pathogens in natural and engineered water systems ... · wastewater wastewater treatment facility...

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Pathogens in Natural and Engineered Water Systems: Emerging Issues

Pathogens in Natural and Engineered Water Systems: Emerging Issues

Tamim YounosResearch Professor of Water Resources

Virginia Water Resources Research Center & Department of Geography

Virginia Tech

AWWA – Virginia Section Research Seminar

October 22, 2007

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Contents of PresentationContents of Presentation

1. Historical Perspective

2. Pathogens in Natural Waters

3. Pathogens in Engineered Water Systems

4. Emerging Issues & Research Needs

5. A Vision for the Future - Paradigm Shift

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Ancient ConcernsAncient Concerns

• Water Clarity

• Water Taste

• Water Odor

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Bacteria as a Public Health Issue:- Nineteenth Century -


• Urbanization –emergence of sewer disposal systems

• Typhoid fever

• Cholera

• Dysentery

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• Presence of microorganisms in water was established (1870s –1880s)

• Scientists focused on securing disease-free drinking water supplies for emerging urban areas

• Scientists started to study the impact of disease-causing microorganisms from privies and sewer systems on sources of drinking water (i.e., natural waters)

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Evolving Water Treatment TechnologiesEvolving Water Treatment Technologies

19th Century

• Sand filtration• Chlorination

20th Century

• Sand filtration• Chlorination• Activated carbon filters• Membrane filtration• Ozone, UV-Radiation• Other

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PathogensPathogens

The term pathogen is derived from theGreek παθογένεια, "that which producessuffering."

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Categories of PathogensCategories of Pathogens

• Waterborne pathogens that are normal inhabitants of water

• Waterborne pathogens introduced from anthropogenic (fecal) sources

• Primary pathogens

• Opportunistic or secondary pathogens

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Pathogenic Health ThreatsPathogenic Health Threats

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Health Threats from Waterborne PathogensHealth Threats from Waterborne Pathogens

• Exposure through ingestion

• Inhalation of aerosols

• Dermal exposure

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Emerging PathogensEmerging Pathogens

Emerging pathogens are microorganisms for whichthere is a lack of data about their transmission,survival, or disinfectant susceptibility.

EPA Contaminant Candidate List (CCL)Does not include microorganisms that have notcaused a waterborne outbreak in the U.S., or thosethat can be removed from water by filtration and/ordisinfection.

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List of Some Candidate Emerging PathogensList of Some Candidate Emerging Pathogens

Pathogen Cited by U.S. EPA, 2005 Listed by Egli and Rust, 2002 Bacteria Aeromonas hydrophila

Helicobacter pylori Mycobacterium avium complex

Pseudomonas aeruginosa Legionella pneumophila Aeromonas hydrophila Campylobacter jejuni Yersinia enterocolitica Chlamydia

Viruses Caliciviruses Adenoviruses Coxsackieviruses Echoviruses

Caliciviruses, i.e. Norwalk virus Other small round structured viruses Rotaviruses Hepatitis A

Protozoa Microsporidia, i.e. Enterocytozoon & Septata

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Pathogens in Natural WatersPathogens in Natural Waters

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SOURCES OF PATHOGENS

IN WATER

Wildlife

Urban Runoff

Landfill

Animal Waste

Sewer Overflow/Leakage

Inadequate Sewer Treatment

EMERGING PATHOGENS

KNOWN PATHOGENS(e.g. E. coli)

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Water Source ProtectionWater Source Protection

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Possible Approach: TMDL PlansPossible Approach: TMDL Plans

• Microbial Source Tracking (MST)Traces the major source of bacterial contamination from the source to surface waters using indicator organisms

• Urban Stormwater Management

• Animal Waste Management

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Pathogens in Engineered Water SystemsPathogens in Engineered Water Systems

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Sanitation Authority

Water Authority

Blacksburg and VT

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Water Distribution SystemsWater Distribution Systems

• Major – Main Lines

• Minor – Home Plumbing

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Minor and Major Water Distribution SystemsMinor and Major Water Distribution Systems

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Little is known about pipelineInteractions among treatmentdisinfectants, chemicalcontaminants, and pipeDeposits (biofilms).

Distribution System Simulator(NRML)

http://www.epa.gov/nrmrl/ews/news092007.html

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BiofilmBiofilm

Biofilm is a complex mixture of microbes,organic and inorganic materialaccumulated amidst a microbiallyproduced organic polymer matrix attachedto the inner surface of distribution system.

