METHOD AND APPARATUS FOR PURIFYING A LIQUID BY MEANS OF FLOTATION

Abstract: Method for purifying a liquid by means of flotation at which the water to be cleaned as well as the dispersion water (or liquid) is fed to the lower part of a first long and narrow standing preferably cylindrical vessel (23) whereby in the water column defined by the form of the vessel a rising water current is achieved which gives to the particles or flocculations in the water column a rising movement which movement increases the flotation effect of the micro bubbles which are created at the dispersion nozzle as well as during the successive decrease in pressure which takes place in the water column and which bubbles by means of the design of the vessel will be given a high concentration, which vessel (23) at least at its upper open end is surrounded by a second open vessel (22) the edges of which reach higher than the upper edge of said first vessel (23) and at the lower part of which a pipe joint (32) is arranged for the outflow of the cleaned liquid and at the upper part of which a third vessel (29) is arranged for receiving of the separated impurities from the second vessel (22).

Description: METHOD AND APPARATUS FOR PURIFYING A LIQUID BY MEANS OF FLOTATION. TECHNICAL FIELD The present invention concerns a method and an apparatus for cleaning of liquids by means of flotation. BACKGROUND ART When cleaning liquids for instance water, drinking water as well as sewage water, flotation plants are used to an increasing extent in order to increase the flowrate of the water through the treatment plants. As a consequence of this it will be possible to decrease the size of the necessary basins and thereby the size of the complete treatment plants. When it comes to drinking water the requirements on the finished product are mainly uniform and the design of a treatment plant is mainly decided by the character of the source of water supply. If the source is a lake or a water course usually separation of larger particles is taking place in a filter chamber whereafter the water is subjected to a chemical and biological treatment carried out by means of the addition of precipitation- and flocculation- agents in a flocculation chamber. The particles which are precipitated are then to be separated from the water which can be done by means of sedimentation which means that the particles through the influence from gravity sink to the bottom in a sedimentation basin. Different types of particles have different density and therefore different sinking velocity. In order to make it possible for particles having low density, "light particles", to sediment while the water passes a sedimentation basin a long time is required which means that the surface of the sedimentation basin must be large. The time for the particle separation could be considerably reduced by means of flotation which means that microscopic air bubbles are added to the water which adhere to particles or particle flocculations in the water and also contribute in the process of creating particle flocculations and keeping them together. The air bubbles raise fast towards the water surface bringing adhered particles and particle flocculations to the water surface where they create a stable sludge cover which can be removed by means of scrapers or by intermittently raising the water level in the flotation basin so that the sludge cover transfers to a sludge channel arranged at one of the edges of the basin. The microscopic air bubbles which are used in the flotation procedure have a diameter of 30-80 micrometer and can not be generated for instance by means of the injection of air directly in the water. Microscopic air bubbles for flotation of drinking water are usually generated by means of dissolving air in clean water in a pressured vessel under overpressure whereby the biggest possible saturation level is aimed at. This water which usually is called dispersion water is fed with maintained high pressure to the inlet for the flocculated water in the flotation basin positioned at its bottom where it is added to said water via special nozzles or sprayers which are designed to give an instantaneous decrease in the pressure at which the air dissolved in the water is released in the form of microscopic bubbles creating a dispersion of air in the water. The pressure vessels which are used for the generation of high pressure dispersion water, saturated with air to the greatest possible extent, are usually kept half filled with water by means of a control unit including sensors and the in- and outlets for the water are positioned below the water surface in the pressured vessel. The addition of compressed air takes place in the space filled with air above the water surface and the vessel is rather big in order to make the contact surface between air and water large. In another known embodiment the pressure vessel which often is as tall as a man has a reduced diameter and the consequently reduced contact surface between the water and air has been compensated by the feeding of the water through a nozzle arranged in the side wall of the vessel in the air filled space through which nozzle the water is spraying against the opposite wall of the vessel where the water jet to some extent is splitted up. From the International patent application WO 96/22248, the description of which is herewith incorporated in the present application it is already known a very efficient apparatus for the generation of air saturated water for use in flotation plants. From the International patent application WO 96/22249 the description of which is also herewith incorporated in the present application it is further known a dispersion nozzle having a very high efficiency for use in flotation plants. The flotation method has considerably decreased the time for water treatment and the decreased time of flow through the plant means that water treatment plants could be made smaller keeping the same capacity. When the space requirement of the treatment plants decreases new application fields are opened. Such a development requires that all the devices are made smaller and if possible are given improved characteristics. This also goes for the apparatus for producing dispersion water in which already treated, recirculated, water is used. An increase of the saturation level in the dispersion water decreases the need of recirculated water whereby the efficiency