The closer the usage draw off point is to the top of the tank (aerobic zone) the better the quality of water. The best system has two outtake points – one 100-300mm or up to 1/3 the way up the tank for use inside the home, and the other at the bottom of the tank (in the anaerobic zone) for use outside the home and overflow. Tank overflows that simultaneously take excess water and vacuum out sludge from the bottom of the tank are best.