NO300838B1 - Process and plant for biological treatment comprising nitrification and wastewater denitrification - Google Patents

Description

The present invention relates to the biological treatment of waste water as well as from ordinary households such as industrial plants, and of water that is to be processed into drinking water.

It is well known that sewage water and sometimes also water that is to be processed into drinking water can contain various materials in suspension, not only mineral substances, but also organic and nitrified substances. Such types of water also contain dissolved materials and colloidal particles (ie particles with a diameter of less than one micron).

Different biological treatment processes have been developed where a biomass is produced that contains bacterial colonies that will break down the organic material or the nitrified material present.

Biological treatment processes can be divided into two groups.

The first group includes biological treatment processes where activated sludge is used after pre-treatment. In a so-called "activated sludge tank" where untreated water is added, favorable aeration conditions are maintained, which adds oxygen to the micro-organisms present. The sludge is also maintained in an expanded state due to suitable aeration devices, e.g. perforated pipe at the bottom of the tank, or an aeration turbine placed on the surface.

The water from such a tank is then transferred to a clarification tank, and at the bottom of this, some sludge is taken out, which is recycled to the first tank to maintain a biological environment. The excess sludge is removed from the system. If v. denotes the biomass content in the water, this quantity will usually be kept at between 2 and 6 g/litre in the activated sludge tank, e.g. about. 5 g/litre.

Usually, the thickening of the sludge will be achieved without adding granular material. In the so-called "activated sludge" process, you essentially only want to operate with free bacteria or other microorganisms.

However, a variation has been proposed where powdered activated charcoal is added as an additional cleaning agent. This procedure is, among other things, described in patent CH-545,254 and in the article "PAC Process" by John A. Meidl in Water/Engineering and Management, June 1982, pp. 33-36. Charcoal is used because this material's high absorption capacity, which means that non-biodegradable toxins will be absorbed. If one takes into account the high price of activated charcoal, it is important to be able to develop the most efficient regeneration cycle possible, which in practice is complex and energy consuming (especially if one takes into account that between each cycle the activated charcoal must heated to 200°C).

In another variation, it is proposed to use calcium carbonate as a sludge additive, or one can use carbon black which is stated to be lost in the excess sludge taken out of the process.

The operation of such a process using activated sludge is dependent on the ratio between the mass content of the supplied water (measured as kg/litre) expressed as the biological oxygen demand in a given period, in relation to the biomass present in the activated the sludge tank (usually one is only interested in the parameter B.0.D.5, which corresponds to the "biological" oxygen demand for a given weight of biomass for 5 days). This ratio, which says something about the nutrient supply in relation to the decomposition capacity in the biomass, is measured in kg/kg per day. The higher the ratio, the greater the amount of water that can be treated in a given period of time, and the lower the investment required. In practice, the ratio will vary between 0.1 and 1.

Procedures that use activated sludge also aim to treat nitrified pollutants, and the important parameter in this case is the weight of treated nitrogen per kg biomass per day, and this parameter has the same dimension as mentioned above.

The activated sludge technique is limited by whether the activated sludge can be decanted, either in a loaded or unloaded state, and this property is usually characterized by the so-called MOHLMANN index, which corresponds to the volume occupied by one gram of sludge, and the lower the index, the the better, the decanting will be able to be carried out. In practice, indices will vary between a value of 1000 (very poor) and a value of 100 (very good). It is understood by the treatment process can be maintained when the content y_ of biomass is as high as the corresponding MOHLMANN index is low.

In order to increase the efficiency of the plant, one tries in advance to keep the biomass content in a stable state, i.e. as high as possible, and if possible to reach up to or exceed 10 g/litre, but in practice there are no known processes that exceed 5 to 6 g/litre. In advance, one also tries to reduce the MOHLMANN index as much as possible.

