13    BIOFILM CHARACTERISTICS IN A FLUIDIZED-BED
BIOREACTOR
S. M. Rao Bhamidimarri, Coordinator
Pollution & Waste Treatment Program
Department of Biotechnology
Massey University
Palmerston North, New Zealand
P. F. Greenfield, Chairman
P. R. F. Bell, Senior Lecturer
Department of Chemical Engineering
University of Queensland
St. Lucia, Australia 4067
INTRODUCTION
Fluidized-bed biofilm reactors have been increasingly considered for microbial applications, especially in biological wastewater treatment, in view of their superiority over other conventional systems.
Such systems have been investigated by a number of researchers18 for biological treatment of wastewaters. Removal of organic carbon and nitrogen have been accomplished employing fluidized-bed
reactors, which provide much higher productivities than conventional suspended growth activated
sludge or attached growth biological trickling filters. This comes about because of the large surface
area available for growth resulting in high biomass concentrations.
Effective substrate diffusivities into the biofilm and biofilm dry density, the most important of the
biofilm characteristics, which determine the performance of the process, need to be determined
accurately for an accurate quantitative analysis of the process.
Effective Diffusivity in Floes and Films
The significance of diffusion of oxygen into submerged pellets of aerobic microorganisms has been
recognized for some time. However, no convenient technique for the measurement of the effective
diffusivity has yet been reported.
Yano et al.9 were the first to analyze the diffusion of oxygen into microbial pellets and its influence
upon growth. However, they assumed the transport mechanism to be molecular diffusion. The model
was subsequently modified by Phillips10 introducing effective diffusivity in place of molecular diffusivity. Effective diffusivity of oxygen was experimentally estimated by Yoshida et a/.11 in Basidiomy-
cetes pellets to be about 4 to 12 times the value of molecular diffusivity depending upon the density of
the pellets. Matson and Characklis12 suggested a method involving the compaction of the microbial
aggregates into a filter medium. The physical compaction, however, is bound to alter the original
structure of the aggregates including the porosity and the tortuosity and, therefore, accurate values of
effective diffusivity can not be expected from this method. Miura et al.,n Ngian4 and Mulcahy14
evaluated the effective diffusivities from the substrate mass balance equations and observed substrate
utilization data via numerical integration. Huang and Bungay's measured oxygen concentration in
and near the mycelial pellets using a polarographic oxygen microelectrode and estimated a more
realistic value of the effective diffusivity to be 1.9 x 10~6 cm2/s.
The determination of substrate effective diffusivities in biofilms confronts two major problems.
Firstly, in view of the fragile nature of the biofilms, none of the reactor configurations popularly used
in the experimental determination of diffusivities in porous solids16 is satisfactory. Secondly, the
commonly used tracers such as the electrolytes (e.g. KC1 and NaCl) and compounds with low dissociation constant such as methanol are either consumed by or are toxic to the microorganisms.
Trypan blue, a bluish-gray vital strain soluble in water forming a deep blue solution with a violet
tinge, has the unique characteristic of being excluded by living cells.17 In this work trypan blue was
103