EP2307314A1

EP2307314A1 - Process for extracting the ammonia nitrogen from liquid waste

Info

Publication number: EP2307314A1
Application number: EP09804612A
Authority: EP
Inventors: Tiziano Alan Reboldi
Original assignee: Niamex Srl
Current assignee: Niamex Srl
Priority date: 2008-08-07
Filing date: 2009-08-06
Publication date: 2011-04-13
Also published as: IT1391102B1; ITMI20081490A1; WO2010015928A1

Abstract

The invention describes a process for extracting the ammonia nitrogen from liquid waste, comprising the following operating steps: a) placing the waste in contact with a basifying agent in a reactor; b) generating an ionized gaseous flow above the waste and in particular above the free surface of the waste and driving a gaseous current comprising ammonia outside the reactor; c) recovering the ammonia from the gaseous current exiting from step b) with the formation of a purified gaseous current.

Description

PROCESS FOR EXTRACTING THE AMMONIA NITROGEN FROM LIQUID WASTE_.

The present invention refers to a process for extracting the ammonia nitrogen from liquid waste. The present invention falls within the technical field of treatment of waste containing nitrogen, in particular zootechnical waste containing animal defecation deriving from pig farms, cattle farms, poultry farms and farms with other animals, civil and industrial waste, and landfill leachate.

With the passing of time, national and international environmental standards have and continue to set increasingly restrictive limits to the quantity of nitrogen that it is possible to distribute into the environment through the spreading of animal excrement on agricultural land. In particular, there are increasingly restrictive limits set on the spreading of excrement in agricultural areas where the nitrate concentration in the earth has reached such high values as to make them "nitrate vulnerable zones" .

With the quantity of excrement produced and its nitrogen content remaining the same, the overall available land surface for spreading thus becomes progressively insufficient to ensure the disposal of the excrement apart from by reducing the nitrogen content .

Similarly, the disposal of civil, industrial and landfill leachate waste represents a big environmental problem due to the large quantities produced, and also due to the heavy costs involved.

In the state of the art different processes for treating liquid waste containing nitrogen aimed at completely removing or at least reducing the nitrogen content are known and used. In the case of zootechnical waste coming from pig farms, for example, nitrogen is present in concentrations that vary from 1000 to 6000 mg/1, according to the specific diet used at the farm and the dilution caused by the more or less frequent washing carried out to clean the piggery.

A medium-sized pig farm (3000-5000 units) produces an amount of such waste that can vary between 30 and 50 m³/day.

An almost complete removal of nitrogen (98-99%) can be obtained using biological waste treatment processes. These processes, which comprise at least one biological oxidation step of the waste followed by a denitrification step, are however very complex to manage, as well as requiring very expensive plants to be made .

Since the final destination of zootechnical waste is mainly spreading on farming land, the removal of nitrogen does not necessarily have to be complete or, for that matter, very high. Indeed, a reduction in nitrogen content of a waste equal to 50% by weight with respect to the total nitrogen would already be sufficient to allow double the amount of waste to be spread over the same farming surface (with respect to the original waste) . Equally important, moreover, is the cost-effectiveness of treatment, i.e. the ratio between the treatment cost and its effectiveness.

From this point of view, a type of process for treating waste that allows the nitrogen content to be reduced to acceptable levels and, at the same time, at low cost with respect to processes known in the state of the art, is that based on the extraction (or stripping) of the nitrogen contained in the waste in the form of ammonia. This type of process, which is typically carried out in tray or Raschling rings towers, foresees the stripping of the ammonia nitrogen through blowing of an air current (stripping current) in countercurrent with respect to the falling of the liquid waste consequently taking away the gaseous ammonia. In order to promote the development of the gaseous ammonia, the waste is generally heated and added to with basifying agents, like NaOH.

