[labawi]

Why would you store it, instead of mixing it back in the sea?

I guess you would need some pipes and pumps to move it away from your inlets, but that seems comparatively cheap.

[walrus01]

because shoving all that brine back into the sea in the general area of the desalination plant causes a localized ecological disaster and dead zone of marine life

[labawi]

As said, use some pipes and pumps to move it away. Don't forget to dilute (again, use pipes and pumps or whatever you prefer).