Removal of Chemical Oxygen Demand and Color from Simulated

Jul 8, 2013 - Anjali Yadav, Suparna Mukherji, and Anurag Garg*. Centre for Environmental Science and Engineering, Indian Institute of Technology Bomba...
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Removal of Chemical Oxygen Demand and Color from Simulated Textile Wastewater Using a Combination of Chemical/ Physicochemical Processes Anjali Yadav, Suparna Mukherji, and Anurag Garg* Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India, 400076 ABSTRACT: The treatment of simulated textile wastewater [chemical oxygen demand (COD) = 3360 mg/L] was performed using three chemical/physicochemical processes, viz., coagulation, Fenton oxidation, and adsorption. Coagulation of the wastewater was performed in a jar test apparatus using inorganic and organic coagulants. The best color removal (∼96%) was obtained with FeSO4 while FeCl3 and MgCl2 caused ∼75% and ∼53% color removal, respectively. The optimum dose for ironbased coagulants was 3 g/L, whereas it was 6 g/L for MgCl2. FeSO4-treated wastewater showed the best results with regard to the settling of flocs. Chitosan was effective for the decolorization of wastewater at low dose (=10 mg/L) and acidic pH (∼3.0) showing ∼20% and 30% COD and color reductions, respectively. For coagulation with FeSO4 followed by adsorption, overall COD and color removal of 85% and 99%, respectively, could be achieved. The raw wastewater treated by Fenton’s process (H2O2 dose = 10.5 g/L and H2O2/Fe2+ molar ratio = 20) exhibited COD and color removal of ∼80% and 99%, respectively. Using Fenton oxidation as a post-treatment step after coagulation with FeSO4, the COD removal was enhanced to ∼83%, although the color removal remained unaffected. The sequential treatment also increased the BOD5 (biochemical oxygen demand) to COD ratio of the treated wastewater to 0.51.

1. INTRODUCTION The textile industry is one of the oldest and largest industrial sectors in India, contributing around 14% to total industrial production in India.1 The characteristic feature of this sector is high water consumption, and generally 200 L of water is required to produce 1 kg of product.2 In India, the Ministry of Environment and Forests (MoEF) has categorized the textile mills as a “red category” industrial sector, since it is considered a heavily polluting sector and is covered under the Central Action Plan.3 The effluent generated from a textile mill is highly colored and contains several organic and inorganic constituents. These include sizing agents, wetting chemicals, dyes, pigments, softening agents, surfactants, oils, and other additives that are added in the various unit operations of the wet processing technique. Due to the presence of the above compounds, the wastewater is characterized by high pH, suspended solids (SS), chemical oxygen demand (COD), color, and alkalinity. The effluent is not considered suitable for direct biological treatment processes due to its color and the presence of recalcitrant compounds. In a recent review article, several physicochemical methods, such as coagulation−flocculation, ion exchange, adsorption, membrane technology, irradiation, and chemical oxidation, and also biological processes have been suggested for the removal of synthetic dyes from textile mill effluents. 4 Generally, physicochemical treatment processes alone are not preferred, despite their effectiveness, due to the high costs involved. Hence, the application of suitable physicochemical methods in conjunction with a biological process may offer a cost-effective and acceptable solution. Recently, Verma et al.5 highlighted the efficacy of chemical coagulation/flocculation technologies using various inorganic coagulants and natural coagulants for the removal of color from textile wastewater. © XXXX American Chemical Society

In the past, several researchers have studied the coagulation of synthetic and real textile wastewater using various inorganic chemicals (e.g., FeCl3, FeSO4, alum, lime, and MgCl2) as well as components of biological origin, such as chitosan and HOC100A (a patented product).6−13 It has been reported that FeSO4 and MgCl2 have potential of 85−100% color removal from dye wastewater at highly alkaline pH.6,7,14,15 The optimum pH for the coagulation process with MgCl2 is reported to be 11−12. FeSO4 has been found effective at a pH around 9.5, although pH values of 12.5 and 5.7−6.5 have also been reported to be favorable for color removal from wastewater.7,16 Another group of researchers have reported chitosan (a natural coagulant) as an excellent coagulant for color removal from wastewater having different dyes in both acidic as well as alkaline environments.12,13 Fenton’s process has also been used to remove color and nonbiodegradable COD from textile wastewater.8,17−21 At an Fe2+/H2O2 molar ratio of 1.21, around 96.3% decolorization could be achieved from a synthetic wastewater containing polyvinyl alcohol and Remazol Turquoise Blue G-133 dye.20 The initial COD of the wastewater was 213 mg/L. Rodrigues et al.22 studied treatment of textile mill effluent (dissolved organic carbon = 239−247 mg/L) using Fenton’s oxidation and biological processes alone and in combination. It was found that around 97% color removal could be achieved through combined treatment. Post-treatment by adsorption on activated carbon is also an effective method for removal of residual color and COD from pretreated textile wastewater. The process Received: March 17, 2013 Revised: July 5, 2013 Accepted: July 8, 2013

