May 8, ASCE Journal Department 1801 Alexander Bell Dr. Reston, VA 20191, U.S.A. Dear Sirs,

Transcription

1 Professor Václav Tesař Process Fluidics Group Department of Chemical and Process Engineering The University of Sheffield Mappin Street Sheffield S1 3JD United Kingdom May 8, 2005 ASCE Journal Department 1801 Alexander Bell Dr. Reston, VA 20191, U.S.A. Dear Sirs, Please find attached two copies of manuscript Fluidic Aerator a New Concept for Aerobic Water Treatment by authors Václav Tesař and William Zimmerman. I would like to have this manuscript reviewed by the Journal of Environmental Engineering. Allow me to emphasise that - while a particular laboratory model and its experimental results are described - the main contribution of this paper is qualitative. It describes a wholly new approach to the basic problem of improvement the present unsatisfactory transport rate of oxygen to the aerobic micro-organisms in waste water processing. Should you need to contact me, please use the above postal address or fax me at or contact me by v.tesar@aheffield.ac.uk. Sincerely, Václav Tesař 1

2 Separate abstract page Fluidic Pulsating Aerator New Concept for Aerobic Water Treatment V. Tesař and W. B. Zimmerman Abstract: A novel aerator for biological treatment of waste water has been recently tested in laboratory model form. It uses a simple no-moving-part fluidic oscillator in the air supply to produce air flow pulsation which controls bubble growth. The generated bubbles are nearly an order of magnitude smaller size than the air bubbles commonly produced by steady flow aerators. This alleviates the main problem limiting the efficiency of water treatment, which is the insufficient transport of oxygen to the aerobic micro-organisms. Because of the substantially larger ratio of total surface area to total volume of the smaller bubbles, and also due to their lower rising velocity, the oxygen mass transfer rate across the air/water interface is much more intensive. This decreases the amount of air needed for the aeration and saves energy. CE Database subject headings: Aeration, Aerobic treatment, Bubbles 2

3 Fluidic Pulsating Aerator New Concept for Aerobic Water Treatment V. Tesař and W. B. Zimmerman Abstract: A novel aerator for biological treatment of waste water has been recently tested in laboratory model form. It uses a simple no-moving-part fluidic oscillator in the air supply to produce air flow pulsation which controls bubble growth. The generated bubbles are nearly an order of magnitude smaller size than the air bubbles commonly produced by steady flow aerators. This alleviates the main problem limiting the efficiency of water treatment, which is the insufficient transport of oxygen to the aerobic micro-organisms. Because of the substantially larger ratio of total surface area to total volume of the smaller bubbles, and also due to their lower rising velocity, the oxygen mass transfer rate across the air/water interface is much more intensive. This decreases the amount of air needed for the aeration and saves energy. CE Database subject headings: Aeration, Aerobic treatment, Bubbles Introduction The usual method of breaking down the major pollutants in waste water - organic matter, nitrates and phosphates [1] is their aerobic biological decomposition. Efficiency of this process is limited by availability of oxygen needed by the aerobic micro-organisms for their growth and activity. The air is usually provided by aeration - supplying air - the most direct approach being injection of bubbles into the water through orifices located near the bottom of the water treatment tank. This is a relatively expensive process. Ref. [2] estimates that its requirements represent more than 80 % of total energetic consumption in typical municipal installations. Unfortunately, the effect is often dis-. Fig. 1. The basic idea. Breaking an object (an air bubble here schematised as a cube) into n smaller objects of similar shape and of the same total volume increases n- times the available surface. proportionately poor. In fact, the authoritative monographs on the subject, e.g. [2], say that majority of current [waste water treatment] plants suffer from oxygen deficiency. At the core of the problem is the relatively large size of the air bubbles, usually between 8 and 10 mm in diameter, and their fast motion towards the surface, since the rising speed increases with size. It has been always recognized that the oxygen transfer would be much higher if the bubbles could be made small. The smaller they are, the longer they stay in the water and the larger is their ratio of total surface area across which the transfer takes place to the total air volume. The latter is presented in Fig. 1 for a symbolic form of cubic shape, where the added surface is easier to recognise: all the contact surfaces of the neighbouring elements become available as diffusion transfer surfaces in the expanded volume. In an attempt to make the bubbles small, aerator designers choose very small size of the air injection orifices, in the final stages this trend leading to air percolation through submillimetre pores (Fig. 2) in sintered porous materials. Unfortunately, the small size does not help much. Quite common is the agonising experience of the bubble size remaining nearly the same no matter how small are the pores, so that the net outcome is essentially just an unwelcome increase of required supply pressure and operational problems due to pores clogging by various debris and particles carried with the air. The 3

