Some screw press donc't use mixing tank, only injecting polymer in the pipe and using an in-line mixer.
If you want to add a mixing tank, it's size will depend on the load your screw can take, the type and concentration of your sludge, the chemical you want to use, etc.
We have a Pulsa Feeder, it has two chambers. One is a static mixer and the other is mechanical. Now, these mix the polymer with water prior to injecting them into the Centrifuge at the same point sludge is brought in. Is that what your asking? This type is overly complex(P.F) I will admit but it does the trick.