The effectiveness of RBC is mainly limited by the area available for growing biofilm.

Once the rotation speed is sufficient to satisfy the oxygen and COD supply to the biofilm required for maximal cell growth  any further increase in speed will not have any effect on the COD removal. 

The rotation serves for mixing the wastewater and the oxygen transfer . The speed is weak and depends of hydraulic load. In strong polluting load, increase the rotation improves the transfer of the pollutant in the biofilm

This is a simple but very high impact question. Traditionally one would accept the textbook explanation of "higher turning speed will give more mass transfer and hence improve the performance by minimising oxygen limitation. However, I am convinced the situation is more complext than that and that if done correctly over long periods, slower turning speeds can be more effective. I am interested to demonstrate and quantify this effect in collaboration with an RBC operator or researcher.

any further increase in speed will not have any effect on the COD removal.i think  getting oxygen is not increas by increasing the rotating velocity.

It depends on COD content of waste water. There is an optimum speed that will satisfy the oxygen requirement to achieve desired removal. Increasing the speed after this point will no matter increase the transfer rate of oxygen but there is also a particular time requirement for bio degradation by micro organisms which would in turn decrease on increasing the speed.

So suggested is the optimization of the system for speed , bio film area and other parameters.

The rotation speed should be just enough to provide sufficient oxygen and COD to the biofilm. It depends on how fast the bacterial growth is, and what is the Ks value for the COD. 

If the biofilm growth is optimal, there is no need to further increase the speed as it means COD uptake rate is highest.