Environmental Engineering Reference
In-Depth Information
taBle 26.4
comparison of the Properties of open Pond raceway culture systems and closed
Photobioreactors
open Ponds
closed Photobioreactors
Species range
Most species, but control of contamination
by other algae and/or predators is a
potential problem, but this can be
controlled for a number of species
(e.g., in all of the ones listed below)
Can only grow species that are shear-tolerant
because the need for circulation (air-lifts,
pumps) damages many species. “Sticking” of
some algae to the walls of the reactor is also a
problem
Species that have been
shown that they can
be cultured reliably
for extended (>3
months) periods
Chlorella
,
Spirulina
,
Scenedesmus
,
Dunaliella
,
Phaeodactylum
,
Pleurochrysis
,
Monodus
,
Nitzschia
,
Nannochloropsis
Spirulina
,
Chlorella
,
Tetraselmis
,
Isochrysis
,
Phaeodactylum
,
Nannochloropsis
key Factors limiting
productivity
Light
Some capacity to optimize light
environment for cells by controlling pond
depth and/or cell density
Can control light environment by controlling
diameter of tubing or width of plate reactor as
well as cell density. Sticking of the algae to the
bioreactor surface also limits light availability
and growth in many species. For maximal
productivities, the light path (reactor thickness)
should not exceed ~400 mm)
Temperature
Limited capacity to control temperature in
large ponds; however, maximum pond
temperature does not exceed ~30-35°C
because of evaporation, so no cooling is
required.
In high light the system heats up rapidly and
there is a need to cool system. Requires energy
(and large amounts of freshwater if evaporative
cooling system is used)
CO
2
Can be added
Can be added
O
2
(high O
2
inhibits
photosynthesis)
O
2
built up during the day by
photosynthesis is lost from system by
exchange on pond surface
Requires efficient degassing system. In tubular
photobioreactors O
2
limits length of tubes that
can be used.
cost factors
Capital cost
Relatively high because of the need for
pond liners
Very high
Power requirement
Mixing using paddlewheels is very
energy-efficient.
Power requirements for circulating the culture
are high. Power may also be required for
cooling during the day.
See also:
Grobbelaar, J.U.,
J Appl Phycol
., 21, 489-492, 2009.
Genetic engineering also presents a new, and as yet little explored option for overcoming some
of the limitations of growth and lipid production. One potential strategy that has been proposed is
to engineer diatoms, and possibly other algae, to secrete some of their lipids (Ramachandra et al.
2009). For example, in corals the symbiotic dinoflagellates have been shown to apparently secrete
lipid-like osmiophilic material (Crossland et al. 1980). The green alga
B. braunii
also accumulates
hydrocarbons extracellularly.
Another possibility is to enhance the light utilization efficiency of the algae. For example,
Melis et al. (1999) found that mutants of
D. salina
with small chlorophyll antenna size showed
higher photosynthetic productivities and photon use efficiencies (see also Mitra and Melis 2008). A
mutant of
Chlamydomonas reinhardtii
(
tla1
) with truncated light harvesting antenna size also had
enhanced photosynthetic productivity in laboratory and outdoors in a greenhouse (Nakajima and
