NO347340B1

NO347340B1 - Degasser and method for increased efficiency in removal of CO2 from water

Info

Publication number: NO347340B1
Application number: NO20210691A
Authority: NO
Inventors: Kari Johanne Kihle Attramadal
Original assignee: NFT Newco AS
Priority date: 2021-06-01
Filing date: 2021-06-01
Publication date: 2023-09-25
Also published as: NO20210691A1

Description

TITLE: Degasser and method for increased efficiency in removal of CO2 from water

The present invention relates to a degasser and a method for increased efficiency in removal of CO2 from water, a bed to be used in such degasser, and use of a degasser for removal of CO2 in water used to farm or keep live aquatic organisms, according to the preamble of the independent claims.

Background of the invention

In order to farm or keep live aquatic organisms in a closed water volume, the water must be treated and/or exchanged continuously to remove CO2 produced by the organisms. How much CO2 the organisms produces are related to the amount of feed and O2 used, and thus the biomass, size of the fish, how active it is, and to the temperature in the water. As the concentration of CO2 in the water increases, it becomes harder for the fish to dispose of the CO2 and take up O2, because the concentration difference between the water and the fish is reduced. Increased CO2 concentration in the fish reduces the ability of the blood to transport oxygen, and affects the fish health.

If the concentration of CO2 in the water becomes too high, it is toxic to the animals and affects not only the respiration, but also the pH balance, sensory and nervous system negatively, and may result in unconsciousness and death at high levels. It also makes the fish more vulnerable for other qualities of the water, such as low concentration of O2 or high concentration of ammonium. At sublethal levels of elevated CO2 in the water, the growth is also affected, and that there is a near linear negative correlation between CO2 concentration in the rearing water and the growth of Atlantic salmon within the tolerance limit of the fish. The CO2 concentration is thus directly affecting the economics of the aquaculture system.

The gas-liquid equilibrium is determining the transport of CO2 between gas and water, and the direction and driving force is determined by the difference in degree of saturation of CO2 in gas and liquid. CO2 will be transported from high to lower concentration, and thus the driving force increases as the concentration difference increases. If both the gas and liquid are equally saturated with CO2, then no net transport will occur. Air at atmospheric pressure has a concentration of about 410 mg/L CO2, and is the preferred gas to be used in degassers. Water in equilibrium with air at atmospheric pressure and room temperature will contain about 0,55 mg/L CO2. As CO2 from the water is transferred to a volume of air, the concentration in the volume of air increases, and in order to remove more CO2 from the water it is important to ensure sufficient replacement of the air to maintain the driving force of net transport of the CO2 from the CO2/enriched water to new air with atmospheric concentration of CO2.

The efficiency of transporting CO2 from water with elevated concentration will increase with increasing surface area between gas and water, concentration difference of CO2 between the gas and water and with decreasing pH. The latter is a result of the pH-dependent carbonate system equilibrium chemistry, where relatively more of the total amount of inorganic carbon is in the CO2 degassable form at lower pH. At high pH on the other hand, relatively more of the total amount of the inorganic carbon in the water is in the form of bicarbonate or carbonate ions, that cannot be degassed. The concentration of CO2 in the water is thus dependent both on the gasliquid balance and on the pH of the water. The gas CO2 is quite soluble in water in which more than 99% exists as the dissolved gas and less than 1% as carbonic acid H2CO3 (Eq.1), which partly dissociates to give H<+>, HCO3<– >(Eq.2) and CO3<2- >(Eq.3). Carbonate and bicarbonate are dissolved in the water and not toxic to the aquatic organisms. At relevant pH levels for aquatic organisms (6-8,5) the inorganic carbon is mainly in the forms of toxic CO2 and harmless bicarbonate. In this pH range small shifts in pH makes large changes in the relative amounts of CO2 and bicarbonate.

Eq. 1: CO2 H2O = H2CO3

Eq. 2: H2CO3 = HCO3<- >+ H<+>

Eq. 3: HCO3<- >= CO3<2- >+ H<+ >

When the concentration difference and the flow ratio between water and air in an aerator/degasser is optimal, the reaction from bicarbonate to H2CO3, that is Eq.2 will be the rate limiting reaction for removal of CO2, as transformation of H2CO3 to CO2 is fast.

