In Australia there are ...

In Australia there are manufactures and distributers of what we call biological system septic systems. I'm sure that the manufactures would have worked out what size you need to the number of people in the household. A Google search may help. We had one called Bio-Septic at our house that was on a 1.6 Ha block when we lived in a rural area. Every drop of water that went through the house went onto the block and I was able to grow over 350 trees during one of the driest times in living memory of what became known as the millennium drought.