## 5 Role of Macrophyte in CW-MFC Contaminant Removal  
In many wastewater effluents, high COD levels, nutrients such as ammonium-nitrogen, nitrate-nitrogen, and phosphorus, and heavy metals are common pollutants. CWMFC as an eco-friendly technology can remove, transform, and immobilize these and many other wide ranges of contaminants [62]. The efficacy of CWMFC in wastewater treatment, however, depends on multifaceted interacting processes that can be narrowly divided into three major categories--physical, chemical, and biological--as shown in Figure 5 below [21; 35; 54; 62]. The macrophyte component plays a versatile role in all of these processes to ensure the effective removal of wastewater contaminants. For example, in the bioaccumulation and phytoremediation process, aquatic plants have an important role in the absorption, assimilation, and storage of nutrients in their biomass, either directly or indirectly. They sequester soluble inorganic nutrients as a means of building their biomass [21; 24; 62; 63]. Since these nutrients are necessary for their growth, the amount of nutrients from wastewater effluent can be maximized by choosing wetland plants with a high capacity for inorganic absorption and their subsequent conversion to organic plant biomass [57]. In addition, besides the release of oxygen (oxygenation) from plant roots to maintain a redox condition necessary to facilitate nitrification by aerobic microbes, the root system of submerged and emergent aquatic macrophytes releases exudates through a process called _\"rhizode-position\"_[18; 25; 33; 64]. These rhizodeposits act as an energy source for denittifiers for the denitrification processes for the effective removal of nitrate in wastewater [18; 64; 65]. Organic rhizodeposits range from low molecular weight exudates to full roots used as energy sources for microorganisms in the CWMFC [17; 62]. In a study conducted by [PERSON] et al. [64] to assess the influence of macrophytes in ammonium nitrogen (NH4+-N), nitrate, and total nitrogen removal, they used _Canna indica_, a typical wetland plant, and their results showed that the planted CW-MFC was 38% higher in removal efficiency than unplanted CW-MFC. This was mainly because the wetland plant enhanced the nitrification process. In a similar study, comparing planted CWMFC with unplanted CWMFC to investigate the role of plants (_Canna indica_) in the treatment of antibiotics and NH4+-N, the presence of plants  
Figure 5: Interacting processes involved in contaminant removal. was observed to have accelerated the accumulation of sulfamethoxazole, tetracycline, and NH\({}_{4}\)+-N [59]. In addition to their inimitable role as bio accumulators, wetland plants also play an essential filtering and adsorption role by reducing wastewater's velocity and allowing finer particles to adhere to plants' biofilm surfaces. It is undoubtedly proven that the wetland plant species significantly affect pollutant removal in CWMFC [17; 62; 63; 64]. In a study conducted by [PERSON] et al. [48] to enhance the efficiency of CWMFC using plant photosynthate, the root exudates of _Ipomoea aquatica_ were utilized as a part of the fuel in photosynthetic MFC to enhance denitrification [48]. _Phragmites australis_ had better removal of NH\({}_{4}\)-N than _Iris pseudacorus_ and was able to enhance the rhizosphere's nitrification process due to the more robust radial oxygen loss (ROL). A recent study by [PERSON] et al. [54] using _Elodea nutalli_ CWMFC operated under artificial aeration attained 98% COD removal efficiency. In another recently conducted experiment in South Africa by [PERSON] et al. [66], to examine the contribution of macrophytes used in CWMFC, they compared three indigenous South African species: _Phragmites australis_, _Coperus profiler_, and _Wachedorfia thyrsiflora_ with an unplanted system as control. They obtained a COD removal efficiency of 97 \(\pm\) 1% for _Coperus profiler_, which was higher than _P.