TREATMENT TECHNOLOGY BACKGROUND DOCUMENT
Larry Rosengrant
Section Chief
Laura Lopez
Project Officer
U.S. Environmental Protection Agency
Office of Solid Waste
401 M Street, SW
Washington, DC 20460
January 1991
'iVl Printed on Recycled Paper
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TABLE OF CONTENTS
Section Page No.
EXECUTIVE SUMMARY viii
Applicability xi
Underlying Principles of Operation xiii
Description of the Treatment Process xiv
Waste Characteristics Affecting Performance (WCAPs) ... xv
Design and Operating Parameters xvii
I. TREATMENT TECHNOLOGIES
A. DESTRUCTION TECHNOLOGIES
1. BIOLOGICAL TREATMENT 1
1.1 Applicability 1
1.2 Underlying Principles of Operation ... 1
1.3 Description of Biological Treatment Processes ... 3
1.4 Waste Characteristics Affecting Performance (WCAPs). 8
1.5 Design and Operating Parameters 11
1.6 References 16
2. CHEMICAL OXIDATION 17
2.1 Applicability '. . . 17
2.2 Underlying Principles of Operation 18
2.3 Description of Chemical Oxidation Processes .... 21
2.4 Waste Characteristics Affecting Performance (WCAPs). 23
2.5 Design and Operating Parameters 24
2.6 References 28
3. CHEMICAL REDUCTION 29
3.1 Applicability 29
3.2 Underlying Principles of Operation 29
3.3 Description of Chemical Reduction Process 30
3.4 Waste Characteristics Affecting Performance (WCAPs). 32
3.5 Design and Operating Parameters 33
3.6 References 37
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TABLE OF CONTENTS (CONTINUED)
Section Page No.
4. ELECTROLYTIC OXIDATION OF CYANIDE 38
4.1 Applicability 38
4.2 Underlying Principles of Operation 38
4.3 Description of Electrolytic Oxidation of Cyanide
Process 39
4.4 Waste Characteristics Affecting Performance (WCAPs). 39
4.5 Design and Operating Parameters 40
4.6 References 43
5. INCINERATION 44
5.1 Applicability 44
5.2 Underlying Principles of Operation 45
5.3 Description of Incineration Process 46
5.4 Waste Characteristics Affecting Performance (WCAPs). 54
5.5 Design and Operating Parameters 57
5.6 References 63
6. WET AIR OXIDATION 64
6.1 Applicability 64
6.2 Underlying Principles of Operation 66
6.3 Description of Wet Air Oxidation Process 66
6.4 Waste Characteristics Affecting Performance (WCAPs). 67
6.5 Design and Operating Parameters 70
6.6 References 73
B. TECHNOLOGIES THAT REDUCE THE SOLUBILITY OR LEACHABILITY
OF METALS
1. AMALGAMATION 74
1.1 Applicability 74
1.2 Underlying Principles of Operation 74
1.3 Description of Amalgamation Treatment Processes . . 75
1.4 Waste Characteristics Affecting Performance (WCAPs). 75
1.5 Design and Operating Parameters 77
1.6 References 80
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TABLE OF CONTENTS (CONTINUED)
Section Page No.
2. CHEMICAL PRECIPITATION 81
2.1 Applicability 81
2.2 Underlying Principles of Operation 82
2.3 Description of Chemical Precipitation Process ... 83
2.4 Waste Characteristics Affecting Performance (WCAPs). 85
2.5 Design and Operating Parameters 89
2.6 References 93
3. ENCAPSULATION 94
3.1 Applicability 94
3.2 Underlying Principles of Operation 94
3.3 Description of Encapsulation Processes 94
3.4 Waste Characteristics Affecting Performance (WCAPs). 96
3.5 Design and Operating Parameters 98
3.6 References .. 101
4. STABILIZATION OF METALS 102
4.1 Applicability 102
4.2 Underlying Principles of Operation 102
4.3 Description of Stabilization Process 104
4.4 Waste Characteristics Affecting Performance (WCAPs). 104
4.5 Design and Operating Parameters 106
4.6 References 110
5. VITRIFICATION Ill
5.1 Applicability Ill
5.2 Underlying Principles of Operation 112
5.3 Description of Vitrification Processes 113
5.4 Waste Characteristics Affecting Performance (WCAPs). 115
5.5 Design and Operating Parameters 119
5.6 References 124
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TABLE OF CONTENTS (CONTINUED)
Section Page No.
II. REMOVAL TECHNOLOGIES
A. RECOVERY, REUSE, AND/OR SEPARATION TECHNOLOGIES FOR
ORGANICS
1. CARBON ADSORPTION 125
1.1 Applicability 125
1.2 Underlying Principles of Operation 125
1.3 Description of Carbon Adsorption Process 125
1.4 Waste Characteristics Affecting Performance (WCAPs). 129
1.5 Design and Operating Parameters 130
1.6 References 133
2. DISTILLATION TECHNOLOGIES 134
2.1 Applicability 134
2.2 Underlying Principles of Operation 135
2.3 Description of Distillation Processes 137
2.4 Waste Characteristics Affecting Performance (WCAPs). 146
2.5 Design and Operating Parameters 150
2.6 References 156
3. EXTRACTION TECHNOLOGIES 157
3.1 Applicability 157
3.2 Underlying Principles of Operation 157
3.3 Description of Extraction Processes 158
3.4 Waste Characteristics Affecting Performance (WCAPs). 164
3.5 Design and Operating Parameters 166
3.6 References 170
4. FUEL SUBSTITUTION 171
4.1 Applicability 171
4.2 Underlying Principles of Operation 174
4.3 Description of Fuel Substitution Process 174
4.4 Waste Characteristics Affecting Performance (WCAPs). 177
4.5 Design and Operating Parameters 179
4.6 References 183
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TABLE OF CONTENTS (CONTINUED)
Section Page No,
B. RECOVERY AND/OR SEPARATION TECHNOLOGIES FOR METALS
1. ACID LEACHING 184
1.1 Applicability 184
1.2 Underlying Principles of Operation 184
1.3 Description of Acid Leaching Process 185
1.4 Waste Characteristics Affecting Performance (WCAPs). 186
1.5 Design and Operating Parameters 188
1.6 References 190
2. FILTRATION TECHNOLOGIES 191
2.1 Applicability 191
2.2 Underlying Principles of Operation 191
2.3 Description of Filtration Processes 192
2.4 Waste Characteristics Affecting Performance (WCAPs). 194
2.5 Design and Operating Parameters 196
2.6 References 200
3. HIGH TEMPERATURE METALS RECOVERY 201
3.1 Applicability . . 201
3.2 Underlying Principles of Operation 202
3.3 Description of High Temperature Metals Recovery
Process 202
3.4 Waste Characteristics Affecting Performance (WCAPs). 205
3.5 Design and Operating Parameters 206
3.6 References 210
4. ION EXCHANGE 211
4.1 Applicability 211
4.2 Underlying Principles of Operation 211
4.3 Description of Ion Exchange Process 212
4.4 Waste Characteristics Affecting Performance (WCAPs). 213
4.5 Design and Operating Parameters 217
4.6 References 220
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TABLE OF CONTENTS (CONTINUED)
Section Page No,
5. RETORTING 221
5.1 Applicability 221
5.2 Underlying Principles of Operation 221
5.3 Description of Retorting Process 222
5.4 Waste Characteristics Affecting Performance (WCAPs). 225
5.5 Design and Operating Parameters 227
5.6 References 228
III. TECHNOLOGIES APPLICABLE TO HIXED WASTE
(RADIOACTIVE/HAZARDOUS)
INTRODUCTION 229
A. TECHNOLOGIES FOR WASTES CONTAINING ORGANICS AND
INORGANICS OTHER THAN METALS 231
1. Carbon Adsorption 231
2. Distillation Technologies 231
3. Extraction Technologies 232
B. TECHNOLOGIES FOR NIXED WASTES CONTAINING METALS 233
1. Acid Leaching 233
2. Chemical Precipitation 233
3. Filtration 233
4. High Temperature Metals Recovery 233
5. Ion Exchange 234
6. Stabilization of Metals 234
C. TECHNOLOGIES FOR MIXED WASTES IN SPECIAL TREATABILITY
GROUPS 235
1. Amalgamation 235
2. Encapsulation 235
3. Vitrification 236
4. Incineration 237
REFERENCES 238
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TABLE OF CONTENTS (CONTINUED)
Section Page No.
APPENDIX A List of Best Demonstrated Available Technology (BOAT)
Background Documents and Associated BDAT(s) .... A-l
LIST OF FIGURES
Figure Page No.
1 Activated Sludge System 4
2 Trickling Filter System 7
3 Anaerobic Digestion 9
4 Continuous Chemical Reduction System 31
5 Liquid Injection Incineration System 49
6 Rotary Kiln Incineration System 50
7 Fluidized Bed Incineration System 52
8 Fixed Hearth Incineration System 53
9 Continuous Wet Air Oxidation System 68
10 Continuous Chemical Precipitation System 84
11 Circular Clarifier Systems 86
12 Inclined Plate Settler System 87
13 Carbon Adsorption Systems 127
14 Plot of Breakthrough Curve 128
15 Batch Distillation System- 138
16 Fractionation System 140
17 Steam Stripping System 142
18 Thin Film Evaporation System 143
19 Screw Flight Dryer 145
20 Two-Stage Mixer-Settler Solvent Extraction System .... 160
21 Packed and Sieve Tray Solvent Extraction Columns 162
22 High Temperature Metals Recovery System 203
23 Two-Step Cation/Anion Ion Exchange System 214
24 Retorting Process (Without a Scrubber and Subsequent
Wastewater Discharge) 223
25 Retorting Process (With a Scrubber and Subsequent
Wastewater Discharge) 224
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EXECUTIVE SUMMARY
This document provides a discussion of the treatment technologies
applicable to wastes that are subject to the Land Disposal Restrictions (LDR),
which were mandated by Congress as part of the 1984 Hazardous and Solid Waste
Amendments (HSWA) to the Resource Conservation and Recovery Act (RCRA). This
document does not include every possible technology that may be used to treat
wastes subject to the LDR, but discusses those most commonly used. The
technologies discussed include those that are demonstrated (commercially
available) and have been proven to substantially diminish the toxicity of
hazardous constituents and/or to reduce the likelihood of migration of such
constituents from the waste of concern. These technologies include those that
treat wastewaters* and those that treat nonwastewaters.
Treatment performance data from technologies discussed in this document
were the basis for the determination of the Best Demonstrated Available
Technology (BOAT), from which the Land Disposal Restrictions treatment standards
were promulgated (as numerical standards or methods of treatment). Information
supporting the development of these treatment standards may be found in the
applicable background documents prepared for EPA's Office of Solid Waste, Waste
Treatment Branch. These BOAT background documents contain information on the
following: industries affected and waste characterization, applicable and
demonstrated treatment technologies for the wastes of concern, determination of
BOAT, and treatment performance data with design and operating conditions.
*For the purpose of the Land Disposal Restrictions rule, wastewaters are
defined as wastes containing less than or equal to 1 percent (weight
basis) filterable solids and less than or equal to 1 percent (weight
basis) total organic carbon (TOC). Wastes not meeting this definition
are classified as nonwastewaters.
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The methodology EPA used to develop the BOAT treatment standards is
described in two separate documents: Generic Quality Assurance Project Plan for
Land Disposal Restrictions Program ("BOAT") (USEPA 1990) and Methodology for
Developing BOAT Treatment Standards (USEPA 1990). The second document also
discusses the petition process to be followed in requesting a variance from the
BDAT treatment standards.
Appendix A lists the background documents developed for the BDAT program,
identifying the technologies and the types of constituents treated (e.g.,
organics, inorganics, metals, etc.) when present in actual treatment performance
data.
The treatment performance data and the design and operating data (see
Section 5 of this executive summary for a discussion of design and operating
data) contained in these documents may be used to help determine whether a
treatment system has been designed and operated properly. The treatment
performance data from these background documents may be used to compare the
performance of a treatment system of concern treating the same or similar
constituents (provided the waste matrices are not significantly dissimilar).
Comparisons can be made of the destruction or removal efficiencies, the level of
treatment achieved, and the design and operating parameters of a system (for
example, feed rate, temperature, and oxygen concentration for incineration
systems or concentration of nutrients, dissolved oxygen, residence time, etc. for
biological treatment systems).
One document that may be particularly helpful is the F039 (Multisource
Leachate) background document, Volumes A through E (USEPA 1990). This document
contains comprehensive treatment performance data covering the complete list of
P and U waste code constituents. The nonwastewater data, for the most part, were
derived from 11 incineration test burns for the organics and from stabilization
treatment for the metals. The wastewater data were derived from various sources
such as EPA's Office of Water's Industrial Technology Division (ITD), the
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National Pollutant Discharge Elimination System (NPDES), Water Engineering
Research Laboratory (WERL), and other data sources available to the Agency.
These above-mentioned data were generally considered to represent long-term
sampling, with many data sets meeting EPA's criteria for quality assurance/
quality control. Hence, treatment performance data from the F039 background
document and other BOAT documents may be used as a basis for comparison when
evaluating the performance of other treatment systems.
The Agency wishes to point out that additional information on treatment
technologies may be obtained through EPA's Alternative Treatment Technology
Information Center (ATTIC), operated by the Office of Environmental Engineering
and Technology Demonstration. The ATTIC is an information retrieval network that
provides up-to-date technical information on innovative treatment methods for
hazardous wastes. The ATTIC provides information on innovative treatment
technologies, cost analysis, and migration and sampling data bases; provides
information on experts who can be contacted for assistance; and is a source for
locating and ordering documents.
Also, the EPA Office of Waste Programs Enforcement has tapes and manuals
for Inspectors of Incinerators (only available to EPA personnel). Information
include an introduction to incinerator design, air pollution control, and
regulatory guidance. Further information may be obtained by contacting the
Regional RCRA training coordinator.
The treatment technologies presented in this document have been organized
into three separate groups according to how they treat waste and to what types
of waste they are applicable: (1) treatment technologies that destroy or reduce
the migration of wastes; (2) removal technologies that separate or recover
constituents for recycle/reuse or treatment; and (3) technologies that treat
mixed waste, i.e., hazardous/radioactive waste. Although some of the
technologies can belong to more than one group, each technology has been placed
in the group where it is most commonly used.
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The remainder of this introduction describes the purpose and contents of
each of the various elements presented in each technology section:
applicability, underlying principles of operation, description of the technology,
waste characteristics affecting performance, and design and operating parameters.
Specifically explained are what information is provided in each technology
subsection, how the Agency uses this information as part of its BOAT program,
and, when applicable, how the Agency intends to modify the treatment technology
discussions as more treatment data and information become available.
1. Applicability
(a) Information Provided. The applicability section contains information
on the general application of the treatment technology to various wastes. EPA's
analysis of applicability is performed in consideration of the parameters and
constituents known to affect treatment selection. That is, in order to identify
whether a treatment technology was applicable to any particular waste stream, the
Agency considered all parameters and characteristics associated with a hazardous
waste that could affect the selection of a treatment technology. EPA uses the
acronym PATS (i.e., parameters affecting treatment selection) for these
parameters.
EPA's list of PATS, shown in Table 1, identifies all known constituents and
parameters that are needed to select a technology or technology train for a given
waste.
It is important to point out that in developing this list, EPA's goal was
to determine the demonstrated technology or treatment technology train that would
best treat all of the constituents in the waste (i.e., achieve the lowest
concentration in the residual). Accordingly, EPA will add components to the
treatment train as long as they are demonstrated and will achieve statistically
significant reduction in the concentrations of the constituents. (Note: the
statistical analysis that the Agency uses to determine whether a significant
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Table 1 List of Parameters Affecting Treatment Selection
BOAT list metals content
BOAT list organics content
Content of other BOAT constituents (sulfides and fluorides)
Biological oxygen demand (BOD)
Btu content
Presence of complexed metals
Cyanide content
Filterable solids content
Oil and grease content
Oxidation state
pH
Total organic carbon (TOC) content
Total organic halides (TOX) content
Viscosity
Water content
Selectivity value
Sublimation temperature
Ash fusion temperature
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reduction in constituents has occurred is referred to as Analysis of Variance
(ANOVA). Refer to the Methodology for Developing BOAT Treatment Standards
document for further information.)
(b) EPA's Use of This Information. EPA uses the PATS as the basis for
making decisions regarding whether a particular technology is "demonstrated" for
a particular waste. The Agency is performing treatment tests on specific wastes
to "demonstrate" the treatment of the waste using the technology, to develop
treatment standards, and to set the criteria by which other wastes can be
regulated using the demonstrated technology.
The PATS analysis will sometimes show that several technologies are
applicable for treating the waste. EPA will prioritize these technologies based
on its analysis of available data and information regarding the effectiveness of
the various technologies. For organics, EPA's preference will be technologies
that recover or destroy the compounds; for metals, the Agency's preference will
be technologies that result in recovery of the metals.
In some instances, analysis of a particular parameter affecting treatment
selection will not be possible or would be meaningless with regard to selecting
a treatment system. For example, the BOD of a contaminated soil is essentially
meaningless since the soil is not likely to be treated in a biological wastewater
treatment system.
(c) Use of New Data and Information. As data become available either
from industry or through EPA's BOAT data collection program, EPA will modify the
PATS analysis to further refine specific concentration ranges for which various
technologies can handle various parameters.
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2. Underlying Principles of Operation
(a) Information Provided. For each treatment technology EPA provides the
fundamental theory of operation of the technology. This section is not meant to
provide a detailed discussion on the specific complex physical, chemical, or
biological mechanisms by which treatment occurs. Instead, the discussion
describes the mechanisms involved so that one can understand what characteristics
can affect the performance of the technology. For example, the underlying
principle of chemical precipitation is that metals in wastewater are removed by
adding a treatment chemical to form a metal precipitate that is generally
insoluble and therefore settles out of solution. Formation of the precipitate
and the settling mechanism are the principles controlling technology performance.
(b) Use of Information Provided. The theory of operation is necessary
as the precursor to analyzing which waste characteristics can affect the
performance of a given treatment technology. That is, the principles of
treatment operation are used as the rationale and basis on which the Agency
determines what waste characteristics can affect performance. Accordingly, waste
characteristics affecting performance (WCAPs), which are discussed below, must
be understood from the standpoint of how each waste characteristic inhibits or
interferes with the fundamental operation of the treatment technology.
3. Description of the Treatment Process
This section provides a brief description of the principal components of
a treatment technology. It is intended to provide a basic understanding of the
equipment used and the treatment sequence. This presentation relates the
discussion on underlying principles of operation with the later discussion of the
design and operating parameters important to effective waste treatment.
Simplified block diagrams are provided in this section. The information
presented is general in nature to avoid attempting to show all combinations and
permutations that are possible in describing a particular technology.
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4. Waste Characteristics Affecting Performance fWCAPs)
(a) Information Provided. The WCAPs are based on the underlying
principles of the treatment technology. For example, the underlying principle
of chemical precipitation is formation of the insoluble metal precipitate and the
settling of the precipitate out of solution. WCAPs for this technology are those
parameters that affect the chemical reaction of the metal compound or the
solubility of the metal precipitate (i.e., formation) and/or affect the ability
of the precipitated compound to settle. The WCAPs include (a) the concentration
and type of metals, (b) the concentration of total dissolved solids, (c) the
concentration of complexing agents, and (d) the concentrations of oil and grease.
The first three parameters can affect precipitate formation, and the last
parameter can affect settling rate.
EPA's selection of WCAPs for the various treatment systems is based on a
literature review, as well as information obtained during plant sampling visits
that test the treatment technology. EPA is aware that in some cases analytical
methods are not available to measure the specific waste characteristics that
affect treatment. In such instances, EPA measures the parameter that most
closely approximates the actual parameter of concern. For example, a WCAP for
steam stripping is the relative volatility of a component. Because relative
volatility cannot be measured, EPA uses boiling point as the best measure of
assessing the WCAP of relative volatility.
The analysis of waste characteristics affecting performance is conducted
where EPA already has determined from a PATS analysis that an untested waste
could be treated effectively by the same BOAT technology as another waste for
which treatment performance data are available. The WCAP analysis is then used
to determine whether the treatment standards from the tested waste can be
transferred to the untested waste. Simply stated, this analysis compares, for
two separate wastes, the characteristics that affect the level of treatment that
can be achieved. Where this comparison shows that the untested waste can be
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treated as well or better, the Agency will transfer the treatment standards to
the untested waste.
For example, for wastes that can be treated using chemical precipitation,
if the Agency tests a waste containing lead at a given level, it will transfer
the treatment standards developed from the test to a second lead-containing waste
if the second waste meets the following WCAP-based conditions:
• The lead concentration is similar to that of the tested waste;
• The second waste has a lower total dissolved solids level;
• The concentration and type of other metals are similar to those of the
tested waste; and
• The untested waste has a concentration of oil and grease similar to or
less than that of the tested waste.
(b) 'Use of Information Provided. In applying the WCAP analyses, it is
important to remember that the technology itself and the associated design and
operating conditions are fixed (i.e., they are fixed at the values that existed
for the already tested waste). EPA does not assume that, where WCAPs for the
untested waste show that it would be significantly harder to treat, the addition
of another treatment component or a change in design and operating conditions
will result in the untested waste achieving the same performance as that of the
previously tested waste. EPA is aware that modifying the treatment system or
operating conditions could result in similar treatment. However, the Agency is
not certain, without demonstrating through testing, that treatment levels
associated with BOAT standards (normally in the low (<10) part per million or
part per billion range) can be achieved by simply changing treatment conditions.
Accordingly, EPA is very cautious in its decisions to transfer treatment
standards where the WCAP analysis shows that treatment modifications are needed
from the previously tested waste.
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(c) Use of New Data and Information. In developing the WCAPs for the
various treatment technologies, EPA found little and, in some cases, no
information on the relative importance of the various WCAPs for a given
technology (i.e., which waste characteristics have the most significant impact
on treatment performance). Where information was available with regard to the
specific WCAPs that are important when assessing the performance of a particular
technology, EPA was unable to find any supporting data that would quantify the
relative impact of each of the WCAPs on treatment performance.
As data become available either from industry or through EPA's BOAT data
collection program, EPA may modify the WCAP analysis to further refine the
parameters identified as affecting treatment technology performance. As
previously noted, before it adopts a new WCAP, the Agency must understand how the
parameters affect treatment relative to the underlying principles of operation.
5. Design and Operating Parameters
EPA uses design and operating information in several ways. First, this
information is used, in conjunction with the Agency's BOAT data collection
program, in determining whether a particular treatment system is well designed.
That is, the Agency assesses design data prior to determining whether to test a
particular piece of equipment or treatment train for potential use in the
development of BOAT treatment standards. EPA obtains operating data during the
course of an Agency-sponsored treatment test to ensure that the treatment system
is well operated. Secondly, EPA expects facilities to provide the appropriate
design and operating data when submitting treatment data to be considered in the
development of the BOAT treatment standards. Again, the Agency assesses these
data to determine whether the treatment system was well designed and well
operated.
For example, some of the critical design and operating parameters for steam
stripping include the number of equilibrium stages in the column, the temperature
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at which the unit is designed to operate, and how well the design temperature is
controlled. In evaluating performance data from a steam stripping operation, the
Agency would examine the design specifications (e.g., the basis for selecting the
number of stages and design temperature) for the treatment unit in order to
determine the extent to which the hazardous constituents could be expected to
volatilize. After the design specifications are established, the Agency would
collect data (e.g., hourly readings of the column temperature) throughout the
operation of the treatment process demonstrating that the unit was operating
according to design specifications. If the data collected vary considerably from
the design requirements, that variation could form the basis for determination
that the treatment unit was improperly operated. If the temperature data show,
for example, that for significant periods of time the temperature varied
considerably from the design requirements, the Agency would not use these data
to determine the levels of performance achievable by BOAT.
Finally, EPA requires facilities petitioning for a treatment variance to
submit the plant's design values and the operating data during the time of the
treatment period to ensure that the treatment system was well designed and well
operated.
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I. TREATMENT TECHNOLOGIES
A. DESTRUCTION TECHNOLOGIES
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1. BIOLOGICAL TREATMENT
1.1 Applicability
Biological treatment can typically be divided into two classifications:
aerobic biological treatment and anaerobic biological treatment. Aerobic
biological treatment takes place in the presence of oxygen, while anaerobic
biological treatment is an oxygen-devoid process.
Aerobic biological treatment is a treatment technology applicable to
wastewaters containing biodegradable organic constituents and some nonmetallic
inorganic constituents including sulfides and cyanides. Four of the most common
aerobic biological treatment processes are (a) activated sludge, (b) aerated
lagoon, (c) trickling filter, and (d) rotating biological contactor (RBC). The
activated sludge and aerated lagoon processes are suspended-growth processes in
which microorganisms are maintained in suspension with the liquid. The trickling
filter and the RBC are attached-growth processes in which microorganisms grow on
any inert medium such as rocks, slag, or specifically designed ceramic or plastic
materials. This section discusses these four processes as well as the powdered
activated carbon (PAC) adsorption process, which is a variation of the activated
sludge treatment.
Anaerobic digestion is best suited to wastes with a moderate to high pH,
nonhalogenated hydrocarbons, moderate to low organic loadings, and low to zero
biological oxygen demand. The waste should also be in a semisolid or sludge
form. Anaerobic biological treatment typically takes place in an anaerobic
digester.
1.2 Underlying Principles of Operation
The basic principle of operation for aerobic biological treatment processes
is that living, oxygen-requiring microorganisms decompose organic and nonmetallic
inorganics constituents into carbon dioxide, water, nitrates, sulfates, simpler
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low-molecular-weight organic byproducts, and cellular biomass. Wastes that can
be degraded by a given species or genus of organisms may be very limited. A
mixture of organisms may be required to achieve effective treatment, especially
for wastes containing mixtures of organic compounds. Nutrients such as nitrogen
and phosphorus are also required to aid in the biodegradation process.
The aerobic biodegradation process can be represented by the following
generic equation:
Cx", * °2 +
Microorganisms produce enzymes that catalyze the biodegradation reactions,
degrading the organic waste to obtain energy for cell metabolism and cell growth.
Aerobic biological treatment of wastewaters containing organic constituents
results in the net accumulation of a biomass of expired microorganisms consisting
mainly of cell protein. However, the cellular biomass or sludges may also
contain entrained constituents from the wastewater or partially degraded
constituents. These sludges must be periodically removed ("wasted") to maintain
proper operation of the aerobic biological treatment system.
The basic principle of operation of anaerobic biological treatment is that
microorganisms in the absence of oxygen transform organic constituents and
nitrogen-containing compounds into carbon dioxide and methane gas. Often other
nutrients such as nitrogen and phosphorus are necessary to aid digestion. The
anaerobic biological treatment process can be described by the following
equation:
r u microorganisms . rn , ru
Cx Hy + nutrients > C02 + CH4
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1.3 Description of Biological Treatment Processes
1.3.1 Activated Sludge
The activated sludge process is currently the most widely used biological
treatment process. This is partly the result of the fact that recirculation of
the biomass, which is an integral part of the process, allows microorganisms to
adapt to changes in wastewater composition with a relatively short acclimation
time and also allows a greater degree of control over the acclimated bacterial
population.
An activated sludge system consists of an equalization basin, a settling
tank, an aeration basin, a clarifier, and a sludge recycle line. Wastewater is
homogenized in an equalization basin to reduce variations in the feed, which may
cause process upsets of the microorganisms and diminish treatment efficiency.
Settleable solids are then removed in a settling tank.
Next, wastewater enters an aeration basin, where an aerobic bacterial
population is maintained in suspension and oxygen, as well as nutrients, is
provided. The contents of the basin are referred to as the mixed liquor. Oxygen
is supplied to the aeration basin by mechanical or diffused aeration, which also
aids in keeping the microbial population in suspension. The mixed liquor is
continuously discharged from the aeration basin into a clarifier, where the
biomass is separated from the treated wastewater. A portion of the biomass is
recycled to the aeration basin to maintain an optimum concentration of acclimated
microorganisms in the aeration basin. The remainder of the separated biomass is
discharged or "wasted." The biomass may be further dewatered on sludge drying
beds or by sludge filtration (which is further discussed in Section II.B.2) prior
to disposal. The clarified effluent is discharged. A schematic diagram of an
activated sludge treatment system is shown in Figure 1.
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OXYGEN
AND
NUTRIENTS
WASTEWATER
INFLUENT
EQUALIZATION
TANK
8ETTLINO
TANK
I
AERATION
BASIN
BASIN
EFFLUENT
SLUDGE RECYCLE
TREATED
EFFLUENT
TO DISPOSAL
WASTE SLUDGE
TO SLUDGE
FILTRATION
AND DISPOSAL
Figure 1 Activated Sludge System
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The recycled biomass is referred to as activated sludge. The term
"activated" is used because the biomass contains living and acclimated
microorganisms that metabolize and assimilate organic material at a higher rate
when returned .to the aeration basin. This occurs because of the low
food-to-microorganism ratio in the sludge from the clarifier.
An important variation on the activated sludge process is the Powdered
Activated Carbon Treatment (PACT) process. This process offers a combined
treatment and pretreatment system in which noncompatible and toxic constituents
are adsorbed onto activated carbon, while microorganism-compatible waste remains
in solution. Powdered activated carbon is added directly to the aeration basin
of the activated sludge treatment system. Overall removal efficiency is improved
because compounds that are not readily biodegradable or that are toxic to the
microorganisms are adsorbed onto the surface of the powdered activated carbon.
The carbon is removed from the wastewater in the clarifier along with the
biological sludge. Usually, the activated carbon is recovered, regenerated, and
recycled.
1.3.2 Aerated Lagoon
Like an activated sludge system, an'aerated lagoon is a suspended-growth
process. The aerated lagoon system consists of a large pond or tank that is
equipped with mechanical aerators to maintain an aerobic environment and to
prevent settling of the suspended biomass. Initially, the population of
microorganisms in an aerated lagoon is much lower than that in an activated
sludge system because there is no sludge recycle. Therefore, a significantly
longer residence time is required to achieve the same effluent quality. However,
this longer residence time may be an advantage when complex organic chemicals are
to be degraded. Also, the microorganisms in aerated lagoons are more resistant
to process upsets caused by feed variations than those in activated sludge
systems because of the larger tank volumes and longer residence times used. The
effluent from the aerated lagoon may flow to a settling tank for removal of
suspended solids. Alternatively, the mechanical aerators in the system may be
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shut off for a period of time to facilitate settling prior to discharge of the
effluent. The settled solids are generally dewatered prior to disposal.
1.3.3 Trickling Filters
A trickling filter is an attached-growth biological treatment process. The
system consists of an equalization basin, a settling tank, a filter medium, an
influent wastewater distribution system, an under drain system, a clarifier, and
a recirculation line. The filter medium consists of a bed of an inert material
to which the microorganisms attach themselves and through which the wastewater
is percolated. Rocks or synthetic material such as plastic rings and saddles are
typically used as filter media. Following equalization and settling of
settleable solids in the wastewater, it is distributed over the top of the filter
medium by a rotating distribution arm or a fixed distributor system. The
wastewater forms a thin layer as it flows downward through the filter and over
the microorganism layer on the surface of the medium. As the distribution arm
rotates, the microorganism layer is alternately exposed to a flow of wastewater
and a flow of air. In the fixed distributor system, the wastewater flow is
cycled on and off at a specified dosing rate to ensure that an adequate supply
of oxygen is available to the microorganisms. Oxygen from air reaches the
microorganisms through the void spaces in the medium. Figure 2 is a diagram of
a trickling filter system.
A trickling filter system is typically used as a roughing filter to reduce
the organic loading on a downstream activated sludge process. Trickling filters
can be used for the treatment of wastewaters that could potentially produce
"bulking" sludge (i.e., a sludge with poor settling characteristics and poor
compactability in an activated sludge process) because the microbial solids that
slough off the trickling filter medium are relatively dense and can be readily
removed in a clarifier.
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WASTEWATER
INFLUENT
1
WASTE DISTRIBUTOR
11 11
PLASTIC OR ROCK
TREATED
EFFLUENT
TO CLARIFIER
Figure 2 Trickling Filter System
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1.3.4 Rotating Biological Contactor
A rotating biological contactor (RBC) consists of a series of closely
spaced, parallel disks that are rotated at an average rate of 2 to 5 revolutions
per minute while submerged to 40 percent of their diameters in a contact tank
containing wastewater. The disks are constructed of polystyrene, polyvinyl
chloride, or similar materials. Each disk is covered with a biological slime
that degrades dissolved organic constituents present in the wastewater. As the
disk is rotated out of the tank, it carries a film of the wastewater into the
air, where oxygen is available for aerobic biological decomposition. As excess
biomass is produced, it sloughs off the disk and is separated from the treated
effluent in a clarifier. The sloughing off process is similar to that which
occurs in a trickling filter. There is no recycle of sludges or recirculation
of treated effluent in an RBC process. Several RBCs are often operated in
series, with the effluent from the last RBC being discharged. Biological solids
are usually dewatered prior to disposal.
1.3.5 Anaerobic Biological Treatment
In anaerobic biological treatment, the influent sludge is settled and
equalized, then pumped to the anaerobic digester (Figure 3) along with an
alkaline adjustment additive. There may or may not be mechanical agitation of
the digester. After an adequate residence time to allow for proper digestion,
the digester contents are allowed to settle. The supernatant is pumped to an
aerobic treatment area (typically to an activated sludge unit), while the sludge
is taken to disposal areas or subjected to additional treatment, such as drying
or incineration.
1.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether biological treatment will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
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INFLUENT
SLUDGE
SLUDGE
EQUALIZATION
TANK
ALKALINITY
ADJUSTMENT
ADDITIVE
FLAME
GAS COMPRESSION
FLOATING I
COVER •
MIXERS
MEAT
SUPERNATANT TO
ACTIVATED SLUDGE
SLUDGE
EFFLUENT
PUMP
EXCHANGER DIGESTER CONCRETE)
RECYCLE SLUDGE
SLUDGE
HOLDING
BASIN
PUMP
Figure 3 Anaerobic Digestion
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waste characteristics: (a) the ratio of the biological oxygen demand to the
total organic carbon content, (b) the concentration of surfactants, and (c) the
concentration of toxic constituents and waste characteristics.
1.4.1 Ratio of Biological Oxygen Demand to Total Organic Carbon Content
Because organic constituents in the waste effectively serve as a food
supply for the microorganisms, it is necessary that a significant percentage be
biodegradable. If they are not, it will be difficult for the microorganisms to
successfully acclimate to the waste and achieve effective treatment. The
percentage of biodegradable organics can be estimated by the ratio of the
biological oxygen demand (BOD) to the total organic carbon (TOC) content. Since
the biological oxygen demand is a measure of the amount of oxygen required for
complete microbial oxidation of biodegradable organics, the BOO analysis is
mostly relevant to aerobic biological treatment. (In anaerobic biological
treatment, BOD is one of the main restrictive characteristics in that BOD must
be relatively low or zero.) If the ratio of BOD to TOC in an untested waste is
significantly lower than that in the tested waste, the system may not achieve the
same performance and other, more applicable technologies may need to be
considered for treatment of the untested waste.
1.4.2 Concentration of Surfactants
Surfactants can affect biological treatment performance by forming a film
on organic constituents, thereby establishing a barrier to effective
biodegradation. If the concentration of surfactants in an untested waste is
significantly higher than that in the tested waste, the system may not achieve
the same performance and other, more applicable technologies may need to be
considered for treatment of the untested waste.
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1.4.3 Concentration of Toxic Constituents and Waste Characteristics
A number of constituents and waste characteristics have been identified as
potentially toxic to microorganisms. Specific toxic concentrations have not been
determined for most of these constituents and waste characteristics. The
constituents and waste characteristics found to be potentially toxic to
microorganisms include metals and oil and grease, ammonia, and phenols. High
concentrations of dissolved solids are treated more effectively by anaerobic
treatment than by aerobic treatment. If the concentration of toxic constituents
and waste characteristics in an untested waste is significantly higher than that
in the tested waste, the system may not achieve the same performance and other,
more applicable technologies may need to be considered for treatment of the
untested waste.
1.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a biological
treatment system, EPA examines the following parameters: (a) the amount of
nutrients, (b) the concentration of dissolved oxygen, (c) the food-to-
microorganism ratio, (d) the pH, (e) the biological treatment temperature,
(f) the mean cell residence time, (g) the hydraulic loading rate, (h) the
settling time, and (i) the degree of mixing.
For many hazardous organic constituents, analytical methods are not
available or the constituent cannot be analyzed in the waste matrix. Therefore,
it would normally be impossible to measure the effectiveness of the biological
treatment system. In these cases EPA tries to identify measurable parameters or
constituents that would act as surrogates in order to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the biological treatment system
indicates removal of organic constituents. Hence, TOC analysis is likely to be
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an adequate surrogate analysis where the specific organic constituent cannot be
measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden (i.e., the TOC
analysis may not be sensitive enough to detect changes at the milligrams per
liter (mg/1) level in matrices where total organic concentrations are hundreds
or thousands of mg/1). In these cases other surrogate parameters should be
sought. For example, if a specific analyzable constituent is expected to be
treated as well as the unanalyzable constituent, the analyzable constituent
concentration should be monitored as a surrogate.
1.5.1 Amount of Nutrients
Nutrient addition is important in controlling the growth of microorganisms
because an insufficient amount of nutrients results in poor microbial growth with
poor biodegradation of organic constituents. The principal inorganic nutrients
used are nitrogen and phosphorus. In addition, trace amounts of potassium,
calcium, sulfur, magnesium, iron, and manganese are also used for optimum
microbial growth. The percent distribution of nitrogen and phosphorus added to
microorganisms varies with the age of the organism and the particular
environmental conditions. The total amount of nutrients required depends on the
net mass of organisms produced.
EPA monitors the amount of nutrients added and their method of addition to
the wastewater to ensure that a sufficient supply is provided to achieve an
effective growth of microorganisms.
1.5.2 Concentration of Dissolved Oxygen
A sufficient concentration of dissolved oxygen (DO) is necessary to
metabolize and degrade dissolved organic constituents in aerobic treatment. The
DO concentration is controlled by adjusting the aeration rate. The aeration rate
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must be adequate to provide a sufficient DO concentration to satisfy the BOD
requirements of the waste, as well as to provide adequate mixing to keep the
microbial population in suspension (for activated sludge and aerated lagoon
processes). The reverse is true for anaerobic treatment, in that DO must be
absent for anaerobic treatment to occur. EPA monitors the DO concentrations
continuously, if possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
1.5.3 Food-to-Microorganisra Ratio
The food-to-microorganism (F/M) ratio, which applies only to activated
sludge systems, is a measure of the amount of biomass available to metabolize the
influent organic loading to the aeration unit. This ratio can be determined by
dividing the influent BOD concentration by the concentration of active biomass,
also referred to as the mixed liquor volatile suspended solids (MLVSS). The F/M
ratio is controlled by adjusting the wastewater feed rate or the sludge recycle
rate. If the F/M ratio is too high, too few microorganisms will be available to
degrade the organics. EPA periodically analyzes the influent BOD and the
aeration unit's MLVSS concentrations to ensure that the system is operating at
the appropriate design condition.
1.5.4 pH
Generally, neutral or slightly alkaline pH favors microorganism growth.
The optimum range for most microorganisms used in biological treatment systems
is between 6 and 8. Treatment effectiveness is generally insensitive to changes
within this range. However, pH values outside the range can lower treatment
performance. EPA monitors the pH continuously, if possible, to ensure that the
system is operating at the appropriate design condition and to diagnose
operational problems.
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1.5.5 Biological Treatment Temperature
Microbial growth can occur under a wide range of temperatures, although the
majority of the microbial species used in aerobic biological treatment processes
are active between 20 and 35*C (68 to 95*F). For anaerobic systems, the
temperature is typically between 30 and 70*C (86 to 158*F). The rate of
biochemical reactions in cells increases with temperature up to a maximum above
which the rate of activity declines and microorganisms either die off or become
less active. EPA monitors the biological treatment temperature continuously, if
possible, to ensure that the system is operating at the appropriate design
condition and to diagnose operational problems.
1.5.6 Mean Cell Residence Time
In activated sludge, aerated lagoon, and anaerobic digestion systems, the
mean cell residence time (MCRT) or sludge age is the length of time organisms are
retained in the unit before being drawn off as waste sludge. By controlling the
MCRT, the growth phase of the microbial population can be controlled. The MCRT
must be long enough to allow the organisms in the unit to reproduce. The MCRT
is determined by dividing the total active microbial mass in the unit (MLVSS) by
the total quantity of microbial mass withdrawn daily (wasted). EPA monitors the
MCRT to ensure that a sufficient number of microorganisms are present in the
unit.
1.5.7 Hydraulic Loading Rate
The hydraulic loading rate determines the length of time the organic
constituents are in contact with the microorganisms and, hence, the extent of
biodegradation that occurs. In trickling filters, the hydraulic loading rate
also determines the shear velocities on the microbial layer. Excessively high
hydraulic loading rates may wash away the microbial layer faster than it can grow
back. However, the hydraulic loading rate must be high enough to keep the
microbes moist and to remove dead or dying microbes that have lost their ability
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to cling to the filter media. For all aerobic biological treatment processes,
the hydraulic loading rate is controlled by adjusting the wastewater feed rate.
In addition, for RBCs, the hydraulic loading rate can be controlled by changing
the disk speed or adjusting the submersion depth. EPA monitors the wastewater
feed rates to ensure that the hydraulic loading provides sufficient time to
achieve an effective biodegradation of organic constituents in the wastewater.
1.5.8 Settling Time
Adequate settling time must be provided to separate the biological solids
from the mixed liquor. Activated sludge systems cannot function properly if the
solids cannot be effectively separated and a portion returned to the aeration
basin. EPA monitors the settling time to ensure effective solids removal.
1.5.9 Degree of Mixing
Mixing provides greater uniformity of the wastewater feed in the
equalization basin to reduce variations that may cause process upsets of the
microorganisms and diminish treatment efficiency. For activated sludge and
aerated lagoon systems, sufficient aeration in the aeration unit provides mixing
to ensure adequate contact between the microorganisms and the organic
constituents in the wastewater. The quantifiable degree of mixing is a complex
assessment that includes, among other factors, the amount of energy supplied, the
length of time the material is mixed, and the related turbulence effects of the
specific size and shape of the mixing unit. The degree of mixing is beyond the
scope of simple measurement. EPA, however, evaluates the degree of mixing
qualitatively by considering whether mixing is provided and whether the type of
mixing device is one that could be expected to achieve uniform mixing of the
wastewater.
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1.6 References
Barley, J., and Ollis, D. 1977. Biochemical engineering fundamentals. New
York: McGraw-Hill Book Co.
Burchett, M., and Tshobanoglous, G. 1974. Facilities for controlling the
activated sludge process by mean cell residence time. Journal Water
Pollution Control Federation. Vol. 46, no. 5 (May 1974), p. 973.
Clark, J.W., Viessman, W., and Hammer, M. 1977. Water supply and pollution
control. New York: Harper & Row, Publishers.
Eckenfelder Jr., W.W., Patoczka, J., and Watkins, A. 1985. Wastewater
treatment. Chemical engineering. September 2, 1985.
Johnson, S.J. 1978. Biological treatment. Unit operation for treatment of
hazardous industrial wastes. Park Ridge, N.J.: Noyes Data Corporation.
Kobayashi, H., and Rittmann, B. 1982. Microbial removal of hazardous organic
compounds. Environmental Science and Technology. Vol. 16, no. 3,
pp. 170-183.
