Pollutant Removal from Sewage in Tropical Climate by Constructed Wetland System: An Asset for Irrigation

. In the global outlook, letting of untreated sewage in existing river bodies deteriorates the water quality. The seepage likely depreciates the quality of ground water too. The quality of groundwater with special reference to India has tremendously gone down in the past twenty years leading to sour taste. On the other hand, agriculture sector is deprived of water in many places of India. A solution can be arrived concurrently by treating sewage and consuming the effluent in agricultural sector. First order kinetics was applied in constructed wetland system at different flow rates and optimised. At optimised HLR, effluent met the standards of discharge that can be utilized for agricultural/ irrigational purpose. The emanating major pollutants can be effectively treated using constructed wetland system under tropical climate. A few clippings at the onsite treatment illustrated the diversity of species thus adjoining sustainable biodiversity and treatment. Thus in tropical countries like India, constructed wetland system might pave solution not only for the treatment of sewage but in deploying the effluent in agricultural sector. A clean ecosystem can be achieved with sustainability.


Introduction
Wastewater discharge from diverse sector releases a wide range of contaminants seeking profound attention of the environmentalists worldwide. Organic pollution of the rivers have a serious negative impact on human health and ecosystem. In the Indian scenario, about 62,000 MLD of domestic wastewater (sewage) is generated while the treatment capacity is only 37% of it. Remaining 63% of untreated sewage is let into rivers across the country including the Ganga River basin which supports almost 45% of living population (Times of India 2015). Agriculture being the backbone of the country strives due to water depletion. Henceforth, providing an effective solution correlating with environmental concern is focussed.
The sources that contribute to contamination of water bodies are bounteous and majorly anthropogenic. The River Yamuna in Delhi, India is highly polluted by domestic wastewater with elevated levels of ammonium concentration making it unfit for human consumption (Groeschke et al. 2017). Water quality of the upstream and downstream of the River Mandzoro was studied and reported by Baloyi et al. (2014) that there was deterioration in the water quality at the downstream due to the discharge of poor quality effluent from the sewage treatment plant. Indirect source of contamination may be due to urbanization, seepage of storm water, agricultural run-off and precipitation of atmospheric contaminants released owing to industrial evolution (Naderizadeh et al. 2016).

List of pollutants in domestic wastewater
The major pollutants in domestic wastewater that deteriorates the water quality when released in water bodies are cited.

Organic pollutants
The organic pollutants emanate majorly from domestic sewage, storm water, industrial effluent and agricultural run-off. Decomposition of organic pollutants results in the depletion of oxygen thus deeming it unfit for the survival of biotic life (Sharma & Gupta 2014). The organic pollutants from wastewater discharges had seriously affected the macro invertebrates in aquatic system. Globally the number of people affected by organic pollution of (Biological Oxygen Demand) BOD > 5 mg/L due to contamination of rivers was projected to be 2.5 billion in 2015 (Wen et al. 2017).

Nutrients
The major nutrients such as phosphorus and nitrogen are released in disagreeable amounts in domestic wastewater. The bounteous supply of nutrient leads to eutrophication. Excess nitrogen results in toxic algal blooms that gains entry via food chain and poses threat to aquatic life and humans leading to economic loss (Naden et al. 2016).

Heavy metals
Domestic wastewater contains heavy metals zinc, iron, cadmium, copper, aluminium, lead and manganese. A study conducted in Japan revealed that an average of 0.2-0.3 Cd, 1.6-1.9 Ni, 3.5-6.8 Pb, 0.  Zn and 111-293 Fe mg/ day/ person is released in domestic wastewater. Intrusion of sewage contamination has led to heavy metal concentration of lead, chromium, cadmium and nickel in underground water, surface water, soil and crop plants (Chino et al. 1991). The sources of heavy metal contamination in sewage are rainfall and soil erosion. These heavy metal containing aerosols usually accumulate on leaf surfaces in the form of fine particulates and can enter the leaves via stomata. Some of the human sources of heavy metals in wastewater effluents are metal finishing and electroplating, mining and extraction operations, textiles activities and nuclear power. Metal finishing and electroplating involve the deposition of thin protective layers into prepared surfaces of metal using electrochemical processes (Oghenerobor et al 2014)

