Integrated Phytobial Remediation of Dissolved Pollutants from Domestic Wastewater through Constructed Wetlands: An Interactive Macrophyte-Microbe-Based Green and Low-Cost Decontamination Technology with Prospective Resource Recovery

Abstract

Macrophytes have the potential to withstand pollutant-induced stress and can be used to clean contaminated water using phyto-extraction, phyto-degradation, phyto-filtration, phyto-stimulation, and phyto-volatilization technique(s). Phytoremediation through constructed wetlands (CWs) for eliminating inorganic and organic pollutants from household sewage and wastewater has attracted scientific attention. CWs are artificially engineered treatment systems that utilize natural cycles or processes involving soils, wetland vegetation, and plant and soil-associated microbial assemblages to remediate contaminated water and improve its quality. Herein, we present a detailed assessment of contaminant removal effectiveness in different CW systems, i.e., free-water surface or surface-flow constructed wetlands (FWSCWs/SFCWs), subsurface-flow constructed wetlands (SSFCWs), and hybrid constructed wetlands (HCWs). Several wetland floral species have been reported as potential phytoremediators, effectively reducing aquatic contamination through biodegrading, biotransforming, and bioaccumulating contaminants. Water hyacinth (Pontederia crassipes) is one of the most resistant macrophytes, capable of tolerating high nitrate (NO₃⁻) and phosphate (PO₄²⁻) concentrations. Other aquatic weeds also effectively alleviate biological oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), and pathogen levels and ameliorate the impact of different ionic forms of nitrogen (N), phosphorus (P), and trace elements (TEs). The review primarily focuses on using hydrophyte(s)-microbe(s) associations in different CWs as an essential phytoremediation tool for sustainable management of freshwater ecosystems, ecorestoration, and prospective resource recovery, favoring a circular bioeconomy (CBE).

1. Introduction

Aquatic ecosystems contaminated by inorganic and organic pollutants are a significant concern. These contaminants are often challenging to completely degrade or deplete biologically, but they can be biologically converted from highly or moderately toxic to less or non-toxic [1]. The water quality of the world’s significant freshwater and saltwater resources is deteriorating due to increasing population, industrialization, urbanization, tourism, and the overuse of natural land and water resources (Table 1). Contaminated aquatic environments disrupt ecosystems, altering the lives of humans, animals, plants, and microbes. Water pollution has adverse implications at the community and species levels of aquatic ecosystems. Eutrophication (also referred to as “algal bloom”) of freshwater and marine water ecosystems is caused by various ionic forms of plant-essential nutrients, such as nitrogen (N) and phosphorus (P), the critical components of chemical fertilizers that are carried with agriculture runoff, which is another potential contributor to water pollution [2].

Among the physicochemical and biological strategies developed for wastewater clean-up, phytoremediation using aquatic floral species and their associated microflora is the most desirable, green, and affordable. The word ‘phytoremediation’ is composed of the Greek prefix “phyto” (plant) and the Latin root “remedium” (to remedy) [3]. The initial step in this plant-based decontamination method is to screen and identify aquatic plants that have demonstrated high efficiency in accumulating dissolved nutrients and other inorganic and organic pollutants from contaminated water [4]. Aquatic macrophytes suitable for phytoremediation of wastewater should be resistant, fast-growing, and easy to handle and harvest. Elevated pollutant levels in soils or wastewater inhibit plant nutrient uptake, photosynthesis, growth and development, enzymatic activities, and survival. Some macrophytes can tolerate pollutant-induced abiotic stress (even under high concentrations of dissolved inorganics and organics) and can be employed for the phytoremediation of contaminated water. Several aquatic floral species have been reported as phytoremediators in CWs, including but not limited to Acorus calamus L., Canna indica L., Chrysopogon zizanioides (L.) Roberty, Colocasia esculenta (L.) Schott, Dracaena sanderiana Sander ex Mast., Eichhornia crassipes (Mart.) Solms, Heliconia, Phragmites australis (Cav.) Trin. Ex Steud., Scirpus grossus L.f., and Typha latifolia L. [5]; they are also highly effective in minimizing water contamination through biotransforming and bioaccumulating inorganic and organic pollutants in their tissues [5].

Constructed wetland (CW) systems, a passive biological treatment technology that mimics natural wetland (NW) ecosystems, are designed and built to facilitate wastewater treatment by utilizing natural cycles involving soils, wetland plants, and their associated microbiota and ameliorating water quality [6]. CWs have been a popular alternative to conventional wastewater treatment strategies since the 1960s because of their low cost, environmental friendliness, straightforward operation, and ease of maintenance [6,7,8]. Furthermore, in CWs, various types of contaminant removal processes (e.g., adsorption, evaporation, filtration, precipitation, sedimentation, root uptake, followed by accumulation and translocation in plants and a variety of microbial processes) typically occur directly and indirectly under different external and internal environmental conditions like availability of dissolved oxygen (DO) and organic carbon (C) sources, handling strategies, pH, redox conditions, and temperature [9,10,11]. Other biological activities of plants, such as photosynthesis, health, and nutrition, are critical for the long-term sustainability of aquatic environments. A particular phytoremediation system’s effectiveness is also determined by aspects connected to the severity of the contamination [12].

In the past few decades, CWs have been widely employed in treating a variety of wastewaters, including agricultural wastewater, domestic sewage, effluents from the livestock industry, industrial effluents, landfill leachates, mine drainage, polluted lake or river water, stormwater runoff, and urban runoff [10,11,13]. Numerous studies (both research and review) have been conducted on CWs’ design, development, and performance. It has been suggested that CW systems may be effective in eliminating various environmental pollutants, including organic matter (OM), dissolved nutrients [14,15], trace elements (TEs) [16], pharmaceutical contaminants (e.g., antibiotics) [17], emerging pollutants (e.g., flame retardants, food additives, hormones, nanoparticles, persistent organic pollutants {like polychlorobiphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs)}, pesticides [18], pathogens [19], and so on, from wastewater.

Significant progress has been made in pollutant removal mechanisms within CW systems. However, a vacuum remains in our comprehension of these artificially engineered wetland systems, limiting our ability to produce long-term improvements in water quality. Using Google Scholar and Web of Science, we used the terms in the title and keywords to identify and synthesize relevant literature [20]. This literature review starts by defining and reviewing phytoremediation techniques, presenting examples of highly polluted riverine systems, and differentiating among the six leading phytoremediation technologies. Next, the types and sources of pollutants in domestic wastewater are identified. The study extensively reviews how phytoremediation through different CW systems using macrophytes and their associated microbial assemblages with plant growth-promoting (PGP) traits has emerged as an innovative, integrated, green, and low-cost eco-restorative strategy to facilitate the elimination of dissolved inorganic and organic nutrients and pollutants from domestic sewage and the consequent recovery of value-added by-products. The study also examines recent improvements in CWs, including aquatic weed selection and operational parameter optimization for the sustainability of wastewater treatment. The paper discusses the pros and cons of using CWs in wastewater phytoremediation. Future research considerations for improving CW systems as an integrated phytobial wastewater decontamination technology are also highlighted.

2. Phytoremediation: An In Situ Plant-Based Clean-Up Technology

According to [20], phytoremediation refers to the efficient utilization of plants to detoxify, immobilize, or remove environmental contaminants. This rapidly evolving, plant-based, environmentally beneficial, affordable, in situ remediation technology is used for treating inorganic and organic pollutants present in the air, freshwater (groundwater and surface water), brackish water, marine water, wastewater, stormwater, sediments, soils, sludge, and salt marshes [21,22]. In this process, absorption of contaminants by roots, accumulation in plant tissues, transformation, and decomposition of pollutants occur, resulting in cleaner water [23]. Fast-growing plants, which can be easily handled and harvested, are selected and used in phytoremediation [24]. Climate conditions have a significant impact on plant development. Proper precipitation timing, optimal amounts of sunlight, shade, wind, appropriate air temperature, and growing season length are needed to ensure maximum growth. Phytoremediation, however, necessitates a commitment of money and time. Still, it has shown the prospects of being a cost-effective and green alternative to traditional remediation strategies at appropriate locations.

Table 1. Some of the World’s most polluted lakes/Rivers.

Sl. No.	Country/Province Name	Length of the River/Lack (km)	World’s Most Polluted Lakes/River	Source of Pollutant	References
1.	BANGLADESH	27 km	Buri ganga River	Mountains of plastic, sewage-related trash, and residential and industrial waste	[25]
2.	PHILIPPINES	12.42 km	Marilao River	Open landfills, metal refining, lead-acid batteries, and other harmful metals.	[26]
3.	JORDAN, ISRAEL, and PALESTINE	251 km	Jordan River	Untreated sewage and Agricultural Runoff.	[27]
4.	NORTHEASTERN AFRICA	6650 km	Nile River	Intensive load of urban, agricultural, and industrial wastewater	[28]
5.	RUSSIA	0.75 km	Lake Karachay	Radioactive wastes	[29]
6.	ITALY	24 km	Sarno River (stream)	Untreated industrial waste and trash from agriculture	[30]
7.	KENYA	390 km	Nairobi River	Untreated sewage and industrial waste.	[31]
8.	TASMANIA	13 km	King River	Acid drainage from the old mines.	[32]
9.	ARGENTINA	64 km	Matanza River (stream)	Sewage and waste water treatment, farming and fossil fuel power plants	[33]
10.	SOUTH OF CHICAGO	25.7 km	Grand Calumet River	Industrial waste and municipal effluent.	[34]
11.	ARGENTINA AND URUGUAY	499 km	Plate River	Urban runoff.	[35]
12.	AUSTRALIA	2508 km	Murray-Darling River	Tillage, cultivation, fertilizers, and animal droppings.	[36]
13.	BRAZIL	853 km	Doce River	Agriculture (e.g., fertilizers and pesticides) and inadequate disposal of municipal waste.	[37]
14.	NORTHERN NEW JERSEY	130 km	Passaic River	Industrialization Watershed. Layers of dioxin, mercury, PCBs, and other harmful chemicals.	[38]
15.	EUROPE	2857 km	Danube River	Organic material, nutrients, hazardous substances, and plastics.	[39]
16.	INDIA	1376 km	Yamuna	Untreated wastewater, etc.	[40]
17.	INDIA	2525 km	Ganges	Sewage dumped industrial waste, agricultural runoff, partially burned or unburned bodies from funeral pyres, and animal carcasses.	[41]
18.	TIBET, INDIA and PAKISTAN	3180 km	Indus River	Municipal, industrial wastewater discharges and, return-agriculture flows through drainage structures.	[42]
19.	CHINA	6300 km	Yangtze River	Industrial wastewater, agricultural chemical fertilizer, ship garbage and acid rain	[43]
20.	CHINA	5464 km	Yellow River	Factory discharges and sewage.	[44]
21.	CHINA and MYANMAR	3289 km	Salween River	Textile factories’ waste.	[45]
22.	INDONESIA	297 km	Citarum River	Industrial waste and high organic waste from cattle.	[46]
23.	INDONESIA	320 km	Brantas River	Micro-plastics pollution	[47]
24.	USA	3034 km	Rio Grande River	High concentrations of salts, microorganisms, and industrial and agricultural contaminants.	[48]
25.	USA	3766 km	Mississippi River	Agricultural runoff	[49]

Table 1. Some of the World’s most polluted lakes/Rivers.

