CHAPTER 7 7. DEVELOPMENT OF A SINGLE STAGE ELECTROCOAGULATION INDUCED SETTLING TANK REACTOR (EISTR) FOR DYE AND TEXTILE WASTEWATER TREATMENT

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1 CHAPTER 7 7. DEVELOPMENT OF A SINGLE STAGE ELTROCOAGULATION INDUCED SETTLING TANK REACTOR (EISTR) FOR DYE AND TEXTILE WASTEWATER TREATMENT 7.1. INTRODUCTION Chemical coagulation (CC) is one of the primary treatment steps commonly used in treating textile wastewater. To eliminate the need for the addition of chemical coagulants, reduced sludge generation and to achieve overall cost reduction, electro-coagulation () process has been investigated (Daneshvar et al. 27, Emamjomeh and Sivakumar, 29). In the past couple of years alone, for example, response surface models have been used to optimize the operating parameters for process (Amani-Ghadim et al. 213, Olmez-Hanci et al. 212). The effect of different dye molecules and molecular mixtures on dye removal efficiency has been studied (Balla et al. 21, Pajootan et al. 212). Improvements in cell design (Balla et al. 21) and the use of cheaper electrode materials (Wei et al. 212) have been evaluated. The presence of adsorbents during was found to improve the dye removal efficiency (Secula et al. 212). The energy (Parsa et al. 211) and cost (Dalvand et al. 211) of process were found to be comparable to CC (Merzouk et al. 211a). In spite of these developments, is yet to find a place in large scale textile and dye wastewater treatment. Wastewater treatment units in the textile industry handle thousands of cubic meters of water every day. The processes following coagulation, namely floc formation and sedimentation kinetics of the flocs play a major role in deciding overall process viability. These stages have been extensively investigated and frequently reviewed for CC processes. These include studies on coagulation step (Duan and Gregory 23), floc structural characteristics (Jarvis et al. 25a) and measurement techniques for floc size estimation (Jarvis et al. 26, Jarvis et al. 25b). The colloidal aggregates formed under diffusion controlled conditions are found to be loosely held. Such diffusion limited colloidal aggregates (DLCA) are low density porous particles, which are prone to reorganization. In contrast, 118

2 compact and dense flocs are formed under conditions where the chemical interactions between colloidal particles control the overall kinetics (Tang et al. 2). The measurement of mass fractal dimension (Bushell et al. 22) and its effect on solid liquid separation (Gregory 1997) have also been studied. The sedimentation kinetics of metal hydroxide flocs formed by CC have also been modeled (Font et al. 1999, Vanderhasselt and Vanrolleghem 2) and experimentally verified (Janczukowicz et al. 21, Katja and Mika 27). The formation of floc and sedimentation rates of coagulants formed in on the other hand, has received attention only in the recent times. It was shown that results in loosely held porous aluminium hydroxide flocs, while CC under comparable conditions lead to smaller compact flocs (Harif and Adin 211, 27, Harif et al. 212). The sedimentation rates were also found to be different for and CC generated sediments (Larue and Vorobiev 23). Specific studies on settling characteristics of chemical mechanical polishing wastewater (Lai and Lin 26, 24, Larue and Vorobiev 23, Wang et al. 29) and oil mill wastewater (Khoufi et al. 27) after were also reported. Recently, the -sedimentation coupled process was modeled (Zodi et al. 29) and evaluated for the treatment of textile wastewater (Merzouk et al. 211b, Zodi et al. 213, 21, 29). All the sedimentation kinetic studies from reported above (Khoufi et al. 27, Lai and Lin 26, 24, Larue and Vorobiev 23, Wang et al. 29, Zodi et al. 213, 21, 29) have handled and sedimentation as two separate steps in reactors and sedimentation columns respectively. This study address the following questions (i) Can these two steps be combined into a single unit, by setting up the electrode pack on the top-portion of the sedimentation column? (ii) What are the optimum conditions for such a single step process? (iii) How do different dye molecules respond to this process? (iv) How does the real wastewater from dye industry respond to this process? (v) Which electrode performs better in this process?. 119

