Introduction

Lack of appropriate wastewater management practices are contributing to both scarcity and decline of fresh water quality worldwide (Almuktar et al. 2018). The situation is posing serious threat to ecosystems especially in developing countries (Wu et al. 2017). Discharge of majority of raw wastewater directly into rivers has become a common practice due to lack of suitable and effective technologies, operational failures of larger treatment plants, and higher cost involved in setting new treatment units (Kumwimba et al. 2017). The constructed wetlands (CWs) are engineered systems that have evolved as an inventive approach to tackle wastewater from domestic sources mainly because of their reliable efficiency, ecological benefits, easy operation, and less maintenance cost (He et al. 2018: Kumar and Dutta 2019). They use natural functions of macrophytes, soil, and microorganisms to treat different water streams (Ilyas and Masih 2017). The use of this technique has grown-up over recent decades with various successful examples (Zhang et al. 2014). CWs are being used to treat almost all types of wastewater such as domestic sewage, stormwater runoff, agricultural runoff, industrial drainage, and polluted rivers water (Li et al. 2017). There are many co-benefits of CWs together with wastewater treatment and recycling as they also provide important ecological services such as valuable wildlife habitat, aquaculture, groundwater recharge, carbon sequestration, fisheries, flood control, silt capture, recreational uses, and add aesthetic values to the surroundings.

Classification of constructed wetlands

CWs are characterized generally into three categories, namely, subsurface flow constructed wetlands (SSFCWs), surface flow constructed wetlands (SFCWs), and hybrid system. Further, on the basis of the flow path, SSFCWs are differentiated into vertical flow constructed wetlands (VFCWs) and horizontal flow constructed wetlands (HFCWs) (Wang et al. 2018). According to the macrophytic growth, they are categorized into emergent, free-floating, submerged, and floating-leaved macrophytes (Vymazal 2010).

Constructed wetland microcosms (CWMs)

A working model of a CWM (Fig. 1) possesses various types of supporting media and aquatic macrophytes depending upon target pollutants. In general, wastewater reaches the treatment chamber, runs all the way through the supporting media, and is released out of the chamber from an outlet system. A CWM unit has following five major components: basin (or chamber), substrate/media materials, vegetation (mostly macrophytes), and inlet and outlet system (Sudarsan et al. 2015).

Fig. 1
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CWM unit planted with emergent macrophytes

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A number of researchers across the world have published their review articles on the use of CWs for wastewater treatment (Liu et al. 2015; Haynes 2015; Almuktar et al. 2018). However, there are somewhat few studies detailing the treatment dynamics, rather the information is meant to provide onsite domestic wastewater treatment that are site specific. Recent investigation on CWs has principally provided information on wastewater decontamination (Avila et al. 2014), suitable working models and appropriate choice of macrophytes (Wang and Sample 2013), retention time (HRT), hydraulic loads (HLR) (Dzakpasu et al. 2015), and variety of supporting media (Ge et al. 2015) (Fig. 2).

Fig. 2
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a, b CWM units designed under net house of Department of Environmental Science, Babasaheb Bhimrao Ambedkar University, Lucknow, India. c CSIR- Institute of Minerals and Materials Technology, Bhubaneswar, Odisha. d International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru, Telangana, India. e Constructed wetland for wastewater treatment for a colony in Andhra Pradesh, India. f CWs working successfully in Georgia treating runoff from a plant nursery

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Treatment mechanisms involved in CWMs

Treatment mechanisms involved in CWMs are biogeochemical transformations and solid/liquid separations. Transformation possesses reduction, oxidation, acid/base reactions, biochemical reactions, flocculation, and precipitation. Separation includes adsorption, absorption, gravity separation, stripping, leaching, filtration, and ion exchange (Choudhary et al. 2011).

