A water treatment method and system of synchronous hardening and silicon removal coupled with silicon carbide membrane filtration

Abstract

This invention discloses a water treatment method and system for simultaneous hardening and silicon removal coupled with silicon carbide membrane filtration. The method includes: adding sodium hydroxide to wastewater for a first mixing reaction; subsequently adding sodium carbonate for a second mixing reaction, with the two reactions spaced apart in time and space to ensure preferential formation of magnesium hydroxide flocs. High-efficiency silicon removal is achieved by utilizing magnesium hydroxide's adsorption of silicate ions and co-precipitation of calcium silicate. The effluent is then introduced into a flocculation reaction tank, where 5%–12% of precipitated sludge is added via recirculation to promote floc growth. The effluent then enters an inclined tube sedimentation tank for solid-liquid separation, reducing the effluent turbidity to 10–50 NTU. Finally, a silicon carbide membrane is used to filter the effluent, achieving a turbidity not exceeding 1.0 NTU. This invention controls the sedimentation growth path through stepwise chemical addition, avoiding the problem of poor silicon removal efficiency caused by large amounts of calcium carbonate encapsulating magnesium hydroxide. Simultaneously, without adding coagulants, the recirculated sludge generates flocculation, coupled with filtration using a silicon carbide membrane resistant to strong acids and alkalis, achieving highly efficient hardening and silicon removal from wastewater, as well as short-process, low-cost treatment.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a water treatment method and system for simultaneously removing hardness and silicon and coupling silicon carbide membrane filtration for high-hardness, high-silicon industrial wastewater. It is applicable to high-calcium, magnesium, and silicon wastewater treatment scenarios such as circulating cooling wastewater, desulfurization wastewater, reverse osmosis concentrate, and mine water. Background Technology

[0002] In the treatment of industrial wastewater such as circulating cooling wastewater, desulfurization wastewater, reverse osmosis concentrate, and mine water, hardening and silicon removal are crucial steps to ensure the stable operation of subsequent membrane treatment systems. Hardening refers to removing calcium and magnesium ions from the wastewater, while silicon removal refers to removing silicon from the wastewater.

[0003] Currently, the mainstream industrial processes for hardening and silicating removal typically employ chemical softening combined with flocculation sedimentation and organic membrane filtration. The specific process involves adding sodium hydroxide (NaOH) to the raw water to remove magnesium ions and sodium carbonate (Na2CO3) to remove calcium ions. Simultaneously, the generated Mg(OH)2 adsorbs silicate ions (some processes also add additional magnesium agents) to achieve silicating removal. After the reaction, coagulants (including flocculants and coagulant aids) are added to initiate flocculation, followed by solid-liquid separation, ultimately achieving both hardening and silicating removal. This process requires a large amount of coagulant to improve sedimentation efficiency, resulting in high reagent costs, large sludge production, and the potential for excessive coagulant residue to cause organic membrane fouling. Furthermore, when NaOH and Na2CO3 are added simultaneously, the large amount of CaCO3 precipitate generated coats the surface of Mg(OH)2, hindering the adsorption of silicate ions by Mg(OH)2, leading to poor silicating removal.

[0004] In the prior art, Chinese Patent Application No. 202310839931X discloses a pretreatment method suitable for high-salinity wastewater treatment processes. This method involves parallel arrangement of a hardening removal tank, a silica removal tank, a flocculation tank, and a sedimentation tank within an integrated hardening and silica removal unit. The hardening removal tank removes hardness from the wastewater through methods such as lime, soda ash softening, or caustic soda softening. In the silica removal tank, magnesium and sodium aluminate are added to remove silica. A coagulant is added to the flocculation tank to promote the flocculation and growth of precipitates generated in the hardening and silica removal tanks. Subsequently, solid-liquid separation is performed in the sedimentation tank to obtain chemical sludge and a supernatant after hardening and silica removal. This patented method involves the stepwise addition of different agents to achieve hardening and silica removal, requiring large amounts of silica removal and coagulant, resulting in high agent costs and large sludge production. Summary of the Invention

