Hybrid Treatment Strategies Based on Hydrodynamic Cavitation

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Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Hybrid Treatment Strategies Based on Hydrodynamic Cavitation, Advanced Oxidation Processes, and Aerobic Oxidation for Efficient Removal of Naproxen Pooja Thanekar, Sakshi Garg, and Parag R. Gogate*

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Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 40019, India ABSTRACT: Removal of naproxen (NAP) present in wastewater has been studied using an improved approach based on hydrodynamic cavitation (HC), combined with ozone or hydrogen peroxide as pretreatment for biological oxidation. Initially, the operating conditions for pretreatment based on HC reactor has been optimized as an inlet pressure of 4 bar and pH 3, where the maximum extents of degradation of NAP (28.9%) and chemical oxygen demand (COD) reduction (11.3%) were achieved using the approach of HC operated individually. Combined approaches of HC with hydrogen peroxide (HC+ H2O2) and ozone (HC + O3) were also investigated for maximizing the removal of NAP from wastewater. Almost 100% NAP degradation with a COD reduction of 40% was obtained for the combined approach of HC + O3 in 40 min, whereas HC + H2O2 resulted in 80% degradation with COD reduction of 24% within 120 min of treatment. The effluent obtained from the best pretreatment approach of HC + O3 was further treated using aerobic oxidation based on activated sludge, and it was observed that ∼89.5% COD reduction was achieved in the subsequent operation. Use of only aerobic oxidation resulted in 36.7% as the COD reduction and 20.4% as the biochemical oxygen demand (BOD) reduction. The biodegradability index (BI) was calculated for the raw effluent without any pretreatment, as well as for effluent subjected to HC + O3 pretreatment. The increased value of biodegradability index, BI (from 0.35 to 0.70) and also the kinetic analysis of biological process revealed the improvement in biological oxidation using pretreatment based on HC and ozone. Also, the operational costs for different treatment approaches were calculated based on the power consumption. Overall, significant benefits using combination of ozone and hydrodynamic cavitation with aerobic oxidation have been demonstrated. facility, NAP concentrations as high as 11 μg/L can be seen.6 It is also reported that the conventional biological treatment could only remove NAP over the range of 40%−78%,7,8 making the need for using alternate processes for treatment very important. Ultraviolet (UV) irradiation-based processes can be offered as an alternative approach for the removal of emerging contaminants, such as NAP from wastewater. For example, TiO2 photolysis,4 ultraviolet photolysis (UV) coupled with vacuum ultraviolet photolysis (VUV),3 as well as UV combined with H2O2,9 have been shown to be effective in the removal of NAP. However, UV-based treatment processes are costly and energy-intensive operations, especially at larger scales of operation. Therefore, it is still important to study new approaches such as those based on cavitation to remove pharmaceutically active compounds such as NAP in order to avoid its adverse effects on the environment.

1. INTRODUCTION The large usage of pharmaceuticals for medical applications, for both human and animals, also results in their exposure to the environmental sources such as groundwater, surface water, soil, and sediments badly affecting human life as well as aquatic life. Even if the concentration of these pharmaceuticals is very low (in the nanogram (ng) to microgram (μg) range), its presence offers a serious health problem to aquatic and terrestrial organisms. In the past few years, a new classification of “emerging pollutants”, for the wastewater and aquatic environment, has been included in the discharge limit standards.1Most of these compounds are recalcitrant or biorefractory, causing a detrimental effect on living organisms and reducing the treatment efficiency of approaches used in conventional effluent treatment plants (ETPs). These compounds require more sophisticated analytical instruments for their detection and are becoming the attraction of many researchers for developing methods of analysis, as well as treatment methods for the complete removal of the pollutant.2 Naproxen (NAP) belongs to class of nonsteroidal antiinflammatory drug (NSAID) widely used for treating rheumatoid arthritis and skeleton-muscle pain and also is a common veterinary medicine.3−5 The maximum concentration of NAP detected in natural water streams is typically in the range of 0.03−1.5 μg/L whereas, in a typical sewage treatment © XXXX American Chemical Society

Special Issue: Characterization and Applications of Fluidic Devices without Moving Parts Received: Revised: Accepted: Published: A

March 13, 2019 May 29, 2019 June 3, 2019 June 3, 2019 DOI: 10.1021/acs.iecr.9b01395 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

was reported to be obtained for all of the effluents. Lafi and AlQodah et al.18 studied the removal of pesticide from aqueous solution using AOPs coupled with the biological treatment process. It was reported that more than 95% of COD removal was observed using the combination of an O3/UV system coupled with biological oxidation. Similarly, Ramteke and Gogate19 studied the removal of ethylbenzene and pnitrophenol from synthetically prepared wastewater, using the combination of ultrasound-assisted Fenton approach with the traditional aerobic process. The enhancement of the biodegradability index (BI) from 0.15 to 0.36 was reported to be achieved within 40 min of the pretreatment, based on the ultrasound-assisted Fenton process. Under optimized conditions of the Fenton process as loading of Fe2+ as 2.0 g/L, and H2O2 as 1.5 g/L, maximum COD removals of 69.3% and 57.6% were obtained in the pretreatment for effluents containing ethylbenzene and p-nitrophenol, respectively. Many literature reports have dealt with the degradation of NAP using different combined treatment approaches based on AOPs. Arany et al.3 studied the degradation of NAP using different approaches such as UV rays at a wavelength of 254 nm alone, VUV rays at a wavelength of 172 nm, and the combined approach of UV/VUV. It was reported that complete degradation of NAP is achieved after 20 min of UV treatment, 10 min of VUV treatment, or 8 min of UV/ VUV photolysis. The VUV method was found to be most effective treatment strategy for the mineralization of NAP because almost complete mineralization was achieved after 2 h of irradiation. However, treatment of NAP using UV and UV/ VUV resulted in 80% and 85% of mineralization, respectively, after 2 h of irradiation, indicating that a combination approach does not necessarily give the best results for mineralization, although it gave a faster removal of NAP. Kanakaraju et al.4 investigated the application of UV/TiO2 photocatalysis for the removal of NAP from different water matrices. It was reported that the treatment of TiO2 photocatalysis resulted in 30% reduction in the dissolved organic carbon (DOC) in the case of distilled water matrix and 19% in drinking water matrix after 180 min of treatment. Kim et al.9 also studied the removal of pharmaceuticals (containing ∼12 different types of antibiotics and 10 types of analgesics) from sewage treatment plant effluent using UV and UV/H2O2 processes. It was reported that the 90% removal efficiency was achieved for present pharmaceuticals including NAP at UV dose of 923 mJ/cm2. The residual DOC concentration for treated effluent using a combination of UV/H2O2 processes was lower than that seen for the treatment using individual UV process as the combination resulted in the generation of more OH radicals, because of UV photolysis. From the literature analysis, it can be concluded that limited studies are available involving the application of advanced oxidation processes for efficient removal of NAP. Specifically, no studies have been reported for removal of NAP using HC either alone or combined with ozone or hydrogen peroxide or biological oxidation, justifying the novelty of this study. The present work mainly focuses on the removal of NAP using combined approaches based on HC/ozone/hydrogen peroxide as pretreatment, followed by biological oxidation. Typically, the effluent obtained from pretreatment as well as the raw effluent was treated with aerobic oxidation using activated sludge. The efficacy of the different combined approaches used in pretreatment and later combined with biological oxidation was quantified in terms of the extent of

