&EPA
United States
Environmental Protection
Agency
Preliminary Study of the
Metal Finishing Category:
2015 Status Report
June 2016
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U.S. Environmental Protection Agency
Office of Water (43 03 T)
1200 Pennsylvania Avenue, NW
Washington, DC 20460
EPA-821-R-16-004
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Table of Contents
TABLE OF CONTENTS
Page
1. INTRODUCTION 1-1
2. EXISTING METAL FINISHING ELGs 2-1
2.1 Metal Finishing Industry Profile at Promulgation 2-2
2.2 Metal Finishing Process Operations at Promulgation 2-3
2.3 Treatment of Metal Finishing Wastewater at Promulgation 2-7
2.3.1 Common Metals Treatment 2-10
2.3.2 In-process Cadmium Reduction 2-10
2.3.3 Treatment of Complexed Metals, Cyanide-Bearing and
Chromium-Bearing Wastewaters, and Wastewater Containing
Oily Wastes 2-11
2.3.4 Control and Treatment of Toxic Organics 2-12
2.4 Applicability of the Existing ELGs to Current Operations 2-14
2.4.1 Changes to Industry Profile 2-14
2.4.2 Scope of Metal Finishing Operations 2-14
2.4.3 Process Technology Modifications and Alternative Chemistries 2-15
2.4.4 Technological Advances in Wastewater Treatment 2-15
3. STUDY ACTIVITIES AND PROGRESS 3-1
3.1 Literature Review 3-1
3.2 Technical Conferences 3-2
3.3 Industry Experts 3-2
3.3.1 Metal Finishing Facilities and Technology Vendors 3-2
3.3.2 Pretreatment Coordinators 3-3
3.3.3 Other Stakeholders 3-3
3.4 Industry and Trade Organizations 3-3
3.5 Site Visits to Metal Finishing Facilities 3-4
3.6 Metal Products and Machinery (MP&M) Rulemaking 3-4
4. PRELIMINARY STUDY FINDINGS 4-1
4.1 Changes in the Metal Finishing Industry Profile 4-1
4.2 Process Technologies 4-2
4.2.1 Cleaning and Rinsing Operations 4-3
4.2.2 Coating and Plating Processes 4-7
4.2.3 Polishing 4-9
4.2.4 Summary of Process Technologies 4-9
4.3 Alternative Chemistries 4-10
in
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Table of Contents
TABLE OF CONTENTS (Continued)
Page
4.3.1 Cadmium Plating Alternatives 4-10
4.3.2 Hexavalent Chromium Plating, Conversion Coating, Primer,
Sealer, and Cleaner Alternatives 4-12
4.3.3 Phosphate Conversion Coating and Cleaning Alternatives 4-17
4.3.4 Cyanide Plating Solution Alternatives 4-19
4.3.4 Summary of Alternative Chemistries 4-19
4.4 Wastewater Treatment Technologies 4-20
4.4.1 Technologies Evaluated for the 1983 Metal Finishing ELGs 4-20
4.4.2 Chemical Precipitation 4-22
4.4.3 Sorption 4-24
4.4.4 Membrane Filtration 4-34
4.4.5 Flotation 4-40
4.4.6 Electrochemical Treatment 4-41
4.4.7 Biological Treatment 4-44
4.4.8 Summary of Wastewater Treatment Technologies 4-45
4.5 Applicability and Other Regulatory Considerations 4-45
4.5.1 Rule Implementation 4-45
4.5.2 Applicability of ELGs to New or Modified Metal Finishing
Operations 4-46
4.5.3 Considerations for Other Regulations 4-47
5. NEXT STEPS 5-1
5.1 Review Pollutant Discharge and Release Data 5-1
5.2 Evaluate Changes to Industry Profile 5-2
5.3 Review Literature from Conferences and Other Industry Sources 5-2
5.4 Continue Discussions with Industry Experts on Key Topics 5-2
5.5 Conduct Site Visits to Metal Finishing Facilities 5-3
5.6 Investigate the Impacts of Other Regulations on the Industry 5-3
6. QUALITY ASSURANCE 6-1
6.1 Project Objectives 6-1
6.2 Data Sources 6-1
6.3 Data Quality Objectives and Criteria 6-1
6.4 Data Quality Evaluation 6-3
6.4.1 Conference Proceedings, Peer-Reviewed Journal Articles, Other
Academic Literature 6-3
6.4.2 Data and Information Obtained from Industry, Vendors, and
Trade Associations 6-4
IV
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Table of Contents
TABLE OF CONTENTS (Continued)
Page
6.4.3 Existing Government Publications and Supporting Information 6-4
7. REFERENCES 7-1
APPENDIX A: DESCRIPTIONS OF THE 46 UNIT OPERATIONS IN THE METAL FINISHING
INDUSTRY
APPENDIX B: KEYWORD SEARCH TERMS FOR THE METAL FINISHING INDUSTRY
APPENDIX C: WASTEWATER TREATMENT TECHNOLOGIES EVALUATED FOR METALS
REMOVAL IN THE 1983 METAL FINISHING ELGs
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List of Tables
LIST OF TABLES
Page
Table 2-1. Unit Operations Regulated by ELGs for the Metal Finishing Category 2-6
Table 2-2. Regulated Pollutants and ELG Limits for the Metal Finishing Category 2-8
Table 4-1. Process Descriptions of Cadmium-Free Alternative Coatings 4-11
Table 4-2. Commercially Available Hard Chrome Plating Alternatives 4-13
Table 4-3. Advantages and Disadvantages of Hard Chrome Plating Alternatives 4-14
Table 4-4. Chromate Conversion Coating Alternatives 4-15
Table 4-5. Regulatory Options Considered in the 1983 ELGs and their Daily Maximum,
Monthly Average, and Long-Term Average Concentrations 4-21
Table 4-6. Chelated Metals Precipitation Treatment Identified in EPA's Literature
Review- Summary of Treatment Results 4-25
Table 4-7. Low-Cost Adsorbents Identified in EPA's Literature Review 4-26
Table 4-8. Low-Cost Alternatives to Coal-based Activated Carbon Treatment Identified
in EPA's Literature Search - Summary of Treatment Results 4-29
Table 4-9. Biosorbent Treatment Identified in EPA's Literature Review - Summary of
Treatment Results 4-31
Table 4-10. Nanomaterials Identified in EPA's Literature Review 4-32
Table 4-11. Ion Exchange Treatment Identified in EPA's Literature Review - Summary
of Treatment Results 4-33
Table 4-12. Ultrafiltration Treatment Identified in EPA's Literature Review - Summary
of Treatment Results 4-37
Table 4-13. Nanofiltration Treatment Identified in EPA's Literature Review - Summary
of Treatment Results 4-39
Table 4-14. RO Treatment Identified in EPA's Literature Review - Summary of
Treatment Results 4-39
Table 4-15. Electrocoagulation Treatment Identified in EPA's Literature Review -
Summary of Treatment Results 4-42
Table 4-16. Electroflotation Treatment Identified in EPA's Literature Review - Summary
of Treatment Results 4-43
Table 6-1. Data Quality Criteria Summary 6-3
Table 6-2. Data Acceptance Criteria for the Preliminary Study of the Metal Finishing
Category 6-5
VI
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List of Figures
LIST OF FIGURES
Page
Figure 2-1. Metal Finishing Process Application 2-7
Figure 2-2. BPT/BAT/PSES Technology Basis 2-9
Figure 2-3. NSPS/PSNS Technology Basis (Equivalent to BPT/BAT/PSES with
Additional Cadmium Reduction) 2-9
Figure 4-1. Original Process at the Bronze Cyanide Plating Facility 4-4
Figure 4-2. New Process at the Bronze Cyanide Plating Facility 4-4
Figure 4-3. Potential Rinse Systems in Facility with Metal Finishing Processes 4-6
Figure 4-4. Plaforization System for Batch Spraying Equipment 4-7
Figure 4-5. Traditional Coating Systems 4-17
Figure 4-6. Illustration of Membrane Filtration Technologies 4-34
Figure 4-7. Illustration of Micellar-Enhanced Ultrafiltration 4-36
vn
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Abbreviations
Acronym
ACS
ACWA
AFM/AFMmm
APCVD
BAT
BMR
BPT
CFR
CIUs
CPR
CWA
DAF
DCN
DDTC
DMR
DoD
DOE
EDTA
ELGs
ELY
EPA
ERG
ESTCP
EU
FR
GACT
GBP
gpm
HRS
HVOF
IONMET
IPS
IPCB
IVD
IWC
LDH
LTA
MEUF
MF
mg/L
ABBREVIATIONS
Definition
American Chemical Society
Association of Clean Water Administrators
Abrasive Flow Machining/that Incorporates a Movable/Rotatable Mandrel
Atmospheric Pressure, Chemical Vapor Deposition
Best Available Technology Economically Achievable
Baseline Monitoring Report
Best Practicable Control Technology Currently Available
Code of Federal Regulations
Categorical Industrial Users
Cleaner Phosphoric Recycling
Clean Water Act
Dissolved Air Flotation
Document Control Number
Di ethyl dithi ocarb am ate
Discharge Monitoring Report
Department of Defense
Department of Energy
Ethylenediaminetetraacetic Acid
Effluent Limitations Guidelines and Standards
End of Life Vehicle
Environmental Protection Agency
Eastern Research Group, Inc.
Environmental Security Technology Certification Program
European Union
Federal Register
Generally Available Control Technology
Glass Bead Peening
gallons per minute
Hot Rolled Steel
High Velocity Oxygen Fuel
New Ionic Liquid Solvent Technology to Transform Metal Finishing
Products and Processes
Integrated Profitable Pollution Prevention Technologies
Independent Printed Circuit Board
Ion Vapor Deposition
International Water Conference
Layered Double Hydroxide
Long-term Average
Micellar-Enhanced Ultrafiltration
Microfiltration
milligrams per Liter
Vlll
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Abbreviations
ABBREVIATIONS
Acronym Definition
MP&M Metal Products and Machinery
NACWA National Association of Clean Water Agencies
NAICS North American Industry Classification System
NASF National Association for Surface Finishing
NAVAIR Naval Aviation Systems Command
NESHAP National Emission Standards for Hazardous Air Pollutants
NF Nanofiltration
NPDES National Pollutant Discharge Elimination System
NSPS New Source Performance Standards
OAR Office of Air and Radiation
ORCR Office of Resource Conservation and Recovery
OW Office of Water
PAM Polyacrylamide
PEUF Polymer-Enhanced Ultrafiltration
PFOs Perfluorooctane Sulfonate
POTW Publicly Owned Treatment Works
PPI Pollution Prevention Institute
PQAPP Programmatic Quality Assurance Project Plan
PSES Pretreatment Standards for Existing Sources
PSNS Pretreatment Standards for New Sources
PVD Physical Vapor Deposition
QA Quality Assurance
RCRA Resource Conservation and Recovery Act
RO Reverse Osmosis
RoHS Restriction of Hazardous Substances
SERDP Strategic Environmental Research and Development Program
SIC Standard Industrial Classification
SRB Sulfate-Reducing Bacteria
TDD Technical Development Document
TOC Total Organic Compounds
TOMP Toxic Organic Management Plan
TRI Toxic Release Inventory
TSS Total Suspended Solids
TTO Total Toxic Organics
UF Ultrafiltration
UV Ultraviolet
VOC Volatile Organic Compounds
WAFS Wetting Agents/Fume Suppressants
WEFTEC Water Environment Federation's Annual Technical Exhibition and
Conference
IX
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Section 1- Introduction
1. INTRODUCTION
The Clean Water Act (CWA) requires Environmental Protection Agency (EPA) to review
existing effluent guidelines annually. EPA reviews all point source categories subject to existing
effluent guidelines and pretreatment standards to identify potential candidates for revision,
consistent with CWA sections 304(b), 301(d), 304(m)(l)(A), and 304(g). The CWA also requires
EPA to revise existing Effluent Limitations Guidelines and Standards (ELGs) when appropriate.
EPA promulgated ELGs for the Metal Finishing Category (Metal Finishing ELGs),
codified at 40 Code of Federal Regulations (CFR) Part 433, in 1983. EPA revisited the Metal
Finishing ELGs during the development of the Metal Products and Machinery (MP&M)
rulemaking in the late 1990s and early 2000s. More recently, EPA conducted a preliminary
category review of the Metal Finishing Category as part of its annual effluent guideline review
process. In the Final 2014 Effluent Guidelines Program Plan, EPA announced plans to conduct a
preliminary study of the Metal Finishing Category to assess the current state of the industry,
including an updated industry profile, descriptions of new and traditional process technologies
and techniques, potential new pollutants of concern, advances in wastewater treatment
technologies, and strategies used to achieve zero liquid discharge (U.S. EPA, 2015a, 2015b).
This study extends EPA's efforts beyond the 304(m) annual review to better understand changes
in metal finishing operations, wastewater characteristics, and wastewater treatment technologies
since EPA promulgated the 1983 ELGs. The study seeks to answer the following key questions:
• How is the metal finishing industry now different from when EPA first
promulgated the Metal Finishing ELGs? Specifically:
— What is the distribution of captive facilities and job shops that currently
make up the industry?
— Which types of facilities are conducting metal finishing operations?
— What products are metal finishing facilities producing?
• Since the promulgation of the Metal Finishing ELGs, what process technology
changes have been implemented and how have the primary sources of wastewater
changed?
• Since the promulgation of the Metal Finishing ELGs, what changes to chemical
formulations have been implemented and how have these changes affected the
volume of wastewater and the concentrations and types of pollutants generated
and discharged?
• What are the best available technologies for pollution prevention and wastewater
treatment, and to what levels do they reduce discharges of pollutions of concern?
— What are the concentrations and loadings of pollutants currently being
discharged (i.e., baseline concentrations)?
— Which pollutant discharges require additional controls?
— How will industry discharges change if facilities implement these best
available technologies/practices to control pollutants?
1-1
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Section 1- Introduction
• What challenges do metal finishing facilities or regulatory authorities face in
applying the Metal Finishing ELGs?
This interim status describes the preliminary study and presents EPA's findings to date.
The remainder of the report is organized as follows:
• Section 2 summarizes the existing metal finishing regulations, the state of the
industry in 1983 when the ELGs were promulgated, and a general discussion of
changes to the industry since promulgation.
• Section 3 describes activities that EPA plans to conduct as part of the preliminary
study and the current status of these activities.
• Section 4 details EPA's study findings (to date) in five general categories:
industry profile changes, advancements in process technologies, advancements in
alternative chemistries, advancements in wastewater treatment technologies, and
existing regulatory issues for consideration.
• Section 5 presents EPA's next steps for the study.
• Section 6 summarizes EPA's quality assurance (QA) procedures for reviewing
existing information presented in this report.
• Section 7 is the list of references cited in the report.
1-2
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Section 2- Existing Metal Finishing ELGs
2. EXISTING METAL FINISHING ELGs
This section provides a brief history of the existing ELGs as background and context for
EPA's continued review of the metal finishing industry. Metal finishing involves changing the
surface of an object to improve its appearance and/or durability. Wastewater discharges from
metal finishing operations are regulated primarily under ELGs for the Electroplating Point
Source Category (40 CFR Part 413) and the Metal Finishing Point Source Category (40 CFR
Part 433). 40 CFR Part 413 includes pretreatment standards for existing sources (PSES) for the
Electroplating Category, and 40 CFR Part 433 include effluent limitations based on best
practicable control technology currently available (BPT) and best available technology
economically achievable (BAT), new source performance standards (NSPS), pretreatment
standards for new sources (PSNS), and PSES for the Metal Finishing Category.l
EPA promulgated PSES for the Electroplating Category in 1979; limitations and
standards for new sources and direct discharges were not established under this rule. The PSES
apply to facilities that perform one or more of six electroplating operations and that indirectly
discharge wastewater to surface water via publically owned treatment works (POTW). The
pretreatment standards differ for discharges less than 10,000 gallons per day (gpd) of wastewater
compared to discharges greater than 10,000 gpd of wastewater (U.S. EPA, 1979). The National
Association of Metal Finishers and the Institute of Interconnecting and Packaging Electronic
Circuits challenged the 1979 rule for the Electroplating Category. On March 7, 1980, EPA
entered into a settlement agreement with these two organizations, and agreed to amend the final
electroplating pretreatment standards. These amendments were implemented on January 28,
1981 (U.S. EPA, 1981). As a result of the agreement, EPA promulgated in 1983 a new regulation
for the Metal Finishing and Electroplating Categories which established BPT, BAT, NSPS,
PSNS, and PSES. Following these amendments, the applicability of the Electroplating Category
ELGs (40 CFR 413) is limited to facilities (both independent (job) platers and captive
operations) that apply metal coatings via electrodeposition, began operation before July 15,
1983, and discharge wastes to POTWs. All other facilities performing electroplating or metal
finishing operations are subject to regulations under the Metal Finishing Category (40 CFR Part
433).2
During the development of the MP&M rulemaking (40 CFR Part 438, promulgated in
2003), EPA evaluated all industries involved in the "manufacture, rebuild or maintenance of
metal parts, products, or machines," including facilities regulated under 40 CFR Parts 413 and
433; however, EPA did not revise the existing limitations and standards for Parts 413 and 433 in
the final MP&M rule. Therefore, the 1983 regulations continue to be the guidelines set for the
industry for the indirect and direct discharge of metal finishing wastewater.3
1 Discharges from facilities performing metal finishing operations may also be regulated under other ELGs (e.g.,
Aluminum Forming, Iron and Steel) that take precedence over the Metal Finishing ELGs.
2 Facilities include electroplaters discharging to surface water, electroplaters that began operation after July 15,
1983, and metal finishers that do not perform one or more of six electroplating operations.
3 EPA promulgated the Electroplating Point Source Category regulation in 1974 and amended it in 1977, 1979,
1981, and 1983. The 1983 amended rule continue to regulate facilities in the Electroplating Point Source Category
in operation prior to July 15, 1983 and that discharge wastes to POTW. Therefore, all other facilities operating on or
after that date that discharge wastewater (indirect and/or direct discharge) are subject to the Metal Finishing Point
Source Category.
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Section 2- Existing Metal Finishing ELGs
This remainder of section describes the existing 1983 Metal Finishing ELGs, a profile the
industry at the time of promulgation (section 2.1), metal finishing process operations (section
2.2), wastewater treatment of metal finishing wastewater (section 2.3), and the applicability of
the existing ELGs to current metal finishing operations (section 2.4).
2.1 Metal Finishing Industry Profile at Promulgation
The applicability of the Metal Finishing Category is defined by process operations rather
than by industry sectors; therefore, a facility subject to the Metal Finishing ELGs may belong to
one or more of a variety of metal processing and metal forming industry classifications. The
industries covered by the Metal Finishing ELGs perform one of 46 unit operations, discussed in
section 2.2 below, and are generally included in the following two-digit Standard Industrial
Classification (SIC) codes (U.S. EPA, 1983a):4
• 34: Fabricated Metal Products, Except Machinery and Transportation.
• 35: Machinery, Except Electrical.
• 36: Electrical and Electronic Machinery, Equipment and Supplies.
• 37: Transportation Equipment.
• 38: Measuring, Analyzing and Controlling Instruments: Photographic, Medical,
and Optical Goods; Watches and Clocks.
• 39: Miscellaneous Manufacturing Industries.
Metal finishing facilities are categorized as either captive facilities or job shops, which
EPA defined as follows (U.S. EPA, 1984):
• Captive facility. A facility that in a calendar year owns more than 50 percent (by
surface area) of the materials undergoing metal finishing. Captive facilities were
categorized as integrated or non-integrated to characterize the wastewater
discharges generated. Integrated facilities are those which, prior to treatment,
combine electroplating waste streams with significant process waste streams not
covered by the Electroplating Point Source Category. Non-integrated facilities are
those which have significant wastewater discharges only from operations
addressed by the Electroplating Point Source Category.
• Job shop. A facility that in a calendar year owns less than 50 percent (by surface
area) of the materials undergoing metal finishing. During development of the
regulation, approximately 97 percent of job shops were found to be non-
integrated.
4 Although facilities performing metal finishing operations generally fall under these SIC codes, not all facilities
under the codes are subject to the Metal Finishing ELGs. For example, the Metal Finishing ELGs are not
applicable to facilities that do not perform any of the six electroplating operations. Instead, these facilities may be
subject to other metal ELGs that take precedence over the Metal Finishing ELGs.
2-2
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Section 2- Existing Metal Finishing ELGs
At promulgation of the 1983 Metal Finishing ELGs, the Metal Finishing and
Electroplating Categories included a total of 13,470 facilities, consisting of 10,000 captive
facilities and 3,470 job shops and independent printed circuit board (IPCB) manufacturers5 (U.S.
EPA, 1984). The facilities varied greatly in size, age, number of employees, and number and
type of operations performed, ranging from small shops with less than 10 employees to large
facilities employing thousands of employees (U.S. EPA, 1983a).
During the Metal Finishing rulemaking development, EPA identified 10,561 out of
13,470 facilities (78 percent) that indirectly discharged to surface water via POTWs. These
facilities were evenly distributed between job shops, non-integrated captive facilities, and
integrated captive facilities. The remaining 2,909 facilities (22 percent) discharged directly to
surface water, with captive facilities (both integrated and non-integrated) predominantly
performing this practice (U.S. EPA, 1983a). The 1983 rule did not capture the number of
facilities in the industry that reused wastewater.
2.2 Metal Finishing Process Operations at Promulgation
Metal finishing is the process of changing the surface of an object by creating a thin layer
of metal or metal precipitate on its surface to impart the desired surface characteristics to the
final product, such as corrosion resistance, wear resistance, and hardness. The operations
performed and the sequence of operations at a metal finishing facility can vary and depend on
numerous factors (e.g., raw materials used, industry sector, product specifications), and may
generate significant volumes of wastewater (U.S. EPA, 2000a).
The Metal Finishing ELGs regulate wastewater discharges from six primary metal
finishing operations. Additionally, at facilities where at least one of these primary operations is
being conducted, the ELGs also cover wastewater discharges resulting from 40 additional metal
finishing operations. If a facility does not perform any of the six primary metal finishing
operations, it is not subject to the Metal Finishing ELGs (U.S. EPA, 1984). The six primary
operations and associated waste streams are described below (U.S. EPA, 1983a).
• Electroplating. The application of a thin surface coating of one metal upon
another by electrodeposition. This surface coating is applied to provide corrosion
protection, wear or erosion resistance, or anti-frictional characteristics, or for
decorative purposes. Cathodic surfaces are plated by reducing metal ions in either
acid, alkaline, or neutral solutions. Metal ions in the plating solution are
replenished by the dissolution of metal from anodes, small pieces contained in
inert wire or metal baskets, or metal salts. Hundreds of different electroplating
solutions have been adopted commercially, but only two or three types are used
widely for a specific metal or alloy. Electroplating baths contain metal salts,
acids, alkalines, and various bath control compounds which contribute to the
wastewater stream through dragout, batch dumping, or floor spills. The waste
5 Indirect discharging job shops and IPCB manufacturers that existed under Part 413 continued to comply with Part
413 Electroplating Pretreatment Standards and were exempt from the Part 433 Metal Finishing ELGs (U.S. EPA,
1984).
2-3
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Section 2- Existing Metal Finishing ELGs
from the electroplating process can include common metals, precious metals,
chromium (hexavalent), and cyanide.
Electrolessplating. A chemical reduction process that depends on the catalytic
reduction of a metallic ion in an aqueous solution containing a reducing agent and
the subsequent deposition of metal without the use of external electrical energy.
Electroless plating has several advantages over electroplating. It provides a
uniform plating thickness on all areas of a part, and a dense and virtually non-
porous plate on properly prepared surfaces. Copper and nickel electroless plating
are the most common. Electroless plating baths can contain precious metals,
complexed metals, and cyanide, which can enter the wastewater stream through
dragout or batch dumping of process baths. The basic ingredients in an electroless
plating solution are:
— A source of metal, usually a salt.
— A reducer, to reduce the metal to its base state.
— A complexing agent, to hold the metal in solution.
— Various buffers and other chemicals designed to maintain bath stability
and increase bath life.
Anodizing. An electrolytic oxidation process that converts the surface of the metal
to an insoluble oxide. These oxide coatings provide corrosion protection,
decorative surfaces, a base for painting and other coatings, and special electrical
and mechanical properties. Aluminum is the most frequently anodized material,
while some magnesium and limited amounts of zinc and titanium are also treated.
For aluminum parts, the formation of oxide occurs when the parts are made
anodic in dilute sulfuric acid or dilute chromic acid solutions. The oxide layer
begins formation at the extreme outer surface, and as the reaction proceeds, the
oxide grows into the metal. The last-formed oxide, the boundary layer, is at the
interface between the base metal and the oxide. The boundary layer is extremely
thin and nonporous. The wastewater from anodizing processes can contain the
base material being anodized (typically aluminum or magnesium) and
constituents of the processing baths (such as sulfuric or chromic acid). It is also
common to dye or color anodized coatings. Many of the dyes contain chromium
and other metals which can enter the wastewater stream.
Coating. The process of chromating, phosphating, metal coloring, and
passivating. These coatings are applied to previously deposited metal or basis
material (i.e., the materials onto which metal finishes are applied) for increased
corrosion protection, lubricity, and preparation of the surface for additional
coatings or formulation of a special surface appearance. In chromating, a portion
of the base metal is converted to a component of the film by reaction with
aqueous solutions containing hexavalent chromium and active organic or
inorganic compounds. Most coatings are applied by chemical immersion although
a spray or brush treatment can be used.
Phosphate coatings are used to provide a good base for paints and other organic
coatings, to condition the surfaces for cold forming operations by providing a
2-4
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Section 2- Existing Metal Finishing ELGs
base for drawing compounds and lubricants, and to provide corrosion resistance to
the metal surface (by the coating itself or by providing a suitable base for rust-
preventative oils or waxes). Phosphate conversion coatings are formed by the
immersion of iron, steel, or zinc-plated steel in a dilute solution of phosphoric
acid plus other reagents. Metal coloring by chemical conversion converts the
metal surface into an oxide or similar metallic compound, producing a variety of
decorative finishes. Passivation refers to forming a protective film on metals by
immersion in an acid solution. Numerous pollutants may enter the wastewater
through coating operations by rinsing and batch dumping of process baths. The
baths usually contain metal salts, acids, bases, and dissolved basis materials and
various additives.
• Chemical Etching and Milling. Methods of producing specific design
configurations and tolerances on metal parts by controlled dissolution with
chemical reagents or etchants. This classification includes chemical milling,
chemical etching, and bright dipping. Chemical etching is the same process as
chemical milling, but with much lower rates and depths of metal removal. Typical
solutions for etching and chemical milling include ferric chloride, nitric acid,
ammonium persulfate, chromic acid, cupric chloride, hydrochloric acid, and
combinations of these reagents. Bright dipping is a specialized form of etching,
used to remove oxide and tarnish from ferrous and nonferrous materials, and can
produce a range of surface appearances from bright clean to brilliant. This unit
operation also includes the stripping of metallic coatings. The wastewater from
etching and chemical milling operations mainly contains dissolved basis
materials, such as stainless steel, aluminum, and copper. Zinc and cadmium,
frequently subjected to bright dipping, may also be present in wastewater.
• Printed Circuit Board Manufacturing. The formation of a circuit pattern of
conductive metal (usually copper) on nonconductive board materials such as
plastic or glass. It usually involves cleaning and surface preparation, catalyst and
electroless plating, pattern printing and masking, electroplating, and etching.
There are three main production methods for printed circuit boards: additive,
which uses pre-sensitized, unclad material as the starting board; semi-additive,
which uses unclad, unsensitized material as the starting board; and subtractive,
which begins with copper clad, unsensitized material. Wastewater is generated in
the manufacturing of printed circuit boards primarily from rinsing and cleaning
during surface preparation, electroless plating, pattern plating, etching, tab
plating, and immersion plating. Additionally, rinsing away spills, air scrubbing
water, equipment washing, and dumping spent process solutions can contribute to
the wastewater. The most common constituents of the waste streams are
suspended solids, copper, fluorides, phosphorus, tin, palladium, and chelating
agents.
As stated above, if any of the six core electroplating operations is present at a facility, the
Metal Finishing ELGs also apply to wastewater discharges from 40 additional unit operations
(listed in Table 2-1). Appendix A further describes the 40 additional metal finishing operations.
2-5
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Section 2- Existing Metal Finishing ELGs
Table 2-1. Unit Operations Regulated by ELGs for the Metal Finishing Category
Six Primary Operations
40 Additional Metal Finishing Unit Operations
Electroplating
Electroless plating
Anodizing
Coating
Etching and chemical milling
Printed circuit board manufacturine
Cleaning
Machining
Grinding
Polishing
Barrel finishing
Burnishing
Impact deformation
Pressure deformation
Shearing
Heat treating
Thermal cutting
Welding
Brazing
Soldering
Flame spraying
Sand blasting
Abrasive jet machining
Electrical discharge machining
Electrochemical machining
Electron beam machining
Laser beam machining
Plasma arch machining
Ultrasonic machining
Sintering
Laminating
Hot dip coating
Sputtering
Vapor plating
Thermal infusion
Salt bath descaling
Solvent degreasing
Paint stripping
Painting
Electrostatic painting
Electropainting
Vacuum metalizing
Assembly
Calibration
Testing
Mechanical plating
Source: 40 CFRPart 433.
As discussed in supporting documentation for the Metal Finishing ELGs, metal finishing
operations usually begin with raw stock materials (rods, bars, sheets, castings, forgings, etc.)
which can progress through the simplest or most sophisticated surface finishing operations.
Production facilities vary in size and processes, and are custom-tailored to the specific needs of
each individual plant. Figure 2-1 illustrates the variation in the number of unit operations that can
be performed in facilities within the metal finishing industry, depending upon the complexity of
the product. A complex product could require the use of nearly all unit operations, while a simple
product might require only a single operation (U.S. EPA, 1983a). The Metal Finishing ELGs
would apply to wastewater discharges from the complex product in Figure 2-1 because at least
one of the six core electroplating operations is taking place; they would not apply to wastewater
discharges from the simple product as shown, because none of the six primary operations are
performed.
Many different raw materials are used by facilities in the Metal Finishing Category.
During the development of the 1983 Metal Finishing ELGs, the basis materials were almost
exclusively metals which range from common copper and steel to extremely expensive high
grade alloys and precious metals, but may also include glass, plastic, and other non-conductive
materials. The raw materials used in metal finishing unit operations to coat these basis materials
can contain acids, bases, cyanide, metals, complexing agents, organic additives, oils, and
detergents. All of the basis materials and raw materials used in metal finishing can potentially
enter wastewater streams during production and subsequently be discharged as metal finishing
wastewater (U.S. EPA, 1983a).
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Section 2- Existing Metal Finishing ELGs
COMPLEX PRODUCT
Riming —- tlectiopUting —; Riming
Adapted from (U.S. EPA, 1983a)
Figure 2-1. Metal Finishing Process Application
2.3 Treatment of Metal Finishing Wastewater at Promulgation
As described in Section 2.2, Metal Finishing ELGs apply to wastewater discharges from
the six primary electroplating operations, including any discharges from the additional metal
finishing unit operations listed in Table 2-1. At promulgation, 78 percent of facilities indirectly
discharged metal finishing wastewater to receiving water via POTWs and 22 percent directly
discharged to surface waters (U.S. EPA, 1984). The Metal Finishing ELGs established one set of
concentration-based discharge limits that apply across a single subpart (Subpart A: Metal
Finishing), summarized in Table 2-2. Direct dischargers comply with BPT/BAT discharge
limitations and NSPS, whereas indirect dischargers comply with PSES and PSNS. As the table
shows, the limitations and standards are the same for new and existing sources of metal finishing
wastewater discharges, except for cadmium, which has a lower NSPS and PSNS discharge
standard (U.S. EPA, 1983a).
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Section 2- Existing Metal Finishing ELGs
Table 2-2. Regulated Pollutants and ELG Limits for the Metal Finishing Category
Process Operations Covered
See Table 2-1, for the list of 46 unit operations3
For industrial facilities with cyanide treatment, and
upon agreement between a source subject to those
limits and the pollution control authority, the
following amenable cyanide limit may apply in
place of the total cyanide limit.
Pollutant
Cadmium (T)
Chromium (T)
Copper (T)
Lead (T)
Nickel (T)
Silver (T)
Zinc (T)
Cyanide (T)b
Total Toxic
Organics
(TTO)C
Oil and Greased
Total
Suspended
Solids (TSS)d
pHd
Cyanide
amenable to
alkaline
chlorination
BPT/BAT/PSES
Daily Max
(Monthly Average)
(mg/L)
0.69 (0.26)
2.77(1.71)
3.38 (2.07)
0.69 (0.43)
3.98(2.38)
0.43 (0.24)
2.61 (1.48)
1.20 (0.65)
2.13
52 (26)
60(31)
Within 6.0 to 9.0
0.86 (0.32)
NSPS/PSNS
Daily Max
(Monthly Average)
(mg/L)
0.11(0.07)
2.77(1.71)
3.38 (2.07)
0.69 (0.43)
3.98 (2.38)
0.43 (0.24)
2.61 (1.48)
1.20 (0.65)
2.13
52 (26)
60(31)
Within 6.0 to 9.0
0.86 (0.32)
Source: 40 CFRPart 433.
a The provisions of this subpart apply to discharges from six electroplating operations on any basis material:
electroplating, electroless plating, anodizing, coating (chromating, phosphating, and coloring), chemical etching
and milling, and printed circuit board manufacturing. If any of these six operations are present, the provisions of
this subpart also apply to discharges from 40 additional metal finishing operations, listed in Table 2-1. These
limits do not apply to (1) metallic platemaking and gravure cylinder preparation conducted within or for
printing and publishing facilities or (2) existing indirect discharging job shops and independent printed circuit
board manufacturers, which are covered by 40 CFR Part 413.
b Anti-dilution provisions are stipulated in 40 CFR Part 433, which require self-monitoring for cyanide after
cyanide treatment and before dilution with other waste streams. In general, the practice of diluting rinse water
as a partial or total substitute for adequate treatment to achieve compliance with discharge limits is in violation
of the National pretreatment standards: Categorical standards (40 CFR Part 403.6(d)).
