EP1414751A1

EP1414751A1 - Method and apparatus for purifying a polluted medium

Info

Publication number: EP1414751A1
Application number: EP02762266A
Authority: EP
Inventors: Poul Erik Bundgaard Jensen
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-07-12
Filing date: 2002-07-12
Publication date: 2004-05-06
Also published as: WO2003006384A1

Abstract

The invention concerns a method and an apparatus for purifying a polluted medium containing ammonium, where the medium in a first chemical process is supplied with at least a first reagent, causing a chemical reaction between the ammonium in the medium and added reagents, which results in precipitation of at least a first ammonium-containing salt, where a subsequent second mechanical process separates medium from precipitation products. The purpose of the invention is to remove ammonium from a polluted medium and simultaneously produce a nitrogenous product that may be recycled. This may be achieved in that precipitation products are decomposed into a precipitate and ammonium in a third liquid based thermo-chemical process in which the precipitate and ammonium are released to the surrounding liquid, where the precipitate is recirculated to the first chemical process, where the precipitate is used as the first reagent. Hereby is achieved that a reagent may be reused and form part of the same process again. At the same time, great resource savings are attained by using the method. Nitrogenous products are formed at the same time by the process.

Description

Method and Apparatus for Purifying a Polluted Medium

The invention concerns a method and an apparatus for purifying a polluted medium containing ammonium, where the medium in a first chemical process may be supplied with a first reagent, causing a chemical reaction between the ammonium in the medium and added reagents, which may result in precipitation of at least a first ammonium- containing salt, where a subsequent second mechanical process can separate medium from precipitation products, where precipitation products in a third chemical process may be decomposed into a precipitate and ammonium.

From chemical reaction plants it is commonly known to added reagents to a medium and subsequently to separate precipitate products from the medium. It is known to remove ammonium by nitrification/denitrification where ammonium is released as free nitrogen to the atmosphere.

From EP 490 396 is known a method and an apparatus for purifying polluted medium containing ammonium, where the medium is supplied with a first reagent in a first chemical process, where the first reagent may contain magnesium. The process causes a chemical reaction between the ammonia of the medium and added reagents. By reaction, at least a first ammonium-containing salt is precipitated, and by a subsequent sec- ond mechanical process medium is separated from precipitation products. In a third chemical process, the precipitation products are decomposed into a precipitate and ammonium, and the precipitate is recirculated to the first chemical process, where it is used as the first chemical reagent. The decomposition of precipitation products into precipitate and ammonium occurs at a high temperature, where ammonium together with water vapour is separated by drying or by steam stripping.

From EP 0 915 058 A2 is known a similar method for purifying ammonium-containing waste water. The decomposition of precipitation products may also occur here by vapour/air precipitation.

The technique described in the cited documents concerns energy demanding ammonium separation. Either drying processes are used where a large amount of water is removed by evaporation, or steam separation of ammonium is used. Both of the cited methods will be expensive in use and therefore not suited for continual purification of large amounts of waste water.

The object of the invention is to achieve an effective and cheap removal of ammonium from a polluted medium and at the same time to produce a nitrogenous product that may be recycled.

This may be achieved if the third chemical process is effected as a liquid based thermo- chemical process in which the precipitate and ammonium are released to the surrounding liquid, where the precipitate is recirculated to the first chemical process, where the precipitate is used as reagent and where the ammonium content of the surrounding liquid may be discharged for subsequent utilisation.

Hereby may be achieved an effective purification of waste water, where the nitrogen content of the waste water is removed very efficiently. This may have great significance for final purification of sewage water after passing mechanical filters and biological processes, where the waste water has high content of nitrogen. Simultaneously it may be achieved that a reagent can be reused and form part of the same process again. Thereby may be achieved great resource savings as supply of reagent from outside is reduced. Simultaneously nitrogenous products may be formed which advantageously may be used as fertiliser. Large amounts of polluted medium may be purified efficiently and cheaply because the process can be performed with low energy consumption. Since the process can be performed continuously, a plant can be constructed in connection with a final purification of waste water after the waste water having passed a biological purifying plant.

