Biological waste water purification reactor and method

Abstract

A biological reactor for treating wastewater. The reactor includes a gas injection system and a system for directing wastewater into the reactor. Further, the reactor includes a biological filter comprised of a packed bed of biofilm carriers and a volume of moveable biofilm carriers. During the method of treating the wastewater, the wastewater moves upwardly through the reactor and through the biological filter while gas is emitted from the gas injection system.

Claims

1. A method of biologically treating wastewater and removing suspended solids from the wastewater in a bioreactor, comprising: in a purification mode: directing the wastewater into a bottom portion of a bioreactor via a wastewater injection system that includes an array of openings that permits the wastewater to be injected into the bioreactor; injecting air into the bioreactor; directing the wastewater and air upwardly through an expansion space containing moveable hollow biofilm carriers and through flowthrough passages in the moveable hollow biofilm carriers; directing the wastewater and air upwardly from the expansion space through a packed bed of biofilm carriers generally disposed over the expansion space and having a density less than the density of the moveable hollow biofilm carriers contained in the expansion space and wherein the biofilm carriers of the packed bed are larger than the flowthrough passages of the moveable hollow biofilm carriers; retaining the packed bed of biofilm carriers under a perforated screen in the bioreactor; forming a biofilter with the packed bed of biofilm carriers and the moveable hollow biofilm carriers; as the wastewater moves upwardly through the expansion space and the packed bed of biofilm carriers, collecting suspended the solids in the biofilter and collecting treated wastewater in a space in the bioreactor; and in a backwashing mode: removing the suspended solids retained by the biofilter through backwashing where at least some of the treated water held in an upper space in the bioreactor is employed as a backwash; directing the backwash downwardly through the packed bed of biofilm carriers and through the expansion space and the moveable hollow biofilm carriers contained therein and expanding the packed bed of biofilm carriers into the expansion space; entraining the suspended solids retained by the biofilter in the backwash; discharging the backwash containing the suspended solids from the bioreactor by directing the backwash and entrained suspended solids through the array of openings in the wastewater injection system; preventing the discharge of the moveable hollow biofilm carriers with the backwash; and forming a protection grid in the bioreactor with the moveable hollow biofilm carriers and generally preventing the biofilm carriers of the packed bed from passing through the protection grid of the moveable hollow biofilm carriers and being discharged from the bioreactor with the backwash through the array of openings in the wastewater injection system.

2. The method of claim 1 including nitrifying and denitrifying the wastewater by: forming an anoxic zone in the bioreactor; wherein injecting air into the bioreactor includes injecting air into the bioreactor at a point above the anoxic zone; denitrifying the wastewater by passing the wastewater through the anoxic zone and providing no air to the anoxic zone; and nitrifying the wastewater by passing the wastewater through a nitrification zone disposed over the anoxic zone and injecting air into the wastewater passing through the nitrification zone.

3. The bioreactor of claim 2 wherein the biofilm carriers of the packed bed have a density of 60-90 kg/m.sup.3 and wherein the biofilm carriers of the packed bed are expanded particles having a granulametric size of 2-6 mm.

4. The bioreactor of claim 2 wherein the wastewater injection system includes a series of elongated and spaced apart channels extending adjacent the bottom of the bioreactor and wherein the array of openings of the wastewater injection system are formed in the series of channels.

5. The method of claim 1 further including backwashing the biofilter through a series of steps in the following order: Step 1: loosening the suspended solids held in the biofilter by instituting a pre-wash phase wherein the pre-wash phase includes directing only water downwardly through the packed bed of biofilm carriers and the moveable hollow biofilm carriers; Step 2: after the pre-wash phase, further loosening the suspended solids held in the biofilter by injecting air into the biofilter; Step 3: after Step 2, settling the suspended solids by implementing a pause phase for a selected period of time where neither air nor the treated wastewater is directed through the biofilter; Step 4: cleaning the biofilter by employing an alternating process comprising a treated wastewater backwashing process and an air injection process where in the first phase of Step 4, the treated wastewater is directed downwardly through the biofilter cleaning suspended solids therefrom, and in a second phase of Step 4, only air is injected into the biofilter, and wherein as a part of Step 4 the first and second phases are repeated; and Step 5: after Step 4, rinsing the biofilter with the treated wastewater.

