Biomedical Engineering Reference
In-Depth Information
Liquid pulsing flow has been also proposed to improve the performance of
anaerobic/anoxic bioreactors. This kind of flow allows removal efficiency and stability to be
enhanced in upflow anaerobic filters (UAF) and UASB reactors, as it favours degasification
avoiding flotation and wash-out of the biomass, and decreases the formation of preferential
pathways (Franco et al., 2002; Franco et al., 2003; Franco et al., 2007). Performance
improvement of bioreactors caused by pulsation is due to a combined effect on both biomass
characteristics and the hydrodynamic conditions (Franco et al.
,
2003). From the point of view
of hydraulics, pulsation promotes biogas-sludge separation which is positive for controlling
biomass wash out at high loading rates, when a quite significant gas flow is produced, and
minimises the formation of preferential pathways. Both factors lead to a higher useful volume
of the reactor. The high porosity of biomass obtained in pulsed reactors could be one of the
causes of the better degasification of the sludge bed due to the enhancement of the internal
gas transfer rate (Alphenaar et al.
,
1992). Improvement in biogas liberation from the inner
part of the granule, due to hydraulic stress, may cause a compensation convective flow of
liquid within the granule, improving mass transport (van den Heuvel et al.
,
1997). In addition,
a better degasification increases the mixing in anaerobic/anoxic bioreactors (Kato et al.
,
1994).
Pulsing flow also favours biomass aggregation and granule development in UASB
reactors (Franco et al., 2002; Franco et al., 2003). Pulsations promote aggregation of
flocculent anaerobic biomass, shortening the granulation process which can last several
months in an UASB reactor (Franco et al.
,
2002). The hydraulic stress or selection pressure
generated by pulsation is beneficial to the self-aggregation of particles and also favours the
wash out of flocculent particles no susceptible to form aggregates of higher size (Hulshoff
Pol, 1989; Alphenaar et al., 1993).
Wastewater
composition
Wastewater
composition
Reactor
type
Reactor
type
Microbial
population
Microbial
population
Specific biomass
activity
Specific biomass
activity
Reactor
performance
Reactor
performance
Shear
stress
Shear
stress
Physical characteristics
of biomass
Physical characteristics
of biomass
EPS accumulation
EPS accumulation
Mass transfer rate
Mass transfer rate
Figure 5. Interactions among wastewater composition, reactor type and shear stress.
The application of pulsing flow to anoxic denitrifying USB reactors results in a more
stable and more efficient operation due not only to the granulation of the flocculent sludge but
also to the increment in the overall mass transfer rate (Franco et al., 2006). High shear stress
produced in pulsed bioreactors generates small-size granules with a high active surface and
porosity compared with those formed in non-pulsed reactors. Franco et al. (2008) studied the
effect of the pulsing flow on the performance of UAF and UASB reactors. These authors
found that in both kinds of systems external mass transfer resistance was decreased by
application of the pulsing flow. Nevertheless, Damköhler numbers obtained (maximum
kinetic rate to maximum external mass transfer rate ratio) showed that the systems were still
