BOD influent is reduced by 70% to 90%. The
solids produced will partially decompose in
anaerobic sludge banks in facultative lagoons,
but the completely mixed effluents usually
require additional treatment, such as clarifica-
tion or polishing pond treatment.
Trickling Filters
Trickling filters reduce BOD and SS by
bacterial action and biological oxidation as
wastewater passes in a thin layer over sta-
tionary media (usually rocks) arranged
above an overdrain. Biological degradation
occurs almost exactly as in the activated
sludge process, except that the filter is a
three-phase system in which the biofilm is
fixed on the solid medium (stones or plastic).
Aeration is accomplished by exposing large
surface areas of wastewater to the atmos-
phere. Layers of zoogloea (filter sludge)
grow on and attach to the medium surface.
Primary treatment should precede this
process if the wastewater suspended solids
concentration exceeds 100 mg/L.
The efficiency of trickling filters is
affected through temperature, waste charac-
teristics, hydraulic loading rate, characteris-
tics of the filter media, and depth of the
filter. Media characteristics such as size, void
space, and surface area, as well as hydraulic
loading rates, tend to affect the performance
of trickling filters more than do other fac-
tors. Removal efficiency is relatively inde-
pendent of surface organic loading rate
within broad ranges. Incorporation of plas-
tic media with more surface area and void
space than rock filter media has permitted
improvements in design and efficiency. This
treatment method is considered more rugged
in operation and easier to maintain than
activated sludge plants.
Activated Sludge
The activated sludge process is widely used
for wastewater treatment. It requires a reac-
tor that is an aeration tank or basin, a clari-
fier, and a pumping arrangement for return-
ing a portion of the settled sludge to the
reactor and discharging the balance to waste
disposal. Primary treatment is optional. A
portion of the clarifier-settled sludge is
returned and mixed with wastewater enter-
ing the reactor. The resulting biological
solids concentration is much higher than
what could be maintained without the recy-
cle. The term “activated sludge” applies
because this returned sludge has microorgan-
isms that actively decompose the waste being
treated. This mixture of influent wastewater
and returned biological suspended solids is
termed the mixed liquor. The activated
sludge process is frequently called the fluid-
bedbiological oxidation system, whereas the
trickling filter is referred to as a fixed-bed
system.
The conventional activated sludge system
has been designed for continuous secondary
treatment of domestic sewage. It is not effec-
tive in treating inorganic dissolved solids but
is very effective for the removal of all organic
matter in the wastewater. This process may
incorporate either surface aerators or air dif-
fusers to achieve mixing. The influent organ-
ics are mixed with the activated sludge, and
undergo biological decomposition as they
pass from the influent end of the reactor to
the discharge end. The detention time in the
reactor can vary from 6 hours to 3 days or
more, depending on the strength of the
wastewater and the method of operation
selected. When the activated sludge contacts
the influent waste, there is a short period
(less than 30 minutes) when influent particu-
late matter is rapidly absorbed onto the
gelatinous matrix of the returned sludge.
Absorption removes a large portion of the
influent BOD. The aeration mechanical and
electrical equipment components of an acti-
vated sludge system are relatively expensive,
and the energy costs are relatively high. ThisWaste Product Handling 227