1993; Raskin et al., 1994; Salt et al., 1998).
1.7.7 Submerged Aquatic Plants
Ceratophyllaceae (coontails or hornworts) and Callitrichaceae (waterstarwort) are submerged plants typical of lentic systems, and they develop easily in ponds as well as in surface-flow CWs. They are slightly attached on the bottom and develop in the water column until the surface. Their photosynthetic rate decreases when the flow velocities increase (Madsen and Søndergaard, 1983). Their seasonal growth is the result of the balance between the energy from photosynthesis and the energy needed to maintain the plants (Best and Visser, 1987).
1.7.8 Mixture of Macrophytes and Microalgae
Oberholster et al. (2017) tested the feasibility of a biological hybrid treatment system to treat sulfur using AMD water treated by a CW and macro-algae from a pond system. The study was conducted under laboratory conditions to determine the bioaccumulation of S and other important algal growth elements such as Ca, Mg, and P from AMD water and treated CW water at different pH values. Following exposure of the microalgae to AMD and treated CW water for 192 h, Microspora tumidula showed the highest bio-accumulation of S and P which occurred at a pH of 5. Oedogonium crassum showed the highest bioaccumulation of Ca and Mg at a pH of 7. M. tumidula appears to be a good candidate for the treatment of sulfur-rich AMD using a hybrid biological system (Oberholster et al., 2017).
1.8 Phycoremediation
Phycoremediation is a biological clean-up technology involving the use of microalgae (in fact, algae and cyanobacteria) for the biological transformation of contaminants, including nutrients such as mineral and organic C, P, N, S, heavy metals, and emerging contaminants (Guleri et al., 2020; Leong and Chang, 2020). During the process of phycoremediation, algae utilize nutrients (N and P), C, and other salts from the wastewater for their growth. The clean-up process has been a research subject for many decades (Oswald, 1995). Algae are also used for their ability to oxygenate the environment due to photosynthesis (Hernandez et al., 2006). Macrophytes and algae are often associated, voluntarily or not, in SFS-CWs. In some cases, algae are considered a public health concern due to the secretion of harmful toxins, cf. § 1.12.5 (Lee et al., 2017) and the algae over-development should be controlled (West et al., 2017).
1.8.1 Carbon and Nutrients (N and P) Removal
The photosynthetic capabilities of algae are particularly attractive, converting solar energy into useful biomass while incorporating mineral nutrients including nitrogen and phosphorus (Zhou, 2014). According to Ruiz-Martinez et al. (2012) a mixture of the microalgae Chlorococcales and cyanobacteria isolated from a submerged anaerobic bioreactor reduced N-NH4 by 62% and P-PO4 to 97% after a 42 day culturing period in WWTP effluent (Ruiz-Martinez et al., 2012). In a study by Silva-Benavides and Torzillo (2011), the authors compare the removal efficiency of Chlorella and a Chlorella-Planktothrix co-culture grown in municipal wastewater. The co-culture of Chlorella-Planktothrix removed the highest nitrogen concentration (80%) over the 2-day exposure period (Silva-Benavides and Torzillo, 2012).
Oberholster et al. (2019) demonstrated the effectiveness of nutrient removal from domestic wastewater by mass inoculating specifically selected strains of algae (Chlorella spp.) into the maturation ponds of a sewage treatment plant. The total phosphorus (Ptot) reduction in the water was 74.7% and 76.4%, and the total nitrogen (Ntot) reduction was 43.1% and 35.1%, in the last two maturation ponds. For maximum treatment results, the algae biomass in the upper surface water layer must be harvested (Oberholster et al., 2019).
Chlorella spp., such as Chlorella sorokiniana and Chlorella vulgaris, are especially popular due their ability to efficiently remove nitrogen and phosphorous from effluents (Hernandez et al., 2006). The large-scale use of algae is mainly due to a good understanding of large-scale culture systems as such systems are used to produce highly valued products including pharmaceuticals and genetically engineered products (Shahid et al., 2020a). Diatoms are microalgae rich in silica and with a higher photosynthetic activity than green algae: they develop either as single cells or embedded in colonies (biofilms) fixed on any surface (including plants) in fresh and saline waters. They are also good candidates for metal removal (Marella et al., 2020).
Nitrogen removal in CWs occurs in a two-step process: nitrification and denitrification. Ammonia-oxidizing bacteria and nitriteoxidizing bacteria convert total ammonia to nitrate. In contrast, in anoxic environment, denitrifiers reduce nitrate and nitrite into nitrogen gas. Microalgae releases oxygen, creating a favorable environment for the oxidation of nitrogen. In addition to oxygen-carbon dioxide exchange, interactions between microalgae and bacteria also include other aspects such as nutrient cycling, growth promotion and EPS production. This indicates the important role the microalgae-bacteria consortia play in the wastewater treatment processes (Chindah et al., 2007; Cho et al., 2015).
1.8.2 Micropollutant Removal
Either live or dead, algae are able to accumulate metals; however, only the accumulation by live algae is discussed here. The extraction of heavy metals by microalgae takes place in two stages. A first stage of rapid extracellular passive adsorption (biosorption), occurring in both living and non-living cells. The presence of peptide and polysaccharide polymers (cellulose and alginate) on the cell wall of the microalgae provides numerous nonspecific adsorption sites, allowing the metal biosorption. The second stage is a metabolism-dependent process of slow intracellular diffusion and accumulation (bioaccumulation). After active transport through the cell membrane, peptides and proteins, such as glutathione, metallothionein proteins, oxidative stress reducing agents, and phytochelatins, bind to the metals (Leong and Chang, 2020). During the slow and generally irreversible bioaccumulation process, heavy metals accumulate inside the cell and bind to intracellular compounds, such as polyphosphate bodies, and/or inside vacuoles (Suresh Kumar et al., 2015).
Oberholster et al. (2014) have studied the bioaccumulation potential of selected filamentous macro-algae species at different pH ranges for possible treatment of AMD. The bioconcentration of metals (mg/kg dry weight) measured on the field in the filamentous macroalgae mats based on Oedegonium crissum, Klebsormidium klebsii, and Microspora tumidula was generally higher for Al and Fe than for Mn and Zn.
Persistent Organic Pollutants (POPs) are synthetic chemicals capable of long-range transport, persistent in the environment and with a potential to bio-magnify and accumulate in ecosystems. The most widely occurring POPs in water systems are related to agriculture runoff [pesticides], to industry [polychlorinated biphenyls (PCBs)], to urban wastewater with flame retardants and surfactants [including PFOS (Perfluoro-octanesulfonic acid)–based products, polychlorinated dibenzo-p-dioxins (PCDDs), and dibenzofurans (PCDFs), commonly known as “dioxins”], and to domestic pollutants (e.g., detergents, pharmaceuticals, and personal care products). As with all wetland techniques, the effectiveness of phyco-remediation in eliminating organic pollutants depends on a series of processes such as photo-degradation, adsorption, bioaccumulation, biodegradation, and volatilization (Gaur et al., 2018).
1.9 Phytoremediation
A large number of aquatic plants such as water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes L.), duckweed (Lemna, Spirodela, Wolffia, and Wolfiella), bulrush (Typha), common reed (P. australis), and vetiver grass (Chrysopogon zizanioides) are used in the elimination of contaminants and therefore have been studied by various researchers (Mkandawire and Dudel, 2007).
1.9.1 Carbon and Nutrients (N and P) Removal
Macrophyte uptake of N and P is one of the processes involved in the removal of nutrients from wastewater. Removal of