United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Technology Transfer
Capsule Report
Disposal of Flue Gas
Desulfurization Wastes
Shawnee Field
Evaluation
-------
Technology Transfer EPA 625/2-80-028
Capsule Report
Disposal of Flue Gas
Desulfurization Wastes
Shawnee Field
Evaluation
October 1980
This report was developed by the
Industrial Environmental Research Laboratory
Research Triangle Park NC 27711
-------
Typical filling operation of waste disposal evaluation pond
-------
1. Introduction
This capsule report summarizes
activities and results of the U.S.
Environmental Protection Agency
(EPA) Shawnee Flue Gas Desulfur-
ization (FGD) Field Disposal Evalua-
tion Project. As a result of the
Resource Conservation and
Recovery Act of 1976 (Public Law
94-580), guidelines and criteria
for FGD wastes are being developed.
Current regulatory development
efforts are based, in part, on data
derived from the Shawnee project.
The Shawnee project was initiated in
September 1974 to evaluate
methods and costs for disposing of
byproducts from wet, nonregener-
able FGD systems. The effects of
various disposal techniques,
scrubber reagents and operations,
weather, and field operation
procedures on the environmental
quality of the disposal site are
being studied to determine environ-
mentally sound disposal methods.
Because water quality and land
reclamation are of principal interest,
periodic sampling, analyses, and
assessments are being conducted of
leachate, supernate, runoff, ground
water, and soil and waste cores.
The Aerospace Corporation is
responsible for the project planning,
coordination, selected water and
solids analysis, performance assess-
ment and evaluation, and reporting.
Site construction, maintenance,
coring, water sampling, and water
analysis are performed by the
Tennessee Valley Authority (TVA),
Completion of the technical effort is
scheduled for September 1980.
Project Description
The project is located at the TVA
Shawnee Steam Plant near Paducah,
Kentucky. Of the 10 field sites
currently being evaluated, 8 are small
ponds up to 0,1 acre (0.04 ha) in
area with a waste depth of 3 to 4 ft
(0.9 to 1.2 m), and 2 are surface
disposal sites that measure up to
70 ft X 70 ft (21 m X 21 m). Waste
materials for the project are
produced by two scrubber sys-
tems—either a turbulent contact
absorber (TCA*, of UOP, Inc.) or
a venturi and spray tower (VST)—
which operate as an EPA/TVA test
facility at the Shawnee plant.
Using lime or limestone slurries as
the sulfur dioxide (S02) absorbent,
each scrubber is capable of treating
flue gas from a system producing
up to 60 X 106 Btu/h (10 MWe
equivalent). The Shawnee project
provides a broad data base for
evaluating the control of flue gas
S02 by combining the results of
field disposal operations and
laboratory analyses.
This report evaluates FGD wastes
that were either chemically treated,
left untreated, or force-oxidized
to gypsum. Figure 1 illustrates the
relationship of the FGD waste to the
four disposal alternatives that
are being evaluated at Shawnee—
landfill, pond disposal, underdrained
pond disposal, and surface stacking.
Disposal of FGD wastes in coal
mines and the oceans is being
evaluated in other EPA projects.
FGD Waste Characteristics
The disposal ponds were filled
with FGD wastes representing a
cross section of scrubber effluent
conditions. The various waste
disposal sites are discussed by
treatment category: chemically
treated, untreated, and force-oxidized.
Table 1 lists all the project disposal
sites, provides pertinent information,
and gives the current status of
each site.
The chemical composition of FGD
waste input liquor, water from
pond runoff, supernate, leachate/
underdrainage, and ground water
from 14 wells in and around the
disposal area is analyzed for a
variety of chemical species. The
composition of the waste input liquor
(before treatment, if any) is sum-
marized in Table 2, which shows
the wide variation in the concen-
tration of chemical species among
different FGD wastes.
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Landfill
- -J-fl" *•*• fVFiriV^'.v::-.:.:.
Figure 1.
FGD Waste Disposal Alternatives
The physical properties considered
in the disposal of FGD wastes
include viscosity, bulk density,
moisture content, bearing strength,
porosity, and permeability. Viscosity
is particularly important in the
transport of the waste to a disposal
site; the other properties concern
the weight and volume of the disposal
material, as well as the suitability
of the waste as a load-bearing
material and as a means of preventing
seepage from a disposal site.
The physical properties of FGD
wastes depend on the characteristics
and interaction of the liquid and
the solid constituents. These wastes
contain finely divided particulate
matter in an aqueous medium.