Usually the waste must also be sieved or filtered, possibly after the addition of flocculant agents, because the presence of solids, even if small in size, can block the holes of the plates or dirty the Raschling rings, causing the plants to stop. Moreover, in order to allow acceptable extraction speeds the waste must be heated to a temperature of above 80⁰C with very high energy consumption. The stripping current containing the gaseous ammonia is generally treated in washing towers, with acidic solutions. Once the gaseous ammonia has been absorbed in the washing solution, the resulting gaseous current without ammonia is released into the atmosphere.

The treatment based on stripping, although sufficiently effective in removing the nitrogen contained in the waste, does however involve very high energy consumption and substantial costs linked to the use of chemical additives. Indeed, in order to carry out the process large quantities of basifying and flocculant agents are necessary, as well as energy to move the mass of air of the stripping current and to heat up the waste. All of this makes it not very cost- effective to make treatment plates at the farms, particularly at small farms. One type of waste treatment process that allows the aforementioned drawbacks to be overcome and that is more energy-efficient and cost-effective than the processes of the prior art is the one described in Italian application MI2007A0002448. This process foresees extracting the nitrogen through the generation of a gaseous flow that, passing over the free surface of the waste with added basifying agent, manages to drive a gaseous current comprising ammonia outside the reactor. It has now been found that by combining the aforementioned extraction process with the use of a special ionized air system it is possible to obtain further increase in the overall extraction yield, in the sense of both extraction speed, and the limit to the final concentration of ammonia in the treated waste .

Therefore, the object of the present invention is a process for extracting the ammonia nitrogen from liquid waste, comprising the following operating steps: a) placing the waste in contact with a basifying agent in a reactor; b) generating an ionized gaseous flow above the waste and in particular above the free surface of the waste and driving a gaseous current comprising ammonia outside the reactor; c) recovering the ammonia from the gaseous current exiting from step b) with the formation of a purified gaseous current.

Preferably, the process is carried out using, in step b) , an inserted ionized gaseous flow formed from one or more gases and/or vapors selected among: air, nitrogen and argon.

The process according to the present invention applies to waste coming from various kinds of farms, such as pig farms, cattle farms, poultry farms, etc., and more generally to all wastes that contain nitrogen in ammonia form, for example those produced after anaerobic digestion treatment (industrial waste, civil waste, landfill leachates) .

The waste collected at the production sites at which it is generated (e.g. in one or more farms, purification plants, landfills, etc.) is stored in a collection tank. They are generally in a liquid or liquid-muddy state and they can be moved by means of the pumping systems typically used in the field. Sometimes, according to the nature of the waste, before being subjected to treatment, the waste can be mixed together and/or diluted with water until the fluidity most suitable for moving it with the aforementioned pumping systems is reached.

The process according to the present invention is generally carried out in a single reactor if it is in discontinuous mode, or else in a suitable system comprising many reactors with preferably vertical extension if it is in continuous mode. Within such a system the waste is loaded by means of pumping systems. In a preferred embodiment of the invention, the loading of the waste takes place in a completely automated manner . Step a) of the process according to the present invention consists of placing the waste characterized by a neutral pH, which according to the chemical- physical conditions is subjected to the necessary preliminary treatments (e.g. filtration, flocculation) , in contact with a quantity of basifying agent capable of raising the pH value up to a value that can vary between 7.2 and 13, preferably between 10.0 and 13.

Should the pH already have a value of over 10.0 it is possible to proceed directly to the nitrogen extraction treatment.

The Applicant has observed that the basifying action is preferably carried out by a solid basifying agent selected among CaO, MgO and/or their mixtures or else selected among aqueous solutions of Ca(OH)₂,

Mg(OH)₂ and/or their mixtures. Preferably, the basifying agent is in solid granular form. Unlike the basifying agents used in the processes of the state of the art

(e.g. NaOH), these additives have a much lower cost. The preferred basifying agent is quick lime, i.e. a CaO/MgO mixture. This is for dealing with the treatment of zootechnical waste, which after treatment must be distributed (spread) on farming land; should one be dealing with landfill leachates, conventional basifying agents can also be used (e.g. soda or caustic potash) .

