Decolorization and COD reduction of disperse and

Desalination 249 (2009) 1159–1164

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Decolorization and COD reduction of disperse and reactive dyes wastewater using chemical-coagulation followed by sequential batch reactor (SBR) process F. El-Gohary, A. Tawfik ⁎ National Research Center, Water Pollution Research Department, El-Behouth St., Dokki, P. Box 12622, Cairo, Egypt

a r t i c l e

i n f o

Article history: Accepted 5 May 2009 Available online 9 October 2009 Keywords: Dyes wastewater Coagulation Inorganic coagulants Cationic polymer Color COD Sludge SBR

a b s t r a c t This paper summarizes the results of disperse and reactive dyes wastewater treatment processes aiming at the destruction of the wastewater's color and chemical oxygen demand (COD) reduction by means of coagulation/flocculation (CF) followed by sequential batch reactor (SBR) process. The color removal efficiency of magnesium chloride aided with lime [MgCl2/CaO] was compared with that of alum [Al2 (SO4)3] and lime [Cao]. The experimental results showed that treatment with lime alone (600 mg/l) at pH value of 11.7 proved to be very effective. Color removal reached 100% and COD was reduced by 50%. Treatment with magnesium chloride aided with lime at pH value of 11 removed color completely and reduced the COD value by 40%. However, lime or lime in combination with magnesium chloride produced high amounts of sludge (1.84 kg/m3 for lime & 1.71 kg/m3 for MgCl2 aided with lime). Also, the pH of the treated effluent was around 11 and needs correction prior to discharge into sewer network. The use of 200 mg/l alum without pH adjustment removed 78.9% of the color. To improve the effectiveness of alum, the cationic polymer namely cytec was used as a coagulant aid. This significantly increased color removal from 78.9 up to 94% and COD reduction was around 44%. Moreover, sludge production was only 0.36 kg/m3. Chemically pre-treated effluent was subjected to SBR process at an HRT of 5.0 h. Residual CODtotal, total biochemical oxygen demand (BOD5 total) and total suspended solids (TSS) in the final effluent were 78 ± 7.7; 28 ± 4.2 and 17 ± 4.2 mg/l, corresponding to the removal efficiency of 68.2; 76.3 and 61.4% respectively. Furthermore, almost complete removal of CODparticulate and BOD5 particulate has been achieved. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Textile and dyeing mills use large amounts of water and discharge colored wastewaters that are heavily polluted with dyes, textile auxiliaries and other chemicals [1]. Furthermore, the composition of wastewater from dyeing and textile processes varies greatly from day to day and even from hour to hour, depending on the dyestuff, fabric and concentration of fixing compounds which are added [2]. Unfixed dye releases large doses of color to the end of pipe effluents. Biological treatment processes are frequently used to treat textile effluents. These processes are generally efficient for biochemical oxygen demand (BOD5) and suspended solids (SS) removal, but they are largely ineffective for removing color which is visible even at low concentrations [3,4]. Initial environmental efforts with dyes dealt with color pollution, which has a strong psychological effect. More recently, this interest has shifted to the potential toxicity of dyes and their degradation products, especially the suspected carcinogenicity of potential intermediate products. Recently, many studies have been demonstrated that fungi are able to degrade and decolorize dyes in

⁎ Corresponding author. Tel./fax: +20233351573. E-mail addresses: Tawfi[email protected], tawfi[email protected] (A. Tawfik). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.05.010

wastewater [5–8]. However the application of white-rot fungi on a large scale is limited by the difficulty of achieving sterilized conditions in open-air reactors slow fungal degradation [9], loss of the extracellular enzymes and mediators with discharged water [10], and excessive growth of fungi [11]. Although dyes in wastewater can be effectively destroyed by advanced chemical oxidation using ozone [12] or H2O2/UV [13] and adsorption using activated carbon [14], the costs of these techniques are still high. In Egypt, the textile industry is one of the most important export industries. It discharges large amounts of wastewater during processing, especially in the coloring and washing steps. The wastewater contains high concentrations of both organic matter and colorants (dyes). Most of these mills (over 3000) discharge their wastewater without any treatment into the sewerage system exerting negative impact on sewer fabric and the performance of sewage treatment plants (STP). Meric et al., [15] investigated the effectiveness of Fenton's oxidation (FO) process and ozone (O3) oxidation compared with a coagulation– flocculation (CF) process to remove color and COD from textile wastewater industry. They found that, the FO process removed a higher value of COD (59%) as compared to O3 (33%) while; color reduction was almost similar (89 and 91% respectively). The CF process achieved COD and color removal efficiency similar to the FO process. Therefore, in this

