The regime of constructed wetlands in greywater treatment

An era of boundless innovations with technical advancement has been achieved by the human race every day. The introduction of machines has evoked new trades, production policies, more mechanized manual production, and reduction in production time. Both the developing and developed nations were pushed to face social and environmental disbalances after the industrialization. Redclift (2009) researched upon the insignificance of economic progression with non-economic factors. Although urbanization and rapid industrialization been the marks of an emerging country, the consequences like water demand and water scarcity are on the rise in a progressive gradient. By 2050 around the globe there will be a 55% upsurge in the water necessity according to the report of the United Nations World Water Development 2015. The well-being of humans, survival of ecology, and economic pursuits are wholly dependent upon the accessibility of water (Kounina et al. 2013). Of the total surface of earth about 70% constitutes water where only 3% of it is available in the form of fresh water making it a most valuable resource. According to a report from 1966, throughout global dimension a gigantic decrease of 55% is observed in the availability of freshwater. (UNU-INWEH 2017). Contrastingly, as a consequence of uncontrollable population increase, most conveniently designed units of wastewater treatment are flowing out of their usual abilities. These planned treatment divisions are capable of handling merely 20% of sewage stock while the leftover 80% are untreated and will be discharged into water bodies acceding to the reports of UNU-INWEH (2017). While on the other side of operation at the global scale into the atmosphere the sewage treatment plants (STPs) liberate 1,400 tons of CO₂ (Muga & Mihelcic 2008). For a better human health and sustainable practices, the long-distance transportation of water should be alleviated. The impediment in the usage of typical driven gravity sewer system around the globe is due to the economic and demographic trends. A shift in paradigm practice is essential for effective utilization of resources and a steady sustainable progress. A significance in reducing the water demand around the globe and the burden on conventional wastewater treatment units are achieved by an alternative source like greywater.

Greywater is said to be one among the other types of wastewaters that emerges from non-toilet wastewater in households. Categorized wastewater under low to a medium category of greywater supposed to have some ups and downs in its own chemical, physical, and biological factors (Friedler & Hadari 2006). A standard of around 100–150 L per capita per day (lpcd) of wastewater has been generated by each and every individual. Of the total produced domestic wastewater, greywater accounts up to 65–85% further can be improved up to 90% by fixing vacuum toilets (Hernández-Leal et al. 2011; Penn et al. 2012; Ghaitidak & Yadav 2013). After recycling greywater, a decrease of 30–10% was achieved in the portable demand for flushing (Karpiscak et al. 1990; Eriksson et al. 2002; Vuppaladadiyam et al. 2018). When compared to centralized treatment plants, a reduction in the energy load from 37.5% to 11.7% and the minimisation of 25.71% emissions of CO₂ is said to be achieved using greywater decentralized treatment units (Matos et al. 2014). The reason to make reprocessing of greywater an interesting and emotive issue is their ability to improve water anxiety and its contents on traditional STPs.

Treatment of greywater before its reuse is very necessary to improve its aesthetical appearance and quality as its stances a greater threat to both plant and human lifespan. Microalgae is suited to be one of the supportable choices for greywater treatment through the world. When compared with other technologies the algae reactors were more economic and easier to operate placing it in a commendable choice. The goal of the current work is to check the efficiency of various treatment that was carried out in greywater treatment.

Greywater in common is recycled water that originates from lavatory, laundry, and kitchen (Friedler & Hadari 2006; Gross et al. 2007; Eriksson et al. 2009; Saumya et al. 2015). Also, the water that flows without any interaction from lavatory leftover is also termed as greywater (Ghaitidak & Yadav 2013). It is also mentioned as sullage and light wastewater (Morel & Diener 2006). The characteristics of transformation of shade into grey even for a shorter storing period were entitled to its name ‘greywater’. Load greywater were additional classified into low pollutant greywater (LGW) and high pollutant greywater (HGW) depending on the pollutant (Boyjoo et al. 2013). In the domain of wastewater recycling application greywater is considered to be a valuable source in the likes of high volume and low pollutant load (Oteng-Peprah et al. 2018).