4 Basics of bubble theory Fig. 2. Percolation of air into water through pores, which are often made (e.g. as voids in sintered material) much more irregular than shown here. Disappointingly, the size of the generated bubbles does not decrease in proportion to the smaller size. The process in unstable: bubble formation tends to concentrate into several pores while others remain inoperative. bubble formation is found to concentrate into just a few locations, no matter how large is the area of the sintered surfaces. According to [2], usually less than 60 % of the sintered aerator surface area actually produces the bubbles. This is sometimes due to the clogging, but a more fundamental reason for the poor uneven distribution is hydrodynamic instability of bubble formation process. A remedy has been sought in moving the aerators such as e.g. by placing them on a rotating arm, [3]. The shear stress on the flow past the aerator surface limits the bubble growth. An additional benefit is the stirring. The advantage claimed in [3] of a single row of orifices on the arm being able to cover quite large volume is hardly convincing, the bubbles being simply more diluted in the larger volume. Serious disadvantage of this approach is the cost of the driving motor and gearbox (held on expensive stiff struts above the surface), bearings, and rotating seals (which tend to become worn and need maintenance. Of course, the consumed power for the driving motor increases the operation cost. The paper presents a novel solution to the problem, capable of producing bubbles practically an order of magnitude smaller and at the same time using no moving components. The aim is to describe the qualitative aspects: the reasons for the failure of the aerators used so far, the new idea which circumvent the existing limitations as well as experience obtained in laboratory feasibility study of the new approach. a) Bubble shape The property of the liquid which governs the bubble formation is the surface tension σ [N/m] - the proportionality constant between the force exerted when the air/water phase boundary surface is increased and the circumference length of the enlarged boundary. For clean distilled water on one side and air on the other side, the value of the surface tension at T = 25 o C is σ = 72.3 mn / m. A useful working idea is to imagine a thin elastically deformed skin on the surface, though physically more appropriate picture is that of the energy required for moving additional liquid molecules from inside of the liquid volume to the newly formed surface, where they are exposed to one-sided pull of the attractive forces of the internal molecules. The basic law governing the behaviour of the liquid/gas interfaces is the Young-Laplace relation [4], [5] between the surface tension force and the force due to the pressure difference across the boundary, presented in Fig.3. The shape of stationary bubbles, especially if they are attached to a solid wall, is visibly influenced by gravitational forces. Classical solutions were derived for the two cases: the shape elongated by the lift force (Fig. 4) acing on a bubble which is held at its bottom (this case Fig. 3 The law dictating the bubble shape is the Young- Laplace equation of the balance between the surface tension and the pressure difference across the interface. 4