6. The method of claim 1 wherein during the purification mode the moveable hollow biofilm carriers are not fluidized.

7. The method of claim 1 wherein the biofilter is backwashed at a washing speed of 30-100 m/h.

8. A bioreactor operative in a first mode to treat wastewater and remove suspended solids from the wastewater and operative in a second mode to perform a backwashing operation comprising: a packed bed of biofilm carriers disposed in the bioreactor; an expansion space in the bioreactor below the packed bed of biofilm carriers; moveable hollow biofilm carriers disposed in the expansion space below the packed bed of biofilm carriers and having a density greater than the density of the packed bed of biofilm carriers: wherein the moveable hollow biofilm carriers include flowthrough passages smaller than the biofilm carriers of the packed bed; an air injection system for injecting air into the bioreactor; a wastewater injection system including an array of openings and configured to inject wastewater into the reactor wherein the wastewater and air move upwardly through the expansion space, the flowthrough passages of the moveable hollow biofilm carriers, and the packed bed of biofilm carriers; wherein the array of openings of the wastewater injection system are smaller than the moveable hollow biofilm carriers; wherein the packed bed of biofilm carriers forms at least a part of a biofilter for removing suspended solids from the wastewater moving upwardly through the biofilter; a protection grid formed in a lower portion of a bioreactor by the moveable hollow biofilm carriers and configured to generally prevent the packed bed of biofilm carriers from passing through the protection grid and being discharged with a backwash through the array of openings in the wastewater injection system; wherein in the first mode wastewater enters the wastewater injection system and moves upwardly through the packed bed of biofilm carriers that remove suspended solids from the wastewater; and wherein in the second mode the wastewater injection system and the array of openings thereof are also configured to discharge the backwash and the suspended solids entrained in the backwash during the backwashing operation and wherein the protection grid generally prevents the packed bed of biofilm carriers from passing through the protection grid and being discharged with the backwash.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a sectional view of a bioreactor according to prior art EP0504065.

(2) FIG. 2 shows a sectional view of a bioreactor according to one embodiment of present invention.

(3) FIG. 3 shows a sectional view of a preferred embodiment of the hollow carriers as used to form the volume of movable particles in present invention.

(4) FIGS. 4A and B show a perspective view of the bottom of the bioreactor and its alternative fluid injection systems.

(5) FIG. 5 shows a sectional view of a bioreactor according to one embodiment of present invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) According to the figures provided, the prior art bioreactor 1 shown in FIG. 1 and the bioreactors according to two preferred embodiments of present invention 1 and 1 shown in FIGS. 2 and 5 comprise in their lower part the space 2 for the expansion and removal of the sludge, the fluid injection system 3, the gas injection system 4 and the packed bed 5 retained by the perforated plate 6 acting as a ceiling; and, finally, the free upper zone 7 acting as a washing reserve where the treated water is removed via outlet 8.

(7) The fluid injection system 3 serves at the same time as the system for sludge out take during backwashing operation of the bioreactor as indicated by the arrows in both directions in FIGS. 1, 2, 4A, 4B and 5.

(8) The liquid to be treated arrives via inlet 9 and is introduced via valve 12 into the zone 2 through the fluid injection system 3 beneath the gas injection device 4. When the gas is introduced by the gas injection device 4, an intensive exchange is obtained between the gas, the water to be treated and the biofilm that clings to the particles. During this operation, the packed bed 5 stays in a non-turbulent state. It is therefore a fixed bed.

(9) Now referring to FIGS. 2 and 5, in the bioreactor of present invention the space 2 for the expansion and removal of the sludge is filled partly with hollow carriers 10. These form what is in present invention referred to as a volume of movable particles. However, other than described above for the packed bed 5, this volume of movable particles does not form a fixed bed, but the hollow carriers 10 can move freely. This means that when the gas is introduced at the base by the gas injection system 4, the volume of movable particles will be in a turbulent state and the hollow carriers 10 will be moved around in the space 2 by the current. Furthermore, due to the fact that these hollow carriers 10 have flow through passages, not only does the water and gas move the carriers around in the space 2, but it also flows through the hollow carriers 10 so that all inner surface areas of these hollow carriers 10 will be in contact with the water and the gas. This maximizes the contact of the water to be treated with the biofilm surface that is present on all surfaces, outer and inner surfaces, of the hollow carriers 10. The inner surfaces of the hollow carriers are referred to as the protected surface area to emphasize the fact that these surfaces do not get harmed by the free movement of the carriers in the water and their resulting collisions. In contrast to that there is the total surface area which refers to the total surface area that is available for biofilm formation on the hollow carriers 10 and thus includes all inner and outer surfaces.

