All land treatment concepts are very efficient at removal of biodegradable organics, typically characterized as biochemical oxygen demand (BOD ₅). Removal mechanisms include filtration, adsorption, and biological reduction and oxidation. Most of the responses in slow rate (SR) and rapid infiltration (RI) occur at the ground surface or in the near surface soils where microbial activity is most intense. Part of the reason for the intermittent or cyclic wastewater applications on these systems is to allow the restoration of aerobic conditions in the soil profile, and infiltration capacity at the soil surface. Essentially all of the responses in overland flow (OF) occur at the soil surface or in the mat of plant litter and microbial material. Settling of most particulate matter occurs rapidly in OF systems as the applied wastewater flows in a thin film down the slope. Algae removal is an exception, since the detention time on the slope is not usually sufficient to permit complete removal by physical settling.46 The biological growths and slimes which develop on the OF slope are primarily responsible for ultimate pollutant removal. These growths are similar to those found in other fixed film processes (i.e., trickling filters, RBCs, etc.), and the presence of adjacent aerobic and anaerobic zones or microsites within the slime layer is to be expected. In a properly managed system, with acceptable loadings, the aerobic zones dominate. However, there are still numerous anaerobic sites which contribute to the breakdown of the more refractory organics and to...