Combined hydrodynamic cavitation based processes as an efficient treatment option for real industrial effluent

Ultrasonics - Sonochemistry 53 (2019) 202–213

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Combined hydrodynamic cavitation based processes as an efficient treatment option for real industrial effluent Pooja Thanekar, Parag R. Gogate

T

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Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 40019, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Real industrial effluent Hydrodynamic cavitation Ultrasonic reactors Advanced oxidation processes Cavitational yield Total treatment cost

In the present work, hydrodynamic cavitation (HC) operated alone and in combination with chemical oxidants has been applied for the treatment of real industrial effluent obtained from a local industry. Initially, the analysis of literature related to the hybrid methods involving hydrodynamic cavitation has been presented along with recommendations for the selection of important operating conditions for the HC operated individually and in combination with oxidation processes based on hydrogen peroxide (H2O2), ozone (O3) and persulphate (KPS). Subsequently, the treatment of real industrial effluent has also been investigated in details using HC alone and in combined mode with other oxidation processes focusing on the main objective of COD reduction. The reduction in the COD achieved using individual treatment of HC under the optimized conditions of inlet pressure as 4 bar and pH as 4 was only 7.9%. The application of different hybrid approaches based on HC such as HC + H2O2, HC + O3, HC + KPS and HC + H2O2 + O3 established higher COD reduction as compared to only HC. The maximum extent of COD reduction as 60.8% was achieved using HC + H2O2 + O3 combination whereas, relatively lower extent of COD was achieved for operations of HC + H2O2, HC + O3 and HC + KPS with the actual COD reduction being 40.3%, 38.7% and 8.5% respectively. It was also observed that 30.4%, 28.2%, 15.6%, and 4.7% of TOC reduction was obtained for the combined process of HC + H2O2 + O3, HC + H2O2, HC + O3, and HC + KPS respectively. Based on the kinetic study, it was established that the degradation fitted the first order kinetics for all the approaches. The combined processes of HC with oxidants were also compared with ultrasound reactors (both individual and combined operation) in terms of COD reduction, cavitational yield calculations, and treatment cost. HC reactors were established to be more energy efficient and also yielded treatment costs significantly lower than ultrasonic reactors.

1. Introduction The treatment of wastewater discharged from industries is a major concern for environmental protection especially considering the large quantities of effluent, number of industries and the presence of new refractory molecules which are having very long half life and are not easily degradable. Industry requires a huge quantum of water for processing and also produces effluent that is extremely harmful to people and the environment. The chemical pollutants present in effluents from industries such as heavy metals, solvents, dyes, pesticides, pharmaceutical drugs, aromatic compounds etc. are major threats to water quality. Also if these pollutants enter into the aquatic system, water gets contaminated and it becomes extremely difficult to treat completely the contaminated water [1,2]. Thus, it is imperative to treat the effluents at source in the industry or in the common effluent treatment plants. Typically, a majority of new generation molecules are recalcitrant or

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bio-refractory, and it is very difficult to degrade these completely using any kind of the conventional techniques. Therefore, there is a urgent need to develop efficient processes to degrade these compounds in a proper manner and bring the concentration to a certain minimum level so as to comply with the environmental laws [3]. In recent years, cavitation has shown good promise for oxidation of various pollutants present in wastewater. Typically, three different steps occur during cavitation viz. cavity formation, growth and their subsequent collapse over micro-scale duration leading to release of large amount of energy [4,5]. Cavitation results in production of free radicals, local hot spots as well as intense turbulence, which can all be extremely suitable for oxidation of pollutants. Cavitation based on the use of ultrasound (frequency in the range of 16 kHz–2 MHz) to induce the pressure variations in the liquid has been most commonly applied for wastewater treatment but the use of ultrasonic reactors for large scale operations have significant problems in terms of lack of energy

Corresponding author. E-mail address: [email protected] (P.R. Gogate).

https://doi.org/10.1016/j.ultsonch.2019.01.007 Received 28 September 2018; Received in revised form 30 December 2018; Accepted 7 January 2019 Available online 08 January 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.

Ultrasonics - Sonochemistry 53 (2019) 202–213

P. Thanekar, P.R. Gogate

Table 1 Overview of earlier work on HC and combined AOPs for degradation of pollutants. Model Pollutant

Operating Conditions

Important Results

References

2,4,6-trichlorophenol (2,4,6 TCP)

Initial concentration = 20 mg/L; Inlet pressure = 4 bar; Solution pH = 5; Reaction volume = 7L; temperature = 30 ± 2 °C; O3 = 400 mg/h; H2O2 = molar ratio of 2,4,6 TCP: H2O2 as 1:5

[25]

4-chloro 2-aminophenol (4C2AP)

Initial concentration = 20 mg/L; Inlet pressure = 4 bar; Solution pH = 6; Reaction volume = 7L; Temperature = 30 °C; UV = 8 W (two 4 W UV light tube) at 254 wavelength; O3 = 400 mg/h

reactive blue 13 (RB13)

Initial concentration = 30 mg/L; Inlet pressure = 0.4 MPa; Solution pH = 2; Reaction volume = 6L; temperature = 30 °C; Reaction time = 120 min; H2O2 = molar ratio of RB13 as 1:20; oxygen = 2 L/min; Fenton = molar ratio of H2O2: FeSO4·7H2O as 1:3; O3 = 3 g/h

potassium ferrocyanide (K4Fe(CN)6)

1. Ultrasonic horn: Frequency = 22 kHz; Rated power = 750 W; Operating volume = 100 ml; Initial concentration = 20 mg/L, pH = 2; Reaction time = 80 min; H2O2 = molar ratio of K4Fe(CN)6: H2O2 as 1:5 2. HC: Initial concentration = 20 mg/L; Inlet pressure = 6 bar; Solution pH = 2; Reaction volume = 5L; temperature = 30 °C; Reaction time = 120 min; H2O2 = molar ratio of K4Fe(CN)6: H2O2 as 1:5 Initial concentration = 50 mg/L Inlet pressure = 5 bar; pH = 5; Reaction volume = 6L; Reaction time = 120 min; H2O2 = molar ratio of RO4: H2O2 as 1: 1:30; Ozone = 3 g/h

