Treatment of soaking effluent from a tannery using membrane separation processes

Desalination 216 (2007) 160–173

Treatment of soaking effluent from a tannery using membraneseparation processes

Chandan Das, Sunando DasGupta, Sirshendu De*Department of Chemical Engineering, Indian Institute of Technology, Kharagpur, Kharagpur 721302, India

Tel. +91 (3222) 283926; Fax: +91 (3222) 2755303; email: [email protected]

Received 26 March 2006; Accepted 8 January 2007

Abstract

A hybrid separation process involving gravity settling, coagulation by alum followed by nanofiltration and reverseosmosis is attempted for treatment of soaking effluent of a tannery. The optimum alum dose is identified. Thefertilizer value of the dried sludge is also evaluated. The membrane separation experiments are conducted in acontinuous cross flow mode. The hydrodynamics in the membrane cell is altered by conducting experiments underlaminar, laminar with turbulent promoters and turbulent flow regimes. The effects of different operating parameterson permeate flux improvement are experimentally observed. A simple resistance-in-series model is proposed toquantify the flux decline during membrane separation processes. The steady state polarized layer resistance is relatedto the operating conditions, namely, transmembrane pressure and Reynolds number. The transient flux decline isquantified by a first order kinetic model and the kinetic constant is evaluated. The performance criteria are evaluatedin terms of COD, BOD, TDS, TS, pH ,Ca2+ concentration, Cl! concentration and conductivity of the permeate.

Keywords: Alum coagulation; Nanofiltration; Reverse osmosis; Turbulent promoter; Kinetic model

1. Introduction

The first tannery operation is soaking, whichis treatment of hides and skins with water. Smalland thin leather (before tanning) is called skinand big, fat and strong leather (before tanning) iscalled hide. During curing, hides and skins loselarge quantity of its physiological content of

*Corresponding author.

water and unless the former regains this waterduring soaking operation, good quality leathercannot be produced. The objectives of soakingare, therefore, to rehydrate the skin proteins, toopen up the contracted fibrous structure of theskin, to remove the curing salt and to clean thesurface filth. Thus, the soaking operation pro-duces huge amount of effluent containing highBOD and COD loading.

0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V.doi:10.1016/j.desal.2007.01.006

C. Das et al. / Desalination 216 (2007) 160–173 161

About 30–40 l of water are used for theproduction of 1 kg leather. Soaking unit employsabout 25% of total water consumed in tannery[1]. As a result, recycling of effluent is needed toreduce the water consumption. Membrane-basedseparation processes may be an attractive alter-native and they are gradually emerging astechnically significant and commercially viable‘cleaner technology’ for the treatment of waste-water from various process industries, namely,textile, leather, paint, paper and pulp industriesetc. [2–6]. In recent years, membrane technolo-gies have been developing rapidly and their costis continuing to reduce while the applicationpossibilities are ever extending [7,8]. The mainadvantage of a membrane based process is thatconcentration and separation are achieved withouta change of phase and without use of additionalchemicals or thermal energy, thus making theprocess energy-efficient and ideally suited forrecovery applications [9].

Cassano et al. presented the possibility ofapplication of various membrane separation pro-cesses, e.g., microfiltration, ultrafiltration (UF),nanofiltration (NF) and reverse osmosis (RO) totreat effluent coming out of different units of atannery [10]. Application of NF to the effluent ofliming [11] and degreasing [12] is reported. Useof NF and RO for treatment of chromium richtanning effluent is widely studied [13]. Use of UFand RO to treat the soaking effluent is con-ceptually proposed by Cassano et al. [10].

In the present work, a scheme is proposed totreat the soaking effluent using a hybrid process,including gravity settling, alum coagulation,nanofiltration and reverse osmosis. The optimumalum dose is established. The fertilizer value ofthe sludge produced is tested. The supernatantliquor is subjected to continuous cross flownanofiltration followed by reverse osmosis.Effects of operating pressure and change inhydrodynamics (laminar, laminar with turbulentpromoter and turbulent flow regime) on thepermeate flux are observed. A resistance-in-seriesmodel is proposed to quantify the flux declineusing a first order kinetic model for the growth ofthe polarized layer. The treatment performance isfinally evaluated in terms of various propertieslike BOD, COD, TS, conductivity, etc. Theproposed scheme of the treatment process ispresented in Fig. 1.