3 7.2. MATERIALS AND METHODS Electrocoagulation experiments An 8 cm internal diameter and 59 cm height graduated polypropylene cylinder, with provisions for hanging two 13.5 x 5.2 x.5 cm Fe or Al electrodes, was used as the electrocoagulation induced settling tank reactor (EISTR Fig.2.2a). The electrodes were positioned parallel to each other with an inter electrode gap of 2 cm with provision for a stainless steel (SS) stirrer between the electrodes. A 25 cm SS rod fixed to a vertical agitator (Model RQ-122/D, REMI Laboratory Instruments, India) with 9 rpm was employed for stirring. The SS rod had three planar blades at the bottom for stirring the solution. A volume of 3 liters of synthetic or real dye wastewater was added to the cylinder. The composition of synthetic wastewater solution was varied by changing dye concentration ( mg/l) and the type of dye (Amido Black 1B (AB), Methyl Violet (MV), Eosin yellow (EY), Malachite Green (MG), Methylene Blue (MB) and Rhodamine 6G (R6G). See Table. 7.1 for the molecular structure, solubility and absorbance wavelength of each dye molecule). NaCl concentration and initial ph (phi) are in the ranges 1-4% and respectively. The initial ph was adjusted wherever necessary using dilute NaOH or HCl. Experiments were conducted at constant current density in the range of 7.1 to 21.3 ma cm -2 for a period of 45 or 9 min. The charge loading (Q e ) and energy consumption (E dye ) per gram of dye molecule were calculated using equation number (7.1) and (7.2). After electrolysis, the sediments were allowed to settle in the same reactor. Q = (7.1) E = (7.2) where Q e is charge loading per gram of dye (Ahg -1 ), E dye - specific electrical energy consumption per gram of dye (Whg -1 ), C - initial dye concentration (g), I - current (A), U - cell voltage (V), t - electrolysis time (h). 12

4 Table 7.1 Characteristics of the dye molecules used in this study. Details of the dye molecules Molecular structure Amido Black 1B (AB) Molecular Formula Molecular Weight (g/mol) Type λ max (nm) Solubility in water(2ºc) g/l Methyl Violet (MV) Molecular Formula Molecular Weight(g/mol) Type λ max (nm) Solubility in water(2ºc) g/l Eosin Yellow (EY) Molecular Formula Molecular Weight(g/mol) Type λ max (nm) Solubility in water(2ºc) g/l Malachite Green (MG) Molecular Formula Molecular Weight(g/mol) Type λ max (nm) Solubility in water(2ºc) g/l Methylene Blue (MB) Molecular Formula Molecular Weight(g/mol) Type λ max (nm) Solubility in water(2ºc) g/l Rhodamine 6G (R6G) Molecular Formula Molecular Weight(g/mol) Type λ max (nm) Solubility in water(2ºc) g/l : C 22 H 14 N 6 Na 2 O 9 S 2 : : Diazo dye : : 3 : C 25 H 3 N 3 Cl : : Basic dye : 585 : 1 : C 2 H 6 Br 4 Na 2 O 5 : : Fluorescence dye : : 1 : C 25 H 54 N 4 O 12 : : Basic : : 1 : C 16 H 18 N 3 ClS : : Basic : : 4 : C 22 H 31 N 2 O 3 Cl : : Fluorescence : : 2 121

