Keywords

3.1 Introduction

Water is the most essential component of the environment any form of life is not possible without water. It is undoubtedly a great solvent and carries variety of chemicals with it in dissolved and suspended forms. Therefore, the water gets polluted easily contaminated. It governs almost all our activities be it domestic need or developmental activities. Metabolic processes of the living organisms are governed by the water, as most of the biochemical mediated reactions within bodies of organisms takes place in aqueous medium. Water has a unique property to dissolve materials in it without altering the chemical properties, so it is excellent in transporting the materials in the body. Water shortage is probably going to turn out to be trickier soon because of fast populace development, expanding per capita water utilization and topographical differences between focuses of populace development and accessibility of water. The increasing population concentrating in urban locations is causing increased generation of wastewater. Lakes, waterways, reservoirs of the world are currently getting contaminated by disposal of wastewater into them and thereby presenting danger to the existence of life forms in water bodies. Wastewater treatment is an issue that has tormented man since the time it was revealed that releasing the waste into inland waters could prompt numerous environmental challenges. The treatment of the wastewater before its disposal into the inland water bodies is one of the solutions to protect water bodies. The choice of a wastewater treatment method is crucial, it depends on various factors like availability of space/land, type of wastewater, degree of treatment required, centralized or decentralized system, energy, and chemical requirements. The conventional wastewater treatment systems involve combination of physical, chemical, and biological treatment processes such as screening, sedimentation, chemical coagulation, filtration, and microbiological activities for the degradation of organics present into the wastewater. The conventional wastewater treatment processes are highly mechanized, concrete, and iron structure that consumes electrical energy to run them. Therefore, it is called energy consuming process of wastewater treatment. In the last 50 years, significant interest has been communicated in the possible utilization of ecological systems to assist wastewater treatment in a controlled way. The natural methods for wastewater treatment like aerobic/anaerobic ponds, constructed wetlands (CWs) and waste stabilization ponds differentiate themselves with the conventional systems as they are combination of natural components like light, gravity, plants and microorganisms working in co-action for treatment of wastewaters. Under these natural systems the natural forces are utilized in managed environment to perform the task, rather using heavy pumps to lift water and motors to run machines.

In the recent decades, constructed wetlands have been tested at laboratory, mesocosm and at field levels. Its diverse applications and treatment efficiency into wastewater treatment, with additional environmental benefits has presented it as promising eco-technology in front of decision makers, scientists, engineers and wastewater treatment solution providing companies. This chapter is presenting the technological outline for its design, operation, treatment mechanisms, environmental advantages, drawbacks and need for future research.

3.2 Background

The natural wetlands are unique reservoir of large volume of water, that release the water slowly into the environment. It plays a significant role in climate regulation, ground water recharge, water purification, habitat for variety of biodiversity (wetlands regulating services). The community living close to the wetlands, depends on it for food, fibre, biomass, medicinal plants and freshwaters (wetlands provisioning services). The close association with wetlands is for its ethical purposes, recreation and ecotourism (Cultural values). The wetlands also support for cycling of the nutrients, soil formation and production biomass and gaseous exchange through photosynthesis (Supporting services).

The wetlands are the natural bioreactors as many bio-mediated transformations take place in these systems, like bacteria degrade the organic matter into simpler forms available to the consumers. The combined physical, chemical and biological actions help in water purification process in natural wetlands. The physical process involves sedimentation, chemical processes are chelation, adsorption and precipitation, etc. Prevalent biological processes are nitrification, de-nitrification and ammonification in the presence of variety of microorganism with or without oxygen. The potential of natural wetlands has been recognized by many workers for the purification of water/wastewater (Verhoeven and Meuleman 1999; Gopal 1999). The unique characters of the wetlands system like large amount of water (causing dilution), oxic and anoxic soil substrate responsible for nitrification/de-nitrification and deep rooted emergent macrophytes responsible for biomass accumulation through nutrient uptake, make them to be considered for wastewater disposal and treatment (Nichols 1983). At the same time addition of excess nutrients to the wetlands through agricultural runoff, municipal and industrial sewage cause eutrophication of wetlands (Liu and Diamond 2005).

CWs are the artificial wetlands, ecologically engineered incorporating water, tankage, substrate, flow distributors and macrophytes. CWs may be developed for several reasons part from the most common one, i.e., wastewater treatment. Some legitimate applications of CWs are

  • Constructed wetlands habitat: these are developed with a reason to provide habitat to the wildlife. Major habitat wetlands are the freshwater swamps and marshes, salt marshes, saltwater swamps, freshwater marshes and swamps.

  • Flood control CWs: flood control wetlands are developed in a large area with native vegetation. These areas are used for the impoundment of runoff and further slow release of the collected waters.

  • Constructed aquaculture wetlands: these are mostly used for growing food like fish, prawn chestnut, fox nut, etc.

3.3 Constructed Treatment Wetlands

Despite engineered wetlands have been created for fulfilment of multiple functions around the globe. Their wastewater treatment abilities presented them as an alternative option for treatment of range of wastewaters (municipals, landfill, industrial, mining and agricultural) before the researchers. While these systems are space intensive, they provide an effective means of integrating wastewater treatment and resource optimization. Often at costs that are competitive with traditional wastewater treatments. This chapter provides brief descriptions on deign, operation, maintenance and advantages of constructed treatment wetland systems for wastewater treatment.

