WATER POLLUTION CONTROL RESEARCH SERIES • 14010 FJX 12/71
Dewatering of Mine
Drainage Sludge
U.S. ENVIRONMENTAL PROTECTION AGENCY
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WATER POLLUTION CONTROL RESEARCH SERIES
The. Water Pollution Control Research Series describes the
results and progress in the control and abatement of pollution
in our Nation's waters. They provide a central source of
information on the research, development, and demonstration
activities in the Environmental Protection Agency, through
inhouse research and grants and contracts with Federal, State,
and local agencies, research institutions, and industrial
organizations.
Inquiries pertaining to Water Pollution Control Research Reports
should be directed to the Head, Publications Branch (Water),
Research Information Division, R&M, Environmental Protection
Agency, Washington, D.C. 20460.
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Dewatering of Mine Drainage Sludge
by
Coal Research Bureau
West Virginia University
Morgantown, West Virginia 26506
for the
ENVIRONMENTAL PROTECTION AGENCY
Project 14010 FJX
December, 1971
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price $1.00
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EPA Review Notice
This report has been reviewed by the Environmental Protection
and approved for publication. Approval does not signify
that the contents necessarily reflect the views and policies
of the Environmental Protection Agency nor does mention of
trade names or commercial products constitute endorsement or
recommendation for use.
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ABSTRACT
This report is a literature review on thickening and dewatering of
sludge resulting from lime or limestone neutralization of coal mine
drainage.
The effects of mine water constituents and methods of treatment on the
physical and chemical characteristics of the resulting sludge are
described. Such current practices as aeration, recirculation and
neutralization are discussed. Additional techniques at various stages
of development, such as thickening, conditioning, and dewatering are
evaluated for use in coal mine drainage treatment.
The most promising coal mine sludge dewatering technique appears to be
vacuum filtration. Other methods such as sand bed filtration, pressure
filtration and centrifugation may also be applicable.
Recommendations are made as to the areas in coal mine drainage treat-
ment and sludge densification that need further research.
This report is submitted in partial fulfillment of Grant 14010 FJX under
the sponsorship of the Environmental Protection Agency and the Coal
Research Bureau of West Virginia University.
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TABLE OF CONTENTS
SECTION PAGE
I. Conclusions I
II. Recommendations 3
III. Introduction 5
IV. The Formation and Characteristics of Mine Water Sludge 7
A. General Characteristics 7
1. Physical-Chemical 7
2. Chemical Analyses of Coal Mine
Drainage and Sludge 9
B. Comparison with Other Sludges 10
1. Pickle Liquor 10
2. Municipal Waste and Water
Treatment 15
3. Plating Waste 15
V. The Effect of Raw Water Chemistry on Sludge Formation
and Characteristics 17
VI. The Effect of Treatment Processes on Sludge Formation and
Characteristics 19
A. Lime Neutralization 19
1. Conventional 19
2. Non-Aeration 19
3. Mechanical Aeration 20
4. Lagoon Aeration 20
5. pH 22
6. Sludge Recirculation 26
7. High Density Sludge Process 29
8. Densator ® Process 30
9. Elpo Treatment Process 33
10. Magnetic Sludge 34
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TABLE OF CONTENTS
(Continued)
SECTION
B. Limestone Neutralization
1. Conventional (Coarse Size Stone)
2. Aeration
3. Sludge Recirculation
4. Biochemical Oxidation
5. Rotating Drum
6. Ground Limestone
C. Limestone-Line Neutralization
VII. Sludge Settling
A. Sedimentation
B. Sedimentation Basin Design
VIII. Sludge Conditioning
A. Thickening
B. Chemical
C. Freezing
D. Ultrasonic
E. Heating
F. Artificial Seeding
G. Other
H. Summary of Sludge Conditioning
IX. Sludge Dewatering
A. Vacuum Filtration
B. Vacuum Filtration and Filter Aids
C. Porous Bed Drying
D. Pressure Filtration
E. Cycloning
F. Centrifugation
G. Thermal Drying
H. Screening
I. Flotation
J. Lagooning
K. Summary of Sludge Dewatering
PAGE
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TABLE OF CONTENTS
(Continued)
SECTION PAGE
X. Cost Comparison Between Methods for Dewatering 71
XI. Methods of Sludge Disposal 73
XII. Regulations Concerning Sludge Disposal 75
XIII. Uses for Sludge 77
XIV. Acknowledgments 79
XV. References 81
XVI. Glossary of Terms 89
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FIGURES
No. Page
1 Influence of Iron Oxidation on Pickle Liquor Sludge 13
2 Influence of Treatment pH.on Pickle Liquor Sludge 14
3 Influence of Aeration on Sludge Volume and Settling
Characteristics 21
4 Influence of Temperature on Iron Oxidation Rate 23
5 Oxygen Diffusion Into a Still Pond at 60°F 24
6 Influence of pH on Iron Oxidation Rate 25
