Water is normally either freshwater or saltwater. Anything in between is brackish.
Brackish water is common in places where freshwater and saltwater meet such as estuaries, mangrove plantations, and deltas. Its salinity is less than that of seawater but more than that of freshwater. Brackish water doesn't host a diverse array of wildlife as rivers or oceans because the conditions are unideal for both freshwater and saltwater creatures.
Nonetheless, due to their reduced salinity compared to seawater, brackish water is a less costly source of potable water. The degree of water treatment required for brackish water isn't as extensive as seawater, helping desalination facilities save on costs. In fact, some desalination plants like the Thames Water Desalination Plant in London draw their water from estuaries.
In the U.S., there's an ongoing effort to develop energy-efficient desalination solutions, which only require up to 6.9 kWh to process a kilogallon (or 1.8 kWh to process a cubic meter) of water. This is the goal of the Affordable Desalination Collaboration, which includes reverse osmosis membrane manufacturers.
Reverse osmosis systems are widely used in desalination due to their function of capturing the smallest particles in the water. Sea salt can range anywhere between 0.035 and 0.05 microns. Reverse osmosis membranes can easily trap sea salt due to its effective trapping range of 0.0001 to just above 0.001 microns, well within the radius of a conventional atom.