FINAL
TREATMENT TECHNOLOGY BACKGROUND DOCUMENT
Larry Rosengrant
Project Manager
U.S. Environmental Protection Agency
Office of Solid Waste
401 M Street, SW
Washington, DC 20460
May 1990
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TREATMENT TECHNOLOGY BACKGROUND DOCUMENT
TABLE OF CONTENTS
Section Page No.
EXECUTIVE SUMMARY viii
INTRODUCTION viii
1. Applicablity viii
2. Underlying Principles of Operation xi
3. Description of the Treatment Process xii
4. Waste Characteristics Affecting Performance (WCAP) ... xii
5. Design and Operating Data xv
TREATMENT TECHNOLOGIES
1. AEROBIC BIOLOGICAL TREATMENT 1-1
1.1 Applicability 1-1
1.2 Underlying Principles of Operation 1-1
1.3 Description of Aerobic Biological Treatment Processes 1-2
1.4 Waste Characteristics Affecting Performance (WCAPs).. 1-8
1.5 Design and Operating Parameters 1-9
1.6 References 1-14
2. BATCH DISTILLATION 2-1
2.1 Applicability 2-1
2.2 Underlying Principles of Operation 2-1
2.3 Description of Batch Distillation Process 2-3
2.4 Waste Characteristics Affecting Performance (WCAPs).. 2-3
2.5 Design and Operating Parameters 2-7
2.6 References 2-8
3. CARBON ADSORPTION 3-1
3.1 Applicability 3-1
3.2 Underlying Principles of Operation 3-1
3.3 Description of Carbon Adsorption Process 3-2
3.4 Waste Characteristics Affecting Performance (WCAPs).. 3-5
3.5 Design and Operating Parameters 3-6
3.6 References 3-9
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4. CRITICAL FLUID EXTRACTION 4-1
4.1 Applicability 4-1
4.2 Underlying Principles of Operation 4-1
4.3 Description of Critical Fluid Extraction 4-1
4.4 Waste Characteristics Affecting Performance (WCAPs).. 4-2
4.5 Design and Operating Parameters 4-4
4.6 References 4-7
5. FRACTIONATION 5-1
5.1 Applicability 5-1
5.2 Underlying Principles of Operation 5-1
5.3 Description of Fractionation Process 5-3
5.4 Waste Characteristics Affecting Performance (WCAPs).. 5-5
5.5 Design and Operating Parameters 5-8
5.6 References 5-11
6. FUEL SUBSTITUTION 6-1
6.1 Applicability 6-1
6.2 Underlying Principles of Operation 6-4
6.3 Description of Fuel Substitution Process 6-4
6.4 Waste Characteristics Affecting Performance (WCAPs).. 6-7
6.5 Design and Operating Parameters 6-9
6.6 References 6-12
7". INCINERATION 7-1
7.1 Applicability 7-1
7.2 Underlying Principles of Operation 7-1
7.3 Description of Incineration Process 7-2
7.4 Waste Characteristics Affecting Performance (WCAPs).. 7-11
7.5 Design and Operating Parameters 7-14
7.6 References 7-20
8. SOLVENT EXTRACTION 8-1
8.1 Applicability 8-1
8.2 Underlying Principles of Operation 8-1
8.3 Description of Solvent Extraction Process 8-2
8.4 Waste Characteristics Affecting Performance (WCAPs).. 8-7
8.5 Design and Operating Parameters 8-8
8.6 References 8-11
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9. STEAM STRIPPING 9-1
9.1 Applicability 9-1
9.2 Underlying Principles of Operation 9-1
9.3 Description of Steam Stripping Process 9-2
9.4 Waste Characteristics Affecting Performance (WCAPs) 9-4
9.5 Design and Operating Parameters 9-7
9.6 References 9-9
10. THIN FILM EVAPORATION 10-1
10.1 Applicability 10-1
10.2 Underlying Principles of Operation 10-1
10.3 Description of Thin Film Evaporation Process 10-2
10.4 Waste Characteristics Affecting Performance (WCAPs) 10-4
10.5 Design and Operating Parameters 10-6
10. 6 References 10-8
11. ACID LEACHING 11-1
11.1 Applicability 11-1
11.2 Underlying Principles of Operation 11-1
11.3 Description of Acid Leaching Process 11-2
11.4 Waste Characteristics Affecting Performance (WCAPs) 11-3
11.5 Design and Operating Parameters 11-5
11.6 References 11-7
12. CHEMICAL PRECIPITATION 12-1
12.1 Applicability 12-1
12.2 Underlying Principles of Operation 12-1
12.3 Description of Chemical Precipitation Process 12-2
12.4 Waste Characteristics Affecting Performance (WCAPs) 12-5
12.5 Design and Operating Parameters 12-10
12 .6 References 12-13
13. ELECTROLYTIC OXIDATION OF CYANIDE 13-1
13.1 Applicability 13-1
13.2 Underlying Principles of Operation 13-1
13.3 Description of Electrolytic Oxidation of Cyanide Process. 13-2
13.4 Waste Characteristics Affecting Performance (WCAPs) 13-2
13.5 Design and Operating Parameters 13-3
13.6 References 13-7
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14. HEXAVALENT CHROMIUM REDUCTION 14-1
14.1 Applicability 14-1
14.2 Underlying Principles of Operation 14-1
14.3 Description of Hexavalent Chromium Reduction Process .... 14-2
14.4 Waste Characteristics Affecting Performance (WCAPs) 14-4
14.5 Design and Operating Parameters 14-5
14.6 References 14-7
15. HIGH TEMPERATURE METALS RECOVERY 15-1
15.1 Applicability 15-1
15.2 Underlying Principles of Operation 15-2
15.3 Description of High Temperature Metals Recovery Process . 15-2
15.4 Waste Characteristics Affecting Performance (WCAPs) 15-5
15.5 Design and Operating Parameters 15-7
15.6 References 15-10
16. ION EXCHANGE 16-1
16.1 Applicability 16-1
16.2 Underlying Principles of Operation 16-1
16.3 Description of Ion Exchange Process 16-3
16.4 Waste Characteristics Affecting Performance (WCAPs) 16-5
16.5 Design and Operating Parameters 16-8
16.6 References 16-10
17. RETORTING 17-1
17.1 Applicability 17-1
17.2 Underlying Principles of Operation 17-1
17.3 Description of Retorting Process 17-2
17.4 Waste Characteristics Affecting Performance (WCAPs) 17-5
17.5 Design and Operating Parameters 17-7
17.6 References 17-9
18. STABILIZATION OF METALS 18-1
18.1 Applicability 18-1
18.2 Underlying Principles of Operation 18-1
18.3 Description of Stabilization of Metals 18-3
18.4 Waste Characteristics Affecting Performance (WCAPs) 18-3
18.5 Design and Operating Parameters 18-5
18. 6 References 18-9
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19-1
19.1" Applicability . .v. .". 19-1
19.2 Underlying Principles of Operation 19-1
19.3 Description of Stabilization of Metals Process 19-5
19.4 Waste Characteristics Affecting Performance (WCAPs) 19-7
19.5 Design and Operating Parameters 19-9
19.6 References 19-12
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) 20-3
20.5 Design and Operating Parameters 20-4
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 21-4
21.6 References 21-8
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 22-5
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 23-7
23.6 References 23-10
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TREATMENT TECHNOLOGY BACKGROUND DOCUMENT
TABLE OF CONTENTS
Section Page No.
TREATMENT TECHNOLOGIES
24. HIGH TEMPERATURE STABILIZATION TECHNOLOGIES 24-1
24.1 Applicability 24-1
24.2 Underlying Principles of Operation 24-2
24.3 Description of High Temperature Stablization
Technologies 24-4
24.4 Waste Characteristics Affecting Performance (WCAPs).... 24-6
24.5 Design and Operating Parameters 24-9
24.6 References 24-14
25. ENCAPSULATION 25-1
25.1 Applicability 25-1
25.2 Underlying Principles of Operation 25-1
25.3 Process Description 25-2
25.4 Waste Characteristics Affecting Performance (WCAPs).... 25-2
25.5 Design and Operating Parameters 25-3
25.6 References 25-4
26. CHEMICAL REDUCTION 26-1
26.1 Applicability 26-1
26.2 Underlying Principles of Operation 26-1
26.3 Description of Chemical Reduction Process 26-2
26.4 Waste Characteristics Affecting Performance (WCAPs).... 26-2
26.5 Design and Operating Parameters 26-3
26.6 References 26-7
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LIST OF TABLE
Table Page No.
1 List of Parameters Affecting Treatment Selection ix
LIST OF FIGURES
Figures Page No,
1-1 Activated Sludge System 1-4
1-2 Trickling Filter System 1-7
2-1 Batch Distillation System 2-4
3-1 Carbon Adsorption Systems 3-3
3-2 Plot of Breakthrough Curve 3-4
5-1 Fractionation System 5-4
7-1 Liquid Injection Incineration System 7-6
7-2 Rotary Kiln Incineration System 7-7
7-3 Fluidized Bed Incineration System 7-9
7-4 Fixed Hearth Incineration System 7-10
8-1 Two-Stage Mixer-Settler Solvent Extraction System 8-4
8-2 Packed and Sieve Tray Solvent Extraction Columns 8-6
9"-l Steam Stripping System 9-3
10-1 Thin Film Evaporation System 10-3
12-1 Continuous Chemical Precipitation System 12-4
12-2 Circular Clarifier Systems 12-6
12-3 Inclined Plate Settler System 12-7
14-1 Continuous Hexavalent Chromium Reduction System 14-3
15-1 High Temperature Metals Recovery System 15-3
16-1 Two Step Cation/Anion Ion Exchange System 16-4
17-1 Retorting Process (Without a Scrubber and Subsequent
Uastewater Discharge) 17-3
17-2 Retorting Process (With a Scrubber and Subsequent
Wastewater Discharge) 17-4
23-1 Continuous Wet Air Oxidation System 23-5
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EXECUTIVE SUMMARY
INTRODUCTION
This section of the treatment technology document provides a
discussion of the purpose and contents of each of the various elements
presented in each technology section. Specifically, this section explains
what information is provided in each technology subsection, how the Agency
intends to use this information as part of its BOAT program, and, when
applicable, how the Agency intends to modify the treatment technology
discussions as more treatment data and information becomes available.
Below are discussions of the following elements of the technology
write-ups: applicability, underlying principles of operation, description
of the technology, waste characteristics affecting performance, and
design and operating parameters.
1. Applicability
(a) Information Provided. This applicability section contains
information on the general application of the treatment technology to
various wastes. EPA's analysis of applicability is performed in
consideration of the parameters and constituents known to affect
treatment selection. That is, in order to identify whether a treatment
technology was applicable to any particular waste stream, the Agency
considered all parameters and characteristics associated with a hazardous
waste that could affect the selection of treatment technology. EPA uses
the acronym PATS for these parameters (i.e., parameters affecting
treating selection).
EPA's list of parameters affecting treatment selection (PATS), shown
in Table 1-1, identifies all known constituents and parameters that are
needed to select a technology or technology train for a given waste.
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Table 1 List of Parameters Affecting Treatment Selection
BOAT metals
BOAT organics
Other BOAT constituents (sulfides and fluorides)
Biological oxygen demand (BOD)
BTU content
Complexed metals
Cyanide
Filterable solids
Oil and grease content
pH
Total organic carbon (TOG)
Total organic halides (TOX)
Viscosity
Water content
Selectivity value
Ash fusion temperature
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It is important to point out that in developing this list, EPA's goal
was to determine the demonstrated technology that would best treat all of
the constituents in the waste (i.e., achieve the lowest concentration in
the residual). Accordingly, EPA will add components to the treatment
train as long as they are demonstrated and will achieve statistically
significant reduction in the concentrations of the constituents. (Note:
the statistical analysis that the Agency uses to determine whether a
significant reduction in constituents has occurred is referred to as
Analysis of Variance (ANOVA).
(b) EPA's Use of this Information. EPA uses the PATS as the basis
for making decisions regarding whether a particular technology is
"demonstrated" for a particular waste. The Agency is performing
treatment tests on specific wastes to "demonstrate" the treatment of the
waste using the technology, develop treatment standards, and to set the
criteria by which other wastes can be regulated using the demonstrated
technology.
The PATS analysis will sometimes show that several technologies are
applicable for treating the waste. EPA will prioritize these technologies
based on its analysis of available data and information regarding the
effectiveness of the various technologies. For organics, EPA's preference
will be technologies that recover or destroy the compounds; for metals,
the Agency's preference will be technologies that result in recovery of
the metals.
In some instances, analysis of a particular parameter will not be
possible or would be meaningless with regard to selecting a treatment
system (for example, a percent water analysis for a U or P waste spill
resulting in contaminated soil).
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(c) Use of New Data and Information. As data become available
either from industry or through EPA's BOAT data collection program, EPA
will modify the PATS analysis to further refine specific concentration
ranges for which various technologies can handle various parameters.
2. Underlying Principles of Operation
(a) Information Provided. For each treatment technology EPA
provides the fundamental theory of operation of the technology. This
section is not meant to provide a detailed discussion on the specific
complex physical, chemical, or biological mechanisms by which treatment
occurs. Instead, the discussion describes the mechanisms involved so
that one can understand what characteristics can affect the performance
of the technology. For example, the underlying principle of chemical
precipitation is that metals in wastewater are removed by adding a
treatment chemical to form a metal precipitate which is generally
insoluble and thereby settles out of solution. Formation of the
precipitate and the settling mechanism are the principles controlling
technology performance.
(b) Use of Information Provided. The theory of operation is
necessary as the precursor to analyzing which waste characteristics can
affect the performance of a given treatment technology. That is, the
principles of treatment operation are used as the rationale and basis
upon which the Agency determines what waste characteristics can affect
performance. Accordingly, waste characteristics affecting performance
(WCAPs) which are discussed below, must be understood from the standpoint
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of how each waste characteristic inhibits or interferes with the
fundamental operation of the treatment technology.
3. Description of the Treatment Process
Information Provided and Use. This section provides a brief
description of the principal components of a treatment technology. It is
intended to provide a basic understanding of the equipment used and the
treatment sequence. This presentation serves to relate the discussion on
underlying principles of operation with the later discussion of the
design and operating parameters important to effective waste treatment.
Simplified block diagrams are provided in this section. The information
presented is general in nature to avoid attempting to show all potential
combinations and permutations that are possible in describing a
particular technology.
4. Waste Characteristics Affecting Performance (WCAP)
(a) Information Provided. The WCAP are based on the underlying
principles of the treatment technology. For example, the underlying
principle of chemical precipitation, presented above, is formation of the
insoluble metal precipitate and the settling of the precipitate out of
solution. WCAP for this technology are those parameters which affect the
chemical reaction of the metal compound or the solubility of the metal
precipitate (i.e., formation), and/or affect the ability of the
precipitated compound to settle. The WCAPs include: (a) the
concentration and type of metals, (b) the concentration of total
dissolved solids, (c) the concentration of complexing agents, and (d) the
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concentrations of oil and grease. The first 3 parameters can affect
precipitate formation and the last parameter can inhibit settling.
EPA's selection of WCAP's for the various treatment systems will be
based on a literature review, as well as information obtained during
plant sampling visits that test the treatment technology. EPA is aware
that in some cases analytical methods are not available to measure the
specific waste characteristics that affect treatment. In such instances,
EPA will measure the parameter that most closely approximates the actual
parameter of concern. For example, a WCAP for steam stripping is the
relative volatility of a component. Because relative volatility cannot
be measured, EPA uses boiling point as the best measure of assessing the
WCAP of relative volatility.
The analysis of waste characteristics affecting performance is
conducted where EPA already has determined from a PATS analysis that an
untested waste could use the same BOAT technology as another waste for
which treatment performance data are available. The WCAP analysis is
then used to determine whether the treatment standard from the tested
waste can be transferred to the untested waste. Simply stated, this
analysis compares, for two separate wastes, the characteristics that
affect the level of treatment that can be achieved. Where this comparison
shows that the untested waste can be treated as well or better, the
Agency will transfer the treatment standards to the untested wastes.
For example, for wastes that can be treated using chemical
precipitation, if the Agency tests a waste containing lead at a given
level, it will transfer the treatment standards developed from the test
to a second lead-containing waste, if the second waste meets the following
WCAP-based conditions:
the lead concentration is similar to the tested waste,
the second waste has a lower total dissolved solids level,
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the concentration and type of other metals is similar to the
tested waste, and
the untested waste has a concentration of oil and grease similar
to or less than the tested waste.
(b) Use of Information Provided. In applying the WCAP analyses,
it is important to remember that the technology itself, and the
associated design and operating conditions, are fixed (i.e., they are
fixed at the values that existed for the already tested waste). EPA does
not assume that, where WCAPs for the untested waste show that it would be
significantly harder to treat, the addition of another treatment
component or a change in design and operating conditions will result in
the untested waste achieving the same performance as the previously
tested waste. EPA is aware that modifying the treatment system or
operating conditions could result in similar treatment. However, the
Agency is not certain, without demonstrating through testing, that
treatment levels associated with BOAT standards (normally in the low part
(<10) per million or part per billion range) can be achieved by simply
changing treatment conditions. Accordingly, EPA will be very cautious in
its decisions to transfer treatment standards
where the WCAP analysis shows that treatment modifications are needed
from the previously tested waste.
(c) Use of New Data and Information. In developing the WCAPs for
the various treatment technologies, EPA found little and, in some cases,
no information on the relative importance of the various WCAPs for a
given technology (i.e., which waste characteristics have the most
significant impact on treatment performance). Where information was
available with regard to the specific WCAPs which are important when
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assessing the performance of a particular technology, EPA was unable to
find any supporting data which would quantify the relative impact of each
of the WCAPs on treatment performance.
As data become available either from industry or through EPA's BOAT
data collection program, EPA may modify the WCAPs analysis to further
refine the parameters identified as affecting treatment technology
performance. As noted above, before it adopts a new WCAP, the Agency
must understand how the parameters affect treatment relative to the
underlying principles of operation.
5. Design and Operating Data
(a) Information Provided and Use. EPA will use design and
operating information in several ways. First, this information is used,
in conjunction with the Agency's BOAT data collection program, in
determining whether a particular treatment system is well designed. That
is, the Agency assesses design data prior to determining whether to test
a particular piece of equipment or treatment train for potential use in
the development of BOAT treatment standards. EPA obtains operating data
during the course of an Agency sponsored treatment test to ensure the
treatment system is well-operated. Secondly, EPA will expect facilities
to provide the appropriate design and operating data when submitting
treatment data to be considered in the development of the BOAT treatment
standards. Again, this data will be assessed by the Agency to determine
if the treatment system was well-designed and operated. Finally, EPA
will require facilities petitioning for a treatment variance to submit
the plant's design values and the operating data during the time of the
treatment period to ensure the treatment system was well designed and
operated.
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1. AEROBIC BIOLOGICAL TREATMENT
1.1 Applicability
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.
1.2 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.
The aerobic biodegradation process can be represented by the
following generic equation:
^ i, ^ microorganisms . ,, _ __ ,, ,
CxHy + °2 nutrients ' V + C°2 + cellular biornas..
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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.
1.3 Description of Aerobic Biological Treatment Processes
1.3.1 Activated Sludge
The activated sludge process is currently the most widely used
biological treatment process. This is partly the result of the fact that
re'circulation of the biomass, which is an integral part of the process,
allows microorganisms to adapt to changes in wastewater composition with
a relatively short acclimation time and also allows a greater degree of
control over the acclimated bacterial population.
An activated sludge system consists of an equalization basin, a
settling tank, an aeration basin, a clarifier, and a sludge recycle
line. Wastewater is homogenized in an equalization basin to reduce
variations in the feed, which may cause process upsets of the
microorganisms and diminish treatment efficiency. Settleable solids are
then removed in a settling tank.
