Development of a benchmarking methodology for evaluating oxidation ditch control strategies

A.A.A. Abusam

Research output: Thesisinternal PhD, WU

Abstract

<strong><u><p>Keywords:</strong></u> wastewater, oxidation ditch, carrousel, modeling, activated sludge, <em>ASM</em> No. 1, oxygen transfer rate, aeration, parameter estimation, calibration, sensitivity analysis, uncertainty analysis, sensors, horizontal velocity, benchmark, benchmarking, control strategies, simulation.</p><p> </p><p>The purpose of this thesis was to develop a benchmarking methodology for evaluating control strategies for oxidation ditch wastewater treatment plants. A benchmark consists of a description of the plant layout, a basic simulation model (reactor, settler, sensors and actuators models) and definitions of (controller) performance criteria. The goal was achieved by outlining the procedure for developing such a benchmark for a specific full-scale WWTP, using available process data. For other WWTP's, the same procedure can be followed.</p><p>In developing the basic simulation model, first a loop-of- <em>CSTR's</em> model, without back-flows, was chosen for modeling oxidation ditches because it is simple and can be used for control purposes. Based on this model, a new method for estimating the standard oxygen transfer rate ( <em>SOTR</em> ), under clean water conditions, was developed and tested successfully. The new method estimates the <em>SOTR</em> on the bases of the aeration constant, which is the product of <em>K <sub>L</sub> a</em> and volume of aerated compartment, because neither <em>K <sub>L</sub> a</em> nor the volume of aerated compartment can be individually identified. Under process conditions, <em>C</em> -oxidation and nitrification processes were assumed to take place in the aerated zones, whereas the denitrification process was assumed to occur in the anoxic zones. For modeling the biochemical processes, <em>ASM No. 1</em> was used, whereas for modeling the secondary settler the non-reactive double-exponential settling velocity model was used. Based on influent-effluent concentrations, it was found that hydraulics of oxidation ditches can be approximated by 10 to 15 <em>CSTR's</em> . The oxidation ditch model was then calibrated successfully using a novel calibration strategy, which is based on response surface analysis. Prior to a formal parameter estimation step, the response surface analysis provides insight in the parameter sensitivity and initial estimates. Because the study was limited to <em>C</em> and <em>N</em> removal processes, only models of <em>DO</em> and <em>N</em> sensors were developed. The actuators, pumps and valves, were assumed to work perfectly, that is: dynamics and time delays of these actuators were neglected.</p><p>Evaluation criteria were then developed by modifying the criteria proposed by both COST 624 Working Group and IWA Task Group on Respirometry. Modifications were mainly made in the aeration and pumping energy equations, because oxidation ditches use mechanical aerators that are different from air diffusers adopted by COST 624 and IWA Working Groups. In addition, long-term evaluation criteria were also developed.</p><p>Further, sensitivity analysis was carried out to determine parameters of <em>ASM No. 1</em> that require special attention from the benchmark user. Sensitivity analysis was carried out using the factorial sensitivity analysis methodology. The main advantage of this methodology is that more information about the interactions (non-linearities) can be obtained. Also, the effect of the various sources of uncertainty on the performance indices was investigated. Estimation of the uncertainty contribution of the various sources is very important because it enables the benchmark user to make an appropriate selection among different control strategies. It is equally important for designing experimental or monitoring programs with the aim of reducing the uncertainty.</p><p>Finally, the benchmarking procedure was described and demonstrated by using it to evaluate some basic and advanced control strategies. Basic control strategies studied were (i) splitting the influent flow between the aerated compartments, (ii) rate of activated sludge recirculated and (iii) aeration patterns. The benchmark was also used in studying the effect of the horizontal (recirculation) velocity on nitrogen removal process. Here, the horizontal velocity was considered as a manipulated control variable, to obtain maximum <em>TN</em> removal efficiency.</p>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • van Straten, Gerrit, Promotor
  • Keesman, Karel, Promotor
  • Spanjers, H., Promotor, External person
Award date17 Sep 2001
Place of PublicationS.l.
Print ISBNs9789058084224
Publication statusPublished - 2001

Keywords

  • waste water treatment
  • oxidation ditches
  • automatic control

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