Integrating microalgae growth in biomethane plants: Process design, modelling, and cost evaluation

Area of Science:

• Biotechnology and Bioengineering
• Environmental Science
• Sustainable Chemistry

Background:

• Anaerobic digestion (AD) plants generate nutrient-rich digestate and CO2, ideal inputs for microalgae cultivation.
• The biostimulant market is rapidly expanding, creating demand for sustainable and cost-effective biomass sources.
• Microalgae can efficiently utilize waste streams, offering a circular economy approach to resource management.

Purpose of the Study:

• To design and assess a full-scale biorefinery integrating microalgae cultivation with anaerobic digestion.
• To evaluate the technical and economic feasibility of producing microalgal biomass for the biostimulant market.
• To identify barriers hindering the development of microalgae-based biorefineries.

Main Methods:

• A three-stage microalgae cultivation process was designed: inoculation in tubular photobioreactors (PBRs), cultivation in raceway ponds, and heterotrophic cultivation in fermenters.
• The biorefinery was designed to treat liquid digestate, CO2 from biogas upgrading, and sugar-rich by-products.
• Economic assessment was performed to calculate biomass production cost.

Main Results:

• The proposed biorefinery can produce approximately 250 t·y⁻¹ of microalgal biomass.
• Biomass production cost was calculated at 2.8 ± 0.3 €·kg DW⁻¹, compatible with biostimulant production costs.
• The nutrient recovery route was deemed technically and economically feasible.

Conclusions:

• Microalgae cultivation integrated with AD plants is a viable strategy for nutrient and CO2 recovery.
• The designed biorefinery demonstrates a cost-effective method for producing microalgal biomass for biostimulants.
• Addressing process, bioproduct, and policy barriers is crucial for advancing microalgae biorefineries.