The hand-wash tank (clean water) must be independently plumbed so there is no way it can become contaminated. It should have a dedicated fresh water supply fitting that is different to any of the other liquid connections on the sprayer. 

Carefully consider the volume of water this tank will hold, many safety data sheets state that eyes must be flushed with running water for 10 to 15 minutes if product accidently splashes into them. It is also ideal to have a soap dispenser next to the hand wash container.