methane yield could be determined. This biomass provided the
highest theoretical methane yield due to its high lipids content and
the highest specific methane yield of this macromolecule. As it can
be seen in Fig. 5A, the methane yield of this biomass increased
along with time until a certain point at which methane production
remained stable (approx. 15 days of digestion). Similar to this
behaviour, Gonzalez-Fernandez et al. [7] also reported a mismatch
of 64% between theoretical and experimental methane yield of
microalgae biomass grown in urban wastewater under the same
non-favourable conditions. Both biomasses exhibited the highest
lipids content (>30%) and therefore it can be concluded that despite
of the specific methane yield of this macromolecule, a high content
of lipids seems to hamper an efficient bioconversion to methane. In
accordance to this trend, the biomass with the second highest lipid
content (35% VSS, biomass grown under non-favourable conditions
and low NH4
þ) evidenced a mismatch of 62%. Opposite to that,
biomasses grown under favourable conditions of light hours and
temperature offered an experimental methane yield of 46 and 52%
for high and low NH4
þ load of that achievable theoretically. Therefore,
biomass grown under favourable conditions supported a
better conversion of microalgae biomass into methane. Likewise,
this value was even higher for biomass grown in semicontinuous
mode under favourable conditions. As it can be seen in Fig. 5B, this
biomass achieved the highest methane yield, reaching almost 60%
of the theoretical methane yield achievable according to the
macromolecular distribution of this biomass. Overall, it can be
concluded that even though non favourable conditions increased
the lipid content which in principle would entail high specific
methane yield, the fact is that experimentally these biomasses
exhibited the lowest bioconversion into biogas. Furthermore,
semicontinuous cultivation mode supported higher methane yield
at the expenses of a biomass enriched in proteins. These results
confirmed the need of further testing promising results not only in
semicontinuous cultivation mode but also evaluating the biomass
generated in continuously operated anaerobic digester to fully
asses the potential of this new feedstockง