The Ultrafiltration of Wool-scouring Effluents

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37THE ULTRAFILTRATION OFWOOL-SCOURING EFFLUENTS

P. E. Chisnalla& B. Robinson

b

aWool Research Organisation of New Zealand Inc. ,

Christchurch, New ZealandbInternational Wool Secretariat, Technical Centre ,

Ilkley, West Yorkshire

Published online: 01 Dec 2008.

To cite this article:P. E. Chisnall & B. Robinson (1979) 37THE ULTRAFILTRATION OF

WOOL-SCOURING EFFLUENTS, The Journal of The Textile Institute, 70:9, 397-406

To link to this article: http://dx.doi.org/10.1080/00405007908658873

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3777ie Vltrafiltration o f Wool-scouring EffluentsChisnall and Robin. ion 39737THE ULTRAFILTRATION OF WOOL-SCOURINGEFFLUENTS

By P. E. CHISN ALL and B. ROBINSON

It is shown that ultrafiliration is a feasible technique for producing an effluent completely frMfrom suspended solids and wool grease together with a concentrated sludge Ironi a strong wool-scouring liquor at a mod eratecost The process would be particularly applicable in areas where thereare stringent effluent standards.1 INTRODUCTION

The scouring of raw wool produces a complex, highly polluting, aqueous effluent con-taining wool grease, suint, and dirt. The removal of settlable solids, screening, centrifuging,and recycling the clarified effluent to the scour as in the WRONZ Comprehensive ScouringSystem^ result in significant economies in water, detergent, and heat-energy usage, an in-crease in the recovery of wool grease, and a reduction in the total pollution load of theeffluent.

Although the water consumption can be reduced to 2 litre per kg of scoured wool(excluding rinse water, which is little contaminated and can be discharged without treat-ment), the final effluent is still highly polluting, with a chemical oxygen demand (COD) ofthe orde r of 80 000 mg/litre. The treatm ent of this effluent to reduce the su spended solidsand the C O D has been achieved by chemical des tabilization followed by iiltration^- *, bu tthis treatment has many disadvantages. It uses large amounts of expensive chemicals andheat energy and gives variable results related to inevitable fluctuations in the feed.The con cept of using a m emb rane-separa tion process to give a physical separation of the

suspended matter (grease and dirt) regardless of liquor characteristics is very attractive inpromising a reliable performance. This relatively new filtration process depends on circu-lating a process liquid at a high velocity past an extremely thin membrane with a pressuredifferential of 200-300 kPa across it. Rapid filtration through the membrane occurs at analmost constant rate, since there is little or no blinding of the membrane by retained solids,as occurs in static filtration. Any build-up on the membrane surface can usually be removedat intervals with suitable cleaning solutions. The membrane pore size is commonly 0.2-0.4 nm, which is much smaller than the particle size, so physical plugging of the pores isunlikely. The liquor is thus divided into an ultrafiltrate (UF), containing suint but nosuspended solids, and a stream in which the suspended solids are concentrated.The application of ultrafiltration to wool-scouring effluents has been examined byPearson^, using a Dorr-Oliver 6CL pilot plant, and by Beaton^, using a larger Dorr-OliverIO PO R test unit. Mozes and T ur pie have also investigated the process by using a hollow-fibre membrane configuration, while EarlyS has operated pilot plants with both hollow-fibreand tube modules. The study reported in the present paper was undertaken to evaluate theUF process on the final effluent from a fine-wool scour involving the use of the WRONZScouring System. The process was thus tested under commercial scouring conditions, thelargest available pilot plant, with full-scale membrane cartridges, being used. Tests weredevised to establish the optimum operating parameters and the ability of the membranesystem to be opera ted for extended perio ds. ^

2 OBJECTIVESIt was desired to obtain data on the following:

(a) the rate of filtration and how this was affected by the process-liquor concentra-tion and its velocity of flow past the membrane, the pressure differential, and thetime between cleaning cycles;J. Text. Inst. 1979 No. 9 I

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^^^ 37TheUltrafiltrationofWool-scouring b) the resistance of the membrane to abrasive wear and blockage; c) the characteristics of the ultrafiltrate;cT) the effectiveness of cleaning; and{e ) the characteristics of the concentrate.