For HC + O3 process, 97.1% as extent of degradation, 94.4% as TOC removal and 78.5% as COD removal For O3 + H2O2 process, 95.5% as extent of degradation, 94.8% as TOC removal and 76.2% as COD removal For HC + O3 + H2O2 process, 100% as extent of degradation, 95.6% as TOC removal and 80.9% as COD removal HC + O3 + H2O2 established as most efficient approach for complete removal of 2,4,6-TCP Extent of degradation for HC + UV as 79.3%, HC + O3 as 73.38%, UV + O3 as 91.17% and HC + UV + O3 as 96.85% HC + UV + O3 scheme was found to be the best treatment strategy for effective removal of4C2AP from the wastewater. Extent of decolorization for HC + oxygen as 66%; HC + Fenton as 66%; HC + H2O2 as 91%; TOC removal for the process of HC + O3 as72%; TOC reduction and complete decolorization in 15 min The combined HC and ozone was found to be best process for the decolorization and mineralization of RB13 Extent of degradation for US as 54.17%, HC as 44.2%; US + H2O2 as 61.22%; HC + H2O2 as 51.29% Cavitational yield in the case of hydrodynamic cavitation as 1.88 × 10−10 (gmol/J); acoustic cavitation as 6.96 × 10−12 (gmol/J) The hydrodynamic cavitation was found to be more energy efficient and gives higher degradation as compared to acoustic cavitation for equivalent power/energy dissipation. Extent of mineralization for HC alone as 14.67%; HC + H2O2 as 31.90%; HC + O3 as 76.25% HC with ozone proves to be the most energy efficient method for the degradation of RO4 Extent of decolorization for HC + H2O2 as 54%; HC + O3 (3 g/h of ozone) as 100% with 84% as TOC removal. HC with ozone established as most promising technique for complete mineralization of Rh6G HC alone gives 41.4% degradation and 27.9% TOC reduction; HC + O3 gives 84.8% degradation and TOC reduction as 39.4%. Only biological oxidation gives 14.4% TOC reduction; HC pre-treatment with biological oxidation gives 50.6% TOC reduction; HC + O3 pre-treatment with biological oxidation gives 86.1% TOC reduction Extent of degradation for HC + H2O2 + O3 as 100%; HC + O3 as 91.4%,HC + H2O2 as 58.3%, and HC + UV as 52.9% Synergetic index obtained for HC + H2O2 + O3 as 3.2, HC + O3 as 2.2, HC + H2O2 as 1.01, and HC + UV as 0.9 Total treatment cost for HC + H2O2 + O3 as 0.29Rs./L The treatment method of HC + H2O2 + O3 established as most effective treatment for removal of CBZ from wastewater Extent of degradation for HC alone as 12.2%; Photocatalysis alone as 28.1%; HC + photocatalysis as 78.2% The rate constants obtained for HC + photocatalysis was 1.5–3.7 times than the rate constants obtained for the individual processes. HC alone resulted about 60% decolorization and 28% TOC reduction in 3 h HC + H2O2 resulted almost 100% decolorization and 60% reduction in TOC The degradation of RR120 was reported to depend on the solution pH and higher degradation was achieved in acidic medium and addition of H2O2 further enhances the degradation rate. Ultrasound (with stirring) alone gives 11% TOC removal; addition of iron powder (0.12 g) to ultrasound treatment resulted in TOC removal as 18%; Ultrasound + advanced Fenton process resulted in about 60% TOC reduction whereas HC + advanced Fenton process resulted in about 70% TOC removal HC was found to be more suitable for treating effluent at a much larger scale of operation as compared to acoustic cavitation

[44]

Orange 4 dye

Rhodamine 6G (Rh6G)

Initial concentration = 10 mg/L; Inlet pressure = 5 bar; Solution pH = 10; Reaction volume = 6L; reaction time = 120 min; Ozone = 3 g/h; H2O2 = molar ratio of Rh6G: H2O2 as 1:30

Dichlorvos

Initial concentration = 50 mg/L; Inlet pressure = 5 bar; Solution pH = 4; Reaction volume = 4L; reaction time = 120 min; Ozone = 400 mg/h Biodegradation analysis as per OECD:301E

Carbamazepine

Initial concentration = 10 mg/L; Inlet pressure = 4 bar; Solution pH = 4; Reaction volume = 4L; reaction time as 120 min; H2O2 = molar ratio of CBZ: H2O2 as 1:5; O3 = 400 mg/h; UV = 16 W

Tetracycline

Initial concentration = 30 mg/L; pH = 10; Temperature = 30 ± 3 °C; Reaction volume = 4L; Treatment time = 90 min; UV254 irradiation provided by a 9 W mercury lamp

Reactive red 120 (RR120)

Initial concentration = 34 µM 50 mg/L; Reaction volume = 6 L; Temperature = 35 °C; Inlet pressure = 5 bar; pH = 2; H2O2 = 2040 µM

2,4-dichlorophenoxyacetic acid

1. Acoustic cavitation: A titanium probe with frequency as 20 kHz operating in a pulse mode of 4 s on and 2 s off; Intensity of the irradiation adjusted to 50%; Initial concentration = 0.235 g/L; Temperature = 22 ± 2 °C; Treatment time = 60 min; hydrogen peroxide (30%; 1.7 ml) and powdered iron (0.12 g) 2. HC: Initial concentration = 0.235 g/L; Reaction volume = 8L; Temperature = 18 ± 2 °C; Inlet pressure = 1500psi; flow rate = 5.2 L/min; Treatment time = 90 min

[21]

[24]

[43]

[45]

[46]

[40]

[47]

[22]

[48]

(continued on next page) 203

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Table 1 (continued) Model Pollutant

Operating Conditions

Important Results

References

Imidacloprid

Initial concentration = 25 mg/L; Reaction volume = 5L; Inlet pressure = 15 bar; Temperature = 32 ± 4 °C; pH = 2.7; H2O2 = Molar ratio of Imidacloprid: H2O2 as 1:40

[20]

Alachlor

Initial concentration = 50 mg/L; Reaction volume = 25 L; Flow rate = 3.6 m3/h; pH 5.9 Inlet pressure = 0.6 MPa; swirling cavitation chamber with a length of100mm and diameter of 10 mm (injection ports length 4.5 mm and width 0.2 mm) Initial concentration = 200 µg/L; pH = 6; Temperature = 25 °C; Inlet pressure = 0.35 MPa; Inlet pressure = 5 bar; pH = 2; HC + H2O2 (MB: H2O2 as 1:20); Treatment time = 60 min; Photo catalyst (Bismuth compounded TiO2) = 200 mg/L

Extent of degradation of HC alone resulted 26.5% with rate constant of 2.565 × 10−3 min−1 HC + H2O2 resulted 100% degradation in 45 min with first order reaction rate constant of 122.216 × 10−3 min−1 and 36.1% TOC removal; synergetic coefficient was 22.79 HC + H2O2 established as efficient process for degradation of imidacloprid Degradation follows pseudo-first-order kinetics and the degradation rate constant was found to be 4.90 × 10−2 min−1.

Ibuprofen (IBP) Methylene blue dye (MB)

Methomyl

Initial concentration = 25 mg/L; Reaction volume = 5L; Reaction time = 120 min; pH = 2.5; Inlet pressure = 5 bar; H2O2 = mole ratio of methomyl: H2O2 as 1:05; Fenton = mole ratio of Fe2+: H2O2 as 1:20; O3 = 2 g/h

More than 60% of ibuprofen was degraded within 60 min with an electrical energy per order of 10.77 kWhm−3 HC alone resulted 32.32% decolorization with rate constant of 3.42 × 10−3 min−1, HC + photocatalysis gives 64.58% decolorization, HC + H2O2 resulted almost 100% decolorization in 60 min Extent of mineralization for HC alone as 9.46, HC + photocatalysis as 12.68, HC + H2O2 as 18.41 HC + H2O2 has shown higher synergetic effect than HC + photocatalysis process Extent of degradation for HC alone as 13.9%, HC + H2O2 as 59.86% in 60 min treatment time; HC + Fenton as 100% in 30 min, HC + O3 as 69.87% in 60 min Extent of mineralization for HC alone as 5.45%, HC + H2O2 as 15.4%, HC + Fenton as 35.79% and HC + O3 as 70.79 Synergetic coefficient for processes such as HC + H2O2, HC + Fenton, HC + O3 was found to be 5.8, 13.41 and 47.6 respectively HC + Ozone process established as the most effective process giving highest synergetic coefficient, energy efficiency and the extent of mineralization

[27]

[49] [50]

