As soon as u put any tap water through di resin, it will take it to 000tds, wether its 5 or 500, as the other guy said u could have not bothered with the ro unit, but i suppose it will still take the tds down from 55 to 12ppm, which will save u money on resin. Remember the whole point of the ro unit is 2 save u money on the di resin at the cost of time it takes to make it, ie. if your tap water was say 300tds and u went straight into your di vessel it would take it to 000tds but it would cane your resin and cost u a bomb, thats were the ro comes in! but if u was in a rush to produce the water fast one day, it would not be the end of the world at 55tds to whack it straight though the di vessel for instant water. hope that helps