Natural and constructed wetlands

A wetland is defined as any saturated zone of fresh or saltwater, temporary or permanent, with typical vegetation suitable for saturated soils (^{Vymazal & Brix, 1998}). Riparian ecosystems act as a nutrient sink for upland runoff. As a result, these wetlands are important buffer zones between farmland and watercourses. There are mitigation techniques commonly used in the field to prevent the introduction of pesticides into water bodies, such as hedgerows, vegetative barriers (dense vegetation strips, serving as a porous filtering barrier for runoff water) (^{Vymazal & Březinová, 2015}), vegetated ditches and Constructed Wetlands (CW). Constructed wetlands provide a surface for the growth of the micro-organisms responsible for the filtration and absorption of pollutants and inhibit the growth of algae.

Constructed wetlands are classified as Free Water Surface (FWS) wetlands or Floating Aquatic Plant (FAP) systems and SubSurface Flow (SSF) wetlands or Rooted Emergent Plants systems, which are divided in turn into Vertical Flow systems (VSSF) and Horizontal Flow systems (HSSF).

Free Water Surface wetlands are ponds or canals with floating root plants in which the surface of the water is exposed to the atmosphere. On the other hand, SubSurface Flow wetlands are ponds with emergent plants and a porous substrate (sand, gravel or any type of coarse-textured soil). There are two types: Horizontal Flow and Vertical Flow wetlands, depending on how the effluent feeds the wetland. The feeding of the Vertical Flow wetlands is carried out at intervals of time, in order to allow percolation of the tributary into the substrate.

Although constructed wetlands are intended for various types of wastewater, they have been documented as natural methods of water treatment since 1912. The United States Environmental Protection Agency (^{EPA, 1993}), was the first to take samples from 1960 to 1970. World-wide projects from 1972 to 1983 have also been documented and studies on the removal of phosphorus and nitrogen (N and P) from constructed wetlands have been conducted since 1970. Constructed wetland-based studies to eliminate pesticides also began in the 1970s. And in the 1990s, studies were conducted on nutrient treatment in constructed wetlands on rivers (^{Vymazal & Březinová, 2015}).

In 1967 a scale Free Water Surface system was developed. In Germany, in the 1970s Dr. Kichuth created a system called the “Root Zone Method”, which consisted of a Horizontal Flow wetland in which he planted Phragmites australis, adding a combination of calcium and iron or aluminum to cause precipitation of phosphorus. In the United States, the study on wetlands is based on the results of European studies, particularly on natural wetlands. This green technology has also been studied by NASA, which has developed its own system called “Hybrid Treatment System” because it uses anaerobic microorganisms and emerging plants (Phragmites communis) (^{Rodríguez, 2003}).

Advantages and disadvantages

Constructed wetlands have great technical, economic, social and aesthetic advantages over conventional wastewater treatment. In accordance with the regulations, quality treated water is obtained which is suitable for agricultural irrigation as it eliminates pathogenic micro-organisms. The costs of implementation, operation and maintenance compared to traditional wastewater treatment systems are lower because they have a useful life of 25 years and minimal energy requirements because gravity is used to bring water to the wetlands. The construction characteristics of wetlands make it difficult to develop imbalances due to changes in pollutant concentrations or oscillations in wetland tributaries (^{Miranda & Luna-Pabello, 2001}). In addition, these systems, where the water flows under the surface of the support, do not favor the development of mosquitoes and unpleasant odors, and even offer a thermal protection to the system (plants, substrate and water), allowing the installation of wetlands in places with heavy frosts (^{Llagas, Wilmer, & Gómez, 2006}).

Although having many advantages, this type of wastewater treatment system has the main disadvantage of requiring a lot of ground for its installation. Consideration should also be given to the disadvantages of SubSurface Flow wetlands, such as the cost of the media used, as well as its routing and installation (^{Rodríguez, 2003}).

Types of constructed wetlands

The establishment of constructed wetlands is a sensitive issue because of the variability in wastewater composition that often changes treatment needs. Plant selection is an important issue in this type of design as it must survive the potential toxic effects of wastewater (^{Calheiros, Rangel, & Castro, 2007}).

Treatment processes

The constructed wetlands are primarily used for the retention of nutrients and organic matter in domestic and municipal wastewater, rainwater and agricultural runoff. Fertilizers used for agriculture mainly contain phosphate, which seeps into groundwater from agricultural runoff (^{Zhang, Moon, Myneni, & Jaffé, 2017}). Although P is important for ecosystems, large amounts can lead to eutrophication of water bodies, caused by excess nitrogen (N) and phosphorus (P), which causes algal growth and reduction of oxygen levels. Within constructed wetlands, physical, chemical and biological processes are carried out, removing pollutants from wastewater. These processes include sedimentation, adsorption on soil particles, plant uptake and microbial transformation (^{Rodríguez, 2003}).

Selecting a suitable substrate is an important step in promoting biofilm development within the wetland (^{Wu, Kuschk, Brix, Vymazal, & Dong, 2014}). The microflora that grows in a wetland is varied, including bacteria, protozoa and even small animals, with bacteria constituting the basic purifier group of wastewater treatment processes.

