Biological treatment of water contaminated by hydrocarbons in three-phase gas-liquid-solid fluidized bed

Biological treatment has been carried out in two different systems: aerated closed and threephase
fluidized bed reactors for hydrocarbons removal from refinery wastewaters. For the two
systems, hydrodynamic study allowed the determination of operating conditions before
treatment experiments. Then, in a second time, biological treatments have been conducted in
the same operating conditions. The obtained results showed that in the three-phase fluidized
bed we can degrade hydrocarbons more rapidly than in a closed aerated bioreactor.
Among the different appropriate techniques available to create efficient contacts between
phases, the three-phase fluidization G/L/S where carrier particles are moving inside the
reactor seems very interesting. It allows an intimate contact between phases and present
many advantages concerning hydrodynamic and mass transfer phenomena. In fact,
depending on operating conditions and the bubble flow behaviour, the three-phase fluidized
bed could display different flow regimes
In these systems called bioreactors the solid particles covered with a biofilm are fluidized by
two ascending flows of air and contaminated water. With favourable operating conditions,
from a hydrodynamic and mass transfer point of view, the pollutant can be biologically
degraded up to 90%.
Until this date, the three-phase bioreactors modelling remains very complex because it
required taking into account several factors: the pollutant biodegradation rate in the biofilm,
the bioreactor hydrodynamic characteristics, and the reactant interfacial gas-liquid and liquidsolid
mass transfer. Thus the essential purpose of modelling is to integrate the microbial
kinetics with the reactor hydrodynamics. We can notice that a few models have incorporated
both bioreactor hydrodynamics and microbial kinetics.
For the steady state bioreactor model, we generally assume that the particles are uniform in
size, the biofilm is uniform in thickness, and the biofilm can be considered as homogeneous
matrix through which oxygen and substrate diffuse and are consumed by the microbes. The
liquid phase in the bioreactor substrate is considered to be axially dispersed while the gas
phase is assumed to be in plug flow [2]. Rittmann (1997) proposed a model based on wake
theory for predicting bed expansion and phase hold-ups for three-phase fluidized bed
bioreactors. In this model he modified the correlation for the computation of the bioparticles
drag coefficient CD [3]. He also attempted to explain the biofilm detachment which can occur
with three broad patterns: erosion, sloughing and scouring and assumed that the factors
affecting detachment rates can be grouped into two categories (physical forces and microorganisms
physiology in the biofilm).