CN101979339A - Method and device for removing arsenic and fluorine in water using flocculant combined with nanofiltration membrane system - Google Patents

Abstract

The invention discloses a method and a device for removing arsenic and fluorine from water by using a flocculant and nano filter membrane combined system, which belongs to the field of water treatment by removing harmful anions from water with a membrane system. The device for removing arsenic and fluorine from water by using the flocculant and nano filter membrane combined system is formed by connecting a raw water flocculation water tank, a primary plate cross flow type nano filter membrane component, a secondary relled nano filter membrane component. In the device, calcium oxide is used as the flocculant, raw water containing suspended matters such as calcium hydroxide colloid, calcium arsenate and calcium fluoride passes through a plate membrane horizontally and the suspended matters are unlikely to settle onto the surface of the membrane; the secondary membrane component designed with a relled structure deepens a water quality conditioning effect; the part of raw water, which remains above the two nano filter membranes, is called concentrated water and returns to a raw material tank from a return pipe to react with arsenic and fluorine in raw water to form complexes; and the molecular diameter of the calcium arsenate and calcium fluoride, which are formed by the complexation of the flocculant and arsenic and fluorine elements, is far greater than the average aperture of the nano filter membrane layer, so the removal rate of arsenic and fluorine reaches over 90 percent.

Description

Method and device for removing arsenic and fluorine in water using flocculant combined with nanofiltration membrane system

technical field

The invention belongs to the field of water treatment for membrane systems to remove harmful anions in water, in particular to a flocculant combined with nanofiltration membrane system for water treatment of carcinogenic substances such as arsenic and fluorine in water, which uses calcium oxide as the main component. A method and device for removing arsenic and fluorine from water.

Background technique

Both arsenic and fluorine are naturally occurring elements. Long-term geological processes such as weathering and dissolution have gradually migrated arsenic and fluorine from rock ore deposits into the groundwater environment, resulting in varying degrees of arsenic and fluorine pollution in groundwater. If groundwater containing arsenic and fluorine is used as a source of drinking water, it will become an important way for arsenic and fluorine to produce and release toxic effects on the human body. Ingestion of arsenic by the human body can cause skin depigmentation, keratinization, and canceration. At present, more than 20 countries and regions, including my country, have reported endemic arsenic poisoning incidents. The number of people exposed to arsenic has exceeded 50 million. . Fluorine is one of the essential trace elements for the human body, but the concentration of fluorine in drinking water should be in the range of 0.5-1.0 mg/L. When drinking water lacks fluorine, the human body is prone to dental caries; but the concentration of fluorine in drinking water for a long time is higher than 1.0 When drinking water with a fluoride concentration of 3.0 mg/L, it will cause dental fluorosis; drinking water with a fluoride concentration higher than 3.0 mg/L will cause bone fluorosis. The groundwater of more than 20 provinces, municipalities and autonomous regions in my country contains high fluoride, especially in the arid regions of Northwest China, where groundwater is the main source of drinking water, causing about 70 million people to be threatened by endemic fluorosis. In order to control the harm of arsenic and fluorine in water to the human body, my country has strictly regulated the concentration limits of arsenic and fluorine in water in the form of standards: "Drinking Water Hygienic Standards" (GB5749-2006) stipulates that the concentration of arsenic in drinking water must be If it is lower than 0.01mg/L, the fluorine concentration must be lower than 1.0mg/L; "Groundwater Quality Standard" (GB/T 14848-93) requires that the concentration of arsenic in groundwater of Class III and above should be lower than 0.05mg/L, and the fluorine concentration Should be lower than 1.0mg/L.

