INTRODUCTION
Introduction
Dyeing wastewater, particularly that containing CI AR 114 dye, poses significant environmental challenges due to its high pollutant concentration, which adversely affects both the ecosystem and human and animal health CI AR 114 is recognized for its carcinogenic properties, with sufficient evidence from studies on experimental animals indicating its potential threat to humans as well Despite various treatment methods available for dyeing wastewater, including adsorption, irradiation, membrane processes, and biological treatments, conventional systems often fail to effectively remove brightly colored, water-soluble reactive, and acidic dyes like CI AR 114 This highlights the urgent need for more efficient treatment solutions to address the environmental and health risks associated with such hazardous dyes.
35, 39, 27, 7, 8, 38, 15, 19] Although these methods presented effectiveness on CI
The treatment of CI AR 114 dye often generates significant secondary solid waste, incurs high operational costs, and requires lengthy reaction times, making it unsuitable for large volumes and high concentrations of polluted wastewater Consequently, further research is essential to explore alternative methods for CI AR 114 dye removal that can achieve higher efficiency and address the limitations of existing approaches.
The electrocoagulation method, established for over a century, has emerged as a promising electrochemical technology in the 21st century Recent studies utilizing aluminum and iron electrodes demonstrate high dye removal efficiencies ranging from 83% to 100% Key advantages of this method include reduced sludge generation and excellent adaptability to varying volumes and pollution loads, making electrocoagulation a viable option for treating wastewater contaminated with CI AR 114 dye.
In addition to electrocoagulation, ozone effectively decomposes aromatic rings found in textile dyes, azo dyes, and other organic pollutants in wastewater This method offers significant benefits, including no increase in wastewater volume and the elimination of secondary solid waste production, such as sludge Consequently, ozonation presents a viable solution for wastewater treatment.
In some of the previous studied showed that the treatment efficiency by ozonation was able to be enhanced by combined with other substances such as H 2 O 2 , UV, TiO 2 [47,
Research indicates that ozone improves the electrocoagulation process when using iron electrodes, yielding positive outcomes Consequently, a system that integrates ozonation with electrocoagulation utilizing aluminum electrodes presents a promising yet unexplored alternative for wastewater treatment This combined approach could harness the benefits of both ozonation and electrocoagulation effectively.
For these reasons, the ozonation, electrocoagulation and the combined system were carried out for the treatment of the dyeing wastewater containing AR 114 dye.
Objectives of this study
The specific objectives of this study were:
This study investigates the effectiveness of ozonation for treating wastewater contaminated with AR 114 dye, utilizing a bubble column reactor in both batch and continuous operation modes The research examines treatment efficiency through co-current and counter-current flow configurations, aiming to optimize the degradation of the dye and enhance wastewater management practices.
This study assesses how pH, gas flow rate, reaction time, temperature, and initial dye concentration influence the efficiency of dye removal via ozonation, as measured by color, dye concentration, and COD parameters The goal is to identify optimal operating conditions for enhancing this process.
+ Evaluate the volumetric mass transfer coefficient, enhancement factor, gas holdup, and reaction rate coefficient that will be useful for ozonation reactor design
+ Assess the dyeing wastewater treatment ability of the electrocoagulation method using aluminum electrodes in the batch and continuous reactors
+ Evaluate the effects of current density, pH value, reaction time, conductivity and initial dye concentration on the treatment efficiency by electrocoagulation and suggest the suitable operating condition
+ Determine the reaction order, reaction rate coefficient of this process
+ Evaluate the energy consumption, volume and mass of generated sludge
+ Assess the treatment efficiency by the ozonation and electrocoagulation combined system through the dye, color, COD removal efficiency
+ Determination of the suitable HRT in each reactor, volume and mass of generated sludge, energy consumption
+ Compare the combined system with the separate ozonation, electrocoagulation treatment based on the treatment performance, treatment time, energy consumption, volume and mass of generated sludge.
Scope of this study
+ This study was carried out with bench scale in experimental room
+ Wastewater used in this study was synthesized from CI AR 114 dye powder and tap water Dye concentration was controlled in range of 40 - 100 mg/L.
Research Methods
To get background and research orientation for this study, some related documents (books, articles…) were collected, selected and studied, including:
+ Characteristics of dyeing wastewater and CI AR 114 dye
+ The methods can be used to treat the dyeing wastewater
+ Mechanism, reaction kinetics of ozonation and electrocoagulation
+ Published articles which studied on CI AR 114 dye treatment and the dyeing wastewater treatment by the ozonation, the electrocoagulation
+ Each experiment was repeatedly carried out 3 times to ensure the reliability of collected data
+ Experiments were performed in both the batch and continuous reactors
+ Dedicated equipments were applied to analyze COD, color, dye concentration in wastewater before and after treatment pH, conductivity, temperature were also observed during experimental processes
COD, color, dye concentration, pH, conductivity were observed, analyzed and collected during experimental processes Collected data was statistically analyzed by statistical function - ANOVA with α = 0.05 in Microsoft Excel
Scientific and practical significance of this study
1.5.1 Scientific significance of this study
This study presents results derived from experimental data that is supported by clear scientific evidence To ensure the scientific integrity of the research, calculations and data analyses are conducted using mathematical statistics.
1.5.2 New features of this study
This study explores the treatment of dyeing wastewater containing AR 114 dye using three innovative methods: electrocoagulation, ozonation, and a combined ozonation-electrocoagulation system with aluminum electrodes This approach represents a novel technique in wastewater treatment, as the integration of ozonation and electrocoagulation has not been previously studied The findings from this research not only provide effective solutions for treating AR 114 dye but also have the potential to be applied to other dyes in dyeing wastewater, paving the way for further advancements in wastewater treatment methodologies.
1.5.3 Practical significance of this study
The results of this study can be applied to design the treatment plant of real wastewater containing AR 114 dye or other dyes that have same properties with AR
Dye 114 presents a viable option for wastewater treatment in developing countries, meeting essential treatment requirements Although high-tech methods may currently be impractical, Dye 114's advantages—including minimal space requirements, low or negligible secondary waste production, and high efficiency—position it as a promising solution for the future, aligning with the evolving trends in wastewater treatment technology and societal needs.
BACKGROUND
Characteristics of the dyeing wastewater and CI Acid Red 114 dye
The textile industry is a significant global sector that generates substantial wastewater rich in both organic and inorganic pollutants The composition and concentration of these contaminants vary based on the types of fabrics manufactured and the dyes employed During dyeing, a portion of the applied dyes remains unfixed to the fabrics and is subsequently washed away, contributing to the pollution.
The percentages of unfixed dyes vary when dyeing different fabrics with various dyes, as illustrated in Table 2.1 [3] High concentrations of these unfixed dyes are commonly present in textile effluents.
Fiber Dye type Unfixed dye (%)
Acid dyes/reactive dyes for wool 7 – 20
Source: adapted from AE Ghaly (2014) [3]
Dyeing wastewater, a significant component of textile wastewater, is distinguished by its elevated levels of salts, total solids (TS), total dissolved solids (TDS), total suspended solids (TSS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), heavy metals, and color The BOD/COD ratio typically hovers around 1:4, suggesting a prevalence of non-biodegradable contaminants For detailed characteristics of textile effluent, refer to Table 2.2.
Table 2.2 Characteristics of typical untreated textile wastewater
Trace elements (mg/l) (Fe, Zn, Cu, As,
Ni, B, F, Mn, V, Hg, PO4 3-, Cn)
Source: adapted from AE Ghaly (2014) [3]
Wastewater discharged from different technological steps in the textile process has different capacity, various pollutant concentrations and kinds of contaminant Characteristics of each processing were described in Table 2.3
Table 2.3 Characteristic of textile wastewater from each processing
Singering, Desizing High BOD, high TS, neutral pH
Scouring High BOD, high TS, high alkalinity, high temperature Bleaching, Mercerizing High BOD, high TS, alkaline wastewater
Heat-setting Low BOD, low solids, alkaline wastewater
Wasted dyes, high BOD, COD, solids, neutral to alkaline wastewater
Source: adapted from Naveed S et al., 2006 [50]
Textile effluents, particularly those from dyeing, printing, and finishing processes, pose significant environmental concerns due to the presence of harmful and toxic dyes These untreated dyes can persist in the environment for extended periods, with some, like hydrolyzed Reactive Blue 19, having a half-life of approximately 46 years The presence of dyes in water not only causes unsightly discoloration but also leads to serious health issues, including skin irritation, ulcers, and nausea Additionally, they obstruct sunlight penetration, hindering photosynthesis, and increase the biochemical oxygen demand in water, which negatively impacts the growth of photoautotrophic organisms.