5 After discontinuing the electrical current, a distinct sedimentation boundary layer (Fig.2.2b) was noticed in the electrolyte solution between the colourless or less colored aqueous layer above and the dark colored layer below this boundary. This may occur sometimes even during, depending on favorable experimental conditions. The change of location of this boundary with time was monitored for 3h starting from the termination of electric current. The column reactor also has a provision for the removal of the supernatant liquid to determine colour removal efficiency (CRE) and turbidity at 1 cm from the reactor bottom (Fig. 2.2a). In addition to the compressed sludge at the bottom of the reactor (Fig. 2.2b), a floating floc layer was also noticed on the top of electrolyte solution, especially when was carried out without stirring (Fig. 2.2b). All the and settling studies were carried out at 25 ± 2 C. During the initial stages of sedimentation, the interfacial boundary layer height seemed to change linearly with time, in almost all the experiments. The slope of the straight line describing the location of the boundary layer as a function of time can be considered as an approximate sedimentation velocity. The boundary layer generally moves below the 1 cm mark from the bottom (below the collection tap) in less than 6 min, leading to the formation of a condensed sludge layer at the bottom of the reactor (Fig. 2.2b). Denser sludge material would settle down into a compact sludge layer, resulting in a lower sludge volume according to sedimentation kinetics models (Larue and Vorobiev 23). Hence, the height of the sludge after 3h of sedimentation was measured as an indicator of sludge density (Fig. 2.2b) Data analysis Analytical grade methyl violet from Merck, India was used in this study. All other dye molecules are from LOBA Chemicals, India. The dye concentration in the supernatant liquid collected after sedimentation from the side tap was monitored using UV-visible spectrophotometer (JASCO V-67). For dye mixtures and real textile wastewater, the organic content was determined using COD analysis (APHA 1998).The turbidity of the solution collected from the tap immediately after and 122

6 at regular intervals of 3 min was also measured using ELICO CL52D Nephlometer. The nature of the flocs settling at the bottom of the reactor and the flocs floating on the top surface was evaluated using scanning electron microscopy (JEOL JSM-639 SEM) and X-ray diffraction (SHIMADZU XRD-6) RESULTS AND DISCUSSION Effect of operating parameters on sedimentation rate using Fe electrodes Amido Black 1B (AB) was taken as the sample dye molecule for initial feasibility studies and optimization of chemical and electrochemical operating parameters in induced stirred tank reactor (EISTR). The effect of current density, electrolysis time, dye concentrations, initial ph and NaCl concentration on the movement of interfacial boundary layer was investigated. For this investigation, the dye solutions were electrolyzed without stirring. The effect of AB concentration on the sedimentation behavior of dye solution containing 2 % NaCl, after at a current density of 14.3 macm -2 for 45 min is shown in Fig. 7.1a. For dye solutions containing less than 5 mg/l of AB, during the initial phase the rate of sedimentation of flocs generated by was quite fast, as indicated by a sharp decline in the height of boundary layer with time. For these concentrations the boundary layer reached a level of less than 15 cm when the electrolysis was completed. When sedimentation is completed, the sludge height reaches around 5 cm which is well below the side tap level of 1 cm marked in Fig. 7.1a. At dye concentrations of 75 mg/l and above, under identical operating conditions, the boundary layer remains well above the 1 cm tap level even after 3 h of settling (Fig. 7.1a). The final boundary layer height increases with increasing dye concentration (Fig. 7.1a). A series of experiments were carried out to identify the columbic charge (Ah) required to bring the flocs below the 1 cm level of the EISTR by varying the reactant concentration between 25 mg/l and 125 mg/l, electrolysis time between 45 min and 9 min and the current density between 7.1 ma cm -2 and 21.3 ma cm

7 (a) 25 mg L mg/l 5mg/l L -1 75mg/l L -1 1 mg/l L mg/l L -1 Intereface height (cm) (b) Final interface height (cm) Charge loading (Ah g -1 ) of dye Fig. 7.1 Sludge settling behavior of electrocoagulated synthetic dye wastewater with Fe electrodes. (a) Effect of initial dye concentration with current density = 14.3 macm -2, electrolysis time = 45 min, phi and C NaCl = 2 %; (b) Effect of charge loading on the final interfacial height (See Table 7.2). Individual experimental parameters and the final interfacial height values after 3h of settling time are summarized in Table 7.2.The columbic charge per gram of dye is also plotted as a function of final interfacial height after 3h (Fig. 7.2b). 124