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constituents of the wastewater were completely mixed in the appropriate amount of tap water, and the wastewater was stored below 4 °C temperature to prevent any changes in the wastewater characteristics. The simulated wastewater composition was as follows (mg/L): wetting agent, Cottoclarin 63 (500), dextrin (800), sucrose (640), sodium hydroxide (1200), sodium silicate (2.5), softening agent, Belsoft 200 (400), acetic acid (165), reactive orange 16 dye (500), sodium carbonate (700), sodium chloride (5000), EDTA (300), and SDS (a detergent) (300). 2.2. Experimental Methods. 2.2.1. Coagulation Studies. The coagulation studies were conducted in a jar test apparatus (Cintex Floculatior, Cintex Industrial Corp., Mumbai, India) with six beakers of 500 mL capacity each. Typically, 300 mL of wastewater was taken in each beaker and a predetermined amount of coagulant was added. Flash mixing (stirring speed = 150 rpm) for 5 min was followed by slow stirring at a speed of 30 rpm for another 15 min so as to achieve good floc formation. The G values for fast and slow steps were 300 and 45.6 s−1, respectively. Subsequently, the flocs were allowed to settle for 1 h. The supernatant was separated and filtered through a Whatman filter paper (grade 4) before examining pH, COD, biochemical oxygen demand (BOD5), and color of the treated wastewater. The sludge settling pattern in the treated suspensions (with inorganic coagulants only) was also observed in a 2 L capacity measuring cylinder. 2.2.2. Fenton’s Oxidation. To perform Fenton’s oxidation, 250 mL of the simulated wastewater or the supernatant obtained after coagulation was added to a 500 mL capacity glass vessel. The wastewater was acidified using 10% H2SO4 to adjust the initial reaction pH in the range of 3.0−3.5. Subsequently, predetermined amounts of H2O2 (30% v/v) and Fe2+ (as FeSO4) were added to the glass vessel and mixed vigorously by means of a magnetic stirrer for 1 h duration at ambient temperature (ca. 30 °C). The agitation speed was maintained at 1050 rpm. The first sample was withdrawn within 1 min after the start of mixing. Later, the samples were withdrawn periodically during the oxidation reaction to study the process performance with time. The withdrawn samples were centrifuged at 5000 rpm for 15 min and filtered using Whatman filter paper (grade 4) before using them for determination of pH, COD, BOD5, and color. 2.2.3. Adsorption Tests. Adsorption studies were performed in batch flasks kept in an orbital shaker cum incubator (Trishul Equipments, Mumbai, India). Activated carbon was used as an adsorbent for the process. The particle size of activated carbon was in the range of 104−295 μm. During a typical run, 50 mL of pretreated wastewater (either by coagulation or Fenton’s oxidation) was added to 100 mL conical flasks having varying doses (1−10 g/L) of activated carbon. The shaker was operated at 200 rpm shaking speed for 1 h duration. The treated samples were filtered by passing through a Whatman filter paper (grade 4) before analyzing for the above-mentioned wastewater quality parameters. 2.3. Analytical Methods. The wastewater parameters such as COD, BOD5, turbidity, alkalinity, conductivity, and solids were determined using the guidelines given in American Public Health Association (APHA) handbook.31 COD was determined in a Hach COD reactor (DRB200, Hach Co., Loveland, CO) using the closed reflux method. To eliminate the effect of residual H2O2 from the samples obtained after Fenton’s process, MnO2 was added as a catalyst.32 In the presence of MnO2, residual H2O2 is rapidly decomposed into O2 and H2O