5 corresponds, with inverted sign, to the shape of pendant liquid drops), and the shape in which the hydrostatic lift tends to press the bubble upwards against the solid wall at which it is held by its upper pert (- the classical solution of the sessile drops). Essential length parameter in problems associated with surface tension in balance with gravitation forces is the capillary length scale (1) - with the specific volume of air v a [m 3 /kg] = r T / P (where T temperature, P pressure, and the gas constant r for humid air taken r = 288 J/kgK), specific volume of water v w = 10-3 m 3 /kg and g = 9.81 m 2 /s gravity acceleration. For the water/air interface the capillary length is l cap = 2.72 mm (2) The problem with bubbles influenced by the gravity forces is their aspherical shape. Instead of the radius r, a more complex form of the Young-Laplace equation with two principal radii r 1 and r 2 in two mutually orthogonal planes is to be used (Fig. 3). Only at the apex (the uppermost point) the shape possesses a single curvature; it is the apex radius r A for which from the Laplace-Young equation (Fig. 3) follows the pressure difference (3) across the top of the elongated bubble as shown in Fig. 4 (or across the bottom of the compressed, sessile drop, bubble). Elsewhere, the pressure varies over the bubble height with the changing hydrostatic pressure dependent on vertical position. The resultant non-linear differential equation for the bubble shape contains a parameter which may be usefully non-dimensionalised to p = 2 l cap / r A (4) - called pressure parameter, since it is through eq.(3) determined by the apex pressure. Several numerical solutions of the bubble shapes for different values of the pressure parameter are plotted in Fig. 5. It is evident that stationary air bubbles in water of diameter ~ 1.2 mm or smaller are practically spherical. If the bubble size is increased, the shape in this pendant drop case Fig. 4 The extreme shape of a vertically elongated air bubble made by careful expulsion of air from a syringe. Note that if vertically inverted, this shape corresponds to that of a pendant drop. Fig. 5 Shapes of top part of bubbles computed for a range of bubble sizes and hence a range of the corresponding values of the pressure parameter p eq. (4). Also shown are the osculation circles passing through the apex points (their radius is equal to the apex radius r A ). 5

6 becomes significantly deformed by the hydrostatic pressure difference so that it is more and more elongated with the extreme of realisable stationary shapes corresponding to Fig. 4 of diameter ~ 8 mm. b) Instability of parallel bubble growth Let us now consider the formation of two bubbles according to Fig. 6. This is a reasonable representation of a section of the currently developed aerator [3], but may also represent two neighbouring pores of a sintered aerator. In the initial stages of their formation the two bubbles are small, of submillimetre size, so that their shape (cf. Fig. 5) is practically spherical, fully determined by the radius r, Fig. 6, which is equal to the apex radius r = r A The air pressure inside the bubble is equal to the apex pressure given by eq. (3). The fact of paramount importance is that eq. (3) shows this pressure to decrease gradually as the bubble radius r grows. This pressure decrease during the growth, or the negative slope of the pressure-radius dependence eq. (3), is at the core of the problems. Suppose some small external disturbance (or even the small hydraulic resistance of the part of the manifold between the two exits) causes the downstream bubble D in Fig. 6 to be slightly retarded in its growth. Its internal pressure becomes higher than that the pressure in the in the upstream bubble U. This, of course, will make the pressure may drive air away. Indeed, even the hydraulic pressure loss in the manifold between the Fig. 6 Simultaneous generation of two bubbles in parallel in an aerator corresponding to the currently developed device [2] with a row of parallel holes connected to the single large-diameter hole manifold. The balance between the two bubbles is unstable. air entry into the upstream bubble U more favourable, air meeting there less resistance. Bubble U will grow at the expense of the downstream neighbouring exits may produce enough disbalance to destabilise the growth. In the line of parallel air injection nozzles it usually the first upstream bubble which starts growing uncontrollably until it separates. Strictly speaking as will be shown below this instability mechanism takes place only for bubbles beyond a certain critical, hemispherical shape. However, with steady air flow into the bubble there is no mechanism that can produce bubbles and stop them growing before the unstable regime is reached. Due to the instability it is impossible to produce many small bubbles in parallel. Instead, the general tendency is to concentrate the air flow into a single injection port, the bubble at which grows unstoppably to a large size c) Separation instability If a growing bubble is supplied by a steady air flow, its growth passes through the progression of shapes shown in Fig. 5. Pressure parameter p gradually decreases towards the limiting value p = 2. The solutions in Fig. 5 were not computed up to the small values of parameter p 2 and especially avoided the bottom part of the bubbles there. This is because the numerical procedure there experiences problems. The solution loses uniqueness and becomes numerically unstable. The non-uniqueness means appearance of two solution values. These represent the necking of the bubble, local contraction at its bottom part. The numerical instability, which makes it progressively difficult to perform the computations of the neck shape at p 2, reflects in fact a mechanical instability of the bubble shape. The shape become instable, the diameter of the neck decreases rapidly to zero. The result is separation of the bubble from its orifice through which it is supplied. Exact details of the process, due to the instability and extreme sensitivity, are easily influenced even by small perturbations.. In theory, the separation takes place at the bubble size reaching the value for which p = 2. In practice the bubbles may continue growing slightly beyond this limit, is supplied by a powerful air flow. On the other hand, if the bubble is generated by a 6

