Control of ammonia concentration

I want to control the concentration of ammonia in a biological treatment tank based on the recycle flow rate. There is no control valve in BLOCK LIBRARY to fix it in a recycle line. I tried using flow sensor, but it is not worked out. How should I do that?. Please reply me.

[quote]
to control the concentration of ammonia .. based on the recycle flow rate
[/quote]
I would use a [b]control block[/b] (different models are available, e.g. "PI") that receives the flow rate ("Q_Out2_Actual") as input signal and controls the "kLa" of the ASU tank.

[quote]There is no control valve ..[/quote]
Control valves are called "[b]fraction[/b] flow splitters" in the Block Library.

I don't understand what you mean by:
[quote]I tried using flow sensor, but it is not worked out[/quote]
Can you elaborate?