WO2009157779A1

WO2009157779A1 - Method and device for aeration of water and purification of aeration media

Info

Publication number: WO2009157779A1
Application number: PCT/NO2009/000236
Authority: WO
Inventors: Yngve Ulgenes; Bjørnar Eikebrokk
Original assignee: Biofarm Systems As
Priority date: 2008-06-24
Filing date: 2009-06-24
Publication date: 2009-12-30
Also published as: NO329300B1; NO20082878L

Abstract

An aeration unit (1) and a method for aeration of water, in which aeration media (20) periodically are cleaned within the aeration unit (1). The control of the transition between the ordinary aeration phase and the cleaning phase takes place by means of a hydraulic switch, in which the water that is used for actuating the hydraulic switch is also used for cleaning the aeration media (20).

Description

METHOD AND DEVICE FOR AERATION OF WATER AND PURIFICATION OR AERATION MEDIA.

Introduction

The present invention relates to a method for aeration of a liquid. More specifically the invention relates to a method by which water sprinkles an aeration media so as for the water to be provided with oxygen. It is a well-known problem in such processes that the aeration media are subject to biological fouling and not before long get clogged, incurring that the aeration efficiency is reduced and resulting in that the aeration media must be cleansed. This may be particularly relevant when waste water from fish tanks shall be aerated, because water from fish tanks often contain much gas which has to be ventilated and at the same time a large proportion of biological and easily decomposable organic material. Several aeration media and aeration tanks are available but the above problem is common to them all and not limited to one particular media. The present application provides a method for a simplified and automated cleansing of such aeration media so as for the aeration to be conducted without unnecessary interruption. The invention provides equipments for employing the process as described below and thus gives a complete technical description of a solution to the problem.

Background art It is known that water from fish tubs contain pollution, among others in the form of dissolved gases and decomposable organic material. When such waste water shall be purified one of several possible methods is to sprinkle the water over an aeration media so as for bringing the water in contact with a gas of desired composition, usually air or oxygen, so as for components to be transferred from the gas to the gas or from the water to the gas phase. as described below.

The transfer of gas from the gas phase to the liquid phase or vice versa is controlled by mechanisms known per se, and it is desirable to transfer much gas in each water cycle so that the pumping energy required for the water becomes low. There are several different methods for aeration of water; gas may be bubbled through the liquid phase, reactors may be provided wherein a finely distributed liquid phase flows in the counter-current direction relative to a gas phase, or a sprinkling device may be applied in which the liquid phase flows over aeration units so as for creating an as large as possible contact surface between gas and liquid. Different methods are used in the different processes and the selection of which process unit to be used depends on which type of gas transfer process is desired to be made.

The purpose of an aeration process as described is generally to achieve a balance between the partial pressures prevailing in the liquid phase and the partial pressures of the same gases in the air of which the liquid is in contact with. The direction of which the mass transport has in this regard is of little importance as long as balance is achieved. This balance is determined by the local conditions for equilibrium in the reactor, conditions which to a certain degree may be controlled in a desired direction. Upon aeration as described here, oxygen may be supplied to water, or CO₂ removed from water, or similar.

The present invention relates to a process in which water sprinkles an aeration media, of which said aeration media is arranged for distributing the water over a generally as large as possible contact surface, and in which an excess of gas is used for aerating the water. Such aeration media prevail in several forms, among others, the Norwegian company Kaldnes has developed a proprietary media with particular density properties. While the water runs downwards in the aeration unit containing the aeration media, gas will be exchanged according to Henry's law, among other physical - chemical equilibrium laws. These mechanisms are well known and we refer to technical literature on the topic such as Cussler, 2nd. ed. (1997), "Mass transfer in fluid systems", for discussion of different process mechanisms in such systems.

Several studies have shown and described how such media function and also how they should be designed and dimensioned. There are large differences between systems with relation to where the aeration media are arranged in a sequential way (so-called structured packings), and systems in which the aeration media are distributed in a random manner in the aeration unit (so-called random packings). Again, relevant literature on the topic is available.