Cavitation typically belong to the class of advanced oxidation processes (AOPs), among the new class of technologies, which can effectively degrade nonbiodegradable pollutants.10 Cavitation (hydrodynamic or acoustic) alone or in combination with the hydrogen peroxide, UV irradiation, catalysts, persulfates, Fenton process, and ozonation have been applied and reported to be effective.11,12 It is important to note that acoustic cavitation offers significant problems for scale up, because of significantly higher energy requirements and higher treatment costs. On the other hand, significantly higher energy efficiencies and low cost of operation makes hydrodynamic cavitation (HC) reactors an effective alternative. The degree of mineralization of wastewater treatment, especially for complex compounds using HC alone, is low, because of an insufficient generation of oxidizing radicals.13 The efficiency of the treatment based on HC can be increased by the combination of HC with AOPs, since the combination generates a higher quantum of oxidizing radicals, which ultimately increases the degradation rate of the pollutant.12 Gagol et al.14 studied degradation of different organic compounds such as different derivatives of BTEX and phenol as well as organosulfur compounds using HC as well as acoustic cavitation coupled with ozone, hydrogen peroxide, and peroxone under basic pH conditions. Complete oxidation of organic compounds using a combination of HC with AOPs revealed the effectiveness of HC-based combinations, compared to individual operations. Boczkaj et al.15 studied the degradation of volatile organic compounds (VOCs) using a combined method based on HC and AOPs. It was reported that a maximum of 40% chemical oxygen demand (COD) reduction and 50% biochemical oxygen demand (BOD) reduction is achieved using HC aided by ozonation. The results confirmed that the HC, in combination with AOPs, can effectively degrade VOCs. However, the combination of HC and AOPs consumes a large quantity of chemicals for the complete removal of pollutant, which ultimately increases the total treatment cost. Therefore, the usage of HC-based processes as a pretreatment to biological oxidation is an interesting approach. Aerobic oxidation is a conventional cost-effective approach that refers to the degradation of pollutants by the microorganisms which use the pollutant for their metabolic activity. However, the removal of complex nonbiodegradable pollutants is a significant limitation for the conventional biological treatments. One way to eliminate these drawbacks of individual biological treatments and individual AOPs is the combination of AOPs with biological oxidation to be applied for removal of complex nonbiodegradable pollutants.16 The pretreatment using AOPs such as cavitation, applied alone or in combination with hydrogen peroxide and ozone, can help in converting the nonbiodegradable molecules into a readily biodegradable molecule, which can ultimately improve traditional biological treatment, in terms of higher COD reduction, along with reducing the treatment cost and energy requirements.17 Some literature records depict the application of AOPs as a pretreatment for biological oxidation processes for the degradation of toxic pollutants present in wastewater.16,18 Trapido et al.16 studied the removal of bio recalcitrant pollutant from industrial effluent using the combined approach of Fenton and biological oxidation. Three different effluents from landfill leachate, industrial hazardous waste landfill, and plywood manufacturing factory were subjected to treatment using Fenton as pretreatment, followed by traditional biological treatment. More than 90% of COD and BOD reduction B

DOI: 10.1021/acs.iecr.9b01395 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Initially, the effect of operating parameters such as inlet pressure (over a range of 2−5 bar) and pH (over a range of 3− 9) was studied. For each run, 4 L of aqueous solution of 10 ppm concentration of NAP was taken and subjected to the treatment for 120 min. For the combined approach of HC and H2O2, different molar ratios of NAP:H2O2 as 1:50, 1:500, 1:1000, 1:1500, and 1:2000 have been used to study the effect of oxidant loading. Ozone generator procured from Eltech Engineering, Mumbai (Model el-oz-O-10g/h) has been used for the approaches involving ozone. For the generation of oxygen to be used for ozone generation, oxygen generator (Model Nuvo8) was used. Ozone was introduced into the feed tank using a ceramic diffuser at the flow rate of 2 g/h. For the analysis of progress of treatment, samples were collected at particular interval or at the end of desired treatment time for analysis. To the withdrawn sample, sodium sulfite (1M) was added immediately as a quenching agent for residual oxidant in the reaction mixture, in order to avoid any changes in the final COD. All the experiments for the pretreatment were repeated at least couple of times to establish reproducibility. The experimental errors were within ±2% of the reported average value. 2.2.2. Biological Oxidation. After the pretreatment, effluent was further subjected to the traditional biological treatment using activated sludge as an inoculum. Also, a blank run was performed using the effluent directly, i.e., without any pretreatment. The initial concentration of NAP as 50 ppm was used for this set of pretreatment experiments using HC alone, as well as in combination with other AOPs and also for the blank run of biological oxidation in order to obtain the distinguishable changes in the COD reduction. The pH of the pretreated effluents was adjusted to range of 7−7.5, which is optimum for biological processes. The biological oxidation was performed in a glass reactor with a capacity of 500 mL, with the use of 300 mL of effluent supplemented with necessary nutrients. The reactor was placed on an orbital shaker operated at 150 rpm over a period of 7 days. The activated sludge was acclimatized to the experimental environment for 4 days prior to the actual oxidation process. The acclimatized inoculum at 10% loading was added into the reactor. During the treatment, the sampling was performed after every 24 h to quantify the COD reduction and generation of biomass, in terms of mixed liquor suspended solids (MLSS) for total treatment time of 7 days. The biodegradation was performed for pretreatment approaches of HC alone, combination of HC + O3 as well as for the untreated effluent in order to check the efficiencies of different treatment approaches. 2.3. Analysis. The extent of degradation of NAP was analyzed using ultra-high-performance liquid chromatography (U-HPLC) equipment obtained from Thermo-Fisher (Model Ultimate 3000). The mobile phase of acetonitrile and deionized water (DI) in 60:40 ratio was used as an eluent for separation under reverse-phase conditions at a flow rate of 0.8 mL/min. The NAP concentration was detected at a wavelength of 210 nm. The limit of detection (LOD) and limit of quantification (LOQ) were determined using the standard deviation and slope of the calibration curve (Figure 3). LOD and LOQ for calibration curve of NAP were estimated as 0.07 and 0.21 ppm respectively, which are in the acceptable range. All the pretreatment samples were analyzed using the same HPLC method of quantification and the concentrations established using the calibration charts. COD analysis was

