Method and system for phosphate recovery from a stream

Abstract

The invention relates to a method and system for phosphate recovery from a stream such as waste flow, sewage or another sludge stream. The method comprises the steps of: providing an incoming stream comprising an initial amount of phosphate; dosing/controlling iron salt to the stream such that precipitates are formed in the stream, wherein the precipitates comprise vivianite like structures comprising more than 60% of the initial amount of phosphate in the incoming stream, and preferably also the steps of: separating the vivianite like structures from the stream; and recovering the phosphates from the separated vivianite like structures.

Claims

1. A method for phosphate recovery from a stream such as waste flow, sewage or another sludge stream, the method comprising the steps of: providing an incoming stream comprising an initial amount of phosphate; dosing and controlling iron salt to the stream such that precipitates are formed in the stream, wherein the precipitates comprise vivianite like structures comprising more than 60% by weight of the initial amount of phosphate in the incoming stream; and separating the vivianite like structures from the stream, wherein separating the vivianite like structures from the stream comprises magnetic separation of the structures with a magnetic and/or electromagnetic separator.

2. The method according to claim 1, wherein the vivianite like structures comprise more than 70% by weight of the initial amount of phosphate in the incoming stream.

3. The method according to claim 1, wherein the iron salts comprise one or more of iron chloride and iron sulphate.

4. The method according to claim 1, wherein dosing iron salt comprises adding an amount of iron with a molar ratio iron:phosphorus of at least 1.25.

5. The method according to claim 1, further comprising the step of controlling the dosing of iron salt in response to a measurement of the initial amount of phosphate in the incoming stream.

6. The method according to claim 1, wherein separating the vivianite like structures from the stream comprises separating the structures with a gravity separator.

7. The method according to claim 1, wherein the step of recovering the phosphates comprises treating the vivianite like structures to produce iron oxide precipitates.

8. The method according to claim 7, wherein treating the vivianite like structures comprises performing an alkaline treatment to produce a potassium phosphate solution.

9. The method according to claim 7, further comprising the step of treating the iron oxide with hydrochloric acid to produce iron chloride.

10. The method according to claim 9, further comprising the step of recycling the iron chloride in the step of dosing iron salt.

11. The method according to claim 1, wherein the stream is a flow to an anaerobic treatment system.

12. The method according to claim 1, wherein the stream comprises of waste flow and/or sewage sludge and/or industrial sludge and/or any other type of sludge.

13. The method according to claim 1, wherein the pH of the stream is in the range of 6-10.

14. The method according to claim 1, wherein the stream is a flow to a digester.

15. A method for phosphate recovery from a stream such as waste flow, sewage or another sludge stream, the method comprising the steps of: providing an incoming stream comprising an initial amount of phosphate; dosing and controlling iron salt to the stream such that precipitates are formed in the stream, wherein the precipitates comprise vivianite like structures comprising more than 60% by weight of the initial amount of phosphate in the incoming stream; separating the vivianite like structures from the stream, wherein separating the vivianite like structures from the stream comprises magnetic separation of the structures with a magnetic and/or electromagnetic separator.

16. The method according to claim 15, wherein the step of recovering the phosphates comprises treating the vivianite like structures to produce iron oxide precipitates, and wherein treating the vivianite like structures comprises performing an alkaline treatment to produce a potassium phosphate solution, and wherein the pH of the stream is in the range of 6-10.

17. The method according to claim 13, wherein the pH of the stream is in the range of 7-10.

18. The method according to claim 17, wherein the pH of the stream is in the range of 7-9.

19. The method according to claim 18, wherein the pH of the stream is in the range of 7-8.

Description

(1) Further advantages, features and details of the invention are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

(2) FIG. 1 shows the method according to the present invention:

(3) FIG. 2 shows an embodiment of the system capable of performing the method of FIG. 1;

(4) FIG. 3 shows a magnetic separator that can be used in the system of FIG. 2; and

(5) FIG. 4 shows vivianite with a SEM-EDX.