0 No monthly average TTO limitation.
d Parameter is regulated for BPT and NSPS only.
EPA based BPT, BAT, and PSES on the treatment of metal finishing wastewater using
hydroxide precipitation, clarification, and sludge dewatering for common metals treatment, with
pretreatment steps for chromium reduction, cyanide oxidation, complexed metals removal, and
oil and grease removal, where the wastewater contains these components. This wastewater
treatment technology is depicted in Figure 2-2 (U.S. EPA, 1983a).
EPA based NSPS and PSNS on the BPT/BAT/PSES technology, adding in-process
treatment modifications for controlling the discharge of cadmium, as illustrated in Figure 2-3.
The modifications for controlling cadmium employ evaporative recovery or ion exchange on
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Section 2- Existing Metal Finishing ELGs
cadmium-bearing wastewater before it mixes with other wastewater (U.S. EPA, 1983 a). The
following subsections discuss in detail the technology basis for BPT/BAT/PSES and
NSPS/PSNS.
Oily Raw Waste Raw Waste Raw Waste Raw
J 1 r-, 1
Emu sion Precioi
r Breaking Metal
Oil
V\
C
'Th$ cSsrifi®,* step tnyolvQs a Cff^vstv s&Win
achieved with a sedim&niatton -basin or ctr
js j Cyanide
§ j Oxidation
ry |
1 Common
~ Metals
ithout
Waste Raw Waste
Chemical
Precipitation
Cfar
\
Treated
Effluent
j process fftar car, be
ojlar c ariher.
Chromium
Reduction
Raw\
Completed
Metals
Lime
jge Slu
,
Sludge
Dewatering
1
Contractor
Removal
iVaste Tc
^ Chemical
Precipitation
dQe "Ion
fier*
I
Treated
Effluent
>xic Organics
Hau ed or
Reclaimed
Source: (U.S. EPA, 1983a)
Figure 2-2. BPT/BAT/PSES Technology Basis
Oily Raw Waste Raw Waste Raw Waste Raw
1 J -, 1
Skimmed Oils gmu s
Break
on Precious Cyanide
ng Metals Oxidation
. j
Waste Raw Waste Raw Waste Raw
I Cadmium
Chromium
Reduction
Common
Metals
Evaporative
Recovery or
Ion Exchange
Complexed
Metals
| Solid Waste
^ Chemical
Precip tation
Treated Clarifier
Pffhipnt
* Lime
Sludge ^ Rlyrtnf»
*The darifier step involves a gravity settling process that can be
achieved with a sedimentation basin or circular darifier.
Dewateri
Waste Toxic Organics
Hauled or
Reclaimed
Lime — » Chemical
Precipitation
Clarified Treated
Sludge
19
1
Contractof
Removal
Source: (U.S. EPA, 1983a)
Figure 2-3. NSPS/PSNS Technology Basis
(Equivalent to BPT/BAT/PSES with Additional Cadmium Reduction)
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Section 2- Existing Metal Finishing ELGs
2.3.1 Common Metals Treatment
BAT is based on BPT of metal finishing wastewater, which reflects treatment and control
practices at metal finishing plants of various sizes, ages, and manufacturing processes. Pollutant
control focuses on end-of-pipe treatment rather than process changes or internal controls, except
where such are common industry practice. The control technologies for treatment of common
metals include hydroxide precipitation, clarification, and sludge dewatering, discussed in the
subsections below. Treatment modifications for controlling the discharges of cadmium (as
required for NSPS/PSNS), including evaporative recovery and ion exchange, are also discussed.
2.3.1.1 Hydroxide Precipitation
Hydroxide precipitation is used to remove dissolved metals and phosphates from metal
finishing wastewater by converting the dissolved pollutants into solid form (precipitates) and
coagulating suspended precipitates into larger, faster settling particles. Precipitation is achieved
by adding lime, caustic, sodium carbonate, or acid to reach a favorable pH (typically 8.8 - 9.3).
Any recovery of precious metals, reduction of hexavalent chromium, removal of oily wastes, or
destruction of cyanide must be performed before metals and phosphates are removed via
hydroxide precipitation.
2.3.1.2 Clarification
Following precipitation, the waste streams flow through a clarifier, where solids are
removed by gravity. Clarifiers are used for sedimentation to reduce space requirements and
retention time, making solids removal more efficient. Coagulants or flocculants are typically
added to the waste stream to enhance solids settling (U.S. EPA, 1983a).
2.3.1.3 Sludge Dewatering
Precipitation and clarification generate large quantities of sludge requiring disposal.
These sludges are dewatered prior to disposal to reduce their volume. Sludge dewatering
techniques include gravity sludge thickening, pressure filtration, vacuum filtration,
centrifugation, and sludge bed drying. Once the sludge is dewatered, it is generally disposed of at
an onsite landfill or hauled away by a contractor to an off-site landfill or reclamation facility.
Other less common disposal options include chemical containment, encapsulation, fixation, and
thermal conversion, all of which require landfilling, but reduce the potential for groundwater
contamination (U.S. EPA, 1983a).
2.3.2 In-process Cadmium Reduction
In addition to the BPT technology of hydroxide precipitation followed by clarification
described above, NSPS/PSNS limitations require in-process treatment modifications for
controlling the discharge of cadmium. The in-process cadmium reduction technologies may
include ion exchange or evaporative recovery to provide near zero discharge of heavy metals
from the raw waste stream. Although both ion exchange and evaporation are used in treatment of
metal finishing wastewaters for removing a variety of precious metals from bath concentrates
and rinse waters, the technology basis for NSPS and PSNS specifically uses these techniques for
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Section 2- Existing Metal Finishing ELGs
removing of cadmium before mixing with other metal-bearing wastewater for common metals
treatment (U.S. EPA, 1983a).
The ion exchange process consists of ions, held by electrostatic forces to charged
functional groups on the surface of the ion exchange resin, exchanged for ions of similar charge
from the solution in which the resin is immersed. It is a sorption process because the exchange
occurs on the surface of the resin. The cadmium is adsorbed onto the resin and exchanged for the
harmless ions of the resin. Eventually, when the resin cannot exchange any more cadmium ions,
it must be regenerated. The ion exchange resin is regenerated with regenerant solution containing
hydrochloric or sulfuric acid. The regenerant flows through the ion exchange resin and replaces
each cadmium ion with one or more hydrogen ions. The used regenerant solution is then treated,
reused, and/or disposed.
Evaporation of water from a solution increases the concentration of solute in the
remaining solution. Evaporation techniques include atmospheric evaporation (boiling the liquid)
and vacuum evaporation (the evaporation pressure is lowered to cause the liquid to boil at
reduced temperature). The vaporized water resulting from these processes humidifies the air in
the system and is either blown out of the system as hot air or collected, condensed, and reused as
waste process heat source for the system. The concentrate may be hauled off-site for recovery
and resale or further processed on site for recovery and reuse.
2.3.3 Treatment of Complexed Metals, Cyanide-Bearing and Chromium-Bearing
Wastewaters, and Wastewater Containing Oily Wastes
The following subsections discuss additional treatment requirements for facilities
generating complexed metals, cyanide-bearing, chromium-bearing, or oily wastewaters.
Complexed metals wastewater requires separate treatment using high pH chemical precipitation.
Cyanide-bearing, chromium-bearing, or oily wastewaters require pretreatment using cyanide
oxidation, chromium reduction, or emulsion breaking prior to common metals treatment. These
constituents in the wastewater may hinder hydroxide precipitation, clarification, and sludge
dewatering.
2.3.3.1 High pH Chemical Precipitation
High pH precipitation is particularly applicable to waste streams containing complexing
agents (or chelating agents), which hinder conventional precipitation. These agents are used
during metal finishing operations to maintain heavy metals in solution. The complexing agent
produces a stable composition of non-metal molecules or ions that are covalently bonded to
metal atoms or ions, which keeps the metal atoms or ions in solution during metal finishing
operations, but also hinders their precipitation during wastewater treatment. Waste streams
containing complexing agents (or complexed metals) are segregated and treated separately.
Precipitation of complexed metals is characteristically accomplished at a high pH (11.6 - 12.5)
to induce a shift in the complex dissociation equilibrium. This produces uncomplexed metal ions
which can then be precipitated out of solution by available hydroxide ions. The pH is adjusted by
the addition of chemicals such as calcium hydroxide, lime, calcium chloride, or calcium sulfate
(U.S. EPA, 1983a).
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Section 2- Existing Metal Finishing ELGs
2.3.3.2 Cyanide Oxidation
Waste streams containing cyanide are segregated for separate treatment prior to common
metals treatment. Cyanides are introduced through metal salts for plating and conversion
coatings, and plating and cleaning baths. Cyanide is generally destroyed by oxidation, and
chlorine is typically used as the oxidizing agent. If the cyanide is not removed before further
treatment, it will prevent efficient removal of metals during common metals treatment (U.S.
EPA, 1983 a).
2.3.3.3 Chromium Reduction
Waste streams containing hexavalent chromium are segregated for separate treatment
prior to common metals treatment. Hexavalent chromium-containing wastewaters are generated
in several ways including chromium electroplating and chromate conversion coatings. Chemical
reduction requires the addition of a chemical such as sulfur dioxide, sodium bisulfite, sodium
metabisulfite, and ferrous sulfate, which form strong reducing agents in aqueous solutions that
reduce hexavalent chromium to trivalent chromium. Trivalent chromium can then be removed
from wastewater using precipitation in common metals treatment (U.S. EPA, 1983a).
2.3.3.4 Emulsion Breaking
Metal finishing wastewater may contain oily wastes from process coolants and lubricants,
wastes from cleaning operations, wastes from painting processes, and machinery lubricants. If
oily wastes are generated, they should be separated and pretreated to remove the oils before
commingling for common metals treatment. Emulsion breaking removes emulsified oils from
oil/water mixtures. Chemicals such as acids, salts, or polymers are added to the wastewater and
agitated to break the oil/water emulsion bond. The oily residue rises to the surface where it is
skimmed off or decanted from the remaining wastewater. The skimmed oily residue is typically
stored in tanks for further processing or removal by a contractor, and the remaining wastewater is
sent to common metals treatment (U.S. EPA, 1983a).
2.3.4 Control and Treatment of Toxic Organics
The Metal Finishing ELGs also establish discharge limitations and pretreatment standards
on TTO. EPA defines TTO as the sum of the masses or concentrations of a specific list toxic
organic compounds detected exist in the industrial user's process discharge at a concentration
higher than 0.01 mg/L.6 The primary source of TTO is from waste solvents. Although TTO may
be present in combined wastewater and concentrated oily wastes generated during metal
finishing operations, EPA determined that TTO concentrations from these sources were small
and generally treatable by the technology basis for common metals treatment and the treatment
of oily wastes (see Sections 2.3.1 and 2.3.3). The established TTO limit in the Metal Finishing
ELGs was intended to prevent the dumping of concentrated toxic organic wastes from waste
solvents, such as solvent degreasers and paint strippers. In addition to the TTO limit, EPA also
established monitoring requirements for TTO in the Metal Finishing ELGs (U.S. EPA, 1983a,
1983b).
'As listed at 40CFR433.il.
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Section 2- Existing Metal Finishing ELGs
2.3.4.1 Monitoring Requirements
Facilities choosing to measure ongoing compliance with the TTO limit via self-
monitoring need to report the results in the baseline monitoring report (BMR) and 90-day
compliance report, which are submitted to the Control Authority.7 In lieu of self-monitoring for
TTO, facilities can choose the certification alternative discussed in Section 2.3.4.2. Subsequent
self-monitoring for TTO is required following the guidelines outlined in the General
Pretreatment Regulations (40 CFR Part 403) for indirect dischargers and National Pollutant
Discharge Elimination System (NPDES) permitting requirements for direct dischargers. If self-
monitoring is required to measure compliance with the limit, the facility needs to report
analytical data only those for pollutants reasonably expected to be present in the wastewater
(U.S. EPA, 1983b, 1985). Ongoing TTO monitoring is generally performed by those facilities
that continue to dump waste solvents in the wastewater as a disposal practice. As indicated in the
1983 Metal Finishing ELGs, EPA anticipated very few facilities with these practices because
waste solvents have sufficient reclaim value to be recovered, and thus, EPA anticipated few
facilities that would need to conduct periodic self-monitoring to comply with the regulations
(U.S. EPA, 1983a).
2.3.4.2 Toxic Organic Management Plan and Certification Process
As an alternative option to self-monitoring, a facility may be allowed to comply with the
requirements through the certification process, which requires that the discharger submit a Toxic
Organic Management Plan (TOMP).8 Indirect dischargers certify in their semi-annual
compliance reports to the control authority that they are implementing the TOMP and are not
dumping toxic organics into the wastewater since the previous filing. For direct dischargers, the
TOMP is incorporated as a condition of the NPDES permits and is reported in the comment
section of the Discharge Monitoring Reports (DMRs) (U.S. EPA, 1983b, 1985).
To determine whether a TOMP is a feasible alternative to TTO monitoring, the facility is
required to complete a process engineering analysis to identify potential sources of TTO and then
evaluate pollution control options to reduce or eliminate TTO in wastewater discharge. The
facility may decide whether a TOMP is a feasible alternative to TTO monitoring after weighing
the costs for implementing the pollutant control options with those for monitoring. If the TOMP
is feasible, the facility can make a request to the control or permitting authority to implement the
TOMP and certification process in lieu of the monitoring requirements. Specifically, the TOMP
must generally include (U.S. EPA, 1985):
• Identification of all the toxic organic compounds used by the facility.9
• Method of disposal for all wastes associated with TTO (e.g., reclamation,
incineration, and/or contract hauling).
7 The Control Authority is considered the POTW, if it has an approved pretreatment program. Otherwise, the reports
are submitted to the state, if it has an approved state pretreatment program, or to the EPA Region.
8 Also referred to in the Metal Finishing ELGs as the solvent management plan.
9 As defined in 40 CFR 433.12.
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Section 2- Existing Metal Finishing ELGs
• Procedures followed by facility to assure that toxic organic compounds do not
routinely spill or leak into any process wastestream that may be discharged.
The TOMP is submitted to the facility's control or permitting authority with baseline
monitoring report to meet the requirements of the certification process (U.S. EPA, 1985).
2.4 Applicability of the Existing ELGs to Current Operations
The Metal Finishing ELGs reflect facilities, process operations, and wastewater
management practices in 1983. Over the ensuing 32 years, process technologies and chemistries
of metal finishing operations have evolved, and the industry has implemented more advanced
treatment technologies than those described in Section 2.3. Additionally, many Metal Finishing
facilities have developed waste minimization techniques leading to zero discharges or to the
recovery/reuse of metal finishing wastewater. With these changes, stakeholders have raised
questions regarding the applicability of certain operations to the Metal Finishing ELGs. These
advances are discussed further in the following subsections.
2.4.1 Changes to Industry Profile
There is uncertainty regarding how the industry profile has changed since 1983. Based on
a 2008 National Center for Manufacturing Sciences review of the surface finishing industry
(including metal finishing) and additional discussions with industry representatives, EPA
believes the industry has moved towards a more global market over the years. In addition, some
U.S. firms may have more recently attempted to concentrate the industry (i.e., incorporate the
smaller job shops into larger companies) to achieve economies of scale, expand niche markets,
and provide a larger range of finishing services in a global market. Many firms may have also
shifted surfacing operations to non-U.S. locations (such as Asia, India, Mexico, Canada, and
Europe) to further reduce costs. Outsourcing metal finishing operations has generally been more
cost effective than operating captive metal finishing facilities; therefore, EPA anticipates a shift
in the number of captive facilities versus job shops as well as a decrease in the number of
domestic job shops (ERG, 2016; U.S. EPA, 2015a).
2.4.2 Scope of Metal Finishing Operations
EPA has received comments from the Association of Clean Water Administrators
(ACWA) urging it to consider the applicability of the Metal Finishing ELGs to current metal
finishing operations (U.S. EPA, 2015c). Specifically, stakeholders identified a need for clarifying
descriptions of metal finishing operations listed in the ELGs, including:
• Guidance to distinguish between metal finishing operations in which the same
acid is used for different functions, such as etching and chemical milling, acid
cleaning, chemical conversion coating, and similar cases.
• Clarification of how the Metal Finishing ELGs apply to current industry practices,
including metal finishing processes or chemical alternatives that are not
specifically identified in the ELGs.
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Section 2- Existing Metal Finishing ELGs
• Clarification of how the Metal Finishing ELGs apply to newer manufacturing
operations that use metal finishing processes not identified in the ELGs, such as
solar panel manufacturing and cell phone manufacturing.
2.4.3 Process Technology Modifications and Alternative Chemistries
Since 1983, numerous modifications and process alternatives have been developed for
processes conducted at metal finishing facilities such as cleaning and rinsing operations, coating
and plating processes, and polishing. These modifications may impact the overall wastewater
volume generated (and subsequently treated) as well as the general wastewater characteristics of
metal finishing wastewater. Section 4.2 of this report further discusses updates to metal finishing
process technologies. Additionally, alternatives to traditional chemistries used in the metal
finishing operations have been developed, primarily alternatives to chemistries using cadmium,
hexavalent chromium, phosphate, and cyanide. See Section 4.3 of this report for further
discussion on alternative chemistries.
2.4.4 Technological Advances in Wastewater Treatment
At promulgation, it was evident that other treatment technologies, while not widespread
in the industry, did exist and could be used to meet the discharge limitations. Since then,
advanced treatment technologies and zero-discharge or reuse practices have emerged within the
industry for the treatment and/or recycling of metal finishing wastewater.
Based on more recent observations from the regional EPA pretreatment coordinators and
industry sources, emerging technologies are being used to some extent, but are still not
widespread within the industry (U.S. EPA, 2015a). Some improvements are being applied to the
technologies identified in the 1983 rule, including new chemical additives for improved
precipitation and sludge generation, and alternative filtration techniques to replace or supplement
clarification. Section 4.4 of this report further discusses treatment technologies that have
emerged in the industry.
Waste minimization technologies for reuse and zero discharge strategies have a small
footprint within the metal finishing industry. Vendors have described potential application of
closed-loop processes for metal finishing operations to collect, treat, and return process
wastewater for reuse (U.S. EPA, 2015a). Section 4.2 of this report further describes waste
minimization technologies and practices available to reduce the volume of wastewater
discharged from metal finishing operations and to recover other process waste streams to be
reused in the process.
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Section 3- Study Activities and Progress
3. STUDY ACTIVITIES AND PROGRESS
This study seeks to answer the key questions described in Section 1. Historically, EPA
has used various approaches for collecting information to further inform Agency actions and
decisions related to effluent guidelines development and revision for select industries. Typically,
a preliminary study profiles an industry category, gathers information about the hazards posed by
its wastewater discharges, collects information about availability and cost of treatment and
pollution prevention technologies, assesses the financial status of the facilities in the category,
and investigates other factors to determine whether revisions to the current effluent guidelines
are warranted. As the study evolves, EPA may decide to conduct a more detailed study, which is
a more rigorous examination of the industry and its practices, and may include primary data
collection activities (such as industry questionnaires and wastewater sampling and analysis) to
fill data gaps (U.S. EPA, 2015a).
As part of this study, EPA is evaluating facilities that make up the metal finishing
industry, their size, and the major markets they serve; the types of metal finishing operations and
alternative chemistries used that may potentially introduce new pollutants of concern that are not
currently regulated; new sources of metal finishing wastewater that contribute to the overall
wastewater characteristics, such as wet air pollution controls or new operations in metal
finishing; and advanced technologies that facilities employ for the treatment and subsequent
recycle or discharge of metal finishing wastewater. The study initially focuses on examining
existing information that EPA collected or will collect through literature reviews, technical
conferences, and discussions with industry experts and stakeholders. EPA also plans to conduct
site visits at metal finishing facilities, analyze current available discharge data, and review
information collected on the industry during the MP&M Rulemaking development. This section
describes these activities planned for the preliminary study of the Metal Finishing Category and
the current status of those planned activities. Section 4 of this report further details EPA's
preliminary findings, which will lay the foundation for EPA's decisions on how to proceed with
the study.
3.1 Literature Review
As an initial step in answering the key study questions, EPA collected and reviewed
technical literature about the metal finishing industry. Using a defined list of key words, listed in
Appendix B, EPA identified technical papers and reports, studies, peer-reviewed journal articles,
and industry publications on metal finishing operations and wastewater management. EPA used
several research collections and search engines10 to identify over 130 documents that were
subject to EPA's quality assurance standards and procedures for inclusion in a literature review
(these standards are described in Section 6 of this report). EPA categorized the documents into
three topic areas:
10 Research collections and search engines used include Kirk-Othmer Encyclopedia of Chemical Technology,
Google Scholar, Dialog, The Strategic Environmental Research and Development Program (SERDP)-Environmental
Security Technology Certification Program (ESTCP), Rowan Technology Group, Science Direct, ACS Publications,
Academic Search, Directory of Open Access Journals, and PLOS One.
3-1
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Section 3- Study Activities and Progress
• Process technology documents, addressing improvements in metal finishing
process techniques and technologies that lower air emissions, reduce chemical
use, or decrease wastewater pollutant discharges.
• Alternative chemistries documents discuss innovative metal finishing chemistries
that can be used to reduce toxicity, wastewater volume, and pollutant discharges.
• Wastewater treatment technology documents discuss innovative technologies to
reduce wastewater pollutant concentrations and strategies to achieve zero liquid
discharge at metal finishing facilities.
A significant portion of the preliminary findings of this study to date (presented in
Section 4 of this report) is based on the data and information gathered from the literature review.
3.2 Technical Conferences
EPA attended the Water Environment Federation's Annual Technical Exhibition and
Conference (WEFTEC) on September 26-30, 2015. WEFTEC provides water quality education
and training by offering technical sessions and workshops on a variety of topics and provides
access to information from exhibitors on water management technologies and services. EPA
attended presentations and obtained papers from proceedings relevant to the general categories
listed in Section 3.1. EPA plans to review publications from WEFTEC regarding metal finishing
wastewater management practices, to support the preliminary study of the industry.
EPA also attended the Engineers' Society of Western Pennsylvania's International Water
Conference (IWC) on November 15-19, 2015. IWC discusses the most recent scientific advances
and practical applications for treatment, use, and reuse of water for engineering purposes,
industry or otherwise. Presenters and attendees of the IWC include researchers, practicing
engineers, managers, educators, suppliers, contractors, government workers, and end users. EPA
plans on reviewing IWC presentations and papers relevant to the metal finishing industry in
support of the preliminary study.
EPA plans to continue seeking other technical conferences to further inform the Agency
on current industry practices.
3.3 Industry Experts
EPA contacted a variety of experts to improve its understanding of the metal finishing
industry and to gain different perspectives on the 1983 regulations and current industry
operations.
3.3.1 Metal Finishing Facilities and Technology Vendors
EPA contacted personnel from metal finishing facilities and technology vendors
advertising zero discharge systems for the treatment of metal finishing wastewater (ERG, 2016).
EPA identified and selected facilities and vendors based on: recommendations by other industry
experts and trade groups; directories and/or marketing databases; information from previous
EPA data collection efforts; and other publicly available information on metal finishing
operations and wastewater treatment technologies of interest to the study. Information collected
3-2
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Section 3- Study Activities and Progress
from vendor and facility contacts fulfilled several objectives of the preliminary study: 1) to
obtain information that helps answer key study questions; 2) to identify potential candidate
facilities for future EPA site visits; 3) to obtain industry perspectives on the existing 1983 Metal
Finishing ELGs; and 4) to initiate assessment of the technical and economic feasibility of waste
minimization practices. EPA plans to continue calling additional metal finishing facilities and
technology vendors to pursue these objectives further.
3.3.2 Pretreatment Coordinators
As discussed in Section 2, the majority of metal finishing facilities are indirect
dischargers subject to pretreatment standards under the Metal Finishing ELGs. As part of the
Metal Finishing Category Review under EPA's annual review process (CWA §304m), EPA had
discussions with the federal pretreatment coordinator and regional pretreatment coordinators who
have direct experience with metal finishing wastewater issues at POTWs. These personnel
provided insight on the applicability of the Metal Finishing ELGs. The coordinators described
key issues encountered by POTWs receiving metal finishing wastewater, primarily from new or
modified metal finishing processes. Issues included misapplication of limits in permit
applications, applicability of the 46 metal finishing unit operations, and new source criteria
development (U.S. EPA, 2015a). As part of the preliminary study, EPA will continue to have
discussions with the coordinators to identify metal finishing scenarios for which the applicability
of the regulations is unclear.
3.3.3 Other Stakeholders
EPA held meetings with the Association of Clean Water Administrators (ACWA) and
several pretreatment coordinators in November 2015 to gather different perspectives on the metal
finishing category (U.S. EPA, 2015d). EPA also plans to initiate discussions with other
organizations, such as the National Association of Clean Water Agencies (NACWA), to
understand their perspective on the implementation of the 1983 regulations.
3.4 Industry and Trade Organizations
In response to EPA's published plans to further review the Metal Finishing Category
under CWA §304m authority, the National Association for Surface Finishing (NASF) reached
out to EPA to support EPA's information gathering efforts for the preliminary study (U.S. EPA,
2015b, 2015c). NASF is a trade association representing the interests of the North American
surface finishing industry, including metal finishing. At NASF's invitation, EPA spoke at the
NASF Washington Forum on April 14-16, 2015, to discuss the Agency's plans to review the
metal finishing industry. EPA also met with NASF in August and November 2015 to discuss the
preliminary study of the Metal Finishing Category (NASF, 2015; U.S. EPA, 2015e, 2015f). EPA
and NASF will continue discussions on approaches for collecting information on the industry to
answer key study questions and to explore potential opportunities to collaborate with industry
and provide additional outreach activities.
3-3
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Section 3- Study Activities and Progress
3.5 Site Visits to Metal Finishing Facilities
To date, EPA has accompanied pretreatment coordinators on visits to the Bureau of
Printing and Engraving (Washington, D.C.), Bethesda (Maryland) Art Metal Works, and Metro
Plating and Polishing (Kensington, Maryland).
EPA will continue site visits to metal finishing facilities to observe operations and
wastewater management practices. EPA may also request information, such as historical data on
raw and treated wastewater samples, general process design, and typical operating conditions.
EPA may also obtain information on the wastewater treatment technologies and treatment
chemicals used on site. EPA will use information gathered through industry experts (as described
in Section 3.3) to identify potential site visit candidates for the coming year.
3.6 Metal Products and Machinery (MP&M) Rulemaking
EPA will continue to review supporting documentation from the MP&M proposed
rulemaking (proposed in 2000), which evaluated facilities covered under the Metal Finishing
ELGs in the 1980's and 1990's. As part of that proposed rulemaking, EPA extensively reviewed
the changes to the metal finishing industry, process and wastewater technology improvements
made by the industry, and metal finishing wastewater characteristics. EPA will evaluate any
significant changes to the industry since its review during the MP&M proposed rulemaking of
2000, which will further define the scope of this study.
3-4
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Section 4 - Preliminary Study Findings
4. PRELIMINARY STUDY FINDINGS
This section describes the information EPA gathered to date for the preliminary study of
the Metal Finishing Category, which will help answer the key study questions outlined in Section
1 of this report. EPA summarized the findings into five main categories that are presented in the
following subsections:
• Changes in the metal finishing industry profile (Section 4.1),
• Process technologies (Section 4.2),
• Alternative chemistries (Section 4.3),
• Wastewater treatment technologies (Section 4.4), and
• Applicability and other regulatory considerations (Section 4.5).
4.1 Changes in the Metal Finishing Industry Profile
As discussed in Section 2.1, at promulgation of the 1983 Metal Finishing ELGs, the
Metal Finishing and Electroplating Categories included a total of 13,470 facilities, consisting of
10,000 captive facilities and 3,470 job shops and IPCB manufacturers. The existing captive
facilities ultimately fell into the Metal Finishing Category (after the final compliance date) and
the 3,470 job shops and IPCB manufacturers remained in the Electroplating Category (U.S. EPA,
1984). EPA has not fully evaluated the changes to the number of facilities in the metal finishing
industry as part of this preliminary study. However, several EPA efforts have collected recent
information on this population.
In the MP&M proposed rulemaking (published in 2000), EPA estimated that
approximately 12,700 facilities performed metal finishing operations. EPA classified the
facilities into four subcategories: general metals, metal finishing job shops, non-chromium
anodizing, and printed wiring boards (U.S. EPA, 2000a). The general metals category may have
included facilities that did not conduct any of the six primary metal finishing operations that
define the applicability of the Metal Finishing ELGs and, therefore, the number of facilities may
be an overestimation of those covered under 40 CFR Part 433. These estimates were primarily
based on responses to industry surveys sent to MP&M facilities in 1989 and 1996 (U.S. EPA,
2000a).
In 2012, EPA revised the National Emission Standards for Hazardous Air Pollutant
(NESHAP) Emissions: Hard and Decorative Chromium Electroplating and Chromium
Anodizing Tanks (SubpartN).11 EPA estimated 1,339 existing U.S. facilities performing metal
finishing operations that involved chromium. Outside California, there were 188 large and 394
11 EPA also revised the NESHAP for Steel Pickling-HCl Process Facilities and Hydrochloric Acid Regeneration
Plants (Subpart CCC) in 2012; however, steel pickling facilities (estimated at 100 facilities) are subject to the Iron
and Steel Category (under 40 CFR Part 420) and therefore, would not be regulated under the Metal Finishing
Category (40 CFR Part 433).
4-1
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Section 4 - Preliminary Study Findings
small hard chromium electroplating facilities.12 The distribution of large and small facilities was
unknown in California, but there are approximately 70 additional hard chromium electroplating
facilities there. There are an estimated 170 chromium anodizing plants and the 517 decorative
chromium electroplating facilities in the U.S. (77 FR 58220). These estimates are a subset of the
facilities comprising the metal finishing industry but provide a more recent look at the population
of chromium electroplaters.
As part of EPA's 2014 Annual Effluent Guidelines Review, EPA searched for recent data
to determine the number of facilities that currently fall into the Metal Finishing Category. The
2007 Economic Census provides a general industry description for each North American
Industry Classification System (NAICS) code under which these facilities may fall; however, it
does not detail facility-specific process operations or wastewater discharge practices, which form
the basis for determining whether the Metal Finishing ELGs would apply to specific facilities. In
the 2011 Annual Effluent Guidelines Review, EPA identified 166,356 facilities included in the
2007 Economic Census for the 200 NAICS codes. However, this number includes establishments
that are distributors or sales facilities, not just manufacturers (U.S. EPA, 2012a). It may also
include facilities that do not conduct any of the six primary operations and thus, would not be
regulated under the Metal Finishing ELGs. In previous annual reviews, EPA has identified the
number of facilities submitting discharge monitoring reports (DMRs) and reporting to EPA's
Toxic Release Inventory (TRI). However, EPA determined that these data sources include only a
fraction of the facilities that would fall under the Metal Finishing Category ELGs, due to the
limitations of the data sets. Therefore, these data sources do not adequately provide a complete
picture of the metal finishing industry (U.S. EPA, 2015a).
After discussions with regional and state pretreatment coordinators, EPA learned that
some EPA regions and states have maintained lists of industrial users that discharge metal
finishing wastewater to POTWs and are subject to pretreatment standards under 40 CFR Part
433; however, a national inventory of metal finishing facilities does not exist (U.S. EPA, 2015a).
As discussed in Section 2.4.1, there is uncertainty in how the metal finishing industry profile has
changed. EPA suspects that the industry has trended toward a wider market since 1983, with
market share dispersed among many
companies. Recent discussions with metal
finishing facilities also suggest some growth
of the U.S. metal finishing industry due to
, , , ,-, , , • . • finishing facilities conducting any of the six primary
increased product quality demands in certain operati(fns: electroplating, electrons plating
markets that cannot be met elsewhere (ERG,
2016).
4.2 Process Technologies
EPA identified papers reporting
advances in metal finishing process
Six Primary Metal Finishing Operations
40 CFR Part 433 applies to discharges from metal
anodizing, coating (chromating, phosphating, and
coloring), chemical etching and milling, and printed
circuit board manufacturing. If any of these six
operations are conducted at a facility, then the
provisions also apply to any of the 40 ancillary
processes (which include cleaning and polishing)
listed in 40 CFR Part 433, if any are also being
conducted at the facility.
12 As defined in the NESHAP, large hard chromium electroplating facilities are any facility with a cumulative
annual rectifier capacity equal to or greater than 60 million ampere-hours per year (amp-hr/yr). Small hard
chromium electroplating facilities are defined as any facility with a cumulative annual rectifier capacity less than 60
million amp-hr/yr.
4-2
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Section 4 - Preliminary Study Findings
techniques and technologies. The papers cover cleaning and rinsing operations, coating and
plating processes, and polishing. Numerous modifications and process alternatives have been
developed for these processes since the 1983 Metal Finishing ELGs were promulgated. The
following subsections discuss the technological advancements in each process category.