7 single exit orifice, it is possible to obtain a smaller bubble size by using a small air flow rate. This leads to slow bubble growth and a (quire reliably occurring) chance that some disturbances will cause the bubble to form the neck and then separate even while its pressure parameter is still at p > 2. This, however, is not usable as a practical way of generating small bubbles, since a single orifice would be uneconomic and multiple orifices would switch the flow into a large bubble due to the bubble growth instability. Fluidic aerator The present paper presents a solution to the problem of improving the aeration efficiency by generation of much smaller bubbles. Essentially, bubbles are kept small by operating the aerator in periodic oscillatory regime. The bubble size, instead being governed by the surface tension necking mechanism, is limited by terminating the growth at the end of each oscillation half-cycle. This solution is based on utilising a combination of several ideas. 1) In the first half of the cycle, bubbles are not left to grow (or. At leas, not substantially) beyond the initial stable stage of their formation. This makes it possible to produce simultaneously a large number of small bubbles of practically the same size. 2) In the second half of the oscillation cycle the shear-flow induced separation mechanism is applied to remove the bubbles from their injection nozzles. 3) The oscillation is generated by using quite simple fluidic oscillation generator in the air supply. This device has no moving components and consists essentially of just a specially shape bifurcation cavity. As shown schematically in Fig. 7, the fluidic oscillator is of the diverting type, with the output flow alternating between the two exit branches, A and B. The air flow in A reaches up to the aeration orifices at the bottom of the water treatment tank. Due to the bleeding exit, water is left to fill the branch B. the alternating air pressure there acts on surface of water, generating water jets issuing from the second set of parallel orifices. These water jets act on the small bubbles and separate them from the air orifices in the second half of each oscillation cycle, while the bubbles are still to small and normally would not separate. Fig. 7 Schematic representation of the investigated model of the pulsating flow aerator. The no-moving-part fluidic oscillator in the air supply operates in diverter mode. During the first half of the cycle, it directs the air flow into A and to the air orifices producing the bubbles. In the next part of the cycle, the air acts on the water in the vertical pipe B and generates water jets that separate the bubbles from their air nozzles. a) The fluidic oscillator Although the last item in the above the list, availability of a suitable oscillator is the key factor for the new approach. Without a no-moving-part generator the operation in sustained oscillation would be impractical, due to inevitable wear of mechanical components. This is no problem in the device based on employing purely aerodynamic phenomena in a cavity with fixed walls. The fluidic oscillator is so simple and easy to manufacture that it does not cause any significant increase to aerator complexity or 7

