Scaling procedure for biofilm processes

This paper presents a scaling procedure for biofilm processes based on three critical parameters derived from mechanistic models and used as similitude criteria. The advantage of using similitude criteria is that the prototype and scaled processes can have the same performance (e.g., effluent concentration) when the similitude criteria are made equal, even though all the mechanistic parameters are not known and the full kinetic models have not been solved. This scaling procedure, limited to situations for which the same microorganisms and process type can be assumed, is based on making equal the three similitude criteria needed in biofilm kinetics: (1) biofilm surface concentration, Φ₁, (2) biofilm shear loss-rate, Φ₂ and (3) mass balance, Φ₃. The three similitude criteria are necessary to give, for the prototype and scaled processes, the same flux into the biofilm (Φ₁), the same accumulation of biofilm (Φ₂), and the same mass balance on the substrate (Φ₃). When all thre criteria are equal, both proceses give the same effluent quality. One key finding of the similitude procedure is that the influent concentration, S₀, cannot remain constant if all three similitude criteria are to be equal simultaneously. The need to alter S₀ is most important when external mass transport resistance and biofilm shear loss are significant. It is only when external mass transport resistance or biofilm shearing is unimportant that similitude can occur with the same S₀ in both processes. This procedure was successfully applied to scale-down of the nitrification filter of the Croissy water treatment plant (France) for laboratory studies.