8 This figure shows that approximately 3.8 Ah of electric charge is required for the sedimentation of each gram of dye molecule AB. The experimental results in Table.7.2 also show that in all the experiments where charge loading is greater than or equal to 4.2 Ah per gram of dye molecule, the final interfacial height lies well below 1 cm. The charge loading with lower current and longer electrolysis time (Exp. No.7) proceeds at a lower cell voltage, resulting in lesser energy consumption (compare Exp. no.7 and Exp. no.11 in Table 7.2). The energy consumption under optimum condition is found to be around 24±3 Wh per gram of dye (Experiments: 2, 7, 8, 11in Table.7.2). 5 Intereface height (cm) Fig.7. 2 Sludge settling behavior of electrocoagulated synthetic dye wastewater with Fe electrodes for different initial ph with current density = 17.8 macm -2. (C AB = 1 mg/l; C NaCl = 2%; electrolysis time = 6 min). The actual ph value of typical textile dye wastewater can vary widely between ph 5 and 1. Hence, the effect of ph of the synthetic dye solution under otherwise identical experimental conditions was also investigated (Fig. 7.2). The sedimentation rate was found to decrease with decreasing ph of dye solution. The final sludge height achieved is also smaller in alkaline solution (Fig. 7.2). It appears that stronger dye adsorption on iron hydroxide colloidal particles and denser floc 125

9 formation occurs at higher ph values, resulting in faster sedimentation behavior. Similar ph effect for electrocoagulation using Fe electrodes has been reported earlier (Zodi et al. 29). In acidic ph media, formation of dissolved Fe 2+ and Fe 3+ species tends to retard the formation of dense iron hydroxide colloidal particles leading to poor settling behavior. Table 7.2 Effect of Amido black 1B dye concentration, electrocoagulation time and current on the performance of EISTR Exp. No C AB (g) t (h) Current (A) Cell voltage (V) Charge loading (Ah g -1 ) Energy consumption (Whg -1 ) Final Interface height (cm) The effect of salt concentration of synthetic dye wastewater on the sedimentation behavior under otherwise identical experimental conditions at initial ph-7.5 is shown in Fig. 7.3a. In this ph, lower salt concentrations lead to good sedimentation rate and dense sludge formation. At higher salt concentrations, with increasing density of the aqueous phase, the relative difference in density between 126

10 aqueous phase and the flocs formed appears to narrow down. This may be the cause of slow sedimentation and floc flotation when the initial ph value is 7.5 (Fig. 7.3a). Since the floc density increases with ph, good sedimentation a behavior was noticed at higher ph values (Fig. 7.3b). At the ph value of 9.5 good sedimentation behaviors has been noticed even for 4 % NaCl solution (Fig. 7.3b) Effect of dye molecules Six dyes were selected (Table. 7.1) for studying the influence of different dye molecules on the sedimentation behavior and CRE using Fe electrodes. The molecules varied widely in terms of molecular weight ( g/mol), solubility (1-4 g/l), chemical properties (diazo, basic and fluorescence dye) and molecular structure. In addition to interfacial boundary layer height, the CRE was also measured in these studies. During without stirring, some flocs containing hydrogen gas bubbles generated at the cathode tend to float on the top of dye solution in the reactor (Fig. 2.1b). During, the stirring (7 rpm) was found to be very efficient in breaking down these floating flocs and enhancing sedimentation rate. Hence in all subsequent studies presented here, the interfacial boundary layer heights have been reported both under unstirred and stirred conditions. In these studies 2 ml of sample solution was collected from the side tap at 3 min interval for measuring both turbidity and CRE. The sludge settling data for three dye molecules, namely AB, MV and EY both with and without stirring during the step are presented in Fig. 7.4a. All the three molecules exhibit good settling behavior after. The interfacial boundary layer moves down at a faster rate when the is carried out with stirring. The sludge height was also found to be lesser under these stirring conditions (Fig.7.4a). However, there are noticeable differences among these molecules in turbidity reduction and CRE. When is carried out with stirring, the initial turbidity of stirred solutions was found to remain high at the beginning of the settling process (Fig.7. 4b). However, the turbidity reduction was found to improve significantly when is carried out with stirring. The CRE was found to be above 9% for a number of dye molecules (Table. 7.3). For fluorescent dyes, namely R6G and EY, 127

