Separation and Purification Technology 50 (2006) 291–299

Treatment of tanning effluent using nanofiltrationfollowed by reverse osmosis

Chandan Das, Piyush Patel, Sirshendu De, Sunando DasGupta ∗Department of Chemical Engineering, Indian Institute of Technology, Kharagpur-721302, India

Received 28 May 2005; received in revised form 21 November 2005; accepted 28 November 2005

Abstract

An investigation on the recovery of chromium from the effluent of a chrome-tanning bath has been performed using nanofiltration (NF) followedby reverse osmosis (RO). The experiments are conducted using a rectangular cross flow cell under laminar and turbulent regimes. Significant fluxenhancement is achieved using thin wires as turbulent promoters. The performance criteria are evaluated in terms of the concentration of chromium,COD, BOD, TDS, TS, pH, and conductivity of the permeate. The effects of different operating parameters on permeate flux and observed retentionof chromium are evaluated experimentally. The retention of chromium is found to be 91–98% for NF and 98.8–99.7% for RO for the experimentalconditions of this study. Concentrations of chromium and COD of the final permeate are well within the permissible limits.©

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2005 Elsevier B.V. All rights reserved.

eywords: Chromium recovery; Nanofiltration; Reverse osmosis; Retention; Turbulent promoter

. Introduction

Almost all leather made from lighter-weight cattle hides androm the skin of sheep, lambs, goats, and pigs is chrome tanned.hrome tanning is performed using a one-bath process that isased on the reaction between the hide and a trivalent chromiumalt, usually a basic chromium sulfate. In the typical one bathrocess, the hides are in a pickled state at a pH of 3 or lower, thehrome tanning materials are introduced, and the pH is raised.ollowing tanning, the chrome-tanned leather is piled down,rung, and graded for the thickness and quality, split into flesh

nd grain layers, and shaved to the desired thickness. The graineathers from the shaving machine are then separated for retan-ing, dyeing, and fat liquoring [1].

Extensive studies have been made to recycle the spent tannediquor [2,3]. But recycling of chromium solution for tanningeads to accumulation of neutral salts, which reduces the uptakef chromium during tanning [4]. Electrodialysis may be use-ul for selective separation of neutral salts from spent tannediquor. However, the economic viability of the technique iset to be established [5]. The exhausted bath from chromium

mally sent to a cleaning-up plant. Here chromium salts endinto the sludge creating serious problems for their disposal[6]. The traditional method for chromium recovery is basedon the precipitation of chromium salt with NaOH followed bythe dissolution of Cr(OH)3 in sulfuric acid [7,8]. However, thequality of the recovered solutions is not always optimal due tothe presence of metals, lipidic substances, and other impuri-ties. In order to improve the quality of the recycled chromium,an alternative method using membrane processes was studied[9–11].

Membrane-based separation processes may be an attractivealternative and they are gradually emerging as a technically sig-nificant and commercially viable ‘cleaner technology’ for thetreatment of wastewater from textile industries, leather indus-tries, paint industries, paper and pulp industries, petrochemicalindustries, etc. [12–16]. In recent years, membrane technolo-gies have been developing rapidly and their cost is continuingto reduce while the application possibilities are ever extending[17,18]. The main advantage of a membrane based process isthat concentration and separation is achieved without a change ofstate and without use of chemicals or thermal energy, thus mak-

annage contains about 30–40% of initial salt and it is nor-

∗ Corresponding author. Tel.: +91 3222 283922; fax: +91 3222 275303.E-mail address: sunando@che.iitkgp.ernet.in (S. DasGupta).

ing the process energy-efficient and ideally suited for recoveryapplications [19].

The possibility of applying membrane processes in the treat-ment of the exhausted bath of a single step offers interestingperspectives for the survival of this industry and for recovering

383-5866/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.seppur.2005.11.034

292 C. Das et al. / Separation and Purification Technology 50 (2006) 291–299

and recycling of primary resources [6]. Application of RO forremoval of unreacted chromium from spent tanning effluent wasstudied in pilot scale [20]. This was followed by a study ofnanofiltration for the recovery of Cr(III) from tannery effluentby Ortega et al. [21]. Cassano et al. have discussed in detaila general overview on the potential of membrane processesinvolving microfiltration (MF), ultrafiltration (UF), nanofiltra-tion (NF), and reverse osmosis (RO) in the treatment of aqueouseffluents from leather industry. The experimental results demon-strated that it is feasible to recuperate Cr (III), depending on themembrane type, as well as reusing the water in the process.Laboratory and industrial pilot scale experiments have demon-strated the economic advantages of recovering chromium usingmembrane processes [1,21,22]. However, application of dual-membrane systems including MF/NF, UF/RO, and NF/RO fortreatment of tannery waste and recovery of chrome and/or otherchemicals is challenged by the presence of considerable BODload and proteins, which may cause fouling and subsequent sys-tem failure temporarily or permanently [23].

