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.
An alternative to a VFD-only system is adaptive control with system-splitting. Adaptive control analyzes a system and provides accurate up-to-date feedback, while system-splitting combines fixed speed machines and VFD machines within the same operation. In a conventional system, there are three or more identical units, all with variable frequency drives. In an adaptive control system, there are three or more units but only one or two have variable frequency drives. The remaining blowers are fixed speed machines with reduced current starting and auto-dual control (load/unload as well as start/stop). By reducing the number of VFDs needed, initial costs are reduced. Furthermore, adaptive control uses real-time system measurement to match the exact output needed. By system-splitting, the auto-dual control on fixed speed blowers allows those units to be placed on idle where air is passed through at no pressure, then shut off after a specified time period. Having the blowers “off” is more efficient than turning the blowers down with a VFD when less air flow is needed. Adaptive control maximizes the time that the blowers are turned off and matches the capacities of the online blowers to the system demand. In this way, the constant speed blowers are always operating at their best efficiency.
Adaptive control systems operating to provide variable flow are considerably more efficient than VFD systems operating at the same pressures. Another consideration is that most wastewater facilities are oversized to compensate for future growth in the community they serve and for “worst case” scenarios. The equipment is usually selected for best efficiency at the specified flow. Because of conservative sizing considerations a wastewater treatment plant may never reach the specified flow and these worst case days often only occur a handful of times per year. That means for the most part, treatment facilities are not operating near their maximum efficiency, leaving adaptive control to be the most efficient option. Since adaptive control uses fixed speed units to carry most of the load and one or two variable speed units to trim the system, performance is more optimal and more reliable. Adaptive control systems have a lower initial cost, lower cost of redundancy, higher reliability and higher system performance. The potential savings from using an adaptive control system can add up, especially considering the expected life cycle of the wastewater treatment system.