The use of pH and oxidation-reduction potential (ORP) as control parameters
for the nitrification-denitrification process is compared and discussed. T
he signals were measured in a sequencing batch reactor activated sludge sys
tem and in a completely mixed activated sludge reactor. The termination of
denitrification was clearly identified not only by an ORP signal, but also
by a pH signal. However, in some cases the ORP measurement failed (e.g. in
the case of high organic substrate concentrations in the system), pH measur
ement failed in the case of low substrate concentration or under the condit
ions of endogenous denitrification. The combined measurement of pH and ORP
signals is proposed to indicate the end of denitrification as a result.