effect of cod/no3−-n ratio on the performance of a hybrid uasb reactor treating phenolic...

Desalination 232 (2008) 128–138

Presented at the Symposium on Emerging Trends in Separation Science and Technology — SESTEC 2006Bhabha Atomic Research Centre (BARC), Trombay, Mumbai, India, 29 September – 1 October 2006

*Corresponding author.

Effect of COD/NO3–-N ratio on the performance of a hybrid

UASB reactor treating phenolic wastewater

Anushuya Ramakrishnan, Sudhir Kumar Gupta*Centre for Environmental Science and Engineering, Indian Institute of Technology,

Bombay. Powai, Mumbai 400 076, IndiaTel. +91 (22) 25767853; Fax +91 (22) 25767853; email: [email protected]

Received 28 March 2007; accepted revised 18 September 2007

Abstract

The effect of COD/O3–-N ratio on the biodegradation of complex phenolic mixture was studied in bench scale

hybrid upflow anaerobic sludge blanket (HUASB) reactors. HUASB reactor is a combination of a UASB unit atthe lower part and an anaerobic fixed film at the upper end. The aim of this study was to evaluate the biodegradabilityof phenolic mixture (from synthetic coal wastewater) as the only carbon and energy source in continuous experimentsusing nitrate as the final electron acceptor. Synthetic coal wastewater contained phenol (490 mg/L); m-,o-,p-cresols(123.0 mg/L, 58.6 mg/L, 42 mg/L); 2,4-, 2,5-, 3,4- and 3,5 dimethyl phenols (6.3 mg/L, 6.3 mg/L, 4.4 mg/L and21.3 mg/L) as major phenolic compounds representing the complex phenolic mixture. Nitrate nitrogen loading wasincreased from 0.11 g/m3/d to 0.5 g/m3/d in order to keep COD/NO3

–-N ratio as 20.1, 14.85, 9.9, 6.36 and 4.45. Aninput phenolics concentration of 752 mg/L and hydraulic retention time (HRT) of 24 h was maintained through outthe study. Removal of phenolic mixture was found to increase with the lowering of COD/NO3

–-N ratio. Maximumphenolics removal of 98% was achieved at a COD/NO3

–-N ratio of 6.36. However, phenolics removal got adverselyaffected when COD/NO3

–-N ratio was reduced below 6.36. A nitrogen production efficiency of 78% was obtainedaccording to nitrate consumption. Simultaneous denitrification and methanogenesis was observed in all the reactorsthroughout the study, demonstrating that denitrification is a feasible alternative for the treatment of coal wastewater.Granules degrading complex phenolic mixture were of diameter 1.6–2.25 mm.

Keywords: Phenolics; Biodegradation; Coal wastewater; C/N ratio; Methane

0011-9164/08/$– See front matter © 2007 Elsevier B.V. All rights reserveddoi:10.1016/j.desal.2007.09.016

A. Ramakrishnan, S.K. Gupta / Desalination 232 (2008) 128–138 129

1. Introduction

Phenols are the major organic constituentsfound in effluents of coal conversion processes,coke ovens, petroleum, refineries, phenolic resinmanufacturing, herbicide manufacturing, fiber-glass manufacturing and petrochemicals. Theterms “phenols” or “phenolics” in wastewatertreatment technology are used interchangeablyeither to denote simple phenol or a mixture ofphenols in effluents [1]. The concentration ofphenols in effluents varies from 10 mg/L to 17×103 mg/L. Coal conversion process effluents andcoke oven effluents contain on the average 60%phenol and 30% cresols [2–4]. Phenols are toxic,carcinogenic, mutagenic and terratogenic [5].Stringent effluent discharge limits of less than0.5 mg/L have been imposed for the release ofphenolics into water bodies [6]. Removal ofphenols thus is a necessity to preserve the envi-ronmental quality.

