The invention relates to the biotechnological removal of a volatile nitrogen compound, in particular ammonia, from a gas containing this nitrogen compound, for example a waste gas from an intensive cattle farm stable or an industrial waste gas. A known method of removing ammonia from a waste gas is that this waste gas is conducted through a biofilter. A biofilter usually comprises a bed of massed material, for example fine compost or peat, to which microorganisms attach themselves, which bed is supported by a material of coarser structure. Since microorganisms in a biofilter are active only under conditions of high humidity, a waste gas is usually saturated with water vapour before being passed through the biofilter.

The removal of nitrogen compounds from a waste gas by means of a biofilter has a number of disadvantages. The reaction conditions in a biofilter are usually difficult to control, so that a biofilter is prone to malfunctions and unreliable in practice. Fluctuations in the concentration of the nitrogen compound in the waste gas interfere with the correct operation of the biofilter. Acidification occurring in the biofilter owing to HNO ₃ formation may stop the activity of the microorganisms. Ammonia is removed only partly from a waste gas in a biofilter. A major disadvantage of a device with a biofilter is that the active microorga- nisms must be present on a very large surface area and the pressure drop across the gas in a biofilter must remain limited, which requires a wide and open structure of a carrier to which the biomass adheres, as a result of which a biofilter occupies a large volume.

It is an object of the invention to provide a method and a device for the removal -of a volatile nitrogen compound, for example ammonia, from a waste gas in which the above disadvantages do not manifest themselves.

According to the invention, this object is achieved by a method for the biotechnological removal of a volatile nitrogen compound from a gas containing this compound, which method comprises

1) a first step carried out in a first device, whereby the mtrogen compound is dissolved in a liquid transport medium, followed by

2) a second step carried out in a second device separate from the first device, whereby the nitrogen compound dissolved in the transport medium is oxidized by means of microorganisms.

In the removal of a mtrogen compound from a waste gas in accordance with the invention, the mtrogen compound is fully absorbed in the liquid transport medium in the first step, and subsequently the nitrogen compound absorbed in the transport medium, i.e. dissolved therein, is oxidized in a second step. The separate implementation of an absorption step and of an oxidation step in two separate devices has the major advantage that the device required for the oxidation step may be comparatively small and compact. The dimensions of such an oxidation device with a given processing capacity, for example, are considerably smaller than those of a biofilter as discussed above with the same capacity. In an embodiment of the method according to the invention, the nitrogen compound is oxidized by a biomass present in the second device in a quantity of at least 10 kg dry matter/m ³ volume of the device, preferably between approxima¬ tely 20 and approximately 30 kg dry matter/m ³ volume of the device.

These biomass density values are substantially higher than those obtaining in the biomass in a biofilter according to the present art for the removal of a nitrogen compound from a waste gas.

The method according to the invention is particularly suitable for the removal from a waste gas of ammonia, which is dissolved in a transport medium comprising mainly water in the first step, and which is substantially oxidized into a nitrate in the second step. Full oxidation of the ammonia, i.e. oxidation into a nitrate, is important because an incomplete oxidation leads to nitrite which destabilizes the further oxidation process.

Preferably, the method according to the invention is characterized in that a third step is carried out in a third device separate from the first and second devices, whereby the nitrogen compoimd oxidized during the second step is substantially reduced to nitrogen.

The third step, in which the oxidized nitrogen compound is substantially

reduced to nitrogen, is referred to hereinafter as denitrification step. Among the advantages of the addition of a denitrification step are the following. Nitrogen (N ₂ ) is more attractive as a waste product than is nitrate. Any nitrite accumulated in the transport medium owing to imperfect or disturbed nitrification is removed during the denitrification step. This is advantageous because, as noted above, nitrite inhibits the nitrification of ammonia. A third major advantage of the addition of a denitrification step is that a pH-neutral product is obtained thereby, so that no pH control by means of titration agents or large quantities of buffers is necessary. Denitrification takes place under anoxic conditions, nitrate being used as an electron acceptor. An electron donor is addded in the form of, for example, methanol or an organic compound originating from waste water. Denitrification takes place at a pH value of between 5.8 and 9.2, preferably at a pH value of between 7 and 8.2.