Metcalf & Eddy, Inc. 1979. Wastewater engineering: treatment, disposal, reuse.
New York: McGraw-Hill Book Co.
Nemerow, N.L. 1978. Industrial water pollution origins, characteristics, and
treatment. Reading, Mass.: Addison-Wesley Publishing Company.
Perry, R., and Green, D. 1984. Chemical engineers' handbook. 6th ed. New York:
McGraw-Hill Book Co. ""
Rittmann, B. 1987. Aerobic biological treatment. Environmental Science and
Technology. February 1987. p. 128.
USEPA. 1985. U.S. Environmental Protection Agency. Final report. Assessment
of treatment technologies for hazardous waste and their restrictive waste
characteristics. Vol 1. Washington, O.C.: U.S. Environmental Protection
Agency.
USEPA. 1986. U.S. Environmental Protection Agency. Best demonstrated available
technology background document for F001-F005 spent solvents. Vol. 1,
EPA/530-SW-86-056. pp. 4-43. Washington, D.C.: U.S. Environmental
Protection Agency.
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2. CHEHICAL OXIDATION
2.1 Applicability
Chemical oxidation is a treatment technology used to treat wastes
containing organics. It is also used to treat sulfide wastes by converting the
sulfide to sulfate. The destruction of cyanides in wastes is usually
accomplished by chemical oxidation. Chemical oxidation can also be used to
change the oxidation state of metallic compounds to valences that are less
soluble, such as converting arsenic in wastes to the relatively insoluble
pentavalent state.
Chemical oxidation is applicable to dissolved cyanides in aqueous
solutions, such as wastewaters from metal plating and finishing operations, or
to inorganic sludges from these operations that contain cyanide compounds. For
cyanides, chemical oxidation is most applicable to solutions containing less than
500 mg/1 of cyanides when the cyanides are in a form that can be easily
disassociated in water to yield free cyanide ions. If cyanides are present in
water as a tightly bound complex ion (e.g., ferrocyanide), only limited treatment
may occur. If the waste contains greater than 500 mg/1 of cyanide, but no more
than about 100,000 mg/1, electrolytic oxidation may be more appropriate. See
Section I.A.4.
Chemical oxidation may also be used for destruction of the organic
component of organometallic compounds in wastes, thus freeing the metal component
for treatment by chemical precipitation or stabilization. Organic compounds such
as EDTA, NTA, citric acid, glutaric acid, lactic acid, and tartrates are often
used as chelating agents to prevent metal ions from precipitating out in
electroless plating solutions. When these spent plating solutions require
treatment for metals removal by chemical precipitation, the organic chelating
agents must first be destroyed. Chemical oxidants, potassium permanganate in
particular, are effective in releasing metals from complexes with these organic
compounds.
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2.2 Underlying Principles of Operation
The basic principle of operation for chemical oxidation is that inorganic
cyanides, some dissolved organic compounds, and sulfides can be chemically
oxidized to yield carbon dioxide, water, salts, simple organic acids, and, in the
case of sulfides, sulfates. Metallic ions such as arsenites can be oxidized to
higher, less soluble valences such as arsenates. The principal chemical oxidants
used are hypochlorite, chlorine gas, chlorine dioxide, hydrogen peroxide, ozone,
and potassium permanganate. The reaction chemistry for each is discussed below.
2.2.1 Oxidation with Hypochlorite or Chlorine (Alkaline Chlorination)
This type of oxidation is carried out using sodium hypochlorite (NaOCl),
calcium hypochlorite (Ca(OCl)2), chlorine gas (C12), or sometimes chlorine
dioxide gas (C102). The reactions are normally conducted under slightly or
moderately alkaline conditions. Alkaline chlorination of cyanide is a two-step
process usually operated at a pH of 10 to 11.5 for the first step and 8.5 for the
second step. The toxic gas cyanogen chloride (CNC1) is formed as a reaction
intermediate in the first step of this process and may be liberated if the pH is
less than 10 and incomplete reaction occurs. Example reactions for the oxidation
of cyanide, phenol, and sulfide using sodium hypochlorite are shown below:
Cyanide: CN" + NaOCl -> OCN" + NaCl (Step 1)
20CN" + 3NaOCl -» CO/" + C02 + N2 + 3NaCl (Step 2)
Phenol: C6HSOH + 14NaOCl - 6C02 + 3H20 + 14NaCl
Sulfide: S" + 4NaOCl -» SO/ + 4NaCl
Arsenic: H3As03 + NaOCl -* H3As04 + NaCl
Chlorine dioxide also oxidizes the same pollutants under identical
conditions. Chlorine dioxide first hydrolyzes to form a mixture of chlorous
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(HC102) and chloric (HC103) acids. These acids act as the oxidants, as shown in
the equations below, for phenol:
2C102 + H20 - HC102 + HC103
C6H5OH + 7HC102 - 6C02 + 3H20 + 7HC1
3C6H5OH + 14HC103 - 8C02 + 9H20 + 14HC1
2.2.2 Peroxide Oxidation
Peroxide oxidizes the same constituents that alkaline chlorination
oxidizes under similar conditions. The relevant reactions are the following:
Cyanide: 2CN' * 5H202 - 2C02 + N2 + 4H20 + 20H'
Phenol: C6H5OH + 14H202 -» 6C02 •»• 17H20
Sulfide: S" + 4H202 -» SO/ + 4H20
2.2.3 Oxidation with Ozone (Ozonation)
Ozone is an effective oxidizing agent for the treatment of organic
compounds and for the oxidation of cyanide to cyanate. Cyanogen gas (C2N2) is
a reaction intermediate in this reaction. Further oxidation of cyanate to carbon
dioxide and nitrogen compounds (N2 or NH3) occurs slowly with ozone. The
oxidation of cyanide to cyanate proceeds by the following reaction:
CN" + 03 -» CNO~ + 02
The rates of ozonation reactions can be accelerated by supplying
ultraviolet (UV) radiation during treatment. Some literature sources indicate
that even the cyanide complexes most difficult to treat, the iron-cyanide
complexes, can be oxidized completely.
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2.2.4 Oxidation with Potassium Permanganate
Potassium permanganate can also be used to oxidize the same constituents
as the other chemical oxidants. The reactions of potassium permanganate with
phenol and sulfide at acidic pHs and with cyanide at pH 12 to 14 are as follows:
Phenol: 3C6H5OH + 28KMn04 + 28H* - 18C02 + 28Mn02 + 23H20 + 28K*
Sulfide: 5S" + 8KMn04 + 24H* -> 5S04" + 8Mn*2 + 12H20 + 8K*
Cyanide: CN" + 2KMn04 + Ca(OH)2 -> CNO" + K2Mn04 + CaMn04 + H20
In cyanide oxidation using potassium permanganate, cyanide is oxidized only to
cyanate. Further oxidation of cyanate can be accomplished by acid hydrolysis or
by the use of another oxidizing agent.
2.2.5 S02/Air Oxidation
Cyanide can be oxidized to cyanate in an aqueous solution by bubbling air
containing from 1 to 10 percent S02 through the waste. The S02 is also oxidized
to sulfate in this reaction. This treatment process occurs by the following
reaction:
CN' + S02 + 02 + H20 -» CNO" + H2S04
This oxidation reaction requires the use of a soluble copper salt catalyst.
Copper sulfate (CuS04) is most often used. S02/air oxidation is used frequently
in the treatment of wastewaters from gold production, which contain both cyanide
and thiocyanate, because S02/air oxidizes cyanide more strongly than thiocyanate
while alkaline chlorination and other common oxidizing agents oxidize thiocyanate
more strongly than cyanide. As with potassium permanganate, further oxidation
of cyanate can be accomplished by acid hydrolysis or by the use of another
oxidizing agent.
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2.3 Description of Chemical Oxidation Processes
2.3.1 Alkaline Chlorination
Alkaline chlorination can be accomplished by either batch or continuous
processes. For batch treatment, the wastewater Is transferred to a reaction
tank, where the pH 1s adjusted and the oxidizing agent Is added. In some cases,
the tank may be heated to increase the reaction rate. For oxidation of most
compounds, a slightly to moderately alkaline pH is used. It is important that
the tank be well mixed for effective treatment to occur. After treatment, the
wastewater is either directly discharged or transferred to another process for
further treatment.
In the continuous process, automatic instrumentation is used to control pH,
reagent addition, and temperature. An oxidation-reduction potential (ORP) sensor
is usually used to measure the extent of reaction.
In both types of processes, agitation is typically provided to maintain
thorough mixing. Typical residence times for these and other oxidation processes
range from 1 to 2 hours.
•
2.3.2 Peroxide Oxidation
The peroxide oxidation process is run under similar conditions, and with
similar equipment, to those used in the alkaline chlorination process. Hydrogen
peroxide is added as a liquid solution.
2.3.3 Ozonation
Ozonation can be conducted in a batch or continuous process. The ozone for
treatment is produced onsite because of the hazards of transporting and storing
ozone as well as its short shelf life. The ozone gas is supplied to the reaction
vessels by injection into the wastewater. The batch process uses a single
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reaction tank. As with alkaline chlorination, the amount of ozone added and the
reaction time used are determined by the type and concentration of the oxidizable
contaminants, and vigorous mixing should be provided for complete oxidation.
In continuous operation, two separate tanks may be used for reaction. The
first tank receives an excess dosage of ozone. Any excess ozone remaining at the
outlet of the second tank 1s recycled to the first tank, thus ensuring that an
excess of ozone is maintained and also that no ozone is released to the
atmosphere. As with alkaline chlorination, an ORP control system is usually
necessary to ensure that sufficient ozone is being added.
2.3.4 Permanganate Oxidation
Permanganate oxidation is conducted in tanks in a manner similar to that
used for alkaline chlorination, as discussed previously. Potassium permanganate
is normally dissolved in an auxiliary tank and added as a solution. As with the
other oxidizing agents, ORP (for continuous processes) and excess oxidizing agent
(for batch processes) are monitored to measure the extent of reaction.
2.3.5 S02/Air Oxidation
S02/air oxidation of cyanide depends on efficient mixing of air with the
waste to ensure an adequate supply of oxygen. Because of this factor, the
equipment requirements for this process are similar to those for ozonation. S02
is sometimes supplied with the air by using flue gas containing S02 as the air
source. Otherwise, sulfur in the +4 oxidation state can be fed as gaseous
sulfur dioxide (S02), liquid sulfurous acid (H2S03), sodium sulfite (Na2S03)
solution, or sodium bisulfite (NaHS03) solution. Sodium bisulfite solution, made
by dissolving sodium metabisulfite (Na2S205) in water, is the most frequently
used source of S02. This process is usually run continously, with the addition
of oxidizing agent and acid/alkali being controlled through continuous monitoring
of ORP and pH, respectively.
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2.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether chemical oxidation will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the concentration of other oxidizable contaminants
and (b) the concentration of metal salts.
2.4.1 Concentration of Other Oxidizable Compounds
The presence of other oxidizable compounds in addition to the constituents
of concern will increase the demand for oxidizing agents and, hence, potentially
reduce the effectiveness of the treatment process. As a surrogate for the amount
of oxidizable organics present, EPA analyzes for total organic carbon (TOC) in
the waste. Inorganic reducing compounds such as sulfide may also create a demand
for additional oxidizing agent; EPA also attempts to identify and analyze for
these constituents. If TOC and/or inorganic reducing compound concentrations in
the untested waste are significantly higher than those in the tested waste, the
system may not achieve the same performance. Additional oxidizing agent may be
required to effectively oxidize the waste and achieve the same treatment
performance, or other, more applicable treatment technologies may need to be
considered for treatment of the untested waste.
2.4.2 Concentration of Metal Salts
Metal salts, especially lead and silver salts, will react with the
oxidizing agent(s) to form metal peroxides, chlorides, hypochlorites, and/or
chlorates. These reactions can cause an excessive consumption of oxidizing
agents and potentially interfere with the effectiveness of treatment.
An additional problem with metals in cyanide solutions is that
metal-cyanide complexes are sometimes formed. These complexes are negatively
charged metal-cyanide ions that are extremely soluble. Cyanide in the complexed
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form may not be oxidizable, depending on the strength of the metal-cyanide bond
in the complex and the type of oxidizing agent used. Iron complexes (for
example, the ferrocyanide ion, Fe(CN)6"4) are the most stable of the complexed
cyanides.
If the concentrations of metal salts and/or metal-cyanide complexes in the
untested waste are significantly higher than those in the tested waste, the
system may not achieve the same performance. Additional oxidizing agent and/or
a different oxidizing agent may be required to effectively oxidize the waste and
achieve the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
2.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a chemical
oxidation system, EPA examines the following parameters: (a) the residence time,
(b) the amount and type of oxidizing agent, (c) the degree of mixing, (d) the pH,
(e) the oxidation temperature, and (f) the amount and type of catalyst.
For many hazardous organic constituents, analytical methods are not
available or the constituent cannot be analyzed in the waste matrix. Therefore,
it would normally be impossible to measure the effectiveness of the chemical
oxidation treatment system. In these cases EPA tries to identify measurable
parameters or constituents that would act as surrogates to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the chemical oxidation treatment
system will indicate removal of organic constituents. Hence, TOC analysis is
likely to be an adequate surrogate analysis where the specific organic
constituent cannot be measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden (i.e., the TOC
f:\document\15254031.01.004 24
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analysis may not be sensitive enough to detect changes at the milligrams/liter
(mg/1) level in matrices where total organic concentrations are hundreds or
thousands of mg/1). In these cases other surrogate parameters should be sought.
For example, if a specific analyzable constituent is expected to be treated as
well as the unanalyzable constituent, the analyzable constituent concentration
should be monitored as a surrogate.
2.5.1 Residence Time
The residence time impacts the extent of volatilization of waste
contaminants. For a batch system, the residence time is controlled by adjusting
the treatment time in the reaction tank. For a continuous system, the waste feed
rate is controlled to make sure that the system is operated at the appropriate
design residence time. EPA monitors the residence time to ensure that sufficient
time is provided to effectively oxidize the waste.
2.5.2 Amount and Type of Oxidizing Agent
Several factors influence the choice of oxidizing agents and the amount to
be added. The amount of oxidizing agent required to treat a given amount of
oxidizable constituent(s) will vary with the agent chosen. Enough oxidant must
be added to ensure complete oxidation; the specific amount depends on the type
of oxidizable compounds in the waste and the chemistry of the oxidation
reactions. Theoretically, the amount of oxidizing agent to be added can be
computed from oxidation reaction stoichiometry; in practice, an excess of oxidant
should be used. Testing for excess oxidizing agent will determine whether the
reaction has reached completion. In continuous processes, oxidizing agent is
added by automated feed methods. The amount of oxidizing agent needed is usually
measured and controlled automatically by an oxidation-reduction potential (ORP)
sensor. EPA examines the amount of oxidant added to the chemical oxidation
system to ensure that it is sufficient to effectively oxidize the waste and, for
continuous processes, examines how the facility ensures that the particular
addition rate is maintained. EPA also tests for excess oxidizing agent for batch
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processes and continuously monitors the ORP for continuous processes to ensure
that excess oxidizing agent, if possible, is supplied.
2.5.3 Degree of Mixing
Process tanks must be equipped with mixers to ensure maximum contact
between the oxidizing agent and the waste solution. Proper mixing also limits
the production of any solid precipitates from side reactions that may resist
oxidation. Mixing also provides an even distribution of tank contents and a
homogeneous pH throughout the waste, improving oxidation of wastewater
constituents. The quantifiable degree of mixing is a complex assessment that
includes, among other factors, the amount of energy supplied, the length of time
the material is mixed, and the related turbulence effects of the specific size
and shape of the tank. This is beyond the scope of simple measurement. EPA,
however, evaluates the degree of mixing qualitatively by considering whether
mixing is provided and whether the type of mixing device is one that could be
expected to achieve uniform mixing of the waste solution.
2.5.4 pH
Operation at the optimal pH maximizes the chemical oxidation reactions and
may, depending on the oxidizing agent being used, limit the formation of
undesirable reaction byproducts or the escape of cyanide from solution as HCN,
CNC1, or C2N2 gas. The pH is controlled by the addition of caustic, lime, or
acid to the solution. In most cases, a slightly or moderately alkaline pH is
used, depending on the type of oxidizing agent being used and the compound being
treated (see Section 2.2, Underlying Principles of Operation). In alkaline
chlorination treatment of organics, a slightly acidic pH may be selected as an
optimum. In permanganate oxidation, a pH of 2 to 4 is often selected. EPA
monitors the pH continuously, if possible, to ensure that the system is operating
at the appropriate design condition and to diagnose operational problems.
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2.5.5 Oxidation Temperature
Temperature affects the rate of reaction and the solubility of the
oxidizing agent in the waste. As the temperature is increased, the solubility
of the oxidizing agent, in most instances, is increased and the required
residence time, in most cases, is reduced. EPA monitors the oxidation
temperature continuously, if possible, to ensure that the system is operating at
the appropriate design condition and to diagnose operational problems.
2.5.6 Amount and Type of Catalyst
Adding a catalyst that promotes oxygen transfer and thus enhances oxidation
has the effect of lowering the necessary reactor temperature and/or improving the
level of destruction of oxidizable compounds. For waste constituents that are
more difficult to oxidize, catalyst addition may be necessary to effectively
destroy the constituent(s) of concern. Catalysts typically used for this purpose
include copper bromide and copper nitrate. If a catalyst is required, EPA
examines the amount and type added, as well as the method of addition of the
catalyst to the waste, to ensure that effective oxidation is achieved.
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2.6 References
Gurnham, C.F. 1985. Principles of industrial waste treatment. New York: John
Wiley and Sons.
Gurol, M.D., and Holden, I.E. 1988. The effect of copper and iron complexation
on removal of cyanide by ozone. Ind. Enq. Chem. Res. 27(7):1157-1162.
McGraw-Hill. 1982. Encyclopedia of science and technology. Vol. 3, p. 825.
New York: McGraw-Hill Book Co.
Metcalf & Eddy, Inc. 1986. Briefing: Technologies applicable to hazardous
waste. Prepared for U.S. Environmental Protection Agency, Hazardous Waste
Engineering Research Laboratory. Cincinnati, Ohio: U.S. Environmental
Protection Agency.
Nutt, S.G., and Zaidi, S.A. 1983. Treatment of cyanide-containing wastewaters
by the copper-catalyzed S02/air oxidation process. In Proceedings of the
38th Industrial Waste Conference. Purdue University, West Lafayette,
Indiana, May 10-12, 1983. Stoneham, Mass.: Butterworth Publishers.
Patterson, J.W. 1985. Industrial wastewater treatment technology. 2nd ed.
Stoneham, Mass.: Butterworth Publishers.
Schroeter, J., and Painter, C. 1987. Potassium permanganate oxidation of
electroless plating wastewater. Carus Chemical Company, 1001 Boyce
Memorial Drive, P.O. Box 1500, Ottawa, Illinois 61350.
Weast, R.C., ed. 1978. Handbook of chemistry and physics. 58th ed. Cleveland,
Ohio: CRC Press.
f:\document\15254031.01.004 28
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3. CHEMICAL REDUCTION
3.1 Applicability
Chemical reduction is a treatment technology used to reduce hexavalent
chromium and selenate ions to the less soluble trivalent chromium ion and
elemental selenium, respectively. It is also used to treat oxidizing wastes
containing reducible organics and inorganic oxidizers such as calcium
hypochlorite, hydrogen peroxide (and other peroxides), and nitric acid. Because
this technology frequently requires that the pH be in the acidic range, it would
not normally be applicable to wastes that contain significant amounts of cyanide
or sulfide. In such wastes, lowering of the pH can result in the release of
toxic gases such as hydrogen cyanide or hydrogen sulfide.
Hexavalent chromium is usually present in wastes from the plating industry,
metal surface preparation processes, the chromium pigments industry, and leather
tanning processes. Selenates are frequently found in some mining and ore
processing wastes. Organic and inorganic oxidizers are found in propel 1 ant
explosives and in the chemical manufacturing industries.
3.2 Underlying Principles of Operation
The basic principle of chemical reduction is to reduce the valence of the
oxidizer in solution. "Reducing agents" used to effect the reduction include the
sulfur compounds sodium sulfite (Na2S03), sodium bisulfite (NaHS03), sodium
metabisulfite (Na2S205), sulfur dioxide (S02), and sodium hydrosulfide (NaHS).
The ferrous form of iron (Fe*2) is a popular reducing agent in many cases.
Elemental magnesium (Mg), zinc (Zn), and copper (Cu) are also effective reducing
agents. Frequently, hydrazine (N2H2) is used as a reducing agent also. Typical
reduction reactions are as follows:
f:\document\15254031.01.005 29
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H202 +
Hydrogen Peroxide
2H*
2Fe>2 -
Ferrous iron
2H20
*3
2Fe
Ferric Iron
4Zn
N0
10H* -*
Metallic Zinc Nitrate Ion
3Na2S03
Sodium
Sulfite
Chromic Acid
3H20
Zinc Ion Ammonium Ion
3H2S04 - (Cr*3)2(S04)3 + 3Na2S04
Sulfuric Chromium Sodium
Acid Sulfate Sulfate
4H20
H2Se03
2S0
Selenic Acid Sulfite Ion
-» 2S04 +
Sulfate Ion
H20
Se
Elemental Selenium
These reactions are usually accomplished at pH values from 2 to 3
3.3
Description of Chemical Reduction Process
The chemical reduction treatment process can be operated in a batch or
continuous mode. A batch system consists of a reaction tank, a mixer to
homogenize the contents of the tank, a supply of reducing agent, and a source of
acid and base for pH control. A continuous chemical reduction treatment system,
as shown in Figure 4, usually includes a holding tank upstream of the reaction
tank for flow and concentration equalization. It also typically includes
instrumentation to automatically control the amount of reducing agent added and
the pH of the reaction tank. The amount of reducing agent is controlled by the
use of a sensor called an oxidation-reduction potential (ORP) cell. The ORP
sensor electronically measures, in millivolts, the level to which the
oxidation/reduction (redox) reaction has proceeded at Figure 4 any given time.
It must be noted, however, that the ORP reading is very pH dependent.
Consequently, if the pH is not maintained at a steady value, the ORP will vary
somewhat, regardless of the level of chemical reduction.
f:\document\15254031.01.005
30
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-------
When chemical reduction is used for treating hexavalent chromium, the
trivalent chromium that is formed is either reused or further treated by
stabilization and land disposed. Likewise, for selenium reduction, the
precipitated elemental selenium may be recovered or stabilized and disposed of.
3.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether chemical reduction will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste,
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the concentration of other reducible compounds and
(b) the concentration of oil and grease.
3.4.1 Concentration of Other Reducible Compounds
The presence of other reducible compounds (also called oxidizers) in
addition to the BOAT list constituents of concern will increase the demand of
reducing agents, thereby potentially reducing the effectiveness of the treatment
process. As a surrogate for the amount of organic oxidizers present in the
waste, EPA might analyze for total organic carbon (TOC) in the waste. Inorganic
oxidizing compounds such as ionized metals (e.g., silver, selenium, copper,
mercury) may also create a demand for additional reducing agent. EPA might
attempt to identify and analyze for these metal constituents. If TOC or
inorganic oxidizer concentration in the untested waste is significantly higher
than that in previously tested wastes, the system may not achieve the same
performance as that achieved previously. Additional reducing agent may be
required to effectively reduce the untested waste and achieve the same treatment
performance, or other, more applicable treatment technologies may need to be
considered for treatment of the untested waste.
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3.4.2 Concentration of Oil and Grease
EPA believes that oil and grease compounds could cause monitoring problems
because of fouling of instrumentation (e.g., electrodes for pH and ORP sensors).
If the concentration of oil and grease in an untested waste is significantly
higher than that in a tested waste, the untested system may not achieve the same
performance. Therefore, other, more applicable treatment or pretreatment
technologies may need to be considered for treatment of the untested waste.
3.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a chemical
reduction system, EPA examines the following parameters: (a) the residence time,
(b) the amount and type of reducing agent, (c) the degree of mixing, (d) the pH,
and (e) the reduction temperature.
For many hazardous organic constituents, analytical methods are not
available or the constituent cannot be analyzed in the waste matrix. Therefore,
it would normally be impossible to measure the effectiveness of the chemical
reduction treatment system. In these cases EPA tries to identify measurable
parameters or constituents that would act as surrogates to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the chemical reduction treatment
system indicates removal of organic constituents. Hence, TOC analysis is likely
to be an adequate surrogate analysis where the specific organic constituent
cannot be measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden (i.e., the TOC
analysis may not be sensitive enough to detect changes at the milligrams per
liter (mg/1) level in matrices where total organic concentrations are hundreds
or thousands of mg/1). In these cases other surrogate parameters should be
f:\document\15254031.01.005 33
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sought. For example, If a specific analyzable constituent is expected to be
treated as well as the unanalyzable constituent, the analyzable constituent
concentration should be monitored as a surrogate.
3.5.1 Residence Time
The residence time affects the extent of reaction of waste contaminants
with reducing agents. For a batch system, the residence time is controlled by
adjustment of the treatment time in the reaction tank. For a continuous system,
the waste feed rate is controlled to make sure that the system is operated at the
appropriate design residence time. EPA monitors the residence time to ensure
that sufficient time is provided to effectively reduce the waste.
3.5.2 Amount and Type of Reducing Agent
Several factors influence the choice of reducing agents and the amount to
be added. The amount of reducing agent required to treat a given amount of
reducible constituent will vary with the agent chosen. Enough reducing agent
must be added to ensure complete reduction; the specific amount will depend on
the type of reducible compounds in the waste and the chemistry of the reduction
reactions. Theoretically, the amount of reducing agent to be added can be
computed from reduction reaction stoichiometry. In practice, however, an excess
of reducing agent should be used. Testing for excess reducing agent, if
possible, will determine whether the reaction has reached completion.
In continuous processes, the addition of reducing agent is usually
accomplished by automated feed methods. The amount of reducing agent needed is
usually metered and controlled automatically by an oxidation-reduction potential
(ORP) sensor. EPA examines the amount of reduction agent added to the chemical
reduction system to ensure that it is sufficient to effectively reduce the waste
and, for continuous processes, examines how the facility ensures that the
particular addition rate is maintained. EPA may also test for excess reducing
agent in the system effluent. For continuous processes, EPA monitors the ORP to
f:\document\15254031.01.005 34
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ensure that enough reducing agent is supplied. EPA may also monitor pH to
determine whether it is controlled to the appropriate optimum range. For
continuous systems, pH is monitored to ensure that it is kept steady to avoid
changes in ORP caused by pH variations.
3.5.3 Degree of Mixing
Process tanks must be equipped with mixers to ensure maximum contact
between the reducing agent and the waste oxidizing solution. Proper mixing also
homogenizes any solid precipitates that may be present, or that may form from
side reactions, so that they can also be reduced if necessary. Mixing provides
an even distribution of tank contents and a homogeneous pH throughout the waste,
improving reduction of wastewater constituents. The quantifiable degree of
mixing is a complex assessment that includes, among other factors, the amount of
energy supplied, the length of time the material is mixed, and the related
turbulence effects of the specific size and shape of the tank. This is beyond
the scope of simple measurement. EPA, however, evaluates the degree of mixing
qualitatively, using engineering judgment, by considering whether mixing is
provided and whether the type of mixing device is one that could be expected to
achieve uniform mixing of the waste solution.
3.5.4 pH
For batch and continuous systems, the pH affects the reduction reaction.
The reaction speed is usually significantly reduced at higher pH values
(typically above 4.0). It is worth noting that some reduction reactions may
proceed better under alkaline conditions, in which case pH must be properly
controlled to the appropriate alkaline range. For a batch system, the pH can be
monitored intermittently during treatment. For a continuous system, the pH must
be continuously monitored because it affects the ORP reading. EPA monitors the
pH to ensure that the system is operating at the appropriate design condition and
to diagnose operational problems.
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3.5.5 Reduction Temperature
Temperature affects the rate of reaction and the solubility of the
reactants in the waste. As the temperature is increased, the solubility of the
reactants, in most instances, is increased and the required residence time, in
most cases, is reduced. EPA may monitor the reduction temperature to ensure that
the system is operating at the appropriate design condition and to diagnose
operational problems.
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3.6 References
McGraw-Hill. 1982. Encyclopedia of science and technology. Vol. 9, pp.
714-718. New York: McGraw-Hill Book Co.
Nebergall et al. General chemistry. 5th ed. Lexington, Mass.: D.C. Heath and
Company.
Patterson, J.W. 1985. Industrial wastewater treatment technology. 2nd ed.
Stoneham, Mass.: Butterworth Publishers.
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4. ELECTROLYTIC OXIDATION OF CYANIDE
4.1 Applicability
Electrolytic oxidation is a treatment technology with demonstrated
applicability to the treatment of wastes containing high concentrations of
cyanide in solution. Because of excessive retention time requirements, the
process is often applied as preliminary treatment for highly concentrated cyanide
wastes, prior to more conventional chemical cyanide oxidation.
This treatment technology is used in industry for the destruction of
cyanide in (a) concentrated spent plating solutions and stripping solutions,
(b) spent heat treating baths, (c) alkaline descalers, and (d) metal passivating
(rust-inhibiting) solutions. Electrolytic oxidation has been demonstrated
successfully for treatment of wastes containing concentrations of cyanide up to
100,000 mg/1. However, for concentrations of cyanide lower than 500 mg/1,
chemical oxidation treatment may be more efficient (see Section I.A.2).
4.2 Underlying Principles of Operation
The basic principle of operation for electrolytic oxidation of cyanide is
that concentrated cyanide waste subject to an electrolytic reaction with
dissolved oxygen in an aqueous solution is broken down to the gaseous products
carbon dioxide (C02), nitrogen (N2), and ammonia (NH3). The process is conducted
at elevated temperatures for periods ranging from several hours to over a week,
depending on the initial cyanide concentration and the desired final cyanide
concentration. The theoretical destruction process that takes place at the
anodes is described by the following reaction:
Electricity
2CN' + 202 > 2C02 + N2 + 2e~
cyanide oxygen carbon nitrogen electrons
ion dioxide
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The effectiveness of electrolytic oxidation is dependent on the
conductivity of the waste, which is a function of several waste characteristics
including the concentration of cyanide and other ions in solution. As the
process continues, the waste becomes less capable of conducting electricity as
the cyanide concentration is reduced, causing the electrolytic reaction to be
much less efficient at longer retention times.
4.3 Description of Electrolytic Oxidation Process
Typically, electrolytic destruction of cyanide takes place in a closed
cell. This cell consists of two electrodes suspended in an aqueous solution,
with direct current (DC) electricity supplied to drive the reaction to
completion. The temperature of the bath containing the cyanide waste is
maintained at or above 52*C (125'F). Sodium chloride may be added to the
solution as an electrolyte (conductor) to increase the conductivity of the waste
being treated. Since the reaction may take days or weeks, water is usually added
to the tank periodically to make up for losses due to evaporation from the heated
tank. This is necessary to ensure that the electrodes remain fully submerged so
that a full flow of current is maintained in the solution during treatment.
Following treatment, the treated waste is usually further treated in a
conventional chemical oxidation system to destroy residual cyanides.
4.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether electrolytic oxidation will achieve the same level
of performance on an untested waste that it achieved a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the concentration of other oxidizable materials and
(b) the concentration of reducible metals.
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4.4.1 Concentration of Other Oxidizable Materials
The presence of oxidizable organic* (such as oil and grease and
surfactants) and the presence of inorganic ionic species in a reduced state (such
as trivalent chromium or sulfide) may increase the treatment time required to
achieve destruction of cyanide because these materials may be oxidized
preferentially to the cyanide in solution. If concentrations of other oxidizable
materials are significantly higher in the untested waste than in the tested
waste, the system may not achieve the same performance. Longer reaction time may
be required to oxidize cyanide and achieve the same treatment performance, or
other, more applicable treatment technologies may need to be considered for
treatment of the untested waste.
4.4.2 Concentration of Reducible Metals
The electrolytic process may cause some of the more easily reduced metals
in the waste, such as copper, to plate out onto the cathode as the pure metal.
The plating of metals onto the cathode may result in changes in current density
and, hence, may change the rate of cyanide oxidation. If the concentration of
reducible metals in the untested waste is significantly higher than that in the
tested waste, the system may not achieve the same performance and other, more
applicable treatment technologies may need to be considered for treatment of the
untested waste.
4.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an
electrolytic oxidation system, EPA examines the following parameters: (a) the
oxidation temperature, (b) the residence time, (c) the pH, (d) the electrical
conductivity, (e) the electrode spacing and surface area, and (f) the degree of
mixing.
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4.5.1 Oxidation Temperature
For the electrolytic process, elevated temperatures are used. Normal
temperatures range from 52 to 93'C (125 to 200*F). The temperature can be raised
by increasing the flow of steam to the coils or jacket supplying heat to the
reactor contents. EPA monitors the oxidation temperature to ensure that the
system is operating at the appropriate design condition and to diagnose
operational problems.
4.5.2 Residence Time
Electrolytic oxidation is usually a batch process. The time allowed to
complete the reaction is an important factor in electrolysis and is dependent on
the initial concentration of the waste and the desired final cyanide
concentration. The rate of cyanide destruction decreases as the cyanide
concentration decreases (i.e., the rate of cyanide destruction asymptotically
approaches zero). Typical residence times range from periods of several hours
to more than a week. EPA observes the residence time to ensure that sufficient
time is provided to effectively destroy the cyanides in the wastes.
4.5.3 pH
Typical solutions for electrolytic oxidation have a pH ranging from 11.5
to 12.0. The pH must be maintained in the alkaline range to prevent liberation
of toxic hydrogen cyanide. Typically, pH is controlled by the addition of
caustic or lime. EPA monitors the pH to ensure that the treatment system is
operating at the appropriate design condition and to diagnose operational
problems.
4.5.4 Electrical Conductivity
The solution must have an electrical conductivity high enough to allow the
reaction to proceed at an acceptable rate. If the conductivity is not high
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enough, it can be improved by adding an electrolyte such as sodium chloride. The
conductivity of the waste during the reaction is normally determined by
monitoring both the current and voltage of the cell. EPA monitors the electrical
conductivity to ensure that the treatment system is operating at the appropriate
design condition.
4.5.5 Electrode Spacing and Surface Area
The spacing and surface area of the electrodes directly impact the current
flowing through the waste. The reaction rate is increased by both closer
electrode spacing and more electrode surface area because each increases the
current density in the cell. EPA observes the electrode spacing and surface area
to ensure that sufficient current density is provided to effectively destroy the
cyanides in the waste.
4.5.6 Degree of Mixing
Electrolytic destruction of cyanides requires good mixing in the reaction
vessel. Mixing helps ensure an adequate supply of oxygen (from the air) for the
electrochemical reaction (see Section 4.2, Underlying Principles of Operation),
enhances mass transfer to promote the oxidation reaction, and keeps suspended
solids in suspension. Mixing may be provided by the bubbling of air from the
bottom of the reactor, or an external source of mixing may be provided. The
quantifiable degree of mixing is a complex assessment that includes, among other
factors, the amount of energy supplied, the length of time the material is mixed,
and the related turbulence effects of the size and shape of the reaction vessel
used. The degree of mixing is beyond the scope of simple measurement. EPA,
however, evaluates the degree of mixing qualitatively by considering whether
mixing is provided and whether the type of mixing device is one that could be
expected to achieve uniform mixing of the waste.
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4.6 References
Cushnie, G.C., Jr. 1985. Electroplating wastewater pollution control tech-
nology, p. 205. Park Ridge, N.J.: Noyes Publications.
Easton, J.K. 1967. Electrolytic decomposition of concentrated cyanide plating
wastes. Journal Water Pollution Control Federation 39(10):1621-1626.
Patterson, J.W. 1985. Industrial wastewater treatment technology. 2nd ed., pp.
123-125. Stoneham, Mass. Butterworth Publishers.
Pearson, G.J., and Karrs, S.R. 1984. Electrolytic cyanide destruction. In
Proceedings for plating and surface finishing, pp. 2 and 3.
Roy, C.H. 1981. Electrolytic wastewater treatment. In a series of American
Electroplaters Society, Inc., illustrated lectures, pp. 8-10.
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5. INCINERATION
5.1 Applicability
Incineration is a treatment technology applicable to the treatment of
wastes containing a wide range of organic concentrations and low concentrations
of water, metals, and other inorganics. The four most common incineration
systems are liquid injection, rotary kiln, fluidized bed, and fixed hearth.
Liquid injection incineration is applicable to wastes having sufficiently
low viscosity values (less than 750 saybolt universal seconds (Sus) such that the
waste can be atomized in the combustion chamber. However, viscosity is
temperature dependent so that while liquid injection may not be applicable to a
waste at ambient conditions, it may become applicable when the waste is heated.
In addition, the number of waste particles and the concentration of suspended
solids need to be sufficiently low to avoid clogging of the burner nozzle (or
atomizer openings).
Rotary kiln, fluidized bed, and fixed hearth incineration are applicable
to wastes having a wide range of viscosity, particle size, and suspended solids
concentration.
Incineration of highly explosive constituents may require treatment in
units that are specially designed and fitted with certain explosion-proof
equipment. Incinerators for corrosive waste should be equipped, if necessary,
with pollution control devices to remove corrosive gases that may be generated
from the burning of corrosive waste. The incineration of hazardous waste must
be performed in accordance with the incinerator design and emissions regulations
in 40 CFR 264, Subpart 0.
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5.2 Underlying Principles of Operation
The basic principle of operation for incineration is the thermal
decomposition of organic constituents via cracking and oxidation reactions at
high temperatures (usually between 760 and 1550*C (1400 and 3000'F) to convert
them into carbon dioxide and water vapor along with nitrite oxides, nitrates, and
ammonia (for nitrogen-containing wastes); sulfur oxides and sulfate (for sulfur-
containing wastes); or halogen acids (for halogenated wastes).
In liquid injection incineration the organic constituents in the waste are
volatilized and the chemical bonds are destabilized. Once the chemical bonds are
broken, these constituents react with oxygen to form carbon dioxide, water vapor,
and other aforementioned compounds.
In rotary kiln and fixed hearth incineration, two chambers are involved in
the incineration process. In the primary chamber, the organic constituents in
the waste are volatilized. During this volatilization process, some of the
organic constituents oxidize to form carbon dioxide and water vapor. In the
secondary chamber, the chemical bonds are destabilized, resulting in the organic
constituents reacting with oxygen to form carbon dioxide, water vapor, and other
aforementioned compounds.
In fluidized bed incineration, a single chamber contains the fluidizing
sand and a freeboard section above the sand. The fluidized bed aids in the
volatilization and combustion of the organic waste constituents. The sand in the
bed provides a sufficient heat capacity to volatilize organic constituents. The
forced air used to fluidize the bed provides sufficient oxygen and turbulence to
enhance the reactions of organics with oxygen to form carbon dioxide, water
vapor, and other aforementioned compounds. Additional time for conversion of the
organic constituents is provided by the freeboard above the fluidized bed.
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5.3 Description of Incineration Processes
The physical form of the waste determines the appropriate feed method into
the incineration system. Liquids are pumped into the combustion chamber through
nozzles or via specially designed atomizing burners. Wastes containing suspended
particles may need to be screened to avoid clogging of small nozzle or atomizer
openings. Sludges and slurries are fed using positive displacement pumps and
water-cooled injection ports. Bulk solid wastes may require shredding for
control of particle size. Wastes may be fed to the combustion chamber via rams,
gravity feed, air lock feeders, vibratory or screw feeders, or belt feeders.
Containerized waste is gravity-fed or ram-fed.
Although sustained combustion is possible with waste heat content as low
as 2230 kcal/kg (4010 Btu/lb), wastes are typically blended to a net heat content
of 4450 kcal/kg (8000 Btu/lb) or higher or auxiliary fuel is used in the
combustion chambers to raise the heat content to a level sufficient to sustain
the combustion process.
Following incineration of wastes, fly ash particulates, acid gases (halogen
acids), and other gaseous pollutants (nitric oxides (NOJ and sulfur oxides
(SOJ) are further treated in an air pollution control system. Particulate
emissions from most waste combustion systems generally have particle diameters
less than 1 micron and require high-efficiency collection systems to minimize air
emissions. The most common air pollution control system used includes a quench
(for gas cooling and conditioning), followed by a high-energy venturi scrubber*
(for particulate and acid gas removal), a packed bed or plate tower absorber (for
acid gas removal), and a demister (for vapor mist plume elimination).
Packed bed or plate tower scrubbers are commonly used without venturi
scrubbers in the air pollution control systems at liquid injection incinerator
*A venturi is a short tube or duct with a tapering constriction that causes an
increase in the velocity of fluid flow and a corresponding decrease in pressure.
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facilities, where absorption of gaseous pollutants is more important than
particulate control. (Wastes burned in liquid injection incinerators typically
have a low ash content and, hence, generate a low level of fly ash particulates.)
Some liquid injection incinerator facilities do not employ any air
pollution control system because the wastes burned contain low levels of both ash
and halogen content, thereby generating low (i.e., untreatable) levels of fly ash
particulates and acid gases.
Many air pollution control system designs in recent years have begun to
incorporate waste heat boilers as a substitute for gas quenching as a means of
energy recovery. Wet electrostatic precipitators (ESP), ionizing wet scrubbers
(IWS), and fabric filters are also being incorporated into newer systems because
these devices have high removal efficiencies for small particles and a lower
pressure drop than that of venturi scrubbers.
The inorganic constituents of wastes (noncombustible ash) are not destroyed
by incineration. These materials, depending on their composition, exit the
incinerator as either bottom ash from the combustion chamber or fly ash
particulates suspended in the combustion gas stream. Bottom ash is typically
either air-cooled or quenched with water following discharge from the combustion
chamber. This waste is then stabilized (if levels of Teachable metal
constituents of concern are found) and/or land disposed.
Fly ash particulates, as well as acid gases and other gaseous pollutants,
are entrained in the scrubber waters of the air pollution control system and
collected in the sumps of recirculation tanks. Here the acids are neutralized
with caustic and much of the water is returned to the air pollution control
system. Eventually, a portion or all of these scrubber waters are discharged for
treatment and disposal when the total dissolved solids level becomes excessively
high. Scrubber waters are discharged either to a settling tank or lagoon or to
a chemical precipitation system (if treatable levels of soluble metal con-
stituents of concern are found) to remove these solids prior to their land
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disposal. Depending on the nature of the remaining dissolved constituents and
their concentrations, treated scrubber waters may be returned to the air
pollution control system, further treated in filtration processes, or discharged.
Below are descriptions of the incineration processes for the four most
common incinerator systems.
5.3.1 Liquid Injection
In a liquid injection incineration system, a burner or nozzle atomizes the
waste and injects it into the combustion chamber, where it is incinerated in the
presence of air or oxygen. A forced-draft system supplies the combustion chamber
with air to provide oxygen for combustion and turbulence for mixing. The
combustion chamber is usually a cylinder lined with refractory (i.e., heat-
resistant) brick and can be fired horizontally, vertically upward, or vertically
downward. Figure 5 illustrates a liquid injection incineration system.
5.3.2 Rotary K1ln
A rotary kiln is a slowly rotating, refractory-lined cylinder that is
mounted at a slight incline from the horizontal. Solid wastes enter at the high
end of the kiln, and liquid or gaseous wastes generally enter through atomizing
nozzles in the afterburner section of the kiln. A forced-draft system supplies
the kiln with air to provide oxygen for combustion and turbulence for mixing.
