A method for removing nitrogen and phosphorus from sludge

Abstract

The application provides a method for removing nitrogen and phosphorus in sludge, and relates to the technical field of sewage treatment. The method comprises the following steps: (1) sludge dewatering treatment; (2) sludge pretreatment; (3) hydrolysis acidification treatment; a mixture of divalent salt and trivalent salt is added in the hydrolysis acidification treatment, and it is unexpectedly found that the crystallinity of the layered double hydroxide formed by co-precipitation of the mixed salt solution is better, the specific surface area is larger, more carriers can be provided for microbial reproduction, the reproduction and metabolism of microorganisms are strengthened, and therefore the hydrolysis acidification efficiency of the sludge is better improved, and the nitrogen and phosphorus removal effects are obviously improved.

Description

Technical Field

[0001] This application relates to the field of wastewater treatment technology, specifically to a method for removing nitrogen and phosphorus from sludge. Background Technology

[0002] Conventional activated sludge nitrogen and phosphorus removal systems often face competition for carbon sources between polyphosphate-accumulating bacteria and denitrifying bacteria. Therefore, one of the key factors for the successful operation of biological nitrogen and phosphorus removal systems is the availability of suitable carbon sources, primarily in the form of volatile fatty acids (VFAs). When the dissolved COD content is very low, it is necessary to supplement carbon sources such as methanol and acetic acid.

[0003] With the dwindling natural resources and increasingly severe pollution, sludge resource utilization methods have become a research hotspot for water treatment workers. Meanwhile, the problem of insufficient carbon sources in urban wastewater treatment plants remains widespread. Short-chain fatty acids produced during the low-oxygen hydrolysis and acidification of sludge can provide an additional carbon source for nitrogen and phosphorus removal bacteria. However, the alkaline anaerobic acidification liquid of sludge contains not only a large amount of dissolved organic matter but also a large amount of nitrogen and phosphorus. Directly using it as a supplementary carbon source would increase the load on the wastewater treatment system and weaken or even worsen the nitrogen and phosphorus removal efficiency.

[0004] Studies have found that directly reusing anaerobic acidification sludge as a carbon source for denitrification in wastewater treatment plants without removing nitrogen and phosphorus can lead to poor denitrification and increase the burden on the wastewater treatment system for nitrogen and phosphorus removal. Currently, the struvite process is widely studied, which removes nitrogen and phosphorus simultaneously by adding magnesium salts such as MgCl2 to generate magnesium ammonium phosphate precipitate. However, its small crystals are difficult to precipitate, and solid-liquid separation is challenging, making large-scale implementation difficult in practice.

[0005] Chinese patent application 201210066435.7 discloses a method for removing nitrogen and phosphorus from anaerobic acidification liquid of sludge. The method involves adjusting the pH of the acidification liquid to 10-12 with dilute NaOH, using chemical reactants CaO and FeCl3 as coagulants and PAM as a coagulant aid, to remove phosphorus through coagulation and sedimentation, and then removing nitrogen using an ammonia stripping process. This invention has a high removal efficiency for nitrogen and phosphorus in acidification liquid. The acidification liquid treated by this invention has a high capacity to serve as a carbon source for denitrification and can be used as an external carbon source for denitrification in wastewater treatment plants.

[0006] Chinese patent application 202011149354.4 discloses a method for simultaneously realizing the resource utilization of carbon, nitrogen, and phosphorus in sludge. The method includes first thickening the sludge to increase its solids content; then, adding agents to raise the sludge pH to 9-11, disrupting the microbial cell structure and effectively releasing intracellular organic matter; and then, by breaking down sludge fermentation, converting the organic matter in the sludge into easily biodegradable soluble substances, while transferring some nitrogen and most phosphorus to the less biodegradable sludge. The liquid after solid-liquid separation can be reused as a carbon source in wastewater treatment plants, and the recovered precipitated sludge, rich in nitrogen and phosphorus, can be recycled into nitrogen and phosphorus fertilizer, thus simultaneously realizing the resource utilization of carbon, nitrogen, and phosphorus in the sludge. This technology can be used for the treatment of sludge from wastewater treatment plants, bottom sludge from water environment remediation, and sludge from urban pipe network drainage.

