Reverse osmosis of edible vegetable oil industry effluent

Journal of Membrane Science 205 (2002) 83–90

Reverse osmosis of edible vegetable oil industry effluent

S. Sridhar, Aniket Kale, A.A. Khan∗,1

Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology,Hyderabad 500007, Andhra Pradesh, India

Received 4 September 2001; received in revised form 30 January 2002; accepted 13 February 2002

Abstract

Pilot-scale reverse osmosis (RO) studies were carried out to determine the feasibility of the process for treating the wastewaterfrom a vegetable oil industry. The effect of varying feed pressure (0–69 bar) and feed TDS concentration (0.54–5.2% w/v)on separation performance of the thin film composite (TFC) polyamide RO membrane was determined. At original effluentcomposition and feed pressure of 55.2 bar, high rejection of TDS (99.4%) and COD (98.2%) along with complete rejectionof color and BOD was achieved with a reasonably high flux of 52.5 l/(m2 h). The flux and percent rejection of pollutantsimproved significantly with increasing transmembrane pressure at constant feed concentration but decreased with increasingfeed TDS concentration. RO was found to be very suitable for treating oil effluents having TDS concentrations upto 52215 ppm.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Reverse osmosis; Composite membrane; Oil effluent; Water treatment; Percent rejection

1. Introduction

In a vegetable oil industry, the effluent mainlycomes from the degumming, deacidification anddeodorisation steps[1]. The blowdown of the boilerand wash water from de-oiling of the bleaching earthalso contribute to the effluent in small amounts.

Previously, effluent from a vegetable oil industryused to be discharged directly into soil or groundwater. But, due to the emergence of environmentalconsciousness, the Pollution Control Boards havebecome stricter and impose very stringent norms. Thescarcity of water also is another incentive for recov-ering pure water from effluents. For the treatment ofan effluent by conventional methods like aerobic oranaerobic digestion the ratio of BOD to COD should

∗ Corresponding author. Tel.:+91-40-7193626;fax: +91-40-7190387/7190757.E-mail address:[email protected] (A.A. Khan).

1 IICT Communication Number: 4868.

be >0.6[2]. However, an effluent from the vegetableoil industry usually has its BOD/COD ratio around0.2 which could cause destruction of micro-organismsuseful for the biodegradation. Other methods likemultiple effect evaporation or incineration are highlyenergy intensive and hence, very expensive. Thesedisadvantages emphasizes the need for furtherresearch using novel separation methods.

Membrane separation techniques such as reverseosmosis (RO) yield excellent results when applied ju-diciously in such cases[3,4]. RO has been applied fortreating a wide variety of industrial effluents[5–9].

In the present study, RO experiments were carriedout on a pilot-scale system incorporating an effectivearea of 2.5 m2 of commercially available thin filmcomposite (TFC) polyamide membrane for treatingthe oil effluent. The effect of process variables suchas feed pressure and feed concentration on membraneperformance with respect to percent rejection of TDS,COD, BOD and color was extensively studied.

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0376-7388(02)00065-0

84 S. Sridhar et al. / Journal of Membrane Science 205 (2002) 83–90

2. Experimental

2.1. Materials

The commercial TFC polyamide RO membranein spiral wound configuration was purchased fromPermionics, Baroda, India. The membrane was speci-fied for seawater desalination with catalogue designa-tion Perma PPT-9908, effective area 2.5 m2, modulelength 40 inches and diameter 2.4 inches. The purewater flux was specified to be 90 l/(h m2) at 55.2 barfeed pressure and 25◦C and the standard NaClsalt rejection under these conditions was 99.4% for32,000 ppm feed concentration with pH range for op-eration being 2–11. The effluent sample was collectedfrom M/s Ramcharan Oil Mills, Katedan, Hyderabad.Tetrasodium EDTA, hydrochloric acid and sodiumbisulphite and sodium hexametaphosphate for mem-brane cleaning and storage were supplied by LobaChemie, Mumbai. Deionised water for cleaning andfeed preparation purposes was generated from thesame RO system.

2.2. Feed pretreatment

The liquid effluent feed to be treated by RO wasanalyzed for the dissolved solids, pH, hardness, quan-tity of microbial organisms present and accordinglypretreatment of the feed was carried out. The effluentfrom the oil industry was highly acidic with a pH ofaround 2. Hence, it had to be neutralized using soda

Fig. 1. Schematic view of pilot-scale RO system.

ash. The neutralized solution had a lot of suspendedsolids impairing its clarity. Hence a coagulant, alum,was added and the solution was transferred to a sep-arating funnel. After about 2 h the solution separatedinto two layers, a clear yellow colored upper layer anda gray colored lower layer which contained the alumand all the coagulated material. The lower layer wasfiltered using a vacuum filter and the filtrate along withthe clear upper layer were mixed and used as feed forthe membrane separation unit. Prefiltration with the0.2�m pore size PTFE filter was then carried out toremove any minute suspended solids. The silt densityindex (SDI) of the treated feed was found to be 3 whichis within the permissible limits to minimize standardmembrane fouling and extend membrane life.

