What do you mean by rejected DI water? Do you mean the "blowdown" or the stuff that hasn't been deionized? That is concentrated in whatever was in the incoming water, including, potentially, chlorides, nitrates, and others.
If you mean DI water, but somehow past its "expiration date" or somesuch, DI water is notoriously corrosive. Water is the universal solvent, and ultra-pure DI water wants to have something in it. If it isn't properly blanketed or inerted, it will absorb CO2, and acidify as carbonic acid. high quality DI water will slowly dissolve stainless steel, PVC, viton, or whatever. The piping material suggested for high quality DI was is non-pigmented polyethylene. That is why "old" DI water is rejected or sewered.