[0007] However, the existing technology has poor nitrogen and phosphorus removal effects and cannot meet the requirements. Furthermore, the precipitated sludge contains a large amount of heavy metals, which will affect the reuse of the sludge. Therefore, it is necessary to develop a method that can better remove nitrogen and phosphorus from the supernatant of sludge hydrolysis and acidification. This method can not only better remove nitrogen and phosphorus from the supernatant of sludge hydrolysis and acidification, but also stabilize the heavy metals in the precipitated sludge, so that the precipitated sludge can be better utilized. Summary of the Invention

[0008] Based on the shortcomings of the existing technology, this application aims to provide a method for removing nitrogen and phosphorus from sludge. The method involves adding salt to the sludge hydrolysate under an alkaline environment. In the alkaline environment, the salt solution containing different metal cations undergoes a co-precipitation reaction to form crystals with good crystallinity, thereby achieving phosphorus precipitation.

[0009] To solve the above-mentioned technical problems, this application provides the following technical solution:

[0010] A method for removing nitrogen and phosphorus from sludge, the method comprising the following steps:

[0011] (1) Sludge dewatering treatment: The sludge is separated into solid and liquid, the supernatant enters the sewage treatment system, and the dewatered sludge enters the pretreatment unit;

[0012] (2) Sludge pretreatment: Add alkali to the concentrated sludge, adjust the pH to 9-11, and pump it into the hydrolysis acidification unit after ultrasonic treatment.

[0013] (3) Hydrolysis acidification treatment: Add alkali and salt to the pretreated sludge, react for 8-15 days, and then transfer to the separation unit. The separated liquid is used as a carbon source, and the recovered precipitated sludge can be further utilized for resource recovery.

[0014] The alkali mentioned in steps (2) and (3) above is one or more of NaOH, KOH, Cu(OH)2, Fe(OH)3, Zn(OH)2, Na2CO3, NaHCO3, urea and ammonia water; preferably NaOH.

[0015] The concentration of the alkali is 1 mol / L to 8 mol / L.

[0016] The ultrasonic frequency mentioned in step (2) above is 40KHz-120KHz, and the time is 5-30min.

[0017] The salt mentioned in step (3) above is a mixture of divalent and trivalent salts;

[0018] The metal ion M of the divalent salt 2+ Selected from Ni 2+ Co 2+ Zn 2+ Mg 2+ and Mn 2+ One or more of the following; preferably Zn 2+ or / and Mg 2+ ;

[0019] The metal ion M of the trivalent salt 3+ Selected from Al 3+ Cr 3+ Fe 3+ V 3+ Co 3+ Ga 3+ and Ti 3+ One or more of the following; preferably Al 3+ or / and Fe 3+ .

[0020] The anion of the divalent or trivalent salt is selected from SO4. 2- or / and Cl - .

[0021] In some preferred embodiments, the trivalent salt is selected from FeCl3 or AlCl3; the divalent salt is selected from MgCl2.

[0022] In some preferred embodiments, the molar ratio of the metal ions of the divalent salt to the metal ions of the trivalent salt is 2-4:1; preferably 2:1, 3:1, or 4:1.

[0023] The molar ratio of the sum of the metal ions of the divalent and trivalent salts to P in the hydrolysis acidification solution is 2-6:1; preferably 2:1, 3:1, or 4:1.

[0024] The P in the hydrolysis acidification liquid mentioned here refers to the concentration of P released into the hydrolysis acidification liquid after the sludge has been in the hydrolysis acidification process for a period of time.