Naturally, greywater accords up to 60–85% of total household wastewater (Abed et al. 2020), and it can also reach a high percentage of 90–100% in houses practicing dry toilets system (Sheng et al. 2020). Developed nations of the USA, Asia, and Germany tend to generate the greywater up to 200 lpcd, 72–225 lpcd, and 35–150 lpcd (Morel & Diener 2006; Mandal et al. 2011). Espoused eco-village tradition in various parts of Holland, Germany, Sweden, and Norway impede Germany as one of the lowest greywater producers compared to Asia and the USA. Moreover, in some Africa regions, greywater production is noticed to be on a minimal note of only 14 lpcd due to water scarcity a low economy (Al-Hamaiedeh & Bino 2010; Shafiquzzaman et al. 2020). However, numerous desalination plants in Oman (an arid region) contribute up to 160 lpcd, almost 82% conversion of domestic wastewater into greywater is observed (Oh et al. 2018). Due to advanced technology and proper water supply, developed nations tend to produce more greywater (Figure 1) (Elhegazy & Eid 2020; Eriksson et al. 2002).

Quality and quantity of domestic water supply, distribution systems for both drinking water and greywater, lifestyle activities of the menage, and source of greywater are the prolific influencers of the greywater characteristics across the world (Ghaitidak & Yadav 2013; Oteng-Peprah et al. 2018). Areas with substantial quantity and quality of water supply tend to dilute the greywater resulting in a low level of pollutants. Biological and chemical degradation in the biofilm formed, leaching pollutants into the water, and also depends on the type of sewer system used to transport water media. Usage of chemicals in diurnal activities tends to vary with the lifestyle of the menage resulting in variation of characteristics of the greywater across the globe.

The general temperature of greywater is examined to vary from 18 to 38 °C. In general, TSS concentration in greywater varies from 23 to 1,140 mg/L. Turbidity values in the greywater usually vary from (20 to 619 NTU), laundry being the predominant source. pH and alkalinity of the household water supply affect the pH of the grey wastewater. Detergents in cleansing powders elevate the pH of greywater. In general, the pH of grey tends to vary from (6 to 8.2). Low values of pH in greywater are due to the occurrence of amino acids. COD and BOD in the greywater usually vary in the range of (25–2,878 mg/L and 14–1,240 mg/L), respectively. Excessive chemicals for cleansing purposes and food waste are the main reason for the high noticeable concentration of COD and BOD in greywater (Table 1).

Compared with domestic wastewater, greywater deficits in total nitrogen concentration, mainly due to absences of urine in the greywater, however proteins in food and meat make up to a nominal concentration of (1.7–36 mg/L) TN in greywater. Total phosphorus in the greywater is primarily originated from laundry sources due to the extravagant usage of phosphorous containing chemicals. However, in some nations, a shallow concentration of total phosphorus is noticed due to prohibiting phosphorus-containing chemicals (Pinto & Maheshwari 2015). In general, greywater manifests TP variation of 0.11–48 mg/L. Direct contact of greywater after using toilets and washing babies’ nappies is the prolific reason for the elevated levels of total coliform and faecal coliform in greywater. In general, total TC, FC values vary in the range of (56 − 2.67E + 07) and (0.1 − 1.23E + 05/100) mL, respectively.

Wetlands are naturally occurring distinct eco-system that is temporally or permanently inundated with water (Keddy 2010). These natural bodies serve various purposes, starting from water treatment to home for the diverse nature of animals and birds by maintaining ecological balance (Dorney et al. 2018). Contrastingly, constructed wetlands are engineered to mimic the natural phenomena, prolifically treating wastewater in a controlled demeanour. Griffiths & Mitsch (2017) expounded constructed wetlands as a designed, human-made complex of submerged and emergent vegetation along with the saturated substrate, animal life, and water that vivifies wetlands for social benefits and uses. Constructed wetlands are traditionally developed treatment systems with a blend of physical, biological, and chemical mechanisms for sustainable treating of wastewater (Kadlec & Wallace 2008; Zhang et al. 2015). Constructed wetland plays a prolific role in water treatment, mainly in areas of tropical and subtropical climates of developing economies (Ali 2018). Compared with conventional centralized sewage treatment plants, wetlands generate a minimal carbon footprint, very economical, and easy to operate (Wang et al. 2020). Despite the eco-friendly nature of the CWs, they are widely substituted for secondary and tertiary treatment units in the conventional municipal treatment units and even for greywater treatment. Organic matter, nutrients, and heavy metals are conventionally detached in the wetlands. Even though CWs provide indirect advantages of wildlife habitats, green space, and water purification at nominal operation and maintenance costs, they lack the proper understanding of design aspects.