The pollutants in water originating from e.g. fish trays are generally easily biologically decomposable and bacteria will thus establish in and on the aeration media and grow there during conversion of the organic material. Further, particles contained in the water are usually trapped in the aeration media, which is a desirable property as such, but those two properties will incur a clogging of the aeration media, and that the aeration unit and thus the gas transfer will work in a less efficient way. The flow through of water may thus after a certain time be completely or partially blocked so that the contact surface will become reduced and incurring that the aeration process becomes less efficient and may ultimately halt. In order to remedy these problems stated the aeration media must be washed.

Previous methods for washing of aeration media are based on shutting down the aeration unit and that the aeration media are removed for consecutively being washed. This results in an interrupted operation for the unit in question. Additionally, such cleansing or washing is a cumbersome and inefficient method which usually involves devices for removal of the aeration medial which may damage them.

It is thus a purpose of the present invention to provide a solution to at least some of the above mentioned problems, and to provide devices which will remedy those problems. Brief summary of the invention

The present invention relates to an aeration unit for aerating a liquid of which said aeration unit comprises aeration media, - in which said aerating media are arranged for providing an efficient mass transport / gas transfer between a gas and the liquid in the aerating unit,

- in which the aerating media are arranged for providing a large contact surface between a gas and the liquid in the aerating unit,

- in which the aerating unit is arranged for setting up a counter-current movement of liquid and air (the counter-flow principle) and thus increase the mass exchange rate between the gas and the liquid,

- in which a sluice is arranged in the aerating unit's lower part, through which the liquid after completed aeration escapes to an outlet pipe,

- in which the sluice is controlled by a hydraulic element, - in which said hydraulic element is arranged for being actuated by pressurized water supplied through an inlet for pressurized water,

- in which said pressurized water is arranged for cleansing said aerating media while the aerating unit is in a cleansing phase or mode.

The invention relates also to a method for aerating a liquid with subsequent maintenance cleansing of aerating media used during the aeration, in which said aerating phase comprises

- introduction of the liquid to be aerated to an upper section of an aerating unit wereby the liquid flows through a volume of aerating media,

- in which said liquid is subject to a counter-flow mass transfer with an air stream being led in through an air intake,

- whereby the liquid runs out through a sluice in the aerating unit's lower end; in which the cleansing phase comprises

- that the sluice is actuated and closed by a hydraulic unit, said hydraulic unit controlled by a flow of cleansing water supplied through a cleansing water pipe, - said cleansing water led into the aeration unit through a nozzle, said nozzle arranged for supplying the cleansing water as a jet with high pressure,

- whereby the water level rises,

- while the cleansing water jet cleanses the aerating media,

- while the cleansing water is removed from the aerating unit through a waste water outlet pipe, - the supply of cleansing water is shut off at the end of the cleansing phase, whereby said sluice is opened, the water level in the aerating unit is lowered, and the aerating phase is resumed.

The invention further relates to a use of an aerating unit as above described for aerating water from aquaculture production units (such as fish trays, fish ponds, etc.).

Figure captions

The present exemplary drawings are in no way intended to illustrate all aspects of the invention, but have as their sole purpose to illustrate some embodiments of the invention. Other embodiments which will reside within reach of the natural understanding of the person skilled in the art shall be considered as being part of the invention

Fig. 1 shows an aerating unit (1) according to an embodiment of the invention. Fig. 2 shows an aerating unit (1) according to the invention when it is in an aerating phase.

Fig. 3 shows an aerating unit (1) according to the invention when it is in a cleansing phase.

Fig. 4a shows details of an exemplary embodiment of a hydraulic unit (8) arranged for controlling the sluice (5). Fig. 4b shows another detail embodiment of a hydraulic unit (8) arranged for controlling the sluice (5) in which a four way valve is used for controlling the hydraulic unit

(8).

Fig. 5 shows details of the sluice (5) of an exemplary embodiment of the invention.

Exemplary embodiments

The invention will be described below in further detail, with reference to the attached drawings.

As shown in Fig. 1 the aeration unit (1) comprises in an embodiment of the invention an aerating unit (1) being open at the top for receiving water to be aerated. The aerating unit (1) is provided with a tight bottom (2). In an embodiment of the invention an upper part of the aeration unit (1) is provided with a perforated grating (3). This grating (3) is arranged for distributing the liquid provided to the unit (1) over a large proportion of the units surface. The grating may also be arranged for holding back aerating media when the aerating unit is in a cleansing phase. The perforated grating (3) main in an embodiment of the invention have a light opening of about 50 %, and should not have grating apertures larger than the aerating media (20). In an embodiment of the invention the grating (3) may be substituted by a net section, and in such an embodiment the net should be arranged on a frame so as for the net section to be plane.