Azo dyes, comprising 60-70% of all dye groups, are known for their bright and high-intensity colors, but they also pose significant health risks due to their acute and chronic toxicity These dyes can be absorbed through the gastrointestinal tract, skin, and lungs, leading to the formation of toxic amines and disturbing blood formation by creating hemoglobin adducts The reported LD50 values for aromatic azo dyes range from 100 to 2000 mg/kg body weight, indicating their potential for harm Moreover, certain azo dyes are linked to DNA damage, increasing the risk of malignant tumors, particularly when electron-donating substituents are present in ortho- and para- positions The toxicity of these compounds can be reduced through the protonation of aminic groups CI AR 114, a synthetic bright red bisazo dye, is produced by converting 3,3’-dimethylbenzidine into a tetraazonium salt, which is then coupled with 2-naphthol-6,8-disulfonic acid and phenol.
CI AR 114 dye has chemical formula as C 37 H 28 N 4 O 10 S 3 2Na, structure as below:
CI AR 114 dye is utilized for dyeing wool, silk, jute, and leather, with direct printing also possible on wool and silk However, this dye has been evaluated for its carcinogenic potential, showing evidence of carcinogenic activity in both humans and animals While human evidence remains inadequate, experimental studies have demonstrated sufficient carcinogenicity in animals According to the US National Toxicology Program (1991), CI AR 114 dye tested in rats via drinking water led to increased occurrences of benign and malignant tumors in various organs, including the skin, Zymbal gland, liver, clitoral gland, lungs, oral cavity, and intestines, particularly in female rats.
2.2 Dyeing wastewater treatment methods overview
Adsorption involves the use of highly porous solids to capture soluble organic compounds, particularly dyes found in dyeing wastewater, with the adsorption capacity varying based on the type of dye Once an adsorbent reaches its critical adsorption time, it becomes saturated and can no longer absorb additional contaminants, necessitating its replacement These saturated adsorbents become secondary waste solids, which can either be regenerated or incinerated The efficiency of the adsorption process is influenced by several factors, including dye type, concentration in the influent, surface area of the adsorbent, particle size, temperature, pH, and contact time Activated carbon is the most commonly used adsorbent, derived from various materials such as coconut shells, rice husks, peat, wood, and nutshells, effectively removing large, negatively charged, or polar dye molecules due to its specific surface area of approximately 500.
Activated carbon exhibits a surface area of 1500 m²/g, a pore volume ranging from 0.3 to 1 cm³/g, and a bulk density between 300 and 550 g/L, making it effective for color removal Research by Anjaneyulu et al (2005) indicates that granular activated carbon (GAC) columns and powdered activated carbon (PAC) in batch treatments demonstrate high removal rates for cationic mordant and acid dyes However, the effectiveness is slightly lower for dispersed, direct, vat, pigment, and reactive dyes, as noted by Cooper (1995) and Nigam et al (2000).
The study on AR 114 dye removal using KTC (activated carbon derived from Kattamanakku tree leaves) achieved an impressive removal rate of 83.2% at a dye concentration of 90 mg/L, utilizing 250 mg of activated carbon over a contact time of 5 hours Despite its high efficiency, the adsorption process with activated carbon presents several drawbacks, including the need for pretreatment, high operational costs, dependency on the type of activated carbon and wastewater characteristics, and significant expenses associated with treating saturated activated carbon.
Irradiation treatment, utilizing gamma rays or electron beams, effectively removes organic compounds and disinfects harmful microorganisms in wastewater The process is particularly efficient for eliminating organic dyes when a high concentration of dissolved oxygen is present The use of catalysts like TiO2 significantly enhances irradiation efficiency Research by Manouchehr Nikazar et al (2008) demonstrated that with a mixed photocatalyst (10 wt% TiO2/CP) and UV irradiation, 92.9% of CI AR 114 dye was degraded in just 2.5 hours, compared to only 6.21% degradation without TiO2.
Membrane processes are innovative technologies used for wastewater treatment aimed at water reuse These pressure-driven methods effectively clarify, concentrate, and separate dyes from effluent, as highlighted by Xu & Lebran (1991) They are capable of removing color, BOD, salts, and microorganisms from dyeing wastewater Key membrane processes include Micro-Filtration (MF), Ultra-Filtration (UF), Nano-Filtration (NF), and Reverse Osmosis (RO), with the selection of the membrane type based on the desired quality of the treated wastewater.
The chemical oxidation process effectively transforms complex pollutants into less harmful or more biodegradable substances, particularly addressing the challenge of refractory dye pollutants Powerful oxidizing agents like chlorines, ozone, Fenton reagents, and UV treatments are essential for dye removal Chemical oxidation is a preferable option over biological treatment for small volumes of wastewater containing non-biodegradable contaminants This method significantly reduces effluent COD, color, and toxicity, and is capable of removing both soluble and insoluble dyes.
Coagulation-flocculation is a wastewater treatment process that involves adding coagulants such as ferrous sulfate, ferric sulfate, and polyaluminum chloride to facilitate the aggregation of colored colloids and small suspended particles into larger flocs that can settle through gravity To enhance floc formation, anionic and non-ionic polymers are also utilized However, this method presents challenges including difficult process control, the potential for varying precipitation rates, the presence of small flocs in the effluent, and the costs associated with sludge treatment.
Electrocoagulation (EC) is an advanced electrochemical treatment that effectively removes dyes and color from wastewater through the formation of metal hydroxide flocs at the anodes The EC process involves electrolytic reactions, flocculation, and the adsorption of soluble or colloidal pollutants onto the flocs, followed by sedimentation or flotation Previous studies have demonstrated the high efficiency of EC in treating dyeing wastewater, achieving over 98% color removal for textile effluents containing Orange II and AR 14 dyes, and a 74.2% dye removal rate overall.
The treatment of Yellow 86 dye-containing wastewater using an iron anode has proven effective, operating for just 30 minutes (Zaharia et al., 2005) This method offers significant advantages, including reduced solid waste generation and the ability to adapt to varying volumes and pollution loads.
Electrocoagulation treatment
Electrocoagulation (EC) has been recognized for over a century as an effective electrochemical technology for water and wastewater treatment This method effectively removes heavy metals, suspended solids, organic compounds, chemical oxygen demand (COD), biological oxygen demand (BOD), and color During the EC process, a direct current voltage applied to iron or aluminum electrodes generates coagulants from the anode, leading to the formation of aluminum or iron hydroxides and polyhydroxides These compounds flocculate and coagulate suspended solids while adsorbing organic compounds and color Additionally, gas generated at the cathode aids in floating the flocs to the water's surface, enhancing the treatment process.
EC requires simple equipments and low initial investment cost
Easily operation with low operating cost
EC requires less maintenance because of no moving equipment
The EC process operates without the addition of chemicals, effectively eliminating the need to neutralize excess chemicals and addressing secondary pollution caused by these substances This characteristic stands out as a significant advantage of EC technology.
Sludge formed by EC is low and tends to readily settable and easy to dewater It also can be separated faster by filtration
EC produces less TDS in the effluent than chemical treatment lead to lower water recovery cost when this water is reused
EC process has ability to remove the smallest colloidal particles
The gas bubbles produced in electrolysis can float the contaminants in medium to the surface of water It is easy to concentrate, collect and remove them
EC technique requires less energy A solar panel can provide sufficiently energy to carry out the process
In some EC systems, oxides film can be formed on the cathode and affect the efficiency of electrocoagulation cell
Requiring high conductivity of water and wastewater
Gelatinous hydroxide may solubilize in some cases
The anode needs to be regularly replaced
In some cases, the use of electricity may be expensive
The electrocoagulation process involves generating metal cations, specifically iron or aluminum, from metal anodes when an electric current is applied to the metal electrodes.
Simultaneously, water is reduced to hydrogen gas and the hydroxyl ions
At the cathode, metal cations react with hydroxyl ions (OH-) to create highly charged coagulants, which adsorb pollutants and form insoluble flocs These cations neutralize the negative charges of particles by generating poly hydroxide complexes with strong adsorption properties The production of hydrogen gas promotes mixing, flocculation, and the flotation of some flocs, resulting in a foam layer at the liquid surface in the EC reactor The remaining flocs are separated from the liquid through sedimentation.
The interaction of substances in aqueous medium may be occurred follow to various ways as [22]:
Movement of ions to an oppositely charge electrode and accumulation of cations with negative charged particles
Some pollutants can be precipitated by combining with the hydroxyl ion (OH - )
Hydroxides, formed from metallic cations and the hydroxyl ion, have high adsorption properties The pollutant is adsorbed on their surface by bridge coagulation bonding
Sweep coagulation is occurred by formation of larger hydroxide structure and sweep through the water
Oxidation of pollutants to less toxic species
Removal by electro-flotation and adhesion to bulk
The amount of dissolved metal generated during the electrochemical (EC) process at the anode is directly related to the amount of electricity flowing through the electrode This relationship can be expressed through the current density and the quantity of dissolved metal, as outlined in equation (2.1) [49].
