System and method for treating tap water with excess nitrate

Abstract

The application discloses a treatment system and method for tap water with excessive nitrate. The treatment system comprises a nitrate separation device (1), a frequently reversed electrode electrodialysis device (2), a nanofiltration salt separation device (3) and a water production tank (5). The treatment method comprises the following steps: performing nitrate separation treatment on raw water to obtain concentrated water and pre-production water; passing the concentrated water into the frequently reversed electrode electrodialysis device (2) to obtain low-salt wastewater and high-salt solution; passing the high-salt solution into the nanofiltration salt separation device (3) to obtain permeate and separation liquid; and mixing the separation liquid with the pre-production water as production water. The application is mainly used for treating tap water with excessive nitrate in a tap water plant, and subsequent treatment of concentrated water after removal of nitrate, adopts a salt separation reflux mode, refluxes divalent ions beneficial to human health and pipe network stability in the concentrated water into tap water production water, and makes the TDS content of the nanofiltration concentrated water meet environmental protection requirements after treatment.

Description

Technical Field

[0001] This invention specifically relates to a system and method for treating tap water with excessive nitrates. Background Technology

[0002] When the nitrate level in the raw water of a water treatment plant exceeds the standard, conventional treatment and ozone activated carbon deep treatment cannot remove inorganic salts. It is necessary to use dense nanofiltration membranes or reverse osmosis processes to remove nitrates and ensure that the effluent water quality meets the standards.

[0003] As high-quality permeate water continuously passes through the nanofiltration membrane during the nanofiltration process, the feed water is continuously concentrated, eventually forming concentrate (effluent) with high salt and organic matter content. If the nanofiltration or reverse osmosis recovery rate is 85% for the purpose of removing nitrates, the TDS content in the concentrate will be 6-7 times higher than that in the feed water.

[0004] In some areas, the total dissolved solids (TDS) content in raw water is high. After nanofiltration or reverse osmosis treatment, the TDS content in the concentrate often exceeds 300 mg / L, failing to meet local environmental emission standards. This creates a conflict between the need to improve drinking water quality and the environmental protection industry's emission regulations. While dense nanofiltration membranes or reverse osmosis systems remove nitrates, they also remove divalent ions that should not be removed, are beneficial to human health, and are related to the chemical stability of the effluent. This can actually be detrimental to the quality of tap water, and tap water with excessively removed ions will have poor water quality stability after entering the corresponding pipe network.

[0005] For the removal of TDS from nanofiltration or reverse osmosis concentrate, industrial wastewater often employs salt separation crystallization, where the crystallized salt can be reused. However, this method typically involves high equipment investment and energy consumption, and the disposal of the crystallized salt needs to be addressed. For water treatment plants, even if the TDS content of nanofiltration effluent does not meet local environmental emission standards, its salinity is much lower than that of industrial wastewater. Therefore, salt separation crystallization is energy-intensive and unsuitable. Even if salt separation crystallization is used to treat the concentrate, the disposal of the crystallized salt is often difficult to resolve, hindering the widespread adoption of salt separation crystallization in water treatment plants and limiting the promotion of nanofiltration technology. Summary of the Invention

[0006] The technical problem this invention aims to solve is to overcome the shortcomings of existing nanofiltration or reverse osmosis methods for removing TDS from concentrated water, and to provide a treatment system and method for tap water with excessive nitrate. This invention is mainly used for the subsequent treatment of concentrated water after nanofiltration nitrate removal in water treatment plants. It employs a salt separation and reflux method, returning divalent ions beneficial to human health and pipeline stability from the concentrated water to the tap water production water, ensuring that the TDS content of the nanofiltration concentrated water meets environmental protection requirements after treatment.

[0007] This invention specifically separates Ca from nanofiltration concentrate. 2+ Mg 2+SO4 2- CO3 2- The divalent ions that do not need to be removed are returned to the tap water, and the Na+ in the concentrated water is also removed. + K + NO3 - Cl - This invention separates monovalent ions and dissolved organic matter, facilitating their classification and disposal, and ensuring that the TDS discharged from the nanofiltration stage of the water plant meets local environmental emission standards. It offers advantages such as precise targeting, low energy consumption, high economic efficiency, and high feasibility, providing a novel approach to the treatment of concentrated wastewater from nanofiltration or reverse osmosis in water treatment plants.

[0008] The objective of this invention is achieved through the following technical solutions.

[0009] In a first aspect, the present invention provides a treatment system for tap water with excessive nitrate, comprising a nitrate separation device, a frequent reverse electrodialysis device, a nanofiltration desalination device, and a product water tank.

[0010] The nitrate separation device has a raw water inlet, a concentrated water outlet and a pre-product water outlet, and is used to separate raw water into concentrated water and pre-product water.

[0011] The frequent polarity reversal electrodialysis device has a concentrated water inlet, a high-salt solution outlet and a low-salt wastewater outlet, used to separate the concentrated water after nitrate removal into a high-salt solution and a low-salt wastewater; the concentrated water inlet is connected to the concentrated water outlet.

[0012] The nanofiltration salt separation device has a high-salt solution inlet, a separation liquid outlet, and a permeate outlet, used to separate the high-salt solution into a separation liquid containing divalent or higher ions and a permeate containing monovalent ions; the high-salt solution outlet is connected to the high-salt solution inlet; the separation liquid outlet and the pre-product water outlet are respectively connected to the product water tank, and the product water tank is used to combine the separation liquid and the pre-product water as product water.

[0013] In this invention, the nitrate separation device can be a nanofiltration device or a reverse osmosis device.

[0014] A pretreatment unit may also be installed upstream of the raw water inlet. The pretreatment unit may include one or more combinations of sedimentation tanks, air flotation machines, ozone contact tanks, advanced oxidation devices, activated carbon adsorption devices, and ultrafiltration membrane devices.

[0015] In some specific embodiments, the pretreatment unit includes a sedimentation tank, an advanced oxidation device, an activated carbon adsorption device, and an ultrafiltration membrane device connected in sequence.

[0016] In some specific embodiments, the sedimentation tank is a flocculation sedimentation tank.