degradation obtained from HPLC analysis, as well as the percentage COD removal obtained from COD analysis. Kinetic analysis has also been performed in order to quantify the important design parameters for the biological oxidation process.

2. MATERIALS AND METHODS 2.1. Materials. A commercially available NAP (98.5% purity as provided by supplier) was purchased from High Media Pvt. Ltd., Mumbai. The chemical structure of NAP has been depicted in Figure 1. The chemicals, such as analytical-

Figure 1. Chemical structure of naproxen (NAP).

grade (AR) grade hydrogen peroxide (30%, w/v) and highperformance liquid chromatography (HPLC)-grade acetonitrile were procured from Thomas Baker (Chemicals) Pvt. Ltd., Mumbai, India. Deionized water (DI) was obtained freshly in the laboratory using the Millipore Milli-Q Gradient water purification unit. The pH adjustments were done using 0.1 N NaOH and 0.1 N H2SO4 solutions. The characteristics of the activated sludge supplied by a sewage treatment plant (STP) in Mumbai, India and used in the biological oxidation are given in Table 1. The Table 1. Characteristics of Activated Sludge parameter pH total solids, TS (%) volatile solids, VS (%) chemical oxygen demand, COD (ppm)

activated sludge 7 2 65 250

± ± ± ±

2 0.5 0.1 5

other chemicals of AR-grade required for COD analysis were obtained from Thomas Baker, Ltd., Mumbai, and were used without any further purification. The reagents required for the analysis of BOD were procured from Lovibond Aqualytic, Mumbai. 2.2. Methodology. 2.2.1. Pretreatment Using Hydrodynamic Cavitation (HC). Although the ultrasound reactors offer significantly higher intensity for degradation, its operation at a commercial scale is difficult, bceasue of high energy requirements, as well as high treatment costs. HC offers an energy-efficient alternative with good scale-up potential and, hence, all of the experiments were performed using the HC reactor in order to lower the treatment cost as well as to study the possibility of treatability at large-scale operation. The HC reactor configuration has been depicted in Figures 2a and 2b. The setup consists of a holding tank for the wastewater, a 1.1 kW capacity positive displacement pump, different control valves, and flanges to hold the cavitating device. The circulation loop consists of the main and bypass lines to allow for effective control of the flow of solution through the cavitating device. The geometric details of the slit venturi used as a cavitating device have been depicted in Figure 2c and Table 2. The reciprocating pump (model SI-104) of HC reactor configuration was procured from Water supply specialties, Mumbai, India. C


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Figure 2. (a) Schematic representation of HC reactor. (b) Photograph of the actual HC reactor setup. (c) Schematic representation of the venturi used as cavitating device.

performed using COD digester supplied by Hanna Equipment’s Pvt. Ltd. (Mumbai, India). The reduction in COD was

performed using the standard protocol of ISO 6060:1989. The sample vials containing effluent were processed in a COD D


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Industrial & Engineering Chemistry Research Table 2. Dimensions of the Slit Venturi Used as Cavitating Device parameter

dimensions

dimension of throat venturi length length of the convergent section length of the divergent section half angle of the convergent section half angle of the divergent section

W = 6.0 mm; H = 1.9 mm; L = 1.9 mm 87 mm 20 mm 65 mm 23.5° 5.5°

Table 3. Effect of Inlet Pressure and Cavitation Number on the Extent of Degradation of NAP and Kinetic Rate Constants inlet pressure (bar)

flow rate (LPH)

velocity (m/s)

cavitation number, Cv

extent of degradation (%)

2 3 4 5

271.2 381.9 411.3 449.4

21.0 29.6 31.9 34.8

0.43 0.22 0.19 0.16

8.8 9.2 12.2 11.7

rate constant (min−1) 0.8 0.9 1.2 1.2

× × × ×

10−3 10−3 10−3 10−3

Figure 3. HPLC standard calibration curve for NAP.

digester with potassium dichromate and sulfuric acid reagents at 150 °C for 2 h. The calibration of COD was performed using 300 ppm standard solution of glucose. Biochemical oxygen demand (BOD) analysis was performed using a BOD digester procured from Analytical Laboratory (Mumbai, India). The reduction in BOD5 was measured using the standard protocol of NEMI5210D.

