@nedreynolds Hi there mate Great question as nobody likes to see wasted nutrients . heres what I do when I'm running run to waste or Hydro grows.
I have a large drum that holds 100 litres . I half fill it with water and simply tip all of the unwanted plants nutrient solution or runoff into the barrel of water.
Once it becomes full I'll use it to water all of my other Garden projects. It is important to dilute this with water as your average garden plant would be burnt by full strength nutrients plus it's a waste as they don't usually need a lot of fertilising like canna plants do . Hope this helps mate