Another type of treatment process uses solid bacteria, e.g. on a granular material in a fluidized bed through which the contaminated water is led from bottom to top. Maintaining the layer in a fluidized state without the material from the layer flowing out requires very strict precautions with regard to the supply of liquid, and this often turns out to be incompatible with the variations you have in the daily supplied water volume. In addition to these, such fluidized bed processes often have initial difficulties. Furthermore, a regeneration of the granular material in the layer will require a partial recirculation between two levels in the layer, and this is energy-intensive. Examples of such processes are described in US Patent 3,855,120 or in an article on the OXITRON system published by Cheming-Tech, vol. 51, No. 6, June 1979, pp 549-559 published by Verlag Chemie Gmbh Weinheim, Germany.

It is an aim of the present invention to avoid the aforementioned disadvantages and to improve the treatment of contaminated water, particularly in cases where the water contains nitrated compounds such as ammonium hydroxide, and where the space requirement that is necessary to have a flexibility that is compatible with variations is reduced in the supply of the contaminated water, and where the investment costs and operating costs are lower than what has previously been the case for similar facilities.

The present invention thus provides a method for the biological control of waste water containing nitrogenous pollution, in which the waste water is supplied to a mixing zone containing at least one room, where by means of turbulent mechanical stirring a homogeneous suspension of said waste water is produced and a low-consumable and low-soluble granular material added to biomass, and the suspension is maintained at a substantially constant mass concentration,

said homogeneous suspension of the water and granular material is circulated to a separation zone, from which on the one hand purified water is taken out, and on the other hand granular material containing active biomass is taken out, and essentially all of the active biomass attached to it the granular material is recycled to the mixing zone.

It should be noted that in the mixing zone you get an internal stirring which is produced in a completely different way to what is done in methods where fluidized layers are used.

In addition to this, all granular material is transferred to a separation zone.

One thus avoids the strict operational limitations associated with fluidized beds, while at the same time achieving a purification that is far better than what is found for equivalent mass loadings in the absence of granular material (activated sludge), and where the present method entails a moderate investment and moderate operating costs. Furthermore, cheap mineral material can be used as granular material, e.g. sand that can be easily recycled without too much energy consumption.

In a preferred embodiment, when the nitrogenous contaminant includes ammonium hydroxide, oxygen is added to the mixing zone, e.g. in the form of air in suitable quantities, so that a nitrifying biomass is developed which is attached to the granular material,

and in the event that the nitrogenous pollution is in the form of nitrates, then adding a source of organic carbon into the mixing zone, in which there is an anaerobic

environment (ie without injection of free oxygen),

and in the event that the nitrogenous pollution is in the form of one or more organic compounds or ammonium hydroxide, the mixing zone consists of two zones, a nitrifying zone (aerobic) and a denitrifying (anaerobic), or vice versa with recycling of the sludge and the granular the material between these two zones,

and where the granular material present in homogeneous suspension is of a mineral type, preferably fine sand,

and where the granular material present in a non-recycled residual part of the sludge is recovered in a separation step either of a physical or biochemical type (non-thermal), and re-injected together with active biomass attached to the granular material, while poorly attached sludge is separated from this granular material and removed from the system,

and where the concentration of granular material in the mixing zone is kept at between 5 and 100 g/litre, preferably between 5 and 50 g/litre,

and if the granular material is fine sand, then the concentration is kept at between 5 and 50 g/litre, preferably approx. 20 g/litre,

and wherein the turbulent stirring induces an internal recirculation of between 5 and 100 times the incoming supply of waste water, preferably 10 to 50, and between 10 and 30 times the supplied amount of water,

and where the granular material carrying the solid mass is returned to the mixing zone by forced recirculation in an amount that is between 10 and 500% of the supplied water amount,

and where almost all of the biomass-containing granular material is fed to a point in front of the mixing zone or directly to the latter,

and where the homogeneous suspension is set aside for degassing before it is taken to the separation zone.

It should be noted that in the case where the water contains a nitrogenous contaminant in the form of ammonium hydroxide, and where a residual part of the sludge to be evacuated is completely or partially produced, then all nitrifying bacteria which develop slowly but which attach strongly to the granular material, and which is thereby not separated during the separation step, even when this consists of hydrocyclones.

