PROPOSED
BEST DEMONSTRATED AVAILABLE TECHNOLOGY (BDAT)
BACKGROUND DOCUMENT FOR D006
CADMIUM WASTES
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
Larry Rosengrant, Chief
Treatment Technology Section
Monica Chatman-McEaddy
Project Manager
November 1989
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TABLE OF CONTENTS
Page No.
1. INTRODUCTION 1-1
2. INDUSTRIES AFFECTED AND WASTE CHARACTERIZATION 2-1
2.1 Industries Affected and Process Descriptions 2-1
2.1.1 Production of Inorganic Cadmium Compounds.... 2-1
2.1.2 Users of Inorganic Cadmium Compounds 2-3
2.2 Waste Characterization 2-4
2.3 Determination of Waste Treatability Group 2-5
3. APPLICABLE AND DEMONSTRATED TREATMENT TECHNOLOGIES 3-1
3.1 Applicable Treatment Technologies 3-1
3.1.1 Applicable Technologies for Nonwastewaters.. 3-1
3.1.2 Applicable Technologies for Cadmium-
Containing Batteries 3-3
3.1.3 Applicable Technologies for Wastewaters 3-4
3.2 Demonstrated Treatment Technologies 3-4
3.2.1 Demonstrated Technologies for
Nonwastewaters 3-4
3.2.2 Demonstrated Technologies for Wastewaters ... 3-5
4. PERFORMANCE DATA BASE 4-1
4.1 Performance Data for Nonwastewaters 4-2
4.1.1 Performance Data for High-Temperature
Metals Recovery 4-2
4.1.2 Performance Data for Stabilization 4-3
4.2 Performance Data for Wastewaters 4-3
5. IDENTIFICATION OF BEST DEMONSTRATED AVAILABLE
TECHNOLOGY (BDAT) 5-1
5.1 BDAT for Nonwastewaters 5-1
5.2 BDAT for Wastewaters 5-2
6. DEVELOPMENT OF BDAT TREATMENT STANDARDS 6-1
7. REFERENCES 7"1
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LIST OF TABLES
Page No
Table 1-1 Proposed BDAT Treatment Standards for D006 -
Wastewaters 1-3
Table 1-2 Proposed BDAT Treatment Standards for D006 -
Nonwastewaters 1-4
Table 1-3 Proposed BDAT Treatment Standards for D006 -
Cadmium Batteries 1-5
Table 4-1 Treatment Performance Data for High-Temperature
Metals Recovery of K061 Waste: Waelz Kiln
(EPA-Collected Data) 4-5
Table 4-2 Treatment Performance Data for High-Temperature
Metals Recovery of K061 Waste: Plasma Arc Reactor . 4-7
Table 4-3 Treatment Performance Data for High-Temperature
Metals Recovery of K061 Waste: Rotary Hearth/
Electric Furnace 4-8
Table 4-4 Treatment Performance Data for High-Temperature
Metals Recovery of K061 Waste: Molten Slag System . 4-9
Table 4-5 Treatment Performance Data for Stabilization of
K061 Waste (EPA-Collected Data) 4-10
Table 4-6 Treatment Performance Data for Stabilization of
F006 Waste 4-12
Table 4-7 Treatment Performance Data for Chemical Precipitation
and Vacuum Filtration 4-14
Table 6-1 Calculation of Treatment Standards for D006
Nonwastewaters 6-3
Table 6-2 Proposed BDAT Treatment Standards for D006
Nonwastewaters 6-4
Table 6-3 Proposed BDAT Treatment Standards for D006
Wastewaters 6-5
Table 6-4 Proposed BDAT Treatment Standards for D006 -
Cadmium Batteries 6-6
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1. INTRODUCTION
Pursuant to section 3004(m) of the Resource Conservation and Recovery
Act (RCRA) as enacted by the Hazardous and Solid Waste Amendments (HSWA)
on November 8, 1984. the Environmental Protection Agency (EPA) is
establishing treatment standards based on the best demonstrated available
technology (BDAT) for cadmium-containing wastes. These wastes are
identified in 40 CFR 261.24 under the waste code D006. Compliance with
these treatment standards is a prerequisite for placement of these wastes
in facilities designated as land disposal units according to 40 CFR
Part 268. The effective date of final promulgated treatment standards
for D006 wastes will be May 8, 1990.
This background document presents the Agency's rationale and
technical support for developing regulatory standards for these wastes.
Sections 2 through 6 present waste-specific information for the D006
wastes. Section 2 presents the number and location of facilities
affected by the land disposal restrictions, the waste-generating process,
and waste characterization data. Section 3 discusses the technologies
used to treat the waste (or similar wastes), and Section 4 presents
available performance data, including data on which treatment standards
are based. Section 5 explains EPA's determination of BDAT. Proposed
treatment standards for cadmium wastes are determined in Section 6.
The BDAT program and promulgated methodology are more thoroughly
described in two additional documents: Methodology for Developing BDAT
Treatment Standards (USEPA 1989a) and Generic Quality Assurance Project
Plan for Land Disposal Restrictions Program ("BDAT") (USEPA 1988a). The
petition process to be followed in requesting a variance from the BDAT
treatment standards is discussed in the methodology document.
From a partial analysis of responses to EPA's 1986 National Survey of
Hazardous Waste Generators, the Agency has information indicating that at
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least three .facilities presently generate D006 wastes from the production
of cadmium pigments and that at least seven other facilities produce the
wastes from the manufacture of other cadmium compounds. The battery and
metal finishing industries also include a large number of generators of
cadmium-containing wastes.
D006 wastes are generated as either nonwastewaters or wastewaters.
For the purpose of BDAT, a wastewater is defined by the Agency as a waste
containing less than 1 percent (weight basis) total suspended solids* and
less than 1 percent (weight basis) total organic carbon (TOC). Wastes
not meeting this -definition must comply with the treatment standards for
nonwastewaters. The Agency is proposing to group all D006 wastes into
three treatability groups: (a) wastewater, (b) nonwastewater, and (c)
cadmium-containing batteries,
BDAT for D006 nonwastewaters (other than cadmium-containing
batteries) is stabilization. The proposed BDAT treatment standard for
D006 nonwastewaters is 0.14 mg/1 based on a transfer of treatment data
from stabilization of K061 waste. The proposed treatment standard for
cadmium-containing batteries is recovery as a method of treatment, BDAT
for D006 wastewaters is chemical precipitation followed by filtration.
The proposed BDAT treatment standard for D006 wastewaters is 0.20 mg/1
based on a 24-hour composite sample. The proposed BDAT treatment
standards for cadmium are summarized in Tables 1-1 through 1-3.
* The term "total suspended solids" (TSS) clarifies EPA's previously used
terminology of "total solids" and "filterable solids." Specifically
the quantity of total suspended solids is measured by Method 209c
(Total Suspended Solids Dried at 103 to 105"C) in Standard Methods
for the Examination of Water and Wastewater, 16th Edition (APHA AWWA
and WPCF 1985).
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Table 1-1 Proposed BDAT Treatment Standards for D006 - Wastewaters
Regulated
Maximum for
anv 24-hour
comDosite samDle
constituent
Total
composition
(mg/1)
Cadmium
0.20
3038s
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Table 1-2 Proposed BDAT Treatment Standards for D006 - Nonwastewaters
Regulated
Maximum for anv single erab sample
constituent
TCLP (mg/1)
Cadmium
0.14
3039g
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Table 1-3 Proposed BDAT Treatment Standards for D006 - Cadmium Batteries
Thermal Recovery as a Method of Treatment
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2. INDUSTRIES AFFECTED AND WASTE CHARACTERIZATION
As defined in 40 CFR 261.24, D006 wastes are wastes that exhibit the
characteristic of EP Toxicity for cadmium. In other words, D006 wastes
have a cadmium concentration of greater than 1.0 mg/1, as measured by the
EP Toxicity Leaching Procedure. Section 2.1 describes the industries
affected by the land disposal restrictions for D006 wastes and describes
the processes identified by EPA that may generate these wastes.
Section 2.2 summarizes the available waste characterization data for
these wastes. Section 2.3 uses the Agency's analysis of the sources of
D006 wastes and waste composition to divide D006 wastes into several
waste treatability groups.
2.1 Industries Affected and Process Descriptions
The industries affected by the land disposal restrictions for D006
wastes are (1) the inorganic chemicals industry, which produces various
inorganic cadmium compounds, and (2) several industries that use cadmium
compounds to manufacture various products. Processes in these industries
that may generate cadmium-containing wastes are discussed below.
2.1.1 Production of Inorganic Cadmium Compounds
The major manufactured cadmium compounds may be classified into three
groups: (1) cadmium pigments, (2) soluble cadmium salts used in the
electroplating and battery industries, and (3) high-purity cadmium
sulfide and cadmium oxide used in the electronics industries. These
classes of materials are discussed separately below.
(1) Cadmium pigments. The term "cadmium pigments" refers to a
group of cadmium sulfide-based pigments that range in color from light
yellow to deep red. Cadmium pigments are used for the colorization of
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plastics, artists' colors, and other quality paints. Differences in
color are achieved by adding zinc, selenium, and barium salts to the
process.
To produce cadmium pigments, cadmium metal is digested in sulfuric
acid to form a cadmium sulfate solution. Often nitric acid is added to
increase the reaction rate. A yellow pigment is produced by adding zinc
sulfate to this solution and mixing with an aqueous solution of sodium
sulfide. A red or orange pigment is made by adding selenium. Barium
sulfide is used in place of the sodium sulfide to produce less intense
pigments. A few plants also add barium sulfate, in suspension, along
with the sodium sulfide as an alternative procedure for generating less
intensely colored pigments.
After addition of the sulfide-containing solution (or suspension),
the pigment precipitate (chiefly cadmium sulfide) is recovered by
filtration, washed, and calcined. The pigment is then ground, rewashed
to remove residual traces of sodium sulfate and other soluble materials,
redried, and packaged for sale.
Treatment of the wastewaters from this process generates wastewater
treatment sludges that contain high levels of cadmium compounds. Other
D006 nonwastewaters are also comprised of high levels of cadmium metal,
such as the residuals of K061 and zinc mining wastes.
(2) Soluble cadmium salts. Cadmium chloride, sulfate, and nitrate
are all produced by dissolving cadmium metal in the appropriate mineral
acids (Versar 1980). The resulting solutions are then evaporated to
recover the desired products. The majority of soluble cadmium salts are
used in electroplating operations and as active anode material in
silver-cadmium and nickel-cadmium batteries. The small amounts of
wastewaters from these processes are generally combined with wastewaters
3107g
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from other production areas prior to chemical treatment before
discharge. This chemical treatment normally generates wastewater
treatment sludges containing cadmium.
(3) High-puritv cadmium sulfide and cadmium oxide. High-purity
cadmium sulfide and cadmium oxide are produced by proprietary processes
for use in component fabrication operations in the semiconductor and
electronics industries. A limited amount of information on these
processes has been published (Parker 1978). The available information
indicates that cadmium-containing wastes (both nonwastewaters and
wastewaters) are likely to be generated by these processes.
2.1.2 Users of Inorganic Cadmium Compounds
The largest single use of cadmium salts is in electroplating
operations. Use of cadmium salts in electroplating rinse waters results
in the generation of a number of cadmium-containing wastes. Many of
these wastes are identified as the electroplating wastes F006, F007,
F008, and F009 and are addressed in the Best Demonstrated Available
Technology (BDAT) Background Document for F006 (USEPA 1988c) and the Best
Demonstrated Available Technology (BDAT) Background Document for Cyanide
Wastes (USEPA 1989c).
The second largest use of cadmium compounds is the use of cadmium
pigments for colorization of plastics, in artists' colors, and in other
quality paints. Use of cadmium pigments involves mechanical formulation
operations from the production of paints, inks, solvents, and resins.
The processes used to manufacture these products generate organo-cadmium
wastes including rinse waters and spilled and off-specification products,
that contain cadmium.
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The third largest use of cadmium compounds is in the battery-
industry. Cadmium hydroxide is used as the active anode material in
silver-cadmium and nickel-cadmium batteries. Manufacture of these anodes
#
includes a number of mechanical pasting operations that may generate
spilled materials containing cadmium. Subsequent incorporation of the
anodes into finished batteries may generate additional wastes consisting
of off-specification anodes and batteries. Washdown of production areas
generates wastewaters containing cadmium. Chemical treatment of the
wastewaters generates cadmium-containing waste sludges.
The use of high-purity cadmium oxide and cadmium sulfide in the
production of electronic components generates cadmium-containing wastes
from component fabrication operations. Most of these wastes consist of
particulates and scrap solids high in cadmium content.
Cadmium is also used in small quantities to produce specialty
alloys. These operations do not generate significant amounts of waste.
2.2 Waste Characterization
The Agency has waste composition information from 3007 Questionnaire
responses from the three current manufacturers of cadmium pigments. The
wastewater treatment sludges generated by these facilities contain about
50 percent cadmium sulfide, with smaller amounts of cadmium selenide and
zinc sulfide. The Agency also has detailed composition information on
the wastewaters from which these sludges were generated from past
effluent guidelines studies (USEPA 1982).
The Agency also has information from 145 facilities generating D006
wastes. Sixty-three of these facilities generated nonwastewater forms of
D006, 76 generated wastewaters, and the remainder generated both (or it
was indeterminable which form of waste they generated). Data on these
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facilities were obtained from the 1986 National Survey of Hazardous Waste
Generators (USEPA 1986a). Cadmium concentrations in the wastes were as
follows:
Below 1 ppm
1-10 ppm
24 plants
46 plants
37 plants
11 plants
6 plants
6 plants
2 plants
2 plants
2 plants
10-100 ppm
100-500 ppm
500-1,000 ppm
10,000-10,000 ppm
10% to 25%
25% to 50%
Over 50%
Twenty-five generators reported generation of wastes that contained
organics. Levels of organics present in these wastes were as follows:
The organics present were those normally associated with paint or
plastics manufacturing or paint removal (e.g., methyl ethyl ketone,
methyl isobutyl ketone, toluene, acetone, methylene chloride, and
phthalate esters).
Several facilities showed cyanides to be present. Process
descriptions from those facilities indicated the source of the cyanides
to be electroplating operations. Most plants reported the presence of
other EP toxic metals in the wastes in addition to cadmium. The metals
most frequently identified were lead, zinc, and chromium.
EPA believes that wastes with the same waste code produced in
different processes in an industry or in different industries may, in
some cases, not be treated to similar concentrations using the same
technologies. In these instances, the Agency may subdivide a waste code
Less than 10%
10% to 50%
Over 50%
16 plants
8 plants
1 plant
2.3 Determination of Waste Treatability Group
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into several treatability groups. This is done when the chemical forms
of the wastes are different and the wastes require different treatments
or combinations of treatments. For example, inorganic and organometallic
compounds containing the same metals frequently require different types
of treatment.
Based on a careful review of available information on the generation
of D006 wastes and available waste characterization data, the Agency has
determined that D006 wastes comprise three treatability groups:
nonwastewaters, wastewaters, and cadmium-containing batteries. The last
treatability group includes cadmium-mercury and cadmium-silver cells in
addition to nickel-cadmium batteries.
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3. APPLICABLE ANO DEMONSTRATED TREATMENT TECHNOLOGIES
Section 2 established three treatability groups for D006 wastes.
This section identifies the treatment technologies that are applicable to
these groups and determines which, if any, of the applicable technologies
can be considered demonstrated for the purpose of establishing BDAT.
To be applicable, a technology must be theoretically usable to treat
the waste in question or to treat a waste that is similar in terms of the
parameters that affect treatment selection. For detailed descriptions of
the technologies applicable for these wastes, or for wastes judged to be
similar, see EPA's Treatment Technology Background Document (USEPA
1989b). To be demonstrated, the technology must be employed in
full-scale operation for the treatment of the waste in question or a
similar waste. Technologies available only at pilot- and bench-scale
operations are not considered in identifying demonstrated technologies.
3.1 Applicable Treatment Technologies
3.1.1 Applicable Technologies for Nonwastewaters
EPA has identified technologies applicable to nonwastewater forms of
D006 wastes. These are discussed separately in the following subsections.
(1) High-temperature metals recovery. Presently, two types of
high-temperature metals recovery operations are in use for reclamation of
cadmium for wastes. The first type is used at commercial primary zinc
smelters, which accept wastes with high concentrations of cadmium and
zinc sulfides. These wastes are blended with sulfide ores and fed to
reverbatory furnaces along with coke. Zinc and cadmium metals volatilize
from the roasting operation and are collected together by condensation.
3109s
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The crude zinc is then refined electronically to separate the cadmium,
which is recovered separately as the metal.
Commercial smelters generally accept cadmium and zinc-bearing
residues only if they are competitive in zinc and cadmium content with
the ores normally processed. For this reason, this version of high-
temperature metals recovery is limited to wastes with cadmium or zinc
contents equivalent to the amounts found in common ores.
A second form of high - temperature metals recovery in use for waste
treatment is a modification of the standard process for production of
zinc oxide. In this process, the waste is blended with coke, silica,' and
lime and fed to a high-temperature furnace. Chemical reactions occur
between the zinc and cadmium sulfides present, oxygen, and carbon to form
elemental zinc and cadmium. For cadmium, these reactions are:
2CdS + 302 - 2S02 + 2CdO
2CdO + C - 2Cd + C02
Elemental zinc and cadmium volatilize and react with oxygen to
re-form the oxides. The oxides are then collected from the hot gases
emerging from the furnace in baghouses. The impure zinc oxide generated
is then purified to separate out cadmium, which is recovered and
purified. High-temperature metals recovery is discussed in the Treatment
Technology Background Document (USEPA 1989b).
(2) Stabilization technologies. Stabilization technologies
involve mixing the waste with lime, fly ash mixtures, cement, or concrete
mixtures. Water is then added, and the mixture sets into a solid mass
that can be land disposed. Stabilization technologies are discussed in
detail in the Treatment Technology Background Document (USEPA 1989b).
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(3) Chemical precipitation. Chemical precipitation technologies
are normally used to treat wastewaters. They may also be used, however,
to treat dilute suspensions of solids in water. These technologies will
be discussed in Section 3.1.3 on wastewaters.
(4) Incineration. Incineration is applicable to treatment of D006
wastes containing organics. Incineration treatment destroys the organic
compounds contained in the wastes. Some of the cadmium present in the
waste volatilizes and is removed as the metal or the oxide in the
scrubber water; the rest remains in the solid residue (ash). These
residuals (scrubber water and ash) can be treated by the technologies
applicable to treatment of wastewaters and nonwastewaters containing no
organics.
3.1.2 Applicable Technologies for Cadmium-Containing Batteries
Recovery has been identified as an applicable technology for
treatment of cadmium battery wastes. Cadmium may be recovered from these
wastes using pyrometallurgical techniques or smelting techniques, as
described in Section 3.1.1(1).
Europe started recovering cadmium from cadmium-containing batteries
in the 1970s. The metal is removed from liquid wastes by precipitation,
flocculation, sedimentation, flotation, and separation and dewatering of
the sludge. A recycling system follows, where pure cadmium metal is
obtained by separation by electrolysis (Barring 1983). Japan has also
developed recycling operations to recover cadmium from cadmium-containing
batteries, starting in the 1970s. Cadmium is obtained by feeding the
waste materials containing cadmium into a rotary kiln. In the kiln,
cadmium compounds are converted to cadmium oxide which is then sent to
the cooling zone and collected in powder form (Ohira, 1986).
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3.1.3 Applicable Technologies for Wastewaters
(1) Chemical precipitation.Chemical precipitation is applicable to
wastewater forms of D006. This technology typically involves addition of
lime, caustic, or a sulfide compound to the wastewater solution with pH
adjustment. Cadmium hydroxide or cadmium sulfide precipitates from
solution and is collected by filtration. The collected solids may then
be further treated by high- temperature metals recovery or stabilization
prior to disposal. The treated wastewater may be further treated or
discharged. Chemical precipitation is discussed in the Treatment
Technology Background Document (USEPA 1989b).
(2) Ion exchange. This technology is applicable to treatment of
wastewaters containing relatively low concentrations of dissolved
metals. The metal must be in a soluble ionic form in order to be removed
by this technology. The waste is passed through a bed of ion exchange
resin beads. The resin adsorbs the soluble ions, thus removing them from
solution. Ion exchange produces both a wastewater residual (from
regeneration of the ion exchange resin) and a nonwastewater residual (the
spent ion exchange resin). The spent regenerate solutions (usually acid
solutions) are more concentrated than the original untreated waste
(though much lower in volume), and must be treated for metals removal by
chemical precipitation followed by filtration if the regenerate solution
is not recyclable. Ion exchange is discussed in detail in the Treatment
Technology Background Document (USEPA 1989b).
(3) Electrolytic Recovery.Electrolytic recovery is applicable for
treatment of wastewaters containing certain dissolved metals. In
electrolytic recovery treatment, an electric current is passed between
two submerged electrodes through a solution containing the metal to be
recovered. An electrolytic oxidation-reduction reaction occurs in the
solution. The dissolved metal is reduced to its pure metallic state and
is deposited on one electrode (the anode).
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3.2 Demonstrated Treatment Technologies
3.2.1 Demonstrated Technologies for Nonwastewaters
According to 3007 Questionnaire responses on file at the Agency, the
three facilities manufacturing cadmium pigments send their wastewater
treatment sludges to primary zinc smelters for metals recovery. The
second type of high-temperature metals recovery discussed in
Section 3.1.1(1) is currently in commercial use treating K061 wastes that
have high zinc, lead, and cadmium content. The operations at this
facility are described in more detail in the K061 background document
(USEPA 1988b). Both types of high-temperature metals recovery are
demonstrated for wastes similar to cadmium-containing batteries. EPA is
requesting data on the recovery of cadmium-containing batteries during
the comment period for the Third Third proposed rule.
Stabilization has been used on a commercial basis to treat the listed
wastes K061 and F006. Some F006 wastes contain high levels of cadmium,
particularly if they originate from electroplating operations producing
cadmium-coated products. The commercial use of stabilization to treat
F006 wastes is described in the F006 background document (USEPA 1988c).
Incineration is demonstrated for treatment of many hazardous wastes
containing organic and metal constituents.
3.2.2 Demonstrated Technologies for Wastewaters
Chemical precipitation has been in use in the cadmium pigments
industry for over a decade as a method for the removal of cadmium from
process wastewaters (Versar 1980, USEPA 1982). Data on the demonstrated
effectiveness of this process are given in the effluent guidelines
development document for the inorganic chemicals industry (USEPA 1982).
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4. PERFORMANCE DATA BASE
This section presents relevant data available to EPA on the
performance of demonstrated technologies in treating listed wastes
containing cadmium in concentrations similar to concentrations expected
to be found in D006 wastes. These data are used elsewhere in this
document for determining which technologies represent BDAT (Section 5)
and for developing treatment standards (Section 6). Eligible data, in
addition to full-scale demonstration data, may include data developed at
research facilities or obtained through other applications at less than
full-scale operation, as long as the technology is demonstrated in
full-scale operation for a similar waste or wastes as defined in
Section 3.