Better understanding of anaerobic microbiol-ogy has led to the application of anaerobic pro-cesses for treating various types of industrialwastewaters. Previous reports on the upflow ana-erobic sludge blanket reactors suggested that thereactors could be modified by adding an anaero-bic filter in the upper zone to replace a gas–liq-uid–solid separator device [7]. In view of enhanc-ing the biomass holding capacity and retentiontime of a system, two phases, namely, suspendedgrowth and attached growth are combined to forma hybrid reactor. The lower part consisting of sus-pended growth acts as a buffer zone to all toxicand inhibitory materials in the feed and thus al-lows upper part of attached growth easier to handlea relatively harmless and mostly acidified efflu-ent. Hybrid reactors have been reported to be ef-ficient in treating phthalic wastewaters [8], acidicpetrochemical wastewater [9] and sugar waste-water [10]

Literature review on hybrid reactors revealsthat there is a need to expand the database of suchreactors for the treatment of toxic industrial waste-water. Since anaerobic treatment of complex phe-

nolic wastewater has not been studied, this re-search was designed to evaluate the performanceof a specially built hybrid reactor in treating highstrength complex phenolic coal wastewater (con-taining 752 mg/L complex phenolics) represent-ing 8 phenolic compounds under denitrifying con-ditions in continuous experiments. Specific ob-jectives of the present study includes (1) the ef-fect of varying COD/NO3

–-N ratio on the perfor-mance of a hybrid UASB reactor towards the bio-degradation of complex phenolic mixture underdenitrifying conditions. (2) the simultaneous deni-trification and methanogenesis in the hybridUASB reactor and (3) the characteristics of granu-lar sludge [diameter (mean and median), sludgevolume index (SVI) and specific methanogenicactivity (SMA)] at different COD/NO3

–-N ratios.

2. Materials and methods

2.1. Materials, experimental set-up

Investigations were carried out in four identi-cal bench scale hybrid reactors (R1, R2, R3 andR4), each of the working volume 13.5 L. The sche-matic diagram of the hybrid reactor is given inFig. 1. The reactors were constructed from trans-parent acrylic plastic sheets with inner dimensionsof 0.1 m × 0.1 m, the length of 1.5 m and the wallthickness of 6 mm. The reactors were providedwith a hopper bottom of 0.15 m length and a feedinlet pipe of 2.5 cm diameter to avoid chokingduring operation. An outlet was provided at thetop (1.5 cm), which was connected to the effluenttank. The gas–solid separator (GSS) device was asquare pyramid with the bottom dimensions 80mm × 80 mm. The reactors were provided withsix equidistant ports along their height to facili-tate sampling. A filter media of 30.48 cm lengthwas provided in the middle of the reactor. Thefilter media consisted of polyvinyl chloride (PVC)rings. About 215 rings were packed in all the fourreactors for consistency. The surface area of eachring was 6.28 cm2 and the total surface area occu-pied by the packing was 1350.2 cm2. The reactors

130 A. Ramakrishnan, S.K. Gupta / Desalination 232 (2008) 128–138

were seeded with a mixture of digested and par-tially granulated sludge (3:1). Digested sludge(median bio-particle diameter of 0.025 mm andtotal solids content of 24.2 g/L) was obtained fromM/s. Mahanadanda Dairy Works, Goregaon,Mumbai. Partially granulated sludge (total solidscontent of 62.1 g/L) was obtained from the benchscale UASB reactor treating phenolic compounds.The reactors were operated at mesophilic tempera-ture (27 ± 5°C).

2.2. Chemicals

Phenolic compounds (phenol, m-cresol, o-cresol, p-cresol, 2, 4-dimethyl phenol, 2, 5-dim-ethyl phenol, 3, 4-dimethyl phenol and 3, 5-dim-ethyl phenol) used in the study were analyticalreagents (99.9%) from Sigma-Aldrich, Germany.Sodium acetate was procured from West CoastLaboratories, India. Standard gases for methane

Fig. 1. Schematic diagram of the hybrid UASB reactor.

(2.5% v/v); CO2 and N2 were purchased from SpanGas, India. All other chemicals were procuredfrom E. Merck, India.

