v>EPA
United States
Environmental Protection
Agency
Municipal Environmental Researc
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-116 Aug. 1981
Project Summary
Physical Properties and
Leach Testing of
Solidified/Stabilized Flue
Gas Cleaning Wastes
This study examines the effective-
ness of five methods of treating flue
gas cleaning (FGC) wastes to restrict
the release of constituents to contact-
ing waters. Five different FGC sludges
were treated using commercially
available sludge stabilization technol-
ogy.
The effects of solidification on
several physical and engineering prop-
erties of the sludges were determined.
These properties were thought to be
important in assessing the stabilization
techniques, but they were found to be
of only moderate predictive value.
The treated and untreated sludges
were exposed to leaching conditions
in specially designed columns for up to
2 years in a controlled environment,
and the resulting leachates were anal-
yzed chemically. Average concentra-
tions of many of the leachate con-
stituents were reduced by most sludge
treatment processes, but no single
process uniformly reduced the con-
centrations of all potential pollutants
for all sludge types tested.
Two problems occurred regarding
the leachate analysis. One was that
additional teachable materials were
added by means of a reagent during
treatment. Thus losses of some con-
stituents were greater than the amount
originally present in the sludge. The
second problem was that some treat-
ment processes increased the solubility
of certain constituents so that they
were lost from the treated sludge at
higher rates than from the untreated.
Extrapolations of results to field
conditions should not be made because
the small sample size (with larger
surface/volume ratio) and continuous
submersion in a carbon-dioxide-satu-
rated solution produced a rigorous
leaching condition that would not be
present in actual landfills. Thus wastes
would probably be more effectively
contained in a field situation. Larger-
scale projects are needed for closer
duplication of the landfill environment.
This Project Summary was devel-
oped by EPA's Municipal Environmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Disposing of the power generation
industry's waste byproducts is becoming
increasingly difficult, expensive, and
dangerous to the environment. Existing
Federal air pollution control regulations
and standards for new, stationary air
pollution sources make it virtually
impossible to operate a power genera-
tion plant using untreated coal (even of
moderate sulfur content) without an
efficient flue gas desulfurization (FGD)
system. Only power plants using very
low-sulfur coal are able to operate
without flue gas cleaning (FGC) systems.
Most FGC systems now being installed
or planned are nonregenerative sys-
tems that combine the SOX in the stack
gas with scrubbing materials to form
insoluble calcium sulfite and calcium
sulfate, which are collected as FGD
sludge. Note that the term "FGC sludge"
-------
usually refers to a mixture of fly ash,
bottom ash, and FGD sludge. Annual
production of FGC sludges (50 percent
solids) for new power plants burning
commonly available grades of coal was
estimated to be 20 million tons (wet
weight) in 1980; 80 million tons are
projected by 1990, and 155 million tons
by 1998.
In the past, the predominant method
of disposing of these FGC sludges has
been to dewater them by ponding and to
give them subsequent burial in a landfill.
Such disposal methods have a high
potential for long-term loss of constit-
uents into adjacent ground and surface
waters.
A technology currently being devel-
oped and used to address this problem is
that of chemical solidification and/or
stabilization of the sludge. This treat-
ment attempts to improve the waste's
handling characteristics and decrease
its potential for leaching undesirable
constituents. In this process, additives
are combined with the wet sludge and
allowed to react with the water and
minerals in the sludge to form a solid
mass or soil-like material. Most stabili-
zation processes decrease the perme-
ability of the sludge and produce chemi-
cal conditions (increased pH) that lower
the solubility of sludge constituents.
This study was established to assess
the effectiveness of five different meth-
ods of treating FGC wastes to restrict
the release of constituents to contacting
waters. Five different FGC sludges were
treated using commercially available
sludge stabilization technology. The
effects of solidification on several physi-
cal and engineering properties of the
sludge were determined. The treated
and untreated sludges were exposed to
leaching conditions in specially designed
columns for up to 2 years in a controlled
environment, and the resulting leachates*
were analyzed chemically. The chemical
leaching data were then used to assess
the effectiveness of the stabilization
processes in containing waste constit-
uents in the FGC sludge tests.
Materials and Methods
Sludges
The three sludges used in this study
were selected to be representative of
the major scrubber techniques and the
major types of coal presently being
used: limestone sludge produced with
calcium carbonate as a scrubbing rea-
gent, lime sludge produced with calcium
hydroxide, and double or dual alkali
sludge produced with sodium sulfite.
