Study on kinetic parameter in real yarn dyed wastewater treatment using electrocoagulation-ozonation process

a

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

C_O3ⁱ

concentration of dissolved ozone in interface, g/cc

di

impeller diameter (cm)

dp

average buble diameter (cm)

D_O3

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)

k_L

individual liquid phase mass transfer coefficient, cm/sec

μ

viscosity of water (g/cm sec)

N

impeller speed (rpm)

P_O3

partial pressure of ozone (KPa)

ρ_L

liquid density (g/cc)

ρ_g

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)

V_G

superficial gas velocity (cm/sec)

X_O3ⁱ

ozone fraction in interface

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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)

At temperature of 298 K, the value for diffusivity of ozone in water is D_O3 = 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 V_G = 25.48 cm/minute.

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.

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 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.

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.