3.4 Development of Constructed Wetlands: Historical Approach

Though Constructed Wetlands technology is new to India, but it has been successfully practiced in many countries like Australia, Denmark, Germany, Japan, U.K., USA, Switzerland and many other western countries for the last 60 years. The CWs has been developed from the 1950s. Sub-surface flow treatment wetlands were developed in Germany in the 1960s through the work of Prof. Kickuth at Kassell University (Conley et al. 1991). Thousands of natural and constructed wetlands across the world are receiving and treating a variety of wastewaters (Kadlec et al. 2000).

The worldwide spread of constructed wetlands started from Europe with pioneer work by Seidel and Kickuth (Max Planck Institute in Plon, Germany) during 1950s (Vymazal 2005). Applications of CWs get momentum during 1980s and started being recognized around the world.

Early development work in the USA commenced in the early 1980s. The Sub-surface constructed wetlands using sand and/or gravel, supporting emergent aquatic plants (Typha, Scirpus and Phragmites) were used, calming excellent removal of BOD, TSS, Nitrogen, Phosphorus and more complex organics from wastewater.

Kickuth suggested the use of cohesive soils instead of sand/gravel, using phragmites, in horizontal flow path. According to Kickuth’s theory, growth and development of plant roots and rhizomes in soil media will open up the flow channels up to a depth of about 0.6 m. It helps in increasing the hydraulic conductivity of the soil substrate at par with sandy soil (Mucha et al. 2017). Such phenomenon permits a reasonable flow rate through soil media that bears a good adsorptive capacity for phosphorus removal.

North America witnessed its first engineered CW in 1973, that was a pilot system constructed at Brookhaven National Laboratory near Brookhaven, NY. Currently, Florida has many large CWs, like Lakeland and Orlando CWs, started in 1987. Each having an area of 500 hectares for advanced treatment of municipal wastewater. The largest constructed treatment wetland is the 1800-hectare Kiss–Balaton project in Hungarg, started in 1985. The CWs were providing a low cost natural option for water quality improvement in varied climatic conditions worldwide. Although this technology is still somewhat innovative, long-term operational information now exists from many full scales engineered wetlands.

CWs in North America were designed basically to treat large volumes (1 lakh to 15 million gallons per day) of municipal wastewater. A survey of 300 wetlands was conducted in North America, treating primarily municipal wastewater, documented the performance for pollution reduction of BOD (53%), TSS (72%), total nitrogen (53%) and total phosphorous (56%) (Brown and Reed 1994).

In the community of Houghton Lake, located in the Central Lower Peninsula of Michigan (USA), with a population of nearly 5000, the wetland provides additional treatment to the wastewater from STP. The CW system can also promote subsurface through a shallow permeable substratum in which aquatic plants are established (sub-surface flow, SF) (Wood 1995). Sub-surface flow wetlands have received popularity in northern Europe and USA. The attraction towards subsurface system in comparison to free water surface and overland flow systems, was because of their decreased risk of flies and odour nuisance and occupying less area (Reed and Brown 1995). In Europe, the thrust was on application of decentralized CWs for domestic wastewater treatment. Hans Brix a noted plant ecologist of Denmark, who is an authority on CWs, reported that Denmark alone has 150 CWs, mostly in isolated villages treating domestic wastewaters. The use of Reed beds is widespread in northern Europe says Brix. Brix in collaboration with Polish engineers were working in Poland to transfer the technology. Poland developed more than 100 CWs (Brix 1994).

In India in recent past good work is done by National Environmental Engineering Research Institute (NEERI) and Central Pollution Control Board (CPCB), New Delhi. CBCB has developed guidelines and manuals for the constructed wetlands applications. NEERI has developed phytorid technology that has been patented too. A good amount of work has been done in central India with regard to treatment of Municipal Wastewater by sub-surface based constructed wetland (CW) using Phragmites karka as a decentralized system (Prashant et al. 2013).

3.5 Classification of Constructed Wetlands

The CWs are classified on several bases, the classifications and the bases are presented in Figure 3.1, despite all sorts of classification (Kadlec and Wallace 2008), the main types are based on the flow direction/pattern.

Fig. 3.1
figure 1

Types of constructed wetlands

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Free water surface flow (FWS) wetlands:

This type of CWs resembles very much to the natural wetlands (Fig. 3.2a). In FWS the flow of wastewater is across the media bed. The substratum here is soil layer (35–45 cm). There is a standing water column of 25–45 cm, exposed to the sunlight. The soil layer remains heavily planted with common native macrophytes (Typha, Phragmites, Scirpus, etc.). The FWS are used for the tertiary treatment of different kinds of wastewater such as domestic, agricultural, storm water, highway runoff, etc. (Vymazal 2013). The dominant microbial activities mediated by bacteria and fungi are responsible for the treatment of wastewater in FWS. The free waters and the root hair assemblages provide micro-habitats for bacteria and fungi in the wetlands. Here the principal treatment processes are sedimentation, filtration, oxidation, reduction, adsorption and precipitation. Sudden increase in the channel width and standing vegetation reduces the flow velocity thereby enhancing the sedimentation of particles from the wastewater. Major components of FWS are Inlet and outlet arrangements, dykes, berm, soil substrate, impermeable geo textile lining in the bottom (to prevent percolation of wastewater) and emergent macrophytes. The FWS mimic the natural wetlands so, it attracts the wildlife too (Kadlec et al. 2000). The planners choose the FWS mostly for tertiary treatment that receives treated wastewater from activated sludge process trickling filters of lagoons. The nitrification and denitrification processes occurring in aerobic/anaerobic pockets of the CW are responsible for nitrogen removal. Phosphorous (P) is removed via adsorption process, P removal rate is not satisfactory in FWS systems. It a good choice for treatment of low organics wastewater like urban storm water runoff, agricultural runoff, etc. as it also withstands the water fluctuations and flood shocks. The operating costs are low and suitable for all types of climates at lower efficiency during colder periods of the year (Wang et al. 2017). The identifiable feature is water column above the soil substrate. The standing water column attracts the breeding of mosquitos to in tropical/sub-tropical climate.

