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Zero-drain wastewater desalination process introduction of concentrated water treatment process!

来源:环保小蜜蜂 作者: 阅读:1775 网友评论0 Updated: 2019-09-17 10:52 Source: Green Bee Author: Read: 1775 2010 0 comments

A large amount of concentrated brine will inevitably be produced in the process of zero discharge of industrial wastewater. Agents, acids and other reaction products, the treatment of concentrated brine has been a key technology restricting zero discharge of industrial wastewater from various industries.

The concentrated brine treatment technology includes concentrated brine concentration technology and concentrated brine final disposal technology. On the one hand, in order to recover water resources as much as possible, on the other hand, because concentrated brine is the most expensive and the final disposal cost is high, it is necessary to first concentrate the concentrated brine so that the salt concentration (TDS) mass concentration reaches 50,000 to 80,000 mg / L, in order to reduce Small final disposal process scale reduces investment and saves energy.

At present, GE and Dow have developed an ultra-high-concentration reverse osmosis membrane. As long as the front end is properly pretreated, the TDS of concentrated brine can be increased to 130,000 to 150,000 mg / L, which greatly reduces the amount of concentrated brine that needs to be disposed of. .

1 Final disposal technology of concentrated brine

At present, there are various methods for the final disposal of concentrated brine according to the actual conditions of various enterprises and the surrounding water environment capacity.

1.1 Direct emissions

(1) Direct discharge of surface water Because national standards such as the Comprehensive Wastewater Discharge Standard (GB8978-1996) and the Pollutant Discharge Standard of Urban Sewage Treatment Plants (GB18918-2002) do not restrict TDS, there are still some After the concentrated brine is processed, as long as the content of various pollutants defined in the standard does not exceed the standard, it is directly discharged into surface water or seawater.

The direct discharge of concentrated brine requires attention to the following issues:

① If the water circulation capacity at the exit is very strong, the high TDS concentration of concentrated water will be quickly dispersed, but if a large amount of concentrated water is discharged into lakes, lagoons, shellfish breeding seas, or high-quality fish that have less salinity, Water bodies will cause greater poisoning.

② The structure of the drainage outlet must ensure that the mixing conditions are reached, and no damage will be caused to the receiving water body, such as aquatic life, wildlife and surrounding areas. Under high turbulence, a simple nozzle discharge is generally used. Draining concentrated water into a large amount of receiving water is sufficient to ensure dilution and mixing. However, in most cases a diffuser-type drain is used to improve mixing conditions.

(2) Deep well injection Deep well injection is a simple and effective way to continuously process a large amount of concentrated RO water, and it is not affected by weather conditions. However, the process of deep well injection is very complicated, and the requirements for geographical conditions are quite special. The selected location must be isolated from the aquifer suitable for drinking, so the injection location should be lower than all adjacent aquifers, and the permeability of rock and soil should be relatively Higher for easy injection. Deep well injection has high costs, high design and construction requirements, and the possibility of contaminating groundwater sources. Therefore, this method needs to be carefully selected.

(3) Discharged into municipal sewage treatment system

The direct discharge of concentrated brine into the municipal sewage pipe network is only a transfer of responsibility, which increases the burden on the municipal sewage treatment process. Excessively high TDS may adversely affect the biological treatment section of the municipal sewage treatment plant, and even affect the stable operation of the biochemical tank. Therefore, it is necessary to seek permission from the municipal department and the sewage treatment plant.

1.2 Reuse of concentrated brine

(1) Production water According to the characteristics of the production process, concentrated brine can be used for slag washing and humidity control.

For example, in the coal chemical industry, concentrated brine is usually used for dust removal and sprinkling in coal storage yards and ash and slag yards. Iron and steel enterprises usually use concentrated brine for water spraying in raw material yards, blast furnace slag treatment, or steelmaking slag treatment. However, most of the slag yards or coal yards currently require closed systems, and the amount of water consumed through humidity control is limited.

In addition, the high concentration of chloride ions in the concentrated brine makes it easy to corrode the equipment when entering the raw coal; the concentrated brine entering the ash and slag yard is likely to cause secondary pollution and also affect the quality of the ash and slag comprehensive utilization products. Although concentrated brine is used for dust removal and water spraying in coal yards and ash and slag yards, the above problems still exist, but because of its low cost, it is still widely used in some industries, especially the steel industry.

However, when concentrated brine contains pollutants that are preferentially controlled by the environment, it must be used with caution. A steel company has used concentrated brine from coking wastewater to flush slag. Because coking wastewater contains a large amount of volatile and semi-volatile organic compounds, the odor is serious, causing Local residents' complaints and strikes by corporate workers.

(2) Irrigation water Concentrated saline can be used as fertilizer for plants and seeds that have high salinity, and can also be used as supplemental water in ecological scenic spots, which can save water resources, especially in areas with poor water resources, so irrigation is also a kind of water. Method of resource protection. However, the use of concentrated saline for irrigation may cause pollution to soil and surface water. Therefore, this method is not an ideal concentrated saline treatment method, and only works in a few occasions.

1.3 concentrated brine solidification

(1) The evaporation of concentrated brine in evaporation ponds was basically the earliest natural evaporation and crystallization of evaporation ponds. Fat ponds were dug a depression on the ground to act as a storage tank for concentrated concentrated water. Under appropriate climate conditions, sufficient water can be effectively used. Solar energy, store concentrated brine in an evaporation pond to allow it to gradually evaporate, and periodically remove residual solid salts.

In response to the shortcomings of natural evaporation ponds, such as the slow evaporation rate and large evaporation area, a mechanical evaporation pond has been developed in recent years. A mechanical atomizing evaporator is used to spray various kinds of water into the atmosphere, increasing the ratio of atomized water droplets in the atmosphere. Surface area and "suspension time" accelerate the evaporation of water. The rate of mechanical evaporation of the same pond area is more than 14 times that of natural evaporation ponds.

Evaporation ponds have the advantages of easy construction, low maintenance and operation costs, and the use of solar energy. However, on May 27, 2015, the Ministry of Environmental Protection issued a draft of "Guidelines on Enhancing Environmental Protection in Industrial Parks", which clearly stated: "Various types of parks must not replace standard sewage treatment facilities such as drying ponds and evaporation ponds. "With the frequent occurrence of various types of evaporation pond environmental protection accidents, evaporation ponds will be phased out.

(2) Evaporative crystallizer Evaporative crystallizer mainly refers to a crystallizer that uses various processes to make the solution reach supersaturation under production conditions and then crystallize. At present, the enhanced evaporation crystallization technology mainly includes mechanical compression evaporation process (MVR), multiple effect evaporation (MED), multi-stage flash evaporation (MSF), and membrane distillation (MD).

High-salt-containing wastewater treated by evaporation technology is easily condensed when it exceeds the saturation solubility limit during evaporation in the evaporator, and the salt in the water is easily crystallized and adhered to the surface of the heat exchange tube to form scale. It will affect the efficiency of the heat exchanger and block the heat exchange tube in severe cases. The "seed method" technology solves the scaling problem of the evaporator heat exchanger tube, and is successfully applied to the treatment of various salty industrial wastewater, and is widely used.

The application of evaporation crystallizers in the fields of salt production and seawater desalination is relatively common, and the technology is mature. In the zero discharge of industrial wastewater, there are also successful cases of application, such as the thick salt crystallization of Datang Ketang Coal and Gas Co., Ltd. The forced circulation and vacuum negative pressure three-effect low-temperature low-pressure decompression evaporation crystallizer, the concentrated solid content Up to 300,000 mg / L or more, it is discharged to the two-stage piston pusher centrifuge through the third effect discharge port for direct curing, and the solid content rate after curing is as high as 95% or more. Since the project was put into production in 2013, the operation has been effective. The investment and operation cost of the evaporative crystallizer are extremely high, and the operation management is complicated. These disadvantages restrict the development and application of the evaporative crystallization process.

2 Conclusion

Not only is the high salt content in the concentrated brine, but it may contain a variety of substances harmful to humans or the ecological environment, which must be properly disposed of to prevent secondary pollution. Various disposal technologies of concentrated brine have certain applicable conditions and some problems that need to be solved urgently. It is the current bottleneck technology to limit the zero discharge of industrial wastewater, and it is also the research focus of environmental workers in the future.

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