oEFA
United States
Environmental Protection
Agency
Office of Water
(4607)
EPA815-R-99-010
April 1999
Guidance Manual for Compliance
with the Interim Enhanced Surface
Water Treatment Rule:
Turbidity Provisions
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DISCLAIMER
This manual provides public water systems and drinking water primacy agencies
with guidance for complying with the turbidity provisions found within the
Interim Enhanced Surface Water Treatment Rule (IESWTR).
This document was issued in support of EPA regulations and policy initiatives
involving development and implementation of the IESWTR. This document is
EPA guidance only. It does not establish or affect legal rights or obligation. EPA
decisions in any particular case will be made applying the laws and regulation on
the basis of specific facts when permits are issued or regulations promulgated.
Mention of trade names or commercial products does not constitute an EPA
endorsement or recommendation for use.
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ACKNOWLEDGMENTS
The Environmental Protection Agency gratefully acknowledges the assistance of the
members of the Microbial and Disinfection Byproducts Federal Advisory Committee and
Technical Work Group for their comments and suggestions to improve this document.
EPA also wishes to thank the representatives of drinking water utilities, researchers, and
the American Water Works Association for their review and comment. Finally the
Agency would like to recognize the individual contribution of the following:
Randy Goss, City of Austin
Ray Letterman, SyracuseCUniversity
Mike Pickel, City of Philadelphia Water Department
Paul Gilbert Snyder, California Department of Health Services
Brian Tarbuck, Maine Department of Human Services
John Miller, Los Angeles Department of Water and Power
Blake Atkins, U.S. EPA Region VI
Eric Bissonette, U.S. EPA, OGWDW, TSC
Jon Bender, U.S. EPA, OGWDW, TSC
Ralph Flournoy, U.S. EPA Region VII
Jeff Robichaud, U.S. EPA, OGWDW
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CONTENTS
1. INTRODUCTION 1-1
1.1 Purpose of Document 1-1
1.2 Summary of Chapters 1-1
2. TURBIDITY REQUIREMENTS: IESWTR 2-1
2.1 Introduction 2-1
2.2 Regulatory Requirements 2-3
2.2.1 Applicability ...2-3
2.2.2 Combined Filter Effluent Monitoring 2-3
2.2.3 Individual Filter Monitoring 2-8
2.3 Reporting and Recordkeeping 2-9
2.3.1 System Reporting Requirements 2-9
2.3.2 State Reporting Requirements 2-11
2.3.3 System Recordkeeping Requirements 2-12
2.3.4 State Recordkeeping Requirements 2-12
2.4 Additional Compliance Issues 2-12
2.4.1 Schedule 2-12
2.4.2 Individual Filter Follow-up Action : 2-13
2.4.3 Notification 2-15
2.4.4 Variances and Exemptions 2-18
2.5 References 2-19
3. TURBIDITY METHODS & MEASUREMENT 3-1
3.1 Introduction 3-1
3.2 Approved Turbidity Methods 3-1
3.2.1 EPA Method 180.1 3-1
3.2.2 Standard Method 2130B 3-1
3.2.3 Great Lakes Instrument Method 2 (GLI2) 3-2
3.3 Turbidimeters 3-2
3.3.1 Bench Top Turbidimeters 3-2
3.4 Quality Assurance/Quality Control 3-6
3.4.1 Quality Assurance Organization and Responsibilities 3-7
3.4.2 Quality Assurance Objectives 3-7
3.4.3 Standard Operating Procedures 3-7
3.4.4 Sampling Strategy and Procedures 3-9
3.4.5 Calibration 3-12
3.4.6 Data Screening, Validation, and Reporting 3-15
3.4.7 Performance and System Audits 3-16
3.4.8 Preventative Maintenance 3-16
3.5 Data Collection and Management 3-17
3.5.1 Data Collection Methods 3-17
3.5.2 Data Management 3-19
3.6 References 3-20
4. APPROACH FOR COMPLIANCE 4-1
4.1 Compliance Approach for Turbidity Requirements 4-1
4.2 System Evaluation & Plant Optimization 4-1
4.2.1 Coagulation/Rapid Mixing 4-2
4.2.2 Flocculation .1 4-4
4.2.3 Sedimentation 4-6
4.2.4 Filtration 4-7
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4.3 Process Enhancements/Technologies 4-8
4.4 References 4-9
5. INDIVIDUAL FILTER SELF ASSESSMENT 5-1
5.1 Introduction 5-1
5.2 Developing A Filter Run Profile .-. 5-4
5.3 Assessing Hydraulic Loading Conditions Of Filter 5-6
5.4 Assessing Condition & Placement Of Filter Media 5-7
5.5 Assessing Backwash Practices 5-8
5.6 Assessing Condition Of Support Media/Underdrains 5-12
5.7 Assessing Rate-Of-Flow Controllers & Filter Valve Infrastructure 5-14
5.8 References , 5-14
6. COMPREHENSIVE PERFORMANCE EVALUATION 6-1
6.1 Introduction 6-1
6.2 Background OryThe CPE 6-1
6.3 Components of a CPE 6-4
6.3.1 Performance Assessment 6-4
6.3.2 Major Unit Process Evaluation 6-7
6.3.3 Factors Limiting Performance 6-8
6.4 Activities During a CPE 6-10
6.5 CPE Quality Control 6-13
6.6 Next Steps : •. 6-15
6.7 References 6-16
7. IMPORTANCE OF TURBIDITY 7-1
7.1 Overview 7-1
7.2 Turbidity: Definition, Causes, and History as a Water Quality Parameter 7-1
7.3 Turbidity's Significance to Human Health 7-4
7.3.1 Waterborne Disease Outbreaks 7-5
7.3.2 The Relationship Between Turbidity Removal and Pathogen Removal 7-7
7.4 References 7-10
8. PARTICLES CONTRIBUTING TO TURBIDITY 8-1
8.1 Introduction 8-1
8.2 Characteristic Properties of Particles 8-1
8.2.1 Particle Settling 8-2
8.2.2 Particle Density and Size Distribution 8-3
8.3 Inorganic Particles 8-4
8.3.1 Naturally Occurring Minerals 8-4
8.4 Organic Particles 8-5
8.4.1 Synthetic Organics 8-5
8.4.2 Natural Organic Matter (NOM) 8-5
8.4.3 Total Organic Carbon (TOC) 8-6
8.4.4 Organic Disinfection By-products (DBFs) .- 8-6
8.5 Particles of Biotic Origin 8-7
8.5.1 Protozoans...,. 8-7
8.5.2 Viruses 8-8
8.5.3 Algae 8-9
8.5.4 Bacteria 8-9
8.6 Particles Added or Created During Treatment 8-13
8.6.1 Coagulants 8-13
8.6.2 Powdered Activated Carbon (PAC)....; 8-14
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8.6.3 Recycle Flows 8-14
8.7 Electrokinetic Properties of Particles 8-14
8.7.1 Electrical Potential 8-15
8.7.2 Electrical Double Layer Theory ..... 8-15
8.8 References 8-18
9. TURBIDITY IN SOURCE WATER 9-1
9.1 Introduction 9-1
9.2 Occurrence of Turbidity in Surface Water Supplies 9-1
9.2.1 Rivers .' '. ......9-1
9.2.2 Lakes and Reservoirs 9-7
9.3 Ground Water Under the Direct Influence (GWUDI) .....9-9
9.4 Additional Watershed Considerations 9-9
9.5 Water Sources Occurrence in the United States .9-10
9.6 References 9-11
10. TURBIDITY THROUGH THE TREATMENT PROCESSES 10-1
10.1 Introduction 10-1
10.2 Intake Facilities/Raw Water Screening 10-2
10.2.1 Intake Location 10-2
10.2.2 Intake Depth 10-2
10.2.3 Effect on Turbidity 10-3
10.3 Pre-sedimentation 10-3
10.3.1 Effect on Turbidity .'....:.' 10-3
10.4 Coagulation 10-4
10.4.1 Chemicals ; 10-4
10.4.2 Rapid Mixing 10-6
10.4.3 Effect on Turbidity 10-6
10.5 Flocculation 10-7
10.5.1 Slow Mixing 10-7
10.5.2 Detention Time 10-7
10.5.3 Effect on Turbidity „ 10-8
10.6 Sedimentation/Clarification 10-8
10.6.1 Effect on Turbidity 10-11
10.7 Filtration ...... .10-11
10.7.1 Conventional Rapid Sand Filters 10-11
10.7.2 Slow Sand Filters , 10-14
10.7.3 Pressure Filters .....10-15
10.7.4 Precoat/Diatomaceous Earth Filters 10-16
10.7.5 Effect on Turbidity .r. 10-16
10.8 Membrane Processes 10-17
10.8.1 Effect on Turbidity , 10-20
10.9 Recycle Streams 10-20
10.9.1 Sources of Recycle Streams 10-21
10.9.2 Recycle Stream Quantity and Quality 10-21
10.9.3 Point of Recycle Stream Return 10-22
10.9.4 Effect on Turbidity 10-22
10.10 References 10-22
11. BASIC TURBIDIMETER DESIGN AND CONCEPTS 11-1
11.1 Introduction 11-1
11.2 Turbidimeter Measuring Principles 11-1
1 L2.1 Light Source 11-2
11.2.2 Sample Volume '. 11-3
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11.2.3Photodetector 11-3
11.3 Turbidimeter Design Configurations , 11-3
11.3.1 Design Standards 11-4
11.3.2 Single Beam Design 11-5
11.3.3 Ratio Design 11-6
11.3.4 Modulated Four Beam Design 11-7
11.3.5 Surface Scatter Design 11-7
11.3.6 Transmittance Design 11-8
11.4 Types of Turbidimeters 11-8
11.4.1 Bench Top Turbidimeters 11-9
11.4.2 Portable Turbidimeters 11-9
11.4.3 On-Line Turbidimeters 11-10
11.5 References 11-11
APPENDIX A LIST OF DEFINITIONS
APPENDIX B DETERMINATION OF TURBIDITY BY NEPHELOMETRY
APPENDIX C TURBIDITY STANDARD METHOD
APPENDIX D TURBIDITY GLI METHOD 2
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Figures
Figure 2-1. Flowchart of IESWTR General Requirements 2-2
Figure 2-2. Flowchart of IESWTR Combined Filter Provisions for Conventional and Direct Filtration
Systems 2-5
Figure 2-3. Flowchart of IESWTR Combined Filter Provisions for Alternative Filtration Technologies 2-6
Figure 2-4. Flowchart of SWTR Combined Filter Provisions for Slow Sand and Diatomaceous Earth
Filtration 2-7
Figure 2-5. Flowchart of IESWTR Individual Filter Provisions 2-10
Figure 2-6. Example Filter Profile 2-13
Figure 3-1. Calibration Checklist 3-16
Figure 5-1. Filter Run Profile - Turbidity (NTU) vs. Time 5-5
Figure 5-2. Box Used for Excavation 5-7
Figure 5-3. Box Excavation Demonstration 5-8
Figure 5-4. Pipe Bed Expansion 5-11
Figure 5-5. Bed Expansion Device 5-11
Figure 5-6. On-Line Turbidimeters Showing Performance Problems Due to Inoperable
Rate-of-Flow Controllers ; 5-14
Figure 6-1. Capable Plant Model 6-2
Figure 6-2. An Example of Performance Assessment Using Historical Data 6-6
Figure 6-3. An Example of Individual Filter Data Collected During CPE 6-7
Figure 6-4. Example Performance Potential Graph 6-8
Figure 6-5. Activities During a CPE 6-12
Figure 7-1. Typical Sources of Turbidity in Drinking Water 7-2
Figure 7-2. Jackson Candle Turbidimeter 7-3
Figure 7-3. Nephelometric Turbidimeter 7-5
Figure 7-4. Particles of Turbidity May Provide Protection for Microorganisms 7-6
Figure 7-5. Relationship Between Removal of Giardia and Turbidity 7-8
Figure 7-6. Relationship Between Removal of Cryptosporidium and Turbidity 7-8
Figure 8-1. Particle Size Spectrum 8-2
Figure 8-2. Cryptosporidium Life Cycle 8-8
Figure 8-3. Plankton and Other Surface Water Algae 8-10
Figure 8-4. Filter Clogging Algae 8-11
Figure 8-5. Examples of Bacteria and Fungi Forms 8-12
Figure 8-6. Double Layer Theory (Guoy-Stern Colloidal Model) 8-16
Figure 9-1. The Natural Hydrologic Cycle 9-2
Figure 9-2. Typical Watershed 9.3
Figure 9-3. Sources of Contaminants in Raw Water Supplies 9-4
Figure 9-4. Turbidity Increase Event on the Missouri River at Omaha 9-6
Figure 9-5. Turbidity Increase Due to Forest Fire in Buffalo Creek 9-8
Figure 9-6. Raw Water Source by Water System Size 9-10
Figure 10-1. A Typical Conventional Water Treatment System 10-1
Figure 10-2. The Alum Coagulation Diagram and Its Relationship to Zeta Potential 10-5
Figure 10-3. Circular Radial-flow Clarifier 10-9
Figure 10-4. Plate Settlers Used for High Rate Sedimentation 10-10
Figure 10-5. Accelerator Solids Contact Unit 10-10
Figure 10-6. Typical Rapid Sand Filter 10-12
Figure 10-7. Typical Covered Slow Sand Filter Installation 10-14
Figure 10-8. Cross Section of a Typical Pressure Filter 10-15
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Figure 10-9. Typical Filter Run Showing Progress of Floe Penetration and Effluent Turbidity 10-17
Figure 10-10. Pressure-Driven Membrane Process Application Guide 10-18
Figure 10-11. Typical Spiral-Wound Reverse Osmosis Membrane Module Driven 10-19
Figure 10-12. Typical Hollow Fine-Fiber Reverse Osmosis Membrane Module 10-19
Figure 10-13. Representation of hollow-fiber UF module 10-20
Figure 11-1. Scattered Light at 90°... 11-1
Figure 11-2. Angular Patterns of Scattered Light from Particles of Different Sizes 11-2
Figure 11-3. Basic Nephelometer 11-5
Figure 11-4. Ratio Turbidimeter 11-6
Figure 11-5. Modulated Four-Beam Turbidimeter 11-7
Figure 11-6. Surface Scatter Turbidimeter 11-8
Figure 11-7. Bench Top Turbidimeter 11-9
Figure 11-8. Portable Turbidimeter 11-9
Tables
Table 5-1. Individual Filter Self Assessment Worksheet 5-3
Table 5-2. WTP Performance Deviation Trigger Events 5-5
Table 5-3.. General Guide to Acceptable Filter Hydraulic Loading Rates 5-6
Table 5-4. Guidelines Regarding Acceptable Backwashing Practices 5-9
Table 5-5. Example Filter Support Gravel Placement Grid 5-13
Table 6-1. CCP Optimized Performance Goals 6-3
Table 6-2. Evaluation Team Capabilities 6-13
Table 6-3. Quality Control Checklist for Completed CPEs 6-14
Table 7-1. Cryptosporidium Outbreaks vs. Finished Water Turbidity 7-7
Table 7-2. Studies on the Relationship between Turbidity Removal and Protozoa Removal 7-9
Table 10-1. Typical Feed Pressures for Pressure Driven Membrane Processes 10-18
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CONTENTS
ACRONYMS
AIDs Acquired Immune Deficiency Syndrome
ASCE American Society of Civil Engineers
AWWA American Water Works Association
AWWARF American Water Works Association Research Foundation
CB Calibration Blank
CCP Composite Correction Program
CDC Centers for Disease Control
CDHS California Department of Health Services
CFR Code of Federal Regulations
CPE Comprehensive Performance Evaluation
CSO Combined Sewer Overflow
CTA Comprehensive Technical Assistance
CWSS Community Water System Survey
DCS Distributed Control Systems
DBF Disinfection Byproduct
DE Diatomaceous Earth
EPA Environmental Protection Agency
FR Federal Register
FTU Formazin Turbidity Units
GLI Great Lakes Instrument
GWUDI Ground Water Under the Direct Influence
HIV Human Immunodeficiency Virus
IESWTR Interim Enhanced Surface Water Treatment Rule
IPC Instrument Performance Check Solution
ISO International Organization for Standardization
JTU Jackson Turbidity Units
LAN Local Area Network
LCR Linear Calibration Range
LRB Laboratory Reagent Blank
LED Light Emitting Diode
MCL Maximum Contaminant Level
MSDS Material Safety Data Sheet
NAS National Academy of Sciences
NCEPI National Center for Environmental Publications and Information
NOM Natural Organic Matter
NPDES National Pollution Discharge Elimination System
NPDWR National Primary Drinking Water Regulation
NSF National Science Foundation
NTU Nephelometric Turbidity Units
PAC Powdered Activated Carbon
PC1 Personal Computer
PCAL Primary Calibration Standard
PCBs Polychlorinated Biphenyls
PWS Public Water System
QA Quality Assurance
QC Quality Control
QCS Quality Control Sample
RIA Regulatory Impact Analysis
SCADA Supervisory Control and Data Acquisition
SCAL Secondary Calibration Standards
SDWA Safe Drinking Water Act
SOP Standard Operating Procedure
SS Secondary Calibration Standard
SSS Stock Standard Suspension
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SWTR
TOC
THM
Surface Water Treatment Rule
Total Organic Carbon
Trihalomethane
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1. INTRODUCTION
1.1 Purpose of Document
The Interim Enhanced Surface Water Treatment Rule (ffiSWTR) establishes a number of
provisions related to the performance of filters in drinking water treatment. These
provisions include treatment technique requirements restricting turbidity levels in the
combined filter effluent, as well as monitoring requirements for individual filters at
conventional and direct filtration plants. These requirements are designed to decrease the
risk from waterborne microbial pathogens by limiting levels of paniculate material in
finished water.
The objective of the guidance manual is to provide public water systems (PWSs) with
guidance for complying with the turbidity provisions found within the ffiSWTR. The
primary audience of the guidance manual is utility personnel at public water systems
which utilize filtration andthe staff of state drinking water programs that work with
PWSs to protect water quality.
The document is divided into two sections. The first section contains technical
information regarding specific requirements of the IESWTR relating to turbidity and is
intended for experienced operators and others in the regulated community. The second
section of the document provides background on concepts surrounding turbidity and
serves as a primer for less experienced operators and individuals.
1.2 Summary of Chapters
As noted, the document is broken up into two sections. The first section of the manual
outlines the specific requirements of the rule and includes detailed information specific to
the rule. Section 1 consists of Chapters 2 through 6:
Chapter 2 - Turbidity Requirements: IESWTR
Chapter 2 outlines the regulatory requirements, reporting and recordkeeping
requirements, and additional compliance aspects of the IESWTR related to
turbidity. Flow charts are provided which graphically demonstrate the
requirements.
Chapter 3 - Turbidity Methods & Measurement
Chapter 3 provides information regarding approved turbidity methods, analytical
issues associated with turbidimeters and turbidity measurement, quality assurance
and quality control issues, and data collection and management issues.
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1. INTRODUCTION
Chapter 4 - Approach for Compliance
Chapter 4 provides information on EPA's suggested approach for compliance
with turbidity requirements of the IESWTR. Plant optimization is the focus of
this chapter, and areas are highlighted which, in the experience of the Agency and
other water professionals, most often can be improved to optimize water treatment
at systems. Two programs, the Composite Correction Program and the
Partnership for Safe Drinking Water, are briefly discussed as systems are
encouraged to utilize these programs to optimize plant performance.
Chapter 5 - Individual Filter Self Assessment
Chapter 5 provides detailed guidance on conducting a filter self assessment.
Necessary components are discussed including conducting filter profiles,
assessing hydraulic loading conditions, and assessing support media and
underdrains. Systems may be required to conduct an individual filter self
assessment based on individual filter monitoring results.
Chapter 6 - Comprehensive Performance Evaluation
Chapter 6 provides a general overview of the Composite Correction Program
(CCP) and specifically the first component of the CCP, the Comprehensive
Performance Evaluation (CPE). Fundamental concepts are discussed including
major CPE components, standard CPE activities and CPE quality control
measures. Systems may be required to arrange for a CPE based on individual filter
monitoring results.
The second section of the manual provides background in order to provide readers with
an understanding of basic concepts that underlie turbidity and the provisions found in the
DESWTR.
Chapter 7 - Importance of Turbidity
Chapter 7 provides an introduction into the importance of turbidity and includes
background on turbidity as a water quality parameter. It discusses the significance
of turbidity to human health, provides a brief discussion of waterborne disease
outbreaks, and the relationship between turbidity removal and pathogen removal.
Chapter 8 - Particles Contributing to Turbidity
Chapter 8 provides an overview of the characteristics of particles which contribute
to turbidity. The section provides brief discussions of organic, inorganic, and
biotic particles, particles created during the treatment process, and a brief
introduction into the electrokinetic properties of particles.
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1. INTRODUCTION
Chapter 9 - Turbidity in Source Water
Chapter 9 describes the various factors that effect turbidity in rivers, lakes and
reservoirs, and groundwater under the direct influence (GWUDI). The chapter
also includes information on other watershed considerations that effect turbidity.
Chapter 10 - Turbidity Through the Treatment Process
Chapter 10 provides a general description of the typical treatment processes
intended to remove suspended solids and reduce turbidity as well as information
on the level of turbidity reduction that is commonly achieved through each.
Chapter 11 - Basic Turbidimeter Design and Concepts
Chapter 11 provides readers with basic information on turbidimeter designs,
measuring principals, design configurations, and various types of turbidimeters.
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2. TURBIDITY REQUIREMENTS: IESWTR
2.1 Introduction
Under the 1996 Safe Drinking Water Act (SDWA) Amendments, EPA must supplement
the existing 1989 Surface Water Treatment Rule (SWTR) with the Interim Enhanced
Surface Water treatment Rule (IESWTR) to improve protection against waterborne
pathogens. Key provisions established in the IESWTR include (USEPA, 1998):
. A maximum contaminant level goal (MCLG) of zero for Cryptosporidium; 2-
log (99 percent) Cryptosporidium removal requirements for systems that filter;
• Strengthened combined filter effluent turbidity performance standards;
« Individual filter turbidity monitoring provisions;
• Disinfection benchmark provisions to assure continued levels of microbial
protection while facilities take the necessary steps to comply with new
disinfection byproduct standards;
• Inclusion of Cryptosporidium in the definition of ground water under the
direct influence of surface water (GWUDI) and in the watershed control
requirements for unfiltered public water systems;
Requirements for covers on new finished water reservoirs; and
Sanitary surveys for all surface water systems regardless of size.
Figure 2-1 presents the general IESWTR requirements.
The following chapter outlines the regulatory requirements, reporting and recordkeeping
requirements, and additional compliance aspects of the IESWTR related to turbidity.
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2. TURBIDITY REQUIREMENTS: IESWTR
Figure 2-1. Flowchart of IESWTR General Requirements
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2. TURBIDITY REQUIREMENTS: IESWTR
2.2 Regulatory Requirements
As described above, the Interim Enhanced Surface Water Treatment Rule contains several
key provisions including strengthened combined filter effluent turbidity performance
standards and individual filter turbidity monitoring.
2.2.1 Applicability
Entities potentially regulated by the IESWTR are public water systems that use surface
water or ground water under the direct influence of surface water and serve at least
10,000 people (including Industries, State, Local, Tribal, or Federal governments). To
determine whether your facility may be regulated by this action, you should carefully
examine the applicability criteria in subpart H (systems subject to the Surface Water
Treatment Rule) and.subpart P (subpart H systems that serve 10,000 or more people) of
the final rule.
Systems subject to the turbidity provisions of the IESWTR are a subset of systems subject
to the IESWTR, which utilize rapid granular filtration (i.e., conventional filtration
treatment and direct filtration) or other filtration processes (excluding slow sand and
diatomaceous earth filtration).
2.2.2 Combined Filter Effluent Monitoring
Under the SWTR, a subpart H system which provides filtration treatment must monitor
turbidity in the combined filter effluent. Turbidity measurements must be performed on
representative samples of the system's filtered water every four hours (or more frequently)
that the system serves water to the public. A public water system may substitute
continuous turbidity monitoring for grab sample monitoring if it validates the continuous
measurement for accuracy on a regular basis using a protocol approved by the State.
The turbidity performance requirements of the IESWTR require that all surface water
systems which use conventional treatment or direct filtration and serve a population
^ 10,000 people must meet two distinct combined filter effluent limits: a maximum limit
and a 95% limit. These limits, set forth in the IESWTR, are outlined below for the
different types of treatment employed by systems.
Conventional Treatment or Direct Filtration
For conventional and direct filtration systems (including those systems utilizing in-line
filtration), the turbidity level of representative samples of a system's filtered water
(measured every four hours) must be less than or equal to 0.3 NTU in at least 95 percent
of the measurements taken each month. The turbidity level of representative samples of a
system's filtered water must not exceed 1 NTU at any time.
Conventional filtration is defined as a series of processes including coagulation,
flocculation, sedimentation, and filtration resulting in substantial particulate removal.
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2. TURBIDITY REQUIREMENTS: IESWTR
Direct filtration is defined as a series of processes including coagulation and filtration but
excluding sedimentation resulting in substantial particle removal. Figure 2-2 presents a
flowchart of the combined filter provisions for conventional and direct filtration systems.
Other Treatment Technologies (Alternative Filtration)
For other filtration technologies (those technologies other than conventional, direct, slow
sand or diatomaceous earth filtration), a system may demonstrate to the State, using pilot
plant studies or other means, that the alternative filtration technology, in combination
with disinfection treatment, consistently achieves 99.9 percent removal and/or
inactivation ofGiardia lamblia cysts and 99.99 percent removal and/or inactivation of
viruses, and 99 percent removal of Cryptosporidium oocysts. For a system that makes
this demonstration, then representative samples of a system's filtered water must be less
than or equal to a value determined by the State which the State determines is indicative
of2-log Cryptosporidium removal, 3-log Giardia removal, and 4-log virus removal in
at least 95 percent of the measurements taken each month and the turbidity level of
representative samples of a system's filtered water must at no time exceed a maximum
turbidity value determined by the State. Figure 2-3 presents a flow chart of combined
filter provisions for alternative filtration technologies. Examples of such technologies
include bag or cartridge filtration, microfiltration, and reverse osmosis. EPA recommends
a protocol similar to the "Protocol For Equipment Verification Testing for Physical
Removal of Microbiological and Particulate Contaminants," prepared by NSF
International with support from EPA. Information regarding this protocol may be found
at NSF's website at: http://www.nsf.org/verification/verification.html.
Slow Sand & Diatomaceous Earth Filtration
The IESWTR does not contain new turbidity provisions for slow sand or diatomaceous
earth (DE) filtration systems. Utilities utilizing either of these filtration processes must
continue to meet the requirements for their respective treatment as set forth in the SWTR
(1 NTU 95%, 5 NTU max). Figure 2-4 presents a flowchart of combined filter provisions
for slow sand and diatomaceous earth filtration.
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2. TURBIDITY REQUIREMENTS: IESWTR
Turbidity Performance Requirements
Systems must meet the following provisions:
- measurements are taken every 4 hours of
representative samples of the system's filtered
water
- turbidity must at no time exceed 5 NTU
turbidity must be taken less than or equal to 1.0
NTU in at least 95 percent of measurements
taken each month.
Was
monitoring
conducted
every 4
hours?
MONITORING
VIOLATION
Reporting and Recordkeepino Requirements
Within 10 days after the end of the month, the system must
provide a report of turbidity measurements to the State which
includes:
- Total number of measurements taken during the month
- Number and percentage of measurements taken less than or
equal to 0.3 MTU
- Date and value of any measurements taken during the month
which exceed 1 NTU.
Were monitoring
esults recorded?
Did the
system report to
the State within 10
days after the
end of the
month?
REPORTING/RECORDKEEPING
VIOLATION
Did turbidity
exceed 1 NTU at
any time?
TREATMENT TECHNIQUE
VIOLATION
Was turbidity
less than or equal to 0.3
NTU in at least 95 percent of the
measurements taken
each month?
TREATMENT TECHNIQUE
VIOLATION
System is in compliance with the IESWTR.
YES
Figure 2-2. Flowchart of IESWTR Combined Filter Provisions for
Conventional and Direct Filtration Systems
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2. TURBIDITY REQUIREMENTS: IESWTR
Alternative Filtration Requirements
System must demonstrate to the
State that the alternative filtration
technology, In combination with
disinfection consistently meets:
- 3-log Giardia and 4-log virus
inactivatlon
• 2-log Cryptosporldium removal
TREATMENT TECHNIQUE
VIOLATION
Alternative Turbidity Performance Requirements
- The State will set turbidity performance
requirements that the system must meet 95
percent of the time (95TH PERCENTILE)
- Th« State will set turbidity performance
requirements that the system may not exceed at
any time (MAXIMUM)
These performance require/nents will be set at a
level that consistently achieves 3-log Giardia
removal/inactivation, 4-log virus removal/
Inactivation and 2-log Cryptosporidium removal
Reporting and Recordkeepinq Requirements
Within 10 days after the end of the month,
system must provide a report of turbidity
measurements to the State which includes:
- Total number of measurements taken during
the month
- Number and percentage measurements
less than or equal 95TH PERCENTILE
- Date and value of any measurements taken
during the month which exceed MAXIMUM
Were monitoring
results recorded?
Did
system report
to State within
10 days after the
end of the
month?
REPORTING/RECORDKEEPING
VIOLATION
Did
turbidity exceed
tate set maximum
ny time?
TREATMENT TECHNIQUE VIOLATION
Was turbidity
less than or equal to state
set 95 percent in at least 95
ercent of the measurements
taken each month?
TREATMENT TECHNIQUE VIOLATION
System in compliance with the IESWTR
^
Figure 2-3. Flowchart of IESWTR Combined Filter Provisions for Alternative
Filtration Technologies
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2. TURBIDITY REQUIREMENTS: IESWTR
Turbidity Performance Requirements
Systems must meet the following
provisions:
- measurements are taken every 4
hours of representative samples of the
system's filtered water (as required
under SWTR)
- turbidity must at no time exceed 5
NTU
- turbidity must be less than or equal to
1.0 NTU in at least 95 percent of
measurements taken each month.
Was
monitoring
conducted
every 4
hours?
YES
Reduced Sampling
State may reduce
sampling frequency if
it determines that
less frequent
monitoring is
sufficient to indicate
effective filtration
performance.
Reporting and Recordkeepino Requirements
Within 10 days after the end of the month, the
system must provide a report of turbidity
measurements to the State which includes:
- Total number of measurements taken during
the month
- Number and percentage of measurements
taken less than or equal to 1 NTU
- Date and value of any measurements taken
during the month which exceed 5 NTU.
Did State
reduce
sampling
requency?
Were
reduced
monitoring
requirements
met?
YES ^ Were monitoring
esults recorded?
Did the
system report to
the State within 10
days after the
end of the
month?
REPORTING/RECORDKEEPING
VIOLATION
Did turbidity
exceed 5 NTU at
any time?
TREATMENT TECHNIQUE
VIOLATION
Was turbidity
less than or equal to 1
NTU in at least 95 percent of the
measurements taken
each month?
TREATMENT TECHNIQUE
VIOLATION
System is in compliance with the IESWTR.
Figure 2-4. Flowchart of SWTR Combined Filter Provisions for Slow Sand
and Diatomaceous Earth Filtration
April 1999
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Systems which Utilize Lime Softening
Systems which practice lime softening may experience difficulty in meeting the turbidity
performance requirements due to residual lime floe carryover inherent in the process.
EPA is allowing such systems to acidify turbidity samples prior to measurement using a
protocol approved by States. The chemistry supporting this decision is well documented
in environmental chemistry texts.
EPA recommends that acidification protocols lower the pH of samples to <8.3 to ensure
an adequate reduction in carbonate ions and corresponding increase in bicarbonate ions.
Acid should consist of either hydrochloric acid or sulfuric acid of Standard Lab Grade.
Care should be taken when adding acid to samples. Operators should always follow the
sampling guidelines outlined in Section 3.4.4 of this document.
If systems choose to use acidification, EPA recommends systems maintain documentation
regarding the turbidity with and without acidification as well as pH values and quantity of
acid added to the sample.
2.2.3 Individual Filter Monitoring
In addition to the combined filter effluent monitoring discussed above, those systems that
use conventional treatment or direct filtration (including in-line filtration) must conduct
continuous monitoring of turbidity for each individual filter using an approved method in
§141.74(a) and must calibrate turbidimeters using the procedure specified by the
manufacturer. Systems must record the results of individual filter monitoring every 15
minutes. If the individual filter is not providing water which contributes to the combined
filter effluent, (i.e., it is not operating, is filtering to waste, or recycled) the system does
not need to record or monitor the turbidity for that specific filter.
Systems which utilize filtration other than conventional or direct filtration are not
required to conduct individual filter monitoring although EPA recommends such
systems consider individual filter monitoring.
If there is a failure in continuous turbidity monitoring equipment, the system must
conduct grab sampling every four hours in lieu of continuous monitoring, but must return
to 15 minute monitoring no more than five working days following the failure of the
equipment.
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2. TURBIDITY REQUIREMENTS: IESWTR
2.3 Reporting and Recordkeeping
There are distinct reporting and recordkeeping requirements for the turbidity provisions
of the IESWTR for both systems and States.
2.3.1 System Reporting Requirements
Under the DBSWTR, systems are tasked with specific reporting requirements associated
with combined filter effluent monitoring and individual filter effluent monitoring.
Combined Filter Effluent Reporting
Turbidity measurements as required by §141.173 must be reported within 10 days after
the end of each month the system serves water to the public. Information that must be
reported includes:
1. The total number of filtered water turbidity measurements taken during the month.
2. The number and percentage of filtered water turbidity measurements taken during
the month which are less than or equal to the turbidity limits specified in
§141.173. (0.3 NTU for conventionaTand direct and the turbidity limit established
by the State for other filtration technologies)
3. The date and value of any turbidity measurements taken during the month which
exceed 1 NTU for systems using conventional filtration treatment or direct
filtration and the maximum limit established by the State for other filtration
technologies.
This reporting requirement is similar to the reporting requirement currently found under
the SWTR.
Individual Filter Requirements
Systems utilizing conventional and direct filtration must report that they have conducted
individual filter monitoring in accordance with the requirements of the IESWTR within
10 days after the end of each month the system serves water to the public. Figure 2-5
presents a flowchart of individual filter requirements.
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2. TURBIDITY REQUIREMENTS: IESWTR
Figure 2-5. Flowchart of IESWTR Individual Filter Provisions
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2. TURBIDITY REQUIREMENTS: IESWTR
Additionally, systems must report individual filter turbidity measurements within 10 days
after the end of each month the system serves water to the public only //"measurements
demonstrate one of the following:
• Any individual filter has a measured turbidity level greater than 1.0 NTU in
two consecutive measurements taken 15 minutes apart. The system must
report the filter number, the turbidity measurement, and the date(s) on which
the exceedance occurred. In addition, the system must either produce a filter
profile for the filter within 7 days of the exceedance (if the system is not able
to identify an obvious reason for the abnormal filter performance) and report
that the profile has been produced or report the obvious reason for the
exceedance.
• Any individual filter has a measured turbidity level of greater than 0.5 NTU in
two consecutive measurements taken 15 minutes apart at the end of the first
four hours of continuous filter operation after the filter has been backwashed
or otherwise taken offline. The system must report the filter number, the
turbidity, and the date(s) on which the exceedance occurred. In addition, the
system must either produce a filter profile for the filter within 7 days of the
exceedance (if the system is not able to identify an obvious reason for the
abnormal filter performance) and report that the profile has been produced or
report the obvious reason for the exceedance.
• Any individual filter has a measured turbidity level of greater than 1.0 NTU in
two consecutive measurements taken 15 minutes apart at any time in each of
three consecutive months. The system must report the filter number, the
turbidity measurement, and the date(s) on which the exceedance occurred. In
addition, the system shall conduct a self-assessment of the filter.
• Any individual filter has a measured turbidity level of greater than 2.0 NTU in
two consecutive measurements taken 15 minutes apart at any time in each of
two consecutive months. The system must report the filter number, the
turbidity measurement, and the date(s) on which the exceedance occurred. In
addition, the system shall contact the State or a third party approved by the
State to conduct a comprehensive performance evaluation.
2.3.2 State Reporting Requirements
Under §142.15, each State which has primary enforcement responsibility is required to
submit quarterly reports to the Administrator of the EPA on a schedule and in a format
prescribed by the Administrator, which includes:
1. New violations by public water systems in the State during the previous quarter
with respect to State regulations adopted to incorporate the requirements of
national primary drinking water regulations.
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2. TURBIDITY REQUIREMENTS: IESWTR
2. New enforcement actions taken by the State during the previous quarter against
public water systems with respect to State regulations adopted to incorporate the
requirements of national primary drinking water standards.
Any violations or enforcement actions with respect to turbidity, would be included in the
quarterly report noted above. EPA has developed a State Implementation guidance
manual which includes additional information on State reporting requirements.
2.3.3 System Recordkeeping Requirements
Systems must maintain the results of individual filter monitoring taken under §141.174
for at least three years. These records must be readily available for State representatives
to review during Sanitary Surveys or other visits.
2.3.4 State Recordkeeping Requirements
Records of turbidity measurements must be kept for not less than one-year. The
information retained must be set forth in a form which makes possible comparison with
limits specified in §§141.71, 141.73, 141.173, and 141.175.
Records of decisions made on a system-by-system and case-by-case basis under
provisions of part 141, subpart H or subpart P, must be made in writing and kept by the
State (this includes records regarding alternative filtration determinations). EPA has
developed a State Implementation guidance manual which includes additional
information on State recordkeeping requirements.
2.4 Additional Compliance Issues
The following section outlines additional compliance issues associated with the IESWTR.
These include Schedule, Individual Filter Follow-up Action, Notification, and Variances
and Exemptions.
2.4.1 Schedule
The IESWTR was published on December 16, 1998, and became effective on February
16, 1999.
The SDWA requires, within 24 months following the promulgation of a rule, that the
Primacy Agencies adopt any State regulations necessary to implement the rule. Under
Sec. 1413, these rules must be at least as stringent as those required by EPA. Thus,
primacy agencies must jpromulgate regulations which are at least as stringent as the
IESWTR by December 17, 2000.
Beginning December 17, 2001, systems serving at least 10,000 people must meet the
turbidity requirements in §141.173.
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2. TURBIDITY REQUIREMENTS: IESWTR
2.4.2 Individual Filter Follow-up Action
Based on the monitoring results obtained through continuous filter monitoring discussed
in Section 2.3 of this chapter, a system may have to conduct one of the following follow-
up actions due to persistently high turbidity levels at an individual filter:
• Filter profile
• Individual filter self assessment
• Comprehensive Performance Evaluation.
These specific requirements are found in §141.175(b) (l)-(4).
•-J
Abnormal Filter Operations- Filter Profile
A filter profile must be produced if no obvious reason for abnormal filter performance
can be identified. A filter profile is a graphical representation of individual filter
performance based on continuous turbidity measurements or total particle counts versus
time for an entire filter run, from startup to backwash inclusively that includes assessment
of filter performance while another filter is being backwashed. The run length during this
assessment should be representative of typical plant filter runs. The profile should include
an explanation of the cause of any filter performance spikes during the run. An example
filter profile is included in Figure 2-6.
Turbidity (NTU)
8:20 A 12:20 A 4:20 P 8:20 P 12:20 A 4:20 8:20 A 12:20 A 4:20
Time
Figure 2-6. Example Filter Profile
April 1999
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2, TURBIDITY REQUIREMENTS: IESWTR
Additional information regarding filter profiles is found in Chapter 5, Individual Filter
Self Assessment. Examples of possible abnormal filter operations which may be obvious
to operators include the following:
• Outages or maintenance activities at processes within the treatment train
• Coagulant feed pump or equipment failure
• Filters being run at significantly higher loading rates than approved
It is important to note that while the reasons for abnormal filter operation may appear
obvious they could be masking other reasons which are more difficult to identify. These
may include situations such as:
• Disruption in filter media
• Excessive or insufficient coagulant dosage
• Hydraulic surges due to pump changes or other filters being brought
on/offline.
Systems need to use best professional judgement and discretion when determining when
to develop a filter profile. Attention at this stage will help systems avoid the other forms
of follow-up action described below.
Individual Filter Self-Assessment
A system must conduct an individual filter self-assessment for any individual filter that
has a measured turbidity level of greater than 1.0 NTU in two consecutive measurements
taken 15 minutes apart in each of three consecutive months. The system must report the
filter number, the turbidity measurement, and the dates on which the exceedances
occurred. Chapter 5 discusses how to conduct an individual filter assessment or self-
assessment.
Comprehensive Performance Evaluation
A system must conduct a comprehensive performance evaluation (CPE) if any individual
filter has a measured turbidity level of greater than 2.0 NTU in two consecutive
measurements taken 15 minutes apart in two consecutive months. The system must
report the filter number, the turbidity measurement, and the date(s) on which the
exceedance occurred. The system shall contact the State or a third party approved by the
State to conduct a comprehensive performance evaluation.
Chapter 6 briefly discusses how to conduct a Comprehensive Performance Evaluation.
Additionally, EPA has developed a guidance document called, Handbook: Optimizing
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2. TURBIDITY REQUIREMENTS: JESWTR
Water Treatment Plant Performance Using the Composite Correction Program
(EPA/625/6-91/027, Revised August 1998).
2.4.3 Notification
The IESWTR contains two distinct types of notification: State and public. It is important
to understand the differences between each and the requirements of each.
State Notification
Systems are required to notify States under §141.31. Systems must report to the State
within 48 hours, the failure to comply with any national primary drinking water
regulation. The system within 10 days of completion of each public notification required
pursuant to §141.32,,must submit to the State a representative copy of each type of notice
distributed, published, posted, and/or made available to persons served by the system
and/or the media. ,
The water supply system must also submit to the State (within the time stated in the
request made by the State) copies of any records required to be maintained under §141.33
or copies of any documents then in existence which the State or the Administrator is
entitled to inspect pursuant to the authority of section 1445 of the Safe Drinking Water
Act or the equivalent provisions of the State Law.
Public Notification
The IESWTR specifies that the public notification requirements of the Safe Drinking
Water Act (SOWA) and the implementation regulations of 40 CFR §141.32 must be
followed. These regulations divide public notification requirements into two tiers. These
tiers are defined as follows:
. TIER1
- Failure to comply with MCL
- Failure to comply with prescribed treatment technique
— Failure to comply with a variance or exemption schedule '
• TIER 2
— Failure to comply with monitoring requirements
- Failure to comply with a testing procedure prescribed by a NPDWR
- Operating under a variance/exemption. This is not considered a violation
but public notification is required.
The IESWTR classifies violations of §§141.70, 141.71(c), 141.72 and 141.73, and
141.173 (i.e., treatment technique requirements as specified in §141.76) as Tier 1
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2. TURBIDITY REQUIREMENTS: IESWTR
violations and violations of §§141.74, and 141.174 as Tier 2 violations. Violations of
§§141.75 and 141.175 (reporting requirements) do not require public notification.
There are certain general requirements which all public notices must meet. All notices
must provide a clear and readily understandable explanation of the violation, any
potential adverse health effects, the population at risk, the steps the system is taking to
correct the violation, the necessity of seeking alternate water supplies (if any) and any
preventative measures the consumer should take. The notice must be conspicuous, and
not contain any unduly technical language, unduly small print, or similar problems. The
notice must include the telephone number of the owner or operator or designee of the
public water system as a source of additional information concerning the violation where
appropriate. The notice must be bi- or multilingual if appropriate.
Tier 1 Violations
In addition, the public notification rule requires that when providing notification on
potential adverse health effects in Tier 1 public notices and in notices on the granting and
continued existence of a variance or exemption, the owner or operator of a public water
system must include certain mandatory health effects language. For violations of
treatment technique requirements for filtration and disinfection, the mandatory health
effects language is:
The United States Environmental Protection Agency (EPA) sets drinking
water standards and has determined that the presence of microbiological
contaminants are a health concern at certain levels of exposure. If water is
inadequately treated, microbiological contaminants in that water may cause
disease. Disease symptoms may include diarrhea, cramps, nausea, and
possibly jaundice, and any associated headaches and fatigue. These
symptoms, however, are not just associated with disease-causing organisms
in drinking water, but also may be caused by a number of factors other than
your drinking water. EPA has set enforceable requirements for treating
drinking water to reduce the risk of these adverse health effects. Treatment
such as filtering and disinfecting the water removes or destroys
microbiological contaminants. Drinking water which is treated to meet EPA
requirements is associated with little to none of this risk and should be
considered safe.
Further the owner or operator of a community water system must give a copy of the most
recent notice for any Tier 1 violations to all new billing units or hookups prior to or at the
time service begins.
The medium for performing public notification and the time period in which notification
must be sent varies with the type of violation and is specified in § 141.32. For Tier 1
violations, the owner or operator of a public water system must give notice:
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2. TURBIDITY REQUIREMENTS: 1ESWTR
1. By publication in a local daily newspaper as soon as possible but in no case later
than 14 days after the violation or failure. If the area does not have a daily
newspaper, then notice shall be given by publication in a weekly newspaper of
general circulation in the area, and
2. By either direct mail delivery or hand delivery of the notice, either by itself or
with the water bill no later than 45 days after the violation or failure. The Primacy
Agency may waive the requirement if it determines that the owner or operator has
corrected the violation within 45 days.
Although the IESWTR does not specify any acute violations, the Primacy Agency may
specify some Tier 1 violations as posing an acute risk to human health; examples might
include:
• A waterborne outbreak in an unfiltered supply
• Turbidity of a filtered water exceeds 1.0 NTU at any time
• Failure to maintain a disinfectant residual of at least 0.2 mg/L in the water
being delivered to the distribution system.
