How to treat water?

This paper introduces a professional knowledge of sewage treatment - industrial application and market demand of secondary reverse osmosis + EDI water treatment and EDI deionized water equipment.

1、 Multi media filtration

The main task of this stage is to roughly filter the tap water, prepare for entering the reverse osmosis membrane, and ensure a certain water quality before entering the reverse osmosis membrane, so as to protect the service effect and service life of the reverse osmosis membrane. The process is to filter the tap water of the raw water tank through fine sand, activated carbon and precision filter, remove the impurities, organic matter, colloid and suspended solids in the water, and prevent these large particle impurities from blocking the reverse osmosis membrane after entering the reverse osmosis membrane. After coarse filtration, the water quality has been improved to a certain extent. And allow to enter the next link.

2、 Primary reverse osmosis

The coarse filtered water passes through the reverse osmosis membrane, which is the primary reverse osmosis. The reverse osmosis membrane is a semi permeable membrane, which can prevent the passage of Ga2 +, Mg2 +, FE-2, SO4-2, CL-1, Na + and other large ions. In order to ensure the effect of reverse osmosis and protect the reverse osmosis membrane, scale inhibitor must be continuously added to the reverse osmosis container, and the water temperature must be above 25 ℃, (use steam heat exchanger in winter) and ensure a certain water pressure, (use vertical pump) under a certain pressure, ionic water is squeezed through the reverse osmosis membrane to form two kinds of water. All water passing through the reverse osmosis membrane, i.e. finished water, enters the next link, while the water not passing through the reverse osmosis membrane is discharged, The water treated by primary reverse osmosis enters the next link - secondary reverse osmosis.

3、 Secondary reverse osmosis

The principle of secondary reverse osmosis is the same as that of primary reverse osmosis. Its function is to further remove the salt in the water (Ga2 +, Mg2 +, FE-2, SO4-2, CL-1, Na + plasma) to further improve the water quality. After secondary reverse osmosis, the water conductivity can be close to 1m Ω CM。 After the primary and secondary reverse osmosis pretreatment, the water retained at last becomes the feed water of EDI, while the water (concentrated water) without reverse osmosis membrane is discharged in time, and its ratio is generally 1:3, that is, about 3 tons of concentrated water (reclaimed water) must be discharged for every ton of qualified water produced.

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4、 EDI (electro deionization) processing

The water after secondary reverse osmosis is stored in the intermediate water tank, and more than 99% of its ions have been removed. However, in order to further improve the water quality, produce ultra pure water and remove the trace elements and CO2 dissolved in the water, it must also be treated by electrodialysis, i.e. EDI. Its principle is as follows. EDI, i.e. continuous electric desalting, uses mixed ion exchange resin to adsorb the anion and cation in the feed water, At the same time, these adsorbed ions are removed through the cation-anion exchange membrane under the action of DC voltage. In this process, the ion exchange resin is continuously regenerated by electricity, so it does not need to be regenerated by acid and alkali. This technology can replace the traditional ion exchange device and produce a resistivity of up to 18 m Ω Cm ultrapure water. This technology is known as the revolution of water treatment industry. Compared with traditional ion exchange, EDI has the following advantages: EDI does not need chemical regeneration; No shutdown is required during EDI regeneration; Provide stable water quality; Low energy consumption; Convenient operation and low labor intensity; Low operation cost.

（1） Feed water treatment of EDI

Feedwater pretreatment is important for EDI and its. The service life, performance and maintenance quantity of components depend on the impurity content in feedwater. If EDI is provided with better pretreatment water, the cleaning rate of components will be reduced. One part of EDI concentrated water is recycled (when the feed water hardness is low and conductivity is low, it can not be recycled), and the other part can be returned to the reverse osmosis feed water, recycled for other purposes or directly discharged to the sewer.

（2） Component structure of EDI

1. Fresh water chamber: fill ion exchange resin between anion and cation exchange membranes to form fresh water unit.

2. Concentrated water chamber: separate each EDI unit with a mesh to form a concentrated water chamber.

3. Extreme water chamber.

4. Insulating plate and pressing plate.

5. Power supply and water connection.

EDI can be operated in parallel to obtain greater flow.

（3） EDI process

Sodium, calcium, magnesium, chloride, nitrate, bicarbonate and other dissolved substances exist in general urban water sources. These compounds are composed of negatively charged cations and negatively charged anions. More than 99% of ions can be removed by reverse osmosis pretreatment. In addition, the raw water may also include other trace elements, dissolved gases (such as CO2) and some weak electrolytes (such as boron and silica). These impurities must be removed from the industrial demineralized water. However, the reverse osmosis process has a poor effect on the removal of these impurities.

These ion exchange membranes do not allow water to pass through. Therefore, they can isolate fresh and concentrated water flows.

The working principle of ion exchange membrane is similar to that of ion exchange resin, which can make specific ions migrate. Anion exchange membrane only allows anions to pass through, not cations; The cation exchange membrane is just the opposite. Filling mixed ion exchange resin between a pair of anion and cation exchange membranes forms an EDI unit. The space occupied by mixed ion exchange resin between anion and cation exchange membrane is called fresh water chamber. A certain number of EDI units are listed together, anion exchange membrane and cation exchange membrane are arranged alternately, and each EDI unit is separated by a mesh to form a concentrated water chamber. Driven by the given DC voltage, in the fresh water chamber, the positive and negative ions in the ion exchange resin migrate to the positive and negative poles respectively under the action of electric field, and enter the concentrated water chamber through the positive and negative ion exchange resin. At the same time, the ions in the feed water are adsorbed by the ion exchange resin and occupy the vacancy flowing down by ion electromigration. In fact, ion migration and adsorption occur simultaneously and continuously. Through this process, the ions in the feed water enter the concentrated water chamber through the ion exchange membrane and are removed to become demineralized water.

Negatively charged anions (such as 0h -, Cl -) are attracted by the positive (+) and enter the adjacent concentrated water chamber through the anion exchange membrane. After that, these ions encounter the adjacent cation exchange membrane when they continue to migrate to the positive electrode, and the cation exchange membrane does not allow them to pass through. These ions are blocked in concentrated water. In concentrated water, the ions passing through the anion and cation membrane maintain electrical neutrality.

The current of EDI module is directly proportional to the ion migration. The current is composed of two parts, one is from the migration of removed ions, and the other is from the H + and 0h - produced by the ionization of water itself. These locally produced H + and 0h - continuously regenerate the ion exchange resin.

The ion exchange resin in EDI components can be divided into two parts. One part is called working resin and the other part is called polishing resin. The boundary between the two is called working front. The working resin is mainly conductive, while the polishing resin is continuously exchanged and regenerated. The working resin undertakes the task of removing most ions, while the polishing resin undertakes the task of removing ions that are difficult to remove, such as weak electrolytes.

（4） Power supply of EDI

The DC power supply used shall be within the operating voltage range of the air conditioner and can provide the voltage required for regeneration. The power of DC power supply shall meet the requirements of EDI maximum current (6a). The ripple rate of DC power supply shall not exceed 30%. Too high ripple rate will make EDI components bear higher than the apparent effective current / voltage in an instant, resulting in damage to the components. When multiple EDI components share one DC power supply, each EDI voltage / current shall be independently adjustable. Equipped with voltmeter and ammeter. At the same time, current limiting devices shall be equipped. In order to protect EDI components, when the water flow through EDI components is lower than a certain point, the power supply shall be turned off.

（5） Instruments for EDI

1. Pressure gauge: measure the feed water pressure and outlet pressure of EDI pure water, concentrated water and extreme water.

Secondary reverse osmosis + EDI water treatment.

2. Flowmeter: measure the flow of pure water outlet, concentrated water inlet, polar water inlet and concentrated water make-up.

3. Conductivity meter: measure the conductivity of EDI feed water and concentrated water inlet.

4. Resistivity meter: measure the resistivity of EDI pure water.

5. Flow switch: if the flow of pure water, concentrated water and polar water flowing into EDI components is too low, the flow switch will cause the system to shut down.

5、 Preservation and supply of pure water

The water after EDI treatment can be finished water and stored in the pure water tank. In order to ensure the water quality, nitrogen sealing method is generally adopted, that is, nitrogen is filled from the top of the pure water tank. After water supply, the liquid level solenoid valve cooperates with the PLC. When the water level of the pure water tank is lower than the minimum water level, the PLC starts the water making program, and the whole system starts water making until the water level of the pure water tank reaches the maximum water level, and the system stops water making. In this cycle, there is a certain water level in the pure water tank.

Deionized water equipment for electroplating is characterized by stable water quality and relatively low cost. Deionized water equipment is a water treatment device that removes anions and anions from water by reverse osmosis, ion exchanger, EDI and other methods. Deionized water equipment has stable performance and is widely used in medicine, electronics, chemical industry, glass, coating, boiler, laboratory and other industries.

Process flow of deionized water equipment for electroplating

1. Ion exchange is adopted, and the process is as follows

Raw water → raw water booster pump → sand filter → activated carbon filter → precision filter → cation resin filter bed → anion resin filter bed → anion resin mixed bed → microporous filter → water point

2. Reverse osmosis is adopted, and its process is as follows:

Raw water → raw water booster pump → sand filter → activated carbon filter → precision filter → reverse osmosis → pure water tank

3. Reverse osmosis and mixed bed are adopted, and the process is as follows:

Raw water → raw water booster pump → sand filter → activated carbon filter → precision filter → reverse osmosis → pure water tank → booster pump → Cation Anion mixed bed → precision filter → water point

6、 Industrial application and market demand of EDI deionized water equipment

In recent years, EDI deionization has been paid more and more attention in various industrial fields. Many industrial systems begin to use electrodeionization as the upgrading technology of their water treatment systems, such as power industry, pharmaceutical industry, microelectronics industry, electroplating and metal surface treatment.

1. Power industry

It is estimated that the operating cost of water treatment units in the power industry accounts for about 10% of the power cost, and using electric deionization instead of ion exchange resin can reduce the cost of treating 1000 gallons of water from $11 to $1.75.

2. Pharmaceutical industry

Although the characteristic of pharmaceutical water is that it does not require a high degree of deionization, the electro deionization system has the characteristics of simultaneous desalination and microbial index control. Therefore, many enterprises have adopted Ro / EDI integrated system. It is said that such systems have stable performance, continuous computer monitoring of the whole process, full-automatic operation and unattended.

3. Electronic industry

The electronic industry has extremely high requirements for water quality, and the water resistivity should be stable more than 18m Ω, while the EDI effluent is generally about 15-17m Ω. Therefore, EDI + polishing resin system is mostly used in the production process of electronic grade water, that is, ion exchange is added after EDI. Although ion exchange is still required in this project, because EDI has removed most ions, the polishing resin hardly needs to be regenerated, Therefore, the cost of water treatment is still very low.

4. Electroplating and metal surface treatment

Electrodeionized water equipment can be used for electroplating wastewater treatment, which can reuse water and recover heavy metal ions. The United States already has experimental devices for this type of system.