Treatment of high-strength phenolic wastewater by a new two-step method

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Abstract

Treatment of high-strength phenolic wastewater by a novel two-step method was investigated in the present study. The two-step treatment method consisted of chemical coagulation of the wastewater by metal chloride followed by further phenol reduction by resin adsorption. The present combined treatment was found to be highly efficient in removing the phenol concentration from the aqueous solution and was proved capable of lowering the initial phenol concentration from over 10,000 mg/l to below direct discharge level (1 mg/l). In the experimental tests, appropriate conditions were identified for optimum treatment operation. Theoretical investigations were also performed for batch equilibrium adsorption and column adsorption of phenol by macroreticular resin. The empirical Freundlich isotherm was found to represent well the equilibrium phenol adsorption. The column model with appropriately identified model parameters could accurately predict the breakthrough times.

Introduction

Phenol and phenolic derivatives are the organic chemicals that appears very frequently in the wastewater from almost all heavy chemical, petrochemical and oil refining industries. Because of their toxicity to human and marine life, increasingly stringent restrictions have been imposed on the concentrations of these compounds in the wastewater for safe discharge [1]. In Taiwan, for instance, the phenol concentration in the industrial wastewater for safe discharge has been reduced from 2 to 1 mg/l due primarily to health consideration. Hence, treatment of industrial wastewater containing phenolic compounds is a necessity.

Traditionally, activated sludge process has been the most widely used method to deal with phenolic wastewater because of its simplicity and relatively low cost [2]. However, the microorganisms in an activated sludge system, even well acclimated, can only deal with chemical wastewater containing relatively low concentration of phenolic compounds, usually less than 100 mg/l due to low biodegradability and inhibitory effects of these compounds [3]. Unfortunately, the chemical wastewaters from many heavy chemical and petrochemical industries contain phenolic compounds exceeding this concentration level. Hence, treatment of high concentration phenolic wastewaters by chemical or physical alternatives is crucial in these industries.

There are various chemical or physical methods for dealing with industrial wastewaters containing phenol over hundreds of mg/l. They included, for example, extraction by liquid membrane [4], [5], [6], [7], adsorption by activated carbon or activated carbon fiber [8], [9], macroreticular resin [10], [11], [12], [13] and organoclays [14], [15], wet air oxidation [1], [16], [17], [18], [19], [20], chemical decomposition by Fenton’s reagent [1], ozonation [31] and electrochemical method [22]. In a number of wastewaters from petrochemical or heavy chemical industries, the phenol concentration could reach as high as tens of thousand mg/l level. Among the methods mentioned above, wet air oxidation (WAO) or its catalytic variation (catalytic WAO) is one that can handle this high-strength phenolic wastewaters [1], [16], [17], [18], [19], [20], [21]. But as reported in the previous investigations [1], [16], [17], [18], [19], [20], [21], the WAO method has several drawbacks. This method is capable of rapidly decomposing the phenolic compounds in the wastewater and the phenolic decomposition is invariably accompanied by a generation of many small molecule organic compounds [1], [16], [17], [18], [19], [20], [21]. These small molecule organic compounds can not be effectively removed in the WAO process and a significant amount of chemical oxygen demand (COD) remains in the WAO effluent. Therefore, the WAO process is effective in destroying the phenolic compounds, but not in removing the wastewater COD. Furthermore, because of its operating high temperatures and high pressures, the WAO method has relatively high capital investment and operating costs. To remedy these disadvantages, effective treatment alternative operating under atmospheric conditions would be necessary.

The objective of this work is to present a new two-step method for dealing with phenolic wastewater containing phenol over tens of thousand mg/l. The present method consists of first step chemical precipitation of phenol by metal chloride. In this step, the metal chloride converts phenol under alkaline conditions to metal phenolate that precipitates out of the aqueous solution due to its very low solubility and the phenolate is thus recovered for reuse. The efficiency of this chemical precipitation, when properly operated and with the right kind of metal chloride, can far exceed 90%. The remaining wastewater containing considerably reduced amount of phenol is then treated using adsorption resin which is able to further lower the phenol concentration to below the discharge level of 1 mg/l. The previous investigations of the phenol adsorption by resin [10], [11], [12], [13] focused primarily on the practical aspects only. In the present work, several theoretical aspects of resin adsorption of phenol were examined using the observed data. Again, the phenol is recovered in this adsorption step when the adsorption resin is regenerated. The present two-step method was found to be highly efficient, as would be elaborated later, enabling essentially total phenol recovery from the wastewater for reuse.

Section snippets

Materials and methods

In the first step of the present treatment process, stock solution was prepared with a phenol concentration of 50,000 mg/l. The metal chlorides chosen included barium, calcium, copper, ferrous and ferric chlorides. All chlorides were GR grade and obtained from Aldrich (Milwaukee, Wisconsin, USA). 0.5 N solutions for all chlorides were prepared. The test runs were conducted in a 250 ml Erlenmeyer flask. 100 ml of stock phenol solution was placed in the flask and the pH of the solution was adjusted

Chemical precipitation by metal chlorides

The chemical reactions (barium chloride, for example) taking place in the aqueous solution in the precipitation stage were as follows:C6H5OH+NaOHC6H5ONa+H2O2C6H5ONa+BaCl2→(C6H5O)2Ba↓+2NaClIn the above chemical reaction and precipitation, the important issues to be addressed included the initial pH of the aqueous solution, the kind and the amount of metal chloride needed to effect a good chemical precipitation and the effect of temperature. Fig. 2 shows the changes of phenol

Conclusions

The high-strength phenol wastewater was treated by a novel two-step process in the present study. The two-step method consists of chemical precipitation of the high-strength phenolic wastewater by metal chloride followed by phenol adsorption using macroreticular resin. The two-step treatment method offers an excellent means for dealing with the high-strength phenolic wastewater. Based on the results obtained from tests of individual step, the optimum operating conditions were identified. The

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