[0017] In some specific embodiments, the activated carbon adsorption device is an upward flow activated carbon adsorption device or a downward flow activated carbon filter.

[0018] In some specific embodiments, the advanced oxidation device is an ultraviolet-hydrogen peroxide advanced oxidation device.

[0019] In some specific embodiments, the ultrafiltration membrane device is an immersion ultrafiltration membrane device or a pressure ultrafiltration membrane device.

[0020] In this invention, such as Figure 3 The frequent reversal electrodialysis device may include a negative electrode and a positive electrode, as well as an anolyte and an anion membrane alternately arranged between the negative electrode and the positive electrode;

[0021] An electrode water chamber may be provided between the negative electrode or the positive electrode and the cation membrane or the anion membrane; a concentration chamber and a desalination chamber may be provided between the cation membrane and the anion membrane, and the concentration chamber and the desalination chamber may be arranged alternately;

[0022] The concentration chamber and the desalination chamber may be equipped with nitrate-free concentrated water inlets; the concentration chamber may be equipped with high-concentration salt solution outlets; and the desalination chamber may be equipped with low-salt product water outlets.

[0023] In this invention, the frequent reverse-polarity electrodialysis device employs EDR, or frequent reverse-polarity electrodialysis technology. Utilizing the selectivity of the ion exchange membrane for ions, approximately 90% of the ions can be concentrated in a high-concentration salt solution. While approximately 20% of the organic matter in the nitrate concentrate can enter the high-concentration salt solution, the remaining approximately 10% of the ions and approximately 80% of the organic matter are separated into low-salt permeate, thus achieving the separation of organic matter and ions.

[0024] In this invention, the nanofiltration salt separation device may be equipped with a nanofiltration membrane.

[0025] The nanofiltration membrane can be made of a polypiperazine composite film.

[0026] The nanofiltration membrane can separate ions, including nitrates, in the water, reducing the nitrate content in the effluent. Divalent and higher ions that do not need to be removed in the effluent are also removed at the same time. Meanwhile, the concentrated water has a high TDS content or exceeds the discharge standard limit, so it cannot be discharged directly and must be further treated.

[0027] In this invention, the nanofiltration desalination device exhibits good separation performance for monovalent and divalent and higher-valence ions, meaning it has different selective permeability to solutes of different valence states, with a higher rejection rate for divalent and higher-valence ions than for monovalent ions. The separated liquid, with a high content of divalent and higher-valence ions, is mixed with pre-processed water to achieve the TDS (Total Discharge Stabilization) standard for the final discharge from the water plant. The nanofiltration desalination device can employ multi-stage nanofiltration, and the nanofiltration membrane array used is a porous nanofiltration membrane capable of separating monovalent, divalent, and higher-valence ions.

[0028] In this invention, the permeate outlet can be connected to a wastewater treatment device. This allows for the separation of Na-containing substances from the concentrated water. + K + NO3 - Cl - The permeate containing monovalent ions can be used partially or entirely for miscellaneous purposes such as landscaping and road watering, depending on the water quality, thus saving water resources and further reducing the TDS in the discharged industrial wastewater.

[0029] In this invention, an external discharge branch can be provided on the pipeline between the separated liquid outlet and the product water tank. This allows the separated liquid outlet to be connected to the wastewater treatment device for the separation of Ca-containing substances from the concentrated water. 2+ Mg 2+ SO4 2- CO3 2- Separation liquid containing divalent or higher ions is partially or completely recycled back to the tap water production device; on the one hand, this reduces the TDS concentration in the discharged production wastewater to meet local environmental emission standards, and on the other hand, it improves the chemical stability of the effluent from the tap water production device.

[0030] In this invention, the low-salt wastewater outlet can be connected to the wastewater treatment device.

[0031] In this invention, an organic matter removal device may be installed on the connecting pipeline between the separated liquid outlet and the pre-produced water outlet. Since the separated liquid often contains some charged organic matter, an organic matter removal device can be used to remove the organic matter before recirculation, thereby further improving the effluent quality of the tap water production device.

[0032] An external discharge branch may also be provided on the pipeline between the separation liquid outlet and the organic matter removal device.

[0033] The organic matter removal device may be an ultraviolet light-hydrogen peroxide advanced oxidation device or an activated carbon adsorption device.

[0034] Secondly, the present invention provides a method for treating tap water with excessive nitrates, comprising the following steps:

[0035] The raw water is treated to separate nitrates, yielding concentrated water and pre-product water respectively.

[0036] The concentrated water is passed into a frequent reverse electrodialysis device to obtain low-salt wastewater and high-salt solution, respectively.

[0037] The high-salt solution is passed into a nanofiltration salt separation device to obtain permeate and separated liquid, respectively.

[0038] The separated liquid is mixed with the pre-product water to obtain the product water.

[0039] In this invention, the nitrate separation process can be nanofiltration or reverse osmosis.

[0040] In this invention, a pretreatment process may be included before the nitrate separation treatment. The pretreatment process is used to remove pollutants such as turbidity and organic matter from the water.

[0041] The pretreatment process includes one or more combinations of sedimentation, flotation, filtration, ozone contact, advanced oxidation, activated carbon adsorption, and ultrafiltration.

[0042] The advanced oxidation can be ultraviolet-hydrogen peroxide advanced oxidation.

[0043] In some specific embodiments, the pretreatment process sequentially includes precipitation, advanced oxidation, activated carbon adsorption, ultrafiltration, and deep separation.

[0044] In some specific embodiments, the UV light intensity of the ultraviolet-hydrogen peroxide advanced oxidation can be 300-500 mJ / cm². 2 For example, 400mJ / cm 2 .

[0045] In some specific embodiments, the UV transmittance of the UV-hydrogen peroxide advanced oxidation can be ≥85%.

[0046] In some specific embodiments, the maximum dosage of H2O2 from the ultraviolet-hydrogen peroxide advanced oxidation can be 6 mg / L.

[0047] In some specific embodiments, the ultrafiltration membrane has a flux of 30-40 L / h, for example, 35 L / h;

[0048] In some specific embodiments, the maximum permissible transmembrane pressure difference of the ultrafiltration membrane can be 0.06 MPa.