For these violations or any others defined by the Primacy Agency as 'acute' violations, the
system must furnish a copy of the notice to the radio and television stations serving the
area as soon as possible but in no case later than 72 hours after the violation. Depending
on the circumstances particular to the system/as determined by the Primacy Agency, the
notice may instruct that all water be boiled prior to consumption.
Following the initial notice, the owner or operator must give notice at least once every
three months by mail delivery (either by itself or with the water bill), or by hand delivery,
for as long as the violation or failures exist.
There are two variations on these requirements. First, the owner or operator of a
community water system in an area not served by a daily or weekly newspaper must give
notice within 14 days after the violation by hand delivery or continuous posting of a
notice of the violation. The notice must continue for as long as the violation exists.
Notice by hand delivery must be repeated at least every three months for the duration of
the violation.
Secondly, the owner or operator of a noncommunity,water system (i.e., one serving a
transitory population) inay give notice by hand delivery or continuous posting of the
notice in conspicuous places in the area served by the system. Notice must be given
within 14 days after the violation. If notice is given by posting, then it must continue as
long as the violation exists. Notice given by hand delivery must be repeated at least every
three months for as long as the violation exists.
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2, TURBIDITY REQUIREMENTS: IESWTR
Tier 2 Violations
For Tier 2 violations (i.e., violations of 40 CFR §§141.74 and 141.174) notice must be
given within three months after the violation by publication in a daily newspaper of
general circulation, or if there is no daily newspaper, then in a weekly newspaper. In
addition, the owner or operator shall give notice by mail (either by itself or with the water
bill) or by hand delivery at least once every three months for as long as the violation
exists. Notice of a variance or exemption must be given every three months from the date
it is granted for as long as it remains in effect.
If the area is not served by a daily or weekly newspaper, the owner or operator of a
community water system must give notice by continuous posting in conspicuous places in
the area served by the system. This must continue as long as the violation exists or the
variance or exemption remains in effect. Notice by hand delivery must be repeated at
least every three months for the duration of the violation or the variance or exemption.
For noncommunity water systems, the owner or operator may give notice by hand
delivery or continuous posting in conspicuous places; beginning within three months of
the violation or the variance or exemption. Posting must continue for the duration of the
violation or variance or exemption, and notice by hand delivery must be repeated at least
every three months during this period.
The Primacy Agency may allow for owner or operator to provide less frequent notice for
minor monitoring violations (as defined, by the Primacy Agency if EPA has approved the
Primacy Agency's substitute requirements contained in a program revision application).
2.4.4 Variances and Exemptions
As with the SWTR, no variances from the requirements in §141 are permitted for subpart
H systems.
Under Section 1416(a), EPA or a State may exempt a public water system from any
requirements related to an MCL or treatment technique of an NPDWR if it finds that (1)
due to compelling factors (which may include economic factors such as qualification of
the PWS as serving a disadvantaged community), the PWS is unable to comply with the
requirement or implement measures to develop an alternative source of water supply; (2)
the exemption will not result in an unreasonable risk to health; and (3) the PWS was in
operation on the effective date of the NPDWR, or for a system that was not in operation
by that date, only if no reasonable alternative source of drinking water is available to the
new systems; and (4) management or restructuring changes (or both) cannot reasonably
result in compliance with the Act or improve the quality of drinking water.
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2. TURBIDITY REQUIREMENTS: IESWTR
2.5 References
1. AWWA. 1991. Guidance Manual for Compliance with the Filtration and
Disinfection Requirements for Public Water Systems Using Surface Water Systems.
Denver, CO.
2. Logsdon, G., M.M. Frey, T.D. Stefanich, S.L. Johnson, D.E. Feely, J.B. Rose, and M.
Sobsey. 1994. "The Removal and Disinfection Efficiency of Lime Softening
Processes for Giardia and Viruses." AWWARF, Denver, CO.
3. Sawyer, C.N., P.L. McCarty, and G.F. Parkin. 1994. Chemistry for Environmental
Engineering. Fourth Edition. McGraw Hill, New York, NY.
4. USEPA. 1998. "National Primary Drinking Water Regulations: Interim Enhanced
Surface Water Treatment Rule; Final Rule." 63 FR 69477. December 16.
5. Viessman, W. and M.J. Hammer. 1993. Water Supply and Pollution Control. Fifth
Edition. Harper Collins, New York, NY.
6. Von Huben, H. 1995. Water Treatment: Principles and Practices of Water Supply
Operations. Second Edition. AWWA.
7. Von Huben, H. 1995. Basic Science Concepts and Applications: Principles and
Practices of Water Supply Operations. Second Edition. AWWA.
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3. TURBIDITY METHODS & MEASUREMENT
3.1 Introduction
The IBS WTR requires systems to measure the turbidity of combined filter effluent and
individual filter effluent. Because these measurements are used for reporting and
compliance purposes (as described in Chapter 2), accurate measurement and strict
adherence to approved methods is of paramount importance. The following chapter
describes approved methods, analytical issues associated with turbidimeters, quality
assurance and quality control issues, and data collection and management.
3.2 Approved Turbidity Methods
Currently, the Agency has approved three methods for the measurement of turbidity as
described in §141.74. Systems must utilize turbidimeters which conform to one of the
following methods for compliance purposes. If the instrument does not conform, then it
may not be used for monitoring under the requirements of the IESWTR. A brief
description of each of the methods is found below.
3.2.1 EPA Method 180.1
EPA method 180.1, "Determination of Turbidity by Nephelometry", is found in the
Agency's publication, Methods for Chemical Analysis of Water and Wastes. The method
is based upon a comparison of the intensity of light scattered by the sample under defined
conditions with the intensity of light scattered by a standard reference suspension. The
higher the intensity of scattered light, the higher the turbidity. Readings, in NTUs, are
made in a nephelometer designed according to specifications laid out in the method. A
primary standard suspension is used to calibrate the instrument. A secondary standard
suspension is used as a daily calibration check and is monitored periodically for
deterioration using one of the primary standards. See Appendix B for EPA Method 180.1.
3.2.2 Standard Method 2130B
Standard Method 2130B, found in Standard Methods (1995), is similar to EPA Method
180.1. The method is also based on a comparison of the intensity of light scattered by the
sample under defined conditions with the intensity of light scattered by a standard
reference suspension under the same conditions. The higher the intensity of scattered
light, the higher the turbidity. Formazin polymer is used as the primary standard
reference suspension. See Appendix C for Standard Method 21 SOB.
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3.2.3 Great Lakes Instrument Method 2 (GLI 2)
Great Lakes Instruments Method 2 is an instrument specific, modulated four beam
method using a ratiometric algorithm to calculate the turbidity value from the four
readings that are produced. The comparison is also based on a comparison of light
scattered by the sample under defined conditions with the intensity of the light scattered
by the reference suspension. The higher the intensity of the scattered light, the higher the
turbidity. Readings in NTUs, are made in a nephelometer designed according to
specifications in the method. See Appendix D for Great Lakes Instrument Method 2.
3.3 Turbidimeters
As noted, turbidimeters must conform to one of the three approved methods for
measuring turbidity. For regulatory reporting purposes, either an on-line or a benchtop
turbidimeter may be used. A system may find it appropriate to utilize on-line
turbidimeters to monitor individual filter effluent, while utilizing either a benchtop or on-
line turbidimeter for combined filter effluent. If a system chooses to utilize on-line units
for monitoring combined filter effluent, they must validate the continuous measurements
for accuracy on a regular basis using a protocol approved by the State.
3.3.1 Bench Top Turbidimeters
Bench top units are used exclusively for grab samples and include glass cuvettes for
holding the sample. Measurement with bench top units requires strict adherence to the
manufacturer's sampling procedure to reduce errors from dirty glassware, air bubbles in
the sample, and particle settling. Plant operators should read and be fully familiar with
the operation manuals for all bench-top turbidimeters used in the plant. Many •
maintenance and operational issues are specific to turbidimeter make and model, and
instruments are usually supplied with a thorough user's manual.
Bench-top Basics
Although durable, turbidimeters need to be stored and operated in a safe and protected
environment. Moisture and dust need to be prevented from entering and accumulating
inside turbidimeters. Humidity also needs to be controlled to prevent condensation
inside the instrument. Turbidimeters should be located where they will not be exposed to
corrosive chemicals or fumes. Chemicals such as chlorine and acids can ruin
instrumentation. Finally, turbidimeters should be located in an environment that is
temperature controlled, at a consistent temperature between 0°and 50°C.
Generally the instrument should be left on at all times (unless otherwise specified in the
user's manual). If any instrument is not left on at all times, it may require a warm-up
period before sample analysis.
The length of the sample piping or tubing from the sampling location to the point where
the sample is drawn off should be minimized. Long sample lines can lead to problems
with biological fouling and scaling which can impact turbidity values. It is best to limit
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3. TURBIDITY METHODS & MEASUREMENT
sample lines to ten feet or less. Long sample lines can also cause confusion due to the lag
time as the sample travels through the piping. The longer the lag time, the more difficult
it is to correlate turbidity fluctuations to actual process changes that might be occurring.
Sample taps in piping should be located on the sides of pipes. Samples taken from the
top or bottom will not accurately represent the turbidity of the water. Samples taken from
the bottom will often times contain sediment while samples from the top may contain a
greater number of air bubbles. Ideally, sample taps should be angled into the water flow
at an angle of 0-45 degrees and extend into the center of the flow channel. Sample taps
should be located away from items which disturb flow such as fittings, bends, meters, or
pump discharges. '•-
Operation and Maintenance
Preventative and routine maintenance should be carried out according to manufacturers'
instructions. Do not make repairs to the instrument unless specified in the instruction
manual. Even if a repair can be made, consider sending the unit back to the
manufacturer. Keep track of maintenance and repair on a log sheet located next to the
unit.
Maintain benchtop instruments in accordance with manufacturer recommendations.
Inspect the cleanliness of bulb and lenses daily. Clean lenses, light sources, and other
glassware with appropriate materials to avoid scratches and dust accumulation. Avoid the
use of chemicals or other materials when cleaning unless instructed by the manufacturer.
Do not touch the optical components with bare hands (soft cotton gloves are
recommended). Recalibrate the instrument after any significant maintenance or cleaning
procedure.
Bench-top turbidimeters, just like most instruments, have an effective service life.
Various elements within the instrument can deteriorate over time and with repeated use.
Daily usage can result in wear on electronics due to movement and temperature.
Microprocessor based electronics are also prone to memory loss during power supply
fluctuations. Service personnel can often provide insight on instrument life and can make
recommendations for specific maintenance items. Since turbidimeters have become
integral parts of a water treatment plant operation and reporting, it is imperative to
maintain instruments and budget for replacements.
Replace incandescent turbidimeter lamps annually, or more frequently if recommended
by the manufacturer. Recalibrate the instrument whenever optical components (e.g.,
lamp, lens, photodetector, etc.) of the turbidimeter are replaced or cleaned.
Calibration
Calibration is an essential part of accurate turbidity measurement, and as such, instrument
calibration should be verified on a daily basis. Calibration verification can be completed
using primary or secondary standards. If verification indicates significant deyiation from
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the standard (true) value (greater than ±10%), thoroughly clean and recalibrate the
instrument using a primary standard. If problems persist the manufacturer should be
contacted. At a minimum regardless of calibration results, turbidimeters should be
thoroughly cleaned and calibrated with primary standards at least quarterly.
After calibration, performance of the turbidimeter should be verified with a secondary
standard. If the instrument has internal electronic diagnostics designed to assist in
determining proper calibration, the operator should use these tools to verify proper
calibration and operation. Calibration is discussed in significant detail in Section 3.4.5.
3.3.2 On-Line Turbidimeters
On-line turbidimeters are process instruments which sample a side stream split-off from
the treatment process. The sample flows through the on-line instrument for measurement
and then wasted to a drain or recycled through the treatment process.
Selection of the flow rate through on-line turbidimeters should be in accordance with
manufacturer specifications. The sample flow should be constant without variations due
to pressure changes or surges. Installation of a flow control device such as a rotameter on
the sample line can eliminate fluctuations in flow rate.
To the extent possible, turbidimeter samples should be obtained directly from the process
flow and not pumped to a remote instrument location. Pumped samples can be
non-representative of the process flow due to changes in the character of particles caused
by the pump or the addition of bubbles due to rapid pressure changes. If pumping is
required, the use of peristaltic pumps are desirable, as they have the least amount of
impact on particles in the sample.
Several of the on-line turbidimeters available today have various sample chamber sizes.
It is important to note that the size of the sampling chamber will affect the instrument
response. The path length of the light passing through the sample is inversely
proportional to resolution of the instrument. Therefore, the larger the sample size, the
more likely that the turbidity reading will be dampened.
Installation
On-line turbidimeters should be installed in accordance with manufacturer instructions.
The goal of proper installation is to ensure proper operation, easy access for maintenance
and calibration procedures that should be performed, and obtain an accurate,
representative and timely sample.
Carefully consider the location of the sample tap. The tap should provide a representative
sample of the water being monitored. If an individual filter is being monitored, locate the
sample tap as close to the filter as possible. The tap should provide a sample from the
centerline of the pipe, as opposed to the bottom or top of the pipe where sediment or air
bubbles may interfere with sample integrity. Ideally, the sample will flow by gravity from
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the sample tap to the turbidimeter without a sample pump. Sample pumps may have an
effect on turbidimeter measurements.
The length of conduit between the sample tap and the instrument should be minimized, to
the extent possible. Lengthy sample runs can delay instrument response time and may
cause changes in sample quality (i.e., settling of particulate matter, increased opportunity
for biological growth). In selecting sample tubing or pipe, the required sample flow rate
and pressure should be considered. Sample lines of insufficient diameter may not provide
adequate flow to the instrument and may be prone to clogging. Excessively large
diameter sample lines will delay the instrument response and may permit settling of
paniculate matter. Line flushing valves and ports may be necessary depending on the
water being sampled. Carefully consider these items when installing an on-line
turbidimeter.
A good sample tap location and plumbing arrangement will minimize the potential for
bubble formation. Most on-line turbidimeters have the capability to eliminate minor
bubble interference through baffles and/or degassing chambers, but if the problem is
severe, the turbidity measurements may be affected.
The turbidimeter should be installed in a location that provides easy access for routine
maintenance and calibration procedures. It should be protected from direct sunlight,
extreme temperatures (<32°F/0°C and >104°F/40°C), and rapid temperature fluctuations.
It should also be firmly mounted so as to avoid vibrations, which may interfere with the
accuracy of turbidity measurements.
The turbidimeter drain should provide easy access for flow verification and collection of
calibration verification samples. Flow rate and calibration verification samples are
important in establishing data validity. Therefore, hard piping the turbidimeter drain
without an airgap is not recommended.
Operation and Maintenance
Preventative and routine maintenance should be carried out according to manufacturer's
instructions. A regular cleaning schedule is necessary to ensure proper operation of on-
line turbidimeters. A weekly inspection is recommended, but this frequency may vary
depending on the instrument location and raw water quality. Warm or turbid samples '
may dictate more frequent cleaning. An instrument mounted in a dusty environment may
also require more frequent cleaning. Items to inspect and clean include, but are not
limited to, lenses, light sources, sample reservoirs, air bubble traps, and sample lines.
Clean lenses, light sources, and other glassware with appropriate materials to avoid
scratches and dust accumulation. During maintenance, care needs to be taken not to
touch the surface of any "bulbs or detectors without proper covering on the fingers. Soft
cotton gloves should be worn when changing bulbs or detectors. Recalibrate the
instrument after any significant maintenance or cleaning procedure.
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On-line turbidimeters, just like most instruments, have an effective service life. Various
elements within the instrument can deteriorate over time and with repeated use. Daily
usage can result in wear on electronics due to movement and temperature.
Microprocessor based electronics are also prone to memory loss during power supply
fluctuations. Many on-line units with unsealed sensor electronics are vulnerable to
damage by outside contamination and splashing. Service personnel can often provide
insight on instrument life and can make recommendations for specific maintenance items.
Since turbidimeters have become integral parts of a water treatment plant operation and
reporting, it is imperative to maintain instruments and budget for replacements.
Incandescent turbidimeter lamps should be replaced annually or more frequently if
recommended by the manufacturer. The instrument should be recalibrated whenever
optical components (e.g., lamp, lens, photodetectors, etc.) of the turbidimeter are
replaced.
Systems should consider verifying sample flow rates on a weekly basis. Flow rates
should be within a range specified by the manufacturer.
Calibration
EPA recommends that on-line turbidimeters have calibration verified on a weekly basis,
if being utilized for combined filter effluent monitoring. Less frequent verification may
be more appropriate for turbidimeters monitoring individual filter turbidity, but EPA
recommends verification be conducted with a frequency of at least once per month.
Calibration verification can be completed using primary standards, secondary standards,
or by comparison to a properly calibrated turbidimeter. If verification indicates
significant deviation from the standard (true) value (greater than ±10%), the instrument
should be thoroughly cleaned and recalibrated using a primary standard. If problems
persist, the manufacturer should be contacted. Regardless of calibration results,
turbidimeters should be thoroughly cleaned and calibrated with primary standards at least
quarterly.
EPA does not recommend calibrating on-line instruments by comparison with a
bench-top turbidimeter. It has been determined that this procedure is likely to introduce
unacceptable levels of error into the calibration.
After calibration, verify instrument performance with a secondary standard or by
comparison with another properly calibrated instrument. If the instrument has internal
electronic diagnostics designed to assist in determining proper calibration, the operator
should use these tools to verify proper calibration and operation. For additional
information on calibration see Section 3.4.5.
3.4 Quality Assurance/Quality Control
Although using proper techniques and equipment is an important part of conducting
proper turbidity measurements, it is imperative that operators are aware of factors in the
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processes which may lead to poor quality data. Such factors include poor lab techniques,
calculation mistakes, malfunctioning or poorly functioning instrumentation, and out-of-
date and deteriorated chemicals. Development of a Quality Assurance and Quality
Control (Q'A/QC) Program ensures that lapses do not occur which allow inaccurate
measurements or erroneous reporting. Systems may want to establish plans to provide
assurance that measurements are being made accurately and consistently.
3.4.1 Quality Assurance Organization and Responsibilities
A good QA/QC plan provides clear organization and defines who is responsible for each
of the aspects laid out in the plan and what their responsibilities are. This section should
include a list of the positions (by title) that have responsibilities and what those
responsibilities are. The appropriate training or skills necessary for each of the positions
listed should also be included.
/
3.4.2 Quality Assurance Objectives
The objectives of the Quality Assurance Program need to be laid out and understood by
the staff members. Objectives should be succinct, and clear. SOPs should be developed
with input from staff, enabling them to effectively conduct work activities in compliance
with applicable requirements. Systems may wish to include one primary objective,
followed by a number of goals which all relate to the objective. An example might look
like the following:
The primary objective of the Quality Assurance Program is to ensure that turbidity
measurements are accurate and consistent. Based on this, the goals of the Quality
Assurance Program at a generic water treatment plant are the following:
• To adhere to proper sampling techniques as set forth in the Standard Operating
Procedures.
• To maintain and operate all turbidimeters at the plant properly in accordance
with manufacturer instructions and Standard Operating Procedures.
• To perform calibration of instruments on a routine and as-necessary basis.
• To communicate and report all, malfunctions, abnormalities, or problems
which may compromise the ability to accurately and consistently measure
turbidity.
3.4.3 Standard Operating Procedures
Standard Operating Procedures (SOPs) are a way to ensure that activities are
accomplished in a consistent manner, and that each activity is understood by all involved.
SOPs should be kept as simple as possible in order to ensure that each operator is
consistent in undertaking the task at hand. The title of the procedure should be clear,
concise, and descriptive of the equipment, process, or activity. Systems should consider
adopting SOPs for any of the following activities:
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Cleaning turbidimeters
Creating Formazin Standards
Calibrating Turbidimeters
Referencing Index Samples
Instructional steps should be concise and precise, using the following guidelines:
• Steps should contain only one action.
• Commands should be written with an action verb at the beginning.
• Limits/and or tolerances for operating parameters should be specific values
and consistent with the accuracy of the instrumentation. Procedures should
not include mental arithmetic.
• "Cautions" should be used to attract attention to information that is essential
to safe performance.
• "Notes" should be used to call attention to supplemental information. Notes
present information that assists the user in making decisions or improving task
performance.
• Documentation methods should be incorporated as part of the procedure
including what data needs to be recorded, if the individual needs to sign or
date data, etc.
After developing an SOP, the author(s) should consider the following questions:
• Can the procedure be performed in the sequence it is written?
• Can the user locate and identify all equipment referred to in the procedure?
• Can the user perform the procedure without needing to obtain direct assistance
or additional information from persons not specified by the procedure?
• Are words, phrases, abbreviations, or acronyms that have special or unique
meaning to the procedure adequately defined?
• Is there a need for special controls on data collection and recordkeeping?
After completing the SOP it should be tested to the extent possible. It is also a good idea
to ask a technical reviewer to verify the accuracy of the procedure. SOPs should be
reviewed at least once every 2 years to determine if the procedure and requirements are
still accurate.
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The following is a simplified example of an SOP written for the development of
Formazin.
Creating a 4000 NTU Formazin Stock Suspension
1. Dissolve 1.000 g of ACD grade hydrazine sulfate, N2H4 H2SO4 in ultra filtered
deionized water and dilute to 100 mL in a Class A, 100 mL volumetric flask.
2. Dissolve 10.00 g of analytical grade hexamethylenetetramine, (CH2)6N4, in ultra
filtered deionized water and dilute to 100 mL in a Class A, 100 mL volumetric.
flask.
3. Combine the equal volumes of the hydrazine sulfate solution and the
hexamethylenetetramine solution into a clean, dry flask and mix.
4. Let the mixture stand for 48 hours at 24-26 °C.
5. Store the suspension in a bottle that filters ultraviolet light.
3.4.4 Sampling Strategy and Procedures
The procedure for conducting sampling should be laid out clearly and concisely,
preferably in SOPs (discussed in Section 3.4.3). It should include information such as
sampling location "and'.frequency, collection methods, sample handling, and any logistical
considerations or s'afety precautions which are necessary. Adherence to proper techniques
is an important step in minimizing the effects of instrument variables and other
interferences (Sadar, 1996). Measurements will be more accurate, precise, arid repeatable
if operators follow and incorporate the techniques listed in this section.
All turbidimeter manufacturers emphasize proper techniques and include detailed
instructions in their literature. Water treatment plant operators responsible for conducting
turbidity measurements are urged to review these instructions and incorporate them into
their SOPs, Specific instruction for securing samples and measuring turbidity will differ
for the various instrument manufacturers and models, but there are certain universally
accepted techniques that should be utilized when conducting measurements. The
following paragraphs highlight some of these techniques.
Handling of Cuvettes/Sample Tubes
Sample cells must be handled with absolute care to avoid contamination or damage, such
as marks and scratches, which might change the optical characteristics of the glass.
Scratches, fingerprints, and water droplets on the sample cell or inside the light chamber
can cause stray light interference leading to inaccurate results. Cells can be acid washed
periodically and coated with a special silicone oil to fill small scratches and mask the
imperfections in the glass. Since the silicone oil required for this application should have
the same refractive characteristics as glass, it is recommended that the oil be obtained
from the instrument manufacturer. Care should be taken to not apply excessive oil that
could attract dirt or contaminate the sample chamber in the instrument. Once the oil has
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been applied to the cell, the excess oil should be removed with a lint-free cloth. The
result should be a sample cell surface with a dry appearance, but with all imperfections
filled with oil. Sample cells should always be handled at the top of the cell or by the cap
to avoid fingerprints or smudges. After a cell has been filled with a sample and capped,
the outside surface should be wiped with a clean, lint-free absorbent cloth until it is dry.
Store cells in an inverted position on clean surfaces to reduce contamination by dirt or
dust or store capped and filled with low turbidity water.
Orientation and Matching of Sample Cells
Since imperfections in the sample cell glass can influence light scattering, the cell should
be inserted in the turbidimeter with the same orientation each time it is used. At the
Philadelphia Water Department, new cells are indexed and are not allowed to vary by
more than 0.01 NTUs. Philadelphia reports that as many as one quarter of the cells are
never used due to imperfections in sample cells (Burlingame, 1998).
Matched sample cells are required to minimize the effects of optical variation among
cells. If possible, it is better to use a single sample cell for all measurements to minimize
the variability due to cell-to-cell imperfections. Once the orientation of a cell has been
established, the operator should always use the same orientation when placing the sample
cell into the instrument. Techniques for matching and indexing are provided below.
Indexing Cells (Steps 1-2) Matching Cells (Steps 1-3)
Step 1. Pour ultra-pure dilution water into a sample cell (several cells if
performing matching) that has been cleaned according to the
techniques described previously in this section.
Step 2. Select sample cell, and place it into the turbidimeter. Rotate the
cell within the instrument until the display reads the lowest value.
Record the reading. Using a marker or pen, place a mark on the
top of the sample cells neck. Do not put the mark on the cap.
Use this mark to align sample cells each time a measurement is
made.
Step 3. Select another sample cell, place it into the turbidimeter and rotate
the cell slightly until the reading matches that of the first sample
cell (within 0.01 NTUs). Using a marker or pen, place a mark on
the top of the sample cells neck. If unable to match the readings
select a different sample cell. Repeat the process until the
appropriate number of cells have been matched.
c
Degassing of the Sample
Water samples almost always contain substantial amounts of entrained gases that can be
released during turbidity measurement. Bubbles are either generated during the filling of
a sample container, occur due to temperature fluctuations resulting in a reduced solubility
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3. TURBIDITY METHODS & MEASUREMENT
of the gas in a liquid, or are due to chemical and/or biological processes. Bubbles within
a sample act much like particles and can scatter light resulting in an incorrect
measurement. Many on-line turbidimeters contain apparatuses inside the instrument that
serve to trap, collect, and vent air bubbles. Usually these consist of baffled entries or
membranous chambers. Some vendors also manufacture add-on units which can be
placed in the sample line before the on-line turbidimeter. There are several other options
for removing bubbles from water (degassing) to reduce the effect they have on
measurements. The most commonly used methods include, addition of a surfactant,
application of a partial vacuum, and use of an ultrasonic bath.
Addition of a surfactant compound to a water sample lowers the surface tension of the
water and allows entrained gases to readily escape. There are a variety of surfactants used
in turbidity measurements today. Because of the variety in chemical composition, it is
difficult to provide guidance for their use. It is important to note that some surfactants
may have constituents which serve as a coagulant and cause particles to aggregate and
settle out. Other chemicals might contain constituents with an ionic charge that cause
particles to rise to the surface. The use of surfactants is more appropriate for
measurement of highly turbid waters such as raw water. The most appropriate
instrument-specific advice regarding the use of surfactants can be obtained by contacting
the instrument manufacturer.
Application of a partial vacuum to a sample lowers the partial pressure above the liquid
surface and allowsxentrained gases to escape. Partial vacuums can be created by a simple
syringe or by use of a vacuum pump. Some instrument manufacturers and suppliers
provide pre-made vacuum kits that include syringes for degassing samples. The most
common arrangement is the use of a syringe and a stopper sized for the opening of the
sample cell or test tube.
The use of an ultrasonic bath creates vibrations in the sample to facilitate the escape of
gases. Ultrasonics is a specialty field/science that utilizes an inaudible spectrum of sound
frequencies ranging from about 20,000 cycles per second to 100,000 cycles per second.
Ultrasonic baths are used for thoroughly cleaning supplies in the medical, electronic, and
metals industries. When high frequency sound waves are passed through a cleaning fluid,
such as water with suitable detergent additive, many millions of microscopic bubbles
form and then rapidly collapse. The bubbles are the result of the stretch and compress
phases of the sound waves within the fluid, a process known as cavitation. Ultrasonic
devices may be most effective in severe turbidity conditions or with viscous samples,
however if used for degassing samples, samples should be sonified for no more than 1
to 2 seconds. Sonification can change particle size ranges, affecting a turbidimeters
response if improperly utilized (Burlingame, 1998).
Timeliness of Sample
Samples should be measured expeditiously after being secured to prevent changes in
particle characteristics due to temperature and settling. Temperature can affect particles
by changing their behavior or creating new particles if precipitates are created. Dilution
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water may dissolve particles or change their characteristics (Sadar, 1996). Operators are
encouraged to draw samples only when turbidimeters are ready to be operated. Do not
draw a sample and allow it to sit while the instrument warms up or is being readied.
Other important Sampling Techniques
• Samples should not be violently agitated as particles can be broken apart or air
may be entrained into the fluid. Gentle agitation such as swirling the sample
cell is advisable to reduce particle settling.
• Sample cells should be used only with the instruments for which they were
intended. Do not mix and match.
• Perform a visual observation of the sample cell every time a measurement is
made. Verify that there are no visible bubbles in the sample and the cell is
clean and'free of scratches.
• Samples entering the turbidimeters should be at the same temperature as the
process flow samples. Changes in temperature can cause precipitation of
soluble compounds and affect readings.
• Sample cells should be evaluated with a low turbidity water (after cleaning) to
determine if cells remain matched. If the evaluation determines that a cell is
corrupted, discard the cell. Systems should consider conducting this
evaluation weekly.
• When in doubt, throw it out - If you have a question as to whether a sample
cell is too scratched or stained get rid of it.
3.4.5 Calibration
Turbidimeters, like all instrumentation, need to be calibrated periodically to ensure that
they are working properly and provide true and accurate readings.
Calibration should always be conducted according to manufacturer
instructions.
Determine the appropriate technical requirements for calibration based on the following
• Manufacturer
• Model name and/or number
• Parameters to be calibrated
• Range to be calibrated
\
• Acceptance criteria
• Mandatory calibration procedures or standards
• Required calibration program
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After calibration, performance of the turbidimeter should be verified with a secondary
standard. If the instrument has internal electronic diagnostics designed to assist in
determining proper calibration, the operator should use these tools to verify proper
calibration and operation.
Calibration Standards
A calibration standard must be used to conduct a calibration. Standards are materials
with a known value which, when placed in the instrument, should be used to adjust the
instrument to read the known value.
There are a variety of standards on the market today which are used to calibrate
turbidimeters. They are most often characterized as Primary, Secondary, or Alternative
standards. Standard Methods describes Primary Standards as a standard which is
prepared by the user'from traceable raw materials, using precise methodologies and under
controlled environmental conditions. (Standard Methods, 1995) Standard Methods
defines Secondary Standards as those standards a manufacturer (or an independent testing
organization) has certified to give instrument calibration results equivalent (within certain
limits) to results obtained when an instrument is calibrated with a primary standard.
Standard Methods and EPA differ in their definitions of each of these standards. EPA
recognizes the following three Standards for approved use in the calibration of
turbidimeters.
• FORMAZIN (user prepared and commercially produced)
• AMCO-AEPA-1®MICROSPHERES
• STABLCAL® (STABILIZED FORMAZIN)
Users need to realize that some instruments have been designed and calibrated on specific
primary standard(s) listed above. For optimal results, users should contact the
manufacturer of the instrument to determine the recommended primary standard to be
used for calibration.
Additionally, EPA recognizes Secondary Standards for use in monitoring the day-to-day
accuracy of turbidimeters by checking the calibration. This check is used to determine if
calibration with a Primary Standard is necessary. Secondary Standards are used to check
whether an instrument produces measurements within acceptable limits around a nominal
value (typically 10%). Examples of SECONDARY STANDARDS include:
• GELEX®
• GLASS/CERAMIC CUBES
• MANUFACTURER PROVIDED INSTRUMENT SPECIFIC SECONDARY
STANDARDS
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The need to reconcile the definitions and differences among Primary and Secondary
Standards will be a continuing issue. It has been recognized that the standards need to be
unbiased, easy to use, safe, available for a range of turbidities, and reproducible. Future
efforts of the Agency, in concert with other organizations and manufacturers, will focus
on ensuring the most appropriate, variation-free, and technologically feasible standards
are available and used for calibration of turbidimeters.
Conducting the Calibration
All reputable turbidimeters have been factory-calibrated before leaving the manufacturer.
As described previously, turbidimeters, like most instrumentation, tend to lose accuracy
over time due to a variety of factors, making periodic calibration very important to
maintain accurate measurements. The most important point to remember is:
Calibration should always be conducted according to manufacturer instructions.
Manufacturers differ in the steps to conduct a calibration, but the following points are
applicable to all calibrations.
• Standards should be checked to ensure they have not expired. Never pour a
standard back into its original container.
• Care should be taken when preparing Formazin. If a spill occurs, clean up
immediately according to the Material Safety Data Sheets (MSDSs) provided
with your chemicals. Make sure to inspect the tube/cuvette for scratches and
chips prior to pouring the solution in.
• Check to make sure the tube/cuvette is lined up properly according to the
indexing. Be sure not to scratch the tube when inserting, and ensure that the
tube/cuvette is free of dust, smudges, and scratches.
• When obtaining the reading, write the value legibly onto a form similar to the
one found in Figure 3-1. Make sure to record the date of the calibration, the
individual conducting the calibration, the value, and any peculiar situations or
deviations from normal calibration procedures (e.g., switch to a new lot of
Formazin, switch in standards, use of a new tube/cuvette, etc.) These
measurements will allow for an understanding of whether the performance of
a turbidimeter is in question. For example, if for 6 months a turbidimeter
reads approximately 20.152 when calibrated using polystyrene beads and one
morning it reads 25.768, this could be an indication that the bulb in the
turbidimeter has a problem. Conversely, if the standard in use was switched
that morning, the resulting change might be due to change in standards.
• Conduct the^calibration the same way each time. Variations in how the
calibration is conducted could yield inaccurate measurements.
• It is extremely important that individuals who conduct the calibration have
been trained to do so. Systems should consider creating Standard Operating
Procedures to be read, learned, and followed by operators at the plant.
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Frequency of Calibration
EPA recommends that the calibration of units be verified daily with secondary standard
and recalibration occur at least quarterly with primary standards. Specific calibration
procedures should be developed for each individual instrument location. Listed below are
several guidelines for selecting calibration frequencies and procedures:
• Select a frequency for checking instrument calibration with secondary
standards and for full re-calibration of instrument with primary standards .
• Establish the acceptable deviation from the primary standard during secondary
verifications. Readings in excess of the deviation should trigger immediate re-
calibration of the instrument. (± 10% is recommended by EPA)
• Choose a time of day when full attention can be devoted to the calibration.
Calibration at the end of a shift or right before a break can often lead to
mistakes and sources of error. A calibration time should be established when
operators are fully alert and focused on completing the task.
• Identify and schedule in advance the dates for full turbidimeter calibration oh
the plant calendar or work scheduling chart.
• Make preparations and maintain adequate supplies to prevent delays in the
calibration schedule. It is important to keep an appropriate stock of standards.
Due to the limited shelf-life of various standards, the age of the stored
standards should be monitored so they can be replaced or reformulated as
needed.
• Assign calibration duties to a select group of individuals, and make it one of
their standard activities. Train all appropriate individuals/operators in
conducting a calibration in the event that one of the regular individuals is not
available.
• Create a Standard Operating Procedure for conducting a calibration and post
next to the turbidimeter.
3.4.6 Data Screening, Validation, and Reporting
The methods for data screening, validation, and reporting should be detailed to ensure
that measurements are recorded calculated and reported correctly. These methods should
be designed to meet the Quality Assurance Objectives. Again, the development and
implementation of SOPs will facilitate those goals.
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Month
Year
Date
Initials
CALIBRATION CHECKLIST
Value
Standard
Comments
Figure 3-1. Calibration Checklist
3.4.7 Performance and System Audits
Performance and system audits should be conducted periodically to determine the
accuracy of the total measurement system(s) or component parts thereof. Performance
audits may include review of documentation and log books for legibility and
completeness. The systems audit consists of evaluation of all components of the
sampling and measurement systems to determine their proper selection and use. This
audit includes a careful evaluation of both field and laboratory quality control procedures
and can include verification of written procedures and analyst(s) understanding,
verification and documentation of procedures, as well as adherence to any SOPs.
3.4.8 Preventative Maintenance
Preventive maintenance should be conducted on all instrumentation. The maintenance
program consists of scheduled (preventive) and non-scheduled maintenance procedures.
All maintenance performed and the results of calibrations should be documented.
Maintenance procedures and schedules for equipment used should be available to the
appropriate staff. Adherence to maintenance schedules and procedures may be
investigated during a systems audit. A preventive maintenance schedule recommended
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by the respective manufacturers should be followed for each instrument. This preventive
maintenance will include regular battery checks and maintenance of a sufficient stock
spare parts and supplies. Manufacturers' procedures identify the schedule for servicing
critical items to minimize downtime of the measurement system.
3.5 Data Collection and Management
The final steps in turbidity measurement deal with the collection of data and management
of collected data. The advent of the personal computer revolution has provided much
needed assistance to tasks that were once time consuming, although automation still
requires operators who are skilled and trained in the use of sometimes sophisticated
equipment. This section describes the several methods available to systems for the
collection of data and provides a brief description of the management of that data.
Data obtained from Supervisory Control and Data Acquisitions (SCADAs), data
recorders, or strip charts should be verified on a weekly basis by comparing the
turbidimeter reading with the data recording device reading. If verification indicates
greater than ±10% deviation, the electronic signal should be recalibrated according to
manufacturer instructions.
3.5.1 Data Collection Methods
Acquisition of data from turbidimeters is an important step in the turbidity measurement
process. With the individual filter turbidity requirements, systems will be required to
continuously monitor each filter. Each of the methods discussed below are typically used
for on-line turbidimeters. Readings using benchtop units are typically recorded by hand
or entered into a PC without the use of the data collection equipment listed below.
Systems may have experience using these methods in monitoring other water quality
parameters.
Strip Recorders and Circular Chart Recorders
Strip Chart and Circular Chart Recorders are a relatively established technique for
recording data. The units are set to obtain a reading at a timed interval. A pen records
the reading on paper at the interval. As additional readings are taken, the pen moves back
and forth (or up and down in the case of a circular recorder) recording the values that are
being monitored.
Newer models include digital readouts as well as the capability to transfer data to data
loggers or other data acquisition systems. The greatest disadvantages to using chart
recorders is the difficulty in incorporating data into electronic format and archiving such
data. Recorders also require the purchasing of replacement pens and charts.
Data Loggers
Data Loggers are "black boxes" which store data which is received from input channels.
The box records the data in memory which can then be downloaded at a future time.
Data loggers consist of two distinct components: hardware and software.
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Hardware
The units themselves typically consist of a device containing solid state memory encased
in a plastic weatherproof enclosure. Units have a varying number of inputs that can be
either analog (records actual numbers) or digital (records a series of Os and Is), as well as
an output to download data. Systems most often are battery powered, but some can be
connected to existing power supplies. Nearly all systems contain lithium or other
batteries to keep memory active in the event of a power failure.
Software
Two software components are important to data loggers/acquisition devices. First,
specialized software is necessary to configure the logging unit. This configuration
specifies the unit frequency at which to obtain turbidity readings. The second part of the
software is used to retrieve the data from the logger and import it into a usable format on
a PC. Most companies offer integrated packages that allow users to import the data and
immediately plot and graph the data to depict trends or produce reports. Data should be
downloaded at regular intervals, as data loggers cannot store data indefinitely.
Several methods exist to transfer data from the logger into the PC. Data acquisition
systems are often equipped to be compatible with telemetry to upload data to PCs via
telephone, cellular telephone, or radio. Alternatively, either a laptop or palmtop can be
connected to the unit to download information, or the data logger can be brought into the
office where the PC is located and plugged into one of the input/output ports on the PC.
The better method could necessitate utilizing a second data logger to take the place of the
first logger when it is being downloaded. Systems may wish to schedule downloads to
occur at times when a filter may not be in operation (when off-line or being backwashed).
SCADA
SCAD A systems are devices used for industrial measurement and control. They consist
of a central host (base unit), one or more field gathering and control units (remotes), and a
collection of standard and/or custom software used to monitor and control remotely
located field data elements. The base unit and the remote units are linked via telemetry,
and the base unit receives data and provides instructions as specified in the software.
SCADA systems at treatment plants are also often times referred to as Distributed
Control Systems (DCS). DCSs function the same as SCADA systems except that field
gathering and control units are located in a more confined area and communications may
be via a local area network (LAN) as opposed to remote telemetry.
SCADA systems can take inputs from a variety of sources and instruments. These
systems collect and display the data produced by a variety of instruments so that the plant
operator can monitor the entire treatment process from one location. SCADA systems are
typically used for a variety of functions at a water treatment plant including flow control,
pH and temperature monitoring, automated disinfection dosing, and a host of other
functions. Control may be automatic or initiated by operator commands. The inclusion
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of continuous turbidity monitoring could be incorporated into the regime of items being
measured and controlled by a SCAD A/DCS system at a treatment plant.
SCADA systems can also be used to log and store data for recording purposes. Signals
sent from remote instruments located on the plant site are interpreted at the base unit.
This unit provides the logic to interpret all of the different signals and display real-time
measurements. The central unit can be programmed to automatically transfer historical
data to other storage media such as a tape drive or Zip-drive.
3.5.2 Data Management
There are two distinct objectives to management of turbidity data: (1) Regulatory
Compliance, and (2) Checking Process Control and Treatment Plant Optimization. The
turbidity reporting and monitoring requirements set forth in Chapter 2 establish the types
of data which must be collected and the analysis which must be done to meet the
requirements of the rule. In order to meet these requirements, operators must understand
three areas of data management:
• Data Format;
• Data Storage; and
• Data Analysis.
Format
Storage of the data in a usable format is the first step to effective data management.
Operators should have the ability to download data from their acquisition equipment into
a usable and manageable format. Data is typically placed in one of many different
formats such as Excel, Access, dBASE, and Lotus 123. Data should be converted into a
format that can be used by the facility. Many systems currently utilize software such as
those listed above. The key to selecting a format is the ease at which the data can be
viewed, manipulated, and or converted. Certain software packages allow users to create
reports, tables, or graphs based on the data.
Storage
Storage of the data is the next step in effective data management. Maintaining these data
points for future analysis may pose a problem due to the amount of disk space required.
Systems should consider the use of Zip-Drives or tape-drives for storage of data. Hard
drives can be used to store data while manipulating or evaluating. Tape and Zip-Drive
backups are also recommended due to the possibility of a PC crashing.
Interpreting and Analyzing Data
Data Analysis is the last step in effective data management/Systems are encouraged to
utilize the Data Collection Spreadsheets and Macros developed for the Partnership for
Safe Water. A description of the Partnership for Safe Water is found in Chapter 4.
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Spreadsheets were prepared for the Partnership for Safe Water to assist utility partners in
collecting performance data. The spreadsheets were developed to capture turbidity data
from raw water, sedimentation basin effluent and filter effluent, but can be used to
measure repetitive data of any kind, from any point in the process for up to 365 days.
Macros have been written to generate frequency distributions on a monthly and annual
basis, to help evaluate trends and summarize large amounts of data. Graphics capabilities
of the spreadsheets are also built in to automatically plot trend charts and frequency
distributions. There are also capabilities for generating summaries of the data to report as
background information. Other data summaries within the capabilities of each
spreadsheet software version could be generated as well. A disk containing the software
along with guidance for using the software is found in the Composite Correction Program
Handbook published by EPA.
The software provided with many of the data acquisition systems, which can be custom
designed for SCAD A/DCS systems, also allow operators to trend and analyze data. Easy-
to-use software provides clear graphics for operators to evaluate. Typically, data can be
exported to various spreadsheets or database programs for later analysis. Software is>
typically interactive, with the ability to change colors, and graph sizes.
Systems should analyze turbidity data to check process control and treatment plant
optimization. Systems may wish to evaluate backwash turbidity spikes for individual
filters, how storm events affect the filtration capabilities, or the effect of various chemical
dosages on filtered effluent. Analysis could be undertaken to compare different filters
within a system or the effect of different flow rates. Chapter 5 provides information on
conducting a Filter Self Assessment and analysis which systems may wish to implement.
3.6 References
1. AWWARF. 1998. Treatment Process Selection for Particle Removal, AWWARF
International Water Supply Association.
2. Burlingame, G.A., MJ. Pickel, and J.T. Roman. 1998. "Practical Applications of
Turbidity Monitoring." J. AWWA. 90(8):57-69.
3. CDHS (California Department of Health Services). 1998. Turbidity Monitoring
Guidelines, June 18.
4. Great Lakes Instruments, Inc. 1992. Turbidity. GLI Method 2. Milwaukee, WI.
5. Great Lakes Instruments, Inc. "Turbidity Measurement." Technical Bulletin Number
Tl Rev 2-193. Milwaukee, WI.
6. Hach Company. 1997. "Low Level Turbidity Measurement." Loveland Colorado,
September.
7. Hach Company. 1995. "Excellence in Turbidity Measurement."
8. Hart, Johnson, and Letterman. 1992. "An Analysis of Low-level Turbidity
Measurements". J. AWWA. 84(12):40.
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3. TURBIDITY METHODS & MEASUREMENT
9. International Organization for Standardization (ISO). 1990. ISO 7027 Water
Quality-Determination of Turbidity.
10. King, K. 1991. "Four-Beam Turbidimeter For Low NTU Waters." /. of the
Australian Water and Wastewater Association, October.
11. Lex, D. 1994. "Turbidimeter Technology Turns on the High Beams." Intech. 41(6).
12. Meeting Notes. 1997. Turbidity Workshop, September 15-16.
13. Sadar, M. 1996. Understanding Turbidity Science, Technical Information
Series-Booklet No. 11, Hach Company.
14. Sadar, M. Turbidity Standards, Technical Information Series-Booklet 12, Hach
Company, Loveland, CO.
15. Sethi, Patanaik, Biswas, Clark, and Rice. 1997. "Evaluation of Optical Detection
Methods for Waterborne Suspensions." J. AWWA. 89(2):98-l 12.
