& EPA
United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park, NC 27711
EPA-453/R-93-022
August 1993
Chromium Emissions from
Industrial Process Cooling
Towers - Background Information
for Proposed Standards
NESHAP
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CHROMIUM EMISSIONS FROM INDUSTRIAL PROCESS COOLING TOWERS-
BACKGROUND INFORMATION FOR PROPOSED STANDARDS .
August 1993
U. S. Environmental Protection Agency
Office of Air and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
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TABLE OF CONTENTS
LIST OF FIGURES vn
LIST OF TABLES viii
CHAPTER 1.0 SUMMARY 1-1
1.1 CONTROL OPTIONS 1-1
1.2 ENVIRONMENTAL IMPACT .......... 1-3
1.3 ECONOMIC IMPACT ......... 1-3
CHAPTER 2.0 .INTRODUCTION 2-1
2.1 BACKGROUND AND AUTHORITY FOR
STANDARDS 2-1
2.2 SELECTION OF POLLUTANTS AND
SOURCE CATEGORIES 2-5
2.3 PROCEDURE FOR DEVELOPMENT OF
•NESHAP 2-6
2.4 CONSIDERATION OF COSTS ........... 2-9
2.5 CONSIDERATION OF ENVIRONMENTAL
IMPACTS . . 2-10
2.6 RESIDUAL RISK STANDARDS . 2-11
.CHAPTER 3.0 INDUSTRIAL PROCESS COOLING TOWERS 3-1
3 .1 GENERAL 3-1
3.2 DEFINITION OF SOURCE CATEGORY .•. . . . 3-1
3.3 INDUSTRIAL COOLING SYSTEM COMPONENTS . . 3-2
3.3.1 Cooling Tower 3-2
3.3.2 Heat Exchangers and Cooling
Water Cycle 3-6
3.4 CHEMICAL TREATMENT PROGRAMS . 3-8
3.4.1 Purpose ' 3-8
3.4.2 Corrosion 3-10
3.4.3 Scaling and Fouling 3-18
3.4.4 Microbiological Control 3-19
3.4.5 Cooling System Control 3-20
3.5 MAINTENANCE REQUIREMENTS 3-22
3.6 COMPOSITION AND FORMATION OF DRIFT . . . 3-23
3.7 DRIFT EMISSION RATE 3-25
3.8 REFERENCES FOR CHAPTER 3 ........ 3-27
CHAPTER 4.0 EMISSION CONTROL TECHNIQUES 4-1
4.1 CONTROL TECHNIQUES 4-1
4.1.1 Nonchromate Treatment Programs . . 4-1
4.1.2 Reduction of Drift 4-15
4.2 REFERENCES FOR CHAPTER 4 4-20
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TABLE OF CONTENTS (continued)
CHAPTER 5.0 MODEL COOLING TOWERS 5_1
5.1 MODEL IPCT SYSTEMS 5-1
5.1.1 Model Cooling Towers 5-2
5.2 HEXAVALENT CHROMIUM EMISSION FACTOR ... 5-8
5.2.1 Baseline Emission Factor 5-12
5.2.2 Controlled Emission Factor .... 5-13
5.2.3 Modeled Hexavalent Chromium
Emissions ..... 5-15
5.3 DISTRIBUTION OF MODEL TOWERS AND
BASELINE EMISSIONS BY INDUSTRY .... 5-16
5.3.1. Petroleum Refineries . 5-16
5.3.2 Chemical Manufacturing Plants . . 5-18
5.3.3 Primary Metals Industry 5-20
5.3.4 Miscellaneous Industries 5-20
5.4 NATIONWIDE EMISSIONS SUMMARY . 5-26
5.5 REFERENCES FOR CHAPTER 5 5-32
CHAPTER 6.0 REGULATORY ALTERNATIVES 6-1
6.1 DEVELOPMENT OF REGULATORY ALTERNATIVES . 6-1
6.2 APPLICATION OF ALTERNATIVES TO MODEL
TOWERS 6-1
6.3 MACT FOR THE IPCT SOURCE CATEGORY . . . . 6-3
CHAPTER 7.0 ENVIRONMENTAL AND ENERGY IMPACTS . . 7-1
7.1 AIR POLLUTION IMPACT ..... 7-1
7.1.1 Existing Sources 7-1
7.1.2 New Sources •.... 7-6
7.2 WATER POLLUTION IMPACT 7-9
7.2.1 Chromium Discharges 7-9
7.2.2 Phosphorus Discharges ...... 7-9
7.2.3 Zinc Discharges 7-14
7.2.4 Molybdate Discharges 7-1.4
7.2.5 New Sources . . 7-14
7.3 SOLID WASTE DISPOSAL ..... 7-14
7.4 ENERGY IMPACT :....' 7-17
7.5 STATE REGULATIONS ! ! ! 7-18
7.5.1 Air Emissions 7-18
7.5.2 Water Discharges 7-21
7.6 POLLUTION PREVENTION 7-22
7.7 REFERENCES FOR CHAPTER 7 '. 7-25
CHAPTER 8.0 COST ANALYSIS OF CONTROL OPTIONS . 8-1
8.1 INTRODUCTION 8-1
8.2 ANNUALIZED COST OF CONTROL TECHNIQUES . . 8-1
IV
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TABLE OF CONTENTS (continued)
8.2.1 High-Efficiency Drift Eliminator
Retrofits
8.2.2 Nonchromate Water Treatment
Programs
Summary of Costs and Cost
Effectiveness of Regulatory
Alternatives
8.2.3
8.3 REFERENCES FOR CHAPTER 8
CHAPTER 9.0 ECONOMIC IMPACTS
9.1 INDUSTRY PROFILES
9.2
9.1.1
1.2
.1.3
9.1.4
1.5
1.6
.1.7
9.1.8
The Supply and Demand of Chrornate
Corrosion Inhibitors .
Chemical Manufacturing ,
Petroleum Refining
Primary Metals . . .
Tobacco Products Industry ...
Textile Finishing
Tire and Rubber Products ....
Glass Products ....
ECONOMIC IMPACT ANALYSIS .
9.2.1 Methodology .
9.2.2 Percentage Price Increases . . .
9.2.3 Percentage Reductions in the.
Quantity Demanded
9.2.4 Further Analysis of the Chemical
Manufacturing Industry . . . . .
9.3 SMALL BUSINESS IMPACTS
9.4 REFERENCES FOR CHAPTER 9
Page
8-1
8-4
8-7
8-18
9-1
9-2
9-2
9-4
9-13
9-20
9-26
9-33
9-39
9-44
9-49
9-49
9-53
9-55
9-59
9-62
9-64
APPENDIX A.
APPENDIX B.
EVOLUTION OF THE BACKGROUND INFORMATION
DOCUMENT
INDEX TO ENVIRONMENTAL IMPACT
CONSIDERATIONS
APPENDIX C. SUMMARY OF TEST DATA
C.I DESCRIPTION OF TESTS
C.I.I Department of Energy, Gaseous
Diffusion Plant, Paducah,
Kentucky
A-l
B-l
C-l
C-l
C-l
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TABLE OF CONTENTS (continued)
C.I.2 National Institute of Standards
and Technology,
Gaithersburg, Maryland C-5
C.I.3 Exxon Refinery, Ethylene
Production, Baytown, Texas .... C-7
C.I.4 Exxon Refinery, Lube Oil
Production, Baytown, Texas .... C-ll
C.I.5 A Southeastern Manufacturing
Facility C-14
C.I.6 National Institute of Standards
and Technology, Gaithersburg,
Maryland C-19
C.I.7 Allied Fibers, Moncure, North
Carolina C-20
C.2 SUMMARY OF TEST DATA C-22
C.3 ANALYSIS OF TEST METHOD C-23
C.3.1 Procedure for Screening Elements . C-24
C.3.2 Precision of Test Method ..... C-25
c.4 "REFERENCES FOR APPENDIX c c-88
APPENDIX D. EMISSION MEASUREMENT OF COOLING
TOWERS D-l
D.I INTRODUCTION • D-l
D.2 EMISSION MEASUREMENT METHODS D-3
D.2.1 Scope of Test Programs ...... D-3
D.2.1.1 Facility Selection '..... D-3
D.2.1.2 Types of Samples and Data
Collected D-5
D.2.1.3 Emission and Process
Sampling Locations .... D-7
D.2.2 Selection of Sampling and
Analytical Methods D-8
D.2.2.1 Total Chromium and Hexavalent
Chromium Emissions .... D-8
D.2.2.2 Minerals D-9
D.2.2.3 Particle Size D-13
D.2.2.4 Ambient Air D-14
D.3 COOLING TOWER OPERATIONS AND
MONITORING . D-15
D.4 .QUALITY ASSURANCE/QUALITY CONTROL
FOR TEST PROGRAMS D-15
D.4.1 QA/QC for Previous EPA Cooling
Tower Tests D-16
D.4.l.l Title Page and Table
of Contents D-17
VI
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TABLE OF CONTENTS (continued)
D.4.
D.4.
D.4.
D.4.
D.4.
D.4.
D.4.
D.4.
D.4.
D.4.
D.4.
D.4.
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
1.13
D.4.1.
D.4.1.
Project Description ... D-17
Project Organization . . D-17
Quality Assurance
Objectives D-17
Sampling Procedures . . . D-17
Sample Custody D-18
Calibration Procedures . D-18
Analytical Procedures . . D-18
Data Reduction, Validation
and Reporting D-18
Internal QC Checks ... D-19
11 Performance and Systems
Audits D-19
Preventive Maintenance . D-22
13 Assessment of Data
Reduction, Accuracy and
Completeness ...... D-22
14 Corrective Action .... D-22
15 QA Reports to Management D-22
D.4.2 Recommended QA/QC for Performance
Testing D-23
VI1
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LIST OF FIGURES
Page
Figure 3-1. Internals of crossflow and counterflow
cooling towers 3-4
Figure 3-2. Details of splash fill and film fill 3-5
Figure 3-3. Operating principles of various drift
eliminators '3-7
Figure 3-4. Corrosion mechanism on carbon steel
surface 3-11
Figure C-l. Tower C-637-2A of Department of Energy
Gaseous Diffusion Plant . , C-27
Figure C-2. Cooling tower at NIST facility in
Gaithersburg, Maryland C-28
Figure C-3. Tower 68 at Exxon-Baytown refinery C-29
Figure C-4. Tower 84 at Exxon-Baytown refinery C-30
'Figure C-5. Cooling tower No. 22-900 at a
Southeastern manufacturing facility C-31
Figure C-6. Cooling tower No. 22-901 at a
Southeastern manufacturing facility ... . . . C-32
Figure C-7. Tower TW-3 at Allied Fibers . . . C-33
Figure D-l. Schematic of Marley comfort cooling tower
with Munters D-15 high-efficiency drift
eliminators . . D-6
Figure D-2. Schematic of CTD emission test method
sampling train D-ll
Vlll
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LIST OF TABLES
Page
TABLE 1-1. SUMMARY OF CONTROL OPTIONS 1-2
TABLE 1-2 ASSESSMENT OF ENVIRONMENTAL AND ECONOMIC
* IMPACTS OF CONTROL OPTIONS ... 1-4
TABLE 1-3. NATIONWIDE CAPITAL AND ANNUALIZED COSTS OF
CONTROL OPTIONS FOR INDUSTRIAL PROCESS.