[0025] In the sludge pretreatment process, the main purpose of adding alkaline solution and ultrasonic treatment in this unit is to destroy the cell walls of microorganisms in the sludge. Alkaline solution can accelerate the dissolution of microbial cells, and ultrasonic treatment can accelerate the rupture of microbial cells. The preferred alkaline solution is a strong or weak alkali containing sodium ions, as sodium ions will increase the osmotic pressure of the cells, thereby accelerating the rupture of the cells.

[0026] In the hydrolysis and acidification process, alkaline solution is continuously added in the initial stage of the reaction until the pH reaches 9-11. This aims to further dissolve and break down any unbroken microbial cells. Additionally, the addition of alkaline solution helps prevent the methanation side reaction of the sludge. Subsequent addition of brine further increases the osmotic pressure of the microbial cells, accelerating cell wall rupture. Compared to conventional sludge microbial cell disruption technologies on the market, such as hot water hydrolysis, alkaline hydrolysis, and ultrasonic technology, this application employs a multi-enhanced cell disruption technology, which can improve the extraction rate of carbon sources from the sludge.

[0027] In the hydrolysis acidification treatment, the system startup procedure for the sludge hydrolysis acidification unit is as follows: The system temperature is controlled at 35℃. After sludge is fed into the system, the alkali tank is first turned on to adjust the pH value of the sludge hydrolysis acidification unit to 9-11 and maintain this pH value. Then, the salt tank is turned on, and the dissolved salt is added at a uniform rate. If the pH value drops below 9-11, the alkali tank is restarted, and the pH value is adjusted back to 9-11. The hydrolysis acidification treatment time is 10-15 days.

[0028] Furthermore, in the hydrolysis and acidification unit, the increased residence time of sludge hydrolysis and acidification also provides reaction conditions for the preparation of layered bimetallic hydroxides—hydrotalcite. The co-precipitation method for preparing hydrotalcite is the most commonly used method due to its simplicity and wide applicability. A salt solution containing metal ions is mixed with an alkaline solution, and the metal ions undergo co-precipitation at a specific pH to achieve a nucleation reaction, resulting in crystals with good crystallinity. In this application, a mixture of divalent and trivalent salts is used as the salt solution, and the molar ratio of metal ions in the divalent salt to the trivalent salt is controlled at 2-4:1. The resulting layered bimetallic hydroxides have better crystallinity and a larger specific surface area, providing more carriers for microbial reproduction, enhancing the reproductive and metabolic capabilities of microorganisms, thereby improving the hydrolysis and acidification efficiency of the sludge and significantly improving nitrogen and phosphorus removal.

[0029] In addition, it should be noted that this application enables the metal ions to co-precipitate and form hydrotalcite at a lower temperature (35°C) by controlling the type and molar ratio of metal ions. If the temperature is too high, the metabolic activity of microorganisms will decrease or even die, thereby affecting the reaction efficiency of sludge hydrolysis and acidification.

[0030] In addition, the layered bimetallic hydroxides formed during the hydrolysis and acidification process can adsorb phosphate ions through ion exchange and precipitate them better, thereby achieving phosphorus separation.

[0031] Secondly, layered bimetallic hydroxides can further exchange with heavy metal ions in the sludge, thereby fixing and stabilizing the heavy metals in the sludge. At the same time, the anion layers of the layered bimetallic hydroxides can also adsorb some small molecule acids in the interlayer through ion exchange reactions, ultimately allowing the separated sludge to be used as a slow-release fertilizer.

[0032] In the separation unit, alkali solution needs to be added to adjust the pH value to 10-11.

[0033] During operation, the ammonia stripped from the hydrolysis acidification unit and the separation unit needs to be recovered. That is, the ammonia gas emitted with the air is collected into an absorption device containing acid to prevent secondary pollution to the environment.

[0034] This application achieves ammonia recovery by adjusting the pH and using a stripping method.

