Environmental Engineering Reference
In-Depth Information
the wastewater (NSFC 1996). The aerobic system employs biological processes
for wastewater treatment. During these processes, the microbes in the wastewater
feed on the organic substances and convert them to non-polluting substances. Dis-
solved and solid pollutants are also converted to cell mass, non-degradable mate-
rial and gases such as carbon dioxide, hydrogen and methane (Lesikar 1999). Ac-
cording to NSFC (1996), the ATUs are very useful in environmentally sensitive
areas that are less suitable for proper use of the conventional septic system. Such
areas include; those with inappropriate soil conditions where the water table is too
high to allow the drain field to operate effectively, areas of insufficient availability
of land for septic systems and areas where high level of treatment is required by
regulation.
5.2.1 How the ATUs Work
There are many types of ATU, but the most common household units use a proc-
ess called suspended growth. These units have a main compartment (aeration
chamber) in which air is forced and mixed with the wastewater (
Figure 5.1
)
. This
creates an environment where bacteria are free-floating in the liquid and grow as
they digest the solids (Lesikar 1999). Many units include a second chamber (set-
tling chamber) where solids that the bacteria are unable to digest, settle. The two
chambers are connected, so these undigested solids can be returned to the aeration
chamber, either by gravity or a pump. It is this process of return and mixing that is
important for effective operation.
Fig. 5.1.
Sketch of a typical aerobic treatment unit, aerator chamber (adapted from: Fried-
man 2010)
The wastewater leaving these two chambers is not ready to be returned to the envi-
ronment and must receive final treatment or disinfection in the disinfection cham-
ber. Methods for final treatment include discharge to a soil absorption field, a sand
filter or an evapotranspiration bed.
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