The average characteristics of the primary treated sewage effluent (PTSE) is given in Table 3. The pH was moderately alkaline (8.35) with considerable amount of salts (2.0 dS m-1). The effluent had the total dissolved and suspended solids of 1376 and 306 mg L-1, respectively. The BOD (256 mg L-1) and COD (506 mg L-1) were marginally higher than prescribed for direct irrigation. The alkalinity of sewage water was due to higher concentrations of sodium and bicarbonates as indicated by higher residual sodium carbonate concentration (5.56 mg L-1). Among nitrogen forms, ammoniacal nitrogen predominated (13.11 mg L-1) followed by organic nitrogen (10.02 mg L-1) and least was nitrate nitrogen (1.81 mg L-1). The mean total phosphorus and potassium concentrations were 10.30 and 43.03 mg L-1 respectively. The boron concentration was low (0.28 mg L-1).
The physico-chemical parameters of the treated sewage effluent after 120 days from start were assessed for evaluating the performance of filterbeds and macrophytes.
pH
The lowest effluent pH was recorded in ‘gravel-sand-(charcoal+brick)-gravel’ filterbed (7.18). Constructed wetland vegetated with Phragmites was most efficient in reducing pH compared to other macrophytes (Table 4).
Across treatments, the reduction in pH after 120 days was 11.4% compared to PTSE. Similar observation was made by Rajimol et al. (2016). The observed pH reduction was attributed to CO2 production from decomposing plant litter, dissolved organic matter and other sewage effluent components trapped in the root mat and nitrification of ammonia (Arivoli and Mohanraj 2013). Presence of considerable calcium+magnesium (SAR < 5) in PTSE (Table 3) and its alkaline pH favour precipitation of these alkaline metals as their carbonates and phosphates when it is stranded in the wetland. That might be the reason for general lowering of pH of treated sewage effluents. Similar reasoning was reported by Priya et al. (2013). They opined that the effluent pH between 7.5 and 8.5 could be ideal for the chemical precipitation of various forms of calcium phosphates. However, the removal of calcium+magnesium through precipitation was only marginal so that the SAR was not increased rather it decreased possibly due to lowering of sodium also through adsorption on filterbed materials and uptake by macrophytes. The reduction in EC of treated sewage effluent over PTSE supported this fact (Table 3). The presence of brick and charcoal as filterbed materials in addition to sand and gravel might have favoured such reactions.
Electrical conductivity (EC)
The EC values for filterbeds and macrophytes varied only slightly (Table 4). Among filterbeds, ‘gravel-sand-(charcoal+brick)-gravel’ reduced more EC and among macrophytes, Canna and Phragmites recorded low EC values. The constructed wetland with ‘gravel-sand-(charcoal+brick)-gravel’ + Canna and ‘gravel-sand-charcoal-gravel’ + Phragmites combination significantly reduced EC (0.67 dS m-1). There was a substantial reduction (50.5 %) in EC compared to PTSE (2.00 dS m-1). The decrease in conductivity was attributed to uptake of micro and macro elements and ions by plants and bacteria, and their removal through adsorption to plant roots, litter and settleable suspended particles (Vera et al. 2011 and Arivoli and Mohanraj 2013) and also due to precipitation.
The ‘gravel-sand-(charcoal+brick)-gravel’ filterbed caused greater reduction in EC compared to others. Looking to the composition of this filterbed, it seemed presence of charcoal and brick with possible micro-porosities could bring more adsorption of ions and thereby lower EC. Among the macrophytes, Phragmites and Canna favoured greater reduction in EC. The average EC reduction after 120 days was 50.5% compared to the mean values of PTSE for the same period (Table 3).
Total dissolved solids (TDS)
Among filterbeds, ‘gravel’ (742 mg L-1) reduced more TDS whereas among macrophytes, Brachiaria (727 mg L-1) was more efficient compared to others (Table 4). The combination of ‘gravel’ and Brachiaria recorded significantly lower TDS (673 mg L-1). Greater reduction (43.2%) in TDS was observed due to wetland treatments over PTSE (Table 3).
The solid portion may be in suspended, dissolved and colloidal states which impart turbidity to the sewage water. Efficiency of constructed wetland in the removal of turbidity is reported to depend largely on the size of sand/ bedding particles and the depth of the bed (Jing et al. 2001).