effectiveness largely depends upon several factors that may include the type of dye, wastewater temperature, COD, reaction pH, and contact time.16,23−25 A significant increase in color and COD removal has been reported by use of adsorption as a post-treatment step.26−29 Kurniawan and Lo29 reported that combined treatment by Fenton’s oxidation and granular activated carbon (GAC) adsorption could reduce the overall COD and NH3−N by 82% and 59%, respectively. Most of the above treatability studies have been performed on synthetic wastewater containing a particular dye, and therefore, color has been used as the main indicator of process performance. COD and biodegradability of the wastewater have not been reported in many of the studies. When physicochemical processes are applied to textile wastewater either singly or in combination, color, COD, and biodegradability should be evaluated after each process to get a comprehensive idea on treatment effectiveness. The determination of biodegradability is necessary so as to determine the feasibility for incorporation of biological processes in the treatment train. Since the wastewater characteristics may vary widely across textile units due to the use of different dyestuffs and other chemicals in the manufacturing process, there is need for conducting more laboratory scale studies before implementation of a treatment scheme. In the present study, an attempt is made to find an appropriate treatment scheme using different nonbiological processes for a simulated textile wastewater. In this research, the efficacy of two physicochemical processes, coagulation and Fenton’s oxidation, was investigated for COD and color removal from a simulated textile wastewater containing reactive orange 16 dye along with other chemicals typically present in textile mill effluents. Adsorption process was used as a posttreatment step to determine if further enhancement in COD and color removal from the wastewater could be achieved. Biodegradability of the treated wastewater was also determined.

2. MATERIALS AND METHODS 2.1. Materials. 2.1.1. Chemicals. The chemicals used for preparation of synthetic wastewater, i.e., such as dextrin, sucrose, sodium dodecyl sulfate (SDS), sodium silicate, sodium carbonate, sodium chloride, ethylenedinitrilotetraacetic acid (EDTA) and sodium hydroxide were purchased from Merck Chemicals (Mumbai, India). The two other constituents of the simulated wastewater, Cottoclarin 63 and Belsoft 200, were purchased from Pulcra Chemicals (Mumbai, India). Reactive orange 16 dye was obtained from a local textile industry. Among other chemicals, activated carbon, H2O2 (30% v/v), and hydrated salts of iron and magnesium (FeCl3, FeSO4, and MgCl2) were also procured from Merck Chemicals (Mumbai, India). All these chemicals were of analytical grade. Chitosan was supplied by Lotto Chemicals (Mumbai, India). During coagulation studies, FeCl3, FeSO4, MgCl2, and chitosan were used as coagulants. Fe and Mg salts were added in solid form as obtained from the supplier. A stock solution of chitosan was prepared using the method suggested by Szygula et al.12 According to this method, 1 g of powdered chitosan was hydrated overnight in 98 mL of distilled water. Subsequently, 1 g of acetic acid was added with mixing, and the solution was left undisturbed for 24 h. The resulting viscous solution was then used for coagulation studies. 2.1.2. Simulated Wastewater Preparation. Simulated wastewater was prepared in the laboratory using a composition similar to that used by Korbahti and Tanyolac.30 The various B

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COD ratio (=0.35). Besides, chloride content was also high in the simulated wastewater (3035 mg/L). It can be noted that TS concentration was very high (ca. 11000 mg/L) mostly in the form of dissolved impurities (TDS ∼ 95% of TS) while the rest contributed to TSS. The sum of experimental TDS and TSS is slightly lower than the measured TS (∼1.7%), which may be acceptable. The inorganic sodium salts are the major contributors to the dissolved solids. 3.2. Treatment of the Simulated Wastewater by the Coagulation Process. The coagulation studies were carried out with FeCl3, FeSO4, MgCl2, and chitosan as coagulants. The effect of coagulant dose on COD and color removal from the simulated wastewater was observed, as illustrated in Figure 1. To determine the efficacy of FeCl3, the coagulant dose was varied from 1 to 6 g/L (corresponding Fe3+ dose = 340−2071 mg/L). During these runs, no adjustment in pH was made. Since FeCl3 is a Lewis acid, its addition reduced pH of the wastewater. Wastewater pH dropped to 2.31 after addition of 6 g/L FeCl3 although alkalinity present in the wastewater resisted change in pH at low coagulant dose. A sharp drop in pH was observed with an increase in coagulant dose. Maximum color removal of ∼75% was observed at a coagulant dose of 3 g/L, whereas the COD reduction was only around 47.6% (final COD = 1760 mg/L). Further addition of FeCl3 reduced decolorization of the wastewater such that only ∼61% color removal was observed at the maximum coagulant dose, as shown in Figure 1a. The wastewater treated with the optimum coagulant dose of 3 g/L was subjected to the BOD5 test and BOD5 was determined as 660 mg/L (BOD5 reduction ∼ 44%). To predict the contribution of various constituents to COD in the simulated raw and treated wastewater, a solution of individual compounds was prepared and the initial pH was adjusted to ∼12 (original pH of the simulated wastewater). The concentration of each constituent was kept the same as that present in the simulated wastewater. COD of the individual compounds before and after the coagulation process was determined experimentally using the closed reflux method.31 Dextrin and sucrose were identified as the two major constituents contributing to COD (=42%). The other three constituents, i.e., detergent, dye, and wetting agent, contributed 45% to the overall COD. It was observed that COD of the simulated wastewater (3360 mg/L) was almost the same as the sum of COD values of all individual constituents (3374 mg/L). The COD removal of all individual constituents (adjusted pH 12) was studied using an FeCl3 dose of 3 g/L. After the reaction, the pH was reduced (2.25−2.40) in all the solutions. The results obtained are illustrated in Figure 2. Significant removal of detergent, dye, and wetting agent could be achieved through coagulation. From the results shown in Figure 2, it can be suggested that about three-fourth of the total percent COD reduction from the simulated wastewater is due to the removal of these three components. About 50% COD reduction was observed for acetic acid solution containing acetic acid as the sole constituent. However, the major components, dextrin and sucrose, that contributed around 42% of the total COD in the simulated wastewater could not be removed effectively by coagulation process and showed only ∼14% COD reduction when they were present as sole constituent. Thus, the overall COD reduction in the simulated wastewater was less than 50% after the coagulation process (Figure 1a). The effect of alkalinity on pH, COD, and color of the treated wastewater was also investigated. This set of experiments was performed with a coagulant (i.e., FeCl3) dose of 6 g/L at which

