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.
