The general idea behind aeration is to bring the water into intimate contact with the air. Either the water may be discharged into free air or the air may be forced into a body of water. Apparatus used includes: low cascades, multiple jet fountains throwing water to considerable heights, multitudinous spray nozzles discharging not far above the surface of a reservoir, superimposed trays or shelves, submerged perforated pipes, and porous tubes and plates. Motivation has been by gravity head for water, pumping head for water and pumping head for air. Chronologically, working installations consisted, first, of cascades and gravity operated multiple-jet fountains, then forced aeration for a few years of commercial exploitation, followed by low-throwing spray nozzles, and finally, diffusion of air through porous tubes and plates in water.