GPS-X Technical Reference

Appendix C –Respirometry C-7
SHORT-TERM BIOCHEMICAL OXYGEN DEMAND
In assessing the Short-term Biochemical Oxygen Demand (BODst), small amounts
of wastewater are typically mixed with larger amounts of activated sludge that is
grown on that specific wastewater. Batch methods to determine BODst consist of the
addition of a wastewater sample to the activated sludge in a respirometer and the
monitoring of respiration rate as a function of time. The BODst is obtained from the
cumulative oxygen consumption. Continuous methods bring flows of sludge and
wastewater together in a respiration chamber and derive the BODst from the
resulting respiration rate by using a measurement of the endogenous respiration rate
as the base or reference value. Under certain conditions it is possible to assess the
effluent BODst by using a continuous method. Some methods (used in BOD-meters)
do not explicitly involve calculation of respiration rate: BODst is derived from
changes in oxygen content. For example BOD-probes, which consists of
immobilized living cells, a membrane and a DO-sensor, use the signal of the DO
probe. The signal is a measure of activity of the cells, which is, if the probe is
submerged in a wastewater, a measure of the substrate concentration in the water.
BOD-probes and other BOD-meters should be calibrated in solutions with known
BOD. Under certain conditions a correlation between BODst and BOD 5 might be
found, irrespective of the different nature of these two variables, so that instead of a
BODst -meter a BOD 5 -meter can be obtained.
ESTIMATION OF MODEL PARAMETERS
Since oxygen consumption is associated with substrate utilization and biomass
growth, respirometry is useful for estimation of activated sludge model parameters.
Continuous respirometric experiments are appropriate for the estimation of
stoichiometric coefficients, while kinetic parameters and wastewater concentrations
can be obtained from batch experiments. For the estimation of kinetic parameters of
a given activated sludge it is essential that the substrate to biomass ratio is small
enough to guarantee that biomass growth and shift in biomass population are
negligible. The parameters can be assessed by fitting a model to the respirometric
measurement time series, i.e. the respirogram. It must be emphasized that from
respiration rate only, not all parameters can be uniquely estimated.
RESPIRATION RATE AS A CONTROL VARIABLE
The activated sludge process aims to achieve, at minimum energy costs, a
sufficiently low concentration of biodegradable matter in the effluent together with a
low sludge production. Therefore, the process has to be controlled. Common control
strategies maintain certain levels of biomass and dissolved oxygen in the aeration
tank by manipulating waste sludge flow, return sludge flow and aeration capacity.
Such strategies are based on measurements of the concentrations of mixed liquor
suspended solids and dissolved oxygen.
As a control variable, the respiration rate of activated sludge has generated much
interest, since it is an essential variable in the activated sludge process and provides
information on activity and concentration of the biomass, influent waste
concentration, toxicity and concentration of biodegradable matter in the effluent. For
respirometry-based control the following variables can be deduced from respiration
rate and used in the control system:
GPS-X Technical Reference