CN113023845A

CN113023845A - Electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in struvite form

Info

Publication number: CN113023845A
Application number: CN202110455798.9A
Authority: CN
Inventors: 刘耀兴; 吴晓云; 戴丽萍; 丁建国; 柯雄; 陈日耀
Original assignee: Fujian Normal University
Current assignee: Fujian Normal University
Priority date: 2021-04-26
Filing date: 2021-04-26
Publication date: 2021-06-25
Anticipated expiration: 2041-04-26
Also published as: CN113023845B

Abstract

The invention discloses an electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite, which is characterized in that a bipolar membrane electrodialysis system is connected with a magnesium-air battery system in series, and NH in the wastewater is treated by using the bipolar membrane electrodialysis system₄ ⁺And PO₄ ^3‑Separating, enriching and concentrating, and then utilizing a peristaltic pump to concentrate the enriched NH₄ ⁺And PO₄ ^3‑Pumping to magnesium-air battery system, and Mg generated by magnesium-air battery system²⁺Reacting to produce struvite while using OH produced by bipolar membrane^‑Ca enriched by electrodialysis with bipolar membranes²⁺Reaction to form Ca (OH)₂Precipitation ofRemoving Ca (OH) by filtration₂Reduction of Ca²⁺Influence on the purity of struvite and simultaneously obtain electric energy. The invention can obtain low-concentration NH₄ ⁺(2mmol/L) and PO₄ ^3‑Struvite with the purity of 86.2 percent is recovered from the wastewater (1mmol/L), and electric energy with the output power of 3.56 +/-0.37 mW is obtained. From NH₄ ⁺、PO₄ ^3‑And Ca²⁺Struvite with the purity of 89.1 percent is recovered from wastewater with the initial concentration of 10, 5.0 and 2.0mmol/L respectively, and electric energy with the output power of 2.39 +/-0.85 mW is obtained.

Description

Electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in struvite form

Technical Field

The invention relates to the field of wastewater treatment, in particular to an electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in a struvite form.

Background

Nitrogen and phosphorus become main pollution indexes of surface water in China, and emission of the nitrogen and phosphorus causes pollution events such as water bloom, red tide and the like, thereby causing great harm to the environment, the economy and the human health. On the other hand, nitrogen and phosphorus are essential elements of all living bodies, and play an important role in cell life activities. In recent years, with global economic development and population increase, the demand for nitrogen and phosphorus has been increasing. However, phosphate rock is a non-renewable resource, with limited reserves and extremely uneven distribution throughout the world. Compared with phosphorus, the nitrogen resource amount is rich, a closed cycle can be formed in the natural environment, but the use and the loss of agricultural nitrogen fertilizers cause serious imbalance of global nitrogen cycle. Therefore, in order to respond to the theory of "circular economy" and realize global sustainable food supply, it is necessary to recover nitrogen and phosphorus resources from wastewater.

At present, struvite (MgNH)₄PO₄.6H₂O) crystallization is considered to be an effective method for recovering nitrogen and phosphorus in water, which can simultaneously recover ammonia Nitrogen (NH) in wastewater₄ ⁺) And Phosphate (PO)₄ ^3-) (equation 1), and struvite is a good agricultural slow release fertilizer. In recent years, research and development workThe inventors have successfully recovered struvite from nearly a hundred kinds of wastewater, and made a lot of research works on the influencing factors and formation mechanism of struvite crystallization, leading to some important conclusions. But recovering NH from different phosphorus-containing wastewaters in the form of struvite₄ ⁺And PO₄ ^3-Many challenges remain. First, struvite crystallization is performed in an alkaline environment, however, according to equation (1), the reaction process continuously generates H⁺The pH value of the solution is reduced, and the struvite crystallization is further influenced; in addition, relative NH in general wastewater₄ ⁺And PO₄ ^3-，Mg²⁺Low concentration, and thus a large amount of alkali source (OH) needs to be added during the struvite crystallization process^-) And magnesium (Mg)²⁺) Sources, which greatly increase the cost of the process, limiting its engineering applications. Secondly, struvite crystallization is not suitable for low NH concentrations₄ ⁺And PO₄ ^3-Waste water when PO₄ ^3-When the concentration is lower than 50mg/L, the purity of the struvite is reduced, and the economic benefit is poor. Third, metal ions (particularly Ca) in the wastewater that are easily coprecipitated with struvite²⁺) To NH₄ ⁺And PO₄ ^3-Removal and struvite formation and purity are significantly affected, while almost all contain NH₄ ⁺And PO₄ ^3-The water bodies all contain Ca with different concentrations²⁺. Therefore, there is a need to develop a method for recovering NH from low concentration wastewater in the form of struvite₄ ⁺And PO₄ ^3-And decrease Ca²⁺Method for influencing the purity of struvite by plasma.

