Growth parameters of microalgae tolerant to high levels of carbon dioxide in batch and continuous-flow photobioreactors

Microalgae can be cultured in photobioreactors to sequester carbon dioxide and produce potentially valuable biomaterials. The goal of the present study was to identify and utilize microalgal strains that are capable of tolerating up to 20% CO(2) (gas phase) concentrations under variable light or flue-gas blend conditions and reactor configurations to produce biomass. Scenedesmus sp. and Chlorella sp., both cultured from a Sonoran desert mineral spring, grew well and tolerated exposure to a gas mixture containing up to 20% CO(2) applied continuously in batch reactors to the culture. Experiments were conducted with simulated coal-powered acidic flue gases containing SO(x)/NO(x) at concentrations of 200 to 350 ppmV. Microalgae did not grow well without pH control, and high levels (> 250 mM) of nitrite or sulphite in the liquid media inhibited algal growth. Pseudo steady-state experiments were also conducted using helical tubular and flat-plate photobioreactors with continuous flow (water and gas) and with artificial or natural sunlight. With a 2 d hydraulic residence time (HRT), the helical tubular photobioreactor produced 0.50 +/- 0.11 g C d(-1) (0.056 +/- 0.012 g C L(-1) d(-1)) dry-weight cell mass during continuous fluorescent-lamp irradiance and 0.048 +/- 0.018 g C L(-1) d(-1) during 12 h light/darkness cycling. The flat-plate photobioreactor (2 d HRT) produced 0.42 +/- 0.28 g C L(-1) d(-1) with artificial lighting and with natural sunlight; a 4 d HRT produced 0.14 +/- 0.02 g C L(-1) d(-1). Reactor modelling indicated that a threshold of reactor size (i. e. HRT) and reactor depth (path-length of light) exists based upon the optical density of the cells in the water column and their growth rates.