Performance data, to the extent that they are available to EPA,
include the untreated and treated waste concentrations for a given
constituent, the values of operating parameters that were measured at the
time the waste was being treated, the values of relevant design
parameters for the treatment technology, and data on waste
characteristics that affect performance of the treatment technology.
Where data are not available on the treatment of the specific wastes
of concern, the Agency may elect to transfer data on the treatment of a
similar waste or wastes, using a demonstrated technology. To transfer
data from another waste category, EPA must find that the wastes covered
by this background document are no more difficult to treat (based on the
waste characteristics that affect performance of the demonstrated
treatment technology) than the treated wastes from which performance data
are being transferred.
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4.1 Performance Data for Nonvastewaters
Presented in this section are data collected by EPA and submitted to
EPA on treatment of various F-code and K-code wastes containing cadmium.
These data include performance data for high-temperature metals recovery
of K061 wastes and stabilization of K061 and F006 wastes. The Agency
believes that K061 is similar in waste characteristics to many D006
wastes that may be generated (such as off-specification cadmium chemicals
and incinerator ash residues containing cadmium). Additionally, EPA
believes that F006 wastes are similar to D006 wastewater treatment
sludges in terms of waste characteristics and cadmium concentrations.
4.1.1 Performance Data for High-Temperature Hetals Recovery
The Agency has 11 data sets for treatment of K061 waste by high-
temperature metals recovery. Tables 4-1 to 4-4 at the end of this
section summarize the treatment performance data collected for high-
temperature metals recovery for each of the 11 data sets. Seven of the
data sets represent data that the Agency collected on a rotary kiln unit
(presented in Table 4-1); all other data were submitted by industry and
include two data sets from plasma arc furnace treatment (see Table 4-2),
one from a rotary hearth electric furnace (see Table 4-3), and one from a
molten slag reactor (see Table 4-4).
Table 4-1 presents total composition data for the untreated waste and
total composition and TCLP leachate data for the treated nonwastewater
residual, as well as design and operating data for each sample set.
Table 4-2 presents total composition data for the untreated waste,
treated nonwastewater, and treated wastewater, as well as, for sample set
No. 2, TCLP leachate data for the treated nonwastewater. Table 4-3
presents TCLP leachate data for both the untreated waste and the treated
nonwastewater, and Table 4-4 presents EP Toxicity Procedure leachate data
for the untreated waste and the treated nonwastewater.
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For high-temperature metals recovery, treatment performance is
measured by the reduction in the concentration of metal constituents from
the untreated waste and also the reduction of leachability of the metals
in the residual as compared to the untreated waste.
4.1.2 Performance Data for Stabilization
EPA has performance data for treatment of cadmium-containing
nonwastewaters using stabilization, shown in Tables 4-5 and 4-6. The
data presented in Table 4-5 are performance data developed from
stabilization of K061 waste, while the data in Table 4-6 represent
treatment of F006 waste. Both data sets present untreated waste total
composition and TCLP data and treated waste TCLP data. Table 4-5 also
presents design and operating parameters for this test.
4.2 Performance Data for Wastewaters
Performance data on chemical precipitation for removal of toxic
metals, including cadmium, from wastewaters were developed as part of the
effort to establish effluent limitations guidelines for various
industries. The effluent guidelines development document for the
inorganic chemicals point source category (USEPA 1982) contains a large
amount of data on the effectiveness of various precipitation processes
for removal of cadmium from solution, as well as the results of
engineering site visit studies at cadmium pigment plants. These data
clearly show that with the use of sulfide precipitation, residual cadmium
levels in the effluent are reduced to 0.25 to 0.5 ppm, which is well
below the EP toxicity limit of 1.0 ppm.
EPA also has performance data for treatment of cadmium-containing
wastewaters using chemical precipitation, shown in Table 4-7. This data
set presents untreated waste composite concentration and treated
wastewater concentration.
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Additional wastewater treatment data, primarily from EPA's Office of
Water, have been analyzed for the development of concentration-based
treatment standards for D006 and other wastewaters. Further information
on these data, including the sources of the data and the treatment
technologies used, can be found in the preamble to the third proposed
rule and in the Best Demonstrated Available Technology (BDAT) Background
Document for Wastewaters Containing BDAT List Constituents (USEPA 1989d).
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Table 4-1 Treatment Performance Data for High-Temperature
Metals Recovery of K061 Waste: Waelz Kiln
(EPA-Collected Data)
Const ituentd
Concentration (units)
Untreated Treated Treated
concentration concentration TCLP
(ppm) (ppm) (mg/1)
Sample Set #1
Cadmium
737
<15
<0.060
Sample Set »2
Cadmium
345
<15
<0.060
Sample Set #3
Cadmium
394
<15
<0.060
Sample Set #4
Cadmium
808
<15
<0.060
Sample Set »5
Cadmium
857
<15
<0.060
Sample Set #6
Cadmium
298
<15
<0.060
Sample Set #7
Cadmium
290
<15
<0.060
Some of the design and operating data associated with these data have
been claimed to be confidential. The remaining design and operating data
associated with these data are shown at the end of this table.
Source: USEPA 1987a.
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3040g
Table 4-1 (continued)
Waste Characteristics Affecting Performance3
Boiling Point (in increasing order) -
Mercury 356"C
Cadniia 765*C
Zinc 909"C
Lead 1760*C
Chromium 2672*C
Boiling Point of Metal
metal) purity and use.
Thermal Conductivity''
28 Btu/hr-ffF.
Design and Operating Data for Rotary Kiln High-Teroerature Hetals Recovery
Operating value
Nominal 6/2/87 6/3/87
Parameter value6 SS #1 SS #2 SS #3 SS #4 SS #5 SS #6 SS #7
- Ho low boiling point metals are present in concentrations that could impact product (recovered
- The thermal conductivity of K061 waste has been estimated to be approximately
Kiln temperature ("C)d
Feed rate (ton/hr)
Rate of rotation (min/i
Zinc content (X)
Moisture content (X)
Carbon content (X)
Calcium/silica ratio
700-800 760-840e
) 1.5
13.3
3.48
730-820e 740-840e
1.5 1.3
10.8 11.2
3.33 9.84
720-840f 6Q0-1065f
1.5 1.1
14.7 11.4
5.6 5.89
575-740f 575-740f
1.1 1.1
14.4 9.2
8.54 8.8
aThe waste characteristics affecting performance for high-tenperature metals recovery are relative volatility and the heat
transfer characteristics of the waste. As the best approximate measure of the parameters, EPA is using boiling point and
thermal conductivity.
''Therma 1 conductivity was calculated based on Major constituents present in the waste and their respective thcnsal
conductivities. This calculation can be found in the Adninistrative Record for K061.
cThis system was built in the 1920s and was not originally designed for treatment of K061 waste. Nominal values were
developed by the plant in lieu of design values.
^eaperature strip chart is included in Appendix C.
eValues reflect those for kiln #2.
fValues reflect those for kiln >3.
- * This information is considered Confidential Business Information.
Reference: USEPA 1987.
4-6
-------
Table 4-2 Treatment Performance Data for High-Temperature Metals Recovery
of K061 Waste: Plasma Arc Reactor
BDAT constituents detected
Untreated Treated Treated Treated
waste3 slag slag waste*,
(ppm) (ppm) TCLP (mg/1) (mg/lj
Sample Set #1 [Stainless Steel)
Cattoium 100-5DC <2 - <0.032-0.004
Sample Set t2 (Carbon Steel|
Cadmium ?00-900 <10-500 <0.005 <0.005-0.016
- * No data.
a For the untreated waste, EPA has values for ranges only. Data were
not available on the specific untreated values that corresponded to the
treated values.
Garments:
1. Data were not provided showing the specific operating conditions at
the time the wastes were treated.
2. No data were provided on treatment characteristics that affect
performance.
Source: SKf Plasmadust 1987.
3197g
4-7
-------
3040g
Table 4-3 Treatment Performance Data for High-
Temperature Hetals Recovery of K061 Vaste:
Rotary Hearth/Electric Furnace
Untreated
Treated
waste
aaste
TCLP
TCLP
Constituent
(rag/1)
(mg/1)
Lead
365
0.38
Ziftc
4,973
0.94
Cacksium
56
0.05
Chromiian
<0.1
<0.1
Caments:
1. Data were not provided on untreated total concentrations.
2. Data Mere not provided on the design and operating values.
3. Data were not provided on waste characteristics that affect
performance.
Reference: INMETCD 198/ (Sample Set #3).
A-8
-------
3040g
Table 4-4 Treatment Performance Data for High-
Taqierature Metals Recovery of K061 Waste:
Molten Slag Systaa
BOAT constituents detected
EP Tox (mg/1)
Untreated
waste
Treated
slag
EP Tox (mg/1)
CacfcriiM
20.2-30.0
0.01-0.07
- * No data.
Consents:
1. Data vara not provided on total Mast* concentrations.
2. Data were not provided on the design and operating values.
3. Data were not provided on waste characteristics that affect
performance.
Reference: Suaitono 1987 (Sanple Set 12).
4-9
-------
Table 4-5 Treatment Performance Data for Stabilization
of K061 Waste (EPA-Collected Data)
Test #1 - Binder: Cement
Untreated waste
BOAT Total TCLP
constituents (ppm) (mg/1)
Cadmium
Untreated waste
BOAT Total TCLP
constituents (ppm) (mg/1)
Cadmium
Untreated waste
BOAT Total TCLP
constituents (ppm) (mg/1)
Treated waste - TCLP (mq/1)
Run Run Run
#1 #2 #3
3.38
Treated waste - TCLP (mq/1)
Run Run Run
#4 #5 #6
0.508
Treated waste - TCLP (ma/1)
Run Run Run
#7 #8 #9
481 12.8 2.86 3.64
Test #2 - Binder: ki In Dust
481 12.8 2.92 1,80
Test #3 - Binder: Lime/Fly Ash
Cadmium 481 12.8 0.033 0.049 0.073
Note: Design and operating data associated with these data can be found
at the end of this table.
Source: USEPA 19B8d.
3197g
4-10
-------
304Og
Table 4-5 (continued)
Pes ion and Operating Data:
Stabilization process/binder
Cement
Kiln dust
Lime and flv asha
Parameter
Run #1
Run #2
Run #3
Run #4
Run #5
Run #6
Run #7
Run #8
Run #9
Binder-to-waste ratio
0.05
0.05
0.05
0.05
0.05
0.05
0.10
0.10
0.10
Uater-to-waste ratio
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
Mixture pH
10.9
11.5
10.5
11.5
11.6
11.1
12.1
12.0
12.0
Cure time (days)
28
28
28
28
28
28
28
28
28
Unconfined compressive
strength (psi)
29.7
88.8
95.7
133.0
167.2
141.2
54.8
58.0
50.7
Waste Characteristics Affecting Performance
Fine particulates - 9OX of the waste copposed of particles <63 i*» or less than 230 mesh sieve size
Oil and grease - 282 ppn
Sulfates - 8,440 ppm
Chlorides - 19,300 ppm
Total organic carbon - 4,430 ppm
a This binder consisted of equal parts of lime and fly ash.
Reference: USEPA 1980d.
4-11
-------
Table 4-6 Treatment Performance Data for Stabilization of F006 Waste
rat io
Const ituent
Concentrat ion
Untreated waste
Total TCLP
(mg/kg) (mg/1)
Treated waste - TCLP (mo/11
Binder-to-waste
0.2 0.5 1.0 1.5
Sample Set #1
(Source-unknown)
Cadmium
0i1 and grease
TOC
1.3
1,520
14,600
0.01
0.01
NR
NR
NR
Sample Set #2
(Source-auto parts
manufacturing)
Cadmium 31.3
Oi1 and grease 60
TOC 1,500
Sample Set #3
(Source-aircraft over-
hauling faci 1 ity)
Cadmium 67.3
Oil and grease 37,000
TOC 137,000
Sample Set #4
(Source-aerospace manufacturing-
mixture of F006 & F007)
Cadmium 1.69
Oi1 and grease 3,870
TOC 8,280
2.21
1.13
0.66
0.50
0.01
NR
NR
0.06 0.02
NR
NR
NR
NR
<0.01 0.01
Sample Set #5
(Source-zinc plating)
Cadmium 1.30
Oil and grease 1,150
TOC 21,200
0.22
0.01
0,01
NR
NR
Sample Set #6
(Source-unknown)
Cadmium
Oi 1 and grease
TOC
720
20,300
28,600
23.6
3.23
0.01
NR
NR
3197g
-------
Table 4-6 (continued)
Concentration
Untreated waste Treated waste - TCLP (mq/1)
Total TCLP Binder-to-waste ratio3
Constituent (mg/kg) (mg/1) 0.2 0.5 1.0 1.5
Sample Set *7
(Source-small engine
manufacturing)
Cadmium 7.28 0.3 0.02 0.01 NR NR
Oil and grease 2,770 - -
T0C 6,550 - ... -
Sample Set #8
(Source-circuit board
manufacturing'3)
Cadmium 5.39 0.06 0.01 0.01 NR NR
Oil and grease 130 - ....
TOC 550 - ....
Sample Set #9
(Source-unknown)
Cadmium 5.81 0.18 0.01 0.01 NR NR
Oil and grease 30 - -
TOC 10,700 - ....
Sample Set #10
(Source-unknown)
Cadmium 5.04 0.01 <0.01 <0.01 NR NR
Oil and grease 1,430 - -
TOC 5,960 - ....
= Not applicable.
NR » Results of tests at this binder-to-waste ratio were not reported.
weiaht of binder material
a(3inder-to-waste ratio * ,
weight of waste
^Oil and grease and total organic carbon (TOC) have been identified by EPA as
waste characteristics that affect the performance of stabilization.
cCircuit board manufacturing waste is not in its entirety defined as F006;
however, an integral part of the manufacturing operation is electroplating.
Treatment residuals generated from treatment of these electroplating wastes are
F006.
Source: CWM 1987.
3197g
-------
Table 4-7 Treatment Performance Data for Chemical Precipitation
and Vacuum Filtration
Concentrat ion
Untreated waste , Treated
<3
composite concentration wastewater concentration
Constituent (mg/1) (mg/1)
Cadmium
13
<0.5
SamDle Set *2
Cadmium
10
<0.5
Sample Set *3
Cadmium
5
<0.5
Samole Set #4
Cadmium
<5
<0.5
Sample Set *5
Cadmium
<5
0.5
Samole Set 16
Cadmium
<5
<0.5
SamDle Set #7
Cadmium
10
<0.5
SamDle Set *8
Cadmium
<5
<0.5
Sample Set #9
Cadmium
•-5
<0.5
SamDle Set #10
Cadmium
<5
<0.5
SamDle Set #11
Cadmium 23 <5
a Untreated waste is a composite of K062, F006, F019, and for D002 waste streams.
Reference: USEPA 1966b.
319 7g
-------
5. IDENTIFICATION OF BEST DEMONSTRATED
AVAILABLE TECHNOLOGY (BOAT)
This section presents the Agency's rationale for determining best
demonstrated available technology (BDAT) for D006 nonwastewaters and
wastewaters.
To determine BDAT, the Agency examines all available performance data
on technologies that are identified as demonstrated to determine (using
statistical techniques) whether one or more of the technologies perform
significantly better than the others. The technology that performs best
on a particular waste or waste treatability group is then evaluated to
determine whether it is "available." To be available the technology must
(1) be commercially available to any generator and (2) provide
"substantial" treatment of the waste, as determined through evaluation of
accuracy-adjusted data. In determining whether treatment is substantial,
EPA may consider data on the performance of a waste similar to the waste
in question provided that the similar waste is at least as difficult to
treat. If the best technology is found to be not available, then the
next best technology is evaluated, and so on.
5.1 BDAT for Nonwastewaters
The most desirable waste management technology is one that results in
no residual streams or a residual stream with no hazardous properties.
For nonwastewaters (including cadmium batteries), recovery of cadmium may
eliminate the waste stream entirely and is therefore the preferred
management method, where applicable. However, the Agency realizes that
this option is available only to generators of wastes with recoverable
quantities of cadmium. EPA has been unable to quantify a recoverable
concentration for cadmium, but believes that recovery is demonstrated for
treatment of all wastes in the cadmium batteries subcategory.
3111g
5-1
-------
High-temperature metals recovery (HTMR), such as is demonstrated for
treatment of K061 wastes, is also demonstrated for nonwastewaters. As a
recovery technology, this is the best technology for treatment of wastes
with high cadmium concentrations. The HTMR residuals will contain much
lower concentrations of cadmium than does the untreated waste.
Stabilization can be used to treat residuals from either recovery or
incineration as well as D006 nonwastewaters that do not have recoverable
concentrations of cadmium. Stabilization is the best treatment for these
wastes. In the absence of such a cutoff concentration for recovery of
cadmium from nonwastewaters, stabilization has been determined to be the
best technology for treatment of D006 nonwastewaters other than
cadmium-containing batteries.
5.2 BDAT for Wastewaters
EPA has two data sets for chemical precipitation treatment of
wastewaters. One data set, presented in Table 4-7, represents treatment
of K062, F006, and other mixed wastewaters containing cadmium. The other
data set was obtained from EPA's Office of Water (see USEPA 1989d).
Chemical precipitation is the only technology for which the Agency has
treatment data for cadmium-containing wastewaters; is therefore the best
technology for treatment of wastewater forms of D006. The Agency
recognizes that chemical precipitation is a well-established method for
removal of cadmium from wastewaters. Thus, this technology is available
and represents BDAT for D006 wastewaters.
31Ug
5-2
-------
6. DEVELOPMENT OF BOAT TREATMENT STANDARDS
In Section 5, the best demonstrated available technology was
identified for each of the three treatability groups (nonwastewaters,
cadmium batteries, and wastewaters) identified in Section 2.3. In this
section, BDAT treatment standards are developed for these technologies
based on the performance data presented in Section 4.
6.1 BDAT Treatment Standards for Nonwastewaters
The Agency has data on the stabilization of K061 nonwastewaters
indicating that cadmium can be stabilized to a level of 0.14 mg/1, as
measured by TCLP extraction. Additionally, the Agency has data
indicating that the stabilization of F006 can achieve a TCLP level of
0.066 mg/1. Based on these available data, the Agency believes that all
cadmium nonwastewater can be stabilized to low leachate concentrations.
The Agency also has data on recovery of K061 nonwastewaters. The data
sets for the K061 nonwastewaters show that recovery technologies generate
a residual for which cadmium leachate concentrations are less than
0,06 mg/1 in all cases.
The Agency is proposing a concentration-based constituent standard
for D006 nonwastewaters of 0.14 mg/1 cadmium in the TCLP leachate based
on the transfer of data from stabilization of K061 nonwastewater.
Calculation of the treatment standards from K061 stabilization data is
shown in Table 6-1. The Agency proposes this standard based on the fact
that the K061 matrix appear to be more difficult to stabilize than the
F006 wastes because cadmium in these wastes is present as cadmium oxide,
which is more difficult to stabilize than the cadmium hydroxide and
cadmium sulfide compounds normally present in F006 wastes. Table 6-2
presents the proposed treatment standards for D006 nonwastewaters.
3112g
6-1
-------
6.2 BDAT Treatment Standards for Wastewaters
The Agency has data from cadmium in K062 wastewaters and from
analysis of treated wastewaters under the Effluent Guideline Program
(USEPA 1989d). Based on statistical analyses of these data, the Agency
determined that data from the Effluent Guideline Program is better than
the data from K062, Therefore, the proposed treatment standard was
calculated from the Effluent Guidelines data.
Based on these treatment data, the Agency is proposing a treatment
standard of 0.20 mg/1 cadmium for all D006 wastewaters. The details of
this calculation, including accuracy adjustment of treatment data and
calculation of variability factor, are shown in the Best Demonstrated
Available Technology (BDAT) Background Document for Wastewaters
Containing BDAT List Constituents (USEPA 1989d). Table 6-3 presents the
wastewater standards.
6.3 BDAT Treatment Standards for Cadmium Batteries
Because the Agency does not have data on the performance of recovery
technologies for the treatment of cadmium battery wastes, it is unable to
establish a concentration-based standard. The Agency is proposing
recovery as a method of treatment for cadmium-containing batteries.
Table 6-4 presents the cadmium battery standards.
3112g
6-2
-------
Table 6-1 Calculation of Treatment Standards
for D006 Nonwastewaters
Accuracy-adjusted Mean of
treated waste accuracy-adjusted Treatment
TCLP concentrations TCLP concentrations Variability standard
Constituent (mg/1) (mg/1) factor (mg/kg)
Cadmium 0.036 0.056 2.5 0.14
0.053
0.080
3112g
6-3
-------
Table 6-2 Proposed BDAT Treatment Standards
for D006 - Nonwastewaters
Regulated Maximum for anv single 24-hour composite sample
constituent Total composition (mg/1)
Cadmium 0.20
3112g
6-4
-------
Table 6-3 Proposed BDAT Treatment Standards
for D006 - Wastewaters
Regulated
constituent
Maximum for anv sinele 24-hour composite sample
Total composition (mg/1)
Cadmium
0.20
3112«
6-5
-------
Table 6-5 Proposed BDAT Treatment Standards
for D006 - Cadmium Batteries
THERMAL RECOVERY AS A METHOD OF TREATMENT
3112g
6-6
-------
7. REFERENCES
APHA, AWWA, and WPCF. 1985. Americal Public Health Association, American
Water Works Association, and Water Pollution Control Federation.
Standard methods for the examination of water and wastewater. 16th
ed. Washington, D.C.: American Public Health Association.
Barring, N.E. 1983. Recycling of nickel-cadmium batteries and process
wastes - processes and operation of the new SAB NIFE plant. In
Proceedings Fourth International Cadmium Conference - San Francisco,
California.
CWM. 1987. Chemical Waste Management. Technical report no. 87-117:
Stabilization treatment of selected metal-containing wastes.
September 22, 1987. Chemical Waste Management, 150 West 137th Street,
Riverdale, IL.
INMETCO. 1987. Description of INMETCO's operations and identification
of the materials that it processes. (Industry-submitted data.)
Ohira, Y. 1986. Current status concerning the recycling of sealed
nickel-cadmium batteries in Japan. In Proceedings Fifth International
Cadmium Conference, San Francisco, California. London: Cadmium
Association, New York: Cadmium Council, International Lead Zinc
Research Organization.
Parker, P.D. 1978. Cadmium compounds. In Kirk Othmer Encyclopedia of
Chemical Technology. Vol. 4, pp.297-411. New York: John Wiley and
Sons.
SKF Plasmadust. 1987. Key data for the Scandust Plant for treating EAF
flue dust (K061). August 1987. (Industry-submitted data.)
SRI. 1989. Stanford Research Institute. Directory of chemical
producers, United States of America. Menlo Park, California: Stanford
Research Institute.
Sumitomo Corporation of America. 1987. On-site treatment of EAF dust
via the NMD system using sensible heat from molten slag. (Industry-
submitted data.)
