Ideally into your sewage system, as they are now concentrated wastewater (although if you live in a developed region with centralized water treatment, there won't be that much waste in the water to begin with). RO systems designed for processing seawater have a much harder time of it, as their waste is ultra-salty brine which will kill any local marine ecosystems if you decide to just dump it back into the sea.

Since the flow of deionized water is only 1/10 of total water flow... I think the increase in "waste" concentration is also only about 10% even of the filtered water is perfectly pure. So if you had 400ppm total dissolved solids it would now be *10/9=444ppm. Not much of a concern.

That would not be ideal for me (Bay Area resident) where water is crazy expensive. I'd want to keep it for watering plants, flushing toilets, etc.

Yes, the only problem is that most homes are not plumbed for grey water and it's a very expensive retrofit. If you're building a new home, ask about that and structured wiring. It's unfortunately usually a way more economical idea to just get those toilets that have a sink built into the top (popular in countries like Japan) that use the hand washing water to flush the toilet vs trying to store and/or pump it around.

Seems like thie brine from desalination plants should be pumped into tailing ponds and dried out to get useful minerals out of it that could then be sold to help offset the cost and environmental impact of desalination. https://news.mit.edu/2019/brine-desalianation-waste-sodium-h... looks like this is not an original idea :)

Can’t they be used as a source of non-potable water? Like toilets, irrigation, maybe laundry?

Sure. I've lived on the countryside as a kid, it was before the village had a sewage system built. We not only had RO, but also a tiny waste water processing plant in the backyard, you'd put a spoonful of some powder in the tank once a month or so, and the "cleaned up" grey water would slowly trickle out and irrigate the vegetables in the garden. This was around 2002. I'd like to live like that one day again.

They’re usually discarded into the drain line, so into the sewer system

God, what a waste.

Why? most people only have it attached to their drinking/cooking water, it ain't that bad. that's worrying about the 1% problem when the 99% problem is stuff like industrial usage and watering lawns in areas that were once arid scrub lands.

I've an aversion to paying for potable water and then running it right down the drain with a very slight increased concentration in contaminants, other conversations regarding filter efficiency in this thread were rather eye opening. It feels like basic "turn the tap off while you brush your teeth" wastage writ large. At the very least, plumb it to grey water uses first.

Yes, in the big scheme of things its a rounding error against industrial uses but that still doesn't mean I'm going to leave my hose bib running 24/7.

Essentially you can think about them as "water which washes the filter", i.e. contaminants removed from the drinkable water are in this waste stream. Without this filter this fine would clog up in couple days. In typical home install this water is just dumped into the sewer, if you have use for gray water, you can capture it and re-use it.

This wastewater isn't even gray water, it could be simply reused as non-potable. I think of it as "light gray water" - you wouldn't want to drink it but it would be better than gray water for plants, etc.

any relevant research on how much of that higher-than-tap-water TDS content makes it into the plants? I'm not sure I'd want to use this brine (plants also aren't huge fans of saltwater) to water an edible herb garden, for example.

It's only about 50% more concentrated than normal tapwater. It's taking the junk dissolved in 3 liters and condensing it down to 2 liters. Different cities probably vary more than that. Probably still pretty safe to drink, although that defeats the point of having a filter.