Zest it can be used over and over again, but once you find that it has come to the end of its useable life it can be disposed off quickly and safely.
Firstly, unscrew the cap from your zest-it bottle and put to one side. The open Zest-it should be lift in an open, well ventilated area (preferably outside) to evaporate the excess fluid. Once the fluid has been evaporated you should be left with a thick sediment.
To dispose of this sediment fill a bag with sand or grit. Place your Zest-It sediment onto this sand/grit to drain out any excess oils. The sediment and used grit can both be bagged up and disposed of with regular household waste.