Next wastewater enters an aeration basin, where an aerobic bacterial
population is maintained in suspension and oxygen, as well as nutrients,
is provided. The contents of the basin are referred to as the mixed
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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.
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.
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OXYGEN
AND
NUTRIENTS
1
WASTEWATER
INFLUENT
EQUALIZATION
TANK
SETTLING
TANK
AERATION
BASIN
BASIN
EFFLUENT
SLUDGE RECYCLE
TREATED
EFFLUENT
TO DISPOSAL
WASTE SLUDGE
.TO SLUDGE
FILTRATION
AND DISPOSAL
Figure 1-1. Activated Sludge System.
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1.3.2 Aerated Lagoon
Like an activated sludge system, an aerated lagoon is a suspended-
growth process. The aerated lagoon system consists of a large pond or
tank that is equipped with mechanical aerators to maintain an aerobic
environment and to prevent settling of the suspended biomass. Initially,
the population of microorganisms in an aerated lagoon is much lower than
that in an activated sludge system because there is no sludge recycle.
Therefore, a significantly longer residence time is required to achieve
the same effluent quality. However, this longer residence time may be an
advantage when complex organic chemicals are to be degraded. Also, the
microorganisms in aerated lagoons are more resistant to process upsets
caused by feed variations than those in activated sludge systems because
of the larger tank volumes and longer residence times used. The effluent
from the aerated lagoon may flow to a settling tank for removal of
suspended solids. Alternatively, the mechanical aerators in the system
may be shut off for a period of time to facilitate settling prior to
discharge of the effluent. The settled solids are generally dewatered
prior to disposal.
1.3.3 Trickling Filters
A trickling filter is an attached-growth biological treatment
process. The system consists of an equalization basin, a settling tank,
a filter medium, an influent wastewater distribution system, an under
drain system, a. clarifier, and a recirculation line. The filter medium
consists of a bed of an inert material to which the microorganisms attach
themselves and through which the wastewater is percolated. Rocks or
synthetic material such as plastic rings and saddles are typically used
as filter media. Following equalization and settling of settleable
solids in the wastewater, it is distributed over the top of the filter
medium by a rotating distribution arm or a fixed distributor system. The
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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 is available to the microorganisms.
Oxygen from air reaches the microorganisms through the void spaces in the
media. Figure 1-2 presents a diagram of a trickling filter system.
A trickling filter system is typically used as a roughing filter to
reduce the organic loading on a downstream activated sludge process.
Trickling filters can be used for the treatment of wastewaters that could
potentially produce "bulking" sludge (i.e., a sludge with poor settling
characteristics and poor compactability in an activated sludge process)
because the microbial solids that slough off the trickling filter medium
are relatively dense and can be readily removed in a clarifier.
1.3.4 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
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often operated in series, with the effluent from the last R£C being
discharged. Biological solids are usually dewatered prior to disposal.
1.4 Waste Characteristics Affecting Performance (WCAPs).
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.
1.4.1 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
(BOD) 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 BOD to
TOC in an untested waste is significantly lower than that in the tested
waste, the system may not achieve the same performance and other, more
applicable technologies may need to be considered for treatment of the
untested waste.
1.4.2 Concentration of Surfactants
Surfactants can affect aerobic biological treatment performance by
forming a film on organic constituents, thereby establishing a barrier to
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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.
1.4.3 Concentration of Toxic Constituents and Waste Characteristics
A number of constituents and waste characteristics have been
identified as potentially toxic to microorganisms. Specific toxic
concentrations have not been determined for most of these constituents
and waste characteristics. The constituents and waste characteristics
found to be potentially toxic to microorganisms include metals and oil
and grease, 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.
1.5 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
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.
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1.5.1 Amount of Nutrients
Nutrient addition is important in controlling the growth of
microorganisms because an insufficient amount of nutrients results in
poor microbial growth with poor biodegradation of organic constituents.
The principal inorganic nutrients used are nitrogen and phosphorus. In
addition, trace amounts of potassium, calcium, sulfur, magnesium, iron,
and manganese are also used for optimum microbial growth. The percent
distribution of nitrogen and phosphorus added to microorganisms varies
with the age of the organism and the particular environmental
conditions. The total amount of nutrients required depends on the net
mass of organisms produced.
EPA monitors the amount of nutrients added and their method of
addition to the wastewater to ensure that a sufficient supply is provided
to achieve an effective growth of microorganisms.
1.5.2 Concentration of Dissolved Oxygen
A sufficient concentration of dissolved oxygen (DO) is necessary to
metabolize and degrade dissolved organic constituents. The DO
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
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.
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1.5.3 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.
1.5.4 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.
1.5.5 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
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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.
1.5.6 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 is present in the
aeration unit.
1.5.7 Hydraulic Loading Rate
The hydraulic loading rate determines the length of time the organic
constituents are in contact with the microorganisms and, hence, the
extent of biodegradation that occurs. In trickling filters, the
hydraulic loading rate also determines the shear velocities on the
microbial layer. Excessively high hydraulic loading rates may wash away
the microbial layer faster than it can grow back. However, the hydraulic
loading rate must be high enough to keep the microbes moist and to remove
dead or dying microbes that have lost their ability 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
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loading provides sufficient time to achieve an effective biodegradation
of organic constituents in the wastewater.
1.5.8 Settling Time
Adequate settling time must be provided to separate the biological
solids from the mixed liquor. Activated sludge systems cannot function
properly if the solids cannot be effectively separated and a portion
returned to the aeration basin. EPA monitors the settling time to ensure
effective solids removal.
1.5.9 Degree of Mixing
Mixing provides greater uniformity of the wastewater feed in the
equalization basin to reduce variations that may cause process upsets of
the microorganisms and diminish treatment efficiency. For activated
sludge and aerated lagoon systems, sufficient aeration in the aeration
unit provides mixing to ensure adequate contact between the
microorganisms and the organic constituents in the wastewater. The
quantifiable degree of mixing is a complex assessment that includes,
among other 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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1.6 References
Barley, J., and Ollis, D. 1977. Biochemical engineering fundamentals.
New York: McGraw-Hill Book Co.
Burchett, M., and Tshobanoglous, G. 1974. Facilities for controlling the
activated sludge process by mean cell residence time. Journal Water
Pollution Control Federation. Vol. 46, no. 5 (May 1974), p. 973.
Clark, J.W., Viessman, W., and Hammer, M. 1977. Water supply and
pollution control. New York: Harper & Row, Publishers.
Eckenfelder Jr., W.W., Patoczka, J., andWatkins, A. 1985. Wastewater
treatment. Chemical engineering. September 2, 1985.
Johnson, S.J. 1978. Biological treatment. Unit operation for
treatment of hazardous industrial wastes. Park Ridge, N.J.: Noyes
Data Corporation.
Kobayashi, H., and Rittmann, B. 1982. Microbial removal of hazardous
organic compounds. Environmental Science and Technology. Vol. 16,
No. 3, pp. 170-183.
Metcalf & Eddy, Inc. 1979. Wastewater engineering: treatment, disposal.
reuse. New York: McGraw-Hill Book Co.
Nemerow, N.L. 1978. Industrial water pollution origins.
characteristics, and treatment. Reading, Mass.: Addison-Wesley
Publishing Company.
Perry, R., and Green, D. 1984. Chemical engineers handbook. 6th ed.
New York: McGraw-Hill Book Co.
Rittmann, B. 1987. Aerobic biological treatment. Environmental Science
and Technology. February 1987. p. 128.
USEPA. 1986. U.S. Environmental Protection Agency. Best demonstrated
available technology background document for F001-F005 spent solvents.
Vol 1, EPA/530-SW-86-056. pp. 4-43. Washington, D.C.: U.S.
Environmental Protection Agency.
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2. BATCH DISTILLATION
2.1 Applicability
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.
2.2 Underlying Principles of Operation
As with other forms of distillation, the basic principle of operation
for 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 condensed and either reused
or further treated by liquid injection incineration; 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
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.
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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 components can be expressed
as relative volatility, which is 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
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 liquid mixture is called
ail azeotrope. Separation becomes easier as the value of the relative
volatility becomes increasingly different from unity. As more of the
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.
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2.3 Description of Batch Distillation Process
A batch distillation unit consists of a steam-jacketed vessel, a
condenser, and a product receiver. Figure 2-1 is a schematic showing the
major components of a batch distillation unit. The steam jacket provides
the heat required to vaporize the volatile constituents in the liquid
fraction of the waste. The rising vapor is collected in the condenser,
cooled, and condensed. The liquid product stream is then routed to the
product receiver.
It is important to note that this technology treats wastes by
vaporizing constituents, not destroying them. Accordingly, an integral
part of this technology is a condensation system to collect the organics,
as well as an air emission control system to collect those organics that
are not condensed. The cooling load of the condenser is calculated in
the design to ensure that the product recovery rate is maximized and
emissions from condenser venting are minimized.
The "bottoms," which are the least volatile constituents of the
waste, are withdrawn from the bottom of the batch still. Because batch
distillation is used Co 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.
2.4 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.
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VENT OF NON-CONDENSED VAPORS
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
WASTE
INFLUENT'
i
CONDENSER
Y///////////S
HEATED
JACKET
PRODUCT
RECEIVER
RECOVERED
ORGANICS
TO REUSE
OR FURTHER
TREATMENT
STILL BOTTOMS
TO REUSE OR
INCINERATION
Figure 2-1. Batch Distillation System.
2-4
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2.4.1 Thermal Conductivity of the Waste
A major factor determining whether a particular constituent will
volatilize is the transfer of heat through the waste. For 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
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 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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2.4.2 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
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.
2.4.3 Concentration of Volatile Components
The concentration of volatile components is a measure of the maximum
fraction of the waste that can be expected to volatilize in the batch
still. 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 components and achieve the same treatment
performance, or other, more applicable treatment technologies may need to
be considered for treatment of the untested waste.
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2.5 Design 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.
2.5.1 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 volatilities will be
removed from the waste.
Pressure is integrally related to the boiling point of the waste and
the subsequent vaporization of the organic constituents. As the pressure
is lowered below atmospheric (i.e., as vacuum is increased), the boiling
point of the waste will also be lowered, thereby requiring less heat
input to volatilize waste constituents. EPA monitors the distillation
temperature as well as the pressure (if pressures other than atmospheric
are used) to ensure that the system is operating at the appropriate
design conditions and to diagnose operational problems.
2.5.2 Residence Time
The residence time determines the necessary energy 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.
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2.6 References
DeRenzo, D.J., ed. 1978. Unit operation for treatment of hazardous
industrial wastes. Park Ridge, N.J.: Noyes Data Corporation,
Kirk-Othmer. 1965. Encyclopedia of chemical technology. 2nd ed.,
Vol. 7, pp. 204-248. New York: John Wiley and Sons.
McCabe, W.L., Smith, J.C., and Harriot, P.. 1985. Unit operations of
chemical engineering, pp. 533-606. New York: McGraw-Hill Book Co.
Perry, R.H. and Chilton, C.H. 1973. Chemical 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.
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3. CARBON ADSORPTION
3.1 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 CAC process; the PAC process is discussed in
Section 9 of this report, Aerobic Biological Treatment.
3.2 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.
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3.3 Description of Carbon Adsorption Process
In GAG systems, the carbon is packed in a column and Che wascewater
is passed through the carbon bed(s). The flow can be either down or up
through the vertical column(s). Figure 3-1 shows two carbon adsorption
systems. In the carbon adsorption process, the wastewater is passed
through a stationary bed of carbon. The contaminants in the wastewater
are adsorbed most rapidly and effectively by the upper layers of carbon
during the initial stages of operation. These upper layers are in
contact with the wastewater at its highest concentrations of
contaminants. The small amounts of the contaminants that are not
adsorbed in the first few layers of the activated carbon bed are removed
from solution in the lower or downstream portion of the bed. Initially,
none of the contaminants escapes from the carbon bed.
As the wastewater flows down the column (or the location in the
column where the majority of adsorption is occurring), and the adsorption
capacity is reached in the top layers, the adsorption zone moves down the
column. As the adsorption zone approaches the end of the carbon bed, the
concentration in the effluent rapidly approaches the influent
concentration. This point in the process is referred co as
breakthrough. A breakthrough curve (Figure 3-2) shows the plot of the
ratio of effluent to influent concentrations versus time of process
operation. At breakthrough, the adsorptive capacity of the carbon bed
is exhausted, and little additional removal of contaminants occurs.
Treated wastewater is then either treated further in another carbon
adsorption column, if necessary, or disposed of.
Once the carbon bed is spent and can no longer remove contaminants
from the waste, it is taken off-line. The activated carbon is then
either regenerated by thermal or chemical methods for further use or
treated by incineration and disposed of. If carbon adsorption is used to
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WASTEWATER
INFLUENT
TREATED
EFFLUENT
TO DISPOSAL
GRANULAR
ACTIVATED
CARBON
DOWNFLOW SERIES ARRANGEMENT
TREATED
EFFLUENT WASTEWATER
TO INFLUENT
DISPOSAL
UPFLOW SERIES ARRANGEMENT
Figure 3-1. Carbon Adsorption Systems.
-------
WASTEWATER
INFLUENT
\J U \J \J
i J J i
ADSORPTION
ZONE
C
EFFLUENT
°B CE
1.0
RATIO OF
EFFLUENT TO INFLUENT
CONCENTRATIONS 0.5
WITH RESPECT TO TIME
TIME
Figure 3-2. Plot of Breakthrough Curve.
-------
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 l,700eF) 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 chat the carbon can be regenerated is determined
by the extent of its physical erosion and the loss of its adsorptive
capacity. Isotherm tests can be performed on the regenerated carbon to
determine adsorptive capacity; such tests can thus aid in predicting the
number of times the carbon can be regenerated.
3.4 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 and type of
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adsorbable contaminants and (b) the concentrations of suspended solids
and oil and grease.
3.4.1 Type and Concentration and Type 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 in 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 in an untested waste are
significantly less adsorbable and higher, respectively, than in the
tested waste, the system may not achieve the same performance.
3'.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.
3.5 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.
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3.5.1 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.
3.5.2 Adsorption Temperature
As the temperature increases, the solubility of the contaminants
generally increases as well, which results in less effective adsorption.
EPA monitors the temperature continuously, if possible, to ensure that
the system is operating at the appropriate design condition and to
diagnose operational problems.
3.5.3 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.
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3.5.4 Hydraulic Loading Kate
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.
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3.6 References
Authur D. Little, Inc. 1977. Physical, chemical and biological treatment
techniques for industrial wastes. Vol. I - NT1S, 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 halogenated organics. Prepared For USEPA, Contract
68-01-6871, 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. , Butter-worth Publishers, pp. 329-340.
Touhill, Shuckrow & Assoc. 1981. Concentration technologies for hazardous
aqueous 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, D.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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4. CRITICAL FLUID EXTRACTION
4.1 Applicability
Critical fluid extraction is a treatment technology 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.
4.2 Underlying Principles of Operation
The basic principle of operation for critical fluid extraction the
enhanced solubilities of various organic compounds in hydrocarbons and
other solvents in the near critical state (i.e., high pressure) aids in
their removal from a waste. The solvents used are compounds that are
Uftvutis
usually gaoco 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.
4.3 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
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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 components. The
solvent-rich stream is fed to a pressurized evaporation 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
(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.
4.4 Waste Characteristics Affecting Performance (UCAPs)
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.
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4.4.1 Concentration of Extractable Hydrocarbons
The process is designed to extract hydrocarbon components from mixed
oily and organic waste liquids and sludges. For this process to be
economically applied, the waste should contain at least a few percent by
weight of extractable hydrocarbons. The process has been demonstrated on
wastes containing from 5 to 34 percent hydrocarbons. For lower
concentrations, batch distillation or conventional solvent extraction may
be more economical. Also, critical fluid extraction is not economical
for wastes containing high concentrations of extractable hydrocarbons
(higher than 95 percent). These wastes are more amenable to
fractionation treatment. 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
implies that most of the waste may become wastewater or waste solids
(i.e., is nonextractable). Total extractable hydrocarbon content can be
estimated by measurement of the total organic carbon (TOC) content. If
the TOC of an untested waste is significantly lower than that of the
tested waste, the 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.
4.4.2 Alkalinity of the Waste
When carbon dioxide is used as the extraction fluid, high alkalinity
will interfere with the process because carbon dioxide will react to form
carbonates and bicarbonates. This will result in excessive carbon
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dioxide consumption. For wastes having high alkalinity 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 may not achieve the same
performance. Use of another extraction fluid may be required to achieve
the same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested
waste.
4.4.3 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.
4.5 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 initial waste mixing,
(c) the degree of mixing during extraction, (d) the extraction
temperature, (e) the residence time, and (f) the settling time.
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4.5.1 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, if
possible, to ensure that the system is operating at the appropriate
design condition and to diagnose operational problems.
4.5.2 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 well-designed and
well-operated system should have such waste-blending equipment. The
quantifiable degree of mixing is a complex assessment that includes,
among other things, the amount of 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.
4.5.3 Degree of Mixing During Extraction
During the fluid extraction, the solvent and waste need to be mixed
to ensure maximum contact and, hence, efficiency of extraction. Any
well-designed system should include a pressurized extraction vessel that
is equipped with an operable mixing system. EPA evaluates this mixing in
the same manner as that previously described for initial waste mixing.
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4.5.4 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
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.
4.5.5 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.
4.5.6 Settling Time
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.
The critical temperature of a substance is that temperature above
which the substance cannot be liquified, no matter how high the
operating pressures.
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4.6 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.
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5. FRACTIONATION
5.1 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.
5.2 Underlying Principles of Operation
As with other forms of distillation, the basic principle 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
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
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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 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 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
i't, 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
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.
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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 the numerical value is 1,
then separation is impossible because the constituents have the same
concentrations in the vapor and liquid phases. When the relative
volatility is 1, the liquid mixture is called an azeotrope. Separation
becomes easier as the value of the relative volatility becomes
increasingly different from unity.
5.3 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 5-1 is a schematic showing the major components of a fractionation
unit. The reboiler is a device that provides the heat required to
vaporize the liquid fraction of the waste. It supplies enough heat to
maintain the liquid in the column at its boiling point. The stripping
and rectifying sections are composed of a set of trays in a vertical
column. The discrete trays may be replaced by loose packing consisting
of plastic, metal, or ceramic geometric shapes that provide surface area
for the continuous boiling/condensing that takes place in the column. In
the stripping section, vapor rising from the boiler is contacted with the
downflowing liquid feed. Through this contact, the constituents with
lower boiling points (i.e., those that are more volatile) are
concentrated in the vapor. In the rectification section, the vapor
rising above the feed tray is contacted with downflowing condensed liquid
product (reflux). Through this contact, 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 in a condenser. The liquid product stream,
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VENT OF NON-CONDENSED VAPORS
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
i
CONDENSER
REFLUX
WASTE
INFLUENT"
T
I
RECTIFIER
SECTION
STRIPPER
SECTION
PRODUCT
RECEIVER
RECOVERED ORGANICS
TO REUSE
I
f(
'(
BOTTOMS
TO REUSE OR
INCINERATION
REBOILER
Figure 5-1. Fractionation System.
5-4
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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.
5.4 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 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.
5.4.1 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 generally treated by
fractionation. 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
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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 waste characteristic
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.
5.4.2 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 suspended solids and
achieve the same treatment performance, or other, more applicable
technologies may need to be considered for treatment of the untested
waste.
5.4.3 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
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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.
5.4.4 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 be required to reduce
foaming and achieve the same treatment performance, or other, more
applicable treatment technologies may need to be considered for treatment
o'f the untested waste.
5.4.5 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
technologies may need to be considered for treatment of the untested
waste.