(6) Process 2 (FIG. 1) starts with supply of stream 4 that comprises an amount of phosphate. In the illustrated embodiment measurement step 6 measures the amount of phosphate and/or sulphide. Calculation step 8 determines the optimal amount of iron or iron salt to be dosed into the stream. In dosing step 10 iron is added to the stream to enable forming 12 of precipitates comprising vivianite like structures. In separation step 14 the vivianite like structure is separated and removed from the stream. The vivianite like structures undergo a post treatment to recover the phosphorus components, such as an alkaline treatment 16. For example, this treatment may provide a potassium phosphate solution that can be used 18 as fertilizer. The iron oxide precipitates can be treated 20 with hydrochloric acid resulting in recycle iron stream 22 that can be used in dosing step 10. Recycle stream 22 may even obviate the need for external iron or at least significantly reduce this need.

(7) Recovery system 24 (FIG. 2) comprises reactor 26 that receives incoming stream 4. In the illustrated embodiment in reactor 26 anaerobic/anoxic conditions are maintained. From reactor 26 flow 28 is directed towards separator 30. Sludge/waste 32 leaves system 24.

(8) In the illustrated embodiment the resulting vivianite like structures 34 are provided to alkaline reactor 36 to enable posttreatment, or vivianite like structures 34 are directly applied. The recovered phosphate, for example in the form of potassium phosphate, leaves reactor 36 in flow 38 and can be used as fertilizer, for example. The iron oxide leaves reactor 36 as flow 40 and can be treated in acid reactor 42, or can be used as such. This treatment enables a recycling flow 22 of iron salt that provides iron to dosing device 44 enabling dosing stream 46 to reactor 26. Therefore, dosing device 44 receives iron or iron salts from recycle flow 22 and from external incoming flow 48.

(9) In one of the preferred embodiments the desired ratio is determined, the actual concentrations are measured and the desired dosage is calculated in order to dose the required and optimal amount of the iron salt to maintain and/or achieve the preferred ratio.

(10) In the illustrated embodiment sensor 50 measures the composition of the stream in reactor 26, for example the amount of phosphate. Measurement signal 52 is provided to controller 54 that determines control setting(s) 56 of dosing device 44. This may involve periodic sampling of the sludge, for example weekly, and analysing the sample. It will be understood that also other components can be measured with one or more of sensors 50, such as the amount of sulphide that is preferably measured in a digester. Preferably, controller 54 also provides control settings 58 to acid dosing device 60 that provides acid, such as hydrochloric acid, to acid reactor 42.

(11) In a sewage treatment plant reactor 26 comprises a receiving reactor (a sewage treatment plant, a waste water treatment plant) that receives the phosphate rich influent. Preferably, phosphate poor effluent leaves the system through an exit (not shown). The phosphate rich effluent 28 is provided to an anaerobic digester that in the illustrated embodiment is part of reactor 26. Iron is optionally added to the receiving reactor and/or the anaerobic digester.

(12) It will be understood that other configurations implementing the present invention could also be envisaged in accordance to the invention. For example, as mentioned, iron may be added before the anaerobic stage to reach the preferred Fe:P range to further reduce the downflow Phosphorus level(s).

(13) Separator 30 (FIG. 3) comprises frame or housing 62 that is preferable made of steel or another magnetisable material for guiding the magnetic flux, first magnet 64 and, advantageously, second magnet 66. Magnets 64, 66, are provided at a distance wherein assembly 68 is provided. Assembly 68 comprises first (magnetisable) plate 70 and second plate 72 that in the illustrated embodiment are provided with profile 74. In the illustrated embodiment the serrated profile 74 has a height H of about 1-2 mm and a width W of about 3-4 mm. Plates 70, 72 are provided at distance D in the range of 0.1-1 mm, preferably in the range of 0.2-0.4 mm. It will be understood that another configuration for separator 30 and/or other separator techniques can be applied in accordance with the invention.

