Environmental Engineering Reference
In-Depth Information
For environmental sustainability, the algae should be grown in saline medium to minimize the
need for freshwater.
The lowest cost culture system for microalgae production at this time is the raceway. The best
annual average productivity for long-term (>6 months) continuous culture in a raceway reported
in the literature is approximately 20 g ash-free dry weight/m
2
per day for the coccolithophorid
alga
Pleurochrysuis caterae
in Perth, Western Australia (Moheimani and Borowitzka 2006). In
current commercial-scale operations, this is not achieved. In summer, very high productivities of
41-47 g dry weight/m
2
per day have been achieved. The lipid content of this alga is approximately
30-35% of dry weight in log-phase growing cultures. However, it should be noted that the growth
conditions or the lipid productivity for this alga have not, as yet, been optimized. To achieve
such productivity, the density of the algal culture needs to be controlled to optimize the available
light reaching the cells, and for most species CO
2
needs to be added (Borowitzka 1998). The
system should also be operated as a continuous culture. This means the algal strain used and the
operating regime must be able to exclude, or at least control, the growth of contaminating species
and predators such as protozoa. This is possible for quite a number of species such as
D. salina
(because of the very high salinity),
Spirulina
spp. (because of the high alkalinity),
P. tricornutum
,
and
Chlorella
spp. (because of high growth rate),
Pleurochrysis caterae
(possibly because of the
production of acrylic acid by the alga) and a range of other fast-growing marine algae such as
Nannochloropsis
,
Tetraselmis
,
Nannochloris
,
Pheodactylum
,
and
Chlorococcum
(Ben Amotz,
personal communication).
For the production of algal lipids to produce biodiesel, it is not the biomass productivity but the
lipid productivity that is important (Griffiths and Harrison 2009). Most algae achieve their maximal
lipid (oil) content in the stationary phase of growth; however, continuous culture for maximal
productivity means that the cells should always be in the log phase of growth. This requires algal
strains that have a high lipid content while actively growing so as to have a high lipid productivity.
Lipid productivity is a function of the algal productivity and the lipid content. The effect of
productivity and cell lipid content on lipid productivity is shown in Figure 26.1. At the potentially
achievable scenario of a productivity of 30 g dry weight/m
2
per day with a lipid content of 40% of
the biomass, 12 kg oil/ha per day are produced.
The algae also need to have a wide temperature tolerance and grow well outdoors. In the high
solar irradiation environments, where high average annual productivities are possible, temperatures
in open ponds can reach up to approximately 35°C in summer, and they may cool down to less than
5°C at night in winter. Similarly, closed photobioreactors heat up rapidly during the day on sunny
22
20
18
16
14
12
10
8
6
4
50
45
40
35
30
25
24
26
28 30 32
Productivity (g dry wt.m
-2
.d
-1
)
34
36
38
40
42
FIGure 26.1
Effect of productivity and cell lipid content (numbers at end of lines are percent lipid content)
on lipid productivity. The fine line shows the potentially achievable target of an annual average productivity
of 30 g dry weight/m
2
per day at a lipid content of 40%.