Mg²⁺+NH₄ ⁺+H_nPO₄ ^n-3+6H₂O→MgNH₄PO₄·6H₂O+nH⁺N-0, 1, or 2 (1)

Bipolar Membrane Electrodialysis (BMED) is a novel electrodialysis system consisting of the introduction of bipolar membranes (BPM) into Electrodialysis (ED). The BPM is combined to lead water molecules (H) in the middle layer to be capable of being absorbed under the action of a direct current field₂O) dissociation to H⁺And OH^-(figure 1) and ED separates and enriches the anions and cations in the wastewaterPerformance, BMED is commonly used in the remediation of saline wastewater and converts the salt to the opposite acid and base (figure 2). A magnesium-air battery system (MAC) is a galvanic cell system composed of magnesium metal (Mg) or Mg alloy as a negative electrode and inert metal as a positive electrode, wherein in the discharging process of the battery, Mg loses electrons through an equation (2) and is changed into Mg²⁺And O dissolved in the solution₂Acceptance of electrons at the anode by equations (3) and H₂Reaction of O to OH^-. The system can spontaneously generate Mg²⁺And OH^-And can generate electric energy. In a MAC system, Mg produced by equation (2)²⁺Can serve as a magnesium source for the struvite reaction, while the OH "produced by equation (3) can provide an alkaline environment for the struvite reaction.

Mg–2e^-→Mg²⁺ (2)

O₂+4e^-+2H₂O→4OH^- (3)。

Disclosure of Invention

The invention aims to provide an electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite, aiming at recovering low-concentration NH in wastewater₄ ⁺And PO₄ ^3-Avoiding adding alkali source and magnesium source required by traditional struvite crystallization and reducing Ca²⁺The influence of metal ions which are easy to coprecipitate with struvite on the purity of struvite is obtained, and electric energy is obtained simultaneously to realize NH in wastewater₄ ⁺And PO₄ ^3-The resource utilization is realized, and clean energy is obtained.

The technical scheme adopted for realizing the purpose of the invention is as follows: the self-made electrolytic bath with a cuboid groove-shaped structure is combined with BPM and an anion-cation exchange membrane, ruthenium iridium titanium plates are respectively taken as electrodes to form a BMED system, and NH in wastewater is treated by the BMED system₄ ⁺、PO₄ ^3-And Ca²⁺Separating and enriching Ca by using metal ions²⁺OH with BPM^-Formation of Ca (OH)₂And removing the NH after filtration to realize low-concentration NH₄ ⁺And PO₄ ^3-Enrichment of and Ca²⁺Removal of(see FIG. 1 for details of the bipolar membrane electrodialysis System), while NH is enriched in the form of ammonia and phosphoric acid₄ ⁺And PO₄ ^3-Delivered to the MAC system by a peristaltic pump. The self-made cylindrical container, the magnesium strip, the titanium plate, the resistor and the universal meter form an MAC system, and OH spontaneously generated by the MAC is utilized^-And Mg²⁺Providing Mg for struvite crystallization²⁺And an alkaline environment, the problem of cost increase caused by adding a magnesium source and an alkaline reagent in the traditional struvite crystallization process is solved, and electric energy is obtained by utilizing the self-discharge process of MAC.

