The primary difference between the types of water mentioned above is in the amount of total dissolved solids (TDS) they contain. Brackish water typically contains TDS in concentrations ranging from 1,000 milligrams per liter (mg/l) to 10,000 mg/l. Saline water or salt water has more than 10,000 mg/l TDS. Seawater typically is very salty (TDS >35,000 mg/l). Brine contains dissolved minerals and other contaminants that are removed from the water during desalination. TDS concentration of brine is also very high (>10,000 mg/L).

In a reverse-osmosis system, the greater the TDS concentration of the water, the higher the pressure needed for the pumps to push water through the membranes, and consequently, the higher the energy costs. Desalinating seawater is, therefore, usually more costly than desalinating brackish water.