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5.5 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.
5.5.1 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 determined through the
use of empirical tray efficiency data typically supplied 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.
5.5.2 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, in turn, 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 more volatile components from less
volatile components.
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5.5.3 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 volatilize at lower
temperatures at reduced pressures. EPA monitors the temperature and
pressure in a fractionation column continuously, if possible, to ensure
that the system is operating at the appropriate design conditions and to
diagnose operational problems.
5.5.4 Differential Pressure
Measuring the differential pressure between the top and bottom of the
column indicates whether the flow rate of either the liquid or the vapor
phase is excessive. For instance, a high pressure drop across the column
may indicate a condition of "flooding," in which the liquid phase cannot
flow down through the column as fast as feed is entering, causing backing
up or "flooding" to occur. EPA monitors the pressure drop across the
fractionation column continuously, if possible, to ensure that the system
is operating at the appropriate design condition and to diagnose
operational problems.
5.5.5 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
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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 theoretical stage, being
more resistant to corrosive materials, having a lower capital cost for
small-diameter columns (less than approximately 3 feet), and being less
susceptible to foaming because of a more uniform flow distribution (i.e.,
lower local variations in flow rates). EPA examines the internal column
design of a fractionation column to ensure that the system is designed to
handle potential operational problems (e.g., corrosion, foaming,
channeling, etc.).
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5.6 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 oreanics. PB84-232628. pp. 1-1 to F4. Prepared for the Industrial
Environmental Research Laboratory, Office of Research and Development,
U.S. Environmental Protection Agency.
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6. 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.
6.1 Applicability
Fuel substitution has been used with industrial waste solvents,
refinery wastes, synthetic fibers/petrochemical wastes, and waste oils.
It can also be used when combusting other waste types produced during the
manufacture of pharmaceuticals, pulp and paper, and pesticides. These
wastes can be handled in a solid, liquid, or gaseous form.
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 heat and/or products by
burning fuels. They include blast furnaces, smelters, 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 applicability of fuel substitution are
the:
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);
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Filterable solids concentration (for liquids); and
Sulfur content.
6.1.1 Halogen Content of the Waste
If halogenated organics are burned, halogenated acids and free
halogen are among the products of combustion. These released corrosive
gases may require subsequent treatment prior to venting to the
atmosphere. Also, halogens and halogenated acids formed during
combustion are likely to severely corrode boiler tubes and other process
equipment. To minimize such problems, halogenated wastes are blended
into fuels only at very low concentrations. High chlorine content can
also lead to the incidental production (at very low concentrations) of
other hazardous compounds such as polychlorinated biphenyls (PCBs),
chlorinated dibenzo-p-dioxins (CDDs), chlorinated dibenzofurans
(CDFs), and chlorinated phenols.
6.1.2 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.
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
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partitioning of the heavy metals to these residual streams primarily
depends on the volatility of the metal, waste matrix, and furnace design.
6.1.3 Heating Value of the Waste
The heating value of the waste must be high enough (either alone or
in combination with other fuels) to maintain combustion temperatures
consistent with efficient waste destruction and operation of the boiler
or furnace. For many applications, only supplemental fuels having
minimum heating values of 4,400 to 5,600 kcal/kg (8,000 to 10.000 Btu/lb;
are considered to be feasible. Below this value, the unblended fuel
would not be likely to maintain a stable flame, and its combustion would
release insufficient energy to provide needed steam generation potential
in the boiler or the necessary heat for an industrial furnace. Some
wastes with heating values of less than 4,400 kcal/kg (8.000 Btu/lb) can
be used if sufficient auxiliary fuel is 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.
6.1.4 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, u
viscosity of 165 centistokes (750 Saybolt Seconds Universal (SSU)) or
less is typically required.
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6.1.5 Filterable Solids Concentration
If filterable material suspended in the liquid fuel prevents or
hinders pumping or atomization, it will be unacceptable.
6.1.6 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 emission regulations for
certain new source industrial boilers (51 FR 22385). Air pollution
control devices are available to remove sulfur oxides from the stack
gases.
6.2 Underlying Principles of Operation
For a boiler and most industrial furnaces there are two distinct
principles of operation. Initially, energy in the form of heat is
transferred to the waste to achieve volatilization of the various waste
constituents. For liquids, volatilization energy may also be supplied by
using pressurized atomization. The energy used to pressurize the liquid
waste allows the atomized waste to break into smaller particles, thus
enhancing its rate of volatilization. The volatilized constituents then
require additional energy to destabilize the chemical bonds, allowing the
constituents to react with oxygen to form carbon dioxide and water
vapor. The energy needed to destabilize the chemical bonds is referred
to as the energy of activation.
6.3 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
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applications. The following section, however, provides a general
description of industrial kilns (one form of industrial furnace) and
industrial boilers.
6.3.1 Kilns
Combustible wastes have the potential to be used as fuel in kilns
and, for waste liquids, are often used with oil to co-fire kilns.
Coal-fired kilns are capable of handling some solid wastes. In the case
of cement kilns, there are usually no residuals requiring land disposal.
since any ash formed becomes part of the product or is removed by
particulate collection systems and recycled back to the kiln. The only
residuals may be low levels of unburned gases that escape with combustion
products. If this is the case, air pollution control devices may be
required.
Three types of kilns are particularly applicable: cement kilns, lime
kilns, and lightweight aggregate kilns.
(1) Cement kilns. The cement kiln is a rotary furnace, which is a
refractory-lined steel shell used to calcine a mixture of calcium,
silicon, aluminum, iron, and magnesium-containing minerals. The kiln is
normally fired by coal or oil. Liquid and solid combustible wastes may
then serve as auxiliary fuel. Temperatures within the kiln are typically
between 1,380 and 1,540°C (2,500 to 2,800°F). To date, only
liquid hazardous wastes have been burned in cement kilns.
Most cement kilns have a dry particulate collection device (i.e..
either an electrostatic precipitator or a baghouse), with the fly ash
collected 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 thac most
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halogenated liquid hazardous wastes currently can be burned in cement
kilns. Available information shows that scrubbers are not used.
(2) Lime kilns. Quick-lime (CaO) is manufactured in a calcination
process using limestone (CaCO,) or dolomite (CaCO, and MgCO,).
These raw materials are also heated in a refractory-lined rotary kiln.
typically to temperatures of 980 to 1,260°C (1,800 to 2,300°F).
Lime kilns are less likely to burn hazardous wastes than are cement kilns
because product lime is often added to potable water systems. Only one
lime kiln in the United States currently burns hazardous waste. That
particular facility sells its product lime for use as flux or as
refractory in blast furnaces.
As with cement kilns, any fly ash collected is recycled back to the
lime kiln; thus, no residual streams are produced by the kiln. Available
information shows that scrubbers are not used.
(3) Lightweight aggregate kilns. Lightweight aggregate kilns heat
clay to produce an expanded lightweight inorganic material used in
Portland cement formulations and other applications. The kiln has a
normal temperature range of 1,100 to 1,150°C (2,000 to 2,100°F).
Lightweight aggregate kilns are less amenable to combustion of hazardous
wastes as fuels than are the other kilns described above because these
kilns lack material to adsorb halogens. As a result, burning of
halogenated organics in these kilns would likely require afterburners to
ensure complete destruction of the halogenated organics and scrubbers to
control acid gas production. Such controls would produce a wastewater
residual stream subject to treatment standards.
6.3.2 Industrial Boilers
A boiler is a closed vessel in which water is transformed into stemn
by the application of heat. Normally, heat is supplied by the combustion
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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
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.
6.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether fuel substitution will achieve the same level
of performance on an untested waste as on a previously tested waste, and
whether performance levels can be transferred, EPA examines the following
waste characteristics: (a) the component boiling points, (b) the thermal
volutivily of the waste, and (c) the component bond dissociation energies.
6.4.1 Component Boiling Points
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.
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EPA recognizes that the relative volatilities cannot be measured or
calculated directly for the types of wastes generally treated in an
industrial boiler or furnace. The Agency believes that the best measure
of relative volatility is the boiling point of the various hazardous
constituents and will, therefore, use this parameter in assessing
volatility of the organic constituents.
6.4.2 Thermal Conductivity of The Waste
Consistent with the underlying principles of combustion in aggregate
kilns or boilers, a major factor with regard to whether a particular
constituent will volatilize is the transfer of heat through the waste.
In the case of industrial boilers burning solid fuels, heat is
transferred through the waste by three mechanisms: radiation,
convection, and conduction. For a given boiler, it can be assumed that
the type of waste will have a minimal impact on the heat transferred from
radiation. With regard to convection, EPA believes that the range of
wastes treated would exhibit similar properties with regard to the amount
of heat transferred by convection. Therefore, EPA will not evaluate the
radiation or convection heat transfer properties of wastes in determining
similar treatability. For solids, the third heat transfer mechanism.
conductivity, is the one principally operative 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 the measurement
essentially reflects heat flow through regions having the greatest
conductivity (i.e., the path of least resistance) and not heat flow
through all parts of the waste. Nevertheless, EPA has not identified a
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better alternative to thermal conductivity, even for wastes that are
nonhomogeneous.
6.4.3 Component Bond Dissociation Energies
Given an excess of oxygen, an organic waste in an industrial furnace
or boiler would be expected to convert to carbon dioxide and water vapor
provided that the activation energy is achieved. Activation energy is
the quantity of heat (energy) needed to destabilize molecular bonds so
that the oxidation (combustion) reaction will proceed to completion. As
a measure of activation energy, EPA is using bond dissociation energies.
In theory, the bond dissociation energy would be equal to the activation
energy; in practice, however, this is not always the case because of
molecular interacdions bonds. In some instances, bond dissociation
energies will not be available and will have to be estimated, or other
energy effects will have a significant influence on activation energy.
6.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a fuel
substitution system, EPA examines the following parameters:
(a) combustion temperature, (b) residence time, (c) the degree of mixing,
(d) air feed rate, (e) fuel feed rate, and (f) steam pressure/rate of
production.
(a) Combustion temperature. Industrial boilers are generally
designed based on their steam generation potential (Btu output). This
factor is related to the design combustion temperature, which, in turn.
depends on the amount of fuel burned and its Btu value. The fuel feed
rates and combustion temperatures of industrial boilers are generally
fixed based on the Btu values of fuels normally handled (e.g.. No. 2
versus No. 6 fuel oils). When wastes are to be blended with fossil fuels
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for combustion, the blending, based on Btu values, must be such that the
resulting Btu value of the mixture is close to that of the fuel value
used in the design of the boiler. Industrial furnaces also are designed
to operate at specific ranges of temperature 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.
(b) Residence time. A sufficient residence time of combustion
products is normally necessary to ensure that the hazardous substances
being combusted (or formed during combustion) are completely oxidized.
Residence times on the order of a few seconds are generally needed at
normal operating conditions. For industrial furnaces as well as boilers.
the residence time is a function of the size of the furnace and the fuel
feed rates. For most boilers and furnaces the residence time usually
exceeds a few seconds.
(c) Degree of Mixing. Boilers are designed so that fuel and air
are intimately mixed. This helps to ensure that complete combustion
takes place. The shape of the boiler and the method of fuel and air feed
influence the turbulence required for good mixing. Industrial furnaces
also are designed for turbulent mixing where fuel and air are mixed.
(d) 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 the flue gas is
a meaningless variable.
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(e) Fuel feed rate. The rate at which fuel is injected into the
boiler or industrial furnace will determine the thermal output of the
system per unit of time (Btu/hr). If steam is produced, steam pressure
monitoring will indirectly determine whether the fuel feed rate is
adequate. However, various velocity and mass measurement devices can be
used to monitor fuel flow directly.
(f) 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
decreases in steam pressure can be effected by increasing or decreasing
the fuel and air feed rates within certain operating design limits. Most
industrial furnaces do not produce steam; instead they produce a product
(e.g., cement, aggregate) and monitor the rate of production.
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6.6 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 81-2^8163. 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.
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7. INCINERATION
7.1 Applicability
Incineration is a treatment technology applicable to the treatment of
wastes containing a wide range of organic concentrations and low
concentrations of water, metals, and other inorganics. The four most
common incineration systems are liquid injection, rotary kiln, fluidized
bed, and fixed hearth.
Liquid injection incineration is applicable to wastes having
sufficiently low viscosity values (less than 750 Saybolt Seconds
Universal (SSU) such that the waste can be atomized in the combustion
chamber. However, viscosity is temperature dependent so that while
liquid injection may not be applicable to a waste at ambient conditions,
it may become applicable when the waste is heated. In addition, the
waste particles and the concentration of suspended solids need to be
sufficiently low to avoid clogging of the burner nozzle (or atomizer
openings).
Rotary kiln, fluidized and fixed hearth incineration are applicable
to wastes having a wide range of viscosity, particle size, and suspended
solids concentrations.
7.2 Underlying Principles of Operation
The basic principle of operations for incinerations is the thermal
decomposition of organic constituents via cracking and oxidation
reactions at high temperatures (usually between 760 and 1650'C (1400
and 3000T) to convert them into carbon dioxide, water vapor, nitrite
oxides, nitrates, and ammonia (for nitrogen-containing wastes), sulfur
oxides and sulfate for sulfur containing wastes and halogen acids (for
halogenated wastes).
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In liquid injection incineration the organic constituents in the
waste are volatilized and the chemical bonds are destabilized. Once the
chemical bonds are broken, these constituents react with oxygen to form
carbon dioxide, water vapor, and other aforementioned compounds. In
rotary kiln and fixed hearth incineration, two chambers are involved in
the incineration process.
In the primary chamber, the organic constituents in the waste are
volatilized. During this volatilization process, some of the organic
constituents oxidize to form carbon dioxide and water vapor. In the
secondary chamber, the chemical bonds are destabilized resulting in the
organic constituents reacting with oxygen to form carbon dioxide, water
vapor, and other aforementioned compounds.
In fluidized bed incineration, one chamber contains the fluidizing
sand and a freeboard section above the sand. The fluidized bed aids in
the volatilizations and combustion of the organic waste constituents.
The sand in the bed provides a sufficient heat capacity to the
Volatilization of organic constituents. The forced air used to fluidize
the bed provides sufficient oxygen and turbulence to enhance the
reactions of organics with oxygen to form carbon dioxide, water vapor,
and other aforementioned compounds. Additional time for conversion of
the organic constituents is provided by the freeboard above the fluidized
bed.
7.3 Description of Incineration Processes
The physical form of the waste determines the appropriate feed method
into the incineration systems. Liquids are pumped into the combustion
chamber through nozzles or via specially designed atomizing burners.
Wastes containing suspended particles may need to be screened to avoid
clogging of small nozzle or atomizer openings. Sludges and slurries are
fed using positive displacement pumps and water cooled injection ports.
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Bulk solid wastes may require shredding for control of particles of
particle size. They may be fed to the combustion chamber via rams,
gravity feed, air lock feeders, vibratory or screw feeders, or belt
feeders. Containerized waste is gravity- or ram-fed.
Although sustained combustion is possible with waste heat content as
low as 2230 kcal/kg (Btu/lb), wastes are typically blended to a net heat
content of 4450 kcal/kg (8000 Btu/kg) or higher or auxiliary fuel used in
the combustion chambers to raise the heat content to appropriate levels
to sustain the combustion process.
Following incineration of wastes, flyash particulates, and acid gases
(halogen acids) and other gaseous pollutants (nitric oxides (NO ) and
X
sulfur oxides (SO )) are further treated in an air pollution control
system. Particulate emissions from most waste combustion systems
generally have particle diameters less than one micron and require high
efficiency collection systems to minimize air emissions.
The most common air pollution control system used includes a quench
(for gas cooling and conditioning), followed by a high-energy venturi
scrubber* (for particulate and acid gas removal), a packed bed or plate
tower absorber (for acid gas removal), and a demister (for vapor mist
plume elimination).
Packed bed or plate tower scrubbers are commonly used without venturi
scrubbers in the air pollution control systems at liquid injection
incinerator facilities, where absorption of gaseous pollutants is more
important than particulate control (because wastes burned in liquid
injection incinerators typically have it low ash content and, hence,
generate a low level of fly ash particulates).
*A venturi is a short tube or duct with a tapering constriction that
causes an increase in the velocity of fluid flow and a corresponding
decrease in pressure.
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Some liquid injection incinerator facilities do not employ any air
pollution control system because the wastes burned contain low levels of
both ash and halogen content, thereby generating low levels (i.e.,
untreatable) of fly ash particulates and acid gases.
Many air pollution control system designs in recent years have begun
to incorporate waste heat boilers as a substitute for gas quenching as a
means of energy recovery. Wet electrostatic precipitators (ESP),
ionizing wet scrubbers (IWS), and fabric filters are also being
incorporated into newer systems because of their high removal
efficiencies for small particles and their lower pressure drop than
venturi scrubbers.
The inorganic constituents of wastes (noncorabustible ash) are not
destroyed by incineration. These materials, depending on their
composition, exit the incinerator as either bottom ash from the
combustion chamber, or as fly ash particulates suspended in the
combustion gas stream. Bottom ash is typically either air-cooled or
quenched with water following discharge from the combustion chamber.
This waste is then stabilized (if levels of leachable metal constituents
or concern are found) and/or land disposed.
Fly ash particulates, as well as acid gases and other gaseous
pollutants, are entrained in the scrubber waters of the air pollution
control system and collected in sumps of recirculation tanks. Here the
acids are neutralized with caustic and much of the water returned to the
air pollution control system. Eventually, a portion or all of these
scrubber waters is discharged for treatment and disposal when the total
dissolved solids level becomes excessively high. Scrubber waters are
discharged to either a settling tank or lagoon or to a chemical
precipitation system (if treatable to levels of soluble metal
constituents of concern are found) to remove these solids prior to their
land disposal. Depending upon the nature of the remaining dissolved
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constituents and their concentrations allowing treatment, treated
scrubber waters may be returned to the air pollution control system,
further treated in filtration processes, or discharged.
Below are descriptions of the incineration processes for the four
most common incinerator systems.
(a) Liquid injection. In a liquid injection incineration system,
a burner or nozzle atomizes the waste and injects it into the combustion
chamber where it is incinerated in the presence of air or oxygen. A
forced draft system supplies the combustion chamber with air to provide
oxygen for combustion and turbulence for mixing. The combustion chamber
is usually a cylinder lined with refractory (i.e., heat resistant) brick
and can be fired horizontally, vertically upward, or vertically
downward. Figure 7-1 illustrates a diagram of a liquid injection
incineration system.
(b) Rotary kiln. A rotary kiln is a slowly rotating,
refractory-lined cylinder that is mounted at a slight incline from the
horizontal. Solid wastes enter at the high end of the kiln, and liquid
or gaseous wastes generally enter through atomizing nozzles in the
afterburner section of the kiln. A forced draft system supplies the kiln
with air to provide oxygen for combustion and turbulence for mixing.
Rotation of the kiln enhances the exposure of the solids to the heat,
thereby aiding their volatilization as well as principles for additional
mixing of the solids with air combustion. In addition, the rotation
causes the ash to move to the lower end of the kiln where it is removed.
Rotary kiln systems usually have a secondary combustion chamber or
afterburner following the kiln for further combustion of the volatilized
waste constituents. Figure 7-2 presents a diagram of a rotary kiln
incineration system.
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FLY ASH PARTICULATES
AND COMBUSTION GASES
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
AUXILIARY FUEL »|BURNER
AIR
LIQUID OR GASEOUS.
WASTE INJECTION
BURNER
1
PRIMARY
COMBUSTION
CHAMBER
AFTERBURNER
(SECONDARY
COMBUSTION
CHAMBER)
BOTTOM ASH TO STABILIZATION
AND/OR LAND DISPOSAL
Figure 7-1. Liquid Injection Incineration System.