The homemade cuboid BMED groove-shaped structure electrolytic cell and the MAC cylindrical container are made of nylon materials. The BMED system electrolytic cell is characterized in that a cathode and an anode are arranged at the left end and the right end of the BMED system electrolytic cell and are respectively connected with a negative pole and a positive pole of a power supply, a bipolar membrane (BPM), a Cation Exchange Membrane (CEM), an Anion Exchange Membrane (AEM) and the bipolar membrane (BPM) are sequentially arranged between the cathode and the anode, and a cathode chamber, an ammonia water chamber, a waste water chamber, a phosphoric acid chamber and an anode chamber are sequentially formed between adjacent membranes from left to right. The magnesium strip and the titanium plate in the MAC system are respectively a negative electrode and a positive electrode and respectively form a closed loop with an external resistor, and the two electronic multimeters are used for measuring the output current and the voltage of the MAC system.

The MAC system is placed in a constant-temperature water bath stirring device and keeps NH by stirring₄ ⁺And PO₄ ^3-Uniformity of concentration, constant temperature of the aqueous solution was maintained by a water bath.

The titanium plate is a ruthenium iridium titanium plate. The magnesium strip is magnesium metal with the purity of more than 97 percent.

And the effluent of the magnesium-air battery system respectively enters an ammonia water chamber and a phosphoric acid chamber in the bipolar membrane electrodialysis system to form a circulating system, so that the zero discharge of the effluent in the magnesium-air battery system is realized.

NH enriched by bipolar membrane electrodialysis system₄ ⁺In the process of entering the magnesium-air battery system, a filter is arranged for filtering Ca (OH)₂Precipitation, prevention of Ca (OH)₂Entering a magnesium-air battery system to affect the struvite purity.

In the process that the effluent of the magnesium-air battery system enters an ammonia water chamber and a phosphoric acid chamber in the bipolar membrane electrodialysis system, a filter is arranged for filtering struvite which is not completely precipitated, and preventing the struvite from entering the bipolar membrane electrodialysis system again.

The magnesium-air battery system is externally connected with a 50-ohm resistor, and the output current and the output voltage of the magnesium-air battery system are respectively recorded by an ammeter and a voltmeter and are used for calculating the electricity generation performance of the magnesium-air battery system.

Separation and enrichment of NH Using BMED according to the invention₄ ⁺And PO₄ ^3-And recovering NH from the MAC as struvite₄ ⁺And PO₄ ^3-The treatment process comprises the following steps:

in a BMED system, a peristaltic pump is used to form a circulation loop between a wastewater tank and a wastewater chamber in the BMED, so that NH in the wastewater tank₄ ⁺And PO₄ ^3-Continuously separated and enriched into an ammonia water chamber and a phosphoric acid chamber in a BMED system, and NH enriched in the form of ammonia water₄ ⁺And PO enriched in the form of phosphoric acid₄ ^3-And continuously transmitted to the MAC. When NH in MAC₄ ⁺And PO₄ ^3-When the concentration is more than 3mmol/L, connecting the negative electrode and the positive electrode in the MAC system, and then generating OH through the reaction of equations (2) and (3) in MAC^-And Mg²⁺And in turn struvite is produced by equation (1). Ca in wastewater²⁺Will react with OH in an ammonia chamber^-The binding forms a precipitate that is removed by filtration, thereby reducing its effect on struvite formation and purity in the MAC. The operating principles of the BMED and MAC systems are detailed in fig. 1 and 2.