The pilot plant was operated in three modes to enable appropriate measurements andobservations to be made.3 , E Q U I P M E N T

The trials were carried out on a skid-mounted pilot plant supplied by Dorr-Oliver Inc.,nme IOPOR-type FRP membrane cartridges being used. These cartridges consist of a seriesof fiat membrane-coated leaves, stacked in parallel and 3.2 mm apart. The ultrafiltrate istaken off via the porous interior of each leaf to a central header point. Each cartridge has1.9 m2 of mem brane area, and three cartridges are located inside a G R P pressure housin gto form a module. In the pilot plant, three such modules were connected in series in themain process loop, which also contained a variable-speed recirculating pump, a shell-and-tube heat exchanger, and a magnetic flowmeter and had suitable drain, inlet, and outletconnexions. The u nit was equipped with a pressure sensor with high- and iow-pressure cut-outs and alarms to prevent possible membrane damage.

Tt was necessary to provide sotne ancillary components to complete a flexible, reliablepilot plant. These included a i500-litre storage tank, a slow-speed positive-displacementscrew pump to feed and pressurize the UF loop, and a 200-litre steam-heated cleaning-solution tank. The complete plant is illustrated diagrammatically in Fig. 1.

4. E X P E R I M E N T A L4.1 Operating ModesThe three operating modes are depicted in Fig. 2.

Fig. 1Diagram of the pilot plant: 1)scour-liquor feed; 2) storage tank1500 litre) ; 3) positive-disptacement feed pu mp ; 4) cleaning-solution tank 200 litre) ; 5) variable-speed recirculation pum p;6) variable-speed cleaning-solution feed pum p; 7) ultrafiltrationmo dules; 8) pressure gauge; 9) ultrafiltrate sample valves;10) heat exchanger; II ) tempera ture-control valve ; 12) drainvalve; 13) pressure-relief valve; 14) steam supply; 15) concen-trate discharge; 16) ultrafiltrate discharge; 17) flowmeter

Fig. 2Diagram showing the three operating mode s: 1) storage tank ;2) feed pum p; 3) recirculation p um p; 4) mem hranes; 5) heatexchangerJ. Text. Inst,,1979,No. 9

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Effluents hisnallan dRobinson 3994.2 Batch concentration M ode

After preliminary tlux determinations in which cleaning solution was used at 50"C,206 kPa, and a velocity of 2.14 m/sec, the loop was drained and immediately refilled withprocess liquor (scour effluent) by the screw feed-pump. Duritig this stage, the ultrafiltrateports were closed to prevent static filtration (i.e., with no liquor movement over the surfaceof the mem bran e), and care was taken to bleed at air from the loo p while it wa s being filled.The loop was then pressurized by throttling the bleed valve returning liquor to the storagetank until the pressure was above the lower cut-out value of 75kP a. This allowed the mainrecirculation pump to be started and its speed adjusted to give the required velocity ofllow, whereupon valves could be opened to withdraw ultrafiltrate from the system.Provided that the bleed rate from the loop to the storage tank was about ten times theultrafiltrate flow-rate, the contents of the loop and the storage tank were maintained at thesame (but increasing) concentration. The ultrafiltrate and the concentrate were sampledand th e ultrafiltrate flow rates m easured at regular inte rvals of either 15 or 30 min. Th etemperature was kept at 5O''C, the maximum permissible for extended operation, sincePearson^ and Beaton* had shown that the flux increased with temperature.

