is achieved by adding chemical coagulating substances and energy to the system.
The most common coagulating substances are aluminum and iron salts. Recent
research has shown that pH alterations can increase the flocculation process speed
(Kemmer and McCallion 1995), other investigations have worked on cationic
starch obtaining good results with freshwater species but not with marine species
(Vandamme et al. 2009, 2011).
Nevertheless, not all microalgae species need a flocculant agent, some can form
floccules in a natural and spontaneous way, others form floccules under nitrogen
stress, pH, and dissolved oxygen. This is known as autoflocculation (Gouveia 2011).
Filtration Is a method applied successfully in the laboratory, but on a major scale
some problems that appear are membrane clogging, formation of compressible
filter cakes, and in particular high maintenance costs. On a large scale, the energy
consumption of the filtering process is equivalent to the centrifugation process so
this is only recommended for filamentous microalgae or colony forming microalgae
(Molina-Grima et al. 2003; Gouveia 2011).
Two of the primary screening devices in microalgae harvesting are microstrainer and vibrating screen filters, excellent due to their mechanical simplicity
and availability in large unit sizes. Microstrainers are rotative filters with a very
thin net submitted to frequent washings in order to avoid clogging of the filter.
Microstrainers are simple, economic, easy-to-handle devices. However, for smallsized microalgae flocculation is recommended before microstraining (MolinaGrima et al. 2003; Gouveia 2011). Some variables involved in the process like type
of filtering membrane can increase costs but can optimize the process.
Flotation The process of flotation is a gravity separation in which air or gas
bubbles are attached to solid particles and then carry them to the liquid surface. It
is a commonly used approach to remove microalgae from reservoir water prior to
its use as drinking water. The water is first ozonated, after which the sensitized
cells are treated with about 10 ppm polyelectrolyte salts, prior to being subjected
to flotation. Flotation is more beneficial and effective than sedimentation with
regard to harvesting microalgae (Chen et al. 1998).
Electrolytic separation An electric field drives charged microalgae to move
out of the solution. Water electrolysis generates hydrogen that adheres to the
microalgal floccules and carries them to the surface. The mechanism involves
consecutive stages: Generation of coagulants by electrolytic oxidation of the
electrode, destabilization of particulate suspension, and breaking of emulsion and
aggregation of the destabilized to form floccules (Gouveia 2011).
Microwave-synthesized magnetic microparticles Prochazkova et al. (2013)
report some interesting results related to a microwave system where a low-cost
oxidized iron suspension is used to harvest C. vulgaris. The separation efficiency
was tested against several variables as culture media, pH, among others. This study
shows that iron oxide amount, pH level, and ions present in the culture medium are
important elements for good harvesting results. Phosphorous ions were identified
as the element that interferes with iron oxide during the separation of magnetized
cells in the system.
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M. Vanthoor-Koopmans et al.
The most common coagulating substances are aluminum and iron salts. Recent
research has shown that pH alterations can increase the flocculation process speed
(Kemmer and McCallion 1995), other investigations have worked on cationic
starch obtaining good results with freshwater species but not with marine species
(Vandamme et al. 2009, 2011).
Nevertheless, not all microalgae species need a flocculant agent, some can form
floccules in a natural and spontaneous way, others form floccules under nitrogen
stress, pH, and dissolved oxygen. This is known as autoflocculation (Gouveia 2011).
Filtration Is a method applied successfully in the laboratory, but on a major scale
some problems that appear are membrane clogging, formation of compressible
filter cakes, and in particular high maintenance costs. On a large scale, the energy
consumption of the filtering process is equivalent to the centrifugation process so
this is only recommended for filamentous microalgae or colony forming microalgae
(Molina-Grima et al. 2003; Gouveia 2011).
Two of the primary screening devices in microalgae harvesting are microstrainer and vibrating screen filters, excellent due to their mechanical simplicity
and availability in large unit sizes. Microstrainers are rotative filters with a very
thin net submitted to frequent washings in order to avoid clogging of the filter.
Microstrainers are simple, economic, easy-to-handle devices. However, for smallsized microalgae flocculation is recommended before microstraining (MolinaGrima et al. 2003; Gouveia 2011). Some variables involved in the process like type
of filtering membrane can increase costs but can optimize the process.
Flotation The process of flotation is a gravity separation in which air or gas
bubbles are attached to solid particles and then carry them to the liquid surface. It
is a commonly used approach to remove microalgae from reservoir water prior to
its use as drinking water. The water is first ozonated, after which the sensitized
cells are treated with about 10 ppm polyelectrolyte salts, prior to being subjected
to flotation. Flotation is more beneficial and effective than sedimentation with
regard to harvesting microalgae (Chen et al. 1998).
Electrolytic separation An electric field drives charged microalgae to move
out of the solution. Water electrolysis generates hydrogen that adheres to the
microalgal floccules and carries them to the surface. The mechanism involves
consecutive stages: Generation of coagulants by electrolytic oxidation of the
electrode, destabilization of particulate suspension, and breaking of emulsion and
aggregation of the destabilized to form floccules (Gouveia 2011).
Microwave-synthesized magnetic microparticles Prochazkova et al. (2013)
report some interesting results related to a microwave system where a low-cost
oxidized iron suspension is used to harvest C. vulgaris. The separation efficiency
was tested against several variables as culture media, pH, among others. This study
shows that iron oxide amount, pH level, and ions present in the culture medium are
important elements for good harvesting results. Phosphorous ions were identified
as the element that interferes with iron oxide during the separation of magnetized
cells in the system.
270
M. Vanthoor-Koopmans et al.