Carbon dioxide removal from air by microalgae cultured in a membrane-photobioreactor

Elevated CO2 levels in a closed space or room are of big concerns in many situations. Controlling the CO2 level within a certain range is one of the most important tasks in a life support system. In this paper, a 10 l photobioreactor integrated with a hollow fiber membrane module was constructed to remove CO2 from air by using the photosynthetic microalga, Chlorella vulgaris. The effects of the inlet CO2 concentration and the introduction of the membrane module on microalgal CO2 fixation were investigated. The results showed that the proper inlet CO2 concentration was determined to be 1.0%, based on the description of the growth characteristics of the microalga, and the gas exchange efficiency was improved greatly when the membrane module was adopted. Compared with an ordinary photobioreactor, not only the retention time of the smaller and more uniform gas bubbles in the new membrane-photobioreactor increased from 2 s to more than 20 s, but also the dissolved oxygen (DO) dropped by a factor of 30, resulting in the enhancement of the CO2 fixation rate from 80 to 260 mg l−1 h−1. When the operating conditions were controlled at cell density of 2.0 × 107 cells ml−1, inlet gas flow rate of 3 l min−1, and light intensity of 157.6 μEm−2 s−1 at 25–30 °C, the 1.0% (v/v) CO2 in the input aeration gas could be reduced to 0.3% in the discharged gas. Using normal room air (0.04% CO2) as feed, the CO2 concentration in the discharged gas could be decreased to the boundary value of 0.015%, indicating that the novel membrane-photobioreactor by intensifying the process of CO2 conversion and fixation during the microalgal photosynthesis may be a promising solution to CO2 removal in a closed space or room.