11 Microalgae-Based Processes for Pigments Production
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11.6.1 Open Systems
11.6.1.1 Raceways Ponds
Raceway ponds have so far been the most widely used method for commercially
producing microalgae biomass owing to their flexibility and ease of scaling up. It
is also known as high-rate lagoons or Oswald lagoons, a tribute to W.J. Oswald,
who described the technology for the first time, which was initially devised for
wastewater treatment (Oswald and Golueke 1968; Acién et al. 2017). Currently,
the industrial outdoor culture of microalgae and cyanobacteria in raceways is well
established, especially for Arthrospira and Dunaliella since these species require
growth media with high alkalinity and salinity, respectively, making it unsuitable for
other competing species (Zitelli et al. 2013).
In raceway ponds, as the name suggests, the microalgae suspension is recirculated
around a racetrack loop (Suparmaniam et al. 2019). At large-scale, the surface area
of the pond could range between 1000 and 5000 m
2 . Although pond configurations
with multiple channels and bends are available, raceways with a minimum number of
bends are more common because they imply less pressure drop and, consequently,
less energy expenditure. The length of the channels should be proportional to the
width, where proportions between 10 and 20 are generally adequate (Chisti 2012;
Zhou et al. 2020).
Light is the driver of the photosynthesis reaction; thus, it is the primary factor
that most influences the microalgal growth rate. In this sense, a key variable of the
raceways is the depth of the channels, which should be shallow, in the range of 0.25
to 0.30 m, to allow the penetration of light inside the culture. As both area and depth
are limited, larger facilities are implemented by multiplying the number of ponds
(Acién et al. 2017).
In commercial microalgae production, these ponds generally are constructed in
compacted soil covered with a PVC membrane. This type of construction carries
a higher risk of contamination, so in some cases, ponds made of concrete with a
plastic liner can be used, which are more expensive but guarantee a lower risk of
contamination (Maroneze and Queiroz 2018).
To provide the energy required for the circulation and mixes the nutrients and
cells along the reactor are utilized paddlewheel. It consists of a wheel with ten to
twelve paddles, with a total diameter of four times the water depth, working at
velocities between 10 and 20 rpm. In addition to what has already been mentioned,
mixing is responsible for keeping cells into suspension, removal of photosynthetically
generated oxygen, and to provide the periodic exposure of cells to sunlight, which
is important to avoid photolimitation and photoinhibition and enhances the light
utilization efficiency. However, excessive energy application in the mixing should
be avoided, mainly to minimize the operational cost since this operation represents
up to 69% of the total costs of microalgal cultures in raceways (Kumar et al. 2015).
In addition to light, the temperature is another determining parameter in biomass
productivity. The optimal temperature for growth of microalgae varies widely with
255
11.6.1 Open Systems
11.6.1.1 Raceways Ponds
Raceway ponds have so far been the most widely used method for commercially
producing microalgae biomass owing to their flexibility and ease of scaling up. It
is also known as high-rate lagoons or Oswald lagoons, a tribute to W.J. Oswald,
who described the technology for the first time, which was initially devised for
wastewater treatment (Oswald and Golueke 1968; Acién et al. 2017). Currently,
the industrial outdoor culture of microalgae and cyanobacteria in raceways is well
established, especially for Arthrospira and Dunaliella since these species require
growth media with high alkalinity and salinity, respectively, making it unsuitable for
other competing species (Zitelli et al. 2013).
In raceway ponds, as the name suggests, the microalgae suspension is recirculated
around a racetrack loop (Suparmaniam et al. 2019). At large-scale, the surface area
of the pond could range between 1000 and 5000 m
2 . Although pond configurations
with multiple channels and bends are available, raceways with a minimum number of
bends are more common because they imply less pressure drop and, consequently,
less energy expenditure. The length of the channels should be proportional to the
width, where proportions between 10 and 20 are generally adequate (Chisti 2012;
Zhou et al. 2020).
Light is the driver of the photosynthesis reaction; thus, it is the primary factor
that most influences the microalgal growth rate. In this sense, a key variable of the
raceways is the depth of the channels, which should be shallow, in the range of 0.25
to 0.30 m, to allow the penetration of light inside the culture. As both area and depth
are limited, larger facilities are implemented by multiplying the number of ponds
(Acién et al. 2017).
In commercial microalgae production, these ponds generally are constructed in
compacted soil covered with a PVC membrane. This type of construction carries
a higher risk of contamination, so in some cases, ponds made of concrete with a
plastic liner can be used, which are more expensive but guarantee a lower risk of
contamination (Maroneze and Queiroz 2018).
To provide the energy required for the circulation and mixes the nutrients and
cells along the reactor are utilized paddlewheel. It consists of a wheel with ten to
twelve paddles, with a total diameter of four times the water depth, working at
velocities between 10 and 20 rpm. In addition to what has already been mentioned,
mixing is responsible for keeping cells into suspension, removal of photosynthetically
generated oxygen, and to provide the periodic exposure of cells to sunlight, which
is important to avoid photolimitation and photoinhibition and enhances the light
utilization efficiency. However, excessive energy application in the mixing should
be avoided, mainly to minimize the operational cost since this operation represents
up to 69% of the total costs of microalgal cultures in raceways (Kumar et al. 2015).
In addition to light, the temperature is another determining parameter in biomass
productivity. The optimal temperature for growth of microalgae varies widely with