In a desalination system, water is treated, for example by reverse osmosis, to produce desalinated product water and a reject stream. Salt from the reject stream is transferred to a flow of low salinity wastewater, such as effluent from a municipal wastewater treatment plant. For example, the reject stream may be fed to the high salinity cell in an electrodialysis (ED) unit. The adjacent low salinity cell receives the low salinity wastewater. The high salinity and low salinity cells are separated by ion exchange membranes. The salty wastewater is discharged, for example to the ocean. The salt-reduced reject stream is treated to extract more product water from it, for example by recycling it to the feed water. Less feed water is needed to produce a given volume of product water. If a thermal desalination unit is combined with divalent ion selective ion exchange membranes, then scaling is reduced.