WWTP with a membrane

I everyone, I have to admit I'm kind of a newbie in WEST.
I'm testing an activated sludge plant with a membrane separation, instead of the classic secondary clarifier.
I'm struggling with the fact that the parameters of the membrane set a constant Qout.
I'm using the ideal membrane (without fouling)
I tried to use a control to set a constant concentration in the SST of the concentrate, but it gave me an error (system error: Model contains one or more algebraic loops)
I have put a loopbreaker and this happens only if the PI_control is present.
If I don't put the control, though, I struggle with the fact that the membrane works in the opposite way of a clarifier (you set the Qout instead of the underflow).
If anyone could help me I'd find it very useful, thank you.

Hi Mirko,
there are a couple of things in your post ...
[list type=decimal]
[li][b]Loop-breaker[/b]: indeed, it may happen that an algebraic loop be detected due to a controller, i.e. the controlled variable is directly dependent on the measured variable.
Unfortunately, there is no such thing as a loop breaker block for signals (red lines); and the only way to solve the issue is to place a normal loop breaker on a blue connection (flux) in a position such that the data loop is also broken[/li]
[li][b]flow through the membrane[/b]: it's not entirely correct that Qout is constant. The outflow is regulated by the flux through the membrane (J12) which is a manipulated variable, i.e. can be controlled[/li]
[/list]
If you need / prefer to directly control Qout, the only way is to re-write the model somehow in reverse, so that it exposes directly Qout (as an input) and e.g. the flux as an output.