How to Size a Commercial Sewer Pump Station: Flow, Static Head and Friction Loss
A commercial sewer pump station is sized by finding one duty point, the peak flow the pumps must deliver and the total head they must deliver it against, then choosing a wet well that holds enough active volume to limit pump starts and enough storage above the alarm to ride out a failure. Total head is static lift plus friction loss in the rising main plus any residual pressure at the discharge. Everything else in the design follows from those numbers.
This guide works through each step with real figures, then shows which packaged dual station fits the result. It is written for builders, plumbing and civil contractors and hydraulic consultants who need a station specified and supplied, not a textbook.
Have the drawings already? Send them to us and we will size the station, pumps and controls and return a quote, usually within 24 to 48 hours. For a quick first pass, the pump station sizing calculator gives an indicative tank and duty point.
Step 1: Establish the peak design flow
The peak design flow is the highest inflow the station must pass without surcharging, and it comes from the hydraulic design rather than from the pump catalogue.
For a building connection, peak flow is normally derived from the fixture loading in the sanitary drainage design under AS/NZS 3500.2. Where the station serves a reticulation catchment or a group of buildings, the water authority's code usually sets the method, typically average dry weather flow multiplied by a peaking factor, plus an allowance for infiltration. The water authority's requirements govern where they apply, so confirm the method with the authority or the hydraulic designer before fixing the number.
The pump duty flow must then satisfy two conditions:
- It must equal or exceed the peak design inflow, so the wet well cannot fill faster than the pumps empty it.
- It must produce a self cleansing velocity in the rising main, commonly taken as around 0.7 to 0.8 m/s, so solids do not settle and the main does not slowly block.
The second condition often governs on small commercial sites. A building may only generate 0.5 L/s at peak, but a 50mm main will not self cleanse at 0.5 L/s. The pump is then sized to the velocity, not the inflow.
Step 2: Calculate static head
Static head is the vertical distance from the pump cut out level in the wet well to the highest point the sewage must reach, which is usually the discharge invert at the receiving manhole or the crest of the rising main, whichever is higher.
Use the pump off level, not the inlet invert or the bottom of the tank, because that is the lowest water level the pumps work from. Where the rising main climbs over a high point and then falls to the discharge, the high point sets the static head. The head height guide covers the measurements in more detail for smaller domestic jobs.
Step 3: Calculate friction loss in the rising main
Friction loss in a sewer rising main is calculated from pipe internal diameter, length, flow and pipe roughness, and the Hazen Williams equation is the usual method for PE pipe at these flows.
The Hazen Williams form in SI units is: hf = 10.67 × L × Q1.852 ÷ (C1.852 × D4.87), where hf is friction loss in metres, L is pipe length in metres, Q is flow in m³/s, D is internal diameter in metres and C is the roughness coefficient. A C value of 140 is a reasonable design figure for PE pipe allowing for some ageing and slime build up; confirm the value the designer or authority uses.
Use internal diameter, not nominal size. A DN63 PE100 SDR11 pipe has an internal diameter of about 51mm, not 63mm, and the difference matters because friction varies with diameter to the power of 4.87.
| Pipe (PE100 SDR11) | Internal diameter | Flow | Velocity | Friction per 100m (C = 140) |
|---|---|---|---|---|
| DN50 | approx. 41mm | 1.0 L/s | 0.76 m/s | 1.8m |
| DN50 | approx. 41mm | 2.0 L/s | 1.52 m/s | 6.6m |
| DN63 | approx. 51mm | 1.5 L/s | 0.72 m/s | 1.3m |
| DN63 | approx. 51mm | 3.0 L/s | 1.45 m/s | 4.6m |
| DN75 | approx. 66mm | 2.6 L/s | 0.75 m/s | 1.0m |
Calculated with Hazen Williams, C = 140, straight pipe only. Add an equivalent length or K factor allowance for bends, valves and fittings. Indicative figures for early sizing, not a substitute for the hydraulic design.
Add the losses through the station pipework, non return valve, isolation valve and bends. On a short main these local losses can rival the pipe friction, so do not drop them because the main is only 30 metres long.
