Figure 2-22. Step aeration.
(b) Two-stage treatment. One aeration and settling tank (first stage) is
followed by second identical set (second stage). Sludge is either recycled or wasted
within each stage, or excess sludge from one stage is recycled to the other. This
procedure offers flexibility in exploiting the quality of both sludges. Figure 2-23
illustrates the two-stage process.
Figure 2-23. Two-stage aeration.
(c) Contact stabilization. An overloaded conventional activated sludge
plant is upgraded by converting the existing aeration tank to two separate tanks, one for
stabilization of returned sludge and the second as the contact tank for the raw
wastewater. Contact stabilization is normally accomplished in two stages, as shown in
Figure 2-24. Detention time in the first stage is usually limited to 20-40 minutes. The
organisms which develop on the contact plates (see Figure 2-20) are quite similar to
those which make up the zoogleal film (schmutzdecke) in the trickling filter and identical
with activated sludge produced by other types of aeration equipment. The settlement
sludge is removed in a settling tank following first-stage aeration. The effluent flows into
a second aerator, to which digester supernatant (see para 2-18) has been added as a
nutrient for the further oxidation of soluble organics (see Figure 2-25). Normally, only
the sludge from the second-stage unit is returned to the incoming wastewater influent.