
The format looks correct. You should be able to load the data file in W2012 (and in W2014 soon).
What version are you using?
Also: please check that tabs are actually used as column separators and not e.g. spaces.

Correct: the default value is indeed 1.
However, f_X_Out = 1 translates in ALL solids leaving the digester. You may want to "decouple" SRT and HRT and therefore rather use f_X_Out <1 (which by the way was the very meaning of the parameter in the original ADM1 implementation, if I recall well).

I am not aware of any "approved" biosorption process description  but other users may.
If you have such mathematical process description, it can certainly be inputted through the Gujer Matrix editor. The outcome will be a new Model Category  whether you define a new set of Components that include EPS, or use more conventional ones.

In the Model Editor:
 load a valid BlockLib.xml (or create a new one)
 select a Category in the Home  Category menu: this will enable the Gujer Matrix button
 click the Gujer Matrix button
See also: Ch. 8.3.2 of the User Guide.

You also ask about the specific equation.
f_X_Out is used to calculate the "outflux" (the outgoing flux) of solids, as follows:
OutFlow[j] = f_X_Out * ( C[j] * Q )
where:
 'j' denotes the jth particulate component
 Q the flow rate

1) the ObjValue is computed as the weighted sum of the individual subobjectives.
If you have 2 vars, 'v1' and 'v2', the obj for 'v1' is e.g. the mean of 'v1' multiplied by it own weight + the 95%ile by its weight. Same for 'v2'.
The overall obj is the sum of the obj for 'v1' multiplied by the weight of 'v1' and the obj of 'v2' by the weight of 'v2'
The error is the difference of the obj to the reference, measured or imposed (e.g. zero)
2) not sure I understand this  can you elaborate?

You cannot use the Qin of the influent as a scenario parameter  because it's not a parameter. What you would probably want to do is to run a SA where each run uses a different input file.
This cannot be done through the WEST user interface directly though.
What you may consider dose a bypass that diverts a fraction 'f' the influent  and rather use the fraction f as parameter for the SA.

I guess the error has to do with an algebraic loop. But you can actually check this by looking at the Logging, as the error message suggests  you may need to scroll up in the Logging pane to spot the line that issued the message.
To answer your question: if you want to have DS as an input (interface var) as opposed to as an output (state var), you need to rewrite the model in a suitable way, e.g. compute Q_Concentrate based on DS instead of the other way round.

Do you mean: "change a [b]parameter[/b] into a manipulated variable"? This essentially allows for changing the value of a 'model parameter' dynamically (e.g. via a controller or an input). A variable is computed (~ output) thus changing it into a manipulated variable (~ input) may require considerable changes to the model.
What you need to do is:
1) change the OBJ definition to interface variable, use the annotation 'manip=1' and set the causality to 'CIN'
2) replace all the occurrences of 'parameters.<OBJName>' by 'interface.<OBJName>'
Which files you need to work on depends on the 'level' at which the parameter is defined:
a) if at a submodel's level, you may need to only work on one file
b) if it is one of the categoryspecific parameters, you will need to work on many of the matrixrelated mal files: *interface.msl, *parameters.msl, ..
The cpp.msl file are of no relevance in this context.

Certainly not. But the main (only) thing you need to look at is the flow rate: is QUnder = 0 all the time? If so, you need to figure out why.
A couple of suggestions: look at the periods (if they are controlled and T5 happens to be zero ...); at the volume (is there sufficient volume to allow for extraction?). Also: I assume you are looking at a plot of QUnder. Is the communication interval sufficiently small to capture the expected change of QUnder during T5? The integrator and its settings may also contribute to conceal the change in QUnder.