Biomedical Engineering Reference
In-Depth Information
turbulent conditions which increase the transfer of substrates, but shear stress higher than a
certain value provokes the loss of the activity. The management of shear stress in wastewater
treatment systems was also used as a selective pressure to separate the desired biomass. In
this way, those biomass particles that are dense can be retained inside the reactor, while the
light ones are washed out (Franco et al., 2006).
Reactors used for treating wastewater could be considered heterogeneous catalytic
reactors in which mass transfer rate completely or significantly controls the overall rate of the
process being promoted in the reactor. Figure 2 shows several physical and biochemical steps
involved during the process: oxygen is firstly transferred from the gas phase (in the case of
aerobic processes) to the bulk liquid, then through the bulk liquid to the surface of the
aggregate (a floc or granule), through the aggregate to the cell surface and, finally, into the
cell where biological reaction occurs.
Boundary layer
liquid-solid
Boundary layer
liquid-solid
Boundary layer
gas-liquid
Boundary layer
gas-liquid
Gas
phase
Gas
phase
S
S
S
S
Biomass
Biomass
Biomass
Biomass
1
1
Mass transfer gas-liquid
Mass transfer gas-liquid
2
2
Mass transfer liquid-solid
Mass transfer liquid-solid
S
S
S
S
S
Biomass
Biomass
Biomass
Biomass
Biomass
3
3
Internal difussion
Internal difussion
4
4
Reaction
Reaction
Figure 2. Steps involved during the reaction (S: Substrate).
External mass transfer describes the movement of substrate from the gas phase to the
bulk liquid and from the bulk liquid to the aggregate surface. This occurs in the regions where
the convective fluid velocity decreases. In this region, often referred to as the external
boundary layer, diffusive mass transfer increasingly dominates as the surface is approached,
but advection (transport due to fluid motion) can still play a significant role. Internal mass
transfer describes the diffusive movement of substrates within the biological aggregate, to the
site of the biochemical reaction (at or within the cell).