Rotation of the kiln enhances the exposure of the solids to the heat, thereby
aiding their volatilization as well as providing additional mixing of the solids
with air combustion. In addition, the rotation causes the ash to move to the
lower end of the kiln, from which it is removed. Rotary kiln systems usually
have a secondary combustion chamber or afterburner following the kiln for further
combustion of the volatilized waste constituents. Figure 6 is a diagram of a
rotary kiln incineration system.
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FLY ASH PARTICIPATES
AND COMBUSTION GASES
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
AUXILIARY FUEL ^BURNER
AIR-
LIQUID OR GASEOUS
WASTE INJECTION
-HBURNER
t
PRIMARY
COMBUSTION
CHAMBER
AFTERBURNER
(SECONDARY
COMBUSTION
CHAMBER)
I
BOTTOM ASH TO STABILIZATION
AND/OR LAND DISPOSAL
Figure 5 Liquid Injection Incineration System
-------
FLY ASH PARTICULARS
AND COMBUSTION GASES TO
AIR POLLUTION CONTROL
UOUIO OR GASEOUS
WASTE INJECTION
AUXILIARY
FUE
SOLID
WASTE
INFLUENT
BOTTOM ASH TO
STABILIZATION
AND/OR LAND DISPOSAL
Figure 6 Rotary Kiln Incineration System
50
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5.3.3 Fluidized Bed
A fluidized bed incinerator consists of a column containing inert particles
such as sand, which is referred to as the bed. Air, driven by a blower, enters
the bottom of the bed to fluidize the sand. Air passage through the bed provides
oxygen for combustion and promotes rapid and uniform mixing of the injected waste
material within the fluidized bed. The fluidized bed has an extremely high heat
capacity (approximately three times that of flue (combustion) gases at the same
temperature), thereby providing a large heat reservoir. The injected waste
reaches incineration temperature quickly and transfers the heat of combustion
back to the bed. The freeboard above the fluidized bed provides additional time
for combustion of organic constituents. Because of the excellent mixing
properties associated with fluidized bed incinerators, they can operate at lower
temperatures more effectively than other incinerators. Figure 7 is a diagram of
a fluidized bed incineration system.
5.3.4 Fixed Hearth Incineration
Fixed hearth (also called controlled air or starved air) incineration is
a two-stage combustion process. Waste is ram-fed into the first stage, or
primary chamber, and burned at less than stoichiometric conditions (not enough
oxygen for complete combustion). The resultant smoke and pyrolysis products,
consisting primarily of volatile hydrocarbons and carbon monoxide, along with the
typical products of combustion, pass to the secondary chamber. Here additional
air is injected to complete the combustion process. This two-stage process
generally yields low fly ash particulate and carbon monoxide (CO) emissions. The
primary chamber combustion reactions and resultant combustion gas velocities are
maintained at low levels by the starved air conditions so that particulate
entrainment and carryover in the combustion gases are minimized. Figure 8 is a
diagram of a fixed hearth incineration system.
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BOTTOM ASH TO STABILIZATION
AND/OR LAND DISPOSAL
FLY ASH
PARTICIPATES
AND COMBUSTION
GASES TO AIR
POLLUTION
CONTROL SYSTEM
MAKE-UP
SAND
AIR
Figure 7 Fluidized Bed Incineration System
52
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AIR
FLY ASH PARTICIPATES
AND COMBUSTION GASES
TO AIR POLLUTION
CONTROL SYSTEM
AIR
in
WASTE
INJECTION
BURNER
i
PRIMARY
COMBUSTION
CHAMBER
I
SECONDARY
COMBUSTION
CHAMBER
BURNERM-
AUXILIARY
FUEL
BOTTOM ASH TO STABILIZATION
AND/OR LAND DISPOSAL
Figure 8 Fixed Hearth Incineration System
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5.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether incineration will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the thermal conductivity of the waste, (b) the
component boiling points, (c) the component bond dissociation energies, (d) the
heating value of the waste, (e) the concentration of explosive constituents, and
(f) the concentration of noncombustible constituents.
5.4.1 Thermal Conductivity of the Waste
A major factor determining whether a particular constituent will volatilize
is the transfer of heat through the waste. For rotary kiln, fluidized bed, and
fixed hearth incineration, heat is transferred through the waste by radiation,
convection, and conduction.
EPA examined all three methods of heat transfer. For a given incinerator,
heat transferred through various wastes by radiation and convection is more a
function of the design and type of incinerator than of the waste being treated;
therefore, EPA believes that conduction would be the primary cause of heat
transfer differences between wastes. Heat flow by conduction is proportional to
the temperature gradient across the material. The proportionality constant,
referred to as the thermal conductivity, is a property of the material to be
incinerated.
Thermal conductivity measurements, as part of a treatability comparison for
two different wastes to be treated by a single incinerator, are most meaningful
when applied to wastes that are homogeneous (i.e., uniform throughout). As
wastes exhibit greater degrees of nonhomogeneity, thermal conductivity becomes
less accurate in predicting treatability because the measurement essentially
reflects heat flow through regions having the greatest conductivity (i.e., the
path of least resistance) and not heat flow through all parts of the waste.
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Nevertheless, EPA believes that thermal conductivity may provide the best measure
of performance transfer. If the thermal conductivity of an untested waste is
significantly lower than that of the tested waste, the system may not achieve the
same performance. Higher temperatures may be required to improve heat transfer
through the waste and achieve the same treatment performance, or other, more
applicable treatment technologies may need to be considered for treatment of the
untested wastes.
5.4.2 Component Boiling Points
Following transfer of heat to a constituent within a waste, the
constituent's removal depends on its volatility. EPA recognizes, however, that
volatilities are difficult to measure or calculate directly for the types of
wastes generally treated by incineration because the wastes usually consist of
a mixture of components. However, because the volatilities of components are
usually inversely proportional to their boiling points (i.e., the higher the
boiling point, the lower the volatility), EPA uses the boiling points of waste
components as a surrogate waste characteristic for volatility. If the boiling
points of waste components in the untested waste are significantly higher than
those in the tested waste, the system may not achieve the same performance.
Higher temperatures may be required to volatilize less volatile components and
achieve the same treatment performance or other, more applicable treatment
technologies may need to be considered for treatment of the untested wastes.
5.4.3 Component Bond Dissociation Energies
The activation energy is the amount of heat energy needed to destabilize
molecular bonds so that the exothermic combustion reactions can proceed.
Typically, the activation energy required for incineration of solids is greater
than that required for liquids, and the activation energy required for liquids
is higher than that required for gases. However, the activation energies for
components are difficult to measure or calculate directly and usually must be
determined empirically. The bond dissociation energy is the amount of energy
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needed to break each of the individual bonds in a molecule. Theoretically, the
bond dissociation energy and the activation energy are equivalent. However,
other energy effects including interactions between different molecular bonds may
have a significant influence on the activation energy. Nevertheless, EPA
believes that bond dissociation energy is the best indicator for activation
energy and therefore uses it as a surrogate waste characteristic. If the bond
dissociation energies of waste components in an untested waste are significantly
higher than those in the tested waste, the system may not achieve the same
performance. Higher temperatures may be required to destabilize molecular bonds
for waste components with high bond dissociation energies and achieve the same
treatment performance, or other, more applicable treatment technologies may need
to be considered for treatment of the untested waste.
5.4.4 Heating Value of the Waste
The amount of heat released from the exothermic combustion reactions of a
waste is referred to as its heating value. To maintain combustion, the heating
value of a waste must be sufficient to heat incoming waste up to the appropriate
design incineration temperature and provide the necessary activation energy for
additional combustion reactions to occur. Higher waste heating values are
required for higher incineration temperatures and higher amounts of excess oxygen
to sustain combustion without auxiliary fuel consumption. Low heating values for
an organic waste are usually caused by high concentrations of water or
halogenated compounds. If the heating value of an untested waste is
significantly lower than that of the tested waste, the system may not achieve the
same performance. Auxiliary fuel may be required to provide the necessary heat
to maintain combustion and achieve the same treatment performance, or other, more
applicable treatment technologies may need to be considered for treatment of the
untested waste.
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5.4.5 Concentration of Explosive Constituents
Explosive constituents may interfere with the smooth operation of an
incinerator, causing process upsets resulting in ineffective treatment of the
waste. If the concentration of explosive constituents in an untested waste is
significantly higher than that in the tested waste, the system may not achieve
the same performance and other, more applicable treatment technologies may need
to be considered for treatment of the untested waste.
5.4.6 Concentration of Noncombustible Constituents
Noncombustible constituents Include water, metals, and other inorganics.
High concentrations of noncombustible constituents result in low waste heating
values, requiring auxiliary fuel; large quantities of bottom ash, requiring
stabilization treatment (if treatable levels of Teachable metal constituents of
concern are present) and/or disposal; and large quantities of fly ash
particulates and volatile metals in the combustion gases, requiring removal in
an air pollution control system and treatment in a settling tank or lagoon or by
means of a chemical precipitation system (if treatable levels of soluble metal
constituents of concern are present) followed by disposal. Volatile metals such
as arsenic may also fuse to the refractory walls in the combustion chamber,
inhibiting effective operation of the incinerator. If the concentration of
noncombustible constituents in an untested waste is significantly higher than
that in the tested waste, the system may not achieve the same performance and
other, more applicable treatment technologies may need to be considered for
treatment of the untested waste.
5.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an
incineration system, EPA examines the following parameters: (a) the incineration
temperature, (b) the concentration of excess oxygen in the combustion gas,
(c) the concentration of carbon monoxide in the combustion gas, (d) the waste
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feed rate, and (e) the degree of waste/air mixing. In addition, incineration of
hazardous waste must be performed in accordance with the incineration design and
emissions regulations in 40 CFR 264, Subpart 0. For many hazardous organic
constituents, analytical methods are not available or the constituent cannot be
analyzed in the waste matrix. Therefore, it would normally be impossible to
measure the effectiveness of the incineration treatment system. In these cases
EPA tries to identify measurable parameters or constituents that would act as
surrogates to verify treatment.
For organic constituents, each constituent contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the incineration treatment system
indicates removal of organic constituents. Hence, TOC analysis is likely to be
an adequate surrogate analysis where the specific organic constituent cannot be
measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden (i.e., the TOC
analysis may not be sensitive enough to detect changes at the milligrams per
liter (mg/1) level in matrices where total organic concentrations are hundreds
or thousands of mg/1). In these cases other surrogate parameters should be
sought. For example, "if a specific analyzable constituent is expected to be
treated as well as the unanalyzable constituent, the analyzable constituent
concentration should be monitored as a surrogate.
5.5.1 Incineration Temperature
Temperature provides an indirect measure of the energy available (i.e.,
Btu/hr) to both volatilize organic waste constituents and overcome their
activation energy. As the design temperature in the combustion chamber
increases, more constituents with lower volatilities and higher activation
energies will be destroyed. EPA monitors the incineration temperature
continuously to ensure that the system is operating at the appropriate design
condition and to diagnose operational problems.
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5.5.2 Concentration of Excess Oxygen in the Combustion Gas
A sufficient supply of oxygen must be supplied to the combustion chamber
to effectively incinerate the organic waste constituents. The stoichiometric or
minimum theoretical oxygen requirement to completely combust the waste is based
on the amounts of combustible constituents in the waste. If perfect mixing could
be achieved and the waste burned instantaneously, only the stoichiometric amount
of oxygen would be needed for complete combustion. However, neither of these
phenomena occur in commercial incinerators. The amount of excess oxygen in a
given incinerator depends on the degree of waste/air mixing achieved in the
combustion chamber and the desired degree of combustion gas cooling. Because
excess air (oxygen) supplied to the combustion chamber acts as a diluent in the
combustion process, it reduces the overall temperature in the incinerator (i.e.,
maximum theoretical incineration temperatures are achieved at zero percent excess
air). This temperature reduction might be desirable to limit refractory
degradation when high-heating value wastes are being burned. However, when low-
heating-value wastes are being burned, excess air should be minimized to keep the
system temperature as high as possible. Even with high-heating-value waste, it
is desirable for equipment design considerations to limit the amount of excess
air to some extent so that combustion chamber volume and downstream air pollution
control system sizes can be limited.
An inadequate supply of oxygen, on the other hand, will lead to ineffective
incineration with higher concentrations of carbon monoxide (CO) and organic
products of incomplete combustion (PICs), resulting in a heavier load on the air
pollution control system. Typically, 20 to 60 percent excess oxygen is required
to provide adequate waste/oxygen contact and, subsequently, effective
incineration of the waste. EPA monitors the concentration of excess oxygen in
the combustion gas continuously to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
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5.5.3 Concentration of Carbon Monoxide 1n the Combustion Gas
The concentration of carbon monoxide in the combustion gas provides an
indication of the extent to which organic waste constituents are converted to
carbon dioxide and water vapor. Higher carbon monoxide levels are an indication
of ineffective incineration with the presence of greater amounts of unreacted or
partially reacted organic waste constituents in the combustion gas, resulting in
a heavier load on the air pollution control system. Higher carbon monoxide
levels can result from an insufficient incineration temperature; an inadequate
supply of excess oxygen; a waste feed rate that is too high, resulting in an
insufficient residence time for the waste; and improper mixing in the combustion
chamber. EPA monitors the concentration of carbon monoxide in the combustion gas
continuously to ensure that the system is operating at the appropriate design
conditions and to diagnose operational problems.
5.5.4 Waste Feed Rate
The waste feed rate determines the residence time of the waste in the
combustion chamber. Sufficient residence time must be provided to allow for
volatilization of the organic waste constituents, mixing with oxygen in the
combustion chamber, and completion of the combustion reactions. The total
residence time required for these processes to occur depends on the incineration
temperature, the amount of excess oxygen provided in the combustion chamber, the
degree of waste/air mixing in the combustion chamber, the size and type of
incinerator used, the physical form of the waste, and the waste particle size
(which determines the amount of waste surface area available for heat transfer).
Typical incinerator residence times range from approximately 2 seconds for liquid
and gaseous wastes in liquid injection incinerators and the afterburner,
freeboard, and secondary combustion chambers of rotary kiln, fluidized bed, and
fixed hearth incinerators to 30 minutes to 1 hour for solid wastes in a rotary
kiln incinerator. EPA monitors the waste feed rate continuously to ensure that
sufficient residence time is provided to effectively incinerate organic waste
constituents.
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5.5.5 Degree of Waste/Air Mixing
The incineration temperature, the amount of excess oxygen, and the
residence time required to achieve effective incineration of a waste all depend
to some extent on the degree of waste/air mixing in the combustion chamber. For
liquid injection incinerators, the degree of waste/air mixing is primarily
determined by the specific burner design and the degree of atomization of the
waste/air mixture achieved. The degree of atomization is dependent on the
viscosity of the liquid waste and the amount of solid impurities present. If the
waste viscosity exceeds 750 Sus or the amount of solid impurities is too high,
the degree of atomization may not be fine enough, resulting in ineffective
incineration.
For rotary kiln incineration, the degree of waste/air mixing is determined
by the airflow rate into the kiln as well as the rate of rotation (revolutions
per minute (RPM)) of the kiln. As the airflow rate is increased, the degree of
waste/air mixing is improved although this also reduces the incineration
temperature and residence time of the combustion gases. Increasing the rate of
rotation of the kiln also improves waste/air mixing; however, the residence time
of the waste solids is also reduced. Typical rotation rates for rotary kilns
range from 0.3 to 1.5 meters per minute (1 to 5 feet per minute).
For fluidized bed incineration, the degree of waste/air mixing is indicated
by the pressure drop through the bed caused by the airflow rate used to fluidize
the bed. The higher the pressure drop, the greater the degree of mixing in the
fluidized bed. However, this also reduces the incineration temperature and the
residence time of the waste solids and combustion gases.
For fixed hearth incineration, the degree of waste/air mixing is determined
by the airflow rate into the combustion chamber. As the airflow rate is
increased, the degree of waste/air mixing is improved. However, this also
reduces the incineration temperature and the residence time of the combustion
gases.
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The quantifiable degree of waste/air mixing is a complex assessment that
is difficult to express in absolute terms. However, for liquid injection
incineration, EPA evaluates the degree of mixing qualitatively by examining the
burner design and determining whether it could be expected to achieve effective
atomization of the waste/air mixture. For rotary kiln incineration, EPA
estimates the degree of mixing by monitoring the airflow rate into the kiln as
well as the kiln's rate of rotation to ensure that effective mixing of the waste
and air is achieved. For fluidized bed incineration, EPA estimates the degree
of mixing by monitoring the pressure drop through the bed to ensure that
effective mixing of waste and air is achieved. For fixed hearth incineration,
EPA estimates the degree of mixing by monitoring the airflow rate into the
combustion chamber to ensure that effective mixing of the waste and air is
achieved.
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5.6 References
Ackerman, D.G., McGaughey, J.F., and Wagoner, D.E. 1983. At sea incineration
of PCB-containinq wastes on board the M/T vulcanus. 600/7-83-024. U.S.
Environmental Protection Agency, Washington, D.C.
Bonner, I.A., et al. 1981. Engineering handbook for hazardous waste
incineration. Prepared by Monsanto Research Corporation for U.S.
Environmental Protection Agency. PB 81-248166.
Moller, J.J., and Christiansen, O.B. 1984. Dry scrubbing of hazardous waste
incinerator flue gas by spray dryer absorption. In Proceedings of the
77th annual APCA meeting.
Novak, R.G., Troxler, W.L., and Dehnke, T.H. 1984. Recovering energy from
hazardous waste incineration, Chemical engineering progress 91:146.
Oppelt, E.T. 1987. Incineration of hazardous waste, JAPCA vol. 37, no. 5, May
1987.
Santoleri, J.J. 1983. Energy recovery—a by product of hazardous waste
incineration systems. In Proceedings of the 15th Mid-Atlantic Industrial
Waste Conference on Toxic and Hazardous Waste.
Vogel, G., et al. 1983. Composition of hazardous waste streams currently
incinerated. Prepared by Mitre Corp. for U.S. Environmental Protection
Agency.
Vogel, G., et al. 1986. Incineration and cement kiln capacity for hazardous
waste treatment. In Proceedings of the 12th Annual Research Symposium on
Incineration and Treatment of Hazardous Wastes. Cincinnati, Ohio.
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6. WET AIR OXIDATION
6.1 Applicability
Wet air oxidation is a treatment technology applicable to wastewaters
containing organics and oxidizable inorganics such as cyanide. The process is
typically used to oxidize sewage sludge, regenerate spent activated carbon, and
treat process wastewaters. Wastewaters treated using this technology include
pesticide wastes, petrochemical process wastes, cyanide-containing metal
finishing wastes, spent caustic wastewaters containing phenolic compounds, and
some organic chemical production wastewaters.
This technology differs from other treatment technologies generally used
to treat wastewaters containing organics in the following ways. First, wet air
oxidation can be used to treat wastewaters that have higher organic
concentrations than are normally handled by biological treatment, carbon
adsorption, and chemical oxidation but may be too dilute to be effectively
treated by thermal processes such as incineration. Wet air oxidation is most
applicable for waste streams containing dissolved or suspended organics in the
500 to 50,000 mg/1 range. Below 500 mg/1, the rates of wet air oxidation of most
organic constituents are too slow for efficient application of this technology.
For these more dilute waste streams, biological treatment, carbon adsorption, or
chemical oxidation may be more applicable. For more concentrated waste streams
(above 50,000 mg/1), thermal processes such as incineration may be more
applicable. Second, wet air oxidation can be applied to wastes that have
significant concentrations of metals (roughly 2 percent), whereas biological
treatment, carbon adsorption, and chemical oxidation may have difficulty in
treating such wastes.
It is important to point out that wet air oxidation proceeds by a series
of reaction steps and the intermediate products formed are not always as readily
oxidized as are the original constituents. Therefore, the process does not
always achieve complete oxidation of the organic constituents. Accordingly, in
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applying this technology it is important to assess potential products of
incomplete oxidation to determine whether further treatment is necessary or
whether this technology is appropriate at all.
Studies of the wet air oxidation of different compounds have led to the
following empirical observations concerning a compound's susceptibility to wet
air oxidation based on its chemical structure:
1. Aliphatic compounds, even with multiple halogen atoms, can be
destroyed within conventional wet air oxidation conditions.
Oxygenated compounds (such as low-molecular-weight alcohols,
aldehydes, ketones, and carboxylic acids) are formed, but these
compounds are readily biotreatable.
2. Aromatic hydrocarbons, such as toluene, acenaphthene, or pyrene, are
easily oxidized.
3. Halogenated aromatic compounds can be oxidized provided there is at
least one nonhalogen functional group present on the ring (e.g.,
pentachlorophenol (-OH) or 2,4,6-trichloroaniline (-NH2)).
4. Halogenated aromatic compounds, such as 1,2-dichlorobenzene, and PCBs,
such as Aroclor 1254, are resistant to wet air oxidation under
conventional conditions.
5. Halogenated ring compounds, such as the pesticides aldrin, dieldrin,
and endrin, are expected to be resistant to conventional wet air
oxidation.
6. DDT can be oxidized, but results in the formation of intractable oils
in conventional wet air oxidation.
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7. Heterocyclic compounds containing oxygen, nitrogen, or sulfur are
expected to be destroyed by wet air oxidation because the 0, N, or S
atoms provide a point of attack for oxidation reactions to occur.
6.2 Underlying Principles of Operation
The basic principle of operation for wet air oxidation is that the enhanced
solubility of oxygen in water at high temperatures and pressures aids in the
oxidation of organics. The typical operating temperature for the wet air
oxidation treatment process ranges from 175 to 325*C (347 to 617*F). The
pressure is maintained at a level high enough to prevent excessive evaporation
of the liquid phase at the operating temperature, generally between 300 and
3000 psi. At these elevated temperatures and pressures, the solubility of oxygen
in water is dramatically increased, thus providing a strong driving force for the
oxidation. The reaction must take place in the aqueous phase because the
chemical reactions involve both oxygen (oxidation) and water (hydrolysis). The
wet air oxidation process for a specific organic compound generally involves a
number of oxidation and hydrolysis reactions in series, which degrade the initial
compound by steps into a series of compounds of simpler structure. Complete wet
air oxidation results in the conversion of organic compounds into carbon dioxide,
water vapor, ammonia (for nitrogen-containing wastes), sulfate (for
sulfur-containing wastes), and halogen acids (for halogenated wastes).
However, treatable quantities of partial degradation products may remain
in the treated wastewaters from wet air oxidation. Therefore, effluents from wet
air oxidation processes may be given subsequent treatment including biological
treatment, carbon adsorption, or chemical oxidation before being discharged.
6.3 Description of Wet Air Oxidation Process
A conventional wet air oxidation system consists of a high-pressure liquid
feed pump, an oxygen source (air compressor or liquid oxygen vaporizer), a
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reactor, heat exchangers, a vapor-liquid separator, and process regulators. A
basic flow diagram is shown in Figure 9.
A typical batch wet air oxidation process proceeds as follows. First, a
copper catalyst solution may be mixed with the aqueous waste stream if
preliminary testing indicates that a catalyst is necessary. The waste is then
pumped into the reaction chamber. The aqueous waste is pressurized and heated
to the design pressure and temperature. After reaction conditions have been
established, air is fed to the reactor for the duration of the design reaction
time. At the completion of the wet air oxidation process, suspended solids or
gases are removed and the remaining treated aqueous waste is either discharged
directly to disposal or fed to a biological treatment, carbon adsorption, or
chemical oxidation treatment system if further treatment is necessary prior to
discharge to disposal.
Wet air oxidation can also be operated in a continuous process. In
continuous operation, the waste is pressurized, mixed with pressurized air or
oxygen, preheated in a series of heat exchangers by the hot reactor effluent and
steam, and fed to the reactor. The waste feed flow rate controls the reactor
residence time. Steam is fed into the reactor column to adjust the column
temperature. The treated waste is separated in a gas-liquid separator, with the
gases treated in an air pollution control system and/or discharged to the
atmosphere, and the liquids either further treated, as mentioned above, and/or
discharged to disposal.
6.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether wet air oxidation will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the chemical oxygen demand and (b) the concentration
of interfering substances.
f:\document\15254031.01.008 67
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PRESSURIZED
WASTEWATER
INFLUENT
t
PRESSURIZED
AIR OR
OXYGEN
CO
FEED HEAT
EXCHANGERS
REACTOR
STEAM
VENT GASES TO
AIR POLLUTION
CONTROL SYSTEM
AND/OR THE
ATMOSPHERE
STEAM
GAS-LIQUID
SEPARATOR
TREATED
EFFLUENT
TO FURTHER
TREATMENT
AND/OR
DISPOSAL
Figure 9 Continuous Uet Air Oxidation System
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6.4.1 Chemical Oxygen Demand
The chemical oxygen demand (COD) of the waste is a measure of the oxygen
required for complete oxidation of the oxidizable waste constituents. The limit
to the amount of oxygen that can be supplied to the waste is dependent on the
solubility of oxygen in the aqueous waste and the rate of dissolution of oxygen
from the gas phase to the liquid phase. This sets an upper limit on the amount
of oxidizable compounds that can be treated by wet air oxidation. Thus, high-COO
wastes may require dilution for effective treatment to occur. If the COD of the
untested waste is significantly higher than that of the tested waste, the system
may not achieve the same performance. Pretreatment of the waste or dilution as
part of treatment may be needed to reduce the COD to within levels treatable by
the dissolved oxygen concentration and to achieve the same treatment performance,
or other, more applicable treatment technologies may need to be considered for
treatment of the untested waste.
6.4.2 Concentration of Interfering Substances
In some cases, addition of a water-soluble copper salt catalyst to the
waste before processing is necessary for efficient oxidation treatment (for
example, for oxidation of some halogenated organics). Other metals have been
tested and have been found to be less effective. Interfering substances for the
wet air oxidation process are essentially those that cause the formation of
insoluble copper salts when copper catalysts are used. To be effective in
catalyzing the oxidation reaction, the copper ions must be dissolved in solution.
Sulfide, carbonate, and other negative ions that form insoluble copper salts may
interfere with treatment effectiveness if they are present in significant
concentrations in wastes for which copper catalysts are necessary for effective
treatment. If an untested waste for which a copper catalyst is necessary for
effective treatment has a concentration of interfering substances (including
sulfide, carbonate, or other anions that form insoluble copper salts)
significantly higher than that in a tested waste, the system may not achieve the
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same performance and other, more applicable treatment technologies may need to
be considered for treatment of the untested waste.
6.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a wet air
oxidation system, EPA examines the following parameters: (a) the oxidation
temperature, (b) the residence time, (c) the excess oxygen concentration, (d) the
oxidation pressure, and (e) the amount and type of catalyst. For many hazardous
organic constituents, analytical methods are not available or the constituent
cannot be analyzed in the waste matrix. Therefore, it would normally be
impossible to measure the effectiveness of the wet air oxidation treatment
system. In these cases EPA tries to identify measurable parameters or
constituents that would act as surrogates to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the wet air oxidation treatment
system indicates removal of organic constituents. Hence, TOC analysis is likely
to be an adequate surrogate analysis where the specific organic constituent
cannot be measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden (i.e., the TOC
analysis may not be sensitive enough to detect changes at the milligrams per
liter (mg/1) level in matrices where total organic concentrations are hundreds
or thousands of mg/1). In these cases other surrogate parameters should be
sought. For example, if a specific analyzable constituent is expected to be
treated as well as the unanalyzable constituent, the analyzable constituent
concentration should be monitored as a surrogate.
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6.5.1 Oxidation Temperature
Temperature is the most important parameter affecting the system. The
design temperature must be high enough to allow the oxidation reactions to
proceed at acceptable rates. Raising the temperature increases the wet air
oxidation rate by enhancing oxygen solubility and oxygen diffusivity. The
process is normally operated in the temperature range of 175 to 325*C (347 to
617*F), depending on the hazardous constituent(s) to be treated. EPA monitors
the oxidation temperature continuously, if possible, to ensure that the system
is operating at the appropriate design condition and to diagnose operational
problems.
6.5.2 Residence Time
The residence time impacts the extent of oxidation of waste contaminants.
For a batch system, the-residence time is controlled directly by adjusting the
treatment time in the reaction tank. For a continuous system, the waste feed
rate is controlled to make sure that the system is operated at the appropriate
design residence time. Generally, the reaction rates are relatively fast for the
first 30 minutes and become slow after 60 minutes. Typical residence times,
therefore, are approximately 1 hour. EPA monitors the residence time to ensure
that sufficient time is provided to effectively oxidize the waste.
6.5.3 Excess Oxygen Concentration
The system must be designed to supply adequate amounts of oxygen for the
compounds to be oxidized. An estimate of the amount of oxygen needed can be made
based on the COD content of the untreated waste; excess oxygen should be supplied
to ensure complete oxidation. The source of oxygen is compressed air or a
high-pressure pure oxygen stream. EPA monitors the excess oxygen concentration
(the concentration of oxygen in the gas leaving the reactor) continuously, if
possible, by sampling the vent gas from the gas-liquid separator to ensure that
an effective amount of oxygen or air is being supplied to the waste.
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6.5.4 Oxidation Pressure
The design pressure must be high enough to prevent excessive evaporation
of water and volatile organics at the design temperature. This allows the
oxidation reaction to occur in the aqueous phase, thereby improving treatment
effectiveness. Typical oxidation pressures range from 900 to 3000 psig. EPA
monitors the oxidation pressure continuously, if possible, to ensure that the
system is operating at the appropriate design condition and to diagnose
operational problems.
6.5.5 Amount and Type of Catalyst
Adding a catalyst that promotes oxygen transfer and thus enhances oxidation
has the effect of lowering the necessary reactor temperature and/or improving the
level of destruction of oxidizable compounds. For waste constituents that are
more difficult to oxidize, the addition of a catalyst may be necessary to
effectively destroy the constituent(s) of concern. Catalysts typically used for
this purpose include copper bromide and copper nitrate. If a catalyst is
required, EPA examines the amount and type added, as well as the method of
addition of the catalyst to the waste, to ensure that effective oxidation is
achieved.
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6.6 References
Detrich, M.J., Randall, T.L., and Canney, P.J. 1985. Wet air oxidation of
hazardous organics in wastewater. Environmental Progress 4:171-197.
Randall, T.L. 1981. Wet oxidation of toxic and hazardous compounds, Zimpro
technical bulletin 1-610. Presented at the 13th Mid-Atlantic Industrial
Waste Conference, June 29-30, 1981, University of Delaware, Newark, Del.
USEPA. 1990. U.S. Environmental Protection Agency, Office of Solid Waste. Best
demonstrated available technology (BOAT) background document addendum for
acrvlonitrile wastes. Washington, D.C.: U.S. Environmental Protection
Agency.
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I. TREATMENT TECHNOLOGIES (CONTINUED)
B. TECHNOLOGIES THAT REDUCE THE SOLUBILITY OR
LEACHABILITY OF METALS
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1. AMALGAMATION
1.1 Applicability
The term amalgamation refers to two types of processes that can be used as
treatment for mercury wastes. Neither amalgamation process significantly reduces
the Teachability of mercury; both processes only convert it into a more easily
processed form. In both processes, a solid alloy of mercury and a base metal,
such as zinc, is formed. This alloy can be subsequently processed by retorting
to recover mercury.
The two processes differ from each other in the types of wastes managed.
The first process is applicable only to solutions containing dissolved mercury
salts. The principal current use for the process is in treatment of wastewaters
containing organomercury salts. The second process is usable only for wastes
rich in elemental mercury. Since the use of this second process is merely a
convenience to avoid handling mercury in liquid form, its use is negligible.
Both processes are applicable only to wastes containing mercury where selective
recovery of the mercury is deemed viable.
1.2 Underlying Principles of Operation
The amalgamation processes depend on the ability of mercury to form low-
melting-point solid alloys with metals such as copper and zinc, which have the
thermodynamic capability of simultaneously reducing mercuric and mercurous salts
to elemental mercury. Basically, an excess of the less noble metal (zinc or
copper) is contacted with a waste containing mercury or mercury salts. The
chemical reaction reducing the mercury in the mercury salts occurs and the
elemental mercury liberated forms an alloy with the excess metal added. For zinc
and mercuric nitrate, the reaction may be written:
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Hg(N03)2 + Zn - Zn(N03)2 + Hg
Hg + Zn •» mercury zinc amalgam
1.3 Description of Amalgamation Treatment Processes
The two types of amalgamation processes are operated as follows:
1. The aqueous replacement process (solution process) involves addition
of excess base metal, such as zinc, to a wastewater solution
containing dissolved mercury salts. The elemental zinc, or other base
metal, reacts with the mercuric or mercurous salts to form elemental
mercury, which subsequently alloys with the excess base metal to form
a solid amalgam (or alloy) that can be recovered by filtration and
then sent for mercury recovery if applicable. Generally, finely
divided zinc (zinc dust) is used. The high surface area of this
material allows for rapid reaction and alloy formation.
2. The nonaqueous process, which is seldom used, involves contacting
waste liquid mercury with finely divided zinc powders. The mass
rapidly solidifies into a solid amalgam, which may be more easily
managed than liquid mercury. This type of process is typically
limited in utility only to waste scrap elemental mercury.
1.4 Waste Characterization Affecting Performance fWCAPs)
In determining whether amalgamation will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the chemical form of the mercury, (b) the solution
pH, (c) the presence of solids, and (d) the presence of other interfering
materials.
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1.4.1 Chemical Form of the Mercury
The solution process requires that the mercury be present in the form of
a dissolved mercury salt. As a result, the process is not applicable to wastes
containing insoluble mercury compounds. The nonaqueous process is applicable
only to waste scrap mercury in elemental form.
1.4.2 Solution pH
The solution process involves reactions of finely divided zinc with mercury
salts in solution. Reactions such as those consuming zinc by reaction with the
acidic content of wastewaters are undesirable. Therefore, the pH of the
wastewater solution should be adjusted to near neutral value before the process
is used. This characteristic does not apply to the second process, where scrap
liquid mercury is a required feedstock.
1.4.3 Presence of Sol Ids
The solution process is aimed at recovery of a solid amalgam that can be
shipped to mercury reprocessors. The presence of other solids in the amalgam may
be undesirable. Therefore, if mercury is to be recovered, wastewater solutions
should be filtered to remove extraneous solids before the process is used. This
characteristic does not apply to the second process.
1.4.4 Presence of Interfering Materials
The solution process requires a clear solution as a feedstock. Solutions
containing oils, greases, and emulsified or suspended materials need to be
pretreated before use of the process. The presence of many other dissolved
salts, such as those of copper, which can react with zinc, is also undesirable,
but does not exclude use of the process. The presence of such salts will
increase the amounts of zinc required for process operation.
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The nonaqueous process requires elemental mercury as a feedstock. Scrap
mercury contaminated with oils, greases, and emulsion is better managed by
retorting directly since these interfering substances may inhibit amalgam
formation.
1.5 Design and Operating Parameters
1.5.1 Solution Process Design and Operating Parameters
In assessing the effectiveness of the design and operation of a solution
process, EPA examines the following parameters: (a) temperature, (b) degree of
mixing, (c) zinc usage, (d) solution pretreatment, and (e) residual mercury in
the treated wastewater.
1.5.1.1 Temperature
The solution process is normally operated at ambient temperature but can
be run at temperatures up to about 60*C to enhance the reaction rate between zinc
and dissolved mercury salts. EPA will examine the basis for the choice of
operating temperature and temperature monitoring equipment to ensure proper
process operation.
1.5.1.2 Degree of Mixing
Finely divided zinc powder is normally added to the solution containing the
mercury compound. To ensure adequate contact between the zinc particles and
mercury ions present in solution, good agitation is needed. EPA will examine the
equipment used to determine that the treatment vessel contains the proper
stirring equipment.
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1.5.1.3 Zinc Usage
Sufficient zinc must be added to the solutions to ensure that all of the
mercury present is converted to an amalgam. To achieve this end, the
concentration of mercury in the solution to be processed must be known to
estimate the minimum amount of zinc to be added. EPA will examine a facility's
calculations for zinc addition for individual batches to ensure proper operation.
1.5.1.4 Solution Pretreatment
In a properly operated unit, the mercury-containing solutions processed
should be clear and free of oil, grease, and suspended solids when introduced to
the processing equipment. EPA will inspect the solutions entering the process
to ensure that proper pretreatment is being used.
1.5.1.5 Residual Mercury Levels in Treated Liquid Phase
One final design and operating parameter of importance is the concentration
of mercury in the treated water after amalgam removal. For a properly designed
and operated unit, both the entering and final mercury concentrations should be
determined on a batch-by-batch basis. EPA will examin'e these records to ensure
proper operation. EPA will also determine proper operation by examination of
capabilities for posttreatment to precipitate any residual mercury from the
treated wastewaters prior to their discharge. Amalgamation processes are
normally used as part of treatment trains devoted to recovering elemental
mercury. The reaction of zinc with mercury salts in solution occurs rapidly at
higher mercury salt concentrations but may become very slow as the concentration
falls to low levels. As a result, treated solutions are likely to contain some
residual mercury salts requiring separate treatment.
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1.5.2 Nonaqueous Process Design and Operating Parameters
The nonaqueous (solid) process Is very rarely used on a significant scale.
Generally, waste mercury is either retorted or redistilled as such without
formation of amalgams. There are, however, a few instances in which it might be
desirable to amalgamate elemental mercury, the most important of which is when
the mercury 1s contaminated by radioisotopes. The solid process involving
amalgamation with zinc is basically simple, and there are two design and
operating parameters of concern: (a) the amount of zinc used and (b) the adequacy
of the mixing.
1.5.2.1 Amount of Zinc Used
The properties of the final amalgam prepared will depend on the ratio of
zinc powder to mercury used. Mercury and zinc form amalgams over a very wide
composition range. However, it is necessary to add sufficient zinc so that an
alloy that is solid at room temperature is obtained. This entails using
sufficient zinc to ensure that an amalgam with at least 25 percent zinc content
is obtained.
1.5.2.2 Adequacy of Mixing
To form uniform zinc-mercury amalgams, the constituents need to be
uniformly mixed. Historically, such amalgams have been prepared in small
quantities. If a facility desires to process mercury by this route in higher
volumes, a specially designed mixing system may be needed. EPA will examine the
design and operation of the mixing system to determine whether adequate mixing
is likely to be achieved.
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1.6 References
Mellor, J.W., 1948. Comprehensive treatise on theoretical and inorganic
chemistry. Vol. 4, pp. 1022-1049. London, U.K.: Longmans Green.
Signer, W., and Nowak M., 1978. Mercury compounds. In Kirk-Othmer encyclopedia
of chemical technology. Vol. 15, pp. 157-171. New York: John Wiley
Interscience.
Rissmann, E.F., and Schwartz, S., 1989. Treatment of wastes containing arsenic,
selenium, thallium, and mercury compounds. In Proceedings of the 44th
Industrial Waste Conference. Purdue University, pp. 643-648. Ann Arbor,
Mich.: Lewis Publishers.
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2. CHEMICAL PRECIPITATION
2.1 Applicability
Chemical precipitation is a treatment technology applicable to wastewaters
containing a wide range of dissolved and other metals, as well as other inorganic
substances such as fluorides. This technology removes these metals and
inorganics from solution in the form of insoluble solid precipitates. The solids
formed are then separated from the wastewater by settling, clarification, and/or
polishing filtration.
For some wastewaters, such as chromium plating baths or plating baths
containing cyanides, the metals exist in solution in a very soluble form. This
solubility can be caused by the metal's oxidation state (for example, hexavalent
chromium wastewaters) or by complexing of the metals (for example, high
cyanide-containing wastewaters). In both cases, pretreatment, such as hexavalent
chromium reduction or oxidation of the metal-cyanide complexes, may be required
before the chemical precipitation process can be applied effectively. In the
case of arsenic, the arsenic-containing solution is normally treated first with
oxidizing agents such as alkali hypochlorite solution to convert the lower-
valence arsenic compound to arsenate. The arsenic ion is then typically
precipitated out as ferric arsenate. Some compounds must be reduced prior to
precipitation. For instance, selinites and selenates are oxidants and are
readily reduced to elemental selenium, which is insoluble in aqueous solutions.
Sulfur dioxide, sulfides, sulfites, and ferrous ion are all effective for this
reduction reaction.
Chemical precipitation may also be applicable to mixed waste for separating
radionuclides from other hazardous constituents in wastewaters. Specific
conditions of pH, temperature, and precipitating reagent addition are required
to selectively remove part or all of the radioactive component as a precipitate.
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2.2 Underlying Principles of Operation
The basic principle of operation of chemical precipitation is that metals
and inorganics in wastewater are removed by the addition of a precipitating agent
that converts the soluble metals and inorganics to insoluble precipitates. These
precipitates are settled, clarified, and/or filtered out of solution, leaving a
lower concentration of metals and inorganics in the wastewater. The principal
precipitation agents used to convert soluble metal and inorganic compounds to
less soluble forms include lime (Ca(OH)2), caustic (NaOH), sodium sulfide (Na2S),
and, to a lesser extent, soda ash (Na2C03), phosphate (P04"), and ferrous sulfide
(FeS).
The solubility of a particular compound depends on the extent to which the
electrostatic forces holding the ions of the compound together can be overcome.
The solubility changes significantly with temperature, with most metal compounds
becoming more soluble as the temperature increases. Additionally, the solubility
is affected by other constituents present in the wastewater, including other ions
and complexing agents. Regarding specific ionic forms, nitrates, chlorides, and
sulfates are, in general, more soluble than hydroxides, sulfides, carbonates, and
phosphates.
Once the soluble metal and inorganic compounds have been converted to
precipitates, the effectiveness of chemical precipitation is determined by how
successfully the precipitates are physically removed. Removal usually relies on
a settling process; that is, a particle of a specific size, shape, and
composition will settle at a specific velocity, as described by Stokes' Law. For
a batch system, Stokes' Law is a good predictor of settling time because the
pertinent particle parameters essentially remain constant. In practice, however,
settling time for a batch system is normally determined by empirical testing.
For a continuous system, the theory of settling is complicated by such factors
as turbulence, short-circuiting of the wastewater, and velocity gradients, thus
increasing the importance of empirical tests to accurately determine appropriate
settling times.
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2.3 Description of Chemical Precipitation Process
The equipment and instrumentation required for chemical precipitation vary
depending on whether the system is batch or continuous. Both systems are
discussed below.
For a batch system, chemical precipitation requires a feed system for the
treatment chemicals and a reaction tank where the waste can be treated and
allowed to settle. When lime is used, it is usually added to the reaction tank
in a slurry form. The supernatant liquid is generally analyzed before discharge
to ensure that settling of precipitates is adequate.
For a continuous system, additional tanks are necessary, as well as the
instrumentation to ensure that the system is operating properly. A schematic of
a continuous chemical precipitation system is shown in Figure 10. In this
system, wastewater is fed into an equalization tank, where it is mixed to provide
more uniformity, thus minimizing the variability in the type and concentration
of constituents sent to the reaction tank.
Following equalization, the wastewater is pumped to a reaction tank where
precipitating agents are added. This is done automatically by using
instrumentation that senses the pH of the system for hydroxide precipitating
agents, or the oxidation-reduction potential (ORP) for nonhydroxide precipitating
agents, and then pneumatically adjusts the position of the treatment chemical
feed valve until the design pH or ORP value is achieved. (The pH and ORP values
are affected by the concentration of hydroxide and nonhydroxide precipitating
agents, respectively, and are thus used as indicators of their concentrations in
the reaction tank.)