The present invention can nevertheless also be made with the same advantages, although in limited form, with other basifying agents such as Na₂O, K₂O, NaOH, KOH, Na₂CO₃, K₂CO₃ and/or their solid mixtures or else with the respective aqueous solutions and/or their mixtures .

In general, the basifying agent can be added to the waste to be treated in solid form or else as aqueous solution. Typically, when the basifying agent is added in solid form, its dose varies from 5 to 20 kg of agent per m₃ of waste to be treated. When it is added in aqueous solution form, typically 30% by weight, the dose varies from 30 to 50 liters/m³.

Due to the raising of the pH, the ammonia nitrogen contained in the waste, (in which it is present mainly in the form of NH4⁺ ions) , tends to separate from the waste in the form of gaseous NH₃. The concentration of the ammonia in the liquid waste increases until, at the specific operative temperature and pressure conditions, it spontaneously tends to come out of the waste until an equilibrium state is reached. By generating a gaseous flow inside the reactor that takes away the ammonia that develops from the waste, such an equilibrium is altered and new ammonia is produced in the waste, which will tend to come out of it to restore the equilibrium. It has been observed that such a phenomenon is particularly accentuated if the gaseous flow is generated by inserting an ionized gaseous flow over the free surface of the waste, since in this way as well as quickly removing the exiting ammonia the very strong electrostatic attraction that the ionized gaseous flow exerts on the polar molecule of the ammonia is exploited. The exit speed of the ammonia is thus increased and the quick development of new ammonia from the waste is promoted. The pressure of the gaseous flow generated to extract the ammonia can vary between -100 mbar (reactor in depression - drawn gaseous current) and 100 mbar (inserted gaseous current), i.e. the gaseous flow that allows the gaseous current comprising ammonia to be extracted can be generated both by inserting a gaseous current that runs over the head of the waste and by placing the reactor in depression. In a preferred embodiment the pressure of the gaseous flow is selected between -10 and +10 mbar.

In both of the operative modes, i.e. using an inserted gaseous current or else placing the reactor in depression, the gaseous flow generated is present above the surface of the waste without bubbling inside it.

By not making enormous masses of air bubble in the waste, as usually occurs in the processes according to the state of the art, the formation of foam is avoided, with a consequent reduction in the management costs deriving from the use of anti-foaming products.

The gaseous flow draws the ammonia, which gradually separates from the waste, outside of the reactor. The gaseous current comprising the ammonia is then sent to the subsequent step c) of the process.

It has been observed that the process for reducing nitrogen proceeds faster when the removal of the gaseous phase is carried out by generating the gaseous flow by means of an inserted gaseous current instead of by placing the inside of the reactor in depression.

It has also been observed that the extraction of the ammonia from the waste is faster in reactors having a shape such as to make a large mass exchange area between the waste and the gaseous current. Preferably, the reactors consist of one or more long and narrow rectangular tanks, with entry of the ionized air flow on the long side. The gaseous flow generated over the waste and in particular on the free surface of the waste inside the reactor, being produced by means of a low pressure inserted gaseous current or by placing the reactor under slight depression, involves a modest energy consumption. The energy consumption for the ionization of said flow is also modest.

The process object of the present invention, therefore, makes it possible to remove the ammonia nitrogen from the waste with economically favorable conditions with respect to the processes known in the state of the art. Moreover, to make the process according to the present invention, modestly sized means for pumping gases and/or vapors and ionization systems that are easy to make or easy to find on the market at low cost are sufficient.

In order to promote the process for extracting nitrogen from waste and decrease the treatment time, the waste can be heated and kept at a temperature of about 50⁰C, for example, through special heating elements, such as coils immersed in the body of the waste . At this temperature one avoids both the formation of foam, due to the decomposition of the bicarbonates, and the precipitation of the insoluble carbonates with consequent formation of calcareous incrustation on the walls of the reactor and on the heating coils. In the case of zootechnical waste, the heating can be particularly useful in the winter time where the collected waste is at an average temperature of 15-20⁰C, whereas in the summer time the temperature of 40-50⁰C of the waste can easily be reached. Moreover, due to the addition of the basifying agent there is an increase in the temperature of 4-5°C due to the exothermicity of the dissolution reaction.