Where: W: the amount of dissolution of electrode (g/cm 2 ) i: current density (A/cm 2 ) t: reaction time (s)
M: Molecular weight of metal n: number of electrons in oxidation/reduction reaction F: Faraday’s constant, 96.500 C/mol
The coefficient of dissolved metal can be calculated follow equation (2.2) [37]: m m T η = (2.2)
Where: mT: actual quantity of dissolved metal (kg) m: theoretical quantity of dissolved metal (kg) The electrode working time can be calculated according to equation (2.3) [37]: m b
Sa: total anode surface area (m 2 ) b: electrode thickness (m) ρ: specific weight of electrode material, (kg/m 3 ) η * : electrode usage coefficient, 0.8
Aluminum and iron are general material used to produce electrodes With iron electrodes, there are two mechanisms that are presented as following [49]:
At Anode: 2Fe(s) ặ 2Fe 2+ (aq) + 2e- (2.8)
Fe 2+ + 2OH - (aq) ặ 2Fe(OH)2(s) (2.9)
Overall: 2Fe(s) + 2H2O(l) ặ 2Fe(OH)2(s) + H2(g) (2.11) With aluminum electrodes, electrolysis mechanism is following:
At Anode: Al(s) ặ Al 3+ (aq) +3e- (2.12)
When Al 3+ (aq) ions are introduced to water, they undergo immediate hydrolysis, resulting in the formation of various hydroxides and polyhydroxides This process generates a range of monomeric and polymeric aluminum compounds, including Al(OH)2+, Al(OH)2+, Al2(OH)24+, Al(OH)4-, Al6(OH)153+, Al7(OH)174+, and Al8(OH)204+.
2.3.3 Previous studies of the electrocoagulation in the generally dyeing wastewater and the namely CI AR 114 dye containing wastewater treatment
Recent studies on electrocoagulation using aluminum and iron electrodes have demonstrated high dye removal efficiencies, ranging from 83% to 100% Notably, Yuksel et al (2013) employed iron and stainless steel electrodes to effectively treat Reactive Orange 84 at an initial concentration of 300 mg/L, showcasing the potential of this method for dye removal.
The study demonstrated that with a conductivity of 3000 µS/cm and current densities of 130 A/m² and 110 A/m², the dye removal efficiency achieved was 89% and 99.8% for iron and stainless steel electrodes, respectively, after an operating time of 40 minutes (Khandegar et al., 2013).
Electrochemical treatments of small-scale dyeing unit effluent containing Reactive Yellow 86, Indanthrene Blue RS, Basic GR 4, and Reactive Yellow 145 dyes were conducted using aluminum electrodes, achieving over 97% color removal efficiency after 120 minutes at a current density of 0.0625 A/cm² Khandegar et al (2013) also reported high removal efficiencies for AR 131, Reactive Black B, Orange 3R, and Yellow GR dyes, reaching 98%, 93.5%, 92%, and 91.1% respectively, under similar conditions Additional studies by Korbahti et al (2011), Arslan Alaton et al (2009), and Charoenlarp and Choyphan (2009) confirmed that electrocoagulation with aluminum or iron electrodes achieved dye removal efficiencies of 95% or higher, demonstrating the effectiveness of electrochemical methods in treating dye-laden effluents.
In studies conducted by Aleboyeh et al (2008), Daneshvar et al (2007), Kashefialasl et al (2006), Parsa et al (2011), and Yang et al (2005), various dyes such as Acid Yellow 23, Acid Yellow 36, Acid Brown 14, and AR 226 were analyzed The research highlighted that the decolorization efficiency of AR 14 exceeded 91% when utilizing the electrocoagulation process.
The electrocoagulation process using iron electrodes was investigated for the removal of dyes, specifically Acid Yellow 23 and Acid Yellow 36, from solutions with an initial concentration of 50 mg/L The experiments were conducted at varying pH levels and current densities, with results showing that at pH 6 and a current density of approximately 112.5 A/m², nearly 98% of Acid Yellow 23 was removed after 5 minutes of electrolysis Additionally, under optimal conditions of pH 8, a reaction time of 6 minutes, and a current density of 102 A/m², 85% of Acid Yellow 36 dye was eliminated.
Parsa and colleagues investigated the effectiveness of electrocoagulation using an aluminum anode and stainless steel cathode for treating Acid Brown 14 dye Their study demonstrated that 91% of the dye was removed in bench-scale experiments, while 80% removal was achieved in pilot-scale tests The electrolysis duration was 18 minutes for the bench scale and 200 minutes for the pilot scale, conducted at a pH of 6.4 and an applied voltage of 2V with an initial dye concentration of 0.5 g/L.
Electrocoagulation has demonstrated high efficiency in treating various dye types; however, its effectiveness on wastewater containing CI AR 114 dye remains unexplored Based on previous research findings, electrocoagulation could be a promising method for treating wastewater contaminated with CI AR 114 dye.
Ozonation process
Ozone has been utilized in drinking water treatment for over 120 years, serving various purposes such as disinfection, controlling taste and odor, removing color, oxidizing iron and manganese, managing turbidity, and inhibiting algal growth Additionally, it effectively controls disinfection by-products, stabilizes biological components, and manages organic compounds and chlorination by-products.
Since its initial use in the 1950s for disinfection and odor control, ozone has evolved in application In the 1960s, it was specifically utilized in France and Germany to oxidize iron and manganese, while Scottish and Irish plants began using it for color removal due to the high color and low turbidity of their highland water By the late 1970s, France leveraged ozone's capability to manage algal growth during blooms Recently, ozone applications have expanded to include disinfection by-product control and biological stabilization, with proposals for its effectiveness in managing certain organic compounds and chlorination by-products, drawing parallels to chlorine's electrophilic reactions.
Ozonation effectively decomposes aromatic rings in textile dyes, azo dyes, and other organic pollutants in wastewater without increasing its volume or producing secondary solid waste However, a notable drawback is its short half-life, typically around 20 minutes, which can vary based on pH, temperature, and salt presence The mechanism of ozonation is complex and will be detailed further.
Previous studies by Hoigné and Bader (1977a, 1977b, 1978a) indicate that ozone interacts with different compounds in aqueous solutions through two primary mechanisms: a direct reaction with ozone molecules and an indirect reaction involving radical species generated from ozone decomposition in water at elevated pH levels.
Ozone has characteristics as a dipole, electrophilic and nucleophilic agents It is reactions in organic solvent following three mechanisms:
Cyclo addition (Criegee mechanism) [42] With dipole property, the ozone molecule may add 1,3 dipolar cyclo on unsaturated bonds to form primary ozonide as following reaction:
In a protonic solvent like water, the primary ozoide breaks down into a carbonyl compound, either an aldehyde or a ketone, and a zwitterion This zwitterion rapidly transitions to a hydroxy-hydroperoxide stage, which subsequently decomposes into another carbonyl compound and hydrogen peroxide.
Electrophilic reactions primarily occur at molecular sites with high electronic density, particularly in certain aromatic compounds Aromatics with electron donor groups like -OH and -NH2 exhibit increased reactivity with ozone at the ortho and para positions due to their elevated electronic density Conversely, aromatics containing electron-withdrawing groups such as -COOH and -NO2 show reduced reactivity with ozone, with initial attacks occurring mainly at the less deactivated meta position Consequently, aromatic compounds with electron donor groups, such as phenol and aniline, react rapidly with ozone.
The initial reaction of ozone with organic molecules results in the creation of ortho- and para-hydroxylated by-products, which are highly reactive and prone to further ozonation This process subsequently generates quinoid compounds and, through the disruption of the aromatic cycle, leads to the formation of aliphatic products containing carbonyl and carboxyl functional groups.
Nucleophilic reaction [42]: The nucleophilic reaction is found locally on molecular sites showing an electronic deficit and, more frequently, on carbons carrying electron- withdrawing groups
So, with direct mechanisms, the molecular ozone reactions are extremely selective and limited to unsaturated aromatic and aliphatic compounds as well as to specific functional groups
CI AR 114 features two unsaturated bonds (-N=N-) and six aromatic rings, making it susceptible to oxidation by ozone through both Criegee and electrophilic mechanisms In the context of wastewater treatment, ozonation may primarily rely on the direct pathway for effectively degrading CI AR 114 dye.
