Anaerobic digestion of microalgae residues: toxicity by ammonia

Abstract

Microalgae are considered a valuable resource for extraction of different high value metabolites as well as for biofuels production. This study aims to evaluate microalgae residues as substrate for biogas production. These residues were generated as a result of processing microalgae by enzymatic hydrolysis and subsequent amino acids extraction. Biochemical methane potential assays of microalgae residues and raw microalgae were performed. Moreover, a semi-continuous assay was carried out to determine possible inhibition processes and optimum OLR and HRT when microalgae residues were digested. Microalgae residues increased its potential biogas and methane productions compared to raw biomass by 126% and 94.4%, respectively, up to 401.2±17.7 Lbiogas kgVS-1 and 272.8±7.3 LCH4 kgVS-1. During the semi-continuous assay two different OLR were tried. Highest biogas and methane productions obtained were 559.4±8.7 Lbiogas kgVS-1 and 283.9±63.5 LCH4 kgVS-1, at 1.1gVS L-1 d-1 and 118 days HRT. Toxicity problems were observed due to high ammonia and VA concentrations at higher OLR. Microalgae residues can be used for anaerobic digestion at low OLR, otherwise co-digestion with high carbon content substrates is necessary.