The conventionally espoused treatment systems for treating either wastewater or greywater are activated sludge process (ASP), sequential batch reactor (SBR), membrane bioreactor (MBR), an up-flow anaerobic sludge blanket (UASB). The significant shortcomings of all this conventional system are the excessive generation of sludge, the involvement of enormous mechanical and electrical components, which eventually ameliorates the high operation and maintains cost and odor and nuisance problem. Even though sludge volume accounts for up to only 1–3% of the total volume of wastewater, the cost involved in the handling and managing sludge may exceed 50% of the total cost. Contrarily, constructed wetlands are cost-efficient treatment technologies with almost similar or less economic inputs. On a global scale, the operation cost incurred during the conventional electrical or mechanical treatment units is 90% more expensive than that of CWs. Unlike conventional treatment units, there is no usage of chemicals, eventually resulting in low sludge volumes. The only sludge generated in CWs is the plant biomass, which can be cleaned more easily and can also be used for biomass production and eventually for biofuel. Moreover, CWs pumps consume an appreciable quantity of energy, which can be alleviated by adjusting the bed slope.

CWs are predominantly classified as four types in the view of water treatment, as shown in Figure 2. The classification (Figure 2) is majorly based on flow directions.

In general, FWS CWs replicates the natural wetlands to a more considerable extent with considerably larger land requirements. FWS CWs are shallow basin engineered wetlands with 10–50 cm of the water surface above the 30–40 cm of substrate media. Usually, soil, the bottom of the soil, is substantially coated with clay material or geomembrane/geotextile to avoid water leaking. Cattails (Typha spp.), common reeds (Phragmites australis), bulrush (Scirpus spp.), and herbs (Juncus spp.) are commonly employed plants in FWS CWs for treating the greywater and wastewater (Vymazal 2013; Stefanakis et al. 2014). Due to complete water contact with different parts of the plants, a satisfactory removal of pollutants is expected through physical, chemical, and biological methods. The magnificent elimination efficacy of BOD and TSS and appreciable elimination of pathogens and TN is observed for FWS CWS but limits its efficiency in TP removal (Kadlec & Wallace 2008; Vymazal 2013). Mostly FWS CWs are employed in the polishing of treated effluent, agriculture effluents, highway storms, and occasionally for treating hydrocarbons in the water (Kotti et al. 2010). Due to practical and uniform utilization of substrate along with breadth, considerable high removal efficiency is observed in comparison with other wetlands.

Initially, HFCs are mostly practiced in high-income countries like the USA and Germany due to high end of operation cost involved for the substrate. Gravel and rock of different composite and mixture are widely used as a substrate for this type of wetland along with plant species like Typha (e.g., latifolia, angustifolia), common reeds (Phragmites australis), and Scirpus (e.g., lacustris, californicus). The soil's bottom is substantially coated with clay material or geomembrane/geotextile to avoid leaking of water, and a mild slope of 1–3% is maintained. In HFCWs substrate layer varies from 30 to 100 cm in depth with water level maintained well beneath the 5–10 cm from the substrate's free surface, henceforth avoiding free contact of water to the atmosphere. Horizontal flowing water in HFCWs, through the substrate's pores, undergoes physical, chemical, and biological deterioration. HFCWs are very efficient in the removal of organic matter but limits in nutrient removal. Principally, HFCWs have widely opted for industrial effluents, landfill leachate, and contaminated groundwater.

Vertical flow constructed wetlands (VFCWs) are commonly more expensive in comparison with other types of wetlands. Water flows vertically through a substrate comprised of a different layer of sand, gravel, and stones organized in an increasing gradient trend and gets at the bottom through a perforated tube. Due to this perforation, better oxygen transfers across the substrate of VFWCs and results in better effluent than other CWs. VFCWs are usually composed of filter media depth varying from 30 to 180 cm with plant cattails (Typha latifolia) and common reeds (Phragmites australis) as the typical vegetation. The soil's bottom is substantially coated with clay material or geomembrane/geotextile to avoid leaking of water, and a mild slope of 1–2% is maintained. To avoid the clogging of perforated tubes, intermittent loading operation is performed, and surface areas of VFCWs are small compared to FWSCWs and HFCWs. Due to adequate oxygen transfer across the substrate, fabulous sauce removal trends are noticed in organic matter and ammoniacal nitrogen. VFCWs have widely opted for municipal, domestic and industrial wastewater. Inevitably water level sub surface flow (SSF) is maintained well below the media, which eventually results in evasion of mosquitoes, public contact, and odor for SSF. In general, SSF is very useful and requires fewer footprints in comparison with FWS. However, the substrate's cost in SSF is the major shortcoming for its applicability, primarily in the developing nations (Kadlec & Wallace 2008).