(14) The method according to the invention is applied to difference incoming streams with different characteristics/composition. In the experiments the sludge remains under anaerobic or anoxic conditions for several days, for example for about 20-30 days. The amount of vivianite like structures has been determined by MOssbauer Spectroscopy and semi quantitative XRD measurements. The table illustrates that the vivianite bound phosphorus increases with the molar Fe:P in the sludge. For example, a molar ratio of about 1.11 results in more than 60% of the incoming phosphorus being bound in vivianite, while higher percentages of above 80% are measured at higher ratio's. This indicates an effective and efficient removal/recovery of phosphorus from a stream. Results are shown in table 1. Table 1 shows that the fraction of the phosphate present as vivianite, after digestion, depends on the molar ratio between iron and phosphate in the waste activated sludge, and thus on the amount of iron that was added in the activated sludge plant prior to anaerobic digestion. The digested sludge was then fed into a magnetic separator to separate the magnetic vivianite-like structures from the rest of the digested sludge (largely organics). Table 3 and 4 show the fraction of phosphate recovered by the magnetic separation versus the total amount of phosphate in the digested sewage sludge.

(15) TABLE-US-00001 TABLE 1 Percentage of phosphate present as vivianite or vivianite-like structures in digested waste activated sludge with different Fe:P ratio Molar ratio XRD- Mössbauer- Sample reference Fe:P measurement (%) measurement (%) Sludge sample 1 0.14 0 Sludge sample 2 0.50 15.3 13 Sludge sample 3 0.82 49.7 30 Sludge sample 4 1.11 63.7 Sludge sample 5 1.62 82.1 Sludge sample 6 1.57 83.6 61 Sludge sample 7 2.36 102.3 89

(16) The table indicates that according to Mossbauer (at 300 K), for the highest Fe:P ratio of 2.36, a significant amount of P is bound in vivianite.

(17) In a further experimental setup, the vivianite like structures are separated by a separator (FIG. 3). This separator has six channels with a radius of about 1.3 mm and a length of 40 mm with a cavity volume of about 0.41 cm.sup.3. The stream has a viscosity of about 0.003 Pa.Math.s, an estimated vivianite susceptibility of 1.0.Math.10.sup.−7 m.sup.3/kg, and a vivianite density of about 2300 kg/m.sup.3. During the separation the magnetic field intensity in the cavity was about 1.Math.10.sup.+06 A/m with a field gradient of about 500 T/m. Particles are subjected to two competing forces, a friction force as a result of the magnetic force that pushes/attracts the vivianite like structure towards the wall of the channel, and a drag force produced by the stream. Non-magnetic particles will only feel the drag force and are flushed out of the cavity. On the other hand, vivianite-like particles, which are magnetic, will resist the drag and stick to the walls. The vivianite-like particles that are thus collected are flushed from the cavity at a later stage. A number of measurements and calculations were performed for different settings. Results are shown in table 2.

(18) TABLE-US-00002 TABLE 2 Effect of flow rate and cavity size on drag and magnetic stick forces for digested waste activated sludge passing a magnetic separator (e.g. FIG. 3) Drag + gravity force on particles near wall Flowspeed in channel Reynolds At 10 At 20 Liquidhead Pressure Central number Total flow micron micron [mm] [Pa] [mm/s] Average [—] [cm.sup.3/s] [N] [N] 0.5 5 18 9 3.81 0.09 4.50E−11 2.07E−10 1 10 35 18 7.63 0.18 8.33E−11 3.60E−10 1.5 15 53 26 11.44 0.27 1.22E−10 5.13E−10 2 20 70 35 15.26 0.36 1.60E−10 6.66E−10 2.5 25 88 44 19.07 0.45 1.98E−10 8.19E−10 3 30 106 53 22.89 0.54 2.36E−10 9.72E−10 Drag + gravity force on particles near wall Magnetic stick force on wall At 30 At 10 At 20 At 30 Ratio of drag and stick micron micron micron micron At 10 At 20 At 30 [N] [N] [N] [N] micron micron micron 5.25E−10 3.01E−11 2.41E−10 8.13E−10 1.49 0.86 0.65 8.69E−10 3.01E−11 2.41E−10 8.13E−10 2.77 1.49 1.07 1.21E−09 3.01E−11 2.41E−10 8.13E−10 4.04 2.13 1.49 1.56E−09 3.01E−11 2.41E−10 8.13E−10 5.31 2.77 1.92 1.90E−09 3.01E−11 2.41E−10 8.13E−10 6.58 3.40 2.34 2.25E−09 3.01E−11 2.41E−10 8.13E−10 7.85 4.04 2.77

