Environmental Engineering Reference
In-Depth Information
floc formation. Secreting polysaccharides and mucoproteins that are
absorbed by bacteria—which make the bacteria “sticky” through bio-
logical agglutination (biological gluing together)—allows them to stick
together and, more importantly, to stick to floc. Thus, large quantities
of floc are prepared for removal from secondary effluent and are either
returned to aeration basins or wasted. The crawlers and stalked ciliates
succeed the free swimmers.
Note:
Stalked ciliated protozoa are attached directly to the activated
sludge solids by a stalk. In some cases, the stalk is rigid and fixed in
place; in others, the organism can move (contract or expand the stalk) to
change its position. The stalked ciliated protozoa normally have several
cilia that are used to create currents that carry bacteria and organic
matter to it. The stalked ciliated protozoa predominate when the dis-
persed bacteria population decreases and does not provide sufficient
food for the free swimmers. Their predominance indicates a stable pro-
cess, operating at optimum conditions.
The free swimmers are replaced in part because the increasing level
of mature floc retards their movement. Additionally, the type of environ-
ment that is provided by the presence of mature floc is more suited to
the needs of the crawlers and stalked ciliates. The crawlers and stalked
ciliates also aid in floc formation by adding weight to floc particles, thus
enabling removal.
8.13.2.5.2 rotifers
Rotifers are a higher life form normally associated with clean,
unpolluted waters. Significantly larger than most of the other organ-
isms observed in activated sludge, rotifers can utilize other organisms,
as well as organic matter, as their food source. Rotifers are normally the
predominant organism; the effluent will usually be cloudy (pin or ash
floc) and will have very low BOD
5
.
8.13.2.5.3 filamentous organisms
Filamentous organisms (bacteria, fungi,
etc.) occur whenever the environment of the acti-
vated sludge favors their predominance. They are
normally present in small amounts and provide
the basic framework for floc formation. When the environmental condi-
tions (e.g., pH, nutrient levels, DO) favor their development, they become
the predominant organisms. When this occurs, they restrict settling,
and the condition known as bulking occurs.
Key Point:
Microscopic examination
of activated sludge that reveals a pre-
dominance of amoebas indicates that
the activated sludge is very young.
Note:
Microscopic examination of activated sludge is a useful control tool.
When attempting to identify the microscopic contents of a sample, the
operator should try to identify the predominant groups of organisms.