According to the invention, an installation is furthermore provided for the biotechnological removal of a volatile mtrogen compound from a gas which contains this compound, comprising at least a first device for dissolving said gas in a liquid transport medium, a second device separate from the first for oxidi¬ zing, by means of microorganisms, the nitrogen compound dissolved in the transport medium, and a first line for transporting the transport medium from the first to the second device.

In an embodiment, the installation is characterized in that the first device comprises a membrane along which the transport medium is to be conducted, which membrane is formed substantially by hollow fibres provided with micropo- res, with the object of having the gas containing the volatile nitrogen compound flow through said fibres such that the volatile component can be absorbed by the transport medium.

In an embodiment of the invention, the second device contains a biomass whose density is at least 10 kg dry matter/m ³ , and preferably lies between approximately 20 and approximately 30 kg dry matter/m ³ . The second device comprises, for example, an oxidation bed which is substantially composed of a plurality of pieces of synthetic resin foam, preferably polyurethane foam, supported by a carrier structure.

In another embodiment, the second device comprises a fluidized-bed reactor. Such a reactor comprises water-filled columns in which granules of carrier material float, surrounded by a biomaterial. The reactor is aerated with compressed air, whereby a column of bubbles is created. The reactor is compact, has a high activity and is particularly suitable for large-scale applications.

Alternatively, the second device may comprise a so-called airlift reactor.

An installation according to the invention is preferably characterized by a third device, separate from the first and the second device, for reducing to nitrogen the nitrogen compound dissolved in the transport medium and oxidized in the second device, and by a second line for transporting the transport medium from the second to the third device.

Still more preferably, the first line is a circulation line for circulation of the transport medium through the first and the second device in that order, and the second line together with the third device forms part of a bypass line which issues into the circulation line in two locations, such that the flowrate through the bypass line is lower than that through the circulation line.

A major advantage of a bypass line through which the flowrate is lower than through the circulation line is that the concentration of dissolved nitrate in the bypass line is comparatively high compared with the concentration of dissol- ved oxygen in front of the denitrification device. The quantity of dissolved oxygen which is kept comparatively small against the quantity of dissolved nitrate in the bypass line in front of the denitrification device leads to a reduced require¬ ment for an electron donor necessary for the denitrification process.

According to the invention, the third device is formed by a denitrification reactor, while means are present for providing this reactor with an additional substance, for example an electron donor or a nutrient for a denitrifying bacteri¬ um. The electron donor is, for example, methanol or an organic compound derived from waste water.

In an advantageous embodiment, the denitrification reactor contains a plurality of pieces of polyurethane foam. Polyurethane foam blocks were found to be particularly suitable for providing a good retention of the biologically active slurry in a denitrification reactor. The foam has a large specific surface area

which offers a sufficient adhesion surface for the denitrifying materials, while it also has a filtering action owing to its foam structure and the presence of a system of pores between the foam blocks.

The invention will now be explained in more detail with reference to the drawing, in which Fig. 1 is a diagrammatic picture of an installation according to the invention.

Fig. 1 diagrammatically depicts an installation 1 according to the inventi¬ on, with an absorption device 2 and a nitrification device 3 which are intercon¬ nected by a circulation line 4. A denitrification device 6 is included in a bypass line 5 which issues into the circulation line 4. Circulation pumps 7, 8 are included in the lines 4, 5, respectively. An electron donor and a nutrient are stored in vessels 9, 10, respectively, which substances are supplied to the denitrification device 6 by a dispensing pump 11 via a dispensing line 12, which issues into the bypass line in front of said device. The Figure also shows a sprinkler 13, an effluent line 14, an effluent reservoir 15, and a thermostat 16. Arrows 17-20 indicate the direction of flow of the transport medium (water) in the circulation line 4, bypass line 5, dispensing line 12, and effluent line 14, respectively. Water is admitted into the circulation line 4 through an inlet 23; the absorber 2 has an inlet 21 and an outlet 22 for waste gas. The operation of the installation is as follows.