Water moves slowly through the wetland, due to the laminar flow of these systems and the resistance provided by the roots and vegetation, which serves as sediment traps, while constituting the main means of elimination of phosphorus (^{Pavlineri, Skoulikidis, & Tsihrintzis, 2017}). Although wetlands are effective in purifying organic compounds and suspended solids to further eliminate nitrogen, it is essential to consider feeding, wetland type (^{Vymazal, 2018}) and recirculation (^{Wu et al., 2014}).

The biological processes of elimination are certainly the most important for the elimination of pollutants. These processes begin with the uptake of oxygen by the plant. Some of the pollutants are nutrients for plants, such as nitrate, ammonium and phosphate ions, which are absorbed by plants. The elimination of pollutants by plants can vary depending on the age of the plant. Bacteria and other soil micro-organisms also provide, capture and store short-term nutrients, as well as some other pollutants. Microorganisms in wetlands remove organic compounds. Treatment in this type of system is usually due to the presence of microbial populations adhering to the surface of the plants and the support. Microbial metabolism also produces the removal of inorganic nitrogen, namely nitrate and ammonium in wetlands. Nitrogen removal begins with the transformation of organic nitrogen (N_Org) to ammonia nitrogen (N-NH₄ ⁺) by ammonification. The N-NH₄ ⁺ can be assimilated by plant roots or microorganisms and volatilized if the pH of the system is greater than 9 or oxidized to nitrate (NO3^-) by the action of ammonium-oxidizing and nitrite-oxidizing bacteria (nitrification). The last step is the reduction of NO3^- in molecular nitrogen (N2) under anoxic/anaerobic conditions (denitrification). Transformations suffered by nitrogen in wetlands vary according to whether their structure is horizontal or vertical (^{Torres-Bojorges et al., 2017}).

The most important chemical removal process in soil is absorption, resulting in short-term retention and long-term immobilization of various types of pollutants. Another important process in wetlands is adsorption, which involves binding ions to soil particles by cation exchange on the surface of clay particles and soil organic matter.

It is from the joint action of removal and metabolic processes carried out in the wetland by the microflora, that the removal of Total Suspended Solids (TSS) and some of the biodegradable organic matter (BOD) occurs by sedimentation and filtration of water. While biodegradable organic matter (BOD) is purified through microbial degradation (aerobic, anaerobic, and facultative). To eliminate nitrogen, the mechanisms of removal are ammonification, followed by bacterial nitrification-denitrification, volatilization of ammonia, in addition to the consumption of the plant.

Phosphorus in turn exhibits adsorption-precipitation reactions with aluminum, iron, calcium and the minerals characteristic of clay, in addition to a portion consumed by the plant. The removal of heavy metals is achieved by sedimentation and adsorption on the surface of the plant and debris.

Constructed wetlands encourage the removal of pathogens, through sedimentation and filtration, ultraviolet radiation, excretion of antibiotics from plant roots and natural death (^{Rodríguez, 2003}). Plants within the wetlands provide the necessary surface for microbial growth, allow filtration and adsorption of pollutants in wastewater, inhibit the growth of algae, and promote the circulation of oxygen, which plants capture through their stems and leaves to be used by the roots.

Similarly, in constructed wetlands, several aerobic and anaerobic biogeochemical processes govern the retention and removal of pollutants. Carbon and the efficiency of the removal of toxic organic compounds are obtained by measuring the oxygen in the soil or in the water of the effluent. Although the removal of nitrates can be verified by measuring dissolved organic carbon, phosphorus retention can also be verified by the availability of reactive iron and aluminum in acid soils or in alkaline soils for Ca and Mg (^{Reddy & D’Angelo, 1997}).

Nitrogen removal in constructed wetlands includes processes such as NH₃ volatilization, nitrification, denitrification, N₂ fixation, plant and microbial uptake, mineralization, nitrate reduction to ammonium, anaerobic oxidation of ammonia, fragmentation, adsorption, desorption, landfilling, leaching, etc. (^{Gao, Zhang, Gao, Jia, & Yang, 2018}).

Plants indirectly influence the removal of nitrogen by nitrification and denitrification. These processes affect oxygen concentration, particularly in the rhizosphere, and increase the supply of potentially denitrifying bacteria, by limiting organic carbon and nitrates.

The transport of oxygen to the rhizosphere through the plants promotes the oxygenation points adjacent to the root zone and triggers the nitrification process (^{Ye & Li, 2009}).

The presence of plants in wetlands promotes the elimination of nutrients by decreasing flow velocity, increasing hydraulic loading, and reducing resuspension of sediments (^{Laterra, Booman, Picone, Videla, & Orúe, 2018}). In the same way, intermittent aeration promotes nitrification and denitrification (^{Llyas & Masih, 2017}).

The removal of phosphorus from wetlands begins with the process of adsorption, precipitation, plant uptake and peat accumulation, fragmentation, leaching, mineralization and sedimentation. However, the limited adsorption capacity of the substrate and the inhibitory effect of microbes and plants at low temperatures are serious obstacles preventing constructed wetlands from effectively removing nitrogen and phosphorus.

