Marra, Luigi (2024) Strategies for techno-economic feasibility and sustainability of microalgae industrial applications. [Tesi di dottorato]

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Abstract

Despite significant investment, commercial microalgae production remains limited to a few high-value products like Omega-3 fatty acids and astaxanthin, with economic and environmental challenges persisting. Microalgae’s versatility in producing lipids, proteins, pigments, and bioactive compounds makes them promising for various industries, but market adoption is hindered by high nutrient costs, suboptimal cultivation methods, and a narrow focus on a few strains. Biorefinery approaches, which extract multiple products from a single biomass, are gaining traction as a way to enhance economic feasibility and sustainability. This PhD thesis investigates strategies to integrate microalgae into large-scale industrial processes by addressing techno-economic and sustainability challenges. It emphasizes the importance of strain selection, operating conditions, and substrate optimization in improving productivity and key sustainability metrics, including water and carbon efficiency. The work highlights the potential of microalgae in diverse sectors such as pharmaceuticals, nutraceuticals, and biofuels, while underlining the need for continued research to overcome technological, regulatory, and market barriers for broader commercialization. A first study analysed 28 strains of Chlorophyta, belonging to six species, cultivated in closed photobioreactors under uniform semi-continuous conditions. The strains were grouped into two categories: those with a "common kinetic behavior," showing a linear relationship between growth rate and nitrogen uptake, and "extreme strains," which deviated significantly. The latter group exhibited higher growth rates relative to nitrogen uptake but lower protein fractions, suggesting that nitrogen uptake could limit protein metabolism. This highlights potential constraints on protein accumulation and cell duplication. Metabolite analysis revealed that as growth rates increased, protein fractions remained constant, lipid fractions increased, and carbohydrate fractions peaked. Optimizing metabolite production requires balancing metabolite fractions with productivity to enhance process sustainability. Two strains stood out: C. saccarophylum (ACUF 050), characterized by high protein and lipid production rates, and C. vulgaris (ACUF 058), notable for a high carbohydrate production rate (0.25 g/L/day) and a 42% accumulation rate. The thesis demonstrated significant advancements in the continuous autotrophic cultivation of Galdieria sulphuraria within a flat-panel photobioreactor, optimizing biomass and metabolite productivity. The alga exhibited resilience to photon flows up to 600 molPh/m², achieving a maximum biomass concentration of 1.65 g/L at a CO₂ mole fraction above 1%v. Optimal productivity (0.1–0.2 g/L/day) was reached at 250 molPh/m², where light intensity influenced metabolite profiles: lipid accumulation increased, while protein and chlorophyll levels decreased. Adaptation to light variations involved pigment ratio adjustments, including reduced chlorophyll A and stable carotenoids and phycobiliproteins. Phycobiliprotein purity was identified as a key factor for productivity. Integration into a biorefinery process, modelled using ASPENPLUS® yielded sustainability and productivity insights. Autotrophic cultivation achieved a biomass flow of 1.2 g/h with optimal CO₂ fixation at 1% CO₂, while mixotrophic conditions maximized biomass production at 2.4 g/h but increased CO₂ emissions. Water reuse strategies reduced cultivation losses, achieving 98% overall water removal efficiency and producing algal pastes with concentrations of 22–70 g/L. Downstream processing optimized phycobiliprotein purification using low-energy ATPS, recovering over 50% with a purity index of 4.1, suitable for pharmaceuticals. 90% of extracted protein (not phycobiliprotein) quality can be tuned in enzymatic hydrolysis keeping a phycobiliprotein content feasible for food market, while biomass disruption yielded 98% efficiency in extracting proteins and 87% for carbohydrates. Harvesting energy requirements were minimized to 50 kWh/kg with 81% biomass recovery. These findings highlight G. sulphuraria as a promising candidate for high-value metabolite production. Autotrophic cultivation offers reduced carbon and water footprints, while mixotrophic regimes maximize productivity at the cost of CO₂ sustainability. This work establishes a strong foundation for scalable, sustainable microalgal biorefineries. The study demonstrated significant progress in reducing the water footprint of autotrophic microalgae cultivation through the reuse of exhausted culture media. For Pseudococcomyxa simplex, reusing 70% of the medium sustained stable growth and chlorophyll content over three batch cycles, while Chlorella vulgaris showed growth inhibition after the first cycle. In semi-continuous cultivation, purging 30% of the exhausted medium ensured stable operation for 30 days, reducing the water footprint from 1000 to 320 kg per kg of biomass and lowering medium costs to 0.05 €/g biomass. Growth limitations in fully reused media were linked to reduced chlorophyll synthesis, though no adverse effects from accumulated sugars were observed. Using ASPENPLUS® stoichiometric modeling, biomass productivity and substrate fixation efficiency were effectively predicted, highlighting the need for improvements in modeling nitrogen uptake. These results emphasize that optimized medium reuse strategies, with tailored purge ratios and dilution rates, can significantly enhance water and nutrient efficiency, offering a pathway to more sustainable and cost-effective microalgae cultivation systems.

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