process can be operated at high efficiency (95
to 98%) and can be modified to remove
nitrogen and phosphorus without the use of
chemicals.
Theextended aeration processis a modifi-
cation of the activated sludge plant. A typi-
cal application is the Pasveer and Carrousel
type of oxidation ditches used in Europe and
in other countries that serve a large popula-
tion. The term extended aerationwas given
to this process because it is operated to min-
imize waste sludge production. This results
in a lengthening of the aeration time to
maintain the mixed liquor suspended solids
at a concentration that will still settle effi-
ciently in the clarifier. This sludge is suffi-
ciently mineralized, and the excess quantity
does not require any further treatment in a
digester before dewatering. However, more
power is consumed in extended aeration sys-
tems because all organics are stabilized aero-
bically. The major advantage of this process
is that it is generally capable of giving high
BOD removal efficiency (95 to 98%) while
minimizing waste sludge handling. This
process is operated without primary treat-
ment.
The aerobic digestion of sludge achieves
volatile solids stabilization similar to that of
aerobic digestion if mechanical or pneu-
matic aeration is provided. This approach is
sometimes used to stabilize surplus biologi-
cal sludges generated in the activated sludge
process and in its modifications, or in trick-
ling filtration. It can also be used to stabilize
primary sludges generated by settling prior
to biological treatment.
Thecontact stabilization processis another
modification of the activated sludge process,
where advantage is taken of the fact that
substrate removal occurs in two stages. The
first stage, which lasts 0.5 to 1.0 hour,
involves rapid adsorption of the colloidal,
finely suspended and dissolved organic com-
pounds in the sewage by the activated sludge
solids. In the second phase, the adsorbed
organic material is separated by gravity sedi-
mentation, and the concentrated mixed
liquor is oxidized in 3 to 6 hours. The first
step occurs in the contact tank and the sec-
ond in the stabilization tank. Therefore, the
adsorption phase is separated from the oxi-
dation decay phases.
In systems based on the use of stabiliza-
tion ponds, a solid layer is formed in the sur-
face of the ponds, increasing the sludge
formation in the bottom, reducing the deten-
tion time (reduction of useful volume), and
hindering the operation of the system.Oxidation Ditch
This treatment technique has been devel-
oped as an efficient, easy-to-operate, and
economical process for treating wastewater.
The process maintains waste materials in
contact with the sludge biomass for 20 to 30
hours under constant mixing and aeration.
After the biological reactor step, the stabi-
lized suspended solids enter a clarification
step, which removes them from the water by
settling. An oxidation ditch can accommo-
date BOD loadings of from 200 to 500 g/day
applied for each cubic meter of available
aeration space. Sludge solids should have a
16- to 20-day turnover (i.e., solids retention
time or sludge age). For each kg of BOD
applied, approximately 200 to 300 g of new
sludge solids can be produced, with an
expected BOD reduction of 90 to 95%. Tem-
perature can have a significant influence on
the waste removal performance of the oxida-
tion ditch. Pinpoint biological flocs may
develop and be discharged with the clarifier
effluent, decreasing the performance efficiency
under cold-weather operating conditions.
The typical, oxidation ditch aeration basin
design is either a single closed-loop channel
or multiple closed-loop channels with serial
flow. An attractive feature of oxidation
ditches is that a minimum of operation