WO2014003554A1

WO2014003554A1 - Reactor and method for the anaerobic treatment of sludge containing phosphorous and nitrogen

Info

Publication number: WO2014003554A1
Application number: PCT/NL2013/050456
Authority: WO
Inventors: Sjoerd Hubertus Jozef Vellinga
Original assignee: Paques I.P. B.V.
Priority date: 2012-06-26
Filing date: 2013-06-26
Publication date: 2014-01-03
Also published as: NL2009064C2

Abstract

Thin sludge can be anaerobically digested with improved phosphate removal in the form of struvite by using a downflow anaerobic digestion reactor (1) which is operated with crystal seeding at the top and enhanced crystal removal at the bottom. The reactor has liquid inlet means (2) at the top, and an exit means (5) at the bottom, the bottom part of the reactor (1) being provided with a rotating scraper (70) capable of collecting solid material from the bottom of the reactor (1) and moving said collected material to said exit means. Magnesium (3) is added as necessary for optimum struvite formation.

Description

REACTOR AND METHOD FOR THE ANAEROBIC TREATMENT OF SLUDGE CONTAINING PHOSPHOROUS

AND NITROGEN

[0001] The present invention in is the field of waste treatment and is particularly

concerned with the treatment of thin sludge containing phosphorus (e.g. as phosphate)

5 and nitrogen (e.g. as ammonia), and with a reactor for carrying out this treatment.

Background

[0002] Anaerobic fermentation of waste such as waste water, sludge, manure etc.

results in the production of phosphate originating from spent cellular material

0 (nucleotides, proteins and dissolved phosphate), as well as in the production of

ammonia (proteins, and dissolved ammonia and amine compounds). In addition,

phosphates and nitrates and other nitrogen compounds present in the waste material add to the phosphorus and nitrogen load of the fermentation process, wherein most of the nitrogen is eventually in the form of ammonia as a result of denitrification.

5 [0003] Phosphate and ammonia can form poorly soluble salts, including the very stable

struvite (magnesium ammonium phosphate, MAP,

When produced in waste water and sludge treatment processes, the crystalline struvite can be deposited as salt crusts on reactor walls or remain in the reactor content as very fine crystals. The salt crusts will eventually be released and will obstruct pumps, mixers and other

0 downstream handling, while separation of fine crystals is very difficult and thus the

struvite cannot be effectively collected in a form suitable for further use.

[0004] Processes for removing phosphates and/or ammonia from waste water and

sludge through the formation and subsequent separation of struvite are well-known in the art. For example, WO 2006/028372 discloses a process for the simultaneous

5 removal of BOD and phosphate by aerobic oxidation of the BOD in the presence of

ammonia and magnesium, while suppressing nitrification of the ammonia though

manipulation of the biomass retention time. WO 2008/115758 discloses a process for the removal of phosphorus and ammonia from an aqueous stream involving adding magnesium and forming and separating struvite in a multi-stage precipitation process

0 with increasing pH.

[0005] The prior art does not provide a solution for the problem of struvite formation in anaerobic waste treatment. Description of the invention

[0006] It was found that anaerobic fermentation of liquid or semi-liquid waste material such as thin sludge, containing nitrogen and phosphorus, can be performed without problems due to precipitation of struvite crystals, by introducing crystalline material comprising struvite into the fermentation reactor together with the introduction of the waste material.

[0007] The invention relates to a process for the anaerobic treatment of thin sludge containing phosphorus and nitrogen comprising:

(a) introducing sludge at the top of an anaerobic digestion reactor;

(b) introducing crystalline material comprising struvite at the top of the reactor;

(c) if necessary, adding a magnesium source to the reactor content at a rate so as to produce a molar magnesium level between 50% and 200% of the molar phosphorus level and/or nitrogen level in the reactor;

(d) collecting effluent comprising crystalline material comprising struvite at the bottom of the reactor;

(e) recycling crystalline material collected in step (d) to step (b).