8 Fig. 8 Laboratory fluidic amplifier model of 1.4 mm main nozzle width used in the tests. It consists of a stack of five Perspex sheets, each 1.2 mm thin, with laser-cut cavities, covered by 10 mm thick top and bottom Perspex cover plates. The bottom plate contains the ferrules for the feedback loop and output tubings. price. The actually tested version consists of two components: a fluidic no-moving-part jet-deflection amplifier (Fig. 8) and a single feedback loop tube. The principle of the amplification in the first component, the fluidic diverter valve, is based on the sensitivity of a jet deflecting actions of flows admitted into the perpendicularly oriented control nozzles. The deflection changes the proportion of the jet fluid captured in two collectors opposing the nozzle from which issues a jet of supplied air. Geometry of the model used in the present tests was based upon successful early valve design [7] later used by Perera and Syred [8]. The flow amplification gain of this model in steady state is approximately This means that to terminal into the other it is sufficient to apply a control flow rate equal to approximately 7 % of the supplied flow. In actual operation, the flow in the amplifying valve is switched by control pulses which owing to their sudden shock character may be even weaker. The version of the amplifier used in the tests, as shown in Fig. 8, was originally a laboratory model designed for a different purpose ref. [6], where there are all the details of its geometry and full description of experimentally found behaviour. This valve operate din bistable regime: without any acting control signal it remains in one of two alternative full jet deflection stable states. This is achieved by using the Coanda effect of jet attachment to walls. The model was made by laser cutting of the cavities in plastic material (transparent polymethylmetacrylate). In fact, this inexpensive and fast manufacturing method may be suitable even for the operational version of the device. The second component, which converts the amplifier into an oscillator, is a de-stabilising fluidic feedback loop. Instead of the classical connecting of the output and control terminals, a simpler version of feedback actually used (Fig. 7) uses a single feedback loop connecting the two control terminals. This, together with experimental data, is also described in ref. [6]. The mechanism of the feedback utilises the fact that pressure levels in the control terminals are unequal. There is much lower pressure on the side to which the jet is deflected. In fact the Coanda attachment to the wall is associated with the low pressure on the concave side of the deflected jet. The pressure difference gives rise to a fluid flow in the loop towards the lower-pressure control nozzle. This flow gradually gains a sufficient momentum for producing a substantial outflow from this nozzle, sufficient to switch the jet to the opposite attachment wall when it reaches the 7% limit. The pressure Fig. 9 Dependence of the oscillation frequency f on the length l of the feedback loop. Results of experimental investigations of the oscillator [6], Fig. 8, with the 10 mm i.d. plastic tube loop and air supply flow rate 10 litres/min. Very low oscillation frequency is obtainable, though with inconveniently long feedback loop. 8

9 difference then changes sign, leading to reversal of the flow direction in the loop. Essential for the oscillation generation is a delay of the flow in the loop Because of fluid inertia, it takes some time for the fluid to stop and then begin flowing back. The jet remains for a certain short time attached to the opposite attachment wall, sufficiently long to produce the inverted pressure levels in the control nozzles. The delay depends on the length of the feedback loop tube. As a result, the oscillation frequency may be adjusted by the feedback tube length. To give some idea about the length and frequency range, experimental data are given in Fig. 9 for the amplifier from Fig. 8. b) Utilising the stable regime The instability of parallel bubble formation, described above in association with Fig. 6, is not affecting the initial stage of bubble formation - before the bubble assumes the hemispherical shape. In these initial stages the bubble radius r actually decreases with growing bubble size so that the pressure inside the bubble increases, making the conditions stable. This initial regime is utilised in the novel aerator, with stable formation of bubbles in a large number of parallel air exit orifices during the first half-period of the oscillation generated by the fluidic oscillator. The growth is terminated - since the air flow pulse comes to an end before the bubble shape reaches the hemispherical limit. Of course, the bubbles below this limit are too small for their Fig.11. Dependence of the pressure difference across the bubble water/air interface, computed for 0.6 mm and 1 mm exit hole diameters, on the volume V of the spherical cap in Fig. 10, which is roughly commensurable with the bubble volume. Note that the hemispherical shape divides the growth into two regimes the initial stable growth with decreasing r and the later unstable one.. separation from the air exit orifice taking place naturally, at least in steady states. At high oscillation frequency, a dynamic separation regime was observed, which occurs due to the discrepancy between the accelerated water around the bubble on one hand, tending to move away from the orifice, while the air flow changes sign on the other hand and tends to return the air flow back into the exit. At low operating frequencies, this dynamic separation mechanism is insufficient and has to be assisted by another, flow shear mechanism which, of course, may become the dominant mechanism in some frequency range. Fig. 10 Initial stage of bubble formation are stable, the pressure difference across the water/air interface increasing with bubble size until the hemispherical shape is obtained. From then on, the slope of the dependence changes sign and the instability of Fig. 6 takes place. c) Bubble release by water jet Removing of the small, sub-hemispherical bubbles from their air exit orifices according to the present solution uses the principle of interaction of perpendicular flows, analogous to the interaction of perpendicular flows utilised in the fluidic amplifying valve. The system of parallel air orifices is connected 9

