Biomedical Engineering Reference
In-Depth Information
A)
B)
C)
Effluent
Effluent
Effluent
Effluent
Feeding
Feeding
Feeding
Feeding
Effluent
Effluent
Effluent
Effluent
Effluent
Effluent
Effluent
Effluent
Feeding
Feeding
Feeding
Feeding
Gas Input
Gas Input
Gas Input
Gas Input
Pulsing
device
Pulsing
device
Valve
Valve
Valve
Valve
Gas input
Gas input
Gas input
Gas input
Feeding
Feeding
Feeding
Feeding
Figure 4. Examples of A) Impact stress: Stirred tank with mechanical stirring; B) Hydraulic stress
generated by gas: Fluidized bed; C) Hydraulic stress generated by liquid: Anaerobic reactor with pulsed
feeding.
Shear Stress and Mass Transfer Coefficients
In wastewater systems, shear stress can be generated by (Figure 4):
•
Impact stress:
due to collisions between the impeller/baffles/tank shell and the
particles or between the particles.
•
Hydraulic stress generated by gas:
due to the aeration (aerobic systems) or to the
recycling gas (anaerobic/anoxic systems).
•
Hydraulic stress generated by liquid:
by maintaining a pulsed flow or by a high
upflow velocity of influent (by recycling).
The effect of shear stress on the biomass characteristics depends on the type of reactor
used (Cao and Alaerts, 1995). Pereboom (1997) found that abrasion rate in stirred tanks was
higher than in bubbles columns for similar shear rates. This author observed attrition of
granules in bubbles columns while in stirred tanks also granule breakage occurs. Arrojo et al.
(2008) also found a higher decrease of biomass specific activity in a recycled gas flow system
compared to a mechanical stirred system both operating at similar shear rates values. These
authors concluded that the decrease of biomass specific activity could be related not only to
the effect of shear stress itself but also to other factors such as the loss of intermediates by gas
sparging. The importance of the type of system used to generate shear stress is discussed in
the following sections.
Shear stress is an important parameter in bioreactors not only by its effects on biomass
characteristics but also by its influence on the average apparent viscosity of non-Newtonian
