70 Marine Macro- and Microalgae: An Overview
and feasible one. The harvested algae biomass is dried so that the product can be stored without spoilage
when algae are used as aquaculture feed. The drying process accounts for 20% of total algal production
costs. Spray drying, freeze-drying, drum drying, and sun drying are used to dry microalgae. Spray drying
is used for high-value products, but it may lead to degradation of pigments and vitamins in algae (although
these may be protected by the addition of antioxidants before drying).
Flotation is a process in which a gas or air is bubbled through the liquid to be clarified. The particles
are adsorbed by the rising bubbles and are removed when they reach the surface of the liquid. The bubbles
can attract smaller particles easily. This process can be used for the separation of algae with particle
diameter of less than 500 μm (Uduman et al. 2010). Air flotation techniques may be either dissolved or
induced. This process is among several new harvesting methods proven to be efficient (Chen et al. 1998).
The efficiency of dewatering microalgae by sedimentation depends on the time of removal of algae. It
is influenced by the intercellular interactions of algae in the different growth phases during cultivation.
The optimum harvesting time is determined by the zeta potential of the microalgal culture. It was found
that algae cultures harvested during the stationary phase had a higher rate of settling than those harvested
during the exponential phase. Algae cells in the exponential phase are highly stable and electrostatically
repel each other. It was found that algal cultures stored in darkness settled faster compared to daylight
conditions (Danquah et al. 2009).
Flocculation
Algae have high negative surface charges during the logarithmic phase of their growth. They remain
widely dispersed in water because of these repulsive charges. Algae settle and forms clusters after
longer residence times when they approach decay phase. This process is called, auto-flocculation. The
other factors responsible for auto-flocculation include nutrient (nitrogen and phosphorus) limitation,
restricted CO 2 supply, and co-precipitation of magnesium, calcium and carbonate salts (Becker 2004).
Auto-flocculation is also found to take place by changing the conditions of algae cultivation. Algal flocs
are formed when pond agitation stops and the CO 2 supply is cut off, causing an increase in pH. On
flocculation with various pH values, Scenedesmus sp. showed no flocculation for pH values between 5.0
and 7.5, while at pH values above 8.5, almost 95% of the algal biomass was found to be removed (Becker
2004).
The algae cells form precipitates with the addition of chemicals. Multivalent metal salts like ferric
chloride [FeCl 3 ], aluminum sulfate [Al 2 (SO 4 ) 3 , alum] and ferric sulfate [Fe 2 (SO 4 ) 3 ] are used to flocculate
algae (Grima et al. 2003). Alum is an efficient flocculant for Scenedesmus and Chlorella to produce fuel.
The metal salts are not intended for flocculation when considering the algae for use in some aquaculture
applications (Golueke and Oswald 1965). Algae can also be flocculated by using cationic polymers or
polyelectrolytes (Tenney et al. 1969). The negative charge on algae is attracted to the positive charge of
cationic flocculants. Algae particles are bound together by polymer flocculants through a process called
bridging. This process fails to occur at high ionic strengths. Dosage between 1 and 10 mg/L of the
polymer is required to flocculate fresh water algae (Bilanovic et al. 1988). Flocculation by polymeric
flocculants was mainly found to depend on the molecular mass, ionic strength and dose of the polymer
and also the pH and concentration of the algae culture (Chisti 1999). Chitosan has several advantages
over other conventional flocculants: It does not produce any toxic effects; it is required in very low
concentrations (Becker 2004). Algae can also be immobilized in a chitosan matrix and used in tertiary
treatment of wastewater (Kaya and Picard 1996).
Economic importance of algal biomass
Health supplements
Microalgae can increase the larval production though the exact mechanism of action is unclear till date.
