Aquatic Life RO Buddie system (cheap!)

[gregzz4]

Both ... DI only catches what RO has missed, which is TDS but not Organics.
Your Prefilter and Carbon cartridges do that job to some degree.
And if you leave exhausted DI resin in the system it will leach TDS back into your filtered water.

My Burnaby source water is between 5-10 TDS throughout the year. My RO membrane reduces it to 0 TDS. I acheive this by flushing the unit down the drain until the output is 0 (10-20 mins), then start collecting water. This regular cleaning of the membrane helps make it last for many years.

So why bother with the extra cost and maintenance?

[GQsmooth]

that unit wastes a lot of water with its 1:3 ratio