- Email: [email protected]
Hydrodynamic cavitation as a novel approach for wastewater treatment in wood finishing industry
Separation and Purification Technology 106 (2013) 15–21
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Hydrodynamic cavitation as a novel approach for wastewater treatment in wood finishing industry Mandar Badve a, Parag Gogate a, Aniruddha Pandit a, Levente Csoka b,⇑ a b
Department of Chemical Engineering, Institute of Chemical Technology, Mumbai 400 019, India University of West Hungary, Institute of Wood and Paper Technology, 9400 Sopron, Hungary
a r t i c l e
i n f o
Article history: Received 26 October 2012 Received in revised form 22 December 2012 Accepted 31 December 2012 Available online 7 January 2013 Keywords: Hydrodynamic cavitation Wood finishing industry Cavitational yield COD reduction Hydroxyl radicals
a b s t r a c t In the present work, hydrodynamic cavitation has been evaluated as a novel treatment scheme for wastewater generated from wood finishing industry. The earlier work on hydrodynamic cavitation has been mostly with synthetically prepared pollutants (typically less volatile compounds) and the current work is the first to report a treatment strategy for the real wastewater generated by wood finishing industry, which contains high concentration of volatile organic compounds. Hydrodynamic cavitation reactor is basically a stator and rotor assembly, where the rotor is provided with some indentations and cavitation events are expected on the surface of rotor as well as within the indentations. The effect of various operating parameters such as speed of rotor, loading of H2O2 and the residence time of wastewater in the cavitating device, on the extent of COD reduction of the wastewater have been investigated. It has been observed that the rate of degradation of wastewater is dependent on the speed of rotor, H2O2 concentration and the liquid phase residence time. Addition of H2O2 resulted in enhanced degradation till an optimum concentration which is dependent on the type of wastewater stream. It has been observed that the cavitational yield increases by 46% at optimum loading of H2O2. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction During recent years, there has been increasing awareness and concern about water pollution caused by industrial wastewater all over the world. It has been globally accepted that effective utilization of available water resources is a need of the time. Treatment of polluted wastewater is a global concern since wastewater collected from various resources such as municipalities, communities and industries must ultimately be returned to the receiving water bodies or to the land. Prevention of pollution at source and prior licensing of wastewater discharges by competent authorities have become key elements of successful policies for preventing, controlling and reducing discharge of hazardous substances, nutrients and other water pollutants from sources into aquatic ecosystems [1]. Wood finishing requires a combination of several operations such as sanding, scraping and planning, which are used to obtain good surface of the wood for further processing. Once the surface is prepared, various quotes of finishes are applied. Commonly used finishes are lacquers, varnish and various paints. Various adhesives are used to bind the pieces of wood together. Lacquers and varnishes are used to make wood surface glossy. Anti-bacterial paints
⇑ Corresponding author. Tel.: +36 99 518 305; fax: +36 99 518 302. E-mail address: [email protected] (L. Csoka). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.12.029
are used to protect wood from any bacterial degradation. UV stabilizers and preservatives are used to protect the wood from sunlight. Wastewater from wood finishing industry is typically generated from such processes and process equipment cleaning. The wood finishing process is a significant source of volatile organic compounds (VOCs) and other hazardous pollutants. In order to reduce the VOC emission, sometimes low VOC finishes are used. A lot of research has been done to reduce air pollution caused by wood finishing industry by minimizing the VOCs as well as hazardous air pollutants (HAPs) emission [2–4]. To reduce the VOCs and HAPs emissions, water based lacquers, varnishes and adhesives are also being commonly used as alternatives. Use of water based chemicals certainly reduces the air pollution caused by organic solvents but at the same time the issue of water pollution arising due to the use of water based additives is still a concern. No significant literature has been found, to be best of our knowledge, on a treatment strategy developed for wastewater generated specifically by the wood finishing industry. The present work proposes a treatment methodology for the wastewater generated by wood finishing industry based on the use of hydrodynamic cavitation reactors either operated individually or in combination with hydrogen peroxide. Cavitation is a phenomenon of nucleation, growth and implosion of vapor or gas filled cavities, which can be achieved by the passage of ultrasound (acoustic cavitation) or by alterations in
16
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
the flow and pressure (hydrodynamic cavitation). In the case of hydrodynamic cavitation, flow geometry is altered in such a way that the kinetic energy is increased by having a flow constriction which results into a reduction in the local pressure of the liquid considerably with a corresponding increase in the kinetic energy. When the pressure of the liquid falls below the vapor pressure of the same liquid, millions of vapor cavities are created, which are subjected to turbulent conditions of varying pressure fields downstream of the constriction. Life time of these cavities is very small (few micro seconds). The cavities finally implode violently and result in generation of very high pressures (up to 1000 bar) and temperatures (10,000 K) intensifying the chemical reactions and/or promoting radical formation and its subsequent reactions [5]. Extreme shear forces are generated which coupled with shock waves generated by cavitation events helps in breaking down of the pollutant molecules especially complex large molecular weight compounds. The intermediate compounds are more prone to OH radicals attack and biological oxidation, which further enhances the overall rate of degradation/mineralization of the wastewater. Under such extreme conditions water molecule within the cavity dissociates into OH and H radicals. OH radicals diffuse into the liquid and react with pollutant molecules resulting into oxidation/ mineralization [6]. In the past, ultrasonic cavitation has been used most commonly for the degradation of various organic pollutants though generally at a laboratory scale operation [7–9]. One of the major drawbacks of ultrasonic reactors is the energy intensive operation due to much lower energy transfer efficiencies (10–30%) and also the active cavitation zone is restricted to near the transducers which is quite low as compared to the total reactor volume and. Also, scale up of such reactors is very difficult due to local nature of the cavitational activity very close to the transducer area [10,11]. On the other hand, hydrodynamic cavitation reactors are relatively easy to scale up and are energy efficient compared to ultrasonic reactors [12,13]. It has been reported that energy transfer efficiency of hydrodynamic cavitation reactors is about 60–70% depending on the operating conditions whereas for ultrasonic reactors it is only about 10–40%. Sivakumar and Pandit [14] have studied the application of hydrodynamic cavitation for the degradation of rhodamine B dye using multiple hole orifice plates and reported that hydrodynamic cavitation devices are more energy efficient than acoustic cavitation devices. Quantitatively, the energy efficiency of hydrodynamic cavitation (multiple hole orifice plate) was two times greater than the acoustic cavitation device (dual frequency flow cell with a capacity of 1.5 L). Jyoti and Pandit [15] studied disinfection of water using different techniques and have reported that hydrodynamic cavitation is an economically attractive alternative compared to techniques such as ozonation and heat sterilization for reducing the bacterial counts in the potable water. In addition, few other papers have also discussed the applications of hydrodynamic cavitation [6,16,17] for KI oxidation. In hydrodynamic cavitation, generally venturi or orifice plates have been used in the past to create cavitation in liquid under circulation. In addition the high pressure and high speed homogenizers also generate cavitating conditions. In the present work, a different type of hydrodynamic cavitating device comprising of a stator and rotor assembly has been used to create the same effect. A rotor rotates at very high speed in a confined annular space and liquid is passed through the gap between the stator and the rotor. Due to high speed of rotation, very high surface velocities are generated. Some indentations are also provided on the surface of rotor. Liquid at such velocities enters the indentation, due to rotary action of the rotor and when liquid comes out of the indentation due to centrifugal flow, a low pressure region/vacuum is created near the upper surface of the indentations resulting into cavitation. Pressure drop across the component is main driving force for cav-