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• Source water may contain high levels of calcium carbonate, iron, or manganese, creating “hard water.”If the hardness is not removed at the water treatment plant, biofilm and scale can form on the inner walls of the distribution pipes.

• The formation of biofilm is dependent on the type of pipes, whether iron, poly vinyl chloride (PVC), or concrete lined. In turn, biofilm and scale can affect the life span of the pipes, thus playing a role in the cost of the distribution system infrastructure. http://www.epa.gov/nrmrl/news/news092007.html

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• Disinfectants (typically chlorine) introduced during the treatment process may react with organic carbons naturally present in the source water to form unwanted disinfection by-products (DBPs).

• Biofilm and scale can decrease concentrations of disinfectants. They can also cause taste and odor problems and harbor disease-causing microorganisms or chemical pollutants, possibly releasing them into drinking water in distribution systems.

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Pathogens in Water Distribution SystemsPathogens in Water Distribution Systems

USEPA REPORT 2002Health Risks from Microbial Growth and Biofilms inDrinking Water Distributions Systems

The report lists 11 primary bacterial pathogens and 11opportunistic bacterial pathogens detected in waterdistribution systems and/or biofilms

http://www.epa.gov/safewater/disinfection/tcr/pdfs/whitepaper_tcr_biofilms.pdf

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Factors Affecting Pathogen Survival and Growth in Distribution Systems

Factors Affecting Pathogen Survival and Growth in Distribution Systems

• Environmental factors• Presence of nutrients• Microbial interactions• Pipe materials• System hydraulics• Disinfectant type and residuals• Sediment accumulation


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Measures for Controlling Biofilm DevelopmentMeasures for Controlling Biofilm Development

• Nutrient control• Control of contamination from materials and equipment• Control and mitigation of system hydraulic problems• Cross-connection control and backflow prevention• Disinfectant residuals• Corrosion control• Infrastructure replacement and repair• Storage vessel management and alteration


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Emerging Pathogens in Water Distribution Systems


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• Mycobacterium avium (M. avium)• Hydrophobic, resistant to disinfecatnts

• Legionella pneumophila (Legionella)• Warm water promotes Legionella growth

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Research NeedsResearch Needs

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Research Needs: Natural Water Systems Research Needs: Natural Water Systems

• A better understanding of and rapid detection methods for emerging waterborne pathogens that are normal inhabitants of water and also found in drinking water systems (e.g., M. avium)

• Quantitative risk assessment for a wide variety of waterborne pathogens

• Identification of suitable indicators for waterborne microbes and viruses

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Research Needs: Engineered Water SystemsResearch Needs: Engineered Water Systems

• Link between organisms present in distribution system biofilmsand human health impacts

• Identification of pipe surface treatments to reduce M. Avium and Legionella numbers

• Potential problems created by cleaning deteriorated pipes

• Level of public health protection provided by various types of disinfectants in distribution systems

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A Vision for the Future: Shift in Paradigm

A Vision for the Future: Shift in Paradigm

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Urban Sprawl and Water DistributionUrban Sprawl and Water Distribution

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Decentralized Water Supply Systems

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RAINFALL

Rooftop Impervious Surfaces

Gutter

DomesticUse

Wastewater

WastewaterTreatment

FacilityStream

StormwaterRunoff (MS4)

StormwaterDetention

Pond

Stream

WaterTreatment

Plant

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RAINFALL

Rooftop Impervious Surfaces

Rainfall Harvesting

DomesticUse

Irrigation & Other

Wastewater

WastewaterTreatment

Facility

Stream

StormwaterRunoff (MS4)

LID, BMP

Stream

Groundwater

DecentralizedWastewater

Systems

Greywater

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Thank you!

Questions/Discussion

Thank you!

Questions/Discussion

With increased water demand, it is expected that by theyear 2025 humans will tap 100 percent of available glob-al fresh water resources. However, assessment and im-provement of water quality for various uses remains a se-rious challenge because of the detection of increased lev-els of pathogens and nonbiological contaminants such asestrogens and pharmaceuticals in the natural and engi-neered water systems. Recreational water quality issuesare also increasing, with many of the etiological agentsemerging or unknown because of the difficulty of conta-minant and source identification.

