Static Ozone Contacting by Ceramic Membranes Peter Janknecht , Peter A. Wilderer Technische Universität München Am Coulombwall, 85748 Garching, Germany Abstract An approach was made to design a bubble free ozone contacting device with ceramic membranes in order to avoid formation of foam in surfactant loaded waste water. In the run of the project advantages of the process became obvious that make it also attractive in other ozone contacting applications. The constant separation of the two phases by a membrane facilitates a continuous recycling of the carrier gas, so that a high ozone concentration can be maintained which favors the transfer performance. The transfer method provides a constant dissolved ozone concentration independent of the volume flow rate. Introduction Conventional ozone contacting methods like bubble columns, injectors, impellers, and others, function through the creation of small bubbles that provide the transfer surface, and of shear forces within the water that perform both the mixing and the continuation of the bubbles. While this creation of bubbles is the simplest method to create a contact surface between the two phases it also displays a few drawbacks. The generation and continuation of bubbles constantly consumes energy, and the surface itself is subject to many factors which thus indirectly influence the gas transfer. One example for the latter problem is the ozonation of water containing surfactants (Döllerer, 1998). Surfactants are substances that tend to interfere with the inherent force of attraction between the dipolar water molecules and accumulate near the inter-phase surfaces. This accumulation in turn disturbs the forces between the dipoles of the water molecules and can reduce the surface tension considerably, which leads to a drastically decreased size of bubbles within the water and stabilizes the bubbles after reaching the surface, so that a layer of foam develops. 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. Its mechanical destruction is not always possible, while chemical foam inhibtors may solve the problem at the expense of forming an additional pollution to the water.

Triggered by difficulties in a research project that was dealing with such surfactant loaded wastewater, and with the concept of a membrane process comparable to the biological respiration, an approach was made to achieve better control over the interphase surface by means of membranes. Ceramic membranes were selected for that purpose, since most polymer materials though being practical for oxygen transfer (Müller, 1986), are not suited for application with ozone. Experiments carried out on ozonation with non-porous silicone membranes led to problems with membrane hardening and successive failure in the presence of certain pollutants (Shanbhag, 1998). In the run of the project advantages of the process became obvious that make it also attractive in other ozone contacting applications. In the new process, porous ceramic material separates the ozone delivering gas phase from the ozone receiving liquid phase, but allows for transfer of ozone between them. The surface of phase contact thus is statically fixed and the flow conditions in the gas as well as in the liquid can be controlled independently. Thus they can be easily adapted to changing process conditions and ozone demands. This feature constitutes a considerable advantage over conventional contacting methods based on bubble formation, in which the phase surface area depends on dynamic flow conditions and the process efficiency range is limited to a certain gas and liquid flow rate. The continuous separation of the phases favors the ozone transfer in that the carrier gas can be continuously recycled through the ozone generator and the transferred ozone is immediately replaced in the gas. Thus the effective ozone concentration at the contact surface is kept at a high level. The recycling in turn is facilitated by the transfer acceleration from the high ozone concentration, which decreases the uptake of moisture and volatile substances into the carrier gas. Favorable conditions in the gas circuit are maintained this way. The recycling process also safes ozone carrier gas by avoiding its squandering in the bubble formation, which is of particular concern when enriched or pure oxygen is deployed as carrier gas. Another feature of the possibility to easily recycle the carrier gas is the avoidance of both ozone loss in the off gas and the necessary gas treatment. Materials and Methods A flow scheme of the experimental setup is given in Figure 1. The membrane type was a single channel tubular ceramic membrane with fine layer on the inner surface. The dimensions were 9 or 10 mm outer diameter, 5 or 7 mm inner diameter, and a length of 200 or 400 mm, respectively. A special module was designed to allow for a turbulent water flow through the inner membrane volume, while the ozone containing gas was applied to the space formed by the outer membrane surface and an enveloping glass tube. The water was circulated within PVC tubing by means of a radial pump, a side stream of the circulating flow passed through a photometer cuvette for aqueous ozone determination. 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. Technical oxygen was used in this circuit, gas

tubing consisted of stainless steel (1.4571 standard), borosilicate glass, Polytetrafluorethylene (PTFE), and a stainless steel metal bellows compressor. Ozone measurement and regulation in the gas phase were carried out in a side stream by direct UV absorption at a wavelength of 253.7 nm (ANSEROS Model GM-6000OEM). The aqueous ozone measurement represented a special difficulty, because the characteristical decay of ozone in the gas phase corresponds to an even faster decay of dissolved ozone in the aqueous phase. Since in the laboratory scale experiments only small membrane samples could be applied, the ozone transfer was limited and demanded for an especially adapted method. After a series of tests with different other techniques (Janknecht, 2000), a process developed by Hoigné and Bader and described by Langlais (1991) and in the German Standard Method DIN 38 408-G3-3 was compressor ozone generator exsiccator

