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How to size a Septic Tank

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How to calculate the size of a domestic septic tank.


According to BS6297 and Flows and Loads by British Water this is the formula: 

C is the size of the septic tank in litres.

C= (150 P + 2,000)   

P is the population to be served - this is potential not actual inhabitants. (Formula assumes de-sludging on an annual basis, and children count as adults.)

So, you start off with 2000 litres and add 150 litres per person that could live in the property, not how many people actually live in the property.

Let's look at an example: 4 bedroom house maximum occupancy 6 = 2000 litres + 900 (150 x 6) = 2900 litres would be the minimum size tank required.

At present the smallest septic tank size for a domestic property is 2700 litres.

For information on how to size a sewage treatment plant CLICK HERE

if you require further information on getting the right sized tank please feel free to call us on: 01296 326111

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