Samples using both eastern (high-
sulfur) and western (low-sulfur) coals
are used in this study for the lime and
double alkali process sludges; only one
sample of limestone process FGC sludge
using eastern coal is included, however.
All sludges studied here are FGC
sludges containing variable amounts of
FGC wastes and ash. The major charac-
teristics and chemical composition of
the sludges are given in Table 1. All are
alkaline, inorganic sludges that are 20
to 30 percent denser than water, have
less than 50 percent solids, and contain
large amounts of calcium, iron, sulfate,
and chloride. The most prevalent heavy
metals are chromium, manganese, and
zinc. Also present at potentially hazard-
ous but highly variable levels are arsenic,
beryllium, cadmium, copper, lead, and
nickel. The major source of these ele-
ments is the coal burned, but significant
amounts can also come from the scrub-
bing materials and the makeup water.
Stabilization Techniques
The stabilization or fixation processes
used in this project generated two types
of products: a soil-like material that was
highly variable in particle size, and a
material capable of being cast into a
solid, monolithic block. The solidification
procedure used with the first group
called for casting the treated sludge into
square molds (122 x 122 * 9 cm), cover-
ing the molds, curing them for 30 days,
breaking the treated samples into smaller
pieces (about 5 cm in diameter), and
loading them into the leaching columns
without further packing. The second
group of samples was molded in 7.6-
cm-diameter, paraffin-coated tubes 122
cm long. After curing, the tubes were
removed, and the resulting cores were
used for chemical and physical testing.
The five processes used in this study
(designated Processes A, B, E, F, and G)
are described here briefly, and their
applications to various sludges are
noted in Table 1. Process A is a patented
procedure that uses fly ash and a lime
additive to produce a pozzolan product.
Process B is also patented and uses two
additives to produce a material of soil-
like consistency. Process E uses cement
and fly ash (which are readily available
commercial materials) as additives to
convert waste sludge into a hardened
product similar to concrete. Process F
mixes a patented additive with a sludge
at a pH adjusted to settle the solids in
the slurry. The additive is a cementitious
product derived from basic, glassy blast
furnace slag. Process G is a stabilization
technique in which waste sludge is
mixed with cement kiln dust (a waste
product from the cement industry), and
the pH is lowered with either waste
sulfuric or phosphoric acid.
Table 1. Major Characteristics, Chemical Constituents, and Stabilization Process Code* of Flue Gas Cleaning Wastes Studied
Waste Description
Lime Process,
Eastern Coal
Limestone Process,
Eastern Coal
Double Alkali Process,
Eastern Coal
Lime Process,
Western Coal
Double Alkali Process,
Western Coal
Sludge
ID No.
100
400
500
600
1000
1978
Production
(wet tons)
1.160.0OO
1,440,000
850.000
2,600.000
600,000
Solids
<%)
36
36
44
20
39
Density
(kg/m*)
1280
1280
1330
1120
1330
Sludge
Liquor
pH
10.3
10.0
13.1
7.5
12.7
Constituents
>10.000
mg/kg (dry)
Ca.Fe.
S0t. Cl
Ca, SOt,
Cl
Ca. Fe,
S04, Cl
Ca. Fe.
S0t. Cl
Ca. S04,
Cl
Constituents
100-10.000
mg/kg (dry)
Cr, Mg,
Mn, Zn
Cr, Fe.
Mg, Zn
Cr. Mg.
Mn.Zn
Cr. Mg,
Mn.Zn
Fe.Mg
Constituents
1-100
mg/kg (dry)
As, Be, Cd.
Cu, Pb. Ni
As, Be, Cd,
Cu, Pb, Mn, Ni
As, Be, Cd,
Cu, Pb. Ni
As, Be. Cd,
Cu, Pb, Ni
Be, Cd, Cr, Cu,
Pb, Mn, Ni, Zn
Process
Designation
A, B, £.
F. G
A.B,
E, G
A.B,
E. G
A, B, E,
F, G
A.B,
E, G
^Processes designated by code letter only (see text for generic description of the processes).