Sl. No.	Country/Province Name	Length of the River/Lack (km)	World’s Most Polluted Lakes/River	Source of Pollutant	References
1.	BANGLADESH	27 km	Buri ganga River	Mountains of plastic, sewage-related trash, and residential and industrial waste	[25]
2.	PHILIPPINES	12.42 km	Marilao River	Open landfills, metal refining, lead-acid batteries, and other harmful metals.	[26]
3.	JORDAN, ISRAEL, and PALESTINE	251 km	Jordan River	Untreated sewage and Agricultural Runoff.	[27]
4.	NORTHEASTERN AFRICA	6650 km	Nile River	Intensive load of urban, agricultural, and industrial wastewater	[28]
5.	RUSSIA	0.75 km	Lake Karachay	Radioactive wastes	[29]
6.	ITALY	24 km	Sarno River (stream)	Untreated industrial waste and trash from agriculture	[30]
7.	KENYA	390 km	Nairobi River	Untreated sewage and industrial waste.	[31]
8.	TASMANIA	13 km	King River	Acid drainage from the old mines.	[32]
9.	ARGENTINA	64 km	Matanza River (stream)	Sewage and waste water treatment, farming and fossil fuel power plants	[33]
10.	SOUTH OF CHICAGO	25.7 km	Grand Calumet River	Industrial waste and municipal effluent.	[34]
11.	ARGENTINA AND URUGUAY	499 km	Plate River	Urban runoff.	[35]
12.	AUSTRALIA	2508 km	Murray-Darling River	Tillage, cultivation, fertilizers, and animal droppings.	[36]
13.	BRAZIL	853 km	Doce River	Agriculture (e.g., fertilizers and pesticides) and inadequate disposal of municipal waste.	[37]
14.	NORTHERN NEW JERSEY	130 km	Passaic River	Industrialization Watershed. Layers of dioxin, mercury, PCBs, and other harmful chemicals.	[38]
15.	EUROPE	2857 km	Danube River	Organic material, nutrients, hazardous substances, and plastics.	[39]
16.	INDIA	1376 km	Yamuna	Untreated wastewater, etc.	[40]
17.	INDIA	2525 km	Ganges	Sewage dumped industrial waste, agricultural runoff, partially burned or unburned bodies from funeral pyres, and animal carcasses.	[41]
18.	TIBET, INDIA and PAKISTAN	3180 km	Indus River	Municipal, industrial wastewater discharges and, return-agriculture flows through drainage structures.	[42]
19.	CHINA	6300 km	Yangtze River	Industrial wastewater, agricultural chemical fertilizer, ship garbage and acid rain	[43]
20.	CHINA	5464 km	Yellow River	Factory discharges and sewage.	[44]
21.	CHINA and MYANMAR	3289 km	Salween River	Textile factories’ waste.	[45]
22.	INDONESIA	297 km	Citarum River	Industrial waste and high organic waste from cattle.	[46]
23.	INDONESIA	320 km	Brantas River	Micro-plastics pollution	[47]
24.	USA	3034 km	Rio Grande River	High concentrations of salts, microorganisms, and industrial and agricultural contaminants.	[48]
25.	USA	3766 km	Mississippi River	Agricultural runoff	[49]

2.1. Different Plant Strategies Used for Phytoremediation

The techniques of phytoremediation (often referred to as phytotechniques or phytotechnologies) are categorized into six main types: (a) phytoextraction, (b) phytodegradation, (c) phytofiltration, (d) phytostabilization, (e) phytostimulation, and (f) phytovolatilization [50,51]. These processes must be defined to clarify and understand the detoxification, immobilization, and elimination mechanisms that may occur naturally due to soil and wetland vegetation. Numerous strategies are involved in phytoremediation (Figure 1).

Besides phytoextraction, macrophyte-mediated decontamination of domestic wastewater is primarily achieved through four main phytotechniques: (a) phytodegradation, (b) phytofiltration, (c) phytostimulation, and (d) phytovolatilization. Each one of these phytotechniques is briefly discussed below.

2.1.1. Phytoextraction

Phytoextraction (also known as phytoaccumulation or phytosequestration) refers to the natural contaminant-removal ability (e.g., metal(loid)s or other inorganics) of algae or plants from ambient soil or water into a harvestable pollutant-rich plant biomass [53]. Hyperaccumulators (i.e., organisms capable of high concentrations of pollutant uptake) are typically regarded as ideal candidates for phytoextraction. Hyperaccumulators uptake high amounts of metal(loid)s, TEs, or other inorganics through their roots, shoots, and leaves from surrounding soil and water, transport, and accumulate or concentrate them above ground in their foliage or other aerial vegetative and reproductive plant parts [53]. However, most hyperaccumulators are slowly growing, increasing the overall phytoremediation time [54]. Notably, accumulator species like low-metal-accumulating, fast-growing, and high-biomass-producing plants or perennial grasses (with deep and extensive roots) can also perform phytoextraction efficiently and remove significant amounts of soil contaminants; moreover, they can thrive in moist soils [55].

Phytoextraction has become increasingly popular worldwide as a phytoremediation strategy. More than 400 hyperaccumulator plant species are known of late for HM hyperaccumulation based on innumerable phytoextraction trials that have been carried out thus far, and many new species are being constantly discovered [52,56]. Hyperaccumulation emerged as an adaptative technique to thrive in or withstand metalliferous environments since high metal toxicity could be detrimental to plant growth and survival [52,56]. For instance, most higher plants are unable to tolerate chromium (Cr) concentrations > 100 μM kg⁻¹ dry weight [57].

Furthermore, interactive plant-microbe association at the endosphere and rhizosphere accelerates phytoremediation activity by increasing the bioavailability of otherwise insoluble metals through various microbial secondary metabolite secretions like biosurfactants, exopolymers, organic acids, and siderophores [56]. To minimize the risk of incorporating toxic substances into the food chain from soil and efficient recovery, the concept and practice of phytomining (the mining of phytoextracted metals) have been evolving rapidly, where pollutants are usually concentrated in a relatively smaller biomass volume than the initial contaminated sediment or soil during disposal. As a low level of contaminating inorganics remains in the soil after harvest, the process of growth and harvest usually needs to be repeated via several rounds of cropping to achieve considerable clean-up [54]. Some researchers have reported on the phyto-management of the biomass of ornamental plants used in composting compaction, biogas production, mat making, and fly ash brick production [58].

Hyperaccumulation’s genetic basis lies in the fact that the roots of accumulators and hyperaccumulators secrete acidic exudates containing different metal-mobilizing substances like carboxylates, organic acids, and phytosiderophores, which ultimately increase metal solubility within the soil, thereby enhancing the metal-uptake process [59]. Moreover, in phytoextraction, the rhizomicrobiota play a crucial role in accelerating plant metal uptake by increasing their solubility for absorption. Rhizomicrobes secrete chelators (biosurfactants, exopolymers, and siderophores) and enzymes into the rhizospheric region that aid in forming chelator-metal(loid) conjugates, enhancing their uptake and transport. Siderophores are low-molecular-weight ion-chelating compounds that display a strong affinity towards ferric ions (Fe³⁺) while having variable affinity towards other metal(loid) ions, including arsenite (As³⁺), arsenate (As⁵⁺), cadmium (Cd²⁺), lead (Pb²⁺), and nickel (Ni²⁺) among others.

2.1.2. Phytodegradation

Phytodegradation (otherwise referred to as phytotransformation) is the breakdown or disintegration of organic molecules by phytoenzymes or plant-associated microbial enzymes into TCA cycle intermediates, CO_2, and H₂O [60]. For the degradation and transformation of organic compounds, plants (including hydrophytes) primarily use three stepwise mechanisms: (a) transformation (where organic compounds are activated and transformed); (b) conjugation (where organic compounds are conjugated with amino acids, D-glucose, glutathione, and malonic acid to minimize their mobility and toxicity); and (c) removal or storage (uptake and accumulation of the partially-degraded transformants in organelles like apoplasts, cell walls, and vacuoles or removal of the wholly degraded products, CO₂ and H₂O) [61]. Through stomata (openings present in plant leaves), a few volatile photodegradable by-products can quickly vaporize and enter the ambient atmosphere. Due to the metabolic cascade’s similarity to mammalian metabolism, it is referred to as the “green liver model” [62]. In phytodegradation, the rhizobacteria degrade the dissolved organic pollutants; after their uptake, the endophytes degrade the degradable ones.

2.1.3. Phytofiltration

Phytofiltration (also called rhizofiltration) refers to phytoextraction in water (instead of soil) performed by aquatic plants (also known as hydrophytes or macrophytes). Hydrophytes can treat and clean up domestic sewage or wastewater, like wastewater treatment plants. In this phytotechnique, hydrophytes absorb and uptake dissolved toxic substances like metal(loid)s, TEs, or excess dissolved nutrients from contaminated water and precipitate them onto plant parts or adsorb them onto roots [63]. The metal(loid) and radionuclide absorption abilities of seaweeds and other aquatic plants through rhizofiltration have long been known. However, large-scale testing has yet to be done because there are no affordable cultivating, collecting, and handling techniques. The evolution of aquaponic or hydroponic systems facilitates using hydroponically grown terrestrial flora to clean up contaminated wastewater. Hydroponic systems possess high-shoot biomass and extensive root systems to absorb and uptake a significant concentration of TEs from wastewater. Three different categories of kinetics were reported to be involved in the metal movement from wastewater to the plant roots, including (a) physicochemical processes like adsorption, chelation, and ion exchange; (b) biological processes that depend on plant metabolism and gradual transfer of the metal from wastewater to plant cells through intracellular uptake and transport to the shoots and vacuolar deposition within the cells; and (c) root exudate-mediated metal precipitation from the wastewater as insoluble compounds (like phytostabilization) [63]. Another attribute or factor commonly found in aquatic and wetland flora (including mangrove-associated flora, salt marshes, and submerged macrophytes) that accounts for selective metal(loid) or TE ion sequestration, followed by their consequent uptake and translocation, is iron (Fe) or manganese (Mn) plaques (IPs, MPs). Ips and MPs are ferric and manganese hydr(oxide) precipitates (reddish or brown) enveloping the plant roots, formed near the rhizospheric zone. Ips and MPs may act as a barrier or buffer and hence may decrease or increase metal(loid)/TE uptake by wetland plants. In the former case, when IPs and MPs act as barriers, low IP and MP-producing aquatic flora may serve as better metal accumulators and be applied in CWs for pollutant remediation. In contrast, high IP and MP-forming edible floral species would render them safe for human consumption.

On the contrary, in the latter case, when IPs and MPs act as buffers for minerals, nutrients, and toxicants, then even though those plant species may become nutrient-rich, they may also be metal(loid) rich, resulting in toxicity upon animal and human consumption [51]. The formation of root IPs and MPs, in turn, may increase root bioavailability and decrease mobility of metal(loid) and TE species through their adsorption or sequestration (e.g., As sequestration by IP on cattail (Typha latifolia) roots) or immobilize and stabilize them via their coprecipitation (e.g., Fe, Mn, Pb, and Zn precipitation on reed canary grass (Phalaris arundinacea L.) roots occasionally through complexation with organics, induced by rhizobacteria). [64] For example, Christensen and Sand-Jensen (1998) [65] demonstrated that precipitated IPs and MPs restrict P uptake by Dortmann’s cardinalflower (Lobelia dortmanna L.) roots and its accumulation when grown in reduced Fe and Mn-rich sediments through their adsorption onto oxidized Fe and Mn in the plaques. In another experiment, ref. [66] showed the impacts of Ips and MPs on As uptake and transport by hydroponically-grown rice (Oryza sativa L.) seedlings supplemented with As⁵⁺ and As³⁺.

In the phytofiltration technique, the plants are first grown hydroponically in clean water (instead of soil) to develop a branched root system under precise greenhouse conditions [67]. After creating an extensive root system, the clean water is replaced with the effluent to be remediated, and the plants are allowed to acclimate to the HM pollutants. Then, the pollutant-acclimated plants are transferred to the contaminated area, where root uptake of contaminants occurs along with the polluted water. Once the plant roots get saturated with contaminated HMs, they are harvested and disposed of following safe and stringent measures. Through such repeated treatments, it is possible to reduce the contamination of metal-impacted sites to appropriate levels [68,69].

2.1.4. Phytostimulation

Phytostimulation (also known as rhizodegradation or rhizoremediation) refers to the degradation/transformation of organic pollutants in the plant’s rhizosphere (i.e., the microbe-rich region of the soil in close contact with the plant’s root system) with the help of rhizosphere-associated microbiota and by exploiting microbial functions that are being enhanced by the plant root exudates rich in acids and carbohydrates [70,71]. Rhizodegradation is an essential tool for the phytoremediation of PAHs, PCBs, and total petroleum hydrocarbons (TPHs) [72]. The fate of organic pollutants, including PAHs, in the environment relies on abiotic (chemical oxidation) and biotic (bioaccumulation and biotransformation) processes. Microbial transformation (or complete chemical degradation) is considered the most important and influential process of environmental PAH removal [73]. In an in-field experiment, [74] utilized a combination of plant microbiota for TPH rhizodegradation and removal. The study used plants belonging to Poaceae (Dactylis glomerata L. and Festuca arundinacea Schreb.) and Fabaceae (Lotus corniculatus L. and M. sativa), which are already noted for rhizodegradation stimulation and hydrocarbon accumulation [75], along with the inoculation of the endomycorrhizae consortium for bioaugmenting the indigenous bacterial flora in the rhizosphere. Similar studies have been carried out using macrophytes. Aquatic plants with functional populations of organic-pollutant-degrading microbes also display phytostimulation ability, as in the case of stimulating hornwort-mediated degradation of atrazine (herbicide) [76].

2.1.5. Phytovolatilization

Phytovolatilization refers to pollutant removal from contaminated soil or water with subsequent release into the ambient air, occasionally because phytodegradation converts it to a more volatile or less toxic form(s). In this phytotechnique, aquatic plants uptake toxicants. As water gets transported to the leaves from roots and stems via the plant’s vascular system, it evaporates into the atmosphere through transpiration [63]. Direct phytovolatilization (a well-studied form) involves the volatilization of contaminants through plant leaves and stems, while in indirect phytovolatilization, pollutants are volatilized from the plant roots [77]. Heavy metal(loid)s and TEs like arsenic (As), selenium (Se), and mercury (Hg) and VOCs are often eliminated from soil and water through the process of phytovolatilization [56,72]. Studies on hydroponically treated Indian mustard plant (Brassica juncea (L.) Czern.) with HgCl₂ demonstrated that post-HM uptake, volatilization of Hg occurs in roots as opposed to shoots, owing to meager HM translocation rates, partially influenced by the algal and microbial communities; this is an example of indirect phytovolatilization [63]. Plants with high rates of transpiration usually display efficient phytovolatilization.