By the bottom (2) of the aerating unit a sluice or sluice valve (5) is arranged according to an embodiment of the invention, wherethrough the aerated water flows out while the aerating unit (1) is in an aerating mode. The sluice (5) is in an embodiment of the invention connected to an outlet pipe (6) which ends up at an outlet (7) of the aerating unit. The sluice (5) may in an embodiment of the invention be controlled by means of a hydraulic unit (8). The hydraulic unit (8) is in this embodiment of the invention connected to the sluice (5) through a rod (9), said rod (9) arranged to move an inner pipe (10) in the sluice (5), either upwards or downwards for opening or shutting the sluice (5). Details of such an exemplary embodiment of such a hydraulic unit (8) is shown in Fig. 4.

The hydraulic control unit as shown in Fig. 4 is a simple embodiment of a hydraulic control unit and comprises a sleeve (26) with a piston (12). The sleeve is provided with plugs (27, 28) in either end. The piston (12) is connected to a rod (9), said rod (9) further connected to an inner pipe (10) controlling the sluice (5). When the aeration unit (1) is in an aerating mode, the piston (12) and the rod (9) is in such a position that the sluice is open, and water flows out through a gap (11) in the sluice (5) and out through the outlet pipe (6). The piston may in an embodiment be held in a desired position by means of pressurized water, in Fig. 4 that pressurized water being provided through a pressurized water pipe (13). When the aerating unit (1) shall convert to the cleansing mode the supply of pressurized water through pressurized water pipe (13) be closed using a valve (15). Cleansing liquid, usually, but not only, water of high pressure is then provided to the opposite face of the piston through a pipe (14), and the piston is thus displaced from a first end of the sleeve to a second end. Whether the piston (12) shall be moved upwards or downwards should be selected according to the designer of the hydraulic unit; in Figs. 4a and 4b it is shown that the piston (12) is moved downwards when the aerating unit (1) shall convert to a cleansing phase. In stead of closing the valve (15) one may also provide for the pressure of the cleansing water or cleansing liquid being higher than the pressure in the pressurized water pipe (13) so as for the piston (12) to be displaced accordingly. In an embodiment as shown in Fig. 4b a four way valve (31) is applied for controlling pressurized water to the desired face of the piston (12) in order for displacing the piston. In such an embodiment there is no need for valves (15) and (30). However the latter valves (15) and (30) in this embodiment may be arranged in place but always left open. The cleansing water may be led in to the piston via a cleansing water pipe (14), and may also in an embodiment of the invention be controlled by means of a valve (30) on this pipe (14). When the cleansing phase is rounded off the supply of cleansing water through pipe (14) is closed by closing valve (30) while valve (15) is opened and pressurized water via pipe (13) makes the piston (12) to be displaced to its original position, the sluice (5) is opened, and the aeration phase may be resumed.

In stead of pressurized water being used for actuating the hydraulic unit other technical solutions may be employed. One may simply imagine pressurized air is used, supplied through the pressurized water pipe (13) or other solutions which will be obvious to a person skilled in the technical field.

The cleansing water not only has the purpose of actuating the piston (12) in the hydraulic unit (8) but shall also contribute to cleansing of the aeration media (20) in the aeration unit's (1) cleansing phase. The cleansing phase will be further be described below, but it will be emphasized that for the cleansing phase to work properly, the cleansing water must be supplied to the aeration media (20) under high pressure. A high pressure may be about 4 bars or more, but will in some embodiments be lower, depending on the range of application. This implies that the cleansing water in an embodiment of the invention is supplied through a nozzle (16), said nozzle having a sufficiently small cross-section so as for the cleansing to take place in a proper manner.

One of the advantages of applying a hydraulic unit as here explained is that this will reduce the number of moveable mechanical components in the aeration unit, and implies that the process may be controlled exclusively by means of pressurized air and pressurized water. Empirically it is known that alternative control units such as sensors and other kinds of mobile mechanical parts, such as motors, easily are negatively affected by the wet environment in which the aeration unit (1) shall operate, the operational reliability provided by such a hydraulic control is of major importance.

