We have a RO system here at the house, so I get RO for free. However the drawback is I can only get about 2 gallons of water out of it at a time. If I need more than 10-15 gallons to do a
nutrient change, I need to start collecting water a day or more before I need it. Sometimes I forgot, and needed to use some tap water as well to fill up the reservoir enough to get by for a day.
I originally planed to buy a large volume RO system (700 gallons a day) instead of building the filter system for the greenhouse. But after thinking over the fact that a RO system uses 2-3 times more water than you get from it, and the fact I now have three 275 gallon reservoirs to fill. I just couldn't justify dumping 1,800 2,700 gallons of water down the drain each time I need to get the 900 gallons of RO water to fill my 3 reservoirs. So I made a system that wont dump water down the drain out of necessity.