[0005] To reduce wastewater treatment costs, and especially to improve the efficiency of hardness and silica removal in wastewater without using any coagulants (including flocculants and coagulant aids), this invention provides a water treatment method and system for simultaneous hardness and silica removal coupled with silicon carbide membrane filtration.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a water treatment method for simultaneous hard and silicon removal coupled with silicon carbide membrane filtration, comprising the following steps: S1. Add sodium hydroxide to the wastewater to carry out the first mixing reaction; S2. Add sodium carbonate to the effluent of the first mixed reaction to carry out the second mixed reaction; S3. The effluent from the second mixed reaction is subjected to flocculation and sedimentation treatment, including: Add precipitated sludge to the effluent of the second mixed reaction to carry out a flocculation reaction; take the effluent of the flocculation reaction for sedimentation treatment to obtain precipitated sludge and supernatant; add a portion of the precipitated sludge back to the effluent of the second mixed reaction, and discharge the supernatant as effluent; S4. The effluent after sedimentation is filtered through a silicon carbide membrane to obtain treated water.

[0007] Furthermore, in step S1, the pH of the reaction system of the first mixed reaction is 11.0~12.0.

[0008] Furthermore, in step S1, the reaction time of the first mixed reaction is 10-15 minutes.

[0009] Furthermore, in step S2, the reaction time of the second mixed reaction is 5 to 10 minutes.

[0010] Further, in step S3, 5% to 12% of the precipitated sludge obtained from the sedimentation treatment is recycled and added to the effluent of the second mixing reaction for flocculation reaction, and the flocculation reaction time is 20 to 30 minutes.

[0011] Further, in step S3, the turbidity of the effluent from the sedimentation treatment is 10~50 NTU; in step S4, the turbidity of the treated water is ≤1.0 NTU.

[0012] Furthermore, the wastewater treatment method further includes the following steps: When the transmembrane pressure difference of the silicon carbide membrane filtration reaches the set threshold, an air-water mixture formed by mixing acidified membrane permeate with compressed air at pH 2-3 is introduced into the silicon carbide membrane from the permeate side for backwashing.

[0013] Secondly, the present invention provides a water treatment system for simultaneous hard and silicon removal coupled with silicon carbide membrane filtration, comprising the following components connected sequentially along the water flow direction: The first mixing reaction tank is equipped with a sodium hydroxide dosing device; The second mixing reaction tank is connected to the first mixing reaction tank and is equipped with a sodium carbonate dosing device. The flocculation reaction tank is connected to the second mixing reaction tank; An inclined tube sedimentation tank, connected to the flocculation reaction tank, is used to perform sedimentation treatment on the effluent from the flocculation reaction tank and to return the resulting sludge to the flocculation reaction tank. An ultrafiltration device is connected to the inclined tube sedimentation tank.

[0014] Furthermore, the water treatment system also includes a backwashing device for backwashing the ultrafiltration unit; the backwashing device includes a backwash tank, an air storage tank, and an acid addition component; the inlet of the backwash tank is connected to the product water end of the ultrafiltration unit; the acid addition component is used to adjust the pH of the water in the backwash tank to 2-3; the outlet of the air storage tank is connected to the inlet of the backwash tank; the outlet of the backwash tank is connected to the product water end of the ultrafiltration unit, for backwashing the ultrafiltration unit with an acidic gas-water mixture.

[0015] Furthermore, the ultrafiltration device adopts an internal pressure multi-channel tubular membrane module or a flat-plate membrane module, the membrane material is silicon carbide, and the filtration pore size is 10~50 nm; the inclined tube sedimentation tank has an inclined tube inclination angle of 60° and a tube diameter of 35~80 mm.