[0035] Compared with the prior art, the beneficial effects of this application are as follows:

[0036] (1) In the process of preparing carbon source by sludge hydrolysis acidification, this application adds alkaline solution and salt solution to the sludge hydrolysis acidification unit, which can better accelerate the dissolution of microbial cells, thereby accelerating cell rupture, allowing organic matter to be fully released, and thus better removing nitrogen and phosphorus from sludge acidification liquid.

[0037] (2) In the implementation process of this application, a mixture of divalent and trivalent salts was used as the salt solution, and the molar ratio of metal ions of divalent salt to metal ions of trivalent salt was controlled to be 2-4:1. It was unexpectedly found that the layered bimetallic hydroxides formed by co-precipitation of the mixed salt solution had better crystallinity and larger specific surface area, which could provide more carriers for microbial reproduction, enhance the reproduction and metabolism of microorganisms, thereby better improving the hydrolysis and acidification efficiency of sludge and significantly improving the nitrogen and phosphorus removal effects. Attached Figure Description

[0038] Figure 1 This is a flowchart of the present invention. Detailed Implementation

[0039] The features mentioned above, or the features mentioned in the embodiments, of this invention can be combined arbitrarily. All features explained in this specification can be used with any methodological form, and each feature disclosed in the specification can be replaced by any alternative feature that provides the same, equivalent, or similar purpose. Therefore, unless otherwise specified, the disclosed features are merely general examples of equivalent or similar features.

[0040] The present invention will be further illustrated below with reference to specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. The following embodiments are implementation methods to indicate specific conditions, generally performed under conventional conditions or as recommended by the manufacturer.

[0041] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those well known to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to the methods of this invention. The preferred embodiments and materials described herein are for illustrative purposes only.

[0042] Under initial conditions of fresh sludge with a mass concentration of 80 g / L, the TN content was 170.46 mg / L, the ammonia nitrogen content was 104.92 mg / L, and the TP content was 12.35 mg / L.

[0043] Example 1

[0044] Fresh sludge, after dewatering to a moisture content of 80%, was diluted with water to a sludge concentration of 80 g / L. The pH of the dewatered sludge was adjusted to 10 with NaOH, and the mixture was ultrasonicated at 25 kHz for 60 minutes before being introduced into the sludge hydrolysis and acidification unit. During this process, organic nitrogen and organic phosphorus in the cells were released, and the TN and TP concentrations in the sludge increased. The TN content was measured to be 1369.24 mg / L, the TP content to be 172.33 mg / L, and the ammonia nitrogen content to be 417.89 mg / L. The pH of the sludge was adjusted to 9.5 with NaOH, and then a mixed salt solution of MgCl2 and AlCl3 was added. 2+ And Al 3+ The molar ratio is 2:1, Mg 2+ And Al 3+ The molar ratio of the total TN to the P in the phosphate supernatant after sludge hydrolysis and acidification is 4:1. Under the action of microorganisms, most of TN is converted into ammonia nitrogen and most of TP is converted into phosphate. NaOH is added to the separation unit to adjust the pH to 11. The ammonia nitrogen in the filtrate after anaerobic hydrolysis and acidification of sludge is 51.15 mg / L and the phosphate is 15.57 mg / L.

[0045] Example 2

[0046] The dewatered sludge, with a moisture content of 80%, was diluted with water to a sludge concentration of 80 g / L. NaOH was added to adjust the pH to 10.5, and the mixture was ultrasonicated at 25 kHz for 60 minutes. The sludge was then introduced into a sludge hydrolysis and acidification unit (the TN content was measured to be 1426.75 mg / L, TP content 181.37 mg / L, and ammonia nitrogen content 421.71 mg / L). The pH was adjusted to 10 using NaOH and Zn(OH)₂, and then a mixed salt solution of MgCl₂ and FeCl₃ was added. 2+ and Fe 3+ The molar ratio is 3:1, Mg 2+ and Fe 3+ The molar ratio of the total molar amount of sludge to the phosphate in the supernatant after sludge hydrolysis and acidification is 4:1; NaOH is added to the separation unit to adjust the pH to 11, and the ammonia nitrogen in the filtrate after anaerobic hydrolysis and acidification of sludge is 54.39 mg / L and the phosphate is 13.64 mg / L.

