This passive filtration system was developed in the United States and is an outcome of a decade (1999 – 2009) of research by Drizo and co-workers on the use of steel slag aggregates (SSA) for P removal from wastewaters. The system is accepted in five states in the USA as the first and only conservation practice especially developed to reduce the amount of P from subsurface drain (tile) flows and other subsurface and surface P containing runoff outflows. Sources of agricultural outflows may include agricultural tile drains, ditches and animal heavy use areas such as milk-house wastewater, feed bunks, and silage leachate runoff.