Use of highly alkaline conditions to improve cost-effectiveness of algal biotechnology

Abstract

Phototrophic microorganisms have been proposed as an alternative to capture carbon dioxide (CO2) and to produce biofuels and other valuable products. Low CO2 absorption rates, low volumetric productivities, and inefficient downstream processing, however, currently make algal biotechnology highly energy intensive, expensive, and not economically competitive to produce biofuels. This mini-review summarizes advances made regarding the cultivation of phototrophic microorganisms at highly alkaline conditions, as well as other innovations oriented toward reducing the energy input into the cultivation and processing stages. An evaluation, in terms of energy requirements and energy return on energy invested, is performed for an integrated high-pH, high-alkalinity growth process that uses biofilms. Performance in terms of productivity and expected energy return on energy invested is presented for this process and is compared to previously reported life cycle assessments (LCAs) for systems at near-neutral pH. The cultivation of alkaliphilic phototrophic microorganisms in biofilms is shown to have a significant potential to reduce both energy requirements and capital costs.

Robust, high-productivity phototrophic carbon capture at high pH and alkalinity using natural microbial communities

“…High volumetric productivities were observed for a mixed microbial community dominated by a cyanobacterium closely related to P. kuetzingianum (maximum 1022 g m −3  day −1 , average 862.91 g m −3  day −1 ) grown at pH 9, 0.5 mol L −1 total carbonates and 80 µmol photon m −2  day −1 . This high productivity combined with the predicted fourfold lower cost of a high pH, high alkalinity system [9] has the potential to reduce the overall cost of phototrophic carbon capture systems. Fatty acid content and profiles were consistent with previously reported axenic cultures of cyanobacteria.…”

“…Studies haves shown that the cost of carbon dioxide supply can constitute approximately 50% of the cost of biomass production in raceway pond systems [8]. The use of high pH and alkalinity can improve transfer of CO 2 into the culture medium and uncouples CO 2 absorption and biological uptake, leading to lower energy requirements and costs [9–11]. Haloalkaliphilic eukaryotic algae and cyanobacteria can use bicarbonate instead of CO 2 at high pH [12, 13].…”

“…Haloalkaliphilic eukaryotic algae and cyanobacteria can use bicarbonate instead of CO 2 at high pH [12, 13]. Thus, CO 2 could be transferred to the growth medium in a separate trickling filter with low pressure drop and driven by the pH equilibrium, reducing energy requirements and operational costs of the carbon supply [9, 10]. The growth of algae using bicarbonate as a method to sequester carbon dioxide has previously been suggested [9–11].…”

“…Thus, CO 2 could be transferred to the growth medium in a separate trickling filter with low pressure drop and driven by the pH equilibrium, reducing energy requirements and operational costs of the carbon supply [9, 10]. The growth of algae using bicarbonate as a method to sequester carbon dioxide has previously been suggested [9–11]. Growth in biofilms reduces energy and costs of biomass growth and harvesting as compared to suspended cultivation due to the lower cultivation volume and higher biomass densities [14, 15].…”

Direct capture and conversion of CO2from air by growing a cyanobacterial consortium at pH up to 11.2

“…However, with only 400 ppm carbon dioxide in air, such direct capture of CO 2 from air would lead to excessive energy costs and evaporation (Davis, Aden, & Pienkos, 2011). The ability of such media to dissolve a large amount of carbon dioxide as bicarbonate, would also enable implementation of CO 2 capture and growth as two separate unit operations (Sharp, Canon-Rubio, Strous, Bergerson, & De la Hoz Siegler, 2015;van Loosdrecht, Daelman, Strous, Sorokin, & Kruse, 2016), saving costs and energy. Alternatively, as previously proposed, we could avoid an energy-expensive concentration step by using growth media with high pH and alkalinity.…”