USEPA. 1982. Development document for effluent limiting guidelines
(BATEA). New source performance standards and pretreatment standards
for the inorganic chemicals manufacturing point source category.
Washington, D.C.: U.S. Environmental Protection Agency.
USEPA. 1986a. U.S. Environmental Protection Agency, Office of Solid
Waste. 1986 National Survey of Hazardous Waste Generators.
Washington,, D.C.: U.S. Environmental Protection Agency.
3113g
7-1
-------
USEPA. 1986b. U.S. Environmental Protection Agency, Office of Solid
Waste. Onsite engineering report of treatment technology performance
and operation for Envirite Corporation, York, Pennsylvania.
Washington, D.C.: U.S. Environmental Protection Agency.
USEPA. 1987. U.S. Environmental Protection Agency, Office of Solid
Waste. Onsite engineering report for Horsehead Development Company for
K061. Draft report. Washington, D.C.: U.S. Environmental Protection
Agency.
USEPA. 1988a. U.S. Environmental Protection Agency, Office of Solid
Waste. Generic qualitv assurance project plan for Land Disposal
Restrictions Program i,"BDAT"). Washington, D.C.: U.S. Environmental
Protection Agency.
USEPA. 1988b. U.S. Environmental Protection Agency, Office of Solid
Waste. Best demonstrated available technology (BDAT) background
document for K061. Washington, D.C.: U.S. Environmental Protection
Agency.
USEPA. 1988c. U.S. Environmental Protection Agency, Office of Solid
Waste. Best demonstrated available technology (BDAT) background
document for F006. Washington, D.C.: U.S. Environmental Protection
Agency.
USEPA. 1988d. Environmental Protection Agency. Onsite engineering
report for Waterways Experiment Station for K061. Draft report.
Washington, D.C.: U.S. Environmental Protection Agency.
USEPA. 1989a. U.S. Environmental Protection Agency, Office of Solid
Waste. Methodology for developing BDAT treatment standards.
Washington, D.C.: U.S. Environmental Protection Agency.
USEPA. 1989b. U.S. Environmental Protection Agency, Office of Solid
Waste. Treatment technology background document. Washington, D.C.:
U.S. Environmental Protection Agency.
USEPA. 1989c. U.S. Environmental Protection Agency, Office of Solid
Waste. Best demonstrated available technology (BDAT) background
document for cyanide wastes. Washington, D.C.: U.S. Environmental
Protection Agency.
USEPA. 1989d. U.S. Environmental Protection Agency, Office of Solid
Waste. Best demonstrated available technology (BDAT) background
document for wastewaters containing BDAT list constituents.
Washington, D.C.: U.S. Environmental Protection Agency.
Versar Inc. 1980. Multimedia assessment of the inorganic chemicals
industry. Task 4, Contract No. 68-03-2604, final report for the
Industrial Environmental Research Laboratory, Vol. 3. Cincinnati,
Ohio: U.S. Environmental Protection Agency.
7-2
3113g
-------
Q -^-OOOfC
: I. i Z. Reus le'.t s K::ess:r. to.
t
; 1
t£:hNjlOGv BACKSROUND DOCUMENT (3RD 3RD5)
5. Report Date
JUNE 1989
6.
". Authorts)
BERlOW,'VORBACH /'OSUi
B. Performing Organisation Rept. Nc
9. Performing 0"?ani:ation Name ana Address
U.S. EF'A
Office o* Solid Waste
401 K. Street SN
Washington, DC 20460
10. Project/Task/Work Drat to.
11. Contract(Cj or Grant(Si No.
1C)
(G)
12. Sponsoring Organization Nane and Address
13. Type o-f Report & Period Covered
14.
15. Supplements") Motes
It. Abstract (Limit: 200 words*
This document, consisting o-f descriptions 0+ 23 treatment technologies, includes inforwation relevant to the use and
understanding c* the background documents for eacr, grour of listed hazardous wastes subject to the R'CRA Land Ban
restrictions. These treatment tecnnology descriptions represent a revision to previously published versions. Typically
the revisions nace mere editorial in nature. The reader should note that this document has not been peer reviewed.
17, Document Analysis a. Descriptors
b. Identifjers/Open-Ended Terms
c. COSATI Field/fcroup
1£. Availability Statement
19. Security Class (This Report)
21. No. o+ Pages
UNCLASSIFIED
0
RELEASE UNLIMITED
20. Security Class (This Page)
22. Price
UNCLASSIFIED
e
(See ANSI—239.18)
OPT
ONAL FORT 272 <4-771
(Foroeriy NTI=-35i
-------
q.-$1-000%£> ^
63©-sui-s«j -anxfit
TREATMENT TECHNOLOGY BACKGROUND DOCUMENT
James R. Berlow, Chief
Treatment Technology Section
Jerry Vorbach
Project Manager
U.S. Environmental Protection Agency
Office of Solid Waste
401 N Street, S.W.
Washington, O.C. 20460
r
1
o
o
June 1989 o
&
-------
TREATMENT TECHNOLOGY BACKGROUND DOCUMENT
This document, consisting of descriptions of 23 treatment
technologies, includes information relevant to the use and understanding
of the background documents for each group of listed hazardous wastes
subject to the RCRA "Land Ban" restrictions. These treatment technology
descriptions represent a revision to previously published versions.
Typically, the revisions made were editorial in nature. The reader
should note that this document has not been peer reviewed.
New technology descriptions may be added to those Included herein as
appropriate to describe adequately technologies being considered for Best
Demonstrated Available Technology (BDAT) selection for Third Third waste.
2699g
1
-------
TABLE OF CONTENTS
1. ACID LEACHING 1-1
1.1 Applicability 1-1
1.2 Underlying Principles of Operation 1-1
1.3 Description of Acid Leaching Process 1-2
1.4 Waste Characteristies Affecting Performance (WCAPs).. 1-3
1.5 Design and Operating Parameters 1-5
1.6 References 1-7
2. CHEMICAL PRECIPITATION 2-1
2.1 Applicability 2-1
2.2 Underlying Principles of Operation 2-1
2.3 Description of Chemical Precipitation Process 2-3
2.4 Waste Characteristics Affecting Performance (WCAPs)... 2-8
2.5 Design and Operating Parameters 2-11
2.6 References 2-14
3. ELECTROLYTIC OXIDATION OF CYANIDE 3-1
3.1 Applicability 3-1
3.2 Underlying Principles of Operation 3-1
3.3 Description of Electrolytic Oxidation of Cyanide
Process 3-2
3.4 Waste Characteristics Affecting Performance (WCAPs)... 3-3
3.5 Design and Operating Parameters 3-4
3.6 References 3-7
4. HEXAVALENT CHROMIUM REDUCTION 4-1
4.1 Applicability... 4-1
4.2 Underlying Principles of Operation 4-1
4.3 Description of Hexavalent Chromium Reduction Process.. 4-2
4.4 Waste Characteristics Affecting Performance (WCAPs)... 4-4
4.5 Design and Operating Parameters 4-5
4.6 References 4-8
5. HIGH TEMPERATURE METALS RECOVERY 5-1
5.1 Applicability 5-1
5.2 Underlying Principles of Operation 5-2
5.3 Description of High Temperature Metals Recovery
Process 5-2
5.4 Waste Characteristics Affecting Performance (WCAPs)... 5-5
5.5 Design and Operating Parameters 5-8
5.6 References 5-11
2699g
2
-------
6. ION EXCHANGE 6-1
6.1 Applicability 6-1
6.2 Underlying Principles of Operation 6-1
6.3 Description of Ion Exchange Process 6-3
6.4 Waste Characteristics Affecting Performance (WCAPs)... 6-5
6.5 Design and Operating Parameters 6-9
6.6 References 6-11
7. RETORTING 7-1
7.1 Applicability 7-1
7.2 Underlying Principles of Operation 7-1
7.3 Description of Retorting Process 7-3
7.4 Waste Characteristics Affecting Performance (WCAPs)... 7-6
7.5 Design and Operating Parameters 7-8
7.6 References 7-10
8. STABILIZATION 8-1
8.1 Applicability 8-1
8.2 Underlying Principles of Operation 8-1
8.3 Description of Stabilization Process 8-3
8.4 Waste Characteristics Affecting Performance (WCAPs)... 8-4
8.5 Design and Operating Parameters 8-6
8.6 References 8-9
9. AEROBIC BIOLOGICAL TREATMENT 9-1
9.1 Applicability 9-1
9.2 Underlying Principles of Operation 9-1
9.3 Description of Aerobic Biological Treatment Process... 9-2
9.4 Waste Characteristics Affecting Performance (WCAPs)... 9-8
9.5 Design and Operating Parameters 9-9
9.6 References 9-15
10. BATCH DISTILLATION 10-1
10.1 Applicability 10-1
10.2 Underlying Principles of Operation 10-1
10.3 Description of Batch Distillation 10-3
10.4 Waste Characteristics Affecting Performance (WCAPs)... 10-5
10.5 Design and Operating Parameters 10-8
10.6 References 10-10
11. CRITICAL FLUID EXTRACTION 11-1
11.1 Applicability 11-1
11.2 Underlying Principles of Operation 11-1
11.3 Description of Critical Fluid Extraction Process 1.1-2
11.4 Waste Characteristics Affecting Performance (WCAPs)... 11-3
11.5 Design and Operating Parameters 11-5
11.6 References 11-8
3
2699g
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12. FRACTIONATION 12-1
12.1 Applicability 12-1
12.2 Underlying Principles of Operation 12-1
12.3 Description of Fractionation Process 12-3
12.4 Waste Characteristics Affecting Performance (WCAPs)... 12-6
12.5 Design and Operating Parameters 12-9
12.6 References 12-12
13. FUEL SUBSTITUTION 13-1
13.1 Applicability 13-1
13.2 Underlying Principles of Operation 13-4
13.3 Description of Fuel Substitution Process 13-5
13.4 Waste Characteristics Affecting Performance (WCAPs)... 13-8
13.5 Design and Operating Parameters 13-12
13.6 References 13-18
14. INCINERATION 14-1
14.1 Applicability 14-1
14.2 Underlying Principles of Operation 14-2
14.3 Description of Incineration Technologies 14-3
14.4 Waste Characteristics Affecting Performance (WCAPs)... 14-10
14.5 Design and Operating Parameters 14-14
14.6 References 14-21
15. SOLVENT EXTRACTION 15-1
15.1 Appl icabil ity... i 15-1
15.2 Underlying Principles of Operation 15-1
15.3 Description of Solvent Extraction 15-2
15.4 Waste Characteristics Affecting Performance (WCAPs)... 15-7
15.5 Design and Operating Parameters 15-9
15.6 References 15-12
16. STEAM STRIPPING 16-1
16.1 Applicability 16-1
16.2 Underlying Principles of Operation 16-1
16.3 Description of Steam Stripping Process 16-3
16.4 Waste Characteristics Affecting Performance (WCAPs)... 16-5
16.5 Design and Operating Parameters 16-8
16.6 References 16-11
17. THIN FILM EVAPORATION 17-1
17.1 Applicability 17-1
17.2 Underlying Principles of Operation 17-1
17.3 Description of Thin Film Evaporation Process 17-2
17.4 Waste Characteristics Affecting Performance (WCAPs)... 17-4
17.5 Design and Operating Parameters 17-7
17.6 References 17-9
4
2699s
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18. CARBON ADSORPTION 18-1
18.1 Applicability 18-1
18.2 Underlying Principles of Operation 18-1
18.3 Description of Carbon Adsorption Process 18-3
18.4 Waste Characteristics Affecting Performance (WCAPs)... 18-6
18.5 Design and Operating Parameters 18-7
18.6 References 18-9
19. CHEMICAL OXIDATION 19-1
19.1 Applicability 19-1
19.2 Underlying Principles of Operation 19-2
19.3 Description of Chemical Oxidation Process 19-5
19.4 Waste Characteristics Affecting Performance (WCAPs)... 19-7
19.5 Design and Operating Parameters 19-9
19.6 References 19-13
20. POLISHING FILTRATION 20-1
20.1 Applicability 20-1
20.2 Underlying Principles of Operation 20-1
20.3 Description of Polishing Filtration Process 20-2
20.4 Waste Characteristics Affecting Performance (WCAPs)... 20-4
20.5 Design and Operating Parameters 20-5
20.6 References 20-8
21. SLUDGE FILTRATION 21-1
21.1 Applicability 21-1
21.2 Underlying Principles of Operation 21-1
21.3 Description of Sludge Filtration Process 21-2
21.4 Waste Characteristics Affecting Performance (WCAPs)... 21-3
21.5 Design and Operating Parameters 21-5
21.6 References 21-9
22. THERMAL DRYING 22-1
22.1 Applicability 22-1
22.2 Underlying Principles of Operation 22-1
22.3 Description of Thermal Drying Process 22-1
22.4 Waste Characteristics Affecting Performance (WCAPs)... 22-2
22.5 Design and Operating Parameters 22-4
22.6 References 22-6
23. WET AIR OXIDATION 23-1
23.1 Applicability 23-1
23.2 Underlying Principles of Operation 23-3
23.3 Description of Wet Air Oxidation Process 23-4
23.4 Waste Characteristics Affecting Performance (WCAPs)... 23-6
23.5 Design and Operating Parameters 23-8
23.6 References 23-11
5
2699g
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LIST OF FIGURES
Figure No. Paoe No.
2-1 Continuous Chemical Precipitation 2-4
2-2 Inclined Plate Settler 2-6
2-3 Circular Clarifiers 2-7
4-1 Continuous Hexavalent Chromium Reduction System 4-3
5-1 High Temperature Metals Recovery System 5-3
6-1 Schematic of Two Step Cation/Anion Exchange System 6-4
7-1 Retorting Process (without wastewater discharge) 7-4
7-2 Retorting Process (with wastewater discharge) 7-5
9-1 Activated Sludge 9-4
10-1 Batch Distillation 10-4
12-1 Fractionation Unit 12-4
14-1 Liquid Injection Incineration 14-4
14-2 Rotary Kiln Incineration 14-6
14-3 Fluidized Bed Incineration 14-7
14-4 Fixed Hearth Incineration 14-9
15-1 Two-Stage Mixed Settler Solvent Extraction System 15-4
15-2 Extraction Columns with Nonmechanical Agitation 15-6
16-1 Steam Stripping 16-4
17-1 Thin Film Evaporation 17-3
18-1 Carbon Adsorption 18-4
18-2 Plot of Breakthrough Curve 18-5
23-1 Wet Air Oxidation Process Flow Diagram 23-5
2699g
6
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I. ACID LEACHING
1.1 Add!icabilitv
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
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.
1.2 Underlying Principles of Operation
Acid leaching is a technology that takes advantage of the enhanced
solubilities of various metals in acid solutions. The process
concentrates the constituent(s) leached by the acid solutions. These
constituents can then be filtered to remove residual solids and
neutralized to precipitate solids containing high concentrations of the
constituents of Interest, which can be further treated 1n 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 (H.SOJ, hydrochloric
Z 4
(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.
1-1
2366
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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 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 to proceed to completion. The
treated solids are then usually separated from the acid by filtration and
further treated using stabilization and/or land disposed.
2368
1-2
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1.4 Waste Characteristics Affecting Performance (WCAPs^
In determining whether acid leaching 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 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 for 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 in order 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
2368
1-3
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solids; therefore, highly alkaline wastes require more acid or a stronger
acid to maintain pH during treatment. If 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 Acid
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 certain 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^} 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 leachable metals 1s 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 leachable metals implies that most of
2368
1-4
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the waste will remain in the solid or slurry waste residues (i.e.,
nonleachable). If the concentration of leachable 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-leachable components and
achieve the same treatment performance, or other, more applicable
treatment 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 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 Acid 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 1s necessary to
form soluble species, then the appropriate acid, as well as the
2368
1-5
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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 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 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 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.
2368
1-6
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1.6 References
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.
2368
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2. CHEMICAL PRECIPITATION
2.1 AddIicabilitv
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
hexavalent chromium wastewaters) or by complexing of the metals (for 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.
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
2-1
1616g
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forms include: lime (Ca(OH)^), caustic (NaOH), sodium sulfide (Na^S),
z
and, to a lesser extent, soda ash (Na„C0,), phosphate (PO, ), and
Z 3 4
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 1s determined
by how successfully they are physically removed. Removal usually relies
on a settling process; that is, a practicle 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, which
increases the importance of empirical tests to accurately determine
appropriate settling times.
2-2
16l6g
-------
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 1s 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 2-1. 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 1s 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
non-hydroxide 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 non-hydroxide precipitating agents,
2-3
I6I69
-------
IREAIMENI
CHEMICAL
FEED
SVSIEM
COAGUtANI OR
FtOCCMANI fEEO SVSIEM
EfftUtNl IO
OlSCIIAMtit Oil
SUBSEOUI Nl
IREAIMENI
HON
UONIIOR
TANK
EtECIMCAL COMIROiS
WAS1EWAIER ElOW
MI1ER
SLUOGE IO
OEWAIEHING
FIGURE 2-1
CONTINUOUS CHEMICAL PRECIPITATION
-------
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 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 2-2 and 2-3. 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.
This technology is discussed in Section 21 of this report.
2-5
I6I69
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INFLUENT
SLUDGE
EFFLUENT
FIGURE 2-2
INCLINED PLATE SETTLER
2-6
-------
EFFLUENT
SLUOGE -
CENTER FEED CLARIFIER WITH SCRAPER SLUDGE REMOVAL SUSTEM
INFLUENT
SLUOGE
RIM PEED - CENTER TAKEOFF CLARIFIER WITH
HYDRAULIC SUCTION SLUOGE REMOVAL SYSTEM
SLUOQC
RIM FEED - RIM TAKEOFF CLARIFIER
FIGURE 2"3
CIRCULAR CLARIFIERS
2-7
-------
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 20 of
this report.
2.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether chemical precipitation 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 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.
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
2-8
-------
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 (TDS)
High concentrations of total dissolved solids 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 1n the treated wastewater
residuals. If the TDS concentration in an untested waste is
significantly higher than 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
Metal complexes consist 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 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
2-9
I616g
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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 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 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.
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16l6g
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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 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, 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.
2-11
16169
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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 from the
wastewater.
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 non-ionic polyelectrolytes also are effective. Typical doses
2-12
16160
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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.
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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1616g
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2.6 References
Cherry, K.F. 1982. Plating waste treatment, pp. 45-67. Ann Arbor,
Mich.: Ann Arbor Science Publishers, Inc.
Cushnie, G.C., Jr. 1985. Electroplating wastewater pollution control
technology, 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. 55-97. 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. Treatabilitv manual.
Vol. Ill, Technology for control/removal of pollutants,
pp. 111.3.1.3.2. EPA-600/2-82-001C, January 1983.
1616g
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3. ELECTROLYTIC OXIDATION OF CYANIDE
3.1 Add!icabil itv
Electrolytic oxidation is a treatment technology applicable to 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 (Cushnie 1985).
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 (Easton 1967).
However, for concentrations of cyanide lower than 500 mg/1, chemical
oxidation treatment may be more efficient.
3.2 Underlying Principles of Operation
The basic principle of 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 (CO ), nitrogen (N ), and ammonia (NH ).
b b ¥
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
3-1
I062g
-------
theoretical destruction process that takes place at the anodes is
described by the following reaction:
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 cyanide concentration is reduced, causing the
electrolytic reaction to be much less efficient at longer retention times.
3.3 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 (OC) 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 bel-ng 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 generally further treated in a
conventional chemical oxidation system to destroy residual cyanides.
2CN" + 202
nectHciti' > 2C02 t N2 + 2e~
cyanide oxygen
ion
carbon nitrogen electrons
dioxide
3-2
1062g
-------
3.4 Waste Characteristies Affecting Performance (WCAPs)
In determining whether electrolytic oxidation 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 other
oxidizable materials and (b) the concentration of reducible metals.
3.4.1 Concentration of Other Oxidizable Materials
The presence of oxidizable organics (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.
3.4.2 Concentration of Reducible Metals
The electrolytic process may cause some of the more easily reduced
metals 1n the waste, such as copper, to plate out onto the anode as the
pure metal. The plating of metals onto the anode 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
3-3
1062g
-------
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.
3.5 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.
3.5.1 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 1s operating at the appropriate
design condition and to diagnose operational problems.
3.5.2 Residence Time
Electrolytic oxidation is usually a batch process. The time allowed
to complete the reaction 1s 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
3-4
1062g
-------
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.
3.5.3 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.
3.5.4 Electrical Conductivity
The solution must have a high enough electrical conductivity to allow
the reaction to proceed at an acceptable rate. If the conductivity is
not high 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.
3.5.5 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.
3-5
1062g
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3.5.6 Degree of Mixing
Electrolytic destruction of cyanide requires good mixing in the
reaction vessel. Mixing helps ensure an adequate supply of oxygen (from
the air) for the electrochemical reaction (see Section 3.2, 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 things, 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. 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.
1062g
3-6
-------
3.6 References
Cushnie, G.C. Jr. 1985. Electroplating wastewater pollution control
technology, 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.
1062g
3-7
-------
4. HEXAVALENT CHROMIUM REDUCTION
4.1 Ado!icabilitv
Hexavalent chromium reduction is a treatment technology applicable to
wastes containing hexavalent chromium wastes, including plating
solutions, stainless steel acid baths and rinses, "chrome conversion"
coating process rinses, and chromium pigment manufacturing wastes.
Because this technology requires that the pH be in the acidic range, it
would not be applicable to a waste that contains significant amounts of
cyanide or sulfide. In such cases, lowering of the pH can result in the
release of toxic gases such as hydrogen cyanide or hydrogen sulfide. It
is important to note that additional precipitation treatment is required
to remove trivalent chromium from the solution following reduction of the
hexavalent chromium.
4.2 Underlying Principles of Operation
The basic principle of hexavalent chromium reduction is to reduce the
valence of chromium in solution (in the form of chromate or dichromate
ions) from the hexavalent state to the trivalent state. "Reducing
agents" used to effect the reduction include sodium sulfite (Na^S^O^),
sodium bisulfite (NaHSO,), sodium metablsulfite (Na S 0 ), sulfur
3 Z Z 5
dioxide (SOJ, sodium hydrosulfide (NaHS), and the ferrous form of iron
rc +2x
(Fe ).
A typical reduction reaction, using sodium sulfite as the reducing
agent, is as follows:
H„(Cr+6)_0, + 3Na.SO. + 3^0, - (Cr+3W$0J, + SNa^SO, + 4H„0
2 2 7 2 3 2 4 '2 4 3 2 4 2
4-1
089 Sg
-------
The reaction is usually accomplished at pH values from 2 to 3.