2.3. Description of RO pilot system

A pilot-scale skid mounted system was built incor-porating a commercial TFC polyamide RO membraneinside an FRP cylindrical pressure vessel (Fig. 1). Afeed tank of 30 l capacity made of stainless steel 316was provided for storage and supply of the effluent tothe system as well as collection of recycled concen-trate. A cooling coil was installed inside the feed tankfor circulating cold water to maintain a constant feedtemperature within the range of 27–29◦C. A high pres-sure pump (Hironisha, Japan) capable of maintaininga pressure up to 100 bar was installed for transportingthe feed liquid throughout the system. The pump wasrun by a 2hp single phase motor (Crompton, India).

S. Sridhar et al. / Journal of Membrane Science 205 (2002) 83–90 85

A PTFE prefilter cartridge of 0.2�m pore size wasinstalled upstream of the spiral membrane module toprevent entry of suspended solid particles capable ofdamaging the membrane. A restricting needle valvewas provided on the concentrate outlet of the mem-brane pressure vessel to pressurize the feed liquid to adesired value indicated by a pressure gauge installedupstream of the valve. The permeate and concentrateflow rates were measured by means of two glass ro-tameters containing metal floats. An adjustable safetyvalve was fixed on the pump discharge line to preventthe system pressure from exceeding the maximumdesired value (70 bar). All the accessories were con-nected by 1/2 inches o.d. stainless steel 316 pipingin the high pressure region whereas the low pressureoutlets from concentrate and permeate sections weremade of PVC braided tubing.

2.4. Experimental procedure

Before introducing the feed into the RO system themembrane was cleaned and wetted with de-ionizedwater for about 15 min to ensure that it was at its max-imum strength before actual separation. Thirty litresof pretreated feed was then charged into the stain-less steel feed vessel. The high pressure pump wasemployed to transport the feed to the spiral woundmembrane module and the system pressure was ad-justed at a value greater than the osmotic pressure bymeans of the restricting needle valve. The retentateflow rate was maintained constant throughout the ex-periments at 10 l/min to ensure similar hydrodynamicconditions inside the membrane module. The studywas a single stage batch process with complete re-tentate recycle throughout the experiment and initialrecirculation of permeate for 10 min until a steadyflux was obtained. The feed pressure was varied from0 to 69 bar, keeping the feed TDS concentration con-stant at 1.04% (w/v) (10,411 ppm). Permeate sampleswere collected after every 6.9 bar increase in feedpressure.

In another set of experiments, the feed pressure waskept constant at 55.2 bar and feed TDS concentrationwas varied from 0.54 to 5.22% (w/v). The flux wasnoted by collecting the permeate in a measuring jar(1 l) for a period of 2–3 min. A total of 250–300 mlof permeate was collected in all cases to enable thor-ough analysis of the same. Each experiment was

repeated twice with every feed sample to check forreproducibility of permeate characteristics.

2.5. Analytical

The feed and permeate samples were analyzedfor TDS, COD, BOD and color (platinum–cobaltprocedure) according to APHA methods[10]. Theconductivity of above samples was determined us-ing a digital conductivity meter. To ensure that thechloride ions present in the effluent do not interferewith COD analysis, the feed and permeate sampleswere treated with mercuric sulphate (HgSO4) whichconverts the chlorides to insoluble mercuric chloride(HgCl) precipitate and gets removed from the titra-tion medium. The method was standardized for highchloride contents.

2.6. Percent rejection, flux and concentrationpolarization modulus

In RO membrane process the separation perfor-mance of the membrane is denoted in terms of %rejection of TDS, COD or any other feed componentwhich is calculated as[3]:

R(%) =(

1 − CP

CF

)× 100

where CP represents the concentration of the par-ticular component in permeate whileCF is its feedconcentration.