In the present work, nanofiltration followed by reverse osmo-sis technique has been used to treat the chrome tanning effluent ina cross flow cell. Retentate stream of NF may be recycled to thetanning chamber after make up of the required chromium con-centration. The permeate stream of NF (which contains most ofthe natural salts) is passed through a reverse osmosis unit to getclean water and concentrated salt solution for reuse. The effectsodatm

2

2

soospbI

2

dflmnwsmks

Fig. 1. Membrane module assembly.

Sixteen equispaced thin wires of diameter 0.19 mm are placedlaterally (along the width of the channel) in between the twogaskets (shown in Fig. 1d), as turbulent promoters. The spacingbetween the turbulent promoters is 14.0 mm. The two flanges aretightened to create a leak proof channel. The effective length andwidth of the membrane available for flltration are 26 and 4.9 cm,respectively. The height of the flow channel is determined by thethickness of the gaskets after tightening the two flanges and isfound to be 0.72 mm. The obstruction in the flow path due to thewires promotes localized turbulence.

f different operating conditions, e.g., transmembrane pressurerop and cross flow on permeate flux and observed retentionre studied. The experiments are conducted in both laminar andurbulent regimes as well as using thin wires as turbulent pro-

oters.

. Experimental

.1. Membranes

The component of interest in the effluent is basic chromiculphate (2Cr(OH)(SO4) + Na2SO4) with a molecular weightf 472. Therefore, an organic thin film composite membranef molecular weight cut off 400, consisting of a polyamidekin over a polysulphone support is used for nanofiltration. Aolyamide membrane is used for reverse osmosis. The mem-ranes are supplied by Genesis Membrane Sepratech, Mumbai,ndia.

.2. Experimental set up

A rectangular cross flow cell, made of stainless steel, isesigned and fabricated. The cell consists of two matchinganges as shown in Fig. 1a. The inner surface of the top flange isirror polished. The bottom flange is grooved, forming the chan-

els for the permeate flow. The channels in the bottom flangeith the internal grid structure are shown in Fig. 1b. A porous

tainless steel plate is placed on the lower plate that providesechanical support to the membrane. Two neoprene rubber gas-

ets are placed over the membrane; the top view of which ishown in Fig. 1c.

C. Das et al. / Separation and Purification Technology 50 (2006) 291–299 293

Fig. 2. Experimental setup for cross flow membrane module.

The schematic of the experimental setup is shown in Fig. 2.The leather effluent is placed in a stainless steel feed tank of10 l capacity. A high pressure reciprocating pump is used tofeed the leather effluent into the cross flow membrane cell. Theretentate stream is recycled to the feed tank routed through arotameter. The permeate stream is also recycled to maintain aconstant concentration in the feed tank. A bypass line from thepump delivery to the feed tank is provided. The two valves inthe bypass and the retentate lines are used to vary the pressureand the flow rate through the cell, independently.

2.3. Operating conditions

Nanofiltration runs are conducted at three pressures (828,966, and 1104 kPa) with cross flow velocities of 0.47 m/s (flowrate of 1.0 lpm), 0.71 m/s (flow rate of 1.5 lpm), and 0.94 m/s(flow rate of 2.0 lpm) in laminar regime both with and with-out promoter. Cross flow velocities of 3.29 m/s (flow rate of7.0 lpm), 3.76 m/s (flow rate of 8.0 lpm), and 4.23 m/s (flow rateof 9.0 lpm) are used in turbulent regime. Reverse osmosis exper-iments are conducted at four different pressures of 1380, 1518,1725, and 1932 kPa with cross flow velocities of 3.29 (flow rateof 7.0 lpm), 3.76 (flow rate of 8.0 lpm), and 4.23 m/s (flow rate of9.0 lpm) in turbulent regime. In laminar regime, with and with-out promoter, the RO experiments are conducted at 1725 kPapressure and at 0.47 and 0.71 m/s of cross flow velocities.