In the reaction tank, the wastewater and precipitating agents are mixed to
ensure commingling of the metal and inorganic constituents to be removed and the
precipitating agents. In addition, effective dispersion of the precipitating
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WASIEWATCR
INFLUENT
CO
^A^yLA^
*4
COUALIIATION
TANK
ELECTHICAL CONTROLS
UIIER
rRECIPIIAIMa
AQfHT
FEED
• tlTCM
COAGULANT OR
riOCCIAANT rCCO tVSICM
IREATEO
tmUEHT TO
fOLISMIHO
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ANO/OR
OIIPOtAL
BOIIOHS
fO tLUOOE
FILTRATION
AND DISPOSAL
Figure 10 Continuous Chemical Precipitation System
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agents throughout the tank is necessary to properly monitor and thereby control
the amount added.
Following reaction of the wastewater with the stabilizing agents,
coagulating or flocculating compounds are added to chemically assist the settling
process. Coagulants and flocculants increase the particle size and density of
the precipitated solids, both of which increase the rate of settling. The
coagulant or flocculating agent that best improves settling characteristics
varies depending on the particular precipitates to be settled.
Settling can be conducted in a large tank by relying solely on gravity or
can be mechanically assisted through the use of a circular clarifier or an
inclined plate settler. Schematics of the two settling systems are shown in
Figures 11 and 12. Following the addition of coagulating or flocculating agents,
the wastewater is fed to a large settling tank, circular clarifier, or inclined
plate settler, where the precipitated solids are removed. These solids are
generally further treated in a sludge filtration system to dewater them prior to
disposal. Sludge filteration is discussed in Section II.B.2 of this document.
The supernatant liquid effluent can be further treated in a polishing
filtration system to remove precipitated residuals both in cases where the
settling system is underdesigned and in cases where the particles are difficult
to settle. Polishing filtration is discussed in Section II.B.2 of this document.
2.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether chemical precipitation will achieve the same level
of performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the concentration and type of metals, (b) the
concentration of total dissolved solids (TDS), (c) the concentration of
complexing agents, and (d) the concentration of oil and grease.
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BOTTOMS TO SLUDGE
FILTRATION AND DISPOSAL
TREATED EFFLUENT
TO POLISHING
FILTRATION
ANO/OR DISPOSAL
WASTTWATIR
INFLUINT
CENTER FEED CLARIF1ER WITH SCRAPER SLUDGE REMOVAL SUSTEM
WASTEWATCR
INFLUENT
RIM FEED - CENTER TAKEOFF CLARIF1ER WITH
HYDRAULIC SUCTION SLUDGE REMOVAL SYSTEM
TREATED
EFFLUENT
•TO POUSMING
FILTRATION
ANO/OR DISPOSAL
BOTTOMS TO
SLUDGE FILTRATION
AMD DISPOSAL
WASTEWATER
INFLUENT
TREATED
EFFLUENT
TO POLISHING
FILTRATION
ANO/OR DISPOSAL
BOTTOMS TO SLUDGE FILTRATION AND DISPOSAL
RIM FEED - RIM TAKEOFF CLARIF1ER
Figure 11 Circular Clar1f1er Systems
86
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WASTEWATER
INFLUENT
TREATED
EFFLUENT
TO POLISHING
FILTRATION
AND/OR DISPOSAL
BOTTOMS TO
SLUDGE FILTRATION
AND DISPOSAL
Figure 12 Inclined Plate Settler System
87
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2.4.1 Concentration and Type of Metals
For most metals, there is a specific pH at which the metal precipitate is
least soluble. As a result, when a waste contains a mixture of many metals, it
is not possible to operate a treatment system at a single pH or ORP value that
is optimal for the removal of all metals. The extent to which this affects
treatment depends on the particular metals to be removed and their respective
concentrations. One alternative is to operate multiple precipitations, with
intermediate settling, when the optimum pH occurs at markedly different levels
for the metals present. If the concentration and type of metals in an untested
waste differ from and are significantly higher than those in the tested waste,
the system may not achieve the same performance. Additional precipitating
agents, alternate pH/ORP values, and/or multiple precipitations may be required
to achieve the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
•
2.4.2 Concentration of Total Dissolved Solids
High concentrations of total dissolved solids (TDS) can interfere with
precipitation reactions, as well as inhibit settling. Poor precipitate formation
and flocculation are results of high TDS concentrations, and higher
concentrations of solids are found in the treated wastewater residuals. If the
TDS concentration in an untested waste is significantly higher than that in the
tested waste, the system may not achieve the same performance. Higher
concentrations of precipitating agents may be required to achieve the same
treatment performance, or other, more applicable treatment technologies may need
to be considered for treatment of the untested waste.
2.4.3 Concentration of Complexing Agents
A metal complex consists of a metal ion surrounded by a group of other
inorganic or organic ions or molecules (often called ligands). In the complexed
form, metals have a greater solubility. Also, complexed metals inhibit the
f:\document\15254031.01.010 88
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reaction of the metal with the precipitating agents and therefore may not be
removed as effectively from solution by chemical precipitation. However, EPA
does not have analytical methods to determine the concentration of complexed
metals in wastewaters. The Agency believes that the best indicator for complexed
metals is to analyze for complexing agents, such as cyanide, chlorides, EDTA,
ammonia, amines, and methanol, for which analytical methods are available.
Therefore, EPA uses the concentration of complexing agents as a surrogate waste
characteristic for the concentration of metal complexes. If the concentration
of complexing agents in an untested waste is significantly higher than that in
the tested waste, the system may not achieve the same performance. Higher
concentrations of precipitating agents may be required to achieve the same
treatment performance, or other, more applicable treatment technologies may need
to be considered for treatment of the untested waste.
2.4.4 Concentration of Oil and Grease
The concentration of oil and grease in a waste inhibits the settling of the
precipitate by creating emulsions that require a long settling time. Suspended
oil droplets in water tend to suspend particles such as chemical precipitates
that would otherwise settle out of solution. Even with the use of coagulants or
flocculants, the settling of the precipitate is less effective. If the
concentration of oil and grease in an untested waste is significantly higher than
that in the tested waste, the system may not achieve the same performance.
Pretreatment of the waste may be required to reduce the oil and grease
concentration and achieve the same treatment performance, or other, more
applicable treatment technologies may need to be considered for treatment of the
untested waste.
2.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a chemical
precipitation system, EPA examines the following parameters: (a) the pH/ORP
value; (b) the precipitation temperature; (c) the residence time; (d) the amount
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and type of precipitating agents, coagulants, and flocculants; (e) the degree of
mixing; and (f) the settling time.
2.5.1 pH/ORP Value
The pH/ORP value in continuous chemical precipitation systems is used as
an indicator of the concentration of precipitating agents in the reaction tank
and thus is used to regulate their addition to the tank. The pH/ORP value also
affects the solubility of metal precipitates formed and therefore directly
impacts the effectiveness of their removal. EPA monitors the pH/ORP value
continuously, if possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
2.5.2 Precipitation Temperature
The precipitation temperature affects the solubility of the metal
precipitates. Generally, the lower the temperature, the lower the solubility of
the metal precipitates and vice versa. EPA monitors the precipitation
temperature continuously, if possible, to ensure that the system is operating at
the appropriate design condition and to diagnose operational problems.
2.5.3 Residence Time
The residence time impacts the extent of the chemical reactions to form
metal precipitates and, as a result, the amount of precipitates that can be
settled out of solution. For batch systems, the residence time is controlled
directly by adjusting the treatment time in the reaction tank. For continuous
systems, the wastewater feed rate is controlled to make sure that the system is
operating at the appropriate design residence time. EPA monitors the residence
time to ensure that sufficient time is provided to effectively precipitate
metals and inorganics from the wastewater.
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2.5.4 Amount and Type of Precipitating Agents, Coagulants, and Flocculants
The amount and type of precipitating agent used to effectively treat the
wastewater depends on the amount and type of metal and inorganic constituents in
the wastewater to be treated. Other design and operating parameters, such as the
pH/ORP value, the precipitation temperature, the residence time, the amount and
type of coagulants and flocculants, and the settling time, are determined by the
selection of precipitating agents. The addition of coagulants and flocculants
improves the settling rate of the precipitated metals and inorganics and allows
for smaller settling systems (i.e., lower settling time) to achieve the same
degree of settling as a much larger system. Typically, anionic polyelectrolyte
flocculating agents are most effective with metal precipitates, although cationic
or nonionic polyelectrolytes also are effective. Typical doses range from 0.1
to 10 mg/1 of the total influent wastewater stream. Conventional coagulants,
such as alum (aluminum sulfate), are also effective, but must be dosed at much
higher concentrations to achieve the same result. Therefore, these coagulants
add more to the settled sludge volume requiring disposal than do the
polyelectrolyte flocculants. EPA examines the amount and type of precipitating
agents, coagulants, and flocculants added, and their method of addition to the
wastewater, to ensure effective precipitation.
2.5.5 Degree of Mixing
Mixing provides greater uniformity of the wastewater feed and disperses
precipitating agents, coagulants, and flocculants throughout the wastewater to
ensure the most rapid precipitation reactions and settling of precipitate solids
possible. The quantifiable degree of mixing is a complex assessment that
includes, among other things, the amount of energy supplied, the length of time
the material is mixed, and the related turbulence effects of the specific size
and shape of the tank. This is beyond the scope of simple measurement. EPA,
however, evaluates the degree of mixing qualitatively by considering whether
mixing is provided and whether the type of mixing device is one that could be
expected to achieve uniform mixing of the wastewater.
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2.5.6 Settling Time
Adequate settling time must be provided to make sure that removal of the
precipitated solids from the wastewater has been completed. EPA monitors the
settling time to ensure effective solids removal.
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2.6 References
Cherry, K.F. 1982. Plating waste treatment, pp. 45-62. Ann Arbor, Mich.: Ann
Arbor Science Publishers, Inc.
Cushnie, G.C., Jr. 1985. Electroplating wastewater pollution control tech-
nology, pp. 48-62, 84-90. Park Ridge, N.J.: Noyes Publications.
Cushnie, G.C., Jr. 1984. Removal of metals from wastewater; neutralization and
precipitation, pp. 55-92. Park Ridge, N.J.: Noyes Publications.
Gurnham, C.F. 1955. Principles of industrial waste treatment, pp. 224-234. New
York: John Wiley and Sons.
Kirk-Othmer. 1980. Flocculation. In Encyclopedia of chemical technology. 3rd
ed., vol. 10, pp. 489-516. New York: John Wiley and Sons.
USEPA. 1983. U.S. Environmental Protection Agency. Treatability manual: Vol.
Ill, Technology for control/removal of pollutants, p. 111.3.1.3.2.
EPA-600/2-82-001C. Washington, D.C.: U.S. Environmental Protection
Agency.
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3. ENCAPSULATION
3.1 Applicability
Encapsulation processes refer to a family of processes wherein high-solids
nonwastewaters are mixed with an organic polymeric substance or with asphalt.
The mixture is then allowed to cure into a solid mass prior to disposal. These
processes are applicable to a wide variety of wastes. They are not applicable
to wastes containing materials that decompose at high temperatures or to wastes
containing oxiders that can react with the binder materials. Also, some aromatic
solvents such as xylene and toluene can diffuse through the encapsulating agent.
Oils, greases, and some chelating agents may interfere with the hardening and
solidification of the encapsulating material. EPA believes that encapsulation
technologies are applicable primarily to wastes containing hazardous metal
constituents. Encapsulation may immobilize hazardous organics as well as metals;
however, incineration is more applicable to organics since incineration destroys
organics completely, whereas encapsulation can only immobilize them.
3.2 Underlying Principles of Operation
There are a number of very similar encapsulation processes that differ from
each other only in the encapsulating agent used. In all of the processes, the
waste is first dried to remove moisture. The waste is then usually reheated and
mixed with hot asphalt or thermoplastic material such as polyethylene. The
mixture is then cooled to solidify the mass. The ratio of matrix (fixative or
encapsulating agent) to waste is generally high (i.e., 1:1 or 1:2 fixative to
waste on a dry basis). The matrix, once solidified, coats the waste to minimize
leaching.
3.3 Description of Encapsulation Processes
Encapsulation processes can take the form of macroencapsulation,
microencapsulation, or both. Microencapsulation is the containment of individual
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waste particles in the polymer or asphalt matrix. Macroencapsulation is the
encasement of a mass of waste in a thick polymer coating. The waste mass may
have been microencapsulated prior to macroencapsulation.
3.3.1 Hicroencapsulation
Microencapsulation processes typically involve the following unit
operations in series:
• Predrying of the waste to remove entrained moisture.
• Mixing of the heated waste with molten encapsulating agent (asphalt,
polyethylene, thermosetting resins).
• Cooling of the hot mixture to allow the mixed mass to harden into a
solid mass.
Generally, ratios of matrix to waste used are high compared to those of
pozzolanic stabilization processes (i.e., for encapsulation the ratio is in the
1:1 to 1:2 range). Mixing is generally done at 120 to 130°C depending on the
melting characteristics of the matrix and the type of equipment used for mixing.
A few processes differ from the above description in that polymerization of
monomers mixed with waste is conducted at ambient or near ambient temperatures
in the presence of catalysts. The monomer (or monomeric mixture) then
polymerizes at room temperature, coating the individual waste particles.
3.3.2 Macroencapsulation
The macroencapsulation process of hazardous waste solids usually involves
two steps. In the first step, the hazardous wastes may be chemically treated by
using low-cost dehydrating agents such as lime, kiln dust, or Portland cement.
This operation does not increase the volume of the solids significantly because
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only a small amount of dehydrating agent is needed to dewater the solids. The
resulting mixtures are friable, and they can easily be ground.
In the second step, the dehydrated sludges are ground and the particles may
be microencapsulated, typically by a polybutadiene binder. Then the mass is
macroencapsulated, or coated, typically by high-density polyethylene.
The typical apparatus for macroencapsulation processes features heated or
cooled molds, a method of waste and hardened product manipulation, and hydraulics
for mold actuation. The molds typically contain electrical band heaters and
water cooling channels. After the polymer coating hardens, the mold is split to
facilitate product demolding.
3.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether encapsulation will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the water content of the waste; (b) the presence of
oxiding agents in the waste; (c) the presence of certain organic solvents;
(d) the presence of oils, greases, and chelating agents; and (e) the presence of
thermally unstable materials in the waste. These characteristics are described
below.
3.4.1 Water Content
The processes normally require a dry solid waste. Wet solids need to be
dried at elevated temperature, or chemically treated, to remove entrained
moisture before they can be mixed with molten polymers, asphalt, or monomeric
mixtures. The presence of free water interferes with the elevated temperature
encapsulation processes by forming steam during the mixing and preventing a
smooth coating of the particles. In ambient temperature processes, water may
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react with the polymerization catalyst, preventing it from initiating the
polymerization reactions.
3.4.2 Presence of Oxidizing Agents
Strong oxidization salts such as nitrites, chlorates, or perchlorates will
react with the organic matrix materials and slowly deteriorate the matrix. These
oxidants may also react spontaneously with the heated matrix material to cause
a fire or explosion. Wastes containing such strong oxidants should not be
managed by polymer encapsulation processes.
3.4.3 Presence of Organic Solvents
Organic solvents, particularly aromatic solvents, may be capable of
dissolving the polymer matrix material. They will also cause significant air
emissions due to volatilization during the mixing of the heated waste and matrix
materials. The process should not be used on wastes containing such solvents
unless the wastes have been pretreated to remove such materials.
3.4.4 Presence of Oils, Greases, and Chelating Agents
Oils, greases, and organic chelating agents are likely to dissolve in and
migrate through the polymeric matrices used for encapsulation. They may also,
to some extent, coat individual waste particles and prevent firm
microancapsulation binding between waste particles and the polymer matrix
surrounding them. Because of the ability of these organics to diffuse through
polymeric matrices, wastes containing oils, greases, and organic chelating agents
should not be managed by polymer encapsulation.
3.4.5 Presence of Thermally Unstable Materials
Hydrated salts are likely to decompose during the initial hot mixing of
matrix and waste materials, liberating water vapor. These salts will also
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rehydrate during cooling and thus interfere with the uniform coating of waste
particles by the matrix polymer. Wastes containing hydrated salts should not be
managed by this process. Salts that are known to cause such problems include
hydrated sodium sulfate, magnesium sulfate, and hydrated metal chlorides. The
soluble hydrates need to be removed from the waste by chemical or physical means
before the wastes are managed by polymer microencapsulation.
3.5 Design and Operating Parameters
There are five design and operating parameters for this set of processes:
(a) the choice of the polymeric matrix material, (b) the mixing equipment used,
(c) the ratio of polymer to waste used, (d) process temperature control, and (e)
air emissions control. These are described in the following paragraphs.
3.5.1 Choice of Polymeric Matrix
Several polymeric matrices are available for use in these encapsulation
processes. These include asphalt, polyethylene, thermosetting plastics (such as
urea formaldehyde type resins), and resins that can be polymerized under ambient
temperature in the presence of a catalyst. The choice of the polymeric matrix
determines the temperature and equipment to be used and strongly influences the
needed cure time. It also determines to some degree the choice of wastes that
can be accepted because of differences in the properties of the polymers. EPA
will examine the basis for the choice of polymer matrix, available test data, and
the wastes being accepted for processing.
3.5.2 Mixing Equipment Used
For microencapsulation, polymers or monomers and catalysts must be well
mixed with wastes to ensure that uniformly coated waste particles are produced.
EPA will examine the design and operating characteristics of the mixing system
to ensure that it is capable of generating uniform mixes.
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3.5.3 Ratio of Polymer to Waste Used
Sufficient polymer must be used to ensure that the waste is uniformly
coated. Generally, a 1:1 or 1:2 polymer-to-waste ratio on a weight basis is
used. EPA will examine the basis for the choice of polymer-to-waste ratio used
to ensure that sufficient polymer is being added.
3.5.4 Process Temperature Control
To achieve uniform mixing, the encapsulation matrix must be in a liquid
state. The melting points of the different thermoplastic polymers will differ
from each other. During mixing of the molten polymer and waste, the temperature
needs to be maintained above the thermoplastic polymer melting point and below
the point at which polymer vaporization or thermal decomposition begins. Thus,
for a given thermoplastic polymer, there is only a certain temperature range in
which the process can be satisfactorily operated. For thermosetting resins the
temperature selected must be appropriate to cause polymerization (hardening)
after thorough mixing. EPA will examine data on the properties of the polymers,
including decomposition, to ensure that a proper operating temperature was
selected. EPA will also examine the temperature control instrumentation in use
to ensure that it is operating properly.
3.5.5 Air Emissions Control
Heating of mixtures of waste with polymers or asphalt can cause the release
of hydrocarbon air emissions that may include volatile components of the waste,
plasticizers present in the polymers, polymer decomposition products, and low-
molecular-weight ingredients of the asphalt. In a properly operating system, air
emissions testing may be needed during at least initial operation to determine
levels of emissions and to assess the need for air emissions control. EPA will
examine such air monitoring data to determine whether air emissions control
systems are appropriate. The Agency will also examine the air emissions control
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systems in use to ensure that they are operating and in proper operating
condition.
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3.6 References
Pierce, J., and Vesilind, P.A. 1981. Hazardous waste management, pp. 82-4. Ann
Arbor, Mich.: Ann Arbor Science Publishers.
Rich, G., and Cherry, K. 1987. Hazardous waste treatment technologies. Chap-
ter 6, pp. 9-3. Northbrook, 111.: Purdue Publishing Company.
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4. STABILIZATION OF METALS
4.1 Applicability
Stabilization is a treatment technology applicable to wastes containing
Teachable metals and having a high filterable solids content, low total organic
carbon (TOC) content, and low oil and grease content. This technology is
commonly used to treat residuals generated from treatment of wastes such as
electroplating wastewaters, incineration ash residues, and characteristic D-metal
wastes such as cadmium (0006), chromium (0007), lead (0008), and mercury (0009).
For wastes with recoverable levels of metals, high temperature metals recovery
and retorting technologies may be applicable.
Stabilization refers to a broad class of treatment processes that
immobilize hazardous constituents in a waste. Related technologies are
encapsulation and thermoplastic binding. However, EPA considers stabilization
technologies to be distinct from the other technologies in that their operational
principles are significantly different. Stabilization typically requires a
chemical reaction between the waste and the stabilizing agent for effective
treatment, while thermoplastic binding and encapsulation usually employ physical
containment of hazardous particulates by the encapsulating agent.
4.2 Underlying Principles of Operation
The basic principle of operation for stabilization is that leachable metals
in a waste are immobilized following the addition of stabilizing agents and other
chemicals. The Teachability is reduced by the formation of a lattice structure
and/or chemical bonds that chemically bind the metals to the solid matrix and
thereby limit the amount of metal constituents that can be leached when water or
a mild acid solution comes into contact with the waste material. Stabilization
is most effective when the waste metal is in its least soluble state, thereby
decreasing the potential for leaching. For example, hexavalent chromium is much
more soluble and more difficult to stabilize than trivalent chromium.
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The two principal stabilization processes used are cement-based and
lime/pozzolan-based processes. A brief discussion of each is provided below.
In both cement-based and lime/pozzolan-based techniques, the stabilizing process
can be modified through the use of additives, such as silicates, that control
curing rates, reduce permeability, and enhance the properties of the solid
material.
4.2.1 Portland Cement-Based Process
Portland cement is a mixture of powdered oxides of calcium, silica,
aluminum, and iron, produced by kiln burning of materials rich in calcium and
silica at high temperatures (i.e., 1,400 to 1,500*C (2,552 to 2,732*F)}. When
the anhydrous cement powder is mixed with water, hydration occurs and the cement
begins to set. The chemistry involved is complex because many different
reactions occur depending on the composition of the cement mixture.
As the cement begins to set, a colloidal gel of indefinite composition and
structure is formed. Over time, the gel swells and forms a matrix composed of
interlacing, thin, densely packed silicate fibrils. Constituents present in the
waste slurry (e.g., hydroxides and carbonates of various metals) are incorporated
into the interstices of the cement matrix. The high pH of the cement mixture
tends to keep metals in the form of insoluble hydroxide and carbonate salts. It
has been hypothesized that metal ions may also be incorporated into the crystal
structure of the cement matrix, but this hypothesis has not been verified.
4.2.2 Lime/Pozzolan-Based Process
Pozzolan, which contains finely divided, noncrystalline silica (e.g., fly
ash or components of cement kiln dust), is a material that is not cementitious
in itself, but becomes so upon the addition of lime. Metals in the waste are
converted to insoluble silicates or hydroxides and are incorporated into the
interstices of the binder matrix, thereby inhibiting leaching.
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4.3 Description of Stabilization Process
The stabilization process consists of a weighing device, a mixing unit, and
a curing vessel or pad. Commercial concrete mixing and handling equipment is
typically used in stabilization processes. Weighing conveyors, metering cement
hoppers, and mixers similar to concrete batching plants have been adapted in some
operations. When extremely dangerous materials are treated, remote-control and
in-drum mixing equipment, such as that used with nuclear waste, is employed.
In most stabilization processes, the waste, stabilizing agent, and other
additives, if used, are mixed in a mixing vessel and then transferred to a curing
vessel or pad and allowed to cure. The actual operation (equipment requirements
and process sequencing) depends on several factors including the nature of the
waste, the quantity of the waste, the location of the waste in relation to the
disposal site, the particular stabilization formulation used, and the curing
rate. Following curing, the stabilized solid formed is recovered from the
processing equipment and disposed of.
4.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether stabilization will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the concentration of fine particulates, (b) the
concentration of oil and grease, (c) the concentration of organic compounds,
(d) the concentration of sulfate and chloride compounds, and (e) the solubility
of the metal compound.
4.4.1 Concentration of Fine Particulates
For both cement-based and lime/pozzolan-based processes, very fine solid
materials (i.e., those that pass through a No. 200 mesh sieve (less than 74 /j m
particle size)) weaken the bonding between waste particles and the cement or
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lime/pozzolan binder by coating the particles. This coating inhibits chemical
bond formation, thereby decreasing the resistance of the material to leaching.
If the concentration of fine particulates in an untested waste is significantly
higher than that in the tested waste, the system may not achieve the same
performance. Pretreatment of the waste may be required to reduce the fine
particulate concentration and achieve the same treatment performance, or other,
more applicable treatment technologies may need to be considered for treatment
of the untested waste.
4.4.2 Concentration of 011 and Grease
Oil and grease in both cement-based and lime/pozzolan-based systems result
in the coating of waste particles and the weakening of the bond between the
particle and the stabilizing agent, thereby decreasing the resistance of the
material to leaching. If the concentration of oil and grease in the untested
waste is significantly higher than that in the tested waste, the system may not
achieve the same performance. Pretreatment may be required to reduce the oil and
grease concentration and achieve the same treatment performance, or other, more
applicable treatment technologies may need to be considered for treatment of the
untested waste.
4.4.3 Concentration of Organic Compounds
Organic compounds in the waste interfere with the stabilization chemical
reactions and bond formation, thus inhibiting curing of the stabilized material.
This interference results in a stabilized waste having decreased resistance to
leaching. If the total organic carbon (TOC) content of the untested waste is
significantly higher than that of the tested waste, the system may not achieve
the same performance. Pretreatment may be required to reduce the TOC and achieve
the same treatment performance, or other, more applicable treatment technologies
may need to be considered for treatment of the untested waste.
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4.4.4 Concentration of Sulfate and Chloride Compounds
Sulfate and chloride compounds interfere with the stabilization chemical
reactions, weakening bond strength and prolonging setting and curing time.
Sulfate and chloride compounds may reduce the dimensional stability of the cured
matrix, thereby increasing Teachability potential. If the concentration of
sulfate and chloride compounds in the untested waste is significantly higher than
that in the tested waste, the system may not achieve the same performance.
Pretreatment may be required to reduce the sulfate and chloride concentrations
and achieve the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
4.4.5 Solubility of the Metal Compound
The metal to be stabilized should be in its least soluble state, or the
stabilized waste may exhibit a potential for increased Teachability.
Pretreatment may be required to chemically reduce or oxidize the metal to a lower
solubility state and achieve maximum stabilization performance. For example,
hexavalent chromium is much more soluble and more difficult to stabilize than
trivalent chromium.
4.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a
stabilization system, EPA examines the following parameters: (a) the amount and
type of stabilizing agent and additives, (b) the degree of mixing, (c) the
residence time, (d) the stabilization temperature and humidity, and (e) the form
of the metal compound.
4.5.1 Amount and Type of Stabilizing Agent and Additives
The stabilizing agent and additives used will determine the chemistry and
structure of the stabilized material and therefore its Teachability. Stabilizing
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agents and additives must be carefully selected based on the chemical and
physical characteristics of the waste to be stabilized. To select the most
effective type of stabilizing agent and additives, the waste should be tested in
the laboratory with a variety of these materials to determine the best
combination.
The amount of stabilizing agent and additives is a critical parameter in
that sufficient stabilizing materials are necessary to properly bind the waste
constituents of concern, making them less susceptible to leaching. The
appropriate weight ratios of stabilizing agent and additives to waste are
established empirically by setting up a series of laboratory tests that allow
separate leachate testing of different mix ratios. The ratio of water to
stabilizing agent (including water in waste) will also impact the strength and
leaching characteristics of the stabilized material. Too much water will cause
low strength; too little will make mixing difficult and, more important, may not
allow the chemical reactions that bind the hazardous constituents to be fully
completed. EPA evaluates the amount of stabilizing agent, water, and other
additives used in the stabilization process to ensure that sufficient stabilizing
materials are added to the waste to effectively immobilize the waste constituents
of concern.
4.5.2 Degree of Mixing
Mixing is necessary to ensure homogeneous distribution of the waste,
stabilizing agent, and additives. Both under-mixing and overmixing are
undesirable. The first condition results in a nonhomogeneous mixture;
consequently, areas will exist within the waste where waste particles are neither
chemically bonded to the stabilizing agent nor physically held within the lattice
structure. Overmixing, on the other hand, may inhibit gel formation and ion
adsorption in some stabilization systems. Optimal mixing conditions are usually
determined through laboratory tests. The quantifiable degree of mixing is a
complex assessment that includes, among other factors, the amount of energy
supplied, the length of time the material is mixed, and the related turbulence
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effects of the specific size and shape of the mix tank or vessel. The degree of
mixing is beyond the scope of simple measurement. EPA, however, evaluates the
degree of mixing qualitatively by considering whether mixing is provided and
whether the type of mixing device is one that could be expected to achieve
homogeneous distribution of the waste, stabilizing agent, and additives.
4.5.3 Residence Time
The residence time or duration of curing ensures that the waste particles
have had sufficient time in which to incorporate into lattice structures and/or
form stable chemical bonds. The time necessary for complete stabilization
depends on the waste and the stabilization process used. The performance of the
stabilized waste (i.e., the levels of waste constituents in the leachate) will
be highly dependent on whether complete stabilization has occurred. Typical
residence times range from 7 to 28 days. EPA monitors the residence time to
ensure that sufficient time is provided to effectively stabilize the waste.
4.5.4 Stabilization Temperature and Humidity
Higher temperatures and lower humidity increase the rate of curing by
increasing the rate of evaporation of water from the stabilization mixtures. If
temperatures are too high, however, the evaporation rate can be excessive,
resulting in too little water being available for completion of the stabilization
reaction. EPA monitors the stabilization temperature and humidity continuously,
if possible, to ensure that the system is operating at the appropriate design
conditions and to diagnose operational problems.
4.5.5 Form of the Metal Compound
Ideally, the waste metal to be stabilized should be in its least soluble
state to reduce leaching of the stabilized waste. Pretreatment such as chemical
oxidation or chemical reduction may be required to oxidize or reduce the metal
to a state of lower solubility. Additionally, the solubility of the metal can
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be decreased by precipitating it with an appropriate compound. For example,
ferric arsenate is less soluble and less Teachable than calcium arsenate. EPA
can monitor solubility of the metal by performing a standard leaching test
(Extraction Procedure (EP) or Toxicity Characteristic Leaching Procedure (TCLP)).
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4.6 References
Ajax Floor Products Corp. n.d. Product literature: technical data sheets,
Hazardous Waste Disposal System. P.O. Box 161, Great Meadows, N.J. 07838.
Austin, G.T. 1984. Shreve's chemical process industries. 5th ed. New York:
McGraw-Hill Book Co.
Bishop, P.L., Ransom, S.B., and Grass, D.L. 1983. Fixation mechanisms in
solidification/stabilization of inorganic hazardous wastes. In
Proceedings of the 38th Industrial Waste Conference. May 10-12, 1983, at
Purdue University, West Lafayette, Indiana.
Conner, J.R. 1986. Fixation and solidification of wastes. Chemical
Engineering. Nov. 10, 1986.
Cullinane, M.J., Jr., Jones, L.W., and Malone, P.G. 1986. Handbook for
stabilization/solidification of hazardous waste. U.S. Army Engineer
Waterways Experiment Station. EPA report no. 540/2-86/001. Cincinnati,
Ohio: U.S. Environmental Protection Agency.
Electric Power Research Institute. 1980. FGD sludge disposal, annual. 2nd ed.
Prepared by Michael Baker Jr., Inc. EPRI CS-1515 Project 1685-1. Palo
Alto, Calif.: Electric Power Research Institute.
Malone, P.G., Jones, L.W., and Burkes, J.P. Application of solidification/
stabilization technology to electroplating wastes. Office of Water and
Waste Management. SW-873. Washington, D.C.: U.S. Environmental Pro-
tection Agency.
Mishuck, E., Taylor, D.R., Telles, R., and Lubowitz, H. 1984. Encapsulation/
fixation (E/F) mechanisms. Report no. DRXTH-TE-CR-84298. Prepared by
S-Cubed under Contract no. DAAK11-81-C-0164.
Pojasek RB. 1979. Sol id-waste disposal: solidification. Chemical Engineering
86(17): 141-145.
USEPA. 1980. U.S. Environmental Protection Agency. U.S. Army Engineer
Waterways Experiment Station. Guide to the disposal of chemically
stabilized and solidified waste. Prepared for MERL/ORD under Interagency
Agreement No. EPA-IAG-D4-0569. PB 81 181 505. Cincinnati, Ohio: U.S.
Environmental Protection Agency.
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5. VITRIFICATION
5.1 Applicability
Vitrification technologies include glass and slag vitrification and
elevated-temperature calcination processes. Vitrification processes involve
dissolving the waste at high temperatures into glass or a glass!ike matrix.
Calcination involves merely heating the material at high temperatures.
High-temperature vitrification is applicable to nonwastewaters containing
arsenic* or other characteristic toxic metal constituents that are relatively
nonvolatile at the temperatures at which the process is operated. This
technology is also applicable to many wastes containing organometallic compounds,
where the organic portion of the compound can be completely oxidized at
process-operating conditions. Afterburners may be required to convert unburned
organics to carbon dioxide.
The process is not generally applicable to volatile metallic compounds or
to wastes containing high levels of constituents that will interfere with the
vitrification process. High levels of chlorides and other halogen salts should
be avoided in the wastes being processed because they interfere with glassmaking
processes and cause corrosion problems.
Calcination processes are applicable to inorganic wastes that do not
contain volatile constituents.
*Volatile arsenic compounds are usually converted to nonvolatile arsenate salts
such as calcium arsenate prior to use of this process. In ordinary glass-
making, arsenic volatilization problems are minimized by adding arsenic
as arsenate salts.
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5.2 Underlying Principles of Operation
The basic principles of operation for vitrification technologies depend on
the technology used. In glass and slag vitrification processes, the waste con-
stituents become chemically bonded inside a glasslike matrix in many cases. In
all instances, the waste becomes surrounded by a glass matrix that immobilizes
the waste constituents and retards or prevents their reintroduction into the
environment. Arsenates are converted to silicoarsenates, and other metals are
converted to silicates.
High-temperature calcination processes remove water of hydration from the
toxic metal-bearing solids, convert hydroxides present to oxides, and sinter the
material, reducing its surface area to a minimum. Conversion of hydroxides to
oxides and minimization of available surface area retard surface reactions that
would reintroduce the material into the environment. Calcination may also be
accompanied by chemical reaction if a material such as lime is blended with the
waste before it is heated. For example, lime will react with arsenic oxides at
higher temperatures to form calcium arsenate, and this material will then be
sintered at elevated temperatures. Brief descriptions of each of the
high-temperature processes are given in the following subsections.
5.2.1 Glass Vitrification
In the glass vitrification process, the waste and normal glassmaking
constituents are first blended together and then fed to a glassmaking furnace,
where the mixed feed materials are introduced into a pool of molten glass. The
feed materials then react with each other to form additional molten glass, in
which particles of the waste material become dissolved or suspended. The molten
glass is subsequently cooled. As it cools, it solidifies into a solid mass that
contains the dissolved and/or suspended waste constituents. Entrapment and
chemical bonding within the glass matrix render the waste constituents
unavailable for reaction.
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5.2.2 Slag Vitrification
Slag vitrification differs from glass vitrification in that finely ground
slag from metal-refining processes and waste are premixed and fed to the same
type of furnace as that used for glassmaking. The slag liquifies at the process
temperature (1100 to 1200'C), and the waste constituents either dissolve or
become suspended in the molten slag. Subsequent cooling of the slag causes it
to solidify, trapping the waste inside a glasslike matrix and rendering it
unavailable for chemical reaction or migration into the environment.
5.2.3 High-Temperature Calcination
In the high-temperature calcination process, the waste is heated in a
furnace or kiln to between 400 and 800'C. In some instances, the waste may be
blended with lime prior to heating. In those cases, chemical reaction may occur
during the calcining process. Water present as either free water or water of
hydration is evaporated, and hydroxides present are thermally decomposed to the
corresponding oxides and water vapor. At the higher temperatures, the surface
area of the dehydrated material is decreased by thermal sintering. Conversion
of hydroxides to oxides and substantial losses of surface area render the
material less reactive in the environment and lower the Teachability of
characteristic toxic metals present. In general, the higher the calcination
temperatures used, the more complete the loss of water and the greater the
accompanying loss of surface area, resulting in lower Teachability potential.
5.3 Description of Vitrification Processes
This chapter discusses three types of high-temperature stabilization
processes, which differ considerably from each other. Individual process
descriptions are given in the following subsections.
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5.3.1 Glass Vitrification
Soda ash, lime, silica, boron oxide, and other glassmaking constituents are
first blended with the waste to be treated. The amount of waste added to the
blend is dependent on the waste composition. Different metal oxides have
differing solubility limits in glass matrices. The blended waste and glass raw
material mixture is then fed to a conventional, heated glass electric furnace.
The introduced material typically is added through a port at the top of the
furnace and falls into a pool of molten glass. The glass constituents dissolve
in the molten glass and form additional glass. Molten glass is periodically
withdrawn from the bottom of the furnace and cooled. This material then
solidifies on cooling into solid blocks of glasslike material. Organics present
in the feed mixture undergo combustion at the normal operating temperatures of
1100 to 1400'C and are fully oxidized to carbon dioxide and water vapor.
The top of the furnace is normally cooled so that volatile materials, such
as arsenic oxides, that are present in the feed mixtures can condense on the
cooled surface and fall back into the melt, where they can undergo chemical
reaction to form silicoarsenates involved in the glassmaking process. Most of
the arsenic used in making glass by this method is present as salts such as
calcium arsenate. This approach was introduced into the glass industry to
minimize fugitive arsenic losses.
Gases, such as carbon dioxide, that are liberated during the glassmaking
process exit the furnace through the top and are generally wet-scrubbed prior to
reentering the atmosphere.
5.3.2 Slag Vitrification
The slag vitrification process is basically similar to glass vitrification
except that granulated slag, instead of the normal glassmaking constituents, is
blended with the waste for feed to the system. A pool of liquid slag is present
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in the furnace, and the blended raw material mix typically is introduced at the
top of the furnace and falls into this molten slag. The granulated slag-waste
mixture liquifies to form additional slag. Slag is periodically withdrawn from
the slag pool and cooled into blocks.
The type of furnace used for glass vitrification can also be used for slag
vitrification. The operating parameters are similar.
5.3.3 High-Temperature Calcination
In the high-temperature calcination process, wastes containing inorganic
compounds are fed to ovens or kilns, where they are heated to high temperatures
(i.e., 500 to 900*C) to drive off water of hydration and to convert hydroxides
present to the corresponding oxides. This process is primarily applicable to
inorganic wastes that contain nonvolatile constituents.
The waste is heated in the oven or kiln to the desired temperature,
moisture and water of hydration are driven off; and, at the 500 to 900*C
temperature range, hydroxides decompose to the corresponding oxides and water
vapor. The high-temperature treatment also significantly reduces the surface
areas of the oxides formed by sintering, thereby reducing the reactivity of the
material. After the waste material has been calcined at an elevated temperature,
it is withdrawn from the oven or kiln, cooled, and either land disposed or
forwarded to another process, such as stabilization, for further treatment.
5.4 Haste Characteristics Affecting Performance fWCAPs)
The waste characteristics affecting performance are different for the two
vitrification processes and the calcination process. Accordingly, they are
discussed separately in the following subsections.
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5.4.1 Waste Characteristics Affecting Performance (WCAPs)
In determining whether vitrification will achieve the same level of
performance on an untested waste as on a previously tested waste, and whether
performance levels can be transferred, EPA examines the following waste
characteristics that affect performance of the vitrification processes:
(a) organic content of the waste, (b) concentrations of specific metal ions in
the waste, (c) concentrations of compounds in the waste that interfere with the
glassmaking process, and (d) moisture content of the waste.
5.4.1.1 Organic Content
At process operating temperatures (1100 to 1400'C), organics are combusted
to carbon dioxide, water, and other gaseous products. The combustion process
liberates heat, reducing the external energy requirements for the process.
The amount of heat liberated by combustion is a function of the Btu value
of the waste. The Btu content merely changes the energy input needs for the
process and does not affect waste treatment performance. The amount of material
that may not oxidize completely is a function of the organic halogen content of
the waste. The presence of these halogenated organics does impact process
performance because sodium chloride has a low solubility in glass. The presence
of high chlorides results in a porous glass that is undesirable. If the
halogenated organic content of an untested waste is the same as or less than that
present in an already tested waste, the system should achieve the same
performance for organic destruction.
5.4.1.2 Concentrations of Specific Metal Ions
Most metal oxides have solubility limits in glass matrices. Hence, their
concentration determines the amount of glass-forming materials or slag with which
they must be reacted in this process to generate a nonleaching slag or glass.
The solubility limits of most common metal oxides and salts in glass are found
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in the Handbook of Glass Manufacture and other treatises on glass production.
Oxides for which extensive solubility information is available are alumina,
antimony oxide, arsenic oxides, barium oxide, cadmium oxide, chromium oxides,
copper oxides, cobalt oxides, iron oxides, lead oxides, manganese oxides, nickel
oxides, selenium oxides, tin oxides, and zinc oxides. Analysis for individual
metal concentrations in the waste can be performed according to EPA-approved
methods. If the concentrations of specific metals in an untested waste are less
than those in a tested waste, then the same ratio of slag or glass raw materials
to waste may be used for vitrification purposes. If, however, the concentration
of metal is greater than that in the tested waste, a different formulation must
be used.
5.4.1.3 Concentrations of Deleterious Materials
Some waste constituents, such as chlorides, fluorides, and sulfates,
interfere with the vitrification process if they are present at high levels.
These salts have limited solubilities in glass; therefore, when they are present,
additional glass-forming raw materials must be added to compensate for their
presence. The solubility limits of various salts in glasses are discussed in
references on glass production such as the Handbook of Glass Manufacture.
Generally, if the concentrations of such materials in an untested waste are lower
than those in a tested waste, then the same ratio of glass-forming constituents
to waste may be used. Reducing agents such as carbon or ferrous salts reduce
arsenates and selenates to lower valence compounds that are more volatile. These
compounds should not be present in significant quantities in arsenic- or
selenium-containing wastes to be vitrified.
5.4.1.4 Moisture Content
Materials fed to the vitrification process should be reasonably dry (i.e.,
contain less than 5 percent free moisture). If a waste has excess moisture above
this level, it should be thermally dried before it is blended with glass-forming
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materials; otherwise, it may react violently when introduced to the molten glass
or slag pool.
5.4.2 Waste Characteristics Affecting Performance of High-Temperature
Calcination
In determining whether high-temperature calcination will achieve the same
level of performance on an untested waste as on a previously tested waste and
whether performance levels can be transferred, EPA examines the following waste
characteristics that impact the performance of the high-temperature calcination
process: (a) the organic content of the waste, (b) the moisture content of the
waste, and (c) the inorganic composition of the waste. These characteristics are
discussed below.
5.4.2.1 Organic Content
Calcination temperatures normally used are too low to initiate combustion
of some types of organic compounds. However, they are high enough to cause
volatilization of organics, which have to be removed from process off-gases. For
these reasons, the presence of significant levels of organics is undesirable but
can be handled with appropriate air pollution controls. Organics, per se, do not
interfere with the conversion of oxides to hydroxides or with sintering
processes. Afterburners may be required on vitrification units managing high-
organic-content wastes to ensure complete combustion of the organics present.
5.4.2.2 Moisture Content
Excess water that has to be removed from the waste by heating increases the
amount of time needed to bring the waste to the calcination temperature. For
this reason, wastes with high moisture content should be dewatered prior to the
use of this process.
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5.4.2.3 Inorganic Composition
Calcination temperatures are normally selected based on the temperatures
at which hydroxides are thermally decomposed to the corresponding oxides and
water vapor. To select an optimum operating temperature, one should know the
approximate composition of the waste. A few toxic metal oxides have fairly low
volatilization temperatures. Arsenic oxide, selenium dioxide, and mercuric oxide
all volatilize below 500*C. High-temperature calcination should not be used for
wastes that contain these volatile constituents unless the wastes are blended
with materials such as lime, which will react with the constituents before they
can vaporize. Nonvolatile arsenic compounds such as ferric and calcium arsenates
can be calcined without concern for vaporization of material.
