Separation and Purification Technology 25 (2001) 341– 346www.elsevier.com/locate/seppur Ozone–water contacting by ceramic membranes P. Janknecht a,*, P.A. Wilderer a, C. Picard b, A. Larbot b a b Technische Uni6ersita¨t Mu¨nchen Am Coulombwall, 85748 Garching, Germany Institut Europe´en des Membranes, CNRS, 1919 route de Monde, Montpellier, F-34293, France Abstract A common process in water treatment is the wet oxidation for the removal of certain organic and inorganic pollutants. The strongest oxidant technically applied in this process is ozone, which is an unstable gas under normal conditions, and therefore is produced from oxygen on site, usually by electrical discharge. After that the ozone has to be transferred from that gas into the water to be treated. Conventionally ozone transfer is achieved by bringing the gas and water in direct contact by means of bubble columns, injectors or other similar devices. Under unfavorable conditions, however, these methods suffer from excessive formation of foam requiring an extra treatment and a high-energy demand for pumping gas or water. This project’s approach was to improve the transfer by better control of gaseous and aqueous phase’s conditions at the contact surface. This was achieved by means of a membrane both separating the two phases and allowing for an ozone transfer between them. Due to ozone’s high oxidation potential, chemically inert ceramic membranes were chosen for that purpose. In experiments, it was found that the transfer of the unstable ozone molecules is not obstructed by ceramic membrane material. Transfer rates between gaseous ozone and model water were measured for conventional ceramic membranes, as well as specially designed ones. They are comparable to conventional methods or better on the base of mass transfer per reactor volume. In conventional oxide membranes, water enters the pores because of capillary effects in the hydrophilic material [Burggraaf, A.J. and Cot, L., 1996, Fundamentals of inorganic Membrane Science and Technology Elsevier Science, The Netherlands]. The water in the pores raises the diffusion resistance for the ozone thus decreasing the transfer itself. Consequently, the modification of the hydrophilic material features into a hydrophobic behavior was one promising approach for the optimization of the process. It was achieved through the application of a hydrophobic coating to the membrane surface, which greatly improved the transfer efficiency. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ceramic membranes; Ozonation; Gas contacting; Gas transfer; Mass transfer 1. Introduction Common membrane processes are characterized by the feature of the utilized membrane to * Corresponding author. Tel.: + 49-89-28913717; fax: + 4989-28913718. E-mail address:
[email protected] (P. Janknecht). hold back certain components of a fluid mixture, while allowing others to pass through. In the most common case of membrane filtration, the permeating component is the solvent of the mixture, whereas the non-permeating components are pollutants that desirably should be removed from it. Here the media that is to be purified is forced through the membrane by application of pressure 1383-5866/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 1 ) 0 0 0 6 1 - 2 since the ozone will be readily consumed in the water. and others. injectors. by reacting with macromolecular organic pollutants ozone can improve flocculation and successive removal of these substances. a similar variation of this process. The novelty is in the fact that ozone instead of oxygen is transferred and the challenge is formed by the peculiar features of this substance. or by additional agitation of the liquid performed by mechanical stirring that also keeps the bubbles from coalescing. This instability accounts for two peculiarities that make technical ozone applications special: first ozone can not be stored or transported like other gases. In high doses ozone can even affect a complete wet oxidation in which all present organic matter is wholly mineralized. the delivering fluid is air and the permeation of oxygen through the membrane to the liquid fluid on the other side constitutes the aim of the process. Ozone in water treatment Ozone is one of the strongest oxidants technically applied. While this concept has been utilized and optimized for millions of years biologically and also technically ([1]). it can only be produced at the place and the time of consumption from oxygen or oxygen containing gases like air. Conventional ozone contacting methods include bubble columns. Thus. either as ultraviolet radiation or as an electrical discharge. since when filled into containers it continuously decays until only oxygen is left. / Separation/Purification Technology 25 (2001) 341–346 to the mixture. the second peculiarity of ozone in technical application is the fact that it cannot be applied as a pure gas. In other cases the purification process involves the retaining of the fluid to be purified. which is the subject of this work is much newer and still