The process for extracting nitrogen from waste according to the present invention proceeds at an acceptable speed even without stirring of the waste.

In order to further increase the extraction speed, it is nevertheless useful to carry out an intermittent stirring by insufflation of the gaseous flow used directly in the waste itself so as to allow better homogenization of the mass or, alternatively, by mechanical stirring means, preferably paddle stirrers.

The stirring can also be obtained by the application of ultrasound to the waste or by the relaunching of a part of the waste itself, taking it from the bottom of the reactor and reinserting it at a second point of the reactor, located above the head of the waste, so as to create turbulence inside it.

Typically, the waste recirculation flow rate varies from 0.2 to 3.5 recircles/hour, i.e. it is such as to recirculate the volume of waste subjected to treatment between 0.2 and 3.5 times in an hour (for example if the volume of the waste to be treated is 5000 liters, 2 recircles/hour correspond to a recirculated volume equal to 10.000 liters/hour) . In a further embodiment of the invention, in addition to the effect of the aforementioned recircles, the waste can also be kept under stirring by insufflation of air captured from the outside.

The stirring of the waste according to the modes described above allows the efficiency of the process to be increased, increasing the ammonia extraction speed.

In order to reduce the overall energy consumption of the process and to limit the emissions into the atmosphere deriving from it, at least a part of the gaseous current coming out from the reactor, once purified of the ammonia in step c) (even only partially) , can be used as inserted gaseous current in step b) . Preferably, all of the purified gaseous current is recycled to step b) of the process so as to make a closed recovery loop.

At the end of the process, the waste is discharged from the reactor and stored in tanks waiting to be sent for spreading on farming land and/or it is partially used to pre-basify the new waste to be treated. If the waste is spread over land, it may be necessary to correct its pH in order to make its characteristics adhere to the criteria established by the relevant environmental standards .

In a preferred embodiment of the invention, the end of the treatment is determined automatically by means of a continuous analysis system that, continuously determining the concentration of the nitrogen still present in the waste (for example the ammonium ions) or else the nitrogen extracted in gaseous phase (for example gaseous ammonia) , is able to indicate the level of nitrogen reduction reached by the process. The process according to the present invention can be carried out discontinuously or batch-wise, i.e. treating predetermined amounts of waste inside a single reactor until the end of the process or continuously. For the treatment of zootechnical waste the discontinuous version is preferable, whereas for industrial waste, civil waste and leachates the continuous version is preferable. In practice, the operative modes depend heavily upon the concentration of the ammonia in the waste and upon the daily flow rate to be treated, as well as upon the required end concentration . In the discontinuous configuration (figure 1) the gaseous flow (air) is sent by a fan to the ionization system, generating an ionized gaseous flow, and from here it is made to pass with horizontal flow over the free surface of the waste to be treated. Typically, the ionized gaseous flow rate (air) sent varies from 1500 to 2000 Nm³/h for every m³/h of waste to be treated.

The ionization system consists of apparatuses available on the market, and preferably a system consisting of charge generators with both negative and positive polarity, with voltage adjustable from 0÷40 KV and 5OmA (max) in direct current, produced by the firm Martignoni Elettrotecnica srl (Vanzaghello-MI) , is used.

The ionization system of the gaseous flow (air) substantially consists of the following: there is a charge generator (which can be negative or positive) that is a high voltage direct current transformer. Coming out from the generator there is then the connection to a charge bar or ionizing bar, said bar consisting of a series of metallic points one beside the other, embedded in an insulating resin with a special electronic adjustment circuit that prevents electrical discharge from being released (for example when the air flow becomes particularly conductive due to the increased humidity) ; again in output from the generator there is also the grounding that closes the circuit (in the case of the present invention it is connected to the liquid to be treated) .