2. Theory

2.1. Flux decline analysis

The permeate flux at any instant of time isexpressed as

(1)( )w

m p

PvR R tΔ

=⎡ ⎤μ +⎣ ⎦

where, vw is the permeate flux, μ is the viscosity

Fig. 1. Proposed scheme for the treatment of soaking effluent.

C. Das et al. / Desalination 216 (2007) 160–173162

of permeating solution, Rm is the membraneresistance and Rp is the polarized layer resistance,which is a function of time. The development ofpolarized layer resistance is assumed to obey afirst order kinetics as follows:

(2)( )dd

p sp p

Rk R R

t= −

where k is the first order rate constant and isspR

the steady-state polarized layer resistance. Theabove equation can be integrated with the initialcondition, Rp = 0 at t = 0:

(3)[ ]1 exp( )sp pR R kt= − −

Therefore, a plot of

lnsp

sp p

RR R

⎛ ⎞⎜ ⎟⎜ ⎟−⎝ ⎠

vs time should be a straight line passing throughthe origin with a slope equal to k.

3. Experimental3.1. Effluent

The soaking effluent is collected from M/s,Alison Tannery, Kolkata. India. The charac-terization of the effluent has been carried out andthe results are presented in Table 1.

3.2. Materials

Commercial alum is used for coagulation andis acquired from the local market. All thechemicals, required for the determination of CODand BOD, are acquired from Merck (India) andLoba Chemie (India) and are of analytical grade.

3.3. Membranes

An organic thin film composite membrane ofmolecular weight cut off 400, consisting of a thinfilm polyamide skin over a polysulphone supportis used for nanofiltration. A thin film composite(TFC) membrane is used for reverse osmosis. Themembranes are supplied by M/s, Genesis Mem-brane Sepratech (Mumbai, India). The hydraulicresistance of the NF membrane is 38.5×1012 m!1

and that for RO is 12.2×1013 m!1 (correspondingvalues of membrane permeability are 2.6×10!11

m/Pas and 8.19×10!12 m/Pas respectively).

3.4. Coagulation by alum

The soaking effluent is brought from the plantand kept for five days for gravity settling. Thesupernatant is siphoned out and subjected tocoagulation by alum. Coagulation study usingcommercial alum has been carried out in eightgraduated cylinders of 50 ml capacity withdifferent dosages of alum, namely, 0.02, 0.1, 0.3,0.5, 1, 2, 3 and 4% (weight by volume) for 24 h.It may be pointed out that the rate of coagulationremains almost unchanged beyond half an hour.

Table 1Characterization of effluent

pH Conductivity(S/m)

TS(ppm)

TDS(ppm)

COD(ppm)

BOD(ppm)

Cl!(ppm)

Ca++

(ppm)

Feed 10.5 5.38 56,800 35,100 9,280 3,569 20,590 1,002Feed after gravity settling 10.4 5.15 46,500 33,900 8,080 3,108 19,738 1,006After alum dose 7.25 4.85 43,100 32,310 4,120 1,585 22,400 1,175


The optimum alum dose is established byexamining various properties (e.g., pH, TDS,conductivity, TS, COD, turbidity) of supernatantsolutions. The effluent is subjected to coagulationat an optimum alum dosing in a 40 l bucket. Thesludge produced is sun dried and pulverized topowder form and stored.

3.5. Membrane experiments

A rectangular cross-flow cell, made of stain-less steel, is designed and fabricated. The cellconsists of two rectangular matching flanges. Theinner surface of the top flange is mirror polished.The bottom flange is grooved, forming the chan-nels for the permeate flow. A porous stainlesssteel plate is placed on the lower plate that pro-vides mechanical support to the membrane whichis placed over the porous plate. Two silicongaskets are placed over the membrane formingthe flow channel.

