I can't imagine that the waste side makes any difference - as long as you don't significantly restrict flow. My RO unit started out life as a low end 5 stage from Lowes with a 25gpd membrane and a standard under sink installation with the waste going to the sink drain. Since then I have added a tee, a long (20-30 ft) tube through the sill plate and a shutoff valve so that I can waste to the yard instead of to the septic tank (at least when the line to the outside is not frozen). I use a lot of water (20+ G/week) and didn't want all of the waste going through the septic system. That line to the outside might be a bit more rigid than the stuff I used internally.