Biomedical Engineering Reference
In-Depth Information
aeration also causes a decrease of the biomass particle size, favouring cell debris and
macromolecules which can have an important contribution to the membrane fouling.
However, the net effect observed is the improved membrane filtration with the increased
aeration. On the contrary, Ueda et al. (1997) reported that an optimum aeration rate exits
beyond which a further increase has no significant effect on membrane fouling suppression.
Hydraulic Stress Generated by Liquid
The upflow velocity of influent could be manipulated to acts as a hydraulic selection
pressure on microbial community. A high upflow velocity of influent selects for the growth of
fast settling bacteria and the sludge with a poor settleability is washed out. Alphenaar et al.
(1993) reported that biomass granulation in an upflow anaerobic sludge blanket (UASB)
reactor is favoured by the combination of high-liquid upflow velocity and short hydraulic
retention time (HRT). However, for successful start-up and stable operation of UASB
reactors, the reactor HRT cannot be reduced below 6 h and upflow velocities must be lower
than 1 m/h to avoid both disintegration of granules due to shearing and the wash out of the
resulting fragments (Kosaric et al., 1990). Mishima and Nakamura (1991) used the so called
Aerobic Upflow Sludge Blanket (AUSB) to obtain granular biomass under aerobic
conditions.
In anaerobic immobilized fixed bed reactors shear stress by the liquid flow rate caused
the biofilm loss and a relationship between shear stress and both biofilm loss rate and biofilm
thickness can be established (Nandy and Kaul, 2002). García-Morales et al. (2003) found that,
in spite of the biofilm detachment, the increase of the upflow velocities in anaerobic fluidized
bed reactors caused a positive effect on the specific biomass activity due to the reduction of
the substrate diffusional limitations to the microorganism growth inside the support pores.
Venkata Mohan et al. (2007) also found the recirculation of liquid improved the efficiency of
the treatment because it promoted the mass transfer of the substrate from the bulk liquid to the
biomass aggregates.
Pulsing flow has been applied to a great number of chemical (Lemay et al., 1975; Lerou et
al., 1980; Boelhouwer et al., 2001) and biochemical processes (Serieys et al., 1978; Ghommidh
et al., 1982; Murakami et al., 2000) for improving mass transfer, performance and efficiency of
the equipment employed (Beeton, 1991; Hewgill et al., 1993; Ni et al., 1995; Dondé et al.,
1987). Pulsing flow punctually increases upflow velocity, so shear stress (i.e., selection
pressure) may be enhanced under pulsing operation. However, it generates lower mechanical
stress than conventional stirring, which prevents disintegration of the granules although it still
generates enough to facilitate the formation of aggregates, especially when no carrier is
employed. Moreover, the implementation of pulsing systems is easier than other systems and
involves a much lower energy cost both at the pilot and industrial scale (Ni et al., 2003).
One type of pulsing bioreactor, the “reciprocating jet bioreactor” was successfully
employed for the removal of organic matter and ammonia from wastewater at pilot scale
(Brauer and Hennig, 1986) at very short residence times (only 5% of the employed in a
conventional system). The turbulence generated by the pulsing movement caused the
dispersion of gas bubbles and reduced the size of sludge particles, which implied an increase
in the interphase area gas-sludge, favouring mass transfer. Pulsation frequency was in this
case directly related to oxygen transfer within biomass particles, and therefore, with its size.
