With in-tank settling

The question is:
For T type oxidation ditch,  the ditch on the two sides will be used for sedimentation during a particular period, can we use WEST to simulate this process with biological degradation followed by sedimentation in one tank?

I am not familiar with the peculiarity of a "T type" oxidation ditch, but I have seen implementations of oxidation ditches in WEST, yes.
You could probably use a Continuous Flow-Intermittent Decanter (CFID) block and a Timer/Controller to switch phases. It may also need some careful modeling of the phase change to avoid hard switches that would slow down the integrator.

For switching between reaction phases and settling phases you should use an continuous flow continuous (CFCD) model. This is a model that has two phases (reaction and settling) cf an SBR, but with a continuous feed.

If you want to consider reactions during the settling phase, you have to modify the model (I don't think it is standard available in WEST).