The flux (J) is the volume of permeate (V) collectedper unit membrane area (A) per unit time (t):

J = V

At

The concentration polarization ratio (Cmembrane/Cbulk) depends on the flux, rejection and mass trans-fer coefficient[3] and is estimated from the followingequation:

Cmembrane

Cbulk= exp

(Jk

)Ro + (1 − Ro) exp

(Jk

)where Ro is the observed rejection which becomesequal to 1 when the solute is completely retainedby the membrane. The mass transfer coefficient (k)is determined by usingRo and J data taken at dif-ferent pressures but constant feed concentration and


feed rate to plot a straight line of best fit betweenln[(1 − Ro)J/Ro] versus J of slope 1/k and inter-cept ln(Dk/δ) whereD is diffusion coefficient andδrepresents boundary layer thickness.

2.7. Membrane fouling and its prevention

The concentration of calcium in the form of carbon-ate and sulfate as well as hardness in the oil effluentwas reasonably high which could cause scale forma-tion on membrane surface and subsequent fouling. Adosage of 10 ppm of sodium hexametaphosphate an-tiscalant to the RO feed helped in preventing CaSO4scaling. CaCO3 scaling was controlled by treatmentof membrane at the end of the day’s study with acidsolution (HCl, pH= 2) which converts the carbonateto CO2 and also removes other metal precipitates andmineral scales. Washing with 1% aqueous solution oftetrasodium EDTA, a chelating agent useful in remov-ing organics and silt, was done on alternate days. Toprevent biological fouling the membrane was washedthoroughly with deionized water and stored in a 0.5%solution of sodium bisulphite (NaHSO3) at the end ofexperimental runs. The above methods were efficientin restoring water flux. However, over a period of 6months of batch operation with the oil effluent, theflux decreased to about 96% of its original value.

3. Results and discussion

3.1. Variation of permeate characteristicswith feed pressure

Figs. 2–4 describe the effect of feed pressure(0–69 bar) on flux and rejection characteristics ofthe RO membrane for original pretreated feed con-centration of 10,411 ppm TDS, 2550 ppm COD and421 ppm BOD (Table 1). A comparison of pure waterflux with feed flux in Fig. 2 clearly emphasizes theeffect of osmotic pressure in RO transport. The feedflux was zero below 8.9 bar while significant flux wasobtained for pure water. The flux for oil effluent feedincreased almost linearly from 9 l/(m2 h) at 13.8 barto 63.0 l/(m2 h) at 69 bar.Figs. 3 and 4, respectively,show that the amount of TDS in permeate reducedabout three times and COD decreased to seven times.The % TDS rejection increased from 98.4 to 99.4%

Fig. 2. Comparison of pure water flux with feed flux at varyingfeed pressure (feed concentration: 104,11 ppm TDS).

whereas that of COD increased more significantlyfrom 95.1 to 99.3%. The color and BOD contents ofpermeate were negligible. Both the solution–diffusionphenomenon and pore size of the selective top layerof the membrane govern the RO separation of so-lute molecules from the solvent[11,12]. The watermolecules are more strongly bound to the hydrophilicmembrane surface and are preferentially absorbedin the pore walls which have diameter in nanometer(molecular) range. The larger solute (TDS) moleculeswhich are mostly inorganic in nature show lesser in-teraction and cannot enter the fine pores easily dueto steric repulsion. As the transmembrane pressureincreases, the solvent flux increases proportionately

Table 1Characteristics of pretreated oil effluent

Number Parameters Feed characteristics

1 Color Yellowish green(5500 Pt–Co units)

2 pH 5.03 Conductivity (�S/cm) 125004 Oil content (ppm) <505 Total dissolved solids (ppm) 104116 Total suspended solids (ppm) 4007 BOD (ppm) 4218 COD (ppm) 25509 Calcium (ppm) 300

10 Chlorides (ppm) 40011 Sulphates (ppm) 13012 Hardness (ppm) 120013 SDI 3


Fig. 3. Effect of feed pressure on permeate TDS rejection (feed concentration: 104,11 ppm TDS, temperature 28◦C).

Fig. 4. Variation of permeate COD content with feed pressure (feed concentration: 2550 ppm COD, 10,411 ppm TDS).


Table 2Effect of feed concentration on permeate characteristics

Feed characteristics Permeate characteristics Cmembrane/Cbulk

Conductivity(mS/cm)

TDS(ppm)

Osmotic pressurea

�π (�P–�π )BOD(ppm)

Conductivity(mS/cm)

TDS(ppm)

BOD(ppm)

Flux(l/(m2 h))

9.2 5407 4.6 (50.6) 180 0.054 26 0 60 2.3812.5 10411 8.9 (46.3) 421 0.086 62 0 52.5 2.1415.7 13956 11.9 (43.2) 574 0.208 154 0 46.4 1.9518.3 21869 18.7 (36.5) 1068 0.339 350 10 41.5 1.8122.5 33042 28.3 (26.9) 1128 0.443 595 13 30.3 1.5424 41344 35.4 (19.8) 1254 0.48 1116 18 18 1.2928.3 52215 44.6 (10.5) 1624 0.60 1984 30 7 1.10