5.5 Design and Operating Parameters
The design and operating parameters for the vitrification processes are
similar to each other, but differ considerably from those for high-temperature
calcination. The following subsections discuss these two technologies
separately.
5.5.1 Design and Operating Parameters for Vitrification Processes
In assessing the effectiveness of the design and operation of a
vitrification system, EPA examines the following parameters: (a) the composition
of the vitrifying agent, (b) the operating temperature, (c) the residence time,
and (d) the vitrification furnace design.
5.5.1.1 Composition of the Vitrifying Agent
Slag and various glassmaking formulations are used as vitrifying agents.
The choice of the vitrifying agent is determined by the solubility of the waste
constituents to be vitrified. Different inorganic oxides have differing
solubilities in various glass matrices.
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For slags, the presence of carbon or other reducing agents is undesirable
when arsenic-bearing or selenium-bearing wastes are vitrified. Carbon or ferrous
salts in the slag reduce arsenates in the waste to arsenic trioxide, which has
a low volatilization temperature. In a similar manner, these same reducing
agents reduce selenates to elemental selenium, which also has a low
volatilization temperature. When slags are used for vitrifying arsenic-bearing
and selenium-bearing wastes, EPA monitors the composition of the granulated slags
to ensure that they do not contain significant concentrations of carbon or
ferrous salts.
In glass vitrification, various glassmaking formulations can be used. EPA
examines the proposed formulations to ensure that the toxic metal ion
concentrations of the final product do not exceed solubility limits. Hence, EPA
examines the material balances based on waste composition and glassmaking
additives and the published solubility limits for metal oxides in various glasses
to ensure that the vitrified product is indeed a glass containing the solubilized
toxic waste constituents.
5.5.1.2 Operating Temperature
Vitrification furnaces are normally operated in the 1100 to 1400*C range.
The exact operating temperature is usually selected based on the desired
composition of the final product. Furnaces are normally equipped with automatic
temperature control systems. EPA examines the basis of choice for operating
temperatures and the nature and physical condition of the temperature monitoring
and control equipment to ensure that the system is being properly operated.
5.5.1.3 Residence Time
Sufficient time must be allowed for the materials added to glass furnaces
to reach operating temperatures and then undergo the chemical reactions needed
to produce glasses. Residence times are normally on the order of 1 to 2 hours
for processes operated at 1100 to 1200*C. For glasses or slags requiring
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slightly higher temperatures, slightly longer residence times are usually
selected. EPA examines the basis for the facility's choice of residence times
to ensure that the system is well operated.
5.5.1.4 Vitrification Furnace Design
Vitrification furnaces normally incorporate the following design features:
• Withdrawal of the product in liquid form from the base of the furnace.
• Maintenance of a liquid pool of product in the furnace.
• Addition of product constituent mix at the top of the furnace.
• Design of the top area of the furnace in a manner that allows for
cooling of this area. (This is important because volatile constituents
of the input feed may vaporize from the melt. The cool top area allows
these constituents to condense and fall back into the melt.)
• Presence and proper operation of an air emissions control afterburner
and scrubbing system to manage vent gas emissions from the system such
as volatilized noncombusted organics and hydrogen chloride vapors from
combustion of any chlorinated organics present.
EPA examines the furnaces to be used for waste vitrification to ensure that
the design features mentioned above are present since they are important for
proper operation of the systems.
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5.5.2 Design and Operating Parameters for High-Temperature Calcination
Systems
In assessing the effectiveness of the design and operation of a
high-temperature calcination process, EPA examines the following parameters:
(a) the operating temperature, (b) the residence time, and (c) the air emission
control units in place on the ovens or kilns used.
5.5.2.1 Operating Temperature
Calcination temperatures of from 500 to 900*C have been used in industry.
The calcination temperature selected is generally a temperature above which metal
hydroxides present will decompose to the corresponding oxides. Data on
decomposition temperatures of some metal hydroxides are given in Table 2. The
temperature chosen is normally high enough to cause extensive sintering (surface
area loss) of the oxides formed, while at the same time not volatilizing these
materials. EPA examines the technical basis for selection of the calcining
temperature to determine whether the system is properly operated. EPA also
examines the temperature monitoring and control systems in place to determine
whether they are properly operated and reliable.
Table 2 Metal Hydroxide Decomposition Temperatures
Metal hydroxide Decomposition temperature (*C)
Cadmium hydroxide 300
Chromic acid 400
Lead hydroxide 145
Nickel hydroxide 230
Zinc hydroxide 125
Source: Weast 1977.
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5.5.2.2 Residence Time
Calcination is generally a batch process, and sufficient time must be
allowed for samples to be brought to the operating temperature. Residence times
of several hours are normally used to minimize the effects of heat-up time. EPA
examines the technical basis for the choice of residence time to ensure that
sufficient heating at the required temperature is allowed to complete the
dehydration and sintering processes. Residence time is a function of the time
needed to bring the calcination furnace or kiln to the desired temperature and
the time needed to complete the dehydration and sintering processes at the
selected temperature.
5.5.2.3 Air Emission Control Systems
During the calcination process, water vapor is driven off as it is formed
by the decomposition of hydroxides present in the waste. These hot gases exit
the calcination furnace or kiln as they are formed. Some particulates from the
waste material, as well as organics present in the waste, may become entrained
in these vent gases. Therefore, for air pollution control purposes, the
calcination units must be equipped with wet or dry particulate collection systems
that are properly designed and operated. If wastes containing organics are
processed by high-temperature calcination, the calcination furnaces need to be
equipped with afterburners to combust organic vapors emitted. EPA examines the
design basis, physical condition, and maintenance of these air emission control
units to determine whether they are properly designed, maintained, and operated.
EPA also examines the system of management in use for handling and subsequent
treatment of the collected particulates to ensure that these finely divided waste
particles are properly managed.
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5.6 References
Penberthy, L. 1984. Electric melting of glass. In Handbook of glass
manufacture. Vol. I, pp. 387-399. New York: Ashlee Publishing Co.
Penberthy, L. 1981. Method and apparatus for converting hazardous material to
a relatively harmless condition. U.S. Patent 4299.611. November 10,
1981.
Steitz, W.R., Hibscher, C.W., and Mattocks, G.R. 1984. Electric melting of
glass. In Handbook of glass manufacture. Vol. I, pp. 400-428. New York:
Ashlee Publishing Co.
Tooley, F.V. 1984. Raw materials. In Handbook of glass manufacture. Vol. I,
pp. 19-56. New York: Ashlee Publishing Co.
Twidwell, L.G., and Mehta, A.K. 1985. Disposal of arsenic bearing copper
smelter flue dust. In Nuclear and chemical waste management. Vol. 5, pp.
297-303. Butte, Mont.: Montana College of Mineral Science and
Technology.
Weast, R.C., ed. 1978. Handbook of chemistry and physics. 58th ed. Cleveland,
Ohio: CRC Press.
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II. REMOVAL TECHNOLOGIES
A. RECOVERY, REUSE*, AND/OR SEPARATION TECHNOLOGIES
FOR ORGANICS
*The Agency encourages pollution prevention efforts such as those in recovery and reuse practices (see 40 CFR
Part 261.1 for the definition of reuse, reclaimation, and recycling). However, the Agency recognizes that
currently there is some confusion over the definition of what constitutes a solid waste that has led to the
proliferation of so-called "sham recycling" by some facilities. Sham recycling are illegitimate waste
management practices whereby hazardous waste materials having no saleable value (no market for reuse) are
claimed to be recycled/reused in order to escape the stringent RCRA hazardous waste definitions.
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1. CARBON ADSORPTION
1.1 Applicability
Carbon adsorption is a treatment technology used to treat wastewaters
containing dissolved organics at concentrations less than about 5 percent and,
to a lesser extent, dissolved metal and other inorganic contaminants. The most
effective metals removal is achieved with metal complexes.
The two most common carbon adsorption processes are the granular activated
carbon (GAC), which is used in packed beds, and the powdered activated carbon
(PAC), which is added loosely to wastewater. This section discusses the GAC
process; the PAC process is discussed in Section I.A.I of this document, i.e.,
Biological Treatment.
1.2 Underlying Principles of Operation
The basic principle of operation for carbon adsorption is the mass transfer
and adsorption of a molecule from a liquid or gas onto a solid surface.
Activated carbon is manufactured in such a way as to produce extremely porous
carbon particles whose internal surface area is very large (500 to 1,400 square
meters per gram of carbon). This porous structure attracts and holds (adsorbs)
organic molecules as well as certain metal and inorganic molecules.
Adsorption occurs because (a) the contaminant has a low solubility in the
waste, (b) the contaminant has a greater affinity for the carbon than for the
waste, or (c) a combination of the two. The amount of contaminants that can be
adsorbed by activated carbon ranges from 0.10 to 0.15 gram per gram of carbon.
1.3 Description of Carbon Adsorption Process
In GAC systems, the carbon is packed in a column and the wastewater is
passed through the carbon bed(s). The flow can be either down or up through the
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vertical column(s). Figure 13 shows two carbon adsorption systems. In the
carbon adsorption process, the wastewater is passed through a stationary bed of
carbon. The contaminants in the wastewater are adsorbed most rapidly and
effectively by the upper layers of carbon during the initial stages of operation.
These upper layers are in contact with the wastewater at its highest
concentrations of contaminants. The small amounts of the contaminants that are
not adsorbed in the first few layers of the activated carbon bed are removed from
solution in the lower or downstream portion of the bed. Initially, none of the
contaminants escapes from the carbon bed.
As the wastewater flows down the column (or the location in the column
where the majority of adsorption is occurring) and the adsorption capacity is
reached in the top layers, the adsorption zone moves down the column. As the
adsorption zone approaches the end of the carbon bed, the concentration in the
effluent rapidly approaches the influent concentration. This point in the
process is referred to as breakthrough. A breakthrough curve (Figure 14) shows
the plot of the ratio of effluent to influent concentrations versus time of
process operation. At breakthrough, the adsorptive capacity of the carbon bed
is exhausted, and little additional removal of contaminants occurs. Treated
wastewater is then either treated further in another carbon adsorption column,
if necessary, or disposed of.
Once the carbon bed is spent and can no longer remove contaminants from the
waste, it is taken off-line. The activated carbon is then either regenerated by
thermal or chemical methods for further use or treated by incineration and
disposed of. If carbon adsorption is used to treat very toxic or hazardous
materials, the spent carbon usually is incinerated and disposed of directly.
Regeneration is accomplished thermally by heating the carbon to a
temperature (between 1,500 and 1,700'F) at which most of the adsorbed
contaminants are volatilized and destroyed but which is not high enough to burn
the surface of the carbon. About 4 to 9 percent of the carbon is lost in this
process. Steam can also be used to regenerate carbon by volatilizing adsorbed
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WASTEWATER
INFLUENT
ro
-•4
TREATED
EFFLUENT
TO DISPOSAL
GRANULAR
ACTIVATED
CARBON
;•»«.:
OOWNFLOW SERIES ARRANGEMENT
TREATED
EFFLUENT WASTEWATER
TO INFLUENT
DISPOSAL
UPFLOW SERIES ARRANGEMENT
Figure 13 Carbon Adsorption Systems
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WASTEWATER
INFLUENT
ro
oo
u
1
ADSORPTION
ZONE
EFFLUENT
\J
I
V*
1
B
\f
J
1.0
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EFFLUENT TO INFLUENT
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Figure 14 Plot of Breakthrough Curve
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organics for subsequent condensation, recovery, and reuse or for treatment and
disposal. Chemical regeneration involves the use of an acid, alkali, or organic
solvent to redissolve contaminants for subsequent recovery and reuse or for
further treatment and disposal. There is a loss of performance with each
regeneration of spent carbon because metals (such as calcium, magnesium, and
iron) plug small pores in the carbon and prevent some organic contaminants from
being desorbed at the thermal regeneration temperature. In each thermal
regeneration process, some carbon becomes spent, requiring treatment and
disposal. As a result, makeup carbon has to be added to the regenerated carbon
being placed back in service.
The number of times that the carbon can be regenerated is determined by the
extent of its physical erosion and the loss of its adsorptive capacity. Isotherm
tests can be performed on the regenerated carbon to determine adsorptive
capacity; such tests can thus aid in predicting the number of times the carbon
can be regenerated.
1.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether carbon adsorption will achieve the same level of
performance on an untested waste as it achieved on a previously tested waste and
whether performance levels can be transferred, EPA examines the following waste
characteristics: (a) the waste type and concentration of adsorbable contaminants
and (b) the concentrations of suspended solids and oil and grease.
1.4.1 Waste Type and Concentration of Adsorbable Contaminants
The concentration of adsorbable contaminants is a measure of the fraction
of the wastewater constituents that can be expected to be adsorbed by a carbon
adsorption column. Concentrations of organics in the wastewater greater than
about 5 percent result in excessive activated carbon consumption requiring
frequent regeneration. At the higher organic concentrations other treatment
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technologies, such as wet air oxidation, solvent extraction, or incineration, may
be more appropriate.
Although all organics can be adsorbed to some degree, activated carbon has
a greater affinity for aromatic than for aliphatic compounds and for nonpolar
than for polar compounds. If the type and concentration of adsorbable
contaminants in an untested waste are significantly less adsorbable and higher,
respectively, than in the tested waste, the system may not achieve the same
performance.
1.4.2 Concentrations of Suspended Sol Ids and Oil and Grease
Suspended solids and oil and grease can reduce the effectiveness of carbon
adsorption by clogging and coating the pores, as well as by competing for
adsorption sites, thereby interfering with the treatment of contaminants of
concern. EPA has information showing that this treatment interference occurs
with suspended solids at levels greater than about 50 mg/1 and with oil and
grease at above 10 mg/1.
1.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a carbon
adsorption system, EPA examines the following parameters: (a) the type and pore
size of the carbon particles, (b) the adsorption temperature, (c) the pH, and
(d) the hydraulic loading rate.
For many hazardous organic constituents, analytical methods are not
available or the constituent cannot be analyzed in the waste matrix. Therefore,
it would normally be impossible to measure the effectiveness of the carbon
adsorption treatment system. In these cases EPA tries to identify measurable
parameters or constituents that would act as surrogates in order to verify
treatment.
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For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the carbon adsorption treatment
system will indicate removal of organic constituents. Hence, TOC analysis is
likely to be an adequate surrogate analysis where the specific organic
constituent cannot be measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden (i.e., the TOC
analysis may not be sensitive enough to detect changes at the milligrams/liter
(mg/1) level in matrices where total organic concentrations are hundreds or
thousands of mg/1). In these cases, other surrogate parameters should be sought.
For example, if a specific analyzable constituent is expected to be treated as
well as the unanalyzable constituent, the analyzable constituent concentration
should be monitored as a surrogate.
1.5.1 Type and Pore Size of Carbon Particles
Activated carbon is made from a variety of substances (e.g., coal, wood),
ground to many different sizes, and manufactured with a number of different pore
sizes. The pore size determines the surface area available for adsorption and,
hence, the carbon's adsorptive capacity. The type and pore size of carbon
particles exhibit different adsorptive capacities for different contaminants.
Another property that is important in assessing the effectiveness of carbon
particles is the iodine number; this value is an indicator of the adsorptive
capacity for low-molecular-weight-organics. Laboratory bench testing is used to
determine the most effective type and pore size of carbon particles for treating
particular wastewaters. EPA examines the type and pore size of the carbon
particles used to ensure that effective adsorption is achieved.
1.5.2 Adsorption Temperature
As the temperature increases, the solubility of the contaminants generally
increases as well, which results in less effective adsorption. EPA monitors the
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temperature continuously, if possible, to ensure that the system is operating at
the appropriate design condition and to diagnose operational problems.
1.5.3 pH
The pH impacts both the solubility of the various contaminants and the
potential for chemical bonding to occur. EPA monitors the pH continuously, if
possible, to ensure that the system is operating at the appropriate design
conditions and to diagnose operational problems.
1.5.4 Hydraulic Loading Rate
The amount of time that the waste contaminants are in contact with the
carbon particles (i.e., residence time) impacts the extent to which adsorption
occurs. Higher residence times generally improve adsorption performance but
require longer carbon beds to maintain the same overall throughput. Typical
residence times for GAC adsorption systems range from 30 to 100 minutes. For a
given size carbon bed, the residence time can be determined by the hydraulic
loading rate. Typical hydraulic loading rates for downflow adsorption systems
range from 0.5 to 8.0 gal/min-ft2, while upflow systems typically operate around
15 gal/min-ft2. EPA monitors the hydraulic loading rate to ensure that
sufficient time is provided to effectively adsorb contaminants.
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1.6 References
Authur D. Little, Inc. 1977. Physical, chemical and biological treatment
techniques for industrial wastes. Vol. I., pp. 1-1 to 1-18 and 1-37 to
1-41. NTIS, PB275-054.
DeJohn, P. B. 1975. Carbon from lignite or coal: Which is better? Chemical
Engineering. .April 28.
Eckenfelder, W.W. Jr., Patoczka, J., and Watkins A. 1985. Wastewater treatment.
Chemical Engineering. September 2, 1985.
GCA Corp. 1984. Technical assessment of treatment alternatives for wastes
containing halogenated orqanics. Prepared for U.S. Environmental Pro-
tection Agency, Contract No. 68-01-6871, pp. 150-160.
Metcalf & Eddy, Inc. 1985. Briefing: Technologies applicable to hazardous
waste. Prepared for U.S. Environmental Protection Agency, Office of
Research and Development, Hazardous Waste Engineering Research Laboratory.
Section 2.13.
Patterson, J. W. 1985. Industrial wastewater treatment technology. 2nd ed.
Stoneham, Mass.: Butterworth Publishers, pp. 329-340.
Touhill, Shuckrow & Assoc. 1981. Concentration technologies for hazardous
aqueous waste treatment. NTIS, PB81-150583. pp. 53-55.
USEPA. 1973. U.S. Environmental Protection Agency. Process design manual for
carbon adsorption. NTIS, PB227-157. pp. 3-21 and 53.
USEPA. 1982. U.S. Environmental Protection Agency. Development Document for
the Porcelain Enameling Point Source Category, pp. 172-241. Washington,
D.C.: U.S. Environmental Protection Agency.
USEPA. 1986. U.S. Environmental Protection Agency. Best demonstrated
available technology (BOAT) background document for F001-F005 Spent
Solvents. Vol. 1, EPA/530-SW-86-056, p. 4-4. Washington, D.C.: U.S.
Environmental Protection Agency.
Versar. 1985 . Versar, Inc. An Overview of Carbon Adsorption. Draft final
report prepared for U.S. Environmental Protection Agency, Exposure
Evaluation Division, Office of Toxic Substances. Contract No. 68-02-3968,
Task No. 58.
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2. DISTILLATION TECHNOLOGIES
2.1 Applicability
Distillation is a thermal treatment technology applicable to the treatment
of wastes containing organics that are volatile enough to be removed by the
application of heat. Constituents that are not volatilized may be reused or
incinerated as appropriate. The four most common distillation processes are
batch distillation, fractionation, steam stripping, and thin film evaporation.
Thermal drying is also included in this section because of its similarity to
distillation processes. Thermal drying uses heat to volatilize water from
wastes.
Batch distillation can be used to treat wastes having a relatively high
percentage of volatile organics. In general, batch distillation is applied to
spent solvent wastes where the wastes are highly concentrated in the solvent and
yield significant amounts of recoverable materials upon separation. Batch
distillation is particularly applicable for wastes that have both very volatile
and very nonvolatile components since the separation of that combination of
components is amenable to the relatively unsophisticated batch distillation
equipment.
Fractionation is typically applied to wastes containing greater than about
7 percent organics. It is designed to achieve the highest degree of distillate
purity of the separated components. Fractionation can be operated to produce
multiple product streams for recovery of more than one organic constituent from
a waste while generating relatively small amounts of residue to be land disposed.
In general, this technology is used where recovery of multiple constituents is
desired and where the waste contains minimal amounts of suspended solids.
Steam stripping is a form of distillation applicable to the treatment of
wastewaters containing organics that are volatile enough to be removed by the
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application of heat using steam as the heat source. Typically, steam stripping
is applied where the waste contains less than 1 percent volatile organics.
Thin film evaporation is typically applied to wastes containing greater
than 40 percent organics. However, the feed stream to the thin film evaporator
must contain low concentrations of suspended solids.
Thermal drying is a treatment technology applicable to solid wastes
typically having a filterable solids content of approximately 40 percent or
greater. Thermal drying removes water and volatile organics from a solid waste
through evaporation. Thermal dryers operate in the range of 300 to 700*F and
usually have mechanical agitation to improve heat transfer. Use of this
technology results in a smaller volume of waste with reduced concentrations of
water and volatile organics.
A thermal dryer or "sludge dryer" is an enclosed device that is typically
used to dehydrate sludge. The dryer's input (from wastes and auxiliary fuel)
usually does not exceed 1500 Btu per pound of wastes treated, compared to 3,300
to 19,000 Btu per pound for incineration.
2.2 Underlying Principles of Operation
The basic principle of operation for distillation processes, i.e., batch
distillation, fractionation, thin film evaporation, and steam stripping, is the
volatilization of hazardous components through the application of heat. The
components that are volatilized are then condensed and typically either reused
or further treated, usually by liquid injection incineration. In thermal drying,
the basic principle of operation for drying is the removal of a liquid from a
solid waste by evaporation. This is similar to distillation in that
volatilization of organic constituents also occurs. However, the primary purpose
of thermal drying is to volatilize water. Liquid constituents will vaporize as
a result of applied heat. In thermal drying, the rate at which liquid
evaporation occurs depends on the thermal conductivity of the solid waste to be
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dried and the boiling points of the volatile liquid constituents to be
evaporated.
An integral part of the theory of the distillation process is the principle
of vapor-liquid equilibrium. When a liquid mixture of two or more components is
heated, the vapor phase present above the liquid phase becomes more concentrated
in the more volatile constituents (those having higher vapor pressures). The
vapor phase above the liquid phase is then removed and cooled to yield a
condensate that is also more concentrated in the more volatile components. The
degree of separation of components depends on the relative differences in the
vapor pressures of the constituents; the larger the difference in the vapor
pressures, the more easily the separation can be accomplished.
If the difference between the vapor pressures is extremely large, a single
separation cycle or a single "equilibrium stage" of vaporization and condensation
may achieve a significant separation of the constituents. In such cases, batch
distillation or thin film evaporation would be used. Typically, batch
distillation units and thin film evaporation units contain only one equilibrium
stage and are thus limited in the degree of separation by the relative
volatilities of the constituents.
If the difference between the vapor pressures of volatile components is
small, then multiple equilibrium stages are needed to achieve effective
separation. In practice, the multiple equilibrium stages are obtained by
stacking "trays" or placing "packing" into a column. Essentially, each tray
represents one equilibrium stage. In a packed steam stripping column or
fractionation column, the individual equilibrium stages are not discernible, but
the number of equivalent trays can be calculated from mathematical relationships.
The vapor phase from a tray rises to the tray above it, where it condenses; the
liquid phase falls to the tray below it, where it is again heated and separated.
The vapor-liquid equilibrium of the waste components can be expressed as
relative volatility, which is the ratio of the vapor-to-liquid concentrations of
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a constituent divided by the ratio of the vapor-to-liquid concentrations of
another constituent. The relative volatility is a direct indicator of the ease
of separation. If the numerical value is 1, then separation is impossible
because the constituents have the same concentrations in the vapor and liquid
phases. When the relative volatility is 1, the liquid mixture is called an
azeotrope. Separation becomes increasingly easier as the value of the relative
volatility becomes increasingly different from unity.
2.3 Description of Distillation Processes
2.3.1 Batch Distillation
A batch distillation unit usually consists of a steam-jacketed vessel, a
condenser, and a product receiver. As the name implies, it is a batch process,
not a continuous process. Figure 15 is a schematic showing the major components
of a batch distillation unit. The steam jacket provides the heat required to
vaporize the volatile constituents in the liquid fraction of the waste. The
rising vapor is collected in the condenser, cooled, and condensed. The liquid
product stream is then routed to the product receiver.
It is important to note that this technology treats wastes by vaporizing
constituents, not destroying them. Accordingly, an integral part of this
technology is a condensation system to collect the organics, as well as an air
emission control system to collect or destroy those organics that are not
condensed. The cooling load of the condenser is calculated in the design to
ensure that the product recovery rate is maximized and emissions from condenser
venting are minimized. The "bottoms," which are the least volatile constituents
of the waste, are withdrawn from the bottom of the batch still. Because batch
distillation is used to remove the volatile organics from wastes, the bottoms are
reduced in volatile organic content. However, prior to disposal, the bottoms
generally require additional treatment, such as incineration, for residual, less
volatile organics.
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VENT OF NON-CONDENSED VAPORS
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
WASTE
INI-LUbNI
i
i
J
(
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OR FURTHER
TREATMENT
STILL BOTTOMS
TO REUSE OR
INCINERATION
Figure 15 Batch Distillation System
138
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2.3.2 Fractionation
Fractionation is a continuous process conducted in a unit that consists of
a reboiler, a column containing stripping and rectification sections, a
condenser, and a "reflux" system. Figure 16 is a schematic showing the major
components of a fractionation unit. The reboiler is a device that provides the
heat required to vaporize the liquid fraction of the waste. It supplies enough
heat to maintain the liquid in the column at its boiling point. The stripping
and rectifying sections are composed of a set of trays in a vertical column. The
discrete trays may be replaced by loose packing consisting of plastic, metal, or
ceramic geometric shapes that provide surface area for the continuous
boiling/condensing that takes place in the column. In the stripping section,
vapor rising from the boiler is contacted with the downflowing liquid feed.
Through this contact, the constituents with lower boiling points (i.e., those
that are more volatile) are concentrated in the vapor. In the rectification
section, the vapor rising above the feed tray is contacted with downflowing
condensed liquid product (reflux). Through this contact, the vapor is further
enriched in the constituents with lower boiling points (i.e., the more volatile
constituents). The rising vapor is collected at the top of the column and
condensed in a condenser. The liquid product stream, except for the portion
returned to the column as reflux, is then routed to a product receiver. The
"bottoms," which are the least volatile components (i.e., those with the highest
boiling points), are continuously withdrawn from the reboiler. Because the
liquid composition varies slightly from one equilibrium stage to the next, it is
also possible to withdraw streams of differing quality (sometimes called
"fractions") from different locations throughout the column. This is typically
done in refining petroleum, resulting in different grades or "cuts" of petroleum
products.
2.3.3 Steam Stripping
Steam stripping is a continuous process conducted in a unit that consists
of a boiler, a stripping column, a condenser, and a collection tank, as shown in
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VENT OF NON-CONDENSED VAPORS
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
i
REFLUX
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WASTE
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t
RECTIFIER
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STRIPPER
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RECEIVER
RECOVERED ORGANICS
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I
REBOIL
BOTTOMS
TO REUSE OR
INCINERATION
REBOILER
Figure 16 Fractional!on System
140
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Figure 17. The boiler provides the heat required to vaporize the liquid fraction
of the waste. The stripping column is composed of a set of trays or packing
placed in a vertical column.
The steam stripping process uses multiple equilibrium stages, with the
initial waste mixture entering at the top, the uppermost equilibrium stage. The
boiler is located beneath the lowermost equilibrium stage, allowing the vapor to
move upward in the column, coming into contact with the falling liquid. As the
vapor comes into contact with the liquid at each stage, the more volatile
components are removed or "stripped" from the liquid by the vapor phase. The
concentration of the emerging vapor is enriched in the more volatile
constituents, and the liquid exiting the bottom of the boiler ("bottoms"), which
is predominantly water, contains high concentrations of the lower vapor pressure
constituents. This effluent from the bottom of the stripper is reduced in
organic content but may still require additional treatment such as carbon
adsorption or biological treatment. The steam and organic vapors exiting the top
of the column are condensed. As condensed liquids, the organic components are
usually immiscible with water. The two immiscible phases are then separated in
a product receiver. Organics in the organic phase are typically recovered or
disposed of in a liquid injection incinerator, while the aqueous condensate is
recycled to the stripper.
2.3.4 Thin Film Evaporation
Thin film evaporation is a continuous process conducted in a unit that
typically consists of a steam-jacketed cylindrical vessel and a condenser.
Figure 18 is a schematic showing the major components of a thin film evaporator.
The steam-heated surface of the cylindrical vessel provides the heat required to
vaporize the volatile constituents in the waste. The evaporator walls are heated
from the outside as the feed trickles down the inside walls in a thin film.
Unique to this form of distillation is the distribution device that spreads the
thin film over the heated surface. The feed rate of waste is controlled to allow
the more volatile material adequate time to vaporize. The heat transfer from the
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STEWATER
INFLUENT
VENT OF NON-CONDENSED VAPORS
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
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TO FURTHER
TREATMENT
ANO/OR DISPOSAL
Figure 17 Steam Stripping System
142
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ROTATING
DRIVE
WASTE
INFLUENT
LIQUID
FILM
VENT OF
NON-CONDENSED VAPORS TO
AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
1
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HEATED
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PRODUCT
RECEIVER
RECOVERED
ORGANICS
TO REUSE
BOTTOMS TO
REUSE OR
INCINERATION
Figure 18 Thin Film Evaporation System
143
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heating medium (steam) to the waste is determined by their relative temperatures,
the heat transfer rate of the vessel materials, and the thermal properties of the
waste stream forming the film. The rising vapor is collected at the top of the
column, cooled, and condensed in a condenser. The condensed liquid product
stream is then routed to a product receiver. The "bottoms," which are the least
volatile components of the waste, are continuously withdrawn from the bottom of
the thin film evaporator. Because thin film evaporation is used to remove the
volatile organics from wastes, the bottoms are reduced in volatile organic
content. However, prior to disposal, the bottoms usually require additional
treatment, such as incineration, for residual, less volatile organics.
2.3.5 Thermal Drying
A wide range of batch and continuous dryers is available. One commonly
used continuous type is the screw-flight dryer, the major components of which are
shown in Figure 19. The screw-flight dryer consists of a screw surrounding a
hollow shaft enclosed in a trough. Heat transfer fluid is heated to temperatures
as high as 750*F and circulated, usually countercurrent to the flow of waste,
through the hollow shaft. Heat transfers from the shaft to the screw blades, and
then into the feed material, causing water and organics to be driven off in a
vapor form. The dried cake is discharged from the dryer.
The dryer is designed to create good contact between the screw and feed
material. The screw is usually equipped with breaker bars to ensure proper
shearing of the input materials and to prevent the screw surfaces from fouling.
Vapors emerging from this system are managed in one of two ways, depending
on their composition. If the vapors contain only water, they are usually
directly vented to the atmosphere. However, if the vapors contain volatile
organics, they are generally passed through a water-cooled condenser system. The
recovered organic liquids from the condenser unit are then forwarded to another
process for treatment or recovery.
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FEED IN
BREAKER BARS
STEAM IN
SCREWS
in
HOLLOW
SHAFT
DRIED
MATERIAL
Figure 19 Screw Flight Dryer
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2.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether distillation technologies will achieve the same
level of performance on an untested waste as on a previously tested waste and
whether performance levels can be transferred, EPA examines the following waste
characteristics: (a) the component boiling points, (b) the thermal conductivity,
(c) the concentration of volatiles, (d) the concentration of suspended solids,
(e) the surface tension, and (f) the oil and grease content. The above-mentioned
waste characteristics affecting performance vary for each distillation
technology.
The WCAPs for each specific distillation process are as follows:
• Batch distillation: thermal conductivity, component boiling points, and
concentration of volatiles.
• Fractionation, steam stripping, and thin film evaporation: concen-
tration of suspended solids, component boiling points, concentration of
volatiles, surface tension, and oil and grease content.
• Thermal drying: thermal conductivity and component boiling points.
2.4.1 Component Boiling Points
As noted earlier, the greater the ratio of volatility of the waste
constituents, the more easily the separation of these constituents can proceed.
This ratio is called relative volatility. EPA recognizes, however, that relative
volatilities cannot be measured or calculated directly for the types of wastes
generally treated by distillation processes because such wastes usually consist
of a myriad of components, all with different vapor pressure-versus-temperature
relationships. However, because the volatility of components is usually
inversely proportional to their boiling points (i.e., the higher the boiling
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point, the lower the volatility), EPA uses the boiling point of waste components
as a surrogate waste characteristic for relative volatility. If the differences
in boiling points between the more volatile and less volatile constituents are
significantly smaller in the untested waste than in the tested waste, the system
may not achieve the same performance and more rigorous operating conditions or
other, more applicable treatment technologies may need to be considered for
treatment of the untested waste.
2.4.2 Thermal Conductivity of the Waste
A major factor determining whether a particular constituent will volatilize
is the potential for transfer of heat through the waste. For batch distillation,
heat transfer is accomplished principally by conduction from the external source
of heat (with some convection within the waste). For thermal drying, heat
transfer is accomplished primarily by conduction rather than convection since the
wastes are usually not liquids. Thermal conductivity is not as critical a
consideration with other forms of distillation because the wastes are broken up
into thin films, which conduct heat relatively rapidly.
Heat flow by conduction is proportional to the temperature gradient across
the material. The proportionality constant, referred to as the thermal
conductivity, is a property of the material to be distilled (or dried in the case
of thermal drying). With regard to convection, EPA believes that the amount of
heat transferred by convection is not dependent on the waste's thermal
conductivity. Convection will usually be more a function of the system design
than of the waste itself (i.e., how well the waste can be mixed during the
process).
Thermal conductivity measurements, as part of a treatability comparison for
two different wastes to be treated by a single batch distillation unit, are most
meaningful when applied to wastes that are homogeneous (i.e., uniform
throughout). As wastes exhibit greater degrees of nonhomogeneity, thermal
conductivity becomes less accurate in predicting treatability because the
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conductivity becomes less accurate in predicting treatability because the
measurement essentially reflects heat flow through regions having the greatest
conductivity (i.e., the path of least resistance) and not heat flow through all
parts of the waste. Nevertheless, EPA believes that thermal conductivity may
provide the best measure of performance of heat transfer. If the thermal
conductivity of an untested waste is significantly lower than that of the tested
waste, the system may not achieve the same performance, and other, more
applicable treatment technologies may need to be considered for treatment of the
untested waste.
2.4.3 Concentration of Volatile Components
The concentration of volatile components is a measure of the maximum
fraction of the waste that can be expected to volatilize during a distillation
process. A relatively low concentration of volatile components implies that most
of the waste may become bottoms (i.e., relatively nonvolatile). If the
concentration of relatively volatile components in the untested waste is
significantly lower than that in the tested waste, the untested system may not
perform adequately. More rigorous operating conditions, such as higher
temperatures, higher residence times, etc., may be required to volatilize less
volatile components and achieve the same treatment performance, or other, more
applicable treatment technologies may need to be considered for treatment of the
untested waste.
2.4.4 Concentration of Suspended Solids
Wastes containing large amounts of suspended solids—organic or
inorganic—may have an adverse effect on distillation treatment systems. For
example, in thin film evaporation, large amounts of suspended solids may coat
heat transfer surfaces, thereby disturbing the uniform film and inhibiting
volatilization of constituents. In fractionation and steam stripping, large
amounts of suspended solids may clog column internals and coat heat tranfer
surfaces, thereby inhibiting heat transfer and mass transfer of constituents
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between the vapor and liquid phases. If the concentration of suspended solids
in the untested waste is significantly higher than that in the tested waste, the
system may not achieve the same performance. Filtration may be required prior
to distillation processes to reduce the concentration of suspended solids and
achieve the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
2.4.5 Surface Tension
The surface tension of the waste is a measure of the tendency of the waste
to foam and to "wet" the heat and mass transfer surfaces. The higher the surface
tension of the liquid, the higher its tendency to foam. Also, higher surface
tensions will not allow a film of the waste to coat heat and mass transfer
surfaces as well as low surface tensions will. The likelihood of foaming
requires special column design or the incorporation of defoaming compounds.
Packed columns are usually less susceptible to foaming than are tray columns.
Waste with high surface tensions that cannot effectively wet column packing or
evaporator walls will not form the thin films that enhance heat and mass
transfer. If the surface tension of the untested waste is significantly higher
than that of the tested waste, the system may not achieve the same performance.
Defoaming compounds and/or the use of a packed column may be required to reduce
foaming and achieve the same treatment performance, or other, more applicable
treatment technologies may need to be considered for treatment of the untested
waste.
2.4.6 Concentration of Oil and Grease
High concentrations of oil and grease may clog steam stripping and
fractionation equipment, thereby reducing their effectiveness. In thin film
evaporation, high concentrations of oil and grease may result in the waste
coating the evaporator walls, preventing a uniform film from forming and
inhibiting volatilization of waste components. If the concentration of oil and
grease in the untested waste is significantly higher than that in the tested
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applicable treatment technologies may need to be considered for treatment of the
untested waste.
2.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of distillation
technologies, EPA may examine the following parameters: (a) the temperature and
pressure, (b) the differential pressure, (c) the liquid and vapor flow rates,
(d) the internal column design, (e) the number of separation stages, (f) the
residence time, (g) the surface area, and (h) the mechanical system design.
The above-mentioned design and operating parameters vary for each
distillation technology. The design and operating conditions for each specific
distillation process are as follows:
• Batch distillation: temperature and pressure, and residence time.
• Fractionation: number of separation stages, liquid and vapor flow
rates, temperature and pressure, differential pressure, and internal
column design.
• Steam stripping: number of separation stages, liquid and vapor flow
rates, temperature and pressure, and internal column design.
• Thin film evaporation: evaporator surface area, temperature and
pressure, residence time, and the speed and design of the waste disposal
mechanism.
• Thermal dryer: temperature and pressure, residence time, and the design
and speed of the screw mechanism.
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To evaluate the effectiveness of design and operating variables, it is
necessary to measure the concentrations of the components to be separated in both
the feed system and the process residuals (e.g., overheads and bottoms). For
many hazardous organic constituents, analytical methods are not available or the
constituent cannot be analyzed in the waste matrix. Therefore, it would normally
be impossible to measure the effectiveness of treatment of distillation systems.
In these cases EPA tries to identify measurable parameters or constituents that
would act as surrogates to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the distillation treatment system
will indicate removal of organic constituents. Hence, TOC analysis is likely to
be an adequate surrogate analysis where the specific organic constituent cannot
be measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden; that is, the TOC
analysis may not be sensitive enough to detect changes at the milligrams per
liter (mg/1) level in matrices where total organic concentrations are hundreds
or thousands of mg/1. In these cases other surrogate parameters should be
sought. For example, if a specific analyzable constituent is expected to be
treated as well as the unanalyzable constituent, the analyzable constituent
concentration might be monitored as a surrogate.
2.5.1 Temperature and Pressure
These parameters are integrally related to the vapor-liquid equilibrium
conditions. In steam stripping and fractionation, the temperature at any point
in the column is an indicator of the constituent concentrations at that point,
thus revealing whether the separation of components is taking place as expected.
Overall column pressure influences the boiling point of the liquid at any
location in the column. For example, through application of a partial vacuum to
the column, the temperatures required to achieve the desired separation can be
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reduced because liquids volatilize at lower temperatures when pressures are
reduced. EPA monitors the temperature and pressure of a steam stripping column
and a fractionation column continuously, if possible, to ensure that the system
is operating at the appropriate design conditions and to diagnose operational
problems.
In thin film evaporation, to achieve the desired volatilization, the
evaporator may be operated at pressures below atmospheric (slight vacuum). At
vacuum conditions, lower temperatures can be used, requiring less heat input,
because boiling points decrease as pressure decreases. EPA monitors the thin
film evaporator temperature as well as the pressure (if pressures other than
atmospheric are used) continuously, if possible, to ensure that the system is
operating at the appropriate design condition and to diagnose operational
problems.
In batch distillation and in thermal drying, temperature provides an
indirect measure of the energy available (i.e., Btu/hr) to vaporize the waste
constituents. As the design temperature increases, more constituents with lower
volatilities will be removed from the waste. Pressure is integrally related to
the boiling point of the waste and the subsequent vaporization of the organic
constituents. As the pressure is lowered below atmospheric (i.e., as vacuum is
increased), the boiling point of the waste will also be lowered, thereby
requiring less heat input to volatilize waste constituents. EPA monitors the
distillation temperature as well as the pressure (if pressures other than
atmospheric are used) to ensure that the system is operating at the appropriate
design conditions and to diagnose operational problems.
2.5.2 Differential Pressure
In fractionation, measuring the differential pressure between the top and
bottom of the column indicates whether the flow rate of either the liquid or the
vapor phase is excessive. For instance, a high pressure drop across the column
may indicate a condition of "flooding," in which the liquid phase cannot flow
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down through the column as fast as feed Is entering, causing backing up or
"flooding" to occur. EPA monitors the pressure drop across the fractionation
column continuously, if possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
2.5.3 Liquid and Vapor Flow Rates
For steam stripping and fractionation, the vapor-liquid equilibrium data
are also used to determine the liquid and vapor flow rates that provide
sufficient contact and residence time between the liquid and vapor streams. The
appropriate flow rates are affected by the column diameter. EPA monitors the
liquid and vapor flow rates to ensure that sufficient contact time between the
liquid and vapor streams is provided to effectively separate the organics from
the wastewater, and to ensure that the column does not "flood" (i.e., the vapor
or liquid flow is so high that the vapor cannot exit the top of the column as
rapidly as it is generated and/or the liquid cannot run out the bottom as rapidly
as it is fed into the column). Also see Differential Pressure, above.
2.5.4 Internal Column Design
In steam stripping and fractionation, column internals are designed to
accommodate the physical and chemical properties of the waste to be fractionated.
Two types of internals may be used in steam stripping and fractionation: trays
and packing. Tray types include bubble cap, sieve, valve, and turbo-grid. Trays
have several advantages over packing. They are less susceptible to blockage by
solids, they have a lower capital cost for large-diameter columns (more than
approximately 3 feet), and they accommodate a wider range of liquid and vapor
flow rates. Compared to trays, packing has the advantages of having a lower
pressure drop per theoretical stage, being more resistant to corrosive materials,
having a lower capital cost for small-diameter columns (less than approximately
3 feet), and being less susceptible to foaming because of a more uniform flow
distribution (i.e., lower local variations in flow rates). EPA examines the
internal column design of a steam stripping or fractionation column to ensure
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that the system is designed to handle potential operational problems (e.g.,
corrosion, foaming, channeling).
2.5.5 Number of Separation Stages
The number of separation stages in a steam stripping or fractionation
column required to achieve the desired separation of the more volatile
constituents from the less volatile constituents is calculated from vapor-liquid
equilibrium data, which are determined empirically. Using the theoretical number
of stages, one can determine the actual number of stages through the use of
empirical tray efficiency data, typically supplied by equipment manufacturers.
EPA examines the actual number of stages in the steam stripping or fractionation
column to ensure that the system is designed to achieve an effective degree of
separation of organics from the wastewater stream.
2.5.6 Residence Time
The residence time determines the necessary energy input into the system
as well as the degree of volatilization achieved for organic constituents. The
residence time requirement is dependent on the distillation or evaporation
temperature (or dryer temperature in the case of thermal drying) and the thermal
conductivity of the waste. EPA observes the residence time to ensure that
sufficient time is provided to effectively volatilize organic constituents from
the waste. In the case of thermal drying, the residence time must be sufficient
to effectively evaporate the volatile liquid constituents and, hence, to dry the
waste.