Depending on the particulate size
distribution and crystal structure,
these particles—the majority being
calcium sulfite hemihydrate
(CaS03 • '/2H20), calcium sulfate
dihydrate (CaS04 • 2H20), and fly
ash—influence the physical
properties of the wastes. Both
calcium sulfite and calcium sulfate
scrubber waste products tend to
have particles in the same size
range as fly ash, that is, between
Table 1.
Description of Shawnee Disposal Sites
Site
A
A1
6
C
D
E
F
G
H
H
J
K
Fill date
Oct. 8, 1974
May 10, 1976
Apr. 15, 1975
Apr, 23, 1975
Feb. 5, 1975
Dec. 7, 1974
Feb. 3, 1977
Oct. 5, 1976
Sept. 2, 1977
Sept. 30, 1977
Dec. 31, 1978
Mar. 29. 1979
Scrubber
type-
VST
VST
TCA*
TCA*
TCA«
TCA*
TCA*
VST
VST
VST
VST
VST
FGD waste
Lirne, filter cake
Lime, filter cake
Limestone, clarifier underflow
Lime, centrifuge cake
Limestone, clarifier underflow
Limestone clarifier underflow
Limestone, clarifier underflow, fly ash
remixed
Lime, centrifuge cake, fly ash remixed and
layered
Limestone, gypsum clarifier underflow
Limestone, gypsum filter cake
Limestone, gypsurn filter cake
Limestone, gypsum filter cake
Waste
treatment
None
None
Chemical
Chemical0
None
Chemical^
None
None
Oxidation
Oxidation
Oxidation
Oxidation
Remark
Out of service on Apr. 15, 1976
Control pond, transferred from Site A
Underwater disposal
Pond converted to runoff mode in Mar. 1 979
Control pond
Covered in Nov. 1977
Underdrained pond, covered in Nov. 1977
Underdrained pond, covered in Apr. 1980
Underdrained during filling
Surface site, unreacted limestone (13% by
dry weight)
Surface site (adipic acid used in scrubber)
Surface site, unreacted limestone (25% by
dry weight)
aVST= venturi and spray tower; TCA® of UOP, Inc. = turbulent contact absorber,
Dravo Corporation.
C(U Conversion Systems, Incorporated.
Chemfix Corporation.
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Table 2.
Chemical Analysis of Disposal Site Input Liquor
Concentration
Site"
A
B
C
D
E
F
G
H
H
J
K
pH
8.3
8.9
8.9
9.2
94
12.2
7.8
7.1
• ("I
5 8
6.9
Calcium
2,100
1,060
2,720
1,880
1,880
1,990
150
1,300
1,510
1,250
550
Sulfate
1,525
1,875
1,575
1,500
1,400
1,100
6.600
1,930
1,875
1,438
2,250
Chloride
4,600
1.850
4,700
2,950
2,700
2,000
3,600
3,500
6,600
3,500
2,450
Sulfite
4
3
45
56
32
Total
dis-
solved
solids
8,560
5,160
9,240
6,750
6,190
6,700
1 4,000
9,200
10,756
9,398
6,694
Arsenic
0.024
0.004
0.002
0.24
0.004
0.002
0.14
<0.003
(b)
0.09
0.03
Boron
44
97
34
93
80
76
93
120
140
105
95
(mg/l)
Lead
0.49
<0.02
<0,01
<0.02
<0.01
<0.01
<0.01
<0.01
Sb)
0.67
0.13
Mag-
nesium
290
2.5
33
50
12
0.3
5,000C
540
1,100
681
764
Sodium
(b)
17
46
56
41
70
12
62
116
107
68
Sele-
nium
0.005
0.020
0.018
0.014
0.014
0.042
0.63
0.14
(b)
0.008
0.035
Mercury
<0.0001
0.00024
<0.0001
0.0003
0.00033
<0.0002
<0.0002
<0,0002
Ib)
0.002
0.0007
Chemical
oxygen
demand
-------
2. FGD Waste
Disposal Methods
Chemically Treated Waste
The physical stabilization of FGD
wastes by chemical treatment
can prevent water pollution and
permit site reclamation. Chemical
treatment has the following
characteristics:
• Converts the waste to a structural
material
• Decreases the waste's coef-
ficient of permeability
• Reduces the initial concentration
of soluble salt constituents in
the leachate
• Permits disposal in a pond or a
landfill above or below grade
• Allows contouring of waste to
promote the runoff of rainwater
Sites B, C, and E contain wastes
that were chemically treated during
late 1974 and early 1975 by the
Dravo Corporation, ID Conversion
Systems, Incorporated, and Chemfix
Corporation, respectively. Site B
simulates a disposal site in which
the waste cures underwater and
remains there, except for periods
of extended drought. Site C initially
represented a depression in a
landfill where rainwater collects,
and Site E initially represented a
landfill that traps rainwater that
collects in a sump at its lower end.