itation. At such high surface velocities of the liquid on the surface of the rotor, liquid pressure drop across the surface of the rotor and indentation is sufficient enough for cavitation to occur. 2. Materials and methods 2.1. Wastewater The wastewater was collected from the actual wood finishing industry (source not revealed due to confidentiality agreement). Experiments were performed to study the efficacy of hydrodynamic cavitation for the reduction in the chemical oxygen demand (COD) of the wastewater sample. COD indicates the concentration of organic compounds which can be oxidized chemically by using strong oxidizing agents. Majority of earlier wastewater treatment studies have been performed with simulated wastewater whereas the current study deals with an actual industry wastewater and hence carries lot of significance in the context of the actual industrial wastewater treatment using hydrodynamic cavitation. Wastewater used in the present study consists of a complex mixture of adhesives, UV stabilizers, lacquers, paints, varnishes. Initial pH of the wastewater was 6.18 and initial COD of the wastewater stream was 38,000 mg/l. 2.2. Experimental setup Fig. 1 gives the schematic of the experimental setup, which basically contains a storage tank for liquid of maximum capacity 15 L. Liquid from the tank is pumped into the cavitating device by using an open impeller type pump with 3 blades and flow capacity of 5–15 L/min with a pressure head of 4.5 bar. Heat exchanger was added between the cavitating device and storage tank to cool the liquid coming out of the cavitating device. Fig. 2a shows schematic details of the actual cavitating device, a stator and rotor assembly. Rotor is attached to a gear assembly, which is connected to a variable frequency drive (VFD: YASKAWA J1000, type: CIMR-JC 4A0011BAA). With the help of VFD, rotor can be rotated at different speeds of rotation. Rotor is solid cylinder which has indentations on its surface. There are a total of 204 indentations equidistant from each other. Each indentation is 12 mm in diameter, 20 mm deep. Schematic of the single indentation has been given in Fig. 2b, which also shows a likely liquid circulation pattern inside single indentation. Due to high speed of rotation of the rotor, very high surface velocities are created at the surface of the rotor. Liquid at this high velocity enters the indentation and when it comes out of it a low pressure region/vacuum gets created near the surface of the indentation. Surface velocity of the liquid is so high such that pressure near the surface falls below or equal to liquid vapor pressure, which is responsible for cavitation. Gap between stator and rotor is 1 cm, and it is fixed. There is no valve present in the line between tank, pump and cavitating device. Only a valve is present at the outlet of the cavitating rotor. Outlet and inlet flow rate is the same as there is no accumulation in the system. Outlet valve at the outlet of cavitating device is used to control the residence time of the liquid inside the cavitating device. 2.3. Experimental and analytical procedure Hydrodynamic cavitation based treatment of wastewater was carried out at different operating conditions by using actual wastewater sample collected from wood finishing industry. For all the experiments, fixed wastewater volume of 4 L and a circulation time through the device of 20–30 min was maintained. In all the cases, temperature was maintained around 20–25 °C by controlling the cooling water flow rate through the heat exchanger. Samples were
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
17
Fig. 1. Schematic of the hydrodynamic cavitation setup.
(a)
(b)
Fig. 2. (a) Schematic of the cavitating device (stator and rotor assembly). (b) Schematic of the single indentation, showing liquid flow pattern.
taken at regular intervals and were analyzed for COD of the wastewater. Chemical oxygen demand (COD) of the wastewater was measured by conventional dichromate method [18]. 3. Results and discussion All the experimental results have been obtained by using different operating parameters viz. rotation speed of the rotor, concentration and protocol for the addition of hydrogen peroxide and residence time of the wastewater in the cavitating device. Objective of all the experiments was to understand the effect of the different operating parameters and arrive at a set of parameters which will give maximum COD reduction of the wastewater. 3.1. Hydraulic characteristics The hydraulic characteristics of the cavitating device were studied initially by calculating velocity of the liquid on the surface of rotor and by quantifying a dimensionless parameter, cavitation number. Extent of cavitation taking place in any system is explained in terms of the cavitation number. Cavitation number is simply derived from Bernoulli’s theorem and can be expressed by the following equation [19]:
Cv ¼
p2 pv 1 2
qv 2
where, p2 is downstream pressure, pv is the vapor pressure of the liquid and v is the velocity at constriction where cavitation takes place. Under ideal conditions, when no cavitation takes place, the sum of velocity head is equal to pressure head. Liquid has a ten-
dency to flash into vapor when subjected to pressure less than or equal to its vapor pressure. When pressure of the liquid falls below the vapor pressure, cavitation occurs. In this case, sum of the pressure head is slightly less than velocity head, which means some of the energy is utilized for the generation of vapor phase. Thus, lower the cavitation number, higher is the energy utilized for the cavitation process and more is its intensity. So ideally cavities are generated at a condition Cv 6 1. But in some cases cavitation is observed at Cv P 1 due to the presence of some dissolved gases and suspended particles which are responsible for nuclei generation required for cavitation to get initiated. The condition at which first cavity is observed is called as the cavitation inception number. As cavitation number decreases, number density of cavities increases. More number of cavities are formed and the overall collapse intensity of cavities increases. But after certain value of cavitation number (dependent on the specific reactor configuration), the number density of cavities is very high, so these cavities start coalescing with each other and form a cavity cloud. Energy produced by the collapse of some cavities is taken up by the neighboring cavities and this condition of cavity cloud formation is called as choked cavitation [19]. Cavitation device must be operated between these two limits, i.e. cavitation inception and choked cavitation to get the maximum effect. In the present stator–rotor assembly, the maximum velocity will be observed at the surface of the rotor due to its rotation at higher speed (assuming no slip) and the diameter. Liquid velocity at the surface of the rotor (Vs) is calculated by considering radius of the rotor and angular velocity of rotation of the rotor (x). Cavitation number is calculated based upon this surface velocity. The velocity of liquid on the surface of the rotor was changed by changing rotation speed of rotor. Fig. 3 shows the effect of rotation speed
18
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
on cavitation number. It can be seen from the Fig. 3 that surface velocities created due to rotation of the rotor are sufficient for cavitation to occur. As the rotation speed of the rotor increases, cavitation number decreases. Sample calculations are shown in Appendix A. 3.2. Effect of rotation speed Effect of speed of rotation of the rotor on change in COD of wastewater sample has been investigated. Fig. 4 shows the effect of speed of rotation on COD of the wastewater sample. From the Fig. 4 it is evident that, a maximum reduction in the COD of the wastewater was obtained at 2200 RPM (approximately 49%). As the speed of rotation increases, the extent of reduction in COD also increases up to 2200 RPM, beyond which a further increase in the speed resulted in lesser reduction in COD (approximately 42% COD removal at 2700 RPM). The acceleration in the degradation of organic contaminants with an increase in the rotation speed of the rotor can be attributed to enhancement of hydroxyl radical generation due to the enhancement in intensity of cavitation or higher number of cavitation events. Cavitation bubble dynamics studies have indicated that increase in the cavitational intensity generated by the collapse of the cavity increases with an increase in the speed of rotation, as also justified by a decrease in the cavitation number values [20]. In other words, the local energy dissipation rate and the intensity of turbulence increase with an increase in the speed of rotation, thereby increasing the collapse intensity. Due to an increase in the collapse intensity, degradation rate of wastewater increases, thus more reduction in COD is observed. At a speed higher than 2200 RPM, degradation rate decreases. The observed results can be attributed to the fact that as the speed of rotation increases, the slip between the water layer and the rotor increases causing reduction in the extent of cavitation. As the speed of rotation increases, intensity of cavitation increases. At higher speeds very large numbers of cavities are generated. Such large numbers of cavities tend to dampen the collapse energy released by the neighboring cavities, which is described by a condition called as choked cavitation [19]. Saharan et al. [6] have done photographic analysis of cavitaing device and clearly established the conditions of cavitation inception and choked cavitation with a variation in the inlet pressure. Photographic as well as experimental study confirmed that the degradation rate of a dye increases with an increase in inlet the pressure up to certain value and after reaching the condition of choked cavitation, the degradation rate decreases. The obtained results relating to increased degradation with higher inlet pressures are consistent with other literature reports.
