Method for recovering phosphorus from sludge and plant thereof

Abstract

A method, and an installation thereof, for recovering phosphorus from sludge to be treated, said method including: a stage of pre-acidification of said sludge to be treated including a step of adding an acid, preferably carbon dioxide into said sludge to be treated; a stage of bio-acidification including a step of acidogenesis and carried out in a reactor having a hydraulic retention time comprised between 1 day to 8 days and, wherein the acidified sludge has a pH comprised between 3.5 to 5.5; and a stage of treatment including: a step of solid/liquid separation; and a step of recovery of phosphates in liquid phase by sorption and/or crystallization, giving a phosphorus depleted water.

Claims

1. Method for recovering phosphorus from sludge to be treated, said method including: a stage of pre-acidification of said sludge to be treated, giving a pre-acidified sludge, wherein the stage of pre-acidification includes a step of adding an acid into said sludge to be treated, said step of adding an acid into said sludge including injecting carbon dioxide into said sludge to yield a pre-acidified sludge having a pH equal to or less than 6.3; a stage of bio-acidification of said pre-acidified sludge, giving an acidified sludge, wherein the stage of bio-acidification includes a step of acidogenesis and is carried out in a reactor having a hydraulic retention time comprised between 1 day to 8 days and, at a pH comprised between 3.5 to 5.5; and, a stage of treatment of said acidified sludge including: a step of solid/liquid separation of said acidified sludge, giving a slurry and an acidified water; and, a step of recovery of phosphates in liquid phase by sorption and/or crystallization, giving a phosphorus depleted water, said step of recovery of phosphates being carried out after said step of solid/liquid separation.

2. Method according to claim 1, wherein the stage of pre-acidification is performed in a flotation reactor.

3. Method according to claim 1, wherein the stage of pre-acidification and/or the stage of bio-acidification include(s) a step of adding an additional acid chosen among strong acid or organic co-substrate to further control the pH.

4. Method according to claim 1, wherein the carbon dioxide added during the stage of pre-acidification is recycled from cogeneration or from incineration off-gas.

5. Method according to claim 1, wherein the stage of treatment of said acidified sludge further includes a step of digestion in liquid phase giving a biogas and a digested liquid and, wherein said step of digestion is a methanization and is carried out prior or after said step of recovery of phosphates.

6. Method according to claim 1 wherein the stage of treatment of said acidified sludge further includes a step of digestion of said slurry giving a biogas and a digested slurry.

7. Method according to claim 6, wherein said phosphorus depleted water is added to said slurry for the step of digestion of said slurry.

8. Method according to claim 6, wherein the digested slurry is, at least partly, recycled and mixed with the pre-acidified sludge.

9. Method according to claim 1, wherein said acidified water is sent to a mainstream wastewater treatment biological system and, wherein said step of recovery of phosphates in liquid phase is carried out downstream of said mainstream wastewater treatment biological system.

10. Method according to claim 1, wherein the stage of treatment of said acidified sludge includes a step of post-acidification, and wherein said step of post-acidification is carried out prior to said step of recovery of phosphates in liquid phase.

11. Method according to claim 1, wherein said method includes a stage of advanced control of pH of said stage of pre-acidification and/or said stage of bio-acidification and, wherein said stage of advanced control takes into account at least one parameter, said at least one parameter being chosen among: the pH for said stage of pre-acidification, the pH for said stage of bio-acidification and the phosphorus recovery performance for said step of recovery of phosphates.

12. Method according to claim 11 wherein the at least one parameter can be further chosen among: the pH for a step of digestion and a biogas recovery performance.

13. Plant for recovering phosphorus from sludge to be treated according to a method according to claim 1, wherein said plant includes: a contact chamber and means for injecting acid into the sludge to be treated, said acid being carbon dioxide and pre-acidified sludge being formed in said contact chamber having a pH equal to or less than 6.3, a sludge reactor adapted for bio-acidification with a hydraulic retention time comprised between 1 day to 8 days, means of solid/liquid separation, and means of phosphorus recovery adapted to recover phosphorus from a liquid phase.

14. Plant according to claim 13 further including a digestor or a methanizer reactor.

15. Method for recovering phosphorus from sludge to be treated, said method including: a stage of pre-acidification of said sludge to be treated, giving a pre-acidified sludge, wherein the stage of pre-acidification includes a step of adding an acid into said sludge to be treated, said step of adding an acid into said sludge including injecting carbon dioxide into said sludge to yield a pre-acidified sludge having a pH equal to or less than 6.3; a stage of bio-acidification of said pre-acidified sludge, giving an acidified sludge, wherein the stage of bio-acidification includes a step of acidogenesis and is carried out in a reactor having a hydraulic retention time comprised between 1 day to 8 days and, at a pH comprised between 3.5 to 5.5; and, a stage of treatment of said acidified sludge including: a step of solid/liquid separation of said acidified sludge, giving a slurry and an acidified water; a step of recovery of phosphates in liquid phase by sorption and/or crystallization, giving a phosphorus depleted water, said step of recovery of phosphates being carried out after said step of solid/liquid separation, wherein said acidified water is sent to a mainstream wastewater treatment biological system and wherein said step of recovery of phosphates in liquid phase is carried out downstream of said mainstream wastewater treatment biological system.

