Novel Bioreactors for Culturing Marine Organisms 12.4 Membrane Bioreactors (MBR) 349
Part B | 12.4
microplantlet cultures, however since their large tissue
size makes it difficult for them to pass through the tubing without settling. Continuous bubbling aeration of
the culture with CO 2 in air simultaneously serves four
purposes, viz. (1) transfer of CO 2 to the culture; (2)
maintenance of the dissolved inorganic carbon level in
the culture medium; (3) pH control; and (4) removal of
dissolved O 2 produced by photosynthesis.
Although macroalgal suspension cultures grow
rather slowly (with specific growth rates less than 0:20
day
1 ) and k L a values in aerated PBRs are high, it
is challenging to supply CO 2 at flow rates that can
avoid the CO 2 limited growth rate condition at high cell
density unless CO 2 is added to the aeration gas. This requires CO 2 to be present in the aeration gas at 10 times
its normal ambient concentration. If the culture suspension is continuously supplied with light and CO 2 , then
the cumulative biomass production will be limited ultimately by available macronutrients dissolved in the
liquid medium, mainly N in the form of nitrate, and P in
the form of phosphate. Macronutrient delivery depends
on the mode of cultivation. In batch mode, macronutrients are initially provided in the growth medium at
culture inoculation. Nitrate and phosphate concentrations in the medium decrease with time until one (or
both) becomes zero. At this point, the limiting nutrient is exhausted, and cell growth stops even if light and
CO 2 are still being continuously supplied. In perfusion
cultures, liquid medium containing dissolved macronutrients is continuously supplied to the bioreactor culture
suspension. The spent medium leaves the bioreactor but
the biomass is retained inside. Under these conditions,
cumulative biomass production continues, but its rate is
ultimately limited by the rate of CO 2 delivery, or the
attenuation of light through the dense culture suspension. The final limit to biomass production is space.
If the tissues completely fill up the bioreactor control
volume, then biomass production will stop even in the
presence of an infinite supply of nutrients (light, CO 2 ,
and macronutrients).
12.4 Membrane Bioreactors (MBR)
An MBR is basically a membrane filtration unit (membranes ranging between microfiltration, and ultrafiltration) coupled or integrated with a suspended phase
bioreactor (Table 12.5). Depending on whether the
membrane module is located inside or outside the
bioreactor, MBRs are categorized as internal (i. e., submerged-type) or external (i. e., cross-flow type). In
a cross-flow MBR medium is recirculated between the
membrane module and the BR by pumping it through
the membranes. In a submerged MBR, the membrane
filtration unit is immersed inside the culture medium in
the BR (or sometimes in a separate tank connected to
the BR). Some common membrane configurations employed in MBRs are:
i) Hollow fiber
ii) Spirally wound
iii) Plate-and-frame (i. e., flat sheet)
iv) Tubular.
Insofar as wastewater treatment applications are
concerned, MBRs provide a host of advantages compared to traditional activated sludge processes that include the total separation of hydraulic retention time
(HRT) and solids retention time (SRT), high efficiency
of pollutant removal and no requirement of a secondary
clarification unit.
Zamalloa et al. [12.39] employed an anaerobic
membrane bioreactor (AnMBR) for biomethanation of
the marine microalgae P. tricornutum. In fact, this report was claimed as the pioneering study to demonstrate
the feasibility of an AnMBR for digestion of algal
biomass. In conventional, continuously stirred anaerobic digesters, where the SRT is identical to the HRT,
a high SRT required for effective destruction of volatile
solids necessitates a large, often prohibitively high reactor volume. Now, in an AnMBR, the solids are
separated from the sludge suspensions by a membrane
so that biomass wasting rates are low – therefore, an
AnMBR routinely allows operation at SRTs as high
as 50 d or even higher. This facilitates the growth of
slow-growing microorganisms, e.g., methanogens, and
also increases the fraction of fermented organic matter. The AnMBR employed in this study consists of
(a) a reactor (working volume 8 L) made of acrylic
panels and (b) a microfiltration membrane module arranged in parallel. Two peristaltic pumps were used
to feed influent into the anaerobic reactor and separately withdraw permeate from there. For mixing and
membrane scouring to control cake formation, biogas recirculation was carried out using a diaphragm
gas pump through a diffuser located just below the
membrane module. Biogas production was measured
Part B | 12.4
microplantlet cultures, however since their large tissue
size makes it difficult for them to pass through the tubing without settling. Continuous bubbling aeration of
the culture with CO 2 in air simultaneously serves four
purposes, viz. (1) transfer of CO 2 to the culture; (2)
maintenance of the dissolved inorganic carbon level in
the culture medium; (3) pH control; and (4) removal of
dissolved O 2 produced by photosynthesis.