Indirect reactions with significant oxidants involve radical species, which are generated during the decomposition of dissolved ozone This process is influenced by factors such as pH levels, UV light exposure, ozone concentration, and the presence of radical scavengers.
Hoigné, Staehelin, and Bader mechanism: the ozone decomposition occurs in a chain process including initiation step 1, propagation steps 2 to 6, and break in chain reaction steps 7 and 8
As a result, all species can consume hydroxyl radicals without regenerating the superoxide radical ion will produce a stabilizing effect on the ozone molecule in water
Initiators, promotors and inhibitors of free – radical reactions [42]:
Initiators are compounds that can generate superoxide (O2-) from ozone molecules, including various inorganic compounds like hydroxide (OH-), hydroperoxide (HO2-), and certain cations, as well as organic compounds such as glyoxylic acid, formic acid, and humic substances Additionally, ultraviolet radiation at a wavelength of 253.7 nm can also trigger the free radical process.
Promotors are compounds that facilitate the regeneration of superoxide (O2-) from hydroxyl radicals The reaction rate of O2- with ozone (O3) significantly exceeds that of other aqueous solutes in water, allowing O2- to effectively promote ozone decomposition Common organic promotors include aryl groups, formic acid, glyoxylic acid, primary alcohols, and humic acids.
Inhibitors: are compounds that can consume OH radicals without regenerating the superoxide anion O 2 - Some more common inhibitors include bicarbonate, carbonate ions, alkyl groups, tertiary alcohols and humic substances
Discussion with CI AR 114 removal by indirect way : at high pH values (pH>8), Acid
The ozonation treatment of Red 114 dye can occur through both direct and indirect mechanisms However, the effectiveness of color removal in an alkaline medium remains uncertain and requires further testing.
2.4.2.1 Solubility of ozone in water
The solubility of ozone in water is crucial for effective water and wastewater treatment, as the concentration of dissolved ozone (CO3) significantly influences the rate and extent of pollutant oxidation This relationship is quantified by a specific equation that governs the solubility dynamics.
Ozone has limited solubility in water at low pressure Assuming ozone behaves as an ideal gas and disregarding gas-phase transfer resistance, its solubility in water can be described by Henry's law.
He values (atm mole fraction -1 ) can determined from a function of temperature and pH of water:
In ozonation process, there are two reactions:
Reaction of ozone with inorganic or organic compounds presented in solution products zM
Decomposition of ozone: products OH
The order rate law for ozonation reactions varies under different conditions Research by Gurol and Singer (1982), Yurteri and Gurol (1988), and Tomiyasu et al (1985) indicates that the first-order rate law is not universally applicable in the ozonation of natural water Specifically, at pH levels between 8 and 11, a combined first and second-order rate law more accurately represents the observed results (Tomiyasu et al 1985).
However, in special cases, namely, scavenger present in aqueous solution (e.g Na 2 CO 3 , t-butanol), the second – order is not observed The rate law becomes nearly first order
The decomposition rate, measured in the presence of excess radical scavengers, which prevent secondary reactions, is expresses by pseudo first-order kinetic equation [42]:
Previous studies on CI AR 114 dye treatment overview
The previous studies related to CI AR 114 dye removal are concentrated on the adsorption method, biological method and photo-degradation treatment They were summarized as follows:
A study conducted by N Rajamohan et al in 2013 investigated the kinetic modeling and isotherm studies on the batch removal of Acid Red 114 dye using activated plant biomass, specifically Acid-Activated Eichornia Crassipes The research identified optimal conditions for dye adsorption, including a pH of 1.5, an adsorbent dosage of 1.25 g/L, and an equilibrium time of 3 hours The findings revealed a maximum adsorption capacity of 112.34 mg/g of adsorbent and an activation energy for the sorption process of 9.722 kJ/mol.
N Thinakaran et al., 2008, studied on the removal of CI AR 114 dye from aqueous solutions using activated carbons prepared from seed shells - agricultural waste such as gingelly (sesame), Cotton and Pongam seed shells The optimum conditions for AR
At pH 3, with an adsorbent dosage of 3 g/L and an equilibrium time of 4 hours, the adsorption capacities of activated carbons derived from pongam seed, cotton, and gingelly seed shells were found to be 204 mg/g, 153 mg/g, and 108 mg/g, respectively The adsorption process of AR 114 dye adhered to both the Langmuir and Freundlich isotherm models Additionally, the data on adsorption kinetics aligned well with the pseudo-second order rate expression These activated carbons present a cost-effective alternative for the removal of dyes from wastewater.
Another study, the removal of CI AR 114 dye by the adsorption treatment, is
A study conducted by G Revathi et al in 2010 investigated the adsorptive removal of AR 114 dye using activated carbon derived from Kattamanakku tree leaves (KTC) The findings indicated that KTC is a cost-effective adsorbent for AR 114 removal, exhibiting a monolayer adsorption capacity of 450.02 mg/g The adsorption process adhered to the Langmuir isotherm model, and the kinetics of adsorption were consistent with both pseudo-first order and pseudo-second order models.
The adsorption of AR 114 dye by microbial biomass was carried out in the research
In a 1994 study by Dr Joseph A and Laszlo, the removal of acid dyes from textile wastewater was explored using biomass for effective decolorization The researchers utilized fungal biomass rich in chitin and chitosan to effectively bind the AR 114 dye, achieving a binding capacity of 0.11 mol/kg.
Manouchehr Nikazar et al., 2006, studied about “Photocatalytic degradation of azo dye
The study investigated the photodegradation of AR 114 dye in water using TiO2 supported on clinoptilolite (CP) as a catalyst Results indicated that the highest efficiency for degrading 20 ppm AR 114 was achieved with a photocatalyst composition of 10% TiO2 and 90% CP It was found that increasing the initial concentration of AR 114 dye negatively impacted the conversion efficiency Additionally, pH played a significant role in the process, with an optimal pH of around 4 identified The degradation kinetics of AR 114 dye followed a pseudo-first-order reaction, with a rate constant (K) of 0.0127 min^-1.
In the study by Jong – Min Lee et al (2002), the photochemical removal of AR 114 dye was achieved using ferric ions, TiO2 particles, and H2O2 under UV radiation, with the highest removal efficiency observed at pH 2.5 The removal rate was influenced by the concentrations of ferric ions, TiO2, and H2O2, as well as the air flow rate The average removal rate reached 0.307 mg/L/min at an air flow rate of 10 L/min, with 130 mg Fe3+/L, 100 mg H2O2/L, an initial dye concentration of 100 mg/L, pH 3.5, and a temperature of 30°C Additionally, the study indicated that the relationship between the removal rate and the concentrations of the added chemicals followed second-order equations.
In their 1983 study, Brown D and Laboureur P investigated the decolourization of AR 114 dye through anaerobic treatment, achieving an average decolorization rate of 62% over 42 days at an initial concentration of 100 mg/L Further research in 1987 by Brown D and Hamburger B focused on the aerobic degradation of AR 114 dye, which was largely degraded within 7 days, resulting in the identification of metabolites such as 4-4’-diamino-3,3’-dimethylbiphenyl and 4-methylbenzenesulphonic acid-(4’-aminophenyl) ester.
In conclusion, the presence of CI AR 114 dye in wastewater poses a significant environmental threat, necessitating effective treatment before discharge This dye not only contributes to high pollutant concentrations in receiving waters but also endangers human and animal health due to its toxicity and carcinogenic properties Traditional treatment methods such as adsorption, biological treatment, and photochemical oxidation have notable drawbacks, including prolonged processing times, substantial secondary waste generation, operational challenges, and high costs However, ozonation and electrocoagulation present promising alternatives, offering high efficiency and advantages that align with the treatment requirements for wastewater containing CI AR 114 dye.
EXPERIMENTS
Characteristics of the synthetic wastewater
Synthetic wastewater was created using tap water and CI AR 114 dyestuff powder from Tokyo Chemical Industry Co., Ltd To regulate the initial electrical conductivity, sodium chloride (NaCl) was added in varying amounts based on the desired conductivity level The initial pH of the wastewater was adjusted using sodium hydroxide (NaOH) and sulfuric acid (H2SO4) The characteristics of the synthetic wastewater are detailed in Table 3.1.
Table 3.1 Characteristics of the synthetic wastewater
Parameters Units Synthetic wastewater pH 4 ∼ 10
Equipments and instruments
A schematic diagram of the ozonation used in this study was shown in Fig 3.1 below This system includes main instruments and equipments as:
The polyacrylic bubble column reactor has a total volume of 11.78 liters, featuring a diameter of 100 mm and a height of 1500 mm, with an effective working volume of 10 liters It is equipped with six sampling ports, which are evenly distributed throughout the reactor.
