A treatment process for gradually concentrating circulating water and blowdown water to end zero discharge

Abstract

The application discloses a kind of circulating water sewage gradually concentrated to end zero discharge processing technology, comprising the following steps;Step one, pretreatment and preliminary softening;Step two, primary reverse osmosis concentration;Step three, secondary reverse osmosis concentration;Step four, nanofiltration salt separation and tertiary concentration;Step five, end evaporation crystallization.The application is by setting primary, secondary reverse osmosis and tertiary high-pressure reverse osmosis, the amount of concentrated water finally entering evaporation system is reduced to 1 / 50 to 1 / 20 of original water amount, greatly reduces the processing load of high-energy consumption evaporation unit, makes the investment and operating cost of evaporation crystallization reduce more than 60%.Nanofiltration salt separation step is introduced before tertiary concentration, combined with online monitoring and shunt determination algorithm, effectively avoids the crystallization and precipitation of calcium sulfate and other insoluble salts on the surface of high-pressure membrane, at the same time, divalent ion is enriched in nanofiltration concentrated water side for early treatment, protects the core high-pressure reverse osmosis membrane element, prolongs the overall cleaning cycle and service life of system.

Description

Technical Field

[0001] This invention relates to the field of industrial wastewater treatment technology, specifically to a treatment process that progressively concentrates circulating water discharge wastewater to zero discharge at the end. Background Technology

[0002] Circulating cooling water systems are widely used in industrial fields. During operation, a certain amount of wastewater needs to be continuously discharged to control water quality indicators. This wastewater is characterized by high salt content, high hardness, and the presence of scale inhibitors and organic matter.

[0003] Existing treatment technologies mostly employ reverse osmosis for desalination and reuse, but their recovery rate is limited by scaling risks, typically reaching only 75%-85%. The remaining 15%-25% of concentrate still needs to be discharged, failing to achieve true zero discharge. To achieve zero discharge, subsequent processes often employ evaporation crystallization to treat all the concentrate, but direct evaporation involves large water volumes, extremely high energy consumption, and high investment and operating costs. Furthermore, existing processes lack precise control over the dynamic balance of ions during the staged concentration process, leading to scaling and fouling of subsequent membrane systems by sparingly soluble salts such as calcium sulfate and magnesium silicate. This results in poor system stability, making it difficult to achieve long-term, continuous, cost-effective, and efficient zero-discharge goals. Summary of the Invention

[0004] Therefore, this invention provides a treatment process for progressively concentrating circulating water wastewater to achieve zero discharge at the end, in order to solve the problem in the prior art that lacks precise control over the dynamic balance of ions, which leads to scaling and fouling of subsequent membrane systems by sparingly soluble salts such as calcium sulfate and magnesium silicate, resulting in poor system stability and difficulty in achieving the goal of long-term continuous, economical and efficient zero discharge.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] A treatment process for progressively concentrating circulating water wastewater to achieve zero discharge at the end of the process includes the following steps;

[0007] Step 1: Pretreatment and preliminary softening. Adjust the pH of the circulating water discharge to 6.5-8.0, add flocculants and coagulants to coagulate and settle, remove suspended solids and colloids, and then remove temporary hardness and most of the calcium and magnesium ions through a sodium ion exchanger or chemical softening method to obtain pretreated permeable water.

[0008] Step 2: First-stage reverse osmosis concentration. The pretreated permeate is pressurized and fed into the first-stage reverse osmosis system. The operating pressure is controlled at 0.8-1.5 MPa, and the system recovery rate is set at 50%-60%. After separation, first-stage permeate and first-stage concentrate are obtained. The first-stage permeate is reused in the circulating water system.

[0009] Step 3: Secondary reverse osmosis concentration. The primary concentrate is collected in the first intermediate water tank. After adding scale inhibitor and non-oxidizing bactericide, it is introduced into the secondary reverse osmosis system. The operating pressure is controlled at 2.0-3.5 MPa, and the system recovery rate is set at 60%-70%. After separation, secondary permeate and secondary concentrate are obtained. The secondary permeate is reused.