Microbial contamination
Most of the rivers are polluted with fecal indicators such as total coliform, fecal coliform, Escherichia coli and fecal Streptococci due to contamination of excreta by humans and warm blooded animals. A study conducted in the rural sectors of Odisha, India by Schriewer et al. (2015) revealed human fecal markers in community water resources such as ponds (8%), tube wells (2%) and stored water (20%).
Bacterial analysis of River Ganges revealed that it is highly contaminated with coliforms, Enterococcus faecalis, Actinomyces sp., Aerobacter aerogenes Staphylococcus aureus, Shigella sp., Bacillus sp., Salmonella sp. and Clostridium perfringens. Hence consumption of Ganga water may lead to serious health risks (Bilgrami & Kumar 1998). Coliform infection, especially E. coli causes bloody diarrhea, nausea, vomiting, dehydration, fever and loss of appetite.

Raw sewage-Source and analysis
Raw sewage was collected from the sewage treatment plant (STP) in Anna University, CEG campus, Chennai. Samples were collected on weekly basis for ten weeks and subjected to physico-chemical and biological analysis. All the analysis were carried out according to the standard procedures (APHA 2017).

Plants chosen for the study
Based on the literature study, the sedge Cyperus alternifolius was chosen for the study with the idea of converting the harvested biomass into some useful product. Cyperus alternifolius commonly known as umbrella sedge belongs to the family Cyperaceae. It is a perennial plant capable of growing to a height of 4-6 feet with higher percentage of fine fibrous root biomass. Healthy plantlets were purchased from Sri Venkateshwara farm, Injambakkam, Chennai. The culms can be used in making mats, hats and thatching. The species is perennial and locally available.

Experimental setup for vertical flow constructed wetland system
The site for the experimental set up was chosen near the sample source, explicitly in the sewage treatment plant, Anna University, CEG campus, Chennai. The constructed wetland was built with 5mm acrylic sheets with the dimension of 1.2x0.6x 0.7 m provided with a slope of 0.5 and an outlet for effluent collection.
The reed bed consisted of three layers: pebbles (10 cm), blue metal chips (5 cm), coarse sand (15 cm) and fine sand (30 cm) from bottom to top. Blue metal chips was sandwiched between the sand and pebbles to avoid the percolation of sand into the gaps created in the pebble layer. The porosity of the wetland was calculated from the formulae Void volume Porosity % = X 100 (Cresswell and Hamilton 2002) Total volume The wetland tank was covered with black cloth to prevent growth and interference of algae in the efficiency of the system. The experimental set up is shown in Figure 2.1.
Twelve plantlets were planted in the reed bed of each unit at equal intervals and saturated with diluted sewage for a period of 4 weeks for acclimatization and establishment of roots. The saturation aids in establishment of compact bed and association of microbial growth in reed bed and rhizome (Sehar et al. 2013).

Figure 2.1 Experimental set up
Sewage was passed through grit chambers to segregate the debris and pumped into storage tank with a capacity of 210 liters. From the storage tank the wastewater was supplied into constructed wetland units. The influent was distributed evenly at the top through perforated pipe lines. Valves were set in the PVC pipes for regulation of water flow. Preliminary studies were conducted to monitor the plant growth of both species and compared with respective control.

Optimisation of HLR
The effect of HLR on removal efficiency was examined over a period of 16 weeks at four different hydraulic loading rates: 28  Volume of wetland x porosity HRT = Flow rate Influent and effluent were collected at sampling interval of 7 days. All samples were analysed for BOD, COD, TKN, TSS, TDS, phosphate, and heavy metals. However, microbiological analysis for total coliforms, faecal coliforms and E. coli were performed on fortnight basis for the same period of study. All the analysis were carried out in triplicates and averaged. The optimal performance of the wetland was evaluated based on mass removal rate, removal efficiency of the pollutants, areal removal rate constant and volumetric removal rate constant.