Chakinala et al. [21] performed the experiments with actual industrial wastewater and reported that the extent of degradation of wastewater increases with an increase in the inlet pressure of the system. Sivakumar and Pandit [14] and Vichare et al. [16] have also reported the existence of optimum inlet pressures to the cavitating device for the degradation of rhodamine B and potassium iodide respectively. 3.3. Effect of residence time As explained in the section of experimental methodology, the liquid wastewater is pumped into the cavitating device and hence depending on the speed of rotor, the liquid flow rate will be fixed (the flow characteristics are given in Fig. 3). Studies were undertaken to check if the residence time of the liquid inside the cavitating device has any effect on the cavitational degradation efficiency. The residence time inside the cavitating device is changed by changing the outlet flow rate of liquid through the cavitating device. Since the total volume of liquid in the cavitator (stator–rotor assembly) is constant, change in flow rate ultimately affects the residence time. Flow rate is adjusted by closing/opening of the valve present at the outlet of the cavitating device. In first case, outlet valve was fully opened, in second it was half open and in the last case it was 1/3rd open. All the experiments were carried out at an optimized rotor speed of 2200 RPM. Fig. 5 shows the effect of residence time on the change in COD of the wastewater sample. Residence time is expressed in terms of number of passes, which signifies the total number of times the entire liquid passes through the cavitating device during the total operation time. Sample calculation for the number of passes is shown in Appendix A. It is clear from the figure that when valve is half open (number of passes 195), change in COD is higher (56%) as compared to the other cases. The initial increase in the extent of degradation with an increase in the number of passes can be explained by the fact that as the residence time of the liquid inside the cavitating device increases, exposure time of pollutant molecule to collapsing cavities increases and hence enhanced degradation is observed in this case compared to when valve is 1/3rd open (number of passes = 160). At condition of number of passes = 160, only 37% reduction in COD was observed. In this case, where the outlet valve position was changed from half open to only 1/3rd open, it is also possible that the overall liquid flow rate decreases which makes it difficult to generate cavitation and hence overall cavitational events are likely to be less and hence the extent of COD removal decreases. Due to a decrease in the flow rate, entire cavitating device is filled with water, which increases the static pressure inside the
Fig. 3. Effect of rotation speed on surface velocities and cavitation number.
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
19
Fig. 4. Effect of rotation speed on reduction of COD of wastewater.
Fig. 5. Effect of residence time on reduction of COD of wastewater.
cavitating device, which results in incomplete or early collapse of the cavities. If cavity is not allowed to grow to its maximum size, energy released by collapse of such cavities will be less and may not be sufficient to bring about the mineralization of the wastewater. At condition of fully open valve (maximum number of passes), liquid is possibly exposed to too much cavitation events which reduces the degree of collapse (conditions similar to super cavitation where a cavity cloud is formed instead of single cavities in isolation). Similar results have also been reported by Ambulgekar et al. [22] with the experimental work on oxidation of toluene using aqueous potassium permanganate under heterogeneous condition in the presence of hydrodynamic cavitation. It has been reported that with an increase in the discharge pressure (by throttling the outlet valve) from pressure of 1 kg/cm2 to 3 kg/ cm2, the cavitational yield increases but beyond the optimum pressure of 3 kg/cm2 any further increase in the pressure results in reduced cavitational yields. 3.4. Effect of hydrogen peroxide concentration In hydrodynamic cavitation, free radicals are generated (OH) which are responsible for oxidation (degradation) of organic impurities present in the wastewater. Thus addition of oxidant like H2O2 can enhance the effect of degradation due to the formation of enhanced OH radicals due to the dissociation of hydrogen peroxide in the presence of cavitating conditions. In this study, combined ef-
fect of hydrodynamic cavitation with H2O2 has been studied. Effect of total H2O2 loading as well as stepwise addition of H2O2 has been investigated. All the experiments were performed at 2200 RPM and the outlet valve of the cavitating device was only half open. Experiments were done for 20 min and the samples were collected at regular intervals of time. The obtained results with different loadings of hydrogen peroxide have been shown in Fig. 6. It can be seen that with an increase in the concentration of H2O2 the reduction in the COD of the wastewater also increases till an optimum loading. Also, the maximum degradation in COD was obtained at a loading of H2O2 as 5 g/l (approximately 80%). However, at higher concentration the increase in the rate of degradation is only marginal (almost constant). This is consistent with the previous literature reports [6,21], which also indicates that at higher concentrations of H2O2 recombination reaction of OH radicals is predominant and also the scavenging of OH radicals by H2O2 takes place. It is also possible that at such high concentration of H2O2, intensity of cavitation is likely to be less because of the presence of vaporous cavities [23]. It is important to note that, optimum value of H2O2 concentration is dependent on the type of wastewater, operating conditions (pH, Inlet pressure, additives or catalysts) and its initial concentration. In the case of stepwise addition of H2O2 experiments were performed for 30 min. 5 g/l of H2O2 was added after 10 min and 20 min respectively. It was observed that when H2O2 is added after 10 min, rate of degradation of COD increases (almost 85% after 20 min). When H2O2 is further added after 20 min no
20
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
Fig. 6. Effect of H2O2 concentration on reduction of COD of wastewater.
significant change in COD was observed even after 30 min. It can be explained by the fact that after certain time, the intermediate oxidation products formed during the process are resistant to the hydroxyl radical attack and hence adding more of H2O2 will only result in scavenging of OH radicals produced by cavitation [6,21,24]. In a recent study by Teo et al. [25], it has been reported that initial rate of degradation of 0.4 mM p-chlorophenol in an aqueous solution increases with an increase in the hydrogen peroxide concentration. Rate of degradation is enhanced by 100% for an initial increase in the concentration of H2O2 to 20 mM. Further increase in the concentration of H2O2 to 40 mM has only a marginal effect on degradation and beyond this concentration, the rate actually decreased. It has been also observed that with only H2O2 without any cavitation no degradation was observed, which indicates that cavitation controls the overall degradation process and H2O2 acts as a catalyst for the formation of enhanced quantity of free radicals. Saharan et al. [6] have also reported similar observation for the degradation of reactive red 120 dye, which was found to increase with an increase in the concentration of H2O2 till an optimum concentration. Almost 100% decolorisation and 60% reduction in TOC was observed at optimum H2O2 concentration of 2.04 mM. 3.5. Cavitational yield calculations Cavitational yield is defined as the ratio of the observed cavitational effect to the energy supplied to the system [12,22,26].
