A typical septic tank has a volume of 750-1500 gallons, and may be made of concrete, steel, or fiberglass. They are watertight chambers that promote the growth of anaerobic bacteria for the biological decomposition of sewage, and the separation of grease and organic particles from the wastewater. Septic tanks are constructed with baffles or sanitary tees to prevent the flow-through of floating solids, and to minimize the possibility of the raw sewage from leaving the tank, and disturbing the settled sludge and floating scum.