Performance of Filterbeds and Macrophytes in Vertical Constructed Wetland for Treating Domestic Sewage E�uent

An experiment with different �lterbeds and macrophytes was carried-out to study their phytoremediation capacity on the e�ciency of domestic wastewater treatment through constructed wetland (CW) during November to March, 2017-18 at


Introduction
Water, food and energy securities are emerging as increasingly important and indispensable issues for India and the world. Water is vital yet, a constrained resource in most of the developing nations. The average availability of potable water is dwindling steadily and India may become a water-scarce country by 2025. Thus, recycle and reuse water needs greater attention. About 38,354 million litres per day (MLD) of sewage water is generating in major cities of India. However, the total sewage treatment capacity in these cities is only 22,963 MLD (Annon., 2020). A large portion of this surplus sewage has the potential to cause widespread water pollution.
About 80 countries and regions, representing 40 per cent of the world's population, are experiencing water stress and about 30 of these countries are facing water scarcity during a large part of the year (Kharraz et al., 2012). To compensate for water shortage, many countries have begun exploiting reserves that are not su ciently being replenished. This short-term strategy is likely to have detrimental long-term effects on the availability of freshwater for human communities and native eco-systems. The consequences of regional and national water scarcity will lead to a depletion of reserves. This scarcity will also give rise to competition for water between nations and regions, as well as among sectors such as agriculture, industry and municipalities.
Globally, agriculture is the dominant user of water, accounting for 70 per cent of total freshwater for irrigation. India's agriculture sector, which is the backbone of Indian economy, right now utilizes around 90 per cent of total water resources. However, with the increasing competition between agriculture, industry and domestic sectors, agriculture is beginning to receive less share of freshwater. Moreover, fast depletion of groundwater reserves coupled with severe water pollution, has placed India in di cult position to provide su cient freshwater for irrigation. In India, the evident shortage of fresh water coupled with a considerable increase in the volume of urban wastewater production from the growing cities have made the problem worse and di cult to manage.
Sewage irrigation is an age-old farming practice and reuse of wastewater in agriculture is gaining wider acceptance in many parts of the world. Sewage water offers an alternative irrigation water source, as well as the chance to recycle plant nutrients. Wastewater also additionally contains expansive range of taints viz., bio-degradable organic compounds, toxic metals, suspended solids, micro-pathogens and parasites (Montaigne and Essick 2002; Pedrero and Alarcon 2009) which restrict its direct application to eld.
In developing nations like India, the issues related with wastewater reuse arise from its lack of treatment. Energy and skill intensive wastewater treatment technologies are most often costlier and not feasible alternatives in areas where electricity supply is scarce and unreliable. The challenge thus is to nd such low-cost, low-tech, user friendly methods of wastewater treatment, which on one hand abstain from debilitating our substantial wastewater dependent livelihoods and on the other hand prevent degradation of our valuable natural resources. It is an advantageous time to refocus on approaches to treat wastewater and reuse it for irrigation and different purposes. Utilization of treated wastewater offers new vistas in improving water accessibility and keeps up water quality prerequisites for crop production (Azize dogan Demir and Ustum Sahin 2017) .
Natural processes have always cleansed water as it owed through rivers, lakes, streams, and wetlands in nature. In developing countries natural treatment systems are considered more suitable which can be built with locally available materials and thus become cost-effective. Natural treatment systems are considered one of the best treatment options, particularly in warm climates (Duenas et al., 2003). Wetlands with hydrophytes are one of the many types of natural systems that can be used for treatment of municipal wastewater.
The major nutrient removal mechanisms associated with constructed wetland system include biodegradation, precipitation and ltration (Vymazal 2011; Dash 2012). The choice of materials for lterbed and their vertical arrangement, thickness/depth-wise, should aim at maximizing the e ciency of these foresaid processes and minimizing the treatment cost. Keeping these in mind, the present study (column study) was executed with locally available materials such as gravel, sand, charcoal and brick materials as lterbed for treating the domestic sewage e uent.
The vertical ow wetland was constructed using PVC pipes (100 cm length and 15 cm dia.), supported in position by iron stands. The top 20 cm in each column was left for planting the macrophyte and ponding purposes and the remaining 80 cm height was lled with different lter bed materials ( Figure 1).
The bottom end of the pipe was closed with end cap tted with a valve. To facilitate easy entry and surface non-clogging, the top 25 cm layer in all the treatments was lled with gravels (basaltic stone pieces) of ~ 20 mm size. Similarly, the bottom 25 cm was lled with gravel of ~ 20 mm size for free downward discharge. The middle 30 cm in the column (except in 'Gravel' lterbed where the entire column was lled with gravel) was lled with sole or combinations of different lter bed materials. In 'Gravel-Sand-Gravel' lterbed, the middle 30 cm was lled with sand (0.02-2.0 mm). In 'Gavel-Sand-Brick-Gravel' lterbed, the mid layer was subdivided into two; the top 15 cm lled with sand and the lower 15 cm with brick (~ 20 mm) while in 'Gravel-Sand-Charcoal-Gravel' lterbed, the top 15 cm was lled with sand and the lower 15 cm with charcoal (~ 20 mm). In 'Gravel-Sand-(Charcoal+Brick)-Gravel' lterbed, the top 15 cm was lled with sand and the lower 15 cm with the equal (50:50 by w/w) mixture of charcoal and brick material ( Figure 1). The physical properties of the lterbed materials are given in Table 1. The hydraulic retention time was worked out using the formula given below: The total storage (porosity volume, cc) was calculated from the porosities of the proportionate contents of lterbed materials lled in each column, which differed among lterbed treatments ( Table 2). The hydraulic retention time was set uniformly at 2.5 days by regulating In uent ow rate using the valve tted at the bottom of each column.
The planting materials of all the four macrophytes were collected from waterlogged / marshy areas around University campus, Dharwad and reared in plastic trays with minimum soil. Young plants of macrophytes raised using sand medium were transplanted in the top layer of gravel in each column after washing off the sand adhering to roots. The columns were irrigated with primary treated sewage e uent (PTSE). Every day, the treated water collected in drain-can was decanted and once in 15 days, the treated water was collected and stored in refrigerator for physico-chemical analysis.
The PTSE from the sedimentation tank in the ow stream in the premises of the University was used for this study. The sewage water from this sedimentation tank was collected regularly and fed to the columns to have ponded condition. The quality of this PTSE was monitored at fortnightly intervals while, the treated sewage e uent samples from each column were analyzed at 120 days from the start of the study. The water quality parameters were analyzed following standard methods: pH, EC (Sparks 1996); total phosphorus and BOD (Anon. 1975); COD, sodium, ammoniacal nitrogen, nitrate nitrogen, total nitrogen, SAR, RSC and bicarbonates (Tandon 1998); total dissolved solids, total suspended solids, total solids and boron (Gupta 2007).
The macrophytes were cut/ pruned at 10 cm height from the base at 30 days intervals and dried in hot air oven at 65˚ C until two consecutive weights were constant. At the end, the total dry shoot biomass of each macrophyte over a period of 120 days was obtained by the summing all the biomass yields of each column and expressed as g column -1 . The N, P and K concentration in this dry shoot biomass was estimated as per standard methods (Jackson 1973). The N,P and K uptake by plant was worked out using the following equation:

Statistical analysis
The statistical interpretation of the experimental data was done by following the Fischer's variance analysis technique as given by Gomez and Gomez (1984). The experimental data were analyzed as per Factorial CRD to compare among the lterbeds, macrophytes and the interaction between the two. The results were computed at ve per cent (P = 0.05) level of signi cance. Critical differences (CD) were worked out whenever 'F' test was signi cant and treatment means were compared by applying Duncan's multiple range test (DMRT).

Results And Discussion
The average characteristics of the primary treated sewage e uent (PTSE) is given in Table 3. The pH was moderately alkaline (8.35) with considerable amount of salts (2.0 dS m -1 ). The e uent 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 e uent after 120 days from start were assessed for evaluating the performance of lterbeds and macrophytes.

pH
The lowest e uent pH was recorded in 'gravel-sand-(charcoal+brick)-gravel' lterbed (7.18). Constructed wetland vegetated with Phragmites was most e cient 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 CO 2 production from decomposing plant litter, dissolved organic matter and other sewage e uent components trapped in the root mat and nitri cation 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 e uents. Similar reasoning was reported by Priya et al. (2013). They opined that the e uent 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 lterbed materials and uptake by macrophytes. The reduction in EC of treated sewage e uent over PTSE supported this fact ( Table 3). The presence of brick and charcoal as lterbed materials in addition to sand and gravel might have favoured such reactions.

Electrical conductivity (EC)
The EC values for lterbeds and macrophytes varied only slightly (Table 4). Among lterbeds, '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 'gravelsand-charcoal-gravel' + Phragmites combination signi cantly 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' lterbed caused greater reduction in EC compared to others. Looking to the composition of this lterbed, 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).
The solid portion may be in suspended, dissolved and colloidal states which impart turbidity to the sewage water. E ciency 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 e cient in TSS removal among macrophytes while Brachiaria among macrophytes ( Table 4). The interaction of 'gravel-sand-(charcoal+brick)-gravel' and Brachiaria recorded signi cantly 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 ltration and sedimentation. The 'gravel-sand-brick-gravel' lterbed removed more TSS than others. Among macrophytes, Brachiaria performed better compared to others in terms of TSS removal. The

