quite a lot of urban areas have gobs of unused spaces, derilect buildings, overgrown lots, etc.

progressive cities like portland and oakland and austin are started to recognize the potential of such spaces.

also, we're not talking conventional wetlands, but perhaps greenhouse-sized systems comprised of various tanks that might house plants, filter material, invertebrates...even fish in the final stages. such options take note of the space being at a premium, and are designed as such with layer stacking, etc.

www.anacostiaws.org
www.earthfirstjournal.org/