The present invention also provides a facility for the biological treatment of pre-treated wastewater containing a nitrogenous pollutant, comprising: a mixing zone, with at least one room, into which a pipe with wastewater to be treated exits, and which contains a granular material that is not consumable and poorly soluble, filled with associated biomass, and which zone has mechanical devices for turbulent mixing and is capable of holding the wastewater and granular material in homogeneous suspension down to a downstream separation zone;

which separation zone downstream of the mixing zone has a channel for the withdrawal of purified water and a channel for the withdrawal of the filled granular material; and

a recirculation pipe with pumping devices that starts from the outlet channel for the sludges and the granular material filled with biomass and ends at the mixing zone or upstream of it.

According to a preferred embodiment, the plant consists of the following: a mixing zone containing at least one mixing reactor (or compartment) which also has devices for the supply of oxygen,

where said devices for supplying oxygen are formed as an air or oxygen-containing supply manifold, where the mixing zone consists in series of an aerobic nitrification reactor containing a device for supplying oxygen, and an anaerobic denitrification reactor containing a device for supplying organic carbon,

where the mixing zone in series contains an anaerobic denitrification reactor, and then an aerobic nitrification reactor equipped with devices for supplying oxygen, and where a

recirculation pipes are connected between the outlet of the aerobic nitrification reactor and the anaerobic denitrification reactor,

where the mechanical means for stirring is an ordinary stirrer,

where said tubes have blades which stand along a driven axis and are shaped in such a way as to produce currents in opposite directions along this axis,

where this stirrer is arranged vertically,

where the granular material is of a mineral type, e.g. fine sand,

where the granular material in the mixing zone has a concentration of between 5 and 100 g/litre, preferably between 5 and 50 g/litre,

where the granular material has a size between

20 and 500 fira,

where the granular material is sand with a size between 80 and 200 [ xm,

where the facility includes a pipeline for physical separation, and where the pipeline starts from the drainage channel for the loaded granular material, and where said pipeline runs parallel to the recycling pipe and ends in a re-injection pipe for biomass-containing granular material in the mixing zone or in front of it,

where the facility includes a degassing zone between the mixing zone and the separation zone,

where the separation zone is a de-sloping facility which can contain laminar de-sloping elements, and where the separation can also be carried out by means of other suitable devices such as centrifugation, screening or filtration over a membrane.

Properties and advantages of the present invention will be apparent from the following description with reference to the attached drawings where: figure 1 is a simplified drawing of a biological treatment plant according to the present invention;

figure 2 is a partial schematic section of a variant of

said facility;

figure 3 is a simplified section of another embodiment according to the present invention, where one has a combined nitrification/denitrification treatment; and where Figure 4 is an embodiment of the plant in Figure 1 and which, in the same way as shown in Figure 3, has devices for a nitrification/denitrification treatment.

The biological treatment plant shown in Figure 1 basically contains a mixing zone'A and a separation zone

B.

The mixing zone A contains at least one mixing reactor 1 (here in the drawing it is shown 1) which contains a granular material mixed with biomass, and where there is a supply pipe 2 for water to be treated, and a recycling pipe 3 for the supply of almost all biomass-containing granular material, while separation zone B consists of a chamber 4 which is in communication with the mixing reactor 1, a vertical degassing chamber 5, and where chamber 4 is equipped with collection devices 6 which end in a drain pipe 7 for purified water, as well as devices 8 for the withdrawal of granular material mixed with biomass, which happens through a drain pipe 9. A control valve is placed on said pipe from which the above-mentioned recycling channel 3 is started. The plant is also equipped with pumping devices 11.

In a preferred embodiment where the nitrogenous contaminant is ammonium hydroxide, aeration devices are placed in the mixing reactor 1, shown in the drawing as an aeration manifold consisting of perforated pipes 12 located at the bottom of the reactor, and where said pipes are connected with an air or oxygen supply. The ventilation devices can also be in the form of a ventilation turbine. The air supply itself is preferably chosen so that between 4 and 5 kg of oxygen is transferred to the water per kg (N-NH4) for nitrification, and between 0.5 and 1.5 kg of oxygen per kg (B.O.D.) for elimination of the latter.