7 Influence of Treatment pH on Sludge Volume 27
8 Mine Drainage Treatment Plant Utilizing Sludge
Recirculation 28
9 High Density Sludge Process vs. Conventional Process 31
10 Densator ® Treatment Plant 33
11 Settling Rate of Sludges from Limestone, Limestone
and Lime, and Lime Treated Mine Water 37
12 Flow Diagram of Complete Biochemical Oxidation and
Limestone Neutralization Process 38
13 Heights of Precipitated Sludge From Lime and
Limestone Neutralized Water 40
14 Comparison of Settling Rate for Lime, Limestone and
Soda Ash Sludges 42
15 Batch Sedimentation 46
16 Settling Curve 46
17 Effect of Treatment With Flyash and Calgon 240 on
Settling of Fe(OH)2 Sludge 53
18 Flow Chart of Future AMD Treatment System 59
19 Sketches of Drum Vacuum Filters 61
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TABLES
No,
1 Thompson Shaft Borehole Sludge Analysis - Hydrated
Line Treatment 7
2 Comparative Analysis of Coal Mine Drainage, Pickle
Liquor and Rinse Water 11
3 Influence of Ferrous Iron Content on Sludge Settled
Solids 29
4 Percent Solids of Lime, Limestone and Soda Ash
Sludges 41
5 Effect of Coagulant Aids on Fe(OH)2 52
6 Effect of Coagulant Aids on Fe(OH)2 and Fe(OH)3 54
7 Effect of Evaporation on Sludge Volume 69
8 Coal Mine Sludge Dewatering Attempts 70
9 Costs of Sludge Handling System for Sewage Sludge 71
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CONCLUSIONS
The major conclusions that can be drawn from this review are:
1. Coal mine water constituents and treatment methods strongly affect
the resulting sludge characteristics. The most important sludge charac-
teristics affected are volume, density, and the ability to flow, settle
and dewater.
2. New coal mine drainage treatment methods, such as the Densator
Process and High Density Sludge Process, are available that produce
sludges with high concentrations of solids. However, these new treat-
ment methods are not applicable to the full spectrum of coal mine waters.
3. Where applicable, limestone neutralization produces a sludge that
generally settles faster, and settles to a smaller volume with a higher
density, as compared to conventional line neutralization methods.
4. From the research reported to date, vacuum filtration alone or
used in conjunction with a precoat appears to be the most feasible
method of dewatering coal mine sludge. Other processes such as sand
bed filtration, centrifugation and pressure filtration may also be
applicable.
5. Mine drainage sludge can be slightly densified by the use of a clari-
fier thickener or Hydraulic Rake™ Rapid settling of the sludge solids
can be achieved by chemical conditioning with organic polyelectrolytes.
Sludge conditioning by artificial freezing has been attempted on other
sludges with reasonable success. Further application of freezing to
mine drainage sludge could be possible if the cost of the freezing pro-
cess were reduced.
6. When land is available, lagooning is the method most commonly used
in the treatment of water for clarification and sludge disposal.
However, plain lagooning does not allow the sludge to undergo significant
concentration.
7. When feasible, treatment plant operators are disposing sludge to
inactive, deep coal mines. This disposal method allows for the construc-
tion of smaller clarification basins which leads to reduced construction
costs. The ecological effects of deep mine disposal have not been
determined.
8. There is currently no practical use for coal mine sludge nor is there
any practical method for the recovery of by-products.
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RECOMMENDATIONS
More research is needed to:
1. Develop a better understanding of the effects of raw water
chemistry and treatment methods on sludge characteristics. Raw
water chemistry is so variable that the resultant sludge can
change from day to day. It is recommended, therefore, that
research be directed toward studying the effects of different
treatment methods and the raw water chemistry variability on
the resultant sludge.
2. Emulate, with coal mine sludge, the initial success achieved
with the dewatering and volumetric reduction of sewage and water
treatment sludge using freezing and thawing techniques.
3. Determine the feasibility of using organic polyelectrolytes
on coal mine drainage sludge to aid vacuum filtration and to
increase the rate of water clarification. Similar applications
of organic polyelectrolytes to waste treatment processes have
proven highly successful.
4. Further enhance the advantages of sludge recirculation combined
with limestone and lime neutralization processes. These processes
increase the percent solids, reduce sludge volume, and increase
treatment efficiency.
5. Amplify the potential of tube settling which appears to be
a feasible clarification technique for certain mine waters.
6. Study the dissolution of coal mine sludge in underground mines.
7. Evaluate pressure filtration and centrifuging which seem to have
been prematurely disregarded as feasible methods for dewatering coal
mine sludges.