At the completion of the chromium reduction step, the trivalent
chromium compounds are precipitated from solution by raising the pH
above 8. The insoluble trivalent chromium (in the form of chromium
hydroxide) is then allowed to settle from solution. The precipitation
reaction is as follows:
Cr2(s°4)3 + 3Ca(0H)2 - 2Cr(OH)3 + 3CaS04
4.3 Description of Chromium Reduction Process
The chromium reduction treatment process can be operated 1n 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 chromium reduction treatment system, as shown in
Figure 4-1, usually includes a holding tank upstream of the reaction tank
for flow and concentration equalization. It also 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
redox reaction has proceeded at 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 chromate reduction. Following chromium
089 5g
4-2
-------
)
1«J
HEXAVALENT
CHROMIUM
CONTAINING
WASTEWATER
tin
ORP pH
SENSORS
pH
SENSOR
ACID
FEED
SYSTEM
REDUCING
AGENT
FEED
SYSTEM
ALKALI
FEED
SYSTEM
TO SETTLING
REDUCTION
PRECIPITATION
ELECTRICAL CONTROLS
CD
«=b
MIXER
FIGURE 41
CONTINUOUS HEXAVALENT
CHROMIUM REDUCTION SYSTEM
-------
reduction, the trivalent chromium is precipitated and settled out of the
solution, which is further treated and/or disposed. Precipitated
trivalent chromium is either reused or further treated by stabilization
and land disposed.
4.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether hexavalent chromium reduction 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 oil and grease
and (b) the concentration of other reducible metals.
4.4.1 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 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.
4.4.2 Concentration of Other Reducible Metals
Ionized metals (such as silver, copper, and mercury) can compete with
chromium for reducing agents, thereby requiring greater amounts of
reducing agents to completely reduce the chromium. 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
089 Sg
4-4
-------
performance. Additional reducing agents may be required to reduce the
chromium and achieve the same treatment performance, or 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 a
hexavalent chromium reduction system, EPA examines the following
parameters: (a) the amount and type of reducing agent, (b) the pH,
(c) the residence time, and (d) the degree of mixing.
4.5. Amount and Type of Reducing Agent
The choice of a reducing agent establishes the chemical reaction upon
which the chromium reduction system is based. The amount of reducing
agent must be monitored and controlled 1n both batch and continuous
systems to ensure complete reduction. In batch systems, reducing agent
is usually controlled by analysis of the hexavalent chromium remaining in
solution, but it may also be controlled by using an ORP monitoring
system. For continuous systems, the ORP reading is used to monitor and
control the addition of reducing agent.
The ORP will slowly change until the reduction reaction is completed
at which point the ORP will change rapidly. The set point for the ORP
monitor is approximately the reading just after the rapid change has
begun. The reduction system must then be monitored periodically to
determine whether the selected set point needs further adjustment. EPA
monitors the hexavalent chromium remaining in solution for batch systems
4-5
0895g
-------
and monitors the ORP continuously, if possible, for continuous systems
to ensure that an effective amount of the reducing agent has been added
to the system.
4.5.2 pH
For batch and continuous systems, pH affects the reduction reaction.
The reaction speed is significantly reduced at pH values above
approximately 4.0. For a batch system, the pH can be monitored
intermittently during treatment. For a continuous system, the pH must be
continuously monitored because of its effect on the ORP reading. EPA
monitors the pH to ensure that the system 1s operating at the appropriate
design condition and to diagnose operational problems.
4.5.3 Residence Time
The residence time impacts the extent to which the hexavalent
chromium reduction reaction goes to completion. For batch systems, the
residence time is controlled directly by adjusting the treatment time in
the reaction tank. For continuous systems, the feed rate is controlled
4:
to make sure that the system is operated at the appropriate design
residence time. EPA monitors the residence time to ensure that
sufficient time 1s provided to effectively reduce the waste.
4.5.4 Degree of Mixing
The reduction system should be designed to provide adequate mixing in
order to ensure uniform distribution of the reducing agent and chromium
throughout the reactor. The quantifiable degree of mixing is a complex
assessment that includes, among other things, the amount of energy
06959
4-6
-------
supplied, the length of time the material is mixed, and the related
turbulence effects of the specific size and shape of the reaction
vessel. 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.
089 Sg
4-7
-------
4.6 References
Aldrich, J.R. 1985. Effects of pH and proportioning of ferrous and
sulfide reduction chemicals on electroplating waste treatment sludge
production. In Proceeding of the 39th Purdue Industrial Waste
Conference. May 8-10, 1984. Stoneham, Mass.: Butterworth Publishers.
Cherry, K.F. 1982. Plating waste treatment. Ann Arbor, Mich.: Ann Arbor
Science Publishers, Inc.
Lanouette, K.H. 1977. Heavy metals removal. Chemical Engineering.
October 17, 1977, pp. 73-80.
Patterson, J.W. 1985. Industrial wastewater treatment technology. 2nd
ed. Stoneham, Mass.: Butterworth Publishers.
Rudolfs, W. 1953. Industrial wastes, their disposal and treatment.
Valley Stream, N.Y.: L.E.C. Publishers Inc.
089 Sg
4-8
-------
5. HIGH TEMPERATURE METALS RECOVERY
5.1 AddIicabilitv
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 7.
The HTMR process has been demonstrated on wastes such as baghouse
dusts and dewatered scrubber sludge from the production of steels and
ferroalloys. Z1nc, 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
5-1
?370g
-------
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.
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.
5.2 Underlying Principles of Operation
The basic principle of operation for this technology 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
5.3 Description of Hioh 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 5-1.
5-2
2370g
-------
AIR OR O2
EXHAUST GAS
TO ATMOSPHERE
I
UNTREATED
WASTE
CARBON
(REDUCING
AGENT)
FLUXES
(LIMESTONE,
SAND)
MIXING
UNIT
HIGH
TEMPERATURE
PROCESSING
UNIT
1
PRODUCT
COLLECTION
UNIT
(CONDENSOR OR
CONDENSOR AND
BAGHOUSE)
REUSE OF
VOLATILE
METAL
PRODUCTS
OR FURTHER
REFINEMENT
PRIOR TO
REUSE
m
1
RESIDUAL
COLLECTION
(QUENCH TANK)
REUSE OF NON-VOLATILE METAL PRODUCTS,
FURTHER RECOVERY IN A FURNACE,
STABILIZATION FOLLOWED BY LAND DISPOSAL,
OR DIRECTLY TO LAND DISPOSAL
FIGURE b-1 HIGH TEMPERATURE METALS RECOVERY SYSTEM
-------
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.
The blended waste materials are fed to a furnace, where they are
heated to temperatures ranging from 1100 to 1400'C (2012 to
25520F), 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, Figure 15-1
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,
2370g
5-4
-------
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); or, If the
material has no recoverable value, (c) stabilized, to immobilize any
remaining metal constituents, and land disposed or (d) directly land
disposed as a slag.
5.4 Waste Characteristics Affecting Performance (WCAPsl
In determining whether high temperature metals recovery 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 concentrations of
undesirable volatile metals, (b) the metal constituent boiling points,
and (c) the thermal conductivity of the waste.
5.4.1 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
5-5
2370g
-------
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.
5.4.2 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
heir 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 differences in
boiling points between the more volatile and less volatile constituents
are significantly lower 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.
5-6
23709
-------
5.4.3 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
on 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.
2370g
5-7
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5.5 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 residence time, (c) the degree of mixing, (d) the carbon content
of the feed, and (e) the calcium-to-silica ratio of the feed.
5.5.1 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 at least 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 excessively high so as to volatilize other
unwanted constituents) and to diagnose operational problems.
5.5.2 Residence Time
The residence time impacts 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.
5-8
2370g
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5.5.3 Degree of Mixing
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 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.
5.5.4 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.
5.5.5 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.
5-9
Z370g
-------
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 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.
2370g
5-10
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5.6 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.
23709
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6. ION EXCHANGE
6.1 Add!icabilitv
Ion exchange is a treatment technology applicable to (1) metals in
wastewaters where the metals are present as soluble ionic species (e.g.,
Cr+^ and CrO, *); (2) nonmetallic anions such as halides, sulfates,
4
nitrates, and cyanides; and (3) water-soluble, ionic organic compounds
including (a) acids such as carboxylics, sulfonics, and some phenols, at
a pH sufficiently alkaline to yield ionic species, (b) amines, when the
solution acidity is sufficiently acid to form the corresponding acid
salt, and (c) quaternary amines and alkysulfates.
6.2 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.
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.
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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 (SO -) and carboxylic (COO ) groups. Anion
exchange resins have immobile basic ions, such as amine (NH^ ), to
which the mobile anions, such as hydroxyl (OH ) or chloride (CI ),
are attached.
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.
M+x" + r"n+ - 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+ + 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 1ons 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
2138g
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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+).
6.3 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 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 1s presented in Figure 6-1. The ion exchange system
2136g
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USED BACKFLUSH WATER
TO TREATMENT SYSTEM
USED BACKFLUSH WATER
TO TREATMENT SYSTEM
INFLUENT
WASTEWATER
RINSE WATER
ACID REGENERANT
BACKFLUSH WATER
BACKFLUSH
WATER
RINSE WATER
CAUSTIC
REGENERANT
CATION
EXCHANGE
SYSTEM
ANION
EXCHANGE
SYSTEM
TREATED
WASTEWATER
USED REGENERANT
SOLUTION AND RINSES
TO STORAGE TANK
USED REGENERANT
SOLUTION AND RINSES
TO STORAGE TANK
FIGURE 6-1 SCHEMATIC OF TWO STEP CATION/ANION 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.
6.4 Waste Characteristics Affecting Performance fWCAPs)
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(s), (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.
6.4.1 Concentration and Valence of the Contaminant(s)
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
2136g
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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(s) 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(s) and 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 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(s) of concern. Other ions in the wastewater with the same
charge as the contaminant(s) 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(s) of concern may
be readily removed from a solution with a low concentration of other
similarly charged ionic species, the contaminant(s) may not be removed as
efficiently from solutions where high concentrations of similarly charged
2138g
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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 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.
6.4.3 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
+2 +4
instance, Mn (manganese) may oxidize to the Insoluble Mn 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.
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6.4.4 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
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.
6.4.5 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.
2136g
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6.5 Oesign 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.
6.5.1 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. Host 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
-2 -2
copper cyanide (Cu(CN). ), chromates (CrO. ), and arsenates
-3 4 4
(As04 ), 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 1on exchange resin is
provided to effectively exchange the metal ions of concern.
213eg
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6.5.2 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 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.
6.5.3 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
2
gal/day-ft . EPA monitors the hydraulic loading rate to ensure that
sufficient time is provided to effectively exchange contaminants.
6.5.4 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.
6-10
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6.6 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., and Klei, H.E. 1979. Wastewater treatment. Englewood
Cliffs, N.J.: Prentice-Hall, Inc.
USEPA. 1983. U.S. Environmental Protection Agency. Treatabilitv
manual: Vol. III. Technology for control/removal of pollutants.
EPA-600/2-82-001c. Washington, D.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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7. RETORTING
7.1 Add!icabi lily.
Retorting is a treatment technology applicable to wastes containing
elemental mercury, as well as mercury present in the oxide, hydroxide,
and sulfide 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 is 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.
7.2 Underlying Principles of Operation
Retorting is a process similar to 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
2366g
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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 leachability than a slag if mercury is the only
constituent of concern present in the untreated waste.
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 + 2^0 + 0^
7-2
23669
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7.3 Description of Retorting Process
The retorting process generally consists of a retort (typically an
oven) in which the waste is heated to volatilize the metal constituents,
a condenser, a metals collection system, and an air pollution control
system. Figures 7-1 and 7-2 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 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 either 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 (SO^), 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.
?366g
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CONDENSER
AIR
POLLUTION
STACK
COOLING
WATER
RETORT
PREHEATED
AIR
TRAVS OF
WASTE
MERCURY
COLLECTION
FIGURE 7-1
RETORTING PROCESS (WITHOUT WASTEWATER DISCHARGE)
-------
WATER
STACK
DECANTER
~ WASTEWATER
o*
RETORT
PREHEATEO
AIR
TRAVS OF
WASTE
MERCURY TO RECOVERY
FIGURE 1-1
RETORTING PROCESS (WITH WASTEWATER DISCHARGE)
-------
7.4 Waste Character!stics Affecting Performance (WCAPsi
In determining whether retorting 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 undesirable volatile
constituents in the waste and (b) the thermal conductivity of the waste.
7.4.1 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 froro 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 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 J 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
7-6
23669
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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.
7.4.2 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 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 1s
significantly lower than that of the tested waste, the system may not
?366g
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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.
7.5 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.
7.5.1 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 at least 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. 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.
7.5.2 Residence Time
The residence time impacts the amount of volatile metal(s)
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
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7-8
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residence time to ensure that sufficient time is provided to effectively
volatilize all of the mercury and/or other metal(s) to be removed from
the waste.
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7.6 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 Laboratory by Versar Inc.,
Contract No. 68-01-7053.
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8. STABILIZATION OF METALS
8.1 Ado!i cab11itv
Stabilization is a treatment technology applicable to wastes
containing leachable 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 electroplating wastewaters and incineration ash residues.
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. Solidification and
fixation are other terms that are sometimes used synonymously for
stabilization or to describe specific variations within the broader class
of stabilization. Related technologies are encapsulation and
thermoplastic binding. However, EPA considers these technologies to.be
distinct from stabilization in that their operational principles-are
significantly different.
8.2 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 reduced 1eachabi1ity is accomplished by
the formation of a lattice structure and/or chemical bonds that 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.
0946g
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The two principal stabilization processes used are cement-based and
1ime/pozzolan-based processes. A brief discussion of each is provided
below. In both cement-based or 1ime/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.
8.2.1 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.
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8-2
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8.2.2 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.
8.3 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 is 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 1s recovered from the
processing equipment and disposed of.
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8-3
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8.4 Waste Characteristics Affecting Performance (MCAPsl
In determining whether stabilization 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 fine particulates,
(b) the concentration of oil and grease, (c) the concentration of organic
compounds, and (d) the concentration of sulfate and chloride compounds.
8.4.1 Concentration of Fine Particulates
For both cement-based and 1ime/pozzolan-based processes, very fine
solid materials (i.e., those that pass through a No. 200 mesh sieve (less
than 74 um particle size)) weaken the bonding between waste particles and
the cement or 1ime/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 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.
8.4.2 Concentration of Oil and Grease
Oil and grease in both cement-based and 1ime/pozzolan-based systems
results 1n the coating of waste particles and the weakening of the
bonding between the particle and the stabilizing agent, thereby
0946g
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decreasing the resistance of the material to leaching. If the
concentration of oil and grease in the untested waste is significantly
higher than 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.
8.4.3 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 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.
8.4.4 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 leachability
potential. If the concentration of sulfate and chloride compounds in the
untested waste is significantly higher than in the tested waste, the
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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.
8.5 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, and (d) the stabilization temperature and
humidity.
8.5.1 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 leachability.
Stabilizing 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
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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.
8.5.2 Degree of Mixing
Mixing is necessary to ensure homogeneous distribution of the waste,
stabilizing agent, and additives. Both undermixing and overmixing are
undesirable. The first condition results in a nonhomogeneous mixture;
therefore, 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 generally are determined through laboratory tests. 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 mix tank or vessel. This 1s beyond the scope of
simple measurement. EPA, however, evaluates the degree of mixing
Q946g
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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.
8.5.3 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 upon 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 upon
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.
8.5.4 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 and result 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.
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8.6 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. 10-12 May 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 Malohe, 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 .anual.
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, O.C.:
U.S. Environmental Protection 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(171: 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/0R0 under
Interagency Agreement No. EPA-IAG-D4-0569. PB 81 181 505.
Cincinnati, Ohio.
09469
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9. AEROBIC BIOLOGICAL TREATMENT
9.1 Add!icabilitv
Aerobic biological treatment is a treatment technology applicable to
wastewaters containing biodegradable organic constituents. 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.
9.2 Underlying Principles of Operation
The basic principal of operation for aerobic biological treatment
processes is that living, oxygen-requiring microorganisms decompose
organic constituents into carbon dioxide, water, nitrates, sulfates,
simpler 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.
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The aerobic biodegradation process can be represented by the
following generic equation:
cx Hy * °2 S1£StStfS"> h2° + C02 * ce^lular biomjss-
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.
9.3 Description of Biological Treatment Processes
9.3.1 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 darifier, and a sludge recycle
line. Wastewater is homogenized in an equalization basin to reduce
9-2
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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 21) prior to disposal. The clarified effluent is discharged. A
schematic diagram of an activated sludge treatment system is shown in
Figure 9-1.
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.
9-3
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Oxygen
and
Nutrients
Waste
Influent
Equalization
Tank
Aeration
Basin
Basin
Effluent ^
Clarifier
Sludge Recycle i
Treatec
Effluent
Waste
Sludge
Figure *"1
Activated Sludge
9-4
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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.
9.3.2 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
1906g
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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 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.
9.3.3 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 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 1s available to the microorganisms.
Oxygen from air reaches the microorganisms through the void spaces in the
media.
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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.
9.3.4 Rotating Biological Contactor (RBC)
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.
9-7
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9.4 Waste Characteristics Affecting Performance fWCAPs).
In determining whether aerobic biological treatment 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 ratio of the biological oxygen
demand-to-total organic carbon content, (b) the concentration of
surfactants, and (c) the concentrations of toxic constituents and waste
characteristics.
9.4.1 The Ratio of the Biological Oxygen Demand-to-Total Organic
Carbon Content
Because organic constituents in the waste effectively serve as a feed
supply for the microorganisms, it is necessary that a significant
percentage be biodegradable. If they are not (i.e., a significant
fraction of the organic constituents are refractory), 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
(BOO) to the total organic carbon (TOC) content. The biological oxygen
demand is a measure of the amount of oxygen required for complete
microbial oxidation of biodegradable organics. If the ratio of BOO 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.
9-8
19069
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9.4.2 Concentration of Surfactants
Surfactants can affect aerobic biological treatment performance by
forming a film on organic constituents, thereby establishing a barrier to
oxygen transfer and 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.
9.4.3 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, as well as high concentrations of total dissolved solids,
ammonia, and phenols. 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.
9.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an
aerobic biological treatment system, EPA examines the* following
parameters: (a) the amount of nutrients, (b) the concentration of
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dissolved oxygen, (c) the food-to-microorganism ratio, (d) the pH,
(e) the aerobic biological treatment temperature, (f) the mean cell
residence time, (g) the hydraulic loading rate, (h) the settling time,
and (i) the degree of mixing.
9.5.1 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.
9.5.2 Concentration of Dissolved Oxygen
A sufficient concentration of dissolved oxygen (00) is necessary to
metabolize and degrade dissolved organic constituents. The 00
concentration is controlled by adjusting the aeration rate. The aeration
rate must be adequate to provide a sufficient DO concentration to satisfy
the BOD requirements of the waste, as well as to provide adequate mixing
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to keep the microbial population in suspension (for activated sludge and
aerated lagoon processes). EPA monitors the DO concentrations
continuously, is possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
9.5.3 Food-to-Microorganism Ratio
The food-to-microorganism (F/M) ratio applies only to activated
sludge systems and 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.
9.5.4 pH
Generally, neutral or slightly alkaline pH favors microorganism
growth. The optimum range for most microorganisms used in aerobic
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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9.5.5 Aerobic 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 (69 to 95°F).
The rate of biochemical reactions in cells increases with temperature up
to a maximum above which the rate of activity declines as enzyme
denaturation occurs and microorganisms either die off or become less
active. EPA monitors the aerobic biological treatment temperature
continuously, if possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
9.5.6 Mean Cell Residence Time
In activated sludge and aerated lagoon systems, the mean cell
residence time (MCRT) or sludge age is the length of time organisms are
retained in the aeration 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
«•
aeration unit to reproduce. The MCRT is determined by dividing the total
active microbial mass in the aeration unit (MLVSS) by the total quantity
of microbial mass withdrawn daily (wasted). EPA monitors the MCRT to
ensure that an effective amount of microorganisms 1s present in the
aeration unit.
9.5.7 Hydraulic Loading Rate
The hydraulic loading rate determines the length of time the organic
constituents are in contact with the microorganisms and, hence, the
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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 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.
9.5.8 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.
9.5.9 Degree of Hixing
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
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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 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 mixing unit. 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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9.6 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 Technoloov. 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, 0. 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. 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, O.C.: U.S.
Environmental Protection Agency.
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10. BATCH DISTILLATION
10.1 Add!icabilitv
Batch distillation is one of several thermal treatment technologies
applicable to the treatment of wastes containing organics that are
volatile enough to be removed by the application of heat. This
technology can be used to treat wastes having a high percentage of
organics. Use of this technology results in an organic product stream
that may be reusable directly or after further treatment and a bottom
stream that is often incinerated.
10.2 Underlying Principles of Operation
As with other forms of distillation, the basic principle of batch
distillation is the separation of a liquid mixture into various
components by a process of vaporization-condensation. The more volatile
constituents, which are vaporized, are then conden d and either reused
or further treated by liquid injection incineratic •. the less volatile
constituents, which do not vaporize significantly, may also be reused,
but are more often incinerated.
An integral part of the theory of batch distillation 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 (i.e., those
having higher vapor pressures). The vapor phase above the liquid phase
is then cooled to yield a condensate that is also more concentrated in
the more volatile components. The remaining liquid phase is richer in
10-1
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the less 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 equilibrium stage of vaporization and condensation may achieve a
significant separation of the constituents. (Refer to the fractionation
technology, Section 12, for a discussion of equilibrium stages.)
Typically, batch distillation units contain only one equilibrium stage
and are thus limited in the degree of separation by the relative
volatilities of the constituents. The greater the difference in
component volatilities, the more likely it is that batch distillation
will be effective.
The vapor-liquid equilibrium of the waste comp ients can be expressed
as relative volatility, which is the ratio of the apor-to-1iquid
concentrations of 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 using distillation is impossible because the
constituents have the same concentrations in the vapor and liquid
phases. When the relative volatility is 1, the licuid mixture is called
an azeotrope. Separation becomes easier as the value of the relative
volatility becomes increasingly different from unity. As more of the
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volatiles are removed, the temperature must be continually raised to
vaporize the remaining waste.
In batch distillation, pressurized steam is usually the source of the
heat. The process usually takes place at temperatures lower than
approximately 340"F (corresponding to steam at a pressure of
120 psig) when atmospheric pressure exists in the distillation unit. For
batch distillation units operating under a vacuum, the constituents of
concern that can be volatilized could have boiling points up to 450"F
at atmospheric pressure.
10.3 Description of Batch Distillation Process
A batch distillation unit consists of a steam-jacketed vessel, a
condenser, and a product receiver. Figure 10-1 is a schematic showing
the major components of a batch distillation unit. The steam jacket
provides the heat required to vaporize the vol at i1 constituents in the
liquid fraction of the waste. The rising vapor is ;ollected 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 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.
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Vent of
Non (Condensed
Vapors
~
Waste
Influent
T
Still Bottoms
to Reuse
or Incineration
Condenser
Product
Receiver
Batch
Still
Heated
Jacket
Figure i(M
Batch Distillation
0-4
-------
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, the bottoms
generally require additional treatment, such as incineration for
residual, less volatile organics, prior to disposal.
10.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether batch distillation 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 thermal conductivity of the waste,
(b) the component boiling points, and (c) the concentration of volatile
components.
