From an operational standpoint, the main difference between
open and closed systems is the costs for building and operating the
systems. Closed systems consist of expensive parts and control units
and typically require much more energy for operation than open
systems. Closed systems typically reach higher areal productivity
than open systems and this can offset some of the extra costs.
4 Harvesting
Even in dense algal cultures >90% of the weight is water and
harvesting the microalgae is a challenge for the economic feasibility
of large scale algal cultivation. There are ways of course to effectively harvest the biomass quickly such as centrifugation, but these
consume a lot of energy and are thus expensive. Current harvesting
methods include chemical, mechanical, electrical, and biological
harvesting methods [6] (Table 3).
Most of these methods aim at flocculation and sedimentation
of the microalgae. Flocculation is the process of small particles
binding together forming larger particles, which then sink quicker.
The sinking material can then be collected and served as a first step
in the harvesting. Flocculation can be induced by addition of a
chemical (flocculating agent) or may also be self-induced, e.g., by
stopping the CO 2 supply and let the pH increase to levels inducing
flocculation [16].
Harvesting may also be less challenging depending on the
cultivation system and the application of the biomass. Algal turf
scrubbers are for example used to grow a biofilm which can be easily
scraped of mechanically [17] or harvested by vacuuming [18]. If
the biomass is used for biogas production, the optimal water content is typically much higher than for production of lipids for
biodiesel.
There are also technologies for circumventing the traditional
harvesting by getting the algae to release the desired product, for
example lipids, into the water [19]. The lipids will separate from the
water and can easily be collected. There would thus be no need for
collecting the actual biomass, which could be left producing more
of the desired product.
6
Jacqueline Jerney and Kristian Spilling
open and closed systems is the costs for building and operating the
systems. Closed systems consist of expensive parts and control units
and typically require much more energy for operation than open
systems. Closed systems typically reach higher areal productivity
than open systems and this can offset some of the extra costs.
4 Harvesting
Even in dense algal cultures >90% of the weight is water and
harvesting the microalgae is a challenge for the economic feasibility
of large scale algal cultivation. There are ways of course to effectively harvest the biomass quickly such as centrifugation, but these
consume a lot of energy and are thus expensive. Current harvesting
methods include chemical, mechanical, electrical, and biological
harvesting methods [6] (Table 3).
Most of these methods aim at flocculation and sedimentation
of the microalgae. Flocculation is the process of small particles
binding together forming larger particles, which then sink quicker.
The sinking material can then be collected and served as a first step
in the harvesting. Flocculation can be induced by addition of a
chemical (flocculating agent) or may also be self-induced, e.g., by
stopping the CO 2 supply and let the pH increase to levels inducing
flocculation [16].
Harvesting may also be less challenging depending on the
cultivation system and the application of the biomass. Algal turf
scrubbers are for example used to grow a biofilm which can be easily
scraped of mechanically [17] or harvested by vacuuming [18]. If
the biomass is used for biogas production, the optimal water content is typically much higher than for production of lipids for
biodiesel.
There are also technologies for circumventing the traditional
harvesting by getting the algae to release the desired product, for
example lipids, into the water [19]. The lipids will separate from the
water and can easily be collected. There would thus be no need for
collecting the actual biomass, which could be left producing more
of the desired product.
6
Jacqueline Jerney and Kristian Spilling