By adopting the technical scheme, the invention aims at recovering NH in wastewater by traditional struvite crystallization₄ ⁺And PO₄ ^3-The existing problem, combining the performance characteristics of BMED and MAC system, provides an electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite, and NH is treated by BMED₄ ⁺And PO₄ ^3-Separating, enriching and removing Ca²⁺Isoeasily coprecipitates metal ions with struvite via MACProviding OH^-(alkali source) and Mg²⁺(magnesium source), thereby solving the background technical problem existing in the traditional struvite crystallization process and simultaneously obtaining electric energy.

The invention has the following beneficial effects:

1. NH in the wastewater can be recovered in the form of struvite without adding a magnesium source and an alkali source₄ ⁺And PO₄ ^3-The method saves the addition cost of chemical reagents and solves the problem of adding magnesium source and alkali source in the traditional struvite crystallization method.

2. Solves the problem that the traditional struvite crystallization method can not recover low-concentration NH in wastewater in the form of struvite₄ ⁺And PO₄ ^3-And (5) problems are solved.

3. Solves the problem that Ca is generated in the traditional struvite crystallization process²⁺The formation and influence of struvite crystals.

4. In the method, clean energy, namely electric energy, can be recycled.

Drawings

Fig. 1 is a schematic diagram of the structural principle of the BMED serial MAC system according to the present invention.

Fig. 2 is a schematic diagram of the operation principle of the MAC system according to the present invention.

FIG. 3 is a graph showing the results of XRD analysis of the precipitate obtained in example 1 and commercial struvite.

FIG. 4 is a graph showing the results of electron microscopy-EDS analysis of the precipitate obtained in example 1 and commercial struvite, (a) the precipitate obtained in the experiment, and (b) commercial struvite.

Fig. 5 is a graph of the power output of the MAC system of embodiment 1.

FIG. 6 is a plot of the XRD analysis results of commercial struvite from example 2, the precipitate from the MAC system, and the precipitate from the ammonia chamber.

FIG. 7 is an image of a precipitation electron microscope in the MAC system of example 2.

Fig. 8 is a system output power diagram of example 2.

Detailed Description

For a better understanding of the present invention, reference will now be made to the following descriptions taken in conjunction with the accompanying drawings.

The system of the method is mainly formed by connecting a BMED system and an MAC system in series, wherein the BMED system is mainly used for separating and enriching NH in wastewater₄ ⁺、PO₄ ^3-And Ca²⁺The main working principle is shown in figure 1, under the action of electric field force, NH₄ ⁺And Ca²⁺Migrate to the cathode through the cation exchange membrane, respectively with the OH generated by the BPM^-Combined synthesis of ammonia and Ca (OH)₂Using a filter pair Ca (OH)₂Filtering and removing, and conveying ammonia water into the MAC system by a peristaltic pump. PO (PO)₄ ^3-Migration to the anode through the anion exchange membrane into the phosphate compartment and BPM generated H⁺Combined into phosphoric acid, a peristaltic pump delivers the phosphoric acid into the MAC system. The current density of the BMED system is 2-6 mA/cm²The Hydraulic Retention Time (HRT) of the ammonia water chamber and the phosphoric acid chamber is 15-60 min, and constant current is provided by a direct current stabilized power supply.

As shown in FIG. 2, the MAC system can spontaneously generate OH^-And Mg²⁺Can provide Mg for struvite crystallization²⁺And an alkaline environment, and obtains electric energy. To ensure higher NH in MAC system₄ ⁺And PO₄ ^3-Concentration in NH₄ ⁺And PO₄ ^3-During recovery, BMED was working continuously, while MAC was working intermittently. When NH in the MAC system₄ ⁺And PO₄ ^3-Starting MAC system (connecting cathode and anode) when the concentration is more than 3mmol/L, and when PO is in use₄ ^3-And when the concentration is lower than 0.5mmol/L, closing the MAC system (disconnecting the cathode and the anode), wherein the HRT in the MAC system and the struvite precipitation chamber is 45-180 min, and the water bath temperature of the MAC is 15-35 ℃.