COOLING TOWERS ...... 1-5
TABLE 3-1. QUALITATIVE CLASSIFICATION OF CORROSION
RATES 3-14
TABLE 3-2. ANODIC AND CATHODIC CORROSION INHIBITOR . . 3-15
TABLE 3-3. CHROMATE BASED CORROSION INHIBITORS .... 3-17
TABLE 4 -1. RANGES OF WATER QUALITY PARAMETERS FOR
REPRESENTATIVE NONCHROMATE COOLING
WATER TREATMENT PROGRAMS 4-3
.TABLE 4-2. HIGH PROCESS TEMPERATURE CHEMICALS 4-11
TABLE 4-3. SUMMARY OF EPA-SPONSORED AND EPA-APPROVED
EMISSION TESTS' 4-19
TABLE 5-1. MODEL TOWER PARAMETERS ..... 5-3
TABLE 5-2. RECIRCULATION RATES FOR MODEL TOWERS AS
DEVELOPED FROM INFORMATION REQUESTS .... 5-6
TABLE 5-3. EMISSION FACTORS FROM EPA- AND
INDUSTRY-SPONSORED TESTS 5-10
6
TABLE 5-4. SOURCE CATEGORY STANDARD INDUSTRY
CLASSIFICATION (SIC) 5-17
TABLE 5-5. CHROMIUM EMISSION ESTIMATES FOR
PETROLEUM REFINERIES ............ 5-19
TABLE 5-6. CHROMIUM EMISSION ESTIMATES FOR CHEMICAL
MANUFACTURING 5-21
TABLE 5-7. CHROMIUM EMISSION ESTIMATES FOR PRIMARY
METALS INDUSTRY 5-22
TABLE 5-8. CHROMIUM EMISSION ESTIMATES FOR TOBACCO
INDUSTRY . 5-24
IX
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LIST OF TABLES (continued)
TABLE 5-9.
TABLE 5-10.
TABLE 5-11.
TABLE 5-12.
TABLE 5-13.
TABLE 6-1.
TABLE 6-2.
TABLE 7-1.
TABLE 7-2.
TABLE 7-3.
TABLE 7-4.
t
TABLE 7-5.
TABLE 7-6.
TABLE 7-7.
TABLE 7-8.
TABLE 7-9.
CHROMIUM EMISSION ESTIMATES FOR TIRE AND
RUBBER INDUSTRY
CHROMIUM EMISSION ESTIMATES FOR TEXTILES
FINISHING INDUSTRY .
CHROMIUM EMISSION ESTIMATES FOR GLASS
PRODUCTS INDUSTRY
NATIONWIDE HEXAVALENT CHROMIUM EMISSIONS
SUMMARY
REFERENCES FOR RESPONSES TO SECTION 114
INFORMATION REQUESTS
EMISSION ESTIMATES FOR ALL REGULATORY
ALTERNATIVES
DISTRIBUTION OF REGULATORY ALTERNATIVES
AMONG EXISTING IPCT'S
BASELINE EMISSION RATE ESTIMATES FOR
MODEL TOWERS
HEXAVALENT CHROMIUM, PHOSPHORUS, AND
PARTICULATE MATTER (PM) EMISSION ESTIMATES
(1991) FOR EACH REGULATORY ALTERNATIVE . .
CHROMATE-BASED CORROSION INHIBITORS .'. .
EMISSION ESTIMATES FOR 1998 (FIFTH YEAR
OF STANDARD)
ESTIMATED WATER DISCHARGES (1991) OF
HEXAVALENT CHROMIUM FOR ALL REGULATORY
ALTERNATIVES
ESTIMATED PHOSPHORUS DISCHARGES (1991) FOR
EACH REGULATORY ALTERNATIVE
SELECTED PHOSPHORUS DISCHARGE SOURCES
ESTIMATED WATER DISCHARGES OF HEXAVALENT
CHROMIUM FOR ALL REGULATORY ALTERNATIVES
IN 1998 (FIFTH YEAR OF STANDARD) ....
ESTIMATED PHOSPHORUS DISCHARGES FOR EACH
REGULATORY ALTERNATIVE IN 1998 (FIFTH
YEAR OF STANDARD)
5-25
5-27
5-28
5-29
5-30
6-2
6-5
7-2
7-3
7-5
7-8
7-10
7-11
-7-13
7-15
7-16
x
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LIST OF TABLES (continued)
TABLE 7-10
TABLE 7-11.
TABLE 7-12
TABLE 8-1.
TABLE 8-2.
TABLE 8-3.
TABLE 8-4.
'TABLE 8-5.
TABLE 8-6.
TABLE 8-7.
TABLE 8-8.
TABLE 8-9.
TABLE 8-10
TABLE 9-1
TABLE 9-2
TABLE 9-3
ESTIMATED ANNUAL REDUCTIONS IN ENERGY
CONSUMPTION FOR REGULATORY ALTERNATIVE II
(HEDE RETROFIT) ,
ESTIMATED ANNUAL INCREASE IN ENERGY
CONSUMPTION FOR REGULATORY ALTERNATIVE III
(NONCHROMATE)
REPRESENTATIVE STATE WATER QUALITY
CRITERIA
HEDE RETROFIT COSTS FOR MODEL COOLING
TOWERS . . . .
NONCHROMATE COSTS FOR MODEL COOLING
TOWERS .........
CONTROL COST ESTIMATE FOR PETROLEUM .
REFINERIES ,
CONTROL COST ESTIMATE FOR CHEMICAL
MANUFACTURERS
CONTROL COST ESTIMATE FOR PRIMARY METALS
INDUSTRY . .
CONTROL COST ESTIMATE FOR TOBACCO
INDUSTRY :
CONTROL COST ESTIMATE FOR TIRE AND
RUBBER INDUSTRY
CONTROL COST ESTIMATE FOR TEXTILES
FINISHING INDUSTRY
CONTROL COST ESTIMATE FOR GLASS PRODUCTS
INDUSTRY .........
SUMMARY OF COSTS AND COST EFFECTIVENESS
OF IMPROVED CONTROL
Pac
7-19
7-20
7-23
8-8
8-9
8-10
8-11
8-12
8-13
8-14
8-15
8-16
8-17
VALUE OF SHIPMENTS FOR CHEMICAL MANUFACTURING
INDUSTRIES, 1980-1989 .... 9-7
U.S. PRODUCTION OF KEY CHEMICALS,
1986-1989 ' . . . . 9-10
PRODUCTION INDEXES FOR THE U.S. CHEMICAL
INDUSTRY, 1980-1990 9-11
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LIST OF TABLES (continued)
Page
TABLE 9-4 VALUE OF SHIPMENTS FOR THE PETROLEUM
TABLE
TABLE
TABLE
9
9
9
-5
-6
-7
AVERAGE U.S. PRODUCTION OF PETROLEUM
PRODUCTS, 1981-1990
AVERAGE APPARENT CONSUMPTION IN THE U.S.
OF PETROLEUM PRODUCTS, 1981-1990
VALUE OF SHIPMENTS FOR THE PRIMARY METALS
INDUSTRIES, 1980-1989
9
9
9
-17
-18
-21
TABLE 9-8
TABLE 9-9
TABLE 9-10
TABLE 9-11
TABLE 9-12
TABLE 9-13
TABLE 9-14
TABLE 9-15
TABLE 9-16
TABLE 9-17
TABLE 9-18
TABLE 9-19
U.S. MINE PRODUCTION OF PRIMARY METALS,
(FERROUS AND NONFERROUS) 1986-1990 . .
U.S. APPARENT CONSUMPTION OF PRIMARY
METALS, 1986-1990
VALUE. OF SHIPMENTS FOR TOBACCO PRODUCTS,
1980-1989 ......
U.S. PRODUCTION OF TOBACCO PRODUCTS,
1980-1988
U.S. PER-CAPITA CONSUMPTION OF TOBACCO
PRODUCTS, 1981-1989
VALUE OF SHIPMENTS FOR THE TEXTILE
FINISHING INDUSTRIES, 1980-1989
TEXTILE MILL PRODUCTION INDEX,
1980-1989
VALUE OF SHIPMENTS FOR THE TIRE AND INNER
TUBE INDUSTRY, 1980-1989
U.S. PRODUCTION OF CAR, TRUCK, AND BUS
TIRE, 1980-1989
VALUE OF SHIPMENTS FOR SIC 3211 AND
SIC 3221
U.S. PRODUCTION OF FLAT GLASS AND
GLASS- CONTAINERS, 1986-1990 . . .
ANNUAL CONTROL COSTS (1991 DOLLARS)
FOR THE REGULATED INDUSTRIES . . .
9-24
9-25
9-28
9-30
9'-32
9-35
9-37
9-41
9-43
9-46
9-47
9-54
XI1
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LIST OF TABLES (continued)
Page
TABLE'9-20 REVENUE FIGURES FOR PRICE INCREASE
SCENARIOS I AND II 9-56
TABLE 9-21 PERCENTAGE PRICE INCREASES FOR
SCENARIO I 9-57
TABLE 9-22 PERCENTAGE PRICE INCREASES FOR
SCENARIO II • 9-58
TABLE 9-23 PERCENTAGE REDUCTIONS IN QUANTITY DEMANDED
FOR SCENARIO I 9-60
TABLE 9-24 PERCENT REDUCTIONS IN QUANTITY DEMANDED
FOR SCENARIO II 9-61
TABLE A-l. EVOLUTION OF THE BACKGROUND INFORMATION
DOCUMENT A- 4
TABLE B-l CROSS-INDEXED REFERENCE SYSTEM TO
HIGHLIGHT ENVIRONMENTAL IMPACT PORTIONS
OF THE DOCUMENT B-2
TABLE C-l. SUMMARY OF OPERATING PARAMETERS AND
METEOROLOGICAL DATA DURING TESTS AT
DEPARTMENT OF ENERGY, GASEOUS DIFFUSION- '
PLANT, PADUCAH, KENTUCKY '. . . C-34
TABLE C-2. SUMMARY OF OPERATING PARAMETERS AND
METEOROLOGICAL DATA DURING TESTS AT NATIONAL
INSTITUTE OF STANDARDS AND TECHNOLOGY,
GAITHERSBURG, MARYLAND (FIRST TEST) .... C-36
TABLE C-3'. SUMMARY OF OPERATING PARAMETERS AND
METEOROLOGICAL DATA DURING TESTING OF
TOWER 68 AT EXXON REFINERY, BAYTOWN,
TEXAS C-37
TABLE C-4. SUMMARY OF OPERATING PARAMETERS AND
METEOROLOGICAL DATA DURING TESTING OF
TOWER 84 AT EXXON'S REFINERY, BAYTOWN,.
TEXAS C-39
TABLE C-5.
TABLE C-6.
SUMMARY OF OPERATING PARAMETERS AND
METEOROLOGICAL DATA DURING TESTS AT A
SOUTHEASTERN MANUFACTURING FACILITY .
OBSERVATIONS DURING EMISSION TESTS AT
A SOUTHEASTERN MANUFACTURING FACILITY,
JULY 13, 1987, THROUGH JULY 17, 1987 .