[0010] Step 4: Nanofiltration desalination and tertiary concentration. The secondary concentrate is passed into a nanofiltration system to selectively separate divalent and monovalent ions, obtaining nanofiltration permeate enriched with monovalent ions and nanofiltration concentrate enriched with divalent ions. The nanofiltration permeate is then passed into a tertiary high-pressure reverse osmosis system, with the operating pressure controlled at 5.0-7.0 MPa and the system recovery rate set at 50%-60%, to obtain tertiary permeate and tertiary concentrate. The tertiary permeate is reused.

[0011] Step 5: Terminal evaporation and crystallization. The tertiary concentrate is transported to the evaporation and crystallization system, where it is concentrated using mechanical vapor recompression technology to obtain crystalline salt and evaporation condensate. The evaporation condensate is then reused.

[0012] Preferably, the chemical softening method in step one adopts a lime-soda ash softening process. The lime dosage is calculated to remove carbonate hardness at 100-300 mg / L, and the soda ash dosage is calculated to remove non-carbonate hardness at 200-500 mg / L. After softening, the total hardness of the effluent is reduced to below 50 mg / L, and a tubular microfiltration membrane is used for solid-liquid separation, replacing the traditional sedimentation tank.

[0013] Preferably, in step two, the primary reverse osmosis system uses a fouling-resistant spiral wound reverse osmosis membrane element with a feed water channel width of 34 mil. During operation, it performs low-pressure flushing for 3 minutes every 120 minutes to flush away trace amounts of organic matter and particles adhering to the membrane surface. The concentrate circulation ratio of the primary reverse osmosis system is controlled at 1.2-1.5 times.

[0014] Preferably, in step four, the nanofiltration system is used to achieve efficient separation of chloride and sulfate ions. The nanofiltration membrane has a sulfate rejection rate greater than 98% and a chloride rejection rate less than 20%. An online ion monitoring and diversion judgment algorithm is set up. When the ratio of sulfate concentration in the nanofiltration concentrate to chloride concentration in the nanofiltration permeate is detected to be greater than a preset threshold K, a portion of the nanofiltration concentrate is triggered to flow back to the nanofiltration system inlet. The preset threshold K ranges from 2.5 to 4.0. The diversion judgment algorithm is as follows:

[0015] Calculate the ratio M = (nanofiltration concentrate flow rate × sulfate concentration in nanofiltration concentrate) / (nanofiltration permeate flow rate × chloride concentration in nanofiltration permeate).

[0016] If M > K, then the reflux valve is opened, and the reflux flow rate ratio R = [(M / K)-1] × 0.3, and R does not exceed 0.5.

[0017] Preferably, the three-stage high-pressure reverse osmosis system adopts a disc tube reverse osmosis membrane module or a high-pressure spiral wound reverse osmosis membrane module with a maximum pressure resistance of 9.0 MPa. An energy recovery device is installed in the system to recover the pressure energy of the three-stage concentrate for driving the feed pump of the three-stage high-pressure reverse osmosis system, with an energy recovery efficiency of not less than 90%.

[0018] Preferably, the total concentration factor N of the overall process is predicted and controlled by the following calculation formula: N = initial circulating water discharge volume / final tertiary concentrate flow rate into the evaporation crystallization system. By adjusting the recovery rates set in the first, second, and third stage reverse osmosis systems, N is controlled between 20 and 50 times.

[0019] Preferably, before the operation of the three-stage high-pressure reverse osmosis system in step four, the pH of the nanofiltration permeate is adjusted to 7.5-8.5, and a high-efficiency silica-inhibiting dispersant is added at a dosage of 2-6 mg / L to prevent the precipitation and scaling of magnesium silicate or calcium silicate under high-pressure concentration conditions. The maximum pressure drop of a single membrane element in the three-stage high-pressure reverse osmosis system does not exceed 0.15 MPa.