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FLY ASH PARTICULATES
AND COMBUSTION GASES TO
AIR POLLUTION CONTROL
LIQUID OR GASEOUS
WASTE INJECTION
AUXILIARY
FUE
SOLID
WASTE
INFLUENT
BOTTOM ASH TO
STABILIZATION
AND/OR LAND DISPOSAL
Figure 7-2. Rotary Kiln Incineration System.
7-7
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(c) Fluidized bed. A fluidized bed incinerator consists of a
column containing inert particles such as sand which is referred to as
the bed. Air, driven by a blower, enters the bottom of the bed to
fluidize the sand. Air passage through the bed provides oxygen for
combustion and promotes rapid and uniform mixing of the injected waste
material within the fluidized bed. The fluidized bed has an extremely
high heat capacity (approximately three times that of flue (combustion)
gases at the same temperature), thereby providing a large heat
reservoir. The injected waste reaches incineration temperature quickly
and transfers the heat of combustion back to the bed. The freeboard
above the fluidized bed provides additional time for combustion of
organic constituents. Because of the excellent mixing properties
associated with fluidized bed incinerators, they can operate at lower
temperatures more effectively than other incinerators. Figure 7-3 shows
a diagram of a fluidized bed incineration system.
(d) Fixed hearth incineration. Fixed hearth (also called
controlled air or starved air) incineration is a two-stage combustion
process. Waste is ram-fed into the first stage, or primary chamber, and
burned at less than stoichiometric conditions (not enough oxygen suitable
for complete combustion). The resultant smoke and pyrolysis products,
consisting primarily of volatile hydrocarbons and carbon monoxide, along
with the typical products of combustion, pass to the secondary chamber.
Here additional air is injected to complete the combustion process. This
two-stage process generally yields low fly ash particulate and carbon
monoxide (CO) emissions. The primary chamber combustion reactions and
resultant combustion gas velocities are maintained at low levels by the
starved air conditions so that particulate entrainment and carryover in
the combustion gases are minimized. Figure 7-4 is a diagram of a fixed
hearth incineration system.
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FLY ASH
PART1CULATES
AND COMBUSTION
GASES TO AIR
POLLUTION
CONTROL SYSTEM
MAKE-UP
SAND
AIR
BOTTOM ASH TO STABILIZATION
AND/OR LAND DISPOSAL
Figure 7-3. Fluidized Bed Incineration System.
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AIR
WASTE
INJECTION
BURNER
AIR
FLY ASH PARTICULATES
AND COMBUSTION GASES
TO AIR POLLUTION
CONTROL SYSTEM
PRIMARY
COMBUSTION
CHAMBER
I
SECONDARY
COMBUSTION
CHAMBER
BURNER
AUXILIARY
FUEL
BOTTOM ASH TO STABILIZATION
AND/OR LAND DISPOSAL
Figure 7-4. Fixed Hearth Incineration System.
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7.4 Waste Characteristics Affecting Performance (WCAPS)
In determining whether incineration 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, (c) The component bond dissociation
energies, (d) The heating value of the waste, (e) The concentration of
explosive constituents, and (f) The concentration of noncombustible
constituents.
(a) Thermal conductivity of the waste. A major factor determining
whether a particular constituent will volatilize is the transfer of heat
through the waste. For rotary kiln, fluidized bed, and fixed hearth
incineration, heat is transferred through the waste by radiation,
convection, and conduction.
EPA examined all three methods of heat transfer. For a given
incinerator, heat transferred through various wastes by radiation and
convection is more a function of the design and type of incinerator that
the waste being treated, therefore EPA believes that conduction would be
the primary cause of heat transfer differences between wastes. Heat flow
by conduction is proportional to the temperature gradient across the
material. The proportionality constant is referred to as the thermal
conductivity and is a property of the material to be incinerated.
Thermal conductivity measurements, as part of a treatability
comparison for two different wastes to be treated by a single
incinerator, are most meaningful when applied to wastes that are
homogeneous (i.e., uniform throughout). As wastes exhibit greater
degrees of non-homogenity, thermal conductivity becomes less accurate in
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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. Higher temperatures may be
required to improve heat transfer through the waste and achieve the same
treatment performance, or other more applicable treatment technologies
may need to be considered for treatment of the untested wastes.
(b) Component boiling points. Following transfer to a constituent
within a waste, its removal depends on its volatility. EPA recognizes,
however, that volatilities are difficult to measure or calculate directly
for the types of wastes generally treated by incineration. This is
because the wastes usually consist of a mixture of components. 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 of waste components as a surrogate
waste characteristic for volatility. If the boiling points of waste
components in the untested waste are significantly higher than in the
tested waste, the system may not achieve the same performance. Higher
temperatures may be required to volatilize less volatile components and
achieve the same treatment performance or other, more applicable
treatment technologies may need to be considered for treatment of the
untested wastes.
(c) Component bond dissociation energies. The activation energy
is the amount of heat energy needed to destabilize molecular bonds so
that the exothermic combustion reactions can proceed. Generally, the
activation energy required for incineration of solids is greater than
required for liquids; and the activation energy for liquids is higher
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than for gases. However, the activation energies for components are
difficult to measure or calculate directly, and generally have to be
determined empirically. The bond dissociation energy is the amount of
energy needed to break each of the individual bonds in a molecule.
Theoretically, the bond dissociation energy and the activations energy
are equivalent. However, other energy effects including interactions
between different molecular bonds may have a significant influence on the
activation energy. Nevertheless, EPA believes that bond dissociation
energy is the best indicator for activation energy and therefore, uses it
as a surrogate waste characteristic. If the bond dissociation energies
of waste components in an untested waste are significantly higher than in
the tested waste, the system may not achieve the same performance.
Higher temperatures may be required to destabilize molecular bonds for
waste components with high bond dissociation energies and achieve the
same treatment performance, or other, more applicable treatment
technologies may need to be considered for treatment of the untested
waste.
(d) Heating value of the waste. The amount of heat released from
the exothermic combustion reactions of a waste is referred to as its
heating value. To maintain combustion, the heating value of a waste must
be sufficient to heat incoming waste up to the appropriate design
incineration temperature and provide the necessary activation energy for
additional combustion reactions to occur. Higher waste heating values
are required for higher incineration temperatures and higher amounts of
excess oxygen to sustain combustion without auxiliary fuel consumption.
Low heating values for an organic waste are usually caused by high
concentrations of water or halogenated compounds. If the heating value
of an untested waste is significantly lower than of the tested waste, the
system may not achieve the same performance. Auxiliary fuel may be
required to provide the necessary heat to maintain combustion and achieve
the same treatment performance, or other, more applicable treatment
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technologies may need Co be considered for treatment of the untested
waste.
(e) Concentration of explosive constituents. Explosive
constituents may interfere with the smooth operation of an incinerator
causing process upsets resulting in ineffective treatment of the waste.
If the concentration of explosive constituents in an untested waste is
significantly higher than 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.
(f) Concentration of noncombustible constituents. Noncombustible
constituents include water, metals, and other inorganics. High
concentrations of noncombustible constituents result in low waste heating
values requiring auxiliary fuel large quantities of bottom ash requiring
stabilization treatment (if treatable levels of leachable metal
constituents of concern are present) and/or disposal, and large
quantities of fly ash particulates and volatile metals in the combustion
gases requiring removal in an air pollution control system and treatment
in a settling tank or lagoon, or a chemical precipitation system (if
treatable levels of soluble metal constituents of concern are present)
followed by disposal. Volatile metals such as arsenic may also fuse to
the refractory walls in the combustion chamber inhibiting effective
operation of the incinerator. If the concentration of noncombustible
constituents in an untested waste is significantly higher than 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.
7.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an
incineration system, EPA examines the following parameters: (a) the
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incineration temperature, (b) the concentration of excess oxygen in the
combustion gas, (c) the concentration of carbon monoxide in the
combustion gas, (d) the waste feed rate, and (e) the degree of waste/air
mixing.
(a) Incineration temperature. Temperature provides an indirect
measure of the energy available (i.e., Btu/hr) to both volatilize organic
waste constituents and overcome their activation energy. As the design
temperature in the combustion chamber increases, more constituents with
lower volatilities and higher activation energies will be destroyed. EPA
monitors the incinerations temperature continuously, to ensure that the
system is operating at the appropriate design conditions and to diagnose
operational problems.
(b) Concentrations of excess oxygen in the combustion gas. A
sufficient supply of oxygen needs to be supplied to the combustion
chamber to effectively incinerate the organic waste constituents. The
stoichiometric or minimum theoretical oxygen requirement to completely
eombust the waste is based on the amounts of combustible constituents in
the waste. If perfect mixing could be achieved and the waste burned
instantaneously, only the stoichiometric amount of oxygen would be needed
for complete combustion. However, neither of these phenomena occur in
commercial incinerators. The amount of excess oxygen in a given
incinerator depends on the degree of waste/air mixing achieved in the
combustion chamber and the desired degree of combustion gas cooling.
Because excess air (oxygen) supplied to the combustion chamber acts as a
diluent in the combustion process, it reduces the overall temperature in
the incinerator (i.e., maximum theoretical incineration temperatures are
achieved at zero percent excess air). This temperature reduction might
be desirable to limit refractory degradation when high heating value
wastes are being burned. However, when low heating value wastes are
being burned, excess air should be minimized to keep the system
34018
7-15
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temperature as high as possible. Even with high heating value waste, it
is desirable for equipment design considerations to limit the amount of
excess air to some extent so that combustion chamber volume and
downstream air pollution control system sizes can be limited.
Although, an inadequate supply of oxygen will lead to ineffective
incineration with higher concentrations of carbon monoxide (CO) and
organic products of incomplete combustion (PICs), resulting in a heavier
load on the air pollution control system. Typically, 20 to 60 percent
excess oxygen is required to provide adequate waste/oxygen contact and,
subsequently, effective incineration of the waste. EPA monitors the
concentration of excess oxygen in the combustion gas continuously to
ensure that the system is operating at the appropriate design condition
and to diagnose operational problems.
(c) Concentration of carbon monoxide in the combustion gas. The
concentration of carbon monoxide in the combustion gas provides an
indication of the extent to which organic waste constituents are
converted to carbon dioxide and water vapor. Higher carbon monoxide
levels ineffective incineration with the presence of greater amounts of
unreacted or partially reacted organic waste constituents in the
combustions gas, resulting in a heavier load on the air pollution control
system. Higher carbon monoxide levels can result from an insufficient
incineration temperature, an inadequate supply of excess oxygen, a waste
feed rate that is too high resulting in an insufficient residence time
for the waste, and improper mixing in the combustions chamber. EPA
monitors the concentration of carbon monoxide in the combustion gas
continuously to ensure that the system is operating at the appropriate
design conditions and to diagnose operational problems.
(d) Waste feed rate. The waste feed rate determines the residence
time of the waste in the combustion chamber. Sufficient residence time
34018
7-16
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must be provided to allow for volatilization of the organic waste
constituents, mixing with oxygen in the combustions chamber, and
completion of the combustion reactions. The total residence time
required for these processes to occur depends on the incineration
temperature, the amount of excess oxygen provided in the combustion
chamber, the degree of waste/air mixing in the combustion chambers, the
size and type of incinerator used, the physical form of the waste, and
the waste particle size (which determines the amount of waste surface
areas available for heat transfer). Typical incinerator residence times
range from approximately 2 seconds for liquid and gaseous wastes in
liquid injection incinerators and the afterburner, freeboard, and
secondary combustions chamber of rotary kiln, fluidized bed, and fixed
hearth incinerators, respectively, to 30 minutes to 1 hour for solid
wastes in a rotary kiln incinerator. EPA monitors the waste feed rate
continuously to ensure that sufficient residence time is provided to
effectively incinerate organic waste constituents.
(e) Degree of vaste/air mixing. The incineration temperature, the
amounts of excess oxygen, and the residence time required to achieve
effective incineration of a waste all depend to some extent on the degree
of waste/air mixing in the combustion chamber. For liquid injection
incinerators, the degree of waste/air mixing is primarily determined by
the specific burner design and the degree of atomization achieved of the
waste/air mixture. The degree of atomization is dependent on the
viscosity of the liquid waste and the amount of solid impurities
present. If the waste viscosity exceeds 750 SSU or the amount of solid
impurities is too high, the degree of atomization may not be fine enough
resulting in ineffective incineration.
For rotary kiln incineration, the degree of waste/air mixing is
determined by the air flow rate into the kiln as well as the rate of
rotation (revolutions per minute (RPM)) of the kiln. As the air flow
3401g
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rate is increased, the degree of waste/air mixing is improved. Although
this also reduces the incineration temperature and residence time the
combustion gases. Increasing the rate of rotations of the kiln also
improves waste/air mixing. However, the waste solids' residence time is
also reduced. Typical rotation rates for rotary kilns range from 0.3 to
1.5 meters per minute (1 to 5 feet per minute).
For fluidized bed incineration, the degree of waste/air mixing is
indicated by the pressure drop through the bed caused by the air flow
rate used to fluidize the bed. The higher the pressure drop, the greater
the degree of mixing in the fluidized bed. However, this also reduces
the incineration temperature and the residence time of the waste solids
and combustion gases.
For fixed hearth incineration, the degree of waste/air mixing is
determined by the air flow rate into the combustion chamber. As the air
flow rate is increased, the degree of waste/air mixing is improved.
However, this also reduces the incineration temperature and the residence
time of the combustion gases.
The quantifiable degree of waste/air mixing is a complex assessment
which is difficult to express in absolute terms. However, for liquid
injection incineration, EPA evaluates the degree of mixing qualitatively
by examining the burner design and determining whether it could be
expected to achieve effective atomization of the waste/air mixture. For
rotary kiln incineration, EPA estimates the degree of mixing by
monitoring the air flow rate into the kiln as well as the kiln's rate of
rotation to ensure that effective mixing of the waste and air is
achieved. For fluidized bed incineration, EPA estimates the degree of
mixing by monitoring the pressure drop through the bed to ensure that
effective mixing of waste and air is achieved. For fixed hearth
3401s
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9. STEAM STRIPPING
9.1 Applicability
Steam stripping is a form of distillation applicable to the treatment
of 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.
9.2 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 stripping is the principle of
vapor-liquid equilibrium. When a liquid mixture of two or more
components is heated, the vapor phase present above the liquid phase
becomes more concentrated in the more volatile constituents (those having
higher vapor pressures). The vapor phase above the liquid phase is Chen
cooled to yield a condensate that is also more concentrated in the more
volatile components. The degree of separation of components depends on
the relative differences in the vapor pressures of the constituents; the
larger the difference in the vapor pressures, the more easily the
separation can be accomplished.
If the difference between the vapor pressures is extremely large, a
single separation cycle or a single equilibrium stage of vaporization and
condensation may achieve a significant separation of the constituents
9-1
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(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 is again heated and separated.
The vapor-liquid equilibrium of the waste components can be expressed
as relative volatility, which is the ratio of the vapor-to-liquid
concentrations of a constituent divided by the ratio of the
vapor-to-liquid concentrations of another constituent. The relative
volatility is a direct indicator of the ease of separation. If the
numerical value is 1, then separation is impossible because the
constituents have the same concentrations in the vapor and liquid
phases. When the relative volatility is 1, the liquid mixture is called
an azeotrope. Separation becomes easier as the value of the relative
volatility becomes increasingly different from unity.
9.3 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 9-1. 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.
9-2
3390s
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WASTEWATER
INFLUENT
VENT OF NON-CONDENSED VAPORS
TO AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
x
NSTRIPPING^;
COLUMN \
i
CONDENSER
RECYCLE
RECEIVER
RECOVERED
ORGANICS
TO REUSE
OR TREATMENT
TREATED EFFLUENT
TO FURTHER
TREATMENT
AND/OR DISPOSAL
Figure 9-1. Steam Stripping System.
9-3
-------
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 in the column, coming into contact with
the falling liquid. As the vapor comes into contact with the liquid at
each stage, the more volatile components are removed or "stripped" from
the liquid by the vapor phase. The concentration of the emerging vapor
is enriched in the more volatile constituents, and the liquid exiting the
bottom of the boiler ("bottoms") contains high concentrations of the
lower vapor pressure constituents, often predominately water. This
effluent from the bottom of the stripper is reduced in organic content
but may still require additional treatment such as carbon adsorption or
biological treatment. The steam and organic vapors exiting the top of
the column are condensed 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.
9.4 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.
9.4.1 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,
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however, that relative volatilities cannot be measured or calculated
directly for the complex types of wastes generally 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
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.
9.4.2 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 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.
9-5
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9.4.3 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 Che untested waste is significantly higher than
that in the tested waste, the 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 compounds
and achieve the same treatment performance, or other, more applicable
technologies may need to be considered for treatment of the untested
waste.
9.4.4 Surface Tens ion
The surface tension of Che waste i;; 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 defoaraing compounds. If the surface
tension of the untested waste is significantly higher than chat 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.
9.4.5 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
9-6
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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.
9.5 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.
9.5.1 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 examines the actual number of
stages in the steam stripping column to ensure that the system is
designed to achieve an effective degree of separation of organics from
the wastewater.
9.5.2 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.
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9.5.3 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.
9.5.4 Internal Column Design
Column internals are designed to accommodate the physical and
chemical properties of the wastewater to be stripped. Two types of
internals may be used in steam stripping: trays or 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 theoretical stage, being
more resistant to corrosive materials, having a lower capital cost for
small-diameter columns (less than approximately 3 feet), and being less
susceptible to foaming because of a more uniform flow distribution
(i.e., lower local variations in flow rates). EPA examines the internal
column design of a steam stripping column to ensure that the system is
designed to handle potential operational problems (e.g., corrosion,
foaming, channeling, etc.).
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9.6 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 sneineers' 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 oreanics. PB84-232628. pp. 1-1 to F4. Prepared for the Industrial
Environmental Research Laboratory, Office of Research and Development,
U.S. Environmental Protection Agency.
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. 10. THIN FILM EVAPORATION
10.1 Applicability
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 low 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.
10.2 Underlying Principles of Operation
As with other forms of distillation, the basic principle of operation
for thin film evaporation is the separation of a liquid mixture into
various components by a process of vaporization-condensation. The
constituents 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
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.
10-1
-------
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, 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 ratio 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,
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.
10.3 Description of Thin Film Evaporation Process
Typically, thin film evaporation consists of a steam-jacketed
cylindrical vessel and a condenser. Figure 10-1 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
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
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WASTE
INFLUENT
LIQUID
FILM
ROTATING
DRIVE
REFLUX
VENT OF
NON-CONDENSED VAPORS TO
AIR POLLUTION CONTROL SYSTEM
AND/OR THE ATMOSPHERE
i
I
CONDENSER
HEATED
JACKET
PRODUCT
RECEIVER
RECOVERED
ORGANICS
TO REUSE
BOTTOMS TO
REUSE OR
INCINERATION
Figure 10-1. Thin Film Evaporation System.
10-3
-------
the vessel materials, and Che 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.
10.4 Waste Characteristics Affecting Performance (WCAPs)
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.
10.4.1 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
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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.
10.4.2 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 is significantly
higher than that in the tested waste, the system may not achieve the same
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.
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 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.
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10.4.4 Concentration of Oil and Grease
High concentrations of oil and grease in 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 in che 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.
10.5 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 waste feed rate.
10.5.1 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.
10.5.2 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
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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.
10.5.3 Waste Feed Rate.
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. It is dependent on the distillation temperature and the boiling
points of waste components. EPA monitors the waste feed rate continuously
to ensure that sufficient residence time is provided to effectively
volatilize organic components from the waste.
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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-Othraer. 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.
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11. ACID LEACHING
11.1 Applicability
Acid leaching is a treatment technology used to treat wastes in solid
or slurry form containing metal constituents that are soluble in a strong
acid solution or can be converted by reaction with a strong acid to a
soluble form. This process has been used to recover metals such as
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.
