THE MECHANISM AND DESIGN
OF THE CONTACT STABILIZATION
ACTIVATED SLUDGE PROCESS
DAVID JENKINS and DERIN ORHON
Sanitary Engineering Research Laboratory, University of California,
Berkeley, U.S.A.
INTRODUCTION
Contact stabilization activated sludge employs two aerated reactors separated by a
sludge separator. Sewage and activated sludge are aerated for between 0.5-1.5 hours in
the contact basin. The sludge, separated by sedimentation, is re-aerated for 1.5—8 hours
in a stabilization basin, then returned to the contact tank.
Contact stabilization and other activated sludge modifications where return sludge is
aerated in the absence of wastewater, have been used extensively. Indeed, the first
full-scale activated sludge plant in the U.S.A. had re-aeration capability (Haseltine, 1961).
Older designs differ from current ones in allotting about 10 per cent of total aeration
volume to sludge re-aeration (currently 75 per cent is common). Ullrich and Smith (1951)
first proposed very short contact times (15—20 min.) and longer (90 min.) stabilization
times. Pilot plant studies of their "biosorption process" indicated 90—95 per cent BOD
and suspended solids removals.
The use of a contact stabilization in place of a conventional flow scheme greatly
increases volumetric loading capacity without decreasing BOD removal efficiency. For
example, at Austin, Texas, a 6 MGD conventional activated sludge plant was modified to
treat 16 MGD in a contact stabilization mode without the bulking problems encountered
prior to modification (Ullrich & Smith, 1957).
Contact stabilization appears more resilient to load, flow and toxicant transients than
conventional activated sludge. Transients only affect the 10-25 per cent of the total
sludge that is in the contact basin, while in the conventional process all the sludge is
exposed (Grich, 1961).
Contact stabilization research has been largely empirical or directed toward the
solution of specific practical problems. General observations are that initial organic
matter removal from wastewater by activated sludge occurred rapidly and was followed
by a longer period during which little further dissolved organic matter reduction
occurred. Most investigators (Ullrich & Smith, 1951; Jones, 1970) postulated that during
contact, activated sludge sorbs or forms "storage products" from wastewater which are
later metabolized during stabilization. Jones et al (1962) interpreted the stabilization
process as a "reactivation period" in which the activated sludge adsorption potential is
generated by the exo-enzymatic hydrolysis and solubilization of sorbed materials.
Siddiqui (1965) attributes increased organic matter removal potential of stabilized
activated sludge to enzymes released during stabilization and returned to the contact
basin. In a conventional scheme these water-soluble enzymes are lost in the effluent.
In contrast to these studies, Miller (1970) showed that storage of metabolites did not
occur during the contact phase. His experiments, combined with the observations of
Weddle and Jenkins (1970) that:
i) standard rate activated sludge viable fraction was 10—20 per cent
ii) viability increased as sludge growth rate increased, and
353
OF THE CONTACT STABILIZATION
ACTIVATED SLUDGE PROCESS
DAVID JENKINS and DERIN ORHON
Sanitary Engineering Research Laboratory, University of California,
Berkeley, U.S.A.
INTRODUCTION
Contact stabilization activated sludge employs two aerated reactors separated by a
sludge separator. Sewage and activated sludge are aerated for between 0.5-1.5 hours in
the contact basin. The sludge, separated by sedimentation, is re-aerated for 1.5—8 hours
in a stabilization basin, then returned to the contact tank.
Contact stabilization and other activated sludge modifications where return sludge is
aerated in the absence of wastewater, have been used extensively. Indeed, the first
full-scale activated sludge plant in the U.S.A. had re-aeration capability (Haseltine, 1961).
Older designs differ from current ones in allotting about 10 per cent of total aeration
volume to sludge re-aeration (currently 75 per cent is common). Ullrich and Smith (1951)
first proposed very short contact times (15—20 min.) and longer (90 min.) stabilization
times. Pilot plant studies of their "biosorption process" indicated 90—95 per cent BOD
and suspended solids removals.
The use of a contact stabilization in place of a conventional flow scheme greatly
increases volumetric loading capacity without decreasing BOD removal efficiency. For
example, at Austin, Texas, a 6 MGD conventional activated sludge plant was modified to
treat 16 MGD in a contact stabilization mode without the bulking problems encountered
prior to modification (Ullrich & Smith, 1957).
Contact stabilization appears more resilient to load, flow and toxicant transients than
conventional activated sludge. Transients only affect the 10-25 per cent of the total
sludge that is in the contact basin, while in the conventional process all the sludge is
exposed (Grich, 1961).
Contact stabilization research has been largely empirical or directed toward the
solution of specific practical problems. General observations are that initial organic
matter removal from wastewater by activated sludge occurred rapidly and was followed
by a longer period during which little further dissolved organic matter reduction
occurred. Most investigators (Ullrich & Smith, 1951; Jones, 1970) postulated that during
contact, activated sludge sorbs or forms "storage products" from wastewater which are
later metabolized during stabilization. Jones et al (1962) interpreted the stabilization
process as a "reactivation period" in which the activated sludge adsorption potential is
generated by the exo-enzymatic hydrolysis and solubilization of sorbed materials.
Siddiqui (1965) attributes increased organic matter removal potential of stabilized
activated sludge to enzymes released during stabilization and returned to the contact
basin. In a conventional scheme these water-soluble enzymes are lost in the effluent.
In contrast to these studies, Miller (1970) showed that storage of metabolites did not
occur during the contact phase. His experiments, combined with the observations of
Weddle and Jenkins (1970) that:
i) standard rate activated sludge viable fraction was 10—20 per cent
ii) viability increased as sludge growth rate increased, and
353