A mathematical model of a fixed-film biological waste water processor has been developed for system level simulations of the Trickle Filter Bioreactor (TPB) currently operating at the NASA Johnson Space Center (JSC Hybrid Regenerative Water Recovery System (HRWRS) laboratory. The TFB model has been compared against HRWRS lab data as well as an independently developed mathematical model of a waste water processing biofilm.On a microscopic level, the fundamental equations which describe the simultaneous diffusion and reaction within the biofilm are simplified by use of polynomial substitution to provide a solution which can be solved much more rapidly than traditional finite difference solution methods. Phenomenological transport equations are used to couple the biofilm and liquid phases as well as the gas and liquid phases. These mathematical simplifications provide an algebraic solution with the simplicity and speed ideal for inclusion in a large scale multi-component system level model.On a macroscopic level the reactor is divided into a series of small cross-sectional volumes within each conditions are assumed homogeneous. The liquid flow over the biofilm and the support media upon which it grows is assumed to be as a thin film. Information from the microscopic biofilm reaction and mass transport equations along with fluid flow characteristics in each cross-sectional volume are used to determine macroscopic phenomena such as biomass growth and propagation, microbial species competition, and biofilm sloughing. Coupling of dynamic bulk fluid flow equations from each cross-sectional volume provides a description of the behavior of the overall TFB hydraulics.