[0016] The present invention has the following beneficial effects: (1) By setting the time and space intervals between the first and second mixing reactions, this invention ensures that magnesium hydroxide flocs are preferentially generated and maintain their activity, thereby fully adsorbing silicate ions. Simultaneously, excess Ca²⁺ in the reaction tank… + The silicate ions react to form CaSiO3 precipitate. Sodium carbonate is then added to form calcium carbonate precipitate. This specific order of dosing effectively avoids the problem in traditional processes where calcium carbonate precipitate encapsulates the active sites of magnesium hydroxide, thus enabling the silica removal process to achieve the dual effects of magnesium hydroxide adsorption and calcium silicate co-precipitation. Simultaneously, a portion of the sludge obtained after this sequential reaction is recycled as floc nuclei, promoting the contact growth of fine flocs and further enhancing the aforementioned dual effects, significantly improving the silica removal rate and process stability.

[0017] (2) This invention requires no addition of any coagulants (including flocculants and coagulant aids). It utilizes only the returned sludge at the bottom of the sedimentation tank as crystal nuclei, promoting floc growth through the flocculation effect generated by the sludge return. This completely avoids membrane pore blockage caused by residual coagulants, significantly reduces irreversible fouling of the membrane system, and lowers reagent costs. Furthermore, this invention couples the high-precision filtration of a silicon carbide membrane resistant to strong acids and alkalis, enabling long-term stable operation under high pH conditions and maximizing the overall treatment effect. Experimental verification shows that using the method described in this invention, the calcium removal rate is ≥95%, the magnesium removal rate is ≥98%, the silicon removal rate is ≥85%, and the turbidity of the treated water is ≤1.0 NTU.

[0018] (3) The present invention uses a silicon carbide membrane, which can directly filter the high pH effluent from the sedimentation tank without prior acid neutralization, thereby simplifying the process and helping to reduce the calcium, magnesium, and silicon content in the effluent after membrane filtration. The extremely high mechanical strength of the silicon carbide membrane enables it to withstand frequent acidic gas-water mixed fluid scouring. This mixed fluid can quickly dissolve calcium carbonate, magnesium hydroxide, and calcium silicate scale on the membrane surface and in the pores, resulting in good backwashing effect and significant water saving. Therefore, there is no need for frequent shutdowns for complex chemical cleaning, which can ensure the continuous and stable operation of the membrane filtration system and extend the membrane's service life. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of a water treatment system that simultaneously removes hard and silicon particles and couples them to a silicon carbide membrane for filtration.

[0021] The following components are marked in the diagram: Wastewater pump 1, First mixing reaction tank 2, Sodium hydroxide dosing device 3, Second mixing reaction tank 4, Sodium carbonate dosing device 5, Flocculation reaction tank 6, Inclined tube sedimentation tank 7, Intermediate water tank 8, Sludge pump 9, Sludge return pump 10, Ultrafiltration feed water pump 11, Ultrafiltration inlet valve 12, Silicon carbide membrane ultrafiltration device 13, Ultrafiltration product water valve 14, Ultrafiltration discharge valve 15, Ultrafiltration backwash discharge valve 16, Backwash valve 17, Ultrafiltration product water tank 18, Acid addition component 19, Backwash tank inlet valve 20, Backwash tank makeup water pump 21, Backwash tank 22, Backwash tank exhaust valve 23, Backwash tank air inlet valve 24, Air storage tank 25. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0023] In the treatment of high-hardness, high-silica wastewater, the existing removal route involves: adding sodium hydroxide to remove magnesium ions; adding sodium carbonate to remove calcium ions; and, if necessary, adding magnesium agents (such as MgCl2, MgSO4) or aluminum salts to adsorb silicate ions to achieve silica removal. This route suffers from poor treatment efficiency and high reagent costs, specifically: firstly, the large amount of CaCO3 precipitate generated coats the surface of Mg(OH)2, hindering its adsorption of silicate ions, resulting in low silica removal efficiency; secondly, the flocculation reaction stage requires the addition of large amounts of coagulants (such as PAC, PFS, PAM, etc.), which not only increases treatment costs but also easily causes membrane fouling; and thirdly, the sedimentation tank has a large volume, and subsequent chemical cleaning of the membrane system is frequent, limiting the membrane's lifespan.