Site B remains as originally con-
figured. Site C was converted in
March 1979 to a runoff configuration,
and in November 1977 Site E was
covered with clay, which was
contoured and planted with grass.
Chemically treated Site E, containing trapped water, supporting drilling rig
during waste coring operation
-------
Typical chemical characteristics of
leachates from the wastes that were
treated chemically are presented
in Figure 2. Chemical treatment
did not achieve substantial reduc-
tions in the concentration of trace
elements in the leachates. In
some instances, the concentrations
of these minor constituents in
the leachates are somewhat higher
than the concentration in the
untreated input liquors. These
higher concentrations probably
result from trace elements present
in the treatment additives.
Because FGD wastes are subject to
any State and local ordinances
(they are temporarily exempt from
Federal regulation under the new
Hazardous Waste Management
System), the following data are
presented. Of more than 600
leachate analyses of chemically
treated wastes analyzed for trace
elements from these sites, 2 showed
concentrations greater than 10 times
the level of the National Interim
Primary Drinking Water Regulations
(i.e., 13 and 20 times the regulation
for selenium). The TDS concen-
tration of leachate from treated ponds
initially ranged from 2,500 to
5,000 mg/l (approximately half the
concentration'of the untreated
liquor), and after approximately
2 years the concentration decreased
to 2,000 to 3,000 mg/l.
A general procedure for managing
rainfall runoff from a full-scale
chemically treated waste site is to
collect the runoff in a peripheral
ditch, which directs the water to a
settling pond. Depending on the
quality of the water [concentration
of TDS and total suspended solids
(TSS)] in the pond, it can be
decanted to a stream or returned
to the scrubber system. After closure
of part or all of the site, it is capped
with soil to support the growth of
vegetation and to prevent erosion.
All the chemically treated materials
(Table 4) demonstrate high ultimate
bearing capacities. For example,
Site B exhibits ultimate bearing
capacities between 150 and
(a)
g
cc
H
Z
LU
(J
O
(J
(b)
z
o
z
LU
O
Z
O
(J
6,000 |-
5,000
4,000
3,000
2,000
1,000
1975 1976
1977
YEAR
1978 1979
Note.—Total dissolved solids of input liquor before treatment = 9,530 mg/l.
102 .-
10'
10°
10"
10"
10"
1975
1976
1977
YEAR
1978
1979
Legend:
* Total dissolved solids
Sulfate
Chloride
Boron
Lead
Arsenic
Mercury
Figure 2.
Concentrations in Typical Chemically Treated Pond Leachate: (a) Total
Dissolved Solids and Major Species and (b) Minor Species
-------
Table 4,
Ultimate Bearing Capacity of FGD Wastes
Site
B
c
E
F
G
H .
Waste treatment
Chemicalb
Chemical'
None (underdrained)
Ultimate
bearing capacity
(Ib/in2)
1 50 to 300
Greater than 330
300 to greater than 330
60 to 75
1 80 to 240
1 35 to greater than 330
aDravo Corporation.
bIU Conversion Systems, Incorporated,
cChemfix Corporation.
dClarifier underflow.
Note.—Shawnee clay soil has an ultimate bearing capacity of 240 to 300 Ib/in2.
300 Ib/in2 (1,034 and 2,068 kPa),
and Sites C and E exceed 330 Ib/in2
(2,275 kPa), which is near the upper
limit of the field testing device.
The pollution potential of waste
effluent seeping into ground waters
is governed by the mobility of
leaching waters. This mobility is
limited by the coefficient of
permeability of the various media
through which the leachate must
pass. Laboratory analyses of core
materials from these three chemically
treated ponds (see Table 5) show
widely varying coefficients of
permeability as a result of cracks
that appear in some core samples.
The coefficient of permeability is
approximately the same for the
wastes in Sites C and E, which are
somewhat less permeable than Site B.
Although the materials in.Sites B,
C, and E have a permeability of
about 4 X 10~6 in/s (10~5 cm/s).
which is at least one order of
magnitude less than the untreated
wastes, several samples from Sites C
and E showed coefficients in the
range of 4 X 10~7 to 4 X 1£T8 in/s
(10"6 to 10~7 cm/s). A time-
dependent trend in the permeability
of core samples of the wastes
is not evident.