The sluice (5) may in an embodiment of the invention be arranged as a slide valve or other kind of valve which may be controlled via the rod (9) from the hydraulic unit (8), please see Fig. 5. Fig. 5 shows an exemplary embodiment of the sluice (5) comprising a gap (11) wherethrough the water which has been aerated runs, an inner bushing (10) connected to the rod (9), of which said inner bushing is arranged for closing the gap (11) during the aeration unit's cleansing phase. The sluice comprises in the embodiment shown in Fig. 5 also a bottom (2) in the lower part of the sluice (5) to which the outlet pipe (6) is connected. Fig. 5 shows the sluice (5) in open or closed position. Even though the sluice (5) here is shown in a specific embodiment it is obvious that alternatives may be envisaged. The advantage of this embodiment of the sluice (5) is that it may be controlled in cooperation with the transition between the aeration phase and the cleansing phase in the aeration unit (1). In an embodiment of the invention an overflow line (17) may be connected to the outlet pipe (6) for partial or complete recycling of the water from the outlet (7). The overflow line (17) is arranged for being connected a generally horizontal spillway (18) extending over the top portion of the aeration unit or through the upper wall part of the aeration unit (1), in which the horizontal spillway (18) is provided with perforations (19) arranged for releasing the water which shall be recycled over the aeration media (20).

In an embodiment of the invention the aeration unit (1) is provided with a second grating (4) in a lower position, said second grating (4) arranged for supporting the aeration media (20) while the aeration unit (1) being in the aerating phase. This second grating serves a purpose of forming a lower space below the aeration media, whereto this space is connected to the sluice (5) and the outlet.

The aeration media themselves may in an embodiment of the invention have a specific weight somewhat lower than that for water, so as those will float in water. This is of importance during the cleansing phase such as will be described below. There are several different kinds of aeration media (20) which may be used, e.g. one example may be KMT/K3 of Kaldnes, which have a density of about 0.96 and a specific surface of about 600 m²/m³.

Other aerating media are obviously available for use such as would be evident for a person skilled in the art. The amount of aeration media used depends on the size of the aerating unit (1) and the parameter may be calculated depending on the amount of water to be aerated, the properties of the aerating media (20) and other process parameters.

The aerating unit further comprises a pipe (22) arranged for providing air under pressure to the aeration unit (1). the inlet pipe (22) for pressurized air should be arranged below the aerating media (20) so as for the aeration to occur at an as well as intended. It is expected that the air will be distributed evenly throughout the entire aeration media (20) but it is similarly possible to arrange an air distributor below the aerating media so as no preferred air paths are formed through the aerating media (20), and thus achieve a contact surface as large as possible formed between air and water.

Another important aspect to the aerating process according to the invention is the amount of energy required for conducting the method. If an aerating media (20) is employed, it provides high porosity in its packed state, in other words it provides low flow resistance for both air and water.

During aeration according to the present invention the aeration occurs counter- flowwise by means of the liquid sprinkles the aeration media from the top section of the aerating unit and in which pressurized air is introduced at a lower level in the aeration unit so that the air will flow upwards through the tank while the water flows downwards. There are several advantages to such a counter-flow principle, those are exhaustively discussed in literature and comprises among others a high mass transfer in every location in the reactor.

If the feed water is oversaturated with e.g. a gas such as CO₂ at the current conditions (temperature, pressure, salinity) at the inlet to the aeration unit, this gas (CO₂) will be transferred from the downwards flowing liquid flow to the air which flows upwards.

Whereas it may be difficult to obtain equilibrium during one single passage the system according to the invention may be provided with recycling devices (17) which permit the liquid flow to be returned up to the top of the aerating unit (1) for consecutive sprinkling over the aeration media (20). The number of recycling rounds required may be calculated, and will depend of the amount and the state of the feed water provided to the aerating unit (1). The ratio of supplied air to the system per time unit to the amount of liquid provided

per time unit may be expressed as ^luft > c , in which c depends on which liquids and which

gases are employed. By aeration of e.g. CO₂ from water it is experienced that c should be above 5 in order to have a good mass exchange in the system. At the same time it is important to achieve a large surface load with respect to water in order to make the aerating unit (1) compact.