4.3 Total recycle ModeWhere it was necessary to have a process liquor of constant composition in order totest for variables such as the efiect of flow velocity over the membrane and the trans-membrane pressure, all the ultrafiltrate was recycled to the storage tank. The loop-liquorcomposition was thus held nearly constant, though some minor changes occurred becauseof evaporation and splashing. The start-up procedure was identical to that described above.A total recycle for up to 36 hr before cleaning was also used to test the decay of flux

with time because of slow fouling of the membrane.4.4 Continuous concentration Mode

This simulated the operation of a commercial system using a cascaded series of loops.The pilot plant was operated as if it were one of the loops of a cascaded system, both con-centrate and ultrafiltrate being discharged. An operating point was selected from the pre-vious experiments, the required concentrate-discharge rate being calculated from a know-ledge of the input mem brane-retained-solids con centration (M RS ), the conce ntrate M R S ,and the ultrafiltrate flow-rate. The plant was started up as before and the calculated con-centrate flow for the selected operating cotiditions obtained by adjustment of the loop bleedvalve. Sampling and flow measurements of both the ultrafiltrate and the concentrate weredone at 30-min intervals.4.5 Membrane cleaning

A prop rietary cleaning solution developed by D orr-OH ver In c. was used for all cleaningoperations. The exact procedure was considered to be important and was as follows.(i) All pumps were switched off, the main feed valve was closed, and the contents

of the loop were drained,(ii) Th e loop was imm ediately refilled with a flushing solution contain ing 0 . 1 %non-ionic detergent at 50C.(iii) The flushing solution was circulated around the loop by the main recirculatingpump, and suflicient solution was allowed through the membranes to flush theultrafiltrate lines.

(iv) The unit was shut down, drained, and immediately refilled with cleaning solu-tion at 50C../. Text. Inst. 1 979 No. 9

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400 37TfteUltrafiltrationofWool scouring(v) The initial cleaning cycle providedforflow past the me mb rane surface with noultrafiitrate draw-off. Ifthe initial flux was notrecovered within 1.5 hr theultrafiltrate valves were opened toallow cleaning solution th roug h the m em-branes.

This procedure was evaluated bym easurin g the u ltrafiltrate fluxatregular intervalsduring theclean ing cycle. Before allscour-liq uor trials, ultrafiltrate-flux det erm inat ion swere made on each membrane by using cleaning solution at50C,avelocity of 2.14 m/sec,andaloop pressure of 206 kP a. This allowed an evalua tion of the degree of fouling occ urringduring the trials and enabled the cleaning cycle necessary to restore the fluxto the pre-testconditionstobe decided.The modules were dismantled after every 50-hr operation and the membrane cartridgesexamined for any signsofabrasive wearorphysical b locka ge.

4.6 AnalysesSamples were analysed for total solids (TS), total greas e, dir t, suin t, andC O Dby

methods applicable to wool-scour liquors^-**.4.7 Examinationof the Concentrate

Destabilizationofthe con cen trate was effected by add ing conc entr ated sulph uric acidto pH2and boiling for 30 m in. The destabilized sludge w as neutralized with lime and de-watered inaSharpies P600 horizontal decan ter. A polyelectrolyte was added to the d ecanterfeedtoimprove solids c aptur e.Samplesofthe con centrate were senttoSolvent Beige for ev aluatin g the po tentia lforgrease recovery prior to final disposal. i

5. RESULTS AND DISCU SSION5.1 Effectof Process ConditionsS.I.I Process-liquor Coneentration

The ultrafiltrate flux was found to behighly dependent onthe concentration ofth eliquor in the loop,as is typically shown inFig.3. Fluxes as high as 135 litre hr~^ m -at4.2% total solids and down to 11 litre hr-i m- at26 % total solids were recorded. Theresults wereingeneral ag reeme nt w ith those reported by Pearson^ and Beaton* .5 0 .

CJtY 2.InLOOP HIESEUat i 10 HP*

Eiiril 50 C

Fig.3VariationofUF flux w ith total solidsin the loop in batch concentration 10 15T r iT A L S O L l P S , I - .

/ . Text. Inst. 1979 No.