In a preferred embodiment, a pipe 14 is provided for injecting granular material into the reactor, and where this granular material is obtained by taking out a portion of granular material mixed with biomass and taken out from a non-recycled part of the material that leaves the separation chamber 4 by means of pipe 9. This non-recycled part circulates via valve 10 in an extraction/separation plant which consists of a container C for receiving biomass-containing granular material, and which is equipped with a stirrer 15 and an extraction/separation plant D. A pipe 16 from container C is equipped with pump devices 17, and leads into device D.

In practice, these devices will be hydrocyclones 18, from which sludge without granular material is taken out via pipe 19, while material containing biomass is taken out via pipe 14 in a suitable physical state so that it can be re-injected into the mixing reactor 1. As a variant not shown , the recovery can be carried out by screening or by ultrasound, by centrifugation or by other chemical or biological means (by e.g. allowing the granular material to rest for a certain period of time without nourishment and/or oxygen supply).

The mixing reactor 1 is equipped with mechanical devices 20 for vigorous stirring, shown in the drawing in the form of a stirrer consisting of a vertical shaft 21 with blades 22, and which is driven by the motor 23.

Separation chamber 4 is equipped with separation devices 25, shown in the drawing in the form of a static plant, e.g. of laminar slope blocks 26.

In Figure 1, the supply pipe 2 is placed halfway up the wall in the reactor. However, due to the vigorous stirring in the reactor, the feed pipe can be placed anywhere in the reactor.

As an example, the drawing also shows how the water or suspension from the reactor is fed into the upper part of the degassing chamber 5 and falls down to the lower part of this, while of course you can also feed the flow into this chamber from bottom to top or horizontally from the side of the reactor 1 into chamber 4. Chamber 5 can optionally also be omitted completely.

The water to be treated biochemically and supplied via pipe 2 has in practice undergone a standard physical pre-treatment to remove larger particles (i.e. > 1 mm) that are present in suspension (in practice the size will be greater than or equal to for the granular material). This pre-treatment will usually be a form of screening, and desanding is preferably carried out completely by an oil treatment or by a primary decantation. Except in certain cases (this applies in particular to the removal of phosphorus) this pre-treatment will not include a complete chemical treatment before the water enters the reactor, but it may however be necessary to add additives just before entering the reactor.

In a preferred embodiment, the granular material in the mixing reactor will have a grain size of between 20 and 500 µm. The material is preferably fine sand which is relatively cheap, and which is a suitable substrate for fixing and producing a biomass, and the grain size is preferably such that it lies between 80 and 200 µm.

The concentration of the granular material in the reactor is kept at a predetermined level, e.g. between 5 and 100 g/litre, preferably between 5 and 50 g/litre (e.g. approx. 20 g/litre4 when using sand.

The vigorous stirring carried out by means of the stirrer 20 will keep the granular material in a homogeneous suspension.

The blades 22 of the stirrer are preferably placed so that they are located at several levels, and are preferably shaped so that they produce vertical currents in opposite directions along the shaft 21.

The vigorous stirring will in practice inside the reactor produce an internal recirculation of from 5 to 100 times in relation to the supply of the water.

The biomass that develops on the granular material will depend on the content of organic material in the water to be treated, and whether or not there is an oxygen supply in the reactor. In the case where there is a supply of oxygen (see Figure 1), the presence of a higher organic load (B.O.D.) in the supplied water will promote the development of bacteria that consume organic particles rather than bacteria that are able to consume ammonium hydroxide. If, on the other hand, the content of organic compounds is low, bacteria will develop which, although they develop slowly, are strongly attached to the material and can convert ammonium hydroxide into nitrates.