Cavitational effect can be measured in terms of COD destruction in the present study. Thus, cavitational yield for the present case is expressed as,
Cavitational yield ¼
COD reductionðmgÞ total energy suppliedðJÞ
Fig. 7 shows the cavitational yield obtained at different speeds of rotation of the rotor. Total energy supplied to the system is the sum of pumping energy as well as energy required for driving the rotor (sample calculation is shown in Appendix A). It can be seen that cavitational yield decreases marginally up to 2200 RPM and after that it decreases rapidly. Cavitational yield at 1800 RPM is highest, but at 1800 RPM reduction in COD (only 42%) is much less as compared to the rotational speed of 2200 RPM (49%). After 2200 RPM cavitational yield decreases rapidly. It can be explained by the fact that at higher rotational speed, energy consumption is considerably higher but at the same time reduction is COD is much less (40% at 2700 RPM). It clearly indicates that the supplied increased energy is not necessarily translated into useful cavitational events to facilitate degradation. Addition of H2O2 enhances the rate of COD reduction significantly (from 49% to 89% at 2200 RPM). Cavitational yield at 2200 RPM when H2O2 is added at an optimum loading is 0.0084 mg of COD reduced/J of energy supplied (as against 0.0046 mg of COD reduced/J of energy in the absence of H2O2), which shows that the addition of H2O2 enhances the yield of cavitation by approximately 45%.
Fig. 7. Effect of speed of rotation on cavitational yield of the hydrodynamic cavitation treatment.
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
4. Conclusions In this work, hydrodynamic cavitation induced treatment of wastewater generated by wood finishing industry has been investigated. Effect of different operating parameters such as rotation speed, addition of H2O2 combined with hydrodynamic cavitation and the liquid phase residence time of the wastewater in the cavitating device on the COD reduction of the wastewater sample was established. Following important conclusions can be established from the present work: 1. The extent of COD removal of the wastewater was found to be dependent on the speed of rotor. As the speed increases, reduction of COD also increases till an optimum speed as 2200 RPM. After 2200 RPM, further increase in the speed results in a decrease in the extent of reduction possibly due to the slip conditions and occurrence of choked cavitation leading to reduced cavitational intensity. Hence it is recommended that the cavitation device should be operated at speeds lower than or equal to 2200 RPM for better results. 2. Addition of H2O2 enhances the COD removal due to generation of additional hydroxyl radicals available for the oxidation of wastewater. Also the extent of degradation increases with an increase in the H2O2 concentration till an optimum concentration of H2O2. 3. Increase in the residence time of the wastewater inside the cavitating device increases the rate of degradation up to a optimum value, beyond which any further increase in the residence time decreases the degradation due to a reduction in the cavitational activity.
Appendix A Sample calculation for surface velocity (Vs) of the liquid on the surface of rotor: Rotation speed = 2700 rpm. Vs = r x (r = radius of rotor, x = angular velocity). Vs = velocity of liquid at the surface of the rotor. x = 2 P n (n = rotations per second) x = 2 3.14 2700/60 = 282.74 rad/s Vs = (0.193/2) 282.74 (diameter of the rotor = 0.193 m) = 27.3 m/s
Sample calculation for cavitation number:
C VN ¼
p2 pv 1 2
qv 2
P2 is the downstream pressure = 101325 Pa (atmospheric pressure), Pv the vapor pressure of liquid (water) = 2350 Pa at 25 °C, V the velocity at indentation (surface velocity) = 27.3 m/s at 2700 RPM, q the density of water = 1000 kg/m3, CVN is the (101325– 2350)/0.5 1000 27.32 = 0.266. Sample calculation for number of passes: Number of passes = (volumetric flow rate/total volume) time of operation.
21
At 2200 RPM, outlet valve half open. Volumetric flow rate = 39 LPM. Total volume = 4 L. Total time = 20 min. Number of passes = (39/4) 20 = 195.
Sample calculation for cavitational yield: At 2700 RPM. Energy consumed by gear assembly for driving the rotor: 4.7 kW h. For 20 min (1200 s) operation energy consumed = 4.7 1200 = 5640 kJ. Power consumed by pump for the pumping of effluent over 20 min = 1 hp = 0.746 1200 = 895.2 kJ. Total energy consumed = 5640 kJ + 895.2 kJ = 6535.2 kJ. Total COD reduced in 20 min of operation at 2700 RPM = 16360 mg.
Cavitational yield ¼
amount of COD reductionðmgÞ total energy suppliedðJÞ
¼ 16360=6535200 ¼ 0:00253 mg=J References [1] F.N. Spon, Water Pollution Control – A Guide to the Use of Water Quality Management Principles, WHO, 1997. [2] S. Kim, J. Hazard. Mater. 176 (2010) 14–19. [3] S. Kim, Bioresour. Technol. 100 (2009) 744–748. [4] F. Werner, K. Richter, Int. J. LCA. 12 (2007) 470–479. [5] R. Dewil, J. Baeyens, R. Goutvrind, Environ. Prog. 25 (2006) 121–128. [6] V.K. Saharan, M.P. Badve, A.B. Pandit, Chem. Eng. J. 178 (2011) 100–107. [7] M.R. Hoffmann, I. Hua, R. Hochemer, Ultrason. Sonochem. 3 (1996) S163–S172. [8] M. Goel, H. Hongqiang, A.S. Mujumdar, M. Bhowmick Ray, Water Res. 38 (2004) 4247–4261. [9] H. Shemer, N. Narkis, Ultrason Sonochem. 12 (2005) 495–499. [10] T.J. Mason, Ultrason. Sonochem. 7 (2000) 145–149. [11] P.M. Kanthale, P.R. Gogate, A.B. Pandit, A.M. Wilhelm, Ultrason. Sonochem. 10 (2003) 331–335. [12] P.R. Gogate, I.Z. Shirgaonkar, M. Sivakumar, P. Senthilkumar, N.P. Vichare, A.B. Pandit, AIChE J. 47 (2001) 2526–2538. [13] P.R. Gogate, A.B. Pandit, Ultrason. Sonochem. 12 (2005) 21–27. [14] M. Sivakumar, A.B. Pandit, Ultrason. Sonochem. 9 (2002) 123–131. [15] K.K. Jyoti, A.B. Pandit, Biochem. Eng. J. 14 (2003) 9–17. [16] N.P. Vichare, P.R. Gogate, A.B. Pandit, Chem. Eng. Technol. 23 (2000) 683–690. [17] P. Senthil Kumar, M. Siva Kumar, A.B. Pandit, Chem. Eng. Sci. 55 (2000) 1633– 1639. [18] Guide manual: Water and Effluent Analysis, Central Pollution Control Board (GOI).. [19] S.S. Sawant, A.C. Anil, V. Krishnamurthy, C. Gaonkar, J. Kolwalkar, L. Khandeparker, D. Desai, A.V. Mahulkar, V.V. Ranade, A.B. Pandit, Biochem. Eng. J. 42 (2008) 320–328. [20] P.R. Gogate, A.B. Pandit, AIChE J. 46 (2000) 1641–1649. [21] A.G. Chakinala, P.R. Gogate, A.E. Burgess, D.H. Bremner, Ultrason. Sonochem. 15 (2008) 49–54. [22] G.V. Ambulgekar, S.D. Samant, A.B. Pandit, Ultrason. Sonochem. 11 (2004) 191–196. [23] J. Rae, M. Ashokkumar, O. Eulaerts, C. von Sonntag, J. Reisse, F. Grieser, Ultrason. Sonochem. 12 (2005) 325–329. [24] A.G. Chakinala, P.R. Gogate, A.E. Burgess, D.H. Bremner, Chem. Eng. J. 152 (2009) 498–502. [25] K.C. Teo, Y. Xu, C. Yang, Ultrason. Sonochem. 8 (2001) 241–246. [26] P.N. Patil, P.R. Gogate, Sep. Purif. Technol. 95 (2012) 172–179.
Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Hydrodynamic cavitation as a novel approach for wastewater treatment in wood finishing industry Mandar Badve a, Parag Gogate a, Aniruddha Pandit a, Levente Csoka b,⇑ a b
Department of Chemical Engineering, Institute of Chemical Technology, Mumbai 400 019, India University of West Hungary, Institute of Wood and Paper Technology, 9400 Sopron, Hungary
a r t i c l e
i n f o
Article history: Received 26 October 2012 Received in revised form 22 December 2012 Accepted 31 December 2012 Available online 7 January 2013 Keywords: Hydrodynamic cavitation Wood finishing industry Cavitational yield COD reduction Hydroxyl radicals
a b s t r a c t In the present work, hydrodynamic cavitation has been evaluated as a novel treatment scheme for wastewater generated from wood finishing industry. The earlier work on hydrodynamic cavitation has been mostly with synthetically prepared pollutants (typically less volatile compounds) and the current work is the first to report a treatment strategy for the real wastewater generated by wood finishing industry, which contains high concentration of volatile organic compounds. Hydrodynamic cavitation reactor is basically a stator and rotor assembly, where the rotor is provided with some indentations and cavitation events are expected on the surface of rotor as well as within the indentations. The effect of various operating parameters such as speed of rotor, loading of H2O2 and the residence time of wastewater in the cavitating device, on the extent of COD reduction of the wastewater have been investigated. It has been observed that the rate of degradation of wastewater is dependent on the speed of rotor, H2O2 concentration and the liquid phase residence time. Addition of H2O2 resulted in enhanced degradation till an optimum concentration which is dependent on the type of wastewater stream. It has been observed that the cavitational yield increases by 46% at optimum loading of H2O2. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction During recent years, there has been increasing awareness and concern about water pollution caused by industrial wastewater all over the world. It has been globally accepted that effective utilization of available water resources is a need of the time. Treatment of polluted wastewater is a global concern since wastewater collected from various resources such as municipalities, communities and industries must ultimately be returned to the receiving water bodies or to the land. Prevention of pollution at source and prior licensing of wastewater discharges by competent authorities have become key elements of successful policies for preventing, controlling and reducing discharge of hazardous substances, nutrients and other water pollutants from sources into aquatic ecosystems [1]. Wood finishing requires a combination of several operations such as sanding, scraping and planning, which are used to obtain good surface of the wood for further processing. Once the surface is prepared, various quotes of finishes are applied. Commonly used finishes are lacquers, varnish and various paints. Various adhesives are used to bind the pieces of wood together. Lacquers and varnishes are used to make wood surface glossy. Anti-bacterial paints
⇑ Corresponding author. Tel.: +36 99 518 305; fax: +36 99 518 302. E-mail address: [email protected] (L. Csoka). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.12.029
are used to protect wood from any bacterial degradation. UV stabilizers and preservatives are used to protect the wood from sunlight. Wastewater from wood finishing industry is typically generated from such processes and process equipment cleaning. The wood finishing process is a significant source of volatile organic compounds (VOCs) and other hazardous pollutants. In order to reduce the VOC emission, sometimes low VOC finishes are used. A lot of research has been done to reduce air pollution caused by wood finishing industry by minimizing the VOCs as well as hazardous air pollutants (HAPs) emission [2–4]. To reduce the VOCs and HAPs emissions, water based lacquers, varnishes and adhesives are also being commonly used as alternatives. Use of water based chemicals certainly reduces the air pollution caused by organic solvents but at the same time the issue of water pollution arising due to the use of water based additives is still a concern. No significant literature has been found, to be best of our knowledge, on a treatment strategy developed for wastewater generated specifically by the wood finishing industry. The present work proposes a treatment methodology for the wastewater generated by wood finishing industry based on the use of hydrodynamic cavitation reactors either operated individually or in combination with hydrogen peroxide. Cavitation is a phenomenon of nucleation, growth and implosion of vapor or gas filled cavities, which can be achieved by the passage of ultrasound (acoustic cavitation) or by alterations in
16
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
the flow and pressure (hydrodynamic cavitation). In the case of hydrodynamic cavitation, flow geometry is altered in such a way that the kinetic energy is increased by having a flow constriction which results into a reduction in the local pressure of the liquid considerably with a corresponding increase in the kinetic energy. When the pressure of the liquid falls below the vapor pressure of the same liquid, millions of vapor cavities are created, which are subjected to turbulent conditions of varying pressure fields downstream of the constriction. Life time of these cavities is very small (few micro seconds). The cavities finally implode violently and result in generation of very high pressures (up to 1000 bar) and temperatures (10,000 K) intensifying the chemical reactions and/or promoting radical formation and its subsequent reactions [5]. Extreme shear forces are generated which coupled with shock waves generated by cavitation events helps in breaking down of the pollutant molecules especially complex large molecular weight compounds. The intermediate compounds are more prone to OH radicals attack and biological oxidation, which further enhances the overall rate of degradation/mineralization of the wastewater. Under such extreme conditions water molecule within the cavity dissociates into OH and H radicals. OH radicals diffuse into the liquid and react with pollutant molecules resulting into oxidation/ mineralization [6]. In the past, ultrasonic cavitation has been used most commonly for the degradation of various organic pollutants though generally at a laboratory scale operation [7–9]. One of the major drawbacks of ultrasonic reactors is the energy intensive operation due to much lower energy transfer efficiencies (10–30%) and also the active cavitation zone is restricted to near the transducers which is quite low as compared to the total reactor volume and. Also, scale up of such reactors is very difficult due to local nature of the cavitational activity very close to the transducer area [10,11]. On the other hand, hydrodynamic cavitation reactors are relatively easy to scale up and are energy efficient compared to ultrasonic reactors [12,13]. It has been reported that energy transfer efficiency of hydrodynamic cavitation reactors is about 60–70% depending on the operating conditions whereas for ultrasonic reactors it is only about 10–40%. Sivakumar and Pandit [14] have studied the application of hydrodynamic cavitation for the degradation of rhodamine B dye using multiple hole orifice plates and reported that hydrodynamic cavitation devices are more energy efficient than acoustic cavitation devices. Quantitatively, the energy efficiency of hydrodynamic cavitation (multiple hole orifice plate) was two times greater than the acoustic cavitation device (dual frequency flow cell with a capacity of 1.5 L). Jyoti and Pandit [15] studied disinfection of water using different techniques and have reported that hydrodynamic cavitation is an economically attractive alternative compared to techniques such as ozonation and heat sterilization for reducing the bacterial counts in the potable water. In addition, few other papers have also discussed the applications of hydrodynamic cavitation [6,16,17] for KI oxidation. In hydrodynamic cavitation, generally venturi or orifice plates have been used in the past to create cavitation in liquid under circulation. In addition the high pressure and high speed homogenizers also generate cavitating conditions. In the present work, a different type of hydrodynamic cavitating device comprising of a stator and rotor assembly has been used to create the same effect. A rotor rotates at very high speed in a confined annular space and liquid is passed through the gap between the stator and the rotor. Due to high speed of rotation, very high surface velocities are generated. Some indentations are also provided on the surface of rotor. Liquid at such velocities enters the indentation, due to rotary action of the rotor and when liquid comes out of the indentation due to centrifugal flow, a low pressure region/vacuum is created near the upper surface of the indentations resulting into cavitation. Pressure drop across the component is main driving force for cav-
