A few observations and very broad generalizations
The equivalent integral time, Ti, from the dependent form of the PID (cf. here) is over a quarter hour, so the current update time of 100ms could probably be increased by one or two orders of magnitude, which would require decreasing the Ki parameter by the same amount, and would also allow for some smoothing of the PV. That said, if the controller performance is acceptable now, there may be no incentive to do this.
There are two effects here:
- Flowrate - downstream measured DO response to changes in flowrate is a first-order process, and are reasonably-well handled (i.e. the customer is happ) in the PIDE with Kc and Ki alone.
- Influent DO load - downstream measured DO response to changes in influent DO load will be a deadtime process, assuming the both linear flowrate is small, and there is minimal diffusion, along the ditch. Controlling for variation in influent DO load would require some non-zero Kd be added to the PIDE (and even then it might not work).
That being the case, the current control system (PIDE), which compensates for changes in both flowrate and influent DO load, is trying to control a FOPDT (First-Order Plus Dead Time) process.
There is currently no allowance (Kd is zero) in the system for the deadtime i.e. influent DO load. However, that does not matter with the current tuning because
- the flow is so variable it is masking any variation in the influent DO load,
AND/OR
- the influent DO load is not varying significantly.
If we change the control scheme to scale the PIDE output CV by influent flowrate, then the CV will be following the difference in DO load between the influent and the setpoint, and the PIDE will be trying to control what is essentially entirely a dead time process, and I doubt the PIDE could do the trick.
Those observations/assumptions beg a few more queries
What are typical flowrates, and typical ranges of flowrates?
How long is the aerator (I assume it extends for some distance along the direction of flow in the ditch)?
What is the cross-sectional area of the ditch i.e. the area perpendicular to the direction of flow in the ditch?