FTWs integrate CWs with ponds, a material with a relative density less than one typically used for growing the plants on the water surface, with plant roots expanding into the water. Therefore, the system visually resembling like a floating island and is unaffected by variation of water levels. Unlike the conventional CWs, roots in FTWs are free and have a greater contact area (Walker 2017). Agrostis alba, Iris ensata, Carex strictavirgatum, and Panicum are few opted plants for FTWs (Spangler et al. 2019). The developed bio-film on hanging roots enhances bacterial growth, thereby enhancing the nitrogen sequestration and adsorption of total phosphorus (Lucke et al. 2019) and, by retarding the flow FTWs, promote sedimentation and filtration (Shahid et al. 2018). Due to their floating characteristics, FTWs are widely used in eutrophicated water bodies.

Huge land acquisition for almost all the constructed wetlands restricts their use to mostly rural and urban fringes. To overcome the above shortcomings in the conventional paradigm practice and increase the adaptability of cost-efficient and eco-friendly treatment processes in highly dense areas, green roof and green walls have been developed (Manso et al. 2021). These systems (green walls and green roof) are directly grown on the walls and roof of the buildings directly and they also serve for decorative purpose (Xu et al. 2020). Due to their direct growth on walls and roof of the buildings they acquire less space in comparison to other conventionally practiced wetlands (Pradhan et al. 2019). In the study of Fowdar et al. (2017); treated greywater using green walls with different ornamental plants reported a removal of >90% of BOD >80% of TSS, >99% of phosphorus and >70% of nitrogen over an operating period of one year. In 2020, Xu et al. (2020), was successful in treating greywater with green walls, in the study over an operation period of 8 days, green wall system is effective in removing BOD₅ COD, turbidity and anionic surfactants, and turbidity were 97%, 81%, 75%, and 88%, respectively.

Apart from treating greywater, this system reduces noise pollution, improves air quality, promotes storm water quality, and improves the energy-efficient of the buildings (Berndtsson 2010; Pugh et al. 2012). Green walls are usually long vertical growing systems on the interior or exterior walls of the system. The green walls’ roots were generally grown over the entire façade (Manso & Castro-Gomes 2015), whereas green roofs are horizontally grown on the top of the roof, and are normally modular systems incorporated with a waterproof membrane, insulation layer, and a vegetation layer rooted in a growing substrate. Depending on the type of vegetation and weight holding capacity of the holding roof, the height of the substrate in the green roof varies from 50 mm to 1 m (Francis & Jensen 2017). Canna lilies, Carex appressa, Lonicera japonica, and Ornamental grapevine are commonly used plants in green roofs and green walls. The operational phenomena of this novel system are similar to that of conventional CWs but have the additional advantage of a small footprint.

Wetland was established in Germany in the 1950s by Kaethe Seidel, who planned the HFCWs, using granular constituents as a grubbing medium. In the 1960s, Reinhold Kickuth investigated granular soil with more clay matter and named the arrangement the ‘Root Zone Method.’ In the early 1980s, the HFCWs technologies was introduced to Denmark, and by 1987 approximately 100 soil based units were set in process. In the course of the late 1980s, the HFCWs were likewise familiarized to further nations, such as Austria and the UK. In the 1990s, this scheme extended into furthermost Africa, Australia, Asia, European nations, and North America.

One of the significant shortcomings affecting the efficiency of CWs is inadequate knowledge in designing aspects mainly for prodigious CWs (Schulz et al. 2019). Construction and design of CWs are usually overlooked and considered simple, resulting in improper and non-sustainable design. As a result of improper design, improper oxygen transfer tends to affect nitrification and denitrification by forming anaerobic bio-film. To date, there are no unanimous sets of guidelines for the design of wetlands; they tend to change from place to place, and depend upon the type of end-use. However, there two basic models developed.

CWs are generally designed based on the removal efficiency of limited parameters like BOD, nutrients removal, TSS, or disinfection performance. The final dimension of the treatment unit is evaluated based on the limiting parameters. Theoretically, the first-order plug model is applied, CWs as attached growth system in steady-state condition with an expected outcome in an exponential manner. Contrastingly, few rules of thumb are developed for designing the small CWs (Vergeles et al. 2015). ‘Reed’ model and ‘Kadlec and Knight’ model are the most widely adopted models in designing the CWs; both models are predominantly distinguished based on constant rate selection in the plug flow model. In the Reed model, the rate constant is volume and temperature dependant. In the later model rate, the constant is independent of temperature and area-based.

