TREATMENT OF TEXTILE MILL WASTEWATER

Abstract

Textile industry is one of the most polluting industries. Treating textile effluent is a challenging task as it contains a variety of chemicals, organics, fibers, etc. Major pollutants in cotton textile wastewater come from natural impurities extracted from the fiber, the processing chemicals and dyes. The removal of color from textile mill wastewater is of great environmental concern. More than half of the nearly 90 dyestuffs used in textile mills (e.g. reactive blue 21, direct blue 80 and vat violet etc. at specific COD/BOD ratios) are non-biodegradable. There are about 706 cotton textile mills in India mainly located in Mumbai, Ahmedabad, Coimbatore and Kanpur. Out of these, 291 are composite mills and 415 are spinning mills. The composite effluent discharged from these textile mills varies from 1 to 10 million liters per day depending upon the quantity of cloth produced and the manufacturing processes employed (Swaminathan and Subhramanyam, 1982; Verma and Mishra, 2006; Toor et al, 2006; Sirianuntapiboon and Srisornsak, 2007). Among the various treatment methods, catalytic thermolysis (thermal treatment) and coagulation are the most promising ones. Thermolysis is a chemical process by which a substance is decomposed in to other substances by use of heat. During catalytic thermolysis, two mechanisms, both in parallel but complementary to each other, take place simultaneously. The organic molecules, both small and large, present in the effluent undergo chemical and thermal breakdown and complexation, forming insoluble particles, which settle down. Further, during thermolysis larger molecules also undergo breakdown into smaller molecules, which are soluble. Work has already been done to reduce COD and color of other effluents like distillery wastewater and pulp and paper mill waste in sufficient amount using thermolysis. A maximum reduction in COD and BOD observed for alcohol distillery effluent were 70% and 83% respectively (Chaudhari et al., 2005). For pulp and paper mill effluent, the COD and color n reductions were 63.3% and 92.5%, respectively (Garg et al., 2005, 2007). After the success of thermolysis process in treating other wastewaters, the present work is aimed to undertake treatment of textile mill wastewaters using thermolysis, coagulation and thermolysis accompanied with coagulation for the three different effluents; desizing waste, dyeing waste and composite waste. The objectives set for the present work were (a) to conduct thermolysis of the effluents at moderate temperatures and atmospheric pressure conditions in presence of different catalysts and study the effect of treatment parameters such as pH, time, temperature and catalyst concentration on COD and color removal (b) to study the kinetics of thermolysis process (c) to study the effectiveness of various coagulants for COD and color reduction (d) to study the settleability of the solid residue and the filterability of treated effluent and the effect of pH on these processes (e) the thermal degradation characteristics and characterization of the solid residue by FTIR, CHNS and proximate analysis to see their suitability of being used as solid fuel and (f) to study the mechanism of coagulation process. The experimental studies for thermolysis were carried out in a 0.5 1 threenecked glass reactor. The pH of the waste water was adjusted to the desired value by adding 0.1 N HC1 or 0.1 N NaOH solution before it was transferred to the three-necked glass reactor. Thereafter, the catalyst was added to the solution. The temperature of the reaction mixture was raised using a hot plate to the desired value by a P.I.D. temperature controller, which was fitted in one of the necks through the thermocouple. The raising of the temperature of the waste water from ambient to 95 °C took about 30 min. A vertical water-cooled condenser was attached to the middle neck of the reactor to prevent any loss of vapor. The time taken to attain the desired temperature was the heating time. Further heating was done at the desired temperature and the time was measured by subtracting heating time from the total time. Thus, heating time was taken as zero for further heating. The reaction mixture was agitated using a magnetic stirrer. The samples were withdrawn at periodic intervals for the measurement of COD, in color and pH. The samples were centrifuged to decant the supernatant. The final pH of the solution after the reaction was also observed. The treated effluent including sludge was then rapidly mixed and the slurry so formed was used to study the settling and filterability characteristics. The thermolysis operation of composite wastewater (CODo=1960 mg/1, color = 2250 PCU) was carried out in presence of catalysts CuS04, FeS04, FeCl3, CuO, ZnO and PAC. Among these copper sulfate was found to be most active in giving 77.9% COD and 92.85% color reductions, at a catalyst concentration of 6 kg/m3, pH 12 and temperature 95 °C. During coagulation aluminum potassium sulfate is found to be the most active among other coagulants (commercial alum, FeS04, FeCb and PAC) tested resulting in 88.62% CODreduction and 95.4% color reduction, at pH 8 and a coagulant concentration of 5 kg/m3. Coagulation of clear fluid (supernatant) obtained after catalytic thermolysis (at above mentioned operating conditions except at a lower coagulant concentration of 3 kg/m ) resulted in a reduction of 97.3% COD and close to 100% of color. Thermolysis followed by coagulation, thus, is found to be the most effective process in reduction of COD as well as color at a lower dose (3 kg/m ) of coagulant. The residual COD was found to be 11.7 mg/1 in the final effluent, whereas, the COD/BOD3 ratio was 1.67. A value of less than 2 of COD/BOD3 ratio is desirable to make the effluent biodegradable. The sludge produced from the coagulation unit would contain lower inorganic mass coagulant, requiring reduces disposal problems, whereas the solid residue from the thermolysis, which is rich in organics, can be used as a solid fuel with high calorific value of about 16 MJ/kg, close to that of Indian coal. During catalytic thermolysis of the fresh dyeing wastewater (COD0 = 5744 mg/1, color = 3840 PCU) copper sulfate has been found to be most active catalyst giving 66.85% COD as well as 71.4% color reductions, at a catalyst concentration of 5 kg/m3, pH 8 and 95 °C temperature. During treatment by coagulation process, commercial alum was