Circuit board wastewater reuse reverse osmosis system technology

The circuit board production process mainly includes cutting, grinding, circuit, etching, development, film release, chemical copper deposition, gold plating, circular electroplating and other processes. In addition to heavy metal ions such as Cu, Ni and Sn, the wastewater discharged from each production process also contains complexing agents such as EDTA and sodium tartrate. At the same time, it also contains high-concentration organic wastewater and waste liquid such as developer, degreaser, and expansion agent. In addition, a large number of patented commercial reagents are also used in the production process, so that the wastewater of circuit board production often contains hundreds of organic substances.

Due to the particularity of circuit board wastewater reuse, in order to prevent organic pollution, a large amount of ozone is added to the raw water, resulting in a substantial increase in investment and operation costs. Reverse osmosis membrane technology cannot be adapted to treat this kind of sewage alone because of its high pollution rate. Among the many successful cases that have been implemented, most of them use strict pretreatment combined with reverse osmosis membrane technology. According to the pilot test results of several months, this project found that by optimizing the design of reverse osmosis, the pollution rate can be completely reduced, and the desalination rate and flow rate of the reverse osmosis membrane of the wastewater reuse system can be guaranteed to be within the design range.

1.1 Design a reasonable membrane flux

The reverse osmosis water flux design is too high, the reverse osmosis membrane is overloaded, and the possibility of pollution will greatly increase, resulting in a decrease in water production, an increase in the frequency of cleaning the reverse osmosis membrane, and an increase in the cost of maintaining the normal operation of the reverse osmosis membrane. However, the design of reverse osmosis water flux is too low, which leads to the fact that the number of membranes remains unchanged, the more the number of membranes, the less water is distributed to each membrane, and the corresponding amount of concentrated water is reduced, and the lower reverse osmosis concentrated water is not enough. The pollutants in the water are taken out of the reverse osmosis membrane, so as to scale on the concentrated water side of the reverse osmosis membrane, affecting the service life of the reverse osmosis membrane.

According to previous engineering experience and pilot test data, when circuit board wastewater is used as the water source, the higher the potential pollutants, the more conservative the water flux design is. When the design flux is selected to be 20L/m2.h, the reverse osmosis membrane can be reduced. pollution speed, while ensuring lower investment costs.

1.2 Design a reasonable arrangement of reverse osmosis membranes

Increasing the tangential flow velocity on the water supply side of the reverse osmosis membrane is one of the effective ways to reduce reverse osmosis pollution. Under the condition that other conditions (water production, recovery rate, membrane flux) remain unchanged, the size of the tangential flow velocity depends on the arrangement. . The conventional membrane arrangement structure of the water treatment system is multi-stage arrangement, while the concentrated water circulation single-stage arrangement should be adopted when the wastewater is reused. The purpose of adopting a single-stage arrangement of concentrated water circulation is to increase the flow rate of concentrated water and increase the tangential flow rate on the surface of the reverse osmosis membrane to slow down the accumulation of pollutants. When adopting the multi-stage arrangement design, one stage of reverse osmosis concentrated water is the second stage reverse osmosis influent. When the salt content increases, the influent pressure and water volume decrease, and the speed of pollution will be greatly accelerated.

The single-stage arrangement design of concentrated water circulation is adopted. Although the total salt content of the reverse osmosis influent increases after the raw water is mixed with the concentrated water, due to the increased influent flow, the pollution and load of each reverse osmosis membrane are equal, reducing the system pressure difference. , increase the physical flushing efficiency, reduce the pollution and cleaning times of the reverse osmosis membrane, and prolong the service life of the reverse osmosis membrane.

1.3 Design a reasonable recovery rate

Recovery rate refers to the ratio of produced water to influent flow, and is an important parameter for reverse osmosis design and operation. The determination of recovery rate is closely related to the quality of raw water. Generally, the recovery rate is designed as high as possible, which can reduce the amount of supply water and reduce the cost of pretreatment. However, the limit value should be that the reverse osmosis will not precipitate due to the supersaturation of impurities such as salts, otherwise it will have the following effects on the reverse osmosis:

(1) When the pressure is constant, the recovery rate is increased, the concentration polarization phenomenon on the surface of the reverse osmosis membrane is more serious, and the effective pressure is relatively reduced, which leads to a decrease in water production and a decrease in the desalination rate.

(2) When the insoluble salts are continuously concentrated in the membrane element and exceed their solubility limit, they will scale on the surface of the reverse osmosis membrane. In order to meet the relevant requirements, circuit board manufacturers always expect more recovery rates, but high recovery rates lead to extremely high operational risks. According to engineering experience, when the recovery rate is 60%~70%, the system is safer. When the recovery rate of many circuit board wastewater reuse projects is greater than 80%, the frequency of pollution and cleaning increases significantly, and the service life of reverse osmosis membranes is greatly reduced. If you want to improve the recovery rate, it is recommended to design an independent concentrated water recovery system, and adopt ion exchange, frequent reverse electrode electrodialysis and other processes to combine with the reverse osmosis process.

1.4 Choose a reverse osmosis membrane with strong anti-pollution ability

1.4.1 Select reverse osmosis membrane with wide inlet flow channel

Among the many anti-fouling mechanisms, the width of the feed water channel of the reverse osmosis membrane element has been recognized as the most important indicator of the anti-fouling performance of the membrane element, and it is also the key point that users need to consider when selecting products. The wider the inlet flow channel, the more relaxed the system’s requirements for the inlet water quality and the abnormal working conditions of the pretreatment equipment. The wider inlet flow path allows for more efficient membrane cleaning, and the system maintains a low pressure drop even under high fouling conditions. However, when the thickness of the inlet screen increases, since the flow rate of the feed water entering the membrane element remains unchanged, the flow rate of the feed water in the flow channel decreases, which strengthens the concentration polarization phenomenon on the membrane surface, thereby making it easier for pollutants to be adsorbed on the membrane surface. Therefore, there is an optimal range for the thickness of the inlet screen, and it varies with the application fields of the membrane elements. In the recycling of circuit board wastewater, a reverse osmosis membrane element with a channel width of 34 mil should be selected.

1.4.2 Adopt reverse osmosis membrane with smooth membrane surface

The latest research from Yale University in the United States proves that the rougher the membrane surface, the easier it is to be contaminated. Atomic microscopy images showed that fouling particles would first accumulate in the troughs of the rough membrane surface, causing the troughs to become blocked, resulting in severe membrane flux drop.

Selecting a reverse osmosis membrane with a smooth membrane surface, such as Dow’s BW30FR series reverse osmosis membrane, can significantly reduce or delay the occurrence of particle and microbial contamination, reduce the operating pressure of the system, and prolong the service life of membrane elements.

Conclusion

After one-month debugging and one-year stable operation, the measured average water quality of the wastewater reuse system is basically consistent with the designed water quality. When the influent CODcr is 180mg/L, the conductivity is 2800μs/cm, and the Cu2+ is 50mg /L, the effluent quality and reuse rate meet the owner’s requirements. The operation of the system is relatively stable, and the reverse osmosis cleaning frequency is controlled to be cleaned once in February. It shows that through the optimized design of the reverse osmosis system, the pollution rate of the reverse osmosis membrane is reduced, and the ideal treatment effect can be achieved. s

Leave a Comment

Your email address will not be published. Required fields are marked *