11.2 Underlying Principles of Operation
The basic principle of operation for acid leaching is that
solubilities of various metals in acid solutions aids in their removal
from a waste. The process concentrates the constituent(s) leached by the
acid solutions. These constituents can then be filtered to remove
residual solids and neutralized to precipitate solids containing high
concentrations of the constituents of interest, which can be further
treated in metals recovery processes. Alternatively, the acid solutions
can be electrolysed 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
(H2SO,), hydrochloric (HC1), and nitric (HNO-). Although any
acidic pH can theoretically be used, acid leaching processes are normally
run at a pH from 1 to 4.
11-1
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11.3 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.
11.3.1 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.
11.3.2 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.
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11.4 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.
11.4.1 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.
11.4.2 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
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
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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.
11.4.3 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(s) of concern and
achieve the same treatment performance, or other, more applicable
treatment technologies may need to be considered for treatment of the
untested waste.
11.4.4 Concentration of Leachable Metals
The amount of leachable metals is a measure of the maximum fraction
of the waste that can be expected to leach in the acid leaching system.
A relatively low concentration of leachable metals implies that most of
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.
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11.5 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.
11.5.1 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.
11.5.2 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 is necessary to
form soluble species, then the appropriate acid, as well as the
appropriate concentration of the acid, must be used to ensure effective
leaching of the metal constituents. EPA examines the type and
concentration of acid used to ensure that the acid solution selected is
capable of effectively leaching the metal constituents from the waste.
11.5.3 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
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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.
11.5.4 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.
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11.6 References
HcCabe, L., and Smith, J. 1976. Unit operations of chemical engineering.
3rd ed., pp. 607-610. New York: McGraw-Hill Book Co.
Perry, R. H., and Chilton, C. H. 1973. Chemical engineers' handbook.
5th ed., pp. 19-41 to 19-43. New York: McGraw-Hill Book Co.
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12. CHEMICAL PRECIPITATION
12.1 Applicability
Chemical precipitation is a treatment technology applicable to
wastewaters containing a wide range of dissolved and other metals, as
well as other inorganic substances such as fluorides. This technology
removes these metals and inorganics from solution in the form of
insoluble solid precipitates. The solids formed are then separated from
the wastewater by settling, clarification, and/or polishing filtration.
For some wastewaters, such as chromium plating baths or plating baths
containing cyanides, the metals exist in solution in a very soluble
form. This solubility can be caused by the metal's oxidation state (for
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.
12.2 Underlying Principles of Operation
The basic principle of operation of chemical precipitation is that
metals and inorganics in wastewater are removed by the addi'tion of a
precipitating agent that converts the soluble metals and inorganics to
insoluble precipitates. These precipitates are settled, clarified,
and/or filtered out of solution, leaving a lower concentration of metals
and inorganics in the wastewater. The principal precipitation agents
used to convert soluble metal and inorganic compounds to less soluble
forms include: lime (Ca(OH)2), caustic (NaOH), sodium sulfide (Na-S),
and, to a lesser extent, soda ash (Na^CO-^), phosphate (P0,~), and
ferrous sulfide (FeS).
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The solubility of a particular compound depends on the extent to
which the electrostatic forces holding the ions of the compound together
can be overcome. The solubility changes significantly with temperature,
with most metal compounds becoming more soluble as the temperature
increases. Additionally, the solubility is affected by other
constituents present in the wastewater, including other ions and
complexing agents. Regarding specific ionic forms, nitrates, chlorides,
and sulfates are, in general, more soluble than hydroxides, sulfides,
carbonates, and phosphates.
Once the soluble metal and inorganic compounds have been converted to
precipitates, the effectiveness of chemical precipitation is determined
by how successfully 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
i-s 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.
12.3 Description of Chemical Precipitation Process
The equipment and instrumentation required for chemical precipitation
vary depending on whether the system is batch or continuous. Both
systems are discussed below.
For a batch system, chemical precipitation requires a feed system for
the treatment chemicals and a reaction tank where the waste can be
treated and allowed to settle. When lime is used, it is usually added to
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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 12-1. 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 is pumped to a reaction tank
where precipitating agents are added. This is done automatically by
using instrumentation that senses the pH of the system for hydroxide
precipitating agents, or the oxidation-reduction potential (ORP) for
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,
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
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WASTEWATER
INFLUENT
ro
PRECIPITATING
AGENT
FEED
SYSTEM
COAGULANT OR
FLOCCULANT FEED SYSTEM
EQUALIZATION
TANK
ELECTRICAL CONTROLS
MIXER
TREATED
EFFLUENT TO
POLISHING
FILTRATION
AND/OR
DISPOSAL
BOTTOMS
TO SLUDGE
» FILTRATION
AND DISPOSAL
Figure 12-1. Continuous Chemical Precipitation System.
-------
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 12-2 and 12-3. 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.
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.
12.4 Waste Characteristics Affectine 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.
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BOTTOMS TO SLUDGE
FILTRATION AND DISPOSAL
WASTEWATER
INFLUENT
TREATED EFFLUENT
TO POLISHING
FILTRATION
AND/OR DISPOSAL
CENTER FEED CLARIFIER WITH SCRAPER SLUDGE REMOVAL SUSTEM
WASTEWATER
INFLUENT
TREATED
EFFLUENT
TO POLISHING
FILTRATION
AND/OR DISPOSAL
BOTTOMS TO
SLUDGE FILTRATION
AND DISPOSAL
RIM FEED - CENTER TAKEOFF CLARIFIER WITH
HYDRAULIC SUCTION SLUDGE REMOVAL SYSTEM
WASTEWATER
INFLUENT
TREATED
EFFLUENT
TO POLISHING
FILTRATION
AND/OR DISPOSAL
BOTTOMS TO SLUDGE FILTRATION AND DISPOSAL
RIM FEED - RIM TAKEOFF CLARIFIER
Figure 12-2. Circular Clarifier Systems.
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WASTEWATER
INFLUENT
TREATED
EFFLUENT
TO POLISHING
FILTRATION
AND/OR DISPOSAL
BOTTOMS TO
SLUDGE FILTRATION
AND DISPOSAL
Figure 12-3. Inclined Plate Settler System.
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12.4.1 Concentration and Type of Metals
For most metals, there is a specific pH at which the metal
precipitate is least soluble. As a result, when a waste contains a
mixture of many metals, it is not possible to operate a treatment system
at a single pH or ORP value that is optimal for the removal of all
metals. The extent to which this affects treatment depends on the
particular metals to be removed and their respective concentrations. One
alternative is to operate multiple precipitations, with intermediate
settling, when the optimum pH occurs at markedly different levels for the
metals present. If the concentration and type of metals in an untested
waste differ from and are significantly higher than those in the tested
waste, the system may not achieve the same performance. Additional
precipitating agents, alternate pH/ORP values, and/or multiple
precipitations may be required to achieve the same treatment performance,
or other, more applicable treatment technologies may need to be
considered for treatment of the untested waste.
12.4.2 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 in the treated wastewater
residuals. If the TDS concentration in an untested waste is
significantly higher than in the tested waste, Che 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.
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12.4.3 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
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.
12.4.4 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
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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.
12.5 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.
12.5.1 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.
12.5.2 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.
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12.5.3 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.
12.5.4 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 created. 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
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
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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.
12.5.5 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.
12.5.6 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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12.6 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.
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13. ELECTROLYTIC OXIDATION OF CYANIDE
13.1 Applicability
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.
13.2 Underlying Principles of Operation
The basic principle of operation for electrolytic oxidation of
cyanide is that concentrated cyanide waste subject to an electrolytic
reaction with dissolved oxygen in an aqueous solution is broken down to
the gaseous products carbon dioxide (002), nitrogen (N~), and ammonia
(NH,). The process is conducted at elevated temperatures for periods
ranging from several hours to over a week, depending on the initial
cyanide concentration and the desired final cyanide concentration. The
theoretical destruction process that takes place at the anodes is
described by the following reaction:
2CN- + 202 Electricity ^ ^ + ^ + ^
cyanide oxygen carbon nitrogen electrons
ion dioxide
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The effectiveness of electrolytic oxidation is dependent on the
conductivity of the waste, which is a function of several waste
characteristics including the concentration of cyanide and other ions in
solution. As the process continues, the waste becomes less capable of
conducting electricity as cyanide concentration is reduced, causing the
electrolytic reaction to be much less efficient at longer retention times.
13.3 Description of Electrolytic Oxidation Process
Typically, electrolytic destruction of cyanide takes place in a
closed cell. This cell consists of two electrodes suspended in an
aqueous solution, with direct current (DC) electricity supplied to drive
the reaction to completion. The temperature of the bath containing the
cyanide waste is maintained at or above 52°C (125°F). Sodium
chloride may be added to the solution as an electrolyte (conductor) to
increase the conductivity of the waste being treated. Since the reaction
may take days or weeks, water is usually added to the tank periodically
to make up for losses due to evaporation from the heated tank. This is
necessary to ensure that the electrodes remain fully submerged so that a
full flow of current is maintained in the solution during treatment.
Following treatment, the treated waste is generally further treated in a
conventional chemical oxidation system to destroy residual cyanides.
13.4 Waste Characteristics 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.
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13.A.I 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.
13.4.2 Concentration of Reducible Metals
The electrolytic process may cause some of the more easily reduced
metals in the waste, such as copper, to plate out onto the 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
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.
13.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an
electrolytic oxidation system, EPA examines the following parameters:
(a) the oxidation temperature, (b) the residence time, (c) the pH,
(d) the electrical conductivity, (e) the electrode spacing and surface
area, and (f) the degree of mixing.
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13.5.1 Oxidation Temperature
For the electrolytic process, elevated temperatures are used. Normal
temperatures range from 52 to 93"C (125 to 200°F). The temperature
can be raised by increasing the flow of steam to the coils or jacket
supplying heat to the reactor contents. EPA monitors the oxidation
temperature to ensure that the system is operating at the appropriate
design condition and to diagnose operational problems.
13.5.2 Residence Time
Electrolytic oxidation is usually a batch process. The time allowed
to complete the reaction is an important factor in electrolysis and is
dependent on the initial concentration of the waste and the desired final
cyanide concentration. The rate of cyanide destruction decreases as the
cyanide concentration decreases (i.e., the rate of cyanide destruction
asymptotically approaches zero). Typical residence times range from
periods of several hours to more than a week. EPA observes the residence
time to ensure that sufficient time is provided to effectively destroy
the cyanides in the wastes.
13.5.3 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.
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13.5.4 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.
13.5.5 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.
13.5.6 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 13.2, 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,
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however, evaluates the degree of mixing qualitatively by considering
whether mixing is provided and whether the type of mixing device is one
that could be expected to achieve uniform mixing of the waste.
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13.6 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.
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14. HEXAVALENT CHROMIUM REDUCTION
14.1 Applicability
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.
14.2 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 dichroraate
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 metabisulfite (Na2S205), sulfur
dioxide (SO), sodium hydrosulfide (NaHS), and the ferrous form of iron
A typical reduction reaction, using sodium sulfite as the reducing
agent, is as follows:
The reaction is usually accomplished at pH values from 2 to 3 .
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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(S04)3 + 3Ca(OH)2 - 2Cr(OH)3 + 3CaS04
14.3 Description of Chromium Reduction Process
The chromium reduction treatment process can be operated in a batch
or continuous mode. A batch system consists of a reaction tank, a mixer
to homogenize the contents of the tank, a supply of reducing agent, and a
source of acid and base for pH control.
A continuous chromium reduction treatment system, as shown in
Figure 14-1, usually includes a holding tank upstream of the reaction
tank for flow and concentration eqvialization. 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 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.
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REDUCING
AGENT
FEED
SYSTEM
ACID
FEED
SYSTEM
WASTEWATER
INFLUENT
ALKALI
FEED
SYSTEM
r
DD
ORPandpH
TREATED EFFLUENT
TO SETTLING,
FURTHER
TREATMENT,
AND/OR DISPOSAL
REDUCTION
REACTION TANK
PRECIPITATION
TANK
ELECTRICAL CONTROLS
MIXER
Figure 14-1. Continuous Hexavalent Chromium Reduction System.
-------
14.4 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.
14.4.1 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.
14.4.2 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
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.
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14.5 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.
14.5.1 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 in 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
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.
14.5.2 pH
For batch and continuous systems, pH affects the reduction reaction.
The reaction speed is significantly reduced at pH values above
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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 is operating at the appropriate
design condition and to diagnose operational problems.
14.5.3 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
to make sure that the system is operated at the appropriate design
residence time. EPA monitors the residence time to ensure that
sufficient time is provided to effectively reduce the waste.
14.5.4 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
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.
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14.6 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.
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15. HIGH TEMPERATURE METALS RECOVERY
15.1 Applicability
High temperature metals recovery (HTMR) is a technology applicable to
wastes containing metal oxides and metal salts (including cadmium,
chromium, lead, nickel, and zinc compounds) at concentrations ranging
from 10 percent to over 70 percent with low levels (i.e., below 5
percent) of organics and water in the wastes. There are a number of
different types of high temperature metals recovery systems, which
generally differ from one another in the source of energy used and the
method of recovery. These HTMR systems include the rotary kiln process,
the plasma arc reactor, the rotary hearth electric furnace system, the
molten slag reactor, and the flame reactor.
HTMR is generally not used for mercury-containing wastes even though
mercury will volatilize readily at the process temperatures present in
the high temperature units. The retorting process is normally used for
mercury recovery because mercury is very volatile and lower operating
temperatures can be used. Thus, the retorting process is more economical
than HTMR for mercury-bearing wastes. Retorting is discussed in
Section 7.
The HTMR process has been demonstrated on wastes such as baghouse
dusts and dewatered scrubber sludge from the production of steels and
ferroalloys. Zinc, cadmium, and lead are the metals most frequently
recovered. The process has not been extensively evaluated for use with
metal sulfides. The sulfides are chemically identical to natural
minerals ordinarily present in ores used as feedstocks by primary
smelters. Some sulfide-bearing wastes from the chrome pigments industry
have been sent to such primary smelters. However, with sulfides, a
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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.
15.2 Underlying Principles of Operation
The basic principle of operation for HTMR is that metal oxides and
salts are separated from a waste through a high temperature thermal
reduction process that uses carbon, limestone, and silica (sand) as raw
materials. The carbon acts as a reducing agent and reacts with metal
oxides to generate carbon dioxide and free metal. The silica and
limestone serve as fluxing agents. This process yields a metal product
for reuse and reduces the concentration of metals in the residuals and,
hence, the amount of waste that needs to be land disposed. An example
HTMR reaction is the recovery of zinc, which proceeds as follows:
2 ZnO + C - 2 Zn + C02
15.3 Description of High Temperature Metals Recovery Process
The HTMR process consists of a mixing unit, a high temperature
processing unit (kiln, furnace, etc.), a product collection system, and a
residual treatment system. A schematic diagram for a high temperature
metals recovery system is shown in Figure 15-1.
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
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AIR OR O0
EXHAUST AIR OR GAS
TO THE
ATMOSPHERE
1
1
WASTE INFLUENT ^
CARBON ^
(REDUCING AGENT)
FLUXES ^
II IMPSTONF SANnt
MIXING
UNIT
^
HIGH
TEMPERATURE
PROCESSING
UNIT
PRODUCT
COLLECTION
UNIT
(CONDENSOR OR
CONDENSOR
AND BAGHOUSE)
VOLATILE
METAL
PRODUCTS
TO REUSE
OR FURTHER
REFINEMENT
PRIOR TO
REUSE
RESIDUAL
COLLECTION
(QUENCH TANK)
NON-VOLATILE METAL PRODUCTS TO REUSE,
FURTHER RECOVERY IN A FURNACE,
STABILIZATION FOLLOWED BY LAND DIPOSAL,
OR DIRECTLY TO LAND DISPOSAL
Figure 15-1. High Temperature Metals Recovery System.
-------
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
2552T), 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,
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
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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.
15.4 Waste Characteristics Affecting Performance (WCAPs)
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.
15.4.1 Concentration of Undesirable Volatile Metals
Because HTMR is a recovery process, the product must meet certain
purity requirements prior to reuse. If the waste contains other volatile
metals, such as arsenic or antimony, which are difficult to separate from
the desired metal products and whose presence may affect the ability to
reuse the product or refine it for subsequent reuse, HTMR may not be an
appropriate technology. If the concentration of undesirable volatile
metals in the untested waste is significantly higher than that in the
tested waste, the system may not achieve the same performance and other,
more applicable treatment technologies may need to be considered for
treatment of the untested waste.
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15.4.2 Metal Constituent Boiling Points
The greater the ratio of volatility of the waste constituents, the
more easily the separation of these constituents can proceed. This ratio
is called relative volatility. EPA recognizes, however, that the
relative volatilities cannot be measured or calculated directly for the
types of wastes generally treated by high temperature metals recovery.
This is because the wastes usually consist of a myriad of components, all
with different vapor pressure-versus-temperature relationships. However,
because the volatility of components is usually inversely proportional to
their boiling points (i.e., the higher the boiling point, the lower the
volatility), EPA uses the boiling point of waste components as a surrogate
waste characteristic for relative volatility. If the 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.
15.4.3 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
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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.
15.5 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.
15.5.1 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
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being recovered, but not excessively high so as to volatilize other
unwanted constituents) and to diagnose operational problems.
15.5.2 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.
15.5.3 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.
15.5.4 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.
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15.5.5 Calcium-to-Silica Ratio of the Feed
The calcium-to-silica ratio in the feed blend must be controlled to
limit precipitation of metallic iron in the high temperature processor.
The iron forms as solid calcium iron silicate, which is very difficult to
subsequently process into any useful material. Aluminum oxide will also
undergo reactions with lime and silica to form calcium aluminosilicates,
which will lower the density and increase the volume of slag generated.
Precipitates and modified slags affect the reduction, volatilization,
and recovery of volatile metals by changing heat flow characteristics in
the system and by undergoing secondary, high temperature chemical
reactions with metal oxides in the feed, converting them to the
previously noted inert metal silicates or silicoaluminates. The ratio of
calcium to silica to be used is dependent on the waste composition.
Generally, a one-to-one silica-to-calcium oxide ratio is highly desired,
so amounts of limestone and sand need to be adjusted based on the calcium
and silica content of the waste to achieve this ratio.
Excess lime may also be added to fix sulfur in the feed as calcium
sulfate. This will prevent the volatile metals from reacting with sulfur
to form metal sulfides, thereby lowering the recovery of metals or
oxides. EPA monitors the amounts of limestone and sand added to the
waste to ensure that the calcium-to-silica ratio selected to maximize
metal or oxide recovery is maintained during treatment.
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15.6 References
Center for Metals Production. 1985. Electric arc furnace dust--
disposal. recycle and recovery. Pittsburgh, Pa.
Duby, P. 1980. Extractive metallurgy. In Kirk-Othmer encyclopedia of
chemical technology. 3rd ed. Vol. 9, p. 741. New York: John Wiley
and Sons.
Lloyd, T. 1980. Zinc compounds. In Kirk-Othmer encyclopedia of
chemical technology. 3rd ed. Vol. 24, p. 824. New York: John Wiley
and Sons.
Maczek, H., and Kola, R. 1980. Recovery of zinc and lead from
electric furnace steelmaking dust at Berzelius. Journal of Metals
32:53-58.
Price, L. 1986. Tensions mount in EAF dust bowl. Metal Producing.
February 1986.
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16. ION EXCHANGE
16.1 Applicability
Ion exchange is a treatment technology applicable to (1) metals in
wastewaters where the metals are present as soluble ionic species (e.g.,
+3 -2
Cr and CrO, ); (2) nonmetallic anions such as halides, sulfates,
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.
