True, but the purification plant is most likely producing brine, not salt, because crystallized salt in a desalinization plant is a no go. One would need a second process to convert that brine into salt, such as solar stills. With the amount of water we are talking though, its likely that most of this brine makes its way back into the sea.
1 water = 192 Gallons. It makes 30000 waters a year. Sea water has a salinity of roughly 3%. So we are talking 23,100 ft^3 of salt a year or 831.6 tons a year. Its a good bet that most of that makes it back into the sea.