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study, coagulation–flocculation (CF) has been chosen, for its simplicity to remove color and reduce the COD value of dyes wastewater. The optimal coagulation conditions (pH and coagulant dose) were determined. The chemically pre-treated wastewater was subjected to aerobic biological treatment via sequential batch reactor (SBR) process. The SBR process of wastewater treatment has received considerable attention because it is compact, easy to operate and maintain and capable of eliminating organic matter from dyes wastewater [16].

solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) of the inoculum were 5500 and 4000 mg/l respectively. The sludge volume index (SVI) amounted to 70 ml/gMLSS. Initially, the SBR was fed with only domestic wastewater for 10 days. From 11 to 20 d, 50% of the influent wastewater was mixed with 50% of chemically pretreated effluent. From 21 to 30 d, the chemically pretreated effluent was gradually increased up to 75% and remaining 25% was sewage. Finally, only chemically pretreated wastewater using alum aided with cytec was directly fed to the reactor for a period of 50 days (30–80 d).

2. Materials and methods 2.1. Wastewater characteristics Composite wastewater samples discharged from four textile and dyes companies into a collective manhole constitute the subject of this study. Mean characteristics of the wastewater are presented in Table 1. Physico-chemical analyses of the wastewater were carried out for one month to cover variations in wastewater characteristics. 2.2. Coagulation–flocculation (CF) experiments Chemical–coagulation–flocculation (CF) experiments using different inorganic coagulants namely, lime (CaO), magnesium chloride (MgCl2. 6 H2O) and alum (Al2SO4)3. (18 H2O) were carried out (Table 2). To improve the performance of the different coagulants, lime was used with magnesium chloride and the linear poly-acrylamide-based cationic polyelectrolyte (cytec) was tested with alum. The polyelectrolyte had a high molecular weight of 8.106 g/mol; degree of charge (+24%) and calculated chain length (37 µm). The standard jar-test apparatus (Phipps & Bird Stirrer, Model 7790-400) was used to determine optimum operating conditions (Table 2). For the jar test, the wastewater of 1.0 l was transferred into the jar. The samples in the jar were rapidly mixed at a paddle speed of 267 rpm (Gradient velocity G = 1000 s− 1) and then coagulants were instantaneously added. The rapid mixing was for 1 min followed by slow mixing for 20 min at 24 rpm (G = 50 s− 1) and settling for 60 min. After settling, COD and color were measured in the supernatant. Also characterization of the sludge produced was carried out. The experiments were repeated four times, only a selected set of results is shown. 2.3. Sequencing batch reactor (SBR) The 5.0 l SBR is operated in a cyclical manner, with each cycle comprising periods of fill, reaction with aeration, settlements, draw and idle. The influent from the feed tank (32 l) was injected into the reactor at the flow rate of 24 l/d using a peristaltic pump. A 60 rpm agitating motor was used to stir the wastewater in the reactor. An aerator and solenoid valve inside the reactor were installed for aeration and removal of clarified water. A timer controlled automatic operation of the reactor was installed. The order of processes during 1 cycle in the SBR was aerobic (230 min.); settlement and draw (70 min). The pH of the influent was 6.6 which is an appropriate pH for biological treatment. The temperature ranged from 22 to 29 ºC. The dissolved oxygen was set at less than 4–6 mg O2/l for ensuring oxidation of organic matter. 2.3.1. Reactor start up The SBR reactor was seeded with aerobic sludge obtained from sewage treatment plant (STP) of the Cairo city. Mixed liquor suspended

2.3.2. Batch aerobic biodegradability 5 l laboratory column was fed at t = 0 with 1 l sludge withdrawn from the SBR reactor and 1 l of the chemically pretreated effluent and aerating them for 6.0 h at 20 °C. The column was covered with laboratory Parafilm to prevent evaporation. For determination of optimum reaction time for degradation of organic matter, the mixed liquor samples (100 ml) were taken from the reactor at t = 0, 1, 2, 3, 4, and 5 h. The collected samples were allowed to settle for 1 h. The supernatant was withdrawn for analysis of COD. Batch experiments were performed in duplicate. 2.4. Analytical methods The biochemical oxygen demand (BOD5), total suspended solids (TSS), volatile suspended solids (VSS), total Kjeldahl nitrogen (TKN), total phosphorous (TP), oil & grease and sludge composition were determined according to the methodology described in the APHA [17]. HACH method was used for determination of COD. Heavy metals (Cupper; Nickel; Chromium; Cadmium) were determined by Varian Atomic Absorption Spectra AA220. The reduction in color concentration at the λmax was measured by using a UV–Visible Spectrophotometer (Systronics 118). The color removal was calculated using the following formulae:

M

M

M

Color removal ð%Þ = 100½ABS0  ABS  = ABS0

where ABSM is the average of absorbance values as it is maximum absorbency visible wavelength. ABSM 0 the value before coagulation, ABSM the value after coagulation–flocculation (CF) process. 3. Results and discussion 3.1. Effect of pH on color and COD removal Color removal is highly dependent on the pH [18]. The effect of pH on the dye reduction could be explained by the combined effect of (i) the ionization of amino, hydroxy and sulpho-groups in the dye molecules which increases with pH in alkaline range, and (ii) the decrease in the concentration of dissolved hydrolysis products [14]. To study the effect of pH on color removal, the pH of the wastewater was changed from 8.0 to 12 using sodium hydroxide (10%). The results have shown that at a pH value of 11.7, the color was removed by a value of 97%. This indicates that the pH value is the main factor affecting color removal. However, maximum COD removal was only 29% and the settleability of the produced sludge was fairly poor. To improve the performance of the treatment, different coagulants have been investigated.

Table 1 Characteristics of the investigated wastewater. pH-value

COD (mg/l)

BOD5 (mg/l)

TSS (mg/l)

TKN (mg/l)

TP (mg/l)

O&G⁎ (mg/l)

Cu (mg/l)

Ni (mg/l)

Cr (mg/l)

Cd (mg/l)

8.8 – 9.4

595 ± 131

379 ± 110

276 ± 76

16 ± 3

3.5 ± 0.8

41 ± 4.6

0.08 ± 0.03

0.08 ± 0.03

0.05 ± 0.0

0.04 ± 0.02

O&G⁎: Oil and grease.

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Table 2 Chemical coagulation flocculation (CF) experiments. Coagulant type

Alum [Al2(SO4)3. 18H2O] Alum + cationic polymer (cytec) Lime (CaO) Magnesium chloride aided with lime (MgCl2. 6H2O/CaO) a

Effect of pH at constant coagulant dose

Effect of coagulant dose at constant pH value

pH-values

pH-adjustment

Coagulant dose mg/l

Coagulant doses (mg/l)

pH-value

(3.24; 4.22; 5.75; 6.46; 7.7 and 7.9) – (9.44; 9.76; 10.4; 10.57; 11 and 11.57) (9.3; 9.8; 10.5; 10.62; 10.82; 11.02 and 11.35)

NaOH (10%) No Lime Lime

200 200 – 120

(50;75;100;175;200 and 300) (1;2;3;4 and 5)a (100; 200; 300; 400; 500 and 600) (10; 30; 50; 70; 100 and 120)

5.3 6.6 – 11.0

Cationic polymer (cytec) dose.

3.2. Coagulation studies 3.2.1. Use of alum 3.2.1.1. Effect of pH. To study the effect of pH on color removal, the alum dose was kept constant at 200 mg/l, while varying the pH of the wastewater with Na (OH) solution. The results presented in Fig. 1 show that color removal efficiency of alum increased with increasing pH from 3.0 to 6, after which a decrease was observed. Color removal could be attributed to the fact that within the optimum pH range, dye particles retain net negative charges which facilitate the performance of cationic coagulant [19]. Similar results were obtained by Song et al., [20] who found that the optimum pH value for color removal from textile wastewater using alum ranged from 5.0 to 6.0. 3.2.1.2. Effect of alum dose. Variation in color removal as a function of coagulant dose at the predetermined optimum pH value (5.3) is shown in Fig. 2. The results obtained indicated that the use of 200 mg/l alum; reduced the color intensity by a value of 78.9%. However, increasing the alum dose from 200 to 300 mg/l exerted limited improvement (only 4%). Similar results have been achieved by Can et al., [21] at pH value of 5.5 and alum dosage of 320 mg/l. In another study carried out by Kumar et al., [22], who found that the coagulation process using a higher alum dose of 5000 mg/l and lower pH value of 4.0, COD removal and color reduction were 58.6 and 74% respectively. 3.2.1.3. Effect of cationic polymer (cytec). The impact of the addition of different doses of cationic polymer (cytec) ranging from 1.0 to 5.0 mg/ l in combination with constant dose of alum (200 mg/l) has been investigated at pH value of 6.6. The results presented in Fig. 3 indicate that the optimum dose of cytec 1.0 mg/l. Color reduction and COD removal reached to 94 and 44%, respectively. However, the results obtained indicated that increasing the dose of cationic polymer (cytec) above 1 mg/l, the improvement of color removal was negligible. This can be due to the bridging mechanism [23,24]. COD removal was dropped (34%) at higher doses of cationic polymer as shown in Fig. 3. This is due to the presence of some organic polymer remains in the treated wastewater. Therefore it is recommended to use cationic polymer dose not exceeding 1 mg/l. The combination of cationic polymer with alum salts has the following advantages; rapid aggregation velocity, larger and heavier flocs and lower required

