Ammonium Based Aeration Control in Wastewater Treatment Plants

121 Pages·2017·2.9 MB·English

ITLicentiatetheses
2018-002
Ammonium Based Aeration Control in
Wastewater Treatment Plants - Modelling and
Controller Design
TATIANA CHISTIAKOVA
UPPSALAUNIVERSITY
DepartmentofInformationTechnology
AmmoniumBasedAerationControlinWastewaterTreatmentPlants-
ModellingandControllerDesign
TatianaChistiakova
[email protected]
April2018
DivisionofSystemsandControl
DepartmentofInformationTechnology
UppsalaUniversity
Box337
SE-75105Uppsala
Sweden
http://www.it.uu.se/
DissertationforthedegreeofLicentiateofPhilosophyinElectricalEngineeringwith
SpecializationinAutomaticControl
c TatianaChistiakova2018
�
ISSN1404-5117
PrintedbytheDepartmentofInformationTechnology,UppsalaUniversity,Sweden

Abstract
Wastewatertreatmentinvolvesmanyprocessesandmethodswhich
make a treatment plant a large-scaled and complex system. A
fundamental challenge is how to maintain a high process ef-
ficiency while keeping the operational costs low. The variety
in plant configurations, the nonlinear behaviour, the large time
delays and saturations present in the system contribute to mak-
ing automation and monitoring a demanding task.
The biological part of a wastewater treatment process includes
anaerationofthewaterandthisprocesshasbeenshowntooften
result in the highest energy consumption of the plant. Oxygen
supplyisafundamentalpartoftheactivatedsludgeprocessused
for aerobic microorganisms growing. The concentration of the
dissolved oxygen should be high enough to maintain a su�cient
level of biological oxidation. However, if the concentration is too
high the process e�ciency is significantly reduced leading to a
too high energy consumption. Hence, there are two motivations
behindtheaerationcontroltask: processe�ciencyandeconomy.
One of the possible strategies to adjust the dissolved oxygen
level in a nitrifying activated sludge process is to use ammonium
feedback measurements.
In this thesis, an activated sludge process is modelled and ana-
lysedintermsofdissolvedoxygentoammoniumdynamics. First,
thedataobtainedfromasimplifiedBenchmarkSimulationModel
no.1 was used to identify the system. Both linear and nonlinear
models were evaluated. A model with a Hammerstein structure
where the nonlinearity was described by a Monod function was
chosen for a more thorough study. Here, a feedback controller
was designed to achieve -stability. The stability region was
2
L
pre-computed to determine the maximum allowed time delay for
the closed loop system. Finally, a feedforward controller was
added to the system, and shown to significantly improve the dis-
turbance rejection properties.
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Acknowledgments
First of all, I would like to thank my supervisors Prof. Bengt
Carlsson and Adj. Prof. Torbj¨orn Wigren for their guidance
and support. Your ideas and enthusiasm helped me to start this
journey. Yourknowledgeande�ciencyarereallyimpressiveand
encouraging!
I am grateful to my external supervisor, Doc. Jesu´s Zambrano,
for his help with the knotty BSM1. Your pure interest in any
research topic and willingness to share ideas are very much ap-
preciated.
During the first year, I had a joyful time in a shared o�ce, I
would like to give thanks to my former o�ce-mates for it and
for introducing me to fika concept. A big thanks also goes to all
former and present SysCon-ers for a pleasant ambience during
and after work. My dear friends outside Sweden (i.e. academia),
thank you for constantly linking me up with a real world!
Lastbutnotleast, Iwouldliketoexpressmysincereandbiggest
gratitude to my family. This whole adventure is possible only
due to a support and faith of my parents who always get behind
my aspirations, thank you! Jonas, thank you for your endless
patience and the greatest encouragements throughout the time.
Uppsala, April 2018
Tania
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Glossary and Notation
Abbreviations
WWTP wastewater treatment plant
ASP activated sludge process
DO dissolved oxygen
MPC model predictive control
PI proportional integral
BSM1 Benchmark Simulation Model no.1
ASM1 Activated Sludge Model no.1
SISO single-input-single-output
MISO multiple-input-single-output
MIMO multiple-input-multiple-output
LOE linear output error
PEM prediction error method
RPEM recursive prediction error method
PRBS pseudorandom binary sequence
RAS return activated sludge
WAS waste activated sludge
Notation
NH ammonium
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Contents
1 Introduction 3
1.1 Research Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4 Thesis Outline . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Wastewater Treatment Plants 7
2.1 General Description . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 The Activated Sludge Process . . . . . . . . . . . . . . . . . . 8
2.3 The Benchmark Simulation Model no.1 . . . . . . . . . . . . 9
2.3.1 Reduced Simulation Models . . . . . . . . . . . . . . . 10
3 System Identification, Control and Stability Analysis 11
3.1 System Identification . . . . . . . . . . . . . . . . . . . . . . . 11
3.2 Model Structures and Identification
Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2.1 Linear Models . . . . . . . . . . . . . . . . . . . . . . 13
3.2.2 Output Error Identification . . . . . . . . . . . . . . . 13
3.2.3 Nonlinear Models . . . . . . . . . . . . . . . . . . . . . 14
3.2.4 The Hammerstein Model Structure . . . . . . . . . . . 15
3.2.5 Gauss-Newton Algorithm for Parameter Estimation . 16
3.2.6 Hammerstein Model Based RPEM . . . . . . . . . . . 17
3.2.7 Static Nonlinearities . . . . . . . . . . . . . . . . . . . 18
3.3 Ammonium-based Aeration Control
in WWTPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3.1 Feedback Controller Design . . . . . . . . . . . . . . . 20
3.3.2 Feedforward Controller Design . . . . . . . . . . . . . 20
3.4 Stability Analysis of the Simplified ASP Model . . . . . . . . 21
3.4.1 Popov Criterion . . . . . . . . . . . . . . . . . . . . . 22
3.4.2 Precomputation of the Stability Region . . . . . . . . 22
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