The MBR literature indicates that the alpha factor decreases exponentially with increasing MLSS. There is an interaction with sludge age, as increasing MLSS tends to increase sludge age, and increasing sludge tends to increase the alpha factor. If you need web links let me know, but a web search engine should do the job.

Plase check the paper to easily understand the issue:

Halil Hasar, Cumali Kınacı, Ayhan Ünlü, Viability of Biomass into Submerged Membrane Bioreactor, Desalination, 150(3), 263-268, 2002

a) The process is prone to bulking of solids and the treatment system can become overloaded.

b) This can cause the dissolved oxygen content to drop; this may reduce the efficiency of nitrification and the settleability of the sludge.

c) Excessive aeration will be required which wastes electricity

One of the easiest control procedures for activated sludge systems is the Constant Mixed Liquor Suspended Solids method. In this method, the operator selects a certain MLSS concentration or range of mix liquor concentrations that produces the best effluent and the highest removal efficiencies. This specific value or range must be determined experimentally. When the operator finds the optimum MLSS concentration for each plant, he attempts to maintain this value by adjusting the sludge wasting rate. One rule of thumb for activated sludge systems is that for every pound of BOD removed in the secondary system a half a pound of new solids is generated through reproduction of the organisms and addition of new organisms from the influent wastes. So, the operator tries to waste the proper amount of solids to keep his selected optimum mix liquor concentration constant. If the MLSS concentration is above the desired concentration, the wasting of the excess solids will have to be started or increased. If the MLSS concentration is below the desired concentration level, wasting should be decreased or stopped.