Calculations
The formulas used for the calculation are presented (Lee et al 2015).
Where C represents concentration in mg/L.
Mass removal rate was calculated using the formula r = q (C (influent) -C (effluent)) Where q is the hydraulic loading rate in m d -1 . Mass removal rate is expressed in g m -2 d -1 . q = Q/ A. where Q is the flow rate through the wetland and A is the area of the wetland. The geo coordinate of sample collection was Latitude: 13.0127 Longitude: 80.2364.

First order kinetics
First order degradation approach was used to predict the removal efficiency of the system. Where Kv is the volumetric removal rate constant in d -1 and t is the hydraulic retention time in the wetland.

Characteristics of sewage
The results of characterisation of the sewage with an average and standard deviation of sample size 10 are provided in Table 3.1. The pH range of domestic sewage from various cities of India ranges from 7 to 7.5 (CPCB 2005). The range of pH for domestic wastewater is 5.5 -8 (Metcalf & Eddy 2003). But present study reveals that sewage is slightly alkaline in nature. Similar alkaline pH range is reported by Sonune et al. (2015). Fresh sewage is alkaline in nature. However, near neutral pH were also reported. If biological treatment is preferred the pH should be in the range of 6-8 for efficient action of microbes.
Electrical conductivity (EC) is a measure of the suitability of water for irrigation. Higher EC values indicates salinity. Irrigation with sewage contaminated water increases the electrical conductivity of the soil (Shresta et al 2017). The range of electrical conductivity in the present study revealed sewage as medium strength.  Biodegradability is a good index for organic degradation and calculated from BOD/COD ratio. Generally it ranges from 0.4-0.8 in raw sewage and differs for different types of wastewater. In our study, the BOD/COD ratio is 0.7 indicating that it can be best treated by biological means rather than chemical process.

ILNS Volume 82
The TKN (Total Kjeldahl Nitrogen) is the combination of ammoniacal nitrogen and organic nitrogen with a range of 20-85 mg/ L. In municipal wastewater, it ranges from 35 to 60 mg/L. TP (Total Phosphate) in domestic wastewater ranges from 5 to 10 mg/L. Sulphate concentration in sewage is 20-50 mg/L. Typical untreated municipal wastewater had a TSS (Total Suspended Solids) of 100-360 mg/L (Metcalf & Eddy 2003). Correlation between electrical conductivity and TDS was evaluated by Uwidia & Ukulu (2013) and our results well agree with it. Prerequisite changes in characterisation of wastewater is likely to occur from one location to another. Even in specific location, the composition varies from time to time.
The allowable limits of heavy metals by CPCB, India are presented in Table 3.2. Results revealed that heavy metals iron and lead exceeded the permissible limit. Hence, these 2 heavy metals were taken into consideration for further studies and other heavy metals zinc, aluminium, copper, cadmium, chromium and nickel were neglected as they are well below the allowable limits. However, the metals Cu, Zn, Ni are considered as micronutrients required for plant growth. Soil irrigated with sewage is contaminated with various heavy metals and long term application of sewage in irrigation may lead to piling up of heavy metal concentration in cultivable land and pass on to the ecosystem via food, posing threat to all living beings (Usharani & Vasudevan 2014).

