When a basement WC backs up or a pump station alarm goes off at 2am, the first thing anyone wants to know is how the system is meant to work. How does a sewage pump work, and why do some installations run for decades while others block every few months? The answer usually comes down to choosing the right type of pump and sizing the pipework around it.
This guide explains how sewage pumps and pumping stations work, the main types of sewage pump (vortex, grinder, cutter and macerator units), and how to choose and size one for a commercial or residential building in the UK. It is written for facilities managers, contractors and specifiers, and includes a calculator for the two figures that catch most people out: rising main velocity and 24-hour storage.
What a Sewage Pump Does
A sewage pump lifts foul water, including toilet waste, from a point below the sewer to a level where it can flow away by gravity. You need one wherever part of a building sits lower than the public sewer, such as a basement flat, a lower ground floor WC, a plant room toilet or a new building on a low plot.
Approved Document H treats pumping as the fallback when gravity drainage is not practical. For pumping installations inside buildings it points to packaged units that conform to BS EN 12050, installed in line with BS EN 12056-4. Those two standards shape most of the decisions further down this guide.
Sewage Pumps vs Drainage and Sump Pumps
A drainage or sump pump is built for clear or lightly dirty water and has small clearances that toilet waste, wipes and rags would block quickly. A sewage pump has a larger free passage or a cutting mechanism designed to handle solids and fibres. Using a sump pump on foul water is one of the most common causes of repeat call-outs. Our comparison of sewage pumps and sump pumps goes into the differences in detail.
How Does a Sewage Pump Work?
Almost every sewage pump installation follows the same cycle. Waste flows by gravity into a sealed chamber, often called a wet well or sump. A level switch starts the pump when the chamber fills to a set point, the pump pushes the contents up a pressurised pipe called the rising main, and the switch stops the pump when the level falls. A non-return valve stops the waste in the rising main draining back into the chamber.
Inside the Pump
Most sewage pumps are submersible centrifugal pumps. A sealed electric motor drives an impeller in a volute casing, and the spinning impeller throws the liquid outwards and up the discharge pipe. What separates a sewage pump from a clean water pump is how the impeller deals with solids: either it lets them pass through untouched, or it cuts them up first. That design choice is what defines the main types below.
Controls and Alarms
Simple installations use a float switch attached to the pump, which starts and stops it as the level rises and falls. Larger stations use separate level sensors or floats wired to a control panel, which can alternate duty between two pumps and raise a high level alarm if the level keeps rising. An alarm is essential on any foul installation, because a failed pump is otherwise only discovered when the waste backs up into the building. Our float switch guide covers the options.
Types of Sewage Pump
Choosing the right type matters more than any other decision. The table compares the main designs.
| Type | How it handles solids | Rising main | Best for |
|---|---|---|---|
| Vortex (recessed impeller) | Impeller sits back from the inlet; solids pass through without touching it | Larger bore, typically 50 mm and above | General foul water, short to medium runs, waste with rags and fibres |
| Grinder | Rotating cutter at the inlet shreds solids into a slurry | Small bore, often 32 to 50 mm | High heads, long runs, pressure sewers |
| Cutter | Cutting edges at the impeller chop fibrous material | Standard bore | Higher flows with fibrous waste, commercial and industrial sites |
| Macerator WC unit | Blades in a small tank behind the WC break up waste | Small bore | Adding a single WC or en-suite where gravity drainage is impossible |
| Packaged pump station | Tank, pump, valves and controls supplied as one unit | Depends on the pump fitted | New basement drainage and whole-building foul pumping |
Vortex Sewage Pumps
A vortex pump has its impeller recessed into the back of the casing. It creates a swirling vortex that draws liquid and solids through the pump, so most solids never touch the impeller. That makes vortex pumps very tolerant of rags, wipes and stringy material, at the cost of some efficiency. They are the default choice for general foul water. Examples include the JS SV single-phase sewage vortex pumps, and the stainless steel Pentair Jung Priox range.
How a Grinder Pump Works
A grinder pump has a hardened cutter at the inlet that spins against a fixed cutting ring. Everything entering the pump is shredded into a fine slurry before it reaches the impeller. Because the slurry carries no large solids, it can be pumped through a small-bore rising main, over long distances and against high heads.
