1.1 Definition of Process
Activated sludge consists of biological flocs that are matrices of microorganisms,
nonliving organic matter, and inorganic materials. The microorganisms include
bacteria, fungi, protozoa, and higher forms of animals such as rotifers, insect larvae,
and worms. An activated sludge process can be defined as a system in which
biological flocs are continuously circulated to come into contact and to oxidize the
organic substances in the presence of oxygen. The fact that an “active” mass of
biological forms is maintained in the system for continuous and successful biological
oxidation explains why the process is designated “activated sludge” treatment.
The objectives of activated sludge treatment are twofold: (1) to obtain the
maximum possible removal of organic substances with the shortest possible time
and (2) to produce flocculant biological flocs having a good settling characteristic.
Both are essential in controlling the secondary effluent quality. From the economic
point of view, it is also desirable to meet both objectives since small aeration tank
(s) and final clarifier(s) can be used. The two objectives, however, are not compatible. Biological flocs that are very efficient in removing organic substances at a rapid
rate are flocs that normally settle poorly and vice versa. The tradeoff is manifested in
the performance of various activated sludge processes. Design engineers and plant
operators should be fully aware of the incompatibility of these two objectives for
proper design and operation of a plant so that certain specific treatments can be
accommodated and optimization of treatment performance can be planned
intelligently.
It is important to recognize the capability as well as the limitations of activated
sludge processes. In the aeration tank, biodegradable organics are converted to
inorganics. A complete oxidation of organics can be expressed as:
Organics C, H, O, N, P, S
ð
Þ þ O 2 ! CO 2 þ H 2 O þ NO
À
3 þ PO 4
3À
þ SO 4
2À
þ H
þ
The equation above assumes an infinite period of aeration time, and plentiful
microorganisms needed to carry out the complete oxidation, including nitrosomonas
and nitrobacters, are present. Economic constraints do not allow sufficient time for
complete oxidation even for an extended aeration process. Nor is it feasible to
maintain a steady population of the less competitive organisms (e.g., nitrosomonas
and nitrobacters) in the aeration tank with the present operational scheme. Most
noticeable is the lack of nitrification in the process, and a significant amount of
ammonia nitrogen exists in the effluent as result. A typical activated sludge treatment
process may yield the following:
Ammonia nitrogen 12 mg/L as N
Phosphate
10 mg/L as PO 4
3À
Nitrate
0.1 mg/L as N
Nitrite
0.01 mg/L as N
BOD
20 mg/L
78
L. K. Wang et al.
Activated sludge consists of biological flocs that are matrices of microorganisms,
nonliving organic matter, and inorganic materials. The microorganisms include
bacteria, fungi, protozoa, and higher forms of animals such as rotifers, insect larvae,
and worms. An activated sludge process can be defined as a system in which
biological flocs are continuously circulated to come into contact and to oxidize the
organic substances in the presence of oxygen. The fact that an “active” mass of
biological forms is maintained in the system for continuous and successful biological
oxidation explains why the process is designated “activated sludge” treatment.
The objectives of activated sludge treatment are twofold: (1) to obtain the
maximum possible removal of organic substances with the shortest possible time
and (2) to produce flocculant biological flocs having a good settling characteristic.
Both are essential in controlling the secondary effluent quality. From the economic
point of view, it is also desirable to meet both objectives since small aeration tank
(s) and final clarifier(s) can be used. The two objectives, however, are not compatible. Biological flocs that are very efficient in removing organic substances at a rapid
rate are flocs that normally settle poorly and vice versa. The tradeoff is manifested in
the performance of various activated sludge processes. Design engineers and plant
operators should be fully aware of the incompatibility of these two objectives for
proper design and operation of a plant so that certain specific treatments can be
accommodated and optimization of treatment performance can be planned
intelligently.
It is important to recognize the capability as well as the limitations of activated
sludge processes. In the aeration tank, biodegradable organics are converted to
inorganics. A complete oxidation of organics can be expressed as:
Organics C, H, O, N, P, S
ð
Þ þ O 2 ! CO 2 þ H 2 O þ NO
À
3 þ PO 4
3À
þ SO 4
2À
þ H
þ
The equation above assumes an infinite period of aeration time, and plentiful
microorganisms needed to carry out the complete oxidation, including nitrosomonas
and nitrobacters, are present. Economic constraints do not allow sufficient time for
complete oxidation even for an extended aeration process. Nor is it feasible to
maintain a steady population of the less competitive organisms (e.g., nitrosomonas
and nitrobacters) in the aeration tank with the present operational scheme. Most
noticeable is the lack of nitrification in the process, and a significant amount of
ammonia nitrogen exists in the effluent as result. A typical activated sludge treatment
process may yield the following:
Ammonia nitrogen 12 mg/L as N
Phosphate
10 mg/L as PO 4
3À
Nitrate
0.1 mg/L as N
Nitrite
0.01 mg/L as N
BOD
20 mg/L
78
L. K. Wang et al.