C-41
C-43
Xlll
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LIST OF TABLES (continued)
Page
TABLE C-7. SUMMARY OF OPERATING PARAMETERS AND
METEOROLOGICAL DATA DURING TESTS AT
NATIONAL INSTITUTE FOR STANDARDS AND
TESTING, GAITHERSBURG, MARYLAND
(SECOND TEST) c_47
TABLE C-8. SUMMARY OF OPERATING PARAMETERS AND
METEOROLOGICAL DATA DURING TESTS AT
ALLIED FIBERS, MONCURE, NORTH CAROLINA ... C-48
TABLE C-9. SUMMARY OF EMISSION TEST RESULTS--
DEPARTMENT OF ENERGY GASEOUS DIFFUSION
. PLANT, PADUCAH, KENTUCKY C-49
TABLE C-10. SUMMARY OF EMISSION TEST RESULTS--NATIONAL
INSTITUTE OF STANDARDS AND TECHNOLOGY,
GAITHERSBURG, MARYLAND . C-51
TABLE C-ll. SUMMARY OF EMISSION TEST RESULTS FOR
TOWER 68 AT EXXON REFINERY, BAYTOWN,
TEXAS c_53
TABLE C-12. SUMMARY OF EMISSION TEST RESULTS FOR
TOWER 84 AT EXXON REFINERY, BAYTOWN,
TEXAS c_55
TABLE C-13. SUMMARY OF EMISSION TEST RESULTS--
SOUTHEASTERN MANUFACTURING FACILITY ... . '. C-57
TABLE C-14. SUMMARY OF SENSITIVE PAPER DRIFT
MEASUREMENTS C-68
TABLE C-15. SUMMARY OF EMISSION TEST RESULTS--NATIONAL
INSTITUTE OF STANDARDS AND TECHNOLOGY'S
STEAM AND WATER CHILL PLANT, GAITHERSBURG,
MARYLAND (SECOND TEST) , c-69
TABLE C-16. SUMMARY OF EMISSION TEST RESULTS--
ALLIED FIBERS, MONCURE, NORTH CAROLINA ... C-81
TABLE D-l. SUMMARY OF EPA-SPONSORED COOLING TOWER
TEST PROGRAMS . D_4
TABLE D-2 SUMMARY OF ACCEPTANCE CRITERIA, CONTROL
LIMITS, AND CORRECTIVE.ACTIONS ....... D-20
xiv
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1.0 SUMMARY
National emission standards for hazardous air pollutants
(NESHAP) are established under Section 112 of the Clean Air Act
(CAA) (P.L. 101-459), as amended in 1990. Section 112(b) of the
CAA contains a list of hazardous air pollutants (HAP's), which
are the specific air toxics to be regulated by NESHAP. Emission
standards established under Section 112 apply to both new and
existing sources and are to achieve the maximum degree of
reduction in HAP emissions achievable, taking into consideration'
the cost of achieving such emission reductions, any non-air
.quality health and environmental impacts, and energy
requirements. These standards have been termed the maximum
achievable control technology (MACT) standards.
Chromium'compounds are included on the list of HAP's
established in Section 112(b) of the CAA. Section 112(c)
requires the Administrator to use this pollutant list to develop
and publish a list of source categories for which NESHAP are to
be developed. The source category list was published on July 16,
1992. Industrial process cooling towers (IPCT's), which emit
chromium compounds, are included on the source category list.
This background information document (BID) supports proposed
standards for chromium emissions from IPCT's.
1.1 CONTROL OPTIONS
Table 1-1 is a summary of the control options for chromium
emissions from IPCT's. ' Three control options have been .
identified. Option I is the "no action," or baseline, option;
Option II involves the use of high-efficiency drift eliminators
(HEDE's) on IPCT's; and Option III involves the use of
nonchromate-based water treatment programs and represents MACT
l-l
-------
TABLE 1-1. SUMMARY OF CONTROL OPTIONS
Control option
Option I (no action)
Option II
Option III
Control technique ''••• :; •:}- ' •" ""r
Existing (baseline) level of control
-nonchromate- based water treatment
programs used in 90 percent of IPCT's
-high-efficiency drift eliminators
used on 5 percent of IPCT's
-IPCT's on chromate use 13 parts per
million chromate in cooling water
High- efficiency drift eliminators that
reduce uncontrolled emissions by
67 percent
Nonchromate -based water treatment
programs that eliminate all chromium
emissions
1-2
-------
for both new-and existing IPCT's. These options form the basis of
the proposed regulatory alternatives for controlling chromium
emissions from IPCT's.
1.2 ENVIRONMENTAL IMPACT
Table 1-2 summarizes the environmental and economic impacts
associated with the control options for IPCT's. Control Option I
represents no.change to existing conditions. Control Option II
(HEDE retrofits) would reduce nationwide chromium emissions from
the baseline level of 22.9 megagrams per year (Mg/yr) (25.3 tons
per year [tons/yr]) to 7.8 Mg/yr (8.6 tons/yr). Control
Option III (nonchromates) would completely eliminate chromium
emissions from IPCT's.
As shown in Table 1-2, the reduction in nationwide chromium
emissions associated with either of the control options would
result in minimal adverse environmental impacts. .Control
Option III would have a negligible negative water impact;
chromium discharges would decrease, but phosphate discharges
would increase under this control option. Control Option III
also would have a negligible positive impact on solid waste
resulting from the elimination of chromium from the waste stream.
A negligible positive energy impact would be attributable to
Control Option II due to the lower energy demands to operate
cooling towers equipped with HEDE's. Under Control Option III,
energy demands would increase slightly, resulting in a negligible
negative energy impact.
1.3 ECONOMIC IMPACT
An overview of the economic impacts of the options for
controlling chromium emissions from IPCT's is presented in
Table 1-2. Table 1-3 summarizes the nationwide capital and
annualized costs associated with each of the control options.
Analyses of the costs and economic impacts are presented in
Chapters 8 and 9.
1-3
-------
OPTIONS
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1-4
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TABLE 1-3.'- NATIONWIDE CAPITAL AND ANNUALIZED COSTS OF CONTROL
OPTIONS FOR INDUSTRIAL PROCESS COOLING TOWERSa
Control option
Control Option I
Baseline
Control Option II
High-efficiency drift
eliminators
Control Option III
Nonchromat e - based
water treatment
programs
Capital costs,
$ million
0
39.3
6.3
Net annual i zed : :
cos ts •, ;" ;?-•••: ';':.. j::.,'.'/v
$ million/yr "
0
11.3
14.0
aCosts above baseline in 1991 dollars.
1-5
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2. INTRODUCTION
2.1 BACKGROUND AND AUTHORITY FOR STANDARDS
According to industry estimates, more than 2.4 billion
pounds of toxic pollutants were emitted to the atmosphere in 1988
(Implementation Strategy for the Clean Air Act Amendments of
1990. EPA Office of Air and Radiation, January 15, 1991). These
emissions may result in a variety of adverse health effects,
including cancer, reproductive effects, birth defects, and
respiratory illnesses. Title III of the Clean Air Act Amendments
of 1990 provides the tools for controlling emissions of these
pollutants. Emissions from both large and small facilities that
contribute to air toxics problems in urban and other areas will
be regulated. - The primary consideration in establishing national
industry standards must be demonstrated technology. Before
national emission standards for hazardous air pollutants (NESHAP)
are proposed as Federal regulations, air pollution prevention and
control methods are examined in detail with respect to their
feasibility, environmental impacts, and costs. Various control
options based on different technologies and degrees of efficiency
are examined, and a determination is made regarding whether the
various control options apply to each emissions source or if
dissimilarities exist between the sources. In most cases,
regulatory alternatives are subsequently developed that are then
studied by EPA as a prospective basis for a standard. The
alternatives are investigated in terms of their impacts on the
environment, the economics and well-being of the industry, the
national economy, and energy and other impacts. This document
summarizes the information obtained through these studies so that
2-1
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interested persons will be able to evaluate the information
considered by EPA in developing the proposed standards.
National emission standards for hazardous air pollutants for
new and existing sources are established under Section '112 of the
Clean Air Act as amended in 1990 [42 U.S.C. 7401 et seg., as
amended by PL 101-549, November 15, 1990], hereafter referred to
as the Act. Section'112 directs the EPA Administrator to
promulgate standards that "require the maximum degree of
reduction in emissions of the hazardous air pollutants subject to
this section (including a prohibition of such emissions, where
achievable) that the Administrator, taking into consideration the
cost of achieving such emission reductions, and any non-air
quality health and environmental impacts and energy requirements,
determines is achievable ... ." The Act allows the Administrator
to set standards that "distinguish among classes,.types, and
sizes of sources within a category or subcategory."
• The Act differentiates between major sources and area
sources. A major source is defined as "any stationary source or
group of stationary sources located within a contiguous area and
under common control that emits or has the potential to emit
considering controls, in the aggregate, 10 tons per year or more
of any hazardous air pollutant or 25 tons per year or more of any
combination of hazardous air pollutants." The Administrator,
however, may establish a lesser quantity cutoff to distinguish
between major and area sources. The level of the cutoff is based
on the potency, persistence, or other characteristics or factors
of the air pollutant. An area source is defined as "any
stationary source of hazardous air pollutants that is.not a major
source." For new sources, the amendments state that the "maximum
degree of reduction in emissions that is deemed achievable for
new sources in a category or subcategory shall not be less
stringent than the emission control that is achieved in practice
by the best controlled similar source, as determined by the
Administrator." Emission standards for existing sources:
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may be "less stringent than the standards for new
sources in the same category or subcategory but shall
not be less stringent, and may be more stringent than--
(A) the average emission limitation achieved
by the best performing 12 percent of the
existing sources (for which the Administrator
has emissions information), excluding those
sources that have, within 18 months before
the emission standard is proposed or within
30 months before such standard is
promulgated, whichever is later, first
achieved a level of emission rate or emission
reduction which complies, or would comply if
the source is not subject to such standard,
• with the lowest achievable emission rate (as
defined by Section 171) applicable to the
source category and prevailing at the time,
in the category or subcategory for categories
and subcategories with 30 or more sources, or
(B) the average emission limitation achieved
by the best performing five sources (for
which the Administrator has or could
reasonably obtain emissions information) in
the category or subcategory for categories or
subcategories with fewer than 30 sources.
The Federal standards are also known as "MACT" standards and
are based on the maximum achievable control technology previously
discussed. The MACT standards apply to both major and area
sources, although the existing source standards may be less
stringent than the new source standards, within the constraints
presented above. The1MACT is considered to be the basis for the
standard, but the Administrator may promulgate more stringent
standards than the MACT floor, which have several advantages.
First, they may help achieve long-term cost savings by avoiding
the need for more expensive retrofitting to meet possible future
residual risk standards, which may be more stringent (discussed
in Section 2.7). Second, Congress was clearly interested in
providing incentives for improving technology. Finally, in the
Clean Air Act Amendments of 1990, Congress gave EPA a clear
mandate to reduce the health and environmental risk of air toxics
emissions as quickly as possible.
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For area sources, the Administrator may "elect to promulgate
standards or requirements applicable to sources in such
categories or subcategories which provide for the use of
generally available control technologies or management practices
by such sources to reduce emissions of hazardous air pollutants."
These area source standards are also known as "GACT" (generally
available control technology) standards, although MACT may be
applied at the Administrator's discretion, as discussed
previously.
The standards for hazardous air pollutants (HAP's), like the
new source performance standards (NSPS) for criteria pollutants
required by Section 111 of the Act (42 U.S.C. 7411), differ from
other regulatory programs required by the Act (such as the new
source review program and the prevention of significant
deterioration program) in that NESHAP and NSPS are national in
scope (versus site-specific). Congress intended for the NESHAP
and NSPS programs to provide a degree of uniformity to State
regulations to avoid situations where some States may attract
industries by relaxing standards relative to other States.
States are free under Section 116 of the Act to establish
standards more stringent than Section 111 or 112 standards.
Although NESHAP are normally structured in terms of
numerical emissions limits, alternative approaches are sometimes
necessary. In some cases, physically measuring emissions from a
source may be impossible or at least impracticable due to
technological and economic limitations. Section 112 (h) of the
Act allows the Administrator to promulgate a design, equipment,
work practice, or operational standard, or combination thereof,
in those cases where it is not feasible to prescribe or enforce
an emissions standard. For example, emissions of volatile
organic compounds (many of which may be HAP's, such as benzene)
from storage vessels for volatile organic liquids are greatest
during tank filling. The nature of the emissions (i.e., high
concentrations for short periods during filling and low
concentrations for longer periods during storage) and the
configuration of storage tanks make direct emission measurement
2 - 4. -'
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impractical."- Therefore, the MACT or GACT standards may be based
on equipment specifications.
