EXPERIMENTAL STUDY OF RO MEMBRANE ORGANIC FOULING FOR WASTEWATER RECLAMATION ZHAO YAN NATIONAL UNIVERSITY OF SINGAPORE 2009 EXPERIMENTAL STUDY OF RO MEMBRANE ORGANIC FOULING FOR WASTEWATER RECLAMATION ZHAO YAN (M.E., Harbin Institute of Technology, Harbin, P.R. China) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING FACULTY OF ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2009 ACKNOWLEGEMENT ACKNOWLEDGEMENT I would like to take this opportunity to acknowledge and thank all those who have helped me along the way. First and foremost, I would like to express sincere gratitude to my supervisor, Professor Say Leong ONG and co-supervisor Associate Professor Lianfa SONG, for their patient guidance and critical comments throughout the course of this study. Without their encouragement and support, the work would not have been completed. I would also like to thank my thesis committee members Associate Professor Jiangyong HU and Assistant Professor How Yong NG for their valuable advice and time to serve in my committee. I owe my special thanks to the staff in Center of Water Research, Mr. Tan Eng Hin, Michael, and Mr. S.G. Chandrasegaran, Ms. Lee Leng Leng and Ms. Tan Xiaolan for their kind assistance and help in handling miscellaneous laboratory matters. Appreciation also goes to Mr Chua Seng Chye and Mr Lee Siew Chor for their kind coordination during sample collection at Bedok NEWater Plant. Thank all the former and current members of our research group for their invaluable discussions, help, and friendship. And also thank Quek Li Wen and Tan Chye Leong for their assistance in part of this work during their Final Year Project. I wish to express my special appreciation to National University of Singapore for providing me the Ph.D scholarship and many opportunities towards my academic and professional pursuit. i ACKNOWLEGEMENT Last but not the least, I extend my heartfelt gratitude to my family and my fiancé, for their everlasting love and support throughout these years. Without them, I would not have been here today. ii TABLE OF CONTENTS TABLE OF CONTENTS ACKNOWLEDGEMENT . i SUMMARY vi TABLE OF CONTENTS iii LIST OF TABLES . iii LIST OF FIGURES x LIST OF ABBREVIATIONS vii CHAPTER ONE INTRODUCTION 1.1 Need for advanced municipal wastewater reclamation technologies 1.2 Limitations of RO membrane process for wastewater reclamation . 1.3 Research needs . 10 1.4 Research objectives 17 1.5 Organization of thesis 18 CHAPTER TWO LITERATURE REVIEW . 20 2.1 Application of RO membrane process for wastewater reclamation 20 2.2 Advantages of RO Process for wastewater reclamation . 26 2.3 Fouling of RO process for wastewater reclamation 28 2.3.1 Definition of fouling 29 2.3.2 Major types of foulants and associated fouling mechanisms . 30 2.3.2.1 Colloidal fouling . 30 2.3.2.2 Scaling . 34 2.3.2.3 Biofouling . 35 2.3.2.4 Organic fouling . 39 2.3.3 Fouling potential assessment and foulant characterization 39 2.4 Organic fouling of RO process used in wastewater reclamation . 48 2.4.1 Effect of composition and characteristics of EfOM . 49 2.4.1.1 Composition and characteristics of NOM . 49 2.4.1.2 Composition and characteristics of SMP or EPS 55 2.4.2 Effect of membrane structure and surface properties 63 2.4.3 Effect of hydrodynamic parameters . 66 2.4.4 Effect of solution chemistry . 68 2.5 Fouling control strategies and associated costs . 71 iii TABLE OF CONTENTS CHAPTER THREE MATERIALS AND METHODS 77 3.1 Materials . 77 3.1.1 Feed water 77 3.1.2 RO membrane 80 3.2 Isolation and fractionation of EfOM 80 3.3 EPS extraction 83 3.4 EfOM characterization . 84 3.4.1 Apparent MWD . 84 3.4.2 Non-purgable organic carbon and specific ultraviolet absorbance measurement . 84 3.4.3 Polysaccharide and protein concentration measurement . 85 3.5 Laboratory-scale cross-flow membrane filtration unit 87 3.6 Experimental protocols of RO fouling experiments . 88 3.6.1 Phase I: Fouling behavior observation and foulant characterization . 88 3.6.2 Phase II: Fouling potential investigation of individual EfOM fractions . 90 3.6.3 Assessment of clean water flux reversibility . 91 3.6.4 Feed water fouling potential assessment 90 3.7 Membrane foulant characterization . 93 3.7.1 Membrane morphology and elemental composition analysis 93 3.7.2 Foulant organic ratio quantification . 93 3.7.3 Foulant organic functional group analysis . 94 3.7.4 Foulant inorganic composition analysis . 95 3.8 Membrane surface chemistry characterization 95 3.8.1 Membrane surface charge measurement 95 3.8.2 Membrane contact angle measurement 96 CHAPTER FOUR RESULTS AND DISCUSSIONS 97 4.1 Part – Fouling behavior and foulant characteristics of RO membranes for treated secondary effluent reclamation 97 4.1.1 Feed water characteristics 98 4.1.2 Fouling behavior with increasing permeate recovery 100 4.1.3 Fouling layer characteristics 106 4.1.3.1 Organic composition of fouling layer . 