[51]

initial COD of 17000 mg/L) was studied to investigate the performance of combined operations. Under the optimized operating parameters as dilution ratio of 50, inlet pressure of 1500 psi and addition of hydrogen peroxide in two cycles (each of 1900 mg/L loading), the maximum TOC removal as 60% was achieved within 150 min of treatment time for industrial effluent 1 while for industrial effluent 2, the maximum TOC removal of 70% was achieved after a similar treatment time. Saxena et al. [13] explored the use of hydrodynamic and acoustic cavitation along with pre-treatment strategy of alum coagulation for the treatment of wastewater obtained from tanning process (initial COD as 6720 mg/ L). It was reported that the BI increased from 0.14 to 0.57 using approach of coagulation followed by HC whereas, the application of US treatment combined with coagulation resulted in an increase in BI to 0.41. The higher cavitational yield (almost six times) was reported for the treatment of coagulation followed by HC as compared to the treatment of coagulation followed by US. Padoley et al. [14] studied the treatment of complex biomethane distillery wastewater (B-DWW) having initial COD of 35000 mg/L using HC and reported that under optimized conditions, enhanced BI as 0.32, COD reduction of 31.44%, TOC reduction of 32.24% and color reduction of 48% was observed. Rajoriya et al. [15] investigated the application of HC for treatment of textile dyeing industry (TDI) effluent having initial COD in the range of 2560–4640 mg/L. Combination of HC with air, ozone, oxygen, and Fenton’s reagent was also studied as a hybrid treatment approach. Approach of HC + Fenton resulted in maximum reduction in TOC and COD as 48% and 38% respectively along with near complete decolorization, which was much higher as compared to the individual treatment using HC. Boczkaj et al. [16] studied combination of HC with other oxidation processes (O3/H2O2/Peroxone) for the treatment of effluent obtained from the production of bitumen (initial COD over the range of 8000–12000 mg/L). The studies revealed that the maximum reduction in COD as 40% and BOD as 60% was obtained using the combination of HC with ozonation process. Overall, analysis of literature reports based on the application of the hybrid methods involving

efficient operation, and higher costs of operation. To overcome these limitations, approach based on inducing pressure variations based on changes in geometry of the flow, also described as hydrodynamic cavitation, has been considered as an efficient alternative approach having large potential even for scale up [6,7]. Among the different techniques for producing cavitation, HC has been demonstrated to offer maximum active zones within the entire capacity and also higher energy efficiency. In addition, there is a higher ease to construct the configuration including the constrictions or the cavitating devices such as venturi and orifice plates [8,9]. However, for the wastewater treatment applications, lower degradation rates for mineralization of complex pollutants can be obtained when HC is applied individually due to lower intensity of cavitation. The efficacy of HC can be enhanced using hybrid treatment methods where HC is coupled with other oxidants such as H2O2, Fenton’s reagent and ozone [10]. Cavitational effects can also help in lowering the drawbacks associated with other oxidation processes giving synergism, which is very important for industrial wastewater treatment where cost is a major consideration. Many studies are reported on the removal of single pollutant present in aqueous solution mostly at low concentration ranges (i.e. 10–100 mg/L) using HC operated alone and coupled with additives. However, only few efforts can be seen for the application of combined oxidation processes based on HC for the treatment of actual industrial effluent taken from any industry. Raut-Jadhav et al. [11] studied the use of HC alone and in combination with ozone and H2O2 for the treatment of real effluent containing pesticides with an initial COD in the range of 17000–18000 mg/L. The efficacy was quantified in terms of changes in the biodegradability index (BI) which was demonstrated to increase from 0.121 to 0.324 due to treatment using optimized appraoch based on HC. Chakinala et al. [12] investigated the treatment of industrial wastewater containing phenolic derivatives using HC induced by a liquid whistle reactor (LWR) coupled with advanced Fenton process (AFP). The treatment of two different industrial wastewater samples (effluent 1 with initial COD of 42000 mg/L and effluent 2 with 204

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favoured for degradation as compared to the alkaline conditions. Use of lower pH favours the formation of hydroxyl radicals and also the rate of recombination of hydroxyl radicals is lower. Due to this mechanism, higher amount of hydroxyl radicals are available which results in the enhanced degradation. It is also important to note that, the enhancement depends on dissociation constant (pKa) for the pollutants which decides whether the pollutant will be present in molecular or ionic state. The optimum pH value and the extent of variation in the degradation have been observed to be different for different pollutants. The extent of the change in degradation with pH will also decide whether pH adjustment from the natural pH to the optimum conditions should be considered as it is important to understand that the discharge is only allowed under neutral pH conditions [24]. Barik and Gogate [25] studied the effect of solution pH on the degradation of 2, 4, 6-trichlorophenol by varying pH over the range of 3–11 and reported that the maximum extent of degradation was 32.13% obtained at an optimum pH of 5. The effect of pH over the range of 2–8 on p-nitrophenol degradation was studied by Pradhan and Gogate [26] and maximum degradation as 63.2% was reported at optimum pH of 3.75. Saharan el al. [22] reported that the extent of degradation of reactive red 120 dye using HC reactor was the maximum (60% as the decolorization with 28% as TOC reduction) at pH of 2. Barik and Gogate [21] also reported similar existence of optimum pH (different value as 4) for the degradation of 4-chloro 2-aminophenol (4C2AP) using combined process of HC + UV + ozone. The maximum degradation of 4C2AP as 39% was reported at the optimum conditions. The case studies presented here have clearly established that the dependency of extent of degradation on pH is indeed specific to the system.

HC reveals that very few literature reports are available on the treatment of industrial effluents procured actually from industry. The present work focuses on giving a critical review and guidelines on the use of HC and subsequent experimental data for the treatment of actual effluent procured from a specialty chemical manufacturing industry. The work also compares the energy efficiency as well as treatment cost of HC reactor based different treatment approaches with the approaches based on ultrasound to establish the efficacy of HC for the real effluent. 2. Overview of literature Overview of earlier work based on the applications of HC operated individually and in combination with other oxidation process for wastewater treatment has been depicted in Table 1. The main objective was to provide guidelines on optimum operating conditions for both individual operation and combined approaches. 2.1. Guidelines about operating parameters of HC 2.1.1. Inlet pressure An increase in the inlet pressure gives enhanced generation of cavities based on higher energy input into the system and also the cavitationally active volume increases. The higher inlet pressures enhance the intensity of collapse of the cavities yielding higher quantum of hydroxyl radicals which in turn results in higher extent of degradation of the pollutant or higher extents of mineralization [17]. However, it is also observed that an optimum value of pressure exists, which usually depends on configuration and beyond the optimum, the beneficial effects are not observed attributed to the super cavitation (formation of large vapour pocket as a result of the generation of numerous cavities) giving reduced intensity of cavitation [18]. Thus, it is expected that an initial increase till an optimum will be beneficial giving enhanced degradation and mineralization whereas beyond the optimum, lower extent of degradation/mineralization are likely to be observed. Laboratory scale studies need to be performed to establish the optimum pressure as the actual value depends on the reactor configuration and the wastewater to be treated. Saharan et al. [17] studied the effect of inlet pressure for different cavitating devices in HC reactor and reported that the optimum inlet pressure depends on the cavitating devices with the actual value being 5 bar for circular venturi (CV = 0.15) and orifice plate (CV = 0.24) and 3 bar (CV = 0.29) for slit venturi. Boczkaj et al. [16] also reported the existence of optimum pressure with study at various inlet pressures (range of 6–10 bar) and flow rates over the range of 470–590 L/h. The maximum extent of COD reduction as 13% was obtained at the optimum pressure of 8 bar. Suresh Kumar et al. [19] studied the effect of inlet pressure on ternary dye wastewater treatment over pressure range of 2–8 bar. It was reported that the maximum decolorization of 28.23% was achieved at optimum pressure of 6 bar. Raut-Jadhav et al. [20] reported that the optimum value of the inlet pressure was 15 bar for the case of circular venturi applied for treatment of insectiside containing wastewater. The maximum extent of degradation as 26.5% was reported at the optimum conditions and beyond 15 bar pressure (actual value of studied operating pressure as 20 bar), lower extent of degradation as 25.07% was reported to be obtained. Based on literature illustrations, it can be said that though the trend for the extent of degradation at varying pressure is similar, the optimum value is different making detailed study into understanding the effect of inlet pressure for a specific system very important.