Example 1

The experimental device of the present example is shown in FIG. 1.

First, use BMED to treat low NH concentration₄ ⁺(2mmol/L) and PO₄ ^3-(1mmol/L) and pumped to the MAC system (NH)₄ ⁺And PO₄ ^3-Initial concentrations of 2 and 1mmol/L, respectively) when PO is present in the MAC system₄ ^3-A concentration higher than 3mmolL, turning on the positive and negative poles of the MAC system, and when PO is in the MAC system₄ ^3-When the concentration is lower than 0.5mmol/L, the anode and the cathode are disconnected, and when PO in the MAC system₄ ^3-When the concentration rises to more than 3mmol/L again, the MAC anode and the MAC cathode are connected to form an intermittent treatment system, HRT of a BMED (water vapor ion exchange) aqua and a phosphoric acid chamber is 15min, and the BMED current density is 4mA/cm². NH in the wastewater tank and MAC System after 199h₄ ⁺And PO₄ ^3-The total removal rate is 94 percent and 93 percent respectively, struvite with the purity of 86.2 percent can be recovered, and electric energy with the output power of 3.56 +/-0.37 mW is obtained. X-ray diffraction (XRD), electron microscopy-EDS (SEM-EDS) analysis and system output power results for precipitated and commercial struvite obtained with the MAC system are shown in FIGS. 3, 4 and 5.

NH-containing gas used in this example₄ ⁺And PO₄ ^3-The wastewater is simulated wastewater prepared in a laboratory.

Example 2

The experimental device of the present example is shown in FIG. 1.

To verify Ca resistance of the coupled system²⁺Interference capability, preparing electrolyte Na₂SO₄Concentration 2mmol/L, NH₄ ⁺、PO₄ ^3-And Ca²⁺The concentrations of the simulated wastewater are respectively 10, 5.0 and 2.0mmol/L, and the BMED current density is 4mA/cm²System NH after 124h under HRT 15min of ammonia water chamber and phosphoric acid chamber₄ ⁺And PO₄ ^3-The removal rates are respectively 95 percent and 76 percent, and the wastewater tank Ca²⁺The removal rate is 83 percent, and struvite with the purity of 89.1 percent and electric energy with the output power of 2.39 +/-0.85 mW are obtained in the MAC system. The results of X-ray diffraction (XRD), electron microscopy spectroscopy (SEM-EDS) analysis, and power output of the system for the precipitate obtained with the MAC system, the precipitate obtained with the ammonia chamber, and commercial struvite are shown in FIGS. 6, 7, and 8.

NH-containing gas used in this example₄ ⁺、PO₄ ^3-And Ca²⁺The wastewater is simulated wastewater prepared in a laboratory.

Claims

1. An electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite is characterized in that a bipolar membrane electrodialysis system is used for treating NH in wastewater₄ ⁺And PO₄ ^3-Separating, enriching and concentrating, and then utilizing a peristaltic pump to concentrate the enriched NH₄ ⁺And PO₄ ^3-Pumping to magnesium-air battery system, and Mg generated by magnesium-air battery system²⁺Reacting to produce struvite while using OH produced by bipolar membrane^-Ca enriched by electrodialysis with bipolar membranes²⁺Reaction to form Ca (OH)₂Precipitating, filtering to remove Ca (OH)₂Reduction of Ca²⁺Influence on the purity of struvite and simultaneously obtain electric energy.

2. The electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite according to claim 1, wherein the bipolar membrane electrodialysis system is implemented by combining an electrolysis cell with a bipolar membrane and an anion-cation exchange membrane, and using ruthenium iridium titanium plates as electrodes, the left and right ends of the electrolysis cell are provided with a cathode and an anode, which are respectively connected with a cathode and an anode of a power supply, and the bipolar membrane, the cation-exchange membrane, the anion-exchange membrane and the bipolar membrane are sequentially arranged between the cathode and the anode, so that a cathode chamber, an ammonia water chamber, a wastewater chamber, a phosphoric acid chamber and an anode chamber are sequentially formed between adjacent membranes from left to right.