Under Section 112 (h) (3), the Act also allows the use of
alternative equivalent technological systems: "If, after notice
and opportunity for comment, the owner or operator of any source
establishes to the satisfaction of the Administrator that an
alternative means of emission limitation" will reduce emissions
of any air pollutant at least as much as would be achieved under
the design, equipment, work practice, or operational standard,
the Administrator shall permit the use of the alternative means.
Efforts to achieve early environmental benefits are
encouraged in Title III. For example, source owners and
operators are. encouraged to use the Section 112(i) (5) provisions,
which allow a 6-year compliance extension of the MACT standard in
exchange for the implementation of an early emission reduction
program. The owner or operator of an existing source must
demonstrate a 90 percent emission reduction of HAP's (or
95 percent if the HAP's are particulates) and meet an alternative
emission limitation, established by permit, in lieu of the
otherwise applicable MACT standard. This alternative limitation
must reflect the 90 (95) percent reduction and is in effect for a
period of 6 years from the compliance date for the otherwise
applicable standard. The 90 (95) percent early emission
reduction must be achieved before .the otherwise applicable
standard is first proposed, although the reduction may be
achieved after the standard's proposal (but before January 1,
1994) if the source owner or operator makes an enforceable
commitment before the proposal of the standard to achieve the •
reduction. The source must meet several criteria to qualify for
the early reduction standard, and Section 112 (i) (5) (A) provides
that the State may require additional reductions.
2.2 SELECTION OF POLLUTANTS AND SOURCE CATEGORIES
As'amended in 1990, the Act includes a list of 190 HAP's.
Petitions to add or delete pollutants from this list may be
submitted to EPA. Using this list of pollutants, EPA will
publish a list of source categories (major and area sources) for
2-5
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which emission standards will be developed. Within 2 years of
enactment (November 1992), EPA will publish a schedule
establishing dates for promulgating these standards. Petitions
may also be submitted to EPA to remove source categories from the
list. The schedule for standards for source categories will be
determined according to the following criteria:
(A) The known or anticipated adverse effects of such
pollutants on public health and the environment;
(B) The quantity and location of emissions or
reasonably anticipated emissions of HAP's that each
category or subcategory will emit; and
(C) The efficiency of grouping categories or
subcategories according to the pollutants emitted or
the processes or technologies used.
After the source category has been chosen, the types of
facilities within the source category to which the standard will
apply must be determined. A source category may have several
facilities that cause air pollution, and emissions from these
facilities may vary in magnitude and control cost. Economic
studies of the source category and applicable control technology
may show that air pollution control is better served by applying
standards to the more severe pollution sources. For this reason,
and because there is no adequately demonstrated system for
controlling emissions from certain facilities, standards often do
not apply to all facilities at a source. For the same reasons,
the standards may not apply to all air pollutants emitted. Thus,
although a source category may be selected to be covered by
standards, the standards may not cover all pollutants or
facilities within that source category.
2.3 PROCEDURE FOR DEVELOPMENT OF NESHAP
Standards for major and area sources 'must (1) realistically
reflect MACT or GACT; (2) adequately consider the cost, the
non-air quality health and environmental impacts, and the energy
requirements of such control; (3) apply to new and existing
sources; and (4) meet these conditions for all variations of
industry operating conditions anywhere in the country.
2-6
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The objective of the NESHAP program is to develop standards
to protect the public health by requiring facilities to control
emissions to the level achievable according to the MACT or GACT
guidelines. The standard-setting process involves three
principal phases of activity: (1) gathering information,
(2) analyzing the information, and (3) developing the standards.
During the information-gathering phase, industries are
questioned through telephone surveys, letters of inquiry, and
plant visits by EPA representatives. Information is also
gathered from other sources, such as a literature search. Based
on the information acquired about the industry, EPA selects
certain plants at which emissions tests are conducted to provide
reliable data that characterize the HAP's emissions from
' well-controlled existing facilities.
In the second phase of a project, the information about the
industry, the pollutants emitted, and the control options are'
used- in analytical studies. Hypothetical "model plants" are
defined to provide a common basis for analysis. The model plant
definitions, national pollutant emissions data, and existing
State regulations governing emissions from the source category
are then used to establish regulatory alternatives. These
regulatory alternatives may be different levels of emissions
control or different degrees of applicability or both.
The EPA conducts studies to determine the cost, economic,
environmental, and energy impacts of each regulatory alternative.
From several alternatives, EPA selects the single most plausible
regulatory alternative as the basis for the NESHAP for the source
category under study.
In the third phase of a project, the selected regulatory
alternative is translated into standards, which, in turn, are
written in the form of-a Federal regulation. The Federal
regulation limits emissions to the levels indicated in the
selected regulatory alternative.
As early as is practical in each standard-setting project,
EPA representatives discuss the possibilities of a standard and
the form it might take with members of the National Air Pollution.
2-7
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Control Techniques Advisory Committee, which is composed of
representatives from industry, environmental groups, and State
and local air pollution control agencies. Other interested
parties also participate in these meetings.
The information acquired in the project is summarized in the
background information document (BID). The BID, the proposed
standards, and a preamble explaining the standards are widely
circulated to the industry being considered for control,
environmental groups, other government agencies, and offices
within EPA. Through this extensive review process, the points of
view of expert reviewers are taken into consideration as changes
are made to the documentation.
A "proposal package" is assembled and sent through the
offices of EPA Assistant Administrators for concurrence before
the proposed standards are officially endorsed by .the EPA
Administrator. After being approved by the EPA Administrator,
the preamble and the proposed regulation are published in the
Federal Register.
The public is invited to participate in the standard-setting
process as part of the Federal Register announcement of the
proposed regulation. The EPA invites written comments on '.the
proposal and also holds a public hearing to discuss the proposed
standards with interested parties. All public comments are
summarized and incorporated into a second volume of the BID. All
information reviewed and generated in studies in support of the
standards is available to the public in a "docket" on file in
Washington, D.C. Comments from the public are evaluated, and the
standards may be altered in response to the comments.
The significant comments and EPA's position on the issues
raised are included in the preamble of a promulgation package,
which also contains the draft of the final regulation. The
regulation is then subjected to another round of internal EPA
review and refinement until it is approved by the EPA
Administrator. After the Administrator signs the regulation, it
is published as a "final rule" in the Federal Register.
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2.4 CONSIDERATION OF COSTS
The requirements and guidelines for the economic analysis .of
proposed NESHAP are prescribed by Presidential Executive Order
12291 (EO 12291} and the Regulatory Flexibility Act (RFA). The
EO 12291 requires preparation of a Regulatory Impact Analysis
(RIA) for all "major" economic impacts. An economic impact is
considered to be major if it satisfies any of the following
criteria:
1. An annual effect on the economy of $100 million or more;
2. A major increase in costs or prices for consumers;
individual industries; Federal, State, or local government
agencies; or geographic regions; or
3. Significant adverse effects on competition, employment,
investment, productivity, innovation, or on the ability of United
States-based enterprises to- compete with foreign-based
enterprises in domestic or export markets.
An RIA describes the potential benefits and costs of the •
proposed regulation and explores alternative regulatory and
nonregulatory approaches to achieving the desired objectives. If
the analysis identifies less costly alternatives, the RIA
includes an explanation of the legal reasons why the less-costly
alternatives could not be adopted. In addition to requiring an
analysis of the potential costs and benefits, EO 12291 specifies
that EPA, to the extent allowed by the CAA and court orders,
demonstrate that the benefits of the proposed standards outweigh
the costs and that the net benefits are maximized. .
The RFA requires Federal agencies to.give special
consideration to the impact of regulations on small businesses,
small organizations, and small governmental units. If the
proposed regulation is expected to have a significant impact on a
substantial number of small entities, a regulatory flexibility
analysis must be prepared. In preparing this analysis, EPA takes
into consideration such factors as the availability of capital
for small entities, possible closures among small entities, the
increase in production costs due to compliance, and a comparison
2-9
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of the relative compliance costs as a percent of sales for small
versus large entities.
The prime objective of the cost analysis is to identify the
incremental economic impacts associated with compliance with the
standards based on each regulatory alternative compared to
baseline. Other environmental regulatory costs may be factored
into the analysis wherever appropriate. Air pollutant emissions
may cause water pollution problems, and captured potential air
pollutants may pose a solid waste disposal problem. The'total
environmental impact of an emission source must, therefore, be
analyzed and the costs determined whenever possible.
A thorough study of the profitability and price-setting
mechanisms of the industry is essential to the analysis so that
an accurate estimate of potential adverse economic impacts can be
made for proposed standards. It is also essential to know the
capital requirements for pollution control systems already placed
on plants so that the additional capital requirements
necessitated by these Federal standards can be placed in proper
perspective. Finally, it is necessary to assess the availability
of capital to provide the additional control equipment needed to
meet the standards.
2.5 CONSIDERATION OF ENVIRONMENTAL IMPACTS
Section 102(2)(C) of the National Environmental Policy Act
(NEPA) of 1969 requires Federal agencies to prepare detailed
environmental impact statements on proposals for legislation and
other major Federal actions significantly affecting the quality
of the human environment. The objective of NEPA is to build into
the decision-making process of Federal agencies a careful
consideration of all environmental aspects of proposed actions.
In a number of legal challenges to standards for various
industries, the United States Court of Appeals for the District
of Columbia Circuit has-held that environmental impact statements
need not be prepared by EPA for proposed actions under the Clean
Air Act. Essentially, the Court of Appeals has determined that
the best system of emissions reduction requires the Administrator
to take into account counterproductive environmental effects of
2-10
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proposed standards as well as economic costs to the industry. On
this basis, therefore, the Courts established a narrow exemption
from NEPA for EPA determinations.
In addition to these judicial determinations, the Energy
Supply and Environmental Coordination Act of 1974 (PL-93-319)
specifically exempted proposed actions under the Clean Air Act
from NEPA requirements. According to Section 7(c)(1), "No action
taken under the Clean Air Act shall be deemed a major Federal
action significantly affecting the quality of the human
environment within the meaning of the National Environmental
Policy Act of 1969" : (15 U.S.C. 793(c) (1)) .
Nevertheless, EPA has concluded that preparing environmental
impact statements could have beneficial effects on certain
regulatory actions. Consequently, although not legally required
to do so by Section 102(2) (C) of NEPA, EPA has adopted a policy
requiring that environmental impact statements be prepared for
various regulatory actions, including NESHAP developed under
Section 112 of the Act. This voluntary preparation of
environmental impact statements, however, in no way legally
subjects the EPA to NEPA requirements.
To implement this policy, a separate section is included in
this document that is devoted solely to an analysis of the
potential environmental impacts associated with the proposed
standards. Both'adverse and beneficial impacts in such areas as
air and water pollution, increased solid, waste disposal, and
increased energy consumption are discussed.
2.6 RESIDUAL RISK STANDARDS
Section 112 of the Act provides 'that 8 years after MACT
standards are established (except for those standards established
2 years after enactment, which have 9 years), standards to
protect against the residual health and environmental risks
remaining must be promulgated, if necessary. The standards would
be triggered if more than one source in a category or subcategory
exceeds a maximum individual risk of cancer of I1 in 1 million.
These residual risk regulations would be based on the concept of
providing an "ample margin of safety to protect public health."
2-11
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The Administrator may also consider whether a more stringent
standard is necessary to prevent--considering costs, energy,
safety, and other relevant factors--an adverse environmental
effect. In the case of area sources controlled under GACT
standards, the Administrator is not required to conduct a
residual risk review.