(10) Now referring again to both FIGS. 1, 2 and 5, due to the accumulation of suspended solids and the biological growth within the packed bed 5 and the hollow carriers 10, the material gradually gets clogged. The increase in the load loss may be followed by manometrical measurement or by the rising of the level of liquid in the loading or load loss measuring column at inlet 9.

(11) When a predefined load loss value is reached, the washing of the bed is started. Washing means the removal of excess sludge from the particles of the biofilter, which leaves the bioreactor through the pipe/channel system located at the bottom of the reactor. This pipe system is connected to a flush valve 11. To start the washing, valve 12 is closed and valve 11 is opened to a predefined position until the desired washing speed is obtained. The rapid outflow, in a counter-current flow direction, of the liquid treated and stored in the upper part 7 of the reactor enables the expansion of the material of the packed bed 5. For the granulometric size and density of the material of the packed bed 5 as defined here above, a washing speed of 30 to 100 m/h is chosen. This washing speed is equally suitable for the hollow carriers 10 located in the space for expansion and sludge removal 2.

(12) The volume of the normal expansion zone needed for the packed bed 5 during backwash is less than the volume of the space for expansion and sludge removal 2.

(13) This means that during backwash the freely moving hollow carriers 10 will move around going mostly towards the bottom of the reactor and thereby leaving enough space for the particles of the packed bed to be fluidized without being limited in their movement. The movement of the hollow carriers 10 towards the bottom can furthermore behave as an additional protection grid, in those very rare cases where particles from the packed bed 5 located above will move further down towards the sludge outlet system 3.

(14) As already described above it is to be understood that the fluid injection system 3 serves at the same time as the system for sludge out take during backwashing operation of the bioreactor as indicated by the arrows in both directions in FIGS. 1, 2, 4A, 4B and 5.

(15) The space 2 is generally provided with a relatively high volume compared to the total volume under the perforated retention ceiling of the biological purifying reactor of around 30-50% in the prior art solutions to avoid loss of particles during the backwashing process. In present invention, however, this space is being used more efficiently by filling 20-70%, preferably 30-65%, of this volume with the hollow carriers 10 thus providing for more biologically active surface while keeping the same total volume of the bioreactor. Taking into account the prior art solutions, one would expect that minimizing the volume of the free space 2 would lead to a higher loss of particles of the packed bed during backwashing, since this space is generally considered to be required for the expansion of the packed bed particles during backwash as described above. However, according to present invention the hollow carriers 10 are retained in the reactor by the fluid injection system 3, due to the fact that the size of the holes 15 (shown in FIGS. 4A and B) of the fluid injection system 3 is chosen to be smaller than the smallest diameter of the hollow carriers used. The fluid injection system 3 thus functions simultaneously as a protection grid preventing the hollow carriers 10 from being washed out of the reactor. The preferred size for the holes 15 of the fluid injection system 3 ranges from 6 to 60 mm in diameter.

(16) At the same time the hollow carriers 10 that move downwards during backwash act as an additional protection grid for the particles of the packed bed 5 preventing them from reaching the sludge outlet system 3. It is very important for the smooth running of the reactor of present invention that the hollow carriers provide for this extra barrier preventing the particles of the packed bed 5 to exit the reactor. In this regard it is extremely important to carefully select the right combination of hollow carriers and particles for the packed bed. The particles of the packed bed have to be chosen so that they cannot enter the inner flow passages of the hollow carriers, which would lead to clogging of the hollow carriers and a decrease in efficiency of the reactor. This means that the size of the particles of the packed bed has to be larger than the largest inner flow passage present in the hollow carriers, or vice versa the hollow carriers have to be chosen so that their inner flow passages are smaller than the smallest particle of the packed bed.