Fig. 3.2
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(a) Free water surface flow Constructed Wetlands, (b) Horizontal subsurface flow constructed wetlands, (c) Vertical subsurface flow constructed wetlands, (d) Artificial floating islands

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Sub-Surface Flow Constructed wetlands (SSFCW):

The SSFCW has a different flow regime than the FWS, here the wastewater flows below the substrate the supports the vegetative growth. The SSFCW can be classified further into Horizontal Sub-Surface Flow Constructed Wetlands (HSSFCW), Vertical Sub-Surface Flow Constructed Wetlands (VSSFCW), and Hybrid systems.

Horizontal Sub-Surface Flow Constructed Wetlands (HSSFCW):

As the name suggests, in this type of CW the flow of water is parallel to the datum below the substrate (Fig. 3.2b). One can find the water by digging the substrate 5–10 cm deep. The wastewater flows horizontally through the pores of the filter media (substrate). The wastewater is retained in the CW tankage for a period (hydraulic retention time—HRT) ranging from 24 to 72 h. During the passage of the wastewater from inlet to the outlet it comes into the direct contact with the filter media, biofilm and the roots of the vegetation. The aerobic/anaerobic and facultative pocket of the HSSFCW treats the wastewater by biologically mediated chemical processes (Kadlec et al. 2000). The CW tankage is sealed with impervious strata (clay/geo-membranes) to avoid the possibilities of leakage of wastewater to the below ground. The substrate provides anchorage to the emergent macrophytes. Gravel/sand-based substrates used in these systems have an advantage over the soil-based substrates for better hydraulic conductivity. The HSSFCWs are mostly planted with native aquatic macrophytes such as Phragmites (australis, karka, aurundo), Typha (latifolia, angustfolia), and scirpus (lacustris, californicus). Thickness of the filter media varies between 60 and 80 cm. The vegetation roots and filter media provide space for development of biofilms. From inlet to the outlet zone, a slope is maintained for removal of the treated water from outlet. The outlet system is fitted with mechanism to control the water level below the substrate in the CW. These systems are extensively used for grey water, industrial, municipal and agricultural wastewater treatment. The primary treated wastewater (after removal of suspended solids) is fed to HSFCW in order to avoid the clogging of the filter media/substrate. A screening and sedimentation chamber is pre-requisites for the HSSFCWs. Such types of systems are good for cold climates as here the wastewater is not exposed to the atmosphere. The insulator layer of substrate keeps the water flowing and restrict it from thermal cold shocks. In comparison to the FWS systems the land area requirement is less in HSSFCW, however, the higher capital cost has been reported as a challenge for its application (Kadlec 2009). For better treatment performances these systems have been modified on the basis of its HRTs, wastewater recirculation, batch feeding, ion exchanging filter medias and fluctuating water levels for originating the bed. The applications of HSSFCWs have been reported for several types of wastewaters like industrial, animal husbandry, municipal, storm water and landfill leachates (Billore et al. 2001; Sharma et al. 2013; Lucas et al. 2014; Bakhshoodeh et al. 2020).

Vertical Flow Constructed Wetlands (VFCW):

VFCWs are the recent addition to the constructed wetlands treatment technology. In VFCWs the wastewater flow direction is top to bottom (perpendicular to the datum) so it is called as vertical flow CW (Fig. 3.2c). It has similarities with the conventional trickling filters. The main components of the VCFWs are the tankage, substrate (filter media), inlet and outlet arrangements, sealing material like geo-membranes and emergent macrophytes. The arrangement is made in a closed vessel usually above ground. The vessel is filled with the graded filter media (sand and/or gravel). The depth of filter media varies from 30 to 180 cm (Brix and Arias 2005; Vymazal 2014). The filter media is arranged on the basis of its size, the lower part is filled with comparatively bigger size gravel and the upper part contains the smaller size sand/gravel. This arrangement of the filter media shall be done keeping in consideration of hydraulic conductivity and short circuiting of the flow. If not arranged properly, small channels may develop from top to bottom within the filter media for vertical the passage of the wastewater and even distribution of the wastewater throughout the filter bed is compromised. The top of the VFCWs is planted with suitable emergent macrophytes (mostly Phragmites and Typha) (Calheiros et al. 2009). The wastewater is applied on the top surface of the VFCWs through perforated pipes. At the bottom of the tankage, a network of perforated pipes is laid and connected to a single outlet for removal of the treated water. A provision of forced aeration through vertical perforated pipes is also applied keeping the system in aerobic mode (Stefanakis and Tsihrintzis 2012). The tankage is made watertight by applying the geo-membranes at the bottom and side walls of the VFCWs. A slope of 1–2% towards the outlet holds good for efficient collection of the treated water at the outlet. The VFCWs functions in aerobic process for the degradation of organics present in the wastewater. The aerobic condition is maintained by batch application of the wastewater, between two cycles of the application of wastewater, the substrate gets refilled with the air into its pores. This condition supports the vigorous microbial mediated nitrification of the constituents of wastewater (Platzer 1999). A pre-treatment of wastewater for removal of the suspended solids is recommended to avoid the clogging of the filter bed. The VFCWs exhibit a god removal of suspended solids, biodegradables organics and ammonia. Due to the presence of air in the pores VFCWs are efficient on BOD5 removal (Cooper 2005). The phosphorous removal efficiency is at lower side that may be enhanced by mixing certain media having high adsorption capacities (Brix et al. 2001). These are the compact systems and require lesser area in comparison to the FWS and HSSFCWs but high manual supervision for its maintenance (Stefanakis et al. 2014). The application is mostly reported for purification of domestic wastewater; still it is used for treatment of landfill leachates, dairy effluents and airport runoffs (Branchu et al. 2014; Pelissari et al. 2014; Bakhshoodeh et al. 2020).

