The typical septic tank is a large buried container made of concrete, fiberglass or polyethylene. Wastewater from your home flows into the tank. Heavy solids settle to the bottom where bacterial action partially decomposes them. Most of the lighter solids, such as grease and oils, rise to the top and form a scum layer.
The wastewater leaving the septic tank is a liquid called effluent. It has been partially treated but still contains disease causing bacteria and other pollutants. From the tank, the effluent flows by gravity into the pump chamber.