Valorisation of dairy industry by-products through mixotrophic and heterotrophic cultivation of Galdieria sulphuraria: a techno-economic perspective

The valorisation of industrial by-products represents a key strategy for improving the sustainability and economic viability of biotechnological processes. In this context, the thermo-acidophilic microalga Galdieria sulphuraria is of particular interest because of its metabolic versatility and its ability to grow under both heterotrophic and mixotrophic conditions. This thesis investigates the use of dairy industry by-products, namely milk permeate and second cheese whey (SCW), as substrates for the cultivation of G. sulphuraria, combining laboratory-scale experimentation with process design and preliminary techno-economic assessment. The experimental work focuses on the continuous cultivation of G. sulphuraria on milk permeate under heterotrophic and mixotrophic conditions. In parallel, milk permeate and SCW are characterised in terms of carbon, nitrogen, and phosphorus content to support process calculations. Under the tested conditions, heterotrophic cultivation achieves a biomass productivity of 2.83 ± 0.05 g/(L d), whereas the highest productivity obtained under mixotrophic conditions is 1.56 ± 0.16 g/(L d), indicating that heterotrophic cultivation is more favourable for biomass production under the investigated conditions. The experimental results are then used as the basis for conceptual plant design, mass and energy balances, process simulation, and techno-economic assessment. Three process configurations are evaluated: a large-scale heterotrophic process based on milk permeate, a small-scale heterotrophic process based on SCW, and a sequential small-scale heterotrophic and mixotrophic process based on SCW with C-phycocyanin recovery. Among the analysed scenarios, the large-scale heterotrophic permeate configuration proves to be the most favourable, with a biomass production of 453 kg/h and an NPV of 65.5 M€. By contrast, the small-scale heterotrophic SCW scenario is not economically feasible in its current form, with an NPV of −0.15 M€. However, the sequential heterotrophic and mixotrophic SCW configuration improves the economic performance by enabling the recovery of 0.18 kg/h of C-phycocyanin as a higher-value product and reaches an NPV of 0.83 M€. Overall, the results show that dairy by-products can be valorised through the cultivation of G. sulphuraria, although the final outcome depends strongly on cultivation regime, process scale, and product portfolio.