2.2. Sources and Types of Pollutants in Domestic Wastewater

Domestic wastewater encompasses two major waste streams: greywater and blackwater (Scheme 1). The former represents water emanating from the bath, kitchen sink, and laundry; the latter represents water from toilets, including feces, perspiration, and urine. The pollutant content in domestic sewage is innumerable, varied, and complex and has been discussed in depth by [78]. The four principal sources of pollutants found in residential sewage are water supply, materials used in household infrastructure (plumbing, fixtures, and equipment used for water delivery and wastewater collection), waste produced by humans, and household practices and products used in daily activities at home. House cleaning (disinfectants, bleaches, etc.), laundry (laundry detergents, laundry softeners, and other laundry products), personal care (deodorants and body lotions, oral hygiene, soaps, shampoo and conditioners, sunscreens, etc.), and infrastructure (appliances and activities, copper pipes, geysers, materials, polyethylene, polyvinyl chloride or PVC, vitrified clay, etc.) products are all examples of residential sewage pollution.

Passive movement or migration of contaminants from domestic items to water, as well as corrosion of metal components, can cause contamination of household infrastructure. Human or anthropogenic sources represent human activities that result in wastewater generation, including waste excretion through metabolic processes (sweat, urine, and feces) and waste generation through residents’ actions and behaviors like food preparation, grooming, cleaning, and bathing. Household infrastructure, water consumption, and activities all influence contaminant levels within each household. The type of home products used, their quantity and frequency of use, the habits of the homeowner, their diet, and the use of domestic appliances like dishwashers and washing machines, as well as their age, all influence the anthropogenic pollutants in wastewater [78].

Chemically, domestic wastewater consists of inorganic (30%) and organic (70%) substances and different gases. Organic compounds are primarily made up of proteins (65%), carbohydrates (25%), and fats (10%), reflecting people’s diets. Inorganic compounds encompass acids, bases, chlorides, trace elements (TEs)/heavy metals (HMs)/metalloids, N, P, sulfur (S), toxic compounds, etc. However, as sewage contains more dissolved solids or particles than suspended ones, a greater percentage of the total inorganic components (around 85–90%) and the organic components (around 55–60%) are dissolved. Ammonia (NH₃), oxides of C like carbon dioxide (CO₂) and carbon monoxide (CO), chlorine (Cl₂), hydrogen sulfide (H₂S), methane (CH₄), nitrogen (N₂), oxygen (O₂), and sulfur dioxide (SO₂) are harmful gases that are frequently dissolved in wastewater. CH₄, H₂S, and NH₃ are produced because of sewage OM decomposition [19,79].

The different types of abiotic and biotic constituents and pollutants present in domestic wastewater include (1) biodegradable organic materials (animal wastes, excretory wastes, food ingredients, garden wastes, vegetable wastes, etc.); (2) other organic constituents (coloring agents, cyanides (CN), detergents, fats, oil and grease, pesticides, phenols, solvents, etc.); (3) minerals or nutrients {ammonium (NH₄⁺), bicarbonate (HCO₃⁻), chlorine (Cl⁻), chlorine compounds, N, NO₃⁻, P, PO₄²⁻, sodium salts, sulfate (SO₄²⁻), etc.}; (4) toxic HMs/metalloids/TEs {arsenic (As), boron (B), cadmium (Cd), chromium (Cr), copper (Cu), fluorine (F), lead (Pb), mercury (Hg), nickel (Ni), zinc (Zn), etc.}; (5) other inorganic materials (acids and bases, H₂S); and (6) pathogenic and nonpathogenic macro- and microorganisms (algae, archaea, bacteria, fungi, plankton, protozoa, viruses, worms’ eggs/larvae, etc.) [19,80]. These domestic wastewater pollutants have several environmental implications for sewage discharge into nearby waterbodies. They are as follows: (1) biodegradable organic materials cause O₂ depletion in lakes and rivers, resulting in malodours and fish deaths; (2) other organic materials cause aesthetic inconveniences, harmful effects, and food chain bioaccumulation and biomagnification; (3) excess nutrients lead to eutrophication, O₂ depletion, and toxicological effects; (4) HMs exert toxic effects in aquatic (macro/micro) flora and fauna through their bioaccumulation in the organism’s body tissues; (5) other inorganic materials produce foul odors, cause corrosion, and exert toxic effects; and (6) pathogenic macro/microorganisms pose health risks to humans during bathing, drinking, and eating finfish, shellfish, and other aquatic creatures [80]. The standard acceptable limits of physicochemical and biological parameters in domestic sewage vary (

Table 2. Standard permissible limits of different physicochemical and biological analysis parameters in domestic wastewater.

Sl. No.	Physico-Chemical and Biological Analysis Parameters	Unit	Permissible Standard Limit(s)	References
1.	Alkalinity	eqv/m3 **	37 *	[19,79,80]
2.	Conductivity	mS/m ***	120 *
3.	pH	–	5.5–9.0
4.	Temperature	°C	22–32 #
5.	Ammoniacal nitrogen (NH3-N)	mg/L	50
6.	BOD3 (3-days Biological Oxygen Demand at 27 °C)	mg O2/L	350
7.	BOD5 (5-days Biological Oxygen Demand at 27 °C)		350 *
8.	COD (Chemical Oxygen Demand)		740 *
9.	Fats, oil, and grease	g/m3	100 *
10.	Oil and grease	mg/L	20
11.	Phenolic compounds		5.0
12.	Suspended Solids (SS)	g SS/m3	450 *
13.	Total Nitrogen (TN)	g N/m3	80 *
14.	Total Organic Carbon (TOC)	g C/m3	250 *
15.	Total Phosphorus (TP)	g P/m3	23 *
16.	Total Suspended Solids (TSS)	mg/L	600
17.	Volatile Suspended Solids (VSS)	VSS/m3	320 *
18.	Arsenic (As)	mg/L	0.01
19.	Cadmium (Cd)		1.0
20.	Copper (Cu)		3.0
21.	Cyanide (CN)		2.0
22.	Fluoride (F)		15.0
23.	Iron (Fe)		3.0
24.	Lead (Pb)		1.0
25.	Manganese (Mn)		2.0
26.	Mercury (Hg)		0.01
27.	Nickel (Ni)		3.0
28.	Selenium (Se)		0.05
29.	Total chromium (Cr)		2.0
30.	Vanadium (Vn)		0.2
31.	Zinc (Zn)		15.0
32.	Bioassay	–	90% fish survival after 96 h in cent percent effluent

Notes: * Domestic wastewater type: concentrated; ** 1 eqv/m³ = 1 m eqv/L = 50 mg CaCO₃/L; *** mS/m = 10 μS/cm = 1 m mho/m; ^# Should not exceed 5 °C above the incoming/receiving water temperature.

). Hence, domestic sewage treatment through primary, secondary, and tertiary processes is crucial before discharging into freshwater or brackish waters to safeguard animal, plant, and human health. Scheme 2 depicts the analysis parameters and domestic wastewater’s primary, secondary, and tertiary treatment processes.

2.3. Phytoremediation of Domestic Sewage/Wastewater

Phytoremediation techniques eliminate dissolved nutrients, heavy metals, and other contaminants through macrophytes. Vegetation is used to degrade/transform/remove pollutants as an inorganic and organic constituent cycle. Phytoremediation techniques are used extensively in CWs for wastewater treatment [81]. Several studies have been conducted on eco-friendly and low-cost domestic sewage treatment and recycling systems [82,83]. Nonetheless, the efficacy of wetlands for wastewater treatment varies depending on the design process and the hydrological environment [81]. CWs have been examined for combining sewer water treatment and reuse and have proven to be an effective strategy [83]. In the CW system for household sewage treatment, the typical removal rates of ammoniacal nitrogen (NH₃-N), chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) are 70–83 percent, 50–80 percent, 40–80 percent, and 50–70 percent, respectively [84,85,86]. For nutrient removal, many plant species, such as A. calamus, Arundo donax L., C. indica, Imperata cylindrica (L.) Raeusch., P. australis, Senna tora (L.) Roxb. (basionym: Cassia tora L.) and T. latifolia have been used [87].

3. Constructed Wetlands (CWs) in Wastewater Decontamination

CWs are artificially engineered secondary treatment systems designed with ecological principles of the natural processes of soils, wetland vegetation, and associated microbial assemblies to remove BOD, COD, N, and P, among others [58,88,89]. CW systems are utilized for treating domestic wastewater, greywater, industrial wastewater, agricultural run-offs, stormwater run-offs, mining wastewater, and landfill leachates [90]. Such treatment systems usually have a filter bed composed of substrate materials, contamination-endurable planted vegetation, a water column, and a free-living, root-associated microbial population. The substrate media can be clay, gravel, limestone, peat, sand, etc., where wetland plants are grown [10.82]. CWs are often referred to as “artificial wetlands”, “engineered wetlands”, “man-made wetlands”, “reed beds”, “soil infiltration beds”, and “treatment wetlands” [91].

The type of wastewater to be treated must be considered when designing a CW. CWs have been utilized in “centralized and decentralized wastewater systems” alike. When there are significant amounts of suspended solids (SS) or dissolved OM (estimated as BOD and COD), primary treatment of wastewater is advised before channeling the CW system [92]. Although not their primary function, some CWs could provide a niche for local and migratory birds and wildlife. Owing to the environmental treatment (or ecorestorative) benefits and economic and social benefits, a CW-based sewage treatment system is a potential or developing sewage treatment method [93]. This review delves into CWs, their fundamental classifications, and the significance of various components and process(ing) parameters in domestic wastewater treatment. The following sections will summarize the types of CWs and nutrient removal by plant (macrophyte) species. Finally, we discussed future research prospects for using aquatic plants and the types of CWs.

3.1. Types and Modes of Action of Constructed Wetlands (CWs)

The two major types of CWs are free-water surface (FWS) or surface-flow (SF) and subsurface-flow (SSF) CWs. Different categories of CWs can be connected in a so-called combined or hybrid system for specific advantages of each system. CWs can be classified based on diverse parameters, but the two most vital parameters are the water flow path or regime and the kind of aquatic plant growth (Figure 2 and Figure 3) [81,94,95,96].

3.1.1. Surface-Flow Constructed Wetlands (SFCWs)

Surface-flow CWs (SFCW) are also known as free-water surface CWs (FWSCWs) or free-water surface-flow CWs (FWSFCWs). In this treatment system, wastewater flows upwards through a substrate medium. In addition, it contains areas of open water and vegetation, such as free-floating, floating, submerged, and emerging plants. Factors like slow flow velocity, shallow water depth, and vegetation litter and stalks regulate water flow, especially in long, narrow channels under plug-flow conditions [97]. These wetland systems are primarily used for treating domestic sewage [98]. Additionally, FWSCWs provide advanced treatment for secondary-treated wastewater and stormwater run-off [99]. Furthermore, all climates are suitable for SFCWs, including those in the far north [100].

Figure 2. Classification of constructed wetland (CW) systems for wastewater treatment. Image concept adopted from Vymazal (2001b, 2007) [95,96] with modifications and created with Biorender.com.

Figure 2. Classification of constructed wetland (CW) systems for wastewater treatment. Image concept adopted from Vymazal (2001b, 2007) [95,96] with modifications and created with Biorender.com.

Figure 3. Types of constructed wetland (CW) systems for wastewater treatment and their experimental designs [adopted from Vymazal [95,96] (2001b, 2007) with modifications]. 1, 2: Sand and Gravel; 3: Root Uptake; 4: Translocation from roots to shoot in Macrophytes; 5: Microbial degradation; 6: Second Bed of Sand and Gravel; 7, 8: Effluent Outlet (Height variable).

Figure 3. Types of constructed wetland (CW) systems for wastewater treatment and their experimental designs [adopted from Vymazal [95,96] (2001b, 2007) with modifications]. 1, 2: Sand and Gravel; 3: Root Uptake; 4: Translocation from roots to shoot in Macrophytes; 5: Microbial degradation; 6: Second Bed of Sand and Gravel; 7, 8: Effluent Outlet (Height variable).

3.1.2. Subsurface-Flow Constructed Wetlands (SSFCWs)

Subsurface-flow CWs (SSFCWs) can be further subdivided into horizontal subsurface-flow CWs (HSSFCWs) and vertical subsurface-flow CWs (VSSFCWs), depending on the direction of water flow.