[0008] As used herein, "thin sludge" denotes any liquid or semi-liquid aqueous waste material containing organic material and suspended solids. The solids in particular comprise cell material, such as microbial biomass, plant residues, or residues, e.g. faecal residues, from animal, including human, origin. Examples include residual sludge of activated sludge plants treating municipal, agricultural or industrial waste streams, anaerobic treatment plants, manure, and the like. Mixtures of e.g. municipal sludge and manure can also be effectively treated. The solid content of the thin sludge when introduced into the process of the invention is preferably between 1 and 15 g dry matter per 1. The total phosphorus content of the incoming sludge is preferably at least 50 mg/1, more preferably at least 100 mg/1, up to e.g. 2000 mg, preferably up to 1000 mg/1, especially between 100 and 1000 mg/1, typically between 200 and 500 mg/1 (as phosphorus). The nitrogen content of waste sludges, manure and similar material will typically be in a stoichiometric excess compared to the phosphorus content (more than 45% on an N/P weight basis).

[0009] Prior to being fed to the fermentation reactor as an influent, the thin sludge is mechanically pre-treated so as to remove crude solid material, including fibrous material. If necessary, the sludge can be diluted. This pretreatment prevents formation of floating layers and excessive viscosity in the reactor. [0010] The anaerobic degradation processes of cell material of the anaerobic biomass, and possibly also from biomass present in the incoming sludge, produces phosphate from nucleic acids, proteins and other phosphate sources. At the same time, the degradation processes also result in the release of ammonia, which adds to any ammonia resulting from nitrogen-containing material in the feed sludge, after denitrification. Together with magnesium sources present in the influent or added to the fermentation reactor, this leads to the formation of struvite. The struvite will precipitate as crystals.

[0011] The presence of the added struvite crystals in the fermentation reactor prevents or reduces the formation of deposits on reactor walls, and ensures that any struvite formed during the fermentation will be produced in a form which allows effective separation of struvite crystals, which can be further used, for example as nutrient for agricultural uses. The purity of the separated struvite can be as high as 90% or more. The crystals can be harvested downstream of the fermentation process by using cyclones or other separators commonly used for separating crystalline material from an aqueous stream.

[0012] It is preferred that the crystalline material comprising struvite introduced in step (b) of the process described above contains at least 30%, preferably at least 50%, most preferably at least 70% struvite, and has an average particle size of between 0.2 and 5, more preferably 0.5 and 3 mm. As struvite crystals are usually in the form of needles, particle size typically refers to the length of such needles.

[0013] The addition of the crystals of the proper size allows the crystals to move downward through the reactor content at such a rate that phosphate released during the fermentation process is continuously faced with sufficient struvite crystals, causing the crystals to grow and keeping the phosphate level below the nucleation threshold and thus preventing formation of new crystallisation nuclei. Deposition on reactor walls is strongly reduced.

[0014] The descending speed of the crystals will be in the order of 1-5 cm/sec, which is faster than the descending speed of the treated water. This implies a feed of crystals at the top of between 2 and 10 litre per s (on a 15 vol.% concentration basis), preferably between 4 and 8 1/s.

[0015] The fermenter/crystalliser reactor is preferably a vertical cylindrical reactor. The reactor is preferably of sufficient height to provide a long descending path for the crystalline material and thus to provide a sufficient amount of crystal surface area per unit volume of reactor content. Preferred heights of the reactor are between 4 and 32 m, most preferred between 10 and 25 m. Typical reactor diameters are between 1 and 25 m, more preferably between 1.5 and 15 m, resulting in a reactor cross-section area between about 2 and 180 m², and a reactor capacity of between 20 and 4500 m³.

[0016] The fermenter is preferably operated as a downward plug-flow reactor, without mixers being provided in the reactor. Intimate contact between the reactor content and the input streams (thin sludge and/or crystalline material) is provided by introducing the influent sludge in step (a) of the process in a stream which forms an angle with the surface level of the reactor content. This produces a circular (spiral) downward flow in the anaerobic reactor. The angle is less than 75°, and at least 0°, preferably at least 15° with the surface. More preferably, the angle with the surface is preferably between 30° and 60°, most preferably between 45° and 60°. In order to promote a circulating movement within the reactor, the incoming stream is directed along the walls of the reactor rather than towards the centre. Preferably the influent is introduced at an angle of at least 30°, more preferably between 60 and 90° with a diagonal of the surface of the reactor content. The influent can be introduced above the reactor liquid surface, or, preferably, just below the liquid surface of the reactor content. Preferably, the influent is introduced through multiple nozzles, e.g. 2-20, in particular 3-12 to promote the rotating movement of the reactor content, preferably the entire reactor content. The multiple nozzles can e.g. be arranged in a line along the radius of the reactor.