16. Standard Methods. 1995. Standard Methods for the Examination of Water and
Wastewater. Nineteenth Edition. Franson, M.H., Eaton, A.D., Clesceri, L.S., and
Greenberg, A.E., (editors). American Public Health Association, AWWA, and Water
Environment Federation. Port City Press, Baltimore, MD.
17, USEPA. 1998. Handbook: Optimizing Water Treatment Plant Performance Using
the Composite Correction Program. EPA/625/6-91/027.
18. USEPA. 1997. "National Primary Drinking Water Regulations: Interim Enhanced
Surface Water Treatment Rule Notice of Data Availability; Proposed Rule." 62 FR
59486. November 3.
19. USEPA. 1993. Methods for the Determination of Inorganic Substances in
Environmental Samples, EPA-600/R-93-100.
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4.1 Compliance Approach for Turbidity Requirements
While many systems already meet or will meet turbidity requirements prior to
compliance deadlines, some systems will need to evaluate their treatment plants to
determine what changes, if any, are needed to comply with the requirements. Utilities
which determine they may have difficulty complying with the turbidity requirements of
the IESWTR should first evaluate the system and begin to optimize plant performance.
Section 4.2 outlines the Agency's suggested approach for utilities to evaluate their
systems, and identifies key areas which systems should evaluate.
Although it is anticipated that compliance with the IESWTR will generally be possible
through adjustments to existing treatment processes, additional treatment processes or
other treatment technologies or enhancements may be required in some cases. It is not
anticipated that systems will need to make major capital improvements, but those
systems considering capital improvements in order to meet requirements of the IESWTR
should conduct an optimization activity similar to the Composite Correction Program to
assess the real need of construction. Section 4.3 briefly outlines many of the process
enhancements that, in the opinion of the Agency and other water professionals, are the
most likely to be employed by systems, if optimization alone does not permit a system to
comply with turbidity requirements.
4.2 System Evaluation & Plant Optimization
A thorough treatment plant evaluation and improvement program is the best way to
ensure pathogen-free drinking water. With an emphasis on improved performance at
minimal cost, optimization is an economical alternative for compliance with the
turbidity requirements. Currently, two programs serve as excellent resources for systems
wishing to follow a systematic and proven approach to optimizing water treatment plant
performance. These are:
• Composite Correction Program
The Composite Correction Program (CCP) is a systematic action oriented
approach that federal or state regulators, consultants, or utility personnel can
implement to improve performance of existing water treatment plants. The
Comprehensive Performance Evaluation phase of the CCP is described in
greater detail hi Chapter 6. The Agency has developed a guidance manual
that may be obtained by calling the EPA Safe Drinking Water Hotline at
1-800-426-4791.
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• Partnership for Safe Water
The Partnership is a voluntary cooperative
effort between the EPA, AWWA and other ^ vi C n
drinking water organizations, and over 186 %. ^ S *^
surface water utilities representing 245 '
water treatment plants throughout the
United States. The goal of this common
sense cooperation is to provide a new
measure of safety to millions of Americans
. .
by implementing prevention programs
where legislation or regulation does not
exist. The preventative measures are based
around optimizing treatment plant
performance and thus increasing protection
against microbial contamination in America's drinking water supply.
Information regarding the Partnership is found at AWWA's website
http://www.awwa.org/paitnerl.htm or may be obtained by calling (303) 347-
6169.
Systems are strongly encouraged to utilize one of the above noted programs if
intending to optimize plant performance.
While systems should consider the above noted programs, this section provides
information utilities may find useful in evaluating their system and optimizing their
plant's performance. It is important to remember that the items listed in this chapter may
or may not apply to all systems. Optimizing water treatment plants is a site-specific
endeavor. As such, this section does not seek to serve as a recipe for how to optimize
water treatment for lowered turbidity. It does however highlight the areas which, in the
experience of the Agency and other water professionals, most often can be improved to
optimize water treatment at PWSs. The items discussed in this section are addressed in
greater detail in the Composite Correction Program and the Partnership for Safe Water.
4.2.1 Coagulation/Rapid Mixing
Coagulation is the process by which small particles are combined to form larger
aggregates and is an essential component in water treatment operations. Evaluation and
optimization of the Coagulation/Rapid Mixing step of the water treatment process
includes a variety of aspects. Optimal coagulant dosages are critical to filter performance.
Maintaining the proper control of these chemicals can mean the difference between an
optimized surface plant and a poorly run surface plant. Inadequate mixing of chemicals
or their addition at inappropriate points within the treatment plant can limit performance.
The following issues may be evaluated as they may improve the performance of this step
in the treatment process.
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Chemicals
An evaluation of the chemicals used in the treatment process can identify the
appropriateness of the coagulation chemicals being used. A thorough understanding of
coagulation chemistry is necessary, arid changes to coagulation chemicals should not be
made without careful consideration. The following aspects relating to coagulation
chemicals may be considered by systems:
• Operating procedures should not call for the shutting off of coagulant
chemical addition when raw water is less than 1 NTU.
• Are chemicals being dosed properly, paying special attention to pH? Is dose
selection based on frequent jar testing or other testing methods such as
streaming current monitoring, zeta potential, or pilot filters? Relying
exclusively on past practice is not good practice.
• Do Standard Operating Procedures exist for coagulation controls? Systems
should develop SOPs, and establish a testing method that is suited to the plant
and personnel.
• Are the correct chemicals being utilized? Is the best coagulant being used for
the situation? Changing coagulant chemicals or adding coagulant aids may
improve the settleability of the flocculated water and in turn optimize
performance. Coagulants may also be changed seasonally.
• Do operators have the ability to respond to varying water quality by adjusting
coagulation controls to ensure optimum performance? Systems should
provide operators with such learning opportunities so that they can react to
various conditions with understanding and confidence.
• Are solutions used promptly? Most solutions should be utilized within 48
hours of their formulation. Are chemicals utilized before manufacturer
recommended expiration or use-by dates?
• Is pH a consideration? Measurement of pH is a key aspect in coagulation
chemistry. Do not dilute coagulant solutions to pH levels higher than 3.3 for
alum and 2.2 for iron salts. Manufacturers instructions should be followed
when diluting polymers.
• Are chemicals being added in the correct order? The order of chemicals is
very important, as certain chemicals interfere with others. Jar tests should be
utilized to develop optimal sequences.
• Is the chemical feed system operating properly? Operators should consider
checking the accuracy of systems at least once daily or once per shift.
\
Feed Systems
Feed systems are another important aspect of the coagulation step in typical treatment
processes. These systems are responsible for delivering coagulants into the system at
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rates necessary for optimal performance. The following aspects should be evaluated
regarding feed systems.
• Is redundancy a consideration? Redundancy may be built into the feed
systems so that proper feeding of chemicals can be maintained in the event of
failure or malfunction of primary systems.
• Is the feed system large enough? Feed systems should be sized so that
chemical dosages can be changed to meet varying conditions.
• Are chemical pumping equipment and piping checked on a regular basis?
Maintenance of these systems should be a priority and incorporated into
routine maintenance performed at the system.
• Is a diaphragm pump utilized? A continuous pump allows coagulants to be
added in such a way as to avoid pulsed flow patterns.
Satisfactory Dispersal/Application Points
Finally, proper coagulation and mixing also depends on satisfactory dispersal of
coagulation chemicals and appropriate application points. Coagulants should be
adequately dispersed so that optimal coagulation may occur. A sufficient number of feed
points should exist such that chemicals have the opportunity to mix completely. Utilities
should evaluate the following items:
• Is adequate dispersion taking place? If chemicals are added at a hydraulic
pump, ensure that the chemicals are distributed across the width of the flow
stream and at the location where turbulence is greatest. The rapidity of
coagulation necessitates even dispersal as soon as possible.
• Are coagulants being added at the proper points? Metal salts should be
introduced at the point of maximum energy input. Low molecular weight
cationic polymers can be fed with metal salts at the rapid mix or to second
stage mixing following the metal salt. High molecular weight
nonionic/anionic floe/filter aids should be introduced to the process stream at
a point of gentle mixing.
• Is rapid mixing equipment checked frequently? Systems should check the
condition of equipment, and ensure that baffling provides for adequate, even-
flow.
4.2.2 Flocculation
Flocculation is the next step in most treatment plants (in-line filtration plants being the
exception). It is a time-dependent process that directly affects clarification efficiency by
providing multiple opportunities for particles suspended in water to collide through gentle
and prolonged agitation. The process takes place in a basin equipped with a mixer that
provides agitation. This agitation should be thorough enough to encourage interparticle
contact but gentle enough to prevent disintegration of existing flocculated particles.
Effective flocculation is important for the successful operation of the sedimentation
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process. Several issues regarding flocculation should be evaluated by utilities to ensure
optimal operation of flocculation basins.
Flocculation Mixing and Time
Proper flocculation requires long, gentle mixing. Mixing energy should be high enough
to bring coagulated particles constantly into contact with each other, but not so high as to
break up those particles already flocculated. Utilities should consider evaluating:
• How many stages are present in the flocculation system? Three to four are
appropriate to create plug flow conditions and allow desired floe formation.
• Is the mixing adequate to form desired floe particles? Tapered mixing is most
appropriate. "G" values should be variable through the various stages from 70
sec"1 to 1£ sec"1.
• Are mechanical mixers functioning properly? Are flocculator paddles rotating
at the correct rates?
• If flow is split between two flocculators, are they mixing at the same speed
and "G" value? If the flocculators have different characteristics, dosages may
be proper for one, but not both.
Flocculator inlets and Outlets
If water passes through the flocculation basin in much less time than the volumetric
residence time, the influent stream has short circuited. Inlet and outlet turbulence is
oftentimes the major source of destructive energy in flocculation basins that contributes
to short circuiting. Utilities should evaluate the following:
• Do basin outlet conditions prevent the breakup of formed floe particles?
Basin outlets should avoid floe breakup. Port velocities should be <0.5 fps.
The velocity gradient at any point from the flocculation basin to the
sedimentation basin should be less than the velocity gradient in the last
flocculation stage.
• Do inlet conditions prevent the breakup of formed floe particles? Inlet
diffusers improve the uniformity of the distribution of incoming water.
Secondary entry baffles across inlets to basins impart headless for uniform
water entry.
• What size are the conduits between the rapid mix basin and the flocculation
basin? Larger connecting conduits help reduce turbulence which can upset
floe.
Flocculator Basin Circulation
Baffles are used in flocculator basins to direct the movement of water through the basin.
Baffling near the basin inlet and outlets improves basin circulation and achieves more
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uniform circulation. A system may think about the following items when evaluating
flocculation.
• Is current baffling adequate? Can baffling be added to improve performance
or does existing baffling require repair? Serpentine baffling is better than
over/under. Baffling should allow headloss through opening to prevent short-
circuiting and to allow plug flow conditions.
• Induced velocity in floe chambers should vary from 2 fps in first stage to 0.25
fps in the last stage. Velocity through openings in the baffle should be slightly
less than the induced velocities.
4.2.3 Sedimentation
Sedimentation is the next step in conventional filtration plants (direct filtration plants
omit this step). The purpose of sedimentation is to enhance the filtration process by
removing particulates. Sedimentation requires that water flow through the basin at a slow
enough velocity to permit particles to settle to the bottom before the water exits the basin.
Utilities should consider the following items when evaluating sedimentation basins.
• Is sludge collection and removal adequate? Inadequate sludge collection and
removal can cause particles to become re-suspended in water or upset
circulation. Systems should disrupt the sludge blanket as little as possible.
Sludge draw-off rates can effect the sludge blanket. Sludge draw off
procedures should be checked periodically, making sure sludge levels are low;
and sludge should be wasted if necessary. Sludge pumping lines should be
inspected routinely to ensure that they are not becoming plugged. These lines
should also be flushed occasionally to prevent the buildup of solids.
• Do basin outlet conditions prevent the breakup of formed floe particles?
Settling basin inlets are often responsible for creating turbulence that can
break up floe. Improperly designed outlets are also often responsible for the
break-up of floe. Finger launders (small troughs with V-notch weir openings
that collect water uniformly over a large area of the basin) can be used to
decrease the chance of short-circuiting.
• Is the floe the correct size and density? Poorly formed floe is characterized by
small or loosely held particles that do not settle properly and are carried out of
the settling basin. This is the result of inadequate rapid mixing, improper
coagulant dosages, or improper flocculation. Systems should look to previous
steps in the treatment train to solve this problem.
• Is the basin subject to short circuiting? If the basin is not properly designed,
water bypasses the normal flow path through the basin and reaches the outlet
in less time than the normal detention time. The major cause of short-
circuiting is poor influent baffling. If the influent enters the basin and hits a
solid baffle, strong currents will result. A perforated baffle can successfully
distribute inlet water without causing strong currents. Tube or plate settlers
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also improve efficiency, especially if flows have increased beyond original
design conditions. Tube settlers can result in twice the basin's original
settling capacity.
• Are basins located outside and subject to windy conditions? Wind can create
currents in open basins that can cause short-circuiting or disturbances to the
floe. If wind poses a problem, barriers lessen the effect and keep debris out of
the unit.
• Are basins subject to algal growth? A problem that occurs in open, outdoor
basins is the growth of algae and slime on the basin walls.
• In solids contact clarifiers, is the sludge blanket maintained properly?
Operators should be able to measure the sludge depth and percent solids to
ensure the sludge blanket is within the manufacturer's recommendations.
/
4.2.4 Filtration
Filtration is the last step in the particle removal process. Improperly designed, operated,
or maintained filters can contribute to poor water quality and sub-optimal performance.
There are a host of items which systems will need to evaluate regarding filters that may
be contributing to poor performance. Many of the items listed below are detailed in
Chapter 5, Individual Filter Self-Assessment.
Design of Filter Beds
It is important to verify that the filters are constructed and maintained according to design
specifications. Utilities should consider the following items when evaluating the design
of filter beds.
• Media - Is the correct media being used? Issues such as size, uniformity
coefficient, and depth need to be evaluated.
• Underdrains - Are underdrains adequate or have they been damaged or
disturbed?
Filter Rate and Rate Control
The rate of filtration and rate control is another important aspect of filters that should be
evaluated. Without proper control, surges may occur which would force suspended
particles through the filter media.
• Are surges in flow an issue of concern? Systems should avoid sudden changes
to filter rate. Systems should minimize plant flow rate changes, throttle filter
control valves slowly, and bring a filter on-line when one is being
backwashed).
• Is the plant operating at the appropriate flow rate? At some plants (typically
smaller systems), the flow is operated at a level that hydraulically overloads
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unit processes. Operating at lower flow rates over longer periods of time
prevents overloading and increases plant performance.
• At what flows are the filters rated? Make sure not to exceed flow rates on
remaining in-service filters when taking filters off-line or out of service for
backwash.
Filter Backwashinq
Filter backwashing has been identified as a critical step in the filtration process. Many of
the operating problems associated with filters are a result of inadequate backwashing.
Utilities should consider the following items when evaluating filter backwash practices.
• Is the rate of filter backwash appropriate for the filter? Filters can be either
underwashed or overwashed. Utilities need to determine the appropriate flow
that will clean the filter and prevent mudballs, but will not upset the filter
media to the extent that the underdrain is damaged or filter media is lost. (20-
50 percent bed expansion is typical)
• Are criteria set for initiating backwash? Systems should establish criteria such
as time, headloss, turbidity, or particle counts for initiating backwash
procedures.
• How are filters brought back on-line? Media should be allowed to settle after
backwashing before bringing filters back on-line. Filters should be brought
back on line slowly. Several filters should not be brought on line at the same
time. Filters should not be brought back on-line without backwashing first.
• When a filter is backwashed, is more water diverted to the remaining filters,
causing them to be overloaded during backwash? During the backwash, flow
going to the remaining filters may need to be cut back to ensure the filters are
not overloaded or "bumped" with a hydraulic surge causing particle pass
through.
4.3 Process Enhancements/Technologies
As noted at the beginning of this chapter, some systems may need to provide additional
treatment processes or make enhancements to existing processes to meet the requirements
of the EESWTR. The Agency stresses that utilities need to first fully evaluate their
systems, specifically utilizing either the CCP or Partnership for Safe Water
programs, prior to installing new treatment or equipment. EPA believes that most
systems will be able to meet requirements through process optimization.
EPA expects that systems might use a combination of equipment modifications and
process enhancements or treatment processes to meet requirements if process
optimization alone does not bring the system into compliance. The Agency developed a
Cost and Technology Document for the Interim Enhanced Surface Water Treatment Rule,
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which discusses these treatment processes/enhancements. Treatment process
enhancements fall into the following categories:
• Chemical Modifications
• Coagulant Improvements
• Rapid Mixing Improvements
• Flocculation Improvements
• Settling Improvements
• Filtration Improvements
• Hydraulic Improvements
• Laboratory Modifications
• Process Control Modifications
By no means is this list exhaustive or do the process enhancements which fall under each
category represent the only modifications available to systems. They represent
enhancements that, in the opinion of the Agency and other water professionals, are the
most likely to be employed by systems. For further information regarding these
enhancements, the reader is directed to the Cost and Technology Document for the
Interim Enhanced Surface Water Treatment Rule, dated July 28, 1998, which was
developed in support of the Regulatory Impact Analysis (RIA) for the IESWTR.
_/'
Certain technologies, especially those involving large financial expenditures, should only
be implemented with appropriate engineering guidance, and should consider factors such
as the quality and type of source water, turbidity of source water, economies of scale and
potential economic impact on the community being served, and treatment and waste
disposal requirements. An engineering study should be conducted, if needed, to select a
technically feasible and cost-effective method to meet the unique needs of each system
for improved filter effluent quality to comply with the ffiSWTR. Some situations may
require more extensive water quality analyses or bench and/or pilot scale testing. The
engineering study may include preliminary designs and estimated capital, operating and
maintenance costs for full-scale treatment.
4.4 References
1. AWWA. 1994. "Preventing Waterborne Disease: How to Optimize Treatment,
Participant Guide." AWWA Satellite teleconference.
2. Bucklin, K., A. Amirtharajah, and K. Cranston. 1998. "Characteristics of Initial
Effluent Quality and its implications for thd Filter to Waste Procedure." AWWARF,
Denver, CO.
3. Logsdon, G. 1987. "Evaluating Treatment Plants for Particle Contaminant Removal "
J. AWWA. 79(9):82-92.
APril 1999 4-9 EPA Guidance Manual
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4. APPROACH FOR COMPLIANCE
4. Partnership for Safe Water. 1995. Voluntary Water Treatment Plant Performance
Improvement Program Self Assessment Procedures. October.
5. Huben, H. 1995. Water Treatment. Second Edition. AWWA.
6. USEPA. 1998. Regulatory Impact Analysis for the Interim Enhanced Surface Water
Treatment Rule. Office of Ground Water and Drinking Water, Washington, D.C.
7. USEPA. 1998. Handbook: Optimizing Water Treatment Plant Performance Using the
Composite Correction Program. EPA/625/6-91/027.
8. USEPA. 1989. Technologies for Upgrading Existing or Designing New Drinking
Water Treatment Facilities. EPA/625/4-89/023, Center for Environmental Research
Information, Cincinnati, OH.
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5, INDIVIDUAL FILTER SELF ASSESSMENT
5.1 Introduction
Based on individual filter monitoring requirements in the IESWTR, some systems may be
required to conduct an individual filter self assessment. Specifically, a system must
conduct an individual filter self-assessment for any individual filter that has a measured
turbidity level greater than 1.0 NTU in two consecutive measurements taken 15 minutes
apart in each of three consecutive months. The system must report the filter number, the
turbidity measurement, and the dates on which the exceedances occurred.
Filters represent the key unit process for the removal of particles in surface water
treatment. Although filters represent only one of the "barriers" in a treatment process
their role is the most critical as the final physical "barrier" to prevent passage of chlorine
resistant pathogenic microorganisms into distribution systems. Properly designed filters
used in conjunction with coagulation, flocculation and sedimentation processes (if in
use), when in proper physical and operational condition, are capable of treating raw water
sources.
For any situation regarding a single poor performing filter, or a bank of poor performing
filters:
Performance limitations observed at the start of a filter run'are most often
attributed to improper chemical conditioning of the filter;
• Limitations observed during the filter run are most often attributed to changes
in hydraulic loading conditions; and
• Limitations observed at: the end of the filter run are most often related to
excessive filter runs.
Filter performance issues may only be apparent during excessive hydraulic loading and
care should be taken to not attribute all turbidity spikes to hydraulic bumping or
overloading. In some circumstances performance "symptoms" for other causes may only
be evident during these hydraulic episodes. Oftentimes disrupted filter media may cause
filter performance problems. The following chapter describes the process of an
individual filter self assessment and is intended to provide clarity regarding which of
these areas are limiting the performance of a filter.
The following chapter will include each of the following components of an individual
filter assessment:
• A general description of the filter including size, configuration, placement of
washwater troughs and surface wash type (if applicable) and filter media design
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5. INDIVIDUAL FILTER SELF ASSESSMENT
(e.g., type, depth and placement) and if filter-to-waste is present and/or used and if
any special conditions exist regarding placing a filter back into service (i.e., is the
filter rested, polymer or coagulant added prior to placement into service, etc.).
Table 5-1 provides a worksheet to assist the evaluator in collecting this
information.
The development of a filter run profile of continuous turbidity measurements or
total particle counts versus time for an entire filter run from start up to backwash,
including assessment of filter performance while another filter is being washed.
The run length during this assessment should be representative of typical plant
filter runs. The profile should include explanations of the cause of performance
spikes during the run.
An assessment of the hydraulic loading conditions of the filter which includes:
the determination of the peak instantaneous operating flow for the individual
filter, an assessment of the filter hydraulic loading rate at this peak instantaneous
operating flow, and an assessment whether plant flow is distributed evenly among
all the filters.
An assessment of the actual condition and placement of the media with a
comparison to the original design specifications. The filter bed should be
investigated for surface cracking, proper media depth, mudballs and segregation
of media in dual media filters. The media should be examined (using coring
and/or gross excavation techniques as appropriate) at several locations to
determine the depth of the different media layers in dual and multi-media filters.
A description of backwash practices including length, duration, presence of and
type of surface wash or air scour, and method for introducing wash water (i.e., via
pump, head tank, distribution system pressure, etc.) and criteria for initiating the
wash (i.e., degraded turbidity or particle counts, head loss, run time, etc.), the
backwash rate, and bed expansion during the wash.
An assessment of the condition of the support media/underdrains including a
filter grid detailing placement of support media, as well as a summary of
inspection of the clearwell for the presence of filter media and any observances of
boils or vortexing during backwash.
An assessment of the filter rate-of-flow controllers and filter valving
infrastructure adequacy. The rate-of-flow controllers and ancillary valving
related to the filter can also have an impact on filter performance and should be
visually inspected to assure proper operation.
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5. INDIVIDUAL FILTER SELF ASSESSMENT
Table 5-1. Individual Filter Self Assessment Worksheet
Topic
General Filter Information
Hydraulic Loading
Conditions
Media Design Conditions
Actual Media Conditions
Support Media/Underdrain
Conditions
Description
Type (mono, dual, mixed)
Number of filters
Filter control (constant, declining)
Surface wash type (rotary, fixed, none)/Air Wash
Configuration (rectangular, circular, square)
Dimensions (length, width, diameter)
Filter-to-waste (capability/specify if used)
Surface area per filter (ft2)
Average operating flow (mgd)
Peak instantaneous operating flow (mgd)
Average hydraulic surface loading rate (gpm/ft2)
Peak hydraulic surface loading rate (gpm/ft2)
Depth, type
Media 1 - Sand
Media 2 (if applicable) - Anthracite
Media 3 (if applicable) - Garnet
Depth
Media 1 - Sand
Media 2 (if applicable) - Anthracite
Media 3 (if applicable) - Garnet
Presence of rnudballs, debris, excess chemical,
cracking, worn media
Is the support media evenly placed (deviation <2
inches) in the filter bed?
Evidence of media in the clearwell or plenum
Evidence of boils/vortexing during backwash
Information
April 1999
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5. INDIVIDUAL FILTER SELF ASSESSMENT
Table 5-1. Individual Filter Self Assessment Worksheet (continued)
Topic
Backwash Conditions
Other Considerations
Description
Backwash initiation (headloss, turbidity/particle
counts, time)
Sequence (surface wash, air scour, flow ramping,
filter-to-waste)
Duration (minutes)
Introduction of wash water (via pump, head tank,
distribution system pressure)
Backwash rate (gpm/ft2)
Bed expansion (percent)
Coagulant or polymer added to wash water
Filter rested prior to return to service
Information
5.2 Developing A Filter Run Profile
The profile for the filter being evaluated shall include a graphical summary of filter
performance for an entire filter run from start-up to backwash inclusively. Performance
is typically represented by turbidity although total particle counts may be used in addition
to, or in lieu of, turbidity measurements. Use of particle counting in conjunction with
turbidity monitoring of filter effluents may offer additional insights to filter performance,
however, care should be taken in the interpretation of particle count results. The
interpretation should focus on the change in count levels as opposed to the discreet
particle count numbers. Plotting the performance data versus time on a continuous basis
is the desirable approach for development of the filter profile. However, time increments
less than a continuous basis may be used with the understanding that the intent is to
identify and minimize ''real" deviations in performance. The filter run should be
representative of a typical run and should encompass the time period when another filter
is being washed. The profile shall include an explanation of the cause of performance
spikes during the run.
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5. INDIVIDUAL FILTER SELF ASSESSMENT
Example
A utility has plotted total turbidity data versus.time for a filter that cannot meet
requirements for individual filters. The filter run is typically 24 to 28 hours with a resting
period after backwash that varies from 8 to 10 hours. The generated filter profile is
shown below in Figure 5-1. The review of turbidity data showed an inordinate number of
spikes occurring during the filter run. This data corroborated with turbidity data that
triggered the filter assessment. These spikes corresponded to changes in hydraulic
loading rates made by the staff and may be indicative of greater problems within the filter
itself. The significant increases in turbidity passing the filters occurred when the plant
staff initiated recycle of treated backwash water to the head of the plant and when plant
loading rates were modified during the evening to take advantage of off-peak electrical
costs (represented by item B&D). Table 5-2 provides explanations for turbidity spikes.
.4
.3
1-2
12:00 am 6:00 am
12:00 pm 6:00 pm 12:00 am
Time
6:00 am 12:00 pm
Figure 5-1. Filter Run Profile - Turbidity (NTU) vs. Time
Table 5-2. WTP Performance Deviation Trigger Events
Event
A
B
C
D
E
F
G
H
Performance Deviation Trigger Explanation
Pump change
Backwash water decant recycle to head of
plant initiated
Backwash water decant recycle completed
Pumping rate increased to take advantage
of off-peak electrical costs
Immediately following backwash of adjoining filter
Filter backwash
Filter taken out of service
Filter placed back in service
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5. INDIVIDUAL FILTER SELF ASSESSMENT
5.3 Assessing Hydraulic Loading Conditions Of Filter
Filters with properly chemically pretreated influents are most vulnerable to pass particles
(including pathogenic microorganisms) at peak loading rates in excess of filter design or
during sudden changes in hydraulic loading rates. Table 5-3 presents a summary of
acceptable filter loading rates for various filters. However, filters may exhibit capable
performance at loading rates in excess of those presented in Table 5-3; these values
are rule-of-thumb and provide a basis for evaluating excessive filter hydraulic
loading. State requirements may differ from acceptable industry loading rates and should
be considered during the assessment.
Since the filters are most vulnerable during excessive loading rates, it is critical to
determine the peak flow on an instantaneous basis that filters are performing under and to
minimize incidences when filters are expected to perform at these higher loading rates.
Peak instantaneous Operating flow rate is identified through review of operating records
and observations of operational practices and flow control capability.
A review of plant flow records can be misleading in determining the peak instantaneous
operating flow. For example, peak daily water production can only be used when those
values are generated during a 24-hour operational day during specific conditions. If
values are used that were generated for a day when the plant only was in operation for 12
hours, the peak instantaneous operating flow would be D of the true value. Additionally,
if pumps are used in multiple combinations throughout the operational day, care should
be taken to determine the actual peak loading on the filters during the day. The peak
instantaneous operating flow should be determined based on the flow distributed to the
filters on a maximum daily minute. The peak instantaneous operating flow that each
filter sees is dependent on the minimum number of filters used per day at the plant's peak
instantaneous operating flow.
Table 5-3. General Guide to Acceptable Filter Hydraulic Loading Rates
Filtration Type
Sand Media
Dual/Mixed Media
Deep bed
(anthracite > 60 in.)
Air Binding
None
Exists
None
Exists
None
Exists
Loading Rate
-2.0 gpm/ft2
~1.0-1.5gpm/fl2
-4.0 gpm/ft2
-2.0 - 3.0 gpm/ft2
-6.0 gpm/ft2
-3.0 - 4.5 gpm/ft2
Example 1
A plant which operates 24 hours per day uses three 5-mgd pumps in various
combinations throughout the year to meet system demand. The pumps are capable of
being throttled to reduce individual flow to 80 percent of capacity. The average daily
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5. INDIVIDUAL FILTER SELF ASSESSMENT
production is 8 mgd while the peak production day over a previous 2-year period has been
12 mgd. The plant in the previous two years has never run all three pumps
simultaneously for an entire day. However, for a 2-hour period each evening, all three
pumps are used to fill on-site storage. Two pumps are used for the first hour with the
third pump only used with the other two pumps for the last 30 minutes of the 2-hour
period. During that 30 minute period plant flow increases to 14 mgd. The peak
instantaneous operating flow that go onto the filters is 14 mgd. The plant's six dual
media filters (each 425 ft2) would have a loading rate of 3.8 gpm/ft2 at this 14 mgd peak
flow.
Example 2
A plant with 8 dual media filters and a constant high service pumping rate of 8 mgd
operates 24 hours per day and is unable to consistently meet the filter requirements. Each
filter has 175 ft2 of surface area and typically has a flow rate of 1 mgd. However, two
filters are backwashed per day at the same time with no reduction in plant flow. During
backwash the two filters are out of service for 40 minutes. During that 40 minute period
the entire plant flow of 8 mgd is handled by just six filters. The peak instantaneous
operating flow for each filter becomes 1.33 mgd. The hydraulic loading rate in gpm/ft2
for each 175 ft" filter at this peak flow becomes 5.3 gpm/fr (1.33 mgd converted to gpm
divided by the filter surface area) which is at the upper end of the acceptable loading rates
for a dual media filter and may be contributing to the unacceptable performance.
5.4 Assessing Condition & Placement Of Filter Media
Assessment of the condition and placement of the filter media is an integral step in
identification of factors limiting
performance of the filtration process.
The presence of mudballs, surface
cracking, or displaced media may
often be attributed to excessive use of
coagulant chemicals, inadequate
backwashing or a more serious
problem related to the underdrain
system. The assessment of the
condition and placement of the filter
media should include a physical
inspection of, the filter bed and a ,
comparison of the actual media
findings to the original specifications.
The filter bed should be investigated
for surface cracking, proper media
depth, presence of mudballs and
segregation of media.
Figure 5-2. Box Used for Excavation
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5. INDIVIDUAL FILTER SELF ASSESSMENT
The filter inspection should begin by
draining the filter. The filter should be
drained enough to allow for excavation
of the media to assess the depths of each
media type, as well as, each media
interface (e.g., just below the
anthracite/sand interface in a dual media
filter). Deeper excavation of the filter
may be necessary if evidence suggests
disrupted support gravels or an
inadequate underdrain system (see
Section 5.6). Care should be taken not
to disrupt the support gravel/media.
Filter media assessments may be
conducted using a gross excavation of
media technique or application of a
variety of coring devices (typically a ID
to 2 inch pipe). Coring methods offer
the advantage of being able to apply the Floe Retention Analysis procedure, if conditions
warrant (see to Section 5.5). Evaluators should place small pieces of plywood on the
media prior to getting on the filters to avoid sinking into the media. The excavation
technique may be conducted using a gross excavation of the media or a plexiglass box
shown in Figures 5-2 and 5-3. The box excavation consists of sinking a plexiglass box
into the media and excavating inside the box down to the support media. The box
excavation technique allows for visual observation of the media depths and interfaces
after the excavation is completed.
Whatever media excavation technique is used, the evaluators should note the depth of
each media type, (comparing this to the original specifications), the general condition of
the media interface, the presence of any mudballs or excess chemical. After the
excavation is completed, the excavation team should make certain that the media is
placed back in the excavations in the same sequence that it was removed. The filter
should be backwashed after completion of the excavation prior to return to service.
Figure 5-3. Box Excavation
Demonstration
5.5 Assessing Backwash Practices
Proper maintenance of filters is essential to preserve the integrity of the filter as
constructed. Limitations of poor performing filters relating to filter media degradation or
disruption of support gravel placement can often be attributed to inadequate backwashing
or excessive backwashing rates. The duration of the backwash, if excessive, may also be
detrimental. Different facilities have had different experiences in how clean the filters
should be after backwashing. Consideration should be given to site-specific
circumstances in the application of any recommendations regarding filter backwash
procedures with the focus always being on filter effluent water quality. Table 5-4
summarizes guidelines for acceptable backwashing practices.
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5. INDIVIDUAL FILTER SELF ASSESSMENT
Table 5-4. Guidelines Regarding Acceptable Backwashing Practices
Area of Emphasis
Basis for initiating backwash
Backwash flow
Backwash flow rate
Bed expansion during backwash
Guideline
focus on filter performance (turbidity,
particle counts) degradation versus
headloss or time
slowly ramped to peak rate
15 - 20 gpm/ft2
20 - 25 percent
The assessment of the filter backwash procedure should include the following:
• A collection of general information related to the backwash;
. Verification of the adequacy of the backwash SOP;
, • A visual inspection of a backwash; and
• Determination of the backwash rate and expansion of the filter media during
the wash.
The individual filter assessment worksheet (Table 5-1) can be used to collect general
information regarding the backwash.
An adequate backwash SOP should describe specific steps regarding when to initiate
backwash, how flows are ramped during the wash, when to start and stop surface wash or
air scour, and duration of the wash.
Backwash rates are designed to provide adequate cleaning of the filter media without
washing media into the collection troughs or causing disruption of the support gravels.
Table 5-4 identifies generally acceptable backwash rates. These values are to be used as a
guide when assessing adequacy of the backwash procedures. Backwash rates in gpm/ft2
may be determined by simple calculation if backwash pump rates or backwash flows are
available and known to be accurate. If these values are unavailable or suspect, backwash
rates can be determined by performing a rise rate test of the filter. Periodic rise rate tests
can also be used to verify the backwash flow measurement instruments. The rise rate test
entails determining the amount of time it takes backwash water to rise a known distance
in the filter bed. Typically, a metal rod marked at 1-inch intervals is fixed in the filter to
enable measurement of the distance that water rises during the wash. The rise rate test
should be conducted such that measurements are taken without the interferences of the
wash water troughs in the rise volume calculation. Extreme care and great attention to
safety should be followed while conducting the rise rate test. Rise rate is used to
calculate backwash rate by dividing the rise volume for a known time period by the filter
area as follows:
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5. INDIVIDUAL FILTER SELF ASSESSMENT
rise-volume(gal) = filter • surface -area(ft2) x rise • distance(ft) x lA8(gal I ft )
backwash-rate(gpm/ft2)=
rise • volume(gal) I rise • time(min)
filter • surface • area(ft~)
Example backwash rate calculation: A filter having a 150 ft2 surface area has wash water
rise 10.7 inches in 20 seconds during the rise rate test. The backwash rate would be 20
gpm/ft2.
rise-volume = 150/f2 x 10.7/n x // / 12in x TASgal I /?3 =
2 1000ga//(20secx(min/60sec))
backwash • rate(gpm I ft ) =
150/f2
backwash • rate = 2Qgpm I ft'
In addition to backwash rate, it is also extremely important to expand the filter media
during the wash to maximize the removal of particles held in the filter or by the media.
Bed expansion is determined by measuring the distance from the top of the unexpanded
media to a reference point (e.g., top of the filter wall) and from the top of the expanded
media to the same reference point. The difference between these two measurements is
bed expansion. A proper backwash rate should expand the filter 20 to 25 percent
(AWWA and ASCE, 1990). Attention should be given to the influence of seasonal
temperature changes on bed expansion during application of this procedure. Percent bed
expansion is given by dividing the bed expansion by the total depth of expandable media
(i.e., media depth less support gravels) and multiplied by 100 as follows:
expanded -measurement = depth -to-top-of -media -during -backwash(inches)
unexpanded -measurment = depth -to-top -of -media -before -backwash(inches)
bed -expansion = unexpanded -measurement(inches) - expanded -measurement(inches)
bed -expansion(percent) =
bed -expansion -measurement(inches)
total -depth -of -expandable -media(inches)
XlOO
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5. INDIVIDUAL FILTER SELF ASSESSMENT
A variety of apparatus can be used to
measure bed expansion. The apparatus
can vary from a metal shaft with a
white disk attached on one end and a
steel measuring tape fitted along the
shaft to a metal pole with an attached
collection of pipe segments of varying
lengths each plugged at the bottom.
The pipes are arranged like a set of
church organ pipes with each pipe 1-
inch longer than the next (see Figure
5-4). The unit is solidly affixed,
resting on the top of the media.
During backwashing, the expanded.
media fills each successive piece of
pipe until the rise stops. Care should
be taken to affix the pipe organ apparatus such that it can easily be determined where bed
expansion ended because during certain situations, all of the pipe segments will be filled
with expanded media making it impossible to accurately determine media expansion.
During this situation, the apparatus should be emptied, affixed higher in the filter above
the media and the bed expansion test repeated. The disk unit is used by placing the disk
on the unexpanded media prior to backwash and recording the length of the metal rod to
the reference point. The disk unit is then removed and backwashing is initiated. After
the backwash is allowed to reach its peak rate the disk is lowered slowly into the
backwashing filter until media is observed on the disk. The measurement of the expanded
media is then recorded and percent bed expansion may then be determined. Figure 5-5
depicts the disk bed expansion apparatus. The key attribute of any method is that
determination of the top of the expanded media be accurately characterized.
Figure 5-4. Pipe Bed Expansion
Figure 5-5. Bed Expansion Device
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5. INDIVIDUAL FILTER SELF ASSESSMENT
Example bed expansion calculation:
The backwashing practices for a filter with 30 inches of anthracite and sand is being
evaluated. While at rest, the distance from the top of the media to the concrete floor
surrounding the top of the filter is measured to be 41 inches. After the backwash has
been started and the maximum backwash rate is achieved, a probe containing a white disk
is slowly lowered into the filter bed until anthracite is observed on the disk. The distance
from the expanded media to the concrete floor is measured to be 34D inches. The
resultant percent bed expansion would be 22 percent.
unexpanded measurement = 41 inches
expanded measurement = 34.5 inches
bed expansion = 6.5 inches
bed expansion (percent) = (6.5 inches/30 inches) * 100 = 22 percent
Use of the Floe Retention Analysis procedure may be warranted if the filter is meeting
backwash expansion and backwash rate guidelines while still not achieving turbidity
performance criteria. (Kawamura, 1991, Wolfe &Pizzi, 1998.) The Floe Retention
Analysis procedure allows for an extremely in-depth analysis of backwash practices.
5.6 Assessing Condition Of Support Media/Underdrains
Maintaining the integrity of the support gravels and underdrains is extremely important to
the performance of a rapid granular filter. Disrupted or unevenly placed support media
can lead to rapid deterioration of the filtered water quality noticeable by quick turbidity
breakthroughs and excessively short filter runs (Peck, Smith). Should disruption of the
support media be significant, the impacted area of the filter may act as a "short-circuit"
allowing particulates and any microbial pathogens which are present to pass directly into
the clearwell. Filter support gravels can become disrupted by various means including
sudden violent backwash, excessive backwashing flow rates, or uneven flow distribution
during backwash.
The condition of the support gravel is assessed in three steps. The first step consists of
visually inspecting the filter during a backwash for the presence of excessive air boiling
or noticeable vortexing as the filter is drained. The second step entails determining
whether filter media has ever been found in the clearwell. This should be determined
visually or by reviewing recent clearwell maintenance records. Clearwell inspections
should be only be conducted following appropriate safety procedures while minimizing
negative impacts on necessary plant operations. Clearwells containing a significant
amount of filter media may indicate a greater problem than just disrupted support gravels.
The problem may be attributed to a severe issue with the filter underdrain system. An in-
depth assessment of the underdrains typically involves excavation of the entire filter bed.
Systems should use best professional judgement and seek additional guidance if
undertaking an underdrain assessment, as it is outside the scope of "typical" filter self
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5. INDIVIDUAL FILTER SELF ASSESSMENT
assessment. The third step is the most common method of assessing the placement of
filter support media. This method involves "mapping" of the filter using a steel or solid
probe. The mapping procedure involves a systematic probing through the filter media
down to the support gravels of a drained filter at various locations in a grid-like manner.
At each probe location, the depth of penetration into the filter is measured against a fixed
reference point such as the wash water troughs. The distance from the fixed reference
point to the top of the support gravels should deviate less than 2 inches. Care should be
taken during the filter probing not to disrupt the support gravel.
Example
Operators, while draining a poor performing filter, observed vortexing occurring at the far
end of the filter. The operators constructed a support gravel placement grid by probing
through the media down to the support gravel every 2 feet throughout the filter using a 6
feet long aluminum rod that had been marked at 1-inch intervals. The operator using the
probe measured the depth of probe penetration against the wash water trough.
Examination of the grid (shown in Table 5-5) indicated that the support gravels were
extremely disrupted at the far end of the filter.
Table 5-5. Example Filter Support Gravel Placement Grid
Depth of Filter Support Gravels (in inches)
Measured from the Wash Water Trough
2ft
4ft
6ft
8ft
10ft
12ft
14ft
16ft
18ft
2ft
41
40.75
41
40.75
41
41
40.75
41
40.75
4ft
40.75
40.5
41.25
41
41
46
46
39
41.25
Filter
Control
Panel
6ft
41
41
40.75
41
40.5
46.5
46.25
38.75
40.75
8ft
41
41
41
40.75
40.5
41
39
37
41
10ft
41
40.75
41
40.75
40.75
41
40.75
40.75
41
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5.7 Assessing Rate-Of-Flow Controllers & Filter Valve
Infrastructure
The rate-of-flow controllers and ancillary valving related to the filter can also have an
impact on filter performance. Hydraulic changes (such as those caused by filter bumping)
cause filters to shed particles. Maintaining the rate-of-flow controllers is important in
minimizing hydraulic changes in the filter. Figure 5-6 shows on-line turbidity
measurements for two filters in a treatment plant. Each of the two filters had rate-of-flow
controller problems that became more evident as headless built up in the filters. Just
prior to initiating backwash in filter 4 the rate-of-flow controllers were opening and
closing constantly "seeking" the correct position. This was first apparent to the filter
evaluation team who observed constant turbidity fluctuations of the filter effluent during
a filter performance review. Improperly seated valves can also have similar impacts on
filter performance. Backwash valves may leak and allow wash water to compromise
filter effluent coming from the filters remaining in operation. All filter assessments
should include an evaluation of all the rate-of-flow controllers and filter valving.
Erratic Performance From Rate Control Valve
Filter Returned to Service After Backwash
1.0
0.0
Time (15 Mln Increments)
Figure 5-6. On-Line Turbidimeters Showing Performance Problems Due to
Inoperable Rate-of-Flow Controllers
5.8 References
1. AWWA. 1998. "How to Do A Complete Examination of Your Filters (Without
Incurring the Wrath of the Filter Gods)." Annual Conference Workshop Summary.
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5. INDIVIDUAL FILTER SELF ASSESSMENT
2. AWWA and ASCE (American Society of Civil Engineers). 1990. Water Treatment
Plant Design. Second edition. McGraw-Hill, New York.
3. Bender, J.H., R.C. Renner, B.A. Hegg, E.M. Bissonette, and R.J. Lieberman. 1995.
"Voluntary Treatment Plant Performance Improvement Program Self-Assessment
Procedure." Partnership for Safe Water, USEPA, AWWA, AWWARF, Association
of Metropolitan Water Agencies, Association of State Drinking Water
Administrators, and National Association of Water Companies.
4. James M. Montgomery Consulting Engineers, Inc. 1985. Water Treatment
Principles and Design. John Wiley & Sons, Inc.
5. Kawamura, S. 1991. Integrated Design of Water Treatment Facilities. John Wiley
& Sons, Incorporated, New York, NY.
6. Peck, B., T. Tackman, and G. Crozes. No date specified. Testing the Sands - The
Development of a Filter Surveillance Program.
1. Smith, J.F., A. Wilczak, and M. Swigert. No date specified. Practical Guide to
Filtration Assessments: Tools and Techniques.
8. USEPA. 1998. Handbook: Optimizing Water Treatment Plant Performance Using
the Composite Correction Program. EPA/625/6-91/027.
9. USEPA. 1998. "National Primary Drinking water Regulations: Interim Enhanced
Surface Water Treatment Rule; Final Rule." 63 FR 69477. December 16.
10. Wolfe, T.A. and N.G. Pizzi. 1998. "Optimizing Filter Performance."
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EVALUATION
6.1 Introduction
Based on individual filter monitoring requirements in the IESWTR, some systems may be
required to arrange for a Comprehensive Performance Evaluation (CPE). Specifically,
systems must conduct a CPE if any individual filter has a measured turbidity level of
greater than 2.0 NTU in two consecutive measurements taken 15 minutes apart in two
consecutive months. The system must report the filter number, the turbidity
measurement, and the date(s) on which the exceedance occurred. The system shall
contact the State or a third party approved by the State to conduct a CPE.