2

1

2

3

4

are determined from the slopes of cumulative volume versustime plot. Permeate samples are collected at different timesfor analysis. The duration of the cross flow experiment is 1 h.

5. Determination of new permeability: Once an experimentalrun is over, the membrane is thoroughly washed, in situ, withdistilled water for 15 min applying a maximum pressure of200 kPa. The cross flow channel is dismantled next and themembrane is dipped in acid solution for 3 h. Next it is washedcarefully with distilled water to remove traces of acid. Thecross flow cell is reassembled and the membrane permeabilityis again measured. It is observed that the membrane perme-ability remains almost constant between successive runs.

2.5. Sample analysis

2.5.1. Measurement of chromium concentrationChromium present in the effluent and supernatant of sub-

sequent NaOH treatment is estimated by measuring the pinkviolate complex formed by diphenyl carbazide at 541 nm accord-ing to APHA [24].

2.5.2. Measurements of chemical oxygen demandCOD is the measure of oxygen consumed during the oxida-

tion of the organic matter in water by a strong oxidizing agent.COD value of each stream is measured by gravimetric analysis[25].

2s

bt

2

p

2

prdcoi

2

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3

tp

.4. Experimental procedure

The steps used in the experiments are as follows:

. The tannery effluent is cloth-filtered first to remove sus-pended impurities.

. Compaction of membranes: A fresh membrane is compactedat a pressure of 690 kPa for 3 h using distilled water.

. Determination of membrane permeability: Membrane per-meability is determined using distilled water. Flux values atvarious operating pressures are measured and the slope offlux versus pressure plot gives the permeability.

. Conduction of the experiments: By regulating the valve inthe bypass line, the pressure is set in the cross flow cell.Controlling the valve before the rotameter, the flow rate iscontrolled independently. Cumulative volumes of permeateare collected during the experiment. Values of permeate flux

.5.3. Measurement of conductivities and total dissolvedolid

The conductivities and TDS of all the streams are measuredy an auto ranging conductivity meter (Chemito 130, manufac-ured by Toshniwal Instruments, India).

.5.4. Measurement of pHAfter each experiment, pH of the sample is measured by a

H meter, supplied by Toshniwal Instruments, India.

.5.5. Measurement of biological oxygen demandBOD is the measure of the degradable organic material

resent in a sample, and can be defined as the amount of oxygenequired by the microorganisms in stabilizing the biologicallyegradable organic matter under aerobatic condition. The prin-iple of the method involves, measuring the difference of thexygen concentration between the sample and after incubatingt for 5 days [25].

.5.6. Measurement of total solidsTotal solids (TS) of all the samples are measured by weighing

known volume of sample in a petri dish and keeping it inn oven maintained at 105 ± 2 ◦C, till complete drying of theample.

. Results and discussions

This section is divided into two parts. In the first part, nanofil-ration of the effluent is conducted to measure the variations ofermeate flux, permeate quality at different pressures and cross

294 C. Das et al. / Separation and Purification Technology 50 (2006) 291–299

Table 1Characterization of effluent

Feed After NF After RO

Chromium concentration (ppm) 2300 57.33 0.2COD (ppm) 1150 402 60.5BOD (ppm) 442 154 23.3pH 4.1 4.44 5.6Conductivity (mho/cm) 75 53.9 4.0TDS (ppm) 49500 35.6 1.4TS (ppm) 126100 64.2 1.4

flow velocities are discussed with and without turbulent pro-moter in laminar flow and in purely turbulent flow conditions.In the second part, the performance of reverse osmosis in termsof flux and permeate quality is discussed.

3.1. Characterization of effluent

Forty litres of effluent of the basic chrome sulphate processof tanning is colleted from Alison Tannery, Kolkata, India. Thecharacterization of the pretreated effluent has been performedand the results are presented in Table 1.