3. RESULTS AND DISCUSSIONS 3.1. Effect of Pressure and Cavitation Number. In hydrodynamic cavitation, the inlet pressure and cavitation number play a crucial role in determining the cavitational intensity and, hence, the degradation of the pollutant.12 Effect of inlet pressure and cavitation number on the degradation of NAP was investigated by changing the inlet pressure from 2 bar to 5 bar at the natural pH of the solution (actual value as 7). Cavitation number (Cv) was calculated based on the pressure drop between the throat and extreme downstream region of constriction, as well as the kinetic head at the throat,20 where the cavitation inception is seen. A sample calculation for the cavitation number is shown in Appendix I. Typically, cavities are generated at Cv≤ 1 and much greater cavitational activity is observed at cavitation numbers of ∼0.1− 0.3.21 The variation in cavitation number for different inlet pressures over the range of 2−5 bar has been depicted in Table 3. The trends indicate that, with an increase in the inlet pressure from 2 bar to 5 bar, the cavitation number decreased from 0.43 to 0.16, typically because of the higher volumetric flow rate through mainline and velocity at the throat. The results for the extent of degradation at 120 min as the treatment time, and rate constants at different values of the inlet pressure, are depicted in Figures 4a and 4b, respectively. For an increase in the inlet pressure from 2 bar to 4 bar, the extent of degradation was found to increase and for further

Figure 4. (a) Effect of inlet pressure on the extent of degradation of NAP (initial concentration of NAP = 10 ppm). (b) Kinetic data fitting for the degradation of NAP at different inlet pressures (initial concentration of NAP = 10 ppm).

increase in pressure, the degradation reduced. A maximum degradation of ∼12.2% was achieved at an operating pressure of 4 bar (Cv = 0.19) with a rate constant of 1.2 × 10−3 min−1 (see Table 3). At higher pressures up to 4 bar, the violent collapse of cavities results in a higher pressure pulse, which ultimately breaks water molecules and the quantum of the hydroxyl radicals generated increases.22 Thus, increasing the inlet pressure up to the optimum level increases the degradation of pollutant. Beyond the optimum level, lesser degradation is observed, which is attributed to the lower cavitational activity based on the cavity cloud formation.23 Based on the interpretation obtained from analysis of these results, 4 bar was selected as the optimum pressure for further experiments. It is worthwhile to compare the obtained results with the literature illustrations. Saharan et al.24 studied the effect of inlet pressure and cavitation number on the extent of degradation of E


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Industrial & Engineering Chemistry Research Orange-G dye. It was reported that ∼92% decolorization was achieved at inlet pressure of 3 bar using a slit venturi as the cavitating device. Comparatively lower extent of decolorization as 76% and 45% were reported using circular venturi and orifice plate, respectively, for the same number of passes through the cavitating devices. The results were attributed to the fact that, in the slit venturi, a comparatively higher volumetric flow rate is obtained at a constant pressure decrease, giving a lower cavitation number and higher intensity. The study indicates that the cavitational intensity is ultimately dependent on the geometry of cavitating device. In the present work, the slit venturi apparatus has been used as the cavitating device, selection of which is also substantiated based on these reported results. Rajoriya et al.25 studied the effect of inlet pressure (over the range of 3−10 bar) on the treatment of a textile dye effluent using a slit venturi as the cavitating device. COD and TOC reduction was reported to increase as the inlet pressure increased from 3 bar to 5 bar. The maximum reduction obtained in TOC, COD, and color was 17%, 12%, and 25%, respectively, at an optimum inlet pressure of 5 bar (optimum Cv = 0.07) within 120 min, whereas comparatively lower TOC and COD reduction (9.3% and 3.5%, respectively) were obtained at an operating pressure of 10 bar. Bhagat et al.26 reported that the maximum extent of degradation of 4nitrophenol (12%) at an inlet pressure of 5 bar using a circular venturi as the cavitating device, whereas the maximum extent of degradation of imidacloprid as 26.5% was reported at a optimum inlet pressure of 15 bar by Raut-Jadhav et al.27 In the present work, the maximum extent of degradation of NAP using the slit venturi as the cavitating device was obtained at inlet pressure of 4 bar. From these different literature reports and by comparison with the present work, it can be confirmed that the optimum value of inlet pressure is dependent on the effluent or targeted pollutants and, hence, justifies the current investigations for establishing the optimum value of the inlet pressure. 3.2. Effect of pH. Operating pH changes the chemical properties of the solution and the pollutant location28 ultimately affecting the progress of degradation significantly. In this work, experiments were performed over the range of pH 3−9 to study the effect of pH at an optimized inlet pressure of 4 bar. The obtained results for the effect of operating pH on the extent of degradation of NAP has been shown in Figure 5, whereas the final extent of degradation and kinetic rate

constant of degradation has been presented in Table 4. The maximum degradation of ∼28.9% and a COD reduction of Table 4. Effect of pH on the Extent of Degradation of NAP and Kinetic Rate Constants pH

extent of degradation

3 4 7 9

28.9 26.4 12.2 6.7

rate constant (min−1) 3.1 2.6 1.2 0.7

× × × ×

10−3 10−3 10−3 10−3

∼11.3% with the rate constant of 3.1 × 10−3 min−1 was obtained at pH 3, whereas, under alkaline conditions (pH 9), the extent of degradation was only 6.68% with a much lower rate constant of 0.7 × 10−3 min−1. The extent of degradation and rate constants obtained under acidic conditions were higher, compared to alkaline conditions, because the oxidation potential of hydroxyl radicals under acidic conditions is high and the rate of recombination of hydroxyl radicals is low.20,29 Kumar et al.30 studied the effect of solution pH on the decolorization by varying the pH (over a range of 2−10) at a constant pressure of 5 bar. It was reported that the rate of decolorization was higher under acidic conditions, compared to that obtained under basic conditions. Quantitatively, the maximum extent of decolorization of 32.32% with the rate constant of 3.41 × 10−3 min−1 was reported at pH 2, whereas only 3.89% decolorization was reported at pH 10. Jawale et al.31 also reported that the acidic conditions are favorable for the degradation of atrazine. The kinetic rate constants obtained for degradation of atrazine decreased from 1.5 × 10−3 min−1 to 5 × 10−3 min−1 with an increase in pH from 3 to 7.3. Saharan et al.28 studied the degradation of Reactive Red 120 dye using HC and reported that the extent of decolorization of ∼60% and 28% TOC reduction was obtained in 3 h of treatment at pH 2. It is also important to note that, in some cases, pH is only marginally affecting the extent of degradation of pollutant. The pH of the effluent after treatment must be readjusted in order to comply with final discharge norms and, hence, pH adjustment for the treatment may not be always recommended and should be decided on a case-by-case basis. In the present situation, a significant increase in the extent of degradation of NAP was observed at an operating pH of 3; hence, pH 3 was chosen as the optimum pH to perform further experiments involving combinations with oxidants. 3.3. Effect of Initial Concentration. Degradation of NAP was studied at different initial concentrations of 10−50 ppm, using only HC under an optimized pressure of 4 bar and pH 3. The obtained results for the kinetic rate constant are represented in Table 5, whereas the data for the extent of degradation have been illustrated in Figure 6a. The extent of degradation increased from 13.7% to 28.9% with a decrease in Table 5. Effect of Initial Concentration on the Extent of Degradation and Kinetic Rate Constants

