Overland flow as a potable water and wastewater treatment method has received considerable attention in recent years. Research conducted at several facilities has shown overland flow to be an efficient process for removal of biochemical oxygen demand (BOD) and suspended solids (SS), with lesser removals of nitrogen and phosphorus. This article presents many of the chemical principles associated with iron and removal in an overland-flow water treatment system. A brief case history of an operating overland-flow water treatment system in Salo, Finland is also presented. The Salo system achieves 97 percent iron and manganese removal. Includes 8 references, tables, figures.