Major constituents involved in treatment mechanisms

Wetland vegetation (macrophytes)

In CWMs, macrophytes are primary vegetation. They are essentially grouped in four categories, namely, emergent, submerged, floating-leaved, and free-floating macrophytes (Kumar and Dutta 2019). Growth characteristics and nutrient uptake capacity of some frequently used macrophytes are presented in Table 1. The macrophytes relocate oxygen and provide dissolved organic matter and supporting media for microbial attachment (Meng et al. 2014). They are also contributing to enhance porosity and permeability of the substrate, act as a catalyst, and promote a number of biological and chemical reactions (Yahiaoui et al. 2018). More than 150 species of macrophytes have been reported that are used in CWMs worldwide; however, only a few of them are commonly used. It is observed that emergent aquatic macrophytes are preferred choice because they have high contaminant removal efficiency (Vymazal 2013). The choice of macrophytes must be indigenous which can grow naturally in wetlands. They should be also capable to withstand with short dry periods as well as shocks generated by wastewater loads. Macrophytes which have well developed root and rhizome systems inside the supportive material are most preferable.

Table 1 Growth characteristics of some frequently used aquatic macrophytes in CWMs treating municipal wastewater
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Supporting media

Currently, available and frequently used supportive media are the industrial by-products, natural and artificial or synthetic materials (Yan and Xu 2014). Some frequently used supporting media in CWMs are presented in Table 2. They must be chosen according to their capacity to absorb/adhere wastewater contaminants and their permeability. It is generally observed that reduced hydraulic conductivity greatly influenced adsorption ability (Wang et al. 2010). Ultimately, the long-lasting applications of the treatment system are highly affected by the chosen media materials (Wang et al. 2010).

Table 2 Frequently used supportive media in CWMs (Revised from Wu et al. 2015)
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Microorganisms

The principal microorganisms concerned with wetlands system are bacteria, yeasts, protozoa, fungi, and algae. Collectively, all these microorganisms participate in the degradation of nearly all of the wastewater contaminants into insoluble or harmless substances. The well-established microbial communities are attached to the supporting media, plant roots, and/or in leaves in the form of biofilms (Faulwetter et al. 2009). The complex microbial communities in the form of biofilms formed by interactions with wastewater are primarily responsible for the breakdown of the wastewater pollutants and increase the overall treatment performance of the CWMs (Sleytr et al. 2009). Several previous studies have identified and characterized microbial communities in full-scale constructed wetlands and laboratory scale units under specific environments (Calheiros et al. 2009; Krasnits et al. 2009; Sleytr et al. 2009; Dong and Reddy 2010; Zhang et al. 2010). However, in case of domestic wastewater, there is lack of information about how the microbial communities and diversity change during long-term operations (Adrados et al. 2014). Comprehensive information about the structure of these communities must be attained by suitable design improvisation in order to understand the biological developments that are taking place inside them (Dong and Reddy 2010). It is observed that the rhizosphere region of the CWMs is capable of providing unique add-on sites for microbial connection and release root exudates and oxygen which helps in estimating the role of the microbial cosmos (Zhang et al. 2016; Lv et al. 2017). Different design and operational parameters undertaken to treat various wastewater in several countries are presented in Table 3.

Table 3 Wetland design and operational parameters considered for different wastewater in several countries
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Removal of organics

Biodegradation of organics takes place by both aerobic as well as anaerobic microorganisms depending upon the availability of oxygen. For aerobic degradation, oxygen can be added from convection, atmospheric dispersal and through root organization of macrophytes (Cooper et al. 1996), while pores of supporting media are sites responsible for anaerobic biodegradation. Settleable organics are removed rapidly under gravitational forces by filtration and sedimentation whereas soluble organics are removed by attached or suspended microbial growth. Degradation of organics by aerobic processes mainly proceeds by aerobic chemoheterotrophs because they have a faster metabolic rate as compared to chemoautotrophs. These chemoheterotrophic bacteria oxidize organic compounds using oxygen and release carbon dioxide (CO2), ammonia (NH3), and other stable compounds (Garcia et al. 2010). Sufficient supply of oxygen greatly enhances degradation of organic matter by increasing biochemical oxidation (Vymazal and Kropfelova 2009). Anaerobic degradation of organic matter by anaerobic heterotrophic bacteria involves two processes namely methanogenesis and fermentation. In methanogenesis, methanogens (methane-producing bacteria) convert organic compounds into methane (CH4) and CO2 and produce new bacterial cells whereas fermentation utilizes acid-forming bacteria to convert organic matter into organic acids and alcohols. These two processes continue in anaerobic zone of wetland system (Kadlec and Knight 1996).