The novelty of CWs in treating greywater includes integrating two primary treatment mechanisms of physical and biological degradation of the pollutants. The active substrate in CWs entraps pollutants, and various biotic and abiotic organisms in CWs perform biological degradation (Garcia et al. 2010; Jasper et al. 2013). From the past few decades of the research, there exists a predilection gap on selecting appropriate greywater treatment from various physical, chemical, and biological processes (Kadlec & Wallace 2008; De Gisi et al. 2016). Contrastingly, there is a possibility of occurrence of all the three processes in an eco-friendly manner in CWs. In the present section, an attempt was made to understand the adaptability of CWs for greywater treatment. Systems like VFCWs, HFCWs, hybrid systems, and even novel systems like GROW are also examined in treating greywater.

Freezing the water's top surface during the winter is a significant issue while treating the cold countries’ wastewater. In such conditions, CWs have opted as secondary treatment. Wang et al. (2017) examined the adaptability of HFCWs in a cold region like Sweden. In the study (Wang et al. 2017), greywater is treated with HFCWs is preceded by aerobic filters, and good removal efficiency is noticed even during the winter due to long retention time of 6–7 days for HFCWs. The appreciable removal efficiency of 78%, 81.9%, and 50% are observed for TP, BOD7, and TN respectively for CW, which is very high compared to the respective efficiency of the preceding bio-filter. Recycled water in the study (Wang et al. 2017) meets the European swimming pool reuse standards.

In the studies of El Hamouri et al. (2008), a comparison was performed between low-cost HFCWs supported with multilayer vertical filter system and extravagant technologies in conventional wastewater treatment units like SBR and MBR with capacities of 600 L/day and 550 L/day, respectively, for treating bathroom greywater of a sports complex. The study (El Hamouri et al. 2008) elucidated low-cost HFCWs system planted with Phragmites can reduce the turbidity, BOD, COD, TN, and TP by 93%, 85%, 75%, and 50% respectively, even an appreciable removal efficiency of 90% is noticed for surfactants. The performance of HFCWs along with the filter system is on par SBR and MBR in treating the bathroom greywater for the reuse of toilet flushing for 15 continuous months.

Abdel-Shafy et al. (2009) elucidated the efficiency of HFCWs as a secondary treatment unit preceded by UASB in treating the greywater from urban households of Egypt. In comparison with UASB, HFCWs exhibited superior removal efficiency for the integrated system. Noticed removal efficiency for CWs with Phragmites australis are 83.5%, 86.4%, 89%, 90%, 95.2%, 69.3%, 56.2% and 99.999% for COD, BOD, TSS, turbidity, sulfides, TKN, TP, and faecal coliform, respectively. Abdel-Shafy et al. (2009) also investigated a different fraction of COD, and HFCW effectively decreased suspended COD by 17% is on par with UASB.

Laaffat et al. (2015) examined the potential of HFCWs in treating greywater emerging from primary schools in morocco. In the study (Laaffat et al. 2015), HFCWs with Typha latifolia is preceded by a coarse screen unit for preliminary treatment, and monitoring of the treatment is performed for 100 days. Results of the study manifested a removal efficiency of 92% BOD 5, 85% of COD, 45% of TN, 41% of TP, and 99% of E. coli from a respective influent value of 44.2 mg/L of BOD5, 77.2 mg/L of COD, 7.1 mg/L of TN, 0.8 mg/L of TP and 5 × 10³ of E. coli. Reclaimed greywater is successfully recycled for toilet flushing.

Arden & Ma (2018) reviewed many literature on constructed wetlands for treating greywater with an outcome of average removal efficiency through various analyses. According to the literatures average BOD, TSS, turbidity, TN and TP removal efficiency are 87%, 64%, 47%, 44% and 24% respectively, for the average influent values of 196 mg/L, 52 mg/L, 89 NTU, 7.2 mg/L and 2.7 mg/L. The average removal efficiency of HFCWs is equivalence to VFCWs but less than the hybrid system. Mars et al. (2003) conducted various experiments with different HRT of 20 days and 10 days. Various studies were conducted to understand the effect of ponding, HRT, and vegetation in nutrient uptake (Mars et al. 2003). Final outcome was compared with SSF and surface flow for a HRT 10 days and all the plant leaves, roots and tubers were digested at end of four months to analyses the phosphorus and nitrogen uptaken by the plants condition after three months of the study. Studies depicted tanks planted with Triglochinhuegelii shown better uptake of nutrients, and the subsurface system with vegetation shown appreciable uptake of nutrients in comparison with the surface system with vegetation at the same HRT.