For experiments with turbulent promoters, 16equispaced thin wires of diameter 0.19 mm areplaced laterally (along the width of the channel)in between the two gaskets. The spacing betweenthe turbulent promoters is 14.0 mm. The twoflanges are tightened to create a leak proofchannel. The effective length and width of themembrane available for Wltration are 26.1 cm and4.9 cm, respectively. The height of the flow chan-nel is determined by the thickness of the gasketsafter tightening the two flanges and is found to be3.4 mm. The obstruction in the flow path due tothe wires promotes localized turbulence. Thesame set up is used for NF as well as RO usingsuitable membranes. The schematic of the experi-mental set-up is available elsewhere [14].

3.5.1. NanofiltrationNanofiltration studies are conducted at three

pressures (828, 966, 1104 kPa) with cross flowvelocities of 0.1 m/s (Re = 680), 0.15 m/s (Re =1020) and 0.2 m/s (Re = 1360) in laminar regimeboth with and without a promoter. Cross flow

velocities of 0.7 m/s (Re = 4762), 0.8 m/s (Re =5442) and 0.9 m/s (Re = 6122) are used in aturbulent regime.

3.5.2. Reverse osmosisReverse osmosis experiments are conducted at

four different pressures of 1518, 1725, and1932 kPa with cross flow velocities of 0.7 m/s(Re = 4762), 0.8 m/s (Re = 5442) and 0.9 m/s(Re = 6122) in turbulent regime. In a laminarregime, with and without a promoter, the ROexperiments are conducted at 1518, 1725 and1932 kPa pressure and at 0.15 m/s (Re = 1020) ofcross flow velocity.

The steps used in the experiments are asfollows:C Compaction of membranes: A fresh mem-

brane is compacted at a pressure higher thanits operating pressure for 3 h using distilledwater.

C Determination of membrane permeability:Membrane permeability is determined usingdistilled water. Flux values at various ope-rating pressures are measured and the slope offlux vs pressure plot gives the permeability.

C Conduction of the experiments: The effluentis placed in a stainless steel feed tank of 10 lcapacity. A high pressure plunger pump isused to feed the effluent into the cross-flowmembrane cell. The retentate stream is re-cycled to the feed tank routed through a rota-meter. The permeate stream is also recycled tomaintain a constant concentration in the feedtank. A by-pass line from the pump deliveryto the feed tank is provided. The two valves inthe bypass and the retentate lines are used tovary the pressure and the flow rate through thecell, independently. Cumulative volumes ofpermeate are collected during the experiment.Values of permeate flux are determined fromthe slopes of cumulative volume versus timeplot. Permeate samples are collected atdifferent times for analysis. The duration ofthe cross-flow experiment is 1 h.


C Determination of new permeability: Once anexperimental run is over, the membrane isthoroughly washed, in situ, with distilledwater for 15 min applying a maximum pres-sure of 200 kPa. The cross-flow channel isthen dismantled and the membrane is dippedin 0.12 (N) hydrochloric acid solution forthree hours. Next, it is washed carefully withdistilled water to remove traces of acid. Thecross-flow cell is reassembled and the mem-brane permeability is again measured. It isobserved that the membrane permeabilityremains almost constant between successiveruns.

3.6. Analysis

Calcium and chloride present in various sam-ples are estimated by Orion AplusTM BenchtopIon Meter (supplied by M/s, Thermo ElectronCorporation, Beverly, MA, U.S.A.) using ion-specific electrodes. COD and BOD values of eachstream are measured by standard techniques [15].The conductivities and TDS of all the streams aremeasured by an auto ranging conductivity meter(Chemito 130, manufactured by ToshniwalInstruments, India). pH of the samples is mea-sured by a pH meter, supplied by ToshniwalInstruments, India. Total solids (TS) of all thesamples are measured by weighing a knownvolume of sample in a petri dish and keeping it in

a vacuum oven maintained at 105±2EC till com-plete drying of the sample. The powdered form ofthe sludge from coagulation is analyzed for itsfertilizer value. The samples are sent for analysisto the Agronomy Laboratory of the Agricultureand Food Engineering Department, IndianInstitute of Technology, Kharagpur.