Figure 5. Effect of pH on the extent of degradation of NAP (inlet pressure = 4 bar, initial concentration of NAP = 10 ppm). F

initial concentration (ppm)


10 20 30 40 50

28.9 22.7 18.3 14.7 13.7

rate constant (min−1) 3.1 2.4 1.9 1.4 1.2

× × × × ×

10−3 10−3 10−3 10−3 10−3


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1500:1, and 2000:1. The obtained results are depicted in Figure 7 and Table 6. It was noticed that an increase in the

Figure 7. Effect of oxidant loading in the combined treatment of HC + H2O2 on the extent of degradation of NAP (inlet pressure = 4 bar, pH of NAP feed solution = 3, initial concentration of NAP = 10 ppm).

Table 6. Effect of H2O2 Loading on the Extent of Degradation of NAP and Kinetic Rate Constants HC + H2O2 (NAP:H2O2) 1:50 1:500 1:1000 1:2000 only H2O2 (NAP:H2O2 as 1:1000)

Figure 6. (a) Effect of initial concentration of NAP on the extent of degradation using only HC (inlet pressure = 4 bar, pH of NAP feed solution = 3). (b) Kinetic data fitting for the degradation of NAP at different concentrations.

extent of degradation (%) 34.5 64.8 80.0 33.5 35.1

rate constant (min−1) 3.5 9.4 15 3.9 3.9

× × × × ×

10−3 10−3 10−3 10−3 10−3

extent of degradation from 34.4% to 80% is obtained as the H2O2:NAP molar ratio increases from 50 to 1000, whereas further increasing the ratio to 2000 resulted in lower degradation (33.5%). The first-order kinetics was found to be suited for explaining the removal of NAP using a hybrid treatment method of HC and H2O2. The maximum rate constant as 15 × 10−3 min−1 was observed at a H2O2:NAP molar ratio of 1000. Also, a maximum COD reduction of 24% was obtained at the same H2O2:NAP molar ratio of 1000, which has been established as the optimum. Using a higher loading of H2O2 beyond the optimum results in the lower degradation due to scavenging action of residual H2O2.34 It was also observed in the present work that not much effect of pH has been observed in the case of HC+ H2O2 treatment at optimum H2O2:NAP molar ratio of 1000. The combined treatment of HC + H2O2 resulted in 76.9% as the extent of degradation of NAP without any adjustment in pH of the solution. The extent of degradation of NAP and rate constant using only H2O2 (1000:1 as the molar ratio) without HC was 35.1% and 3.9 × 10−3 min−1, respectively, confirming the role of cavitation in enhancing the extent of degradation. The value of the synergetic index (f) was determined based on the kinetic rate constant using the following equation:

the initial concentration from 50 ppm to 10 ppm. The maximum extent of degradation of 28.9% was obtained at 10 ppm, with the rate constant being 3.1 × 10−3 min−1, whereas only 13.7% degradation with rate constant being 1.2 × 10−3 min−1 was obtained for an initial NAP concentration of 50 ppm (Figure 6b). The observed results are attributed to the fact that an increase in the initial concentration increases the number of pollutant molecules, whereas the total generated OH radicals remains the same, which ultimately reduces the extent of degradation.32 A similar trend of decrease in the extent of degradation with an increase in initial concentration was reported by Jawale at al.33 for the case of potassium ferrocyanide over the range of initial concentration as 20−200 ppm. The maximum extent of degradation of 20.76% was obtained at a concentration of 20 ppm, whereas only 8.02% degradation was obtained at 200 ppm under the constant power supplied for a reaction volume of 100 mL. Rajoriya et al.20 also reported a similar trend of higher degradation under conditions of lower pollutant concentration with the degradation increasing from 19% to 47% with a decrease in the concentration of Reactive Blue dye over the range from 60 ppm to 30 ppm. As in the present work, the extent of degradation was found to be maximal at 10 ppm, the remaining set of experiments were performed using 10 ppm as the initial concentration of NAP, unless stated otherwise. 3.4. Effect of the Combination of HC/H2O2. The hybrid treatment method of HC and H2O2 was applied for degradation of NAP at varying loadings of H2O2, using the different molar ratios of H2O2:NAP as 50:1, 500:1, 1000:1,

f=

KHC + H2O2 KHC + K H2O2

=

15 × 10−3 = 2.9 1.2 × 10−3 + 3.9 × 10−3 (1)

The observed synergistic index confirms the better efficiency of the hybrid treatment method of HC and H2O2 than the G


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Industrial & Engineering Chemistry Research individual treatment methods. The cavitation generates local turbulence, which helps to eliminate the mass-transfer resistance for the utilization of H2O2 and also helps in the generation of additional hydroxyl radicals, which ultimately increases the extent of degradation of the pollutant. Thanekar et al.35 studied the degradation of the antiepileptic drug carbamazepine, using combined treatment methods based on hydrodynamic cavitation and H2O2 (different molar ratio over the range of 6−34) at an inlet pressure of 4 bar and pH 4. It was reported that the degradation increased from 31% to 58.3% as the molar ratio increases from 6 to 27. However, a further increase in the H2O2 molar ratio to 34 resulted in lower degradation. Bagal and Gogate36 also studied the degradation of an anti-inflammatory drug, diclofenac sodium, using hybrid methods. The effect of the addition of H2O2 was studied by varying the molar ratio of H2O2 to diclofenac sodium from 23 to 140 (0.05−0.3 g/L), using a hybrid method of HC/UV/ TiO2/H2O2. It was reported that the maximum extent of degradation of ∼95% and a TOC reduction of 76% was achieved at an optimum molar ratio of H2O2 to diclofenac sodium as 93 (0.2 g/L), whereas a further increase in the molar ratio up to 140 resulted in only a marginal increase in the extent of degradation. Comparison with literature reveals that, although trends for the effect of H2O2 loading are similar, the actual value of optimum molar ratio and the degree of observed intensification are different for each pollutant, clearly establishing the importance of the current work. The differences in the extent of increase with the addition of external oxidant are typically dependent on the type of structure of the pollutants and the reactivity, with respect to the hydroxyl radicals or the added chemical oxidant. Thus, it is recommended to justify the selection of oxidants and the optimum dosage based on the laboratory-scale investigations. 3.5. Effect of Combination of HC/O3. The combination of HC and ozone can yield enhanced degradation due to higher utilization of ozone based on higher rates of mass transfer and dual oxidation mechanisms of direct attack of ozone and attack of hydroxyl radicals formed by dissociation of ozone molecules under the effects of cavitation.20 Combined approach of HC + O3 was studied at a constant flow rate of 2 g/h introduced into the holding tank containing an aqueous solution of NAP at the previously optimized pH of 3. Almost 100% degradation and 40% COD reduction with a rate constant of 0.15 min−1 was achieved within 40 min, using the combined treatment of HC + O3 as per the data shown in Figure 8 and Table 7. Using a combination of HC + O3 at natural pH of feed solution resulted in a 92% extent of degradation, confirming the better trends under acidic conditions. The extent of degradation in 120 min of treatment and rate constant obtained for degradation of NAP using only ozone were 72.5% and 0.011 min−1respectively. The efficiency of the combined process of HC and O3 was compared with individual processes using the synergetic index f, which is calculated based on the kinetic rate constants as follows: f=