2.5.7 Evaporator Surface Area
In thin film evaporation, the evaporator surface area required to achieve
the desired volatilization of organic components from the waste is calculated
from the vapor-liquid equilibrium data, which are determined empirically, and
from waste liquid flow rates. EPA examines the surface area of the evaporator
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to ensure that sufficient surface area is provided to achieve effective
volatilization of the more volatile organic components.
2.5.8 Mechanical System Design
Both thin film evaporation and thermal drying have rotating parts. For the
evaporator, the speed and design of the feed distributor will help determine how
well the feed will coat the evaporator surfaces. For the dryer, the size, screw
flight, spacing, and speed of the screw mechanism will affect residence time and
heat transfer. Typically, higher rotational speed will distribute feed more
effectively in an evaporator and will provide more mixing turbulence in a dryer
(for improved heat transfer). However, increased speed will reduce residence
time in the dryer and will cause earlier mechanical failure in both evaporators
and dryers (i.e., the bearings and shafts will wear more rapidly).
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2.6 References
DeRenzo, D.J., ed. 1978. Unit operation for treatment of hazardous industrial
wastes. Park Ridge, N.J.: Noyes Data Corporation,
Kirk-Othmer. 1965. Encyclopedia of chemical technology. 2nd ed., vol. 7, pp.
204-248. New York: John Wiley and Sons.
McCabe, W.L., Smith, J.C., and Harriot, P. 1985. Unit operations of chemical
engineering, pp. 533-606. New York: McGraw-Hill Book Co.
Perry, R.H. and Chilton, C.H. 1973. Chemical engineers7 handbook. 5th ed., pp.
13-1 to 13-60. New York: McGraw-Hill Book Co.
Rose, L.M. 1985. Distillation design in practice. pp. 1-307. New York:
Elsevier.
Van Winkle, M. 1967. Distillation, pp. 1-684. New York: McGraw-Hill Book Co.
Water Chemical Corporation. 1984. Process design manual for stripping of
organics. PB84-232628. pp. 1-1 to F4. Prepared for the Industrial
Environmental Research Laboratory, Office of Research and Development,
U.S. Environmental Protection Agency.
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3. EXTRACTION TECHNOLOGIES
3.1 Applicability
Extraction technologies (i.e., solvent extraction and critical fluid
extraction) are used to treat wastes containing a variety of organic constituents
and a broad range of total organic content. Extraction technologies have been
demonstrated for treatment of API separator sludges and other hydrocarbon-bearing
wastes generated by the petroleum and petrochemicals industries. In theory,
these technologies are also applicable to wastes of similar composition generated
by other industries such as the organic chemicals industry.
In solvent extraction, the selection of an extraction fluid (solvent) is
dependent on the solubility of the organic waste constituents in the extraction
fluid.
Critical fluid extraction is applicable to wastes containing organics that
are soluble in pressurized fluids such as carbon dioxide, propane, butane, or
pentane (extraction fluids). Compounds that have been successfully extracted
from wastes by this process include aliphatic hydrocarbons, alkenes, simple
aromatics such as benzene and toluene, polynuclear aromatics, and phenols.
3.2 Underlying Principles of Operation
The basic principle of operation in extraction technologies is that
constituents are removed from a waste by mixing the waste with an extraction
fluid (solvent) that will preferentially dissolve the waste constituents of
concern from the waste. In the simplest extraction systems, two components are
mixed: (a) the waste stream to be extracted and (b) extraction fluid. The
extraction fluid and waste stream are mixed to allow mass transfer of the
constituent (the solute) from the waste stream to the extraction fluid.
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Except for the waste constituents that are to be extracted, the waste must
be immiscible in the extracted fluid, so that after mixing, the two immiscible
phases can physically separate by gravity. Separation of the extraction fluid
phase and the waste stream phase occurs under quiescent conditions, relying on
the density differences between the two phases.
The extraction fluid, which now contains the extracted contaminants, is
called the extract; the extracted waste stream from which the contaminants have
been removed is called the raffinate. The extract can be either the heavy (more
• dense) phase or the light (less dense) phase. In theory, the maximum degree of
separation that can be achieved is provided by the selectivity value, which is
the ratio of the equilibrium concentration of the contaminants in the extraction
fluid to the equilibrium concentration of the contaminants in the waste. The
solvent extraction process can be either batch or continuous.
In critical fluid extraction, the extraction fluids used are compounds
that are usually gases at ambient conditions. Since the extraction fluid being
used is a gas, it is first pressurized, which converts it to a liquid. As a
liquid, it leaches (dissolves) the organic constituents out of the complex waste
with which it is mixed. The enhanced solubilities of various organic compounds
in hydrocarbons and other extraction fluids at high pressure aid in their removal
from a waste. The process is usually carried out at or near the extraction
fluid's "critical pressure," which is the pressure above which the liquid form
of the extraction fluid cannot be gasified, no matter how much the fluid is
heated. (In fact, above the critical pressure liquid and gas phases become
physically indistinguishable, in that two phases do not really exist.)
3.3 Description of Extraction Process
3.3.1 Solvent Extraction
The simplest, least effective solvent extraction unit is a single-stage
system (mixer-settler system). The solvent and the liquid waste stream are mixed
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together; the raffinate and extract are separated by settling without further
extraction.
The more effective multistage contact extraction is basically a series of
batch mixer-settler units. The waste stream is contacted with solvent in a
series of successive steps or stages. Raffinate from the first extraction stage
is contacted with fresh solvent in a second stage, and so on.
In countercurrent, multistage extraction columns, fresh solvent and the
waste stream continuously enter at opposite ends of a column consisting of a
series of extraction stages. Extract and raffinate layers pass continuously and
countercurrently from stage to stage through the system.
Several types of extraction systems are used for contact and separation.
Three of the most common systems--mixer-settler systems, extraction columns, and
centrifugal contactors — are discussed below.
3.3.2 Mixer-Settler Systems
Single-stage mixer-settler systems are extraction systems composed of a
mixing chamber for phase dispersion, followed by a settling chamber for phase
separation. Mixer-settler systems are typically used to treat solids or highly
viscous wastes and can handle difficult-to-mix components. The vessels may be
either vertical or horizontal. Dispersion in the mixing chamber occurs by pump
circulation, nonmechanical in-line mixing, air agitation, or mechanical
stirring. In a two-stage mixer-settler system (a simple multistage contact
extractor), as shown in Figure 20, the extract from the first stage is sent to
a recovery unit to separate the solvent from the remaining extract containing the
organic constituents of concern. The recovered solvent is used, and the
remaining extract is either reused or further treated in an incineration unit.
The raffinate from the first stage is sent to the second-stage unit for
additional extraction. Recycled solvent from recovery of the first-stage extract
and/or fresh solvent makeup is added to the first-stage raffinate before it is
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RECYCLED SOLVENT FROM
RECOVERY/FRESH SOLVENT
MAKEUP
en
o
RECYCLED
SOLVENT
EXTRACT TO RECOVERY
RECYCLED
TO PROCESS
TO
DISPOSAL
RAFFINATE
*-^^--
EXTRACT
RAFFINATE
^—^"
EXTRACT
STAGE 2
EXTRACT TO
RECYCLE OR
DISPOSAL
Figure 20 Two-Stage Mixer-Settler Solvent Extraction System
-------
mixed and sent to the second stage. The extract from the second stage,
containing mainly solvent, is recycled to the first-stage unit as the solvent
stream. The resulting raffinate from the second stage may require filtering to
remove solids before it is sent to further treatment (if required). If solids
collected during filtration contain treatable levels of hazardous constituents,
they will require further treatment, such as stabilization (for metals) and/or
incineration (for organics) prior to disposal.
Parameters such as the density or specific constituent concentrations in
the extract may be monitored to determine when the second-stage extract must be
sent to solvent recovery and when fresh or recycled solvent must be added to the
first-stage unit.
3.3.3 Extraction Columns
Extraction columns are continuous flow, countercurrent, multistage contact
systems. Two types of extraction columns are packed extractors and sieve-tray
extractors. Figure 21 presents schematics of these two types of extraction
columns. A packed extractor contains plastic or ceramic materials in various
geometric shapes or structured packings of wire gauze or mesh. Mass transfer of
the contaminants from the waste to the extract is promoted because of breakup and
distortion of both phases as they contact the packing, resulting in the intimate
mixing of the waste and the solvent.
The sieve-tray extractor is similar to the sieve-tray column used in
fractionation distillation. Tray perforations cause the formation of liquid
droplets that aid the mass transfer process by allowing for more intimate contact
between the solute and the solvent.
3.3.4 Certrifugal Contactors
Centrifugal contactors are based on the application of centrifugal force
to increase rates of countercurrent flow and enhance separation of the phases.
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SOLVENT
at
IS)
WASTE
INFLUENT
TREATMENT AND/OR
DISPOSAL
SOLVENT.
\PACKINQ
SUPORT
HAhHNAIt TO
TREATMENT AND/OR
DISPOSAL
EXTRACT AND SOLVENT
TO SEPARATOR
, AND RECOVERY UNIT
PACKED EXTRACTOR
EXTRACT AND SOLVENT
TO SEPARATOR
AND RECOVERY UNIT
B. SIEVE TRAY EXTRACTOR
Figure 21 Packed and Sieve Tray Solvent Extraction Columns
-------
Centrifugal units are used when short contact times are required, such as when
unstable materials are being processed. One type of centrifugal contactor
consists of a drum that rotates around a shaft equipped with annular passages at
each end for feed and raffinate. The light phase is injected under pressure
through a shaft and is then routed to the periphery of the drum through
perforations. The heavy phase is also charged through the shaft, but it is
channeled to the center of the drum through perforations. Centrifugal force
acting on the phase-density difference promotes dispersion as the phases are
forced through the perforations. Centrifugal extractors provide short contact
time, have minimal space requirements, and easily handle emulsified materials and
fluids with small density differences.
3.3.5 Critical Fluid Extraction
A critical fluid extraction system consists of a blending tank, one or more
extraction vessels, one or more decanters, one or more filters, and an
evaporation unit. Organic wastes such as oil refinery sludges are first combined
in a blending tank and mixed well to yield a homogeneous, pumpable mixture. This
mixture is then pumped under pressure to an extraction vessel filled with a
liquified gas such as carbon dioxide, propane, butane, or pentane and is mixed
under pressure to extract (dissolve) hydrocarbon components from the waste into
the liquified gas extraction fluid. After the hydrocarbon components of the
waste dissolve in the pressurized liquid, the resulting solution is
gravity-separated in a decanter into a wastewater (or waste solids) stream and
an extraction fluid-rich stream. The wastewater stream, containing inorganic
solids, is sometimes filtered under pressure to remove the insoluble components.
The extraction fluid-rich stream is fed to a pressurized evaporation unit. The
extraction fluid (carbon dioxide, propane, butane, or pentane) is evaporated by
dropping the pressure and is subsequently recovered, repressurized, and
recondensed for reuse. The residue from the evaporation, consisting of a liquid
hydrocarbon mixture, is then reprocessed or reused, blended with fuels for heat
recovery, or incinerated. If the extracted waste stream still exceeds treatment
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requirements, it may be extracted again with fresh extraction fluid at the high
pressure conditions.
If inorganic residuals (or waste solids) filtered from the waste/solvent
mixture contain treatable levels of hazardous constituents such as certain metals
(e.g., chromium, lead) or organics, they will require further treatment such as
stabilization or incineration prior to disposal.
3.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether extraction processes will achieve the same level of
performance on an untested waste as on a previously tested waste and whether
performance levels can be transferred, EPA examines the following waste
characteristics: (a) the concentration of extractable hydrocarbons, (b) the
solubility of the waste constituents of concern in the extraction fluid, (c) the
surface tension, and (d) the alkalinity of the waste.
3.4.1 Concentration of Extractable Hydrocarbons
The concentration of extractable hydrocarbons in the waste feed is a
measure of the maximum fraction of the waste that can be expected to be extracted
in the process. A relatively low concentration of extractable hydrocarbons
implies that most of the waste may become wastewater or waste solids (i.e., is
nonextractable). Total extractable hydrocarbon content can be estimated by
measurement of the total organic carbon (TOC) content. If the TOC of an untested
waste is significantly lower than that of the tested waste, the untested system
may not remove as much hydrocarbon as the tested system. More rigorous
extraction conditions such as higher temperatures and pressures, additional
mixing, and longer settling times may be required to extract less extractable
components and achieve the same treatment performance, or other, more applicable
treatment technologies may need to be considered for treatment of the untested
waste.
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Critical fluid extraction processes are designed to extract hydrocarbon
components from mixed oily and organic waste liquids and sludges. For critical
fluid extraction to be economically applied, the waste should contain at least
a few percent by weight of extractable hydrocarbons. This process has been
demonstrated on wastes containing from 5 to 34 percent hydrocarbons. For lower
concentrations, batch distillation or conventional solvent extraction may be more
economical. Also, critical fluid extraction is not economical for wastes
containing high concentrations of extractable hydrocarbons (higher than
95 percent). These wastes are more amenable to fractionation treatment.
3.4.2 Solubility of the Haste Constituents of Concern in the
Extraction Fluid
The constituents in the waste feed that are to be extracted determine the
type of extraction fluid that is best suited for the process. For example, polar
organic molecules (e.g., phenol) in an organic feed can be extracted with an
aqueous solvent. Organic constituents in aqueous feeds can be extracted with
various organic solvents. Metal-containing wastes can be extracted with acids
(e.g., trialkylphosphoric and carboxylic acids) or amine solvents. If the
solubility of the waste constituents of concern in the extraction fluid in the
untested waste is significantly lower than that in the tested waste, the untested
system may not achieve adequate performance. Use of another extraction fluid may
be required to increase the solubility of the waste constituents of concern and
achieve the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
3.4.3 Surface Tension
The surface tension of the waste is a measure of the tendency of the waste
to foam. The higher the surface tension of the liquid, the higher its tendency
to foam. If foaming is likely, the system design must be modified or defoaming
compounds may be required. For column extractors, packed columns are less
susceptible to foaming than are tray columns. If the surface tension of the
untested waste is significantly higher than that of the tested waste, the
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untested system may not achieve adequate performance. Defearning compounds and/or
the use of a packed column may be required to reduce foaming and achieve the same
treatment performance, or other, more applicable treatment technologies may need
to be considered for treatment of the untested waste. Foaming has not been found
to affect the performance of critical fluid extraction processes.
3.4.4 Alkalinity of the Waste
In critical fluid extraction, when carbon dioxide is used as the extraction
fluid, high alkalinity will interfere with the process because carbon dioxide
will react to form carbonates and bicarbonates. This will result in excessive
carbon dioxide consumption. For wastes having high alkalinity levels, an
extraction fluid other than carbon dioxide should be used in the critical fluid
extraction process. The same problem does not arise if a hydrocarbon fluid
(propane, butane, or pentane) is used. If carbon dioxide was the extraction
fluid used on a tested waste and the alkalinity of the untested waste is
significantly higher than that of the tested waste, the system may not achieve
the same performance. Use of another extraction fluid may be required to achieve
the same treatment performance, or other, more applicable treatment technologies
may need to be considered for treatment of the untested waste. Alkalinity of the
waste has not been found to affect the performance of conventional solvent
extraction processes.
3.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of extraction
systems, EPA examines the following parameters: (a) the number of separation
stages, (b) the extraction temperature and pH, (c) the degree of mixing, (d) the
residence time, (e) the settling time, and (f) the extraction pressure.
For many hazardous organic constituents, analytical methods are not
available or the constituent cannot be analyzed in the waste matrix. Therefore,
it would normally be impossible to measure the effectiveness of the extraction
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treatment system. In these cases, EPA tries to identify measurable parameters
or constituents that would act as surrogates to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the extraction treatment system
implies removal of organic constituents. Hence, TOC analysis is likely to be an
adequate surrogate analysis where the specific organic constituent cannot be
measured.
However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden; that is, the TOC
analysis may not be sensitive enough to detect changes at the milligrams per
liter (mg/1) level in matrices where total organic concentrations are hundreds
or thousands of mg/1. In these cases, other surrogate parameters should be
sought. For example, if a specific analyzable constituent is expected to be
treated as well as the unanalyzable constituent, the analyzable constituent
concentration should be monitored as a surrogate.
3.5.1 Number of Separation Stages
For solvent extraction columns, the number of theoretical stages required
to achieve the desired separation of hazardous constituents from a liquid waste
into the selected extraction fluid is calculated from solvent equilibrium data,
which are determined empirically. Using the theoretical number of stages, one
can then determine the actual number of stages through the use of empirical tray
efficiency data typically supplied by equipment manufacturers. EPA examines the
actual number of stages in a solvent extraction column system to ensure that the
system is designed to achieve an effective degree of extraction.
3.5.2 Extraction Temperature and pH
Temperature and pH changes can affect equilibrium conditions (pH will
affect equilibrium when the waste or the extraction fluid is aqueous) and,
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consequently, the performance of the solvent extraction system. Critical fluid
extraction is normally performed at ambient temperature although it is possible
to use higher temperatures when necessary. In all cases, temperatures used must
be below the critical temperature* of the extraction fluid used to ensure that
the fluid is present in the extraction vessel in the liquid phase (so that
maximum contact with the waste can be achieved). EPA monitors the temperature
and pH (as applicable) continuously, if possible, to ensure that the system is
operating at the appropriate design conditions and to diagnose operational
problems.
3.5.3 Degree of Mixing
For mixer-settler solvent extractors, mixing determines the amount of
contact between the two immiscible phases and, accordingly, affects the degree
of mass transfer of the constituents to be extracted. Intense agitation to
provide high rates of mass transfer, however, can produce solvent waste
dispersions that are difficult to separate into distinct phases. The waste must
be premixed before introduction into the critical fluid extraction system to
ensure a uniform, pumpable blend of material. The waste is normally mixed in
tanks or other vessels. Any well-designed and well-operated system should have
such waste blending equipment. The quantifiable degree of mixing is a complex
assessment that includes, among other things, the amount of mixing energy
supplied, the length of time the material is mixed, and the related turbulence
effects of the specific size and shape of the tank vessel. This is beyond the
scope of simple measurement. During the fluid extraction, the extraction fluid
and waste need to be mixed to ensure maximum contact and, hence, efficiency of
extraction. Any well-designed system should include a pressurized extraction
vessel that is equipped with an operable mixing system. EPA, however, evaluates
the degree of mixing qualitatively by considering whether the type of mixing
*The critical temperature of a substance is that temperature above which the
the substance cannot be liquified, no matter how high the operating pressures.
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device provided is one that could be expected to achieve adequate uniform mixing
of the waste.
3.5.4 Residence Time
The residence time in the extraction vessel impacts the extent of
extraction of organic contaminants from the waste. Residence time will vary with
the solubility of the organic contaminants in the solvent and the degree of
mixing. For a batch solvent extraction system, the residence time is controlled
directly by adjusting the treatment time in the extraction vessel. For a
continuous solvent extraction system, the waste feed rate is controlled to ensure
that the system is operating at the appropriate design residence time. In
critical fluid extraction, as with conventional solvent extraction, the residence
time in the extraction vessel impacts the extent of extraction of organic
contaminants from the waste. EPA monitors the residence time to ensure that
sufficient time is provided to effectively extract the organic contaminants from
the waste.
3.5.5 Settling Time
For mixer-settler solvent extractors and critical fluid extractors,
adequate settling time must be provided to make sure that separation of the
phases has been effectively completed. EPA monitors the settling time to ensure
effective phase separation.
3.5.6 Extraction Pressure
Critical fluid extraction systems operate at pressures at which the
extraction fluids will be liquids at ambient temperatures. Pressure is normally
monitored by means of gauges and recorders attached to the extraction vessel.
EPA monitors the extraction pressure continuously, if possible, to ensure that
the system is operating at the appropriate design condition and to diagnose
operational problems.
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3.6 References
CF Systems Corporation. 1988. CF Systems units to render refinery wastes
non-hazardous.
De Renzo, D.J., ed. 1978. Unit operations for treatment of hazardous industrial
wastes. Park Ridge, N.J.: Noyes Data Corporation.
Gallacher, L.V. 1981. Liquid ion exchange in metal recovery and recycling.
Third Conference on Advanced Pollution Control for the Metal Finishing
Industry. USEPA 600/2-81-028. pp. 39-41.
Hackman, E. 1978. Toxic organic chemicals, destruction and waste treatment.
pp. 109-111. Park Ridge, N.J.: Noyes Data Corporation.
Hanson, C. 1968. Solvent extraction theory, equipment, commercial operations,
and economics. Chemical Engineering. August 26, 1968, p. 81.
Humphrey, J.L., Rocha, J.A., and Fair, J.R. 1984. The essentials of extraction.
Chemical Engineering. September 17, 1984, pp. 76-95.
Johnston, K. 1978. Supercritical fluids. In Kirk-Othmer encyclopedia of
chemical technology. Supplement Vol. I, pp. 872-983. New York:
Wiley-Interscience.
Lo, Teh C., Baird, M.H.I., and Manson, C., eds. 1983. Handbook of solvent
extraction, pp. 53-89. New York: John Wiley and Sons.
Perry, R.H., and Chilton, C.H. 1973. Chemical engineer's handbook. 5th ed.,
pp. 15-1 to 15-24. New York: McGraw-Hill Book Company.
Weast, R.C., ed. 1978. Handbook of chemistry and physics. 58th ed. Cleveland,
Ohio: CRC Press.
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4. FUEL SUBSTITUTION
Fuel substitution involves using hazardous waste as a fuel in industrial
furnaces or in boilers. The hazardous waste may be blended with other
nonhazardous wastes (e.g., municipal sewer sludge) and/or fossil fuels.
On December 31, 1990, EPA promulgated regulations to control emissions from
industrial furnaces and boilers from treatment of hazardous waste. Permit
requirements are similar to those for incineration, in that controls limit the
emissions of total organic carbons, toxic metals, HC1, and wastewaters. Like the
incinerator regulations (40 CFR 264, Subpart 0), the boiler and furnace
regulations (40 CFR 266, Subpart H) require 99.99 percent destruction and removal
efficiency of principal organic hazardous constituents (POHCs). These
regulations will also minimize products of incomplete combustion (PICs),
including carbon monoxide (CO). These regulations will also minimize particulate
emissions by regulating the metal emissions.
4.1 Applicability
Fuel substitution has been used with industrial waste solvents, refinery
wastes, synthetic fibers/petrochemical wastes, and waste oils. It can also be
used when combusting other waste types produced during the manufacture of
Pharmaceuticals, pulp and paper, and pesticides. These wastes can be handled in
a solid, liquid, or gaseous form.
Industrial furnaces include a variety of industrial processes that produce
heat and/or products by burning fuels. They include blast furnaces, electric arc
smelting furnaces, cement kilns, lime kilns, smelters, coke ovens, and halogen
acid furnaces (HAFs). Industrial boilers are units wherein fuel is used to
produce steam for process and plant use. Industrial boilers typically use coal,
oil, or gas as the primary fuel source.
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The parameters that affect the applicability of fuel substitution are the
following:
• Halogen content of the waste;
• Inorganic solids content (ash content) of the waste, particularly heavy
metals;
• Heating value of the waste;
• Viscosity of the waste (for liquids);
• Filterable solids concentration (for liquids); and
• Sulfur content.
4.1.1 Halogen Content of the Waste
If halogenated organics are burned, halogenated acids and free halogen are
among the products of combustion. These released corrosive gases may require
subsequent treatment prior .to venting to the atmosphere. Also, halogens and
halogenated acids formed during combustion are likely to severely corrode boiler
tubes and other process equipment. To minimize such problems, halogenated wastes
are blended into fuels only at very low concentrations. High chlorine content
can also lead to the incidental production (at very low concentrations) of other
hazardous compounds such as polychlorinated biphenyls (PCBs), chlorinated
dibenzo-p-dioxins (CDDs), chlorinated dibenzofurans (CDFs), and chlorinated
phenols.
4.1.2 Inorganic Solids Content of the Waste
High inorganic solids content (i.e., ash content) of wastes may cause two
problems: (1) scaling in the boiler and (2) particulate air emissions. Scaling
results from deposition of inorganic solids on the walls of the boiler.
Particulate emissions are produced by noncombustible inorganic constituents that
flow out of the boiler with the gaseous combustion products. Because of these
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problems, wastes with significant concentrations of inorganic materials are not
usually handled in boilers unless the boilers have an air pollution control
system.
Industrial furnaces vary in their tolerance to inorganic constituents.
Heavy metal concentrations, found in both halogenated and nonhalogenated wastes
used as fuel, can cause environmental concern because they may be emitted in the
gaseous emissions from the combustion process, in the ash residues, or in any
produced solids. The partitioning of the heavy metals to these residual streams
primarily depends on the volatility of the metal, waste matrix', and furnace
design.
4.1.3 Heating Value of the Waste
The heating value of the waste must be high enough (either alone or in
combination with other fuels) to maintain combustion temperatures consistent with
efficient waste destruction and operation of the boiler or furnace. For many
applications, only supplemental fuels having minimum heating values of 4,400 to
5,600 kcal/kg (8,000 to 10,000 Btu/lb) are considered to be feasible. Below this
value, the unblended fuel would not be likely to maintain a stable flame, and its
combustion would release insufficient energy to provide needed steam generation
potential in the boiler or the necessary heat for an industrial furnace. Some
wastes with heating values of less than 4,400 kcal/kg (8,000 Btu/lb) can be used
if sufficient auxiliary fuel is used to support combustion or if special designs
are incorporated into the combustion device. Occasionally, for wastes with
heating values higher than virgin fuels, blending with auxiliary fuel may be
required to prevent overheating or overcharging of the combustion device.
4.1.4 Viscosity of the Waste
In combustion devices designed to burn liquid fuels, the viscosity of the
liquid waste must be low enough that it can be atomized in the combustion
chamber. If the viscosity is too high, heating the storage tanks may be required
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prior to combustion. For atomization of liquids, a viscosity of 165 centistokes
(750 Saybolt Universal Seconds (Sus)) or less is typically required.
4.1.5 Filterable Solids Concentration
Filterable materials suspended in the liquid fuel may prevent or hinder
pumping or atomization and if so other technologies may need to be considered.
4.1.6 Sulfur Content
A waste's sulfur content may affect whether it can be burned or not because
the waste may emit sulfur oxide into the atmosphere. For instance, the EPA has
promulgated sulfur oxide emission regulations for certain new source industrial
boilers (40 CFR 266, Subpart H). Air pollution control devices are available to
remove sulfur oxides from the stack gases.
4.2 Underlying Principles of Operation
For a boiler and most industrial furnaces there are two distinct principles
of operation. Initially, energy in the form of heat is transferred to the waste
to achieve volatilization of the various waste constituents. For liquids,
volatilization energy may also be supplied by using pressurized atomization. The
energy used to pressurize the liquid waste allows the atomized waste to break
into smaller particles, thus enhancing its rate of volatilization. The
volatilized constituents then require additional energy to destabilize the
chemical bonds, allowing the constituents to react with oxygen to form carbon
dioxide and water vapor. The energy needed to destabilize the chemical bonds is
referred to as the energy of activation.
4.3 Description of Fuel Substitution Process
Since a number of industrial applications can use fuel substitution, there
is no one process description that will fit all of these applications. The
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following section, however, provides a general description of industrial kilns
(one form of industrial furnace) and industrial boilers.
4.3.1 Kilns
Combustible wastes have the potential to be used as fuel in kilns and, for
waste liquids, are often used with oil to co-fire kilns. Coal-fired kilns are
capable of handling some solid wastes. In the case of cement kilns, there are
usually no residuals requiring land disposal since any ash formed becomes part
of the product or is removed by particulate collection systems and recycled back
to the kiln. The only residuals may be low levels of unburned gases that escape
with combustion products. If this is the case, air pollution control devices may
be required.
Three types of kilns are particularly applicable: cement kilns, lime kilns,
and lightweight aggregate kilns.
4.3.1.1 Cement Kilns
The cement kiln is a rotary furnace, which is a refractory-lined steel
shell used to calcine a mixture of calcium, silicon, aluminum, iron, and
magnesium-containing minerals. The kiln is normally fired by coal or oil.
Liquid and solid combustible wastes may then serve as auxiliary fuel.
Temperatures within the kiln are typically between 1,380 and 1,540*C (2,500 to
2,800'F). To date, only liquid hazardous wastes have been burned in cement
kilns.
Most cement kilns have a dry particulate collection device (i.e., either
an electrostatic precipitator or a baghouse), with the fly ash collected and
recycled back to the kiln. Buildup of metals or other noncombustibles is
prevented through their incorporation into the product cement. Many types of
cement require a source of chloride so that most halogenated liquid hazardous
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wastes currently can be burned in cement kilns. Available information shows that
scrubbers are not used.
4.3.1.2 Lime Kilns
Quick lime (CaO) is manufactured in a calcination process using limestone
(CaC03) or dolomite (CaC03 and MgC03). These raw materials are also heated in
a refractory-lined rotary kiln, typically to temperatures of 980 to 1,260*C
(1,800 to 2,300'F). Lime kilns are less likely to burn hazardous wastes than are
cement kilns because product lime is often added to potable water systems. Only
one lime kiln in the United States currently burns hazardous waste. That
particular facility sells its product lime for use as flux or as refractory in
blast furnaces.
As with cement kilns, any fly ash collected is recycled back to the lime
kiln; thus, no residual streams are produced by the kiln. Available information
shows that scrubbers are not used.
4.3.1.3 Lightweight Aggregate Kilns
Lightweight aggregate kilns heat clay to produce an expanded lightweight
inorganic material used in Portland cement formulations and other applications.
The kiln has a normal temperature range of 1,100 to 1,150*C (2,000 to 2,100'F).
Lightweight aggregate kilns are less amenable to combustion of hazardous wastes
as fuels than are the other kilns described above because these kilns lack
material to adsorb halogens. As a result, burning of halogenated organics in
these kilns would likely require afterburners to ensure complete destruction of
the halogenated organics and scrubbers to control acid gas production. Such
controls would produce a wastewater residual stream subject to treatment
standards.
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4.3.2 Industrial Boilers
A boiler is a closed vessel in which water is transformed into steam by the
application of heat. Normally, heat is supplied by the combustion of pulverized
coal, fuel oil, or gas. These fuels are fired into a combustion chamber with
nozzles and burners that provide mixing with air. Liquid wastes, and granulated
solid wastes in the case of grate-fired boilers, can be burned as auxiliary fuel
in a boiler. However, few grate-fired boilers burn hazardous wastes. For
liquid-fired boilers, residuals requiring land disposal are generated only when
the boiler is shut down and cleaned. This is generally done once or twice per
year. Other residuals from liquid-fired boilers would be the gas emission
stream, which would consist of any products of incomplete combustion, along with
the normal combustion products. For example, chlorinated wastes would produce
acid gases. If this is the case, air pollution control devices may be required.
For solid-fired boilers, an ash normally is generated. This ash may contain
residual amounts of organics from the blended waste/fuels, as well as
noncombustible materials. Land disposal of this ash would require compliance
with applicable BOAT treatment standards.
4.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether fuel substitution will achieve the same level of
performance on an untested waste as on a previously tested waste and whether
performance levels can be transferred, EPA examines the following waste
characteristics: (a) the component boiling points, (b) the thermal conductivity
of the waste, and (c) the component bond dissociation energies.
4.4.1 Component Boiling Points
The term relative volatility refers to the ease with which a substance
present in a solid or liquid waste will vaporize from that waste upon application
of heat from an external source. Hence, it bears a relationship to the
equilibrium vapor pressure of the substance.
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EPA recognizes that the relative volatilities cannot be measured or
calculated directly for the types of wastes generally treated in an industrial
boiler or furnace. The Agency believes that the best measure of relative
volatility is the boiling point of the various hazardous constituents and will,
therefore, use this parameter in assessing volatility of the organic
constituents.
4.4.2 Thermal Conductivity of the Waste
Consistent with the underlying principles of combustion in aggregate kilns
or boilers, a major factor with regard to whether a particular constituent will
volatilize is the transfer of heat through the waste. In the case of industrial
boilers burning solid fuels, heat is transferred through the waste by three
mechanisms: radiation, convection, and conduction. For a given boiler, it can
be assumed that the type of waste will have a minimal impact on the heat
transferred from radiation. With regard to convection, EPA believes that the
range of wastes treated would exhibit similar properties with regard to the
amount of heat transferred by convection. Therefore, EPA will not evaluate the
radiation or convection heat transfer properties of wastes in determining similar
treatability. For solids, the third heat transfer mechanism, conductivity, is
the one principally operative and most likely to change between wastes.
Using thermal conductivity measurements as part of a treatability
comparison for two different wastes through a given boiler or furnace is most
meaningful when applied to wastes that are homogeneous. As wastes exhibit
greater degrees of nonhomogeneity, thermal conductivity becomes less accurate in
predicting treatability because the measurement essentially reflects heat flow
through regions having the greatest conductivity (i.e., the path of least
resistance) and not heat flow through all parts of the waste. Nevertheless, EPA
has not identified a better alternative to thermal conductivity, even for wastes
that are nonhomogeneous.
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4.4.3 Component Bond Dissociation Energies
Given an excess of oxygen, an organic waste in an industrial furnace or
boiler would be expected to convert to carbon dioxide and water vapor provided
that the activation energy is achieved. Activation energy is the quantity of
heat (energy) needed to destabilize molecular bonds so that the oxidation
(combustion) reaction will proceed to completion. As a measure of activation
energy, EPA is using bond dissociation energies. In theory, the bond
dissociation energy would be equal to the activation energy; in practice,
however, this is not always the case because of molecular interaction bonds. In
some instances, bond dissociation energies will not be available and will have
to be estimated, or other energy effects will have a significant influence on
activation energy.
4.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a fuel
substitution system, EPA examines the following parameters: (a) the combustion
temperature, (b) the residence time, (c) the degree of mixing, (d) the air feed
rate, (e) the fuel feed rate, and (f) the steam pressure/rate of production.
For many hazardous organic constituents, analytical methods are not
available or the constituent cannot be analyzed in the waste matrix. Therefore,
it would normally be impossible to measure the effectiveness of the fuel
substitution treatment system. In these cases, EPA tries to identify measurable
parameters or constituents that would act as surrogates to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the fuel substitution treatment
system will indicate removal of organic constituents. Hence, TOC analysis is
likely to be an adequate surrogate analysis where the specific organic
constituent cannot be measured.
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However, TOC analysis may not be able to adequately detect treatment of
specific organics in matrices that are heavily organic-laden (i.e., the TOC
analysis may not be sensitive enough to detect changes at the milligrams/liter
(mg/1) level in matrices where total organic concentrations are hundreds or
thousands of mg/1). In these cases, other surrogate parameters should be sought.
For example, if a specific analyzable constituent is expected to be treated as
well as the unanalyzable constituent, the analyzable constituent concentration
should be monitored as a surrogate.
4.5.1 Combustion Temperature
Industrial boilers are generally designed based on their steam generation
potential (Btu output). This factor is related to the design combustion
temperature, which, in turn, depends on the amount of fuel burned and its Btu
value. The fuel feed rates and combustion temperatures of industrial boilers are
generally fixed based on the Btu values of fuels normally handled (e.g., No. 2
versus No. 6 fuel oils). When wastes are to be blended with fossil fuels for
combustion, the blending, based on Btu values, must be such that the resulting
Btu value of the mixture is close to that of the fuel value used in the design
of the boiler. Industrial furnaces also are designed to operate at specific
ranges of temperature to produce the desired product (e.g., lightweight
aggregate). The blended waste/fuel mixture should be capable of maintaining the
design temperature range.
4.5.2 Residence Time
A sufficient residence time of combustion products is normally necessary
to ensure that the hazardous substances being combusted (or formed during
combustion) are completely oxidized. Residence times on the order of a few
seconds are generally needed at normal operating conditions. For industrial
furnaces as well as boilers, the residence time is a function of the size of the
furnace and the fuel feed rates. For most boilers and furnaces, the residence
time usually exceeds a few seconds.
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4.5.3 Degree of Mixing
Boilers are designed so that fuel and air are intimately mixed. This helps
to ensure that complete combustion takes place. The shape of the boiler and the
method of fuel and air feed influence the turbulence required for good mixing.
Industrial furnaces also are designed for turbulent mixing where fuel and air are
mixed.
4.5.4 Air Feed Rate
An important operating parameter in boilers and many industrial furnaces
is the oxygen content in the flue gas, which is a function of the air feed rate.
Stable combustion of a fuel generally occurs within a specific range of
air-to-fuel ratios. An oxygen analyzer in the combustion gases can be used to
control the feed ratio of air to fuel to ensure complete thermal destruction of
the waste and efficient operation of the boiler. When necessary, the air feed
rate can be increased or decreased to maintain proper fuel-to-oxygen ratios.
Some industrial furnaces do not completely combust fuels (e.g., coke ovens and
blast furnaces); hence, oxygen concentration in the flue gas is a meaningless
variable.
4.5.5 Fuel Feed Rate
The rate at which fuel is injected into the boiler or industrial furnace
will determine the thermal output of the system per unit of time (Btu/hr). If
steam is produced, steam pressure monitoring will indirectly determine whether
the fuel feed rate is adequate. However, various velocity and mass measurement
devices can be used to monitor fuel flow directly.
4.5.6 Steam Pressure or Rate of Production
Steam pressure in boilers provides a direct measure of the thermal output
of the system and is directly monitored by use of in-system pressure gauges.
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Increases or decreases in steam pressure can be effected by increasing or
decreasing the fuel and air feed rates within certain operating design limits.
Most industrial furnaces do not produce steam; instead they produce a product
(e.g., cement, aggregate) and monitor the rate of production.
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4.6 References
Bonner, T.A., et al. 1981. Engineering handbook for hazardous waste
incineration. Prepared by Monsanto Research Corporation for U.S.
Environmental Protection Agency PB 81-248163.
Castaldini C., et al. 1986. Disposal of hazardous wastes in industrial boilers
or furnaces. Park Ridge, N.J.: Noyes Publications.
USEPA. 1990. Office of Solid Waste. Standards for owners and operators of
hazardous wastes in boilers and industrial furnaces; proposed and
supplemental proposed rule, technical corrections, and request for
comments. 55 FR 17862, April 27, 1990.
Versar. 1984. Estimating PMN incineration results. EPA Contract No.
68-01-6271, Task No. 66. Draft report for Exposure Evaluation Division,
Office of Toxic Substances. Washington, D.C.: U.S. Environmental
Protection Agency.
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II. REMOVAL TECHNOLOGIES (CONTINUED)
B. RECOVERY AND/OR SEPARATION TECHNOLOGIES
FOR METALS
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1. ACID LEACHING
1.1 Applicability
Acid leaching is a treatment technology used to treat wastes in solid or
slurry form containing metal constituents that are soluble in a strong acid
solution or can be converted by reaction with a strong acid to a soluble form.
This process has been used to recover metals such as mercury, copper, nickel,
silver, and cadmium from inorganic wastes generated in the primary metals and
inorganic chemicals industries.
The acid leaching process is most effective with wastes having high (over
1,000 ppm) levels of metal constituents. Wastes containing lower levels of such
contaminants are more difficult to process because the low metal concentrations
require longer contact times.
Acid leaching can also be used to extract heavy metals or radionuclides
from mixed wastes, principally soils, separating the material into its hazardous
and radioactive components. This process has greater utility when combined with
unit operations such as ion exchange, solvent extraction, chemical precipitation,
and filtration.
Another type of leaching process, alkaline leaching, is used to treat
wastes containing metal constituents that are soluble in a strong caustic or
alkaline solution. This process is mainly useful in recovering aluminum from
bauxite ore. Generally, the following information on acid leaching would be
applicable to alkaline leaching.
1.2 Underlying Principles of Operation
The basic principle of operation for acid leaching is that solubilities of
various metals in acid solutions aid in their removal from a waste. The process
concentrates the constituent(s) leached by the acid solutions. These
constituents can then be filtered to remove residual solids and neutralized to
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precipitate solids containing high concentrations of the constituents of
interest, which can be further treated in metals recovery processes.
Alternatively, the acid solutions can be electrolyzed to recover pure metals.
An acid leaching system usually consists of a solid/liquid contacting unit
followed by a solid/liquid separator. The most frequently used acids include
sulfuric (H2S04), hydrochloric (HC1), and nitric (HN03). Although any acidic pH
can theoretically be used, acid leaching processes are normally run at a pH from
1 to 4.
1.3 Description of Acid Leaching Process
Acid leaching processes can be categorized into two major types:
(a) treatment by percolation of the acid through the solids or (b) treatment by
dispersion of the solids within the acid solution. Both treatments are followed
by subsequent separation of the solids from the liquid. In both types of
systems, sufficient acid must be supplied to keep the pH at the level needed to
effectively leach the metals from the waste.
1.3.1 Percolation Processes
Percolation is typically conducted in batch tanks. Batch percolators are
large tanks ranging in size up to 50,000 gallons. First, the solids are placed
in the tank, and then acid is added. The acid percolates through the solids and
drains out through screens or a porous medium in the tank bottom. Following
treatment, the solids are removed and further treated using stabilization, and/or
they are land disposed.
1.3.2 Dispersed-Solids Processes
Acid leaching by dispersion of fine solids into the acid is performed in
batch tanks. The untreated waste and the acid are mixed in the reaction tank to
ensure effective contact between the solids and the acid. Following mixing, the
suspension may be pumped to stirred holding tanks, where the leaching is allowed
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to proceed to completion. The treated solids are then usually separated from the
acid by filtration and further treated using stabilization, and/or they are land
disposed.
1.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether acid leaching will achieve the same level of
performance on an untested waste that it achieved on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the solid waste particle size, (b) the alkalinity of
the waste, (c) the solubility of metal constituents in the acid, and (d) the
concentration of leachable metals.
1.4.1 Solid Waste Particle Size
Both the solubility reaction rate of the acid with the hazardous metal
constituents in the waste and the rate of transport of acid to and from the site
of the hazardous constituents are affected by the size of the solid waste
particles. The smaller the particles, the more rapidly they will leach because
of the increased surface area of the waste that is exposed to the acid. If the
particle size of the untested waste is greater than that of the tested waste, the
system may not achieve the same performance. Grinding the untested waste may be
required to reduce the particle size and achieve the same treatment performance,
or it may be necessary to consider other, more applicable treatment technologies
for treatment of the untested waste.
1.4.2 Alkalinity of the Waste
The neutralizing capacity (or alkalinity) of the waste solids affects the
amount of acid that must be added to the waste to achieve and/or maintain the
desired reactor pH. In addition to dissolving the waste contaminants, the acid
will also dissolve some of the alkaline bulk solids; therefore, highly alkaline
wastes require more acid or a stronger acid to maintain pH during treatment. If
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the alkalinity In an untested waste is greater than that in a tested waste, the
system may not achieve the same performance. Use of additional acid or a
stronger acid may be required to compensate for the increased alkalinity and
achieve the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
1.4.3 Solubility of the Metal Constituents in the Add
The metal constituents must dissolve in the acid to form soluble salts for
the process to be effective. Thus, the acid selected should be one that forms
soluble salts for all of the constituents to be removed. If the solubility of
a metal constituent(s) of concern in an untested waste is less than that of
another constituent(s) of concern in a previously tested waste in the same acid,
or less than the solubility of the same metal(s) tested with a different acid,
the system may not achieve the same performance. Use of another acid may be
required to increase the solubilities of the metal constituent(s) of concern and
achieve the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
1.4.4 Concentration of Leachable Metals
The amount of Teachable metals is a measure of the maximum fraction of the
waste that can be expected to leach in the acid leaching system. A relatively
low concentration of Teachable metals implies that most of the waste will remain
in the solid or slurry waste residues (i.e., it is nonleachable). If the
concentration of Teachable metals in the untested waste is significantly less
than that in the tested waste, the system may not achieve the same performance.
