Cover Page

Table of Contents

Title Page


About the Authors



Chapter 1: Introduction

1.1 The Importance of Principles

1.2 The Importance of Sustainability


Chapter 2: Water Quality and Public Health

2.1 Relationship between Water Quality and Public Health

2.2 Source Waters for Municipal Drinking Water Systems

2.3 Regulations of Water Treatment in the United States

2.4 Evolving Trends and Challenges in Drinking Water Treatment

2.5 Summary and Study Guide


Chapter 3: Process Selection

3.1 Process Selection Based on Contaminant Properties

3.2 Other Considerations in Process Selection

3.3 Sustainability and Energy Considerations

3.4 Design and Selection of Process Trains

3.5 Summary and Study Guide

Homework Problems


Chapter 4: Fundamental Principles of Environmental Engineering

4.1 Units of Expression for Chemical Concentrations

4.2 Chemical Equilibrium

4.3 Chemical Kinetics

4.4 Reactions Used in Water Treatment

4.5 Mass Balance Analysis

4.6 Introduction to Reactors and Reactor Analysis

4.7 Reactions in Batch Reactors

4.8 Hydraulic Characteristics of Ideal Flow Reactors

4.9 Reactions in Ideal Flow Reactors

4.10 Measuring the Hydraulic Characteristics of Flow Reactors with Tracer Tests

4.11 Describing the Hydraulic Performance of Real Flow Reactors

4.12 Reactions in Real Flow Reactors

4.13 Introduction to Mass Transfer

4.14 Molecular Diffusion

4.15 Diffusion Coefficients

4.16 Models and Correlations for Mass Transfer at an Interface

4.17 Evaluating the Concentration Gradient with Operating Diagrams

4.18 Summary and Study Guide

Homework Problems


Chapter 5: Coagulation and Flocculation

5.1 Role of Coagulation and Flocculation in Water Treatment

5.2 Stability of Particles in Water

5.3 Principles of Coagulation

5.4 Coagulation Practice

5.5 Principles of Mixing for Coagulation and Flocculation

5.6 Rapid-Mix Practice

5.7 Principles of Flocculation

5.8 Flocculation Practice

5.9 Energy and Sustainability Considerations

5.10 Summary and Study Guide

Homework Problems


Chapter 6: Sedimentation

6.1 Principles of Discrete (Type I) Particle Settling

6.2 Discrete Settling in Ideal Rectangulor Sedimentation Basins

6.3 Principles of Flocculant (Type II) Particle Settling

6.4 Principles of Hindered (Type III) Settling

6.5 Conventional Sedimentation Basin Design

6.6 Alternative Sedimentation Processes

6.7 Physical Factors Affecting Sedimentation

6.8 Energy and Sustainability Considerations

6.9 Summary and Study Guide

Homework Problems


Chapter 7: Rapid Granular Filtration

7.1 Physical Description of a Rapid Granular Filter

7.2 Process Description of Rapid Filtration

7.3 Particle Capture in Granular Filtration

7.4 Head Loss through a Clean Filter Bed

7.5 Modeling of Performance and Optimization

7.6 Backwash Hydraulics

7.7 Energy and Sustainability Considerations

7.8 Summary and Study Guide

Homework Problems


Chapter 8: Membrane Filtration

8.1 Classification of Membrane Processes

8.2 Comparison to Rapid Granular Filtration

8.3 Principal Features of Membrane Filtration Equipment

8.4 Process Description of Membrane Filtration

8.5 Particle Capture in Membrane Filtration

8.6 Hydraulics of Flow through Membrane Filters

8.7 Membrane Fouling

8.8 Sizing of Membrane Skids

8.9 Energy and Sustainability Considerations

8.10 Summary and Study Guide

Homework Problems


Chapter 9: Reverse Osmosis

9.1 Principal Features of a Reverse Osmosis Facility

9.2 Osmotic Pressure and Reverse Osmosis

9.3 Mass Transfer of Water and Solutes through RO Membranes

9.4 Performance Dependence on Temperature and Pressure

9.5 Concentration Polarization

9.6 Fouling and Scaling

9.7 Element Selection and Membrane Array Design

9.8 Energy and Sustainability Considerations

9.9 Summary and Study Guide

Homework Problems


Chapter 10: Adsorption and Ion Exchange

10.1 Introduction to the Adsorption Process

10.2 Adsorption Equilibrium

10.3 Adsorption Kinetics

10.4 Introduction to the Ion Exchange Process

10.5 Ion Exchange Equilibrium

10.6 Ion Exchange Kinetics

10.7 Fixed-Bed Contactors

10.8 Suspended-Media Reactors

10.9 Energy and Sustainability Considerations

10.10 Summary and Study Guide

Homework Problems


Chapter 11: Air Stripping and Aeration

11.1 Types of Air Stripping and Aeration Contactors

11.2 Gas–Liquid Equilibrium

11.3 Fundamentals of Packed Tower Air Stripping

11.4 Design and Analysis of Packed-Tower Air Stripping

11.5 Energy and Sustainability Considerations

11.6 Summary and Study Guide

Homework Problems


Chapter 12: Advanced Oxidation

12.1 Introduction to Advanced Oxidation

12.2 Ozonation as an Advanced Oxidation Process

12.3 Hydrogen Peroxide/Ozone Process

12.4 Hydrogen Peroxide/UV Light Process

12.5 Energy and Sustainability Considerations