(according to eq 1) within 5 min duration. Thus, further degradation of residual organic compounds in the treated sample by H2O2 and consequent error in COD estimation were prevented. Control studies revealed that 7 g/L H2O2 added to the wastewater sample elevated the COD by 7% and this rise in COD could be avoided by addition of a MnO2 as catalyst. MnO2

2H 2O2 ⎯⎯⎯⎯⎯→ O2 + 2H 2O

(1)

BOD5 of the wastewater samples was determined using the modified Winkler’s method.31 Wastewater pH was measured using a digital pH meter (Polmon, LP-1395). For measuring total organic carbon, a TOC analyzer (TOC-VCSH, Shimadzu, Kyoto, Japan) was used. To determine color removal, the absorbance of the untreated and treated samples was measured at 494 nm wavelength using UV−vis spectrophotometer (HEλIOS Thermo spectronic, Mumbai, India). The optimum wavelength was obtained by scanning the untreated sample. Iron and magnesium in the treated water was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Utltima 2000, HORIBA, JobinYvon). Total solids (TS) in the effluent were determined by drying the raw or treated sample (of known volume) at 105−110 °C temperature in an oven for 24 h. To determine total suspended solids (TSS), the liquid sample was filtered through a Whatman filter paper (grade 4). The oven-dried residue (at 105−110 °C temperature) left on the filter paper was weighed and reported as TSS.

3. RESULTS AND DISCUSSION 3.1. Characteristics of the Simulated Wastewater. The physical and chemical characteristics of the simulated textile wastewater are presented in Table 1. The highly alkaline (pH = Table 1. Characteristics of the Simulated Industrial Textile Wastewater

a

parameter

value

pH color turbidity, NTUa conductivity, mS/cm COD, mg/L TOC, mg/L BOD5, mg/L chlorides, mg/L TS, mg/L TSS, mg/L TDS, mg/L alkalinity, mg/L as CaCO3

12.1−12.3 maroon 2.72 15.7 3360 1390 1176 3035 11016 540 10292 1770

NTU, nephelometric turbidity units.

12.1−12.3) wastewater was maroon in color. BOD5 and COD of the wastewater were 1176 mg/L and 3360 mg/L, respectively. The buffering capacity of the wastewater was significantly high (alkalinity = 1770 mg/L as CaCO3). The wastewater is not suitable for discharge into inland water bodies due to noncompliance of these parameters with the standards prescribed by the Central Pollution Control Board (CPCB), India, for textile mill wastewater33 and can be considered as unsuitable for aerobic biological process due to low BOD5/ C

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Figure 1. COD and color removal from simulated textile wastewater with (a) FeCl3, (b) FeSO4, (c) MgCl2, and (d) chitosan as coagulant.

Figure 3. Effect of addition of alkalinity on pH, COD, and color for 6 g/L FeCl3 as coagulant.