In conclusion, a negative or positive ionization is generated (according to the generator that is used) , whereas the bar is always the same one, placed about 15-20 cm from the surface of the liquid and at the inlet of where the air or gaseous flow to be ionized is inserted, which (since the liquid is connected to the ground of the generator) is immediately ionized.

Preferably, bars of variable length according to the size of the reactor are used, placed about 15-20 cm over the free surface of the waste whereas the circuit is closed directly in the liquid itself; in this way, a much greater ionization and extraction effect is obtained than what can be obtained using a counter electrode for closing the metallic circuit (in practice, it has been seen that such a metallic counter electrode is effectively replaced by using the waste itself as "counter electrode") .

The ionized gaseous flow is diffused inside the reactor by inserting the gas in an upper channel made by creating a slot on the long side of the reactor. In the upper part of the slot for the entire length of the tank is where the charge bar is positioned (bars can even be made of a length of 4m) . Just above the bar a closure for the tank is made through a slab of plastic material (given the high voltage used, all parts of the reactor must strictly be made from plastic) that thus determines the formation of a "channel" in which the air runs along the width of the tank; on the opposite side of the tank (again on the long side) an air outlet slot is made that is exactly the same as the inlet one. The closing of the circuit (as already stated earlier) takes place directly on the liquid to be treated so as to have as high as possible degree of ionization, located a few cm from the free surface itself.

The waste is fed to the reactor that preferably consists of a long and narrow rectangular tank, with ionized air flow inlet on the long side, so as to reduce as much as possible the distance between air inlet and outlet and thus maintain a high speed. Moreover, at regular intervals, the waste is stirred for a certain amount of time, in order to keep the uniformity of the ammonia concentration.

In the continuous configuration (figure 2) the process is carried out by connecting two or more reactors in series . In this case the ionization system can consist of the charge bars described earlier placed a few centimeters away from the free surface of the waste. Preferably, the charge bars system is adopted.

In practice, a charge bar is positioned on each tank along the side of insertion/entry (or passage) of the air with the closure of the circuit (also in this case) in the liquid itself.

The charge bar has proven more effective than the mesh and also has the advantage of allowing adjustment through an electronic system of the real power necessary according to the ammonia extraction speed in both extraction systems (continuous-discontinuous) .

In the continuous process (figure 2) , the waste is pumped into the upper part of a special reactor with stacked rectangular tanks. The tanks containing the waste are alternated with the tanks for absorbing the ammonia containing acidulated water through acids of different kinds (organic or inorganic) .

The gaseous current coining out from a first tank, once purified of the ammonia, is introduced into the next tank, increasing the nitrogen removal speed from the waste with respect to that of the process carried out in a single reactor (for the same amount of waste treated) and at the same time decreasing the specific energy consumption of the process.

The number of tanks and their sizes are selected by the man skilled in the art so as to obtain an optimal "evaporation surface/waste volume" ratio to maximize the ammonia extraction yield and minimize the energy costs and the costs of chemical additives to be used, also according to the specific chemical composition of the waste.

In Figure 3 a possible embodiment of a tank for treating waste (slurry tank) and of a tank for absorbing ammonia (ammonia absorption tank) are given as examples . The reactors can be made from plastic or composite material, stainless steel, alloy steel with anti- corrosion treatment or else waterproof reinforced cement .

The energy and material (chemical additives) saving permitted by the process according to the present invention is even clearer if one considers that the processes according to the state of the art foresee the insufflation of substantial masses of clean air into the waste, which must necessarily be purified before being able to be emitted into the atmosphere in the conditions foreseen by environmental standards, with a very high energy and material cost. The recovery of gaseous ammonia in step c) of the process according to the present invention can be carried out with different techniques. For example, the gaseous current coming from step b) can be made to bubble in an acidic solution (neutralization) . Alternatively, the gaseous current is sent to a washing tower (scrubber) that uses a similar acidic solution as washing liquid.