16.2 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.
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 (C00~) groups. Anion
exchange resins have immobile basic ions, such as amine (NH2~), to
16-1
34008
-------
which the mobile anions, such as hydroxyl (OH ) or chloride (Cl ),
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 ions back into solution. This
relatively low volume solution, now highly concentrated with the
contaminant ions, must then be treated prior to disposal for recovery or
removal of the hazardous cation or anion contaminants. There will
continue to be a high concentration of the regenerant ion (sodium in the
above example) in the used regenerant solution because excess regenerant
ion is necessary to force the contaminant ions back into solution. The
direction and extent of the completion of the exchange reaction depend
upon the equilibrium that is established between the ions in the solution
(MX) and those in the exchange material (R N ).
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16.3 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 is presented in Figure 16-1. The ion exchange system
shown in this schematic includes a series treatment with separate cation
and anion exchange systems. Some systems contain both anion and cation
exchange resins in the same vessel.
The pressure vessels used for ion exchange generally range in size
from 2 to 6 feet in diameter for prepackaged modular systems, which
typically handle 25- to 300-gpm flow rates, to a maximum custom size of
12 feet in diameter, which can handle flow rates up to 1,150 gpm. The
height of these vessels varies between 6 and 10 feet to provide adequate
resin storage, distribution nozzle layout, and freeboard capacity for bed
expansion during backwashing. The nominal surface loading area of the
ion exchange vessels ranges from 8 to 10 gpm per square foot.
16-3
3*008
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USED BACKFLUSH WATER
TO TREATMENT
USED BACKFLUSH WATER
TO TREATMENT
WASTEWATER
INFLUENT
OJ
BACKFLUSH WATER
RINSE WATER
ACID REGENERANT
BACKFLUSH
WATER"
CATION
EXCHANGE
SYSTEM
USED REGENERANT
SOLUTION AND RINSES
TO RECOVERY
AND/OR TREATMENT
RINSE WATER
CAUSTIC
REGENERANT
ANION
EXCHANGE
SYSTEM
USED REGENERANT
SOLUTION AND RINSES
TO RECOVERY
AND/OR TREATMENT
TREATED
EFFLUENT
TO FURTHER
TREATMENT
AND/OR DISPOSAL
Figure 16-1. Two Step Cation/Anion Ion Exchange System.
-------
16.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether ion exchange will achieve the same level of
performance on an untested waste as on a previously tested waste and
whether performance levels can be transferred, EFA 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.
16.4.1 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
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.
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16.4.2 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
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.
16.4.3 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,
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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.
16.4.4 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.
16.4.5 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.
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16.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of an ion
exchange system, EPA examines the following parameters: (a) the amount
and type of resin, (b) the amount and type of regenerant solution,
(c) the hydraulic loading, and (d) the exchange temperature.
16.5.1 Amount and Type of Resin
The main design parameter that affects the performance of ion
exchange systems is the amount and type of resin used. Numerous cationic
and anionic resins are commercially available. The selection of a resin
is based on a variety of factors. Different resins have different
exchange capacities, and some have greater affinity than others for
specific ions. Certain resins are designed to tolerate corrosive,
oxidizing, or high temperature solutions, so their exchange capacity does
not degrade as rapidly with use. Most resins will effectively remove
contaminant ions from solution until they become exhausted. However, if
resin bed exhaustion occurs too frequently, or if regeneration requires
excessive volumes of the regenerant, the type and/or amount of resin
might need to be changed. In some instances, pretreatment technologies
may be required prior to ion exchange. For most metals removal, cation
resins are usually required. However, some metal complexes, such as
-2 -2
copper cyanide (Cu(CN), ), chromates (CrO, ), and arsenates
(AsO, ), are anionic and require the use of anion exchange resins.
EPA examines the amount and type of ion exchange resin in the treatment
system to ensure that a sufficient amount of ion exchange resin is
provided to effectively exchange the metal ions of concern.
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16.5.2 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.
16.5.3 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.
16.5.4 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.
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16.6 References
De Renzo, D.J., ed. 1978. Unit operations for treatment of hazardous
industrial wastes. Park Ridge, N.J.: Noyes Data Corporation.
Dorfner, K. 1972. Ion exchangers: properties and applications.
Ann Arbor, Mich.: Ann Arbor Science Publishers, Inc.
Metcalf & Eddy, Inc. 1979. Wastewater engineering: treatment disposal
reuse. 2nd ed. New York: McGraw-Hill Book Co.
Patterson, J.W. 1985. Industrial waste treatment technology. 2nd ed.
Stoneham, Mass. Butterworth Publishers.
Perry, R.H., and Chilton, C.H. 1973. Chemical engineers' handbook.
5th ed. New York: McGraw-Hill Book Co.
Sundstrom, D.W., andKlei, H.E. 1979. Wastewater treatment. Englewood
Cliffs, N.J.: Prentice-Hall, Inc.
USEPA. 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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17. RETORTING
17.1 Applicability
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 10008F) 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.
17.2 Underlying Principles of Operation
The basic principle of operation for retorting is a process similar
to that for high temperature metals recovery in that it provides for
recovery of metals from wastes primarily by volatilization and subsequent
collection and condensation of the volatilized components. Retorting
yields a metal product for reuse and significantly reduces the
concentration of metals in the waste residual, and, hence, the amount of
treated waste that needs to be land disposed. This technology is
different from HTMR in that HTMR includes a reduction reaction involving
the use of carbon, while retorting does not use a reducing agent.
Additionally, this process differs with regard to the form and, possibly,
the leachability of the residue generated; HTMR generates a slag, while
retorting generates a granular solid residue that may have lower
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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 + 2H20 + Oj.
17.3 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 17-1 and 17-2 show a retort system without and with a
scrubber-type air pollution control system.
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PREHEATED
AIR
RETORT
~t
T
CONDENSER
COOLING
WATER
WASTE
INFLUENT
TREATED RESIDUAL WASTE TO STABILIZATION
AND/OR LAND DISPOSAL
METAL
COLLECTION
VOLATILE METALS
TO REUSE
EXHAUST AIR
TO THE ATMOSPHERE
| AIR |
CONTROL
| SYSTEM |
r
)
V FAN
>**« , *
STACK
Figure 17-1. Retorting Process (Without a Scrubber and Subsequent Wastewater Discharge).
-------
WATER
VENTURI
SCRUBBER
PREHEATED
AIR
RETORT
1
T
DECANTER
WASTE
INFLUENT
TREATED RESIDUAL WASTE TO STABILIZATION
AND/OR LAND DISPOSAL
EXHAUST AIR
TO THE ATMOSPHERE
STACK
-*» WASTEWATER TO TREATMENT
VOLATILE
METALS
TO REUSE
Figure 17-2. Retorting Process (With a Scrubber and Subsequent Wastewater Discharge).
-------
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 (S0~), 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.
17.4 Waste Characteristics Affecting Performance (WCAPs)
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.
17.4.1 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
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metal(s) to be recovered, they will be volatilized and condensed along
with the desired metal(s) present in the waste. These constituents may
be difficult to separate from the recovered product and may affect the
ability to reuse the product metal or refine it for subsequent reuse.
Undesirable volatile constituents sometimes present in mercury-bearing
wastes include (a) mercury chlorides, which distill unchanged in the
retorting process, and other volatile metal halides; (b) arsenic oxide
and arsenic trichloride; and (c) organomercury compounds, such as
phenylmercuric acetate, which are not decomposed to elemental mercury by
the retorting process.
For wastes containing significant levels (i.e., above 1 percent) of
these contaminants, retorting may not be an appropriate technology. If
the concentration of undesirable volatile constituents in the untested
waste is significantly higher than that in the tested waste, the system
may not achieve the same performance. Chemical pretreatment may be
required to convert mercury chlorides and organomercury compounds to
mercuric sulfide and/or elemental mercury and achieve the same treatment
performance, or other, more applicable treatment technologies may need to
be considered for treatment of the untested waste.
17.4.2 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
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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.
17.5 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.
17.5.1 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.
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17.5.2 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
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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17.6 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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18. STABILIZATION OF METALS
18.1 Applicability
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.
18.2 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 leachability 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.
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The two principal stabilization processes used are cement-based and
lime/pozzolan-based processes. A brief discussion of each is provided
below. In both cement-based or lime/pozzolan-based techniques, the
stabilizing process can be modified through the use of additives, such as
silicates, that control curing rates, reduce permeability, and enhance
the properties of the solid material.
18.2.1 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.
18.2.2 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
18-2
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in the waste are converted to insoluble silicates or hydroxides and are
incorporated into the interstices of the binder matrix, thereby
inhibiting leaching.
18.3 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 is recovered from the
processing equipment and disposed of.
18.4 Waste Characteristics Affecting Performance (WCAPs)
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.
18-3
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18.4.1 Concentration of Fine Participates
For both cement-based and lime/pozzolan-based processes, very fine
solid materials (i.e., those that pass through a No. 200 mesh sieve (less
than 74 um particle size)) weaken the bonding between waste particles and
the cement or lime/pozzolan binder by coating the particles. This
coating inhibits chemical bond formation, thereby decreasing the
resistance of the material to leaching. If the concentration of fine
particulates in an untested waste is significantly higher than 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.
18.4.2 Concentration of Oil and Grease
Oil and grease in both cement-based and lime/pozzolan-based systems
results in the coating of waste particles and the weakening of the
bonding between the particle and the stabilizing agent, thereby
decreasing the resistance of the material to leaching. If the
concentration of oil and grease in the untested waste is significantly
higher than 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.
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18.4.3 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. Fretreatment 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.
18.4.4 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
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.
18.5 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.
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18.5.1 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
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.
18.5.2 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;
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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 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 deice is one that could be expected to achieve
homogeneous distribution of the waste, stabilizing agent, and additives.
18.5.3 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.
18.5.4 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
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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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18.6 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
Eneineerine. Nov. 10, 1986.
Cullinane, M.J., Jr., Jones, L.W., andMalone, 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, D.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. Solid-Waste Disposal: Solidification. Chemical
engineering 86(17): 141-145.
USEPA. 1980. U.S. Environmental Protection Agency. U.S. Army Engineer
Waterways Experiment Station. Guide to the disposal of chemically
stabilized and solidified Waste. Prepared for MERL/ORD under
Interagency Agreement No. EPA-IAG-D4-0569. PB 81 181 505.
Cincinnati, Ohio.
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19. CHEMICAL OXIDATION
19.1 Applicability
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 EDTA, NTA, citric acid, glutaric acid,
lactic acid, and tartrates are often used as chelating agents to prevent
metal ions from precipitating out in electroless plating solutions. When
these spent plating solutions require treatment for metals removal by
chemical precipitation, the organic chelating agents must first be
destroyed. Chemical oxidants, potassium permanganate in particular, are
effective in releasing metals from complexes with these organic compounds.
19.2 Underlying Principles of Operation
The basic principle of operation for chemical oxidation is that
inorganic cyanides, some dissolved organic compounds, and sulfides can be
chemically oxidized to yield carbon dioxide, water, salts, simple organic
acids, and, in the case of sulfides, sulfates. The principal chemical
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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 (CaCOClK), chlorine gas (CU) , or
sometimes chlorine dioxide gas (CIO-). 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 + 14NaCl
Sulfide: S" + 4NaOCl - S04" + 4NaCl
Chlorine dioxide also oxidizes the same pollutants under identical
conditions. Chlorine dioxide first hydrolyzes to form a mixture of
chlorous (HC102) and chloric (HClOj) acids. These acids act as the
oxidants, as shown in the equations below, for phenol:
2C102 + H20 - HC102 -i- HC103
C6H5OH + 7HC102 -» 6C02 + 3H20 + 7HC1
3C6H5OH + 14HC103 -* 8C02 + 9H20 + 14HC1
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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: C6H5OH + 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
. 2 2
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 +0 -» CNO +0
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.
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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 + 28Mn02 + 23H20 + 28K+
Sulfide: 5S~ + 8KMn04 + 24H+ - 5S04" + 8Mn+2 + 12H20 + 8K+
Cyanide: CN" + 2KMn04 + Ca(OH)2 - CNO' + K2Mn04 + CaMn04 + H20
In cyanide oxidation using potassium permanganate, cyanide is oxidized
only to cyanate. Further oxidation of cyanate can be accomplished by
acid hydrolysis or by the use of another oxidizing agent.
19.2.5 S02/Air Oxidation
Cyanide can be oxidized to cyanate in an aqueous solution by bubbling
air containing from 1 to 10 percent SOj through the waste. The S02
is also oxidized to sulfate in this reaction. This treatment process
occurs by the following reaction:
CN' + S02 + 02 + H20 - CNO' + H2S04
This oxidation reaction requires the use of a soluble copper salt
catalyst. Copper sulfate (CuSO,) is most often used. S0~/air
oxidation is used frequently in the treatment of wastewaters from gold
production, which contain both cyanide and thiocyanate, because SO^/air
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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 Alkaline Chlorination
Alkaline chlorination can be accomplished by either batch or
continuous processes. For batch treatment, the wastewater is transferred
to a reaction tank, where the pH 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.
19.3.2 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.
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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 is maintained and also that no
ozone is released to the atmosphere. As with alkaline chlorination, an
ORP control system is usually necessary to ensure that sufficient ozone
is being added.
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
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.
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19.3.5 S02/Air Oxidation
S02/air oxidation of cyanide depends on efficient mixing of air
with the waste to ensure an adequate supply of oxygen. Because of this
factor, the equipment requirements for this process are similar to those
of ozonation. S0« is sometimes supplied with the air by using flue gas
containing S0~ as the air source. Otherwise, sulfur in the +4 oxidation
state can be fed as gaseous sulfur dioxide (S0~), liquid sulfurous acid
(H«SO^), sodium sulfite (Na^SOO solution, or sodium bisulfite
(NaHSO.) solution. Sodium bisulfite solution, made by dissolving
sodium metabisulfite (Na2S20,) 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 (WCAPs)
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.
19.4.1 Concentration of Other Oxidizable Compounds
The presence of other oxidizable compounds in addition to the
constituents of concern will increase the demand for oxidizing agents
and, hence, potentially reduce the effectiveness of the treatment process.
As a surrogate for the amount of oxidizable organics present, EPA analyzes
for total organic carbon (TOC) in the waste. Inorganic reducing compounds
such as sulfide may also create a demand for additional oxidizing agent;
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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.
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.
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.
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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 chat sufficient time is provided to effectively
oxidize the waste.
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
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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
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 C^N gas. The pH is controlled by the addition of
caustic, lime, or acid to the solution. In most cases, a slightly or
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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 chlorination treatment of organics,
a slightly acidic pH may be selected as an optimum. In permanganate
oxidation, a pH of 2 to 4 is often selected. EPA monitors the pH
continuously, if possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
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.
19.5.6 Amount and Type of Catalyst
Adding a catalyst that promotes oxygen transfer and thus enhances
oxidation has the effect of lowering the necessary reactor temperature
and/or improving the level of destruction of oxidizable compounds. For
waste constituents that are more difficult to oxidize, catalyst addition
may be necessary to effectively destroy the constituent(s) of concern.
Catalysts typically used for this purpose include copper bromide and
copper nitrate. If a catalyst is required, EPA examines the amount and
type added, as well as the method of addition of the catalyst to the
waste, to ensure that effective oxidation is achieved.
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19.6 References
Gurnham, C.F. 1985. Principles of industrial waste treatment. New
York: John Wiley and Sons.
Gurol, M.D., and Holden, I.E. 1988. The effect of copper and iron
complexation on removal of cyanide by ozone. Ind. Eng. 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 SC^/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. Cams Chemical Company, 1001 Boyce
Memorial Drive, P.O. Box 1500, Ottawa, Illinois 61350.
Weast, R.C., ed. 1978. Handbook of chemistry and physics. 58th ed.
Cleveland, Ohio: CRC Press.
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20. POLISHING FILTRATION
20.1 Applicability
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
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
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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 are 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
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.
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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.
20.4 Waste Characteristics Affecting Performance fWCAPs).
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
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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 in an untested waste are significantly more gelatinous
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.
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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 in 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.
20.5.2 Filtration Pressure
Pressure impacts both the design pore size of the filter media and
the design feed flow race (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.
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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. Diatomaceous earth is the most commonly used
filter aid. The use of coagulants, flocculants, and filter aids
significantly increases che 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 co 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.
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20.5.5 Pore Size of the Filter Media
The pore size of che 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.
Grain, R.W. 1981. Solids removal and concentration. In Third
Conference on Advanced Pollution Control for the Metal Finishing
Industry, pp. 56-62. Cincinnati, Ohio: U.S. Environmental Protection
Agency.
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 Applicability
Sludge filtration, also known as sludge dewatering or cake-formation
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.
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
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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 solids "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 is
accomplished by taking the unit off-line, opening the filter, and using
an adjustable knife mechanisms to scrape the solids off (a batch
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. Dewatered
solids are further treated in processes such as incineration and solvent
extraction, if treatable levels of organics are present, stabilization
(if treatable levels of leachable metals are present), are/or disposal.
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the liquid filtrate is further treated in processes such as polishing
filtration, carbon adsorption and aerobic biological treatment, and/or
disposal.
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
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
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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 filterability
significantly. Finally, precoating the filter with diatomaceous earth
prior to sludge filtration assists in dewatering 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 precoating 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.
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
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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 is
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,
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 is 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 is operating at the appropriate
design conditions and to diagnose operational problems.
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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
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.
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
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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
Grain, R. W. 1981. Solids removal and concentration. In Third
Conference on Advanced Pollution Control for the Metal Finishing
Industry, pp. 56-62. Cincinnati, Ohio: U.S. Environmental
Protection Agency.
Eckenfelder, W.W., Jr., Patoczka, J., and Watkins, A. 1985. Wastewater
treatment, Chemical Engineering. September 2, 1985.
Kirk-Othmer. 1980. Encyclopedia of chemical technology. 3rd ed.,
Vol. 10. New York: John Wiley and Sons.
Perry, R. H., and Chilton, C. H. 1973. Chemical engineers'handbook.
5th ed., Section 19. New York: McGraw Hill Book Co.
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22. THERMAL DRYING
22.1 Applicability
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
The basic principle of operation for drying is 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 liquid constituents to be evaporated.
22.3 Description of Thermal Drying 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
material, causing water and organics to be driven off in a vapor form.
The dried cake is discharged from the dryer.
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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 (WCAPs)
In determining whether thermal drying 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 and
(b) the volatile liquid constituent boiling points.
22.4.1 Thermal Conductivity of the Waste
A major factor determining whether a particular constituent will
volatilize is the transfer of heat through the waste. For thermal
drying, heat transfer is accomplished by both convection and conduction.
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
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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.
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.
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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 lower volatilities will be
removed from the waste.
Pressure is integrally related to the boiling point of the waste and
the subsequent vaporization of the water and/or 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
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 as well as the degree of volatilization of water and volatile
organics. 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. Chemicalengineers' 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.
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23. WET AIR OXIDATION
23.1 Applicability
Wet air oxidation is a treatment technology applicable to wastewaters
containing organics and oxidizable inorganics such as cyanide. The
process is typically used to oxidize sewage sludge, regenerate spent
activated carbon, and treat process wastewaters. Wastewaters treated
using this technology include pesticide wastes, petrochemical process
wastes, cyanide-containing metal finishing wastes, spent caustic
wastewaters containing phenolic compounds, and some organic chemical
production wastewaters.
This technology differs from other treatment technologies generally
used to treat wastewaters containing organics in 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
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.
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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
Co assess potential products of incomplete oxidation to determine whether
further treatment is necessary or whether this technology is appropriate
at all.
Studies of the wet air oxidation of different compounds have led to
the following empirical observations concerning a compound's
susceptibility to wet air oxidation based on its chemical structure:
1. Aliphatic compounds, even with multiple halogen atoms, can be
destroyed within conventional wet air oxidation conditions.