2-12
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3.0 INDUSTRIAL PROCESS COOLING TOWERS
3.1 GENERAL
This section provides a description of the source category
and the major users of industrial process cooling towers
(IPCT's), the cooling tower system, the heat exchangers served by
cooling towers, cooling water chemical treatment programs, the
mechanism by which hexavalent chromium (Cr+6) is emitted, and the
emission control techniques.
3.2 DEFINITION OF SOURCE CATEGORY
All cooling towers that are used to remove heat from an
industrial process or chemical reaction are included in the IPCT
source category. Towers that are used to cool both industrial
processes and heating, ventilating, and air conditioning (HVAC)
and refrigeration systems also are included in the source -
category. Only towers devoted exclusively to cooling HVAC and
refrigeration systems are excluded from the source category.
Major users of IPCT's that also use chromium-based water
treatment chemicals are chemical manufacturing plants, petroleum
refineries, primary metals facilities, and several miscellaneous
manufacturing industries (textiles, tobacco products, tire and
rubber products, and glass products). Other major users of
IPCT's, which do not use chromium-based water treatment
chemicals, include the electrical utility, food processing, and
pulp and paper industries. The number of plants in each
industry, the total number of IPCT's in each industry, and the
number of IPCT's that are using chromates were estimated by
procedures presented in Chapter 5. From the information '
presented in detail in Chapter 5, it is estimated that IPCT's are
used at approximately 189 petroleum refineries, 1,824 chemical
3-1.
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manufacturing plants, 210 primary metals plants, and 599 plants
in the miscellaneous industries.1"3 In addition, the percentage
of cooling towers using chromium-based water treatment chemicals
in each industry is estimated as 20 percent at petroleum
refineries, 10 percent at chemical manufacturing plants,
10 percent at primary metals facilities, 7.5 percent at plants in
the tire and rubber industry, and 2.5 percent at plants in the
other miscellaneous industries.4"7 These estimates result in a
total of about 790 IPCT's using chromium-based water treatment
chemicals: 136 at petroleum refineries, 510 at chemical plants,
97 at primary metals plants, and 47 at miscellaneous plants.8"15
The nationwide baseline Cr+6 emissions from these towers are
estimated to be 22.9 megagrams per year (Mg/yr) (25.3 tons per
year [tons/yr]).
3.3 INDUSTRIAL COOLING SYSTEM COMPONENTS
3.3.1 Cooling Tower
Cooling towers are devices that cool warm water by
contacting it with ambient air that is drawn or forced through
the tower. Typically, about 80 percent of the cooling occurs
from evaporation of water as the air flowing through the tower
contacts water cascading from the top to the bottom of the tower.
The remaining 20 percent of the cooling is the result of sensible
heat transfer that raises the air temperature.16 Most .tower
systems are designed with recirculating water systems to conserve
water resources or reduce costs of purchasing water. The major
cooling tower components include the fan(s), fill material, water
distribution deck, or header, drift eliminator, structural frame,
and cold water basin. Other components that affect tower
operation include the pumps and pipes necessary to circulate the
cooling water through the cooling tower and heat exchanger loops.
Most IPCT's are designed with induced-draft airflow, but
many have forced-draft airflow, and some (especially in the
utilities industry) have natural-draft airflow. Induced draft is
provided by a propeller-type axial fan located in the stack at
the top of the tower. Forced-draft towers are usually smaller
than induced-draft towers and have either centrifugal fans
3-2
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located at the base of the tower, which is constructed as a
plenum to provide positive-pressure airflow through the fill
material, or axial fans located on the side of the tower.
Natural-draft airflow relies on air currents created by
temperature differences between the air in the tower and the
atmosphere. When the cooling demands are minimal and the air
temperature is low enough, water can be circulated through the
tower and cooled sufficiently without using the fans. In these
instances, a natural draft is created in the cooling tower. 'In
addition, the direction of the airflow through a mechanical draft
tower is either crossflow or counterfldw. Crossflow refers to
horizontal airflow through the fill, and counterflow refers to
upward vertical airflow. Schematics of counterflow and crossflow
cooling towers are presented in Figure 3-1.
Fill material is used to maintain an even distribution of
water across the horizontal plane of the tower and to create as
much water surface as practical to enhance evaporation and
sensible heat transfer. The fill material improves the water-to-
air interface by creating either a large number of water droplets
or many thin vertical sheets (or tubes) of water. Splash fill is
constructed as successive layers of staggered impact surfaces in
the form of bars. Small droplets are formed as warm water falls
through the fill and splashes off each layer. Splash fill
typically is constructed of wood, polyvinyl chloride (PVC),
polystyrene, polypropylene, or asbestos cement board. Film fill
is constructed of sheets of material in a "honeycomb"
configuration. The fill is oriented such that water enters the
open end of the honeycomb .and flows vertically in sheets along
the surface of the fill.material. Typically, film fill materials
are PVC, polystyrene, or polypropylene. Schematics of both
splash and film fill are presented in Figure 3-2..
The mechanism by which the warm water is distributed over
the fill material depends on the type of tower. In crossflow
towers, there is a water distribution deck above the fill
material at the top of the tower. The floor of this deck
contains gravity flow nozzles, and the water level in the deck
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AIR
OUTLET
WATER
INLET
FAN
WATER
INLET
WATER OUTLET
MECHANICAL DRAFT
CROSS-FLOW TOWER
AIR
OUTLET
WATER OUTLET
MECHANICAL DRAFT
COUNTER-FLOW TOWER
Figure 3-1. Internals of crossflow and counterflow
cooling towers.
(Reprinted from Drift Technology for Cooling Towers,
The Marley Company, 1973.)
3-4
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SPLASH FILL
FILM FILL
Figure 3-2. Details of splash fill and film fill.
(Reprinted from Custodis-Cottrell product brochure.)
3-5
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controls the-rate of water flow onto the fill. In counterflow
towers, the water distribution system is constructed of a series
of header pipes connected to pressure flow nozzles placed above
the fill material. In both systems, the nozzles are arranged to
provide even water distribution over the fill material.
Water droplets entrained in the air and the dissolved and
suspended solids contained in the droplets that are emitted from
cooling towers are referred to as drift. (Drift is. discussed in
Section 3.6.) Drift eliminators can be installed at the exit of
the fill sections to reduce the amount of drift in the exiting
airflow. The efficiency of a drift eliminator is a function of-
its design. Figure 3-3 presents schematics of the three major
drift eliminator designs: herringbone (blade-type), waveform,
and cellular (or honeycomb). In general (with a few exceptions),
herringbone units are the least efficient, cellular units are the
most efficient, and waveform units achieve an intermediate
efficiency. Drift eliminators are constructed of wood, PVC,
metal, asbestos-cement, polystyrene, or cellulose. The material
most often specified is PVC. Drift eliminators installed in
towers built in recent years are more likely to be high-
efficiency waveform or cellular units, but a large number of
older towers still have low-efficiency eliminators.
The structural frame of cooling towers can be wood,
concrete, masonry, steel-, and combinations of these materials.
The cold water basins (reservoirs) typically are located directly
below the fill material at the base of the cooling tower. Basin
size is affected by the size of the tower and by the necessity to
accommodate any short-term fluctuations in the water volume of
the system.
3.3.2 Heat Exchangers and Cooling Water Cycle
In a typical IPCT system, cooling water is pumped from the
cooling tower basin to the heat exchanger(s) served by the tower,
and the heated water flows back to the cooling tower water
distribution deck. The cooling water loop may include numerous
separate heat exchangers of various designs. Heat exchangers are
designed to transfer heat from one fluid to another. The
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Wood Latrh
Bfades
BLADE-TYPE
ELIMINATOR
Plemtle
WAVEFORM
ELIMINATOR
CELLULAR
ELIMINATOR
Figure 3-3. Operating principles of various drift eliminators
(Reprinted from Reference No. 17.)
3-7
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transfer can'- occur directly by mixing the hot and cold materials
or indirectly through a device separating the hot and cold
materials. Indirect heat exchanger types include shell-and-tube,
flat plate, and spiral designs. In most industries, heat
transfer is accomplished with shell-and-tube heat exchangers that
typically are constructed from carbon steel.16 However, metals
such as stainless steel,- copper, and admiralty brass are also
used. Graphite blocks, titanium, Hastelloy, and other very
corrosion-resistant metals are used in severe environments.
Although maximum process fluid temperatures may exceed
1000°C (1832°F), the bulk fluid process temperatures at the
inlets to the heat exchangers are much less than the process
fluid temperatures due to heat losses between the process reactor
and heat exchangers. In addition, because the volume of cooling
water is much greater than the volume of process fluid that is
cooled per unit time, and because of other losses in heat, the
temperature of the cooling water within the heat exchanger (bulk
water temperature) rarely exceeds 70°C (160°F) in most IPCT
systems. The temperature of the heat exchanger material (skin
temperature) on both the process side and the water side of the
exchanger is less than the process fluid temperature and greater
than the bulk water temperature.
The side of the heat exchanger providing the cooling is
referred to as the water side, and the side with the process
fluid is referred to as the process side. The water side of the
heat exchanger is of primary interest for the cooling tower
source category because of the effects of cooling water treatment
programs on operating and maintaining the heat exchangers. Water
velocities in heat exchangers vary but normally range between 0.9
to 2.4 meters per second (m/s) (3 to 8 feet per second tft/s]).18
3.4 CHEMICAL TREATMENT PROGRAMS
3.4.1 Purpose
Chemicals are added to the recirculating cooling water to
inhibit the corrosive effects of the water, to control the rate
of scaling and fouling, and to control the growth of
microorganisms in both the cooling tower and the heat exchangers.
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As evaporation occurs during cooling, the chemical constituents
of the water become concentrated. A percentage of the
recirculating water is intentionally discharged (blowdown) to
limit the concentrations of suspended and dissolved solids to
acceptable levels. Also, as water cascades through the tower,
some is entrained and emitted from the stack as drift. Fresh
water (makeup) is added to make up for the losses resulting from
evaporation, blowdown, and drift.
Typical water treatment program chemicals include (1) a -
corrosion inhibitor, (2) an antiscalant, (3) an antifoulant,
(4) a dispersant, (5) a surfactant, (6) a biocide, and (7) an
acid and/or caustic soda for pH control. Chromium-based
chemicals are corrosion inhibitors. The quality of the cooling
tower water supply directly affects the type and quantity of
chemicals required to maintain a satisfactory chemical treatment
program. The three problems--corrosion, scaling and fouling, and
microbiological growth--and the chemicals used to control them
are discussed later in this section.
Physical parameters that affect the selection of chemical
treatment programs include heat exchanger material, bulk water
and water-side skin termperatures, water velocity, and holding
time index.19'20 The holding time index is usually defined as
the half-life of the recirculating water, or the time required
for half of the recirculating water to be replaced.
Major water chemistry parameters that affect the selection
of chemical treatment programs include pH, calcium hardness
(calcium ion concentration), alkalinity (bicarbonate, carbonate,
and' hydroxide ions), chloride, sulfate, silica, iron, dissolved
solids (conductivity), and suspended solids.19'20 Water quality
also directly affects the number of cycles of concentration that
can be maintained. The number of cycles of concentration is •
-defined as the ratio of either the conductivity or calcium
hardness of the recirculating water to that of the makeup water.
The maximum allowable conductivity or calcium hardness is
established based on the chemical treatment program and the
acceptable rates of corrosion and scaling. In general, limits
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are set'so as not to exceed the mineral solubility and cause
scale formation. Generally, the higher the water hardness or
recirculating water pH, the fewer the cycles of concentration
allowable. Recently developed calcium dispersants allow more
cycles of concentration or higher pH in IPCT operations.