Horizontal Subsurface-Flow Constructed Wetlands (HSSFCWs)

In HSSFCWs, wastewater flows horizontally below the substrate surface through porous media and the pores of plant roots. It is collected at the outlet before being released through a water-level control structure. The substrate is usually gravel or a mixture of gravel and sand, supporting vegetation growth. The substrate bed depth typically varies between 0.3 and 0.8 m, which depends on the plant roots [101]. In HSSFCWs, the wastewater to be treated contacts a network of oxic and anoxic zones while passing through the wetland bed. These systems play a crucial role in anaerobic processes, as oxygen transport capacity is insufficient for ensuring aerobic decomposition. Even though the HSSFCW system effectively treats secondary-treated sewage, several other applications have been documented in the literature [91].

Vertical Subsurface-Flow Constructed Wetlands (VSSFCWs)

VSSFCW comprises a flatbed of gravel/sand along with aquatic plants. Wastewater is fed from the top through the perforated network and collected by an outlet system at the base. In general, the substrate bed depth varies from 45 to 120 cm. CWs with VSSF are fed intermittently in a large batch mode. Wastewater is fed vertically to the bed surface, which may cause a clogging problem, but vegetation assists in preventing the clogging of the bed [102]. In a recently developed influent-fed batch (“fill and drain”) mode operation, better contact of influent wastewater with the microbes present in the media is ensured, which dramatically increases the purification processes [81]. VSSFCWs are mainly used for domestic, dairy, landfill leachate, and food processing wastewater treatments [103]. Unlike horizontal flow (HF) systems, vertical flow (VF) systems usually require less space. VSSFCW can be further subdivided into vertical up-flow CW (VUFCW) and vertical down-flow CW (VDFCW) systems [94].

3.1.3. Hybrid Constructed Wetlands (HCWs)

HCWs are different wetland types that occur in series, where horizontal and vertical SSF systems, or vice-versa, are placed adjacent to each other in combination. CWs with HSSF-VSSF and VSSF-HSSF combinations are the most usual HCW systems; in general, any CW can be combined to achieve high treatment effects. The VF-HF HCWs were primarily designed for treating domestic sewage where nitrified effluents occur [96], but they were also used for other types of wastewater treatment.

3.2. Operation and Maintenance of Constructed Wetland (CW) Systems

Generally, CWs have relatively simple operational requirements. Nevertheless, the systems will always require some degree of maintenance for the length of their lives, which is crucial for economic considerations. Weeds that could outcompete the newly planted wetland vegetation should be removed during the start-up season. The buildup of particles and microbial film will cause the gravel to become clogged over time. Every 10 to 15 years, the filter material at the inlet zone must be replaced. Priority should be given during maintenance to ensure that primary treatment effectively minimizes solid concentrations in the sewage before it reaches the wetland system. In addition, maintenance should prevent the growth of trees in the area whose roots could damage the liner. Only when the water level rises above the ground level may there be odor issues, which indicates anaerobic conditions. In this situation, the filter must be rested, and the loads must be readjusted. Pre-treatment facilities must be inspected regularly, emptied frequently, and sludge discharged accurately to ensure they function correctly. It is always a wise design decision to have multiple line plants for maintenance and climate change considerations [104]. Peak overloading should not impact the CW’s performance, but ongoing overloading might reduce the treatment capacity due to excess fats, sludge, or SS. For SSFCWs, routine inspections of the pretreatment procedure, pump usage, influent loads, and their distribution over the filter bed are necessary maintenance tasks [105].

4. Contaminant Removal in Different Constructed Wetland (CW) Types

As discussed in Section 3.1, CW treatment systems are typically categorized as FWSCWs, SFCWs, SSFCWs, and HCWs. Depending on the flow direction, SSFCWs can be further classified into HSSF and VSSF systems [106]. The selection of flow regimes mainly relies on the available area, geographical location, targeted treatment constituents, treatment objectives, and treatment costs [107].

4.1. Role of Macrophytes or Vegetation

Planted vegetation is one of the essential components of CW technology, which helps remove pollutants from domestic wastewater. The presence of plants in CW systems is the only reason they are called “green technologies”. Plant (macrophyte) species used in CWs are usually the same species that live in the NWs. Plants suitable for CW use in CWs must meet the following criteria listed below [108].

• Plants must adapt to local environmental conditions.
• Plants must be practicable under local climatic conditions and may tolerate/resist potential pests, insects, and diseases.
• Plants should tolerate various contaminants (e.g., N, OM, P, etc.) in the wastewater.
• Plants should be easily adjusted in local CW environments to show relatively fast growth and spreading.
• Plants should have a high pollutant elimination capacity.

The wetland vegetation’s roots, stems, and leaves act as substrates for the growth of microbes as they decompose OM [109]. This microbial community, called the “periphyton”, comprises “a complex mixture of algae, cyanobacteria, heterotrophic microbes, and detritus attached to submerged surfaces in most aquatic ecosystems”. It can absorb pollutants, eliminate them from the water column, and prevent further spreading of such contaminants. This periphyton and natural chemical processes accomplish about 90% of contaminant removal and waste breakdown. The wetland plants eliminate around 7–10% of the contaminants and serve as a C source for the microorganisms once they die and decay. Notably, the choice of vegetation for a CW should consider the varied rates at which different aquatic plant species absorb/uptake HMs/TEs from polluted ambient media.

4.2. Role of Substrate Materials

The filter bed of CWs plays an equally significant part in the overall performance of an artificial secondary wastewater treatment system. Selecting substrate materials for the filter bed requires a crucial design parameter that can significantly impact the bed’s performance. Growth media provide a physical basis for vegetation growth, additional sites for biofilm growth and nutrient absorption, and promote sedimentation and filtration of contaminants [110]. At present, most media have gravel layers of various types of origin in the filter media, mainly with a sand layer at the top. The media play several functions, as highlighted below.

• The media supports the growth of planted vegetation.
• It stabilizes the bed (contact effect with the roots of developed plants).
• It provides a media filtration effect.
• It ensures high permeability and reduces possible clogging problems.
• It provides an attractive attachment area for many microbes (biofilm formation) that are involved in pollutant removal processes.
• It supports many transformation and elimination processes.

4.3. Role of (Plant Root-Associated) Microbes

As a significant component of CWs, microbes are crucial in decontamination processes, including nutrient transformation and organic pollutant degradation [111]. Microbes can also remove HMs (that cannot be biodegraded) from domestic sewage through their bioaccumulation, biosorption, and biotransformation (also called speciation, i.e., transformation of species/valence states) [112]. Microbes can even utilize antimicrobial compounds (antibiotics) as their sole C source [113,114,115]. Additionally, microbes in CWs can ameliorate abiotic and biotic stress tolerance and improve pollutant-removal efficiency by augmenting phytoremediation processes [116].

For comprehending CWs’ performance patterns and exploring optimized strategies, a thorough examination of the microbial community structure and their diversity, particularly for active/functional microbes in CWs, is necessary. With the recent developments in molecular biotechnology techniques, it is now feasible to monitor/study and analyze “microbial communities and species composition” in intricate ecosystems or environments [117], in their systematic review, summarized the primary functional microbes of CW systems engaged in the elimination of antibiotics, emerging pollutants, HMs, N, and P and investigated the impacts of these contaminants on microbial diversity. Their findings indicated that Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria are the chief phyla of the functional microbes in CWs. These active microbes help eliminate contaminants from CWs through diverse processes, including biodegradation, biosorption, catalyzing chemical reactions, and stimulating plant growth and development. HMs and high N and P concentrations in wastewater substantially impact microbial richness and diversity, while antibiotics result in considerable fluctuations in microbial alpha (α) diversity.

Plants harbor a vast repertoire of beneficial microbes (actinomycetes, bacteria, fungi, etc.) in their endosphere (internal plant tissues), rhizosphere (region of the soil/water surrounding the plant roots), and phyllosphere (area surrounding the plant leaves). These free-living microbes or microbial root symbionts secrete various PGP secondary metabolites, including biocontrol agents, biosurfactants, chelating agents, exopolysaccharides, nitrogenous compounds, organic acids, phytoenzymes, phytohormones, and volatile compounds, all of which directly or indirectly promote plant health and nutrition and alleviate abiotic (pollutant) stress in soil and water. These microbes with plant growth-promoting (PGP) traits are usually called plant growth-promoting microbes (PGPM). When found associated with plant roots (including internal root tissues), they are called plant growth-promoting rhizomicrobes (PGPRM) or endophytes. The PGPM/PGPRM, in turn, derive their nutrients from the plant root exudates and photosynthates, which are rich sources of sugars and amino acids [52,56,71]. The antimicrobial products secreted by PGPM inhibit or kill a broad spectrum of disease-causing phytopathogens, such as pathogenic bacteria, fungi, nematodes, and viruses [118]. Therefore, the indigenous PGPM associated with the planted vegetation in CWs could indirectly facilitate (phyto)pathogen removal and phytoremediation by promoting plant growth.

4.4. Role of Influent-Feeding Mode

Another critical design parameter of CW is the influent-feeding mode [119]. The method of influent feeding in the CWs (for example, continuous, batch, and intermittent) significantly affects the contaminant’s removal efficiency. As usual, batch-feeding modes (alternating filler and drain cycles) can achieve better performance by promoting more oxidizing conditions than continuous operation. In particular, N and P removal efficiency can improve in this wetland [119]. Effect of physico-chemical pretreatment on the removal efficiency of horizontal subsurface-flow constructed wetlands was studied by Caselles-Osorio et al. [120].

4.5. Role of Constructed Wetland (CW) as a Catalyst in Phytoremediation

In CW systems, physicochemical and biological processes remove toxic substances, including inorganic and organic contaminants. A thorough knowledge of these processes is essential for designing different CW systems and comprehending pollutants’ fate once they reach the wetlands. Theoretically, wastewater treatment in CWs occurs as it moves through the wetland substrates and the rhizosphere of the plants. Notably, owing to the loss and release of O₂ from the plant’s root systems (rhizomes, roots, and rootlets), a thin oxic layer surrounds each root hair [121]. Both aerobic and anaerobic microbes aid in OM decomposition. Gaseous nitrogen (N₂) is released into the atmosphere due to microbial nitrification and subsequent denitrification processes. P is coprecipitated with compounds of aluminum (Al), calcium (Ca), and Fe present in the root-filter bed media [121]. In SFCWS, SSs get filtered out while settling down in the water column, whereas in SSFCWS, they are physically filtered out by the wetland media. In SSF and VF systems, bacterial and viral pathogens are minimized through adsorption and filtration by the microbial biofilms present on the gravel or sand layers.

Plant growth, death, and microbial degradation contribute to the biogeochemical cycle occurring within a CW ecosystem. Overall, CWs provide a safe and beneficial environment for plants to remove pollutants from wastewater without endangering their health. During plant growth, aquatic macrophytes remove most pollutants while also providing a suitable environment for the proliferation of microbes. Therefore, this integrated CW technology treats wastewater contaminants more effectively than conventional treatment technology. The mechanism of removal of various pollutants through the CWs is described below.

4.5.1. Removal of Total Dissolved Solids (TDS)

Total Dissolved Solids (TDS) are the number of materials dissolved in water and wastewater like carbonate (CO₃²⁻), HCO₃⁻, NO₃⁻, PO₄²⁻, SO₄²⁻, chloride (Cl⁻), sodium (Na⁺), Ca²⁺, magnesium (Mg²⁺), organic ions, and other ions. These materials may not be regarded as pollutants contributing to dissolved solids. A few ions in water are necessary for sustaining aquatic life, and they are biologically utilized or chemically reactive in CWs. Aquatic macrophytes show a coordinated action to increase the durability/treatment of high TDS stress, and halophytes can reduce wastewater TDS content through their accumulation in plant tissues [122].

4.5.2. Removal of Biological Oxygen Demand (BOD)

Biological oxygen demand (BOD) indicates the DO amount aerobic macro/microorganisms require to decompose dissolved organic materials in water at a specific temperature over a particular time. BOD is the most critical parameter for measuring O₂ demand by microorganisms to degrade OM present in domestic wastewater. Its value is typically expressed in milligrams (mg) of O₂ consumed per liter (l) of water sample during incubation at 20 °C for 5 days. BOD is frequently employed as a proxy for the level of organic contamination in domestic wastewater [123]. BOD removal followed the “first order plug flow approach” described by Kadlec and Knight (1996) [124]. This approach is designed for certain pollutants extracted mainly through biological processes [125] (Akinbile and Yusoff 2012). In CWs, aerobic and anaerobic degradations of soluble organic compounds are equally suitable for removing BOD. However, during the sequential “fill and drain” process, the performance of BOD elimination was significantly better than during the previous traditional operating period [126].

