Abstract
The search for effective and sustainable techniques for the decontamination of polluted water bodies has led to significant progress over the last two decades with the emergence of the concept of bioremediation, i.e., the use of nature-based solutions (NBSs) to eliminate pollution. The sustainability of these processes is based on the availability of low-cost resources and community-wide acceptance of NBSs. The chapter begins presenting (1) the basic concepts of bioremediation in freshwater ecosystems, based on NBSs, and (2) the details about aquatic bioremediation structures used. It discusses (3) the different techniques and plants used, with published results in phycoremediation (4) and phytoremediation (5), followed by improvement of bioremediation (6), with physical-chemical and microbial activity stimulation techniques, and the development of electro- bioremediation based on a passive redox control of microorganisms. Then, it deals with the maintenance and biodiversity of constructed wetlands (CWs), (7 and 8) and the possible nuisances to be controlled. Finally, it deals with monitoring (9) and modeling of CWs (10). The chapter ends with the social acceptance of the installations in the landscape and the concepts used to integrate them at catchment scale (12). Case studies of applications in the field are used to illustrate the various points of the topic. The conclusion summarizes the important points and traces the directions for future progress. A bibliography, mostly published over the last 20 years but not exhaustive, completes the chapter.
Keywords: Self-purification, eco-hydrology, constructed wetland, phytoremediation, bank filtration
1.1 Introduction
Modern global lifestyle contaminates almost all compartments of the water cycle, both surface and groundwater, with organic matter (OM), nutrients, metals, as well as synthetic chemicals. Domestic and industrial wastewater discharges many endocrine disruptors as well as metals and pharmaceutical residues. Thus, minerals and organic components from domestic, agricultural, or industrial activities pollute water bodies. At the end of the 20th century, environmental degradation due to human activities led to awareness about the existence of societal benefits derived from ecosystems: ecosystem services. The impacts on ecological services could be ignored as long as the resilience of the ecosystems allowed it. However, the ecological footprint of human activity continues to grow. Local and reversible impacts have become global and difficult to reverse, revealing the limits of ecological systems to support human activity, with negative cascading effects, when alteration on one ecosystem service has negative consequences on one or more other services. A well-known example is that of water resources and their pollution. The European Water Framework Directive 2000/60/EC was a first level of response aimed at reducing the ecological footprint (WFD, 2000).
In order to face these socio-environmental challenges, without aggravating the situation through the introduction of disruptive technologies, the European Commission promotes the use of management methods inspired by natural processes: nature-based solutions (NBS). The European Commission defines NBS as: “Solutions that are inspired and supported by nature, which are cost-effective, simultaneously provide environmental, social, and economic benefits and help build resilience” (Faivre et al., 2017). The chapter focuses mainly on publications from the last 20 years devoted to the NBS implementation in bioremediation in water environment.
1.2 Basic Principles
1.2.1 Bioremediation
Strictly speaking, the term bioremediation encompasses a set of remediation technologies based on the use of living organisms to degrade or extract pollutants from the waterbodies. Bioremediation technologies stimulate the natural processes of biodegradation (self-purification) and clean the polluted environment. They can be applied directly on site in the case of in-situ bioremediation, treating the contaminant on site, or remotely in the case of ex-situ bioremediation, where the contaminated soil or water is extracted for treatment at a facility near the polluted site, or elsewhere after transportation (EPA, 2013). Bioremediation techniques are sustained by natural processes of a physical-chemical and/or biological character by exploiting the natural purification capacities of living systems: NBSs, applied separately or in synergetic way (Daghio et al., 2017; Lofrano et al., 2017).
Whether they are physical-chemical, microbial- or plant-assisted, or both, all bioremediation techniques involve oxidation-reduction reactions. Indeed, the common denominator for all NBSs applied to bioremediation is the addition of sufficient electron acceptors or donors to oxidize the pollutant or to stimulate the living organisms that will oxidize the pollutant.
1.2.2 Self-Purification
Self-purification is a natural biogeochemical process occurring in any ecosystem, and leading to elimination or assimilation of OM, mineral nutrients, or other pollutants by the natural activity of its resident biological communities (Namour, 1999; Marmonier et al., 2012). It is particularly active in the river underflow (hyporheic zone) where large contact surfaces develop and a redox gradient naturally installs (Namour, 1999; Namour and Le Pimpec, 2001). Its effectiveness depends on several factors such as the amount and toxicity of the contaminant, its ability to be degraded or “biodegradability” according to the surrounding physical-chemical conditions.
Biodegradability is the ability of substances to be decomposed into simple chemical elements, by the enzymatic activity of living organisms, mainly microorganisms. In fact, biodegradation is a dynamic balance between more or less complex chemical structures of varying resistance (more or less biodegradable), and the action of physical-chemical and biological agents. The quality and quantity of OM present and the existence of specific enzymes dictate the nature and intensity of biodegradation. Thus, biopolymers such as lignins or geopolymers such as humic substances are refractory to biodegradation due to the absence of specific degradation enzymes and require the prior intervention of redox enzymes that produce free radicals capable of opening aromatic cycles (Lipczynska-Kochany, 2018).
1.2.2.1 Redox Processes
Sediment is generally oxygen poor (low diffusion and rapid consumption by microorganisms) and overloaded in OM, so microbial metabolism maintains reducing conditions in sediment where the biodegradation reactions take place according to a redox gradient (Figure 1.1) (Bertrand et al., 2011). Biodegradation effectiveness is often limited by the low availability (presence and mobility) in electron acceptors [e.g.,
Figure 1.1 Schematic organization of different microbial metabolic pathways in the water systems according the redox potential. The order of terminal electron acceptors (TEA) displayed in an idealized system is