Artificial Floating Islands (AFIs):

AFIs or Artificial Floating Reed Beds (AFRB) are a new alternative to CWs meant for onsite treatment of water bodies (Billore et al. 2009). The AFIs evolved on the principal of CWs and natural floating islands that are similar to hydroponics. Free-floating aquatic plants or sediment-rooted emergent riparian vegetation in subsurface flow or surface flow CWs were the traditional land based designs. The AFIs are recent edition of CW, here rooted emergent plants (reed, typha, etc.) are grown on a floating mat over water surface rather than rooted in the sediments (Fig. 3.2d). The floating features of AFIs amid the water column make them suitable for in situ treatment applications, without being affected by the fluctuating water column of lakes or reservoirs. The AFIs are designed in a way that the roots of the vegetative mat hang in the water column for a depth of at least 0.75–0.80 m. The Eco-engineered Artificial Floating Islands (AFIs) are initiative in the area of onsite treatment of degrading freshwater systems including ponds, lakes, reservoirs, stagnant rivers, artificial lagoons or oxidation ponds.

AFIs are floating superstructures composing emergent plants specially reed (Phragmites karka) (Billore et al. 2009) floating on waterscape. The major components of the AFIs are floating mat usually made up of coconut coir, bamboo pieces, emergent macrophytes and floats (Prashant and Billore 2020). The AFIs remove the pollutants from open water surface through nutrient uptake from macrophytes, sedimentation process, root attached particulate matter (RAPM) accumulation and microbial activities (Wang et al. 2020). The AFIs has been applied for the removal of pollutants from storm waters (Headley and Tanner 2012) and for aesthetic purposes.

Hybrid Constructed wetlands:

This type of wetlands are combination of both horizontal flow type and vertical flow type CWs. The purpose of the combination is to achieve the high degree of treatment of the wastewater. The uniqueness of HSSFCW and VFCW combination gives a better result for NH4–N and Total Nitrogen removal (Vymazal 2005). These CWs have also better treatment efficiency for BOD, COD, suspended solid, pathogens, heavy metals and also for the emerging pollutants (Masi and Martinuzzi 2007). Successful treatment of variety of wastewater, such as dairy (Sharma et al. 2013), leachate treatment (Saeed et al. 2021) has been reported from the hybrid CWs. Some disadvantages are embedded with this treatment system like large area demand, more complex design mechanism and difficulty in working in cold climate.

Components of the Constructed wetlands:

Major components of the constructed wetlands that play important role in the treatment of wastewater are the filter media/substrate and vegetation (Wu et al. 2015).

Media for Constructed wetland:

Substrate media means the filter which is used to trap significant pollutant in sewage through various treatment processes. These processes are sedimentation, filtration, adsorption, etc. Substrate media provides a path through which wastewater can flow and surface on which microbes can live. These microbes feed on waste material present in wastewater and remove them. Surface type and size of substrate provides the special site for biofilm development and adsorption of nutrients too. The physical and chemical property of substrate media can affect the treatment efficiency of whole CW system. If the filter media is made up of porous material having large surface area, then it can improve the hydraulic and mechanical property of treatment system. The hydraulic conductivity of the media is a key parameter in deciding the performance of CW system (Sundaravadivel and Vigneswaran 2001). Hydraulic retention time depends on it. The particle size distribution is the main parameter that influences the soil hydraulics (Stottmeister et al. 2003). According to previous study fine sand and soil-based substrates have less hydraulic conductivity while coarse substrates (sand/gravel) have high conductivity. The substrate media plays a significant role in nutrient removal mechanism also (Prochaska and Zouboulis 2006). Adsorption is the key process for the removal of the phosphorous which occurs through the filter media. In N2 removal mechanism, substrate media plays an important role also. Nitrification–denitrification reaction is the main N2—removal procedure. For these reactions the possible microbial environment can be possible in suitable filter media (Reddy and D’Angelo 1997). The role of filter media in governing the various treatment processes are mentioned in Table 3.1.

Table 3.1 Role of media in wastewater treatment under CWs
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The media is very essential and growth and development of the vegetation. The type and size of the media is an important consideration. The size of the media particles can affect the ability of treatment of the CW.

  • Substrate media particle of small size has small pores. These small pores can filter smaller particles from the wastewater effluent.

  • With large surface area of media particle, large amount of microbial assemblage can be possible which enhances the degradation of waste materials present in effluent.