8. Realize by-product recovery or direct utilization of coal mine
sludge.
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INTRODUCTION
The coal mining industry, under severe pressure from states such as
the Commonwealth of Pennsylvania, has in some cases adopted effective
measures in dealing with the drainage resulting from active coal
mines. Currently, acid mine drainage treatment employs the use of
lime or limestone as neutralizing agents. While being effective in
mine drainage treatment, these neutralizing agents contribute to the
production of substantial quantities of sludge or insoluble precipitates.
In some cases as much as 33 percent of the treatment plant inflow can
remain as sludge.(1)
This precipitated sludge may be composed of over 99 percent water
which compounds handling and disposal problems. In West Virginia
alone, enough sludge is currently produced to require an estimated
400 acres of land a year on which to construct storage ponds.
Further, this creates a situation where large land areas in northern
West Virginia and other sections of the State are put to inefficient
use.(2)
Sludge is one of the products of an expensive water treatment process
and must be handled by the coal mine operators. Since sludge is
valueless, it is desirable to dispose of this waste product in the
cheapest manner possible. Currently, sludge is either perpetually
stored in ponds, pumped underground into mined-out workings, trucked
to abandoned mines for disposal, or dumped into drying lagoons.
This situation may be altered in the future. Land areas for storage
lagoons may be too expensive or not available and underground disposal
may be undesirable. Coupled with this, the national alarm over eco-
logical disturbances due to waste disposal has motivated industry, the
Federal and State governments, and universities to make an indepth re-
view of the coal mine sludge question.
To this end, this report reviews the current status of coal mine
sludge densification and dewatering as a means of obtaining a better
understanding of the coal mine drainage treatment system and methods
of improvement.
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THE FORMATION AND CHARACTERISTICS OF MINE WATER SLUDGE
General Characteristics
Physical-Chemical
There are two effluents from the mine drainage neutralization process:
(1) treated water and (2) sludge.
Sludge formed from the neutralization process is generally affected
first, by the mine water composition and second, by the neutralization
method.
The chemical composition of acid mine drainage sludge is highly vari-
able and non-uniform in nature. LovellO) reports that sludge is
generally composed of hydrated ferrous or ferric oxides, gypsum,
hydrated aluminum oxide, varying amounts of sulfates, calcium, car-
bonates, bicarbonates, and trace amounts of silica, phosphate, manga-
nese, titanium, copper and zinc. A typical sludge analysis is shown
in Table 1.
Table 1
Thompson Shaft Borehole Sludge Analysis - Hydrated Lime Treatment
Calcium Sulfate CaSO^ — 40%
Magnesium Sulfate MgSO^ — 5%
Free Lime CaO — 3%
Magnesia MgO — 1%
Ferric Oxide Fe2°3 ~~ 15%
Manganese Oxide Mn203 — 4%
Silica Si02 — 20%
Aluminum Oxide A12°3 ~~ 12%
(4)
After Young and Steinman
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As mine drainage typically contains high iron concentrations, the pre-
cipitation of ferric hydroxide (Fe(OH)3) or "yellow boy", accounts for
the generally high concentrations of this compound that occur in coal
mine sludge. Because of the captured water within this iron compound,
much of the sludge settleability and final volume will be influenced
by ferric hydroxide as opposed to the other hydrous oxides present within
the sludge. The formation of ferric hydroxide occurs by the oxidation
of the ferrous iron to the ferric state followed by hydrolysis or by
the formation of the ferrous hydroxide followed by oxidation to the
ferric state. Iron III, (ferric iron) in the presence of most alkaline
agents, begins to precipitate at approximately pH 4 and approaches
residual concentrations at pH values between 6 and 7.^)
Ferrous hydroxide (Fe(OH)2) also exhibits control over sludge properties.
Being similar in nature to its ferric counterpart, ferrous hydroxide
"binds" considerable amounts of water and also affects sludge settling
and final volume.
The sulfate concentration in sludge varies with the amount of sulfate
present in the parent acid mine water. When high concentrations of
lime or limestone are needed to neutralize highly acidic waters,
the solubility limit of calcium sulfate (approximately 2000 mg/1)
may be exceeded and precipitation would then occur. The precipi-
tation of calcium sulfate not only changes sludge properties, but
creates problems by formation of scale on sludge withdrawal ports,
overflow pipes and channels.