2.1.3. Initial concentration of pollutant The extent of degradation decreases as the pollutant concentration or the COD loading of the effluent increases, which is due to the fact that an increased loading of pollutants cannot be oxidized effectively by the fixed quantum of generated OH radicals available based on the generated cavitational intensity giving lower rates [18]. Rajoriya et al. [24] studied the effect of initial concentration of reactive blue 13 (RB13) on the extent of decolorization and reported that extent of decolorization increased from 19% to 47% by lowering the initial concentration of RB13 from 60 to 30 mg/L. Wang and Zhang [27] also reported that the degradation rate constant decreased from 5.22 × 10−2 min−1 for a initial concentration as 10 mg/L to 3.87 × 10−2 min−1 for initial concentration as 150 mg/L. It is important to understand that though lower concentrations are preferred for higher destruction, dilution of the effluent cannot be thought as an effective option as the quantum of the effluent to be treated increases. Focus should be more on increasing the oxidation capacity say by using combination approaches. 2.1.4. Temperature It is expected that the extent of degradation increases with an increase in the operating temperature due to the favorable kinetics and also nuclei formation which ultimately increases cavitational intensity. It is also important to understand that beyond a certain temperature, too many cavities will lead to cushioned collapse reducing the intensity of cavitation and hence the rates of treatment can be negatively affected [28]. Thus, an optimum temperature exists typically specific for the system. Bagal and Gogate [21] studied the effect of temperature on degradation of 4-chloro 2-aminophenol (4C2AP) over the range of 30–38 °C. It was reported that up to an optimum vale of temperature as 35 °C, the extent of degradation increased and then decreased with a further increase in the temperature to 38 °C. At an optimum temperature of 35 °C, maximum degradation of 24.6% was obtained whereas minimum degradation of 18.6% was obtained at 38 °C. Wang and Zhang [27] also reported the existence of optimum temperature with actual value being 40 °C where maximum degradation rate constant as

2.1.2. pH Another important parameter that affects the extent of degradation is the pH of wastewater as it affects the nature of the pollutants and the state/exposure to the cavitating conditions finally affecting the extent of reaction of the free radicals with the complex pollutants [18,21–23]. Analysis of literature revealed that the acidic conditions are generally 205

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4.90 × 10−2 min−1 was obtained. It is important to note that some researchers have reported a continuous increase in the rate constant over the studied temperature range. Wang and Zhang [29] reported that the degradation of rhodamine B was favoured over the entire temperature range of 30–50 °C. Patil et al. [23] also reported similar results of continuous increase in the degradation of methyl parathion with an increase in operating temperature of the system from 32 °C to 39 °C. The rate constant was reported to increase from 2.24 × 10−3 to 2.50 × 10−3 min−1 with an increase in operating temperature from 32 °C to 39 °C. The observed existence of the optimum temperature is typically due to the counteracting effects of higher cavitational activity and kinetic rate constants with the enhanced solvent vapour pressure that gives cushioned collapse of cavities and hence less intensity of cavitation, though both these effects are strongly dependent on the system. Depending on the liquid and constituents of the effluent, the optimum temperature may not exist or the actual value may be different that need to be established using specific laboratory studies.

effluent management as ozone has high oxidation potential of 2.08 eV which means that it can oxidize the organic pollutants efficiently [24]. The ozonation follows two types of mechanism based on the direct attack of ozone and oxidation by OH radicals obtained from dissociation of ozone in the presence of energy or favorable pH conditions. During direct ozonation, three types of reaction can occur depending on type of organic contaminants in the effluent. For contaminants with dipolar structure, ozone undergoes a 1–3 dipolar cyclo addition with saturated bonds giving an intermediate called ‘ozonide’, that is subsequently converted to aldehydes or ketones. Ozone can also react with pollutants having high electron density based on the electrophilic reaction based mechanism. Ozone attacks on the electron donating groups (for example –OH or –NH2) present in the ortho and para positions of the aromatic compounds which leads to opening of the aromatic ring. In the case of third type of nucleophilic reaction based mechanism, ozone reacts with pollutant containing electron withdrawing groups (for example –COOH or –NO2) [16]. The indirect mechanism of oxidation by ozone involves formation of hydroxyl radicals based on pH/energy supply. In the presence of cavitating conditions also, ozone dissociates into nascent oxygen (O) that reacts with water to form OH radicals [15], giving two simultaneously acting mechanisms for the combination of HC + ozone. Rajoriya et al. [24] studied the decolorization of RB13 dye using combination of HC and O3 at various loadings of ozone (range of 1–4 g/h). It was reported that with an increase in the ozone feed rate from 1 g/h to 3 g/h, the extent of mineralization increased and remained constant afterward for a further increase to 4 g/h. At optimized flow rate of 3 g/h, the maximum TOC reduction of 72% was achieved in 120 min. Gore et al. [30] reported that the combined treatment approach of HC and ozone was effective for degradation of reactive orange 4 dye (RO4). Use of ozone feed rate of 3 g/h combined with HC resulted in maximum TOC reduction as 76.25% in treatment time of 60 min whereas, HC and ozone alone resulted in TOC reduction of 14.67% and 20.75% respectively in same treatment time. Jawale et al. [31] also reported similar observations for the treatment of wastewater containing potassium ferrocyanide though in combination with ultrasound. Significantly lower extent of degradation (4.7%) was reported when ozone was operated individually whereas, the combination of ultrasound with ozone resulted in the maximum extent of degradation as 82.41%. Patil and Gogate [32] also reported that the combined treatment of ultrasound and ozone resulted in complete degradation of dichlorvos whereas only 60.4% of degradation was obtained using only ozone at a flow rate of 0.576 g/h. The discussed case studies have clearly established that using combination of cavitation and ozone is more favorable for effluent treatment as compared to only cavitation or ozone.

2.2. Combination approaches of HC with other oxidants Due to the limited efficacy of individual treatment of HC especially for complex effluents or highly loaded streams, HC has been combined with other oxidants for achieving enhanced treatment efficacy. We now present overview of different combination approaches. 2.2.1. HC + H2O2 Hydrogen peroxide, mainly acts as an additional source of hydroxyl radicals as it undergoes dissociation under cavitating conditions. Combination of HC with H2O2 increases the quantity of generated hydroxyl radicals, due to the dissociation induced by the extreme temperature conditions, which ultimately intensifies the degradation [20]. However, any excess H2O2 can also react with the free radicals as represented in Eq. (1) making them unavailable for the main degradation reaction.

H2 O2 + OH. →. HO2 + H2 O

(1)

Thus it can be said that as the loadings of H2O2 increases, the degradation would only be favoured till an optimum value (in most cases) beyond which the beneficial effects of adding extra oxidant will not be observed. The degree of obtained intensification and the required optimum H2O2 loading is generally different and sometimes optimum may not be observed over the investigated loading. Raut-Jadhav et al. [11] reported that the combination of HC and H2O2 was the most effective approach for the treatment of effluent containing pesticides. A detailed study into the effect of using diffrent loadings of H2O2 as 2, 5 and 10 g/ L on the TOC reduction established that use of loading 10 g/L was best giving the maximum TOC reduction as 54.87%. Chakinala et al. [12] investigated the effect of combination of HC and H2O2 on the treatment of the two different industrial effluents to evaluate the efficacy of combined technique. Under the optimum conditions of operating parameters such as dilution ratio as 50 and pressure of 1500 psi, the stepwise addition of H2O2 (each of 1900 mg/L loading) resulted in maximum TOC removal of about 60% and 70% for the two effluents after a treatment time of 150 min for both cases. Suresh Kumar et al. [19] reported that the extent of decolorization of dye wastewater increased for the combined treatment of HC and H2O2 as compared to only HC with 100% decolorization being achieved in 40 min at molar ratio of ternary dye: H2O2 as 1:40. Higher concentrations of H2O2 beyond 40 mol ratio resulted in a decrease in decolorization rate establishing the 1:40 ratio as the optimum. The case studies discussed here again confirm that the existence of optimum and the specific value indeed depend on the type of wastewater to be treated making laboratory investigations important.