106 4.1.3.2 Inorganic Composition of Fouling Layer 110 iv TABLE OF CONTENTS 4.1.3.3 Changes in membrane surface morphology 113 4.1.3.4 Changes in membrane surface chemistry 115 4.1.4 Fouling Potential with Increasing Recovery 116 4.1.5 Summary – Part . 118 4.2 Part – Fouling of RO membranes by EfOM: relating major components of EfOM to their characteristic fouling behaviors . 119 4.2.1 Distribution, composition and characteristics of EfOM fractions . 120 4.2.2 Membrane fouling by various EfOM fractions 125 4.2.2.1 Flux decline rate and extent 125 4.2.1.2 The influence of calcium ions . 128 4.2.3 Mass of fouling layer deposited . 130 4.2.4 Affinity of fouling layer with the membrane . 132 4.2.5 Summary – Part . 134 CHAPTER FIVE CONCLUSIONS AND RECOMMENDATIONS . 136 5.1 Conclusions 136 5.2 Recommendations for future studies 138 REFERENCES 142 Appendix A Publications and conference papers from this project 157 v SUMMARY SUMMARY Despite considerable research effort undertaken by the scientific community, membrane fouling still remains as a major technical hurdle that needs to be addressed to enhance the cost-effectiveness of reverse osmosis (RO) systems for wastewater reclamation. The residual organic matter in treated secondary effluent, known as effluent organic matter (EfOM), has been implicated as an important RO foulant. However, our current understanding of this category of foulant is still very limited towards its efficient control. This study first investigated the fouling behaviors of wastewater reclamation RO membranes fed with ultrafiltration (UF) prefiltered secondary effluent from a tropical area wastewater reclamation plant at two operationally important permeate recovery levels. The associated fouling mechanisms were delineated with the aid of microscopic analysis of the fouling layer characteristics. There was a remarkable correlation between the different fouling behaviors observed and the characteristics of fouling layers developed. Organic fouling by carbohydrates and protein-like matters was found to be primarily responsible for the flux loss observed at the first-stage RO operating with a recovery of 55%. At the second-stage RO with a higher recovery of 70%, the greatly enhanced deposition of organic foulants together with inorganic precipitation led to the formation of a thicker and irreversible composite fouling layer. Calcium phosphate and calcium carbonate were found to be the major precipitates at this recovery. The enhanced organic vi SUMMARY deposition along with increasing permeate recovery was also corroborated by the significantly increasing organic fouling potential measured after inhibition of scaling. This study clearly demonstrated the need for more effective organic fouling control at the second-stage RO. As EfOM is a mixture of structurally complex poorly-defined organic compounds, this study further investigated the interactions between fractional components of EfOM and RO membranes and attempted to identify the most influential fraction(s) or physico-chemical properties governing the fouling process. Organic fouling behavior of EfOM was observed to closely correlate to the composition and physico-chemical properties of its components. Both hydrophilic in nature and strongly negatively-charged, the hydrophilic acid fraction resulted in minimal flux decline. Regardless of the presence of calcium ions, the hydrophilic neutral fraction, mainly composed of small size carbohydrates, resulted in the highest flux decline and exhibited highest affinity towards the membrane. EPS biopolymers, to which great importance has been associated with regard to causing RO organic fouling, resulted in less fouling than hydrophilic carbohydrates. Although EPS biopolymers tended to accumulate on the membrane in much higher quantities, the cake layer formed was found to constitute a much lower resistance towards filtration and have a much lower membrane affinity, probably due to their large molecular sizes. Therefore, it should be expected to impose a weaker fouling threat for longterm RO operation. 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(2007). “Effect of solution chemistry on the fouling potential of dissolved organic matter in membrane bioreactor systems”. J. Membr. Sci., 310 (1-2), pp. 503-511. 2. Zhao Y., Song L. and Ong S.L. (Submitted). “Fouling Behavior and Foulant Characteristics of Reverse Osmosis Membranes for Treated Secondary Effluent Reclamation”. J. Membr. Sci. 3. Zhao Y., Song L. and Ong S.L. (Submitted). “Fouling of RO membranes by effluent organic matter (EfOM): relating major components of EfOM to their characteristic fouling behaviors”. J. Membr. Sci. A.2 Conference papers 1. Zhao Y. and Song L. (2007). “Organic fouling potential characterization of secondary effluent as feed water to RO membranes”. The 18th North American Membrane Society Annual Meeting, 12-16 May 2007, Orlando, Florida, USA. 2. Zhao Y. and Song L. (2006). “Characterization of foulants on RO membrane treating secondary effluent after UF pretreatment”. The 17th North American Membrane Society Annual Meeting, 12-17 May 2006, Chicago, Illinois, USA. 157 [...]