KHC + O3 KHC + K O3

=

0.15 = 12.3 1.2 × 10−3 + 0.011

Figure 8. Comparison of combined treatment of HC + O3 and ozone, in terms of the extent of degradation of NAP (inlet pressure = 4 bar, pH of NAP feed solution = 3, initial concentration of NAP = 10 ppm).

Table 7. Comparison of Combined Approach of HC + O3 and Ozone in Terms of the Extent of Degradation of NAP and Kinetic Rate Constants treatment approach


rate constant (min−1)

only ozone HC + O3

72.5 100

0.011 0.15

suspension. The combined HC + O3 approach resulted in almost-complete (99%) removal of algae within only 10 min, whereas individual methods of HC and ozone resulted in the removal of algae in amounts of ∼15% and 35%, respectively. Rajoriya et al.25 also studied the combined effect of HC + O3 by varying the ozone flow rate from 1 g/h to 5 g/h with ozone injected at the throat of the venturi for the treatment of textile dyeing effluent. It was reported that the maximum extent of TOC and COD reduction (48% and 27.2%, respectively) was obtained at an optimum ozone flow rate of 3 g/h for 120 min as the treatment time, whereas only HC resulted in much lower TOC and COD reduction as 17% and 12%, respectively. The result indicates that the HC, combined with ozone, resulted in ∼3.2 times higher mineralization than that obtained using HC alone. The combination approach reduces mass transfer (gas−liquid) resistance and enhances the quantum of hydroxyl radicals. It is important to note that the mass ratios of ozone to COD or the pollutant used in different literature illustrations are different; hence, exact effectiveness of ozone in lowering the pollutant concentration or COD cannot be examined. However, the literature reports and observed trend from current work clearly established that the combined method of HC and ozone is significantly beneficial, compared to the individual treatment method. 3.6. Effect of Different Pretreatments on Biological Oxidation and Comparison with the Conventional Approach. The pretreated effluents using different approaches based on HC were also further treated with aerobic biological oxidation for 7 days. The oxidation rates for different pretreated effluents were compared with the oxidation rate obtained for only biological oxidation of effluent without any pretreatment. The obtained COD reduction and biological oxidation rates have been depicted in Figures 9a and 9b, respectively. Significant COD reduction of ∼89.4% was obtained for the approach of HC + O3 pretreatment, followed

(2)

The obtained synergetic index indicated that the efficiency of the combined method is more than the additive effect and also better than the individual method. A similar additive effect of the combined method of HC and ozone was reported by Wu et al.37 for the removal of blue green algae from aqueous H


Article


maximum reduction in BOD (73%) for pretreatment of HC + O3, followed by biological oxidation, indicates that the intermediates formed were readily biodegradable. Kinetic analysis of COD removal was performed for all the pretreatment approaches using the following model equation: dS = kSn dt

For the first-order mechanism (n = 1), the above equation becomes ln

by biological oxidation, whereas HC pretreatment followed by biological oxidation resulted in a COD reduction of 45%. The untreated effluent of NAP (without pretreatment) subjected to biological oxidation gave in a COD reduction of only 36.7% (Table 8). An additional set of experiments was performed Table 8. Biological Oxidation Rates for Different Combined Approaches treatment approach approach of only aerobic oxidation approach of pretreatment of HC, followed by aerobic oxidation approach of pretreatment of HC + O3, followed by aerobic oxidation

rate constant (day−1)

36.7 45

0.08 0.10

89.4

0.32

(3)

where S represents the residual COD of the substrate, S0 represents the initial COD, n represents the order of reaction, K is the kinetic rate constant, and t is the time of treatment. Figure 9b shows that the biodegradation follows a first-order kinetics model. A maximum biological oxidation rate constant of 0.32 day−1 was obtained for the effluent pretreated with HC + O3, whereas comparatively lower oxidation rate constants of 0.08 and 0.10 day−1 were obtained for the effluent without any pretreatment and HC pretreated effluent, respectively. The biodegradability index (BI), i.e., BOD5:COD ratio was also estimated for the two representative cases of untreated effluent and pretreatment using the best pretreatment approach. The obtained values were 0.35 for initial effluent (without any treatment) and 0.70 for HC + O3 pretreatment. The results clearly confirmed that the HC + O3 pretreatment resulted in increased BI value, which proves that the pretreatment results in the conversion of complex, nonbiodegradable pollutants to readily biodegradable intermediates, which are easily degraded in the biological treatment step.18,19 A similar trend was observed in a few literature reports, but with different quantitative results, justifying the need to perform a set of experiments related to improving biological oxidation using pretreatments based on HC for the specific system under question. Thanekar et al.38 reported an improvement in biological oxidation of Dichlorvos using HC as a pretreatment with a much higher TOC reduction of 86.1% being achieved using HC + O3 pretreatment, followed by biological oxidation, compared to 14.4% obtained for untreated effluent being subjected to direct biological oxidation. Padoley et al.39 also studied HC as a pretreatment for the intensified treatment of biomethanated distillery wastewater and reported that pretreatment of HC resulted in an increase in the biodegradability index (BI) from 0.13 to 0.32. It was reported that the use of HC results in favorable breakage of the complex molecules into easily digestible compounds, leading to better efficacy of the subsequent biological oxidation. Similar favorable changes in the biodegradability (increased in BI from 0.17 to 0.39) has been reported by Ramteke and Gogate40 for the use of Fenton/ultrasound applied for oxidation of pollutants, such as toluene, benzene, naphthalene, and xylene. About 80%−95% of COD reduction was achieved using the combination of ultrasound/Fenton and biological oxidation process under optimized conditions such as pH 3−3.5, a Fe2+ dosage of 2.0 g/L, and a H2O2 dosage of 1.0 g/L. 3.7. Aerobic Degradation Kinetics and Evolution of Biomass. The evaluation of biomass growth kinetics in the biological oxidation of untreated effluent (without any pretreatment) and effluent obtained for best pretreatment approach of HC + O3 was also studied. During aerobic