3.4.2 Corrosion
3.4.2.1 Description. Corrosion is the oxidation of a metal
by some oxidizing agent in the environment. The area over which
metal is oxidized (corroded) is called the anode; the area over
which the oxidizing agent is reduced is called the cathode. In
water, an electrochemical cell is formed in which cations migrate
toward the cathode, anions move toward the anode, and electrons
flow through the metal from anode to cathode. Water is the
conducting fluid or electrolyte, and the metal surface develops
anodic and cathodic areas as the result of differences in
electrochemical potential, temperature, and the concentration of
dissolved oxygen or solids. In equipment made from a single type
of metal, differences in electrochemical potential occur as the
result of impurities in the metal, localized stresses, grain size
or composition differences, and discontinuities on the surface.21
An electrochemical potential difference also is created when two
different metals are in contact both with each other and the same
solution. This electrochemical potential difference causes
galvanic corrosion in the active (or least noble) metal that
serves as the anode.21'22 For both single and multiple metal
systems, local differences in dissolved oxygen and solids
concentrations and in temperature also cause anodic and cathodic
areas in the metal to form. Anodic surfaces will exist in
regions of low oxygen concentration, low temperature, and high
salt concentrations.16'23 As the anode corrodes and releases
cations to solution, electrons are released that flow through the
metal to the cathode. The flow of electrons through the metal is
the corrosion current, which is limited by the rate at which
electrons are accepted by the oxidizing agent at the cathode.
Figure 3-4 illustrates the mechanisms for corrosion in a single
metal and presents the reactions at the anode and cathode.
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WATER(ELECTROLYTE)
ANODIC REACTIONS
Fe * Fe+++2e~
-*• Fe(OH)2 (soluble form)
Fe(OH)2+H2O+f02 -»• 2Fe(OH)3 (insoluble red-brown deposit)
CATHODIC REACTIONS
iO2+H2O+2e~ '* 2OH~ (at pH >4)
2e"+2H+ * H2gas (at pH <4)
l|H2+02gas -^ H20+OH~
Figure 3-4. Corrosion mechanism on carbon steel surface.
(Reprinted from Reference 21.)
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Two primary types of corrosion occur in cooling water
systems depending on the equipment design and the system
operation: general and localized corrosion.16'18'21'22 in
general corrosion (the less harmful type of corrosion), the metal
dissolves uniformly. General corrosion occurs when the local
concentration or temperature difference along the metal surface
causes differences in electrochemical potential and creates
corrosion cells. The movement of these cells from place to place
on the metal surface as concentrations and temperatures fluctuate
cause uniform corrosion along the surface of the metal.21
However, the corrosion rate is enhanced by high concentrations
and temperature. Fouling from corrosion products is likely to be
the major problem when general corrosion occurs.21'22
Localized corrosion, typically in the form of pitting,
occurs when the anodic sites remain stationary and corrosion
proceeds rapidly at localized points. Corrosion of this type, is
. a more serious problem than general corrosion in industrial
systems because it can cause metal to perforate in a very short
time.21'22 Pitting corrosion is the formation of cavities or
holes in the metal and can occur in crevices within a metal, at
joints between metal sections, and under deposits of suspended
^ solids and precipitating chemical species. Conditions that
promote these deposits are low-velocity water flow, high
concentrations of dissolved solids, and high pH.21 Pitting can
also occur when a dissolved cathodic (less active) metal plates
out in spots on an anodic (active) surface. This most serious
form of galvanic corrosion typically occurs in systems containing
both copper and steel components. The copper can plate out on
steel surfaces, causing galvanic corrosion of the steel. Once
the plating occurs, it is very difficult to inhibit the resulting
corrosion.21 Severe localized galvanic corrosion also can result
if the anodic area is small relative to the cathode; when the
electron flow is from a small anodic area to a large cathode
(which serves as an electron sink), the anode corrodes rapidly
and a pit develops. However, when the anodic surface area is
substantially larger than the cathodic surface, the corrosion is •
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general in nature because only a thin layer of metal will be
removed over time. Pitting corrosion can also occur when small
anodic areas are left unprotected by inhibitors, as discussed
below.
3.4.2.2 Corrosion Inhibitors. Inhibitors are added
primarly to protect the heat transfer surfaces from corrosion.
These surfaces are the most critical metal components in the
system. Corrosion can be .retarded, but not totally, prevented,
and the rate of corrosion that is acceptable varies among
systems. The terms and corrosion rates presented in Table 3-1
are generally used to describe the severity of.carbon steel and
copper corrosion.
Corrosion inhibitors used in a recirculating system limit
the rate of reaction at the anode, the cathode, or both. The
mechanisms by which these inhibitors protect the metal are
passivation, precipitation, and adsorption. Anodic and cathodic
inhibitors produce a barrier film, or deposit, on the anodic and
cathodic metal surfaces, respectively. The barrier created by
passivation is an oxide that forms on the metal surface, and the
barrier created by precipitation is an insoluble precipitate that
coats the metal surface. Typically, anodic inhibitors are
passivators, and cathodic inhibitors are precipitators.
Table 3-2 lists various anodic and cathodic inhibitors.
Molecules that have polar properties provide a barrier by
adsorbing on the entire metal surface. Adsorption inhibitors are
usually organic compounds.
18,21,22
The rate of corrosion at the anode is controlled by the rate
of the cathodic reaction (i.e., the same mass of the anode
corrodes for a given cathodic reaction rate). Thus, pitting may
occur if only small spots of the anodic surface remain
unprotected and the cathodic reaction is not controlled by
cathodic inhibitors and/or organic adsorption inhibitors.
Although chromate is an excellent anodic inhibitor, the
likelihood that small spots will remain unprotected increases as
the concentration of chromate maintained in the recirculating
water decreases.18'21'22
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TABLE 3-1'-. QUALITATIVE CLASSIFICATION OF CORROSION RATES18
Description
Negligible
Mild
Moderate
Severe
Corrosion rates, pm/yr (mils/yr)a
Carbon steel
<25.4-50.8 (254.0 (>10)
Copper alloy
<2.54 (<0.1)
3.81-5.08 (0.15-0.2)
5.08-8.89 (0.2-0.35)
12.7-25.4 (0.5-1)
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TABLE 3-2.
ANODIC AND CATHODIC CORROSION
INHIBITORS
Anodic
Cathodic
Chromate
Polyphosphate
Molybdate
Zinc
Orthophosphate
Polysilicate
Nitrite
Orthosilicate
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3.4.2.2"-.! Chromium-based inhibitors. Chromates
historically have provided the best protection against corrosion
for the money and effort. Less stringent monitoring and control
of chemical concentration and pH are required with chromates than
with alternative treatments. Even if the inhibitor feed is
temporarily interrupted, the existing barrier film will continue
to provide protection for several days.
Chromate concentration typically must be maintained above
200 parts per million (ppm) if the chromates are used alone in
the recirculating water. However, chromates (which are anodic
inhibitors) typically are used in combination with cathodic
inhibitors. In these combinations, smaller concentrations of
chromate provide the same corrosion protection that is provided
by high concentrations of chromate alone. Vendors provide many
chromium-based formulations for use in IPCT's. The chromium
concentrations in these formulations generally range from 3 to
30 ppm as chromate. However, very few plants use chromate in
concentrations of less than 10 ppm, and the results obtained with
these "low" chromate water treatment programs have been mixed.
Programs that contain less than 10 ppm of chromate require
tighter chemical control and more expensive chemical additives
than are required for formulations that use higher chromium
concentrations. Plants that use chromates prefer to use higher
chromate concentrations (i.e., 10 ppm or more) to avoid the
additional costs for additives and tighter control requirements.
The inhibitor chemicals most commonly added to chromate-
based formulations are zinc and phosphate; but organic compounds,
polysilicates, and molybdates also are used. Some of the effects
that these compounds have on the treatment program are discussed
below, but because the same chemicals are used in nonchromate
treatment programs, they are discussed in greater detail in
Section 4.1.1. Table 3-3 provides concentrations and typical
operating conditions of some of the corrosion inhibitor
formulations discussed in various publications.18'21'22
Molybdates are anodic inhibitors that protect all metals but
are most effective on steel. Zinc is an effective cathodic
3-16
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TABLE 3-3."- CHROMATE-BASED CORROSION INHIBITORS18'21'22'24'25
Chromate/zinc
Chromate/orthophosphate
Qiromate/orthophosphate/ziiic
Ohromate/polyphosphate/zinc
Chromate/zinc/phosphonate
Chromate/phosphonate/specified
Chromate/phosphonate/dispersant
Chromate/dispersant
Chromate/polysilicate
Chromate/zinc/dispersant
Chromate/molybdate
5-20/2.5-10
20-25/3-3.5
5-10/10
15-25/2-5/2-5
15-25/2-5/2-5
10-30/3-5/3-5
20-25/5-10/2.5-3.0
15-25/2-4/3-5
2-3/2-3/5-10
20-25/2-4/3-5
5-10/3-5
5-15/2-6/2-6
3-5/30
5-10/3-6
5-10/5-10
10-20/1-2/1.5-10
10-15/1-2/5-10
10-30/1-5
pH 6.5-7.0
pH 7.0-7.5
pH 6.2-6.8
pH 6.2-6.8
pH 6.0-7.0
pH 6.5-7.0 CaH 100-600 ppmc
pH 6.0-6.5 CaH <400
pH 6.5-7.5
pH 6.5-7.0
pH 6.5-7.0
Not specified
pH 7.5-8.5
pH 7.5-8.0
pH 7.5-8.0
pH >7.5
Si < 10 ppm
pH 7.0-9.0
pH 7.5-9.0
pH >7.5
aln all combinations except chromate/polysilicate, the organic triazole corrosion inhibitors should be included at
1 to 10 ppm when the system contains copper.
''The components of some combinations can be formulated differently for different applications.
cCaH is calcium hardness.
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inhibitor for steel when the pH is below about 8.0; with
increasing pH, the solubility of zinc decreases, and fouling may
result. Polyphosphates also are effective cathodic inhibitors
for steel, but they may react with copper or aluminum.
Consequently, organic azoles are added to formulations containing
phosphate that are used in systems containing components made
with copper alloys. In addition, because of the nutrient value
of polyphosphate, it is especially important to monitor
microbiological activity and maintain the recommended types and
concentrations of microbiocides in the recirculating water when
polyphosphate is used. Orthophosphates are effective anodic
inhibitors, but they typically must be used in small quantities
or in conjunction with phosphate-specific dispersants to reduce
the likelihood of fouling.
3.4.3 Scaling and Fouling
3.4.3.1 Description. Scale formation occurs when dissolved
solids and gases in cooling water reach their limit of solubility
and precipitate onto piping and heat transfer surfaces. Fouling
occurs when deposits of dirt, leaves, and/or flocculations of
insoluble salts or hydrous oxides produced by corrosion
agglomerate in the heat exchanger tubes. Scaling and fouling
reduce the flow rate and heat transfer in any heat exchanger.
These conditions also contribute to pitting-type corrosion by
creating corrosion cells and preventing the corrosion inhibitor
from contacting the surface of the metal. Calcium carbonate is
the most common scale found in cooling water systems, but calcium
sulfate and calcium phosphate also can be formed in many systems.
All three types of scale become less soluble and, therefore, more
likely to form at high temperatures. Calcium sulfate is more
likely to form at low pH, and the other two scales are more
likely to form at high pH.
3.4.3.2 Antiscalants and Antifoulants. Control of scaling
and fouling is achieved by controlling deposition onto surfaces.
Deposition can be affected by changing the solubility of scale-
forming salts, reducing the crystalline growth capacity of scale-
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forming salts, and dispersing constituents that form fouling-
related flocculations.
