Page 203 - Refining Biomass Residues for Sustainable Energy and Bioproducts

172 Refining Biomass Residues for Sustainable Energy and Bioproducts

faces challenges making the current growth of the microalgae biorefinery industry
still economically unviable. The assurance of sustainable and cheap supplies of
water, energy, and quality nutrients by using wastewaters to cultivate the biomass is
a strategy that can improve the economics of the biomass production process.
Moreover, the phycoremediation of the liquid wastes could bring along the eco-
nomic advantage of wastewater treatment cost offsets. However, the variability in
the composition of liquid wastewaters makes it difficult to predict the outcomes.
The tolerance limits of the selected strain to the waste streams need to be known.
The minimum wastewater content to sustain cells growth, as well as the maximum
wastewater concentration allowed before the growth is inhibited, must be first
investigated. Between these limits, the optimal percentage of wastewater in the cul-
ture medium must be defined depending on the projected goal of the cultivation,
since optimal conditions for maximum biomass productivity, biomass yield, nutri-
ents removal, and coproducts content in the biomass are rarely the same.
In In addition, the composition of the targeted wastewater would determine the
production mechanism and the cultivation system to be employed, as well as the
adequacy of the use of consortia of microalgae and bacteria to attain the projected
results.
Toxicity levels of NH 3 to the cells and its volatility from the cultures depend on
the pH of the culture medium. In turn, the pH is easily affected by several phenom-
ena occurring during the growth (CO 2 supply/uptake, main source of N being used
by the cells, and the presence of organic carbon). LLs and liquid digestates are par-
ticularly high in NH 3 , which causes a high pH that might also cause the precipita-
tion of PO 4 from the culture media. These wastewaters are generally characterized
by their dark color which hinders light penetration promoting heterotrophy, where
organic carbon becomes essential to support the growth of the microalgae. The
effects of heavy metals on the cells are strain dependent and are mainly affected by
light intensity. The best growth values found in the literature in liquid digestates are
around a dilution ratio of 1 10, where lower dilutions have been used to produce
value-added coproducts. The adequate ratio of C:N:P rather than a high concentra-
tion of nutrients would yield better biomass productivities. Moreover, the synergis-
tic effect of bacteria and microalgae has been shown to have positive effects on
nutrient removals.
The mechanisms involved in nutrients removal by microalgae from wastewaters
have been identified. For N removal, the mechanisms involved are the assimilation
2
of both NO and NH 3 , and stripping of NH 3 due to the increased pH during micro-
3
algal growth. Reduction in P concentrations is due to bioassimilation, adsorption,
and chemical precipitation due to high pH values. Organic carbon is consumed by
microalgae depending on the regulation/acclimation of both heterotrophic and pho-
tosynthetic pathways.
Improved biomass production and nutrients removal can be achieved by adjust-
ing the composition of culture media prepared from wastewaters. This can be done
by the addition of nutrient limiting the growth and/or by the dilution of the waste-
waters to attain optimum nutrient contents and ratios. However, the addition of