2.3. Synthetic wastewater

The synthetic wastewater used in the experi-ment contained 490 mg/L phenol; 58.6 mg/L o-cresol; 123.0 mg/L of m-cresol; 42.0 mg/L of p-cresol; 42.0 mg/L of 2,4-dimethyl phenol; 42 mg/Lof 2,5-dimethyl phenol; 6.3 mg/L of 3,4-dimethylphenol and 6.3 mg/L of 3,5-dimethyl phenol asmajor phenolic compounds. Volatile fatty acidsin the synthetic wastewater were represented by28.0 mg/L acetic acid; 16.0 mg/L propionic acid;5 mg/L butyric acid and 3.5 mg/L valeric acid re-spectively. Major nutrients in the feed included110-500 mg/L sodium nitrate; 28 mg/L ammo-nium chloride; 10 mg/L calcium chloride; 25 mg/Ldi potassium hydrogen ortho phosphate; 10 mg/Lmagnesium sulphate; 2 mg/L iron sulphate; and1100 mg/L sodium bi carbonate. Trace metal so-lution was prepared in distilled water by dissolv-ing per liter 0.05 mg zinc chloride; 0.05 mg cu-pric chloride; 0.03 mg manganous sulphate;0.05 mg ammonium molybate; 0.05 mg alu-minium chloride; 0.05 mg cobaltous chloride and0.05 mg nickel chloride. 1 ml of this solution wasadded per liter of the feed solution. The influentalkalinity and pH were 1100 ± 50 mg/L (asCaCO3) and 7.2 respectively.

2.4. Analytical methods

The analytical procedures for all tests were asoutlined in the Standard Methods for the Exami-nation of Water and Wastewater [11], unless speci-fied otherwise. Daily measurements were takenfor the influent and effluent pH, COD, rate of gasproduction and alkalinity. Influent and effluentsamples were analyzed for phenolic compoundson alternate day. Volatile fatty acids (VFA), alka-linity, and gas composition were analyzed once aweek. The sludge samples were analyzed bi-weekly for suspended solids (SS) and volatile sus-

Feed tank

Peristaltic pump

Influent

Effluent GLSS

Bio gas

Sludge blanket

Filter media

Sludge bed

Sampling ports


pended solids (VSS). The daily biogas produc-tion was measured by water displacement method.Analysis of phenolic compounds, and VFA in theeffluent and methane in biogas was carried out asper the procedure outlined in Anushyaa and Gupta[12].

2.5. Operational method

2.5.1. Start up and acclimatization

During the acclimatization phase (day 0 – day120), the hybrid UASB reactors were operatedcontinuously with a hydraulic retention time(HRT) of 24 h. Details of the operational proce-dure during the start up and acclimatization andthe results obtained during different phases of thestart up and acclimatization are outlined inAnushyaa and Gupta [12]. The reactors at the endof the start-up and acclimatization phase wereacclimatized to 752 mg/L of the complex phenolicmixture representing 10% of the synthetic coalwastewater.

2.5.2. COD/NO3–-N ratio study

All the four reactors were operated at 24 hHRT, keeping influent COD concentration as 2240± 10 mg/L (organic loading rate 2.24 ± 0.04 kgCOD/m3.d). Phenolic mixture was used as car-bon source and sodium nitrate was used as NO3

–-N source. The reactors were fed with 752 mg/Lof complex phenolic mixture. Among the fourreactors, Reactor R1 was maintained as a controland was fed with a constant amount of sodiumnitrate. Nitrate nitrogen loading in the other threereactors was increased from 0.11 g/m3/d to0.55 g/m3/d in order to keep COD/NO3

–-N ratiosas 20.3, 14.85, 9.9, 6.36 and 4.48. The reactorswere operated for 20–25 days under pseudo-steady state conditions at each COD/NO3

–-N ra-tio. Pseudo-steady state was arbitrarily consideredas variation of phenolics and COD concentrationin the effluent and biogas production within ±5%of average value.

3. Results and discussion

3.1. Effect of COD/NO3–-N ratio on the perfor-

mance of the reactors

The hybrid reactors were operated for 150 days(day 300–450) in order to investigate the continu-ous treatment of phenolic mixture under denitri-fying conditions. The operational parameters andtreatment efficiencies obtained during continuousoperation of hybrid reactors are shown in Fig. 2and Table 1.

During phase I (days 300–325), the reactorreceived nitrate nitrogen loading of 110 mg/L anda complex phenolic mixture at a concentration of752 mg/L corresponding to a COD of 2240 mg/L.The COD/NO3

–-N ratio for this period was 20.1.During the first 10 days of operation, the reactorgot acclimatized to take up the nitrate fed waste-water. Once the seed sludge mixture was adaptedto the nitrate loading in the influent, the reactorshowed high removal efficiencies by day 320. Ascan be seen in Figs. 3 and 4, phenolics and CODremoval efficiencies of 90% could be achieved.Nitrogen production in this stage was 210 ml rep-resenting 84.18% of theoretical nitrogen produc-tion (TNP). At this COD/NO3

–-N ratio, the perfor-mance of the reactors towards the simultaneousbiodegradation of complex phenolic mixture wassatisfactory. Methane was produced at the end ofphase I indicating that a simultaneous denitrify-ing/methanogenesis process was taking place.Nitrite was not detected in the effluent through-out the phase. Residual nitrate nitrogen in the ef-fluent averaged 20 mg/L, recording a nitrate re-moval efficiency of 82% at the end of phase I(Fig. 5).