[0020] Preferably, in step three, the secondary reverse osmosis system uses a seawater desalination membrane element and a security filter is installed at its inlet. The filter accuracy is 5 microns. The total dissolved solids content of the concentrate from the secondary reverse osmosis system is controlled within the range of 15,000-25,000 mg / L to ensure that the nanofiltration system operates under optimal influent water quality conditions.

[0021] Preferably, in step five, the evaporation crystallization system is equipped with a crystal slurry circulation device and a centrifugal dehydrator. By controlling the evaporation temperature at 80-100℃ and the solid content of the crystal slurry at 15%-25%, industrial-grade sodium chloride crystal salt with a purity greater than 98.5% is produced. The final discharge of mother liquor from the system is less than 0.5% of the total influent, achieving zero discharge at the end of the entire process.

[0022] The present invention has the following advantages:

[0023] By setting up primary, secondary, and tertiary high-pressure reverse osmosis systems, the amount of concentrate entering the evaporation system is reduced to 1 / 50 to 1 / 20 of the original amount, which greatly reduces the processing load of the high-energy-consuming evaporation unit and reduces the investment and operating costs of evaporation crystallization by more than 60%.

[0024] By introducing a nanofiltration salt separation step before the third-stage concentration, combined with online monitoring and diversion judgment algorithms, the crystallization and precipitation of sparingly soluble salts such as calcium sulfate on the high-pressure membrane surface is effectively avoided. At the same time, divalent ions are enriched on the nanofiltration concentrate side for early treatment, protecting the core high-pressure reverse osmosis membrane element and extending the overall cleaning cycle and life of the system.

[0025] By using a precise concentration factor control formula to couple and optimize the recovery rates at each stage, the system can operate stably under different water quality conditions, ultimately producing high-quality crystalline salt and reusable fresh water, truly achieving the goal of resource utilization and zero discharge of circulating water wastewater. Attached Figure Description

[0026] To more intuitively illustrate the prior art and this application, exemplary drawings are provided below. It should be understood that the specific shapes and structures shown in the drawings should not generally be regarded as limiting conditions for implementing this application; for example, based on the technical concept disclosed in this application and the exemplary drawings, those skilled in the art are able to easily make conventional adjustments or further optimizations to the addition / reduction / classification, specific shapes, positional relationships, connection methods, size ratios, etc. of certain units (components).

[0027] Figure 1 This is a process diagram of a treatment process for gradually concentrating circulating water wastewater to zero discharge at the end, as provided in an embodiment of this application. Detailed Implementation

[0028] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. It should be understood that these embodiments are merely for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Technical engineers in the field can make some non-essential improvements and adjustments to the present invention based on the above-described content. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Please see Figure 1 A treatment process for progressively concentrating circulating water wastewater to achieve zero discharge at the end, comprising the following steps;

[0030] Step 1: Pretreatment and preliminary softening. Adjust the pH of the circulating water discharge to 6.5-8.0, add flocculants and coagulants to coagulate and settle, remove suspended solids and colloids, and then remove temporary hardness and most of the calcium and magnesium ions through a sodium ion exchanger or chemical softening method to obtain pretreated permeable water.

[0031] Step 2: First-stage reverse osmosis concentration. The pretreated permeate is pressurized and fed into the first-stage reverse osmosis system. The operating pressure is controlled at 0.8-1.5 MPa, and the system recovery rate is set at 50%-60%. After separation, first-stage permeate and first-stage concentrate are obtained. The first-stage permeate is reused in the circulating water system.

[0032] Step 3: Secondary reverse osmosis concentration. The primary concentrate is collected in the first intermediate water tank. After adding scale inhibitor and non-oxidizing bactericide, it is introduced into the secondary reverse osmosis system. The operating pressure is controlled at 2.0-3.5 MPa, and the system recovery rate is set at 60%-70%. After separation, secondary permeate and secondary concentrate are obtained. The secondary permeate is reused.

