Electrode Potentials and Electrolytic
Control in the Anaerobic Digestion Process
FREDERIC C. BLANC
Assistant Professor of Civil Engineering
Northeastern University
Boston, Massachusetts
ALAN H. MOLOF
Associate Professor of Civil Engineering
New York University
Bronx, New York
INTRODUCTION
With improved treatment methods and greater organic removals in the sewage
treatment processes, the problems of sludge stabilization and disposal have greatly
increased. One of the most extensively used methods of sewage sludge stabilization
is anaerobic digestion. However, a lack of understanding of the process mechanism
and control has caused the anaerobic digestion process to be operated at less than the
maximum possible loading rate. It is, therefore, necessary to find better ways of
monitoring and controlling the process. The electrode potential measurement is such
a monitoring parameter which can be used effectively in the anaerobic digestion
process.
ELECTRODE POTENTIALS
An electrode potential is the difference between the electrical potential of the
terminal lead of an indicating electrode in an electrolytic solution and the electrical
potential of a reference electrode in contact with the same solution. Figure 1 depicts
the elements of an electrode potential measuring system used in work on laboratory
digestion units. The indicating electrode is constructed of some metal such as
platinum which will not take part in the electrode potential determining electrochemical reactions but will merely act as a conductor of electrons to the reactions.
Electrical contact between the anaerobic waste solution and the reference electrode
is maintained by means of a salt bridge and a salt reservoir. This type of electrical
connection prevents the calomel reference electrode from becoming coated with a
fouling layer. A conventional pH meter with a millivolt scale has an impedance on
the order of 10^ ohms and is an excellent potentiometer for measuring the electrode
potentials developed in the anaerobic digestion system.
The saturated calomel electrode is perhaps the most commonly used of all
reference electrodes and is quite suitable for use in biological systems. An electrode
potential value reported relative to a saturated calomel electrode is often indicated
by the notation Ec. The notation Ejj indicates that the electrode potential value is
being reported relative to the universal standard hydrogen reference electrode. It
should be pointed out that most Ejj electrode potential values reported for biological
systems are not measured values but are calculated from the measured values obtained
using some other reference electrode.
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