[0024] To simplify processes and reduce costs, existing technologies attempt to combine magnesium removal and calcium removal into a single reaction tank. For example, Chinese patent application No. 202310839931X provides a pretreatment method for high-salinity wastewater treatment. This method involves parallel arrangement of a hardening tank, a silicon removal tank, a flocculation tank, and a sedimentation tank within an integrated hardening and silicon removal unit. The hardening tank removes hardness from the wastewater using methods such as lime, soda ash softening, or caustic soda softening. Silicon is removed from the wastewater by adding magnesium and sodium aluminate agents in the silicon removal tank. A flocculant is added to the flocculation tank to promote the growth of precipitates generated in the hardening and silicon removal tanks. Finally, solid-liquid separation is performed in the sedimentation tank to obtain chemical sludge and a supernatant after hardening and silicon removal. However, even with this method combining magnesium and calcium removal in the same reaction tank to streamline the hardening process, the reagent costs are not significantly reduced. Large amounts of reagents are still required in the subsequent silicon removal and flocculation tanks, and the silicon removal effect is unsatisfactory.

[0025] The inventors discovered that the main reason for the poor silicon removal effect was that placing the removal of calcium and magnesium in the same reaction tank resulted in poor silicon removal efficiency: a large amount of CaCO3 precipitate settled and coated the surface of Mg(OH)2, hindering its adsorption of silicate ions. Therefore, this patented method had to add a silicon removal tank and a flocculation tank after the hardening process, and add a large amount of silicon removal agent and flocculant to assist in silicon removal, further increasing the processing cost.

[0026] In view of this, in order to improve the hardness and silicon removal effect while reducing the cost of wastewater treatment, the present invention provides a water treatment method and system for simultaneous hardness and silicon removal coupled with silicon carbide membrane filtration.

[0027] In one embodiment, a water treatment method is provided that simultaneously removes hardness and silicon dioxide and couples to a silicon carbide membrane for filtration, comprising the following steps: S1. Add sodium hydroxide to the wastewater to carry out the first mixing reaction; S2. Add sodium carbonate to the effluent of the first mixing reaction to carry out a second mixing reaction; the second mixing reaction is spaced apart from the first mixing reaction in time and space to ensure that magnesium hydroxide flocs are formed first and calcium carbonate precipitates later. S3. The effluent from the second mixed reaction is subjected to flocculation and sedimentation treatment, including: Add precipitated sludge to the effluent of the second mixed reaction to carry out a flocculation reaction; take the effluent of the flocculation reaction for sedimentation treatment to obtain precipitated sludge and supernatant; add a portion of the precipitated sludge back to the effluent of the second mixed reaction, and discharge the supernatant as effluent; S4. The effluent after sedimentation is filtered through a silicon carbide membrane to obtain treated water.

[0028] By setting time and spatial intervals between the first and second mixing reactions, the preferential formation and activity of Mg(OH)₂ flocs are ensured, thereby fully adsorbing silicate ions. Simultaneously, during this process, excess Ca in the reaction tank... 2+ The magnesium hydroxide precipitate is formed by reacting with silicate ions, followed by the addition of sodium carbonate to form calcium carbonate precipitate. This specific order of dosing effectively avoids the problem of calcium carbonate precipitate encapsulating the active sites of magnesium hydroxide in traditional processes, thus enabling the silicon removal process to have the dual effects of magnesium hydroxide adsorption and calcium silicate co-precipitation. Simultaneously, a portion of the sludge obtained after this sequential reaction is recycled as floc nuclei, promoting the contact growth of fine flocs and further enhancing the aforementioned dual effects, significantly improving the silicon removal rate and process stability.

[0029] The water treatment method provided by this invention does not require the addition of any coagulants (including flocculants and coagulant aids). It utilizes only the returned sludge at the bottom of the sedimentation tank as crystal nuclei, promoting floc growth through the flocculation effect generated by the sludge return, thereby completely avoiding membrane pore clogging caused by residual coagulants and reducing reagent costs. Furthermore, the high-precision filtration effect coupled with a silicon carbide membrane resistant to strong acids and alkalis enables long-term stable operation under high pH conditions, maximizing the overall treatment effect. In one embodiment, experimental verification shows that the above-mentioned water treatment method with simultaneous hardness and silicon removal coupled with silicon carbide membrane filtration achieves a calcium removal rate ≥95%, a magnesium removal rate ≥98%, a silicon removal rate ≥85%, and a product water turbidity ≤1.0 NTU.