[0017] The crystalline material, optionally together with further reactor content collected at the bottom of the reactor, is preferably also introduced at an angle with the reactor content surface and with the reactor diagonal. These angles can be the same as those of the sludge influent, or, less preferred, different angles. While not preferred, it is well feasible to combine the incoming thin sludge and the crystalline material before introducing them into the reactor.

[0018] In order to assist the formation of struvite from the phosphate and/or nitrogen released in the fermentation process, sufficient magnesium should be provided. If necessary, magnesium is added separately to the reactor or combined with the influent before its introduction into the reactor. When introduced separately as a liquid, the magnesium stream can be introduced at an angle with the reactor content surface, thus assisting in effecting a rotating movement of the reactor content in cooperation with the influent sludge stream. The angle can be same as the angle at which the sludge stream is introduced.

[0019] The amount of magnesium is such that optimum struvite formation is possible. As the stoichiometric molar ratio of phosphorus (phosphate), nitrogen (ammonia) and magnesium in struvite is 1 : 1 : 1, the amount of magnesium, after addition of magnesium where necessary, is at least 0.5 times (50%), preferably 0.75 times, more preferably at least 1.0, most preferably at least 1.2 times, up to e.g. 2.0 times (200%), the molar level of either phosphate or ammonia, in particular of the component which is at the stoichio- metrically lower level, which is usually phosphorus. Magnesium can be added as a suitable salt, such as magnesium hydroxide, oxide, chloride or sulphate, as such, i.e. as a dry or semi-dry material, or after dissolution or slurrying in water. If desired, ammonia or phosphate can also be added to increase the stoichiometric ratio of the added component compared to the component which is present in a stoichiometric excess. However, addition of phosphorus is generally not necessary or not desired.

[0020] The fermenter (anaerobic reactor) can be operated at conditions which are conventional for anaerobic digestion reactions. The fermentation temperature is advantageously in the mesophilic range, i.e. between 20 and 45°C, preferably between 25 and 40°C, in particular between 30 and 38°C. However, thermophilic operation, at temperatures up to e.g. 60°C, is also feasible. The anaerobic biomass can be derived from common anaerobic digestion plants. The hydraulic residence time for the sludge is preferably at least 12 days, more preferably between 15 and 30 days, especially between 20 and 25 days. The hydraulic residence time for the aqueous phase is preferably at least 8 days, more preferably at least 12 days, especially 15-30 or even 20- 25 days for mesophilic operation. This means that no sludge retention is required. Reduced residence times of at least 4 days, preferably 8-20 days, can apply in case of thermophilic operation.

[0021] The anaerobic treatment of the invention, apart from producing struvite, results in conversion of organic (biological) material to harmless and/or useful small organic molecules, in particular methane and carbon dioxide, and a cleared aqueous effluent. The anaerobic treatment is preferably performed in a single reactor, i.e. the reactor according to the invention. The reactor can advantageously be provided with gas collection and exit means at the top for discharging (bio)gas, which can be used for heating or other purposes, optionally after purification, e.g. desulphurisation. [0022] The crystalline material comprising struvite is collected at the bottom of the reactor. In order to facilitate collection and subsequent transport of the crystalline material, the crystalline material is continuously cleared from the bottom of the reactor by concentrating to one or more exits by scraping or equivalent means such as shoving or brushing. It is preferably drained off together with at least a part of the treated water and further reactor content, i.e. the total reactor effluent.

[0023] The scraper or other device provided for clearing and collecting the crystalline material from the bottom of the reactor is preferably arranged in such a manner that a temporary excess of crystalline material can be accommodated. Such temporary excess can be caused by fluctuations in the fermentation process or malfunction of the circulation mechanism. This can be done by making the scraper blades forming the scraper or collecting slides vertically movable, driven by the height of the solid material at the bottom of the reactor, and thus avoiding overloading of the collector.