12.6 Summary and Study Guide

Homework Problems


Chapter 13: Disinfection

13.1 Disinfection Agents and Systems

13.2 Disinfection with Free and Combined Chlorine

13.3 Disinfection with Chlorine Dioxide

13.4 Disinfection with Ozone

13.5 Disinfection with Ultraviolet Light

13.6 Disinfection Kinetics

13.7 Disinfection Kinetics in Real Flow Reactors

13.8 Design of Disinfection Contactors with Low Dispersion

13.9 Disinfection By-products

13.10 Residual Maintenance

13.11 Energy and Sustainability Considerations

13.12 Summary and Study Guide

Homework Problems


Chapter 14: Residuals Management

14.1 Defining the Problem

14.2 Physical, Chemical, and Biological Properties of Residuals

14.3 Alum and Iron Coagulation Sludge

14.4 Liquid Wastes from Granular Media Filters

14.5 Management of Residual Liquid Streams

14.6 Management of Residual Sludge

14.7 Ultimate Reuse and Disposal of Semisolid Residuals

14.8 Summary and Study Guide

Homework Problems


Appendix A: Conversion Factors

Appendix B: Physical Properties of Selected Gases and Composition of Air

B.1 Density of Air at Other Temperatures

B.2 Change in Atmospheric Pressure with Elevation

Appendix C: Physical Properties of Water

Appendix D: Periodic Table

Appendix E: Electronic Resources Available on the John Wiley & Sons Website for This Textbook


Title Page

About the Authors

Dr. Kerry J. Howe is an associate professor in the Department of Civil Engineering at the University of New Mexico. His career in water treatment spans both consulting and academia. He has a B.S. degree in civil and environmental engineering from the University of Wisconsin-Madison, an M.S. degree in environmental health engineering from the University of Texas at Austin, and a Ph.D. degree in environmental engineering from the University of Illinois at Urbana-Champaign. After a stint at CH2M-Hill, he worked for over 10 years at MWH, Inc., where he was involved in the planning, design, and construction of water and wastewater treatment facilities up to 380 ML/d (100 mgd) in capacity. He has experience with conventional surface water treatment and other treatment technologies such as membrane treatment, ozonation, and packed-tower aeration. At the University of New Mexico, his teaching and research focuses on membrane processes and desalination, physicochemical treatment processes, water quality, sustainability, and engineering design. Dr. Howe is a registered professional engineer in Wisconsin and New Mexico and a Board Certified Environmental Engineer by the American Academy of Environmental Engineers.

Dr. David W. Hand is a professor of civil and environmental engineering at the Michigan Technological University. He received his B.S. degree in engineering at Michigan Technological University, an M.S. degree in civil engineering at Michigan Technological University, and a Ph.D. in engineering from Michigan Technological University. His teaching and research focuses on water and wastewater treatment engineering with emphasis on physicochemical treatment processes. He has authored and co-authored over 130 technical publications including six textbooks, two patents, and eight copyrighted software programs. He received the ASCE Rudolf Hering Medal, an outstanding teaching award and publication award from the Association of Environmental Engineering and Science Professors, and a publication award from American Water Works Association. He is a Board Certified Environmental Engineering Member of the American Academy of Environmental Engineers.

Dr. John C. Crittenden is a professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology and the director of the Brook Byers Institute for Sustainable Systems. In this position, he leads the creation of an integrated initiative in Sustainable Urban Systems. He is a Georgia Research Alliance (GRA) Eminent Scholar in Sustainable Systems and occupies the Hightower Chair for Sustainable Technologies. Dr. Crittenden is an accomplished expert in sustainability, pollution prevention, physicochemical treatment processes, nanotechnology, air and water treatment, mass transfer, numerical methods, and modeling of air, wastewater, and water treatment processes. He has received multiple awards for his research in the treatment and removal of hazardous materials from drinking water and groundwater. He has four copyrighted software products and three patents in the areas of pollution prevention, stripping, ion exchange, advanced oxidation/catalysis, adsorption and groundwater transport. The American Institute of Chemical Engineers (AIChE) Centennial Celebration Committee named Dr. Crittenden as one of the top 100 Chemical Engineers of the Modern Era at their 100th annual meeting in 2008. He is a member of the National Academy of Engineering.