  • Large media particles have good ability to reduce transmission of odours and vectors (disease causing). These large media particles have large openings that allow better air exchange which can allow the exit bad odour and vectors too.

  • The media particles should be off suitable size because very small sized particle can cause the clogging problem more often and extra-large particle can allow the wastewater to flow through the bed without proper treatment.

Characteristics of different media particle used in CW

  • Bulk porosity—It is the amount of space between media particles. These spaces are filled with air or water. The bulk porosity should be about 30% which allows the wastewater to flow adequately through the wetland media. This amount of porosity provides sufficient time to the effluent for the better contact with media particle.

  • Stability—The media should be made up of such kind of particle which are strong enough for their long-time stability.

  • Particle size—The media particles should be off suitable size because very small sized particle can cause the clogging problem more often and extra-large particle can allow the wastewater to flow through the bed without proper treatment.

  • Surface area—With large surface area of media particle, large amount of microbial assemblage can be possible which enhances the degradation of waste materials present in effluent.

  • Uniformity—The particle size along the whole media surface should be same. With similar size of particles, the pore size between them is also same, which maintain the uniform permeability throughout the media surface.

3.6 Frequently Used Media

The sand, gravel and soil are the commonly used filter media in many CWs. Sand is readily available in the nearby rivers and construction material suppliers. Soil substrates helps in ammonia removal through interaction with the strata, humic substances (Kadlec 2009). The peanut size gravel is the most suitable media in the CWs. Now a days alternative media is also being used into CWs. The major classes of the alternative media are natural (zeolite, limestone, bauxite, etc.) artificial/by products of industries (coal fly ash, red mud and slag etc.) and man-made products (light weight aggregates) (Valipour and Ahn 2016). The role of substrate in phosphorous and nitrogen removal is presented in Table 3.2.

Table 3.2 Comparison of different filter media for removal of phosphorous and nitrogen
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Vegetation for Constructed Wetlands:

The wetland vegetation is an essential component of the CWs that play key role in treatment of wastewater. The major classes of the vegetation used in CWs are emergent macrophytes, submerged vegetation, floating plants. Important functions of the vegetation are to stabilize the surface of filter media, manage the suitable condition for filtration process, provide large surface area for microbial assemblage, biofilm development for better degradation of wastewater and translocation of the oxygen to the gravel bed (Brix 1997). The wetlands vegetation shall be selected on the basis of certain properties.

  • The vegetation shall not be a weed into the area of reason for any disease.

  • It shall be a locally available ecologically acceptable vegetation.

  • It shall be tolerant to the high organic and nutrient loads.

  • The pollutant removal capacity shall be high.

  • Vegetation should have large sized roots and rhizomes for providing large space for microbial assemblage and for oxygenation process.

  • Vegetation should have high aboveground cover for insulation during winters.

Major role of vegetation in CWs:

The presence of vegetation reduces the flow velocity of effluent into the CW, which creates better condition for the sedimentation process (Persson et al. 1999). It also stabilizes the substrate media through compact binding of media particle with the help of their root system. In temperate areas the vegetation covers provides insulation cover to the wetland surface and keep the media substance free of frost during winter season. The roots and rhizomes of macrophytes provide the water channel through the soil pore in the substrate media which maintain the conductivity of wastewater through it. The roots and rhizomes of macrophytes enhance the loosening of soil and after their death they leave the tubular pores and channels made by them. Therefore, they not only maintain the hydraulic conductivity but also stabilize it. Vegetation in CW enhances the concentration O2 in the wetland environment by leaking O2 into it. The rate of leakage of O2 is highest in the root region. The rich oxygen environment helps in oxidation of the harmful pollutants in the rhizosphere and supporting nitrification process. Macrophytes used in CW have large root system, which provide the rhizosphere, centre of microbial activity. It enhances the microbial density and activity by increased root surface for their growth. These grown microbes are responsible for the degradation of wastewater pollutants. Therefore, by enhancing the surface area, macrophytes help in the treatment process, the sole purpose of CW. Like other plants wetland plants also require nutrients for their growth. They take nutrients through their root system. In CW, wastewater effluent has large amount of nutrient concentration in the form of nitrate and phosphate compound. Macrophytes take these nutrients for their growth and reproduction and make the effluent nutrient free. The uptake of nutrient is stored as plant biomass (Brix 2003). Macrophytes not only uptake nutrients but also heavy metals and other harmful chemical compound present in wastewater effluent. These are accumulated into different plant parts like stem, leaves, etc. Different parts of the macrophytes have been recognized to play some particular functions. The aerial plant tissues of the vegetation attenuate sun light and reduce the growth of algae that cause clagging in the inlet and outlet regions. It also provides insulation to the bed. It has an aesthetic appearance apart from the main role of nutrient storage. The plant tissue that remains present in the water helps in filtration by removing larger debris, reduce the flow velocity thereby increasing the rate of sedimentation, providing space for growth of surface attached microorganisms, nutrient uptake and photosynthesis. The submerged part includes the roots and rhizome that has special role in preventing the erosion of substrate, release of oxygen into the bed, nutrient uptake and release of root antibiotics.

Siting criteria and design consideration of the CWs:

The siting of the full-scale field level CWs and the design criteria is an important aspect, beforehand the actual installation of the CWs.