At the passage of the medium of the precipitation unit, at least one first reagent may be added at a first time, where at least one second reagent is added at a second time, where the acidity of the medium may be adjusted at a third time by adding an basic product to the medium. Hereby may be achieved that reagents are supplied in dependence of the actual need for increasing the concentration of a certain reagent. Thereby may be achieved resource savings simultaneously with the chemical processes running optimally.

The first reagent may advantageously contain at least one salt at least containing phos- phate and a metal. Hereby may be achieved that a chemical reaction between phosphate, a metal and ammonium may result in precipitation of an ammonium-containing salt.

The invention may advantageously be performed by the metal containing magnesium. Hereby may be achieved precipitation of a magnesium salt.

Advantageously, the invention may be performed by the metal containing manganese. Hereby, precipitation of a manganese salt may be achieved.

Advantageously, the ammonium of the liquid surrounding the regenerating unit may be released by heating and be supplied to an ammonia washer, where ammonia vapours may react with an ammonium sulphate solution, whereby supersaturation of ammonium sulphate occurs, crystallising as a salt. Hereby appears a fertiliser product which advantageously may be used in agriculture. In this way, excess sulphuric acid from e.g. smoke purification in power plants together with nitrogen removed from a polluted medium may react into a fertiliser product.

The invention also concerns an apparatus for purifying a polluted medium where the apparatus can contain a first precipitation unit where the medium is advantatageously admixed with at least a first reagent, where subsequently the medium may be supplied to a separation unit separating purified medium from precipitation products, where precipitation products may be supplied to a regenerating unit in which reagents may be regenerated, and where the regenerated reagents may be added to the first precipitation unit, where the first precipitation unit advantageously may include at least one pipe system in which flowing through of a polluted medium occurs, where reagents may be supplied through at least one branch of the pipe system, where the addition of reagents are timely controlled by a control unit, where the separation unit may include means for separating at least one liquid, where the regenerating unit may include at least one thermal heating unit, where precipitation products go through a liquid based thermo- chemical process, whereby a precipitate and ammonium can be released to the surrounding liquid, from which the precipitate may be supplied to the first precipitation unit via a pipe.

Hereby may be achieved an apparatus with a constant medium flow, where an effective removal of ammonium from the through-flowing waste water is effected concurrently. At the same time, a fertiliser product is formed that may be used in agriculture. Reagents may be reused by the process whereby consumption of reagents is minimised.

Advantageously, the separation unit may include at least one hydrocyclone. Hereby may be achieved a rapid and effective separation between liquid and precipitation products.

The regenerating unit may include an ammonia washer where at least a pipe connection between heating unit and ammonia washer conducts ammonium vapours to the ammonia washer, where a pump pumps air from the ammonia washer to a ventilation system included in the heating unit, where the ammonia washer transforms ammonium into salt by a chemical process. Hereby may be achieved a very efficient separation of ammo- nium form the surrounding liquid, where the liquid surface, by means of the ventilation system and stirring, attains a very large surface. If the temperature is adapted to a temperature at which the solubility of ammonium is minimal, the large liquid surface will result in a very rapid and efficient ammonium separation. At the same time, in the ammonia washer there may be achieved a very rapid and efficient reaction between am- monium and sulphuric acid, whereby ammonium is bound in a salt which advantageously may be used as fertiliser.

The invention may advantageously be designed with means for measuring the ammonia contents of the medium in the medium supply, where reagents are dosed to the first reaction tank on the basis of the ammonium content of the supplied medium. Hereby may be achieved a demand depending dosing of the amount of reagents whereby large savings may be achieved by the reduced consumption, and simultaneously chemical reactions may proceed optimally. The apparatus may include means for measuring the phosphate content of the medium in the medium inlet, where reagents are dosed on the basis of the phosphate content of the supplied medium. Hereby may be achieved a demand dependent dosing of the amount of phosphate added. Reduction of the used amount to the actual need in the chemical reactions chemical bonding secures the added phosphate, and thus ensures against unnecessary discharge of excess amounts.

