I have an airless paint sprayer (Wagner Procoat) which I clean by running water through it, until all the paint is gone. The paint I used was Valspar Ultra Zero VOC.
I collect the water+paint in a bucket. I know the proper way of disposing latex paint is to let it dry and harden and take it to the disposal centre. In this case, it will take years to dry since it's mostly water. What's the best way to dispose it?