  • Location and size of the land: A decentralized system shall be adopted and the site of constructed shall be close to the wastewater generation point. The commercial lands shall be avoided to keep the cost low. The strategic location helps in developing a green patch for aesthetic look. The location shall be easily approachable. There shall be a reuse plan of the treated water or availability of the water bodies for final disposal of the treated water. A natural gradient from the points of wastewater generation and CWs location is favourable, in order to avoid the usage of pumps for lifting the wastewater. The size of the CW depends on the quantity of wastewater to be treated, pollution load of the wastewater and desired quality of treatment. The most common criteria for the sizing, is unit area required (m2) per person equivalent (PE). Before designing one must learn what are the legal compliance/treated water quality standards of the local government. Major heads for the cost estimation of the CWs are as follows.

  • Size and per unit price of the land.

  • Development of sewerage system (civil work).

  • Requirement of pumps (capacity and unit price).

  • Excavation of the land for preparing the tankage.

  • Volume of the filter media to be filled in the tankage and unit volume price.

  • Area of the sealing material (geo-membrane/LDPE liner) required and unit area price.

  • Plant material: the native vegetation is grown in nursery then planed in the CWs. Usually, such plants are not commercially available in the local market. The cost of plant material preparation involves nursery development, polybags for the growth of saplings, sand and soil mixture to be filed in the poly bags and irrigation for the growth of the plants.

  • Skilled and semiskilled labours requirement for tank excavation (can be done by machines also), nursery work, filling substrate into the tankage, etc. can be hired on the man days basis.

Design criteria for HSSFCWs:

The HSSFCWs are the secondary wastewater treatment process that focus on BOD and TSS removals. Usually, the pollutant removal efficiency for BOD and TSS varies from 60 to 85% and 60 to 90%, respectively (Vymazal and Kröpfelová 2009; Prashant et al. 2013). Type of the wastewater, pollutants concentration and hydraulic retention period of the CWs are the critical parameters to decide the efficiency. Due to the presence of anaerobic zones in the bed, the HSSFCWs are efficient in de-nitrification. Many workers have put forward the sizing/design guidelines that vary from place to place depending on the local climatic conditions, local materials and plants. The specific surface area requirement method is most comfortable one keeping the depth around 0.6–0.8 m. The population equivalent (PE) is a guiding factor for determining the size of CW. The PE is the ratio of total BOD (kg/day) and per capita BOD (kg/day). In a Czech Republic review the population equivalent was reported between 4 and 1200 (Vymazal 2002). For Indian conditions the average daily per person BOD load is considered 45 g for the estimation of population equivalent. The studies carried out in Denmark and UK evaluated that for pre-treated wastewater size of HSSFCW has value of about 5 m2/PE (Vymazal 2002). A length and width ratio of 2:1 is ideal for secondary treatment; it helps to maximize the cross-sectional area of flow and minimize clogging problems. The vegetation root penetration is around 0.6 m so ideally the depth of filter media is kept as same. A simpler way of sizing the HSSFCW bed can also be done one the basis of porosity (ɳ) of filter media, HRT (time in days), and discharge (Q= m3/day) (Billore et al. 1999). Based on the BOD5 concentration and inlet (Cin—mg/L), desired BOD5 concentration at the outlet (Cout—mg/L) average flow rate per day (Qd—m3/day), the size (Ah—m2) of the HSSFCW is determined by the equation (Ah = Qd (ln Cin − ln Cout)/KBOD) for pre-treated domestic wastewater where KBOD is the rate constant (day−1) (Kickuth 1977).

Design criteria for VFCWs:

The vertical flow (VF) systems require less area in comparison to the HSSFCWs. The parameters for designing of the VF systems are very much similar to the horizontal flow systems. The population equivalent is the most commonly (many national guidelines are suggested) used parameter for the sizing of the VFCWs. Apart from the population equivalent (PE), the daily organic load (gBOD 5/m2 day or gCOD/m2 day) can also be used. The reported PE for VFCW range from 1.2 to 5 m2/PE (Brix and Arias 2005; Kadlec and Wallace 2008; Molle et al. 2008).

Pre-treatment:

The secondary treatment based CWs shall receive pre-treated wastewater. Here the meaning of pre-treatment is removal of suspended particulate materials, floating substances and removal of oil and grease. Insufficient pre-treatment cause clogging of the VFCWs and HSSFCWs. Based on the characteristics of the wastewater, the pre-treatments like screens, sedimentation tanks, grit removal basins and skimming tanks may be selected. For the municipal wastewater, the effluents of the anaerobic pre-treatments (septic tanks, imhoff tanks, baffle reactors, etc.) can also be fed to the CWs.

Inlet and outlet arrangements:

The inlet of the CWs is important as it governs the flow distribution of the water in the wetland cell. A proper inlet arrangement helps in avoiding the short circuiting of the flow from inlet to the outlet. The inlet and outlet structures of the surface flow wetlands/free water surface flow wetlands are simple, just to release the water in the cell and collect it from the cell. But in subsurface flow systems (HSSFWs and VFCWs) uniform distribution across the filter media is essential for better treatment. A higher length-width ration (3:1 or more) supports uniform flow distribution. The inlet is kept above the filter media for proper distribution, cleaning and maintenance of the inlet. The inlets can be termed as, single point inlet, spreader trench or perforated horizontal pipe along the width. Provision of water sample collection shall be there for monitoring and analysis. The perforated inlet pipe above the filter media is exposed to the sunlight that causes development of algae in the orifices. An inlet perforated pipe covered with the filter media reduce the chances of development of algal slimy layer. Coarse filter media or bigger size gravel/stone shall be used at the inlet for proper distribution of the wastewater being fed.