3.2. Nanofiltration

3.2.1. Transient flux declineFigs. 3 and 4 represent the flux decline behavior of the efflu-

ent at 828 and 1104 kPa pressures, respectively. It can be clearlyseen from these two figures that the time required to reach steadystate decreases with increase in cross flow velocity. For example,it can be observed from Fig. 3 that the steady state is attainedin about 1520 s, at 7.0 lpm and 828 kPa pressure, whereas at thesame pressure but at 9.0 lpm, the steady state is attained within1300 s. In addition, the flux decline is about 36% of the initialvalue for 7.0 lpm, about 33% with increase in velocity to 8.0 lpm,and 24% at 9.0 lpm. As the cross flow velocity increases, thegsssiaf

Fig. 4. Transient flux data at 1104 kPa pressure in NF.

state is attained faster with an increase in operating pressure.For example in Fig. 4, steady state is attained in about 1214 sfor an operating condition of 7.0 lpm and at 1104 kPa pressure.Whereas at the same cross flow velocity, the time required toattain steady state is about 1520 s for an operating pressure828 kPa. It is also observed that the steady state is achievedfaster using turbulence promoter compared to laminar flow. Forexample, in Fig. 3, at 1.0 lpm and 828 kPa, the steady state isattained in about 2675 s without promoter and about 2046 s withpromoter at the same operating condition. The flux decline isabout 26% without promoter at 1.5 lpm and 828 kPa pressure; butonly 18% using promoter at the same operating condition. Sim-ilar trends are observed in Fig. 4 as well. Use of the turbulencepromoters creates local turbulence, thus reducing the concen-tration polarization at the membrane surface and the growth ofthe concentration boundary layer is checked quickly, establish-ing steady state earlier than the case of without promoter. Sincethe concentration polarization is reduced due to the presence ofthe promoters, the flux decline is also less than the no promotercase.

3.2.2. Steady stateThe variations of steady state permeate flux with pressure

at different cross flow rate under laminar flow and with turbu-lent promoters are shown in Fig. 5. The variations of permeateflapflioTralTs

rowth of the concentration boundary layer over the membraneurface is arrested faster. This leads to the onset of the steadytate at an earlier time. For the above reason, the resistance to theolvent flux also decreases with the cross flow velocity, result-ng in higher permeate flux. Therefore, the flux decline is lowert higher cross flow velocities as observed. It can also be seenrom Figs. 3 and 4 that at a fixed cross flow velocity, the steady

Fig. 3. Transient flux data at 828 kPa pressure in NF.

ux under turbulent regimes at different operating conditionsre shown in Fig. 6. The figures show the usual trend that theermeate flux increases with the operating pressure and crossow rate. Higher flux at higher pressure is due to enhanced driv-

ng force. The increase in flux with cross flow rate is becausef decreasing concentration polarization as discussed earlier.he percentage enhancements of the permeate flux in laminar

egime with turbulent promoters for all the operating conditionsre presented in Fig. 7. All the increases are calculated taking theaminar flow results under same operating conditions as the base.he concentration boundary layer over the membrane surface isignificantly disturbed in presence of the turbulent promoters.

C. Das et al. / Separation and Purification Technology 50 (2006) 291–299 295

Fig. 5. Variation of permeate flux with pressure drop in laminar regime in NF.

This causes reduction in the membrane surface concentrationand thereby increase in the effective driving force (�P–�π, �π

is the osmotic pressure) and hence an increase in permeate flux.It may be observed from Fig. 7 that the flux increment is in therange of 31–57% for laminar flow with promoter. The results forturbulent flow regime cannot be directly compared with laminar

Fig. 6. Variation of permeate flux with pressure drop in turbulent regime in NF.

Fr

flow results for flux enhancement calculations, as the operatingconditions are different. However, it can be clearly seen fromFigs. 5 and 6 that the permeate flux in turbulent flow at a spe-cific pressure is considerably higher than either the laminar orthe turbulent promoter enhanced cases.

A resistance in series type of model is used to analyze theexperimental results to observe the effects of the hydrodynamicconditions. Uses of resistance in series approach involving sim-ilar resistances are quite common in the membrane literature.Aimar et al. [26] have postulated that the decrease in flux duringultrafiltration is a result of two resistances, namely (i) liquidmacromolecular concentration polarization (osmotic pressureresistance) and (ii) the deposition on the membrane (gel-typeresistance). This deposited layer cannot go on increasing like acake in dead-end filtration. In cross flow mode, scouring willlimit the thickness of the deposited layer. A general approachto resistance in series model for the flux decline in membranefiltration is available (Song [27]; Ho and Zydney [28]).