[0033] Step 4: Nanofiltration desalination and tertiary concentration. The secondary concentrate is passed into a nanofiltration system to selectively separate divalent and monovalent ions, obtaining nanofiltration permeate enriched with monovalent ions and nanofiltration concentrate enriched with divalent ions. The nanofiltration permeate is then passed into a tertiary high-pressure reverse osmosis system, with the operating pressure controlled at 5.0-7.0 MPa and the system recovery rate set at 50%-60%, to obtain tertiary permeate and tertiary concentrate. The tertiary permeate is reused.

[0034] Step 5: Terminal evaporation and crystallization. The tertiary concentrate is transported to the evaporation and crystallization system, where it is concentrated using mechanical vapor recompression technology to obtain crystalline salt and evaporation condensate. The evaporation condensate is then reused.

[0035] This embodiment describes the treatment of wastewater from the circulating cooling water system of a petrochemical enterprise, with a designed treatment capacity of 100 m³. 3 / h. The main parameters of the raw water quality are: total hardness (calculated as CaCO3) 800mg / L, total dissolved solids 1800mg / L, sulfate 450mg / L, chloride ion 600mg / L, silica 30mg / L, pH value 8.2.

[0036] First, pretreatment and preliminary softening steps are performed. The circulating wastewater is collected in an equalization tank, where sulfuric acid is added to adjust the pH to 7.5. It is then introduced into a high-density sedimentation tank, where 50 mg / L of polyaluminum chloride (PAC) is added as a flocculant and 1 mg / L of polyacrylamide (PAM) as a coagulant aid for coagulation and sedimentation to remove most suspended solids and colloids. The effluent from the sedimentation tank enters a sodium ion exchanger, reducing the total hardness of the effluent to 30 mg / L, thus obtaining pretreated permeable water.

[0037] The second step involves feeding the pretreated permeate into the primary reverse osmosis system. The system uses a fouling-resistant spiral wound reverse osmosis membrane (34mil wide channel), with an operating pressure controlled at 1.2MPa and a system recovery rate set at 55%. After separation, primary permeate (55mil) is obtained. 3 / h) and primary concentrate (45m 3 / h). The conductivity of the primary product water is less than 50μS / cm, and it is directly reused in the circulating cooling water tower.

[0038] The third step involves collecting the primary concentrate in the first intermediate water tank, adding 5 mg / L of organophosphorus scale inhibitor and 10 mg / L of isothiazolinone non-oxidizing bactericide, and then pumping it into the secondary reverse osmosis system via a high-pressure pump. The secondary system uses seawater desalination membrane elements, with the operating pressure increased to 3.0 MPa and the system recovery rate set at 65%. A 5 μm security filter is installed before the inlet. Secondary permeate (29.25 m³) is then separated. 3 / h) and secondary concentrate (15.75m) 3 / h). Secondary permeate is reused, and the total dissolved solids content of the secondary concentrate reaches approximately 21,000 mg / L.

[0039] The fourth step involves nanofiltration for salt separation and three-stage concentration. The secondary concentrate enters the nanofiltration system, using a nanofiltration membrane with a sulfate rejection rate higher than 98% and a chloride rejection rate lower than 15%. During system operation, online ion monitoring and flow separation algorithms are implemented. The measured nanofiltration concentrate flow rate is 6.5 m³ / s. 3 The sulfate concentration is 1850 mg / L; the nanofiltration permeate flow rate is 9.25 m³ / h. 3 / h, chloride ion concentration is 2200mg / L. Preset threshold K is 3.0.

[0040] The calculated ratio M = (6.5 × 1850) ÷ (9.25 × 2200) = 12025 ÷ 20350 ≈ 0.59. 0.59 is less than 3.0, therefore the reflux start condition is not met, and the system operates normally. Nanofiltration permeate (enriched with monovalent ions) enters the third intermediate water tank, where the pH is adjusted to 8.0, and a high-efficiency silica-inhibiting dispersant of 4 mg / L is added. It is then fed into a three-stage high-pressure reverse osmosis system. This system uses disc-tube reverse osmosis membrane modules with a maximum pressure resistance of 9.0 MPa and is equipped with an energy recovery device. The operating pressure is controlled at 6.2 MPa, and the system recovery rate is set at 55%. Tertiary permeate (approximately 5.09 m³) is obtained after separation. 3 / h) and tertiary concentrate (approximately 4.16m) 3 / h). The third-stage permeate is reused, and the third-stage concentrate is the final concentrate that needs to be evaporated.