In phase II (days 326–351), the nitrate nitro-gen loading was increased to 151 mg/L at an in-fluent phenolics concentration of 752 mg/L (cor-responding COD concentration of 2240 mg/L-d.).The COD/NO3

–-N ratio for this period was adjustedto 14.85. The reactors took 7 day time to adapt toincreased nitrate loading, as the seed sludge wasnot exposed to nitrates prior to this study. After


Table 1Experimental data obtained under steady state conditions at five COD/NO3

–-N ratios

Abbreviations: HRT = hydraulic retention time, SRT = sludge retention time

Effluent COD/NO3-N ratio

Parameters Influent

4.45 ± 0.8 6.36 ± 0.2 9.9 ± 0.5 14.85 ± 0.5 20.1 ± 1.4 Operating conditions — — — — — — HRT, h — 24 24 24 24 24 SRT, d — 124 104 63 58 47 pH 7.2 ± 0.2 7.57 ± 0.2 7.8 ± 0.2 7.452 ± 0.3 7.39 ± 0.4 7.30 ± 0.3 COD, mg/L 2240 ± 20 181.6 ± 2.4 185.2 ±1.1 179.2 ± 5.8 111.18 ± 1.2 233.6 ± 1.5 COD removal, % — 11.07 95.75 95.03 92 89.57 Biogas (total), L/d 5.533 ± 2.30 11.95 ± 2.1 10.5 ± 1.2 10.155 ± 1.1 9.644 ± 1.8 Alkalinity (as calcium carbonate), mg/L

1100±25 1438.8 ± 3.6 1625 ± 4.2 1600 ± 2.2 1588.8 ± 3.4 1543.3 ±3.4

Total phenolics, mg/L 752 391± 2.5 1.72 ± 1.5 37.6 ± 2.5 53.7± 3.8 75.2 ± 1.2 Total phenolics removal, % — 48.00 ± 1.8 99.77 ± 0.1 95.00± 0.5 92.85± 2.8 90.00± 3.5 Nitrogen, ml 154 245 232 220 210 SS in effluent, mg/L — 26.28 ± 1.4 56 ± 3.05 58 ± 2.2 69.14 ± 4.09 49.71 ± 5.9 Sludge characteristics — — — — — — Median diameter, mm — 1.6 1.7 1.8 2.1 2.25 Mean diameter (± S.D.), mm — 1.56 ± 0.38 1.7 ± 0.65 1.82 ± 0.35 2.12 ± 0.8 2.18 ± 0.5 SVI, ml/g.SS — 14 12 12 11 10

day 334 of phase II, the phenolics and COD re-moval efficiencies increased to 92%. At this COD/NO3

–-N ratio, the performance of the reactors to-

wards the simultaneous biodegradation of com-plex phenolic mixture was satisfactory Simulta-neous methanogenesis and denitrification was ob-

Fig. 2. Operational parameters during COD/NO3–-N ratio study.


served in the reactors. Methane was produced atthe end of phase II indicating the active role ofmethanogens in the biodegradation of phenolicmixture. Nitrogen production in this stage was220 ml representing 88.19% of TNP. Nitrite wasnot detected in the effluent throughout the phaseResidual nitrate nitrogen in the effluent averaged

Fig. 3. Variation of influent and effluent phenolics at varying COD/ NO3–-N ratios.

Fig. 4. Variation of influent and effluent COD at varying COD/ NO3–-N ratios.

to 10 mg/L recording a nitrate removal efficiencyof 93.3% at the end of phase II.