Removal of nitrogen

The contribution of macrophytes in terms of nitrogen removal varies among several species such as Typha latifolia contributing 1.73 to 8.81%, Canna indica 0.98 to 17.95%, and for Phragmites australis, it ranges from 7.15 to 17.04% (Jesus et al. 2018). In CWMs, the different macrophytes offer oxygen and surface which is necessary for the development of microbes in the root zone, consequently enhancing nitrification. In addition, there is supply of carbon from root system (5–25%, fixed photosynthetically) and optimization of denitrification process (Wang et al. 2012). Wastewater stream has typically inorganic and/or organic form of nitrogen (Stefanakis et al. 2014). Major nitrogen elimination pathways which are engaged with CWMs are classified into two broad categories—novel (new) and classical (traditional) nitrogen removal pathways (Saeed and Sun 2012). Traditional nitrogen removal pathways in CWMs include ammonification, ammonia volatilization, nitrification, denitrification, and adsorption. In the CWM system, ammonification is more in the upper aerobic facultative zone as compared to the bottom obligate anaerobic zone. Both ammonification and ammonia volatilization are pH-dependent process. The suggested pH value to get good results from ammonification ranges from 6.5–8.5 (Saeed and Sun 2012), while a notable rise in pH (> 9.3) converts ammonium ions into ammonia gas (Bialowiec et al. 2011). Adsorption takes place mostly in the form of ammonia into the supporting media (Tsihrintzis 2017) which is used to encourage cation exchange capacity. Supporting media with greater cation exchange capacity has been employed due to their enhanced nitrogen removal efficiency (Saeed and Sun 2012). Biochar is a potential material which supports the denitrification process and removal of NO3− by providing organic carbon source. A short description of novel nitrogen removal pathways is provided below:

Novel nitrogen removal pathways

Recently, some new and more efficient nitrogen exclusion routes are pointed out which comprises of partial nitrification-denitrification, anaerobic ammonium oxidation (Anammox), and completely autotrophic nitrite removal (Canon). The main operating factors of partial nitrification processes (i.e., Anammox and Canon) include temperature, pH, free ammonia, free nitrous acid, HRT, dissolved oxygen, salt, organic compounds, and hydroxylamine (Wang and Yang 2004; Lee et al. 2009). They are described briefly in the following section.

Partial nitrification-denitrification

This process involves translation of NH4–N to NO2–N which is called nitrification (Eq. 1) after that the denitrification of NO2–N to N2 gas (Eq. 2) takes place.

$$ {{\mathrm{NH}}_4}^{+}+1.{5\mathrm{O}}_2\to {{\mathrm{NO}}_2}^{-}+{\mathrm{H}}_2\mathrm{O}+{2\mathrm{H}}^{+} $$
(1)
$$ {{\mathrm{NO}}_2}^{-}+1/{2\mathrm{CH}}_3\mathrm{OH}+{\mathrm{H}}^{+}\to 1/{2\mathrm{N}}_2+1/{2\mathrm{CO}}_2+1.{5\mathrm{H}}_2\mathrm{O} $$
(2)

Jianlong and Ning (2004) reported that this process needs approximately 40% and 25% lower organics and oxygen respectively, as compared to other available nitrogen removal methods.

Anammox

Oxidation of ammonium anaerobically (anammox) is a recently revealed nitrogen removal pathway in which ammonium changes into nitrogen gas with the assistance of Planctomycetes bacterial group under anaerobic environment. The anammox process is more advantageous than another treatment system as it requires external carbon in negligible amount. Further, oxygen and energy requirements are also very low and nitrogen is removed at greater speed (Saeed and Sun 2012).