Total suspended solids (TSS)
The ‘Gravel-sand-brick-gravel’ was more efficient in TSS removal among macrophytes while Brachiaria among macrophytes (Table 4). The interaction of ‘gravel-sand-(charcoal+brick)-gravel’ and Brachiaria recorded significantly lower TSS (39 mg L-1). A substantial reduction in TSS (50.7 %) was observed due to wetland treatment over PTSE. The mean reduction in TSS was greater than in TDS. Similar observation was made by Vymazal (2011) who opined that suspended solids are retained predominantly by filtration and sedimentation. The ‘gravel-sand-brick-gravel’ filterbed removed more TSS than others. Among macrophytes, Brachiaria performed better compared to others in terms of TSS removal. The constructed wetland system acted as a mechanical and biological filter and removed suspended particles from the water (Zurita et al. 2009; Vera et al. 2011; Suhad et al.2018).
Total solids (TS)
The TS at 120 days varied relatively among filterbeds (901 to 966 mg L-1) and macrophytes (838 to 999 mg L-1). The ‘gravel-sand-charcoal-gravel’ among filterbeds while, Brachiaria among macrophytes was more efficient in lowering TS (Table 4). The interaction between the filterbeds and macrophytes was significant. The combination of ‘gravel-sand-(charcoal+brick)-gravel’ and Brachiaria recorded significantly lower TS (744 mg L-1). A considerable reduction in total solids (44.5 %) was recorded over PTSE due to physical and biological filtration processes (Table 3).
Biological oxygen demand (BOD5) and chemical oxygen demand (COD)
The BOD5 concentration of treated effluent was significantly reduced due to filterbeds and macrophytes (Table 4). The filterbed ‘gravel-sand-(charcoal+brick)-gravel’ (92.1 mg L-1) reduced more BOD5 concentration compared to other filterbeds. The BOD5 concentration of macrophytes varied slightly in the range of 106-107 mg L-1 though, Brachiaria showed statistical superiority over others. The combination of ‘gravel-sand-(charcoal+brick)-gravel’ and Canna recorded significantly lower BOD5 (88.3 mg L-1).
Among the filterbeds, ‘gravel-sand-(charcoal+brick)-gravel’ (212 mg L-1) was more efficient in COD reduction compared to others (Table 5). Among Macrophytes, Canna (224 mg L-1) topped with higher reduction in COD compared to others. The COD concentration of macrophytes ranged from 224 to 231 mg L-1. The interaction of ‘gravel-sand-(charcoal+brick)-gravel’ and Canna significantly reduced COD in treated sewage effluent (208 mg L-1).
Compared to the average BOD concentration of PTSE, the reduction in BOD was 58.6 % due to constructed wetland treatments (Table 3). As like in case of BOD, the COD reduction was 55.3 % due to wetland treatment over PTSE. Similar reductions through wetland were also reported by Jizheng et al. (2012). Based on the mean BOD value of 256 mg L-1(Table 3), the raw sewage effluent was unsuitable for irrigation when compared to permissible limits of 100 mg L-1 (Anon. 1985). After allowing the raw sewage effluent to flow through wetland system, there was a reduction in its BOD5. Zurita et al. (2009) reported a higher BOD5 removal efficiency in a vertical flow constructed wetland system because of the better oxygen transfer from the atmosphere. The presence of multiple plant species as a bio-filter with varied root-phenotypic traits has been reported to provide a more propitious habitat for the development of a great microbial diversity leading to higher removal efficiencies. Similar findings were reported by Li et al. (2008), Vera et al. (2011) and Kelvin and Tole (2011). The same reason could be attributed to the notable decline in the COD of treated sewage effluent since both COD and BOD measures the organic matter present in sewage effluent and the same principles of removal inside the constructed wetlands would apply to them. The reduction of COD might be due to higher dissolved oxygen in rhizosphere meeting the oxygen demand for chemical oxidation of organic constituents. According to MoEF standards, COD of maximum 250 mg L-1 is allowed for inland surface water disposal and as well for irrigation. In the present study, the treated effluent COD was reduced to less than 250 mg L-1 making it suitable for irrigation.
Sodium
A greater reduction in sodium concentration in treated effluent was observed in ‘gravel-sand-charcoal-gravel’ among filterbeds and Typha among macrophytes (Table 5). The ‘gravel-sand-charcoal-gravel’ wetland vegetated with Brachiaria significantly reduced sodium in the treated effluent. The mean sodium concentration of the PTSE was 10.64 meq L-1 which reduced to 4.57 meq L-1 due to wetland treatment with the magnitude of reduction of 57.0% (Table 3).
