GPS-X Technical Reference

2 Appendix A – Prefermenter Petersen Matrix
respiration rate can be operationally defined as the oxygen consumption rate in the
absence of substrate from external sources. According to this definition the
endogenous respiration not only includes the oxidation of all the substrate that is
externally released from decay of bacteria and internally released for maintenance
but also the oxygen consumption of protozoa.
The graphical representation of the respirometric response of activated sludge on the
addition of substrate is called respirogram. If the substrate concentration is high
enough the oxygen uptake rate will approximate its maximum value: the maximum
respiration rate. During maximum respiration the biomass grows at its maximum
rate.
The extra oxygen used in addition to the endogenous oxygen consumption is
referred to as the Short-Term Biochemical Oxygen Demand (BODst ). For practical
purposes, the BODst is equivalent to the area between the curve and the imaginary
underlying endogenous respiration rate. If a respirogram is obtained by mixing
known volumes of sewage and activated sludge, then the BODst of the sewage can
be calculated (see further). Since respiration is the result of the oxidation of multiple
substrates by a heterogenous population of micro-organisms, one type of wastewater
will generate different BODst if combined with different types of sludge. In other
words, the obtained sewage BODst is a function of the sewage-sludge combination
used. The principal difference between BOD 5 and BODst is that BODst is obtained
with a much lower substrate to biomass ratio. Therefore, during a BODst test, the
net growth of biomass is negligible so that the endogenous respiration rate can be
considered constant. BODst mainly represents readily biodegradable substrate and,
if nitrification occurs, ammonium. Since an extra amount of slowly hydrolysable
material is introduced, some readily biodegradable substrate released from
hydrolysis will also be included. However, most of the slowly biodegradable
material is on a long term hydrolysed as a part of the endogenous process and does
not contribute to the BODst. Finally it should be noted that BODst is based on the
same biomass as that of the activated sludge plant under consideration, which is not
the case for BOD5 .
MEASURING PRINCIPLES
A respirometer is an instrument for measuring the respiration rate, that is the oxygen
consumption per unit of volume and unit of time. Many respirometers have been
developed although they rely on only a few principles. The principles can be
classified according to two criteria:
the phase where oxygen is measured, and
the dynamics of either phases.
In a respirometer, the oxygen concentration can either be measured in the gas phase
or in the liquid phase. Irrespective of the location of oxygen measurement, the
phases can be flowing or static. The operation of all existing respirometers can be
explained in terms of these criteria.
GPS-X Technical Reference