DESIGN OF ACTIVATED SLUDGE SYSTEMS-
REACTION RATES AND HYDRAULIC INTEGRITY
Arthur W. Busch, Environmental Engineering Consultant
Dallas, Texas 75230
INTRODUCTION
As has been shown in previous work [1,2], hydraulic residence time is not fundamentally involved in the design of biological process reactors. However, reaction time is
important and must coincide with hydraulic residence time for optimum competence in
both design and performance. Reaction time is defined as the time for removal of soluble
substrate and depends on initial concentrations of substrate and bacteria in a given reaction system. Hydraulic integrity is defined as the capability to predict degree of mixing
and residence time distribution for a reactor. This capability is vital in scale-up and in
proper assessment of process performance in order to avoid penalizing a process for inadequate hydraulic performance of the system housing the process. In short, we must be able
to determine whether lack of process efficiency is due to hydraulic deficiency [3]. Of
basic significance is that all operations and processes currently used in biological treatment
systems are adversely affected by excessive hydraulic residence time, as weU as by inadequate residence time. Evidence for detrimental effects of excessive hydraulic residence time
includes anaerobic primary and secondary clarifier and production of biological solids
which do not settle readdy. A recent paper [4]  discussed the issue of solids separation
in meeting effluent objectives.
DISCUSSION
The role of reaction time in establishing hydraulic integrity can be dlustrated by reference to Figure 1. Consider that, in a nominally plug flow biological reactor, backmixing
would be indicated by lack of significant concentration gradients in the reaction mixture.
If concentration gradients disappeared at, for example, the midpoint of the reactor, the
conclusion could be reached that the last half of the reactor was "completely mixed."
Unless, however, the reaction time had been shown to be greater than half of the
YNTHETIC  RUBBER WASTE
DISTANCE ALONG AERATION BASIN
Figure 1. Reactant concentration gradients in
conventional activated sludge.
IOO 200 300 400 500
SOLUBLE  TOD   CONCENTRATION , mg/l.
Figure 2. Unit rates of substrate removal for
various industrial wastes showing linear portion of curves [1].
896