C, N, and S cycles (Sanchez, 2017). Experimental results show that various microbial consortia are able to produce same power levels, demonstrating their flexibility and resilience (Lyon et al., 2010). The pathogen fate in a BMFC remains unclear (Morato et al., 2014). Some studies show its high disinfecting potential (Gajda et al., 2016; Ieropoulos et al., 2017). Therefore, BMFC in-field implementing requires further studies on sanitary hazards and biodiversity loss related to the electro-bioremediation.
1.10.7 Maintenance of Aquatic Bioremediation Systems
Like all natural systems, nature-based systems evolve over time. During the evolution of CWs, plants grow, develop, and spread, producing a biomass of increasing importance. With environment closure, the number of plant strata increases and a tendency toward afforestation is observed (climax formation). In order to conserve the purifying properties and biodiversity of CWs, management of the biomass produced and the accumulated sediments is necessary. Management measures are aimed primarily at keeping the system open and ecologically diverse. Annual operating and maintenance costs for CWs are in the range of 3% to 6% of the construction cost (Weiss et al., 2007).
1.10.8 Biomass Management
Operators use different approaches to aquatic vegetation management. The most common option for SSF-CWs is to harvest the biomass in spring and remove it from the system. The second option is to mow the biomass in late fall, keep it on the surface to isolate it and remove it in early spring. However, if the mowed biomass remains on the ground, then the rain slowly leaches out the accumulated nutrients, organic pollutants, and metals and returns them into the system. Finally, in the absence of autumn mowing, the reeds that remain standing are also subject to decomposition and leaching by rain. Moreover, in common reed, the leaves fall into the wetland and enrich the system with nutrients and pollutants (Vymazal, 2020). The bioaccumulation of metals by aquatic plants is not sufficient to ensure the bioremediation of a waterbody on its own, if the biomass produced is not harvested and exported out of the aquatic environment.
Mowing and harvesting aquatic plants at the end of the growing season allows exporting pollutants out of the waterbody. Otherwise, the biomass, enriched with metals during the growing season, will perish at the end of the season and decompose on site, releasing the metals trapped in the aquatic environment. It will thus participate in the over-contamination of the waterbody. As seen above, phytoremediation uses some slightly invasive species such as bulrush T. latifolia, common reed P. australis, water hyacinth E. crassipes, water lettuce P. stratiotes, or duckweeds Lemna spp. They have vigorous growth and produce more biomass than non-invasive ones. Also, after the harvesting of the biomass produced, a valorization can generate income and reduce the overall cost of the bioremediation system through the production of useful by-products such as compost or mulch for soil improvement or livestock feeding. Alternatively, if the contamination level is too high, then the production of non-food byproducts can be considered such as fuel (e.g., briquettes, biodiesel, ethanol, and biogas) for energy production or building materials (e.g., fiber board, insulating fibers, and chipboard).
In any case, the size of the system should be taken into account: what is possible on small CWs, in particular SSF-CWs, is much more difficult and expensive in shallow FSF-CWs covering several hectares.
1.10.9 Sediment Management
Sludge and sediment accumulated over time in CWs must be extracted regularly to maintain bioremediation efficiency: over time, their accumulation reduces the residence time and treatment capacity by filling the volume of deep basins. The monitoring of the performance of a CW, carried out over 7 years, showed a decrease in efficiency over time, linked to soil saturation. In fact, this CW saw its performance decrease after 2 to 3 years of operation, particularly in terms of purification performance for Kjeldahl nitrogen and Ptot. Also, it is recommended to dredge the bottom at least every 5 years, in order to keep at minima the hydraulic operation in the CW, but also, at the same time, to extract non-biodegradable micro-pollutants such as phosphorus and metals, and to avoid the accumulation of OM leading to a reducing environment favorable to the production of GHGs. Shoreline restructuring can be carried out at this time to restore habitat diversity (Basilico et al., 2017). Removing sediment already trapped in CWs uses a variety of mechanical techniques as excavation or dredging.
The low oxygen availability in wetland sediment is the main obstacle to the degradation of organic contaminants. To overcome oxygen limitation, a basic biostimulation technique consists in cultivating the sediment, either in situ, during a grazing transitional phase after drying-out: traditional practice of “assecs” or ex situ after spreading the dredged sediment in a thin layer to facilitate oxygenation, with optional nutrient additions (Ockenden et al., 2014). Droughts of very high intensity (1 year in case of assec) kill any vegetative part of aquatic plants. Faced with such disturbances, the resilience of aquatic flora depends only on a persistent pool of seeds or natural seed inputs (Arthaud et al., 2012). Finally, the assec technique shows a positive effect on the biodiversity of macrophytes and macroinvertebrates in the littoral and on oligochaetes in the center of the CW (Arthaud et al., 2013).
The periodic regeneration of aquatic facies caused by the management of CWs seems positive on their biodiversity. In a network of nested wetlands, the specific biodiversity decreases over time after a disturbance for all CWs. However, there is a significant interaction with the location of the CW within the network. It does not seem to influence species richness during the first 2 years after disturbance, but from the third year and afterward, the CW linked to a high number of upstream CWs showed a constant decrease in richness, in contradiction with the idea of an increase in species richness due to increasing plant dispersal elements (diaspores) inputs from upstream (Arthaud et al., 2013).
1.11 Animal Biodiversity
In addition of improving water quality and flood abatement, through appropriate management, CWs, especially large-scale FSF-CWs, can support a variety of wildlife species and thus enhance biodiversity. Also, they provide an opportunity for water and urban managers and other stakeholders to conserve and promote aquatic biodiversity in generally ecologically poor environments. Work of Hsu et al. (2011) suggest that the surface areas of the wetland and its macrophytes covert, as well as water quality play a major role on the biodiversity hosted by a CW. However, these factors are of varying weight according to the various taxonomic groups. Briefly, the richness, abundance, and diversity of birds increase with the CW area. Fish richness and abundance increase with CW area and dissolved oxygen, while diversity decreases with increasing Ptot concentration. Aquatic macroinvertebrate richness and density increase with aquatic macrophyte cover, while diversity increases with CW area. Therefore, a CW, installed to improve water quality and enhance biodiversity, must have a large surface area and significant macrophyte coverage (Hsu et al., 2011). Habitat diversity can be increased with the creation of floating islands (FT-CWs), resulting in an overall increase in edge habitat (Becerra-Jurado et al., 2012).
1.11.1 Biodiversity Management
Without proper management, CWs can promote biological invasion or act as an “ecological trap” for native species, i.e., trapping them in poor-quality habitat, jeopardizing individuals or the local or regional population (Sievers et al., 2018). Integrated basin-wide management and connection to natural wetlands can partially mitigate these negative impacts on the long-term biodiversity (Zhang et al., 2020a).
In CWs, the existence of suitable habitat and predatory pressure are the two factors influencing aquatic macroinvertebrate community structure. Fish are often the main predators in CWs and have an influence on the richness, density, diversity, and structure of aquatic macroinvertebrate communities (Hsu
