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Membrane-integrated hybrid system for the effectivetreatment of ammoniacal wastewater of coke-makingplant: a volume reduction approachRamesh Kumara & Parimal Palaa Environment and Membrane Technology Laboratory, Chemical Engineering Department,National Institute of Technology Durgapur, Mahatma Gandhi Avenue, Durgapur, 713209 WestBengal, IndiaPublished online: 10 Mar 2014.

To cite this article: Ramesh Kumar & Parimal Pal (2014) Membrane-integrated hybrid system for the effective treatment ofammoniacal wastewater of coke-making plant: a volume reduction approach, Environmental Technology, 35:16, 2018-2027,DOI: 10.1080/09593330.2014.889760

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Environmental Technology, 2014Vol. 35, No. 16, 2018–2027, http://dx.doi.org/10.1080/09593330.2014.889760

Membrane-integrated hybrid system for the effective treatment of ammoniacal wastewaterof coke-making plant: a volume reduction approach

Ramesh Kumar∗ and Parimal Pal

Environment and Membrane Technology Laboratory, Chemical Engineering Department, National Institute of Technology Durgapur,Mahatma Gandhi Avenue, Durgapur, 713209 West Bengal, India

(Received 24 October 2013; accepted 26 January 2014 )

Nanofiltration (NF) of ammoniacal wastewater containing phenol and cyanide has been investigated for effective separation ofthese hazardous pollutants and for the subsequent downstream chemical treatment resulting in valuable by-product generation.Four different types of composite polyamide commercial NF membranes (Sepro, USA) were tested under different operatingconditions including transmembrane pressure and recovery rate (RR). At a transmembrane pressure of 15 bar, the achievedrejection of cyanide and phenol were 95% and 93%, respectively (concentrated stream) when the permeate contained 85% ofammonium-N. A high flux of 120 L m−2 h−1 was achieved during NF at a concentrated mode, with a volumetric cross-flowrate of 800 L h−1 at a pH of 10.0. The RR was 60% for the NF1 membrane. Fenton’s reagents (7.0 and 3.75 g L−1 H2O2 andFeSO4 · 7H2O, respectively) were used to degrade more than 99% of pollutants present in the concentrated stream. In thepermeate side, 97% of NH+

4 −N was precipitated out as struvite by using Mg2+ : NH4 : PO+4 in 1:1:1 molar ratio at pH 9.0.

Keywords: coke wastewater; nanofiltration; ammonium-N; struvite; membrane-integrated system

IntroductionWastewater from coke-manufacturing plants containsammonium-N, cyanide and phenolic compounds. There areother industrial processes as well such as oil refineries,petrochemical plants, ceramic plants, steel plants, coal con-version processes and coal-based power plants which alsogenerate aqueous effluents containing high concentrationsof ammonium-N, cyanide and phenolic compounds.[1]Effective treatment of wastewater from coke plants isextremely difficult due to the highly complex nature of thewaste-bearing stream. Besides the very high NH+

4 −N con-tents, there are also toxic compounds such as cyanide andphenol that render the frequently used treatments for ammo-niacal water by nitrification/denitrification as the biologicaltreatment [2,3] very inefficient. Due to toxicity associatedwith cyanide, phenol and NH+

4 −N, several countries andenvironment protection agencies have set limiting standardsfor their discharging. In India, the Central Pollution Con-trol Board has set a minimal national standard limit forindustrial effluent as 0.2, 0.5 and 50 mg L−1 for cyanide,phenol and NH+

4 −N, respectively (Indian Standard).[4]Individual biodegradation of phenol, cyanide, thiocyanateand NH+

4 −N is well proven, when wastewater containscombinations of all these compounds, biological removalof individual compound can be affected by the presenceand concentration of other pollutants in wastewater. Phe-nol and cyanide when present in combination each had a

∗Corresponding author. Emails: rameshibt@gmail.com, rameshnit13@gmail.com

negative effect on the others removal in a rotating biologicalcontactor.[5] Attempts have been made for the removalof these pollutants from wastewater by several methodssuch as biological treatments combined with steam/airstripping,[6] reverse osmosis [7] and ozonation.[8] How-ever, treatments based on these methods have often failedin successful removal of cyanide and the other majorpollutants from wastewater and the drawbacks are welldocumented.[9,10] For example, the main drawback ofalkaline chlorination, one of the widely used processes,is the formation of highly toxic cyanogen chloride whichalong with the residual chlorine, creates even more haz-ardous environmental pollutants. The use of activatedcarbon is limited due to its high cost and requirement ofwell-treated and high-quality activated carbon. Althoughbiological methods are gaining importance as potentiallyinexpensive and environmental friendly alternatives, it maynot be possible to treat wastewaters with high concentra-tions of cyanide, phenol and NH+