Figure 9. (a) Effect of different pretreatment approaches on the biological oxidation of the effluent containing NAP. (b) Kinetic data fitting for combination of different pretreatment approaches and biological oxidation.

extent of COD degradation (%)

S0 = kt S

based on the treatment of artificially prepared solution of the same concentration of NAP obtained after only HC directly using biological oxidation (without HC pretreatment). A COD reduction of 18.7% was obtained after 7 days, using the activated sludge as an inoculum. From this result, it can be seen that the pretreatment of HC results in biodegradable intermediates, which can be easily further degraded, compared to the artificially prepared solution of the same concentration of NAP obtained after only HC being subjected to biological oxidation (without HC pretreatment). Also, only 20.4% of BOD5 removal was achieved using untreated effluent of NAP (without pretreatment), whereas significant reduction in BOD5 of 73% was achieved by subjecting effluent to approach of pretreatment of HC + O3 followed by biological oxidation. The I


Article

Industrial & Engineering Chemistry Research oxidation, samples were collected at regular intervals. The collected samples were analyzed for COD reduction, as well as the MLSS levels. In order to describe the relationships between utilization of substrate and microbial growth, different kinetic models have been proposed for aerobic degradation. The widely used Monod’s model for aerobic degradation did not give acceptable fits of experimental data obtained in the present work. The Contois model was then applied to the present case in order to investigate the relationships between substrate utilization and biological growth. The Contois model was found to fit well with the obtained experimental data. Beltran et al.41 also reported that the Contois model fits well for the treatment of high-strength distillery wastewater, using the combination of aerobic oxidation and ozonation. Similarly, Ramteke and Gogate40 also reported that the Contois model of microbial growth kinetics was a good fit for the combined approach of Fenton/ultrasound pretreatment and aerobic oxidation. According to the Contois model, the specific growth rate (μ) can be expressed as μ=

i S 1 dX = μmax jjj X dT k αX +

yz zz S{

Figure 10. Determination of growth coefficient Y for biological oxidation of the effluent containing NAP.

By plotting the graph of

1 t

(

ln

β − YS X0

) against ln( ), the 1 t

S0 S

values for μmax and α were obtained from the slope (αY) and intercept (μmax) of a straight line. The slope (αY) and intercept (μmax) were calculated for the approaches of HC + O3 pretreatment, followed by biological oxidation as well as direct biological treatment. Figure 11 depicts the representative graph

(4)

where μmax represents the maximum specific growth rate of micro-organisms, S represents the limiting substrate concentration, X represents the biomass concentration, and α is a dimensionless parameter that is related to the inhibition of the degradation because of the overpopulation of micro-organisms. The Contois model of microbial growth kinetics describes the relationship between specific growth rate (μ) and the limiting substrate concentration in a batch process.42 In order to express this model, the limiting substrate is measured in terms of COD and the biomass concentration is measured in terms of MLSS. The yield coefficient (Y) can be expressed in terms of change in substrate degradation with cell growth for a finite time interval as Y=−

X − X0 dX = dS S0 − S

Figure 11. Determination of slope (αY) and intercept (μmax) using a plot of

(5)

where X0 represents the MLSS concentration at time t0 = 0, X represents the MLSS concentration at time t = 7 days, S0 represents the COD at time t0 = 0, and S represents the COD at time t = 7 days. The yield coefficients (Y) were determined to be 18.61 and 17.10 for the combined treatment approach of HC + O3, followed by biological oxidation and untreated material (i.e. only biologically treated effluent), respectively as shown in Figure 10. The rearranging and combining eqs 4 and 5 yields the following expression: ÄÅ ÉÑ ÅÅ ÑÑ β−X Å ÑÑ μ = μmax ÅÅÅ Ñ ÅÅÇ β + (αY − 1)X ÑÑÑÖ (6)

of

β − YS X0

ln

(

β − YS X0

ln

1 t

S0 S

3

pretreatment.

) versus ln( ) for HC + O 1 t

S0 S

3

pretreatment,

followed by a biological oxidation approach. Table 9 represents the obtained values for μmax and α for different combinations based on pretreatment approaches. It can be seen that the pretreatment using HC + O3 resulted in a significant increase in the value of μmax, as well as a slight increase in the value of Y, justifying the improvement in biological degradation after pretreatment attributed to the formation of readily biodegradable pollutants due to the cavitational effects (as also confirmed earlier based on the increase in the biodegradability index). Ramteke and Gogate40 also reported that the data obtained from biological oxidation was well-fitted to the Contois model kinetics and the obtained higher values of Y for Fenton-treated samples justified the improved biodegradability of pollutants when treated with Fenton. 3.8. Operational Cost of the Treatment. In hydrodynamic cavitation reactors, the reciprocating pump is the major energy dissipation source for generating cavitation. The power dissipated by the pump obtained on the basis of flow rate was 34.5 W. Also, in the case of ozonation, ozonator

where β = YS0 + X0. Integrating eq 6, the final model equation can be expressed as αY ij S0 yz 1 ijj β − YS yzz lnj lnjj zz z = μmax + t jjk X 0 zz{ t kS{