The apparatus may contain means for measuring the pH value of the medium in the first reaction tank, where the first reaction tank includes means for pH regulation. Optimal precipitation conditions for ammonium are hereby achieved, whereby up to 99% of the ammonium content in sewage water reacts, with the possibility of later separation.

The apparatus may include means for adding at least one polymer to the first reaction tank in dependence of the measured value of ammonium in the medium supply. Hereby, better separation properties are attained.

The apparatus may advantageously include means for measuring and regulating the pH value in the thermal reactor. Hereby may be achieved that the pH value in the thermal reactor is optimised.

In the following, the invention may be described with reference to Figures, where:

Fig. 1 shows a first possible embodiment of the invention seen from the side, where

Fig. 2 shows the same invention from above, where

Fig. 3 shows a schematic drawing of a first possible alternative embodiment, where

Fig. 4 shows the chemical reactions schematically, where

Fig. 5 shows a schematic drawing of a second possible alternative embodiment of the invention.

Figs. 1 and 2 show a possible embodiment of the invention, designed for placing in a low container, i.e. a compact and highly efficient plant which efficiently removes am- monium from a polluted medium. On Fig. 1 is shown a supply pump 1 for the untreated polluted medium. The pump 1 has connection to a reaction tank 2, from which the medium is conveyed to a precipitation tank 3. From here the medium is pumped to a separation unit 4. A supply pump 5 for chemical sludge (POψMgNHt, 6H₂O) conveys the sludge further on to a regenerating and heating unit for chemical sludge 6. From here a supply pump 7 performs chemical pumping of sludge (2 MgHPO₄) to reaction tank 2. All components of the plant may advantageously be incorporated in a standard container.

The flow of the polluted medium and the effect is shown on Fig. 3. The polluted medium with a high content of ammonium (e.g. percolate from fillings, waste water from sludge concentrations or highly polluted industrial waste water) are led into the plant via a pump 29 that may accurately dose the amount of polluted medium.

The concentration of ammonium in the medium is measured on-line by a spectropho- tometer 55. The measured concentration of ammonium (mg/1) is multiplied with the supplied amount of medium (1/min) in order to reach the value of supplied ammonium per time unit (mg ammonium/min). A phosphate of e.g. PO₄ and a metal (Me: e.g. Mg, Mn, Fe, K, Ca) is added for achieving a 1 : 1 :1 ratio between NH4, Me and PO₄. When this ration is present, a precipitate will appear:

Nϊf4 + MgO + H3PO4 + 5 H₂O → PO4MgNH4, ^β HjO + H Reaction 1, see Fig. 4.

Dosing pumps 30 and 31 connected to the pipe system 34 add the metal 23 and phosphate 24 to the medium. The processed medium is discharged into a double tank system. This system consists of an internal reaction tank 22a and an external precipitation tank 22b. The medium is conducted to the centre oaf the bottom of the internal tank 22a. In order to adjust pH, base 26 or acid is supplied to the tank by means of a dosing pump 32. A pH sensor 38 that automatically regulate the dosing from the pump 32 measures the pH-value. The internal tank 22a and the external tank 22b are mutually connected by means of holes in the upper tank section. By means of the force of gravity, a flow of medium from the internal tank 22a to the external tank 22b thereby oc- curs for further chemical reaction. The medium in the internal tank 22a and the external tank 22b is carefully stirred by an air mixing unit 57. This unit consists of a blower and membrane making small air bubbles in the medium.

