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

Simone Rossi; Davide Carecci; Francesca Marazzi; Francesca Di Benedetto; Valeria Mezzanotte; Katia Parati; Davide Alberti; Ignazio Geraci; Elena Ficara

doi:10.1016/j.heliyon.2023.e23240

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

Heliyon. 2023 Dec 3;10(1):e23240. doi: 10.1016/j.heliyon.2023.e23240. eCollection 2024 Jan 15.

Authors

Simone Rossi¹, Davide Carecci¹, Francesca Marazzi², Francesca Di Benedetto¹, Valeria Mezzanotte², Katia Parati³, Davide Alberti⁴, Ignazio Geraci⁴, Elena Ficara¹

Affiliations

¹ Politecnico di Milano, DICA - Department of Civil and Environmental Engineering, 2, P.zza Leonardo da Vinci, 20133 Milano, Italy.
² University of Milano - Bicocca, DISAT - Department of Earth and Environmental Sciences, 1, P.zza della Scienza, 20126 Milano, Italy.
³ Istituto Sperimentale Italiano Lazzaro Spallanzani, Aquaculture division, 26027 Rivolta d'Adda, Italy.
⁴ A2A s.p.a., 230, v. Lamarmora, 25124, Brescia, Italy.

Abstract

The integration of microalgae cultivation in anaerobic digestion (AD) plants can take advantage of relevant nutrients (ammonium and ortho-phosphate) and CO₂ loads. The proposed scheme of microalgae integration in existing biogas plants aims at producing approximately 250 t·y^-1 of microalgal biomass, targeting the biostimulants market that is currently under rapid expansion. A full-scale biorefinery was designed to treat 50 kt·y^-1 of raw liquid digestate from AD and 0.45 kt·y^-1 of CO₂ from biogas upgrading, and 0.40 kt·y^-1 of sugar-rich solid by-products from a local confectionery industry. An innovative three-stage cultivation process was designed, modelled, and verified, including: i) microalgae inoculation in tubular PBRs to select the desired algal strains, ii) microalgae cultivation in raceway ponds under greenhouses, and iii) heterotrophic microalgae cultivation in fermenters. A detailed economic assessment of the proposed biorefinery allowed to compute a biomass production cost of 2.8 ± 0.3 €·kg DW^-1, that is compatible with current downstream process costs to produce biostimulants, suggesting that the proposed nutrient recovery route is feasible from the technical and economic perspective. Based on the case study analysis, a discussion of process, bioproducts and policy barriers that currently hinder the development of microalgae-based biorefineries is presented.

Keywords: Anaerobic digestion; Biorefinery; Heterotrophic and autotrophic microalgae cultivation; Mathematical modelling; Resource recovery; Techno-economic assessment.