11 the CRE was found to be lower (Fig. 7.5). For R6G and other molecules the CRE increases with increasing settling time (Fig.7. 5). Both turbidity reduction and CRE were also found to improve for electrocoagulated samples with stirring, obviously due to improved adsorption of dye molecules on colloidal particles with stirring (a) 1% 2% Intereface height (cm) % 4% (b) 2% 3% 4% Intereface height (cm) Settling time(min) Fig. 7.3 Sludge settling behavior of electrocoagulated synthetic dye wastewater with Fe electrodes. Effect of NaCl concentration (C NaC l) at initial ph =7.5 (a) and phi =9.5 (b). (C AB = 1 mg/l, current density = 17.8 macm -2 ; electrolysis time = 6 min) 128

12 Table 7.3 Sedimentation behaviour, CRE and turbidity reduction for electrocoagulated simulated dye wastewater with different dye molecules in the presence of C Dye = 1 mg L -1 using Fe electrodes (C NaCl = 2%; Current density = 17.8 macm -2 ; electrolysis time = 6 min) Name Sludge settling velocity (cm min -1 ) Final sludge height (cm) CRE (%) Turbidity reduction (%) of the Dye without with without with without with without with AB MV EY MG MB R6G For the fluorescent dye molecule EY, CRE was found to decrease slightly with time during the sedimentation process (Fig. 7.5). These fluorescent molecules with excited energy levels may be weakly adsorbed on iron hydroxide colloidal particles. The cleavage of weakly adsorbed dye molecule from the iron hydroxide particles may lead to the reappearance of colour of the dye molecule. The presence of such loosely bound low density particles may also be the cause for the observation of higher turbidity levels for this molecule (Fig. 7.4b). Iron hydroxide (Black particle) + EY (Red solution) Iron hydroxide-ey (Black particle) This interesting behavior by fluorescent molecules which remain at excited state for longer period of time however deserves further investigation. The sedimentation data collected for all the six molecules under identical experimental conditions is summarized in Table

13 6 5 (a) without stirring ( ) with stirring ( ) Intereface height (cm) 4 3 AB MV EY (AB) (MV) (EY) (b) without stirring ( ) with stirring ( ) 35 AB (AB) 3 MV (MV) Turbidity (NTU) EY (EY) Fig. 7.4 (a) Sludge settling behavior of electrocoagulation with Fe electrodes for three different dye wastewater and (b) Turbidity measurement. (C AB = C MV = C EY = 1 mg/l; C NaCl = 2%; current density = 17.8 macm -2 ; electrolysis = 6 min; phi =7.5; without (solid line) and with (dashed line) stirring during ). The sludge settling velocity (initial slope of boundary layer height versus time) increases significantly with stirring during for all these molecules. The final sludge height, which is an indicator of sludge density also decreases significantly. The CRE also improves with stirring during. The final CRE was found to be more than 97% for all the molecules except fluorescence dyes namely 13

14 EY (81%) and R6G (9%). It appears that fluorescent molecules form loosely held flocs due to photo-adsorption, resulting lower CRE Colour removal efficiency (%) AB MV EY (AB) (MV) (EY) without stirring ( ) with stirring ( ) 65 R6G R6G Fig Color removal efficiency for the electrocoagulated synthetic dye wastewater with Fe electrodes. (C AB = C MV = C EY = 1 mg/l; C NaCl = 2%; current density = 17.8 macm -2 ; electrolysis time = 6 min; phi =7.5; without (solid line) and with (dashed line) stirring during ). Turbidity removal was also found to improve if the solution is stirred during. In general, stirring during was prevents the floc floatation significantly by reducing gas bubbles in the flocs, enhancing sedimentation rate and minimizing the sludge volume. It was also noted that turbidity removal efficiency was always lower (7-85%) than the CRE (79-99%) under the present experimental conditions. Smaller suspended particles always remain in the solution to some extent after sludge settling and color removal Dye mixtures and industrial dye wastewater Industrial dye wastewaters normally contain mixtures of dye molecules, surfactants and decomposed products. For such systems dye or organic removal is generally monitored by measuring COD levels both before and after wastewater treatment. Fig.7.6a shows the sedimentation behavior of two-component dye 131

