The Kraus Model of the Kinetics of
Activated Sludge Bulking
WESLEY O. PIPES, Associate Professor
Department of Civil Engineering
Northwestern University
Evanston, Illinois
INTRODUCTION
The most serious operational problems with the activated sludge process are
all manifest by impairment of the settling characteristics of the sludge. Unfortunately, in many cases any deterioration in the settling characteristics of activated
sludge is called "bulking" without any attempt to describe what is actually happening. The lack of discrimination between phenomena which result in poor separation of sludge and effluent in the secondary clarifier has led to a very confused
situation. In order to approach a solution to the operational problems with activated sludge it will be necessary to make a clear distinction between these different phenomena and to relate each to operational parameters.
DESCRIPTION OF SETTLING PROBLEMS
Normal Sludge
The most common method of measuring sludge settling characteristics quantitatively is the Sludge Volume Index (SVI) determination (1) in which the volume
(in ml) occupied by the sludge after settling 30 mins in a one liter graduated
cylinder is divided by the mixed liquor suspended solids (MLSS in gm/l). Activated sludge in good condition settles rapidly as a uniform mass giving a SVI of
less than 100 ml/gm. The supernatant remaining after the sludge has settled is
clear and there is a distinct demarkation between the sludge and supernatant.
Filamentous Bulking
One of the settling problems which was recognized very early in the history of
the activated sludge process is filamentous bulking which occurs when large numbers of filamentous organisms are present in addition to the normal bacterial
clumps (2). The first organism isolated and properly identified as a causative organism of filamentous bulking was sphaerotilus nutans (3). Despite the fact that
Lackey and Wattie (4) stated that a number of other organisms could be the cause
of filamentous bulking most succeeding investigators have assumed that any filamentous organism in activated sludge is sphaerotilus natans without any attempt
at identification. So far only Sphaerotilus nutans and Geotrichum candidum have
been definitely established as causative organisms of a specific case of filamentous
bulking in a full scale activated sludge plant but there probably are a large number
of other organisms which are involved in filamentous bulking (5).
When filamentous bulking occurs the sludge settles much more slowly than
normal sludge does. Consequently, the SVI is very high. However, there is still
a definite demarkation between the sludge and the supernatant and the supernatant
is lower in turbidity and BOD than it is with normal sludge (6,7,8). Thus, the only
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