[0049] In some specific embodiments, the ultrafiltration membrane may be made of PVDF material.

[0050] In some specific embodiments, the reverse osmosis flux may be 15-20 L / h, for example 19.98 L / h.

[0051] In some specific embodiments, the inlet pressure of the reverse osmosis can be 1-2 MPa.

[0052] In this invention, the inlet water pressure of the frequent polarity reversal electrodialysis device can be 0.2-0.3 MPa, for example 0.25 MPa.

[0053] In this invention, the effective membrane area in the frequent polarity reversal electrodialysis device can be 250-350 m². 2 For example, 300m 2 Effective membrane area refers to the actual effective membrane area in a frequently reversed electrodialysis device.

[0054] In this invention, the reversal cycle of the frequent reversal electrodialysis device can be 15-25 minutes, for example, 20 minutes.

[0055] In this invention, the inlet water pressure of the nanofiltration desalination device can be 0.5-0.8 MPa, for example 0.6 MPa.

[0056] In this invention, the nanofiltration salt separation device may be equipped with a nanofiltration membrane.

[0057] The nanofiltration membrane can be made of a polypiperazine composite film.

[0058] The maximum pressure drop of the nanofiltration membrane can be 0.1 MPa.

[0059] The nanofiltration membrane flux can be 15-20 L / h, for example 19 L / h.

[0060] In this invention, the separated liquid can be mixed with the pre-produced water after organic matter has been removed.

[0061] The organic matter removal method may include advanced oxidation or activated carbon adsorption. The advanced oxidation may be ultraviolet-hydrogen peroxide advanced oxidation.

[0062] In this invention, 80%-100% of the separated liquid can be mixed with the pre-product water.

[0063] In this invention, the removal of organic matter can further improve the quality of the reflux water. The removal of organic matter can be achieved through various methods, such as advanced oxidation and activated carbon adsorption. Advanced oxidation technology is an existing technology; for example, see "Research on the Removal of Typical Odor-Causing Substances from Drinking Water Using Ultraviolet Light-Hydrogen Peroxide Advanced Oxidation Technology".

[0064] In this invention, the volume ratio of the low-salt wastewater to the high-salt solution can be 18000:(3100-3200), for example, 18000:3176.

[0065] In this invention, the volume ratio of the permeate to the separation liquid can be 2700:(450-500), for example 2700:476.

[0066] In this invention, the nitrate content in the raw water can exceed 10.0 mg / L, preferably 15.0-20.0 mg / L, for example 19.0 mg / L.

[0067] In this invention, the organic matter COD in the raw water... Mn The content can be 4.50-5.00 mg / L, for example 4.61 mg / L.

[0068] In this invention, the TDS content in the raw water can exceed 250 mg / L, preferably 350.0-400.0 mg / L, for example 397.6 mg / L. The mass percentage of monovalent and divalent or higher ions can be 48-52%, for example 50%.

[0069] In this invention, the TDS content in the concentrated water can be 2500-2700 mg / L, for example 2605 mg / L.

[0070] In this invention, the concentrated water contains organic matter COD Cr The concentration can be 40-60 mg / L, for example, 50 mg / L.

[0071] In this invention, the nitrate content in the concentrated water can be 110-120 g / L, for example 115.7 g / L.

[0072] In this invention, the TDS content in the pre-production water can be 150-200 mg / L, for example 163 mg / L.

[0073] In this invention, the organic matter COD in the pre-production water... Mn The concentration can be 0.9-1.1 mg / L, for example, 1 mg / L.

[0074] In this invention, the nitrate content in the pre-production water can be less than 10 g / L, preferably 8-9 g / L, for example 8.76 g / L.

[0075] In this invention, the TDS content in the low-salt wastewater can be 300-350 mg / L, for example, 306 mg / L.

[0076] In this invention, the organic matter COD in the low-salt wastewater is... Cr The concentration can be 40-60 mg / L, for example, 50 mg / L.

[0077] In this invention, the TDS content in the high-salt solution can be 15000-16000 mg / L, for example 15630 mg / L.

[0078] In this invention, the high-salt solution contains organic matter with a COD of [missing information]. Cr The concentration can be 40-60 mg / L, for example, 50 mg / L.

[0079] In this invention, the TDS content in the permeate can be 8200-8300 mg / L, for example 8275 mg / L.

[0080] In this invention, the organic matter COD in the permeate is... Cr The content may not exceed 5 mg / L, for example, 0 mg / L.

[0081] In this invention, the TDS content in the separation liquid can be 55,000-60,000 mg / L, for example 57,310 mg / L.

[0082] In this invention, the organic matter COD in the separation liquid is... Cr The content can be 300-400 mg / L, for example 333 mg / L.

[0083] In this invention, the nitrate content in the separation liquid can be 20-25 g / L, for example 20.8 g / L.

[0084] In this invention, the TDS content in the product water can be 250-350 mg / L, for example 299 mg / L.

[0085] In this invention, the organic matter COD in the produced water... Mn The content may be less than 3 mg / L, preferably less than 1.8 mg / L, and more preferably less than 1.24 mg / L.

[0086] In this invention, the nitrate content in the produced water can be 8-9 g / L, for example 8.79 g / L.

[0087] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0088] The reagents and raw materials used in this invention are all commercially available.

[0089] The positive and progressive effects of this invention are as follows:

[0090] (1) In this invention, monovalent ions, divalent and higher ions, and organic matter in the concentrated water are separated and classified accordingly: monovalent ions containing nitrates are discharged, divalent and higher ions are recycled back to the product water, and most of the organic matter in the concentrated water is discharged, that is, it is discharged with the low-salt wastewater in the EDR stage, and a small part can be further removed in the divalent and higher ion separation liquid using other technologies. Thus, the goal of achieving high-quality effluent and meeting discharge standards is achieved.

