GPS-X Technical Reference

MEASUREMENT IN THE LIQUID PHASE
Appendix C –Respirometry C-3
Respirometers that are based on measuring Dissolved Oxygen (DO) concentration in
the liquid phase all use a polarographic DO probe. The DO probe generally consists
of two electrodes in an internal electrolyte solution covered with a semi-permeable
membrane. Dissolved oxygen molecules diffuse from the liquid through the
membrane into the internal solution. The molecules are reduced on the cathode,
which generates an electrical current. This current is proportional to the diffusion
rate of the oxygen molecules through the membrane, which, in turn, is proportional
to the DO concentration in the solution. The relationship between electrical current
and DO concentration is established by calibrating the DO probe. It can be shown
that for a DO probe, water saturated air is equivalent to oxygen saturated water. This
is used to calibrate the DO-meter at 100% DO.
All respirometric principles based on measurements of the DO concentration use a
DO mass balance of activated sludge in some well-defined measurement system to
derive the respiration rate. Consider a system consisting of a liquid phase, containing
activated sludge, and a gas phase both having an input and an output. It is assumed
that the DO concentration in the liquid phase can be measured. The DO mass
balance over the liquid phase is:
dC
dt
L
where:
Q
( CLin
CL
) K
V
L
L
a
x
C
L C
r
CL = DO concentration of activated sludge in the system
CLin = DO concentration of activated sludge entering the system
KLa = oxygen mass transfer coefficient
Q = flow rate of activated sludge into the system
r = respiration rate of the activated sludge in the system
VL = volume of the liquid phase
Notice that, since it is a mass balance over the liquid phase, it does not contain gas
flow terms. The first and second terms on the right hand side represent mass flow of
DO in the activated sludge. In most systems Q will be equal to the flow leaving the
system. The second term describes the mass transfer of oxygen from the gas phase
to the liquid phase. This process should not be limiting. The last term is the
respiration rate to be derived from the mass balance. Therefore, C L must be
measured and all other coefficients be known or neglected. In practice, the
determination of r can be simplified in several ways.
One approach is to prevent liquid flow and gas transfer. In practice, this means that a
certain amount of activated sludge is kept in a vessel and that entry of oxygen is
avoided by sealing the liquid-gas interface or keeping the entry of oxygen at a
minimum. In that case, the mass balance reduces to:
L
GPS-X Technical Reference