Coupled wastewater treatment and CO₂ mitigation by microalgal (Chlorella Vulgaris) cultures

Thesis Type: Postgraduate

Institution Of The Thesis: Middle East Technical University, Graduate School of Natural and Applied Sciences, Earth System Science, Turkey

Approval Date: 2017

Thesis Language: English

Student: Direniş Çaylı

Principal Consultant (For Co-Consultant Theses): Göksel Niyazi Demirer

Co-Consultant: Tuba Hande Bayramoğlu


Eutrophication, ecosystem damage and poor water quality predominantly are among the major problems which are brought about by excess nitrogen and phosphorus discharged to receiving environments by different wastewaters. Nutrient (mainly nitrogen and phosphorus) removal from wastewaters is still an unsolved problem in many countries, including Turkey. For example, the ratio of wastewaters which are subjected to tertiary (advanced) wastewater treatment is around 38.3% (Turkish Statistical Institute, 2012). Therefore, there has been an increasing interest for seeking better and more feasible nutrient removal techniques. Microalgal cultures have been effectively used in nutrient removal from wastewaters for a long time. Global warming has reached to an alarming level because of the elevated CO2 levels in the atmosphere. United Nations promoted Kyoto Protocol which is an international commitment to reduce carbon emission of more than 170 countries. Like nutrient removal, there are different physical, chemical, and biological technologies to mitigate CO2. One of the biological methods for CO2 mitigation is the algae based CO2 uptake. This research has focused on investigating the possibility of parallel nutrient removal from domestic and industrial wastewaters and CO2 mitigation from industrial flue gas. The aim of this study is to understand (i) the tolerance of high CO2 levels by microalgae, (ii) removal rate of nutrients from wastewater, (iii) the amount of CO2 biotransformed by microalgae, and (iv) the performance of microalgae cultivation in reactors fed by real flue gas collected from iron and steel industry. In this study, batch and fed-batch reactors were operated for investigation of nutrient removal efficiency of Chlorella vulgaris. Maximum N and P removal efficiencies were achieved in these reactors between 82% and 99% at the end of experiment. CO2 biofixation rates were normalized dry biomass of microalgae. The results indicated that CO2 biofixation performance of these reactors were in the range of 6.44 and 0.32 g CO2/TS. The results of this study indicated that microalgal biotechnology is a feasible alternative for integrated nutrient removal and CO2 biofixation applications.