A number of aerators/degassers are used in fish farms, aquaria, fish tanks in well boats and other types of transport of live aquatic organisms to remove the CO2 from the water, by exposing the water to fresh air. A large gas - liquid interface is used to increase the amount of CO2 being transferred and supply of fresh air with a low concentration of CO2 is used to draw CO2 from the water to the air. The maximum difference between the concentration in water and air is given by the acceptable or tolerance limits of the animals in the system, and this varies with the type of animal, and time of exposure, such as cultivation or transportation. The maximum recommended limit to secure good growth and welfare of Atlantic salmon in cultivation is 15 mg/L in the water, however, the limit varies with different species.

The recommended limit of CO2 in water is commonly the determinant factor for the amount of water that has to be moved in flow through systems and moved and treated in recirculating aquaculture systems (RAS) and is the dimensioning criteria for the degassers in RAS. To maintain the CO2 concentration below the recommended limit in the fish tanks of RAS with high fish density and production, both the water flow, the degasser volume and the interface between water and gas must be very large. To achieve this the flows should typically be 5:1 to 10:1 of air:water v/v. The necessary space for achieving sufficient reduction of CO2, is particularly a problem when the degasser is used offshore on a barge, or on a boat.

Systems for removal of CO2 from water in an aquaculture plant are for instance known from KR102037631 and CN211631413.

Another way of removing CO2 from the water surrounding the fish, is to transform CO2 to bicarbonate HCO3- and carbonate CO3<2->. Carbonate and bicarbonate are dissolved in the water and not poisonous to the fish. However, and as said above, the amount of carbonate and bicarbonate are influenced by the pH of the water, and at the relevant pH range for aquatic organisms (6-8,5) the inorganic carbon is mainly in the forms of toxic CO2 and harmless bicarbonate.

It is known, for instance from WO 2019/212359, that the enzyme carbonic anhydrase (hereinafter referred to as CA enzyme) increases the speed of Eq.2, identified as the rate limiting step above, and that the enzyme may be immobilized on a gas separation membrane transferring CO2. Gas transferring membranes are however expensive and less suitable for treating large amounts of water.

US 2015010453 and US 2015231561 describes processes for removal of CO2 from gas by bringing the gas in contact with a liquid and CA enzymes. However, the process equipment and operating conditions for removing CO2 from gas are very different from the processes and conditions for removing CO2 from liquid.

An object of the invention is to provide a degasser and method to remove CO2 from water without the problems described above. Further there is an object that the CO2 should be removed at a rate and an extent which significantly reduces the needed size of the degasser. Yet another object is that it should be possible to adapt the degasser and method into existing systems, often referred to as retrofit systems. Yet another object is that the removal of CO2 should be stable and reliable. Further, the degasser should be of a size and construction making it possible to install it at or in already existing systems.

The invention

The above said objects are solved by a degasser, a bed for the degasser, a method and use of the degasser according to the characterizing part of the independent claims. Further advantages features are given in the corresponding dependent claims.

A degasser for removal of CO2 from water according to the invention, is arranged to create a flow of water and gas through at least one bed, wherein the bed comprises immobilized carbonic anhydrase (CA) enzyme.

In a preferred embodiment, the gas is air surrounding the degasser, also referred to as ambient air.

The degasser for removal of CO2 from water according to the invention,comprises a housing having a separate inlet for water and another inlet for gas, and a separate outlet for water and another outlet for gas. The the outlets and inlets are arranged for a crossing, concurrent or counter current flows of water and gas through the at least one bed comprising immobilized CA enzyme.

The expression "removal of CO2" should herein be interpreted as meaning not only completely removal but also partially removal, and thereby reduction of the amount or concentration of CO2.

The inlets and outlets for water and gas are to and from the housing, respectively, and the bed is arranged in the housing.