Sludge physical and chemical properties are affected by both mine
water chemistry and process parameters which include the sequence
of unit operations, reaction rates, completeness of the reactions,
temperatures, agitation, catalytic effects, and bacteriological
influences. '•*)
The more important physical sludge properties are settleability,
density, "dewaterability", particle characteristics, particle surface
properties, and sludge flow properties such as viscosity.™)
The general term "sludge settleability" now seems to have the conno-
tation of combining into one term the aspects of sludge settling
rate and final sludge volume formed from a given amount of treated
water. Results from a study by Sanmarful^5' on the effects of dif-
ferent neutralizing agents (using synthetic acid mine drainage) on
sludge settling rates, densities and compaction levels showed that
carbonates yield a granular, dense sludge as compared to the semi-
gelatinous form resulting from hydroxide neutralizing agents.
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Sludge density is another physical property of interest. Sludge
densities are generally reported as percent solids by weight.
Densities ranging from 0.9 to 4.98 percent from clarifier underflows
when using hydrated lime as the neutralizing agent have been reported
by Charmbury and Maneval(°). Wilmoth and Hill") conducted limestone
neutralization studies and found sludge solids as high as 9.5 percent.
Properties of coal mine drainage sludge such as viscosity have not
been reported in any depth in the literature. The ability of sludge
to flow is of considerable importance since sludge is frequently pumped
or otherwise moved from one location to another. Viscosity, therefore,
is important in treatment plant design.
Surface properties, particularly electrostatic charge, influence the
coagulation of sludge floes. The ability of sludge floes to aggregate
affects the sludge settling rate which in turn is related to sedimen-
tation basin size. Use of coagulating aids to reduce the zeta potential
can greatly enhance settling rate.
Sludge particle size, like electrostatic charge, is a physical property
that influences coagulation and sedimentation processes. Factors that
affect floe size and the relative importance of floe size are discussed
in later sections of this report.
Sludge "dewaterability" connotates the ability of sludge to be con-
centrated into a more manageable form. The success of coal mine sludge
concentration, as will be shown later in this report, depends upon the
method of concentration and the type of sludge being studied.
Chemical Analyses of Coal Mine Drainage and Sludge
The concentrations of cations such as ferrous iron, ferric iron, calcium,
magnesium, aluminum, manganese and the sulfate anion as well as pH,
acidity and alkalinity properties are commonly analyzed in coal mine
drainage. Alkalinity refers to the concentrations of hydroxide, carbonate
and bicarbonate anions present in solution, while acidity is derived from
a combination of the free acidity normally resulting from the hydrolysis
of iron sulfate plus acidity due to cation oxidation as, for example, the
oxidation of ferrous iron. Acidity and ferrous iron determinations can be
used to predetermine the quantity of alkaline agent and aeration, re-
spectively, which would be required to neutralize the mine water.
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Water characterization is generally carried out by chemical analyti-
cal techniques specifically designed for acid mine water analysis or
adapted from other water analysis methods. Analytical kits produced
by Hach Chemical Company and others can be used for chemical analysis
of mine water in the field. Laboratory techniques, using both wet
chemical and instrumental methods, have been published which mav be
directly used or adapted for coal mine drainage analysis. * » ^
Coal mine sludge analysis has traditionally involved determination
of percent solids, settled volume and settling rate. Very little
work is being done in the area of chemical analysis of sludge. How-
ever, some of the conventional chemical analysis techniques such as
atomic absorption spectroscopy can be readily adapted to coal mine
sludge analysis. Some of the more important elements in coal mine
sludge that can be determined by absorption spectroscopy are iron,
calcium, silicon, magnesium and aluminum.
Comparison With Other Sludges
Acid wastewaters have been discharged in large quantities for many
years. Some of these acidic wastes have been treated with various
neutralizing agents. Since lime and limestone are relatively
cheap neutralizing agents their use has been extensive. However,
their use creates problems due to the production of substantial
amounts of sludge.
A number of sludges produced from lime or limestone treatment are
reviewed in this section with the intention of presenting an over-
view of sludges and their properties. This overview will illustrate
the variability of sludges and sludge properties and describe other
waste sludges which often undergo some comparable form of treatment
which could be applicable to coal mine drainage sludge.
Pickle Liquor
Steel fabrication processes produce a surface scale that must be
removed prior to finishing operations. The most common method of
scale removal is by immersing (pickling) the steel product in a
dilute sulfuric acid solution (15-25 percent by weight). After
immersion in the pickling solution the steel is rinsed in clean
water for removal of sulfuric acid. The pickling solution, after
repeated usage, will contain large amounts of ferrous sulfate
resulting from reaction of iron oxide and the sulfuric acid. As the
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ferrous sulfate content is increased, the cleaning action is decreased
to the point of ineffectiveness and the pickling solution must be dis-
carded. Water used to rinse the steel following the pickling operation
eventually becomes acidic and must be replaced. This rinse water
represents another waste that must be dealt with. A comparative analysis
of coal mine drainage, pickle liquor and rinse water is illustrated
in Table 2.