A CPE is the evaluation phase of the Composite Correction Program (CCP). The CCP,
including detailed CPE procedures and qualifications for CPE providers, is described in a
separate handbook (USEPA, 1998). This chapter's goal is to present a fundamental
discussion of CPE concepts and provide a general understanding of what a plant should
expect when a CPE is completed. Detailed CPE procedures are not included in this
guidance manual. Detailed CPE procedures should be obtained from the CCP Handbook
(available by calling the EPA Safe Drinking Water Hotline at 1-800-426-4791).
6.2 Background On The CPE
The CCP is a systematic, comprehensive procedure that identifies and corrects the unique
combination of factors, in the areas of design, operation, maintenance and administration,
that limit the performance of a filtration plant. It was developed to improve performance
at filtration plants using existing facilities thereby minimizing construction alternatives.
The capable plant model, presented in Figure 6-1, shows conceptually how the CCP
considers the various aspects of the operation, design, maintenance, and administration of
a filtration plant. A plant is considered capable when it has treatment processes of
sufficient size with adequate mechanical equipment to meet current water demand,
adequate administrative support including funding and policies, and a maintenance
program that keeps key equipment operational. Once these components are in place,
proper operations capabilities are required for the plant to achieve its performance goals,
whether for regulatory compliance or treatment optimization.
At the core of the CCP is the assumption that if a filtration plant cannot achieve specific
performance, there is a unique combination of interrelated factors with respect to the
design, maintenance, administration and/or operations of the filtration plant that are
limiting its performance. The purpose of the CPE is to identify these factors and
prioritize them with respect to their relative importance in preventing compliance and/or
optimized performance. Once the factors are identified and prioritized they can be
corrected so that performance can improve and compliance can be achieved. During a
CPE, the historic performance of the plant is assessed with respect to pathogen removal
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6. COMPREHENSIVE PERFORMANCE EVALUATION
and inactivation. The design, administration, and maintenance of the plant are completely
reviewed to determine if they properly support a capable plant. If they are not supporting
a capable plant, the root causes are identified as to how they are contributing to the
performance problem. Operational practices are also reviewed to assess if operators have
the necessary skills to achieve required performance and compliance when provided with
a capable plant.
Performance Goals
Compliance with the IESWTR
Operations
(Process Control)
Capable Plant
Administration
Design
Maintenance
Figure 6-1. Capable Plant Model
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In using the CPE/CCP it is important to understand that the approach has applications in
addition to achieving regulatory compliance and should be applied as appropriate for
meeting desired performance needs. All of the CPE procedures are designed to focus a
plant toward meeting the compliance requirements and performance goals described in
Table 6-1.
Table 6-1. CPE Treatment Performance Goals
Minimum Data Monitoring and/or
Reporting Requirements
Individual Sedimentation Basin
Performance Criteria
Individual Filter Performance Criteria
Combined Filtered Water
Performance. Criteria
Disinfection Performance Criteria
IESWTR Compliance Requirements
Continuous individual filter turbidity
monitoring with values recorded at 15
minute intervals (conventional and direct
filtration systems).
Representative filtered/finished water
effluent turbidity every 4 hours.
Not applicable.
Maximum filtered water turbidity of 1 NTU in
two consecutive measurements taken 15
minutes apart (conventional and direct
filtration systems).
Maximum filtered water turbidity 4 hours
following backwash of less than 0.5 NTU in
two consecutive measurements taken 15
minutes apart (conventional and direct
filtration systems).
Representative filtered/finished water
turbidity less than 0.3 NTU 95 percent of the
time based on 4-hour measurements
(conventional and direct filtration systems).
Maximum filtered/finished water turbidity of
1 NTU based on 4-hour measurements
(conventional and direct filtration systems).
CT values to achieve required log
inactivation of Giardia and viruses.
CCP Optimized Performance Goals
Daily raw water turbidity.
4-hour settled water turbidity from each
sedimentation basin.
On-line continuous turbidity from each
filter.
Settled water turbidity less than 1 NTU
95 percent of the time when raw water
turbidity is less than or equal to 10 NTU.
Settled water turbidity less 2 NTU 95
percent of the time when raw water
turbidity is less than or equal to 20 NTU.
Filtered water is less than 0.1 NTU 95
percent of the time (excluding 15 minute
period following backwashes) based on
maximum values recorded during 4-hour
increments.
Maximum filtered turbidity measurement
of 0.5 NTU.
Maximum filtered water turbidity
following backwash of less than 0.3
NTU.
Maximum backwash recovery period of
15 minutes (e.g., return to less than 0.1
NTU).
Maximum filtered water measurement of
less than 10 total particles per millileter
(>30m) of particle counts are available.
CT values to achieve required log
inactivation of Giardia and viruses.
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6.3 Components of a CPE
A CPE consists of the following three components:
• Performance assessment (evaluates historical plant performance);
• Major unit process evaluation (for assessing the physical plant capabilities);
and
• Factors limiting performance.
The following subsections discuss each of these components; detailed procedures are
provided in the CCP Handbook.
6.3.1 Performance Assessment
The performance assessment component of the CPE determines the status of a facility
relative to achieving'compliance requirements and performance goals and verifies the
extent of any performance problems at the plant. This information also provides the CPE
evaluators with some initial insights on possible causes of performance problems. These
insights are then used to focus other activities during the CPE to assess the design,
operation, maintenance and administration of the plant. Historical turbidity data from
plant records is used, supplemented by data collected during the CPE.
To achieve desired performance levels (compliance or optimized), a water treatment plant
should demonstrate that it can take a raw water source of variable quality and produce a
consistent, high quality finished-water. Further, the performance of each unit process
should demonstrate its capability to act as a barrier to the passage of particles at all times.
The performance assessment determines if major unit treatment processes consistently
perform at optimum levels to provide maximum multiple barrier protection. If perform-
ance is not optimized, the assessment also provides valuable insights into possible causes
of the performance problems and serves as the basis for other CPE findings.
During the performance assessment, historical turbidity data for the raw, settled, and
finished water is collected from the plant records and trends are charted as shown in
Figure 6-2. From this example data the CPE evaluator can see that the plant treats a raw
water that varies moderately throughout the year. The settled and finished water
performance indicates that this plant has a performance problem since turbidity levels
produced for treatment processes are significantly above compliance requirements and
performance goals described in Table 6-1.
Figure 6-2 also shows how the CPE evaluator can use the performance assessment to gain
some insights into the causes of the poor performance. In reviewing this data it is
apparent that a spike in raw water turbidity on March 9th carried through the plant
resulting in finished water turbidities close to 1 NTU. These pass through variations and
spikes provide some insight into the root cause of these performance problems that the
CPE evaluators will use to direct the subsequent portions of the CPE. Typically, these
types of performance problems are related to the process control skills of the plant staff,
but other design and/or administrative issues or raw water events may also make a
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significant contribution to the problem. During their review of the design, operation and
administration of the plant, the CPE evaluators will use these insights to focus the
discussions they have with the plant staff. Information on the possible causes of this
spike will be investigated until the evaluators are sure they understand the root cause.
Additional data is collected during the CPE to confirm the historical performance data,
further assess the performance of individual treatment processes, and confirm insights on
possible causes of poor performance. Typically additional data is collected through
special studies including the following:
• Verification of filtered turbidity results by independently comparing a system's
measurements with measurements from a continuous turbidimeter brought by the
CPE evaluators. If the plant is not already individually measuring turbidity from
each filter, the CPE team can select the filter which the operators believe has the
most problems and collect individual filter data on that filter.
. Filter inspections for media depth and media condition.
• Filter media expansion during backwash.
• Verification of chemical dosages to be sure plant staff are actually adding the
amount of chemicals they are intending to add.
• Verification of the benchtop turbidimeter in the plant laboratory with a unit brought
by the CPE evaluators.
Additional data on the performance of individual sedimentation basins may also be
collected depending on the needs of the CPE evaluators. Continuous monitoring of
individual filters during the CPE allows for an in-depth assessment of the filter '
performance during critical periods of startup, backwash, and/or changes in plant flow
rates. Figure 6-3 shows the performance of a filter during a CPE immediately after start-
up following a backwash. Backwash spikes of this magnitude also indicate a possible
problem with the plant's process control procedures.
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Raw Water
120.00
0.00
•I nn nn Example of Pass Through
1UU.UU -- Event on 3/9/96
K 80.00 ••
•jj 60.00 -•
I2 40.00 ••
20.00 --I
Sep-94 Oct-94 Nov«94 Dec-94 Jan-95 F«b-95 Mar-95 Apr-95 May-95 Jun-95 Jut-95 Aug-95
Settled Water
25.00
20.00
15.00 •-
10.00 -•
5.00
0.00
Sop-94 Oci-94 Nov-94 Dae-94 Jan-95 Fab-95 Ma/-95 Apr-95 May-95 Jun-95 Jul-95 Aug-9!
Finished Water
p-94 Oel-94 Nov-94 Dec-94 Jan-95 Feb-9S Mar-95 Apr-95 May-9S
Figure 6-2. An Example of Performance Assessment Using Historical Data
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1.0
= 0.8-
0>
iO.6
a
I
Jo.4-
£
1 0.2
0.0
10
15
Time after Filter Startup
20
25
30
Figure 6-3. An Example of Individual Filter Data Collected During CPE
6.3.2 Major Unit Process EEvaluation
After the performance assessment, the CPE begins to focus on the causes of the identified
performance problems. The major unit process evaluation determines if the various key
existing treatment processes in the plant, if properly operated, are of sufficient size to
meet the performance goals at the plant's current peak instantaneous operating flows. If
the evaluation indicates that the major unit processes are of adequate size, then the
opportunity for the existing facility to achieve compliance by addressing operational,
maintenance or administrative limitations is available. If, on the other hand, the
evaluation shows that major unit processes are too small, then construction of new or
additional processes may be required to obtain compliance or optimize performance.
The major unit process evaluation only considers if the existing treatment processes are of
adequate size to treat current peak instantaneous operating flows and to meet the desired
performance levels. The intent is to assess whether existing facilities, in terms of
concrete and steel, are adequate. This evaluation does not review the adequacy or
condition of existing mechanical equipment. The evaluation assumes that if the concrete
and steel are not of adequate size then major construction may be warranted, and the
pursuit of purely operational approaches to achieve performance may not be prudent. The
condition of the mechanical equipment around the treatment processes is an important
issue, but in this part of the CPE it is assumed that the potential exists to repair and/or
replace this equipment without the disruption of the plant inherent to a major construction
project. These types of issues are addressed in the factors limiting performance
component of the CPE. It is also presumed in the major unit process evaluation that the
necessary process control procedures are in place and practiced to meet performance
goals. By assuming that the equipment limitations can be addressed and that operational
practices are optimum, the evaluator can project the performance potential or capability
of a unit process to achieve performance goals.
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27.5
Peak Row- 12MGD
12.2
Flocculation Sedimentation Filtration
Unl Processes
Disinfection
Flocculation criteria: Hydraulic detention time = 30 minutes: total volume = 202,500 gal; single stage, tapered flocculation
Sedimentation criteria: Surface loading rate = 0.7 gpm/ft2; total surface area = 13,440 ft2; swd=15 ft
Filtration criteria; Surface loading rate = 4 gpm/ft2; 6 filters in service; 30 inches mixed media
Disinfection criteria; Total Giardia inactivation = 3 log, 0.5 log required by disinfection; available volume = 900,000 gallons @
depth = 10 ft; pH = 7.5; temp = 0.5 C; chlorine residual = 1.5 mo/L; T10/T = 0.7
Figure 6-4. Example Performance Potential Graph
During the CPE, a performance potential graph similar to that shown in Figure 6-4 is
developed. The four treatment processes included in this major unit process evaluation
are flocculation, sedimentation, filtration and disinfection. The CPE evaluators determine
the peak instantaneous operating flow that the plant has seen over the last year and collect
data on the sizes of the various basins. To prepare the performance potential graph, the
CPE evaluators should select loadings for each process that they consider adequate for the
plant to achieve the performance goals. The assumptions and loadings used in this
example are shown at the bottom of the graph. Based on these loadings a projected
capacity is calculated and shown as a bar on the performance potential graph. Bars above
the dashed line in Figure 6-4 represent unit processes that have the capacity to treat the
peak instantaneous flow. Bars below the dashed line indicate processes where major or
minor changes may be necessary.
6.3.3 Factors Limiting Performance
The last and most significant component of a CPE is the identification of factors that limit
the filtration plant's performance. All information collected during the CPE is reviewed
and the root causes of any performance problems are identified and prioritized. This step
is critical in defining the future activities that the plant will need to focus on to achieve
the compliance or optimized performance goals. To assist in factor identification, a list of
50 different factors and definitions that could potentially limit water treatment plant
performance is provided in the CCP Handbook. These factors are divided into the four
broad categories of administration, design, operation, and maintenance. This list and
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definitions are based on the results of more than 70 water treatment plant CPEs.
Definitions are provided for the convenience of the user as a reference to promote
consistency in the use of factors from plant to plant and to assist others in interpreting the
CPE results.
While the definitions for the administrative, operation and maintenance factors
adequately explain when these factors are identified, the plant staff may find several of
the design factors confusing when reviewing the CPE findings. Design factors are
included for each of the treatment processes in the major unit process evaluation. If any
of the treatment processes in the major unit process evaluation were classified as marginal
or inadequate, they would be identified in the CPE findings as a factor limiting the plant's
performance. Treatment processes that were identified as adequate in the major unit
process evaluation can also be identified as a factor when there are equipment related
problems that are limiting performance. This would occur when key equipment (e.g.,
filter rate-of-flow control valves) needs to be repaired and/or replaced before desired
performance can be achieved.
A CPE is intended to be a performance-based evaluation and therefore factors should be
identified only if they impact performance. A proper CPE does not contain factors that
are primarily observations that a utility does not meet a particular "industry standard"
(e.g., utility does not have a documented preventive maintenance program or does not
practice good housekeeping) unless a clear link is made between the practice and the
identified performance problem.
The major challenge in identifying a plant's unique list of factors is making sure that the
root causes are identified. This is difficult because the actual problems in a plant are
often masked. This concept is illustrated in the following example:
Example
A review of plant records revealed that a conventional water treatment plant was
periodically producing finished water with a turbidity greater than 0.5 NTU. The utility,
assuming that the plant was operating beyond its capability, was beginning to make plans
to expand both the sedimentation and filtration unit processes. Field evaluations
conducted as part of a CPE revealed that settled water and finished water turbidities
averaged about 5 NTU and 0.6 NTU, respectively. Filtered water turbidities peaked at
1.2 NTU for short periods following a filter backwash.
Conceivably, the plant's sedimentation and filtration facilities were inadequately sized.
The major unit process evaluation, however, showed that these processes were capable to
handle the plant's current peak flows.
A review of the plant's operation procedures revealed that the poor performance was
caused by the operator adding coagulants at excessive dosages, leading to formation of a
pin floe that was difficult to settle and filter. The operators did not have an adequate
process control program or equipment to allow them to identify and set the proper
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chemical doses. Additionally, the plant was being operated at its peak capacity for only 8
hours each day, further aggravating the washout of solids from the sedimentation basins.
The CPE evaluators assessed that by implementing proper process control of the plant
(e.g., jar testing for coagulant control, calibration and proper adjustment of chemical feed)
and operating the plant at a lower flow rate for a longer time period would allow the plant
to continuously achieve the desired performance.
When the operator and administration were questioned about the reasons that the plant
was not operated for longer periods of time, it was identified that it was an administrative
decision to limit the plant staffing to one person. This limitation made additional daily
operating time as well as weekend coverage difficult.
It was concluded that three major factors contributed to the poor performance of the
plant:
1. Application of Concepts and Testing to Process Control: Inadequate
operator knowledge existed to determine proper coagulant doses and to set
chemical feed pumps to apply the correct chemical dose.
2. Administrative Policies: A restrictive administrative policy existed that
prohibited hiring an additional operator to allow increased plant operating
time at a reduced plant flow rate.
3. Process Control Testing: The utility had inadequate test equipment and an
inadequate sampling program to provide process control information.
In this example, pursuing the perceived limitation regarding the need for additional
sedimentation and filtration capacity would have led to improper corrective actions.
Completing a plant expansion without correction of the operation and administrative
factors probably would not have solved the performance problems. The limitations in
process control would have remained even with a new plant. Administrative policies that
led to insufficient staffing of the old plant could have remained with a new plant. The
CPE, however, indicated that addressing the identified operational and administrative
factors would allow the plant to achieve the desired performance on a continuous basis
without major expenditures for construction. The funds that initially were directed
towards construction could then be directed towards other factors that truly are limiting
the plant's performance.
This example illustrates that a comprehensive analysis of a performance problem is
essential to identify the actual performance limiting factors. The CPE emphasis of
assessing factors in the broad categories of administration, design, operation, and
maintenance helps to ensure the identification of root causes of performance limitations.
6.4 Activities During a CPE
When a plant is required or decides to have a CPE conducted, there are several activities
that they should expect to occur. In general, if all of the following activities do not occur,
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the plant should question whether the evaluators are following the procedures in the CCP
Handbook.
A CPE involves numerous activities conducted within a structured framework. A
schematic of CPE activities is shown in Figure 6-5. Initial activities are conducted prior
to on-site efforts and involve notifying appropriate plant personnel to ensure that they, as
well as other necessary resources, will be available during the CPE. The kick-off meeting,
conducted on site, allows the evaluators to describe forthcoming activities, to coordinate
schedules, and to assess availability of the materials that will be required.
Following the kick-off meeting, a plant tour is conducted by the superintendent or process
control supervisor. During the tour, the evaluators ask questions regarding the plant and
observe areas that may require additional attention during data collection activities. For
example, an evaluator might make a mental note to investigate more thoroughly the flow
splitting arrangement prior to flocculation basins if one basin appeared to receive more
flow than the other units (e.g., flooding).
Following the plant tour, data collection activities begin. Depending on team size, the
evaluators split into groups to facilitate simultaneous collection of the administrative,
design, operations, maintenance, and performance data. Appropriate forms are provided
in Appendix F of the CCP Handbook to facilitate the data collection activities. After data
are collected, the performance assessment and the major unit process evaluation are
conducted. It is noted that often the utility can provide the performance data prior to the
site visit. In this case the performance graphs can be completed prior to the on-site
activities. However, it is important to verify the sources of the samples and quality of the
data during field efforts.
Field evaluations are also conducted to continue to gather additional information
regarding actual plant performance and confirm potential factors. This activity may
typically include a special study focusing on an individual filter or filters. Once all of this
information is collected, a series of interviews are completed with the plant staff and
administrators. Initiating these activities prior to the interviews provides the evaluators
with an understanding of current plant performance and plant unit process capability,
which allows interview questions to be more focused on potential factors.
After all information is collected, the evaluation team meets at a location isolated from
the utility personnel to review findings. At this meeting, factors limiting performance of
the plant are identified and prioritized. The prioritized list of factors, performance data,
field evaluation results, and major unit process evaluation data are then compiled and
copied for use as handouts during the exit meeting.
An exit meeting is held with appropriate operations and administration personnel where
all evaluation findings are presented and the plant staff are given the opportunity to ask
questions. The evaluation team answers clarifying questions during the exit meeting
but does not make recommendations or offer solutions to the factors identified. A
CPE report is then generated off-site by the CPE providers which formalizes the
information presented in the exit meeting. It is intended that all of the CPE findings
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I
Initial Activities
J
| ~ Kick-Off Meeting [
Plant Tour
Data Collection Activities
Operations Data
Field Evaluations
Conduct Interviews
Identify and Prioritize
Factors
Assess Applicability of
CTA
Exit Meeting
CPE Report
Location
Off-Site
On-Site
Off-Site
Figure 6-5. Activities During a CPE
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are presented in the exit meeting and it is critical that the report not present any
additional findings. The CPE provider should not save any controversial findings for
the report.
A CPE is typically conducted over a three to five-day period by a team consisting of at
least two personnel. A team approach is necessary to allow a facility to be evaluated in a
reasonable time frame, and for evaluation personnel to jointly develop findings on topics
requiring professional judgment. Professional judgment is critical when evaluating
subjective information obtained during the on-site CPE activities. For example, assessing
administrative versus operational performance limiting factors often involves the
evaluators' interpretation of interview results. The synergistic effect of two people
making this determination is a key part of the CPE process.
Because of the wide range of areas that are evaluated during a CPE, the evaluation team
needs to have a broad range of available skills. This broad skills range is another reason
to use a team approach in conducting CPEs. Specifically, persons should have capability
in the areas shown in Table 6-2.
Table 6-2. Evaluation Team Capabilities
Technical Skills/Knowledge
• Water treatment plant design
• Water treatment operations and
process control
• Regulatory requirements
• Maintenance
• Utility management (rates,
budgeting, planning)
Leadership Skills
• Communication (presenting, listening,
interviewing)
• Organization (scheduling, prioritizing)
• Motivation (involving people, recognizing
staff abilities)
• Decisiveness (completing CPE within time
frame allowed)
• Interpretation (assessing multiple inputs,
• making judgments)
Regulatory agency personnel with experience in evaluating water treatment facilities,
consulting engineers who routinely work with plant evaluation, design and start-up, and
utility personnel with design and operations experience represent the types of personnel
with appropriate backgrounds to conduct CPEs. Other combinations of personnel can be
used if they meet the minimum experience requirements outlined above. Although teams
composed of utility management and operations personnel associated with the CPE
facility can be established, it is often difficult for an internal team to objectively assess
administrative and operational factors. The strength of the CPE is best represented by an
objective third party review.
6.5 CPE Quality Control!
It is important for CPE providers and recipients of CPEs to be aware of appropriate CCP
concepts and expectations of the process. The providers should maintain the integrity of
the program and the recipients should make sure they receive the full benefit of the CPE.
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This is accomplished by following the protocols described in the CCP Handbook.
However, to assure effective and consistent CPE results, quality control considerations
have been developed. Table 6-3 presents a checklist for CPE providers and recipients to
assess the adequacy of a CPE relative to the guidance provided in the CCP Handbook.
The following discusses some of the key areas of concern in more detail.
Table 6-3. Quality Control Checklist for Completed CPEs
Checklist
Findings demonstrate emphasis on achievement of compliance and/or
optimized performance goals (i.e., performance emphasis is evident in
the discussion of why prioritized factors were identified).
Lack of bias associated with the provider's background in the factors
identified (e.g., all design factors identified by a provider with a design
/background or lack of operations or administrative factors identified by
the utility personnel conducting a CPE).
Emphasis in the CPE results to maximize the use of existing facility
capability.
All components of the CPE completed and documented in a report
(i.e., performance assessment, major unit process evaluation,
identification and prioritization of factors, and assessment of CTA
application).
Fewer than 15 factors limiting performance identified (i.e., excessive
factors indicates lack of focus for the utility).
Specific recommendations are not presented in the CPE report, but
rather, clear examples that support the identification of the factors are
summarized.
Identified limitations of operations staff or lack of site-specific
guidelines instead of a need for a third party-prepared operation and
maintenance manual.
Findings address administrative, design, operation and maintenance
factors (i.e., results demonstrate provider's willingness to iden-
tify/present all pertinent factors).
A challenging area for the CPE provider is to maintain the focus of the evaluation on
performance and public health protection. Often, a provider will tend to identify
limitations in a multitude of areas which may not be related to the performance criteria.
Typical areas may include poor plant housekeeping practices, lack of preventive
maintenance, or lack of an operation and maintenance manual. Limitations in these areas
are easily observed and do not challenge the capability of the operations staff. While they
demonstrate a thoroughness by the provider to identify all issues, their identification may
cause the utility to focus resources on these areas while ignoring areas more critical to
achievement of performance goals. The evaluator should be aware that a utility may take
the CPE results and only address those factors that are considered relatively easy to
correct without consideration of priority or the inter-relatedness of the factors.
Another significant challenge in conducting an effective CPE is the tendency for
providers to identify limitations that are non-controversial rather than real factors that
may challenge the plant personnel's roles and responsibilities. For example, it is often
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6, COMPREHENSIVE PERFORMANCE EVALUATION
easy to identify a design limitation, since the utility could not be expected to achieve
desired performance with inadequate facilities. It,is much more difficult to identify "lack
of administrative support" or an operator's "inability to apply process control concepts"
as the causes of poor performance. This may be especially a problem when the CPE
findings tend to criticize the administrators that have hired the CPE providers. Failing to
appropriately identify these difficult factors is a disservice to all parties involved. A
common result of this situation is the utility addressing a design limitation without
addressing existing administrative or operational issues. Ultimately, these
administrative and operational issues remain and impact the utility's ability to achieve
desired performance. Understanding this concept allows the CPE provider to present the
true factors, even though they may not be well received at the exit meeting. CPE
recipients should be suspicious when a plant has a performance problem and no
operations or administrative factors are identified.
A final consideration when implementing a CPE, is to understand the importance that
specific recommendations involving plant modifications or day-to-day operational
practices should not be made by the CPE provider or accepted without question by the
recipient. For example, direction on changing coagulants or chemical dosages is not
appropriate during the conduct of a CPE. These types of changes should be evaluated to
determine if they are truly appropriate for the specific plant. A coagulant that worked for
the CPE provider at one plant may not work for the plant being evaluated; causing
unnecessary costs and/or poor performance. There is a strong bias for providers to give
specific recommendations and for recipients to want specific checklists to implement.
CPE providers should focus their observations during the evaluation on two key areas:
1. Identification of factors limiting the facility from achieving desired
performance goals (compliance or optimized); and
2. Providing specific examples to support these factors.
Recipients should, also, not request specific guidance from the providers and, if this
guidance is provided, they should make sure that the information provided is truly
appropriate to their plant.
6.6 Next Steps
The results of the CPE provide systems and States with a thorough evaluation of
processes at a treatment plant. CPE results identify factors which may be limiting
performance and subsequently causing compliance problems. The CPE affords systems
the opportunity to achieve improvements largely through administrative and operational
changes. Most systems can implement any necessary changes through a self-
improvement program, but if assistance is necessary facilities should work closely with
EPA, the States, and technical assistance programs geared towards improving treatment
plant performance.
The second phase of the CCP, the Comprehensive Technical Assistance (CTA), may be
used to improve performance in a more formal and structured setting. During the CTA
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6. COMPREHENSIVE PERFORMANCE EVALUATION
phase, the system, with assistance from the State, identifies and systematically addresses
plant-specific factors. The CTA is a combination of utilizing CPE results as a basis for
follow-up, implementing process control priority-setting techniques, and maintaining
long-term involvement to systematically train staff and administrators.
6.7 References
1. USEPA. 1998. Handbook: Optimizing Water Treatment Plant Performance Using
the Composite Correction Program. EPA/625/6-91/027.
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7. IMPORTANCE OF TURBIDITY
7.1 Overview
Section 2 of this guidance manual is included to present an overview on the definition
and sources of turbidity. Understanding turbidity, its causes and sources, and the
significance to human health will provide the background on which the new turbidity
standards are based.
7.2 Turbidity: Definition, Causes, and History as a
Water Quality Parameter
Turbidity is a principal physical characteristic of water and is an expression of the optical
property that causes light to be scattered and absorbed by particles and molecules rather
than transmitted in straight lines through a water sample. It is caused by suspended
matter or impurities that interfere with the clarity of the water. These impurities may
include clay, silt, finely divided inorganic and organic matter, soluble colored organic
compounds, and plankton and other microscopic organisms. Typical sources of turbidity
in drinking water include the following (see Figure 7-1):
• Waste discharges;
• Runoff from watersheds, especially those that are disturbed or eroding;
• Algae or aquatic weeds and products of their breakdown in water reservoirs,
rivers, or lakes;
• Humic acids and other organic compounds resulting from decay of plants,
leaves, etc. in water sources; and
• High iron concentrations which give waters a rust-red coloration (mainly in
ground water and ground water under the direct influence of surface water).
• Air bubbles and particles from the treatment process (e.g., hydroxides, lime
softening)
Simply stated, turbidity is the measure of relative clarity of a liquid. Clarity is important
when producing drinking water for human consumption and in many manufacturing uses.
Once considered as a mostly aesthetic characteristic of drinking water, significant
evidence exists that controlling turbidity is a competent safeguard against pathogens in
drinking water.
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7. IMPORTANCE OF TURBIDITY
Treatment Plant Discharges
Runoff from
Plowed Land
Colored Industrial,
Wastes
Erosion of Rocks and
Mineral Deposits
Figure 7-1. Typical Service of Turbidity in Drinking Water
The first practical attempts to quantify turbidity date to 1900 when Whipple and Jackson
developed a standard suspension fluid using 1,000 parts per million (ppm) of
diatomaceous earth in distilled water (Sadar, 1996). Dilution of this reference suspension
resulted in a series of standard suspensions, which were then used to derive a ppm-silica
scale for calibrating turbidimeters.
The standard method for determination of turbidity is based on the Jackson candle
turbidimeter, an application of Whipple and Jackson's ppm-silica scale (Sadar, 1996).
The Jackson candle turbidimeter consists of a special candle and a flat-bottomed glass
tube (Figure 7-2), and was calibrated by Jackson in graduations equivalent to ppm of
suspended silica turbidity. A water sample is poured into the tube until the visual image
of the candle flame, as viewed from the top of the tube, is diffused to a uniform glow.
When the intensity of the scattered light equals that of the transmitted light, the image
disappears; the depth of the sample in the tube is read against the ppm-silica scale, and
turbidity was measured in Jackson turbidity units (JTU). Standards were prepared from
materials found in nature, such as Fuller's earth, kaolin, and bed sediment, making
consistency in formulation difficult to achieve.
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7. IMPORTANCE OF TURB/D/TY
Eye
A
-*•>
fflf-
Scattered Light is as
Intense as Transmitted
• Light-Image of Flame
Disappears at this Depth
•Scattered Light
•Scattered Light Weak-
Transmitted Light Strong
Length of Arrow
Proportional to Intensity
of Beam of Light
Source: Sadar, 1996.
Figure 7-2. Jackson Candle Turbidimeter
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7. IMPORTANCE OF TURBIDITY
In 1926, Kingsbury and Clark discovered formazin, which is formulated completely of
traceable raw materials and drastically improved the consistency in standards
formulation. Formazin is a suitable suspension for turbidity standards when prepared
accurately by weighing and dissolving 5.00 grams of hydrazine sulfate and 50.0 grams of
hexamethylenetetramine in one liter of distilled water. The solution develops a white hue
after standing at 25 °C for 48 hours. A new unit of turbidity measurement was adopted
called formazin turbidity units (FTU).
Even though the consistency of formazin improved the accuracy of the Jackson Candle
Turbidimeter, it was still limited in its ability to measure extremely high or low turbidity.
More precise measurements of very low turbidity were needed to define turbidity in
samples containing fine solids. The Jackson Candle Turbidimeter is impractical for this
because the lowest turbidity value on this instrument is 25 JTU. The method is also
cumbersome and too dependent on human judgement to determine the exact extinction
point.
Indirect secondary methods were developed to estimate turbidity. Several visual
extinction turbidimeters were developed with improved light sources and comparison
techniques, but all were still dependent of human judgement. Photoelectric detectors
became popular since they are sensitive to very small changes in light intensity. These
methods provided much better precision under certain conditions, but were still limited in
ability to measure extremely high or low turbidities.
Finally, turbidity measurement standards changed in the 1970's when the nephelometric
turbidimeter, or nephelometer, was developed which determines turbidity by the light
scattered at an angle of 90° from the incident beam (Figure 7-3). A 90° detection angle is
considered to be the least sensitive to variations in particle size. Nephelometry has been
adopted by Standard Methods as the preferred means for measuring turbidity because of
the method's sensitivity, precision, and applicability over a wide range of particle size and
concentration. The nephelometric method is calibrated using suspensions of formazin
polymer such that a value of 40 nephelometric units (NTU) is approximately equal to 40
JTU (AWWARF, 1998). The preferred expression of turbidity is NTU.
7.3 Turbidity's Significance to Human Health
Excessive turbidity, or cloudiness, in drinking water is aesthetically unappealing, and may
also represent a health concern. Turbidity can provide food and shelter for pathogens. If
not removed, turbidity can promote regrowth of pathogens in the distribution system,
leading to waterborne disease outbreaks, which have caused significant cases of
gastroenteritis throughout the United States and the world. Although turbidity is not a
direct indicator of health risk, numerous studies show a strong relationship between
removal of turbidity and removal of protozoa.
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7. IMPORTANCE OF TURBIDITY
\J/
Glass
Sample Cell
Lamp
Lens Aperture
Transmitted
Light
90° Scattered
Light
Detector
Source: Sadar, 1996; photo revised by SAIC, 1998.
Figure 7-3. Nephelometric Turbidimeter
The particles of turbidity provide "shelter" for microbes by reducing their exposure to
attack by disinfectants (Figure 7-4). Microbial attachment to paniculate material or inert
substances in water systems has been documented by several investigators (Marshall,
1976; Olson et al., 1981; Herson et ah, 1584) and has been considered to aid in microbe
survival (NAS, 1980). Fortunately, traditional water treatment processes have the ability
to effectively remove turbidity when operated properly.
7.3.1 Waterborne Disease Outbreaks
Notwithstanding the advances made in water treatment technology, waterborne pathogens
have caused significant disease outbreaks in the United States and continue to pose a
significant problem. Even in developed countries, protozoa have been identified as the
cause of half of the recognized waterborne outbreaks (Rose et al., 1991). The most
frequently reported waterborne disease in the United States is acute gastrointestinal
illness, or gastroenteritis (Huben, 1991). The symptoms for this disease include fever,
headache, gastrointestinal discomfort, vomiting, and diarrhea. Gastroenteritis is usually
self-limiting, with symptoms lasting one to two weeks in most cases. However, if the
immune system is suppressed, as with the young, elderly and those suffering from HIV or
ADDS, the condition can be very serious and even life threatening. The causes are usually
difficult to identify but can be traced to various viruses, bacteria, or protozoa.
April 1999
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7. IMPORTANCE OF TURBIDITY
Protected
Micro-organisms
Exposed
Micro-organisms
Participates
Source: LeChevallier and Norton, 1991.
Figure 7-4. Particles of Turbidity May Provide Protection for
Microorganisms
Giardia and Cryptosporidium are the two most studied organisms known to cause
waterborne illnesses. These two protozoa are believed to be ubiquitous in source water,
are known to occur in drinking water systems, have been responsible for the majority of
waterborne outbreaks, and treatments to remove and/or inactivate them are known to be
effective for a wide range of waterborne parasites (LeChevallier and Norton, in Craun,
1993). Giardia and Cryptosporidium have caused over 400,000 persons in the United
States to become ill since 1991, mostly due to a 1993 outbreak in Milwaukee, Wisconsin.
Giardia and viruses are addressed under the 1989 SWTR. Systems using surface water
must provide adequate treatment to remove and/or inactivate at least 3-log (99.9%) of the
Giardia lamblia cysts and at least 4-log (99.99%) of the enteric viruses. However,
Cryptosporidium was not addressed in the SWTR due to lack of occurrence and health
effects data. In the mid-1980's, the United States experienced its first recognized
waterborne disease outbreak of cryptosporidiosis (D1 Antonio et ah, 1985). It was soon
discovered that the presence of Cryptosporidium in drinking water, even in very low
concentrations, could be a significant health hazard (Gregory, 1994). In 1993, a major
outbreak of cryptosporidiosis occurred even though the system was in full compliance
with the SWTR. Several outbreaks caused by this pathogen have been reported (Smith et
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7. IMPORTANCE OF TURBIDITY
si., 1988; Hayes at al., 1989; Levine and Craun, 1990; Moore et al., 1993; Craun, 1993).
The ESWTR's primary focus is to establish treatment requirements to further address
public health risks from pathogen occurrence, and in particular, Cryptosporidium.
Table 7-1 displays several instances of past outbreaks of cryptosporidiosis in systems
using surface water as a source, along with general information about the plant and
turbidity monitoring. In three out of four of the cases displayed in the table (Milwaukee,
Jackson County, and Carrollton), turbidity over 1.0 NTU was occurring in finished water
during the outbreaks.
Table 7-1. Cryptosporidium Outbreaks vs. Finished Water Turbidity
Location of Outbreak
Las Vegas, Nevada
(CDC, 1996)
/
Milwaukee, Wisconsin
(CDC, 1996,
Logsdon, 1 996)
1
Jackson County,
Oregon
(USEPA, 1997)
Carrollton, Georgia
(USEPA, 1997,
Logsdon, 1996)
Year
1993-
1994
1993
1992
1987
General Plant Information
No apparent deficiencies or problems
with this community system; SWTR
compliant; system performed pre-
chlorination, filtration (sand and carbon),
and filtration of lake water; outbreak
affected mostly persons infected with
the human immunodeficiency virus
(HIV)
Community system; SWTR compliant;
however, deterioration in source (lake)
raw-water quality and decreased
effectiveness of the coagulation-
filtration process
Poor plant performance (excessive
levels of algae and debris); no pre-
chlorination before filtration
Conventional filtration plant; sewage
overflowed into water treatment intake,
followed by operational irregularities in
treatment; filters were placed back into
service without being backwashed.
Turbidity Information
The raw water averaged 0.14
NTU between January 1 993 and
June 1995, with a high of 0.3
NTU; the maximum turbidity of
finished water during this time
was 0.17 NTU.
Dramatic temporary increase in
finished water turbidity levels;
reported values were as high as
2.7 NTU. (Turbidity had never
exceeded 0.4 NTU in the
previous 10 years.)
Earlier in the year when outbreak
occurred, filtered water had
averaged 1 NTU or greater.
Filtered water turbidity from one
filter reached 3 NTU about three
hours after it was returned to
service without being washed.
7.3.2 The Relationship Between Turbidity Removal and Pathogen
Removal
Low filtered water turbidity can be correlated with low bacterial counts and low
incidences of viral disease. Positive correlations between removal (the difference
between raw and plant effluent water samples) of pathogens and turbidity have also been
observed in several studies. In fact, in every study to date where pathogens and turbidity
occur in the source water, pathogen removal coincides with turbidity/particle removal
(Fox, 1995).
As an example, data gathered by LeChevallier and Norton (in Craun, 1993) from three
drinking water treatment plants using different watersheds indicated that for every log
removal of turbidity, 0.89 log removal was achieved for the parasites Cryptosporidium
and Giardia (Figures 7-5 and 7-6). Of course, this exact relationship does not hold for all
treatment plants. Table 7-2 lists several other studies in addition to LeChevallier and
Norton's, and their conclusions on the relationship of turbidity to protozoan removal.
April 1999
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7. IMPORTANCE OF TURBIDITY
All studies in Table 7-2 show turbidity as a useful predictor of parasite removal
efficiency. This evidence suggests that although a very low turbidity value does not
completely ensure that particles are absent, it is an excellent measure of plant
optimization to ensure maximum public health protection.
.S 4 -
"S
•2 o_j
O 3
| ^
o>
1 -
D)
O
o-
logY = 0.892(logx) + 0.694
r = 0.780
-1.0 0.0 1.0 2.0 3.0 4.0
Log Removal Turbidity
Source: LeChevallier and Norton, 1991.
Figure 7-5. Relationship Between Removal of Giardia and Turbidity
E 4.0
"S 3.0 H
Q.
CO
f 2.H
o
1.1-1
-0.9
logY = 0.886(logx) + 0.494
r = 0.771
-1.0 0.0 1.0 2.0 3.0 4.0
Log Removal Turbidity
Source: LeChevallier and Norton, 1991.
Figure 7-6. Relationship Between Removal of Cryptosporidium and
Turbidity
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April 1999
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7. IMPORTANCE OF TURBIDITY
Table 7-2. Studies on the Relationship between Turbidity Removal
and Protozoa Removal
Reference/Study
Patania et al., 1995*
Nieminski and Ongerth,
1995*
Ongerth and Pecoraro,
1995*
LeChavallier and Norton
(in Craun, 1993)
Nieminski, 1992
Ongerth, 1990
LeChavallier etal., 1991*
LeChavallier and Norton,
1992*
Foundation for Water
Research, 1994*
Hall etal., 1994
Gregory, 1994
Anderson et al., 1996
Discovery/Conclusion on Turbidity
Four systems using rapid granular filtration, when treatment conditions were optimized for
turbidity and particle removal, achieved a median turbidity removal of 1.4 log and median
particle removal of 2 log. The median cyst and oocyst removal was 4.2 log. A filter effluent
turbidity of less than 0.1 NTU or less resulted in the most effective cyst removal, by up to 1 .0
log greater than when filter effluent turbidities were greater than 0.1 NTU (within the 0.1 to
0.3 NTU range).
Pilot plant study: Source water turbidity averaged 4 NTU (maximum - 23 NTU), achieving
filtered water turbidities of 0.1-0.2 NTU. Cryptosporidium removals averaged 3.0 log for
conventional treatment and 3.0 log for direct filtration, while Giardia removals averaged 3.4
log for conventional treatment and 3.3 log for direct filtration.
Full scale plant study: Source water had turbidities typically between 2.5 and 1 1 NTU (with
a peak level of 28 NTU), achieving filtered water turbidities of 0.1 -0.2 NTU. Cryptosporidium
removals averaged 2.25 log for conventional treatment and 2.8 log for direct filtration, while
Giardia removals averaged 3.3 log for conventional treatment and 3.9 log for direct filtration.
Using very low-turbidity source waters (0.35 to 0.58 NTU), 3 log removal for both cysts were
obtained, with optimal coagulation. (With intentionally suboptimal coagulation, the removals
were only 1 .5 log for Cryptosporidium and 1 .3 log for Giardia.)
Data gathered from three drinking water treatment plants using different watersheds
indicated that for every log removal of turbidity, 0.89 log removal was achieved for
Cryptosporidium and Giardia.
A high correlation (r2=0.91) exists between overall turbidity removal and both Giardia and
Cryptosporidium removal through conventional water treatment.
Giardia cyst removal by filtration of well-conditioned water results in 90% or better turbidity
reduction, which produces effective cyst removal of 2-log (99%) or more.
In a study of 66 surface water treatment plants using conventional treatment, most of the
utilities achieved between 2 and 2.5 log removals for both Cryptosporidium and Giardia, and
a significant correlation (p=0.01 ) between removal of turbidity and Cryptosporidium existed.
In source water turbidities ranging from 1 to 120 NTU, removal achieved a median of 2.5 log
for Cryptosporidium and Giardia at varying stages of treatment optimization. The probability
of detecting cysts and oocysts in finished water supplies depended on the number of
organisms in the raw water; turbidity was a useful predictor of Giardia and Cryptosporidium
removal.
Raw water turbidity ranged from 1 to 30 NTU, and Cryptosporidium removal was between 2
and 3 log. Investigators concluded that any measure which reduces filter effluent turbidity
should reduce risk from Cryptosporidium.
Any measure which reduces filtrate turbidity will reduce the risk from Cryptosporidium; a
sudden increase in the clarified water turbidity may indicate the onset of operational
problems with a consequent risk from cryptosporidiosis.
Maintaining the overall level of particulate impurities (turbidity) in a treated water as low as
possible may be an effective safeguard against the presence of oocysts and pathogens.
In a pilot plant study, the removal of particles > 20m was significantly related to turbidity
reduction r=0.97 (p<0.0001); the removal of Cryptosporidium oocysts may be related to the
removal of Giardia, r=0.79 (p<0.14); the reduction of turbidity may be related to the removal
of Giardia cysts, r=0.67 (p<0.1 3) and Cryptosporidium oocysts (p<0.08)
as discussed in EPA's Notice of Data Availability (USEPA, 1 997)
April 1999
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7. IMPORTANCE OF TURBIDITY
7.4 References
1. Anderson, W.L., et al. 1996. "Biological Particle Surrogates for Filtration
Performance Evaluation."
2. CDC (Centers for Disease Control). 1996. "Surveillance for Waterborne-Disease
Outbreaks - United States, 1993-1994." Morbidity and Mortality Weekly Report,
45(SS-1).
3. D'Antonio, R.G., R.E. Winn, J.P. Taylor, et al. 1985. "A Waterborne Outbreak of
Cryptosporidiosis in Normal Hosts." Annals of Internal Medicine. 103:886-888.
4. Fox, K.R. 1995. "Turbidity as it relates to Waterborne Disease Outbreaks."
Presentation at M/DBP Information Exchange, Cincinnati, Ohio. AWWA white
paper.
5. Gregory, J. 1994. "Cryptosporidium in Water: Treatment and Monitoring
Methods." Filtration & Separation. 31:283-289.
6. Hall, T., J. Presdee, and E. Carrington. 1994. "Removal of Cryptosporidium
oocysts by water treatment processes." Foundation for Water Research.
7. Herson, D.S., D.R. Marshall, and H.T. Victoreen. 1984. "Bacterial persistence in
the distribution system." J. AWWA. 76:309-22.
8. LeChevallier, M.W., W.D. Norton, and R.G. Lee. 1991. "Giardia and
Cryptosporidium in Filtered Drinking Water Supplies." Applied and Environmental
Microbiology. 2617-2621.
9. LeChevallier, M.W. and W.D. Norton. 1992. "Examining Relationships Between
Particle Counts and Giardia, Cryptosporidium, and Turbidity." J. AWWA.
10. LeChevallier, M.W. and W.D. Norton. "Treatments to Address Source Water
Concerns: Protozoa." Safety of Water Disinfection: Balancing Chemical and
Microbial Risks. G.F. Craun, editor. ILSI Press, Washington, D.C.
11. Marshall, K.C. 1976. Interfaces in microbial ecology. Harvard University Press,
Cambridge, MA.
12. NAS (National Academy of Sciences). 1980. National Research Council: drinking
water and health, Volume 2. National Academy Press, Washington, D.C.
13. Nieminski, E.G. 1992. "Giardia and Cryptosporidium - Where do the cysts go."
Conference proceedings, AWWA Water Quality Technology Conference.