PATHOGENS IN NATURAL WATERS

Pathogenic microorganisms (bacteria, viruses, andprotozoa) in natural waters are contributed from agricul-tural and domestic animals (animal waste), urban runoff,sewer and septic system (human waste) inputs, andwildlife. Some are normal inhabitants of natural waters.Fecal indicator bacteria such as fecal coliform, Es-cherichia coli and enterococci, are a major cause of watercontamination in the United States (U.S.). Microbialsource tracking (MST) is a thriving scientific field thattraces bacterial contamination from its source to surfacewaters (Keeling et al., 2005). Recognized indicator bacte-ria such as E. coli and Enterococcus spp. are used formany MST methods, while alternative indicators such asBacteroides sp. are also employed.

Emerging pathogens are newly discovered microor-ganisms and/or newly recognized pathogens. Typically,little data are available about their transmission routes,

virulence, minimum infective dose, survival outside ofhost, or disinfectant susceptibility. The U.S. Environ-mental Protection Agency (USEPA) periodically updatesthe drinking water Contaminant Candidate List (CCL)and it was last updated in February 2005 (USEPA, 2005).Several examples of candidate microbial pathogens arelisted in the left column of Table 1, and scientists haveidentified several other emerging pathogens (right col-umn of Table 1) for which limited information is available(Egli and Rust, 2002).

PATHOGENIC HEALTH THREATS

Pathogens pose a risk to human health through var-ious uses of water, particularly drinking water (Figure 1).Water treatment technologies for disinfection were gener-ally considered effective for removing pathogens fromwater, but in 1993, Cryptosporidium parvum, a disinfec-tant-resistant protozoan pathogen, was the cause of thelargest waterborne (drinking water) disease outbreak in U.S. history. This outbreak affected more than400,000 people in Milwaukee, Wisconsin, and causedmore than 100 deaths (Lindquist, 1999). Giardia intesti-nalis (previously named Giardia lamlia) is a frequent

Volume 9 • Number 3 Water Resources IMPACT • 11

PATHOGENS IN NATURAL AND ENGINEEREDWATER SYSTEMS: EMERGING ISSUES

Tamim Younos, Valerie J. Harwood,Joseph O. Falkinham III, and Hua Shen

... utilities are required to identify the best avail-

able technology for the treatment of contaminated

source waters; however, while treated water is

considered safe, less is known about microbial

pollution in water distribution systems

Table 1. Some Candidate Emerging Pathogens.

Pathogen Cited by USEPA (2005) Listed by Egli and Rust (2002)

Bacteria Aeromonas hydrophila Pseudomonas aeruginosaHelicobacter pylori Legionella pneumophilaMycobacterium avium complex Aeromonas hydrophila

Campylobacter jejuniYersinia enterocoliticaChlamydia

Viruses Caliciviruses Caliciviruses, i.e. Norwalk virusAdenoviruses Other small round structured virusesCoxsackieviruses RotavirusesEchoviruses Hepatitis A

Protozoa Microsporidia, i.e., Enterocytozoonand Septata

cause of waterborne outbreaks of acute gastroenteritis(symptoms include diarrhea, cramps and fatigue). Bothof these obligate parasites produce cysts (or oocytsts)that are resistant to disinfection.

The USEPA recommends a multiple barrier approachfor drinking water treatment to reduce the impact ofpathogenic and chemical contaminants in the drinkingwater and has enacted a number of rules that specifical-ly address pathogens and source water quality (USEPA,2007). Current USEPA regulations under the Safe Drink-ing Water Act require that drinking water utilities identi-fy and quantify a specific set of microbial contaminantsin source waters. In addition, utilities are required toidentify the best available technology for the treatment ofcontaminated source waters. However, while treatedwater is considered safe, less is known about microbialpollution in water distribution systems. As discussedlater, recent research indicates the potential for adversehealth effects associated with bacterial growth in waterdistribution pipes and home plumbing systems. Aero-solization of some types of pathogenic bacteria, such asLegionella pneumophila, in home-heated water system(bathing showers and hot tubs) are also considered ahealth risk factor (pulmonary disease).

Pathogens pose a risk to human health through var-ious uses of water as well. Waterborne disease outbreaksassociated with recreational water are attributed to theuse of public pools, hot tubs, rivers, lakes, beaches, andwater fountains (Dzuiban et al., 2006). Many outbreaksoccur in pools and hot tubs that have been chlorinated.

Shellfish contaminated via waterborne routes can impacthuman health through the food chain. For example, a re-cently reported outbreak of hepatitis A was caused byoyster consumption (Bialek et al., 2007). Also recently,contaminated irrigation or runoff water was speculatedas possible pathways to a recent, nationwide outbreakcaused by E. coli O157:H7 (CDC, 2006).