Optimisation of HLR -First order kinetics
Published literature on optimisation studies revealed that application of first order kinetics fitted well for removal efficiency of BOD, COD, TSS, TKN and TP. The theoretical HRT for the respective HLR with flow rate is presented in Table 3.3. The mass removal rate, k values of areal removal rate constant and volumetric removal rate constant of the present study is presented in Table 3.4. In the present study, first order kinetics fitted well for removal of organics and nutrients till 84 mm /d. The removal efficiency at the highest HLR of 112 mm /d is slightly less. It may be due to little over loading and spillage of influent. Higher values were obtained at HLR 84 mm /d for organics (BOD and COD) and nutrients (TKN and TP) removal confirming the optimal load of existing system. The quality of influent corresponds to typical municipal wastewater and the quality of effluent remained consistent after a period of 9 weeks at all hydraulic loading rate. Stabilization of the system was attained after maximal growth of plants to a height of 5-6 feet in 9 weeks. The removal efficiency remains constant in a fully matured VFCWS (Stefanakis & Tsihrintzis 2012).

Removal of organics in VFCWS
The concentration of BOD in the influent ranged from 216-252 mg/L. As the HLR increases, the removal rate of organics increased till 84 mm /d with HRT of 4.32 days. Maximum removal efficiency of organics was achieved at 84 mm /d. The BOD concentration in the influent and effluent with removal efficiency of VFCWS is depicted in Figure 3.1. In the present study, a maximum of 80 -88% removal of BOD was achieved at 84 mm /d. BOD removal of 50-56%, 54-63% and 64-73% was achieved for 28 mm/d, 56 mm/d and 112 mm/d respectively. The respective concentration of BOD in the effluent at maximal removal % was 95, 78, 25 and 58 mg/L for 28 mm/d, 56 mm/d, 84 mm /d and 112 mm/d respectively. The removal of BOD increased steadily till 9 weeks. However, the removal % tends to remain stable and constant after 9 weeks. The reason might be due to time period to attain the stabilisation of the system. Moreover, the plants reached a maximum height of 5 feet within that time duration indicating a fully matured VFCWS. In a fully matured VFCWS as the HLR increases, the rate of organic removal increases (Stefanakis & Tsihrintzis 2012).
According to Klomjek (2016) when HLR was increased from 2 cm/d to 5 cm/d the removal efficiency of BOD increased from 86 ± 4% to 94 ±1% in CWS planted with giant Napier grass. Transformation of pollutants vary with depth and is a crucial factor in determining contaminant removal by affecting the redox status and dissolved oxygen level in CWS. The microbial community is highly active near the root zone favouring organic removal (Prajapati et al. 2017). Yang et al. (2017) reported that as the OLR increases, the removal rate of BOD and COD in aerobic bio filters increased. The efficiency of wetland in organic removal is higher in summer than other seasons (Ramakrishna Rao et al. 2013). The removal efficiency of VFCWS for organic removal is reported to be above 90% in several studies (Luederitz et al. 2001). Removal of 95.3-99% of BOD was achieved in hybrid CWS (Lee et al. 2015).

Removal of nutrients in VFCWS
The concentration of TKN in the influent ranged from 24-30 mg/L. As the HLR increases, the removal of nutrients increased till 84 mm /d with HRT of 4.32 days. The influent and effluent concentration and TKN removal efficiency of VFCWS is shown in Figure 3.3. From the results it can be inferred that maximum removal of 82-84% of TKN was accomplished at 84 mm /d and removal efficiency of 63-70%, 60-76% and 65-72% was attained for 28 mm/d, 56 mm/d and 112 mm/d respectively. The respective concentration of TKN in the effluent at maximal removal % was 7.5, 6, 4 and 7.2 mg/L for 28 mm/d, 56 mm/d, 84 mm /d and 112 mm/d respectively. At the maximum HLR 112 mm /d, a slight decrease in removal efficiency might be due to over loading. The stability of the system in nutrient removal is attributed by an increase in the number of lateral roots. Accomplishment of lateral root production might be achieved when the plant attains maximal growth.
Nitrogen is up taken by plants, stored in sediments and apart from that microbial nitrification and denitrification process takes place. Biological process of nitrification involves 2 steps: The conversion of ammonia into nitrite and conversion of nitrite into nitrate. E. coli is reported to reduce nitrate to ammonia (Gonzalez et al. 2006). Denitrification was retarded at higher salinity of 15 ppm (Wu et al. 2008). The wastewater characteristics of the present study confine salinity of medium strength which does not have much interference with denitrification process.
In a fully matured VFCWS as the HLR increases, the rate of nitrogen removal increases (Stefanakis & Tsihrintzis 2012). Lower HRT in CWS is associated with incomplete denitrification of wastewater because N removal requires longer HRT than organic removal (Lee et al. 2015). As OLR increases, the removal rate of ammoniacal nitrogen in aerobic biofilters increased (Yang et al. 2017). Nitrogen removal is higher in vertical flow because it provides the suitable conditions for nitrification process and removal of 19-53.3% of TKN was achieved in hybrid constructed wetland system (Lee et al. 2015).

