GPS-X Technical Reference

6 Appendix A – Prefermenter Petersen Matrix
technique enables deduction of the respiration rate from the electrolysis current. In
both cases, temperature and pressure must be kept constant.
APPLICATIONS
The respiration rate is a key variable in the activated sludge process because it is
directly related to both growth and substrate utilization. Therefore, respiration rate
measurement can be used to increase our understanding of the process, which is
essential for good control. The previous section described the different principles on
which respiration rate measuring techniques are based. This section presents some
useful applications of respirometry.
RESPIRATION RATE IN THE AERATION TANK
The respiration rate in the aeration tank, or actual respiration rate, provides
information on the loading of the tank and the biomass activity. Actual respiration
rate can be measured by using a direct method, based on off-gas analysis or
estimator principles, or by using a respirometer. The latter requires sampling of the
activated sludge prior to measurement. It is essential to keep the time between
sampling and measurement as short as possible because of oxygen and substrate
limitation. In most cases this would mean that the correct actual respiration rate
could only be approximated if the respirometer is located in or very close to the
aeration tank and if the meter is operated fast. With a flow-through respirometer the
exact actual respiration rate can be measured if the load in the respiration chamber is
equivalent to the load in the aeration tank. This condition can be satisfied when
sample streams of wastewater and of activated sludge are mixed in the respiration
chamber in such a way that the ratio of wastewater flow and chamber volume equals
the ratio of influent flow and aeration tank volume.
TOXICITY
Since respiration rate is a direct measure of the primary function of the activated
sludge process it is suitable for indicating toxic effects on the activated sludge. The
actual respiration rate might be used to indicate toxic effects. However, since this
rate also depends on the loading it is difficult to discriminate between effects
originating from a change in the loading and from a toxic input. Therefore, a
respiration variable should be used that is less sensitive to loading of the activated
sludge, such as endogenous respiration rate and maximum respiration rate. Because
the latter reflects both potential activity and growth it is a suitable indicator for toxic
effects on the biomass. If the sludge in the respirometer is loaded with an excess of
wastewater the substrate concentration becomes high and consequently the
maximum respiration rate can be approached. This rate serves as a reference level in
the absence of toxicity. At the same time the exposure of the biomass to a possible
toxicant will be higher than in the plant so that toxic wastewater is detected before it
inhibits the plant. If it is not possible to obtain a stable maximum respiration rate
because of a varying wastewater composition, a synthetic wastewater may be used.
In general, for a respirometer to be used as an early warning system, the wastewater
must be sampled upstream of the treatment plant.
GPS-X Technical Reference