Membrane separation is a new separation technology that appeared in the early 20th century and rose rapidly after the 1960s.

Membrane separation technology is widely used in the process of "separation and concentration of substances" because of its advantages of high separation efficiency, no phase change, no chemical reaction, small volume, low energy consumption and easy operation.

The application of membrane separation technology in zero-discharge wastewater technology has obvious technical advancement and investment economy.

Membrane separation technology is divided according to the morphology, property, structure and separation mechanism of the membrane. According to the separation mechanism, membrane separation technology can be divided into microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), etc.

First, microfiltration

Microfiltration, also known as microfiltration, is a porous membrane (microporous filter membrane) as the filtration medium.

Under the pressure of 0.1~0.3MPa, the particles such as gravel, silt, clay and Giardia, Cryptosphaeria, algae and some bacteria in the solution are trapped, and a large number of solvents, small molecules and a small number of macromolecular solutes can pass through the membrane separation process.

The microfiltration technique removes these substances through mechanical, physical or adsorption, bridging and internal interception of network membranes.

In the zero-emission technology, microfiltration technology can be used in the pretreatment stage.

The content of suspended matter in desulfurization wastewater, industrial wastewater and domestic sewage is high. Usually, most of the suspended matter is removed after softening, coagulation and precipitation to meet the wastewater discharge standard, but it still contains a small amount of suspended matter.

A small amount of suspended matter will hinder the subsequent concentration treatment process and may cause mechanical damage to the subsequent membrane treatment process. The use of microfiltration technology to strengthen the pretreatment stage can improve the inlet water quality and ensure the safety of the subsequent treatment process.

How to reduce microfiltration membrane pollution?

In the process of microfiltration, the phenomenon of membrane contamination is inevitable, which reduces the water yield. In order to reduce the adverse effects of membrane pollution, the degree of membrane pollution can be reduced through the following aspects:

1. In terms of operation mode, adopting a reasonable feeding liquid mode and optimizing the operating conditions of the membrane can improve the flow state and hydraulic conditions of the membrane surface, which is the main means to prevent and control membrane pollution.

2. In the selection of membrane materials and structure, suitable membrane components can be selected through small tests, the membrane surface can be modified, and the additional field can be used to prevent membrane contamination. In the actual use process, the membrane surface can not be modified, so it is particularly important to select the appropriate membrane components and optimize the operating conditions.

Second, ultrafiltration

Ultrafiltration is one of the membrane separation technologies driven by pressure.

That is, under a certain pressure, the small molecule solute and solvent pass through the special film with a certain aperture, so that the large molecule solute cannot pass through and remains on one side of the membrane, so that the large molecule substance is partially purified. When the water passes through the ultrafiltration membrane, most of the colloidal silicon contained in the water can be removed, and a large amount of organic matter can be removed.

Ultrafiltration has certain requirements for wastewater quality during use, turbidity < 50 NTU, CODCr < 200mg/L, SS < 100mg/L, temperature 5~45℃, pH 2~13, residual chlorine 200mg/L. Therefore, it is often necessary to increase the pretreatment means before ultrafiltration, and the selection of pretreatment methods is based on the principle of simple operation, favorable for improving the effluent quality, low cost, and will not destroy the membrane material.

At present, in the domestic research on low-pressure membrane filtration technology, the most feasible and practical pretreatment methods mainly fall into three categories: coagulation, adsorption, oxidation, and their combination processes. The pretreatment before ultrafiltration membrane also includes pre-filtration technology, which generally uses a filter column or filter tank composed of granular filter material. By removing particles larger than the pore size in advance, the pollution on the membrane surface may be reduced and the filtration period of the membrane may be prolonged. Removing small particles will also reduce internal membrane contamination.

Physical and chemical methods can be used to clean the pollution of ultrafiltration membrane. There are two kinds of physical cleaning: positive washing and backwashing. The cleaning is mainly divided into water flushing, air flushing and gas water combined washing. When the long-term accumulation of pollutants causes the physical cleaning ability to weaken, it is necessary to carry out chemical cleaning of the ultrafiltration membrane.

Third, nanofiltration

Nanofiltration membrane (Nanofiltration) originated in the 1970s, with the birth of low pressure reverse osmosis membrane and the development of a new membrane technology.

Nanofiltration membrane has two typical characteristics: one is that the molecular weight of the interception is between the reverse osmosis membrane and the ultrafiltration membrane, and the other is that the surface separation layer of nanofiltration membrane is usually charged.

