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 N and P will depend on the growth rate of the plant, the culture density, and environmental parameters such as solar radiation and temperature (Reddy and Tucker, 1983). Results from applications involving emergent plants on domestic effluent treatment have had varying results. A study from Van De Moortel et al. (2010) on the treatment of raw domestic effluent had N (including NH4) and P removal between 40% and 50%. This may be due to the low coverage that the vegetation had in the system (Van de Moortel et al., 2010). Weragoda et al. (2012) applied the FT-CW principles with two aquatic plant species (Typha angustifolia and Canna iridiflora). NH4 removal from domestic effluent varied between 50% and 85% for T. angustifolia and 58% and 81% for C. iridiflora (Weragoda et al., 2012). Total N (Ntot) removal in the CW types studied by Vymazal (2007) varies between 40% and 50%, with the load removed ranging between 250 and 630 g N/(m2 yr) depending on the CW type and inflow loading. In fact, the total elimination of nitrogen pollution requires both aerobic and anaerobic conditions at the same time on contiguous areas. Single flow type CWs cannot achieve high Ntot removal due to their inability to provide these conditions at the same time. Vertical flow CWs successfully remove NH4, but their denitrification capacity is very limited. On the other hand, horizontal flow CWs provide good denitrification conditions, but their ability to nitrify ammonia is very limited. Therefore, it is necessary to combine different flow types of CWs in a hybrid system in order to exploit the specific advantages of each flow type individually and ensure maximum elimination of nitrogen pollution (Vymazal, 2007). In SSF-CWs, nutrients removal by plants is usually considered to be negligible (10% of the N load and 5% for the P load). However, in case of low inflow loadings (i.e., 200–500 g N/m2/yr and 50–100 g P/m2/yr, with high stands of emergent biomass, much higher removal (around 60% for N and P) can be achieved (Vymazal, 2020). A full-scale 20-year-old surface-flow CWs treating agricultural was monitored over a 2-year-long period by (Lavrnić et al., 2020b). In this well-established system, high retention was observed for TSS (82%) and N (78%), but it was rather mediocre for P (27%). This negative value became positive when the input of nutrients by rainfall was included in the mass balance.