That makes grinder pumps the right choice when the discharge point is a long way off or well above the pump, or where a small pipe is all that can be run through the building. Models such as the Pentax DTR reach heads of over 50 metres. The trade-offs are higher power draw for the flow delivered, cutters that wear and need replacing, and sensitivity to grit and sand. Grinders are generally not recommended upstream of a septic tank, because the tank relies on solids settling out.
Cutter Pumps and Macerator Units
Cutter pumps use cutting edges on the impeller to chop fibrous material while keeping a normal bore and higher flows, which suits commercial premises and industrial wastewater. Macerator units, such as the DAB Genix range, are compact tanks that sit behind a WC and pump its waste away through a small pipe. They are a good fix for adding a single WC or en-suite, but they are not a substitute for a pumping station serving a whole building.
How to Choose a Sewage Pump
Work through these steps in order. Each one narrows the choice before you look at any pump curves.
Step 1: Define What the Pump Will Handle
Start with the waste itself. Toilet waste and general foul water suit a vortex pump. Long or high discharge routes suit a grinder. Commercial kitchens add fat and food waste, which call for grease management upstream of the pump. Sites where wipes, paper towels or sanitary products are likely, such as offices, care homes and public toilets, need a pump with a generous free passage or a cutter.
Step 2: Work Out the Flow and Storage
Approved Document H says the receiving chamber should hold 24 hours of inflow, so waste can still be contained if a pump or the power supply fails. For homes the minimum daily discharge is taken as 150 litres per person per day. For other buildings it is based on the calculated daily water demand. The pump itself must clear the peak inflow comfortably, which on a building with several WCs and showers is much higher than the daily average suggests.
Step 3: Calculate the Head
Total head is the vertical lift from the stop level in the chamber to the highest point of the rising main, plus the friction losses in the pipe, valves and bends. Friction depends on the flow, the pipe bore and the length of the run, so it cannot be estimated with a simple rule of thumb. Our guide to calculating pump sizing walks through total dynamic head step by step, and how to read a pump curve shows how to check a pump delivers your flow at that head.
Step 4: Size the Rising Main for Self-Cleansing Velocity
This is the step most often missed. Waste in the rising main must move fast enough to keep solids in suspension. BS EN 12056-4 sets a flow velocity of 0.7 to 2.3 m/s in the rising main for lifting plants in buildings. Below 0.7 m/s, solids and grease settle and the pipe slowly blocks; well above the range, friction losses and water hammer rise sharply.
Inside buildings, the same standard calls for a minimum rising main size of DN 80 for sewage containing faeces, unless the pump has a cutting or grinding device. That is why grinders are chosen where only a small pipe can be run. Use the calculator below to check your velocity and storage figures. For more on pipe sizing generally, see our guide to sizing pump discharge pipes.
Sewage pumping calculator
Check the velocity in your rising main and the minimum 24-hour storage for a domestic installation.
Velocity range from BS EN 12056-4. Storage based on Approved Document H: 150 litres per person per day for domestic use. Non-domestic buildings should use their calculated daily water demand. A guide only; a full station design should be checked by a drainage engineer.
Step 5: Decide Between a Single Pump and Duty/Standby
A single pump is acceptable where a failure would be an inconvenience rather than a disaster, and where the 24-hour storage gives time to get it repaired. Where a building depends entirely on pumped drainage, such as a block of basement flats, offices or a care home, a duty/standby pair is the safer choice. The control panel alternates the pumps to share wear, runs both together on very high inflow, and switches automatically to the standby pump if the duty pump fails.
Step 6: Check the Power Supply
Smaller sewage pumps run on 230 V single-phase, which suits homes and small commercial buildings. Larger pumps and those for high heads or flows are usually 400 V three-phase, such as the JST SV 400 V vortex range. On construction sites, 110 V versions are available for some ranges. Confirm the supply before you choose a pump, because it can rule out half the options.
Common Mistakes When Specifying a Sewage Pump
We see the same problems again and again on installations that fail early:
- Using a drainage or sump pump on foul water, which blocks on the first wipes or rags.
- Oversizing the rising main to reduce friction, so velocity drops below 0.7 m/s and the pipe silts up.
- No high level alarm, so a pump failure is only discovered when waste backs up into the building.
