Ind. Eng. Chem. Res. 2001, 40, 3973-3978
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SEPARATIONS Water Reclamation in the Textile Industry: Nanofiltration of Dye Baths for Wool Dyeing B. Van der Bruggen,*,† I. De Vreese,‡ and C. Vandecasteele† Laboratory for Environmental Technology, Department of Chemical Engineering, University of Leuven, W. de Croylaan 46, B-3001 Heverlee, Belgium, and Centexbel-Gent, Technologiepark 7, 9052 Zwijnaarde, Belgium
The textile industry is a large water consumer: dyeing, rinsing, and follow-up treatment of textiles use large amounts of freshwater. As regulations become more and more stringent and the cost of freshwater increases, reclamation of wastewater becomes more and more attractive. This paper explores the possibility of using nanofiltration to improve the wastewater quality to the standards that are used for the dyeing of wool. Four different samples from the wastewater treatment sequence of a textile factory were filtrated with three different nanofiltration membranes (NF70, UTC-20, and NTR 7450). The samples were a used and untreated metal complex dye bath, a used and untreated acid dye bath, a sample from the storage tank (containing a diluted mixture of the previous baths), and the effluent of the biological treatment. Nanofiltration was possible for all samples, but the biologically treated dye baths showed a more efficient color removal. For direct nanofiltration of used dye baths, two membrane passages would be needed to provide the required permeate quality. Flux decline due to adsorption of organic material on the membrane decreased the membrane capacity by up to 73%, but the process water flux reached a stable value in all experiments. The flux decline was less important for the biologically treated water. The effect of flux decline was only partly reversible; the effect of osmotic pressure on the process water flux is fully reversible. It was found that flux decline is largely concentration-dependent: higher concentrations of organic compounds always caused lower process water fluxes. Introduction Water reclamation is a key topic in the textile industry. In West Flanders, the province where most Belgian textile factories are located, the groundwater level is dropping to an alarming depth. For quality reasons, textile factories are large consumers of (ground)water. They are only allowed to use groundwater up to a certain level, which is determined in advance for a period of (usually) 2 years. In addition, the permit is often conditional and is related to the factory’s efforts to find possibilities for water reuse. In the future, many of these factories will face the requirement of reusing a significant part of all incoming freshwater. This involves an improvement of the wastewater quality to the standards used for freshwater (groundwater). In this context, nanofiltration can be the technique needed to meet the future regulations.1-6 Dye baths usually contain high concentrations of organic compounds as well as inorganic compounds.7 Environmental problems with used dye baths are related to the wide variety of different components * To whom all correspondence should be addressed. Tel.: +32 16 32 27 26. Fax: +32 16 32 29 91. E-mail:
[email protected]. † University of Leuven. ‡ Centexbel-Gent. Tel.: +32 9 220 41 51. Fax: +32 9 220 49 55. E-mail:
[email protected].
added to the dye bath, often in relatively high concentrations. Several additives remain in solution and are thus discharged in the wastewater. Of course, the dye itself is the most “visible” problem. Degradation of dyes as well as additives in biological systems is problematic, and the remaining fraction in the wastewater after biological degradation is still high. Standards for wastewater discharge can differ significantly from factory to factory. So far, the general standards in Flanders8 do not include a regulation for the presence of color. For discharge in surface water, the chemical oxygen demand (COD) norm is 400 mg/L and the biochemical oxygen demand (BOD) norm is 25 mg/L. The environmental legislation stipulates that individual permits can be more rigid, which is usually the case (e.g., 150 mg/L of COD). European guidelines for urban and industrial wastewater9 mention a maximal COD of 125 mg/L and a maximal BOD of 25 mg/L. The requirements for reuse as process water are less obvious. In the textile industry, the required water quality is generally not determined. A few basic conditions are related to water turbidity, which should be comparable to the groundwater that is used as freshwater, and the water hardness, which should be in the normal range for relatively soft groundwater (not higher than 40 mg/L Ca or 10 °Fr). Of course, all color should be removed before reuse. Furthermore, no concentration
10.1021/ie010104y CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001
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Ind. Eng. Chem. Res., Vol. 40, No. 18, 2001
Table 1. Composition of the SA and MC Dye Bathsa Additives component foam-reducing agent egaliser antimoth wetting agent salt (Na2SO4) acid bleaching agent lubricant acid spender
SA bath (g/L)
MC bath (g/L)
0.25 0.85 0.15 0.7 7.8 2.4 1.55 1.7 na
0.25 0.85 0.15 0.7 a 2.4 na 1.7 0.6
manufacturer Textilcolor Bohme Bayer Bayer Boucquillor Boucquillor Clariant Bohme Textilcolor
Dyes Sandolan Jaune E2 GL 133% AY17 Sandolan Rubinol E3 GSL 230% AR37 Bleu Alizarine 110% GS AB 45 (CI Acid Blue 45)
Clariant Clariant Stiernon Lanasyn Jaune S2 GL Isolan Rot SRL (azochromium complex with CI Acid Red 414) Lanasyn Bleu SDNL Lanasyn Bordeaux SD Erionyl Black MR (CI Acid Black 172)
Clariant Dyestar Clariant Clariant Ciba-Geigy
a Salt is generally not used for MC dye baths, but sometimes a combination of SA and MC dyeing is applied. The salt concentration is then 7.8 g/L. na ) component not used.