[0030] In a preferred embodiment, in step S1, the pH of the reaction system of the first mixed reaction is 11.0~12.0.

[0031] In a preferred embodiment, in step S1, the reaction time of the first mixing reaction is 10-15 minutes.

[0032] In a preferred embodiment, in step S2, the reaction time of the second mixing reaction is 5 to 10 minutes.

[0033] In a preferred embodiment, in S3, 5% to 12% of the precipitated sludge obtained from solid-liquid separation is recycled and added to the effluent of the second mixing reaction for flocculation reaction, and the flocculation reaction time is 20 to 30 minutes.

[0034] In a preferred embodiment, in step S3, the turbidity of the effluent from the solid-liquid separation is 10~50 NTU; in step S4, a silicon carbide membrane is used to filter the effluent from the solid-liquid separation.

[0035] In a preferred embodiment, the wastewater treatment method further includes the following steps: When the transmembrane pressure difference of the silicon carbide membrane filtration reaches a set threshold, a gas-water mixture formed by mixing acidified membrane permeate with compressed air at a pH of 2-3 is introduced into the silicon carbide membrane from the permeate side for backwashing. The silicon carbide membrane used in this application is resistant to strong acids and alkalis and can withstand pH values ​​from 0 to 14. Online acid backwashing directly dissolves inorganic scale, avoiding scale accumulation. The combined gas-water backwashing significantly reduces backwash water consumption, and the physical scouring of compressed air is more effective for dense filter cake layers. This extends the chemical cleaning cycle of the membrane system from 7-20 days to more than 40 days, with a TMP recovery rate of over 95%, and a membrane lifespan of more than 6 years under high pH operating conditions.

[0036] Furthermore, the threshold value for the transmembrane pressure difference of the silicon carbide membrane is preferably set to 100~200 kPa.

[0037] In another embodiment, combined Figure 1 As shown, a water treatment system with simultaneous hardening and silicon removal coupled to silicon carbide membrane filtration is provided, comprising a first mixing reaction tank 2, a second mixing reaction tank 4, a flocculation reaction tank 6, an inclined tube sedimentation tank 7, and an ultrafiltration device 13 connected sequentially along the water flow direction. Wherein: The first mixing reaction tank 2 is equipped with a sodium hydroxide dosing device 3; The second mixing reaction tank 4 is connected to the first mixing reaction tank 2 and is equipped with a sodium carbonate dosing device 5; The flocculation reaction tank 6 is connected to the second mixing reaction tank 4; Inclined tube sedimentation tank 7 is connected to the flocculation reaction tank 6 and is used to perform solid-liquid separation on the effluent of the flocculation reaction tank 6, and to return a portion of the separated sludge to the flocculation reaction tank 6. The ultrafiltration device 13 is connected to the inclined tube sedimentation tank 7.

[0038] In a preferred embodiment, the surface loading of the inclined tube sedimentation tank 7 is 10~25 m³ / (m²·h).

[0039] 5% to 12% of the sludge obtained from the inclined tube sedimentation tank 7 is recycled and added to the effluent of the second mixing reaction, and the reaction time is 20 to 30 minutes.

[0040] In a preferred embodiment, the water treatment system further includes an intermediate water tank 8, which is disposed between the inclined tube sedimentation tank 7 and the ultrafiltration device 13 and is connected to both the inclined tube sedimentation tank 7 and the ultrafiltration device 13.

[0041] In a preferred embodiment, the water treatment system further includes an ultrafiltration product water tank 18, which is connected to the product water end of the ultrafiltration device 13.