Other physical parameters of core
materials include solids content,
porosity (or void fraction), and
unconfined compressive strength.
Typical solids contents for the
cores from the three ponds are
45 percent for Site B, 61 percent
for Site C, and 52 percent for Site E.
The average void fractions are 0.75,
0.63, and 0.71 for samples from
Sites B, C, and E, respectively.
There are wide variations in the
unconfined compressive strength of
free-standing samples of these
materials (Table 5). These large
variations may be attributed in part
to random cracks that occurred
in some of the test samples.
Untreated Waste
Pond Disposal, Disposal of untreated
material in a pond is usually the least
costly method of FGD waste disposal.
If the pond does not have a base
material considered to be im-
permeable, a liner must be added
to prevent seepage. Clay or synthetic
liners may be placed in the base
and on the slopes of such ponds.
All three types of liners—indigenous
clay, purchased clay, and synthetic—
are in use today. Any pond con-
tinually exposed to the elements,
however, eventually is subject
to a degree of seepage because
liners are not completely imperme-
able and long-term durability is
uncertain. FGD wastes are thixotropic
in nature; therefore, ponds are
nonstructural sites that usually are
difficult to reclaim, except possibly
in areas of low rainfall and high
evaporation.
Control ponds for untreated lime
and limestone waste disposal were
installed at the Shawnee site and
are being monitored principally
for the determination of chemical
characteristics of the leachate. These
ponds, which are identified in
Table 1 as Sites A, A1, and D, are
totally saturated (except for
periods of extreme'drought).
The initial leachates of the untreated
waste ponds contained TDS concen-
trations ranging from 5,000 to
14,000 mg/l. The depletion of TDS
from the leachate as a function of
Table 5.
Physical Characteristics of Impounded, Chemically Treated FGD Waste Core Samples
Characteristic
Solids content (% by weight) , . .
Unconfined compressive strength, wet (Ib/in )
Density (g/cm3):
Wet
Dry
Permeability coefficient (cm/s)
Site B
45
28 to 84
1,37
0.63
0.75
2.1 X 10~4to3.7X 10~S
Site C
61
40 to 996
1.52
0.92
0.63
5.2 X 1 0^S to 3.2 X 1 0~7
Site E
52
24 to 260
1.36
0.70
0.71
1.1 X 10~4to6,9X 10"~7
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time is depicted in Figure 3a, which
shows that the chloride content
of the waste was essentially depleted
after approximately 2 years.
Thereafter, the leachates were
saturated with gypsum and have re-
mained so after 5 years. Analyses
have been made for the concentration
of the more significant minor
species present in these wastes
(Figure 3b). Except for boron, which
decreases steadily with time,
the minor species have shown only
slight reductions after 5 years of
seepage. Of nearly 900 analyses
of untreated leachate from these
sites, only 4 showed concentrations
of trace elements greater than
10 times the National Interim Primary
Drinking Water Regulations. Three
samples were 12, 14, and 20
times the regulation for arsenic,
and one sample was 11 times the
regulation for selenium.
The dewatering characteristics of
FGD wastes are important to the
various disposal techniques because
they affect the volume of the
disposal basin, the waste handling
methods, and the condition of the
wastes in their final disposal state.
The effectiveness of the dewatering
method used and the ability of a
waste to be dewatered depend on a
number of solids characteristics—
including the size and distribution of
particles and the crystalline
structure of the particles—that are
principally a function of the absorbent
and scrubber operating parameters.
In typical laboratory tests four
dewatering methods were evaluated:
settling and decanting, settling by
free drainage, centrifugation, and
vacuum filtration (see Table 6).
The highest density was obtained
by vacuum-assisted filtration.
However, there were relatively small
density differences between filtra-
tion and centrifugation. For most
FGD wastes, settling by free
drainage yields a slightly greater
density than dewatering by settling
only. This slight gain, coupled
with the associated higher solids
content, significantly increases
load-bearing strength. Table 6 shows
6,000 |-
5,000
1 4,000
<
EC
3,000
LU
Z 2.000
O
u
1,000
1975
1976
1978
1977
YEAR
Note.—Input liquor total dissolved solids = 5.373 mg/l.
1979
(b)
102
10'
^ 10"
<
EC
LU
U
8
10-
1975
1976
1977
YEAR
1978
1979
Legend:
Total dissolved solids
Sulfate
Calcium
Chloride
Boron
Arsenic
Lead
Selenium
Mercury
Figure 3.
