The present invention relates to an air scrubber for scrubbing ammonia- containing air, in particular derived from excretion products such as dung and/or urine from animals, for example cows, pigs or goats, wherein the air scrubber comprises:

- a washing liquid reservoir for receiving an acidic washing liquid, particularly with a pH value less than 3,

- an acid reservoir for receiving a concentrated acid with a pH value that is lower than the pH value of the acidic washing liquid in the washing liquid reservoir, wherein the acid reservoir is connected by means of a valve device to the washing liquid reservoir,

- a pipe system with a washing liquid pump for pumping the washing liquid,

- an air scrubbing chamber for bringing the ammonia-containing air into contact with washing liquid derived from the washing liquid reservoir, supplied via the pipe system by the washing liquid pump,

- a control system with an acidity determination system for determining the pH value of the washing liquid, wherein the control system is configured for controlling the valve device for regulating the supply of concentrated acid from the acid reservoir to the washing liquid reservoir on the basis of the determined pH value of the washing liquid.

An air scrubber and a method for scrubbing ammonia-containing air are known from NL2001538C. Ammonia (NH3) -contaminated air, for example from a barn, can be cleaned prior to emission to the environment by means of an air scrubber. The air scrubber comprises an air scrubbing chamber, a washing liquid reservoir and an acid reservoir. The ammonia-containing air is led, inside an air scrubbing chamber of the air scrubber, through a filter pack while the latter is sprayed with a washing liquid. The washing liquid is preferably an acidic to very acidic solution, for example a solution of sulfuric acid (H2SO4) or nitric acid (HNO3) in water. Through contact between the ammonia-containing air and the washing liquid, ammonia is withdrawn from the air and dissolved in the washing liquid. The thus ammonia-free, cleaned air can now be discharged to the environment.

The dissolution of ammonia in the washing liquid leads to a decrease in acidity (increase in pH value) of the washing liquid. The washing liquid becomes less acidic through use. The reason for this will be clear from the reaction equation that describes the dissolution of ammonia:

NH ₃ (g) + H ₃ O ⁺ (aq) NH ₄ ⁺ (aq) + H ₂ O (I) The increase in the pH value means that the solubility of ammonia in the washing liquid decreases. To guarantee effective operation of the air scrubber it is necessary for the pH value of the washing liquid to remain low enough. For this purpose, an acidity sensor is fitted in the washing liquid reservoir. The acidity sensor measures the acidity in the washing liquid reservoir continuously or periodically. If a control system determines that the pH value of the washing liquid exceeds a predetermined limit value, the control system controls a valve in order to supply a concentrated acid from the acid reservoir to the washing liquid reservoir, so thatthe solubility of ammonia in the washing liquid is restored. Being able to determine the pH value accurately is therefore important for efficient operation of the air scrubber.

Although the aforementioned publication does not describe the acidity sensor further as such, pH-meters are generally known. A known pH-meter comprises a combined electrode, which is dipped in the liquid to be measured. The electrode comprises a pH electrode and a reference electrode. The pH electrode is for example made of pH-sensitive glass, whose potential depends on the activity of the oxonium ions in the liquid. The reference electrode is usually a silver/silver chloride electrode, whose potential has a fixed value. The potential difference is measured by a voltmeter, which converts the signal to a pH value. However, these pH-meters are not always reliable. For accurate pH measurements, the pH-meters must be calibrated regularly by means of calibration solutions. This is time-consuming and expensive. Furthermore, the glass electrodes are fragile.

An object of the invention is to provide an improved air scrubber, particularly an air scrubber wherein the pH value of the washing liquid can be determined more easily.

This object is achieved according to the invention by providing the acidity determination system with sensors for measuring at least one process variable that is representative of the density of the washing liquid and at least one process variable that is representative of the electrical conductivity of the washing liquid, and the control system is configured for determining the pH value of the washing liquid on the basis of these measured process variables. The air scrubber according to the invention does not need to use a pH-meter with electrodes according to the prior art, and also does not provide a further developed embodiment of the known pH-meters, but instead determines the pH value on the basis of process variables that are easy to measure, which form a measure of the density and electrical conductivity of the washing liquid. The invention is based on the insight that an unambiguous pH value can be derived accurately and reliably from the combination of values for the density and the electrical conductivity of the washing liquid at low pH values such as are employed in air scrubbers, particularly pH values less than 3, particularly less than 2. In other words, if the density and the electrical conductivity of the washing liquid have been determined, the pH value is therefore also established. Consequently, the pH value of the washing liquid according to the invention can be determined simply and reliably without needing to use the known pH-meters, with their associated drawbacks.

