Treatment of real yarn dyed wastewater using hybrid electrocoagulation (EC)-ozonation process has been carried out to solve the non biodegradable wastewater. The work aimed to treat the real yarn dyed wastewater under different ozone concentration and agitation speed and to estimate the kinetic parameter of ozonation. The effect of ozone concentration, agitation speed were studied to give the best performance of color and chemical oxygen demand (COD) removal. The result indicated 38.12% of COD removal and 92.53% of color removal using EC with Al/Al electrodes for 10 minutes. The effluent was pumped to ozonation process for further COD removal. The result showed that 1 mg/L ozon was needed to destroy 4.73 mg/L COD. At ozonation process, the COD removal was 87.4% using 5.8% mol ozone at 400 rpm for 60 minutes. The kinetic parameter was estimated based on the experimental data. The reaction rate constant was 173.5 cm3/(g sec).

NOMENCLATURE

  • a

    average specific interfacial surface for mass transfer, area/volume, cm2/cm3

  • CO3i

    concentration of dissolved ozone in interface, g/cc

  • di

    impeller diameter (cm)

  • dp

    average buble diameter (cm)

  • DO3

    diffusivity of ozone in water (cm2/sec)

  • E

    enhancement factor

  • g

    acceleration of gravity, cm/sec2

  • k

    rate constant of direct reaction of O3 with compound in water, cm3/(g sec)

  • kL

    individual liquid phase mass transfer coefficient, cm/sec

  • μ

    viscosity of water (g/cm sec)

  • N

    impeller speed (rpm)

  • PO3

    partial pressure of ozone (KPa)

  • ρL

    liquid density (g/cc)

  • ρg

    gas density, density of ozone (g/cc)

  • r

    rate of COD degradation, g/(cm3 sec)

  • rate of ozone reaction, g/(cm2 sec)

  • V

    reaction volume (cc)

  • Vt

    terminal settling velocity of single buble (cm/sec)

  • VG

    superficial gas velocity (cm/sec)

  • XO3i

    ozone fraction in interface

INTRODUCTION

Yarn dyed wastewater as effluent from a yarn dyed processing industry contains different dyes and pigments. The wastewater from a yarn dyed manufacture is located in an industrial area of Surabaya city, East Java province, Indonesia. The wastewater has intense color, high chemical oxygen demand (COD), suspended solids, which has been known as non-biodegradable wastewater since it can't be treated biologically. Discharge of the untreated wastewater may hinder photochemical activities in aquatic systems since it reduces light penetration (McMullan et al. 2001). Mostly of the dyes are carcinogen and mutagenic which can cause severe dysfunctional of brain and nerve systems (Kadirelu et al. 2003). Conventional treatments of such wastewater include adsorption was not effective for great amount of the wastewater. Advance oxidation processes including UV/H2O2, TiO2/UV, Fenton's reagent and photo-Fenton have been introduced for wastewater treatment (Selcuk 2005). In addition, electrocoagulation (EC) process is effective for decolorizing dyes and also for removing suspended solids (Riadi et al. 2014). However, the removal of COD by EC will not be enough to make the COD content meet the standard of wastewater based on regulation by East Java Governor number 72/2013. Ozone is known as a very reactive agent in both water and air. Ozone molecule has high reactivity due to its electronic configuration. Hence, a hybrid method of EC-Ozonation was introduced to treat this colored and high strength wastewater. The mechanism of ozone reaction follow a direct path corresponding to the action of molecular ozone and indirect path resulting from the decomposition of ozone radicals (Sarasa et al. 1998). The aim of this work was to experimentally investigate the effect of system parameters in decolorisation and degradation of COD for yarn dyed wastewater by hybrid EC-ozonation. The evaluation of treatment efficiency was made using the parameters of COD and color, and the kinetic parameter from the study was also estimated.

MATERIALS AND METHODS

Electrocoagulation

A flexi glass reactor with 1,500 cm3 volume was used. The real wastewater was taken form yarn industry located in Surabaya, East Java province, Indonesia. Anode and Cathode Al/Al was used with a distance between two electrodes was 2 cm. The electrodes were connected to DC power supply. The size of Alumunium plate was 8 cm × 8 cm, with a thickness of 2 mm. The EC process was run for 10 minutes and used a method as described by Riadi et al. (2014). The effluent from EC process was then used as influent for ozonation process.

Ozonation

A 1.5 L stainless steel reactor equipped with stirrer, sparger, thermocouple and cooling jacket was used for the experiment. The cooling jacket was used to maintain the experiment at 25°C. The equipment used in the experiment can be seen at Figure 1. Ozone was generated from oxygen by Ozone generator. The exit port of ozone generator is connected with deep tube and the ozone gas is delivered to the reactor through tube sparger at the bottom. The reactor outlet is connected to 2% potassium iodide solution trap hence excess ozone will be decomposed. Effluent from EC treatment was fed to a stainless steel reactor. The experiment was carried out at room temperature. Samples were withdrawn periodically up to 60 minutes of experiment, and aerated for 5 minutes to remove residual ozone (Selcuk 2005). The ozone trapped in container was measured using sodium thiosulfate tritation procedure.
Figure 1

schematic of EC-ozonation process.