Description

V. BRIEF DESCRIPTION OF THE DRAWINGS OF THE DRAWINGS

(1) FIG. 1 represents a schematic view of the three stages of the method according to the present invention.

(2) FIG. 2 represents a schematic view of the essential steps of the stage of treatment of the acidified sludge according to the method of FIG. 1.

(3) FIGS. 3a and 3b represent a schematic view of the steps of the stage of treatment according to the method of FIG. 1, including a step of digestion in liquid phase carried out prior to (cf. FIG. 3a) or after (cf. FIG. 3b) the step of recovery of phosphates in liquid phase.

(4) FIG. 4 represents a schematic view of the steps of the stage of treatment according to the method of FIG. 1, including a step of digestion of the slurry.

(5) FIG. 5 further represents the embodiment according to which the phosphorus depleted water is added to the slurry for digestion of the slurry.

(6) FIG. 6 represent a schematic view of the steps of the stage of treatment according to the method of FIG. 1, in which the acidified water is sent to a mainstream wastewater treatment biological system.

(7) FIG. 7 represents a pilot semi-industrial setup in which have been conducted acidification experiments.

(8) FIG. 8 represents the detail of the CO.sub.2 injection system used in the pilot semi-industrial setup of FIG. 7.

(9) FIG. 9 represents the anions and cations concentration in function of time for a pH controlled at 5.

(10) FIG. 10 represents the anions and cations concentration in function of time for a pH controlled at for a pH being alternatively equal to 6 and 4.5.

VI. DESCRIPTION OF EXEMPLARY EMBODIMENTS

(11) With reference to FIG. 1, the method 1 for recovering phosphorus from a sludge to be treated 2 includes: a stage of pre-acidification 10 of the sludge to be treated 2, giving a pre-acidified sludge 4; a stage of bio-acidification 20 of said pre-acidified sludge 4, giving an acidified sludge 5; and, a stage of treatment 30 of said acidified sludge 5, giving a phosphorus depleted water 8 and recovered phosphorus 9.

(12) The sludge to be treated 2 contains water, organic matter and phosphorous-based matter. It can originate directly from a production line, as for example industrial sludge, or from a wastewater treatment plant, as for example a primary settling sludge, a biological sludge, or a mix of both types of sludge.

(13) The stage of pre-acidification 10 includes a step of adding carbon dioxide (CO.sub.2) 3 into the sludge to be treated 2. The resulting sludge is called pre-acidified sludge 4.

(14) The stage of pre-acidification 10 can be carried out in a contact chamber in which the carbon dioxide (CO.sub.2) is injected to the sludge to be treated 2. The stage of pre-acidification 10 can also be carried out in a flotation reactor. The injection of carbon dioxide 3 in a flotation reactor enables to thicken the treated sludge during the stage of pre-acidification 10.

(15) Carbon dioxide 3 can be recycled from cogeneration or from incineration off-gas of the wastewater treatment plant and is significantly cheaper than strong acids.

(16) The stage of bio-acidification 20 includes a step of acidogenesis. Acidogenesis is a part of an anaerobic digestion in which the biomass transforms the organic matter into fatty acids, mainly into volatile fatty acids. Fatty acids are saturated or unsaturated carboxylic acids with an aliphatic chain. They are said “volatile” when their aliphatic chain comprises between two to six carbon atoms. Because of the production of fatty acids during acidogenesis, the pH of the pre-acidified sludge 4 further decreases during the stage of bio-acidification 20. The pH obtained at the end of the stage of bio-acidification 20 is comprised between 3.5 and 5.5, preferably between 4 and 4.5 and more preferably equal to 4. These values of pH are particularly favorable to the release of phosphates in the sludge.

(17) The resulting sludge is called acidified sludge 5.

(18) As the acidogenesis is carried out at a pH comprised between 3.5 and 5.5, which are optimum conditions for the biomass to convert organic matter into fatty acids, the transformation of organic matter into fatty acids is both complete and fast kinetically. Therefore, the hydraulic retention time of the reactor in which the bio-acidification is carried out, can be particularly short while ensuring a high conversion of organic matter into fatty acids. The hydraulic retention time is comprised between 1 day to 8 days, depending on the temperature.