2.2.3. HC + Persulphate The persuphate radicals (SO4−%) like hydroxyl radicals have also been used for the treatment of wastewater containing pollutants such as pesticides, pharmaceuticals, dyes, halogenated compounds etc. based on the oxidation mechanisms [33]. Like hydroxyl radicals, persulphate is also non-selective, having high oxidation potential as 2.60 V. Activation of persulphate via heat, UV light, and ultrasound results in cleavage of peroxide bond generating sulphate radicals as shown in following equation

S2 O82 − + Activator(Heat, UV light, and US) → 2SO·4

(2)

Activated persulphate has still higher oxidation potential and hence the degree of removal of the pollutant will be more than that obtained without activated persulphate anion. Typically the extent of degradation increases with an increase in the persulfate loading. HC can also provide the activation effect responsible for dissociation of persulphate ions due to which higher activity radicals are generated which can result in the faster degradation of pollutant [34]. Hence the combined approach of HC and persulphate can give enhanced degradation/

2.2.2. HC + O3 Ozonation has proven to be one of the efficient treatments for 206

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ultrasound induced treatment was studied using US reactor equipped with a single large transducer (25 kHz, rated power of 1 kW, calorimetric efficiency of 25%) at the bottom of reactor as shown in Fig. 2B. The reactor is a stainless steel tank (dimensions of 35 × 12 × 20 cm) having 7 L capacity (actual treatment volume used was 4 L). The constant operating temperature of 30 ± 2 °C was achieved with the help of cooling arrangements provided externally. The cooling water was circulated continuously through pump. The actual power dissipated into the system was found to be 250 W using calorimetric studies [39].

mineralization as compared to any individual treatment approach though not many studies have reported such study involving HC.Some available studies demonstrating the activation using ultrasound, UV, metal complexes or Fenton have now been discussed. Prajapat and Gogate [35] studied the effect of KPS loading (over the range of 0–0.1%) on degradation of guar gum using ultrasound and reported that an increase in KPS loading from 0% to 0.1% resulted in increased degradation from 24.18% to 98.27%. The observed enhanced degradation was attributed to generation of additional radicals as a result of the dissociation of KPS in the presence of UV/ultrasonic irradiations. Gokulakrishnan et al. [36] reported that for the KPS activation using Ni based perchlorate complex, the maximum degradation of malachite green was obtained at optimum KPS loading of 1 g/L. Oh and Shin [37] investigated the treatment of spent caustic using activated persulphate using zero valent iron/Fe2+ based oxidation processes. The maximum TOC reduction of 95% was reported using persulphate activated by Fe (0) with a Fe (0) to persulphate molar ratio as 1:5 and temperature of 95 °C. However, only 4.3% of TOC reduction was obtained using direct oxidation (without persulphate). The extent of TOC reduction was enhanced marginally (7–10% greater) when Fe (0) was used instead of Fe2+. The presented overview leads to understanding that the efficacy of persulphate increases with different activation mechanisms though use of HC for activation has not been demonstrated. The present work concentrates on the use of KPS in combination with both HC and ultrasound for effluent treatment, thus presenting novel insights into the treatment of industrial effluents.

3.1.3. Methodology For all the treatment studies, the effluent used was diluted by a factor of 10 using tap water. Initially, the effluent characteristics were analysed and the obtained data has been depicted in Table 2. The effluent was mainly a complex mixture containing phenolic derivatives, resins and sodium acetate with a pH around 9, chemical oxygen demand (COD) of 14,400 mg/L and total dissolved solids (TDS) content in the range of 9800–9850 mg/L. During the treatment, the suspended particles present in the effluent (the presence of solids is attributed to the earlier processing in the plant) were removed by coagulation (the removed solids after filtration will have to be sent to hazardous disposal facilities) using alum at dosage of 0.8 g/L. During coagulation, the reaction mixture was stirred for 1 h at a speed of 500 rpm. After subsequent 1 h of settling, the supernatant was collected and analysed for initial COD. The collected supernatant was used for all the cavitation based treatment. For all the experiments using HC reactor, 4 L effluent was taken in the reactor. All the experimental runs were performed for the 120 min and samples were withdrawn at periodic intervals of 30 min for analysis of COD reduction. The pH of the effluent was adjusted using the required amount of 0.1 M aqueous H2SO4 solution as required for the studies. The degradation rate constants were obtained using an integral method of analysis. The cavitational yield and total treatment costs were also calculated for different approaches including the combination and the individual approaches in order to compare the efficacy of the process. Under the optimized conditions of inlet pressure of 4 bar and pH of effluent as 4, the combined approach of HC + H2O2 was studied at varying H2O2 loadings over the range of 2–9 g/L. Similarly, the effect of using potassium persulphate (KPS) on the extent of degradation was investigated by using varying KPS loadings over the range of 0.5–2.5 g/ L. For treatment involving ozone, ozone generator procured from Eltech ozone, Mumbai was used. Ozone gas was introduced using a ceramic diffuser into the feed tank. In order to compare efficiencies of HC reactor with US reactor, the combined approach of US reactors with oxidising agents was also studied using previously optimized oxidant loading for HC reactor. All the experiments using US horn were performed in a glass reactor containing 100 ml of effluent. The effect of the combination of US horn and H2O2 was investigated at an optimum H2O2 concentration as 5 g/L. Similarly, the effect of the addition of KPS on the extent of COD reduction was investigated at an optimum loading of KPS as 2 g/L. For combined approach of ultrasound and ozone, as the reaction volume was very low (as compared to reaction volume used for HC reactor), the ozone flow rate of 400 mg/h was used for the case of ultrasonic horn. Also, set of experiments were performed using large-scale operation of ultrasound for the treatment of 4 L aqueous solution of effluent using the US reactor equipped with large longitudinal transducer. The obtained results for individual and combined process involving ultrasonic reactors were compared with results obtained for treatment using HC reactor.

3. Case study of real industrial effluent treatment using combination of HC and oxidants 3.1. Experimental 3.1.1. Materials The real industrial effluent was obtained from a local industry around Mumbai. The effluent mainly contained phenolic derivatives, resins, and sodium acetate as per details on the raw materials used and the typical intermediates and products manufactured in the industry. All the other chemicals such as hydrogen peroxide, potassium persulphate and aluminium sulphate used in the work were obtained from S.D. Fine Chem. Pvt. Ltd. Mumbai, India. 3.1.2. Experimental setup 3.1.2.1. HC reactor. The HC reactor consisting of slit venturi as a cavitating device used in the work has been depicted in Fig. 1. HC reactor configuration was mainly operated in recirulation mode with the liquid being pumped from a holding tank using a reciprocating pump (1.1 kW) and passed through different flow lines (main and bypass) housing valves to control flow rates. Slit venturi was used as a cavitating device and the geometric details of slit venturi are available in earlier work [18]. The temperature of the wastewater was maintained constant at 30 ± 2 °C with the help of cooling water circulated through the jacket. 3.1.2.2. US horn. The US horn (frequency of 20 kHz and supplied power of 120 W) represented in Fig. 2A was obtained from Dakshin, Mumbai, India. The duty cycle was adjusted at constant value of 70% (7 s ON and 3 s OFF) in the present operation. The actual efficiency of the US horn used in the work as calculated using calorimetric study [38] was observed to be 15.7%, which means that actual power dissipated in the solution and available for cavitation is 18.8 W for the rated power as 120 W. Another US horn (frequency of 22 kHz, supplied power of 700 W and calorimetric transfer efficiency of 38%) procured from Sonics Inc. USA was also used for investigating the treatment efficacy. 3.1.2.3. Longitudinal

ultrasonic

horn. Large-scale

operation

3.1.4. Analysis The withdrawn samples after each 30 min of treatment were analysed for the chemical oxidation demand (COD) using the COD digester procured from Hanna Equipments Pvt. Ltd., Mumbai, India. COD was measured as per the standard protocol given by ISO 6060:1989. During

of 207

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Fig. 1. Schematic representation of HC reactor.

effluent was observed to reduce to 12,480 mg/L and pH also reduced to 6.6. All the experiments were performed under optimized operating conditions of HC reactor as inlet pressure of 4 bar and pH of 4 (acidic range) established in earlier work [40].

the measurements involving the treatment with hydrogen peroxide, sodium sulfite (1 M) was added as quenching agent in the withdrawn samples for scavenging the residual H2O2 (unreacted) to avoid interference in the measurements of COD. The total organic carbon (TOC) reduction was analysed using a TOC analyser, model TOC-L-H564054 procured from Shimadzu. The total dissolved solids (TDS) content was analysed using TDS meter (model-254312-A01) procured from Thermoscientific Ltd.