Concentrations in Untreated Leachate at Site D: (a) Total Dissolved Solids
and Major Species and (b) Minor Species
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Table 6.
Bulk Densities of Untreated FGD Wastes
Dewatering method3
Fly ash
Shawnee source and sampling date (%bydry
weight)
Limestone:
Feb. 1, 1973
June 15, 1974
Lime:
Mar. 19, 1974
Sept. 8, 1 976
Sept. 8, 1976
20
40
40
40
~0
Settling and
decanting
Solids
(%by
weight)
49
53
42
45
47
Density
(g/cm3)
1.45
1.46
1.34
1.34
1.37
Settling by free
drainage
Solids
(%by
weight)
56
58
43
58
51
Density
(g/cm3)
1.51
1.53
1.36
1.50
1.41
Centrifugation
Solids
(%by
weight)
60
63
50
53
48
Density
(g/cm3)
1.56
1.60
1.44
1.44
1.38
Vacuum
Solids
(%by
weight)
65
66
56
61
57
filtration
Density
(g/cm3)
1.65
1.64
1.51
1.54
1.49
"Using laboratory equipment.
that the wet-bulk densities of lime-
stone FGD wastes ranged from
a low of approximately 90.5 Ib/ft3
(1.45 g/cm3) for settled wastes
to a high of 103 Ib/ft3 (1.65 g/cm3)
for vacuum-filtered wastes. Values
for lime FGD wastes were approxi-
mately 7 percent less than for
limestone wastes.
Figure 4 presents load-bearing
strengths of untreated FGD
wastes—including gypsum—as a
function of moisture, fly ash content,
and waste origin (power plant,
type of absorbent). Among other
considerations, the data highlight
the criticality of solids content
on the load-bearing strength of
untreated wastes. Solids content is
particularly important in the disposal
of slurried gypsum, gypsum filter
cake, and untreated wastes that
are underdrained because these
types of disposal depend on
dewatering to attain a desired
material-bearing strength. The data
indicate that these wastes may
Chemically treated Site C, which sheds water after conversion to a runoff configuration
-------
250 r-
200
150
cc
I-
(fl
O
<
O
100
50
60 70
SOLIDS CONTENT (% by weight)
80
90
Curve Power plant
Absorbent
Fly ash
(% solids
by dry
weight)
Sampling
date
Legend:
Carbide lime absorbent
Soda ash absorbent, double alkali
Lime absorbent
Limestone absorbent
Limestone absorbent (extrapolated)
Paddy's Run Carbide lime 12 July 1976
Scholz Soda ash, double alkali <1 June 20, 1976
Scholz Soda ash, double alkali 30 June 27, 1976
Shawnee Lime <1 Sept. 8, 1976
Shawnee Limestone <1 Nov. 30, 1976
Shawnee Lime 40 Sept. 8, 1976
RTPa Limestone <1 Dec. 4, 1975
Gadsby Soda ash, double alkali 9 Aug. 9, 1974
Shawnee Limestone 40 Nov. 30, 1976
Phillips Lime 60 June 17, 1974
RTPa Limestone 40 Sept. 30,1975
Shawnee6 Limestone <1 Sept. 1977
Cholla Limestone 59 Apr. 1,1974
Shawnee0 Limestone <1 May 2, 1979
'Gypsum, contains 5% sulfite.
bGypsum slurry.
°Gypsum cake with 13% unreacted calcium carbonate.
Figure 4.
Load-Bearing Strength as a Function of Moisture, Fly Ash Content, and Waste Origin
-------
be dewatered to a narrow range
of solids content, above which
the load-bearing strengths increase
rapidly to values well above
the minimum for safe access of
personnel and equipment. In addi-
tion, the critical concentration
appears to be unique for each type
of waste tested.
Figure 4 also illustrates the effect
of the absorbent and fly ash on
dewatering characteristics. Lime-
stone FGD wastes are capable of
being dewatered to higher solids
contents, whereas lime wastes
yield a higher load-bearing
strength at lower solids contents.
The presence of fly ash enhances
dewatering in both wastes; however,
for any specific solids content of a
given waste, the load-bearing
strength is less with fly ash than
without.
The permeability coefficient of
untreated wastes (lime and lime-
stone) containing fly ash is approxi-
mately 8 X 1CT5 in/s (2 X 1CT4 cm/s),
which is comparable to typical
values of 4 X 10~5 in/s (1CT4 cm/s)
for silty sand.