14. Olson, B.H., H.F. Ridgway, and E.G. Means. 1981. "Bacterial colonization of
mortar-lined and galvanized iron water distribution mains." Conference
proceedings, AWWA National Conference. Denver, CO.
15. Ongerth, J.E. 1990. "Evaluation of Treatment for Removing Giardia Cysts." J.
AWWA. 82(6):85-96.
16. Sadar, M.J. 1996. Understanding Turbidity Science. Hach Company Technical
Information Series - Booklet No. 11.
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7. IMPORTANCE OF TURBIDITY
11. USEPA. 1997. Occurrence Assessment for the Interim Enhanced Surface Water-
Treatment Rule, Final Draft. Office of Ground Water and Drinking Water,
Washington, D.C.
18. USEPA. 1983. Turbidity Removal for Small Public Water Systems. Office of
Ground Water and Drinking Water, Washington, D.C.
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8. PARTICLES CONTRIBUTING TO
TURBIDITY
8.1 Introduction
To address turbidity removal during treatment processes, an understanding of the physical
characteristics and properties of particles in raw water is required. This chapter provides
an overview of the inorganic, organic and biotic particles, as well as particles created
during typical treatment processes that contribute to turbidity. Because the stability of
particles in water is dominated by the electrokinetic properties, a discussion of
electrokinetic properties is included to provide information concerning how these
properties affect the removal of particle contamination during the treatment process.
8.2 Characteristic Properties of Particles
Particles in a raw water supply may be composed of inorganic materials, pathogens, or
toxic materials. These particles may also provide sorbent sites for pesticides and other
synthetic organic chemicals and heavy metals. Particles are undesirable not only for the
cloudy appearance they impart to finished water, but because they also have the ability to
shelter microorganisms from inactivation by disinfectants. Consequently, a principal
element in supplying quality drinking water is the maximum removal of particles. To
establish or optimize a particle removal process, it is important to understand the physical
properties of particles.
Particles suspended in water can be categorized into three classes based on their origin:
1. Inorganic materials, such as silt or minerals;
2. Living or dead organic matter; and
3. Biotic material including algae, viruses and bacteria.
Due to the range of small sizes for common particles in water, it is common to find sizes
termed in "microns" within the water industry. A micron, or micrometer, is equal to 1 x
10"6 meters, or 0.00004 inches. Generally, particulate contaminants to be removed from a
raw water source range from the larger macro sized particles visible to the naked eye, to
the ionic particles viewed only by scanning electron microscopes.
Figure 8-1 illustrates ,some common particles found in raw water sources and indicates
where, within the size range, these particles would typically be detected.
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8. PARTICLES CONTRIBUTING TO TURBIDITY
MACRO
MICRO
Red Blood
MACRO
MOLECULAR
MOLECULAR
Asbestos
Sugar
IONIC
Sand
Yeast Cells
Pollen
Granular
Activated
Carbon
I
1
Virus
Salts
II.
B" a.
Bacteria
3
R-
5e
5'
Hair
Colloids
Giardia
Molecules
SUSPENDED PARTICLES
DISSOLVED PARTICLES
Micron
Scale
1000
Source: Osmonics, Inc., 1996; AWWA, 1990.
Figure 8-1. Particle Size Spectrum
8.2.1 Particle Settling
Particle settling, or sedimentation, may be described for a singular particle by the Newton
equation for terminal settling velocity of a spherical particle. A knowledge of this
velocity is basic in the design and performance of a sedimentation basin.
The rate at which discrete particles will settle in a fluid of constant temperature is given
by the equation:
= [(4g(ps-p)d)\(3Cdp)]
0.5
where
V = terminal settling velocity
g = gravitational constant
ps = mass density of the particle
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8. PARTICLES CONTRIBUTING TO TURBIDITY
p = mass density of the fluid
d = particle diameter
Ca = Coefficient of drag (dimensionless)
The terminal settling velocity is derived.by equating the drag, buoyant, and gravitational
forces acting on the particle. At low settling velocities, the equation is not dependent on
the shape of the particle and most sedimentation processes are designed so as to remove
small particles, ranging from 1.0 to 0.5 micron, which settle slowly. Larger particles
settle at higher velocity and will be removed whether or not they follow Newton's law, or
Stokes' law, the governing equation when the drag coefficient is sufficiently small (0.5 or
less) as is the icase for colloidal products (McGhee, 1991).
Colloids are very fine solid particles, typically between 10 and 0.001 microns in diameter,
which are suspended'in solution. Colloidal particles are not visible even with the aid of
high-powered microscopes (Sawyer and McCarty,1978). Colloids will not settle out by
gravitational forces and may not be removed by conventional filtration alone. The
removal of colloidal particles is typically achieved by coagulation to form larger particles,
which then may be removed by sedimentation and/or filtration. Coagulation, as defined
by Kawamura (1991), is the "destabilization of (the) charge on colloids and suspended
solids, including bacteria and viruses," and is further discussed in Section 8.7,
"Electrokinetic Properties of Particles."
8.2.2 Particle Density and Size Distribution
Typically, a large range of particle sizes will exist in the raw water supply. Type 1
settling is the designation given to discrete particles of various sizes, in a dilute
suspension, which settle without flocculating. Dilute suspensions of flocculating
particles, where heavier particles overtake and coalesce with smaller and lighter particles,
are given the designation of Type 2. As there is no mathematical equation which can be '
applied to the relationships of Type 1 and 2 sedimentation, statistical analysis is applied
to predict the settling velocities for particles in water having a broad range of size and
density. Particle size distribution analysis (Type 1) or settling-column analysis (Type 1
or 2) is applied and a settling velocity cumulative frequency curve is obtained and used in
settling basin design. An excellent resource for understanding the use of settling column
analysis, and discrete particle settling is given by Gregory and Zabel (1990).
Type 3 a and 3b, or hindered settling, occur when high densities of particles in suspension
result in an interaction of particles. The displacement of water produced by the settling of
one particle affects the relative velocities of its neighbors (McGhee, 1991). A zone is.
formed in which more rapidly-settling particles act as a group with a reduced settling
velocity. However, even at fairly high concentrations, the reduction in settling velocity is
not significant. The following equation from McGhee (1991) gives an estimate of the
magnitude for hindered settling:
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8. PARTICLES CONTRIBUTING TO TURBIDITY
Vh/V = (1 - C
^4.65
where Vh = hindered settling velocity
V = free settling velocity
Cv = volume of particles divided by total volume of the suspension.
8.3 Inorganic Particles
Inorganic particles in water are produced by the natural weathering of minerals, including
both suspended and dissolved materials. Inorganic particles may consist of iron oxides,
salts, sulfur, silts and clays such as bentonite or muscovite. Depending on the
concentration of inorganic particles present in raw water sources, human health effects
can vary from beneficial to toxic.
8.3.1 Naturally Occurring Minerals
Naturally occurring minerals find their way into raw water sources either naturally
through the breakdown of minerals in rock, or through industrial process discharges
which have contaminated a raw water source. Industrial contributors can include mining,
smelting, coal burning power producers, oil and gas companies, and electroplating
operations.
Clays, metal hydroxides, and other particles originating from mineral sources typically
vary from several nanometers to several microns in diameter, with a continuous size
distribution over this range. In surface waters, the majority of these particles are within a
0.1 to 1 micron size range. As a result of their settling characteristics, particles in this
size range have the ability to remain in suspension in moving water. Particles of this size
range scatter visible light efficiently, due to the larger surface areas which are created as
particles decrease in size. This scattering gives the water a turbid, or cloudy, appearance
at very low concentrations. However, Wiesner and Klute (1998) suggest that the real
threat of these particles is the adsorptive properties. The large surface areas created by
even a small mass concentration of the colloid particles provide abundant adsorption sites
for natural and synthetic organic matter, metals, and other toxic substances. Bacteria and
viruses can also attach to these particles, and there is some concern that inorganic
particulate contamination has the ability to shield microorganisms from inactivation by
disinfectants.
Dissolved inorganics known to have adverse health effects on humans when ingested
include aluminum, arsenic, cadmium, copper, fluoride, lead, and mercury. The EPA has
established maximum contaminant levels (MCLs) for a variety of inorganic contaminants
and is in constant review of health advisories to determine the health effects from
inorganics ingested in drinking water (Tate and Arnold, 1990). The inorganic materials
for which MCLs have been established are toxic to humans in some form.
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8.4 Organic Particles
Organic materials are compounds, natural or manmade, having a chemical structure based
upon the carbon molecule. Millions of organic compounds containing carbon have been
identified and named, including; hydrocarbons, wood, sugars, proteins, plastics,
petroleum-based compounds, solvents, pesticides and herbicides.
Both naturally-occurring and synthetic organics are present in surface waters and typically
originate from the following sources (Tate and Arnold, 1990):
1. The decomposition of naturally occurring organic materials in the environment;
2. Industrial, agricultural and domestic activities; and
3. Reactions occurring during the treatment and distribution of drinking water.
/
Organics may have adverse human health impacts, such as toxicity, or as carcinogens
when ingested. In addition, naturally occurring organics, most widely referred to as
natural organic matter (NOM), can give raw water a characteristic color, taste, or odor.
Furthermore, organics in water can be altered by treatment processes resulting in
disinfection byproducts (DBFs). In the following sections, a description of the organic
constituents in raw water is provided.
8.4.1 Synthetic Organics
Artificial organics, or synthetic organics, can infiltrate raw water supplies through
overland flow of contaminated urban and agricultural rainwater; direct discharge from
industries and wastewater treatment plants, and, as leachate from contaminated soils.
Most contaminants found in water supplies that have adverse health effects are synthetic
organics including: herbicides and pesticides; solvents; and, polychlorinated biphenyls
commonly known as PCBs (Tate and Arnold, 1990). The EPA has set MCLs for many
synthetic substances, both in industrial waste discharge and within the primary drinking
water standards. '
8.4.2 Natural Organic Maltter (NOM)
In the majority of raw water sources, the largest fraction of all organic particles is due to
NOM originating from the degradation of plant or animal materials (Wiesner and Klute,
- 1998). NOM is undesirable in raw water for a variety of reasons, ranging from
undesirable color to providing adsorption site for toxic substances. NOM will also
adsorb to inorganic particles present in raw water, reducing the settling properties of
those particles. Aiken and Cotsaris (1995) recognized numerous studies supporting the
importance of NOM in mobilization of hydrophobic organic species; of metals (lead,
cadmium, copper, zinc, mercury, and chromium); and radionuclides through the treatment
process. Elevated levels of certain NOM constituents require additional coagulation in
order to destabilize the particles and remove them in sedimentation and/or filtration
basins.
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NOM is also present in raw water supplies as colloidal organic carbon in the form of
humic materials. Humic substances have generated considerable attention due to their
disinfection by-products (DBF) formation potential (Amirtharajah and O'Melia, 1990).
8.4.3 Total Organic Carbon (TOC)
TOC is a composite measure of the overall organic content, in a water sample. TOC is
measured by the amount of carbon dioxide produced when a water sample is atomized in
a combustion chamber (Standard Methods, 1985). Total organic halogen (TOX) indicates
the presence of halogenated organics, and is a proper indication of synthetic chemical
contamination. Either of these methods are more economical than testing for any, or all,
individual organic compounds likely to be in a raw water supply.
8.4.4 Organic Disinfection By-products (DBFs)
The use of oxidants for disinfection, taste and odor removal, or for decreasing coagulant
demand also produces undesirable organic by-products. These by-products are difficult
to analyze and remove from the treatment process. Organic contaminants formed during
water treatment include trihalomethanes (THMs) and haloacetonitriles. Surveys
conducted since the mid-1970s have determined that chloroform and other THMs are the
organic chemicals occurring most consistently, and at overall highest concentrations, of
any organic contaminant in treated drinking water (Wiesner and Klute, 1998).
THMs are formed in water when chlorine being used as a disinfectant reacts with NOM,
such as humic acids from decaying vegetation. Chloroform is one of the most common
THMs formed in this type of reaction. The THMs include trichloromethane or
chloroform; dibromochloromethane; dichlorobromomethane; and bromoform.
Water chlorination not only produces THMs, but also a variety of other organic
compounds. Alternative disinfectants such as, chloramines, chlorine dioxide, and ozone
can also react with source water organics to yield organic by-products. Exactly which
compounds are formed, their formation pathways, and their health effects are not well
known. To complicate matters, many of the DBFs are not susceptible to even highly
sophisticated methods of extraction and analysis.
8.5 Particles of Biotic Origin
Four categories of waterborne microorganisms exist as particles contributing to the
turbidity of raw water:
• Protozoans ;N
• Enteric viruses;
• Algae; and
• Bacteria.
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Microorganisms are living organisms that are invisible or barely visible to the naked eye.
While many microorganisms commonly found in source waters do not pose health risk to
humans, others such as Cryptosporidium can be sources of infectious and communicable
diseases.
Isolation and identification of a specific organism such as Cryptosporidium may prove
difficult due to the volume and variety of other microorganisms in the sample. Most
municipal water plant labs do not possess the equipment required for testing and
identification of specific pathogens. Indicator organisms are frequently used to assess
contamination by a biotic constituent. Total coliforms are the widely used indicator for
pathogens. While the presence of coliforms is not proof that the water contains harmful
pathogens, the absence of them is often used as evidence that it is free of pathogens.
8.5.1 Protozoans
Protozoans are organisms that can exist in colonies or as single cells. Some protozoans
are capable of producing spores, a small reproductive body capable of reproducing the
organism under favorable conditions. In water, most spores resist adverse conditions that
would readily destroy the parent organism.
Of the tens of thousands of species of protozoa, the principal protozoan pathogens of
concern in potable water are Cryptosporidium, Giardia lamblia, and E. histolytica. When
these organisms are ingested by humans, they can cause symptoms including; stomach
cramps, diarrhea, fever, vomiting, arid dehydration. These parasites are typically more
resistant to traditional chlorine disinfection than coliforms.
Cryptosporidium is a disease-causing protozoan housed in a hard-shelled oocyst
(pronounced o-he-sist). The oocyst is typically 2 to 5 microns in diameter, round to egg
shaped, colorless and nearly transparent. Human and animal feces are sources of
Cryptosporidium in surface water. Normally, oocysts are found dormant in the
environment. When ingested, the oocyst splits open to release sporozoites. In this new
form, a complex reproductive cycle begins. Figure 8-2 describes the lifecycle of
Cryptosporidium.
The sporozoites invade the lining of the gastrointestinal tract and can cause an illness
called Cryptosporidiosis. The disease can be fatal to people with suppressed immune
systems, including persons with acquired immune deficiency syndrome (AIDS), those
undergoing chemotherapy, children and the elderly (Current and Garcia, 1991). Human
Cryptosporidiosis was first reported in 1976, and outbreaks in public water systems have
motivated numerous studies and regulatory attention on the effectiveness of filtration and
chemical disinfection in the removal and inactivation of these protozoans. Limited data
suggests that Cryptosporidium oocysts are resistant to disinfection at levels practiced in
the U.S. at the time of D3SWTR promulgation. While research is underway to identify
more appropriate inactivation techniques, removal by filtration is currently the most
effective means of dealing with Cryptosporidium. , '
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MEROZOITES
29-060497-028
Source: Ewing, 1986.
Figure 8-2. Cryptosporidium Life Cycle
Human and animal feces are also sources of Giardia in surface waters. Giardia exists as
either a flagellated trophozite of approximately 9 to 21 microns, or ovoid cysts,
approximately 10 microns long and 6 microns wide. Cysts can survive in water from 1 to
3 months. Objects of this size are easily removed by packed bed filters, provided that
coagulation and flocculation pretreatment are properly controlled.
Unlike Giardia and Cryptosporidium, mammals are not a source of E. histolytica to water
supplies and potential contamination of surface water is considered to be low (Wiesner
and Klute, 1998). The size range for the protozoan is 15 to 25 microns for a trophozite
and 10 to 15 microns for the cyst, and are effectively removed by filtration. Additional
information regarding Cryptosporidium may be found in Occurrence Assessment for the
Interim Enhanced Surface Water Treatment Rule (USEPA, 1997).
8.5.2 Viruses
A virus is a parasitic, infectious microbe, composed almost entirely of protein and nucleic
acids that can cause disease in humans and other living organisms. Viruses can reproduce
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8. PARTICLES CONTRIBUTING TO TURBIDITY
only within living cells, and typically range from 0.004 to 0.1 micron in diameter. The
principal viral pathogens of concern in potable water are the Enteric viruses: hepatitis A,
Norwalk-type viruses, rotaviruses, adenoviruses, enteroviruses, and reoviruses. Enteric
viruses infect the gastrointestinal tracts of humans and are transmitted through public
water supplies. It appears that many viruses have an attraction for the surfaces of larger
colloidal particles and, if aggregated, may increase the effective size of these pathogens to
promote their removal (Wiesner arid Klute, 1998).
8.5.3 Algae
Algae are common and normal inhabitants of surface waters and are encountered in every
water supply that is exposed to sunlight (Tarzwell, undated). Algae typically range in
size from 5 to 100 microns. Figure 8-3 presents common types of algae which can be
found within source water and in the water treatment process.
Algae are not typically a threat to public health in a drinking water supply. Concerns in
potable water treatment arising from the presence of algae include; the ability to create
large quantities of organic matter; the production of turbidity, tastes and odors in source
water, and; the physical impact on the water treatment plant processes. Some species of
blue-green algae are known to produce endotoxins which may affect human health.
Algae can clog filters, resulting in reduced run times and an increase in the volume of
backwash water needed for cleaning. Examples of filter clogging algae are seen in Figure
8-4. In slow sand filters and biologically active filters, algae will produce oxygen for
bacteria that actively degrade organic compounds.-. They may also release biopolymers
that aid in the destabilization of fine colloidal riaatdrials (Wiesner and Klute, 1998).
8.5.4 Bacteria
Bacteria are single-celled organisms that lack well-defined nuclear membranes and other
specialized functional cell parts. Bacterial cells typically range from 1 to 15 microns in
length. They vary in shape from simple spheres to filamentous threads. Figure 8-5
presents various bacterial and fungal forms. Bacteria and fungi are decomposers that
break down the wastes and bodies of dead organisms to make their components available
for reuse. Bacteria can exist almost anywhere on earth and in almost any medium. Some
are beneficial to man while others are harmful, or even fatal. The principal bacterial
pathogens of concern in water treatment are the Salmonella, Shigella, Yersinia
enterocolitica, enteropathogenie E. coli, Campylobacter jejuni, Legionella, Vibrio
cholerae, and Mycobacterium.
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Fragilaria
Gomphosphaeria
Scenedesmus
Cylinclrospermium
Stauroneis
Source: Standard Methods, 1985.
Figure 8-3. Plankton and Other Surface Water Algae
Pediastrum
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8. PARTICLES CONTRIBUTING TO TURBIDITY
Anacystis
Dinobryon
Anabeana
Diatoma
Source: Standard Methods, 1985.
Figure 8-4. Filter Clogging Aigae
Gymbella
Palmella
ApriM999
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a)
c)
d)
e)
Bacteria
micrococcus
0,03
b) ©CQGCOS streptococcus
sarcina
bacillus
vibrio
Fungi
Leptomitus, showing
zoospores and cellulin
plugs (diameter 8.5-16/nm)
Tetracladium (diameter 2.5-
3.5 |im)
Zoophagus, showing
mycelial pegs
Zoophagus, with rotifer
impaled on mycelial peg
(diameter 3 |0.m)
Achlya, showing oospores
f)
spirillum
Achlya, showing encysted
zoospores (Oogonia 50-
60|im, oospores 18.5-22 (im,
encysted zoospores 3-5 (im)
Source: Standard Methods, 1985.
Figure 8-5. Examples of Bacteria and Fungi Forms
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8.6 Particles Added or Created During Treatment
Several steps in the water treatment process may contribute to turbidity. As discussed in
Section 10, water treatment is provided to remove undesirable constituents from raw
water, and many of these processes are intended to remove suspended solids and reduce
turbidity. However, this section identifies those chemicals and practices used in water
treatment which are known to increase turbidity. Specifically, the addition of
pretreatment chemicals for coagulation-flocculation-sedimentation or filter aids prior to
filtration can substantially increase the particulate materials loading of sedimentation
basins, filters and other processes used in water treatment. Moreover, increases in
turbidity may occur when any aspect of the water treatment process fails.
8.6.1 Coagulants
The coagulation of water generally involves the chemical addition of either hydrolyzing
electrolytes or organic polymers for the destabilization of colloids in suspension. Some
common coagulants are those based on aluminum, such as aluminum sulfate and alum;
and those based on iron, such as feme and ferrous sulfate. The action of metallic
coagulants is complex and is dependent on the fact that colloid particles are charged
entities in water solution. More discussion of the electrokinetic properties of colloids is
included in section 8-7. Additionally, the use of bentonite, and activated silica for
coagulation enhancement will increase the particle loading in the treatment stream
(Wiesner and Klute, 1998).
Polymers
Natural and synthetic coagulant aids are known as "polyelectrolytes," because they have
characteristics of both polymers and electrolyte. Polyelectrolytes are long-chain, high-
molecular-weight molecules which bear a large number of charged groups. The net
charge on the molecule may be positive, negative, or neutral. The chemical groups on the
polymer are thought to combine with active sites on the colloid, combining them into a
larger particles which will then settle by gravitational force. Both the molecular weight
of the polymer and charge density influence the effectiveness of polyelectrolytes.
Polyelectrolytes may be used alone or in tandem with metallic coagulants. Optimal doses
for polymeric coagulant are typically determined in bench scale or pilot scale plant testing
utilizing source water. Use of quantities over the optimal dose will not increase
coagulation and instead will create unnecessary loading of particles to be removed.
Lime
Lime is a calcinated chemical material used in lime or lime and soda ash water treatment
processes to add alkalinity to the water and adjust the pH. Lime treatment has the
incidental benefits to remove iron, aid in clarification of turbid waters, and minimal
bactericidal benefit (Logsdon et al., 1994). Lime has a tendency to deposit solids at
changes in directions and will precipitate out of solution at areas where velocity decreases
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8. PARTICLES CONTRIBUTING TO TURBIDITY
or where changes in velocity occur. The precipitates formed in the lime-soda softening
process consist principally of calcium carbonate and magnesium hydroxide with size
ranges from 15 to 20 microns. If lime is dosed in quantities greater than the water supply
requires, residual lime particles will increase the turbidity in treated water effluent.
8.6.2 Powdered Activated Carbon (PAC)
PAC adsorption is generally used for the removal of organics, radon, color, and taste and
odor treatment. Activated carbon is produced from bituminous coal, or cellulose-based
substances like wood or coconut shells, by a destructive distillation process that drives off
the volatile components of the material. A highly porous, adsorbent material is created
which possesses a large surface area per unit volume.
PAC is generally less than 0.075 millimeters in size and has an extremely high ratio of
surface area to volume. Nonpolar compounds of high molecular weight are attracted and
held, or adsorbed, to this surface. The effectiveness of organic removal by PAC is
dependent on the pH, temperature, contaminant concentration, molecular weight of the
particles to be adsorbed, type of PAC used, and the contact time of the PAC with the
water.
The relative capacity of different carbons to attract and adsorb particles to their surfaces is
best assessed by bench or pilot scale testing of the raw water supply. Therefore, the
addition of PAC for the removal of organic materials, or to control tastes and odors,
creates an additional loading of materials to the downstream processes, as it is slow to
settle because of its small size and low density.
8.6.3 Recycle Flows
Filtration treatment processes require frequent, intermittent backwash cycles to remove
particles from the media. The backwash water is a concentrate of particles and
pretreatment chemicals added prior to the filters. Some plants capture and return this
concentrate to a location in the treatment process as a recycled flow. The properties of
the backwash concentrate depend on the type and quantity of particles present in the
source water, and pretreatment chemicals and treatment processes used earlier in the
treatment train. The practice of returning spent backwash water to the treatment system
has become a concern due to the potential for returning pathogens to the treatment train.
8.7 Electrokinetic Properties of Particles
Colloidal particles comprise a large portion of the turbidity-producing substances in
waters. Examples of colloidal particles include color compounds, clays, microscopic
organisms and organic matter from decaying vegetation or municipal wastes. Colloidal
dispersions are stable in water, as they posses a large surface area relative to their weight.
Therefore, gravitational forces alone will not remove colloids during sedimentation.
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Effective removal of these colloidal dispersions is greatly impacted by the electrokinetic
properties on the surface of the colloids.
Each colloid carries a similar electrical charge that produces a force of mutual
electrostatic repulsion between adjacent particles. If the charge is high enough, the
colloids will remain discrete and in suspension. The addition of coagulants or polymers
reduces or eliminates this charge and colloids will begin to agglomerate and settle out of
suspension or form interconnected matrices which can then be removed during filtration.
This agglomeration causes the characteristics of the suspension to change by creating new
particle viscosity, settling rates and effective size properties for the colloids.
Colloids are classified as hydrophobic (resistant to water bonding) or hydrophilic (affinity
for water bonding). Hydrophilic colloids are stable because their attraction to water
molecules will overcome the slight charge characteristic they possess. This attraction
makes hydrophilic colloids difficult to remove from suspension. Examples of hydrophilic
colloids include soaps and detergents, soluble starches, soluble proteins and blood serum.
On the other hand, hydrophobic particles are dependent on electrical charge for their
stability in suspension. The bulk of inorganic and organic matter in a turbid raw water is
of this type.
8.7.1 Electrical Potential
Most colloidal particles in water are negatively charged as a result of differences in
electrical potential between the water and the particle phases. This charge is due to an
unequal distribution of ions over the particle surface and the surrounding solution.
The charge on a colloidal particle can be controlled by modifying characteristics of the
water which holds the particles in suspension. Modifications include changing the
liquid's pH or changing the ionic species in solution. Another, more direct technique is to
use surface-active agents, such as coagulants, that directly adsorb to the surface of the
colloid and change its characteristics.
8.7.2 Electrical Double Layer Theory
The double layer model explains the ionic environment surrounding a charged colloid and
explains how the repulsive forces are set up around a colloid. Figure 8-6 illustrates the
resulting colloidal state.
A single negative colloid will initially attract some of the positive ions in the solution to
form a firmly attached layer around the surface of the colloid, known as the Stern layer.
Additional positive ions are still attracted by the negative colloid, but are also repelled by
the Stern layer as well as by other positively charged ions trying to get close to the
negatively charged colloid. This constant attraction and repulsion results in the formation
of a diffuse layer of charged ions surrounding the colloid and Stern layers.
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The diffuse layer can be visualized as a charged atmosphere surrounding the colloid.
Together, the attached positively charged ions in the Stern layer and the charged
atmosphere in the diffuse layer is referred to as the double layer. The charge is a
Stem layer
Diffuse layer
Surface shear
Stem potential
Nerst Potential
Rigid or Stern "/ " Diffuse layer
layer attached /
to particle —'
Source: McGhee,1991.
Figure 8-6. Double Layer Theory (Guoy-Stern Colloidal Model)
maximum at the particle surface and decreases with distance from the surface. The
thickness of this layer depends on the type and concentration of ions in solution.
The DLVO Theory (for Derjaguin, Landau, Verwey and Overbeek) is the classic model
which describes the balance of forces between charged colloid particles. Amirtharajah
and O'Melia (1990) provide a thorough discussion of the electrostatic theory of colloidal
stability from the DLVO model and other works.
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When two similar colloidal particles with similar primary charge approach each other,
their diffuse layers begin to interact. The similar primary charges they possess result in
repulsive forces. The closer the particles approach, the stronger the repulsive forces.
Repulsive forces which keep particles from aggregating are counteracted to some degree
by an attractive force termed van der Waals attraction. All colloidal particles possess this
attractive force regardless of charge and composition. As van der Waals forces tend to be
relatively weak attractions, the force decreases rapidly with an increasing distance
between particles.
The balance of the two opposing forces, electrostatic repulsion and van der Waals
attraction, explains why some colloidal systems agglomerate while others do not. As
particles with similar charge approach one another, the repulsive electrostatic forces
increase to keep them separated. However, if they can be brought sufficiently close
together to get past this energy barrier, the attractive van der Waals force will
predominate, and the particles will remain together. The random motion of colloids
caused by the constant collisions with water molecules, termed Brownian Movement, will
bring particles in close proximity and aggregation may occur. However, the addition of
coagulant and polymers is typically used to lower the energy barriers between particles
and provide efficient agglomerations for settling.
Zeta Potential
The Stern layer is considered to be rigidly attached to the colloid, while the diffuse layer
is a dynamic layer of charged particles. The Nernst Potential is the measurement of
voltage (in the order of millivolts) in the diffuse layer. The potential is a maximum at the
Stern layer and drops exponentially through the diffuse layer. The zeta potential is the
electrical potential representing the difference in voltage between the surface of the
diffuse layer and the water. It is important to know the magnitude of the zeta potential, as
it represents the strength of the repulsion between colloid particles and the distance which
must be overcome to bring the particles together.
The primary charge on a colloid cannot be measured directly. However, the zeta potential
can be computed from measurements of particle movement within an electrical field
(electrophoretic mobility). Therefore, the zeta potential, £, is defined by the equation:
C = 47t5q
D
where q = charge of the particle
5 = thickness of the zone of influence of the charge on the particle
D = dielectric constant of the liquid
Zeta potential measurements can be made using a high-quality stereoscopic microscope to
observe colloidal particles inside an electrophoresis cell (Zeta-Meter 1998). An electric
field is created across the cell and charged particles move within the field. Their velocity
and direction are then related to the zeta potential. Measurements of zeta potential can
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8. PARTICLES CONTRIBUTING TO TURBIDITY
give an indication of the effectiveness of added electrolytes in lowering the energy barrier
between colloids, and can direct the optimization of coagulant dose in water treatment.
The destabilization of colloids is accomplished by the reduction of the zeta potential with
coagulants such as alum, ferric chloride and/or cationic polymers. Once the charge is
reduced or eliminated, no repulsive forces exist. Gentle agitation in a flocculation basin
will cause numerous, successful colloid collisions. Chapter 10 further discusses the
mechanics of coagulation and flocculation in the water treatment process.
Streaming Current
As discussed in the previous section, a charged particle will move with fixed velocity
through a voltage field under the physical phenomenon known as electrophoresis.
Streaming current is a measurable electric current that is generated when particles in
water are temporarily immobilized and the bulk liquid is forced to flow past the particles.
A streaming current monitor is a continuous, online sampling instrument which measures
the charge on particles. A streaming current detector, or monitor, is a cylinder and piston.
The up and downward motion of the piston draws a sample of water into the annular
space between the piston and cylinder. An alternating current is read by the electrodes
attached to the ends of the cylinder (Amirtharajah and O'Melia, 1990). Charged particles
are temporarily immobilized by the piston and cylinder, and the motion of charged
particles in the double layer passing these immobilized particles creates the streaming
current (ChemTrac, 1997).
8.8 References
1. Aiken, G. and C. Evangelo. "1995. Soil and Hydrology: their effect on NOM." /.
AWWA. 1:36-37.
2. Amirtharajah, A. and C.R. O'Melia. 1990. "Coagulation Processes: Destabilization,
Mixing, and Flocculation." Water Quality and Treatment, A Handbook of
Community Water Supplies. Fourth Edition. AWWA. F.W. Pontius, editor. McGraw-
Hill, New York.
3. AWWA. 1990. Water Quality and Treatment. Fourth Edition. McGraw-Hill, Inc.,
New York.
4. Standard Methods. 1985. Standard Methods for the Examination of Water and
Wastewater, Sixteenth Edition. Franson, M.H., Eaton, A.D., Clesceri, L.S., and
Greenberg, A.E., (editors). American Public Health Association, AWWA, and Water
Environment Federation. Port City Press, Baltimore, MD.
5. ChemTrac Systems Inc. 1997. Optimizing Particle Removal with Streaming Current
Monitors and Particle Counters. Atlanta, GA.
6. Current, W.L. and L.S. Garcia. 1991. Cryptosporidiosis. Clinical Microbiological
Reviews. 4(3):325.
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8. PARTICLES CONTRIBUTING TO TURBIDITY
7. Ewing, R.B. 1986. Microbiological Reviews. American Society of Microbiology.
50:458.
8. Gregory, R. and T.F. Zabel. 1990. "Sedimentation and Flotation. " Water Quality
and Treatment, A Handbook of Community Water Supplies. Fourth Edition. AWWA.
Ed. F.W. Pontius, editor. McGraw-Hill, New York.
9. Jancangelo, J.G., et al. 1992. Low Pressure Membrane Filtration for Particle
Removal. AWWARF, Denver, CO.
10. Kawamura, S. 1991. Integrated Design of Water Treatment Facilities. John Wiley &
Sons, New York.
11. Logsdon, G., M.M. Frey, T.D. Stefanich, S.L. Johnson, D.E. Feely, J.B. Rose, and
M. Sobsey. 1994. "The Removal and Disinfection Efficiency of Lime Softening
Processes for Giardia and Viruses." AWWARF, Denver, CO.
/ *
12. McGhee, T.J. 1991. Water Resources and Environmental Engineering. Sixth
Edition. McGraw-Hill, New York.
13. Sawyer, C.N. and P.L. McCarty. 1978. Chemistry for Environmental Engineering.
Third Edition. McGraw-Hill, New York.
14. Tarzwell, C.M, editor, undated. "Algae - Taste and Odor Control. WT-138." Robert
A. Taft Sanitary Engineering Center, Cincinnati, OH.
15. Tate, C.H. and K.F. Arnold. 1990. Health and Aesthetic Aspects of Water Quality.
Water Quality and Treatment, A Handbook of Community Water Supplies. Fourth
Edition. F.W. Pontius, editor. AWWA, McGraw-Hill, New York.
16. USEPA. 1997. Occurrence Assessment for the Interim Enhanced Surface Water
Treatment Rule, Final Draft. Office of Ground Water and Drinking Water,
Washington, D.C.
17. Wiesner, M.R. and R. Klute. 1998. Properties and Measurements of Particulate
Contaminants in Water. Treatment and Process Selection for Particle Removal. J.B.
McEwen, editor. AWWARF and International Water Supply Association, Denver,
CO.
18. Zeta-Meter Inc., 1998. Zeta Potential: A Complete Course. Internet Access:
www.zeta-meter.com.
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9, TURBIDITY IN SOURCE WATER
9.1 Introduction
The characteristics of turbidity in surface water supplies are a function of many factors.
Watershed features, such as geology, human development (i.e., agricultural uses or urban
development), topography, vegetation, and precipitation events can all greatly influence
raw water turbidity. In addition, reservoirs and ponds can often dampen the impact of
increased turbidity events by acting as points in a stream or river where particles can
settle before being drawn into the intake of a treatment plant. Wells and infiltration
galleries along streams or rivers can also reduce the impact of turbidity increases in
streams by their use of a natural aquifer as a filter. This chapter will discuss the turbidity
in surface water and ground water under the direct influence (GWUDI) of surface water,
and other source water characteristics as they relate to turbidity in raw water supplies.
9.2 Occurrence of Turbidity in Surface Water Supplies
There are many natural processes by which turbidity is created and conveyed to a raw
water intake for a water treatment plant. The following discussion focuses on the origins
of turbidity in surface water supplies including rivers, lakes and reservoirs.
9.2.1 Rivers
The largest component comprising the mix of particles creating turbidity found in rivers
is caused by erosion of materials from the contributing watershed. Turbidity may be
created from a wide variety of eroded materials, including clay, silt, or mineral particles
from soils, or from natural organic matter created by the decay of vegetation. Particles
may capture and hide, or mask, other inorganic and organic constituents that are present
in the watershed.
To help understand the formation of turbidity in rivers, it is important to understand the
natural hydrologic cycle. The diagram shown in Figure 9-1 illustrates the hydrologic
cycle. Natural evaporation occurs from water bodies, such as oceans and lakes, and
forms clouds, which then condense into precipitation. As precipitation falls to the earth,
it first infiltrates the soil and replaces soil moisture and eventually recharges ground water
aquifers. Runoff occurs when the rate of precipitation exceeds the rate of water
infiltrating the soil. As runoff flows over the land surface, the water can cause soil and
other materials to erode, which results in increased turbidity. Runoff then collects in
streams and rivers that flow back into water bodies such as lakes and oceans. From there,
the hydrologic cycle begins again.
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Source: AWWA, 1990.
Figure 9-1. The Natural Hydrologic Cycle
Thus, during precipitation events, river turbidities are higher than during periods between
precipitation events when the ground water may be supplying nearly all flow to the
stream.
Watersheds have great influence on the resulting stream turbidity during a precipitation
event. Figure 9-2 illustrates the various components of a watershed. Watersheds can be
very large, such as the Mississippi River drainage basin, which drains most of the central
United States. A large drainage basin usually consists of runoff that has large amounts of
turbidity from the various sources of urban and agricultural runoff and the differing soil
types present. Smaller watersheds may be highly urbanized and create high turbidity
runoff. However, areas which have been protected from development can produce lower
levels of turbidity even during runoff events.
Mountainous watersheds generally contain turbidity particles that are largely colloidal
rock matter. In glacial areas, the grinding of mountain glaciers often produces rock
particles that add a blue or green color to the mountain rivers. Mountain watersheds that
contain little development usually have much lower turbidities than those that may be
from an agricultural area in the plains or seacoast areas. Although low in turbidity, these
type of watersheds may still be contaminated by Giardia and Cryptosporidium and other
enteric viruses due to wildlife.
Rivers in the plains or coastal areas generally have watersheds which have farming and
urban runoff that add turbidity from topsoil and organic matter. The topsoil particles are
largely clay, and can hide or carry many different types of contaminants which are a
concern to water utilities. These trapped contaminants can include agricultural chemicals
and compounds such as fertilizers and pesticides.
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9. TURBIDITY IN SOURCE WA TER
'SPRING
(GROUND WATER
SEPAGE)
Mouth of Watershed
Source: AWWA, 1990.
Figure 9-2. Typical Watershed
One agricultural practice of particular concern is confined animal feeding operations,
such as feed lots. This type of development can produce runoff with high concentrations
of organic material and nutrients. In addition, many microorganisms, including Giardia
and Cryptosporidium which are known to be present in animal wastes, can be carried
downstream.
Because of the high percentage of impermeable surfaces in urban areas, the magnitude of
runoff often increases leading to the increased levels of turbidity and related
contaminants.- In addition to eroded soils, runoff can include domestic and industrial
wastes, fertilizers, road de-icing salts, overflows from combined sewers, eroded material
from construction sites, and contaminants from roadways and parking lots. Significant
industrial development can result in high levels of toxic materials, and power generating
facilities can cause elevated water temperatures.
Figure 9-3 shows the possible sources of contaminants that can comprise or be found in
turbidity particles.
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9. TURBIDITY IN SOURCE WATER
Point
Sources
Diffuse Sources
Power plants
Heated water
Feedlots
Organics
Solids
Nutrients
Microorganisms
Industries
Organics
Chemicals
Color and
Foam
Salts
Toxins
Heated water
Municipalities
Domestic and Industrial
Wastes
©
Legend
Treatment Plant
Agricultural
Land Drainage
Soil from erosion
Fertilizers
Pesticides
Organics
Microorganisms
Suspended solids
Acid mine drainage
Urban Storm Runoff
Industrial and
residential dust, dirt,
and litter
Bypassed water
Microorganisms
Color and foam
Nitrogen
Phosphorus
Outflow from combined sources
Source: Hammer, 1997.
Figure 9-3. Sources of Contaminants in Raw Water Supplies
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Organic materials reaching the rivers from both undeveloped and urbanized land can
serve as a food source for bacteria and other microorganisms. Inorganic nutrients such as
nitrogen and phosphorus from wastewater and agricultural runoff can provide nutrients to
stimulate the growth of algae, resulting in increased turbidity during low flow times.
The turbidity in streams and rivers is a constantly changing phenomenon. During dry
periods, when no precipitation occurs, turbidity levels usually drop to a somewhat stable
value for the stream. A precipitation event in the watershed can then bring additional
suspended material into the stream and greatly increase the turbidity. Generally, the more
intense the precipitation event, the higher the turbidity values experienced in the stream.
In addition, turbidity levels are typically found to be higher further downstream in a
watershed due to the amount contributed from upstream, the variety of contributing
factors it contains, and biological growth that accumulates in the stream as the water
moves through the basin.
Low Turbidity in Rivers
Low turbidity streams and rivers (less that 20 NTU) are those which are usually located at
the upper reaches of a undeveloped watershed. These watersheds include high mountain
areas, as well as those watersheds with little or no development. Characteristics of these
watersheds generally include:
• Little or no development;
• No agricultural activity;
• Heavy natural vegetation along streambanks; and
• Little streambank erosion.
Although these streams are usually low in turbidity, Total Organic Carbon (TOG) levels
can be high during runoff events resulting in an increase in turbidity from biological
growth or the presence of significant natural color. In addition, higher mountain streams
with low turbidities are often low in alkalinity due to the lack of natural buffering
materials in the water. This makes the treatment of these waters subject to swings in pH
with the addition of coagulants. A pH adjustment chemical and the resulting increase in
alkalinity is usually needed to achieve good coagulation and produce stabilized water.
High Turbidity in Rivers
High turbidity streams and rivers tend to be located in watersheds which have erodible
soils and/or significant agricultural fanning activity. They can also be streams which
receive runoff from urban and industrialized areas. Large rivers such as the Missouri,
Ohio and Mississippi, have consistently high levels of turbidity in the lower reaches of its
watersheds.
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Rapidly Changing Turbidity Case Study
There are many rivers which experience rapid changes to turbidity in response to precipitation
events. One such example of rapidly changing turbidity involves the Metropolitan Utilities
District (MUD) of Omaha, Nebraska. The Florence Water Treatment Plant in Omaha is located
on the Missouri River. In the spring, the combination ofsnowmelt and rainwater runoff, can
cause turbidity levels in the Missouri River to rise rapidly. The upstream drainage area is
largely agricultural with several urban centers. The Florence Water Treatment Plant receives
water directly from the Missouri River through a surface intake. The plant has pre-
sedimentation using cationic polymer followed by split-treatment with lime softening and alum.
Disinfection and filtration complete the treatment.
During the period between March 5th and March 9th of 1995, the plant experienced a
severe increase in the raw water turbidity associated with spring precipitation. Turbidity values
increased from under 100 NTU on March 5th to over 1,000 NTU on March 8th. A graph of the
turbidity increases is shown on Figure 9-4. While the Florence Plant has experienced similar
episodes in the past, the 1995 event was one of the largest swings in turbidity due to runoff in
recent years.
During the four day time period, the plant operators were able to manage the increasing
turbidity levels by increasing chemical dosages while monitoring raw water quality parameters
such as color, hardness, and turbidity. Over the years, the plant operators have noticed the raw
water color will begin to increase and hardness will begin to drop prior to the arrival of the high
turbidity water from snow melts or precipitation events. The increased color results from the
suspension of decayed organic matter in the spring runoff. The reduction in hardness
experienced is typical of runoff.
By observing color and hardness during a runoff event, the plant adjusted polymers and
coagulants slightly ahead of the turbidity event so that optimal treatment could be maintained
through the plant. In fact, no appreciable increase in turbidity from the pre-sedimentation
basins were recorded throughout the event. The quality of the finished water effluent was
unitnpacted.
1800
1600
5
Figure 9-4. Turbidity increase Event on the Missouri River at Omaha
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9. TURBIDITY IN SOURCE WA TER
High turbidity streams and rivers are usually subject to large increases in turbidity as the
result of precipitation events. Larger amounts of precipitation cause erosion by bringing
topsoil and decaying organic matter into the rivers. Increases in turbidities can sometimes
occur within short periods of time as large flushes of precipitation runoff pass through the
watershed.
9.2.2 Lakes and Reservoirs
Lakes and reservoirs capture water either naturally or through man-made impoundments
through the construction of dams on streams or rivers. Because of the quiescent nature in
a reservoir, turbidity levels are generally lower than streams or rivers due to the settling of
particles.
Low Turbidity in Lakes and Reservoirs
In general, larger reservoirs or lakes have lower turbidity levels. For example, the Great
Lakes usually have turbidity levels below 100 NTU, whereas rivers can have turbidities
reaching over 1000 NTU. Lakes and reservoirs provide longer detention times, allowing
for adequate settling of the larger turbidity particles and suspended solids. Of course,
intakes near a river inlet to the lake may be subject to greater swings in turbidity since
they may experience carryover of the river turbidity into, the lake before settling can
occur.
Low turbidity levels can also be found in smaller reservoirs receiving drainage from
higher quality streams serving non-agricultural watersheds such as those found in
mountainous areas. These reservoirs have watersheds which have streams carrying low
sediment loads due to a great deal of snow melt runoff. They usually produce relatively
stable sources of supply with little swings in turbidity. However, because they are small
they sometimes do not have the capacity to deal with the extremes caused by severe
intense precipitation or destruction of vegetation by fire. Such was the case experienced
by the Denver Water Department in the spring of 1996.
High Turbidity in Lakes and Reservoirs
High turbidity events in reservoirs usually occur in smaller reservoirs or lakes that receive
water from a agricultural watershed or urban drainage area. Larger reservoirs also
experience high turbidity as a result of water quality changes during annual thermal
changes in the lake, and may experience high turbidity events associated with severe
flooding. During periods of heavy watershed runoff, smaller reservoirs are usually not
large enough to effectively settle out the turbidity particles before they reach the intake
location of a water treatment plant. In addition, if a reservoir or lake is shallow, wave
action created by high winds can stir up sediments from the bottom and re-suspend
particles.
Reservoir turbidity events can also be caused by seasonal turnover of the thermal
stratification levels which form in the reservoirs. In deep reservoirs (i.e., those over 20
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feet) de-stratification often occurs in the fall when the upper levels of the reservoir
become cooler and denser, drop to the lower levels, and destroy the stratification. This
"overturning" effect can happen quickly and may bring anoxic water that is nutrient-rich
from the lower depths to the surface where algae is present. Sudden algal blooms can
then severely raise turbidity levels. The resulting impacts on raw water supplies can be
minimized through the construction of intake structures with multiple level draw-off
points.