4. Results and discussion

4.1. Pretreatment

Various properties of the supernatant ofgravity settled liquor are presented in Table 1.The supernatant of the settled liquor is tested foroptimum alum dosing for coagulation. Variousproperties of the clear liquid after coagulation atdifferent alum concentrations are presented inTable 2. It may be observed from the table thatbeyond an alum concentration of 2%, TDS,conductivity and TS concentration increasesignificantly. The COD of the clarified liquordecreases with alum concentration and beyond2%, the change is gradual. It may also be notedthat with increasing concentration of alum, theturbidity of the solution decreases (with moresettling of solids) and beyond 2% the turbidityincreases rapidly. It is also observed from Table 2that the pH of the clear solution is close to thenormal pH (~7.25) at 2% alum concentration andit decreases further with increase in alum dose.

Table 2Determination of optimum alum dose

Alum dose (wt %) 0.02 0.1 0.3 0.5 1 2 3 4

pH 10.38 10.12 9.83 9.35 8.90 7.25 4.95 3.12TDS (g/l) 28.07 28.16 28.57 29.95 30.27 32.31 34.34 41.42Conductivity (S/m) 4.289 4.297 4.363 4.568 4.625 4.85 5.24 6.273TS (g/l) 46.1 45.9 45.4 43.9 43.4 43.1 44.2 67COD (mg/l) 7760 7120 6480 6000 5760 4120 3200 2560Turbidity (NTU) 502 467 359 221 237 239 242 268


Table 3Fertilizer quality of sludge

Sample pH Organic carbon(wt %)

Nitrogen (wt %)

Phosphorous (wt %)

Potassium(wt %)

Sludge from soaking 6.8 11.25 1.28 0.11 0.44Vermi-compost 7.1–7.8 9.97–10.62 1.80 0.90 0.40

From these observations, 2% is selected as theoptimum concentration of alum for coagulation.Using alum treatment, the quantity (TS–TDS) hascome down to 10.8 g/l from 12.6 g/l (correspond-ing to the effluent after gravity settling). Thusappreciable quantities of TS are present in theeffluent after alum treatment. The clarified liquorafter optimum alum dosing is subjected to mem-brane filtration after a fine cloth filtration. Thedried and pulverized sludge is analyzed for itsfertilizer value and compared with vermi-compost. Vermi-compost is a stable, organicmanure produced as the vermicast by earthwormfeeding on biological waste materials. It is a pre-ferred balance nutrient source for organic farmingand is eco-friendly. The results are presented inTable 3. It is observed from Table 3 that theproperties of the sludge are close to those ofvermi compost. Therefore, the sludge produced(1.2 kg from 40 l of effluent) can be used as agood fertilizer.

4.2. Nanofiltration

4.2.1. Transient flux declineFig. 2 represents the flux decline behavior of

the effluent at 828 kPa pressure. It can be clearlyseen from the figure that the time required toreach steady state decreases with an increase inReynolds number. For example, it can be ob-served from Fig. 2 that the steady state is attainedin about 558 s for Re = 4762 and 828 kPa pres-sure, whereas at the same pressure but at Re =5442 and Re = 6122, the steady states are attained

Fig. 2. Transient flux at 828 kPa pressure in NF.

within 436 s and 328 s, respectively. The fluxdecline is about 25% of the initial value for Re =4762, about 20% with increase in Re = 5442, and15% at Re = 6122. Similar trends can be observedfor flux decline in laminar regime with andwithout promoters. As the cross flow velocityincreases, the growth of the polarized layer overthe membrane surface is slower because ofenhanced forced convection. This leads to the


onset of steady state at an earlier time. For theabove reason, the resistance to the solvent fluxalso decreases with the cross flow velocity,resulting in higher permeate flux. Therefore, theflux decline is lower at higher cross flow velo-cities. It is also observed that the steady state isachieved faster using turbulence promoter com-pared to laminar flow. For example, in Fig. 2, atRe = 680 and 828 kPa, the steady state is attainedin about 1025 s without promoter and about 762 swith promoter at the same operating condition.The flux decline is about 31% without a promoterat Re = 680 and 828 kPa pressure; but only 21%using a promoter at the same operating condition.