200 mm A ceramic diffuser (ứ60 mm) was installed at the bottom of reactor
The PC-57 ozone generator from Ozonetech Co., Ltd in South Korea supplies gaseous ozone to a bubble column reactor, with the flow rate adjustable via a flowmeter on the device.
Synthetic wastewater is created in a 200-liter mixing tank and then transferred to a 100-liter storage tank To ensure uniformity and prevent sedimentation of suspended solids, both tanks are equipped with agitators.
Synthetic wastewater can be delivered to the bubble column reactor via two methods: in a countercurrent flow, it is pumped from the top to the bottom of the tank, while in a cocurrent flow, it is introduced from the bottom to the top.
Ozone analyzers: Aeroqual series 200 (New Zealand) and Portable Dissolved Ozone Meter (OZ – 21P, DKK – TOA Corporation, Japan) were used to analyze gaseous and soluble ozone concentrations
The electrocoagulation was performed in the batch and continuous reactors, set up as Fig 3.2 and Fig 3.3 below, respectively a The electrocoagulation treatment in the batch reactor:
The main instruments and equipments in the eletrocoagulation treatment operated with the batch operation mode (BOM) were:
A 200 liter - mixing tank was used to store the synthetic wastewater before distributing to the electrocoagulation reactor
A reaction tank made of polyacrylic has total volume of 3.6 liters, the working volume of 2.5 liters Dimensions of the length, width, height were 15 cm, 12 cm and 20 cm, respectively
This study utilized cylindrical aluminum electrodes in a polyacrylic rectangular reaction tank, with external and internal dimensions measuring 50 mm in diameter and 100 mm in height, and 95 mm in diameter and 100 mm in height, respectively The electrodes were positioned 2.25 cm apart, resulting in a total geometric surface area of 455.3 cm².
The SC15-30A laboratory DC power supply from Sunchang Electronic Co., Ltd in South Korea was utilized to provide direct current and regulate voltage during the electrochemical (EC) treatment process, featuring a current range of 0 to 30 A.
The magnetic stirrer (MS300HS, Korea) operated at a speed of 150 rpm to enhance the interaction between aluminum ions and pollutants in the wastewater reactor This process is part of the electrocoagulation treatment conducted within a continuous reactor system.
The main instruments and equipments in the electrocoagulation treatment operated with the continuous operating mode (COM) were:
A 200 liter - mixing tank was used to store the synthetic wastewater before distributing to the electrocoagulation reactor
The study utilized an electrocoagulation reactor featuring a pair of cylindrical aluminum electrodes connected to a poly-acrylic distribution part The dimensions of the aluminum electrodes are detailed in Table 3.2, with an electrode gap of 2.25 cm The total geometric surface area of the electrodes measures 2425.65 cm², while the total working surface area is 2276.5 cm².
Table 3.2 Dimensions of the cylindrical aluminum electrodes
External Cylinder Internal Cylinder Hydraulic height H (mm)
Volume (V) of reaction region (liter)
These experiments used the same laboratory DC power supply utilized in the batch reactor
A pump with capacity of 5 L/min and a flow meter were used to control supplied wastewater into the electrocoagulation reactor with fixed flow rate.
The mixing tank, with a total volume of 3.6 liters, is positioned after the EC reactor In this tank, treated water is agitated using a magnetic stirrer at a speed of 50 rpm to improve the flocculation and flotation processes.
Fig 3.1 Schematic diagram of the ozonation system
Fig 3.2 Schematic diagram of the electrocoagulation treatment in the batch reactor
Fig 3.3 Schematic diagram of the electrocoagulation in the continuous reactor
Figure 3.4 illustrates a schematic diagram of the combined ozonation-electrocoagulation (EC) system This system utilizes equipment and instruments that are largely the same as those employed in continuous ozonation and electrocoagulation treatments.
Fig 3.4 Schematic diagram of the combined system
This study utilized various instruments and equipment, including a pH meter (Denver/UB-10, Germany), a thermocouple (OKAYA Handy Thermo T200, Japan), a conductivity meter (ORION Model 130, Germany), and two spectrophotometers (DR 2800, USA, and Humas Co., Ltd., South Korea) to accurately measure pH value, temperature, conductivity, color, dye concentration, and chemical oxygen demand (COD).
Experimental procedure
All experiments were repeatedly carried out three times to ensure the reliability of the experimental results
The ozonation treatment was carried out with the batch operating mode, the counter current and cocurrent continuous operation modes
3.3.1.1 The ozonation treatment in the batch reactor
In the batch operation mode, the bubble column reactor was filled 10 liter-synthetic wastewater The experimental processes were set up as follows:
The gas holdup was assessed by measuring the variations in wastewater levels before and after gas bubbling at flow rates between 0.3 and 1.1 L/min The wastewater, which had a concentration of 100 mg/L AR 114 dye and a pH of 7, was monitored using a digital camera to observe the generated bubbles.
To determine the optimal gas flow rate for treating synthetic wastewater with a dye concentration of 100 mg/L, conductivity of 1500 µS/cm, temperature between 14 to 16°C, and a pH of 7, gaseous ozone was supplied from an ozone generator through a gas diffuser at varying flow rates of 0.3, 0.5, 0.7, and 0.9 L/min for a reaction time of 20 minutes The removal efficiency was evaluated by measuring the color, dye concentration, and COD before and after treatment, leading to the identification of the optimal gas flow rate based on the collected data.
The volumetric mass transfer coefficient and enhancement factor were assessed by alternating the reactor's fill between tap water and synthetic wastewater Optimal gas flow rates were utilized to supply gaseous ozone into the reactor, while concentrations of gaseous ozone in both inlet and outlet gas flows, as well as dissolved ozone levels in the water and wastewater, were monitored at five-minute intervals.
The study investigated the impact of varying initial dye concentrations in wastewater, specifically at levels of 100 mg/L, 80 mg/L, 60 mg/L, and 40 mg/L, with a conductivity of 1500 µS/cm, temperature ranging from 16 to 18 °C, and a pH of 7 Ozonation was conducted at a gas flow rate of 0.7 L/min for a duration of 20 minutes Measurements of color, dye concentration, and Chemical Oxygen Demand (COD) were collected before and after the ozonation process to evaluate its effectiveness.
The study investigated the impact of pH on the ozonation process for dye removal, using an initial dye concentration of 100 mg/L, conductivity of approximately 1500 µS/cm, and a temperature range of 16 to 18 °C Ozonation was conducted at pH levels of 4, 7, and 10, with a gas flow rate of 0.7 L/min The removal efficiency of color, dye, and Chemical Oxygen Demand (COD) was assessed to determine the optimal pH for effective ozonation.
The impact of temperature on wastewater treatment was assessed by controlling the temperature at 10°C, 20°C, and 30°C The experiments were conducted under initial conditions of a dye concentration of 100 mg/L, a conductivity of 1500 µS/cm, and a pH value of 7.
20 min reaction time, the removal efficiency of color, dye, COD were determined
+ Determine the removal efficiency of ozonation treatment : the gaseous ozone was supplied from the ozone generator into column containing 10 liters of 100 mg/L
The study investigated the ozonation of AR 114 dye wastewater at an optimal flow rate, utilizing various reaction times (5, 10, 15, 20, 30, 40, and 50 minutes) with an initial dye concentration of 100 mg/L, a conductivity of 1500 µS/cm, and a pH of 7 Key parameters such as color, dye concentration, and chemical oxygen demand (COD) were measured, alongside continuous monitoring of pH and conductivity throughout the process.
3.3.1.2 The ozonation treatment in the continuous operation mode (COM)
Ozonation was conducted using continuous operation with both countercurrent and cocurrent flow configurations These experiments utilized the optimal gas flow rate established in the batch reactor, alongside an initial dye concentration of
The study evaluated the removal efficiency of a solution with a concentration of 100 mg/L, a conductivity of 1500 µS/cm, and a pH of 7 Hydraulic retention times (HRT) were varied at 5, 10, 20, and 40 minutes, corresponding to liquid flow rates of 2 L/min, 1 L/min, 0.5 L/min, and 0.25 L/min Key parameters such as color, dye concentration, and chemical oxygen demand (COD) were measured to assess the effectiveness of the removal process.
The electrocoagulation treatment was operated with the batch and continuous reactors to determine the optimal condition and assess the removal efficiency
3.3.2.1 The electrocoagulation treatment in the batch reactor
All experiments adhered to the schematic diagram in Fig 3.2, with samples collected every 5 minutes throughout the reaction process The generated sludge underwent a two-step separation process involving 90 minutes of settling followed by 5 minutes of centrifugation at 400 rpm Subsequently, treated water was sampled to analyze the remaining COD, color, dye concentration, pH value, and conductivity.