Step 4: Fix the duty point
The duty point is the design flow paired with the total head at that flow, and the selected pump curve must pass through or just above it.
Total head = static head + rising main friction + local losses + residual pressure at discharge. For a free discharge into a gravity manhole the residual is zero. For a discharge into a pressurised system the residual is set by that system.
Plot the system curve, not just the single point. Static head is fixed, friction rises roughly with the square of flow, and the pump runs where its curve crosses the system curve. A pump selected exactly on the duty point with no margin will fall short once the main ages or a valve partly closes.
Worked example: small commercial building to a gravity manhole
This worked example sizes a station for a small commercial building discharging to a gravity sewer manhole, and arrives at a duty point of 1.5 L/s at about 10 metres of total head.
| Input | Value | Note |
|---|---|---|
| Peak design inflow | 0.8 L/s | From the hydraulic design (assumed for this example) |
| Rising main | DN63 PE100 SDR11, 120m | Internal diameter approx. 51mm |
| Duty flow | 1.5 L/s (90 L/min) | Set by velocity: 0.72 m/s, not by the 0.8 L/s inflow |
| Static head | 6.5m | Pump off level to manhole discharge invert |
| Pipe friction | 1.8m | 120m plus 20m equivalent length for fittings |
| Station pipework and valves | allow 1.0m | Non return valve, ball valve, bends |
| Residual at discharge | 0m | Free discharge to gravity manhole |
| Total head at duty | 9.3m, design to 10m | Rounded up for margin |
Velocity governed the duty flow here. Had the designer simply matched the 0.8 L/s inflow, the main would have run at under 0.4 m/s and accumulated solids. Had the main been DN50, friction at 1.5 L/s would have been about 5.4 metres instead of 1.8, and velocity 1.14 m/s, which is also workable but pushes the pump further up its curve.
A duty point of 90 L/min at 10 metres sits well inside the published envelope of the Aline SABRE ALG1500-1 grinder pump (1.5kW, 240V single phase, maximum head 30m, maximum flow 240 L/min). The final selection is confirmed against the pump curve at the duty point, not against the maximum figures.
Step 5: Size the wet well
The wet well is sized for two volumes: the active volume between pump on and pump off, which limits starts per hour, and the emergency storage above the high level alarm, which buys time when the station fails.
Active volume
The minimum active volume for a single pump cycling is V = Q × T ÷ 4, where Q is the pump flow and T is the minimum cycle time, because the worst case for starts occurs when inflow is exactly half the pump rate.
For the worked example, a pump at 90 L/min limited to 10 starts per hour has a minimum cycle time of 6 minutes, so V = 90 × 6 ÷ 4 = 135 litres between the on and off levels. Check the pump manufacturer's permitted starts per hour; smaller motors usually tolerate more starts than larger ones.
Emergency storage
Emergency storage is the volume between the high level alarm and the lowest inlet or overflow point, and the required figure is set by the water authority or the hydraulic designer, often expressed as hours of average dry weather flow.
This is usually what pushes a commercial station from a 900 litre tank to a 1,200 or 2,000 litre tank. Duty and standby pumps reduce the chance of a failure; storage covers the response time when one happens anyway.
Step 6: Select pumps, controls and cover
A commercial sewer pump station normally needs duty and standby pumps, a dual pump controller with alternation and alarms, and an access cover rated for the traffic above it.
- Pump type: grinder pumps suit small diameter, long or high head rising mains; cutter pumps suit shorter, larger mains with fibrous solids. Both are in our submersible sewage pump range.
- Duty and standby: most commercial sites, apartment buildings and civil works specify two pumps so one failure does not cause an overflow. See our dual pump station guide for where a second pump is justified.
- Power: 240V single phase pumps suit lighter duties; heavier duties move to 415V three phase, such as the SABRE ALG2200-3 (2.2kW, 28m maximum head, 240 L/min) or the SABRE ALG3700-3 (3.7kW, 34m maximum head, 360 L/min).
- Cover: a station under a driveway, car park or loading area needs a trafficable cover. Our Class D pit lids page covers the options, including the 600 x 600mm Class D cast iron trafficable cover that suits a 600mm access opening.
