Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperationbetween Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the EuropeanDesalination Society and Office National de l’Eau Potable, Marrakech, Morocco, 30 May–2 June, 2004.

0011-9164/04/$– See front matter © 2004 Elsevier B.V. All rights reserved

Desalination 167 (2004) 387–392

Nanofiltration of biologically treated textile effluentsusing ozone as a pre-treatment

A. Bes-Piá*, A. Iborra-Clar, J.A. Mendoza-Roca, M.I. Iborra-Clar,M.I. Alcaina-Miranda

Department of Chemical and Nuclear Engineering, Universidad Politécnica of Valencia,Camino de Vera s/n, 46071 Valencia, Spain

Tel. +34 (96) 387-9633; Fax +34 (96) 387-7639; email:mbespia@iqn.upv.es

Received 29 January 2004; accepted 12 February 2004

Abstract

Water scarcity in Mediterranean areas such as Comunidad Valenciana (Spain) makes water reuse necessary in highwater-consuming industries. Previous studies by our research group showed that nanofiltration (NF) membranepermeates could be reused in some processes of finishing, dyeing and printing in the textile industry. In this work,biologically treated textile wastewaters were subjected to ozonation as a pre-treatment stage to NF. The aim was toreduce organic matter in order to prevent membranes from fouling and to oxidize organic wastewater compounds thatcould damage the membrane material. NF experiments were carried out in a laboratory plant equipped with a pressurevessel containing one spiral-wound membrane element (2.51 m2 of active surface). With ozonation, wastewater CODwas reduced up to three different levels (160, 135 and 82 mg/L). NF experiments with wastewaters of different organicmatter concentrations were carried out studying the effect of increasing the feed concentration periodically. Con-ductivity retentions higher than 65% were achieved, with no significant flux decay observed during the experiments.

Keywords: Wastewater; Textile industry; Membrane; Ozone; Reuse

1. Introduction

Recycling of wastewater in industries withhigh water consumption is especially important in

*Corresponding author.

areas with water shortages such as ComunidadValenciana (Spain). Several authors have pro-posed integrated treatment systems in order torecycle wastewater in textile factories. All thesesystems include a conventional treatment (mainly

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biological) and membrane technologies [ultra-filtration (UF), nanofiltration (NF) or reverseosmosis (RO)], which produce permeate streamsof sufficient quality to be reused [1,2].

In 2003 Krull et al. proposed a method basedon a two-stage biological anaerobic–aerobicprocess followed by a membrane bioreactor andfinally a chemical stage to remove the remainingcolouring of the process water with ozone [3].Other investigators developed integrated processschemes where that membranes are the firsttreatment stage. In this way, Ambruster et al. in2001 described a textile wastewater treatmentplant consisting of the following steps: sieving,conventional filtration, UF and NF. NF permeatewas recycled into the production process. Con-centrated streams from the UF and NF stageswere treated in a fixed-bed bioreactor so that theycould be discharged into the municipal waste-water treatment plant [4]. Lee et al. proposed asimilar scheme in 2001. However, these authorsomitted the UF stage previous to NF and includedadvanced oxidation processes. These processeswere applied both to NF permeate streams beforetheir recycling and to concentrate streams fromNF before and after the biological treatment. Theaim was to increase biodegradability and finalquality, respectively [5].

In summary, it can be said that there are twopossibilities of combining biological treatmentswith membrane technologies to obtain water to bereused: either direct wastewater NF (with theappropriate pre-treatment) and biological treat-ment of the NF concentrate or biological treat-ment of raw wastewater and NF or ROafterwards.

In this work, NF of biologically treated textilewastewater was studied, including chemical oxi-dation with ozone before NF in order to reducethe organic matter in the membrane feedingstream. This is an advantage not only for NFmembranes (fouling is reduced) but also for theconcentrate stream treatment (evaporation stage).

