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

Microfiltration can trap particles greater than 0.1-1 microns, allowing macromolecules and dissolved solids (inorganic salts) to pass, but will trap suspended solids, bacteria, and large molecular weight colloids and other substances. The operating pressure of the microfiltration membrane is generally 0.3-7bar.

The separation mechanism of microfiltration membrane is mainly sieve interception, which has the advantages of low operating pressure and high membrane flux, but the general microfiltration membrane is easy to be polluted and has a low service life.

Ultrafiltration is used in medicine, chemical industry, water treatment and other fields. Microfiltration is mostly used for water supply pretreatment, and is also used in medicine, chemical industry, electronics and other fields. Ultrafiltration and microfiltration are also used in the treatment of high-salt wastewater, but are generally used as pretreatment.

Reverse osmosis (RO)

Reverse osmosis is also known as reverse osmosis, a membrane separation operation that uses the pressure difference as a driving force to separate the solvent from the solution.

At present, reverse osmosis technology has achieved good results in pre-desalting treatment. After reverse osmosis treatment, it can remove 99.5% of the magnesium and calcium components and 99% of the salt in the water. The load of ion exchange resin can be reduced by more than 90%, and the amount of regenerant of resin can also be reduced by 90%.

Therefore, it not only saves costs, but also benefits environmental protection. Reverse osmosis technology can also be used to remove particles, organic substances and colloids in water, which has a good effect on reducing the pollution of ion exchange resin and extending the service life.

In the case of membrane production technology is becoming more and more mature and the cost is gradually reduced, reverse osmosis also plays a great role in the treatment of high-salt wastewater. However, when the conductivity of high-salt wastewater is greater than 25000us/cm, the membrane flux will decay rapidly, and the scaling phenomenon of membrane parts is serious.

It is worth mentioning that in the reverse osmosis process with efficient crystallization technology, you can improve the amount of water treated by reverse osmosis, extend the service life of the membrane, and treat more high-salt wastewater.

Positive penetration (FO)

Because the operation principle of positive osmosis is different from that of traditional membranes, it has special advantages.

For example, the membrane device is simple in composition and easy to operate; The positive permeable membrane exerts low or even no pressure, saving energy consumption and reducing operating costs; Positive osmosis has a strong ability to separate pollutants and a high salt cutting rate. The pollution to the forward osmotic membrane is almost reversible, and the cleaning efficiency is relatively high.

Under ideal conditions, the forward permeable membrane needs to have an active layer with high retention rate, good hydrophilicity and high water flux, while the supporting layer should have the characteristics of thin thickness, low tortuous factor, high porosity and high mechanical strength. At the same time, it also needs to have strong anti-pollution ability and can be applied in many fields.

The forward osmosis membranes used in early studies were mainly reverse osmosis membranes and modified nanofiltration membranes. With the deepening of research, it is found that the concentration polarization of reverse osmosis is very large due to its thick porous supporting layer, resulting in a rapid decrease in water flux.

Membrane distillation

Membrane distillation technology is a membrane separation technology which combines distillation and membrane method.

The separation principle of vacuum membrane distillation is that one side is pumped into a vacuum state to achieve mass transfer of steam with the pressure difference at both ends, and other substances in the solution are trapped through the membrane, and the liquid is condensed after distillation to achieve separation or concentration.

The process of vacuum membrane distillation is that the operating temperature can be lower than other membrane distillation processes, and the permeability can be larger, so that it is convenient to use cheap heat sources such as geothermal, solar energy and waste heat.

In recent years, there have been more and more researches on the treatment of concentrated brine by vacuum membrane distillation.

Some scholars have studied the vacuum membrane distillation of RO seawater desalination concentrated brine by using polyethylene and polypropylene microporous membranes respectively. According to the research, the maximum retention rate of the membrane can be as high as 99.999%, so the concentration of RO desalination brine can be realized efficiently through this technology.

This technology uses the pressure difference on both sides of the membrane to generate the driving force, and has the advantages of low mass transfer resistance, high heat utilization efficiency, high separation efficiency, large membrane flux, and no evaporation of permeants. But at the same time, this process also has scaling problems and membrane pollution problems when dealing with concentrated brine.