PID & DO control

[panelman]

Anyone have any experience using PID controls to control DO via airation?

 

[saxon]

panelman, Yes, I have experience with PID Control loops. Three questions: What do you mean by "DO"? What do you mean by "airation" and What do you want the control loop to do?

saxon

 

[panelman]

DO = dissolved oxygen, Aeration = blowing air through the sewage and the loop controls the aeration to maintain the DO at set point

But it is a spectacularly unstable process, non of my normal PID tuning methods seem to be working well enough to let me get a P band of less than 50% which is better than the previous controller was managing but I feel I should be able to improve it.

I was hoping for someone who has done it already, it’s quite a common process in water treatment

 

[jonelv]

Panelman

PID controls very common but you often use two or more to control the process. For example use a loop to control actual DO against DO setpoint in each aeration zone. You want to use a different loop to then control the blowers against an aeration set point. I have also been involved with systems where separate flow set points are used which can provide a more stable DO band. Basically the design depends on the diurnal variation oxygen demand and the rate of change which is a function of the sewage works catchment.

Jonelv

 

[panelman]

Cheers for that...in the system we have the blowers pid loop is used to maintain a constant pressure in the air main. This then feeds actuated valves controlled to match a DO set point in each zone