A gas containig ammonia is admitted through inlet 21 into the absorption device 2 where the ammonia is absorbed from the waste gas by water in the circulation line 4. The waste gas divested of its ammonia is discharged through outlet 22. Circulation water containing ammonia passes through circulation line 4 and is sprinkled by sprinkler 13 onto the biomass (not shown) in the nitrification device 3, where the dissolved ammonia is oxidized to nitrate, during which small quantities of nitrite may be formed in the case of an incomplete oxidation reaction. The nitrate (and possibly nitrite) formed are conducted through the circulation line 4 and the bypass line 5 to the denitrification device 6, where these substances are reduced to nitrogen. An electron donor, for example methanol, ethanol, acetic acid, or an organic compound derived from waste water, is fed to the denitrification device 6 from vessel 9, while vessel 10 supplies a mixture of

nutrients, for example a solution containing KH ₂ PO ₄ , K ₂ HPO ₄ , CaCl ₂ , MgSO ₄ , FeSO ₄ and or a yeast extract. The flowrate in the bypass line 5 is lower than in the circulation line 4, so that the nitrate to oxygen ratio in the solution in the bypass line immediately in front of the denitrification device is comparatively great. A comparatively higher nitrate concentration is sought in order to reduce the required quantity of electron donor: the electron donor binds itself first to oxygen and then to nitrate. hen methanol is used, the following reactions take place 1. NH ₃ + H ₂ O -> NH ₄ ⁺ + OH- 2. NH ₄ + 2O ₂ -> NO ₃ - + 2H ⁺ + H ₂ O

3. NO ₃ - + 5/6 CH ₃ OH -> 1/2 N ₂ + 5/6 CO ₂ + 7/6 H ₂ O + OH ^"

It follows from these chemical equations that the proton balance becomes neutral, i.e. no titration agents or large buffer quantities are necessary. The circulation water may be re-used internally for a long time and contains hardly any nitrogen compounds when discharged, for example through effluent line 14 into effluent reservoir 15. The temperature of the circulation water may be kept at an optimum temperature by means of the thermostat 16, for example at 30° C.

Example 1

In a test installation with a configuration as in Fig. 1, waste gas with a concentra- tion of between 15 and 30 mg NH ₃ /m ³ is cleaned. The installation comprises a cleaning device ("scrubber") which is known per se with a volume of 7.5 1, an oxidation bed with a volume of 10 1, and a denitrification reactor with a volume pf 2.2 1. The removal capacity of the installation is 0.23 g NH ₄ 7h at a temperature of 30° C. The concentrations in the circulating liquid at a measured pH value of 7.2 are as follows (mg/1): NH ₄ - N:28 NO ₃ - N:15 NO ₂ - N:2.6 Result: 99% NH ₃ has been removed from the waste gas.

The removed NH ₃ has been converted into (%):

N ₂ : 98.5 nitrate in effluent: 0.6 ammonium in effluent: 0.9

Assuming a typical value for the waste gas flow from an intensive pig farm to be, for example, 8000 m ³ /h, the NH ₃ concentration being 15 mg/m ³ , the dimensions of an installation in accordance with Fig. 1 will be the following: scrubber volume: 3 m ³ oxidation bed volume: 4 m ³ denitrification vessel volume: 1 m ³

Example 2

A membrane is installed as the absorption device for the installation of Fig. 1, with a membrane surface area of 0.1 m ² and consisting of 40 microporous capillary polypropylene tubes of 50 cm length (ND 020 CP 2N, microdyn, Wuppertal, Germany). The gas flowrate is 1.2 m ³ /h, the ammonia concentration in the incoming gas is 31 mg/m ³ , and the gas and liquid flows have opposite directions (counterflow principle). Concentration measurements of the incoming and outgoing gases yield the result that the ammonia present in the gas is taken up by the membrane with an efficiency of 83%.

Example 3 The gas flowrate in the absorption device of Example 2 is set for 0.4 m ³ . Concen¬ tration measurements of the incoming and outgoing gases now show an absorpti¬ on efficiency of 99% of the ammonia present in the gas.