(17) Another important parameter to keep in mind when selecting suitable hollow carriers and particles for the packed bed is the density of the particles. As density is depended on temperature and pressure the density ranges of this application are determined for 4 C. and normal atmospheric pressure. As described above the density of the hollow particles ranges from 900 to 1200 kg/m.sup.3, preferably between 920 to 980 kg/m.sup.3. Furthermore the density of the packed bed particles is below 900 kg/m.sup.3, more preferably below 500 kg/m.sup.3. This will minimize the mixing of the two types of particles during normal operation and backwashing.

(18) In a preferred embodiment the density of the packed bed particles ranges from 15-100 kg/m.sup.3, preferably from 35-90 kg/m.sup.3, more preferably from 60-90 kg/m.sup.3. This low density ensures that the particles of the packed bed float upwards to the water surface if not retained. The particles thus return very quickly, within seconds, to their location under the retention ceiling after backwash, whereas the more dense hollow particles are kept suspended and moving in the water under the packed bed. In addition, due to the low density of the particles of the packed bed the upward force of these particles is very high. The packed bed is therefore very compact and an almost completely fixed bed. The filtration capacity of this packed bed is thus very high. Furthermore the density difference between the two different kinds of particles ensures that the mixing of the two kinds of particles during normal operation of the reactor is extremely limited.

(19) In present invention to achieve the organization of the two types of particles and the formation of a highly compact packed bed there is no additional upflow of air or water necessary. It is therefore not necessary to control and adjust the flow to maintain the bioreactor itself in a functional state. The flow can thus be purely adjusted to achieve optimal water treatment efficiency. In contrast to that, conventional prior art reactors containing a fixed and fluidized bed of particles having a density closer to the density of water, usually require an additional upflow of air or water to keep the lower bed fluidized and the upper bed packed. Furthermore, the reorganization of the two beds after backwash is not achieved as quickly as in the case of the packed bed and the movable carriers in present invention.

(20) A further disadvantage of using particles that have a density closer to the density of water and thus having a less compact fixed bed is that when injecting air for purification of the wastewater, the injected air can form pathways in the fixed bed. These pathways will decrease the treatment efficiency of the fixed bed. In present invention this does not happen. Furthermore, due to the fact that the packed bed is so compact, the air bubbles take longer to move through the packed bed. This increases the time for oxygen transfer from the air to the water thus increasing the activity of the biofilm.

(21) Now referring back to the operation of the bioreactor shown in FIGS. 2 and 5, the rapid change to a counter-current flow makes it possible to carry along the solids stored in the interstitial spaces and dislodge the excess biomass that has collected on the surface of the materials, but the above-mentioned range of speed makes it possible to preserve the active biofilm on the materials. After the draining of the reserve 7 and the closing of the valve 11, it is possible to restart the feeding by opening valve 12 with a load similar to the one used before washing.

(22) Another advantage of using a counter-current backwash is that the particles in the upper part of the packed bed do not come into contact with the pollutants, since during operation only purified water reaches these parts of the packed bed, whereas the main part of the pollutants stays in the lower part during operation. Then during backwash, the pollutants are moved downwards again so that the upper part of the packed bed will not get into contact with the pollutants during backwash either. In contrast to that a co-current backwash brings all of the packed bed particles in contact with the total pollutants, thus decreasing the efficiency of the packed bed. In addition, pollutants reach and can clog the retention ceiling when co-current backwash is used. During counter current backwash the retention ceiling is spared.

(23) A recycling of the purified effluent by a pump may, if necessary, enable the distribution to be improved or may enable the supply of nitrates in the prefiltration zone.

(24) To extend the periods of time between the washings, very brief flushing operations, by the opening of the valve 11, may be done periodically to loosen the material and enable a deeper penetration of the impurities into the filtering bed. These mini-washing operations will further unclog the lower part of the filter, which is more charged with suspended solids. The fast flushing operations may be implemented in such a way as to provide for a balanced load loss throughout the height of the filtering beds.

(25) Sequential gas injection may be maintained during the washing to aid with the unclogging of the packed bed as well as the hollow carriers. Short sequences of air during backwashing will shake up the hollow carriers and prevent their clogging. The sequences of air can be introduced during a pause in the wash water inlet as for example described in the preferred embodiment below, or can be introduced in sequences while the wash water is running continuously.

