upflow anaerobic sludge blanket reactor: modelling

60 Pages·2011·5.1 MB·English

KUNGLIGA TEKNISKA HÖGSKOLAN
(KTH)

KTH Chemical Science
and Engineering

UPFLOW ANAEROBIC SLUDGE BLANKET REACTOR:
MODELLING

RAÚL RODRÍGUEZ GÓMEZ

LICENTIATE THESIS IN CHEMICAL ENGINEERING

SCHOOL OF CHEMICAL SCIENCE AND ENGINEERING
DEPARTMENT OF CHEMICAL ENGINEERING AND TECHNOLOGY
ROYAL INSTITUTE OF TECHNOLOGY
STOCKHOLM, SWEDEN 2011
UPFLOW ANAEROBIC SLUDGE BLANKET REACTOR: MODELLING

Raúl Rodríguez Gómez
Licentiate Thesis in Chemical Engineering

Department of Chemical Engineering and Technology
School of Chemical Science and Engineering
Royal Institute of Technology
Stockholm, Sweden

TRITA-CHE Report 2011:4
ISSN 1654-1081
ISBN 978-91-7415-849-6

Copyright © 2011 by Raúl Rodríguez Gómez

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ABSTRACT

Anaerobic treatment is widely used around the world as a biological stage in both
domestic and industrial wastewater-treatment plants. The two principal
advantages of anaerobic over aerobic treatment are the production of biogas,
which can be used as fuel, and the lower rate of biomass production, which results
in lower maintenance costs for the plant. The upflow anaerobic sludge blanket
(UASB) reactor is an attractive alternative for regions in hot climates since it
works better under mesophilic conditions and it does not need any supporting
structure for the development of microorganisms, which grow in the form of
granules.
In this thesis, a model describing the UASB reactor behaviour with respect to
substrate degradation, microorganism growth and granule formation was
developed. The model is transient and is based on mass balances for the substrate
and microorganisms in the reactor. For the substrate, the processes included in the
model are dispersion, advection and degradation of the organic matter in the
substrate. The reaction rate for the microorganisms includes the growth and decay
of the microorganisms. The decay takes into account the microorganism dying
and the fraction of biomass that may be dragged into the effluent. The
microorganism development is described by a Monod type equation including the
death constant; the use of the Contois equation for describing the microorganism
growth was also addressed.
An equation considering the substrate degradation in the granule was required,
since in the UASB reactor the microorganisms form granules. For this, a
stationary mass balance within the granule was carried out and an expression for
the reaction kinetics was then developed. The model for the granule takes into
account the mass transport through the stagnant film around the granule, the intra-
particle diffusion, and the specific degradation rate. The model was solved using
commercial software (COMSOL Multiphysics). The model was validated using
results reported in the literature from experiments carried out at pilot scale.
A simplified model was also developed considering the case in which the
microorganisms are dispersed in the reactor and granules are not formed. The
UASB reactor is then described as formed by many well-stirred reactors in series.
The model was tested using experimental results from the literature and the
sensitivity of the processes to model parameters was also addressed.
The models describe satisfactorily the degradation of substrate along the height in
the reactor; the major part of the substrate is degraded at the bottom of the reactor
due to the high density of biomass present in that region. This type of model is a
useful tool to optimize the operation of the reactor and to predict its performance.

Keywords: Wastewater, anaerobic treatment, modelling, UASB reactor.

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LIST OF PAPERS

This thesis is based on the followings papers, referred to by Roman numerals I to III:

I- Rodriguez R., L. Moreno (2009), Modeling of Substrate Degradation and
Microorganism Growth in an UASB Reactor, In Proceedings of the
International Conference on Chemical, Biological & Environmental
Engineering (CBEE 2009), Singapore, October 9-11, 2009, Singapore; Kai
Ed., pp 76-80.

II- Rodriguez R., L. Moreno (2010), Modelling of a Upflow Anaerobic
Sludge Blanket reactor, In Proceedings of the Tenth International
Conference on Modelling, Monitoring and Management of Water
Pollution, Bucharest, June 9-11 2010, Brebbia C. (Ed), WIT Press, pp 301-
310.

III- Rodriguez R., L. Moreno (2011), Simulation of a UASB reactor.
Manuscript submitted for publication. January 2011.

