 excl VAT£ 1,116.00 incl VATView Details -
6000L Millennium Septic Tank
£ 1,230.00 excl VAT£ 1,476.00 incl VATView Details
