RO units waste TONS of water. I'm sure there are probably exceptions, but MOST home RO systems never shut off the incoming water supply feed. The system is most efficient as it fills the holding tank. After the holding tank is full, the water is switched to bypass flowing through the membrane and runs directly into the drain. A bucket to catch wastewater is not an option, unless you have a 600 gallon bucket. lol
The wastewater line on an RO system HAS to be plumbed into a drain line. It's hard for me to imagine a system that needs no plumbing. It is actually quite easy to attach the wastewater line to your sink's drain. You only need to drill a hole big enough for the wastewater line in your drain. RO systems come with a collar that fits around the drain to keep the wastewater line from leaking.