A septic tank is designed for human waste, both black water and gray water. Normally, a bio-additive is introduced into the tank that consumes the waste and the by-product is inert. French drains (pipes with holes in them) are attached to the tank and the clean inert fluid is discharged over a large surface area and absorbed by the soil. The nutrients in the discharge are actually beneficial to the plant life in the area.
A cistern tank is designed for below ground storage of potable (drinking) water. The resins used in a cistern tank are FDA approved for contact with products intended for human consumption, septic tanks are not. Cistern tanks are heavier duty than septic tanks as well, meaning they can be routinely filled and emptied without worrying about sidewall integrity. Septic tanks are designed to be full at all times. Cistern tanks can be used as septic tanks, but septic tanks should NOT be used as cistern tanks.