or stirred tank reactors with a Rushton turbine, which represents a considerable saving in
agitation-aeration energy (Serieys et al., 1978).
A piston pulsator was coupled to an extractive fermentation system for alcoholic
fermentation, where liquid-liquid extraction permits the continuous separation of ethanol.
Pulsation was shown useful to increase the contact between aqueous and organic phases,
thus improving ethanol extraction, and also minimising gas hold-up (Minier and Goma,
1982). Good results were also obtained in ethanol fermentation employing a pneumatic
pulsator, shown in Figure 11.1-c (Navarro and Goma, 1980). The use of a perforated
plates column with a pneumatic pulsing system also improved the effectiveness of the
aeration and, additionally, it was responsible for a reduction of gas hold-up and the
enhancement of the mass transfer coefficients (Dondé et al., 1987).
The reciprocating jet bioreactor consists of a cylindrical vessel with a height/diameter
ratio of 4 to S and contains an assembly of sieve plates attached to a vertical rod (similar
to Figure 11.1-a). The reciprocating motion is achieved by an electric motor and a special
crank gear connected to the package of sieve plates by means of a central axis. The
turbulence generated, due to the movement of the plates, causes the dispersion of the gas
bubbles and reduces the pellet size, which implies an increase of the interfacial area
between bubbles and pellets, this enhancing mass transfer.
This reactor was successfully applied to the anaerobic wastewater treatment (Brauer
and Sucker, 1979). A system of three reciprocating jet bioreactors provided with settlers
for biomass retention, was effectively applied to remove carbon and nitrogen from highly
polluted wastewaters (Brauer and Annachhatre, 1992a,b). Reciprocating bioreactors have
also been employed for the production of antibiotics (by Cyathus striatus), citric acid (by
Aspergillus niger) and ethanol (by Zymomonas mobilis) (Brauer, 1991).
One important point in wastewater treatment in fixed-bed systems based on anaerobic
(associated to the production of CH 4 and CO 2 ) or aerobic processes (in which apart from
the air which is introduced, there is production of CO 2 ) is degassing. The gas retained
between the bioparticles reduces the effective reactor volume and, also, impedes an
efficient contact between the dissolved organic matter and cells. Pulsation helps
degassing due to shearing off the gas bubbles and favours their evacuation outside the
reactor. Furthermore, it facilitates the generation of a large interfacial surface, as well as
the periodical renewal of the interphase and the uniform distribution of the air bubbles.
Other types of pulsing devices (membrane pumps) have been applied to anaerobic
wastewater treatment aiming to minimise the formation of preferential pathways and
clogging at the exit of the reactor in anaerobic filters, fluidised-bed reactors and loop
reactors. Pulsation improves degassing and increases stability of the pH, mainly because
of an improvement in the distribution of substrate and of the slow movements of the bed,
which generates new exchange surfaces (Etzold and Stadlbauer, 1990).
Each type of the above mentioned pulsators is particularly suitable to the resolution of
particular problems, although all of them attempt to increase mixing inside the system,
this being undesirable in the cases of processes inhibited by product. In order to make the
concepts of plug flow and pulsed flow compatible, a device, the “elastic membrane
pulsator” (EMP), was proposed (Lema et al., 1995).
Multiphase bioreactor design 332