If the content of organic matter is zero, and there is no oxygen supply, a third type of bacteria will develop which converts the nitrates into nitrogen. When taking into account the disadvantages associated with the formation of nitrates, it may be advantageous to provide two successive mixing reactors which decompose the ammonium hydroxide into nitrogen. One can actually make a choice with regard to bacteria by appropriate addition, and one can also improve nitrification by injecting ammonium hydroxide into the water. It should be noted that the nitrifying bacteria remain attached to the granular material during extraction/separation.

Because of the vigorous stirring and because all water is fed to the L~separation chamber, there will be no accumulation of granular material in the reactor, so that the concentration of this remains more or less constant. This means that there is also an approximately constant concentration of biomass, because all granular material leaving the separation chamber via pipe 9 is re-injected into the reactor again via pipe 3, while pipe 14 only brings a first initial injection.

In practice, one will re-inject at least 3/4, if not 90% (or even more) of the granular material leaving chamber 4 via pipe 9. Due to the fact that this "sludge" is very suitable for pumping, it is necessary to re-inject this material in an amount that is between 10 and 500% in relation to the amount of water added, because if the recycling rate is lower, you will get a concentration of granular material. In practice, the recirculation rate will be defined as a function of the maximum concentration that can be maintained in the chamber.

Instead of ending in the reactor, the recycling pipe can, as a variant not shown, enter pipe 2 just outside reactor 1.

By using a degassing chamber 5, the mixture will be freed from air bubbles that could be trapped during stirring and that would affect the separation in chamber 8, even if the granular material is heavier than the water.

Table 1 contains data exemplifying two systems of ammonium hydroxide nitrification (after elimination of B.O.D.) which were successively obtained in a pilot plant of the type shown in Figure 1, and where

the mixing reactor was of a cylindrical shape with a capacity of 30 liters and a height of 1 m,

the supply of untreated water to the reactor was controlled by means of a pump which supplied between 10 and 30 litres/hour, and where the supply varied with time as a function of the volume of water to be treated,

and there was a supply of approx. 20 and 150 litres/hour of water via recirculation pump 11, i.e. between 100 and 500% of the supplied water,

and where the sloping surface 25 was inclined by 60°

(simulating a laminar slope), and where the water was supplied via the bottom, while the treated water flowed out via an overflow.

The inflowing water has a so-called "E" level which, according to French standards, means that it has a maximum content of suspended matter of 30 mg/litre and a chemical oxygen demand (C.O.D.) of 90 mg/litre. In the samples, the content of ammonium hydroxide in the water was between 30 and 90 mg/litre N-NH4.

The figures in table 1 correspond to a system where there was a content of 0.8 kg N-NH4 per m<3> of the reactor per day and night. Two series of numbers are given for each system, and each series contains both the inlet and the outlet values respectively.

In both systems, the content of granular material in the suspension is 40 g/litre, the biomass content is 0.7 g/litre - labeled VMS (volatile material in suspension) - for a total air supply of 0.5 m<3>/hour which in a real plant can be reduced to approx. 0.1 m<3>/hour.

In the first system, 11 liters of water were supplied per hour, and this water has a high content of ammonium hydroxide (between 83 and 89 mg of nitrogen per litre), and a C.O.D. of 42 to 50 mg/liter, less than 10 mg/liter of MIS and an NO3 content of 24 to 26 mg/liter expressed as N-NO3. The system had a recycle rate of 400 to 500%, and a residence time in the reactor of 3 hours, a velocity of 20 m/h in the de-sloping plant and a HAZEN (characteristic property of the de-sloping process) velocity of 0.26 m/h. The content of ammonium hydroxide was reduced by between 66 and 72%.

In the second system, the water supply was higher (28 l/hour), the content of ammonium hydroxide was only 40 to 56 mg of nitrogen per litres, while the content of C.O.D. and MIS was the same. The recycling rate was less (200%) and the residence time was shorter (one hour). The reduction of ammonium hydroxide was also better (75 to 85%) than in the first system.