Lake Reservoir Turbidity Case Study
A forest fire in the watershed area of Buffalo Creek near Denver has caused some long-lasting
impacts on the tributary area turbidities. The Foothills Water Treatment Plant receives water
from this watershed via a storage reservoir in Watertown Canyon. In May 1996, the Buffalo
Creek watershed experienced a forest fire which destroyed many trees and ground vegetation.
Prior to the fire, the highest turbidities observed were around 40 NTH (Denver Water). Usually,
the turbidities from the reservoir were less than 20 NTU.
During and after the fire, increased erosion has caused the turbidity to reach as much as 400
NTU during peak runoff events. Figure 9-5 shows a comparison of the maximum average day
turbidities before and after the fire. The watershed is still recovering from the fire and so these
higher turbidities may still occur during precipitation events. Since the fire, Denver Water
Department has installed a turbidimeter on the intake tower at the Strontious Springs Dam, as
well as instituting an observation system of spotters who live in Buffalo Creek that will inform
the Department when heavy rainfalls are occurring so that appropriate flow and chemical
adjustments can be made at the downstream plant so that finished water quality is not effected.
Figure 9-5. Turbidity Increase Due to Forest Fire in Buffalo Creek
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9.3 Ground Water Under the Direct Influence (GWUDI)
GWUDI is also a water supply source that can be subject to changes in turbidity. These
sources usually consist of a well or infiltration gallery located adjacent to or under a
stream, river or lake. However, wells that are located significant distances away from
surface water sources can also be influenced by surface water runoff due to geologic
conditions that transport water quickly from the ground surface to the well.
The classification of a water source as GWUDI is usually the result of a Microscopic
Particulate Analysis (MPA) test that examines the source for certain surface water
organisms in the water supply. The presence of these organisms is an indication that the
supply is influenced by surface water. In addition, if raw water turbidities or
temperatures rise and fall with the source water this can also be an indicator of a
connection between the surface water source and the well.
The degree to which these types of raw water sources are subject to changes in turbidity
are usually a function of how far the well is from the surface water source and the type of
linking aquifer. Wells in very close proximity to a river with a coarse gravel aquifer may
mirror the turbidity changes in the river very closely. On the other hand, wells that are
several hundred feet from a surface water source in a tight sandy aquifer may not
experience noticeable changes in turbidity in response to a surface water turbidity event.
9.4 Additional Watershed Considerations
Water facilities should also be aware of the nature of their watershed and any
contamination sources. Some sources of contamination may be present which are not a
concern under normal circumstances, but may become a problem during a large
precipitation event. For example, some U.S. cities have combined sanitary and storm
sewers which can result in surface water contamination during large rain events.
During dry periods, combined sewer systems collect wastewater and convey it to the
wastewater treatment plant. However, during periods of high precipitation these systems
also collect storm water which also travels to the wastewater treatment plant. When
extreme precipitation events occur, wastewater plant can be overloaded by the high
combined flows. This causes sewerage to bypass the plant and undergo only preliminary
treatment prior to discharge to a river. If flows are extremely high, preliminary treatment
may also be bypassed and raw sewage may be discharged directly to the river. These
events, termed combined sewer overflows (CSOs), can cause increases in river turbidity.
However, at times turbidity levels in the receiving stream may actually drop due to the
large amount of sewage flow, but the levels of bacteria and protozoa in the raw water
supply may rise significantly.
Another similar situation may exist where water treatment plants are situated immediately
downstream of livestock operations. These feeding operations can be sources for runoff
containing elevated levels of Giardia and Cryptosporidium during runoff events.
Downstream utilities should be aware of these situations and take the necessary treatment
precautions to ensure adequate treatment.
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9.5 Water Sources Occurrence in the United States
Turbidity occurrence in raw water supplies is generally associated with surface water
sources or GWUDI. An indication of the percentage of utilities using surface water
sources of raw water supply was presented in the 1997 Community Water System Survey
(USEPA, 1997). Figure 9-6 illustrates the percentage of utilities using ground water
versus surface water sources by water system size. These percentages were developed
from the data presented in the 1997 study, which is a representative sample of systems in
the U.S.
The figure indicates that smaller systems typically use ground water while larger systems
typically use surface water. Since turbidity occurrence is generally greater with surface
water sources, larger water systems may be more concerned with potential difficulties
arising from high turbidity levels.
D Primarily Ground Water Source
B Primarily Surface Water Source
<100
101-500 501-1,000
1,001-
3,300
POPULATION
3,301-
10,000
10,001-
50,000
50,001-
100,000
>100,000
Source: USEPA, 1997.
Figure 9-6. Raw Water Source by Water System Size
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9.6 References
1. AWWA. 1990. Water Quality and Treatment. Fourth Edition. McGraw-Hill, New
York.
2. Hammer, M.J. 1977. Water and Wastewater Technology. First Edition. John Wiley
&Sons.
3. LeChevallier, M.W. and W.D. Norton. 1992. "Examining Relationships Between
Particle Counts and Giardia, Cryptosporidium, and Turbidity." J. AWWA. 84(12) :54-
60.
4. USEPA. 1997. Community Water System Survey (CWSS), Volumes I and II. Office of
Water, Washington, D.C. EPA/815-R-97-001 a and -00Ib.
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TREATMENT PROCESSES
10.1 Introduction
In the arena of public water supply, water treatment is provided to remove constituents
from raw water which may pose a risk to public health or are undesirable in finished
water. Turbidity is a characteristic related to the concentration of suspended solid
particles in water and has been adopted as an easy and reasonably accurate measure of
overall water quality. Turbidity can be used to measure the performance of individual
treatment processes as well as the performance of an overall water treatment system.
Common water treatment processes intended to remove suspended solids and reduce
turbidity, or aid in this removal and reduction process, include:
• Raw water screening;
• Pre-sedimentation;
• Coagulation;
• Flocculation;
• Sedimentation; and
• Filtration.
This chapter provides a general description of each of these processes and information on
the level of turbidity reduction that is commonly achieved through each. A typical water
treatment system is shown schematically in Figure 10-1.
COAGULATION
FLOCCULATION
SEDIMENTATION
DISTRIBUTION
PUMPING
, i
TREATED WATER
STORAGE
FILTRATION
Figure 10-1. A Typical Conventional Water Treatment System
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10.2 Intake Facilities/Raw Water Screening
Systems which obtain water from surface supplies such as lakes and rivers employ intake
facilities to allow water to be withdrawn from the source. Most surface water intake
facilities are equipped with some type of screening device to prevent large rocks, sticks,
and other debris from entering the treatment system. Large bar racks with openings of 1
and 3 inches apart are commonly used for this purpose. They are designed specifically to
prevent large materials that could damage the intake structure or downstream equipment
from entering the treatment system. Bar racks are usually designed for manual cleaning.
When the raw water source is a river and a bar rack is used, the rack is usually oriented to
take advantage of the hydraulics of the river to keep the rack cleaned. Although trash
racks have little effect on turbidity, they do serve an important function in keeping large
solids out of the treatment system.
In other cases, intake screens are employed to perform the same function. Intake screens
are generally similar to bar racks except they have smaller openings (typically 3/16 to 3/8
inch openings) and are usually equipped with mechanical or hydraulic cleaning
mechanisms. Because intake screens remove particles much smaller than those generally
removed with bar racks, screens may provide some turbidity reduction by removing larger
solids that may be the source of smaller particles further in the treatment train.
10.2.1 Intake Location
Intake facilities are typically the very first process in the water treatment system. When
the water source is a lake or reservoir, substantial "pre-sedimentation" may occur within
the reservoir itself. This can serve to reduce the turbidity of water entering the treatment
system as well as dampen the impact of fluctuations in source water turbidity resulting
from storm events and other environmental phenomenon. If the intake facility is located
away from the reservoir's water source (i.e., the river feeding the reservoir) this pre-
sedimentation may substantially reduce the turbidity of the water entering the treatment
system. On the other hand, if the intake facility is located near the point where the supply
stream enters the reservoir, the benefit of the pre-sedimentation occurring within the
reservoir can be significantly reduced or lost.
When the water source is a river, the quality of water withdrawn may be impacted by the
location of the intake in relationship to sources of pollution entering the river. For
example, an intake structure located upstream from a municipal or industrial wastewater
discharge may supply substantially higher quality water than if it were located
downstream from the discharge.
10.2.2 Intake Depth
\
For many lakes and reservoirs the water surface elevation varies seasonally due to
environmental factors or reservoir management practices. In such cases, it is essential to
be able to withdraw water from the source under a variety of different water surface level
conditions. Even in cases where the source water surface elevation does not vary
significantly, the intake structure may be designed to allow withdrawal from the surface,
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or from different levels beneath the surface. This capability can have a significant impact
on the quality of water entering the treatment system. Many times stratification within a
lake or reservoir exists continually or occurs on a seasonal basis. In these cases, the
quality of water may vary significantly from near the surface to tens or hundreds of feet
below .the surface. For example, if algae growth is a problem, water withdrawn from
several feet beneath the surface may have substantially lower turbidity than water
withdrawn from the surface, as algae needs sunlight to survive and is typically found near
the water surface.
10.2.3 Effect on Turbidity
Intake screens are not intended to reduce the turbidity of the water entering the treatment
system. The solids removed by intake screens are large enough that they typically do not
directly impact turbidity, though subsequent deterioration and break-up of these solids
could contribute to increased levels of turbidity later in the treatment process. The
physical location of the intake structure and the flexibility it provides for varying the
depth from which source water is withdrawn can significantly influence the turbidity of
the water entering the treatment system,
10.3 Pre-sedimentatlon
Pre-sedimentation is commonly used for water supplies where raw water turbidity is
continually high, is high on a seasonal basis, or is high sporadically due to storms or other
environmental events within the watershed. Pre-sedimentation may also be used in
situations where substantial amounts of sand and gravel may be present in the source.
water. Depending on the purpose of the pre-sedimentation process, the pre-sedimentation
basins may be relatively large settling ponds or small concrete basins. When ponds are
utilized, they are generally designed to remove large quantities of silt from the raw water
and typically provide hydraulic detention times ranging from a few hours to a few days.
Smaller concrete basins that provide less than 20 minutes detention time are sometimes
used to provide grit removal. The larger settling ponds are generally not equipped with
mechanical sludge removal facilities and must be periodically cleaned by dredging or
other means. The concrete pre-sedimentation basins may be equipped with mechanical
equipment to remove solids from the basin bottom, or they may be designed to promote
manual cleaning using a fire hose or other equipment.
When pre-sedimentation is intended to remove silt and other fine suspended solids,
chemical addition is often used to enhance process performance. Organic polymers are
the chemicals most commonly added prior to pre-sedimentation to enhance solids
removal, but alum and ferric chloride are also sometimes used. The chemicals are added
to the raw water as it enters the pre-sedimentation basin to promote solid separation.
10.3,1 Effect on Turbidity
Pre-seclimentation facility performance depends largely on facility design. Factors such
as the ability to provide low velocity plug flow through the pre-sedimentation facility and
the capability to add chemicals are critical to achieving optimal system performance. The
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characteristics of the suspended solids in the raw water also play a key role in facility
performance. In cases where well-designed pre-sedimentation facilities are available and
adequate hydraulic detention times are provided (generally greater than 12 hours),
significant turbidity reduction can be achieved through the pre-sedimentation process.
In cases where pre-sedimentation is used primarily to remove grit from the raw water
before-it enters subsequent treatment processes, detention times are generally limited to
10 to 20 minutes and very little turbidity reduction is achieved.
10.4 Coagulation
Coagulation is the process of conditioning suspended solids particles to promote their
agglomeration and produce larger particles that can be more readily removed in
subsequent treatment processes. In many cases, dissolved organic substances are
adsorbed on the surface of suspended solids particles and effective coagulation can be an
effective step in their removal as well (AWWA and ASCE, 1990). The particles
suspended in raw water typically vary widely in size. Typical sizes for different types of
particles commonly found in water supplies can be seen in Figure 8-1.
Colloidal size particles typically carry an electrical charge (AWWA and ASCE, 1990).
When the suspended particles are similarly charged, the resulting repulsive forces
between particles tend to "stabilize" the suspension and prevent particle agglomeration.
The process of coagulation is complex and may involve several different mechanisms to
achieve "destabilization", which allows particle agglomeration and enhances subsequent
removal.
Coagulation is typically accomplished through chemical addition and mixing. Following
coagulation, the processes of flocculation, sedimentation, and filtration are used to
remove the "destabilized" particles from suspension.
10.4.1 Chemicals
Chemicals commonly used in the coagulation process include aluminum or iron salts and
organic polymers. The most common aluminum salt used for coagulation is aluminum
sulfate, or alum. Alum may react in different ways to achieve coagulation. When used at
relatively low doses (<5 mg/1), charge neutralization (destabilization) is believed to be the
primary mechanism involved. At higher dosages, the primary coagulation mechanism is
entrapment. In this case, aluminum hydroxide (A1(OH)2) precipitates forming a "sweep-
floe" which tends to capture suspended solids as it settles out of suspension.
Solution pH plays an important role when alum is used for coagulation since the
solubility of the aluminum species in water is pH dependent. If the pH value of a mixed
solution^is between 4 and 5, alum is generally present in the form of positive ions (i.e.,
Al(OH)-*, A18(OH)44", and A13+). However, optimum sweep and sweep coagulation occur
when negatively-charged forms of alum predominate, which occurs when the pH is
between 6 and 8. Figure 10-2 depicts the solubility of some of these aluminum species
present during a typical coagulation process. Figure 10-2 depicts some of the aluminum
EPA Guidance Manual 1CM " " ~ " AD,iMqqa
Turbidity Provisions April 1999
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
species involved in alum coagulation and the conditions of aluminum concentration and
pH under which they occur (AWWA and ASCE, 1990).
When alum is used and charge neutralization is the primary coagulation mechanism,
effective flash mixing is critical to the success of the process. When the primary
mechanism is entrapment, effective flash mixing is less critical, and flocculation is the
more important process.
s!
o
(3
O
UJ
0.
g
N
CHARGE NEUTRALIZATION
TO ZERO ZETA
POTENTIAL WITH
RESTABILIZATION ZONE
OPTIMUM SWEEP
SWEEP COAGULATION
-6
CHARGE NEUTRALIZATION
CORONA TO ZERO ZETA
POTENTIAL WITH
AI(OHWs)
AI(OH);
AI(OH)3(s)
I
IEP (ISOELECTRIC POINT)
COLLOID COATED
WITH(AI(OH)3(s))"*
10
12
pH OF MIXED SOLUTION
Source: AWWA and ASCE, 1990.
100
30
1°
3
1
0.3
O)
E
6
nr
CO
CO
Figure 10-2. The Alum Coagulation Diagram and Its Relationship to
Zeta Potential
Ferric Chloride (FeCl3) is the most common iron salt used to achieve coagulation. Its
reactions in the coagulation process are similar to those of alum, but the relative solubility
and pH ranges differ significantly from those of alum.
Both alum and ferric chloride can be used to generate inorganic polymeric coagulants that
have been used for coagulation. These coagulants are typically generated by partially
April 1999
10-5
EPA Guidance Manual
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
neutralizing concentrated solutions of alum or ferric chloride with a base such as sodium
hydroxide prior to their use in the coagulation process (AWWA and ASCE, 1990). The
resulting inorganic polymers may have some advantages over alum or ferric chloride in
cold waters or in low alkalinity waters.
Organic polymers tend to be large molecules composed of chains of smaller "monomer"
groups (AWWA and ASCE, 1990). Because of their large size and charge characteristics,
polymers can promote destabilization through bridging, charge neutralization, or both.
Polymers are often used in conjunction with other coagulants such as alum or ferric
chloride to optimize solids removal.
10.4.2 Rapid Mixing
Mixing is utilized as part of the coagulation process to distribute the coagulant chemicals
throughout the wate^stream. When alum or ferric chloride are used to achieve
destabilization through charge neutralization, it is extremely important that the coagulant
chemical be distributed quickly and efficiently because it is the intermediate products of
the coagulant reaction that are the destabilizing agents. The life of these intermediate
species is short and they must contact the solids particles in the water if destabilization is
to be achieved. When other mechanisms are predominant in the coagulation process, or
when organic polymers are being used as the coagulant chemical, immediate distribution
of the coagulant chemical is not as critical and less intense mixing may be acceptable, or
even desirable. In some cases, excessive mixing may serve to break-up coagulant
molecules or floe particles, thereby reducing the effectiveness of subsequent solids
removal processes.
Mixing intensity is typically quantified with a number known as the "velocity gradient" or
"G value". The G value is a function of the power input into the mixing process and the
volume of the reaction basin. Typical G values for coagulation mixing range from 300 to
8000 sec'1 (Hudson, 1981).
The time required to achieve efficient coagulation varies, depending on the coagulation
mechanism involved. When charge neutralization is the mechanism involved, the
detention time required may be one second or less. When sweep floe or entrapment is the
mechanism involved, longer detention times on the order of 1 to 30 seconds may be
appropriate (Kawumara, 1991; AWWA and ASCE, 1998; Hudson, 1981).
10.4.3 Effect on Turbidity
Coagulation by itself does not achieve turbidity reduction, in fact turbidity may increase
during the coagulation process due to the additional insoluble compounds generated
through chemical addition. The subsequent processes of flocculation, sedimentation, and
filtration are used in conjunction with coagulation to achieve suspended solids and
turbidity reduction.
EPA Guidance Manual ~ 10^6
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
10.5 Flocculation
Flocculation is the physical process of agglomerating small particles into larger ones that
can be more easily removed from suspension. Flocculation is almost always used in
conjunction with, and preceded by coagulation. During the coagulation process the
repulsive forces between solids particles are reduced or eliminated. Flocculation is the
process of bringing the destabilized particles into contact with one another to form larger
"floe" particles. These larger particles are more readily removed from the water in
subsequent processes.
Flocculation is generally accomplished by mixing the destabilized suspension to provide
the opportunity for the particles to come into contact with one another and stick together.
10.5.1 Slow Mixing
Mixing is a key aspect of the flocculation process. Often the intensity of mixing is
reduced as the water, proceeds through the flocculation process to achieve optimum
performance.
At the beginning of the process, the mixing is fairly intense to maximize the particle
contact opportunities. Mixing intensity values (G values) in this area are typically in the
range of 60 to 70 sec"1 (Kawamura, 1996).
Toward the end of the flocculation process, mixing intensity is generally reduced to
minimize the potential for breaking up the floe particles that have begun to form. In this
portion of the process, G values are commonly in the 10 to 30 sec"1 range (Kawamura,
1996). Many times mixing intensity is tapered through several different stages of the
flocculation process to optimize process effectiveness.
A wide variety of flocculation mixing mechanisms have been used in water treatment.
These include vertical shaft mechanical mixers, horizontal shaft mechanical mixers, and
hydraulic mixing systems.
10.5.2 Detention Time
The amount of time the water spends in the flocculation process is a key performance
parameter. Adequate time must be provided to allow generation of particles sufficiently
large to allow their efficient removal in subsequent treatment processes. The optimum
particle size may vary significantly depending on the downstream treatment processes
utilized. For example, when sedimentation is used, large floe particles are typically
desirable because they tend to settle out of suspension readily. If filtration directly
follows the flocculation process, smaller floe particles may be the most desirable since
they tend to be stronger and less susceptible to break-up by the shear forces encountered
within the filters. Overall detention time in the flocculation process typically ranges from
10 to 30 minutes and is generally provided in several different basins or basin segments.
This allows the mixing intensity to be varied through the process.
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
10.5.3 Effect on Turbidity
As with coagulation, the purpose of the flocculation process is not to directly reduce
turbidity or suspended solids levels, but rather to prepare the solids for subsequent
removal. The reduction in number of suspended solids particles in suspension is typically
achieved in the flocculation process as the smaller particles are combined to form larger
ones. This process may, or may not result in a reduction in turbidity.
10.6 Sedimentation/Clarification
Sedimentation is the process by which solids are removed from the water by means of
gravity separation. In the sedimentation process, the water passes through a basin in
which relatively quiescent conditions prevail. Under these conditions, the floe particles
formed during flocculation settle to the bottom of the basin while the "clear" water passes
out of the basin over an effluent baffle or weir. As shown in Figure 10-3, the solids
collect on the basin bottom and are removed, typically by a mechanical "sludge
collection" device. The sludge collection device scrapes the solids (sludge) to a
collection point within the basin from which it is pumped directly to disposal or to a
sludge treatment process.
Conventional sedimentation typically involves one or more basins. These "clarifiers" are
relatively large open tanks, either circular or rectangular in shape. In properly designed
clarifiers, velocity currents are reduced to the point where gravity is the predominant
force acting on the water/solids suspension. Under this condition, the difference in
specific gravity between the water and the solids particles causes the solids particles to
settle to the bottom of the basin.
High rate sedimentation is similar to conventional sedimentation except that the
sedimentation basin has been modified through the addition of some mechanical or other
device to aid in the settling process. These mechanical devices typically consist of plates
or tubes intended to reduce the distance the solids particles must settle through the water
before they reach the bottom of the basin and can be removed. Figure 10-4 illustrates a
plate settler used for high rate sedimentation.
Another high rate clarification process employs an "adsorption clarifier" and is designed
to provide flocculation and clarifications within a single process. These clarifiers consist
of a basin filled with adsorption media, generally small particles of either plastic or rock,
about the size of pea gravel. As the water passes through the media, hydraulic mixing
promotes flocculation and the flocculated particles adhere to the surface of the media
particles. The media is cleaned periodically using an air or air and water backwash
process to remove the solids.
Solids Contact clarifiers represent an entirely different approach to high rate clarification.
They consist of a basin similar to that used for a conventional clarifier but with a sludge
recycle system to promote development of a dense sludge blanket as depicted in Figure
10-5.
EPA Guidance Manual 10-8 April 1999
Turbidity Provisions
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w. TURBIDITY THROUGH THE TREATMENT PROCESSES
FEEDWELL
EFFLUENT DROP-OUT
LAUNDER
WEIR
WALKWAY
INFLUENT PIPE
SLUDGE DRAW-OFF PIPE
Source: AWWA and ASCE, 1990.
Figure 10-3. Circular Radial-flow Clarifier
April 1999
10-9
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
ADJUSTABLE WEIR
OUTLET TROUG
INLET BOTTOM
INLET FLUME
INLET ORIFICE-
OUTLET
BOTTOM
Source: AWWA and ASCE, 1998.
Figure 10-4. Plate Settlers Used for High Rate Sedimentation
Effluent
Orifice
Chemical
. Clarified v
\ Water \
X Return X
Flow ;?
Zone /?
Hood
" /'
>A
^ V
f~\ Seconder'
Impeller
Drive ~
f~m-i
Chemical
- wcownucai'y mixing
'and Reaction Zone/
Rotor Impeller ; MX -•—
A*
I^TSI&v^
/ / / A \ \ \ ^
Walkway
Oraft Tubes
/ / /
3
- Influent
^^
Blow-off
and Drain
Source: AWWA and ASCE, 1998.
Figure 10-5. Accelerator Solids Contact Unit
EPA Guidance Manual
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10-10
April 1999
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
As the water enters the bottom of the basin and passes upward through the sludge blanket,
the flocculated solids in the blanket tend to contact and capture or adsorb the solids from
the water.
10.6.1 Effect on Turbidity
Suspended solids removal and turbidity reduction rates achieved through sedimentation
may range from about 50 to 90 percent, depending on the nature of the solids, the level of
pretreatment provided, and the design of the clarifiers. Common values are in the 60 to
80 percent range (Hudson, 1981). A primary function of the sedimentation or clarification
process is to reduce the load of solids going to the filters. Optimization of the
clarification process will minimize the solids loading on the filters and contribute to
enhanced filter performance and better overall treated water quality.
10.7 Filtration
Like clarification, filtration is a process in which solids are removed from water and
substantial turbidity removal is achieved. Optimization used prior to the filtration process
will control loading rates while allowing the system to achieve maximum filtration rates.
In fact, filtration is the final step to achieve turbidity reduction in most water treatment
operations. The water leaving the filtration process should be well within turbidity limits.
In the filtration process, the water passes through a bed of granular filter media or other
filtering material and solids are physically retained on the media. After passing through
the filter media, the "filtered" water is collected and removed from the filter. The solids
retained on the media are also periodically collected and removed. As with the
sedimentation process, the performance of most filters depends largely on the preparatory
treatment processes of coagulation and flocculation. Without effective use of these
processes, only marginal filter performance can be expected.
Filters are classified according to the type of media used and the operational conditions
employed. The primary types of filters used in domestic water treatment include:
* Rapid Sand Filters;
• Pressure Filters;
• Slow Sand Filters; and
• Precoat Filters.
10.7.1 Conventional Rapid Sand Filters
Rapid sand filters are the most commonly used type of filters in water treatment systems
today. They get their name from the type of media employed (sand) and from the rate at
which they are hydraulically loaded. A sectional drawing of a typical rapid sand filter is
shown in Figure 10-6.
April 1999 10-11 EPA Guidance Manual
Turbidity Provisions
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
WASH TROUQI
UNDEHDHAIN BLOCKS
Source: AWWA and ASCE, 1998.
Figure 10-6. Typical Rapid Sand Filter
Water enters the filter unit above the media and flows by gravity downward through the
filter media to the underdrain or collection system, where it is removed from the filter.
When the filter media becomes clogged with solids it is cleaned through a "backwash"
process. In the backwash process water, and in some instances, air is introduced to the
filter at a relatively high rate through the underdrain system. The water and air flow
upward through the media, expanding the media bed and creating a scrubbing or scouring
action which removes solids accumulated on the media surface and in inter-particle sites
within the media bed. After passing through the media bed, the backwash water and the
solids it contains are removed from the filter with a series of collection troughs.
Media
A variety of different types of media are used in rapid sand filters. As the name implies,
the primary media is sand. In some cases all of the sand is the same size, but more
commonly the media consists of particles of varying composition, size, and density.
Filters with more than one type and size of media are referred to as dual media, mixed
media, or multi-media filters, depending on the media provided. As the backwashing
process in these filters concludes and the media particles settle back into position in the
filter bed, the particles become stratified due to their differing sizes and densities. The
largest and least dense media particles accumulate near the top of the media bed and the
smallest and most dense particles migrate to the bottom. With this media stratification,
when the filter is placed back into service and the water passes down through the bed of
media, it first encounters large particles and then finer and finer sand until it reaches the
EPA Guidance Manual
Turbidity Provisions
10-12
April 1999
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
underdrain system. This stratification tends to minimize the "blinding" effect that occurs
when solid particles accumulate at the very top of the media bed. It also provides a much
greater volume for solids storage within the filter bed, which allows longer filter runs
between backwash operations.
The top, or coarsest layer of media is often composed of anthracite coal rather than sand.
The relatively light coal remains at the top of the filter bed after filter cleaning
(backwashing), even though its particle sizes are relatively large. Conversely, high
density garnet sand is used for the smallest layer of filter media. The high density of this
material causes it to settle quickly to the bottom of the filter bed following backwash
operations, even though its particles are relatively small.
Hydraulics
Filter performance is affected significantly by its hydraulic characteristics. Typical rapid
sand filter loading rafes range from 2 to 8 gallons per minute per square foot of filter bed
surface area. As the filter bed becomes dirty and clogged with solids, the resistance to
flow increases. Ultimately flow will cease when the resistance to flow is greater that the
gravitational force compelling it. As the "head" required to push water through the filter
increases, the rate of flow tends to decrease and solids particles are pushed further and
further into the bed of media. Ultimately, if sufficient head is available, solids will be
driven completely through the bed and appear in the filtered water, a condition known as
"breakthrough". The filter run should be terminated and backwash initiated before
breakthrough occurs.
Controls
Typical filter control parameters include:
• Filter loading rate;
• Filter run length;
• Headloss;
• Filtered water turbidity;
• Backwash rate; and
• Backwash duration.
Control of these parameters gives the operator a great deal of influence in determining the
performance of a filter. Often, however, there are conflicting objectives associated with
filter operation. For example, the goal of maximizing water production may conflict with
the objective of minimizing treated water turbidity. The operator must use good '
judgement in establishing operational goals and exercising process control to achieve
optimal finished water quality and production.
April 1999 . 10-13 EPA Guidance Manual
Turbidity Provisions
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
10.7.2
Slow Sand Filters
Slow sand filters have been used for nearly two centuries and have been proven to an
effective "low-tech" method of treating some waters (AWWA and ASCE, 1990). A slow
sand filter consists of a bed of uniform, relatively fine grain sand underlain by an
underdrain system as depicted in Figure 10-7.
SLOW SAND FILTER
FINISHED WATER
STORAGE
SOURCE WATER
- — j 1
SUPERNATANT
WATER DRAIN
„, ft- 1
•" 10
FILTER TO WASTE
AND BACKWASJHING
V HEAD SPACE
SUPERNATANT WATER
SCHMUTZDECKE
— 1 SAND MEDIA
SU
[ftafl£WSp*SItSfKSS3fi*S#K8XSK*
-* — •»- ® iiiiiiiiiiiiiiii mi illinium
3PORT GRAVEL
i HIM i inn i ii i
FLOW METER
»
[_)-&-,-
VENT
^7
UNDERDRAIN
EFFLUENT
TO
DISTRIBUTION
Source: AWWA and ASCE, 1998.
Figure 10-7. Typical Covered Slow Sand Filter Installation
Water is introduced at the top of the bed and under the influence of gravity it passes
downward through the bed to the underdrain system. Slow sand filters are loaded at
much lower rates than rapid sand filters, with typical hydraulic loading rates ranging from
0.04 to 0.1 gallons per minute per square foot of filter bed area (AWWA and ASCE,
1998). Since the sand used in slow sand filters is relatively uniform in size and fine
grained, most of the solids removal and turbidity reduction occurs at the very top of the
sand bed. As Operation of the filter continues, a layer of dirt and micro-organisms builds
up at the surface of the bed. This layer is known as the "schmutzdecke" and contributes
to the effectiveness of the filter in removing suspended solids and reducing turbidity.
After a period of operation the headless through the filter becomes excessive and the
filter must be cleaned. Cleaning is accomplished by letting the water level drop below
the top of the filter media and then physically removing the schmutzdecke along with the
top 0.8 to 1.2 inches of sand. Typical slow sand filter runs between cleaning are one to
six months (Kawamura, 1996).
Generally slow sand filters are not proceeded by coagulation or flocculation processes,
making them one of the simplest water filtration processes available. However, relatively
large land areas are required and, with their simplicity comes little operational flexibility.
The only basic operational controls available to the slow sand filter operator are hydraulic
loading and frequency of cleaning.
EPA Guidance Manual
Turbidity Provisions
10-14
April 1999
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
Slow sand filters can be very effective in removing suspended solids and reducing
turbidity, depending on the nature of the solids particles involved, however, they have
been found to be limited in their capability to remove clay particles and color (AWWA
and ASCE, 1990).
10.7.3
Pressure Filters
Pressure filters are essentially a variation of the conventional rapid sand filter. They
employ the same types of media and function in much the same way. The primary
difference is that pressure filters are contained within a pressurized vessel, usually made
of steel, and pressure is used to push the water through the filter bed rather than gravity as
depicted in Figure 10-8.
Since pressure filters function much like conventional rapid sand filters, their capability
to remove suspended solids and reduce turbidity is similar.
The primary advantage of pressure filters is that they do not require the vertical space for
several feet of water above the filter bed and the water leaves the filter under pressure,
thus eliminating the potential for air binding associated with conventional rapid sand
filters. Disadvantages include the lack of access for visual observation of the filter bed
and the possibly greater potential to experience turbidity "breakthrough" due to the higher
pressure of force driving the filtration process.
BAFFLE PLATE
FILTERING SURFACE
oooooooooo a o o o
INFLUENT
EFFLUENT
SAMPLE
FAUCET-
Source: AWWA and ASCE, 1990.
Figure 10-8. Cross Section of a Typical Pressure Filter
MANIFOLD AND
LATERALS
April 1999
10-15
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
10.7.4 Precoat/Diatomaceous Earth Filters
Precoat filters represent an entirely different kind of filtration mechanism. Instead of
using sand or other granular material as the filtration medium, precoat filters use a thin
layer of "diatomaceous earth" or similar material to form a thin layer of filter media over
a supporting fabric or "septum", which in turn is supported by a rigid filter support
structure. The filter coat material is applied in a slurry before the filtration cycle begins.
When water passes through the filter under pressure, usually supplied by a pump but
sometimes by gravity, the solids in the water are captured on the surface of the filter
media. As the filtration process proceeds, additional filter media is added to the water
going to the filter. This supplemental media or "body feed", like the suspended solids in
the water, accumulates on the surface of the filter coat, increasing the depth of the media
and preventing the surface blinding effect that would otherwise occur. When the pressure
loss through the filter becomes excessive, filtration is discontinued and the filter media
coat is washed off through a backwash process, a new pre-coat is applied, and the
filtration process begins again.
Precoat filters have the capability to remove particles down to about one micron in size.
Hydraulic loading rates are typically in the range of 0.5 to 2 gallons per minute per square
foot of coated filter surface (ASCE, 1990). Advantages include relatively low capital
cost and no need for the preliminary processes of coagulation and flocculation.
Disadvantages include the inability to handle high turbidity water, the potential for
particle pass-through if the precoat process is not effective or cracking occurs during filter
operation, and the relatively poor capability to remove color and taste and odor causing
compounds.
10.7.5 Effect on Turbidity
Conventional and direct filtration processes have the capability of producing water with
turbidity below the proposed SDWA turbidity of 0.3 NTU and even below the 0.1 NTU
Partnership for Safe Water finished water optimization goal if properly operated and
maintained.
Filter performance depends largely on the characteristics of the solids particles entering
the filter and on the characteristics of the filter itself. Generally treated water turbidity
will be relatively high for a short period immediately following the backwash cycle of
rapid sand filter operation, commonly known as post backwash turbidity spiking, but will
then improve rapidly to a level near the highest quality level the filter can produce. The
filter will then operate at or near this level for an extended period. Figure 10-9 shows
typical filtered water turbidity as a function of filter run time.
EPA Guidance Manual 10-16 April 1999
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES ^
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
• Microfiltration;
• Ultrafiltration;
• Nanofiltration; and
• Reverse Osmosis.
Figure 10-10 shows the size of particles generally removed by these different membrane
processes.
Source: AWWA and ASCE, 1998.
Figure 10-10. Pressure-Driven Membrane Process Application Guide
Table 10-1 provides information on the typical pressure operating ranges for the different
types of pressure-driven membrane processes. Most pressure driven membrane processes
utilize either cellulose acetate or synthetic organic polymer membranes (AWWA and
ASCE, 1998). Standard pressure membrane configurations include spiral wound
membrane units and hollow fiber membrane units. Figures 10-11, 10-12, and 10-13 show
the standard membrane configurations commonly used for potable water treatment
systems.
Table 10-1. Typical Feed Pressures for Pressure Driven Membrane
Processes
Membrane Process
Reverse Osmosis - Brackish Water Application
Low Pressure
Standard Pressure
Reverse Osmosis - Seawater Application
Nanofiltration
Ultrafiltration
Microfiltration
Typical Feed Pressure (psi)
125 to 300
350 to 600
800 to 1,200
50 to 150
20 to 75
15 to 30
Source: AWWA and ASCE, 1998.
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Turbidity Provisions
10-18
April 1999
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
Processed water passes through the membranes
on both sides ot the porous permeate carrier.
Permeate
water
SOURCE WATER
MEMBRANE (cast on fabric backing)
POROUS PERMEATE CARRIER
MEMBRANE (cast on labile backing)
SOURCE WATER 4.
• MEMBRANE LEAF
SOURCE WATER
Adapted from hydranautics
Water Systems diagram.
The permeate flows through the porous material in a
spiral path until it contacts and flows through the holes
in the permeate core tube.
Cutaway view of a spiral membrane module
PRESSURE VESSEL•
,—ANTI-TELESCOPING
/ SUPPORT —v
CONCENTRATE (bnne)
SEAL—;
PERMEATE
WATER
0~
' MEMBRANE
MODULE '
MEMBRANE
MODULE
\
MEMBRANE
MODULE
-rf/S/(^ — SNAP RING
JtjL-— END CAP
1%ir SOURCE
I — — WATER
f^l INLET
^— BRINE SEALS •*" \—
Cross section of pressure vessel with 3 membrane modules
O-RING
CONNECTOR
Source: AWWA and ASCE.1998.
Figure 10-11. Typical Spiral-Wound Reverse Osmosis Membrane
CONCENTRATE
HEADER
(source
water)
- END PLATE
(FEED)
The permeator in this figure is adapted from
E.I. duPont de Namours & Co. (Inc.) sales literature. '
Source: AWWA and ASCE, 1998.
Module Driven
Figure 10-12. Typical Hollow Fine-Fiber Reverse Osmosis Membrane
Module
April 1999
10-19
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
Feed Port
Hollow Fibers In
Epoxy Tube Sheet
Concentrate Port
Permeate Ports
Source: AWWA and ASCE, 1998.
Figure 10-13. Representation of Hollow-fiber UF Module
10.8.1 Effect on Turbidity
Membrane systems provide a positive barrier to particles of a size larger than will pass
through the membrane. Consequently, the turbidity of water produced by membrane
treatment systems is usually well below 0.3 NTU. The size of particles that will pass
through the membrane depends on the structure of the membrane itself. Figure 10-10
contains information on the sizes of particles removed by different types of membrane
systems. As shown in Figure 10-10 all conventional membrane processes will effectively
remove bacteria and other large organisms such as Giardia and Cryptosporidium. Only
the more restrictive membranes are effective for removing viruses, small colloids, and
dissolved constituents. Many times membrane system performance is determined not by
treated water turbidity but by the level of other constituents such as total dissolved solids
that may be of concern in a particular situation. Please note that this assessment is based
on absolute pore sizes outlined in Figure 10-10. It does not reflect microbe pass-through
resulting from nominal pore size membranes or membrane failures (e.g., rupture, seal
leakage).
10.9 Recycle Streams
Recycle streams are waste streams generated during the water treatment process that are
returned to the treatment train with or without prior treatment. Though they are not
related to one particular treatment process, recycle streams may have a deleterious impact
on treated water quality, including turbidity. Consequently, proper management of
recycle streams is an important part of optimizing turbidity reduction in water treatment.
Any discharges of recycle streams must comply with Federal and State regulations,
including the National Pollution Discharge Elimination System (NPDES) program (40
CFR 122) and the Pretreatment program (40 CFR 403).
Waste streams may be handled in several different ways. Historically it was a common
practice to discharge waste streams directly to surface waters. In some cases this may still
be acceptable but state or federal regulations have largely curtailed the practice of
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
discharging water treatment waste streams directly to surface water. If a waste stream is
to be discharged to a surface water, an NPDES permit must be obtained from the
appropriate permitting authority prior to any discharge occurring. Due to these new
regulatory requirements, or to conserve water and reduce wastes, waste streams are now
recycled by many systems, with or without treatment, or discharged to a sanitary sewer
system.
10.9.1 Sources of Recycle Streams
The most common recycle streams found in potable water treatment systems include:
• Filter backwash water;
• Sludge thickener supernatant;
• Filter to waste flow; and
• Sedimentation basin underflow.
These recycle streams can represent concentrated waste flows and contain high levels of
contaminants. The continued recycle of these contaminants may affect treated water
quality. Impacts may include higher turbidity as well as higher concentrations of
pathogens and other contaminants in the plant influent. Because of its potential impact to
finished water quality, the handling of recycle streams should be carefully considered
during design and upgrade of all water treatment systems.
10.9.2 Recycle Stream Quantity and Quality
The quantity and quality of recycle streams varies considerably depending on the quality
of the raw water, the treatment processes employed and their efficiencies, and the type
and amount of chemicals used during treatment. Generally, the composite of all waste
streams generated in a conventional complete treatment system employing coagulation,
flocculation, sedimentation, and filtration will be in the neighborhood of 2 to 10 percent
of the total volume of water treated. Contaminants present in recycle streams may
include:
• Suspended solids;
• Organics;
• Inorganics; and
• Microorganisms.
Reported quality parameters associated with sludges generated from alum coagulation
include (Gulp and Gulp, 197.4):
BOD - * 40 to 150 mg/1
COD - 340 to 5000 mg/1
• TSS - 1100 to 14,000 mg/1
VSS - 600 to 4000 mg/1
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
• pH - near neutral
Levels of Cryptosporidium oocysts ranging from 2,900 to 47,000 counts per mL have
been reported in recycle streams from sedimentation and filtration processes after settling
(Cornwell and Lee, 1994). Reported values of spent filter backwash water turbidity
typically range from 30 to 400 NTU (Kawamura, 1991; Cornwell and Lee, 1993).
Waste streams generated from membrane processes such as reverse osmosis may also
contain significant levels of other contaminants such as dissolved solids and salts.
10.9.3 Point of Recycle Stream Return
The point at which a recycle stream is introduced to the treatment train is also important.
Recycle streams should be introduced at the plant headworks or as close to the beginning
of the treatment system as possible to provide the maximum level of recycle stream
treatment. The point of introduction should also be one where effective mixing is
provided to thoroughly disperse the recycled flow in the raw water stream before it enters
subsequent treatment processes. Studies have shown that the timing or regularity of the
recycle stream introduction is also very important in determining it's impact on the
performance of the treatment process (Goldgrabe-Brewen, 1995). The continuous and
steady introduction of an equalized recycle stream will have much less negative impact
on the water treatment process than sporadic introduction of larger volume recycle flows
that vary in quantity and quality.
10.9.4 Effect on Turbidity
When waste streams are recycled they may have an impact on the amount of solids loaded
to the treatment system. A model, developed by Cornwell and Lee (1993) to predict the
impact of recycled spent filter backwash water on the concentration of Giardia cysts and
Cryptosporidium cysts on water entering a treatment system, demonstrates the significant
impact recycle streams can have on water quality, even when a relatively high level of
treatment is provided for the recycle flow before it is returned to the main flow stream.
The model predicts a greater than three-fold increase in cyst concentration in the water
entering the treatment process as a result of the recycle practice, even when 70 percent of
the cysts initially present in the recycle stream are removed through treatment. Similar
impacts on turbidity can be expected.
10.10 References
1. AWWA. 1990. Water Quality and Treatment. Fourth Edition. McGraw-Hill, Inc.
2. AWWA and ASCE. 1998. Water Treatment Plant Design. Third Edition. AWWA
and ASCE, McGraw-Hill, Inc, New York, NY.
3. AWWA and ASCE. 1990. Water Treatment Plant Design. Second Edition. ASCE
and AWWA, McGraw-Hill, Inc, New York, NY.
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10. TURBIDITY THROUGH THE TREATMENT PROCESSES
4. Cornwell, D.A. and R.G. Lee. 1994. "Waste Stream Recycling: Its Effect on Water
Quality." J. AWWA, 86(11): 50-63.
5. Cornwell, D.A. and R.G. Lee. 1993. Recycle Stream Effects on Water Treatment.
AWWARF and AWWA.
6. Gulp, G.L. and R.L. Gulp. 1974. New Concepts in Water Purification. Van
Nostrand Reinhold Environmental Engineering Series, Litton Educational
Publishing, Inc.
7. Goldgrabe-Brewen, J.C. 1995. "Impacts of Recycle Streams on Water Quality."
Conference proceedings, AWWA Water Quality Conference, Anaheim, CA.
8. Hudson, H.E. Jr. 1981. Water Clarification Processes, Practical Design and
Evaluation. Van Nostrand Reinhold Environmental Engineering Series, Litton
Educational Publishing, Inc.
9. Kawumara, S. 1/991. Integrated Design of Water Treatment Facilities. John Wiley
& Sons, Inc. .
April 1999
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11. BASIC TURBIDIMETER DESIGN AND
CONCEPTS
11.1 Introduction
Turbidity is described in the Standard Methods for the Examination of Water and
Wastewater Method 21 SOB (EPA Method 180.1) for turbidity measurement as, "an
expression of the optical property that causes light to be scattered and absorbed rather
than transmitted in straight lines through the sample" (Standard Methods, 1995). This
chapter includes a detailed summary of the
various types of instruments used to measure
turbidity and include's descriptions of the
physical properties associated with the
measurements of turbidity and design
configurations.
As shown in Figure 11-1, modern
turbidimeters use the technique of
nephelometry, which measures the amount
of light scattered at right angels to an
incident light beam by particles present in a
fluid sample. In general, all modern
turbidimeters utilize the nephelometric
measurement principals, but instrument
manufacturers have developed several
different meter designs and measurement
configurations.
IfeMUring Prtncipto
Li »
12 -
P • sampl*
S» - scatt*r»
GjQ-| » peripheral ray» of Ihe scattered Eght
t»am used tormsaswcmeot
Source: GLI, undated,
Figure 11-1. Scattered Light at 90e
11.2 Turbidimeter Measuring Principles
As light passes through 'absolutely pure' water, the light beams travel along relatively
undisturbed paths. However, some distortion occurs as light is scattered by molecules
present in the pure fluid. As shown in Figure 11-1, when light passes through a fluid
containing suspended solids, the light beam interacts with the particles, and the particles
absorb the light energy and re-radiate light in all directions.
Particle size, configuration, color, and refractive index determine the spatial distribution
of the scattered light intensity around the particle. As shown in Figure 11-2, particles
much smaller than the wavelength of the incident light, which is typically expressed in
nanometers (nm), scatter light of approximately equal intensity in. all directions.