The surface load may be defined as

OVF =Q_water/Area

where:

OVF = surface load (m/hour)

Qva_nn = water flow m³/hour

Area = cross section area of the aerating unit (1) m²

A high surface load provides a compact aerating unit (1) and thus the possibility of reducing the space required, or possibly to increase the throughput of the system. Experiments have shown that speeds over 160 m/h may be used over long periods of time while the temperature is in the range 10 - 16 ⁰C.

The aerating unit (1) further comprises, in an embodiment of the invention a separate waste water outlet pipe (25) used in the cleansing phase to remove cleansing water from the aerating unit (1). As will be further described below the cleanisng water comprises pollutions from the aerating medias (20) that are cleansed off during the cleansing process. In an embodiment of the invention the waste water outlet pipe arranged as a pipe extending into the aerating unit (1), where the pipe is provided with openings sufficiently small for the aerating medias not to be carried away into the waste water outlet pipe (25). The openings must in the same way be large enough for the cleansing water with detached particles from the aerating medias to be removed effectively.

The aerating unit (1) described above has two different operation modes when in use. During ordinary operation, what is considered as an aeration phase, water for being cleaned flows down into the aerating unit e.g. through an inlet pipe (29). The water may either run out directly from a pipe, or if necessary be distributed by means of a coarse grating (3) in the top of the aerating unit (1), or distributed otherwise such as will be obvious to the person skilled in the art. The distribution is of importance whereas it will be an issue that a proportion of the water as large as possible of the aeration media (20) are wetted by the water and may thus work according to the purpose. After the water has been distributed it will flow down onto the aerating means (20) for being subject to a gas transfer / aerating process. In order to increase the efficiency of the aeration there is arranged a gas inlet (22) below the aerating means which will result in that the gas thus flows counter-flowwise with respect to the water, up through the aerating means (20). When the water then has passed the aerating means (20) it is then taken out through the sluice (5) possibly for recycling through the overflow line (17), alternatively for the water to be released, or it may also be led through the outlet (7) back to the tank from which the feed water to be cleansed was taken from.

The water to be cleansed may originate from several kinds of sources if there is a need for such. One of the fields for which the aerating unit (1) is particularly well suited is for employment for aerating water from fish tanks. Fish tanks are used to a significant degree for aquaculture of small fish for later being put out in water or for further cultivation in a fish farm. Such water often has relatively high concentrations of CO₂ and this may constitute a limiting factor to the growth of the fish and its general health state. However, the employment of the aerating unit is not limited to such an embodiment, but may be employed in any field which is applicable for processing by aeration or on every kind of water which is in need for aeration and / or gas transfer.

The aeration process as such is known from literature, and the aeration unit (1) may during ordinary operation be in an aeration phase during a considerably larger proportion of the time of which the plant is operated. However it is a problem as described above that in such units the aerating media (20) are subject to fouling rather quickly, and that the subsequent cleansing process occupies considerable time, that the cleansing may at times may be less efficient and difficult to conduct, and that it may be rather labour intensive. It is thus a purpose to remedy this situation.

As a solution to this problem it is thus suggested a solution whereby the cleansing of the aerating media occurs within the aeration unit (1) as such, and in which this may take place in an automated way. The method for selecting which cleansing process frequency to be applied depends entirely on the condition of the feed water, in other words to which degree it is polluted, what it contains of organic matter, how easily and quickly biodegradable the organic matter is, its temperature, its pH, etc. If the cleansing occurs too early this may incur an needless interruption of operation of the system, whereas if the cleansing occurs too rarely, this will incur that the aeration media (20) get fouled so much that they are difficultly cleansed. Experiments carried out have shown that the temperature is a crucial factor for fouling, as an increasing water temperature implies an increased turnover of biological matter. The cleansing process frequency should however be thoroughly examined for each single application, and to determine this as such is not the main purpose of the present invention.

At a given time, that is, after a given time of operation, or after a sufficently amount of fouling is observed in the aeration unit (1), the cleansing process is started. That process may be initiated e.g. by closing the valve (15), and opening the valve (30) so as for the piston (12) to be moved and the sluice (5) is closed. Alternatively this may be conducted by using a four way valve as described above and in Fig. 4b. The cleansing water will then flow into the aerating unit (1) and it will be filled with water. In stead of closing the valve (15) one may rather provide for the pressure of the cleansing water being higher than the pressure of the pressurized water so that the piston (12) is actuated without shutting down valves.