Dr. R. Rhodes Trussell is a registered Civil and Corrosion Engineer in the State of California with 40 years of water treatment experience. He has a B.S., M.S., and Ph.D. in environmental engineering from the University of California at Berkeley. He founded Trussell Technologies, Inc., a consulting firm specializing in the application of science to engineering, after working for 33 years for MWH, Inc. He has authored more than 200 publications, including several chapters in all three editions of MWH's Water Treatment: Principles and Design. Dr. Trussell has served as Chair of the EPA Science Advisory Board's Committee on Drinking Water, serves on the Membership Committee for the National Academy of Engineering, and as Chair of the Water Science and Technology Board for the National Academies. For the International Water Association, Dr. Trussell serves as a member of the Scientific and Technical Council, the Editorial Board, and on the Program Committee. In 2010, Dr. Trussell was awarded the prestigious A.P. Black Award from the American Water Works Association.

Dr. George Tchobanoglous is a professor emeritus of environmental engineering in the Department of Civil and Environmental Engineering at the University of California at Davis. He received a B.S. degree in civil engineering from the University of the Pacific, an M.S. degree in sanitary engineering from the University of California at Berkeley, and a Ph.D. in environmental engineering from Stanford University. His principal research interests are in the areas of wastewater treatment, wastewater filtration, UV disinfection, wastewater reclamation and reuse, solid waste management, and wastewater management for small systems. He has authored or coauthored over 500 technical publications, including 22 textbooks and 8 reference works. Professor Tchobanoglous serves nationally and internationally as a consultant to both governmental agencies and private concerns. An active member of numerous professional societies, he is a past president of the Association of Environmental Engineering and Science Professors. He is a registered civil engineer in California and a member of the National Academy of Engineering.


Without water, life cannot exist. Thus, securing an adequate supply of fresh, clean water is essential to the health of humankind and the functioning of modern society. Water is also known as the universal solvent—it is capable of dissolving a vast number of natural and synthetic chemicals. Increasing population and the contamination of water with municipal, agricultural, and industrial wastes has led to a deterioration of water quality and nearly all sources of water require some form of treatment before potable use. This textbook is designed to serve as an introduction to the field of water treatment and the processes that are used to make water safe to drink.

The authors of this book have collaborated on two books that are intertwined with each other, both published by John Wiley and Sons, Inc.The other book, MWH's Water Treatment: Principles and Design, 3rd ed. (Crittenden et al., 2012), was the source for a significant portion of the material in this book. The focus of this present book is on principles of water treatment; it is suitable as a textbook for both undergraduate and graduate courses. The other book is an expanded edition, nearly triple the length of this one, that provides more comprehensive coverage of the field of drinking water treatment and is suitable as both a textbook and a reference for practicing professionals. The unit process chapters of MWH's Water Treatment: Principles and Design contain a detailed analysis of the principles of treatment processes as well as in-depth material on design. MWH's Water Treatment: Principles and Design also provides extensive chapters on the physical, chemical, and microbiological quality of water, removal of selected contaminants, internal corrosion of water conduits, and case studies that are not included in this book. Students who use this textbook in a class on water treatment and go on to a career in design of water treatment facilities are encouraged to consult MWH's Water Treatment: Principles and Design on topics that were beyond the scope of this textbook.


The authors gratefully acknowledge the people who assisted with the preparation of this book. Particular credit goes to Dr. Harold Leverenz of the University of California at Davis, who adapted most of the figures for this textbook after preparing them for the companion book, MWH's Water Treatment: Principles and Design, 3rd Ed. Figures for several chapters were prepared by Mr. James Howe of Rice University. Mr. Daniel Birdsell and Ms. Lana Mitchell of the University of New Mexico reviewed and checked the chapters, including the figure, table, and equation numbers, the math in example problems, and the references at the ends of the chapters. Ms. Lana Mitchell also helped prepare the solutions manual for the homework problems. Dr. Sangam Tiwari of Trussell Technologies assisted with the writing of Chap. 2, Dr. Daisuke Minakata of Georgia Tech assisted with the writing of Chaps. 10 and 12, and Dr. Zhongming Lu of Georgia Tech assisted with the writing of Chap. 10.

