3.4. Methane production using the harvested biomass
The biomasses harvested at the end of the batch experiments
and the semi-continuous operation were subjected to AD to study
the effect of illumination periods, temperature and NH4
þ load on
methane yield. Final methane yields ranging 106e146 mL CH4 g
COD in1 were obtained for the microalgal biomasses obtained
from the batch experiments (Fig. 5A). This yieldwas increased up to
171 ± 6 mL CH4 g COD in1 for the biomass cultivated in semicontinuous
mode (Fig. 5B). Taking into account that total anaerobic
degradation corresponds to methane yield of 350 mL CH4 g COD
in1, biodegradabilities were in the range of 30e42% and 49% for
batch and semi-continuous, respectively. This value is in good
agreement with other reported values related with anaerobic biodegradility
of fresh microalgae biomass [28].
The theoretical methane yield was calculated taking into account
the specific methane yields for the different macromolecules
(0.415 for carbohydrates, 0.496 for proteins and 1.014 mL CH4 g VS
in1 for lipids [32]). In order to compare the theoretical values with
the experimental results, the experimental methane yields
(expressed in mL CH4 g COD in1) were converted to mL CH4 g VS
in1 using an estimated ratio COD/VS of 1.8, corresponding to
microalgae biomass [33].When methane yield was correlated with
the macromolecular composition of the biomass subjected to AD,
the results showed theoretical methane yields ranging from 505 to
681 mL CH4 g VS in1. More precisely, the upper value (681 L CH4 g
VS in1) corresponded to the biomass collected under nonfavourable
conditions at high ammonium load. Nevertheless, this
biomass exhibited the lowest value experimentally, and thus a
mismatch of 72% between the theoretical and the experimental