Use of the turbulent promoters creates localturbulence, thus reducing the concentration pola-rization at the membrane surface and the growthof the polarized layer is controlled quickly,establishing steady state earlier than without pro-moters. Since the concentration polarization isreduced due to the presence of the promoters, theflux decline is also less compared to the nopromoter case.

4.2.2. Steady stateThe variations of steady-state permeate flux

with pressure at different Reynolds numbersunder turbulent flow, laminar flow without andwith turbulent promoters are shown in Fig. 3. Thefigure shows the usual trend that the permeateflux increases with operating pressure and Rey-nolds number. Higher flux at higher pressure isdue to enhanced driving force. The increase influx with Reynolds number is because of decreas-ing concentration polarization as discussedearlier. The percentage enhancements of thepermeate flux in laminar regime with turbulentpromoters for all the operating conditions arepresented in Fig. 4. All the increases are cal-culated taking the laminar flow results under thesame operating conditions as the basis. The for-mation of polarized layer over the membranesurface is significantly reduced in presence of the

Fig. 3. Variation of permeate flux with transmembranepressure in NF.

turbulent promoters. This causes a correspondingincrease in permeate flux. It may be observedfrom Fig. 4 that the flux increment is in the rangeof 30 to 43% for laminar flow with promoter.However, it can be clearly seen from Fig.3 thatthe permeate flux in turbulent flow at a specificpressure is considerably higher than either thelaminar or the turbulent promoter enhanced cases.

4.2.3. Analysis of polarized layer resistanceFrom the steady-state flux values obtained

from the experimental results, the polarized layerresistance at the steady state is calculated as

(4)sp ms

w

PR RVΔ

= −μ

for various operating conditions. The variation of


Fig. 4. Flux enhancement with cross flow velocity andpressure in laminar regime with promoter in NF.

dimensionless steady-state polarized layer resis-tance with Reynolds number is presented inFig. 5, for all the hydrodynamic conditions. It isobserved from the figure that the steady-statevalues of Rp decreases with Reynolds number asexpected. For example, for a transmembranepressure of 828 kPa in laminar flow, the ratio ofthe polarized layer and hydraulic resistancereduces from 7.2 to 6.6 with an increase in Rey-nolds number from 680 to 1020. Rp valuesincrease with the transmembrane pressure. Withincrease in pressure, more solutes are convectedtowards the membrane and this enhances theconcentration polarization, resulting in increase inRp values. For the case with the promoters, thepolarized layer resistance decreases significantlydue to the enhanced forced convection near themembrane surface induced by the promoters. Atthe same Reynolds number (680) and transmem-brane pressure (828 kPa), the presence of turbu-lent promoters reduces the resistance to 5.0compared to 7.2 in laminar flow. This reductionin is more than 34% in some of the experi-s

pRments leading to a significant enhancement of thepermeate flux. The figure also shows further

Fig. 5. Variation of the ratio of polarized layer andhydraulic resistances at steady state with Reynoldsnumber during NF.

reductions for the case of purely turbulent flowsfor reasons already discussed. The steady statepolarized layer resistance is correlated with theoperating pressure and Reynolds number as

