Biomedical Engineering Reference
In-Depth Information
covered particles coexist. In fact several works showed the most important factor which
influences the density of the biofilm and its detachment rate, and therefore the biofilm size, is
not superficial gas velocity but collisions between bare and covered particles (Gjaltema et al.
,
1997; Kwok et al., 1998).
Mechanical agitation can be used to improve the mass transfer rate of anaerobic systems
(Michelan et al.
,
2008). These authors determined the apparent kinetic constants of the
process and observed that the increase of the rotor speed resulted in an increase in the
constant values, by increasing the mass transfer rate, and that axial flow in mechanically
stirred reactors was preferable over radial-flow. A similar positive effect of mechanical
agitation on the performance of anaerobic systems was found by Pinho et al. (2004) who
observed the acceleration of suspended COD degradation with the increase of agitation rate.
They attributed these results to the higher velocity of shear of larger particles and major
contact between the particulate organic matter and the extra-cellular enzymes. They found the
apparent first-order kinetic constant for suspended COD rose approximately 360% when the
agitation rate was changed from 500 to 900 rpm, whereas the apparent first-order kinetic
constant for soluble COD did not vary significantly. Although the time required to achieve a
desirable efficiency can be reduced as the agitation rate is increased, high agitation rates may
represent high energy consumption. Therefore, the agitation rate to be applied should be
chosen taking into account both economic and environmental aspects (Ratusznei et al., 2001;
Rodrigues et al., 2003; Cubas et al., 2004).
Membrane fouling control in membrane bioreactor (MBR) could be achieved by means
of mechanical agitation. The major drawback of the membrane bioreactor operation is the
decline in membrane filtration performance with time. This decline is due to deposition of
particles and soluble materials onto membrane pores, attributed to the interaction of between
the biomass components and the membrane. The fouling of membranes is determined, among
other factors, by biomass characteristics and the hydrodynamic environment which are
closely related to shear stress inside the system. Shear stress can be induced either by cross-
flow velocity or by aeration. The cross-flow velocity is key factor in the operation of a side-
stream MBR while bubbling has been the strategy of choice for submerged MBRs to induce
flow circulation and shear stress on the membrane surface. The actual tendency shows an
increase of the submerged membrane systems application in the wastewater treatment field
due to their absence of a high-flow recirculation pump and, therefore, their lower power
requirements compared to external MBRs (Yang et al., 2006).
The increase in the aeration intensity to avoid membrane fouling can be uneconomical
and also affect the biomass growth rate and the microbial community of the sludge (Ji and
Zhou, 2006). Therefore, Khan and Visvanathan (2008) proposed the use of mechanical
agitation to enhance shear stress in addition to aeration intensity for membrane fouling
control in submerged MBRs. These authors observed that the fouling rate in a MBR with
mechanical agitation could be up to five times slower than that of a MBR without mechanical
agitation in spite of the amount of soluble and colloidal matter in the mixed liquor suspension
was higher when mechanical agitation was applied. They attributed this fact to the high stress
intensity and to the reduction of the activity of suspended biomass and a subsequent low EPS
release of the biofilm formed on the membrane.