(26) In a preferred embodiment of present invention the backwash procedure includes the following steps: a) Pre-wash with water alone

(27) This operation, during which the filter is switched out, consists of a pre-wash (with water alone) by opening the wash water drain valves for a predetermined period of time, T0, in order to loosen the sludge before injecting the air for mixing. b) Loosening of sludge with air alone

(28) This step, during which the wash water drain valves are closed, consists of injecting air into the air system to mix the excess sludge and loosen it. This step lasts T2. c) Pausing

(29) Pausing to let loosened material settle for the time T14 d) Cleaning by alternating water and air phases

(30) This step consists of the successive injection of: Water alone for T1 Air alone for T2 Pausing for T14 Water alone for T1 Air alone for T2 Pausing for T14

(31) These phases are designed to loosen all the excess sludge and to evacuate it partially towards the filter medium. Additional water scour phases (pre-set time T1) and an air scour phase (pre-set time T2) and a pause (pre-set time T14) can be added to obtain more thorough washing. e) Rinsing with water alone

(32) This phase consists in evacuating residual excess sludge with a descending flow of water for a time T3.

(33) The backwash sequence terminates when T3 has elapsed. Depending on the actual filtration velocity, the filter is either then put back into filtration mode or put into standby mode.

(34) One of many advantages of the use of freely moving hollow carriers instead of a second fluidized or fixed bed in the reactor are that only minor additional barometrical headloss is introduced during normal operation of the bioreactor by these movable carriers. This leads to less energy consumption for aeration of the bioreactor.

(35) Last but not least the hollow carriers provide for an improved distribution and a slower movement of the gas upwards towards the packed bed, since gas bubbles will be split into smaller bubbles when coming into contact with the freely moving hollow carriers in the lower part of the reactor. This ensures improved supply of the biofilm with gas thus resulting in higher efficiency of the reactor.

(36) The hollow carriers furthermore reduce the clogging of the packed bed with total suspended solids (TSS), since the produced and accumulated biomass on the hollow carriers will be removed during backwashing. Furthermore, since less COD reach the packed bed, the growth of biomass is slower on the packed bed particles than in the conventional bioreactor as shown in FIG. 1 where only a packed bed is used. This minimizes the backwash frequency and therewith also the wash water load needing to be discarded.

(37) In FIG. 3 a side view of an exemplary hollow carrier suitable for use in present invention is shown. The structure shows outer and inner walls of the carrier that are suitable for growth of biofilm. As can be readily understood from this drawing, biofilm that grows on inner surfaces of the carrier will be protected from being harmed via collision with other carriers during operation of the bioreactor.

(38) In FIGS. 4A and B two alternative fluid injection systems are shown. In FIG. 4A a fluid injection system made of concrete is shown which is according to the fluid injection system shown in the bioreactor in FIG. 2. The fluid injection system can for example be made of concrete, or other suitable materials known in the art. At the bottom of the bioreactor 13 inlet channels 14 with holes 15 are formed. In FIG. 4B, which is an alternative solution, pipes 16 with holes 15 are inserted at or above the bottom of the bioreactor. These pipes can for example be made of steel or plastic, or other suitable materials known in the art. In both embodiments the water inlet channels function also as the sludge outlet channels during backwash as indicated by the arrows pointing in both directions. The size of the holes 15 is chosen to be smaller than the chosen size of the movable particles 10 so that the particles cannot pass through the holes 15 and are retained by the fluid injection system 3. During backwash, when the movable particles 10 are pressed down towards the bottom of the reactor, they are retained by the fluid injection system 3 avoiding loss of the valuable movable particles 10.

(39) FIG. 5 shows an alternative embodiment of the reactor of present invention, which functions in the same way as the bioreactor as shown in FIG. 2, with the difference being that it comprises a second air injection system 17 located within the packed bed 5. When the bioreactor is operated and air introduced via the air injection system 4 and the second air injection system 17, the packed bed 5 will comprise an aerated zone 19 and a non aerated zone 18 within the packed bed as shown in FIG. 5. Within the aerated zone 19 nitrification using O.sub.2 from the injected air can take place. When no aeration from the air injection system 4 takes place, an anoxic zone, i.e. a zone with oxygen coming from NO.sub.3N alone, can be assured allowing for removal of nitrates (denitrification) when the oxygen from the NO.sub.3N instead of the oxygen supplied by aeration is used for carbon removal. It is to be understood that the second air injection system 17 does not serve for air injection during backwashing, but only during normal operation of the bioreactor. As described above for the embodiment with only air injection system 4, also in this embodiment, where air is introduced additionally or exclusively through air injection system 17, an intensive exchange is obtained between the gas, the water to be treated and the biofilm that clings to the particles. During this operation, the packed bed 5 stays in a non-turbulent state and is therefore a fixed bed.

