OXYGEN LIMITATION IN HETEROTROPHIC BIOFILMS
Dolores T. M. Owen, Refinery Engineer
Standard Oil Company
Richmond, California 94802
Kenneth J. Williamson, Assistant Professor
Department of Civil Engineering
Oregon State University
Corvallis, Oregon 97331
INTRODUCTION
Aerobic heterotrophic growth of bacterial films is common in aquatic environments
and in many waste treatment reactors such as trickling filters, activated biofilters [1],
and rotating disc filters [2J.   Consequently, a thorough understanding of the kinetics
of organic degradation by biofilms is important for describing wastewater treatment reactors and natural purification processes.
Bacterial-mediated reactions involve electron donor-electron acceptor couples; for
heterotrophic growth, these are specifically organics and dissolved oxygen.   Many investigators have proposed models of heterotrophic biofilm growth [3-6], but in all cases the
concentration of the electron donor has been assumed as limiting the reaction rate.
Williamson and McCarty [7] have predicted that oxygen concentrations may limit the kinetics of heterotrophic biofilms.   Their model was verified with nitrifying biofilms; no experimental data for heterotrophic growth were presented to confirm their oxygen-limiting
prediction.
The purpose of this study was to determine if heterotrophic biofilms under typical
conditions for wastewater treatment reactors are kinetically limited by the dissolved oxygen
(DO)concentrations.   The experimental data obtained were compared to those predicted by
the model proposed by Williamson and Chung [8] using assumed Monod kinetic coefficients.   In addition, the composite diffusion coefficient for soluble domestic sewage organics through heterotrophic biofilms was measured.
BIOFILM MODELS
Single-Species Limitation
Biofilm models based on diffusion of the substrate from the bulk solution into the
biofilm and the metabolism of the substrate within the biofilm by Monod kinetics have
been presented by Atkinson et al. [3] and Williamson and McCarty [71.   The former
model is based on a dimensionless analysis and is limited in application to certain ranges
of the dimensionless parameters.   The latter model is limited to thick biofilms such that
the limiting species approaches a zero concentration deep within the biofilm.   Both models
can only be used if either the electron donor (e.d.) or the electron acceptor (e.a.) limits
the substrate utilization rate at all depths within the biofilm.  The model presented by
Williamson and McCarty [71, which uses the nomenclature (See Appendix A) adopted
by Lawrence and McCarty [9) for their modeling of the activated sludge process, will be
used in this paper.
Conceptually, substrate must diffuse first from the bulk liquid through a stagnant
liquid layer at the biofilm surface, and then into the biofilm (Figure 1).   The biofilm is
assumed to be attached to a flat plate with uniform mixing at the biofilm surface.   The
diffusion process is modeled by Fick's Law and the bacterial metabolisms by Monod
kinetics.   Thus, for the stagnant liquid layer, the flux is:
AcDw(S0—Ss)
'° "        L, + L2 (,)
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