Although macroalgal suspension cultures grow
rather slowly (with specific growth rates less than 0:20
day
1 ) and k L a values in aerated PBRs are high, it
is challenging to supply CO 2 at flow rates that can
avoid the CO 2 limited growth rate condition at high cell
density unless CO 2 is added to the aeration gas. This requires CO 2 to be present in the aeration gas at 10 times
its normal ambient concentration. If the culture suspension is continuously supplied with light and CO 2 , then
the cumulative biomass production will be limited ultimately by available macronutrients dissolved in the
liquid medium, mainly N in the form of nitrate, and P in
the form of phosphate. Macronutrient delivery depends
on the mode of cultivation. In batch mode, macronutrients are initially provided in the growth medium at
culture inoculation. Nitrate and phosphate concentrations in the medium decrease with time until one (or
both) becomes zero. At this point, the limiting nutrient is exhausted, and cell growth stops even if light and
CO 2 are still being continuously supplied. In perfusion
cultures, liquid medium containing dissolved macronutrients is continuously supplied to the bioreactor culture
suspension. The spent medium leaves the bioreactor but
the biomass is retained inside. Under these conditions,
cumulative biomass production continues, but its rate is
ultimately limited by the rate of CO 2 delivery, or the
attenuation of light through the dense culture suspension. The final limit to biomass production is space.
If the tissues completely fill up the bioreactor control
volume, then biomass production will stop even in the
presence of an infinite supply of nutrients (light, CO 2 ,
and macronutrients).
12.4 Membrane Bioreactors (MBR)
An MBR is basically a membrane filtration unit (membranes ranging between microfiltration, and ultrafiltration) coupled or integrated with a suspended phase
bioreactor (Table 12.5). Depending on whether the
membrane module is located inside or outside the
bioreactor, MBRs are categorized as internal (i. e., submerged-type) or external (i. e., cross-flow type). In
a cross-flow MBR medium is recirculated between the
membrane module and the BR by pumping it through
the membranes. In a submerged MBR, the membrane
filtration unit is immersed inside the culture medium in
the BR (or sometimes in a separate tank connected to
the BR). Some common membrane configurations employed in MBRs are:
i) Hollow fiber
ii) Spirally wound
iii) Plate-and-frame (i. e., flat sheet)
iv) Tubular.
Insofar as wastewater treatment applications are
concerned, MBRs provide a host of advantages compared to traditional activated sludge processes that include the total separation of hydraulic retention time
(HRT) and solids retention time (SRT), high efficiency
of pollutant removal and no requirement of a secondary
clarification unit.
Zamalloa et al. [12.39] employed an anaerobic
membrane bioreactor (AnMBR) for biomethanation of
the marine microalgae P. tricornutum. In fact, this report was claimed as the pioneering study to demonstrate
the feasibility of an AnMBR for digestion of algal
biomass. In conventional, continuously stirred anaerobic digesters, where the SRT is identical to the HRT,
a high SRT required for effective destruction of volatile
solids necessitates a large, often prohibitively high reactor volume. Now, in an AnMBR, the solids are
separated from the sludge suspensions by a membrane
so that biomass wasting rates are low – therefore, an
AnMBR routinely allows operation at SRTs as high
as 50 d or even higher. This facilitates the growth of
slow-growing microorganisms, e.g., methanogens, and
also increases the fraction of fermented organic matter. The AnMBR employed in this study consists of
(a) a reactor (working volume 8 L) made of acrylic
panels and (b) a microfiltration membrane module arranged in parallel. Two peristaltic pumps were used
to feed influent into the anaerobic reactor and separately withdraw permeate from there. For mixing and
membrane scouring to control cake formation, biogas recirculation was carried out using a diaphragm
gas pump through a diffuser located just below the
membrane module. Biogas production was measured