Technical Field

[0001] Present invention relates to a device and method for the removal of particles and/or sludge especially from municipal and industrial sewage water.

Background Art

[0002] There are known several methods and devices for removal of particles, sludge, fat and oil from sewage water.

[0003] One method is known as Dissolved Gas/Air Flotation (DGF/DAF). This method uses introduction of gas (such as air) into a large vessel containing the sewage water to be cleaned. The pressure drop of the gas will produce small bubbles that will adhere to the particles and lift them slowly to surface of the vessel. Due to the difference in buoyancy, the clean water will descend to the bottom of the vessel. The particles on the water surface are removed using a scraping device. The vessels used for DGF are generally large tanks with a retention period of 10 minutes or more. The gas is mixed into the water due to pressure drop.

[0004] Traditional use of gas flotation within the oil and gas field has been dependent on the salinity of the water to be treated is higher than a certain threshold in order to achieve small gas bubbles that adheres to the surface of the oil drops. It is the surface properties of the gas bubbles that makes the oil drops to adhere to the gas bubbles. If the water to be treated does not have sufficient salinity, it will not be possible to obtain as small bubbles as necessary to obtain a sufficient separation and one solution to this problem is to supply microbubbles/nanobubbles to obtain a similar separation effect as if the water has a sufficient salinity.

[0005] Norwegian patent NO327190 relates to a flotation device for flotation of fluids containing particles. The device comprises a flotation unit with an inlet tube is situated in the lower part of the flotation unit, and where said flotation unit comprises an inner, open-ended conical container with a large upper cross-section and a relatively smaller lower cross-section. Further the flotation unit is provided with a scraper for removal of material at the upper part of the unit. The conical container has a downward open tube, in said tube it is provided a funnel-shaped unit with a large, upper opening area and close to its lower end, the funnel-shaped unit is closed against the tube but is provided with an outlet tube to remove cleaned liquid from the flotation unit. In one embodiment of the flotation unit, the inlet tube is directed upwards, preferably centrally at the bottom of the flotation unit and is configured to lead the liquid flow against a distributor plate which distributes the liquid centrally over the whole flotation area.

[0006] Typical features for the DGF method are that it requires large volumes of the separation vessels in order to obtain the required retention time and the mixing of water and gas is relatively ineffective. Further the separation is performed in the same, open vessel which means that smaller particles can follow the cleaned water to the bottom of the vessel.

[0007] Norwegian patent NO156975 relates to a flotation device for separating materials in a liquid, said materials having a density which is close to the density of the liquid. The device is especially configured to recover fat and fatty substances and protein and protein like compounds. The device comprises an outer container provided with scraper at the upper part for removal of the material and a device at the bottom to remove precipitated material. Further, the device comprises an inlet tube ending in the lower part of the device, externally of a conical container which is centrally mounted in the outer container. Further, the upper part of the conical container is provided with a liquid opening, preferably directed in a downward direction towards the lower, tapered part of the conical container, which is provided in the outer container, said conical container is provided with some distance to the bottom of the outer container, and an outlet pipe for removal of treated liquid. The lower part of the conical container is provided with a ring with one or more openings, in order to remove water from the feed water through the openings. The other part of the conical container is provided with a tube for emptying the container.

[0008] Norwegian patent 327190 relates to a flotation device with an inlet tube in the lower end of the flotation unit, where said flotation unit comprises an inner, upward open funnel-shaped vessel with a large cross-sectional area in its upper part and a substantially smaller cross-sectional area at its lower end, and where the flotation unit is provided with a scraper for removal of flotate at its upper end. Further, the flotation device is provided with a downward open pipe socket in which a funnel-shaped unit with a large upper opening area and that the lower end of the funnel-shaped unit is closed against the pipe socket but is provided with an outlet pipe to empty cleaned liquid from the flotation unit. The inlet tube is directed in an upward direction centrally at the bottom of the flotation unit and is configured to direct the liquid flow against a distribution disk which is distributing the liquid centrically over the whole flotation area.

[0009] WO9108175A1 describes a method and apparatus for sewage purification using flotation. The apparatus may contain an outer and an inner vessel, where the outer vessel may contain a bell open at the bottom with upward directed leading- away channels for purified liquid. Possible downward directed shells between the vessel further enhance the purification of the vessel. The inlet tube is directed tangentially to the inner vessel.

[0010] Further embodiments of flotation devices can be found in EP 0682985, EP 3018101 , WO9925658A1 , JP3770475, TWI680105 and WO2019/221609A1 .

Summary of invention

[0011 ] The above advantages is achieved by a device for removal of particles and/or sludge from a water body, especially from municipal and industrial sewage water, said device comprising:

- an outer vessel with a conical lower comprising an outlet for sedimented sludge/particles;

- an inlet for water to be treated;

- an outlet for treated water;

- an outlet for separated sludge/particles, wherein the device further comprising:

- a gas/water mixer to mix the water to be treated with a gas;

- an inlet chamber receiving the mixed gas and water from the gas/water mixer, said inlet chamber has an upstanding cylindrical shape with an open upper end and a closed lower end,

- one or more inlet pipes from the gas/water mixer to the inlet chamber, said inlet pipe(s) is/are oriented tangentially in the lower part of the inlet chamber to provide the mixed gas and water with a swirling motion inside the inlet chamber;

- a perforated plate covering a major part of the upper opening of the outer vessel, said upper end of the inlet chamber protruding a distance above upper surface of the perforated plate,

- a sludge funnel provided with a sludge ramp with a sludge outlet for collection and removal of sludge/particles from the upper surface of the perforated plate, and

- one or more scrapers provided with a scraper motor for removal of the sludge/particles from the upper surface of the perforated plate and bring the sludge/particles to the sludge funnel.

