High cell density culture of Neochloris oleoabundans in novel horizontal thin-layer algal reactor: Effects of localized aeration, nitrate concentration and mixing frequency

Abstract

A horizontal thin-layer algal reactor was designed, constructed and tested for high cell density culture of an oil-rich green alga, Neochloris oleoabundans, at a thickness of 6 mm. A hollow fiber membrane sparger was used for localized aeration with CO2-enriched air, which allowed control of culture pH at a narrow range of pH 7.0–7.5. The optimal nitrate concentration under the investigated conditions was 345 mg-N/L with a cell density of ∼6.6 × 108 cells/mL and biomass concentration of ∼8 g/L. The volumetric and areal biomass productivities were 1.81 g/L/day and 10.85 g m− 2/day, respectively, and the maximum volumetric and areal productivities 3.57 g/L/day and 21.42 g m− 2/day, respectively. Intermittent mixing at a frequency of once every two hours (five min mixing each time) produced a cell growth profile similar to that of continuous mixing. Such intermittent mixing resulted in 98.38 % saving on mixing energy and 33.9 % reduction in water evaporation. It was observed that metabolic stress such as high pH and high nitrate concentration could inhibit cell division while causing cell size to enlarge abnormally. The results indicate that it is technically feasible to have high cell density culture of photoautotrophic microalgae with intermittent mixing and no culture circulation, opening the door towards the development of open algal farming systems of low capital, operational, and maintenance costs.