(40) According to an advantageous embodiment of the method of present invention, one or more batteries of bioreactors as described above are set up in parallel in one large water treatment plant. Each battery of parallel bioreactors in one large water treatment plant can contain from 1-20 bioreactors. However, an amount of 4-14 bioreactors per battery is preferred. One to 10 batteries of bioreactors can be implemented in parallel in one water treatment plant.

(41) Each battery of bioreactors has one common water reservoir that feeds the loading columns individually associated with each bioreactor. This way excess pressure in the bioreactors can be prevented when one column is clogged, since the other loading columns can compensate the pressure.

(42) The water reserves for the purified water of each bioreactor are also interconnected and form one big compartment for purified water at the top of each battery. Thus, the purified water of all the bioreactors in operation in one battery supplies the flow of water for the backwashing of the clogged bioreactor that at that time is being backwashed.

(43) It is preferred for the smooth running of the water treatment plant of present invention that only one bioreactor at a time is backwashed while the other bioreactors are in normal water treatment operation. The use of several batteries in parallel allows the backwashing of more than one bioreactor per plant at a time, even though only one bioreactor per battery can be backwashed at a time, which increases the efficiency of the treatment plant.

EXAMPLE

(44) A test run was made to measure the efficiency for removal of total suspended solids (TSS) and soluble COD of the biological purifying reactor of present invention as shown in FIG. 2.

(45) The reactor used for the test-run was a 0.9 m diameter column of 6.5 m height. The reactor had 3.5 m of packed bed using a spherical media with a diameter of 4.5 mm and a density of 55 kg/m.sup.3. The volume below the packed bed having a height of 1.9 m was filled with 35% of hollow carriers with a density of 960 kg/m.sup.3 and protected surface area of 800 m.sup.2/m.sup.3. The reactor was fed with municipal wastewater coming from the primary settler of the St. Thibaut des Vignes WWTP (France) and the total suspended solids (TSS) and Soluble Chemical Oxygen Demand (Filtered COD) content of the wastewater before and after the reactor was measured.

(46) The reactor was seeded during 3 weeks at 1 m/h of influent flow and when sufficient activity had been documented the load to the reactor was increased in several steps. 24 hour average samples were taken during the highest loading of the plant.

(47) The result of the test run is shown in tables 1 and 2 below. The results are shown in comparison to the standard design values and results as expected from the biological purifying reactor as disclosed in the prior art as shown in FIG. 1.

(48) TABLE-US-00001 TABLE 1 Comparison of efficiency for removal of total suspended solids (TSS) by the biological purifying reactors as disclosed in the prior art (FIG. 1) and in present application (FIG. 2). TSS Removal Cycle TSS Load TSS Influent Effluent Rate Duration (kg/m.sup.3/d) (mg/l) (mg/l) (%) (h) Prior art 2.7 100 25 75 24 reactor (FIG. 1) Reactor of 6 200 70 65 24 present invention (FIG. 2)

(49) TABLE-US-00002 TABLE 2 Comparison of efficiency for removal of COD by the biological purifying reactors as disclosed in the prior art (FIG. 1) and in present application (FIG. 2). CODsol CODsol CODsol Removal Cycle Load Influent effluent Rate Duration (kg/m.sup.3/d) (mg/l) (mg/l) (%) (h) Prior art 2.9 200 50 75 24 reactor (FIG. 1) Reactor of 6 170 60 65 24 present invention (FIG. 2)

(50) It should be noted that the St. Thibaut des Vignes WWTP has a high degree of industrial influent coming into the WWTP leading to a relatively large non-degradable fraction of soluble COD in the incoming wastewater. Hence the amount of soluble COD in the effluent is slightly higher than the amount that would be expected from a more classical municipal wastewater, leading to a lower achieved removal rate for this parameter. Such classical municipal wastewater was used for obtaining the efficiency data for the biological purifying reactor as disclosed in the prior art and shown in FIG. 1.