It is noted that the valve device according to the invention may comprise a valve body, which is movable between an open and closed state. In this case the control system regulates the supply of concentrated acid from the acid reservoir to the washing liquid reservoir by opening or closing the valve device. Furthermore, it is possible for the valve device to comprise a valve body that is configured for adjusting the mass flow through the valve device. The control system may then regulate the supply of concentrated acid from the acid reservoir to the washing liquid reservoir by setting the valve body to the desired mass flow.

In one embodiment according to the invention, the control system is configured for determining values for the density and the electrical conductivity of the washing liquid on the basis of the measured process variables. In this case the control system is further configured for determining the pH value of the washing liquid on the basis of these values determined for the density and the electrical conductivity of the washing liquid. In other words, first the measured process variables are converted to values for the density and the electrical conductivity of the washing liquid, and then the control system determines the associated pH value. This leads to an accurate and reliable pH value.

Moreover, it is possible for the control system to comprise a memory, in which for different pH values in each case a relation for the electrical conductivity of the washing liquid as a function of the density of the washing liquid is stored, wherein the control system is configured for determining the pH value of the washing liquid on the basis of the values determined for the density and the electrical conductivity of the washing liquid and the stored functions. In particular for pH values that are applied in air scrubbers, such as pH values less than 3 or even less than 2, it has been found that the graphs of the functions for each pH value do not touch or intersect each other, i.e. at each input value for the density and function value for the conductivity there is an unambiguous pH value. After the values for the density and the conductivity have been calculated on the basis of the measured process variables, the control system is able to determine the pH value. For example, the relationship between the density and the conductivity is stored in the memory of the control system in the form of a table, graph or function. The control system is then able for example to look up the pH value in the table and/or calculate it with the function.

In one embodiment according to the invention, the control system is configured for comparing the pH value determined for the washing liquid with a preset pH value (limit value), and for supplying concentrated acid from the acid reservoir to the washing liquid reservoir on the basis of this comparison. The limit value may for example be entered manually into the control system by a user, or be determined by the control system on the basis of the acid used and/or other variables. As already explained above, the pH value of the washing liquid increases owing to the dissolution of ammonia therein. For effective operation of the air scrubber it is desirable for the pH value of the washing liquid to remain lower than the preset pH value. The pH value is, according to this embodiment of the invention, for example determined continuously or periodically and compared with the preset pH value, which forms a limit value. If the control system establishes, on the basis of the comparison, that the pH value of the washing liquid has reached the limit value, the control system controls the valve device to supply concentrated acid from the acid reservoir to the washing liquid reservoir, so that the pH value of the washing liquid goes down and the solubility of ammonia is restored.

It is preferable, according to the invention, for the process variable that is representative of the density of the washing liquid and the process variable that is representative of the electrical conductivity of the washing liquid to be measurable directly on the washing liquid. The sensors of the acidity determination system, which measure the process variables, may be in contact with the washing liquid during use. These process variables can be measured easily and accurately, so that the pH value can be determined reliably.

Various process variables are possible according to the invention, and the process variables can be measured according to the invention in a plurality of ways and/or with different sensors. For example, the sensors of the acidity determination system comprise a pressure sensor for measuring the pressure in a liquid column of the washing liquid and a level sensor for measuring a filling level of this liquid column of the washing liquid, wherein the control system is configured for determining values for the density of the washing liquid on the basis of the measured pressure and measured filling level. The liquid column may be formed by the washing liquid in the washing liquid reservoir. Preferably, however, the liquid column is measured in a separate measuring reservoir, for example a measuring tube that is more than 2 meters high, in which the washing liquid from the washing liquid reservoir or the pipe system is received. The level sensor then measures the height of the liquid column, whereas the pressure sensor measures the pressure at the bottom of this liquid column. Determination of the density in this way is quick, easy, accurate and reliable. It is, however, also possible for the sensors of the acidity determination system to comprise a density sensor for directly measuring the density of the washing liquid in the washing liquid reservoir. Density sensors of this kind are known per se.