4.2.1 Cleaning and Rinsing Operations
Most metal finishing operations require surface preparation including cleaning and
rinsing. Cleaning and rinsing remove oil and dirt to ensure that the finishes applied during
coating or plating properly adhere to the surface and meet performance expectations. Surface
preparation can range from simple mechanical techniques, such as spray cleaning, abrasive
blasting, buffing, or grinding, to more complex chemical techniques, such as acid washes and
multi-stage acid/alkaline cleaning processes.13 These techniques may be used alone or in
combination to remove dust, grease, oils, oxides, base metal, and other materials from the
surface of the object (U.S. EPA, 1979).
Acidic or alkaline chemicals are used as cleaning agents. According to studies done at the
Rochester Institute of Technology and Wayne State University (Fister, 2010; Xiao, et al., 2012),
cleaning and subsequent rinsing are the primary sources of wastewater from metal finishing
facilities. However, alternative methods of cleaning and rinsing are increasingly used to reduce
water use and chemical consumption (Fister, 2010; Xiao & Huang, 2012). In recent discussions
with industry, EPA learned that cleaning and rinsing operations may be conducted over other
rinse tanks or plating baths to collect and reuse the wastewater. Facilities have also implemented
countercurrent rinse cycles to reduce the replacement frequency of the rinsewater used in rinsing
operations (ERG, 2016).
The Wayne State study reviewed the growing use of "integrated profitable pollution
prevention" (IPS) techniques by metal finishers to reduce water use and chemical consumption
(Xiao & Huang, 2012). That study identified the following five IPS technologies:
• A cleaning and rinse process control method that uses a two-layered hierarchical
dynamic optimization technology to conserve chemical and water use. The lower
layer adjusts local control variables, such as chemical concentration and water
flow rate, to optimal settings. The upper layer uses the optimal processing time in
all the cleaning and rinse tanks to achieve the desired surface characteristics. The
two layers, when used together, maximize cleaning and rinsing efficiency, thereby
minimizing chemical use and conserving water.
• Re-designing water flow patterns through a rinsing system to optimize flow rates.
The water flow patterns can be switched during operation (based on current use)
to maximize water reuse while maintaining rinse water quality.
• A sludge reduction method to classify sludge as avoidable or unavoidable. The
method determines the amount of avoidable sludge and reduces it by altering the
chemical use and identifying unnecessarily long cleaning times.
13
Acid washes are subject to Metal Finishing ELGs (U.S. EPA, 2004).
4-3
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Section 4 - Preliminary Study Findings
• A model-based design and operation strategy to derive an optimal reversed drag-
out network system to recover chemical losses from cleaning and plating systems.
• A dynamic hoist scheduling algorithm to generate a production schedule based on
waste generation, chemical consumption, and energy use.
To demonstrate the effectiveness of IPS technologies, the researchers also described a
manually operated bronze cyanide plating line at a facility that implemented a near-zero
chemical and metal discharge system, intended to reduce chemical consumption and water use
(Xiao & Huang, 2012). Figure 4-1 presents the original configuration of the bronze cyanide
plating line. Each rack of parts is first cleaned (soak cleaning and electro cleaning or de-rusting)
and then rinsed. As shown in the diagram, the initial cleaning and rinsing are followed by an acid
clean step, followed by another rinse before the actual copper and tin plating (Xiao & Huang,
2012).
Evaporation
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Figure 4-1. Original Process at the Bronze Cyanide Plating Facility
Figure 4-2 presents a revised rinsing configuration. The facility installed a new rinse tank
(Tank 11) so chemical solvents from the soak clean and electro clean steps, accumulated in Rinse
Tank 3, could be recovered and reused. The tank before New Rinse Tank 11 is charged for static
rinse, and New Rinse Tank 11 is for flow rinse. Rinse Tank 8 is also charged for static rinse,
while Rinse Tanks 9 and 10 are countercurrent rinse tanks. Two pumps are added to the system,
one to pump the solution from Rinse Tank 3 to the electrocleaning tank, and one to pump water
back to the plating tank from Rinse Tank 8 (Xiao & Huang, 2012).
Water
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Figure 4-2. New Process at the Bronze Cyanide Plating Facility
4-4
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Section 4 - Preliminary Study Findings
After making these changes, the facility reduced use of the derusting chemicals (used in
the electroclean and acid clean steps) by 40 percent, the soak cleaners by 100 percent, and the
acid by 8 percent. Rinse water costs were reduced by 40 percent and wastewater treatment costs
were reduced by 70 percent. According to the study authors, implementation of profitable
pollution prevention techniques typically involve slight modifications of processes, which
require little capital investment but can increase efficiency and reduce waste (Xiao & Huang,
2012).
Traditional rinsing methods include independent rinse tanks, with rinse water flowing in
and out of each tank, and countercurrent rinsing, with relatively clean rinse water flowing from
the second rinse tank to the more contaminated primary rinse tank. The top diagram in Figure
4-3 shows a typical rinse system with four independent rinse tanks; and the bottom diagram
depicts a reduced flow rinse scheme (Fister, 2010).
The study done by the Pollution Prevention Institute (PPI) at the Rochester Institute of
Technology (Fister, 2010) indicates that reactive rinsing can be used to reduce water use in
rinsing processes that include both acid and alkaline rinse tanks. In a reactive rinse system, as
shown in the bottom diagram of Figure 4-3, the acid from the acid rinse is sent to a preceding
alkaline rinse tank where it neutralizes residual alkalinity in the water. In traditional rinsing
methods, the acid contained in the rinse water is sent to be treated. Reactive rinsing allows any
rinse water from the alkaline rinse that is dragged out by parts to the acid tank to now contain
acid that would have otherwise been wasted (Fister, 2010). According to Fister (2010), the total
water use for a rinse system with four independent rinse tanks is 12 gallons per minute (gpm)
(top diagram, Figure 4-3). By incorporating reactive rinsing into a reduced flow rinsing scheme
(bottom diagram, Figure 4-3), the water use is 3 gpm. This results in a savings of $5,400 per year
in water costs (at a rate of $5 per 1,000 gallons for a 2,000 hours-per-year operation) (Fister,
2010).
Powder coating facilities commonly use phosphate-based cleaning and rinsing systems
before painting large fabricated products. Conventional cleaning and rinsing systems are
generally separate and use large amounts of water and chemicals to clean, degrease, and apply a
pretreatment chemical to products (Guidetti, et al., 2009). EPA identified two alternatives to
conventional phosphate-based cleaning and rinsing systems: the Cleaner Phosphoric Recycling
(CPR) System and the plaforization process.
From reviewing vendor literature and a facility case study, EPA found that CPR Systems,
a division of T. George Podell & Co, Inc., has developed the CPR System primarily for
companies that manufacture large fabricated steel and aluminum products. The CPR System
incorporates non-rinse cleaning, degreasing, and pretreatment in one-step. PhosBite 101, a
combined cleaning and coating chemical, is used in the wash water to surface clean and generate
a conversion coating on the metal substrate to improve paint adhesion. Water and chemicals used
in the system are recycled, greatly reducing water and chemical use compared to conventional
systems. The CPR system was introduced in 1989. According to CPR Systems, over 100
facilities in the United States have installed the CPR System (ERG, 2016; Tucker, 2013).
4-5
-------
Section 4 - Preliminary Study Findings
Paris and solution drag-out movement
Water u«»t1!gpm
Figure 1. Rinse system with four independent rinse tanks.
Parts and solution drag-out movement
Water use at 3 gpm
Figure 2. Maximized use of counterflow and reactive rinses.]
Source: (Fister, 2010)
Figure 4-3. Potential Rinse Systems in Facility with Metal Finishing Processes
A powder coating facility in Pennsylvania installed the CPR System in 2013 to clean and
pretreat large hydraulic dump trailers, while also decreasing water and chemical use. Facility
contacts reported that after learning to use the new process, they were able to reduce chemical
and water consumption by 70 percent, primarily through recycling (Tucker, 2013).
From reviewing industry literature, EPA identified the plaforization process, a second
alternative to conventional phosphate-based cleaning and rinsing systems. The technology was
first introduced in the European market by PAI-KOR S.r.l, in Italy, in the late 1960s. As of 2009,
approximately 500 industrial facilities in more than 25 countries had incorporated plaforization
processes. Plaforization is a one-step process with no rinse; it does not use water and creates no
wastewater, sludge, or other contaminants (Guidetti, et al., 2009).
The chemistry is based on organic high-boiling-point fluids and an organic polymeric
resin modified by phosphating groups. Several different processes occur almost simultaneously:
organic fluids dissolve oily contaminants, solid particles are washed off and taken into the
solution, the phosphating acid part of the organic resin attacks the metal surface to clean and
pretreat the metal. Finally, an organic polymeric resin is applied to the part with a thickness of
about one micron (Guidetti, et al., 2009).
According to the developer of plaforization, the process eliminates the need for
continuous chemical analysis, dumping tanks, chemical replenishment, and sludge removal, all
4-6
-------
Section 4 - Preliminary Study Findings
of which are required with conventional phosphate-based cleaning and rinsing systems (Guidetti,
et al., 2009). Plaforization also allows many metals, such as steel, aluminum, and cast iron, to be
treated with the same chemistry (Guidetti, et al., 2009). Figure 4-4 shows an example of a
plaforization system installed for batch spraying equipment. In this system, parts are manually
hung in the washer; the unit is equipped with three risers with nozzles that move during
treatment to completely bathe all the part surfaces.
Source: (Guidetti, et al., 2009)
Figure 4-4. Plaforization System for Batch Spraying Equipment
4.2.2 Coating and Plating Processes
Numerous coating types are used in metal finishing, including transition metal coatings,
hexavalent and trivalent chromium coatings, and graphene nanocomposite coatings. In 2008, the
Strategic Environmental Research and Development Program (SERDP), an environmental
research program under the Department of Defense (DoD),14 completed a study on ultraviolet
(UV)-curable, corrosion-inhibiting primers and high-performance topcoats that protect aluminum
substrates, specifically for aircraft interiors. Conventional coatings used for DoD vehicles release
volatile organic compounds (VOC) during application and curing. Additionally, while the
equipment is in use, flaking and chipping of the coating can cause hazardous substances, such as
hexavalent chromium, to enter the environment. The goal of DoD's study was to test a
combination of chromate-free and VOC-free substrate preparation and coating systems to
identify a coating system that eases compliance with current and anticipated environmental
regulations, and at the same time makes coating application simpler and faster. Results from the
study indicated that all tested coating systems provided good adhesion, fluid resistance, impact
and solvent resistance, and low-temperature flexibility, making them viable alternatives to
conventional coatings. These coating systems do not release VOCs during application or curing,
and minimize flaking and chipping, thereby reducing hazardous materials (such as hexavalent
chromium) in the environment. The process tested can be used in many industries and products,
14 Two environmental research programs exist under the DoD: the Strategic Environmental Research and
Development Program (SERDP), and the Environmental Security Technology Certification Program (ESTCP).
4-7
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Section 4 - Preliminary Study Findings
including aircraft and spacecraft, ground and marine vehicles and equipment for all military
branches, and industrial coatings in chemical and mechanical plants (Phely-Bobin, 2010).
A study performed at the University of Ulster investigated the corrosive response of hot
rolled steel (HRS) after exposure to various pre-powder-coating surface treatments (Tepe, et al.,
2008). Powder-coated HRS is used extensively in the automotive, agricultural, and appliance
industries, among others. The study reviewed conventional pre-powder coating treatments,
including iron and zinc phosphate, as well as newer nano-structured conversion coatings (such as
zirconium-based coatings). The study reported that the nano-structured conversion coatings
perform as well as or better than the conventional coatings, and can enhance the corrosion
resistance of powder coated HRS. However, researchers determined that achieving the optimally
effective corrosion barrier with zirconium-based nano-scaled conversion coatings requires the
removal of oxide scale from the substrate before any pre-powder-coating treatments (Tepe &
Gunay, 2008). Zirconium-based coatings are further discussed in Section 4.3.
According to a study on the use of ionic liquids in metal finishing practices, conventional
coating and plating processes based on aqueous technologies such as cyanide or hexavalent
chromium are well established in the metal finishing industry (Smith, et al., 2010).15 However,
many companies are feeling pressure to develop new coating and plating processes in order to
increase process efficiency, decrease energy use and wastewater discharges, and reduce the use
of toxic chemicals (Smith, et al., 2010). Due to this pressure, the New Ionic Liquid Solvent
Technology to Transform Metal Finishing Products and Processes (IONMET) consortium was
developed in Europe. The IONMET project consists of 33 partners, including industrial
companies, trade associations, and research institutions. The overall objective of the project is to
introduce breakthrough ionic liquid technology into metal finishing processes. Ionic liquids are
ionic materials that are liquid below 100 degrees Celsius and have properties that make them
well-suited for metal finishing, such as high solubility of metal salts, high conductivity, and
unique metal ion coordination chemistry (Smith, et al., 2010).
The IONMET consortium scaled up five room temperature ionic liquid metal deposition
processes:
• Electropolishing;
• Hard chromium electroplating;
• Aluminum plating;
• Immersion silver displacement; and
• Zinc-tin barrel plating.
The IONMET consortium found that in all five processes, ionic liquids could be
considered drop-in replacements for existing aqueous solutions composed of strong inorganic
acids and toxic reagents (i.e., all five processes are effective metal coating processes). Ionic
liquids are easy to handle, inexpensive, and have low toxicity, making them attractive
alternatives to conventional coating and plating solutions. The IONMET consortium stated that
' Ionic liquids are salts in the liquid state, which act as powerful solvents or electrically conducting fluids.
4-8
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Section 4 - Preliminary Study Findings
further research is necessary, but that ionic liquids have a high potential to be incorporated into
metal finishing processes (Smith, et al., 2010).
In the United States, SERDP is researching the application of ionic liquids. The DoD
expects this research to illuminate key mechanisms and process variables for using ionic liquids
in electroplating aluminum, specifically related to weapon systems. The project is expected to be
completed in 2016 (Dai, 2014).
According to industry experts, brush plating, also known as spot plating, is an
electroplating process in which a pad is used to apply aqueous coatings, such as nickel, hard
chromium, silver, etc., to a substrate (Chaix, et al., 2013). Advantages of brush plating include
low costs, limited need for extensive air handling or scrubbing, and portability. However, it can
be messy, with drips and splatters, and is best used for small areas. An alternative to brush
plating is brush electrofmishing, which features a computerized non-drip stylus. According to
industry experts, brush electrofmishing can be performed in any orientation, including overhead,
and it eliminates drips, splashes, and most of the fumes associated with brush plating (Chaix, et
al., 2013). The brush electrofmishing technique can reduce air emissions and replace toxic
materials with less toxic alternatives, while still maintaining process control (Chaix, et al., 2013).
4.2.3 Polishing
Polishing (i.e., smoothing out surface defects) is typically one of the final steps in metal
finishing. According to industry literature, some polishing is traditionally done by hand, making
it one of the most time-consuming and expensive steps in metal finishing (Pusavec, et al., 2014).
An alternative to hand polishing is abrasive flow machining (AFM), in which the flow of a
pressurized abrasive polymer medium removes excess material from part surfaces. Pusavec and
Kenda (2014) researched the performance and energy efficiency of AFM, focusing on an
upgrade of AFM that incorporates a movable/rotatable mandrel (AFMmm). Their study looked
at a computer simulation of AFMmm in the finishing of gear injection molds and performed
fatigue and performance tests. They concluded that the AFMmm finishing system uses less
polishing fluid and energy compared to the traditional AFM system and hand polishing, which
improves the efficiency and environmental impact of the technology (Pusavec & Kenda, 2014).
4.2.4 Summary of Process Technologies
Based on recent literature and industry discussions, there are a few reasons for which
industry would consider process technology advances: 1) to improve the efficiency of the
coating/finishing operation; 2) to implement pollution prevention strategies that would reduce
the impact of the pollutants generated and discharged; and 3) to reduce the operating costs
associated with chemical usage, wastewater treatment, and hazardous solid waste disposal.
Industry discussions have suggested that wastewater management and solid waste
disposal generally play a significant role in the overall operating costs at metal finishing
facilities. Recycling generated wastewater and chemicals back to the process minimize the need
for the facility to replenish with a separate water source or with new chemicals. Minimizing the
chemical usage can also reduce the volume of sludge generated and disposed during wastewater
treatment. EPA learned that several facilities are implementing pollution prevention practices to
minimize the volume of wastewater discharged, including a countercurrent rinsing system and
4-9
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Section 4 - Preliminary Study Findings
replenishing plating baths that have evaporative losses with both untreated and treated rinsewater
(ERG, 2016). To date, EPA has not found information that suggest that the primary sources of
wastewater has changed. A significant portion of the wastewater generated continues to originate
from rinsing and cleaning operations. Improvements to coating and plating processes have not
shown to generate additional waste streams and generally aim to minimize the amount of process
losses associated with the technology. Several facilities and vendors suggest that chemical
recovery operations do occur at metal finishing facilities to minimize the need to treat and
dispose of process losses containing valuable plating chemicals as waste (see Section 4.4 for
information of technologies used for this purpose).
4.3 Alternative Chemistries
At the promulgation of the 1983 Metal Finishing ELGs, facilities covered by the ELGs
operated one of six primary operations, including electroplating, electroless plating, anodizing,
coating, etching and chemical milling, and printed circuit board manufacturing. Generally, these
six operations produce a coating on the surface of the base materials that consist of a metal or
metal oxide. See Section 2 for detailed descriptions of the traditional chemistries used in these
six metal finishing operations, as well as the 40 additional unit operations regulated by the ELGs.
EPA's literature review identified papers reporting on alternatives to the traditional
chemistries used in metal finishing operations, primarily alternatives to chemistries using
cadmium, hexavalent chromium16, phosphate, and cyanide. Exposure to these materials may
result in adverse health and/or environmental impacts. In addition, solid waste and wastewater
generated from these materials result in high costs for hazardous waste disposal. The following
subsections discuss alternatives to the most common metal finishing chemistries: cadmium
plating, the use of hexavalent chromium in plating, conversion coating, primers, sealers, and
cleaners, phosphate conversion coatings and cleaners, and cyanide in plating baths.
4.3.1 Cadmium Plating A Iternatives
Cadmium is widely used in electroplating and electroless plating operations to provide
corrosion protection in applications for the aerospace, military, and aviation fields. Cadmium-
plated high-strength steel is used in aircraft, spacecraft, and components of weapon systems.
According to Keith Legg of Rowan Technology Group, the aerospace and military industries
have successfully replaced most cadmium coatings used on nuts and bolts with a safer zinc-
nickel plating alternative (K. Legg, 2012a). However, industry has not identified replacements
for cadmium plating on structural materials because of the strict requirements for high corrosion
resistance and a narrow margin for material fatigue (Berman, et al., 2009). According to the
literature, the aerospace and military industries are considering potential cadmium alternatives
16 Chromium is a naturally occurring element, and primarily occurs in the environment in two valence states:
trivalent chromium (Cr III) and hexavalent chromium (Cr VI). Trivalent chromium occurs naturally, is an essential
nutrient, and is much less toxic than hexavalent chromium. Hexavalent chromium and metallic chromium are most
commonly produced by industrial processes.
4-10
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Section 4 - Preliminary Study Findings
for protective shells on electrical connectors and steel fasteners, as well as for brush plating
repair solutions.17
Table 4-1 presents the cadmium-free alternatives reported in literature and the processes
used to apply them. Aluminum deposition uses glass bead peening (GBP) (Aguero, et al., 2012),
atmospheric pressure, chemical vapor deposition (APCVD) (Berman & Brooman, 2009), and the
ion vapor deposition (IVD) (K. Legg, 2012a). Electroplating processes use tin-zinc alloys and
zinc-nickel alloys as alternatives to cadmium, and etching electroless plating processes use
nickel alloys (R. Mason, et al., 2010; Orduz, 2008)). According to one industry representative,
commercially available coating systems use 20 to 30 percent zinc in tin-zinc alloys, and 5 to 15
percent nickel in zinc-nickel alloys, to provide the best performance (R. Mason, et al., 2010).
Table 4-1. Process Descriptions of Cadmium-Free Alternative Coatings
Cadmium
Alternative
Aluminum
Tin-Zinc
Alloy
Zinc-Nickel
Alloy
Nickel
Coating Process
Glass bead
peening (GBP)
Atmospheric
pressure
chemical vapor
deposition
^ '
Ion vapor
deposition (IVD)
Electroplating
Electroplating
Etching
Electroless
plating
Process Description
Parts are sand-blasted and vapor-degreased prior to coating.
The coating slurry is applied by brush, immersion, or spray
gun and cured using a proprietary curing process under air,
followed by glass bead peening.
Parts are cleaned, subjected to a tri-isobutyl-aluminum
coating in a deposition chamber, using nitrogen as the carrier
gas and an induction heater to deposit aluminum at
temperatures between 275 and 300 degrees Celsius. After
removing from the chamber the surface may or may not be
treated with a conversion coating.
Parts are placed in a vacuum chamber and subjected to
ionized gas (typically argon), which generates a glow
discharge and acts as a cleaning process prior to coating.
Pure aluminum is added to the chamber and deposited onto
the surface. As the coating is formed, ions from the glow
discharge attract to the aluminum and increase the coating's
density.
Parts are electroplated in a bath of an aqueous solution
containing a tin-zinc alloy composed of 20 to 30 percent
zinc.
Parts are electroplated in a bath of an aqueous solution
containing a zinc -nickel alloy composed of 5 to 15 percent
nickel.
Parts are soaked in an acid cleaner, and acid etched. Parts are
submerged in a nitric acid solution, then a zincate
(Zn(OH)42~) solution, then placed in each solution once more
before plating in an electroless nickel bath. Parts are rinsed
between each step.
Source
(Aguero, et
al., 2012)
(Berman &
Brooman,
2009)
(R. Mason,
etal.,2010)
(R. Mason,
etal.,2010)
(R. Mason,
etal.,2010)
(Orduz, 2008)
When tested against a cadmium coating, the GBP aluminum coating demonstrated
comparable or better performance, exhibiting higher fatigue strength and equal corrosion
resistance. According to the study, the coating process produces significantly less process waste
compared to electroplating, and is therefore more cost-effective (Aguero, et al., 2012). A study
17 Brush plating is a portable process where an electroplated or anodized coating is applied locally to a metal part,
focusing only on the specific areas that require the coating.
4-11
-------
Section 4 - Preliminary Study Findings
on APCVD aluminum coatings by the Air Force Research Laboratory presented several
advantages. The study found that no vacuum chamber, pumps, or ancillary control equipment are
needed for coating. The process allows for higher throughput due to short processing times and
provides consistent and even coating thickness across the part. However, drawbacks to APCVD
aluminum coatings include susceptibility to embrittlement when exposed to hydrogen gas,
unacceptable performance in handling fatigue, and less lubricity than traditional coatings, which
allow parts to operate more smoothly (Berman & Brooman, 2009). The aircraft industry has used
IVD aluminum for years on high-strength steels; however, the vacuum deposition method is
significantly more expensive than cadmium plating (K. Legg, 2012a). An ongoing project by the
Environmental Security Technology Certification Program found that high purity aluminum
plating applied to high strength steel fasteners combined with an electrocoated topcoat was
comparable to cadmium plating in corrosion resistance. Aluminum coating is potentially costlier
than cadmium plating (Scott, 2013).
Non-aluminum alloy coatings are primarily made from tin-zinc and zinc-nickel alloys, of
varying composition. A tin-zinc alloy coating, specifically tin-20 wt. percent zinc, provided
efficient electrochemical protection and superior corrosion protection compared to both cadmium
and zinc-nickel coatings, but also has a slightly higher friction coefficient (Dubent, et al., 2010).
Zinc-nickel coating exhibited sufficient adhesion after bending, adhesion to paint, long-term
corrosion resistance, and resistance to embrittlement when exposed to hydrogen gas; however,
the coatings only performed marginally better when tested for environmentally induced cracking.
Boeing determined that zinc-nickel plating is a commercially acceptable cadmium replacement
for use on component parts made of low strength steel, stainless steel, aluminum, and copper
alloys (Indumathi, et al., 2011; R. Mason, et al., 2010). A zinc coating consisting of an inorganic
metal flake dip-spin coating offered high corrosion resistance with an equal or lower cost than
cadmium (Scott, 2013). A zinc-nickel brush plated coating for repairing weapon systems was
found to have several advantages compared to cadmium, mainly in cost savings - the plating
solution is recycled and reused in a closed-loop process (Slife, 2014).
Lastly, Uyemura Corporation developed a cadmium- and lead-free electroless nickel
coating system that provides a long bath life, corrosion resistance, and passed bend testing
without blistering or flaking the plated aluminum sheet (Orduz, 2008).
Although the chemistries reviewed eliminate the use of cadmium coatings, some may
require post-treatment conversion coatings to improve corrosion resistance and paint adhesion.
For example, the APCVD process described in Table 4-1 may use a hexavalent chromium-based
conversion coating (i.e., chromate conversion coating), which would also present health and
environmental concerns (Berman & Brooman, 2009).
4.3.2 Hexavalent Chromium Plating, Conversion Coating, Primer, Sealer, and Cleaner
Alternatives
Hexavalent chromium finishing processes provide self-healing corrosion protection and
improve adhesion between the paint and metal surface. Hexavalent chromium is applied in hard
chrome plating conversion coating, in primers and paint, and with anodizing processes (K. Legg,
2012b). Hexavalent chromium is hazardous to human health and the environment, and therefore,
4-12
-------
Section 4 - Preliminary Study Findings
researchers are exploring and developing alternatives that would reduce human and
environmental exposure to the chemical during use and disposal.
Hexavalent chromium may be present in wastewater generated by a facility and when
further treated, may end up in wastewater discharged to POTWs or surface waters and
wastewater sludge. Hexavalent chromium is also volatile and likely present in the air emissions
surrounding open tank baths. The Office of the Secretary of Defense in 2009 issued a
memorandum restricting the use of hexavalent chromium unless no cost-effective alternatives
that provide satisfactory performance are available (K. Legg, 2009). These actions pushed
defense and other industries to identify and adopt alternatives to hexavalent chromium.
According to Paul Wynn of MacDermid, for chromium-free alternatives to be widely adopted,
cooperation is required along the entire metal finishing supply chain to achieve low operating
costs, high performance, and best practice techniques (Morose, 2013; Wynn, 2006).
In EPA's review, EPA identified alternatives to hexavalent chromium that replace it with
a less toxic metal or that substitute a mechanical treatment step that imparts similar surface
characteristics, or a combination of both. The following sections discuss hard chrome plating
alternatives, chromate conversion coating alternatives, and alternatives for chromium-based
primers, sealers, and cleaners.
4.3.2.1 Hard Chrome Plating Alternatives
Hard chrome plating applies a thin layer of chromium metal to a base metal by
electroplating in a chromic acid solution. Hard chrome plating18 is used to make industrial
equipment and automobile parts wear- and corrosion-resistant. Since chromic acid contains
hexavalent chromium, researchers are testing alternative treatments for a variety of applications.
Table 4-2 presents examples of commercially available alternatives to hard chrome plating, their
market applications, and descriptions of the chemistries of each method. Commercially available
alternatives include thermal spraying with high velocity oxygen fuel (HVOF), which can replace
hard chrome plating of parts for commercial and military aircraft, off-road vehicles, and
hydraulic systems. The Air Force began implementing HVOF coatings on landing gear
components in 2003. HVOF sprays provide high performance, but according to industry experts,
the spray process is not as forgiving as hard chrome plating and requires more preparation,
training, and process control (K. Legg, 2008; K. Legg, 2010, 2012b; Sartwell, et al., 2004).
Table 4-2. Commercially Available Hard Chrome Plating Alternatives
Alternative
Methods
Thermal spray
(high velocity
oxygen fuel,
etc )
Typical
Applications
Aircraft landing
gear, hydraulic
rods, industrial
rolls
Description of Alternative Chemistries
Thermal spraying can be performed with a wider range of
coating materials than hard chrome plating, including materials
that incorporate non-hexavalent chromium. The most common
materials used in thermal sprays are chrome carbide-nickel
chrome (Cr3C2-NiCr) and tungsten carbide-cobalt (WC-Co or
WC-CoCr). The chromium in these coating materials is not in
hexavalent form.
Sources
(K. Legg,
2012c)
18 Hard chrome plating is used as a protective layer for increased corrosion resistance, while decorative chrome
plating is used to provide metal parts with a bright and shiny finish.
4-13
-------
Section 4 - Preliminary Study Findings
Table 4-2. Commercially Available Hard Chrome Plating Alternatives
Alternative
Methods
Electroplating
and electroless
plating
Heat treating
Vacuum
coatings
Welding
methods
Typical
Applications
Aircraft
engines,
internals
(especially
electroless Ni)
Hydraulic rods,
gears, bearings
Typically small,
high value
items, molds,
and dies
Rebuild of worn
or corroded
items
Description of Alternative Chemistries
The most common chromium-free electroplates are based on
nickel; other available materials are based on nickel-tungsten,
nickel-tungsten-boron, and nickel-tungsten-boron combined
with silicon carbide. Cobalt-based electroplates are also used,
including a nanocrystalline cobalt phosphorous alloy plating.
Electroless nickel plating alternatives to chromium include
nickel-phosphorous and nickel-boron.
Heat treating puts metal through cycles of high temperatures
and cooling in a controlled atmosphere. Processes include
nitriding, carburizing, and nitrocarburizing, which add
nitrogen, carbon, and both, respectively, to the surface of the
metal. The controlled heating and cooling cycles harden the
surface.
Physical vapor deposition (PVD) vacuum coating most
commonly uses titanium nitride, chromium nitride, and various
diamond-like coatings. Chromium nitride contains chromium
in trivalent form.
Welding involves melting the surface of the treated material;
hence, only materials that can withstand the required heating
can be welded successfully. Welding replaces chromium
application with a mechanical method, instead of chemical
alternatives, to impart similar properties (e.g., wear resistance).
Sources
(K. Legg,
2012c;
Prado, et
al., 2010)
(K. Legg,
2012c)
(K. Legg,
2012c)
(K. Legg,
2012c)
The hard chrome plating alternatives presented in Table 4-2 have advantages and
disadvantages, described below in Table 4-3.
Table 4-3. Advantages and Disadvantages of Hard Chrome Plating Alternatives
Alternative
Methods
Thermal spray
(high velocity
oxygen fuel, etc.)
Electroplating and
electroless plating
Heat treats
Vacuum coatings
Welding methods
Advantages
Harder and more wear resistant; can
rebuild; spraying is faster than hard
chrome plating.
Drop-in chrome replacement due to
similar bath process; cobalt-
phosphorous plating exhibited good
wear and corrosion resistance in a lab-
scale study; electroless nickel is a very
flexible process and can apply uniform
coatings to complex parts.
Almost any size and shape; not a
coating process and therefore cannot
come off as coatings can.
Extremely hard and wear resistant, can
be cost effective on small components.
Good for thick coatings, mostly on
externals, can be highly controlled and
automated.
Disadvantages
Require more careful process
control; spraying requires constant
attention.
Generally no better than chrome;
nickel electroplating baths
periodically require complete
replacement; alternatives may be
based on alloys that are usually
more complicated than elemental
coatings.
Cannot be used for rebuild or for
heat-sensitive materials.
Complex and expensive; cannot be
used for rebuild; highly sensitive to
contaminants.
Not for heat sensitive materials,
must be refinished.
Sources
(K. Legg,
2012c)
(K. Legg,
2012c;
Prado, et
al., 2010)
(K. Legg,
2012c)
(K. Legg,
2012c)
(K. Legg,
2012c)
4-14
-------
Section 4 - Preliminary Study Findings
4.3.2.2 Chromate Conversion Coating Alternatives
The metal finishing industry is developing alternatives to hexavalent chromium-based
conversion coatings, also referred to as chromate conversion coatings. Chromate conversion
coatings are widely used in the aircraft and defense industries, on large components as well as on
screws, nuts, and bolts, to provide corrosion resistance and self-healing properties (K. Legg,
2009). Several alternatives are undergoing laboratory testing and some are commercially
available. Table 4-4 lists chromate conversion coating alternatives and descriptions of their
performance.
Table 4-4. Chromate Conversion Coating Alternatives
Alternatives
Trivalent
chromium
coatings
Silicate-based
ceramic
coatings
Polymer-based
coatings
Permanganate -
based coatings
Phytic acid
coatings
Cerium
conversion
coating
Materials
Coated
Aluminum
alloys and
magnesium
alloys
Steel
screws,
nuts, and
bolts
Aluminum
alloys and
magnesium
alloys
Zinc and
zinc alloys
Zinc alloys
Aluminum
alloys
Description
A trivalent chromium process can replace hexavalent
chromium on aluminum alloys with comparable properties
and costs. Tests of a trivalent chromium conversion coating
applied to magnesium alloys displayed better corrosion
resistance than hexavalent chromium.
Elisha Technologies researched and tested a passivation
system that replaces chromium with silicates, which form a
ceramic, glass-like layer on the finished parts. In a lab-scale
salt spray performance test, the ceramic surface treatment
combined with a zinc plate and aluminum-rich epoxy
coating displayed great corrosion resistance.
NEI Corporation's NANOMYTE® PT-60 is commercially
available hexavalent chromium conversion coating
replacement for use on magnesium alloys. Another coating
comprised of zeolites provides a universal solution and
process that can coat several types of aluminum alloys.
John Bibber with Sanchem, Inc. explored alternatives to
chromium-based conversion coatings and found that
permanganate-based coatings duplicate chromium's
chemical and physically properties without heavy metals.
