(168a) Potential Use of Mixed Indigenous Microalgae for Carbon Dioxide Bio-Fixation and Advanced Wastewater Treatment

In the near future, improving the overall efficiency of the Carbon Capture and Storage (CCS) chain will become increasingly important, as the implementation of the various CO₂ existing capture methods from flue gas are not currently economically feasible. On the other hand, it is to be expected that CO₂ will become available from a multitude of different sources. For the GCC (Gulf Cooperation Council) this will include power plants, refineries, various petrochemical processes (methanol, ammonia), and will encompass gas separation at processing plants and at LNG production facilities. CO₂ has to be removed from flue gas (CO₂-N₂) and from natural gas (CO₂-CH₄) mixtures.

The current average costs of CO₂ capture and storage, based on chemical absorption, are estimated to be at least 55 US$ per ton of CO₂ taking a coal-fired power plant as a reference. In the absorption process, the solvent regeneration step accounts for approximately 75% of the energy consumption (and thus costs). The high energy penalty, which for power plants can be as high as 10% (based on the overall power plant efficiency of 35-45%), is one of the main drivers for research into more economically viable, and ultimately sustainable, approaches to be included in the full CCS chain.

One of the emerging approaches is based on so-called CO₂ utilization, in which CO₂ is considered as a feedstock for, for example, biological systems. Of such approaches the one which has garnered the most interest is the use of micro-algae for the biofixation of CO₂, in combination with the production of a range of bioproducts. In a number of papers an overview is given of the amount of CO₂ that can converted by different types of algae. The main way to identify suitable algae is based on the so-called CO₂ fixation rate, the amount of CO₂ converted in mg (milligram) of CO₂ per day per unit of volume (in liters).

At the same time, with the increase on world population the high volumes of wastewater generated by human was always a permanent problem that requires appropriate treatment. While domestic wastewater treatment plants (WWTPs) is considered an effective treatment for removal of biochemical oxygen demand (BOD), suspended solids and bacteria, the removal of inorganic nutrients, still limited and challenging in these processes. With the implementation of the new wastewater discharge regulation, the removal of nutrients, mainly dissolved nitrogen and phosphorus, is becoming one of the essential requirement for the approval of the WWTP performance. The new regulations were based on the fact that the presence of these nutrients in wastewater will leads to eutrophication by stimulating the growth of unwanted plants such as algae and aquatic macrophytes, and increase the effluents toxicity to fishes and aquatic organisms. Moreover, untreated nutrients in wastewater effluent reduce the performance of disinfection step and increase the chlorine demand. As a result, there is a big need to find a treatment process that can remove these nutrients before the effluents are discharged.

So far, limited attention has been paid to the dual role of microalgae as advanced technology for treatment of different kinds of wastewater and as CO₂ bio-fixation technology. Moreover, there are knowledge gaps in understanding the performance of different microalgae strains in the treatment of wastewater and their capacity for CO₂ capturing. In particular, according to our literature review, no studies were carried out using mixed indigenous microalgae cultures as opposed to lab-grown pure culture strains. This knowledge is needed to select and match the right type of microalgae with the right type of wastewater, which is crucial in improving the operation and performance of microalgae-CO₂-biofixiation- wastewater treatment systems

This study evaluated the use of mixed indigenous microalgae (MIMA) as carbon dioxide bio-fixation technology and advanced wastewater treatment. The study follows the growth rate of MIMA, removals of organic matter, removal of nutrients from wastewater and its effectiveness as CO₂ capturing technology from flue gas. A noticeable difference between the growth patterns of MIMA was observed at different CO₂ and different operational temperatures. MIMA showed the highest growth grate when injected with CO₂ dosage of 10% and limited growth was observed for the systems injected with 5% and 15 % of CO2 at 30 â—¦C. Ammonia and phosphorus removals for MIMA were 69%, 75%, and 83%, and 20%, 45% and 75 % for the media injected with 0, 5 and 10% CO₂. The results of this study show that simple and cost-effective microalgae-based wastewater treatment systems can be successfully employed at different temperatures as a successful CO₂ capturing technology even with the small probability of inhibition at high temperatures