The line of tanks was referred to as a cascade, a rather abstract bit of 
engineer's whimsy lost on the tourists who did not see anything snapshot-worthy 
there. All the action took place in the walls separating the tanks, which were 
not really walls but nearly infinite grids of submicroscopic wheels, 
ever-rotating and many-spoked. Each spoke grabbed a nitrogen or water molecule 
on the dirty side and released it after spinning around to the clean side. 
Things that weren't nitrogen or water didn't get grabbed, hence didn't make it 
through. There were also wheels for grabbing handy trace elements like carbon, 
sulfur, and phosphorus; these were passed along smaller, parallel cascades 
until they were also perfectly pure. The immaculate molecules wound up in
reservoirs. Some of them got combined with others to make simple but handy 
molecular widgets. In the end, all of them were funneled into a bundle of 
molecular conveyor belts known as the Feed, of which Source Victoria, and the 
other half-dozen Sources of Atlantis/Shanghai, were the
fountainheads.