worked on. The production consists of applying energy to molecular oxygen (O2). Therefore. The medical application of membrane dialysis represents this kind. but similar ones occur manifoldly in technical and natural processes. . Respiration can also be regarded as a membrane process. Janknecht et al. impellers. The aims of ozonation in water treatment include the removal of color and smell and the disinfecting of biologically polluted water in order to destroy or inactivate microorganisms and even viruses. if ozone is to be applied to water it needs to be transferred from the gas phase mixture into the liquid phase. through which by splitting and recombination three oxygen molecules can be transformed into two ozone molecules. It can also increase the biological accessibility of organic pollutants when toxic substances are present. In chemical view the ozone molecule (O3) consists of three oxygen atoms and is considerably unstable. This process. The necessary condition for that is a surface between the two phases to enable the phase transfer. which otherwise inhibit the activity of microorganisms ([2]). creating the necessary drop in partial pressure from the gas to the liquid phase. 2. thus overlaying a fast convectional transport to the slow diffusion. It is utilized in a variety of chemical processes. however. however. while the undesired pollutants pass through the membrane and thus can be removed. Transport within the two phases generally can be accomplished by diffusion. due to its high ozone demand usually is limited to applications with low water volumes and difficult pollutants. however. is subject to a thermodynamical equilibrium that only allows for a comparatively small portion of the oxygen to be transformed to ozone.342 P. usually prepared by the formation of gas bubbles within the water. This process. Since the diffusion coefficient of ozone in water is relatively low. as well as in the treatment of both drinking and wastewater. The shear forces in the water boundary layer are engendered either within the resting liquid by the floating of the bubbles as in bubble columns. Furthermore. the transfer performance can be and in technical application often is considerably increased by mechanical agitation creating shear forces on the liquid boundary layer close to the surface. The driving force in these processes is a gradient in partial pressure of the respective substance from the mixture side of the membrane to the receiving side. but is always mixed with oxygen and possibly other gases that usually make up more than 90% by mass of the mixture. Thus. The water was circulated within PVC tubing by means of a radial 343 Fig. Flow scheme of the experimental setup. The generation and continuation of bubbles constantly consumes energy. respectively.7 nm (ANSEROS Model GM-6000-OEM). 5 or 7 mm inner diameter. After a series of tests with different techniques ([4]). the ozone transfer was limited and demanded for an especially adapted method. and a stainless steel metal bellows compressor. The membrane type was a single channel tubular ceramic membrane with fine layer on the inner surface. Ceramic membranes were selected for that purpose. while the ozone containing gas was applied to the space formed by the outer membrane surface and an enveloping glass tube. and a length of 200 or 400 mm. borosilicate glass. Ozone measurement and regulation in the gas phase were carried out in a side stream by direct UV absorption at a wavelength of 253. a process developed by Hoigne´ and Bader and described by Langlais [5] and in the German Standard Method DIN 38 408-G3-3 was successfully modified and adopted — a model water with a defined concentration of dissolved indigo trisulfonate (a strong blue dye) is prepared to serve as the receiving aqueous phase. a side stream of the circulating flow passed through a photometer cuvette for aqueous ozone determination. Technical oxygen was used in this circuit. / Separation/Purification Technology 25 (2001) 341–346 While the creation of bubbles is the simplest method to create a contact surface between the two phases it also displays a few drawbacks. 1. because the characteristical decay of ozone in the gas phase corresponds to an even faster decay of dissolved ozone in the aqueous phase. Triggered by difficulties in a research project dealing with such surfactant-loaded wastewater. The dimensions were 9 or 10 mm outer diameter. which leads to a drastically decreased size of bubbles within the water and stabilizes the bubbles after reaching the surface. 