Dissolved Oxygen Control in
Activated Sludge
ROBERT A. RYDER, Director
Pacific Engineering Laboratory
San Francisco, California
INTRODUCTION
The development of electrochemical sensing elements for the measurement of
dissolved oxygen has made possible the automatic control of this important factor
of the activated sludge process. This development has extended for nearly the past
20 years and like simUar sensing and control systems for pH, chlorine residual, conductivity, and selective ion electrodes has become a useful and effective operational
tool in wastewater treatment practice.
Polarographic methods of dissolved oxygen measurement were, at their inception, extensions of the classical dropping mercury electrode system (1).
Quantitative polarographic systems of this type measure the current produced at
constant voltage by an oxidation reduction reaction at the electrodes of the sensing
elements.
The development of an oxygen selective membrane over the electrode sensing
element by Clark (2) resulted in a considerably more functional instrument for laboratory or field dissolved oxygen monitoring. The development of temperature-
compensating thermistors in the electrode assembly resulted in an instrument
system with signal output proportional to dissolved oxygen concentration (3).
Process oxygen monitoring and control systems then become a reality.
The polarographic method of dissolved oxygen analysis can utilize either
galvanic (4, 5, 6), or amperometric electrodes (7); but, the basic quantitative reaction of the reduction of oxygen at the cathode at the required potential is the
same.
O2 + 2H2O + 4e"W 40H"
A concurrent anodic oxidation reaction occurs, and a current proportional to
the amount of oxygen present in the sample is produced.
Other methods of quantitative dissolved oxygen analysis in Uquids have been
developed utiUzing measurement of the partial pressure of oxygen in the gas above
the water surface. An instrument of this type has been developed from flue and
stack gas monitors utilizing a magnetic oxygen analyzer (8).
One of the more recent developments in dissolved oxygen instrumentation has
been the thallium electrode. This does not require the membrane filter of the other
solid electrode polarographic instruments. A more rapid response to changes in dissolved oxygen concentration and avoidance of membrane fouling and periodic replacement result.
Prior to the development of these dissolved oxygen sensing instruments, the
classical Winkler "Wet Chemical" method was all that was available, except for rather
sophisticated manometric devices. To anyone who has extensively analyzed dissolved
oxygen in activated sludge by the time consuming copper sulfate-sulfamic acid
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