[0047] Comparative Example 1

[0048] Fresh sludge, after dewatering to a moisture content of 80%, was diluted with water to a sludge concentration of 80 g / L. The pH of the dewatered sludge was adjusted to 10 with NaOH, and the mixture was ultrasonically reacted at 25 kHz for 60 minutes. The sludge was then introduced into a sludge hydrolysis and acidification unit (the TN content was measured to be 1369.24 mg / L, TP content to be 172.33 mg / L, and ammonia nitrogen content to be 417.89 mg / L). The pH of the sludge was adjusted to 9.5 with NaOH. Without adding salt, the ammonia nitrogen content after hydrolysis and acidification was 262.3 mg / L, and the phosphate content was 131.57 mg / L.

[0049] Comparative Example 2

[0050] The difference from Example 2 is that: Mg 2+ and Fe 3+ The molar ratio is 1:1. After treatment, the ammonia nitrogen in the filtrate from the anaerobic hydrolysis and acidification of the sludge is 168.35 mg / L and the phosphate is 85.40 mg / L.

[0051] Comparative Example 3

[0052] The difference from Example 2 is that: Mg 2+ and Fe 3+ The molar ratio is 5:1. After treatment, the ammonia nitrogen in the filtrate from the anaerobic hydrolysis and acidification of the sludge is 129.76 mg / L, and the phosphate is 72.57 mg / L.

Claims

1. A method for removing nitrogen and phosphorus from sludge, characterized in that: The method includes the following steps: (1) Sludge dewatering treatment: solid-liquid separation of sludge, and dewatered sludge enters the pretreatment unit; (2) Sludge pretreatment: alkali is added to concentrated sludge, ultrasonic treatment is performed, and then pumped into the hydrolysis acidification unit; (3) Hydrolysis acidification treatment: alkali and salt are added to pretreated sludge, and hydrolysis acidified sludge enters the separation filtrate unit; (4) Separation filtrate treatment: the separated liquid is used as a carbon source; the salt mentioned in step (3) is a mixture of divalent and trivalent salts; The pH range of the hydrolysis acidification treatment is 9-11; The frequency of the ultrasound in step (2) is 25 kHz, and the duration is 60 min; The mixture of divalent and trivalent salts is a mixed salt of MgCl2 and AlCl3, Mg 2+ And Al 3+ The molar ratio is 2:1; or the mixture of the divalent and trivalent salts is a mixed salt of MgCl2 and FeCl3, Mg 2+ and Fe 3+ The molar ratio is 3:1; The molar ratio of the sum of the metal ions of the divalent salt and the trivalent salt to the molar ratio of P in the hydrolysis acidification solution is 4:

1. In the hydrolysis acidification treatment, the system startup process of the sludge hydrolysis acidification unit is as follows: after the sludge is fed into the system, the alkali tank is first turned on to adjust the pH value of the sludge hydrolysis acidification unit to 9-11 and maintain the pH value at 9-11; then the salt tank is turned on, and the dissolved salt is added at a uniform rate. If the pH value is lower than 9, the alkali tank is turned on again to adjust the pH value to 9-11.

2. The method according to claim 1, characterized in that: The alkali mentioned in steps (2) and (3) is one or more of NaOH, KOH, Cu(OH)2, Fe(OH)3, Zn(OH)2, Na2CO3, NaHCO3, urea and ammonia water.

3. The method according to claim 1, characterized in that: The base mentioned in steps (2) and (3) is NaOH and / or Zn(OH)2.

4. The method according to claim 1, characterized in that: The hydrolysis acidification treatment is carried out at a temperature of 35°C for 8-15 days.

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