(19) The stage of bio-acidification 20 can be carried out in a sludge reactor designed for bio-acidification with a 1 day to 8 days hydraulic retention time.

(20) An additional acid 3a, 3b can optionally be added during respectively the step of pre-acidification 10 and/or the step of bio-acidification 20 to further control the pH. The acid 3a or 3b can be a mineral acid, such as HCl, H.sub.2SO.sub.4 or HNO.sub.3. These acids can be recovered from industrial waste. The acid 3a and/or 3b can also be an organic acid. The organic acid can be any organic substrate that will be qualified as easily biodegradable with a COD value higher than 0.3 g/g of product. These substrates can be by-product from Food & Beverage industries, additives manufacturers, fine chemicals, biomass residues, etc. The additional acid(s) can be in the form of liquid, gas or solid (powder).

(21) The acid 3a and 3b is preferably a strong acid, namely an acid that is virtually 100% ionized in water. It offers the benefit of efficiently reducing the pH with a minimum amount of chemical added and of being ionized fast kinetically. In a preferred embodiment, the acid 3a is HCl.

(22) The additional acid 3b added during the stage of bio-acidification 20 enables to further regulate the pH in function of the efficiency of acidogenesis in order to ensure optimum pH conditions for the acidogenesis. The additional acid 3b can be added simultaneously or successively with the acidogenesis.

(23) With reference to FIG. 2, the stage of treatment 30 of the acidified sludge 5 includes: a step of solid/liquid separation 40 of the acidified sludge 5, giving a slurry 6 and an acidified water 7; and, a step of recovery of phosphates 60 in liquid phase, giving a phosphorus depleted water 8 and recovered phosphorus 9.

(24) The step of solid/liquid separation 40 can be chosen among any means of sludge dewatering, and preferably press filter, belt filter, or centrifugation.

(25) The step of recovery of phosphates 60 is carried out after the step of solid/liquid separation 40. It is preferably carried out at a pH inferior to 7.5 in order to mitigate the addition of a basis, such as caustic soda.

(26) According to a first embodiment, the step of recovery of phosphates 60 can be carried out by sorption (adsorption, ion exchange, . . . ). The sorption can be on a non-regenerable or regenerable media, in situ or off site.

(27) According to a second embodiment, the step of recovery of phosphates 60 can be carried out by crystallization of phosphates into a phosphate mineral. For the crystallization, calcium or magnesium products can be added in order to obtain a calcium phosphate (such as brushite) or a magnesium phosphate (such as struvite). As a magnesium product, MgCl.sub.2 can be used. As a calcium product Ca(OH).sub.2 can be used. As the acidified sludge and/or water 5 is rich in organic matter, a step of digestion can be included in the stage of treatment 30. The step of digestion is carried out after the step of solid/liquid separation 40.

(28) With reference to FIGS. 3a and 3b, according to a first embodiment, the step of digestion can be carried out in liquid phase before 51 or after 52 the step of phosphorus recovery 60. As the liquid phase contains dissolved organic matter, and in particular low fatty acids, the digestion 51, 52 is a methanization, giving a biogas 510, 520 containing methane and carbon dioxide. The methanization can be carried out in an Upflow Anaerobic Sludge Blanket reactor or an Anaerobic Moving Bed Biofilm Reactor. The methanization features a superior biogas yield compared to a mere methanization carried out without prior stage of pre-acidification 10 and/or stage of bio-acidification 20.

(29) With reference to FIGS. 4 and 5, according to a second embodiment, the step of digestion 53, 54 is carried out on the slurry 6, giving a biogas 530, 540 and a digested slurry 531, 541. The step of digestion 53, 54 can be carried out in a Mesophilic digestor, a Thermophilic Digestor, a Thermal Lysis Digestion Reactor or an Anaerobic Digestion Membrane Reactor. The digestion 53, 54 features a superior biogas yield compared to a mere digestion carried out without prior stage of pre-acidification 10 and/or stage of bio-acidification 20.

(30) As shown by FIG. 5, the phosphorus depleted water 8 can advantageously be mixed to the slurry 6 for the step of digestion 54.

(31) With reference to FIG. 6, when digestion is not available on a treatment plant, the acidified water 7 can be sent to a mainstream wastewater treatment biological system 70. The step of recovery of phosphates 60 in liquid phase is then carried out downstream of the mainstream wastewater treatment biological system 70. In particular, the wastewater treatment biological system 70 can include an activated sludge reactor, a Moving Bed

(32) Biofilm Reactor, a Membrane Bio Reactor or a Sequenced Batch Reactor.

