Septic tanks allow gravity to separate solids from wastewater as heavier solids settle and fats, grease and lighter solids float. You might typically think of solids removal primarily happening via settling but separation of suspended solids by flotation is also very important in the septic tank. The process works best when the flow through the tank is very slow and can be compared to settling in reverse. The flotation process is enhanced by the presence of fats, oil and grease in the wastewater, which congeals on small particle surfaces, making them more buoyant and floatable. The accumulation of scum by
