METHOD AND ASSEMBLY FOR RECOVERING MAGNESIUM AMMONIUM PHOSPHATE

Abstract

The invention relates to a method and an assembly for recovering magnesium ammonium phosphate from slurry that is supplied to a reaction container (10) in which an aerobic milieu is present and in which the slurry is guided in a circuit with the aid of ventilation. Cationic magnesium, such as magnesium chloride, is added to the slurry, and magnesium ammonium phosphate crystals which are precipitated from the slurry are removed via a removal device (30) provided in the base region of the reaction container. Substances which contain magnesium ammonium phosphate crystals collected in the removal device (30) are loosened and/or rinsed.

Claims

1-14. (canceled)

15. Method for recovering magnesium ammonium phosphate from sludge supplied to a reaction tank (10) in which an aerobic environment prevails and in which the sludge is guided in a cycle supported by aeration, with cationic magnesium such as magnesium chloride being added to the sludge and magnesium ammonium phosphate crystals precipitated from the sludge being removed via an extraction device (30) provided in the floor area of the reaction tank, wherein substances collected in the extraction device (30) and containing magnesium ammonium phosphate crystals are loosened up and/or flushed, and wherein gas and liquid are introduced into the extraction device (30) alternatingly, and substances exiting the extraction device (30) on the tank side by means of a guide (31) are passed through a funnel-shaped or conically designed lower section (28) of the reaction tank (10) and subjected in an adjoining cylindrical upper section (26) to the flow there.

16. Method according to claim 15, wherein the gas, such as air, and the liquid, such as water, in particular service water, is introduced in the floor area of the interior of the extraction device (30), which preferably has a hollow-cylindrical internal geometry.

17. Arrangement for recovering magnesium ammonium phosphate from sludge, comprising a reaction tank (10) with a cylindrical upper section (26) that merges into a funnel-shaped or conically designed lower section (28) from which in turn extends an extraction device (30) for magnesium ammonium phosphate crystals, at least one aerator device (48, 50) being provided in the upper section (26), and wherein the extraction device (30) has at least one first connection (96) for gas to be introduced and at least one second connection (94) for liquid to be introduced, and a guide (31) aligned with the extraction device (30) and having a hollow-cylindrical geometry with widening (33) on the extraction device side for passing substances washed upward in the extraction device (30) into the cylindrical upper section (26) is arranged in the lower section (28).

18. Arrangement according to claim 17, wherein in the floor area of the extraction device (30) in particular, several first connections (96) and/or several second connections (94) are provided preferably evenly distributed around the circumference of the extraction device.

19. Arrangement according to claim 17, wherein first and second connections (96, 94) are connected alternatingly to the extraction device (30).

20. Arrangement according to claim 17, wherein the interior of the extraction device (30) collecting magnesium ammonium phosphate crystals has a cylindrical or conical geometry.

21. Arrangement according to claim 17, wherein the extraction device (30) is designed closure-free on the tank side.

22. Arrangement according to claim 17, wherein the extraction device (30) has on the floor side a closure device, such as a shutoff valve or rotary valve (32).

23. Arrangement according to claim 17, wherein the longitudinal axis of the guide (31) coincides with the longitudinal axis of the extraction device (30) and preferably with the longitudinal axis of the reaction tank (10).

Description

[0061] The drawing shows in:

[0062] FIG. 1 an illustration of the principle of an arrangement for recovering magnesium ammonium phosphate,

[0063] FIG. 2 an illustration of the principle of a first reaction tank,

[0064] FIG. 3 an enlarged illustration of an extraction device shown in FIGS. 1 and 2 and

[0065] FIG. 4 a cross-section of the extraction device according to FIG. 3.

[0066] On the basis of the figures, a two-stage method is described by means of which magnesium ammonium phosphate crystals are precipitated from digested sludge, without this being intended to restrict the teachings in accordance with the invention. Instead the teachings also relate to single-stage methods for precipitating magnesium ammonium phosphate crystals.

[0067] The two-stage method is explained purely in principle on the basis of FIG. 1.