itation. At such high surface velocities of the liquid on the surface of the rotor, liquid pressure drop across the surface of the rotor and indentation is sufficient enough for cavitation to occur. 2. Materials and methods 2.1. Wastewater The wastewater was collected from the actual wood finishing industry (source not revealed due to confidentiality agreement). Experiments were performed to study the efficacy of hydrodynamic cavitation for the reduction in the chemical oxygen demand (COD) of the wastewater sample. COD indicates the concentration of organic compounds which can be oxidized chemically by using strong oxidizing agents. Majority of earlier wastewater treatment studies have been performed with simulated wastewater whereas the current study deals with an actual industry wastewater and hence carries lot of significance in the context of the actual industrial wastewater treatment using hydrodynamic cavitation. Wastewater used in the present study consists of a complex mixture of adhesives, UV stabilizers, lacquers, paints, varnishes. Initial pH of the wastewater was 6.18 and initial COD of the wastewater stream was 38,000 mg/l. 2.2. Experimental setup Fig. 1 gives the schematic of the experimental setup, which basically contains a storage tank for liquid of maximum capacity 15 L. Liquid from the tank is pumped into the cavitating device by using an open impeller type pump with 3 blades and flow capacity of 5–15 L/min with a pressure head of 4.5 bar. Heat exchanger was added between the cavitating device and storage tank to cool the liquid coming out of the cavitating device. Fig. 2a shows schematic details of the actual cavitating device, a stator and rotor assembly. Rotor is attached to a gear assembly, which is connected to a variable frequency drive (VFD: YASKAWA J1000, type: CIMR-JC 4A0011BAA). With the help of VFD, rotor can be rotated at different speeds of rotation. Rotor is solid cylinder which has indentations on its surface. There are a total of 204 indentations equidistant from each other. Each indentation is 12 mm in diameter, 20 mm deep. Schematic of the single indentation has been given in Fig. 2b, which also shows a likely liquid circulation pattern inside single indentation. Due to high speed of rotation of the rotor, very high surface velocities are created at the surface of the rotor. Liquid at this high velocity enters the indentation and when it comes out of it a low pressure region/vacuum gets created near the surface of the indentation. Surface velocity of the liquid is so high such that pressure near the surface falls below or equal to liquid vapor pressure, which is responsible for cavitation. Gap between stator and rotor is 1 cm, and it is fixed. There is no valve present in the line between tank, pump and cavitating device. Only a valve is present at the outlet of the cavitating rotor. Outlet and inlet flow rate is the same as there is no accumulation in the system. Outlet valve at the outlet of cavitating device is used to control the residence time of the liquid inside the cavitating device. 2.3. Experimental and analytical procedure Hydrodynamic cavitation based treatment of wastewater was carried out at different operating conditions by using actual wastewater sample collected from wood finishing industry. For all the experiments, fixed wastewater volume of 4 L and a circulation time through the device of 20–30 min was maintained. In all the cases, temperature was maintained around 20–25 °C by controlling the cooling water flow rate through the heat exchanger. Samples were
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
17
Fig. 1. Schematic of the hydrodynamic cavitation setup.
(a)
(b)
Fig. 2. (a) Schematic of the cavitating device (stator and rotor assembly). (b) Schematic of the single indentation, showing liquid flow pattern.
taken at regular intervals and were analyzed for COD of the wastewater. Chemical oxygen demand (COD) of the wastewater was measured by conventional dichromate method [18]. 3. Results and discussion All the experimental results have been obtained by using different operating parameters viz. rotation speed of the rotor, concentration and protocol for the addition of hydrogen peroxide and residence time of the wastewater in the cavitating device. Objective of all the experiments was to understand the effect of the different operating parameters and arrive at a set of parameters which will give maximum COD reduction of the wastewater. 3.1. Hydraulic characteristics The hydraulic characteristics of the cavitating device were studied initially by calculating velocity of the liquid on the surface of rotor and by quantifying a dimensionless parameter, cavitation number. Extent of cavitation taking place in any system is explained in terms of the cavitation number. Cavitation number is simply derived from Bernoulli’s theorem and can be expressed by the following equation [19]:
Cv ¼
p2 pv 1 2
qv 2
where, p2 is downstream pressure, pv is the vapor pressure of the liquid and v is the velocity at constriction where cavitation takes place. Under ideal conditions, when no cavitation takes place, the sum of velocity head is equal to pressure head. Liquid has a ten-
dency to flash into vapor when subjected to pressure less than or equal to its vapor pressure. When pressure of the liquid falls below the vapor pressure, cavitation occurs. In this case, sum of the pressure head is slightly less than velocity head, which means some of the energy is utilized for the generation of vapor phase. Thus, lower the cavitation number, higher is the energy utilized for the cavitation process and more is its intensity. So ideally cavities are generated at a condition Cv 6 1. But in some cases cavitation is observed at Cv P 1 due to the presence of some dissolved gases and suspended particles which are responsible for nuclei generation required for cavitation to get initiated. The condition at which first cavity is observed is called as the cavitation inception number. As cavitation number decreases, number density of cavities increases. More number of cavities are formed and the overall collapse intensity of cavities increases. But after certain value of cavitation number (dependent on the specific reactor configuration), the number density of cavities is very high, so these cavities start coalescing with each other and form a cavity cloud. Energy produced by the collapse of some cavities is taken up by the neighboring cavities and this condition of cavity cloud formation is called as choked cavitation [19]. Cavitation device must be operated between these two limits, i.e. cavitation inception and choked cavitation to get the maximum effect. In the present stator–rotor assembly, the maximum velocity will be observed at the surface of the rotor due to its rotation at higher speed (assuming no slip) and the diameter. Liquid velocity at the surface of the rotor (Vs) is calculated by considering radius of the rotor and angular velocity of rotation of the rotor (x). Cavitation number is calculated based upon this surface velocity. The velocity of liquid on the surface of the rotor was changed by changing rotation speed of rotor. Fig. 3 shows the effect of rotation speed
18
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
on cavitation number. It can be seen from the Fig. 3 that surface velocities created due to rotation of the rotor are sufficient for cavitation to occur. As the rotation speed of the rotor increases, cavitation number decreases. Sample calculations are shown in Appendix A. 3.2. Effect of rotation speed Effect of speed of rotation of the rotor on change in COD of wastewater sample has been investigated. Fig. 4 shows the effect of speed of rotation on COD of the wastewater sample. From the Fig. 4 it is evident that, a maximum reduction in the COD of the wastewater was obtained at 2200 RPM (approximately 49%). As the speed of rotation increases, the extent of reduction in COD also increases up to 2200 RPM, beyond which a further increase in the speed resulted in lesser reduction in COD (approximately 42% COD removal at 2700 RPM). The acceleration in the degradation of organic contaminants with an increase in the rotation speed of the rotor can be attributed to enhancement of hydroxyl radical generation due to the enhancement in intensity of cavitation or higher number of cavitation events. Cavitation bubble dynamics studies have indicated that increase in the cavitational intensity generated by the collapse of the cavity increases with an increase in the speed of rotation, as also justified by a decrease in the cavitation number values [20]. In other words, the local energy dissipation rate and the intensity of turbulence increase with an increase in the speed of rotation, thereby increasing the collapse intensity. Due to an increase in the collapse intensity, degradation rate of wastewater increases, thus more reduction in COD is observed. At a speed higher than 2200 RPM, degradation rate decreases. The observed results can be attributed to the fact that as the speed of rotation increases, the slip between the water layer and the rotor increases causing reduction in the extent of cavitation. As the speed of rotation increases, intensity of cavitation increases. At higher speeds very large numbers of cavities are generated. Such large numbers of cavities tend to dampen the collapse energy released by the neighboring cavities, which is described by a condition called as choked cavitation [19]. Saharan et al. [6] have done photographic analysis of cavitaing device and clearly established the conditions of cavitation inception and choked cavitation with a variation in the inlet pressure. Photographic as well as experimental study confirmed that the degradation rate of a dye increases with an increase in inlet the pressure up to certain value and after reaching the condition of choked cavitation, the degradation rate decreases. The obtained results relating to increased degradation with higher inlet pressures are consistent with other literature reports.
