Variable frequency drive (VFDs) can greatly improve efficiencies when used in pumping stations and aeration systems in wastewater treatment facilities.
Learn how the City of Columbus uses VFDs to significantly decrease power demands and reduce energy costs at their WWTF.
What is a variable speed drive (VSD)?
A variable speed drive (VSD) is any device that is used to change the speed of a motor shaft. These can be viscous couplings, transmissions, belts, eddy current drives, and/or wound motor drives.
What is a variable Frequency drive (VFD)?
A variable frequency drive (VFD) is an electronic solid-state device with an adjustable output that imitates a sinusoidal wave. A VFD is the most practical and common type of VSD device currently used in the water/wastewater industry.
Benefits of VFDs
VFDs can be used in pumping stations and aeration systems in wastewater treatment. For pumping stations, VFDs reduce the speed of the pump motor shaft and the resulting pumping rate. When operating under normal conditions, reducing the pumping rate will lower the total dynamic head (TDH) of the system. This is especially effective for systems with long piping systems and/or high fluid velocities, which cause significant friction losses resulting in steep system curves at peak flow rates.
In addition, for pumping systems with VFDs and appropriate controls (typically a programmable logic controller (PLC)) the pump speed can be controlled to maintain a given water level. This increases the average water level in a wet well, which reduces the TDH of the pumping system.
Specific energy, the energy used to pump a specific volume of water, is useful for measuring the effectiveness of VFDs. Specific energy is typically measured in kWH/MG.
City Of Columbus Case Study
The City of Columbus wastewater treatment facility (WWTF) previously used five lineshaft, constant speed, influent pumps. In 2011, the City replaced three of the pumps with dry pit submersible, variable speed pumps. The replacement pumps had approximately the same hydraulic efficiency as the original pumps. The other two pumps that were not replaced were kept for emergency purposes and typically did not operate.
In addition to the pump replacement, the City upgraded the pump controls and added PLCs to control the pump operation. This allowed the City to raise the average pump “on” level and reduce the static head for the pumps. Note: the influent flow rate was not great enough to set a constant wet well level.
The City monitored electrical usage before and after the pump replacement project. Savings included:
Specific Energy Before Pump Replacement: 259 kWH/MG
Specific Energy After Pump Replacement: 179 kWH/MG
Energy Usage Reduction: 30%
VFDs can also be used to reduce the power demand upon pump start up. The VFDs are a type of reduced voltage starter, which helps to reduce the peak power demand compared to a constant speed pump, which is simply “on” or “off.” Power demands are important, as they make up the vast majority of energy costs for a typical utility monthly invoice.
The City monitored their power demands before and after the pump replacement project. Savings included:
Power Demand Before Pump Replacement: 60kW
Power Demand After Pump Replacement: 30kW
The City also used VFDs as part of an aeration upgrade project to better match the necessary aeration rate for process requirements. Before the upgrade, the City did not have the ability to “dial down” the equipment to the required aeration rate. This resulted in over aerating for biological treatment, an issue that’s not uncommon in wastewater treatment plants.
The project included changing replacement blowers from the multi-stage centrifugal type to the positive displacement type, VFDs for the blowers, updated controls for the blowers, and replacement of the fine-bubble diffusers.
The replacement blowers proved to be more efficient than the original. The City compared utility invoices before and after the project; no other processes were changes and total influent pumping volume for each month was approximately the same as before the improvements.
Savings included:
Before aeration project (July 2013): $8,260
After aeration project (August 2013): $6,074
Energy Cost Reduction: 26%
It’s estimated that approximately half of the above energy cost savings is from the actual blowers/diffusers, while the other half is from the use of VFDs.
ADDITIONAL BENEFITS OF VFDS
The ability to extend pump runtime when the working volume of the wet well cannot be increased due to the need for a higher peak pumping rate. This functionality is used to meet minimum cycle time requirements for sewage pump stations.
The capability to reduce the size of standby generators. VFDs are a type of reduced voltage starter. VFDs reduce the inrush current required by the motor thereby reducing the generator size that would be otherwise required with an across-the-line starter. The Village of Hartland is using VFDs with a pump upgrade project to limit the increased size of their standby generator so that the existing generator building can be re-used.
Variable and reduced pumping rates can be useful for treatment processes such as pumping biosolids to a dewatering device.
Precautions to be aware of when using VFDs
VFDs add complexity to the system. They are a control device added in series to system.
There can be increased electrical losses with VFDs. Typically, these are very minimal.
VFDs sometimes require input and output filters to reduce interference with other power equipment and motors.
The typical life span of a VFD is 8-12 years depending on usage and starts per hour.
Learn More
To learn how your community can use VFDs to save energy and costs, please contact one of our experts today!
About the Author
David W. Arnott, P.E.
Team Leader/Senior Project Manager
Dave has extensive experience in the planning, design, and construction project management of wastewater treatment facility renovation and upgrade projects. Areas of expertise include treatment processes, hydraulics, mechanical equipment, and wastewater disinfection. He also has experience in sanitary and water pumping station design and sanitary sewer wet weather facilities.