Fig. 1. Determination of optimum pH value of alum at a fixed dose of 200 mg/l.

dosage of inorganic coagulant [25]. The use of a higher alum dose of 2000 mg/l aided with 0.5 mg/l flocculant polymer synthesized from cyano-guanidine and formaldehyde under acidic conditions (pH = 5.0) for color removal from textile wastewater has been investigated by Joo et al., [25]. At these operating conditions, the color reduction amounted to only 62%. [22] used ferrous sulfate (1250 mg/l) aided with anionic polyelectrolyte with a concentration of 5.0 mg/l for coagulation of textile wastewater at pH value of 9.5. The reduction of color and COD removal were 90 and 59% respectively. 3.2.2. Use of lime 3.2.2.1. Effect of lime dose. The use of lime at different doses ranging from 100 to 600 mg/l was investigated. The results presented in Fig. 4 proved that the use of lime dose of 600 mg/l is very effective for color removal. Color removal reached 100%. However, the pH value was relatively high (11.7). Corresponding COD reduction was 50%. Lower color removal of 90% was obtained by Georgiou et al., [26] at a higher lime dose of 1000 mg/l. The undesired result of this process was the rise of pH (11.7), which is attributed to the partial dissolution of lime in the wastewater. Neutralization with hydrochloric acid prior to wastewater disposal is necessary. 3.2.3. Use of magnesium chloride (MgCl2. 6H2O) aided with lime 3.2.3.1. Effect of pH. To determine the optimum pH-value for magnesium chloride, a fixed magnesium chloride dose, equivalent to 120 mg/l was used. The pH was changed using lime to cover a range from 9.3 to 11.35. Available data indicates that the optimum pH value for color and COD removal is around 11.0 (Fig. 5). This is consistent with the findings reported by Gao et al., [27]. When the pH value is higher than 11.0, almost all the magnesium ions are converted into perceptible hydroxide. Mg (OH)2 precipitate provides a large adsorptive surface area and its positive electrostatic surface charge enables it to act as a powerful and efficient coagulant [28]. 3.2.3.2. Effect of magnesium chloride dose. To find out the optimum magnesium chloride dose, different doses ranging from 10 to 120 mg/l were used (Fig. 6). The pH was adjusted at the pre-determined value (11.0). The results obtained indicated that removal efficiencies of both

Fig. 2. Determination of optimum dose of alum at a fixed pH value of 5.3.

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Fig. 3. Determination of optimum dose of cationic polymer (cytec) at a fixed dose of alum 200 mg/l and pH value of 6.6.

COD and color increased with the increase of coagulant dosage. The COD was reduced by 40% and color was completely removed (100%) at magnesium chloride dose of 120 mg/l. The most attractive feature of this process is that magnesium can be recovered from the sludge and recycled. This recoverability may significantly reduce the chemical costs and sludge disposal problems associated with chemical coagulation processes [27]. 3.3. Sludge production Variation in the characteristics of the sludge produced using 200 mg/l alum aided with 1 mg/l cytec; lime (600 mg/l) and MgCl2/ CaO are presented in Table 3. Available data indicates that the use of alum and cytec produced only 0.36 kg/m3 sludge. Corresponding values when lime or magnesium chloride aided with lime were 1.84 and 1.71 kg/m3, respectively. However, Georgiou et al., [26] found that the use of lime for textile wastewater treatment produced lower concentration of solids (0.5–0.6 kg/m3) of wastewater. A relatively high volume of sludge generated from chemical treatment of dyes wastewater has mostly hindered the adoption of its use as an alternative wastewater treatment strategy. However, the results obtained in Table 3. reveal that the sludge produced is compact and possesses advantageous qualities over biological sludge. Improved sludge settleability, compatibility, good dewatering characteristics, recirculation, recycling potentiality and sludge stability comprise the major advantages of chemical inorganic coagulants-based sludge. Such advantages could partly off-set the burden of increased sludge volumes [29]. However, further research and experimentation are highly recommended for more comprehensive and critical quantification and characterization of sludge in chemical wastewater treatment systems.

