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