[0024] The scrapers are preferably operated in a circular movement, advantageously using the same rotation direction as the rotation produced by the input streams at the top of the reactor. The tip speed of the scraper is preferably between 0.03 m/sec and 0.2 m/sec. The scraper resistance can be e.g. between 100 and 600 N/m.

[0025] The effluent of the reactor collected at the bottom, containing the crystalline material, can be fed to a separator such as a (hydro)cyclone, or returned to the top of the rector, or both, depending on the mode or the stage of the fermenting process. In the separator, the crystalline material is separated from the main part of the aqueous liquid and from suspended solids. It can be separated in the form of a concentrated aqueous slurry, e.g. at the bottom of the hydrocyclone. A part of it can be recycled to the reactor in step (b) of the process described herein. The remaining part or, alternatively, all of the struvite crystalline material separated off can be further processed, concentrated as desired, and used, e.g. as an agricultural fertiliser.

[0026] The concentration of struvite crystalline material in the aqueous recycle stream (slurry) is preferably between 5 and 25, more preferably between 10 and 20 vol.%. The crystalline material comprising struvite is recycled at a rate of between 1 and 10 m³ (on the basis of a concentration of 15 vol.%) per m² of reactor liquid surface area, preferably between 2 and 5 m³ per m² per hour. The residence time for the crystalline material comprising struvite, as recycled over the anaerobic reactor, can be at least 30 days, preferably at least 60 days, more preferably at least 90 days, up to e.g. 150 or even up to 180 days. The residence time can be increased or decreased by recycling a larger or smaller proportion of the crystalline material collected at the bottom of the reactor.

[0027] In an advantageous embodiment of the process of the invention, the reactor is operated batch-wise. After loading a batch, the entire effluent comprising the crystalline material can be recycled over the reactor. When treatment has sufficiently proceeded, e.g. when growth of the crystals has ceased, the reactor content is separated into a heavy (solid) fraction containing the struvite crystals and a light (liquid) fraction containing cleared waste water optionally containing some residual sludge.

Alternatively, the reactor can be operated continuously, and the effluent is divided into a fraction which is recycled and a fraction which is separated as described above. In both embodiments, the recycle rate is preferably such that the residence of the crystalline material is as described above.

[0028] The liquid effluent issuing from the separator may be further treated, e.g. using a sludge settling device and/or a second stage anaerobic or aerobic post-treatment, or it can be discharged as such. Sludge separated from such post-treatment can be discharged, or returned to the anaerobic reactor of the invention.

[0029] The invention also relates to a reactor for carrying out the process as described above. The reactor is referred to herein as "anaerobic reactor", "digester" or

"fermenter", or even "crystalliser" illustrating that the reactor combines these various functions. In particular, the reactor has a height which is at least twice its diameter, and an inlet means at the top, and an exit means at the bottom. An exit is provided at the bottom part, which can be a opening in the bottom plate, occupying e.g. between 0.5 and 5%, of the bottom surface, or between 20 and 200 dm². For example, the exit can have the form of a slit extending over a part of the whole of the radius of the circular bottom plate. It is also feasible to have two or more exit means.

[0030] A scraper is provided at the bottom part of the reactor for collecting solid material from the bottom of the reactor and moving said collected material to said exit. The movement of the scraper is preferably by rotation, and can be driven by a vertical axis positioned in the centre of the reactor. The rotation direction can be the same as the rotation produced by the input streams at the top of the reactor. The tip speed of the scraper is preferably between 0.03 m/sec and 0.2 m/sec. The scraper resistance can be e.g. between 100 and 600 N/m. The central axis can extend over essentially the height of the reactor, driving e.g. being provided at the top of the reactor, or alternatively, be limited to the height of the scraper, driving being provided at the bottom of the reactor. [0031] The scraper can have various shapes and sizes. For example, the scraper can be in the form of one or more vertical blades mounted to a radial arm connected to the central driving axis. Two or more blades can be mounted in a V-shape or similar shape extending over most or the whole of the radius, and cooperating to concentrate the scraped material to the position of the exit. Two radial arms, each provided with one or more, for example 2-4 scraper blades, can be combined to form a diametrical arm. It is also feasible to provide three, four, six etc. symmetrically arranged radial arms, each provided with one or more scraper blades. The further arrangement and operation of the scraper can be as described above and as depicted in the figures.

