The volume of the sewage ejector tank must be large enough to accommodate any backflow from the pipes and to effectively dose the system.
Not all basements require them, but when municipal sewer lines to the street are at a higher level than the facility, the ejector pump serves to pump both liquids and solids into the sewer line so that it can flow properly. Ejector pumps are also very common in septic drainfield systems, such as those found in rural areas, where the septic drainfield or holding tank may be considerably higher than the basement plumbing fixtures.