Environmental Engineering Reference
In-Depth Information
8.13.2.3 Dissolved Oxygen Profile
All dissolved oxygen profile readings should be >0.5 mg/L. Readings
of <0.5 mg/L indicate inadequate aeration or poor mixing.
8.13.2.4 Mixed Liquor Suspended Solids
The range of mixed liquor suspended solids is determined by the
process modification used. When MLSS levels increase, more solids,
organisms, and an older, more oxidized sludge are typical.
8.13.2.5 Microscopic Examination
The activated sludge process cannot operate as designed without the
presence of microorganisms; thus, microscopic examination of an aera-
tion basin sample to determine the presence and type of microorganisms
is important. Different species prefer different conditions; therefore, the
presence of different species can indicate
process conditions. Table 8.2 lists process
conditions indicated by the presence and
population of various microorganisms.
Key Point:
It is important to point out that,
during microscopic examination, identifying
all of the organisms present is not required,
but identifying the predominant species is.
Routine process control identification can be limited to the general
category of organisms present. For troubleshooting more difficult prob-
lems, a more detailed study of organism distribution may be required
(the knowledge required to perform this type of detailed study is beyond
the scope of this text). The major categories of organisms found in the
activated sludge are:
•
• Rotifers
• Filamentous organisms
Protozoa
Bacteria are the most important microorganisms in the activated
sludge. They perform most of the stabilization or oxidation of the organic
matter and are normally present in extremely large numbers. They are
not, however, normally visible with a conven-
tional microscope operating at the recom-
mended magnification and are not included
in the Table 8.2 list of indicator organisms.
Key Point:
The presence of free-swimming
and stalked ciliates, some flagellates, and
rotifers in mixed liquor indicates a balanced,
properly settling environment.
8.13.2.5.1 Protozoa
Protozoa are secondary feeders in the activated sludge process
(secondary as feeders but nonetheless definitely important to the acti-
vated sludge process). Their principal function is to remove (eat or crop)
dispersed bacteria and help to produce a clear process effluent. To help
gain an appreciation for the role of protozoa in the activated sludge pro-
cess, consider the following explanation.