[0032] The scraper or other device provided for clearing and collecting the crystalline material from the bottom of the reactor is preferably arranged in such a manner that a temporary excess of crystalline material can be accommodated. Such temporary excess can be caused by fluctuations in the fermentation process or malfunction of the circulation mechanism. This can be done by making the scrapers or collecting slides vertically movable, driven by the height of the solid material at the bottom of the reactor, and thus avoiding overloading of the collector.

Description of the Figures

[0033] Fig. 1 schematically depicts a reactor of the invention.

[0034] Fig. 2 shows the top part of the reactor.

[0035] Fig. 3 shows the bottom part of the reactor.

[0036] Fig. 4 shows the scraper in the bottom part of the reactor in more detail.

[0037] In Figure 1, a vertical cylindrical embodiment of fermenter/crystalliser of the invention is schematically shown. The reactor 1 is provided with inlet means 2 for supply of thin sludge, with one or more nozzles 21 for introducing the sludge into the reactor. A second inlet 3 with nozzle 31 can be provided for introducing a magnesium source into the reactor. A third inlet 4 with nozzle 41 is provided for recirculation of crystalline material into the reactor. Alternatively, two or more of the inlets 2, 3 and 4 can be combined to a single inlet (not shown). An opening 5 is provided at the bottom of the reactor connected to exit line 51.

[0038] The material exiting the reactor through line 51 can be separated into a recycle line 53 and a harvesting line 55, both provided with pumps (52, 54). The recycle line recycles crystalline material (as a slurry) to reactor inlet 4. The harvesting line is fed to a separator 6, such as a hydrocyclone, where the material is separated by gravity and rotating forces. The heavier material containing the struvite crystals is collected as underflow and leaves the cyclone at exit 62 and is fed to exit line 63 for further processing the struvite. If desired, a part of the struvite separated in hydrocyclone 6 can be fed to the reactor through 4 for increasing the amount or concentration of crystalline material in the reactor, by a line leading from exit 62 to inlet 4 (not shown). The lighter overflow fraction of separator 6 contains treated waste and possibly sludge and leaves the separator through exit 64 to line 65 for further treatment or disposal.

[0039] A scraper 7, for collecting crystalline material at the bottom and moving it to exit 5, is provided at the bottom. It comprises one or more scraper blades 71 attached to a scraper arm 70. The scraper is mounted to a central axis 78, which in this

embodiment extends through the reactor and is driven by drive gear 79 at the top of the reactor.

[0040] The reactor is further provided with a gas exit 8 where gas, mainly biogas, is discharged and, after optional purification, can be used e.g. for heating purposes.

[0041] Figure 2 shows an embodiment of the top part of the reactor in more detail. Sludge inlet 2 is divided here into two inlet tubes 25 and 26, each provided with two outlets with nozzles 21, 22, 23 and 24. These outlets are positioned at an angle with the reactor content surface (which may be above or below the nozzles, not shown), such that the injected sludge causes the reactor content to rotate (here: clockwise).

Magnesium inlet means 3 is arranged here as a single tube 33, provided with two outlets and nozzles 31 and 32, which have the same direction as the sludge outlets and nozzles 21, 22, 23, 24. Inlet means 4 for introducing struvite seeds is arranged here as a single inlet 41. The central scraper axis 78 and drive mechanism 79, as well as the gas exit 8 are also shown.