VII. EXPERIMENTAL DATA—PRE-ACIDIFICATION

(33) Trials have been both conducted in lab and semi-industrial pilot scales, with the aim i) to assess the impact of environmental parameters (pH, CO.sub.2 injection) and ii) to develop a control strategy to optimize the VFA production and stability and also to release phosphorus as P—PO4 (phosphates).

(34) Therefore, a cascade structure has been developed to control the system by using CO.sub.2 flow as an actuator. This approach is new in the sense that the CO.sub.2 is used as the driver for pH adjustment and production of VFA. Moreover, the control algorithm optimizes the CO.sub.2 consumptions and achieves better performances in terms of VFA production and phosphorous release.

VII.1-PILOT SEMI-INDUSTRIAL SETUP

(35) The pilot semi-industrial setup is presented on FIG. 7. It comprises an anaerobic digestion tank having stirring means. The anaerobic digestion tank can be fed with sludge. CO.sub.2 and HCl can be added in the tank. The injection of CO.sub.2 is part of a recirculation loop/measuring circuit. A biogas analyzer measures the content of biogas (CO.sub.2, CH.sub.4, H.sub.2, H.sub.2S). A VFA analyzer measures the VFA content. The content of cations and anions of the acidified sludge has also been analyzed on filtrate (after sludge separation) by ionic chromatography.

VII.2-CARBON DIOXIDE INJECTION SYSTEM

(36) Details of the carbon dioxide injection system are shown on FIG. 8.

(37) Preliminary tests for estimating the gas-liquid transfer of CO.sub.2 have been carried out on the tank in order to find an efficient mode of diffusion of CO.sub.2. The results of these tests showed that the diffusion of CO.sub.2 through a “fine bubbles” diffuser installed in the tank's recirculation loop/measuring circuit improves the gas-liquid transfer of CO.sub.2 compared with a direct injection of CO.sub.2 into the tank. The control of the CO.sub.2 injection is ensured by a EPFLOW-type mass flowmeter. The CO.sub.2 flow rate to be injected into the reactor is set either at the supervision level (supervisory control system) or locally at the ELFLOW local display.

VII.3-INLET SLUDGE CHARACTERISTICS

(38) The total phosphorous concentration in the inlet sludge to be treated was 0.51 g/L.

VII.4-MEASUREMENTS OF CATIONS AN ANIONS

(39) Sludge retention (SRT) was set on 2 days, and pH conditions was adjusted thanks to CO.sub.2 and/or mineral acid injection along the 10 days period (>240 hr).

(40) More precisely, pH has been adjusted by 2 levers:

(41) CO.sub.2 injection (between 0.3 to 0.8 nL/min); and, HCl at 0.1 mol/L

(42) Cations and anions values have been analyzed in parallel of VFA production (after sludge separation).

(43) FIG. 9, respectively FIG. 10, shows the anion/cation content in function of time for a pH controlled at 5, respectively for a pH being alternatively equal to 6 and 4.5. As can be seen on FIG. 9, the concentration of soluble P—PO4 is steady around 0.3-0.38 g/L at pH equal to 5.

(44) As can be seen on FIG. 10, the concentration of soluble P—PO4 ranges from 0.3 to 0.45 g/L at pH equal to 4.5 and is almost equal to 0 when pH is increased to a pH equal to 6. These values have been consistent over the 10 days testing period. This represents between 60 to 90% of phosphorous being solubilized under lower pH conditions and efficient control of CO.sub.2-acid injection.

VII.5-CONCLUSION

(45) The production increase of VFA and desorption of P from the inlet sludge up to 90% have been observed and monitor during the experiments. The precise mechanism is still to be fully understood and tested, a few conclusions can be drawn: injection of CO.sub.2 with an efficient control system allows decreasing the pH of the digestion close to the pKa of CO.sub.2 (6.3). Carbon dioxide acts as a “pH buffer”, compensating the inlet fluctuation and stabilizing the acidification process. It also helps the speciation of VFA (partly converting some species such as butyric acid to acetic acid), which results to an increase of VFA up to 48%; with a lower and stabilized pH at low values (6.3), the mineral acid consumption (HCl) used to decrease further down to 4.5 is minimized; P—PO.sub.4 solubilization under such conditions reaches an efficiency above 60% (up to 90% measured);

(46) Combined with a VFA increase (which enhances biogas production in anaerobic digester), the phosphorus release is improved by the combination of CO.sub.2 and acidification reactor versus conventional acidification process.

(47) Carbon dioxide being a “by-product” from biogas purification, it can significantly reduce the OPEX involved in existing process, by reducing mineral acid usage and boost the biogas production by increasing VFA production.