Figure 1

schematic of EC-ozonation process.

Assay

COD was measured using closed reflux, colorimetric method based on Standard Methods for the Examination of Water and Wastewater (APHA, AWWA, WEF 1998). Total Suspended solid was measured using dried method, color measurement was conducted using UV-Vis spectrophotometer at 498 nm wavelength based on Standard Methods for the Examination of Water and Wastewater (APHA, AWWA, WEF 1998). The biological oxygen demand (BOD) was measured by respirometer using BOD system BD600, Lovibond. Ozone concentration trapped in KI solutions was measured using sodium thiosulfate tritation procedure. Ozone consumed was calculated as: Ozone produced – Ozone in off gas, with an assumption there is no ozone left to the environment.

RESULTS AND DISCUSSIONS

Characterisation of yarn dyed wastewater and after pretreatment with EC

Wastewater used in this study was supplied from a yarn dyed processing industry located in Surabaya, Indonesia. The characteristic of raw wastewater and after pretreated using EC is presented in Table 1.

Table 1

Characteristic of raw wastewater and after EC pretreatment

Parameter Unit Raw wastewater After EC 
COD mg/L 849.7 525.8 
BOD5 mg/L 100 n/a 
TSS mg/mL 6.22 5.81 
Conductivity mS/cm 7.23 6.98 
Color, Optical Density cm−1 1.66 0.124 
pH  10.5 7.7 
Parameter Unit Raw wastewater After EC 
COD mg/L 849.7 525.8 
BOD5 mg/L 100 n/a 
TSS mg/mL 6.22 5.81 
Conductivity mS/cm 7.23 6.98 
Color, Optical Density cm−1 1.66 0.124 
pH  10.5 7.7 

As can be seen in Table 1, the COD and color removal were 38.12% and 92.53% respectively. EC process created stable floc and unstable floc. The stable floc will settle, whereas the floating unstable floc was removed from the wastewater. After coagulation, the COD in wastewater needs to be further reduced as the concentration is still high enough. The pretreated wastewater was then fed to the ozonation process.

Ozonation

The pretreated wastewater with pH of 7.7 was fed to ozonation reactor. Ozone molecule can react via two different mechanisms namely, direct and indirect ozonation, since the structures presents negative and positively charged oxygen atoms which informs the characteristics of an electrophilic, dipolar and nucleophilic agents. In water, ozone can direct react with organic substances through dipolar, electrophilic reactions (Beltran 2003). Indirect reaction of ozonation is due to reactions of free radical species and hydroxyl radical with organic substances present in water. The free radicals come from reaction mechanisms of ozone decomposition in water due to hydroxyl ion as shown as follows: 
formula
1
 
formula
2

Reactions (1) and (2) are important because these are the initiating steps of the radical mechanism leading to the formation of hydroxyl radicals when ozone decomposes.

Effect of ozone concentration

We investigated the effect of ozone concentration (% mol/mol) on COD removal efficiency in a series of experiments by enhancing the oxygen flow rate (L/minute). Higher concentration of ozone resulted in higher COD removal.

Higher ozone concentration resulted in an increase of the COD removal as can be seen in Figure 2. High ozone output mass flow rate will increase the mass transfer rate of ozone from gas phase to liquid phase (Song et al. 2008).
Figure 2

COD profile at various ozone concentration, ▴ = 4.8%; ▪ = 5.3%, ♦ = 5.8%.

Figure 2

COD profile at various ozone concentration, ▴ = 4.8%; ▪ = 5.3%, ♦ = 5.8%.

The percentage of COD removal, amount of ozone supplied and consumed at various concentration of ozone for 60 minutes experiment is presented in Table 2. It is a significant difference in COD removal efficiency from 44.08% to 87.4% while using 4.8% mol ozone and 5.8% ozone respectively.

Table 2

COD removal at various concentration of ozone at 400 rpm

Concentration of Ozone (% mol) Ozone input (ppm) Ozone consumed (ppm) % removal COD 
4.8% ozone 94.3 50.3 44.08 
5.3% ozone 104 86.2 77.95 
5.8% ozone 113.7 97.15 87.45 
Concentration of Ozone (% mol) Ozone input (ppm) Ozone consumed (ppm) % removal COD 
4.8% ozone 94.3 50.3 44.08 
5.3% ozone 104 86.2 77.95 
5.8% ozone 113.7 97.15 87.45 

The initial concentration of COD used in ozonation process (after coagulation) was 525.8 ppm, and the ozone input was 113.7 ppm. Since the COD removal from the wastewater was 87.45%, hence 4.73 mg/L COD was destroyed per 1 mg/L ozone consumed.

