FIELD OF THE INVENTION

[0001 ] The present invention relates to a water purification method comprising flotation. Especially, the invention relates to a water purification method based on dissolved air flotation combined with filtration. The invention also relates to an apparatus for implementing the method.

BACKGROUND OF THE INVENTION

[0002] Flotation is widely used in the purification of industrial and municipal waste waters for removing of impurities such as fats, oil, grease and suspended solids. Before flotation, the wastewater to be purified is treated with flocculation chemicals, which are as a rule inorganic compounds or organic short-chained polymers. The flotation may be performed as a so-called dissolved air flotation. This process utilizes so-called dispersion water, which is generated by dissolving air in water under pressure. The air is then released from the dispersion water at atmospheric pressure in a flotation tank or basin containing the waste water. The released air forms small bubbles which adhere to suspended matter, which comprises impurities as well as flotation chemicals. The bubbles cause the suspended matter to float to the surface of water, from where the impurities may then be removed for example by a skimming device.

[0003] Flotation has been traditionally carried out in large tanks or basins having a rectangular form. The newest flotation basins, so-called third generation basins, have an almost square form and their depth is much larger than breadth and length. Dispersion water is introduced to the basin from the bottom, forming bubbles which rise upwards and form a thick bubble layer (such as 1 .5-2.5 m) to the wastewater surface. The wastewater flow in these conditions is turbulent and the residence time of the wastewater in the reaction channel is short.

[0004] WO 2006/131593 A1 discloses a water purification apparatus comprising a combination of sedimentation and flotation. The apparatus comprises a sedimentation basin, a flotation basin as well as means for conveying the wastewater into the sedimentation basin and means for conveying the purified water out of the apparatus. Furthermore, the apparatus comprises a vertical mixing channel placed in the flotation basin. The mixing channel contains micro-bubbles, to which the impurities stick to and rise together with the micro-bubbles upwards to the surface of the flotation basin. The micro-bubbles are generated by introducing pressurized dispersion water (usually 4-8 bar) in downward direction through another vertical channel to the bottom of the mixing channel. When arriving into the mixing channel through a nozzle from the vertical channel, the dispersion water is relieved to normal pressure conditions in the mixing channel and hereby forms the micro-bubbles in the mixing channel.

[0005] One of the disadvantages associated with the known arrangements for water purification is that they require a lot a space and the installation thereof may be cumbersome. Consequently, there exists a need for improved processes and arrangements alleviating these disadvantages.

BRIEF DESCRIPTION OF THE INVENTION

[0006] An object of the present invention is to provide a method and an apparatus for implementing the method so as to alleviate the disadvantages discussed above. The objects of the invention are achieved by a method and an apparatus which are characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

[0007] The invention is based on combining flotation and filtration into a compact arrangement, which can be manufactured at a plant and can then be easily transferred to and installed in locations where water purification is needed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 presents a diagrammatic cross-sectional view of an embodiment of the water purification apparatus of the invention.

[0009] Figure 2 presents a cross-sectional top view of the embodiment of Figure 1 .

DETAILED DESCRIPTION OF THE INVENTION

[0010] The invention relates to a method and an apparatus for purifying water containing impurities by utilizing flotation and filtration.

[0011] The method and apparatus of the invention may be applied to the purification of various industrial waste waters as well as municipal drink waters and waste waters. The method can also be applied to surface waters, groundwaters as well as natural mineral waters and spring waters. [0012] It has been found that the method and apparatus of the invention efficiently removes for example manganese, iron and arsenic, which have been as a rule considered problematical in the field of water purification.

Method

[0013] The method of the invention employs a container (1 ) having an essentially cylinder form, which comprises

[0014] at least three coaxially arranged vertical flow channels (2, 3, 4), which comprise an innermost tubular channel (2), an intermediate ring- formed channel (3) and an outer ring-formed channel (4) containing filter material (4'), whereby there is a connection (2e) between the innermost channel (2) and the intermediate channel (3) at the bottom of the container and a connection (4a) between the intermediate channel (3) and the outer channel (4) in the upper part of the container, and

[0015] means (5) for introducing pressurized dispersion water to the lower part (3a) of the intermediate channel,

[0016] wherein the method comprises the steps of:

[0017] flocculation by conveying the water under turbulent flow conditions in downwards direction through the innermost channel (2) via the connection (2e) to the lower part (3a) of the intermediate channel (3),

[0018] flotation by conveying the flocculated water in upwards direction through the intermediate channel (3) while introducing pressurized dispersion water to the lower part (3a) of the intermediate channel, thus forming air bubbles (3c) with adhered impurities, which rise upwards and grow towards to the upper part (3b) of the intermediate channel (3),

[0019] filtration by conveying the flocculation-treated water to and through the outer channel (4) containing filter material (4'), while retaining impurities on the surface of the filter material (4') in the outer channel (4),

[0020] recovering the purified water from the bottom of the outer channel (4), and

[0021] withdrawing the impurities from the surface of the filter material 4' in the outer channel (4).