The study evaluated the impact of current density on wastewater treatment containing a dye concentration of 100 mg/L, with a pH of 7 and conductivity of 1500 µS/cm The electrical current was applied within a range of 0.5 to 2 A, resulting in a current density of 1.1 to 4.4 mA/cm², and the electrolysis process lasted for 20 minutes.
The study investigated the impact of varying initial dye concentrations on synthetic wastewater, utilizing concentrations of 100 mg/L, 80 mg/L, 60 mg/L, and 40 mg/L, while maintaining a consistent pH of 7.
1500 àS/cm The electrocoagulation treatment was performed with the current density of 2.2 mA/cm 2 , reaction time of 20 minutes
The impact of conductivity on treatment performance via electrocoagulation (EC) was assessed through experiments using wastewater samples with varying conductivities of 1500 µS/cm, 2000 µS/cm, and 2500 µS/cm The tests were conducted at a current density of 2.2 mA/cm² and a reaction time of 20 minutes, with each sample initially containing a dye concentration of 100 mg/L and a pH of 7.
+ Determine the effect of pH value : in this case, pH value was changed at 4, 6 and 8
The initial dye concentration, conductivity were maintained at 100 mg/L, 1500 àS/cm, respectively These experiments were done with the current density of 2.2 mA/cm 2 , reaction time of 20 min
The removal efficiency of contaminants through electrocoagulation (EC) treatment in a batch reactor was evaluated under optimal conditions This assessment focused on various reaction times, specifically 5, 10, and 15 minutes, to determine the effectiveness of the treatment process.
20 min) by analyzing COD, color, dye concentration before and after treatment The volume and mass of generated sludge were also determined in this treatment
3.3.2.2 The EC treatment in the continuous operation mode
The electrocoagulation treatment was conducted in continuous operation mode, utilizing optimal conditions identified in batch mode The hydraulic retention times (HRT) were adjusted to 1.278 min, 2.56 min, and 5.12 min, corresponding to liquid flow rates of 2 L/min, 1 L/min, and 0.5 L/min, respectively Following treatment in the electrocoagulation reactor, the processed water was directed to a rectangular tank with a volume of 3.6.
Measurements and analysis methods
During the experiments, samples were collected from the apparatus at a fixed reaction time, and color measurements were conducted using a spectrophotometer (Hatch, DR 2800, USA) at a wavelength of 455 nm The Chemical Oxygen Demand (COD) was assessed following the Standard Methods for the Examination of Water and Wastewater, utilizing the spectrophotometer and analyzer kits from Humas (HS – 3300, water analyzer & spectrophotometer, Korea) Additionally, the concentration of dissolved dye was determined using the same spectrophotometer at a maximum wavelength of 500 nm, based on a calibration curve with an R² value of 0.9999, as illustrated in Fig 3.5.
Fig 3.5 Calibration curve of dissolved CI AR 114 dye concentration
The pH values, temperature and conductivity were measured by using the pH meter (Denver/UB-10, Germany), OKAYA Handy Thermo (T200, Japan) and conductivity meter (ORION Model 130, Germany)
In the electrocoagulation (EC) treatment process, the volume of generated sludge (mL/L) was assessed by allowing 1 liter of treated water to settle in a cylinder for 90 minutes The mass of the generated sludge was then calculated using the suspended solid determination method as outlined in the Standard Methods for the Examination of Water and Wastewater.
Calculation of experimental parameters method
The color, dye concentration and COD removal efficiency (Eff, %) was calculated as:
With: C0: initial color (Pt-Co), dye concentration or COD (mg/L)
Ct: color (Pt-Co), dye concentration or COD (mg/L) with t reaction time
3.5.2 The ozonation process a The volumetric mass transfer coefficient
At low pressure, ozone is assumed as ideal gas Partial pressure of ozone can be calculated follow this function: [ ]
The equilibrium ozone concentration in water can be evaluated by Henry’s law
Henry’s constant was calculated by [12]
When assumed that ozone decomposition is neglected, the volumetric mass transfer coefficient can be determined by graphical analysis when plot ⎟⎟
C versus bubbling time t, slope is –kLa [1, 53] b The gas holdup [9]
In this study, the expansion method was employed to assess gas holdup, determined by measuring the difference in wastewater levels before and after gas passage, which reflects the volume of gas present in the wastewater The gas holdup was subsequently calculated based on these measurements.
Where: S: the cross sectional area of the reactor, m 2
VL is the liquid volume, m 3 Δh is the height difference, m c The experimental enhancement factor: The enhancement factor can be determined from the experimental data by this expression [26]
+ n i , n o are the inlet and outlet concentration of ozone in the gas flow (mg/L) + Qg is gas flow rate (L/min)
+V is the liquid volume (L) d The energy consumption in ozonation process:
The energy consumption in the ozonation process was determined as energy required for the ozone generator to remove a specified quantity of pollutants, namely, COD, color, dye
EO3 is energy consumption, Wh/g (COD, dye) or Wh/color unit (Pt-Co)
P is the power, W t is the reaction time (s)
V is the working volume of reactor (L)
3.5.3 The electrocoagulation process a The current density
The current density was calculated through the equation [13]:
S is total surface area of the electrode (m 2 ) b The energy consumption for COD, color, dye removal [45]
The energy consumption in the electrolysis process is energy required to remove a specified quantity of pollutants, namely, COD, color, dye in this study
E is energy consumption, Wh/g (COD, dye) or Wh/color unit (Pt-Co)
E cell is the cell potential V
I is the total current (A) t is the electrolysis time (s)
V is the working volume of reactor (L) c The quantity of dissolved aluminum
The dissolved aluminum quantity in the EC process was calculated by equation (3.10) [44,
W: the amount of dissolution of aluminum electrode (g) i: current (A) t: reaction time (s)
M: Molecular weight of aluminum n: number of electrons in oxidation/reduction reaction, n Al = 3 F: Faraday’s constant, 96485 C/mol
The data was analyzed by using the ANOVA with α = 0.05 in Microsoft excel.
RESULTS AND DISCUSSION
Characteristics of the dyeing wastewater containing AR 114 dye treatment by the ozonation
Figure 4.1 illustrates the relationship between gas flow rate and gas holdup, demonstrating that gas holdup is directly proportional to the gas flow rate, a finding consistent with existing literature [59].
This study investigated gas flow rates of 0.3 L/min, 0.5 L/min, 0.7 L/min, 0.9 L/min, and 1.1 L/min, corresponding to gas velocities of 0.00106 cm/s to 0.0039 cm/s, all within a homogeneous bubbling regime of less than 5 cm/s As illustrated in Fig 4.2, the size of the generated bubbles increased, and the bubble density in the solution rose with increasing gas flow rates from 0.3 L/min to 1.1 L/min.
Fig 4.1 Gas holdup as a function of gas flow rate
The ANOVA analysis results from Table C.1 in Appendices C indicate that the gas flow rate significantly influences gas holdup, with a P-value of 4.55x10^-9, which is well below the 0.05 threshold The flow rates tested were 0.3 L/min, 0.7 L/min, and 1.1 L/min.
Fig 4.2 Effect of gas flow rates on the generated gas bubbles
4.1.2 The effect of gas velocity
The experimental setup involved a dye concentration of 100 mg/L, a pH level of 7, a color measurement of 780 Pt-Co, and a chemical oxygen demand (COD) of 77.7 mg/L The reaction duration was set to 20 minutes, during which samples were collected at 5-minute intervals to analyze the remaining COD, dye concentration, and color.
The Anova analysis results indicate that gas velocity significantly influences the removal efficiency in ozonation treatment, with a p-value much less than 0.05 However, the interaction between gas velocity and reaction time does not significantly impact removal efficiency, as evidenced by a p-value of 0.0598, which is greater than 0.05.
Increased gas flow rates from 0.3 to 0.7 L/min enhanced color, dye concentration, and COD removal efficiencies due to greater gas holdup, which improved the contact surface area between pollutants and ozone bubbles However, at flow rates exceeding 0.7 L/min, removal efficiencies slightly decreased, with values of 64.3%, 73.96%, and 45.6% at 0.7 L/min compared to 63.7%, 72.85%, and 43.5% at 0.9 L/min This reduction occurred despite higher gas holdup at 0.9 L/min, as larger bubble sizes and faster bubble velocities decreased the contact time and surface area with pollutants Therefore, a gas flow rate of 0.7 L/min is optimal for ozonation treatment in this system.