10.4.1 Thermal Conductivity of the Waste
A major factor determining whether a particula constituent will
volatilize is the transfer of heat through the waste. For batch
distillation, heat transfer is accomplished principally by conduction
when the source of heat is indirect steam and by both convection and
conduction when the heat source is direct steam injection.
EPA examined both methods of heat transfer and 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
:9
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be distilled. With regard to convection, EPA believes that the amount of
heat transferred by convection will generally be more a function of the
system design than of the waste itself.
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 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 transfer. If the thermal conductivity of an
untested waste is significantly lower than that of he tested waste, the
system may not achieve the same performance and oUer, more applicable
treatment technologies may need to be considered for treatment of the
untested waste.
10.4.2 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 batch distillation.
This is because the wastes usually consist of a myriad of components, all
1943g
10-6
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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
differences in boiling points between the more volatile and less volatile
constituents are significantly lower 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.
10.4.3 Concentration of Volatile Components
The concentration of volatile components is a measure.of the maximum
fraction of the waste that can be expected to volat:lize in the batch
still. A relatively low concentration of volatile :omponents implies
that most of the waste may become bottoms (i.e., i. nonvolatile). If the
concentration of volatile components in the untested waste is
significantly lower than that 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 waste.
10.5 Desion and Operating Parameters
In assessing the effectiveness of the design and operation of a batch
distillation system, EPA examines the following parameters: (a) the
distillation temperature and pressure and (b) the residence time.
10-7
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10.5.1 Distillation Temperature and Pressure
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 volatility 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 prob >ms.
10.5.2 Residence Time
The residence time determines the necessary er rgy input into the
system as well as the degree of volatilization of organic constituents.
It is dependent on the distillation temperature 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.
1943q
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10.6 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 engineers' 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.
1943g
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11. CRITICAL FLUID EXTRACTION
11.1 Add!icabi1i t v
Critical fluid extraction is a comparatively new technology. It is
applicable to wastes containing a variety of organics that are soluble in
pressurized fluids such as carbon dioxide, propane, butane, or pentane.
Compounds that have been extracted from wastes by this process include
aliphatic hydrocarbons, alkenes, simple aromatic solvents such as benzene
and toluene, polynuclear aromatics, and phenols. In theory, this process
should also be applicable to a variety of other organic waste
constituents. Critical fluid extraction has been demonstrated for
treatment of API separator sludges and other hydrocarbon-bearing wastes
generated by the petroleum and petrochemicals industries. It may also be
applicable to wastes of similar composition generated by other industries
such as the organic chemicals industry.
11.2 Underlying Principles of Operation
Critical fluid extraction is a technology that takes advantage of the
enhanced solubilities of various organic compounds in hydrocarbons and
other solvents in the near critical state (i.e., high pressure). The
solvents used are compounds that are usually gases at ambient
conditions. The volatile organic solvent is pressurized, which converts
it from a gas to a liquid. As a liquid, it leaches (dissolves) the
organic constituents out of the complex waste with which it is mixed.
2082g
11-1
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11.3 Description of Critical Fluid Extraction Process
A critical fluid extraction process consists of a blending tank, one
or more extraction vessels, one or more decantors, one or more filters,
and an evaporation unit. Oil refinery sludges or other organic wastes
are first blended together in a blending tank and mixed well to yield a
homogeneous, pumpable mixture. This mixture is then pumped 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
solvent. 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 a solvent-rich
stream. The wastewater stream, containing inorganic solids, is sometimes
filtered under pressure to remove the insoluble co :onents. The
solvent-rich stream is fed to a pressurized evapor :ion unit. The
solvent (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 returned to the refinery or other
process plant for reprocessing and/or reuse, blended with fuels for heat
recovery, or incinerated.
The inorganic residuals (or waste solids) filtered from the
waste/solvent mixture are either land disposed (If they contain
nontreatable levels of hazardous constituents such as certain metals
2062g
11-2
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(e.g., chromium, lead)) or further treated by processes such as
stabilization (and/or incineration if waste solids contain treatable
organic levels) prior to land disposal.
11.4 Waste Characteristics Affecting Performance /WCAPs)
In determining whether critical fluid extraction 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 alkalinity of the waste, and (c) the solubility of
the waste constituents of concern in the solvent.
11.4.1 Concentration of Extractable Hydrocarbons
The process is designed to extract hydrocarbon components from mixed
oily and organic waste liquids and sludges. For tKis process to be
economically applied, the waste should contain at east a few percent by
weight of extractable hydrocarbons. The process I- .$ 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. 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 critical fluid extraction
process. A relatively low concentration of extractable hydrocarbons
iCo^g
11-3
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implies that most of the waste may become wastewater or waste solids
(i.e., is nonextractable). Total extractable hydrocarbon content can oe
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 system may not achieve the same performance. 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.
11.4.2 Alkalinity of the Waste
When carbon dioxide is used as the extraction fluid, high alkalinity
will interfere with the process because carbon die ide will react to form
carbonates and bicarbonates. This will result in xcessive carbon
dioxide consumption. For wastes having high alka' nity levels, an
extraction fluid other than carbon dioxide .should be used. 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 rray not achieve the same
performance. Use of another extraction fluid may oe required to achieve
the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested
waste.
"6? g
11-4
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11.4.3 Solubility of the Waste Constituents of Concern in the Solvent
The constituents in the waste feed that are to be extracted determine
the solvent best suited for the extraction process. If the solubility of
the waste constituents of concern in the solvent in the untested waste is
significantly lower than that in the tested waste, the system may not
achieve the same performance. Use of another solvent 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.
11.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a
critical fluid extraction system, EPA examines the following parameters:
(a) the extraction pressure, (b) the degree of ini al waste mixing,
(c) the degree of mixing during extraction, (d) tb extraction
temperature, (e) the residence time, and (f) the settling time.
11.5.1 Extraction Pressure
Critical fluid extraction systems operate at pressures at which the
critical 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, 1f
possible, to ensure that the system is operating at the appropriate
design condition and to diagnose operational problems'.
:082g
11-5
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11.5.2 Degree of Initial Waste Mixing
The waste must be premixed before introduction into the extraction
system to ensure a uniform, pumpable blend of material. The waste is
normally mixed in tanks or other vessels. Any wel1-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 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 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.
11.5.3 Degree of Mixing During Extraction
During the fluid extraction, the solvent and w.ste 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 evaluates this mixing in
the same manner as that previously described for initial waste mixing.
11.5.4 Extraction Temperature
The process is normally operated 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
The critical temperature of a substance is that temperature above
which the substance cannot be liquified, no matter how high the
operating pressures.
11-6
?08?g
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solvent used to ensure that the solvent is present in the extraction
vessel in the liquid phase (so that maximum contact with the waste can be
achieved). EPA monitors the extraction temperature continuously, if
possible, to ensure that the system is operating at the appropriate
design condition and to diagnose operational problems.
11.5.5 Residence Time
The residence time in the extraction vessel impacts the extent of
extraction of organic contaminants from the waste. It is dependent on
the solubility of the waste constituents of concern in the solvent. EPA
monitors the waste feed rate to ensure that sufficient residence time is
provided to effectively extract the organic contaminants from the waste.
11.5.5 Settling Time
Adequate settling time must be provided to make sure that separation
of the phases has been effectively completed. EPA *ionitors the settling
time to ensure effective phase separation.
<;ob£9
11-7
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11.6 References
CF Systems Corporation. 1988. CF Systems Units to Render Refinery
Wastes Non-hazardous.
Johnston, K. 1978. Supercritical fluids. In Kirk-Othmer encyclopedia
of chemical technology. Supplement Vol. I, pp. 872-893. New York.:
Wiley-Interscience.
Weast, R.C., ed. 1978. Handbook of chemistry and physics. 58th ed.
Cleveland, Ohio: CRC Press.
20823
11-8
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12. FRACTIONATION
12.1 Applicability
Fractionation is a form of distillation applicable to wastes
containing organics that are volatile enough to be separated from other
components by the application of heat. It differs from other forms of
distillation, such as batch distillation, steam stripping, and thin film
evaporation, in that 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 minimal 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.
12.2 Underlying Principles of Operation
As with other forms of distillation, the basic arinciple of operation
for fractionation is the volatilization of the more volatile constituents
from the less volatile constituents through the application of heat. The
constituents that are volatilized are then condensed and typically
reused. Constituents that are not volatilized may also be reused or
incinerated as applicable.
An integral part of the theory of fractionation 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
2075s
12-1
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the higher vapor pressures). The vapor phase above the liquid phase is
then cooled to yield a condensate that is also more concentrated in the
more volatile components. The remaining liquid phase is richer in the
less 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.
(Typically, batch distillation or thin film evaporation would be used in
such a case). If the difference between the vapor pressures is small,
then multiple equilibrium stages are needed to ach'eve effective
separation. In practice, the multiple equilibria stages are obtained by
stacking trays or placing packing into a column. Essentially, each tray
represents one equilibrium stage. In a packed fractionation column, the
individual equilibrium stages are not discernible, but the number of
equivalent trays can be calculated from mathematical relationships using
the height of the packing. 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. Fractionation processes use
multiple equilibrium stages, with the initial waste feed entering at a
point between the first and last equilibrium stages. '
The stages at and below the point of entry are called the stripping
section. These stages sequentially "strip" the volatile components from
12-2
zorsg
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the liquid feed (i.e., reduce the quantities of more volatile
constituents). The stages located above the point of feed are called the
rectification section. These stages allow the vapor rising from the
stripping section to become further enriched in the more volatile
components. The vapor leaving the top of the fractionating column is
condensed, and a portion of this condensed liquid is returned to the
uppermost stage to aid in rectification. This step of returning a
portion of the condensed liquid to the column is called reflux. The
remaining condensed liquid is collected in a product receiver and reused.
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 a constituent divided by the vapor-to-liquid
concentrations of another constituent. The relative volatility is a
direct indicator of the ease of separation. If th numerical value is 1,
then separation is impossible because the constitt nts have the same
concentrations in the vapor and liquid phases. When the relative
volatility is 1, the 1iquid mixture is called an azeotrope. Separation
becomes easier as the value of the relative volatility becomes
increasingly different from unity.
12.3 Description of Fractionation Process
A fractionation unit consists of a reboiler, a column containing
stripping and rectification sections, a condenser, and a reflux system.
Figure 12-1 is a schematic showing the major components of a
fractionation unit. The reboiler is a device that provides the heat
2075g
12-3
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Vent of
;¦-Condense
vapors
Waste
Influent
Condenser
Reflux ~
Rectifier
Section
Product
Receiver
Stripper
Section
Reboil
Bottoms
to Treatment
or Reuse
Reboiler
FIGURE 12-1
FRACTIONATION UNIT
iZ-4
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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, further
enrichment of the vapor in the constituents with lower boiling points
(i.e., the more volatile constituents) is achieved The rising vapor is
collected at the top of the column and condensed i 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.
12-5
t075g
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12.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether fractionation 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 characteristies: (a) the component boiling points, (b) the
concentration of suspended solids, (c) the concentration of volatile
components, (d) the surface tension, and (e) the concentration of oil and
grease.
12.4.1 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 complex types of wastes generall. treated by
fractionation. This is because the wastes usuall.. consist of a myriad of
components, all with different vapor pressure-versus-temperature
relationships. Determining relative volatilities is further complicated
by the fact that the relative volatility changes as the temperature
conditions change throughout the fractionation column (the column is
cooler at the top than at the bottom). 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 viaste characteristic
12-6
i 3' Sg
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far relative volatility. If the differences in boiling points between
the more volatile and less volatile constituents are lower 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.
12.4.2 Concentration of Suspended Solids
Wastes containing large amounts of suspended solids, organic or
inorganic, may clog column internals and coat heat transfer surfaces,
thereby inhibiting mass transfer of constituents 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 fractionation to reduce the concentration of si -pended solids and
achieve the same treatment performance, or other, ^re applicable
technologies may need to be considered for treatir-it of the untested
waste.
12.4.3 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 in the
fractionation column. A relatively low concentration of volatile
components implies that most of the waste may become bottoms (i.e., is
nonvolatile). If the concentration of volatile components in the
untested waste is significantly lower than that in the tested waste, the
12-7
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system may not achieve the same performance. Higher temperatures, lower
pressures, and/or an increase in the number of separation stages 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.
12.4.4 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. The likelihood of foaming requires special column
design or the incorporation of defoaming compounds. Packed columns are
usually less susceptible to foaming than tray columns. 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 b- required to reduce
foaming and achieve the same treatment performanc- or other, more
applicable treatment technologies may need to be considered for treatment
of the untested waste.
12.4.5 Concentration of Oil and Grease
High concentrations of oil and grease may clog fractionation
equipment. Consequently, special designs may be required to accommodate
oil and grease. 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 and other, more applicable treatment
12-8
2075g
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technologies may need to be considered for treatment of the untested
waste.
12.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a
fractionation system, EPA examines the following parameters: (a) the
number of separation stages, (b) the liquid and vapor flow rates, (c) the
fractionation temperature and pressure, (d) the differential pressure,
and (e) the internal column design.
12.5.1 Number of Separation Stages
The number of theoretical stages in the fractionation column required
to achieve the desired separation of more volatile components from less
volatile components is calculated from vapor-liquid equilibrium data,
which are determined empirically. Using the theoretical number of
stages, the actual number of stages can then be de rmined through the
use of empirical tray efficiency data typically sullied by an equipment
manufacturer. EPA examines the actual number of stages in the
fractionation column to ensure that the system is designed to achieve an
effective degree of separation of more volatile components from less
volatile components.
12.5.2 Liquid and Vapor Flow Rates
The vapor-liquid equilibrium data are also usee to determine the
liquid and vapor flow rates that provide sufficient contact between the
liquid and vapor streams. These rates are, in turn, iaffected by the
12-9
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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 more volatile components from less
volatile components.
12.5.3 Fractionation Temperature and Pressure
These parameters are integrally related to the vapor-liquid
equilibrium conditions. 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 reduced because liquids volatilir- at lower
temperatures at reduced pressures. EPA monitors t • temperature and
pressure in a fractionation column continuously, i possible, to ensure
that the system is operating at the appropriate design conditions and to
diagnose operational problems.
12.5.4 Differential Pressure
Measuring the differential pressure between the top and bottom of the
column indicates whether the flow rate of either tne 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 down through the column as fast as feed is entering, causing backing
12-10
20/5g
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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.
12.5.5 Internal Column Design
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 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
(greater 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 the -etical stage, being
more resistant to corrosive materials, having a lo-er 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 fractionation column to ensure that the system is designed to
handle potential operational problems (e.g., corrosion, foaming,
channeling, etc.).
2075g
12-11
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12.6 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 engineers' 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. P884-232628. pp. 1-1 to F4. Prepared for the Industrial
Environmental Research Laboratory, Office of Research and Development,
U.S. Environmental Protection Agency.
2075g
12-12
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13. FUEL SUBSTITUTION
Fuel substitution involves using hazardous waste as a fuel in
industrial furnaces or in boilers for generation of steam. The hazardous
waste may be blended with other nonhazardous wastes (e.g., municipal
sludge) and/or fossil fuels.
13.1 AddIicabi1i tv
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.
The most common types of units in which waste fuels are burned are
industrial furnaces and industrial boilers. Industrial furnaces include
a variety of industrial processes that produce hea and/or products by
burning fuels. They include blast furnaces, smelt *s, and coke ovens.
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.
The parameters that affect the selection of fuel substitution are:
• 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);
13-1
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• Filterable solids concentration (for liquids); and
• Sulfur content.
13.1.1 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-dioxi ns (CDOs), chlorinated dibenzofurans
(CDFs), and chlorinated phenols.
13.1.2 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 problems, wastes with
significant concentrations of inorganic materials are not usually handled
in boilers unless they have an air pollution control system.
13-2
OSBlg
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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.
13.1.3 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 (P.000 to 10,000 Btu/lb)
are considered to be feasible. Below this value, ie unblended fuel
would not be likely to maintain a stable flame, an: 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 employed 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.
13-3
068 lg
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13.1.4 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 prior to combustion. For atomization of liquids, a
viscosity of 165 centistokes (750 Saybolt Seconds Universal (SSU)) or
less is typically required.
13.1.5 Filterable Solids Concentration
If filterable material suspended in the liquid fuel prevents or
hinders pumping or atomization, it will be unacceptable.
13.1.6 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 proposed sulfur oxide emissio regulations for
certain new source industrial boilers (51 FR 22385 . Air pollution
control devices are available to remove sulfur oxides from the stack
gases.
13.2 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
13-4
Oedlg
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waste allows the atomized waste to break into smaller particles, thus
enhancing its rate of vol at i1i za t i on. 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,
13.3 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 following section, however, provides a general
description of industrial kilns (one form of industrial furnace) and
industrial boilers.
13.3.1 Kilns
Combustible wastes have the potential to be u ad as fuel in kilns
and, for waste liquids, are often used with oil t. co-f1re 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.
13-5
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(1) Cement kilns. The cement kiln is a rotary furnace, which is a
refractory-1 ined 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 1,5403C (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 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 wastes currently can i burned in cement
kilns. Available information shows that scrubbers are not used.
(2) Lime kilns. Quick-lime (CaO) is manufact red in a calcination
process using limestone (CaCO^) or dolomite (CaCO^ and MgCO^).
These raw materials are also heated in a refractory-1ined rotary kiln,
typically to temperatures of 980 to 1,260BC (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.
13-6
363 lg
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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.
(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 oroduce a wastewater
residual stream subject to treatment standards.
13.3.2 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. Few
grate-fired boilers burn hazardous wastes, however. For liquid-fired
boilers, residuals requiring land disposal are generated only when the
13-7
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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.
13.4 Waste Characteristics Affecting Performance (WCAPs)
For cement kilns, lime kilns, and lightweight aggregate kilns burning
nonhalogenated wastes (i.e., no scrubber is needed to control acid
gases), no residual waste streams would be produce Any noncombustible
material in the waste would leave the kiln in the -oduct stream. As a
result, in transferring standards EPA would not examine waste
characteristics- affecting performance but rather would determine the
applicability of fuel substitution; that is, EPA would investigate the
parameters affecting treatment selection. For kilns, these parameters
(as mentioned previously) are Btu content, percent filterable solids,
halogenated organics content, viscosity, and sulfur content.
Lightweight aggregate kilns burning halogenated organics and boilers
burning wastes containing any noncombustibles will produce residual
13-8
068 lg
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streams subject to treatment standards. In determining whether fuel
substitution is likely to achieve the same level of performance on an
untreated waste as on a previously treated waste, EPA will examine:
(1) relative volatility of the waste constituents, (2) the heat transfer
characteristics (for solids), and (3) the activation energy for
combustion.
13.4.1 Relative Volatility
The term relative volatility (a) 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.
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 :hat the best measure
of relative volatility is the boiling point of the /arious hazardous
constituents and will, therefore, use this parameter in assessing
volatility of the organic constituents.
13.4.2 Heat Transfer Characteristics
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,
13-9
OUSlg
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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 convection heat transfer properties of wastes in determining
similar treatability. For solids, the third heat transfer mechanism,
conductivity, is the one principally operative or 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 e measurement
essentially reflects heat flow through regions ha. ng 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.
Other parameters considered for predicting heat transfer
characteristics were Btu value, specific heat, and ash content. These
parameters can neither better account for nonhomogeneity nor better
predict heat transferability through the waste.
13-10
-88 lg
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13.4.3 Activation Energy
Given an excess of oxygen, an organic waste in an industrial fjrnace
or boiler would be expected to convert to CO^ and H^O provided that
the activation energy is achieved. Activation energy is the quantity of
heat (energy) needed to destabilize molecular bonds and create reactive
intermediates 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.
In some instances, bond energies will not be available and will have
to be estimated, or other energy effects (e.g., vibrational) and other
reactions will have a significant influence on acf;vation energy.
Because of the shortcomings of bond energies in e imating activation
energy, EPA analyzed other waste characteristic p. ameters to determine
whether these parameters would provide a better basis for transferring
treatment standards from an untested waste to a tested waste. These
parameters included heat of combustion, heat of formation, use of
available kinetic data to predict activation energies, and general
structural class. All of these parameters were rejected for the reasons
provided below.
The heat of combustion measures only the difference in energy of the
products and reactants; it does not provide information on the transition
13-11
oeeig
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state (i.e., the energy input needed to initiate the reaction). Heat of
formation is used as a tool to predict whether reactions are likely to
proceed; however, there are a significant number of hazardous
constituents for which these data are not available. The use of
available kinetic data was rejected because these data are limited and
could not be used to calculate dissociation requirements for the wide
range of hazardous constituents. Finally, EPA decided not to use
structural classes because the Agency believes that evaluation of bond
dissociation energies allows for a more direct determination of whether a
constituent will be destabilized.
13.5 Design and Operating Parameters
13.5.1 Design Parameters
Cement kilns, lime kilns, and aggregate kilns burning nonhalogenated
wastes produce no residual streams. Their design d operation is such
that any wastes that are incompletely destroyed wi be contained in the
product. As a result, the Agency will not look at design and operating
values for such devices since treatment, per se, cannot be measured
through detection of constituents in residual streams. In this instance,
it is important merely to ensure that the waste is appropriate for
combustion in the kilns and that the kiln is operated in a manner that
will produce a usable product.
Specifically, cement, lime, and aggregate kilns are demonstrated only
on liquid hazardous wastes. Such wastes must be sufficiently free of
filterable solids to avoid plugging the burners at the hot end of the
13-12
:3Si3
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kiln. Viscosity also must be low enough to inject the waste into the
kiln through the burners. The sulfur content is not a concern unless the
concentration in the waste results in exceeding Federal, State, or local
air pollution standards promulgated for industrial boilers.
The design parameters that normally affect the operation of an
industrial boiler (and aggregate kilns with residual streams) with
respect to hazardous waste treatment are (1) the design temperature,
(2) the design retention time of the waste in the combustion chamber, and
(3) turbulence in the combustion chamber. Evaluation of these parameters
would be important in determining whether an industrial boiler or
industrial furnace is adequately designed for effective treatment of
hazardous wastes. The rationale for selection of these three parameters
is given below.
(1) Design temperature. Industrial boilers e generally designed
based on their steam generation potential (Btu ou 3Ut). 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
13-13
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to operate at specific ranges of temperature in order to produce the
desired product (e.g., lightweight aggregate). The blended waste/fuel
mixture should be capable of maintaining the design temperature range.
(2) Retention time. A sufficient retention time of combustion
products is normally necessary to ensure that the hazardous substances
being combusted (or formed during combustion) are completely oxidized.
Retention times on the order of a few seconds are generally needed at
normal operating conditions. For industrial furnaces as well as boilers,
the retention time is a function of the size of the furnace and the fuel
feed rates. For most boilers and furnaces the retention time usually
exceeds a few seconds.
(3) Turbulence. 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 .el and air feed
influence the turbulence required for good mixing. Industrial furnaces
also are designed for turbulent mixing where fuel and air are mixed.