Total solids (TS)
The TS at 120 days varied relatively among lterbeds (901 to 966 mg L -1 ) and macrophytes (838 to 999 mg L -1 ). The 'gravel-sand-charcoal-gravel' among lterbeds while, Brachiaria among macrophytes was more e cient in lowering TS ( Table 4). The interaction between the lterbeds and macrophytes was signi cant. The combination of 'gravel-sand-(charcoal+brick)-gravel' and Brachiaria recorded signi cantly lower TS (744 mg L -1 ). A considerable reduction in total solids (44.5 %) was recorded over PTSE due to physical and biological ltration processes (Table 3).
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 e uent was unsuitable for irrigation when compared to permissible limits of 100 mg L -1 (Anon. 1985). After allowing the raw sewage e uent to ow through wetland system, there was a reduction in its BOD 5  of maximum 250 mg L -1 is allowed for inland surface water disposal and as well for irrigation. In the present study, the treated e uent COD was reduced to less than 250 mg L -1 making it suitable for irrigation.

Sodium
A greater reduction in sodium concentration in treated e uent was observed in 'gravel-sand-charcoal-gravel' among lterbeds and Typha among macrophytes ( Table 5). The 'gravel-sand-charcoal-gravel' wetland vegetated with Brachiaria signi cantly reduced sodium in the treated e uent. 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, ltration, decomposition, adsorption and plant uptake.

Sodium adsorption ratio (SAR)
Among lterbeds, 'gravel-sand-charcoal-gravel' caused greater reduction in SAR (2.41 mmol 1/2 L -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 signi cantly reduced SAR in TSE (1.84 mmol 1/2 L -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 e uent are ingrained in the cause for reduction of sodium.
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 lterbeds, 'gravel-sand-charcoal-gavel' (RSC 1.94 meq L -1 ) composition was more e cient 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 signi cantly lower RSC (1.40 meq L 1 ). 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 e uent. The processes like sedimentation, ltration, 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 lterbeds whereas the macrophyte Canna consistently proved more e cient 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' lterbed registered signi cantly higher ammoniacal nitrogen (NH 4 + -N) and nitrate nitrogen (NO 3 --N) as compared to other lterbeds. This also registered signi cantly lower organic N. This implied that 'gravel-sand-(charcoal+brick)-gravel' lterbed 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 NH 4 + -N concentration (13.24 mg L -1 ) while phargmites higher NO 3 --N (2.40 mg L -1 ) in treated sewage e uent (Table 5 and 6). The NH 4 + -N concentration in the treated sewage e uent at 120 days remained almost similar to that of PTSE. However, the NO 3 --N concentration was considerably higher by 26.0% in the treated e uent while the organic N was greatly reduced by 92%) due to wetland treatment (Table 3) -N, obviously due to concomitant uptake by macrophytes (Fig.1).
The total N (TN) concentration registered 35.2% reduction as compared to raw sewage e uent. Comparable results were reported by Kelvin and Tole (2011) who reported a removal e ciency of 41 per cent for TN. These reductions were mediated by nitri ers such as nitrosomonas, nitropira, nitrosococcus and nitrobacter in both surface and subsurface ow constructed wetlands (Kadlec and Knight. 1996). The reduction in total nitrogen could also be attributed to the process of adsorption of ammoniacal nitrogen on lterbed materials. Among the macrophytes, Brachiaria and uptake by macrophytes.

Total phosphorus (TP)
The 'gravel-sand-gravel' (4.54 mg L -1 ) lterbed reduced more TP compared to others ( 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 e uent 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 ndings of

Potassium
The lterbeds 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 lterbeds 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' lterbed signi cantly reduced potassium (7.15 mg L -1 ).
Reduction in potassium by 50.7% was observed in treated sewage e uent 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 e uent. The lterbeds 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 lterbeds 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' lterbed signi cantly reduced potassium (7.15 mg L -1 ).
Reduction in potassium by 50.7% was observed in treated sewage e uent 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 e uent.

Boron
The lterbeds comprising brick or charcoal showed higher removal of boron as compared to only gravel and sand. There was no statistical signi cance between the macrophytes in respect of boron removal. The reduction in boron concentration in treated sewage e uent was 60.7% over PTSE. The boron concentration of both PTSE and treated e uent were less than 1 mg l -1 and was suitable for irrigation. A notable fall in boron concentration of treated sewage e uent 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 e uent (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.

Conclusion
The inclusion of brick and/or charcoal as lterbed material in addition to sand and gravel has improved the physical ltration capacity of the wetland system.
Looking to differential biological ltration ability of macrophytes, inclusion of more than one type of macrophytes would seem more bene cial. In case of speci c requirement of remediation of water quality (viz; sodium or boron removal), suitable combination of lterbed and macrophyte may be resolved. The exibility of selection of lterbed and macrophyte allows the wetland to be adapted to different sites. This exibility also allows to adapt suitable macrophyte /s in the primary, secondary or tertiary treatment stage.      Nutrient uptake by macrophytes as in uenced by lterbeds and hydrophytes at 120 days from start