2001_G.ciardelli_Membrane Separation for Wastewater Reuse in the Textile Industry

Resources, Conservation and Recycling 31 (2000) 189–197

Membrane separation for wastewater reuse inthe textile industry

G. Ciardelli *, L. Corsi, M. MarcucciTecnotessile s.r.l., 6ia del Gelso 13, I-59100 Prato, Italy

Received 7 April 2000; accepted 30 June 2000

Abstract

A technical and economical analysis of the application of a membrane separationtechnique for the purification of wastewaters aimed at their reuse is described. The investiga-tion has been carried out by treating wastewaters of a pilot plant, reproducing on a smallerscale a separation system based on ultrafiltration and reverse osmosis. Significant indicationsfor the exploitation of this approach on the fulling industrial scale were gained during thework. The effluent from dyeing and finishing plants, after activated sludge oxidation, wastreated at an 800 l/h by means of sand filtration, followed by a separation in an ultrafiltra-tion membrane module. The last separation step, reverse osmosis at 8 bar pressure, produceda permeate (60% of the inlet flow) that, relying on the analytical screening performed, wasof much better quality with respect to process water presently in use. Therefore the permeateproduced can be re-used in all production steps, including the most demanding onesconcerning water quality such as dyeing with light coloration. A preliminary analysis ofinvestment and operating costs also gave encouraging indications of the economic feasibilityof the approach. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Color removal; Reverse osmosis; Textile wastewaters; Ultrafiltration; Wastewater manage-ment; Water reuse

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1. Introduction

Wet processes in the textile industries require water of very good qualityconcerning the content of dyes, detergents, and suspended solids. Therefore, a

* Corresponding author. Tel.: +39-0574-634040; fax: +39-0574-634045.E-mail address: [email protected] (G. Ciardelli).

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purification treatment to recycle water must have a better performance than forsimple discharge according to the limits imposed by legislation.

In order to meet legislative requirements, textile wastewaters are usually treatedin a chemical–physical, or most commonly, in an activated sludge biochemicalplant. In order to have water that can be recycled in production cycles, especiallydyeing processes, water needs further treatments (Klose, 1993).

In a previous paper (Ciardelli and Ranieri, 1998), we reported the positive resultsconcerning the application of ozone treatment for the purification of textilewastewater for re-use. This research background has been recently translated intothe realization of industrial plants for the full implementation of the technique. Itis interesting to see if this approach can be combined to the application ofmembrane technology for a more efficient management and recycle of textile dyeingand fulling plant water resources. The approach based on: (1) ozonization plant forbioresistant pollutant oxidation; and (2) ultrafiltration and reverse osmosis mem-brane treatment is schematized in Fig. 1 and compared to the present managementof wastewater in the dyeing and finishing textile plant.

This paper is concerned with the results of the experimentation of ultrafiltrationand reverse osmosis techniques on dyeing and fulling textile wastewaters. The pilotplant used was designed to reproduce an industrial treatment facility on a reducedscale. Experiments were run in order to gain information on:� the decrease of several analytic parameters concerning water polluting content;� membrane hydraulic performance during experimentation; and� water recycle possibilities at the operating conditions which guarantee optimum

cost/benefit ratio.The interest in membrane processing various industrial applications is increasingthanks to the more recent technological innovations that render them reliable andeconomically feasible in respect to other alternative systems. The design of theeffluent pre-treatment step (coagulation, sand filtration, disinfection) is crucial to

Fig. 1. Present (a) and proposed (b) wastewater management for a dyeing and finishing textile plant.

G. Ciardelli et al. / Resources, Conser6ation and Recycling 31 (2001) 189–197 191

guarantee a good and constant performance of the membrane’s efficiency (Coste etal., 1996).

Membranes are made of several materials and can be liquid or solid, of naturalor synthetic origin. They can be made of inorganic (ceramic) or organic (polymeric)materials. Polymeric (cellulose acetate, polysulphone, polyamide, polyvinyldenefluoride) membranes, for their characteristics, seem to be the most promising forapplication in the field of textile wastewaters. The state of the art in the field ofmembranes is currently anisotrope (asymmetric) membranes.

