Membrane bioreactors are ...

Membrane bioreactors are generally capable of tolerating wider variations of MLSS concentrations inside the bioreactor than conventional activated sludge plants. However there are some boundaries that need to be respected. Do you know these boundary conditions for your plant?

Generally a too high MLSS increases sludging of your membrane filters and lowers the aeration efficiency in the bioreactor. A too low MLSS may also increase membrane fouling depending on the type of membrane. A loss of biological activity might be another result of a drastic lowering of MLSS concentrations which then would also lead to increased fouling (among other nasty things).

Maybe you should try to keep your MLSS concentration in check by controlling the rate of excess sludge extraction while you gradually and slowly increase the feed from your rainwater tanks.