[0042] In a preferred embodiment, the water treatment system further includes a backwashing device; the backwashing device includes a backwash tank 22, an air storage tank 25, and an acid addition component 19. The inlet of the backwash tank 22 is connected to the ultrafiltration product water tank 18, the acid addition component 19 is used to adjust the pH of the water in the backwash tank 22 to 2-3, the outlet of the air storage tank 25 is connected to the inlet of the backwash tank 22, and the outlet of the backwash tank 22 is connected to the product water end of the ultrafiltration device 13.

[0043] During backwashing, the gas in the gas storage tank 25 enters the backwash tank 22 through the gas outlet, which increases the pressure in the backwash tank 22. The gas mixes with the acidic water in the backwash tank 22 to form an acidic gas-water mixture, which enters the water production end of the ultrafiltration device 13 to backwash the membrane module in the ultrafiltration device 13.

[0044] In a preferred embodiment, the ultrafiltration device 13 is an internal pressure multi-channel tubular membrane module or a flat-plate membrane module, the membrane material is silicon carbide, and the filtration pore size is 10~50 nm; the inclined tube sedimentation tank 7 has an inclined tube inclination angle of 60° and a tube diameter of 35~80 mm.

[0045] In one specific embodiment, wastewater is pumped into the first mixing reaction tank 2 by wastewater pump 1, and flows through the first mixing reaction tank 2 for reaction, the second mixing reaction tank 4 for reaction, and then through the flocculation reaction tank 6 and the inclined tube sedimentation tank 7 for flocculation and sedimentation treatment. Solid-liquid separation occurs in the inclined tube sedimentation tank 7, yielding precipitated sludge and supernatant. Part of the precipitated sludge is discharged by sludge discharge pump 9, and part of the precipitated sludge is returned to the flocculation reaction tank 6 by sludge return pump 10, where it mixes with the water from the second mixing reaction tank 4 for flocculation. The supernatant is sent as the effluent from the inclined tube sedimentation tank 7 to the intermediate water tank 8, and then pumped by the ultrafiltration feed water pump 11 through the ultrafiltration inlet valve 12 into the silicon carbide membrane ultrafiltration device for filtration. The permeate after silicon carbide membrane filtration enters the ultrafiltration permeate tank 18, and acid (hydrochloric acid or sulfuric acid) is added through the acidification component 19 to adjust the pH to 6.5-8.0 before use.

[0046] In one specific embodiment, when the transmembrane pressure difference of the silicon carbide membrane ultrafiltration device 13 reaches a set threshold of 100-200 kPa, the silicon carbide membrane ultrafiltration device 13 is backwashed with an acidic gas-water mixture. Specifically: the ultrafiltration feed water pump 11 is stopped, filtration is stopped, and the ultrafiltration inlet valve 12 and ultrafiltration product water valve 14 are closed; the backwash tank exhaust valve 23 is opened, the backwash tank inlet valve 20 is opened, the backwash tank makeup water pump 21 is started to introduce ultrafiltration product water, and at the same time, acid is added through the acid adding component 19 to adjust the pH of the backwash water to 2-3, and water is added to the set liquid level of the backwash tank (e.g., 80% of the backwash tank volume); then the backwash tank exhaust valve 23 is closed, the backwash tank air inlet valve 24 is opened, and compressed air is introduced from the air storage tank 25 to raise the pressure of the backwash tank to 0.4-0.6 kPa. MPa; Next, open the backwash valve 17 and the ultrafiltration backwash discharge valve 16, and the acidic gas-water mixture is reverse-flowed into the membrane module from the membrane permeate side to flush the membrane surface for 5-15 seconds; close the backwash valve 17 and the backwash tank air inlet valve 24, open the ultrafiltration inlet valve 12 and the ultrafiltration discharge valve 15, start the ultrafiltration feed water pump 11, and perform forward flushing for 30-60 seconds to discharge the stripped contaminants; finally, open the ultrafiltration permeate valve 14 and close the ultrafiltration discharge valve 15 to restore filtration. The water consumption of the entire process does not exceed 3% of the permeate volume. This embodiment can achieve online backwashing.