Li et al. (2004) developed an integrated system by combining VF CWs and photocatalytic oxidation mainly for semi-tropical and tropical climate zones. In the study (Li et al. 2004), preliminary treatments grit chambers with solid grease traps are equipped before VF CWs. VF CWs are able to decrease TOC to 28 mg/L, TN to 5 mg/L, TP to 6.80 mg/L and E. coli to 26,200/100 mL from the respective influent values of 93.8 mg/L, 16.6 mg/L, 9.6 mg/L and 2.6 × 105. On integrating the system with TiO² based on photocatalytic oxidation for 6 h with 10 g/L of TiO². E. coli and TOC concentrations were reduced up to 1/100 mL and 6 mg/L, respectively. Reclaimed water meets European bathing quality standards. However, further investigation should be carried out for optimal utilization of TiO².

Gross et al. (2007) developed a novel recycling VFCWs mainly in the dimension of low tech, economically sound, and easy maintains. The study's system is a combination of VFCWs and trickling filters mainly to treat greywater for households for irrigation purposes. The substrate in VFCWs is composed of 15 cm of planted organic soil as a top layer, followed by 30 cm of plastic or tuff and 5 cm of pebbles as a bottom layer. The long term and short term analysis of the study elucidate the system removal efficiencies by reducing TSS, BOD, COD, TN, TP, surfactants, boron, and fecal coliform from respective influent values of 158 ± 30 mg/L, 466 ± 66 mg/L, 839 ± 47 mg/L, 22.8 ± 1.8 mg/L, 34.3 ± 2.6 mg/L, 4.7–15.6 mg/L, 1.6 ± 0.1 mg/L, 5 × 107 ± 2 × 107 /100 mL to corresponding effluent values of 3 ± 1 mg/L, 0.7 ± 0.3 mg/L, 157 ± 62 mg/L, 6.6 ± 1.1 mg/L, 10.8 ± 3.4 mg/L, 0.4–1.3 mg/L, 0.4–0.8 mg/L and 2 × 105 ± 1 × 105 /100 mL to the corresponding effluent values. The study's cost analysis showed a return over investment (ROI) to be within three years approximately.

In the study of Kadewa et al. (2010), a comparative analysis was performed between a novel unplanted cascading filter with planted VFCWs. VFCWs planted Phragmites australis, a mixture of sand, soil, and organic matter as media depicted removal efficiencies of 81.9% of turbidity, 96.12% of BOD, and 93.9% of COD from the respective influent values of 17.7 mg/L, 43.9 mg/L and 151 mg/L, which were on par with cascading filter system. However, better removal efficiency of 75.5% from the respective influent surfactant concentration of 1.39 mg/L is observed for planted VFCWs in comparison with cascading filter removal efficiency of 37.4%.

Ramprasad & Philip (2016) conducted experiments to understand the potential of VF CWs in treating greywater emerging from student hostel enriched with surfactants and personal care products compared to HF CWs. In the study (Ramprasad & Philip 2016), both the wetlands were planted with Phragmites australis monitored for one year with propylene glycol (PG), sodium dodecyl sulfate (SDS), and trimethylamine (TMA) as selected primary targeted elements. The study results manifested that VFCW's efficiency in treating PG, SDS, and TMA is 95%, 89% and 98%, respectively is marginally high compared to the respective removal efficiency of HFCWs. The removal efficiency of COD, BOD, TSS, TP and TN are 95%, 91.25%, 88.57%, 98.4% and 98.7% respectively from the corresponding influent 160 mg/L, 64 mg/L, 140 mg/L, 2.47 mg/L, and 18.1 mg/L. The reclaimed water from both setups meets the reuse guidelines of USEPA.

A hybrid system combined with HFCW, followed by VFCW, is examined by Paulo et al. (2013) for treating mixed greywater emerging from Brazil's households. In the study (Paulo et al. 2013), HF SSF CW is designed with a mixture of fine and coarse gravel as media which remove 20% of TN and 80% BOD and immediately effluent from HFCW is fed intermittently to VFCW with fine mixture sand, fine gravel, and coarse gravel as media to remove the remaining BOD and TN. The substantial removal efficiency of 95%, 88%, 95%, 58%, 82% and 98% for respective parameters turbidity, COD, BOD, TP, TN and total coliform for the corresponding influent of 254 NTU, 646 mg/L, 435 mg/L, 5.6 mg/L, 8.8 mg/L and 5.4 × 108/100 mL were noticed.