Furthermore, it is possible according to the invention for the sensors of the acidity determination system to comprise a conductivity sensor for measuring the electrical conductivity of the washing liquid. The conductivity sensor may be fitted in the washing liquid reservoir or in the pipe system of the air scrubber. Conductivity sensors are also known per se, and can measure the electrical conductivity of a liquid accurately and reliably. If a conductivity sensor of this kind is employed with the air scrubber according to the invention, accurate and reliable pH values are obtained.

It is preferable according to the invention for the air scrubber to be provided with a washing liquid discharge device for discharging washing liquid from the washing liquid reservoir to a storage tank for storing washing liquid discharged from the washing liquid reservoir, wherein the control system is configured for controlling the washing liquid discharge device for regulating the discharge of washing liquid from the washing liquid reservoir on the basis of the measured process variable that is representative of the density of the washing liquid and/or the measured process variable that is representative of the electrical conductivity of the washing liquid. There is a linear relationship between the density and the salt content. Therefore the control system is preferably configured for determining values for the salt content of the washing liquid on the basis of the density of the washing liquid, and the control system is configured for controlling the washing liquid discharge device for regulating the discharge of washing liquid from the washing liquid reservoir on the basis of the values for the salt content of the washing liquid. Incidentally, the electrical conductivity of the washing liquid may also be used as a measure for the salt content. However, the relationship between the conductivity and the salt content is not linear.

If the salt content of the washing liquid exceeds a limit value, crystallization may occur. On the basis of the measured density, the control system according to the invention controls the washing liquid discharge device in such a way that the washing liquid is drained from the washing liquid reservoir before crystallization occurs. For example the control system is configured to determine whether the salt content, which is preferably calculated on the basis of the density, or the density per se (which increases linearly with the salt content), exceeds a limit value for a predetermined period and, if this exceeding is established, to drain a predetermined partial amount of the washing liquid from the washing liquid reservoir via the washing liquid discharge device. The washing liquid reservoir may then be replenished with water and/or acid to achieve the desired acid solution in the washing liquid reservoir again. Consequently, at least one of the sensors according to this preferred embodiment has a dual function. The control system uses the one or more sensors to determine the density, or the conductivity sensor, not only to determine the pH value accurately, but also to regulate draining of the washing liquid.

In a particular embodiment according to the invention, the sensors of the acidity determination system comprise a temperature sensor for measuring the temperature of the washing liquid in the washing liquid reservoir, wherein the control system is configured for determining values for the density of the washing liquid and/or the electrical conductivity as a function of the measured temperature. The density and the conductivity of the washing liquid are temperature-dependent. By measuring the temperature of the washing liquid, this can be taken into account when calculating the values for the density and the conductivity of the washing liquid. This results in even greater accuracy of the pH value.

In a preferred embodiment according to the invention, the concentrated acid comprises sulfuric acid (H2SO4). If the washing liquid is prepared with sulfuric acid, dissolution of ammonia therein gives a solution of ammonium sulfate ((NH^SC ). However, according to the invention, nitric acid (HNO3) may also be used in place of sulfuric acid for acidifying the washing liquid. In that case a solution of ammonium nitrate (NH4NO3) is formed. Both salt solutions form liquid fertilizer, which can be used in cattle farming. Such a washing liquid, which is discharged from the washing liquid reservoir prior to crystallization and supplied to the storage tank, to be stored therein, as described above, forms valuable liquid fertilizer.

In one embodiment according to the invention, the control system comprises a memory, and the control system is configured for storing a plurality of pH values, determined at different points in time, in the memory. The pH values are for example stored in a log file. With the pH values stored in the memory, the efficiency of the air scrubber can be demonstrated, for example to satisfy government legislation.

The combination of the washing liquid reservoir and the air scrubbing chamber may be configured in various ways according to the invention. In one embodiment, the washing liquid reservoir and the air scrubbing chamber each comprise a separate container or housing, which are connected together by the pipe system. However, it is also possible according to the invention for the air scrubbing chamber to comprise the washing liquid reservoir. In this case the washing liquid reservoir is integrated with the air scrubbing chamber. The air scrubbing chamber comprises for example a washing zone, which is configured to bring the washing liquid into contact with the ammonia-containing air, and a receiving zone, which is configured for receiving the washing liquid that has left the washing zone. In this case the receiving zone forms the washing liquid reservoir.

The invention further relates to a barn system, comprising:

- a barn for keeping animals, such as cows, pigs or goats,

- an air scrubber as described above. The ammonia-containing air is derived in particular from dung and/or urine from the animals. The air scrubber according to the invention has particular advantages when employed in cattle farming.

