Production of the marine microalga Nannochloropsis gaditana in pilot-scale thin-layer cascade photobioreactors using fresh pig slurry diluted with seawater

Abstract

The utilisation of effluents as nutrient sources is an interesting alternative for microalgae production as it reduces production costs and prevents environmental contamination. This work assesses the potential of producing the microalga Nannochloropsis gaditana outdoors using pig slurry as the source of nutrients. The experiments were carried out in a thin-layer reactor operated in semi-continuous mode at dilution rates of 0.30 and 0.40 day−1 during the autumn and summer, respectively. The biomass productivity ranged from 11.6 to 16.2 g·m−2·day−1 in autumn and summer, respectively. The system was effective at removing N-NO3− (0.1–0.2 g·m−2·day−1), N-NH4+ (4.0–5.0 g·m−2·day−1), and P-PO43− (0.15–0.24 g·m−2·day−1) from the pig slurry. The process was also able to reduce the content of pathogenic bacteria, such as total coliforms, sulphite-reducing Clostridia and Salmonella. Despite being able to grow in pig slurry, the use of this nutrient source led to a decrease in biomass productivity when compared to a medium of seawater supplemented with fertilisers. The eicosapentaenoic acid (20:5 n3) concentration in the N. gaditana biomass was 32 g·100 g−1 of total fatty acids, for both autumn and summer, when using pig slurry as the nutrient source. The results suggest that the production of N. gaditana could be coupled with the biological treatment of pig slurry, allowing the recovery of nutrients while producing valuable biomass for the aquafeed industry. The resulting biomass, which has a high market value, complied with the requirements of the aquaculture sector.