Abstract The work analyzes the interaction of a biological reactor and a settler coupled either via mass balance, exocellular polymer production or competition between flocculating and filamentous microorganisms. Pertinent experimental information is reviewed. Simple models are constructed to account for the necessary features of the interaction. The analysis reveals that chemical interaction should be correlated with sludge age and optimal operating conditions are derived. Biological interaction induces steady state multiplicity or coexistence of the two species. Recent observations of populations-shift dynamics are presented.