Sodium was the dominant cation in both treated and PTSE which was well above the permissible level of 4 meq L-1 for irrigation (Anon. 1985). The reduction in sodium concentration was accredited to the processes of sedimentation, filtration, decomposition, adsorption and plant uptake.
Sodium adsorption ratio (SAR)
Among filterbeds, ‘gravel-sand-charcoal-gravel’ caused greater reduction in SAR (2.41 mmol1/2L-1/2) while, Brachiaria, Typha and Canna did the same as compared to Phragmites (Table 5). The interaction of ‘gravel-sand-charcoal-gravel’ and Brachiaria significantly reduced SAR in TSE (1.84 mmol1/2L-1/2). The reduction in SAR after 120 days was 40.4% as compared to PTSE (Table 3). The reasons for the reduction of SAR in the treated sewage effluent are ingrained in the cause for reduction of sodium.
Bicarbonates
The filterbed, 'gravel-sand-charcoal-gavel' (7.08 meq L-1) was found more efficient in bicarbonate reduction (Table 5). Among macrophytes, Brachiaria (9.23 meq L-1) was found less efficient compared to the remaining three; each of them was on par within. The interaction of 'gravel-sand-(charcoal+brick)-gravel' and Typha significantly reduced bicarbonates in TSE (6.10 meq L-1).
The comparison of mean data of bicarbonate concentrations of the treated and PTSE revealed a reduction of bicarbonate concentrations to the extent of 48.2 % due to wetland treatment (Table 3). The bicarbonate concentration was higher in both treated and PTSE making it alkaline, more importantly exceeding the recommended level of 1.5 me L-1 (Anon., 1985).
Residual sodium carbonate (RSC)
Among filterbeds, 'gravel-sand-charcoal-gavel' (RSC 1.94 meq L-1) composition was more efficient in lowering RSC (Table 5). The trend between macrophytes remained similar to that observed under bicarbonate concentrations. Except Brachiaria, the remaining three macrophytes were equally more effective in reducing RSC. The interaction of 'gravel-sand-charcoal-gavel' and Canna recorded significantly lower RSC (1.40 meq L1). After wetland treatment, the mean RSC was reduced by 50.2% as compared to PTSE (Table 3). The RSC is bound to vary depending on the cationic (calcium + magnesium) and anionic (bicarbonate) concentrations in the raw sewage effluent. The processes like sedimentation, filtration, decomposition, adsorption and plant uptake of these ions are reported as possible reasons for the reduction in RSC. In general, inconsistent results were observed in the reduction of RSC by filterbeds whereas the macrophyte Canna consistently proved more efficient in reducing RSC.
Nitrogen forms and total nitrogen
The inorganic nitrogen in wasterwater is largely represented by ammoniacal and nitrate nitrogen. However, in wasterwaters, the organic nitrogen far exceeds the inorganic forms which is concurrently represented by higher BOD values.
In this study, 'gravel-sand-(charcoal+brick)-gravel' filterbed registered significantly higher ammoniacal nitrogen (NH4+ -N) and nitrate nitrogen (NO3- -N) as compared to other filterbeds. This also registered significantly lower organic N. This implied that 'gravel-sand-(charcoal+brick)-gravel' filterbed facilitated higher oxidative conditions resulting in lower levels of organic N and higher levels of inorganic N forms. The same treatment also witnessed greater reduction in BOD. Among macrophytes, Typha registered higher NH4+ -N concentration (13.24 mg L-1) while phargmites higher NO3- -N (2.40 mg L-1) in treated sewage effluent (Table 5 and 6). The NH4+ -N concentration in the treated sewage effluent at 120 days remained almost similar to that of PTSE. However, the NO3- -N concentration was considerably higher by 26.0% in the treated effluent while the organic N was greatly reduced by 92%) due to wetland treatment (Table 3). Similar higher reduction in organic N, amounting to 50.6% was witnessed by Zurita et al. (2009) for a vertical flow constructed wetland.
Vymazal (2011) reported that a lower hydraulic retention time and a greater oxidation in the rhizosphere of macrophytes created more conducive conditions for faster ammonification process leading to conversion of organic N to NO3- -N. A higher reduction in organic N in treated sewage effluent over PTSE was a clear indication of this fact. The higher extent of oxygenation in the rhizosphere due to complementary effect of filterbed and macrophyte favoured chemolithoautotrophic microbial activity which led to conversion of NH4+-N to NO3--N. The percentage reduction in organic nitrogen in the treated sewage effluent did not exactly match with percentage increase in NO3--N, obviously due to concomitant uptake by macrophytes (Fig.1).