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Figure 1: Flow scheme of the experimental setup

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. During circulation in the setup this dye is readily oxidized by the ozone transferred through the membrane, which leads to a bleaching effect on the blue color. Indigo trisulfonate concentration is monitored colorimetrically by ultraviolet absorption at a wave length of 605 nm and from its decrease the stoichiometrical ozone consumption is calculated. Due to a limited absorption linearity the

measurement range of this method is moderate, but precise and reproducible measurements of ozone transfer rates could be carried out with it.

Results with Hydrophilic Membranes In the first experiments conventional alumina membranes were used. 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. The transfer rate displayed a strong dependency on the conditioning of the membrane during experiment startup: once the water circuit was started and water touched the fine layer of the membrane, the capillary effects of the hydrophilic material caused a penetration of water into the pores (Burggraaf, 1996). If no pressure was applied to the surrounding gas phase the membrane was completely penetrated within seconds and no ozone transfer was

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the water during contacting with a tubular hydrophilic alumina membrane (inner membrane diameter 7 mm, effective length 390 mm, ozone concentration in the gas 100 g m-3, liquid flow velocity 2.2 m s-1). If a partially wet membrane was utilized or the conditioning procedure was not observed, the transfer rates decreased considerably. In successive trials the drying time of a wet membrane before an experiment was correlated to the respective transfer rates during the experiment. The result indicates that residual water within the pores constitutes a major limiting factor to the transfer (Figure 3).

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Results with Hydrophobic Membranes It became obvious from these findings, that the penetration of the fine layers, though only covering a distance of a few micrometers, represented a considerable obstacle for a fast ozone transfer through the membrane. Thus keeping the liquid phase from entering the pores, appeared as a promising approach for increasing the transfer performance. It was engendered by application of a thin hydrophobic layer to the ceramic surface by means of a so-called grafting process. With this modification the process' inherent advantages yielded their profits in a surprisingly strong transfer improvement. No direct comparison to the hydrophilic membrane was possible, however, since the membrane geometry also had to be modified. The transfer performance measured under similar conditions exceeded 12 grams ozone per membrane square meter and hour. Figure 4 depicts the accumulation of transferred ozone within the liquid phase during contacting with a hydrophobic membrane (inner membrane diameter 5.2 mm, effective length 190 mm ozone concentration in the gas 100 g m-3, liquid flow velocity 3 m s-1). Results with Surfactant loaded Wastewater

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The advantage that originally was expected from the new contacting process was the avoidance of bubble and foam formation in the ozonation process. In this respect the method fulfilled the expectations: in experiments with surfactant loaded wastewater high ozone doses were transferred without any foam problems. Figure 5 depicts the relation between ozone dose and the development of Biochemical Oxygen Demand and Chemical Oxygen Demand (BOD and COD) during ozonation of biologically pretreated biowaste-fermentation wastewater. The transferred ozone and its specific dose were estimated from previous experiments with model water under identical conditions. It was assumed to be consumed immediately. A considerable bleaching effect of the ozone was observed, but the effect on BOD and COD was not satisfying in this experiment and will need further optimization. Comparison to Conventional Contacting Methods During the research on the new process it was found, that next to foam avoidance it also displays other benefits: One favorable peculiarity is that the high transfer rates with hydrophobic membranes are within a wide range almost linearly related to the liquid flow speed (Figure 6). Thus a constant ozone dosage even with unsteady flow rates is easily achieved.