(5)( )1

2

maxRe

nsnp

m

R PaR P

⎛ ⎞Δ= ⎜ ⎟Δ⎝ ⎠

where ΔPmax is the maximum transmembranepressure (1104 kPa for NF and 1932 kPa for ROexperiments). The values of ln a, n1, n2 arepresented in Table 4 for different hydrodynamicconditions. The positive values of n1 and negativevalues of n2 confirm the trend of polarized layerresistance with the operating conditions as dis-cussed earlier. It may be observed from Fig. 5that Reynolds number has a significant effect onthe polarized layer resistance. For laminar flowwith and without promoter, polarized layerresistance is the major contributing resistance.For example, in case of pure laminar flow, atReynolds number = 680 and transmembrane pres-sure at 1104 kPa, Rm and Rp constitute about 12%and 88% of the total resistance, respectively. In


Table 4Model constants

Operating condition ln a n1 n2

Nanofiltration Turbulent 14.11 ± 0.63 0.14 ± 0.07 !1.59 ± 0.07Laminar 3.37 ± 0.11 0.067 ± 0.04 !0.21 ± 0.02Laminar with promoter 3.28 ± 0.59 0.374 ± 0.004 !0.24 ± 0.08

Reverse osmosis Turbulent 16.02 ± 0.80 0.29 ± 0.003 !1.81 ± 0.09

case of laminar flow with promoter, at the sameoperating condition, contribution of Rp decreasesto 84% of the total resistance. For turbulent flowregime, effects of the Reynolds number are reallyprofound and polarized layer resistance becomescomparable to the membrane hydraulic resistance.For the range of Reynolds number studied herein,

varies between 1.2 to 1.9 times of Rm. AtspR

Reynolds number 4762, Rp contributes about 65%of total resistance, whereas at Reynolds number6122, it is about 55%.

The values of at different operating con-spR

ditions are evaluated from the correlation pre-sented in Eq. (5). Now, with these values ands

pRexperimentally observed Rp values,

lnsp

sp p

RR R

⎛ ⎞⎜ ⎟⎜ ⎟−⎝ ⎠

is plotted at various time points for a fixed set ofoperating conditions, resulting into almost astraight line through the origin. The slope of thesecurves estimates the value of k which is thekinetic rate constant of the growth of polarizedlayer resistance. For laminar flow without a pro-moter, the range of k is 0.005 s!1 to 0.013 s!1; forlaminar flow with promoter it is from 0.007 s!1 to0.014 s!1 and for turbulent flow it is from0.007 s!1 to 0.03 s!1. Therefore, average values ofk are taken for calculating the profile of polarizedlayer resistance. The average value of k is0.008 s!1 for laminar flow, 0.011 s!1 for laminar

Fig. 6. Variation of calculated dimensionless polarizedlayer resistance with time in NF.

flow with a promoter and 0.016 for turbulentflow. Using the average k values and Eq. (4) for

, the profiles of Rp are calculated from Eq. (1)spR

and are presented in Fig. 6 for various operatingconditions. It is observed from Fig. 6 that the


Table 5Permeate analysis after nanofiltration

Sr. no Pressure, kPa Reynolds no. TDS, ppm TS, ppm pH Conductivity, S/m Cl!, ppm Ca++, ppm

Turbulent regime

1 828 4,762 22,700 29,400 8.0 3.60 15,000 4282 828 5,442 22,650 29,250 7.99 3.57 15,300 4873 828 6,122 22,600 29,200 8.05 3.55 15,900 5524 966 4,762 22,600 29,100 8.03 3.64 15,900 5425 966 5,442 22,650 29,000 7.77 3.59 16,500 5926 966 6,122 22,500 28,950 7.93 3.56 17,250 6967 1,104 4,762 22,500 28,900 7.95 3.66 18,000 6368 1,104 5,442 22,400 28,750 7.86 3.63 19,800 6609 1,104 6,122 22,400 28,700 7.88 3.57 20,250 708

Laminar regime

10 828 680 23,600 30,600 7.83 3.72 17,600 38611 828 1,020 23,600 30,500 7.87 3.69 19,900 39412 828 1,360 23,600 30,400 7.93 3.63 18,180 35013 966 680 23,300 30,300 7.82 3.70 16,700 38614 966 1,020 23,200 30,100 7.89 3.65 18,480 44015 966 1,360 23,150 30,050 7.91 3.62 18,160 45416 1,104 680 23,100 30,100 7.84 3.66 18,280 48017 1,104 1,020 23,000 29,900 7.84 3.63 18,180 49618 1,104 1,360 22,950 29,750 7.83 3.60 18,380 402

