Biogas Digestate and Its Electrodialysis Concentrate as Alternative Media Composition for A. platensis Cultivation: A Study on Nutrient Recovery from Dairy Wastewater

The dairy industry generates substantial nutrient-rich wastewater, posing environmental challenges if discharged untreated. This study explores the potential of using the cyanobacterium Arthrospira platensis for nutrient recovery from dairy wastewater, precisely the liquid biogas digestate (BD). The research investigates the feasibility of utilising BD and electrodialysis-concentrated BD (BD concentrate) as alternative media for A. platensis cultivation, with a focus on biomass productivity, nutrient uptake, and high-value product formation. Batch and continuous cultivation modes were employed. In batch experiments, biomass productivity was in the ratio of 0 and 0.27 g L⁻¹ d⁻¹, which was 8–100% lower than simulated values for all five tested media compositions. Phosphate fixation was limited with no fixation during batch cultivation and 8–69% during continuous cultivation, likely due to suboptimal N/P ratios, while ammonium removal remained consistently high (>98%). Phycocyanin yield decreased significantly by 92% at high BD concentrate concentrations compared to standard media. Continuous cultivation with 50% BD concentrate improved biomass productivity to 1.02 g L⁻¹ d⁻¹ and pigment yield to 107.9 mg g⁻¹, suggesting a sufficient supply of nutrients. The findings highlight the potential of BD-based media for nutrient recovery but emphasise the need for optimisation strategies, such as nutrient supplementation and microbial adaptation, to enhance performance.