In raceway ponds, the only mechanism commonly used for removal of oxygen
from the medium is the agitation by the paddle wheel and is not particularly
effective. Even with high surface areas, the oxygen removal is insufficient during
periods of maximum photosynthetic activity. At these photosynthesis peaks, the
performance of the system can decrease up to 35% due to the excess dissolved
oxygen in the medium, which can reach 300% of the air saturation value. Moreover,
the biochemical composition of microalgae biomass can be influenced by the
oxygen level in the pond (Richmond 1990; Chisti 2013).
Culture Contamination
As the raceway ponds are open to the environment, they are easily contaminated by
bacteria, viruses, fungi, and other microalgae species and by predators. An alternative to control the contamination is to place the lakes inside greenhouses with
controlled environmental conditions, but for the production of biofuels on a large
scale, this is economically unfeasible.
However, it is known that not many contaminants can survive under extreme
conditions. In this way, the contamination can be avoided by cultivating some
highly resistant microalgal strains at high pH or high salinity. The species with the
best performance in commercial cultivation in raceways includes Chlorella sp.,
Spirulina sp., and Dunaliella sp., which are cultivated under stringent conditions
that inhibit the growth of other microorganisms or other species of microalgae
(Chang et al. 2017).
2.1.2 Biomass Production in Raceway Ponds
Although biomass productivities of 0.40 g/L/d or higher have already been reported
(Wen et al. 2016), values much lower than these are typically found, as shown in
Table 1. The reported productivities are specific for the reactor designs, operating
conditions, local weather conditions, and algae species. For this reason, the productivities obtained with a specific system cannot be simply extrapolated to other
growth conditions. The biomass productivities in raceways are considered low, but
generally are compensated by high product prices and low construction and operating costs.
2.1.3 Cost of Construction and Operation of Raceway Ponds
In terms of cost of construction, the plastic-lined earthen are the raceway ponds
with the best cost-benefit, as unlined earth ponds are not generally considered
satisfactory for producing algal biomass (Chisti 2013). According to Chisti (2016),
the cost estimated to produce a 100-ha plastic-lined pond of compacted earth was
about US$ 144,830 per ha in 2014. This cost data can be corrected for inflation and
2 Microalgal Production Systems with Highlights …
9
from the medium is the agitation by the paddle wheel and is not particularly
effective. Even with high surface areas, the oxygen removal is insufficient during
periods of maximum photosynthetic activity. At these photosynthesis peaks, the
performance of the system can decrease up to 35% due to the excess dissolved
oxygen in the medium, which can reach 300% of the air saturation value. Moreover,
the biochemical composition of microalgae biomass can be influenced by the
oxygen level in the pond (Richmond 1990; Chisti 2013).
Culture Contamination
As the raceway ponds are open to the environment, they are easily contaminated by
bacteria, viruses, fungi, and other microalgae species and by predators. An alternative to control the contamination is to place the lakes inside greenhouses with
controlled environmental conditions, but for the production of biofuels on a large
scale, this is economically unfeasible.
However, it is known that not many contaminants can survive under extreme
conditions. In this way, the contamination can be avoided by cultivating some
highly resistant microalgal strains at high pH or high salinity. The species with the
best performance in commercial cultivation in raceways includes Chlorella sp.,
Spirulina sp., and Dunaliella sp., which are cultivated under stringent conditions
that inhibit the growth of other microorganisms or other species of microalgae
(Chang et al. 2017).
2.1.2 Biomass Production in Raceway Ponds
Although biomass productivities of 0.40 g/L/d or higher have already been reported
(Wen et al. 2016), values much lower than these are typically found, as shown in
Table 1. The reported productivities are specific for the reactor designs, operating
conditions, local weather conditions, and algae species. For this reason, the productivities obtained with a specific system cannot be simply extrapolated to other
growth conditions. The biomass productivities in raceways are considered low, but
generally are compensated by high product prices and low construction and operating costs.
2.1.3 Cost of Construction and Operation of Raceway Ponds
In terms of cost of construction, the plastic-lined earthen are the raceway ponds
with the best cost-benefit, as unlined earth ponds are not generally considered
satisfactory for producing algal biomass (Chisti 2013). According to Chisti (2016),
the cost estimated to produce a 100-ha plastic-lined pond of compacted earth was
about US$ 144,830 per ha in 2014. This cost data can be corrected for inflation and
2 Microalgal Production Systems with Highlights …
9