The outlet arrangement functions mainly (1) to collect the treated water from the distal end of the CWs, (2) to control the water level in the wetlands and (3) to provide a point for sample collection. In HSSFCWs a perforated pipe is laid at the bottom of the distal end along the width of the cell. An arrangement shall be provided at the outlet to adjust the water level in the CW bed. At the outlet zone again larger/coarser filter material shall be placed for better hydraulic conductivity. A good slope (1%) from inlet to outlet is required. Shade shall be provided at the outlet/treated water collection well shall minimize algal growth by cutting light. In VFCWs a network of perforated pipes shall be laid at the bottom surface of the cell for efficient collection of the treated water.

Sealing of the cell/tankage:

The wastewater fed to the CW bed shall remain into the cell itself. There shall not be leakage of the wastewater into the ground water. Otherwise, the purpose will be defeated if the wastewater percolates and meets to the groundwater. To make the CW cell impervious mostly geo-membrane/LDPE liners are laid to the bottom or sides of the wetlands. After excavation of the tank the following shall be done (in case of HSSFCWs) to make the cell water tight:

  • Clear all the pointed stones from the excavated cell bottom and sides.

  • Clear all the pointed roots, stems that may pierce the liners.

  • Remove the water logging if any.

  • Provide a layer of cushion (sand) at the bottom.

  • Lay the LDPE liners.

  • Interlock and seal the liner with other pieces.

  • Seal the liners with the native soil at the top of the cell.

  • After laying the impervious layer, filter media shall be filled in the cell.

Vegetation development and plantation:

Vegetation is an important component of CWs, it plays a vital role in oxygen transport, nutrient uptake, biomass accumulation and microbial growth in the rhizosphere (Chen et al. 2016). In a CW planning and design establishment of vegetation is very important stage. The various steps of establishment of vegetation include the following.

  • Selection of plant species.

  • The plant species shall be selected on the basis of the local availability of the plant species, wastewater to be treated, rapid growth, dense root system.

  • Looking for such plant species stock on nearby water bodies.

  • Development of vegetation in nursery.

  • Planting the saplings in the CW bed.

  • Decide number of plants/m2 (usually 4/m2).

  • Impound the CW bed with water for few days for vegetation growth.

Operation and maintenance:

The CWs are designed with a focus with minimum maintenance but all the CWs require some timely maintenance for proper functioning of the CWs. The objectives of operation and maintenance are as follows.

  • To validate that the CWs are working as per the design.

  • To increase the treatment efficiency.

  • To increase the life of the CWs.

  • To save cost of the major breakdowns.

The major operation and maintenance issues of concern are regular flow of the water in the bed, maintaining the level of water, maintaining the vegetation cover, regular sampling, monitoring and analysis.

The major problem being faced by the operators/investigators are the clogging of the bed of CWs. The clogging occurs when the pores of the filter media get filled by the solids being bought by the wastewater and the death of the vegetation. Due to the clogging of the bed the waste capacity of the CW decreases. The domestic wastewater contains sizable amount of the suspended solids that need to be removed beforehand feeding to the CW. Usually the maintenance of the septic tank and the sedimentation tanks are compromised that results in feeding high suspended solids laden wastewater in the CWs. CWs shall not be considered as the primary treatment units. Properly washed filter media shall be filled in the tank of the CWs. The filter media needs to be inspected on the regular basis to ensure that clogging is not there. If the clogging has occurred and flow is being interrupted, then the operator shall see for the replacement of the filter media.

  • Hydraulic stress: Sometimes the CWs are designed for the higher capacities of treatment but starts operating at the low quantities of wastewater. In that case the water level remains below the root penetration causing death of vegetation. Such issues can be easily solved by raising the outlet weir or by adding more wastewater into the cells.

  • Flooding: sometimes due to heavy rainfall flooding occurs in the bed. The storm water through the drains and surface run off comes to the CW bed. The flooding causes loss of vegetation and clogging of the bed. Such scenarios may be avoided by provision of bypass channels before the inlet point at the same times dykes around the CW bed will restrict the entry of runoff to the bed from nearby areas.

  • De-weeding: the weeds (that are not a planned vegetation in the CW) shall be removed on the regular basis, especially at the initial period. The weeds compete with the vegetation in focus and efficiency of CW decreases.

  • Temperature: CWs are nature-based wastewater treatment system. Like natural wetland, these are also exposed to atmosphere, thus affected by temperature. Temperature variation also affects the treatment efficiency of the CW. The treatment efficiency of CWs differ in summer and winter season, affecting the water quality parameters like TSS, BOD, TP TN, etc. and both biological and physical activities of the wetland system (Kadlec 2006). The treatment performance of wetland is seasonally cyclic and the biotic reactions are reduced at temperature lower than the optimum range 20–35 °C (Kadlec and Reddy 2001).

3.7 Treatment Mechanism of CWs

The treatment mechanisms identified in CWs are classified physical and biological processes. The CWs perform better than the conventional treatment systems due to the process of biodegradation, photo degradation and plant uptake (Matamoros et al. 2010). The main pollution removal mechanism is presented in Table 3.3.