The charge interaction caused by its surface charge changes the mass transfer process of nanofiltration membrane and the ability of nanofiltration membrane to retain ions of different valence states. Most nanofiltration membranes have negative charges on the membrane surface, and positively charged ions in aqueous solution will be attracted by the membrane surface charge, while negatively charged ions will be repelling away from the membrane surface. This charge effect is called the Donan effect.

The retention rate of dissolved salt in water by nanofiltration is often affected by both the volume size and valence state of salt ions. For example, the standard substance retention order for the three common salt retention tests, Na2SO4, CaCl2, and NaCl, is Na2SO4>CaCl2>NaCl.

In the zero-discharge process of wastewater, crystalline salt is eventually produced. Although the desalination rate of nanofiltration is not as good as reverse osmosis, it has a good retention rate for bivalent ions. Therefore, nanofiltration technology can effectively improve the purity of the final industrial salt and improve the product quality in the pretreatment stage. However, it inevitably leads to the increase of construction costs and operating costs in the pretreatment stage, so for different water plants, it is necessary to understand the specific situation of the treated water quality, and make a preliminary judgment on whether it is necessary to increase nanofiltration technology through the determination of various ion components in the wastewater.

Fourth, reverse osmosis

Reverse osmosis is one of the common water treatment technologies.

The principle is that under the action of higher osmotic pressure than the solution, other substances can not pass through the semi-permeable film and these substances and water leave, effectively removing dissolved salts, colloids, microorganisms, heat sources, organic matter, etc.

In other words, the principle of reverse osmosis salt removal is that in salt water (such as raw water), the pressure is greater than the natural osmotic pressure, so that the penetration is carried out in the opposite direction, and the water molecules in the raw water are pressed to the other side of the membrane, becoming clean water, so as to achieve the purpose of removing the salt in the water.

Reverse osmosis process is a process that is opposite to the natural osmosis phenomenon. It is a membrane separation technology driven by pressure difference, and it is also one of the most advanced membrane separation technologies at present.

Reverse osmosis can not only be used in the preparation of boiler feed water, but also in the treatment of wastewater. Compared with microfiltration, ultrafiltration, nanofiltration and other technologies, reverse osmosis has a higher desalting rate and recovery rate, which can effectively reduce the discharge of concentrated water, thereby reducing the energy consumption of the subsequent evaporation solidification process. However, reverse osmosis technology also has high requirements for inlet water quality, generally speaking, the pH range is between 4 and 10, the temperature is less than 40 ° C, the sludge density index SDI is less than 5, free chlorine is less than 0.1mg/L, turbidity is less than 1, and the iron content is less than 0.1mg/L.

Main causes of reverse osmosis membrane pollution:

Like other membrane treatment technologies, reverse osmosis inevitably produces membrane pollution during operation, the main reasons are as follows:

1. Fouling. Reverse osmosis is a process of concentration of raw water. As the raw water is concentrated, these insoluble salts are precipitated beyond their solubility product, and scale is formed on the surface of the film. In addition, the concentration polarization will aggravate this phenomenon.

2. Microbial contamination. Reverse osmosis water may contain a variety of biological organisms, such as bacteria, algae, fungi, viruses and other higher organisms.

3. Organic pollution. Reverse osmosis water may contain a large variety of organic matter, in the process of system operation, these organic matter will be adsorbed on the surface of the membrane, resulting in the loss of membrane flux, especially in serious cases, irreversible flux loss.

4. Colloid contamination. Colloidal contamination is mainly caused by the concentration of raw water and the change of pH value, and the contaminant components may be organic or inorganic monomers or complexes.

How to prevent reverse osmosis membrane contamination?

In terms of preventing reverse osmosis pollution, the following points can be adopted:

1. Adjust the operating parameters of the reverse osmosis device, and the protective film is safe, mainly pH value, temperature and operating pressure.

2. In order to prevent the occurrence of inorganic salt scaling on the surface of the film, it can be prevented by adding acid and adding scale inhibitor.

3. Prevent colloid contamination through media filtration, coagulation-assisted coagulation process, filter type filtration or comprehensive process to achieve preventive effect.

4. The most effective way to prevent microbial contamination is to chlorinate, but in order to prevent the membrane from being oxidized, it is necessary to consider the reverse osmosis water for dechlorination.