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Effluents Chisnall and Robinson 401The flux of interest to the plant designer is the average value over the desired range ofconcentrations. This can be obtained for batch operation by measuring the volume of ultra-filtrate in a given time or by plotting the flow-rate-time curve and measuring the area underthe curve. A typical curve is shown in Fig. 4, and some fluxes derived from it are given inTable I. Th us a fivefold conc entration of the M RS {3 3%) in this liquor gave an average U F

COM) r I m i 'v ociry 2.7 n/tHtW I LOOP PHtSSUBt JOO eP

Fig.4Variation of UFflow with time inbatch concentration: L = loop total solids; UU F total solids

TableIAverage FluxesforVarying DegreesofC


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402 31The ltrafiltrationofWoohscouringabrasive wear was found after 205hr ofope ration . The long-term resistanceofthe m em-branestoabrasive wear and othe r physical damag e is,ofcours e, c riticaltothe viability ofthe process.

The velocity ofthe liquor was found tohavea large effectonthe U F flux, this beingdoubled by increasing the velocity from 2.15to2.7 m/sec. T his is explained byareductionin the build-upof a boundary layer and the increasing rateatwhich the rejected m atte riscarried away from the membrane surfaceatthe h igher velocity.5.1 J Trans-membrane Pressure

Ultrafiltration theory indicates that, above abou t 100 kP a, the U F flux should becom eindependent ofthe applied pressure becauseofcompaction of th e boundary layer, whichwo uld result in an increase in resistance to flow, cancelling ou t any g ain in fluxatthe higheipressure. This was foundtobe tru eathigh concentrations ( >5 TS) andatrelatively lowvelocities (


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Effluents Chisnall and Robinson 403Typical changes in flux during cleaning are shown in Fig. 6. The highest cleaning-solutionflux achieved was 270 litre hr-^ m"-, but it was found that the degree of cleaning did notmatter as long as the flux was increased to a level above that required in the next trial.

As a result of the trials, a daily 2-hr cleaning cycle is recomm ended, a D orr-O liver pro -prietary cleaning solution being used at pH 10.5. (A higher pH would result in an increasedcleaning flux but could damage the membranes.) The membranes should be left to soak inthe solution when not being used.5.4 Continuous Concentration

Operation in this mode most closely approached the commercial situation. Both ultra-filtrate and concentrate were withdrawn from the system, which was allowed to equilibratebefore sam pling comm enced. Fig. 7 illustrates the results obtained . The d otted line shows abatch-concentration curve under the same conditions, and it is significant that all but twoof the results are better than the batch-concentration curve.

(UODLTIONS:VtLOCITY 2.1 U / EAN UOQP MS SUM 206 P)1lENPtBiTUBE M' C

COHDI t l CNSwELDCtrv 2 . a s /nC M LQ IlP PRESSURE 2*10 KpATEn PEmi u B i SO ' C

5 5U 15 60 7 5 90 1D5I I N E "I K

Fig. 6Cleaning-solution flux

10 12 lit 16S O L I D S . I - V -Fig. 7Variation of UF flux with total solidsin loop in continuous conccn I ratio n; the

broken curve is for batch concentrationunder the same process conditions5.5 Cha racteristics of the Ultrafiltrate

The ultrafiltrate was a clear amber liquid with a strong odour of suint, free from sus-pended matter and with a tendency to foam when shaken. Its concentration, colour, andCOD increased with increasing concentration in the loop, as shown in Fig. 8. The total solidscon tent ranged from 1.55 to 2.86 % and the closely related C O D from 11 400 to 19 400 mg/litre. The maximum solvent-extractable matter observed was 0.07%., and this was consideredto be detergent rather than grease^, since the emulsified grease particles in the originalliquor were probably very much larger than the pores in the membrane.5.6 Cha racteristics of the Conc entrate

Concentrates with 20% total solids were produced as a matter of routine with finalUF fluxes of around 15-20 litre hr~^ m^^ and no problems were encountered in cleaning themembranes afterwards. The highest concentration reached was 26% total solids (16.6%wool grease, 6.7% dirt, and 2.7% suint) at a flux of 10.7 litre hr-^ m-^. This sludge was aJ. Texl. tnst. 1979 No. 9 \