10 to the ext branch A of the fluidic oscillator, Fig. 7. There is yet another system of orifices, connected to the water filled branch B. A water and an air exit orifices form a pair, with their axes mutually perpendicular, as shown in Fig. 12. Fig. 13 presents a cross section passing through the two orifices of the pair. The mutual position of the two nozzles in the pair is such that the water jets formed by the water flow from the water nozzles act form aside on the bubbles formed at the exit of the air nozzle. For ease of manufacture of the test model by the simple drilling, the holes are made to exit into in a 90 deg groove. The filling of the water into the water nozzles is made possible by the bleeding exit, as shown in Fig. 7, which enables the air flow into the branch B to escape into the atmosphere. It should be noted that there is a restrictor in the bleeding exit. The pressure drop on the restrictor, varying with the magnitude of the instantaneous air flow from the oscillator, produces a pressure action on the top of the water column in the branch B. In fact, the restrictor used in the investigated model had a character of small cross section capillary, which may have contributed by yet Fig. 13. Geometry of the groove and the pair of water and air nozzles. Note the asymmetry: the air nozzle is smaller, water nozzle larger and located so that its exit reaches to the bottom of the groove. This helps the water jet to sweep away the air bubble held at the air nozzle exit. another effect the inertance of the flow accelerated in the capillary. This makes possible easy escape of the time mean air flow component (and thus ingress of water into the branch B) while the unsteady air pulsation cannot leave easily and acts quite strongly on the water column. In the second half of the oscillation cycle, while the bubbles are still held outside it air nozzle exit by the accelerated surrounding water, the air pressure pulse in the branch B expulses water from the water nozzles, forming water jets. Note that the geometry Fig.12. The groove with the mutually perpendicular nozzle pairs in the aerator model. The layout was chosen for ease of manufacturing of the holes (e.g. drilling at right angle to the local surface) in the model. A different manufacturing technique - and resultant shape - is envisaged for the operational version. Fig. 14 Drilling of the 0.6 mm air nozzle holes the groove in Fig. 12 makes the drilling at right angle to the local surface easier. 10

11 of the water nozzles causes the jet to follow the grove wall with the air nozzle exit. The water jet impinges upon the bubbles. Experimental results The model of the new aerator was made from two parts, made from the transparent Perspex and mutually connected by transparent Tygon tubes. One of the parts, the oscillator (Fig. 8), was outside the test tank filled with water. Immersed at the bottom of the tank was aerator component (Fig. 14). This was of rectangular shape, with water and air connection ferrules leading from opposing sides to the water and air nozzles, drilled in the groove on the top of this component. In the oscillatory regime, the length of the feedback loop tube was adjusted so that the oscillation frequency was initially low, f = 2 Hz. Because the oscillator was originally made for a different purpose, its size was too large for the mere 38 water nozzles and 38 air nozzles and some Fig. 16. The water jets issuing from the water nozzles in the second half of the oscillation cycle separate the air bubbles and move them away from the aerator surface. generated pulsatile flow was bled not only on the water side, as shown in Fig. 7, but in a similar layout (with an adjustable restrictor) also on the airs side, branch A. This, of course, was only a temporary measure. Fig. 15. The first half of the oscillation cycle. While the bubbles formed by the air flow admitted to the air nozzles.are still small, of the sub-hemispherical size, the distribution of the air flows is stable. All bubbles grow simultaneously to the same size. Fig. 17. Photograph of standard sized ~ 8 mm dia. bubbles produced by steady airflow into the branch A. 11