4 −N.Reverse osmosis involves the use of high pressure and

expensive membranes. Fouling problem is another big hin-drance in adopting the reverse osmosis method. In therecent years, attempts have been made to recover cyanidefrom industrial wastewater by using gas-filled microporousmembranes.[11] But poor flux is the major drawback of suchreported studies. Nanofiltration (NF) appears as an adequatesolution due to the decrease in osmotic pressure associated

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with solutes partial retentions, which is useful for the con-finement of multi-component solutions, like coke wastew-ater containing cyanide, phenol and NH+

4 −N.[12] Bodaloet al.,[13] have applied the NF membranes to reduce the phe-nol concentration up to 80%. By using appropriate NF mem-branes, the NF technique appears to be promising if operatedin a suitable module avoiding fouling.[14] Moreover, sep-aration through NF can be performed at relatively lowtransmembrane pressure and using less-expensive mem-branes. The pressure-driven membrane separation process,namely, NF has been playing a major role in the develop-ment of advanced wastewater treatment and its performanceis highly dependent on the solution composition and charac-teristics, like pH as both sieving as well as Donnan exclusionmechanisms work in effecting separation of components.NF has the unique feature of selective permeation very closeto the level of reverse osmosis without using high operatingpressure.[15] In complex steel-making industry, a treatmentplant of coke wastewater often fails to attract desired atten-tion as it does not directly produce any revenue-earningproduct and the conventional treatment steps are quite slowand involve uncertainty. NF has been noticed considerablyin recent years due to its higher efficiency, consumptionof less energy and application to form point-of-use, whencompared with reverse osmosis membranes.

Whenever there is a chemical treatment of industrialwastewater, a volume reduction of the total wastewaterstream prior to such chemical treatment will always saveon cost for chemicals. In this research work envisagesthe possible use of composite polyamide NF membranes(NF1, NF2, NF3 and NF20) as a fractionation techniquefor coke wastewater for generating two streams, namely astream containing mainly cyanide and phenol and anotherstream containing mainly NH+

4 −N. The treatment of thestream enriched with cyanide and phenol generally involvedthe chemical treatment, whereas the stream enriched withNH+

4 −N was treated with magnesium and phosphatesalts resulting in the formation of magnesium ammonium

phosphate (MAP, MgNH4PO4 · 6H2O) popularly known asstruvite. Struvite is a white inorganic crystalline substanceand is considered as a valuable slow-release fertilizer foragricultural use.[16] So long for recovery of these nutri-ents, only sophisticated techniques are used to be consideredat quite high cost.[17] With emergence of tailor-mademembranes, possibilities of recovering the nutrients at rea-sonably low cost using membrane-based hybrid technologyneed to be explored. The present investigations are towardsthis direction only where an attempt is made to bring downthe cost of chemical treatment through a volume reductionapproach. So in this hybrid treatment scheme, the volume ofwastewater is reduced by microfiltration (MF) and NF andthen subjected to the chemical treatment. In the hybrid treat-ment scheme of industrial wastewater this kind of approachhas not yet been reported in the literature.

Materials and methodsRaw wastewater and characterizationThe real wastewater used in the experiments was collectedfrom the Durgapur Project Limited (ammoniacal liquortank), Durgapur, West Bengal, India. Table 1 gives someparameters of the wastewater sample influent and finaleffluent concentrations with respect to permissible limit.The phenol content was determined by high-performanceliquid chromatography (Agilent Technologies 1200 series,USA) with Zorbax SB-Phenyl column (Germany) havinga mobile phase of methanol:water (70:30) at the flow rate1 mL min−1, residence time of 3.567 min and injection vol-ume of 5 μL. Cyanide, ammonium-N, chloride, fluoride andpH concentrations were determined by Orion 4 Star pH.ion-selective electrodes bench top ion meter (USA) withrespective electrodes. Total dissolved solids (TDS), conduc-tivity and salinity were measured by InoLab Cond 720, withelectrode TetraCon 325, WTW, Germany. Chemical oxy-gen demand (COD) and biochemical oxygen demand weremeasured by COD Vario Tube Test MR (0–1500 mg L1)

Table 1. Physico-chemical characterization of the coke wastewater obtained from Durgapur Project Limited,West Bengal, India.