The viscosity of an FGD waste
indicates its pumpability, which could
affect both the mode and cost of
transport. The results of viscosity
tests for various untreated wastes
define dewatering limits for
certain waste materials if they are
to be pumped. The tests also show
that easily pumpable mixtures [less
than 20 P (2 Pa -s)] range from
a high solids content of approxi-
mately 55 percent by weight to
a low of 30 percent by weight,
depending on waste origin,
absorbent, and ash content (see
Figure 5). This figure also shows
that FGD wastes of a given type are
more pumpable if they contain
fly ash. For example, Shawnee lime-
stone waste with ash is pumpable
[less than 20 P (2 Pa-s)] at a
solids content up to 52 percent.
Typical condition of chemically treated Site B, in which supernate covers
3 ft of stabilized FGD waste
whereas Shawnee limestone waste
without ash is pumpable at 42 per-
cent. Of those tested, the most
difficult to pump would be the GM
Parma and Utah Power and Light
double alkali wastes and the Louisville
Gas and Electric carbide lime wastes,
all of which contain low percentages
of fly ash and are pumpable only
up to solids contents of 32 to
35 percent.
Underdrained Pond Disposal. An
underdrainage system at the disposal
site can collect all seepage for
return to the scrubber system,
thus maintaining control of leachate
during the fill period. Underdrainage
also enhances dewatering, which
results in a material that can
support personnel and construction
equipment. This type of disposal
requires an earthen cap that is
contoured and maintained to shed
water after disposal site closure.
Sites F and G and the initial phase of
Site H are ponds containing
untreated FGD wastes deposited in
an underdrained impoundment.
Perforated plastic pipes are imbedded
in pea gravel under the 1-ft- (0.3-m-)
thick sand layer at the base of each
impoundment. Leachate that seeped
through the waste is fed by the
plastic pipes through a gravity
drain system to a sump. Water is
pumped from the sump to remove all
seepage. At an operational site, the
underdrainage (and all unevaporated
rainfall on the disposal pond)
would be recycled to the scrubber,
thereby reducing the normal amount
of fresh makeup water. Recycling
would increase the concentration
of soluble salts (i.e., chloride) in
-------
140
120
100
£ 80
£
CO
O
O
CO
60
40
20
25
30
35
40 45 50
SOLIDS CONTENT (% by weight)
55
60
65
70
Curve Power plant
Absorbent
Fly ash
(% solids
by dry
weight)
Sampling
date
Legend:
Soda ash absorbent, double alkali
Carbide lime absorbent
Limestone absorbent
Lime absorbent
a Parma Soda ash, double alkali 7 July 18, 1974
b Paddy's Run Carbide lime 12 July 1976
c Gadsby Soda ash, double alkali 9 Aug. 9, 1974
d Shawnee Limestone <1 Sept. 28,1976
e Shawnee Lime <1 Sept. 8, 1976
f Shawnee Lime 40 Mar. 19, 1974
g Scholz Soda ash, double alkali <1 June 20, 1976
h Scholz Soda ash, double alkali 30 June 27, 1976
i Shawnee Lime 40 Sept. 8, 1976
j Phillips Lime 60 June 17, 1974
k Shawnee Limestone 20 Feb. 1,1973
I Shawnee Limestone 40 June 15, 1974
m.... Shawnee Limestone 41 July 11, 1973
Figure 5.
Viscosity of Untreated FGD Wastes
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Untreated underdrained Site G supporting personnel during filling operation
the scrubber loop. Approximate
scrubber design considerations may
be necessary in some instances
of high chloride content.
A low hydrostatic pressure exists
at the pond/subsoil interface
because the hydraulic head is
interrupted at the porous base and
the system is vented to the atmos-
phere. For example, if the subsoil
coefficient of permeability were
4X 1CT7 in/s (1
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Filling lower portion of Site H with scrubber clarifier underflow slurry that has been force-oxidized to gypsum
Force-Oxidized Waste
The oxidation of FGD wastes to
gypsum results in a material that
is readily dewatered by vacuum
filtration or by centrifuging to a solids
content in the range of 75 to 85
percent by weight. When stacked
above grade, filter cake may crack
under freeze-thaw or wet-dry
cycling, thereby allowing the entry
of rainwater. Additionally, it
may erode when exposed to rainfall
and produce a runoff containing
high concentrations of dissolved
solids. These observations indicate
that special site maintenance may
be required on an operational
scale to reconfigure the disposal
pile after weathering and to control
the runoff.