13.5.2 Operating Parameters
The operating parameters that normally affect the performance of an
industrial boiler and many industrial furnaces with respect to treatment
of hazardous wastes are (1) air flow rate, (2) fuel feed rate, (3) steam
pressure or rate of production, and (4) temperature. EPA believes that
these four parameters will be used to determine whether an industrial
boiler burning blended fuels containing hazardous waste constituents is
13-14
068 lg
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operated properly. The rationale for selection of these four operating
parameters is given below. Most industrial furnaces will monitor similar
parameters, but some exceptions are noted below.
(1) 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 flow 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 thp flue gas is a
meaningless variable.
(2) Fuel feed rate. The rate at which fuel 1 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.
(3) 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. Increases or
13-15
0881 3
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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.
(4) Temperature. Temperatures are monitored and controlled in
industrial boilers to ensure the quality and flow rate of the steam.
Therefore, complex monitoring systems are frequently installed in the
combustion unit to provide a direct reading of temperature. The
efficiency of combustion in industrial boilers is dependent on combustion
temperatures. Temperature may be adjusted to design settings by
increasing or decreasing air and fuel feed rate.
Wastes should not be added to primary fuels until the boiler
temperature reaches the minimum needed for destruction of the wastes.
Temperature instrumentation and control should be esigned to stop adding
waste in the event of process upsets.
Monitoring and control of temperature in industrial furnaces (e.g.,
lime, cement, or aggregate kilns) that require minimum operating
temperatures are also critical to the product quality. Kilns have very
high thermal inertia in the refractory and in-process product, high
residence times, and high air flow rates, so that ~ven in the case of a
momentary stoppage of fuel flow to the kiln, organ c constituents are
likely to continue to be destroyed. The main operational control
required for wastes burned in kilns is to stop waste flow in the event of
13-16
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low kiln temperature, loss of electrical power to the combustion air fan.
and loss of primary fuel flow.
13.5.3 Other Operating Parameters
In addition to the four operating parameters discussed above, EPA
considered and then discarded one additional parameter fuel-to-waste
blending ratios. However, while blending is done to yield a uniform Btu
content fuel, blending ratios will vary significantly depending on the
Btu content of the wastes and the fuels being used.
OBeig
13-17
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13.6 References
Castaldini C., et al. 1986. Disposal of hazardous wastes in industrial
boilers or furnaces. Park Ridge, New Jersey: Noyes Publications.
Bonner, T. A. et al. 1981. Engineering handbook for hazardous waste
incineration. PB 81-248163. Prepared by Monsanto Research
Corporation for U.S. EPA. June 1981.
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.
CdSlg
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14. INCINERATION
This section addresses the common!}' used incineration technologies:
liquid injection, rotary kiln, fluidized bed, and fixed hearth. As
appropriate, the subsections are divided by type of incineration unit.
14.1 Add!icabi1i tv
14.1.1 Liquid Injection
Liquid injection is applicable to wastes that have viscosity values
low enough that the waste can be atomized in the combustion chamber. A
range of literature maximum viscosity values are reported, with the low
being 100 Saybolt Seconds Universal (SSU) and the high being 10,000 SSU.
It is important to note that viscosity is temperature dependent so that
while liquid injection may not be applicable to a waste at ambient
conditions, it may be applicable when the waste is heated. Other factors
that affect the use of liquid injection are partic e size and the
presence of suspended solids. Both of these can cause plugging of the
burner nozzle.
14.1.2 Rotary Kiln/Fluidized Bed/Fixed Hearth
These incineration technologies are applicable to a wide range of
hazardous wastes. They can be used on wastes that contain high or low
total organic content, high or low filterable solids, various viscosity
ranges, and a range of other waste parameters. EPA has not found these
technologies to be demonstrated on most wastes that are composed
essentially of metals with low organic concentrations'. In addition, the
Agency expects that the incineration of some of the high metal content
14-1
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wastes may not be compatible with existing and future air emission limits
without emission controls far more extensive than those currently in use.
14.2 Underlying Principles of Operation
14.2.1 Liquid Injection
The basic operating principle of this incineration technology is that
incoming liquid wastes are volatilized and then additional heat is
supplied to the waste to destabilize the chemical bonds. Once the
chemical bonds are broken, these constituents react with oxygen to form
carbon dioxide and water vapor. The energy needed to destabilize the
bonds is referred to as the energy of activation.
14.2.2 Rotary Kiln and Fixed Hearth
There are two distinct principles of operation for these incineration
technologies, one for each of the two chambers involved. In the primary
chamber, energy, in the form of heat, is transferr d to the waste to
achieve volatilization of the various organic waste constituents. During
this volatilization process some of the organic constituent bonds
destabilize and oxidize to carbon dioxide and water vapor. In the
secondary chamber, additional heat is supplied to overcome the energy
requirements needed to destabilize the remaining chemical bonds and allow
the constituents to react with excess oxygen to form carbon dioxide and
water vapor. The principle of operation for the secondary chamber is
similar to that of liquid injection.
14.2.3 Fluidized Bed
The principle of operation for this incinerator technology is
somewhat different from that for rotary kiln and fixed hearth
14-2
1367g
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incineration, in that there is only one chamber, which contains the
fluidizing sand and a freeboard section above the sand. The purpose of
the fluidized bed is to both volatilize the waste and combust the waste.
Destruction of the waste organics can be accomplished to a better degree
in this chamber than in the primary chamber of the rotary kiln and fixed
hearth because of (a) improved heat transfer from fluidization of the
waste using forced air and (b)the fact that the fluidization process
provides sufficient oxygen and turbulence to convert the organics to
carbon dioxide and water vapor. The freeboard volume generally does not
have an afterburner; however, additional time is provided for conversion
of the organic constituents to carbon dioxide and water vapor (and
hydrochloric acid if chlorine is present in the waste).
14.3 Description of Incineration Technologies
14.3.1 Liquid Injection
The liquid injection system is capable of Incinerating a wide range
of gases and liquids. The combustion system has a simple design with
virtually no moving parts. A burner or nozzle atomizes the liquid waste
and injects it into the combustion chamber, where it burns 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 it can be fired
horizontally, vertically upward, or vertically downward. Figure 14-1
illustrates a liquid injection incineration system.
14-3
13 & 7 9
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WATER
AUXILIARY FUEL
¦p»
¦
¦I*
LIQUID OR GASEOUS.
WASTE INJECTION
-M BURNER
AIR-
BURNER
PRIMARY
COMBUSTION
CHAMBER
AFTERBURNER
(SECONDARY
COMBUSTION
CHAMBER)
I
rrn
SPRAY
CHAMBER
GAS TO AIR
POLLUTION
CONTROL
HORIZONTALLY FIRED
LIQUID INJECTION
INCINERATOR
ASH
WATER
FIGURE 141
LIQUID INJECTION INCINERATOR
-------
14.3.2 Rotary Kiln
A rotary kiln is a slowly rotating, refractory-lined cylinder that is
mounted at a slight incline from the horizontal (see Figure 14-2). Solid
wastes enter at the high end of the kiln, and liquid or gaseous wastes
enter through atomizing nozzles in the kiln or afterburner section.
Rotation of the kiln exposes the solids to the heat, vaporizes them, and
allows them to combust by mixing with air. The rotation also causes the
ash to move to the lower end of the kiln, where it can be removed.
Rotary kiln systems usually have a secondary combustion chamber or
afterburner following the kiln for further combustion of the volatilized
components of solid wastes.
14.3.3 Fluidized Bed
A fluidized bed incinerator consists of a column containing inert
particles such as sand, which is referred to as thr bed. Air, driven by
a blower, enters the bottom of the bed to fluidize the sand. Air passage
through the bed 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 gas at the
same temperature), thereby providing a large heat reservoir. The
injected waste reaches ignition temperature quickly in the hot fluidized
bed. Continued bed agitation by the fluidizing air allows larger
particles to remain suspended in the combustion zone. (See Figure 14-3)
14.3.4 Fixed Hearth
Fixed hearth incinerators, versions of which are also called
controlled air or starved air incinerators, are another major technology
14-5
1367g
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GAS TO
AIR POLLUTION
CONTROL
AFTERBURNER
AUXILIARY
FUEL.
AIR
COMBUSTION
GASES
FEED
MECHANISM
ROTARY
KILN
LIQUID OR
GASEOUS
WASTE
INJECTION
ASH
FIGURE W-if
ROTARY KILN INCINERATOR
14-6
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GAS TO
AIR POLLUTION
CONTROL
FREEBOARD
MAKE-UP
SAND
SAND BED
WASTE
INJECTION
BURNER
AIR
ASH
FIGURE
FLUIDIZED BED INCINERATOR
14-7
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used for hazardous waste incineration. Fixed hearth incineration is a
two-stage combustion process (see Figure 14-4). Waste is fed into the
first stage, or primary chamber, and usually burned at less than
stoichiometric conditions (less than the theoretically required amount of
air). The resultant smoke and pyrolysis products, consisting primarily
of volatile hydrocarbons and carbon monoxide, along with the normal
products of combustion, pass to the secondary chamber. Here, additional
air is usually injected to complete the combustion. This two-stage
process generally yields low stack particulate and carbon monoxide (CO)
emissions. The primary chamber combustion reactions and combustion gas
volumes are maintained at low levels by the starved air conditions so
that particulate entrainment and carryover are minimized.
14.3.5 Air Pollution Controls
following incineration of hazardous wastes, cc oustion gases are
generally further treated in an air pollution control system. The
presence of chlorine or other halogens'in some waste requires a scrubbing
or absorption step to remove hydrogen chloride (HC1) and other halo-acids
from the combustion gases. Ash in the waste is not destroyed in the
combustion process. Depending on its composition, ash will exit either
as bottom ash, at the discharge end of a kiln or hearth for example, or
as particulate matter (fly ash) suspended in the combustion gas stream.
Particulate emissions from most hazardous waste combustion systems
generally have particle diameters of less than 1 micron and require
high-efficiency collection devices to minimize air emissions. In
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AIR
WASTE
INJECTION
BURNER
AIR
1
GAS TO AIR
POLLUTION
CONTROL
PRIMARY
COMBUSTION
CHAMBER
GRATE
I
SECONDARY
COMBUSTION
CHAMBER
AUXILIARY
FUEL
2rSTAGE FIXED HEARTH
INCINERATOR
ASH
FIGURE K-4
FIXED HEARTH INCINERATOR
-------
addition, scrubber systems provide an additional buffer against
accidental releases of incompletely destroyed waste products resulting
from poor combustion efficiency or combustion upsets.
14.4 Waste Characteristics Affecting Performance fWCAPsl
14.4.1 Liquid Injection
In determining whether liquid injection 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
dissociation bond energies of the constituents in the untested and tested
wastes. This parameter is being used as a surrogate indicator of
activation energy which, as discussed previously, destabilizes molecular
bonds. In theory, the bond dissociation energy would be equal to the
activation energy; however, in practice this is not always the case.
Other energy effects (e.g., vibrational effects, tr^ formation of
intermediates, and interactions between different molecular bonds) may
have a significant influence on activation energy.
Because of the shortcomings of bond energy calculations in estimating
activation energy, EPA analyzed other waste characteristic parameters to
determine whether these parameters would provide a better basis for
transferring treatment standards from an untested waste to a tested
waste. These parameters include heat of combustion, heat of formation,
use of available kinetic data to predict activation energies, and general
structural class. All of these were rejected for the reasons provided
below.
1267g
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The heat of combustion measures only the difference in energy of the
products and reactants; it does not provide information on the transition
state (i.e., the energy input needed to initiate the reaction). Heat of
formation is used as a tool to predict whether reactions are likely to
proceed; however, there are a significant number of hazardous
constituents for which these data are not available. The use of kinetic
data was rejected because these data are limited and could not be used to
calculate dissociation requirements for the wide range of hazardous
constituents. Finally, EPA decided not to use structural classes because
the Agency believes that evaluation of bond dissociation energies allows
for a more direct determination of whether a constituent will be
destabi1ized.
14.4.2 Rotary Kiln/Fluidized Bed/Fixed Hearth
Unlike liquid injection, these incineration te nnologies always
generate a residual ash. Accordingly, in determining whether these
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 that
affect volatilization of organics from the waste, as well as destruction
of the organics once volatilized. Relative to volatilization, EPA
examines the thermal conductivity of the entire waste and the boiling
points of the various constituents. As with liquid injection, EPA
examines bond energies in determining whether treatment standards for
scrubber water residuals can be transferred from a tested waste to an
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136 7 g
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untested waste. Below is a discussion of how EPA arrived at thermal
conductivity and boiling point as the best means to assess volatilization
of organics from the waste; the discussion relative to bond energies is
the same for these technologies as for liquid injection and is therefore
not repeated.
(1) Thermal conductivity. Consistent with the underlying principles
of incineration, a major factor with regard to whether a particular
constituent will volatilize is the transfer of heat through the waste.
In the case of rotary kiln, fluidized bed, and fixed hearth incineration,
heat is transferred through the waste by three mechanisms: radiation,
convection, and conduction. For a given incinerator, heat transferred
through various wastes by radiation is more a function of the design and
type of incinerator than of the waste being treated. Accordingly, the
type of waste treated has a minimal impact on the nount of heat
transferred by radiation. With regard to convection, EPA also believes
that the type of heat transfer is generally more a function of the type
and design of incinerator than of the waste itself. However, EPA is
examining particle size as a waste characteristic that may significantly
impact the amount of heat transferred to a waste by convection and thus
may impact volatilization of the various organic compounds. The final
type of heat transfer, conduction, is the one that EPA believes has the
greatest impact on volatilization of organic constituents. To measure
this characteristic, EPA uses thermal conductivity; an explanation of
this parameter, as well as how it can be measured, is provided below.
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Heat flow by conduction is proportional to the temperature gradient
across the material. The proportionality constant is a property of the
material and is referred to as the thermal conductivity. (Note: The
analytical method that EPA has identified for measurement of thermal
conductivity is described in Section 5, High Temperature Metals
Recovery). In theory, thermal conductivity would always provide a good
indication of whether a constituent in an untested waste would be treated
to the same extent in the primary incinerator chamber as the same
constituent in a previously tested waste.
In practice, thermal conductivity has some limitations in assessing
the transferabi1ity of treatment standards; however, EPA has not
identified a parameter that can provide a better indication of heat
transfer characteristics of a waste. Below is a discussion of the
limitations associated with thermal conductivity, s well as other
parameters considered.
Thermal conductivity measurements, as part of a treatability
comparison of 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
(e.g., significant concentration of metals in soil), 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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Btu value, specific heat, and ash content were also considered for
predicting heat transfer characteristics. These parameters can no better
account for nonhomogeneity than can thermal conductivity; additionally,
they are not directly related to heat transfer characteristics.
Therefore, these parameters do not provide a better indication of the
heat transfer that will occur in any specific waste.
(2) Boiling point. Once heat is transferred to a constituent within
a waste, removal of this constituent from the waste depends on its
volatility. As a surrogate for volatility, EPA is using the boiling
point of the constituent. Compounds with lower boiling points have
higher vapor pressures and, therefore, would be more likely to
volatilize. The Agency recognizes that this parameter does not take into
consideration the impact of other compounds in the waste on the boiling
point of a constituent in a mixture; however, the 'gency is not aware of
a better measure of volatility that can easily be determined.
14.5 Design and Operating Parameters
14.5.1 Liquid Injection
For a liquid injection unit, EPA's analysis of whether the unit is
well designed focuses on both the likelihood that sufficient energy is
provided to the waste to overcome the activation level for breaking
molecular bonds and whether sufficient oxygen is present to convert the
waste constituents to carbon dioxide and water vapor. In assessing the
effectiveness of the design and operaton of a liquid injection unit, EPA
examines the following parameters: (a) the temperature, (b) the excess
1367g
14-14
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oxygen concentration, (c) the carbon monoxide concentration, and (d) the
waste feed rate. Below is a discussion of why EPA believes that these
parameters are important, as well as a discussion of how these parameters
are monitored during operation.
It is important to point out, relative to the development of land
disposal restriction standards, that since liquid injection generally
does not produce bottom ash, EPA is concerned with these design
parameters only when a quench water or scrubber water residual is
generated from treatment of a particular waste. If treatment of a
particular waste in a liquid injection unit would not generate a
wastewater stream, then the Agency, for purposes of land disposal
treatment standards, would be concerned only with the waste
characteristics that affect selection of the unit, not with the
above-mentioned design parameters.
(1) Temperature. Temperature provides an indirect measure of the
energy available (i.e., Btu/hr} to overcome the activation energy of
waste constituents. As the design temperature increases, it becomes more
likely that the molecular bonds will be destabilized and the reaction
completed.
The temperature is normally controlled automatically through the use
of instrumentation that senses the temperature and automatically adjusts
the amount of fuel and/or waste being fed. The temperature signal
transmitted to the controller can be simultaneously transmitted to a
recording device and thereby continuously recorded. To fully assess the
14-15
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operation of the unit, it is important to know not only the exact
location in the incinerator at which the temperature is being monitored
but also the location of the design temperature.
(2) Excess oxygen concentration. It is important that the
incinerator contain oxygen in excess of the stoichiometric amount
necessary to convert the organic compounds to carbon dioxide and water
vapor. If insufficient oxygen is present, then destabilized waste
constituents could recombine to form the same or other BOAT list organic
compounds and potentially cause the scrubber water to contain higher
concentrations of BOAT list constituents than would be the case for a
well-operated unit.
In practice, the amount of oxygen fed to the incinerator is
controlled by continuous sampling and analysis of the stack gas. If the
amount of oxygen drops below the design value, the' the analyzer
transmits a signal to the valve or damper controllng the air supply and
thereby increases the flow of oxygen. The analyzer simultaneously
transmits a signal to a recording device so that the amount of excess
oxygen can be continuously recorded. Again, as with temperature, it is
important to know the location from which the combustion gas is being
sampled.
(3) Carbon monoxide concentration. The carbon monoxide
concentration is an important operating parameter because it provides an
indication of the extent to which the waste organic constituents are
being converted to carbon dioxide and water vapor. An increase in the
carbon monoxide level indicates that greater amounts of organic waste
136/9
14-16
-------
constituents are unreacted or partially reacted. Increased carbon
monoxide levels can result from insufficient oxygen, too much oxygen
(causing cooling), insufficient turbulence in the combustion zone, or
insufficient residence time of combustion gases.
(4) Waste feed rate. It is important to monitor the waste feed rate
because it is correlated to the residence time. The residence time is
associated with a specific Btu energy value of the feed and a specific
volume of combustion gas generated. Prior to incineration, the Btu value
of the waste is determined through the use of a laboratory device known
as a bomb calorimeter. The volume of combustion gas generated from the
waste to be incinerated is determined from a waste analysis referred to
as an ultimate analysis. This analysis determines the amount of
elemental constituents present, which include carbon, hydrogen, sulfur,
oxygen, nitrogen, and halogens. Using this analys s plus the total
amount of air added, the volume of combustion gas :an be calculated.
After both the Btu content and the expected combustion gas volume have
been determined, the feed rate can be fixed at the desired combustion gas
residence time. Continuous monitoring of the feed rate determines
whether the unit was operated at a rate corresponding to the designed
residence time.
14.5.2 Rotary Kiln
For this incineration technology, EPA examines both the primary and
secondary chamber in evaluating the design of a particular incinerator.
14-17
!36?g
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Relative to the primary chamber, EPA's assessment of design focuses on
whether it is likely that enough energy is provided to the waste to
volatilize the waste constituents. For the secondary chamber, analogous
to the sole liquid injection incineration chamber, EPA examines the same
parameters discussed previously under liquid injection incineration.
(These parameters will not be discussed again here.)
In assessing the effectiveness of the design and operation of the
primary chamber, EPA examines the following parameters: (a) the kiln
temperature, (b) the residence time of the waste solids, and (c) the
revolutions per minute. Below is a discussion of why EPA believes that
these parameters are important, as well as a discussion of how these
parameters are monitored during operation.
(1) Temperature. The primary chamber temperature is important
because it provides an indirect measure of the ene gy input (i.e.,
Btu/hr) available for heating the waste. The hig^r the design
temperature in a given kiln, the more likely it is that the constituents
will volatilize. As discussed earlier in Section 14.5.1, Liquid
Injection, temperature should be continuously monitored and recorded.
Additionally, it is important to know the location of the temperature
sensing device in the kiln.
(2) Residence time of the waste solids. This parameter is important
in that it affects whether sufficient heat is transferred to a particular
constituent for volatilization to occur. As the time that the waste is
in the kiln is increased, a greater quantity of heat is transferred to
14-18
-------
the hazardous waste constituents. The residence time is a function of
the specific configuration of the rotary kiln, including the length and
diameter of the kiln, the waste feed rate, and the rate of rotation.
(3) Revolutions per minute (RPMh This parameter provides an
indication of the turbulence that occurs in the primary chamber of a
rotary kiln. As the turbulence increases, the quantity of heat
transferred to the waste would also be expected to increase. However, as
the RPM value increases, the residence time of waste solids decreases,
resulting in a reduction of the quantity of heat transferred to the
waste. This parameter needs to be carefully evaluated because it
provides a balance between turbulence and residence time.
14.5.3 Fluidized Bed
As discussed previously in Section 14.2, Underlying Principles of
Operation, the primary chamber accounts for almost all of the conversion
of organic wastes to carbon dioxide and water vapor (and acid gas if
halogens are present). The freeboard volume will generally provide
additional residence time for combustion gases for thermal oxidation of
the waste constituents. Relative to the primary chamber, the parameters
that EPA examines in assessing the effectiveness of the design are
temperature, residence time, and bed pressure differential. The first
two were included in the rotary kiln discussion and will not be discussed
here. The last, bed pressure differential, is important in that it
provides an indication of the amount of turbulence and, therefore,
indirectly the amount of heat supplied to the waste. In general, as the
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1367g
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pressure drop Increases, both the turbulence and heat supplied increase.
The pressure drop through the bed should be continuously monitored and
recorded to ensure that the designed valued is achieved.
14.5.4 Fixed Hearth
The design considerations for this incineration unit are similar to
those for a rotary kiln with the exception that rate of rotation (i.e.,
RPMs) is not an applicable design parameter. For the primary chamber of
this unit, the parameters that EPA examines in assessing how well the
unit is designed are the same as those discussed under Rotary Kiln
(Section 14.5.2); for the secondary chamber (i.e., afterburner), the
design and operating parameters of concern are the same as those
discussed under Liquid Injection (Section 14.5.1.
1367g
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14.6 References
Ackerman, D.G., McGaughey, J.F., and Wagoner, D.E. 1983. At sea
incineration of PCB-containing wastes on board the M/T Vulcanus. USEPA
600/7-83-024. Washington, O.C.: U.S. Environmental Protection
Agency. 1981.
Bonner, T.A., et al. 1981. Engineering handbook for hazardous waste
incineration. SW-899. Prepared by Monsanto Research Corporation for
US EPA. NT IS PB 81-248163.
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.
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. Washingtor D.C.: U.S.
