On-Site Wastewater Treatment and Disposal Systems - Forced ...

across the clogging mat, which can increase the infiltration
rate (2)(18).
If adequate treatment is to be achieved in coarse granular
soils such as sands, wastewater application by gravity flow
requires that a clogging mat exist at the infiltrative surfaces
to prevent saturated conditions in the underlying soil and to
prevent groundwater contamination. The mat develops with con-
tinued application, but groundwater contamination by pathogenic
organisms and viruses can be a danger at first.
2. Dosing can be employed to provide intermittent aeration of the
infiltrative surface. In this method, periods of loading are
followed by periods of resting, with cycle frequencies ranging
from hours to days. The resting phase should be sufficiently
long to allow the system to drain and expose the infiltrative
surface to air, which encourages rapid degradation of the clog-
ging materials by aerobic bacteria.
This method of operation may increase the rate of infiltration,
as well as extend the life of the absorption system, because
the clogging mat resistance is reduced (1)(4)(6)(15)(17). In
sands or coarser textured materials, the rapid infiltration
rates can lead to bacterial and viral contamination of shallow
groundwater, expecially when first put into use (4). There-
fore, systems constructed in these soils should be dosed with
small volumes of wastewater several times a day to prevent
large saturated fronts moving through the soil. In finer tex-
tured soils, absorption, rather than treatment, is the con-
cern. Large, less frequent doses are more suitable in these
soils to provide longer aeration times between doses (4). See
Table 7-4 for suggested dosing frequencies.
3. Uniform Application means applying the wastewater in doses uni-
formly over the entire bottom area of the system to minimize
local-overloading and the depth of ponding fol-lowing each dose.
This is usually achieved with a pressure distribution network.
In this manner, the soil is more likely to remain unsaturated
even during initial start-up when no clogging mat is present.
The minimum depths of ponding during application permit rapid
draining and maximum exposure of the bottom surface to air
which reduces the clogging mat resistance. The sidewall is
lost as an infiltrative surface, but this may be compensated
for by the maintenance of higher infiltration rates through the
bottom surface. See Table 7-4 for suggested dosing frequen-
cies.
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