Figure 3.3 TKN removal in VFCWS
The concentration of phosphate in the influent ranged from 4.2-5.2 mg/L. The influent and effluent concentration and TP removal efficiency of VFCWS is shown in Figure 3 In our study about three fourth of the media is constituted by sand that might have played a major role. River sand has excellent phosphate removal property (Trang et al. 2010). The mechanism for phosphate removal might be due to adsorption and/or precipitation in sand filter, plant uptake and microbial action. In the previous studies a removal of 58.03% and 27.5% of TP was achieved by Phragmites australis in VFCWS and HFCWS respectively indicating that vertical flow achieves higher removal % than horizontal for the same species (Sudarsan et al. 2015, Mesquita et al. 2017). Maximum of 63.2% of TP was reported in hybrid constructed wetland (Lee et al. 2015). The microbial enzyme phosphatase is responsible for phosphate removal in CWS. The microbes in wastewater and soil contributes to 7% and 6% and adsorption in soil contributes to 71%. According to Kumar et al. (2011) a maximum of 64-75% of TP is removed by adsorption process in the system, 9-19% by plant uptake and 7-12% by microbial metabolism. Cyperus papyrus has the ability to uptake 28.5% of nitrogen and 11.2% of phosphate from the system (Kyambadde et al. 2005).
The efficiency of wetland in nutrient removal is higher in summer than other seasons

Removal of coliform in VFCWS
The pathogen removal in CWS of the present study reveals 99.99 % removal of total coliforms (TC), fecal coliforms (FC) and E.coli corresponding to 2-3 log removal. The range of TC in the inlet ranges from 4.3x10 6 -37.8x10 7 MPN/100mL respectively. Consistent removal of TC, FC and E.coli was attained after 8 weeks period of time. Log removal of TC increased from 0.93-2.3, 0.8-1.7, 0.6-1.6 and 0.14-0.5 for 28 mm/d, 56mm/d, 84mm/d and 112 mm /d respectively during the first seven weeks.

Figure 3.5 Reduction of TC at different HLRs
After 8 th week, a maximum of 3.1 log reduction was achieved at 28 mm/d whereas, 2.6 log removal, 2.3 log removal and 0.9 log removal was achieved for 56mm/d, 84mm/d and 112 mm /d respectively. The log reduction of total coliforms in CWS at different HLR is presented in Figure  3.5. The lowest HLR had the retention time of 12 days. Increased retention time might have favoured maximum removal. About 99.99% of pathogen reduction is reported by several authors in different constructed wetlands with different species equivalent to 3-4 log reduction (Tole et al. 2014).
The concentration of FC in the inlet ranged from 4.9x10 5 to 11.5x10 6 MPN/100 mL. The log reduction of fecal coliforms in CWS at different HLR is presented in Figure 3.6.

