SRT in AS-unit

Im modelling in ASM2d and Im trying to understand how to modify the sludge retention time in my activated sludge units. Is it possible and how do I do it?

If it matters: I have 3 separate activated sludge lines with 5 units on each line (to mimic that each activated sludge unit has 5 compartments).

Many thanks :)

You need to 1) compute the SRT and 2) feed the signal to a controller.

To compute the SRT:
[list]
[li]use the "Process Calculator" block (only one model associated, namely: "OperationalParameters")[/li]
[li]feed the relevant inputs: volumes and MLSS of the AS tanks; flow rates and TSS concentrations of the effluent and waste sludge flows. There are some limitations in the number of such tanks and flows, you may need to do some clever manipulation ...[/li]
[li]the sludge age is presented as SRT (state variable) or y_SRT (interface variable). However, this is the [b]instantaneous[/b] SRT which may be fluctuating too much to be given as input to a controller.[/li]
[/list]

You may want to use the "Data Treatment" block (model: "MovingAverage") to compute a more stable (and realistic) SRT:
[list]
[li]feed the y_SRT of the Process Calculator as input[/li]
[li]set an appropriate time window, e.g. 7 days[/li]
[li]the [b]averaged[/b] sludge age is presented as y_M[/li]
[/list]

Now, feed the y_M of the Data Treatment to a suitable control model.

[i]Watch out for algebraic loops[/i]!

Thanks for your answer Enrico. I still have two questions/problems though:

1. I know that the SRT should be around 1,5-2 days. Cant I skip the calculation part and feed an SRT value directly?

2. If Im using a process calculator to get the y_SRT and feeding it to a data treatment block, my SRT becomes the y_M of this block. So I want to feed this to the AS-unit, but the only options I can choose in the interface link are Kla and Temp. How can I let my AS-unit know that the y_M is the SRT?

Thanks a lot!

Now I don't understand what do you have in mind exactly.
The reason why I suggested that way to compute SRT is that I would generally [i]indirectly[/i] control SRT through the rate of sludge wastage. Therefore the chain Calculator block -> Control block -> Pump (e.g. a flow splitter).

Assuming you could directly feed an SRT value to the AS tank (see below), what would be the effect? [b]Can you elaborate?[/b]

From a modeling standpoint, all you can "control" (i.e. feed data to) in a model are its input variables, specifically what is called in WEST the [b]manipulated variables[/b]. If you take a look at the MSL code, these are interface variable that are flagged with: [tt]manip = "1"[/tt] .
So, yes: for the standard AS tanks, there are only 2 manipulated variables, i.e. temperature and KLa.
Now, from a purely technical point of view, if you would like to feed the SRT to an AS tank, all you need to do is:
[list]
[li]define a manipulated variable called e.g. "SRT"[/li]
[li]add (at least) one equation that makes use of the new variable[/li]
[/list]