Environmental Engineering Reference
In-Depth Information
9.2.2 Bubble Column
Photobioreactor
A bubble column PBR consists of a vertically
arranged cylindrical column made of transpar-
ent material (Eriksen
2008
). This PBR has the
highest volumetric mass transfer rates, proper
mixing, and controllable growth conditions
(Eriksen
2008
). The cost of this bioreactor is
very low, and it is compact and easy to operate.
The introduction of gas takes place at the bot-
tom of the column and causes a turbulent
stream to enable optimum gas exchange. At
present, these types of reactors are built with a
maximum diameter of 20-30 cm in order to
ensure the required supply of sunlight. This
type of PBR allows a reduction of harmful
shear forces.
10
Wastewater as a Source
of Nutrients for the Growth
of Microalgae
Wastewater is an excellent, cheap, and readily
available medium for the growth of various
microalgal strains (Schenk et al.
2008
).
Wastewater also contains macronutrients that
help in the growth of microalgae (Raja et al.
2004
; Hosikian et al.
2010
; Rawat et al.
2011
).
The macronutrients present in wastewater are
ammonia, nitrate, phosphate, urea, trace elements
such as vitamins (biotin and thiamine), certain
trace metals, and, rarely, radioisotopes. The
growth of several microalgae is found to be suit-
able in wastewater because of the pH and dis-
solved CO
2
concentration. Large-scale production
of microalgae for biofuels using wastewater has
the possibility to improve the economics of bio-
mass production. Growth of microalgae using
wastewater also has some disadvantages, such as
bacterial and viral contamination that may nega-
tively affect the biomass and production process.
Use of wastewater for the growth of microalgae
might lead to an adequate cleaning of the culture
system (Lam and Lee
2012
) and also help to
reduce the eutrophication in the aquatic environ-
ment. Microalgae can also be used for the
removal of rich nutrients available in wastewater
(Raja et al.
2008
) (e.g.
C. vulgaris
is used for the
removal of nitrogen and phosphorous from
wastewater, with an average removal effi ciency
of nitrogen [72 %] and phosphorus [28 %];
3-8 mg/LNH
4+
and 1.5-3.5 mg/LPO
4
−3
) (Aslan
and Kapdan
2006
). Other microalgae cultures
used for the removal of nutrients are
Chlorella
,
Scenedesmus
,
Spirulina
spp.,
Nannochloris
,
B.
brauinii
, and cyanobacterium
Phormidium boh-
neri
(Lee and Lee
2001
; Olguín et al.
2003
; An
et al.
2003
; Jimenez-Pérez et al.
2004
). Some
companies actively involved in culturing algae
using wastewater for biofuel production include
Algae wheel Technologies (USA), Algal
Scientifi c Corporation (USA), Aquafl ow Binomic
Corporation (New Zealand), Blue Marble Energy
(USA), Community Fuels (USA), Nanoforce
9.3
Two-Step Cultivation System
The two-step cultivation system involves a com-
bination of raceway and PBR designs. This sys-
tem has been traditionally used to develop
inoculum for aquaculture operations. The main
advantage of this system is the production of
inoculum that is free from contamination
(Schenk et al.
2008
). The fi rst step is the fast cul-
tivation of biomass in the PBR; the second step
is stress cultivation in open ponds. The fi rst step
allows for better protection of the growing bio-
mass in the PBR during the early stages, and
CO
2
capture is maximized. The microalgae sus-
pension is transferred in open ponds that have
enough nutrients, are low in nitrogen, and main-
tain high CO
2
. The second step in open raceways
has few problems because a high algal density is
more resistant to contamination, and this phase
is nutrient depleted, avoiding the growth of con-
taminating species. The combination of PBR and
an open pond has proven effi ciency for astaxan-
thin production (Huntley and Redalje
2006
). It is
currently being used by companies that are
developing biofuel applications. Hybrid systems
can produce as much as 20-30 tonne ha
−1
of lip-
ids annually, depending on suitable climatic con-
ditions (Rodolfi et al.
2009
).
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