Several chapters were reviewed by external reviewers and their comments helped improve the quality of this book. The reviewers included:

Ms. Elaine W. Howe, Trussell Technologies Inc.
Dr. Jaehong Kim, Georgia Institute of Technology
Dr. David A. Ladner, Clemson University
Dr. Qilin Li, Rice University
Dr. Edward D. Schroeder, University of California-Davis
Dr. John E. Tobiason, University of Massachusetts-Amherst

We gratefully acknowledge the support and help of the Wiley staff, particularly Bob Hilbert, James Harper, Robert Argentieri, and Daniel Magers.

Kerry J. Howe
David W. Hand
John C. Crittenden
R. Rhodes Trussell
George Tchobanoglous

Chapter 1


1.1 The Importance of Principles

1.2 The Importance of Sustainability



Securing and maintaining an adequate supply of water has been one of the essential factors in the development of human settlements. The earliest communities were primarily concerned with the quantity of water available. Increasing population, however, has exerted more pressure on limited high-quality surface sources, and contamination of water with municipal, agricultural, and industrial wastes has led to a deterioration of water quality in many other sources. At the same time, water quality regulations have become more rigorous, analytical capabilities for detecting contaminants have become more sensitive, and the public has become more discriminating about water quality. Thus, the quality of a water source cannot be overlooked in water supply development. In fact, most sources of water require some form of treatment before potable use.

Water treatment can be defined as the processing of water to achieve a water quality that meets specified goals or standards set by the end user or a community through its regulatory agencies. Goals and standards can include the requirements of regulatory agencies, additional requirements set by a local community, and requirements associated with specific industrial processes.

The primary focus of this book is the principles of water treatment for the production of potable or drinking water on a municipal level. Water treatment, however, encompasses a much wider range of problems and ultimate uses, including home treatment units and facilities for industrial water treatment with a wide variety of water quality requirements that depend on the specific industry. Water treatment processes are also applicable to remediation of contaminated groundwater and other water sources and wastewater treatment when the treated wastewater is to be recycled for new uses. The issues and processes covered in this book are relevant to all of these applications.

This book thoroughly covers the fundamental principles that govern the design and operation of water treatment processes. Following this introduction, the next three chapters provide background information that is necessary to understand the scope and complexity of treatment processes. Chapter 2 describes the relationship between water quality and public health, introduces the types of constituents that are present in various water supplies, and outlines some of the challenges faced by water treatment professionals. Chapter 3 introduces how the physicochemical properties of constituents in water and other factors guide the selection of treatment processes. Chapter 4 introduces the core principles necessary for understanding treatment processes, such as chemical equilibrium and kinetics, mass balance analysis, reactor analysis, and mass transfer. Chapters 5 through 13 are the heart of the book, presenting in-depth material on each of the principal unit processes traditionally used in municipal water treatment. Chapter 14 presents material on the processing of treatment residuals, a subject that can have a significant impact on the design and operation of treatment facilities.

1.1 The Importance of Principles

From the 1850s to about the 1950s, water treatment facilities were frequently designed by experienced engineers who drew upon previous successful design practices. Improvements were made by incremental changes from one plant to the next. Treatment processes were often treated as a “black box,” and detailed understanding of the scientific principles governing the process was not essential in completing a successful design. In recent years, however, significant changes have taken place in the water treatment industry that require engineers to have a greater understanding of fundamental principles underlying treatment processes. Some of these changes include increasing contamination of water supplies, increasing rate of technological development, and increasing sophistication of treatment facilities.

Early treatment practices were primarily focused on the aesthetic quality of water and prevention of contamination by pathogenic organisms. These treatment goals were relatively clear-cut compared to today's requirements. Since about the 1950s, tens of thousands of chemicals have been developed for a wide variety of purposes—about 3300 chemicals are produced in quantities greater than 454,000 kg/yr (1,000,000 lb/yr) in the United States. Some chemicals have leaked into water supplies and have carcinogenic or other negative health impacts on humans. Many water supplies are now impacted by discharges from wastewater treatment plants and urban storm sewers. Engineers may be required to identify and design treatment strategies for chemicals for which no previous experience is available. As will be demonstrated in Chap. 3, treatment processes depend on well-established physicochemical principles. If the scientific principles are understood, it is possible to identify candidate processes based on the expected interaction between the properties of the contaminants and the capabilities of the processes. For instance, by knowing the volatility and hydrophobicity of a synthetic organic chemical, it is possible to predict whether air stripping or adsorption onto activated carbon is a more suitable treatment strategy.

Technology has been accelerating the pace at which treatment equipment is being developed. Engineers are faced with situations in which equipment vendors and manufacturers have developed new or innovative processes, and the engineer is assigned the task of recommending to a client whether or not the equipment should be evaluated as a viable option. Potable water is a necessary part of modern society, properly working processes are a matter of public health, and consumers expect to have water available continuously. Practical knowledge of previous successful design practices may not be sufficient for predicting whether new equipment will work. Understanding the scientific principles that govern treatment processes gives the engineer a basis for evaluating process innovations.

