No, the salt water you're about to discharge it into.

Have a pump that draws in 10L of ocean water for every 1L of brine you need to dispose of, mix 'em up, and discharge the 11L of only-slightly-saltier water back into the sea.

Not sure when it makes more sense to do that vs. having a leach-field type of brine discharge. They both ultimately do the same thing, but one requires more mechanicals, the other requires more piping and "passive" infrastructure.

Or a large amount of regular saltwater.

Sewage treatment outflows maybe? Wouldn't be enough in a community where all the freshwater is coming from desalination, because not all of the freshwater goes into the sewage system, but it might be workable in communities where desalination is augmenting other sources of freshwater.