Table 2
Comparative Analysis of Coal Mine Drainage,
Pickle Liquor and Rinse Water
Comparative Analyses
rag/I or ppm
Mine Drainage
(Maneval 1966)
Morea Marianna
(strip pit) (bore hole)
"Typical"
Steel Mill Pickle Liquors
(FWPCA 1968)
Rinse Water Strong Liquor
Acidity as
CaC00 or
J
Fe
S°4
pH
190
6
290
3.2
4,040
815
10,000
2.6
3,000
4,100
10,800
less
38,000
52,000
125,000
than 2.0
(11)
After Dean
Although the compositions of the rinse water used on pickled steel and
of coal mine drainage are similar, the volume of drainage from coal
mines is much greater than the acid rinse produced from pickling
operations.
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Numerous treatment methods have evolved from the research on pickle
liquor. Various alkaline neutralizing agents have been studied with
high calcium quicklime or hydrate emerging as the most successful. Use
of lime as the neutralizing agent in pickle liquor treatment creates
voluminous amounts of sludges that are similar to the sludge from coal
mine drainage treatment.
A comparison between the chemical composition of mine drainage, rinse
water and strong pickle liquor presented in Table 2 illustrates that
the resulting sludges contain similar chemical constituents. Mine
drainage, however, can contain various amounts of magnesium, manganese,
silica and aluminum, which are usually precipitated during the neutra-
lization process and end up in the sludge.
The effects of these additional elements on mine drainage sludge
are not completely known due to the variability of raw mine waters.
Aluminum alone in the hydrous form can add to total sludge volume.
Parsons' ' summarized the results of research on sludges produced
from acid wastes. The following factors were found to affect sludge
volume and characteristics:
1. Alkaline agent
2. Acid concentration
3. Degree of neutralization
4. Temperature
5. Oxidation state
6. Seed nuclei
The volume of sludge formed from the treatment process represents the
amount of material to be handled; therefore, if an alkaline agent
produces an insoluble product, the ultimate sludge volume can be
directly influenced by that neutralizing agent.
The relationship between acid concentration and sludge volume exhibits
two interesting but contradictory phenomena. First, as acid concentra-
tion of the wastes increases, the percent solids or density of the sludge
tends to increase. Second, when sulfuric acid wastes are diluted with
water they become less concentrated. When this diluted waste is neu-
tralized with calcium based alkalies, the solubility product of calcium
sulfate is not exceeded. This dilution process decreases the calcium
sulfate precipitate which, in turn, reduces the total sludge volume.
The implication is that if a minimum sludge volume is desired there
are two opposing alternatives that can produce similar ends - namely
lower sludge volumes.
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The oxidation state of iron influences sludge density which in turn
affects the resulting sludge volume and settling rates. As illustrated
in Figure 1, the completely oxidized sludge settles quickly, but
compacts to only about 57 percent of the original volume. The un-
oxidized sludge settles slowly, but compacts to a final volume of 30
percent of the original. The sludge that is 65 percent oxidized
exhibits both high compaction and rapid settling.
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Figure 1 - Influence of Iron Oxidation on Pickle
Liquor Sludge. After Levine and Rudolfs
(13)
The hydrogen iron concentration (pH) or degree of neutralization is
another major factor to be considered in the neutralization of pickle
liquor. A pH of 8.5 is generally required for the precipitation of
ferrous iron, but this does not necessarily mean the optimum sludge
volume is achieved. The pH effects on sludge volume are presented
in Figure 2. A slight increase of pH from 8.5 to 9.5 reduces the
final sludge volume 30 percent, although increasing the pH to 11.0
increases the sludge volume.^ '
The amount of neutralizing agent added, therefore, is critical in the
effective treatment of the acid waste and the concurrent production
of a minimum sludge volume.
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UJ
Figure 2 - Influence of Treatment pH on Pickle Liquor
Sludge. After Levine and Rudolfs
Temperature effects on sludge characteristics can be expected due to
changes in viscosity and density of the solutions. Hoak and
Sindlinger'1 ^ have reported pickle liquor studies where the reaction
temperature was kept at 75°C or above and substantial improvement in
the sludge settling rate was observed.
The problem of dilute acid solutions resulting in high sludge volumes
has induced research into seed nucleation. Faust, et al.(13,16,17,18)
found that calcium sulfate could be induced to precipitate on gypsum
crystals which had been previously precipitated or intentionally added
as ground rock gypsum. The addition of either ground rock gypsum or
previously precipitated gypsum dramatically increased the suspended
solids concentration with a corresponding reduction in sludge volume.
Superior sludge properties resulted from the addition of ground rock
gypsum. However, the most practical method to provide previously pre-
cipitated gypsum was by the use of sludge recirculation.
