When we put the big tank in, I plumbed one of the closed loops to the drain in the wet bar, thru the wall next to the tank. RO/DI goes to a 36 gallon RV water storage tank under it. To change water, I just turn the drain handle, let it drain to the line on the tank, then take the hose from the full topoff, and plug in the pump.
BTW, no, the saltwater shouldn't hurt your sink, as long as it's not leaking (to damage some of the hardware that isn't stainless, underneath). Karin, I'm guessing you might have a septic system? If so, the saltwater might not be doing it good. Not sure, but it might affect the bacteria that breaks down the waste. Might check it out.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."