1.1. Oxidation with ozone

Ozone is a powerful oxidizing agent that mayreact with organic compounds either directly orvia radicals formed in a reaction chain as OH-radicals. Ozone can be used in wastewater field toreduce COD, colour, toxicity and pathogens andto improve wastewater biodegradability the andcoagulation–flocculation processes [6,7].

Ledakowicz et al. [8] reported that azo-dyesused in the textile industry react rapidly withozone by direct attack forming decolouredproducts. Ciardelli et al. [9] confirmed significantcolour removal (95–99%) after applying ozone towastewaters from a dyeing and finishing factorypreviously treated in an active sludge plant withfiltrated through sand. Regarding COD removal,ozone treatment with 40 gO3/m3 at contact timesof 15 min and 30 min drove COD reductionsfrom 160 to 53 and 203 to 123 mg/L, respect-ively. The most important parameter to evaluateprocess feasibility is the quotient between thegenerated ozone and the eliminated COD(kg O3/kg COD). This parameter ranged between1 and 4 according to the literature [10]. Valueshigher than 3 could make the oxidation processfor COD elimination economically unfeasible.

1.2. Objectives

The main objective of this work was the studyof a biologically treated textile wastewater withozone as a nanofiltration pre-treatment. First, theoxidation times and ozone doses had to be deter-mined. Then, the effect of remaining organicmatter in the NF experiments had to be studied interms of permeate COD, salt rejection and flux.

2. Materials and methods

This work was carried out in two steps. Thefirst step consisted of the ozonation of the bio-logically treated textile wastewater to reduce the

A. Bes-Piá / Desalination 167 (2004) 387–392 389

organic matter. The main reasons were to preventmembranes from fouling and to oxidize organiccompounds that could affect membrane behav-iour. The next step was NF on pre-treatedwastewater.

2.1. Ozonation experiments

The ozonation experiments were carried out ina laboratory plant equipped with three ozonegenerators (4 g/h each one) fed with pure oxygenand a contact reactor of 25 L. The oxidizedwastewater volume was 50 L in each experiment.The experiments were controlled continuously bythe oxidation–reduction potential (ORP)measurement.

Fig. 1. Photograph of the laboratory plant.

2.2. Nanofiltration experiments

Membrane experiments were performed in alaboratory plant equipped with a pressure vesselthat contained one spiral-wound membraneelement. The operating conditions of the experi-ments were a transmembrane pressure of 12 bar,300 L/h of feed flow rate and a temperature of25°C. NF experiments lasted was 6 days. Fig. 1 isa photograph of the laboratory plant.

Permeate fluxes JP (L/m2h) and salt retentionRSALT (%) were determined periodically. Permeateand reject streams were recycled into the feedtank. However, 5 L of permeate were withdrawnfrom the system every 24 h in order to increasethe feed concentration up to a volume concen-tration factor (VCF) of 2.5. The COD of thesesamples were analyzed.

The membrane tested was DK2040 (2.51 m2

of active area) from Osmonics according to pre-vious results obtained by the research group [2].

3. Results

3.1. Wastewater characterizationCharacterization of the biologically treated

textile wastewater is shown in Table 1. As can beseen, both COD and conductivity are still high toreuse the water in the textile factory.

3.2. Ozonation experiments

Fig. 2 shows the ozonation results in terms ofwastewater COD and ORP variations with the

Table 1Textile wastewater characterization after activated sludgeprocess

Parameter

Conductivity, mS/cm 2.59COD, mg/L 205pH 8.36

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Fig. 2. COD and ORP evolution during ozonation of thebiologically treated wastewater.

ozonation time. It can be seen that CODdecreases rapidly at the beginning of the experi-ment (from 205 to 88 mg/L in 60 min). Thus, ifmaximal ozone production from generators isconsidered (12 g/h), the O3/COD ratio was 2.1.After 1 h, COD remained approximately constantuntil t = 450 min. Then, wastewater COD wentdown to 65 mg/L. From this time on, COD hardlydecreased and the experiment was stopped after650 min of ozonation. The O3/COD ratio forreducing COD from 88 to 65 mg/L was 67.5.Thus, it makes no sense, from an economicalpoint of view, to oxidize with ozone down to65 mg/L COD.