With turbulent promoter

19 828 680 23,000 29,800 7.7 3.62 19,400 39220 828 1,020 22,900 29,600 7.4 3.59 19,800 32421 828 1,360 22,850 29,450 7.6 3.56 20,000 28622 966 680 22,900 29,650 7.38 3.66 15,800 41023 966 1,020 22,800 29,500 7.55 3.61 16,380 40624 966 1,360 22,800 29,400 7.69 3.58 16,840 38625 1,104 680 22,700 29,300 7.59 3.68 16,520 40026 1,104 1,020 22,650 29,150 7.64 3.65 17,000 44227 1,104 1,360 22,600 29,050 7.72 3.59 16,180 406

calculated profiles of Rp match closely with theexperimental values. For all the hydrodynamicconditions, the Rp values are lower at higherReynolds number as expected.

4.2.4. Permeate qualityThe permeate quality after NF, for various

operating conditions, is presented in Table 5. Theconductivity of the permeate is same as the feed

which signifies that almost all the salt present inthe feed solution has permeated through the NFmembrane.

Variations of permeate COD with trans-membrane pressure at the operating Reynoldsnumber in turbulent, laminar and with turbulentpromoter are shown in Fig. 7. It is observed thatwith an increase in transmembrane pressure andReynolds number, the permeate quality improves.


With an increase in pressure, the solvent fluxincreases linearly, while the solute flux is nearlyindependent of pressure for less open membranes(RO and in some cases for NF membranes) [16].This indicates that with increasing pressure, moresolvent passes through the membrane along witha fixed amount of the solute; the permeatebecomes purer and hence the permeate quality(expressed as COD) increases. The similar trendsare observed for laminar flow with promoter andturbulent flow. It can be seen from Fig. 7 that at828 kPa pressure and Re = 1360, COD decreasesby about 13% in presence of promoter comparedto the base case (laminar at same operating con-ditions). Percentage decrease in COD is found tobe about 22% at 966 kPa pressure and Re 1360and about 30% at 1104 kPa pressure andRe 1360. At Re 4762, as the transmembranepressure increases from 828 kPa to 1104 kPa,COD decreases by 38%.

Fig. 7. Variation of COD with transmembrane pressurein NF.

4.3. Reverse osmosis

The permeate from the NF is collected andtreated using RO in the same cross flow cell inlaminar, laminar with turbulent promoters and inturbulent conditions at different operatingconditions.

4.3.1. Transient flux declineFig. 8 presents the flux decline behavior with

time and transmembrane pressure and cross flowvelocity at 1518 kPa in RO. The results clearlyshow that as in the case of NF, the time requiredto reach steady state decreases with increase incross flow velocity and also in the presence ofturbulent promoters. Extent of flux decline alsofollows similar trends for reasons already dis-cussed in Section 4.2.1.

4.3.2. Steady state fluxThe values of flux obtained in the turbulent

Fig. 8. Transient flux at 1518 kPa pressure in RO.


Fig. 9. Variation of permeate flux with transmembranepressure in RO.

Fig. 10. Variation of the ratio of polarized layer andhydraulic resistances at steady state with transmembranepressure in RO.

regime are significantly higher than that of lami-nar and laminar with turbulent promoters due tohigher turbulence induced by higher Reynoldsnumbers. The effects of transmembrane pressureand Reynolds number on steady-state flux in ROare shown in Fig. 9. The figure shows that thepermeate flux increases with operating pressureand Reynolds number as in NF.

4.3.3. Analysis of polarized layer resistanceFig. 10 represents the variation of dimension-

less steady-state polarized layer resistance withtransmembrane pressure for all the hydrodynamicconditions. The steady state values of Rp increasemarginally with the transmembrane pressure anddecrease significantly with increase in Reynoldsnumber as discussed earlier.