[0068] Substantial components of the arrangement for recovering phosphorus, i.e. for precipitating magnesium ammonium phosphate crystals, are a first reaction tank 10 in which an aerobic environment prevails, and a second reaction tank 12 with an anaerobic environment. The first reaction tank 10 is connected via a first line 14 to the second reaction tank 12, which in turn is connected via a second line 16 to the first reaction tank 10 for recirculation of magnesium ammonium phosphate crystals or of sludge containing crystal nuclei. This second line 16 preferably leads into a line 18 via which sludge is supplied from a sludge digestion plant to the first reaction tank 10.

[0069] If required, the digested sludge can be slightly acidified by acid, for example H.sub.2SO.sub.4, before being supplied to the first reaction tank 10, in order to increase the orthophosphate concentration for intensification of subsequent magnesium ammonium phosphate (MAP) crystallization in the inflow to the first reaction tank 10.

[0070] According to the illustration of the principle in FIG. 1, the sludge can optionally flow through a tank 20 to which is supplied an acid, for example H.sub.2SO.sub.4, via a line 22. If such an acidification is not required, a bypass line 24 is provided in order to permit supply of the sludge directly to the first reaction tank 10.

[0071] The first reaction tank 10 consists, according to the illustration of the principle in FIG. 2, of an upper cylindrical section 26 and a lower conical or funnel-shaped designed lower section 28. The funnel-shaped lower section 28 merges into an extraction device 30 referred to as a separator in which MAP crystals are collected, in order to supply them to a container 36 after opening of, for example, a rotary valve 32 or an otherwise secured outlet device via a dewatering screw 34. The dewatering water produced during conveying via the screw 34 is extracted via a line 38.

[0072] Substantial features can be found in self-explanatory form in FIGS. 3 and 4, in which the extraction device 30—referred to in the following as separator—is shown in more detail. The separator 30 has a hollow-cylindrical geometry and is connectable via a flange 35 to the outlet of the conical lower section 28 of the reaction tank 10. On the floor side, i.e. away from the tank, the separator 30 also has a flange 37, so that the separator 30 can be connected for example to the rotary valve 32 or to an otherwise secured outlet device.

[0073] In the floor area of the separator 30, connections 94, 96 are provided for loosening up and flushing MAP crystals of differing sizes and sludge or sludge flocs—referred to overall as substances—collected in the interior 39 of the separator 30. The connections 96 are provided for air and the connections 94 for water. The connections 94, 96 are alternatingly arranged, meaning that an air connection 96 is followed by a water connection 94 and vice versa.

[0074] Loosening up of the collected MAP crystals and flushing are achieved by preferably pulsating and/or alternating and/or simultaneous introduction of air and water. At the same time, the MAP crystals are graded, such that large, i.e. heavy MAP crystals remain in the lower area of the separator 30 while smaller and lighter MAP crystals, in particular microcrystals and sludge particles and flocs, rise and are washed back into the reaction tank 10. This ensures that small MAP crystals, which are hard to separate by the usual methods, are returned to the reaction tank 10 and hence back to the process. This leads to further growth of these small MAP crystals.

[0075] The separator 30 is closable on the floor side, while a closure does not have to be provided on the tank side.

[0076] To ensure that the washed-upward MAP crystals actually reach the upper cylindrical section 26 in order to grow, it is provided in accordance with the invention that above the separator 30 a guide 31, also having in particular a hollow-cylinder geometry, is arranged in the funnel-shaped lower section 28, the longitudinal axis 41 of the guide coinciding with the longitudinal axis 43 of the separator 30 and in particular also with the longitudinal axis 45 of the reaction tank 10.

[0077] The guide 31 has on the separator side a funnel-shaped widening 33, thereby ensuring that the substances washed upward and exiting from the separator 30 pass through the guide 31 to the upper section 26 of the reaction tank 10. It is ensured by these measures that the flushed-out microcrystals and sludge flocs are returned to the process, since the funnel-shaped lower section 28 of the reaction tank 10 can be flowed through vertically upwards. If a such a guide were not provided, the upward-directed flow would be slowed down by the flushing air and the flushing water by the widening of the flow profile in the funnel-shaped lower section 28, and the uplift force would be lost, with the result that the flushed-out substances would not reach the flow cycle, described in the following, in the cylindrical upper section 26 of the reaction tank 10. The flushed-out substances are picked up by the guide 31, also referred to as guide tube, and by the funnel-shaped widening 33 and guided selectively into the cylindrical upper section 26 of the reaction tank 10 or into the influence area of its upward-directed flow.

