Evaluation of the Kraus Model of
Activated Sludge Bulking
WESLEY O. PIPES, Professor of Civil Engineering
Northwestern University
Evanston, Illinois
FRANK S. MEADE, Assistant Project Engineer
Roy F. Weston, Inc.
Newton Square, Pennsylvania
INTRODUCTION
Most of the proposed kinetic models of the activated sludge process interpret
effluent BOD as being caused by organic matter which was originally present in
the influent waste, but was not removed in the aeration tank. This interpretation is obviously incorrect if activated sludge solids are being lost in the effluent
from the secondary settling tank. Respiratory oxygen consumption by sludge organisms in the effluent will cause a high effluent BOD which is not due to unchanged organic matter from the influent, and it is even possible to have an effluent BOD higher than the influent BOD. Further improvements in kinetic models
of the activated sludge process must include methods of predicting when sludge
solids will be lost into the effluent.
It is possible to construct a mathematical model which completely neglects
all the different phenomena which can lead to the loss of sludge solids from an
activated sludge process. However, solids loss is so crucial to the performance of
any activated sludge system that it deserves a great deal of study. The obvious
approach to the development of a method four predicting this problem is to deal
individually with each of the phenomena which can lead to solids loss. Since
there are at least 10 phenomena which can lead to loss of sludge solids in the effluent (1), a large variety of different formulations will be required.
Rising sludge caused by nitrification in the aeration tank and denitrification
in the settling tank is the best understood of any of the biological phenomena occurring in the activated sludge process. The review of Wuhrman (2) demonstrates
clearly how it is possible on the basis of work by Downing et al (3) to predict
sludge loss due to rising sludge from the kinetic parameters of the process. It
probably never will be possible to predict the amount of solids which will be lost
due to rising sludge, but it is possible to design and operate on a rational basis
with the objective of avoiding the occurrence of rising sludge.
Bulking sludge is another occurrence which can lead to the loss of activated
sludge solids in the effluent. The intensity of bulking is measured quantitatively
by the sludge volume index (SVI). Kraus (4) presented equations which could be
developed into a formulation which would predict the amount of sludge loss due
to bulking from the SVI, the mixed liquor suspended solids (MLSS), and operating
parameters of the secondary sedimentation tank. Since bulking sludge does settle
to some extent, it is possible to operate with bulking sludge and still avoid loss of
solids in the effluent if the capacities of the secondary settling tanks and return
sludge pumps are great enough. As a general rule, the higher the SVI, the better the quality of effluent produced as long as loss of sludge solids to the effluent
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