It appears from the results from these two systems that, according to the present invention, a strong degree of removal of nitrogenous pollution is achieved per unit volume of the reactor (0.7 to 1 kg (N-NH4)/m3) . As a comparison, one can indicate that an installation based on an activated sludge principle results in values corresponding to approx. 0.2 kg (N-NH4)/m<3>), (and on the basis of this, one should also not forget that the principle of fluidized beds has more expensive and strict operating conditions).

It is important to note that the present method achieves these results by not following common practice where one has a high biomass content in order to achieve high efficiency. In the present invention, this content is lower than 1 g/litre, and this is lower than the known methods.

Furthermore, the MOHLMANN index here is lower than 50, which gives excellent results.

Figure 2 shows a biological treatment plant which is a variant of that shown in Figure 1 (the extraction sequence is not shown here because it is not necessary when one has a pollutant which is only of the nitrated type (N-NH4 in particular)). This plant differs from the one in figure 1 in that the mixing zone A' is in direct communication with the separation zone B' without a degassing chamber, and furthermore the recirculation is carried out internally.

The mixing zone A' is similar to the one in figure 1 (the numbers in the figure themselves refer to the same numbers as in figure 1). The supply of water to be treated takes place in the upper part of the reactor 1', while the communication with the separation zone is carried out in the lower part by an underflow. The separation of water and granular material takes place here by simple gravity. The bottom of the separation chamber 4' is inclined towards the reactor and equipped with raking devices 8' which force the granulated material into an extraction funnel 9' which is placed below the communication zone by means of an underflow between zones A' and B'. This funnel 9' is equipped with an extraction nozzle 9'A which via a valve 9'B is in communication with a line 3' for recycling granulated material, normally equipped with a pump device 11'. The drawing does not show a possible injection 13' of granular starting material, and this pipeline for withdrawing such recirculating material can either be placed before or after the aforementioned pump.

This facility can be used to break down the organic material in the water, and large quantities of sludge are formed which can easily be taken out for the recovery of granular material and the disposal of surplus biomass.

The plant in figure 2 has the advantage that it is very compact.

It should be noted that on this plant the recirculation nozzle is not attached to reactor 1', but to pipe 2 where the water to be treated is supplied.

Figure 3 is a variant where you have several reactors in series, and where the reactor plant is well suited for a nitrification/denitrification treatment of the water combined with a BOD treatment.

This plant consists of a first reactor 1''A which is similar to that shown in figures 1 and 2, and includes an oxygenation manifold 12'', and this makes it possible to establish an aerobic system which allows bacteria to more easily convert the ammonium hydroxide to nitrates (this can be improved by an initial addition of bacteria). The reactor 1''A is connected by means of an overflow to another mixing reactor 1''B which differs from the one shown in Figures 1 and 2 by the absence of an oxygen supply source (anaerobic environment). A source of organic carbon (e.g. in the form of methanol, molasses, acetic acid, etc.) is injected into this reactor at position 30. It is this injection of substrate that provides a good development of bacteria that convert the nitrates that are formed in mixing reactor 1''A, to gaseous nitrogen. This second reactor is connected by means of an underflow with a degassing chamber 5'' which leads into the separation zone 4''. As before, most of the filled granular material collected in the de-sloping zone B'' will be recycled.

According to another embodiment, which is not shown, and where only a denitrification is carried out, the first reactor 1''A will be missing.

Figure 4 shows a plant which is a variant of the plant in Figure 1~ and which comprises two mixing reactors 1'''A and 1'''B which correspond to the reactors 1''A and 1"B in Figure 3, but the liquid which is to be treated, is passed through these reactors in the opposite order to that shown in figure 3. This means that the liquid to be treated is first passed through reactor 1 ' ''B, where there are anaerobic conditions, and then through reactor 1' "A where there are aerobic conditions. A pipe 40 is provided with pumping devices, not shown, which enable the liquid containing granular material to be recycled from the reactor 1"A to the reactor 1B. This means that the nitrates formed in the reactor 1''A are re-injected into the reactor 1''B, where they are converted into gaseous nitrogen with the help of the bacteria that feed on the organic carbon found in the treated liquid. The recycle quantity that passes from reactor 1'''A to reactor 1'''B is chosen so that it is between 1 and 6 times (advantage weld between 2 and 4

times) the amount of water supplied.