1 t

( ) versus ln( ) for HC + O

1 t

(7) J


Article


Table 9. Calculated Values for the Growth Yield (Y), the Maximum Specific Growth Rate of Micro-organisms (μmax), and the Inhibition Parameter (α) treatment approach

Y (mg MLSS/mg COD)

μmax (day−1)

α (mg COD/mg MLSS)

approach of only aerobic oxidation approach of pretreatment of HC + O3, followed by aerobic oxidation

16.89 18.61

0.013 0.21

0.041 0.023

biological oxidation rate constant (0.32 day−1) was achieved when pretreatment of HC + O3 was coupled with aerobic oxidation. The kinetic evaluation of microbial growth was performed in order to obtain growth yield (Y) and growth rate (μmax). Comparatively, a higher growth yield Y and growth rate μmax of biomass were obtained for the pretreatment of HC + O3, followed by biological oxidation, than the untreated effluent samples (obtained without any pretreatment). The biodegradability index (BI) value increased from 0.35 to 0.70, because of the pretreatment of HC + O3. These results confirmed that the pretreatment based on HC can convert nonbiodegradable pollutants to intermediates that are easily biodegradable via the traditional biological oxidation process. Overall, it can be concluded that the combination of HC with ozone, followed by traditional aerobic oxidation, is the most effective combination, which gives satisfactory results in an efficient removal of NAP as well as chemical oxygen demand (COD) reduction.

consumes ∼200 W of power. The operational cost for different treatment approaches was calculated in terms of power requirements for a specific COD reduction of the effluent. The obtained data are given in Table 10, whereas a sample Table 10. Operational Cost for Different Treatment Approaches scheme HC HC + H2O2 HC + O3 HC pretreatment, followed by biological oxidation HC + O3 pretreatment, followed by biological oxidation


energy required (kWh)

total operation cost related to powera (Rs/L)

11.3 24 40 45

0.029 0.014 0.024 0.038

0.26 0.12 0.21 0.34

89.4

0.056

0.5

■

a

Cost gives an indicative representation and does not compare different methods, since the extents of degradation are different. Only power consumption for HC and ozone are considered in the analysis.

APPENDIX I Sample calculation of the cavitation number for a slit venturi, which is used as a cavitating device at an optimum inlet pressure of 4 bar, can be expressed as follows:

calculation for the best treatment approach of pretreatment of HC + O3, followed by biological oxidation, has been depicted in Appendix II. The best treatment approach of HC + O3, followed by biological oxidation required a treatment cost of only 0.50 Rs/L (where Rs/L denotes Indian Rupees per liter), whereas the pretreatment of HC followed by biological oxidation required 0.33 Rs/L, based on power requirements. Thanekar and Gogate43 also reported the cost estimation based on power consumption for the treatment of real effluent, using combined oxidation processes based on HC. The total cost obtained for the best treatment approach of HC + H2O2 + O3 was 11.5 Rs/L, whereas the estimated cost for HC + H2O2, HC + O3 and HC + KPS were 11.0, 1.1, and 3.6 Rs/L, respectively. It was also reported that the cost obtained for different approaches based on an ultrasound reactor was quite high, compared to that obtained for HC reactors. Raut-Jadhav et al.44 also reported cost estimation for the treatment of pesticide industry effluent using HC combined with H2O2 and ozone. It was reported that the combination of HC and H2O2 was the most energy efficient, because the cost required for the combination of HC and H2O2 was quite low (by a factor of ∼4), compared to that observed for the individual approach of HC (10.25 Rs/L).

cavitation number = Cv p − pv = 2 (1/2)ρv0 2 101325 − 4242.14 = 0.5 × 1000 × (31.87)2 = 0.19

where p2 is the downstream pressure (p2 = 101 325 Pa), pv the vapor pressure of water at 30 °C (pv = 4242.14 Pa), V0 the volumetric flow rate at a pressure of 4 bar (V0 = 411 LPH (where LPH denotes liters per hour) = 1.15 × 10−4 m3/s), a0 the flow area (a0 = 3.61 × 10−6 m2), and V0/a0 the velocity at the throat (V0/a0 = 1.15 × 10−4/3.61 × 10−6 = 31.87 m/s).

■

APPENDIX II A sample calculation for the treatment cost for the best treatment approach of pretreatment of HC + O3, followed by biological oxidation, is described below: • Power dissipated by the HC reactor pump (flow rate basis) = 34.5 W • Power dissipated by the pump (per unit volume in given treatment time) is given as A = (34.5/4) × 120 × 60 = 6.2 × 104 J/L • Power dissipated by the ozonator = 200 W • Power dissipated by the ozonator (per unit volume in given treatment time) is given as B = (200/4) × 120 × 60 = 1.2 × 105 J/L • Total power dissipated by the combined method of pretreatment of HC + O3, followed by biological oxidation, is given as A + B = 1.82 × 105 W s/L

4. CONCLUSIONS The current work established the effective use of a combination of hydrodynamic cavitation (HC) with H2O2 or O3 for the degradation of naproxen (NAP). The use of HC alone resulted in 28.9% degradation of NAP within 120 min, whereas HC + O3 resulted in almost-complete degradation within 40 min, clearly establishing the suitability of combined methods. It has been also observed that the integrated HC− aerobic oxidation process was more efficient for complete mineralization of NAP from aqueous solution. The maximum K


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• Energy required for pretreatment of HC + O3, followed extent of degradation (mg/L) by biological oxidation, is given as = 5.35 × 10−4 mg/J

total power dissipation (W s/L)

• Energy required for complete removal of NAP = 0.056 kWh As a result, considering 1 kWh = 8.78 Rs (using data obtained from Maharashtra State Electricity Distribution Co. Ltd., Mumbai), the cost of treatment is given as 0.056 × 8.78 = 0.5 Rs.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +91 22 3361 2024. Fax: +91 22 3361 1020. E-mail: pr. [email protected]. ORCID

Parag R. Gogate: 0000-0003-0044-7227 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

■

REFERENCES

Authors would like to acknowledge the funding of Department of Science and Technology under the Water Technology initiative scheme (Project Reference No. DST/TM/WTI/ 2K15/126(G)).

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Hybrid Treatment Strategies Based on Hydrodynamic Cavitation

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