The three major resistances to permeate flow are the hydraulic(membrane) resistance, osmotic pressure resistance and a gel-type layer resistance. The hydraulic resistance of the membraneis constant between the experimental runs as has been foundby the nearly constant values of the membrane permeability.An estimate of the magnitude of the osmotic pressure resis-tances is made and it has been found to be small compared tothe overall driving force for the experiments considered herein.Foptsa

hmir

s

v

wtmr

ig. 7. Flux enhancement with cross flow velocity and pressure in laminaregime with promoter in NF.

r

R

mvtff

or example, in laminar flow NF experiments, the maximumsmotic pressure for the worst polarizing conditions (maximumressure, minimum cross flow velocity) is only about 3% of theransmembrane pressure. For turbulent flow, the osmotic pres-ure contribution is about 2%. In RO, this value is even less,bout 1%.

Thus, in the present case, the gel-type layer resistance and theydraulic resistances are considered in the resistance in seriesodel and the gel-type resistances are estimated from the exper-

mental flux values. The effect of cross flow velocity on thisesistance is investigated next.

The steady state permeate flux (vw) can be written using clas-ical cake filtration theory as,

w = �P

µ(Rm + Rsp)

(1)

here Rm is the hydraulic resistance and RsP is the gel-type resis-

ance at steady state. The membrane permeability Lp is experi-entally measured using pure distilled water and the hydraulic

esistance of the membrane is calculated from the followingelation.

m = 1

µLP(2)

The value of Rm is found to be 38.46 × 1012 m−1 for the NFembrane having MWCO 400 and for the RO membrane this

alue is 12.82 × 1013 m−1. RsP are calculated from Eq. (1) using

he experimental steady state values of the permeate fluxes at dif-erent operating conditions. It has been found that Rs

P accountsor about 95% of the total resistance in the case of laminar flow

296 C. Das et al. / Separation and Purification Technology 50 (2006) 291–299

Fig. 8. Variation of the ratio of gel-type and hydraulic resistances at steady statewith the cross flow velocity during NF.

with lowest velocity, whereas even at the highest cross flowvelocity in turbulent flow Rs

P accounts for 90% of the total resis-tance. The variation of gel-type layer resistances with cross flowvelocity for some typical experiments is presented in Fig. 12.As can be observed from Fig. 8, the gel-type layer resistance Rs

Pdecreases with increase in cross flow velocity in all the cases.For example, for a transmembrane pressure drop of 828 kPain laminar flow, the ratio of the gel-type and hydraulic resis-tance reduces from 20 to 17.6 with an increase in cross flowvelocity from 0.47 to 0.71 m/s. Significant reductions in gel-type layer resistance, compared to laminar flow, are achievedusing turbulent promoters at the same cross flow velocity. At thesame operating velocity (0.47 m/s) and transmembrane pressure(828 kPa), the presence of turbulent promoters reduces the resis-tance to 12.38 compared to 20 in laminar flow. For the case withthe promoters, the gel-type layer resistance decreases signifi-cantly due to the enhanced forced convection near the membranesurface induced by the promoters. This reduction in Rs

P is morethan 61% in some of the experiments leading to a significantenhancement of the permeate flux. The figure also shows fur-ther reductions for the case of purely turbulent flows for reasonsalready discussed. Similar behavior has also been observed forthe RO experiments.

3.2.3. Permeate quality

cptivsiawbp

Fig. 9. Variation of observed retention of chromium with pressure drop in lam-inar and with promoter in NF.

rejection) improves. The effects of turbulence promoter are alsoinvestigated in the laminar flow regime. The results are sum-marized in Fig. 9. It can be seen from Fig. 9 that at 828 kPapressure and 2.0 lpm flow rate, chromium retention character-istic improves slightly (by about 3%) in presence of promotercompared to the base case (laminar at same operating condi-tions). Percentage improvement in quality is found to be in aboutthe same range as in the turbulent regime as shown in Fig. 10.Fig. 10 illustrates that the chromium retention increases withcross flow velocity and pressure for reasons already describedearlier. For example, at 1104 kPa, an increase in cross flowvelocity from 7.0 to 9.0 lpm results in an increase in chromiumretention from 95.65 to 97.8%. At 7.0 lpm cross flow rate, as thetransmembrane pressure increases from 828 to 1104 kPa, theobserved retention of chromium increases by 5.1%.