... TWO LITERATURE REVIEW 2.1 Application of RO membrane process for wastewater reclamation The application of membrane processes for wastewater reclamation has been initially developed with RO process In view of the superior rejection capabilities of RO membranes and high loadings of impurities present in treated effluent fed to RO, effective control of membrane fouling has long been considered critical... operation of membrane systems used for wastewater reclamation varies for specific reuse applications and will be determined by the level of treatment required (Water Environment Federation, 2006) 1.2 Limitations of RO membrane process for wastewater reclamation The two important performance indicators for membrane processes are: the quality of product water, which is related to the rejection efficiency of. .. the fouling of RO membranes caused by EfOM, the biggest challenge lies in a lack of understanding in terms of composition and characteristics of EfOM and its associated fouling mechanisms EfOM represents a large group of structurally complex, heterogeneous and poorly defined organic compounds derived from both raw wastewater and microbial metabolic activities Although a myriad of compounds such as proteins,... underlying fouling mechanisms on RO membranes treating highfouling treated effluent is needed The information obtained could facilitate the formulation of effective fouling prevention or mitigation strategies through practical and economic approaches 1.4 Research objectives The overall objective of this study is to develop an improved understanding of EfOM fouling behavior and underlying mechanisms on RO membranes... composition of new and fouled membrane surface by SEM-EDX…………………………………………………………….…….116 Table 4.5 Inorganic composition of physically stripped fouling layer…………………………………………………………….………… 110 Table 4.6 Surface chemistry of fouled membranes as compared to the clean membrane ………………………………….…………………… 113 ix LIST OF FIGURES LIST OF FIGURES Figure 2.1 Overview of schemes of using RO membrane systems for wastewater reclamation. .. EPS, particularly the hydrophilic colloids and macromolecules, have recently been observed as major foulants of NF /RO membranes used for wastewater reclamation (Jarusutthirak et al., 2002; 2006) However, systematic fouling characterization studies of RO membranes receiving MF/UF pretreated 11 CHAPTER ONE INTRODUCTION secondary effluent for reclamation have yet to be conducted Most fouling characterization... sites for sparingly soluble salts leading to irreversible membrane fouling (Schneider et al., 2005) As readily available microbial nutrients, they could also be assimilated by “opportunistic” microorganisms and thereby encourage membrane biofouling the “Achilles heels” of RO membrane processes (Ridgway et al., 1996; Flemming et al., 1997) 7 CHAPTER ONE INTRODUCTION To hinder the occurrence of fouling, ... provide a comprehensive review on previous theoretical and experimental studies on fouling of RO membrane processes used in wastewater reclamation applications with critical technical issues and research gaps being identified This includes an overview of the key types of foulants and their fouling characteristics, with a specific emphasis on EfOM and the resulting organic fouling pertinent to this study. .. emerging micropollutants (Kimura et al., 2004) However, the efficiency of RO membranes in rejecting these organic contaminants has not been fully proven Fouling occurrence during wastewater reclamation might influence the rejection efficiency of membranes towards these organic contaminants and compromise 9 CHAPTER ONE INTRODUCTION the reclaimed water quality It has been reported colloidal fouling could... of operating conditions for membrane systems is also kept far from optimum A higher initial flux will result in a faster decline in specific flux and a much higher cleaning frequency, and the net effect is often far less cumulative water production (Chellam et al., 1998) Fouling phenomenon also set the upper limit of permeate recovery attained by a RO membrane process For MF and UF membranes used for . RO membrane process for wastewater reclamation 20 2.2 Advantages of RO Process for wastewater reclamation 26 2.3 Fouling of RO process for wastewater reclamation 28 2.3.1 Definition of fouling. EXPERIMENTAL STUDY OF RO MEMBRANE ORGANIC FOULING FOR WASTEWATER RECLAMATION ZHAO YAN NATIONAL UNIVERSITY OF SINGAPORE 2009 EXPERIMENTAL STUDY OF RO. control. This study first investigated the fouling behaviors of wastewater reclamation RO membranes fed with ultrafiltration (UF) prefiltered secondary effluent from a tropical area wastewater