Fig. 5. Determination of optimum pH value of magnesium chloride at a fixed dose of 120 mg/l.

[31] reported only 36% color removal efficiency in an activated sludge (AS) system. To decrease the toxicity of dyestuff wastewater, chemical coagulation as a pretreatment step is necessary [26,32]. In this investigation, chemically pre-treated dyes wastewater using 200 mg/l alum in combination with 1.0 mg/l cationic polymer (cytec) was used as a feed for the sequential batch reactor (SBR). Residual CODtotal, BOD5 total and TSS in the treated effluent of SBR were 78 ± 7.7; 28 ± 4.2 and 17 ± 4.2 mg/l, corresponding to the removal efficiency of 68.2; 76.3 and 61.4% respectively (Table 4). Furthermore, almost complete removal of CODparticulate and BOD5 particulate has been achieved. This can be mainly due to entrapment of the particulate matter into the sludge flocs followed by biodegradation process. The results obtained indicated that introducing chemical coagulation step prior to sequential batch reactor offers the following advantages; (1) decreasing the loading rate and consequently reducing the required HRT from 8 days for activated sludge process [26] to 5.0 h (2) eliminating the toxicity of dyes (3) minimizing the fluctuations in the incoming textile wastewater, consequently, the feed to the SBR unit will be more stable than when process wastewaters are directly treated (4) reduce the size, complexity and cost of secondary treatment to conform with environmental regulations and guidelines [33](5) improve the performance of secondary treatment processes [31]. Despite many advantages of introducing chemical-coagulation as a pretreatment prior sequential batch reactor (SBR), the use of anaerobic process instead of chemical coagulation process for treatment of dyes wastewater is a good alternative option [34,35]. The anaerobic process does not consume energy but in fact produce useful energy in the form of biogas. Moreover, the anaerobic pretreatment could reduce aeration costs and resulted in sludge volume reduction [36].

Aerobic treatment processes are not effective for treating dyestuff wastewater because many of commercial dyestuffs are toxic to the organisms being used and result in sludge bulking [30]. Pala and Tokat

3.4.1. Batch aerobic biodegradability Results of the residual COD as a function of reaction time in the batch aerobic column are presented in Fig. 7. The results obtained indicated that, the COD removal efficiency was gradually increased from 18.4 to 66.9% with increase in reaction time from 1 to 5 h. Lu et al., [33] found that pre-oxidation process significantly improved the aerobic

Fig. 4. Determination of optimum dose of lime.

Fig. 6. Determination of optimum dose of magnesium chloride at a fixed pH value of 11.0.

3.4. Sequential batch reactor (SBR)

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Table 3 Sludge characteristics. Samples parameters

Unit

Alum (200 mg/l)+cytec(1.0 mg/l)

Lime (600 mg/l)

Magnesium chloride (120 mg/l) + Lime (500 mg/l) at pH = 11

Sludge volume (SV) Sludge weight (105 °C) Sludge volume index (SVI)

ml/l kg/m3 ml/ gTSS

30 0.36 83.5

76 1.84 41.3

46 1.71 29.6

Table 4 Summary of the results of the SBR process treating chemically pre-treated effluent at a HRT of 5.0 h. Samples parameters

Unit

Chemically pretreated effluenta

SBR- effluent at HRT = 5.0 h

%R

pH-value CODtotal CODsoluble CODparticulate BOD5 total BOD5 soluble BOD5 particulate TSS (105 °C) Oil and Grease

– mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l

6.7 245 ± 22 124 ± 13 112 ± 12 118 ± 12 58 ± 11 60 ± 9 44 ± 11 13 ± 6

7.9 78 ± 7.7 58 ± 6.3 20 ± 3.2 28 ± 4.2 18 ± 3.5 10.0 ± 2.3 17 ± 4.2 3.2 ± 1.2

– 68.2 ± 3 61.3 ± 3.2 80 ± 3.6 76.3 ± 4.1 68.9 ± 2.8 83.3 ± 3.2 61.4 ± 3 75.4 ± 2.2

a

200 mg/l alum + 1.0 mg/l cytec at pH value = 6.7.