In the discontinuous process, the washing system consists (scheme of figure 1) of a tank divided by one or more sectors containing water acidified with acid and/or mixtures of acids (both organic and inorganic) . The gaseous current containing ammonia after having undergone a first washing in a compartment is sent to a second compartment for further washing. The division into sectors allows different types of ammonium salts to be obtained as final co-product according to the acid used in the different compartments.

The water that after absorption of the ammonia has a pH of between 7.0 - 7.5, is sent to a collection reservoir where the pH is taken to values of between 6.0 - 6.5 through addition of acids and once again sent to the washing tank .

In an embodiment of the invention the gaseous current (air) coming from step b) of the discontinuous process, is inserted perpendicularly into the absorption liquid with a head that can vary from 1÷3 cm, so as to allow close contact with the acidic solution that captures its ammonia and neutralizes it forming the corresponding ammonium salt, but limiting as much as possible the load losses to avoid reducing the flow rate to values that are too low that would stop the process.

The diffusion of the gaseous flow (air) inside the absorption solution is obtained by distributing the flow along the entire width of the tank through tubes having a diameter that can vary between 1÷5 cm such as not to generate a reduction in the total section, with respect to the initial insertion diameter (the total section of the main duct is equal to the sum of the individual sections of the insertion tubes on the liquid) .

According to requirements (and therefore the length of the absorption tank) it is possible to perform 2 or more washes of the gaseous current .

In another embodiment of the invention that concerns the continuous extraction system, the ammonia absorption tanks that preferably extend vertically, are spaced apart by the tanks containing the waste to be treated. The tanks, as highlighted in the diagram of figure 2, can be slightly inclined in an opposite way with respect to the next one. The discharge of the sediment possibly present is thus allowed and blockages of the connection channels between one tank and the other are avoided.

In this case, the gaseous flow (air) is inserted from the bottom into the first tank of waste passing over the free surface thereof, where it is ionized by the charge bar located about 15-20 cm over the air inlet; in the next absorption tank, the air coming out from the first tank of waste is inserted on the long side parallel to the absorption liquid (the diffusion system is practically identical to the discontinuous case) and about 1÷2 cm below the free surface, to then be collected on the opposite side of the tank and sent on to the next tank of waste, where it is inserted from the bottom to then pass over the free surface thereof and finally be ionized by the charge bar, as described earlier. The flow of air from the tank of waste can continue towards a further absorption tank until extraction is complete.

The waste that as stated is inserted from the top of the reactor runs towards the bottom helped also by gravity. It is possible to consider roughly making a system with 5 extraction tanks containing waste, each connected to a corresponding absorption tank.

The difference between the continuous system and the discontinuous system is the fact that by operating continuously, the efficiency of the ionization of the air is much greater, since it is generated directly on the waste and thus does not suffer from the length of the route during which the ionization effect runs out.

According to the present invention to absorb ammonia it is preferable to use an aqueous solution of an inorganic acid selected among sulfuric acid (H₂SO₄) , phosphoric acid (H₃PO₄) , nitric acid (HNO₃) , carbonic acid (H₂CO₃) , hydrochloric acid (HCl) and/or their mixtures or else an organic acid like for example citric acid, acetic acid or sulfamic acid.

The resulting solution at the end of the ammonia recovery operation (bubbling or washing saturated solution) is an aqueous solution containing one or more ammonium salts, depending upon the mixture of acids used for neutralization (for example, using H₂SO₄ gives (NH4 J₂SO₄, using HNO₃ gives NH₄NO₃, using H₂CO₃ will give ammonium carbonate and/or bicarbonate) . The ammonium salt solution can be advantageously used as fertilizer.

Typically, the concentration of ammonium salt varies

(according to the type of acid used) from 150 to 350 g/1 and the amount of solution that can be obtained ranges from 1500 to 2000 liters per day, for example making the process according to the present invention in a farm of 10,000 pigs.