3.2.1. Effect of dilution on COD reduction using HC alone The effect of dilution of the effluent on the COD reduction using HC was studied by selecting varying dilutions as no dilution and dilution ratios of 1:5 and 1:10. The obtained results for COD reduction and rate constants for different dilutions of the effluent are depicted in Figs. 3 and 4 respectively. The results depicted in Fig. 3 establish that the maximum COD reduction of 7.9% was achieved using the individual approach of HC for the treatment of effluent diluted for 10 times. However, only 4% degradation was obtained for undiluted effluent. The maximum rate constant as 0.7 × 10−3 min−1 was also obtained at 1:10 dilution. Similar trends of enhanced extent of COD reduction for diluted

3.2. Results and discussion Initially, the effluent characteristics such as COD and pH of the solution were quantified and observed as initial COD of 14,400 mg/L and pH of 9.7. The suspended particles from the effluent were removed by coagulation process and supernatant was used for all the cavitation based treatment experiments. After coagulation pre-treatment, COD of

Horn

Effluent Generator

Fig. 2A. Schematic representation of US horn. 208

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Effluent

Sample

Generator

Fig. 2B. Schematic representation of US reactor equipped with longitudinal horn. 0.09

Table 2 Characteristics of effluent used in the present work.

COD REDUCTION (%)

9.5–9.7 Pale yellow 14,400 mg/L 6119 mg/L 9800–9850 mg/L Phenolic derivatives, resins and sodium acetate

1:10 dilution

0.07 LN (CA0/CA)

pH Color Chemical oxygen demand (COD) Total organic carbon (TOC) Total dissolved solids (TDS) Main constituents

0.05

y = 0.0003x R² = 0.9885

0.04 0.03 0.02

8

0.01

7

0 0

5

1:5 dilution

3

30

60 TIME (MIN)

90

120

Fig. 4. Kinetic data fitting for the COD reduction at different dilutions (inlet pressure = 4 bar; pH = 4).

No dilution

4

y = 0.0006x R² = 0.9915

1:5 dilution

0.06

9

6

y = 0.0007x R² = 0.9868

No dilution

0.08

1:10 dilution

degradation efficiency was very less when HC was operated individually, the combination of HC with oxidants such as H2O2, ozone and KPS was investigated in order to improve degradation efficiency.

2 1 0 0

20

40

60 TIME (MIN)

80

100

120

3.2.2. HC + H2O2 The combined effect of HC and H2O2 was investigated at varying loadings of H2O2 over the range of 2–9 g/L and the results obtained in the study are depicted in Fig. 5. The integral analysis was also performed in order to find rate constant and order of reaction. It was observed that the reaction follows first order kinetics and the rate constants obtained from the analysis are given in Table 3. The maximum COD reduction as 40.30% and TOC reduction as 28.2% was obtained at the H2O2 loading of 5 g/L, which was established as the optimum. The observed COD reduction was lower at higher loadings of H2O2. In order to study the effect of individual treatment of H2O2 on the extent of COD reduction, experiments were also performed by passing flow through bypass line by closing the valve situated at main line to avoid the flow through the cavitating chamber (condition of only H2O2 treatment). Significantly lower extent of COD reduction as 11.7% was obtained using only H2O2 at the same 5 g/L loading. Based on the obtained rate constants, the synergetic index (f) was calculated using the following equation

Fig. 3. Effect of dilution on extent of COD reduction using only HC (inlet pressure = 4 bar; pH = 4).

effluent have been reported [11–13,15]. Rajoriya et al. [15] used diluted effluent from textile industry in the various proportions of 0%, 25%,50% and 75% (V/V). It was reported that with an increase in dilution from 0 to 75% obtained using tap water, the extents of COD and TOC removal increased. Similar trend of increase in mineralization with an increase in dilution of effluent was reported by Raut-Jadhav et al. [11]. It was reported that the maximum COD reduction of 21% was obtained at 10 times dilution. Although an increase in dilution gives maximum degradation, the total quantity of effluent also increases and hence using much diluted effluent for treatment of industrial effluent using HC is not a cost effective method in actual practice. Based on the extent of mineralization obtained in actual operation, an appropriate dilution ratio should be used for the treatment. In the present work, all the further experiments were carried out using 10 times dilution of the original effluent in order to maximize the efficacy of HC. Also, as the

f= 209

kHC + H 2O2 4.4 × 10−3 = = 2.75 kHC +kH 2O2 0.6 × 10−3 + 1 × 10−3

(3)

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P. Thanekar, P.R. Gogate 45

40

HC+H2O2 (5g/L)

HC+H2O2 (7 g/L)

HC+H2O2 (9g/L)

HC+O3

45

Only O3

40

Only H2O2 (5g/L)

COD REDUCTION (%)

COD REDUCTION (%)

35

HC+H2O2 (2g/L)

30 25 20 15 10

35 30 25 20 15 10 5

5

0 0

0 0

30

60 TIME(MIN)

90

Extent of COD reduction (%)

K × 10 (min

HC + H2O2 (2 g/L) HC + H2O2 (5 g/L) HC + H2O2 (7 g/L) HC + H2O2 (9 g/L) Only H2O2 (5 g/L)

22.8 40.3 39.5 37.5 11.8

1.9 4.4 3.8 3.6 1.0

−1

)

R

60 TIME (MIN)

80

100

120

Table 4 Effect of combination of HC with ozone on extent of COD reduction and kinetic rate constants.

Table 3 Effect of combination of HC with H2O2 at different loadings on final extent of COD reduction and kinetic rate constants. Scheme

40

Fig. 6. Effect of combination of HC with ozone on extent of COD reduction of real effluent (inlet pressure = 4 bar; pH = 4; ozone flow rate = 5 g/h).

Fig. 5. Effect of combination of HC with H2O2 on extent of COD reduction of real effluent (inlet pressure = 4 bar; pH = 4).