Oxygenated compounds (such as low molecular weight alcohols,
aldehydes, ketones, and carboxylic acids) are formed, but these
compounds are readily biotreatable.
2. Aromatic hydrocarbons, such as toluene, acenaphthene, or pyrene,
are easily oxidized.
3. Halogenated aromatic compounds can be oxidized provided there is
at least one nonhalogen functional group present on the ring
(e.g., pentachlorophenol (-OH) or 2,4,6-trichloroaniline
(-NH2)).
4. Halogenated aromatic compounds, such as 1,2-dichlorobenzene, and
PCBs, such as Aroclor 1254, are resistant to wet air oxidation
under conventional conditions.
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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 basic principle of operation for wet air oxidation is that the
enhanced solubility of oxygen in water at high temperatures and pressures
aids in the oxidation of organics. The typical operating temperature for
the wet air oxidation treatment process ranges from 175 to 325°C (347
to 617°F). The pressure is maintained at a level high enough to
prevent excessive evaporation of the liquid phase at the operating
temperature, generally between 300 and 3000 psi. At these elevated
temperatures and pressures, the solubility of oxygen in water is
dramatically increased, thus providing a strong driving force for the
oxidation. The reaction must take place in the aqueous phase because the
chemical reactions involve both oxygen (oxidation) and water
(hydrolysis). The wet air oxidation process for a specific organic
compound generally involves a number of oxidation and hydrolysis
reactions in series, which degrade the initial compound by steps into a
series of compounds of simpler structure. Complete wet air oxidation
results in the conversion of organic compounds into carbon dioxide, water
vapor, ammonia (for nitrogen-containing wastes), sulfate (for
sulfur-containing wastes), and halogen acids (for halogenated wastes).
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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 to disposal or fed to a biological treatment, carbon adsorption,
or chemical oxidation treatment system if further treatment is necessary
prior to discharge to disposal.
Wet air oxidation can also be operated in a continuous process. In
continuous operation, the waste is pressurized, mixed with pressurized
air or oxygen, preheated in a series of heat exchangers by the hot
reactor effluent and steam, and fed to the reactor. The waste feed flow
rate controls the reactor residence time. Steam is fed into the reactor
column to adjust the column temperature. The treated waste is separated
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PRESSURIZED
WASTEWATER
INFLUENT
1
to
U)
PRESSURIZED
AIR OR
OXYGEN
FEED HEAT
EXCHANGERS
REACTOR
STEAM
VENT GASES TO
AIR POLLUTION
CONTROL SYSTEM
AND/OR THE
ATMOSPHERE
STEAM
GAS-LIQUID
SEPARATOR
TREATED
EFFLUENT
TO FURTHER
TREATMENT
AND/OR
DISPOSAL
Figure 23-1. Continous Wet Air Oxidation System.
-------
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 on the solubility of oxygen in the aqueous waste
and the rate of dissolution of oxygen from the gas phase to the liquid
phase. This sets an upper limit on the amount of oxidizable compounds
that can be treated by wet air oxidation. Thus, high-COD wastes may
require dilution for effective treatment to occur. If the COD of the
untested waste is significantly higher than that of the tested waste, the
system may not achieve the same performance. Pretreatment of the waste
or dilution as part of treatment may be needed to reduce the COD to
within levels treatable by the dissolved oxygen concentration and to
achieve the same treatment performance, or other, more applicable
treatment technologies may need to be considered for treatment of the
untested waste.
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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 in wastes for which copper catalysts are necessary for
effective treatment. If an untested waste for which a copper catalyst is
necessary for effective treatment has a concentration of interfering
substances (including sulfide, carbonate, or other anions that form
insoluble copper salts) significantly higher than that in a tested waste,
the system may not achieve the same performance and other, more
applicable treatment technologies may need to be considered for treatment
of the untested waste.
23.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a wet
air oxidation system, EPA examines the following parameters: (a) the
oxidation temperature, (b) the residence time, (c) the excess oxygen
concentration, (d) the oxidation pressure, and (e) the amount and type of
catalyst.
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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 is controlled
directly by adjusting the treatment time in the reaction tank. For a
continuous system, the waste feed rate is controlled to make sure that
the system is operated at the appropriate design residence time.
Generally, the reaction rates are relatively fast for the first 30
minutes and become slow after 60 minutes. Typical residence times,
therefore, are approximately 1 hour. EPA monitors the residence time to
ensure that sufficient time is provided to effectively oxidize the waste.
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 COD content of the untreated waste; excess
oxygen should be supplied to ensure complete oxidation. The source of
oxygen is compressed air or a high-pressure pure oxygen stream. EPA
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monitors the excess oxygen concentration (the concentration of oxygen in
the gas leaving the reactor) continuously, if possible, by sampling the
vent gas from the gas-liquid separator to ensure that an effective amount
of oxygen or air is being supplied to the waste.
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, if possible, to ensure that the system is operating at the
appropriate design condition and to diagnose operational problems.
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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24. HIGH-TEMPERATURE STABILIZATION TECHNOLOGIES
24.1 Applicability
High-temperature stabilization technologies include glass and slag
vitrification and elevated-temperature calcination processes.
Vitrification processes involve dissolving the waste at high temperatures
into glass or a glasslike matrix. Calcination involves merely heating
the material at high temperatures.
High-temperature vitrification is applicable to nonwastewaters
containing arsenic* or other characteristic toxic metal constituents that
are relatively nonvolatile at the temperatures at which the process is
operated. This technology is also applicable to many wastes containing
organometallic compounds, where the organic portion of the compound can
be completely oxidized at process-operating conditions.
The process is not generally applicable to volatile metallic
compounds or to wastes containing high levels of constituents that will
interfere with the vitrification process. High levels of chlorides and
other halogen salts should be avoided in the wastes being processed
because they interfere with glassmaking processes and cause corrosion
problems.
Calcination processes are applicable to inorganic wastes that do not
contain volatile constituents.
* Volatile arsenic compounds are usually converted to nonvolatile
arsenate salts such as calcium arsenate prior to use of this process.
In ordinary glassmaking, arsenic volatilization problems are minimized
by adding arsenic as arsenate salts.
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24.2 Underlying Principles of Operation
The basic principles of operation for high-temperature stabilization
technologies depend on the technology used. In glass and slag
vitrification processes, the waste constituents become chemically bonded
inside a glasslike matrix in many cases. In all instances, the waste
becomes surrounded by a glass matrix. This acts to immobilize the waste
constituents and to retard or prevent their reintroduction into the
environment. Arsenates are converted to silicoarsenates, and other
metals are converted to silicates.
High-temperature calcination processes remove water of hydration from
the toxic metal-bearing solids, convert hydroxides present to oxides, and
sinter the material, reducing its surface area to a minimum. Conversion
of hydroxides to oxides and minimization of available surface area retard
surface reactions that would reintroduce the material into the
environment. Calcination may also be accompanied by chemical reaction if
a material such as lime is blended with the waste before it is heated.
For example, lime will react with arsenic oxides at higher temperatures
to form calcium arsenate, and this material will then be sintered at
elevated temperatures. Brief descriptions of each of the high-tempera-
ture processes are given below.
24.2.1 Glass Vitrification
In the glass vitrification process, the waste and normal glassmaking
constituents are first blended together and then fed to a glassmaking
furnace, where the mixed feed materials are introduced into a pool of
molten glass. The feed materials then react with each other to form
additional molten glass, in which particles of the waste material become
dissolved or suspended. The molten glass is subsequently cooled. As it
cools, it solidifies into a solid mass that contains the dissolved and/or
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suspended waste constituents. Entrapment and chemical bonding within the
glass matrix render the waste constituents unavailable for reaction.
24.2.2 Slag Vitrification
Slag vitrification differs from glass vitrification in that finely
ground slag from metal-refining processes and waste are premixed and fed
to the same type of furnace as that used for glassmaking. The slag
liquifies at the process temperature (1100° to 1200°C), and the
waste constituents either dissolve or become suspended in the molten
slag. Subsequent cooling of the slag causes it to solidify, trapping the
waste inside a glasslike matrix and rendering it unavailable for chemical
reaction or migration into the environment.
24.2.3 High-Temperature Calcination
In the high-temperature calcination process, the waste is heated in a
furnace or kiln to between 400" and 800°C. In some instances,
the waste may be blended with lime prior to heating. In those cases,
chemical reaction may occur during the calcining process. Water present
as either free water or water of hydration is evaporated, and hydroxides
present are thermally decomposed to the corresponding oxides and water
vapor. At the higher temperatures, the surface area of the dehydrated
material is decreased by thermal sintering. Conversion of hydroxides to
oxides and substantial losses of surface area render the material less
reactive in the environment and lower the leachability of characteristic
toxic metals present. In general, the higher the calcination
temperatures used, the more complete the loss of water is and the greater
the accompanying loss of surface area is, resulting in lower leachability
potential.
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24.3 Description of the High-Temperature Stabilization Processes
This chapter discusses three types of high-temperature stabilization
processes, which differ considerably from each other. Individual process
descriptions are given in the following subsections.
24.3.1 Glass Vitrification
Soda ash, lime, silica, boron oxide, and other glassmaking
constituents are first blended with the waste to be treated. The amount
of waste added to the blend is dependent on the waste composition.
Different metal oxides have differing solubility limits in glass
matrices. The blended waste and glass raw material mixture is then fed
to a conventional, heated glass electric furnace.
The introduced material typically is added through a port at the top
of the furnace and falls into a pool of molten glass. The glass
constituents dissolve in the molten glass and form additional glass.
Molten glass is periodically withdrawn from the bottom of the furnace and
c'ooled. This material then solidifies on cooling into solid blocks of
glasslike material. Organics present in the feed mixture undergo
combustion at the normal operating temperatures of 1100° to
1400°C and are fully oxidized to carbon dioxide and water vapor.
The top of the furnace is normally cooled so that volatile materials,
such as arsenic oxides, that are present in the feed mixtures can
condense on the cooled surface and fall back into the melt, where they
can undergo chemical reaction to form silicoarsenates involved in the
glassmaking process. Most of the arsenic used in making glass by this
method is present as salts such as calcium arsenate. This approach was
introduced into the glass industry to minimize fugitive arsenic losses.
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Gases, such as carbon dioxide, that are liberated during the
glassmaking process exit the furnace through the top and are generally
wet-scrubbed prior to reentering the atmosphere.
24.3.2 Slag Vitrification
The slag vitrification process is basically similar to glass
vitrification except that granulated slag, instead of the normal
glassmaking constituents, is blended with the waste for feed to the
system. A pool of liquid slag is present in the furnace, and the blended
raw material mix typically is introduced at the top of the furnace and
falls into this molten slag. The granulated slag-waste mixture liquifies
to form additional slag. Slag is periodically withdrawn from the slag
pool and cooled into blocks.
The type of furnace used for glass vitrification can also be used for
slag vitrification. The operating parameters are similar.
24.3.3 High-Temperature Calcination
In the high-temperature calcination process, wastes containing
inorganic compounds are fed to ovens or kilns, where they are heated to
high temperatures (i.e., 500° to 900°C) to drive off water of
hydration and to convert hydroxides present to the corresponding oxides.
This process is primarily applicable to inorganic wastes that contain
nonvolatile constituents.
The waste is heated in the oven or kiln to the desired temperature,
moisture and water of hydration are driven off, and, at the 500° to
900°C temperature range, hydroxides decompose to the corresponding
oxides and water vapor. The high-temperature treatment also
significantly reduces the surface areas of the oxides formed by
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sintering, thereby reducing the reactivity of the material. After the
waste material has been calcined at an elevated temperature, it is
withdrawn from the oven or kiln, cooled, and either land disposed or
forwarded to another process, such as stabilization, for further
treatment.
24.4 Waste Characteristics Affecting Performance (WCAPs)
The waste characteristics affecting performance are different for the
two vitrification processes and the calcination process. Accordingly,
they are discussed separately in the following subsections.
24.4.1 Waste Characteristics Affecting Performance of Vitrification
Processes
In determining whether vitrification will achieve the same level of
performance on an untested waste as on a previously tested waste, and
whether performance levels can be transferred, EPA examines the following
waste characteristics that affect performance of the vitrification
processes: (1) organic content of the waste, (2) concentrations of
specific metal ions in the waste, (3) concentrations of compounds in the
waste that interfere with the glassmaking process, and (4) moisture
content of the waste.
(1) Organic content. At process operating temperatures
(1100° to 1400°C), organics are combusted to carbon dioxide,
water, and other gaseous products. The combustion process liberates
heat, reducing the external energy requirements for the process.
The amount of heat liberated by combustion is a function of the Btu
value of the waste. The Btu content merely changes the energy input
needs for the process and does not affect waste treatment performance.
The amount of material that may not oxidize completely is a function of
the organic halogen content of the waste. The presence of these
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halogenated organics does impact process performance because sodium
chloride has a low solubility in glass. The presence of high chlorides
results in a porous glass that is undesirable. If the halogenated
organic content of an untested waste is the same as or less than that
present in an already tested waste, the system should achieve the same
performance for organic destruction.
(2) Concentrations of specific metal ions. Most metal oxides
have solubility limits in glass matrices. Hence, their concentration
determines the amount of glass-forming materials or slag with which they
must be reacted in this process to generate a nonleaching slag or glass.
The solubility limits of most common metal oxides and salts in glass are
found in the Handbook of Glass Manufacture and other treatises on glass
production. Oxides for which extensive solubility information is
available are alumina, antimony oxide, arsenic oxides, barium oxide,
cadmium oxide, chromium oxides, copper oxides, cobalt oxides, iron
oxides, lead oxides, manganese oxides, nickel oxides, selenium oxides,
tin oxides, and zinc oxides. Analysis for individual metal
concentrations in the waste can be performed according to EPA-approved
methods. If the concentrations of specific metals in an untested waste
are less than those in a tested waste, then the same ratio of slag or
glass raw materials to waste may be used for vitrification purposes. If,
however, the concentration of metal is greater than that in the tested
waste, a different formulation must be used.
(3) Concentrations of deleterious materials. Some waste
constituents, such as chlorides, fluorides, and sulfates, interfere with
the vitrification process if they are present at high levels. These
salts have limited solubilities in glass; therefore, when they are
present, additional glass-forming raw materials must be added to
compensate for their presence. The solubility limits of various salts in
glasses are discussed in references on glass production such as the
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Handbook of Glass Manufacture. Generally, if the concentrations of such
materials in an untested waste are lower than those in a tested waste,
then the same ratio of glass-forming constituents to waste may be used.
Reducing agents such as carbon or ferrous salts reduce arsenates and
selenates to lower valence compounds that are more volatile. These
properties should be avoided when arsenic or selenium is vitrified.
(4) Moisture content. Materials fed to the vitrification
process should be reasonably dry (i.e., contain less than 5 percent free
moisture). If a waste has excess moisture above this level, it should be
thermally dried before it is blended with glass-forming materials,
otherwise, it may react violently when introduced to the molten glass or
slag pool.
24.4.2 Waste Characteristics Affecting Performance of High-Temperature
Calcination
In determining whether high-temperature calcination will achieve the
same level of performance on an untested waste as on a previously tested
w.aste, and whether performance levels can be transferred, EPA examines
the following waste characteristics that impact the performance of the
high-temperature calcination process: (1) the organic content of the
waste, (2) the moisture content of the waste, and (3) the inorganic
composition of the waste. These characteristics are discussed below.
(1) Organic content. Calcination temperatures normally used are
too low to initiate combustion of some types of organic compounds.
However, they are high enough to cause volatilization of organics, which
have to be removed from process off-gases. For these reasons, the
presence of significant levels of organics is undesirable but can be
handled with appropriate air pollution controls. Organics, per se, do
not interfere with the conversion of oxides to hydroxides or with
sintering processes. Afterburners may be required on vitrification units
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managing high organic content wastes to ensure complete combustion of the
organics present.
(2) Moisture content. Excess water that has to be removed from
the waste by heating increases the amount of time needed to bring the
waste to the calcination temperature. For this reason, wastes with high
moisture content should be dewatered prior to the use of this process.
(3) Inorganic composition. Calcination temperatures are
normally selected based on the temperatures at which hydroxides are
thermally decomposed to the corresponding oxides and water vapor. To
select an optimum operating temperature, one should know the approximate
composition of the waste.
A few toxic metal oxides have fairly low volatilization
temperatures. Arsenic oxide, selenium dioxide, and mercuric oxide all
volatilize below 500°C. High-temperature calcination should not be
used for wastes that contain these volatile constituents unless they are
blended with materials such as lime, which will react with them before
they can vaporize. Nonvolatile arsenic compounds such as ferric and
calcium arsenates can be calcined without concern for vaporization of
material.
24.5 Design and Operating Parameters
The design and operating parameters for the vitrification processes
are similar to each other, but differ considerably from those for
high-temperature calcination. The following subsections discuss these
two technologies separately.
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24.5.1 Design and Operating Parameters for Vitrification Processes
In assessing the effectiveness of the design and operation of a
vitrification system, EPA examines the following parameters: (1) the
composition of the vitrifying agent, (2) the operating temperature,
(3) the residence time, and (4) the vitrification furnace design.
(1) Composition of the vitrifying agent. Slag and various
glassmaking formulations are used as vitrifying agents. The choice of
the vitrifying agent is determined by the solubility of the waste
constituents to be vitrified. Different inorganic oxides have differing
solubilities in various glass matrices.
For slags, the presence of carbon or other reducing agents is
undesirable when arsenic-bearing or selenium-bearing wastes are
vitrified. Carbon or ferrous salts in the slag reduce arsenates in the
waste to arsenic trioxide, which has a low volatilization temperature.
In a similar manner, these same reducing agents reduce selenates to
elemental selenium, which also has a low volatilization temperature.
When slags are used for vitrifying arsenic-bearing and selenium-bearing
wastes, EPA monitors the composition of the granulated slags to ensure
that they do not contain significant concentrations of carbon or ferrous
salts.
In glass vitrification, various glassmaking formulations can be
used. EPA examines the proposed formulations to ensure that the toxic
metal ion concentrations of the final product do not exceed solubility
limits. Hence, EPA examines the material balances based on waste
composition and glassmaking additives and the published solubility limits
for metal oxides in various glasses to ensure that the vitrified product
is indeed a glass containing the solubilized toxic waste constituents.
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(2) Operating temperature. Vitrification furnaces are normally
operated in the 1100° to 1400°C range. The exact operating
temperature
is usually selected based on the desired composition of the final product.
Furnaces are normally equipped with automatic temperature control systems.
EPA examines the basis of choice for operating temperatures and the
nature and physical condition of the temperature monitoring and control
equipment to ensure that the system is being properly operated.
(3) Residence time. Sufficient time must be allowed for the
materials added to glass furnaces to reach operating temperatures and
then undergo the chemical reactions needed to produce glasses. Residence
times are normally on the order of 1 to 2 hours for processes operated at
1100° to 1200°C. For glasses or slags requiring slightly higher
temperatures, slightly longer residence times are usually selected. EPA
examines the basis for the facility's choice of residence times to ensure
that the system is well operated.
(4) Vitrification furnace design. Vitrification furnaces
normally incorporate the following design features:
Withdrawal of the product in liquid form from the base of the
furnace.
Maintenance of a liquid pool of product in the furnace.
Addition of product constituent mix at the top of the furnace.
Design of the top area of the furnace in a manner that allows
for cooling of this area. (This is important because volatile
constituents of the input feed may vaporize from the melt. The
cool top area allows these constituents to condense and fall back
into the melt.)
Presence and proper operation of an air emissions control
afterburners and scrubbing system to manage vent gas emissions
from the system such as hydrogen chloride vapors from combustion
of any chlorinated organics present.