Chemical compounds that are commonly used and are the most
effective in controlling the rate of scaling include
polyphosphates, phosphonates, and polymeric and copolymeric
dispersants. These compounds reduce the crystalline growth
capacity of calcium salts. Certain phosphonate compounds affect
the solubility of the calcium salts and reduce the formation of
scale. Polymeric dispersants reduce the potential for
fouling.18'21'22
Phosphonates typically are added to chromate-zinc
formulations as an alternative to phosphate because phosphonates
control scaling better than phosphate does. Also, the amount of
chromate can be reduced when phosphonate is used because the
phosphonate system can be operated at a slightly higher pH. A
disadvantage of using phosphonate is that the powerful oxidizing
potential of chlorine can promote corrosion of copper if
phosphonates are present in the system. Adding benzotriazole (or
other azoles) and dispersants can effectively control this
problem. Phosphonates are also subject to biological oxidation,
which results in the release of. orthophosphonate ions that can
cause fouling as well as reduce the recommended concentration of
phosphonate. However, fouling is much less of a problem in
systems treated with phosphonate than in systems treated with
polyphosphates.18 • . • .
3.4.4 Microbiological Control
Three types of microorganisms are found in cooling tower
water systems: bacteria, fungi, and algae. Bacteria are
dispersed in the water, fungi invade wood components, and algae
attach to surfaces in the tower. Slim'e produced by bacteria can
coat heat exchanger surfaces and aggregate debris on those
surfaces, thereby.reducing the efficiency of heat transfer.
Biological deposits on metal surfaces also can accelerate pitting
corrosion. Fungi can decay wood either by surface attack (soft
rot) or internal attack of the cellulose (white rot) and, thus,
are a threat to the structural integrity of wooden towers. Algal
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growth can coat the fill material and reduce the effectiveness of
the water droplet formation and, thus, the effectiveness of heat
transfer in the tower.
Microbiocides can be classified as oxidizing agents, enzyme
poisons, organic chemical compounds, and miscellaneous compounds.
The oxidizing agents include chlorine, bromine, and iodine.
Chlorine is the most widely used and the least costly
microbiocide. Enzyme poisons include methylene bisthiocyanate,
acrolein, and heavy metals (e.g., copper sulfate, copper citrate,
tin, phenylmercurie acetate, and methyl mercury). Acrolein and
the heavy metals are considered to be outdated technology and are
not known to be currently used in IPCT systems. Organic
compounds include dodecylquamidine hydrochloride and quaternary
ammonium salts and normally require high dosage rates. Most
microbiocides used for treating cooling water are.included in the
categories above, but dithiocarbamates are a class of
miscellaneous compounds that also are effective microbiocides.
However, they reduce chromate and, thus, cannot be used in
chromate-treated systems.18/21
Organic chemical compounds that either hydrolyze to•
relatively nontoxic forms or that can be detoxified are also used
as microbiocides. Hydrolyzable materials include
2,3-dibromo-3-nitrilopropionamide, chlorinated cyanurates,' and
halogenated hydantoins. Chemicals that are both hydrolyzable and
detoxifiable are methylene bis-thiocyanate and bromonitro-
styrene. Isothiazolin is a'widely used biocide that can be
detoxified.23
3.4.5 Cooling System Control
A cooling system chemical feed and monitoring control system
maintains the proper operating conditions of the cooling system
to minimize corrosion and fouling. Addition of treatment
chemicals is regulated to prevent overfeeding and underfeeding
and thereby optimize performance and operating costs. Cooling
system control is achieved by monitoring water quality conditions
and feeding appropriate concentrations of the required treatment
chemicals described previously. Control system components may
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include pH and conductivity monitors, a blowdown control valve,
corrosion inhibitor feed, acid feed, microbiological control
chemical (biocide) feed, and .a makeup water pump. The components
of the control system may be completely automated via an
integrated computer system, automated yet function independently
of one another, controlled by a combination of automated and
manual controls, or operated manually.
A minimum control system consists of pH and conductivity
monitors and some sort of chemical feed system. This type of-
control system, whether manual or automated, maintains the proper
pH for effective corrosion inhibition based on the type of
corrosion inhibitor used. This basic control system also
controls the blowdown through conductivity measurements to limit
conditions that favor scaling or fouling. Computer-based control
systems are available that automate water quality.monitors and
chemical feed pumps. The control system program operates the
chemical feed pumps based on measurements of specific water
quality parameters. Even tighter control of chemical addition is
achieved through computer feedback loops in which the computer
compares the actual water quality to planned water quality
conditions and adjusts chemical feed pumps to achieve the desired
water quality conditions.
The level of control automation required by a cooling system
depends on whether manual control at the plant is adequate to •
maintain acceptable pH and conductivity. A system operated by
experienced personnel may not require as much automated control
as a plant with inexperienced personnel or a plant that lacks the
employee time to adequately monitor the cooling systems.
However, using automated equipment may offer savings in chemical
costs to some cooling systems through quality control and tighter
monitoring conditions, which are possible with computer-run '
systems. Although an automated control system may prove cost
effective, nonchromate- and chromate-based treatment programs
both can be operated successfully without such a system.19'20
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3.5 MAINTENANCE REQUIREMENTS
The maintenance requirements for cooling towers and
associated heat exchangers are a -function of the effectiveness of
the chemical treatment programs. The effectiveness of the
corrosion inhibitor is important to the life expectancy of metal
surfaces exposed to the recirculating water in both the tower and
heat exchanger. Metal structural components of the tower such as
steel columns and beams, connector plates and bolts, piping and
pumps, valves, and controller equipment corrode from exposure to
the cooling tower water. However, the most critical corrosion
occurs in the heat .exchangers. Excessive corrosion, followed by
fouling in heat exchangers, will result in both the need for
excessive tube bundle cleaning to maintain heat transfer and tube
bundle replacements because of pitting corrosion.
Corroded tubes may be discovered during routine inspections
of the heat exchanger, but, in many instances, contamination of
the recirculating water with process fluid (or vice versa) is the
first indication of failed tubes. Typically, the leaking tubes
are identified and plugged. Some loss of efficiency or cooling
capacity because of plugged tubes can usually be tolerated
because cooling systems are often designed with an excess of heat
exchange capacity. However, in some processes, this margin may
not exist because production levels are such that a "capacity
bottleneck" is present at the heat exchanger. A new tube bundle
generally is required when about 10 to 15 percent of the tubes
are plugged.
Scaling and fouling can occur in both the cooling tower and
the heat exchangers. In the cooling tower, scaling and fouling
can reduce tower heat rejection capacity by interfering with
splash or film fill water distribution and the formation of water
droplets. Airflow characteristics also can be altered when
airflow passages in the fill and drift eliminator become blocked;
this blockage increases the pressure drop across the system and
reduces cooling efficiency. The tower fill and drift eliminator
surfaces are inspected periodically and cleaned if necessary.
Usually, this work is performed during a process shutdown.
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However, cle'aning may be required more frequently if the scaling
rate increases because of poor control of the cycles of
concentration, contamination problems, or lack of attention to
the chemical treatment program.
Scaling and fouling in heat exchanger tubes reduce the heat
transfer capacity because of the low thermal conductivity of the
crystalline film created on the tube surfaces and flow
restrictions that result. The level of scaling and fouling in
heat exchangers and, thus, the level of maintenance required, can
vary with changes or variations in the water treatment program.
The scale and foulants can be removed physically by a process
called rodding, by water blasting, or by flushing with acids.
These physical cleaning methods require that the heat exchanger
be taken out of service until the maintenance ,is completed.
Alternatively, side stream filters can be used to continuously
remove silt and other foulants while the system is in service.
In industrial plants that operate nearly continuously, the heat
exchangers may not be inspected and cleaned for 3 years or more
unless a problem occurs. Chemical cleaning also can take place
while the system is in service/ but the effectiveness of this
procedure depends upon the level of scaling or fouling. If
strong acids are to be used, the heat exchanger will be taken out
of service to protect ancillary cooling water system components.
3.6 COMPOSITION AND FORMATION OF DRIFT
Water droplets entrained in the air and the dissolved and
suspended solids contained in the droplets that are emitted from
the stack are referred to as drift. These droplets contain the
chemical constituents, additives, and contaminants present in the
recirculating water. The droplets are formed both from water's
splashing down through the fill material and from the shearing
action of the airflow along the water surfaces within the tower.
In the past, drift was usually associated only with the loss of
water from the tower stack, but the minerals, metals, and other
constituents of'the recirculating water are also emitted in the
drift droplets. The rate at which drift is emitted from the
tower (i.e., the drift rate) is primarily a function of the air
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velocity through the fill material, the recirculation rate, and
the type of fill material and drift eliminator used. Although
air velocity has a greater impact on the drift rate, drift rate
is usually expressed as a percent of the water recirculation
rates for two reasons: most IPCT's are designed so that air
velocities through the fill are comparable, and it is much easier
to measure the water recirculation rate than the air velocity
through the fill. The rate of water flow through the fill
material is referred to as "water loading," and the range of
water loading in IPCT's is typically 81.5 to 410 liters per
minute per square meter of a horizontal cross section of the fill
(L/min/m2) (2 to 10 gallons per minute per square foot
[gal/min/ft2]),26'27 The velocity of the airflow in the fill
typically is from 91 to 213 meters per minute (m/min) (300 to
700 feet per minute [ft/min]). At.91 m/min (300 ft/min), the
airflow will entrain all water droplets smaller than
•370 micrometers (/tm) in diameter (14.6 thousandths-of-an-inch
[mils]). At 213 m/min (700 ft/min), water droplets smaller than
800 jim (31.5 mils) in diameter will become entrained in the
airflow. A drift eliminator manufacturer indicated .that drift
rates are highest when the air velocity is at either end of the
range. Within the overall range, minimum drift rates are
obtained over a much broader range for high-efficiency drift
eliminators than for low-efficiency drift eliminators.
Velocities in the stack are usually between 460 and
550 m/min (1,500 and 1,800 ft/min) and, thus, are capable of
carrying droplets larger than 800 ^m (31.5 mils) in diameter.28
If droplets this large are created in or near the stack, possibly
by collision between droplets in the turbulent air stream or by
impingement and condensation of smaller droplets on structural
members near the fan, they may also become entrained.
Splash fill towers tend to have higher drift rates than film
fill towers, because splash fill is designed to create water
droplets. As mentioned previously, herringbone design drift
eliminators are the least efficient, and cellular drift
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eliminators "are generally the most efficient type of drift
eliminator.
The recirculating water contains the corrosion inhibitors;
other additives; naturally occurring silica and calcium- and
magnesium-containing compounds; and other dissolved solids. The
concentration of these dissolved solids increases in evaporating
drift droplets. According to the theoretical calculations of
evaporation rates used to generate Table 3-4, droplets that are
created at 30 /xm (1.2 mils) and below get much smaller as they
evaporate, and the concentration of the solids in those droplets
increases as evaporation occurs. Tests sponsored by EPA show
that, in many cases, over 90 percent of the Cr+^ emissions are
contained in droplets less than 30 fjaa (1.2 mils) in
diameter.29"31 Thus, the tower emissions may include water
droplets containing various concentrations of dissolved solids,
and the smallest of these droplets (<30 /jm [1.2 mils] in
diameter) may contain a significantly higher concentration of
dissolved solids, including chromate.
3.7 DRIFT EMISSION RATE
As mentioned previously, drift is often expressed as the
percentage of the recirculating water flow rate that is emitted.
Based on test results, a drift eliminator manufacturer claims
that the achievable drift rates range from 0.001 to 0.02 percent
of the recirculating water. The approximate dividing line
between drift rates for high- and low-efficiency drift
eliminators is 0.04 to 0.02 percent. Those achieving a lower
percentage are "high efficiency," and those that cannot achieve
0.02 percent are "lower efficiency."32'33 However, it is
important to note that drift rate results are highly dependent on
the measurement method; therefore, achievable drift rate claims
may not be comparable if they are based on different measurement
methods. According to a cooling tower manufacturer, differences
in drift rate measurement arise when drift is expressed in terms
of a percentage of the water flow rate because drift is more a
function of airflow rate than water flow rate. Therefore, drift
should be expressed as a concentration of tower exhaust air
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rather than 'as a percentage of the recirculating water flow rate.
