Water electrolysis-based microalgae cultivation in wastewater for nutrients removal and carbon capture

Abstract

In the recent years, the implementation of microalgae cultivation as part of wastewater treatment has received a growing interest as means to tackle global water crisis issues while recovering nitrogen and phosphorous contained in wastewater to produce valuable biomass. This approach has also been coupled to biologically fix the ever-increasing CO2 emitted from industrial flue gas, and has been widely implemented as part of carbon capture, utilization, and storage (CCUS) technology. Water electrolysis-based microalgae cultivation system, which has a proven ability to control the pH of algae culture medium and continuously supply bicarbonate as carbon source, was adapted and its performance in improving wastewater treatment was confirmed. As a result of the experiment using the domestic sewage from Daejeon Wastewater Treatment Plant (WWTP), biomass production rate could achieve over than 3 times higher than that of the control group with final biomass concentration of 1.46 g L–1. Bicarbonate was successfully supplemented into the algae culture medium (approximately 76 g L–1 d–1). Additionally, nitrogen and phosphorous was effectively removed with final concentration of TN and TP was 3.3 mg L–1 and 0.3 mg L–1, respectively. These concentrations were able to satisfy the discharge water quality standards. Next, the potential of electrochemical CO2 absorption using sodium hydroxide as absorbent was also confirmed. It was revealed that by applying a sufficient cell voltage to enable continuous OH– generation and maintain high enough critical pH, a target removal of CO2 up to above 90% could be achieved, which is desirable for CCUS technology. Furthermore, bicarbonate produced from the CO2 absorption could effectively be utilized as a mixed solution for algae culture medium since it could yield a high biomass productivity as long as pH is controlled.