Use of a higher concentration of acid or a stronger acid may be required to leach
less Teachable components and achieve the same treatment performance, or other,
more applicable treatment technologies may need to be considered for treatment
of the untested waste.
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1.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an acid
leaching system, EPA examines the following parameters: (a) the residence time,
(b) the type and concentration of acid used, (c) the pH, and (d) the degree of
mixing.
1.5.1 Residence Time
The extent of reaction and dissolution of the contaminants in the acid is
directly related to the contact time. EPA monitors the residence time to ensure
that sufficient time is provided to effectively leach the metal contaminants from
the waste.
1.5.2 Type and Concentration of Add Used
If the hazardous constituents to be removed in the acid leaching system are
already present in the waste in soluble form, or are solubilized by pH reduction,
then any acid that will reduce the pH to the desired value may be used. However,
if chemical reaction is necessary to form soluble species, then the appropriate
acid, as well as the appropriate concentration of the acid, must be used to
ensure effective leaching of the metal constituents. EPA examines the type and
concentration of acid used to ensure that the acid solution selected is capable
of effectively leaching the metal constituents from the waste.
1.5.3 pH
For dispersed solids systems, the feed of acid to the leaching reactor is
based on pH monitoring and control because the reaction rate is highly pH
dependent. Therefore, a pH should be determined, based on the residence time and
amount of hazardous metal constituents in the waste, that provides for complete
dissolution of metal constituents. For percolation systems, pH monitoring of the
acid percolating through the tank (i.e., leaving the system) should ensure that
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enough acid is being added. EPA monitors the pH to ensure that the system is
operating at the appropriate design conditions and to diagnose operational
problems.
1.5.4 Degree of Mixing
Mixing provides greater contact between the acid and the solid waste
particles, ensuring more rapid leaching of metal contaminants from the waste.
The quantifiable degree of mixing is a complex assessment that includes, among
other factors, the amount of energy supplied, the length of time the material is
mixed, and the related turbulence effects of the specific size and shape of the
tank. Although the exact degree of mixing is beyond simple measurement, EPA
evaluates it by considering whether mixing is provided and whether the type of
mixing device is one that could be expected to achieve uniform mixing of the
waste solution.
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1.6 References
Kirk-Othmer. 1978. Encyclopedia of chemical technology. Vol. 2, pp. 140-144.
New York: John Wiley and Sons.
McCabe, L., and Smith, J. 1976. Unit operations of chemical engineering. 3rd
ed., pp. 607-610. New York: McGraw-Hill Book Co.
Perry, R.H., and Chilton, C.H. 1973. Chemical engineers' handbook. 5th ed.,
pp. 19-41 to 19-43. New York: McGraw-Hill Book Co.
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2. FILTRATION TECHNOLOGIES
2.1 Applicability
Filtration technologies (i.e., polishing filtration and sludge filtration)
are used to remove particles from waste streams that are predominantly water or
to remove water from wet solids and sludges.
Polishing filtration is a treatment technology applicable to wastewaters
containing relatively low concentrations of solids (usually less than
1,000 mg/1). This type of filtration is typically used as a polishing step for
the supernatant liquid following chemical precipitation and settling/
clarification of wastewaters containing metal and other inorganic precipitates.
Polishing filtration removes particles that are difficult to settle because of
their size and/or density, as well as precipitated particles from an
underdesigned settling system.
Sludge filtration, also known as sludge dewatering or cake-formation
filtration, is a technology used on wastes that contain high concentrations of
suspended solids, usually higher than 1 percent (10,000 mg/1). Sludge filtration
is commonly applied to waste sludges, such as clarifier solids, for dewatering.
Typically, these sludges can be dewatered to 19 to 50 percent solids
concentration using this technology.
2.2 Underlying Principles of Operation
The basic principle of operation for sludge and polishing filtration is the
removal/separation of particles from a mixture of fluid and particles by a medium
that permits the flow of the fluid but retains the particles. Usually, the
larger the particles, the easier they are to remove from the fluid.
Extremely small particles, in the colloidal size range, may not be removed
or filtered effectively in a sludge or polishing filtration system and thus may
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appear in the wastewater passing through the filter. To mitigate this problem,
the wastewater can be treated prior to filtration to modify the particle size
distribution and/or particle electrostatic charge by using appropriate
precipitants, coagulants, flocculants, and filter aids. The selection of the
appropriate precipitant and coagulant is important because they affect the type
of waste particles formed. For example, lime (calcium hydroxide) precipitation
usually produces larger, less gelatinous particles (which are easier to remove
from aqueous wastes by using filtration) than does caustic soda (sodium
hydroxide) precipitation. For particles that become too small or too highly
charged to remove effectively, the use of coagulants and flocculants both
decreases particle charge and increases the size of the particles. Also, if
pumps are used, shear can be minimized by lowering the pump speed or using a
low-shear type of pump. Excessive shear breaks up agglomerated particles, making
them smaller and more difficult to filter. Filter aids such as diatomaceous
earth are used to precoat cloth-type filter media and to provide an initial
filter cake onto which additional solids can be deposited during the filtration
process. The presence of the precoat aids in the removal of small particles from
the solution being filtered. These particles adhere to, and are actually
filtered by, the precoat solids during the filtration process.
2.3 Description of Filtration Processes
2.3.1 Polishing Filtration Processes
During polishing filtration, wastewater may flow by gravity or under
pressure to the filter. The two most common polishing filtration processes are
cartridge and granular bed filtration. Both processes remove particles that are
much smaller than the pore size of the filter medium by straining, adsorption,
and coagulation/flocculation mechanisms; these processes are also capable of
producing an effluent with a low level of solids (typically less than 10 mg/1).
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2.3.1.1 Cartridge Filtration
Cartridge filters can be used for relatively low waste feed flows. In this
process, a hollow, cylindrically shaped cartridge with a matted cloth-type filter
medium is placed within a sealed vessel. Wastewater is pumped through the
cartridge wall until the flow drops excessively or until the pumping pressure
becomes too high because of plugging of the filter medium. The sealed vessel is
then opened, and the plugged cartridge is removed and replaced with a new
cartridge. The plugged cartridge is then cleaned and/or disposed of. Cartridge
filters can be assembled in a parallel arrangement to increase the overall system
flow.
2.3.1.2 Granular Bed Filtration
For relatively large volume flows, granulated media such as sand, garnet,
or anthracite coal are used singly or in combination to trap suspended solids
within the pore spaces of the media. Dual and multimedia filter arrangements
allow higher flow rates and efficiencies. Typical hydraulic loading rates range
from 2 to 5 gal/sq ft-min for single-medium filters and from 4 to 8 gal/sq ft-min
for multimedia filters.
In this process, wastewater is either gravity-fed or pumped through the
granular bed media and filtered until either the flow drops excessively or the
pumping pressure becomes too high because of plugging of the granular filter
media. Granular media filters are usually cleaned by backwashing with previously
filtered water in an upflow manner to expand the bed, loosen the media granules,
and resuspend the entrapped filtered solids. The backwash water, which may be
as much as 10 percent of the volume of the filtered wastewater, is then returned
to the head of the wastewater treatment system so that the filtered solids in the
backwash water can be settled out of solution and the water refiltered prior to
discharge.
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2.3.2 Sludge Filtration Process
For sludge filtration, waste is pumped through a cloth-type filter medium
(also known as pressure filtration, such as that performed with a plate-and-frame
filter); drawn by vacuum through the cloth or metal mesh medium (also known as
vacuum filtration, such as that performed with a vacuum drum filter); or
gravity-drained and mechanically pressed through two continuous fabric belts
(also known as belt filtration, such as that performed with a belt filter press).
In all cases, the solids "cake" builds up on the filter medium and acts as a
filter for subsequent solids removal.
For a plate-and-frame filter, removal of the solids is accomplished by
taking the unit off-line, opening the filter, and using mechanical or manual
methods to scrape off the solids (a batch process). For the vacuum filter, cake
is removed continuously by using an adjustable knife mechanism to scrape the
sludge from the vacuum drum as the drum rotates. For the belt filter, the cake
is continuously removed by a discharge roller and blade, which dislodge the cake
from the belt. For a specific sludge, the plate-and-frame filter will usually
produce the driest cake (highest percentage of solids). The belt filter produces
a drier cake than does a vacuum filter, but usually not as dry as that produced
by a plate-and-frame filter. Dewatered solids are further treated in processes
such as sludge drying, incineration, solvent extraction (if treatable levels of
organics are present), stabilization (if treatable levels of Teachable metals are
present), and/or disposal. The liquid filtrate that penetrates the filter medium
is further treated in processes such as polishing filtration, carbon adsorption,
and biological treatment, and/or it is disposed of.
2.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether polishing and sludge filtration will achieve the
same level of performance on an untested waste as on a previously tested waste
and whether performance levels can be transferred, EPA examines the following
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waste characteristics: (a) the solid waste particle size and (b) the type of
solid waste particles.
2.4.1 Solid Waste Particle Size
Extremely small particles, in the colloidal range, may not be filtered
effectively in a polishing filter and thus may appear in the filtrate.
In sludge filtration, the smaller the particle size, the more the particles
tend to go through the filter media. This is especially true for a vacuum
filter. For a pressure filter (such as a plate and frame), smaller particles may
require higher pressures for equivalent fluid throughput because the smaller pore
spaces between particles collected on the filter medium create resistance to
flow. If the solid waste particje size distribution of an untested waste is
significantly lower than that of the tested waste, the system may not achieve the
same performance. In practice, it is usually difficult to measure particle size
accurately and representatively. However, it can be said that individual
particles formed by chemical precipitation are typically much smaller than
particles formed mechanically (e.g., by erosion, grinding, or abrasion).
Pretreatment of the waste with coagulants and flocculants may be required to
increase the particle sizes and achieve the same treatment performance, or other,
more applicable treatment technologies may need to be considered for treatment
of the untested waste.
2.4.2 Type of Solid Waste Particles
Some solids formed during metal precipitation are gelatinous in nature
because of high levels of electrostatic charge. Such solids are difficult to
filter by polishing filtration and may be impossible to dewater effectively by
sludge filtration. In most cases, solids can be made less gelatinous by use of
the appropriate coagulants and coagulant dosage prior to settling/clarification,
or after settling/clarification but prior to filtration. In addition, the use
of lime instead of caustic in chemical precipitation of metals reduces the
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formation of gelatinous solids. Also, in the case of sludge filtration, adding
filter aids, such as lime or diatomaceous earth, to a gelatinous sludge increases
its filterability significantly. Finally, precoating filter media with
diatomaceous earth prior to sludge filtration assists in dewatering gelatinous
sludges. If solids in an untested waste are significantly more gelatinous than
those in the tested waste, the system may not achieve adequate performance.
Pretreatment of the waste with coagulants may be required to decrease the
gelatinous nature of the waste and achieve the same treatment performance, or
other, more applicable treatment technologies may need to be considered for
treatment of the untested waste.
2.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a polishing
filtration system, EPA examines the following parameters: (a) the type and size
of the filter; (b) the filtration pressure; (c) the amount and type of
coagulants, flocculants, and filter aids used; (d) the hydraulic loading rate;
and (e) the pore size of the filter media.
2.5.1 Type and Size of Filter
The type and size of the filtration system used is dependent on the rate
of feed, the nature of the particles to be removed/separated, the desired solids
concentration in the filtrate (usually just for polishing filtration) and in the
filter cake (in sludge filtration), the amount and concentration of solids in the
feed, and the required downtime for solids removal and maintenance. As noted
earlier, with polishing filtration, cartridge filtration is limited to lower
volume wastewaters and those with lower solids concentrations as compared to
granular bed filtration. For granular bed filtration, when more than one medium
is used (dual and multimedia filter arrangements such as sand, garnet, and
anthracite coal), a higher capacity can be expected for the same size filter bed.
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In sludge filtration, typically a pressure filter (such as a plate and
frame) will yield a drier cake than a belt or vacuum filter and will also be more
tolerant of variations in influent sludge characteristics. Pressure filters,
however, are characterized by batch processes requiring downtime for solids
removal. When cake is built up to the maximum depth physically possible
(constrained by filter geometry) or to the maximum design pressure, the
filtration system 1s taken off-line while the cake 1s removed. (An alternate
unit can be put on-line while the other is being cleaned.) Belt and vacuum
filters are continuous systems (i.e., cake discharges continuously), but each of
these filters is usually much larger than a pressure filter with the same
capacity.
For all filtration processes, the larger the filter, the greater its
hydraulic capacity (overall throughput) and the longer the filter runs between
solids removal. EPA examines the type and size of the filter chosen to ensure
that it is capable of achieving effective filtration of the wastewater.
2.5.2 Filtration Pressure
Pressure impacts both the design pore size of the filter media and the
design feed flow rate (hydraulic loading rate). The higher the feed pressure,
the longer the run will be prior to solids removal in polishing filtration. In
sludge filtration, at higher pressures the cake will be drier and the runs will
usually be longer prior to cake discharge. However, for gelatinous solids, such
as some metal hydroxides, excessive pressure may cause the solids to clog the
filter pores and prevent additional polishing or sludge filtration. Also, high
pressures may force particles through the filter medium, especially early in a
filter run, resulting in ineffective filtration. In vacuum sludge filtration,
the maximum amount of vacuum typically applied ranges from 20 to 25 inches of
mercury. (The absolute maximum amount of vacuum that can theoretically be
applied is 29.9 inches of mercury, or atmospheric pressure.) For belt filters,
neither hydraulic pressure nor vacuum is applied to the waste feed (although
mechanical pressure is applied). For plate-and-frame and vacuum filtration
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systems, EPA monitors the filtration pressure (or vacuum) applied to the waste
feed continuously, if possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
2.5.3 Amount and Type of Coagulants, Flocculants, and Filter Aids
Coagulants, flocculants, and filter aids may be mixed with the waste feed
prior to filtration. Coagulants and flocculants affect the type and size of the
particles in the waste and, hence, their ease of removal. In vacuum filtration
their effect is particularly significant since they may make the difference
between no cake formation and the forming of a relatively dry cake. In a
pressure filter, coagulants, flocculants, and filter aids can significantly
improve overall throughput and cake dryness. Filter aids, such as diatomaceous
earth, can be precoated on all sludge filters for particularly difficult-to-
filter sludges (those containing a high concentration of gelatinous solids). The
precoat layer acts somewhat like a filter in that sludge solids are trapped in
the precoat pore spaces. In polishing filtration, filter aids both improve the
effectiveness of filtering gelatinous particles and increase the time that the
filter can stay on-line.
Coagulants, flocculants, and filter aids are particularly useful when the
sludge (in the case of sludge filters) and wastewater (in the case of polishing
filtration) have a high percentage of very small particles or when the
concentration of solids in the waste feed or wastewater is low. Inorganic
coagulants include alum, ferric sulfate, and lime. Organic flocculants, which
are called polyelectrolytes, are anionic, cationic, or nonionic in nature.
Diatomaceous earth is the most commonly used filter aid. The use of coagulants,
flocculants, and filter aids increases the amount of solids requiring disposal.
Polyelectrolyte flocculant usage, however, usually does not increase the solids
volume significantly because the required dosage is relatively low. If the
addition of coagulants, flocculants, and filter aids is required, EPA examines
the amount and type added to the waste, along with their method of addition, to
ensure effective dewatering and filtration.
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2.5.4 Hydraulic Loading Rate
Lower hydraulic loading rates generally improve filtration performance.
For sludge filtration, higher hydraulic loading rates for a given size filter
yield greater overall throughput but result in the formation of wetter cakes
(lower percent solids) and, for plate-and-frame filters, shorter cycle times.
For polishing filtration, higher hydraulic loading rates yield greater throughput
but result in shorter cycle times. EPA monitors the hydraulic loading rate to
ensure effective dewatering and filtration of waste sludge and effective
filtration of wastewater.
2.5.5 Pore Size of the Filter Media
For polishing filtration systems, the pore size of the filter media
determines the particle size that will be effectively removed from the
wastewater. EPA examines the pore size of the filter media to ensure effective
filtration of wastewater. For sludge filters, the pore size and type relate to
resistance to abrasion and corrosion.
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2.6 References
Anonymous. 1985. Feature report. Wastewater treatment. Chemical Engineering
92(18):71-72.
Grain, R.W. 1981. Solids removal and concentration. In Third Conference on
Advanced Pollution Control for the Metal Finishing Industry, pp. 56-62.
Cincinnati, Ohio: U.S. Environmental Protection Agency.
Eckenfelder, W.W., Jr., Patoczka, J., and Watkins, A. 1985. Wastewater
treatment. Chemical Engineering, September 2, 1985.
Kirk-Othmer. 1980. Encyclopedia of chemical technology. 3rd ed. Vol. 10. New
York: John Wiley and Sons.
Perry, R.H., and Chilton, C.H. 1973. Chemical engineers' handbook. 5th ed.,
Section 19. New York: McGraw Hill Book Co.
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3. HIGH TEMPERATURE METALS RECOVERY
3.1 Applicability
High temperature metals recovery (HTMR) is a technology applicable to
wastes containing metal oxides and metal salts (including cadmium, chromium,
lead, nickel, and zinc compounds) at concentrations ranging from 10 percent to
over 70 percent with low levels (i.e., below 5 percent) of organics and water in
the wastes. There are a number of different types of high temperature metals
recovery systems, which generally differ from one another in the source of energy
used and the method of recovery. These HTMR systems include the rotary kiln
process, the plasma arc reactor, the rotary hearth electric furnace system, the
molten slag reactor, and the flame reactor.
HTMR is generally not used for mercury-containing wastes even though
mercury will volatilize readily at the process temperatures present in the high
temperature units. The retorting process is normally used for mercury recovery
because mercury is very volatile and lower operating temperatures can be used.
Thus, the retorting process is more economical than HTMR for mercury-bearing
wastes. Retorting is discussed in Section II.B.5.
The HTMR process has been demonstrated on wastes such as baghouse dusts and
dewatered scrubber sludge from the production of steels and ferroalloys. Zinc,
cadmium, and lead are the metals most frequently recovered. The process has not
been extensively evaluated for use with metal sulfides. The sulfides are
chemically identical to natural minerals ordinarily present in ores used as
feedstocks by primary smelters. Some sulfide-bearing wastes from the chrome
pigments industry have been sent to such primary smelters. However, with
sulfides, a possibility exists for formation of either carbon disulfide from
reaction with carbon or sulfur dioxide from reaction with oxygen in the HTMR
processes.
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Metal halide salts are also not directly used in HTMR processes. They,
however, may be converted to oxides or hydroxides, which are acceptable
feedstocks for HTMR processes.
3.2 Underlying Principles of Operation
The basic principle of operation for HTMR is that metal oxides and salts
are separated from a waste through a high temperature thermal reduction process
that uses carbon, limestone, and silica (sand) as raw materials. The carbon acts
as a reducing agent and reacts with metal oxides to generate carbon dioxide and
free metal. The silica and limestone serve as fluxing agents. This process
yields a metal product for reuse and reduces the concentration of metals in the
residuals and, hence, the amount of waste that needs to be land disposed. An
example HTMR reaction is the recovery of zinc, which proceeds as follows:
' 2 ZnO + C •» 2 Zn + C02 •
3.3 Description of High Temperature Metals Recovery Process
The HTMR process consists of a mixing unit, a high temperature processing
unit (kiln, furnace, etc.), a product collection system, and a residual treatment
system. A schematic diagram for a high temperature metals recovery system is
shown in Figure 22.
The mixing unit homogenizes metal-bearing wastes, thus minimizing feed
variations to the high temperature processing unit. Before the wastes are fed
into the high temperature processing unit, fluxing agents and carbon can be added
to the mixing unit and mixed with the wastes. The fluxes used (sand and
limestone) are often added to react with certain metal components, preventing
their volatilization and resulting in an enhanced purity of the desired volatile
metals removed.
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AIR OR O,
1
EXHAUST AIR OR GAS
TO THE
ATMOSPHERE
ro
o
I
WASTE INFLUENT ^
CARBON ^
(REDUCING AGENT)
FLUXES ^
/LIMESTONE. SANDI^
MIXING
UNIT
w
^
HIGH
TEMPERATURE
PROCESSING
UNIT
^.
PRODUCT
COLLECTION
UNIT
(CONDENSOR OR
CONDENSOR
AND BAGIIOUSE)
VOLATILE
METAL
PRODUCTS
TO REUSE
OR FURTHER
REFINEMENT
PRIOR TO
REUSE
RESIDUAL
COLLECTION
(QUENCH TANK)
NON-VOLATILE METAL PRODUCTS TO REUSE.
FURTHER RECOVERY IN A FURNACE.
STABILIZATION FOLLOWED BY LAND DIPOSAL.
OR DIRECTLY TO LAND DISPOSAL
Figure 22 High Temperature Metals Recovery System
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The blended waste materials are fed to a furnace, where they are heated to
temperatures ranging from 1100 to 1400*C (2012 to 2552*F), resulting in the
reduction and volatilization of the desired metals. The combination of
temperature, residence time, and turbulence provided by rotation of the unit or
addition of an air or oxygen stream helps ensure the maximum reduction and
volatilization of metal constituents.
The product collection system can consist of either a condenser or a
combination condenser and baghouse. The choice of a particular system depends
on whether the metal is to be collected in the metallic form or as an oxide.
Recovery and collection are accomplished for the metallic form by condensation
alone, and for the oxide by reoxidation, condensation, and subsequent collection
of the metal oxide particulates in a baghouse. There is no difference in these
two types of metal recovery and collection systems relative to the kinds of waste
that can be treated; the use of one system or the other simply reflects the
facility's preference relative to product purity. In the former case, the direct
condensation of metals allows for the separation and collection of individual
metals in a relatively uncontaminated form; in the latter case, the metals are
collected as a combination of several metal oxides.
The treated waste residual slag, containing higher concentrations of the
less-volatile metals than the untreated waste, is sometimes cooled in a quench
tank and (a) reused directly as a product (e.g., a waste residual containing
mostly iron can be reused in steelmaking); (b) reused after further processing
(e.g., a waste residual containing oxides of iron, chromium, and nickel can be
reduced to metallic form and then recovered for use in the manufacture of
stainless steel); (c) stabilized (material has no recoverable value) to
immobilize any remaining metal constituents and then land disposed; or
(d) directly land disposed as a slag.
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3.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether high temperature metals recovery will achieve the
same level of performance on an untested waste that it achieved on a previously
tested waste and whether performance levels can be transferred, EPA examines the
following waste characteristics: (a) the concentrations of undesirable volatile
metals, (b) the metal constituent boiling points, and (c) the thermal
conductivity of the waste.
3.4.1 Concentration of Undesirable Volatile Metals
Because HTMR is a recovery process, the product must meet certain purity
requirements prior to reuse. If the waste contains other volatile metals, such
as arsenic or antimony, which are difficult to separate from the desired metal
products and whose presence may affect the ability to reuse the product or refine
it for subsequent reuse, HTMR may not be an appropriate technology. If the
concentration of undesirable volatile metals in the untested waste is
significantly higher than that in the tested waste, the system may not achieve
the same performance, and other, more applicable treatment technologies may need
to be considered for treatment of the untested waste.
3.4.2 Metal Constituent Boiling Points
The greater the ratio of volatility of the waste constituents, the more
easily the separation of these constituents can proceed. This ratio is called
relative volatility. EPA recognizes, however, that the relative volatilities
cannot be measured or calculated directly for the types of wastes generally
treated by high temperature metals recovery. This is because the wastes usually
consist of a myriad of components, all with different vapor pressure-versus-
temperature relationships. However, because the volatility of components is
usually inversely proportional to their boiling points (i.e., the higher the
boiling point, the lower the volatility), EPA uses the boiling point of waste
components as a surrogate waste characteristic for relative volatility. If the
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differences in boiling points between the more volatile and less volatile
constituents are significantly smaller in the untested waste than in the tested
waste, the system may not achieve the same performance and other, more applicable
treatment technologies may need to be considered for treatment of the untested
waste.
3.4.3 Thermal Conductivity of the Waste
The ability to heat constituents within an HTMR process feed matrix is a
function of the heat transfer characteristics of the individual feed components
(coke, limestone, untreated waste, etc.). The constituents being recovered from
the waste must be heated to near or above their boiling points in order for them
to be volatilized and recovered. The rate at which heat will be transferred to
the feed mixture is dependent «n the mixture's thermal conductivity, which is the
ratio of the conductive heat flow to the temperature gradient across the
material. Thermal conductivity measurements, as part of a treatability
comparison of two different wastes to be treated by a single HTMR system, are
most meaningful when applied to wastes that are homogeneous (i.e., uniform
throughout). As wastes exhibit greater degrees of nonhomogeneity, thermal
conductivity becomes less accurate in predicting treatability because the
measurement reflects heat flow through regions having the greatest conductivity
(i.e., the path of least resistance) and not heat flow through all parts of the
waste. Nevertheless, EPA believes that thermal conductivity may provide the best
measure of performance of heat transfer. If the thermal conductivity of the
untested waste is significantly lower than that of the tested waste, the system
may not achieve the same performance and other, more applicable treatment
technologies may need to be considered for treatment of the untested waste.
3.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an HTMR
system, EPA examines the following parameters: (a) the HTMR temperature, (b) the
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residence time, (c) the degree of mixing, (d) the carbon content of the feed, and
(e) the calcium-to-silica ratio of the feed.
3.5.1 HTMR Temperature
Temperature provides an indirect measure of the energy available (i.e.,
Btu/hr) to volatilize the metal waste constituents. The higher the temperature
in the high temperature processor, the more likely it is that the constituents
will react with carbon to form free metals and volatilize. The temperature must
be equal to or greater than the boiling point of the metals being volatilized for
recovery. However, excessive temperatures could volatilize less-volatile,
undesirable metals into the product, possibly inhibiting the potential for reuse
of the product. EPA monitors the HTMR processor temperature continuously, if
possible, to ensure that the system is operating at the appropriate design
condition (at or above the boiling point(s) of the metal or metals being
recovered, but not so high that it volatilizes other unwanted constituents) and
to diagnose operational problems.
3.5.2 Residence Time
The residence time affects the amount of volatile metals volatilized and
recovered. It is dependent on the HTMR processor temperature and the thermal
conductivity of the feed blend. EPA monitors the residence time to ensure that
sufficient time is provided to effectively volatilize the volatile constituents
for recovery.
3.5.3 Degree of Nixing
Effective mixing of the waste with coke, silica, and limestone is necessary
to produce a uniform feed blend to the system. The quantifiable degree of mixing
is a complex assessment that includes, among other things, the amount of energy
supplied, the length of time the material is mixed, and the related turbulence
effects of the specific size and shape of the tank or vessel. This is beyond the
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scope of simple measurement. EPA, however, evaluates the degree of mixing
qualitatively by considering whether mixing is provided and whether the type of
mixing device is one that could be expected to achieve uniform mixing of the
waste.
3.5.4 Carbon Content of the Feed
The amount of carbon added to the waste must be sufficient to ensure
complete reduction of the volatile metals being recovered. EPA examines the
basis for calculation of the amount of carbon added to the waste to ensure that
sufficient carbon is being used in the feed blend to effectively reduce metal
compounds.
3.5.5 Calcium-to-Silica Ratio of the Feed
The calcium-to-silica ratio in the feed blend must be controlled to limit
precipitation of metallic iron in the high temperature processor. The iron forms
as solid calcium iron silicate, which is very difficult to subsequently process
into any useful material. Aluminum oxide will also undergo reactions with lime
and silica to form calcium aluminosilicates, which will lower the density and
increase the volume of slag generated.
Precipitates and modified slags affect the reduction, volatilization, and
recovery of volatile metals by changing heat flow characteristics in the system
and by undergoing secondary, high temperature chemical reactions with metal
oxides in the feed, converting them to the previously noted inert metal silicates
or silicoaluminates. The ratio of calcium to silica to be used is dependent on
the waste composition. Generally, a one-to-one silica-to-calcium oxide ratio is
highly desired, so amounts of limestone and sand need to be adjusted based on the
calcium and silica content of the waste to achieve this ratio.
Excess lime may also be added to fix sulfur in the feed as calcium sulfate.
This will prevent the volatile metals from reacting with sulfur to form metal
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sulfides, thereby lowering the recovery of metals or oxides. EPA monitors the
amounts of limestone and sand added to the waste to ensure that the
calcium-to-silica ratio selected to maximize metal or oxide recovery is
maintained during treatment.
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3.6 References
Center for Metals Production. 1985. Electric arc furnace dust—disposal,
recycle and recovery. Pittsburgh, Pa.
Duby, P. 1980. Extractive metallurgy. In Kirk-Othmer encyclopedia of chemical
technology. 3rd ed. Vol. 9, p. 741. New York: John Wiley and Sons.
Lloyd, T. 1980. Zinc compounds. In Kirk-Othmer encyclopedia of chemical
technology. 3rd ed. Vol. 24, p. 824. New York: John Wiley and Sons.
Maczek, H., and Kola, R. 1980. Recovery of zinc and lead from electric furnace
steelmaking dust at Berzelius. Journal of Metals 32:53-58.
Price, L. 1986. Tensions mount in EAF dust bowl. Metal Producing. February
1986.
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4. ION EXCHANGE
4.1 Applicability
Ion exchange is a treatment technology applicable to (a) metals in
wastewaters where the metals are present as soluble ionic species (e.g., Cr*3 and
Cr04~2); (b) nonmetalllc anions such as halides, sulfates, nitrates, and
cyanides; and (c) water-soluble, ionic organic compounds including (I) acids such
as carboxylics, sulfonics, and some phenols, at a pH sufficiently alkaline to
yield ionic species, (2) amines, when the solution acidity is sufficiently acid
to form the corresponding acid salt, and (3) quaternary amines and alkysulfates.
4.2 Underlying Principles of Operation
Ion exchange, when used in hazardous waste treatment, is a reversible
process in which hazardous cations and/or anions are removed from an aqueous
solution and are replaced by nonhazardous cations and/or anions such as sodium,
hydrogen, chloride, or hydroxyl ions. Ion exchange resins are cationic if they
exchange positive ions (cations) and anionic if they exchange negative ions
(anions). When the waste stream to be treated is brought into contact with a bed
of resin beads (usually in a packed column), an exchange of hazardous ions for
nonhazardous ions occurs on the surface of the resin beads. Initially, a
nonhazardous ion is loosely bound to the surface of the resin. When a hazardous
ion is near the resin, it is preferentially adsorbed to the surface of the resin
(based on the differences in ionic potential), releasing the nonhazardous ion.
Cation exchange resins contain mobile positive ions, such as hydrogen (H*)
or sodium (Na*), which are attached to immobile functional acid groups, such as
sulfonic (S03-) and carboxylic (COO") groups. Anion exchange resins have
immobile basic ions, such as amine (NH2~), to which the mobile anions, such as
hydroxyl (OH") or chloride (Cl~), are attached.
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Ion exchange material is contacted with the solution containing the ion to
be removed until the active sites in the exchange material are partially or
completely used up ("exhausted") by that ion. For example, a cation exchange
resin (designated R") to which a mobile positive ion (N*) is attached reacts with
a solution of electrolyte (M*X~) as shown below:
MT + O* - R'M* + N* + X'
After exhaustion, the resin is then contacted with a relatively low volume of a
very concentrated solution of the exchange ion to convert ("regenerate") it back
to its original form. The regeneration reaction may be written as follows:
R~M* -i- N* (high concentration) -* R'N* + M*
For instance, in the case of a sodium-based resin, a strong solution of sodium
chloride is typically the regenerant solution. The regenerant solution forces
the previously removed ions back into solution. This relatively low volume
solution, now highly concentrated with the contaminant ions, must then be treated
prior to disposal for recovery or removal of the hazardous cation or anion
contaminants. There will continue to be a high concentration of the regenerant
ion (sodium in the above example) in the used regenerant solution because excess
regenerant ion is necessary to force the contaminant ions back into solution.
The direction and extent of the completion of the exchange reaction depend upon
the equilibrium that is established between the ions in the solution (M*X~) and
those in the exchange material (R"N*).
4.3 Description of Ion Exchange Process
Most ion exchange operations are conducted in packed columns. The aqueous
solution to be treated is continuously fed to either the top or the bottom of the
column. A typical fixed-bed ion exchange column consists of a vertical
cylindrical pressure vessel with corrosion-resistant linings. If appropriate,
a filter is installed at the inlet of the column to remove suspended particles
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because they may plug the exchange resin. Spargers are provided at the top and
bottom of the column to distribute waste flow. Frequently, a separate
distributor is used for the regenerant solution to ensure an even flow. The
resin bed, usually consisting of several feet of ion exchange resin beads, is
supported by a screen near the bottom distributor or by a support bed of inert
granular material. Externally, the unit has a valve manifold to permit downflow
operation, upflow backwashing (to remove any suspended material), injection of
the regenerant solution, and rinsing of any excess regenerant.
A typical process schematic for a basic two-step cation/anion ion exchange
system is presented in Figure 23. The ion exchange system shown in this
schematic includes a series treatment with separate cation and anion exchange
systems. Some systems contain both anion and cation exchange resins in the same
vessel'. The pressure vessels used for ion exchange generally range in size from
2 to 6 feet in diameter for prepackaged modular systems, which typically handle
25- to 300-gpm flow rates, to a maximum custom size of 12 feet in diameter, which
can handle flow rates up to 1,150 gpm. The height of these vessels varies
between 6 and 10 feet to provide adequate resin storage, distribution nozzle
layout, and freeboard capacity for bed expansion during backwashing. The nominal
surface loading area of the ion exchange vessels ranges from 8 to 10 gpm per
square foot.
4.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether ion exchange will achieve the same level of
performance on an untested waste as on a previously tested waste and whether
performance levels can be transferred, EPA examines the following waste
characteristics: (a) the concentration and valence of the contaminant, (b) the
concentration of competing ionic species, (c) the concentration of interfering
inorganics and organics, (d) the concentrations of dissolved and suspended solids
and oil and grease, and (e) the corrosiveness relative to the resin material.
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USED BACKFLUSH WATER
TO TREATMENT
USED BACKFLUSH WATER
TO TREATMENT
WASTEWATER
INFLUENT
BACKFLUSH WATER
RINSE WATER
ACID REOENERANT
BACKFLUSH
WATER'
CATION
EXCHANGE
SYSTEM
USED REGENERANT
SOLUTION AND RINSES
TO RECOVERY
AND/OR TREATMENT
RINSE WATER
CAUSTIC
REGENERANT
ANION
EXCHANGE
SYSTEM
USED REGENERANT
SOLUTION AND RINSES
TO RECOVERY
AND/OR TREATMENT
TREATED
EFFLUENT
TO FURTHER
TREATMENT
AND/OR DISPOSAL
Figure 23 Two-Step Cation/AnIon Ion Exchange System
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4.4.1 Concentration and Valence of the Contaminant
As the concentration and valence of adsorbable ions In the wastewater
Increase, the size of the resin bed required will increase as well, or,
alternatively, the bed will become exhausted more rapidly. This is because a
given amount of ion exchange resin has a limited number of sites to adsorb
charged Ions. If, for example, the valence is doubled or the concentration of
the adsorbed ions is doubled, the sites will be exhausted twice as quickly.
Hence, very high concentrations of the waste may be inappropriate for ion
exchange because of rapid site exhaustion, which could conceivably require
regenerant volumes to be essentially equal to waste flow volumes. If the
concentration and/or the valence of the contaminant in an untested waste is
significantly higher than that of the tested waste, the system may not achieve
the same performance. A larger exchange bed or more frequent regeneration may
be required to exchange higher concentrations and/or higher valences of the
contaminant and achieve the same treatment performance, or other, more applicable
treatment technologies may need to be considered for treatment of the untested
waste.
4.4.2 Concentration of Competing Ionic Species
The presence of other contaminants or ions in the wastewater can affect the
performance of the ion exchange unit in removing the hazardous contaminant of
concern. Other ions in the wastewater with the same charge as the contaminant
of concern will compete for exchange sites on the resin. Also, ions with a
higher valence will be preferentially adsorbed. While a low concentration of the
contaminant of concern may be readily removed from a solution with a low
concentration of other similarly charged ionic species, the contaminant may not
be removed as efficiently from solutions where high concentrations of similarly
charged ions exist, especially if those ions have a higher valence than that of
the contaminants. If the ions of concern are removed from a solution with high
concentrations of other similarly charged ions, the resin will become exhausted
more rapidly because most resins cannot selectively adsorb one contaminant in a
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solution containing other similarly charged ionic species. If the concentration
of competing ionic species in an untested waste is significantly higher than that
in the tested waste, the system may not achieve the same performance. A larger
exchange bed or more frequent regeneration may be required to exchange higher
concentrations of competing ionic species and achieve the same treatment
performance, or other, more applicable treatment technologies may need to be
considered for treatment of the untested waste.
4.4.3 Concentration of Interfering Inorganics and Organics
Interfering inorganics, such as iron precipitates, can accumulate in the
pores of anion exchangers; these inorganics will physically break down or block
the resin particles. Some organic compounds, particularly aromatics, can be
irreversibly adsorbed by the exchange resins. Also, some ions tend to oxidize
after they are removed from solution. For instance, Mn*2 (manganese) may oxidize
to the insoluble Mn*4 state, thereby permanently fouling the exchange sites and
requiring the premature replacement of the resin. If the concentration of
interfering inorganics and organics in an untested waste is significantly higher
than that in the tested waste, the system may not achieve the same performance
and other, more applicable treatment technologies may need to be considered for
treatment of the untested waste.
4.4.4 Concentrations of Dissolved and Suspended Solids and Oil and
Grease
High concentrations of dissolved and suspended solids and oil and grease
can affect the performance of ion exchange sites. Conventional ion exchange
systems are usually downflow; i.e., the wastewater flows down through the resin
bed. Regeneration is accomplished in either the downflow or upflow mode. If
excessive concentrations of dissolved and suspended solids and/or oil and grease
are present in the wastewater, the bed may clog and require backwashing prior to
exhausting its exchange capacity. Backwashing may prove ineffective in the
removal of some solids or oils. If the concentration of dissolved and suspended
solids and/or oil and grease in an untested waste is significantly higher than
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that in the tested waste, the system may not achieve the same performance and
other, more applicable treatment technologies may need to be considered for
treatment of the untested waste.
4.4.5 Corrosiveness
Some wastewaters are extremely corrosive to ion exchange resin materials,
reducing efficiency or increasing downtime for maintenance and repair. For
instance, strong solutions of chromates may oxidize many resins, requiring
premature replacement. If the corrosiveness of the untested waste is
significantly higher than that of the tested waste, the system may not achieve
the same performance and other, more applicable treatment technologies may need
to be considered for treatment of the untested waste.
4.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an ion
exchange system, EPA examines the following parameters: (a) the amount and type
of resin, (b) the amount and type of regenerant solution, (c) the hydraulic
loading, and (d) the exchange temperature.
For many hazardous organic constituents, analytical methods are not
available or the constituent cannot be analyzed in the waste matrix. Therefore,
it would normally be impossible to measure the effectiveness of the ion exchange
system. In these cases EPA tries to identify measurable parameters or
constituents that would act as surrogates to verify treatment.
For organic constituents, each compound contains a measurable amount of
total organic carbon (TOC). Removal of TOC in the ion exchange system will
indicate removal of organic constituents. Hence, TOC analysis is likely to be
an adequate surrogate analysis where the specific organic constituent cannot be
measured. However, TOC analysis may not be able to adequately detect treatment
of specific organics in matrices that are heavily organic-laden (i.e., the TOC
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analysis may not be sensitive enough to detect changes at the milligrams/I Her
(mg/1) level in matrices where total organic concentrations are hundreds or
thousands of mg/1). In these cases other surrogate parameters should be sought.
For example, if a specific analyzable constituent is expected to be treated as
well as the unanalyzable constituent, the analyzable constituent concentration
should be monitored as a surrogate.
4.5.1 Amount and Type of Resin
The main design parameter that affects the performance of ion exchange
systems is the amount and type of resin used. Numerous cationic and anionic
resins are commercially available. The selection of a resin is based on a
variety of factors. Different resins have different exchange capacities, and
some have greater affinity than others for specific ions. Certain resins are
designed to tolerate corrosive, oxidizing, or high temperature solutions, so
their exchange capacity does not degrade as rapidly with use. Most resins will
effectively remove contaminant ions from solution until they become exhausted.
However, if resin bed exhaustion occurs too frequently, or if regeneration
requires excessive volumes of the regenerant, the type and/or amount of resin
might need to be changed. In some instances, pretreatment technologies may be
required prior to ion exchange. For most metals removal, cation resins are
usually required. However, some metal complexes, such as copper cyanide
(Cu(CN)4~2), chromates (Cr04~2), and arsenates (As04~3), are anionic and require
the use of anion exchange resins. EPA examines the amount and type of ion
exchange resin in the treatment system to ensure that a sufficient amount of ion
exchange resin is provided to effectively exchange the metal ions of concern.
4.5.2 Amount and Type of Regenerant Solution
For hydrogen-based cation exchangers, acid regenerant solutions are used
(e.g., sulfuric, nitric, or hydrochloric acids). For sodium-based cation resins,
sodium chloride is generally used. For anion exchange resins, alkali (commonly
sodium hydroxide) is used to regenerate hydroxide-based resins. Sodium chloride
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is used for chloride-based anion resins. EPA examines the amount and type of
regenerant solution used to ensure that it is compatible with the resin and waste
treated and that effective removal of the contaminant ions from the exchange
resin is achieved.
4.5.3 Hydraulic Loading Rate
The amount of time that the wastewater contaminants are in contact in the
ion exchange resin (i.e., residence time) impacts the extent to which ion
exchange occurs. Higher residence times generally improve exchange performance,
but require larger ion exchange beds to maintain the same overall throughput.
For a given size ion exchange bed, the residence time can be determined by the
hydraulic loading rate. Typical hydraulic loading rates for ion exchange systems
range from 600 to 15,000 gal/day-ft2. EPA monitors the hydraulic loading rate
to ensure that sufficient time is provided to effectively exchange contaminants.
4.5.4 Exchange Temperature
High temperatures reduce resin life, requiring premature replacement. EPA
monitors the temperature in an ion exchange column continuously, if possible, to
ensure that the system is operating at the appropriate design condition and to
diagnose operational problems. EPA has information showing that the high
temperature limit for anionic resins may be approximately 60°C.
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4.6 References
De Renzo, D.J., ed. 1978. Unit operations for treatment of hazardous industrial
wastes. Park Ridge, N.J.: Noyes Data Corporation.
Dorfner, K. 1972. Ion exchangers: properties and applications. Ann Arbor,
Mich.: Ann Arbor Science Publishers, Inc.
Metcalf & Eddy, Inc. 1979. Wastewater engineering: treatment disposal reuse.
2nd ed. New York: McGraw-Hill Book Co.
Patterson, J.W. 1985. Industrial waste treatment technology. 2nd ed. Stoneham,
Mass.: Butterworth Publishers.
Perry, R.H., and Chilton, C.H. 1973. Chemical engineers' handbook. 5th ed.
New York: McGraw-Hill Book Co.
Sundstrom, D.W., andKlei, H.E. 1979. Wastewater treatment. Englewood Cliffs,
N.J.: Prentice-Hall, Inc.
USEPA. 1982. U.S. Environmental Protection Agency. Development document for
the porcelain enameling point source category, pp. 172-241. Washington,
D.C.: U.S. Environmental Protection Agency.