Phytic acid is a non-toxic acid requiring no hazardous waste
treatment. A phytic acid conversion coating and chromate
conversion coating were applied to a zinc-cobalt alloy and
tested for corrosion. According to the study, the phytic acid
with a 15-minute deposition time resulted in superior
corrosion resistance compared to the chromate coating.
A system using a cerium conversion coating and a
multifunctional UV curable coating with inorganic
corrosion inhibitors resulted in aluminum alloys exhibiting
good flexibility, adhesion, and fluid resistance. The coating
system met most aerospace metal finishing requirements.
Sources
(Bhatt, etal.,
2009; La Scala,
2009;
Manavbasi, etal.,
2012;Nickerson,
etal., 2012)
(Winn, etal.,
2008)
(Bhargava, et al.,
2012; Lew, etal.,
20 10; NEI
Corporation,
2014;Yan, 2009)
(Bibber, 2008)
(Bikulcius, etal.,
2010)
(O'Keefe, 2010)
According to Keith Legg of Rowan Technology Group, most industries have already
adopted hexavalent chromium-free conversion coatings for aluminum, however hexavalent
chromium is still used in the aerospace and defense industries due to the increased importance of
high performance corrosion resistance in these industries (Eichinger, et al., 1997; K. Legg,
2011). Out of several non-chromate conversion coatings laboratory tested by the Naval Aviation
4-15
-------
Section 4 - Preliminary Study Findings
Systems Command (NAVAIR), the only coatings based on trivalent chromium were comparable
to hexavalent chromium conversion coatings (Nickerson & Matzdorf, 2012). NAVAIR's
trivalent chromium process requires no additional equipment or operator training when changing
from hexavalent chromium processes (La Scala, 2009). During testing, NAVAIR found that
process parameters, such as coating time, temperature, and chemical concentration required
adjustment for each specific aluminum alloy. Due to trivalent chromium's increased dependence
on process parameters, some facilities may be discouraged to change from conventional
hexavalent chromium processes (Bhatt, et al., 2009). In recent discussions with metal finishing
facilities, EPA learned that some facilities that conduct hexavalent chromium plating operations
may have also installed trivalent chromium plating operations in response to increasing demand
for safer alternatives. However, this practice is not widespread. Trivalent chromium generally
presents a less durable product which can lead to product performance issues. Its use also
requires higher operating costs to maintain the plating baths and equipment used, although
facilities incur less costs for wastewater treatment (ERG, 2016).
NEI Corporation developed and tested NANOMYTE® PT-60, a chromium-free, self-
healing, polymer-based nanocomposite conversion coating, consisting of organic, inorganic,
and/or both polymer types. According to NEI Corporation, the alternative coating applied by
immersion into a bath of PT-60 solution displayed uniform distribution and consistent thickness,
higher electrochemical resistance compared to chromate coatings, and excellent corrosion
resistance and self-healing properties after salt-fog exposure when applied to magnesium alloy
during a lab oratory-scale test (Bhargava & Allen, 2012).
The University of California conducted laboratory-scale research on a zeolite coating
developed to replace military chromium-based coatings. Zeolites are inexpensive, non-toxic,
crystalline inorganic polymers containing silica and aluminum. Zeolite coatings were tested on
hundreds of 1 by 2 inch and 3 by 6 inch metal panels and 99 percent passed corrosion resistance
tests including adhesion, impact, bending, salt-fog, and UV testing. The researchers were able to
develop a universal solution composition and deposition procedure that produces high quality
coatings on several different aluminum alloys, which involved immersing the metal panels into
the solution and then heating in a convection oven at 175 degrees Celsius for 12 to 16 hours.
This results in significant cost savings because no adjustments are needed for coating different
types of alloys (Lew, et al., 2010; Yan, 2009).
An experiment examining several types of conversion coatings concluded that while
some coatings work well in certain areas, none of them compare against hexavalent chromium.
Trivalent chromium had better heat resistance but less corrosion resistance. Permanganate-based
coatings had equal corrosion resistance but was unable to regenerate (Pommiers, et al., 2014).
4.3.2.3 Alternatives for Chromium-based Primers, Sealers, and Cleaners
Most research for replacing hexavalent chromium has focused on alternatives to hard
chrome plating and chromate conversion coatings; however, researchers are also exploring
alternatives for chromium-based primers, sealers, and cleaning solutions. Primers provide
additional adhesion between the surface and topcoat paint. Sealers are mainly used to prevent the
surface from absorbing paint, but may also act as a primer. Figure 4-5 illustrates traditional steel
4-16
-------
Section 4 - Preliminary Study Findings
and aluminum coating systems. As shown, traditional coating systems also use primers
containing hexavalent chromium on top of a chromate conversion coating.
High VOC topcoat
Chromated primer
Ghromate conversion coat
Anodize (or not)
Aluminum alloy
Source: (K. Legg, 2009)
Figure 4-5. Traditional Coating Systems
Researchers at an Air Force facility that anodizes aluminum landing gear investigated
alternatives to their sodium dichromate sealer. They identified two: a commercial off-the-shelf
nickel-based sealer and permanganate-based sealer. The nickel-based sealer is applied at a lower
temperature than the sodium dichromate but seals at a slower rate. Researchers found that the
permanganate-based sealer had exceptional corrosion resistance but exhibited less adhesion than
the sodium dichromate (R. Mason, et al., 2011). In addition to the nickel- and permanganate-
based sealers, the aerospace industry tested a magnesium-rich primer to replace chromate-based
primers used on aluminum alloys. They found this primer was the only chromium-free
alternative that provides equal or superior corrosion protection to conventional chromate
systems, and does not require any pretreatment other than a good cleaning (Bierwagen, et al.,
2010).
Along with sealers and primers, hexavalent chromium is a constituent of acidic cleaning
solutions used to remove impurities and inorganic contaminants from metals before plating. Acid
treatments are also known as pickling. Researchers tested a pickling solution consisting of
phosphoric acid and sodium molybdate and found it produced a more uniform film and less
corrosion when compared to a pickling bath of chromium trioxide and nitric acid (Lei, et al.,
2011).
4.3.3 Phosphate Conversion Coating and Cleaning Alternatives
Phosphate conversion coatings are used to provide a good base for paints and other
organic coatings, to condition the surfaces for cold forming operations by providing a base for
drawing compounds and lubricants, and to impart corrosion resistance to the metal surface by the
coating itself or by providing a suitable base for rust-preventative oils or waxes (U.S. EPA,
1979). Along with phosphate conversion coatings, phosphate cleaners may also be used in order
to remove grease and other contaminants from metal substrates prior to coating (List, et al.,
4-17
-------
Section 4 - Preliminary Study Findings
2012). Note that phosphate cleaners or other alkaline cleaners are also subject to the Metal
Finishing ELGs (U.S. EPA, 2004).
Phosphorous is an essential nutrient for plants and animals. Increased phosphorous levels
in water bodies from the discharge of phosphate-containing wastewater can impact aquatic
ecosystems and lead to fish kill by lowering dissolved oxygen levels in the water (U.S. EPA,
2012b; USGS, 2015). In recent years, metal finishing companies have converted to phosphate-
free processes to address strict environmental regulations on phosphorus discharges in some
localities and high operating costs associated with phosphate sludge removal (List, et al., 2012).
Kaluzny (2012) noted several benefits to alternative phosphorus-free conversion coating
processes, mainly in cost savings by avoiding POTW surcharges due to phosphate restrictions,
eliminating onsite wastewater treatment, minimizing heat required during the surface treatment
process, and significantly reducing the amount of sludge produced. They also identified several
disadvantages of phosphorous-free processes, including the need for additional rinsing and
cleaning between each process step (Kaluzny, 2012).
One alternative to phosphate conversion coatings is the use of ionic liquids containing
ethylene glycol and choline chloride for electropolishing steel in a process that incorporates a
recycling protocol for the liquid waste. Researchers found the ionic liquids were as effective as
aqueous solutions using phosphoric acid mixtures. Additionally, metals are highly soluble in
ionic liquids, so such liquids are suitable for electrodepositing tin, zinc, and zinc alloys. The
study shows the ionic liquid surface finishes for the electropolishing process were at least as
good as conventional aqueous solutions and incorporated a recycling system that allowed for full
metal recovery (Abbott, et al., 2007).
Zirconium-based conversion coatings are an alternative to phosphate conversion coatings.
Klingenberg and Jones (2007) tested zirconium and zirconium-vanadium coatings against iron
phosphate coatings and found several benefits in the zirconium-based alternatives. The
phosphate-free process involved immersing metals into a bath of the zirconium-based solution.
When compared to traditional phosphate conversion coating processes, this process requires less
heat, shorter contact time, and lower coating bath volume. The process also provided thinner
surfaces, produced much less process waste, and exhibited better corrosion-resistance
(Klingenberg, et al., 2007).
Researchers found that phosphorous-free coatings that use Zirconization™ provide
similar benefits. Zirconization eliminates the need for a conditioning step, which further reduces
required chemicals and water use (Moore, et al., 2008). David Schmipff of DuBois Chemicals
was reported to have said that in two years of testing Zirconization™ and iron phosphate
coatings, "less than 5% of results favor an iron phosphate over Zirconization™ or other non-
phosphate technology" (Dunham, 2012).
Atotech provides phosphorous-free cleaners and conversion coatings. According to
Atotech, after a metal finishing company changed to these products, the company reduced costs
by 22 percent and reduced their defect rate from 25 to 11 percent. The phosphorous-free
chemistries improved the company's process by lowering the operating temperatures of the
4-18
-------
Section 4 - Preliminary Study Findings
cleaning and coating stages and eliminating the build-up of significant sludge on their spray line
(List, etal., 2012).
4.3.4 Cyanide Plating Solution Alternatives
Plating operations use cyanide with copper, zinc, brass, cadmium, silver, and gold.
Cyanide is also in chromate and phosphate coatings and etching operations. Cyanide is used as
an additive in plating baths due to its ability to form more fine grained metal deposits and
increase the bath's tolerance to impurities and contaminants. Many cyanide compounds are
highly toxic and can be readily absorbed through the skin and lungs. Some compounds are highly
stable, making them difficult to break down during wastewater treatment processes (U.S. EPA,
2000b). According to industry experts, replacing cyanide in metal finishing operations has
become increasingly popular due to the health and environmental concerns associated with
cyanide (Abdel-Hamid, et al., 2009; Lei, et al., 2010). The literature presents cyanide-free
alternatives for coatings on stainless steel and magnesium alloy surfaces.
Researchers tested non-cyanide alkaline baths on stainless steel and demonstrated
successful electroplating of a copper layer using an alkaline electrolyte (Abdel-Hamid & Abdel-
Aal, 2009). The presence of sorbitol in the plating bath improved efficiency and helped form a
dense copper layer, even in the absence of cyanide (Abdel-Hamid & Abdel-Aal, 2009).
Researchers also studied cyanide-free coating processes for magnesium alloys (Lei, et al.,
2010). Magnesium alloys are widely used in the aerospace, automotive, electronic, and
communication industries due to their low weight; however, their use generally requires a metal
plating finish to improve corrosion resistance, and the plating solutions usually contain cyanide
(Lei, et al., 2010). The study showed successful coating applications on magnesium alloys
without the use of cyanide for the following metal finishes (Lei, et al., 2010):
• Electroless nickel plating
• Electrodeposited nickel
• Electroplated copper
• Electroplated copper/nickel/chromium composite layer plating
The copper/nickel/chromium composite was obtained through multi-electrodeposition.
The magnesium alloys were electroplated in three bath compositions of acid copper, bright
nickel, and chromium, each with different operating conditions (Lei, et al., 2010). Researchers
found nickel and copper-based processes are the most viable alternatives to cyanide-containing
plating solutions (Abdel-Hamid & Abdel-Aal, 2009; Lei, et al., 2010).
4.3.5 Summary of Alternative Chemistries
Another key research focus for the industry is finding and applying less toxic alternatives
than those commonly used in metal finishing operations. In practice, this proves a challenge to
industries that have more stringent product quality specifications that cannot be achieved by
those alternatives to date. There is evidence that these alternatives have been applied
commercially in the metal finishing industry for products with less stringent specifications. The
alternative chemistries show that there may be emerging pollutants of interest because 1) the
-------
Section 4 - Preliminary Study Findings
industry has introduced new chemicals that were not identified in metal finishing wastewater
during the 1983 regulations (e.g., zirconium, platinum, tungsten) or 2) increased use of chemicals
that may have been identified in the 1983 regulations may have increased presence in metal
finishing wastewater (e.g., magnesium, manganese, boron). This may also be evident for other
chemicals that are used in plating processes; however, EPA did not identify in literature any
specific chemicals used outside of the primary plating chemicals.
Alternative chemistries may also require a change to commonly used metal finishing
operations. In recent literature, many of the alternative chemistries that have been commercially
applied use different operations than the typical plating operations, such as thermal spray and
vapor deposition applications. As a result of changes to plating chemistries, new finishing
operations may require EPA to further evaluate them in the context of the existing six primary
metal finishing operations that subject facilities to the Metal Finishing ELGs.
4.4 Wastewater Treatment Technologies
As part of its preliminary study, EPA reviewed literature describing recent technological
advances in removing metals from wastewaters (e.g., metal finishing wastewater). This section
first presents the heavy metals removal technologies that EPA evaluated while promulgating the
1983 Metal Finishing ELGs, and then compares them with the performance of treatment
technologies that EPA identified in its recent literature search.
4.4.1 Technologies Evaluated for the 1983 Metal Finishing ELGs
Metal finishing operations generate wastewater primarily from rinsing the surfaces of
metals (often several times) during cleaning and preparation, and again after coating or plating is
completed. Other sources of metal finishing wastewater include spills, spent process fluids, wash
or quench water from auxiliary operations, and water from air pollution control devices. The
wastewater may include inorganic wastes (primarily heavy metals) and organic wastes (including
cyanide, oils, and toxic organics). As such, the 1983 Metal Finishing ELGs regulate the direct
and indirect discharge of metal finishing wastewater containing heavy metals, cyanide, and TTO
to surface waters of the United States (see Section 2 of this report) (U.S. EPA, 1983a).
In setting the pretreatment standards and discharge limitations for metal finishing
wastewater in the 1983 regulations, EPA evaluated several wastewater treatment technologies for
removal of heavy metals, cyanide, and other organic waste. Table 4-5 summarizes these
treatment technologies and their calculated daily maximum, monthly average, and long-term
average concentrations. Ultimately, EPA selected hydroxide precipitation followed by
sedimentation (regulatory option 1) as both the BPT and the BAT. Hydroxide precipitation
followed by sedimentation with in-plant cadmium controls (regulatory option 3) was selected as
the NSPS/PSNS technology for the ELGs. (See Section 2.3 of this report for a more detailed
description of these technologies). Although the technology considered in regulatory option 2
demonstrated better treatment performance, EPA did not select this option because filtration
presented a very high incremental cost to the industry (U.S. EPA, 1983a).
In identifying these regulatory options, EPA evaluated the performance of individual
treatment technologies for heavy metals removal. Table C-l in Appendix C presents additional
treatment technologies that EPA identified in the 1983 regulations as being used to remove
4-20
-------
Section 4 - Preliminary Study Findings
Table 4-5. Regulatory Options Considered in the 1983 ELGs and their Daily Maximum,
Monthly Average, and Long-Term Average Concentrations
Regulatory Option
1 : Hydroxide precipitation
followed by sedimentation
(the BPT technology basis in
the 1983 ELGs)
2: Hydroxide precipitation
followed by sedimentation
and filtration
(not selected as a technology
basis in the 1983 ELGs)c
3 : Hydroxide precipitation
followed by sedimentation
with in-plant cadmium
controls
(the NSPS/PSNS technology
basis in the 1983 ELGs)
Technology Description
A precipitation technique to form insoluble
metal hydroxides and phosphates that are
removed by gravity settling techniques
including sedimentation basins or circular
clarifier. May require pretreatment of
wastewater containing cyanide, precious
metals, hexavalent chromium and oily
wastes. Wastewater containing complexed
metals are also segregated and treated
separately using this treatment technology.
In addition to Regulatory Option 1, a
filtration device is placed after the primary
solids removal to remove metal hydroxides
that did not settle out via gravity settling.
Filtration devices include granular bed or
diatomaceous earth.
In addition to Regulatory Option 1, in-plant
controls to nearly eliminate cadmium from
the wastewater. Controls can include
evaporative recovery, ion exchange, and
recovery rinses.
Pollutant
TSS
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
Silver
Cyanide
TTO
Oil and Grease
TSS
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
TSS
Cadmium
Chromium
Copper
Lead
Nickel
Zinc
Silver
Cyanide
TTO
Oil and Grease
Daily Maximum
Concentration
(mg/L)
60b
0.69
2.77
3.38
0.69
3.98
2.61
0.43
1.20
2.13
52b
46
0.42
1.55
1.52
0.14
1.94
1.13
60b
0.114
2.77
3.38
0.69
3.98
2.61
0.43
1.20
2.13
52b
Monthly Average
Concentration
(mg/L)
31b
0.26
1.71
2.07
0.43
2.38
1.48
0.24
0.65
NA
26b
24
0.16
0.95
0.93
0.09
1.16
0.67
31b
0.066
1.71
2.07
0.43
2.38
1.48
0.24
0.65
NA
26b
Long-term Average
(LTA) Concentration
(mg/L)a
16.7b
0.13
0.572
0.815
0.2
0.942
0.549
0.096
0.18
0.434
12b
12.8
0.08
0.32
0.37
0.04
0.46
0.25
16.7b
0.058
0.572
0.815
0.2
0.942
0.549
0.096
0.18
0.434
12b
TSS - Total suspended solids; TTO - Total toxic organics; NA - Not applicable
a Based on Table 3.4 of the Guidance Manual for Electroplating and Metal Finishing Pretreatment Standards, unless otherwise noted (U.S. EPA, 1984).
b LTA is based on the daily variability factor of 3.59 mg/L for TSS and 4.36 mg/L for Oil and Grease (U.S. EPA, 1983a).
0 Regulatory option 2 was based on Table 7-32 of the Development Document for Effluent Limitations Guidelines New Source Performance Standards for the
Metal Finishing Point Source Category (U.S. EPA, 1983a).
4-21
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Section 4 - Preliminary Study Findings
heavy metals. The table also includes performance data for each technology used to treat metal
finishing wastewater.
4.4.2 Chemical Precipitation
Chemical precipitation has been used to remove heavy metals from metal finishing
wastewater for many years. In general, chemical precipitation involves the conversion of soluble
heavy metals into insoluble compounds (i.e., precipitates) that are then physically separated from
the water using any of several removal techniques, such as clarification, sedimentation, or
membrane filtration (WEF, 2008). Precipitation techniques commonly used to remove heavy
metals include hydroxide precipitation and sulfide precipitation; complex metals precipitation
and electrochemical precipitation have also been used to remove heavy metals. All of these
precipitation techniques were evaluated during the 1983 rulemaking. Electrochemical
precipitation is further discussed in the electrochemical treatment methods section below (see
Section 4.4.5).
4.4.2.1 Hydroxide Precipitation
As discussed in Section 2.3, hydroxide precipitation is widely used to treat metal
finishing wastewater and is part of the technology basis for the Metal Finishing ELGs (see Table
4-1). The BPT for the Metal Finishing ELGs includes hydroxide precipitation, followed by
clarification and a sludge dewatering step. BPT also includes pretreatment steps for cyanides,
hexavalent chromium, and oil and grease prior to hydroxide precipitation (U.S. EPA, 1983a).
Hydroxide precipitation relies on the solubility curves of targeted heavy metals and uses
hydroxide compounds to manipulate this characteristic in the wastewater. Although lime (CaO)
and caustic (NaOH) are common treatment chemicals, calcium hydroxide (Ca(OH2)) and
magnesium hydroxide (Mg(OH2)) are also effective for treating metal finishing wastewater (Fu,
etal., 2011; U.S. EPA, 1983a).
A disadvantage of hydroxide precipitation is that the process generates large volumes of
low-density sludge that can be difficult and costly to dewater and dispose of. Optimization of
single stage hydroxide precipitation for wastewater containing multiple metals may also be
difficult to attain because metal hydroxides can have dissimilar solubility ranges and pH
adjustment may return metals back into solution. Moreover, complexing agents (which are used
in metal finishing processes) are also present in the wastewater and can inhibit hydroxide
precipitation, thereby making it difficult to remove heavy metals completely (Fu & Wang, 2011;
Islamoglu, et al., 2006). EPA did not identify literature indicating any advances in hydroxide
precipitation treatment of metal finishing wastewater.
4.4.2.2 Sulfide Precipitation
Sulfide precipitation uses treatment chemicals such as sodium hydrogen sulfide (NaSH),
sodium sulfide (Na2S), calcium sulfide (CaS), and iron sulfide (FeS) to precipitate heavy metals
from wastewater. Ferrous and ferric sulfate may also be used in this process (Huisman, et al.,
2006; U.S. EPA, 1983a). Like hydroxide precipitation, sulfide precipitation relies on the
solubility of metals in wastewater; however, metal sulfide precipitates have a much lower
solubility, and therefore, more metal is removed over a broader pH range than metal hydroxide
precipitates. This characteristic of sulfide precipitation allows metal precipitation to be more
4-22
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Section 4 - Preliminary Study Findings
selective, particularly for divalent soluble metals, such as cadmium, copper, zinc, and lead
(WEF, 2008).
During the development of the 1983 Metal Finishing ELGs, EPA identified a few
facilities treating metal finishing wastewater by sulfide precipitation in conjunction with
hydroxide precipitation. EPA's literature search indicates that the chemistry employed by sulfide
precipitation systems remains relatively unchanged; however, the use of multi-stage
precipitation, applying multiple precipitation techniques, was a relatively new approach for metal
finishing wastewater treatment at the time of the 1983 regulations. This approach may be more
widely used now. Sulfide precipitation combined with hydroxide precipitation can improve
removal of several metals over a wide pH range (Fu & Wang, 2011; Huisman, et al., 2006).
Compared to hydroxide precipitation, sulfide precipitation generates a more dense sludge
that is more amenable to thickening and dewatering for disposal. The pH of the system must also
be maintained at a neutral to basic medium because sulfide precipitates in an acidic medium can
generate toxic hydrogen sulfide gas (FbS) (WEF, 2008).
4.4.2.3 Chelated Metals Precipitation
Complexing (or chelating) agents in industrial wastewater can hinder conventional
precipitation techniques, such as hydroxide precipitation. Complexing agents are used in
chemical processes to help maintain metals in solution, and these metals are present in the
wastewater as complexed metals (or chelated metals). Commonly used complexing agents in
metal finishing include phosphates, tartrates, ethylenediaminetetraacetic acid (EDTA), cyanide,
and ammonia. Successful removal of complexed metals in wastewater require adjusting the pH
of the wastewater to very low or very high levels. The technology basis for the 1983 regulations
includes the separate treatment of wastewaters using hydroxide precipitation (lime) to drive up
the pH of the system to disassociate the complexed metals and free the metal ions to allow
hydroxide precipitates to form. Wastewater containing complexed metals can also be treated by
adding chemicals to lower the pH of the wastewater and followed by chemical reduction and
precipitation. In the acidic environment, the complexed metals disassociate and subsequently, a
reducing agent added to reduce the free metal ions to an oxidation state which can then be removed
using hydroxide precipitation. In this process, a suitable cation (e.g., calcium, iron) may be added
to tie up the complexing agents and allow for effective precipitation of the targeted metal using
hydroxide precipitation techniques (U.S. EPA, 1983a; WEF, 2008).
Another approach to treating wastewater containing complexed metals that EPA
identified in the 1983 regulations uses sulfide precipitation (through addition of ferrous or ferric
sulfate) to facilitate metals removal. Similar to the chemical reduction process, the approach
introduces a suitable cation (i.e., iron) into the wastewater, which frees the metal ions targeted
for removal (U.S. EPA, 1983a; WEF, 2008). Recent studies focus on the synthesis of precipitants
that effectively remove heavy metals even in the presence of complexed metals (Fu & Wang,
2011; Li, et al., 2003). The treatment effectiveness of the precipitants in forming metal
precipitates depends on the complexing agents present in the wastewater. In addition to ferrous
and ferric sulfate, three other commercially available precipitants used in treating wastewater
containing complexed metals are trimercaptotriazine, potassium or sodium thiocarbonate, and
sodium dimethyldithiocarbamate. All three are known pesticides, but are effective at removing
4-23
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Section 4 - Preliminary Study Findings
complexed metals from the wastewater. Experimental xanthates, thiol-based compounds, and
dithiocarbamate compounds have also been studied for treating electroplating wastewater
containing complexing agents (Fu & Wang, 2011; Li, et al., 2003). Some of these precipitants,
particularly the pesticides, present their own environmental risks when discharged into surface
waters (U.S. EPA, 2000a). Table 4-6 summarizes the effectiveness of precipitants used in
chelated metals precipitation that EPA identified in recent literature. According to Li et al.
(2003), the concentration of complexed metals can have an impact on the overall metal removal
observed and additional adjustments to the precipitant doses would need to be considered to
optimize the removal by the system. On a bench scale, several precipitants demonstrate the
ability to remove heavy metals to concentrations that are lower than the LTAs for the BPT
technology basis, specifically for copper.
In addition to hydroxide precipitation, EPA also identified chemical reduction and sulfide
precipitation as alternative methods for treating wastewater containing complexed metals in the
1983 regulations, although chemical reduction was much less prevalent in the industry at that
time. Treatment performance data at that time indicated that sulfide precipitation effectively
treated wastewater containing cyanide complexes using ferrous sulfate as the precipitant (see
Appendix C). EPA did not identify recent literature describing treatment performance for
chemical reduction or sulfide precipitation.
4.4.3 Sorption
Recent literature shows increased research interest in sorption technologies for heavy
metals removal. The most common sorption technologies studied involve adsorption and ion
exchange processes. The literature reviewed to date did not identify absorption processes for
heavy metal removal. This section discusses the advances in adsorption and ion exchange
processes in the metal finishing industry.
4.4.3.1 Adsorption
Adsorption involves the physical or chemical binding of a substance that exists in a liquid
solution (e.g., metals in wastewater) onto the surface of a solid resin or adsorbent, thereby
removing the substance from the liquid solution (Kurniawan, et al., 2006; WEF, 2008). The
effectiveness of adsorption (i.e., adsorptive capacity) relies on the number of binding sites
available on the adsorbent and can be greatly affected by how the adsorbent is synthesized. Some
adsorption processes are reversible, which allows for the recovery of metals via desorption
processes. In the 1983 regulations, EPA identified several adsorption technologies as polishing
steps (tertiary treatment steps) for removing organics, dissolved metals, and trace metals. These
technologies included carbon adsorption, integrated adsorption, peat adsorption, and synthetic
resin adsorption. However, the adsorption technologies were relatively new at the time and no
metals removal data were available (U.S. EPA, 1983a). Recent studies apparently continue to
evaluate adsorption as a tertiary treatment step, rather than as a primary technology for heavy
metals removal (Kurniawan, et al., 2006). Adsorbents that have been identified in literature for
the removal of heavy metals from wastewater include activated carbon and low-cost alternatives
to activated carbon, described below.
4-24
-------
Section 4 - Preliminary Study Findings
Table 4-6. Chelated Metals Precipitation Treatment Identified in EPA's Literature Review - Summary of Treatment Results
Scale of
Study
Bench scale,
batch
operation
Uncertain
Uncertain
Uncertain
Type of Wastewater
Synthetic (without
complexing agents)
Synthetic (with
complexing agents)0
Uncertain (synthetic
or industrial)
Uncertain (synthetic
or industrial)
Uncertain (synthetic
or industrial)
Precipitant
Sodium diethyldithiocarbamate
(DDTC), Ferric sulfate
(Fe2(SO4)3); polyacrylamide
(PAM)
Potassium ethyl xanthate
Dipropyl dithiophosphate
1,3,5-
hexahydrotriazinedithiocarbamate
Targeted
Metal
Copper
Copper
Lead
Cadmium
Copper
Mercury
Copper
LTA
Concentration
(mg/L)a
0.815
0.815
0.2
0.13
0.815
NA
0.815
Final Metal
Concentration
(mg/L)b
0.08
0.38
0.40
0.41
5.35
10.3
3
1
0.1
0.5
0.05
0.25
0.35
0.4
Percent
Removal
(%)
99.6
92.3
96.0
98.0
73.3
48.6
94.0-99.7
99.5
100
99.8
100
99.0
99.3
99.6
Source
(Li, et al., 2003)
(Chang et al., 2002)d
(Xu and Zhang,
2006)d
(Fuetal.,2007)d
NA - Not applicable
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the BPT technology basis.
0 The results represent the total copper removal observed for multiple scenarios of complexed copper and total copper concentrations in the sample. Total
copper removals of 73.3 and 48.6 percent present scenarios in which the complexed copper represents at least half of the total copper concentrations.
d As cited in (Fu & Wang, 2011).
4-25
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Section 4 - Preliminary Study Findings
Activated carbon is a widely used adsorbent made from carbonaceous materials such as
coal, wood, lignite, peat, coconut husks, or nutshells. A majority of commercially available
activated carbon is made from coal. Generally, activated carbon is an effective adsorbent because
its microporosity offers a large surface area to which substances can adhere. In EPA's review of
the literature, several articles noted its effectiveness in heavy metal removals (Fu & Wang, 2011;
Haynes, 2014); however, there were limited quantitative results showing the treatment
effectiveness of these adsorbents for heavy metals removal. Sancey et al. (2011) suggested that
the high cost of activated carbon adsorption as a tertiary treatment technology makes it less
desirable for heavy metals treatment. According to the Water Environment Federation, the
effectiveness of activated carbon adsorption for treating inorganic compounds such as metals has
not been well demonstrated (WEF, 2008).
The primary focus of many of the adsorption studies EPA reviewed was the adsorptive
capacities of activated carbon derived from inexpensive alternatives to coal. These alternatives
include agricultural wastes, industrial by-products, natural materials, and many other
carbonaceous materials that can be converted to activated carbon and used for adsorbing
polyvalent metals such as zinc, nickel, cadmium, copper, lead, mercury, and arsenic. Table 4-7
lists low-cost activated carbon alternatives that EPA identified in recent literature.
Table 4-7. Low-Cost Adsorbents Identified in EPA's Literature Review
Adsorbent
Source
Agricultural Byproducts and Natural Materials
Eucalyptus bark
Dried plants
Hulls (peanut, soybean)
Husks (corn, rice, almond, black gram, coffee)
Peels (orange, banana, potato)
Piths (coconut, sugar cane)
Shells (pecan, hazelnut, almond, seed, egg)
Corn cobs
Poultry litter
Sawdust
Modified starch (flour)
(Kongsuwan et al., 2009)a
(Chiban, etal.,2011)
(Periasamy & Namasivayam, 1995)b; (Marshall et al., 1999)b
(Ahmaruzzaman, et al., 2011; Ajmal, et al., 2003; Hegazi, 2013;
Wong, et al., 2003) (Oliveira et al., 2008)a; (Saeed et al., 2005)a;
(Hasar, 2003)b
(Ahmen-Basha, et al., 2008; Ajmal, et al., 2000) (Annadurai et
al., 2002)b
(Namasivayam, et al., 2008); (Khan et al., 2001)c; (Tan et al.,
1993)c
(Jai et al., 2007)a; (Amudaa et al., 2009)a; (Bansode et
al., 2003)b; (Demirbas, et al., 2002)b; (Kobya, 2004)b; (Dakiky
etal.,2002)b
(Ahmaruzzaman & Gupta, 201 1)
(Guoetal.,2010)a
(Agoubordea & Navia, 2009)a
(Sancey, etal.,2011)
Industrial Byproducts
Pulp and paper byproducts
Fly ash
Slag (blast furnace, iron or steel)
Lignin
Lignite, diatomite, clino-pyrrhotite, kyanite
(Sthiannopkao, et al., 2009)
(Ahmaruzzaman & Gupta, 2011; Haynes, 2014; Hegazi, 2013)
(Haynes, 2014)
(Betancur et al., 2009)a; (Reyes et al., 2009)a
(Mohan & Chander, 2006)a; (Sheng et al., 2009)a; (Lu et al.,
2006)a; (Ajmal, etal., 2001)
4-26
-------
Section 4 - Preliminary Study Findings
Table 4-7. Low-Cost Adsorbents Identified in EPA's Literature Review
Adsorbent
Clays (Clinoptilolite, kaolinite,
montmorillonite)
Zeolites, vermiculite, bentonite
Aragonite shells
Peat
Source
(Fu & Wang, 201 1); (Bhattacharyya & Gupta, 2008)a
(Alvarez-Ayuso, et al., 2003); (Apiratikul & Pavasant, 2008)a
(Kohleretal.,2007)3
(Liu et al., 2008)a
Natural Materials
Sodium alginate (natural polymer)
Olive (stone, cake)
Pine needles, cactus
(Abdel-Halima, etal.,2011)
(Dakikyetal.,2002)b
(Dakikyetal.,2002)b
Synthetic Materials
Activated alumina
Synthesized Layered Double Hydroxide
(LDH)
Silicates
(Bishnoi et al., 2004)d
(Lv, etal.,2013)
(Fu& Wang, 2011)
a As cited in (Fu & Wang, 2011).
b As cited in (Kurniawan, et al., 2006).
As cited in (Hegazi, 2013).
d As cited in (Ahmaruzzaman & Gupta, 2011).
The literature describes methods to optimize heavy metals removal by adjusting
adsorbent dose and pH levels (Kurniawan, et al., 2006). Table 4-8 summarizes the optimal
percent removals for heavy metals using various adsorbents and determined by using optimal
adsorbent dose and operating pH ranges. Adsorption of heavy metals was not well demonstrated
at the time of the 1983 regulations. Many of the alternative adsorbents demonstrate at bench
scale the capability to achieve effluent concentrations equal to or lower than the LTAs for the
BPT technology basis.