Moreover, it is possible according to the invention for the barn to be provided with a separating system for substantially separating dung and urine from the animals, wherein the separating system comprises a urine reservoir for receiving the separated urine, and wherein the air scrubber comprises an air supply that is connected to the urine reservoir for supplying ammonia-containing air that is located above the urine in the urine reservoir, to the air scrubbing chamber. The separating system comprises for example a barn floor, which is substantially permeable to urine from the animals and is substantially impermeable to dung from the animals, and wherein the urine reservoir extends under the barn floor and is configured for receiving urine that has passed through the barn floor. Ammonia-containing air is then located above the surface of the urine in the urine reservoir.

Complete separation of urine and dung is impossible in practice - there will always be some amount of dung passing through the barn floor and reaching the urine reservoir, and some amount of urine remaining on the barn floor. Nevertheless, with a barn floor according to this embodiment, the urine is largely, and almost immediately after excretion by the animal, separated from the dung lying on the barn floor. Dung contains enzymes that can quickly convert the urea in the urine to ammonia, which can then easily evaporate. With quick separation of the urine, this conversion may hardly occur.

The barn system may further comprise a dung removal device for removing the dung from the barn floor, for example a manure scraper or autonomous dung removal vehicle. By removing the dung from the barn floor relatively quickly after excretion by the animal, the liquid in the urine reservoir under the barn floor may comprise the maximum possible urine component.

After treatment of the ammonia-containing air by the air scrubber, the air that is now ammonia-free can be discharged to the outside air. The emission of harmful substances is reduced thereby. Furthermore, the washing liquid discharged from the washing liquid reservoir results in a usable fertilizer, which can be stored as liquid fertilizer in a storage tank. Consequently, the air scrubber may be provided with a storage tank for storing washing liquid discharged from the washing liquid reservoir, which forms liquid fertilizer.

Incidentally, the air scrubber according to the invention is also usable in the purification of stripped ammonia derived from digestate from a manure digester or sewage treatment plant. Barn air from an animal house may also be fed to the air scrubber according to the invention. Such barn air also forms ammonia-containing air. In the case of pig houses, for example, the emission of harmful substances to the environment can be reduced by means of the air scrubber according to the invention.

The invention also relates to a method for scrubbing ammonia-containing air, derived in particular from excretion products such as dung and/or urine from animals, for example cows, pigs or goats, by means of an air scrubber such as described above.

The invention will be explained hereafter, referring to the appended figures.

Figure 1 shows a schematic overview of an embodiment of an air scrubber according to the invention.

Figure 2 shows a number of graphs of the electrical conductivity of the washing liquid as a function of the density for different pH values.

Figure 3 shows a schematic overview of an alternative embodiment of an air scrubber according to the invention.

Figure 1 shows a barn system 1 for keeping animals, such as cows, pigs or goats. The barn system 1 comprises a barn 2 for keeping the animals in an animal space. A cowshed is shown, which is configured as a typical loose housing barn for cows with a cubicle part 7 and a dunging passage 8. The animals will substantially do their business on the dunging passage 8, i.e. the excretion products such as dung (solid fraction of the excretion products) and urine (wet fraction of the excretion products) from the animals mostly end up on the dunging passage 8.

The barn system 1 comprises a separating system for substantially separating dung and urine from the animals. In this exemplary embodiment the separating system comprises a barn floor 3 with openings 11 , which are substantially permeable to urine from the animals and substantially impermeable to dung from the animals. Urine trickles through the openings 11 into a manure cellar, which is located under the barn floor 3. In this exemplary embodiment, the manure cellar forms a urine reservoir 4 for receiving substantially urine. After all, the dung 10 remains substantially on the barn floor 3.

It is noted that in practice complete separation of dung and urine is impossible, so that a small amount of dung will always pass through the barn floor 3 and end up in the urine reservoir 4, and some amount of urine will remain on the barn floor 3. Nevertheless, via the barn floor 3, the urine will largely, and almost immediately after excretion by the animal, be separated from the dung that remains on the barn floor 3. In the urine reservoir 4 there is consequently formation of a liquid layer, which in practice consists of urine mixed with other solid and liquid substances derived from the excretion products from the animals. The air above contains high concentrations of ammonia and other volatile substances that evaporate from the liquid, such as harmful hydrogen sulfide.

