Photobioreator technology for carbon capture and nutrients removal

Abstract

Carbon dioxide concentration in the atmosphere is increasing significantly worldwide. Many are debating the influence of increasing carbon dioxide concentration on global climate, but most scientists agreed that the increasing carbon dioxide concentrations will have a deep effect on the environment. Most of the carbon dioxide results from combustion of fossil fuels to fulfill the increasing demand for energy. Meeting this demand without significantly increasing the Carbon dioxide emissions will require more than the conventional carbon capture and storage techniques. There is growing recognition of microalgae as one of the most efficient biological systems to capture industrial CO2 and produce biomass (bio-fuel) at the same time. Algae also has the potential to remove nutrient from wastewater such as nitrogen and phosphorus. Green algae utilize carbon dioxide in their main building blocks in the photosynthesis process, which means that algal have a high potential for CO2 capture and sequestration. Algae can also produce high value products which can boost the revenues to overcome the relatively expensive microalgae culturing. Algae requires sunlight and CO2 to perform the photosynthesis process, to maximize the energy stored in algae and increase the growth rate of algae a large amount of CO2 is required which is available from the discharge of heavy industries. Algal production does not require a high purity CO2 stream, flue gas containing different CO2 concentrations can be fed directly to the photo-bioreactor which will make the CO2 separation from the flue gas much easier and less expensive. The objectives of the study were (1) evaluate the capability of algae to capture CO2 from gaseous streams at different concentrations [5, 10and 15v/v%] and different temperatures [20, 25, 30◦C], (2) ability of algae to remove nutrient from secondary effluent wastewater under different temperatures [25 and 30◦C], and CO2 concentrations [5% and 10%]. Experiments were carried out in lap-scale and pilot scale set up. Lab-scale results showed that the maximum growth rate, biomass productivity and CO2 bio-fixation rate for Spirulina platensis (SP.PL) were obtained at temperature of 25◦C for culture injected with 10 v/v% CO2. Under these conditions, growth rate, biomass productivity and CO2 bio-fixation rate were determined to be 0.772 d-1, 0.15 g.L-1.d-1 and 0.281 g.L-1.d-1, respectively. These values are higher than the values reported in literature for green algae strains grown under similar conditions. Higher growth rate, biomass productivity and CO2 bio-fixation rate were obtained in the experiments carried out using natural solar light in pilot plant PBR. SP.PL under the same previous conditions (25◦C and 10% CO2 injection) was able to achieve biomass productivity and CO2 biofixation rate of 0.153 g.L-1.d-1 and 0.281 g.L-1.d-1, respectively. Experiments carried out to study the performance of SP.PL in removing nutrients from wastewater showed a typical algae growth rate under both temperatures (25 and 30◦C) and CO2 injection dosage (5 and 10%). The growth of algae in wastewater was observed to have lag phase up to 7 days followed by an exponential growth phase. Decay or stationary phase was not observed under the tested operational conditions. Ammonia removals by SP.PL for experiments performed at 25 ◦C and with CO2 injection of 0, 5 and 10 % were 94.5, 92.4 and 84.5%, Respectively. The % phosphorous removals for the same previous conditions were 94.8, 89.3 and 84.2%, respectively. The results of this study show that microalgae-based wastewater treatment systems can be successfully employed at different temperatures as a successful CO2 capturing technology and post-wastewater treatment process