A septic tank is usually only a few thousand litre. In the UK a house with a maximum occupancy of six would have a septic tank capacity of about 3 000 litre, t

A septic tank is usually only a few thousand litre. In the UK a house with a maximum occupancy of six would have a septic tank capacity of about 3 000 litre, that is, 3 cubic metre. A cube of just under 1.5 m on a side. See https://www.ukseptictanks.co.uk/size-septic-tanks https://www.ukseptictanks.co.uk/size-septic-tanks