Physical processes

  • Sedimentation and filtration are two main processes included in the physical treatment of wastewater. The process sedimentation works on the principal of gravity in which the settling of suspended particle is done according to their shape, size and mass. It is the part of primary treatment of wastewater in which a sedimentation tank is added to the CW system. In this sedimentation tank large particulate matters settle down due to the gravitational settling. The process sedimentation is mainly used for the removal of suspended solid from the wastewater effluent and it also helps in reducing the clogging problem. Apart from the primary treatment sedimentation also occurs in the CWs, the sudden increase into the cross-sectional area at the entry point of wastewater causes reduction in the flow velocity causing sedimentation of the suspended particles.

  • Filtration is the physical process in which particulate matters are filtered mechanically when wastewater pass through substrate and root masses. In this process small sized particulate matters adsorbed on the substrate or trapped in root masses (Dotro et al. 2015).

Chemical processes

  • Adsorption , precipitation and chelation are the major chemical processes of the CWs. Phyto-volatilization and Phyto stabilization are other important chemical process used for the removal of heavy metals and other emerging pollutants.

  • Adsorption is the process of deposition and retention of dissolved substances on the surface of substrate media. It is main process responsible for the phosphorous removal from the wastewater (Lin et al. 2008). Heavy metals are also removed by this process. In the removal of heavy metal their adsorption occurs on organic matter present in substrate media and wastewater. Ammonium cation (NH4+) also gets adsorbed to the surface of filter media due to their charged property.

  • Precipitation is the process of wastewater treatment in which the formation of co-precipitant with insoluble compounds occurs and due to gravity, they are precipitated to the bottom of treatment system. It depends on the solubility of metals, wastewater pH, metal ion concentrations and significant anions.

  • Chelation is the chemical process of wastewater treatment in which a reaction takes place between a metal ion and an organic pollutant, which results in the formation of a ring structure that encompasses the metal ion and removes it. It helps in the removal of heavy metal present in the wastewater.

Biological processes

  • Ammonification, nitrification and denitrification are the three main biological processes involved in wastewater treatment in CW. Some other processes are also involved in it like photosynthesis, fermentation, microbial removal, etc.

  • In ammonification process organic nitrogen is converted into NH4+–N (Vymazal 2007). In ammonification process the deamination reaction of amino acid takes place which results to the formation of NH3. The rate of this reaction is high in the upper portion of CW due to the aerobic condition. But in the lower portion the rate of ammonification reaction is low due to the partial anaerobic environment (Reddy et al. 1984). The rate of ammonification reaction also depends on the pH and temperature. The optimum pH for the ammonification reaction is 6.5–6.8.

  • In the biological treatment process of wastewater, the nitrification occurs in two steps. These two steps involve first the conversion of NH4–N to NO2–N and the second is the conversion of NO2–N into NO3–N. The first step takes place in the upper portion of CW due to the requirement of aerobic environment. Some microbes are involved in the first step reaction like nitrosomonas, nitrococcus. The second step of reaction also takes place with the help of microbes like nitrobactor.

  • Denitrification is the process of total nitrogen removal form wastewater in constructed wetland (Koottatep and Polprasert 1997). In this process the conversion of nitrate into nitrogen gas occurs. It needs the anaerobic environment for the smooth generation of nitrogen gas. Therefore, in VFCW the poor denitrification process occurs due to the high concentration of oxygen. Various bacteria are involved in denitrification process such as Bacillus, Enterobacter, Micrococcus, Pseudomonas, etc. (Ottová et al. 1997).

  • Some other biological processes also help the treatment mechanism of CW like photosynthesis, fermentation, etc. Photosynthesis influence C and O2 addition to the wetland. Both C and O2 run the process of nitrification. Fermentation is the anaerobic decomposition of organic carbon and produce compounds like volatile fatty acid. These compounds are of high energy value which is used in microbial degradation process of wastewater.

Table 3.3 Pollutant removal mechanism under constructed wetlands
Full size table

3.8 Advantages and Disadvantages

The advantages of the CWs are counted over the conventional wastewater treatment systems. The CWs need fewer mechanical parts as the major role for treatment is played by the vegetation and media. The operational cost of CWs is also low in comparison to the conventional system. The CWs operate on solar energy (Not PVC), so requirement of conventional energy is low. High volume of sludge is produced in the conventional systems as a by-product, in case of CWs the sludge is not produced in the secondary treatment process. It also provides additional advantages as habitat for birds, wildlife and macroinvertebrates (Prashant and Billore 2020). The appearance of CWs also looks good. At the same time the land area required for the establishment of CWs are high. The treatment capacity changes from place to place as these are natural systems so, the variation in the climatic condition has an effect on it. In conventional treatment process the microbial activity is under control but that is lacking in the CWs.

Conclusion:

CW’s getting popularity as a substitute to traditional wastewater treatment systems. These provide a sustainable decentralized solution for isolated communities, small cities, countryside, individual houses, housing society, that are not connected to wastewater collection and treatment systems. It exhibits multiple benefits that actually need to be analysed in economic terms too, like habitat for birds and wildlife, good scenic beauty, recreational purposes, potential of reuse of treated water for irrigation in nearby areas. In general, CWs are easy to construct, simple to run and offer better treatment efficiency. The CWs are being seen as sustainable systems, providing sanitation, protecting environment and water resources. The CWs have been proved good for domestic wastewater treatment. There is an urgent need to scale up the application of CWs. Environmental entrepreneurs shall look forward to adopt the tested technology for field applications, as well as the certifications and third-party evaluation of the established CWs shall be carried out.