Environmental Engineering Reference
In-Depth Information
days unless cooled, but they cool down rapidly at night unless heated, and cooling and heating
require a significant energy input. The need to reduce the need for freshwater also means that saline
water should be used to make up evaporative losses, and this, in turn, means that the algae should
be able to grow well over an extended range of salinities.
Harvesting and dewatering also presents a major cost to any microalgae production process
(Mohn and Cordero-Contreras 1990; Borowitzka 1999a; Molina Grima et al. 2003) and have been
reviewed by Mohn (1988) and Molina Grima et al. (2004). The solids content of microalgal cultures
in large-scale systems ranges from 0.1 to approximately 1 g/L, meaning that very large volumes of
water have to be processed. Most of the microalgae of interest are also very small (<20 µm dimeter)
and have a density very similar to that of the medium they are growing in. Because the water also still
contains significant quantities of nutrients, the growth medium must be recycled after harvesting
of the algae for economic and environmental reasons. This excludes some types of harvesting. The
cheapest harvesting method is filtration; however, most algae, other than filamentous species such
as
Spirulina
, are too small for effective filtration. The next best method is settling followed by
dewatering, but, where this is not possible, flocculation and flotation (or settling) could be used. The
method of harvesting will depend on the species of alga cultured (Shelef et al. 1984; Mohn 1988)
and may also be affected by the growth phase (Danquah et al. 2009).
Extraction of the lipids from microalgae will probably need to be by solvent extraction (Molina
Grima et al. 1995; Lee et al. 2010).
26.8 oPen Pond culture versus closed PhotoBIoreactors
Closed photobioreactors are often cited as the solution for the production of microalgae for biodiesel
(e.g., Chisti 2007; McCall 2008; Rodolfi et al. 2009), and combined closed photobioreactor/open
pond systems also have been proposed (Huntley and Redalje 2007). However, despite more than
50 years of work on closed photobioreactors, they have as yet not been shown to be commercially
viable for microalgal production, except for the very high-value alga
Haematococcus pluvialis
and
Chlorella
for the health-food market. The various designs of closed photobioreactors recently have
been reviewed by Tredici (2004) and Carvalho et al. (2006). Although closed systems appear to
be a solution for some of the problems encountered in open systems, such as the potential control
of contaminants and greater control of environmental factors, they present other challenges and
problems, and it has been our experience that many species of algae cannot be grown in closed
systems. Closed reactors also require cooling during periods of high irradiance and are more
difficult to scale up (Borowitzka 1996, 1999b). Table 26.4 compares several aspects of open and
closed culture systems.
To date, productivities in large-scale closed systems (calculated on a grams-per-liter basis) are
only approximately 2-3 times those of open systems. However, the capital costs of closed systems
are at least 5-10 times higher and the operating costs, especially energy costs for circulating the
algal culture and for temperature control, are also much higher. Furthermore, open raceway culture
systems have the advantage of being a well-known, proven, and reliable technology. Details of open
pond culture can be found in Borowitzka (2005).
26.9 the Future
The high activity in research and development on the development of new algae and on processes
to produce renewable biofuels from algae will mean that the current economic and technological
challenges will be overcome in time (Stephens et al. 2010b). In many laboratories, there is
ongoing isolation and screening for better strains suited to large-scale cultivation, and improved
lipid productivity will broaden the range of species available. There is also a search for valuable
coproducts that may help to improve the economics of microalgal fuel production (Stephens
et al. 2010a).
