Desalination — the process of making freshwater from saltwater — actually has no effect on the pH of the oceans, even though more salty water is returned to the ocean. That’s because pH, which measures the acidity of a solution, is calculated by the concentration of hydrogen ions, which remain unchanged in the desalination process.
However, desalination does affect salinity. To understand why, let’s take a look at how desalination works. First, high-pressure filters with extremely small pores separate ions of sodium, chloride, and sulfate that are dissolved in seawater. For each gallon of freshwater made, another gallon of doubly concentrated seawater is produced. For example, to make five gallons of freshwater, we would need to filter 10 gallons of seawater. What’s left is five gallons of saltwater containing twice as much salt.
The average ocean salinity is about 35 parts per thousand (ppt), or 3.6% dissolved salt. In other parts of the world, salinity can be higher or lower depending upon rainfall, runoff from rivers, and evaporation. Off San Diego and most of Southern California, the ocean salinity is 33.5 ppt. So, in the case of desalination in Southern California waters, a doubling of the seawater concentration produces 67 ppt.
This doubly salty water poses an environmental threat because it could affect organisms living in the area if it is not diluted before the water is discharged into the ocean. For some desalination operations, it is thought that the disappearance of some organisms from discharge areas may be related to the non-dilution of the salty outflow.
— Jeffrey Graham, research physiologist and marine biologist, Center for Marine Biotechnology and Biomedicine