In this embodiment example the barn system 1 further comprises a dung removal device 9 for removing dung 10. The dung removal device 9 is configured as a dung removal vehicle that can travel autonomously through the animal space. The dung removal vehicle is configured for sucking up the dung 10 and moving the dung 10 to a manure pit (not shown). The dung removal device 9 may, however, also be configured as a mechanical manure scraper, known per se, which is drawn by a chain or rope (not shown). At predetermined points in time, the manure scraper pushes the dung over the dunging passage 8 to a manure pit at the end thereof.

The barn system 1 further comprises an air scrubber 5, which is connected to the urine reservoir 4. The air scrubber 5 is configured for cleaning, at least partially, contaminated ammonia-containing air from the urine reservoir 4. As shown in Figure 1 , the air scrubber 5 is mounted directly above the urine reservoir 4. Figure 1 shows a recessed mix pit 6, which is a component of the urine reservoir 4. However, the mix pit 6 is optional. The walls of the urine reservoir 4 are for example made of sturdy concrete and form a good foundation for the air scrubber 5.

The actual scrubbing of the ammonia-contaminated air takes place in an air scrubbing chamber 26. The air is fed from the urine reservoir 4 via the air supply 12 to the air scrubbing chamber 26. The air scrubber 5 further comprises a barn air inlet 20, which is connected directly to the barn 2. This makes extra purification of the barn air with the same air scrubber 5 possible. After cleaning, the air leaves the air scrubbing chamber 26 via the air discharge 15. The air flow through the air scrubbing chamber 26 is generated and driven by a fan 19. During operation, the fan 19 generates a partial vacuum, which draws the air from the urine reservoir 4 via the air supply 12 through the air scrubbing chamber 26.

The air scrubber 5 comprises a washing liquid reservoir 21 for receiving an acidic washing liquid, particularly a solution of sulfuric acid in water with a pH value less than 3. Other acid solutions may also be used, such as nitric acid in water. Acid increases the solubility of ammonia in the washing liquid. The washing liquid from the washing liquid reservoir 21 is fed via a pipe system 24 by a washing liquid pump 25 for pumping the washing liquid to the air scrubbing chamber 26.

Inside the air scrubbing chamber 26, a filter pack 13 is installed, which for example is made of a material with high porosity and a high specific surface area. The air to be cleaned is led through the filter pack 13, while the filter pack is sprayed with the washing liquid by means of a sprinkler head 18. In the exemplary embodiment shown in Figure 1 , the direction of the air stream and the direction of the washing liquid stream are at an angle relative to one another (cross-current principle). The direction of the airstream and the direction of the washing liquid stream may, however, also be opposed or the same (according to the counter-current or co-current principle).

Through contact between the air to be cleaned and the washing liquid, ammonia is withdrawn from the air and dissolved in the washing liquid. Incidentally, dust from the animal space of the barn that gets into the urine reservoir 4 via the openings 11 can also be trapped by the air scrubber 5. The dissolution of ammonia from the air means that the air is cleaned and can then be discharged to the environment via the air discharge 15.

In the air scrubbing chamber 26, the washing liquid is received in a receiving reservoir 17 and then led back via a line 14 by a pump 16, via a filter 38, for example to trap flies, to the washing liquid reservoir 21. However, dissolution of ammonia in the washing liquid has the result that the pH value of the latter increases. The increase in pH value indicates "exhaustion" of the washing liquid. To guarantee efficient operation of the air scrubber 5, it is possible to acidify the washing liquid with concentrated acid from an acid reservoir 22.

The acid reservoir 22 contains a concentrated acid with a pH value that is lower than the pH value of the washing liquid in the washing liquid reservoir 21 , for example highly concentrated sulfuric acid when using a solution of sulfuric acid in water as washing liquid. The acid reservoir 22 is connected by means of a valve device 23 to the washing liquid reservoir 21 . Optionally, in the line between the acid reservoir 22 and the washing liquid reservoir 21 there is an additional pump for pumping the concentrated acid from the acid reservoir 22 via the valve device 23 into the washing liquid reservoir 21 (not shown).

The valve device 23 is connected operationally to a control system 28. The control system 28 comprises an acidity determination system for determining the pH value of the washing liquid, which will be described in more detail hereunder. The valve device 23 is controllable by the control system 28, i.e. the valve device 23 is movable by means of the control system 28 between an open or closed state. Optionally the valve device 23 is controllable in the sense that the mass flow of concentrated acid can be adjusted by means of the control system 28.

