xkcd

The joke about adding well water back in again at the end is "correct". Reverse osmosis removes 100% of the solids from the water, but drinking water usually contains small quantities of solids - you can see a breakdown on the label of some bottled water. Completely pure water would leach all of the solids that have built up on the insides of water pipes over the decades, and leaches away the protective oxide layer from metal pipework, causing it to corrode surprisingly rapidly. It also tastes pretty shitty - kind of "dead". So a small amount of high-solids water is mixed back in after RO to bring the water back to normal levels.

All that other shit in the diagram? No. Purification and treatment takes place after the mixing step, it would be crazy not to.