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CONTENTS

LIST OF PAPERS ........................................................................................... IV(cid:1)
ABSTRACT ..................................................................................................... III(cid:1)
CONTENTS .................................................................................................... IV(cid:1)
PREFACE ...................................................................................................... VII(cid:1)
1- INTRODUCTION ......................................................................................... 1(cid:1)
1.1(cid:1) BACKGROUND ........................................................................................................................ 1(cid:1)
1.2(cid:1) THE AIM OF THE THESIS ....................................................................................................... 2(cid:1)
1.3(cid:1) THE THESIS OUTLINE ............................................................................................................ 2(cid:1)
2- ANAEROBIC PROCESSES ........................................................................ 4(cid:1)
2.1 ANAEROBIC DIGESTION .......................................................................................................... 4(cid:1)
2.2 ANAEROBIC REACTORS ........................................................................................................... 5(cid:1)
3- THE UPFLOW ANAEROBIC SLUDGE BLANKET (UASB) REACTOR ...... 9(cid:1)
3.1 GRANULE DEVELOPMENT (THEORIES) ............................................................................... 9(cid:1)
3.1.1 PHYSICAL THEORIES ........................................................................................................ 10(cid:1)
3.1.2 MICROBIAL THEORIES ..................................................................................................... 11(cid:1)
3.1.3 THERMODYNAMIC THEORIES ........................................................................................ 12(cid:1)
3.2 MECHANISM OF GRANULE AGITATION INSIDE A UASB REACTOR ............................ 14(cid:1)
3.3 FACTORS INFLUENCING REACTOR PERFORMANCE ...................................................... 16(cid:1)
3.3.1 pH ......................................................................................................................................... 16(cid:1)
3.3.2 TEMPERATURE .................................................................................................................. 16(cid:1)
3.3.3 ORGANIC LOADING RATE (OLR) .................................................................................... 16(cid:1)
3.3.4 HYDRAULIC RETENTION TIME AND UP-FLOW VELOCITY ........................................ 16(cid:1)
3.3.5 SUBSTRATE ......................................................................................................................... 18(cid:1)
3.3.6 SLUDGE INOCULATION ................................................................................................... 19(cid:1)
4- KINETICS MODELS .................................................................................. 20(cid:1)
4.1 THE MICHAELIS-MENTEN MODEL ...................................................................................... 21(cid:1)
4.2 THE MONOD MODEL ............................................................................................................... 22(cid:1)
4.3 THE CONTOIS MODEL ............................................................................................................. 23(cid:1)
5- MODELS DESCRIBING UASB BEHAVIOUR ........................................... 25(cid:1)
5.1 EXISTING MODELS OF A UASB REACTOR ......................................................................... 25(cid:1)
6- PRELIMINARY MODEL ............................................................................. 32(cid:1)
6.1 STUDY CASE FOR THE PRELIMINARY MODEL ................................................................. 33(cid:1)
6.2 RESULTS AND DISCUSSION (PRELIMINARY MODEL) ..................................................... 34(cid:1)
7- THE MAIN MODEL .................................................................................... 36(cid:1)
7.1 MODEL DEVELOPMENT ......................................................................................................... 36(cid:1)
7.2 KINETIC RATE CONSTANT BASED ON THE MONOD MODEL ........................................ 36(cid:1)
7.3 THE GOVERNING EQUATIONS ALONG THE HEIGHT OF THE REACTOR ..................... 36(cid:1)
7.4 KINETIC RATE CONSTANT BASED ON A MASS BALANCE ............................................. 38(cid:1)
7.5 KINETIC RATE CONSTANT IN THE DEVELOPED MODEL ............................................... 40(cid:1)
7.6 MODEL APPLICATION ............................................................................................................ 40(cid:1)
8- CONCLUSIONS ........................................................................................ 45(cid:1)
ACKNOWLEDGEMENT ................................................................................................................. 45(cid:1)
9-REFERENCES ........................................................................................... 46(cid:1)
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List of abbreviations

AM2 Anaerobic Model Number 2
ADM1 Anaerobic Digestion Model Number 1
CSTR Continuous Stirred-Tank Reactor
COD Chemical Oxygen Demand
EGSB Expanded Granular Sludge Bed
ECP Extracellular Polymers
GSS Gas Solid Separator
HRT Hydraulic Retention Time
ODE Ordinary Differential Equation
OLR Organic Loading Rate
PVA Polyvinyl Alcohol
RTD Residence Time Distribution
TBC Turbulent Bed Contactor
TSS Total Suspended Solids
UASB Upflow Anaerobic Sludge Blanket Reactor
VSS Volatile Suspended Solids
VFA Volatile Fatty Acids

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PREFACE

This research was financially supported by the Swedish International Development
cooperation Agency (SIDA/SAREC). It was carried out as a cooperation program
between the Department of Chemical Engineering and Technology in the Royal
Institute of Technology (KTH), Stockholm, Sweden, and the National University of
Engineering (UNI), Managua, Nicaragua.

To be able to do this work, I received help directly and indirectly from many people
I would like to thank:

•• My supervisor, Luis Moreno, for his encouragement and guidance.
•• My parents for their unconditional backing and love.
•• My Nicaraguan friends and partners for their faith and advice.
•• Special thanks to Larisa Korsak and Isabelle Hughes for their support.

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1- INTRODUCTION

1.1 BACKGROUND

Discharges of inadequately treated wastewater may have a great impact on natural
water sources. The treatment of wastewater has been an issue of high priority in most
industrial countries and they have therefore reached a very satisfactory quality of their
wastewater discharges. However, due to economic factors, attitudes and education,
the developing countries have problems in the treatment of residual water. In some
cases, industrial wastewaters are discharged directly into public collection systems,
and this affects the performance of the treatment plants. In many countries,
wastewater is discharged directly into rivers or lagoons with damage to the aquatic
life and to the quality of the water sources. Wastewaters with high amounts of
nutrients and organic material may cause eutrophication. This problem could be
solved by improving the wastewater-treatment system.

