Reclamation of Nutrient Solution from Membrane-Based Microalgal Harvesting Processes for Cultivation of Vegetables in Hydroponic Systems

Abstract

This study investigates membrane-based microalgal harvesting processes for simultaneously concentrating microalgal cells and reclaiming liquid nutrients for cultivating vegetables in hydroponic systems. During both gravity-driven (with microfiltration and ultrafiltration membranes) and crossflow (with microfiltration membranes) filtration, it was found that cake layer fouling was predominant, regardless of microalgal solution properties, membrane type, and filtration condition. Although the gravity-driven membrane process offered lower operation cost, the water productivity was limited due to its lower fluxes (<1 L/m2h). During crossflow microfiltration of the microalgal solutions, increasing periodic water (50 °C) flushing frequency helped in alleviating both irreversible fouling and cake fouling development, especially with an extending filtration cycle. Under the same water productivity condition, optimal periodic water flushing (50 °C, 5 min duration per 15 min filtration) displayed more effectiveness in cake fouling control compared to periodic ultrasonication (60 s per 15 min filtration). As the collected permeate contained phosphorus and nitrogen, it was utilized as a potential cultivation solution to grow lettuce and basil in hydroponic systems. Both plants cultivated with the permeate grew slower than those with the commercial fertilizer solution, possibly associated with the lack of some essential microelements and possible uptake of certain heavy metals. The targeted hazard quotient (THQ) values of all examined heavy metals in the edible part (stem and leaves) of basil and lettuce were 0.79 and 1.58, respectively, and comparable to those with the fertilizer solution (0.81 for basil and 1.35 for lettuce). This implies that the basil cultivated with the reclaimed microalgal nutrient solution did not pose a potential adverse health effect on human consumption.