I would imagine they are filtering the H2O from the cistern then (hopefully) making water adjustments in the kettle.

edit to say, in re reading your post I would agree that they would use a shitton of water that doesn't end up as product....I haven't read anything on this in a few years but I think you could find info that states it takes anywhere from 5-50 gallons of water to produce a gallon of beer. There may be a guy or two here ITB that could chime in. <img src="http://www.traveltalkonline.com/forums/images/graemlins/dine.gif" alt="" />