Treatment plants have gotten more complex. Sometimes facilities fail to work properly and the engineer is called in to identify factors that are preventing the plant from working or to recommend strategies to improve performance. Often, the difference between effective and ineffective performance is the result of scientific principles—a coagulant dose too low to destabilize particles, a change in water density because of a change in temperature, treatment being attempted outside the effective pH range. In these instances, scientific principles can guide the decision-making process regarding why a process is not working and what changes to operation would fix the problem.

As a result, the range of knowledge and experience needed to design water treatment facilities is extensive and cannot be learned in a single semester in college; today's design engineers need both knowledge about the fundamental principles of processes and practical design experience. This book provides a solid foundation in the former; other books focus more on the latter, such as books by Kawamura (2000) and AWWA and ASCE (2004). In addition, a companion book written by the authors, MWH's Water Treatment Principles and Design, 3rd ed. (Crittenden et al., 2012), covers both principles and design. While the coverage of that book is broad, it is nearly triple the length of this book and is difficult to cover in detail in a single engineering course. This book takes a focused approach on principles of water treatment and does so with the perspective of applying principles during design and operation so that it will serve as a useful introduction into the field of water treatment.

1.2 The Importance of Sustainability

Another concept in this book is that sustainability and energy consumption should be considered in selecting treatment processes, designing them, and operating them. There are several reasons for this approach. First, the withdrawal, conveyance, treatment, and distribution of potable water—and subsequent collection, treatment, and discharge of domestic wastewater—is one of the most energy-intensive industries in the United States. Only the primary metal and chemical industries use more energy. A focus on sustainability and energy considerations will help the water treatment industry develop ways to be more efficient while conserving resources.

Water demand has grown in urban areas and adequate supplies of locally available, high-quality water are increasingly scarce. Simultaneously, the ability to detect contaminants has become more sophisticated, negative health effects of some constituents have become more evident, regulations have become more stringent, and consumer expectations of high-quality water have become more strident. The growing trend toward use of poor-quality water sources, coupled with these other effects, has stimulated a trend toward more advanced treatment that requires more energy and resources. Increasing energy and resource use will contribute to greater pollution and environmental degradation; incorporating sustainability and energy consumption into process and design practices will offset that trend and allow higher levels of water treatment without the negative impacts.

Ultimately, the most important reason to consider sustainability in water treatment plant design is an issue of leadership. Environmental engineering professionals—the engineers who design water treatment facilities—ought to be more knowledgeable about environmental considerations than the general public and should demonstrate to other professions that successful design can be achieved when the environmental impacts are taken into account. The section on sustainability and energy considerations at the end of each of the process chapters in this book is a small start in that direction.


AWWA and ASCE. (2004) Water Treatment Plant Design, 4th ed., McGraw-Hill, New York.

Crittenden. J. C., Trussell, R. R., Hand, D. W., Howe, K. J., and Tchobanoglous, G. (2012) MWH's Water Treatment: Principles and Design, 3rd ed., Wiley, Hoboken, NJ.

Kawamura, S. (2000) Integrated Design and Operation of Water Treatment Facilities, Wiley, New York.

Chapter 2

Water Quality and Public Health

2.1 Relationship between Water Quality and Public Health

2.2 Source Waters for Municipal Drinking Water Systems

2.3 Regulations of Water Treatment in the United States

2.4 Evolving Trends and Challenges in Drinking Water Treatment

2.5 Summary and Study Guide



The primary purpose of municipal water treatment is to protect public health. Water can contain a wide array of constituents that can make people ill and has a unique ability to rapidly transmit disease to large numbers of people. The purpose of this chapter is to introduce the relationship between water quality and public health and identify the major sources of contaminants in water supplies. The basic features of drinking water regulations in the United States are introduced. The chapter ends with a description of some of the challenges, competing issues, and compromises that water treatment engineers must balance to successfully design a water treatment system.

2.1 Relationship between Water Quality and Public Health

History of Waterborne Disease

Prior to the middle of the nineteenth century, it was commonly believed that diseases such as cholera and typhoid fever were primarily transmitted by breathing miasma, vapors emanating from a decaying victim and drifting through the night. Serious engagement in treatment of public drinking water supplies began to develop in the last half of the nineteenth century after Dr. John Snow identified the connection between contamination of drinking water and waterborne disease. Snow's discovery was later supported by the advocacy of the germ theory of disease by the French scientist Louis Pasteur in the 1860s and the discovery of important microbial pathogens (microorganisms capable of causing disease) by the German scientist Robert Koch. These developments led to the understanding that gastrointestinal disease spreads when the pathogens in the feces of infected human beings are transported into the food and water of healthy individuals—exposure via the so-called fecal-to-oral route. As a result, a number of strategies were developed to break the connection between drinking water systems and systems for disposal of human waste. These strategies included the use of water sources that are not exposed to sewage contamination, the use of water treatment on contaminated supplies, the use of continuously pressurized water systems that ensure that safe water, once it is obtained, could be delivered to the consumer without exposure to further contamination, and the use of bacterial indices of human fecal contamination.