[0022] The method of the invention is performed in a container (1 ) having an essentially cylinder form, which comprises at least three coaxially arranged vertical flow channels (2, 3, 4). These comprise an innermost tubular channel (2), an intermediate ring-formed channel (3) arranged round the in- nermost channel (2) and an outer ring-formed channel (4) arranged round the intermediate channel (3). The outer channel (4) contains one or more filter materials (4') arranged as one or more layers. The filter materials may be selected case by case.

[0023] In the container (1 ), there is a connection (2e) between the innermost channel (2) and the intermediate channel (3) at the bottom of the container and a connection (4a) between the intermediate channel (3) and the outer channel (4) in the upper part of the container. Connections (2e) and (4a) enable the water flow from one channel to another.

[0024] In a preferred embodiment of the invention, the connection (2e) between the innermost channel (2) and the intermediate channel (3) is arranged as an opening (open space). The height of the opening may vary case by case.

[0025] In one embodiment of the invention, the connection (4a) between the intermediate channel (3) and the outer channel (4) is arranged as an open space above the intermediate channel (3) and the outer channel (4).

[0026] In one embodiment of the invention, the innermost channel (2) widens in downwards direction and the intermediate channel (3) widens in upwards direction in the flow direction (A) of the water. This arrangement assists the floe formation in the flocculation step and the flotation in the flotation step.

[0027] Furthermore, the container (1 ) comprises means (5) for introducing pressurized dispersion water to the lower part (3a) of the intermediate channel.

[0028] The pressurized dispersion water may be introduced by any suitable means comprising appropriate outlets for releasing pressurized gas/water mixture. In one embodiment of the invention, the dispersion water may be conveyed through one or more tubes provided with one or more regulator valves or orifices having outlets (5'), whereby the outlets are arranged in the lower part (3a) of the intermediate channel. The pressurized dispersion water is obtained by dissolving air into water at a pressure in the range of 6-12 kPa, for example 8 kPa, to obtain a pressurized air-water mixture containing small air bubbles. When the pressure of the air-water mixture is released at the atmospheric conditions of the lower part of the intermediate channel, the air bubbles are mixed with the flocculated water, the water impurities adhere to the air bubbles and rise upwards in the intermediate channel (3). [0029] The container (1 ) further comprises means (5a) for withdrawing the purified water or for introducing wash water in the regeneration of the filter material (4') in the outer channel (4), whereby said means (5a) are arranged at the bottom of the container.

[0030] In one embodiment of the invention, the container (1 ) may be provided with means for oxidating the water to be purified.

[0031] In a further embodiment of the invention, the container (1 ) may be provided with a tube flocculator or a chamber flocculator for pretreat- ment of the water to be purified.

[0032] The flocculation step of the method of the invention takes place in the innermost channel (2) (flocculation channel) by conveying the water under turbulent flow conditions in downward direction through the innermost channel. In the flocculation, the water to be treated is subjected to flocculation, i.e. floes are formed from the water impurities, optionally using flocculation chemicals as flocculation aids. When flocculation chemicals are used, the method comprises a pretreatment step of combining the water with flocculation chemicals. Flocculation chemicals are well known in the field. As typical flocculation chemicals can be mentioned inorganic compounds and organic short- chained polymers, for example. Chemicals for adjusting the pH may also be added in addition to or optionally with the flocculation chemicals.

[0033] The turbulent flow conditions in the flocculation may be achieved inherently by appropriate adjusting of the flow velocity in the innermost channel. Optionally, various mixing means, such as paddle mixers may also be applied.

[0034] From the innermost channel (2), the water is conveyed through the connection (2e) to the lower part (3a) of the intermediate channel (3) (flotation channel). The flocculation may continue in the lower part (3a) of the intermediate channel (3) up to the vicinity of the point (5') where pressurized dispersion water is introduced.

