Biokinetic behaviour of Chlorella vulgaris in a continuous stirred bioreactor and a novel circulating loop photobioreactor

Abstract

Capture of CO2 by algae is an important mechanism for removal of this greenhouse gas from the atmosphere. For this reason, Chlorella vulgaris has beeen studied extensively over the years. A high growth rate of the microalgae is essential in order to increase the fixation rate of CO2 through photosynthesis. Though a number of studies have been carried out to optimize growth of C. vulgaris, high growth rates have not been achieved (Kleinheinz & Keffer, 2002).

In this study, a novel circulating loop photobioreactor with enhanced light distribution, gas mass transfer rate and mixing properties was used for the biokinetic study of C. vulgaris. The objective was to study individual effects of light intensity and CO2 concentration on the specific growth rate of C. vulgaris. Studies in this photobioreactor could generate high growth rates of 0.043 h-1. The effect of light intensity on the growth of the C. vulgaris was determined by varying intensity of light to the photobioreactor within a range of 26.945 mW to 431.128 mW, while the flow of air and CO2 were held constant. An increase in light intensity to the photobioreactor resulted in an increase in cell density and variation in the chlorophyll content of the C. vulgaris cells. Enhanced growth rates of C. vulgaris cells in this novel circulating loop photobioreactor could be attributed to the combined effect of the CO2 concentration and the uniform distribution of light throughout the reactor volume.

Interacting effects of dilution rate, light intensity and concentration of carbon dioxide on specific growth rate was studied in an externally illuminated, continuous flow stirred bioreactor. Steady flow of nutrient medium and air flow was maintained in the bioreactor. This study proved that the dilution rate has major significance at high concentrations of CO2 and high intensities of light. At high concentrations of CO2 at 10% (v/v) and 15% (v/v), increasing dilution rate from 0.005 h-1 to 0.01 h-1 could result in increasing growth rate by a factor of 1.45 and 2.16 respectively.