Phosphorus removal in constructed wetlands is carried out in a biological and physicochemical way. Although the plant-nutrient interaction of the wetland is important for streamlining the eutrophic water mass treatment process, nutrient uptake by plants and microbes in wetlands is a complex process that includes series of chemical and biological reactions (^{Zhao, Li, & Chen, 2018}). Biological methods are given by phosphorus uptake in plant tissues and finalized by harvest, while in physico-chemical methods, where the precipitation and adsorption process is carried out, the oxidation-reduction potential and the support are the most important (^{Andrés, Araya, Vera, Pozo, & Vidal, 2018}).

Currently, more attention is being paid to the elimination of pesticides. These products are removed in these systems by physical (sedimentation, flocculation, absorption, coprecipitation, precipitation), chemical (oxidation, reduction, cation exchange, hydrolysis, photolysis), biological (absorption and metabolism of the plant) or biochemical (microbial degradation) processes (^{Vymazal & Březinová, 2015}). Constructed wetlands can be classified according to different criteria: hydrology (surface flow and subsurface flow), types of macrophytes (floating, emergent and submerged) and flow path (horizontal or vertical) (^{Vymazal, 2011}). They can be successfully applied for the decentralized treatment of wastewater in sparsely populated areas (^{Machado, Beretta, Fragoso, & Duarte, 2017}).

In the subsurface flow wetlands, the flow of water runs underground, allowing oxygenation of the system and the treatment of high loads of organic pollutants, while FWS and FIs (Floating Islands) are systems flooded with a water mirror exposed to the environment whose only form of oxygenation of the system is through the vegetation.

The depth of the IFs must allow the vegetation to settle in the bottom, which allows the accumulation of organic remains from plant, animal and human sources.

Although different types of constructed wetlands offer different advantages in terms of wastewater treatment, Table 2 compares the advantages and disadvantages, both operational and functional, of water purification efficiency for nitrogen (ammoniacal, nitrates, nitrites, organic) and phosphorus (phosphates, organic) between different constructed wetlands (Free Water Surface, Subsurface Flow and Floating Islands). In Horizontal Flow wetlands the decomposition of organic matter occurs through aerobic and anaerobic microbial processes, as well as sedimentation and filtration of particulate organic matter. Although the removal of nitrogen is by nitrification, anoxic and anaerobic conditions are suitable for denitrification. Although the removal of phosphorus is produced by adsorption and precipitation (^{Vymazal, 2014}). In Vertical Flow wetlands, oxygen diffusion from the air contributes much more to the oxygenation of the filtration bed.

Higher system oxygenation (> 1.50 mg OD L^-1 for nitrification and < 0.50 mg OD L^-1 for denitrification), produces increased biodegradation and efficiency for the removal of nitrogen and organic matter in the system.

Floating island plants have a good ability to absorb N and P. The following are the most important processes in aquatic models: nutrient uptake by emerging macrophytes, phytoplankton and submerged plants, zooplankton feeding, detritus, sediments, phosphorus and sediment mineralization, nutrient propagation, and diffusion of phosphorus and nitrogen (^{Pavlineri et al., 2017}; ^{Kedlec y Wallace, 2009}; ^{Vymazal, 2013}; ^{Vymazal, 2005}; ^{Foladori et al., 2013}; ^{Cooper & Knight, 1996}).

In Vertical Flow wetlands, the diffusion of oxygen in the air contributes a lot to the oxygenation of the filtration bed. Hybrid wetlands stem from the need to increase the benefits of vertical and horizontal flow wetlands (^{Vymazal, 2014}).

Low treatment efficiency, as well as clogging, are problems caused by conventional substrates, for which the research also focuses on the search for profitable and efficient substrates (oyster shells, pieces of tires, construction waste, expansive clay aggregates, etc.) to increase the purification capacity and to minimize clogging issues. Ideally, substrates have unique physicochemical properties for the efficient removal of pollutants (^{Gao et al., 2018}).

The longer the retention time, the higher the pollutant removal percentage (^{Ramírez-Carrillo et al., 2009}).

In order to remove high rates of organic matter, so-called HIGH-rate constructed WETlands (HIGHWET) are emerging as a new variant of wetlands. These systems use plastic as a support, which promotes biofilm formation and achieves a porosity of up to 94-96% (^{Corzo et al., 2016}).

Gravel and volcanic slag are often used as substrates for the rehabilitation of polluted rivers. By using slag as a substrate, it is possible to further eliminate Total Phosphorus (TP) and to reduce the Chemical Oxygen Demand (COD). However, the adsorption of TP (Total Phosphorus) by slag quickly saturates during the follow-up period (^{Ge et al., 2015}). P concentration is regulated in wetland sediments by adsorbing it and releasing it under certain physicochemical and biological conditions (^{Cui, Xiao Xie, & Zhang, 2018}). To optimize P removal in wetlands, different materials have been used, allowing this process in the long run. The following are some of the media used in wetlands to improve the removal of P: zeolite, dolomite, gravel, sand, limestone and apatite, alunite, red mud, ash and slag (^{Gao et al., 2018}).