(19) Experiments with the separator show that it is possible to separate vivianite and vivianite like structures from a flow with a magnetic separator. This separation is effectively possible due to the vivianite like structures being present relatively pure and as “free” particles. Non-optimised experiments already indicate that more than 60% (expressed as % of P bound in vivanite) of the vivianite and vivianite like structures can be separated. It will be understood that this separation would also be possible with an electromagnetic separator or gravity separator. The separation enables an effective removal and enabling an effective recovery of phosphate/phosphorus from a stream.

(20) A further experiment using wet magnetic separation of vivianite was performed using the following protocol: the sludge is sieved (1 mm), providing a flowrate of 4-20 mL/min for 30 seconds, rinsing with water at 4-20 mL/min for 30 seconds, flushing the material with distilled water followed by vacuum drying. Different sludge types were used, for example Dutch, German and Finnish sludge. Optionally the material could be recirculated, for example to increase the purity. Results are shown in table 3 and 4.

(21) TABLE-US-00003 TABLE 3 Percentage of phosphate present as viviante or vivianite- like structures and percentage of dry weight made up of Volatile Solids (VS), for 3 different digested waste activated sludge's that differ in molar Fe:P ratio NL Ger Fin Initial VS (%) 58.9 56.0 58.7 Vivianite P ~65 ~80 ~90 (% of total P) Molar Fe:P 1.1 1.57 2.36

(22) TABLE-US-00004 TABLE 4 Percentage of the iron and phosphate in digested waste activated sludge that is recovered from the bulk sludge in a magnetic separator (e.g. FIG. 3), operated at different flow rates; the enrichment factor for iron and phosphate by magnetic separation; and the Volatile Solids percentage of the dry weight for the magnetically separated fraction Volatile solids Recovery as percentage of efficiency Enrichment dry weight for Flow Rate (%) factor magnetically (mL/min) Fe P Fe P separated fraction Dutch 4 44.1 36.2 1.8 1.4 50.4 Sludge 8 49.2 40.4 3.4 2.8 31.1 16 33.7 27.7 3.3 2.7 36.0 20 17.0 13.3 2.5 2.0 42.3 German 4 75.3 67.8 1.3 1.2 63.1 Sludge 8 75.6 71.9 1.7 1.6 52.2 16 47.7 43.7 2.2 2.0 46.2 20 35.6 32.6 2.0 1.8 43.8 Finnish 4 98.3 112.5 1.3 1.4 59.8 Sludge 8 78.3 94.2 1.3 1.6 53.7 16 54.2 61.8 1.9 2.1 44.3 20 48.1 52.1 1.6 1.7 45.3

(23) Results show that the concentration of the various materials which can be recovered does not limit the output. Low as well as high concentrations of iron and phosphor can be used.

(24) Other metals can also be recovered. When flow rates are increased 52% to 62.2% vivianite is recovered. Results are shown in tables 5 and 6. Recovery of Fe and P is shown in Table 5.

(25) TABLE-US-00005 TABLE 5 Concentrations of different elements in the magnetically separated fraction recovered from two different types of digested waste activated sludge and pure vivianite Separated fraction Ca (g/kg TS) Fe (g/kg TS) K (g/kg TS) Mg (g/kg TS) Al (g/kg TS) P (g/kg TS) S (g/kg TS) FIN-A 26.7 120.3 10.8 4.0 6.1 28.4 8.6 FIN-B 17.1 236.2 2.0 2.8 4.6 64.6 6.0 NL-A 36.2 64.0 10.3 3.4 6.0 37.4 21.0 NL-B 20.0 195.2 1.8 7.5 4.6 77.2 9.6 Pure vivianite 9.5 308.0 0.0 10.8 0.0 119.0 0.8

