Cultivation of a Starch-rich Microalga Desmodesmus insignis in Dairy Wastewater and Removal of Nitrogen and Phosphorus

Desmodesmus insignis strain JNU24 was cultivated in BG-11 medium containing four different initial NaNO₃ concentrations and dairy wastewater (DWW) diluted into four concentrations to compare its growth, starch production performance, and nitrogen and phosphorus removal capacity in DWW. Results indicated that the optimum nitrate concentrations and DWW concentration for growth were 9.0 mmol/L and 100%, respectively, in which biomass achieved was 6.23 g/L and 10.31 g/L, respectively. Compared with the BG-11 group, however, D. insignis showed a greater ability to grow and accumulate starch when cultivated with 75% DWW, in which the starch content, volumetric starch content, and productivity reached 50.9%, 4.86 g/L, and 405 mg·L^-1·d^-1 respectively. In addition, D. insignis removed 90.8% of nitrogen and 98.7% of phosphorus from DWW. Indoor scaled-up cultivation of D. insignis was carried out in vertical flat-plate glass photobioreactors based on optimum concentrations of BG-11 and DWW. The maximum biomass concentration, volumetric starch content, and productivity of D. insignis were obtained under 75% DWW, reaching 9.75 g/L, 4.75 g/L, and 230 mg·L^-1·d^-1, respectively, which were approximately two times greater than those obtained in the BG-11 group. These results showed that D. insignis could tolerant high nitrate concentrations in DWW, and could be applied on a large scale with cheap media. The D. insignis cells exhibited outstanding settling capacity, as inferred from their ability to settle within 90 min in 75% DWW after stopping aeration. In conclusion, D. insignis is a microalga that has the potential to produce starch and purify wastewater.