Anisotrope membranes present a thin film that avoids the entrapment of sus-pended solids into the membrane body and are therefore less subject to aging andflow reduction than symmetric membranes.

A recent development concerns composite membranes where a thin film withsmall pores is laid on a classical asymmetric membrane. This kind of membrane,originally developed for reverse osmosis, is also currently finding application inultrafiltration.

Membrane configurations are usually classified depending on the kind of modulesadopted. One of the most common is the spiral wound module. Having cylindricalform, it wraps the membrane in itself with a net that avoids membrane-to-mem-brane contact and lets the feed flow. The flow of the concentrate is parallel to theaxis of the membrane module while the permeate flows through the membrane ina radial direction (cross-flow), reaches a collector, and then flows axially in aseparate circuit.

One of the most recurring problems in membrane plants, also in the textile field,is the progressive worsening of the quality of permeate produced. The flowreduction has to be ascribed to a reversible (concentration polarization) or irre-versible (fouling) increased resistance of the membrane to the permeate flow.Membrane disinfection is necessary to avoid biofouling of the membrane surface,but reverse osmosis is not usually resistant to the more common chlorine-baseddisinfecting agents. Chemical cleaning with detergents and acid and basic solutionsis an alternative approach.

Membrane technology has found several industrial applications, supported bysufficient literature references, especially for ultrafiltration. The most importantones concern the treatment of tannery and textile wastes, oily emulsions, andelectrophoretic painting (Denaro, 1993). Membrane processes have been screenedfor the treatment and reuse of effluents mainly from textile dyebaths (Drioli, 1992;Buckley, 1992).

Ultrafiltration allows for water clarification and disinfection, without by-prod-ucts, in a single step and with a constant permeate quality. Ultrafiltration separatesparticles or molecules of dimensions higher than 1 nm and operating pressuresranging from 2 to 10 bar. It removes bacteria, viruses, proteins and some sugarsfrom effluents without possibility of re-growth after treatment (Gadani et al., 1996).

Reverse osmosis is another membrane process with a history in industrialapplications in the removal of salts from solutions, furnishing an almost deionizedwater (Marinas, 1991).


2. Materials and methods

2.1. Industrial effluents tested

Wastewaters treated were coming from two dyeing and finishing plants used todye fabrics, hanks, skeins, tops and flocks of different natural and synthetic fibersand a mixture of both. Effluents were first pre-treated by means of a biologicalactivated sludge plant.

2.2. Membranes

A Trisep 8040-UE50-TXA membrane, of the spiral wound type (with fiberglassouter wrap), was used for the ultrafiltration step. The membrane is 200 mm indiameter and 1000 mm in length and has a filtrating surface of 23 m2. Characteristicmolecular weight cut-off is 100 kDa.

A module, with two Toray polyamide membranes of the spiral wound typeplaced in series, was used for reverse osmosis. Each membrane is of 100 mmdiameter and 1000 mm length and has a filtrating surface of 54 m2.

2.3. Pilot plant

The pilot plant installed consisted in three stages: sand filtration, ultrafiltration,and reverse osmosis.

A part of the effluent from the biological plant was sent to the sand filter (3 barpressure) which had an output of 800 l/h. The filter was washed every 15 h.

Water from sand filtration was stored in a tank and then sent to the ultrafiltra-tion module at 4 bar relative pressure. A total of 10% of total flow was thepermeate of the ultrafiltration step while the rest is sent back to the storage tank.The average flow of the ultrafiltration step was 550–600 l/h. The membrane waschemically washed as soon as the hydraulic performance worsened. Chemicalstested were:� alkaline detergent, containing phosphonated and non-ionic detergents (5%) and

EDTA (5–15%);� alkaline detergent, containing sodium hydroxide;� neutral pH anionic detergent; and� acid detergent with nitric and phosphoric acid.The ultrafiltrated effluent was stored in a second tank and sent at 8 bar pressure tothe reverse osmosis module at an inlet flow of 500 l/h (40% concentrate fordischarge; 60% of permeate for reuse).

