United States
                                 Environmental Protection
                                  Municipal Environmental Researc
                                  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-
                                   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).

                                   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
  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

  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
ID No.
(wet tons)
mg/kg (dry)
S0t. Cl
Ca, SOt,
Ca. Fe,
S04, Cl
Ca. Fe.
S0t. Cl
Ca. S04,
mg/kg (dry)
Cr, Mg,
Mn, Zn
Cr, Fe.
Mg, Zn
Cr. Mg.
Cr. Mg,
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
A, B, .
F. G
E, G
E. G
A, B, E,
F, G
E, G
 ^Processes designated by code letter only (see text for generic description of the processes).


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
                     Note: All Joints and Seams Cemented
                                            10.16 cm
      ^ Teflon
N q
i Vi


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

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 4C 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-
  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-

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

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
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





Solidification Processes








Process E held up to any degree (all
samples withstood wet-dry test cycles
and  half survived freeze-thaw test
  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
  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
  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-
  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
  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-

  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
  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: 11 -757-01Z/7Z67

United States
Environmental Protection
Center for Environmental Research
Cincinnati OH 45268
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