Septic tanks receive raw sewage, allow it to settle, and pass the relatively clear liquid to the adsorption field, which is the next stage of treatment. The remaining solids digest slowly in the bottom of the tank. In septic tank design, environmental engineers must consider the treatment following the septic tank as a part of the septic tank system. Environmental engineers must check soil porosity and base the design of the soil absorption field on the rate of percolation. Septic tanks are suitable only for isolated facilities with low waste flows where the soil can be used as an absorption field. Conventional plants are best identified by what they do not achieve, namely nutrient removal, demineralization, and the removal of trace organics. Settling processes remove settleable solids by gravity settling either prior to or after biological or chemical treatment and between multiple-stage biological or chemical treatment steps.