Environmental Engineering Reference
In-Depth Information
Table 8.2 ProCess CondiTion vs. organisMs PresenT
Process Condition
organism Population
Poor BOD
5
and TSS removal
No floc formation
Very cloudy effluent
Predominance of amoebae and flagellates
Mainly dispersed bacteria
A few ciliates present
Poor-quality effluent
Dispersed bacteria
Some free-swimming ciliates
Some floc formation
Cloudy effluent
Predominance of amoebae and flagellates
Some free-swimming ciliates
Satisfactory effluent
Good floc formation
Good settleability
Good clarity
Predominance of free-swimming ciliates
Few amoebae and flagellates
High-quality effluent
Excellent floc formation
Excellent settleability
High effluent clarity
Predominance of stalked ciliates
Some free-swimming ciliates
A few rotifers
A few flagellates
Effluent high TSS and low BOD
5
High settled sludge volume
Cloudy effluent
Predominance of rotifers
Large numbers of stalked ciliates
A few free-swimming ciliates; no flagellates
The activated sludge process is typified by the successive devel-
opment of protozoa and mature floc particles. This succession can be
indicated by the presence of the type of dominant protozoa present. At
the start of the activated process (or recovery from an upset condition),
the amoebae dominate.
Note:
Amoebae have very flexible cell walls and move by shifting fluids
within the cell wall. Amoebae predominate during process startup or
during recovery from severe plant upsets.
As the process continues uninterrupted or without upset, small
populations of bacteria begin to grow in logarithmic fashion which, as
the population increases, develop into mixed liquor. When this occurs,
the flagellates dominate.
Note:
Flagellated protozoa typically have single hair-like flagella, or
“tails,” that they use for movement. The flagellate predominates when
the MLSS and bacterial populations are low and organic load is high. As
the activated sludge gets older and denser, the flagellates decrease until
they are seldom used.
When the sludge attains an age of about 3 days, lightly dispersed
floc particles begin to form (flocculation “grows” fine solids into larger,
more settleable solids), and bacteria increase. At this point, free-swim-
ming ciliates dominate.