The control system 28 controls the valve device 23 to supply concentrated acid from the acid reservoir 22 to the washing liquid reservoir 21 on the basis of the pH value of the washing liquid, which is determined by the acidity determination system. When the control system 28 finds that the pH value of the washing liquid exceeds a predetermined - possibly adjubarn - pH value, for example a pH value that is between 2 and 3, the control system 28 opens the valve device 23, so that concentrated acid flows from the acid reservoir 22 to the washing liquid reservoir 21. If the pH value drops below the predetermined pH value, the valve device 23 is closed again by the control system 28.

The washing liquid from the washing liquid reservoir 21 that is no longer usable for effective washing of the air in the air scrubber 5 may be discharged via a washing liquid discharge device with a three-way valve 36 to a storage tank 35. The three- way valve 36 is connected operationally to the control system 28. This "spent" washing liquid from the air scrubber 5 may be utilized as liquid fertilizer. In order to reach the desired amount and concentration of the acid solution used as washing liquid, the washing liquid reservoir 21 further comprises a water supply 37 for supplying (mains) water. The water supply 37 is connected operationally to the control system 28.

In this exemplary embodiment, for determining the pH value of the washing liquid accurately and reliably, the acidity determination system comprises a pressure sensor 30 for measuring the pressure of the washing liquid in the washing liquid reservoir 21 , and a level sensor 31 for measuring a filling level of the washing liquid in the washing liquid reservoir 21. Incidentally, the pressure sensor and the level sensor may also be fitted in a separate measuring reservoir, for example a measuring tube, in which a liquid column of the washing liquid from the washing liquid reservoir 21 or pipe system 24 is received. The pressure sensor 30 and the level sensor 31 are connected operationally to the control system 28, which is configured for determining values for the density of the washing liquid (kg/l) on the basis of the measured pressure and measured filling level. In place of or in addition to the pressure sensor 30 and the level sensor, the acidity determination system may comprise a density sensor 33 for directly measuring the density of the washing liquid in the washing liquid reservoir 21 . The density sensor 33 is then also connected operationally to the control system 28 (not shown).

The acidity determination system further comprises a conductivity sensor 32 for measuring the electrical conductivity (EC) of the washing liquid in the washing liquid reservoir 21. The conductivity sensor 32 is also connected operationally to the control system 28. The control system 28 is configured for determining values for the conductivity of the washing liquid (mS cm ² ) on the basis of the signals from the conductivity sensor 32.

In addition, the acidity determination system comprises a temperature sensor 34 for measuring the temperature of the washing liquid in the washing liquid reservoir 21. The temperature sensor 34 is also connected operationally to the control system 28. The control system 28 is configured for determining values for the density of the washing liquid and the electrical conductivity as a function of the measured temperature. The density and the conductivity of the washing liquid are temperaturedependent. By measuring the temperature of the washing liquid, this can therefore be taken into account when calculating the values for the density and the conductivity of the washing liquid.

The control system 28 comprises a memory 29, in which, for different pH values in each case, a relation for the electrical conductivity of the washing liquid as a function of the density of the washing liquid is stored. Figure 2 shows, for two different pH values, a graph of the conductivity as a function of the density (p), in this exemplary embodiment pH=0.8 (line a) and pH=2 (line b). It has been found that for pH values that are employed in air scrubbers, such as pH values less than 3, preferably less than 2, the graphs of the functions for each pH value do not touch or intersect each other. In other words, there is an unambiguous pH value corresponding to each input value for the density and function value for the conductivity.

After the values for the density and the conductivity have been calculated on the basis of the signals from the sensors 30, 31 , 32, 33, 34 of the acidity determination system, the control system 28 can derive the associated pH value by means of the relations stored in the memory 29. Incidentally, the relation between the density and the conductivity can also be stored in some other way in the memory of the control system 28, for example in the form of a table or function. Also in that case, the control system 28 can determine the pH value on the basis of the values for the density and the conductivity of the washing liquid.

In practice it has been found that such a determination of the pH value for air scrubbers is especially accurate and reliable. Moreover, the sensors 30, 31 , 32, 33, 34 used are robust, reliable and easy to use. The air scrubber according to the invention therefore does not need to use a generally known pH-meter with electrodes, which is delicate and must be calibrated regularly.

Moreover, the pH values determined at different points in time are stored in the memory 29 of the control system 28. In other words the pH values are logged over time. With the pH values stored in the memory 29, the efficiency of the air scrubber can be demonstrated, for example to satisfy government legislation.