The water to be treated are led to the inlet, for instance in a pipe, and the treated water are removed from the outlet, for instance in another pipe. The gas, such as air, is preferably blown in or sucked out by a fan, to ensure a flow of gas through the degasser. In one embodiment the inlets and outlets are arranged for a concurrent flow, meaning that the inlet of water and the inlet of gas are arranged at one side of the bed in the degasser, and the outlets are arranged at the the opposite side In another embodiment, the inlets and outlets are arranged for a countercurrent flow, meaning that the inlet of water and the outlet of gas are arranged at one side of the bed in the degasser, and the outlet of water and inlet of gas are arranged at the the opposite side. The construction of the degasser, including the housing, inlets, outlets and pumps may be arranged as a well-known tower degasser, and the operating principals of treating water may be similar.

As mentioned above, the gas may be blown into the degasser, which will create a pressure higher than the surroundings, or sucked out of degasser which will create a pressure lower than the surroundings. Using a pressure lower than the surroundings is preferred as risk for gas bubble disease of the fish receiving the water is reduced, and further as more CO2 gas may be removed. Further, the flow of gas should preferably be higher than the flow of water through the degasser, and preferably the amount of gas is 10:1 or preferably at least 5:1 of the amount of water, meaning that 5 m3 of gas is flowing through the degasser per 1 m3 of water flowing through the degasser.

At least one bed is arranged between the inlet and outlet for water, and between the inlet and outlet for gas, meaning that both the flow of water and the flow of gas must pass through the bed. In a preferred embodiment, the bed fills the cross section of the housing, in the flow direction for water and gas. As the bed comprises immobilized CA enzyme, both flows must pass the CA enzyme. By "immobilized" it is herein meant any fastening which anchor the CA enzyme to the bed, including both coated and covalently bound.

The concentration of CO2 in the water flowing into the inlet of the degasser, will be higher than the concentration at saturation in water in equilibrium with air at atmospheric pressure, while the concentration in the gas flowing into the inlet of the degasser will be at equilibrium for air at atmospheric pressure and, and thus there is a driving force to transfer CO2 from the water to the gas. When the water passes the bed, the CA enzyme immobilized therein will speed the rate of eq 2 above, and carbonate or bicarbonate in the water will be transformed to CO2. In this way, a degasser according to the invention will improve the removal of CO2 gas and the removal of carbonate and bicarbonate dissolved in the water, per volume of gas and water flowing through the degasser, and at a shorter time than degassers according to prior art. This also means that a degasser according to the invention may have a reduced physical size compared to known degassers, and still reduce the amount of CO2 sufficiently to reuse the water for farming aquatic organisms.

In a preferred embodiment, the bed comprises or consists in at least one carrier, and the CA enzyme is immobilized on the carrier. In a more preferred embodiment, a bed comprises several layers of carriers, and thus the flow of water and gas will get in close contact with the enzyme, transforming even more carbonate and bicarbonate to CO2.

In one embodiment, the bed comprises several steel baffles wherein the CA enzyme is immobilized on the baffles. They may be arranged in layers wherein each baffle is arranged to cover only parts of the cross section and the number of baffles will cause a turbulent flow through the bed. Further, the baffles may have holes and allow the water and gas to pass through the holes. Such an embodiment will also contribute to better distribution of water and gas throughout the cross section of the degasser.

In another embodiment, the bed is a submerged moving or fixed bed reactor filter, and the carrier is ceramic or plastic elements for biofilm attachments having a large surface wherein the the CA enzyme may be immobilized. By having a large surface it is possible to immobilize a large amount of CA enzyme, and thus to speed the transformation of carbonate and bicarbonate.

In yet another embodiment, the bed is plates or nets inserted into the degasser, wherein the CA enzyme is immobilized onto the plate or net. In a preferred embodiment, the plate or net is made of stainless steel.

A separate bed may be inserted into a degasser as described above. CA enzyme is immobilized directly to the bed, or to carriers in the bed. In this aspect, the bed is a separate unit and may be replaced. By having the beds as a separate replaceable unit, the bed may be replaced with a minimum time of shutdown of the degasser, and if the degasser comprises more than one bed, the beds may be replaced one by one without shutting down the degasser. Further, as the beds may be removed, maintenance and cleaning of the degasser is considerably easier.