USEPA. 1983. U.S. Environmental Protection Agency. Treatabilitv manual:
Vol. III. Technology for control/removal of pollutants. EPA-600/
2-82-OOlc. Washington, O.C.: U.S. Environmental Protection Agency.
Wheaton, R.M. 1978. Kirk-Othmer encyclopedia of chemical technology. 3rd ed.
Vol. 13. New York: John Wiley and Sons.
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5. RETORTING
5.1 Applicability
Retorting Is a treatment technology applicable to wastes containing
elemental mercury, as well as mercury present in the oxide, hydroxide, and
sulflde forms, at levels above 100 parts per million, provided that the waste has
a low total organic content (I.e., below 1 percent). For metals other than
mercury, the typical retort operating temperatures (700 to 1000'F) are not high
enough to decompose the metal compounds. High temperature metals recovery (HTMR)
processes must be used to recover most other metals when they are not present in
the pure metal form.
For most retorting processes, there 1s an additional requirement that the
waste have a low water content, preferably below 20 percent. Dewatering reduces
energy consumption by minimizing the amount of water evaporated and precludes
problems involving the separation of recovered metals from large quantities of
water.
5.2 Underlying Principles of Operation
The basic principle of operation for retorting is a process similar to that
for high temperature metals recovery in that it provides for recovery of metals
from wastes primarily by volatilization and subsequent collection and
condensation of the volatilized components. Retorting yields a metal product for
reuse and significantly reduces the concentration of metals in the waste
residual, and, hence, the amount of treated waste that needs to be land disposed.
This technology is different from HTMR in that HTMR includes a reduction reaction
involving the use of carbon, while retorting does not use a reducing agent.
Additionally, this process differs with regard to the form and, possibly, the
Teachability of the residue generated; HTMR generates a slag, while retorting
generates a granular solid residue that may have lower Teachability than a slag
if mercury is the only constituent of concern present in the untreated waste.
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The basic principle of operation of retorting is that sufficient heat must
be transferred to the waste to cause elemental metals to vaporize. In the case
of mercury present as an oxide, hydroxide, or sulfide compound, sufficient heat
must be transferred to the waste to first decompose the compounds to the
elemental form and then volatilize the mercury. In mercury wastes that are
wastewater treatment sludges, mercury is most often present in the form of the
sulfide (HgS) as a result of the use of sodium hydrosulfide treatment of
mercury-bearing wastewaters. In a few instances, hydrazine is used to treat
these same wastewaters; in such instances, a mercurous hydroxide sludge is
generated. This latter compound can be more easily treated to yield elemental
mercury because this reaction occurs more readily than the sulfide decomposition
at the temperatures at which the process is normally operated. Preheated air is
provided to the retort to supply the oxygen necessary for the sulfide
decomposition and to enhance the heat transfer to the waste.
The equations for decomposition of both forms of mercury are presented
below:
(a) HgS + 02 - Hg + S02
(b) 2Hg2(OH)2 - 4Hg + 2H20 + 02
5.3 Description of Retorting Process
The retorting process generally consists of a retort (typically an oven,
i.e., multiple hearth furnace or rotary kiln) in which the waste is heated to
volatilize the metal constituents, a condenser, a metals collection system, and
an air pollution control system. Figures 24 and 25 show a retort system without
and with a scrubber-type air pollution control system.
Trays of wastes are placed in the retort, where they are heated, and
decomposition of mercury compounds and volatilization of the metallic mercury and
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EXHAUST AIR
TO THE ATMOSPHERE
ro
PREHEATED
AIR
RETORT
~t
T
COOLING
WATER
I AIR
1 POLLUTION
CONTROL
SYSTEM |
STACK
METAL
COLLECTION
WASTE
INFLUENT
VOLATILE METALS
TO REUSE
TREATED RESIDUAL WASTE TO STABILIZATION
AND/OR LAND DISPOSAL
Figure 24 Retorting Process (Without a Scrubber and Subsequent Wastewater Discharge)
-------
WATER
VENTURI
SCRUBBER
ro
ro
PREHEATED
AIR
RETORT
DECANTER
WASTE
INFLUENT
EXHAUST AIR
TO THE ATMOSPHERE
STACK
WASTEWATER TO TREATMENT
VOLATILE
METALS
TO REUSE
TREATED RESIDUAL WASTE TO STABILIZATION
AND/OR LAND DISPOSAL
Figure 25 Retorting Process (With a Scrubber and Subsequent Wastewater Discharge)
-------
other volatile elemental metals occur. Although most commonly carried out in an
oven, retorting can also be performed in a multiple hearth furnace.
The vapor stream from the retort is cooled in a condenser. If a scrubber
is not used as an air pollution control device, an electrostatic precipitator is
provided after the condenser to remove any residual metal in the exhaust vapor
stream, as well as to control other potential emissions such as sulfur dioxide
(S02), fly ash, and hydrogen chloride (HC1) vapors. Condensed metal is collected
for reuse before the electrostatic precipitator.
Residual solids remaining in the retort, stripped of volatile metal
contaminants, are collected and may be either directly land disposed or
stabilized, to immobilize any remaining metal constituents, and then land
disposed.
5.4 Waste Characteristics Affecting Performance fWCAPs)
In determining whether retorting will achieve the same level of performance
on an untested waste that it achieved on a previously tested waste and whether
performance levels can be transferred, EPA examines the following waste
characteristics: (a) the concentration of undesirable volatile constituents in
the waste and (b) the thermal conductivity of the waste.
5.4.1 Concentration of Undesirable Volatile Constituents
Because retorting is a recovery process, its product must meet certain
purity requirements prior to reuse. If the waste contains other volatile
constituents with boiling points equal to or below that of the metal(s) to be
recovered, they will be volatilized and condensed along with the desired metal(s)
present in the waste. These constituents may be difficult to separate from the
recovered product and may affect the ability to reuse the product metal or refine
it for subsequent reuse. Undesirable volatile constituents sometimes present in
mercury-bearing wastes include (a) mercury chlorides, which distill unchanged in
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the retorting process, and other volatile metal halides; (b) arsenic oxide and
arsenic trichloride; and (c) organomercury compounds, such as phenylmercuric
acetate, which are not decomposed to elemental mercury by the retorting process.
For wastes containing significant levels (i.e., above 1 percent) of these
contaminants, retorting may not be an appropriate technology. If the
concentration of undesirable volatile constituents in the untested waste is
significantly higher than that in the tested waste, the system may not achieve
the same performance. Chemical pretreatment may be required to convert mercury
chlorides and organomercury compounds to mercuric sulfide and/or elemental
mercury and achieve the same treatment performance, or other, more applicable
treatment technologies may need to be considered for treatment of the untested
waste.
5.4.2 Thermal Conductivity of the Waste
The ability to heat constituents within a waste matrix is a function of the
heat transfer characteristics of the waste material. Mercury and other
recoverable metals in the waste must be heated to near or above their boiling
points in order to be volatilized and recovered. The rate at which heat will be
transferred to the waste material is dependent on the material's thermal
conductivity, which is the ratio of the conductive heat flow to the temperature
gradient across the material. Thermal conductivity measurements, as part of a
treatability comparison of two different wastes to be treated by a single retort
system, are most meaningful when applied to wastes that are homogeneous (i.e.,
uniform throughout). As wastes exhibit greater degrees of nonhomogeneity,
thermal conductivity becomes less accurate in predicting treatability because the
measurement reflects heat flow through regions having the greatest conductivity
(i.e., the path of least resistance) and not through all parts of the waste.
Nevertheless, EPA believes that thermal conductivity may provide the best measure
of performance of heat transfer. If the thermal conductivity of the untested
waste is significantly lower than that of the tested waste, the system may not
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achieve the same performance and other, more applicable treatment technologies
may need to be considered for treatment of the untested waste.
5.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a retort
system, EPA examines the following parameters: (a) the retorting temperature and
(b) the residence time.
5.5.1 Retorting Temperature
Temperature provides an indirect measure of the energy available (i.e.,
Btu/hr) to vaporize the metal of concern. The higher the temperature in the
retort, the more likely it is that the metal will volatilize. The temperature
must be equal to or greater than the boiling point of the metal. However,
excessive temperatures could volatilize undesirable constituents into the
product, possibly inhibiting its potential reuse, and also could cause sintering
of the feed material. For mercury retorting, EPA monitors the retort temperature
to ensure that the system is operating at the appropriate design condition (a
temperature at least equal to the boiling point of mercury (674*F) but below
1000'F) and to diagnose operational problems.
5.5.2 Residence Time
The residence time impacts the amount of volatile metal volatilized and
recovered. It is dependent on the retort temperature and the thermal
conductivity of the waste. Typical residence times in retort systems for mercury
range from 4 hours to 20 hours. EPA monitors the residence time to ensure that
sufficient time is provided to effectively volatilize all of the mercury and/or
other metals to be removed from the waste.
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5.6 References
Occidental Chemical Corp. 1987. Delisting petition, chloralkali plant, Muscle
Shoals, Alabama. Submitted to U.S. Environmental Protection Agency,
July 20, 1987.
Perry, R.A. 1984. Mercury recovery from contaminated wastewaters and sludges.
PB 238 600.
Sittig, M. 1975. Resource recovery and recycling handbook of industrial
wastes. Park Ridge, N.J.: Noyes Publications.
Versar. 1987. Waste minimization audit report - case studies of minimization
of mercury bearing wastes from chloralkali plants. Prepared for U.S.
Environmental Protection Agency, Office of Research and Development,
Hazardous Waste Engineering Research Laboratory by Versar Inc., Contract
No. 68-01-7053.
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III. TECHNOLOGIES APPLICABLE TO MIXED WASTE
(RADIOACTIVE/HAZARDOUS)
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Introduction
This section describes the treatment technologies applicable to mixed waste
(i.e., waste that is both radioactive and hazardous). This introduction explains
what mixed wastes are and how they are classified, where they are likely to be
generated, and how they are regulated. Following this introduction are sections
discussing (a) technologies for treating mixed wastes containing organics and
inorganics other than metals, (b) technologies for treating mixed wastes
containing metals, and (c) technologies for treating mixed wastes that cannot be
treated by technologies determined to be BOAT for the corresponding
nonradioactive wastes (i.e., special mixed wastes treatability group). Each of
these three sections includes subsections describing applicable treatment
technologies.
Mixed wastes are those wastes that satisfy the definition of radioactive
waste subject to the Atomic Energy Act (AEA) and that also contain waste that
either is listed as a hazardous waste in Subpart D of 40 Code of Federal
Regulations (CFR) Part 261 or exhibits any of the hazardous waste characteristics
identified in Subpart C of 40 CFR Part 261. Because they are hazardous, mixed
wastes are subject to the Resource Conservation and Recovery Act (RCRA) Land
Disposal Restrictions. All promulgated treatment standards for RCRA listed and
characteristic wastes apply to the RCRA mixed wastes unless EPA has specifically
established a separate treatability group for a specific category of mixed waste
(see following Section C). Although mixed wastes are regulated by EPA under
RCRA, the specific standards for radioactive material management developed under
the AEA are administered by the Department of Energy (DOE) for government-owned
facilities and by the Nuclear Regulatory Commission (NRC) for commercially owned
facilities. The majority of mixed wastes can be divided into the following three
categories based on the radioactive component of the waste: (a) low-level
wastes, (b) transuranic (TRU) wastes, and (c) high-level wastes. Low-level
wastes include radioactive waste that is not classified as spent fuel from
commercial nuclear power plants or as defense high-level radioactive waste from
the production of weapons. TRU wastes are those wastes containing elements with
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an atomic number greater than 92 (the atomic number for uranium). TRU wastes
generally pose greater radioactivity hazards than low-level wastes because they
contain long-lived alpha radiation emitters. However, commercial transuranic
waste is included as low-level radioactive waste. High-level mixed wastes have
high levels of radioactivity and are extremely dangerous to handle. High-level
mixed wastes are generated from the reprocessing of irradiated fuel rods from
commercial and military nuclear reactors.
Mixed low-level wastes are generated principally from nuclear power plants,
the Department of Energy (DOE), and academic and medical institutions. Some
examples of low-level mixed wastes include spent solvents containing suspended
or dissolved radionuclides, scintillation cocktails from diagnostic procedures,
spent Freon used for cleaning protective garments, acetone or solvents used for
cleaning pipes or other equipment, and still bottoms from tha distillation of
chlorofluorocarbon solvents (CFCs). Other wastes in this category include a wide
range of solid materials such as spent ion-exchange resins (contaminated with
various metals), filters used in reclaiming CFCs, adsorbents, residues from the
cleanup of spills, lead shields, lead-lined containers, welding rods, and
batteries. Also, the production of military weapons produces large amounts of
wastes that fall into the low-level and TRU categories of mixed waste.
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A. TECHNOLOGIES FOR MIXED WASTES CONTAINING ORGANICS
AND INORGANICS OTHER THAN METALS
The following technologies have been determined to be applicable for
treating mixed waste. These technologies were described in detail in previous
sections in this document for hazardous wastes that are not mixed. Technologies
for mixed waste operate under the same principles of operation used for treating
nonmixed waste. However, with mixed waste these technologies typically separate
the hazardous waste components from the radioactive components, thereby reducing
the volume of the radioactive component requiring treatment, or immobilize the
waste (in the case of stabilization technologies). If the hazardous component
is also radioactive (e.g., a hazardous solvent whose structure contains
radioactive carbon), that component may sometimes be removed from the waste,
leaving a less hazardous, less radioactive residual.
1. Carbon Adsorption
For organic low-level mixed wastes, carbon adsorption may be used to remove
organics in wastewaters. If the organic component is not radioactive, then this
treatment reduces the quantity of the hazardous waste component in the mixed
waste.
2. Distillation Technologies
A distillation technology such as batch distillation, steam stripping,
fractionation, thin film evaporation, or thermal drying may be applied to a mixed
waste in that it can be used to separate the mixed waste into the hazardous and
radioactive waste components, as long as the hazardous component is not also
radioactive. The separation can be made where the volatility of the hazardous
components is significantly different from the volatility of the radioactive
components. The distillation process will remove the more volatile organic
components, leaving the less volatile constituents in the still bottoms.
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3. Extraction Technologies
As with distillation, extraction technologies (i.e., solvent extraction and
critical fluid extraction) may be used to separate a mixed waste into its
radioactive and hazardous components if either the hazardous or the radioactive
components can be selectively extracted by the extracting solvent.
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B. TECHNOLOGIES FOR MIXED WASTES CONTAINING METALS
1. Add Leaching
Acid leaching may be used to extract hazardous metals (regardless of
whether the metals are radioactive) from mixed wastes, principally soils,
separating the material into its hazardous and radioactive components or removing
the hazardous and radioactive component from the rest of the waste. This process
may have greater utility when combined with unit operations such as ion exchange,
solvent extraction, chemical precipitation, and filtration.
2. Chemical Precipitation
Chemical precipitation may be applicable to mixed wastes for separating
hazardous metals and/or hazardous metal radionuclides from other constituents in
wastewaters. Specific conditions of pH, temperature, and precipitating reagent
addition are required to selectively remove part or all of the radioactive
components as a precipitate.
3. Filtration
Filtration technologies may be used following precipitation and settling
processes to separate low-level mixed waste from high-level mixed waste. Hence,
filtration may concentrate the high-level waste components in the filter cake,
leaving a relatively low-level filtrate.
4. High Temperature Metals Recovery
The high temperature metals recovery (HTMR) process may be applicable to
mixed wastes if the volatility of the radioactive metal component is
significantly different from that of the nonradioactive portion. This technology
is similar to distillation of organics, but instead "distills" more volatile
inorganic components from less volatile ones.
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5. Ion Exchange
Ion exchange treatment may be used to separate a mixed waste into its
radioactive and hazardous constituents if the radioactive components are ionic.
It will also concentrate the radioactive ionic species into a small volume,
leaving a nonradioactive aqueous phase. The principal mixed waste application
of this process is to recover metallic radionuclides from wastewaters or acid
leach liquors. If more than one radionuclide exists in solution, it may be
necessary to use either a mixed bed ion exchange unit or different types of ion
exchange resins to recover all the radionuclides.
6. Stabilization of Metals
Stabilization refers to a broad class of treatment processes that
immobilize hazardous constituents in a waste. For treatment of metals in
low-level mixed wastes and for some TRU wastes containing low-level radioactive
components, stabilization technologies will reduce the Teachability of the
hazardous metal constituents (regardless of whether the metals are radioactive)
in nonwastewater matrices. However, EPA does not believe that stabilization
using cementitious binders is an appropriate treatment for high-level radioactive
mixed wastes generated during the reprocessing of fuel rods. It does not
adequately immobilize the high-level radioactive portion of the mixed wastes.
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C. TECHNOLOGIES FOR NIXED WASTES IN SPECIAL TREATABILITY GROUPS
1. Amalgamation
Amalgamation is applicable to radioactive wastes containing mercury and
particularly to wastes containing radioactive mercury isotopes. Mercury
compounds are converted into a solid mercury-zinc alloy, which is more easily
managed and less mobile than solutions containing radioactive mercury. The
process can be used directly with radioactive wastes provided that additional
precautions are taken to prevent release of radioactive materials and to protect
operations from radioactive emissions.
The Agency has established amalgamation as a method of treatment for mixed
wastes containing elemental mercury. These types of wastes are typically found
in vacuum pumps and related manometers. In the nuclear industry, this form of
mercury has been contaminated with radioactive tritium (a radioisotope of
hydrogen).
EPA has determined that amalgamation not only provides a significant
reduction in air emissions of mercury but also provides a change in mobility from
liquid mercury to a paste-like solid, potentially reducing Teachability. The
required method of treatment, i.e., amalgamation, may be performed using any of
the following elements: zinc, copper, nickel, gold, and sulfur. Further
information on the amalgamation process may be found in Section I.B.I.
2. Encapsulation
Encapsulation processes such as plastic and asphalt encapsulation may be
used for the management of mixed wastes. However, the equipment used for mixing
of the wastes with molten encapsulating agents must be of special design to
protect workers from radioactive hazards. Additional precautions may be needed
to prevent generation of airborne particulates or vapors of radioactive material
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during the processing. More information on encapsulation may be found in the
encapsulation section (see Section I.B.3).
The Agency has established a treatment standard of macroencapsulation
(encapsulating an entire mass, rather than microencapsulation, which coats
individual particles of a waste) as a method of treatment for radioactive lead
solids. This treatment standard applies to all forms of radioactive mixed waste
containing elemental lead (including discarded equipment containing elemental
lead that served as personnel or equipment shielding prior to becoming a RCRA
hazardous waste). These lead solids do not include treatment residuals such as
hydroxide sludges, other wastewater treatment residuals, or incinerator ash,
which are usually amenable to conventional pozzolanic stabilization, nor do they
include organolead materials that can be incinerated and then stabilized as ash.
3. Vitrification
Vitrification is a treatment technology that will provide effective
immobilization of the inorganic constituents (i.e., both radioactive and
hazardous components) of a mixed waste. Vitrification has been demonstrated to
be an effective treatment technology for high-level mixed waste generated during
the reprocessing of fuel rods. Since vitrification is a high-temperature
process, small quantities of organics that may be present will be volatilized
during the vitrification process. If there are gamma-emitting radionuclides
present, the gamma dose rate will be reduced as a result of the increase in
density of the vitrified matrix (going from the density of soil or other solids
to the density of glass). If radium is present, the radon flux rate will also
be reduced because of the change in the density. Both alpha and beta emitters
will be sealed in the glass matrix. More information on the vitrification
process may be found in the vitrification section (see Section I.B.5).
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4. Incineration
Incineration is a technology applicable to nonradioactive elemental mercury
(i.e., retorting) waste containing high levels of organics. The Agency has
determined that incineration is also an applicable technology for
mercury-containing hydraulic oil contaminated with radioactive material
(low-level mixed waste) and has set incineration as the standard (method of
treatment) for that waste. Incineration of radioactive mercury and other
volatile mixed waste components is likely to cause the mercury or other volatile
components to volatilize. Therefore, incinerators must be equipped with air
pollution control devices to ensure that any volatilized hazardous and/or
radioactive components are not released to the atmosphere above permitted levels.
Incineration of organic wastes containing concentrations of the radionuclides
tritium and carbon 14 will cause the escape of radioactive compounds since the
combustion products of tritium (radioactive hydrogen) and carbon 14 are steam and
carbon dioxide, respectively, both of which will usually escape conventional air
pollution control devices.
f:\document\15254031.01.023 237
-------
References
U.S. Congress, Office of Technology Assessment. 1989. Partnerships under
pressure: managing commercial low-level radioactive waste. Washington,
D.C.: U.S. Government Printing Office. OTA-0-426.
USEPA. 1990. U.S. Environmental Protection Agency. Federal Register Notice,
55 FR 22626, June 1, 1990.
U.S. General Accounting Office. 1989. Fact sheet for the Chairman, Environment,
Energy, and Natural Resources Subcommittee, Committee on Government
Operations, House of Representatives; Nuclear Waste, DOE's program to
prepare high-level radioactive waste for final disposal. Washington,
D.C.: U.S. General Accounting Office.
U.S. General Accounting Office. 1989. Report to the Chairman, Committee on
Governmental Affairs, U.S. Senate; Nuclear Regulation, the military would
benefit from a comprehensive waste disposal program.
Westinghouse Electric Corp. 1983. Waste technology reprints: Nuclear waste
management in the United States.
f:\document\15254031.01.023 238
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APPENDIX A
t
LIST OF BEST DEMONSTRATED AVAILABLE TECHNOLOGY (BOAT)
BACKGROUND DOCUMENTS AND ASSOCIATED BDAT(S)
f:\document\15254031.01.024
-------
This appendix contains a list of BOAT background documents prepared for
EPA's Office of Solid Waste, Waste Treatment Branch, in support of the Land
Disposal Restrictions. (The specific Third under which each document was
prepared is also noted.) This appendix is meant to be a quick reference,
however, for details on a specific waste code see the relevant background
document for that waste code. In summary, this appendix presents the following:
• The waste codes, industries, or sources of the waste associated with
each background document.
• The treatment technologies on which the treatment standards were based
i.e., BOAT. The BOAT associated with each background document is also
identified. (Note that BDAT(s) may vary for individual waste codes
contained in a background document. For specific information on the
BOAT applicable to a certain waste code, see the applicable background
document.)
• The source of the treatment data used to set the treatment standard and
whether the data were collected or submitted to EPA (actual data) or
transferred from existing data.
• The category of waste treated (i.e., organics, inorganics, metals).
A-l
f:\document\15254031.01.024
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BDAT(S)
i
ro
Third
1/3
1/3
1/3
Waste Code/
(Background Document)
F001-FOOS (revised for
methylene chloride)
F006
K001
Industry or
Source of Uaste
Pharmaceutical
industry
Electroplating
operations
Wood preserving
industry
BDAT(s)
Steam stripping (wastewaters)
Stabilization (nonwastewaters)
Rotary kiln incineration,
Stabilization (nonwastewaters,
Actual Data
or Transferred
and
Origin of Data
Data/industry-subraitted
Data/ industry-submit ted
Data/EPA test.
transfer/FOOo
Types of
Wastes Regulated
Methylene chloride
Metals, cyanides
Organics, metals
1/3 K01S
1/3 K016, K01B, K019,
K020. K030
1/3 K022
1/3 K024
1/3 K037
1/3 K044, K045, K047
1/3 K046 (nonreactives)
Distillation of benzyl
chloride
Chlorinated organic*
production
Phenol/acetone
production
Phthalic anhydride
production
Disulfoton production
Manufacturing and
processing of
explosives
Lead-based initiating
compounds production
wastewaters); chemical precipitation
(metals in wastewaters)
Liquid injection incineration
(nonwastewaters); chemical
precipitation (wastewaters)
Rotary kiln incineration
(nonwastewaters, wastewaters)
Fuel substitution, stabilization
(nonwastewaters)
Rotary kiln incineration
(nonwastewaters, wastewaters)
Rotary kiln incineration
(nonwastewaters, wastewaters)
Open burning/Open detonation,
incineration (nonwastewaters)
Stabilization (nonwastewaters)
Transfer/KOI9
Data/EPA test
Oata/industry-submitted,
transfer/FOOo
Data/EPA test
Data/EPA test
Data/000
Data/EPA test
Organics, metals
Organics
Organics, metals
Phthalic acid
Disulfoton, toluene
Reactive waste from
K044, K045. 1C047
Lead (metals)
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BOAT(S)
Waste Code/
Third (Background Document)
Industry or
Source of Waste
BOAT(s)
Actual Data
or Transferred
and
Origin of Data
Types of
Wastes Regulated
1/3 K048-KOS2
I
Co
1/3 K061
1/3 K062
1/3 K069
1/3 K071
1/3 K083
1/3 K086
Petrol refining
industry
Primary steel
production industry
Steel finishing from
Iron and steel
production
Secondary lead
smelting operations
Chlorine production
from the mercury
cell process
Distillation bottoms
from aniline production
Solvent washes and
sludges from the
production of inks,
pigments, soaps, and
stabilizers
Solvent extraction, fluidiied bed
incineration, stabilization
(nonwastewaters); incineration,
chromium reduction, chemical
precipitation, and filtering
(wastewaters)
High temperature metal* recovery for
high zinc subcategory, stabilization
for low line subcategory
Chromium reduction, chemical
precipitation, settling, filtering
(nonwastewaters, wastewaters)
Recycling (noncalcium sulfate
category); (nonwastewaters)
Acid leaching, chemical oxidation
sludge dewatering/acid washing
(nonwastewaters), chemical
precipitation and filtering
(wastewaters)
Liquid injection incineration
(nonwastewaters, wastewaters)
Liquid injection incineration
(nonwastewaters, wastewaters);
chromium reduction, chemical
precipitation, settling,
filtering (wastewaters)
Data/fndustry-subnitted
Data/EPA test
Data/EPA test
Data/industry-submitted
Data/industry-submit ted
Data/industry-submitted
Data/EPA test,
transfer/K062
Organics, metals,
inorganics
Metals
Metala
Metals
Mercury
Organics
Organics, metals
t:\docuinani\1 5254031.01 O25
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BDAT(S)
Waste Code/
Third (Background Document)
Industry or
Source of Waste
BDAT(s)
Actual Data
or Transferred
and
Origin of Data
Types of
Wastes Regulated
1/3 K087
1/3 K099
1/3 K101. K102
1/3 K103, KIM
1/3 K106
1/3 K004-K008, K021
K02S, K036, K060.
K100 (no document
prepared)
Tar sludge from
coking operations
2,4-Dichlorophenoxy-
acetic acid (2,4-D)
production
Production of
veterinary
Pharmaceuticals from
arsenic
AniIine/ni trobenzene
production
Sludges from the
mercury cell process
in chlorine production
Pigments, fluoro-
methane, nitrobenzene,
disulfoton, coking
and lead production
Rotary kiln incineration
(nonwastewaters);
chromium reduction, chemical
precipitation, settling,
filtering (wastewaters)
Chemical oxidation (nonwastewaters,
Wastewaters)
Rotary kiln incineration
(nonwastewaters); chemical
precipitation, settling,
filtering (wastewaters)
Solvent extraction, steam stripping,
and activated carbon adsorption
followed by incineration of the
solvent stream from extraction
(nonwastewaters, wastewaters)
Thermal recovery (retorting)
(nonwastewaters)
Chromiun reduction, carbon
adsorption; chemical oxidation
and metal precipitation
(nonwastewaters, wastewaters)
Data/EPA test,
transfer/K062
Data/EPA collected
Data/EPA test,
transfer/K062
Data/EPA test
Data/industry-subrtitted
Data/industry-submitted
Organics, metals
2,4-Dichlorophenoxy-
aceticacid. chlorinated
dibenzo-p-dioxins,
chlorinated dibanzofurBns
Organics, metals
Organics, inorganics
(cyanides)
Mercury
Metals, organics
2/3 F006-F012. F019
Electroplating, heat
treating operations
Alkaline chlorination,
stabilization (nonwastewaters),
alkaline chlorination, chemical
precipitation, settling,
filtering (wastewaters)
Oata/industry-submit ted
Cyanides, metals
f:\documont\15254031.01.025
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BOAT(S)
Waste Code/
Third (Background Document)
Industry or
Source of Waste
BOAT(s)
Actual Data
or Transferred
and
Origin of Data
Types of
Wastes Regulated
I
tn
2/3 K011. K013. KOK
2/3 Cyanide P wastes
(P013. P021. P029
P030, P063. P074, P098.
P099, P104, P106. P121)
2/3 F024
2/3
2/3 K009, K010
Inorganic pigments
(K002-K008)
2/3 K023, K093. K094
U028. U069, U088,
U102, U107. U190
2/3 K027. Kill. K112
K113, IC1H. K115.
K116, K221, U223
2/3 K028, K029, K095
1C 096
Acrylonitrile
production
Cyanide P wastes
Chlorinated aliphatics
production
Pigments production
Acetaldehyde production
Phthalic anhydride
production
Toluene diisocyanate
production
1,1,1,-trichloroethane
production
Rotary kiln incineration
(Nonwastewaters)
Electrolytic oxidation, alkaline
chtori nation, stabilization
(nonuasteuaters), alkaline chlorination,
chemical precipitation, settling and
filtration (uastewaters)
Rotary kfIn.incineration
(nonuasteuaters); rotary kiln
incineration, lime and sulfide
precipitation, vacuum filtering
(uasteuaters)
Not established under 2/3
rule; see 3/3
Rotary kiln incineration
(nonuasteuaters); steam stripping,
biological treatment (wasteuaters)
Rotary kiln incineration
(nonuasteuaters and uastewaters)
Incineration or fuel substitution
(nonuasteuaters); direct incineration,
or carbon adsorption followed by
incineration or fuel substitution,
stabilization (wasteuaters)
incineration, stabilization
(nonuasteuaters); lime and sulfide
precipitation (wasteuaters)
Data/EPA test
Data/industry-submitted,
transfer/F006-F012. F019
Data/industry-submit ted,
transfer/K062
See 3/3
Transfer/K019.
Transfer/data from
Office of Water
Transfer/K024
Transfer/KOIS, K086,
F006
Transfer/K019,
Transfer/K062
Acrylonitriles, cyanides
Cyanides, metals
Organics, metals
See 3/3
Organics
Organics
Organics. metals .
Organics. metals
l:\documont\15254O31 Ol 025
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BDAT(S)
Waste Code/
Third (Background Document)
Industry or
Source of Waste
BOAT(s)
Actual Data
or Transferred
and
Origin of Data
Types of
Wastes Regulated
2/3 K036, K038, K039,
K040, P039. P040,
P041, P043, P044,
P062, P07. P085.
P089, P094, P097.
P109, Pill, U058.
U087. U235
2/3 KOA3
2/3
K069 (Calcium
sulfate)
3/3 0001
3/3 0002
3/3 0003
Organophosphorous
pesticide production,
e.g., disulfoton,
phorates, and others
P and U wastes
2,4-Dichlorophenol
production
Secondary lead
smelting industry
Wastes having the
characteristic of
ignitability
Wastes having the
characteristic of
corrosivity
Wastes having the
characteristic
of reactivity
Rotary kiln incineration
(nonwasteuaters); biological
treatment; carbon adsorption
incineration (wastewaters)
Incinerator (nonwastewaters,
wasteuaters)
Stabilization (nonwastewaters);
chromium reduction, chemical
precipitation, settling, and/or
filtering (yastewaters)
Deactivation* to remove ignitability,
or incineration/fuel substitution.
recovery for high TOC D001
Deactivation* to remove corrosivity
Deactivation* of cyanides; alkaline
except for cyanides; alkaline
chlorination (cyanides)
Transfer/K037.
Data/industry-submitted
Data/industry-submitted
Transfer/FOOo
and K062
Information references
Information references
Information references.
Industry submitted data,
transfer/Office of Water
Organics, organophos-
pSons coipords, dioxirs
and furans
Phenols, dioxins,
and furans
Metals
Waste having the
characteristic
of ignitability
Waste having the
characteristic
of corrosivity
Wastes having the
characteristics
of reactivity
l:\documontU 5254031.01.025
-------
LIST OF BOAT BACKGROUND DOCUNENIS AND ASSOCIATED BOAT(S)
Waste Code/
Third (Background Document)
Industry or
Source of Waste
BOAT(s)
Actual Data
or Transferred
and
Origin of Data
Types of
Wastes Regulated
3/3 Arsenic, selenium, (0004, characteristic and
0010, K031, K084, K101,
K102, P010, P011, P012,
P036. P038, P103, P1U,
P204, P205, U136)
3/3 Barium (0005, P013)
Pill wastes 1 K wastes
for arsenic and
selenium and wastes
from veterinary
Pharmaceuticals
production
Characteristic and
P waste for barium
Vitrification (nonwastewaters);
chemical precipitation (wastewaters)
Stabilization (nonwastewaters);
chemical precipitation
settling, filtration (wastewatert)
Data/industry-submitted
transfer/Office of Water
Arsenic, metals,
inorganics
Data/Office of Water.
Data/industry-submitted
Barium
3/3 Cadmium (0006)
3/3 Chromium (0007, U032)
3/3 Lead (0008, K069,
K100, P110, UUA-UK6)
3/3 Mercury (0009, K071,
K106, P065, P092,
U1S1)
3/3 Silver (0011, P099,
P104)
Characteristic waste
for cadmium
Characteristic and
U wastes for chromium
Characteristic and
U&P wastes for lead,
and K waste from
lead smelting
operations
Characteristic and
U&P wastes for
mercury and K waste
from chlorine
production
Characteristic wastes
for siIver, and P
silver wastes
Stabilization, thermal recovery
(cadmium batteries) (nonwastewaters);
chemical precipitation, filtration
(wastewaters)
Chrcvniin reduction, chemical
precipitation, settling, filtration,
dewatering of solids (nonwastewaters
and wastewaters)
Stabilization/vitrification (non-
wastewaters); chemical precipitation,
flocculation, clarification, filtration,
and sludge thickening (wastewaters)
Data/industry-submitted,
transfer F006, K062
Transfer/K062
Data/industry-submit ted
Thermal processing, incineration
acid leaching, roasting, retorting
amalgamation, stabilization
(nonwastewaters); chemical
precipitation, filtration,
incineration, ion exchange,
carbon adsorption (wastewaters)
Recovery, stabilization
(nonuastewaters);
chemical precipitation, filtration
(wastewaters)
Data/industry-submit ted,
transfer/K071
Cadmium
Chromium
Lead, organometalUc
compounds
Metals. organonetaUic
compounds
Data/industry-submitted,
and transfer/F006/
Office of Water
Metals
f:\documontV1S254031.01.025
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BOAT(S)
Waste Code/
Third (Background Document)
Industry or
Source of Waste
BOAT(s)
Actual Data
or Transferred
and
Origin of Data
Types of
Wastes Regulated
i
00
3/3 Thallium (P113. P1U.
P115. U2K. U215.
U216. U217)
3/3 Vanadium (P119. P120)
3/3 F002. F005
3/3 F006
3/3 F019
3/3 F024
3/3 F025
3/3 F039 and associated
U&P wastes (Vol. A-E)
U&P thallum
wastes
U&P vanadium
wastes
Spent halogenated and
nonhalogenated
solvents
Sludges from
electroplating
operation
Sludges from the
chemical conversion
coating of aluminum
Production of
chlorinated aliphatic
hydrocarbons
Wastes from production
of chlorinated
aliphatic hydrocarbons
having carbon chain
linkage
Hultisource leachate
and U&P wastes
Recovery, stabilization
(nonwastewaters);
chemical oxidation, chemical
precipitation settling, filtration
(wastewaters)
Stabilization (nonwastewaters);
recovery, chemical precipitation
(wastewaters)
Incineration (nonwastewaters);
biological treatment, steam (tripping,
carbon adsorption . w«t air
oxidation/chemical oxidation
liquid extraction (wastewaterc)
Stabilization (nonwastewaters);
alkaline chlorination, chromium
reduction, chemical precipitation
(wastewaters)
Alkaline chlorination, stabilization
(nonwastewaters); alkaline
chlorination, chromium reduction,
chemical precipitation
(wastewaters)
Stabilization, incineration
(nonwastewaters)
Incineration (nonwastewaters)
biotreatment, steam stripping,
carbon adsorption, liquid
extraction (wastewaters)
See Footnote b.
Data/industry-submitted
Data/industry-submitted
Transfer/EPA data from
various sources
Transfer/KOoZ
Transfer/F006-F012
Data/EPA test
Transfer/K019, K001,
F039 (Vol. A)
Data/transfer/EPA data
from various sources and
EPA test
Metals
Metals
Organics
Metals, cyanides
Metals, cyanides
Metals, organics
Organics
Metals, inorganics,
organics
(AdocumanlM S254O31.01.02S
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BOAT(S)
Waste Code/
Third (Background Document)
3/3 K001, U051 (addendum)
3/3 K002-K008
3/3 IC011, K013, KOU
3/3 K015
3/3 K017
Industry or
Source of Waste
Wood preserving
industry
Pigment industry
sludges
Acrylonitrite,
production
Still bottoms from
distillation of
benzyl chloride
Production of
epichlorohydrin
BOAT(s)
Incineration
(nonwastewaters, wastewaters)
Chemical precipitation, stabilization
(nonwastewaters); alkaline
chlorination followed by chromium
reduction (wastewaters)
Wet air oxidation (wastewaters)
Incineration, stabilization
(nonwastewaters)
Incineration (nonwastewaters);
biotreatment (wastewatert)
Actual Data
or Transferred
and
Origin of Data
Data/EPA test
Data/transfer/industry,
K062, transfer/FOOo
Data/industry-sutmitted
Transf«r/K048-52. K087.
transfer/F039 (Volume A)
Transf«r/F039,
(Vol. A and C)
Types of
Wastes Regulated
Organics, metals
Metals, inorganics
Metals, organics
Organics, metals
Organics
3/3 K021
3/3 K073
3/3 K022
3/3 K025
3/3 K026
Aqueous antimony
catalyst front
fluoromethane
production
Chlorine production
Bottoms, tars
from production
of phenol/acetone
Distillation
bottoms from the
production of nitro-
benzene
Tails from the produc-
tion of methyl ethyl
pyridines
Incineration (nonwastewaters);
biotreatment, steam stripping,
lime conditioning, sedimentation,
Filtration (wastewaters)
Incineration (nonwastewaters);
biotreatment, steam stripping,
filtration (wastewaters)
Chemical precipitation, vacuum
filtration (wastewaters)
Incineration (nonwastewaters);
liquid-liquid extraction,
steam stripping, carbon adsorption,
incineration of spent carbon
(wastewaters)
Incineration (nonwastewaters,
wastewaters)
Transfer/K019. K048-52.
F039 (Vol. A)
Transfer/r019,
transfer/FOJ9 (Vol. A)
Transfer/F039 (Vol. A),
K062
Transfer/previous EPA
incineration tests,
transfer/Kl03-K104
T ransfer/prev i ous
incineration tests
Metals, organics
Organics
Metals, organics
Organics
Organics
I :\docum*nt\l 5 254O31.01.0 25
-------
LIST OF BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BOAT(S)
Actual Data
or Transferred
Third
Waste Code/
(Background Document)
Industry or
Source of Waste
BDAT(s)
and
Origin of Data
Types of
Wastes Regulated
I
•—•
o
3/3 K028, K029,
K09S, K096
3/3 K032-K034, K041-K042.
K08S, K097, K098.
K10S. D012-D017
3/3 K035
3/3 K036
3/3 K037
3/3 K044, K045, K047
3/3 KOA6 (reactives)
Wastes from
production of
1,1.1-trichloroethane
Natogenated pesticides
and chlorobenzene
waste
Uastewater treatment
sludges from the
production of
creosote
Still bottoms from
toluene reclamation
distillation in the
production of
disulfoton
Wasteuater treatment
Sludges from the
Production of
Disulfoton
Manufacturing
explosives and lead-
based initiating
compounds
Lead-based initiating
compounds
Stabilization (nonwasteuaters);
biotreatment, steam stripping
carbon adsorption,
filtration (wasteuaters)
Incineration (nonwasteuaters)
biotreatment, steam stripping,
carbon adsorption, liquid
extraction incineration/wet
air oxidation/chemical
oOxidation (wastewaters)
Incineration (nonwastewaterg);
biotreatment (wastewaters)
Incineration (nonwasteuaters for
disulfoton)
Biological treatment (wastewaters)
Deactivation. settling and filtration,
alkaline precipitation (wastewaters)
Deactivation by chemical treatment or
specialized incineration followed
by stabilization (nonwastewaters);
chemical deactivation followed by
alkaline precipitation, settling,
and filtrating (wastewaters)
Transfer/f039 (Vol. A),
transfer/F024
Transfer/EPA data
Transf«r/K087
Transfer/K037
Data/EPA test
Data/industry-subroitted,
EPA
Data/industry-submitted
Transfer/K062
Organics, metals
Various halogenated
pesticides, chlorinated
nonbornane deriva-
tives, other organics
Organics
Organics
Organics, organo-
phosphorus
pesticides
Reactive wastes
Lead
f:\docum.nt\15254031.Or025
-------
LIST Of BOAT BACKGROUND DOCUMENTS AND ASSOCIATED BDAT(S)
Waste Code/
Third (Background Document)
Industry or
Source of Waste
BDAT(s)
Actual Data
or Transferred
and
Origin of Data
Types of
Wastes Regulated
3/3 K048-K052
(addendum)
3/3 K060
3/3 K061
3/3 K083
3/3 K086
Petroleum refining
industry
Ammonia still lime
sludge from coking
operations
Emission control
dust/sludge from the
primary production of
steel in electric
furnaces
Distillation bottoms
from the production
of aniline
Wastes from cleaning
equipment used in
formulation of ink
from pigments, driers,
soaps, and stabilizers
containing chromium
and lead
Incineration, solvent extraction
(nonuastewaters)
Incineration (nonwasteuaters)
biological treatment (wastewaters)
Chemical reduction, chemical
precipitation (wasteuaters)
Incineration, stabilization
(nonwastewaters); liquid-liquid
extraction, steam stripping,
carbon adsorption, biological
treatment (wastewaters)
Incineration, chromium reduction,
chemical precipitation, filtration,
stabilization (nonuastewaters),
incineration, wet air oxidation or
chemical oxidation, carbon
adsorption, biological treatment,
or steam stripping
Data/industry-submit ted
Transfer/Office of Water,
transfer/K087
Transfer/K062.
industry data
Transfer/previous EPA
incineration test. Transfer/
F039 (Vol. A)
Data/EPA test,
transfer/F006-F012.
transfer/F039 (Vol. A)
Organics, metals.
inorganics
Organics, inorganics
Metals
Organics, metals
Organics, cyanides
(inorganic)
'The Agency has promulgated a treatment standard of Deactivation for wastes exhibiting the characteristic of Ignitability (D001), Corrosivity (0002), or
Reactivity (D003). As part of the land disposal restrictions, treaters of these above-mentioned wastes are required to use a deactivation technology that
will remove the characteristic for which the waste is hazardous. Technologies that are applicable and demonstrated for treating these characteristic wastes
are listed in 40 CFR Part 268, Appendix 6.
bThe F039 background document (Volumes A through E) contains treatment performance data from various sources such as the BOAT program, the Office of Water's
Industrial Technology Division, the National Pollutant Discharge Elimination System, the Hazardous Waste Engineering Resource Laboratories, and other
sources. The F039 background document presents numerous technologies as BOAT. For further information, refer to the F039 background document. Volumes A
through E.
l:\docum.nt\16 254031.01.025
------- |