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Figure 6: Correlation of liquid flow velocity and ozone transfer Another benefit is the high ozone transfer per installation volume: due to the utilization of a statically fixed phase border the liquid flow velocity can be accelerated and the turbulence increased far beyond the values practicable with open surfaces such as bubbles. Aditionally, since the recycling of the carrier gas provides a constant high ozone concentration on the gas side the gradient in partial pressure remains favorably

steep, so that the ozone transfer rate per time and per area is extremely high. Values up to 16 grams ozone per square meter and hour have been measured in a tubular membrane with 5.2 mm inner diameter and a mean liquid flow velocity of 4 ms-1. 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, more than 1000 grams ozone per hour could be transferred in an installation volume of one cubic meter. In comparison the performance of a bubble column as is commonly used for ozonation is below 40 g h-1 m-3, that is by a factor of 25 less. This value was calculated from Langlais, 1991. A second advantage of the high ozone transfer performance is the reduction of moisture uptake by the carrier gas. While the gas that is left over in a conventional ozonation process usually is disposed off, the gas phase in the membrane process can be easily recycled back to the ozone generator. Since pure or enriched oxygen gas is often applied in ozonation processes in order to reach higher ozone concentrations, the utilization of this resource can be optimized considerably. Energy Demand One parameter of interest connected to ozonation systems is the energy demand. Ozonation processes were first applied technically more than one hundred years ago, since then the efficiency of ozone production has been increasing continuously and nowadays seems to approach the thermodynamical limits. One gram of ozone today can be produced from oxygen with an energy in the magnitude of 10 Watt-hours, depending on the technology and the desired ozone concentration. The contacting process, however, can consume almost the same amount for pumping and mixing performance: values between 2 and 10 Wh per gram ozone are found in literature (Pschera, 1997). The bulk energy demand of the membrane ozonation process arrises from the turbulent liquid flow along the membrane surface, which widely controls the transfer performance as mentioned above. Due to the turbulence the liquid flow velocity is directly related to the head loss and pumping energy for the process. The pumping energy for the laminar flow of the gas circulation is comparatively small. Though the laboratory scale of the experiments did not allow for a hydrodynamic optimization so far, the head loss of water measured in an ozonation experiment with a reverse membrane corresponded to a specific energy demand of only 7 Wh per gram of ozone, which is well in the order of magnitude of conventional processes.

Conclusions From the findings it can be concluded that the ozone contacting process with ceramic membranes has the potential to become a useful method of treating wastewater. Several advantages in the process design speak in favor of this approach as compared to conventional bubble based contacting procedures, including a constant ozone dosage independent of the liquid flow rate, a potential to easily recycle the oxygen carrier gas and the high transfer performance within a small installation volume. Acknowledgments The authors wish to express their appreciation for the assistance of the staffs of Institute for Water Quality Control and Waste Management, Munich, and Laboratoire des Matériaux et Procédés Membranaires, Montpellier. The financial support of Bayerisches Staatsministerium für 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. References Burggraaf, A.J. and Cot, L. Fundamentals of Inorganic Membrane Science and Technology (Elsevier Science B.V., Netherlands, 1996) Döllerer, J. Biologische Behandlung von Sickerwässern aus Sonderabfalldeponien mittels schubweise beschickter, überstauter Festbettreaktoren (Technische Universität München, Munich, Germany, 1998) Janknecht, P., Wilderer, P.A., Picard, C., Larbot, A., Sarrazin, J.: "Investigations on Ozone Contacting by Ceramic Membranes", Ozone Science & Engineering, accepted for publication Langlais, B., Reckhow, D.A., Brink, D.R., et al. Ozone in Water Treatment: application and engineering (Lewis Publishers, Chelsea, USA, 1991) Müller, N. Berechnungsgrundlagen und Anwendungsbeispiele zum Sauerstoffeintrag in Wasser und Abwasser über nichtporöse Polymermembranen (Technische Universität Hamburg-Harburg, Hamburg, Germany, 1986) Pschera, S. Abwasserbehandlung mit Ozon (R. Oldenbourg Verlag, München, Germany, 1997) Shanbhag, P.V., Guha, A.K. and Sirkar, K.K. "Membrane-Based Ozonation of Organic Compounds", Ind. Eng. Chem. Res., Vol. 37, No. 11: 4388-4398 (1998)

Keywords Ozone; Contacting; Ozone Transfer; Ceramic Membranes; Physico-Chemical Treatment; Leachate Treatment