[0078] In other words, the guide 31 is used to guide flushed-out substances from the separator 30 directly into the interior 46 of the upper section 26 that is enclosed by the cylindrical partition wall 40.

[0079] A partition wall 40 forming a ring in section is installed in the upper section of the first reaction tank and is at a distance to the outer wall 42 of the upper section 26, such that between the partition wall 40 forming a hollow cylinder and the outer wall 42 of the first reaction tank 10 an outer area 44 ring-shaped in section is provided which equates to a cylinder ring section. The upper rim of the partition wall 40 is at a distance from the sludge level 47.

[0080] On the floor side, the partition wall 40 ends just above the area in which the upper section 26 merges into the lower section 28, as illustrated in the drawing.

[0081] Inside the interior 46 enclosed by the partition wall 40 are aerator devices 48, 50, in particular in the form of membrane aerators, for introducing air into the interior 46, which is filled with sludge/water mix.

[0082] The introduction of air must serves three purposes. The introduction of air achieves a directed flow profile of the sludge flowing inside the reaction tank 10 while mixing it. There is also a grading of the MAP crystals, as is explained in the following. Finally, there is a gas exchange in the sludge, which is basically a sludge/water mix, with CO.sub.2 stripping and redissolution of oxygen taking place.

[0083] Mixing in the first reaction tank 10 and creation of the directed flow in the upper part 26 of the reaction tank 10 are generated by the resultant density difference between the unaerated medium inside the outer area 44 and the aerated medium in the interior 46, and by the uplift force of the air bubbles exiting the aerator devices 48, 50. Due to the difference between the “heavy” medium in the ring-shaped outer area 44 and the “lighter” medium in the interior 46, the sludge or sludge/water mix is drawn out of the annular area 44 to the middle of the tank, and accordingly flows around the lower rim of the partition wall 40.

[0084] Inside the interior 46, the sludge is permeated with air, in order to then be driven in the uplift direction in the interior 46 in a vertical flow to the sludge surface 47. At the sludge surface 47, the sludge/water mix degasses and then flows horizontally above the upper rim of the partition wall 40 and outwards to the annular area 44. In the outer and unaerated annular area 44, there is then a vertical downward movement in the direction of the lower section 28.

[0085] The driving force in the cycle described is the energy input by adiabatic compression of air in a compressor and by subsequent polytropic expansion following introduction to the sludge/water mix. The air is supplied to the membrane aerators 48, 50 by means of fans 52, 54 via lines 56, 58.

[0086] To enable MAP crystals to precipitate, the ammonium present in the sludge and cationic magnesium is required, which in the example is supplied in the form of magnesium chloride, onto the sludge surface 47, preferably via the annular area 44. In the illustration of the principle in FIG. 1, the magnesium chloride 5 is added via a line 60.

[0087] The energy input also sets the uplift force in the interior 46 of the upper section 26 of the first reaction tank 10. This grades the precipitating MAP crystal size. The bigger the crystal structure, i.e. the greater the weight of the MAP crystals, the greater the gravity-related sedimentation rate. Above a defined size and hence weight of the crystals, the lifting force in the interior 46 is insufficient to carry the crystals into the vertical upward flow, with the result that the crystals fall in the direction of the lower section 28 and sediment there, collecting in the separator 30. Smaller crystals are by contrast drawn along with the flow and carried in the process cycle until a size is reached that allows settling inside the conical or funnel-shaped lower section 28 and hence inside the separator 30.

[0088] The air introduced via the membrane aerators 48, 50 leads to stripping of CO.sub.2 from the sludge/water mix. This increases the pH value, in particular to a value between 7.5 and 8.2. The pH value increase initiates, as magnesium is simultaneously provided by metering of magnesium chloride or another suitable magnesium compound, MAP crystal formation or crystal growth.

[0089] The digested sludge itself, which is supplied via the line 18 to the first reaction tank 10, is supplied, as shown in FIG. 2, at the sludge level 47 of the first reaction tank 10.