4.5.3. Removal of Total Nitrogen (TN)

Excessive discharge of N into the waterbodies makes them prone to eutrophication and “black-odorous”, which not only deteriorates the overall water quality but eventually poses severe health risks to aquatic flora and fauna as well as humans [111,127]. In wastewater, TN refers to the amount of all N sources, including nitrite (NO₂⁻), NO₃⁻, NH₃, and organic N (org-N), and is usually expressed in mg/L. Biological processes are the main N removal mechanisms in CWs. N elimination in CWs occurs via ammonification, nitrification, denitrification, volatilization, and plant uptake [85,96,117]. N removal through CWs with the help of partial nitrification and denitrification processes involves converting the nitrification process, which oxidizes ammonium (NH₄⁺) to NO₂⁻ and then NO₃⁻, and the denitrification process, which converts NO₃⁻ to N₂ (Figure 4). Ref. [128] has reported that microbes can eliminate around 90% of the N. Wetland plants can also convert inorganic N to org-N through their metabolism.

4.5.4. Removal of Nitrates (NO₃⁻)

NO₃⁻ is an essential parameter for a specific state of decomposition of OM in domestic wastewater. NO₃⁻ uptake can cause a severe health condition in infants because of oxygen deprivation called “methemoglobinemia” or “blue baby syndrome” [129]. In CWs, microbial denitrification and vegetation uptake primarily achieve NO₃⁻ removal from domestic wastewater. This mechanism of NO₃⁻ elimination can occur biologically.

Biological Removal Mechanism(s)

This process of NO₃⁻ removal occurs in two stages: (1) nitrification (where NH₄⁺ is converted to NO₂⁻), and (2) denitrification (where NO₂⁻ is converted to NO₃⁻). The first stage is done resolutely aerobically; the organisms depend on the oxidation of NH₃ for cell growth and energy. The second stage is completed by the facultative chemolithotrophic bacteria that use organics for cellular growth and energy. The removal of NO₃⁻ is typically very high in the wetlands [130]. The central N removal mechanisms by functional microbes in CWs are depicted in Figure 5.

According to most recent studies, microbes in CWs remove N primarily via ammonification, nitrification, and denitrification processes [130,131,132]. Ammonification in wastewater involves converting Org-N into NH₄⁺; the latter is eliminated through other processes, including nitrification, plant uptake, and volatilization [131,133]. Notably, the most common genera of ammonifying bacteria are Bacillus, Chitinophaga, Isoptericola, and Sinorhizobium, as indicated in a review by Wang et al., 2022b [117]. As stated earlier, when it comes to nitrification and denitrification, microbes utilize NH₄⁺ as an electron donor during the process of nitrification and oxidize NH₄⁺ to NO₂⁻ and then to NO₃⁻ before using it as an electron acceptor during the denitrification process and finally reducing it to N₂O or N₂ [127,134]. Moreover, the microbes participating in nitrification are of two types: (1) “ammonia-oxidizing archaea” (AOA) and (2) “ammonia-oxidizing bacteria” (AOB), which convert NH₄⁺ to NO₂⁻, and “nitrite-oxidizing bacteria” (NOB) that transform NO₂⁻ to NO₃⁻ [127,135].

Mainly, AOA is more adaptable to low NH₃ and high salt conditions than AOB [136,137]. As oxidation of NH₃ is the initial and rate-limiting phase in the nitrification process, this may facilitate the AOA becoming the principal microbial group more rapidly and expedite the nitrification process [136]. The Nitrospinae, Nitrospirae, Proteobacteria, and Thaumarchaeota are well-known phyla that participate in nitrification. All the presently recognized AOA are found in the Thaumarchaeota phylum [136]. Regarding denitrification, the popular denitrifying bacterial phyla in CWs include Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria [138]. However, poor abundance and feeble competitiveness are common issues for nitrifying bacteria in CWs’ microbial population [128]. As a result, steady NH₄⁺ oxidation will require a longer start-up time, making the nitrification process a limiting step in N removal [128]. Notably, recent reports have emphasized “heterotrophic nitrification and aerobic denitrification” (HN-AD) bacterial significance in this context [128,138,139]. In the start-up/initial stage of CWs, these bacteria may carry out the transformation of NH₄⁺ and NO₃⁻, converting the N present in the aqueous solution (i.e., dissolved N) to gaseous N (N₂) for complete denitrification [128]. Additionally, they proliferate more quickly and can rapidly take over/dominate [128]. The old belief that only autotrophic bacteria are capable of carrying out nitrification and that denitrification can only occur in anaerobic environments has been disproved by the discovery of HN-AD bacteria, making it more advantageous for OM and N removal [138]. According to the reports by Wang et al. (2022b) [138], the HN-AD bacteria primarily belong to the genera Aeromonas, Dechloromonas, Ferribacterium, Hydrogenophaga and Zoogloea. Moreover, new N removal mechanisms like “sulfur autotrophic denitrification” (SAD) and “denitrifying anaerobic methane oxidation” (DAMO) have been identified in relation to the denitrification process [138,140]. “Sulfur-oxidizing bacteria” (SOB) use elemental S, sulfide (S²⁻), and thiosulfate (S₂O₃²⁻) as electron donors and NO₃⁻ as an electron acceptor in anoxic environments for reducing NO₃⁻ to N₂ during SAD [111,141]. Because of the readily accessible electron donors from S and related S compounds, this pathway may predominate in removing N from water/wastewater with a low C/N ratio [138]. The phylum Proteobacteria contains the majority of “sulfur-autotrophic denitrifying” bacteria, with Sulfurimonas and Thiobacillus as two of its well-known genera. In DAMO, CH₄ is the sole C source and electron donor for reducing NO²⁻ to N² under anaerobic (O₂-deficit) conditions [140,142]. More environmental advantages are made possible due to DAMO’s inherent ability to lessen the “greenhouse effect” and reduce N₂O, the unneeded by-product, during N removal [140,142].

A novel mechanism called “AOA” or anammox exists for N removal apart from the conventional nitrification-denitrification processes [130,143]. Under anaerobic conditions, NO²⁻ is used as an electron acceptor in this pathway for directly transforming NH₃ into N₂ [132,143]. As a result, it provides an alternative denitrification mechanism when the O₂ and C/N ratios are low [130]. Almost every anammox bacteria reported is a member of the phylum Planctomycetes [127,144]. Concerning NO₃⁻, among the various nitrogenous contaminants, NO₃-N is more prone to leaching and subsequently degenerate the quality of water than the others [144]. Thus, removing NO₃⁻ is crucial to safeguarding freshwater systems and subsurface water quality [144]. Besides denitrification, there is another pathway for reducing NO₃, which is called “dissimilatory nitrate reduction to ammonium” (DNRA) [145]. The DNRA pathway converts NO³⁻ to NH⁴⁺, thereby reducing it to available NH⁴⁺ suitable for utilization by other microbes, including ammonia-oxidizing archaea and AOB [127,135]. According to the published reports, it is more advantageous to the denitrification process in S²⁻-rich coastal and marine habitats where salinity levels are high [127]. Numerous investigations have revealed that a few denitrifying bacterial genera, including Clostridium, Desulfovibrio, and Vibrio, can carry out the DNRA process [127,146]. Nevertheless, it is currently challenging to discriminate between DNRA and denitrifying bacteria, necessitating future advancements in molecular biotechnology.

The phylum Proteobacteria has a sizable species number active in N transformation [117]. This species is prevalent in CWs and is the leading phylum in most systems, significantly removing N from various wastewater categories [132,147]. The three genera, viz., Nitrobacter, Nitrosomonas, and Nitrosospira, are related to the process of nitrification, while among denitrifying bacteria, the genera Arenimonass, Tauera, Thermomonas, and Thiobacillus are frequently found. The three prominent classes associated with N removal in CWs are Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. They are rich in nitrifying bacteria, such as AOB and NOB, which are functionally crucial to CW ecology and are primarily responsible for removing N [148].

Additionally, an increasing body of research has linked the functional genes of N-removing microbes to their operational and quantitative analyses [134,135,149].

For instance, the plentitude of nirK- and nrfA-carrying microbes controls how well CWs performed at denitrification [149]; the prevalence of the functional genes for nitrification, viz., amoA-ammonia-oxidizing archaea, amoA-AOB, and nxrA, indicated the nitrifying-bacterial-growth status [21]. A summary of the active gene pools related to the various N removal processes, such as nitrification, denitrification, anammox, and DNRA, has been prepared [128,134,135]. With active genes, one may examine how microorganisms function in a particular habitat or ecosystem and offer a viable method for researchers to continue studying functional microbes in CW systems.

4.5.5. Removal of Phosphate (PO₄²⁻)

P is one of the primary elements contributing to the eutrophication process in waterbodies [150,151]. Excess P discharge into aquatic ecosystems from diverse sources encompassing agricultural, industrial, and residential sources can also adversely affect aquatic organisms by modifying water pH, reducing DO levels, and triggering algal/phytoplankton growth [150,151]. P occurs naturally in inorganic and organic forms/phosphates (PO₄²⁻). Soluble reactive P (SR-P) is the term used to describe the analytical measure of biologically available orthophosphates. In general, insoluble forms of P (inorganic and organic) and dissolved org-P are physiologically inaccessible until they are converted into soluble inorganic forms [152]. The basic phenomenon of removing P in CWs depends on its accumulation by the sediments, substrates, and plants. For removal, P can be processed physically, chemically, or biologically.

Physical Removal Mechanism(s)

In CWs, physical absorption through roots, leaves, and plant parts is usually deficient. Thus, macrophytes account for P removal at the beginning of their growing period, and precipitation of soluble and insoluble P in the influent, followed by sedimentation, is the physical form of removal occurring in the CW. Filtration through solid substances and fallen leaves in CWs reduces P from wastewater [124].

Chemical Removal Mechanism(s)

The main P removal by chemical mechanism in the CWs occurs through adsorption and precipitation (precipitation in the water column and adsorption by porous media) [96]. Adsorption and precipitation within substrates are well-established in performing the most critical roles in the PO₄²⁻ removal process [153]. Other than this, sand, washed gravel, crushed rocks, and peats can also participate in the adsorption process. CW bed fills, on the other hand, have a short-term P sorption capability [96].

Biological Removal Mechanism(s)

The mechanism for removing P is through biological resources, but this process still does not allow much storage. Microbes are crucial for eliminating P from CWs and can regulate the transformation of P into different forms [151]. P uptake through microorganisms is relatively fast since bacteria, fungi, and algae can multiply rapidly. Maximal P transformation mediated by microbes involves the mineralization of organic PO₄²⁻ to inorganic PO₄²⁻ (a process also referred to as “decomposition”) or the conversion of insoluble, mobile, primary PO₄²⁻ that are more readily utilized by organisms [96]. A comprehensive review on microorganism in constructed wetlands list the major functional microorganisms responsible for P removal in CWs [117].

“Phosphorus-accumulating organisms” (PAOs) are primarily responsible for biological P removal in CWs. PAOs can absorb wastewater PO₄²⁻ and store it within their cells under oscillating oxic and anoxic environments [150,154,155]. Under anoxic conditions, PAOs degrade intracellular polyphosphates and uptake volatile fatty acids from the surrounding media. These fatty acids are subsequently stored as polyhydroxyalkanoates/poly-β-hydroxyalkanoates (PHAs) [156].

In contrast, PAOs utilize PHAs under oxic conditions to provide energy and absorb PO₄^2, creating polyphosphate storage [155]. The process of microbial P removal in CWs is generally realized because the amount of P absorbed by PAOs will be higher than that of P discharged [150,154,155]. Pseudomonadota (formerly Proteobacteria), which plays a significant role in P elimination, is the major phylum [112,147,157]. Of these, the majority of the bacterial/microbial species linked to biological P removal are found in the classes Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria [154,157]. TP removal in CWs is facilitated by the Rhizobiaceae and Rhodobacteraceae of the class Alphaproteobacteria, which can uptake volatile fatty acids under oxic conditions and transform them into PHAs [156]. The genera Candidatus, Dechloromonas, and Rhodocyclus constitute the principal members of the class Betaproteobacteria. Among them, the bacterial group Candidatus Accumulibacter is a representative PAO dominant in large-scale wastewater treatment facilities and laboratory-scale reactors. Under anaerobic conditions, Dechloromonas can reduce perchlorate, assemble polyphosphate, and take up C [157]. Additionally, it has been demonstrated that Rhodocyclus significantly contributes to P elimination [158]. Pertinent research has identified three genera, viz., Acinetobacter, Klebsiella, and Pseudomonas, belonging to Gammaproteobacteria [155]. Pseudomonas is an efficient P-removal bacterium due to its considerable capacity to absorb wastewater P and store it as polyphosphates within its cell biomass [157]. According to [155], Pseudomonas can eliminate up to 80.6 percent of TP from household sewage. Notably, the first bacterial isolate from biomass belonged to the genus Acinetobacter, with a solid capacity to remove P [150]. Besides Proteobacteria, other taxa like Gemmatimonadacea can absorb surplus PO₄²⁻ under oxic conditions [151].

