VFDs Can Make a Big Difference in Your WWTP
May 31, 2012 | by: Ron Eisele
Most of us operators know what VFDs are, but for those who have not yet heard of them or for those who haven't had the need to learn more about them, it stands for Variable Frequency Drive. Most new treatment plants use them as a standard and many older treatment plants are retrofitting them into their process. Here’s basically how they work.
In the older plants we had a motor running, say a blower. The motor was running at x amount of horsepower at 100% of the time. The motor that is connected to the blower did not care about how much DO you needed in the aeration basin, all it knew was to deliver x amount of horsepower all the time. So basically you could be sending out tons of air when you didn’t need it and wasting all that energy.
Well along comes the VFDs. We can now control the speed of the motor with the concentration of DO, (or other set points) and lower the speed of the motor to match the DO in the aeration basin. All we need is a DO sensor in the basin reading the DO, send a 4-20 milliamp signal to the VFDs and they will slow the motor down, (or speed up if needed) to match whatever set point you have in your plant.
Greensville Water and Sewer Authority in Greensville County just added two new VFDs to their 150hp motors that run centrifugal blowers for their extended aeration basins at their Three Creek Wastewater Treatment Plant. They were fighting the DO in those basins. The DO would range from 2.0- 9-0. The electrical costs for these blowers were high.
In talking with James Warf, Superintendent of Water and Sewer for the authority, he stated the treatment plant is holding steady at 2.0 mg/l of DO and motors on the blowers are only running a small amount of rated horsepower. They are already seeing the plant operating in the range it was designed to be and at a cost savings with the electricity.
The bottom line is one of the highest costs to a treatment plant, after wages is the electrical cost. As a good operator we always need to be keeping a watchful eye on our budget. If we can operate our plants more efficiently and at a lower cost, this can only be a win/win for everyone. So, take a look at your plant. Do an in-house “energy survey” and see if you can install some VFDs somewhere. See you next time in the RAS/WAS Building.