Environmental Protection Agency.
Vogel, G., et al. Incineration and cement kiln capacity for hazardous
waste treatment. In Proceedings of the 12th Annual Research Symposium.
Incineration and Treatment of Hazardous Wastes. Cincinnati, Ohio,
April 1986.
Mitre Corp. 1983. Guidance manual for hazardous waste incinerator
permits. NTIS PB84-100577.
136 7g
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15. SOLVENT EXTRACTION
15.1 Add!icabilitv
Solvent extraction is a treatment technology used to treat wastes
with a broad range of total organic content, with the selection of a
solvent dependent upon its solubility with the organic compounds to be
removed and the other constituents in the waste. This technology has
been used to treat oil refinery wastes and other organic wastes.
15.2 Underlying Principles of Operation
Solvent extraction is a treatment technology used to remove
constituents from a waste by mixing the waste with a solvent that will
preferentially dissolve the waste constituents of concern from the
waste. The waste and the solvent must be immiscible so that after mixing
the two immiscible phases can physically separate by gravity. In theory,
the maximum degree of separation that can be achi;.ed is provided by the
selectivity value, which is the ratio of the equilibrium concentration of
the constituent in the solvent to the equilibrium concentration of the
constituent in the waste. The solvent extraction process can be either
batch or continuous.
In the simplest extraction systems, three chemical components are
mixed: (1) the solute, or the contaminants in the waste liquid stream to
be extracted; (2) the nonsolute portion of the waste stream; and (3) the
solvent. The solvent and the waste stream are mixed to allow mass
transfer of the constituent(s) (the solute) from the waste stream to the
;D6Jg
15-1
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solvent. Separation of the solvent phase and the waste stream phase
occurs under quiescent conditions, relying on the density differences
between the two phases.
The solvent solution containing the extracted contaminants is called
the extract, and the extracted waste stream with the contaminants removed
is called the raffinate. The extract can be either the heavy (more
dense) phase or the light (less dense) phase.
15.3 Description of Solvent Extraction Processes
The simplest, least effective solvent extraction unit is a
single-stage system (mixer-settler system). The solvent and the liquid
waste stream are mixed together; the raffinate and extract are separated
by settling without further extraction.
The more effective multistage contact extraction is basically a
series of mixer-settler units. The waste stream 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 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.
1964g
15-2
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15.3.1 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 dispersion
systems. 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 15-1, 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 solvent is
recycled, and the remaining extract is either reused or further treated
in an incineration unit. The raffinate from the f -st 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 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. Any solids
collected during filtration are land disposed unless they contain
treatable levels of hazardous constituents. If they do contain treatable
levels of hazardous constituents, they will require further treatment,
such as stabilization (for metals) and/or incineration (for organics).
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lD6Jg
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IT
RECYCLED SOLVENT FROM
RECOVERY/FRESH SOLVENT
MAKEUP
RECYCLEO
SOLVENT
EXTRACT
AND SOLVENT
EXTRACT AND SOLVENt lO RECOVERY
RECYCLED
TO PROCESS
TO
DISPOSAL
WASTE
RAFFINAT
RAFFINATE
SOLVENT
EXTRACT
SOLIDS
SEPARATION
)
O
ui
EXTRACT TO
RECYCLE OR
DISPOSAL
FIGURE 15-1 TWO-STAGE MIXER-SETTLER SOLVENT EXTRACTION SYSTEM
-------
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,
15.3.2 Extraction Columns
Extraction columns are countercurrent, multistage contact systems.
Two types of extraction columns are packed extractors and sieve-tray
extractors. Figure 15-2 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 solute to the extract is promoted because of breakup and
distortion of the dispersed phase as it contacts the packing, resulting
in the intimate mixing of the solute and the solve'
The sieve-tray extractor is similar to the sieve-tray column used in
fractionation distillation. Tray perforations produce the formation of
liquid droplets that aid the mass transfer process by allowing for more
intimate contact between the solute and the solvent.
15.3.3 Centrifugal Contactors
Centrifugal contactors are based on the application of centrifugal
force to increase rates of countercurrent flow and enhance separation of
the phases. 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
15-5
!06
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UAI MUAI E
~ IIAIHIIATE
SOI VI NT
LIQUID'
INTLlll ACE
SOLVENT
tn
o
WASTE
rm
V
SOLVENT
PACKING
LLLt
v.
PACKING
SUPPORT
REDISTRIBUTOR
PACKING
SUPORT
=^1
EXTRACT
SOLVENT
LIQUID
INTERFACE
DOWNCOMER
WASTE
EXTRACT
A. PACKED EXTRACTOR B. SIEVE TRAY EXTRACTOR
Figure ib-2
EXTRACTION COLUMNS WITH NONMECHANICAL AGITATION
-------
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-densi ty
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 having small density differences.
15.4 Waste Characteristics Affecting Performance (WCAPsl
In determining whether solvent extraction 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 cons ituents of concern in
the solvent, and (c) the surface tension.
15.4.1 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 solvent extraction process. A relatively low
concentration of extractable hydrocarbons implies that most of the waste
may become raffinate 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 system may not
achieve the same performance. More rigorous extraction conditions such
15-7
I064g
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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.
15.4.2 Solubility of the Waste Constituents of Concern in the Solvent
The constituents in the waste feed that are to be extracted determine
the type of solvent that is best suited for the extraction process. For
example, polar molecules in an organic feed can be extracted with an
aqueous solvent. Conversely, organic constituents in aqueous feeds can
be extracted with organic solvents. Metal-containing wastes may be
extracted with organic acids (e.g., trialkylphosphoric, sulfuric, and
carboxylic acids) or amine solvents. If the solubility of the waste
constituents of concern in the solvent in the unte.ted waste is
significantly lower than that in the tested waste, the system may not
achieve the same performance. Use of another solvent 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.
15.4.3 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,
15-8
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packed columns are less susceptible to foaming than tray columns. 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.
15.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a
solvent extraction system, 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, and (e) the
settling time.
15.5.1 Number of Separation Stages
For extraction columns, the number of theoretical stages required to
achieve the desired separation of hazardous constituents from a liquid
waste into the selected solvent is calculated from solvent equilibrium
data, which are determined empirically. Using the theoretical number of
stages, the actual number of stages can then be determined through the
use of empirical tray efficiency data typically supplied by an equipment
manufacturer. 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.
:064g
15-9
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15.5.2 Extraction Temperature and pH
Temperature and pH changes can affect equilibrium conditions and.
consequently, the performance of the extraction system. EPA monitors the
temperature and pH continuously, if possible, to ensure that the system
is operated at the appropriate design conditions and to diagnose
operational problems.
15.5.3 Degree of Mixing
For mixer-settler extractors, mixing determines the amount of contact
between the two immiscible phases and, accordingly, 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
quantifiable degree of mixing is a complex assessment that includes,
among other things, the amount of energy supplied, :he 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. EPA, however, evaluates the degree of mixing qualitatively
by considering whether the type of mixing device provided is one that
could be expected to achieve uniform mixing of the waste.
15.5.4 Residence Time
The residence time in the extraction vessel impacts the extent of
extraction of organic contaminants from the waste. It is dependent on
the solubility of the organic contaminants in the solvent. For a batch
system, the residence time is controlled directly by adjusting the
treatment time in the extraction vessel. For a continuous system, the
1064g
15-10
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waste feed rate is controlled to ensure 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 extract the
organic contaminants from the waste.
15.5.5 Settling Time
For mixer-settler 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.
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15.6 References
Oe 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, pp. 76-95.
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-Hil" Book Company.
1064g
15-12
-------
16. STEAM STRIPPING
16.1 ApdIicabilitv
Steam stripping is a form of distillation applicable to the treatment
af wastewaters containing organics that are volatile enough to be removed
by the application of heat using steam as the heat source. Typically,
steam stripping is applied where there is less than 1 percent volatile
organics in the waste.
16.2 Underlying Principles of Operation
The basic principle of operation for steam stripping is the
volatilization of hazardous constituents through the application of
heat. The constituents that are volatilized are then condensed and
typically either reused or further treated by liquid injection
incineration.
An integral part of the theory of steam stripp ng is the principle of
vapor-liquid equilibrium. When a liquid mixture o-" 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
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.
16-1
-------
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
(Typically, batch distillation or thin film evaporation would be used in
such a case.) If the difference between the vapor pressures 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,
the individual equilibrium stages are not discernible, but the number of
equivalent trays can be calculated from mathematical relationships using
the height of the packing. 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 1s again heated and separated.
The vapor-liquid equilibrium of the waste components can be expressed
as relative volatility, which,fs the ratio of the vapor-to-liquid
concentrations of 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 1s 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 easier as the value'of the relative
volatility becomes increasingly different from unity.
1387g
16-2
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16.3 Description of Steam Stripping Process
A steam stripping unit consists of a boiler, a stripping column, a
condenser, and a collection tank, as shown in Figure 16-1. 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 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 ir» 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, anc the liquid exiting the
bottom of the boiler ("bottoms") contains high concentrations of the
lower vapor pressure constituents, often predominately water. This
effluent from the botto'm 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 and separated in a product receiver. Organics
in the organic phase are typically recovered or disposed of in a liquid
injection incinerator. The aqueous condensate is recycled to the
stripper.
: 38 79
16-3
-------
VENT OF
NON CONDENSED
VAPORS
~
CONDENSER
WASTE
NFLUENT
\\
RECYCLE
RECEIVER
STRIPPING
COLUMN
RECOVERED SOLVENT
FOR REUSE
OR TREATMENT
STEAM
TREATED
EFFLUENT
BOILER
FIGURE i*-i
STEAM STRIPPING
ib-4
-------
16.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether steam stripping 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
concentration of suspended solids, (c) the concentration of volatile
components, (d) the surface tension, and (e) the concentration of oil and
grease.
16.4.1 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 meas ;red or calculated
directly for the complex types of wastes general 1;, treated by steam
stripping. This is because the wastes usually consist of a myriad of
components, all with different vapor pressure-versus-temperature
relationships. Determining relative volatilities is further complicated
by the fact that the relative volatility changes as the temperature
conditions change throughout the steam stripping column (the column is
cooler at the top than at the bottom). 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 differences in boiling points between
:3o7g
16-5
-------
the more volatile and less volatile (water) constituents are
significantly lower in the untested waste than those 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.
16.4.2 Concentration of Suspended Solids
Wastes containing large amounts of suspended solids, organic or
inorganic, may clog column internals and coat heat transfer surfaces,
thereby inhibiting mass transfer of constituents 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 steam stripping to reduce the concentration of jspended solids and
achieve the same treatment performance, or other, :>re applicable
treatment technologies may need to be considered f;r treatment of the
untested waste.
16.4.3 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 in the steam
stripping column. An extremely high concentration of volatile components
may imply that adequate separation will not occur. If the concentration
of volatile components in the untested waste is significantly higher than
that in the tested waste, the system may not achieve the same
16-6
-------
performance. Higher temperatures, lower pressures, and/or an increase in
the number of separation stages may be required to volatilize less
volatile compounds and achieve the same treatment performance, or other,
more applicable technologies may need to be considered for treatment of
the untested waste.
16.4.4 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. The likelihood of foaming requires special column
design or the incorporation of defoaming compounds. 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 equired 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.
16.4.5 Concentration of Oil and Grease
High concentrations of oil and grease may clog steam stripping
equipment, thereby reducing its effectiveness. 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 and
other, more applicable treatment technologies may need to be considered
for treatment of the untested waste.
16-7
-------
16.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a steam
stripping system, EPA examines the following parameters: (a) the number
of separation stages, (b) the liquid and vapor flow rates, (c) the
stripping temperature and pressure, and (d) the internal column design.
16.5.1 Number of Separation Stages
The number of theoretical stages in the steam stripping 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, the actual number of stages can then be
determined through the use of empirical tray efficiency data typically
supplied by an equipment manufacturer. EPA examir i the actual number of
stages in the steam stripping column to ensure tha the system is
designed to achieve an effective degree of separat on of organics from
the wastewater.
16.5.2 Liquid and Vapor Flow Rates
The vapor-liquid equilibrium data are also used to determine the
liquid and vapor flow rates that provide sufficient contact between the
liquid and vapor streams. These 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.
16-8
-------
16.5.3 Stripping Temperature and Pressure
These parameters are integrally related to the vapor-liquid
equilibrium conditions. The temperature at any point in the stripping
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 reduced because liquids volatilize at lower
temperatures at reduced pressures. EPA monitors the temperature and
pressure of a steam stripping column continuously, if possible, to ensure
that the system is operating at the appropriate design conditions and to
diagnose operational problems.
16.5.4 Internal Column Design
Column internals are designed to accommodate t-e physical and
chemical properties of the wastewater t
-------
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 column to ensure that the system is
designed to handle potential operational problems (e.g., corrosion,
foaming, channeling, etc.).
12b 7g
16-10
-------
16.6 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 engineers' 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.
136/9
16-11
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17. THIN FILM EVAPORATION
17.1 Add!i cab i1ity
Thin film evaporation is a form of distillation applicable to the
treatment of wastes containing organics that are volatile enough to be
removed by the application of heat. This technology can be used to treat
highly concentrated organic wastes. However, the feed stream to a thin
film evaporator must contain Tow concentrations of suspended solids. Use
of this technology results in an organic product stream, which may be
reused or further treated, and a bottoms stream, which is often
incinerated.
17.2 Underlying Principles of Operation
As with other forms of distillation, the basic principle of thin film
evaporation is the separation of a liquid mixture ^to various components
by a process of vaporization-condensation. The co stituents that are
volatilized are then condensed and either reused or further treated by
liquid injection incineration. The constituents that are not volatilized
may also be reused or incinerated as applicable.
An integral part of the theory of thin film evaporation 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
the higher vapor pressures). The vapor phase above the liquid phase is
then cooled to yield a condensate that is also more concentrated in the
more volatile components. The remaining liquid phase is richer in the
less volatile components. The degree of separation of components depends
17-1
2063g
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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.
Typically, thin film units contain only one equilibrium stage and are
thus limited in the degree of separation by the relative volatilities of
the constituents. The greater the difference in component volatilities,
the more likely it is that thin film evaporation will be effective.
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 a constituent divided by the rat o of the vapor-to-
liquid concentrations of another constituent. The relative volatility is
a direct measure of the ease of separation. If the numerical value is 1,
i
then separation using thin film evaporation 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 easier as the value of the relative
volatility becomes increasingly different from unity.
17.3 Description of Thin Film Evaporation Process
Typically, thin film evaporation consists of a steam-jacketed
cylindrical vessel and a condenser. Figure 17-1 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
17-2
2063g
-------
Vent of
Rotating Non-Condensed
* w ~
Condenser
Waste
Influent
Reflux
Product
Receiver
Heated
Jacket
Liquid
Film
Bottoms to
Treatment
or Reuse
Figure 17-1
Thin Film Evaporation
j.7-3
-------
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 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, the bottoms
generally require additional treatment, such as Incineration for
residual, less volatile organics, prior to disposal.
17.4 Waste Characteristics Affecting Performance (WCAPsl
In determining whether thin film evaporation 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 concentration of suspended solids, (c) the concentration of
volatile components, and (d) the concentration of oil and grease.
2063g
17-4
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17.4.1 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 the relative volatilities cannot be measured or calculated
directly for the types of wastes generally treated by thin film
evaporation. 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 for relative volatility. If the
differences in boiling points between the more volatile and less volatile
constituents are lower 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.
17.4.2 Concentration of Suspended Solids
Wastes containing large amounts of suspended solids, organic or
inorganic, may coat heat transfer surfaces, thereby disturbing the
uniform film and inhibiting volatilization of constituents. If the
concentration of suspended solids in the untested waste 1s significantly
higher than that in the tested waste, the system may hot achieve the same
17-5
Z063g
-------
performance. Filtration may be required prior to thin film evaporation
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.
17.4.3 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 in the thin film
evaporator. A relatively low concentration of volatile components
implies that most of the waste may become bottoms (i.e., is
nonvolatile). If the concentration of volatile components in the
untested waste is significantly lower than that in the tested waste, the
system may not achieve the same performance. Higher temperatures may be
required to volatilize less volatile compounds and achieve the same
treatment performance, or other, more applicable treatment technologies
may need to be considered for treatment of the untested waste.
17.4.4 Concentration of Oil and Grease
High concentrations of oil and grease 1n the waste may result in
coating of the evaporator walls, preventing a uniform film from forming
and inhibiting volatilization of waste components. If the concentration
of oil and grease 1n 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.
17-6
20639
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17.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a thin
film evaporation system, EPA examines the following parameters: (a) the
evaporator surface area, (b) the evaporation temperature and pressure,
and (c) the residence time.
17.5.1 Evaporator Surface Area
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 to ensure that sufficient surface area is provided to achieve
effective volatilization of the more volatile organic components.
17.5.2 Evaporation Temperature and Pressure
These parameters are integrally related to the vapor-liquid
equilibrium conditions. To achieve the desired volatilization, the
temperature of the evaporator must be maintained high enough to
volatilize the volatile components from the waste at the waste flow
rate. If the evaporator is operated at pressures below atmospheric
(slight vacuum), lower temperatures can be used, requiring less heat
input, because boiling points decrease as pressures decrease. 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 conditions
and to diagnose operational problems.
17-7
2063g
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17.5.3 Residence Time.
The residence time is determined by the energy input into the system
as well as the volatility of the components and the degree of purity
desired. EPA monitors the waste feed rate to ensure that sufficient
residence time is provided to effectively volatilize organic components
from the waste.
2063g
17-8
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17.6 References
OeRenzo, O.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. 2nded.,
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 engineers' 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 oroanics. PB84-232628. pp. 1-1 to F4. Prepared for the Industrial
Environmental Research Laboratory, Office of Research and Development,
U.S. Environmental Protection Agency.
2063g
17-9
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18. CARBON ADSORPTION
18.1 Applicability
Carbon adsorption is a treatment technology used to treat wastewaters
containing dissolved organics at concentrations less than 1,000 mg/1 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 9 of this report, Aerobic Biological Treatment.
18.2 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: (1) the contaminant has a low solubility
in the waste, (2) the contaminant has a greater affinity for the carbon
than for the waste, or (3) 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.
18-1
0930
-------
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 generally 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 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
18-2
0930
-------
capacity. Isotherm tests can be used on the regenerated carbon to
determine adsorptive capacity; such tests can thus aid in predicting the
number of times the carbon can be regenerated.
18.3 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 vertical column(s). Figure 18-1 shows two carbon adsorption
systems. In the carbon adsorption process, the wastewater 1s 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 18-2) shows the plot of the
18-3
-------
GRANULAR
ACTIVATED
CARBON
.\vv\w
OUT
DOWNFLOW IN SERIES UPFLOW EXPANDED IN SERIES
CARBON ADSORPTION
FIGUflE 18-1
-------
ADSORPTION
20NE
m
z
o
p
<
cc
t Ul
2 2
uj —
tl)*"
2»
ttlgo
—, u.
It. H
U.HU
uiz ui
Ul CL
u. Din
O-iui
IL CC
cch?
FEED
EFFLUEHT
TIME
FIGIWE 1&-Sf PLOT OF BREAKTHROUGH CUnVF.
-------
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.
18.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether carbon adsorption 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 type and concentration of adsorbable
contaminants and (b) the concentrations of suspended solids and oil and
grease.
18.4.1 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 1,000 mg/1 results 1n excessive
activated carbon consumption requiring frequent regeneration.
While all organics can be adsorbed to some degree, activated carbon
has a greater affinity for aromatic rather than for aliphatic compounds
and for nonpolar rather than for polar compounds. If the type and
concentration of adsorbable contaminants 1n an untested waste are
significantly less adsorbable and higher, respectively, than in the
tested waste, the system may not achieve the same performance.
18-6
0930
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18.4.2 Concentrations of Suspended Solids 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.
18.5 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.
18.5.1 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.
18-7
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18.5.2 Temperature
As the temperature increases, the solubility of the contaminants
generally increases as well, which results in less effective adsorption.
EPA monitors the temperature continuously, if possible, to ensure that
the system is operating at the appropriate design condition and to
diagnose operational problems.
18.5.3 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 to diagnose operational problems.
18.5.4 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
through-put. 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
2
8.0 gal/min-ft , while upflow systems typically operate around
2
15 gal/min-ft . EPA monitors the hydraulic loading rate to ensure that
sufficient time is provided to effectively adsorb contaminants.
18-8
0930
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18.6 References
Authur D. Little, Inc. 1977. Physical, chemical and biological treatment
techniques for industrial wastes. Vol. I - NTIS, PB275-054. pp. 1-1 to
1-18 and 1-37 to 1-41.
DeJohn, P. B. 1975. Carbon from lignite or coal: which is
better? Chemical Engineering. 28 April. Eckenfelder, W., et al.
1985. Wastewater Treatment Chemical Engineering. 2 September.
GCA Corp. 1984. Technical assessment of treatment alternatives for
wastes containing halooenated oroanics. Prepared For USEPA, Contract
58-01-6871, October, pp. 150-160.
Metcalf and Eddy, Inc. 1985. Briefing: technologies applicable to
hazardous waste - Prepared For USEPA ORD/HWERl. Section 2.13.
Patterson, J. W. 1985. Industrial wastewater treatment technology. 2nd
ed., Butterworth Publishers, pp. 329-340.
Touhill, Shuckrow & Assoc. 1981. Concentration technologies for hazardous
aoueous waste treatment - NTIS, PB81-150583. pp. 53-55. February.
USEPA 1973. Process design manual for carbon adsorption - NTIS,
PB227-157, October, pp. 3-21 and 53.
USEPA 1986. Best Demonstrated Available Technology (BOAT) Background
Document For F001-F005 Spent Solvents. Vol. 1, EPA/530-SW-86-056,
November, p. 4-4. Washington, O.C. 20460. U.S. Environmental
Protection Agency
Versar 1985. Versar, Inc. An Overview of Carbon Adsorption. Draft Final
Report. U.S. Environmental Protection Agency: Exposure Evaluation
Division Office of Toxic Substances. Washington, D.C. EPA Contract No.
68-02-3968, Task No. 58.
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19. CHEMICAL OXIDATION
19.1 Add!icabil itv
Chemical oxidation is a treatment technology used to treat wastes
containing organics. In addition, it is used to treat sulfide wastes by
converting the sulfide to sulfate. Also, the destruction of cyanides in
wastes can be accomplished by chemical oxidation.
Chemical oxidation of cyanide is applicable for dissolved cyanides in
aqueous solutions, such as wastewaters from metal plating and finishing
operations, or for inorganic sludges from these operations that contain
soluble cyanide compounds. Chemical oxidation is most applicable to
cyanides that are in a form that can be easily disassociated in water to
yield free cyanide ions. If the cyanide is present in water as a tightly
bound complex ion (e.g., ferrocyanide), only limited treatment may occur.
Chemical oxidation may also be used in treatment of complexed metal
wastes. Organic compounds such as EOTA, 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 1n releasing metals from complexes with these organic compounds
(Schroeter and Painter 1987).