Effect of agitation

The effect of agitation during ozonation was also studied. Since it is a heterogenous reaction, the effect of mass transfer from gas phase to liquid phase is very important. Hence, various agitation speeds were used to study the effect in COD removal. Figure 3 shows the agitation has effect on COD degradation, but the effect of agitation was not as strong as the effect of ozone concentration.
Figure 3

COD profile at different speed of agitation with [O3] = 5.8%, ▴ = 400 rpm, ▪ = 300 rpm, ♦ = 200 rpm.

Figure 3

COD profile at different speed of agitation with [O3] = 5.8%, ▴ = 400 rpm, ▪ = 300 rpm, ♦ = 200 rpm.

According to the result (Tables 2 and 3), the reaction is affected significantly by mass transfer phenomena. The influence of higher ozone concentration means the greater driving force which lead to the greater reaction rate. The faster agitation speed lead to the greater removal of COD which implied the greater mass transfer coefficient. Both of these parameters relate to mass transfer phenomena.

Table 3

COD removal at various agitation speed, using 5.8% ozone

Agitation (rpm) % COD Removal 
200 728,649 
300 811,651 
400 874,463 
Agitation (rpm) % COD Removal 
200 728,649 
300 811,651 
400 874,463 

Estimation of kinetic parameter

The ozonation reaction can be illustrated as follows: 
formula
3

Since 1 mg/L ozone used to destroy 4.73 mg/L COD, hence we use z = 4.7

Ozone balance: 
formula
4
By assuming steady state condition, then we have: 
formula
5
Rate of COD degradation: 
formula
6
 
formula
7
 
formula
8
 
formula
9
 
formula
10
 
formula
11
To simplify the calculation, we assume: 
formula
12
By integration Equation (11), the value of K can be determined: 
formula
 
formula
13

In one hour of ozonation, the COD can be reduced from 525 ppm to 66 ppm, the K value = 0.493 (mg/L)0.5/(minute)

The mol fraction of ozone in interface can be expressed as follows: 
formula
14
where H is Henry's law constant for the ozone-water system, H = 5.58 105 KPa (Sotelo et al. 1989).
The partial pressure of ozone was = 5.88 KPa (5.8% of ozone with pressure of 1.1 atm) 
formula
The ozone concentration in interface is: 
formula
The Diffusivity of ozone in water (Johnson & Davis 1996) is: 
formula
15

At temperature of 298 K, the value for diffusivity of ozone in water is DO3 = 1.8975 × 10−5 cm2/sec

Data from the experiment showed the agitation was 400 rpm, volumetric rate of ozone was 2 liter/minute, the reactor diameter is 10 cm and the impeller diameter is 4 cm. Hence, the linear velocity of gas can be calculated as: volumetric rate of ozone/ surface area of reactor which gave the gas velocity VG = 25.48 cm/minute.

From the literature (Treybal 1981), the terminal settling velocity of single bubble is calculated as: 
formula
16

As we assume the gas diameter is small, with the buble diameter is 1 mm, the terminal velocity is: Vt = 0.061 × 103 cm/sec.

Since the value of (Re)0.7 (N di/VG)0.3 is less than 30.000 (Treybal 1981), we can use the empiric equation to get a value of average specific interfacial surface for mass transfer, ‘a’ (cm2/cm3), 
formula
17
  • PG = power impeller with gas, we get the value of PG = 0.45 P (Treybal 1981)

  • P = agitator power in the reactor = 40 watt.

  • VL = volume of liquid = 1,200 mL

As we need to get the value of average specific interfacial surface for mass transfer, it is necessary for applying Equation (17), hence we can get ‘a’ = 1.0795 cm2/cm3.

The value of kinetic constant is estimated from Equation (12): 
formula
which is k = 173.5 cc/(g sec).

CONCLUSION

The process of hybrid EC and ozonation was investigated for Color and COD removal from real yarn dyed wastewater. The EC process removed color by 92.53% removal. The COD removal in EC was not yet enough to reduce the COD level to achieve the standard required by regional government. The wastewater form EC process was then fed to Ozonation process which showed the COD removal up to 87.45%. The optimum condition in ozonation was at 400 rpm and 5.8% mol of ozone concentration. The kinetic reaction was second order with kinetic constant (k) of the reaction was 173.5 cc/(g sec). It is concluded that ozone is very effective in COD reduction after EC process.

ACKNOWLEDGEMENT

The authors are grateful to the University of Surabaya for financial support for the research. We thank to the reaction engineering laboratory staffs for their help in carrying out this study.

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