You could pipe it to a barrel or other holding tank. If you do so make sure you provide an air gap so there is no chance of backsiphoning back to the RO unit and it meets local and state plumbing codes.
After looking at all the work involved plus pumps or manpower involved to save and use the waste I made the decision to buy a low waste RO/DI which runs at slightly less than 1:1 waste ratio so I save water and send what I do produce to the sanitary sewer where it gets treated and reused over and over.
You would only have a high concentration of fluoride or anything else if it were present in the tap water to begin with.
The waste or more accurately "brine" flow is only 20% to 25% higher TDS than the tap water TDS was to begin with so nothing is at toxic or unhealthful levels unless you were treating non potable water.
The normal waste ratio is 4:1 or 4 gallons of brine to 1 gallon of treated water. The concentrated TDS from that 1 gallon of treated water is diluted in those 4 gallons of waste thus the 20 to 25% higher TDS. Really no big deal and safe to use as long as the source was safe to begin with.
I would watch using the waste water on sensitive plants like orchids, they generally prefer a very clean water to begin with. The washing machine is a great idea, though even for the amount of ro I make for my 180g, at most 100g/mo I just dump it.
I learned a long time ago about flooding things.
My first RO system was in the kitchen and I filled my buckets and jugs from the RO faucet and pressure tank, remember this was about 20 years ago and before the popularity of RO/DI and I didn't know anything about TDS creep in a pressure tank. I forgot and flooded the kitchen a couple times and boy was she mad! We had commercial carpet in the kitchen area and it really stunk when wet.....
I bought a couple small digital kitchen timers, one even has a clip so I can wear it like a pager, so I can now set a timer if I am filling something and need to be reminded. Mostly I use the timers for the BBQ or for my homebrewing but they still come in handy for filling buckets or jugs too. You could do the same when filling the washer.
One bad thing is the new front loaders can't be filled like that.
I too send my waste to the drain and I don't feel guilty about it since it gets recycled and reused over and over. I also have an ultra low waste RO/DI system so I am only wasting about 0.67 gallons for every gallon I treat but it cost me to get to that point, may not be for everyone.
Another option is to keep ALL of it in the basement and use a pump to move the water (either ro/di or brine) wherever you need it. A separate bucket for the waste/brine collection could have an overflow that dumps to a drain or sump, it's not a bad idea to have an overflow on the ro/di water container headed to the same drain/sump. It's a cheap way to avoid self inflicted floods.
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