Breaking the barriers towards large-scale microalgae-based bio-hydrogen production

Microalgae-based biohydrogen (MaBHP) can couple CO 2 mitigation with renewable fuel generation and wastewater remediation, yet deployment is limited by low light-to-H 2 efficiencies and high cultivation and processing costs. This review maps scale-up barriers across cultivation, H 2 induction, and purification, and prioritizes strategies with demonstrated cost or yield impact toward industrial feasibility. The review synthesized quantitative evidence (2000–2025) from techno-economic and life-cycle studies and pilot demonstrations covering wastewater integration, flue-gas CO 2 utilization, immobilized cultivation, hybrid ORP–PBR operation, and biorefinery co-products. Results showed that cultivation dominates the process cost: typical biomass costs are $3.54–$5.78/kg in tubular PBRs versus $3.42–$4.13/kg in ORPs; an automation/modularization case decreased microalgae production cost from $89 to $16/kg at ∼200 t/yr. Today, MaBHP via biophotolysis remains $7.2–$7.6/kg—above green electrolysis ($5–$7/kg) and grey/blue SMR ($1–$3/$1.6–$3.5/kg). Integration levers show tangible gains: secondary-treated wastewater enabled Chlorella growth with 76 % NH 4 + removal and 53 % lipid accumulation; the spent medium yielded 200.8 μmol H2 /mg chlorophyll.a in cyanobacteria; swine-wastewater loops cut freshwater use six-fold with 45.5 mL H2 /g VS ; alginate immobilization raised H 2 ∼40 % (to 2.4 L H2 /L culture ) over five reuse cycles. A CSTR nutrient-recovery line on digested Scenedesmus recovered 68 % N and 72 % P via struvite, reducing synthetic fertilizer ∼35 %; flue-gas CO 2 (12 % v/v) lifted biomass 22 % and reduced carbon-supplement cost 86 %. The results show that combining wastewater/nutrient circularity, CO 2 co-utilization, oxygen/electron-flow control, high-A/V reactors with automation, and co-product valorization can narrow the cost gap and orient MaBHP toward future $1–$2/kg benchmarks. • Scalability of Microalgae-based biohydrogen (MaBHP) production is discussed. • Co-culturing boosted MaBHP up to 16 time compared with monocultures. • Automation and modular design cut cultivation costs from $89/kg to $16/kg. • Biorefinery co-products offset ∼35 % of OPEX, improving overall MaBHP feasibility. • Scavengers and cocultures help reducing Oxygen-sensitive hydrogenases in MaBHP.