Another form of adsorption is biosorption (or bioadsorption), which uses inexpensive
biomass adsorbents (e.g., non-living, algal, and microbial biomasses) to remove toxic heavy
copper, zinc, cadmium, lead, etc. from water. These low-cost alternatives can be available in
large quantities, relative to more common adsorption media, such as coal-based activated carbon
(Ahluwalia, et al., 2007; Fu & Wang, 2011; Lesmana, et al., 2009). Table 4-9 summarizes the
treatment effectiveness of biosorbents in heavy metals removal identified in EPA's literature
review. Many of the studies are bench scale and hence, do not firmly establish applicability of
the treatment to full scale operations. EPA did not identify biosorbents in the 1983 regulations;
however, most of the results show final treated effluent concentrations that do not yet meet the
LTAs for chromium, nickel, or zinc.
Research on nanomaterials is relatively new. None of the studies reviewed to date
investigated the technology for treating metal finishing wastewater. Most of the studies focus on
the adsorption capacities achieved for heavy metals, such as lead, copper, chromium, cadmium,
zinc, and nickel, but few studies presented removal data. Table 4-10 lists the nanomaterials EPA
identified in recent literature. Because the nanomaterials are usually present in fine or ultrafme
particles, their treatment effectiveness can be improved by using structural supports, such as
4-27
-------
Section 4 - Preliminary Study Findings
those incorporating natural materials, metal oxides, or manufactured polymers (Hua, et al.,
2012).
4-28
-------
Section 4 - Preliminary Study Findings
Table 4-8. Low-Cost Alternatives to Coal-based Activated Carbon Treatment Identified in EPA's Literature Search -
Summary of Treatment Results
Scale of
Study
Pilot scale,
batch
operation
Bench scale,
continuous
operation
(column)
Bench scale,
batch
operation
Type of
Wastewater
Electroplating
Synthetic
Electroplating
Metal finishing
Metal finishing
Electroplating
Electroplating
Acid zinc
electroplating
Nickel
electroplating
Chromium
electroplating
Adsorbent Material
Synthesized LDH
Rice husk (tartartic
acid treated)
Orange peels
Lime mud (pulp and
paper byproduct)
Recovery boiler ash
(pulp and paper)
Coconut coir pith
Kyanite (commercial
mineral)
Synthetic zeolite
Targeted Metal
Chromium (VI)
Copper
Lead
Nickel (II)
Chromium
Copper
Lead
Zinc
Chromium
Copper
Lead
Zinc
Chromium
Copper (II)
Zinc (II)
Nickel (II)
Chromium (VI)
Zinc
Nickel
Chromium
Zinc
LTA
Concentration
(mg/L)a
0.572
0.815
0.2
0.942
0.572
0.815
0.2
0.549
0.572
0.815
0.2
0.549
0.572
0.815
0.549
0.942
0.572
0.549
0.942
0.572
0.549
Final Metal
Concentration
(mg/L)b
<0.5
0.007-32.0
0.007-35.5
1.5
2.16-4.06
0.11-3.08
0.10-0.17
0.059-12.33
2.93-36.6
5.90 -62.24
0.327-1.46
0.736-29.4
2.46
1.50
2.70
4.06
39.7
42.5
7.0
9.0
0.08 - 4.77
Percent
Removal
(%)
>99.9
46.9-
100.0
49.4-100.0
89
89.7-93.7
95.4-99.8
93.3-97.1
56.4-99.8
6.61-92.5
6.85-91.2
22.9-90.8
-0.789 -
97.6
80
81.3
15.6
71
5.5
66
86
91
44.2-97.2
Source
(Lv, etal.,2013)
(Wong, etal., 2003)
(Ajmal, etal.,2000)
(Sthiannopkao &
Sreesai, 2009)
(Namasivayam &
Sureshkumar, 2008)
(Ajmal, etal., 2001)
(Alvarez-Ayuso, et
al., 2003)
4-29
-------
Section 4 - Preliminary Study Findings
Table 4-8. Low-Cost Alternatives to Coal-based Activated Carbon Treatment Identified in EPA's Literature Search -
Summary of Treatment Results
Scale of
Study
Type of
Wastewater
Treated surface
finishing
Industrial
Synthetic
Uncertain
Adsorbent Material
Modified starch
(flour)
Dried plants
Eucalyptus bark
Rice husk
Fly ash
Clinoptilolite
Targeted Metal
Iron
Copper
Lead
Cadmium
Nickel
Cadmium
Copper
Lead
Zinc
Chromium (VI)
Iron
Lead
Cadmium
Copper
Iron
Lead
Cadmium
Copper
Lead (II)
Nickel (II)
Zinc
LTA
Concentration
(mg/L)a
NA
0.815
0.2
0.13
0.942
0.13
0.815
0.2
0.549
0.572
NA
0.2
0.13
0.815
NA
0.2
0.13
0.815
0.2
0.942
0.549
Final Metal
Concentration
(mg/L)b
0.04 - 0.66
0 - 0.076
1.03-5.01
0.071-2.18
87.2 - 503
0.004
0.163 - 0.338
0.008 - 0.051
0.44 - 2.75
2
0.09-3.71
0.15-0.93
0.15-0.36
0.1-4.1
1.56-6.34
0.28-0.92
0.13-0.36
0.08-3.40
466
1.6
Negligible
Percent
Removal
(%)
53.5-95.4
66.7 - 100
40.3-77.6
38.3-91.7
41.8-85.9
94.1
84.1-92.4
99.2-99.9
84.1-97.5
99
68.6-99.3
22.2 - 87.2
26.0-67.9
24.5 - 98.2
46.2 - 86.8
21.79-76.1
25.2-73.5
37.4-98.5
55
93.6
100
Source
(Sancey, etal.,
2011)
(Chiban, et al.,
2011)
(Sarin, et al., 2006)
(Hegazi, 2013)
(Inglezakis etal.,
2007)c
(Argun, 2008)c
(Athanasiadis &
Helmreich, 2005)c
NA - Not applicable
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the BPT technology basis.
c As cited in Fu & Wang, 2011.
4-30
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Section 4 - Preliminary Study Findings
Table 4-9. Biosorbent Treatment Identified in EPA's Literature Review - Summary of Treatment Results
Scale of Study
Pilot scale,
continuous
operation
(column)
Bench scale,
batch operation
Type of
Wastewater
Chromium
plating
Electroplating
Synthetic
Biosorbent Material
Dry yeast (Saccharomyces
cervisiae)
Dead fungal biomass (A.
niger)
Dead fungal biomass (A.
sydoni)
Dead fungal biomass (P.
janthinellum)
Bacterial strains isolated from
electroplating effluent (B.
cereus)
Brown seaweed (Ecklonia sp.)
Brown seaweed (F.
vesiculosus)
Green seaweed (Ulva spp.)
Red seaweed (P. palmata)
Targeted Metal
Chromium (VI)
Chromium (VI)
Chromium (VI)
Chromium (VI)
Chromium (VI)
Zinc
Chromium (VI)
Chromium (III)
Chromium (VI)
Chromium (III)
Chromium (VI)
Chromium (III)
Chromium (VI)
Chromium (III)
LTA
Concentration
(mg/L)a
0.572
0.572
0.572
0.572
0.572
0.549
0.572
0.572
0.572
0.572
Final Metal
Concentration
(mg/L)b
154
8.65
10.4
11.2
239
0.1
Negligible
1
795
831
1117
590
930
702
Percent
Removal
(%)
61.5
71.2
65.3
62.6
76
97.6
100
33.3
18.7
NAC
14.7
NAC
18.4
NAC
Source
(Colica, etal.,
2012)
(Kumar, etal.,
2008)
(Naik, etal., 2012)
(Park, et al., 2006)
(Murphy, et al.,
2009)
NA - Not applicable
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the BPT technology basis.
0 Chromium (III) was not present in the untreated wastewater; it was generated through the reduction of chromium (VI) and subsequently biosorbed.
4-31
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Section 4 - Preliminary Study Findings
Table 4-10. Nanomaterials Identified in EPA's Literature Review
Nanosized ferric oxides Nanosized aluminum oxides
Goethite (a-FeOOH)
Hematite (a-Fe2Os) Nanosized titanium oxides
Hydrous ferric oxide
Maghemite (y-Fe2O3) Nanosized zinc oxides
Magnetite (Fe2O3)
Polymer-modified (FesO/i) magnetic nanoparticles Nanosized magnesium oxides
Nanosized manganese oxides Nanosized cerium oxides
Hydrous manganese oxide
Mixed-valence manganese oxides Single- or multi-walled carbon nanotubes
Source: (Fu & Wang, 2011; Ge, et al, 2012; Hua, et al., 2012).
4.4.3.2 Ion Exchange
Ion exchange technology has been used successfully in industry for removing heavy
metals from wastewater, including metal finishing wastewater. It is also effective at treating
wastewater containing complexed metals (WEF, 2008). Ion exchange is a process in which ions,
held by electrostatic forces within charged functional groups on the surface of the ion exchange
resin, are exchanged for ions of similar charge from the solution in which the resin is immersed.
This is classified as a sorption process because the exchange occurs on the surface of the resin,
and the exchanging ion undergoes a phase transfer from solution phase to solid phase (i.e., from
wastewater to the ion exchange resin). Resin performance declines with continued use and
requires regeneration to remove the impurities removed from the wastewater (U.S. EPA, 2000a).
EPA identified ion exchange in the 1983 regulations for the in-process treatment of cadmium-
bearing wastewaters, but the technology was not included as part of the BPT technology basis
(U.S. EPA, 1983a). During the MP&M rulemaking, EPA found that ion exchange technologies
were used as a polishing step to remove trace metals from electroplating wastewater (U.S. EPA,
2000a).
Ion exchange systems have been demonstrated in recovery operations at metal finishing
facilities, particularly to concentrate and purify plating baths; however, they are only part of the
technology basis in the 1983 ELGs for new sources to control cadmium (see Appendix C). Table
4-11 summarizes the results of ion exchange treatment for removing hexavalent chromium—the
only metal identified by EPA in the recent literature on ion exchange. Treatment performance
data EPA collected in 1983 from metal finishing plants operating this technology have
comparable concentrations for hexavalent chromium, as well as effective removals for a number
of other heavy metals including aluminum, cadmium, copper, iron, manganese, nickel, silver, tin,
and zinc. Recent studies in adsorption and ion exchange technologies focus on alternative
sorbents and tend to overlap because sorption processes can include surface interactions
involving both adsorption and ion exchange. Many of the low-cost adsorbents identified
previously in Table 4-3, such as zeolites and silicate materials, have been identified as potentially
effective adsorbents as well as ion exchange resins.
4-32
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Section 4 - Preliminary Study Findings
Table 4-11. Ion Exchange Treatment Identified in EPA's Literature Review - Summary of Treatment Results
Scale of Study
Bench scale,
continuous
operation
Bench scale,
batch operation
Type of
Wastewater
Electroplating
Synthetic
Synthetic
Resin Material
Strongly basic resin ( with
trimethylbenzyl ammonium)
Strongly basic resin (with
electrodialysis)
Acidic resin (based on
hydrophilic polymer)
Targeted
Metal
Chromium
(VI)
Chromium
(VI)
Chromium
(VI)
LTA
Concentration
(mg/L)a
0.572
0.572
0.572
Final Metal
Concentration
(mg/L)b
Negligible
3.67-4.09
0.0005
Percent
Removal
(%)
100
98.7-
98.8
99.5
Source
(Saparietal., 1996)c
(Ahmen-Basha, et
al., 2008)
(Kabay et al., 2003)c
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the BPT technology basis.
c As cited in (Owlad, et al., 2009).
4-33
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Section 4 - Preliminary Study Findings
Based on the studies, ion exchange performance is sensitive to pH, temperature, and
initial metal concentrations (Ahluwalia & Goyal, 2007; Fu & Wang, 2011; Owlad, et al., 2009).
In recent discussions with industry, EPA also learned that penetration of this technology for
treating metal finishing wastewater has been hindered by costs related to resin regeneration and
solids disposal (ERG, 2016).
4.4.4 Membrane Filtration
The 1983 Metal Finishing rulemaking identified a total of 20 membrane filtration systems
used to remove solids from industrial wastewater, including metal finishing wastewater. At that
time, EPA identified seven metal finishing plants using these systems, although the types of
membrane filtration systems were not specified. The research at the time also showed effective
treatment of wastewater containing specific complexing agents on a pilot scale. By 1996, EPA
identified nearly 700 MP&M facilities that used membrane filtration systems, a subset of which
likely comprised metal finishing facilities. The 1983 regulations did not identify filtration
technologies as part of the technology basis (U.S. EPA, 1983a, 2000b).
Membrane filtration technologies are basically physical sieves that separate contaminants
such as metals and oils from the wastewater. They are widely used as a solids removal step due
to their high removal efficiency, easy operation, and minimal space requirements. As a solids
removal step, this technology is preceded by a treatment technique, such as chemical
precipitation, to prepare the wastewater for solids removal (Fu & Wang, 2011; U.S. EPA,
1983a). Commercially available membranes offer a variety of pore sizes to achieve the desired
level of filtration, and each type of membrane can tolerate a range of operating pressures. Figure
4-6 illustrates typical membrane pore sizes and operating pressures used to filter selected
contaminants.
Microfiltration
Membrane pore size: 0.1 - 5 urn
Operating pressure: 0.1 - 3 bar
Suspended particles
Ultrafiltration
Membrane pore size: 20 nm - 0.1 um
Operating pressure: 2-10 bar
Small colloidal matter
Nanofiltration
Membrane pore size: » 1 nm
Operating pressure: 5-30 bar
Multivalent salts (Ca2+, Mg2+
Small solutes, organic matter
Reverse Osmosis
Membrane pore size: 0.1 - 1 nm
Operating pressure: 10 -100 bar
Monovalent salts (Na*, CM
Water
Figure 4-6. Illustration of Membrane Filtration Technologies
4-34
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Section 4 - Preliminary Study Findings
From the literature, EPA identified the following membrane processes used to remove
metals from wastewater: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and
reverse osmosis (RO). These processes differ by the pore size and the differential pressure
required across the membrane for filtration to occur (Fu & Wang, 2011; U.S. EPA, 2000a).
4.4.4.1 Microfiltration
Microfiltration technologies use membranes that reject particle sizes ranging from 0.1 to
5 microns. Microfiltration is an alternative to gravity clarification after chemical precipitation
and has been used to remove precipitates from metal-bearing industrial wastewater, such as
metal finishing wastewater. Microfiltration membranes primarily consist of homogeneous
polymer material and operate between 0.1 and 3 bar, depending on membrane pore size, to drive
the separation of the contaminants across the membrane (U.S. EPA, 2000a). EPA observed use
of microfiltration for solids separation following chemical precipitation at metal finishing
facilities visited as part of the MP&M rulemaking development. Based on these visits, EPA
determined that well-operated chemical precipitation systems followed by microfiltration
removed 99.6 percent of targeted metals, compared to 96.7 percent for chemical precipitation
systems followed by gravity clarification. Absent a preceding chemical precipitation step,
microfiltration does not achieve high removal efficiencies of dissolved heavy metals, due to its
large membrane pore size.
Similar to clarification (which is part of the BPT), microfiltration generates a
concentrated, suspended-solid slurry that requires dewatering. Microfiltration is more expensive
than conventional gravity clarification because it requires periodic membrane regeneration to
maintain treatment effectiveness (U.S. EPA, 2000a).
4.4.4.2 Ultrafiltration
Ultrafiltration technologies reject particles ranging from 0.02 to 0.1 microns (or 20 nm to
100 nm). Ultrafiltration membranes also operate between 2 to 10 bar to drive the separation of
contaminants, particularly dissolved and colloidal material, across the membrane. The pore size
of the membrane does not prevent dissolved metals or some metal complexes from passing
through, but has been shown to remove oil and grease (Fu & Wang, 2011; U.S. EPA, 2000a).
The technology was not well demonstrated for metals removal during the 1983 Metal Finishing
or the 2000 MP&M rulemakings.
Recent literature reports that micellar-enhanced ultrafiltration (MEUF) and polymer-
enhanced ultrafiltration (PEUF) use surfactants and complexing agents, respectively, to aid in the
aggregation of micelles or polymers that bind with metal ions to form a macro-structure that is
large enough to be rejected by ultrafiltration membranes (as illustrated in Figure 4-7 for MEUF)
(Fu & Wang, 2011). In the last decade, researchers have identified several surfactants and
complexing agents that form macromolecules with heavy metals (e.g., copper, chromium, nickel,
cadmium), which are susceptible to this level of filtration.
4-35
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Section 4 - Preliminary Study Findings
Surfactant solution containing micelles
Wastewater containing metal ions
v
x
Ultrafiltration membrane
• •«
• • • •
Permeate
Retentate
Figure 4-7. Illustration of Micellar-Enhanced Ultrafiltration
The metal removal efficiency of MEUF depends on the characteristics and concentrations
of the metals and surfactants used, as well as membrane operating parameters (Fu & Wang,
2011). The effectiveness of PEUF membranes also relies on membrane operating parameters, in
addition to the polymer used to selectively remove the metal, the targeted metal to polymer ratio,
and the presence of other metals in solution (Bakarat, et al., 2010; Fu & Wang, 2011; Owlad, et
al., 2009). Table 4-12 summarizes the treatment effectiveness of Ultrafiltration membranes at
optimal conditions in bench scale experiments that EPA identified in recent literature. Several
MEUF and PEUF systems demonstrate at bench scale the capability to achieve effluent
concentrations equal to or lower than the LTAs for the BPT technology basis.
4-36
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Section 4 - Preliminary Study Findings
Table 4-12. Ultrafiltration Treatment Identified in EPA's Literature Review - Summary of Treatment Results
Scale of Study
Type of
Wastewater
Ultrafiltration Membrane
Targeted
Metal
LTA
Concentration
(mg/L)a
Final Metal
Concentration
(mg/L)b
Percent
Removal
(%)
Ultrafiltration Membranes
Bench scale,
batch operation
Bench scale,
continuous
operation
Synthetic
Synthetic
Carbon
Nitrated carbon
Animated carbon
Polyamide thin-film
Chromium (VI)
Chromium (VI)
Chromium (VI)
Chromium (VI)
0.572
0.572
0.572
0.572
40
160
120
Not reported
96
84
88
77
MEUF and PEUF Systems
Bench scale,
continuous
operation
Synthetic
Ground water
Ceramic
(Dodecylbenzenesulfonic
acid, dodecylamine)
Polysulfone (Sodium dodecyl
sulfate)
Polyethersulfone
(polyethyleneimine)
Polyethersulfone (Carboxy
methyl cellulose)
Amicon 8400
Ceramic (Poly(acrylic acid)
sodium)
Polysulfone (Poly(ammonium
acrylate))
Polysulfone (chitosan,
polyethylenimine, or pectin)
Polyacrylnitrile fibre
(hexadecylpyridine chloride)
Lead (II)
Arsenate
Cadmium (II)
Zinc (II)
Copper (II)
Nickel (II)
Copper (II)
Chromium (III)
Nickel (II)
Chromium (III)
Chromium (VI)
Copper (II)
Cadmium (II)
Chromium (VI)
Chromium (VI)
0.2
NA
0.13
0.549
0.815
0.942
0.815
0.572
0.942
0.572
0.572
0.815
0.13
0.572
0.572
0.044 - 0.076
3.56-6.16
1-4
1-4
3
Negligible
0.24
0.05
0.09
<0.9
<9
0.8
1.12
7
0.02
>99
19
92-98
92-98
94
100
97.6
99.5
99.1
82-100
82-100
99.5
99
30
90
Source
(Pugazhenthi et
al., 2005)c
(Hafiane, 2000)c
(Ferellaetal.,
2007)d
(Huang etal.,
2010)d
(Molinari et al.,
2008)d
(Bakarat &
Schmidt, 2010)
(Korus & Loska,
2009)d
(Camarilloaetal.,
2010)d
(Ennigrou et al.,
2009)d
(Arouaetal.,
2007)c
(Bohdziewicz,
2000)c
NA - Not applicable
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the BPT technology basis.
c As cited in (Owlad, et al., 2009).
d As cited in (Fu & Wang, 2011).
4-37
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Section 4 - Preliminary Study Findings
In the 1983 ELGs, EPA identified ultrafiltration as an effective technology for treating
oily wastes, not as a metals removal technology. These results show the potential of enhanced
membrane filtration systems to treat heavy metals to concentrations lower than those established
in the 1983 ELGs for cadmium, chromium, copper, lead, nickel, and zinc. A disadvantage of this
technology is that surfactants and complexing agents account for a large portion of operating
costs and can become less cost effective if not recovered and reused (Fu & Wang, 2011).
4.4.4.3 Nanofiltration
Nanofiltration membranes retain particle sizes greater than 0.001 microns, which
approach the size of dissolved multivalent metals, such as nickel, chromium, copper, and arsenic.
Similar to microfiltration and ultrafiltration, nanofiltration is a pressure driven process, with
operating pressures typically ranging between 5 and 30 bar (Fu & Wang, 2011). Nanofiltration
membranes are commercially available. Summarizes the information EPA identified on the
treatment effectiveness of nanonfiltration membranes. Nanofiltration membranes are relatively
new; EPA did not identify nanofiltration during the development of the 1983 regulations and
further, did not find recent literature on the use of nanofiltration for treating metal finishing
wastewater.
One disadvantage of nanofiltration membranes is that, because of the small pores, the
membranes are more prone to fouling and may require frequent regeneration to remove
contaminants that impact membrane performance. Frequent regeneration adds to the total
operating costs of the system (Fu & Wang, 2011).
4.4.4.4 Reverse Osmosis (RO)
RO uses a semi-permeable membrane and a pressure differential to drive the wastewater
across the membrane. The membrane rejects contaminants, including particulates and dissolved
contaminants, from passing across the membrane (WEF, 2008). The rejection rate for specific
contaminants is dependent on several operating factors. The feed rate, temperature, pH, as well
as the operating pressure across the membrane, can all impact overall treatment performance
(Qin, et al., 2002; WEF, 2008).
The 1983 ELGs evaluated the performance of RO units for treating oily wastes at several
metal finishing facilities. During the MP&M rulemaking, EPA found that RO applications for
metal recovery in cadmium, copper, nickel, and zinc plating operations rejected 99 percent of
multivalent ions and 90 to 96 percent of monovalent ions. Similar to ion exchange, RO is not
sensitive to the presence of chelating agents in the wastewater. The permeate stream is usually of
sufficient quality to be reused as rinsewater (U.S. EPA, 2000a). As reported by Fu & Wang
(2011) and summarized in Table 4-13, several studies evaluated the performance of RO under
various conditions and found heavy metal removal rates up to 99.5 percent. Fu & Wang (2011)
did not identify whether the treatment performance represented metal finishing wastewater;
however, in a separate study by Qin et al. (2002), RO successfully removed greater than 99.8
percent nickel concentrations from nickel plating wastewater.
4-38
-------
Section 4 - Preliminary Study Findings
Table 4-13. Nanofiltration Treatment Identified in EPA's Literature Review - Summary of Treatment Results
Scale of Study
Bench scale,
continuous
operation
Type of
Wastewater
Synthetic
Nanofiltration
Membrane
Composite polyamide
Polyamide thin-film
Commercial
Targeted Metal
Chromium (VI)
Chromium (VI)
Nickel (II)
Nickel (II)
Nickel
Cadmium
LTA
Concentration
(mg/L)a
0.572
0.572
0.942
0.942
0.942
0.13
Final Metal
Concentration
(mg/L)b
10
60
0.1
20
0.05
0.87
Percent
Removal
(%)
99
94
98
92
98.9
82.7
Source
(Muthukrishnan
& Guha, 2008)c
(Murthy &
Chaudhari,
2008)d
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the BPT technology basis.
c As cited in (Owlad, et al., 2009).
d As cited in (Fu & Wang, 2011).
Table 4-14. RO Treatment Identified in EPA's Literature Review - Summary of Treatment Results
Scale of Study
Bench scale, continuous
operation
Bench scale, continuous
operation
Type of
Wastewater
Uncertain
Nickel plating
Targeted Metal
Copper (II)
Nickel (II)
Copper (II)
Arsenic (V)
Arsenic (III)
Nickel (II)
Zinc (II)
Nickel
LTA
Concentration
(mg/L)a
0.815
0.942
0.815
NA
NA
0.942
0.549
0.942
Final Metal
Concentration
(mg/L)b
2.5
2.5
1-30
0.005-0.045
0.4-0.225
0.31-1.18
0.70 - 1.87
0.009
Percent
Removal
(%)
99.5
99.5
70-95
91-99
20-55
99.3
98.9
>99.8
Source
(Mohsen-Niaetal.,
2007)c
(Zhang et al., 2009)c
(Chan & Dudeney,
2008)c
(Ipek, 2005)c
(Qin, et al., 2002)
NA - Not applicable
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the BPT technology basis.
c As cited in (Fu & Wang, 2011).
4-39
-------
Section 4 - Preliminary Study Findings
The metal plating industry has a growing interest in RO for recovering and reusing
rinsewater (Qin, et al., 2002). Qin, et al. (2002) evaluated RO treatment of spent rinse water for
reuse as alkaline rinsewater. Their study investigated the performance of four different
commercial RO membranes in treating four variations of metal plating rinsewater containing
cyanide and heavy metals such as chromium, copper, zinc, lead, nickel, and iron, among other
contaminants from metal plating. They observed reduced fouling when ultrafiltration preceded
RO; therefore, in the treatability experiments, they pretreated nickel plating wastewater with an
ultrafiltration membrane prior to RO, which resulted in greater than 99 percent removal of nickel
(Qin, et al., 2002). Csefalvay et al. (2009) and Sudilovskiy (2008) also reported greater than 95
percent copper removals, as cited in Fu & Wang (2011), using sequential RO and nanofiltration
technologies.
Due to the operational costs of this technology—particularly costs of high power
consumption, regenerating the membranes, and potential need for softening pretreatment—RO is
not a widely applied technology in the metal plating industry (Fu & Wang, 2011).
4.4.5 Flotation
Since the 1990s, studies have investigated flotation technologies to separate heavy metal
precipitates from wastewater. Common flotation technologies include dissolved air flotation
(DAF), ion flotation, and precipitation flotation (Fu & Wang, 2011).
DAF relies on the physical interaction between bubbles of air introduced to the
wastewater and the suspended particles in the wastewater. The interaction creates agglomerates
of particles that float to the surface of the water and are skimmed off and removed (Fu & Wang,
2011; WEF, 2008). The DAF technology is well demonstrated for treating oily wastes in the
metal finishing industry, and was identified as part of the technology basis for the separate
treatment of oily wastes in the 1983 regulations. The review of metal finishing facilities during
the MP&M rulemaking suggests that this technology is still prevalent in the industry for oily
waste treatment (U.S. EPA, 1983a, 2000b). EPA did not find recent literature describing the
effectiveness of DAF systems in heavy metals removal.
Ion flotation uses surfactants to make the ionic metal species in the wastewater
hydrophobic, and subsequently separates the hydrophobic species from the wastewater using air
bubbles. The wastes are then skimmed off the surface of the water. Several studies investigated
the effectiveness of ion flotation for treating lead, copper, cadmium, silver, zinc, and trivalent
chromium. Yuan et al. (2008), as cited in Fu & Wang (2011), reported use of tea saponin,
sodium dodecyl sulfate, and hexadecyltrimethyl ammonium bromide as complexing agents. Tea
saponin demonstrated bench scale removal of lead (II), copper (II), and cadmium (II) at 90.0,
81.1, and 71.2 percent, respectively. Also reported in Fu & Wang (2011), Polat & Erdogan
(2007) indicated optimal removals of copper (II), zinc (II), chromium (III), and silver reached
approximately 74 percent under acidic conditions and 90 percent under basic conditions (with
the aid of secondary hydroxide precipitation in the high pH range). Fu and Wang (2011) pointed
out that that study did not report treated effluent concentrations (Fu & Wang, 2011).
Precipitate flotation applies the flotation method to remove metal precipitates formed
through common precipitation techniques. Air bubbles introduced into the wastewater carry the
precipitates to the surface of the water, where they are skimmed off. Capponi et al. (2006), as
4^40
-------
Section 4 - Preliminary Study Findings
cited in Fu and Wang (2011), conducted a bench scale study which showed 96.2 percent removal
of trivalent chromium by precipitate flotation of dilute aqueous solutions. The study did not
report treated effluent concentrations (Fu & Wang, 2011). EPA did not identify these
technologies in the 1983 regulations or the MP&M regulations for the treatment of heavy metals.
4.4.6 Electrochemical Treatment
Electrochemical treatment involves the application of an electric potential across a
cathode and anode to facilitate the recovery or precipitation of heavy metals in wastewater (Fu &
Wang, 2011). As part of the 1983 ELGs, EPA identified electrochemical oxidation, reduction,
and regeneration as alternative treatments for chromium and cyanide-bearing wastewaters. EPA
identified several treatment methods using this electrochemical approach, including
electrodialysis, electrocoagulation, electroflotation, and electrodeposition, which are further
described in this section. Electrochemical treatment methods have not found application on a
large industrial scale due to high initial costs and potentially high energy costs (Fu & Wang,
2011).
4.4.6.1 Electrodialysis
Electrodialysis is the process of separating metal ions across a charged membrane,
typically an ion exchange membrane. The use of ion exchange membranes in this process is also
referred to as electrochemical ion exchange. Electrodialysis uses an electric field as the driving
force across the membrane, rather than the typical pressure driven process described for
membranes above, and the literature shows effective treatment of several metals, including
hexavalent chromium and lead, as well as copper and iron from copper electrowinning
operations. In a pilot study, electrodialysis treated hexavalent chromium down to levels as low as
0.1 milligrams per liter, which is lower than chromium limits in the 1983 regulations (Fu &
Wang, 2011; U.S. EPA, 1983a). One study observed the recovery of chromium from plating
rinsewater using a combination of ion exchange and electrodialysis units, which effectively
recovered nearly 99 percent of chromium on a bench scale. The treated effluent had sufficient
quality to be reused in the process. The study did not describe disposal considerations for the
reject stream (Ahmen-Basha, et al., 2008). Further investigations also determined that increasing
voltage and temperature improved the performance of electrodialysis; however, the treatment
effectiveness may be reduced with greater flow rates and concentrations (i.e., conditions that
would apply to a full scale operation) (Fu & Wang, 2011).
4.4.6.2 Electrocoagulation
Electrocoagulation incorporates electric potential into conventional chemical
precipitation processes to improve heavy metals removal from wastewater. The process may also
be referred to as electrochemical precipitation, which involves an electrolytic cell containing
steel plates (one cathode and one anode) submerged in water and between which an electric
current is applied. By applying an electric potential between the cathode and anode, the charges
that hold the heavy metals in solution destabilize and coagulate to form a mass, which can be
easily removed. Additional chemicals are generally not required to facilitate precipitation, just
the anode and cathode. The effectiveness of the process has been found to rely on the electrical
potential, hydraulic retention time, and solution pH. According to Fu & Wang (2011),
electrocoagulation applications have grown over the past two decades. At the promulgation of
-------
Section 4 - Preliminary Study Findings
the 1983 Metal Finishing regulations, its application was evaluated for treating chromium-
containing wastewater; however electrocoagulation could also be effective in removing other
heavy metals from wastewater (U.S. EPA, 1983a). Table 4-15 provides a summary of available
electrocoagulation treatment results that EPA identified in recent literature.
Table 4-15. Electrocoagulation Treatment Identified in EPA's Literature Review -
Summary of Treatment Results
Scale of
Study
Bench
scale,
unknown
operation
Uncertain
Bench
scale,
batch
operation
Type of
Wastewater
Electroplating
Uncertain
Synthetic
Targeted
Metal
Chromium
(VI)
Manganes
e(II)
Nickel (II)
Zinc (II)
Arsenic
(HI)
Arsenic
(V)
Chromium
(VI)
Copper,
Nickel,
Zinc,
Cadmium,
Lead, Iron
LTA
Concentration
(mg/L)a
0.572
NA
0.942
0.549
NA
NA
0.572
0.815,0.942,
0.549,0.13,0.2,
NA
Final Metal
Concentration
(mg/L)
0.12
Not reported
312.9
21.8
Negligible
Negligible
O.022
0.022
Negligible
0.5 - 40
Percent
Removal
(%)
98.5
99.6
77.0-
100
85.1
78.2
100
100
>99
>99
100
90-99
Source
(Owlad, etal.,
2009)
(Shafaeietal.,
2010)c
(Kabdas.li et
al, 2009)c
(Parga et al.,
2005)c
(Olmez, 2009)
(Merzouk, et
al., 2009)
NA - Not applicable
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory
option 1 in Table 4-1).
b Concentrations in bold text indicate lower treatment results than the long-term average concentration for the
BPT technology basis.
c As cited in (Fu & Wang, 2011).
Electrocoagulation studies involving the removal of zinc, copper, chromium, nickel,
silver, lead and dichromate show effective and consistent removals across a broad concentration
range (Akbal, et al., 2011; Fu & Wang, 2011; Merzouk, et al., 2009). Kabdasli et al. (2009) also
investigated the treatability of complexed metals in nickel and zinc plating wastewater using
electrocoagulation. In the study, both zinc and nickel were completely removed using this
process (Fu & Wang, 2011; Kabdasli, et al., 2009). In Kobya et al. (2010) reported that
electrocoagulation showed effective removal (greater than 99 percent) of cadmium, nickel, and
cyanide at optimal conditions. Electrocoagulation may be an alternative treatment option for
removing cyanide by first dissociating the metal complexes in electroplating wastewater,
removing the freed cadmium and nickel ions, and generating a new metal complex with the
metal ions generated by the electrodes (Kobya, et al., 2010). During an electrocoagulation
4-42
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Section 4 - Preliminary Study Findings
process, no additional chemicals are added to the wastewater, in contrast to conventional
precipitation methods. Therefore, this process generates a more compact sludge, which can
significantly reduce disposal costs (Akbal & Camci, 2011).