Canon

Removal of nitrite over nitrate in the complete autotrophic way involves the anammox process and partial nitrification simultaneously; together, these processes remove all available total nitrogen (TN) in a particular region. There is a mutual co-existence between anammox bacteria and ammonium oxidizing bacteria. Sun and Austin (2007) reported that the canon process in a vertical flow constructed wetlands (VFCWs) removed a significant amount of nitrogen (approximately 52%).

Removal of Total phosphate (TP)

A mixture of inert and natural phosphate is available in the wastewater stream, out of which, the most common is orthophosphates (PO43−). The performance of CWMs is reduced due to low phosphorus removal efficiency. The treatment efficiency of CWMs towards phosphate depends on the prevailing ecological situations, type and the number of macrophytes, available form of phosphate, and the loading rates (USEPA 2000). The contribution of macrophytes in removal of phosphate ranges from 4.8 to 74.87% (Jesus et al. 2018). Various macrophytes possess different plant uptake capacity such as Typha latifolia contributing 0.06 up to 74.87%, for Canna indica, 0.43 to 4.17%, and for Phragmites australis, it ranges from 0.56 to 36.7% (Jesus et al. 2018). It is pointed out that the higher water depth with reduced flow velocity advances the removal rate (Guo et al. 2017). Phosphate removal is regulated by immobilization by microorganisms, the adherence capability of a range of filter media used in different seasons, temperature, and growth periods. Dissolved state of phosphorus is taken up by macrophytes or adhered to the substrates when the cations such as Fe, Al, Mg, and Ca are present in excess. The process starts by ligand exchange reactions. Phosphate allocates H2O and OH− ions on the face of iron oxides and aluminum. However, the rate of deletion typically decreases unless an appropriate adsorbent matter is incorporated in the system (Vymazal 2010). Removal of phosphorus through various supporting media is ranging between 40 and 60%. Currently, a number of specialized media materials are used in CWMs to attain enhanced removal performance such as slag (Okochi and McMartin 2011), basic oxygen furnace slag (BOFS), sandstone, zeolite, dolomite bauxite (Stefanakis et al. 2014), and electric arc furnace (EAF) (Barca et al. 2014). It is reported that biochar has huge potential to enhance phosphorus removal by providing maximum adherence sites. Inorganic, organic, dissolved, and insoluble phosphate is not as such taken up by macrophytes until they are transformed into a simple soluble form (Choudhary et al. 2011). It has been observed that magnesium (Mg)-containing materials such as magnesia and magnesite, in the supporting media improves TP removal performance (Lan et al. 2018). In terms of plant uptake, macrophytes have lower phosphorus uptake capacity compared to nitrogen because

  1. a.

    Under aerobic setting, unsolvable phosphate is precipitated with Fe, Ca, and Al ions.

  2. b.

    Organic peat, clay, and Fe and Al hydroxides and oxides have participated in phosphate adsorption.

  3. c.

    Phosphorus is bound up in organic matter through assimilation by bacteria, algae, and macrophytes.

A number of man-made substrates such as zirconium oxide nanoparticle (ZON), magnetic iron oxide nanoparticle (MION), and iron oxide coated granular activated carbon (Fe-GAC) have been identified with improved adsorption capability. Because of high-cost involvement, discharge of secondary contaminants, and complications in manufacturing processes, the use of these materials is limited in full-scale treatment systems (Park et al. 2017). As a result, the selection of right filter media with better adsorption ability is crucial for better performance.

Removal of heavy metals

Wastewater which is contaminated with trace metals has the great impact on biosphere; therefore, the remediation of these trace metals is essential. The presences of such metals greatly affect the flora and fauna of an aquatic system (Parnian et al. 2016). Remediation of wastewater polluted with heavy metals implies various technologies in which adsorption, reverse-osmosis, electrodialysis, and ion exchange are more common. Almost all of such technologies are expensive, energy-intensive, and generally metals-specific. However, macrophytes in the CWMs are known to have the huge potential towards trace metals buildup in their tissues (Mishra and Tripathi 2008). Removal of metals from domestic wastewater through CWMs involves mainly filtration, sedimentation, adsorption, cation exchange, precipitation, complexation, macrophyte uptake, and microbial oxidation/reduction processes. Several biotic, abiotic, and environmental factors like pH and temperature in the CWMs have direct consequences on bioaccumulation of trace metals (Xing et al. 2013). Removal of heavy metals in CWs using aquatic macrophytes by different studies has been shown in Table 4.

