Gravity Sewers & Lift Stations Explained
A collection system's whole job is to move wastewater from thousands of services to the treatment plant — mostly downhill, by gravity, with pumps to lift it over the high spots. Understanding how gravity sewers and lift stations work, and the components an operator runs, is the foundation of the collection system exam.
TL;DR
- Gravity sewers carry flow downhill at a slope designed to keep a self-cleansing velocity (~2 ft/s).
- Manholes provide access for inspection, cleaning, and ventilation, and sit at junctions and grade/direction changes.
- Lift (pump) stations raise wastewater where gravity runs out, then push it through a force main to a higher gravity sewer.
- Force mains are pressurized; gravity sewers are not — a key distinction.
- Pair this with collection system maintenance & I&I.
Gravity sewers — the backbone
Most of a collection system is gravity sewer: pipe laid on a downhill slope so wastewater flows on its own. The slope is engineered to keep velocity in the self-cleansing range (around 2 ft/s minimum) so solids stay suspended and don't settle into blockages. Gravity sewers flow partially full by design — the empty space above the flow carries sewer gases and gives capacity for peak flows. Pipe materials include PVC, vitrified clay, ductile iron, and concrete; older systems have more cracks and joints that admit infiltration.
Manholes
Manholes are the access points of the system, placed at:
- Changes in direction, slope, or pipe size
- Junctions where sewers combine
- Regular intervals on straight runs (so cleaning equipment can reach every segment)
They let operators inspect, clean, and ventilate the system. They're also a major inflow entry point — leaky covers in low-lying or flood-prone spots let stormwater pour straight in.
Lift stations (pump stations)
When the ground flattens or rises and gravity can't carry the flow any farther, a lift station pumps it up to a higher gravity sewer. The core components:
- Wet well — the chamber where incoming wastewater collects until it's pumped.
- Pumps — usually submersible or dry-pit centrifugal pumps; typically a duty pump and a standby (lead/lag) for redundancy.
- Level controls — floats or pressure/ultrasonic sensors that start the lead pump at a high level, bring on the lag pump if flow is heavy, and stop pumps at a low level.
- Force main — the pressurized pipe that carries the pumped flow to its discharge point.
- Controls, alarms, and standby power — SCADA/telemetry, high-level alarms, and a generator or connection for backup power, because a stalled lift station causes overflows fast.
Operators monitor run times, alternate the lead pump to even out wear, and watch for short cycling (which destroys motors — see the collections math guide on cycle time).
Force mains vs gravity sewers
This distinction shows up on exams: a force main is under pressure (the pump pushes flow through it, and it can run uphill), while a gravity sewer is not (flow moves only by slope and is open to atmosphere). Force mains have their own issues — they go septic and produce hydrogen sulfide because the wastewater is enclosed with no reaeration, which drives odor and corrosion at the discharge manhole where the force main dumps back into a gravity sewer.
Hydrogen sulfide, odor, and corrosion
Wastewater sitting in force mains and slow sewers goes anaerobic and generates hydrogen sulfide (H2S) — a gas that's toxic, explosive in the right range, smells like rotten eggs at low levels (and deadens your sense of smell at high levels), and converts to sulfuric acid that corrodes concrete pipe and manholes ("crown corrosion"). Managing H2S — with ventilation, chemical addition, and reducing detention — is a constant collection-system concern and a serious safety hazard (covered in the safety guide on the collections hub).
Practice it
Build on this with collection system maintenance & I&I and the collections operator math guide. Since collection and treatment overlap, the Wastewater Class I practice test covers collection-system basics too.