As before, the recycling of granular material from separation zone 4''' to the mixing zone (preferably reactor 1'''B and not to reactor 1'''A) takes place through pipe 2'''

The plant of the type shown in Figure 4 may in certain cases be preferred for the treatment of waste water, while the plant in Figure 3 may sometimes be preferred for the production of drinking water.

The above descriptions are only given as examples and are thus not intended to be a limitation of the invention.

E.g. it can be mentioned that the granular material described is preferably natural sand, then materials such as clay, pumice stone, kaolinite etc. can be used instead.

Claims (39)

1. Procedure for biological treatment of waste water containing nitrogenous pollution, characterized in that the waste water is supplied to a mixing zone containing at least one room (A, A', A''), where a homogeneous suspension of said waste water and an in-itself low-consumable and low-soluble granular material that is added to biomass is produced by means of turbulent mechanical stirring , and the suspension is maintained at an essentially constant mass concentration, said homogeneous suspension of the water and granular material is circulated to a separation zone (B, B', B''), from which on the one hand purified water is taken out, and on the other hand granular material containing active biomass is taken out, and and substantially all of the active biomass attached to the granular material is recycled to the mixing zone (A, A', A''). 2. Method according to claim 1, characterized in that the nitrogen-containing pollution is in ammonium form, and in that oxygen is injected into the mixing zone in such a large quantity that a biomass is developed which consumes the pollution found in the waste water. 3. Method according to claim 2, characterized in that this oxygen is injected in the form of air. 4. Method according to claim 1, characterized in that the nitrogenous pollution is in nitrate form, and in that an organic carbon source is injected into the mixing zone where an anaerobic environment is maintained. 5. Method according to claim 1, characterized in that the water to be treated contains a nitrogenous pollutant in organic form or in the form of ammonium hydroxide, and in that the mixing zone consists of an aerobic nitrification zone (l''A) into which oxygen is injected, and an anaerobic denitrification zone (l''B ), into which an organic carbon source is injected. 6. Method according to claim 1, characterized in that the water to be treated contains nitrogenous pollution in organic form or in ammonium form, and that the mixing zone consists of an anaerobic denitrification zone (l'''B) and then an aerobic nitrification zone (l'''A), into which oxygen is injected and liquid containing granular material from the aerobic nitrification zone is recycled towards the anaerobic denitrification zone in an amount at least equal to the original supply of wastewater. 7. Method according to any one of claims 1 to 6, characterized in that the granular material present in the homogeneous suspension is fine sand. 8. Method according to any one of claims 1 to 7, characterized in that the granular material that is present in the non-recycled residual part of the sludge is extracted by separation and re-injected together with solid biomass into the mixing zone, while sludge separated from this granular material is sieved away. 9. Method according to any one of claims 1 to 8, characterized in that the concentration of granular material in the mixing zone is kept at between 5 and 100 g/litre. 10. Method according to claim 9, characterized in that the concentration of granular material when this is fine. sand, is kept at between 5 and 50 g/litre. 11. Method according to any one of claims 1 to 10, characterized in that the turbulent stirring results in an internal recirculation of between 5 and 100 times in relation to the original supply of waste water. 12. Method according to any one of claims 1 to 11, characterized in that the suspension is returned to the mixing zone by a forced recycling in an amount of between 10 and 500% in relation to the original supply of waste water. 