The cleansing water is led into the aeration unit (1) through a nozzle (16), said nozzle designed so that the cleansing water is supplied under high pressure. When the water jet impacts the fouled aeration media (20) the fouling matter will be washed off and the aeration media as such cleansed. At the same time, whereas the sluice (5) is closed, the water level (23) in the aerating unit (1) will rise. In a preferred embodiment of the invention the aerating media (20) have a density being lower than that of water, and those will thus float in the cleansing water. Whereas the jet of cleansing water is supplied under high pressure, this will lead to a powerful turbulence in the water and the aeration media (20) will thus also circulate so as for a main proportion of those being exposed to the water jet and thus become cleansed.

The nozze (16) as such may be arranged and designed for setting up a circulation of the aeration media (20) as good as possible when the aeration unit (1) is in its cleansing phase. This may be provided for in that the nozzle (16) is arranged centrally in the unit, or arranged to either side in the aeration unit (1). The positioning of the nozzle (16) should be made based on a technical assessment and may be determined based on the hydrodynamic conditions desired for the aerating unit (1), such as would be obvious to a person skilled in the art.

The cleansing water may be removed through the waste water outlet pipe (25). The waste water outlet pipe (25) may be arranged so as for the aerating media (20) are not carried away into the waste water outlet pipe (25) and escape from the aerating unit (1). This may be arranged for e.g. by having an internal grating in the waste water outlet pipe (25). For enabling the waste water pipe (25) to be capable of collecting as much as possible of the pollutants from the aerating media (20) it is in one embodiment extending into the tank, and extends along a considerable part of the breadth of the aeration unit (1). The fouling matter peeled off the aerating media (20) and possibly particles may to a certain degree be prevented by the lower grating (4) from running to the bottom of the aerating unit (1). The circulation of the cleansing water in the aerating unit will result in the particles being conducted in the direction of the waste water outlet pipe (25) for the particles and the peeled off fouling to be led out of the aeration unit. This will, among other features result in a reduced need for a lower sump valve for removal of sediments. However, such a bottom sump valve may have its purpose in some applications.

The waste water outlet pipe (25) may in an embodiment of the invention have a capacity as large as the supply pipe for cleansing water into the aeration unit (1). In this way an overflow of the aerating unit (1) may be avoided, or that cleansing water is carried with recycling water (17, 18, 19) and then possibly back to the tank from which the liquid to be aerated was taken.

In an embodiment of the invention liquid for being aerated is allowed to be supplied also when the aerating unit (1) is in its cleansing phase. In this embodiment of the invention the liquid will run into the recycling line (17, 18, 19) and then possibly back to the tank from which the liquid was taken. In this way the application of not strictly required valve systems on the inlet pipe (29) are avoided.

The cleansing period's length may significantly be determined based on the degree of fouling on the aerating media (20), and may be time controlled or possibly controlled based on measurements of the quality of the cleansing water from the waste water outlet pipe (25). Such measurements may comprise e.g. turbidity (particle content) or similar measurements. Alternatively a cleansing period may be set to a length being considered sufficient based on experience. The cleansing period is under any circumstances significantly shorter than the aerating phase, and the aeration unit (1) would most likely be in the aerating phase about 95% of the operating time. This is made feasible due to the fact that the cleansing of the aerating media (20) takes place in the aerating unit (1) itself, and implies a considerable increase of the operable aeration time as compared to the operable cleansing time compared to background art methods.

