Hydrolysis of photobioreactor-derived mixed bacterial biomass for recovery of peptides and fermentable sugars: Comparison with waste activated sludge

Biomass produced in anaerobic photobioreactor inoculated with purple phototrophic bacteria (PPB) for wastewater treatment represents a promising source of proteins and carbohydrates for valorization within wastewater-based biorefineries. This study evaluated and compared the hydrolysis of photobioreactor-derived mixed bacterial (PMB) biomass and conventional waste activated sludge (WAS) for the recovery of peptides and fermentable sugars. Different hydrolysis conditions were applied by varying temperature and residence time, using HCl or NaOH (0.5–1 M). Hydrolysis treatments included mild conditions (50ºC, 180 min) and moderate-to-severe thermal conditions (120ºC, 30/60 min). ANOVA identified treatment (temperature/residence time) and reagent type as significant operational factors (p<0.05) controlling component release and recovery, while reagent concentration showed limited influence. 16S rRNA gene amplicon sequencing revealed that PMB biomass was mainly composed of Gram-negative bacteria (≈72%), whereas WAS was mainly composed of Gram-positive bacteria (≈76%), which was consistent with the different hydrolysis responses observed. Under low-to-moderate conditions, PMB biomass released over 70% of proteins, 60% of carbohydrates, 97.1% of lipids, and 87% of humic substances, while WAS showed higher solubilization under alkaline conditions and required more severe conditions (120ºC, 60 min) to reach similar solubilization levels. Average recoveries reached 67.4% (PMB) and 50% (WAS) for proteins as peptides and amino acids, and 57.1% (PMB) and 52.5% (WAS) for carbohydrates as fermentable monosaccharides. These results highlight PMB biomass as a promising feedstock for wastewater-based biorefinery applications.