Indigenous microalgae consortium for producing safe reclaimed water and bio-fertilisers. Batch tests for replication, scale-up and legislative analysis

This study aimed to assess the potential of using indigenous microalgae to treat sewage and produce safe reclaimed water and bio-fertilisers. Cultivation tests were conducted using sole or mixed sewage streams combining primary effluent (PSE), secondary effluent (SSE), and centrate (CEN) to identify the most suitable substrate. Overall, higher nitrogen removal rates, biomass productivity, and carbon biofixation were observed when microalgae were cultivated in PSE compared to SSE, with values ranging from 2.1 to 12.1 mgN·L⁻¹·d⁻¹, 18 to 52 mgVSS·L⁻¹·d⁻¹ (20–53 mgTSS·L⁻¹·d⁻¹), and 10.7 to 27.0 mgCO₂·L⁻¹·d⁻¹, respectively. Adding centrate improved microalgae performance by increasing the availability of nutrients and inorganic carbon, but when 30 % centrate was added, detrimental effects were observed. The costs of the microalgae-based treatment process ranged from 0.081 to 0.098 €·m⁻³, which is significantly lower than those of conventional activated sludge (CAS) processes. The reclaimed water obtained during the tests demonstrated high-quality in terms of pathogen and metal levels, making it suitable (in Spain) for agricultural, recreational, industrial and environmental uses. However, some legal gaps were identified regarding the use of reclaimed water and microalgae biomass for bio-fertiliser production.