A submersible sewage pump is designed to operate while submerged in wastewater or sewage. Instead of requiring a separate dry installation, the pump and motor assembly is installed directly inside a wet well, sump or sewage collection pit.
Submersible sewage pumps are commonly used for applications where wastewater contains suspended solids and other contaminants that would not be suitable for standard clean-water pumps.
Flow Chem Pumps manufactures submersible sewage and sludge pumps for municipal and industrial wastewater applications, along with drainage, wastewater, cutter, slurry and dewatering pump solutions.
Why Pump Selection Matters in an STP
An STP contains several pumping duties, and the wastewater characteristics can change from one stage to another.
A pump may be required for:
- Raw sewage transfer
- Collection sump pumping
- Primary treatment transfer
- Sludge handling
- Secondary treatment applications
- Treated wastewater transfer
- Return sludge movement
- Final discharge
These applications do not necessarily require the same pump configuration.
The correct selection should therefore be based on the actual liquid characteristics and hydraulic requirements rather than selecting a pump based only on motor horsepower.
1. Determine the Required Flow Rate
The first important parameter is the required flow rate.
Flow rate indicates how much wastewater the pump needs to move within a given period. It is normally specified in units such as m³/hr or litres per second.
When determining the required flow, consider:
- Average wastewater flow
- Peak flow
- Influent variation
- Number of pumps operating simultaneously
- Required standby capacity
- Operating hours
For an STP, designing around average flow alone may not be sufficient if the plant experiences significant peak inflows.
2. Calculate the Total Head
Flow rate tells you how much liquid must be moved. Head determines the pressure required to move that liquid through the system.
Total dynamic head can include:
- Vertical lifting height
- Discharge pipe friction losses
- Pipe fittings and valves
- Non-return valve losses
- Required discharge pressure
A pump should be selected using the actual operating point rather than simply choosing a high-head model as a precaution.
3. Check the Solids Handling Requirement
Sewage can contain suspended solids, fibrous material, organic matter and other debris.
The pump must therefore be capable of handling the solids expected in the particular application.
Important considerations include:
- Maximum solids size
- Type of solids
- Fibrous material
- Settling solids
- Sludge concentration
- Risk of clogging
For example, Flow Chem's wastewater pump range includes models designed for suspended solids handling, while its cutter pumps are designed for applications where fibrous waste and materials such as rags and wipes create a higher clogging risk.
4. Decide Whether You Need a Standard Sewage Pump or Cutter Pump
This is an important distinction for sewage applications.
A conventional sewage pump is suitable when the solids can pass through the impeller and discharge passage without creating unacceptable blockage.
However, some sewage streams contain fibrous materials that can wrap around the impeller or cause repeated clogging.
In such applications, a submersible cutter pump can be considered. Cutter pumps use a cutting mechanism to reduce fibrous and solid material before pumping.
The correct choice depends on the actual wastewater characteristics rather than assuming that every sewage application requires a cutter.
5. Consider Motor Power and Operating Duty
Motor power should be selected according to the required flow, head, pump efficiency and operating conditions.
Other motor considerations include:
- Motor voltage
- Single-phase or three-phase supply
- Frequency
- Motor efficiency
- Insulation class
- Continuous or intermittent operation
- Starting requirements
For treatment plants operating for extended periods, motor efficiency and thermal protection become especially important.
Flow Chem's product range includes pumps designed for continuous-duty wastewater applications, with specific configurations available for different pumping requirements.
6. Select the Right Pump Material
Material selection should take into account the wastewater chemistry and operating environment.
Depending on the application, pump construction may involve cast iron, stainless steel or other suitable materials.
Material selection becomes particularly important when pumping industrial wastewater containing corrosive chemicals, high chloride levels or aggressive process contaminants.
For example, Flow Chem's wastewater pump range offers cast iron construction as standard with stainless-steel and special-alloy options for more aggressive effluent conditions.
7. Pay Attention to Mechanical Seals
Mechanical seals protect the motor and other internal components from contact with the pumped liquid.
In submersible applications, seal reliability is particularly important because the pump operates directly in the liquid.
When evaluating a sewage pump, consider:
- Single or double mechanical seal arrangement
- Seal-face material
- Compatibility with the pumped liquid
- Operating temperature
- Expected operating hours
For aggressive wastewater, seal material and configuration should be evaluated against the actual chemical conditions.
8. Consider Pump Speed
Pump speed can affect wear, vibration, noise, hydraulic performance and component life.
For demanding continuous-duty sewage applications, lower-speed configurations may offer advantages in specific operating conditions.
Flow Chem's 4-pole sewage pump range, for example, operates at approximately 1440 RPM and is designed for continuous-duty applications in CETP, STP and ETP environments.
The appropriate speed should still be selected based on the required duty point and pump design rather than speed alone.
9. Consider Automatic Level Control
STP collection sumps and wet wells can experience changing liquid levels.
Automatic level control can help start and stop the pump according to the liquid level and reduce the need for manual operation.
Possible control methods include:
- Float switches
- Conductivity-based level sensors
- Ultrasonic level sensors
- Control panels with automatic pump sequencing
Flow Chem's wastewater pump systems can be configured for automatic level control using appropriate sensing arrangements.
10. Check Wet-Well and Installation Conditions
Pump selection should also consider the physical installation.
Important site parameters include:
- Sump dimensions
- Installation depth
- Discharge pipe size
- Guide rail arrangement
- Available electrical supply
- Access for maintenance
- Operating water level
- Pump lifting arrangements
A correctly selected pump can still perform poorly if the sump, discharge piping or installation arrangement is unsuitable.