Figure 3.6 Reduction of FC at different HLRs
Log removal of FC increased from 0.08-2, 0.06-1.8, 0.04-1.6 and 0.02-1.2 for 28 mm/d, 56 mm/d, 84 mm/d and 112 mm/d respectively during the first seven weeks. After 8 th week, a maximum of 3.3 log reduction was achieved at 28 mm/d while, 3.1 log reduction, 2.9 log reduction and 2 log reduction was achieved for 56mm/d, 84mm/d and 112 mm /d respectively.
Among the four wetland species: Cyperus papyrus, Cyperus alternifolius, Typha latifolia and Phragmites mauritianus; Cyperus alternifolius and Typha latifolia were effective in significant removal of Salmonella and E.coli above 98% followed by Cyperus papyrus. The pathogen removal % of Phragmites mauritianus was least (Kipasika et al. 2016). The highest removal of 96% and 89% total coliforms and E.coli was reported in subsurface flow wetlands (Reinso et al. 2008). Domestic wastewater treatment with Cyperus papyrus attained 99.99% removal of fecal coliforms equivalent to 2 log units (Mburu et al. 2008). Reduction of 1.28, 1.21 and 1.01 log units of total coliform, fecal coliform and E.coli was attained in multilayered substrates of CWS without plantation (Latrach et al. 2015).

Figure 3.7 Reduction of E.coli at different HLRs
After 99.9% removal of E.coli, total coliforms and fecal coliforms, pathogens still persists in the effluent than the permissible limit of STP except membrane bioreactor and further research initiative is needed to achieve complete disinfection (Hendricks & John pool 2012).
The range of E.coli in the inlet ranges from 5.9x10 4 to 37.8x10 5 MPN/100mL. The log reduction of E.coli in CWS at different HLR is presented in Figure 3.7. Log removal of E.coli increased from 0.8-1.9, 0.3-1.4, 0.1-1.6 and 0.1-1.3 for 28 mm/d, 56mm/d, 84mm/d and 112 mm /d respectively during the first seven weeks. After 8 th week a maximum of 2.8 log reduction was achieved at 28 mm/d while, 2.6.1 log reduction, 2.4 log reduction and 1.7 log reduction was achieved for 56 mm/d, 84 mm/d and 112 mm /d respectively. The effectiveness of E.coli removal in CWS majorly depends on the filtering mechanism. Reduction of 4.7 log of E.coli was reported in sand filtration (Seeger et al. 2016). Sand beds are capable of removing 1.2-2.7 log unit of total coliforms and 1.5-3.5 log unit of E.coli (Bohorquez et al. 2016). Fine media had an increased log reduction than coarse media (Albalawneh et al. 2016).
Typha latifolia in CWS potentially removed 96.8-99.7% of fecal coliforms throughout a period of 17 months (Smith et al. 2005). Karathanasis et al. (2003) reported more than 93% removal of fecal coliforms in CWS planted with Typha latifolia. In hybrid CWS with Paspalidium flavidum 98.6% removal of fecal coliform in domestic wastewater was reported by Sehar et al. (2013). The pathogen removal in CWS is not adequate and requires an additional treatment for disinfection. Maximum of 6 log reduction is recommended by WHO (2006) for wastewater reuse in agriculture.

Removal of TSS and TDS in VFCWS
The fate of suspended organic matter under anaerobic condition in CWS is presented in    Manios et al. (2003) reported that among different substrates, gravel reed bed planted with Typha had the best removal of TSS more than 95% with effluent concentration of less than 10 mg/ L. About 95-97% of TSS removal throughout the year was achieved irrespective of season (Smith et al. 2006 The concentration of TDS in the influent ranges from 430-480 mg/L. The results of TDS removal in CWS at different HLR is presented in Figure 3.10.