2
-------
Leaching Column Design
The leaching columns used in this
study were designed to simulate leaching
from sludges buried in an unlined,
water-saturated landfill. Plexiglass
tubing (152 cm long by 10.2 cm ID) was
used to construct the columns (Figure
1), which were approximately 10 liters
in volume and covered to prevent dust
and air contamination. An inlet port was
located 19 cm below the top of the
column, providing space for a fluid head
of 2.5 cm on top of the sample. The
bottoms of the columns were sealed,
and a Teflon stopcock was installed at
the lower end as a leachate drain. A
collecting well was provided by cement-
ing a perforated plate onto the tube 2.5
•1.6 cm
cm above the stopcock. Movement of
sludge into the leachate collection
system was retarded with a 7.6-cm
layer of 0.64-cm-diameter polypropylene
pellets at the bottom of each column.
Flow through the column was regulated
by the stopcock to maintain a fluid
velocity of approximately 1 * 10~5cm/sec
to simulate the flow rate through a raw
sludge or fine silt.
Column Loading and Set Up
The treated sludges that were cast
into cylinders were placed into the
leaching columns, and the space be-
tween the sludge and column wall
(about 1.3 cm) was filled with polypro-
pylene pellets to create a dispersed flow
0.64 cm
^-Plexiglass
Note: All Joints and Seams Cemented
u
5.
10.16 cm
I-
rv
CM
^ Teflon
Stopcock
i
5
* —
\
\
V////////A
N q
i Vi
\\
-?
v
l^-'W^''.^///////^
4.44 cm N N~
""^ X
3//////////////A V/////////////^
0.8 cm
Detail A
Figure 1. Leaching column design and detail.
of liquid around the sludge samples. The
fixed sludges that were not cast into
cylinders were taken from the large
molds, broken into smaller pieces, and
loaded directly into the columns with no
pellet packing. The raw sludges were
poured into the columns as a slurry.
In all cases, the leaching fluid was
back-flooded into each column from the
bottom to remove any air spaces. The
specimens were maintained in a satu-
rated-flow condition throughout the
experiments. All sample columns were
set up in triplicate.
The leaching fluid was deionized
water saturated with carbon dioxide at
pH 4.5 to 5.0. All materials used in the
leaching fluid distribution system were
either polypropylene or Teflon to mini-
mize any contamination during the
testing. The leaching columns were
randomly assigned within a system of
racks, each of which was fed from a
constant head reservoir at one end of
the rack. These reservoirs were in turn
connected in series to a central reservoir
of the leaching fluid in which the carbon
dioxide was equilibrated with the fluid.
One column from each triplicate set of
treated and untreated sludge columns
was selected randomly to be studied
with more sensitive and expensive
analytic techniques. Performances of
the solidification processes were judged
by results from this set of priority
columns. Data from the other columns
were used to confirm the trends and
conclusions noted with the priority
columns.
Chemical Analyses
Leachate flowed from the columns
continuously and was collected in 4.5-
liter plastic bottles. At each sampling
interval, the pH, conductivity, and volume
of the collected leachate were mea-
sured, and each sample was split into
aliquots of appropriate volume. Each
aliquot was preserved using EPA-
accepted techniques. All samples were
held at 4°C until analyzed.
Parameters selected for analysis
included all potential pollutants. High-
resolution metal analysis was performed
on the priority column for each sludge
type, and low-resolution metal analysis
was done with the remaining two repli-
cates.
Leachate samples were collected
from each column at logarithmic time
intervals for at least 2 years, and many
of the priority columns were sampled
over a longer period. An extensive
-------
quality control program was implemented
to ensure precision and accuracy with
the analytical system. These efforts
were concentrated on metals, since
they constituted the major group of
pollutants in this project.
Physical and Engineering
Properties of FGC Sludges
Test Procedures
Tests commonly used for soil and
concrete were performed on the un-
treated and solidified sludges to deter-
mine their physical and engineering
properties. The use of these standard
tests permitted sludge properties to be
compared with those of common mate-
rials. Test procedures were selected on
the basis of the material's appearance
(i.e., soil-like or concrete-like). The
testing schedule is shown in Table 2.
Standard test procedures were modified
as necessary to prevent the alteration of
sludge properties during testing and to
accommodate nonstandard test speci-
mens.
Sludge Characteristics
The FGC sludges prepared for this
study show features typical of this type
of sludge: they are made up largely of
very small particles with high water
content (greater than 50 percent), low
bearing capacity, a specific gravity of 2.4
to 3.0, and low permeability (averaging
3 to 4 x 10~5 cm/sec). These properties
indicate poor handling and dewatering
characteristics.