[0035] The flotation step of the method of the invention takes place in the intermediate channel (3) by conveying the flocculated water in upwards direction through the intermediate channel (3) while introducing pressurized dispersion water to the lower part (3a) of the flotation channel. Growing air bubbles (3c) with adhered impurities are formed, which rise upwards towards to the upper part (3b) of the intermediate channel (3). The floes formed in the flocculation step are thus caused to rise upwards in the intermediate channel (3).

[0036] The intermediate channel (3) may be provided with throttle means (7), such as cones, in the vicinity of the outlets (5') of the means (5) for introducing pressurized dispersion water. The purpose of the throttle means is to assist mixing of the flocculated water with the dispersion water.

[0037] From the intermediate channel (3), the floc-containing water is conveyed to the surface of the filter material (4') in the outer channel (4). The floes containing the impurities as a sludge effluent is retained on the surface of the filter material and can be collected therefrom.

[0038] In the filtration step, the flocculation-treated water is subjected to filtration to remove any remaining impurities from the water. The filtration takes place in the outer channel (4) (filtration channel) containing filter material.

[0039] The purified water is recovered from the bottom of the outer channel through means (5a).

[0040] In the method of the invention, the water level in the outer channel (4) may be adjusted. Consequently, the filtration in the filtration channel may be performed as wet filtration or dry filtration.

[0041] The method of the invention may further comprise regeneration of the filter material (4') in the outer channel (4) by counter-current wash. In the regeneration, wash water is conveyed in upwards direction through the filter material (4'). The impurities entrained in the filter material rise upwards and may then be collected from the surface of the filter material.

[0042] In one embodiment of the invention, the method further comprises a pretreatment step of oxidating the water to be purified. The purpose of the pretreatment is to oxidate the divalent manganese and iron compounds of the water to accomplish more efficient removal of manganese and iron and arsenic from the water. By the oxidation treatment, the use of external floccu- lation chemicals may be avoided.

[0043] It has been found that the method of the invention efficiently removes for example manganese, iron and arsenic, which have been as a rule considered problematical in the field of water purification.

Apparatus

[0044] The invention also relates to an apparatus for implementing the method described above. The apparatus comprises one or more containers (1 ) having an essentially cylinder form, which comprises [0045] at least three coaxially arranged vertical channels (2, 3, 4), which comprise an innermost tubular channel (2), an intermediate ring-formed channel (3) and an outer ring-formed channel (4) containing filter material (4'), whereby there is a connection (2e) between the innermost channel (2) and the intermediate channel (3) at the bottom of the container and a connection (4a) between the intermediate channel (3) and the outer channel (4) in the upper part of the container, and

[0046] means (5) for introducing pressurized dispersion water to the lower part (3a) of the intermediate channel.

[0047] The arrangement of coaxial flow channels described above provides a sufficient length for the water stream in a compact process container to achieve efficient purification.

[0048] In a preferred embodiment of the invention, the wall (2') of the innermost channel (2) is arranged as the inner wall of the intermediate channel (3), the outer wall (3') of the intermediate channel is arranged as the inner wall of the outer channel (4), and the outer wall (1 ') of the container is arranged as the outer wall of the outer channel (4). This arrangement further assists in obtaining a compact container.

[0049] In one embodiment of the invention, the intermediate channel (3) is provided with throttle means (7), such as cones, in the vicinity of the outlets (5') of the means (5) for introducing dispersion water.

[0050] In a further embodiment of the invention, the innermost channel (2) widens in downwards direction and the intermediate channel (3) widens in upwards direction in the flow direction (A) of the water.

[0051] In one embodiment of the invention, the means (5) for introducing pressurized dispersion water comprise one or more tubes provided with one or more regulator valves or orifices having outlets (5'), whereby the outlets are arranged in the lower part (3a) of the intermediate channel.

[0052] In a preferred embodiment of the invention, the connection (2e) between the innermost channel (2) and the intermediate channel (3) is arranged as an opening (open space). The height of the opening may vary. The connection (4a) between the intermediate channel (3) and the outer channel (4) may be arranged as an open space above the intermediate channel (3) and the outer channel (4). [0053] The apparatus of the invention further comprises means (5a) for withdrawing the purified water or for introducing wash water in the regeneration of the filter material (4') in the outer channel (4), whereby said means (5a) are arranged at the bottom of the container. Consequently, the same arrangement may be used for withdrawing the purified water and for the regeneration. The means (5a) may comprise a set of tubes, for example.

[0054] The apparatus of the invention may also be provided with means for oxidating the water to be purified. The apparatus of the invention may also be provided with a tube flocculator or a chamber flocculator for pre- treatment of the water to be purified.