The total N (TN) concentration registered 35.2% reduction as compared to raw sewage effluent. Comparable results were reported by Kelvin and Tole (2011) who reported a removal efficiency of 41 per cent for TN. These reductions were mediated by nitrifiers such as nitrosomonas, nitropira, nitrosococcus and nitrobacter in both surface and subsurface flow constructed wetlands (Kadlec and Knight. 1996). The reduction in total nitrogen could also be attributed to the process of adsorption of ammoniacal nitrogen on filterbed materials. Among the macrophytes, Brachiaria and uptake by macrophytes.
Total phosphorus (TP)
The 'gravel-sand-gravel' (4.54 mg L-1) filterbed reduced more TP compared to others (Table 5). It might be due to lower effective size of filterbed materials which includes sand as major component. It is evident from Table 1 that sand with minimum size had greater surface area among the filterbed materials accounting for greater adsorption. Similar finding was reported by Seo et al. (2005). The most important characteristic of the filterbed material determining its P-removal capacity is its Ca concentration. A considerable Ca concentration in gravel (pH 8.08 indicating the presence of alkaline salts) might have favoured precipitation with P as sparingly soluble calcium phosphates particularly at the slightly alkaline conditions, typical of domestic sewage (Arias et al. 2001).
Brachiaria (5.03 mg L-1), Canna (5.20 mg L-1) and Phragmites (5.13 mg L-1) were on par with each other. The uptake of phosphorus by Brachiaria was the highest among the macrophytes (Fig. 1). Whereas, the TP reduction was less by Typha (5.54 mg L-1) compared to other macrophytes. The TP in treated sewage effluent across treatments was reduced by 49.3% over the mean TP values of PTSE (Table 3). This reduction is ascribed to the processes like precipitation, plant uptake and adsorption on root surface taking place in the wetland treatment system. The results were in accordance with the findings of Neralla et al. (2000), Vera et al. (2011) and Arivoli and Mohanraj (2013).
Potassium
The filterbeds viz., 'gravel' (18.98 mg L-1), 'gravel-sand-brick-gravel' (17.77 mg L-1) and 'gravel-sand-gravel' (19.53 mg L-1) reduced more potassium as compared to others (Table 6). The K removal was less in filterbeds involving charcoal indicating that charcoal might have contributed K during wetland treatment. The Brachiaria (12.40 mg L-1) was found highly effective in K removal compared to other macrophytes which also witnesses the highest K uptake among macrophytes. The Brachiaria planted in ‘gravel-sand-gravel' filterbed significantly reduced potassium (7.15 mg L-1).
Reduction in potassium by 50.7% was observed in treated sewage effluent over PTSE at 120 days (Table 3). The processes like plant uptake and adsorption taking place in the wetland treatment system might be responsible for the reduction in potassium in the treated sewage effluent. The filterbeds viz., 'gravel' (18.98 mg L-1), 'gravel-sand-brick-gravel' (17.77 mg L-1) and 'gravel-sand-gravel' (19.53 mg L-1) reduced more potassium as compared to others (Table 5). The K removal was less in filterbeds involving charcoal indicating that charcoal might have contributed K during wetland treatment. The Brachiaria (12.40 mg L-1) was found highly effective in K removal compared to other macrophytes. The Brachiaria planted in ‘gravel-sand-gravel' filterbed significantly reduced potassium (7.15 mg L-1).
Reduction in potassium by 50.7% was observed in treated sewage effluent over PTSE at 120 days (Table 3). The processes like plant uptake and adsorption taking place in the wetland treatment system might be responsible for the reduction in potassium in the treated sewage effluent.
Boron
The filterbeds comprising brick or charcoal showed higher removal of boron as compared to only gravel and sand. There was no statistical significance between the macrophytes in respect of boron removal. The reduction in boron concentration in treated sewage effluent was 60.7% over PTSE. The boron concentration of both PTSE and treated effluent were less than 1 mg l-1 and was suitable for irrigation. A notable fall in boron concentration of treated sewage effluent was observed, though all the times it was well below the safe limit. Filtration, adsorption and plant uptake might have contributed for the reduction of B in the treated sewage effluent (Vymazal 2011). Though Turker et al. (2014) reported that Phragmites could be used to decontaminate water containing high concentrations of boron; in our case all macrophytes were equally effective in boron removal.