Page 191 - Refining Biomass Residues for Sustainable Energy and Bioproducts

162 Refining Biomass Residues for Sustainable Energy and Bioproducts

explained by the combined effects of biomass growth, and P adsorption and precipi-
tation due to the increase of the pH of the media.
Dense algae cultures are usually poor in N and C and have a limited light pene-
tration. Under these conditions, algae might use only a small fraction of P, with the
remaining portion staying in the culture media. In their work, Mahapatra et al.
(2014) observed that the removal efficiency of orthophosphates was higher than
that of PO 4 . This was explained as an improved assimilation of P by algae by the
conversion of organic P into inorganic forms.
As previously mentioned, the pH greatly affects the uptake of NH 3 by algae.
Depending on the pH value, NH 3 can change its protonated state and it can be vola-
tilized from the culture media. Mahapatra et al. (2014) studied the NH 3 removal in
cultures of microalgae bacteria consortium grown in municipal wastewater. The
major fraction of NH 3 -N was removed by algae assimilation since pH (,8.5) and
temperature (26 C) were moderate. However, a fraction of NH 3 -N removal was due

to NH 3 volatilization. The incomplete removal of NH 3 -N resulted from carbon limi-
tation or light interception in the dense algal cultures. The observed increase in
NO 3 -N and decrease in NO 2 -N concentrations were attributed to nitrification.
Nitrification results from the oxidation of reduced nitrogen compounds (primarily
1
NH 3 /NH ) into NO 2 and NO 3 .
4
When compared to digestate slurries, domestic wastewaters have a greater
amount of NO 3 -N. Lekshmi et al. (2015) evaluated the nutrients uptake in auto-
claved and raw domestic sewage by Chlorella pyrenoidosa. They observed that
when NH 3 -N was present in the culture, the uptake of NO 3 -N was slow. This was
explained by the high energy required for NO 3 assimilation compared to that of
NH 3 . Similarly Ebrahimian et al. (2014) observed that under heterotrophic cultiva-
tion, C. vulgaris preferred to uptake nitrogen from NH 3 -N instead of from NO 3 -N.
They also observed growth continuation after the nitrogen sources were nearly
exhausted. This was explained by the utilization of intracellular nitrogen by the
cells.
He et al. (2013) studied the effect of the NH 3 -N concentrations on the growth
and lipid production of C. vulgaris cultivated in septic and secondary sedimentation
tank effluents. The growth of algae cells declined at NH 3 -N concentrations
higher than 17 mg/L in the cultures. However, the maximum lipid productivity
(23.3 mg/L/day) was achieved at a concentration of 38 mg/L of NH 3 -N. Also,
protein content in the biomass had a positive correlation with the NH 3 -N content
in the range of 17 207 mg/L.
A lipid productivity of 45.5 mg/L/day in C. vulgaris cultures grown in a mixture
of 25% primary and 75% secondary wastewaters, with an initial NH 3 -N concentra-
tion of 11.3 mg/L, was reported by Ebrahimian et al. (2014). In this study a 100%
NH 3 -N and 82% NO 3 -N removals were achieved.
When microalgae cultivation is intended for biodiesel production, the fatty acid
profile of the lipids is an important parameter to be considered. C16 C18 fatty
acids, as well as monounsaturated fatty acids (MUFAs), are highly desirable since
they provide the best compromise between oxidative stability and cold flow proper-
ties of biodiesel (Knothe, 2009; Song et al., 2014a). Calixto et al. (2016) observed a