2.4. Effluents analysis

Chemical oxygen demand (COD), color content, conductivity, detergents, sus-pended solids, microbiological examination, pH, Redox potential were determined.In some cases other specific parameters as anions (chlorides, sulphates), metal-ions,S.D.I. (Silt Density Index), and turbidity were determined.


Fig. 2. Characteristic ultrafiltration permeate water flow.

3. Results

3.1. Membrane hydraulic performance

First trials were focused on the optimization and checking of performances forthe ultrafiltration step which played a decisive role for the success of the treatment.It has in fact a discontinuous operating cycle, because the treatment is interruptedfor discharge and cleaning with chemicals.Even after chemical cleaning, the initialof initial flow (1200 l/h) was not obtained in the subsequent cycles, but the initialvalue was stabilized at �800 l/h. The working cycle observed lasted for �80 hwith a final flow value of 400–450 l/h (mean value 550–600 l/h corresponding to aspecific flow of 20–25 (l/h)/m2). The characteristic evolution of the water flow withits operating time is shown in Fig. 2.

The reverse osmosis process worked with constant mechanical and hydraulicparameters. A total of 40% of the feed was discharged as concentrate as defined inthe water treatment strategy of the plant.

3.2. Contaminant remo6al

To test the performance of the various treatment steps, sampling of the effluentswas performed at the following six points:

(1) sand filtration inlet; and (2) sand filtration outlet;(3) ultrafiltration permeate outlet; and (4) ultrafiltration concentrate discharge;(5) reverse osmosis permeate outlet; and (6) reverse osmosis concentratedischarge.

The analytical data concerning the satisfactory performance of the treatment onCOD removal are reported in Table 1. Mean pH, conductivity and Redox potentialvalues are listed in Table 2.


Suspended solids (starting concentration 45 mg/l) were removed partially by sandfiltration (60%) and completely by ultrafiltration.

An adequate anionic surfactants removal was obtained (�95%). Mean MBASvalue was 3.1 mg/l at sand filtration inlet and 0.2 mg/l at reverse osmosis permeateoutlet.

Further results concerning the efficiency of the reverse osmosis in removingsalinity are given in Table 3. They confirm the optimal trend indicated by aconductivity decrease of the reverse osmosis permeate.

Colour is one of the most important parameters in checking textile wastewaterrecycling. Mean values of the absorbance at 420 nm and percentile removals basedon the integral of the absorbance curve in the whole visible range (400–800 nm) arelisted in Table 4.

A further evaluation, only on samples of the ultrafiltration permeate, was carriedout in order to test the performance of this step towards the reverse osmosis. Thistask is decisive for the success of the overall strategy since it strongly influences theperformance of the reverse osmosis (technical point) and the duration of themembrane (economical point). After ultrafiltration, the turbidity was reduced up to95% of the starting value while S.D.I. mean value was 1.5, showing that thepermeate quality produced by the ultrafiltration step guarantees a good perfor-mance and duration of the reverse osmosis membranes. They also indicate that thereverse osmosis permeate has very good analytical characteristics (almost total

Table 1Mean COD values and % removal at the sampling points

COD (mg/l O2) COD total removal (%)Sampling point COD removal (single phase) (%)

2631 – –164 38 382

471393 17–3034 –

87765 34–6 182 –

Table 2Mean pH, conductivity and Redox potential values and % removal at the sampling positions

Sampling point Redox potential (mV)ConductivitypH(ms/cm)

−551 7.8 36307.92 3610 −57

3 8.0 3550 −60−6836104 8.1

6.35 35 288.16 6780 −66


Table 3Mean potassium, magnesium iron and sulphates concentrations at the sampling positions 1 and 5

MagnesiumPotassiumSampling point Iron ChloridesSulphates(mg/l)(mg/l)(mg/l)(mg/l)(mg/l)

7.021.2 0.28 225 72510.07 75 421.1 4.2

removal of salts and organic content) to be reused in all processes of the textilefactories, even the most demanding concerning water quality such as dyeing yarnsor with light colors.

