microalgae growth due to shadowing effects. In addition, the microorganisms also
contribute to the turbidity of the water, limiting even more the depth of light
penetration. Taking into account these factors that limit light penetration, photosynthesis occurs only in the superficial layers of the culture (USEPA 2011),
influencing the overall treatment efficiency. In order to maximize the light penetration, the mixing degree inside the photobioreactors is an important factor, as all
cells can be exposed to light in a turbulent regime for at least a short period of time,
being possible to achieve high productivity (Yaakob and Fakir 2011).
5 Biofuels Production with Microalgae Cultivated
in Wastewater: Recent Advances and Challenges
The production of microalgal biofuels has two major challenges: (i) production
costs reduction and (ii) identification of the harvesting process. The integration of
biomass production and wastewater treatment reduces the requirements of nutrients
and freshwater. Studies with real wastewaters should be performed to evaluate the
nutrient removal efficiencies (wastewater treatment efficiency) and biomass productivities (possible growth inhibition). With the achieved biomass, the potential
for production of different biofuels (biodiesel, bioethanol, biogas, between others)
should be assessed. Table 2 shows some recent studies focusing on biofuel production with microalgae cultivated in wastewater. Prandini et al. (2016) evaluated
the growth of microalgae Scenedesmus sp. in piggery wastewater and bubbled
swine wastewater-derived biogas (for biogas filtration). Microalgal culture was able
to assimilate N–NH 3 , P–PO 4
3− and CO 2 at a rate of 21 ± 1, 4 ± 3 and
219 ± 5 mg/L/d, respectively. H 2 S in biogas (up to 3000 ppm) was not inhibitory
and it was completely removed. Hernandez et al. (2016) tested a consortium of
microalgae composed by Chlamydomonas subcaudata, Anabaena sp. and Nitzschia
sp. for treatment of slaughterhouse wastewater in two high-rate algal ponds—
HRAPs (indoor and outdoor) during 115 d. High removal efficiencies of chemical
oxygen demand and soluble phosphorus were achieved in both HRAPs. The
maximum productivity was 12.7 g/m
2 /d. High quality of free fatty acids (FFA) was
achieved in a ratio of 142 mg FFA/g. Biogas production was also assessed,
resulting in 195 mL CH 4 /g. Lutzu et al. (2016) evaluated the potential of brewery
wastewater as microalgal culture medium. Adjustments in nitrogen and phosphorus
concentrations were needed to improve biomass and lipid productivities. The
chemical analysis of the fatty acids methyl esters showed that high fractions
(67.24%) are unsaturated ones and they are composed mainly by C16–C18.
Concerning the wastewater treatment, high removal efficiencies were achieved for
nitrogen and phosphorus (>99%) and a significant reduction of chemical oxygen
demand was observed (65%). Despite the recent studies reported in the literature,
further researches are still needed. Due to the natural variability of wastewater
composition, microalgal culture should be tested under environmental stresses in
50
A. P. de Carvalho Lopes et al.
contribute to the turbidity of the water, limiting even more the depth of light
penetration. Taking into account these factors that limit light penetration, photosynthesis occurs only in the superficial layers of the culture (USEPA 2011),
influencing the overall treatment efficiency. In order to maximize the light penetration, the mixing degree inside the photobioreactors is an important factor, as all
cells can be exposed to light in a turbulent regime for at least a short period of time,
being possible to achieve high productivity (Yaakob and Fakir 2011).
5 Biofuels Production with Microalgae Cultivated
in Wastewater: Recent Advances and Challenges
The production of microalgal biofuels has two major challenges: (i) production
costs reduction and (ii) identification of the harvesting process. The integration of
biomass production and wastewater treatment reduces the requirements of nutrients
and freshwater. Studies with real wastewaters should be performed to evaluate the
nutrient removal efficiencies (wastewater treatment efficiency) and biomass productivities (possible growth inhibition). With the achieved biomass, the potential
for production of different biofuels (biodiesel, bioethanol, biogas, between others)
should be assessed. Table 2 shows some recent studies focusing on biofuel production with microalgae cultivated in wastewater. Prandini et al. (2016) evaluated
the growth of microalgae Scenedesmus sp. in piggery wastewater and bubbled
swine wastewater-derived biogas (for biogas filtration). Microalgal culture was able
to assimilate N–NH 3 , P–PO 4
3− and CO 2 at a rate of 21 ± 1, 4 ± 3 and
219 ± 5 mg/L/d, respectively. H 2 S in biogas (up to 3000 ppm) was not inhibitory
and it was completely removed. Hernandez et al. (2016) tested a consortium of
microalgae composed by Chlamydomonas subcaudata, Anabaena sp. and Nitzschia
sp. for treatment of slaughterhouse wastewater in two high-rate algal ponds—
HRAPs (indoor and outdoor) during 115 d. High removal efficiencies of chemical
oxygen demand and soluble phosphorus were achieved in both HRAPs. The
maximum productivity was 12.7 g/m
2 /d. High quality of free fatty acids (FFA) was
achieved in a ratio of 142 mg FFA/g. Biogas production was also assessed,
resulting in 195 mL CH 4 /g. Lutzu et al. (2016) evaluated the potential of brewery
wastewater as microalgal culture medium. Adjustments in nitrogen and phosphorus
concentrations were needed to improve biomass and lipid productivities. The
chemical analysis of the fatty acids methyl esters showed that high fractions
(67.24%) are unsaturated ones and they are composed mainly by C16–C18.
Concerning the wastewater treatment, high removal efficiencies were achieved for
nitrogen and phosphorus (>99%) and a significant reduction of chemical oxygen
demand was observed (65%). Despite the recent studies reported in the literature,
further researches are still needed. Due to the natural variability of wastewater
composition, microalgal culture should be tested under environmental stresses in
50
A. P. de Carvalho Lopes et al.