Fig 4.3 The removal efficiency by ozonation with various gas flow rates
4.1.3 The volumetric mass transfer coefficient
The volumetric mass transfer coefficient is a crucial parameter for characterizing and designing ozone bubble reactors, making its estimation essential for effective reactor design and scaling.
Fig 4.4 Plot of experimental graphical analysis for the volumetric mass transfer coefficient
This study determined the volumetric mass transfer coefficient through experimental graphical analysis, using tap water at a temperature of 14.3°C and an average inlet gaseous ozone concentration of 64.17 mg/L, with a gas flow rate of 0.7 L/min and a pH of 7.05 The results, illustrated in Fig 4.4, indicated a volumetric mass transfer coefficient of 0.021 min⁻¹, assuming negligible automatic ozone decomposition.
The irreversible reaction of absorbed gas with a liquid solute can be quantified by the equation N A a = k L aC A * E, where E represents the enhancement factor In this study, the enhancement factor was derived from experimental data using equation 3.6 The experiments utilized synthetic wastewater containing 100 mg/L of AR 114 dye, with a gas flow rate of 0.7.
L/min, pH 7, temperature in a range of 14~15 o C The results were described in Table
4.1 The experimental enhancement factors were calculated and in a range of 10.22 ~
10.93 during 30 min ozonation The E factor decreased when increased the reaction time due to the reduction of dye concentration in the liquid phase
Table 4.1 Data in the enhancement factor determination
Time (min) 0 5 10 15 20 25 30 n i (mg/L) Average 0 64.17 64.17 64.17 64.17 64.17 64.17 no(mg/l)
The pH value of wastewater significantly influences the effectiveness of ozonation in treatment processes In alkaline conditions, characterized by high pH levels, ozone decomposition produces highly reactive hydroxyl radicals, which can enhance the removal rates of contaminants However, this is not consistently the case, as noted by E Kusvuran et al (2010).
The removal efficiency of Basic Yellow 28 dye is significantly influenced by pH changes, with a notable decrease from 90% to 50% as the pH value rises from 3 to 10, according to research by Chedly Tizaoui et al.
(2011) [12], the pH value also had significant effect on the decolorization efficiency in the treatment of RO16 dye by the ozonation process when increased pH from 7 to
11 The removal percentage was 86% at all pH in a range of 2~7 and 95% at pH 11 However, in the study of A.R Terani-Bagha, et al (2010) [2], the rate of decolorization of organic dye from the colored textile wastewater in alkaline condition was almost same as neutral and acidic mediums Therefore, it needs to check the effect of various pH values on the treatment performance by ozonation in
This study demonstrates that initial pH values ranging from 4 to 10 significantly influence the removal efficiencies of COD, color, and dye An interaction effect between pH and reaction time on removal efficiency was confirmed through ANOVA analysis, with all p-values being less than 0.05 Notably, treatment performance declined as pH increased As illustrated in Figure 4.5, removal efficiencies for COD, dye, and color were highest at pH 4, followed by pH 7, and lowest at pH 10 The order of removal rates during ozonation was pH 4 > pH 7 > pH 10.
With 20 min reaction time, the COD, color, dye removal efficiencies were 57.3%, 77.2%, 85% (pH 4); 55.5%, 62.58%, 74.1% (pH 7) and 51.3%, 50.36%, 63% (pH 10), respectively Maybe the reason of these differences is the ozone decomposition occurred faster to transfer to the hydroxyl radicals at high pH that were interested in reaction with the intermediate products rather than the parent molecules [12] The difference of COD reduction between pH 4, pH 7, pH 10 were less than the differences of color and dye reductions And the difference between the COD, color and dye removal efficiencies at pH 10 was less than others It means the intermediate products were oxidized at pH 10 were more than others In addition, the degree of solubility of ozone gas was able to be decreased when increasing the pH value
Fig 4.5 The effect of pH value on the ozonation treatment efficiency
4.1.6 The effect of initial dye concentration
The initial dye concentration significantly influences the ozonation process, impacting the removal rate of dyes This finding aligns with several related studies Additionally, the interaction between initial dye concentration and reaction time also affects the removal rate Notably, all p-values from the ANOVA analysis were below 0.05, indicating statistical significance.
Fig 4.6 The removal efficiencies by ozonation treatment with various initial dye concentrations
Characteristics of the dyeing wastewater containing AR 114 dye treatment by the electrocoagulation
4.2.1 The effect of pH value
The initial pH value of wastewater significantly influences the removal efficiency in electrocoagulation treatment In this study, pH levels were adjusted to 4, 6, and 8 prior to electrolysis, with results indicating that variations within this range did not notably impact treatment performance Anova analysis confirmed no interaction effect between pH and reaction time on removal efficiency This finding is practically significant, as the pH of textile wastewater can vary greatly depending on discharge timing, product type, and dye used Consequently, controlling pH prior to aluminum electrocoagulation treatment is unnecessary, leading to reduced operating costs and simplified processes.
At pH 4, the removal efficiencies of COD, color, and dye concentration were slightly higher during the initial 10 minutes of reaction time After 20 minutes, treatment efficiencies stabilized around 91%, 91%, and 88%, respectively This improvement is likely due to the rapid increase in pH during electrolysis, reaching an optimal range of 5 to 7 for aluminum ion coagulation Additionally, the acidic environment reduced the concentrations of CO3²⁻ and HCO3⁻, which are known to scavenge hydroxyl radicals (·OH) at the anode, thereby enhancing removal efficiency through the oxidation process.
After 10 minutes of electrolysis, the pH of the solution rose above 7.5, aligning closely with the pH values of the other samples Consequently, the removal efficiencies across all samples showed minimal variation after this duration.
Fig 4.12 The color, dye, COD removal efficiencies with various initial pH values
During electrocoagulation, the pH value exhibited varying gradients based on the initial pH and reaction time Notably, the pH change gradient increased as the initial pH decreased For initial pH values of 4 and 6, there was a rapid pH change within the first 10 minutes, followed by a slower change in the subsequent 10 minutes In contrast, at an initial pH of 8, the pH changed slowly for the first 5 minutes and then stabilized until the 20th minute These observations were attributed to the generation of hydroxide ions at the cathode, along with hydrolysis and polymerization reactions that produced aluminum hydroxide and polyhydroxide complexes.
Fig 4.13 Variation of pH during the electrocoagulation process in the BOM
Fig 4.14 Variation of conductivity with various initial pH values in the BOM
The conductivity results depicted in Fig 4.14 indicate a decrease in conductivity over time during electrocoagulation, with the rate of decrease influenced by the initial pH of the solution This reduction is attributed to the rapid formation of aluminum hydroxide and polyhydroxide in the alkaline conditions present.
4.2.2 The effect of current density
In electrocoagulation treatment, current density is a crucial factor that influences the amount of dissolved metal in the solution, directly impacting both treatment efficiency and energy consumption.
In this study, with the same conductivity, increasing current density led to increasing electrical potential (V) In this experimental condition, their relationship is linearly proportional, shown in Fig 4.15 below
Fig 4.15 Potential is a function of current density with conductivity of 1500 àS/cm
The Anova analysis results, as shown in Table C.7 of Appendices C, indicate that variations in current density significantly impacted color performance, COD, and dye treatment efficiency Additionally, the interaction between current density and reaction time also had a substantial effect on treatment performance, with P-values well below the 0.05 threshold.
The removal efficiencies improved significantly with higher current densities, particularly when the current density increased from 0.0011 A/cm² to 0.0022 A/cm² Beyond this point, the efficiencies continued to rise slightly as the current density increased to 0.0033 A/cm² and 0.0044 A/cm².
Fig 4.16 Comparisons of the color, dye, COD removal efficiencies (a) and the energy consumption (b) vs reaction time between various current densities in the BOM
In these experiments, energy consumption was a key factor in determining the appropriate current density alongside removal efficiency The energy consumption was calculated according to equation 3.9, and the results were plotted against reaction time in Fig 4.16 (b) Notably, energy consumption increased significantly, showing approximately three-fold, seven-fold, and twelve-fold increases compared to the two-fold, three-fold, and four-fold increases in current density, respectively.
Base on the removal efficiency and the energy consumption, the current density of 2.2 mA/cm 2 was suitable for the electrocoagulation process in this experimental condition
4.2.3 The effect of initial dye concentration
The study investigated the impact of initial dye concentration on the removal efficiency during electrocoagulation treatment, using concentrations of 100 mg/L, 80 mg/L, 60 mg/L, and 40 mg/L at a current density of 2.2 mA/cm² for 20 minutes ANOVA analysis indicated that variations in initial dye concentration significantly influenced the removal efficiency of COD, dye, and color Additionally, the interaction between initial dye concentration and reaction time also played a crucial role in treatment performance.
