Discussion by Philip H. Jones
Professor of Civil Engineering, Microbiology and
Chairman, Institute of Environmental Sciences and
Engineering University of Toronto
History of Process
Many sludge reaeration processes have been tested and used in the past. One of the first must have
been the one developed in England in 1921. The first plant was placed in operation in 1930 (Chase,
1944). In this plant, a 30-minute contact period was provided with a return sludge reaeration
(stabilization) period of 8 hours.
The contact stabilization process has not been widely used, and perhaps this is due largely to the
lack of understanding of the mechanisms involved. With only empirical information to go on, process
failures can neither be predicted nor prevented.
Initially the process was known commercially as "biosorption" (Ullrich & Smith, 1951). This name
implied that the process consisted of a physical reaction (adsorption) in the presence of biological floe.
Perhaps it was this fact that caused researchers to assume that the extremely rapid removal of BOD
(15-20 mins.) was almost exclusively the result of a physical reaction.
Postulated Process Theory
It has been postulated (Jones 1970) that the mechanism for removal of BOD consists of three
simultaneous reactions, adsorption (physical), absorption (biological) and hydrolysis (biological). The
authors make the assumption that "in the contact basin only cell growth is significant." This is a
radical and exciting hypothesis, and the authors offer some evidence to prove its validity. Although
not conclusive, the possibility commands further study.
Previous studies (Jones & Brown 1967) have indicated that an important parameter in the contact
stabilization process is the solubility index ( S I Q
= B O D soluble/BOD total). It has been shown (Fig. 1)
that the higher the S I Q , the lower the COD removal in a
x h hour hydraulic detention period and at a
mixed liquor suspended solids concentration (MLSS) of 2000-3500 mg/1.
The same study (Fig. 2) indicated that the S I Q did not affect the degree of treatment when the
MLSS in the contact tank exceeded 3500 mg/1. These findings agree with the authors' results due
undoubtedly to the fact that the MLSS in their study were in excess of 3500 mg/1.
Introducing the concept of cell growth in the contact tank and cell death in the stabilization tank
provides a satisfactory explanation of a phenomenon which has only really been examined empirically.
In addition, it allows for a more critical understanding of the biological mechanism and will lead
logically and ultimately to a more satisfactory design procedure.
Sludge Settling Properties
The authors' data indicating an improved settling sludge is consistent with prior observations.
However, the theory proposed as the reason is not demonstrated and alternative theories could equally
well apply. If in fact organic solids are adsorbed and entrapped in the sludge floe during the contact
period, it is equally conceivable that the sludge would be denser and therefore settle more readily. In
fact, many microbiological growth studies have indicated that flocculation is less likely to occur when
the organisms are in a rapid growth phase; and it is for this reason that biological wastewater treatment
plants are traditionally operated in the declining growth, stationary or even death phase. Further
studies will be required to establish the validity of the polymer theory. If, however, this theory is
validated, then this work can be considered to be a significant discovery and application of this finding
will allow increase of the capacity of all activated sludge plants at nominal cost or allow the
construction of new plants at significant savings.
Stabilization of Organic Carbon & Nitrogen
The combination of the cell growth/death theory and the bacterial polymer flocculation theory
would give adequate cause for all sanitary and environmental engineers to seriously question and
rethink the present design procedures for activated sludge treatment plants. If accelerated growth is
really the reason for the rapid reaction, then the nature of the waste (soluble or particulate) will have
little bearing on the process selection (provided the waste is biologically degradable and nontoxic).
Further studies will indicate if the solubility of the waste has any detrimental effect upon the rate of
the reaction.
The paper shows that the overall removal rate can be increased significantly without affecting the
net growth rate simply by varying the ratio of sludge in the stabilization tank to that in the contact
tank. This is indeed significant for two reasons. First of all, this operating procedure permits a rapid
363
Professor of Civil Engineering, Microbiology and
Chairman, Institute of Environmental Sciences and
Engineering University of Toronto
History of Process
Many sludge reaeration processes have been tested and used in the past. One of the first must have
been the one developed in England in 1921. The first plant was placed in operation in 1930 (Chase,
1944). In this plant, a 30-minute contact period was provided with a return sludge reaeration
(stabilization) period of 8 hours.
The contact stabilization process has not been widely used, and perhaps this is due largely to the
lack of understanding of the mechanisms involved. With only empirical information to go on, process
failures can neither be predicted nor prevented.
Initially the process was known commercially as "biosorption" (Ullrich & Smith, 1951). This name
implied that the process consisted of a physical reaction (adsorption) in the presence of biological floe.
Perhaps it was this fact that caused researchers to assume that the extremely rapid removal of BOD
(15-20 mins.) was almost exclusively the result of a physical reaction.
Postulated Process Theory
It has been postulated (Jones 1970) that the mechanism for removal of BOD consists of three
simultaneous reactions, adsorption (physical), absorption (biological) and hydrolysis (biological). The
authors make the assumption that "in the contact basin only cell growth is significant." This is a
radical and exciting hypothesis, and the authors offer some evidence to prove its validity. Although
not conclusive, the possibility commands further study.
Previous studies (Jones & Brown 1967) have indicated that an important parameter in the contact
stabilization process is the solubility index ( S I Q
= B O D soluble/BOD total). It has been shown (Fig. 1)
that the higher the S I Q , the lower the COD removal in a
x h hour hydraulic detention period and at a
mixed liquor suspended solids concentration (MLSS) of 2000-3500 mg/1.
The same study (Fig. 2) indicated that the S I Q did not affect the degree of treatment when the
MLSS in the contact tank exceeded 3500 mg/1. These findings agree with the authors' results due
undoubtedly to the fact that the MLSS in their study were in excess of 3500 mg/1.
Introducing the concept of cell growth in the contact tank and cell death in the stabilization tank
provides a satisfactory explanation of a phenomenon which has only really been examined empirically.
In addition, it allows for a more critical understanding of the biological mechanism and will lead
logically and ultimately to a more satisfactory design procedure.
Sludge Settling Properties
The authors' data indicating an improved settling sludge is consistent with prior observations.
However, the theory proposed as the reason is not demonstrated and alternative theories could equally
well apply. If in fact organic solids are adsorbed and entrapped in the sludge floe during the contact
period, it is equally conceivable that the sludge would be denser and therefore settle more readily. In
fact, many microbiological growth studies have indicated that flocculation is less likely to occur when
the organisms are in a rapid growth phase; and it is for this reason that biological wastewater treatment
plants are traditionally operated in the declining growth, stationary or even death phase. Further
studies will be required to establish the validity of the polymer theory. If, however, this theory is
validated, then this work can be considered to be a significant discovery and application of this finding
will allow increase of the capacity of all activated sludge plants at nominal cost or allow the
construction of new plants at significant savings.
Stabilization of Organic Carbon & Nitrogen
The combination of the cell growth/death theory and the bacterial polymer flocculation theory
would give adequate cause for all sanitary and environmental engineers to seriously question and
rethink the present design procedures for activated sludge treatment plants. If accelerated growth is
really the reason for the rapid reaction, then the nature of the waste (soluble or particulate) will have
little bearing on the process selection (provided the waste is biologically degradable and nontoxic).
Further studies will indicate if the solubility of the waste has any detrimental effect upon the rate of
the reaction.
The paper shows that the overall removal rate can be increased significantly without affecting the
net growth rate simply by varying the ratio of sludge in the stabilization tank to that in the contact
tank. This is indeed significant for two reasons. First of all, this operating procedure permits a rapid
363