By Stephen Horne
In municipal and industrial wastewater treatment, there has been a shift from manual to automatic process control in order to increase efficiency and improve effluent quality. An increasingly popular approach includes installing a variable frequency drive (VFD) to constantly adjust the amount of air injected into the process to control dissolved oxygen, NH4, and/or NO3 in the wastewater treatment. This approach works well from a process control standpoint. The additional benefits include: lower power cost and extended service life of the blowers. VFDs offer great flexibility, but they have high purchase costs and there are still many applications in which they can be inefficient.