Enhancing hydrogen production and biochar recovery from algal biomass: A novel techno-economic synergism with gelatinous digestate

The fermentation of algal biomass (AB) for H2 production suffers from inefficient acidogenic metabolic pathway activities and the generation of digestate as a source of secondary pollution. This study focuses on the supplementation of AB with gelatinous digestate (GD) rich in hydrolytic enzymes, macro/micro-nutrients, and hydrogen producers, followed by management of the solid fraction digestate for biochar recovery. The dark fermentation of different AB:GD mixing ratios was conducted in batch mode at 20 rpm rotation speed and 32 °C for 320 h. The AB:GD composition of 50: 50 (w/w) exhibited the best hydrogenase enzyme activity of 0.45 ± 0.02 mg MB reduced/min and extracellular polymeric substances (EPS) of 388 ± 23 mg/gVS. Integrating AB with GD facilitated the secretion of hydrolytic enzymes, i.e., α-amylase (128.9 ± 7.7 U/100 mL), protease (217.0 ± 13.7 U/mg), and xylanase (134.0 ± 8.2 U/100 mL), maximizing the conversion efficiencies of carbohydrate and protein into H2 (528 ± 31 mL/g carbohydrate removed and 599 ± 34 mL/g protein removed, respectively). This AB–GD synergistic interaction increased the relative abundances of bacterial populations responsible for the conversion of volatile fatty acids (VFAs) into H2, i.e., Proteobacteria (29.1 %), Firmicutes (17.4 %), Actinobacteria (17.3 %), Chloroflexi (12.9 %), Bacteroidetes (9.4 %), Planctomycetes (8.4 %), and Acidobacteria (2.5 %). The fermented digestate obtained after H2 harvesting was subjected to pyrolysis at 650 °C for 2 h. The produced biochar involved multiple functional groups, 52.3 % crystallinity index, and 0.7 O/C molar ratio, exhibiting a material half-life of <100 yr. Based on the estimated H2 (1.970 mol/mol glucose) and biochar (0.33 g/gbiomass) yields, a waste-to-energy facility was designed, signifying the generation of 49.5 m3 H2/d and 11.8 kg biochar/d from an organic load of 220.5 kg COD/d. The techno-economic estimation of this route revealed profits from H2 and biochar sales and the associated carbon credit revenue with a 6.7-yr payback period. Overall, the AB–GD synergism could maximize green bio-fuel productivity while maintaining a circular economy approach.