Aerobic Sludge Digestion at a Trickling Filter
Waste Treatment Plant
QUINTIN B. GRAVES, Professor Emeritus
DAVID SCOTT, JR., Graduate Student
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma
E. E. COOK, Assistant Professor
Southern Illinois University
Carbondale, Illinois
Y. A. BADRA
Clark, Dietz, and Associates, Inc.
Urbana, Illinois
INTRODUCTION
Aerobic sludge digesters have been used for many years for treating the excess sludge at
activated sludge waste treatment plants. In addition, much aerobic experimental research
has been made on a laboratory scale and on a pilot plant scale and reported in the literature.
H owever, little has been written concerning aerobic sludge digestion at trickling filter waste
treatment plants.
With some changes in the piping at the Stillwater, Oklahoma, trickling filter sewage
treatment plant, three types of sludges were utilized in a group of six identical pilot plants.
The basic objectives of the investigation were to determine the usefulness and
effectiveness of aerobic digestion of sludges from a trickling filter sewage treatment plant.
Comparisons were made of various parameters of aerobically digested sludges. Due to
limitation of time results on only a few parameters will be present in this paper.
DESCRIPTION OF PILOT PLANT AND EQUIPMENT
The Stillwater, Oklahoma, sewage treatment plant consists of a sewage screen at the
main lift station, an aerated grit chamber, a preaeration tank, two primary clarifiers, a
primary trickling filter, an intermediate clarifier, a secondary trickling filter, two final
clarifiers, a primary digester, a secondary digester and sludge drying beds, along with the
auxiliaries that normally go with treatment plants of this type. The piping of the sewage
treatment plant was installed so that the sludge from the intermediate clarifier and from the
final clarifier would be returned to the primary clarifier. Modifications of the piping were
made so that the primary sludge alone could be obtained for the pilot plant. Then valves
were changed so that the primary and secondary sludge could be obtained for the pilot
plant. Finally secondary sludge alone was piped to the pilot plant.
Each unit consisted of one 200 gallon polyethylene tank (180 gallon capacity) placed
inside of a 250 gallon tank. The first tank had a smaller diameter than the second providing
an annular space so that water could be circulated from a constant temperature control unit
to the reactor and back again to the temperature control unit. This arrangement provided
essentially a constant temperature control unit to the reactor and back again to the
temperature control unit. This arrangement provided essentially a constant temperature in
the reactors as soon as the controls were properly adjusted. The sludges were fed to the
aerobic reactors or digesters at specific intervals. Any overflow was discarded. Samples
were taken after the reactors approached equilibrium. The system was operated as a
completely mixed semi-continuous flow operation. The same amount of waste was
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