What is the ruling on desalinating drinking water and selling it, knowing that one-third or one-half of the quantity is wasted in drainage?

If the remaining water after the desalination process cannot be utilized in any way, then there is no harm in discharging it into the drainage pipes. Obtaining desalinated water requires wasting a portion of the salty water, and people are in need of potable water. Some places depend on desalinating seawater.