[0055] Furthermore, the apparatus of the invention is provided with means for introducing the water to be purified to the top of the innermost channel (2). The apparatus of the invention is also provided with means for removing the impurities from the surface of the filter material (4') in the outer channel (4), arranged in the upper part of the outer channel (4).

[0056] The apparatus of the invention has the advantages that it contains all necessary elements, i.e. the flocculation, flotation and filtration means in one "package". The apparatus can be manufactured at a plant and can thereafter be easily transported to and installed for use in the required locations. The impurities as a sludge effluent, including also the impurities from the regeneration, can be easily withdrawn from the surface of the filter material.

Γ0057Ί Embodiments of Figures 1 and 2

[0058] In the following, the method and apparatus of the invention will be described by a reference to the embodiment of Figures 1 and 2, which are not, however, meant to limit the invention in any way.

[0059] The apparatus of the invention comprises one or more containers 1 shown in Figures 1 and 2. Figure 1 shows a horizontal cross- sectional view of a container 1 having a cylinder form. Figure 2 shows a top cross-sectional view of the same container.

[0060] The container 1 comprises an innermost channel 2 (flocculation channel), an intermediate channel 3 (flotation channel) and an outer channel 4 (filtration channel), which are coaxially arranged round each other. The innermost channel 2 has a tubular form and is installed in the vertical direction round the vertical axis 6 of the container. The intermediate channel 3 is installed round the innermost channel and has a ring form. The intermediate channel is provided with throttle means 7 in the vicinity of the outlets 5' for introducing pressurized dispersion water. The outer channel 4 is installed round the intermediate channel 3 and has a ring form. An opening 2e is arranged between the end of the innermost 2 and the beginning of the intermediate channel 3 at the bottom of the container to enable the water flow from one channel to the another.

[0061] In accordance with Figures 1 and 2, the innermost channel 2 and the intermediate channel 3 widen in the flow direction of the water stream shown with an arrow A in Figure 1 .

[0062] Figure 1 also shows means 5 with outlets 5' for introducing dispersion water (an air-water mixture) to the lower part 3a of the intermediate channel. Figure 1 also shows the air bubbles 3c, to which impurities contained in the water are adhered to. The air bubbles with adhered impurities rise and grow upwards in the intermediate channel forming a sludge layer 3d.

[0063] Furthermore, Figure 1 shows a filtration channel 4 filled with appropriate filter materials 4'.

[0064] Figures 1 and 2 also show an inlet 2a for feeding the raw water influent and an outlet 5a for removing the purified water from the apparatus.

[0065] In the method of the invention, the water containing impurities, optionally together with flocculation chemicals, is introduced through inlet 2a to the innermost channel 2 for flocculation. The direction of the process flow is shown by arrow A, which in the innermost (flocculation) channel takes place in downwards direction. Rapid mixing of the water and optional flocculation chemicals takes place in the beginning of the innermost channel in section 2b under the turbulent conditions of the flow. The innermost channel 2 may also be provided with separate mixing devices, such as paddle mixers to enhance the flocculation.

[0066] From the flocculation channel 2, the floc-containing water is conveyed through opening 2e at the bottom of the container upwards to the intermediate channel 3 of the container, where flocculation continues until the flow reaches the outlets 5' at the lower part 3a of the intermediate channel 3. Pressurized dispersion water is introduced through regulator valves or orifices having outlets 5' to the water flow. In the atmospheric conditions of the intermediate flotation channel, the pressure of the dispersion water is released, forming growing air bubbles 3c, to which the floes with impurities adhere to. The growing air bubbles with adhered impurities rise upwards in the interme- diate channel 3 and the impurities are then collected as a sludge effluent 3d from the surface of the filter material 4' in the outer channel 4.

[0067] The flocculation-treated water is further subjected to filtration by conveying the water through the outer channel 4 filled with appropriate filter materials 4'. Most of the impurities are retained on the surface of the filter material 4' and are collected as a sudge effluent therefrom. Any remaining impurities are entrained in the filter. Purified water is removed from the apparatus through outlet 5a.

[0068] For the regeneration of the filter material in the outer filtration channel 4, wash water is introduced from below to the outer channel 4 through means 5a and conveyed upwards through the filter material 4' in a counter- current manner. The impurities entrained in the filter material rise to the surface of the filter material and can be collected therefrom. The regenerated filter material is then ready for use in a new purification cycle. For example the process disclosed in EP 1 428 564 B1 may be used for the regeneration.

[0069] It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.