12 If the oscillation was stopped (by interruption of the feedback loop, the steady air flow admitted into the row of the air nozzles resulted in a stream of air bubbles issuing, as documented by the photograph Fig. 17, from the first air nozzle immediately downstream from the inlet ferrule bringing the air into the immersed aerator component. This provided a graphic demonstration of the bubble formation instability: all air flow concentrated into just a single orifice the one offering the least hydraulic resistance. The generated bubbles were irregular, their size was measured from the photographs and was evaluate to correspond to 8.2 mm spheres. While the large bubbles in Fig. 17 shoot towards the surface, characteristically forming bubble clusters which move together, the motion of the small bubbles in Fig. 18 was slow. This slow rising speed is an advantage, providing more time for the oxygen transport between the bubble and the water. In a later experiment, the length of the feedback loop tube was decreased considerably, which resulted at oscillation frequency measured to be f = 90 Hz.. The mechanism of bubble formation changed towards the dynamic release of the bubbles, which remained in operation albeit with immediately apparent irregularity even when the oscillation of the water tube B (which decreased so as to be practically unrecognisable anyway) was eliminated completely. The obvious advantage of this high-frequency operation mode was the visibly increased production rate of the bubbles from the same number of nozzles. The rising bubbles with the surrounding water formed a clearly visible column, decreasing in cross section in the vertical direction. Fig. 18. Photograph of the small ~ 0.8 mm dia. bubbles leaving the air holes of the aerator during the second half of the period. With the oscillation, the character of the generated bubbles changed essentially. The process of bubble generation takes place in two stages, Figs. 15 and 16, which at the originally set low oscillation frequency were clearly discernible. The bubble size decreased by a decimal order of magnitude, to the mean diameter 0.86 mm (again evaluated from the photographs. As the example in Fig. 18 shows, the bubbles were released progressively along the length of the air nozzle row, in a pattern resembling a result of an acting travelling wave. A striking difference was observed in the rising speed of the bubbles. Conclusions The main contribution of the present paper is qualitatively new approach to the aeration problem. By pulsating the supply air flow preferably using the cheap and reliable fluidic oscillator -: it demonstrates the feasibility of a new method of producing air bubbles in liquids. The size of the bubbles may be demonstrably decreased by an order of magnitude. This change of the size, together with the accompanying decrease in the bubble rising speed, provides a substantially increased opportunity for mass transfer between the injected gas (air) and liquid (water). Acknowledgement The stay of the principal author (Prof. Tesař) at the University of Sheffield during the development of the oscillator as well as the novel aerator was made possible by EPSRC grant to Dr. Zimmerman. The manufacturing of the aerator was made possible by financial support of Food Processing Faraday Partnership. 12

13 References [1] Stevenson D. G.: Water Treatment Unit Processes, London, Imperial College Press, 1997 [2] Hänel K.: Biological treatment of sewage by the activated sludge process, Chichester (Ellis Horwood books in water and wastewater technology): Ellis Horwood, 1988 [3] Diaz M., Komarov S.V., Sano M,: "Bubble Behaviour and Adsorption Rate in Gas Injection through Rotary Lances", ISIJ International, Vol. 37, No. 1, p. 1, 1997 [4] Young T., Philosophical Transactions of the Royal Societ (London), Vol.95, p.65, 1805 [5] Laplace de (Marquis) P. S.: "Traité de Mécanique Céleste", 4th volume, 1st section (Teéorie de l'action capillaire) of the supplement to Book 10 (Sur divers points relatifs au systéme du monde), publ. by Cez Courier, Paris, 1806 [6] Tesař V., Hung C.-H., Zimmerman W.B.J.: No- Moving-Part Hybrid Synthetic Jet Actuator, in review process, Sensors and Actuators A: Physical, Elsevier [7] Tesař V.: A Mosaic of Experiences and Results from Development of High-Performance Bistable Flow- Control Elements, Proceedings of the Conference 'Process Control by Power Fluidics', Sheffield, United Kingdom 1975 [8] Perera P.C., Syred N.: A Coanda Switch for High Temperature Gas Control, Paper 83-WA/DSC- 26, American Society of Mechanical Engineers, Winter Annual Meeting, Boston

14 14