Minimal Solids Aeration Complete
Mixing Activated Sludge
MARK W. TENNEY, Graduate Student
Massachusetts Institute of Technology
Cambridge, Massachusetts
RAYMOND H. JOHNSON, JR., Graduate Student
Massachusetts Institute of Technology
Cambridge, Massachusetts
JAMES M. SYMONS, Assistant Professor of Sanitary Engineering
Massachusetts Institute of Technology
Cambridge, Massachusetts
The complete mixing activated sludge process has received widespread attention and acceptance in recent years (1,2, 3,4,5). This study was undertaken in an
attempt to expand the application and the effective use of the complete mixing
activated sludge process. Some modifications were sought which would allow
even more rapid and economical treatment of high strength organic wastes than is
currently available.
Complete mixing activated sludge systems were tested on a laboratory scale
to observe their performance under high organic volumetric loadings (lb COD/
day/1000 cu ft). Extensive consideration was also given to the problem of the
treatment and/or disposal of the large quantity of solids created in highly loaded
activated sludge systems. Investigations were made on the feasibility of using
either moderate or extended solids aeration complete mixing activated sludge,
chemical treatment, or an oxidation pond, for treatment of the solids wasted
from the highly loaded, highly wasted complete mixing activated sludge pilot
plant.
THEORETICAL CONSIDERATIONS
Complete mixing activated sludge has been defined by McKinney (1) as a
process in which the incoming wastes are completely mixed with the entire contents of the aeration tank. This is generally accomplished by feeding the influent
wastes uniformly along the aeration tank's longer horizontal dimension. This implies that a uniform organic load exists throughout the aeration tank, which results in a uniform oxygen demand and uniform biological growth. McKinney also
pointed out that "the two modifications of complete mixing systems are total oxidation and high sludge synthesis."
In order to more completely differentiate among the biological behaviors of
various activated sludge processes four definitions have been established:
(1) Solids Retention Time (SRT)
_ Total Weight of Solids in a Biological System	
Total Weight of Solids leaving the System/Unit Time (usually one day)
Note:   1/SRT x 100 = Per Cent Solids Wasting
Garrett (6) was one of the first to propose control of activated sludge behavior via solids wasting.    Similarly, based on solids retention time or solids
- 483 -