Introduction
Selecting the correct submersible mixer is an important part of wastewater treatment system design.
A common starting question is:
"How many watts per cubic meter do I need?"
Power density, or W/m³, can be useful for preliminary estimation. However, it should not be used as the only basis for final mixer selection.
The same motor power can produce different hydraulic performance depending on impeller design, diameter, rotational speed, and tank conditions.
For this reason, wastewater mixer selection should consider both power and hydraulic performance.
1. Calculate the Effective Tank Volume
The first step is to determine the operating volume.
For a rectangular tank:
Volume = Length × Width × Water Depth
Example:
20 m × 10 m × 4 m
= 800 m³
The effective operating volume should be used rather than simply the nominal structural volume.
2. Understand W/m³
Power density can be calculated as:
P/V = Mixer Power ÷ Effective Tank Volume
For example:
Mixer power = 4 kW
Tank volume = 800 m³
Therefore:
4 ÷ 800 = 0.005 kW/m³
or:
5 W/m³
This value can be useful for comparing preliminary design alternatives.
However, 5 W/m³ does not automatically mean that a mixer is suitable for every 800 m³ tank.
Why?
Because tank geometry and hydraulic conditions can be very different.
3. Mixer Thrust Is Critical
Motor power tells you how much electrical power the motor consumes.
It does not fully describe the hydraulic performance of the mixer.
For wastewater applications, mixer thrust is an important parameter because it indicates the hydraulic force generated by the propeller or impeller.
When comparing mixers, engineers should review:
Motor power
Mixer thrust
Impeller diameter
Rotational speed
Hydraulic efficiency
Flow characteristics
A high-power mixer with poor hydraulic efficiency may not necessarily provide better mixing than a properly designed lower-power mixer.
4. Tank Geometry Determines the Flow Pattern
Consider two tanks with the same volume:
Tank A: 10 × 10 × 4 m = 400 m³
Tank B: 20 × 5 × 4 m = 400 m³
Although both contain 400 m³, their hydraulic characteristics are different.
Tank length-to-width ratio, inlet and outlet locations, internal structures, and water depth can all affect circulation.
Therefore:
Same volume ≠ Same mixer configuration
The number and position of mixers should be selected according to the required circulation pattern.
5. MLSS and Wastewater Characteristics
Wastewater is not clean water.
Activated sludge may commonly contain approximately:
2,500–3,500 mg/L MLSS
depending on the process.
Higher solids concentrations can affect suspension requirements and hydraulic behavior.
Other characteristics may also be relevant, including:
Sludge concentration
Temperature
Wastewater viscosity
Presence of fibrous materials
Tank process type
Required mixing objective
These factors should be considered when selecting a mixer.
6. Determine the Purpose of Mixing
The mixer should be selected according to its actual purpose.
Anoxic tank
Primary objectives:
Keep MLSS suspended
Maintain circulation
Promote contact between nitrate, carbon source, and biomass
Avoid unnecessary oxygen transfer
Anaerobic tank
Primary objectives:
Maintain solids suspension
Promote uniform conditions
Avoid unwanted oxygen introduction
Equalization tank
Primary objectives may include:
Prevent solids settling
Homogenize wastewater
Reduce concentration fluctuations
Oxidation ditch
Mixers may be used to support continuous circulation through the treatment channel.
Different applications may therefore require different mixer configurations.
7. Determine the Number and Position of Mixers
A common mistake is to select the mixer first and determine the installation arrangement afterward.
The better approach is:
Tank geometry → Hydraulic requirement → Mixer thrust → Mixer quantity → Installation position
Multiple mixers may be required for large or irregular tanks.
Installation should consider:
Flow direction
Tank partitions
Inlet location
Outlet location
Water depth
Obstructions
Required circulation path
The objective is to avoid stagnant areas and achieve sufficient circulation throughout the effective volume.
What Information Should You Provide to a Mixer Supplier?
For preliminary mixer selection, you can try to provide:
Tank length
Tank width
Operating water depth
Effective tank volume
MLSS concentration
Wastewater type
Mixing purpose
Number of tanks
Existing mixer information
Installation restrictions
If available, also provide:
Process flow diagram
Tank drawings
Existing equipment specifications
Required treatment performance
Photographs of the tank
With this information, an engineer can evaluate the appropriate mixer power, thrust, quantity, and installation arrangement.
Submersible Mixer Product/Application
Submersible mixers are commonly used for anoxic tanks, anaerobic tanks, oxidation ditches, equalization tanks, and other wastewater treatment applications requiring continuous circulation and solids suspension.
Depending on the project requirements, mixer configurations can be customized for factors such as:
Stainless steel construction
Corrosion resistance
Voltage and frequency
Cable length
Motor insulation class
Leakage and overheat protection
Installation method
For project-specific selection, the tank dimensions and operating conditions should be reviewed before recommending a model.
FAQ
How do I calculate submersible mixer power?
A preliminary calculation can use power density:
P/V = Mixer Power ÷ Effective Tank Volume
However, W/m³ should only be used as a preliminary reference. Final selection should also consider mixer thrust, tank geometry, MLSS, impeller characteristics, and hydraulic requirements.
Is W/m³ enough to select a wastewater mixer?
No. W/m³ is useful for preliminary comparison, but it does not fully describe the hydraulic performance of a mixer.
Why is mixer thrust important?
Thrust indicates the hydraulic force generated by the mixer and helps engineers evaluate its ability to create circulation and maintain solids suspension.
How many mixers does a wastewater tank need?
There is no universal number. The required quantity depends on tank geometry, effective volume, mixer thrust, installation arrangement, and required circulation pattern.
Can one mixer be used for different wastewater applications?
Potentially, but the model and configuration should be evaluated according to the specific application. An anoxic tank, equalization tank, and oxidation ditch may have different mixing requirements.
Conclusion
Submersible mixer selection should not be reduced to a single W/m³ number.
A reliable selection considers:
Tank volume + Tank geometry + MLSS + Mixer thrust + Impeller characteristics + Mixer quantity + Installation position
The goal is not to install the largest mixer.
The goal is to achieve effective circulation, solids suspension, stable process conditions, and reasonable energy consumption.
For project-specific applications, providing complete tank and process information allows the mixer to be selected based on actual hydraulic requirements rather than a simple power-density estimate.