Chakinala et al. [21] performed the experiments with actual industrial wastewater and reported that the extent of degradation of wastewater increases with an increase in the inlet pressure of the system. Sivakumar and Pandit [14] and Vichare et al. [16] have also reported the existence of optimum inlet pressures to the cavitating device for the degradation of rhodamine B and potassium iodide respectively. 3.3. Effect of residence time As explained in the section of experimental methodology, the liquid wastewater is pumped into the cavitating device and hence depending on the speed of rotor, the liquid flow rate will be fixed (the flow characteristics are given in Fig. 3). Studies were undertaken to check if the residence time of the liquid inside the cavitating device has any effect on the cavitational degradation efficiency. The residence time inside the cavitating device is changed by changing the outlet flow rate of liquid through the cavitating device. Since the total volume of liquid in the cavitator (stator–rotor assembly) is constant, change in flow rate ultimately affects the residence time. Flow rate is adjusted by closing/opening of the valve present at the outlet of the cavitating device. In first case, outlet valve was fully opened, in second it was half open and in the last case it was 1/3rd open. All the experiments were carried out at an optimized rotor speed of 2200 RPM. Fig. 5 shows the effect of residence time on the change in COD of the wastewater sample. Residence time is expressed in terms of number of passes, which signifies the total number of times the entire liquid passes through the cavitating device during the total operation time. Sample calculation for the number of passes is shown in Appendix A. It is clear from the figure that when valve is half open (number of passes 195), change in COD is higher (56%) as compared to the other cases. The initial increase in the extent of degradation with an increase in the number of passes can be explained by the fact that as the residence time of the liquid inside the cavitating device increases, exposure time of pollutant molecule to collapsing cavities increases and hence enhanced degradation is observed in this case compared to when valve is 1/3rd open (number of passes = 160). At condition of number of passes = 160, only 37% reduction in COD was observed. In this case, where the outlet valve position was changed from half open to only 1/3rd open, it is also possible that the overall liquid flow rate decreases which makes it difficult to generate cavitation and hence overall cavitational events are likely to be less and hence the extent of COD removal decreases. Due to a decrease in the flow rate, entire cavitating device is filled with water, which increases the static pressure inside the
Fig. 3. Effect of rotation speed on surface velocities and cavitation number.
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
19
Fig. 4. Effect of rotation speed on reduction of COD of wastewater.
Fig. 5. Effect of residence time on reduction of COD of wastewater.
cavitating device, which results in incomplete or early collapse of the cavities. If cavity is not allowed to grow to its maximum size, energy released by collapse of such cavities will be less and may not be sufficient to bring about the mineralization of the wastewater. At condition of fully open valve (maximum number of passes), liquid is possibly exposed to too much cavitation events which reduces the degree of collapse (conditions similar to super cavitation where a cavity cloud is formed instead of single cavities in isolation). Similar results have also been reported by Ambulgekar et al. [22] with the experimental work on oxidation of toluene using aqueous potassium permanganate under heterogeneous condition in the presence of hydrodynamic cavitation. It has been reported that with an increase in the discharge pressure (by throttling the outlet valve) from pressure of 1 kg/cm2 to 3 kg/ cm2, the cavitational yield increases but beyond the optimum pressure of 3 kg/cm2 any further increase in the pressure results in reduced cavitational yields. 3.4. Effect of hydrogen peroxide concentration In hydrodynamic cavitation, free radicals are generated (OH) which are responsible for oxidation (degradation) of organic impurities present in the wastewater. Thus addition of oxidant like H2O2 can enhance the effect of degradation due to the formation of enhanced OH radicals due to the dissociation of hydrogen peroxide in the presence of cavitating conditions. In this study, combined ef-
fect of hydrodynamic cavitation with H2O2 has been studied. Effect of total H2O2 loading as well as stepwise addition of H2O2 has been investigated. All the experiments were performed at 2200 RPM and the outlet valve of the cavitating device was only half open. Experiments were done for 20 min and the samples were collected at regular intervals of time. The obtained results with different loadings of hydrogen peroxide have been shown in Fig. 6. It can be seen that with an increase in the concentration of H2O2 the reduction in the COD of the wastewater also increases till an optimum loading. Also, the maximum degradation in COD was obtained at a loading of H2O2 as 5 g/l (approximately 80%). However, at higher concentration the increase in the rate of degradation is only marginal (almost constant). This is consistent with the previous literature reports [6,21], which also indicates that at higher concentrations of H2O2 recombination reaction of OH radicals is predominant and also the scavenging of OH radicals by H2O2 takes place. It is also possible that at such high concentration of H2O2, intensity of cavitation is likely to be less because of the presence of vaporous cavities [23]. It is important to note that, optimum value of H2O2 concentration is dependent on the type of wastewater, operating conditions (pH, Inlet pressure, additives or catalysts) and its initial concentration. In the case of stepwise addition of H2O2 experiments were performed for 30 min. 5 g/l of H2O2 was added after 10 min and 20 min respectively. It was observed that when H2O2 is added after 10 min, rate of degradation of COD increases (almost 85% after 20 min). When H2O2 is further added after 20 min no
20
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
Fig. 6. Effect of H2O2 concentration on reduction of COD of wastewater.
significant change in COD was observed even after 30 min. It can be explained by the fact that after certain time, the intermediate oxidation products formed during the process are resistant to the hydroxyl radical attack and hence adding more of H2O2 will only result in scavenging of OH radicals produced by cavitation [6,21,24]. In a recent study by Teo et al. [25], it has been reported that initial rate of degradation of 0.4 mM p-chlorophenol in an aqueous solution increases with an increase in the hydrogen peroxide concentration. Rate of degradation is enhanced by 100% for an initial increase in the concentration of H2O2 to 20 mM. Further increase in the concentration of H2O2 to 40 mM has only a marginal effect on degradation and beyond this concentration, the rate actually decreased. It has been also observed that with only H2O2 without any cavitation no degradation was observed, which indicates that cavitation controls the overall degradation process and H2O2 acts as a catalyst for the formation of enhanced quantity of free radicals. Saharan et al. [6] have also reported similar observation for the degradation of reactive red 120 dye, which was found to increase with an increase in the concentration of H2O2 till an optimum concentration. Almost 100% decolorisation and 60% reduction in TOC was observed at optimum H2O2 concentration of 2.04 mM. 3.5. Cavitational yield calculations Cavitational yield is defined as the ratio of the observed cavitational effect to the energy supplied to the system [12,22,26].
