* The process may not remove sufficient organic matter from the wastewater.
* The sludge age may be too low to enable nitrification.
The typical control band for the concentration of MLSS is 2 to 4 g/L for conventional activated sludge, or up to 15 g/L for membrane bioreactors.
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 calculated depending on COD or BOD load [kg/d]. When the operator finds the optimum MLSS concentration for each plant, they attempt to maintain this value by adjusting the sludge wasting or sludge excess 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 their 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.