Parameter (mg L−1) Influent Final effluent after chemical treatment Tolerance limit [4]

Cyanide 124 BDLa 0.2Phenol 162 BDLa 0.5Ammonium-N 1940 45 50COD 3850 250 250TDS 10420 2500 2100Total carbon 1150 4.2 –Total organic carbon 250 1.1 –Chloride ion 5930 181 <1000Fluoride ion 110 0.3 <1.5Oil & Grease 50 N.D –Conductivity (mS cm−1) 9.12 5.5 –pH 9–9.5 6.5 5.0–9.0

aBelow detection limit.

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of COD analyzer of LoviBond and Oxitop (WTW) kits(Germany), respectively. Dissolved oxygen was measuredby InoLab oxi 730, with electrode Cellox 325. Alkalin-ity (bicarbonate ion), phosphate, magnesium, nitrate, oiland grease were determined following the standard proce-dures described in the standard methods American PublicHealth Association.[18] Total carbon and total organic car-bon were determined by a TOC analyzer (Aurora 1030,USA.). During NF, percentage removal of pollutants wascalculated using the initial concentration (Ci) of that pollu-tants in the feed sample and the residual concentration (Cf )in the permeate, respectively, using the following equation:

Removal of pollutants =(

1 − Cf

Ci

)× 100. (1)

Standards, reagents and membranesAll the chemical reagents were of reagent grade and nofurther purification was done. Cyanide, chloride, fluorideand ammonia standard solutions (1000 mg L−1) were pro-cured from Merck (Germany). The chemicals like methanol(Sigma-Aldrich, WI, USA) for calibrating phenyl column,and phenol standard (Merck) were also used. Their workingsolutions of required concentration were prepared in deion-ized water from the Milli-Q purification system (Waters,MA, USA) by serial dilution for calibrating the cyanide andammonia electrodes and phenyl column. Phenol, sodiumhydroxide pellets, HCl, ferrous sulphate, hydrogen perox-ide and other chemicals used during analysis were procuredfrom Merck India Ltd. Thin-film composite polyamideNF (NF1, NF2, NF3 and NF20) as well as poly (vinyli-dene fluoride) (PVDF) MF membranes were procured fromSepromembranes Inc. of USA. Some major characteristicsof the membranes used are given in Table 2.

SEM-EDS, XRD, FT–IR, TGA analysis for struviteThe content of struvite in precipitates during NH+

4 −Nprecipitation has been confirmed through characterization

by scanning electron microscopy with energy-dispersiveX-ray analysis (SEM-EDS, S-3000, Hitachi Japan), X-raydiffraction (XRD, D/max-RB, Rigaku, Japan), thermo-gravimetric analysis (TGA, DTG-60H, Shimadzu, Japan)and Fourier transform infra-red (FT–IR, Nicolet iS10,Thermo Fischer Scientific, USA).

Experimental set-upThe experimental set-up was made up of SS-316 material toavoid rusting and consisted of a stirred stainless steel feedtank and two cross-flow membrane modules for MF and NFmembranes. The membrane modules were flat-sheet cross-flow type which is known for largely foul-free operation.Each membrane module had an effective membrane surfacearea of 100 cm2. A peristaltic pump (ENER TECH elec-tronics Pvt. Ltd., maximum output 350 L h−1) was used topump the wastewater from the reservoir to the membranemodule. The microfiltrate was collected in the temporaryholding tank which was attached to the NF feed tank. Adiaphragm pump (Milton Roy India (p) limited, maximumoutput 800 L h−1) was used to circulate the feed samplethrough the module for NF. Permeate of NF was collectedin the separate tank containing ammoniacal wastewater. Theretentate containing enriched cyanide and phenol concen-trations was collected in a separate tank. The upstream anddownstream pressure gauges indicated respective pressurevalues in both MF and NF modules. The cross-flow ratethrough the system was regulated by a rotameter and con-trol valves. Schematic diagram of the experimental set-upis shown in Figure 1.

