Water recycling from desalination and purification process of reactive dye manufacturing industry by combined membrane filtration

https://doi.org/10.1016/j.jclepro.2004.02.044Get rights and content

Abstract

In this study, nanofiltration membrane and reverse osmosis membrane were tested to investigate the optimal operating conditions for COD, color, salt rejection and permeate flux and evaluate the feasibility of nanofiltration and reverse osmosis membrane combined process to improve the permeate flux and separation efficiencies in reactive dye manufacturing process wastewater. Effects of pressure, types of membrane and dye solution on flux and rejections were investigated. Experiments showed that the type of membrane had a significant effect on both permeate flux and rejection efficiencies. Variations in the applied pressure also affected the COD, color and salt rejections, which increased with increasing pressure. Finally, the nanofiltration and reverse osmosis membrane combined process was found to be suitable for the removal of residual organics, color, salt and the water recycling from waste stream arising from reactive dye manufacturing process. The permeate can be recycled back into the process, thus offering economical benefits by reducing the water consumption and wastewater treatment cost.

Introduction

A lot of wastewater was produced from the dye manufacturing processes and liquid effluents can contain toxic organic residues. Generally, these wastewater generation rates are of the order of 1–700 l/kg of product. Furthermore, the wastewater characteristics in a particular dye house can be highly variable from day to day and hour to hour, depending on the type and color of dye. Parameters of major interest in the treatment of dye manufacturing process wastewaters concern chemical oxygen demand (COD), salinity and color [1]. Dye compounds generally have low biochemical oxygen demand (BOD) values and high COD/BOD ratios depending on the structural complexity of those compounds.

Effluent treatment of the dye manufacturing process conventionally includes neutralization, flocculation, coagulation, activated carbon adsorption, advanced oxidation using UV systems or H2O2 solutions, and biological treatment. Due to the variable contents of the wastewater, such methods encounter serious difficulties in meeting environmental discharge requirements [2], [3], [4].

Moreover, the former presented treatment technologies are predominantly applied as end-of-pipe processes. Increase of water and wastewater disposal costs as well as more stringent legistrations enforce the establishment of internal water circuits rather than downstream treatment of mixed wastewater and employment of better effluent treatment facilities. Therefore, membrane technology has emerged as a feasible alternative to conventional treatment processes of dye wastewater and has proven to save the operation costs and water consumptions by water recycling. The membrane processes that can meet the requirements are NF and RO membranes, which can retain not only relatively small organic molecules but also ions from a dye wastewater. Removal of auxiliary chemicals, salts and colors from dye wastewaters can thus be achieved in one step [3]. However, high osmotic pressure differences limit the applicability of RO membrane. RO membrane filtration has problems with fouling, which resulted in low fluxes and poor separation efficiency [5], [6].

Thus, the pretreatment process is required such as nanofiltration. NF membranes are used to separate the soluble dye residual, e.g., reactive dye, from spent dye solution while allowing sodium and chlorine ions to pass through the membrane at the permeate stream [7]. NF is proving to be an ideal method for the pretreatment of salt removal [8], [9]. In certain dye manufacturing processes, membranes are used to recover quantities of the finished product from the wastewater. In view of the decrease of the water consumption as well as the wastewater amount, water reuse using membranes is advisable [10].

Generally, dyes are synthesized in a reactor, filtered, dried and blended with other additives to produce the final product in the dye manufacturing industry. The dyes are then separated from the mixture and purified [11]. Unfortunately, salt (NaCl), small molecular weight intermediates and residual compounds are produced from the synthesis process. These salt and residual impurities must be removed before the dyes are dried for sale as powder because they reduce the purity of the dyes. Conventionally, the dye is precipitated from an aqueous solution using salt. The slurry is passed through a filter press. The dye is retained by a filter press, and the filtrate containing salt and small molecular weight residuals from chemical reactions is discharged. The retained dye is collected in trays and dried in ovens. The dried dye is then pulverized to produce a saleable product. The purity of the final dye product from the conventional process is low, about 30% salt content; further, the conventional manufacturing process is carried out in various baths which makes the entire process very labor intensive and causes inconsistency in the production quality.

Membrane filtration is used by many process industries for product purification. The process of dye desalination and purification is currently one of the biggest applications for NF membrane technology, which removes salts and impurities to improve the quality of the product. It not only improves product quality, but also makes spray drying more efficient because the granulation of the dye takes place without the production of dust [1]. This new process has been developed using NF membrane separation technology that is continuous in operation, is not very labor intensive, and produces a high purity product of consistent quality [9].

However, the effluents from the process of dye desalination and purification are normally very large but have low level pollutants. In this respect, the NF and RO combined membrane process can be versatile application potentials to treat the above process wastewater and recycle back into the process in this industry. Reactive dye manufacturing process is among the largest amount of water intensive processes in dye production. Reactive dyes normally have a low rate of fixation and have high loss to the effluent.

The compound structure of dyes can be very complex and impact on the degree of biodegradability. Generally, dye with low solubility (e.g. disperse and vat dyes) can be easily removed by physical means such as flocculation and coagulation. Whereas, since the introduction of water soluble dyes such as reactive dyes, which are used extensively in the industry, coagulation or adsorption cannot be well removed from the soluble dyes. Furthermore, conventional biological treatment processes are also no longer able to achieve adequate color removal [4]. Although these dyestuffs and color materials in wastewater can be effectively destroyed by wet oxidation, advanced chemical oxidation such as H2O2/UV, O3 and adsorption using activated carbon, the costs of these methods are relatively high [12].

Among the commercial textile dyes, reactive dyestuffs are of environmental concern because of their widespread use and high solubility. In the dye industry, it is also effective in water recycling from the desalination and purification process wastewater and in treating effluent prior to final disposal.

The large amount, but low level pollutant of effluents, was produced from the desalting and purification process of the reactive dye manufacturing industry. In this study, the combined process of NF and RO membranes was applied, not only to improve the rejection efficiencies and flux recovery, but also to recycle the permeate back into the process from desalting and purification process of reactive dye manufacturing industry.

Section snippets

Materials and methods

Two types of reactive dye (reactive yellow 145 (R.Y.145) and reactive black 5 (R.B.5)) manufacturing process wastewater from the desalination and purification process using NF membrane filtration (Fig. 1) were obtained from a dye manufacturing company in Gyeonggi, Korea. Fig. 2 shows the molecular structure of the reactive dyes used in this study. The filtration in the module occurs in cross-flow. Cross-flow NF and RO filtration using thin film composite polysulfone membrane was used to recover

Flux declination

The dye manufacturing industry produces a large amount of wastewater that is highly colored with high loading of inorganic salt. Using reactive dye manufacturing wastewater, the study focused on the flux and rejection by varying three main parameters: types of dye, feed pressure and kinds of membrane.

Fig. 4 shows the variation of flux values of pure distilled water for NF membrane (DK) and RO membrane (AG). Membrane permeabilities were determined using pure distilled water. Flux values of pure

Conclusions

The dye manufacturing industry produces a large amount of wastewater that is highly colored, with high loading of inorganic salt. The NF membranes are newly applied to improve the desalination and purification process of dye manufacturing industry. However, the large amount but low level pollutant of effluents was produced from this process. In this study, combined process of NF and RO membranes was applied, not only to improve the rejection efficiencies and flux recovery, but also to recycle

Acknowledgements

This work was supported by the Korea Ministry of Science and Technology (National Research Laboratory Program). The authors deeply appreciate their financial support.

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