Calculating Pump Flow Rate for Sewer and Stormwater Systems

When it comes to the selection of your sewage or stormwater pump station, understanding how to calculate the flow rate of a pump is critical. Properly calculating flow rate ensures that your pump selection aligns with the needs of your project, whether it’s managing sewerage, wastewater or stormwater.

If the Pump Flow Rate is specified

Often engineers will calculate the discharge flow rate for you and include this in their plans. If the flow rate is specified, it can typically be found in the following places: 

1. In the key or legend of the drawing

Note: If dual pumps are required, ensure you’re aware of whether the specifications refer to the ‘total flow rate’ or the ‘flow rate of a single pump’. This is because dual pumps can either run in a duty/standby set-up (only 1x running) or in a duty/assist set-up (2x pumps running concurrently).

If the Pump Flow Rate is not specified

On occasion the engineer may not include the discharge flow rate of the pumps. They may indicate this to be calculated by a third party e.g. pumps to manufacturers recommendations, alternate engineers, or to be detailed by hydraulic/civil engineer. When trying to determine how to calculate the flow rate of a pump on your own, there are two industry accepted methods you can use. 

For sewer pump station applications, there’s a few things you’ll need to know: 

Sewerage type: Understanding the type of sewage—whether it's raw sewage, effluent waste, or trade waste—is crucial for selecting the appropriate pump. Different waste types have varying characteristics, such as the presence of soft solids or corrosiveness, which must be considered to choose the correct pump. 

Inflow: For sewage or effluent, knowing the population of the building helps estimate the average daily flow rate which is a critical first step in calculating the flow rate or 'outflow'. Accepted daily water usage rates vary based on the system's purpose; for example, public ablution blocks and residential living conditions have different expected water demands. Ideally, the discharge flow rate should meet or exceed inflow, aiding in chamber capacity calculations. It should be noted that most wastewater authorities have stringent limits on the sewer flow rate that they permit for a particular site. 

For trade waste, if you understand what creates inflow, such as kitchen waste or industrial machine discharge, you should be able to accommodate this value in your outflow, again noting the wastewater authority limitations.

For stormwater applications, the below are required to estimate the flow rate accurately:

Location: Since stormwater pump stations are designed to collect and discharge rainfall events, understanding the average rainfall values for your site location is essential. Different locations have varying rainfall patterns, often provided by the Bureau of Meteorology. 

Collection area: Knowing how much water will be collected in your pump station is a key factor for understanding inflow, hence sizing pumps. The larger the catchment area your pump station receives run-off from, the more stormwater it will be required to collect and discharge. On your plans your collection area is often roof surface area and/or exposed paved areas. Where commercially viable, the pumped discharge flow rate should meet or exceed inflow. If this is not possible, your pump chamber may need to compensate for the additional inflow. If you’re unsure how to calculate the surface area, check out part 1 of this blog series: How to calculate sump capacity. 

Note: If the stormwater pump station is located in an under-croft carpark, special spark-resistant pumps are required to manage harmful fumes emitted by cars.

Pump Selection

Pumps are selected to achieve a discharge outcome; however, a pumps’ performance will vary depending on the system they are being installed in. Due to system resistance (often referred to as total system head) the pump encounters opposing force that increases or decreases the discharge flow rate a pump can achieve. Therefore, understanding how to calculate pump flow rate and the system parameters is essential for accurate pump selection.

For both sewer and stormwater applications you need to know the following factors:

Discharge Pipework: The diameter and length of the discharge pipe (also known as the rising main) must be known to ensure the pump will be able to transfer the sewage or stormwater effectively. The longer and narrower the pipe, the more resistance is created as the liquid rubs against the internal pipe walls. This increased resistance directly reduces the pump flow rate. This is known as friction loss. 

Site levels: Understanding if there’s any rise in site elevation from the pump station location to the discharge point, also known as the static head or vertical lift, is essential. The pump must be powerful enough to overcome this “lift” to discharge effectively.

These parameters are necessary for using a stormwater or sewer flow rate calculator to effectively select the right pump/s for your project. 

Need help calculating your flow rate? 

Our tool simplifies calculating flow rate and pump station sizing, ensuring optimal efficiency and performance. Whether you're managing stormwater or sewage, our selection program enables you to make informed decisions, delivering a tailored and effective pump station solution. Get started today by creating an account and accessing instant online quotes 24/7.