the mass or volume load. The issue is far more complex
since the “sludge age” is also a decisive parameter to
maintain purification capacities. This is particularly true
of nitrifying autotrophic bacteria, which require a “sludge
age” greater than heterotrophic bacteria to develop.
“Sludge age” is directly linked to the sludge production
resulting from the applied mass load. For important
“sludge ages,” the substrate–microorganisms (C m ) ratio
reaches a maximum value, as well as the biomass concentration; the growth velocity is in balance with mortality.
This limit in mass load C m , due to a more important
“sludge age” (>50 days), magnifies the endogenous respiration phenomenon, which generates a decrease of the
production of sludge. However, the protracted “sludge
age” generates detrimental effects on the sludge biological
activity, which tends to become mineralized.
• Aeration of activated sludge
The biological aeration tank is the core of the wastewater
treatment plant since the suspended aerobic microbial
culture degrades the pollutant load of wastewater in
presence of dissolved oxygen. This oxygen is brought
by an aeration device whose functioning amounts to
50–70 % of the total energetic cost of the wastewater
plant, which corresponds to one-third of the total operational cost approximately. With such costs, optimizing
oxygen transfer or the energetic control of wastewater
treatment processes has become a major challenge.
The efficiency of aerobic bacteria in the wastewater
treatment strongly depends on the oxygen supply and
consequently on its transfer from the gaseous phase
towards the liquid phase, which must not be a restrictive
step of the process operation.
Whatever the medium may be studied, an increase in
the viscosity of the liquid phase or of the suspension
involves a decrease of the oxygenation capacity of the
medium; thus, it generates degradation in the
performances of the aerobic biological systems.
Biological Nitrogen and Phosphorus Removal
The nitrates constituted during the aerobic process in the
activated sludge tank are removed during denitrification by
heterotrophic and chemolithoautrophic autotrophic bacteria
responsible for anammox (cf. Sect. 14.3.3) by recycling
waters of the aeration tank in an anoxic tank (no free oxygen
but presence of bonded oxygen, particularly under the form
of NO 3
À ) upstream of the aerobic reactor (Fig. 16.9a).
The organic or mineral molecules in wastewater will
bring by oxidation the energy necessary to the reduction of
nitrates to nitrogen. In this type of process, the reduction of
nitrates can be incomplete and stopped to the stage N 2 O
(greenhouse gas) which is released to the atmosphere.
In these conditions, a tertiary treatment is necessary. As far
as phosphorus removal is concerned, the phosphate removal
of the effluent is achieved in an anaerobic tank (absence of
free and bonded oxygen, in particular under the form of
NO 3
À ) upstream of the anoxic tank and immediately after
the primary treatment (Fig. 16.9b).
The operators of wastewater treatment used to
describe the different tanks of a WWTP as the expression
of aerated tank (aerobic) to indicate oxygenated environments (presence of free oxygen) and the expression of
anoxic tank to indicate environments without free oxygen
but containing combined oxygen (particularly under the
form of nitrate). The expression of anaerobic tank is only
for environments without free or bonded oxygen (i.e.,
nitrate).
For biologists, these expressions have different meanings.
Indeed, oxic (or aerated) and anoxic, respectively, characterize biotops in which oxygen is present or absent; bonded
oxygen is not taken into account in this definition. Besides,
aerobic and anaerobic, respectively, characterize organisms
(or biological processes), which require the presence or
absence of molecular oxygen to live.
The principle of phosphorus removal is the following
one: in anaerobic conditions (in anaerobic tanks), various
bacterial communities, probably most of them belonging to
Betaproteobacteria, use their polyphosphates to synthetize
from acetate or other volatile organic acids intracellular
polyhydroxyalkanoates (PHA), releasing phosphate in the
tank. These bacteria under aerobic conditions reconstitute
stocks of cellular polyphosphates more important than the
previous ones. They will be eliminated with the cells during
the secondary settling (Fig. 16.10) (Seviour et al. 2003).
Frequently in the WWTP, the aerobic activated sludge
treatment is combined in the same tank to anoxic treatments
(denitrification) and anaerobic treatments (phosphorus
removal). The activated sludge tank is compartmentalized.
Each compartment is intended for a specific treatment. Such
devices can be observed, for example, at the WWTP of
Gre ´asque (France), where the central annular zone of the
tank is only for the anaerobic process and the peripheral zone
for aerobic and anoxic processes (Fig. 16.11). Another
example is represented by the WWTP of Skaekinge
(Denmark), whose tank is divided into three annular
compartments. The central compartment is devoted to phosphorus removal, the most peripheral one to aerobic process,
Table 16.3 Loads according to their order of magnitude
Type of load
F/M
(kg BOD5 ∙d
À1
∙kg MLVSS
À1
)
L v
(kg BOD5 ∙d
À1
∙m
À3
)
High load
0.4 to 1
1.5
Medium load
0.15–0.35
0.5–1.5
Low load
0.1
0.3
Extended aeration
0.07
0.25
672
J.-C. Bertrand et al.
since the “sludge age” is also a decisive parameter to
maintain purification capacities. This is particularly true
of nitrifying autotrophic bacteria, which require a “sludge
age” greater than heterotrophic bacteria to develop.