15 mixtures after with and without stirring. The sedimentation behavior improves significantly for flocs generated by with stirring. The settling time required to reach the side tap level (1 cm), for example, decreases from 15 min to 3 min (Fig.7.6a) due to stirring during. during improves the turbidity reduction efficiency of all the mixed dye systems investigated (Fig.7.6b). The sedimentation behavior of actual mixed dye system containing AB, MV and EY was also compared with a textile effluent sample collected from one of the textile dye industry (Scotts Garments, Tirupur, Tamil Nadu) in South India. The physicochemical properties both before and after the /sedimentation processes for this textile effluent are given in Table The sedimentation behavior for both these wastewaters was found to be similar (Fig. 7.7a). Table 7.4 Characteristic of synthetic dye wastewater and textile effluent collect from Scotts Garments, Tirupur, Tamil Nadu, India Simulate dye wastewater Textile effluent Parameters (AB+MV+EY) Before After sedimentation Before After sedimentation ph Conductivity (ms) Chloride (g/l) Total Hardness Turbidity (NTU) COD (mg/l) On the other hand, the turbidity reduction of real textile effluent was found to be better than synthetic solutions (Fig. 7. 7b). The textile wastewater was quite turbid before (155 NTU) indicating that higher concentration of suspended particles are present in this system even before. These particles probably 132

16 enhance the COD and turbidity reduction during induced settling. Initially, the synthetic dye wastewater possesses low turbidity, however the smaller particles generated during remain suspended during sedimentation leading to a relatively higher turbidity for these solutions. Table 7.5 Sedimentation behaviour, COD and turbidity reduction for electrocoagulated synthetic dye wastewater with Fe electrodes. (C NaCl = 2%; current density = 17.8 macm -2 ; electrolysis time = 6 min) Different combination of dyes Sludge settling velocity (cm min -1 ) without with Final sludge height (cm) without with COD reduction (%) without with Turbidity removal (%) without with (AB+MV) a (MV+EY) a (EY+AB) a (AB+MV+EY) b Textile Effluent a = (5 + 5) mg/l ; b = (4+4+4) mg/l The sedimentation data obtained for mixed dye system as well as real textile effluent are summarized in Table The sludge settling velocity and final sludge height for the textile effluent was found to be quite similar to the synthetic dye solutions. It should be noted that COD reduction values observed for mixed dye molecular systems and actual textile effluent fell in the range of 5-68% (Table. 7.5). This is substantially lower than 81-99% CRE obtained for solutions containing single dye molecules as presented in Table During, some of the dye molecules would have been reduced to colorless entities or cleaved into smaller water soluble entities, contributing higher levels of residual COD. Higher CRE and lower COD reduction have also been reported in the literature for individual dye 133

17 molecules (Wei et al. 212) as well as binary dye molecular mixtures (Pajootan et al. 212). 6 5 (a) without stirring ( ) with stirring ( ) 4 AB+MV (AB+MV) Intereface height (cm) 3 2 MV+EY EY+AB (MV+EY) (EY+AB) (b) without stirring ( ) with stirring ( ) Turbidity (NTU) AB+MV MV+EY EY+AB (AB+MV) (AB+MV) (EY+AB) Fig. 7.6 (a) Sludge settling behavior of electrocoagulated binary dye mixture wastewater with Fe electrodes; (b) Turbidity measurement. (C AB+MV = C MV+EY = C EY+MV = 1 mg/l, C AB+MV+EY = 12 mg/l; C NaCl = 2%; current density = 17.8 macm -2 ; electrolysis time = 6 min; phi =7.5; without (solid line) and with (dashed line) stirring during ). 134

18 6 5 (a) wiithout stirring ( ) with stirring ( ) (AB+MV+EY) Intereface height (cm) (AB+MV+EY) Effluent Effluent Turbidity (NTU) (b) without stirring ( ) with stirring ( ) (AB+MV+EY) (AB+MV+EY) Effluent Effluent Fig. 7.7 (a) Sludge settling behavior of electrocoagulation with Fe electrodes with tertiary mixtures of dye wastewater and textile effluent; (b) Turbidity measurement. (C AB+MV+EY = 12 mg/l; C NaCl = 2%; current density = 17.8 macm -2 ; electrolysis time = 6 min; phi =7.5; without (solid line) and with (dashed line) stirring during ). 135