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Pickle liquor sludge was initially thought to be dewaterable only by
pressure filtration.(19) As dewatering technology advanced other
techniques such as centrifugation were studied.(20) Levine and
Rudolfs(13) studied vacuum filtration of pickle liquor sludges and
found that neutralization with either dolomitic lime or high calcium
lime produces a sludge that could be acceptably dewatered.
A quicklime pickle liquor treatment process developed by Wing( '
produced a sludge that exhibited exceptional filtration properties.
The sludge cake from a rotary vacuum filter averaged about 58 percent
moisture.
It must be understood that the chemical composition of coal mine
drainage is highly variable as compared to the reasonably constant
composition of waste pickle liquor and rinse water. Due to pickle
liquor's extreme concentration of acidity, iron, and sulfates, direct
comparison to the "average" coal mine drainage may not be entirely
valid. However, research performed with lime and limestone neutra-
lization of pickle liquors has laid much of the ground work for
neutralization studies of coal mine drainage and much of the work has
direct application in coal mine drainage neutralization. Moreover,
the dewatering studies conducted on pickle liquor sludge have also
contributed to densification studies on coal mine drainage sludge.
Municipal Waste and Water Treatment
Municipal sludges have characteristics which are largely determined
by the waste treatment process. Being organic in nature, sewage sludge
is completely different in character from inorganic coal mine drainage
sludge.
Since the concentration process primarily thickens sludge and the de-
watering process removes or reduces the water content, the use of gravity
filtration, vacuum filtration and other concentrating and dewatering
techniques has established technology and general cost data which serve
as a guide in coal mine drainage sludge dewatering. Cost data will be
discussed in section X.
Plating Waste
Wastes from the plating industry are many and varied depending upon
the plating operation. In most cases wastewaters result from the
rinsing operations after the various plating processes. The treatment
of these waters is generally accomplished by precipitation, thickening,
and disposal. Lime slurry is a common neutralizer and precipitating
agent. The resulting sludge can contain various amounts of copper,
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nickel, chromium, zinc, aluminum, iron and other impurities depending
on the plating process.'22'
In most cases the quantity of sludge produced from plating waste
treatment is small compared to the sludge produced from coal mine
drainage treatment. Small and Graulich^22' reported that a moderate
sized plating operation creates about 1000 gallons of sludge per day
containing four percent solids. This sludge usually constitutes one
percent or less of the original volume of acid waste.
Dewatering of plating waste sludge is frequently accomplished by a
continuous vacuum filter. Thin filter cloths are used to eliminate
cloth blinding and a strong discharge system is used to remove the
dewatered cake.
In actual practice filtration rates have varied from 1.5 Ib to 30 Ib
of dry solids per hour per square foot of filter area.C22) The wide
variance in filtration rates clearly indicates the need for pilot
plant experimentation prior to construction of an operational vacuum
filtration system.
The problems experienced in treating plating waste sludges, particu-
larly the judgments as to filter size and type of cloth, are the same
problems a coal mine drainage treatment plant operator would encounter
if he were to construct a vacuum filtration dewatering system.
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THE EFFECT OF RAW WATER CHEMISTRY ON SLUDGE
FORMATION AND CHARACTERISTICS
^ J' and Barthauer^' found that coal mine drainage can be
separated into distinct types or classes according to chemical
characteristics. Three general classifications have been reported
and the chemical constituents within these classifications can
contribute to sludge formation and characteristics. It must be
remembered, however, that other factors such as treatment method
affect sludge characteristics and that an attempt has been made
to discount these factors and dwell specifically on the raw water
chemistry effects.
The first type of mine drainage usually has a pH of 6.5 to 7.5 or
greater, very little or no acidity, and contains iron, usually in
the ferrous state, that varies from less than 60 mg/1 up to 1000
mg/1. Barthauert2^) reported that a sludge from this type of water
was light and fluffy in character and settled to 0.5 percent of the
original treated volume.
Mine waters of this type need not necessarily be neutralized due
to the lack of present acidity; however, when there are large amounts of
ferrous iron present, lime may be needed to raise the pH to handle
potential acidity resulting from the oxidation of ferrous iron. If
the water does not need lime addition, the volume of the sludge is
totally dependent upon the iron content and other possible precipi-
table elements present in the water.
A second type of mine drainage is partially oxidized water that
can have a pH in the range of 3.5 to 6.5, acidity values that range
from 0 to 1000 mg/1, iron in both the ferric and ferrous state and
small amounts of aluminum. This type of water can contain both
acidity and alkalinity and the relative amounts of ferric and ferrous
iron depend upon the pH.v^3)
Sludges from this second type of water can vary depending upon the
raw water constituents. Iron content will be a major contributor
to the sludge volume, but when considering this type of water,
neutralization processes have to be introduced which drastically
change sludge properties.