The studies indicate that for electrocoagulation, optimization of metals removal relies on
the electrode positions, electrical potential, and pH of the solution. According to Senturk (2013),
iron electrodes are more effective than aluminum in treating electroplating wastewater
containing zinc and cyanide; however, in a separate study, Akbal & Camci (2011) noted iron-
aluminum electrode pairs to be equally efficient in electroplating wastewater containing nickel,
copper, and chromium (Akbal & Camci, 2011; Senturk, 2013). Olmer (2009) noted stainless
steel electrodes for use in hexavalent chromium removals in hard chrome plating rinsewater.
Several studies have focused on optimization schemes for different types of wastewater
characteristics and suggest that conditions for optimal removal should be identified on a case-by-
case basis. EPA did not identify any full-scale application of electrocoagulation processes for
metal finishing wastewater during the literature review.
4.4.6.3 Electroflotation
Similar to conventional flotation techniques, electroflotation relies on the use of air
bubbles to bind with and transport metal precipitates to the surface of the water. Bubbles
consisting of hydrogen and oxygen gases are generated through the electrolysis of the water on
the surface of the electrodes as an electric potential is applied. These hydrogen and oxygen gases
become the transport media for metal ions. Multiple studies have been reported to show potential
uses in the metal finishing industry for treating iron, nickel, copper, zinc, lead, and cadmium at
up to 99 percent removal efficiency (Fu & Wang, 2011). In a separate study, copper and nickel
removals under optimal conditions reached 98 to 99 percent (Khelifa, et al., 2005). Table 4-16
summarizes the removals of electroflotation that EPA identified in recent literature.
Table 4-16. Electroflotation Treatment Identified in EPA's Literature Review -
Summary of Treatment Results
Scale of Study
Bench scale,
batch operation
Type of
Wastewater
Synthetic
Targeted
Metal
Zinc (II)
Nickel (II)
Copper (II)
LTA
Concentrati
on
(mg/L)a
0.549
0.942
0.815
Final Metal
Concentrati
on (mg/L)
0.8
2
1
Percent
Removal
(%)
96
98-99
99-99
Source
(Casqueira et
al., 2006)b
(Khelifa etal.,
2005)b
a Long-term average concentrations represent total metal concentrations for the BPT technology basis (regulatory
option 1 in Table 4-1).
b As cited in (Fu & Wang, 2011).
4.4.6.4 Electrodeposition
Electrodeposition (or electrophoretic deposition) is a general term used for a process
using an applied current across electrodes to deposit metals onto an electrode. Electrodeposition
can refer to an electroplating process; however, it also covers a number of processes to recover
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Section 4 - Preliminary Study Findings
metals from wastewater. Terms such as 'electrolytic recovery' and 'electrowinning' are also
common terms for recovery processes using electrodeposition (Kirk-Othmer, 2004; U.S. EPA,
1983a). Oztekin & Yazicigil (2006) and Chang et al. (2006), as cited in Fu & Wang (2011)
investigated the recovery of metals from complex wastewaters, which effectively removed up to
90 percent and nearly 96 percent, respectively, of copper from complex wastewater (Fu & Wang,
2011). A disadvantage of electrodeposition—and of other electrochemical methods discussed in
this section—is the high energy requirement (resulting in high operating costs).
4.4.7 Biological Treatment
The literature suggests that biological treatment of metal-bearing wastewater is an
expanding area of research. EPA identified a few biological treatment systems during the 1983
rulemaking; however, biological treatment was not part of the technology basis for the 1983
Metal Finishing ELGs.
Researchers have evaluated the use of biological processes to reduce hexavalent
chromium (Cr (VI)) to trivalent chromium (Cr (III)) in industrial electroplating wastewater prior
to conventional chemical precipitation. A pilot study conducted on the ChromeBac™ biological
system in Malaysia showed successful reduction of Cr (VI) down to less than 0.05 milligrams per
liter (or greater than 99.8 percent removal). Total chromium was reduced to 0.7 mg/L (or 98.6
percent removal) (Ahmad, et al., 2010). Chromium removal (as Cr (III)) was completed using
conventional chemical precipitation.
Sulfide precipitation processes have used sulfate-reducing bacteria (SRB) to generate
sulfide on-site. The SRB oxidizes simple organic compounds and reduces sulfates under
anaerobic conditions to form biogenic hydrogen sulfide. Hydrogen sulfide is subsequently used
to precipitate metals. The process was tested on zinc-bearing wastewater (containing 400 mg/L
of Zn) and proved effective at completely removing soluble zinc as well as sulfate and total
organic compounds (TOC) (Fu & Wang, 2011). According to Huisman et al. (2006), Paques BV,
a Netherlands company, is operating full-scale Sulfateq® technology using SRB for wastewater
treatment. They have implemented nearly 500 industrial installations for several industries. The
technology reliably removes sulfate, nitrate, heavy metals, selenium, and fluoride from metal and
mining industry wastewater. Another Pacques BV technology, the Pacques Thioteq process, was
also developed to aid in the on-site generation of biogenic hydrogen sulfide when wastewater
characteristics inhibit its production (Huisman, et al., 2006). EPA did not identify application of
these technologies in metal finishing wastewater.
In another study, Park et al. (2005) evaluated the use of iron-oxidizing bacteria to reduce
iron levels from electroplating wastewater. The researchers used a biological treatment step to
remove iron from the wastewater without removing other heavy metals such as zinc or nickel.
After biological pretreatment, wastewater was treated by sodium hydroxide precipitation to
generate sludge with high concentrations of zinc or nickel. The iron-removal pretreatment
reduced the volume of the hydroxide sludge generated, allowing for more economical recovery
of higher value metals (Park, et al., 2005).
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Section 4 - Preliminary Study Findings
4.4.8 Summary of Wastewater Treatment Technologies
Thus far, EPA's literature review on wastewater treatment has primarily focused on
articles from academic journals, which identified numerous wastewater treatment technologies
for removing heavy metals; however, many are not new technologies or new applications for
metal finishing wastewater treatment. In fact, EPA identified many of these technologies during
the development of the 1983 Metal Finishing ELGs (as summarized in Table 4-5 and in
Appendix C). Although these technologies effectively removed metals from wastewater at
sampled metal finishing facilities, many were relatively new applications for the industry and
likely cost more than the treatment technologies that industry used at the time. In particular, EPA
had considered granular bed and diatomaceous earth filtration, or similar media filtration
technologies, as part of regulatory option 2 (see Table 4-5); however, EPA ultimately did not
select these alternatives because they were not economically feasible at the time.
There were additional technologies that EPA also considered in the 1983 ELGs,
specifically for treating oily wastes and toxic organics, which are now finding application in
heavy metals removal. These included sorption, advanced membrane filtration, flotation, and
electrochemical methods. Advances in heavy metals removal have also included research into
better-performing or more cost-effective chemical additives or materials. Based on the number of
articles EPA identified, sorption and membrane filtration seem to be the focus of recent research
for heavy metals removal. The articles present bench-scale results that suggest promising
advances in treating heavy metals; however, EPA has not determined whether any of these
promising technologies have been implemented at full-scale within the metal finishing industry.
From discussions with pretreatment coordinators and with industry representatives, EPA
learned that most metal finishing facilities continue to use conventional chemical precipitation
technologies, although some facilities have added a polishing step such as membrane filtration or
sorption technologies. EPA also learned that advances in wastewater treatment technologies in
the industry have been slow, and most likely inhibited by the costs of installing and operating
more advanced technologies. However, EPA has not fully evaluated the extent to which the
metal finishing industry is applying technologies beyond the BPT technology basis (ERG, 2016;
U.S. EPA, 2015a).
4.5 Applicability and Other Regulatory Considerations
EPA expects that in the 32 years that have passed since the promulgation of the Metal
Finishing ELGs, metal finishing process technologies and chemistries have evolved and
advanced wastewater treatment technologies have become available for metal finishing
wastewater treatment. EPA has often received requests for official EPA determinations on the
applicability of the ELGs to specific operations or to provide clarification on metal finishing
operations described in the rule. Additionally, stakeholders have also urged EPA to consider
other regulations that may also have a bearing on the industry. This section outlines the key
considerations that have been identified thus far in the Metal Finishing Preliminary Study.
4.5.1 Rule Implementation
Pretreatment coordinators noted that POTWs are still implementing 40 CFR Part 413
(Electroplating) pretreatment standards for some metal finishing facilities. Most metal finishing
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Section 4 - Preliminary Study Findings
facilities should be covered by 40 CFR Part 433 pretreatment standards, and not 40 CFR Part
413 standards. The scope of facilities still regulated under 40 CFR Part 413 is technically limited
to job shops and IPCB manufacturers that were considered existing sources at the time of the
promulgation of the 1983 Metal Finishing ELGs. In a public comment on EPA's 2014 Effluent
Guidelines Program Plan, the Association of Clean Water Administrators (ACWA) urged EPA
to consider merging the facilities still operating under 40 CFR Part 413 regulations into the
Metal Finishing ELGs (U.S. EPA, 2015c).
Unlike wastewater regulations for other metal-related industries (e.g., aluminum forming,
iron and steel), which include production-based limits, the Metal Finishing ELGs are
concentration-based which can make them easier to apply in wastewater permits. Due to
potential overlap of Metal Finishing regulations with other metal related regulations that make
take precedence, pretreatment coordinators suspect that there is confusion on when Metal
Finishing ELGs versus other metals related ELGs may apply at POTWs.
4.5.2 Applicability of ELGs to New or Modified Metal Finishing Operations
EPA and regional pretreatment coordinators have received questions from stakeholders
on a number of topics related to the 46 metal finishing operations that are listed in the 1983
Metal Finishing ELGs. In particular, stakeholders asked for clarification on:
• Whether a newly designed metal finishing operation would fall under the six
primary metal finishing operations that would subject them to the Metal Finishing
ELGs (e.g., revivation, zirconization, citric acid passivation);
• Whether modifications to the metal finishing operation over time (e.g., increasing
the number of finishing lines, expanding plant operations, changing plating
process) would subject them to new source standards (NSPS and PSNS) under 40
CFR Part 433 (instead of BAT and PSES or 40 CFR Part 413 electroplating
standards);
• Whether newer manufacturing industries (e.g., solar panel manufacturing, cell
phone manufacturing) that were not considered during development of the 1983
rule would be subject to the Metal Finishing ELGs.
• How to define current industry practices to determine whether they are subject to
the Metal Finishing ELGs. For example,
— Clarification of the distinction between "cleaning" and "etching"
operations.
— Use of phosphoric acid, chromic acid, or citric acid in "cleaning" versus
"conversion coating" or "etching" operations.
— Use of brighteners during "acid cleaning" vs "bright dipping" operations
where "bright dipping" is mentioned as an example "etching" core process
in the Technical Development Document (TDD) and "cleaning" is not a
core process.
— Clarification on whether facilities performing "powder coating" are
subject to the Metal Finishing ELGs.
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Section 4 - Preliminary Study Findings
Regional pretreatment coordinators have fielded questions from industry on a case-by-
case basis related to these topics. When necessary, EPA plans to develop policy memoranda to
address questions on the applicability of the Metal Finishing ELGs.
4.5.3 Considerations for Other Regulations
Based on discussions with stakeholders and EPA's review of the literature, EPA
identified the following regulations that may impact the characteristics of wastewater generated
from metal finishing operations or inhibit the implementation of advanced wastewater treatment
technologies for the industry.
4.5.3.1 National Emission Standards for Hazardous Air Pollutants (NESHAPs), Office of
Air and Radiation (OAR)
EPA identified the following air regulations that have been promulgated since the 1983
Metal Finishing ELGs. These regulations can impact the overall characteristics of process
wastewater generated from metal finishing operations.
• NESHAP for Chromium Emissions from Hard and Decorative Chromium (40
CFR Part 63, Subpart N), 1995 and 2012. EPA promulgated the original
NESHAP in 1995 affecting all facilities using chromium electroplating tanks. In
2012, EPA amended the rule to tighten emission standards for chromium
electroplating and anodizing operations, which included revised emission limits
and a ban on the use of perfluorooctane sulfonate (PFOS)-based fume
suppressants in air pollution control devices. Based on discussions ith some metal
finishing facilities, EPA learned that wastewater generated from emission control
devices used to control chromium emissions can be commingled with metal
finishing wastewater prior to wastewater treatment.
• NESHAP for Plating and Polishing Operations (40 CFR Part 63, Subpart
WWWWWW), 2008. EPA published the NESHAP in 2008 requiring use of
generally available control technology (GACT) standards at facilities with plating,
polishing or thermal spray processes that contain cadmium, nickel, lead,
manganese and/or chromium (excluding chromium electroplating and
anodizing operations). The rule does not establish emission limits for these
operations. Facilities have several compliance alternatives including use of
wetting agents/fume suppressants (WAFS), air pollution control devices or tank
covers. Platers also need to implement management practices that reduce the
generation of airborne chemicals. These modifications may introduce pollutants to
metal finishing wastewater that are not commonly used in metal finishing
operations.
• NESHAP for Metal Fabrication and Finishing Area Source (40 CFR Part 63,
Subpart XXXXXX), 2008. In 2008, EPA promulgated requirements to reduce air
pollution of compounds of metals such as cadmium, chromium, lead, manganese
and nickel from nine metal fabrication and finishing source categories. This rule
applies to facilities primarily engaged in these nine source categories and covers
the following operations: dry abrasive blasting, dry grinding and dry polishing
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Section 4 - Preliminary Study Findings
with machines, dry machining, spray painting, and welding. The NESHAP may
require the use of wet emission control devices for some operations, which may
introduce pollutants that impact metal finishing wastewater characteristics.
4.5.3.2 Resource Conservation and Recovery Act (RCRA) Hazardous Waste Regulations,
Office of Resource Conservation and Recovery (ORCR)
Based on discussions with wastewater treatment technology vendors, EPA also learned
that hazardous waste regulations can inhibit the advancement of wastewater treatment
technologies for the industry. Specifically, vendors noted the difficulty of applying more
advanced technologies such as ion exchange or RO due to the costs to the facilities for offsite
resin or membrane reclamation and/or disposal. As defined under RCRA, the resins and
membranes may be classified as hazardous waste (most commonly under F006 - wastewater
treatment sludges from electroplating operations). Technologies such as ion exchange and RO
can treat wastewaters to a quality that can be reused in the process, which can significantly
reduce the amount of wastewater discharged; however, the added cost of managing the
hazardous wastes generated by these technologies may have rendered them economically
infeasible for many of metal finishing facilities (particularly, job shops). On January 13, 2015,
EPA published in the Federal Register (FR) a revised definition of solid waste (80 FR 1694).
EPA recently revised the definition of solid waste in 2015 with the objective of encouraging
reclamation of hazardous secondary materials without increasing risk to human health and the
environment from discarded hazardous secondary material. This new definition may pave a way
for the advancement of wastewater treatment and reuse for the metal finishing industry.
4.5.3.3 European Union (EU) End of Life Vehicle (ELV) and Restriction of Hazardous
Substances (RoHS) Directives
The EU issued the End of Life Vehicles Directive in 2000 to address the issue of the
recycling and/or disposal of automobiles at the end of their useful lives to limit waste containing
lead, mercury, cadmium, and hexavalent chromium. The directive bans lead, mercury, and
cadmium and limits hexavalent chromium to 2.0 grams per vehicle for the purpose of corrosion
protection only. Similarly, in 2006, the EU also issued the RoHS Directive banning lead,
mercury, cadmium, and hexavalent chromium, as well as two additional flame retardants, from
electronic products, thereby reducing the amount of these substances disposed at the end of their
useful lives. Based on discussions with the metal finishing industry, these directives may impact
the metal finishes used in the U.S. on products that are then sold abroad.
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Section 5 - Next Steps
5. NEXT STEPS
As discussed in Section 1 of this report, EPA plans to gather sufficient information to
answer the following key questions for the study:
• How is the current metal finishing industry different from the industry as
regulated by the Metal Finishing ELGs?
— What is the current distribution of captive facilities and job shops within
the industry?
— Which types of facilities are conducting metal finishing operations?
• Since the promulgation of the Metal Finishing ELGs, what process technology
changes have been implemented and how have the primary sources of wastewater
changed?
• Since the promulgation of the Metal Finishing ELGs, what chemical formulation
changes have been implemented and how have these changes affected the
characteristics of the raw wastewater, i.e., pollutants, concentrations, flow
rates/volume?
• What are the best available technologies for pollution prevention and wastewater
treatment and to what levels do they reduce pollutants of concern?
— What are the concentrations and loadings of pollutants currently being
discharged (i.e., baseline concentrations)?
— Which pollutant discharges require additional control?
— How will industry discharges change if facilities implement these best
available technologies and practices?
• What challenges do metal finishing facilities face in applying the Metal Finishing
ELGs?
EPA plans to continue gathering and analyzing information relevant to these questions
over the next year, after which it will determine whether additional data collection efforts are
needed and how to proceed with updating the 1983 Metal Finishing ELGs. The information that
EPA has compiled to date identifies the key topics for further investigation, as described in the
subsections below.
5.1 Review Pollutant Discharge and Release Data
EPA plans to review existing industry discharge data to further evaluate and characterize
both direct and indirect metal finishing wastewater discharges. EPA will evaluate discharge
monitoring report (DMR) and Toxics Release Inventory (TRI) data to identify metal finishing
facilities with wastewater discharges, the location of facilities reporting to DMR and TRI, the
pollutants being generated and discharged, and their respective prevalence, magnitude, and
relative toxicity in wastewaters that are not currently regulated by the Metal Finishing ELGs.
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Section 5 - Next Steps
5.2 Evaluate Changes to Industry Profile
Currently, EPA does not have a complete understanding of how the industry profile has
changed since the promulgation of the 1983 regulations. Particularly, EPA is seeking to
understand any changes in metal finishing operations and markets, in wastewater treatment and
discharge practices, and in the number, size, and types of facilities generating and discharging
wastewater. To date, EPA's review of the industry suggests that there have been changes in the
industry since 1983 that would affect wastewater characteristics and discharges from metal
finishing facilities. However, EPA cannot yet adequately define the extent of those changes or
their impacts.
EPA will evaluate options for developing a current profile of metal finishing facilities.
These options may include using a marketing database, reviewing more recent data collected
during the MP&M regulatory development, reviewing TRI and DMR data, and identifying other
data collection efforts that may have been conducted by other EPA offices, such as the Office of
Air and Radiation (OAR) and the Office of Resource Conservation and Recovery (ORCR),
which may have already compiled these types of information. EPA will also collaborate with
EPA regional offices, state and local agencies, and POTW control authorities to obtain POTW
pretreatment reports, which identify categorical industrial users (CIUs) in the POTW's
catchment area.
5.3 Review Literature from Conferences and Other Industry Sources
EPA has conducted an extensive literature review of metal finishing process
technologies, alternative chemistries, and wastewater treatment using industry keywords on
several research collections and search engines.19 In addition, EPA collected and will review
literature on wastewater treatment from the 2015 WEFTEC and 2015 IWC. EPA does not plan to
conduct another extensive literature search on technical papers and reports, studies, peer-
reviewed journal articles, and industry publications on metal finishing operations and wastewater
management; however, EPA will review literature that it identifies from other EPA activities and
industry sources described in this section.
5.4 Continue Discussions with Industry Experts on Key Topics
EPA will continue conversations with federal and regional pretreatment coordinators.
These personnel often have on-site experience at POTWs that receive metal finishing
wastewater. Such discussions will help EPA identify and explore issues with the implementation
of the Metal Finishing ELGs, and to understand the effects of these issues on POTW operations.
EPA held meetings with the ACWA and several pretreatment coordinators in November 2015 to
gather different perspectives on the metal finishing category (U.S. EPA, 2015d). EPA also plans
to initiate discussions with other organizations, such as NACWA and NASF, to understand their
perspective on the implementation of the 1983 regulations.
EPA will also continue to reach out to personnel from metal finishing facilities and
wastewater treatment technology vendors to: 1) obtain information that will help answer key
19EPA used specific industry keywords for the metal finishing category, listed in Appendix B.
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Section 5 - Next Steps
study questions, 2) identify potential candidate facilities for future EPA site visits, 3) gain
industry perspective on the 1983 Metal Finishing ELGs, and 4) generally assess the technical and
economic feasibility of implementing more advanced wastewater treatment technologies and
waste minimization practices at metal finishing facilities. EPA will identify additional facilities
to contact, based on discussion with these industry experts, and based on its review of available
annual pretreatment reports or other sources listing metal finishing facilities that indirectly
discharge wastewater.
5.5 Conduct Site Visits to Metal Finishing Facilities
EPA will visit metal finishing facilities to observe operations and wastewater
management practices first-hand. EPA will seek to visit facilities representing a range of
operational approaches, including facilities that are:
• Operating new or modified finishing processes.
• Using alternative chemicals in metal finishing operations.
• Using pollution prevention practices to maximize the reuse and minimize the
generation and/or discharge of pollutants in wastewater.
• Operating advanced wastewater treatment technologies (aside from conventional
hydroxide precipitation and clarification).
During these visits, EPA may request data on wastewater treatment effectiveness, general
process design, typical operating conditions, market demands, and other topics. EPA may also
request information on the wastewater treatment technologies and treatment chemicals used on
site.
5.6 Investigate the Impacts of Other Regulations on the Industry
EPA will further investigate how other EPA regulations may be affecting the metal
finishing industry. Possible examples of such regulations include NESHAPs (described in
Section 4.5), RCRA hazardous waste disposal regulations, and other EPA efforts. As part of this
investigation, EPA will collaborate with other EPA offices and analyze information regarding
regulatory impacts that the Agency has already collected from the industry.
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Section 6- Quality Assurance
6. QUALITY ASSURANCE
In gathering information to support EPA's preliminary study of the Metal Finishing
Category. EPA evaluated and documented the usefulness and quality of the data collected to date
in accordance with the criteria specified in The Environmental Engineering Support for Clean
Water Regulations Programmatic Quality Assurance Project Plan (PQAPP) (ERG, 2013).
EPA's review of the data sources for this interim study also followed the quality assurance
procedures specified in the PQAPP, with the addition of specific criteria discussed in Section
6.3, below. This section provides detailed information on the data sources used and data quality
evaluation performed.
6.1 Project Objectives
As discussed in Section 1 one of EPA's primary objectives for the preliminary study of
the Metal Finishing Category is to assess the current state of the industry to better understand
how metal finishing operations, wastewater characteristics, and wastewater treatment
technologies have changed since EPA promulgated the 1983 ELGs. This assessment will help
EPA to determine whether additional data collection efforts are needed and how best to address
the 1983 Metal Finishing ELGs. Specifically, the study seeks to answer the key questions listed
in Section 1 and reiterated in Section 5 of this report.
6.2 Data Sources
To date, EPA has used the following types of data sources to support its preliminary
study of the Metal Finishing Category:
• Conference proceedings, peer-reviewed journals, other academic literature.
• Interviews with industry personnel, vendors, trade association representatives, and
pretreatment coordinators.
• Existing government publications and supporting information.
6.3 Data Quality Objectives and Criteria
As described in the PQAPP, EPA ensures that the data collection, processing, and
analyses performed for the preliminary study will meet the data quality objectives of objectivity,
integrity, and utility, as described below:
• Objectivity. The information must be accurate, reliable, and unbiased, and the
manner in which the information is presented must be accurate, clear, complete,
and unbiased.
• Integrity. The information may not be compromised through corruption or
falsification, either by accident, or by unauthorized access or revision.
• Utility. The information must be useful for the intended users.
The sources of the data used will also be made transparent. As the study progresses and
EPA analyzes the data, EPA will also provide information on the various assumptions, analytical
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Section 6- Quality Assurance
methods, and statistical procedures applied throughout the study. EPA prioritized the review of
the data sources described in Section 6.2 to address the key study questions listed in Section 1 of
this report. The criteria that EPA will use to evaluate the quality of literature are accuracy,
reliability, and representativeness, as described in Section 4.3.1 and in Table 4-2 of the PQAPP
(ERG, 2013), and summarized below:
Accuracy. EPA assumed that the underlying data and information contained in state and
federal reports, peer-reviewed journal articles, and industry publications are accurate. Although
industry publications are not usually peer-reviewed, this resource provides useful information for
understanding metal finishing processes and wastes generated.
Relevance. Selected articles must describe process operations, pollutants, or wastestreams
that are representative of the metal finishing industry. Articles that most closely provide answers
to the key questions listed in Section 1 are the most relevant.
Reliability. EPA considered the following factors when evaluating reliability of the data
sources used to support the study: (1) data sources that have been generated by governmental
agencies or are otherwise subject to peer review and assessment are considered to be the most
reliable and useful for understanding industry process operations, quantitatively characterizing
wastewater discharges, and demonstrating treatment system performance; (2) data sources from
entities with established knowledge in the topic area (e.g. studies conducted by industry experts,
academic researchers, data generated by an industrial facility using documented and approved
methods) are also considered to be reliable and useful for understanding industry process
operations, quantitatively characterizing wastewater discharges, demonstrating treatment system
performance, and understanding applicability of the regulations; and (3) data sources that use
unknown collection and data review procedures are less reliable, but may be generally useful for
qualitative understanding of industry process operations and waste streams. In general, EPA
evaluated reliability based on the degree to which sources met the following criteria:
• Scientific work is clearly written, so that all assumptions and methodologies can
be identified.
• Variability and uncertainty (quantitative and qualitative) in the information or in
the procedures, measures, methods, or models are evaluated and characterized.
• Assumptions and methodologies are consistently applied throughout the analysis
as reported in the source.
Representativeness. EPA evaluated whether selected data sources described process
operations, pollutants, or waste streams that are representative of the metal finishing industry.
For the purposes of this study, EPA expanded upon the general criteria set forth in the PQAPP by
establishing data quality acceptance criteria related to the geographic scope and age of the data
(described below):
• Geographic Scope. Data sources must describe the wastewater characteristics for
the metal finishing industry in the United States. EPA also collected additional
information from the data sources to describe the generation of the data, such as
the source of the wastewater, sample collection procedures, analytical methods,
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Section 6- Quality Assurance
units, and relevant data qualifiers to further evaluate its quantitative use in future
analyses. EPA included some international data sources that were relevant for
their descriptions of other potential wastewater treatment technologies or
chemical processes used in metal finishing.
• Age. EPA prioritized data sources published in 2000 or later, as they reflect more
recent industry changes. However, information published prior to 2000 (e.g., 1983
Metal Finishing Technical Development Document, supporting documentation for
the MP&M rulemaking) can provide useful qualitative information regarding the
status of the industry then and the nature of any changes in the years following
promulgation. In addition, EPA notes the year of the data source referenced in the
preliminary study to clearly document the time period.
Table 6-1 summarizes the data quality criteria discussed above.
Table 6-1. Data Quality Criteria Summary
Data Quality Criterion
Accuracy
Reliability
Representativeness
Description
Underlying data in state and federal reports, peer-reviewed journal articles,
and industry publications are accurate.
Clearly written, assumptions and methodologies identified.
Variability and uncertainty in the information are evaluated and characterized.
Assumptions and methodologies are consistently applied.
Process operations, pollutants, or waste streams that are representative of the
metal finishing industry are described.
Wastewater characteristics of the U.S. metal finishing industry are described.
Data sources addressing industry outside of the United States were also
included for descriptions of potential wastewater treatment technologies or
chemical processes.
Data sources published in 2000 or later are prioritized; data sources prior to
2000 were used qualitatively.
6.4 Data Quality Evaluation
This section describes the data sources in detail and how they met the evaluation criteria
listed in Section 6.3. Table 6-2, at the end of this section, summarizes the data sources and
acceptance criteria evaluated. EPA recognizes data sets contain different levels of information
and limitations, therefore, EPA evaluates each type of data set and will apply appropriate
acceptance criteria based on the purpose of each analysis. Table 6-2 presents the applied
acceptance criteria EPA used to evaluate data for the Preliminary Study.
6.4.1 Conference Proceedings, Peer-Reviewed Journal Articles, Other Academic Literature
EPA reviewed selected conference proceedings,20 peer-reviewed journal articles, and
other academic literature in support of its preliminary study of the Metal Finishing Category.
EPA used a list of key words (see Appendix B) to identify peer-reviewed journal articles and
20 For the preliminary study, EPA focused its literature review on peer-reviewed journal articles and other academic
literature, but not on conference proceedings. EPA intends to review conference proceedings and collect additional
industry data.
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Section 6- Quality Assurance
other academic literature. EPA collected over 130 documents from the literature search, recorded
them on a quality evaluation tracking spreadsheet, and documented how each data source met (or
did not meet) the quality criteria described in Section 6.3 (ERG, 2015). EPA applied the data
quality criteria established in the Environmental Engineering Support for Clean Water
Regulations PQAPP (ERG, 2013) and determined that the data and information obtained from
conference proceedings, peer-reviewed journals, and other academic literature were sufficiently
accurate, reliable, and relevant for characterizing metal finishing process operations, chemistries,
wastewater, and treatment technology performance.
6.4.2 Data and Information Obtained from Industry, Vendors, and Trade Associations
EPA obtained information from direct email or telephone communications with industry
personnel, wastewater treatment technology vendors, and trade association representatives to
support its preliminary study of the Metal Finishing Category. This included contacting specific
facilities, vendors, and trade associations to gather information regarding facility-specific process
operations and waste streams. EPA also obtained information from the web sites of metal
finishing facilities, vendors, and trade associations. Web site information included descriptions
of process operations and company profiles, including, for example, the types of products and
services performed. EPA applied the criteria established in the Environmental Engineering
Support for Clean Water Regulations PQAPP (ERG, 2013) and determined this information was
sufficiently accurate, reliable, and representative of the facilities of interest for use in
characterizing industry sector trends and qualitative understanding of process operations and
treatment technologies used.
6.4.3 Existing Government Publications and Supporting Information
EPA obtained information from government publications and supporting documents,
specifically documents supporting the Metal Finishing ELGs and the MP&M proposed
rulemaking. During the MP&M proposed rulemaking, EPA evaluated facilities covered under
the Metal Finishing ELGs in the 1980's and 1990's. EPA applied the criteria established in the
Environmental Engineering Support for Clean Water Regulations PQAPP (ERG, 2013) and
determined this information was sufficiently accurate and reliable for characterizing metal
finishing process operations, chemistries, wastewater characteristics, and wastewater treatment
technologies. However, due to the age of the data, EPA determined that the information may not
be representative of current industry practices, and only used the information qualitatively to
establish a timeline for changes within the industry.
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Section 6- Quality Assurance
Table 6-2. Data Acceptance Criteria for the Preliminary Study of the Metal Finishing Category
Type of Data
Data Not Usable
Data Usable to
Profile Operations
and Wastewater
Treatment
Technologies
Data Usable to
Characterize In-
Process Waste
Streams
Data Usable to Demonstrate Wastewater
Treatment Performance/Efficiency
Conference Proceedings,
Peer-Reviewed Journal
Articles, Other Academic
Literature
Article/paper not peer-
reviewed or otherwise
deemed sufficient for
limited purposes such as
identifying incidental and
qualitative data.
Current and relevant
to the specific
facilities/industry
operations of
interest.
Waste stream
identified and
analytes, units,
analytical methods,
and detection limits
identified.
Geographic scope is
within the United
States.
(1) Represents full-scale system operated at
applicable metal finishing facility.
(2) Influent and effluent data show that
treatment system is well designed and operated.
(3) Detailed description of the treatment system
and operating conditions.
(4) Analytes identified; units, analytical
methods and detection limits included.
Data and Information
Obtained from Industry,
Vendors, and Trade
Associations (e.g., direct
email or telephone
communications with
industry, wastewater treatment
technology vendors, and trade
associations)
(1) The plant has since
changed operations (e.g.,
installed a new treatment
system) since the data
were collected.
(2) Data collected during
upset conditions.
(3) Represents a process
that is not of interest (e.g.,
sanitary wastewater).
Process operations
clearly described.
Waste stream
identified and
analytes, units,
analytical methods,
and detection limits
identified.
(1) Represent full-scale system operated at
applicable metal finishing facility.
(2) Influent and effluent data or percent removal
identified and show that treatment system is
well designed and operated.
(3) Detailed description of the treatment system
and operating conditions.
(4) Analytes identified; units, analytical
methods and detection limits included.
Government Publications and
Supporting Information (e.g.,
documents supporting the
Metal Finishing ELGs, data
collected during the MP&M
Rulemaking, DMR and TRI
databases, other governmental
agency databases/lists)
(1) Data collected by an
unknown method or units
undefined.
(2) Data collected during
upset conditions.
(3) Represents a process
that is not of interest (e.g.,
sanitary wastewater).
Process operations
clearly described.
Waste stream
identified and
analytes, units,
analytical methods,
and detection limits
identified.
(1) Represents full-scale system operated at
applicable metal finishing facility.
(2) Influent and effluent data show that
treatment system is well designed and operated.
(3) Detailed description of the treatment system
and operating conditions.
(4) Analytes identified; units, analytical
methods and detection limits included.
