the remaining part is collected on the base of the
ramp. Percolation can be insignificant because
this system is initially conceived for low-permeability soils. In spite of that, its use has been also
reported in soils with medium permeability and
impermeable underground (USEPA 1981). Sewage application is intermittent and the following
types of feeding can be adopted: (i) high pressure
sprinklers; (ii) low pressure sprinklers; (iii) distribution piping or channels with spaced openings.
The typical configuration of a wastewater
treatment plant consisting of a UASB reactor
and post-treatment by overland flow has a very
simple flowsheet (Figure 3). Besides the preliminary treatment units, the flowsheet comprises
the anaerobic treatment unit, the land treatment
system and the dewatering unit for the sludge
produced in the UASB reactor. The same considerations made for the polishing ponds, regarding
the characteristics of the anaerobic sludge, that is
already thickened and stabilised, are also valid
here. Dewatering units using drying beds can be
used in small-sized plants.
Studies carried out by Coraucci Filho et al.
(2000) and Chernicharo et al. (2001c) with overland
flow systems following anaerobic filters and UASB
reactors, respectively, operated under application
rates varying from 0.10 to 0.50 m3 m)1 h)1, have
conducted to average concentrations in the final
effluent ranging from 98 to 119 mgCOD l)1,
48 to 62 mgBOD l)1 and 17 to 57 mgTSS l)1,
respectively). In relation to the microbiological
quality of the final effluent, an excellent removal
of helminth eggs in the UASB/overland flow system was observed, with an average counting of
0.2 egg l)1 in the final effluent. Regarding faecal
coliforms, the removal was only satisfactory,
with the whole treatment system removing 2–3
log-units.