DESIGN OPTIMIZATION FOR ACTIVATED
SLUDGE AND EXTENDED AERATION PLANTS
JAMES L. BARNARD et al
Chief Scientific Officer, National Institute for Water Research,
Pretoria, Republic of South Africa
The recent shift of emphasis in biological waste treatment to effluents of higher
standards, especially with regard to nitrification, total oxidation and water reuse
generally, justifies a closer examination of the kinetics of sludge synthesis and
endogenous respiration. Nitrification may be resorted to as a measure against oxygen
depletion of the receiving water or as part of a biological denitrifying system for water
reuse or nutrient reduction. The growth rates of the nitrifying organisms are low and are
temperature dependent and the biomass in the activated sludge plant which includes the
nitrifying organisms, should be aerated sufficiently long to insure that these organisms
can grow as fast as they are removed. The rate of removal of the organisms from the
system depends on the rate of sludge production and consequently the rate of sludge
wastage when the biomass in the aeration basin is kept constant. The rate of sludge
production is thus an important parameter in the design of total biological denitrification
systems since a breakdown in the nitrification unit will negate the function of the
denitrification unit.
Sludge handling and disposal facilities are also affected by the aeration system. When
the biomass is under aeration for a relatively short period, (i.e., the sludge age, being the
mass of sludge under aeration divided by the mass of sludge produced per day, is
relatively low) a larger mass of sludge will be produced containing a higher fraction of
degradable and potentially putrescible material; conversely, with higher sludge ages, less
sludge containing a lesser fraction of degradable solids will be produced. The extent and
type of sludge treatment will be dependent on the characteristics of the sludge that is
produced.
A number of relationships have been used to define the organic removal from
municipal and industrial wastewaters using the activated sludge process. Relationships
have also been developed to determine the excess sludge produced in the process and the
oxygen requirements. Most of these relationships have assumed that the volatile
suspended solids content in the aeration basin was directly proportional to the active
biomass in the process. In fact, the fraction of volatile suspended active biomass is related
both to volatile suspended solids present in the raw wastewater and to the sludge age.
Increasing the sludge age increases the inert volatile solids accumulation in the system.
Excess sludge from the activated sludge process has been estimated by various
investigators by the relationship: (1) (2)
ΔΧ ν = aS r - bX v
(1)
The coefficients a and b in Equation (1) relate to total volatile yield and include the
effects of both biological synthesis and volatile solids accumulation from the raw
wastewater.
The sludge produced in activated sludge plants has various origins. When raw screened
sewage is introduced into an extended aeration plant, the sludge that is removed from the
377
SLUDGE AND EXTENDED AERATION PLANTS
JAMES L. BARNARD et al
Chief Scientific Officer, National Institute for Water Research,
Pretoria, Republic of South Africa
The recent shift of emphasis in biological waste treatment to effluents of higher
standards, especially with regard to nitrification, total oxidation and water reuse
generally, justifies a closer examination of the kinetics of sludge synthesis and
endogenous respiration. Nitrification may be resorted to as a measure against oxygen
depletion of the receiving water or as part of a biological denitrifying system for water
reuse or nutrient reduction. The growth rates of the nitrifying organisms are low and are
temperature dependent and the biomass in the activated sludge plant which includes the
nitrifying organisms, should be aerated sufficiently long to insure that these organisms
can grow as fast as they are removed. The rate of removal of the organisms from the
system depends on the rate of sludge production and consequently the rate of sludge
wastage when the biomass in the aeration basin is kept constant. The rate of sludge
production is thus an important parameter in the design of total biological denitrification
systems since a breakdown in the nitrification unit will negate the function of the
denitrification unit.
Sludge handling and disposal facilities are also affected by the aeration system. When
the biomass is under aeration for a relatively short period, (i.e., the sludge age, being the
mass of sludge under aeration divided by the mass of sludge produced per day, is
relatively low) a larger mass of sludge will be produced containing a higher fraction of
degradable and potentially putrescible material; conversely, with higher sludge ages, less
sludge containing a lesser fraction of degradable solids will be produced. The extent and
type of sludge treatment will be dependent on the characteristics of the sludge that is
produced.
A number of relationships have been used to define the organic removal from
municipal and industrial wastewaters using the activated sludge process. Relationships
have also been developed to determine the excess sludge produced in the process and the
oxygen requirements. Most of these relationships have assumed that the volatile
suspended solids content in the aeration basin was directly proportional to the active
biomass in the process. In fact, the fraction of volatile suspended active biomass is related
both to volatile suspended solids present in the raw wastewater and to the sludge age.
Increasing the sludge age increases the inert volatile solids accumulation in the system.
Excess sludge from the activated sludge process has been estimated by various
investigators by the relationship: (1) (2)
ΔΧ ν = aS r - bX v
(1)
The coefficients a and b in Equation (1) relate to total volatile yield and include the
effects of both biological synthesis and volatile solids accumulation from the raw
wastewater.
The sludge produced in activated sludge plants has various origins. When raw screened
sewage is introduced into an extended aeration plant, the sludge that is removed from the
377