[0041] The overall concentration ratio of the process is calculated using the formula: initial influent flow rate 100m³ 3 The final flow rate of the tertiary concentrate entering the evaporation system is 4.16 m³ / h. 3 / h, the concentration factor N = 100 ÷ 4.16 ≈ 24.0 times. By adjusting the recovery rate at each stage, this embodiment controls the concentration factor at 24 times, which is far superior to the 5-6 times of the traditional process.

[0042] Step 5: End-stage evaporation and crystallization. (The remaining text appears to be incomplete and possibly contains errors.3 The tertiary concentrate, produced at a rate of [amount missing] / hour, is fed to the mechanical vapor recompression evaporation crystallization system. The evaporation temperature is controlled at 90°C, and the crystal slurry circulation device maintains a solid content of 20%. The resulting crystallized salt slurry enters a centrifugal dewatering machine, ultimately producing sodium chloride crystals with a purity of 99.1%, at a rate of approximately 0.2 tons per hour. Evaporation condensate (approximately 3.96 m³ / h) is also produced. 3 The conductivity of the liquid is less than 100 μS / cm, and it is recycled to the circulating water system. The entire process involves no liquid discharge, and the final amount of mother liquor produced by the system is extremely low (approximately 0.03 m³ / h). 3 ( / h), which can be periodically sent out for hazardous waste treatment or further drying, fully meeting the requirements for zero emissions at the end of the line.

[0043] In this embodiment, the system operated continuously and stably for 120 days. During this period, the chemical cleaning cycle of the reverse osmosis system was extended from the conventional 30 days to 75 days. No obvious calcium sulfate scaling occurred in the high-pressure membrane system, which proves the effectiveness and advancement of the process and its built-in algorithm of this invention.

[0044] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A treatment process for progressively concentrating circulating water wastewater to achieve zero discharge at the end, characterized in that, Includes the following steps; Step 1: Pretreatment and preliminary softening. Adjust the pH of the circulating water discharge to 6.5-8.0, add flocculants and coagulants to coagulate and settle, remove suspended solids and colloids, and then remove temporary hardness and most of the calcium and magnesium ions through a sodium ion exchanger or chemical softening method to obtain pretreated permeable water. Step 2: First-stage reverse osmosis concentration. The pretreated permeate is pressurized and fed into the first-stage reverse osmosis system. The operating pressure is controlled at 0.8-1.5 MPa, and the system recovery rate is set at 50%-60%. After separation, first-stage permeate and first-stage concentrate are obtained. The first-stage permeate is reused in the circulating water system. Step 3: Secondary reverse osmosis concentration. The primary concentrate is collected in the first intermediate water tank. After adding scale inhibitor and non-oxidizing bactericide, it is introduced into the secondary reverse osmosis system. The operating pressure is controlled at 2.0-3.5 MPa, and the system recovery rate is set at 60%-70%. After separation, secondary permeate and secondary concentrate are obtained. The secondary permeate is reused. Step 4: Nanofiltration desalination and tertiary concentration. The secondary concentrate is passed into a nanofiltration system to selectively separate divalent and monovalent ions, obtaining nanofiltration permeate enriched with monovalent ions and nanofiltration concentrate enriched with divalent ions. The nanofiltration permeate is then passed into a tertiary high-pressure reverse osmosis system, with the operating pressure controlled at 5.0-7.0 MPa and the system recovery rate set at 50%-60%, to obtain tertiary permeate and tertiary concentrate. The tertiary permeate is reused. Step 5: Terminal evaporation and crystallization. The tertiary concentrate is transported to the evaporation and crystallization system, where it is concentrated using mechanical vapor recompression technology to obtain crystalline salt and evaporation condensate. The evaporation condensate is then reused.

2. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 1, is characterized in that... The chemical softening method in step one adopts a lime-soda softening process. The lime dosage is calculated to remove carbonate hardness by 100-300 mg / L, and the soda ash dosage is calculated to remove non-carbonate hardness by 200-500 mg / L. After softening, the total hardness of the effluent is reduced to below 50 mg / L. A tubular microfiltration membrane is used for solid-liquid separation, replacing the traditional sedimentation tank.

3. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 2, is characterized in that... In step two, the primary reverse osmosis system uses anti-fouling spiral wound reverse osmosis membrane elements with a feed water channel width of 34 mil. During operation, it performs low-pressure flushing for 3 minutes every 120 minutes to flush away trace amounts of organic matter and particles adhering to the membrane surface. The concentrate circulation ratio of the primary reverse osmosis system is controlled at 1.2-1.5 times.

4. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 3, is characterized in that... In step four, the nanofiltration system is used to achieve efficient separation of chloride and sulfate ions. The nanofiltration membrane has a sulfate rejection rate greater than 98% and a chloride rejection rate less than 20%. An online ion monitoring and diversion judgment algorithm is set up. When the ratio of sulfate concentration in the nanofiltration concentrate to chloride concentration in the nanofiltration permeate is detected to be greater than a preset threshold K, a portion of the nanofiltration concentrate is triggered to be recirculated back to the nanofiltration system inlet. The preset threshold K ranges from 2.5 to 4.

0. The diversion judgment algorithm is as follows: Calculate the ratio M = (nanofiltration concentrate flow rate × sulfate concentration in nanofiltration concentrate) / (nanofiltration permeate flow rate × chloride concentration in nanofiltration permeate). If M > K, then the reflux valve is opened, and the reflux flow rate ratio R = [(M / K)-1] × 0.3, and R does not exceed 0.

5.

5. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 4, is characterized in that... The three-stage high-pressure reverse osmosis system uses disc tube reverse osmosis membrane modules or high-pressure spiral wound reverse osmosis membrane modules, with a maximum pressure resistance of 9.0 MPa. The system is equipped with an energy recovery device to recover the pressure energy of the three-stage concentrate for driving the feed pump of the three-stage high-pressure reverse osmosis system, with an energy recovery efficiency of not less than 90%.

6. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 5, is characterized in that... The overall concentration factor N of the process is predicted and controlled by the following formula: N = initial circulating water discharge volume / final concentration flow rate of the three-stage concentrate entering the evaporation and crystallization system. By adjusting the recovery rates set in the first, second, and third-stage reverse osmosis systems, N is controlled between 20 and 50 times.

7. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 4, is characterized in that... Before the operation of the three-stage high-pressure reverse osmosis system in step four, the pH of the nanofiltration permeate is adjusted to 7.5-8.5, and a high-efficiency silica-inhibiting dispersant is added at a dosage of 2-6 mg / L to prevent the precipitation and scaling of magnesium silicate or calcium silicate under high-pressure concentration conditions. The maximum pressure drop of a single membrane element in the three-stage high-pressure reverse osmosis system shall not exceed 0.15 MPa.

8. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 3, is characterized in that... In step three, the secondary reverse osmosis system uses seawater desalination membrane elements and a security filter is installed at its inlet. The filter accuracy is 5 microns. The total dissolved solids content of the concentrate from the secondary reverse osmosis system is controlled within the range of 15,000-25,000 mg / L to ensure that the nanofiltration system operates under optimal feed water quality conditions.

9. The treatment process for progressively concentrating circulating water wastewater to zero discharge at the end, as described in claim 1, is characterized in that... In step five, the evaporation crystallization system is equipped with a crystal slurry circulation device and a centrifugal dehydrator. By controlling the evaporation temperature at 80-100℃ and the solid content of the crystal slurry at 15%-25%, industrial-grade sodium chloride crystal salt with a purity greater than 98.5% is produced. The final discharge of mother liquor from the system is less than 0.5% of the total influent, achieving zero discharge at the end of the entire process.

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