Kinetic characteristics of textile wastewater ozonation in fluidized and fixed activated carbon beds

doi:10.1016/S0043-1354(99)00214-6

Water Research

Volume 34, Issue 3, 15 February 2000, Pages 763-772

https://doi.org/10.1016/S0043-1354(99)00214-6 Get rights and content

Abstract

This study was to investigate the ozone oxidation of textile wastewaters in a fluidized or fixed granular activated carbon (GAC) bed. Experiments were conducted to examine the effects of various operating variables on the treatment efficiencies of the combined ozonation and activated carbon adsorption. Ozonation was found to provide very efficient regeneration of exhausted GAC in the reactor, avoiding the costly ex situ GAC regeneration. It was also observed that the GAC acts not only as an adsorbent, but also as a catalyst in promoting ozone oxidation. The combined ozonation and GAC adsorption offers strong synergistic effects on the textile wastewater treatment. A generalized kinetic model consisting of multiple steps of ozonation and adsorption was proposed in this work to represent the combined process. The proposed adsorption/ozonation/desorption mechanism and the generalized kinetic model were shown to describe very well the combined treatment process.

Introduction

In the past several decades, wastewater discharges by dye manufacturing and dyeing/finishing plants have consistently been a major environmental concern (Gurnham, 1965). Those effluent discharges are notorious due primarily to their strong color, high dissolved solids (SS) and other organic contents. Those pollutants in the effluent discharges, if not removed, will cause upset to the ecological system of a receiving water body. Unfortunately, the dyestuff, dyeing additives and sizing agents (such as polyvinyl chloride) in the wastewater effluents are highly structured complex polymers, which are very difficult to decompose biologically. Hence, very little decomposition of those organic molecules takes place in a biological treatment process. Very often, strong color and turbidity of the wastewater effluents are particularly troublesome because of its negative visual impact. In industrial practices, decolorization using hydrogen peroxide (H₂O₂), sodium hypochlorite (NaOCl) or various proprietary decoloring agents have been reported (Beszedits, 1980, Far Eastern Textile Company, 1994). However, polishing step using these decoloring chemicals is expensive. Search for more effective and less expensive methods in decolorization as well as pollutant reduction is in order.

Effective decolorization of dye wastewater by granular activated carbon (GAC) adsorption was reported by McKay, 1987, McKay, 1990 and Paprowicz and Slodczyk (1988). As an alternative, ozonation had become a popular method researched by many investigators (Snider and Porter, 1974, Beszedits, 1980, Green, 1985, Gould and Groff, 1987, Lin and Lin, 1993, Lin and Liu, 1994). This method was found to consistently yield excellent results. Kuo, 1992, Lin and Peng, 1995 considered dye decolorization by Fenton's reagent which is a mixture of hydrogen peroxide (H₂O₂) and ferrous sulfate (FeSO₄). The Fenton's reagent generates, in a sequence of chemical reactions, hydroxyl radical (OH) which is a very strong oxidant and accounts mainly for the decoloring reactions. In a more recent work, Davis et al. (1995) utilized UV light in conjunction with semi-conductor catalyst (TiO₂) in investigating the kinetics of dye decolorization.

Although effective, separate ozonation and granular activated carbon (GAC) adsorption methods mentioned above do possess some inherent disadvantages. Ozonation renders decomposition of highly structured dye molecules into smaller organic molecules. Although the color of the wastewater effluent was effectively removed by this method, but there was a relatively small change in the chemical oxygen demand (COD) concentration of the treated wastewater primarily because of generation of smaller molecule organic compounds (Snider and Porter, 1974, Beszedits, 1980, Green, 1985, Gould and Groff, 1987, Lin and Lin, 1993, Lin and Liu, 1994). Hence the treated wastewater very often still does not meet the discharge standard regulated by the government. In terms of COD removal, the GAC carbon adsorption is much improved method over ozonation. But the GAC can get saturated easily in the process, which requires regeneration or complete replacement. The GAC adsorption process hence can become quite expensive in practices. Combination of both methods into a single process could offer an attractive alternative to remedy the inherent disadvantages of ozonation and GAC adsorption. The purpose of this work is to conduct experimental studies of the combined ozonation and GAC adsorption to address this point. As will be shown later, the combined treatment process does offer considerable advantages unrealizable by each of them.

Section snippets

Experimental section

The experimental set-up consisted of a pyrex reactor which was 6 cm in ID and 100 cm long. The reactor, as shown in Fig. 1, was equipped with an external cooling jacket for temperature control. At the bottom of the reactor, a solution sampling port and an ozone gas input port were provided. The ozone gas was generated by a Sumitomo SG-PSA-01A ozone generator (Sumitomo Electric Co., Osaka, Japan) which was equipped with a PSA unit for air processing. The ozone generator was rated at 30 g h⁻¹ as the

Effects of operating variables

In an experimental test run, the filtered raw wastewater (2 l) placed in pyrex reactor were sufficient to cover a height for a maximum amount of 200 g GAC. Hence experiments were first carried out to test the effects of various amounts of GAC (up to 200 g) on the COD and color removal of the wastewater for a given set of other operating conditions. The test results are demonstrated in Fig. 2. The effect of GAC on the COD removal appears to be considerably more pronounced than that on the color

Conclusions

Experiments were conducted to investigate the treatment of textile wastewater by ozonation in a fluidized or fixed granular activated carbon bed. Emphases were placed on examining the performance characteristics of the combined treatment process and regeneration of exhausted GAC. A mechanism of adsorption/ozonation/desorption and a generalized kinetic model were proposed to describe the combined treatment system. Results from all experimental runs reveal that:

1.
For an ozone gas flow rate of 4 l min

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Kinetic characteristics of textile wastewater ozonation in fluidized and fixed activated carbon beds

Abstract

Introduction

Section snippets

Experimental section

Effects of operating variables

Conclusions

Standard Methods for Water and Wastewater Examination

Ozonation to decolor textile effluents

Am. Dyestuff Rept.

Photocatalytic decolorization of wastewater dyes

Water Environ. Res.

Kinetics of ozonolysis of synthetic dyes

Ozone Sci. Eng.

Using ozone to decolorize dyeing plant wastewater

Am. Dyestuff Rept.

Numerical Methods in Chemistry