Treatment technology of wastewater with zero discharge

Definition of zero emissions

The concept of "zero discharge" refers to the high degree of purification of industrial wastewater after a series of treatment processes, so that the final discharge of wastewater is reduced to a very low level (usually more than 99%), and even no waste liquid is discharged to the external environment. In this process, the salts and pollutants in the wastewater are efficiently recovered or converted into solid form, and then disposed by landfill or resource utilization, so as to realize the recycling of wastewater and minimize the discharge of pollutants.

The realization of zero discharge usually involves two key steps of concentrated water pretreatment and concentrated crystallization, and the treatment techniques commonly used in these two steps are described in detail below.

Second, concentrated water pretreatment technology

In the concentrated water pretreatment stage, the end treatment technology is an important part of it. This technology uses specific physical, chemical or biological methods to further treat contaminated substances in wastewater, creating favorable conditions for subsequent concentration and crystallization processes.

1. Advanced oxidation technology

Advanced oxidation technology utilizes strong oxidizing hydroxyl radical (·OH) to oxidize and decompose organic pollutants in water. The technology includes Fenton oxidation, ozone catalytic oxidation, photocatalytic oxidation and electrochemical oxidation, and has the characteristics of rapid, non-selective and complete oxidation of organic and inorganic pollutants.

(1) Fenton oxidation

Through the reaction of H2O2 and Fe2+ under acidic conditions, the Fenton oxidation process generates hydroxyl radical (·OH) with strong oxidation, so as to achieve effective removal of organic matter in wastewater. The method has the advantages of simple operation, fast reaction speed and good treatment effect, and is widely used in the wastewater treatment of printing and dyeing, oil refining and other industries.

(2) Ozone oxidation + photocatalysis

The combined application of ozonation and photocatalysis technology can significantly improve the removal rate of organic matter in wastewater. Ozone oxidation can destroy the molecular structure of organic matter, and photocatalysis can further oxidize and decompose organic matter by using the surface active material generated by photoexcitation. Practice has shown that the combination of these two technologies can increase the removal rate of DOC by more than 30%.

(3) electrochemical oxidation

Electrochemical oxidation technology uses electrochemical reactions under the action of electric fields to remove organic matter and inorganic salts from wastewater. The technology has the advantages of low energy consumption, good treatment effect and simple operation, and is especially suitable for treating wastewater with high salinity and high COD.

2. Coagulation/adsorption method

By adding coagulant to the wastewater, the suspended particles and colloidal substances in the water condense into larger particles, so as to achieve solid-liquid separation. The adsorption rule uses the adsorption properties of adsorbents (such as activated carbon) to remove dissolved organic matter from wastewater. These two methods are usually used to remove DOC in wastewater and improve the biodegradability of wastewater.

Third, concentrated water pretreatment - reconcentration technology

After concentrated water pretreatment, in order to further improve the treatment efficiency of wastewater, reconcentration technology is usually used to further concentrate wastewater. In this process, it may be necessary to soften the wastewater to reduce the hardness, silicon content and other adverse factors in the water.

1. Electrodialysis

Electrodialysis can be said to be a kind of salt removal technology, because concentrated water contains a certain amount of salt, and the negative and cation components of these salts will move to the opposite direction of the electrode under the action of the direct current electric field. Electrodialysis is suitable for industries such as electroplating, which requires relatively high water intake and requires direct current. Electrodialysis (ED) is a membrane process in which charged ions are separated from aqueous solutions and other uncharged components by using the principle of counter-ion migration of charged ion membranes under the action of direct current electric field. It is a membrane separation process driven by potential difference. Several groups of alternating negative and cation exchange membranes are arranged in the electrodialysis unit. Under the action of direct current electric field, the cation moves through the positive membrane towards the negative electrode. The anion moves through the negative film towards the positive electrode. In this way, a fresh water chamber for removing ions from the water and a concentrated water chamber for concentrating ions are formed, and the concentrated water is discharged, and the fresh water obtained is the brine.

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2. Special membrane technology
With its excellent separation performance, the special membrane can efficiently separate the organic matter and salinity from the water in RO concentrated water, and produce a permeable liquid with excellent water quality. Its COD and salinity removal rates are significantly more than 90%, which enables the permeant of the special membrane to be discharged directly or further treated by biochemical treatment processes. For concentrated liquid, evaporation and crystallization are processed by MVR technology to achieve zero emissions.

Special membrane technology principle:
The special membrane operates under high pressure conditions, breaking through the limitation of the traditional membrane process on the recovery rate of concentrated water through the liquid, thereby improving the recovery rate of water production and reducing the generation of concentrated water. This not only reduces the scale and cost of the subsequent RO concentrated water treatment process, but also improves the efficiency of the entire treatment process.

3. Overclocking vibration film technology
Overclocking vibrating film technology, similar to the working principle of a shaking sieve, generates shear force by vibrating the film barrel, effectively preventing water impurities from adhering to the film surface. This technology significantly extends the service life of the membrane, broadens the range of inlet water quality requirements, and enables it to treat water sources that are difficult to treat with traditional fixed RO membranes.

Overfrequency vibration film features:
The technology has low inlet water quality requirements, long membrane life, and the operating cost is mainly concentrated on power consumption. A complete overclocking vibration film system requires only a vibration power motor of approximately 7.35kw and a feed and liquid pump of 3.65kw, greatly reducing operating and maintenance costs.

Core objectives of RO concentrated water reconcentration technology:
The fundamental purpose of RO concentrated water reconcentration technology is to reduce the amount of water required for MVR evaporation treatment, thereby reducing the cost of zero emission treatment, and achieving efficient use of resources and environmental protection.

4. Concentrated water crystallization technology

1. Membrane distillation technology
Membrane distillation technology (MD) combines traditional distillation and membrane separation technology to achieve near "zero discharge" of RO concentrated water. The technology has low operating temperature and low pressure, and can make full use of cheap energy, such as solar energy and geothermal energy. Membrane distillation technology is particularly good at treating high-concentration wastewater, and has a very high retention rate for inorganic salts, macromolecules and other non-volatile components.

Membrane distillation technology integration application:
Membrane distillation technology is often used in combination with other technologies, such as scale inhibition pretreatment technology, which can significantly improve the treatment effect and keep the conductivity of the produced water at a very low level. At the same time, the membrane distillation technology is combined with the crystallizer to achieve a total recovery rate of up to 95%.

2. Enhanced evaporation technology
Enhanced evaporation technologies include multi-effect evaporation (MEE), multistage flash (MSF), thermal steam recompression (TVR) and mechanical steam recompression (MVR). These technologies improve the treatment efficiency by optimizing the evaporation process, but relatively speaking, the investment is large and the treatment cost is high.

Features of various enhanced evaporation technologies:
Multi-effect evaporation and multi-stage flash evaporation may face scaling problems in the treatment process. Thermal steam recompression and mechanical steam recompression improve the treatment efficiency by compressing steam, especially MVR technology, which compresses secondary steam by mechanical means, achieving significant energy savings.