A review on the occurrence, fate and removal of steroidal hormones during treatment with different types of constructed wetlands

The performance of four types of constructed wetlands (CWs) namely, free water surface CW (FWSCW), horizontal flow CW (HFCW), vertical flow CW (VFCW), and hybrid CW (HCW) was comprehensively and critically evaluated for the removal of steroidal hormones (SHs) based on peer-reviewed literature. The average removal efficiency of 11 widely studied SHs ranged from 55 % to 100 %. Although the effluent concentration of SHs was decreased after the treatment, the environmental risk posed by them was not considerably reduced in most cases. For instance, estimated risk quotient of five out of nine examined SHs (i.e., 17 alpha-ethinylestradiol, 17 beta-estradiol, testosterone, estrone, and estriol) was extremely high for the effluent of CWs. In most of the examined SHs, biodegradation (aerobic and/or anaerobic) is the most dominant removal mechanism followed by plant uptake and adsorption and/or sorption, along with the physicochemical properties of SHs playing an important role in the elimination processes. Among the studied CWs, the VFCW performed better followed by HFCW, HCW and FWSCW. The aerobic biodegradation being more efficient than anaerobic explains the better removal in VFCW compared with HFCW, also because several SHs showed better biodegradation under aerobic conditions (e.g., 17 alpha-ethinylestradiol, estriol, progesterone, and testosterone). Additionally, plants and support matrix as well as other governing factors such as design and operational factors (area, depth, hydraulic loading rate, hydraulic retention time, and organic loading rate), and physicochemical parameters (effluent dissolved oxygen, temperature, and pH) of CWs also play a considerable role in the removal of the examined SHs.