3. The electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite according to claim 2, wherein the magnesium-air battery system is characterized in that magnesium strips and titanium plates are respectively used as a negative electrode and a positive electrode, and respectively form a closed loop with an external resistor, and an ammeter and a voltmeter are simultaneously used for measuring the output current and voltage of the magnesium-air battery system.

4. An electrochemical process for the recovery of low concentrations of nitrogen and phosphorus in wastewater in the form of struvite according to claim 3 wherein the wastewater tank and the pair of peristaltic pumps are used to pump the wastewater tank and pairThe waste water chamber in the polar membrane electrodialysis system forms a circulation loop, so that NH in the waste water tank₄ ⁺And PO₄ ^3-Continuously separated and enriched into an ammonia water chamber and a phosphoric acid chamber in a bipolar membrane electrodialysis system, and NH enriched in the form of ammonia water₄ ⁺And PO enriched in the form of phosphoric acid₄ ^3-Continuously delivering the magnesium alloy to a magnesium-air battery system, wherein Mg in the magnesium-air battery system loses electrons and is changed into Mg²⁺And O dissolved in the solution₂On electron accepting reaction to produce OH^-And further produces struvite, Ca in wastewater, by equation (1)²⁺In an ammonia water chamber with OH^-The binding forms a precipitate, which is removed by filtration.

Mg²⁺+NH₄ ⁺+H_nPO₄ ^n-3+6H₂O→MgNH₄PO₄·6H₂O+nH⁺n is 0,1, or 2 (1).

5. The electrochemical process for the recovery of low concentrations of nitrogen and phosphorus in wastewater as struvite of claim 4, wherein the bipolar membrane electrodialysis system is operated continuously, and the magnesium-air battery system is started intermittently; when the magnesium-air battery system NH₄ ⁺And PO₄ ^3-When the concentration is more than 3mmol/L, connecting the cathode and the anode of the magnesium-air battery system, and spontaneously generating Mg required by the struvite reaction by using the magnesium-air battery system²⁺Form of magnesium source and OH required for struvite reaction^-Basic environment of the form, NH₄ ⁺And PO₄ ^3-With Mg²⁺The reaction produces struvite.

6. The electrochemical process for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite according to claim 4, wherein the current density of the bipolar membrane electrodialysis system is 2-6 mA/cm²。

7. The electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite according to claim 4, wherein the effluent of the magnesium-air battery system enters the ammonia water chamber and the phosphoric acid chamber of the bipolar membrane electrodialysis system respectively to form a circulating system, so as to realize zero discharge of the effluent in the magnesium-air battery system.

8. The electrochemical process for recovering low concentrations of nitrogen and phosphorus in wastewater as struvite of claim 4, wherein the bipolar membrane electrodialysis system is enriched in NH₄ ⁺In the process of entering the magnesium-air battery system, a first filter is arranged for filtering Ca (OH)₂Precipitation, prevention of Ca (OH)₂Entering a magnesium-air battery system to affect the struvite purity.

9. An electrochemical process for the recovery of low concentrations of nitrogen and phosphorus in wastewater as struvite according to claim 4, wherein a second filter is provided for filtering the non-precipitated struvite from entering the ammonia and phosphoric acid compartments of the bipolar membrane electrodialysis system, and preventing the struvite from entering the bipolar membrane electrodialysis system again.

10. The electrochemical method for recovering low-concentration nitrogen and phosphorus in wastewater in the form of struvite according to claim 4, wherein the magnesium-air battery system is externally connected with a resistor, and the output current and voltage of the magnesium-air battery system are respectively recorded by an ammeter and a voltmeter and are used for calculating the electricity generation performance of the magnesium-air battery system.