After precipitation of sludge (PO4MgNH₄, 6 H₂O), the sludge/water mixture is conducted out of the outer tank 22b and into the separation unit 28, e.g. a precipitation tank, a louver separator, a decanting unit, a bag filter or the like, which separates the precipitation product from the water. The precipitation product in the form of sludge is removed from the plant by a volumetric pump 36. An automatic valve system 37 con- trols discharging the sludge either into a container 47 for fertiliser purposes or into the regenerating unit 42. The sludge is automatically conducted to the regenerating unit 42, either continuously or at preselected intervals. The discharged sludge is mixed inside the tank by a stirrer 43 and is heated by a heating unit 46. The sludge is heated to about 70°C. In this way, the first sludge (PO₄MgNH₄, 6 H₂O) is transformed into a second sludge (MeHPO₄). If MgO is used as metal supply, the following reaction occurs:

PO Mg H₄, 6 H₂O + heat → MgHPO₄ + H₃ + 6 H₂O Reaction 2, see Fig. 4

The second sludge may be used in the first reaction 1 as substitution for PO₄ 24 and metal 23 supplied from pump 30 and pump 31. The second sludge is taken out of the regeneration unit 42 by means of a volumetric pump 48 and is conducted either to a tank for storage or for recycling to the supply unit. For possible adjustment of pH in the regeneration unit, base/acid may be added to tank 42 from a pump 41 communicating with a tank 40. Ammonium-containing water is discharged via a pipe 54 disposed in the upper part of the tank 42.

Fig. 4 shows the chemical reactions schematically.

Fig 5 shows a draft in principle of a possible alternative embodiment of the invention. The plant is divided into a precipitation unit 100 and a regenerating unit 200. The medium 101 is firstly supplied to the precipitation unit 100. The precipitation unit 100 is divided into three operation units: a precipitation section containing a pipe 102, a tank

105 and a separation unit 107. Polluted medium 101 is supplied to the precipitation unit 100 by means of a pump 109, whereby a medium flow can be controlled. Detectors 122 and 124 determine the chemical content in the polluted medium 101. The me- dium 101 is supplied to the pipe 102. A reagent 217 is added to the pipe 102 through a pipe branch 114 by means of a pump 216. A magnesium containing reagent 103 may be added by means of pump 110 through a pipe branch 115 to the pipe 102. Subsequently, a phosphate-containing reagent 104 may be added by means of a pump 111 through a pipe branch 116 to the pipe 102. From the pipe 102, the medium 101 is supplied to a tank 105 where a detector determines the chemical composition of the medium. NaOH

106 may be added by means of a pump 112 through a pipe connection. 117. With a pipe 118, the tank 105 is connected to a pump 113 which through a pipe 119 pumps medium further on to a hydrocyclone 107. Purified medium leaves the hydrocyclone through a pipe connection 108. Precipitation products are taken from the hydrocyclone 107 through a pipe 120. A control unit 130 has at least connection to detectors (122,

12, 126, 220) both in precipitation unit 100 and in regenerating unit 200, where the control unit at least controls the pumps 109, 110, 111, 112, 113, 213, 214, 215, 216 both in precipitation unit and in regeneration unit 200.

The function of the precipitation unit 100 is, by means of the additives 217, 103, 104,

106 and the medium 101 content of ammonium, to precipitate ammonium magnesium phosphate (POψMgNHU, 6 H₂O). This chemical process has already been described previously and is prior art. The function of the plant is to precipitate ammonium magnesium phosphate optimally and so that this precipitate subsequently may be separated from the liquid phase of the medium so that ammonium is removed from the medium.

