Hello
Our firm designs as well as operates many municipal wastewater treatment plants. One such plant is undergoing an upgrade and we have identified a Submerged Aeration Filter (SAF) vendor who will provide us with media to increase the capacity of the plant.
As a part of the upgrade, the existing Roots, positive displacement blower will be retrofitted with a VFD to control the blower speed. Secondly, flow control knife gate valves will be introduced into the air line.
The aeration basin has DO analysers and a Pressure transmitter and Flow transmitter will be introduced into the Air line.
Now as per the vendor's philosophy, the position of the knife gate control valve will be interlocked with the DO level in the basin i.e the valve open position changes depending on DO level (higher do, valve closes and vice versa). Flowmeter measures the air flow in air line.
Secondly as per his philosophy the Pressure transmitter set point will be used to control the VFD on the blower with the aim of keeping discharge pressure constant.
Now I'm unclear whether this is ok. I understand that the system needs a constant discharge pressure (Height of water in basin is 4 m in this case and blower has to provide this discharge pressure at all flow rates).
With a PD blower, the changing speed of the blower should only change the flowrate with the discharge pressure being nearly constant in all cases. So how can a Pressure Transmitter control the blower discharge pressure.
However, isn't the main reason to use VFD to reduce the energy consumption by controlling flow (at times of lower BOD load). Usually the Blower is upsized to handle intermittent peak loads in flow or BOD loads thus adding a heavy energy penalty to the process.
If that is the case shouldn't the blower VFD be interlocked with the DO analyser and thus turned down or ramped up depending on DO conditions in the basin? This is what we follow when we buy new blowers with VFD for any of our clients.
Thank you for your Time