In phase III (days 352–377), the nitrate nitro-gen loading was increased to 226 mg/L at an in-fluent phenolics concentration of 752 mg/L (cor-responding COD concentration of 2240 mg/L).The COD/NO3

–-N ratio for this period was adjusted


to 9.9. Increase in nitrate nitrogen loading favoredboth denitrification and methanogenesis. After day5 of phase III, the phenolics and COD removalefficiencies increased to 95%. At this COD/NO3

–-N ratio, the performance of the reactors towardsthe simultaneous biodegradation of complex phe-nolic mixture was satisfactory Methane was pro-duced at the end of phase III which revealed thatmethanogenesis was active in the reactors. Nitritewas not detected in the effluent throughout thephase. Nitrogen production in this stage was 232ml representing 93% of TNP. Nitrate nitrogen inthe effluent was 5 mg/L recording a high nitrateremoval efficiency of 98% at the end of phase III.

In phase IV (days 378–403), the nitrate nitro-gen loading was increased to 352 mg/L at an in-fluent phenolics concentration of 752 mg/L (cor-responding COD concentration of 2240 mg/L).The COD/NO3

–-N ratio for this period was adjustedto 6.36. The increased nitrate loading resulted inan increased removal of COD and phenolics(99%) (Table 1). This COD/NO3

–-N ratio was mostfavourable for the simultaneous biodegradationof complex phenolic mixture. Methane produc-

Fig. 5. Effluent nitrate levels of reactors during different COD/NO3–-N ratios.

tion at the end of phase IV indicated the role ofmethanogens in the phenolics degradation. Nitritewas not detected in the effluent throughout thephase. Nitrogen production in this stage was245 ml representing 98% of TNP. Residual nitratenitrogen content in the effluent was less than5 mg/L at the end of period IV.

In phase V (days 404–450), the nitrate nitro-gen loading was increased to 500 mg/L at an in-fluent phenolics concentration of 752 mg/L (cor-responding COD concentration of 2240 mg/L).The COD/NO3

–-N ratio for this period was adjustedto 4.48. As can be seen from Fig. 5, the perfor-mance of the reactors was disturbed due to theincreased nitrate nitrogen loading, which suggestsan inhibitory effect of the nitrate towards theanaerobic biodegradation of biomass. At thisCOD/NO3

–-N ratio, methane production declined.This showed that denitrifiers out competedmethanogens in the degradation of phenolic com-pounds. COD and phenolics removal efficienciessharply decreased to 11% and 48% respectively.This COD/NO3

–-N ratio was not favourable for thesimultaneous biodegradation of complex phenolic


mixture. Residual nitrate nitrogen content in theeffluent increased to 120 mg/L, with a nitrate re-moval efficiency of 76% at the end of phase V.This was because the wastewater did not havesufficient phenolic compounds for complete deni-trification. Nitrogen production in this stage de-clined to 154 ml representing 62% of TNP.

3.2. Simultaneous denitrification and methano-genesis in hybrid UASB reactors

Denitrification and methanogenesis occurringsimultaneously during the degradation of phenoliccompounds are mediated by different microbialpopulations, which require distinct environmen-tal conditions for an integration of these processes.In reactors treating nitrate containing wastewa-ter, denitrifiers and methanogens compete for elec-trons producing nitrogen and methane, respec-tively. Electron flow to methanogenesis and deni-trification were dependent on the COD/NO3

–-Nratio. The fraction of electron flow to methano-genesis increased with the COD/NO3

–-N. WhenCOD/NO3

–-N ratios were low, meaning a short-age of electron supply, denitrifiers would utilizeall electrons available, and only the leftoverswould be utilized by methanogens. This is becausedenitrification is thermodynamically more favour-able than methanogenesis [13].

Nitrate nitrogen removal was more than 80%when nitrate nitrogen loading was up to 0.35 g/L,

Table 2Denitrifying efficiency of hybrid reactors at different COD/NO3

–-N ratios

Abbreviations: SDR = specific denitrifying rate

COD/NO3–-N ratio NO3-N loading rate

(g/L/d) NO3-N removal (%)

SDR (mg N2/g VSS.d)

20.1 0.11 81.8 2.14 14.85 0.15 93.3 3.20 9.9 0.22 97.7 4.8 6.36 0.35 98.5 7.22 4.48 0.5 76 10

corresponding to a COD/ NO3–-N ratio of 6.36.

