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.
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.
The below is a guide to sizing a septic tank, with worked examples for single and multiple properties.
British water flows and loads 4 document states that:
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.
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)
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.
4 bedrooms = 6 people. 6 x 150 = 900L per day
4 bedroom + 5 Bedroom properties = 12 population: 12 x 150 = 1800L Per day
4 bedrooms = 6 people. 6 x 150 = 900L + 2000L = 2900L
4 Bedroom + 5 Bedroom = 12 people: 12 x 150 = 1800L = 2000L = 3800L
For ease, we have created a simple table.
|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.