Biogas production and biomolecule degradation from Chlorella vulgaris using a lagoon-digester granular sludge versus...

Anaerobic digestion of microalgal biomass is a promising route for biogas production, but its efficiency is constrained by algal properties and by the biomolecule degradation kinetics of the microbial consortia used as inoculum. This study provides the first evaluation of biogas production and biomolecule degradation kinetics by a spontaneously formed granular sludge from an industrial lagoon digester, using Chlorella vulgaris biomass as substrate, in comparison with a silty sludge naturally adapted to the presence of microalgae (chlorophytes). Granular sludge initiated methanogenesis without a lag phase and showed higher "affinity" for the substrate (lower apparent Km) and more rapid protein hydrolysis than silty sludge, yet yielded less biogas overall (∼560 mL·g VS^-1 vs. ∼650 mL·g VS^-1, at STP); methane content was ∼70 % (v/v) in both systems. This partial decoupling between biomolecule hydrolysis and methanogenesis could reflect partial retention of metabolized carbon within microbial biomass and the extracellular polymeric substances matrix, rather than full allocation to methanogenesis. Degradation dynamics of the microalga differed markedly: granular sludge showed more rapid protein hydrolysis and progressive pigment loss, whereas silty sludge displayed a higher degradation rate of chlorophylls. Spearman correlations linked biogas production to biomolecule decline, and analysis of covariance confirmed significant differences in degradation rates between sludge types. Taken together, the findings show that inoculum choice defines the process strategy: the granular sludge to intensify biomolecule degradation and the silty sludge to optimize biogas production. This study confirms that an industrial lagoon digester can provide granular sludge applicable to anaerobic digestion of microalgal biomass.