Table 4 Removal of heavy metals in CWs using aquatic macrophytes
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Sustainability of CWMs

A sustainable design of CWMs for domestic wastewater treatment includes the suitable design of CWMs at proper site with efficient macrophytes and supporting media. Design in a way that it acquires the natural features of the surroundings and to diminish its disturbance. The working model is set by the prevailing landscape, geology, and availability of land. Supply of additional oxygen is via artificial aeration, water depth, optimization of HLR and HRT, bioaugmentation of specific microorganisms, proper plant harvesting; reuse/recycling methods, and the addition of extra organic matters (Fig. 3) (Kadlec and Wallace 2009). Recently, the recirculation of effluent within the CWM system attains huge potential towards enhancement of removal performance through sufficient settling time. The removal performance of CWMs declines considerably when the environmental parameters such as water temperature, pH, and DO are not properly managed (Kadlec and Wallace 2008).

Fig. 3
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Sustainability of CWMs—key criteria (modified from Wu et al. 2015)

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Future concerns and challenges

Firstly, optimization of hydraulics, selection of appropriate macrophytic species and supportive media, mode of operation, and pollutant loading rate are important factors to gain higher removal efficiencies. Suitable plant harvest techniques are vital because when they die and decay, leave nutrients and several other contaminants into the water body. In future research, there is a need to develop techniques to improve treatment efficiencies which could be achieved by microbial augmentation, artificial aeration, a range of supporting media, and supply of additional carbon, tidal action, step feeding, baffled flow, and mixed systems (Wu et al. 2015). CWMs are land intensive, requiring large land area and prone to seasonal weather conditions. Therefore, suitable design improvisation could be done to reduce the overall land requirements. This is also reported by various researchers that the CWMs are by nature prime mosquito habitat. This challenge could be tackled by conserving natural enemies (invertebrates) such as dragonflies, damselflies, beetles, predatory flatworms, true bugs, and crustaceans such as copepods, tadpole shrimp. Fishes, amphibians, spiders, bats, and microbial larvicide Bacillus thuringensis var. israelensis (Bti) are also used to control mosquitoes’ larvae (Mazzacano and Black 2013).

Conclusion

CWMs can be designed as biofilters to imitate the features of natural wetlands for removing nutrients, and other contaminants from the wastewater streams. The focus of this review paper has been on evaluation of treatment performance of CWMs treating domestic wastewater. Both ecological factors such as temperature, pH, DO, and working parameters such as availability of carbon, HLR, HRT, pollutant loads, recirculation, C/N ratios, plant harvesting techniques, addition of extra organic matter, and bioaugmentation of specific microorganisms are vital to achieving sustainable contaminant removal efficiency. Supply of additional oxygen via artificial aeration (mainly intermittent) and effluent recirculation greatly enhances the removal efficiency for organics and nutrients. Novel nitrogen removal pathways have greatly enhanced the nitrogen removal. The removal efficiency increased at influent C/N ratio between 1 and 3 and decreased significantly at the increasing C/N ratios between 3 and 15. The contribution of macrophytes in terms of nitrogen removal varies from 0.98 to 93% and for phosphate ranges from 4.8 to 74.87% depending upon area of the root surface and root oxidizing capacity. Removal of phosphate mostly occurs by adsorption and its efficiency is usually low until a suitable supporting media is not incorporated. Biochar has great potential to support denitrification rate and NO3–N removal by providing carbon source and also enhance phosphorus removal. Typically, the removal of phosphorus from a variety of supportive media ranged from 40 to 60%. Removal of heavy metals from wastewater implies various technologies such as ion exchange, electrodialysis, adsorption, and reverse-osmosis. Almost all these technologies are expensive metals-specific and energy-intensive. However, macrophytes are known to have huge potential towards trace metals buildup in their tissues.