As much as I would like to use RO water to flush my systems, it's just not possible for me to get that much water from our RO system. They do make
High volume RO systems, but run about $400. So I use the tap water that's from a water softener to flush my systems. I flush them multiple times. I'll clean the reservoir and
pump first, then fill it with fresh (plane tap) water, and let it run through the system for about 20 minuets. Then dump the water, and do it again. Depending on how long it's been scene the last
nutrient change, and how much time I have that day, I'll flush the system like that from 2 to 6 times. Then get as much of the "flushing" water out of the system before filling the reservoir with the RO water and
nutrients.
As for collecting RO water, our tap is on the kitchen sink as well. I just use a piece of 1/2 inch tubing, and a five gallon bucket (picture attached). Notice the soap bottle against the water lever holding it up (so I don't need to do it). Our system only puts out 2 to 3 gallons at a time before the accumulator (tank) needs to fill back up again. It takes about 20-25 minutes to get that 2 to 3 gallons. Then I need to wait at least one and a half hours before the accumulator tank's full again. If I keep at it and time things right, I can get about 15 gallons a day out of our RO system. So I usually start collecting water the day before unless I only need 5 or 6 gallons, that way I have enough when I do the nutrient change.
Even though I don't want to, I have used tap water (from the water softener) for my nutrients in emergency's. Like when there was an unexpected leak or overflow that dumped my nutrients on the ground, and I didn't have time to collect enough RO water before the plants would die. Then I would use just enough tap water to allow the water and nutrients to cycle through without running the pump dry, and then finish filling it with RO water and nutrients the next day.