Fe2+ dose = 201−1206 mg/L), pH was reduced to 11.6−7.23. Similar to FeCl3, the maximum COD and color removal from the wastewater were obtained at an FeSO4 dose of 3 g/L (47.6% and 96.3%, respectively). The decrease in FeSO4 performance at higher coagulant dose may be attributed to restabilization of the colloidal particles due to the reversal of charge. Colloidal particles in water are typically negatively charged. Coagulants such as FeSO4 causes generation of Fe2+ and other positively charged Fe species (generated through hydrolysis) that can adsorb on the surface of the colloids to neutralize the charge, reduce repulsive interaction between colloids, and thus cause growth in size and particle settling. However, at high coagulant dose, adsorption of excess positively charged ions on the surface of colloidal particles can cause the development of positive charge, which promotes repulsive interaction and hinders settling. A maximum BOD5 removal of 40% from the simulated wastewater could be obtained at the optimum FeSO4 dose (i.e., 3 g/L). The chloride salt of magnesium (i.e., MgCl2) was found to be the least effective coagulant and showed maximum COD and color reduction of 47.3% and 54.3%, respectively, from the simulated wastewater (Figure 1c). To achieve the highest removal in these two parameters, 6 g/L of coagulant dose (equivalent

Figure 2. COD removal from solutions containing the individual components of simulated textile wastewater using 3 g/L FeCl3.

pH of the original wastewater dropped to 2.3. Alkalinity was imparted to the wastewater by adding sodium bicarbonate (dosage = 1−10 g/L as CaCO3). For the wastewater having an additional alkalinity of 6 g/L (as CaCO3), the pH dropped to 6.82 compared to 2.3 when no additional alkalinity was present. With an increase in alkalinity up to 10 g/L (as CaCO3), the drop in pH was lower (final pH 7.6). However, alkalinity addition had an adverse impact on the performance of the coagulation process. COD reduction was a maximum at an alkalinity dose of 2 g/L showing ∼48% removal, although color reduction was 63.6%, which was slightly lower than that obtained when no additional alkalinity was provided. With a change in solution pH, the nature of hydrolysis products may be altered, which may influence the soluble metal concentration in the wastewater. Beyond 2 g/L of additional alkalinity, the COD removal efficiency of the FeCl3 coagulant decreased rapidly (Figure 3). Another inorganic coagulant, FeSO4, showed much better decolorization of the wastewater than FeCl3, as shown in Figure 1b. With addition of FeSO4 coagulant at 1−6 g/L (equivalent D

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Mg2+ concentration = 708.3 mg/L) was added, which is exactly double the optimum dose obtained for iron-based salts. The wastewater pH dropped to 10.3 after addition of 6 g/L MgCl2. MgCl2 has been found to be effective at higher pH (greater than 10.5) due to the formation of insoluble magnesium hydroxide species.6,14 It can be seen from the figure that addition of 3 g/L MgCl2 dose yielded COD removal of 35.6%, which was further increased by ∼12% with doubling the coagulant dose (i.e., 6 g/L). Although the pH of the wastewater was favorable for coagulation with lower dose of MgCl2, magnesium hydroxide floc formation may not have been adequate for yielding high color and COD removal. The results obtained with MgCl2 are not in agreement with the previous findings.6 Various factors such as pH of the wastewater, nature of the metal species and affinity of various wastewater constituents to the coagulant species may affect the efficacy of the coagulation process. After coagulation, BOD5 of the wastewater was reduced by 49% with 6 g/L coagulant dose. The settling patterns of sludge formed during coagulation at optimum doses with the three inorganic coagulants (FeSO4, FeCl3, and MgCl2) were observed in a 2 L capacity measuring cylinder. The sludge blanket height was recorded with time for each wastewater suspension as shown in Figure 4. The