[0091] (2) In this invention, the treatment method for nitrate-removed concentrated water is different from the traditional treatment method of evaporation and crystallization after salt separation. The treatment system for concentrated water after nitrate removal by nanofiltration or reverse osmosis in waterworks mentioned in this invention is composed of a combination of a frequent-reverse electrodialysis device (EDR) and a nanofiltration membrane module. It separates divalent and higher ions that do not need to be removed and returns them to the water production device, reducing the TDS discharge of concentrated water and meeting environmental protection requirements. The permeate containing nitrate and the low-salt production water containing dissolved organic matter can be used for comprehensive utilization such as greening and irrigation or discharged. This can increase the water recovery rate of nanofiltration or reverse osmosis in waterworks to 90-95%, saving water resources. The energy consumption per cubic meter of concentrated water treated can be reduced by 80% compared to the evaporation and crystallization technology. Under the premise of ensuring the quality of water supply and meeting environmental discharge requirements, the energy consumption for controlling the TDS of concentrated water is greatly reduced, making it green, low-carbon and environmentally friendly.

[0092] (3) In this invention, the treatment process for concentrated water after nitrate removal by nanofiltration or reverse osmosis in waterworks can also have water quality monitoring functions, and can automatically adjust operating parameters according to the salt separation effect. The relatively energy-efficient EDR technology can efficiently separate low-salt permeate containing organic matter for discharge, while the remaining high-concentration salt solution is concentrated 6-7 times relative to the concentrated water, greatly reducing the treatment scale and energy consumption of subsequent nanofiltration salt separation devices. Water quality monitoring indicators may include turbidity, particulate matter, nitrate, and COD. Mn Indicators such as TDS are monitored, and the commercially available monitoring equipment selected can achieve unmanned online monitoring.

[0093] (4) In summary, the present invention uses the separation of dissolved organic matter, divalent and above ions and monovalent ions, and the return of the separated liquid containing divalent and above ions to the tap water production device. In addition to meeting the environmental protection emission requirements, it also has the following three advantages: it can retain the inorganic components in the water that are beneficial to human health, it is more conducive to maintaining the chemical stability in the water supply network system, and it can also avoid the problem of disposing of solid salt after traditional salt separation and crystallization. Attached Figure Description

[0094] Figure 1 This is a schematic diagram of the processing system in Example 1;

[0095] Figure 2 This is a schematic diagram of the processing system in Example 2;

[0096] Figure 3 This is a schematic diagram of the frequent polarity reversal electrodialysis device in Examples 1-2;

[0097] The diagram shows the following components: 1. Nitrate separation unit; 11. Raw water inlet; 12. Concentrate outlet; 13. Pre-product water outlet; 2. Frequent polarity reversal electrodialysis unit; 21. Concentrate inlet; 22. High-salt solution outlet; 23. Low-salt wastewater outlet; 3. Nanofiltration desalination unit; 31. High-salt solution inlet; 32. Permeate outlet; 33. Separated liquid outlet; 4. Organic matter removal unit; 5. Product water tank. Detailed Implementation

[0098] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0099] Generally speaking, organic matter in raw water and drinking water is measured by COD. Mn This indicates that organic matter in wastewater is measured using COD (Chemical Oxygen Demand). Cr It was stated that the reagents and methods used for the two tests were different.

[0100] Example 1

[0101] First, the nitrate-excessive tap water treatment system in this embodiment, such as... Figure 1 The system includes a nitrate separation unit 1, a frequent-reverse electrodialysis unit 2, a nanofiltration desalination unit 3, and a product water tank 5. The nitrate separation unit 1 has a raw water inlet 11, a concentrated water outlet 12, and a pre-product water outlet 13, used to separate the raw water into concentrated water and pre-product water. The frequent-reverse electrodialysis unit 2 has a concentrated water inlet 21, a high-salt solution outlet 22, and a low-salt wastewater outlet 23, used to separate the nitrate-removed concentrated water into a high-salt solution and low-salt wastewater. The concentrated water inlet 21 is connected to the concentrated water outlet 12. The nanofiltration desalination unit 3 has a high-salt solution inlet 31, a separated liquid outlet 33, and a permeate outlet 32, used to separate the high-salt solution into a separated liquid containing divalent or higher ions and a permeate containing monovalent ions. The high-salt solution outlet 22 is connected to the high-salt solution inlet 31. The separated liquid outlet 33 and the pre-product water outlet 13 are respectively connected to the product water tank 5, which is used to combine the separated liquid and the pre-product water as product water.

[0102] Nitrate separation unit 1 is a reverse osmosis unit. A pretreatment unit is also provided upstream of the raw water inlet 11; the pretreatment unit includes a flocculation sedimentation tank, an ultraviolet hydrogen peroxide advanced oxidation unit, an upflow activated carbon adsorption unit and an ultrafiltration membrane unit connected in sequence; the nanofiltration desalination unit 3 is equipped with a nanofiltration membrane; the nanofiltration membrane is made of polypiperazine composite membrane.

[0103] Second, the treatment system described in this embodiment is used to treat the concentrated water after nitrate removal from the raw water at a certain waterworks using reverse osmosis.

[0104] A waterworks in Shandong Province, with a capacity of 200,000 cubic meters.3 The nitrate content in the raw water exceeds 10 mg / L as specified in GB5749-2022. Based on precipitation, advanced oxidation, activated carbon adsorption, and ultrafiltration, further treatment of 120,000 m³ of nitrate is required. 3 The per-day permeate water uses an additional reverse osmosis process to remove nitrates from the raw water.

[0105] 1. The main water quality parameters of the raw water are as follows:

[0106] Nitrate 19.0 mg / L, COD Mn The total salt concentration was 4.61 mg / L, and the total TDS content was 397.6 mg / L. The monovalent ions and divalent and higher ions each accounted for approximately 50%, as detailed in Table 1.

[0107] 2. The specific process flow of the water purification process in the water plant is as follows:

[0108] Flocculation sedimentation tank + UV hydrogen peroxide advanced oxidation device + upflow activated carbon adsorption device + submerged ultrafiltration membrane device + reverse osmosis device.