Continuous chlorination of drinking water as a means for bacteriological control was introduced at the beginning of the twentieth century. In the next four decades, the focus was on the implementation of conventional water treatment and chlorine disinfection of surface water supplies. By 1940, the vast majority of water supplies in developed countries had “complete treatment” and was considered microbiologically safe. The success of filtration and disinfection practices lead to the virtual elimination of the most deadly waterborne diseases in developed countries, particularly typhoid fever and cholera, as depicted on Fig. 2.1 (CDC, 2011).

Figure 2.1 Decline in the incidence of typhoid fever in the United States due to the provision of higher quality drinking water and other sanitation and hygiene practice improvements. [Data from CDC (2011).]


In 1974, however, both in the United States and in Europe, it was discovered that chlorine, the chemical most commonly used for disinfection, reacted with the natural organic matter in the water to produce synthetic organic chemicals, particularly chloroform. Since that time, decades of research have shown that chlorine produces a large number of disinfection by-products (DBPs), and that alternate chemical disinfectants produce DBPs of their own. The challenge to protect the public from waterborne diseases continues as engineers balance disinfection and the formation of treatment by-products.

In the 1970s and 1980s, it became apparent that some waterborne diseases spread by means other than from one human to another via the fecal-to-oral route. First among these are zoonotic diseases, diseases that humans can contract via the fecal-to-oral route from the feces of other animals. Examples of zoonotic pathogens are Giardia lamblia and Cryptosporidium parvum. Second are diseases caused by opportunistic pathogens that make their home in aquatic environments but will infect humans when the opportunity arises. Examples of opportunistic pathogens are Legionella pneumophila, Aeromonas hydrophilia, Mycobacterium avium complex, and Pseudomonas aeruginosa. An opportunistic pathogen is a microorganism that is not ordinarily able to overcome the natural defenses of a healthy human host. Under certain circumstances, however, such organisms are able to cause infection resulting in serious damage to the host. There are two circumstances when opportunistic pathogens are more successful: (a) when the immune response of the host has been compromised [e.g., persons with human immunodeficiency virus (HIV), persons on drugs that suppress the immune system, the very elderly] or (b) when the host is exposed to such high levels of the organism in question that the infection becomes overwhelming before the body can develop a suitable immune response. As a result of the possible presence of zoonotic pathogens, finding a water supply free of sewage contamination does not assure the absence of pathogens and does not obviate the need for water treatment. Also, understanding the role of opportunistic pathogens makes it clear that purifying water and transporting it under pressure does not provide complete protection, and growth of opportunistic pathogens must also be controlled in distribution systems and in water system appurtenances.

Role of Water in Transmitting Disease

A unique aspect of water as a vehicle for transmitting disease is that a contaminated water supply can rapidly expose a large number of people. When food is contaminated with a pathogen, tens to hundreds of persons are commonly infected. If a large, centralized food-packaging facility is involved, thousands might be infected. However, when drinking water is contaminated with a pathogen, typically hundreds of people are infected and occasionally hundreds of thousands are infected. For example, it is estimated that 500,000 people became ill from contaminated drinking water in the 1993 Milwaukee Cryptosporidium incident (MacKenzie et al., 1994).

The principal mechanisms for the transmission of enteric (gastrointestinal) diseases are shown on Fig. 2.2. Suppose that, while infecting an adult, a pathogen causes a severe, debilitating enteric disease that immobilizes and seriously injures the infected person. The route of transmission can be analyzed using Fig. 2.2. If an adult with severe illness is too debilitated to prepare food, the organism cannot get into the food supply. However, the organism does get in the sewer even if the sick person cannot get out of bed. Once in the sewer, the organism is then transported to the wastewater treatment plant. If the organism is not removed or inactivated at the wastewater treatment plant, it enters the receiving watercourse. If that watercourse serves as a water supply and water treatment does not remove or inactivate the organism, both healthy toddlers and adults who drink the water are exposed and may get infected. Thus, the entire population drinking the water supply is potentially exposed to the disease-causing agent. Under these conditions, an organism can successfully reproduce even if it causes a severe disease from which the host rarely recovers. According to some historical accounts, the classic form of Asiatic cholera that appeared in the middle of the nineteenth century behaved in this way. The route of transmission can be interrupted by removing or inactivating the organism from the water either at the wastewater treatment plant or at the drinking water treatment plant.