[0090] There is also the possibility to introduce a defoamer for reducing foaming on the sludge surface 47, via a line 66 or directly into the line 18.

[0091] In the outer area between the partition wall 40 and the outer wall 42, i.e. in the annular area 44, is a discharge shaft 98 which leads into a pipe 70, starting from which the sludge is supplied via the first line 14 to the second reaction tank 12.

[0092] The discharge from the first reaction tank 10 is in accordance with the positive displacement principle. When the first reaction tank 10 is supplied with sludge, sludge in the same volume proportion is simultaneously washed out of the first reaction tank 10.

[0093] The displacement is out of the lower area of the upper section 26 from the annular area 44 into the discharge shaft 98. Discharged sludge/water mix flows inside the discharge shaft 98 upwards—in the drawing illustration matching the direction of the arrow 74—in order to then reach the discharge area over a discharge sill 76, as indicated by the arrow 77.

[0094] The sludge or the sludge/water mix reaching the second reaction tank 12 via the first line 14 is subjected to an anaerobic environment. To ensure this, there is only gentle mixing (agitator 78) without any aeration. If bacteria contained in the digested sludge have absorbed more phosphate in the first reaction tank 10 under aerobic conditions parallel to orthophosphate crystallization, phosphorus redissolution leading to further MAP crystal formation or to crystal growth takes place under anaerobic conditions in the second reaction tank 12.

[0095] A predefined sludge/water mix quantity is then extracted continuously or intermittently, i.e. in batches, from the lower section 80, also having a conical or funnel shape, of the second reaction tank 12, whose upper area should have a cylindrical shape, and is recirculated via the second line 16 into the first reaction tank 10, as already explained above. To do so, there is a pump 84 in the second line 16.

[0096] Discharge from the second reaction tank 12 is achieved using an extraction pump 100. Inflowing sludge/water mix which is not recirculated into the first reaction tank 10 is extracted and discharged via this extraction pump 100. It is possible here to subject the sludge either directly to dewatering via a line 88 or optionally to pass it through a separator 90, such as a hydrocyclone, in order to separate MAP crystals or crystal nuclei still present in the sludge, which are then supplied via a third line 92 to the first reaction tank 10 and/or passed via a fourth line 102 back into the second reaction tank 12. These are substantially microcrystals.

[0097] The MAP crystals separated in the first reaction tank 10 pass into the separator 30, which extends from the lowest point of the lower section 28 of the reaction tank 10.

[0098] Regarding the separator 30, it should be noted that it can be designed closure-free on the tank side for its separation function. However, a closure can be provided that separates the separator from the tank, for example to perform maintenance work, for example on the connections 94, 96.

[0099] The separator 30 can consist for example of special steel and if necessary have an anti-stick coating, in particular on the inside, or can also be designed in steel with anti-stick coating on the inside. Typical diameters of a corresponding separator 30 are between 300 mm and 600 mm with a structural length of between 400 mm and 1500 mm.

[0100] The guide 31 can also consist of special steel or steel and if necessary be provided with an anti-stick coating. Typical diameters should be 300 mm to 600 mm. The maximum length corresponds to the height of the funnel-shaped or conical lower section 28 of the first reaction tank 10. Dimensioning/arrangement must be such that the MAP crystals can flow to the separator 30 without any disruption in the flow.

[0101] The volume of the first reaction tank 10 should correspond to 2 to 20 times the hourly volumetric inflow quantity to the first reaction tank 10. Identical dimensions for the second reaction tank 12 are preferable.

[0102] Regarding the air introduced via the membrane aerators 48, 50, it should be noted that the quantity should be 5 to 35 times the hourly volumetric inflow quantity into the first reaction tank 10.

[0103] The metering of magnesium chloride depends on the concentration of PO4, NH4 and Mg ions in the inflowing sludge.

[0104] An anaerobic environment should prevail in the second reaction tank 12 in accordance with the invention. For that reason only gentle mixing takes place. The energy input by the agitator 78 should be 2-20 Watts per m.sup.3 of the sludge/water mix.

[0105] If the optionally provided pre-acidification is performed, the pH value in the pre-acidification should not drop below 5.0.