have been found to be more suitable than SFS-CWs in the cases where odorous water or wastewater with high solid content is being treated (Reed and Clearinghouse, 1993). In AMD treatments, anaerobic SSF-CWs are effective when the acidity of a water sample is greater than pH 4. Water is neutralized by the microbial usage of Fe3+ as the terminal electron acceptor. Aerobic settling ponds may be required for metal precipitation. Aerobic wetlands (surface flow) are usually used for metal precipitation and are used as a final step in integrated AMD treatment (Fripp et al., 2000).
Hybrid CWs are the combination of various types of CWs in order to exploit the specific advantages of each type and thus provide aerobic and anaerobic conditions at the same time, conducive to the elimination of nitrogen pollution (Rizzo et al., 2020).
There are two important aspects to consider in the design and operation of CWs: the water flow and the vegetation.
1.6.1 Water Flow
Water flow type and retention time strongly influence the physical-chemical parameters (e.g., redox potential and insolation) in CWs. In surface flow systems, sunlight can promote possible photo-degradation of pollutants due to direct exposure of water. FSF-CWs and SSF-CWs have generally a barrier to prevent seepage toward aquifers. In some FSF-CWs, infiltration might be a goal in some cases, such as protecting a bathing area downstream the discharge point. However, the latter strategy can be dangerous as infiltration means the transfer of pollutants to underground aquifers.
The water balance calculation in case of infiltration is tricky because it must also take into account possible evaporation and evapotranspiration. However, it is clear that soil sealing of large CWs is difficult and expensive. Nevertheless, a soil analysis should be conducted prior to CW building to evaluate the infiltration capacity (Bouzouidja et al., 2020). This capacity may be spatially distributed and change overtime due to clogging (Lavrnić et al., 2020a).
Hydraulic loading rate (HLR) is the rate of water flow into a wetland. Hydraulic retention time (HRT) is the measure of time that a soluble compound remains in the system during water treatment. A lower HLR and HRT of about 6 days are required for efficient pollutant removal in CWs. However, HLRs are difficult to categorize as low or high as values from 135 to 5,140 L/(m.d) were reported as high HLRs (Chang et al., 2007). Length-to-width ratio of the wetland needs to be determined so that dimensions are worked out to accommodate inflow of water. One of the greatest causes of failure in CWs was found to be under sizing, thus resulting in inefficient water treatment. Dimensions need to be sufficient to handle water increase levels due to rainfall and adequate inlet and outlet systems to maintain water pre-rainfall levels. Water depth needs to be determined based on whether the type of CWs is FSF or SSF. The wetland depth for SSF-CW types is in the range 60 to 100 cm. In the case of surface flow wetlands, the mean depth is 30 cm (Wong and Geiger, 1997) and also depth of 2 m has been reported (Bois et al., 2017). SSF-CWs are more commonly reported as compared to FSF-CWs in literature, though both have different advantages. As previously mentioned SSF-CWs are beneficial when dealing with odorous polluted water, while FSF-CWs can beneficiate from direct aeration, which can be promoted by wind.
There is no specific design for CWs. In 1993, there have been already approximately 80 types of SCW designs proposed since 1980 (Reed and Clearinghouse, 1993). A majority of European systems are based on rooted emergent macrophyte systems (Haberl et al., 1995). CWs are designed to treat all kinds of urban and industrial wastewater and various runoffs (e.g., urban storm water and agriculture) and to polish already treated waste-water (Vymazal, 2009). Some CWs were even designed for phosphorus removal in natural waterbodies such as Lake Apopka in Florida (Dunne et al., 2012), Everglades in Florida (Zhao and Piccone, 2020), and Albufera in Spain (Martín et al., 2020). Understanding the phenomena taking place in natural wetlands helps to design properly these green (Zhang and Chui, 2019) or turquoise (Childers et al., 2019) urban infrastructure elements that are CWs (Martín et al., 2020). In terms of surface, CWs’ size ranges can differ largely. Zhao and Piccone (2020) report on a 6,600-ha FSF-CW in Florida to reduce phosphorus load from stormwater agricultural runoff and protect the Everglades Protection Area (Zhao and Piccone, 2020). On the other hand, Gaullier et al. (2020) discuss the efficiency of two small FSF-CWs (0.01 and 0.02 ha) built just downstream single agricultural plots (10 and 8 ha, respectively) (Gaullier et al., 2020).
1.6.2 Aquatic Vegetation
Suitable vegetation for water treatment needs to be considered, depending on the type of water being treated, the tolerance of the vegetation, and the climatic conditions. Vegetation serves a variety of functions in CWs. It slows down the HLR, thereby increasing HRT for efficient water treatment. Large success has been reported in the usage of wetlands in farm waste-water treatment plants, with aquatic vegetation such as cattails (Typha spp.), bulrush (Scirpus spp.), and phragmites (P. australis) used to treat water. Phragmites australis is a very popular as emergent aquatic plant for waste-water treatment CWs. In addition, vegetation provides habitat to wildlife and gives the CW an attractive aspect (Brix, 1994).
1.7 Phytoremediation and Constructed Wetlands
Phytoremediation sensu lato (aka plant-assisted bioremediation or phytotechnology, including phycoremediation) uses macrophytes and related phytobiome to clean up environments. These technologies are environmentally friendly and cost-effective and do not require sophisticated equipment, to manage contaminated water (Krzeminski et al., 2019).
1.7.1 Phytoremediation Techniques
Phytoremediation encompasses many phyto-techniques and processes involving macrophytes and the associated phytobiome. Depending on the main process involved, we will refer to phyto-degradation/phyto-transformation, phyto-stabilization/phyto-sequestration, phyto-accumulation/phyto-extraction, phytovolatilization, and rhizo-filtratio/phyto-filtration (Figure 1.5).
Figure 1.5 Various processes involved in phytoremediation of contaminants in aquatic environment interpreted from Ansari et al. (2020).
In aquatic systems, macrophytes promote the oxygenation of sediment and favour purification processes by enhancing and diversifying microbial activities through its rhizosphere. Moreover, various parts in macrophytes hyper-accumulate contaminants by absorption (phytoextraction) (Rulkens et al., 1998). Some macrophytes have the ability to consume and volatilize the contaminants directly into the atmosphere through the leaves. Phyto-volatilization process is cost effective in elimination contaminants from soils, groundwater, residues, and sludge (Girdhar et al., 2014). Rhizo-filtration involves adsorption and precipitation of the metal contaminants into the growth substrate surrounding the root zone. Moreover, macrophytes are in constant interaction with their phytobiome: their roots secrete specific exudates and compounds, which attract some specific microbial communities, and develop a thick coating or biofilm around them (rhizosphere). Because of its various physical- chemical conditions, creating various microbial growth niches, the rhizosphere accommodates various microbial consortia, and it can, even in surrounding aerobic conditions, host some anaerobic consortia, such as denitrifying, sulfate-reducing, and methanotrophic bacteria (Shahid et al., 2020b).
1.7.2 Aquatic Phytobiome
The phytobiome consists in the community of micro-organisms, fungi,