1834g
19-1
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19.2 Underlying Principles of Operation
The basic principle of 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. 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.
19.2.1 Oxidation with Hypochlorite or Chlorine (Alkaline Chlorination)
This type of oxidation is carried out using sodium hypochlorite
(NaOCl), calcium hypochlorite (Ca(0C1)^)» chlorine gas (Cl^), or
sometimes chlorine dioxide gas (CI0^)• 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 - C032" + C02 + N2 + 3NaCl (Step 2)
Phenol: C6H5OH + 14NaOCl - 6C02 + 3H20 + 14N*C1
Sulfide: S" + 4NaOCl - S04" + 4NaCl
19-2
1834g
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Chlorine dioxide also oxidizes the same pollutants under identical
conditions. Chlorine dioxide first hydrolyzes to form a mixture of
chlorous (HCIO^) and chloric (HCIO^) acids. These acids act as the
oxidants, as shown in the equations below, for phenol:
2C10Z + H20 - HC102 + HC103
C6H5OH + 7HC102 - 6C02 + 3H20 + 7HC1
3C5H5OH + 14HC103 - 8C02 + 9H20 + 14HC1
19.2.2 Peroxide Oxidation
Peroxide oxidizes the same constituents that alkaline chlorination
oxidizes under similar conditions. The relevant reactions are:
Cyanide: 2CN" + 5H202 -» 2C02 + N2 + 4H20 + 20H"
Phenol: C5H5OH + 14H202 -» 6C02 + 17H20
Sulfide: S" + 4H202 - S04" + 4H20
19.2.3 Oxidation with Ozone (Ozonation)
Ozone is an effective oxidizing agent for treatment of organic
compounds and for the oxidation of cyanide to cyanate. Cyanogen gas
(C^N^) is a reaction intermediate in this reaction. Further
oxidation of cyanate to carbon dioxide and nitrogen compounds (N^ or
NH ) occurs slowly with ozone. The oxidation of cyanide to cyanate
proceeds by the following reaction:
CN" + 03 - CNO* + 02
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18349
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The rates of ozonation reactions can be accelerated by supplying
ultraviolet (UV) radiation during treatment. Some literature sources
indicate that even the most difficult cyanide complexes to treat, the
iron-cyanide complexes, can be oxidized completely.
19.2.4 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 + 28Hn02 + 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.
19.2.5 SO^/Air Oxidation
Cyanide can be oxidized to cyanate 1n an aqueous solution by bubbling
air containing from 1 to 10 percent SO^ through the waste. The SO^
is also oxidized to sulfate in this reaction. This treatment process
occurs by the following reaction:
CN* + S02 + 02 + H20 - CNO- + H2S04
19-4
18349
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This oxidation reaction requires the use of a soluble copper salt
catalyst. Copper sulfate (CuSO^) is most often used. SO^/air
oxidation is used frequently in the treatment of wastewaters from gold
production, which contain both cyanide and thiocyanate, because SO^/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.
19.3 Description of Chemical Oxidation Processes
19.3.1 A1kaline 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 is 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.
19-5
18349
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19.3.2 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.
19.3.3 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 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 is recycled to the first
tank, thus ensuring that an excess of ozone 1s maintained and also that
no ozone 1s released to the atmosphere. As with alkaline chlorination,
an ORP control system 1s usually necessary to ensure that sufficient
ozone is being added.
19.3.4 Permanganate Oxidation
Permanganate oxidation is conducted in tanks in a manner similar to
that used for alkaline chlorination, as discussed previously. Potassium
19-6
1834g
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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.
19.3.5 SO^/Air Oxidation
SO^/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
of ozonation. SO^ is sometimes supplied with the air by using flue gas
containing SO^ as the air source. Otherwise, sulfur in the +4
oxidation state can be fed as gaseous sulfur dioxide (SO^), liquid
sulfurous acid (H^SO^), sodium sulfite (Na^SO^) solution, or
sodium bisulfite (NaHSO ) solution. Sodium bisulfite solution, made by
dissolving sodium metabisulfite (Na^S^O^) in water, is the most
frequently used source of SO^. This process is usually run
continuously, with the addition of oxidizing agent and acid/alkali being
controlled through continuous monitoring of ORP and pH, respectively.
19.4 Waste Characteristics Affecting Performance (WCAPsl
In determining whether chemical oxidation 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 other oxidizable
contaminants and (b) the concentration of metal salts'.
19-7
lS3'g
-------
19.4.1 Concentration of Other Oxidizable Compounds
The presence of other oxidizable compounds in addition to the BDAT
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.
19.4.2 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.
19-8
18349
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Cyanide in the complexed 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,
-4
Fe(CN) ) are the most stable of the complexed cyanides.
0
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.
19.5 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.
19.5.1 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.
19-9
1834g
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19.5.2 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 will depend 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, the addition
of oxidizing agent is accomplished 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 processes and continuously monitors the ORP for
continuous processes to ensure that excess oxidizing agent, If possible,
is supplied.
19.5.3 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
1834g
19-10
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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 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 waste solution.
19.5.4 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 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 19.2, Underlying Principles of Operation). In alkaline
chlorlnation 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, 1f possible, to ensure that
the system is operating at the appropriate design condition and to
diagnose operational problems.
19-11
1834g
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19.5.5 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.
19.5.5 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, catalyst addition
may be necessary to effectively destroy the constituent^) 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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19-12
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19.6 References
Gurnham, C.F. 1985. Principles of industrial waste treatment. New
York: John Wiley and Sons.
Gurol, M.D., and Holden, T.E. 1988. The effect of copper and iron
complexation on removal of cyanide by ozone. Ind. Ena 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 SOo/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 ohvsics. 58th ed.
Cleveland, Ohio: CRC Press.
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20. POLISHING FILTRATION
20.1 ApdIicabilitv
Polishing filtration is a treatment technology applicable to
wastewaters containing relatively low concentrations of solids (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 shape and/or density, as well as
precipitated particles from an underdesigned settling system.
20.2 Underlying Principles of Operation
The basic principle of operation for polishing filtration is the
removal of particles from a mixture of fluid and particles by a medium
that permits the flow of the fluid but retains the particles. The larger
the particles, the easier they are to remove from the fluid.
Extremely small particles, in the colloidal range, may not be removed
effectively in a polishing filtration system and thus may appear in the
treated wastewater. To mitigate this problem, the wastewater can be
treated prior to filtration to modify the particle size distribution in
favor of the larger particles 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 precipitation
usually produces larger, less gelatinous particles (which are easier to
2420g
20-1
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remove from aqueous wastes using this technology) than does caustic soda
precipitation. For particles that become too small to remove effectively
because of poor resistance to shearing, the use of coagulants and
flocculants both improves shear resistance 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. 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 the precoat solids during the filtration
process.
20.3 Description of Polishing Filtration Processes
During polishing filtration, wastewater may flow by gravity or under
pressure to the filter. The two most common polishing filtration
processes af*e cartridge and granular bed filtration. Both processes
remove particles that are much smaller than the pore size of the filter
media by straining, adsorption, and coagulation/flocculation mechanisms;
they are also capable of producing an effluent with a low level of solids
(less than 10 mg/1).
20.3.1 Cartridge Filtration
Cartridge filters can be used for relatively low waste feed flows.
In this process, a cylindrically shaped cartridge with a matted
cloth-type filter medium, is placed within a sealed vessel. Wastewater
24209
20-2
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is pumped through the cartridge until the flow drops excessively or until
the pumping pressure becomes too high because of plugging of the filter
media. The sealed vessel is then opened and the plugged cartridge is
removed and replaced with a new cartridge. The plugged cartridge is then
disposed of. Cartridge filters may be assembled in a parallel
arrangement to increase the overall system flow.
20.3.2 Granular Bed Filtration
For relatively large volume flows, granulated media such as sand 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 filter media. Granular media filters are cleaned by backwashing
with 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 wastewater treatment system
so that the filtered solids in the backwash water can be settled out of
solution prior to discharge.
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20.4 Waste Characteristics Affecting Performance (WCAPs).
In determining whether polishing 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 waste characteristics: (a) the solid waste particle size and
(b) the type of solid waste particles.
20.4.1 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. If the solid waste particle size distribution of an untested
waste is significantly lower than that of the tested waste, the system
may not achieve the same performance. 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.
20.4.2 Type of Solid Waste Particles
Some solids formed during metal precipitation are gelatinous in
nature and are difficult to filter. 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 formation of gelatinous
solids. If solids 1n an untested waste are significantly more gelatinous
2420g
20-4
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than in the tested waste, the system may not achieve the same
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.
20.5 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.
20.5.1 Type and Size of Filter
The type and size of the polishing filtration system used is
dependent on the nature of the particles to be removed, the desired
solids concentration 1n the filtrate, and the amount and concentration of
solids in the feed. As noted earlier, cartridge filtration is limited to
lower volume wastewaters and/or those with lower solids concentrations
than is granular bed filtration. For granular bed filtration, when more
than one medium is used (dual and multimedia filter arrangements such as
sand and anthracite coal), a higher capacity can be expected for the same
size filter bed. For both filtration processes, the larger the filter
size, 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.
2420g
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20.5.2 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. For
gelatinous solids, such as metal hydroxides, however, excessive pressure
may cause the solids to clog the filter pores and prevent additional
polishing filtration. Also, high pressures may force particles through
the filter medium, resulting in ineffective filtration. EPA monitors the
filtration pressure 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.
20.5.3 Amount and Type of Coagulants, Flocculants, and Filter Aids
Coagulants, flocculants, and filter aids may be mixed with the
wastewater prior to filtration. Coagulants and flocculants affect the
type and size of waste particles in the wastewater and, hence, their ease
of removal. Filter aids both improve the effectiveness of filtering
gelatinous particles and increase the time that the filter can stay
on-line by increasing the surface area available for filtration.
Coagulants, flocculants, and filter aids are particularly useful when the
wastewater contains a high percentage of very small particles and/or when
the concentration of solids in the wastewater is low. Inorganic
coagulants include alum, ferric sulfate, and lime; organic flocculants
are polyelectrolytes. Oiatomaceous earth is the most commonly used
filter aid. The use of coagulants, flocculants, and filter aids
2420g
20-6
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significantly increases the amount of solids requiring removal and
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, as well as
their method of addition to the wastewater, to ensure effective
filtration.
20.5.4 Hydraulic Loading Rate
Lower hydraulic loading rates generally improve filtration
performance. Higher hydraulic loading rates yield greater throughput,
but result in shorter cycle times. EPA monitors the hydraulic loading
rate to ensure effective filtration of the wastewater.
20.5.5 Pore Size of the Filter Media
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 the wastewater.
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20.6 References
Anonymous. 1985. Feature report. Wastewater treatment. Chemical
Engineering. 92(18):71-72.
Crain, 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.
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. pp. 19-57. New York: McGraw Hill Book Co.
Shucosky, A.C. 1988. Feature report. Filtration. Chemical
engineering. January 18, 1988.
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21. SLUDGE FILTRATION
21.1 Add!icabilitv
Sludge filtration, also known as sludge dewatering or cake-format ion
filtration, is a technology used on wastes that contain high
concentrations of suspended solids, generally higher than 1 percent
(10,000 mg/1). Sludge filtration is commonly applied to waste sludges,
such as clarifier solids, for dewatering. These sludges can be dewatered
to 20 to 50 percent solids concentration using this technology.
21.2 Underlying Principles of Operation
The basic principle of operation for sludge filtration is the
separation of particles from a mixture of fluid and particles by a medium
that permits the flow of the fluid but retains the particles. The larger
the particles, the easier they are to separate from the fluid.
Extremely small particles, in the colloidal range, may not be
filtered effectively in a sludge filtration system and may appear in the
filtrate. To mitigate this problem, the waste can be treated prior to
filtration to modify the particle size distribution in favor of the
larger particles 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 precipitation usually produces
larger, less gelatinous particles (which are easier to separate from
waste sludges using this technology) than caustic soda precipitation.
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For particles that become too small to filter effectively because of poor
resistance to shearing, the use of coagulants and flocculants improves
shear resistance in addition to increasing 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. Filter aids such as
diatomaceous earth are used to precoat cloth-type filter media and
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 waste being filtered.
These particles adhere to the precoat solids during the filtration
process.
21.3 Description of 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 medium (also
known as vacuum filtration, such as that performed with a vacuum drum
filter), or gravity-drained and mechanically pressured through two
continuous fabric belts (also known as belt filtration, such as that
performed with a belt filter press). In all cases, the sol Ids "cake"
builds up on the filter medium and acts as a filter for subsequent solids
removal. For a plate and frame type filter, removal of the solids 1s
accomplished by taking the unit off-line, opening the filter, and using
an adjustable knife mechanisms to scrape the solids off (a batch
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process). For the vacuum filter, cake is removed continuously by using
an adjustable knife mechanism to scrape scraping 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 type
filter will usually produce the driest cake (highest percentage of
solids). The belt filter produces a drier cake than a vacuum filter, but
not as dry as that produced by a plate and frame filter.
21.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether 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
waste characteristics: (a) the solid waste particle size and (b) the
type of solid waste particles.
21.4.1 Solid Waste Particle Size
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 particle size distribution
of an untested waste is significantly lower than that of the tested
waste, the system may not achieve the same performance. Pretreatment of
21-3
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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.
21.4.2 Type of Solid Waste Particles
Some solids formed during metal precipitation are gelatinous in
nature and cannot be dewatered well by sludge filtration. In fact, for
vacuum filtration a cake may not form at all. 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
formation of gelatinous solids. Also, adding filter aids, such as lime
or diatomaceous earth, to a gelatinous sludge Increases Its fllterability
significantly. Finally, precoating the filter with diatomaceous earth
prior to sludge filtration assists 1n dewaterlng gelatinous sludges. If
an untested waste is significantly more gelatinous than the tested waste,
the system may not achieve the same performance. Pretreatment of the
waste with coagulants and filter aids or precoatlng of the filter 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.
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21.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a
sludge filtration system, EPA examines the following parameters: (a) the
type and size of filter; (b) the filtration pressure; (c) the amount and
type of coagulants, flocculants, and filter aids used; and (d) the
hydraulic loading rate.
21.5.1 Type and Size of Filter
The type and size of the filtration system used is dependent on the
nature of the particles to be separated, the desired solids concentration
in the cake, the amount and concentration of solids in the feed, and the
required downtime for solids removal and maintenance. Typically, a
pressure-type filter (such as a plate and frame) will yield a drier cake
than a belt or vacuum type filter and will also be more tolerant of
variations in influent sludge characteristics. Pressure-type filters,
however, are batch processes. When cake is built up to the maximum depth
physically possible (constrained by filter geometry) or to the maximum
design pressure, the filtration system is taken off-line while the cake
is removed. (An alternate unit can be put on-line while the other is
being cleaned.) Belt and vacuum type filters are continuous systems
(i.e., cake discharges continuously), but each of these filters 1s
usually much larger than a pressure filter with the same capacity.
For all filter types, the larger the filter, the greater Its
hydraulic capacity (overall throughput) and, for pressure-type filters,
21-5
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the longer the filter runs between cake discharges. EPA examines the
type and size of the filter chosen to ensure that it is capable of
achieving effective dewatering and filtration of the waste sludge.
21.5.2 Filtration Pressure
Pressure impacts both the design pore size of the filter media and
the design feed flow rate. For plate and frame filters, the higher the
feed pressure, the drier the cake will be and the longer the runs will be
prior to cake discharge. However, for gelatinous solids, such as some
metal hydroxides, excessive pressures may cause the solids to clog the
filter pores and prevent additional sludge filtration. Also, high
pressures may force particles through the filter medium, resulting in
ineffective filtration. For vacuum filters, the maximum amount of vacuum
typically applied ranges from 20 to 25 Inches of mercury. (The absolute
maximum amount of vacuum that can be applied 1s 29.9 inches of mercury,
or atmospheric pressure.) For belt filters, neither pressure nor vacuum
is applied to the waste feed (although mechanical pressure is applied).
For plate and frame and vacuum-type filtration systems, EPA monitors the
filtration pressure (or vacuum) applied to the waste feed continuously,
if possible, to ensure that the system 1s operating at the appropriate
design conditions and to diagnose operational problems.
21.5.3 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
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size of waste particles in the waste and, hence, their ease of removal.
Their effect is particularly significant for vacuum filtration since they
may make the difference between no cake and the formating of a relatively
dry cake. In a pressure filter, coagulants, flocculants, and filter aids
significantly improve overall throughput and cake dryness. Filter aids,
such as diatomaceous earth, can be precoated on all 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. Coagulants, flocculants, and filter aids are particularly useful
when the sludge has a high percentage of very small particles and/or when
the concentration of solids in the waste feed is low. Inorganic
coagulants include alum, ferric sulfate, and lime. Organic flocculants
are polyelectrolytes. Diatomaceous earth is the most commonly used
filter aid. The use of coagulants, flocculants, and filter aids
significantly increases the amount of solids in the sludge requiring
disposal, although, polyelectrolyte flocculant usage usually does not
increase sludge 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
sludge, along with their method of addition, and to ensure effective
dewatering and filtration.
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21.5.4 Hydraulic Loading Rate
Lower hydraulic loading rates generally improve filtration
performance. Higher hydraulic loading rates yield greater overall
throughput, but result in the formation of wetter cakes (lower percent
solids) and, for plate and frame filters, shorter cycle times. EPA
monitors the hydraulic loading rate to ensure effective dewatering and
filtration of the waste sludge.
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21.6 References
Crain, 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. 3rded.,
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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22. THERMAL DRYING
22.1 AddIicabilitv
Thermal drying is a treatment technology applicable to solid wastes
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.
22.2 Underlying Principles of Operation
Drying involves the removal of a liquid from a solid waste by
evaporation. Liquid constituents will vaporize as a result of heat
absorbed. In any drying process, assuming an adequate supply of heat,
the rate at which liquid evaporation occurs depends on the thermal
conductivity of the solid waste to be dried and the boiling points of the
volatile 7iquid constituents to be evaporated.
22.3 Description of Thermal Drvina Process
A wide range of batch and continuous dryers is available. One
commonly used continuous-type, the screw-flight dryer, is described below.
The screw-flight dryer consists of a hollow screw and shaft enclosed
in a jacketed trough. Transfer fluid is heated to temperatures as high
as 750*F and circulated, usually countercurrently, through the hollow
screw and shaft. Heat transfers from the screw and shaft into the feed
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22-1
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material, causing water and organics to be driven off in a vapor form.
The dried cake is discharged from the dryer.
The screw is designed to create good contact between the shaft and
feed material. The screw is also 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 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.
22.4 Waste Characteristics Affecting Performance
-------
EPA examined both methods of heat transfer and 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
is referred to as the thermal conductivity and is a property of the
material to be dried. With regard to convection, EPA believes that the
amount of heat transferred by convection will generally be more a
function of the system design than of the waste itself.
Thermal conductivity measurements, as part of a treatability
comparison for two different wastes to be treated by a single dryer 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 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 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.
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22.4.2 Volatile Liquid Constituent Boiling Points
The lower the boiling points of the volatile liquid constituents, the
more easily they will be evaporated and the solid waste dried. If the
boiling points of the volatile liquid constituents in the untested waste
are significantly higher than those in the tested waste, the system may
not achieve the same performance. More rigorous drying conditions
including a higher temperature, lower pressure, and a longer residence
time may be required to evaporate less volatile liquid constituents and
achieve the same treatment performance, or other, more applicable
treatment technologies may need to be considered for treatment of the
untested waste.
22.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a
thermal drying system, EPA examines the following parameters: (a) the
drying temperature and pressure and (b) the residence time.
22.5.1 Drying Temperature and Pressure
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 decreasing volatilities
will be removed from the waste.
Pressure is integrally related to the boiling point of the waste and
the subsequent vaporization of the water and/or organic constituents. As
the pressure is lowered below atmospheric (i.e., as vacuum is increased),
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the boiling point of the waste will also be lowered, thereby requiring
less heat input to volatilize waste constituents. EPA monitors the
drying temperature as well as the pressure (if pressures other than
atmospheric are used) to ensure that the system is operating at
appropriate conditions and to diagnose operational problems.
22.5.2 Residence Time
The residence time determines the necessary energy input into the
system. It is dependent on the dryer temperature and the thermal
conductivity of the waste. EPA observes the residence time to ensure
that the treatment system provides sufficient time to effectively
evaporate the volatile liquid constituents and, hence, dry the solid
waste.
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22.6 References
Perry, R.H., and Chilton, C.H. 1973. Chemical engineers' handbook. 5th
ed. New York: McGraw-Hill Book Co.
Risk Sciences International. 1987. Evaluation of treatment technologies
for listed petroleum industry wastes. Interim Report, pp. 41-45.
22-6
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23. WET AIR OXIDATION
23.1 AddIicabilitv
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 several 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 15,000 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
15,000 mg/1), thermal processes such as incineration may be more
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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 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 al1.
Studies of the wet air oxidation of different compounds have led to
th§ 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 (-0H) or 2,4,6-trichloroaniline
(-NH2)).
4. Halogenated aromatic compounds, such as 1,2-dlchlorobenzene, and
PCBs, such as Aroclor 1254, are resistant to wet air oxidation
under conventional conditions.
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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.
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.
23.2 Underlying Principles of Operation
The wet air oxidation of aqueous wastes occurs at high temperatures
and pressures. The typical operating temperature for the 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 hazardous compounds into carbon
dioxide, water vapor, ammonia (for nitrogen-containing wastes), sulfate
(for sulfur-containing wastes), and halogen acids (for halogenated
wastes).
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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.
23.3 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 reactor, heat exchangers, a vapor-liquid separator, and
process regulators. A basic flow diagram is shown in Figure 23-1.
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,
respectively. 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 or fed to a biological treatment, carbon adsorption, or chemical
oxidation treatment system If further treatment 1s necessary prior to
discharge.
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PRESSURIZED
WASTE FEED
T
PRESSURIZED
AIR OR
OXYGEN
REACTOR
FEED HEAT
EXCHANGERS
STEAM
STEAM
GAS-LIQUID
SEPARATOR
Y
TREATED
WASTE
(TO
FURTHER
TREATMENT
OR
DISPOSAL)
FIGURE 23-1 WET AIR OXIDATION PROCESS FLOW DIAGRAM.
-------
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.
23.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether wet air oxidation 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 chemical oxygen demand and (b) the
concentration of interfering substances.
23.4.1 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 01^ 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'-COD wastes may
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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.
23.4.2 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 1n 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
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insoluble copper salts) significantly higher than that in a 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.
23.5 Desion 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.
23.5.1 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.
23.5.2 Residence Time
The residence time impacts the extent of oxidation of waste
contaminants. For a batch system, the residence time' 1s controlled
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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.
23.5.3 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 COO 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 beiflg supplied to the waste.
23.5.4 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. EPA monitors the oxidation pressure
continuously, 1f possible, to ensure that the system 1s operating at the
appropriate design condition and to diagnose operational problems.
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23.5.5 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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23.6 References
Dietrich, 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.
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