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 <i>Arthrospira platensis</i> 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 <i>A. platensis</i> 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.