Vertical farms use hydroponics and aeroponics, soil-free growing techniques once researched by NASA to grow plants in space. One major problem with conventional "horizontal" farms is loss of water to runoff. But plant cultivation in either a water-based nutrient solution, shown here, or a nutrient-rich mist enables water to be almost completely conserved.

Some hypothetical vertical farms have even been designed to recycle processed sewage (pdf), turning waste into water that meets drinking water standards.