For ORP it was observed that this parameterrose sharply at the beginning of the experiment.Then its increase gradually became lower withtime since oxidation reactions hardly occurred.

According to these results, it was decided tocarry out further ozonation experiments in orderto obtain wastewater samples with three differentCOD levels (after 30, 45 and 60 min of oxidationtime). Wastewater COD values after these experi-ments were 160, 135 and 82 mg/L, respectively.Longer ozonation times were rejected as ex-plained above.

It has to be highlighted that all ozonationexperiments were performed with the samewastewater. Oxidation of samples taken on otherdays, even from the same factory, would produce,to some extent, different results in terms of the

Fig. 3. Evolution of permeate fluxes over the time for theexperiments with different feeding streams.

O3/COD ratio. The main reason is the applicationof different dyes in the factory.

3.3. NF experiments

Fig. 3 shows the evolution of the permeateflux over the time for the experiments carried outwith three different feeding streams (F1, F2 andF3) containing 160 mg/L, 135 mg/L and 82 mg/Lof COD, respectively, at the beginning of the NFexperiments.

It was observed that flux decay was higher forthe experiment with F1 due to higher COD con-centration. At the end of the experiments highpermeate fluxes were achieved with samples F2and F3 (37 L/m2h and 39 L/m2h, respectively).

Fig. 4 details the variation of salts retentionwith the time. In all cases salts rejection remainedpractically constant with a slight tendency toimprove because of the increasing feeding streamconcentration. The values ranged between 64%and 69%.

Table 2 shows the values of permeate fluxes,salt retention and permeate COD after 24 h and144 h (VCF = 1 and VCF = 2.5). For a VCF of 1,the highest salt retention was produced with F1,which corresponds with the highest COD. For aVCF of 2.5, the same tendency was observed.This is due to the influence of organic mattercontent in conductivity rejection and vice versa[11]. High COD values resulted in an increase of

A. Bes-Piá / Desalination 167 (2004) 387–392 391

Fig. 4. Evolution of salt retention over time for theexperiments with different feeding streams.

Table 2Comparison between flux, retention salt and COD in thepermeate at different VCF values

Feeding VCF = 1.0 VCF = 2.5

Flux,L/m2h

Rsalt,%

COD,mg/L

Flux,L/m2h

Rsalt,%

COD,mg/L

1 37.6 66.9 67 32.5 68.8 1222 40.1 64.8 54 36.8 67.0 983 42.5 64.0 <50 39.2 64.6 <50

the layer thickness surrounding the membrane,leading to a decrease in the available area for thesalt passage. As expected, for a given feedingstream, the higher the VCF, the higher thepermeate COD values achieved.

On the other hand, permeates obtained with F3were in all cases lower than 50 mg/L. Therefore,ozonated wastewater samples until a COD of82 mg/L generate permeates with sufficientquality to be reused for either VCF.

4. Conclusions

A COD removal efficiency of 43% wasaccomplished with low ozone doses at 60 minusing three ozone generators of 4 gO3/h, each onefor a biologically treated textile wastewater. Itwas not worth continuing with the COD removal

since too high ozone doses were required. How-ever, prior to an economical feasibility study, itshould be proved that these results are similar fordifferent wastewater samples that include otherdyes and detergents.

NF of the sample with the lowest organicmatter concentration (F3) presented the lowestflux decline during the process. Consequently, acombination of ozonation and NF results in anincrease of membrane life.

Salt retention was very similar for the threefeeding streams, reaching slightly higher valuesfor samples with higher organic matter concen-trations. Solely for the F3, permeate CODremained lower than 50 mg/L even with themaximum VCF (2.5).

References

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