Experimental procedureCoke wastewater collected from a coke-oven unit of a coal-based thermal power plant (Durgapur Projects Limited,Durgapur, West Bengal, India) was used as the feed solu-tion for this study. Raw wastewater from a 25-L reservoirwas continuously passed to the MF membrane module firstand the permeate from this MF was subsequently passed

Table 2. Characteristics of the composite, flat-sheet polyamide NF membranes (Sepro, USA) having thickness 165 μm, used:

Membranes

Characteristics NF1 NF2 NF3 NF20

Solute Rejection (%)MgSO4 99.5 97 98 98NaCl 90.0 50 60 35pH 2–11 2–11 2–11 2–11Maximum temperature (◦C) 50 50 50 50Maximum pressure (bar) 83 83 83 83Material Polyamide Polyamide Polyamide PolyamidePore size (nm) 0.53 0.57 0.55 0.54Membrane surface area used (m2) 0.01 0.01 0.01 0.01Molecular weight cut-off (g mol−1) 150–250 250–300 250–300 200–300Flux (L m−2 h−1) at 15 bar pressure 120 295 133 125

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Figure 1. Schematic diagram for the NF membrane-integrated hybrid treatment system.

to the flat-sheet cross-flow NF membrane module. TheNF experiments were carried out in a concentration mode,where only the concentrate is recirculated to the feed tankwhile permeate is continuously removed. The operating pHis maintained at 10.0 which enables the confinement ofcyanide and phenol-enriched concentration in the retentedside and treated with Fenton’s reagents (effluent 1). Thepermeate side containing ammoniacal water was collectedin the MAP by-product unit having the capacity 25 L andtreated with magnesium and phosphate salts to produce stru-vite (effluent 2). When stirring was stopped the precipitate(struvite) settled down and was recovered by opening thevalve. The permeate fluxes as well separation of pollutantswere monitored in function of wastewater recovery rate(RR), defined as

RR =Permeate volume collectedInitial feed volume

× 100.

To attain a steady state the system was run for 30 min aftercharging the feed sample to the tank. Analysis of cyanide,phenol and NH+

4 −N was done in feed and after NF.

Results and discussionFractionation of pollutants present in coke wastewaterby NFEffect of transmembrane pressure and cross-flow rate onpermeates flux: in the recirculation modeCritical levels of cross-flow rates and transmembrane pres-sure are obtained as these values permit maximization offlux for a given membrane surface area. Effect of trans-membrane pressure and cross-flow rate was determined forthe four membranes with real coke wastewater. The flux

Figure 2. Effect of transmembrane pressure and cross-flowrate on flux for all the four membranes. Operating conditions:feed microfiltrate real coke wastewater, pH = 10, at ambienttemperature.

data presented in Figure 2 show that the permeate flux wasincrease with the increase of transmembrane pressure from5 to 15 bar and cross-flow rate from 350 to 800 L h−1 for

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all four membranes. NF2 membrane exhibited the high-est flux 295 L m−2 h−1 followed by NF3, NF20 and NF1,respectively. NF2 membrane was the loosest type and henceit gave the maximum flux compared with the other threemembranes. Whereas NF1 is the tightest in nature and soflux is the lowest, 120 L m−2 h−1. The flux by the NF mem-brane depends upon on compactness of the membrane andpore size. The pore sizes of the NF1, NF2, NF3 and NF20membranes are 0.53, 0.57, 0.55 and 0.54 nm, respectively.So the flux was achieved at NF2, NF3, NF20 and NF1in order. The cross-flow rate also plays an important rolein reducing membrane fouling and optimization of mini-mizing membrane area requirement. Increase in cross-flowrates increases flux; this is because of greater convectiveforce that minimizes coke formation and concentrationpolarization effects.

Effect of transmembrane pressure on the rejectioncoefficient of pollutants: in the recirculation modeEffect of transmembrane pressure on the rejection ofcyanide, phenol and ammonium-N by the four membraneswas determined with the feed wastewater containing 124,162 and 1940 mg L−1 of cyanide, phenol and NH+

4 −N,respectively, present in the coke wastewater. The natureof the membranes in terms of above pollutants removal isshown in Figure 3, in the recirculation mode, cyanide andphenol rejection increased with increase in applied pressure.There was very little effect of pressure on NH+

4 −N rejection.This may be ascribed to the solution diffusion mechanismthat applies to NF. In the solution diffusion mechanism,solute flux and solvent flux are uncoupled and hence anincrease in solvent flux with an increase in transmembrane