Once used, the bed or the carriers may be recoated by enzyme if necessary, before returned to the degasser. In embodiments where the bed comprises carriers, the carriers may be replaced and the bed may be returned to the carrier. When the bed is a submerged moving bed reactor with free floating carriers, the carriers are removed and replaced before the water treatment is resumed.

The carrier may be made of stainless steel, glass, ceramics or plastics such as polyethylene or polypropylene, possibly coated with polyurea or PDMS to ease the immobilizing of CA enzyme. Such beds and elements are well known to a skilled person. Immobilizing of the CA enzyme on the carrier is also established and well known to a skilled person, and is for instance described in WO 2019/212359 mentioned above.

Another aspect of the invention is related to a method for removing CO2 in water by using a degasser as described above. The method comprises the following steps: - guiding water to be treated into the degasser,

- guiding gas into the degasser,

- allowing the water and gas to flow through the bed of the degasser, and

- removing treated water from the degasser.

In a preferred embodiment, the degasser comprises a housing having inlets and outlets for gas and water. Then, the water to be treated is guided into the inlet of the housing, and gas is pumped into the gas inlet, or drawn out of the gas outlet. Water and gas is allowed to flow through the degasser, and the treated water is removed from the water outlet, and the used gas is removed from the gas outlet.

In a preferred embodiment, the amount of gas flowing through the degasser is larger than the amount of water flowing through at the same time, more preferred the amount of gas is about 5 times the amount of water. As long as new gas at equilibrium for air at atmospheric pressure keeps flowing through the degasser the driving force for removal of CO2 is maintained.

In another preferred embodiment, the air is drawn out of the degasser by a fan, creating a reduced pressure in the degasser. The reduced pressure should preferably be sufficient to avoid gas bubble disease in the organism receiving the water, and will be well known to a skilled person operating vacuum aerators.

In yet another preferred embodiment, the gas is air, and the water is fresh water, sea water and/or brackish water.

Another aspect of the invention relates to use of the invention for removing CO2 in water used to farm aquatic animals, such as water in recirculating aquaculture systems (RAS systems) and/or aquaculture systems such as closed fish cages. The aquatic animals may for instance be molluscus, crustaceans or fish of any type, but is not limited to this. It may also be used in semi-opened systems at sea, or other situations and systems having problems with sufficient exchange of water, such as well-boats and aquariums.

Available space is particularly limited on systems offshore or on a well boat, and may limit the system and thus the desired reduction of CO2. By using a degasser and a method according to the invention, the efficiency of transferring CO2 from water to air is greatly increased as described above and proven in the examples below, and the amount of treated water may be substantially increased, without increasing the physical size of the degasser. This also means that by replacing a degasser according to prior art with a degasser according to the present invention in an existing RAS system, the capacity of the system is greatly increased.

The concentration of CO2 in water used for farming or keeping living aquatic organisms should be is less than 50 mg/l, preferably less than 30 mg/l or even less than 15 mg/l as mentioned above, and thus the water led into the degasser will have a concentration less than 50 mg/l. The concentration in water will depend on the organisms being farmed. When using a degasser according to the invention, the concentration should be substantially reduced, possibly to less than 4 mg, less than 1 mg/l or even to about 0,55 mg/l CO2 per liter of water when ambient air is used as the gas. Once the concentration of CO2 is sufficiently reduced, the water may be recycled to the fish cages.

Description of the figures

The following description of an exemplary embodiment refers to the drawings, and the following detailed description is not meant or intended to limit the invention.

Instead, the scope of the invention is defined by the appended claims.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

Embodiments of the present invention will now be described, by way of example only, with reference to the following figures, wherein:

Figure 1 shows a degasser according to the invention, wherein the flow of water and gas is concurrent,

Figure 2 shows another concurrent degasser according to the invention, Figure 3 and 4 show other degassers according to the invention, wherein the flow of water and gas is countercurrent, and

Figure 5 and 6 are graphics showing the reduction of the CO2 concentration in water.