Flow Chem provides sewage pump installation and commissioning support covering site assessment, pit preparation, electrical setup, pump installation, testing and operator training.
11. Understand Continuous vs Intermittent Operation
Not every sewage pump operates under the same duty cycle.
Before selecting a pump, determine:
- Expected daily operating hours
- Number of starts per hour
- Continuous or intermittent operation
- Peak operating periods
- Standby pump requirements
For plants where pumps operate for long periods, the motor, seals, bearings and hydraulic components should be selected for the expected duty.
12. Consider Maintenance Requirements
Maintenance should be considered before purchasing the pump rather than after installation.
Ask the manufacturer about:
- Ease of pump removal
- Mechanical seal replacement
- Impeller inspection
- Bearing maintenance
- Motor protection
- Availability of spare parts
- Service support
A pump that is difficult to access or maintain can increase the total operating cost of an STP over its service life.
Submersible Sewage Pump Selection Checklist
| Parameter | What to Check |
|---|---|
| Flow | Required flow rate and peak flow |
| Head | Static lift plus friction and system losses |
| Solids | Solids size, concentration and type |
| Fibrous waste | Determine whether cutter action is required |
| Motor | Power, voltage, phase and duty cycle |
| Material | Compatibility with wastewater chemistry |
| Seals | Seal arrangement and material |
| Installation | Sump depth, piping and guide system |
| Controls | Automatic level control and protection |
| Maintenance | Access, spare parts and service support |
Common Mistakes When Selecting an STP Sewage Pump
Choosing Only by HP
Motor horsepower alone does not determine whether a pump is suitable. Flow, head, solids handling and operating conditions must also be considered.
Ignoring Peak Flow
Selecting a pump based only on average flow can create capacity problems during peak inflow conditions.
Ignoring Solids
A pump designed for relatively clean wastewater may not be appropriate for raw sewage containing larger solids or fibrous materials.
Using the Wrong Material
Industrial wastewater may require materials or seal arrangements that differ from standard municipal sewage applications.
Overlooking Installation
Pump performance depends not only on the pump but also on the wet well, piping, valves, electrical system and installation arrangement.
Submersible Sewage Pump vs Wastewater Pump
The terms sewage pump and wastewater pump are sometimes used interchangeably, but the actual application matters.
A sewage pump is generally selected for sewage streams containing solids, while wastewater pumps may be used for treated or partially treated wastewater with different solids and contamination characteristics.
Flow Chem's wastewater pump range is designed for treated and partially treated wastewater, ETP/STP discharge and industrial effluent applications, with models offering solids handling up to 50 mm.
For raw sewage and sludge applications, buyers should review the dedicated sewage/sludge pump range and match the pump to the actual solids and hydraulic requirements.
When Should You Consider a 4-Pole Sewage Pump?
A 4-pole sewage pump can be considered for applications where continuous-duty operation and reduced rotational speed are important design considerations.
Flow Chem's 4-pole sewage pump operates at approximately 1440 RPM and is positioned for CETP, STP and ETP continuous-duty applications. The manufacturer states that the lower speed is intended to reduce wear on components such as seals, bearings and impellers.
The final selection should be based on the required flow-head duty point and site conditions.
Why Work With an Application-Focused Pump Manufacturer?
Pump selection is more than choosing a catalogue model. The manufacturer should understand the liquid, solids, hydraulic conditions and installation environment.
Flow Chem Pumps is an ISO 9001:2015 certified pump manufacturer in Gujarat with a product range covering sewage, sludge, wastewater, drainage, dewatering, cutter, slurry and jet aerator applications. The company states that it has manufactured submersible pumps since 2002.
For a pump enquiry, useful information to provide includes the application type, required flow, head, pit depth, liquid characteristics, solids content, installation conditions and expected operating hours.
Frequently Asked Questions
Which pump is best for an STP?
There is no single pump configuration suitable for every STP duty. The selection depends on flow, head, solids, liquid characteristics, operating hours and installation conditions.
What information is required to select a sewage pump?
Important information includes required flow rate, total head, solids size and concentration, liquid characteristics, installation depth, electrical supply and operating duty.
When is a cutter pump required for sewage?
A cutter pump may be considered when wastewater contains fibrous materials such as rags, wipes or other solids that create a significant clogging risk for conventional sewage pumps.
Can submersible sewage pumps run continuously?
Some sewage pumps are designed for continuous-duty operation. The pump, motor, seals and cooling arrangement must be selected for the intended duty cycle.
How do I select the correct pump HP?
HP should be determined from the required flow, total head, pump efficiency and operating conditions. Selecting a pump based only on horsepower is not sufficient.
Does Flow Chem provide sewage pump installation support?
Yes. Flow Chem provides installation and commissioning support that includes site assessment, pit preparation, electrical setup, pump installation, testing and operator training.
Conclusion
Selecting the right submersible sewage pump for an STP requires a complete understanding of the application. Flow rate, head, solids handling, pump speed, motor duty, materials, seals, controls and installation conditions all influence the final selection.
Instead of choosing a pump only by horsepower or price, evaluate the complete operating requirement and select a configuration designed for the actual wastewater conditions.
For sewage, wastewater, sludge, drainage or dewatering applications, you can review Flow Chem's submersible pump range or contact the team through the Flow Chem enquiry page with your flow, head and application details.