“Sludge age” is directly linked to the sludge production
resulting from the applied mass load. For important
“sludge ages,” the substrate–microorganisms (C m ) ratio
reaches a maximum value, as well as the biomass concentration; the growth velocity is in balance with mortality.
This limit in mass load C m , due to a more important
“sludge age” (>50 days), magnifies the endogenous respiration phenomenon, which generates a decrease of the
production of sludge. However, the protracted “sludge
age” generates detrimental effects on the sludge biological
activity, which tends to become mineralized.
• Aeration of activated sludge
The biological aeration tank is the core of the wastewater
treatment plant since the suspended aerobic microbial
culture degrades the pollutant load of wastewater in
presence of dissolved oxygen. This oxygen is brought
by an aeration device whose functioning amounts to
50–70 % of the total energetic cost of the wastewater
plant, which corresponds to one-third of the total operational cost approximately. With such costs, optimizing
oxygen transfer or the energetic control of wastewater
treatment processes has become a major challenge.
The efficiency of aerobic bacteria in the wastewater
treatment strongly depends on the oxygen supply and
consequently on its transfer from the gaseous phase
towards the liquid phase, which must not be a restrictive
step of the process operation.
Whatever the medium may be studied, an increase in
the viscosity of the liquid phase or of the suspension
involves a decrease of the oxygenation capacity of the
medium; thus, it generates degradation in the
performances of the aerobic biological systems.
Biological Nitrogen and Phosphorus Removal
The nitrates constituted during the aerobic process in the
activated sludge tank are removed during denitrification by
heterotrophic and chemolithoautrophic autotrophic bacteria
responsible for anammox (cf. Sect. 14.3.3) by recycling
waters of the aeration tank in an anoxic tank (no free oxygen
but presence of bonded oxygen, particularly under the form
of NO 3
À ) upstream of the aerobic reactor (Fig. 16.9a).
The organic or mineral molecules in wastewater will
bring by oxidation the energy necessary to the reduction of
nitrates to nitrogen. In this type of process, the reduction of
nitrates can be incomplete and stopped to the stage N 2 O
(greenhouse gas) which is released to the atmosphere.
In these conditions, a tertiary treatment is necessary. As far
as phosphorus removal is concerned, the phosphate removal
of the effluent is achieved in an anaerobic tank (absence of
free and bonded oxygen, in particular under the form of
NO 3
À ) upstream of the anoxic tank and immediately after
the primary treatment (Fig. 16.9b).
The operators of wastewater treatment used to
describe the different tanks of a WWTP as the expression
of aerated tank (aerobic) to indicate oxygenated environments (presence of free oxygen) and the expression of
anoxic tank to indicate environments without free oxygen
but containing combined oxygen (particularly under the
form of nitrate). The expression of anaerobic tank is only
for environments without free or bonded oxygen (i.e.,
nitrate).
For biologists, these expressions have different meanings.
Indeed, oxic (or aerated) and anoxic, respectively, characterize biotops in which oxygen is present or absent; bonded
oxygen is not taken into account in this definition. Besides,
aerobic and anaerobic, respectively, characterize organisms
(or biological processes), which require the presence or
absence of molecular oxygen to live.
The principle of phosphorus removal is the following
one: in anaerobic conditions (in anaerobic tanks), various
bacterial communities, probably most of them belonging to
Betaproteobacteria, use their polyphosphates to synthetize
from acetate or other volatile organic acids intracellular
polyhydroxyalkanoates (PHA), releasing phosphate in the
tank. These bacteria under aerobic conditions reconstitute
stocks of cellular polyphosphates more important than the
previous ones. They will be eliminated with the cells during
the secondary settling (Fig. 16.10) (Seviour et al. 2003).
Frequently in the WWTP, the aerobic activated sludge
treatment is combined in the same tank to anoxic treatments
(denitrification) and anaerobic treatments (phosphorus
removal). The activated sludge tank is compartmentalized.
Each compartment is intended for a specific treatment. Such
devices can be observed, for example, at the WWTP of
Gre ´asque (France), where the central annular zone of the
tank is only for the anaerobic process and the peripheral zone
for aerobic and anoxic processes (Fig. 16.11). Another
example is represented by the WWTP of Skaekinge
(Denmark), whose tank is divided into three annular
compartments. The central compartment is devoted to phosphorus removal, the most peripheral one to aerobic process,
Table 16.3 Loads according to their order of magnitude
Type of load
F/M
(kg BOD5 ∙d
À1
∙kg MLVSS
À1
)
L v
(kg BOD5 ∙d
À1
∙m
À3
)
High load
0.4 to 1
1.5
Medium load
0.15–0.35
0.5–1.5
Low load
0.1
0.3
Extended aeration
0.07
0.25
672
J.-C. Bertrand et al.