Comparison among the three inorganic coagulants reveals that FeSO4 was the most effective coagulant for decolorization of simulated textile mill wastewater. A maximum of ∼96% color removal could be achieved with FeSO4 coagulant, although COD reduction was below 50% with all the three coagulants at their optimum doses. The effectiveness of the three coagulants for color removal from the wastewater can be arranged as follows (in decreasing order): FeSO4 > FeCl3 > MgCl2. The biodegradability (BOD5/COD) of the treated wastewater after coagulation was also slightly improved for iron-based coagulants. For FeCl3-treated wastewater, it was 0.38 compared to 0.40 for the wastewater treated by FeSO4 coagulant at optimal dose. The biodegradability of MgCl2-treated wastewater remained almost unchanged (i.e., ∼0.35). For coagulant addition at optimal dose, ICP-AES analysis showed that FeSO4- and FeCl3-treated wastewaters contained 74.1 and 68.7 mg/L iron, respectively, while a higher concentration of magnesium (=99.5 mg/L) was found in the MgCl2-treated wastewater. This may be attributed to the addition of a higher dose of magnesium salt required for achieving maximum removal (6 g/L compared to 3 g/L for iron-based coagulants). The effect of a natural coagulant, i.e., chitosan, on COD and color removal from the simulated textile industry wastewater was also studied. A much smaller coagulant dose was added compared to the inorganic coagulants used. The dose of natural coagulant was varied from 10 to 60 mg/L and no pH adjustment was necessary. The maximum COD and color removal observed were 20% and 32% for a coagulant dose of 10 mg/L. The effect of varying chitosan dose on COD and color removal is shown in Figure 1d. For dose varying over the range 20−60 mg/L, there was a drop in percent reduction of color, while COD removal remained unchanged. It was noticed that the solubility of chitosan decreased as the pH of the wastewater sample approached the basic region. Hassan et al.11 reported that chitosan was dissolved in an aqueous solution at a pH less than 6.0, although at higher pH it became insoluble and existed as solid particles. Roussy et al.35 have also demonstrated the low efficiency of chitosan at alkaline pH and suggested a higher coagulant dose requirement for achieving efficient decolorization. In order to investigate the effect of pH on coagulation efficiency of chitosan, the pH was varied over the range 3.0−8.0. At pH 3.0, both color and COD removal were increased to 58.7% and 33.3%, respectively, at the optimum chitosan dose (i.e., 10 mg/L). On the basis of the preliminary results obtained with chitosan, it may be suggested that this natural coagulant is very effective at a much smaller dose in the acidic pH range compared to the inorganic coagulants. However, problems with settling of the coagulated mass were observed. 3.3. Combined Treatment with Coagulation Followed by Adsorption Process. For the combined treatment (coagulation followed by adsorption), coagulation was first carried out with FeCl3, FeSO4, and MgCl2 using the optimum dose for each coagulant, i.e., 3, 3, and 6 g/L, respectively. The supernatant left after the separation of sludge was subjected to activated carbon adsorption. For the post-treatment step, the adsorbent dose was varied from 1 to 10 g/L. COD and color removal during the adsorption process for varying dose of activated carbon are illustrated in Figure 5. The overall COD and color removal from the simulated textile wastewater were significantly increased after two-stage treatment (i.e., coagulation followed by adsorption). Combined

Figure 4. Settling curves for suspensions obtained after treatment with inorganic coagulants.

wastewater treated with Fe salts showed better settling compared to MgCl2-treated wastewater. The sludge settling for FeSO4-treated wastewater was marginally better than the wastewater treated with FeCl3. The lower pH of the FeCl3 treated wastewater (∼4.0) may have adversely impacted the settling characteristics due to higher concentration of dissolved iron species. The inferior color removal was possibly due to the higher concentration of dissolved iron. For all the three treated samples, the major portion of sludge was settled within the first 2.5 h. The settling curves can be divided into three segments: rapid settling, transition, and hindered settling. For the wastewater treated by iron-based coagulants, the rapid settling zone and the hindered settling zone were separated by a shorter transition section in contrast to the MgCl2-treated suspension. The sludge volume for MgCl2-treated wastewater was higher compared to the other coagulants, indicating the formation of less compact sludge with slower settling characteristics. These results are consistent with those reported by Bidhendi et al.34 E

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H2O2/Fe2+ was around 20, whereas the H2O2/COD mass ratio was 2.08. The wastewater with pH 12 was finally adjusted to a pH of 3.5 by addition of 10% H2SO4 solution. The process requires highly acidic conditions, since otherwise Fe2+ ions begin to form flocs and precipitate under alkaline conditions. Moreover, H2O2 is unstable at higher pH and may decompose into oxygen and water, thus losing its oxidation ability. The decrease in COD and color with time is shown in Figure 6.

Figure 6. COD and color removal over time during Fenton’s oxidation ([Fe2+] = 604 mg/L, [H2O2] = 7 g/L, pH 3.5).

Figure 5. Effect of adsorbent (activated carbon) dose on (a) COD and (b) color reduction from the wastewater pretreated by coagulation using all three inorganic coagulants, (A) FeCl3, (B) FeSO4, and (C) MgCl2.

The first sample was withdrawn within a minute (after 50 s) of addition of Fe2+ and H2O2 into the wastewater. The reactants were vigorously mixed by means of a magnetic stirrer. Within a minute, the COD and color reduction were found to be ∼30% and 80%, respectively. Around 98% color removal was obtained after 5 min of the reaction, which was further increased to 99.4% at the end of 45 min of reaction. On the other hand, COD reduction was gradually increased to ∼63% after the same reaction period. Rapid color removal may be attributed to the conversion of dye molecules into colorless organic compounds. The results obtained from the present study are consistent with that reported by Bautista et al.36 To study the effect of Fe2+ dose on removal of COD and color, Fe2+ dose was varied from 201 to 1007 mg/L (equivalent to 1−5 g/L FeSO4). The initial pH of the reaction was adjusted to 3−3.5 and the H2O2 dose was kept the same (i.e., 7 g/L). The corresponding H2O2/Fe2+ molar ratio was varied from ∼60 to 12. The reaction was performed for 1 h duration. The effect of varying Fe2+ dose on COD and color removal efficiency is shown in Figure 7a. From the treated wastewater analysis, it was found that around 99% color was removed with 1 g/L of FeSO4 dose ([Fe2+] = 200 mg/L) although COD removal was only 54%. With increase in FeSO4 dose up to 2 g/L ([Fe2+] = 400 mg/L), COD removal was further enhanced to ∼63%, beyond which no change in COD reduction could be obtained. This suggests the presence of low molecular weight carboxylic acids (mainly acetic acid), which may be resistant to further degradation under the reaction conditions used in the present study. A run was also performed without any Fe2+ in the solution. H2O2 concentration in the solution was 7 g/L. After the reaction period of 1 h, only 7% and 21% reduction in COD and color could be obtained, which demonstrates the poor removal