3

20

120

Scheme

Extent of COD reduction (%)

K × 103 (min−1)

R2

HC + O3 (O3 flow rate = 400 mg/h) Only O3 (O3 flow rate = 400 mg/h)

38.7

4.4

0.97

15.3

1.3

0.94

2

0.91 0.94 0.91 0.93 0.97

constant using the following scheme where kHC+H2O2 is the kinetic rate constant for the combined operation of HC and H2O2, kHC is the kinetic rate constant for only HC and kH2O2 is the kinetic rate constant for the operation of only H2O2. The combination of HC and H2O2 gave higher synergetic index which can be attributed to the dissociation of H2O2 due to cavitation which results in the enhanced generation of OH radicals. Also, HC generates turbulence which ultimately enhances mass transfer rates driving higher contact of effluent with oxidant [20]. Barik and Gogate [25] studied the effect of H2O2 addition at different proportions of H2O2 to 2,4,6-trichlorophenol (2,4,6-TCP) as 1:1 to 1:7. It was reported that an increase in the H2O2 loading from 1:1 to 1:5 resulted in a corresponding increase in the degradation extent from 37.5% to 62% whereas a further increase in the ratio to 1:7 resulted in a marginal decrease (actual value as 55.3%). Zupanc et al. [41] reported that significantly higher degradation as 89% was achieved at an optimum loading of 20 ml of H2O2 combined with HC as compared to HC operated individually (24%). Boczkaj et al. [16] also reported similar beneficial trends for the combination of HC and H2O2 with 20% and 49% as the COD and BOD reduction respectively. As discussed earlier and also confirmed in the presented comparison, the actual optimum loading of oxidant is different for specific systems clearly establishing the importance of the current work.

f=

kHC + O3 4.4 × 10−3 = = 2.3 kHC +ko3 0.6 × 10−3 + 1.3 × 10−3

(4)

where kHC+O3 is the kinetic rate constant for the combined operation of HC and O3, kHC is the kinetic rate constant for only HC and kO3 is the kinetic rate constant for the operation of only ozone. The obtained synergetic index reveals that the combined approach is more beneficial than the individual treatment method. The higher extent of degradation for combination of HC + O3 can be attributed to the decomposition of ozone molecules as a result of cavitational effects which ultimately enhances the availability of OH radicals. Raut-Jadhav et al. [11] studied the treatment of effluent obtained from pesticide industry using HC combined with ozone and reported that the approach of HC + ozone (3 g/h) enhanced COD reduction by almost 5 times as compared to HC operated individually. Raut-Jadhav et al. [20] also studied the degradation of the commercial pesticide methomyl in the aqueous solution. It was reported that the highest synergetic coefficient as 47.6 was obtained for HC + ozone combined process as compared to other combined processes of HC with H2O2 where the synergetic index was 5.8 and for HC with Fenton process where the value was 13.41. The rate constants obtained for individual treatment of HC and ozonation as 2.146 × 10−3 min−1 and 17.094 × 10−3 min−1 respectively were reported to be significantly enhanced to 915.94 × 10−3 min−1 for HC combination with ozone at constant ozone rate as 0.75 g/h.

3.2.3. HC + O3 The effect of addition of ozone in the HC was studied by introducing ozone at a loading of 5 g/h directly inside the feed tank. It can be seen from the results given in Fig. 6 and Table 4 that the COD reduction and rate constant as 15.3% and 1.3 × 10−3 min−1 respectively obtained for individual approach of ozone were enhanced for the combined approach of HC + O3 with the actual COD reduction and rate constant being 38.7% and 4.4 × 10−3 min−1 respectively. The values for the combination approach were also higher as compared to the only HC. The TOC reduction obtained for combined approach of HC + ozone (5 g/h) was 15.6%. The synergetic index(f) was calculated in terms of the obtained rate

3.2.4. HC + H2O2 + O3 The combined approach of HC, H2O2 (optimum loading of 5 g/L) and O3 (flow rate = 5 g/h) has also been applied for COD reduction and the COD reduction obtained was 60.8% as represented in Fig. 7. The TOC reduction of 30.4% was also obtained for this combined approach. To compare effectiveness of combined process, the synergetic index(f) was calculated using the following method

210

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70

Table 5 Effect of combination of HC with KPS on the extent of COD reduction of real effluent in 120 min and kinetic rate constants.

COD REDUCTION (%)

60 50

Scheme

40

HC + KPS HC + KPS HC + KPS HC + KPS

30

K × 103 (min−1)

R2

5.7 7.7 8.5 8.0

0.5 0.7 0.8 0.7

0.91 0.97 0.98 0.96

20

the KPS loading of 2 g/L. The synergetic index(f) obtained was 0.9 using following equation based on kinetic rate constants for the degradation.

10 0 0

20

40

60 TIME (MIN)

80

100

f=

120

kHC + KPS 0.8 × 10−3 = = 0.89 kHC +kKPS 0.6 × 10−3 + 0.3 × 10−3

kHC + H2O2 + O3 8.1 × 10−3 = = 2.8 − 3 kHC + kH 2O2 +ko3 0.6 × 10 + 1 × 10−3 + 1.3 × 10−3 (5)

where kHC+ H2O2+O3 is the kinetic rate constant for the combined operation of HC, H2O2 and O3, kHC is the kinetic rate constant for only HC, kH2O2 is the kinetic rate constant for the operation of only H2O2 and kO3 is the kinetic rate constant for the operation of only ozone The obtained synergetic index indicates that the combined approach gives much better efficiency attributed to the enhanced generation of highly reactive hydroxyl radicals and also elimination of the mass transfer resistance.

3.3. Treatment using US reactors The ultrasonic horn was used for studies based on sonochemical oxidation and the results related to the treatment using US (ultrasonic horn) operated individually and in combined approaches have been summarized in Fig. 9. It has been observed that only 10.6% as the COD reduction was obtained using only ultrasound treatment for 120 min. The approach of US/H2O2 resulted in much higher COD reduction as 60.0%, which can be explained on the fact that the ultrasound favours the dissociation of H2O2 leading to the higher quantity of hydroxyl radicals. Also, the combined operation of US/O3, US/H2O2/O3 and US/ KPS gave higher COD reduction as 20%, 61.8%, and 19.2% respectively as compared to the individual ultrasound induced treatment. Also, another US horn with higher supplied power of 700 W (actual power determined calorimetrically was 266 W) was used for the studies based on sonochemical oxidation and results obtained were compared with US horn operated at comparatively lower supplied power of 120 W (calorimetric power of 18.8 W). The combined approach of US/H2O2/ O3 resulted in 73.5% as the extent of degradation whereas treatment using only US horn resulted in 38.7% as the extent of degradation when second horn was operated at higher supplied power of 700 W.

3.2.5. HC + KPS The effect of using KPS at different loading over the range 0.5–2.5 g/L, on the COD reduction using HC was studied with the results depicted in Fig. 8 and Table 5. An increase in KPS loading from 0.5 to 2 g/L resulted in a continuous increase in COD reduction from 5.7% to 8.5% as well as rate constant was enhanced from 0.5 × 10−3 to 0.8 × 10−3 min−1 within 120 min of treatment. Increase in the KPS loading over 2 g/L resulted in no significant change in the extent of COD reduction (8%) as well as rate constant (0.8 × 10−3 min−1). The observed increase in the extent of COD reduction in the case of combined approach based on HC was attributed to the dissociation of KPS due to cavitational effects giving reactive free radicals [35]. The TOC reduction of 4.7% was obtained for combined approach of HC + KPS at 10 HC+KPS(0.5g/L)

8

HC+KPS (2g/L)

7

Only KPS (2g/L)

100

HC+KPS(1.5g/L)

90

HC+KPS (2.5g/L)

ONLY US

US+H2O2+O3

US+H2O2

US+O3

US+KPS

80 COD REDUCTION (%)

COD REDUCTION

9

(6)

where kHC+KPS is the kinetic rate constant for the combined operation of HC and KPS, kHC is the kinetic rate constant for only HC and kKPS is the kinetic rate constant for the operation of only KPS. It can be said that the combination of HC + KPS results in better efficiency that HC or using KPS in the presence of stirring but no synergism is observed. Also the increase in the COD reduction for combined approach is much lower as compared to the other oxidants used in the work, which possibly can be attributed to the lower reactivity of the pollutants towards sulphate radicals.