In order to recover the ammonia in step c) it is also possible to bubble the gaseous current coming out from the reactor in water and/or subject it to washing in water washing towers. The product of this recovery is an ammonium hydroxide solution.

According to a further alternative, it is possible to condense the ammonia through condensation systems (e.g. condensation towers) obtaining anhydrous ammonia.

It is also possible to foresee the combustion of the gaseous current coming out from the reactor and containing the ammonia. The current can be used both as such and in a mixture with other fuels. Preferably, a fraction of such a current is sent for combustion and the remaining fraction is used as reactant to neutralize the nitrogen oxides (NOx) produced by the combustion of the first fraction.

With the ammonia recovered in step c) it is also possible to subject the gaseous current coming out from the reactor to an ammonia disassociation process with formation of H₂ and N₂, then storing the H₂ in a suitable manner, for example using suitable compounds in solid state. A further alternative for the recovery of the ammonia extracted from the waste is liquefaction. By compressing the gaseous current containing the ammonia, following dehydration and using suitable temperature and pressure conditions, it is indeed possible to obtain the separation of the ammonia in liquid form.

In order to dehydrate the gaseous current it is possible to use CaO or MgO and/or their solid mixtures. For example, by making the process according to the present invention in a farm of 10,000 pigs it is possible to obtain about 150÷200 Kg/day of liquid ammonia . Unlike neutralization, the recovery of ammonia through its liquefaction allows a product of high commercial value to be obtained, which is much more valuable than liquid fertilizer and/or ammonium hydroxide, since liquid ammonia can be used as a raw material in other industrial processes.

The process according to the present invention has different advantages with respect to processes for reducing the nitrogen content in zootechnical waste known in the state of the art . Firstly, it allows a further increase in the global nitrogen extraction yield from waste to be obtained. Obtaining treated waste containing a smaller quantity of nitrogen makes it possible to spread a greater quantity thereof on farming land qualified as "nitrate vulnerable zones" . Typically, in a waste originating from pigs the ammonia nitrogen comes to about 50% of the total nitrogen present .

Moreover, the use of a low pressure ionized air flow over the free surface of the waste makes it possible to obtain higher waste treatment speeds combined with low energy consumption required to generate such a flow. These advantages, together with the fact that making the apparatus for actuating the process involves modest costs and its operation can be completely automated, make the process according to the present invention particularly suitable for being exploited industrially in the field of pig farms, cattle farms, poultry farms and other animal farms.

Possible embodiments of the process according to the present invention are shown in the diagrams of figures 1, 2 and 3, which respectively illustrate the discontinuous extraction process, the continuous extraction process and two example embodiments of the tanks for treating the waste and absorbing the ammonia.

The following example embodiments are provided merely in order to illustrate the present invention and must not be taken to limit the scope of protection defined by the attached claims. Example 1 4000 liters of a waste coming from pig farms and containing 4500 mg/1 of nitrogen in ammonia form, (equal to 68% of the total nitrogen of the waste) , corresponding to a total amount of 18 Kg of nitrogen in ammonia form, was treated for 1.5 hours in a single reactor plant. The waste was added to with 40 kg of a mixture of CaO and MgO in granules until the pH was brought to a value equal to 11.2. The pressure of the inserted gaseous current was equal to 5 mbar and the flow rate was equal to 4000 Nm³/h. The air inserted on the long side of the reactor was ionized through an electrostatic generator (mod.50 mA- 40KV dc from the firm Martignoni Elettrotecnica srl) and relative ionizing bar positioned on the long side for insertion of air, and with closure of the circuit on the waste itself.

The flow rate of the waste recirculation was fixed at 800 liters/hour. The gaseous current containing ammonia coming out from the reactor was sent into a dual stage washing system positioned above the extraction tank itself, and more specifically, inserted 2÷3 cm below the free surface of the acidic solution (having a volume of 500 liters) of H₂SO₄ at 9.14% by weight. At the end of the treatment the concentration of ammonium in the waste was about 300 mg/1; this means that 4200 mg/1 of ammonium have been extracted from the waste, which have formed 500 liters of 12.5% solution by weight of ammonium sulfate.