The first part of the precipitation section is formed in that the reaction chamber is designed as a pipe system 102 through which the flow occurs with a speed enabling that particles/precipitation products are not precipitated in the pipe system. The system is made as a "plug-flow" system so that the addition of additives 21, 103, 104, 106 may be controlled in time and in relation to the most optimal sequence for adding the additives: Mg₃(PO₄)₂ 217, Mg 103, PO₄ 104, NaOH 106. The addition of NaOH 106 either occurs lastly in the reaction chamber or outside the "plug-flow" system in a fully stirred tank 105. As the first additive for the medium is added Mg₃(PO₄)₂ 217, which is regenerated in the regeneration unit 200. In order to satisfy the requirement that the mole ratio between Mg : NH4 : PO₄ be 1 : 1 : 1 in order for the process to operate, the additive magnesium 103 is added at first in the said sequence, in the form of e.g. MgO or MgCl₃, then the additive phosphate 104 in the shape of e.g. H₃PO₄ or KH PO₄ to the medium, and the additives react before pH regulation by means of NaOH 106. After pH regulation, ammonium magnesium phosphate (PO₄MgNH4, 6 H₂O) is crystallised and subsequently separated off in the separator 107.

The function of the separator 107 is to separate the precipitate (PO₄MgNH4, 6 H O) from the liquid phase of the medium. The unit is formed as a hydrocyclone which, by means of the centrifugal force (50-300 G) induced by the pressure of the medium pumped in, brings the particles/precipitation products out at the sides of the cyclone and down into the bottom, where they are concentrated and wherefrom the parti- cles/precipitation products are taken out. The precipitation product normally constitutes between 0.1 and 3 % of the inlet flow, depending on the ammonium concentration. The function of hydrocyclones is known from other sides.

The precipitation product (POφMgNFLt, 6 H O) removed has resulted in removing 90 - 99% of the ammonium nitrogen of the medium in laboratory and full scale experiments.

The total hydraulic stay time in the precipitation unit 100 is 3 - 7 minutes. The precipitation product (PO^gNFLt, 6 H₂O) may subsequently be dried and used as fertiliser product in agriculture or be added to the regenerating unit 200 of the plant, so that magnesium and phosphate may be reused in the precipitation unit 100.

The regeneration unit 200 is made up of a thermal heating unit 209 and an ammonium washer 212. The pipe 120 adds precipitation products to the heating unit 209 containing a stirrer 210, where air is supplied through a ventilation system 211 that communi- cates with an air pump 215 sucking from the ammonia washer 212. Above the liquid level of the heating unit there is a pipe 218 that forms connection to the ammonia washer 212. A detector 220 determines the chemical composition of the content of the heating unit. Ammonia vapour together with air are conducted through the pipe 218 to the ammonia washer 212. A pump 213 pumps sulphuric acid from the bottom of the ammonia washer 212 to a sprinkler 221 at the top of the ammonia washer. Sulphuric acid may be supplied to the ammonia washer with a pump 214. A fertiliser salt may be taken out at the bottom 219 of the ammonia washer.

The function of the regeneration unit 200 is, by means of a thermal process to regenerate magnesium and phosphate from the precipitation product (PO₄MgNH₄, 6 H₂O) produced in the precipitation unit 100. Compared with continuous addition of magnesium 103 and phosphate 104 to the precipitation unit 100, the purpose of the regenera- tion unit 200 is to attain a substantial improvement of the economy of the entire process by regenerating magnesium 103 and phosphate 104 from the precipitation product 6 H₂O) produced in the precipitation unit 100, so that it may be reused in the precipitation unit 100.

It is known that when the precipitation product (PO₄MgNH₄, 6 H₂O) in an aqueous solution is heated to about 70°C, ammonium is released from the precipitation product to the liquid phase. And a secondary salt, Mg₃(PO₄)₂, is formed, consisting of two of the chemical components which are added to the precipitation unit 100.

This property is utilised in the thermal heating unit 209 which is a closed container designed so that addition of precipitation product from the separator 107 to the heating unit 209 occurs continually or intermittently. The aqueous solution of precipitation product is kept in suspension in the heating unit 209 by means of partly by a mechanical stirrer 210 with scrapers fitted, continuously scraping deposits off the inner sides of the tank, and partly by a ventilation system 211 which is connected to the ammonia washer 212.