Raw mine waters of the second type frequently contain bicarbonate which
can lead to a unique sludge problem. Lombardo'2^) reported on one
such water that was very close to alkaline with a pH ranging from 5 -
6.5, but contained a large amount of ferrous iron and varying concen-
trations of bicarbonate. When this water was treated with lime and
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aerated, the relatively tight floe formation that normally occurs did
not develop and extended retention time was required for complete
sedimentation. This problem was solved by eliminating the aeration
step and allowing the iron to remain in the unoxidized ferrous hydroxide
form. Lime consumption increased in this treatment process due to the
higher pH (8.8 - 9.3) required to properly remove ferrous iron as
ferrous hydroxide. In this case the bicarbonate content of the
raw water played a deciding role in affecting the sludge settling
characteris tics.
Another water reported by Wilmoth and Hill''7' falls into this
second classification. This water is characterized by a low pH
(2.8), a low aluminum content (31 mg/1) and a low iron content
(93 mg/1), which was primarily in the ferric state. The low iron
and aluminum content of the water would normally produce a small volume
of sludge. In this case the water was treated with three different kinds
of neutralizers and each neutralizing agent had a different effect on
the sludge settling rate and final settled volume.
The third type of water is highly acidic. This water frequently
contains large amounts of acidity (1000 - 15,000 mg/1), large
amounts of iron (500 - 10,000 mg/1). mostly in the ferrous state,
and aluminum (0 - 2000 mg/l).(2->)
The volume of sludge formed from this water is generally high. The
high iron content will naturally contribute to the sludge volume,
but in this case the presence of aluminum in the form of hydrous
oxide adds to the sludge volume. The fact that aluminum III reaches
residual concentrations at approximately pH 4-4.5. and then resolu-
bilizes above pH 8.0 has been well documented.'"' The total effect
of aluminum on the sludge volume will, therefore, be dependent upon
the treatment pH.
Girard and Kaplan^2**' found that silica present with alumina aided
sludge sedimentation due to the formation of large floes which in
turn aided the coagulation of the iron hydroxide.
Lovell^ ' reported that calcium sulfate precipitate can change sludge
properties. The exact nature of this change is not reported, but
calcium sulfate is known to contribute to scaling on sludge with-
drawal ports and other equipment.
To summarize, coal mine drainage can generally be classified into
three types depending upon chemical characteristics. In addition,
the resulting sludges can also vary as the raw water and treatment
methods change. Generally iron, aluminum, sulfates, bicarbonates,
silicon, calcium, and acidity are the major chemical constituents
in coal mine drainage that affect sludge properties.
18
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THE EFFECT OF TREATMENT PROCESSES
ON SLUDGE FORMATION AND CHARACTERISTICS
The chemical constituents of raw mine water greatly influence
sludge formation and characteristics; however, unit operations and
type of neutralizing agent used during treatment exert the greatest
influence on sludge properties.
Lime Neutralization
Conventional
Calcium hydrate (Ca(OH) ) is the most widely used neutralizing agent
in the treatment of coal mine drainage. In some cases, such as
Jones and Laughlin's Shannopin Airshaft Number 1 Treatment Plant.
quicklime (CaO) is used instead of calcium hydrate. Steinman(27)
reported an economic advantage from the slaking of the quicklime;
however, treatment operators have generally depended upon the hydrated
form due to the high cost of the slaking equipment required with
quicklime as well as handling problems.
In a report on hydrated versus quicklime, Bisceglia stated that
most quicklime manufacturers leave a small amount (2-3 percent) of
unburned limestone or "core" in the quicklime. This "core" is
generally removed from hydrated lime. Thus considerably less sludge
is produced from hydrated lime neutralization as compared to quicklime
treatment.
Sanmarful^ ' studied sludge characteristics resulting from a synthetic
mine water using various alkaline agents. Examining Ca(OH>2 as the
neutralizing agent, the sludges produced were light, gelatinous in
nature and were very voluminous as compared to sludge formed by the
use of CaCO-j, Na2CO-j and NaOH neutralizing agents.
Dolomitic lime has been extensively studied in the treatment of waste
pickle liquor. Reactivity of the magnesia component of doloraitic
lime is required in the presence of pickle liquor. Therefore, more
lime is required for complete neutralization. As more lime is added,
the unreacted magnesium oxide and calcium sulfate add to the normal
sludge production. Depending upon the reactivity of the dolomitic
lime the sludge volume and solids content can be appreciably higher as
compared to high calcium lime neutralization.