At present, the methods for treating groundwater containing arsenic and fluorine mainly include coagulation co-precipitation, adsorption/ion exchange, electrochemical treatment, membrane treatment, etc. The coagulation and co-precipitation method is easy to operate, but it occupies a large area and generates a large amount of arsenic-containing and fluorine-containing sludge; adsorption/ion exchange has high requirements on the water quality of the treated water, and the regeneration of the adsorbent is complicated; the equipment for electrochemical treatment is expensive and difficult to operate. High level requirements. Compared with the above treatment methods, the membrane treatment method has unique advantages in the removal of arsenic and fluorine from groundwater: the membrane treatment equipment occupies a small area, the application is flexible, and the water quality after treatment is good. There are many types of membranes used in membrane treatment methods, among which nanofiltration membrane (Nanofiltration Membrane, NF) is a pressure-driven membrane between reverse osmosis membrane and ultrafiltration membrane. Much higher than the ultrafiltration membrane, the requirement for transmembrane pressure is much lower than that of the reverse osmosis membrane, so the nanofiltration membrane has the advantages of cost-effectiveness. The membrane pore diameter of the nanofiltration membrane is about 1nm, but the diameters of arsenate ion, arsenite ion, and fluorine ion in water are 0.78nm, 0.38nm, and 0.26nm, respectively, which are smaller than the average pore diameter of the nanofiltration membrane layer, resulting in nanofiltration The range of membrane rejection rate for arsenic and fluorine is between 40-60%. Usually, the above-mentioned treatment effect can only be achieved after the optimal combination of multi-stage and multi-stage nanofiltration membranes. At the same time, factors such as metal cations and hardness in groundwater will accelerate the formation of membrane scale on the surface of nanofiltration membranes and reduce the retention efficiency of arsenic and fluorine.

Therefore, in view of arsenic and fluorine pollution in groundwater, it is important to develop a membrane system that can give full play to the advantages of flexible application of the membrane system and good water quality, and can effectively improve the retention rate of small ions such as arsenic and fluorine, prolong the service life of membrane components, and reduce economic costs. Difficulties to be solved urgently in the current research and application of arsenic and fluoride removal from groundwater.

Contents of the invention

The object of the present invention is to aim at the pollutants of arsenic and fluorine in water, especially the pollutants of arsenic and fluorine in groundwater, to provide a solution that uses calcium oxide as a flocculant combined with a nanofiltration membrane system to remove arsenic and fluorine in water with high performance, economical feasibility and flexible application. The method and device are characterized in that the method for removing arsenic and fluorine in water by using a flocculant combined with a nanofiltration membrane system includes:

1) The membrane system composed of the first-level flat cross-flow nanofiltration membrane and the second-level roll-type nanofiltration membrane is used to reduce the generation of membrane scale and improve the recovery rate of the system through the self-circulation of concentrated water;

2) Calcium oxide is used as a flocculant, and calcium oxide is added to the raw water flocculation tank to obtain calcium hydroxide colloid. At this time, arsenic, fluorine and calcium hydroxide colloid react to form calcium arsenate and calcium fluoride slightly soluble compounds;

3) After the booster pump presses the raw water after flocculation into the above-mentioned membrane system composed of the first-level flat cross-flow nanofiltration membrane and the second-level roll-type nanofiltration membrane, the membrane system intercepts calcium arsenate, calcium fluoride, etc. in the water. For insoluble compounds, since calcium arsenate and calcium fluoride are not easy to settle in water, solid-water separation is carried out by nanofiltration membrane, which greatly improves the efficiency of arsenic and fluorine removal;

4) The fresh water that has been separated from solid water is output after passing through the series device of a stop valve, a flow meter, an online conductivity meter and a pressure gauge.

The dosage range of the calcium oxide is 0.1-10mmol/L; in order to prevent the carbon dioxide in the air from consuming the calcium hydroxide in the water, the raw water flocculation water tank and the membrane system water tank are both designed to be closed.

The device for removing arsenic and fluorine in water by the flocculant combined with the nanofiltration membrane system is to pass the raw water flocculation tank 1 through the series device of the booster pump 3, the flow meter 6, the online conductivity meter 7 and the pressure gauge 8, and the first-stage flat plate The lower layer of the flow nanofiltration membrane module 2 is connected, and the upper layer of the first-level flat cross-flow nanofiltration membrane module 2 is connected to the upper end of the second-level roll-type nanofiltration membrane module 4 through the flow meter 6, and the upper end of the second-level roll-type nanofiltration membrane module 4 The right side is connected to the series device of stop valve 5, flow meter 6, online conductivity meter 7 and pressure gauge 8 to output fresh water 9; After the lower layer of the nanofiltration membrane module 2 is connected, it is connected to the upper end of the raw water flocculation tank 1 through the series device of the flow meter 6, the online conductivity meter 7 and the pressure gauge 8.

The raw water flocculation tank 1 has a stirring device.