[0109] For the UV-hydrogen peroxide advanced oxidation unit: Due to the high concentration of bromide ions in the raw water, UV-hydrogen peroxide advanced oxidation is used instead of ozone oxidation in the process. The UV light intensity for the UV-hydrogen peroxide advanced oxidation unit is designed to be 400 mJ / cm². 2 The ultraviolet transmittance is ≥85%; the maximum dosage of H2O2 is 6mg / L, and it should be diluted to a concentration of about 1wt% before addition.

[0110] For submerged ultrafiltration membrane devices: the submerged ultrafiltration membrane is equipped with 12 membrane cells, with a design flux of 35 L / h and a maximum allowable transmembrane pressure difference of 0.06 MPa. The ultrafiltration membrane is made of PVDF material.

[0111] For reverse osmosis units: the required feed sludge density index (SDI) is <3, the design flux is 19.98 L / h, the number of membrane modules is 12, and the permeate flow rate per module is 10,000 m³. 3 / d, inlet water pressure 1MPa, adopts two-stage series; each membrane module is equipped with one security filter with a filtration accuracy of 5μm; high pressure pump head 100m; inter-stage booster pump head 20m; 85 membrane housings (6 cores), containing a total of 510 membrane cores, with a final water recovery rate of approximately 85%.

[0112] 3. 120,000 m 3 The water quality of the pre-permeate and concentrate after reverse osmosis is as follows:

[0113] (1) Pre-production water: nitrate 1.93 mg / L, total dissolved solids (TDS) 6.81 mg / L, and another 80,000 m³ 3 After mixing with ozone and activated carbon, the effluent contained 8.76 mg / L of nitrate, 163 mg / L of total dissolved solids (TDS), and COD.Mn Approximately 1 mg / L, see Table 1 for details.

[0114] (2) Concentrated water: Nitrate 115.7 mg / L, Total Salt Content (TDS) 2605 mg / L, Organic Matter Content (COD) Cr The concentration of the concentrated wastewater is approximately 50 mg / L, as detailed in Table 1. This concentrated wastewater does not meet Shandong Province's environmental protection discharge requirement of no more than 1600 mg / L for TDS. The volume is approximately 21176 m³. 3 / d.

[0115] Table 1. Main water quality parameters of raw water, pre-production water, and concentrate.

[0116] index raw water Pre-production water Concentrated water Nitrate mg / L 19.0 8.76 115.7 Divalent and higher salts mg / L 199 80 1325 TDS mg / L 397.6 163 2605 organic matter mg / L <![CDATA[COD Mn Approximately 4.61 <![CDATA[COD Mn Approximately 1.0]]> <![CDATA[COD Cr Approximately 50

[0117] Third, according to the process route of this invention, a frequent-reversal electrodialysis device is first used to separate organic matter and ions from the concentrated water. Organic matter, colloids, etc., flow out and are discharged with the product water. The retained high-concentration salt solution then enters a nanofiltration desalination device for the separation of monovalent, divalent, and higher-valent ions. The process includes the following steps: The concentrated water is passed into the frequent-reversal electrodialysis device 2 to obtain low-salt wastewater and a high-salt solution; the high-salt solution is passed into the nanofiltration desalination device 3 to obtain permeate and separated liquid; the separated liquid is mixed with the pre-product water to obtain the product water. The specific treatment details are as follows:

[0118] 1. Water quality analysis of influent and effluent from a frequent polarity reversal electrodialysis device

[0119] Frequent electrode reversal electrodialysis devices can achieve a desalination rate of 85%-95% and a water production rate of 80%-95%. In this embodiment, the frequent electrode reversal electrodialysis device uses two sections connected in series, with an inlet water pressure of 0.25 MPa and an effective membrane area of ​​approximately 300 m². 2 The reversal cycle is 20 minutes, and automatic control is used. In this embodiment, the frequent reversal electrodialysis device achieves a desalination rate of 90% and a water production rate of 85%. After passing through the frequent reversal electrodialysis device, the wastewater is separated into two parts: 18,000 m³ of low-salt wastewater containing organic matter and colloids. 3 / d, salt content 306mg / L, organic matter content 50mg / L; 3176m of high-salt solution was retained. 3 / d, salt content 15630mg / L, organic matter content (COD) Cr Approximately 50 mg / L, see Table 2 for details.

[0120] Table 2 Water quality parameters of inlet and outlet water for frequent polarity reversal electrodialysis devices

[0121]

[0122] 2. Water quality analysis of influent and effluent from nanofiltration desalination unit

[0123] 3176m of high-salt solution was retained. 3 The water then enters the nanofiltration desalination unit. The nanofiltration desalination unit achieves a removal rate of 20%-50% for monovalent ions and as high as 80%-90% or even higher for divalent and higher ions and organic matter, with a comprehensive desalination efficiency of 70%-85%. For the nanofiltration desalination unit in this embodiment, a polypiperazine composite membrane material with high desalination efficiency for monovalent and divalent and higher ions is selected. The maximum pressure drop is 0.1 MPa, with 10 membrane stacks, each employing a three-stage nanofiltration unit, a membrane flux of 19 L / h, and an inlet pressure of 0.6 MPa. The water recovery rate of the nanofiltration desalination unit in this embodiment is 85%, the removal rate for monovalent ions is 20%, the removal rate for divalent and higher ions is 90%, and organic matter is almost completely removed.

[0124] After passing through the nanofiltration desalination unit, the high-salt solution is divided into two parts: the permeate, which mainly contains monovalent ions, is mixed with the permeate from the frequent-reversal electrodialysis unit and then discharged at a flow rate of 2700 m³ / h. 3 / d, TDS is 8275mg / L; the separated liquid is to be refluxed and mixed with the pre-product water, mainly containing divalent and higher ions, as well as a small amount of organic matter not removed by the frequent reverse-polarity electrodialysis unit, with a flow rate of 476m³ / d. 3 / d, TDS was 57310 mg / L (of which nitrate content was estimated to be approximately 20.8 mg / L), organic matter content (COD) Cr Approximately 333 mg / L, see Table 3 for details.