Removal of Heavy Metals in VFCWS
The concentration of lead in the influent ranged from 0.156 -0.263 mg/L. With respect to the removal of heavy metals Fe and Pb in our study, the effluent collected from outlets at different HLR confined the limits of discharge. However, the removal % varied at different HLR involved in the study. A maximum of 90-94% removal of lead was accomplished at 84 mm /d confronting effluent  Figure 3.11. Efficient removal of heavy metals by biological treatment is achieved best at pH 8.8 (Rajasulochana & Preethty 2016). The alkaline pH of the sewage might be one of the reason for efficient removal. The lateral roots of Cyperus play a major part in heavy metal uptake. Metals may be retained in the sediments either in oxidized/ reduced soil conditions (Sinicrope et al. 1992). A study conducted on the comparison of three species of Cyperus: Cyperus alternifolius, Cyperus prolifer and Cyperus textilis for uptake and tolerance of heavy metals aluminium and iron revealed that, Cyperus alternifolius was the best fit for phytoremediation. However few reports are available about the genus Cyperus in heavy metal removal (Ayeni 2016).

Figure 3.11 Lead Removal in VFCWS
Phragmites australis could efficiently remove 88% lead and 92% iron. Typha could proficiently remove 87% lead and 95% iron (Gikas et al. 2013). The removal of lead by substrates is influenced by the concentration of iron in the system (Ren et al. 2016). An average of 60. 6% removal of lead was observed in HFCWS planted with Phragmites australis .
The concentration of iron in the influent ranged from 3.28 to 5.82 mg/L. A maximum of 82-85% removal was attained at 84 mm/d confronting discharge quality. Iron removal of 70-72%, 77-78% and 76-79% was achieved for 28 mm/d, 56 mm/d and 112 mm/d respectively. The respective concentration of lead in the effluent at maximal removal % was 1.5, 1.2, 0.8 and 1.1 mg/L for 28 mm/d, 56 mm/d, 84 mm/d and 112 mm/d respectively. Heavy metals are removed as their bicarbonate due to bacterial production of bicarbonate alkalinity and as insoluble sulphide. The reduction of metals to non-mobile forms is achieved by microbial activity in wetlands and the reducing conditions are afforded by sulphate reducing bacteria. Iron and lead are precipitated into insoluble sulphides in CWS (Sheoran & Sheoran 2006).The influent and effluent concentration with iron removal efficiency at different HLR is presented in Figure 3.12.
Iron uptake by plants might be favoured as it is one of the critical component involved in many physiological process of plant: DNA synthesis, respiration, photosynthesis, mitochondrial and chloroplast metabolism. When iron enters the xylem, it complexes with citrate (Rout & Sahoo 2015).

Figure 3.12 Iron Removal in VFCWS
Plant cell wall has cationic exchange sites named phytochelatins that detoxify and balance homeostasis of heavy metal uptake. Metal reduction can be tolerated in the body mass of plants without showing negative effects on its growth (Sheoran & Sheoran 2006). Heavy metals present in wastewater are effectively removed by the wetland mesocosms at different hydroperiods (Sinicrope et al. 1992).

Conclusion
Among the diverse existing wastewater treatments schemes, CWS secures a peculiar place for its aesthetic value in addition to low cost maintenance. The optimal HLR for VFCWS planted with Cyperus alternifolius was 84 mm/d (60L/d) with discharge quality which met the standards of discharge for agriculture and inland water. Hence the effluent can be utilized either for irrigation or used for ground water recharge. Vegetation is the major factor contributing the removal of organics and nutrients. The dynamic role of microbes in removal of organics, nutrients and heavy metal removal is inevitable and secures a noteworthy place in CWS. CWS not only aids in treatment but also biodiversity thus favouring a successful ecological balance apart from sustainable treatment of sewage. A few clippings of visitors indicating biodiversity in CWS clicked at the time of sample collection is presented in Figure. 4.1. The different species such as caterpillar, grasshopper, snail etc were observed in the constructed wetland system indicating it as a clean green way in sewage treatment without affecting the ecosystem and biodiversity.

Figure 4.1 Visitors at CWS indicating biodiversity
In a nutshell it can be concluded that for tropical countries CWS is not only the evergreen technology that provides solution for wastewater treatment but the best for deterrence of exploitation of diversity. The harvested plants can be left decayed because the oxidation of the heavy metals were changed such that it remains less toxic. The findings related to this are continued in the further studies.