3.4 Current Practice in Process Design and Scale-Up
To design and scale up microalgae-based wastewater treatment processes properly,
the diverse characteristics of wastewater composition as well as the large volume of
wastewater needs to be considered (Cuellar-Bermudez et al. 2017). The color or
turbidity of wastewater is one of the limiting factors for algal growth, diminishing
light transmittance into the culture (Kai et al. 2010). Many studies on microalgaebased wastewater treatment processes include a pretreatment step (e.g., anaerobic
digestion, filtration, autoclaving, or water dilution) to eliminate any potential of the
growth of competitive microorganisms and decrease water turbidity (CuellarBermudez et al. 2017). However, this practice has been limited to laboratory-scale
tests with synthetic wastewater. Finding a cost-effective harvesting method is
another barrier for scale-up of the processes due to the high cost of algae cultivation.
The most common large-scale algae-based wastewater treatment systems
use HRAPs, WSPs as a suspended algal growth, and ATS as an attached algal
growth (Gross et al. 2013; Shoener et al. 2014). Although PBR as a closed cultivation system has been used successfully for producing large quantities of microalgal
biomass (Chisti 2007), it is not fully yet explored for large-scale wastewater treatment because of their high operation costs (Xu et al. 2009).
3.4.1 Suspended Algal Treatment Systems
A HRAP is an open raceway pond that encourages suspended algal growth
for treating wastewater. HRAP systems developed by Oswald in the 1950s (Oswald
1991) essentially consist of four main unit processes arranged in series: (1) wastewater solid removal anaerobic digestion, (2) aerobic treatment by algal growth,
(3) algal removal and conversion to biofuel, and (4) further polishing of the treated
effluent (Craggs et al. 2014). Although HRAPs are much more cost-effective than
traditional mechanical wastewater treatment systems, the primary disadvantage of
HRAPs is the difficulty of harvesting suspended algae with small size (< 20 μm) and
the cost of land required for the pond systems (Hoffmann 1998). Waste stabilization
ponds (WSP) are large and shallow basins that consist of anaerobic and aerobic
ponds for organic removal and maturation ponds for pathogen removal (Santhanam
2009). WSPs are one of the most cost-effective, reliable, and easily operated
methods but also require large land area (Mara 2009).
3.4.2 Attached Algal Treatment Systems
Attached growth treatment systems can play an important role in overcoming problems related to the costs of harvesting biomass, which is a key barrier for the
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To design and scale up microalgae-based wastewater treatment processes properly,
the diverse characteristics of wastewater composition as well as the large volume of
wastewater needs to be considered (Cuellar-Bermudez et al. 2017). The color or
turbidity of wastewater is one of the limiting factors for algal growth, diminishing
light transmittance into the culture (Kai et al. 2010). Many studies on microalgaebased wastewater treatment processes include a pretreatment step (e.g., anaerobic
digestion, filtration, autoclaving, or water dilution) to eliminate any potential of the
growth of competitive microorganisms and decrease water turbidity (CuellarBermudez et al. 2017). However, this practice has been limited to laboratory-scale
tests with synthetic wastewater. Finding a cost-effective harvesting method is
another barrier for scale-up of the processes due to the high cost of algae cultivation.
The most common large-scale algae-based wastewater treatment systems
use HRAPs, WSPs as a suspended algal growth, and ATS as an attached algal
growth (Gross et al. 2013; Shoener et al. 2014). Although PBR as a closed cultivation system has been used successfully for producing large quantities of microalgal
biomass (Chisti 2007), it is not fully yet explored for large-scale wastewater treatment because of their high operation costs (Xu et al. 2009).
3.4.1 Suspended Algal Treatment Systems
A HRAP is an open raceway pond that encourages suspended algal growth
for treating wastewater. HRAP systems developed by Oswald in the 1950s (Oswald
1991) essentially consist of four main unit processes arranged in series: (1) wastewater solid removal anaerobic digestion, (2) aerobic treatment by algal growth,
(3) algal removal and conversion to biofuel, and (4) further polishing of the treated
effluent (Craggs et al. 2014). Although HRAPs are much more cost-effective than
traditional mechanical wastewater treatment systems, the primary disadvantage of
HRAPs is the difficulty of harvesting suspended algae with small size (< 20 μm) and
the cost of land required for the pond systems (Hoffmann 1998). Waste stabilization
ponds (WSP) are large and shallow basins that consist of anaerobic and aerobic
ponds for organic removal and maturation ponds for pathogen removal (Santhanam
2009). WSPs are one of the most cost-effective, reliable, and easily operated
methods but also require large land area (Mara 2009).
3.4.2 Attached Algal Treatment Systems
Attached growth treatment systems can play an important role in overcoming problems related to the costs of harvesting biomass, which is a key barrier for the
12 Microalgae: An Eco-friendly Tool for the Treatment of Wastewaters for. . .
293