Sub-seawater, a source of water between seawater and fresh water, is 1‰ higher than fresh water and 3% lower than seawater, which is between 3000 ~ 8000 us/cm.

The nature

The salt content of sub-seawater is much less than that of seawater, which is usually around 3000mg/L, while sub-seawater is between seawater and fresh water. The salt content of subsea water is more than 1‰ of that of fresh water and less than 3% of seawater. Its conductivity (salinity) is generally between 3000 ~ 8000 us /cm, but the conductivity of seawater is between 30000~40000 us/cm, that is to say, the conductivity of sub-seawater is only about 1/10~1/5 of that of seawater.

There are two main forms of sub-seawater. One is that the water in the coastal river mouth becomes the sub-seawater with low salinity due to the influence of the tide. The other is the water in the coastal tidal flats. The water quality of these two places is characterized by low turbidity, low salt content, high algal organic content and high microbial and bacterial content. Compared with seawater, although the salt content is reduced, the pollution of algae and microorganisms to the reverse osmosis membrane greatly increases the treatment cost of sub-seawater desalination. Therefore, we think that the conventional process of water plants can be used to pretreatment sub-seawater, remove algae, microorganisms, bacteria and other substances that will contaminate the reverse osmosis membrane in the sub-seawater, and then desalination through the reverse osmosis membrane, which can greatly reduce the cost of sub-seawater desalination treatment. The cost of subsea water treatment can also be reduced if some medium scale or largescale engineering projects are invested and operated.

Some countries have rich sub-seawater resources, if rational development and utilization, it can greatly alleviate the problem of water shortage in coastal areas.