The cleansing liquid may in an embodiment of the invention generally be water, but other possibilities are imagined such as water mixed with different chemical agents or even gas. But in the cases in which gas is used the supply of water to be aerated should be maintained so as for the aerating unit to be filled. The cleansing liquid will thereby work as a powerful gas flow contributing to peel off biofilm from the aerating media and for circulating them. The starting of the cleansing phase may be controlled either automatically using a time controlled switch or be controlled based on measurements of the quality of the aerated liquid, of the air resistance or the pressure loss in the flow of water or air through the aerating unit (1), or on other parameters. Experiments have shown that the fouling is strongly temperature dependent, and it would be natural to adjust the aerating device (1) so as for it to initiate the cleansing phase more often, increasing with the temperature of the liquid to be aerated. Several of those control and actuating methods may possibly be combined so as for the unit, as an example, may be set to convert to a cleansing phase at regular time intervals, but with such control overruled if the air resistance becomes too high. Further, the cleansing phase may be initiated on given critical occurrences such as strongly excessive delivery of feedstuff or strong fouling in the fish tank. There are several combinations of occurrences imaginable and those should all be considered covered by the present invention. In an embodiment of the invention the aerating unit (1) comprises sensor devices arranged for measuring the temperature of the liquid to be aerated, and the possibility to change the initiation time for the cleansing phase according to such measurements. Other measurements may similarly be made, such as the amount of dissolved organic material in the liquid to be aerated, or similar. In this way the process may be improved and adapted to each particular liquid to be aerated. Experiments have shown that cleansing may be required each second day when the water temperature is above 16°C, but in order to ascertain that problems shall not arise one may make the tank subject to cleaning every day by temperatures above 12°C. As mentioned above there are, however, several factors influencing, and more or less frequent cleansing may be implemented according to local conditions and experiences. As long as the tank is cleansed sufficiently frequent, few problems will arise.

When the cleansing phase is finished the supply of cleansing water is halted and the piston (12) in the hydraulic unit (8) is actuated and the sluice (5) is opened. The water level (23) in the aeration unit (1) will then sink, and the aeration unit (1) will return to the aeration phase, during the cleansing phase one may choose to turn off the air or one may choose to let it be on. Formation of foam may happen during the cleansing process but this may simply be controlled by means of known skimming techniques. Foam may particularly arise when the liquid contains much proteins.

Despite the cleansing normally being conducted in the aeration unit (1) it may serve a purpose having the possibility for a periodic cleansing of the aeration media outside the tank, either chemically or in other way. Several aeration units (1) may be arranged in parallel so as for the processing of the liquid to be aerated may take place in an uninterrupted way even if one unit is taken out for such cleansing. One may easily imagine two or more units arranged in parallel, of which the tanks are provided by air from one common pressure air unit (such as a fan) and the same liquid to be aerated. In such an embodiment of the invention one may in an uninterrupted way be able to process larger amounts of liquid. The cleansing may either be conducted sequentially so as one aerating unit (1) always being in a cleansing phase, or in an other way.

One of the intended areas of applications for the invention may be for processing water from fish tanks in which cultivation of hatchery produced fish, smolt or fry. The unit has proved to work rather satisfactory for aerating water from tanks containing rainbow trout. This is water which contains relatively much CO₂, a gas affecting the respiration and health condition of the fish and must be removed for the rainbow trout to thrive.

In the above it has been shown an aeration unit (1) for liquid to be aerated, in which the aeration media (20) periodically are cleansed within the aeration unit itself. The control of the transition between the aeration phase and the cleansing phase takes place using a hydraulic unit (8), said water supplied to the hydraulic unit at the same time being applied for cleansing of the aeration media (20). The hydraulic unit (8) allows the aeration unit (1) to be used in a corrosive environment without neither the risk of damaging the aeration unit (1) nor that the operation is affected due to this reason.

Claims

1. An aerating tank (1) for aerating feed water, comprising: a water inlet (29) for the feed water arranged at an upper part of the aerating tank (1), aerating media (20) for being sprinkled by the feed water, arranged between said water inlet

