I agree with Amit Christian. ...

I agree with Amit Christian. One should also take note of the following:

1. Depending on the raw influent characteristics, primary settling tanks remove a large portion of the organic load which now passes to the reactor.
2. The higher organic load in the reactor will exert a higher oxygen demand and if your aeration capacity will not cope resulting in lower than required dissolved oxygen levels. This can be counteracted by shortening the sludge age but may not have a desired effect on the effluent quality.
3. The higher organic load will also result in a higher MLSS to cope with the higher load. This can also be counteracted by shortening the sludge age but may not have a desired effect on the effluent quality.
4. I would not recommend excluding the primary settling tank in an existing system without doing the calculations. If it is a new system it can be designed for the unsettled influent quality without a problem.