The waste water from the RO is salty with what every came threw when it was used. Since it use 1/4 tubing not sure of the 2 to 1. Also RO has to have no Chlorine water, to work. The Chlorine water will kill the RO membrane. We have a water softener before the RO system.
To do what you Thanking would take up a lot of room. You would need gravel, sand , some plant life and a big tanks per media going from one to another to filter it out. No chemicals are use in there system. There is a lot of info on this if you go to to New Zealand on the web,they do it a lot. We looked at it when we were there.
Where we live here we have very hard water. Sometimes up in the 100 grain per gallon , but almost always over 60. I use 9.5 lbs of salt per week to get right at 600 gallons of water. Not sure of what % is wasted but it back flushes for around 35 minutes. Our town is in the process of putting in a Very large RO for 70,000 people and the waste water is pump back into the ground down stream from the plant. A good ideal but you need the space to do it and a lot of weight to take along with you.