3 Microalgae: An Eco-friendly Tool for Advanced
Wastewater Treatment
3.1 Alternative Microalgal Culture and Treatment Systems
Wastewater treatment processes using microalgae are becoming more attractive due
to their photosynthetic capabilities of converting solar energy into useful biomass
and capturing primary nutrients (N and P) which are responsible for eutrophication
and the potentials of lowering operational costs (eliminating mechanical aeration)
compared to conventional wastewater treatment processes (de la Noue and de Pauw
1988; Liu et al. 2013). Algae can be cultivated in a wastewater treatment set up by
injecting flue gas (CO 2 ) discharged from power plants (Hwang et al. 2016).
3.1.1 Hyper-concentrated Cultures
The utilization of microalgae for biological tertiary wastewater treatment has been
studied for five decades (Caldwell 1946; Chevalier and De la Noüe 1985). However,
many problems are still unsolved, and one of the biggest challenges is the recovery
of microalgae from wastewater. A few studies have been reported that algae are
concentrated by settling using flocculants such as chitosan which can overcome algal
cell growing on sewage sludge (Noüe et al. 1992; Sturm and Lamer 2011). A
disadvantage of using flocculants is secondary pollution which is accumulation of
diverse flocculating agents as alum, lime, FeCl 3 , and Ca(OH) 2 in high concentration.
Chitosan is considered as an excellent flocculant because it is a nontoxic natural
polymer and can efficiently produce hyper-concentrated cultures of algae. Hyperconcentrated culture studies have been investigated only in lab-scale tests, and thus,
more studies for scale-up of effective hyper-concentrated culture processes are
required to enhance the efficiency of NH 4
+
, NO
3À , and PO 4
3À removals in realworld applications.
3.1.2 Immobilized Cell Systems
Immobilized microalgal systems are used for biological nutrient removal with their
high removal efficiency (e.g., 93% of nitrate and 80% of P are absorbed by
immobilized cells within 24 h). Compared to conventional suspended systems, the
immobilized microalgal technology offers many advantages such as high biomass,
high metabolic activity, and strong resistance to toxic chemicals (Mallick 2002). The
immobilized cell process of continuous biomass production can have low operation
costs because no further biocatalyst is required (Mrudula and Shyam 2012). Immobilization of biomass also provides protection to cells from metal toxicity; Chlorella
sorokiniana was immobilized on loofa sponge and successfully used as a new
biosorption system for the removal of lead (II) ions from aqueous solutions (Akhtar
288
J.-H. Hwang et al.
Wastewater Treatment
3.1 Alternative Microalgal Culture and Treatment Systems
Wastewater treatment processes using microalgae are becoming more attractive due
to their photosynthetic capabilities of converting solar energy into useful biomass
and capturing primary nutrients (N and P) which are responsible for eutrophication
and the potentials of lowering operational costs (eliminating mechanical aeration)
compared to conventional wastewater treatment processes (de la Noue and de Pauw
1988; Liu et al. 2013). Algae can be cultivated in a wastewater treatment set up by
injecting flue gas (CO 2 ) discharged from power plants (Hwang et al. 2016).
3.1.1 Hyper-concentrated Cultures
The utilization of microalgae for biological tertiary wastewater treatment has been
studied for five decades (Caldwell 1946; Chevalier and De la Noüe 1985). However,
many problems are still unsolved, and one of the biggest challenges is the recovery
of microalgae from wastewater. A few studies have been reported that algae are
concentrated by settling using flocculants such as chitosan which can overcome algal
cell growing on sewage sludge (Noüe et al. 1992; Sturm and Lamer 2011). A
disadvantage of using flocculants is secondary pollution which is accumulation of
diverse flocculating agents as alum, lime, FeCl 3 , and Ca(OH) 2 in high concentration.
Chitosan is considered as an excellent flocculant because it is a nontoxic natural
polymer and can efficiently produce hyper-concentrated cultures of algae. Hyperconcentrated culture studies have been investigated only in lab-scale tests, and thus,
more studies for scale-up of effective hyper-concentrated culture processes are
required to enhance the efficiency of NH 4
+
, NO
3À , and PO 4
3À removals in realworld applications.
3.1.2 Immobilized Cell Systems
Immobilized microalgal systems are used for biological nutrient removal with their
high removal efficiency (e.g., 93% of nitrate and 80% of P are absorbed by
immobilized cells within 24 h). Compared to conventional suspended systems, the
immobilized microalgal technology offers many advantages such as high biomass,
high metabolic activity, and strong resistance to toxic chemicals (Mallick 2002). The
immobilized cell process of continuous biomass production can have low operation
costs because no further biocatalyst is required (Mrudula and Shyam 2012). Immobilization of biomass also provides protection to cells from metal toxicity; Chlorella
sorokiniana was immobilized on loofa sponge and successfully used as a new
biosorption system for the removal of lead (II) ions from aqueous solutions (Akhtar
288
J.-H. Hwang et al.