From the Proceedings of the 1975 Meetings of the Arizona Section - American Water Resources Assn. and the Hydrology Section - Arizona Academy of Science - April 11-12, 1975, Tempe, Arizona / In direct (normal, forward) osmosis water automatically flows through a semipermeable membrane from a "source" solution of low concentration to a "driving" solution with higher solute content. The process requires a membrane which is impermeable to the solutes; hydrostatic pressure differences are not directly involved and can be set equal to zero. In principle, direct osmosis is a low -technology, low-power consumption method for reducing the water volume of industrial effluents or liquid agricultural products, and for reclaiming brackish irrigation water. In the latter application the driving solution may utilize fertilizer as a solute; the source solution is drainage that contains harmful salt components. This type of operation has been experimentally demonstrated. This paper summarizes basic physical principles and introduces some quantitative design factors which must be understood on both a fundamental and on an applications level.