Figure 2.2 Schematic of routes of transmission for enteric disease.


Figure 2.2 can also be used to consider the spread of the disease via the food route. Adults with mild symptoms of the disease, if they do not use adequate hygiene, may contaminate food when they prepare it. Both toddlers and adults who eat the contaminated food may then get infected. Some of those who get infected will be asymptomatic; others may exhibit mild symptoms. Infected adults may again prepare and contaminate food, and some infected toddlers will go to child-care centers. Toddlers in child-care centers will expose other toddlers. Adult caregivers can also expose themselves while handling the sick toddlers.

Debilitating diseases are less likely to spread this way because seriously ill adults are unlikely to be preparing food for others and seriously ill children are unlikely to go to child-care centers. Furthermore, the drinking water has no connection to this route of communication so treating the drinking water will not stop it. The value of a water treatment intervention is much greater where severe, debilitating disease is concerned.

Enteric organisms that cause seriously debilitating disease can be nearly eliminated through water treatment because they depend on this route of exposure for survival. When enteric organisms cause mild disease or asymptomatic infections, water treatment can prevent the largest scale epidemic events but the disease remains in the community. This is because mildly ill or asymptomatic carriers will spread the disease via food preparation and in child-care centers.

2.2 Source Waters for Municipal Drinking Water Systems

Designing on effective water treatment plant is a complex process because of the wide variety of undesireable constituents that can be in the source water. Even waters thought of as “pristine” might contain some constituents that should be removed. The specific constituents in water, the relative concentrations of those constituents, and other water quality parameters that affect treatment depend heavily on local conditions of geology, climate, and human activity. Thus, treatment processes must be tailored to the specific source water. The specific treatment challenges, however, are heavily influenced by the type of source water, which can include groundwater, lakes and reservoirs, rivers, seawater, and wastewater impaired waters. Each type of source will require different treatment processes and present different challenges to the water treatment engineer. Constituents can enter the water supply through several pathways, as depicted on Fig. 2.3. Potential types of contamination and general characteristics of each type of source are described in the following sections.

Figure 2.3 Sources of naturally occurring constituents and contaminants in drinking water supplies.



Groundwater is water that exists in the pore spaces between sand, gravel, and rocks in the earth and can be brought to the surface using wells. About 35 percent of people served by public water systems in the United States are supplied with groundwater; nearly all the rest are supplied with fresh surface water. Undesirable constituents in groundwater can be either naturally occurring or anthropogenic (of human origin). The natural constituents result from dissolution caused by long-term contact between the water and the rocks and minerals. Some natural constituents that might need to be removed by water treatment include:

2.1 Iron and manganese: Depending on local conditions, groundwater can be aerobic (in the presence of oxygen gas) or anaerobic (in the absence of oxygen-containing electron acceptors). In anaerobic conditions, iron- and manganese-containing minerals are relatively soluble and can dissolve into the water. When the water is aerated and/or chlorinated, the iron and manganese react to form insoluble species that precipitate and cause rust- and black-colored stains on laundry and plumbing fixtures.
2.1 Hardness: Hardness is a characteristic of water caused by the presence of calcium and magnesium, which are abundant in the Earth's crust. Hard water does not cause negative health impacts, but it reacts with soap to form a white precipitate (soap scum), leaves water spots on surfaces, and forms precipitates in water heaters, tea pots, heat exchangers, boilers valves, and pipes, clogging them and/or reducing their efficiency.
2.1 Trace inorganics: Minerals can contain many trace elements, including arsenic, barium, chromium, fluoride, selenium, and species that exhibit radioactivity such as radium, radon, and uranium. Many trace inorganics exhibit toxicity, carcinogenicity, or other adverse health effects, if concentrations are too high.
2.1 Salinity: Brackish groundwater with low to moderate salinity, ranging from about 1000 to 5000 mg/L total dissolved solids (TDS), is relatively common. Brackish water is too salty for potable, industrial, or agricultural applications (the United States secondary drinking water standard for TDS is 500 mg/L). Interest in desalinating these sources has increased in areas short on freshwater, such as Florida, Texas, and the Southwest region of the United States.
2.1 Natural organic matter: Most groundwaters have low concentrations of natural organic matter (NOM), but some locations have shallow groundwater that is hydraulically connected with swampy areas. The Biscayne Aquifer in southeast Florida is an example of this type of water source. These waters are highly colored (like weak ice tea), which is not only undesirable aesthetically but can react with chlorine during disinfection to form disinfection by-products that may be carcinogenic.

