4.2. Overland flow system
Sewage treatment by the overland flow method
is the one that presents the lowest relationship
with the type of soil. In this method, the vegetation, associated with the top soil layer, acts as
a filter, removing the nutrients and providing
conditions for the retention and transformation
of the organic matter contained in the sewage.
Besides that, it protects the soil against erosion
and creates a support layer on which the microorganisms settle. The main mechanisms through
which organic matter and solids are removed
are biological oxidation, sedimentation and filtration (USEPA 1981, 1984; Metcalf & Eddy
1991). The main characteristic that differentiates
this method from the others is the fact that the
effluent flows downward on a slightly inclined
vegetated ramp and the remaining water (effluent), which is neither absorbed nor evaporated,
is collected downstream and directed for disposal. For more permeable soils, the process is
similar to that of irrigation, but with the generation of effluent.
Therefore, the method consists in applying
the liquid in the highest part of the ramp. The
effluent then drains all over the slope by gravity,
where part of it is lost by evapotranspiration and