of the whole treatment plant is improved. The method has, however, not only brought advantages. Thus, it has at the same time caused an increase in the cost of the cleaning procedure due to the fact that already treated water is recirculated and this used for the production of dispersion water. In order to make the procedure more cost effective the consumption of dispersion water has to be kept low by means of sophisticated methods on one hand by means of an increase of the saturation level in the dispersion water and on the other hand by making the dispersion more efficient at the inlet to the flotation basin. As mentioned above the dispersion water is according to prior art introduced into the flotation basin by means of a number of sprayers or nozzles. Each of these sprayers is provided with a check valve which creates the instantaneous decrease in the pressure of the dispersion water which is required in order to release the air in the water in the form of microscopic bubbles. The design of the dispersion nozzle according to WO 96/22249 is working with a release of the pressure in two steps. The nozzles are often positioned in a rake-formed arrangement on one side of a distribution tube having a length of several meters, to which tube the feeding is taking place through a feeding tube arranged at the middle of the distribution tube essentially perpendicular relative to the nozzles, which position, if we revert to the comparison with a rake, corresponds to the hole for the handle of the rake. A "rake" can comprise about 25 nozzles which are screwed into sockets welded on to the distribution tube. Complete distribution tubes provided with nozzles are comparatively expensive and if the nozzles are made of plastic they will easily be damaged e. g when cleaning the basin. Dispersion nozzles of the type described in WO 96/22249 could replace a number of nozzles of the other described type. In figure 1 which schematically and in a side view shows a conventionally equipped flotation basin in section the basin is generally designated 1. The basin has an inlet 2 to which water is fed from a flocculation chamber, not shown. By means of an inclined barrier 3 arranged in front of the inlet 2 in the basin 1 the water is caused to flow upwards in the basin 1. At the bottom of the pocket which is created between the wall 4 of the basin and the barrier 3 a set of tubes provided with nozzles or sprayers debouch, one tube 6 of which is shown in the figure. The tube 6 feed air saturated water, dispersion water, to the flocculated water supplied to the basin through the inlet 2 which water is given an increased movement by the dispersion water injected under pressure. The nozzles or sprayers 5 are so designed that the dispersion water is subjected to an instantaneous pressure decrease which releases the air in that water in the form of microscopic bubbles which in their movement towards the water surface adhere particles and particle flocculations and relatively fast carry most of these particles to the water surface where a sludge cover is created which by means of some kind of sludge scraper 7 or in another way is transferred to the sludge channel 8 at the wall 9 of the basin. Heavy particles which the Micro bubbles are not able to lift are collected by the filter 10 which is arranged between the back side of the barrier 3 and the wall 9 of the basin and through which filter the treated water passes on its way via the outlet 11 to a clear water basin, not shown. From this basin part of the treated water is pumped, under high pressure, to the water inlet 13 of the pressure vessel 12. The vessel 12 in which the dispersion water is produced is provided with compressed air via the air inlet 14 from a compressor, not shown, and the air saturated water is fed back via the outlet 15 and its branchings 6 to the nozzles 5 arranged in the flotation basin 1. BRIEF DESCRIPTION OF THE INVENTION The object of the present invention is to provide a method and an apparatus for cleaning of a liquid by means of flotation. The apparatus is characterized in that the water which is to be cleaned as well as the dispersion water is fed to the lower part of a first long and narrow standing preferably cylindrical vessel whereby in the water column defined by the form of the vessel a rising water current is achieved which gives to the particles or flocculations in the water column a rising movement which movement increases the flotation effect of the micro bubbles which are created at the dispersion nozzle as well as during the successive decrease in pressure which takes place in the water column and which bubbles by means of the design of the vessel will be given a high concentration, which vessel at least at its upper open end is surrounded by a second open vessel the edges of which reach higher than the upper edge of said first vessel and at the lower part of which a pipe joint is arranged for the outflow of the cleaned water and at the upper part of which a third vessel is arranged for receiving of the separated impurities from the second vessel. In a special embodiment the water which is to the cleaned is introduced through a separate opening in the lower part of the first vessel. At the same part of this vessel a dispersion nozzle is additionally arranged for example of the type which is described in the document WO 96/ 22249. In another embodiment the dispersion water has already been added to the water which is to be cleaned via a branch conduit to the intake conduit four the water to be cleaned. In another embodiment the third vessel is arranged at the upper edge of the second vessel and on the inside of said vessel. In another embodiment the third vessel is arranged at the upper edge of the second vessel and on the outside of said vessel. In another embodiment the separated impurities from the second vessel are transferred to the third vessel by means of a mechanical scraper. In another embodiment the separated impurities from the second vessel are transferred to the third vessel by means of flooding of the second vessel. The apparatus according to be invention shows high efficiency and combines high capacity with small dimensions. DESCRIPTION OF THE DRAWINGS One embodiment of the invention will be described more in detail with reference to the enclosed drawing on which figure 1 in a side view and schematically shows a conventionally equipped flotation basin in section