Membrane assisted algae cultivation for high productivity and nutrient recovery in horticultural drain water

The excessive use of fertilizer in horticulture leads to high-strength drain water, posing environmental risks in the form of nitrate and phosphate pollution. Conventional treatment options often fail to achieve adequate removal and lack mechanisms for sustainable nutrient recovery. This study presents a membrane-assisted microalgae-based process designed to efficiently reduce nitrate and phosphate levels while supporting high biomass productivity. By decoupling the hydraulic retention time (HRT) from the solid retention time (SRT), the system enables continuous nutrient removal to very low concentrations without impairing algal productivity. A growth model incorporating light and nutrient limitation was used to identify optimal operational parameters and predict system performance. Laboratory-scale trials in a 1.7 L flat panel photobioreactor demonstrated near-complete nutrient recovery of 97 % for NO 3 -N and 99 % of PO 4 -P, thereby meeting legal discharge standards, while maintaining consistent algal productivity of 0.7 g L −1 d −1 . The process was then scaled up to a tubular bioreactor with a liquid volume of 220 L with real greenhouse drain water and fluctuating outdoor light conditions in addition to artificial LED light. In this experiment, continuous nutrient recovery efficiencies of 91 % and 88 % were reached for NO 3 -N and PO 4 -P respectively, at a volumetric productivity of 0.29 g L −1 d −1 . These findings show that membrane assisted microalgae cultivation can effectively mitigate nutrient pollution, while providing valuable algal biomass for applications in the bioindustry, offering a much-needed scalable alternative for sustainable drain water management. Design and scale-up of the membrane assisted algae cultivation system. • Decoupled retention times enable efficient nutrient removal at high productivity. • Recovery of 97 % NO3 and 99 % PO4 in continuous trials, meeting EU discharge limits • Scalability to a 220 L pilot-scale system with real drain water was demonstrated.