6-5
-------
Section 7-References
7. REFERENCES
1. Abbott, A., et al. (2007). Electropolishing and Electroplating of Metals Using
Ionic Liquids Based on Choline Chloride. Ionic Liquids IV, Chapter 13, 186-197.
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2. Abdel-Halima, E. S., & Al-Deyab, S. S. (2011). Removal of Heavy Metals from
Their Aqueous Solutions through Adsorption onto Natural Polymers.
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3. Abdel-Hamid, Z., & Abdel-Aal, A. (2009). New Environmentally Friendly
Noncyanide Alkaline Electrolyte for Copper Electroplating. Surface and Coatings
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6. Ahmad, W. A., et al. (2010). Pilot-scale Removal of Chromium from Industrial
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Adsorbents in Water and Wastewater Treatment. Industrial & Engineering
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8. Ahmen-Basha, C., et al. (2008). Management of Chromium Plating Rinsewater
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9. Ajmal, M., et al. (2000). Adsorption Studies on Citrus reticulata (fruit peel of
orange): Removal and Recovery of Ni(II) from Electroplating Wastewater.
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10. Ajmal, M., et al. (2001). Removal and Recovery of Heavy Metals from
Electroplating Wastewater by using Kyanite as an Adsorbent. Journal of
7-1
-------
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11. Ajmal, M., et al. (2003). Adsorption Studies on Rice Husk: Removal and
Recovery of Cd(II) from Wastewater. Bioresource Technology, 86(2), 147- 149.
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12. Akbal, F., & Camci, S. (2011). Copper, Chromium, and Nickel Removal from
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13. Alvarez-Ayuso, E., et al. (2003). Purification of Metal Electroplating Waste
Waters Using Zeolites. Water Research, 37(20), 4855-4862.
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14. Bakarat, M. A., & Schmidt, E. (2010). Polymer-enhanced Ultrafiltration Process
for Heavy Metals Removal from Inudstrial Wastewaer. Desalination., 256(1- 3),
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15. Berman, E. S., & Brooman, E. W. (2009). Evaluation of APCVD Aluminum
Coatings as an Environmentally Acceptable Alternative to Electroplated
Cadmium Coatings. Metal Finishing, 707(2), 35- 43. doi:10.1016/S0026-
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16. Bhargava, G., & Allen, F. (2012). Self-Healing, Chromate-free Conversion
Coating for Magnesium Alloys. Metal Finishing, 770(4), 32- 38.
doi: 10.1016/80026-0576(13)70127-2. EPA-HQ-OW-2015-0665. DCN MF00017.
17. Bhatt, H., et al. (2009). Trivalent Chromium for Enhanced Corrosion Protection
on Aluminum Surf aces. Metal Finishing, 707(6), 39- 47. doi:10.1016/S0026-
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18. Bibber, J. (2008). Non-chrome-containing Conversion Coatings for Zinc and Zinc
Alloys: Environmentally Friendly Alternatives Provide Equal or Better Adhesion
and Corrosion Resistance as Conventional Methods. Metal Finishing, 106(4), 41-
46. doi: 10.1016/80026-0576(08)80091-8. EPA-HQ-OW-2015-0665. DCN
MF00019.
19. Bierwagen, G., et al. (2010). Active Metal-based Corrosion Protective Coating
Systems for Aircraft Requiring No-Chromate Pretreatment. Progress in Organic
Coatings, 68(1- 2), 48-61. doi: 10.1016/j.porgcoat.2009.10.031. EPA-HQ-OW-
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20. Bikulcius, G., et al. (2010). Ecologically Green Conversion Coating for Zinc-
Cobalt Alloy. Transactions of the Institute of Metal Finishing, 55(3), 163- 165.
doi: 10.1179/174591910X12692576434699. EPA-HQ-OW-2015-0665. DCN
MF00021.
7-2
-------
Section 7 -References
21. Chaix, J. P., et al. (2013). Thinking Outside the Bucket with Non-Drip
Electrochemical Processing. Retrieved from
http://www.rowantechnology.com/wp-
content/uploads/2014/02/SURFIN%202013%20Dalistick%20for%20web.pdf.
EPA-HQ-OW-2015-0665. DCNMF00022.
22. Chiban, M., et al. (2011). Characterization and Application of Dried Plants to
Remove Heavy Metals, Nitrate, and Phosphate Ions from Industrial Wastewaters.
Clean-Soil, Air, Water, 39(4), 376-383. doi:10.1002/clen.201000127. EPA-HQ-
OW-2015-0665. DCNMF00023.
23. Colica, G., et al. (2012). Biosorption and Recovery of Chromium from Industrial
Wastewaters By Using Saccharomyces cerevisiae in a Flow-Through System.
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24. Dai, S. (2014). Alternative Plating for Metal Electorplating using Ionic Liquids.
Retrieved from U.S. DOE Strategic Environmental Research and Development
Program (SERDP) website: https://www. serdp-estcp.org/Program-
Areas/Weapons-Systems-and-Platforms/Surface-Engineering-and-Structural-
Materials/Coatings/WP-2316. EPA-HQ-OW-2015-0665. DCNMF00025.
25. Dubent, S., et al. (2010). Electrodeposition, Characterization and Corrosion
Behaviour of Tin-20 wt.% Zinc Coatings Electroplated from a Non-Cyanide
Alkaline Bath. Materials Chemistry and Physics, 720(2-3), 371-380.
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MF00026.
26. Dunham, B. (2012). Zirconium Pretreatments: Not Just for Early Adopters
Anymore. Metal Finishing, 770(6), 18-21. doi: 10.1016/80026-0576(13)70215-0.
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27. Eichinger, E., et al. (1997). Hexavalent Chromium Elimination: An Aerospace
Industry Progress Report. Metal Finishing, 95 (3), 36, 38, 40-41.
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28. ERG. (2013). Eastern Research Group, Inc. Environmental Engineering Support
for Clean Water Regulations Programmatic Quality Assurance Project Plan
(PQAPP). Chantilly, VA. (October). EPA-HQ-OW-2010-0824-0229.
29. ERG. (2015). Eastern Research Group, Inc. Quality Evaluation Tracking
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30. ERG. (2016). Eastern Research Group, Inc. Summary of Metal Finishing
Facilities and Vendors Contacted During Preliminary Study Activities in 2015.
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7-3
-------
Section 7 -References
31. Fister, D. (2010). Reducing Operational Costs, Environmental Impact Via
Rigorous Plating/Finishing Analyses. Metal Finishing, 108(6), 39-46.
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32. Fu, F., & Wang, Q. (2011). Removal of heavy metal ions from wastewaters: A
review. Journal of Environmental Management, 93(3), 407-418.
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33. Ge, F., et al. (2012). Effective removal of heavy metal ions Cd2+, Zn2+, Pb2+,
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34. Guidetti, G., et al. (2009). Plaforization Process for Cleaning, Degreasing, and
Vhosphating. Metal Finishing, 107(3), 39-43. doi: 10.1016/80026-0576(09)80050-
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35. Haynes, R. J. (2014). Use of Industrial Wastes as Media in Constructed Wetlands
and Filter Beds - Prospects for Removal of Phosphate and Metals from
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36. Hegazi, H. A. (2013). Removal of heavy metals from wastewater using
agricultural and industrial wastes as absorbents. HBRC Journal, 9(3), 276-282.
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37. Hua, M., et al. (2012). Heavy metal removal from water/wastewater by nanosized
metal oxides: A review. Journal of Hazardous Materials, 211-212, 317-331.
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38. Huisman, J., et al. (2006). Biologically produced sulphide for purification of
process streams, effluent treatment and recovery of metals in the metal and
mining industry. Hydrometallurgy, 53(1-4), 106-113.
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39. Indumathi, S. N., et al. (2011). Cadmium- and Chromate-Free Coating Schemes
for Corrosion Protection of 15CDV6 Steel. Metal Finishing, 109(3), 15-21.
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40. Islamoglu, 8., et al. (2006). Development of a Precipitation Based Separation
Scheme for Selective Removal and Recovery of Heavy Metals from Cadmium
Rich Electroplating Industry Effluents. Separation Science and Technology,
41(15), 3367-3385. doi:10.1080/01496390600851665. EPA-HQ-OW-2015-0665.
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41. Kabdasli, L., et al. (2009). Complexing agent and heavy metal removals from
metal plating effluent by electrocoagulation with stainless steel electrodes.
7-4
-------
Section 7 -References
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42. Kaluzny, K. (2012). Roll with the Changes. Metal Finishing, 110(4), 43-46.
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43. Khelifa, A., et al. (2005). Treatment of metal finishing effluents by the
electroflotation technique. Desalination, 757(1-3), 27-33.
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44. Kirk-Othmer. (2004). Electroplating. Krik-Othmer Encyclopedia of Chemical
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45. Klingenberg, C., & Jones, D. (2007). Low-Temperature, Phosphate-Free
Conversion Coatings. Metal Finishing, 28-30. EP A-HQ-OW-2015-0665. DCN
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46. Kobya, M., et al. (2010). Treatment of cadmium and nickel electroplating rinse
water by electrocoagulation. Environmental Technology, 37(13), 1471-1481.
doi: 10.1080/09593331003713693. EP A-HQ-OW-2015-0665. DCNMF00046.
47. Kumar, R., et al. (2008). Biosorption of chromium(VI) from aqueous solution and
electroplating wastewater using fungal biomass. Chemical Engineering Journal,
735(3), 202-208. doi:10.1016/j.cej.2007.03.004. EP A-HQ-OW-2015-0665. DCN
MF00047.
48. Kurniawan, T. A., et al. (2006). Comparisons of low-cost absorbents for treating
wastewaters laden with heavy metals. Science of The Total Environment, 366(2-
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49. La Scala, J. (2009). Non-Chromate/No VOC Coating System for DoD
Applications. WP-1521. Retrieved from http://www.dtic.mil/cgi-
bin/GetTRDoc?AD=ADA608406. EPA-HQ-OW-2015-0665. DCN MF00049.
50. Legg, K. (2008). Rowan Technology Group. Greening DoD Surface Finishing
Operations. Paper presented at the Washington Forum.
http://www.rowantechnology.com/wp-content/uploads/2012/06/Greening-DoD-
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51. Legg, K. (2009). Rowan Technology Group. Hexavalent Chrome Issues and
Options. Paper presented at the DLA Shelf Life Symposium.
http://www.rowantechnology.com/wp-content/uploads/2012/06/Legg-Hexavalent-
chrome-issues-and-options.pdf EPA-HQ-OW-2015-0665. DCN MF00051.
52. Legg, K. (2010). Rowan Technology Group. Eliminating Cr6+, Cd, and Other
Hazardous Materials without Compromising Performance. Paper presented at the
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7-5
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53. Legg, K. (2011). Rowan Technology Group. Overview of Development and
Implementation of Non-Chr ornate Treatments for Aluminum in the U.S. . Paper
presented at the A3TS, Paris, http://www.rowantechnology.com/wp-
content/uploads/2012/06/Legg-A3TS-Paris-overview.pdf. EPA-HQ-OW-2015-
0665. DCN MF00053.
54. Legg, K. (2012a). Rowan Technology Group. Choosing a Cadmium Plate
Alternative. Retrieved from http://www.rowantechnology.com/wp-
content/uploads/2012/06/Cd-Plating-Alternatives.pdf EPA-HQ-OW-2015-0665.
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55. Legg, K. (2012b). Rowan Technology Group. Choosing a Chromate Alternative.
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56. Legg, K. (2012c). Rowan Technology Group. Choosing a Hard Chrome
Alternative. Retrieved from http://www.rowantechnology.com/wp-
content/uploads/2012/06/Hard-Chrome-Plating-Alternatives.pdf EPA-HQ-OW-
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57. Lei, X., et al. (2010). Successful Cyanide Free Plating Protocols on Magnesium
Alloys. Transactions of the Institute of Metal Finishing, 88(2), 75-80.
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MF00057.
58. Lei, X., et al. (2011). A Study of Chromium-free Pickling Process before
Elecroless Ni-P Plating on Magnesium Alloys. Surface and Coatings Technology,
205(16), 4058-4063. doi: 10.1016/j.surfcoat.2011.02.051. EPA-HQ-OW-2015-
0665. DCN MF00058.
59. Lesmana, S. O., et al. (2009). Studies on potential applications of biomass for the
separation of heavy metals from water and wastewater. Biochemical Engineering
Journal, 44(\\ 19-41. doi:10.1016/j.bej.2008.12.009. EPA-HQ-OW-2015-0665.
DCN MF00059.
60. Lew, C. M., et al. (2010). Zeolite Thin Films: From Computer Chips to Space
Stations. Accounts of Chemical Research, 43(2), 210-219.
doi:10.1021/ar900146w. EPA-HQ-OW-2015-0665. DCNMF00060.
61. Li, Y., et al. (2003). Study on the treatment of copper-electroplating wastewater
by chemical trapping and flocculation. Separation and Purification Technology,
37(1), 91-95. doi: 10.1016/81383-5866(02)00162-4. EPA-HQ-OW-2015-0665.
DCNMF00061.
7-6
-------
Section 7 -References
62. List, B., et al. (2012). Transitioning to Phosphorous-Free Paint Pretreatment
Processes: A Comprehensive View. Metal Finishing, 770(6), 12-16.
doi: 10.1016/80026-0576(13)70214-9. EPA-HQ-OW-2015-0665. DCNMF00062.
63. Lv, X., et al. (2013). Use of high-pressure CO2 for concentrating CrVI from
electroplating wastewater by Mg-Al layered double hydroxide. ACS Applied
Materials & Interfaces, 5(21), 11271-11275. doi: 10.102l/am4035009. EPA-HQ-
OW-2015-0665. DCNMF00063.
64. Manavbasi, A., et al. (2012). New Pretreatments and Non-Chromated Chemfilm
for Magnesium Alloys. Metal Finishing, 770(8), 17-22. doi:10.1016/S0026-
0576(13)70144-2. EP A-HQ-OW-2015-0665. DCNMF00064.
65. Mason, R., et al. (2010). Update on Alternatives for Cadmium Coatings on
Military Electrical Connectors. Metal Finishing, 705(3), 12-20.
doi: 10.1016/80026-0576(10)00012-7. EP A-HQ-OW-2015-0665. DCNMF00065.
66. Mason, R., et al. (2011). Alternatives to dichromate sealer in anodizing
operations. Metal Finishing, 709(4-5), 25-32. doi: 10.1016/80026-0576(11)80065-
6. EP A-HQ-OW-2015-0665. DCNMF00066.
67. Merzouk, B., et al. (2009). Removal turbidity and separation of heavy metals
using electrocoagulation-electroflotation technique: A case study. Journal of
Hazardous Materials, 164(\\ 215-222. doi:10.1016/j.jhazmat.2008.07.144. EPA-
HQ-OW-2015-0665. DCNMF00067.
68. Moore, R., & Dunham, B. (2008). Zicronization™: The Future of Coating
Pretreatment Processes: Alternative, phosphate-free, eco-friendly pretreatment
procedure addresses energy and chemical consumption while improving product
quality. Metal Finishing, 706(7-8), 46-55. doi:10.1016/S0026-0576(08)80259-0.
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69. Morose, G. (2013). Evaluation for Alternatives to Hexavalent Chromium
Sealants. Metal Finishing, 777(3), 32-37, 63. doi: 10.1016/80026-0576(13)70232-
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70. Murphy, V., et al. (2009). A novel study of hexavalent chromium detoxification
by selected seaweed species using SEM-EDX and XPS analysis. Chemical
Engineering Journal, 148(2-3), 425- 433. doi: 10.1016/j.cej.2008.09.029. EPA-
HQ-OW-2015-0665. DCNMF00070.
71. Naik, U. C., et al. (2012). Isolation and characterization of Bacillus cereus IST105
from electroplating effluent for detoxification of hexavalent chromium.
Environmental Science and Pollution Research, 79(7), 3005-3014.
doi:10.1007/s!1356-012-0811-6. EP A-HQ-OW-2015-0665. DCNMF00071.
72. Namasivayam, C., & Sureshkumar, M. V. (2008). Removal of chromium(VI)
from water and wastewater using surfactant modified coconut coir pith as a
7-7
-------
Section 7 -References
biosorbent. Bioresource Technology, 99(7), 2218-2225.
doi:10.1016/j.biortech.2007.05.023.EPA-HQ-OW-2015-0665.DCNMF00072.
73. NASF. (2015). Preliminary Response to EPA's Metal Finishing Study Questions
from November 23, 2015 Meeting. EPA-HQ-OW-2015-0665. DCNMF00102.
74. NEI Corporation. (2014). Nanomyte® PT-60, Chromate-Free Self-Healing
Pretreatment for Magnesium. Retrieved from
http://www.neicorporation.com/specs/Nanomyte_PT-60.pdf EPA-HQ-OW-2015-
0665.DCNMF00073.
75. Nickerson, W., & Matzdorf, C. (2012). Non-Chromate Aluminum Pretreatments.
ESTCP Project WP-200025. Retrieved from https://www.serdp-
estcp.org/Program-Areas/Weapons-Systems-and-Platforms/Surface-Engineering-
and-Structural-Materials/CoatingsAVP-200025. EPA-HQ-OW-2015-0665. DCN
MF00074.
76. O'Keefe, M. (2010). Missouri University of Science and Technology. Corrosion
Finishing/Coating Systems for DoD Metallic Substrates Based on Non-Chromate
Inhibitors and UV Curable, Zero Valent Materials. SERDP Project WP-1519.
Retrieved from https://www.serdp-estcp.org/Program-AreasAVeapons-Systems-
and-Platforms/Surface-Engineering-and-Structural-Materials/CoatingsAVP-1519
EPA-HQ-OW-2015-0665. DCNMF00075.
77. Olmez, T. (2009). The optimization of Cr(VI) reduction and removal by
electrocoagulation using response surface methodology. Journal of Hazardous
Materials, 762(2-3), 1371-1378. doi: 10.1016/j.jhazmat.2008.06.017. EPA-HQ-
OW-2015-0665. DCNMF00076.
78. Orduz, M. (2008). High-Performance Characteristics of Lead- and Cadmium-Free
Electrons Nickel. Metal Finishing, 106(1), 22-26. doi:10.1016/S0026-
0576(08)80004-9. EPA-HQ-OW-2015-0665. DCNMF00077.
79. Owlad, M., et al. (2009). Removal of Hexavalent Chromium-Contaminated Water
and Wastewater: A Review. Water, Air, & Soil Pollution, 200(1), 59-77.
doi: 10.1007/sl 1270-008-9893-7. EPA-HQ-OW-2015-0665. DCNMF00078.
80. Park, D., et al. (2005). Metal Recovery from Electroplating Wastewater Using
Acidophilic Iron Oxidizing Bacteria: Pilot-Scale Feasibility Test. Industrial &
Engineering Chemistry Research, 44(6), 1854-1859. doi: 10.102l/ie049015m.
EPA-HQ-OW-2015-0665. DCNMF00079.
81. Park, D., et al. (2006). Biosorption Process for Treatment for Electroplating
Wastewater Containing Cr(VI): Lab oratory-Scale Feasibility Test. Industrial &
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EPA-HQ-OW-2015-0665. DCNMF00080.
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82. Phely-Bobin, T. (2010). U.S. DOE Strategic Environmental Research and
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83. Pommiers, S., et al. (2014). Alternative Conversion Coatings to Chromate for the
Protection of Magnesium Alloys. Corrosion Science, 84, 135-146.
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84. Prado, R., et al. (2010). Optimized Deposition Parameters & Coating Properties of
Cobalt Phosphorus Alloy Electroplating for Technology Insertion Risk Reduction.
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85. Pusavec, F., & Kenda, J. (2014). The Transition to a Clean, Dry, and Energy
Efficient Polishing Process: An Innovative Upgrade of Abrasive Flow Machining
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86. Qin, J. J., et al. (2002). A feasibility study on the treatment and recycling of a
wastewater from metal plating. Journal of Membrane Science, 205(1-2), 213-
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MF00085.
87. Sancey, B., et al. (2011). Heavy metal removal from industrial effluents by
sorption on cross-linked starch: chemical study and impact on water toxicity.
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doi:10.1016/j.jenvman.2010.10.033.EPA-HQ-OW-2015-0665.DCNMF00086.
88. Sarin, V., & Pant, K. K. (2006). Removal of chromium from industrial waste by
using eucalyptus bark. Bioresource Technology, 97(1), 15- 20.
doi:10.1016/j.biortech.2005.02.010. EPA-HQ-OW-2015-0665. DCN MF00087.
89. Sartwell, B., et al. (2004). Naval Research Laboratory. Validation of HVOF
WC/Co Thermal Spray Coatings as a Replacement for Hard Chrome Plating on
Aircraft Landing Gear. EPA-HQ-OW-2015-0665. DCN MF00088.
90. Scott, M. (2013). PPG Industries Inc. Environmentally Friendly Fastener Coating
Demonstartion. ESTCP Project WP-201315. Retrieved from https://www.serdp-
estcp.org/Program-Areas/Weapons-Systems-and-Platforms/Surface-Engineering-
and-Structural-Materials/Coatings/WP-201315. EPA-HQ-OW-2015-0665. DCN
MF00089.
91. Senturk, E. (2013). The treatment of zinc-cyanide electroplating rinse water using
an electrocoagulation process. Water Science and Technology, 68(10), 2220-
2227. doi:10.2166/wst.2013.481. EPA-HQ-OW-2015-0665. DCNMF00090.
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Section 7 -References
92. Slife, R. (2014). Air Force Materiel Command. Cadmium-Free Alternatives for
Brush Plating Repair Operations. ESTCP Project WP-201412. Retrieved from
https://serdp-estcp.org/Program-AreasAVeapons-Systems-and-Platforms/Surface-
Engineering-and-Stmctural-Materials/Coatings/WP-201412/WP-201412. EPA-
HQ-OW-2015-0665. DCNMF00091.
93. Smith, E. L., et al. (2010). Metal Finishing with Ionic Liquids: Scale-up and Pilot
Plants from IONMET Consortium. Transactions of the Institute of Metal
Finishing, 88(6), 285-293. doi:10.1179/174591910X12856686485734. EPA-HQ-
OW-2015-0665. DCNMF00092.
94. Sthiannopkao, S., & Sreesai, S. (2009). Utilization of pulp and paper industrial
wastes to remove heavy metals from metal finishing wastewater. Journal of
Environmental Management, 90(11), 3283-3289.
doi:10.1016/j.jenvman.2009.05.006. EPA-HQ-OW-2015-0665. DCNMF00093.
95. Tepe, B., & Gunay, B. (2008). Evaluation of Pre-Treatment Processes for Hot
Rolled Steel in Powder Coating. Progress in Organic Coatings, 62(2), 134-144.
doi: 10.1016/j.porgcoat.2007.10.004. EP A-HQ-OW-2015-0665. DCNMF00094.
96. Tucker, R. (2013). One Step, Zero Discharge. Metal Finishing, 111(4\ 16-17.
doi: 10.1016/80026-0576(13)70249-6. EPA-HQ-OW-2015-0665. DCN MF00095.
97. U.S. EPA. (1979). Development Document for Existing Source Pretreatment
Standards for the Electroplating Point Source Category. Washington, D.C. EPA-
HQ-OW-20 14-0170-0007.
98. U.S. EPA. (1981). Federal Register Notice: Effluent Guidelines and Standards;
Electroplating Point Source Category Pretreatment Standards for Existing
Sources. Washington, D.C. . EPA-HQ-OW-2014-0170-0138.
99. U.S. EPA. (1983a). Development Document for Effluent Limitations Guidelines
and New Source Performance Standards for the Metal Finishing Point Source
Category. Washington, D.C. EPA-HQ-OW-2004-0032-0110.
100. U.S. EPA. (1983b). Federal Register Notice: Electroplating and Metal Finishing
Point Source Categories; Effluent Limitations Guidelines, Pretreatment
Standards, and New Source Performance Standards. Washington, D.C. EPA-HQ-
OW-2014-0170-0004.
101. U.S. EPA. (1984). Guidance Manual for Electroplating and Metal Finishing
Pretreatment Standards. Washington, D.C. EPA-HQ-OW-2014-0170-0139.
102. U.S. EPA. (1985). Guidance Manual for Implementing Total Toxic Organics
(TTO) Pretreatment Standards. Washington, D.C. EPA-HQ-OW-2015-0665.
DCNMF00107.
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Section 7 -References
103. U.S. EPA. (2000a). Development Document for the Proposed Effluent Limitations
Guidelines and Standards for the Metal Products & Machinery Point Source
Category. Washington, D.C. EPA-HQ-OW-2014-0170-0005.
104. U.S. EPA. (2000b). Capsule Report: Managing Cyanide in Metal Finishing. .
(EPA/625/R-99/009). Washington, D.C. Retrieved from
http://nepis.epa.gOv/Exe/Z vPDF.cgi/30004TAD.PDF?Dockev=30004TAD.PDF.
EPA-HQ-OW-2015-0665. DCNMF00108.
105. U.S. EPA. (2004). "Chemical Etching"Metal Finishing Option. . Retrieved from
https://www.epa.gov/sites/production/files/2015-10/documents/chemical-
etching metal-finishing air-products epa-letter 2004-06-04.pdf EPA-HQ-OW-
2015-0665. DCNMF00106.
106. U.S. EPA. (2012a). The 2011 Annual Effluent Guidelines Review Report. (EPA-
821-R-12-001). Washington, D.C. EPA-HQ-OW-2010-0824-0195.
107. U.S. EPA. (2012b). 5.6 Phosphorous. Water: Monitoring & Assessment.
Retrieved from http://water.epa.gov/type/rsl/monitoring/vms56.cfm. EPA-HQ-
OW-2015-0665. DCNMF00109.
108. U.S. EPA. (2015a). The 2014 Annual Effluent Guidelines Review Report. (EPA-
821-R-15-001). Washington, D.C. EPA-HQ-OW-2014-0170-0209.
109. U.S. EPA. (2015b). Final 2014 Effluent Guidelines Program Plan. (EPA-821-R-
15-002). Washington, D.C. EPA-HQ-OW-2014-0170-0210.
110. U.S. EPA. (2015c). Response to Comments for the Preliminary 2014 Effluent
Guidelines Program Plan. Washington, D.C. EPA-HQ-OW-2015-0665-0208.
111. U.S. EPA. (2015d). Meeting notes with Association of Clean Water
Administrators (ACWA) on November 9, 2015. EPA-HQ-OW-2015-0665. DCN
MF00105.
112. U.S. EPA. (2015e). Meeting Notes with National Association of Surface Finishers
(NASF) on November 23, 2015. EPA-HQ-OW-2015-0665. DCN MF00103.
113. U.S. EPA. (2015f). Meeting Notes with National Association of Surface Finishers
(NASF) on August 19, 2015. EPA-HQ-OW-2015-0665. DCNMF00104.
114. USGS. (2015). The USGS Water Science School: Water Questions & Answers,
What Causes Fish Kills. Retrieved from http://water.usgs.gov/edu/qa-chemical-
fishkills.html. EPA-HQ-OW-2015-0665. DCNMF00110.
115. WEF. (2008). Water Environment Federation. Industrial Wastewater
Management, Treatment, and Disposal, Manual of Practice No. FD-3 (Third ed.).
Alexandria, VA: McGraw-Hill. EPA-HQ-OW-2015-0665. DCNMF00096.
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Section 7 -References
116. Winn, D., & Dalton, W. (2008). Chromium-free Corrosion Solutions: Silica-based
Electrolytic Method Offers Viable Alternative to Both Hex- and Trivalent-
ChmmatePassivates. Metal Finishing, 106(6), 70-74. EPA-HQ-OW-2015-0665.
DCN MF00097.
117. Wong, K. K., et al. (2003). Removal of Cu and Pb from electroplating wastewater
using tartaric acid modified rice husk. Process Biochemistry, 39(4), 437-445.
doi: 10.1016/80032-9592(03)00094-3. EPA-HQ-OW-2015-0665. DCN MF00098.
118. Wynn, P. (2006). Managing the Transition to Hexavalent Chromium Free Anti-
Corrosion Coatings. Transactions of the Institute of Metal Finishing, 84(6), 280-
285. doi: 10.1179/174591906X130266. EPA-HQ-OW-2015-0665. DCN
MF00099.
119. Xiao, J., & Huang, Y. (2012). Technology Integration for Sustainable
Manufacturing: An Applied Study on Integrated Profitable Pollution Prevention
in Surface Finishing Systems. Industrial & Engineering Chemistry Research,
57(35), 11434-11444. doi:10.1021/ie300154n. EPA-HQ-OW-2015-0665. DCN
MF00100.
120. Yan, Y. (2009). Zerolite Coating System for Corrosion Control to Eliminate
Hexavalent Chromium from DoD Applications. SERDP Project WP-1342.
Retrieved from httg^AwiLMcjlI^ EPA-HQ-
OW-2015-0665. DCNMF00101.
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Appendix A
Appendix A
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
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Appendix A
Appendix A:
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
Electroplating is the production of a thin surface coating of one metal upon another by electrodeposition. This
surface coating is applied to provide corrosion protection, wear or erosion resistance, anti-frictional
characteristics, or for decorative purposes. The electroplating of common metals includes the processes in which
ferrous or nonferrous basis material is electroplated with copper, nickel, chromium, brass, bronze, zinc, tin, lead,
cadmium, iron, aluminum or combinations thereof. Precious metals electroplating includes the processes in which
a ferrous or nonferrous basis material is plated with gold, silver, palladium, platinum, rhodium, indium,
ruthenium, iridium, osmium, or combinations thereof.
In electroplating, metal ions in either acid, alkaline, or neutral solutions are reduced on cathodic surfaces. The
cathodic surfaces are the workpieces being plated. The metal ions in solution are usually replenished by the
dissolution of metal from anodes or small pieces contained in inert wire or metal baskets. Replenishment with
metal salts is also practiced, especially for chromium plating. In this case, an inert material must be selected for
the anodes. Hundreds of different electroplating solutions have been adopted commercially but only two or three
types are utilized widely for a particular metal or alloy. For example, cyanide solutions are popular for copper,
zinc, brass, cadmium, silver, and gold. However, non-cyanide alkaline solutions containing pyrophosphate have
come into use recently for zinc and copper. Zinc, copper, tin and nickel are plated with acid sulfate solutions,
especially for plating relatively simple shapes. Cadmium and zinc are sometimes electroplated from neutral or
slightly acidic chloride solutions. The most common methods of plating are in barrels, on racks, and continuously
from a spool or coil.
Electroless Plating is a chemical reduction process which depends upon the catalytic reduction of a metallic ion in
an aqueous solution containing a reducing agent and the subsequent deposition of metal without the use of
external electrical energy. It has found widespread use in industry due to several unique advantages over
conventional electroplating. Electroless plating provides a uniform plating thickness on all areas of the part
regardless of the configuration or geometry of the part. An electroless plate on a properly prepared surface is
dense and virtually non-porous. Copper and nickel electroless plating are the most common. The basic
ingredients in an electroless plating solution are:
1. A source of metal, usually a salt.
2. A reducer to reduce the metal to its base state.
3. A complexing agent to hold the metal in solution (so the metal will not plate out indiscriminately).
4. Various buffers and other chemicals designed to maintain bath stability and increase bath life.
Electroless plating is autocatalytic, i.e., catalysis is promoted from one of the products of a chemical reaction.
The chemistry of electroless plating is best exemplified by examining electroless nickel plating. The source of
nickel is a salt, such as nickel chloride or nickel sulfate, and the reducer is sodium hypophosphite. The most
commonly used complexing agents are citric and glycolic acid. Hypophosphite anions in the presence of water
are dehydrogenated by the solid catalytic surface provided by nickel to form acid orthophosphite anions. Active
hydrogen atoms are bonded on the catalyst forming a hydride. Nickel ions are reduced to metallic nickel by the
active hydrogen atoms, which are in turn oxidized to hydrogen ions. Simultaneously, a portion of the
hypophosphite anions are reduced by the active hydrogen and adsorbed on the catalytic surface, producing
elemental phosphorus, water and hydroxyl ions. Elemental phosphorus is bonded to or dissolved in the nickel
making the reaction irreversible. At the same time hypophosphite anions are catalytically oxidized to acid
orthophosphite anions, evolving gaseous hydrogen. The basic plating reactions proceed as follows:
The nickel salt is ionized in water
There is then a reduction-oxidation reaction with nickel and sodium hypophosphite.
Ni+2 + SO4- 2 + 2NaH2PO2 + 2 H20 = Ni + 2NaH2PO3 + H2 + H2SO4
The sodium hypophosphite also reacts in the following manner:
2NaH2PO2 + H2 = 2P + 2NaOH + 2H2O
As can be seen in the equations above, both nickel and phosphorus are produced, and the actual metal deposited
is a nickel-phosphorus alloy. The phosphorus content can be varied to produce different characteristics in the
nickel plate.
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Descriptions of the 46 Unit Operations in the Metal Finishing Industry
When electroless plating is done on a plastic basis material, catalyst application and acceleration steps are
necessary as surface preparation operations. These steps are considered part of the electroless plating unit
operation.
Immersion plating is a chemical plating process in which a thin metal deposit is obtained by chemical
displacement of the basis metal. Unlike electroless plating, it is not an autocatalytic process. In immersion
plating, a metal will displace from solution any other metal that is below it in the electromotive series of
elements.
The lower (more noble) metal will be deposited from solution while the more active metal (higher in the series)
will be dissolved. A common example of immersion plating is the deposition of copper on steel from an acid
copper solution. Because of the similarity of the wastes generated and the materials involved, immersion plating
is considered part of the electroless plating unit operation.
Anodizing is an electrolytic oxidation process which converts the surface of the metal to an insoluble oxide.