Jokerst et al. (2011) examined the potential hybrid system of wetlands in treating greywater over a period of a year. In the study (Jokerst et al. 2011), FWS CW is planted with Typha latifolia which was followed by SSF HFCW vegetated by Scirpusacutus; FWS CW is mainly used as a preliminary treatment to trap coarse material. The observed removal efficiencies for the treatment unit during the summer season are 93.04%, 92.8%, 94.07%, 80% and 98.7% of BOD, surfactants, TN, TP and E. coli respectively for the corresponding influent 86.3 mg/L, 2.63 mg/L, 13.5 mg/L, 4 mg/L and 543 /100 mL. Hybrid system performance in summer is on balance with fall and spring. However, it is sub standardized during winter with a recorded removal efficiency of 47.2, 74.75, 47.47, 47.5 and 24.8% for respective parameters of BOD, surfactants, TN, TP, and E. coli.

To understand the precipitation effect on removal efficiency of the BOD in a hybrid system, Paulo et al. (2013) experimented a hybrid constructed wetland for treating greywater. The study's treatment unit (Paulo et al. 2013) consists of a grease trap, sedimentation tank followed by HF SSF CW with VFCW. Global removal efficiencies of 89%, 92%, 98%, 98% and 99% of COD removal were noticed for the corresponding precipitation values 2.62 ± 8.44 mm/day, 0.07 ± 0.34 mm/day, 1.60 ± 5.33 mm/day, 4.21 ± 9.49 mm/day and 4.07 ± 9.13 mm/day at a respective HLR vales of 0.2,0.3,0.2,0.3 and 0.3 (m³ m⁻² d⁻¹).

To improve the performance of CWs during the variable loading regime, Comino et al. (2013) proposed a novel recycling hybrid CWs to treat the greywater. The study's treatment unit (Comino et al. 2013) is equipped with two units of VFCWs operating in parallel mechanisms, followed by HF SSF CW. The performance of the system was evaluated at different loading rates and different vegetative conditions. During the operation at optimal flow and standard pollutant load with no vegetation, the treatment unit showed an overall removal efficiency of 75.8% for COD, whereas when operated with double, triple, and quadruple times the standard pollutant load and at the standard design flow rate, treatment unit manifested a removal efficiency of 93.1%, 95%, and 95% of COD, respectively. However, by increasing the flow rates to 10 times the design flow rate, the expected removal efficiencies were not appreciable. On the other hand, almost 100% removal COD is obtained for the vegetated system at all the pollutant loads.

Avery et al. (2007) develop a novel green roof water recycling system (GROW) for treating low strength greywater. The study novel GROW system planted with multiple plants is compared with HFSSF CW, and VFCWs vegetated with Phragmites australis. The removal efficiency of organic matter in GROW is on par with HFSSF and VFCWs, nevertheless, GROW treatment exhibited better performance in the removal of turbidity and suspended solids in comparison to the conventional CWs.

Fowdar et al. (2017), for the first time, examined various operating parameters and design parameters like loading rates, different types of vegetation, inflow concentration, filling material, and modified saturated zone effecting the performance of green walls in treating greywater. The study results manifested prolific removal efficiency mainly for plant species like Canna lilies and Carex appressa in removing organic matter and suspended solids, but limits in nutrients removal remarkably few species of the plant shown negative trends in nutrient removal. To analyze the above short comings, Fowdar et al. (2017) conducted experiments by introducing a carbon source (15 Urea) to promote denitrification in the media. The modified system depicted good removal efficiency for organic matter but falls shorts again in terms of nutrient removal.

To analyze the effect of different growth media on the performance of the green walls, Prodanovic et al. (2017) conducted various experiments. In the study (Prodanovic et al. 2017), river sand, vermiculite, coco coir, rock wool, fyto-foam, grow stone, expanded clay, and perlite are respective substrates used without vegetation to understand media mechanism in pollutant removal. Results of the study showed that delineated coco coir and perlite are incredibly useful in reducing organic and nutrient content. In the extension of research (Prodanovic et al. 2017) performed a study to understand the effect of the mixture of perlite and coir on the removal of the pollutants. The mixture with relatively high percentages of coir resulted in the slow drainage, thereby enhancing the contaminates’ biological removal. On the other hand, composite with high perlite content retains the water for a short duration, resulting in a dominant physical removal mechanism.