The P-removal ability of PAOs primarily relies on their bioaccumulation and utilization of intracellular polyphosphates [155], which are directly correlated with exopolyphosphatase (ppx) and polyphosphate kinase (ppk) activities [150]. For achieving biological P removal, the enzymes ppk and ppx can catalyze aerobic P uptake and anaerobic P release, respectively [134]. Elevated temperatures, however, inhibit their functions; based on an earlier study, the ideal temperature ranges between 20.0 °C and 35.0 °C [150].

Moreover, besides PAOs, “phosphorus-solubilizing bacteria” (PSB) and “denitrifying phosphorus-accumulating organisms” (DNPAOs) have been identified in CW systems. Examples of PSB include the genera Corynebacterium and Enterobacter, which produce organic acids like citric and oxalic acids for converting the insoluble form of soil P into its soluble form for plant uptake [151]. DNPAOs may absorb polyphosphate in anoxic environments using NO₃⁻ or NO₂⁻ as electron acceptors [151]. There have been reports of DNPAOs in Alphaproteobacteria (including the genus Paracoccus) and Anaerolineae [156]. Interestingly, organophosphate hydrolases from the genera Brevundimonas and Chlorobaculum hydrolyze organophosphate esters, and Variovorax utilizes insoluble PO₄²⁻ as a P source for its growth [159].

4.5.6. Removal of Heavy or Trace Metals

HMs are widely dispersed in aquatic ecosystems and regarded as toxic environmental contaminants as they are hard to break down and, if left untreated, can build up in the food web [160], resulting in biomagnification and causing health risks to humans. CWs have been extensively used to eliminate dissolved metal(loid)s or TEs. Although these pollutants are frequently found in mine drainage, treatment wetlands have been built for stormwater, landfill leachates, and other sources (such as leachates/FDG washwater at coal-fired power plants and domestic sewage), which also contain dissolved trace elements [161]. Through the exploitation of diverse processes like biomineralization, biosorption, and biotransformation (valence transformation), microbes in CW systems can efficiently remove HMs/TEs [112]. The principal HM-removal pathways/processes by functional microbes in CWs are shown in Figure 6. The phyla and genera of functional microbes are reviewed by Wang et al., 2022b [117].

Ironically, HM ions usually harm microbes because they break cell membranes, damage DNA, impede enzyme activity, and interfere with cellular functions [160]. For this reason, HM tolerance is crucial for microbes in HM removal in CWs. The genera Sideroxydans and Thiomonas can oxidize Fe²⁺ to Fe³⁺, making its precipitation easy and rendering it less toxic [162]. Additionally, Yu et al. (2020) [160] discovered that the dominant genera Pseudomonas and Serratia displayed resistance to Cd²⁺ and Zn²⁺ when tested utilizing concentration gradients of these two HMs, leading to increases in their removal rates of 10.13 percent and 8.57 percent, respectively. Analysis of subcellular compartments further revealed the presence of bioaccumulated HMs primarily in the microbial cell membrane and cell wall. The exopolymers from Pseudomonas can bind to HMs and prevent their transport inside the biofilm through diffusion, attaining extracellular sequestration, which shields bacterial cells from HM stress [163].

Moreover, the presence of anionic functional groups on the cell surfaces of Pseudomonas and Serratia may further facilitate Cd²⁺ and Zn²⁺ adsorption [164]. These results suggest that culturing resistant bacteria/microbes is a feasible strategy for removing HMs from wastewater and call for further study. However, Serratia minimizes HM toxicity by secreting a variety of enzymes and proteins, including amino acids, histidine-binding proteins, HM-binding proteins, redox enzymes, and transporter proteins capable of effluxing HM ions [162]; this is unable to aid in the elimination of HMs by CWs. Hence, there is a difference between resistant and functional microbes, and further investigation into microbial HM removal mechanisms is necessary before a conclusion can be drawn. According to [160], functional microbes also evolved over a longer time in the control CW system that was not exposed to resistant microbes. The system’s microbial community structure most likely changed spontaneously, facilitating tolerance to HM stress. Contrarily, when exposed to HM-containing environments, systems supplemented with resistant microbial inoculants may display a less prominent microbial community evolution to achieve a dominant strain, saving biofilm stabilization time [145].

The elimination of HMs is also significantly influenced by plant-microbe interactions. Plants and microbes have coexisted for a long time, and microbes have formed intricate relationships with plants [116]. Particularly, endophytic and rhizospheric bacteria (PGPR) can facilitate plant growth and development through nutrient (Ca, Fe, Mg, N, and P) uptake, phytohormone production, and tolerance towards pollutant stress [116]. This, in turn, can reduce harmful metal-induced plant stress and help plant metal accumulation [116]. Conversely, the primary role of macrophytes in CW systems is to supply additional OM and O₂ needed for the growth of microbes [165]. Therefore, healthy plant growth also offers a better habitat for microbial proliferation [166]. These symbiotic interactions enhance HM removal in CWs. For example, Syranidou et al. (2016) [167] showed that inoculating Juncus acutus L. with a particular endophytic bacterial consortium eliminated emerging contaminants and HMs more quickly and effectively than uninoculated plants. In another study by Vassallo et al. (2020) [116], it was shown that eight rhizobacterial isolates from P. australis roots belonging to the genera Bacillus, Planococcus, and Pseudomonas thrived in domestic sewage with high levels of HMs (45 mg/L and 0.09 mg/L of Fe and Se, respectively), and the more HM concentrations present, the more rapid they grew. In conclusion, due to their high levels of HM resistance and ability to improve phytoremediation effectiveness, PGPR has been demonstrated to be a trustworthy functional microbe for HM removal.

4.5.7. Removal of Pathogens

Pathogens commonly found in household/municipal sewage capable of inflicting different acute or chronic diseases in humans encompass varied microbial genera like Escherichia, Salmonella, and Vibrio among bacteria, Ascaris among intestinal nematodes, Enteroviruses and Rotaviruses among viruses, etc. Notably, the genera used as pollution indicators in domestic sewage include Clostridium, Citrobacter, Enterobacter, Escherichia, Klebsiella, and Streptococcus [19]. Severe disease conditions caused by various microbial pathogens and their toxins, along with other contaminants in wastewater, are amoebiasis (Entamoeba histolytica), botulism (Clostridium botulinum), campylobacteriosis (Campylobacter), cholera (Vibrio cholerae), cryptosporidiosis (Cryptosporidium parvum), giardiasis (Giardia lamblia), hemorrhagic diarrhea (Escherichia coli), hepatitis A (Hepatitis A virus), typhoid (Salmonella typhi), scabies, shigella infection (Shigella spp.), and other parasitic (helminthic and protozoan) infections (Endolimax nanus, Entamoeba coli, and whipworm) [19].

In a CW system, all kinds of pathogens, including bacteria, fungi, helminths, protozoa, viruses, etc., are anticipated to be somewhat eliminated; however, an SSFCW is expected to remove pathogens more thoroughly than SFCWs. Usually, a 1-2 log₁₀ reduction in pathogen count can be expected in a FWSCW, but in systems with dense vegetation, the elimination of bacteria and viruses may be <1 log₁₀ reduction. CWs frequently contain flora that helps eliminate other pollutants (nutrients) like N and P. As a result, the role of sunlight in killing bacteria and viruses is diminished in these systems since heavy vegetation prevents the exposure of pathogens to direct sun rays. According to published reports, in a well-designed and adequately operated/maintained FWSCW, pathogen removal is around <1-2 log₁₀, 1-2 log₁₀, 1-2 log₁₀, and <1-2 log₁₀ for bacteria, helminths, protozoa, and viruses, respectively. In contrast, the expected elimination of pathogenic bacteria, helminths, protozoa, and viruses in SSFCWs is relatively higher and is reported to be 1-3 log₁₀, 2 log₁₀, 2 log₁₀, and 1-2 log₁₀, respectively [92].

The removal efficiencies stated above as log₁₀ can alternatively be understood in terms of how removal efficiencies are often/generally reported in terms of percentages (%); for instance, 1 log₁₀ removal corresponds to 90% removal efficiency (RE), 2 log₁₀ removal corresponds to 99% RE; 3 log₁₀ removal corresponds to 99.9% RE; 4 log10 removal corresponds to 99.99% RE, and so on [168]. Various studies have been conducted in the last two decades on domestic wastewater treatment and developing alternative, low-cost, and sustainable strategies that can effectively reduce pollutant concentrations to acceptable/permissible environmental standards (

Table 3. A detailed synopsis of the wastewater operational parameters and macrophyte- and microbe-assisted treatment efficiencies of FWSCWs, SSFCWs, HSSFCWs, VSSFCWs, and HFCWs using selected case studies from the last two decades (2003–2022).