Test Results
Solidification generally produced a
material that resembled either soil-
cement mixture or low-strength con-
crete. This procedure produced no con-
sistent change in specific gravity or
water content of the sludges.
Processes A and B yielded materials
whose bulk and dry unit weights re-
sembled those of soils (and appeared to
be related, as these properties would be
in soil). The remaining processes pro-
duced materials having very small
differences in bulk and dry unit weights
(similar to concrete).
Only Process B produced a product
that could be subjected to Atterberg
limit and compaction tests. Comparison
of Atterberg limit tests for untreated and
Process-B-treated sludges showed no
consistent effect that could be related to
the process. Optimum water contents
determined from the compaction test
were generally higher or equal to values
determined for typical soils.
Process B yielded materials that had
compressive strengths comparable to
those of cohesive or cemented soils.
Processes A and G produced materials
resembling low-strength soil cement,
with two to four times the compressive
strengths of samples from Process B.
Sludges treated by Processes E and F
had compressive strengths in the same
range as low-strength concrete.
Sludge samples solidified using
Processes B and G have permeabilities
resembling those of untreated sludges.
Processes A and E decreased the sludge
permeabilities to one one-hundredth
the value for untreated sludges.
Freeze-thaw and wet-dry testing
generally showed that the solidified
materials had durability properties
similar to soil cement or low-strength
concrete. Only sludges solidified by
Table 2. Test Schedule for Treated and Untreated FGC Sludges
Untreated
Type of Test Sludges
Physical Tests
Grain -size analysis
Specific gravity of solids
Water content
Bulk and dry unit weight
Porosity and void ratio
Liquid limit
Plastic limit
Engineering Tests
15 -blow compaction test
Unconfined compression test
Permeability test
Freeze-thaw test
Wet-dry test
X
X
X
X
X
X
X
X
A
X
X
X
X
X
X
X
X
Solidification Processes
B E F
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
G
X
X
X
X
X
X
X
X
Process E held up to any degree (all
samples withstood wet-dry test cycles
and half survived freeze-thaw test
cycles).
Physical and engineering tests indi-
cate that solidification processes do not
always alter the physical and engineer-
ing properties of the sludges in ways
that enhance their ability to contain
noxious constituents. Testing procedures
adapted specifically to sludges and
solidified sludges might increase our
ability to predict the success of solidifi-
cation processes.
Results of Leachate Testing
The loss of constituents in the leachate
from the experimental columnstypically
followed one of two distinct patterns,
with or without treatment. Those con-
stituents whose concentration in the
sludge greatly exceeded their solubilities
(e.g., calcium, nickel, lead, and sulfate)
were found at relatively constant con-
centrations in the leachates over the
course of the testing. For these con-
stituents, the rate of loss depended on
the volume of leachate produced and
was independent of the length of time
over which the leaching took place. The i
second leaching pattern was seen in "
those constituents whose solubilities
were large compared with their concen-
trations in the sludge (e.g., chloride and
nitrate). These constituents had very
high concentrations in the initial
leachate, followed immediately by an
asymptotic drop in concentration as the
element was depleted from the sludge
exposed to the leaching medium. Chan-
nelization of leachate flow m the un-
treated sludge columns greatly increased
the rate at which the concentration of
the soluble constituents in the leachate
fell off, as this process lessened the
amount of sludge that came in contact
with the leaching medium.
The major pollution problem associ-
ated with the untreated FGC sludges is
that the leachate from the lime and
limestone process is saturated with
calcium sulfate (gypsum) and would be
expected to remain so for long periods of
time. Typically this leachate contains
500 to 600 mg/L calcium and 1200 to
1500 mg/L sulfate. The double alkali
process sludges present an added initial
problem of extremely high sulfate losses
(presumably because of a relatively high
proportion of sodium and/or potassium).
Leachates containing 35,000 to 40,000
mg/L sulfate were found in early sam- |
pies from all double alkali columns. "
-------
Depletion of the monovalent cations
from the double alkali sludges brings
about a lowering of the sulfate levels to
that found for leachate from other
sludge types. Other contaminants in
leachates from all untreated FGC sludges
that consistently exceeded drinking
water standards were arsenic, chro-
mium, and manganese; cadmium and
lead exceeded drinking water standards
in relatively few leachate samples.