Figure 3. Effect of transmembrane pressure on the rejectionof cyanide, phenol and ammonium-N in recirculation mode byusing four NF membranes. Operating conditions: feed real cokewastewater, cross-flow rate = 800 L h−1, pH = 10; at ambienttemperature.

pressure does not result in increase in solute flux. Ratherincrease in solvent flux obstructs the transport of solutethrough the membrane. Transmembrane pressure leads toan increase in solvent flux.[19] The NF1 membrane wasthe best-performing membrane in terms of cyanide andphenol removal followed by NF3, NF20 and NF2, respec-tively. More than 95% of cyanide and 93% of phenol wererejected by the NF1 membrane at 15 bar transmembranepressure and 800 L h−1 cross-flow rate. The rejection pat-tern by four NF membranes (NF1 > NF3 > NF20 > NF2)was not exactly reciprocal from that of flux (NF2 > NF3 >

NF20 > NF1). This is because the rejection by NF mem-branes depends not only on the size exclusion mechanismbut also on the membrane charge density (Donnan exclu-sion). NF1 gives a much better rejection than NF2, NF3 andNF20 because of its higher charge density.[19] At higherpH values, the addition of sodium hydroxide leads to anincrease in osmotic pressure and ionic strength, thus reduc-ing the membrane permeability and increasing rejection.High pH results in an increased thickness of the doublelayer of the charged functional groups over the surface of themembrane thus reducing the apparent pore size and result-ing in greater rejection of the charged solutes.[20] Thus atpH values greater than 7.0 most of the polyamide compositeNF membranes possess negative zeta potential.[21] Whennegatively charged cyanate ions and phenolate ion at pH10 come in contact with the negatively charged membranesurface, charge repulsion occurs and this results in rejectionof cyanide. But at high pH, ammonia was neutral in nature,so rejection was very less.

NF experiments to concentrate the streamAfter optimization of the transmembrane pressure andcross-flow rate, the NF experiments were carried out in theconcentration mode, i.e. only the concentrate was recircu-lated to the feed tank, while the permeate-treated water wascollected. In Figure 4 four different types of NF membranes

Figure 4. Decline of flux with respect to RR (%) in the con-centrate mode by using four NF membranes (NF1, NF2, NF3and NF20). Operating conditions: feed real coke wastewater,cross-flow rate = 800 L h−1, transmembrane pressure = 15 bar,pH = 10; at ambient temperature.

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Figure 5. Variations of cyanide, phenol and ammonia rejec-tion of four NF membranes (NF1, NF2, NF3 and NF20)with the RR (%). Operating conditions: initial concen-tration of cyanide = 124 mg L−1, phenol = 162 mg L−1,ammonium-N = 1940 mg L−1, membrane surface area =0.01 m2, feed circulation rate = 800 L h−1, transmembranepressure = 15 bar and at ambient temperature.

show variation of permeate fluxes and solute rejectioncoefficients up to a maximum recovery ratio (80%). Thepermeate fluxes remain practically constant at the valueof 102, 273, 122 and 108 L m−2h−1 by NF1, NF2, NF3and NF20, respectively, for the RR of 60%, then the fluxesgradually decreases. The variation of rejection coefficientswith RR for four NF membranes is shown in Figure 5, therejection coefficient of cyanide, phenol and ammonium-Nwas constant at values of 92%, 89.5% and 15.5% by NF1;69%, 67% and 11% by NF2; 82.8%, 81.5% and 14.2%by NF3 and 75.6%, 73% and 13% by NF20, respectively.The NF1 membrane was the best-performing membrane interms of cyanide and phenol rejection and permeation ofammonium-N in the flux side up to RR 60%. The pH hasthe significant role on the removal for cyanide, phenol andammonia, as speciation of these three compounds changeswith pH of the medium. In fact, the decrease in pH leads

