Definition - What does Gravity sewer mean?
A gravity sewer is an underground piping system which is sloped downwards away from the source and towards the destination. The product which flows through the pipes is geared by the force of gravity; there are no pumps or pressurized components in the system. Most sewage and storm water sewer systems were installed originally as gravity sewers, although pressurized systems are becoming more common.
Trenchlesspedia explains Gravity sewer
Gravity sewers are sized such that the product does not fill the pipe to prevent airlocks in the system. As pipes age and water ingress occurs through cracks, the product flow through the pipes may increase beyond design causing problems in flow.
Gravity sewers are also vulnerable to blockages, as there are no pumps and associated filters and blades that break up and remove solids. Lift stations are needed within the sewer network so that the pipe does not go too deep over long distances.