Sl. No.	Latin and Common Names of Plant (Macrophyte) Species	Percentage Removal of Inorganic and Organic Pollutant(s) (%)	Percentage Removal of Pollutant Indicator Organism(s)/Pathogen(s) (%)	Microbes Involved in the Removal of Nutrient(s)/Pollutant(s)	Constructed-Wetland-Based Phytoremediation Set-Up(s)/System(s) Used	Type of Treated Wastewater	Reference(s)
1.	Salix atrocinerea Brot. (grey willow) and Typha latifolia L. (cattail)	BOD5 (87.5) COD (89.0) SS (66.5)	Fecal bacteria (99.9)	-	Full–scale, pilot-plant constructed wetland	Domestic wastewater	[169]
2.	Control unit (A): Phragmites mauritianus Kunth (reed grass) and T. latifolia	COD (33.6, 56.3, and 60.7) NH4+-N (11.2, 25.2, and 23.0) NO2-N (23.9, 38.5, and 23.1) NO3−-N (32.2, 40.3, and 44.3)	TC and FC (43.0–72.0)	-	Horizontal subsurface-flow constructed wetland	Domestic wastewater	[170]
3.	Control unit (A): Colocasia esculenta (L.) Schott (taro) and T. latifolia	COD (64.7, 74.8, and 79.4) NH3 (74.0–75.0) P4 (72.0–77.0) SO42− (74.0–75.0)	-	-	Engineered wetland	Domestic wastewater	[171]
	T. latifolia	NH3 (74.0) PO42− (69.0) SO42− (72.0)	-	-
4.	Phragmites sp.	BOD5 (93.0) COD (88.0) TDS (93.0) TSS (93.0)	FC (76.0–99.0) Fecal Streptococci (49.0–85.0) Note: These were removed in two phases in four distinct seasons	-	Subsurface-flow constructed wetland	Municipal wastewater	[172]
	Typha sp.	BOD5 (63.0) COD (50) TDS (58.0) TSS (58.0)	FC (50.0–99.0) Fecal Streptococci (33.0–85.0) Note: These were removed in two phases in four distinct seasons	-
5.	Pontederia crassipes Mart. [formerly Eichhornia crassipes (Mart.) Solms] (common water hyacinth) and Phragmites australis (Cav.) Trin. ex Steud. (common reed)	BOD5 (72.1) COD (67.2) Org-N (59.3) SS (64.6) Settleable solids (91.8) TN (38.0) TP (43.0)	-	-	Surface-flow constructed wetland	Secondary-treated domestic wastewater	[173]
6.	Typha angustifolia L. and Scirpus grossus L.f. (club-rush or bulrush)	BOD5 (68.2) NH4+-N (74.4) NO3−-N (50.0) TP (19.0) TSS (71.9)	-	-	Free-water surface wetland	Secondary-treated municipal wastewater	[174]
7.	Lemna minor L. (common duckweed), P. australis, Schoenoplectus tabernaemontani (C.C. Gmel.) Palla (syn. Scirpus validus Vahl) (softstem bulrush), and Typha orientalis C. Presl (cumbungi)	BOD5 (70.4) NH3-N (40.6) SS (71.8) TP (29.6)	TC and FC (99.7 and 99.6, respectively)	-	Constructed wetland	Sewage water	[175]
8.	A Acorus gramineus Sol. ex Aiton (Japanese sweet flag), B Iris pseudacorus L. (yellow flag)	BOD5 (A 71.3, B 72.5) COD (A 61.71, B 61.5) TN (11.24–21.95) TP (33.15)	-	-	Constructed wetland	Domestic wastewater	[176]
9.	A Iris pseudacorus L. and B Acorus gramineus Soland	BOD5 (72.5, 71.3) COD (61.1, 61.7) TN (70.9, 70.7) TP (86.9, 84.8) A HMs (Cd, Cr, and Pb)—15.3, 21.3, and 24.5, respectively)	-	-	Model wetlands	Rural or urban domestic wastewater	[176]
10.	T. angustifolia	BOD5 (80.78) NH4+-N (95.75) TN (66.5) TP (58.59)	-	-	Free-water surface wetland	Secondary-treated municipal wastewater	[177]
11.	Canna and Heliconia	TSS (88.0) COD (42–83)	-	-	Horizontal subsurface-flow constructed wetland	Domestic wastewater	[178]
12.	P. australis and T. latifolia	BOD5 (>86.0) COD (>86.0)	-	-	Vertical-flow constructed wetland	Domestic wastewater	[179]
13.	Canna	BOD5 (94.0) TN (93.0)	-	-	Vertical subsurface-flow constructed wetland and horizontal subsurface-flow constructed wetland	Sewage water	[180]
14.	Cyperus alternifolius L. (umbrella papyrus)	COD (83.6) NH4+-N (71.4) TN (64.5) TP (68.1) TSS (99.0)	-	-	Hybrid-flow constructed wetland	Municipal wastewater	[181]
15.	Acorus calamus Linn. (sweet flag)	COD (73.0–93.0) TN (46.0–87.0) TOC (40.0–66.0) TP (75.0–90.0)	-	-	Vertical-flow constructed wetland	Domestic wastewater	[182]
16.	Anthurium andraeanum Linden (flamingo flower), Strelitzia reginae Aiton (crane flower), Zantedeschia aethiopica (L.) Spreng. (calla lily), and Agapanthus africanus L. (African lily)	BOD5 (81.94) TN (49.38) TP (50.14) TSS (61.56)	TC (99.30)	-	Vertical subsurface-flow constructed wetland	Secondary-treated municipal wastewater	[183]
17.	Canna, Cyperus papyrus L. (papyrus or Nile grass), and P. australis	BOD5 (90) COD (88.0) TSS (92)	TC, FC, and E. coli (94.0–99.0)	-	Vertical-flow constructed wetland	Municipal wastewater	[184]
18.	Canna indica L. (Indian shot) and T. orientalis	BOD5 (62.8) NH4+-N (80.72) NO3−-N (12.8) TN (51.1)	-	-	Hybrid-flow constructed wetland	Municipal wastewater	[185]
19.	Scirpus alternifolios (umbrella papyrus)	BOD5 (84.9) COD (89.8) NH4-N (82.2) TKN (82.7) TP (76.5) TSS (98.1)	-	-	Vertical subsurface-flow constructed wetland	Wastewater	[186]
20.	T. angustifolia	NH4+-N (95.2) TP (69.6)	-	-	Subsurface-flow constructed wetland	Artificial wastewater	[120]
21.	Canna and P. australis	BOD5 (92.8, 93.6) COD (91.5) NH3 (62.3, 57.1) TSS (92.3, 94.0)	-	-	Vertical-flow and horizontal-flow constructed wetlands	Municipal wastewater	[187]
22.	Alternanthera sessilis (L.) R.Br. ex DC. (Brazilian spinach), C. esculenta, P. australis, Pistia stratiotes L. (water lettuce), Persicaria hydropiper (L.) Delarbre (syn. Polygonum hydropiper L.) (water pepper), and T. latifolia	BOD5 (90.0) NH4-N (86.0) NO3-N (84.0) TDS (78.0) TE (As, Co, Cr, Cu, Mn, Ni, Pb, and Zn—85.0, 49.0, 35.0, 95.0, 87.0, 39.0, 92.0, and 55.0, respectively) TSS (65.0)	-	-	Subsurface-flow constructed wetland	Sewage wastewater	[188]
23.	T. latifolia	COD (53.0–70.0) NH3 (12.0–15.0) P (18.0–25.0)	-	-	Surface, up-flow constructed wetland	Sewage wastewater	[12]
24.	Phragmites	CODCr (75.7) NH3-N (96.8) TN (96.7) TP (90.4)	-	N was removed by Paenibacillus sp., Pseudomonas oleovorans, Pseudomonas pseudoalcaligenes, Pseudomonas stutzeri (LZ-4), Pseudomonas stutzeri (LZ-14), Pseudomonas stutzeri (XP-2), and Pseudomonas pseudoalcaligenes (Note: These microbes remove N from wetlands with processes like adsorption, filtration, precipitation, sedimentation, and volatization); Pseudomonas mendocina LR contributed to the maximal N removal (97.35%)	Laboratory-scale constructed wetland microcosm	River water and domestic wastewater	[189]
25.	Canna and Phragmites	NH4+-N, NO3−-N and TKN (52.99)	-	-	Vertical-flow constructed wetland	Secondary-treated sewage wastewater	[190]
26.	I. pseudacorus, P. australis, and T. latifolia	BOD5 (41.0) Dissolved P (59.0) HM (Pb—98.0) NH4+-N (66.0) TP (46.0) SS (66)	-	-	Constructed wetland	Mine water and Sewage	[191]
27.	Agapanthus africanus (L.) Hoffman. (African lily), Canna ffuses, C. indica, Watsonia borbonica (Pourr.) Goldblatt (Cape bugle lily), and Z. aethiopica	BOD5 (90.0) NH4+ (84.0) PO42− (92.0)	-	-	Horizontal subsurface-flow constructed wetland	Sewage water	[192]
28.	Aquatic plants	BOD5 (87.9) CODCr (90.6) NH3-N (66.7) TN (63.4) TP (92.6)	-	-	New-type, multi-layer artificial wetland	Domestic wastewater	[193]
29.	Canna × generalis	NO3− (51.9) P (8.9) Phenolic compounds (1.0)	-	-	Constructed wetland	Domestic wastewater	[129]
30.	A. calamus, C. indica, Iris japonica Thunb. (butterfly flower), P. australis, T. angustifolia, and Zizania caduciflora (Trin.) Hand.-Mazz. (wild rice)	TP (79.6, 87.9, 90.3, and 93.2)	-	-	Integrated vertical-flow constructed wetland	Synthetic domestic wastewater	[194]
31.	P. australis	BOD5 (84.0) COD (75.0) NH4+ (32.0) TP (22.0) TSS (75.0)	-	-	Vertical subsurface-flow constructed wetland	Sewage water	[195]
32.	C. indica	BOD5 (88.11, 80.51, and 89.78) NH4+-N (94.81, 39.39) TN (56.17, 50.0, and 55.06) TP (94.82, 93.04, and 93.31)	-	N was removed by denitrifying bacteria	Hybrid vertical down-flow constructed wetland	Domestic wastewater	[196]
33.	A. calamus and P. australis	TN (45.2)	-	N was removed by a large number of rhizospheric bacteria (out of that, 17.9–26.8% non-rhizospheric bacteria removed N from the soil)	Horizontal subsurface-flow constructed wetland	Domestic wastewater	[197]
34.	Centella asiatica (L.) Urb. (Indian pennywort), E. crassipes, P. australis, T. latifolia, and Chrysopogon zizanioides (L.) Roberty (vetiver grass)	BOD5 (81.0 and 82.0) TKN (63.0 and 69.0) TSS (79.0 and 89.0)	-	-	Hybrid constructed wetland	Domestic wastewater	[198]
35.	Typha domingensis Pers. (southern cattail)	BOD5 (56.0) TKN (41.0) TP (37.0) TSS (78.0)	-	-	Constructed floating wetland	Domestic sewage	[199]
36.	P. australis	BOD5 (93.0) COD (91.0) TN (67.0) TP (62.0) TSS (95.0)	TC, FC, and fecal Streptococci (64.0, 63.0, and 61.0, respectively)	-	Hybrid constructed wetland	Wastewater	[200]
37.	Typha and Commelina benghalensis L. (Benghal dayflower)	NO3− (84.0) PO43− (77.0)	TC and FC, E. coli, Enterococcus, Clostridium, and Salmonella (65.0–70.0)	-	Horizontal-flow constructed wetland	Primary and secondary-treated sewage	[201]
38.	Pennisetum purpureum Schumach. (Napier grass) and T. latifolia	BOD5 (up to 87) (inlet BOD5 of 748–1642 mg L−1) COD (up to 81) (inlet COD of 835–2602 mg L−1)	-	-	Horizontal subsurface-flow constructed wetlands	Industrial (brewery) wastewater	[202]
39.	C. indica and Typha angustata Bory & Chaub. (accepted name: Typha domingensis Pers.)	BOD, COD, NH3-N, TDS, TKN, TP, and TVS (A 65.0–B 62.0, A 64.0–B 61.0, A 21.0–B 58.0, A 34.0–B 33.0, A 15.0–B 35.0, and A 54.0– B 40.0) [at first stage] and (A 88.0–B 84.0, A 90.0–B 90.0, A 52.0–B 82.0, A 58.0–B 61.0, A 50.0–B 47.0, and A 71.0–B 64.0) [for the second stage reactor] Note: The nutrient removal was measured at two different hydraulic loadings at A 0.150 m day−1 and at B 0.225 m day−1	-	-	Two-stage vertical-flow constructed wetland	Domestic wastewater	[203]
40.	C. papyrus and P. australis	BOD5 (80.69) COD (69.87) NH3-N (69.69) TP (50.0)	TC and FC (98.08 and 95.61, respectively)	-	Vertical-flow subsurface constructed wetlands	Municipal wastewater	[204]
41.	A. calamus and C. indica	BOD5 (78.74 and 81.79) TDS (18.96 and 22.31) TN (56.33 and 60.37) PO42− (79.57 and 81.53)	-	-	Pilot-scale vertical subsurface-flow constructed wetland	Primary-treated domestic sewage	[205]
42.	Myriophyllum elatinoides Gaudich. (water milfoil)	NH4+ (91.35) NO3− (95.16) TN (90.36) TP (96.14)	-	Bacteroides and Firmicutes carried out denitrification; N was removed by Pseudomonas, Dechloromonas, Desulfobacca, and Desulfomicrobium; PO42− was removed by Chlorobaculum, Methanobacterium, and Rhodoblastus	Multi-stage, surface-flow constructed wetland	Domestic sewage	[141]
43.	C. esculenta and Dracaena sanderiana Sander ex Mast. (Chinese water bamboo)	BOD5/ (74.0) NH4-N (90.0) TSS (76.0)	TC (59.0)	-	Novel vertical-flow and free-water surface constructed wetland	Dormitory sewage	[83]
44.	A. calamus and reeds	TN (15.0) TP (18.0)	-	Bioremediation and degradation of diesel, petroleum, and other alkanes could be achieved by Tistrella; N was removed by Achromobacter, Aeromicrobium, Aquicella, Azospirillum, Fluviicola, Halomonas, Limnohabitans, Methylophilacterium, Perlucidibaca, Pseudomonas, Rhodobacter, Rhodospirillaceae, and Variovorax; S compounds were removed by Desulfovibrio and Rhodocista	Constructed wetland	Domestic sewage	[159]
45.	C. indica, C. alternifolius, and Thalia dealbata Fraser ex Roscoe (powdery alligator-flag)	COD (95.2) NH4-N (98.1) PO4−-P (85.3) TN (87.9) TP (86.1)	-	-	Hybrid constructed wetland	Domestic sewage	[206]
46.	Chrysopogon zizanioides L. (vetiver and khus)	BOD5 (83.36) COD (92.34) NH4-N (89.41) NO3-N (90.72) PO4−-P (92.81) TCr (95.20) TN (93.54) TSS (94.66)	-	-	Horizontal subsurface-flow constructed wetland	Tannery wastewater	[207]
47.	A Commelina benghalensis L. (Benghal dayflower) and B T. latifolia	A,B BOD (61.0 and 59.0) A,B COD (58.0 and 53.0) A,B NH4+ (60.3 and 51.5) A,B NO3-N (60.3 and 51.5) A,B PO42−(61.0 and 64.0)	A,B TC (41.0 and 39.0) A,B FC (50.0 and 30.0) A,B E. coli (45.0 and 35.0)	-	Horizontal-flow constructed wetland	Domestic wastewater	[207]
48.	Oenanthe javanica DC. (water celery)	COD (38.65) NH4+-N (28.20) TN (18.82) TP (14.57)	-	Purification of micro-polluted water was improved indirectly by inoculating low temperature-resistant Bacillus spp., via altering the community structure of the wetland microbes (i.e., through stimulating beneficial microbes for treating sewage while inhibiting microbial pathogens)	Vertical subsurface-flow constructed wetland	Micro-polluted lake water	[208]
49.	A Canna indica L. (Indian shot) and B Iris sibrica L. (Siberian flag)	A,B COD (83.6 and 66.3) A,B NH4+-N (82.7 and 44.1) A,B TN (76.8 and 43.8)	-	-	Horizontal subsurface-flow constructed wetland	Domestic wastewater	[162]
50.	P. australis	TP (36.2– 87.5)	-	-	Microbial-enhanced constructed wetlands	P-rich Saline wastewater	[209]

Notes: BOD₅: biochemical oxygen demand measured in a 5-day test; COD: chemical oxygen demand; C: carbon; FC: fecal coliforms; HM: heavy metal(s); NH₃: ammonia; NH₄⁺: ammonium; NO₃⁻: nitrate; Org-N: organic N; PO₄²⁻: phosphate; SO₄²⁻: sulphate; SS: suspended solids; TC: total coliforms; TCr: total chromium; TDS: total dissolved solids; TE: trace element(s); TN: total nitrogen; TP: total phosphorus; TSS: total suspended solids; TVS: total volatile solids.