The permeate quality after NF, for various operating condi-tions, is presented in Table 2. It may be observed from the tablethat for the laminar regime, the retention of chromium variesfrom about 92 to 97.3%. The COD of the permeate remainsquite high, higher than the permissible limit (250 mg/l) in India.The conductivity of the permeate is same as the feed which sig-nifies that almost all the salt present in the feed solution has

Ft

The permeate quality is expressed in terms of retention ofhromium. Variations in permeate quality with transmembraneressure at three different cross flow velocities in laminar andurbulent regimes are shown in Figs. 9 and 10, respectively. Its observed that with increase in pressure drop and cross flowelocity, the permeate quality improves. With increase in pres-ure, the water flux increases in a linear fashion, while solute fluxs nearly independent of pressure for less open membranes (ROnd in some cases for NF membranes) [29]. The result is that,ith increasing pressure, more water passes through the mem-rane along with a fixed amount of the solute; the water is thusurer and hence the permeate quality (expressed as observed

ig. 10. Variation of observed retention of chromium with pressure drop inurbulent regime NF.

C. Das et al. / Separation and Purification Technology 50 (2006) 291–299 297

Table 2Permeate Analysis after nanofiltration

S.No. Pressure (kPa) Flow rate (lpm) TDS (ppm) TS (ppm) pH Conductivity (mS) COD (ppm) BOD (ppm) R0 of chromium

Turbulent regime1 828 7.0 42800 73.0 4.4 64.8 859.1 330.4 91.02 8.0 42500 71.4 4.4 64.4 822.4 316.3 91.53 9.0 42300 71.0 4.4 51.8 797.9 306.9 91.94 966 7.0 42300 72.0 4.4 48.6 675.5 259.8 93.65 8.0 42300 72.1 4.4 46.2 614.4 236.3 94.56 9.0 43000 69.0 4.4 43.1 569.5 219.0 95.17 1104 7.0 43000 68.3 4.4 50 532.8 204.9 95.78 8.0 43000 67.1 4.4 45.8 496.1 190.8 96.29 9.0 43500 67.6 4.4 43.6 381.9 146.9 97.8

Laminar regime10 828 1.0 42900 71.4 4.3 65 797.9 306.9 91.911 1.5 42000 71.8 4.3 58.6 740.8 284.9 92.712 2.0 42000 72.2 4.4 56.6 695.9 267.7 93.313 966 1.0 43000 69.4 4.4 51.6 712.3 273.9 93.114 1.5 42000 70.8 4.4 49.4 622.5 239.4 94.415 2.0 42000 68.2 4.4 46.3 512.4 197.1 95.916 1104 1.0 40900 67.2 4.4 61.8 577.7 222.2 95.017 1.5 40900 68.5 4.4 59.4 512.4 197.1 95.918 2.0 37300 68.2 4.4 56.6 418.6 161 97.3

With turbulent promoter19 828 1.0 37200 68.1 4.4 56.4 691.9 266.1 93.420 1.5 35600 67.7 4.4 51.1 565.4 217.5 95.221 2.0 42300 67.5 4.3 50.9 512.4 197.1 95.922 966 1.0 42600 67.1 4.3 64.5 659.2 253.5 93.923 1.5 36900 65.3 4.4 61.2 504.2 193.9 96.124 2.0 38900 65.1 4.3 58.9 426.7 164.1 97.225 1104 1.0 39200 65.1 4.3 59.4 540.9 208.1 95.526 1.5 37300 64.5 4.4 56.6 475.7 183 96.527 2.0 35600 64.2 4.4 53.9 402.3 154.7 97.5

permeated through the NF membrane. From Table 2, it can beobserved that the presence of turbulent promoters or high crossflow velocity (turbulent regime) does not affect the chromiumretention characteristics of the membrane to a great extent, ratherit contributes to the considerable increase in the permeate flux.

3.3. Reverse osmosis

The permeate from the NF is collected and treated using ROin the same cross flow cell in purely laminar, laminar with turbu-lent promoters and in turbulent conditions at different operatingconditions.

3.3.1. Transient flux declineFig. 11 presents the flux decline behavior with transmem-

brane pressure and cross flow velocity in RO. The results clearlyshow that as in the case of NF, the time required to reach steadystate decreases with increase in cross flow velocity and appliedpressure or in presence of turbulence promoters. Extent of fluxdecline also follows similar trends for reasons already discussedin the section describing NF operations. It can be observed thatthe flux decline is about 26% of the initial value at a crossflow velocity of 1.5 lpm and 1725 kPa pressure and is 20% atthe same operating conditions but with promoters. The systemreaches steady state in about 800 s with promoter compared to

about 1700 s without promoter in laminar regime. It may alsobe observed from Fig. 11 that at lower operating pressure inturbulent regime, flux decline is marginal; whereas at higheroperating pressure, it is significant due to concentration polariza-tion effects. For example, flux decline is about 26% at 1932 kPaand 9.0 lpm flow rate.

