Key constructed wetland design features for maximized micropollutant removal from treated municipal wastewater: A literature study based on 16 indicator micropollutants

The removal of micropollutants from wastewater by constructed wetlands (CWs) has been extensively studied and reviewed over the past years. However, most studies do not specifically focus on the removal of micro-pollutants from the effluent of conventional wastewater treatment plants (WWTP) that still contains micro-pollutants, but on the removal of micropollutants from raw wastewater. Raw wastewater has a significantly different composition compared to WWTP effluent, which positively or negatively affects micropollutant removal mechanisms. To determine the optimal CW design for post-treatment of WWTP effluent to achieve additional micropollutant removal, this review analyzes the removal of 16 Dutch indicator micropollutants for post-treatment technology evaluation from WWTP effluent by different types of CWs. It was concluded that CW systems with organic enhanced adsorption substrates reach the highest micropollutant removal efficiency as a result of adsorption, but that the longevity of the enhanced adsorption effect is not known in the systems studied until now. Aerobic biodegradation and photodegradation are other relevant removal mechanisms for the studied micropollutants. However, a current knowledge gap is whether active aeration to stimulate the aerobic micro-pollutant biodegradation results in an increased micropollutant removal from WWTP effluent. Further knowledge gaps that impede the wider application of CW systems for micropollutant removal from WWTP effluent and allow a fair comparison with other post-treatment technologies for enhanced micropollutant removal, such as ozonation and activated carbon adsorption, relate to i) saturation of enhanced adsorption substrate; ii) the analysis of transformation products and biological effects; iii) insights in the relationship between microbial community composition and micropollutant biodegradation; iv) plant uptake and in-plant degradation of micropollutants; v) establishing design rules for appropriate hydraulic loading rates and/or hydraulic retention times for CWs dedicated to micropollutant removal from WWTP effluent; and vi) the energy-and carbon foot-print of different CW systems. This review finishes with detailed suggestions for future research directions that provide answers to these knowledge gaps.