 Sewers can be laid in flat terrain, and parts may flow uphill (within limits).A saw-tooth profile keeps sewer lines shallow; in frost-free climates, trench depth can be about 1.0 – 1.2 m. By contrast, gravity sewers need a monotonically falling slope of at least 0.5 - 1.0%, which can mean that expensive trenching and pumping stations are needed.Once the wastewater arrives in the vacuum collection tank at the vacuum station, it is pumped to the discharge point, which could be either a gravity sewer or the treatment station.