[0047] The features and performance of the present invention will be further described in detail below with reference to specific embodiments. The following embodiments and comparative examples utilize circulating cooling wastewater from a power plant to implement the wastewater treatment method of the present invention. 2+ =400 mg / L, Mg 2+ =100mg / L, alkalinity 5 mmol / L, SiO2=40 mg / L, pH=7.8.

[0048] Example 1 A water treatment method with simultaneous hard and silicon removal coupled to silicon carbide membrane filtration includes the following steps: S1. The wastewater is introduced into the first mixing reaction tank, and NaOH is added to the first mixing reaction tank at a dosage of about 550-600 mg / L. The pH of the water is adjusted to 11.0-12.0, and the first mixing reaction is carried out for 12 minutes.

[0049] S2. The effluent from the first mixing reaction is fed into the second mixing reaction tank, and Na2CO3 is added at a dosage of approximately 800 mg / L to carry out the second mixing reaction for 8 minutes.

[0050] S3. The effluent from the second mixing reactor is fed into the flocculation reactor, and a portion of settled sludge is added back into the flocculation reactor for a retention time of approximately 25 minutes to promote floc growth. The effluent from the flocculation reactor is then fed into an inclined tube sedimentation tank with a surface loading of 20 m³. 3 / (m 2 In the sedimentation tank (h), solid-liquid separation is achieved by settling, resulting in precipitated sludge and supernatant. Approximately 8% of the precipitated sludge is returned to the flocculation reaction tank using a sludge return pump, and the supernatant is used as the effluent from the inclined tube sedimentation tank.

[0051] S4. The effluent from the inclined tube sedimentation tank is fed into the intermediate water tank, and then the water in the intermediate water tank is filtered through an ultrafiltration membrane. The ultrafiltration membrane module is made of silicon carbide, with a pore size of 30 nm and a membrane flux of 180 L / (m²). 2 ·h), to obtain treated water. The Ca of the treated water 2+ =15 mg / L, Mg 2+ =2 mg / L, SiO2=5 mg / L, turbidity=0.2 NTU.

[0052] The S5 silicon carbide ultrafiltration membrane unit undergoes a combined gas-water backwash every 35 minutes of operation, with a pH of 2.5, a pressure of 0.5 MPa, and a backwash time of approximately 8 seconds. The self-water consumption rate is approximately 2.5%. The chemical cleaning cycle exceeds 40 days.

[0053] Comparative Example 1: Simultaneous addition of alkali and flocculant for flocculation and sedimentation + organic ultrafiltration membrane filtration Process: NaOH and Na2CO3 are added simultaneously until pH=11.0, then flocculant PFS20 mg / L and PAM1 mg / L are added. The sedimentation tank has a retention time of 2 h. The supernatant is filtered through a PVDF ultrafiltration membrane with a membrane flux of 60 L / (m²·h). Pure water is used for backwashing, and the self-use rate is about 10%.

[0054] Processing result: Outflow Ca 2+ =20 mg / L, Mg 2+=5 mg / L, SiO2=12 mg / L, turbidity=0.2 NTU; chemical cleaning is required every 10-15 days; hydrolytic cracking occurred after 1 year of operation.

[0055] Comparative Example 2 Compared to Example 1, the difference is that Na₂CO₃ is added first, followed by NaOH. All other steps are the same as in Example 1. The steps are as follows: S1. The wastewater is introduced into the second mixing reaction tank, and Na2CO3 is added to carry out the second mixing reaction for 8 minutes.

[0056] S2. The effluent from the second mixing reaction is introduced into the first mixing reaction tank, and NaOH is added to the first mixing reaction tank to adjust the pH of the water to 11.0~12.0. The first mixing reaction is then carried out for 12 minutes. The effluent from the first mixing reaction tank is then proceeded to the next step S3.

[0057] S3, Same as Example 1.

[0058] S4, Same as in Example 1.

[0059] The Ca in the treated water 2+ =16 mg / L, Mg 2+ =3 mg / L, SiO2=8 mg / L, turbidity=0.2 NTU.

