the organic carbon, as well as the nutrients in urban wastewater to values lower than
discharge limits (McCarty et al. 2011). Microalgae can be a good approach for the
tertiary treatment of urban wastewaters because they require large amounts of
nitrogen and phosphorus to their growth, including for the synthesis of proteins
(40–60% by dry weight), nucleic acids and phospholipids (Silva-Benavides and
Torzillo 2011). The wastewater treatment based on microalgae can remove N and P
more efficiently than the traditional activated sludge treatment (Lau et al. 1995;
Lavoie and De La Noüe 1983; Tam and Wong 1989). In addition to the removal of
these nutrients, microalgae have the ability to remove heavy metals from
wastewater (Rai et al. 1981). Finally, the microalgae can perform a disinfectant
effect in the effluent due to the pH increase inherent to photosynthesis (De La Noue
and De Pauw 1988). The mentioned advantages make this system an excellent
alternative to the traditional technologies employed for wastewater.
Nutrient removal efficiencies are dependent on the wastewater composition and
environmental conditions, such as light intensity, the N:P ratio, the light/dark cycle
and microalgal species (Aslan and Kapdan 2006). The most studied microalgal
species for the treatment of urban wastewaters are Chlorella, Scenedesmus,
Phormidium, Botryococcus, Chlamydomonas and Spirulina (Chinnasamy et al.
2010; Kong et al. 2010; Olguín 2003; Wang et al. 2010). Taking into account the
potential of microalgae for wastewater treatment, Table 1 presents several studies
that demonstrate the viability of microalgal cultures in the nutrients removal from
different types of wastewater.
3.2 Limitations of Conventional Treatments
The consortium of microorganisms present in activated sludge systems require
phosphorus for their growth, which results in partial removal of phosphate during
the secondary treatment (Yau 2016). However, to achieve discharge limits of
1 mg P/L normally is required the use of inorganic coagulants (such as lime, aluminium sulphate and iron chloride). Besides the increase on the treatment cost, the
addition of these coagulants is less environmentally sustainable than the removal of
phosphorus by microalgae.
Another limitation is the main by-product generated in biological treatment:
activated sludge waste. To treat 1 million litres of wastewater, the biological
treatment produces about 70–100 kg of activated sludge in dry basis (Athanasoulia
et al. 2012). Consequently, the treatment and disposal of this waste requires a
considerable deposition area and a high-energy expense. In addition, the mechanical aeration (necessary in biological treatment) can cause the release of volatile
contaminants into the atmosphere (Jia et al. 2016). The role of microalgae in this
step could reduce or even prevent the release of these contaminants, since
microalgae would produce oxygen and thus reduce the need for aeration.
Finally, greenhouse gases (such as methane—CH 4 , N 2 O and CO 2 ) are released
to the atmosphere in the biological treatment (Campos et al. 2016). Conventional
44
A. P. de Carvalho Lopes et al.
discharge limits (McCarty et al. 2011). Microalgae can be a good approach for the
tertiary treatment of urban wastewaters because they require large amounts of
nitrogen and phosphorus to their growth, including for the synthesis of proteins
(40–60% by dry weight), nucleic acids and phospholipids (Silva-Benavides and
Torzillo 2011). The wastewater treatment based on microalgae can remove N and P
more efficiently than the traditional activated sludge treatment (Lau et al. 1995;
Lavoie and De La Noüe 1983; Tam and Wong 1989). In addition to the removal of
these nutrients, microalgae have the ability to remove heavy metals from
wastewater (Rai et al. 1981). Finally, the microalgae can perform a disinfectant
effect in the effluent due to the pH increase inherent to photosynthesis (De La Noue
and De Pauw 1988). The mentioned advantages make this system an excellent
alternative to the traditional technologies employed for wastewater.
Nutrient removal efficiencies are dependent on the wastewater composition and
environmental conditions, such as light intensity, the N:P ratio, the light/dark cycle
and microalgal species (Aslan and Kapdan 2006). The most studied microalgal
species for the treatment of urban wastewaters are Chlorella, Scenedesmus,
Phormidium, Botryococcus, Chlamydomonas and Spirulina (Chinnasamy et al.
2010; Kong et al. 2010; Olguín 2003; Wang et al. 2010). Taking into account the
potential of microalgae for wastewater treatment, Table 1 presents several studies
that demonstrate the viability of microalgal cultures in the nutrients removal from
different types of wastewater.
3.2 Limitations of Conventional Treatments
The consortium of microorganisms present in activated sludge systems require
phosphorus for their growth, which results in partial removal of phosphate during
the secondary treatment (Yau 2016). However, to achieve discharge limits of
1 mg P/L normally is required the use of inorganic coagulants (such as lime, aluminium sulphate and iron chloride). Besides the increase on the treatment cost, the
addition of these coagulants is less environmentally sustainable than the removal of
phosphorus by microalgae.
Another limitation is the main by-product generated in biological treatment:
activated sludge waste. To treat 1 million litres of wastewater, the biological
treatment produces about 70–100 kg of activated sludge in dry basis (Athanasoulia
et al. 2012). Consequently, the treatment and disposal of this waste requires a
considerable deposition area and a high-energy expense. In addition, the mechanical aeration (necessary in biological treatment) can cause the release of volatile
contaminants into the atmosphere (Jia et al. 2016). The role of microalgae in this
step could reduce or even prevent the release of these contaminants, since
microalgae would produce oxygen and thus reduce the need for aeration.
Finally, greenhouse gases (such as methane—CH 4 , N 2 O and CO 2 ) are released
to the atmosphere in the biological treatment (Campos et al. 2016). Conventional
44
A. P. de Carvalho Lopes et al.