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Septic tank capacity is daily flow times retention time, plus sludge storage, plus a base allowance. The common British rule of 2,000 litres plus 150 litres per person gives a similar answer, and a 2 day retention is the minimum for adequate settlement.
Septic sizing
capacity = occupants x flow per person x retention days x (1 + sludge allowance) + base volume
Solids need time to settle and scum to rise. Too short a retention lets solids carry over into the drainage field and blind it permanently.
Annually is the usual recommendation, or when sludge reaches a third of the depth. Overfull tanks discharge solids that destroy the soakaway.