In addition to these natural constituents, groundwater can contain a variety of anthropogenic contaminants. The potential number of anthropogenic contaminants is vast. In the United States, about 70,000 chemicals are used commercially and about 3300 are considered by the U.S. Environmental Protection Agency (EPA) to be high-volume production chemicals [i.e., are produced at a level greater than or equal to 454,000 kg/yr (1,000,000 lb/yr)]. Anthropogenic contributions to groundwater can come from the following sources:

2.1 Leaking underground storage tanks: Gas stations store gasoline in underground tanks, which can corrode, leak, and contaminate groundwater. Benzene, toluene, ethylbenzene, and xylene (BTEX) are constituents in gasoline that must be removed from groundwater to make it potable, and methyl t-butyl ether (MTBE) is a gasoline additive that is particularly difficult to remove with conventional water treatment processes.
2.1 Leaking residential septic systems: Improperly constructed septic systems can leak nitrate, household chemicals, and other contaminants into the water supply.
2.1 Industrial contamination: Past practices of discharging chemical wastes on the ground, in landfills, in open pits, or into waste disposal wells have contaminated water supplies with many kinds of industrial chemicals. Industrial solvents like trichloroethene (TCE) and tetrachloroethene (PCE) are particularly common contaminants in groundwater. Inadvertent chemical spills also lead to contamination.
2.1 Agricultural contamination: During irrigation, plants uptake some water but excess water can percolate downward and reach the underlying groundwater table. Pesticides, herbicides, and fertilizers applied to the land can travel down with the water and contaminate the groundwater. In addition, irrigation water will contain some dissolved salts and the plants can selectively uptake water, leaving the excess water with higher salinity that can contaminate the groundwater with excess salts.

In aquifers where groundwater withdrawals exceed rates of recharge, seawater migrates inland. This process, called saltwater intrusion, can result in high concentrations of TDS (mainly sodium and chloride) at potable water supply wells. Coastal areas in Florida and California have been affected by saltwater intrusion. The only long-term solution is to balance supply and demand, but saltwater intrusion can be slowed or reversed by injection of water between the supply wells and the ocean, as shown on Fig. 2.4. Such saltwater intrusion barriers typically consist of a network of wells arrayed parallel to the shoreline to form a hydrostatic barrier. In several cases, including four saltwater intrusion barriers in southern California, highly polished reclaimed water has been used to create the groundwater barrier.

Figure 2.4 Saltwater intrusion into a groundwater supply: (a) natural hydrologic condition, (b) saltwater intrusion caused by depression of the water table and reversal of the hydraulic gradient by pumping, and (c) prevention of saltwater intrusion by creation of a hydrostatic barrier.


Despite the potential for many constituents to be in groundwater, an advantage of this type of water supply is that the quality tends to be consistent over time with little or no seasonal variation. Changes due to migration of contaminants tend to happen slowly. Groundwater withdrawn from properly constructed wells is free from pathogenic organisms and does not need to be filtered. A disadvantage, however, is that the quality of the water is not known until the well has actually been drilled and pumped long enough to exert its full zone of influence for some time. While general water quality can often be predicted from the local geology, there have been many cases of wells drilled to different depths or a few hundred meters apart that contain significantly different concentrations of trace constituents such as arsenic, which then affects treatment requirements. The lack of reliable information on the specifics of water quality prior to installing a well complicates the treatment selection and design process in some locations.

It is important to realize that not all groundwaters will exhibit all of these problems. Confined aquifers (isolated from the surface by a zone of lower permeability) can be less susceptible to anthropogenic contamination, depending on where the recharge zone is. Depending on local geology and human activity, many groundwaters might be relatively pure and have essentially no treatment requirements, others might have excessive iron and manganese or high hardness, still others might have contamination from septic tanks or fertilizers, and some will have a combination of these problems. The treatment required will be different in each case, leading to the reality that treatment practices must be tailored to the individual water supply.


The water in rivers often has less mineral content than groundwater but can dissolve natural materials during overland flow after rain or during interaction with groundwater. Surface waters can contain floating and suspended material like sediment, leaves, branches, algae, and other plants or animals that wash into the water during overland flow or live in the water itself. The key element that distinguishes all surface waters from groundwater is the potential for the presence of pathogenic bacteria and other microorganisms that must be eliminated to make water safe to drink. The necessity of removing pathogenic organisms makes surface water treatment dramatically different from groundwater treatment; nearly all surface water treatment plants have filtration systems designed to physically remove microorganisms and engineered disinfectant contact basins to disinfect the water. In contrast, treatment facilities for groundwater have processes focused on removing dissolved contaminants.