However, particles larger than the wavelength of the incident light, form a spectral pattern
that results in greater light scattering in the forward direction (away from the incident
light) than in the other directions. This scattering pattern and intensity of the light beam
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11. BASIC TURBIDIMETER DESIGN AND CONCEPTS
(A) Small Particles
(B) Large Particles
Inent
Beam
Size: Smaller Than Vio
the Wavelength of Light
Description: Symmetric
transmitted through the sample can also be affected by the particles absorbing certain
wavelengths of the transmitted light (Sadar, 1996).
Since the light scattered in the
forward direction is variable
depending on particle size, the
measurement of the light
transmitted through the sample
yields variable results. In
addition, the change in transmitted
light is very slight and difficult to
distinguish from electronic noise
when measuring low turbidities.
High turbidity samples are also
difficult to measure using
transmitted light due to multiple
scatter of the light by many
Incident
Beam
Size: Approximately '/4 the
Wavelength of Light
Description: Scattering Concentrated
in Forward Direction
(C) Larger Particles
Incident t
Beam
Size: Larger Than the Wavelength of Light
Description: Extreme Concentration of Scattering in Forward
Direction; Development of Maxima and Minima of Scattering
Intensity at Wider Angles
Source: Sadar, 1996.
Figure 11-2. Angular Patterns of
Scattered Light from Particles of
Different Sizes
particles in the fluid. To solve
these problems, turbidimeters
primarily measure the scatter of
light at a 90 degree angle to the
incident beam and relate this
reading to turbidity. This angle is considered very sensitive to light scatter by particles in
the sample. As described later in this chapter, additional light sensors are also sometimes
added to detect light scattered at other angles in order to improve the instrument range
and remove errors introduced by natural colors and lamp variability.
11.2.1 Light Source
The basic turbidimeter instrument contains a light source, sample container or cell, and
photodetectors to sense the scattered light. The most common light source used is the
tungsten filament lamp. The spectral output (band of wavelength light produced) of these
lamps is generally characterized by "color temperature," which is the temperature that a
black body radiator must be operated to produce a certain color. The tungsten filament
lamps are incandescent lamps and are termed "polychromatic," since they have a fairly
wide spectral band that includes many different wavelengths of light, or colors. The
presence of the various wavelengths can cause interference in the turbidity measurements
as natural color and natural organic matter in the sample can absorb some specific
wavelengths of light and reduce the intensity of the scattered light (King, 1991).
The tungsten filament lamp is also highly dependent on the voltage of the lamp power
supply. The voltage applied to the lamp determines the spectral output characteristics
produced, making a stable power supplies a necessity. In addition, as with any
incandescent lamp, the output from the lamp decays with time as the lamp slowly "burns
out," making recalibration of the instrument a frequent and necessary requirement.
To overcome some of the incandescent lamp limitations, some turbidimeter designs
utilize monochromatic light sources, such as light emitting diodes (LEDs), lasers,
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11. BASIC TURBID/METER DESIGN AND CONCEPTS
mercury lamps, and various lamp filter combinations. Monochromatic light has a very
narrow band of light wavelengths (only a few colors). By selecting light wavelengths that
are not normally absorbed by organic matter, the monochromatic light source can be less
susceptible to interference by sample color. However, some of these alternate light
sources respond differently to particle size, and are not as sensitive to small sized
particles as the tungsten filament lamp.
11.2.2 Sample Volume
Grab samples are typically introduced into bench top turbidimeter instruments through a
transparent sample cell made of glass. These samples cells, or cuvettes, are usually about
30 milliliters in capacity. Some on-line turbidimeters utilize the glass sample cell, but
most designs use a flow-through chamber with the light source located outside the
sample. Sample chambers in on-line instruments range from 30 milliliters to over two
liters.
11.2.3 Photodetector
In turbidimeters, photodetectors detect the light produced from the interaction of the
incident light and the sample volume and produce an electronic signal that is then
converted to a turbidity value. These detectors can be located in a variety of
configurations depending on the design of the instrument. The four types of detectors
commonly used include photomultiplier tubes, vacuum photodiodes, silicon photodiodes,
and cadmium sulfide photoconductors (Sadar, 1992).
Each of the four types of detectors vary in their response to certain wavelengths of light.
Therefore, if a polychromatic light source is used, the spectral output of the light source
has a direct bearing on the type and design of photodetector selected for an instrument.
The specification of the photodector is not nearly as critical when a monochromatic light
source is used. In general, with the polychromatic tungsten filament lamp as a light
source, the photomultiplier tube and the vacuum photodiode are more sensitive to the
shorter wavelength light in the source, making them more sensitive to the detection of
smaller particles. Conversely, the silicon photodiode is more sensitive to longer
wavelengths in the light source, making it more suited for sensing larger particles. The
sensitivity of the cadmium sulfide photoconductor is between the sensitivity of the
photomultiplier tube and the silicon photodiode.
11.3 Turbidimeter Design Configurations
Several instrument design standards have been developed by various organizations to
attempt to standardize instrument designs and achieve test results that are accurate and
repeatable. These standards govern the design of the various turbidimeter configurations
available today, whichx include the single beam design, modulated four beam design,
surface scatter design, and transmittance design. Only the single beam design, ratio
design, and modulated four beam design are approved by EPA.
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11. BASIC TURBIDIMETER DESIGN AND CONCEPTS
11.3.1 Design Standards
The requirements stated in Standard Methods 21306 (see Appendix D) are similar to the
requirements of EPA Method 180.1 (see Appendix C) for turbidity measurement. The
EPA Method 180.1 lists the following design requirements for turbidimeters:
• "Light Source: Tungsten-filament lamp operated at a color temperature
between 2200 and 3000 degree K.
• Distance traversed by incident light and scattered light within the sample tube
not to exceed 10 cm.
• Angle of light acceptance by detector: Centered at 90 degrees to the incident
light path and not to exceed +/- 30 degrees from 90 degrees. The detector, and
filter system if used, shall have a spectral property between 400 and 600 nm
(Standard Methods, 1995)."
EPA has recognized one additional standard for turbidimeter design called GLI Method 2.
Like EPA Method 180.1, this standard is applicable for turbidities in the 0 to 40 NTU
range, but may be used for higher turbidities by diluting the sample. The GLI Method 2
standard requires that instruments utilize basic nephelometric concepts, but requires the
use of two light sources with a photodetector located at 90-degrees from each source.
This concept, which is often called a modulated four beam design, pulses the two light
sources on and off and utilizes a portion of the scattered light as a reference signal to
arithmetically cancel errors. A full description of the modulated four beam design is
included later in this Chapter.
The specific apparatus requirements listed in the GLI Method 2 standard are as follows:
• The wavelength of the incident radiation shall be 860 nanometers.
• The spectral bandwidth of the incident radiation shall be less than or equal to
60 nanometers.
• There shall be no divergence from parallelism of the incident radiation and
any convergence shall not exceed 1.5 degrees.
• There shall be two light sources and two detectors.
• The measuring angle between the optical axis of the incident radiation and that
of the diffused radiation for light pulsed through the sample by either light
source shall be 90 +\- 2.5 degrees.
An additional turbidimeter design standard was developed by the International
Organization for Standardization. ISO 7027 defines the requirements for a turbidimeter
light source with stricter requirements attempting to produce instruments that have good
repeatability and compare well with other instruments. The specification reads:
"Any apparatus may be used provided that it complies with the following
requirements:
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11. BASIC TURBID/METER DESIGN AND CONCEPTS
• The wavelength , X, of the incident radiation shall be 860 nm;
• The spectral bandwidth , AA,, of the incident radiation shall be less than or
equal to 60 nm;
• There shall be no divergence from parallelism of the incident radiation and
any convergence shall not exceed 2.5 degrees;
• The measuring angle (tolerance on deviation of the optical axis) shall be o +/-
2.5 degrees
• The aperture angle, Qe, should be between 10 and 20 degrees in the water
sample (ISO, 1990)."
ISO 7027 requires the use of monochromatic light sources such as tungsten lamps fitted
with monochromators and filters, diodes, or lasers. However, the standard recognizes
that many older instruments have polychromatic light sources, and allows their use for
water treatment monitoring and control, but not for comparison to readings from other
turbidimeters.
The tighter definition of the light source in ISO 7027 eliminates many of the variables
inherent to the polychromatic sources used in the other standards. However, ISO 7027
does not eliminate the effects of light source decay or electronic gains and drifts inherent
in monochromatic sources such as LEDs (Lex, 1994). ISO 7027 is not accepted by
EPA for turbidity analysis for compliance with the IESWTR.
11.3.2 Single Beam Design
The single beam design configuration, shown in Figure 11-3, is the most basic
turbidimeter design using only one light source and one photodetector located at 90
degrees from the incident light. The single beam design is the oldest of the modern
nephelometers and typically is used with a polychromatic tungsten filament lamp. The
design is still in wide used today and yields accurate results for turbidity under 40 NTU,
provided that samples have little natural color. In fact, many on-line instruments in use'
today still utilize the single beam design.
The single beam design does, however, have limited accuracy at higher turbidities. As
turbidity increases and the
amount of scattered light
increases, multiple scattering can
occur when light strikes more
than one particle as it reacts with
the sample fluid. The resulting
scattered light intensity reaching
the 90 degree detector can
diminish as the instrument
effectively "goes blind." For
source: Sadar, 1996. this reason, a single beam design
Figure 11 -3. Basic Nephelometer conforming strictly to EPA 180.1
does not typically demonstrate
Lens
Glass
Sample Cell
Transmitted
'- Light
90° Scattered
Light
Detector
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11. BASIC TURBIDIMETER DESIGN AND CONCEPTS
stable measurement capability at high turbidities and is generally only applicable for
turbidity readings from 0 to 40 NTU.
The design of the single beam instrument is also limited by the need for frequent
recalibration of the instrument due to the decay of the incandescent light source. Because
of the polychromatic nature of the light source, these instruments also can demonstrate
poor performance with samples containing natural color. Since most treated water
samples have low or no color, use of the single beam design is appropriate.
11.3.3 Ratio Design
The ratio turbidimeter design expands upon the single beam concept, but includes
additional photodetectors located at other angles than 90 degrees from the incident light.
As shown in Figure 11-4, the ratio design utilizes a forward scatter detector, a transmitted
light detector, and for very high turbidity applications, a back scatter detector. The
signals from each of ihese detectors are mathematically combined to calculate the
Light Path Diagram
90° DETECTOR
LAMP
LENS
SAMPLE TRANSMITTED
CELL LIGHT DETECTOR
Source: Sadar, 1996.
Figure 11-4. Ratio Turbidimeter
turbidity of the sample. A typical ratio mathematical algorithm is as follows (Standard
Methods, 1995):
T= I90/ (do * I, + d, * Ifs + d2 * Ibs + d3 * I9o)
Where:
T = Turbidity in NTU
do, di, da, da = Calibration Coefficients
I90 = 90 Degree Detector Current
I, = Transmitted Detector Current
Ifs = Forward Scatter Detector Current
Ibs = Back Scatter Detector Current
The use of multiple photodetectors and the ratio algorithm gives the instrument much
better performance with colored samples. The transmitted light and the 90-degree
scattered light are affected almost equally by the color of the sample because they travel
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11. BASIC TURBIDIMETER DESIGN AND CONCEPTS
PHASE 1
PHASE 2
nearly the same distance through the sample volume. When the ratio of the two readings
is taken, the effects of color absorption on the two readings tend to cancel
mathematically.
11.3.4 Modulated Four Beam Design
Unlike the single beam and ratio turbidimeters, the modulated four-beam instrument
design utilizes two light sources and two photo detectors. The two sources and the two
detectors are used to the implement theory of ratio measurements to cancel errors. As
shown in Figure 11-5, the light sources and detectors are located at 90 degrees around the
sample volume (Great Lakes Instruments, undated).
This design takes two measurements
every 0.5 seconds. In the first phase, light
from source #1 is pulsed directly into
photodetector #2. Simultaneously,
photodetector #1 measures the light
scattered from this pulse at a 90 degree
angle. In the second phase, light from
source #2 is pulsed directly into
photodetector #1. Simultaneously,
photodetector #2 measures the light
scattered from this pulse at a 90 degree
angle. In both phases, the signal from
the photodetector receiving the direct
light signal is the active signal, while
LIGHT
SOURCE
Source: GLI, undated.
Figure 11-5. Modulated Four-Beam
Turbidimeter
the signal from photodetector measuring scattered light is called the reference signal. The
two phase measurements provide four measurements from two light sources: two
reference signals and two active signals.
The turbidity of the sample is calculated from the four independent measurements taken
from the two light sources using a mathematical algorithm similar to the algorithm used
by the ratio instrument design. The result is that errors resulting from sample color
appear in both the numerator and denominator of the mathematical algorithm, and the
errors are mathematically canceled.
Like the ratio design, the mathematical algorithm used in the four beam design allows for
more sensitivity in highly turbid samples and extends the range of the instrument to about
100 NTU. The error cancellation achieved by the ratio algorithm also makes the
instrument very accurate in the 0 to 1 NTU range.
11.3.5 Surface Scatter Design
As turbidity increases, light scattering intensifies and multiple scattering can occur as
light strikes more than one particle as it interacts with the fluid. Light absorption by
particles can also significantly increase. When particle concentration exceeds a certain
point, the amount of transmitted and scattered light decreases significantly due to multiple
scattering and absorption. This point is known as the optical limit of an instrument.
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The surface scatter design utilizes a light
beam focused on the sample surface at an
acute angle. As shown in Figure 11-6, light
strikes particles in the sample and is scattered
toward a photodetector that is also located
above the sample surface. As turbidity
increases, the light beam penetrates less of the
sample, thus shortening the light path and
compensating for interference from multiple
scattering. The reported range of surface
scatter instruments is about 0 to 9999 NTU,
although these instruments are best suited for
measuring high turbidities such as are present
in raw water and recycle streams (Hach
Corporation, 1995). These designs are not
approved by EPA.
Source: Hach Corporation, 1995.
Figure 11 -6. Surface Scatter Turbidimeter
11.3.6 Transmittance Design
Instruments utilizing a transmittance design are often referred to as turbidimeters, but
these instruments do not measure true turbidity of water in NTUs. These instruments are
better termed "absorptometers" as they measure the amount of light transmitted through a
sample rather than the amount of light scattered by a sample. Light transmittance is
measured by introducing a light source to a sample volume and measuring the relative
amount of light transmitted through the sample volume to a photodetector located
opposite the light source. Transmittance values are reported as 0 to 100 percent of the
incident light source transmitted through the sample. The use of absorptometers in water
treatment has generally been restricted to monitoring spent filter backwash water to .
determine relative cleanliness of the filter media (Hach Corporation, 1995). These
designs are not approved by EPA.
11.4 Types of Turbidimeters
There are three common types of turbidimeters employed today. These are referred to as
bench top, portable, and on-line instruments. Bench top and portable turbidimeters are
used to analyze grab samples. Bench top units are typically used as stationary laboratory
instruments and are not intended to be portable. On-line instruments are typically
installed in the field and continuously analyze a sample stream spilt off from a unit
process. Measurement with these units requires strict adherence to the manufacturer's
sampling procedure to reduce errors from dirty glassware, air bubbles in the sample, and
particle settling.
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71. BASIC TURBID/METER DESIGN AND CONCEPTS
11.4.1 Bench Top Turbidimeters
Most bench top turbidimeters are designed for
broad applications and have the capability to
measure highly colored samples as well as
samples with high turbidities. The most popular
bench top turbidimeters used today utilize the
ratio design, but may have options for back
scatter detectors or monochromatic light sources.
Many ratio bench top instruments also have the
capability to turn off the ratio calculation so that
measurements can be made using the single beam
design. Older bench top instruments may be of
the single beam design, and some have analog
rather than digital displays. Bench top units
are used exclusively for grab samples and
require the use of glass cuvettes for holding
the sample volume.
«ft**L
Source: Hach Corporation, 1995.
Figure 11 -7. Bench Top Turbidimeter
11.4.2
Portable Turbidimeters
Source: Hach Corporation, 1995.
Figure 11-8. Portable Turbidimeter
Portable turbidimeters are similar to the bench top
units, except that they are designed for portable use
and are battery operated. Portable turbidimeters
are available in a variety of designs, including, the
single beam and ratio designs. The accuracy of
v portable instruments is comparable to the bench
top units, but the resolution of low turbidity
reading may only be 0.01 NTU as compared to
the 0.001 NTU resolution of bench top units
(Hach Corporation, 1995).
Portable turbidimeters are designed for use in the field with grab samples. These
instruments are designed to be rugged and capable of withstanding the affects of moving
the instrument as well as variable field conditions. However, since these instruments are
inherently susceptible to damage or disturbance from dropping, abuse, or environmental
conditions such as dust, these units are not appropriate for the process monitoring and
reporting tasks normally accomplished by bench top units or on-line turbidimeters.
However, portable instruments are useful for measuring turbidity at remote locations such
as at sampling points in the watershed upstream of a water treatment plant, or at a remote
raw water intake location. Portable instruments are also useful for conducting special
process studies, such as backwash recycle characterization or distribution system analysis
that may be accomplished more readily and accurately in the field rather than conducting
analysis after transporting a sample to a laboratory.
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11.4.2
On-Line Turbidimeters
Source: GLI, undated.
Figure 11-9. On-Line
Turbidimeter
The on-line instruments used in the water
treatment industry typically utilize the single
beam or modulated four beam design. On-
line ratio turbidimeters are also available, but
their use has not been as extensive as the
single beam and modulated four beam
designs. On-line surface scatter
turbidimeters are often used for raw water
monitoring and transmittance-type
absorptometers have been used for filter
backwash monitoring.
On-line instruments typically sample a side
stream split off from/the treatment process.
The sample flows through the on-line
instrument for measurement and then is
wasted to a drain or recycled through the
treatment process. Supervisory Control and
Data Acquisition (SCADA) instrumentation
and remote telemetry can also be connected to
on-line instruments to collect data for trending
analysis or to control automated treatment actions based on the turbidities measured.
use of SCAD A with turbidity measurement is discussed in Chapter 4.
Typical sample flow rates through on-line instruments range from about 0.1 to 1.0 liter
per minute. Some single beam on-line turbidimeters do not contain a glass sample
container. The light source is located above the sample volume, which has an optically
flat surface as it flows over a weir. The photodetector is submerged within the sample
volume and requires frequent cleaning to prevent fouling. Most on-line four beam
instruments used in the water industry contain a sealed flow-through sample volume with
windows at each of the light sources and photodetectors. These surfaces must also be
cleaned frequently to prevent fouling.
Most on-line instruments contain bubble traps to eliminate air bubbles from the sample
that might interfere with the turbidity readings. Bubble traps are typically baffled
chambers that allow air bubbles to rise to the sample surface prior to the sample entering
the measurement chamber. The volume of the sample chamber varies significantly
between the single beam and four beam design due mostly to the design of the bubble
trap. Single beam devices typically include a bubble trap within the sample chamber,
making the sample volume in excess of two liters. Several other on-line instruments use
sample volumes as small as 30 milliliters.
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11.5 References
1. California Department of Health Services. 1996. Particle Counting Guidelines.
California Department of Health Services.
2. GLI (Great Lakes Instruments), undated. Technical Bulletin Number Tl - Turbidity
Measurement. Rev. 2-193, Great Lakes Instruments.
3. Hach Corporation. 1995. Excellence in Turbidity Measurement. Hach Corporation.
4. International Organization for Standards (ISO). 1990. International Standard ISO
7027- Water Quality - Determination or Turbidity. ISO.
5. King, K. 1991. Four-Beam Turbidimeters for Low NTU Waters. Great Lakes
Instruments.
6. Lex, D. 1994. Turbidity Technology Turns on the High Beams. Intech Engineer's
Notebook. 41(6):36
7. Sadar, MJ. 1996. Understanding Turbidity Science. Technical Information Series -
Booklet No. 11. Hach Company.
8. Standard Methods. 1995. Standard Methods for the Examination of Water and
Wastewater, nineteenth edition. America Public Health Association, AWWA,
Water Environment Federation. Franson, M.H., A.D. Eaton, L.S. Clesceri, and A.E.
Greenberg (editors). American Public Health Association, AWWA, and Water
Environment Federation. Port City Press, Baltimore, MD.
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11. BASIC TURBIDIMETER DESIGN AND CONCEPTS
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A.1 List of Definitions
accuracy. How closely an instrument measures the true or actual value of the process
variable being measured or sensed.
acidic. The condition of water or soil which contains a sufficient amount of acid
substances to lower the pH below 7.0.
activated carbon. Adsorptive particles or granules of carbon usually obtained by heating
carbon (such as wood). These particles or granules have a high capacity to selectively
remove certain trace and soluble organic materials from water.
air binding. A situation where air enters the filter media. Air is harmful to both the
filtration and backwash processes. Air can prevent the passage of water during the
filtration process and can cause the loss of filter media during the backwash process.
alarm contact. A switch that operates when some pre-set low, high or abnormal
condition exists.
algae. Microscopic plants which contain chlorophyll and live floating or suspended in
water. They also may be attached to structures, rocks or other submerged surfaces. They
are food for fish and small aquatic animals. Excess algal growths can impart tastes and
odors to potable water. Algae produce oxygen during sunlight hours and use oxygen
during the night hours. Their biological activities appreciably affect the pH and dissolved
oxygen of the water.
alkaline. The condition of water or soil which contains a sufficient amount of alkali
substances to raise the pH above 7.0.
alkalinity. The capacity of water to neutralize acids. This capacity is caused by the
water's content of carbonate, bicarbonate, hydroxide and occasionally borate, silicate, and
phosphate. Alkalinity is expressed in milligrams per liter of equivalent calcium carbonate.
Alkalinity is not the same as pH because water does not have to be strongly basic (high
pH) to have a high alkalinity. Alkalinity is a measure of how much acid can be added to a
liquid without causing a great change in pH.
analog. The readout of an instrument by a pointer (or other indicating means) against a
dial or scale.
Association of Boards of Certification. An international organization representing over
150 boards which certify the operators of waterworks and waste water facilities. For
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information on ABC publications regarding the preparation of and how to study for
operator certification examinations, contact ABC, 4261/2 Fifth Street, P.O. Box 786,
Ames, Iowa 50010-0786.
available expansion. The vertical distance from the sand surface to the underside of a
trough in a sand filter. This distance is also called FREEBOARD.
back pressure. A pressure that can cause water to backflow into the water supply when
a user's water system is at a higher pressure than the public water system.
backflow. A reverse flow condition, created by a difference in water pressures, which
causes water to flow back into the distribution pipes of a potable water supply from any
source or sources other than an intended source. Also see backsiphonage and
cross-connection.
backsiphonage. A form of backflow caused by a negative or below atmospheric
pressure within a water system. Also see backflow and cross-connection.
backvvashing. The process of reversing the flow of water back through the filter media
to remove the entrapped solids.
bacteria. Singular: bacterium. Microscopic living organisms usually consisting of a
single cell. Bacteria can aid in pollution control by consuming or breaking down organic
matter in sewage, or by similarly acting on oil spills or other water pollutants. Some
bacteria in soil, water or air may also cause human, animal and plant health problems.
baffle. A flat board or plate, deflector, guide or similar device constructed or placed in
flowing water or slurry systems to cause more uniform flow velocities, to absorb energy,
and to divert, guide, or agitate liquids (water, chemical solutions, slurry).
best available technology (BAT). The best technology treatment techniques, or other
means which the Administrator finds, after examination for efficacy under field
conditions and not solely under laboratory conditions, are available (taking cost into
consideration). For the purposes of setting MCLs for synthetic organic chemicals, any
BAT must be at least as effective as granular activated carbon.
best management practices (BMPs). Structural, nonstructural and managerial
techniques that are recognized to be the most effective and practical means to control
nonpoint source pollutants yet are compatible with the productive use of the resource to
which they are applied. BMPs are used in both urban and agricultural areas.
bias. An inadequacy in experimental design that leads to results or conclusions not
representative of the population under study.
breakthrough. A crack or break in a filter bed allowing the passage of floe or particulate
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APPENDIX A. LIST OF DEFINITIONS
matter through a filter. This will cause an increase in filter effluent turbidity. A
breakthrough can occur: 1) when a filter is first placed in service, 2) when the effluent
valve suddenly opens or closes, and 3) during periods of excessive head loss through the
filter (including when the filter is exposed to negative heads).
calcium carbonate (CACOs) equivalent. An expression of the concentration of
specified constituents in water in terms of their equivalent value to calcium carbonate.
For example, the hardness in water which is caused by calcium, magnesium and other
ions is usually described as calcium carbonate equivalent.
calibration. A procedure which checks or adjusts an instrument's accuracy by
comparison with a standard or reference.
capital costs. Costs (usually long-term debt) of financing construction and equipment.
Capital costs are usually fixed, one-time expenses which are independent of the amount
of water produced.
carcinogen. Any substance which tends to produce cancer in an organism.
clarifier. A large circular or rectangular tank or basin in which water is held for a period
of time, during which the heavier suspended solids settle to the bottom. Clarifiers are also
called SETTLING BASINS and SEDIMENTATION BASINS.
clear well. A reservoir for the storage of filtered water of sufficient capacity to prevent
the need to vary the filtration rate with variations in demand. Also used to provide
chlorine contact time for disinfection.
coagulant aid. Any chemical or substance used to assist or modify coagulation.
coagulants. Chemicals that cause very fine particles to clump together into larger
particles. This makes it easier to separate the solids from the water by settling, skimming,
draining or filtering.
coagulation. The clumping together of very fine particles into larger particles caused by
the use of chemicals (coagulants). The chemicals neutralize the electrical charges of the
fine particles and cause destabilization of the particles. This clumping together makes it
easier to separate the solids from the water by settling, skimming, draining, or filtering.
cohesion. Molecular attraction which holds two particles together.
colloids. Very small, finely divided solids (particles that do not dissolve) that remain
dispersed in a liquid for a long time due to their small size and electrical charge. When
most of the particles in water have a negative electrical charge, they tend to repel each
other. This repulsion prevents the particles from clumping together, becoming heavier,
and settling out.
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combined sewer. A sewer that transports surface runoff and human domestic wastes
(sewage), and sometimes industrial wastes. Wastewater and runoff in a combined sewer
may occur in excess of the sewer capacity and cannot be treated Immediately. The excess
is frequently discharged directly to a receiving stream without treatment, or to a holding
basin for subsequent treatment and disposal.
i
community water system (CWS). A public water system which serves at least 15
service connections used by year round residents or regularly serves at least 25 persons
year-round
residents. Also see non-community water system, transient water system and
non-transient non-community water system.
complete treatment. A method of treating water which consists of the addition of
coagulant chemicals, flash mixing, coagulation - flocculation, sedimentation and
filtration. Also called CONVENTIONAL FILTRATION.
continuous sample. A flow of water from a particular place in a plant to the location
where samples are collected for testing. This continuous stream may be used to obtain
grab or composite samples. Frequently, several taps (faucets) will flow continuously in
the laboratory to provide test samples from various places in a water treatment plant.
conventional filtration. A method of treating water to remove particulates. The method
consists of the addition of coagulant chemicals, flash mixing, coagulation - flocculation,
sedimentation and filtration. Also called COMPLETE TREATMENT. Also see direct
filtration and in-line filtration.
conventional filtration treatment. A series of processes including coagulation,
flocculation, sedimentation, and filtration resulting in substantial particulate removal.
cross connection. Any actual or potential connection between a drinking (potable) water
system and an unapproved water supply or other source of contamination. For example,
if you have a pump moving nonpotable water and hook into the g water system to supply
water for the pump seal, a cross-connection or mixing between the two water systems can
occur. This mixing may lead to contamination of the drinking water. Also see
backsiphonage and backflow.
CT or CTcalc. The product of "residual disinfectant concentration" (C) in mg/1
determined before or at the first customer, and the corresponding "disinfectant contact
time" (T) in minutes, i.e., "C" x "T". If a public water system applies disinfectants at
more than one point prior to the first customer, it must determine the CT of each
disinfectant sequence before or at the first customer to determine the total percent
inactivation or "total inactivation ratio". In determining the total inactivation ratio, the
public water system must determine the residual disinfectant concentration of each
disinfection sequence and corresponding contact time before any subsequent disinfection
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application point(s). "CT99.9" is the CT value required for 99.9 Percent (3-log)
inactivation of Giardia lamblia cysts. CT99.9 a variety of disinfectants and conditions
appear in Tables 1.1- 1.6, 2.1, and 3.1 of section 141.74(b)(3) in the code of Federal
Regulations. CT99.9 is the inactivation ratio. The sum of the inactivation ratios, or total
inactivation ratio shown as E = (CT calc) / (CT99.9) is calculated by adding together the
inactivation ratio for each disinfection sequence. A total inactivation ratio equal to or
greater than 1.0 is assumed to provide a 3-log inactivation of Giardia lamblia cysts.
degasification. A water treatment process which removes dissolved gases from the
water. The gases may be removed by either mechanical or chemical treatment methods or
a combination of both.
degradation. Chemical or biological breakdown of a complex compound into simpler
compounds.
diatomaceous earth filtration (DE nitration). A filtration method resulting in
substantial particulate removal, that uses a process in which: 1) a "precoat" cake of
diatomaceous earth filter media is deposited on a support membrane (septum), and 2)
while the water is filtered by passing through the cake on the septum, additional filter
media, known as "body feed," is continuously added to the feed water to maintain the
permeability of the filter cake.
direct filtration. A filtration method of treating water which consists of the addition of
coagulant chemicals, flash mixing, coagulation, minimal flocculation, and filtration. The
flocculation facilities may be omitted, but the physical-chemical, reactions will occur to
some extent. The sedimentation process is omitted. Also see conventional filtration and
in-line filtration.
effective range. That portion of the design range (usually upper 90 percent) in which an
instrument has acceptable accuracy. Also see range and span
effective size (E.S.). The diameter of the particles in a granular sample (filter media) for
which 10 percent of the total grains are smaller and 90 percent larger on a weight basis.
Effective size is obtained by passing granular material through sieves with varying
dimensions of mesh and weighing the material retained by each sieve. The effective size
is also approximately the average size of the grains.
effluent. Water or some other liquid-raw, partially or completely treated-flowing from a
reservoir, basin, treatment process or treatment plant.
end point. Samples are titrated to the end point. This means that a chemical is added,
drop by drop, to a sample until a certain color change (blue to clear, for example) occurs.
This is called the END POINT of the titration. In addition to a color change, an end point
may be reached by the formation of a precipitate or the reaching of a specified pH. An
end point may be detected by the use of an electronic device such as a pH meter.
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enteric. Of intestinal origin, especially applied to wastes or bacteria.
entrain. To trap bubbles in water either mechanically through turbulence or chemically
through a reaction.
EPA. United States Environmental Protection Agency.
epidemic. Widespread outbreak of a disease, or a large number of cases of a disease in a
single community or relatively small area. Disease may spread from person to person,
and/or by the exposure of many persons to a single source, such as a water supply.
filtration. A process for removing paniculate matter from water by passage through
porous media.
finished water. Water that has passed through a water treatment plant; all the treatment
processes are completed or "finished". This water is ready to be delivered to consumers.
Also called PRODUCT WATER.
floe. Clumps of bacteria and particulate impurities that have come together and formed a
cluster. Found in flocculation tanks and settling or sedimentation basins.
flocculation. The gathering together of fine particles in water by gentle mixing after the
addition of coagulant chemicals to form larger particles.
garnet. A group of hard, reddish, glassy, mineral sands made up of silicates of base
metals (calcium, magnesium, iron and manganese). Garnet has a higher density than sand.
gastroenteritis. An inflammation of the stomach and intestine resulting in diarrhea, with
vomiting and cramps when irritation is excessive. When caused by an infectious agent, it
is often associated with fever.
Giardia lamblia. Flagellate protozoan which is shed during its cyst stage into the feces
of man and animals. When water containing these cysts is ingested, the protozoan causes
a severe gastrointestinal disease called giardiasis.
giardiasis. Intestinal disease caused by an infestation of Giardia flagellates.
grab sample. A single sample collected at a particular time and place which represents
the composition of the water only at that time and place.
ground water under the direct influence (GWUDI) of surface water. Any water
beneath the surface of the ground with: 1) significant occurrence oflnsects or other
macroorganisms algae, or large-diameter pathogens such as Giardia lamblia or,
2) significant and relatively rapid shifts in water characteristics such as turbidity,
temperature, conductivity, or pH which closely correlate to climatological or surface
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water conditions. Direct influence must be determined for individual sources in
accordance with criteria established by the State. The State determination of direct
influence may be based on site-specific measurements of water quality and/or
documentation of well construction characteristics and geology with field evaluation.
hardness, water. A characteristic of water caused mainly by the salts of calcium and
magnesium, such as bicarbonate, carbonate, sulfate, chloride and nitrate. Excessive
hardness in water is undesirable because it causes the formation of soap curds, increased
use of soap, deposition of scale in boilers, damage in some industrial processes, and
sometimes causes objectionable tastes in drinking water.
head. The vertical distance (in feet) equal to the pressure (in psi) at a specific point. The
pressure head is equal to the pressure in psi times 2.31 ft/psi.
/
head loss. The head, pressure or energy (they are the same) lost by water flowing in a
pipe or channel as a result of turbulence caused by the velocity of the flowing water and
the roughness of the pipe, channel walls or restrictions caused by fittings. Water flowing
in a pipe loses head, pressure or energy as a result of friction losses.
humus. Organic portion of the soil remaining after prolonged microbial decomposition,
hydrogeologic cycle. The natural process recycling water from the atmosphere down to
(and through) the earth and back to the atmosphere again.
influent. Water or other liquid-raw or partially flowing INTO a reservoir, basin,
treatment process or treatment plant.
in-line filtration. The addition of chemical coagulants directly to the filter inlet pipe.
The chemicals are mixed by the flowing water. Flocculation and sedimentation facilities
are eliminated. This pretreatment method is commonly used in pressure filter
installations. Also see conventional filtration and direct filtration.
jar test. A laboratory procedure that simulates a water treatment plant's
coagulation/flocculation units with differing chemical doses and also energy of rapid mix,
energy of slow mix, and settling time. The purpose of this procedure is to ESTIMATE the
minimum or ideal coagulant dose required to achieve certain water quality goals.
Samples of water to be treated are commonly placed in six jars. Various amounts of
chemicals are added to each jar, and the settling of solids is observed. The dose of
chemicals that provides satisfactory settling removal of turbidity and/or color is the dose
used to treat the water being taken into the plant at that time. When evaluating the results
of a jar test, the operator should also consider the floe quality in the flocculation area and
the floe loading on the filter.
legionella. A genus of bacteria, some species of which have caused a type of pneumonia
called Legionnaires Disease.
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linearity. How closely an instrument measures actual values of a variable through its
effective range; a measure used to determine the accuracy of an instrument.
microbial growth. The activity and growth of microorganisms such as bacteria, algae,
diatoms, plankton and fungi micrograms per liter (mg/L) One microgram of a substance
dissolved in each liter of water. This unit is equal to parts per billion (ppb) since one liter
of water is equal in weight to one billion micrograms.
micron. A unit of length. One millionth of a meter or one thousandth of a millimeter.
One micron equals 0.00004 of an inch.
microorganisms. Living organisms that can be seen individually only with the aid of a
microscope.
milligrams per liter (mg/L). A measure of concentration of a dissolved substance. A
concentration of one mg/L means that one milligram ofa substance is dissolved in each
liter of water. For practical purposes, this unit is equal to parts per million (ppm) since
one liter of water is equal in weight to one million milligrams. Thus a liter of water
containing 10 milligrams of calcium has 10 parts of calcium per one million parts of
water, or 10 parts per million (10 ppm).
mudballs. Material that is approximately round in shape and varies from pea-sized up to
two or more inches in diameter. This material forms in filters and gradually increases in
size when not removed by the backwashing process.
National Environmental Training Association (NETA). A professional organization
devoted to serving the environmental trainer and promoting better operation of
waterworks and pollution control facilities. For information on NETA membership and
publications, contact NETA, 8687 Via de Ventura, Suite 214, Scottsdale, AZ 85258
nephelometric. A means of measuring turbidity in a sample by using an instrument
called a nephelometer. A nephelometer passes light through a sample and the amount of
light deflected (usually at a 90-degree angle) is then measured.
nephelometric turbidity unit (NTU). The unit of measure for turbidity.
non-transient non-community water system (NTNCWS). A public water system that
regularly serves at least 25 of the same nonresident persons per day for more than six
months per year.
non-community water system (NCWS). A public water system that is not a community
water system. There are two types of NCWSs: transient and non-transient.
operation and maintenance costs. The ongoing, repetitive costs of operating a water
system; for example, employee wages and costs for treatment chemicals and periodic
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equipment repairs.
organic. Substances that come from animal or plant sources. Organic substances always
contain carbon. (Inorganic materials are chemical substances of mineral origin.)
organics. 1) A term used to refer to chemical compounds made from carbon molecules.
These compounds may be natural materials (such as animal or plant sources) or
man-made materials (such as synthetic organics). 2) Any form of animal or plant life.
overflow rate. One of the guidelines for the design of settling tanks and clarifiers in
treatment plants. Used by operators to determine if tanks and clarifiers are hydraulically
(flow) over- or underloaded. Overflow Rate (GDP/sq ft) = Flow (GPD)/Surface Area (sq
ft) particle count. The results of a microscopic examination of treated water with a special
"particle counter" which classifies suspended particles by number and size.
particulate. A very small solid suspended in water which can vary widely in size, shape,
density, and electrical charge. Colloidal and dispersed particulates are artificially gathered
together by the processes of coagulation and flocculation.
pathogenic organisms. Organisms, including bacteria, viruses or cysts, capable of
causing diseases (typhoid, cholera, dysentery) in a host (such as a person). There are
many types of organisms which do NOT cause disease. These organisms are called
non-pathogenic.
pathogens. Microorganisms that can cause disease in other organisms or in humans,
animals and plants. They may be bacteria, viruses, or parasites and are found in sewage in
runoff from animal farms or rural areas populated with domestic and/or wild animals, and
in water used for swimming. Fish and shellfish contaminated by pathogens, or the
contaminated water itself, can cause serious illnesses.
performance evaluation sample. A reference sample provided to a laboratory for the
purpose of demonstrating that the laboratory can successfully analyze the sample within
limits of performance specified by the Agency. The true value of the concentration of the
reference material is unknown to the laboratory at the time of the analysis.
pH. pH is an expression of the intensity of the basic or acid condition of a liquid.
Mathematically, pH is the logarithm (base 10) of the reciprocal of the hydrogen ion
concentration, [H+]. pH = Log (1/H+) The pH may range from 0 to 14, where 0 is most
acid, 14 most basic, and 7 neutral. Natural waters usually have a pH between 6.5 and 8.5.
plug flow. A type of flow that occurs in tanks, basins or reactors when a slug of water
moves through a tank without ever dispersing or mixing with the rest of the water flowing
through the tank.
polymer. A chemical formed by the union of many monomers (a molecule of low
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molecular weight). Polymers are used with other chemical coagulants to aid in binding
small suspended particles to larger chemical floes for their removal from water. All
polyelectrolytes are polymers, but not all polymers are polyelectrolytes.
pore. A very small open space in a rock or granular material.
precision. The ability of an instrument to measure a process variable and to repeatedly
obtain the same result. The ability of an instrument to reproduce the same results.
public water system. A system for the provision to the public of piped water for human
consumption, If such system has at least fifteen service connections or regularly least 60
days out of the year. Such term includes: 1) any collection, treatment, storage, and
distribution facilities under control of the operator of such system and used primarily in
connection with such system, and 2) any collection or pretreatment storage facilities not
under such control which are used primarily in connection with such system. A public
water system is either a "community water system" or a "non-community water system.
range. The spread from minimum to maximum values that an instrument is designed to
measure. Also see span and effective range.
recarbonation. A process in which carbon dioxide is bubbled into the water being
treated to lower the pH. The pH may also be lowered by the addition of acid.
Recarbonation is the final stage in the lime-soda ash softening process. This process
converts carbonate ions to bicarbonate ions and stabilizes the solution against the
precipitation of carbonate compounds.
reservoir. Any natural or artificial holding area used to store; regulate, or control water.
reverse osmosis. The application of pressure to a concentrated solution which causes the
passage of a liquid from the concentrated solution to a weaker solution across a
semipermeable membrane. The membrane allows the passage of the solvent (water) but
not the dissolved solids (solutes). The liquid produced is a demineralized water.
Safe Drinking Water Act (SDWA). Commonly referred to as SDWA. An Act passed
by the U.S. Congress in 1974. The Act establishes a cooperative program among local,
state and federal agencies to insure safe drinking water for consumers.
sand. Soil particles between 0.05 and 2 .0 mm in diameter.
sand filters. Devices that remove some suspended solids from sewage. Air and bacteria
decompose additional wastes filtering through the sand so that cleaner water drains from
the bed.
sedimentation. A water treatment process in which solid particles settle out of the water
being treated in a large clarifier or sedimentation basin.
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slow sand nitration. A process involving passage of raw water through a bed of sand at
low velocity (generally less than 0.4 m/h) resulting in substantial particulate removal by
physical and biological mechanisms.
standard. A physical or chemical quantity whose value is known exactly, and is used to
calibrate or standardize instruments.
Standard Methods for the Examination of Water and Wastewater. A joint
publication of the American Public Health Association, American Water Works
Association, and the Water Pollution Control Federation which outlines the procedures
used to analyze the impurities in water and wastewater.
standardize. To compare with a standard. 1) In wet chemistry, to find out the exact
strength of a solution by comparing it with a standard of known strength. 2) To set up an
instrument or device to read a standard. This allows you to adjust the instrument so that it
reads accurately, or enables you to apply a correction factor to the readings.
State. The agency of the State or Tribal government which has jurisdiction over public
water systems. During any period when a State or Tribal government does not have
primary enforcement responsibility pursuant to Section 1413 of the Safe Drinking Water
Act, the term "State" means the Regional Administrator, U.S. Environmental Protection
Agency.
surface water. All water naturally open to the atmosphere (rivers, lakes, reservoirs,
streams, impoundments, seas, estuaries, etc.) and all springs, wells, or other collectors
which are directly influenced by surface water.
surfactant. Abbreviation for surface-active agent. The active agent in detergents that
possesses a high cleaning ability.
suspended solids. l)Solids that either float on the surface or are suspended in water or
other liquids, and which are largely removable by laboratory filtering. 2) The quantity of
material removed from water in a laboratory test, as prescribed in STANDARD
METHODS FOR THE EXAMINATION OF WATER AND WASTEWATER.
transient water system. A non-community water system that does not serve 25 of the
same nonresident persons per day for more than six months per year. Also called a
transient non-community water system (TNCWS).
tube settler. A device that uses bundles of small bore (2 to 3 inches or 50 to 75 mm)
tubes installed on an incline as an aid to sedimentation. The tubes may come in a variety
of shapes including circular and rectangular. As water rises within the tubes, settling
solids fall to the tube surface. As the sludge (from the settled solids) in the tube gains
weight, it moves down the tubes and settles to the bottom of the basin for removal by
conventional sludge collection means. Tube settlers are sometimes installed in
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sedimentation basins and clarifiers to improve particle removal.
turbid. Having a cloudy or muddy appearance.
turbidimeter. A device that measures the amount of suspended solids in a liquid.
turbidity. The cloudy appearance of water caused by the presence of suspended and
colloidal matter. In the waterworks field, a turbidity measurement is used to indicate the
clarity of water. Technically, turbidity is an optical property of the water based on the
amount of light reflected by suspended particles. Turbidity cannot be directly equated to
suspended solids because white particles reflect more light than dark-colored particles
and many small particles will reflect more light than an equivalent large particle.
urban runoff. Stormwater from city streets and adjacent domestic or commercial
properties that may carry pollutants of various kinds into the sewer systems and/or
receiving waters.
virus. The smallest form of microorganisms capable of causing disease. Especially, a
virus of fecal origin that is infectious to humans by waterborne transmission.
waterborne disease outbreak. The significant occurrence of acute infectious illness,
epidemiologically associated with the ingestion of water from a public water system that
is deficient in treatment, as determined by the appropriate local or state agency.
water supplier. A person who owns or operates a public water system.
water supply system. The collection, treatment, storage, and distribution of potable
water from source to consumer.
zeta potential. In coagulation and flocculation procedures, the difference in the electrical
charge between the dense layer of ions surrounding the particle and the charge of the bulk
of the suspended fluid surrounding this particle. The zeta potential is usually measured in
millivolts.
A.2 References
Calabrese, E.J., C.E. Gilbert, and H. Pastides, editors. 1988. Safe Drinking Water Act
Amendments, Regulations and Standards. Lewis Publishers. Chelsea, MI.
California State University. 1988. Water Treatment Plant Operation. School of
Engineering, Applied Research and Design Center, Sacramento, CA.
Dzurik, A. A., Rowman, and Littlefield. 1990. Water Resources Planning. Savage, MD.
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APPENDIXA. LIST OF DEFINITIONS
USEPA. 1991a. Code of Federal Regulations, Title 40, Chapter I, Section 141.2. July 1.
USEPA. 1991b. "National Primary Drinking Water Regulations; Lead and Copper Rule;
Final Rule." 56 FR 26547, June 7, 1991. 56 FR 3578, January 30 (Phase 11)
von Huben, H. 1991. Surface Water Treatment: The New Rules. AWWA.
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APPENDIX B.. DETERMINATION OF
TURBIDITY BY
NEPHELOMETRY
METHOD 180.1
DETERMINATION OF TURBIDITY BY NEPHELOMETRY
Edited by James W. O'Dell
Inorganic Chemistry Branch
Chemistry Research Division
Revision 2.0
August 1993
ENVIRONMENTAL MONITORING SYSTEMS LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
180.1-1
April 1999
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APPENDIX B. DETERMINATION OF TURBIDITY
1.0 SCOPE AND APPLICATION
1.1 This method covers the determination of turbidity in drinking, ground,
surface, and saline waters, domestic and industrial wastes.
