Nevertheless, photoreactors require much more energy for mixing typical values
of 55 W m
-3 for flat plates and up to 2,000–3,000 W m
-3 for horizontal tubular
reactors have been reported (Carvalho et al. 2006; Sierra et al. 2008). Raceway
ponds are made of less expensive materials, their construction involves lower costs,
and they require less energy for mixing, of the order of only 3–4 W m
-3 .
8.4.3 Semi-Closed Systems
These systems are similar to open system but are covered by a physical structure to
protect culture from environmental changes. For example, in China most production systems have adopted a semi-closed culture system, where raceway culture
ponds are covered by glass houses or transparent plastic sheets, thereby protecting
culture from frequent summer rains, storms, and/or cold winters. Although
investment costs increase, this system allows for year-round production.
8.5 Microalgae Harvest
Microalgae cultures have enormous water quantities, it is necessary to remove this
water. There are chemical, physical, and biological processes available to do this
(Fig. 8.6). Mainly, these processes are reduced to separate the solid and liquid
components. According to Molina-Grima et al. (2003), the harvesting process is
energy-dependent and represents the main percentage of total production cost
(20–30 %) being still considered as a major limiting factor in this activity. The
harvest is not always easy, mostly due to the cell size of most microalgae, which
makes it necessary to use large forces for separation. However, it is possible to
develop an economic and efficient harvest method according to microalgae specie.
The most common harvesting methods are gravity sedimentation, centrifugation, filtration and microscreening, ultrafiltration, flotation, sometimes with an
additional flocculation step or with a combination of flocculation–flotation, and
electrolytic separation techniques (Uduman et al. 2010; Gouveia 2011). The
selection of harvesting method is dependent on the properties of microalgae, such
as density, size, and value of the desired products (Brennan and Owende 2010).
A clear example of lower cost harvesting are the cyanobacteria of Arthrospira
and Spirulina genera that thrive forming floccules that build thick floating layers;
this situation makes the harvest easy using filtration methods. This is a rapid and
energy efficient way of getting biomass (Benneman and Oswald 1996). A different
situation is related to the chlorophyta H. pluvialis that presents the tendency to
sinking through water column when the agitation process is interrupted; this makes
harvesting easy by collecting the cells from the bottom.
The process of harvest can be divided into two steps (Brennan and Owende
2010): (1) Bulk harvesting, separate microalgal biomass from the bulk suspension.
By this method, the total solid matter can reach 2–7 % using flocculation, flotation,
268
M. Vanthoor-Koopmans et al.
of 55 W m
-3 for flat plates and up to 2,000–3,000 W m
-3 for horizontal tubular
reactors have been reported (Carvalho et al. 2006; Sierra et al. 2008). Raceway
ponds are made of less expensive materials, their construction involves lower costs,
and they require less energy for mixing, of the order of only 3–4 W m
-3 .
8.4.3 Semi-Closed Systems
These systems are similar to open system but are covered by a physical structure to
protect culture from environmental changes. For example, in China most production systems have adopted a semi-closed culture system, where raceway culture
ponds are covered by glass houses or transparent plastic sheets, thereby protecting
culture from frequent summer rains, storms, and/or cold winters. Although
investment costs increase, this system allows for year-round production.
8.5 Microalgae Harvest
Microalgae cultures have enormous water quantities, it is necessary to remove this
water. There are chemical, physical, and biological processes available to do this
(Fig. 8.6). Mainly, these processes are reduced to separate the solid and liquid
components. According to Molina-Grima et al. (2003), the harvesting process is
energy-dependent and represents the main percentage of total production cost
(20–30 %) being still considered as a major limiting factor in this activity. The
harvest is not always easy, mostly due to the cell size of most microalgae, which
makes it necessary to use large forces for separation. However, it is possible to
develop an economic and efficient harvest method according to microalgae specie.
The most common harvesting methods are gravity sedimentation, centrifugation, filtration and microscreening, ultrafiltration, flotation, sometimes with an
additional flocculation step or with a combination of flocculation–flotation, and
electrolytic separation techniques (Uduman et al. 2010; Gouveia 2011). The
selection of harvesting method is dependent on the properties of microalgae, such
as density, size, and value of the desired products (Brennan and Owende 2010).
A clear example of lower cost harvesting are the cyanobacteria of Arthrospira
and Spirulina genera that thrive forming floccules that build thick floating layers;
this situation makes the harvest easy using filtration methods. This is a rapid and
energy efficient way of getting biomass (Benneman and Oswald 1996). A different
situation is related to the chlorophyta H. pluvialis that presents the tendency to
sinking through water column when the agitation process is interrupted; this makes
harvesting easy by collecting the cells from the bottom.
The process of harvest can be divided into two steps (Brennan and Owende
2010): (1) Bulk harvesting, separate microalgal biomass from the bulk suspension.
By this method, the total solid matter can reach 2–7 % using flocculation, flotation,
268
M. Vanthoor-Koopmans et al.