Numerical Simulation of Mixed‐Culture Biofilm

Abstract

Differential equations describing mass balances on solutes and mass fractions in a mixed‐culture biological film within a completely mixed reactor are derived. Simultaneous numerical solution of these equations is not feasible because they are inherently stiff. Dimensional analysis is used to identify characteristic time periods of the phenomena involved. Separation of relatively slow and relatively rapid processes permits solution under nonsteady and sequential steady‐state conditions, respectively. Coupling of these solutions yields a nonsteady‐state biofilm model. The model incorporates external mass transport effects, Monod kinetics with internal determination of limiting electron donor or acceptor, competitive and sequential reactions, and multiple active and inert biological fractions which vary spatially. The system to which the model is applied includes carbonaceous oxidation, nitrification, and denitrification. Results of hypothetical simulations involving competition between heterotrophs deriving energy from an organic solute and autotrophs deriving energy from ammonia and nitrite are presented. The numerical approach used should be applicable to other fixed‐film biological systems and is amenable to modification to increase sophistication.