TABLE OF CONTENTS Overland flow (OF) is defined as the controlled application of wastewater onto grass-covered, uniformly graded, gentle slopes, with relatively impermeable surface soils. The process was first applied in the United States for industrial wastewaters in Napoleon, Ohio,1 and Paris, Tex.2 As described in Chap. 13, there are many OF systems used to treat industrial wastewater, especially food processing. Early application of the process for municipal wastewaters occurred in England,3 where it was termed grass filtration and in Melbourne, Australia.4 Many of these OF systems have been in continuous and successful operation since the late 19th century. Research efforts by EPA5 and the U.S. Army Corps of Engineers6 ,7 and the performance of operational systems8 10 led to modeling efforts and the development of rational design criteria.11 13
Overland flow is best suited for use at sites having surface soils that are slowly permeable (clays) or that have a restrictive layer, such as a hardpan or claypan at depths of 1 to 2 ft (0.3 to 0.6 m). Overland flow can also be used on moderately permeable soils if the subsurface layer is compacted.
Overland flow may be used at sites with grades between 1 and 12 percent. Slopes can be constructed on level terrain by creating a 2 percent slope. Grades steeper than 10 percent should be terraced (slopes of 2 to 8 percent builtup, followed by a steep drop and another terrace) so that erosion from heavy rainfall is minimized.
