Sludge return is one of the basic operating controls in an activated sludge wastewater treatment system. The amount of sludge returned from the secondary clarifier affects the MLSS concentration in the biological tank, the sludge blanket in the clarifier, and the quality of the final effluent.
In practice, there is no single return sludge flow rate that works for every plant. The setting needs to be adjusted according to the actual operating conditions of the system.
What Does a Sludge Return Pump Do?
After biological treatment, activated sludge settles in the secondary clarifier. Part of this settled sludge is pumped back to the biological treatment stage, usually to an anoxic or anaerobic tank, to maintain the required biomass concentration.
The return sludge pump is responsible for this circulation.
One of the most useful parameters for operators is the sludge return ratio (R):
R = Return sludge flow rate ÷ Influent flow rate × 100%
For municipal wastewater treatment and A²O systems, a return ratio of approximately 50%–100% is commonly used as a starting range. Industrial wastewater can be quite different, however, so the actual setting should be based on sludge settling characteristics and plant performance rather than on a fixed number.
The return ratio should also be reviewed as operating conditions change. Influent characteristics, MLSS, SVI, and the sludge blanket level in the secondary clarifier can all affect the appropriate return rate.
How Can the Return Sludge Flow Be Adjusted?
There are several ways to control a return sludge pump. The method used usually depends on the type of pump and the control system installed at the plant.
1. VFD Control
A variable frequency drive (VFD) is generally the most convenient way to adjust the flow of a return sludge pump. Changing the motor frequency changes the pump speed and, consequently, the sludge flow.
The important thing is not to make large changes too quickly.
Before changing the frequency, operators should check the current influent flow, MLSS in the biological tank, and sludge blanket level in the secondary clarifier. A small adjustment of around 0.5–1 Hz at a time is usually more practical than making a large change in one step.
After changing the setting, give the system some time to respond. In many cases, 30–60 minutes is a reasonable period for the first observation. Further adjustment can then be made based on what is actually happening in the tank and clarifier.
VFD control also avoids the need to continually throttle the discharge valve and can provide smoother flow control with lower energy consumption.
2. Discharge Valve Adjustment
For constant-speed pumps without a VFD, the discharge valve can be used to make a basic flow adjustment. A gate valve or butterfly valve is partially closed to reduce the flow.
This method is simple, but it is not ideal for continuous fine control. Closing the valve increases resistance in the discharge line, which means the pump uses more energy. Excessive throttling may also contribute to pipeline wear and pump cavitation.
For this reason, valve adjustment is better regarded as a coarse or auxiliary method when VFD control is not available. Long-term operation with the discharge valve only slightly open should generally be avoided.
3. Using Several Pumps Together
Some treatment plants have two or more return sludge pumps installed in parallel. In this situation, the total return flow can be adjusted by combining pump start/stop control with VFD adjustment.
For example, one pump can handle normal operation and provide the fine adjustment. A second pump can be started when the required flow becomes significantly higher.
It is generally not a good idea to run several pumps at very low frequencies just to achieve a small flow. Depending on the pump characteristics, this can reduce operating efficiency.
A Practical Approach to Pump Adjustment
Before changing the return sludge flow, it is useful to record a few basic operating parameters:
Influent flow rate
MLSS concentration in the biological tank
SVI
Sludge blanket level in the secondary clarifier
Effluent SS concentration
These figures give the operator a picture of what is happening before the adjustment is made.
The next step is to decide what problem needs to be corrected. Is the MLSS too low? Is it too high? Is the sludge blanket rising? Is the effluent becoming cloudy?
Once the objective is clear, adjust the return flow gradually rather than making a large change based on a single measurement.
Another important point is that the effect of return sludge adjustment is not immediate. Sludge has to circulate through the biological process and secondary clarifier before the full effect can be seen. Therefore, making several adjustments within a short period can make the system difficult to control.
As a general operating practice, allow at least about one hour before checking the main parameters again, unless there is an urgent operating problem that requires immediate action.
After the system has had time to respond, check whether the MLSS is moving toward the desired range, whether the sludge blanket is stable, and whether the effluent remains clear.
What Should You Do Under Different Conditions?
When MLSS Is Too Low
If the MLSS concentration in the biological tank is lower than required, increasing the return sludge flow can bring more settled activated sludge back from the secondary clarifier.
However, increasing the return rate indefinitely is not the answer. A very high return flow can increase hydraulic loading and turbulence in the secondary clarifier. This may interfere with settling and increase the risk of sludge being carried out with the treated effluent.
The return rate should therefore be increased gradually while watching the clarifier performance.
When MLSS Is Too High
When MLSS becomes too high and the sludge is excessively aged, reducing the return sludge flow may be appropriate.
However, return sludge control is only one part of controlling the amount of sludge in the system. If the total sludge inventory is too high, excess sludge wasting is also needed.
In other words, reducing return flow should not be used as a substitute for proper sludge wasting.
When the Sludge Blanket Keeps Rising
A continuously rising sludge blanket in the secondary clarifier requires attention.
Increasing the return sludge flow can help remove settled sludge from the bottom of the clarifier and bring the blanket level down. This can be useful when the immediate problem is excessive sludge accumulation.
But the pump setting is not necessarily the root cause.
The operator should also look at influent loading, sludge settling characteristics, SVI, and possible sludge bulking. If the underlying problem is not addressed, simply increasing the return flow may only provide temporary relief.