coagulation, anaerobic and aerobic process. Overall performance of the three step treatment system showed 89% decolorization and 94% COD removal. Comparison of the results obtained from the present study with that published by other investigators indicates a considerable variation depending on the treatment system and operating condition i.e. COD removal efficiencies were found to be 76, 63 and 91% for UASB, CSTR and UASB/CSTR sequential reactor system respectively [38]. Sirianuntapiboon et al., [39] used granular activated carbon-sequential batch reactor (GAC-SBR) system for treatment of textile wastewater containing dyes. The total system achieved 86.2% for COD; 84.2% for BOD5 and 76% for color at a loading rate of 0.36 kgBOD5/m3 d. Ghoreishi and Haghighi [40] investigated a bi-sulfite-catalyzed sodium boro-hydride reduction followed by activated sludge technique for color removal from non-biodegradable textile dying wastewater. Color, BOD5, COD and TSS were removed by a value of 74–88, 76–83 and 92–97%, respectively. In another study, the removal of color and COD from wastewater containing reactive dyes using sequential ozonation and up-flow biological aerated filter process was investigated by Lu et al., [33]. Color and COD removal efficiency were 97% and 90%, respectively.

4. Conclusions From the above study the following can be concluded:

Fig. 7. The effect of reaction time on COD removal in batch aerobic degradability of chemically pretreated effluent (T = 20 °C; pH = 6.7 and VSS = 4.0 g/l).

biodegradability of wastewater containing reactive dyes. The introducing of chemical oxidation process (Fenton reagent) prior biological treatment using SBR process improved the COD and color removal as compared to SBR treating raw wastewater, and SBR process followed by chemical oxidation [37].

3.5. Overall removal efficiency The results presented in Table 5. show that chemical coagulation process using alum aided with cytec followed by SBR for treatment of dyes wastewater at a total HRT of 6.35 h is very effective for removal of color, COD, BOD5, TSS, oil and grease. The total system achieved an overall removal efficiency of 86.9% for COD; 92.6% for BOD5; 93.8% for TSS and 92.2% for oil and grease. These results are comparable to the study reported by Moosvi and Madamwar et al., [36] who investigated an integrated process for the treatment of textile wastewater using

• Lime and magnesium chloride aided with lime proved to be very effective in removing color (97–100%) and part of the COD (40–50%) from the textile wastewater. • The undesired result of using lime or magnesium chloride aided with lime for dyes wastewater was the rise of pH above11.0. Therefore, it has been concluded that, the treatment with lime or lime in the presence of magnesium chloride is not recommended since it led to the production of high amounts of sludge. • Alum and alum aided with cationic polymer removed up to 78.9 and 94% of color respectively. The use of alum aided with cationic polymer offer advantages not only for improvement of color removal but also less sludge was produced. Accordingly, it is recommended to use 200 mg/l of alum in combination with 1.0 mg/l cytec at pH value of 6.6 for color removal from dyes wastewater prior to discharge into the sewer network. If the textile factories are not connected to the sewerage system, further treatment using aerobic biological processes is recommended. • Chemical coagulation process using alum aided with cytec followed by SBR for treatment of dyes wastewater at a total HRT 6.35 h is very effective for removal of color, COD, BOD5, TSS, oil and grease. The total process achieved an overall removal efficiency of 86.9% for COD; 92.6% for BOD5; 93.8% for TSS and 92.2% for oil and grease.

Table 5 Overall performance of the coagulation flocculation (CF) process followed by SBR at a total HRT of 6.35 h. Samples parameters

Unit

Raw wastewater

Chemically pretreated effluent

%R

SBR effluent

%R

Overall removal efficiency

pH -value CODtotal BOD5 total TSS (105 °C) Oil and Grease

– mg O2/l mg O2/l mg/l mg/l

8.8–9.4 595 ± 131 379 ± 110 276 ± 76 41 ± 4.6

6.7 245 ± 22 118 ± 12 44 ± 11 13 ± 6

– 58.8 ± 8.7 68.9 ± 6.4 84 ± 5.8 68.3±

7.9 78 ± 7.7 28 ± 4.2 17 ± 4.2 3.2 ± 1.2

– 68.2 ± 3 76.3 ± 4.1 61.4 ± 3 75.4 ± 2.2

– 86.9 ± 1.3 92.6 ± 1.4 93.8 ± 2.2 92.2 ± 3.1

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