The beneficial effects of the present invention are as follows:

1. Calcium oxide flocculant is a common water treatment agent with low cost;

2. The molecular diameters of calcium arsenate and calcium fluoride produced by the complexation of the flocculant with arsenic and fluorine are micron-sized, which is much larger than the average pore size of the nanofiltration membrane, so the removal efficiency of arsenic and fluorine can reach more than 90%;

3. The flocculant is added into the raw water to increase the pH value of the raw water, so that metal cations such as iron and manganese in the water are precipitated and separated from the water through the nanofiltration membrane, thereby improving the water quality;

4. The membrane system is composed of two-stage membrane modules with simple structure and low energy consumption. The membrane layer is not easy to scale, and the effect of removing arsenic and fluorine is remarkable; the method has high efficiency, low cost and easy operation.

5. The concentrated water self-circulation structure of the membrane system can make full use of the flocculant added and improve the recovery rate of the system at the same time.

Description of drawings

Figure 1 is a schematic diagram of a system for removing arsenic and fluorine in water by a flocculant combined with a nanofiltration membrane system.

Detailed ways

The present invention aims at arsenic and fluorine pollutants in water, especially arsenic and fluorine pollutants in groundwater, and provides a high-performance, economically feasible, and flexible application that uses calcium oxide as a flocculant and adopts a flocculant combined with a nanofiltration membrane system to remove arsenic, fluorine, and arsenic in water. Fluorine method and apparatus. The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

Figure 1 is a schematic diagram of a system for removing arsenic and fluorine in water by a flocculant combined with a nanofiltration membrane system. In Fig. 1, the raw water flocculation tank 1 is connected to the lower layer of the first-stage flat cross-flow nanofiltration membrane module 2 after passing through the series device of the booster pump 3, the flow meter 6, the online conductivity meter 7 and the pressure gauge 8, and the first-stage flat plate The upper layer of the cross-flow nanofiltration membrane assembly 2 is connected to the upper end of the second-stage roll-type nanofiltration membrane assembly 4 through the flow meter 6, and the right side of the second-stage roll-type nanofiltration membrane assembly 4 is connected to the stop valve 5, the flow meter 6, and the online The series connection of the conductivity meter 7 and the pressure gauge 8 outputs fresh water 9; after the left side of the second-stage roll-type nanofiltration membrane module 4 is connected to the lower layer of the first-level flat cross-flow nanofiltration membrane module 2, it passes through the flow meter 6. The series device of the online conductivity meter 7 and the pressure gauge 8 is connected to the upper end of the raw water flocculation tank 1 . The nanofiltration membrane system in the device is a membrane system composed of a first-stage flat cross-flow nanofiltration membrane and a second-stage roll-type nanofiltration membrane. The raw water containing calcium hydroxide colloid, calcium arsenate, and calcium fluoride flocs passes through the flat plate The membrane is not easy to settle on the surface of the membrane; the second-stage membrane module is designed as a roll-type membrane to enhance the water quality treatment effect; the part of the raw water that fails to pass through the two-stage nanofiltration membrane is called concentrated water, which is returned to the raw water tank through the return pipeline. Continue to react and complex with arsenic and fluorine in the raw water, and the molecular diameters of calcium arsenate and calcium fluoride produced after the flocculant complexes with arsenic and fluorine are micron-level, which is much larger than the average pore size of the nanofiltration membrane layer, so the arsenic, The removal efficiency of fluorine can reach more than 90%.

The groundwater containing arsenic and fluorine pollutants is treated by using the method for removing arsenic and fluoride, and the treated water can meet the requirements of the hygienic standards for drinking water in my country. The method for removing arsenic and fluorine can also effectively intercept iron, manganese, organic matter and other substances in groundwater, thereby comprehensively improving the quality of treated water.

Example 1

Take some groundwater, the quality of the groundwater: arsenate ion 53μg/L, arsenite ion 19μg/L, Fe 0.21mg/L, Mn 0.15mg/L, alkalinity 305mg/L, water temperature 14℃, redox potential 8.8mV, conductivity 530μS/cm.

Dosing calcium oxide 0.25mmol/L, the transmembrane pressure of the nanofiltration membrane is 0.5MPa, the recovery rate of the membrane system is 50±10%, the concentration of arsenic in the effluent of the first-level flat nanofiltration membrane is 9.5μg/L, the second-level The retention rate of arsenic in the effluent of the roll-type nanofiltration membrane is 7.1 μg/L, and the concentration of Fe and Mn in the raw water is reduced to below 0.1 mg/L.