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37 The Ultrafiitration of Wool-scouring

L O O P I Q T S L S D U D S . I / wFig. 8Relation between UF tota l solids and loop total solids for three runs wilh different liquors

thick viscous fluid above 40 C and b ecame a greasy solid on co oling. It may not be econo m icto concentrate to this level because ofthe low flux, but this would depend on the subsequenttreatment ofthe sludge.The sludge could be destabilized by boiling with acid as described and dewatered in adec ante r centrifuge to give a solid sludge w ith a total solids con tent of 45 %.The sludge had a high grease content, and reports from Solvent Beige indicated thailaboratory recoveries of 90% of the grease were possible from their experience with theSover solvent-scouring system. The grease could be refined and the degreased sludge driedto a disposable powder in a rotary-drum dryer.

6 . E C O N O M I C SThe cheapest method of effluent treatment is generally in admixture with domesticwastes in a municipal treatment plant. However, large increases in the charges levied onindustry have recently been imposed in many ofthe wool-processing areas, and restrictionson the quality ofthe effluent accepted have become more severe. Both these factors favourthe use of an in-plant treatment, such as ultrafiltration. To be economically viable, thistreatmen t must be cheaper than com peting m ethods or have particular advantag es, such asthe ability to achieve stringent efliuent standards relating lo suspended solids and grease.The cost of an ultrafiltration unit to treat the eflluent from a 2-m-wide scouring train,operating u nder the W R O N Z Scouring System for 5000 hours per year and handling 1000 kgof greasy merino wool per hour, has been estimated as about 2 p per kg of scoured wool, asshown by the analysis of costs given in Table IL It should be noted that it is important incomparing methods of treating wool-scouring effluents to base the cost on the mass of woolprocessed, since the volume of effluent produced can vary by a factor often between plants.The costs of disposal of the ultrafiltrate and the sludge would be additional to this butvery variable depending on the circumstances. Thus it might be possible to lecycle part ofthe ultrafiltrate to the scouring train to make savings in detergent and water usage at the

expense of a build-up of dissolved solids and a possibility of staining the wool. Since theultrafiltrate virtually contains only suint that is rich in potassium, it is a potential source ofpotassium compounds in a fairly pure form.The sludge could probably be disposed of by dumping, or it may be feasible to dewaterit, or to recover the grease from it. Th e last possibility could very well be a viable prop ositio nif the concentrates from several plants could be combined for grease extraction and finaldisposal at a central treatment facility.

J. Text Inst. 1979 N o. 9

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6 37 The Ultrafiltration of fVool-scoitring Effluents Chisnall and RobinsonREFERENCES

R. G. Stewart, G. V. Barker, P. E. Chisnall, and J. L.H oare. WR ONZ Report No. 25, 1974; WoolRec.1975, 127 Text. Mach. SuppL, 1 Feb., 8 ; 18 April, 13.2 R. P. Marker. Chem. Engr,1970, 77 Jan./Feb., CE8.^ J. R. McCracken and M. Chaikin.Text. J. Aust., 1976, 51, N o. 2. 4. J. R. Christoe. J. Water olln Cont. Fed, 1977, 49 848.5 A. J. C. Pearson. /. Water ollnCont. Fed.,1976, 48, 945. N. C. Beaton. Text. Inst. Industr.,1975, 13, 361. T. E. Mo7es and D . W. F. Tiirpie. SAWT RI Technical Repor t No. 349,1977.^ D. B.Early. WRONZ Report No. 48, 1978. R. J. Fearn, W. H. Hetherington, S. M. Jaeckel, and C. D. W ard. J. Soc. Dyers Col.,1967, 83, 146.(P.E.C.) Wool Research Organisation of New Zealand Inc.,Christchurch,New Zealand.(B.R.) International Wool Secretariat,Technical Centre,Ilkley.West Yorkshire.

Received 3\.Accepted for publication 15.2.1979

. Text. Inst., 1979, No. 9

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The Ultrafiltration of Wool-scouring Effluents

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