[0125] Table 3. Water quality parameters of influent and effluent from the nanofiltration salinization unit

[0126] index high salt solution Permeable liquid Separation liquid <![CDATA[Flow rate m 3 / d]]> 3176 2700 476 Nitrate mg / L 694.2 653.3 Approximately 20.8 Divalent and higher salts mg / L 7952 935.5 47712 TDS mg / L 15630 8275 57310 <![CDATA[COD Cr mg / L]]> Approximately 50 Approximately 0 Approximately 333

[0127] 3. Analysis of refluxed liquid and water quality after reflux

[0128] The reflux liquid is the separation liquid from the nanofiltration salt separation unit, with a flow rate of 476 m³ / h. 3 / d, TDS approximately 57310 mg / L (including an estimated total nitrate content of 20.8 mg / L), organic matter content (COD) Cr Approximately 333 mg / L, see Table 5 for details.

[0129] The selection of the reflux ratio should be comprehensively determined based on the water quality of the reflux liquid, pre-processed water, and discharged wastewater, generally ranging from 80% to 100%. It is also important to ensure that the nitrate and COD levels in the refluxed product water are within acceptable limits. Mn The levels of TDS and other substances in the discharged wastewater all meet the relevant requirements of the "Standards for Drinking Water Quality" (GB5749-2022) and maintain chemical stability. CrThe water meets the Shandong Provincial Local Standard "Integrated Discharge Standard for Water Pollutants in River Basins" (DB37 3416.1~5). In this embodiment, all the separated liquid from the nanofiltration desalination device is recycled. Through recycling, the re-entry of divalent and higher ions into the product water makes the TDS of the product water closer to the level of the raw water, which is more conducive to maintaining the chemical stability within the existing pipeline network system, as detailed in Table 4.

[0130] Table 4. Water quality parameters of pre-product water and product water after reflux.

[0131] index Pre-production water water production GB5749-2022 Limits Nitrate mg / L 8.76 8.79 10 Divalent and higher salts mg / L 79.5 192.7 - TDS mg / L 163 299 1000 <![CDATA[COD Mn mg / L]]> Approximately 1.0 <1.8 3

[0132] 4. Wastewater quality analysis

[0133] The low-salt wastewater from the frequent electrode-switching electrodialysis device and the permeate from the nanofiltration desalination device of this invention will be comprehensively utilized or discharged as production wastewater, with a volume of 20,700 m³. 3 / d. Water quality status: organic matter content (COD) Cr The concentration of TDS is approximately 43.5 mg / L, and the concentration of TDS is approximately 1346 mg / L. This meets the requirement of the Shandong Provincial Local Standard "Integrated Discharge Standard for Water Pollutants in River Basins" (DB37 3416.1~5), which stipulates that the maximum allowable discharge concentration of TDS is 1600 mg / L. For details, please refer to Table 5.

[0134] Table 5 Water quality parameters of discharged wastewater

[0135] index Concentrated water low-salinity wastewater Permeable liquid Production wastewater DB37 3416.1~5 Limits <![CDATA[Flow rate m 3 / d]]> 18000 2700 20700 TDS mg / L 2605 306 8275 1346 1600 <![CDATA[COD Cr mg / L]]> Approximately 50 Approximately 50 Approximately 0 Approximately 43.5 50～60

[0136] Example 2

[0137] First, the treatment system for concentrated wastewater after nitrate removal from tap water in this embodiment, such as... Figure 2 The system includes a nitrate separation unit 1, a frequent-reverse electrodialysis unit 2, a nanofiltration desalination unit 3, and a product water tank 5. The nitrate separation unit 1 has a raw water inlet 11, a concentrated water outlet 12, and a pre-product water outlet 13, used to separate the raw water into concentrated water and pre-product water. The frequent-reverse electrodialysis unit 2 has a concentrated water inlet 21, a high-salt solution outlet 22, and a low-salt wastewater outlet 23, used to separate the nitrate-removed concentrated water into a high-salt solution and low-salt wastewater. The concentrated water inlet 21 is connected to the concentrated water outlet 12. The nanofiltration desalination unit 3 has a high-salt solution inlet 31, a separated liquid outlet 33, and a permeate outlet 32, used to separate the high-salt solution into a separated liquid containing divalent or higher ions and a permeate containing monovalent ions. The high-salt solution outlet 22 is connected to the high-salt solution inlet 31. The separated liquid outlet 33 and the pre-product water outlet 13 are respectively connected to the product water tank 5, which is used to combine the separated liquid and the pre-product water as product water.

[0138] The nitrate separation unit 1 is a reverse osmosis unit. A pretreatment unit is also located upstream of the raw water inlet 11; the pretreatment unit includes a flocculation sedimentation tank, a UV hydrogen peroxide advanced oxidation unit, an upflow activated carbon adsorption unit, and an ultrafiltration membrane unit connected in sequence; the nanofiltration desalination unit 3 is equipped with a nanofiltration membrane; the nanofiltration membrane is made of polypiperazine composite membrane. An organic matter removal unit 4 is installed on the connecting pipeline between the separated liquid outlet 33 and the pre-product water outlet 13. The organic matter removal unit 4 is an activated carbon adsorption unit.

[0139] Second, the treatment system in this embodiment is used to treat the concentrated water after nitrate removal in Example 1. Activated carbon adsorption is used to remove organic matter from the nanofiltration retentate containing divalent and higher ion-containing ions, significantly reducing the organic matter concentration in the return water and further improving the quality of the water plant's effluent. The treatment method includes the following steps: The concentrated water is passed through a frequent-reversal electrodialysis device 2 to obtain low-salt wastewater and a high-salt solution; the high-salt solution is passed through a nanofiltration device 3 to obtain permeate and separated liquid; the separated liquid is adsorbed by activated carbon and then mixed with pre-product water to obtain the product water. The specific treatment details are as follows:

[0140] The difference from Example 1 is that this example additionally uses powdered activated carbon adsorption on the separation liquid, which can remove about 70% of the organic matter in the separation liquid, thus reducing the COD of the refluxed water. Mn The concentration will be lower than 1.24 mg / L, which is significantly lower than the 1.8 mg / L organic matter index in the effluent water of Example 1, as shown in Table 6.

