GPS-X Technical Reference

Appendix C –Respirometry C-5
the effect of substrate and DO limitation. However, the effect of limiting substrate
can be eliminated by the continuous addition of substrate (wastewater) to the
respiration chamber.
MEASUREMENT IN THE GAS PHASE
Respirometers that are based on measuring gaseous oxygen obviously always deal
with two phases: the liquid phase containing the activated sludge of which the
respiration rate has to be measured and the gas phase where the measurement takes
place. The advantage of this technique is that the instrument is not affected by
contaminations common in an activated sludge medium. Oxygen can be measured
either directly by using a sensor such as a paramagnetic oxygen analyzer or
indirectly by measuring pressure or volume changes.
Respirometric principles based on measuring gaseous oxygen also use oxygen mass
balances to derive the respiration rate. However, in addition to the mass balance of
the liquid phase, a balance of the gas phase has to be considered:
dC
dt
G
Q
V
L
x
C
C
K aC
L C
r
Lin
L
L
L
dC
dt
L
F
V
in
G
C
Gin
F
V
out
G
C
G
V
V
L
G
L
K a C
x
L
C
L
Where:
CG = O2 concentration in the gas phase
CGin = O2 concentration in the gas entering the gas phase
Fin = flow rate of the gas phase
VL = volume of the gas phase
The term KLa(C x L -CL) represents the dissolution rate of oxygen from the gas phase
to the liquid phase and it comprises the connection between the two phases. For a
correct measurement it has to be assumed that the dissolution rate is identical to the
respiration rate of the activated sludge. If CL is not measured directly, it has to be
related to volume or pressure changes by using the gas law. Additional assumptions
to be made are that the gas behaves ideally and that measured changes are only
caused by changes in oxygen concentration. Because during the activated sludge
process carbon dioxide is produced this gas must be absorbed chemically to avoid
interference with the oxygen measurement.
In respirometers the measurement of r is usually simplified by operating under static
liquid phase. In the simplest case, when both liquid and gas phase are static, the
same restriction as with the simplest DO base principle exists: when the oxygen
becomes exhausted it must be replenished by, for instance, venting the gas phase.
Another possibility is supplying oxygen from an external tank and measuring the
amount of oxygen supplied or generating the oxygen by electrolysis. The latter
GPS-X Technical Reference