13. Method according to any one of claims 1 to 12, characterized in that most of the biomass-containing granular material in the supplied wastewater is returned to a point in front of the mixing zone. 14. Method according to any one of claims 1 to 13, characterized in that the homogeneous suspension is degassed (5, 5'') before it is led to the separation zone. 15. Facilities for biological treatment of pre-treated waste water containing nitrogenous pollution, kara k—t erized in that it includes a mixing zone (A, A', A", A"'), with at least one compartment, into which a pipe (2, 2', 2") of wastewater to be treated exits, containing a granular material that is small consumable and sparingly soluble, filled with associated biomass, and which zone has mechanical devices (20) for turbulent mixing and is capable of holding the wastewater and the granular material in homogeneous suspension down to a downstream separation zone; which separation zone (B, B', B") downstream of the mixing zone has a channel (7, 7', 7") for withdrawal of purified water and a channel (9, 9'A, 9'B, 9") for withdrawal of the filled granular material; and a recirculation pipe (3, 3', 3"A) with pumping devices (11) starting from the outlet channel for the sludges and the granular material filled with biomass and ending at the mixing zone or upstream of it. 16. Installation according to claim 15, characterized in that the mixing zone (A, A', A' ') includes a mixing reactor (1, 1', 1'') containing devices for supplying oxygen (12). 17. Installation according to claim 16, characterized in that the devices for adding oxygen are designed as an oxygenation manifold. 18. Plant according to claim 15, characterized in that the mixing zone consists of an aerobic nitrification reactor (1''A) with a supply pipe for oxygen (12'') and an anaerobic denitrification reactor (1''B) which includes a supply (30) for organic carbon. 19. Plant according to claim 15, characterized in that the mixing zone consists of an anaerobic denitrification reactor (l'''B), and then an aerobic nitrification reactor (l'''A) equipped with a supply pipe for the oxygen, and where a recycling pipe (40) runs between the outlet of the aerobic nitrification reactor (1'' 'A) and the anaerobic denitrification reactor (1'' 'B). 20. Plant according to any one of claims 15 to 19, characterized in that the mechanical devices for turbulent stirring are an ordinary stirrer. 21. Plant according to claim 20, characterized in that this stirrer consists of blades (22) at a certain distance along a drive shaft (21) and shaped in such a way that currents are produced in opposite directions along said shaft. 22. Installation according to claim 20 or 21, characterized in that this stirrer is placed vertically. 23. Plant according to any one of claims 15 to 22, characterized in that the granular material is fine sand. 24. Plant according to any one of claims 15 to 23, characterized in that the concentration of granular material in the mixing zone is between 5 and 100 g/litre. 25. Plant according to claim 24, characterized in that the concentration is between 5 and 50 g/Lj.ter. 26. Plant according to any one of claims 15 to 25, characterized in that the granular material has a grain size of between 20 and 500 µm. 27. Plant according to claim 26, characterized in that the granular material is sand with a grain size of between 80 and 200 ptm. 28. Plant according to any of claims 15 to 27, characterized by including an extraction/separation sequence (C, D) starting from the withdrawal channel for biomass-containing granular material, located after the recycling pipe outlet and ending in a reinjection pipe for biomass-laden granular material, and where the latter pipe is fed into the mixing zone or just before this. 29. Plant according to claim 28, characterized in that the extraction tube includes at least one hydrocyclone (D). 30. Plant according to claim 28, characterized in that the extraction tube includes devices for a separation by means of ultrasound at a moderate energy level. 31. Plant according to claim 28, characterized in that the extraction tube includes devices for separation by means of screening. 32. Plant according to claim 28, characterized in that the extraction tube includes devices for separation by means of centrifugation. 33. Plant according to claim 28, characterized in that the extraction tube includes devices for separation using chemical or biological means. 34. Plant according to any one of claims 15 to 33, characterized in that a degassing zone is placed between the mixing zone and the separation zone. 35. Plant according to any one of claims 15 to 34, characterized in that the separation zone includes slope devices. 36. Plant according to claim 35, characterized in that the separation zone includes laminar slope or overflow elements (26). 37. Plant according to any one of claims 15 to 34, characterized in that the separation zone includes centrifugation devices. 38. Plant according to any one of claims 15 to 34, characterized in that the separation zone includes sieve devices. 39. Installation according to any one of claims 15 to 34, characterized in that the separation zone includes devices for filtering over a membrane.