POWDERED ACTIVATED CARBON BIOLOGICAL TREATMENT:
LOW DETENTION TIME PROCESS
John F. Ferguson, Associate Professor
Department of Civil Engineering
University of Washington
Seattle, Washington 98195
George F. P. Keay, Professional Engineer
City of Johannesburg
Johannesburg, South Africa
M. Steve Merrill, Project Engineer
Brown and Caldwell Consulting Engineers
Seattle, Washington 98104
Arthur H. Benedict, Project Engineer
Howard Edde, Inc.
Bellevue, Washington 98004
INTRODUCTION
Powdered activated carbon (PAC) addition to activated sludge has been practiced now
for several years [1-5].   The previous research suggests that there are several kinds of
benefits in combined PAC-activated sludge treatment in terms of improved process performance compared to activated sludge without carbon addition.   The improvements are
both in efficiency of pollutant removal and improved process stability.   The process is
particularly well suited to wastewaters containing adsorbable, toxic and poorly biodegradable organics.   Research with wastes from organic chemical manufacture, petroleum
refining and textile manufacture best exemplify these benefits.
The objectives in testing carbon addition to activated sludge in Seattle were different.
Though the research reported in this paper did not test each of the potential advantages
of carbon addition, the rationale for the study is important.   Seattle METRO currently
discharges 100 mgd (dry weather) to deep waters of Puget Sound after primary treatment
and chlorination.   It was recognized at the time of secondary effluent quality regulations
that compliance with the July 1977 deadline was not physically possible.   METRO has
undertaken a study of the effects of the waste on the Sound and of various treatment
schemes to provide treatment ranging from advanced primary to advanced secondary.
METRO proposes to make treatment changes that will be instituted by 1983 to meet
the water quality objectives of PL 92-500 by the technology best suited for the particular
pollutants and for the particular characteristics of the sewerage system at Seattle.
The sewerage system presently includes both separate and combined sewers and has a
storm water/infiltration/iriflow problem.   The hydraulic capacity of the trunk sewer to
the West Point treatment plant is 360 mgd.   Flow during a heavy rain may increase from
about 100 mgd to 360 mgd with a decrease in 5-day biochemical oxygen demand (BOD)
and suspended solids (SS) concentrations from 200 mg/1 to considerably less than 100 mg/1
with a corresponding decrease in water temperatures from 60-50 F during a summer rain.
High flow rates and low concentrations may persist for several days.   Biological treatment
to achieve the secondary effluent limitations for concentration and percent removal of
BOD and SS is likely to be very difficult and costly.
In addition, the site of the plant, which was surrounded by a military base, was adequately sized for expansion of sludge digestion, but is not large enough for a conventional
secondary treatment facility.   Expansion must be by encroachment into an unstable bluff
or by fill into shallow water of the Sound.   Much of the military base has been given to
the city for a park, designated as an urban wilderness area.   The existing plant is viewed
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