19 Effect of electrode materials In the present work, Fe electrode was used in all the dye wastewater studies reported so far in EISTR. A few comparative studies on the sedimentation behavior of Al and Fe electrodes were also carried out in the absence and in presence of dye molecule. Typical sedimentation behavior of Fe and Al electrodes in 2% NaCl solution at initial ph (ph i ) values of 4.5 and 7.5 are shown in Fig. 7.8a. Higher sludge settling velocities as well as lower sludge height were observed for Fe electrode when compared to Al electrode. The settling behavior was also found to be independent of ph i at the tested range for both the electrodes (Fig. 7.8a). Similar movement of boundary layers was also observed for these two electrodes, in the presence of dye molecule (Fig.7. 8b). The CRE for 1 mg/l of EY in 2 % NaCl solution on Fe and Al electrodes were found to be 81% and 13% respectively, suggesting a much weaker removal of dye molecules through aluminum hydroxide flocs under the present sedimentation conditions. It may be noted from Fig. 7.8a and Fig. 7.8b that, in the case of Fe electrode, the boundary layer moves below the side tap level within 3 min. On the other hand, the interfacial boundary layer remains well above 1 cm from the bottom (location of side tap) for Al electrodes even after 3h. This resulting in an interesting observation in turbidity measurements made on samples collected from the side tap at regular intervals. For Fe electrodes, the turbidity level decreases with time, since the major portion of the sludge has moved below the side tap level. For Al electrodes, on the other hand turbidity increases with time since the slow sedimentation process of Al flocs continue up to 3h (Fig. 7.9). This is true both in the absence (Fig.7.9a) and in presence of dye molecules (Fig. 7.9b). The above observations indicate that under the present experimental conditions, Fe electrodes produce denser floc particles which settle down faster when compared to more porous low density aluminium hydroxide flocs generated by Al electrodes. Also, large amount of coagulants formed by Al electrodes, either by high current densities or longer electrolysis time result in slow settling of the flocs. 136

20 6 5 (a) (phi-4.5) (phi-7.5) 4 (phi-4.5) (phi-7.5) Intereface height (cm) (b) (phi-4.5) (phi-7.5) 4 (phi-4.5) (phi-7.5) Intereface height (cm) Fig. 7.8 Sludge settling behavior of electrocoagulation using Fe (, ) and Al (, ) electrodes in wastewater: (a) Absence of dye molecule and (b) C EY = 1 mg/l with two different initial ph values (C NaCl = 2%; current density = 17.8 macm -2 ; electrolysis time = 6 min). 137

21 (a) 35 3 Turbidity (NTU) (phi-4.5) (phi-7.5) (phi-7.5) (phi-7.5) (b) 35 3 Turbidity (NTU) (phi-4.5) (phi-4.5) (phi-7.5) (phi-7.5) Fig. 7.9 Turbidity measurement for electrocoagulation with Fe (, ) and Al (, )electrodes in wastewater containing (a) Absence of dye molecule and (b) C EY = 1 mg/l with two different initial ph values (C NaCl = 2%; current density = 17.8 macm -2 ; electrolysis time = 6 min). 138