(26) TABLE-US-00006 TABLE 6 Composition of the magnetically separated fraction for two different digested waste activated sludges Vivianite (as % FeCO.sub.3 (as % Organic (as % Quartz (as % Total (as % Unknown(as % Sludge of total TS) of total TS) of total TS) of total TS) of total TS) of total TS) Finland 52 3.33 21.2 7.81 84.34 15.66 Netherlands 62.6 0.26 21.9 7.32 92.08 7.92

(27) A further experiment using different types of sludge, Dutch sludge sampled in Dokhaven and Finish sludge sampled in Espoo, were used to separate various components present in sludge. Different flow rates have been applied to both types of sludge. The iron (Fe) recovery for Dutch sludge was at 4 mL/min 38%, at 8 mL/min 44%, at 16 mL/min 45% and at 20 mL/min 39%. The phosphor (P) recovery for Dutch sludge was at 4 mL/min 32%, at 8 mL/min 38%, at 16 mL/min 36% and at 20 mL/min 31%. The iron (Fe) recovery for Finnish sludge was at 4 mL/min 53%, at 8 mL/min 49%, at 16 mL/min 31% and at 20 mL/min 31%. The phosphor (P) recovery for Finnish sludge was at 4 mL/min 53%, at 8 mL/min 51%, at 16 mL/min 39% and at 20 mL/min 37%.

(28) The enrichment increases with the flow rate for both type of sludge and elements. The separation becomes more selective with the increase of the flow rate. Higher streams reduce the part of non and/or less magnetic material susceptible to be retained.

(29) An even further experiment using different types of sludge showed that during the magnetic separation the organic content decreases of around half which is corroborated by the reduction of the amorphous bump. Thus, vivianite can be recovered from the sludge by magnetic separation.

(30) Furthermore, the experiments show the effect of the molar Fe:P ratio on the efficiency of the removal/recovery of phosphate from the stream. Furthermore, the experiments show that an effective recovery is possible by effective separation of the vivianite like structures from the stream.

(31) Further separation experiments using a bench-scale vertically pulsating high gradient magnetic separator (VPHGMS) were performed. This separator utilizes a matrix of steel rods with 1 mm diameter placed in a magnetic field of 1 Tesla created by electromagnetic coils. A steady water flow of 4 L/min is created over the matrix and a vertical pulsation is created in the water flow with a frequency of 20 Hz. A 500 gram sludge sample is fed batch wise to the water flow and is consequently carried over the matrix together with the water. Magnetic particles are retained on the matrix while the non-magnetic particles are flushed out of the machine with the water flow. Once the water flow coming out of the machine is clear by sight and no non-magnetic solids are coming out, the flow is stopped, the magnetic field is switched off and the magnetic fraction is then flushed from the matrix and captured. This magnetic fraction is called the concentrate.

(32) The result of the separation experiment is shown in Table 7.

(33) TABLE-US-00007 TABLE 7 Result of VPHGMS separation test. Dry solids Concentrate dry Recovery Feed content Concentrate content Feed sample content mass yield Fe P Fe P Fe P Finnish sludge 2.66% 16.0% 49.0% 56.6% 12.0% 3.0% 36.8% 10.7%

(34) The results show that 57% of the phosphorus is recovered from the feed sludge. The elemental phosphorus content of the concentrate is 10.7%

(35) The concentrate was studied with a scanning electron microscope (SEM) combined with energy-dispersive x-ray spectroscopy (EDX). The SEM-EDX results indicate that the concentrate is homogeneous in composition and mostly comprise vivianite (FIG. 7, wherein needle-like crystal structures which are typical for vivianite were observed).

(36) Pure vivianite has an elemental phosphorus content of 12.35%. If we assume that all the phosphorus in the concentrate is bound to vivianite as indicated by SEM-EDX, the vivianite content of the concentrate is 86.6%

(37) The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are described by the following claims, within the scope of which many modifications can be envisaged.