PATHOGENS IN ENGINEERED SYSTEMS

Bacterial growth in water distribution systems hasbeen investigated for several decades. For example,Baylis (1930, cited in van der Kooij, 2003) reported col-iform growth in sediments accumulating in water distri-bution pipes. Researchers have found that mycobacteri-al numbers were substantially higher in the water distri-bution systems (on average 25,000-fold) than those col-lected immediately downstream from the water treatmentfacilities, indicating that mycobacteria grow in the distri-bution system (Falkinham et al. 2001). In recent years,there has been great concern about the presence ofemerging pathogens such as Legionella spp., Mycobac-terium spp., and Aeromonas spp. and other opportunisticpathogens in water distribution pipes and home plumb-ing systems. It should be noted that both Legionnaires’disease (a serious, life-threatening pneumonia) and Pon-tiac fever (a mild, flue like illness) are caused by membersof the genus Legionella. Two pathogens of concern inwater systems are discussed below.

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Pathogens in Natural and Engineered Water Systems: Emerging Issues ... cont’d.

Figure 1. Uses of Water and Pathogen Pathways in Natural and Engineered Water Systems.

Mycobacterium avium (M. avium) is an environmentalopportunistic bacterial pathogen, a normal inhabitant ofnatural waters and drinking water distribution systems,and has been listed on the USEPA’s Candidate Contami-nant List (CCL) (Table 1). They are opportunistic patho-gens of humans, animals, and fish and are very slowgrowing organisms, reproducing, at the most, 1 genera-tion/day. Slow growth makes them poor competitors forresources against other microorganisms; however, theirdie-off rates are lower and they show evidence of adapta-tion to harsh conditions. Mycobacteria are very hy-drophobic, which makes them resistant to antimicrobialagents. Their hydrophobicity leads to their attachment tosurfaces and proliferation on them, i.e. biofilm formation(attached populations of microbial cells and polysaccha-ride that form at interfaces and surfaces such as pipewalls) because they can grow in waters with relatively loworganic nutrient concentrations (> 50 µg assimilable or-ganic carbon per liter). Biofilm formation increases M. avium persistence in drinking water distribution sys-tems. Hydrophobicity is also a major contributor to M. avium’s aerosolization and concentration in aerosolsabove waters. M. avium numbers increase in recirculat-ing hot water distribution systems in hospitals, officebuildings, and apartment houses. The use of showersand hot tubs (spas) are risk factors for development of M. avium pulmonary disease because of M. aviumaerosolization potential and the entrainment of M. avium- rich biofilms into aerosols upon water flow.

Legionella pneumophila (Legionella), an emergingbacterial pathogen (Table 1), was discovered following anoutbreak of pneumonia amongst attendees at the 1976American Legion convention in Philadelphia, Pennsylva-nia. It is a Gram negative bacterium, commonly found infresh water environments and replicates in protozoa asintracellular parasite. The bacterium Legionella exists inlow numbers in natural waters. However, higher watertemperatures in engineered water systems promote rapidgrowth of Legionella (Fields et al., 2002).

Legionnaires’ disease has been linked to aerosolsgenerated from air conditioners, humidifiers, decorativefountains, whirlpool spas, and industrial or residentialcooling towers. An opportunistic pathogen, the bacteri-um causes disease in individuals with weakened immu-nity. Large outbreaks of disease have been reported innews media. For example: in Virginia, September 1996, awhirlpool spa display at a home improvement storecaused 23 cases of the disease, and killed 2; in Spain in2001, a hospital cooling tower caused 449 cases of thedisease and killed 2; in September 2005, a Toronto out-break in an assisted living facility killed 17 people. Themajority of Legionella pneumonia cases, however, aresporadic community infections which are possibly-underdiagnosed and underestimated. In the U.S., it is estimat-ed that up to 20,000 cases occur annually, and mortali-ty of the disease is up to 26 percent (Benin et al., 2002).In 2003, the Centers for Disease Control (CDC) reportedincreased cases in the Mid-Atlantic region, increasing to178 cases versus 48 cases in 2002. Disease control andprevention depends on a rapid, sensitive, and quantita-tive detection method. Also, since the bacterium is

common in engineered systems, it is important to estab-lish an acceptable level of the bacteria in these water sys-tems.