[0060] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product is in use. These terms are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0061] Furthermore, terms such as "horizontal," "vertical," and "sag" do not imply that components must be absolutely horizontal or suspended, but rather that they can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0062] Unless otherwise specified, the materials and equipment used in this invention are commonly used in the field; the methods used in this invention are conventional methods in the field.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A water treatment method for simultaneous hardening and silicon removal coupled with silicon carbide membrane filtration, characterized in that: Includes the following steps: S1. Add sodium hydroxide to the wastewater to carry out the first mixing reaction; S2. Add sodium carbonate to the effluent of the first mixed reaction to carry out the second mixed reaction; S3. The effluent from the second mixed reaction is subjected to flocculation and sedimentation treatment, including: Add precipitated sludge to the effluent of the second mixed reaction to carry out a flocculation reaction; take the effluent of the flocculation reaction for sedimentation treatment to obtain precipitated sludge and supernatant; add a portion of the precipitated sludge back to the effluent of the second mixed reaction, and discharge the supernatant as effluent; S4. The effluent after sedimentation is filtered through a silicon carbide membrane to obtain treated water.

2. The water treatment method according to claim 1, characterized in that: In step S1, the pH of the reaction system of the first mixed reaction is 11.0~12.

0.

3. The water treatment method according to claim 1 or 2, characterized in that: In step S1, the reaction time of the first mixed reaction is 10-15 minutes.

4. The water treatment method according to claim 1, characterized in that: In step S2, the reaction time of the second mixed reaction is 5 to 10 minutes.

5. The water treatment method according to claim 1, characterized in that: In step S3, 5% to 12% of the precipitated sludge obtained from the sedimentation treatment is recycled and added to the effluent of the second mixing reaction for flocculation reaction, and the flocculation reaction time is 20 to 30 minutes.

6. The water treatment method according to claim 1, characterized in that: In step S3, the turbidity of the effluent from the sedimentation treatment is 10~50 NTU; in step S4, the turbidity of the treated water is ≤1.0 NTU.

7. The water treatment method according to claim 1, characterized in that: The wastewater treatment method further includes the following steps: When the transmembrane pressure difference of the silicon carbide membrane filtration reaches the set threshold, an air-water mixture formed by mixing acidified membrane permeate with compressed air at pH 2-3 is introduced into the silicon carbide membrane from the permeate side for backwashing.

8. A water treatment system that simultaneously removes hardness and silicon dioxide and couples to a silicon carbide membrane for filtration, characterized in that: Including those connected sequentially along the direction of water flow: The first mixing reaction tank is equipped with a sodium hydroxide dosing device; The second mixing reaction tank is connected to the first mixing reaction tank and is equipped with a sodium carbonate dosing device. The flocculation reaction tank is connected to the second mixing reaction tank; An inclined tube sedimentation tank, connected to the flocculation reaction tank, is used to treat the effluent from the flocculation reaction tank by sedimentation, and to return a portion of the sludge obtained from sedimentation to the flocculation reaction tank. An ultrafiltration device is connected to the inclined tube sedimentation tank.

9. The water treatment system according to claim 8, characterized in that: The water treatment system also includes a backwashing device for backwashing the ultrafiltration unit. The backwashing device includes a backwash tank, an air storage tank, and an acid addition component; the inlet of the backwash tank is connected to the product water end of the ultrafiltration device; the acid addition component is used to adjust the pH of the water in the backwash tank to 2-3; the outlet of the air storage tank is connected to the inlet of the backwash tank; the outlet of the backwash tank is connected to the product water end of the ultrafiltration device, and is used to backwash the ultrafiltration device with an acidic gas-water mixture.

10. The water treatment system according to claim 8, characterized in that: The ultrafiltration device is an internal pressure multi-channel tubular membrane module or a flat-plate membrane module, the membrane material is silicon carbide, and the filtration pore size is 10~50 nm; the inclined tube sedimentation tank has an inclined tube inclination angle of 60° and a tube diameter of 35~80 mm.

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