and figure 2 shows partly in section an apparatus according to the invention for producing dispersion water. PREFERRED EMBODIMENT Figure 2 shows a simple drawing of an embodiment of the invention. The raw water, i.e. the water which is to be cleaned is first fed to a conventional flocculation vessel (not shown) in which a suitable flocculation agent is added and the water is kept in movement for instance by means of an agitator or in another way. A flocculation agent can alternatively be added directly via a branch conduit 21 to the conduit 20 for incoming water. Several different chemicals can be added depending on the desired effect. In order to increase the pH value in order to deposit lime a caustic solution can be used but successful results have also been achieved with other additives. After that the water is fed to a flotation vessel 23 which according to the invention could have the form of a vertical tube in which the actual flotation takes place. At the outlet 32 from the apparatus the pH value is then adjusted to the desired level. The vessels 22 and 23 could for example have cylindrical form but could have other forms as well. The efficiency of the apparatus is to an essential part dependent on the form of the vessels. In the figure the first vessel 23 has been shown concentrically arranged inside the second vessel 22 but other arrangements could also be envisaged. Additionally the first vessel does not have to be cylindrical but could have for instance conical form tapering upwards or downwards. Dispersion water is produced in the apparatus 26 which for example could be of the type described in the document WO 96/22248. A pump arrangement 25 pumps clean water from the lower part of the second vessel 22 to the apparatus 26 for generation of dispersion water. The dispersion water is then according to the example shown in figure 2 fed through a conduit 31 to the common inlet 24 for dispersion water and water to be cleaned. A dispersion nozzle is in this case arranged somewhere in the flow path between the apparatus 26 and the inlet 24. The addition of dispersion water could also be made by means of a separate nozzle at the bottom of the flotation tube 23. The impurities lifted by means of the flotation are collected at the liquid surface 28 in the second vessel 22. This liquid surface is kept at a constant level by means of a control system (not shown). A sensor for the level is indicated with the reference numeral 27. When a sufficient amount of sludge has been collected at the surface 28 this surface could for instance be raised by reducing the flow for example by means of a controlled valve, not shown. The sludge is then flowing into a sludge channel which schematically has been indicated with the reference numeral 29 and is disposed of through the outlet 30 from this channel. The transfer of the sludge to the channel could alternatively be carried out by means of a mechanical scraper. At the inside of the external vessel 22 inlet openings 33 for additional dispersion water could be arranged. Such openings could also be arranged in a ring 34 around the upper end of the first tube. This additional dispersion water creates with its raising movement by means of the air bubbles a filter function for the relatively clean water which has a downward movement. The second vessel does not necessarily have to be as tall as the first vessel. The bottom of the second vessel 22 could thus be arranged higher up so that the vessel becomes more shallow and thus gets a smaller volume. The complete apparatus could in this way be given smaller physical dimensions.

Claims: CLAIMS 1. Method for purifying a liquid by means of flotation characterized in that the water which is to be cleaned as well as the dispersion water is fed to the lower part of a first long and narrow standing preferably cylindrical vessel whereby in the water column defined by the form of the vessel a rising water current is achieved which gives to the particles or flocculations in the water column a rising movement which movement increases the flotation effect of the micro bubbles which are created at the dispersion nozzle as well as during the successive decrease in pressure which takes place in the water column and which bubbles by means of the design of the vessel will be given a high concentration, which vessel at least at its upper open end is surrounded by a second open vessel the edges of which reach higher than the upper edge of said first vessel and at the lower part of which a pipe joint is arranged for the outflow of the cleaned water and at the upper part of which a third vessel is arranged for receiving of the separated impurities from the second vessel. 2. Apparatus for purifying a liquid by means of flotation characterized in that the water which is to be cleaned as well as the dispersion water is fed to the lower part of a first long and narrow standing preferably cylindrical vessel whereby in the water column defined by the form of the vessel a rising water current is achieved which gives to the particles or flocculations in the water column a rising movement which movement increases the flotation effect of the micro bubbles which are created at the dispersion nozzle as well as during the successive decrease in pressure which takes place in the water column and which bubbles by means of the design of the vessel will be given a high concentration, which vessel at least at its upper open end is surrounded by a second open vessel the edges of which reach higher than the upper edge of said first vessel and at the lower part of which a pipe joint is arranged for the outflow of the cleaned water and at the upper part of which a third vessel is arranged for receiving of the separated impurities from the second vessel. 3. Apparatus according to claim 2 characterized in that the water to be cleaned is arranged to be fed through a separate opening in the lower part of the first tube and that at the same part of this tube a dispersion nozzle is arranged. 4. Apparatus according to claim 2 characterize in that the dispersion water has already been added to the water which is to be cleaned via a branch conduit to the intake conduit four the water to be cleaned.

Inventors: ERIKSSON HANS, ISAKSSON KENT, ERIKSSON, HANS, ISAKSSON, KENT
Applicants: ERIKSSON HANS, ISAKSSON KENT, ERIKSSON, HANS, ISAKSSON, KENT
Application year: 1998
Main classification: C02F1
IPC codes: C02F1