1. The gas was also pumped in a circuit from the electrical discharge ozone generator (Anseros COM) through the module and by way of a silica gel exsiccator back to the generator. Since in the laboratory scale experiments only small membrane samples could be applied. which thus indirectly influence the gas transfer. this dye is readily oxidized by the ozone transferred through the membrane. This accumulation in turn disturbs the forces between the dipoles of the water molecules at the surface and can reduce the surface tension considerably. and the surface itself is subject to many factors. an approach was made to achieve better control over the inter-phase surface by means of membranes. . This foam can complicate the control of process parameters by consuming reactor space and in the extreme even prevent the whole ozonation process from working. Polytetrafluorethylene (PTFE).4571 standard). Surfactants are substances that due to the inherent force of attraction between the dipolar water molecules tend to accumulate near the inter-phase surfaces. so that a layer of foam develops. Janknecht et al. since most polymer materials are not suited for application with ozone. The aqueous ozone measurement represented a special difficulty. During circulation in the setup.P. gas tubing consisted of stainless steel (1. Materials and methods A flow scheme of the experimental setup is given in Fig. which leads to a 3. One example for the latter problem is the ozonation of water containing surfactants ([3]). pump. A special module was designed to allow for a turbulent water flow through the inner membrane volume. and with the concept of a respiration-like membrane process. the capillary effects of the hydrophilic material caused a penetration of water into the pores. Accumulation of transferred ozone within the liquid phase during contacting with a hydrophilic alumina membrane. represented a considerable obstacle for a fast ozone transfer through the membrane. Due to a limited absorption linearity the measurement range of this method is moderate. the membrane was completely penetrated within seconds and no ozone transfer was observed later. effective length 390 mm. once the water circuit was started and water touched the fine layer of the membrane. liquid flow velocity 2. but precise and reproducible measurements of ozone transfer rates could be carried out with it. Fig. 5. Correlation between membrane drying time and ozone transfer. If a partially wet membrane was utilized or the conditioning procedure was not observed. Janknecht et al. The transfer rates obtained with those were satisfying in that they proved that ozone transfer through ceramic membranes is possible and is not obstructed by the porous ceramic material through which the ozone must pass. 4). ozone concentration in the gas 100 g m − 3. . If no pressure was applied to the surrounding gas phase.344 P. conventional alumina membranes were used. The result indicates that residual water within the pores constitutes a major limiting factor to the transfer (Fig. It was engendered by application of a thin hydrophobic layer to the ceramic surface by means of a so-called grafting process. though only covering a distance of a few micrometers. Results with hydrophobic membranes It became obvious from these findings. Fig.35 g of ozone per square meter and hour was observed reproducibly (Fig. 3). The transfer rate displayed a strong dependency on the conditioning of the membrane during experiment startup. The modification resulted in a surprisingly strong transfer improvement. 2. the transfer rates decreased considerably. Results with hydrophilic membranes In the first experiments. The transfer performance measured under similar conditions was 12 g ozone per membrane square meter and hour and higher (Fig. In successive trials the drying time of a wet membrane before an experiment was correlated to the respective transfer rates during the experiment.2 m s − 1. Thus keeping the liquid phase from entering the pores appeared as a promising approach for increasing the transfer performance. Inner diameter 7 mm. 3. though no direct comparison was possible since the membrane geometry also had to be modified. 4. This gas pressure is higher than the capillary pressure of the support material and thus allowed the water only to penetrate the fine layers. the membrane was dried before the experiment and a gas pressure of 50 kPa was applied before the water circuit was started. that the penetration of the fine layers. / Separation/Purification Technology 25 (2001) 341–346 bleaching effect on the blue color. Under these conditions and in relation to the required membrane surface a transfer of 0. 2). Therefore. Indigo trisulfonate concentration is monitored colorimetrically by ultraviolet absorption at a wavelength of 605 nm and from its decrease the stoichiometrical ozone consumption is calculated. Since ozonation processes first were applied technically more than 100 years ago. While the gas that is left over in a conventional ozonation process usually is disposed off. A second advantage of the high ozone transfer performance is the reduction of moisture uptake by the carrier gas. Due to the turbulence. however. the flow rates are related to the head loss and pumping energy for the process. In comparison the performance of a bubble column as is commonly used for ozonation is below 40 g h − 1 m − 3. Furthermore. the efficiency of ozone production has been increased continuously and nowadays seems to approach the thermodynamical limits. thus a constant ozone dosage even with unsteady flow rates is easily secured (Fig. 6. During the development it was found. more than 1000 g ozone per hour could be transferred in an installation volume of 1 m3. Correlation of liquid flow velocity and ozone transfer in a hydrophobic tubular membrane.P. high ozone doses were transferred to surfactant loaded wastewater without any foam problems. Inner diameter 5. Accumulation of transferred ozone within the liquid phase during contacting with a hydrophobic membrane. Since pure or enriched oxygen gas is often applied in ozonation processes in order to reach higher ozone concentrations. 