control, mixing, and light utilization efficiency. In general, the biomass productivity
is higher in cultivations with lower depth raceways, but this also depends on the
microalgae species used and the dimensions.
In raceway ponds, the mixing serves several purposes such as periodic exposure
of cells to sunlight, keeping cells into suspension, availability of the nutrient to algal
cells, and removal of photosynthetically generated oxygen. In this sense, an ideal
mixing supply can increase productivity by nearly 10 times. Conventionally, the
mixing is conventionally measured by the Reynolds number (Re), which in an ideal
situation is about 257,000, considering a 1.5 m wide channel with a broth depth of
0.3 m and a culture velocity of 0.3 m/s (Chisti 2016).
Carbon Supply and pH
The carbon is the major constituent of microalgal cells, with approximately 50% of
the cell mass. All carbon is photosynthetically assimilated from CO 2 and, this
assimilation is closely related to the pH of the medium, since that, if CO 2 is
consumed rapidly and not replenished, the pH becomes alkaline. In raceway ponds,
generally, the pH is instable, because the CO 2 absorption from atmosphere through
the surface of a raceway is insufficient to support the high photosynthesis rate for a
good part of the day (Chisti 2013). An alkaline pH results in generation of toxic
ammonia from dissolved ammonium salts, lowers the affinity of algae for CO 2 , and
increases the flexibility of mother cells, delaying completion of the cell cycle
(Juneja et al. 2013). For this reason, to obtain better productivities in raceway, it is
necessary to engineer a supply of CO 2 .
Gas diffusers are used in raceways to inject CO 2 in the form of fine bubbles.
According to Li et al. (2014), the CO 2 concentration greater than 73 µmol/L at a pH
of 8.0 is optimal for the normal growth of microalgae. To produce high-value
compounds, commercial pure carbon dioxide has been extensively used in
microalgal cultures. However, this entails in additional economic costs and reduces
the economic viability and sustainability of the process. It is estimated that the cost
of the carbon source in microalgae production ranges from 8 to 27% of the daily
production cost (Li et al. 2014). Furthermore, in this type of system, between 35
and 70% of the pure CO 2 injected into a pond is lost to the atmosphere. As an
alternative, flue gas can be used, which also could contribute to the mitigation of
environmental problems (de Godos et al. 2014).
Oxygen Accumulation
The photosynthesis reaction produces stoichiometrically 1.9 tons of oxygen to
produce 1 ton of microalgal biomass. So, when there is intense microalgal growth,
an excess of oxygen is generated. At high concentrations of O 2 , the productivity of
microalgae reduces considerably due to photorespiration and photoinhibition effects
(Raso et al. 2012).
8
M. M. Maroneze and M. I. Queiroz
is higher in cultivations with lower depth raceways, but this also depends on the
microalgae species used and the dimensions.
In raceway ponds, the mixing serves several purposes such as periodic exposure
of cells to sunlight, keeping cells into suspension, availability of the nutrient to algal
cells, and removal of photosynthetically generated oxygen. In this sense, an ideal
mixing supply can increase productivity by nearly 10 times. Conventionally, the
mixing is conventionally measured by the Reynolds number (Re), which in an ideal
situation is about 257,000, considering a 1.5 m wide channel with a broth depth of
0.3 m and a culture velocity of 0.3 m/s (Chisti 2016).
Carbon Supply and pH
The carbon is the major constituent of microalgal cells, with approximately 50% of
the cell mass. All carbon is photosynthetically assimilated from CO 2 and, this
assimilation is closely related to the pH of the medium, since that, if CO 2 is
consumed rapidly and not replenished, the pH becomes alkaline. In raceway ponds,
generally, the pH is instable, because the CO 2 absorption from atmosphere through
the surface of a raceway is insufficient to support the high photosynthesis rate for a
good part of the day (Chisti 2013). An alkaline pH results in generation of toxic
ammonia from dissolved ammonium salts, lowers the affinity of algae for CO 2 , and
increases the flexibility of mother cells, delaying completion of the cell cycle
(Juneja et al. 2013). For this reason, to obtain better productivities in raceway, it is
necessary to engineer a supply of CO 2 .
Gas diffusers are used in raceways to inject CO 2 in the form of fine bubbles.
According to Li et al. (2014), the CO 2 concentration greater than 73 µmol/L at a pH
of 8.0 is optimal for the normal growth of microalgae. To produce high-value
compounds, commercial pure carbon dioxide has been extensively used in
microalgal cultures. However, this entails in additional economic costs and reduces
the economic viability and sustainability of the process. It is estimated that the cost
of the carbon source in microalgae production ranges from 8 to 27% of the daily
production cost (Li et al. 2014). Furthermore, in this type of system, between 35
and 70% of the pure CO 2 injected into a pond is lost to the atmosphere. As an
alternative, flue gas can be used, which also could contribute to the mitigation of
environmental problems (de Godos et al. 2014).
Oxygen Accumulation
The photosynthesis reaction produces stoichiometrically 1.9 tons of oxygen to
produce 1 ton of microalgal biomass. So, when there is intense microalgal growth,
an excess of oxygen is generated. At high concentrations of O 2 , the productivity of
microalgae reduces considerably due to photorespiration and photoinhibition effects
(Raso et al. 2012).
8
M. M. Maroneze and M. I. Queiroz