The disposal of slurried gypsum
that is allowed to drain or settle in
an impoundment (e.g., the base of
Site H) produces a structurally
stable material. Operationally,
excess moisture would have to be
decanted or underdrained and
returned to the scrubber. Stacking of
settled gypsum slurry may be
superior to the stacking of filter cake,
but this procedure has not yet
been evaluated at the Shawnee site.
Sites J, K, and the stacked portion
of Site H contain limestone scrubbing
FGD wastes that were force-oxidized
to gypsum and filtered. (The
evaluation of the lower portion of
Site H, which contains clarifier
underflow slurry, was discussed in
the preceding subsection on
underdrained pond disposal.) Input
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Gypsum filter cake stacked on the upper portion of Site H
conditions are presented in Tables 1,
2, and 3. The composition of these
filter cake materials represents a
range of oxidation, with unreacted
limestone concentrations at 1, 13,
and 25 percent by dry weight for
Sites J, H, and K, respectively. Tests
at these sites determined the impact
of stacking FGD gypsum on the
ground. Observations have been
made of erosion, runoff water
quality, surface crust formation, crust
bearing strength, and strength
loss when moisture content is
increased.
Chemical analyses have-been made
of leachate collected in the under-
drainage system and of runoff
samples from Site H. The analysis
of underdrainage is similar to that
of an untreated FGD waste leachate
shown in Figure 3. The TDS
concentrations in both underdrainage
and runoff samples decrease with
time. The runoff has a TDS slightly
in excess of 2,000 mg/l and a TSS
that ranges between 4 and 300 mg/l,
indicating the need to control runoff
in this type of disposal to prevent
seepage into underground supplies
of drinking water or direct discharge
into streams.
At Sites H and J, FGD gypsum
filter cake was stacked so that
a natural slope occurred with a
conical shape, producing a surface
of about 35° to the horizontal.
It is likely that erosion would be a
problem with disposal of FGD gypsum
in this manner. For example, at
Site H, after 18 months of weather-
ing, approximately 20 percent of
the mass flowed to the base, produc-
ing a slope of about 45°. At Site J,
which was stacked the following
year, the same erosion pattern is
developing.
Several tests were made with a D-8
Caterpillar tractor at Site J to
determine the maximal angle at
which the vehicle could negotiate
the slope of this material. During
testing, the cleats of the tractor broke
through the crust and the vehicle
lost traction at an angle of 17° to
the horizontal. After approximately
six passes over the same spot, the
material became so moist that it
could not support personnel.
Field penetrometer readings
were taken of ultimate bearing
capacity on the gypsum stacks. During
periods of dry weather, a crust
was observed with a thickness vary-
ing from approximately 2 in (5 cm)
near the peak to 5 in (13 cm) at
midheight, and 11 in (28 cm) at the
base. Bearing capacities of the crust
varied from 60 Ib/in2 (414 kPa)
near the peak to 450 Ib/in2
(3,103 kPa) near the base. The
crust, however, absorbs water and,
during periods of continued rainfall,
reverts to a material character-
istic of the original FGD gypsum
filter cake. When water is allowed
to collect, the gypsum increases in
moisture content and approaches a
slurried condition.
Two sets of FGD gypsum samples
were obtained from the pilot FGD
scrubber at EPA's Industrial
Environmental Research Laboratory
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Gypsum filter cake at Site J, which shows signs of slumping and erosion 6 mo after placement
at Research Triangle Park. After
filtration, one set of samples
that was completely oxidized
exhibited substantially higher
unconfined compressive strength
[60 Ib/in2 (414 kPa)] than the other
set [20 to 25 Ib/in2 (138 to 172 kPa)],
which contained about 5 percent
sulfite. The unconfined compressive
strength of the samples containing
about 5 percent calcium sulfite
was approximately equal to that
of an FGD waste that is predominantly
calcium sulfite. Also, the addition
of fly ash had little effect on any
of these samples.
Typical wet-bulk densities for
gypsum filter cake with 80 percent
solids content are between 81 and
87 Ib/ft3 (1.3 and 1.4g/cm3).
For the Shawnee gypsum clarifier
underflow that is saturated and
settled in the impoundment beneath
the Site H filter cake pile (see
Table 1), the wet-bulk density is
approximately 106 Ib/ft3 (1.7 g/cm3).
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3. FGD Waste Disposal
Costs
Periodic cost estimates are made
for FGD waste disposal by various
methods related to the type
of disposal evaluation being con-
ducted at Shawnee. The cost
estimates for the disposal methods
given in Table 7 are based on the
conditions summarized in Table 8.