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EPA examines the furnaces to be used for waste vitrification to
ensure that the design features mentioned above are present, since they
are important for proper operation of the systems.
24.5.2 Design and Operating Parameters for High-Temperature Calcination
Systems
In assessing the effectiveness of the design and operation of a
high-temperature calcination process, EPA examines the following
parameters: (1) operating temperature, (2) residence time, and (3) air
emission control units in place on the ovens or kilns used.
(1) Operating temperatures. Calcination temperatures of from
500° to 900°C have been used in industry. The calcination
temperature selected is generally a temperature above which metal
hydroxides present will decompose to the corresponding oxides. Data on
decomposition temperatures of some metal hydroxides are given in Table
B-l. The temperature chosen is normally high enough to cause extensive
sintering (surface area loss) of the oxides formed, while at the same
time not volatilizing these materials. EPA examines the technical basis
for selection of the calcining temperature to determine whether the
system is properly operated. EPA also examines the temperature
monitoring and control systems in place to determine whether they are
properly operated and reliable.
Table 24-1 Metal Hydroxide Decomposition Temperatures
Metal hydroxide Decomposition temperature (°C)
Cadmium hydroxide 300
Chromic acid 400
Lead hydroxide 145
Nickel hydroxide 230
Zinc hydroxide 125
Source: Weast 1977.
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(2) Residence time. Calcination is generally a batch process,
and sufficient time must be allowed for samples to be brought to the
operating temperature. Residence times of several hours are normally
used to minimize the effects of heat-up time. EPA examines the technical
basis for the choice of residence time to ensure that sufficient heating
at the required temperature is allowed to complete the dehydration and
sintering processes. Residence time is a function of the time needed to
bring the calcination furnace or kiln to the desired temperature and the
time needed to complete the dehydration and sintering processes at the
selected temperature.
(3) Air emissions control systems. During the calcination
process, water vapor is driven off as it is formed by the decomposition
of hydroxides present in the waste. These hot gases exit the calcination
furnace or kiln as they are formed. Some particulates of the waste
material and organics present in the waste may become entrained in these
vent gases; therefore, for air pollution control purposes, the
calcination units must be equipped with wet or dry particulate collection
systems that are properly designed and operated when processing inorganic
wastes. If wastes containing organics are processed by high-temperature
calcination, the calcination furnaces need to be equipped with
afterburners to combust organic vapors emitted. EPA examines the design
basis, physical condition, and maintenance of these air emissions control
units to determine whether they are properly designed, maintained, and
operated when processing inorganic wastes. If wastes containing organics
are processed by high-temperature calcination, the calcination furnaces
need to be equipped with afterburners to combust organic vapors emitted.
EPA also examines the system of management in use for handling and
subsequent treatment of the collected particulates to ensure that these
finely divided waste particles are properly managed.
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24.6 References
Penberthy, L. 1984. Electric melting of glass. In Handbook of glass
manufacture, Vol. I, pp. 387-399. New York: Ashlee Publishing Co.
Penberthy, L. 1981. Method and apparatus for converting hazardous
material to a relatively harmless condition. U.S. Patent 4299.611.
November 10, 1981.
Steitz, W.R., Hibscher, C.W., and Mattocks, G.R. 1984. Electric melting
of glass. In Handbook of glass manufacture, Vol. I, pp. 400-428.
New York: Ashlee Publishing Co.
Tooley, F.V. 1984. Raw materials. In Handbook of glass manufacture,
Vol. I, pp. 19-56. New York: Ashlee Publishing Co.
Twidwell, L.G., and Mehta, A.K. 1985. Disposal of arsenic bearing
copper smelter flue dust. In Nuclear and chemical waste management,
Vol. 5, pp. 297-303. Butte, Montana: Montana College of Mineral
Science and Technology.
Weast, R.C., ed. 1977. Handbook of chemistry and physics, 58th ed.
Cleveland, Ohio: CRC Press.
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25. ENCAPSULATION
25.1 Applicability
Encapsulation processes are applicable to liquid, semiliquid, or
solid wastes that may leach hazardous constituents. These processes can
be used to treat individual waste particulates or waste particulate
agglomerations. Testing has shown encapsulation to be an effective
method of treatment for a variety of hazardous wastes containing
low-level radioactive contaminants and/or heavy metals. These wastes
include sludges from electroplating operations; ion-exchange resins from
water demineralization; spent activated carbon; pesticides;
nickel-cadmium battery sludge; and pigment production sludge.
25.2 Underlying Principles of Operation
The encapsulation process involves taking waste that has been
previously dewatered and enclosing it in a coating or jacket of inert
material (such as organic polymers for asphalt). The operation of the
process requires that the material be mixed with a hardening resin,
placed in a mold, and heated until the material fuses. The resulting
material is a hard, solid block with reduced leachability potential.
In addition, the encapsulated waste exhibits stability to mechanical,
physical, and chemical stresses of handling; transportation; and final
disposal. An important advantage of encapsulation versus more
conventional stabilization/solidification techniques is the minimal
volume increase for the resin coating relative to the original waste
material. In conventional stabilization, large volumes of stabilizing
agent (e.g., portland cement or lime-pozzolanic material) are required.
The most serious drawback of encapsulation processes is the uncertainty
about their long-term effectiveness.
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25.3 Process Description
The encapsulation process of hazardous waste sludge involves two
steps. In the first step, the hazardous wastes are chemically stabilized
and solidified by using low-cost dehydrating agents such as lime, kiln
dust, or portland cement. This operation does not increase the volume of
the sludges significantly because only a small quantity of agents is
needed to dewater the sludges. The cured mixtures are friable, and they
can easily be ground. The solified particles are then agglomerated and
encapsulated. Solidification converts the sludge into a solid waste that
is free of mobile liquid.
In the second step, the solidified sludges are ground and the
particles are agglomerated, typically by a polybutadiene binder, and
encapsulated, or coated, by a high density polyethylene.
25.4 Waste Characteristics That Affect Performance
In determining whether encapsulation will be an effective method of
treatment, certain waste characteristics should be considered. These
include the compatability of the waste and the encapsulating material,
the viscosity of the waste, and the water content of the waste.
Although encapsulation is an effective treatment on many different
contaminants, some organic materials may act as solvents to the
encapsulating resin and break it down. In addition, liquid or semiliquid
wastes with a low viscosity may tend to run off during the cure stage of
the encapsulation process. Further, water is likely to impede or
interfere with the resin curing process, so wastes should first be
dewatered and perhaps even dried prior to encapsulation.
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25.5 Design and Operating Parameters
The typical apparatus for encapsulation processes features heated
and/or cooled molds, a method of waste and solidified product
manipulation, and hydraulics for mold actuation. The molds for
agglomeration and encapsulation have electrical band heaters and
drilled-in channels for water cooling. The agglomerating mold is a
cylindrical steel shell typically about 60 cm (23.5 in.) in diameter. A
device fits inside the mold body to confine the waste/binder mixture.
The solidified agglomerate is transported to the encapsulation mold,
after which the mold is split vertically to facilitate product demolding.
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25.6 References
Mackie J.A. and Niesen K. August 6, 1984. Hazardous waste management:
the alternatives. Chemical Engineering.
Pierce, J.J. and Vesiland, P.A., eds. 1981. Hazardous waste
management. Ann Arbor, Michigan: Ann Arbor Science Publishers.
Rich, G. and Cherry, K. 1984. Hazardous waste treatment technologies.
Second printing. Northbrook, Illinois: Pudvan Publishing Co.
Unger, S.L., Telles, R.W., and Lubowicz, H.W. Surface encapsulation
process for stabilizing intractable contaminants. Environmental
aspects of stabilization and solidification of hazardous and
radioactive wastes, ASTM STP 1033, P.L. Cote and T.M. Gilliam, Eds.
American Society for Testing and Materials, Philadelphia, 1989,
pp. 40-52.
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26. CHEMICAL REDUCTION
26.1 Applicability
Chemical reduction is a treatment technology used to treat oxidizing
wastes containing reducible organics, and inorganic oxidizers such as
calcium hypochlorite , hydrogen peroxide (and other peroxides), and nitric
acid. Because this technology frequently requires that the pH be in the
acidic range, it would not normally be applicable to a wastes that
contains significant amounts of cyanide or sulfide. In such wastes,
lowering of the pH can result in the release of toxic gases such as
hydrogen cyanide or hydrogen sulfide .
26.2 Underlying Principles of Operation
The basic principle of chemical reduction is to reduce the valence of
the oxidizer in solution. "Reducing agents" used to effect the reduction
include the sulfur compounds sodium sulfite (Na»SO^) , sodium
bisulfite (NaHSO-j) , sodium metabisulf ite (Na-S^Oc) , sulfur
dioxide (SCL) , and sodium hydrosulfide (NaHS) . The ferrous form of
iron (Fe ) is a popular reducing agent in many cases. Elemental
magnesium (Mg) , zinc (Zn) , and copper (Cu) are also effective reducing
agents. Frequently, hydrazine (N?H?) is used as a reducing agent
also.
Typical reduction reactions are as follows:
H202 + 2H+ + 2Fe+2 -» 2H20 + 2Fe+3
Hydrogen Peroxide Ferrous iron Ferric Iron
4Zn + N03" + 10H+ -» 4Zn+2 + NH4+ + 3H20
Metallic Zinc Nitrate Ion Zinc Ion Ammonium Ion
These reactions are usually accomplished at pH values from 2 to 3 .
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26.3 Description of Chemical Reduction Process
The chemical reduction treatment process can be operated in a batch
or continuous mode. A batch system consists of a reaction tank, a mixer
to homogenize the contents of the tank, a supply of reducing agent, and a
source of acid and base for pH control.
A continuous chemical reduction treatment system usually includes a
holding tank upstream of the reaction tank for flow and concentration
equalization. It also typically includes instrumentation to
automatically control the amount of reducing agent added and the pH of
the reaction tank. The amount of reducing agent is controlled by the use
of a sensor called an oxidation reduction potential (ORP) cell. The ORP
sensor electronically measures, in millivolts, the level to which the
oxidation/reduction (redox) reaction has proceeded at any given time. It
must be noted, however, that the ORP reading is very pH dependent.
Consequently, if the pH is not maintained at a steady value, the ORP will
vary somewhat, regardless of the level of chemical reduction.
26.4 Waste Characteristics Affecting Performance (WCAPs)
In determining whether chemical 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 other reducible
contaminants and (b) the concentration of oil and grease.
26.4.1 Concentration of Other Reducible Compounds
The presence of other reducible compounds (also called oxidizers) in
addition to the BOAT list constituents of concern will increase the
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demand of reducing agents, thereby potentially reducing the effectiveness
of the treatment process. As a surrogate for the amount of organic
oxidizers present in the waste, EPA might analyze for total organic
carbon (TOC) in the waste. Inorganic oxidizing compounds such as ionized
metals (e.g., silver, selenium, copper, mercury) may also create a demand
for additional reducing agent. EPA might attempt to identify and analyze
for these metal constituents. If TOC and/or inorganic oxidizer
concentration in the untested waste is significantly higher than in
previously tested wastes, the system may not achieve the same performance
as that achieved previously. Additional reducing agent may be required
to effectively reduce the untested waste and achieve the same treatment
performance, or other, more applicable, treatment technologies may need
to be considered for treatment of the untested waste.
26.A.2 Concentration of Oil and Grease
EPA believes that oil and grease compounds could cause monitoring
problems because of fouling of instrumentation (e.g., electrodes for pH
and ORP sensors). If the concentration of oil and grease in an untested
waste is significantly higher than that in a tested waste, the untested
system may not achieve the same performance. Therefore, other, more
applicable treatment technologies may need to be considered for treatment
of the untested waste.
26.5 Design and Operating Parameters
In assessing the effectiveness of the design and operation of a
chemical reduction system, EPA examines the following parameters:
(a) the residence time, (b) the amount and type of reducing agent,
(c) the degree of mixing, (d) the pH, and (e) the reduction temperature.
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26.5.1 Residence Time
The residence time affects the extent of reaction of waste
contaminants with reducing agents. For a batch system, the resident time
is controlled by adjustment of the treatment time in the reaction tank.
For a continuous system, the waste feed rate is controlled to make sure
that the system is operated at the appropriate design residence time.
EPA monitors the residence time to ensure that sufficient time is
provided to effectively reduce the waste.
26.5.2 Amount and Type of Reducing Agent
Several factors influence the choice of reducing agents and the
amount to be added. The amount of reducing agent required to treat a
given amount of reducible constituent will vary with the agent chosen.
Enough reducing agent must be added to ensure complete reduction; the
specific amount will depend on the type of reducible compounds in the
waste, and the chemistry of the reduction reactions. Theoretically, the
amount of reducing agent to be added can be computed from reduction
reaction stoichiometry. In practice, however, an excess of reducing
agent should be used. Testing for excess reducing agent, if possible,
will determine whether the reaction has reached completion.
In continuous processes, the addition of reducing agent is usually
accomplished by automated feed methods. The amount of reducing agent
needed is usually metered and controlled automatically by an
oxidation-reducing potential (ORP) sensor. EPA examines the amount of
reducing agent added to the chemical reduction system to ensure that it
is sufficient to effectively reduce the waste and, for continuous
processes, examines how the facility ensures that the particular addition
rate is maintained. EPA may also test for excess reducing agent in the
system effluent. For continuous processes, EPA monitors the ORP to
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ensure that enough reducing agent is supplied. EPA may also monitor pH
to determine whether it is controlled to the appropriate optimum range.
For continuous systems, pH is monitored to ensure that it is kept steady,
so as to avoid changes in ORP caused by pH variations.
26.5.3 Degree of Mixing
Process tanks must be equipped with mixers to ensure maximum contact
between the reducing agent and the waste oxidizing solution. Proper
mixing also homogenizes any solid precipitates that may be present, or
that may form from side reactions, so that they can also be reduced if
necessary. Mixing provides an even distribution of tank contents and a
homogeneous pH throughout the waste, improving reduction of wastewater
constituents. The quantifiable degree of mixing is a complex assessment
that includes, among other 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, using engineering judgment, by considering whether mixing
i's provided and whether the type of mixing device is one that could be
expected to achieve uniform mixing of the waste solution.
26.5.4 pH
For batch and continuous systems, the pH affects the reduction
reaction. The reaction speed is usually significantly reduced at higher
pH values (typically above 4.0). It is worth noting that some reduction
reactions may proceed better under alkaline conditions, in which case pH
must be properly controlled to the appropriate alkaline range. For a
batch system, the pH can be monitored intermittently during treatment.
For a continuous system, the pH must be continuously monitored because it
affects the ORP reading. EPA monitors the pH to ensure that the system
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is operating at the appropriate design condition and to diagnose
operational problems.
26.5.5 Reduction Temperature
Temperature affects the rate of reaction and the solubility of the
reactants in the waste. As the temperature is increased the solubility
of the reactants, in most instances, is increased, and the required
residence time, in most cases, is reduced. EPA may monitor the reduction
temperature to ensure that the system is operating at the appropriate
design condition and to diagnose operational problems.
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26.6 References
McGraw-Hill. 1982. Encyclopedia of science and technology. Vol. 9,
pp. 714-718. New York: McGraw-Hill Book Co.
Nebergall, et al. General Chemistry. 5th ed. Lexington, Massachusetts:
D.C. Health and Company.
Patterson, J.W. 1985. Industrial wastewater treatment technology.
2nd ed. Stoneham, Massachusetts: Butterworth Publishers.
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MEMORANDUM
TO: Administrative Record cc: J. Strauss
S. Schwartz
5254 File
FROM: Jim Berkes
DATE: May 8, 1990
SUBJECT: Final Treatment Standards for Nonwastewater and Wastewater
Forms of K044, K045, and K047
This memorandum presents a summary of the technical support and
rationale for the development of the treatment standard for nonwastewater
and wastewater forms of K044, K045, and K047.
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 best demonstrated available technology (BOAT) treatment
standards for the listed wastes identified in 40 CFR 261.32 as K044
(wastewater treatment sludges from the manufacturing and processing of
explosives), K045 (spent carbon from the treatment of wastewater
containing explosives), and K047 (pink/red water from TNT operations).
Compliance with the treatment standards being promulgated will be a
prerequisite for placement of these wastes in units designated as land
disposal units according to 40 CFR Part 268. The effective date of these
treatment standards will be June 9, 1990.
K044, K045, and K047 wastes are listed as hazardous because of the
characteristic of reactivity. The Agency believes that technologies
applicable for treatment of these wastes include those that are capable
of rendering the waste nonreactive. Commenters indicated that K044,
K045, and K047 wastes can be treated by technologies such as chemical
oxidation or incineration. Such treatments permanently remove the
reactive characteristic of these wastes by thermal or chemical
destruction.
In the Second Third final rule (53 FR 31158, August 17, 1988), EPA
promulgated a treatment standard of "No Land Disposal Based on
Reactivity" for K044, K045, and K047 nonwastewaters and wastewaters. In
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Administrative Record Page Two
May 8, 1990
today's final rule (Third Third final rule), the Agency is revoking this
standard and is promulgating a treatment standard of "Deactivation
(DEACT) as a Method of Treatment" for K044, K045, and K047 nonwastewaters
and wastewaters. The treatment standard for K044, K045, and K047 wastes
is presented in Table 1.
Several commenters had questions on the definition of deactivation.
To clarify this point, the Agency is defining deactivation for K044,
K045, and K047 wastes to be the process of removing the characteristic of
reactivity, by technologies such as incineration or chemical oxidation.
See 40 CFR 268 Appendix VI for a list of technologies that used alone or
in combination can achieve this standard. See also 40 CFR 268.42 Table 1
for a description of the technologies referred to by a five letter
technology code in parentheses. This standard should provide the
treaters of K044, K045, and K047 wastes the ability to use the "best"
treatment technology based on the chemical and physical parameters of the
waste, and any safety considerations.
The BOAT program and EPA's promulgated methodology are more
thoroughly described in two additional documents: Methodology for
Developing BOAT Treatment Standards (USEPA 1988) and Generic Quality
Assurance Project Plan for Land Disposal Restrictions Programs (BOAT)
(USEPA 1987). The petition process to be followed in requesting a
variance from the BOAT treatment standards is discussed in the
methodology document. Additional information on reactive wastes can be
found in the BOAT Background Document for Characteristic Ignitable Wastes
(D001), Characteristic Corrosive Wastes (D002), Characteristic Reactive
Wastes (D003), and P and U Wastes Containing Reactive Listing
Constituents (USEPA 1990).
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TABLE 1 BOAT TREATMENT STANDARD FOR K044, K045, AND K047
[Nonwastewaters and Wastewaters]
(REVISED FROM NO LAND DISPOSAL)
DEACTIVATION (DEACT) AS A METHOD OF TREATMENT*
See 40 CFR 268 Appendix VI for a list of applicable technologies that
used alone or in combination can achieve this standard. See also 40
CFR 268.42 Table 1 for a description of the technologies referred to
by a five letter technology code in parentheses.
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REFERENCES
USEPA. 1987. U.S. Environmental Protection Agency. Generic quality
assurance project plan for land disposal restrictions program
(BOAT). EPA/530-SW-87-011. Washington, D.C.: U.S. Environmental
Protection Agency, Office of Solid Waste.
USEPA. 1988. U.S. Environmental Protection Agency. Methodology for
developing BOAT treatment standards. Washington, D.C.: U.S.
Environmental Protection Agency.
USEPA. 1990. U.S. Environmental Protection Agency. Best demonstrated
available technology background document for characteristic ignitable
wastes (D001), characteristic corrosive wastes (D002), characteristic
reactive wastes (D003), and P and U wastes containing reactive
listing constituents. Washington, D.C.: U.S. Environmental
Protection Agency.
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