The heating unit may heat the aqueous solution of precipitation products to 70°C whereby ammonium is released to the ambient liquid. At high temperatures, ammonium has a very low solubility in water why it will further evaporate to the ambient atmosphere. The stirring system and the ventilation system will accelerate evaporation of ammonia from liquid to the atmosphere since the contact liquid/atmosphere will be multiplied by this stirring/ventilation. The air leaving the heating unit for the ammonia washer has a high content of ammonium. The heating unit 209 is connected in a closed circuit with the ammonia washer 212 so that the air ventilated over into the heating unit 209 from the ammonia washer 212 is returned to the ammonia washer 212 via a pipe 218. The ammonium evaporation process may be accelerated by adding NaOH, whereby pH is increased and the solubility of ammonium in the liquid is further reduced. Simultaneously with ammonium being released from the precipitation product (POφMgNHt, 6 H₂O), a secondary salt Mg₃(PO )₂ is formed, which is continuously or intermittently removed from the heating tank 209 by a pump 216 and supplied to the precipitation unit 100.

The ammonia washer 212 is designed as a closed container containing a liquid with a solution of ammonium sulphate. The lower half of the tank contains this liquid. At the top of the tank there is mounted a sprinkling system 221 which is used for discharging the sulphate liquid over the incoming ammonia gas from the heating unit. At the con- tact between the sulphate solution and the ammonia gas, ammonia sulphate is formed and dissolved in the liquid. The sprinkling system operates by the solution at the bottom of the tank being pumped up to the sprinklers 221 at the top of the tank by means of the pump 213. By continually adding sulphuric acid to the system by means of a pump 214 concurrently with consumption of sulphuric acid, the solution may attain supersaturation of ammonium sulphate so that this will be crystallised, and a salt is formed which can be extracted, dried and used a fertiliser in agriculture and horticulture. The washed air is returned via the air pump 215 to the heating unit.

In laboratory experiments, 90-95% regeneration of magnesium and phosphate is at- tained by this method.

Claims

1. A method for purifying a polluted medium containing ammonium, where the medium in a first chemical process is supplied with a first reagent (23, 24, 35, 103, 104, 217), causing a chemical reaction between the ammonium in the medium and added reagents

(23, 24, 35, 103, 104, 217), which results in precipitation of at least a first ammonium- containing salt, where a subsequent second mechanical process (4, 28, 107) separates medium from precipitation products, where precipitation products in a third chemical process (6, 42, 200) are decomposed into a precipitate (53, 217) and ammonium (54, 218), characterised in that the third chemical process (6, 42, 200) is effected as a liquid based thermo-chemical process in which the precipitate (53, 217) and ammonium (54, 218) are released to the surrounding liquid, where the precipitate (53, 217) is recirculated to the first chemical process (2, 3, 22, 102), where the precipitate is used a reagent (35, 217) and where the ammonium content (54, 218) of the surrounding liquid is discharged for subsequent utilisation.

2. A method for purifying polluted medium according to claim 1, characterised in that at least one first reagent (23, 24, 35, 103, 104, 217) is added when the medium passes the precipitation unit (2, 3, 22, 100) at a first time where at least one second reagent (23, 24, 35, 103, 217) is added at a second time where the acidity of the medium is adjusted at a third time by adding an acid regulating product (26, 106) to the medium.

3. A method for purifying a polluted medium according to claim 1 or 2, characterised in that the first reagent (23, 24, 35, 103, 104, 217) containing at least one salt containing at least phosphate and a metal.

4. A method for purifying a polluted medium according to one of claims 1-3, characterised in that the metal contains magnesium.

5. A method for purifying a polluted medium according to one of claims 1 - 4, characterised in that the metal contains manganese.