22 This is because of the gel-like structure of aluminium hydroxides flocs. Earlier sedimentation kinetics measurements of flocs generated from different wastewaters after in a separate sedimentation column have employed both Fe and Al as anode materials (Khoufi et al. 27, Lai and Lin 26, 24, Larue and Vorobiev 23, Merzouk et al. 211b, Wang et al. 29, Zodi et al. 21, 29). A few comparative studies of Fe and Al electrodes (Wang et al. 29, Zodi et al. 29) also indicate better settling characteristics of iron hydroxides flocs generated by. Aluminium hydroxide flocs generated by was recently shown to be loosely held and porous when compared to the same aluminium hydroxide flocs generated by CC (Harif et al. 212). To further substantiate this view, the flocs obtained in the floating top layer and the settled bottom layer of the reactor after in presence of 1 mg/l EY using Al and Fe electrodes were collected, filtered and air-dried. SEM images of these samples showed lumpy particles with an average size of several micrometers for Al electrodes (Fig.7.1a & b). The flocs and sludge material obtained using Fe electrode under similar conditions was found to contain much smaller particles (Fig. 7.1c & d). It appears that smaller particle size and hence higher surface area of iron hydroxide colloids favour dense floc formation when compared to aluminium hydroxide flocs obtained under similar experimental conditions. XRD patterns are presented in Fig for the same samples of Al and Fe floating flocs and sludge after using Al (Fig. 7.11a & b) and Fe (Fig c & d) electrodes respectively. The 2θ values of XRD signals were found to match with the JCPDS values of aluminium hydroxide (JCPDS No:12-457) and iron oxide hydroxide (JCPDS No:89-385) respectively. The XRD signal intensities suggest that iron hydroxide produced under these conditions (Fig c & d) is significantly more amorphous when compared to aluminium hydroxide (Fig a & b). The XRD signal intensities for Al(OH) 3 floating flocs (Fig.7.11a) were observed to be slightly higher than the Al(OH) 3 suspension (Fig.7.11b). The slight decrease in XRD signal intensity in the case of the settling material may be due to better organic adsorption during settling. The decrease in XRD signal intensity was 139

23 substantial in the case of iron hydroxide sediment (Fig.7.11d) when compared to the iron hydroxide floating flocs (Fig.7.11c). In this case, the sediment becomes completely amorphous. These observations support the view that in terms of settling, Fe electrode shows better performances when compared to Al electrode. (a) (b) (c) (d) Fig. 7.1 SEM images of electrocoagulation generated sludge with optimum conditions. (C AB = 1 mg/l, current density = 17.8 macm -2 ; electrolysis time = 6 min; phi =7.5). Using Al electrodes (a) Floated flocs and (b) Sediment flocs. Using Fe electrode (c) Floated flocs and (d) Sediment flocs. 7.4 CONCLUSION The present investigations reveal that a single electrocoagulation settling tank reactor (EISTR) containing Fe electrodes can indeed be used for the removal of dye molecules, dye molecular mixtures and textile effluent. 14

24 The current density and electrolysis time required would be dependent on the concentration of dye molecule. For Fe electrodes, an initial solution of ph above 7.5 produces denser sludge and enables efficient sludge settling, even when the synthetic dye wastewater contains up to 4% NaCl (a) 5 (b) 8 4 Intensity 6 4 Intensity θ(degree) θ(degree) Intensity (c) Intensity (d) θ(degree) θ (degree) Fig X-ray diffraction patterns of electrocoagulation generated sludge with optimum conditions. (C AB = 1 mg/l, Current density = 17.8 macm -2 ; electrolysis time = 6 min; ph =7.5). Using Al electrodes (a) Floated flocs and (b) Sediment flocs. Using Fe electrode (c) Floated flocs and (d) Sediment flocs. More than 3.8 Ah of charge loading and 24±3 Wh of energy consumption per gram of dye molecule was found to be sufficient for achieving up to 99% CRE for AB. 141

25 Similar CRE as well as turbidity reduction was noticed for a wide variety of dye molecules investigated in this work using Fe electrodes. The COD reduction in mixed dye molecular systems and actual textile effluent was found to be slightly lower. The actual CRE, turbidity and COD reduction between individual molecules and molecular mixed systems investigated in the present work were found to vary around ±1% from the average values. This suggests that the optimization process for each dye wastewater composition may have to be worked out individually. SEM and XRD investigations show that Al electrode produces large porous low density flocs under similar experimental conditions. These flocs exhibit much weaker dye adsorption and a slower settling behavior when compared to the Fe electrode under similar experimental conditions. The simple investigation using EISTR indeed suggest the possibility of further studies in this direction. The Lamella clarifier units currently used for wastewater treatment can, for example, be fitted with necessary electrode pack on top of the clarifier and used as EISTR. Some initiatives in this direction would indeed be worthwhile. 142