Leaching column tests indicated that
the average concentrations of many of
the constituents in the leachates col-
lected during this study were reduced by
most of the sludge treatment processes;
however, no treatment process uni-
formly reduced the concentration of all
constituents in the leachate for all the
types of FGC sludges tested. Solidifica-
tion/stabilization does tend to lower the
pollutant potential of FGC sludges to
contacting waters. Reduction of the
highest concentrations of sludge con-
stituents occurring in the leachate was
the most pronounced effect of sludge
treatment. When the proportion of dry
sludge solids contained in the final
solidification/stabilization product is
taken into account, the apparent bene-
icial effect of sludge treatment is re-
duced, however.
Two possible problems in the treat-
ment processes were identified. In
some cases, leachable material appears
to have been added by means of the
reagent that produces the solidifying or
stabilizing reaction. Thus greater
amounts of certain constituents were
sometimes released from the treated
sludge than from the original untreated
wastes. In other cases, the treatment
process appears to have altered the
chemical conditions in the sludge so as
to increase the solubility of certain
constituents. Consequently, more
material was lost from the treated waste
than from the equivalent weight of
untreated waste.
The small sample size (with larger
surface/volume ratio) and continuous
submersion in a carbon-dioxide-satu-
rated leaching solution, as used in this
study, appear to represent very vigorous
leaching conditions. Most landfill oper-
ations would allow the use of much
larger blocks of treated sludge (with
smaller surface/volume ratios) and
would undergo only intermittent satu-
rating conditions in the soil. Thus condi-
tions in an actual landfill would be more
favorable to the containment of the
Treated wastes.
Conclusions and
Recommendations
The results of this study suggest that
solidification/stabilization of FGC
wastes may be a feasible method of
reducing their pollutant potential in
landfilling. A great deal more study is
necessary, however, before the behavior
of treated FGC sludges under actual
field conditions can be adequately
understood.
Two years of leaching data indicate
that the physical and engineering prop-
erties thought to be important in assess-
ing solidification/stabilization techniques
are of only moderate predictive value.
Specific testing procedures should be
developed and standardized to have
direct bearing on the ultimate behavior
of the final product under actual landfill
conditions. But such a step can only be
taken with sufficient understanding of
the important variables affecting the
loss of pollutants from similarly treated
materials under actual landfill condi-
tions.
Though the results of small-scale
leachate testing can be used with
confidence in comparing samples with-
in a small-scale study, extrapolation to
field conditions should not be attempted.
Most landfilled wastes would have far
lower surface-area-to-volume ratios
than the small samples used in any
bench-scale testing. Furthermore, the
test procedure used here requires the
specimens to be constantly immersed in
an aggressive leaching medium con-
sisting of water saturated with carbon
dioxide. These conditions cause reac-
tions such as hydration of the calcium
aluminum silicates in the cement addi-
tives and biological activity that may
accelerate the release of potential
contaminants.
Thus large-scale, controlled tests
using treated sludge samples more
nearly typical of the surface-to-volume
relationship actually encountered in
landfill situations are needed for a more
realistic estimate of treatment benefits.
Intermittent saturation of the treated
samples should also be considered in
any future testing. In addition, treat-
ment benefits should be calculated on
the basis of actual sludge solids in-
corporated into the treated sludge so
that the effects of simple waste dilution
can be separated from those of stabiliza-
tion.
The full report was submitted in ful-
fillment of Interagency Agreement No.
EPA-IAG-D4-0569 by the Waterways
Experiment Station, U.S. Army Corps of
Engineers, Vicksburg, MS 39180, under
the sponsorship of the U.S. Environ-
mental Protection Agency.
This Project Summary was prepared by staff of the Environmental Laboratory,
Waterways Experiment Station. U.S. Army Corps of Engineers. Vicksburg, MS
39180.
Robert E. Landreth is the EPA Project Officer (see below).
The complete report, entitled "Physical Properties and Leach Testing of Solidi-
fied/Stabilized Flue Gas Cleaning Wastes." (Order No. PB 81-217 036; Cost:
$15.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U. S. Environmental Protection Agency
Cincinnati, OH 45268
« US GOVERNMENT PBINTINO OFFICE: 1««1 -757-01Z/7Z67
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
RETURN POSTAGE GUARANTEED
PS 0000329
U S EMVIR PRQThCTIUN
REGION 5 LIBRARY
230 S DEARBORN S'fRfclET
CHICAGO IL 60604
------- |