here are a whole bunch of them-
float switch | Marine Depot
you would drill a hole in the bucket and mount it towards the top, then attach the R.O. output tube. when the water comes up to the top it will lift the float, and shut off the water.
i don't know how else to explain it.
many folks buy 100gpd R.O.D.I. units because they think more is better, but the truth is, you will have considerably more waste water using a 100gpd unit, rather than a 75 or 50 gpd unit. they larger ones just waste more water to get less return.
i have heard of people putting it in their washing machines, and watering plants. what else would you need large amounts of water for? fill your swimming pool?