Microalgae as an Attractive Source for Biofuel Production - Environmental Sustainability: Role of Green Technologies - page 139

Owende 2010 ; Rawat et al. 2013 ) and therefore
have the potential for a large net energy produc-
tion (Rodolfi et al. 2009 ). D. salina is one of the
most commonly cultivated strains in open pond
systems, and, during 2008, the unit price was
about

(Huntley and Redalje 2008 ). In this culture
process, the fi rst step of microalgae production
is controlled by temperature (e.g. sea water bath
[16-18 °C]) PBR. Microalgae are transferred
into an open pond for 5 days in a second culture
step (Huntley and Redalje 2006 , 2008 ). Closed
PBRs have the advantage of high productivity,
low contamination risk, effi cient CO 2 capture,
continuous runs, and controlled growth condi-
tions. The major drawbacks are the higher capi-
tal investment and operating costs.

2.55/kg of dry biomass, which was con-
sidered too high to justify production for biofuels
(Tan 2008 ; Brennan and Owende 2010 ; Rawat
et al. 2013 ).

ᾬ

9.2 Closed System

9.2.1 Tubular Photobioreactor
The tubular PBR is one of the most suitable
bioreactors for large-scale outdoor cultivations
since they expose a large surface to sunlight. A
tubular reactor consists of vertical, horizontal,
inclined, or helix-shaped tubes connected with
a pipe system (Molina et al. 2001 ; Ugwu et al.
2002 ; Brennan and Owende 2010 ). The algae-
suspended fl uid is able to circulate in this tub-
ing. The tubes are generally of transparent
plastics or borosilicate glass and the circulation
is kept constant by a pump at the end of the sys-
tem. The diameter of the tube is 0.1 m or less,
and helpful for the high penetration of light
into the dense culture (Chisti 2007 ). Either a
mechanical pump or an airlift system is used to
recirculate the algal cultures after allowing CO 2
and O 2 to be exchanged between the liquid
medium and aeration gas as well as providing a
mechanism for mixing (Eriksen 2008 ). To
encourage gas exchange in the tubes, agitation
and proper mixing is very important. The intro-
duction of gas takes place at the end or begin-
ning of the tube system. This method of
introducing gas causes defi ciency in CO 2 , a
high concentration of oxygen at the end of the
unit during circulation, and poor effi ciency.
Therefore, cultures are generally reticulated by
pump, passing through a degasser at regular
intervals in order to remove excess oxygen. The
largest closed PBRs are tubular (e.g. the 25 m 3
plants at Mera Pharmaceuticals, Hawaii
[Olaizola 2000 ] and the 700 m 3 plant in Klötze,
Germany [Pulz 2001 ]). Tubular reactors are
currently being used for the culture of high-
value products such as astaxanthin (Fig 2a).

The closed system is designed to overcome all
the problems associated with open ponds (con-
taminations and expected biomass). PBRs are
continuous culture systems that can achieve
expected biomasses of up to 6.7 g/L (Chisti
2007 ; Ranjbar et al. 2008 ; Bai et al. 2011 ). This
technology is used to grow microalgae for the
production of high-value compounds such as
pharmaceuticals, neutraceuticals, and cosmetics
that cannot be grown as a monoculture in open
pond systems. Using PBRs, single species of
microalgae can be grown for long durations
with minimal contamination risk. PBRs have
lower direct exchanges of gases and contamina-
tions (e.g. microorganisms, dust) than open
pond systems. Different PBR models (indoor
and outdoor) have been developed, including
tubular, fl at plate, airlift, bubble column, and
stirred tank (Xu et al . 2009 ). The principle
behind the PBR is reduction of the light path,
thereby increasing the amount of light received
by each cell (Borowitzka 1999 ). The culture is
generally mixed by airlift or mechanical stir-
ring/pumping. Mixing of culture is important
for gaseous exchange within the system
(Brennan and Owende 2010 ; Rawat et al. 2013 ).
High biomass yield, which depend on good con-
trol of culture parameters such as temperature,
pH, and CO 2 concentration, etc., can be achieved
using closed PBRs (Suh and Lee 2003 ), but the
capital costs are ten times higher than those of
open ponds (Carvalho et al . 2006 ). However, the
combination of both can be profi table because
microalgae can be grown in open ponds while
reducing contamination by undesired species