Table 2 shows the denitrifying efficiency of thehybrid reactors at different COD/NO3

–-N ratios.Denitrifying efficiency increased with decreaseof COD/NO3

–-N ratio, which signifies that deni-trification out-competed methanogenesis at lowerCOD/NO3

–-N ratio. Nitrite was never detected inthe effluents. None of the phenolic compounds inthe complex mixture had an adverse effect ondenitrification. They could be removed simulta-neously at COD/NO3

–-N ratios up to 6.65 (Figs. 6and 7). Reduction of COD/NO3

–-N ratio to 4.45resulted in a loss of biomass due to incompletedenitrification and methanogenesis which resultedin the rising of sludge bed.

The stoichiometric COD/NO3–-N ratios for

complete degradation of phenolics was 6.65 inthe present study. As COD/NO3

–-N dropped to4.48, endogenous metabolism was observed,which resulted in substrate insufficiency on thesurface of the biogranules, which drastically re-duced the phenolics removal. Methane produc-tion would cease to take place at a COD/NO3

–-Nless than 4.48 for the degradation of complex phe-nolic mixture, indicating that below this stoichio-metric COD/NO3

–-N ratio, all COD would be con-sumed by denitrifiers.

Denitrification resulted in the generation ofalkalinity. Thus the effluent from all the four re-actors was having alkalinity more than that of theinfluent (Table 1). As the COD/NO3

–-N was re-


duced, alkalinity levels in the effluent started in-creasing. Alkalinity generated per g of NO3

–-Ndenitrification at optimum COD/NO3

–-N ratio of6.65 in the present study was calculated to be 4.61,

Fig. 6. Removal of phenols and cresols at varying COD/NO3-N ratios. Roman numerals in square boxes indicate differentphases of operation.

Fig. 7. Removal of dimethyl phenols at varying COD/NO3-N ratios.

which is higher than the theoretical value of 3.57.Karim and Gupta [14] have reported a value of 4,which is comparable to the value obtained in thepresent study. Higher alkalinity generation can be


attributed to CO2 (byproduct) conversion to bi-carbonate due to high pH condition in the reac-tors. Similar conversion of CO2 (byproduct) to bi-carbonate under high pH condition inside a UASBreactor has been reported by Chui et al. [15].

3.3. Characteristics of granular sludge

Granulation or granule formation is the pro-cess of microbial adhesion which involves attach-ment of a bacterial cell surface to another. Ini-tially granular biomass inside the reactors, weretypical black in colour at COD/NO3

–-N ratios of20.1:1, 14.85:1, 9.9:1, 6.36:1, but got changed tolight brown color at lower COD/NO3

–-N ratio of4.45:1 due to the lack of substrate on the surfaceof the sludge. As COD/NO3

–-N ratio was lowered,a change in granule surface morphology could beobserved. At the COD/NO3

–-N ratio of 4.45:1,granules got disintegrated. The granular sludgedeveloped inside the system had a median diam-eter in the range of 1.6–2.25 mm (typically rang-ing between 1.0–4.00 mm) and was well settle-able with a SVI of 10–14 ml/g.SS.

4. Conclusions

The present study revealed that the removalof complex phenolic mixture from coal wastewa-ter improved with the lowering of COD/NO3

–-Nratio. Specific conclusions are as follows:• The optimum COD/NO3

–-N ratio for maximumCOD and phenolics removal was about 6.36in the hybrid UASB reactor.

• COD removal increased from 89.57% to95.75% with the reduction of COD/NO3

–-Nratio from 20.1 to 6.36, which drastically re-duced to 11.57% upon descent of COD/NO3

–-N ratio to 4.45.

• Phenolics removal increased from 90% to 99%with the reduction of COD/NO3

–-N ratio from20.1 to 6.36, which drastically reduced to 48%upon descent of COD/NO3

–-N ratio to 4.45.• Simultaneous removal of phenolic compounds

could be observed at all COD/NO3–-N ratios.

At the C/N ratio of 4.45, removal efficiencyof phenolic compounds started declining.

• As COD/NO3–-N ratio was lowered, a change

in granule surface morphology could be ob-served. At the COD/NO3

–-N ratio of 4.45, gran-ules got disintegrated. The granular sludgedeveloped inside the system had a median di-ameter in the range of 1.6–2.25 mm (typicallyranging between 1.0–4.00 mm) and was wellsettleable with a SVI of 10–14 ml/g.SS.

Acknowledgement

This research work was supported by Univer-sity Grants Commission, Government of India,New Delhi.

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