Page 182 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Biorefinery of microalgae biomass cultivated in wastewaters 155

Under light-limited conditions, photosynthetic activities are limited, and cells are
able to metabolize starch (and other polyglucans) and lipids to produce energy and
carbon molecules for reproductive processes. This ability allows the cells to become
completely independent of a direct supply of energy and carbon from photosyn-
thetic activity (Zachleder et al., 2016).
Microalgae cells can shift their metabolism toward heterotrophic pathways
depending on the extent of the light constraint and on the availability of organic
carbon in the culture medium. In that sense the adequacy of using turbid liquid
wastes to grow algae is greatly affected by the amount of organic carbon conveyed
in these wastes. Fortunately, slurries coming from AD sites and LLs are usually
rich in organic matters, making them potentially useful as sources of energy and
carbon for microalgae cultivation despite their turbidity.

7.2.3 Heavy metals
One negative impact of population growth is the increase in the production of
anthropogenic waste. The industrial activities lead to the presence of inorganic
contaminants, such as heavy metals (causedbyminingoperationsfor energy
production and consumer goods), in various waste streams that can damage
plant life, wildlife, and human health (Torres et al., 2017). Heavy metals are
not biodegradable. Instead, they are adsorbed by the living cells from aqueous
solutions through bioaccumulation processes (Mishra et al., 2018). Microalgae
cultivation in wastewaters is an attractive bioremediation strategy since they
can uptake important amounts of nutrients (nitrogen, phosphorus, organic
carbon, and sulfur) and they are also able to absorb heavy metals and other
contaminants. Microalgae cell wall possesses high metal-binding capacity due
to the presence of a negative functional group for metal cation uptake, which
allows the biosorption process on the cell surface. This ability can be used
for the tertiary treatment of liquid wastes containing heavy metals
(Kumar et al., 2015).
High removal efficiencies of heavy metals were reported by Mishra et al. (2018)
for Chlorella sp. isolated from a landfill site. The cells, cultivated in domestic treat-
ment plant wastewater, showed removal efficiency for zinc (Zn), copper (Cu), man-
ganese (Mn), and iron (Fe) of 82.6%, 56.5%, 79.8%, and 40%, respectively, after
14 days of cultivation. The initial concentrations of these metals were 0.41, 0.06,
0.052, and 0.371 ppm, respectively.
Cultivation conditions, such as light intensity, can influence the toxicity of heavy
metals to the cells. Torres et al. (2017) investigated the impact of individual con-
taminants at three concentrations, and three light intensities on the growth of
Nannochloropsis salina. Doubling of the biomass productivity was measured when
2
light intensity was increased from 200 to 450 μmol/m /s. This trend was not
2
observed when light intensity was increased from 450 to 800 μmol/m /s. In this
work, five contaminants [mercury (Hg), cadmium (Cd), lead (Pb), selenium (Se),
and cobalt (Co)] had minimal impact on growth at any concentration and light
intensity. Conversely, detrimental effects were observed with Hg at a concentration of