Treatment and reuse of reactive dyeing effluents
Introduction
Even though it appears to be in plentiful supply on the earth's surface, water is a rare and precious commodity, and only an infinitesimal part of the earth's water reserves (approximately 0.03%) constitutes the water resource which is available for human activities. The growth of the world's population and industry has given rise to a constantly growing demand for water in proportion to the supply available, which remains constant. According to the data of the I.F.E.N (Institut Français de l’ENvironnement), the amount of water taken from the natural environment in France was estimated at about 40 billion m3 (1995). On the global level, the question of the supply of fresh water is becoming more acute every day. In the dyeing of textile materials, water is used firstly in the form of steam to heat the treatment baths, and secondly to enable the transfer of dyes to the fibers. Cotton, which is the world's most widely used fiber, is also the substrate that requires the most water in its processing. The dyeing of one kilogram of cotton with reactive dyes demands from 70 to 150 L [2] water, 0.6 to 0.8 kg NaCl and anywhere from 30 to 60 g dyestuff. More than 80,000 tonnes of reactive dyes are produced and consumed each year, making it possible to estimate the total pollution caused by their use. After the dyeing is completed, the various treatment baths are drained out, including the first dye bath, which has a very high salt concentration, is heavily colored and contains a substantial load of organic substances. One solution to this problem consists in mixing together all the different aqueous effluents, then concentrating the pollution and reusing the water either has rinsing water or as processing water, depending on the treatment selected (either nanofiltration or reverse osmosis for the membrane processes). These treatments concern only very dilute dye baths. This is generally not the case of the first dye baths recovered which are the most heavily polluted ones. The wastewater produced by a reactive dyeing contains:
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Hydrolyzed reactive dyes not fixed on the substrate, representing 20–30% of the reactive dyes applied (on average 2 g L−1). This residual amount is responsible for the coloration of the effluents and cannot be recycled.
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Dyeing auxiliaries or organic substances, which are non-recyclable and responsible for the high BOD/COD of the effluents.
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Textile fibres.
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Sixty to one hundred gram per liter electrolyte, essentially sodium chloride and sodium carbonate, which is responsible for the very high saline content of the wastewater.
In addition, these effluents exhibit a pH of 10–11 and a high temperature (50–70 °C). The legal regulations respecting the limit values for the release of wastewater are changing and are becoming increasingly severe, including the limits with respect to salinity. In France, the activities of installations subjected to authorization are delimited by a decree of 2 February 1998.
The objectives of this study were (a) to removal cotton fibres and to control the carbonate concentration which might strongly influence the nanofiltration, (b) the treatment of dye baths by nanofiltration in order to recover and reuse the sodium chloride and the water, (c) to select the NF membranes that were able to operate at high temperatures (50–70 °C), allowed for passage of monovalent salts, while retaining the hydrolyzed reactive dyes and (d) evaluate a RO process that was robust in terms of operating at elevated temperature (40–50 °C), able to operate at 80 × 105 Pa (80 bar) transmembrane pressure (TMP), and achieve greater than 90% of sodium chloride was reused. The simplified Fig. 1 presents the challenges of this study.
The process proposed by this study [1] has the following simultaneous advantages:
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compact: the place available in the dye houses is small;
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progressive: fashions and colors change; treatment must take place at the outlet of the dyeing machine, regardless of the bath, yet be capable of recycling a maximum amount of water and mineral salts;
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flexible: widely variable volumes must be capable of being treated;
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feasible: the quality of the water and of the brine must be constant, so as to be reusable for a new dyeing.
This paper presented the different methods currently used for the treatment of dye house effluents and those which are in the course of development. Then, it evaluated a four-step process to recover only the water and salts, while leaving the spent dyes in the reject stream. Processes evaluated included (1) cartridge filtration to remove textile fibres, (2) acidification to make the brine recovered, suitable for reuse and further dyeing operations, (3) nanofiltration (NF) to concentrate the hydrolyzed dyes and (4) reverse osmosis (RO) to further concentrate the salts for reuse in the dyeing process.
Section snippets
Review of current treatment methods of treating dyeing effluents
Owing to their high BOD/COD, their coloration and their salt load, the wastewaters resulting from dyeing cotton with reactive dyes are very polluted. For example, for Drimaren HF, this ratio is constant and around 0.35 for each dyeing step (bleaching step BOD = 1850 mg L−1, COD = 5700 mg L−1; neutralization step BOD = 290 mg L−1, COD = 830 mg L−1; dyeing step BOD = 500 mg L−1, COD = 1440 mg L−1; soaping step BOD = 310 mg L−1, COD = 960 mg L−1). As aquatic organisms need light in order to develop, any deficit in this respect
Solutions
The effluents which constitute the object of this study result from the dyeing of cotton with reactive dyes by the exhaust process. The reactive dyes used are of the following types: vinyl sulfone, monochlorodifluoropyrimidine, monochlorotriazine, trichloropyrimidine and monofluorotriazine. The hydrolyzed reactive dyes present in the treated effluents comprise the entire range of possible types of reactive dyes. Diagrammatically, a reactive dye has the form of a complex molecule which is
Results and discussion
The results are normalized with respect to temperature.
Conclusion
Textile process who dye cotton by the exhaust method with reactive dyes are facing by increasingly restrictive environmental problems. It consists of different steps (pretreatments, nanofiltration and reverse osmosis). The two pretreatments ensured the efficiency and strength of our process and take into account the industrial requirements. After having determined the cut-off of the prefilter on the laboratory-scale, this choice was validated on industrial site. All cotton fibres were stopped.
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