There are two main types of biological processes for the treatment of wastewaters
with a high content of organic material: aerobic processes and anaerobic processes.
An advantage of the latter type is the small generation of cell mass; around three per
cent of the organic matter present in the wastewater feed is converted to cell mass,
whereas a high amount of organic matter is converted to cell mass in aerobic
processes (Haandel and Lettinga 1994). Anaerobic processes also require less energy
and, in addition, they produce methane that can be used as a source of energy. Some
of the disadvantages of the anaerobic processes are that they require a long time to
start-up, they may need additional alkali to control the pH, and they are more
susceptible to toxic substances (Tchobanoglous et al. 2003).

There are several anaerobic processes for the treatment of wastewater; one of these is
the Upflow Anaerobic Sludge Blanket (UASB) reactor. This type of reactor may, in
many cases, be the optimal solution for wastewater treatment in developing countries
due to its simplicity and low cost. Moreover, this kind of treatment may be a very
attractive alternative for wastewaters (industrial or/and municipal) containing high
amounts of organic material in tropical countries, since the UASB reactor works
better at high temperatures.

In the design and construction of a wastewater-treatment plant or in the optimization
of an existing plant, it is important to study the impact of the operating parameters on
the treatment performance, as well as the consequences of the construction of the
treatment plant for the environment. A helpful tool in the decision-making process is
a simulation of the processes occurring in the treatment plant, but there are, at present,
few models describing the processes in UASB reactors. The existing models include
dispersion, advection and reaction processes to describe the substrate degradation
(Kalyuzhnyi et al. 2006, Wu et al. 2007). The model proposed by Wu and co-workers
(Wu et al. 2005) takes into account only the dispersion and transport terms since their
model studies the distribution of a conservative tracer in the reactor so that there is no
reaction term. Sponza and Uluköy (2008) compared three kinetic models describing
the substrate removal kinetics of UASB reactors. Narnoli and Merhotra (1997)
modelled the sludge distribution on the UASB reactor blanket. The Monod model is
the most common model describing reaction kinetics (Rodriguez and Moreno 2009,
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Sponza and Uluköy 2008, Soto et al. 1993). However, another kinetic model used by
several researchers is the Contois model (Rodriguez and Moreno 2010, Sponza and
I(cid:3)ik 2002, Hu et al., 2002). Korsak et al. (2008) developed a model describing
processes occurring in UASB reactors using the Monod kinetics to describe the
reaction within the granules. The existing models show, in some cases, important
weaknesses or some processes are missing.

The UASB reactor is an adequate alternative for developing countries situated in
tropical zones, since the process is simple, the equipment and operation are not too
expensive, and the process works better at higher temperatures. This type of reactor
could therefore, with advantage, be used in Nicaragua, a developing country located
in Central America. Industries in Nicaragua do not treat their wastewater in a
satisfactory way, in spite of knowledge of the consequences of allowing inadequately
treated water to flow into the water recipients or municipal treatment plants.
Economic aspects and a lack of consciousness are the main factors influencing the
construction and improvement of the wastewater-treatment plants.

Some industries have their own treatment plants, but in the majority of the cases they
are inefficient. There are several anaerobic treatments like UASB reactors handling
municipal, distillery and brewery wastewaters, but they have not achieved the granule
formation of the UASB reactors. However these reactors produce biogas, which in
some cases is utilised as fuel. For these reasons, we decided to study the UASB
reactor, seeking to understand all the processes occurring in a UASB reactor, in order
to be able to develop models to predict the changes in the performance of the UASB
reactor when operating conditions are varied. In the future, the models developed in
this study can hopefully be applied to Nicaragua’’s UASB reactors.

1.2 THE AIM OF THE THESIS

The aim of this work has been to develop a model that takes into account the most
important processes occurring in a UASB reactor. In addition to the advection,
dispersion, and reaction terms, the model should include the sludge distribution as a
function of height in the reactor and describe the change of particle size in the reactor.
The model includes the reactions occurring within the granule in an analytical way,
and this significantly reduces the computation time. The model has been validated
using data from the literature.

1.3 THE THESIS OUTLINE

This thesis is divided into eight parts. The first part is the introduction, in which a
synopsis of the thesis is presented. The second chapter is an overview of anaerobic
processes and it describes the conceptual aspects of the UASB reactor. In the third
chapter several theories of granule formation are explained, and the mechanisms of
granule agitation and factors controlling the reactor behaviour are discussed. The
fourth chapter considers the different kinetics models commonly used to describe the
reactions in a UASB reactor. Chapter five shows and explains the existing models
describing UASB reactors, their weaknesses and strengths. In chapter six, a
preliminary model describing UASB reactor behaviour is developed and tested with
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