20    COMPARISON OF TWO RESPIROMETRIC PRINCIPLES FOR
THE DETERMINATION OF SHORT-TERM
BIOCHEMICAL OXYGEN DEMAND
Peter A. Vanrolleghem, Postdoctoral Fellow
Laboratory of Microbial Ecology, University Gent
Coupure Links 653, Gent, Belgium
Henri Spanjers, Postdoctoral Fellow
Department Environmental Technology, Wageningen Agricultural University,
Wageningen, The Netherlands
INTRODUCTION
Current legislation of effluent discharges from wastewater treatment plants is geared to enforce a
constant low level of pollutant discharged, independent of seasonal variations of the capacity of the
receiving body or variations in wastewater load that enters a treatment plant. While some discussion
has been incited for a long time now1 whether this policy should be maintained, it must be stressed
that the central goal of a treatment plant will always be to guarantee a certain effluent quality under
time-varying influent compositions.
When one considers the variation in the effluent concentration, a treatment plant can be regarded
as a lowpass filter where high frequency disturbances are dampened.2 However, this does not hold for
the required treatment capacity that must follow the changes in incoming load closely to guarantee
stable effluent quality. From a process control and economic perspective it is therefore required that
the process conditions in which the biocatalysts perform their task are set to adjust to these varying
requirements. Feedback control of certain variables is a traditional approach, e.g. dissolved oxygen
control.3 However, certain process time constants are so large that the delay between initiation of a
control action and its effect on the plant may be prohibitively long to make feedback control successful.4 As an alternative for such systems feedforward control has been proposed.3,5 However, it is well-
known that this type of control is sensitive to the quality of the process model.6 Evidently it is essential
to measure the disturbance that is considered important from a control point of view.4,5
In this paper the measurement of probably the main disturbance variable of a wastewater treatment
plant is considered, i.e. the wastewater concentration. Chemical methods such as on-line COD and
TOC monitors that have been proposed for long3 to quantify the concentration of pollutants in
influents suffer from several drawbacks.7 Next to the problems associated with vulnerability of the
devices and the concomitant high maintenance requirements, the major flaw is that these methods
cannot give any information on the treatability of the pollutants. On the other hand, it must be
stressed that these methods can provide the data at a high frequency. Traditional methods that rely on
the monitoring of the biodegradation of the pollutants to obtain an indication of the treatability such
as the BOD520-method are clearly inapt to provide the necessary information for feedforward process
control due to the large time delay between sample introduction and measurement result. However,
the principle of monitoring the oxygen uptake for assessment of the treatability and pollutant concentration of a wastewater is a very powerful one, because most wastewater treatment processes rely on
aerobic degradation of the waste. Therefore, methods have been proposed to decrease the response
time of these biologically medicated methods to such a level that application of the sensor data within
control loops becomes feasible.
In this contribution two such —independently developed — respirometric principles for the assessment of the variations in influent concentration are presented and experimentally compared. Complemented with flow rate information it will be illustrated with some case studies that interesting feedforward control strategies can be devised with this information.
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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