treatment enhanced the COD and color removal to ∼86% and ∼99%, respectively, for an adsorbent dose of 8 g/L (from ∼47.6% and 96% obtained after coagulation with FeSO4). Even the addition of 1 g/L of activated carbon enhanced the overall COD removal to 58.3%. Combined treatment comprising coagulation with FeCl3 followed by adsorption on activated carbon at a dose of 10 g/L enhanced COD reduction to ∼81% (final COD ∼ 640 mg/L) from 47.6% obtained with coagulation alone. The overall color removal was increased to 99%. The effectiveness of adsorption process for MgCl2-treated wastewater was also investigated and it was found that almost 100% color removal was achieved with an activated carbon dose of 9 g/L and above. However, the overall COD removal was only ∼76% with 10 g/L adsorbent. Hence, the combination of coagulation with MgCl2 followed by adsorption process was less efficient for the removal of COD compared to the other combinations. The biodegradability of the treated wastewater (with FeSO4 and activated carbon) was determined. The results revealed that the BOD5/COD ratio was increased from 0.40 to 0.48 after the adsorption process. Since the increase in biodegradability is marginal after the two-step process, it can be suggested that the biological process can be used after the coagulation process, and the residual nonbiodegradable compounds should be removed by employing adsorption after biological treatment. 3.4. Fenton’s Oxidation Process. The change in COD and color of synthetic wastewater (COD = 3360 mg/L) with time was observed during the Fenton’s oxidation process performed at ambient temperature conditions. The oxidative reaction was carried out with a H2O2 concentration of 7 g/L and 604 mg/L Fe2+ (as 3 g/L FeSO4) dose. The molar ratio of F

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The effect of H2O2 dose on COD and color removal is illustrated in Figure 7b. The initial pH of the wastewater was kept constant at ∼3.5, and 604 mg/L of Fe2+ was added to the reaction vessel. The maximum COD and color removal achieved were ∼80% and ∼99%, respectively, with H2O2 dose of 10.5 g/L (H2O2/Fe2+ molar ratio = 30). The results showed a gradual increase in percent COD removal when H2O2 dose was increased from 5 to 10.5 g/L. Further increase in H2O2 had an adverse effect on COD removal. It has been reported that excess H2O2 acts as a scavenger of hydroxyl radicals and produces a less potent perhydroxy radical, which results in lower degradation.36 3.5. Sequential Treatment Comprising of Coagulation Followed by Fenton’s Process. The effectiveness of combined treatment processes (i.e., coagulation followed by Fenton’s oxidation) was also studied for the simulated wastewater. During the coagulation step, the wastewater was pretreated with FeSO4 coagulant at the optimum dose of 3 g/L. The supernatant obtained after the treatment was subjected to Fenton’s oxidation using a H2O2 concentration of 7 g/L. The pH of the pretreated wastewater was adjusted to 3.5 before the oxidation step. During the combined treatment comprising coagulation and Fenton’s oxidation, the following variations were studied during the oxidation step: (a) effect of residual Fe2+ from the pretreatment step on overall COD and color removal, (b) effect of additional Fe2+ ions on overall COD and color removal, and (c) effect of H2O2 dose on overall COD and color removal from the pretreated wastewater. The residual Fe2+ concentration of ∼74 mg/L remaining from the coagulation step was utilized to generate free radicals, and no additional ferrous ions were added to the reaction

Figure 7. Effect of varying Fe2+ and H2O2 dose on COD and color removal efficiency during Fenton’s process: (a) varying Fe2+ dose at a constant H2O2 dose of 7 g/L (pH 3−3.5) and (b) varying H2O2 dose at a constant Fe2+ dose of 604 mg/L (pH 3.5).

efficiency of H2O2 in the absence of Fe2+ catalyst. In another run, only Fe2+ (604 mg/L) was added without using H2O2. Low color removal (∼21%) from the wastewater was observed, which clearly indicates that oxidation of dye is responsible for the removal of color and COD during Fenton’s process.