[0012] Preferably the gas supplied to the gas/water mixer is in the form of microbubbles/nanobubbles.

[0013] Preferably the size of the microbubbles/nanobubbles is between 20 pm and 80 pm, more preferred between 40 pm and 60 pm.

[0014] Preferably, the bottom of the cylindrical inlet chamber has an upward conical shape with an outlet for sedimented particles /sludge at the periphery of the bottom.

[0015] According to one preferred embodiment of the invention, the gas to be mixed with the water to be treated, is air.

[0016] Preferably, the device according to the invention further comprising a level control device , preferably a plate that can be moved up and down to adjust the water level in the device.

[0017] In a preferred embodiment of the invention, the perforated plate is exchangeable with other plates with smaller or larger perforations.

Brief description of drawings

[0018] Figure 1 depicts a vertical cross section of the flotation unit

[0019] Figure 2 depicts the flotation unit seen from above.

Detailed description of the invention

[0020] Figures 1 and 2 depicts the flotation unit according to present invention. The flotation unit comprises an outer vessel (22) with a conical lower part (18) which has an outlet (17) for sedimented sludge/particles. This vessel (22) is open at the top and will thereby operate under ambient pressure. Further, it comprises an inlet chamber (24), which is an upright cylindrical vessel with an upward conical closed bottom (14). This inlet chamber (24) is preferably provided centrally in the outer vessel (22). The water to be cleaned (10) is supplied with a gas (12) and led to a mixer (11 ) to mix the water and the gas. Preferably, the gas is air, but in some embodiments, the gas can also be oxygen or nitrogen. In a preferred embodiment, the gas is in the form of nanobubbles/microbubbles. This water/gas flow is supplied to the lower part of the inlet chamber (24) through one or more tangential inlet pipes (13). Due to the tangential inlet tube(s), the water/gas mixture will obtain a swirling motion in the inlet chamber (24). Heavier particles/sludge will sink down to the bottom of the inlet chamber and due to the upward conical shape of the bottom (14) the particles /sludge can be removed at the periphery of the bottom through an outlet pipe (16) provided with a valve (15). At the upper part of the outer vessel (22) there is provided a perforated plate (19) which covers a substantial sector of the top of the outer vessel. In some embodiments, this perforated plate (19) can be removed in order to provide plates with larger or smaller perforations depending on the size of the particles to be removed. The center of the perforated plate (19) has a circular opening to accommodate the upper part of the inlet chamber (24). The upper part of the inlet chamber (19) protrudes a small distance above the upper surface of the perforated plate (19).

[0021] Further, the top of the outer vessel is provided with a sludge ramp (6) covering a part of the upper open part of the outer vessel (22). This sludge ramp (6) is elevated above the upper surface of the perforated plate (19). Next to the sludge ramp (6) is a sludge funnel (7) for collection and removal of sludge through a sludge outlet (8). The top of the flotation unit is provided with a scraping device comprising a motor (1 ) and one or more scrapers (3). The lower part of the scraper(s) (3) is positioned at the level of the perforated plate (18). By operation of the scraper motor (1 ), the scraper(s) (3) will remove sludge/particles collected on the perforated plate (18) and bring them to the sludge ramp (6), where the sludge/particles will be removed from the scraper(s) (3) and fall down into the sludge/particle funnel (7) , from which they can be removed through at sludge/particle outlet (8) Finally, the flotation unit comprises an outlet chamber (9) for cleaned water with a level control device (5)., The purpose of this level control device (5) is to control the water level in the flotation device in order to have a correct water level despite a reduction or increase of the flow rate. This device (5) can in one embodiment be a plate which can be manually or automatically moved up and down to increase or decrease the water level, respectively.

[0022] The function of the flotation device is as follows:

[0023] Water to be cleaned (10) is mixed with gas (12), preferably air, in a gas/water mixer (11 ) where the gas is dissolved as small bubbles in the water. This gas/water mixture is led to an inlet chamber (24) by one or more tangential inlet pipes (13). Due to the tangential configuration of the inlet pipe(s) (13), the gas/water mixture is set in a swirling motion in the inlet chamber (24). The water/gas mixture is moving upwards in the inlet chamber and the gas bubbles will adhere to the sludge/particles, and the sludge/particles (2) will flow upwards and out of the inlet chamber (24) an onto the perforated plate (19). Due to the perforations, the sludge/particles will collect on the upper surface of the plate (19) and the water will flow down into the outer vessel through the perforations in the plate (19). The collected sludge/particles (2) will finally be scraped off from the perforated plate (19) and fall down into the outlet chute (7). The cleaned water will move downwards in the outer vessel (18) and will finally be removed from the flotation unit via a level control (5) as a stream (9). Remaining heavier sludge/particles will be collected in the lower part of the outer vessel (18) and will be removed through the outlet (17) for sedimented sludge.

[0024] The flotation process according to the invention is a co-current process, which means that there is no contact between the cleaned water and polluted water. Further, since the mixture of water and gas is led into the inlet chamber (24), the retention time of the water in the flotation device can be drastically reduced compared to the prior art solutions discussed above. Due to the reduction of retention time, the actual size and footprint of the flotation device can be highly reduced compared to prior art solutions with the same capacity.

[0025] The flotation device according to present invention has a wide range of applications, such as removal of microplastic, sludge removal in municipal sewage treatment plants, removal of feces and feed residues in onshore based fish farms, particle removal within the construction industry, and several applications within the food industry and maritime sector.