Example 2

Take some groundwater, the quality of the groundwater: F 2.16mg/L, Fe 0.64mg/L, Ca 16.87mg/L, Mg 26.36mg/L, Si 13.5mg/L, total dissolved solids (TDS) in water 356mg/L L, water temperature 9.3°C, conductivity 758μS/cm.

Dosing calcium oxide 0.7mmol/L, the transmembrane pressure of the nanofiltration membrane is 0.5MPa, the recovery rate of the membrane system is about 60%, the concentration of fluorine in the effluent is 0.11mg/L, and the concentration of Fe is 0.12mg/L.

Example 3

Laboratory water distribution experiment, water quality: arsenate ion 100μg/L, F 2.0mg/L, tap water distribution, water temperature 24°C, conductivity 340μS/cm.

Dosing calcium oxide 0.25mmol/L, the transmembrane pressure of the nanofiltration membrane is 0.8MPa, the recovery rate of the membrane system is 70±10%, the concentration of arsenic in the effluent is 5.3μg/L, and the concentration of fluorine is 0.04mg/L L.

Claims (4)

1. A method for removing arsenic and fluorine in water using a flocculant in conjunction with a nanofiltration membrane system, characterized in that the method for removing arsenic and fluorine in water using a flocculant in conjunction with a nanofiltration membrane system comprises: 1) The membrane system composed of the first-level flat cross-flow nanofiltration membrane and the second-level roll-type nanofiltration membrane is used to reduce the generation of membrane scale and improve the recovery rate of the system through the self-circulation of concentrated water; 2) Calcium oxide is used as a flocculant, and calcium oxide is added to the raw water flocculation tank of the raw water to be treated to obtain calcium hydroxide colloid. At this time, arsenic, fluorine and calcium hydroxide colloid react to form calcium arsenate and calcium fluoride. soluble compound; 3) A booster pump reacts arsenic, fluorine and calcium hydroxide colloids to form calcium arsenate and calcium fluoride slightly soluble compounds and presses them into the membrane composed of the first-level flat cross-flow nanofiltration membrane and the second-level roll-type nanofiltration membrane After the system, the membrane system intercepts arsenic and fluorine in water; calcium arsenate and calcium fluoride are insoluble compounds with high solubility products, which are not easy to settle in water, so solid-water separation is carried out by means of nanofiltration membranes; 4) The fresh water that has been separated from solid water is output after passing through the series device of a stop valve, a flow meter, an online conductivity meter and a pressure gauge. 2. The method for removing arsenic and fluorine in water by using a flocculant combined with a nanofiltration membrane system according to claim 1, wherein the dosage range of the calcium oxide is 0.1-10mmol/L; in order to prevent the consumption of carbon dioxide in the air Calcium hydroxide in the water, the raw water flocculation water tank and the membrane system water tank are all designed to be closed. 3. A device for removing arsenic and fluorine in water using a flocculant in conjunction with a nanofiltration membrane system, characterized in that the device for removing arsenic and fluorine in water with a flocculant in conjunction with a nanofiltration membrane system is to pass the raw water flocculation tank (1) through an increasing The serial device of pressure pump (3), flowmeter (6), online conductivity meter (7) and pressure gauge (8) is connected to the lower layer of the first-stage flat cross-flow nanofiltration membrane module (2), and the first-stage flat cross-flow nanofiltration membrane module (2) is connected in series. The upper layer of the flow nanofiltration membrane assembly (2) is connected to the upper end of the second-stage roll-type nanofiltration membrane assembly (4) through the flow meter (6), and the right side of the second-stage roll-type nanofiltration membrane assembly (4) is connected to the stop valve (5), flow meter (6), on-line conductivity meter (7) and pressure gauge (8) output fresh water (9) after the serial device; After the lower layer of the first-stage flat cross-flow nanofiltration membrane module (2) is connected, it is connected to the upper end of the raw water flocculation tank (1) through the series device of the flow meter (6), the online conductivity meter (7) and the pressure gauge (8). . 4. The device for removing arsenic and fluorine in water by using a flocculant combined with a nanofiltration membrane system according to claim 3, wherein the raw water flocculation tank (1) has a stirring device.

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