5). The contacting process. Janknecht et al. 5. effective length 190 mm ozone concentration in the gas 100 g m − 3. as mentioned above. the utilization of this resource can be optimized considerably. since the recycling of the carrier gas provides a constant high ozone concentration on the gas side the gradient in partial pressure is considerably high. Assuming a configuration of a number of these membranes stacked closely together in a hexagonal arrangement with a small gap for gas circulation in between.2 mm inner diameter and a mean liquid flow velocity of 4 ms − 1.2 mm. / Separation/Purification Technology 25 (2001) 341–346 Fig. Values upto 16 g ozone per square meter and hour have been measured in a tubular membrane with 5. values between 2 and 10 Wh per g ozone are found in literature ([2]). one of the most important transfer parameters is the liquid flow velocity along the membrane surface. thus making up the bulk . so that the ozone transfer rate per time and per area is extremely high. the gas phase in the membrane process can be easily recycled back to the ozone generator. Due to the utilization of a statically fixed phase border. liquid flow velocity 3 m s − 1. depending on the technology and the desired ozone concentration. Another point of interest connected to ozonation systems is the energy demand. In the membrane ozonation process. Comparison to conventional contacting methods The advantage that originally was expected from the new contacting process was the avoidance of bubble and foam formation in the ozonation process. 4. Today 1 g of ozone can be produced from oxygen with an energy in the magnitude of 10 Watt-hours (Wh). that it also displays other benefits: with hydrophobic membranes the transfer rate is almost linearly related to the liquid flow speed within a wide range. In this respect the method fulfilled the expectations. that is by a factor of 25 less (value calculated from [5]). however. the liquid flow velocity can be accelerated and the turbulence increased far beyond the values practicable with open surfaces such as bub- Fig. can consume almost the same amount for pumping and mixing performance. 345 bles. Chelsea. Janknecht. Langlais. et al.R. Sarrazin. Acknowledgements The authors wish to express their appreciation References [1] N. for the assistance of the staffs of Institute for Water Quality Control and Waste Management. Lewis. C.). Brink. Oldenbourg Verlag. / Separation/Purification Technology 25 (2001) 341–346 of its energy demand. Ing. USA. 1986. including a constant ozone dosage independent of the liquid flow rate. [5] B. Biologische Behandlung von Sickerwa¨ ssern aus Sonderabfalldeponien mittels schubweise beschickter.A. Though the laboratory scale of the experiments did not allow for a hydrodynamic optimization so far. Larbot. A. Several advantages in the process design speak in favor of this approach as compared with conventional bubble based contacting procedures. Tech. Picard. Mu¨ nchen. D. Chem. it can be concluded that the ozone contacting process with ceramic membranes has the potential to become a useful method of treating wastewater. [3] J. Germany). 7. Montpellier. and Laboratoire des Mate´ riaux et Proce´ de´ s Membranaires. D. Do¨ llerer. which is well in the order of magnitude of conventional processes. 1997. Ozone in Water Treatment: application and engineering. Germany). the energy loss of water measured in an ozonation experiment with a reverse membrane corresponded to a specific energy demand of only 7 Wh per g ozone.346 P. Janknecht et al. J. Wilderer. Germany). Munich. 1998. P. Hamburg. [2] S. . Berechnungsgrundlagen und Anwendungsbeispiele zum Sauerstoffeintrag in Wasser und Abwasser u¨ ber nichtporo¨ se Polymermembranen (Technische Universita¨ t Hamburg-Harburg. [4] P. Blasenfreier Ozoneintrag durch keramische Membranen zur naßoxidativen Abwasserbehandlung. Reckhow. 1991. 72 (2000) 122 –126. (Eds. The financial support of Bayerisches Staatsministerium fu¨ r Landesentwicklung und Umweltfragen and Centre National de la Recherche Scientifique within the framework of cooperation between Bavaria and the French Region Languedoc-Roussillon is gratefully acknowledged. Abwasserbehandlung mit Ozon (R.A. Pschera. Munich. a potential to easily recycle the oxygen carrier gas and the high transfer performance within a small installation volume. Conclusions From the findings. Mu¨ ller. u¨ berstauter Festbettreaktoren (Technische Universita¨ t Mu¨ nchen.