[Feed pressure (�P): 55.2 bar, feed temperature: 28◦C].a Osmotic pressure values calculated from�π = vcRT assuming feed to be NaCl solution.

while solute flux increases less rapidly. The soluterejection characteristics of an RO membrane aredetermined by dimensions of solute passage and wa-ter flux and therefore for a relatively constant solutepassage the rejection must decrease with decreasingwater flux and increase with increasing water flux asobserved in the present case. The mass transfer co-efficient determined as per procedure inSection 2.6was found to be about 0.068 m/h. This value wasapplied to determine the concentration polarizationratio (Cmembrane/Cbulk), which increased from 1.14at initial feed pressure (13.8 bar) to 2.52 at 55.2 bar,owing to increasing flux and rejection.

Fig. 5. Effect of feed concentration on TDS rejection (feed concentration: 55.2 bar, temperature 28◦C).

3.2. Effect of feed concentration on permeatecharacteristics

The effect of feed concentration on permeate char-acteristics was studied keeping feed pressure and tem-perature constant at 55.2 bar and 28◦C, respectively.This is because effluents from different oil industriescontain wide range of TDS which may vary from 5000to 50,000 ppm or more. It is also desirable to studythe upper limit of TDS or COD rejection up to whichRO technology may be feasible. Hence, different setsof feeds were prepared by dilution as well as con-centration of the original oil effluent. The results are


Fig. 6. Variation of permeate COD content with feed concentration (feed concentration: 55.2 bar, temperature 28◦C).

presented inTable 2 and Figs. 5 and 6which showthat the flux and solute rejection decreased propor-tionately with increasing feed concentrations of TDSfrom 0.54 to 5.22% (w/v). The flux lowered from 60to 7 l/(m2 h) while the rejection decreased from 99.5to 96.2% for TDS and from 98.6 to 92.3% for COD.The concentrations of permeate BOD and color werenegligible up to a feed conductivity of 18.3 mS/cm be-yond which it increased to final values of 30 ppm and85 Pt–Co units, respectively. The decreasing flux andrejection is due to rising concentration polarizationof solute which increases the osmotic pressure at themembrane surface and causes a loss in effective trans-membrane pressure[5]. The approximate osmoticpressure (�π ) and corresponding transmembranepressure (�P–�π ) values are reported inTable 2assuming the feed to be solution of predominantlysodium chloride. The mass transfer coefficient valueof 0.068 m/h was utilized to estimate the concentrationpolarization ratio (Cmembrane/Cbulk) which expectedlydecreased from 2.38 to 1.10 with decreasing flux andrejection at varying feed concentration (Table 2).

The membrane performance was reasonably goodup to a feed TDS concentration of 33042 ppm (conduc-tivity 22.5 mS/cm) and the permeate TDS and CODconcentrations remained below the permissible levels(<1000 ppm TDS and 500 ppm COD) as per Indian

Pollution Control Board specifications. The percentrecovery of water was estimated by pressurizing 30 l oforiginal feed containing 10,411 ppm TDS at 55.2 psigand concentrating it until the permeate gave a con-ductivity of more than 0.45 mS/cm and the retentatereached a concentration of about 3.3% TDS. At anaverage flux of 45 l/m2 h, a permeate volume of ap-proximately 21 l was collected which meant that thepercent recovery (permeate volume/feed volume) was70% for a single stage[4].

4. Conclusions

RO has been found to be a very promising separa-tion process for treatment of oil industry effluent andwater recovery due to the high fluxes obtained along-side significant rejection of TDS, COD, BOD andcolor. The absence of heat application and high rateof mass transfer generated by RO shows that a largeamount of water can be permeated economically in-stead of being vaporized by energy intensive evapora-tion. The pollutant levels in permeate were acceptableunder Indian Pollution Control Board specificationsfor effluent discharge (<1000 ppm TDS and 500 ppmCOD) up to feed TDS concentrations of about 3.3%(w/v) with single stage RO. The extent of contami-


nant rejection and percent recovery of water, whichwas 70% in the present case, can be further improvedby means of multi-stage cascade arrangement.

Acknowledgements

The contributions of Mr. Someswara Reddy, B.Sridhar, Anwar Sharief and P.A. Louis of Workshop,Mr. M.R.K. Murthy of Electrical Dept., Dr. VijayKale of Oils & Fats Division and Mr. T. Srinivasanfor analytical works are gratefully acknowledged.

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Reverse osmosis of edible vegetable oil industry effluent