).

5. Design of a Representative Constructed Wetland (CW) System

The current field-scale VSSFCW was built near the existing moving bed biofilm reactor (MBBR)-based domestic sewage treatment plants (STPs) at the Indira Gandhi National Tribal University (IGNTU) campus in Amarkantak, Madhya Pradesh, India. This Indian government-funded university is situated in a tribal region of central India (22° 480′ N and 81° 450′ E) at an elevation of 1048 m above sea level. The campus of IGNTU lies in an entirely rural setting surrounded by forests. The region experiences a mean annual rainfall of 123.5 cm and mean annual temperatures of 31.6 °C (maximum) and 18.2 °C (minimum) (District Groundwater Information Booklet, Anuppur District, 2007). Two different units of VSSFCWs (with a 3.08 m² surface area and 2.48 m × 1.24 m × 1.54 m dimensions) were built on the IGNTU campus (Figure 7). Selected filter media, such as gravel, coarse sand, and fine sand with sizes 26–32 mm, 8–10 mm, and 6–8 mm, respectively, were placed inside each wetland unit. In each bed, the media had a thickness of 30 cm of gravel, 36 cm of coarse sand, and 22 cm of fine sand (Figure 8). Gravel was placed at the bottom of each wetland unit, coarse sand was placed in the center, and fine sand was placed on top. Overall, the wetland medium was 0.88 m thick. After building the wetland chamber, a drainage pipe made of perforated PVC (4 cm in diameter) was positioned at the bottom with two outlets, S2 and S3, to collect the treated effluents. On top of this PVC pipe, a 30 cm layer of gravel was placed, followed by several layers of sand. After media filling, another perforated PVC pipe (“feeding pipe” with a diameter of 2 cm) was laid on top of the media layers for influent wastewater discharge. For aeration, vertical PVC pipes with perforations were installed at regular intervals. The filter media were collected, cleaned, and put into the wetland chamber. In addition, the CW was washed by filling the chamber with water, followed by repeated draining to clean the gravel and sand.

6. Prospective Resource Recovery in Constructed Wetland (CW) Systems for Circular Bioeconomy (CBE)

The enormous demand for energy and derived goods/products, largely dependent on fossil fuels, has seriously affected the environment due to greenhouse gas (GHG) emissions, pollution, and climate change. While several renewable sources, like sunlight, water, wind, etc., can provide energy, they are either inherently impossible or challenging to regulate or exhibit climate dependency, which is becoming increasingly unpredictable daily. In this regard, plant biomass has once again become a viable alternative to increase the options for renewable energy sources [210,211,212].

A circular bioeconomy (CBE) provides a “conceptual framework for using renewable natural capital to transform and manage our land, food, health, and industrial systems”, intending to attain sustainable well-being in balance with nature [213,214,215,216]. The fossil-based economy (FBE) is being replaced by a CBE through better utilization of plant biomass as a feedstock (raw materials) for valued bioenergy/biofuels and chemicals (Figure 9). To achieve CBE, coherent resource strategies along with green technologies that are both effective and sustainable need to be developed.

Additionally, solid waste, wastewater, GHG emissions, and ecosystem damage due to disposal and depletion of natural resources must be minimized to retain the product’s carbon value. This series of procedures serves as the framework for the new CBE, which aims to create a net-zero-carbon society. However, its conceptualization and implementation are still in their early stages. Primarily, the CBE depends on biomass carbon as a fundamental building block, which may be derived from any biodegradable organic sources, and economic, environmental, and social factors act as the principal drivers [210].

Wastewater treatment and disposal, agricultural irrigation and fertilization, and domestic energy production are independent systems. Some communities may be able to maximize these resources, especially in arid places, by incorporating CW into food, energy, and water value chains, resulting in economic and ecological benefits. Wastewater (including sewage) is increasingly used to irrigate crops worldwide [19,217]. The consequences of harvesting for the wetland community must be considered when recovering biomass as an energy source. The CW’s operation and maintenance must also be organized, including logistics, infrastructure, and human resources for energy conversion of wetland biomass and wastewater effluent utilization. This integrated strategy may be very effective in periurban areas, where the interface between urban infrastructure, high population density, and agricultural lands has caused competition for space and resources. CW for wastewater treatment can act as water and energy recovery systems and are intrinsically linked systems.

7. Performance Assessment of Constructed Wetlands (CWs) and Macrophytes for Domestic Sewage Treatment

For domestic sewage treatment, where SSFCWs are more beneficial, FWSCWs are suited for most other wastewater treatments (Table 3). [177] utilized Typha angustifolia L.-planted FWSCW for the treatment of municipal wastewater and achieved removal efficiencies of 80.0%, 95.0%, 66.0%, and 58.0% for BOD, NH₄⁺-N, TN, and TP, respectively. Using Canna, P. australis, and Cyperus papyrus-planted SSFCW, [184] reduced BOD by 90.0% and TSS by 92.0%. This was a pilot-scale SSFCW study. Li et al. (2020) used a multistage surface-flow wetland system for TN and TP removal. TN and TP removed were 90.0% and 96.0%, respectively. With the use of Canna plantations, HSSFCW and VSSFCW were found to have achieved about 94.0% BOD and 93.0% TN removal, which was higher than the other CWs. However, the prevailing oxic conditions in VFCWs are not conducive to denitrification, resulting in incomplete NO₃⁻ to N conversion. HFCWs could efficiently remove 86.0% TN and 87.0% TP [141]. Despite the poor denitrification, these systems had a TN removal rate of 68.0 percent, significantly greater than that of HFCWs. This removal rate could be because nitrification must be followed by denitrification to remove TN altogether. HFCWs with C. indica, Thalia dealbata, and Cyperus alternifolius exhibited maximum removal of 87.0%, 89.0%, and 86.0% of TP, TSS, and TN, respectively, for domestic sewage treatment. However, another hybrid vertical down-flow CW for domestic sewage treatment through C. indica and windmill grass efficiently removed 89.7%, 94.8%, 56.1%, and 94.8% BOD, NH₄⁺-N, TN, and TP, respectively.

The plant’s growth and physiological performance were influenced by several environmental factors like pH, temperature, solar radiation, and salinity [218]. In aquatic environments, numerous aquatic weeds utilized in pollutant removal have been worked out by various researchers; for example, bulrush (T. latifolia), clubrush (S. grossus), common reeds (P. australis), taro (C. esculenta), bach (A. calamus), Indian shot (C. indica), lobster claw (Heliconia), vetiver grass (C. zizanioides), water hyacinth (E. crassipes), and lucky bamboo (D. sanderiana) [219]. The proper growth of the macrophyte is also essential for the efficacy of a phytoremediation system [218]. The foliage and roots of these aquatic floral species have distinct structures that aid in plant stabilization and nutrient absorption [220]. Plants in wetlands play a vital role in increasing the residence period of water, which reduces the velocity and hence increases particle sedimentation and related contaminants. The ability of aquatic plants to reduce eutrophication through N and P assimilation has long been recognized. During the day, plants produce oxygen through photosynthesis. Furthermore, both aquatic plant roots and microbial populations drain nutrients simultaneously.

8. Pros and Cons of Phytoremediation through Constructed Wetland (CW) Systems

Phytoremediation through CWs has several benefits but a few limitations [221].

The benefits include the following: (1) According to research studies, CW systems have considerable potential for reducing water pollution from industrial, residential, and non-point source (NPS) contaminants; (2) Phytoremediation through CWs is an economical (i.e., involving low operating cost), effective, natural, reliable, simple, and widely recognized technology compared to many other conventional wastewater decontamination systems; (3) CW is an effective method for treating residential wastewater in small towns without sewer services and as a tertiary treatment option for dissolved nutrient removal in medium and oversized sewage treatment plants/facilities; (4) Except for a modest electric pump for VF type, CW-based phytoremediation is a natural system without any or low energy/power consumption. Moreover, there is no requirement for chemical reagents. (5) It utilizes common substrate materials, such as gravel, sand, soil, etc.; (6) It shows good BOD, COD, NH₄-N, pathogen, TDS, TE, TKN, TP, TSS, and TVS reduction; (7) It has no odor problems (groundwater flow); (8) Nitrification (vertical); (9) It has minimal impact on the landscape and environment; and (10) Branches of macrophytes, leaf cuttings, and other materials from its maintenance can be utilized for other integrated solutions, i.e., biogas production and composting.

The limitations encompass the following: (1) Regular/frequent maintenance of CWs is required to extend the useful life of the entire treatment plant; (2) Depending on the location and its climate, the success of the phytoremediation process may be seasonal. Moreover, other climatic factors may influence its efficacy; however, it is not suitable for cold climates; (3) Adaptability of the exotic macrophyte species used to the local climate may pose a problem; (4) Considerable surface area is required for its construction; hence, CWs are more common in rural areas compared to urban set-ups due to the cost of building area; (5) Pre-treatment is required for making it ready for the actual wastewater treatment; (6) The overall phytoremediation process is slow with a long start-up time; and (7) Careful laying of substrate materials is vital for its proper functioning.

9. Recommendations and Prospects

Based on this review, a few recommendations are proposed. The current approach to enhancing the phytoremediation of domestic sewage using aquatic macrophytes and CWs still has room for future research and development. For example:

• The role of different types of CWs and environmental conditions needs to be explored, especially in rural areas with no centralized wastewater treatment system.
• When studying the interaction between the ecological environments of plants and local substrates in a CW, the role of the microbial community must be examined.
• Dissolved nutrients and other contaminants assimilated by the wetland vegetation have been reported to be released into the water when plants wither, die, and decay during the winter, potentially resulting in poor removal performances of CWs. As a result, research and development (R&D) on optimal plant-harvesting techniques and the conservation and recycling of plant-based resources in CW systems are critical.
• A few limitations associated with this technology have raised grave concerns about its widespread practice, like climate adaptability, less efficient plant morphological traits, phytodisposal, seasonal dependencies, and time consumption. To overcome such barriers, plant geneticists have recently emphasized the application of plant biotechnology in developing new species of macrophytes with increased capabilities of dissolved nutrient/pollutant extraction through breeding techniques, protoplast fusion, and mutagenesis.

In CWs, the pollutant removal processes are significantly slow, thereby requiring a large land-use footprint for their implementation in treating wastewater. This slow treatment rate may be due to inferior or limited electron acceptors in the deeper anaerobic parts of CWs. A few aerobic zones exist in CWs near the air–water interfaces. Microbial fuel cells (MFCs) need an artificial redox gradient (i.e., aerobic and anaerobic zones) to function correctly. CW-MFCs are a novel merger technology recently developed using CW systems’ naturally occurring stratified redox gradients. Researchers are paying much attention to this integrated technology since it combines the best aspects of both CWs and MFCs. The development and potential applications of the CW–MFC technology have been discussed in detail by Srivastava et al. (2019) [222].

The inoculation of PGPRM-rich fallen leaf litters of healthy (pollutant-free) terrestrial plants (after ex-situ decomposition or composting) in the rhizosphere of the wetland vegetation as a biofertilizer/biopesticide for promoting aquatic macrophyte growth will not only reduce the consumption of commercial synthetic fertilizers but also minimize the use of inorganic pesticides. Moreover, this eco-friendly approach can earn revenues and create job opportunities if successfully commercialized. This is called bioprospecting, the need of the hour for protecting environmental health and promoting sustainable development. Also, CW systems can be clubbed with aquaculture or pisciculture ponds to treat the excess nutrient wastes generated from unused fish feed and NH₃ from fish excreta, followed by nutrient recovery and pond water recycling; this will enhance sustainability in the fishery sector worldwide.

10. Conclusions

A sustainable phytoremediation system requires plants with extensive root systems, rapid growth rates, higher biomass, and high nutrient uptake and accumulation rates. Some native aquatic plants are more tolerant and can be a strong barrier, preventing toxins from entering food webs. These aquatic plants and their associated microbial assemblages can reduce NO₃⁻, PO₄²⁻, SO₄²⁻, TEs, and other essential hydrological parameters like BOD, COD, and TDS. Various macrophytes (emergent, submerged, and free-floating) have been widely used in aquaponics (hydroponics), fields, and CWs. Furthermore, the review illustrates the efficacy of CWs for controlling dissolved nutrient pollution in domestic sewage. Finally, the combination of this microbe-assisted, CW-based phytoremediation strategy with the prospective recovery of valuable resources (as by-products of contaminating organics and inorganics) will enhance the decontamination of large volumes of domestic wastewater and help in developing a CBE framework and sustainable management of wetland ecosystems.