(29) and a sluice (5) for aerated outlet water in a lower part of said aerating tank (1), to an outlet pipe (6), a grating (4) arranged between said aerating media (20) and said sluice (5) arranged for preventing said aerating media (20) to pass said sluice (5), an air inlet (22) in the lower part of said aerating tank (1) arranged for supplying said aerating media with air and for contributing to a counter-current mass exchange between the air and the inlet water, a cleansing water pipe (14) to a nozzle (16) arranged between said sluice (5) and said upper part of said aerating tank (1), said nozzle (16) arranged for flushing cleansing water under pressure towards said aeration media (20), a waste water outlet pipe (25) for waste water and detached particles from the aerating media (20), arranged in an elevation level above said sluice (5), a grid (31) arranged between said aerating media (20) and said waste water outlet pipe (25) for preventing said aerating media (20) from escaping with the waste water, characterized by a cylinder (8) with an inlet from the cleansing water pipe (14) and further conduct for the cleansing water over a piston (12) in said cylinder (8) to an outlet to said nozzle (16), with a piston arm (9) extending down to a closing member (10) in said sluice (5), wherein said piston (12) under elevated pressure of said cleansing water is arranged to displace said piston (12) so as for said closing member (10) to close said sluice (5) and to bring about that the water level in said tank is elevated at least to said waste water outlet pipe (25) while flushing of said aerating media (20) is conducted, and in which said piston (12) under reduced pressure from said cleansing water is arranged for being released together with said piston arm (9) and said closing member (10) and thus open said sluice (5).

2. The aerating tank (1 ) according to claim 1 , further comprising an upper grating (3) arranged for distributing the feed water to be aerated and which is supplied to the aerating tank (1).

3. The aerating tank (1) according to claim 1 , in which said lower grating (4) is arranged for supporting said aerating media (20).

4. The aerating tank (1) according to claim 1 , in which said lower grating (4) is provided with narrow apertures and arranged for collecting particles or other pollutants from said aeration media (20) while the aerating tank (1) being in the aerating mode.

5. The aerating tank (1) according to claim 3, in which said air inlet (22) is arranged below said lower grating (4).

6. The aerating tank (1) according to claim 3, in which said waste water outlet pipe (25) is arranged between said upper and said lower gratings (3, 4).

7. The aerating tank (1) according to claim 1 , in which a recycling line (17, 18, 19) is arranged for recycling a part of the aerated liquid.

8. The aerating tank (1) according to claim 1 , in which said aerating media (20) have a specific weight lower than the specific weight of water.

9. A method for aerating feed water in an aerating tank (1) comprising the following steps:

- letting the feed water run in through a water inlet (29) at an upper part of said aerating tank

(D-

- letting said feed water sprinkle over aerating media (20) arranged between said water inlet (29) and a sluice (5) in a lower part of said aerating tank (1), and in which aerated outlet water is released through an outlet pipe (6) of which a grating (4) prevents said aerating media (20) from passing said sluice (5),

- blowing in air through an air inlet (22) to said lower part of said aeration tank (1) so as for said aeration media (20) to be supplied with air and contributing to a counter-current mass exchange between said air and said feed water during a normal aerating phase in said aeration tank (1),

- leading cleansing water in under pressure through a cleansing water pipe (14) to a nozzle (16) arranged between said sluice (5) and said upper part of said aerating tank (1) and flushing cleansing water onto said aerating media (20) during a cleansing phase, in which said sluice (5) is closed using a closing member (10) whereby waste water and detached particles from said aerating media (20) are released through a waste water outlet pipe (25) with an outlet arranged above said sluice (5), characterized by

- letting said cleansing water in under pressure to a cylinder (8) and further let the cleansing water displace a piston (12) in said cylinder (8) and move a piston arm (9) which extends to a closing member (10) which thus causes closing of the sluice (5) so as for the level of water in the tank to rise to at least said waste water outlet pipe (25) during a cleansing phase flushing process, and

- wherein said piston (12) under a reduced pressure from said cleansing water, upon finishing the cleansing phase, is released to return together with said piston arm (9) and said closing member (10) and thus opens said sluice (5), for the ordinary aerating phase.

10. The method according to claim 9, in which a desired part of the aerated outlet water is recycled through a recycling line (17, 18, 19).

11. The method according to claim 9, in which the point of time for starting is timed as a result of measurements of the aerated liquid, and is subject to being overruled, or in other way controlled.

12. The method according to claim 9, in which said aerating media (20) has a specific weight lower than that of the cleansing water, whereby the aerating media (20) will float during the cleansing phase, whereby the aeration media (20) will circulate in said aeration tank (1) due to the currents in the cleansing water.

13. The method according to claim 9, in which a surface load (OVF) is more than 150 m/h

14. The method according to claim 9 or claim 13, in which several aeration tanks (1) are arranged in parallel for uninterrupted processing of the feed water to be aerated.

15. Application of an aerating tank (1) according to any of the above claims, for aerating of water from fish tanks.