to the displacement of solute equilibria towards the ammo-nium ion (NH+

4 ), HCN and phenol species. Cyanide existsessentially in the form of HCN at pH below 9.3 (pKa)under non-oxidizing conditions.[22] At pH 10, phenol wasconverted into phenolate ion (pKa value of phenol is 9.9)carrying the negative charge and cyanide as the CN− ion. Asthe pH was increased to 10, increased further the retention ofcyanide and phenol due to Donnan exclusion. This enrichedthe cyanide and phenol in the retentate side and allowedpurification of the ammonium-N. The NF membranes aremostly negatively charged, rejection of cyanide and phenolseems to be affected by the charge valences of cyanate ionand phenolate ion in the solution (Donnan exclusion). Theapparent pore size of polyamide NF membranes can alsovary with the solution pH. At the pore surface point of zerocharge (isoelectric point), the membrane functional groupsare minimal in charge and hence open up, as the absenceof repulsion forces contributes to the widening of the mem-brane pores. At high or low pH value, functional groupsof the membrane polymer can dissociate and take on posi-tive or negative charge functions. Repulsion between thesefunctions in the membrane polymer reduces or closes upthe membrane pores. Braghetta [23] has shown the effect ofsolution pH and ionic state on apparent pore size of mem-branes. At high ionic strength and high pH, apparent poresize reduces remarkably. Most of the organic and inorganicsolutes were rejected due to NF, thus lowering the value ofoil and grease, cyanide, phenol, TDS, salinity, conductivityand COD in the permeate side. But just in reverse, with theincrease in pH, there is displacement of solute equilibriatowards the NH3 neutral species from the NH+

4 ion. Thushigh pH leads to displacement towards the NH3 form, whichhas a slightly lower affinity to the NF membranes than theNH+

4 form. Variation of permeate and concentrate compo-sitions of pollutants present in the coke wastewater by fourdifferent NF membranes at 60% recovery ratio at 15 barand cross-flow rate 800 L h−1 at pH 10 are given in Table 3.The NF1 membrane was found to be the best membrane interms of fractionation of cyanide, phenol, TDS and salinityfrom NH+

4 −N.

Table 3. Variation of permeate and concentrate compositions by four different NF membranes at 60% recovery ratio, 15 bar, cross-flowrate 800 L h−1, at pH 10.

NF membranes

NF1 NF2 NF3 NF20

C P C P C P C P

Cyanide (mg L−1) 201 10 150 40 181 21 165 30Phenol (mg L−1) 250 17 215 50 240 30 225 43NH+

4 −N (mg L−1) 250 1650 210 1700 320 1630 250 1680TDS (mg L−1) 19,800 1800 16,020 5450 17,970 3960 16,920 4170Conductivity (mS cm−1) 18.8 2.0 15.5 5.6 17.0 2.6 15.0 4.0COD (mg L−1) 5590 390 3020 1450 4970 760 3930 420Salinity 10.4 1.0 8.3 4.3 9.3 2.5 8.5 3.0pH 9.7 10.5 10.1 10.2 9.9 10.4 9.9 10.1

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Investigation of foulingIt was found that the NF1 membrane suffered a minimumdecline in the flux due to fouling despite sufficient long hourof operation. Such fouling was insignificant and reversiblewhereas in the PVDF MF membranes, concentration polar-ization was significant as evident in substantial drop inthe flux over the same period of operation. During theseparation of suspended particles present in the raw cokewastewater, the possibility of fouling of the membranesby the minor components or contaminants present in thefeed side may change the physical or chemical propertiesof the membranes. If the membrane module is operatedin a dead-end mode, concentration polarizations build uprapidly, resulting in a rapid decrease in the flux. However,the building up of such concentration polarization may besubstantially reduced by the operation of the membranemodule in a cross-flow mode when the fluid travels par-allel to the surface of the membrane imparting a sweepingaction on the membrane surface and thus leaving a very lit-tle scope for the formation of the concentration polarizationlayer.[24,25] But this fouling problem cannot be completelyignored. Chemical cleaning is the most widely practicedmethod for reducing the fouling problem with the chemicalssuch as HCl, NaOH and NaOCl as an acidic, alkaline andalkali oxidizing agents to obtain the flux recovery. Out ofthese three chemical reagents, sodium hypochlorite solution(0.01 M) shows the highest flux recovery.[26] Normally theused membranes are not regenerated, but only ringed withhypochlorite solution to recover the flux, when the flux fallsdrastically.