The costs, given in mid-1980
dollars, are engineering estimates
that are typical for each type
of disposal. Cost-effective and
environmentally sound disposal
methods—namely, chemical treat-
ment, untreated with underdrainage,
and gypsum with an indigenous
liner and surface drainage—cost
between 1.05 and 1.25 mills/kWh.
The disposal cost of chemically
treated waste is an average of costs
derived from data provided by
the three treatment contractors who
participated in the Shawnee project.
This average was updated to
current conditions.
For the indigenous liner case, a soil
permeability coefficient of 4 X 10~8
in/s (10~7 cm/s) was taken to be
representative of clay to be used
for pond lining and, consequently,
no cost was associated with liner
material. The cost of pond construc-
tion for disposing of untreated
(and force-oxidized) wastes
increases if a synthetic liner must
be added. The estimated installed
cost of a synthetic liner is $5/yd2.
For the underdrained pond, a
seepage model (with replenishment)
based on Darcy's Law was created
to derive pipe spacing relationships
with various sand bed depths
for installation beneath a disposal
pond. Using a layer of sand 1 ft
(0.3 m) thick and following the
requirements of a theoretical water
level in the drain of zero, the maximal
spacing between drains in a
The underdrainage system, which drains seepage from the bottom of the pond
and promotes dewatering of the waste (pipes may be spaced 100 ft or
more in an operational site)
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Table 7,
Disposal Cost Estimates
Cost estimate
Disposal method
mills/kWh
Waste, dry
($/short ton)
Coat
($/short ton)
Chemically treated waste landfill 1.25 11.85 3.60
Untreated waste pond:
Indigenous liner 0.65 5.90 1.75
Synthetic liner 1.40 12.50 3.80
Underdrained 1.05 9.40 2.85
Force-oxidized waste with surface drainage:
Indigenous liner 1.20 10.20 3.15
Synthetic liner 1.75 15.05 4.65
Note.—Mid-1980 cost basis. All waste includes ash.
Table 8.
Summary of Base Conditions
horizontal base was found to be
133 ft (41 m). A pond designed for
a minimal sand bed depth is the
lowest cost design for this type of
disposal. Ten 50-acre (20-ha) ponds
are required for this disposal mode
according to the baseline conditions
in Table 8. The cost of the under-
drainage components, including
sand, gravel, pipes, and fittings,
is approximately 20 percent of
the total capital cost.
In the disposal of force-oxidized
FGD waste, it was assumed that a
15-percent slurry is pumped to the
site where it settles to 65 percent
solids and the supernatant water is
recycled. The cost of the oxidation
equipment is included as part of
the disposal costs.
Item
Base condition
Cost basis Mid-1980 dollars
Plant characteristics Two 500-MWe units burning coal at
9,000 Btu/kWh
Coal burned 3.5% sulfur; 12.00O Btu/lb; 14% ash
Annual operating hours 4,250 h, with 48.5% capacity factor for
30-yr life (average)
Plant disposal site lifetime 30 yr
Sulfur dioxide removal 90%
Waste generated:
Untreated limestone pond (settled to 50% solids). . . 4.8 X 105 short tons/yr, dry
Force-oxidized slurry (settled to 65% solids) .. . 4.9 X 10s short tons/yr, dry
Limestone utilization:
Untreated waste 80%
Force-oxidizpd waste 100%
Annual capital charges, 30-yr average ,,,,. 17%
Cost of land used for disposal $5,000/acre
Disposal site location Within 1 mi of plant
Total disposal area requirements (including berrn re-
quirements) for a 30-ft waste depth:
Chemically treated waste (lime/fly ash additive).... 480 acres
Untreated waste 540 acres
Force-oxidized waste 440 acres
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This report was prepared jointly by The Aerospace Corporation of
Los Angeles CA and the Centec Corporation of Reston VA for the U.S.
Environmental Protection Agency. Paul R, Hurt, Paul P. Leo, Jerome
Rossoff, and Jack R. Witz of Aerospace are the principal investigators.
Photographs were provided by The Aerospace Corporation. Julian W,
Jones is the EPA Project Officer.
Comments on or questions about this report or requests for information
regarding the disposal of flue gas desulfurization wastes should be
addressed to:
Emissions/Effluent Technology Branch
Utilities and Industrial Power Division
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency (MD 61}
Research Triangle Park NC 27711
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, Research Triangle
Park NC, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
COVER PHOTOGRAPH: Untreated FGD waste disposal evaluation pond at
Shawnee showing coring locations, access pier, leachate well, and
weather station
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EPA
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