For turbulent flow regime, the Rps/Rm values

are fitted with the operating conditions as given

Fig. 11. Variation of calculated dimensionless polarizedlayer resistance with time in RO.


Table 6Permeate analysis after reverse osmosis

Sr. no. Reynolds no. Pressure, kPa TDS, ppm TS, ppm pH Conductivity, S/m Ca++, ppm Cl!, ppm

Turbulent regime

1 4762 1518 7470 7770 7.05 1.25 40 22002 5442 1518 8300 8630 7.1 1.37 45 22103 6122 1518 7650 7950 7.15 1.31 36 21404 4762 1725 5280 5490 7.12 0.93 49 21805 5442 1725 5170 5380 7.05 0.92 45 22206 6122 1725 5430 5650 7.1 0.93 43 21607 4762 1932 5610 5830 6.98 0.98 33 21408 5442 1932 5930 6170 7.09 1.01 39 21209 6122 1932 5800 6030 7.12 0.99 38 2180

Laminar regime

1 1020 1518 5540 5760 7 0.95 41 22402 1020 1725 5620 5840 7.04 0.97 41 22003 1020 1932 5480 5900 7.11 0.93 45 2120

With turbulent promoter

1 1020 1518 5500 5720 7.05 0.94 36 21602 1020 1725 5820 6050 7.15 0.99 32 21803 1020 1932 5920 6260 7.1 1.01 33 2140

in Eq. (5) and the estimated parameters aretabulated in Table 4. For the growth of Rp/Rm, kvalues are fitted using Eq. (4). Average k valuesare found to be 0.006 s!1 for laminar flow,0.007 s!1for laminar flow with a promoter and0.012 s!1 for turbulent flow. The calculated andexperimental Rp values are presented in Fig. 11for all operating conditions. Fig. 11 shows a closematch between the calculated and experimentaldata.

4.3.4. Permeate qualityThe effects of transmembrane pressure and

cross flow velocity on permeate quality in termsof COD for turbulent regime in RO are shown inFig. 12. The figure illustrates that the decrease ofpermeate quality with decrease in Reynoldsnumber and pressure is significant. The permeatequalities in terms of other properties for variousoperating conditions are presented in Table 6. It

Fig. 12. Variation of COD with transmembrane pressurein RO.


may be observed from Table 6 that COD in thepermeate varies from about 128 to 64 ppm in thepressure range of 1518 to 1932 kPa which issubstantially lower than the permissible limit(250 ppm). From Table 6, it may also be observedthat the conductivity of the permeate is very smallsignifying that significant amount of the saltpresent in the feed has been retained by the ROmembrane. This salt rich retentate stream can berecycled to the pickling process. These data areessential for choosing the operating conditionsand thereby improving the economics of theprocess without loss of product quality.

5. Conclusion

Effluent from a soaking unit has been success-fully treated using a combined process of coagu-lation by alum and membrane separation. Theretentate of the RO, rich in sodium chloride, canbe recycled for the pickling process. The timerequired to reach steady state decreases with anincrease in the Reynolds number and appliedpressure. The use of turbulent promoters inlaminar regime results in substantial increase influx (30–43% for NF and 33–40% for RO) com-pared to the laminar case. The treatment of thepermeate of the NF process by RO successfullyretains most of the dissolved salts. Both in NFand RO, polarization resistance is the majorcontributor to overall resistance to the solventflow. A first-order kinetic model suitablydescribes the growth rate of polarized layer resis-tance. The values of COD (~92.3 ppm) and BOD(~28.5ppm) in the permeate of RO are well belowthe discharge limit of the same.

Acknowledgements

This work is partially supported by a grantfrom the Department of Science and Technology,

New Delhi, Government of India, under thescheme no. DST/TSG/WM/2005/55. Any opin-ions, findings and conclusions expressed in thispaper are those of the authors and do not neces-sarily reflect the views of DST.

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Documents

Treatment of soaking effluent from a tannery using membrane separation processes