Waste Water Treatment
In 2007, Paul Cavanagh, an engineer at Passaic Valley Sewerage Commissioners was having problems with an existing PLC program used to control both the mixed liquor channel level and to evenly split flows between twelve downstream secondary clarifier settling tanks.
Problem: existing control was slow and unstable
With the existing code, the level control was both slow and unstable. The tanks' flow control valves tended to respond slowly and to hunt around their setpoints. But repairing the PLC program on the actual plant was problematic: a high level could overflow into empty tanks or an out-of-control valve could disturb sludge settling.
Solution: real-time VisSim model
To avoid these problems, a dynamic real-time simulation of the plant was developed with VisSim. The simulated plant was interfaced to a PLC, so the program could be tested before trying it on the actual plant. Simulations were run for extreme changes from normal dry weather to sudden high wet weather flows.
Experimenting was now possible without risk
Experimenting was now possible without risk, which resulted in the development of a new PLC control algorithm that demonstrated significant improvement to the channel level and flow splitting controls. After proving the new PLC code on the simulated plant, it was loaded into the actual plant's PLC where it performed identically well. The level control and flow splitting problems were gone. Based on these results, the use of dynamic real-time computer simulation promises to assist PVSC in making dramatic improvements to its automated control systems.
Paul has given a paper on their success with VisSim at the ISA WWAC Symposium (Aug 5, 2009 in Orlando, Fla Session 4) .