Page 185 - Refining Biomass Residues for Sustainable Energy and Bioproducts

158 Refining Biomass Residues for Sustainable Energy and Bioproducts

In this work the adequacy of different wastewater streams to sustain microalgae
growth is reviewed. The liquid streams were classified according to their origin and
composition as follows: digestate slurries, domestic and industrial flows, LLs, and
treatment plant effluents.

7.3.1 Digestates

The AD of organic materials is a way to treat wastes with the production of biogas
(methane, CH 4 ) as clean energy and a liquid digestate. During AD, bacteria trans-
form complex organic wastes into biogas. The transformed wastes are then used as
a green fertilizer because of their high nutrient contents. When lipids, carbohy-
drates, and proteins contained in these wastes are converted to CH 4 , nitrogen is
released, typically as NH 3 , the main source of nitrogen in digestate slurries. This
large content of NH 3 is one of the drawbacks of the use of this stream for algae
cultivation, since NH 3 can be, at high levels, toxic to algae. Therefore the dilution
of these liquid wastes based on the NH 3 concentration is a common practice.
This, at the same time, improves light penetration to the cultures prepared from
digestates, as their characteristic dark-brown color can interfere with light reach-
ing the cells.
Recent works have reported on the use of digestate as a source of nutrients
to grow algae (Table 7.2). These investigations aimed at defining ideal concen-
trations of digestate to be used, at assessing strain tolerances, and at establishing
the conditions for wastewater cleaning, and biomass productivity and
composition.
Ji et al. (2014a) evaluated the growth of Desmodesmus sp. in 2.5%, 5%, and
10% of AD wastewater (ADW) and found that the best growth and removal rate of
nutrients was observed at 10% of ADW. Similarly, Koutra et al. (2018) tested the
performance of seven different microalgae strains to select the most appropriate one
for biomass production and digestate bioremediation. In their study, microalgae
were grown in 10% digestate (derived from agro-industrial wastes) diluted in water,
during a cultivation period of 25 days. The maximum growth was observed in the
cultures of Ponticola kessleri, Anisancylus obliquus, and Chlorella vulgaris with
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specific growth rates of 0.130, 0.145, and 0.161 day , respectively.
Similar values of biomass (0.32 g/L) and lipid production (15.5%) were found
for Chlorella sorokiniana grown in diluted digestate (10%) and in commercial
medium (BBM) (Lizzul et al., 2014).
Culture media formulated from digestates at different dilution ratios have been
used to produce value-added components from the biomass. The growth and pro-
duction of polyhydroxy butyrate (PHB) of a brackish cyanobacteria strain
Synechocystis cf. salina was compared in a 33% digestate diluted in water, and in
an artificial culture medium (BG11) (Meixner et al., 2016). The cultivation in the
diluted digestate achieved 84% of the biomass and 78% of the PHB values obtained
in BG11. However, the cultivation took almost 50% more time in the diluted
digestate.