356
D. Jenkins and D. Orhon
plant, a unique net growth rate is associated with each organic loading rate, because both
sludge growth and decay take place in a single basin. However, when growth and decay
functions are separated—as is partially achieved in contact stabilization—this unique
combination of net growth rate and substrate removal rate is unlocked and one can, by
manipulation of design and operating parameters, exercise control of sludge production
(growth) independent of organic loading.
EXPERIMENTAL
Continuous flow laboratory activated sludge units (Eckhoff and Jenkins, 1966)
maintained at 20 ± 2°C, and the Sanitary Engineering Research Laboratory pilot plant,
both treating Richmond, California settled domestic sewage, were used in conventional
and contact stabilization modes. All data were taken from units that had reached
steady-state (defined as ± 20 per cent variation of daily loading values from desired
organic loading value) for at least 2 mean cell residence times. Analyses were by Standard
Methods (American Public Health Association, 1970) except, activated sludge viability
was by standard plate counting on sonically dispersed activated sludge, using activated
sludge extract (ASE), agar (Prakasam and Dondero, 1967) and incubation for 7 days at
25 ±1°C. Soluble COD determinations were on samples filtered through Whatman GF/C
glass filters. Dehydrogenase activity was by the TTC method of Bucksteeg (1966).
RESULTS AND DISCUSSION
The steady-state cell continuity equation for the contact basin, μ ης = Y c q c — kd c is
linear (Fig.2) over a wide range (q c of 2-22) and gives a yield value of 0.74 g VSS/ g
COD removed. The line intercepts very close to zero, supporting the validity of neglecting
decay for typical contact basin growth rates. Although the contact yield value is greater
than double COD-based yield values reported for activated sludge plants treating domestic
sewage in the standard rate range (Pearson and Haas, 1966; Jenkins and Garrison, 1968),
it is consistent with high rate activated sludge yield values reported by Humenick and
Kaufman (1971); and Menar and Jenkins (1967). The latter authors showed that yield
increases with increasing substrate removal rates, reaching a limiting value of 0.73 g
VSS/ g COD removed.
The stabilization basin net VSS mass decay rate increases with increasing values of
contact basin removal rate, to reach a limiting value of 0.6—0.7 day'
1 at contact substrate
removal rates of 8-10 g COD/ g VSS - day (Fig.3). The observation of a variable solids
decay rate is consistent with previous observations on both contact stabilization and
conventional systems (Stewart, 1962; Weddle, 1970) and means that with a contact
stabilization flow sheet (in which it is possible to operate the contact basin at substrate
removal rates > 5 day"
1
) high overall substrate removal rates can be achieved at
significantly lower net growth rates than in conventional systems. Moreover, having the
flexibility of two aeration basins allows the designer and operator to choose (within
limits) from a range of sludge net growth rates at which the process can be operated at a
given substrate removal rate. Thus, the sludge production rate and the degree of
nitrification—parameters that are directly proportional to net growth rate—can be varied
at a given substrate removal rate. On the basis of Figs.2 and 3 the overall cell continuity
D. Jenkins and D. Orhon
plant, a unique net growth rate is associated with each organic loading rate, because both
sludge growth and decay take place in a single basin. However, when growth and decay
functions are separated—as is partially achieved in contact stabilization—this unique
combination of net growth rate and substrate removal rate is unlocked and one can, by
manipulation of design and operating parameters, exercise control of sludge production
(growth) independent of organic loading.
EXPERIMENTAL
Continuous flow laboratory activated sludge units (Eckhoff and Jenkins, 1966)
maintained at 20 ± 2°C, and the Sanitary Engineering Research Laboratory pilot plant,
both treating Richmond, California settled domestic sewage, were used in conventional
and contact stabilization modes. All data were taken from units that had reached
steady-state (defined as ± 20 per cent variation of daily loading values from desired
organic loading value) for at least 2 mean cell residence times. Analyses were by Standard
Methods (American Public Health Association, 1970) except, activated sludge viability
was by standard plate counting on sonically dispersed activated sludge, using activated
sludge extract (ASE), agar (Prakasam and Dondero, 1967) and incubation for 7 days at
25 ±1°C. Soluble COD determinations were on samples filtered through Whatman GF/C
glass filters. Dehydrogenase activity was by the TTC method of Bucksteeg (1966).
RESULTS AND DISCUSSION
The steady-state cell continuity equation for the contact basin, μ ης = Y c q c — kd c is
linear (Fig.2) over a wide range (q c of 2-22) and gives a yield value of 0.74 g VSS/ g
COD removed. The line intercepts very close to zero, supporting the validity of neglecting
decay for typical contact basin growth rates. Although the contact yield value is greater
than double COD-based yield values reported for activated sludge plants treating domestic
sewage in the standard rate range (Pearson and Haas, 1966; Jenkins and Garrison, 1968),
it is consistent with high rate activated sludge yield values reported by Humenick and
Kaufman (1971); and Menar and Jenkins (1967). The latter authors showed that yield
increases with increasing substrate removal rates, reaching a limiting value of 0.73 g
VSS/ g COD removed.
The stabilization basin net VSS mass decay rate increases with increasing values of
contact basin removal rate, to reach a limiting value of 0.6—0.7 day'
1 at contact substrate
removal rates of 8-10 g COD/ g VSS - day (Fig.3). The observation of a variable solids
decay rate is consistent with previous observations on both contact stabilization and
conventional systems (Stewart, 1962; Weddle, 1970) and means that with a contact
stabilization flow sheet (in which it is possible to operate the contact basin at substrate
removal rates > 5 day"
1
) high overall substrate removal rates can be achieved at
significantly lower net growth rates than in conventional systems. Moreover, having the
flexibility of two aeration basins allows the designer and operator to choose (within
limits) from a range of sludge net growth rates at which the process can be operated at a
given substrate removal rate. Thus, the sludge production rate and the degree of
nitrification—parameters that are directly proportional to net growth rate—can be varied
at a given substrate removal rate. On the basis of Figs.2 and 3 the overall cell continuity