- Too little storage below the inlet, which gives no time to respond to a fault and causes rapid pump cycling.
- Floats that can tangle with each other, the pump cable or the chamber wall, leaving the pump running dry or not starting.
- No isolation valve or union on the discharge, so the pump cannot be lifted out without draining the rising main.
Maintenance and Servicing
A sewage pump that is sized correctly needs relatively little attention, but it should not be ignored. Test the floats and alarm regularly, listen for changes in running noise or longer run times, and keep the chamber free of fat build-up and debris. A professional service should check the pump's insulation resistance, seal condition and impeller or cutter wear. On grinder pumps the cutter is a wearing part and will need replacing over time. If a pump has failed, our repair and rewind team can often get it back into service faster than a replacement can be delivered.
Recommended Sewage Pumps from AES Rewinds
Looking for a sewage pump? At AES Rewinds, we stock a comprehensive range of sewage pumps and pumping stations suitable for industrial, commercial and residential applications.
Browse our full range:
- Sewage and waste water pumps, including vortex, grinder and cutter designs
- JS GS grinder pumps for small-bore rising mains
- JS Box domestic pumping stations and Vacusystem sewage pump stations
- Float switches and controls
Our team can help you select the right pump for your building and discharge route. Contact us for expert advice on sewage pumping.
Frequently Asked Questions
Should a sewage pump run all the time?
No. A sewage pump should run in short cycles, starting when the chamber reaches the start level and stopping when it is emptied. A pump that runs constantly has a fault: a stuck float, a leaking non-return valve letting the rising main drain back, a blockage, or an inflow larger than the pump can handle.
What is the difference between a grinder pump and a vortex pump?
A vortex pump lets solids pass through a large free passage without cutting them, so it needs a larger rising main. A grinder pump shreds solids into a slurry, which allows a small-bore rising main and much higher heads. The types section above compares them in detail.
Can a sewage pump handle toilet paper and wipes?
Sewage pumps handle toilet paper without difficulty. Wet wipes, including those sold as flushable, do not break down and are the most common cause of blocked sewage pumps. Vortex and cutter pumps cope with them better than other designs, but no pump is immune, so signage in commercial washrooms is worthwhile.
How does a sewage pumping station work?
Waste flows by gravity into a chamber. Level switches start a pump when the chamber fills, the pump pushes the waste up a rising main through a non-return valve to a gravity sewer, and the pump stops when the level drops. Larger stations have two pumps and a high level alarm.
What size rising main does a sewage pump need?
Size it so the velocity stays between 0.7 and 2.3 m/s at the pump's duty flow. Inside buildings, BS EN 12056-4 calls for at least DN 80 for sewage containing faeces unless a grinder or cutter pump is used. Use the calculator above to check a pipe size.
How much storage does a sewage pumping station need?
Approved Document H recommends storage for 24 hours of inflow, taken as 150 litres per person per day for homes. A four-person house therefore needs at least 600 litres. Other buildings use their calculated daily water demand.
Can a grinder pump discharge into a septic tank?
It is generally not recommended. A septic tank works by letting solids settle and break down, and the fine slurry from a grinder tends to stay in suspension and carry through to the drainage field. A vortex pump is usually the better choice ahead of a septic tank or treatment plant.
Key Takeaways
- A sewage pump works in a fill, start, pump, stop cycle, lifting foul water up a pressurised rising main through a non-return valve.
- Vortex pumps suit most foul water; grinders suit long or high discharge routes with small-bore pipes.
- Keep rising main velocity between 0.7 and 2.3 m/s, and use at least DN 80 for faecal waste inside buildings unless the pump grinds or cuts.
- Approved Document H calls for 24 hours of storage, at 150 litres per person per day for homes.
- Fit a high level alarm on every foul installation, and use duty/standby pumps where the building depends on pumped drainage.
Related Articles
- Sewage Pump vs Sump Pump: why the two are not interchangeable.
- How to Calculate Pump Sizing: total dynamic head and duty point, step by step.
- How to Size Pump Discharge Pipes: velocity and friction loss explained.
- Everything You Need to Know About Float Switches: choosing level controls for a pump station.
- What Is a Dirty Water Pump?: where dirty water pumps end and sewage pumps begin.