Modeling the Effect of Ozonation on Acid Yellow 17 Dye in a Semi-Batch Bubble Column

Abstract

A pilot-scale ozonation process employing a semi-batch column was used to study the removal of acid yellow dye 17 from synthetic wastewater in a semi-batch bubble column. The effect of initial dye concentration on system performance was evaluated. This study utilized an 11.12-L, clear, PVC column that was filled with 8-L of tap water tainted with the acid yellow 17 dye for each experimental run conducted. Ozone was supplied to the system by sparging the gas through three porous stainless steel filters fixed in the reactor 0.15-m from the bottom of the column. The ozone supply rate was held constant at 3.78 L/min. Acid yellow 17 dye concentration (measured as A400), chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD5) and the quantity of ozone utilized was measured as a function of ozonation time during each experimental run. Results indicate that ozonation is very effective at removing acid yellow 17 dye from synthetic textile wastewater. The biodegradability of the dye in the synthetic wastewater was evaluated by monitoring changes in BOD5 with respect to COD. The initial BOD5:COD ratio was 0.0083 and over time increased by an order of magnitude to a maximum ratio of 0.126 at 30 minutes. These results indicate that the biodegradability of the wastewater increased with an increase in ozonation time. Two-film theory was used to kinetically model the gas-liquid reactions occurring in the reactor. Modeling results indicated that during the first 10 to 15 min of ozonation, the system could be characterized by a fast, pseudo-first order regime. By combining a differential mole balance on the gas phase ozone concentration within the reactor with film theory, this initial period of the ozonation reaction was successfully modeled. With continued ozonation, system kinetics transitioned through a moderate then to a slow regime. Successful modeling of this period required use of a kinetic equation corresponding to a more inclusive condition. Model results are presented. Copyright ASCE 2005 EWRI 2005 Introduction Textile industry wastewater often contains high concentrations of color and therefore poses a wide variety of issues to receiving streams including the introduction of recalcitrant or toxic compounds and a decrease in relative clarity or turbidity. A common treatment for color containing wastewater is ozonation as this type of process has been shown to remove color (Adams and Gorg, 2002) and chemical oxygen demand (COD) (El-Din and Smith, 2002) and to increase the biodegradability (BOD5:COD) of the wastewater (Zhou and Smith, 1997). Acid yellow 17 dye is a common additive found in ordinary household products such as shampoo, bubble bath, shower gel, liquid soap, multi-purpose cleanser, dishwashing liquid and alcoholbased perfumes. As regulations associated with dyestuff are tightened, associated industries are faced with finding economically viable water treatment solutions. Two-film theory (Lewis and Whitman, 1924) is traditionally used to model mass transfer and associated reactions. For simplicity, many reactions are described as either “slow” or “fast” because relatively simple solutions exist for these regimes. For example, for an instantaneous, reversible reaction typified by a Hatta number (Ha) greater than 3, the reaction is assumed “fast” and an analytical solution is available. For a “slow” reaction, (Ha<0.3), it is assumed that the reaction occurs within the bulk fluid and the reaction rate can be determined from standard kinetic theory. Danckwerts (1970) and van Krevelen and Hoftijzer (1948) have developed approximate methods for the transition region characterized by 0.3<Ha<3. The focus of this study can be characterized as follows: 1. to evaluate the effect of ozone on the removal of acid yellow 17 dye from a synthetic wastewater, and 2. to develop a model based on film theory to describe the removal of COD associated with acid yellow 17 dye by ozonation in a semi-batch reactor. Materials and Methods All experiments were conducted in a semi-batch bubble column. The column was fabricated from clear 7.62 cm schedule 40 PVC pipe and was 2.44 m in height. The gas feed was sparged through three stainless steel filters with 10 um porosity. The sparging system entered the side of the reactor 15.24 cm from the bottom of the column. Ozone was produced with the Ozotech, Inc. OZ2BTUSL ozone generator. The air feeding the ozone generator was conditioned using the Power Prep Model Titan (Ozotech, Inc.). The carrier gas flow rate was maintained at a constant of 3.78 L/min resulting in a uniform ozone introduction to the system of 79.2 mg O3/min. Figure 1 shows a schematic of the equipment utilized. Copyright ASCE 2005 EWRI 2005