The control system 28 is further configured for controlling the washing liquid discharge device with the three-way valve 36 for regulating the discharge of washing liquid from the washing liquid reservoir 21 on the basis of the measured density of the washing liquid, or a salt content of the washing liquid, which the control system calculates on the basis of the measured density. There is a linear relationship between the density and the salt content. The density of the washing liquid forms a measure for the salt content. If the salt content of the washing liquid is too high, crystallization may occur.

On the basis of the measured density, or on the basis of the salt content that is calculated from the measured density, the control system 28 controls the washing liquid discharge device with the three-way valve 36 in such a way that an amount of the washing liquid is drained from the washing liquid reservoir 21 to the storage tank 35 before crystallization occurs. For example, the control system 28 is configured to drain a partial amount of the washing liquid from the washing liquid reservoir 21 via the washing liquid discharge device with the three-way valve 36 if the measured density or the calculated salt content exceeds a limit value for a predetermined period.

The washing liquid reservoir 21 may then, under the control of the control system 28, be replenished with water via the water supply 37 and/or with concentrated acid from the acid reservoir 22 to achieve again the desired amount and concentration of the acid solution as washing liquid in the washing liquid reservoir 21. Moreover, the filling level can be regulated by means of the filling level sensor 31.

Consequently, the sensors for determining the density have a dual function. The control system 28 firstly uses these sensors for accurate determination of the pH value, and secondly to control the discharge of the washing liquid from the washing liquid reservoir 21 , via the washing liquid discharge device with the three-way valve 36, to the storage tank 35.

In place of or in addition to the aforementioned sensors, the acidity determination system may comprise other sensors. These sensors may be configured for measuring other process variables than the aforementioned process variables (pressure, filling level, conductivity and temperature), which do, however, form a measure for the density and the electrical conductivity of the washing liquid or are derived therefrom.

Consequently, according to the invention, various process variables are possible, which are preferably measured directly on the washing liquid. The process variables according to the invention may also be measured in a plurality of ways and/or with various sensors. The invention is, in the general sense, applicable if the sensors of the acidity determination system are configured for measuring at least one process variable that is representative of the density of the washing liquid and at least one process variable that is representative of the electrical conductivity of the washing liquid. Then the control system 28 may make use of a relationship as shown in Figure 2, i.e. an unambiguous pH value follows from the measured process variables, so that the control system can determine this pH value on the basis of the measured process variables.

Moreover, it is possible for the control system to convert the measured process variables first to values for the density and the electrical conductivity of the washing liquid, and then determine the associated pH value. However, functions of process variables that are derived from the density and the conductivity may also be stored in the memory 29 of the control system 28. The functions of these derived process variables then also possess the property that the graphs of these functions, for each pH value that is used in air scrubbers, particularly less than 3 or 2, do not touch or intersect each other, i.e. for each input value for the one process variable and function value for the other process variable there is a corresponding unambiguous pH value.

Figure 3 shows an alternative embodiment of an air scrubber according to the invention, in which the same or similar components are indicated with the same reference numbers. The operation of the air scrubber shown in Figure 3 is, in process engineering terms, substantially identical to the operation of the air scrubber according to Figure 1. In place of a separate container or housing for the air scrubbing chamber and washing liquid reservoir as shown in Figure 1 , the air scrubbing chamber 26 and washing liquid reservoir 21 are integrated in the embodiment according to Figure 3, i.e. the receiving reservoir 17 of the air scrubbing chamber 26 forms the washing liquid reservoir 21. In this case the air scrubbing chamber 26 comprises a washing zone with the filter pack 13, in which the washing liquid is brought into contact with the ammonia-containing air, and a receiving zone with the receiving reservoir 17/washing liquid reservoir 21 for receiving the washing liquid that flows down from the filter pack 13. As mentioned, in process engineering terms, operation is substantially the same as described above on the basis of Figure 1.

The invention is not limited to the embodiments described above. For example, in place of the barn floor 3 with relatively small openings 11 for substantially allowing exclusively urine to pass through, the barn floor 3 may be configured as a generally known slotted floor with sufficiently large slot-shaped openings through which urine and dung fall and end up together in a manure cellar 4 in the form of so-called slurry.

The air scrubber according to the invention may also be used for cleaning stripped ammonia derived from digestate from a manure digester or sewage treatment plant, or for cleaning barn air without also scrubbing air from a urine reservoir or manure cellar.