[0042] Figure 3 shows an embodiment of the bottom part of the reactor in more detail. Scraper 7 is provided here as a radial arm 70 provided with vertical blades 71, 72, 73 and 74, mounted to the scraper arm with mountings strips 75. The blades 71 and 73 are positioned in a V arrangement for improved scraping efficacy, and the same applies to blades 72 and 74. The strips 75 are mounted to the radial arm(s) 70 in a rotatable connection, so that the blades can move upward as a result of pressure caused by jamming in case of temporarily excessive amounts of crystals or other irregularities. Scraper arm 70 is mounted to the drive axis 78 by mounting collar 76. As an alternative to the rotatable mounting of the scraper blades to the scraper arm, mounting collar 76 can be vertically movable over range 77 of the axis 78. Figure 3 also shows exit 5, here as a single opening, connected to exit line 51. The scraper and the exit cooperate to effectively remove crystalline material from the bottom of the reactor together with treated waste and resulting liquid.

[0043] Figure 4 gives a side view of the scraper with vertically movable blades 72 (71, 73, 74) and rotatable mounting strips, as well as collar 76 which can be vertically movable over range 77 of the axis 78. It will obviously be sufficient when either the blades 71-74 are rotationally movable or the collar (with arms 70 and blades) are vertically movable.

Claims

1. An anaerobic digestion reactor for producing and collecting struvite, having a height which is at least twice the diameter, the reactor having one or more inlet means at the top, and an exit means at the bottom, wherein one or more of said inlet means at the top are arranged to introduce liquid at such an angle that the incoming liquid causes a rotating movement of the entire reactor content, the bottom part of the reactor being provided with a rotating scraper capable of collecting solid material from the bottom of the reactor and moving said collected material to said exit means.

2. A reactor according to claim 1, wherein the scraper is driven by a vertical axis

positioned in the centre of the reactor.

3. A reactor according to claim 1 or 2, wherein the liquid inlet means at the top is arranged to introduce liquid at an angle of less than 75° with the surface of the reactor content and an angle of more than 30° with a horizontal diagonal of the reactor.

4. A reactor according to claim 3, wherein said angle with the surface of the reactor content is between 30° and 60° and said angle with a horizontal diagonal of the reactor is between 60 and 90°.

5. A reactor according any one of claims 1-4, wherein the rotating scraper is arranged to rotate in the same direction as the rotating movement of the reactor content caused by the incoming liquid.

6. A reactor according to any one of claims 1-5, having a height of between 10 and 25 m and a diameter between 1.5 and 15 m.

7. A reactor according to any one claims 1-6, further being provided independently with one or more of:

a. a return line connecting said exit means at the bottom with one of said inlet means at the top for recycling part of the solid material to the reactor;

b. a separator, separating at least part of said solid material collected at the exit from liquid and suspended solids;

c. scraper blades forming said scarper which scraper blades are vertically movable.

8. A process for the anaerobic treatment of sludge containing phosphorus and nitrogen comprising:

a. introducing sludge at the top of an anaerobic digestion reactor;

b. introducing crystalline material comprising struvite at the top of the reactor; c. if necessary, adding a magnesium source to the reactor content at a rate so as to produce a molar magnesium level between 75% and 200% of the molar phosphorus level and/or nitrogen level in the reactor;

d. collecting effluent comprising crystalline material at the bottom of the reactor; and

e. recycling crystalline material to step (b).

9. A process according to claim 8, wherein the crystalline material is introduced in step (b) in an aqueous stream containing between 5 and 25, preferably between 10 and 20 vol.% of crystalline material.

10. A process according to claim 8 or 9, wherein said crystalline material introduced in step (b) has an average particle size of between 0.5 and 3 mm.

11. A process according to any one of claims 8-10, wherein said crystalline material is recycled at such a rate that the residence time for the crystalline material in the reactor is between 30 and 180 days.

12. A process according to any one of claims 8-11, wherein the sludge is introduced in step (a) in a stream at an angle of less than 75° with the surface level of the reactor content and at an angle of more than 30° with a diagonal of the surface of the reactor content, to produce a circular downward flow in the reactor.

13. A process according to claim 12, wherein the crystalline material is introduced in step (b) in a stream at substantially the same angle as the sludge.

14. A process according to any one of claims 8-13, wherein the phosphorus content of the sludge introduced in step (a) is between 100 and 2000 mg/1.

15. A process according to any one of claims 8-14, wherein solid content of the sludge introduced in step (a) is between 1 and 15 g dry matter per 1.