Maintaining a disinfectant residual in water distribu-tion systems can be an effective way to prevent bacterialgrowth of some pathogens in water distribution pipes.However, the challenge is how to balance the need for ad-equate disinfection while reducing the potential chronichealth effects of chlorination without selecting for otherpotential pathogens. Also, it should be noted that somepathogens are resistant to chlorination (M. avium is re-sistant to chlorine, chloramines, chlorine dioxide, andozone). Recently, across the U.S. there is a trend to use chloramines instead of free chlorine as disinfectant,since there is less potential for generating harmful by-products. However, little is known about potential healtheffects of switching to chloramines that could arise fromincreased microbial survival or growth in the drinkingwater distribution system. Furthermore, a recent studyindicated that a switch from free chlorine to chloraminesdisinfectant triggered lead release from home plumbingpipes (Edward and Dudi, 2004). Exposure to high levelsof lead in drinking water can pose significant health riskto society.

FUTURE DIRECTIONS AND RESEARCH NEEDS

Water quality monitoring and public health assur-ance is routinely performed through enumeration of fecalindicator bacteria in both potable and recreational wa-ters. Fecal coliforms are used as indicators based on theexpectations that the only source of these organisms isdirectly from fecal contamination, and that the fate andtransport of fecal coliforms reflects that of waterbornepathogens. Furthermore, environmental water standardsare based on risk to human health from human sewagecontamination. However, conventional indicator organ-isms such as fecal coliforms and enterococci have manydrawbacks in terms of recreational water quality assess-ment. Fecal indicator bacteria have many sources, in-cluding storm water runoff and environmental reservoirssuch as sediments. Many waterborne pathogens do nothave the same fate and transport characteristics as fecalindicators, and these relationships are unknown forother pathogens.

To help understand the sources, fate, and predictiverelationships of waterborne pathogens, efforts are beingmade in mathematical modeling for prediction, and mi-crobial and fecal source tracking. The latter involvestracking target chemicals found exclusively in sewage in-stead of pathogens or indicator microorganisms. The mi-crobial source tracking target requires the followingcharacteristics: (1) unique to a particular host species orgroup, (2) all host individuals are carriers of the target,(3) it has wide geographic range and it is temporally sta-ble, (4) the assay is inexpensive but reliable, and (5) quantitative data can be generated.

A new candidate indicator for human fecal source de-tection is the human polymavirus, which can be detect-ed by polymerase chain reaction (PCR). To date there hasbeen too little work at exploring the challenges posed by

Volume 9 • Number 3 Water Resources IMPACT • 13


waterborne pathogens that are normal inhabitants ofwater (e.g., pathogenic Vibrio species) and drinking waterdistribution systems (e.g., M. avium). Here, the environ-ment or the water system is the source and the presenceof the pathogen does not represent contamination from afecal source. Achievement of the goal of providing rapid,accurate identification of emerging waterbornepathogens pose the following challenges to microbiolo-gists and epidemiologists: (1) measure risks ascribed todifferent host sources, (2) measure risks posed by differ-ent pathogens, (3) accurately describe the ecology of in-dicators and pathogens, and (4) develop more sensitiveand more rapid detection technologies for indicators andpathogens.

Some candidate detection or measurement methodsfor these emerging pathogens include quantitative PCR(polymerase chain reaction), RT-PCR (reverse transcrip-tase-PCR), multiplex PCR, NASBA (nucleic acid based se-quence amplification) (isothermal method), transcriptionmediated amplification (isothermal and reverse tran-scriptase), and micro arrays. An incomplete list of poten-tial research needs include: (1) quantitative risk assess-ment for a wide variety of waterborne pathogens, (2) iden-tification of suitable indicators for waterborne microbesand viruses, (3) epidemiology studies of the relationshipbetween pathogens and human health outcomes inrecreational waters that are not impacted by pointsources, (4) methods for rapid detection of pathogens andindicators, (5) quantitative microbial source trackingmethods for human and other sources of fecal contami-nation, (6) linkage of indicator bacterial numbers withrisk of human disease, (7) linkage of microbial sourcetracking results with the different indicators, (8) under-standing indicator organism and pathogen survival andgrowth in sediments and sands, (9) improved modeling ofmicrobial transport and fate in the environment, (10) im-proved methods to determine the causes of increased an-tibiotic resistance due to chemical contamination in theenvironment, (11) rapid detection methods for recre-ational marine waters to be able to update swimming ad-visories accurately, (12) identification of pipe surfacetreatments to reduce M. avium and Legionella numbersin biofilms, and (13) identification of cost-effective meth-ods for reducing the number of emerging pathogens inwaters

ACKNOWLEDGEMENT

Parts of this article were presented at the Pathogen Re-search Symposium sponsored by the Virginia Water ResourcesResearch Center at Virginia Tech on November 2, 2006.Authors

REFERENCES

Benin A.L. R. F. Benson, and R.F. Besser, 2002. Trends in Legionnaires’ Disease, 1980-1998: Declining Mortality and New Patterns of Diagnosis. Clin. Infect. Dis. 35(9):1039-1046.