Cavitational effect can be measured in terms of COD destruction in the present study. Thus, cavitational yield for the present case is expressed as,
Cavitational yield ¼
COD reductionðmgÞ total energy suppliedðJÞ
Fig. 7 shows the cavitational yield obtained at different speeds of rotation of the rotor. Total energy supplied to the system is the sum of pumping energy as well as energy required for driving the rotor (sample calculation is shown in Appendix A). It can be seen that cavitational yield decreases marginally up to 2200 RPM and after that it decreases rapidly. Cavitational yield at 1800 RPM is highest, but at 1800 RPM reduction in COD (only 42%) is much less as compared to the rotational speed of 2200 RPM (49%). After 2200 RPM cavitational yield decreases rapidly. It can be explained by the fact that at higher rotational speed, energy consumption is considerably higher but at the same time reduction is COD is much less (40% at 2700 RPM). It clearly indicates that the supplied increased energy is not necessarily translated into useful cavitational events to facilitate degradation. Addition of H2O2 enhances the rate of COD reduction significantly (from 49% to 89% at 2200 RPM). Cavitational yield at 2200 RPM when H2O2 is added at an optimum loading is 0.0084 mg of COD reduced/J of energy supplied (as against 0.0046 mg of COD reduced/J of energy in the absence of H2O2), which shows that the addition of H2O2 enhances the yield of cavitation by approximately 45%.
Fig. 7. Effect of speed of rotation on cavitational yield of the hydrodynamic cavitation treatment.
M. Badve et al. / Separation and Purification Technology 106 (2013) 15–21
4. Conclusions In this work, hydrodynamic cavitation induced treatment of wastewater generated by wood finishing industry has been investigated. Effect of different operating parameters such as rotation speed, addition of H2O2 combined with hydrodynamic cavitation and the liquid phase residence time of the wastewater in the cavitating device on the COD reduction of the wastewater sample was established. Following important conclusions can be established from the present work: 1. The extent of COD removal of the wastewater was found to be dependent on the speed of rotor. As the speed increases, reduction of COD also increases till an optimum speed as 2200 RPM. After 2200 RPM, further increase in the speed results in a decrease in the extent of reduction possibly due to the slip conditions and occurrence of choked cavitation leading to reduced cavitational intensity. Hence it is recommended that the cavitation device should be operated at speeds lower than or equal to 2200 RPM for better results. 2. Addition of H2O2 enhances the COD removal due to generation of additional hydroxyl radicals available for the oxidation of wastewater. Also the extent of degradation increases with an increase in the H2O2 concentration till an optimum concentration of H2O2. 3. Increase in the residence time of the wastewater inside the cavitating device increases the rate of degradation up to a optimum value, beyond which any further increase in the residence time decreases the degradation due to a reduction in the cavitational activity.
Appendix A Sample calculation for surface velocity (Vs) of the liquid on the surface of rotor: Rotation speed = 2700 rpm. Vs = r x (r = radius of rotor, x = angular velocity). Vs = velocity of liquid at the surface of the rotor. x = 2 P n (n = rotations per second) x = 2 3.14 2700/60 = 282.74 rad/s Vs = (0.193/2) 282.74 (diameter of the rotor = 0.193 m) = 27.3 m/s
Sample calculation for cavitation number:
C VN ¼
p2 pv 1 2
qv 2
P2 is the downstream pressure = 101325 Pa (atmospheric pressure), Pv the vapor pressure of liquid (water) = 2350 Pa at 25 °C, V the velocity at indentation (surface velocity) = 27.3 m/s at 2700 RPM, q the density of water = 1000 kg/m3, CVN is the (101325– 2350)/0.5 1000 27.32 = 0.266. Sample calculation for number of passes: Number of passes = (volumetric flow rate/total volume) time of operation.
21
At 2200 RPM, outlet valve half open. Volumetric flow rate = 39 LPM. Total volume = 4 L. Total time = 20 min. Number of passes = (39/4) 20 = 195.
Sample calculation for cavitational yield: At 2700 RPM. Energy consumed by gear assembly for driving the rotor: 4.7 kW h. For 20 min (1200 s) operation energy consumed = 4.7 1200 = 5640 kJ. Power consumed by pump for the pumping of effluent over 20 min = 1 hp = 0.746 1200 = 895.2 kJ. Total energy consumed = 5640 kJ + 895.2 kJ = 6535.2 kJ. Total COD reduced in 20 min of operation at 2700 RPM = 16360 mg.
Cavitational yield ¼
amount of COD reductionðmgÞ total energy suppliedðJÞ
¼ 16360=6535200 ¼ 0:00253 mg=J References [1] F.N. Spon, Water Pollution Control – A Guide to the Use of Water Quality Management Principles, WHO, 1997. [2] S. Kim, J. Hazard. Mater. 176 (2010) 14–19. [3] S. Kim, Bioresour. Technol. 100 (2009) 744–748. [4] F. Werner, K. Richter, Int. J. LCA. 12 (2007) 470–479. [5] R. Dewil, J. Baeyens, R. Goutvrind, Environ. Prog. 25 (2006) 121–128. [6] V.K. Saharan, M.P. Badve, A.B. Pandit, Chem. Eng. J. 178 (2011) 100–107. [7] M.R. Hoffmann, I. Hua, R. Hochemer, Ultrason. Sonochem. 3 (1996) S163–S172. [8] M. Goel, H. Hongqiang, A.S. Mujumdar, M. Bhowmick Ray, Water Res. 38 (2004) 4247–4261. [9] H. Shemer, N. Narkis, Ultrason Sonochem. 12 (2005) 495–499. [10] T.J. Mason, Ultrason. Sonochem. 7 (2000) 145–149. [11] P.M. Kanthale, P.R. Gogate, A.B. Pandit, A.M. Wilhelm, Ultrason. Sonochem. 10 (2003) 331–335. [12] P.R. Gogate, I.Z. Shirgaonkar, M. Sivakumar, P. Senthilkumar, N.P. Vichare, A.B. Pandit, AIChE J. 47 (2001) 2526–2538. [13] P.R. Gogate, A.B. Pandit, Ultrason. Sonochem. 12 (2005) 21–27. [14] M. Sivakumar, A.B. Pandit, Ultrason. Sonochem. 9 (2002) 123–131. [15] K.K. Jyoti, A.B. Pandit, Biochem. Eng. J. 14 (2003) 9–17. [16] N.P. Vichare, P.R. Gogate, A.B. Pandit, Chem. Eng. Technol. 23 (2000) 683–690. [17] P. Senthil Kumar, M. Siva Kumar, A.B. Pandit, Chem. Eng. Sci. 55 (2000) 1633– 1639. [18] Guide manual: Water and Effluent Analysis, Central Pollution Control Board (GOI).