Figure 8. COD and color removal through sequential treatment by coagulation with 3 g/L FeSO4 followed by Fenton’s process at pH 3.5. (a) Removal kinetics with no additional Fe2+ added ([Fe2+] = 74 mg/L) and constant H2O2 dose of 7 g/L. (b) Effect of varying additional Fe2+ dose at a constant H2O2 dose of 7 g/L. (c) Effect of varying H2O2 dose at constant additional Fe2+ of 300 mg/L. G

dx.doi.org/10.1021/ie400855b | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

dose of activated carbon (=8 g/L) may be a concern. When coagulation was combined with adsorption, color removal was almost 100%. On the other hand, Fenton’s oxidation of the untreated wastewater exhibited good removal of COD and color up to ∼80% and 99%, respectively, although high chemical doses were required. The chemical costs can be offset by lower sludge generation and consequently lower sludge disposal problem compared to the coagulation step. Future work may focus on the quantification and characterization of solids and a comprehensive cost analysis of the various treatment options. Sludge disposal may be a major concern with the suggested options, especially those involving the coagulation process. Hence, options for sludge utilization in a beneficial manner need to be explored. The use of natural coagulants, such as chitosan, in textile mill wastewater treatment should also be studied further.

mixture. The results revealed that the overall COD removal was enhanced to 50% after the oxidative treatment (Figure 8a) from 47.6% achieved during the coagulation process. The color removal was also slightly increased to around 98%. It appeared that the degradation of organics was completed within the first 5 min. The minor increase in removal of COD and color was possibly due to the absence of sufficient Fe2+ ions in solution. The pretreated wastewater contains mostly dextrin, sucrose, acetic acid, and EDTA, which may not be amenable to chemical oxidation under these reaction conditions. In order to demonstrate the effect of ferrous ions in the reaction mixture, additional FeSO4 was used. With an increase in Fe2+ ion concentration in the pretreated wastewater, COD removal was enhanced substantially up to an optimum Fe2+ dose, although increase in color removal was not significant (Figure 8b). A significant portion of color-imparting compound (i.e., dye) had already been removed during the first step. The slight increase in decoloration (∼3% enhancement) of the pretreated wastewater was mainly due to the degradation of residual dye. The COD removal was increased to 83% with an additional Fe2+ dose of 300 mg/L and was decreased slightly at higher Fe2+ concentrations. According to eq 2, the excess Fe2+ ions may scavenge the hydroxyl radicals and thus reduce the efficiency of the oxidation process.36 Fe 2 + + OH• → Fe3 + + OH−

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AUTHOR INFORMATION

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*Phone: +91-22-25767861. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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The effect of H2O2 on COD and color removal from the pretreated wastewater was also studied. Additional Fe2+ concentration of 300 mg/L was added to the reaction mixture. The results shown in Figure 8c suggests that COD removal was continuously increased from 65.7% (final COD = 1150 mg/L) to 83.3% (final COD = 560 mg/L) as H2O2 concentration was varied from 3 to 7 g/L, respectively. As expected, the change in color removal was not significant. The biodegradability of the wastewater was also determined after the combined treatment. It was found that BOD5/COD ratio was increased from 0.40 (pretreated wastewater) to 0.51. It can be deduced from this observation that the wastewater treated with coagulation followed by Fenton’s process is suitable for further treatment with the aerobic biological process. After the combined treatment methods, the simulated wastewater can be subjected to conventional biological treatment to achieve the prescribed BOD5 and COD discharge limits.

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4. CONCLUSIONS From the present study, it was found that ferrous sulfate was the most efficient coagulant for the removal of color from the simulated textile wastewater (color removal ∼ 96%), while the maximum COD removal was 47.6% at an optimal dose for both FeSO4 and FeCl3. The COD imparted by the dye, wetting agent, and softening agent was effectively removed by coagulation, while that due to dextrin and dextrose was not removed by coagulation. The application of coagulation followed by Fenton’s process enhanced the COD removal up to ∼83% when an additional Fe2+ concentration of 300 mg/L was provided along with 7 g/L H2O2. The wastewater after such a combined treatment would be more amenable to aerobic biological treatment, since the final BOD5/COD ratio increased to 0.51. Another combined treatment scheme comprising coagulation followed by adsorption on activated carbon was also found to be very effective in further COD reduction and biodegradability enhancement. The overall COD removal was enhanced up to ∼86%; however, the requirement of a high H

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dx.doi.org/10.1021/ie400855b | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX