High salt wastewater in the realization of "zero discharge" path

01 What is high salt wastewater?

Source and water quality characteristics of high salt wastewater

In China, there are three main sources of high-salt wastewater:

1. Concentrated brine produced in the process of seawater desalination

There are two main ways to deal with high-salt wastewater generated by seawater desalination: one is to use waste recycling to generate economic benefits and achieve true "zero emission"; The second is to directly discharge high-salt wastewater into sewage treatment systems, rivers, lakes or oceans.

However, due to the lack of technical and economic costs in most coastal areas, the second treatment method is generally chosen for production.

2. High salt wastewater directly discharged in the industrial production process

Generally speaking, inorganic salts in high-salt wastewater mainly come from production wastewater and domestic sewage (potassium ions, calcium ions, sodium ions, chloride ions, sulfate ions, etc.), and some of the organic substances it contains are mainly glycerol and low carbon chain compounds.

It is worth mentioning that most industrial wastewater in addition to containing the above potassium sodium calcium and other inorganic salt ions, the inorganic salt ions contained in industrial wastewater in different fields are very different, and even some high-salt wastewater also contains some heavy metal elements.

3. Salt water generated by recycling of industrial production wastewater

Such as iron and steel enterprises, coal chemical industry, petroleum and other large displacement of industrial industries, in order to save energy and reduce emissions, in the production process need to recover most of the water reuse, in the reuse process will also have a certain concentration of salt water generated.

If this part of concentrated brine is not treated and then discharged, it will cause great environmental pollution. After treatment, different industrial wastewater will produce high content of wastewater, such as calcium, magnesium, potassium, sodium, chloride ions, carbonate ions and so on.

02 What are the treatment methods of high salt wastewater?

Traditional biological treatment methods are difficult to play

At present, there are dozens of treatment methods for high-salt wastewater, including thermal method, membrane method, ion exchange method, hydrate method, solvent extraction method and freezing method.

Thermal and membrane desalination technologies are the main technologies used in large-scale industrial applications at present.

The thermal method can be divided into multi-stage flash distillation (MSF), multi-effect evaporation (MED) and pressure vapor distillation (VC). In the 1990s, the seawater desalination technology was mainly multi-stage flash evaporation, especially in the Middle East countries, but MSF was greatly challenged by multi-effect evaporation and membrane technology in the later period [3].

Membrane desalination technology represented by RO technology, because it does not require a lot of heat energy, it is suitable for large, medium and small-scale brine desalination.

For the zero discharge treatment of high salt wastewater, direct evaporation and crystallization can achieve the purpose of zero discharge, but it costs a lot of energy and wastes resources.

The use of membrane technology can further concentrate high-salt wastewater into ultra-high salt wastewater, and the freshwater part can be directly reused. The concentrated ultra-high salt wastewater can be evaporated and crystallized to achieve zero discharge, which greatly reduces energy consumption and rationally utilizes part of water resources.

However, membrane technology has certain requirements for the quality of incoming water. Therefore, the high-salt wastewater must be pre-treated (agent softening, filtration, ion exchange, etc.), so that the membrane pollution can be effectively reduced, the service life of the membrane, and the effluent quality can be improved.

03 Key technologies for zero discharge of high salt wastewater

3 stages: pretreatment, film treatment, evaporation crystallization

Combined with the above analysis, the key technologies of zero discharge of high-salt wastewater can be divided into three stages: pretreatment stage, membrane treatment stage and evaporation stage.

1. Pretreatment

Hardness is divided into total hardness, temporary hardness and permanent hardness.

The total hardness refers to the total amount of Ca2+ and Mg2+ in the water.

Temporary hardness is also known as carbonate hardness, the main chemical composition is Ca(HCO3)2, Mg(HCO3)2. Because the salts decompose into precipitates after heating from water, it is called temporary hardness.

Permanent hardness, also known as non-carbonate hardness, mainly refers to CaSO4, MgSO4, CaCl2, MgCl2, Ca(NO3)2, Mg(NO3)2 and other salts in water. This hardness cannot be removed by heating, so it is called permanent hardness.

Hardness is an important indicator of water quality, removing the hardness of water is called the softening of water. At present, water softening mainly includes precipitation softening method, enhanced crystallization technology, adsorption and ion exchange method.