The reuse will drastically reduce the draining from wells furnishing water ofhigher and more constant quality. Moreover, the draining from wells will faceproblems with water shortages and possible taxes by local authority. It is alsoknown that the quality of water from wells tends to worsen with increases in timeand amount of water spilled (Smith and Wang, 1994).

In designing approaches based on membrane technologies, the management ofconcentrates (process by-products) must be considered carefully. The approachconsidered in the case study is to discharge both concentrates to a centraldepuration plant treating mixed civilian and industrial wastes. According to ourknowledge, approaches based on the treatment of membrane concentrates forrecycling are still at a laboratory development stage (Balanosky et al., 1998).

3.3. Economic analysis

On the basis of the results obtained, some economic considerations can be drawnto foresee the economical feasibility of the implementation of the membranetechnique for the treatment of dyehouse effluents for reuse. Some data is reportedin Table 5. Considering investment and operating costs, a value of �1 Euro per m3

treated is obtained, which would be a reasonable cost even for Italy where the costsfor water supply are still under the European average (Antonelli et al., 1998), but

Table 4Mean absorbance values and % color removal at the sampling positions

Sampling point Color removal (%)Absorbance at 420 nm

1 –0.0830.0752 6

3 0.068 16–0.1014

0.0025 950.1136 –


Table 5Operating and investment costs (evaluated in Euro) for ultrafiltration/reverse osmosis membranetreatment of 1000 m3/day (250 000 m3/year) of textile wastewaters

Total costClass of cost Annual cost % of total Cost per m3

(Euro) (Euro)

Cost of the plant 300 000 30 000 21 0.20(investment in 10 years)

34– 0.33Energy 50 000Chemical products 25 000 17 0.17–

40 000 28Membrane 0.27120 000(change every 3 years)

100 0.97420 000Total 145 000

are going to increase in the future. The duration of the membranes has beenestimated to be 3 years from results obtained. In fact, operating time for mem-branes of \1000 h can be considered sufficient to draw conclusions about areasonable life of the filtrating media (Linn et al., 1996; Dittrich et al., 1996).

4. Conclusions

Membrane processes show to be promising methods for purification aimed atreuse of textile wastewaters, resulting in direct environmental and economicbenefits.

The sand filtration furnishes a satisfactory reduction of suspended solid contentand also a reduction of the organic substances content and a slight effect on color.Further efficiency of the filtration could be obtained by adding a chemical coagula-tion step before filtration (Kurbiel and Rybicki, 1991; Crosse et al., 1996) thatwould however cause an increase of treatment costs. The quality of the effluentfrom ultrafiltration, as indicated by the S.D.I. values measured, is in accordancewith the required specification for feeding the reverse osmosis membranes. More-over, the time trend of the ultrafiltration permeate mean flow is satisfactory for acorrect dimensioning of a real industrial plant. No variation in the hydraulic andmechanical parameters of the reverse osmosis step was detected, indicating theabsence of membrane fouling.

The analytical parameters of the reverse osmosis permeate is of a high quality(�95% reduction of salt content, practical absence of COD and color) and can betherefore reused without problems since the quality of water presently used intextile wet processes (usually drained from wells and in part softened) is usuallyworse (conductivity of �800 ms/cm, absence of COD and color). Considering thequality of the effluent produced, the considerable reduction of the need of waterdrains and eliminating the need for water softening, the technique seems to be readyfor implementation on the industrial scale giving indication of technical andeconomic feasibility (Masson and Deans, 1996).


Acknowledgements

Giorgio Palloni (Fildrop, Campi Bisenzio, Italy) is kindly acknowledged fortechnical support, Elena Pini and Emiliano Romagnoli for collection and analyticalscreening of samples, Regione Toscana for covering part of the costs of theexperimental campaign.

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2001_G.ciardelli_Membrane Separation for Wastewater Reuse in the Textile Industry