Nitrogen removal via anammox process in constructed wetland – A comprehensive review

• Role of anammox process in constructed wetland (CW) is discussed. • Nitrogen transformation and removal pathways in CWs via anammox process are discussed. • Various factors affecting N removal in CW via Anammox are discussed. • Anammox bacteria improve the total nitrogen removal efficiency of CW. • Macrophytes support anammox enrichment in CW systems. Constructed wetland (CW) is regarded as one of the major sustainable eco-technologies that offers robust and effective remediation of several types of contaminants. Nevertheless, when viewed from the aspect of nitrogen removal, the remediation efficiency of CWs is quite unsatisfactory and exhibits considerably fluctuating total nitrogen removal efficiencies ranging between 40% and 70%. Moreover, N 2 O production from nitrification and denitrification is inevitable in CWs. Anaerobic ammonium oxidation (Anammox) is an eco-friendly technology whose energy autarky and excellent nitrogen removals are well known. Consequently, its integration in CWs along with different system modifications like biochar addition, microbial fuel cell (MFC) incorporation for bioelectricity production, and degradation of organic pollutants, besides phytoremediation, could be one of the potential solutions to overcome the shortcomings of CWs. Although low temperature and electrode biological reaction can decrease the efficiency of MFC coupled anammox-CW systems. Different studies have discussed the contribution of anammox in various nitrogen transformation and removal pathways like simultaneous partial nitrification, anammox, and denitrification (SNAD), Partial nitrification-anammox (PN/A), Partial denitrification-anammox (PD/AMX) for efficient N removal. Limited nitrification and denitrification in response to low oxygen transfer and carbon source is one of the major limitations of CWs. However, this opens opportunities for anammox integration as it requires low dissolved oxygen (DO) and no carbon source to eliminate nitrogen. Macrophytes including Typha and Phragmites species were found promising for anammox enrichment while studying several CW systems. This provides a way forward for future researchers to investigate associated challenges and put together a cost-effective approach for reducing nitrogen.