Fig. 7. Effect of combination of HC, H2O2 and O3 on extent of COD reduction of real effluent (inlet pressure = 4 bar; pH = 4; ozone flow rate = 5 g/h, H2O2 = 5 g/L).

f=

(0.5 g/L) (1.5 g/L) (2.0 g/L) (2.5 g/L)

Extent of COD reduction (%)

6 5 4 3

70

60 50 40 30 20

2

10

1

0

0

0

0

30

60 TIME(MIN)

90

30

60

90

120

TIME (MIN)

120

Fig. 9. Comparison of different treatment approaches based on US (US horn) in terms of extent of COD reduction (inlet pressure = 4 bar; pH = 4; ozone flow rate = 400 mg/h; H2O2 = 5 g/L; KPS = 2g/L).

Fig. 8. Effect of combination of HC with KPS at different loading on the extent of COD reduction of real effluent (inlet pressure = 4 bar; pH = 4). 211

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Raut-Jadhav et al. [11] also reported cost estimation for real effluent treatment using HC and its combination with additives. It was reported that the electrical cost of HC treatment, when operated individually was almost 4 times higher (10250.99 Rs./m3) than the combined approach of HC and H2O2 (1951.63 Rs./m3). It is important to note here that the cost of treatment presented here is only for the cavitation based treatment and no costs for the primary treatments such as coagulation have been considered in this comparative analysis.

Ultrasonic reactor of larger capacity (7 L) equipped with the longitudinal horn was also used for the treatment of real effluent operated individually and in combination with other oxidation approaches. About 8.5% as the COD reduction was obtained using only US reactor within 120 min. The maximum extent of COD reduction as 58.2% was achieved using the approach of US reactor + H2O2 + O3. The combined operations of US reactor/O3, US reactor/H2O2 and US reactor/KPS resulted in COD reduction as 17.5%, 55.6%, and 15.8% respectively. The COD reduction obtained using ultrasound treatment (US horn) was seen to be higher than the degradation obtained using hydrodynamic cavitation or the high capacity ultrasonic reactor equipped with longitudinal horn. The obtained results can be attributed to higher cavitational intensity in the case of ultrasonic horn type reactors [42] that results in enhanced generation of hydroxyl radicals locally. It is also important to note that the treated volume is much lower in ultrasonic horn and hence a better comparison would be in terms of the cavitational yield as discussed in next section.

4. Conclusions The present work demonstrated the effective use of HC combined with other oxidation processes for the treatment of industrial wastewater. The literature analysis presented initially highlighted the important guidelines for the selection of operating parameters of HC reactor as well as for the combination with oxidants. Subsequently studies with the treatment of real industrial effluent using HC operated individually and in combination with, H2O2, ozone and persulphate have been presented. The maximum extent of COD reduction as 7.9% was obtained using HC alone for the real effluent diluted 10 times using tap water. The extent of COD reduction significantly increased (about 40.3%) by using combination of HC and H2O2 at optimum loading of 5 g/L. Also the subsequent combination with ozone in the approach of HC + H2O2 + O3 showed beneficial effect on COD reduction (actual value being 60.8%). The combination of HC + O3 and HC + KPS resulted in lower extent of COD reduction as compared to combination of HC + H2O2 + O3 with the actual COD reduction being 38.7% and 8.5% respectively. The COD reduction obtained using HC reactor was compared with COD reduction obtained using ultrasonic reactors. The extent of COD reduction obtained using US horn treatment was higher than the reduction obtained using hydrodynamic cavitation. The cavitational yield obtained for best treatment approach of HC + H2O2 + O3 (0.04 mg/J) was significantly greater than US horn operated at supplied power of 120 W (0.00089 mg/J), US horn operated at supplied power of 700 W (0.000095 mg/J) as well as US reactor equipped with longitudinal horn (0.016 mg/J). Also, total treatment cost for combined operation using HC reactor is low (11.5 Rs./L) as compared to treatment cost obtained for combined operation using US reactor operated at low power (44.8 Rs./L), US reactor operated at high power (177.8 Rs./L) as well as US reactor equipped with longitudinal horn (12.7 Rs./L). Considering this analysis, it can be said that HC reactors offer more realistic potential for immediate commercial applications as

3.4. Comparison of US reactor and HC reactor based on cavitational yield and cost of treatment The comparison of HC and ultrasonic reactors was performed in terms of the cavitational yield calculations (COD reduction obtained per unit energy consumed (mg/J)). In US reactor, generator is the main energy consuming source. In the present work, the actual power dissipation (based on energy supply) for ultrasonic horn and US reactor with longitudinal horn is 120 W and 1000 W respectively. In HC reactor, the main energy consuming source is the reciprocating pump. The energy consumption in HC reactor was obtained as 5.87 W/L on the basis of flow rate [40]. The detailed cavitational yield calculations for the HC reactor have been already explained in our earlier work [40]. From Table 6, it can be seen that the cavitational yield for HC + H2O2 + O3 is the maximum (0.04 mg/J) as compared to other processes. The total treatment cost was also calculated based on the obtained cavitational yield. The total treatment cost of HC alone is 1.3 Rs/L which is quite low compared to the total treatment cost of individual operations using US reactor (196.9 Rs./L), US reactor operated at high power rating (137 Rs./L) as well as US reactor equipped with longitudinal horn (12.9 Rs./L). The total treatment cost obtained for best treatment approach of HC + H2O2 + O3 was 11.5 Rs./L whereas, for the approaches of HC + H2O2, HC + O3 and HC + KPS, treatment cost was 11.0 Rs./L, 1.1 Rs./L and 3.6 Rs./L respectively.

Table 6 Comparison of cavitational yield and cost of treatment for different treatment approaches. Scheme

COD reduction (%)

Cavitational yield (mg/J)

Energy required (kWh)

Cost related to power (Rs./L)

Additive cost (Rs./L)

Total treatment cost (Rs./L)

Only HC HC + H2O2 (5 g/L) HC + O3 (5 g/h) HC + H2O2 (5 g/L) + O3(5 g/h) HC + KPS (2.0 g/L) Only US horn (120 W) US horn (120 W) + H2O2 (5 g/L) US horn (120 W) + O3 (400 mg/h) US horn (120 W) + H2O2 (5 g/L) + O3 (400 mg/ h) US horn (120 W) + KPS (2.0 g/L) US horn (700 W) US(700 W) + H2O2 (5 g/L) + O3 (400 mg/h) Only US reactor with longitudinal horn US reactor with longitudinal horn + H2O2 (5 g/L) US reactor with longitudinal horn + O3 (400 mg/ h) US reactor with longitudinal horn + H2O2 (5 g/L) + O3 (400 mg/h) US reactor with longitudinal horn + KPS (2.0 g/ L)

7.9 40.3 38.7 60.8 8.5 10.7 60.1 20.1 61.9

0.023 0.119 0.025 0.040 0.025 0.00015 0.00086 0.00029 0.0008941

0.15 0.029 0.13 0.08 0.14 22.42 3.99 11.9 3.88

1.3 0.25 1.17 0.74 1.21 196.93 35.0 104.8 34.0

– 10.8 – 10.8 2.4 – 10.8 – 10.8

1.3 11.0 1.1 11.5 3.6 196.9 45.8 104.8 44.8

19.2 38.7 73.5 8.5 55.6 17.5

0.00027 0.000095 0.000182 0.0023 0.015 0.004

12.5 36.17 19 1.47 0.22 0.71

109.7 317 167 12.9 1.97 6.27

2.4 – 10.8 – 10.8 –

112.1 317 177.8 12.9 12.7 6.3

58.5

0.016

0.21

1.88

10.8

12.7

15.8

0.004

0.79

6.94

2.4

9.3

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P. Thanekar, P.R. Gogate

compared to the US reactors. [25]

Acknowledgement

[26]

Authors would like to acknowledge the funding of Department of Science and Technology, New Delhi, India under the Water Technology initiative scheme (Project reference: DST/TM/WTI/2K15/126(G)).

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