The removal efficiency of dye increased significantly with higher initial dye concentrations, as illustrated in Fig 4.17 This improvement is attributed to the increased contact opportunities between the dye and aluminum hydroxides, facilitating better flocculation and enhanced adsorption of dye onto the flocs Additionally, in samples with elevated dye concentrations, the resulting flocs were larger and heavier, making them easier to separate from the treated water.
Fig 4.17 Variation of the removal efficiency with various initial dye concentrations
In electrocoagulation treatment, the conductivity of wastewater is closely related to the electrical current and potential applied At a fixed current of 2.2 mA/cm², there is an inverse relationship between the supplied potential and the conductivity of the wastewater, as illustrated in Figure 4.18.
Fig 4.18 Potential as a function of conductivity in the electrocoagulation treatment
Increasing the conductivity of the solution resulted in a reduced supplied potential (V) while maintaining the same current (I), leading to lower energy consumption Consequently, higher conductivity directly influences operational costs.
The results in Fig 4.18 indicate that electrocoagulation at a conductivity of 2500 µS/cm achieved slightly higher removal efficiencies However, the ANOVA analysis presented in Table C.9 of the Appendices shows that changes in conductivity did not significantly impact color, COD, or dye removal efficiencies, as all P-values were above 0.05.
Higher chloride (Cl-) concentrations in samples with increased conductivity led to enhanced generation of OCl- and HOCl during the electrocoagulation process, improving removal efficiencies While conductivity did not significantly impact treatment performance, it remains an important factor to consider Additionally, treating high conductivity wastewater can reduce energy costs.
Fig 4.19 Comparisons of the removal efficiency between various conductivities
4.2.5 The removal efficiency by EC treatment in the BOM
Characteristics of the dyeing wastewater containing AR 114 dye treatment by the ozonation-electrocoagulation combined system
4.3.1 Determine HRT for each reactor in the combined system
To assess the hydraulic retention time (HRT) of ozonation and electrocoagulation reactors in a combined system, experiments were conducted using synthetic wastewater with a dye concentration of 100 mg/L, varying the liquid flow rates through the ozonation column at 3 L/min, 4 L/min, and 5 L/min, corresponding to HRTs of 3.33 min, 2.5 min, and 2 min Ozonation was performed at a gas flow rate of 0.7 L/min and pH 7 The treated water was then directed to the electrocoagulation reactor, where flow rates of 3 L/min, 2.5 L/min, 2 L/min, and 1.5 L/min were tested, resulting in HRTs of 0.85 min, 1 min, 1.28 min, and 1.7 min, with a current density of 2.2 mA/cm² Results indicated that the combined system achieved the highest removal efficiencies at an ozonation HRT of 2.5 min, particularly when paired with 0.85 min and 1.02 min HRTs in the electrocoagulation reactor This enhancement is attributed to the cumulative effects of dye concentration on electrocoagulation efficiency and the influence of HRT on ozonation performance As the HRT in the electrocoagulation reactor increased to 1.71 min, the differences in treatment efficiencies for COD, dye, and color removals diminished, suggesting that the impact of ozonation HRT was mitigated by longer HRTs in the electrocoagulation stage.
Fig 4.28 The removal efficiency by the combination system with various HRT of the ozonation and electrocoagulation reactors
The high performance of the electrocoagulation treatment in the combined system is evident, particularly with sufficient reaction time, which effectively bridges the gap in removal efficiencies observed in previous ozonation processes due to variations in hydraulic retention time (HRT) However, increasing the HRT in electrocoagulation leads to higher energy consumption and increased sludge production, resulting in elevated operational costs and waste sludge treatment expenses Therefore, it is crucial to select an optimal HRT for the electrocoagulation reactor In this combined system, an HRT of 2 to 2.5 minutes for ozonation and 0.85 to 1 minute for electrocoagulation is adequate for treating wastewater containing 100 mg/L of AR 114 dye.
The energy consumption of the combined ozonation and electrocoagulation (EC) system was analyzed, revealing that higher hydraulic retention time (HRT) in both processes resulted in increased energy usage Notably, the relationship between energy consumption and HRT in the EC reactor was identified as a quadratic function, as illustrated in Figure 4.29.
Fig 4.29 Energy consumption of the combined system with various HRT in ozonation and EC reactors
Increasing the hydraulic retention time (HRT) in the ozonation reactor led to a reduction in the mass of generated sludge in the combined system As illustrated in Fig 4.30, the sludge mass decreased with higher HRT, while the volume of sludge remained constant at 30 mL/L when the HRT was adjusted from 2 minutes to 3.33 minutes.
Fig 4.30 Comparisions of the generated sludge mass between various HRT in ozonation reactor
4.3.2 The treatment efficiency by the combined system
The combined ozonation and electrocoagulation system using aluminum electrodes demonstrated significant removal efficiencies in treating synthetic wastewater Under optimal conditions of 2 minutes of ozonation and 0.85 minutes of electrocoagulation at a current density of 2.2 mA/cm², the system effectively reduced color from 808.7 Pt-Co to 56.3 Pt-Co, dye from 100 mg/L to 5.7 mg/L, and COD from 78.4 mg/L to 15.2 mg/L The removal efficiencies achieved were 93% for color, 94.25% for dye, and 80.6% for COD Additionally, the process generated a volume of sludge measuring 30 mL/L and a mass of 77 mg/L.
The initial pH of the wastewater was measured at 7.1, which decreased to 6.9 after 2 minutes of ozonation In contrast, the electrocoagulation process increased the pH to 7.35 within 0.85 minutes, aligning with discharge standards Consequently, the combined treatment system effectively maintains pH levels without the need for additional adjustments post-treatment.
Conductivity in this combined system increased with ozonation step and decreased in
EC treatment However, the change was not too much, less than 1.5%
The total energy consumption of the combined ozone generator and electrocoagulation system was measured at 393.9 Wh/m³, with a hydraulic retention time (HRT) of 2 minutes for ozonation and 0.85 minutes for the electrocoagulation reactor.
4.3.3 Comparisons of the combined system, the separate ozonation and the electrocoagulation treatment
Table 4.2 Comparisons of the combined system, the separate ozonation and the electrocoagulation treatment
Parameters Ozonation Electrocoagulation Combined system
Generated sludge mass (g/L) 0 0.4507 0.077 Energy consumption (Wh/m 3 ) 2133.3 1776.816 393.9
The study compares the combined system of ozonation and electrocoagulation with separate processes based on key parameters such as hydraulic retention time (HRT), removal efficiency, energy consumption, and the volume and mass of generated sludge Color removal efficiency, set at approximately 92%, serves as the benchmark for these comparisons, which are detailed in Table 4.2 All values referenced are derived from the results of the continuous reactors employing the combined system of ozonation and electrocoagulation.
From the Table 4.2, it is obvious that the combined system had many noticeably achievements when compare with the separate ozonation and electrocoagulation treatment as follows:
The combined treatment system demonstrates a significantly lower hydraulic retention time (HRT) compared to other methods, requiring just 2 minutes for ozonation and 0.85 minutes for electrocoagulation In contrast, separate ozonation and electrocoagulation treatments demand 40 minutes and 5.12 minutes, respectively, to achieve a comparable color removal efficiency of approximately 92% This efficiency, coupled with reduced area and construction costs, highlights the combined system's practical advantages, especially as land availability continues to decline.
- The combined system could improve the performance of COD removal when compare with the separate ozonation, about 10% higher than the separate ozonation
The combined electrocoagulation system produced significantly less sludge compared to the separate treatment method, with the volume and mass of generated sludge being only one-third and one-fifth, respectively Specifically, the combined system generated 30 mL of sludge per liter.
The combined system offers significant advantages, including a reduction in waste sludge treatment costs, with concentrations of 77 mg/L instead of 110 mg/L and 450.7 mg/L in separate EC This innovative approach aligns with the evolving trends in wastewater treatment technology, focusing on minimizing and potentially eliminating secondary waste.
The combined treatment system significantly reduces energy consumption compared to separate ozonation and electrocoagulation methods Specifically, the energy required for treating 1 m³ of water using the combined system is only 393.9 Wh, which is approximately 1/4.5 and 1/5.4 of the energy consumed by electrocoagulation (1.8 kWh) and ozonation (2.1 kWh), respectively This demonstrates the efficiency of the combined approach in water treatment.
The combination system of ozonation and electrocoagulation (aluminum electrodes) will be able to become a reasonable selection for treatment of wastewater containing
CI AR 114 dye not only because of the high efficiency but also due to less waste sludge production, too short HRT and less energy consumption.