[0141] Table 6

[0142]

[0143] Comparative Example 1

[0144] In this comparative example, the treatment method includes the following steps: the concentrated water after nitrate removal from the tap water in Example 1 is first passed through the organic matter removal device 4 for activated carbon adsorption to remove organic matter, and then enters the nanofiltration desalination device 3 for desalination of monovalent ions and divalent and above ions. The separated divalent and above ions are returned to the pre-production water, while the monovalent ions are discharged at the same time.

[0145] The processing results of Comparative Example 1 are compared with the processing results of Example 2 as shown in Table 7 below.

[0146] Table 7 Comparison of processing results between Comparative Example 1 and Example 2

[0147]

[0148] In this comparative example, the concentrate volume is 21176 m³. 3 / d, the scale of both the organic matter removal and nanofiltration desalination unit 3 is 21176m³.3 / d, significantly higher than the scale of nanofiltration salt separation device 3 in Example 2 (3176.5m). 3 / d), Organic matter removal unit 4 (476.5m) 3 / d).

[0149] Furthermore, since the equipment investment and operating costs of the frequent polarity reversal electrodialysis device 2 are lower than those of the nanofiltration desalination device 3, the operating cost per ton of water in Example 2, when converted to the scale of the nitrate separation device, is 0.18 yuan / m³ lower than that in Comparative Example 1. 3 Therefore, Example 2 is more efficient and economical than Comparative Example 1.

Claims

1. A system for treating tap water with excessive nitrates, characterized in that, It includes a nitrate separation device (1), a frequent polarity reversal electrodialysis device (2), a nanofiltration desalination device (3), and a product water tank (5). The nitrate separation device (1) has a raw water inlet (11), a concentrated water outlet (12) and a pre-product water outlet (13) for separating raw water into concentrated water and pre-product water; The frequent polarity reversal electrodialysis device (2) has a concentrated water inlet (21), a high-salt solution outlet (22) and a low-salt wastewater outlet (23) for separating the concentrated water after nitrate removal into a high-salt solution and a low-salt wastewater; the concentrated water inlet (21) is connected to the concentrated water outlet (12); The nanofiltration salt separation device (3) has a high-salt solution inlet (31), a separation liquid outlet (33), and a permeate outlet (32) for separating the high-salt solution into a separation liquid containing divalent or higher ions and a permeate containing monovalent ions; the high-salt solution outlet (22) is connected to the high-salt solution inlet (31); the separation liquid outlet (33) and the pre-product water outlet (13) are respectively connected to the product water tank (5), and the product water tank (5) is used to combine the separation liquid and the pre-product water as product water; An organic matter removal device (4) is provided on the connecting pipeline between the separation liquid outlet (33) and the product water tank (5); an external discharge branch is also provided on the pipeline between the separation liquid outlet (33) and the organic matter removal device (4).

2. The nitrate excess treatment system for tap water as described in claim 1, characterized in that, The nitrate separation device (1) is a nanofiltration device or a reverse osmosis device; And / or, the nanofiltration salt separation device (3) is provided with a nanofiltration membrane.

3. The nitrate excess treatment system for tap water as described in claim 2, characterized in that, A pretreatment unit is also provided upstream of the raw water inlet (11); And / or, the nanofiltration membrane is made of a polypiperazine composite film.

4. The nitrate excess treatment system for tap water as described in claim 3, characterized in that, The pretreatment unit includes one or more combinations of sedimentation tank, air flotation machine, ozone contact tank, advanced oxidation device, activated carbon adsorption device and ultrafiltration membrane device.

5. The nitrate excess treatment system for tap water as described in claim 4, characterized in that, The pretreatment unit includes a sedimentation tank, an advanced oxidation device, an activated carbon adsorption device, and an ultrafiltration membrane device connected in sequence.

6. The nitrate excess treatment system for tap water as described in claim 4, characterized in that, The sedimentation tank is a flocculation sedimentation tank; And / or, the activated carbon adsorption device is an upward flow activated carbon adsorption device or a downward flow activated carbon filter. And / or, the advanced oxidation device is an ultraviolet-hydrogen peroxide advanced oxidation device; And / or, the ultrafiltration membrane device is an immersion ultrafiltration membrane device or a pressure ultrafiltration membrane device.

7. The nitrate excess treatment system for tap water as described in claim 1, characterized in that, The organic matter removal device (4) is an ultraviolet light-hydrogen peroxide advanced oxidation device or an activated carbon adsorption device.

8. A method for treating tap water with excessive nitrates, characterized in that, The treatment system for tap water with excess nitrate as described in any one of claims 1-7 includes the following steps: The raw water is treated to separate nitrates, yielding concentrated water and pre-product water respectively. The concentrated water is passed into a frequent reverse electrodialysis device (2) to obtain low-salt wastewater and high-salt solution respectively; The high-salt solution is passed into a nanofiltration salt separation device (3) to obtain permeate and separation liquid respectively; after removing the organic matter from the separation liquid, it is mixed with the pre-product water to obtain the product water.

9. The method for treating tap water with excessive nitrates as described in claim 8, characterized in that, The nitrate separation process is nanofiltration or reverse osmosis; And / or, the inlet water pressure of the frequent polarity reversal electrodialysis device (2) is 0.2-0.3 MPa; And / or, the effective membrane area in the frequent polarity reversal electrodialysis device (2) is 250-350 m². 2 ; And / or, the reversal cycle of the frequent reversal electrodialysis device (2) is 15-25 min; And / or, the inlet water pressure of the nanofiltration desalination device (3) is 0.5-0.8 MPa; And / or, the nanofiltration salt separation device (3) is provided with a nanofiltration membrane.

10. The method for treating tap water with excessive nitrates as described in claim 9, characterized in that, The nitrate separation process includes a pretreatment process.

11. The method for treating tap water with excessive nitrates as described in claim 10, characterized in that, The pretreatment process includes one or more combinations of sedimentation, flotation, filtration, ozone contact, advanced oxidation, activated carbon adsorption, and ultrafiltration.

12. The method for treating tap water with excessive nitrates as described in claim 10, characterized in that, The pretreatment process includes, in sequence, precipitation, advanced oxidation, activated carbon adsorption, ultrafiltration, and deep separation.