Microalgae offer food for zooplankton which helps to advance the quality of the culture. For numerous
freshwater and seawater animals, supplying phytoplankton to the ponds leads to much better results
on survival, growth and transformation index (Muller-Feuga 2000). Due to some excreted biochemical
and feasible one. The harvested algae biomass is dried so that the product can be stored without spoilage
when algae are used as aquaculture feed. The drying process accounts for 20% of total algal production
costs. Spray drying, freeze-drying, drum drying, and sun drying are used to dry microalgae. Spray drying
is used for high-value products, but it may lead to degradation of pigments and vitamins in algae (although
these may be protected by the addition of antioxidants before drying).
Flotation is a process in which a gas or air is bubbled through the liquid to be clarified. The particles
are adsorbed by the rising bubbles and are removed when they reach the surface of the liquid. The bubbles
can attract smaller particles easily. This process can be used for the separation of algae with particle
diameter of less than 500 μm (Uduman et al. 2010). Air flotation techniques may be either dissolved or
induced. This process is among several new harvesting methods proven to be efficient (Chen et al. 1998).
The efficiency of dewatering microalgae by sedimentation depends on the time of removal of algae. It
is influenced by the intercellular interactions of algae in the different growth phases during cultivation.
The optimum harvesting time is determined by the zeta potential of the microalgal culture. It was found
that algae cultures harvested during the stationary phase had a higher rate of settling than those harvested
during the exponential phase. Algae cells in the exponential phase are highly stable and electrostatically
repel each other. It was found that algal cultures stored in darkness settled faster compared to daylight
conditions (Danquah et al. 2009).
Flocculation
Algae have high negative surface charges during the logarithmic phase of their growth. They remain
widely dispersed in water because of these repulsive charges. Algae settle and forms clusters after
longer residence times when they approach decay phase. This process is called, auto-flocculation. The
other factors responsible for auto-flocculation include nutrient (nitrogen and phosphorus) limitation,
restricted CO 2 supply, and co-precipitation of magnesium, calcium and carbonate salts (Becker 2004).
Auto-flocculation is also found to take place by changing the conditions of algae cultivation. Algal flocs
are formed when pond agitation stops and the CO 2 supply is cut off, causing an increase in pH. On
flocculation with various pH values, Scenedesmus sp. showed no flocculation for pH values between 5.0
and 7.5, while at pH values above 8.5, almost 95% of the algal biomass was found to be removed (Becker
2004).
The algae cells form precipitates with the addition of chemicals. Multivalent metal salts like ferric
chloride [FeCl 3 ], aluminum sulfate [Al 2 (SO 4 ) 3 , alum] and ferric sulfate [Fe 2 (SO 4 ) 3 ] are used to flocculate
algae (Grima et al. 2003). Alum is an efficient flocculant for Scenedesmus and Chlorella to produce fuel.
The metal salts are not intended for flocculation when considering the algae for use in some aquaculture
applications (Golueke and Oswald 1965). Algae can also be flocculated by using cationic polymers or
polyelectrolytes (Tenney et al. 1969). The negative charge on algae is attracted to the positive charge of
cationic flocculants. Algae particles are bound together by polymer flocculants through a process called
bridging. This process fails to occur at high ionic strengths. Dosage between 1 and 10 mg/L of the
polymer is required to flocculate fresh water algae (Bilanovic et al. 1988). Flocculation by polymeric
flocculants was mainly found to depend on the molecular mass, ionic strength and dose of the polymer
and also the pH and concentration of the algae culture (Chisti 1999). Chitosan has several advantages
over other conventional flocculants: It does not produce any toxic effects; it is required in very low
concentrations (Becker 2004). Algae can also be immobilized in a chitosan matrix and used in tertiary
treatment of wastewater (Kaya and Picard 1996).
Economic importance of algal biomass
Health supplements
Microalgae can increase the larval production though the exact mechanism of action is unclear till date.
Microalgae offer food for zooplankton which helps to advance the quality of the culture. For numerous
freshwater and seawater animals, supplying phytoplankton to the ponds leads to much better results
on survival, growth and transformation index (Muller-Feuga 2000). Due to some excreted biochemical