3.3.2. Steady state fluxThe values of flux obtained in the turbulent regime are sig-

nificantly higher than that of laminar and laminar with turbulentpromoters due to the higher operating pressure (driving force)and turbulence present near the membrane surface. The effects

Fig. 11. Transient flux data in RO.

298 C. Das et al. / Separation and Purification Technology 50 (2006) 291–299

Table 3Permeate analysis after reverse osmosis

S.No. Pressure (kPa) Flow rate(lpm)

TDS (ppm) TS (ppm) pH Conductivity (mS) COD (ppm) BOD (ppm) R0 of chromium Cr Concentration(ppm)

Turbulent regime1 1380 7.0 5350 5563 6.2 10.1 160.3 61.7 98.9 12 8.0 4800 4991 6.1 9.3 148.7 57.2 99.0 0.9073 9.0 5020 5220 5.9 8.0 127.0 48.8 99.1 0.7334 1518 7.0 5800 6031 5.7 10.9 94.1 36.2 99.4 0.4935 8.0 5740 5969 5.9 10.5 94.1 36.2 99.6 0.366 9.0 5610 5834 6.1 10.2 78.4 30.2 99.6 0.327 1725 7.0 4360 4534 6.1 13.9 94.1 36.2 99.5 0.4678 8.0 5360 5574 6.4 10.9 78.4 30.2 99.6 0.3479 9.0 6360 6614 5.8 9.0 62.7 24.1 99.7 0.29310 1932 7.0 7360 7653 5.7 14.9 77.1 29.7 99.6 0.33311 8.0 8360 8693 5.7 11.8 71.1 27.4 99.7 0.30712 9.0 9360 9733 5.6 9.6 63.8 24.6 99.7 0.227

of transmembrane pressure and cross flow velocity on steadystate flux in RO are shown in Fig. 12. The figure shows that thepermeate flux increases with operating pressure and cross flowvelocity as in NF. At 1932 kPa, an increase in cross flow velocityfrom 7.0 to 9.0 lpm results in about 64% increase in permeateflux.

3.3.3. Permeate qualityThe effects of transmembrane pressure and cross flow veloc-

ity on permeate quality in terms of chromium retention forturbulent regime, in RO, are shown in Fig. 13. The figure illus-trates that the improvement of permeate quality with cross flowvelocity and pressure is marginal. For example, at 1932 kPa, anincrease in cross flow velocity from 7.0 to 9.0 lpm results in anincrease in retention of chromium from 99.62 to 99.74%. Thepermeate qualities in terms of other properties for various oper-ating conditions are presented in Table 3. It may be observedfrom Table 3 that the concentration of chromium in the perme-ate varies from about 1.0 to 0.2 ppm in the pressure range of1380–1932 kPa which is within the permissible limit (1.0 ppm

Fig. 13. Variation of observed retention of chromium with pressure drop inturbulent regime RO.

[1]). Furthermore, the COD of permeate is substantially lowerthan the permissible limit (250 mg/l). From Table 3, it may alsobe observed that the conductivity of the permeate is very smallsignifying that almost all the salt present in the feed has beenretained by the RO membrane. This salt rich retentate stream canbe recycled to the tanning process. The information are essentialfor choosing the operating conditions and thereby improving theeconomics of the process without loss of product quality.

4. Conclusion

Effluent from a chrome tanning unit has been successfullytreated by a combination of NF followed by RO process. Theretentate of the NF, rich in chromium, can be recycled. However,most of the salt is not retained by the process. The time requiredto reach steady state decreases with increase in cross flow veloc-ity and applied pressure. The use of turbulence promoters inlaminar regime results in substantial increase in flux comparedto the laminar case. Effluent quality also increases with increasein pressure and cross flow velocity. Nevertheless, the permeate

Fig. 12. Variation of permeate flux with pressure drop in RO.

C. Das et al. / Separation and Purification Technology 50 (2006) 291–299 299

contains higher than permissible amounts of chromium andhigher COD. The treatment of the permeate of the NF processby RO successfully addresses these problems including retainingmost of the dissolved salts. The permeate RO has less than 1 ppmof chromium with low values of COD and BOD. The gel-typeresistance is evaluated using a resistance in series model fromthe experimental data. It is found that the resistance decreaseswith increase in cross flow velocity and significantly decreasesby the presence of turbulent promoters.

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