Packaged stations that fit this duty
For duties in the range of the worked example, a pre assembled dual station is usually faster and cheaper to install than a site built well, because the pumps, valves, pipework and controls arrive fitted.
| Package | Tank | Pumps | Included | From |
|---|---|---|---|---|
| Kwikflo Dual Sewer Pump Station | 900L, 1,200L or 2,000L poly | Two cutter or grinder pumps, duty and standby | Control panel and floats, ball and non return valves, PVC pipework, poly lid; Class B and Class D cover options | $5,716 |
| BlueSelect 700L Dual Pump Station | 700L poly | Two grinder pumps, 1.1kW or 1.5kW, 240V or 415V | Guide rail lift out kits, high level alarm, 50mm discharge, 600 x 600mm cover | $7,874 |
Prices current as of November 2026, AUD including GST, for the base configuration. Final price depends on tank size, pump selection, cover class and risers.
For the worked example, a Kwikflo dual station in the 1,200 litre tank with grinder pumps covers the 135 litre active volume with plenty of room for emergency storage above the alarm. The whole range is in our packaged pump stations collection.
What is and is not included in a supply quote
A Waternex supply quote covers the station, pumps, controller, floats, internal valves and pipework and the cover; it does not cover excavation, bedding, the rising main, electrical connection or commissioning on site.
Installation is carried out by the site's licensed plumber and electrician. The inlet penetration is cut on site to suit the incoming invert. Payment is required before dispatch, and stations are dispatched promptly and delivered to site Australia wide, with freight quoted per project.
Duties beyond the listed range, such as large commercial stations, high head long rising mains, mine and remote site amenities or slurry and dewatering duties, are sized and supplied as custom packages on a quote basis. The commercial building sewer pump stations and mine site sewer pump stations pages describe those applications.
Frequently asked questions
How do you size a commercial sewer pump station in Australia?
A commercial sewer pump station in Australia is sized by setting a duty flow that meets both the peak design inflow and a self cleansing rising main velocity of around 0.7 to 0.8 m/s, calculating total head as static head plus friction plus local losses plus residual pressure, then sizing the wet well for active volume and emergency storage. The water authority's requirements govern storage and design method where they apply.
What is the duty point of a sewage pump?
The duty point of a sewage pump is the flow and total head at which it must operate in a particular system, for example 1.5 L/s at 10 metres. The selected pump curve must pass through or slightly above the duty point, confirmed on the manufacturer's curve rather than the maximum head and flow figures.
How do you calculate friction loss in a sewer rising main?
Friction loss in a sewer rising main is usually calculated with the Hazen Williams equation, h = 10.67 × L × Q^1.852 ÷ (C^1.852 × D^4.87), using the pipe's internal diameter and a roughness coefficient of around 140 for PE. For example, 140 metres of DN63 PE100 SDR11 at 1.5 L/s loses about 1.8 metres.
How big should a commercial pump station wet well be?
A commercial pump station wet well must hold the active volume between pump on and pump off, at least pump flow × minimum cycle time ÷ 4, plus the emergency storage above the high level alarm required by the water authority or designer. Emergency storage usually drives the tank size, which is why commercial stations commonly use 1,200 or 2,000 litre tanks.
Does a commercial sewer pump station need two pumps?
Most commercial sewer pump stations use duty and standby pumps, because a single pump failure on a commercial or multi unit site causes an overflow and downtime. The requirement for a second pump is set by the water authority, the designer or the site risk, and a dual pump controller alternates the pumps and brings in the standby on failure or high level.
Send us your drawings
Send us the hydraulic drawings, or the peak flow, static head, rising main length and diameter and the cover class needed, and we will size the station, select the pumps and controls and return a supply quote, usually within 24 to 48 hours.
Email sales@waternex.com.au or call 1300 930 847. Browse the packaged pump stations range in the meantime.
This guide is general technical information for Australian builders, contractors and hydraulic consultants. It is not a substitute for a hydraulic design for a specific site. Water authority requirements govern station design, storage and connection where they apply. Plumbing work must be carried out by a licensed plumber and electrical work by a licensed electrician.