I've only been looking into greywater treatment for a few months now, but the most promising g ideas I've found have been through the use of constructed wetlands, plants like cattails absorb heavy metals, where the flowing of water over waterfalls oxygenates it, aerobic bacteria helps remove toxins and voc's, and the running of water through ever finer gradients of gravel to silt removes particulates, and then you can even polish if off with activated charcoal if you want. It should be fine to water plants with after that, especially if you install a pump that delivers it to the roots through a slow drip system. Heck, add a bit of chlorine, and it might even be potable, provide you don't add too much, and allow time for it to dissipate.
