Many people believe that the bodies of water found on the planet are classified into two: freshwater and seawater. However, there are other bodies of water that fall in the middle; they’re saltier than freshwater, but not salty enough to be called seawater. These are known as "brackish water" and can be found in places where freshwater and seawater meet, notably the Baltic Sea and Caspian Sea.
As brackish water still contains sizable amounts of salt, it’s not fit for human consumption without undergoing treatment first. Reverse osmosis water treatment systems are often employed to make it cleaner, but the process involved isn’t always as simple as it needs to be. In addition to salt, brackish water may contain sediments like silica and nitrate which can clog and/or contaminate water systems. These sediments need to be removed using filtration or oxidation during the pre-treatment process.
Brackish water also reacts differently to certain water pressures, which means the water treatment process will vary in result unless the treatment system is properly tweaked to accommodate the pressure variances. In theory, the osmotic pressure needed to treat brackish water should fall between 5 to 75 psi. In practice, most brackish water treatments need up to 250 psi in order to be successful.