Treatment of pulp and paper industry bleaching effluent by electrocoagulant process

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Abstract

The experiments were carried out in an electrocoagulation reactor with aluminum as sacrificial electrodes. The influence of electrolysis time, current density, pH, NaCl concentration, rotational speed of the stirrer and electrode distance on reduction of color, COD and BOD were studied in detail. From the experimental results, 15 mA/cm2 current density, pH of 7, 1 g/l NaCl, 100 rpm, 28 °C temperature and 3 cm electrode distance were found to be optimum for maximum reduction of color, COD and BOD. The reduction of color, COD and BOD under the optimum condition were found to be 94%, 90% and 87% respectively. The electrode energy consumption was calculated and found to be varied from 10.1 to 12.9 kWh/m3 depending on the operating conditions. Under optimal operating condition such as 15 mA/cm2 current density, pH of 7, 1 g/l NaCl, 100 rpm, 28 °C temperature and 3 cm electrode distance, the operating cost was found to be 1.56 US $/m3. The experimental results proved that the electrocoagulation is a suitable method for treating bleaching plant effluents for reuse.

Introduction

The pulp and paper making industry is a water intensive industry; it is ranked third based on water consumption after metals and chemical industries. The Indian pulp and paper industry consuming 100–250 m3 freshwater/ton of paper production depend on the process [1]. The volume of wastewater generated by pulping and bleaching operations is approximately in the range of 75–225 m3/ton of product [2]. The volume of effluent generated and its characteristics are normally dependent on process used for paper making and the effectiveness of the process. The discharge of highly colored wastewater into the ecosystem involves environmental problems like aesthetic pollution and perturbation of aquatic life. The State Pollution Control Board of Tamil Nadu, India, has directed the industries to implement zero discharge facilities [3]. The continuous depletion of groundwater, shortage in rainfall and huge consumption of water by paper and pulp industries has necessitated the recycling and reuse of processed water.

The characteristics of the wastewater generated from the pulp and paper industry depend upon the types of raw material, types of process technology applied, internal recirculation of the effluent and management practices. In order to select and design a suitable treatment technique for reuse of effluent in the process application, a complete knowledge of characteristics of the particular industrial effluent is necessary. The general characteristics of the wastewater produced at various process stages and pollution sources of various industries located in and around Erode district, Tamil Nadu India, where samples were collected are given in Fig. 1.

In wood preparation section, the raw materials are chopped into small pieces using suitable techniques. Then the pieces are sent to the pulping section where sulfate process method is applied to produce pulp. In pulp bleaching section, pulps are bleached with liquid chlorine and liquid oxygen followed by a second bleaching with chlorine dioxide and third bleaching with hydrogen peroxide. The bleached pulp is then passed to paper making section. The waste water from wood preparation sections contains mostly suspended solids, dirt, fibers, etc., so this waste water is filtered and reused again. The effluent from pulping section is called black liquor effluent, which is approximately 20–25 m3/ton of paper production. The black liquor effluent contains more amounts of cooking chemicals such as sodium hydroxide and sodium sulfate, lignin and other extractives. In industries the effluent from pulping section is sent to chemical recovery plant where chemicals are recovered and reused for process. Effluent from pulp bleaching units (80–100 m3/ton paper production) contains dissolved lignin, carbohydrate, color, COD, AOX and inorganic chloride compounds. The wastewater from paper making section contains particulates, organic compounds, inorganic dyes, etc. It is characterized depending on the dyes used and the effluent generation is very low when color papers are produced. Development of unified method for treating effluent from paper making process for reuse is more difficult and uneconomical, whereas a large volume of bleaching effluent has been generated and having the uniform characteristics, it is economically feasible to develop new techniques to recover water from bleaching effluent for reuse.

Physical and chemical processes are quite expensive and generate a considerable amount of sludge which itself needs further treatment [4], [5]. The low biodegradability index of pulp and paper effluent from bleaching process clearly shows that the biochemical method cannot be used [6], [7]. Various treatment methods used for bleaching effluent reported in the literature are given in Table 1. In recent years, electrochemical technologies such as electrocoagulation, electrofloatation and electrooxidation have been used for the treatment of pulp and paper effluent. Among these methods, electrocoagulation emerges as one of the promising techniques due to its unique feature such as complete degradation of pollutants, less sludge generation and ease in operation. Electrocoagulation is a complex and interdependent process where the generation of coagulants takes place in situ by dissolving sacrificial anode. From the literature it is found that most of the researchers successfully used electrocoagulation method for the treatment of black liquor from pulp and paper industries (Table 2). However, very limited works have been reported for the treatment of pulp bleaching effluent using electrocoagulation method and hence in the present paper an attempt is made to study the influence of operating parameters such as time of electrolysis, current density, initial pH, electrolyte concentration, agitation speed and electrode distance on color, COD and BOD reduction and the fundamental and operating variables were optimized.

Section snippets

Experimental set-up and measurements

Bleaching effluent used in this study was obtained from pulp and paper mill located in Erode, Tamil Nadu. Experiments were carried out in a 500 ml reactor made up of glass. Aluminum plates of thickness 3 mm were used as electrodes. Dimensions of electrodes were 50 mm × 60 mm; and area of electrodes dipped into the solution was 50 mm × 50 mm. The total effective surface area of each electrode was 25 cm2; and 1 cm gap was maintained between the bottom of the electrodes and the bottom of the cell is allowed

Effect of current density

Several authors have reported [14], [24] that the current density has significant influence on the efficiency of the electrocogulation process. Experiments were carried out by varying current density and the observations are shown in Fig. 3, Fig. 4, Fig. 5. The removal of color, COD and BOD were increased with increasing current density. This can be attributed due to the fact that the applied current density determines the rates of coagulant and bubble production, which in turn can influence

Conclusion

The influences of current density, pH, rotational speed and NaCl concentration on electrocoagulation using aluminum electrodes have been examined. The results showed that optimal operating conditions were found to be an initial pH of 7, current density of 15 mA/cm2, rotational speed of 100 rpm, NaCl concentration of 1 g/l and electrolysis time of 20 min. This experimental study clearly showed that under the optimal conditions, about 94% color, 90% COD and 87% BOD were successfully removed. The

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