CWs have real potential to reduce the fluxes of agricultural contaminants such as nitrates and pesticides. The implementation of green infrastructure undeniably contributes to reducing the transfer of contaminants in agricultural landscapes. Many pesticide retention processes (e.g., hydrolysis, photolysis, sedimentation, adsorption, biodegradation, or plant uptake) are involved in CWs, the intensity of which depends on local conditions and is attributed to the presence of vegetation. The highest removal rates were achieved for organochlorine, organosphorus and pyrethroid pesticides, while the lowest retention rates were observed for herbicides from the group of triazines and triazinones, aryloxyalkanoic acid, and urea. Pesticide removal is weakly correlated with the KOC value (Vymazal and Březinová, 2015). A reduction target of 50% can be expected by devoting only 1% of the upstream contributory area to a CW. Obviously, purification efficiency will vary greatly depending on the season, hydrological flows and the type of pollutants (Tournebize et al., 2017). The degradation of pesticides involves two types of enzymatic reactions: lysis enzymes operating without any redox cofactor such as hydrolases and esterases; and oxidation and mineralization by redox enzymes such as oxidases, peroxidizes, and oxygenases (Gaur et al., 2018). Lemna minor showed great potential to improve the water quality of textile effluents contaminated with Toluidine Blue dye (Neag et al., 2018). Free floating macrophytes, viz., Salvinia, Lemna, Eichornia, and Pistia showed their potential for the purification of waters containing perchlorate (Bhaskaran et al., 2013).
1.10 Improving Bioremediation Systems
1.10.1 Introduction
As presented above, microorganism capacities to metabolize pollutants and convert them to non-toxic forms by redox reactions support the microbe-assisted or microbial bioremediation technologies. Some recent bibliographical reviews synthesize the knowledge acquired in microbial bioremediation with or without associated plants. Shahid et al. (2020) provide an in-depth insight into the specific role of microbes in bioremediation systems, specifically on rhizosphere-plant interactions in floating wetlands, but also relevant to CWs (Shahid et al., 2020b).
Phytobiome around plant roots play a significant role in organic pollutant biodegradation during bioremediation process. The main ways of mobilizing phytobiome in bioremediation are as follows:
1 Bio-sorption is the passive adsorption of dissolved pollutants on the surface of living or dead cells. The pollutant changes phase but remains available in the environment. Bio-sorption can be a possible preliminary step to a proper biodegradation but it is not an actual removal from the environment. Adsorption intensity varies considerably depending on the hydrophobicity, structure, and functional groups of the pollutant involved. Pollutants with cationic groups are more effectively biosorbed on cell surfaces through electrostatic interactions. Biosorption is used as physical-chemical process, for metal removal (Verma et al., 2008; Fard et al., 2011; Ali et al., 2020);
2 Bio-accumulation is an active metabolic process requiring energy from the living organism to absorb and concentrate pollutants. Harvesting the organisms and exporting them out of the contaminated environment, allow removing the pollutants from the treated environment;
3 Biostimulation by nutrient input, air injection (bioventing), or by microbial fuel cell (inexhaustible TEA setup). Biostimulation consists in increasing the activity of endogenous microbes by feeding them with nutrients (e.g., organic carbon and nitrogen) or TEA (e.g., bioventing and bioelectro-remediation). The nutrient content stimulates the microbial co-metabolism of organic pollutants. Additionally some free radical-producing microbial oxidation-reduction enzymes will non-specifically oxidize the aromatic rings, open them and make them degradable by microbial enzymes. Obviously, biostimulation technologies involving discharge of wastewater in natural water systems should be avoided, and must be limited to stimulation without chemical inputs. Biostimulation is backed up by the phenomenon of co-metabolism, i.e., the simultaneous biodegradation of two organic compounds, in which the degradation of the pollutant depends on the presence of the microorganism’s normal carbon source. The additional carbon source serves not only to support biomass production, but also acts as an electron donor for the metabolism of the non-growthrelated pollutant (Xiong et al., 2018);
4 Bio-augmentation consists of adding microbes to polluted water body to increase the biodegradation of contaminants. Bio-augmentation is possible when the biostimulation fails to meet the objectives, for example, when the indigenous microbes do not have the metabolic capability (gene pool) to degrade the contaminants, or when the contaminant is present at concentrations toxic to the indigenous microbes. Inoculated microbes may be derived from a selection, from a contaminated site, of microbes suited to the contaminant degradation, their lab enrichment, before inoculation on the contaminated site (Hussain et al., 2018). Obviously, on-site bio-augmentation prohibits the use of genetically modified organisms and introducing of recombinant microbial strains in natural environment in field applications. Bio-augmentation makes use of indigenous microbe strains such as microalgae, bacteria, and fungi. Addition of microbial cultures to accelerate biodegradation rate of pollutants. Microorganisms are sampled from contaminated sites where they are already capable of degrading some specific pollutants, depending on the pollutants contaminating the origin site. However, on-site application of bio-augmentation can pose problems of predation, nutritional competition between native and inoculated bacteria, too low inoculations, or ecological balance disturbance due to too large quantities inoculated.
1.10.2 Floating Treatment Constructed Wetlands
FT-CWs use natural water macrophytes on buoyant mats to increase the efficiency of FSF-CWs. The large root surfaces available for habitat for microbial growth and development lead to an efficient removal of pollutants from the water (Colares et al., 2020). Their main function is to remove nutrients (such as N and P) and OM from the water column and thus remedy the eutrophication of waterbodies. FT-CWs are built on the principle of removing unwanted nutrients but do so without the soil compound and roots are directly immersed in the water column (Pavlineri et al., 2017; Rehman et al., 2019).
For instance, yellow iris Iris pseudacorus was planted to remove N from the secondary effluent in three tanks, with or without the addition of an electron donor (acetate or thiosulfate), in order to study heterotrophic and autotrophic denitrification. Ntot removal rates were around 89.4% in autotrophic tank and 88.5% in heterotrophic tank, for a HRT of 1 day in summer. The autotrophic tank showed better nitrification and denitrification rates. The addition of electron donors effectively reduced N2O emissions, particularly in summer and autumn, and they distinctly induced microbial population shift. Dechloromonas, Thiobacillus, and Nitrospira became the predominant genera in heterotrophic, autotrophic and control tanks respectively (Gao et al., 2017). In a small-scale setup (600 inhabitants), a floating mat planted with Typha domingensis and applied to the raw sewage treatment for 12 months, removed in average 55% of COD, 56% of BOD5, and 78% of TSS. Reduction rates were 41% for Kjeldahl nitrogen (Nkj) and 37% for Ptot. Anaerobic condition measured in effluent could explain the low reduction rates (Benvenuti et al., 2018). Comparative pilot trials with pickerel rush Pontederia cordata and soft rush Juncus effusus showed a greater efficiency of P. cordata for Ntot and Ptot, with highest removal rates of Ntot (0.31mg/(L day) and Ptot (0.34mg/(L day) (Chanc et al., 2019). Afzal et al. (2019) used a full-scale (1,858 m2) FT-CW made of 120 coconut mats, on polyethylene rafts, planted with Brachiaria mutica, Canna indica, Leptochloa fusca, P. australis, Rosa indica, T. domingensis, and set up in stabilization ponds receiving 60% sewage and 40% industrial (textiles, food, and chemicals) wastewater. The maximum removal capacities of the system were 79% of COD, 88% of BOD, and 65% of TDS (Afzal et al., 2019).
Almost all the publications are dealing with bench or pilot scale trials, with very few working on a real scale. Therefore, one of the main challenges for FT-CWs is to move from pilot scale to a more realistic scale, in order to assess the behaviour