A sewage pump has to do something a clean-water pump never has to: pass solids without jamming, while still generating useful pressure. Every design decision in a wastewater pump – impeller shape, passage size, motor sealing, wet well geometry – follows from that one requirement, and every blocked pumping station is a case where one of those decisions was made wrong.
What makes a sewage pump different
A clean-water impeller has several narrow vanes and tight clearances, because that is what makes it efficient. Put a rag through it and it stops. A sewage pump trades efficiency for free passage – fewer vanes, wider channels, bigger clearances – so that a solid of a given size goes straight through.
The rated solids-handling size is the diameter of the largest sphere that will pass. For municipal foul sewage it is typically 80 to 100 mm; for a domestic package station, 50 to 65 mm. That figure is the headline specification and you ignore it at your peril.
The impeller types
| Impeller | How it works | Best for | Trade-off |
|---|---|---|---|
| Vortex (recessed) | Recessed into the casing; the solid rides a whirlpool rather than touching the vanes | Rags, fibrous matter, unpredictable inflow | Lowest efficiency, around 45 – 55 per cent |
| Single-channel | One wide passage through the impeller | Municipal foul sewage with large solids | Prone to imbalance if partly blocked |
| Multi-channel | Two or three passages | Larger stations with screened inflow | Smaller passing size |
| Grinder | Cutting head macerates solids before pumping | Small-bore pressure sewer, domestic stations | High power draw, cutter wear |
| Chopper | Cutting action integrated at the impeller | Abattoirs, dairies, heavy industrial effluent | Cost |
| Open / semi-open | Partly exposed vanes | Sludge, abrasive slurries | Clearance wear |
The wet wipe problem
Almost every blockage we are called out to involves textile wipes. They do not disperse like toilet paper; they collect around anything they can catch on and bind into a rope, which then wraps the impeller hub. Vortex and chopper impellers handle them; narrow multi-channel impellers on small stations frequently do not. If a station blocks repeatedly, the impeller type is usually the answer rather than the maintenance schedule.
How a pumping station is built
The wet well
Sewage arrives by gravity into a chamber and the pumps lift it onward. Wet well design is more important than most people expect. Too large and solids settle out, rot and generate hydrogen sulphide, which attacks concrete and corrodes anything metallic in the chamber. Too small and the pumps cycle constantly. A benched, sloped floor that directs solids towards the pump inlet is the single best defence against septicity.
Guide rails and duckfoot bends
A submersible sewage pump sits on a duckfoot bend at the bottom of the well and is lowered down twin guide rails. Its own weight makes the seal against the discharge bend. This is what lets a technician lift the pump out with a chain for service without anyone entering the chamber – confined-space entry into a sewage wet well is a serious hazard and a properly built station never requires it.
Level control
Float switches are the traditional method and still work well on small stations, although floats are exactly the things rags wrap around. Ultrasonic or hydrostatic level sensors avoid that and allow proper level-based control: start level, stop level, high-level alarm and a lag-pump start level.
Duty and standby
Any station that matters has two pumps. A duty-standby controller alternates them on each cycle so both wear evenly and both are proven to work, and starts the second pump as well as the first when inflow exceeds one pump’s capacity. Alternation is not optional – a standby pump that has not run in eight months is not a standby pump.
Sizing a sewage pumping station
- Peak inflow. Work from contributing population or connected fixtures, applying the peak factor in SANS 10252 rather than an average.
- Rising main velocity. Aim for 0.8 to 1.5 m per second. Below that solids settle out in the pipe; above it friction and wear rise sharply. This constraint sets the rising main diameter, and the diameter then sets the required flow.
- Total head. Static lift from the stop level to the discharge point plus friction in the rising main. Long rising mains are friction-dominated.
- Pump selection. Pick a pump that meets the duty with the required solids passage. Passage size is a hard constraint, not a preference.
- Wet well volume. Size for a minimum cycle time – usually 10 to 15 minutes between starts – so the motor is not cycled to death, while keeping retention short enough to limit septicity.
- Emergency storage. Enough volume above the high-level alarm to cover a realistic response time. Load shedding makes this a live design question in South Africa.
What goes wrong, and why
- Ragging. Wrong impeller type for the influent. Fix the impeller, not the roster.
- Grease build-up. Common downstream of restaurants. Grease traps upstream and periodic wet well cleaning.
- Septicity and corrosion. Hydrogen sulphide from long retention. Reduce wet well volume, improve benching, or dose if necessary.
- Settled solids in the rising main. Velocity too low, usually from an oversized main. Every start then has to re-suspend the deposit.
- Float entanglement. Replace with a hydrostatic or ultrasonic level sensor.
- Dry running and burnout. Failed level control. Sewage pumps rely on the pumped liquid for motor cooling exactly as borehole pumps do.
Safety and standards
Sewage pumping stations are confined spaces with a real risk of hydrogen sulphide, which deadens the sense of smell at exactly the concentrations that become dangerous. A properly designed station is serviced entirely from the surface: guide rails, lifting chains, a surface-mounted control panel and a davit or gantry. Electrical installations in and around the wet well must respect the hazardous-area classification, and the whole installation should be built to SANS 10252 and the relevant municipal standard drawings.
Rentals and emergencies
When a station fails, the flow does not stop arriving. Over-pumping with a portable diesel or electric set, a suction hose into the wet well and lay-flat delivery to the next manhole, buys the time to repair properly. We keep sewage-capable units on the rental fleet for exactly this – see equipment rentals.
For the broader sanitation picture, see our water and sanitation solutions, and for the pumping fundamentals behind the head figures above, how to calculate pump head.
Designing or fixing a sewage pumping station? We supply submersible sewage pumps, guide rail systems, control panels and complete package stations, and we service what we supply. Request a quote or call +27 (15) 516-4991.