However, because water flow, rate is more easily measured than
exhaust airflow rate, industry prefers to express drift in terms
of percentage of water flow rate.32
One drift measurement method employs a type of paper
sensitive to the impingement of drift droplets. An impinging
droplet leaves a mark on the paper that, after microscopic
examination, can be correlated to the droplet size; thus, the
total mass of drift water may be calculated. However, this
method aerodynamically -excludes droplets smaller than about 30 /wn
(1.2 mils) in diameter.29'3° Traditionally, the main focus of .
drift control has been to minimize the deposition of droplets and
salts in the vicinity of the tower; drift deposition on plant or
personal property in the immediate area around the tower can be a
nuisance. Droplets less than 30 ^m (1.2 mils) do.not deposit but
evaporate further and remain airborne.34 Thus, the sensitive
paper method measures only the portion of the drift most likely
to cause deposition problems in the immediate area around the
tower.
Other measurement techniques collect drift droplets
isokinetically. A mineral or a tracer in the collected draft
droplets is recovered in a sample train for chemical analysis. '
The chemical analysis performed determines the mass of the
mineral or added tracer recovered in the sample. Then, if it is
assumed that the concentration of the salt or tracer in the drift
is the same as that in the cooling water itself, the drift rate
can be calculated. Minerals and tracers that have been used for
analysis include, but are not limited to, sodium,, calcium,
magnesium, strontium, and chromium. The collection and recovery
efficiencies for this type of method are greater than those of
the sensitive paper method. Over time, several different
sampling train configurations have been developed. For a given
tower, different collection and recovery efficiencies may be
obtained with each of these configurations. Therefore, drift
rates measured with different configurations may not be
comparable.
3-26
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When these isokinetic methods are used to measure drift
rates it is assumed that the mineral or tracer concentration in
the drift is equivalent to that in the cooling water. If,
because of droplet evaporation, the salt concentration is
significantly higher in the drift than in the cooling water, this
method may overestimate the drift water loss rate. However,
regardless of this uncertainty in the drift rate, the emission
rate of the salt is accurately measured with the isokinetic
method. This study is primarily concerned with the chromium '
emission rate, not the actual drift rate of water and salt
combined. Therefore, the EPA emission tests performed in this
study report only the Cr+^ emission rate. For each test, the
emission rate is expressed as a percentage of the Cr+6
recirculating rate. (Further discussion on the EPA test program
is included in Section 4.1.4.) This percentage can equal the
drift rate only if.the equivalent concentration assumption is
made.
3.8 REFERENCES FOR CHAPTER 3
1.
2.
3.
4.
5.
United States Refining Capacity. June 9, 1986. National
Petroleum Refiners Association. Washington, D.C. pp. 5-17.
1987 Census of Manufactures.
Washington, D.C.
U.S. Department of Commerce.
6.
Steel, USA: Into the '80's, Techno-Economic-Report.
Institute for Iron and Steel Studies. Green Brook, NJ.
January 1980.
Letter and attachments. Roti, J., Drew Industrial Division,
Ashland Chemical, to Crowder, J., EPArlSB. July 16, 1991.
Response to questionnaire on cooling water treatment
programs and control equipment.
«
Letter and attachments. James, E., Betz Industrial, to
Crowder, J.,' EPArlSB. July 17, 1991. Response to
questionnaire on cooling water treatment programs and
control equipment.
Questionnaire. Eastin, P., Nalco Chemical Company, to
Myers, R., EPArlSB. . August 5, 1991. Response to
questionnaire on cooling water treatment programs and
control equipment.
3-27
-------
7. Questionnaire. Lutey, R., Buckman Laboratories, to
Myers, R.,_EPA:ISB. August 6, 1991. Response to
questionnaire on cooling water treatment programs and
control equipment.
8. Responses to Section 114 information requests Nos. 1 through
94 listed on Table 5-14.
9. Telecon. Upchurch, M., MRI, with Dean, J., Brown and
Williamson Tobacco Company. October 31, 1986. Number of
cooling towers.
10. Telecon. Upchurch, M., MRI, with Gallo, T., Corning Glass
Works, Inc. October 31, 1986. Number of cooling towers.
11. Telecon. Upchurch, M., MRI, with Mabry, R., Mount Vernon
Mills. October 31, 1986. Number of cooling towers.
12. Telecon. Upchurch, M., MRI, with Durvin, T., PPG
Industries, Inc. October 30, 1986. Number of cooling
towers. .
13. Telecon. Upchurch, M., MRI, with Jordan, S., PPG
Industries, Inc.--Glass Division. October 30, 1986.
of cooling towers.
Number
14. Telecon. Upchurch, M., MRI, with Howard, W., Jr.,
R. J. Reynolds Tobacco Company. December 2, 1986. Number
of cooling towers.
15. Telecon. Upchurch, M., MRI, with Ryan, S., Reynolds
Aluminum. October 23, 1986. Number of cooling'towers.
16. Chemical Engineers Handbook. 5th Edition. J. Perry and
Chilton, eds. New York, NY. McGraw Hill. 1973.
17. Kelly, G. M. A System-Efficient Approach to Cooling Tower
Energy Modifications'. Cooling Tower Institute Technical
Paper No. TP-85-18. New Orleans, Louisiana. January 1985.
18. McCoy, J. W. The Chemical Treament of Cooling Water.
Chemical Publishing Company, Inc. New York, NY. 1983.
pp. 8-12, 21-44, 124-187.
19. Memorandum. Marinshaw, R., MRI, to Myers, R., EPArlSB.
April 3, 1991. Trip report: Nalco Chemical Company,
Naperville, Illinois, on February 22, 1991.
20. Memorandum. Marron, J., MRI, to Myers, R., EPArlSB.
April 3, 1991. Trip report: Betz Industrial, Trevose,
Pennsylvania, on February 26, 1991.
3-28
-------
21. Betz Handbook of Industrial Water Conditioning. 8th
Edition. Betz Laboratories. Trevose, PA. 1980.
pp. 167-190, 202-215.
22. Strauss, S. and P. Puckorius. Cooling-Water Treatment for
Control of Scaling, Fouling, Corrosion. Power Magazine.
June 1984. pp. S-l through S-24.
23. The Nalco Water Handbook. Nalco Chemical Company.
F. Kemmer, ed. McGraw-Hill Book Company. New York, NY.
1988. pp. 20.1-22.22, 38.3-38.30.
24. Letter. Macht, W., Betz Entec, Inc., to Randall, D.', MRI.
April 10, 1987. Biocides and chromate-based water treatment
programs.
25. Letter and attachments. James, E., Betz Laboratories, Inc.,
to Crowder, J., EPA:ESED. March 27, 1987. Comments on
draft background information document Chapters 3-6, 8.
26. Baker, D. Cooling Tower Performance. Chemical Publishing
Company. New York, NY. 1984. pp. 120-129.
27. Telecon. Randall, D., MRI, with Depalma, T., Custodis-
Cottrell. June 4, 1987. Effect of water loading on drift
rate.
28. Telecon. Nicholson, B., MRI, with Kuharic, I., Marley
Cooling Tower Company. October 15, 1986. Cooling tower
parameters.
29. Emission Test.Report: Exxon Company U.S.A., Baytown, Texas.
EMB Report 85-CCT-3. November 1986.
30. Emission Test Report: National Bureau of Standards Steam
and Water Chill Plant, Gaithersburg, Maryland. EMB
Report 85-CCT-4. November 1986.
31. Emission Test Report: Southeastern Manufacturing Facility.
Draft. Prepared for U. S. Environmental Protection Agency,
Research Triangle Park, North Carolina. September 1987.
32. Telecon: Bellin, P., MRI, with Holmberg, J., Marley Cooling
Tower Company. April 2, 1985. Drift eliminator, efficiency.
33. Telecon: Bellin, P., MRI, with Holmberg, J., Marley Cooling
Tower Company. July 19, 1985. Drift eliminator efficiency.
34. Memorandum from Marinshaw, R., MRI, to Industrial Process
Cooling Towers Project Files. September 9, 1991. Draft.
Relationships between evaporation and droplet size.
3-29
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4.0 EMISSION CONTROL TECHNIQUES
4.1 CONTROL TECHNIQUES
This chapter describes techniques that are available to
reduce hexavalent chromium (Cr+^) emissions from industrial
process cooling towers (IPCT's). The control techniques
discussed include substituting nonchromate-based chemicals for
chromate-based water treatment programs and retrofitting towers
with high-efficiency drift eliminators (HEDE's). .
4.1.1 Nonchromate Treatment Programs
In recent years, research efforts in cooling water treatment
technology have focused almost exclusively on alternatives to
chromate for corrosion protection. The result has been the
development of improved polymers that overcome the earlier
scaling and fouling problems with nonchromates and that have made
nonchromate water treatment programs more competitive to
chromate-based programs in cost and effectiveness. Nonchromate
treatment programs typically consist of combinations of cathodic
inhibitors, anodic inhibitors, antiscalants, and dispersants.
Many of the compounds used in nonchromate programs are
essentially the same as those used as additives in the chromate
programs described in Section 3.4.2.2. With all. nonchromate
inhibitors, monitoring and control are critical. It is necessary
to control carefully the recommended pH and inhibitor concen-
trations. Vendors typically recommend some level of automated
control (i.e., automated control of chemical feed, blowdown, pH,
or some combination of these). 2~5 However, most systems do not
require total feed and control automation. Good control of
nonchromate programs can be achieved manually. In addition, when
chlorine is used as a biocide, microbiological control is more
4-1
-------
critical for"many of the nonchromate programs that require
alkaline conditions because chlorine is less effective at higher
pH levels than at the lower levels characteristic of chromate
programs.4
Suitable operating conditions for common nonchromate
treatment programs are shown in Table 4-1. Typical nonchromate
formulations are based on phosphates, molybdates, zinc, and
organics. These and less common programs are discussed below.
• 4.1.1.1 Treatment Formulations and Operating Conditions.
4.1.1.1.1 Phosphates. Phosphate-based programs are widely
used in IPCT's. The constituents of phosphate-based treatment
programs include orthophosphates and/or polyphosphates and
dispersants. The "dispersants" used with phosphate programs are
actually stabilizers that maintain the solubility of phosphate
under conditions in which they would otherwise be-insoluble.
Concentrations of phosphates in water treatment programs
generally range from 4 to 18 parts per million (ppm). If the
system contains copper or copper alloys, azoles are included in
the treatment program to prevent phosphate from reacting with the
metal. With these combinations, effective corrosion control can
be achieved and deposition of calcium carbonate and calcium
phosphate scale can be minimized.
Phosphate inhibitor programs may be modified to treat water
with water quality limitations. Increased use of additives or
other program modifications may be required to operate a
phosphate program successfully under high iron concentrations,
high calcium hardness (>800 to 1,000 ppm CaC03), high water side
skin temperatures (60° to 82.2°C [140° to 180°F]), and high
chloride concentrations (up to 15,000 ppm chloride).5 An
advantage of phosphates over chromates is that they do not react
with reducing agents (contaminants such as H2S and organic
compounds) in the recirculating water and, thus, do not lose
their effectiveness in contaminated water.3'6
4.1.1.1.2 Molybdates. Until recently, molybdate treatments
were rarely used in situations where the cooling water was
contaminated because the cost of molybdate treatments, under even
4-2
-------
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