13. The method for treating tap water with excessive nitrates as described in claim 11, characterized in that, The advanced oxidation is ultraviolet-hydrogen peroxide advanced oxidation; And / or, the conditions for ultrafiltration satisfy at least one of the following: ①The membrane flux of the ultrafiltration membrane is 30-40 L / h; ② The maximum permissible transmembrane pressure difference of the ultrafiltration membrane is 0.06 MPa; ③ The ultrafiltration membrane is made of PVDF material; And / or, the reverse osmosis conditions satisfy at least one of the following: ①The reverse osmosis flux is 15-20 L / h; ②The inlet water pressure for the reverse osmosis is 1-2 MPa; And / or, the inlet water pressure of the frequent polarity reversal electrodialysis device (2) is 0.25 MPa; And / or, the effective membrane area in the frequent reverse electrodialysis device (2) is 300 m². 2 ; And / or, the reversal cycle of the frequent reversal electrodialysis device (2) is 20 min; And / or, the inlet water pressure of the nanofiltration desalination device (3) is 0.6 MPa; And / or, the nanofiltration membrane is made of a polypiperazine composite film.

14. The method for treating tap water with excessive nitrates as described in claim 13, characterized in that, The ultrafiltration membrane has a flux of 35 L / h; And / or, the reverse osmosis flux is 19.98 L / h.

15. The method for treating tap water with excessive nitrates as described in claim 13, characterized in that, The conditions for the UV-hydrogen peroxide advanced oxidation satisfy at least one of the following: ①UV light intensity is 300-500mJ / cm 2 ; ②Ultraviolet transmittance ≥85%; ③The maximum dosage of H2O2 is 6 mg / L.

16. The method for treating tap water with excessive nitrates as described in claim 15, characterized in that, The UV light intensity is 400 mJ / cm. 2 .

17. The method for treating tap water with excessive nitrates as described in claim 9, characterized in that, The maximum pressure drop of the nanofiltration membrane is 0.1 MPa; And / or, the nanofiltration membrane has a membrane flux of 15-20 L / h.

18. The method for treating tap water with excessive nitrates as described in claim 17, characterized in that, The nanofiltration membrane has a flux of 19 L / h.

19. The method for treating tap water with excessive nitrates as described in claim 8, characterized in that, The volume ratio of the low-salt wastewater to the high-salt solution is 18000:(3100-3200); And / or, the volume ratio of the permeate to the separation liquid is 2700:(450-500); And / or, the nitrate content in the raw water exceeds 10.0 mg / L; And / or, in the raw water, the organic matter COD Mn The content is 4.50-5.00 mg / L; And / or, the TDS content in the raw water exceeds 250 mg / L; And / or, the TDS content in the concentrated water is 2500-2700 mg / L; And / or, in the concentrated water, the organic matter COD Cr The content is 40-60 mg / L; And / or, the TDS content in the pre-production water is 150-200 mg / L; And / or, in the pre-production water, the organic matter COD Mn The content is 0.9-1.1 mg / L; And / or, the TDS content in the low-salt wastewater is 300-350 mg / L; And / or, in the low-salinity wastewater, the organic matter COD Cr The content is 40-60 mg / L; And / or, the TDS content in the high-salt solution is 15000-16000 mg / L; And / or, in the high-salt solution, the organic matter COD Cr The content is 40-60 mg / L; And / or, the TDS content in the permeate is 8200-8300 mg / L; And / or, in the permeate, the organic matter COD Cr The content is not greater than 5 mg / L; And / or, the TDS content in the separation solution is 55000-60000 mg / L; And / or, in the separated liquid, the organic matter COD Cr The content is 300-400 mg / L; And / or, the TDS content in the product water is 250-350 mg / L; And / or, in the produced water, the organic matter COD Mn The content is less than 3 mg / L.

20. The method for treating tap water with excessive nitrates as described in claim 19, characterized in that, The volume ratio of the low-salt wastewater to the high-salt solution is 18000:3176; And / or, the volume ratio of the permeate to the separation liquid is 2700:476; And / or, the nitrate content in the raw water is 15.0-20.0 mg / L; And / or, in the raw water, the organic matter COD Mn The content was 4.61 mg / L; And / or, the TDS content in the raw water is 350.0-400.0 mg / L; And / or, in the raw water, the mass percentage of monovalent ions and divalent and above ions is 48-52%; And / or, the TDS content in the concentrate is 2605 mg / L; And / or, in the concentrated water, the organic matter COD Cr The content is 50 mg / L; And / or, the TDS content in the pre-production water is 163 mg / L; And / or, in the pre-production water, the organic matter COD Mn The content is 1 mg / L; And / or, the TDS content in the low-salt wastewater is 306 mg / L; And / or, in the low-salinity wastewater, the organic matter COD Cr The content is 50 mg / L; And / or, the TDS content in the high-salt solution is 15630 mg / L; And / or, in the high-salt solution, the organic matter COD Cr The content is 50 mg / L; And / or, the TDS content in the permeate is 8275 mg / L; And / or, in the permeate, the organic matter COD Cr The content is 0 mg / L; And / or, the TDS content in the separated solution is 57310 mg / L; And / or, in the separated liquid, the organic matter COD Cr The content is 333 mg / L; And / or, the TDS content in the product water is 299 mg / L; And / or, in the produced water, the organic matter COD Mn The content is less than 1.8 mg / L.

21. The method for treating tap water with excessive nitrates as described in claim 20, characterized in that, The nitrate content in the raw water was 19.0 mg / L; And / or, the TDS content in the raw water is 397.6 mg / L; And / or, wherein the mass percentage of monovalent ions and divalent and higher ions is 50%; And / or, in the produced water, the organic matter COD Mn The content is less than 1.24 mg / L.

22. The method for treating tap water with excessive nitrates as described in claim 8, characterized in that, The organic matter removal methods include advanced oxidation or activated carbon adsorption; And / or, 80%-100% of the separated liquid is mixed with the pre-product water.

23. The method for treating tap water with excessive nitrates as described in claim 22, characterized in that, The advanced oxidation in the organic matter removal method is ultraviolet-hydrogen peroxide advanced oxidation.

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