Simultaneous degradation of cyanide and phenol byFenton’s reagentAfter NF the retented side contains high concentrationsof cyanide (201 mg L−1), phenol (254 mg L−1) and over-all COD (5590 mg L−1) up to 60% RR by NF1. To degradethe above-mentioned pollutants, Fenton’s reagent was usedwith varying concentrations of H2O2 and FeSO4 · 7H2Ofrom 1.4 to 8.4 g L−1 and 0.75 to 4.5 g L−1, respectively.Hydrogen peroxide concentration was chosen with the con-centration of COD by keeping a minimum concentration offerrous ion to avoid an additional step to remove the ironcontamination. The results are represented in Figure 6. It hasbeen observed that Fenton’s treatment has a tremendouspotential to degrade this mixed pollutants waste presentin the samples. With increase in the amount of H2O2 andFeSO4 · 7H2O though the ratio is fixed, cyanide, phenol andthe COD percentage reduction increased and the maximumpercentage reductions achieved was about >99%, >99%and 95%, respectively, for 60 min treatment time, when theamount of H2O2 (7.0 g L−1) and FeSO4 · 7H2O (3.75 g L−1)was used. The results obtained can be explained on thebasis of the fact that, with increasing the amount of H2O2and FeSO4 · 7H2O in Fenton’s reagents, the generation of

Figure 6. Degradation of cyanide, phenol and COD by dif-ferent concentrations of Fenton’s reagent during treatment ofconcentrated solution obtained by NF of coke wastewater. Condi-tions: H2O2 = 1.4–8.4 g L−1; FeSO4H2O = 0.75–4.5 g L−1 andpH = 6.0, at ambient temperature.

OH· (hydroxyl) ion increases, which effectively decreasethe pollutants from the wastewater. The reaction mecha-nism for Fenton’s reagent can be presented by the followingequations:

H2O2+Fe2+ → Fe3+ + OH− + OH·. (2)

There is a possibility of auto regeneration of Fe2+ in thissystem and acting as a catalyst

Fe3+ + H2O2 → OH2 + Fe2+ + H+. (3)

Cyanide is first oxidized to cyanate, which is furtheroxidized to ammonium and carbonate ions [27] as follows:

CN− + H2O2 → CNO− + H2O, (4)

CNO− + 2H2OH2O2−−→ NH−

4 CO2−3 . (5)

Phenol was degraded according to following reaction:

C6H6O + 14H2O2 → 6CO2 + 17H2O. (6)

The phenol oxidation produces as cyclic intermediateshydroquinone and catechol and a strong brown colour isobtained at H2O2 dosages that could be due to quinine con-densation. The cyclic intermediates are produced which canbe further oxidized to the short chain acids (acetic, formic,maleic and oxalic acids) and to CO2.[28] In Figure 7 theeffect of pH was studied by varying the pH of the solutionfrom 3.0 to 10.0. The initial concentrations of cyanide, phe-nol and COD were 201, 254 and 5590 mg L−1, respectively.H2O2 and FeSO4 · 7H2O were used at concentrations of7.0 and 3.75 mg L−1, respectively. It was observed that thehigh pH was better for degradation of cyanide but not suit-able for phenol and COD. At pH 10, cyanide was degradedup to 99.9%, but phenol was degraded only 12.5% at time30 min. At pH 3.0 the case was just reversed, 100% phenol

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and 98% COD were degraded but only 4.8% cyanide wasdegraded at the same time. Cyanide oxidation by Fenton’sreagent was highly pH dependent and at high pH cyanideis present as CN− ions so it reacts easily with H2O2 andFe2+ ions but in the acidic condition cyanide is present inHCN gas which is very difficult to oxidize. At high pH, thegeneration of hydroxyl radical (OH•) was reduced becauseof the formation of the ferric hydroxo complexes, which

Figure 7. Effect of pH, varying from 3.0 to 10.0 on thedegradation of cyanide, phenol and COD. Operating conditions:H2O2 = 7.0 g L−1, FeSO47H2O = 3.75 g L−1 and at ambienttemperature.

subsequently form {Fe(OH)4} at higher pH.[29] The crit-ical pH was found to be 6.5 at which 100% phenol andcyanide and 95.5% COD degraded.

Chemical precipitation of NH+4 −N

After NF the permeate side contained high concentrationof NH+

4 −N in the uncontaminated form, as impurities were

Figure 8. Effect of pH on the removal of NH+4 −N and production

of struvite with varying pH, keeping Mg2+ : NH+4 : PO−3

4 ratio1:1:1 fixed.

Figure 9. Surface characterization analysis of struvite (MAP precipitate) obtained during the chemical treatment of ammonium-N.(a) Energy-dispersive X-ray analysis of struvite; (b) scanning microscopy analysis of struvite; (c) XRD pattern of the struvite crystel;(d) TGA and DTGA curves for struvite for heating rate 10◦C min−1 and (e) FT–IR spectroscopy analysis of struvite.