These oxide coatings provide corrosion protection, decorative surfaces, a base for painting and other coating
processes, and special electrical and mechanical properties. Aluminum is the most frequently anodized material,
while some magnesium and limited amounts of zinc and titanium are also treated.
Although most anodizing involves immersion of racked parts in tanks, continuous anodizing is done on large
coils of aluminum in a manner similar to continuous electroplating. For aluminum parts, the formation of the
oxide occurs when the parts are made anodic in dilute sulfuric acid or dilute chromic acid solutions. The oxide
layer begins formation at the extreme outer surface, and as the reaction proceeds, the oxide grows into the metal.
The last formed oxide, known as the boundary layer, is located at the interface between the base metal and the
oxide. The boundary is extremely thin and nonporous. The sulfuric acid process is typically used for all parts
fabricated from aluminum alloys except for parts subject to stress or containing recesses in which the sulfuric
acid solution may be retained and attack the aluminum. Chromic acid anodic coatings are more protective than
sulfuric acid coatings and have a relatively thick boundary layer. For these reasons, a chromic acid bath is used if
a complete rinsing of the part cannot be achieved.
Coating - This manufacturing operation includes chromating, phosphating, metal coloring, and passivating.
These coatings are applied to previously deposited metal or basis material for increased corrosion protection,
lubricity, and preparation of the surface for additional coatings or formulation of a special surface appearance. In
chromating, a portion of the base metal is converted to one of the components of the protective film formed by
the coating solution. This occurs by reaction with aqueous solutions containing hexavalent chromium and active
organic or inorganic compounds. Chromate coatings are most frequently applied to zinc, cadmium, aluminum,
magnesium, copper, brass, bronze, and silver. Most of the coatings are applied by chemical immersion, although
a spray or brush treatment can be used. Changes in the solutions can impart a wide range of colors to the coatings
from colorless to iridescent yellow, brass, brown, and olive drab. Additional coloring of the coatings can be
achieved by dipping the parts in organic dye baths to produce red, green, blue, and other colors.
Phosphate coatings are used to provide a good base for paints and other organic coatings, to condition the
surfaces for cold forming operations by providing a base for drawing compounds and lubricants, and to impart
corrosion resistance to the metal surface by the coating itself or by providing a suitable base for rust-preventative
oils or waxes. Phosphate conversion coatings are formed by the immersion of iron, steel, or zinc plated steel in a
dilute solution of phosphoric acid plus other reagents. The method of applying the phosphate coating is dependent
upon the size and shape of the part to be coated. Small parts are coated in barrels immersed in the phosphating
solution. Large parts, such as steel sheet and strip, are spray coated or continuously passed through the
phosphating solution. Supplemental oil or wax coatings are usually applied after phosphating unless the part is to
be painted.
Metal coloring by chemical conversion methods produces a large group of decorative finishes. This operation
covers only chemical methods of coloring in which the metal surface is converted into an oxide or similar
metallic compound. The most common colored finishes are used on copper, steel, zinc, and cadmium.
Application of the color to the cleaned basis metal involves only a brief immersion in a dilute aqueous solution.
The colored films produced on the metal surface are extremely thin and delicate. Consequently, they lack
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Appendix A:
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
resistance to handling and the atmosphere. A clear lacquer is often used to protect the colored metal surface. A
large quantity of copper and brass is colored to yield a wide variety of shades and colors. Shades of black, brown,
gray, green and patina can be obtained on copper and brass by use of appropriate coloring solutions. The most
widely-used colors for ferrous metals are based on oxides which yield black, brown, or blue colors. A number of
colors can be developed on zinc depending on the length of immersion in the coloring solution. Yellow, bronze,
dark green, black and brown colors can be produced on cadmium. Silver, tin, and aluminum are also colored
commercially. Silver is given a gray color by immersion in a polysulfide solution such as ammonium polysulfide.
Tin can be darkened to produce an antique finish of pewter by immersion in a solution of nitric acid and copper
sulfate.
Passivation refers to forming a protective film on metals, particularly stainless steel and copper, by immersion in
an acid solution. Stainless steel is passivated to dissolve any imbedded iron particles and to form a thin oxide film
on the surface of the metal. Typical solutions for passivating stainless steel include nitric acid and nitric acid with
sodium dichromate. Copper is passivated with a solution of ammonium sulfate and copper sulfate forming a blue-
green patina on the surface of the metal.
Etching and Chemical Milling - These processes are used to produce specific design configurations and
tolerances or surface appearances on parts (or metal-clad plastic in the case of printed circuit boards)-by
controlled dissolution with chemical reagents or etchants. Included in this classification are chemical milling,
chemical etching, and bright dipping. Chemical etching is the same process as chemical milling except the rates
and depths of metal removal are usually much greater in chemical milling. Typical solutions for chemical milling
and etching include ferric chloride, nitric acid, ammonium persulfate, chromic acid, cupric chloride, hydrochloric
acid, and combinations of these reagents. Bright dipping is a specialized form of etching and is used to remove
oxide and tarnish from ferrous and nonferrous materials and is frequently performed just prior to anodizing.
Bright dipping can produce a range of surface appearances from bright clean to brilliant, depending on the surface
smoothness desired for the finished part. Bright dipping solutions usually involve mixtures of sulfuric, chromic,
phosphoric, nitric, or hydrochloric acids. This unit operation also includes the stripping of metallic coatings.
Printed Circuit Board Manufacturing involves the formation of a circuit pattern of conductive metal (usually
copper) on nonconductive board materials such as plastic or glass. There are five basic steps involved in the
manufacture of printed circuit boards: cleaning and surface preparation, catalyst and electro less plating, pattern
printing and masking, electroplating, and etching.
After the initial cutting, drilling and sanding of the boards, the board surface is prepared for plating electroless
copper. This surface preparation involves an etchback (removal of built-up plastic around holes) and an acid and
alkaline cleaning to remove grime, oils, and fingerprints. The board is then etched and rinsed. Following etching,
the catalyst is applied and rinsing operations following catalyst application. The entire board is then electroless
copper plated and rinsed.
Following electroless copper plating, a plating resist is applied in non-circuit areas. Following application of a
resist, a series of electroplates are applied. First the circuit is copper plated. A solder electroplate is applied next
followed by a rinse. For copper removal in non-circuit areas, an etch step is next. After the etch operation, a
variety of tab plating processes can be utilized depending on the board design requirements. These include nickel
electroplating, gold electroplating, rhodium electroplating, and tin immersion plating. There are presently three
main production methods for printed circuit boards: additive, semi-additive, and subtractive. The additive method
uses pre-sensitized, unclad material as the starting board; the semi-additive method uses unclad, unsensitized
material as the starting board; and the subtractive method begins with copper clad, unsensitized material.
Cleaning involves the removal of oil, grease, and dirt from the surface of the basis material using water, with or
without a detergent or other dispersing agent. Alkaline cleaning (both electrolytic and non-electrolytic) and acid
cleaning are both included.
Alkaline cleaning is used to remove oily dirt or solid soils from workpieces. The detergent nature of the cleaning
solution provides most of the cleaning action; agitation of the solution and movement of the workpiece are
secondary. There are three types of alkaline cleaners: soak, spray, and electrolytic. Soak cleaners are used on
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Appendix A:
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
easily removed soil; they are less efficient than spray or electrolytic cleaners. Spray cleaners combine the
detergent properties of the solution with the impact force of the spray, which mechanically loosens the soil. The
most effective conventional alkaline cleaning method is electrolytic cleaning. The strong agitation of the solution
by gas evolution, and oxidation-reduction reactions that occur during electrolysis produce the cleanest surface of
the three methods. Also, certain dirt particles become electrically charged and are repelled from the surface.
Direct current (cathodic) cleaning uses the workpiece as the cathode; reverse current (anodic) cleaning uses the
workpiece as the anode. In periodic reverse current cleaning, the current is periodically reversed from direct
current to reverse current. Periodic reverse cleaning gives improved smut removal, accelerated cleaning, and a
more active surface for any subsequent surface finishing operation.
Acid cleaning employs a combination of a wetting agent or detergent with a solution of an inorganic (mineral)
acid, organic acid, or an acid salt to remove oil, dirt, or oxide from metal surfaces. Depending on the acid
concentration, acid cleaning may be referred to as pickling, acid dipping, descaling, or desmutting. The solution
may or may not be heated and can be an immersion or spray operation. Agitation is normally required with
soaking, and spray is usually used with complex shapes. An acid dip operation may also follow alkaline cleaning
prior to plating. Phosphoric acid mixtures are also commonly used to remove oils and light rust while leaving a
phosphate coating that provides a paint base or temporary resistance to rusting. Strong acid solutions are used to
remove rust and scale prior to surface finishing.
Machining removes stock from a workpiece by forcing a cutting tool through the workpiece and removing a chip
of basis material. Machining operations such as turning, milling, drilling, boring, tapping, planing, broaching,
sawing and cutoff, shaving, threading, reaming, shaping, slotting, bobbing, filing, and chamfering are included in
this definition.
Grinding removes stock from a workpiece with abrasive grains held by a rigid or semirigid binder. The grinding
tool is usually in the form of a disk (the basic shape of grinding wheels), but may also be in the form of a
cylinder, ring, cup, stick, strip, or belt. The most commonly used abrasives are aluminum oxide, silicon carbide,
and diamond. The processes included in this unit operation are sanding (or cleaning to remove rough edges or
excess material), surface finishing, and separating (as in cut-off or slicing operations).
Polishing is an abrading operation used to remove or smooth out surface defects (scratches, pits, tool marks, etc.)
that adversely affect the appearance or function of a part. Polishing is usually performed with either a belt or
wheel to which an abrasive, such as aluminum oxide or silicon carbide, is bonded. Both wheels and belts are
flexible and will conform to irregular or rounded areas where necessary. The operation usually referred to as
buffing is included in the polishing operation.
Barrel Finishing or tumbling is a controlled method of processing parts to remove burrs, scale, flash, and oxides
to improve surface finish. Widely used as a finishing operation for many parts, it obtains a uniformity of surface
finish not possible by hand finishing. For large quantities of small parts it is generally the most economical
method of cleaning and surface conditioning. Parts to be finished are placed in a rotating barrel or vibrating unit
with an abrasive medium, water, or oil, and usually some chemical compound to assist in the operation. As the
barrel rotates slowly, the upper layer of the work is given a sliding movement toward the lower side of the barrel,
causing the abrading or polishing action. The same results may also be accomplished in a vibrating unit, in which
the entire contents of the container are in constant motion.
Burnishing is the process of finish sizing or smooth finishing a workpiece (previously machined or ground) by
displacement, rather than removal, of minute surface irregularities. It is accomplished with a smooth point or line-
contact and fixed or rotating tools.
Impact Deformation is the process of applying an impact force to a workpiece such that the workpiece is
permanently deformed or shaped. Impact deformation operations include shot peening, peening, forging, high
energy forming, heading, and stamping.
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Appendix A:
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
Pressure Deformation is the process of applying force (at a slower rate than an impact force) to permanently
deform or shape a workpiece. Pressure deformation includes operations such as rolling, drawing, bending,
embossing, coining, swaging, sizing, extruding, squeezing, spinning, seaming, staking, piercing, necking,
reducing, forming, crimping, coiling, twisting, winding, flaring, or weaving.
Shearing is the process of severing or cutting a workpiece by forcing a sharp edge or opposed sharp edges into
the workpiece, stressing the material to the point of shear failure and separation.
Heat Treating is the modification of the physical properties of a workpiece through the application of controlled
heating and cooling cycles. Such operations as tempering, carburizing, cyaniding, nitriding, annealing,
normalizing, austenizing, quenching, austempering, siliconizing, martempering, and malleabilizing are included
in this definition.
Thermal Cutting comprises cutting, slotting, or piercing a workpiece with an oxyacetylene oxygen lance or
electric arc cutting tool.
Welding refers to joining two or more pieces of material by applying heat, pressure, or both, with or without filler
material, to produce a localized union through fusion or recrystallization across the interface. Included are gas
welding, resistance welding, arc welding, cold welding, electron beam welding, and laser beam welding.
Brazing joins metals by filling a thin (capillary thickness) layer of nonferrous filler metal into the space between
them. Bonding results from the intimate contact produced by the dissolution of a small amount of base metal in
the molten filler metal, without fusion of the base metal. The term brazing is used when the temperature exceeds
425°C (800°F).
Soldering is the process of joining metals by filling a thin (capillary thickness) layer of nonferrous filler metal
into the space between them. Bonding results from the intimate contact produced by the dissolution of a small
amount of base metal in the molten filler metal, without fusion of the base metal. The term soldering is used
when the temperature range falls below 425°C (SOOT).
Flame Spraying is the application of a metallic coating to a workpiece using finely powdered fragments of wire,
together with suitable fluxes, projected through a cone of flame onto the workpiece.
Sand Blasting removes stock, including surface films, from a workpiece by the use of abrasive grains
pneumatically impinged against the workpiece. The abrasive grains used include sand, metal shot, slag, silica,
pumice, or natural materials such as walnut shells.
Abrasive Jet Machining is a mechanical process for cutting hard brittle materials. It is similar to sand
blasting, but uses much finer abrasives carried at high velocities (500-3,000 fps) by a liquid or gas stream. Uses
include frosting glass, removing metal oxides, deburring, and drilling and cutting thin sections of metal.
Electrical Discharge Machining is a process which can remove metal with good dimensional control from any
metal. It cannot be used for machining glass, ceramics, or other nonconducting materials. The machining action is
caused by the formation of an electrical spark between an electrode, shaped to the required contour, and the
workpiece. Since the cutting tool has no contact with the workpiece, it can be made from a soft, easily worked
material such as brass. The tool works in conjunction with a fluid such as mineral oil or kerosene, which is fed to
the work under pressure. The function of this coolant is to serve as a dielectric, to wash away particles of eroded
metal from the workpiece or tool, and to maintain a uniform resistance to flow of current.
Electrical discharge machining is also known as spark machining or electronic erosion. The operation was
developed primarily for machining carbides, hard nonferrous alloys, and other hard-to-machine materials.
Electrochemical Machining is a process based on the same principles used In electroplating except the
workpiece is the anode and the tool is the cathode. Electrolyte is pumped between the electrodes and a potential
applied with the result that metal is rapidly removed.
A-5
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Appendix A
Appendix A:
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
In this process, electrode accuracy is important since the surface finish of the electrode tool will be reproduced in
the surface of the workpiece. While copper is frequently used as the electrode, brass, graphite, and copper-
tungsten are also used. The tool must be an electrical conductor, easy to machine, corrosion resistant, and able to
conduct the quantity of current needed. Although there is no standard electrolyte, sodium chloride is more
generally used than others.
Electron Beam Machining is a thermoelectric process. In electron beam machining, heat is generated by high
velocity electrons impinging on part of the workpiece. At the point where the energy of the electrons is focused, it
is transformed into sufficient thermal energy to vaporize the material locally. The process is generally carried out
in a vacuum. While the metal-removal rate of electron beam machining is approximately 0.01 milligrams per
second, the tool is accurate and is especially adapted for micro-machining. There is no heat affected zone or
pressure on the workpiece and extremely close tolerances can be maintained. The process results in X-ray
emission which requires that the work area be shielded to absorb radiation. At present the process is used for
drilling holes as small as 0.0508 mm (0.002 in.) in any known material, cutting slots, shaping small parts, and
machining sapphire jewel bearings.
Laser Beam Machining uses a highly focused monochromatic collimated beam of light is to remove material
from a workpiece at the point of impingement. Laser beam machining is a thermoelectric process, and material
removal is largely accomplished by evaporation, although some material is removed in the liquid state at high
velocity. Since the metal removal rate is very small, they are used for such jobs as drilling microscopic holes in
carbides or diamond wire drawing dies, and for removing metal in the balancing of high-speed rotating
machinery.
Lasers can vaporize any known material. They have small heat-affected zones and work easily with nonmetallic
hard materials.
Plasma Arc Machining is the process of material removal or shaping of a workpiece by a high velocity jet of high
temperature ionized gas. A gas (nitrogen, argon, or hydrogen) is passed through an electric arc causing it to
become ionized and raised to temperatures in excess of 16,649°C (30,000°F). The relatively narrow plasma jet
melts and displaces the workpiece material in its path. Because plasma machining does not depend on a chemical
reaction between the gas and the work material and because plasma temperatures are extremely high, the process
can be used on almost any metal, including those that are resistant to oxygen-fuel gas cutting. The method is of
commercial importance mainly for profile cutting of stainless steel and aluminum alloys.
Ultrasonic Machining is a mechanical process designed to effectively machine hard, brittle materials. It removes
material by the use of abrasive grains which are carried in a liquid between the tool and the work and which
bombard the work surface at high velocity. This action gradually chips away minute particles of material in a
pattern controlled by the tool shape and contour. A transducer causes an attached tool to oscillate linearly at a
frequency of 20,000 to 30,000 times per second at an amplitude of 0.0254 to 0.127 mm (0.001 to 0.005 in.). The
tool motion is produced by being part of a sound wave energy transmission line which causes the tool material to
change its normal length by contraction and expansion. The tool holder is threaded to the transducer and
oscillates linearly at ultrasonic frequencies, thus driving the grit particles into the workpiece. The cutting
particles, boron carbide and similar materials, are of a 280-mesh size or finer, depending upon the accuracy and
the finish desired. Operations that can be performed include drilling, tapping, coining, and the making of
openings in all types of dies. Ultrasonic machining is used principally on materials such as carbides, tool steels,
ceramics, glass, gem stones, and synthetic crystals.
Sintering is the process of forming a mechanical part from a powdered metal by fusing the particles together
under pressure and heat. The temperature is maintained below the melting point of the basis metal.
Laminating is adhesive bonding of layers of metal, plastic, or wood to form a part.
Hot Dip Coating is defined by coating a metallic workpiece with another metal by immersion in a molten bath to
provide a protective film. Galvanizing (hot dip zinc) is the most common hot dip coating.
A-6
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Appendix A
Appendix A:
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
Sputtering covers a metallic or nonmetallic workpiece with thin films of metal. The surface to be coated is
bombarded with positive ions in a gas discharge tube, which is evacuated to a low pressure.
Vapor Plating is the decomposition of a metal or compound upon a heated surface by reduction or decomposition
of a volatile compound at a temperature below the melting point of either the deposit or the basis material.
Thermal Infusion applies a fused zinc, cadmium, or other metal coating to a ferrous workpiece by imbuing the
surface of the workpiece with metal powder or dust in the presence of heat.
Salt Bath Descaling is the process of removing surface oxides or scale from a workpiece by immersion of the
workpiece in a molten salt bath or a hot salt solution. Molten salt baths are used in a salt bath—water quench—
acid dip sequence to clean hard-to-remove oxides from stainless steels and other corrosion-resistant alloys. The
work is immersed in the molten salt (temperatures range from 400 to 540 degrees C), quenched with water, and
then dipped in acid. Oxidizing, reducing, and electrolytic baths are available, and the particular type needed
depends on the oxide to be removed.
Solvent Degreasing removes oils and grease from the surfaces of a workpiece by the use of organic solvents, such
as aliphatic petroleums (e.g., kerosene, naptha), aromatics (e.g., benzene, toluene), oxygenated hydrocarbons
(e.g., ketones, alcohol, ether), halogenated hydrocarbons (e.g., 1,1,1-trichloroethane, trichloroethylene, methylene
chloride), and combinations of these classes of solvents. Solvent cleaning can be accomplished by either the
liquid or vapor phase of these solvents. Solvent vapor degreasing is normally quicker than solvent liquid
degreasing. However, ultrasonic vibration is sometimes used with liquid solvent so as to decrease the immersion
time required with complex shapes. Solvent cleaning is often used as a precleaning operation, e.g., prior to the
alkaline cleaning that precedes plating, as a final cleaning of precision parts, or as a surface preparation for some
painting operations.
Emulsion cleaning is a type of solvent degreasing that uses common organic solvents (e.g., kerosene, mineral oil,
glycols, and benzene) dispersed in an aqueous medium with the aid of an emulsifying agent. Depending on the
solvent used, cleaning is done at temperatures from room temperature to 82°C (180°F). This operation uses less
chemical than solvent degreasing because a lower solvent concentration is employed. The process is used for
rapid superficial cleaning and is usually performed as emulsion spray cleaning.
Paint Stripping is the removal of an organic coating from a workpiece. The stripping of such coatings is usually
performed with caustic, acid, solvent, or molten salt.
Painting refers to the application of an organic coating to a workpiece. The application of coatings such as paint,
varnish, lacquer, shellac, and plastics by spraying, dipping, brushing, roll coating, lithographing, and wiping are
included. Spray painting is by far the most common and can be used with nearly all varieties of paint. The paint
can be sprayed manually or automatically, hot or cold, and it may be atomized with or without compressed air to
force the paint through an orifice. Other processes included under this unit operation are printing, silk screening,
and stenciling.
Electrostatic Painting is the application of electrostatically charged paint particles to an oppositely charged
workpiece followed by thermal fusing of the paint particles to form a cohesive paint film. Usually the paint is
applied in spray form and may be applied manually or automatically, hot or cold, and with or without compressed
air atomization. Both waterborne and solvent-borne coatings can be sprayed electrostatically.
Electropainting is the process of coating a workpiece by making it either anodic or cathodic in a bath that is
generally an aqueous emulsion of the coating material. The electrodeposition bath contains stabilized resin,
dispersed pigment, surfactants, and sometimes organic solvents in water. Electropainting is used primarily for
primer coats because it gives a fairly thick, highly uniform, corrosion resistant coating in relatively little time.
Vacuum Metalizing coats a workpiece with metal by flash heating metal vapor in a high-vacuum chamber
containing the workpiece. The vapor condenses on all exposed surfaces.
A-7
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Appendix A
Appendix A:
Descriptions of the 46 Unit Operations in the Metal Finishing Industry
Assembly is the fitting together of previously manufactured parts or components into a complete machine, unit of
a machine, or structure.
Calibration is the application of thermal, electrical, or mechanical energy to set or establish reference points for a
component or complete assembly.
Testing is the application of thermal, electrical, or mechanical energy to determine the suitability or functionality
of a component or complete assembly.
Mechanical Plating is the process of depositing metal coatings on a workpiece using a tumbling barrel, metal
powder, and usually glass beads for the impaction media. The operation is subject to the same cleaning and
rinsing operations that are applied before and after the electroplating operation.
Source: U.S. EPA, 1983a
A-8
-------
Appendix B
Appendix B
Keyword Search Terms for the Metal Finishing Industry
-------
Appendix B
Appendix B: Keyword Search Terms for the Metal Finishing Industry
Master Terms
Metal Finishing
Industrial Wastewater
Industrial Wastewater Treatment
Metals Removal
General Terms
Case study
Effluent
Elimination
Green
Pilot (scale)
Full/Field (scale)
Improvement
Influent
Nano
Percent (%)
Performance
Pollution Prevention
Recovery/recycle
Reduce/Reducti on
Removal
Rinse Water
Replacement
6 Primary Metal Finishing Operations
Electroplating
Electroless Plating
Anodizing
Coating
Chromating
Phosphating
Coloring
Chemical Etching and Milling
Printed Circuit Board Manufacture
Pollutants
Cadmium
Chromium
Hexavalent Chromium
Phosphate
Trivalent Chromium
Cyanide
B-l
-------
Appendix B
Process Operations
Aluminum Coating
Bright Dipping
Brighteners
Cadmium Free Technologies
Cell Phone Screen Manufacturing
Chemical Etching and Milling
Dry Technologies
Hexavalent Chromium Free Technologies
New Wastewater Treatment Technologies
Nickel Fluorocarbon
Phosphate free technologies
Phosphoric Acid
Powder Coating
Reuse
Replacement for Phosphating
Solar Panel Manufacturing
Transition Metal Coatings
Transportation equipment cleaning
Zero Liquid Discharge
Zinc-Nickel Alloy Coating
Zirconization
B-2
-------
Appendix C
Appendix C
Wastewater Treatment Technologies Evaluated
for Metals Removal in the 1983 Metal Finishing ELGs
-------
Appendix C
Appendix C:
Wastewater Treatment Technologies Evaluated
for Metals Removal in the 1983 Metal Finishing ELGs
Treatment
Technology
Sulfide
precipitation
HighpH
precipitation
Chemical
reduction
Technology Description
A precipitation technique using
hydrogen sulfide or soluble sulfide salts
to precipitate metal sulfides. May also
use ferrous or ferric sulfates to
precipitate metals, specifically for
complex wastewater.
Full-scale application in metal finishing.
Precipitation technique operating at high
pH levels for complex wastewater
treatment.
Full-scale application in metal finishing.
Precipitation technique operating at low
pH levels for complex wastewater
Main Treatment Purpose
Inorganics Removal;
Complex Metals Removal
Complex Metals Removal
Inorganics Removal;
Complex Metals Removal
Type of
Wastewater
Aluminum
anodizing
wastewater
Metal finishing
wastewater
Metal finishing
wastewater
Industrial
wastewater
Metal finishing
wastewater
(cyanide bearing)
No data available
No data available
Pollutant
Aluminum
Chromium
(VI)
Chromium
Chromium
(VI)
Chromium
Iron
Zinc
Chromium
(VI)
Chromium
Copper
Zinc
Chromium
(VI)
Chromium,
total
Iron
Nickel
Zinc
Cyanide, total
Average
Effluent
Concentration
(mg/L)
0.112
Non-detect
Non-detect
0.01
O.04
0.1
0.07
O.005
O.005
0.003
0.009
0.02
O.I
0.6
0.1
0.1
0.024
Average
Percent
Removal
(%)
97.3
100
100
>99.9
>99.9
80.8
>99.8
>99.9
>99.9
89.7
85
>9.1
>95.8
99.4
>85.3
>99.7
99.2
C-l
-------
Appendix C
Appendix C:
Wastewater Treatment Technologies Evaluated
for Metals Removal in the 1983 Metal Finishing ELGs
Treatment
Technology
Electrochemical
Chromium
Reduction
Oxidation by
Chlorination
Oxidation by
Ozonation
Oxidation by
Ozonation with
UV Radiation
Technology Description
treatment. Addition of reducing agent to
reduce metals oxidation state to allow
precipitation at high pH. Commonly
used for hexavalent chromium reduction.
Full-scale application in metal finishing
for chromium reduction.
Selective reduction of hexavalent
chromium by applying an electric current
across iron electrodes to free ferrous ions
that further react with chromate in
solution to form chromic and ferrous
hydroxides. Also effective with zinc.
Full-scale application in metal finishing.
Addition of chlorine in an alkaline
environment to convert cyanide into
carbon dioxide and nitrogen.
The generation of ozone via an electrical
discharge process that is dissolved in the
wastewater to convert cyanide
compounds into cyanates that are
subsequently decomposed by other
methods.
Full-scale application in metal finishing.
In addition to oxidation by ozone,
simultaneous application of ultraviolet
light. Commonly used to treat complex
cyanide salts.
Main Treatment Purpose
Inorganics Removal
Inorganics Removal;
Complex Metals Removal
Inorganics Removal
Inorganics Removal;
Complex Metals Removal
Type of
Wastewater
Metal finishing
wastewater
Metal finishing
wastewater
Cyanide bearing
wastewater
No data available.
Pollutant
Chromium
(VI)
Zinc
Cyanide
Cyanide, total
Cyanide,
amenable
Average
Effluent
Concentration
(mg/L)
0.05
0.1
Non-detect
0.083
0.082
Average
Percent
Removal
(%)
99.5
96.7
>99
91.6
91.7
C-2
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Appendix C
Appendix C:
Wastewater Treatment Technologies Evaluated
for Metals Removal in the 1983 Metal Finishing ELGs
Treatment
Technology
Oxidation by
Hydrogen
Peroxide
Electrochemical
Cyanide
Oxidation
Flotation
Membrane
Filtration
Technology Description
Full-scale application in metal finishing.
Addition of peroxides to convert
cyanides to cyanates. Applicable to
cyanide-bearing wastewater containing
zinc or cadmium.
Full-scale application in metal finishing
for cyanide destruction and metal
recovery.
Conversion of cyanides to cyanates by
applying an electric current across iron
electrodes to generate ions that further
react with cyanide via ozonation and
direct oxidation.
No full-scale application for metal
finishing in 1983.
Solids removal process using air bubbles
to carry suspended solids to the surface
where they are skimmed off and
disposed. Commonly used to treat
wastewaters with suspended solids near
the specific gravity of 1.0, which make
gravity settling difficult. Also used for
oily waste treatment.
Full-scale application in oily waste
removal. Bench scale studies in heavy
metals removal.
Solids removal process by recirculating
wastewater through a tubular membrane
Main Treatment Purpose
Inorganics Removal;
Metal/Chemical Recovery
Inorganics Removal
Inorganics Removal
Complex Metals Removal;
Polishing
Type of
Wastewater
No data available.
No data available.
No data available.
Metal finishing
wastewater
Pollutant
Chromium
(VI)
Average
Effluent
Concentration
(mg/L)
0.008
0.038
Average
Percent
Removal
(%)
98.9
99.8
C-3
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Appendix C
Appendix C:
Wastewater Treatment Technologies Evaluated
for Metals Removal in the 1983 Metal Finishing ELGs
Treatment
Technology
Diatomaceous
Earth Filtration
Peat Adsorption
Insoluble
Starch Xanthate
Technology Description
filter to enhance removal of heavy
metals. May also be used to treat
complex wastewater.
Full-scale application in metal finishing.
Solids separation device to further
enhance suspended solids removal.
Equipment includes multiple leaf screens
coated with diatomaceous earth, filter
housing, and pumps.
Full-scale application in metal finishing.
Solids removal through chemical
bonding between naturally occurring
functional groups in peat moss and
transition metals and polar organic
molecules in wastewater.
Pilot scale studies in metal finishing.
Full-scale application in textile,
newsprint, and metal reclamation
industries.
Ion exchange resin material that can treat
wastewater in a batch or continuous
process or as a filter precoat.
Full-scale application in metal finishing.
Main Treatment Purpose
Polishing
Polishing
Polishing; Metal/Chemical
Recovery
Type of
Wastewater
Unclarified
chemical
precipitation
effluent (metal
finishing)
Industrial
wastewater
Treated metal
finishing effluent
Metal finishing
rinse water
Pollutant
Chromium
Copper
Iron
Lead
Nickel
Zinc
TSS
TSS
Cadmium
Chromium
Copper
Lead
Nickel
Tin
Zinc
Antimony
Chromium
(VI)
Copper
Cyanide
Lead
Mercury
Nickel
Silver
Zinc
All metals
Copper
Iron
Lead
Nickel
Average
Effluent
Concentration
(mg/L)
0.13
0.28
0.01
0.18
0.05
4.05
35.6
0.01
0.19
1.39
0.06
0.20
0.08
1.68
0.9
O.04
0.24
0.7
0.025
0.02
0.07
0.05
0.25
0.02
0.26
0.13
0.27
273.5
Average
Percent
Removal
(%)
98.5
99.3
97.5
99.8
98.4
69.2
94.2
59.1
97.4
94.4
70.9
87.1
87.1
89.0
64.0
>99.9
99.9
98.1
99.9
>98.0
97.2
>95.0
83.3
98.0
3.9
31.6
5.4
0.9
C-4
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Appendix C
Appendix C:
Wastewater Treatment Technologies Evaluated
for Metals Removal in the 1983 Metal Finishing ELGs
Treatment
Technology
Electrolytic
Recovery
Evaporation
Ion Exchange
Technology Description
Selective reduction of a metal by
applying an electric current across
electrodes and allowing a plating process
to occur to recover the metal.
Full-scale application in metal finishing.
Concentration process where water
evaporates from solution and increases
concentration of solute in remaining
solution. May also involve a
condensation process.
Full-scale application in metal finishing.
Exchange of ionic contaminants in
wastewater with ions of the resin.
Commonly used to recover process
chemicals and metals from rinsewater to
allow reuse. Many metal finishers use
the technology to concentrate and purify
plating baths.
Full-scale application in metal finishing.
Main Treatment Purpose
Metal/Chemical Recovery
Metal/Chemical Recovery;
In-plant Controls
Metal/Chemical Recovery;
In-plant Controls
Type of
Wastewater
Photographic
processing
wastewater
No data available
Electroplating
rinse water
Printed circuit
board
Pollutant
Tin
Zinc
Silver
Aluminum
Cadmium
Chromium
(III)
Chromium
(VI)
Copper
Cyanide
Iron
Manganese
Nickel
Silver
Tin
Zinc
Copper
Cyanide
Average
Effluent
Concentration
(mg/L)
0.75
0.063
21
0.2
0.00
0.01
0.01
0.09
0.04
0.01
0.00
0.00
0.00
0.00
0.4
0.1
0.09
Average
Percent
Removal
(%)
25.0
16.4
95.6
96.4
100
99.7
99.9
98.0
99.6
99.9
100
100
100
100
97.3
99.8
97.4
C-5
-------
Appendix C
Appendix C:
Wastewater Treatment Technologies Evaluated
for Metals Removal in the 1983 Metal Finishing ELGs
Treatment
Technology
Technology Description
Main Treatment Purpose
Type of
Wastewater
manufacturing
rinse water
Metal finishing
rinse water
Photographic
processing
wastewater
Pollutant
Gold
Lead
Nickel
Silver
Sulfate
Tin
Chromium
Copper
Nickel
Silver
Average
Effluent
Concentration
(mg/L)
0.1
0.01
0.01
0.01
2
0.1
0.03
0.16
0.003
0.093
Average
Percent
Removal
95.7
99.4
99.4
99.9
99.0
90.9
76.9
99.8
89.6
90.3
C-6
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