Optimizing calcium and magnesium in seawater medium with high bicarbonate concentration for efficient growth and self-flocculation harvesting of Chlorella sp
Xiang Du, Chenba Zhu, Zhanyou Chi
Abstract
Microalgae cultivation using high bicarbonate concentration in seawater medium triggers CaCO 3 precipitation due to pH elevation, contaminating the photobioreactor and reduce biomass productivity. It is necessary to control Ca 2+ and Mg 2+ concentration in appropriate range to allow microalgae rapid growth without precipitation, followed by effective self-flocculation for harvesting. This study optimized this concentration range as 0.5–1.0 mmol L -1 for Ca 2+ and 2.5–5.0 mmol L -1 for Mg 2+ . With this concentration, subsequent pH adjustment to 11 induced > 90 % self-flocculation efficiency. Also, outdoor cultivation in floating photobioreactors utilized treated seawater with optimized Ca 2+ and Mg 2+ concentration achieved productivity of 9.36 g m −2 day −1 , which is 112 % higher than untreated seawater (4.42 g m −2 day −1 ). The optimized process reached the goal of higher biomass productivity without precipitation and efficient harvesting. On the other hand, microalgae-induced precipitation may serve as potential carbon sink, contributing to ocean-negative carbon emissions.