The figures are for illustrating the invention only, and the different parts may not be in the same scale. To ease the understanding of the Figures, corresponding parts are given the same reference numbers in different embodiments and figures.

Figure 1 shows a longitudinal cross section of a degasser constructed similar to aerating fountains. The shown degasser according to the invention, comprises a submerged moving bed 2 of carriers 3, wherein CA enzyme is immobilized onto the carriers. An inlet 1 of water is arranged through the bed 2, and water is flowing upwards and into the surrounding air. The inlet 1 is preferably arranged above the bed 2, and thereby the water will flow through the inlet, mix with the surrounding air, and air and water will flow into and through the bed. The flow of water is indicated by dark arrows, and the flow of air is indicated by light arrows. A tank 4 receiving water from the bed 2 is arranged at a distance below the bed, and air drawn into the bed, may leave the degasser below the bed. In this way, a flow of water and a flow of air will pass through the bed concurrently.

Figure 2 shows a longitudinal cross section of another embodiment of a degasser, constructed similar to a trickling tower, where the flow of water and gas is concurrent. The degasser according to the present invention shown in Fig.2 comprises a housing 5 having an inlet 1 for water to be treated and another inlet for air 6, and an outlet 7 for treated water and another outlet for air 8. Four beds 2 with immobilized carbonic anhydrase enzyme is arranged between the inlet 1 and outlet 7 for water, and between the inlet 6 and outlet 8 for air. Each bed is a separate unit which may be replaced, and are are arranged to cover the whole cross section of the housing. All water and air passing through the degasser must thus pass through all four beds between inlet and outlet.

In the embodiment shown in Figure 2, a plate 9 of stainless steel with holes is arranged in the housing, downstream of the inlet for water 2, but upstream of the inlet 6 for air, above the beds 2. The holes of the plate is indicated by a dotted line in the figure.

When water is added to the degasser it will flow through the inlet 1 and onto the plate 9 before it flows through the holes. In this way the water is evenly distributed over the whole cross section of the degasser, regardless of the positioning of the inlet 1. In Figure 2 a pool 10 of water is shown above the plate, and the flow of water is indicated by dark grey arrows. The flow of air is indicated by light grey arrows.

Figure 3 shows a degasser similar to the degasser shown in Figure 2, but the flow of air is countercurrent to the flow of water, and the bed is a random packing media filter with carriers 3 wherein the CA enzyme is immobilized. Degassers having this design are also known as trickling towers in prior art.

Figure 4 shows another embodiment of the degasser shown in Figure 3, wherein bed is submerged with free floating (moving bed) or fixed random packing carriers 3. The water inlet 1 is at the top and the water outlet 7 is at the bottom, while the gas inlet 6 is at the bottom and the gas outlet 8 is at the top, creating a countercurrent flow through the bed. Degassers having this design are also known as fluicized fixed bed biofilter in prior art.

The amount of air flowing from the inlet 6 through the bed and out through the outlet 8 is preferably 5 times higher than the flow of water flowing from the inlet 1 through the bed and out through the outlet 7.

Water to be treated will be added to the inlet, distributed by the plate 9, and mixed with the gas and led into contact with the CA enzyme immobilized in the beds. The CA enzyme will drive the equations mentioned above towards CO2 gas, and the CO2 gas will be transferred from the water to the gas, and transported out of the degasser. Thus the CO2 concentration of the water will be reduced.

The following examples are given to show the effect on the rate of reducing the concentration of CO2 in water, when using a degasser according to the invention.

Experiment 1 - CA immobilized on silica particles in a moving bed.

CA enzyme was immobilized on a carrier of silica particles and the carriers were added to 200 ml water containing 15 mg/L CO2 at an amount corresponding about a total of 18 mg CA enzyme. The carriers were only added to the water, and constituted a moving bed. Then air was bubbled through the water at a rate of 300 cm3 pr min. The time and CO2 concentration of water was registered continuously until the concentration was about 3 mg/L CO2.