To investigate the GROW system's adaptability in different climatic conditions, flow rates, and hydraulic loading rates, Ramprasad et al. (2017) conducted studies over 17 months in treating greywater generated from a college hostel. In the study (Ramprasad et al. 2017), GROW system was operated mainly in four stages (1. Startup stage, 2. Seasonal variation stage, 3. Flow rate variation, 4. Organic load variation). In all the phases of operation, the average BOD and COD concentration in the effluent is continuously less than 10 mg/L and 20 mg/L, respectively, appreciable high removal efficiencies are observed of organic matter during summer. TSS removal efficiency of 85–90% is noticed during all the operation phases. A maximum average removal percentage of 99% and 92% were noticed for nitrogen and phosphorous compounds during the summer seasonal regime (Table 2).

Figure 3 shows the resulting relationships of water quality indicators such as BOD, COD, E. coli, ammonia, TSS, and turbidity as an HRT function (Arden & Ma 2018). The plot between the following variables indicates little or no significant inter-relationship. Therefore, the variation can also be based on the influence of various other factors, such as the GW's temperature and strength. Hence the correlation of many factors is detrimental to the treatment performance.

As indicated in Figure 3(a), BOD achieved the highest removal of 95% in VFSW at an HRT of 14 h. The average removal rates for BOD were obtained as 87.5, 89.8, 65.53, and 42.6% for HSSF, VFSW, GROW, and FTW respectively. The VFSW system continues to achieve the maximum probably due to continuous circulation leading to oxygenation. Constructed wetlands being a biological process, it was observed that COD removal rates were much lower compared to BOD. COD removal% using FTW was consistently low due to the floating roots’ insufficient filtration of particulate organics. Various mechanisms, such as filtration, adsorption, and oxidation, aids in removing suspended solids from the system (United States Environmental Protection Agency (USEPA) 2000). The trend of decreasing TSS as a function of increasing HRT was observed in VFSW and FTW. As in Figure 3(f), none of the systems achieved a removal rate as per USEPA guidelines for unrestricted reuse in the case of turbidity removal. The removal percentage was high in the case of VFSW compared to other systems. A much higher removal of 97% was observed in RVF (Paulo et al. 2013). The ammonia removal percentage varied based on the amount of phosphorous linked to the plant biomass. As oxygenation was higher in the case of VFSW, biomass formation increased, which led to a maximum ammonia removal rate of 89%. Based on the trend observed in Figure 3(c), E. coli removal fluctuations cannot be taken as a predictable indicator for producing an effluent for reuse.

It is important to conduct comprehensive studies of greywater treatment, which can assess and analyze the impact on future development.

• Pretend and evaluate by various scenarios for treating greywater in an open environment.

• CWs operation is often complicated and challenging, therefore, skilled labour is needed to avoid short circuiting, hence, advance technology should be designed with an auto intelligence system.

• Advanced GW treatment studies are required to optimize results based on influent water quality, turbidity, water stress, loading, and chemical requirements to set regulation standards to control the quality of treated for various usage purposes.

• Most of the studies regarding CWs are done across the globe with different climatic conditions, hence issues such as evapotranspiration must be considered while optimizing the data.

• Moreover, studies related to biomass growth regarding various wetlands are minimal; hence, such research can improve working conditions within wetlands for improved quality of treated water.

Greywater makes it an attractive option in water recycling projects because of its low pollution and high volume. The average developed country produces a large amount of gravel compared to developing countries. It is noteworthy that recycling greywater can easily reduce water pressure by 30–40%, however, reusing greywater without proper purification has been shown to have negative effects on both human and animal/plant life. Built-in wetlands offer a low-cost and easy-to-operate solution to purify gravel. Typically designed wetlands operate primarily in the form of secondary or tertiary and organic matter-reducing cable and preferably suspended solids. Hybrid systems show excellent removal capacity for nutrient enrichment. To alleviate the space crisis of the swamps, new purification systems such as green roofs and green walls were developed. CWs should be optimally integrated with the appropriate disinfectant option to further reduce TC and FC in the crucible to desirable limits.

Authors are thankful to CSIR India and ICWR (International Consortium for Water Researchers) for giving support to this work through CSIR-Summer Research Training Programme 2020, by CSIR-NIEST, Jorhate.

All relevant data are included in the paper or its Supplementary Information.