It just goes down the drain, there is no need for any special treatment or disposal.
The waste RO water simply carries off the dissolved solids from the tap water, while the pure water is squeezed through the RO membrane. So there's nothing toxic about it, it just has a higher mineral content than the original tap water, but these are the same minerals that came from the tap water in the first place.
Imagine you filled a jug with tap water, then allowed 1/3 of it to evaporate. What you have left is exactly the same as the RO waste water. All of the minerals are still dissolved in it, but it is more concentrated because some of the water has evaporated.
Actually, I have connected my waste outlet to our garden sprinkler, I though that would be better than just wasting it, so it waters the plants and the extra minerals will probably help a bit, who knows!
-Philip
PS connecting a remote device to the omnitrolley, or any other trolley for that matter is very easy. If you're interested, PM me and I'll tell you how I did it with my trolley and from where I got the components.