Claims

1. Process for extracting the ammonia nitrogen from liquid waste, comprising the following operating steps: a) placing the waste in contact with a basifying agent in a reactor; b) generating an ionized gaseous flow above the waste and in particular above the free surface of the waste and driving a gaseous current comprising ammonia outside the reactor; c) recovering the ammonia from the gaseous current exiting from step b) with the formation of a purified gaseous current.

2. Process according to claim 1, wherein the gaseous flow in step b) is generated by means of an inserted gaseous current, preferably composed of- one or more gases and/or vapors selected from the group comprising air, nitrogen and argon.

3. Process according to any one of the preceding claims, wherein the inserted gaseous flow has a pressure in the range of -100 to 100 mbar, preferably in the range of - 10 to +10 mbar.

4. Process according to any one of the preceding claims, wherein the basifying agent is added in a quantity such to bring the pH to a value that can vary from 7.2 to 13, preferably from 10.0 to 13.

5. Process according to any one of the preceding claims, wherein the basifying agent is selected from among CaO, MgO and/or their mixtures or from among aqueous solutions of Ca(OH)₂, Mg(OH)₂ and/or their mixtures, and preferably it is composed of a CaO/MgO mixture.

6. Process according to any one of the preceding claims, wherein the purified gaseous current exiting from the reactor in step (c) is completely recirculated as inserted gaseous current in step (b) .

7. Process according to any one of the preceding claims, wherein the waste is maintained under stirring in the reactor by means of paddle stirrers, or by means of ultrasound application, or by means of insufflation of the employed gaseous flow directly into the body of the waste itself, or by means of insufflation of air obtained from outside, or by extracting a part of said waste from the bottom of the reactor and re-inserting it in a second point of the reactor above the head of the waste.

8. Process according to any one of the preceding claims, wherein the ionized gaseous flow is generated by a fan, which sends the flow to an ionization system.

9. Process according to any one of the preceding claims, wherein the ionized gaseous flow rate varies from 1500 to 2000 Nm³/h for every m³/h of waste to be treated.

10. Process according to claims 8 or 9, wherein the ionization system is composed of both positive and negative polarity charge generators, with voltage adjustable from 0÷40 KV and 5OmA (max) in direct current, which transfer the voltage generated to the bars.

11. Process according to claim 10, wherein the bars of the ionization system are placed at about 10-20 cm from the free surface of the waste while the closure of the circuit is made directly in the liquid itself.

12. Process according to any one of the preceding claims characterized in that it is simultaneously carried out in two or more reactors or it is continuously achieved by connecting two or more reactors in series .

13. Process according to claim 12, wherein the purified gaseous current exiting from a first reactor is used as inserted gaseous current in the process carried out in the next reactor.

14. Process according to any one of the preceding claims, wherein the recovery of the ammonia in step c) is achieved by making the gaseous current coming from step b) pass onto the free surface of an acidic solution or by making the gaseous current coming from step b) pass onto a system composed of a tank divided by one or more sectors containing water acidified with acid and/or mixtures of acids, or by perpendicularly inserting the gaseous current coming from step b) into the acidified water with a head that can vary from 1÷3 cm.

15. Process according to any one of the preceding claims, wherein two or more ammonia recovery processes are achieved.

16. Process according to any one of the preceding claims, wherein the recovery of the ammonia in step c) is achieved by bubbling the gaseous current exiting from step b) in water and/or subjecting it to washing in water washing towers, or via condensation of the gaseous current exiting from step b) , or via combustion of the gaseous current exiting from step b) , or via liquefaction of the gaseous current exiting from step b) , upon dehydration.

17. Process according to any one of the preceding claims, wherein part of the treated waste is used for pre-basifying the waste to be treated.