The CA enzyme used was expressed recombinantly in Escherichia coli high cell density cultivations and purified to above 60% purity by heat precipitation of host proteins. The silica particles were particles with amino groups, having an average surface area of ca 440 m2/g, and an average about 0,64 µmol NH2/g.

The CA enzyme was immobilized on the silica particles in an established two step method, wherein the first step comprised activation of the amino groups with glutaraldehyde. 5 mmol glutaraldehyde was added per g particles in phosphate buffer of pH 7.7, and the reaction time was 4 hours at room temperature. In the second step 500 µg carbonic anhydrase was added per mg particles in phosphate buffer of pH 7.7, and the reaction time was 20 hours at room temperature.

Several test were performed; pure water, water with and without silica particles with immobilized CA enzyme and with and without buffer. The result is given in table 1, and in Figure 5.

Table 1

The result is given in Fig.5.

As can be seen above, the addition of buffer and particles with CA enzyme will more than halve the time needed to reduce the concentration of CO2 in the water from 15 to 3 mg/L. To treat 1 liter water, 0.09 g CA enzyme is needed to obtain 2.2 times reduction of the CO2 stripping time.

Experiment 2 - CA immobilized on a steel net constituting a fixed bed

CA enzyme was immobilized on a a steel net, 0,35 mm thick, which was easy to bend and provided a surface area of 2,3 cm2 for each 1cm x 1cm net. The CA enzyme was immobilized on steel net in a two step method, as described in example 1 above, and constituted a fixed bed in the degasser.

100 µg CA enzyme from Sigma Aldrich was immobilized on 0,49 m2 steel net, the amount of CA enzyme was determined by a standard assay test (Wilbur-Anderson). The steel net was fitted into a thermostated glass reactor with glass diffuser and magnetic stirrer. The temperature was 12,9 degrees Celcius, and 250 ml water to be treated was added. Then air was bubbled through the water at a rate of 300 cm3 pr min. The time and CO2 concentration of water was registered continuously until the concentration was about 3 mg/L CO2.

Several test were performed; pure water, water with steel net with and water with steel net with immobilized CA enzyme. The result is given in Figure 6. As can be seen from Fig.6, 100 μg coupled CA Sigma enzyme to 0.49 m2 steel net have a substantial effect by reducing the CO2 stripping time: from 64 to 41 min.

By using a degasser according to the invention, having at least one bed comprising immobilized CA enzyme, and arranged to create a flow of water and gas through the bed, the amount of treated water may be doubled, without increasing the physical size of the degasser.

Claims

1. Degasser for removal of CO2 from water, the degasser comprises at least one bed (2) with immobilized carbonic anhydrase enzyme, wherein the degasser comprises a housing arranged to create a flow of water and gas through at least one bed, the housing comprises a separate inlet (1) for water and another inlet (6) for gas, a separate outlet (7) for water and another outlet (8) for gas, the outlets and inlets are arranged for a crossing, concurrent or counter current flow of water and gas through the at least one bed (2).

2. Degasser according to claim 1, wherein the bed (2) is arranged to fill the crosssection of the housing, in the flow direction for water and gas.

3. Degasser according to any one of the preceding claims, wherein the bed (2) comprises a number of carriers (3), and the CA enzyme is attached to the carriers.

4. Degasser according to any one of the preceding claims, wherein the bed (2) is a submerged moving or fixed bed reactor and in that the CA enzyme is immobilized to carriers (3) in the bed.

5. Degasser according to any one of the preceding claims, wherein the carbonic anhydrase enzyme is anchored by being coated and/or covalently bound to the bed.

6. Method for reducing an amount of CO2 in water by using a degasser according to claims 1-5, characterized by the following steps

- guiding water to be treated into the degasser,

- guiding gas into the degasser,

- allowing the water and gas to flow through the bed with immobilized carbonic anhydrase enzyme, and

- removing treated water from the degasser.

7. Method according to claim 6, wherein the flow of gas is 5 times higher than the flow of water.

8. Use of an method according to claims 6 or 7, for reducing the amount of CO2 in water used to farm aquatic organisms, such as water in RAS systems, and/or aquaculture systems.