1.2 The applicable range is 0 to 40 nephelometric turbidity units (NTU).
Higher values may be obtained with dilution of the sample.
2.0 SUMMARY OF METHOD
2.1 The method is based upon a comparison of the intensity of light scattered
by the sample under defined conditions with the intensity of light scattered
by a standard reference suspension. The higher the intensity of scattered
light, the higher the turbidity. Readings, in NTUs, are made in a
nephelometer designed according to specifications given in sections 6.1
and 6.2. A primary standard suspension is used to calibrate the
instrument. A secondary standard suspension is used as a daily calibration
check and is monitored periodically for deterioration using one of the
primary standards.
2.1.1 Formazin polymer is used as a primary turbidity suspension for
water because it is more reproducible than other types of standards
previously used for turbidity analysis.
2.1.2 A commercially available polymer primary standard is also
approved for use for the National Interim Primary Drinking Water
Regulations. This standard is identified as AMCO-AEPA-1,
available from Advanced Polymer Systems.
3.0 DEFINITIONS
3.1 CALIBRATION BLANK (CB) - - A volume of reagent water fortified
with the same matrix as the calibration standards, but without the analytes,
internal standards, or surrogates analytes.
3.2 INSTRUMENT PERFORMANCE CHECK SOLUTION (IPC) - - A
solution of one or more method analytes, surrogates, internal standards, or
other test substances used to evaluate the performance of the instrument
system with respect to a defined set of criteria.
3.3 LABORATORY REAGENT BLANK (LRB) - - An aliquot of reagent
water or other blank matrices that are treated exactly as a sample including
exposure to all glassware, equipment, solvents, reagents, internal
standards, and surrogates that are used with other samples. The LRB is
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APPENDIX B. DETERMINA TION OF TURBIDITY
3.4
3.5
3.6
3.7
3.8
3.9
used to determine if method analytes or other interferences are present in
the laboratory environment, the reagents, or the apparatus.
LINEAR CALIBRATION RANGE (LCR) - - The concentration range
over which the instrument response is linear.
MATERIAL SAFETY DATA SHEET (MSDS) - - Written information
provided by vendors concerning a chemical's toxicity, health hazards,
physical properties, fire, and reactivity data including storage, spill, and
handling precautions.
PRIMARY CALIBRATION STANDARD (PCAL) - - A suspension
prepared from the primary dilution stock standard suspension. The PCAL
suspensions are used to calibrate the instrument response with respect to
analyte concentration.
QUALITY CONTROL SAMPLE (QCS) - - A solution of the method
analyte of known concentrations that is used to fortify an aliquot of LRB
matrix. The QCS is obtained from a source external to the laboratory, and
is used to check laboratory performance.
SECONDARY CALIBRATION STANDARDS (SCAL) , - Commercially
prepared, stabilized sealed liquid or gel turbidity standards calibrated
against properly prepared and diluted formazin or styrene divinylbenzene
polymers.
STOCK STANDARD SUSPENSION (SSS) - - A concentrated suspension
containing the analyte prepared in the laboratory using assayed reference
materials or purchased from a reputable commercial source. Stock
standard suspension is used to prepare calibration suspensions and other
needed suspensions.
4.0 INTERFERENCES
4.1
4.2
4.3
The presence of floating debris and coarse sediments which settle out
rapidly will give low readings. Finely divided air bubbles can cause high
readings.
The presence of true color, that is the color of water which is due to
dissolved substances that absorb light, will cause turbidities to be low,
although this effect is generally not significant with drinking waters.
Light absorbing materials such as activated carbon in significant
concentrations can cause low readings.
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APPENDIX B. DETERMINATION OF TURBIDITY
5.0 SAFETY
5.1 The toxicity or carcinogenicity of each reagent used in this method has not
been fully established. Each chemical should be regarded as a potential
health hazard and exposure should be as lows as reasonably achievable.
5.2 Each laboratory is responsible for maintaining a current awareness file of
OSHA regulations regarding the safe handling of the chemicals specified
in this method. A reference file of Material Safety Data Sheets (MSDS)
should be made available to all personnel involved in the chemical
analysis. The preparation of a formal safety plan is also advisable.
5.3 Hydrazine Sulfate (7.2.1) is a carcinogen. It is highly toxic and may be
fatal if inhaled, swallowed, or absorbed through the skin. Formazin can
contain residual hydrazine sulfate. Proper protection should be employed.
6.0 EQUIPMENT AND SUPPLIES
6.1 The turbidimeter shall consist of a nephelometer, with light source for
illuminating the sample, and one or more photo-electric detectors with a
readout device to indicate the intensity of light scattered at right angles to
the path of the incident light. The turbidimeter should be designed so that
little stray light reaches the detector in the absence of turbidity and should
be free from significant drift after a short warm-up period.
6.2 Differences in physical design of turbidimeters will cause differences in
measured values for turbidity, even though the same suspension is used for
calibration. To minimize such differences, the following design criteria
should be observed:
6.2.1 Light source: Tungsten lamp operated at a color temperature
between 2200-3000°K.
6.2.2 Distance traversed by incident light and scattered light within the
sample tube: Total not to exceed 10 cm.
6.2.3 Detector: Centered at 90° to the incident light path and not to
exceed ± 30° from 90°. The detector, and filter system if used,
shall have a spectral peak response between 400 and 600 nm.
6.3 The sensitivity of the instrument should permit detection of a turbidity
difference of 0.02 NTU or less in waters having turbidities less than 1 unit.
The instrument should measure from 0 to 40 units turbidity. Several
ranges may be necessary to obtain both adequate coverage and sufficient
sensitivity for low turbidities.
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APPENDIX B. DETERMINA TION OF TURBIDITY
6.4 The sample tubes to be used with the available instrument must be of
clear, colorless glass or plastic. They should be kept scrupulously clean,
both inside and out, and discarded when they become scratched or etched.
A light coating of silicon oil may be used to mask minor imperfections in
glass tubes. They must not be handled at all where the light strikes them,
but should be provided with sufficient extra length, or with a protective '
case, so that they may be handled. Tubes should be checked, indexed and
read at the orientation that produces the lowest background blank value.
6.5 Balance - - Analytical, capable of accurately weighing to the nearest
0.0001 g.
6.6 Glassware - - Class A volumetric flasks and pipettes as required.
7.0 REAGENTS AND STANDARDS
7.1 Reagent water, turbidity-free: Pass deionized distilled water through a
0.45^ pore size membrane filter, if such filtered water shows a lower
turbidity than unfiltered distilled water.
7.2 Stock standard suspension (Formazin):
7.2.1 Dissolve 1.00 g hydrazine sulfate, (NH2)2.H2SO4, (CASRN 10034-
93-2) in reagent water and dilute to 100 mL in a volumetric flask
CAUTION--CARCINOGEN
7.2.2 Dissolve 10.00 g hexamethylenetetramine (CASRN 100-97-0) in
reagent water and dilute to 100 mL in a volumetric flask. In a 100
mL volumetric flask, mix 5.0 mL of each solution (7.2.1 + 7.2.2).
Allow to stand 24 hours at 25 + 3 °C, then dilute to the mark with
reagent water.
7.3 Primary calibration standards: Mix and dilute 10.00 mL of stock standard
suspension (7.2) to 100 mL with reagent water. The turbidity of this
suspension is defined as 40 NTU. For other values, mix and dilute
portions of this suspension as required.
7.3.1 A new stock standard suspension (7.2) should be prepared each
month. Primary calibration standards (7.3) should be prepared
daily by dilution of the stock standard suspension.
7.4 Formazin in commercially prepared primary concentrated stock standard
suspension (SSS) may be diluted and used as required. Dilute turbidity
standards should be prepared daily.
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APPENDIX B. DETERMINATION OF TURBIDITY
7.5 AMCO-AEPA-1 Styrene Divinylbenzene polymer primary standards are
available for specific instruments and require no preparation or dilution
prior to use.
7.6 Secondary standards may be acceptable as a daily calibration check, but
must be monitored on a routine basis for deterioration and replaced as
required.
8.0 SAMPLE COLLECTION. PRESERVATION AND STORAGE
8.1 Samples should be collected in plastic or glass bottles. All bottles must be
thoroughly cleaned and rinsed with turbidity free water. Volume collected
should"be sufficient to insure a representative sample, allow for replicate
analysis (if required), and minimize waste disposal.
8.2 No chemical preservation is required. Cool sample to 4°C.
8.3 Samples should be analyzed as soon as possible after collection. If storage
is required, samples maintained at 4°C may be held for up to 48 h.
9.0 QUALITY CONTROL
9.1 Each laboratory using this method is required to operate a formal quality
control (QC) program. The minimum requirements of this program
consist of an initial demonstration of laboratory capability and analysis of
laboratory reagent blanks and other solutions as a continuing check on
performance. The laboratory is required to maintain performance records
that define the quality of data generated.
9.2 INITIAL DEMONSTRATION OF PERFORMANCE.
9.2.1 The initial demonstration of performance is used to characterize
instrument performance (determined of LCRs and analysis of
QCS).
9.2.2 Linear Calibration Range (LCR) - - The LCR must be determined
initially and verified every 6 months or whenever a significant
change in instrument response is observed or expected. The initial
demonstration of linearity must use sufficient standards to insure
that the resulting curve is linear. The verification of linearity must
use a minimum of a blank and three standards. If any verification
data exceeds the initial values by + 10%, linearity must be
reestablished. If any portion of the range is shown to be nonlinear,
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Turbidity Provisions
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APPENDIX B. DETERMINATION OF TURBIDITY
sufficient standards must be used to clearly define the nonlinear
portion.
9.2.3 Quality Control Sample (QCS) - - When beginning the use of this
method, on a quarterly basis or as required to meet data-quality
needs, verify the calibration standards and acceptable instrument
performance with the preparation and analysis of a QCS. If the
determined concentrations are not within + 10% of the stated
values, performance of the determinative step of the method is
unacceptable. The source of the problem must be identified and
corrected before continuing with on-going analyses.
9.3 ASSESSING LABORATORY PERFORMANCE
9.3.1 Laboratory Reagent Blank (LRB) - - The laboratory must analyze
at least one LRB with each batch of samples. Data produced are
used to assess contamination from the laboratory environment.
9.3.2 Instrument Performance Check Solution (IPC) - - For all
determinations, the laboratory must analyze the IPC (a midrange
check standard) and a calibration blank immediately following
daily calibration, after every tenth sample (or more frequently, if
required) and at the end of the sample run. Analysis of the IPC
solution and calibration blank immediately following calibration
must verify that the instrument is within + 10% of calibration.
Subsequent analyses of the IPC solution must verify the calibration
is still within + 10%. If the calibration cannot be verified within
the specified limits, reanalyze the IPC solution. If the second
analysis of the IPC solution confirms calibration to be outside the
limits, sample analysis must be discontinued, the cause determined
and/or in the case of drift the instrument recalibrated. All samples
following the last acceptable IPC solution must be reanalyzed. The
analysis data of the calibration blank and IPC solution must be kept
on file with the sample analyses data. NOTE: Secondary
calibration standards (SS) may also be used as the IPC.
9.3.3 Where additional reference materials such as Performance
Evaluation samples are available, they should be analyzed to
provide additional performance data. The analysis of reference
samples is a valuable tool for demonstrating the ability to perform
the method acceptably.
10.0 CALIBRATION AND STANDARDIZATION
April 1999 B^ - EPA Guidance Manual
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APPENDIX B. DETERMINATION OF TURBIDITY
10.1 Turbidimeter calibration: The manufacturer's operating instructions
should be followed. Measure standards on the turbidimeter covering the
range of interest. If the instrument is already calibrated in standard
turbidity units, this procedure will check the accuracy of the calibration
scales. At least one standard should be run in each instrument range to be
used. Some instruments permit adjustments of sensitivity so that scale
values will correspond to turbidities. Solid standards, such as those made
of lucite blocks, should never be used due to potential calibration changes
caused by surface scratches. If a pre-calibrated scale is not supplied,
calibration curves should be prepared for each range of the instrument.
11.0 PROCEDURE
11.1 Turbidities less than 40 units: If possible, allow samples to come to room
temperature before analysis. Mix the sample to thoroughly disperse the
solids. Wait until air bubbles disappear, then pour the sample into the
turbidimeter tube. Read the turbidity directly from the instrument scale or
from the appropriate calibration curve.
11.2 Turbidities exceeding 40 units: Dilute the sample with one or more
volumes of turbidity-free water until the turbidity falls below 40 units.
The turbidity of the original sample is then computed from the turbidity of
the diluted sample and the dilution factor. For example, if 5 volumes of
turbidity-free water were added to 1 volume of sample, and the diluted
sample showed a turbidity of 30 units, then the turbidity of the original
sample was 180 units.
11.2.1 Some turbidimeters are equipped with several separate scales. The
higher scales are to be used only as indicators of required dilution
volumes to reduce readings to less than 40 NTU.
NOTE 1: Comparative work performed in the Environmental
Monitoring Systems Laboratory - Cincinnati (EMSL-
Cincinnati) indicates a progressive error on sample
turbidities in excess of 40 units.
12.0 DATA ANALYSIS AND CALCULATIONS
12.1 Multiply sample readings by appropriate dilution to obtain final reading.
12.2 Report results as follows:
EPA Guidance Manual B-8 APril 19"
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APPENDIX B. DETERMINA TION OF TURBIDITY
NTU
0.0-1.0
.1-10
10-40
40 - 100
100-400
400-1,000
> 1,000
13.0 METHOD PERFORMANCE
Record to Nearest
0.05
0.1
1
5
10
50
100
13.1 In a single laboratory (EMSL-Cincinnati), using surface water samples at
levels of 26,41, 75 and 180 NTU, the standard deviations were ± 0.60, ±
0.94, ±1.2 and ± 4.7 units, respectively.
13.2 The interlaboratory precision and accuracy data in Table 1 were developed
using a reagent water matrix. Values are in NTU.
14-° POLLUTION PREVENTION
14.1
14.2
14.3
Pollution prevention encompasses any technique that reduces or eliminates
the quantity or toxicity of waste at the point of generation. Numerous
opportunities for pollution prevention exist in laboratory operation. The
EPA has established a preferred hierarchy of environmental management
techniques that places pollution prevention as the management option of
first choice. Whenever feasible, laboratory personnel should use pollution
prevention techniques to address their waste generation. When wastes
cannot be feasibly reduced at the source, the Agency recommends
recycling as the next best option.
The quantity of chemicals purchased should be based on expected usage
during its shelf life and disposal cost of unused material. Actual reagent
preparation volumes should reflect anticipated usage and reagent stability.
For information about pollution prevention that may be applicable to
laboratories and research institutions, consult "Less is Better: Laboratory
Chemical Management for Waste Reduction," available from the
American Chemical Society's Department of Government Regulations and
Science Policy, 1155 16th Street, N.W., Washington, DC 20036, (202)
872-4477.
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APPENDIX B. DETERMINATION OF TURBIDITY
15.0 WASTE MANAGEMENT
15.1 The U.S. Environmental Protection Agency requires that laboratory waste
management practices be conducted consistent with all applicable rules
and regulations. Excess reagents, samples and method process wastes
should be characterized and disposed of in an acceptable manner. The
Agency urges laboratories to protect the air, water and land by minimizing
and controlling all releases from hoods, and bench operations, complying
with the letter and spirit of any waste discharge permit and regulations,
and by complying with all solid and hazardous waste regulations,
particularly the hazardous waste identification rules and land disposal
restrictions. For further information on waste management consult the
"Waste Management Manual for Laboratory Personnel," available from
the American Chemical Society at the address listed in Sect. 14.3.
16.0 REFERENCES
1. American Society for Testing and Materials (ASTM). 1993. Annual Book
of ASTM Standards, Volume 11.01. Water (1), Standard D1889-88A, p.
359. West Conshohocken, PA,
2. Standard Methods. 1992. Standard Methods for the Examination of
Water and Waste-water. Eighteenth Edition, pp. 2-9, Method 2 BOB.
APHA, AWWA, and WEF. Port City Press, Baltimore, MD.
17.0 TABLES. DIAGRAMS. FLOWCHARTS AND VALIDATION DATA
Table 1. Intel-laboratory Precision And Accuracy Data
Number of
Values
Reported
373
374
289
482
484
489
640
487
288
714
641
True Value
(T)
0.450
0.600
0.65
0.910
0.910
1.00
1.36
3.40
4.8
5.60
5.95
Mean
(X)
0.4864
0.6026
0.6931
0.9244
0.9919
0.9405
1.3456
3.2616
4.5684
5.6984
5.6026
Residual for
X
0.0027
-0.0244
0.0183
0.0013
0.0688
-0.0686
-0.0074
-0.0401
-0.0706
0.2952
-0.1350
Standard
Deviation
(S)
0.1071
0.1048
0.1301
0.2512
0.1486
0.1318
0.1894
0.3219
0.3776
0.4411
0.4122
Residual for
S
-0.0078
-0.021 1
0.0005
0.1024
-0.0002
-0.0236
0.0075
-0.0103
-0.0577
-0.0531
-0.1078
Regressions: X = 0.955T + 0.54, S = 0.074T + 0.082
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APPENDIX C. TURBIDITY STANDARD
METHOD
2130 TURBIDITY*
2130 A. Introduction
1. Sources and Significance
Clarity of water is important in producing products destined
for human consumption and in many manufacturing operations.
Beverage producers, food processors, and potable water treat-
ment plants drawing from a surface water source commonly rely
on fluid-particle separation processes such as sedimentation and
filtration to increase clarity and insure an acceptable product.
The clarity of a natural body of water is an important determinant
of its condition and productivity.
Turbidity in water is caused by suspended and colloidal matter
such as clay. silt, finely divided organic and inorganic matter.
and plankton and other microscopic organisms. Turbidity is an
expression of the optical property that causes light to be scattered
and absorbed rather than transmitted with no change in direction
or flux level through the sample. Correlation of turbidity with
the weight or particle number concentration of suspended matter
is difficult because the size, shape, and refractive index of the
particles affect the light-scattering properties of the suspension.
When present in significant concentrations, particles consisting
of light-absorbing materials such as activated carbon cause a
negative interference. In low concentrations these panicles tend
to have a positive influence because they contribute to turbidity.
The presence of dissolved, color-causing substances that absorb
light may cause a negative interference. Some commercial in-
struments may have the capability of either correcting for a slight
color interference or optically blanking out the color effect.
' Approved h> Standard Methods Committee.
2. Selection of Method
Historically, the standard method for determination of tur-
bidity has been based on the Jackson candle turbidimeter; how-
ever, the lowest turbidity value that can be measured directly on
this device is 25 Jackson Turbidity Units (JTU). Because'tur-
bidities of water treated by conventional fluid-particle separation
processes usually fail within the range of 0 to 1 unit, indirect
secondary methods were developed to estimate turbidity. Elec-
tronic nephelometers are the preferred instruments for turbiditv
measurement.
Most commercial turbidimeters designed for measuring low
turbidities give comparatively good indications of the intensity
of light scattered in one particular direction, predominantly at
right angles to the incident light. Turbidimeters with scattered-
light detectors located at 90C to the incident beam are called
nephelometers. Nephelometers are relatively unaffected by small
differences in design parameters and therefore are specified as
the standard instrument for measurement of low turbidities. In-
struments of different make and model may vary in response. *
However, interinstrument variation may be effectively negligible
if good measurement techniques are used and the characteristics
of the particles in the measured suspensions are similar. Poor
measurement technique can have a greater effect on measure-
• Nephelometers that instrument manufacturers claim meet the design specifica-
tions of this method ma> not give the same reading for a given suspension, even
»;hcn each instrument has been Calibrated using the manufacturer's manual. This.
dinerenlial performance is cspcciaih important when measurements are made for
regulator;, purposes. Consult regulator) authorities when selecting a nephelomcter
to be used for making measurements that will be reported for regulator purposes
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TURBIDITY (2130)/Nephelometric Method
meni error than small differences in instrument design. Turbi-
dimeters of nonstandard design, such as forward-scattering de-
vices, may be more sensitive than nephelometers to the presence
of larger particles. While it may not be appropriate to compare
their output with that of instruments of standard design, they
still may be useful for process monitoring.
An additional cause of discrepancies in turbidity analysis is the
use of suspensions of different types of paniculate matter for
instrument calibration. Like water samples, prepared suspen-
sions have different optical properties depending on the particle
size distributions, shapes, and refractive indices. A standard ref-
erence suspension having reproducible light-scattering properties
is specified for nephelometer calibration.
Its precision, sensitivity, and applicability over a wide turbidity
range make the nephelometric method preferable to visual meth-
ods. Report nephelometric measurement results as nephelo-
metric turbidity units (NTU).
3. Storage of Sample
Determine turbidity as soon as possible after the sample is
taken. Gently agitate all samples before examination to ensure
a representative measurement. Sample preservation is not prac-
tical; begin analysis promptly. Refrigerate or cool to 4°C, to
minimize microbiological decomposition of solids, if storage is
required. For best results, measure turbidity immediately with-
out altering the original sample conditions such as temperature
or pH.
2130 B. Nephelometric Method
1. General Discussion
a. Principle: This method is based on a comparison of the
Intensity of light scattered by the sample under defined conditions
with the intensity of light scattered by a standard reference sus-
pension under the same conditions. The higher the intensity of
scattered light, the higher the turbidity. Formazin polymer is
used as the primary standard reference suspension. The turbidity
of a specified concentration of formazin suspension is defined as
4000 NTU.
b. Interference: Turbidity can be determined for any water
sample that is free of debris and rapidly settling coarse sediment.
Dirty glassware and the presence of air bubbles give false results.
"True color," i.e., water color due to dissolved substances that
absorb light, causes measured turbidities to be low. This effect
usually is not significant in treated water.
2. Apparatus
a. Laboratory or process nephelometer consisting of a light
source for illuminating the sample and one or more photoelectric
detectors with a readout device to indicate intensity of light scat-
tered at 90s to the path of incident light. Use an instrument
designed to minimize stray light reaching the detector in the
absence of turbidity and to be free from significant drift after a
short warmup period. The sensitivity of the instrument should
permit detecting turbidity differences of 0.02 NTU or less in the
lowest range in waters having a turbidity of less than 1 NTU.
Several ranges may be necessary to obtain both adequate cov-
erage and sufficient sensitivity for low turbidities. Differences in
instrument design will cause differences in measured values for
turbidity even though the same suspension is used for calibration.
To minimize such differences, observe the following design cri-
teria:
1) Light source—Tungsten-filament lamp operated at a color
temperature between 2200 and 3000°K.
2) Distance traversed by incident light and scattered light within
the sample tube—Total not to exceed 10 cm.
3) Angle of light acceptance by detector—Centered at 90° to
the incident light path and not to exceed ±30° from 90°. The
detector and filter system, if used, shall have a spectral peak
response between 400 and 600 nm.
b. Sample cells: Use sample cells or tubes of clear, colorless
glass or plastic. Keep cells scrupulously clean, both inside and
out, and discard if scratched or etched. Never handle them where
the instrument's light beam will strike them. Use tubes with
sufficient extra length, or with a protective case, so that they
may be handled properly. Fill cells with samples and standards
that have been agitated thoroughly and allow sufficient time for
bubbles to escape.
Clean sample cells by thorough washing with laboratory soap
inside and out followed by multiple rinses with distilled or deion-
ized water; let cells air-dry. Handle sample cells only by the top
to avoid dirt and fingerprints within the light path.
Cells may be coated on the outside with a thin layer of silicone
oil to mask minor imperfections and scratches that may contrib-
ute to stray light. Use silicone oil with the same refractive index
as glass. Avoid excess oil'because it may attract dirt and con-
taminate the sample compartment of the instrument. Using a
soft, lint-free cloth, spread the oil uniformly and wipe off excess.
The cell should appear to be nearly dry with little or no visible
oil.
Because small differences between sample cells significantly
impact measurement, use either matched pairs of cells or the
same cell for both standardization and sample measurement.
3. Reagents
a. Dilution water: High-purity water will cause some light scat-
tering, which is detected by nephelometers as turbidity. To obtain
low-turbidity water for dilutions, nominal value 0.02 NTU, pass
laboratory reagent-grade water through a filter with pore size
sufficiently small to remove essentially all particles larger than
0.1 p.m;* the usual membrane filter used for bacteriological ex-
aminations is not satisfactory. Rinse collecting flask at least twice
with filtered water and discard the next 200 mL.
Some commercial bottled demineralized waters have a low
turbidity. These may be used when filtration is impractical or a
good grade of water is not available to filter in the laboratory.
Check turbidity of bottled water to make sure it is lower than
the level that can be achieved in the laboratory.
• Nuclepore Corp.. 7035 Commerce Circle, Pleasamon. Calif., or equivalent.
-------
b. Stock primary standard formazin suspension:
1) Solution I—Dissolve 1.000 ghydrazine sulfate. (NH,)2-H2SO4.
distilled water and dilute to 100 mL in a volumetric flask.
UTION: Hydra:ine sulfale is a carcinogen; avoid inhalation,
Ingestion. and skin contact. Formazin suspensions can contain
residual hydrazine sulfate.
2) Solution II — Dissolve 10.00 g hexamethylenetetramine.
(CH:)6Nj. in distilled water and dilute to 100 mL in a volumetric
flask.
3) In a flask, mix 5.0 mL Solution I and 5.0 mL Solution II.
Let stand for 24 h at 25 ± 3°C. This results in a 4000-NTU
suspension. Transfer stock suspension to an amber glass or other
UV-light-blocking bottle for storage. Make dilutions from this
stock suspension. The stock suspension is stable for up to 1 year
when properly stored.
c. Dilute turbidity suspensions: Dilute 4000 NTU primary
standard suspension with high-quality dilution water. Prepare
immediately before use and discard after use.
d. Secondary standards: Secondary standards are standards
that the manufacturer (or an independent testing organization)
has certified will give instrument calibration results equivalent
(within certain limits) to the results obtained when the instrument
is calibrated with the primary standard, i.e., user-prepared for-
mazin. Various secondary standards are available including:
commercial stock suspensions of 4000 NTU formazin, commer-
cial suspensions of microspheres of styrene-divinylbenzene co-
polymer,t and items supplied by instrument manufacturers, such
as sealed sample cells filled with latex suspension or with metal
oxide particles in a polymer gel. The U.S. Environmental Pro-
tection Agency1 designates user-prepared formazin, commercial
formazin suspensions, and commercial styrene-divinylben-
ie suspensions as "primary standards," arid reserves the term
secondary standard" for the sealed standards mentioned above.
Secondary standards made with suspensions of microspheres
of styrene-divinylbenzene copolymer typically are as stable as
concentrated formazin and are much more stable than diluted
formazin. These suspensions can be instrument-specific: there-
fore, use only suspensions formulated for the type of nephelo-
meter being used. Secondary standards provided by the instru-
ment manufacturer (sometimes called "permanent" standards)
may be necessary to standardize some instruments before each
reading and in other instruments only as a calibration check to
determine when calibration with the primary standard is nec-
essary.
All secondary standards, even so-called "permanent" stand-
ards, change with time. Replace them when their age exceeds
the shelf life. Deterioration can be detected by measuring the
turbidity of the standard after calibrating the instrument with a
fresh formazin or microsphere suspension. If there is any doubt
about the integrity or turbidity value of any secondary standard,
check .instrument calibration first with another secondary stand-
ard and then, if necessary, with user-prepared formazin. Most
secondary standards have been carefully prepared by their man-
ufacturer and should, if properly used* give good agreement with
formazin. Prepare formazin primary standard only as a last re-
sort. Proper application of secondary standards is specific for
each make and model of nephelometer. Not all secondary stand-.
ards have to be discarded when comparison with a primary stand-
'AMCO-AEPA-l Standard. Advanced Polymer Systems. 3696 Haven Ave.. Red-
wood Cit\. Calif., or equivalent.
ard shows that their turbidity value has changed. In some cases,
the secondary standard should be simply relabeled with the new
turbiditv value. Alwavs follow the manufacturer's^directions.
4. Procedure
a. General measurement techniques: Proper measurement tech-
niques are important in minimizing the effects of instrument
variables as well as stray light and air bubbles. Regardless of the
instrument used, the measurement will be more accurate, pre-
cise, and repeatable if close attention is paid to proper meas-
urement techniques.
Measure turbidity immediately to prevent temperature changes
and particle flocculation and sedimentation from changing sam-
ple characteristics. If flocculation is apparent, break up aggre-
gates by agitation. Avoid dilution whenever possible. Particles
suspended in the original sample may dissolve or otherwise change
characteristics when the temperature changes or when the sample
is diluted.
Remove air or other entrained gases in the sample before
measurement. Preferably degas even if no bubbles are visible.
Degas by applying a partial vacuum, adding a nonfoaming-type
surfactant, using an ultrasonic bath, or applying heat. In some
cases, two or more of these techniques may be combined for
more effective bubble removal. For example, it may be necessary
to combine addition of a surfactant with use of an ultrasonic bath
for some severe conditions. Any of these techniques, if misap-
plied, can alter sample turbidity; use with care. If degassing can-
not be applied, bubble formation will be minimized if the samples
are maintained at the temperature and pressure of the water
before sampling.
Do not remove air bubbles by letting sample stand for a period
of time because during standing, turbidity-causing particulates
may settle and sample temperature may change. Both of these
conditions alter sample turbidity, resulting in a nonrepresentative
measurement.
Condensation may occur on the outside surface of a sample
cell when a cold sample is being measured in a warm, humid
environment. This interferes with turbidity measurement. Re-
move all moisture from the outside of the sample cell before
placing the cell in the instrument. If fogging recurs, let sample
warm slightly by letting it stand at room temperature or by par-
tially immersing it in a warm water bath for a short time. Make
sure samples are again well mixed.
b. Nephelometer calibration: Follow the manufacturer's op-
• crating instructions. Run at least one standard in each instrument
range to be used. Make certain the nephelometer gives stable
readings in all sensitivity ranges used. Follow techniques outlined
in 1is 2b and 4a for care and handling of sample cells, degassing,
and dealing with condensation.
c. Measurement of turbidity: Gently agitate sample. Wait until
air bubbles disappear and pour sample into cell. When possible.
pour well-mixed sample into cell and immerse it in an ultrasonic
bath for 1 to 2 s or apply vacuum degassing, causing complete
bubble release. Read turbidity directly from instrument display.
d. Calibration of continuous turbidity monitors: Calibrate con-
tinuous turbidity monitors for low turbidities by determining
turbidity of the water flowing out of them, using a laboratory-
model nephelometer. or calibrate the instruments according to
manufacturer's instructions with formazin primary standard or
appropriate secondary standard.
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5. Interpretation of Results
Report turbidity readings as follows:
Turbiditv Range
ATI/
0-1.0
1-10
10-40
40-100
100-400
400-1000
>1000
Report to the
Nearest
,\'TU
0.05
0.1
1
5
10
50
100
When comparing water treatment efficiencies, do not estimate
turbidity more closely than specified above. Uncertainties and
discrepancies in turbidity measurements make it unlikely that
results can be duplicated to greater precision than specified.
6. Reference
I. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1993. Methods for
Determination of Inorganic Substances in Environmental Samples.
EPA-600.'R'93/100 - Draft. Environmental Monitoring Systems Lab..
Cincinnati, Ohio.
7. Bibliography
HACH. C.C.. R.D. VANOUS & J.M. HEER. 1985. Understanding Tur-
bidity Measurement. Hach Co.. Technical Information Ser.. Book-
let 11. Loveland. Colo.
KATZ. E.L. 1986. The stability of turbidity in raw water and its rela-
tionship to chlorine demand. J. Amer. Water Works Assoc. 78:72.
McCov. W.F. & B.H. OLSON. 1986. Relationship among turbidity.
particle counts and bacteriological quality within water distribution
lines. Water Res. 20:1023.
BUCKLIN. K.E.. G.A. MCFETERS & A. AMIRTHARAJAH. 1991. Pene-
tration of coliform through municipal drinking water filters. Water
Res. 25:1013.
HERNANDEZ. E.. R.A. BAKER & P.C. CRANDALL. 1991. Model for
evaluating turbidity in cloudy beverages. J. Food Sci. 56:747.
HART, V.S.. C.E. JOHNSON & R.D. LETTERMAN. 1992. An analysis of
low-level turbidity measurements. J. Amer. Water Works Assoc.,
84(12):40.
LECHEVALLIER. M.W. & W.D. NORTON. 1992. Examining relationship
between particle counts and Ciardia, Cryptosporidium, and turbid-
ity. J. Amer. Water Works Assoc. 84(12):54.
2150 ODOR*
2150 A. Introduction
1. Discussion
Odor, like taste, depends on contact of a stimulating substance
with the appropriate human receptor cell. The stimuli are chem-
ical in nature and the term "chemical senses" often is applied to
odor and taste. Water is a neutral medium, always present on
or at the receptors that perceive sensory response. In its pure
form, water cannot produce odor or taste sensations. Man and
other animals can avoid many potentially toxic foods and waters
because of adverse sensory response. These senses often provide
the first warning of potential hazards in the environment.
Odor is recognized1 as a quality factor affecting acceptability
of drinking water (and foods prepared with it), tainting of fish
and other aquatic organisms, and esthetics of recreational waters.
Most organic and some inorganic chemicals contribute taste or
odor. These chemicals may originate from municipal and indus-
trial waste discharges, from natural sources such as decompo-
sition of vegetable matter, or from associated microbial activity.
and from disinfectants or their products.
The potential for impairment of the sensory quality of water
has increased as a result of expansion in the variety and quantity
of waste materials, demand for water disposal of captured air
pollutants, and increased reuse of available water supplies by a
growing population. Domestic consumers and process industries
such as food, beverage, and pharmaceutical manufacturers re-
quire water essentially free of tastes and odors.
* Approved by Standard Methods Committee. 1991.
Some substances, such as certain inorganic salts, produce taste
without odor and are evaluated by taste testing (Section 2160).
Many other sensations ascribed to the sense of taste actually are
odors, even though the sensation is not noticed until the material
is taken into the mouth. Because some odorous materials are
detectable when present in only a few nanograms per liter, it is
usually impractical and often impossible to isolate and identify
the odor-producing chemical. The ultimate odor-testing device
is the human nose. Odor tests are performed to provide quali-
tative descriptions and approximate quantitative measurements
of odor intensity. The method for intensity measurement pre-
sented here is the threshold odor test, based on a method of
limits.1 This procedure, while not universally preferred.3 has def-
inite strengths.4
Sensory tests are useful as a check on the quality of raw and
finished water and for control of odor through the treatment
process. They can assess the effectiveness of different treatments
and provide a means of tracing the source of contamination.
Section 6040B provides an analytical procedure for quantifying
several organic odor-producing compounds including geosmin
and methylisoborneol.
2. References
1. U.S. ENVIRONMENTAL PROTECTION AGENCY. 1973. Proposed Criteria
for Water Quality. Vol. 1. Washington. D.C.
2. AMERICAN SOCIETY FOR TESTING AND MATERIALS COMMITTEE E-18.
1968. STP 433. Basic principles of sensory evaluation: STP 434. Man-
ual on sensory testing methods: STP 440. Correlation of subjective-
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APPENDIX D. TURBIDITY GLI METHOD 2
Revision Date: November 2,1992
1. SCOPE AND APPLICATION
1.1 This method is applicable to drinking water samples in the range of
turbidity from 0 to 40 nephelometric units (NTU). Higher values may be
obtained with dilution of the sample. A method detection limit of. 100
NTUs is recommended for this procedure.
NOTE 1: NTUS are considered comparable to the previously reported
Formazin Turbidity Units (FTU).
2. SUMMARY OF METHOD
2.1 The method is based upon a comparison of the intensity of light scatters by
the sample under defined conditions with the intensity of the light
scattered by a standard reference suspension. The higher the intensity of
the scattered light, the higher the turbidity. Readings, in NTUs, are made
in a nephelometer designed according to the specifications outlined in
Apparatus. A standard suspension of Formazin, prepared under closely
defined conditions, is used to calibrate the instrument.
2.1.1 Formazin polymer is used as the turbidity reference suspension for
water because it is more reproducible than other types of standards
previously used for turbidity standards.
3. SAMPLE HANDLING AND PRESERVATION
3.1 Collect each sample in a soft or hard plastic, or soft or hard glass
container. Immediately refrigerate or ice the sample to 4°C and analyze
within 48 hours.
4. Physical Characteristics
4.1 The presence of floating debris and coarse sediments which settle out
rapidly will give low readings. Finely divided air bubbles will affect the
results in a positive manner.
4.2 The presence of color, that is the color of water which is due to dissolved
substances which absorb light, will cause turbidities to be low, although
this effect is generally not significant.
April 1999 D-1 EPA Guidance Manual
Turbidity Provisions
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APPENDIX D. TURBIDITY GLI METHOD 2
5. APPARATUS
5.1 The turbidimeter shall consist of a nephelometer with two light sources for
illuminating the sample and two detectors with a readout device to indicate
the intensity of light scattered at right angles to the path of the incident
light. The turbidimeter shall accomplish two measurement phases every
0.5 seconds.
In the first phase, light source shall pulse a beam of light directly into
photodetector two. Simultaneously, photodetector one shall measure the
light scattered at a 90 degree angle. Diffused light measured by
photodetector two shall be called the reference signal while scattered light
measured by photodetector one shall be called an active signal.
/
In the second phase, light source two shall pulse a beam of light directly
into photodetector one. Simultaneously, photodetector two shall measure
the light scattered at a 90 degree angle. This time, the diffused light
measured by photodetector one shall be called a reference signal and
scattered light measured by photodetector two shall be called an active
signal. The two phase measurement shall provide four independent
measurements from the two light sources; two reference signals and two
active signals. A ratiometric algorithm will then be used to calculate the
turbidity value from these four readings.
The turbidimeter should be designed that little stray light reaches the
detector in the absence of turbidity and should be free from significant
drift after a short warm-up period.
5.2 The sensitivity of the instrument should permit detection of a turbidity
difference of 0.02 unit or less in waters with turbidities less than 1 unit.
The instrument should measure from 0 to 40 units turbidity. Several
ranges will be necessary to obtain both adequate coverage and sufficient
sensitivity for low turbidities.
5.3 The sample tubes to be used with the available instrument must be of
clear, colorless optical glass. They should be kept scrupulously clean, both
inside and out, and discarded when the become scratched or etched. They
must not be handled at all where the light strikes them, but should be
provided with sufficient extra length, or with a protective case, so that they
may be handled.
5.4 Any apparatus may be used provided that it complies with the following
requirements:
5.4.1 the wavelength of the incident radiation shall be 860 nm;
EPA Guidance Manual D-2 April 1999
Turbidity Provisions
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APPENDIX D. TURBIDITY GU METHOD 2
5.4.2 the spectral bandwidth of the incident radiation shall be less than or
equal to 60 nrn;
5.4.3 there shall be no divergence from parallelism of the incident
radiation and any convergence shall not exceed 1.5 degrees;
5.4.4 There shall be two light sources and two detectors
5.4.5 the measuring angle between the optical axis of the incident
radiation and that of the diffused radiation for light pulsed through
the sample by light source one shall be 90 ± 2.5 degrees;
5.4.6 the measuring angle between the optical axis of the incident
radiation and that of the diffused radiation for light pulsed through
the sample by light source two shall be 90 ± 2.5 degrees;
The narrow definition of the light source makes it unnecessary to specify
sensitivity of the photodetector.
6. REAGENTS
6.1 Turbidity-free water: Pass distilled water through a membrane filter having
precision-sized holes of 0.2 ^m; the usual membrane filter used for
bacterial examinations is not satisfactory (Sec. 3.1, EPA-approved
Standard Method 214A, 16th edition).
6.2 Stock formazin turbidity suspension:
Solution 1: Distilled 1.00 g hydrazine sulfate (NH2)H2SO4,, in distilled
water and dilute to 100 ml in a volumetric flask.
Solution2: Dissolve 10.00 g hexamethylenetetramine in distilled water and
dilute to 100 ml in a volumetric flask. Li a 100 ml volumetric flask, mix
5.0 ml Solution 1 with 5.0 ml Solution 2. Allow to stand 24 hours at 24 ±
degrees C, then dilute to the mark and mix.
6.3 Standard formazin turbidity suspension: Dilute 10.00 ml stock turbidity
suspension to 100 ml with turbidity-free water. The turbidity of this
suspension is defined as 40 units. Dilute portions of the standard turbidity
suspension with turbidity-free water as required.
7. PROCEDURE
7.1 Turbidimeter calibration: The manufacturer's operating instructions should
be followed. Measure standards on the turbidimeter covering the range of
interest. If the instrument is already calibrated in standard turbidity units,
this procedure will check the accuracy of the calibration scales. At least
one standard should be run in each instrument range to be used. Some
instruments permit adjustments of sensitivity so that scale is not supplied,
then calibration curves should be prepared for each range of the
April 1999 D-3 EPA Guidance Manual
Turbidity Provisions
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APPENDIX D. TURBIDITY GLI METHOD 2
8.
instrument.. If a pre-calibrated scale is not supplied, then calibration
curves should be prepared for each range of the instrument.
7.2 Turbidities less than 40 units: Shake the sample thoroughly to disperse the
solids. Wait until air bubbles disappear then pour the sample into the
turbidimeter tube . Read the turbidity directly from the instrument scale or
from the appropriate calibration curve.
7.3 Turbidities exceeding 40 units: Dilute the sample with one or more
volumes of turbidity-free water until the turbidity falls below 40 units.
The turbidity of the original sample is then computed from the turbidity of
the diluted sample and the dilution factor. For example if five volumes of
turbidity-free water were added to 1 volume of sample, and the diluted
sample showed a turbidity of 30 units, then the turbidity of the original
sample was 180 units.
CALCULATIONS
8.1 Nephelometric turbidity units (NTU)
= Ax(B+C)
C
where: A = NTU found in the diluted sample
B = volume of the dilution water, mL and
C = sample volume taken for dilution, mL
8.2 Report results as follows:
NTU
0.0-1.0
1-10
10-40
40-100
100-400
400-1,000
>1,000
Record to
Nearest
0.05
0.1
1
5
10
50
100
EPA Guidance Manual
Turbidity Provisions
D-4
April 1999
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APPENDIX D. TURBIDITY GLI METHOD 2
9.
PRECISION AND ACCURACY
9.1
9.2
In a single laboratory, using surface waters filtered to minimum turbidity
with 0.04// filters and dosed with formazin to levels of 0.47, 0.91, 5.6,
9.8, 39.3, 82.7 and 99.4 NTU, the + standard deviations were 0.007,
0.014, 0.1, 0.22, 0.45, and 0.83 units respectively.
Accuracy of the great Lakes Turbidity Method
The range of the mean percent recoveries of turbidity from 10 fortified
drinking water samples, each analyzed in triplicate by the Great Lakes
Instruments Turbidity Method, was as follows:
Turbidity Added to Sample
4.5
9.5
34.5
Range of Percent Recovery
98.1-112.2
96.8-111.1
93.7-114.3
PRECISION OF THE GREAT LAKES TURBIDITY METHOD
The range of the standard deviations and percent relative standard deviations (or percent
coefficient of variations) associated with the triplicate observations of the total theoretical
concentration of 10 fortified drinking water samples by the Great Lakes Instruments
turbidity Method were as follows:
Total Theoretical
Turbidity (NTU)
0.5
5.0
10.0
35.0
Standard Deviation
(NTU)
0.01-0.06
0.00-0.10
0.00
0.00-0.58
Relative Standard
Deviation (%)
1.1-10.7
0.0-2.0
0.0-5.6
0.0-1.8
9.3 Accuracy and precision should be checked on a routine basis to monitor
the overall performance of the instrument. A series of reagent blanks and
check standards should be run to validate the quality of sample data.
These checks should occur at frequency that is required for regulatory
compliance.
April 1999
D-5
EPA Guidance Manual
Turbidity Provisions
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APPENDIX D. TURBIDITY GLI METHOD 2
10. SAFETY
10.1 Operators handling reagents should wear safety glasses, rubber gloves and
appropriate protective clothing. Consult the Material Safety Data Sheets
for additional safety information before working with reagents.
11. QUALITY ASSURANCE
11.1 Each laboratory using this method in regulated environmental monitoring
is required to operate a formal quality assurance/control program. The
minimum initial requirements of this program consist of the
demonstration of the laboratory's capability with this method. On a
continuing basis, the laboratory should check its performance (accuracy
and precision) by analyzing reagent blanks and check standards, fortified
blanks, and/or fortified samples, preferably at a minimum frequency of
10% of the total samples analyzed by the method. The laboratory should
maintain the performance records that define the quality of the data
generated with the method.
12. POLLUTION PREVENTION
12.1 Solution samples should be used, collected and disposed of in accordance
with all Federal, state and local regulations.
13. WASTE MANAGEMENT
13.1 In case of spill or release: Dilute with water. Dispose of in accordance
with all Federal, state, and local regulations.
14. REFERENCES
1. Book of ASTMStandards. 1976. Part 31. "Water." Standard D1889-71.
2. Methods for the Examination ofWater and Wastewater. 1975. Fourteenth
Edition. Method 214A.
3. Standard, Ref. No. ISO 7027-1984 (E).
4. Methods for the Examination of Water and Wastewater. Sixteenth Edition.
Method 214A.
5. "Standard Methods for the Certification of Laboratories Analyzing Drinking
Water: Criteria and Procedures, Quality Assurance." 1990. EPA/570/9-90/008.
EPA Guidance Manual D-6 April 1999
Turbidity Provisions -&U.S. GOVERNMENT PRINTING OFFICE:1999-720-077/93733
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