found to be the best among the other coagulants tested, resulting -1 in 58.57% COD and 74% color reductions, at pH 4 and a coagulant dose of 5 kg/m . IV Coagulation of the supernatant obtained after catalytic thermolysis at above mentioned operating conditions (except at a lower coagulant concentration of2 kg/m3) resulted in a reduction of 89.91% COD and 94.4% color. The final COD and color was found to be 192 mg/1 and 61.5 PCU, respectively in the treated effluent. The COD/BOD3 ratio was 1.88. In case of desizing wastewater (CODo = 2884 mg/1, color = 520 PCU) the catalytic thermolysis at moderate temperatures (60-95 °C) and atmospheric pressure in the presence of a CUSO4 catalyst showed a maximum removal of 71.6% of COD and 87.2% of color at a catalyst concentration of 4 kg/m3, 95 °C and initial pH 4. The results on the effect of variation of catalyst mass loading revealed that 4 kg/m is the critical catalyst concentration. The maximum COD and color reductions by coagulation were 58.34% and 85 %, respectively, at a coagulant dose of 5 kg/m and pH 4. The application of coagulation to the supernatant obtained after thermolysis show a removal of 87.96 % COD and 96.0 % color at above mentioned conditions except at a coagulant dose of 1 kg/m3. The amount of inorganic sludge obtained due to the addition of coagulant is, thus, drastically reduced due to the reduced amount (almost 25%) of coagulant. The COD and color of the final effluent were found to be 98.6 mg/1 and 2.67 PCU, respectively and the COD/BOD3ratio was 1.36. The thermogravimetric studies (TGA, DTA and DTGA) were conducted for the precipitated sludges of the dyeing, desizing and composite waste waters in order to study the combustion characteristics and the prospect of being used as a solid fuel. The information on the temperature of dehydration and volatilization (removal of volatiles), their rates and heats evolved/consumed during the process were obtained. The kinetic analysis and reaction mechanisms of thermal degradation and oxidation have been proposed for residue degradation using thermogravimetry. For a one step irreversible reaction model, differential and integral methods have been suggested for the determination of kinetic parameters, the order of reaction, frequency factor, the specific reaction rate constant, activation energy, change in entropy, enthalpy and Gibb's free energy, steric factor and its dependence on temperature using Arrhenius equation (Jalan and Srivastava, 1994,1999; Gangavati et al., 2005). The chemical aspects of coagulation has been explained by an examination of the hydrolysis of metal ions. The hydrolytic reactions of metal ions have been used extensivelyto explain coagulation mechanisms (Hundt and O'Melia, 1988; Dempsey et al, 1984; Edwards and Amirtharajah, 1985; Mangravite et al, 1975). The particular form of the metal present at a given pH is assumed to be responsible for the actual coagulation process. The effect of pH0 on the color reduction of dyeing waste, for example, could be explained by the combined effect of (i) the ionization of amino, hydroxy and sulpho groups present in the dye molecules which increases with pHin the acidic range, and (ii) the decrease in the concentration of dissolved hydrolysis products. Aluminum based coagulants show better results than that of iron based coagulants in the removal ofCOD and color of the dyeing wastewater. Kinetic studies of catalytic thermolysis process of composite, dyeing and desizing wastewater reveal that the kinetics, in general, can be represented adequately by the first order processes. A two-step reaction mechanism was suggested. The activation energy, E for the first and second steps for composite wastewater were found to be 8.34 and 19.24 kJ/mol, respectively. The corresponding values of the frequency factor were 0.098 and 0.065 m"1 respectively. The values ofEfor first and second step in case of dyeing wastewater were found to be 23.00 and 39.57 kJ/mol; and for desizing wastewater 36.64 and 42.36 kJ/mol respectively. The values of the frequency factors for first and second steps were 12.59 and 429.92 m"1 for dyeing and 614.96 and 1057.17 m"1 for desizing wastewater, respectively. The FTIR of the dried wastewater and the residues obtained after treatment showed useful information on the type and intensity of various bonds and groups present. Composite wastewater after drying at -38 °C for 36 h exhibits a broad band due to v(OH), 6(OH), conjugated C=C bond and a weak bond of CH2 groups in waste. The waste material also exhibits medium intensity band of sulphate group. On vi treatment with copper sulphate by thermolysis most of the IR peaks become intense. The sharp band of sulphato group was possibly present in the coordinated form, v(OH), 8(OH), v(C=C) and v(CH2). The presence of two to three medium intensity bands in the 400-600 cm"1 in both the above samples possibly suggest that hydroxyl groups are coordinated to metal ions present in the sample. Similar results were also obtained and explained with other effluents before and after treatment. In order to study the effectiveness of various process parameters, the settling and filtration characteristics of the treated effluent were studied. The parameters such as sedimentation velocity (uc) and sedimentation flux and area of sedimentation tank in case of settling experiments and specific cake resistance and filter medium resistance in case of filterability studies were calculated. The treatment variables have strong effect on the quality and texture of the particles present in final slurry vis-a-vis their settling and filtration characteristics. On the basis of above results, it is concluded that the catalytic thermolysis followed by coagulation is an effective process for the treatment of textile mill wastewaters to removeCOD and color. The advantages include : (i) The treatment temperature of 95°C during catalytic thermolysis can be easily maintained in case of dyeing and desizing waste as these streams evolve at temperatures close to this from the plant, (ii) Substantial reduction of COD and color during thermolysis results in reduced coagulant requirement in the subsequent process, thus producing much less inorganic sludge to be disposed, (iii) The final liquid effluents are within the disposable norms as prescribed by Central Pollution Control Board, India, (iv) Thermolysis sludge containing organics can be used as a solid fuel (after drying) with good calorific value.