Figure 1. Schematical representation of the wastewater treatment (MC, metal complex bath; SA, acid bath; BT, buffer tank; AS, activated sludge system) with indication of the positions where samples were taken (1, MC bath; 2, SA bath; 3, BT; 4, AS system).
of other components such as heavy metals can be allowed in the water cycle. In this paper we aimed at investigating the possibilities of nanofiltration to decrease the quantity of the wastewater and simultaneously decrease the amount of freshwater that is needed, by reusing the treated wastewater. The wastewater of a textile company was nanofiltrated at different stages of the discharge path (Figure 1). At first, two processed dye baths (without any further treatment) used for dyeing of wool were separately nanofiltrated. The traditional treatment sequence consists of mixed storage in a buffer tank (BT), followed by an activated sludge (AS) treatment. The effluent of the AS treatment is eventually discharged. The wastewater in the storage tank, as well as the biologically treated wastewater, was also nanofiltrated. Methods and Materials Wastewaters Origin. The dye baths were obtained from a textile factory that is specialized in wool processing. For the dyeing of wool, mainly acid dyeing and metal complex dyeing are used.10 Both were treated using nanofiltration. They are indicated as MC (metal complex) and SA (strong acid). The compositions of the dye baths are summarized in Table 1. The dyes that are used are indicated with their commercial names.11 Sandolan is an acid dye with exceptionally good lightfastness and a good penetration.12 Lanasyn12 is a MC dye with good fastness properties that is often used in
combination with other dyes. Isolan Rot SRL13 is a red MC dye that contains 3.1% chromium. Erionyl Black MR14 is a black MC dye with high wet fastness and a shade shift in artificial light toward red. Alizarin Brilliant Blue GS is an acid dye. The concentration of the dyes was 8 g/L for the MC and SA baths. In the factory, the dye baths are collected in a BT in order to supply a continuous feed to the AS system. The composition of the wastewater at this point corresponds more or less to the weighed average of the SA and MC baths, where the weighing factor is the fraction of each dye bath that had been used. On the average, this is 25% for MC and 75% for SA. Usually, it is necessary to rinse the dyeing machines after each dyeing, especially when dark colors are followed by lighter ones. The concentrations in the BT will be lower than the weighted average because of the dilution effect. The preparation tanks are also periodically rinsed with boiled water in order to clean the tanks; this double rinsing results in a lower final concentration. Additionally, some smaller wastewater streams are added, such as test baths. In this way, also other components can end up in the BT, usually in small concentrations. The process scheme for the water treatment in the factory considered is represented in Figure 1. The AS system was operated at a retention time of 72 h. The sludge concentration was 5-8 g/L; the COD of the influent was 1600-2000 mg/L; the COD of the effluent was 200-300 mg/L. A typical composition of the biologically treated wastewater (AS) is shown in Table 2. The amount of settleable solids was low because the wastewater was settled out for 24 h prior to the analysis and experiments. Dyebath Characterization. The following techniques or methods were used for the analysis of the dye baths: BOD (measured after 5 days) and COD were determined according to their respective definitions;15 total phosphorus was determined using the molybdene blue spectrophotometric method, after oxidation to orthophosphate; total nitrogen was determined as ni-
Ind. Eng. Chem. Res., Vol. 40, No. 18, 2001 3975 Table 2. Composition of the Effluent of the AS System parameter
value
conductivity (µS/cm at 20 °C) pH BOD (mg of O2/L) COD (mg of O2/L) total P (mg of P/L) total N (mg of N/L) chlorides (mg/L) suspended solids (mg/L) settleable solids (mg/L) arsenic (µg/L) cadmium (µg/L) chromium, total (µg/L) chromium(VI) (µg/L) copper (µg/L) mercury (µg/L) nickel (µg/L) lead (µg/L) silver (µg/L) zinc (µg/L) PCBs (µg/L)
7410 8.35 11 231 1.54 21.7 146 37