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2026 R. Kumar and P. Pal

rejected. The NH+4 −N was recovered as struvite through the

chemical precipitation method by adding external sourceof magnesium and phosphate salts. The combination ofMgCl2 · 6H2O + Na2HPO4 · 12H2O was the most efficientin the ammonium-N removal,

MgCl2 · 6H2O + NH+4 + Na2HPO4 · 12H2O

→ MgNH4PO4 · 6H2O ↓ +2NaCl. (7)

An important factor for struvite formation is pH. The opti-mum pH was determined for maximum NH+

4 −N removalas well as struvites formation. The effect of pH on theremoval of the ammonium-N was studied by keeping molarratio of Mg2+ : NH+

4 : PO3−4 fixed at 1:1:1. As shown in

Figure 8, with the increase in the pH, NH+4 −N removal was

increased sharply as well as struvite formation. When thepH reached 9.0, maximum NH+

4 −N 97% was removed and19.0 g L−1 of struvite was produced. With further increasein the pH, ammonia removal was decreased, therefore pH9.0 may be considered as the optimum range to achievemaximum NH+

4 −N removal. When the pH was lower thanthe optimum point, i.e. pH 9.0, hydrogen ion in the reactionsolution would inhibit the struvite formation. As a result, theremoval of NH+

4 −N was lower. When the pH was higherthan the optimum point, Mg3(PO4)2 was formed insteadof struvite, resulting in the decrease in NH+

4 −N removal.Struvites formed during chemical precipitation reaction canbe used as a slow-release fertilizer for agricultural use.

The MAP precipitate was formed rapidly and settledquickly at the bottom after stopping stirring. As shown inFigure 9, the content of struvite in precipitates has beenconfirmed through characterization by SEM-EDS, XRD,TGA and FT–IR analyses. The FT–IR analysis, SEM-EDSand XRD patterns showed that the infra-red spectrum ofthe precipitate and elemental profile were close to that ofthe MAP as reported elsewhere.[30,31] The TGA and dif-ferential thermo-gravimetric analysis (DTGA) curves forstruvite at 10◦C min−1 indicate that the mass loss begins ata temperature around 55◦C and is essentially complete whentemperature reached above 250◦C and ∼51% correspondsto the following decomposition for the struvite:

MgNH+4 PO4 · 6H2O → MgHPO4 + NH3 ↑ +6H2O ↑ .

(8)

The DTGA curve of the struvite for heating rate 10◦C min−1

shows a single peak which is attained at a temperature of103◦C due to simultaneous loss of both ammonia and watermolecules which indicates that the precipitate material isstruvite.

ConclusionsDischarge of enormous quantity of complex wastewaterfrom coal-based industries results in the contamination ofsoils and natural water bodies. Existing technologies often

fail to adequately treat such complex wastewater effectively.This suggests that novel processes are required to allevi-ate this serious problem of environmental pollution. Thepresent study shows that with MF pre-step, coke wastew-ater can be effectively treated with the NF membrane. Outof four different types of NF membranes, NF1 has shownhigh efficiency in fractionating the pollutants in two dif-ferent streams with reduced volume. The effect of RR upto 80% on rejection coefficient of pollutants and flux wasalso investigated. The RR up to 60% shows little varia-tion of rejection and flux. Cyanide and phenol rejectionwas 95% and 93%, respectively, by the used NF mem-branes. In the optimized chemical process, the retentatestreams representing concentrated solutions of phenol andcyanide were successfully treated for degradation of over99% of phenol and cyanide. Over 97% of NH+

4 −N presentin the permeate side (stream 2) was precipitated out as stru-vite fertilizer by adding magnesium and phosphate salts.Thus the present study has demonstrated that coke-ovenwastewater treatment plants could be turned operationallyfast, economically more viable and environmentally benignthrough judicious integration of both micro and nano mem-brane modules to the conventional treatment units. Thefindings are expected to be very useful in design and oper-ation of the industrial scale coke wastewater treatmentplant as these are results of experimental investigationsusing actual industrial wastewater rather than syntheticsolution.

FundingThis work was supported by the Department of Science and Tech-nology, Government of India under Start-up research grant foryoung scientist (SERB) [grant number SB/FTB/ETA-59/2013].

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