6. A method according to one of claims 1 - 5, characterised in that the ammonium of the surrounding liquid is released by heating and is added an ammonia washer (212) where ammonium vapours react with an ammonium sulphate solution, whereby super- saturation of ammonium sulphate occurs, crystallising as a salt.

7. An apparatus for purifying a polluted medium where the apparatus includes a first precipitation unit (2, 3, 22, 100) where the medium is admixed with a first reagent (23, 24, 35, 103, 104, 217), where subsequently the medium is supplied to a separation unit (4, 28, 107) separating purified medium from precipitation products, where precipitation products are supplied to a regenerating unit (6, 42, 200) in which reagents are regenerated, and where the regenerated reagents are added to the first precipitation unit (2, 3, 22, 100), characterised in that the first precipitation unit (2, 3, 22, 100) includes at least one pipe system (102) in which flowing through of a polluted medium (21, 101) occurs, where reagents (23, 24, 35, 103, 104, 217) are supplied through at least one branch (50, 51, 114, 115, 116) of the pipe system, where the addition of reagents (23, 24, 35, 103, 104, 217) are timely controlled by a control unit (130), where the separation unit (4, 28, 107) includes means for separating at least one liquid, where the regenerating unit (42, 200) includes at least one thermal heating unit (209), where precipitation products go through a liquid based thermo-chemical process, whereby a precipitate (53, 217) and ammonium are released to the surrounding liquid, from which the precipitate (53, 217) is supplied to the first precipitation unit (2, 3, 22, 100) via a pipe.

8. An apparatus for purifying a polluted medium according to claim 7, characterised in that the precipitation unit (22, 100) includes at least a first pipe branch (51, 115) through which a pump (31, 110) adds at least one first reagent (23, 103) containing magnesium, where the precipitation unit (22, 100) includes at least a second pipe branch (50, 116) through which a pump (30, 111) supplies at least a second reagent (24, 104) containing phosphate, where the precipitation unit (22, 100) includes a reaction tank (22, 105) in which acidity regulation occurs by supplying NaOH (26, 106) through a pipe connection (117) and at least one pump (32, 112).

9. An apparatus for purifying a polluted medium according to claims 7 or 8, characterised in that the separation unit includes at least one hydrocyclone (107).

10. An apparatus for purifying a polluted medium according to one of claims 7 - 9, characterised in that the regenerating unit (200) includes an ammonia washer (212), where at least a pipe connection (218) between heating unit (209) and ammonia washer (212) conducts ammonium vapours to the ammonia washer (212), where a pump (215) pumps air from the ammonia washer (212) to a ventilation system (211) included in the heating unit (209), where the ammonia washer (212) transforms ammonium into salt by a chemical process.

11. An apparatus for purifying a polluted medium according to one of claims 7 - 10, characterised in that the apparatus includes means (55, 122) for measuring the ammonium content of the medium in the medium inlet, where reagents (23, 35, 103, 104, 217) are dosed to the first reaction unit on the basis of the ammonium content of the polluted medium.

12. An apparatus for purifying a polluted medium according to one of claims 7 - 11, characterised in that the apparatus includes means (56, 124) for measuring the phos- phate content of the medium in the medium inlet, where reagents (24, 35, 104, 217) are dosed on the basis of the phosphate content of the supplied medium.

13. An apparatus for purifying a polluted medium according to one of claims 7 - 12, characterised in that the apparatus includes means (39, 126) for measuring the pH value of the medium in the first reaction tank, where the first reaction tank (6, 42, 102,

105) includes means for pH regulation.

14. An apparatus for purifying a polluted medium according to one of claims 7 - 13, characterised in that the apparatus includes means (27, 33) for adding at least one polymer to the first reaction tank (22, 102) in dependence of the measured value for ammonium in the medium supply.

15. An apparatus for purifying a polluted medium according to one of claims 7 - 9, characterised in that the apparatus includes means (45, 220) for measuring and controlling the pH value in the thermal reactor (42, 209).