Potential of algal biofuel production in a hybrid photobioreactor

Abstract

Hybrid bioreactors are equipment featuring characteristics of pneumatic bioreactors and stirred tanks. The production of lipids and carbohydrates was evaluated in a concentric draft tube stirred airlift photobioreactor. Mixotrophic cultivation was carried out using the microalgae Chlorella vulgaris grown in vegetable waste. The response surface methodology was used for the experiment optimization. A central composite rotatable design was used to evaluate the effects of light intensity, CO2 concentration and stirring (independent variables) on the lipid and carbohydrate production. The exponential phase of growth occurred mainly in the first seven days of cultivation, and the stationary phase was achieved under all experimental conditions (occurring between the 9th and 14th day). The increase in cellular concentration was obtained mainly in experiments performed with high light intensity and CO2 concentration. At a rotational speed of 800rpm, growth was affected due to excessive shearing forces caused by increased turbulence. On the other hand, in the absence or at low CO2 concentrations there was a considerable increase in lipid and carbohydrate production. Through response surfaces, we observed that higher percentages of these metabolites may be obtained without adding CO2 and operating the bioreactor with high light intensity and with rotation speed higher than 800rpm or at low rotational speeds as well as conventional airlift. From an economic point of view, stirring can cause an increase in power consumption; however, if the bioreactor is operated at low rotational speeds, the difference in power consumption with airlift bioreactors is minimal, while the productivity of lipids and carbohydrates increases, which makes the use of this equipment very attractive.