Bialek S.R., P.A. George, G.L. Xia, M.B. Glatzer, M.L. Motes, J.E. Veazey, R.M. Hammond, T. Jones, Y.C. Shieh, J. Wamnes, G. Vaughan, Y. Khudyakov, and A.E. Fiore, 2007. Use of Molecular Epidemiology to Confirm a MultiState Outbreak of Hepatitis A Caused by Consumption of Oysters. Clin Infect Dis. 44:838-840.

CDC (Centers for Disease Control and Prevention), 2006. Ongo-ing Multi-State Outbreak of Escherichia coli Serotype O157:H7 Infections Associated With Consumption of Fresh Spinach – United States, September 2006. MMWR Morb Mortal Wkly Rep.55:1045-1046.

Dziuban E.J., J.L. Liang, G.F.Craun, V. Hill, P.A.Yu, J. Painter, M.R. Moore, L.R. Calderon, S.L. Roy, and M.J. Beach, 2006. Surveillance for Waterborne Disease and Outbreaks Associat-ed With Recreational Water – United States, 2003-2004. Centers for Disease Control and Prevention (CDC), MMWR Surveill Summ. 55:1-30.

Edward, M. and A. Dudi, 2004. Role of Chlorine and Chlo-ramines in Corrosion of Lead-Bearing Plumbing Materials. Jour. Amer. Water Works Assoc. 96(10):69-81.

Egli, T. and A. Rust, 2002. Pathogens in (Drinking) Water? In:Risk Factors in Water. EAWAG News 53e, pp. 26-28. http://e-collection.ethbib.ethz.ch/ecol-pool/journal/eawag_ news_e/53_2002.pdf, accessed March 17, 2007.

Falkinham, J.O., C.D. Norton, and M.W. LeChevallier, 2001. Factors Affecting Numbers of Mycobacterium avium, Mycobac-terium intracellulare, and Other Mycobacteria in Drinking Water Distribution Systems. Applied and Environmental Microbiology 67(3):1225-1231.

Fields, B.M, R.F. Benson, and R.E. Besser, 2002. Legionella and Legionnaires’ Disease: 25 Years of Investigation. Clinical Microbiology Reviews 15(3):506-526.

Keeling, W.G., C. Hagedorn, B. A. Wiggins, and K.R. Porter, 2005. Bacterial Source Tracking: Concept and Application to TMDL. In: Total Maximum Daily Load: Approaches and Challenges, T. Younos (Editor). PenWell Books, Tulsa, Okla-homa, pp. 207-246.

Lindquist, H.D.A., 1999. Emerging Pathogens of Concern in Drinking Water. EPA-600-R-99/070, U.S. Environmental Protection Agency, Washington, D.C.

USEPA (U.S. Environmental Protection Agency), 2005. Drinking Water Contaminant List and Regulatory Determinations. http://www.epa.gov/safewater/ccl/index.html, accessed March 17, 2007.

USEPA (U.S. Environmental Protection Agency), 2007. Setting Standards for Safe Drinking Water. http://www.epa.gov/safewater/standard/setting.html, accessed February 23, 2007.

van der Kooji, D., 2003. Managing Regrowth in Drinking Water Distribution Systems. In: Heterotrophic Plate Counts and Drinking Water Safety, J. Bartram et al. (Editors). World Health Organization (WHO). IWA Publications, London, United Kingdom.

Tamim YounosResearch Professor of Water ResourcesVirginia Tech, Department of GeographyBlacksburg, VA 24061-0444(540) 231-8039 / Fax: (540) 231-6673

[email protected]@[email protected]@vsu.edu

Tamim Younos is a Research Professor of Water Re-sources in the Geography Dept. and Associate Directorfor Research in the Virginia Water Resources ResearchCenter at Virginia Tech. His areas of interest include en-vironmental hydrology, watershed monitoring and as-sessment, and water quality management.

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