Recycled runoff from plants can absolutely be reused, we do it very frequently in large commercial vegetable and flower greenhouse system worldwide (you can save between 40-70% on fertilizer costs, which is huge to a 250 acre greenhouse operation).
The problem with UV and chemical (ozone, hydrogen peroxide, chlorine, etc) treatment is that the oxidizing process destroys chelates, and virtually all iron content in the treated runoff, which means the runoff solution is now missing just those elements and not others, making it hard to inject the right fertilizers at the right concentrations when switching back and forth between mixing runoff with fresh water and using only fresh water. It’s doable, but tricky.
There’s a new system that uses hemo-dialysis filtration technology to actually physically filter out bacteria, fungal spores, and viruses (it filters down to 0.0034 microns, and the smallest dangerous microbes, viruses, only go down to 0.02 microns). Not sure about availability in the US right now, since it’s a newly developed system from Israel, but we can get one here if anyone is interested.
If you’re talking about using grey water (completely different than recycled runoff), then I would say the sterilization system would also need, at the very least, a carbon filter after it to remove any detergents or any other nasty chemical that may get washed down whatever drain this water is collected from. RO would be even better to ensure nothing makes it through.
Hope that helps!