Non-Aeration
The basic chemical reactions related to lime treatment of coal mine
19
-------
drainage are as follows:
Ca(OH)2 + H2S04-»CaS04 + 2H20
Ca(OH)2 + FeS04-»Fe(OH)2 + CaS04
One of the more important aspects of the above equations is that
without additional oxygen, ferrous sulfate reacts with lime to form
ferrous hydroxide. Non-aerated ferrous hydroxide sludges have con-
siderably different sludge properties compared to sludges that
contain iron in the ferric state. Settling curves in Figure 3
illustrate that non-aerated sludge settles slightly faster than
aerated sludge, but produces a larger settled volume. However, in
order to remove ferrous iron from mine drainage the pH must be
increased to a much higher level (approximately pH 9) than is
required for ferric iron removal (pH 5).
Mechanical Aeration
Mechanical aeration is the most common method employed to oxidize
ferrous hydroxide to ferric hydroxide in the treatment of coal mine
drainage. The following equation represents the approximate chemical
reaction that takes place:
4FeOH+ + 40H~ + 6H20 + 02-»4Fe(OH)3 + 4H20
Research was conducted at West Virginia University^ ' on the aeration
of lime neutralized coal mine drainage and the following conclusions
were drawn:
1. When iron is in the ferric state, the volume of settled sludge
is less than that for ferrous iron.
2. A savings in lime costs can be realized due to the lower treat-
ment pH required if the iron is in the ferric state.
Different methods of aeration and aeration devices were also studied
and the smallest possible air bubbles were found to be the most
desirable for aeration due to increased bubble surface area. Bubble
size was dependent upon the type of dispersion device used.
Lagoon Aeration
The use of a lagoon or a large pond to complete the iron oxidation
is applicable in some cases. Factors affecting lagoon oxidation
20
-------
to
1000 •
800
600
(/)
fc
ui
400
200
ANALYSIS OF AMD
Acidity
Total Iron
pH (initial)
pH (final)
2082 ppm
652 ppm
582 ppm
3.3
9.5
NON-AERATED LIME SLUDGE
AERATED LIME SLUDGE
—i—
6
8 10
TIME (HOURS)
12
14 16
18
Figure 3 - Influence of Aeration on Sludge Volume and Settling Characteristics. After Pudlo
(29)
-------
have been reported by Barthauer *• ' to be temperature, rate of oxygen
diffusion and pH of the discharge.
The influence of temperature on oxidation rate of ferrous iron is
illustrated in Figure 4. As the water temperature varies from 73°F
to 40°F, the time required to oxidize the ferrous iron ranges from
4.5 days to more than 14 days.
Water temperature of an underground mine discharge is reasonably
constant (approximately 54°F) and would generally not approach the
lower temperatures indicated in Figure 4. Under certain conditions,
however, evaporative cooling takes place and affects the oxidation rate.
A mine water holding pond is a case where cold, windy winter weather
can drastically reduce the iron oxidation rate. Another example
would be a treatment plant that is located some distance from the dis-
charge point and water which is exposed to the prevailing atmospheric
conditions while traveling to the treatment plant.
Another factor found to be important in this study of aeration by
lagooning was the rate of oxygen diffusion into the pond. The re-
lationship between oxygen diffusion and pond depth is presented in
Figure 5. Even under ideal conditions, the diffusion rate of oxygen
into the pond is very slow.
The pH of the discharge was the last factor mentioned by Barthauer
that affects oxidation rate. Sturom and Lee^31' have found that the
oxidation rate of ferrous iron is strongly influenced by pH as
illusted in Figure 6.
The use of the lagoon as the sole means of aeration is practical only
with certain types of coal mine waters, generally alkaline waters
having a low ferrous iron content. Sludges produced under these con-
ditions are generally slow settling and settle to a small final volume.
Barthauer neglected to mention the importance of another factor,
natural bacteriological oxidation of iron. This phenomenon is impor-
tant and cannot be dismissed when discussing lagoon oxidation.
studied the effects of treatment pH on sludge volume and
found that with certain mine waters a higher treatment pH resulted
in a larger sludge volume. A mine water with an analysis of 1041
mg/1 acidity, 326 rag/1 total iron, 291 mg/1 ferrous iron was treated
22
-------
( PPM)
\
\
2 3 4 5 6 7 8 9 10 II 12 13 14
TIME (DAYS)
Figure 4 - Influence of Temperature on Iron
Oxidation Rate. After
23
-------
PPM- 0
9
8
7
6
5
2 4
3
2
f FT DEPTH
3 FT. DEPTH
S FT. DEPTH
10 FT. DEPTH
20 FT. DEPTH
10 20 30 40 50 60 TO 80 90 100
TIME (DAYS)
Figure 5 - Oxygen Diffusion into a Still Pond at 60°F,
After Phelps<3°)
24
-------
3.0
10
20 30 40
TIME (WIN.)
50
Figure 6 - Influence of pH on Iron Oxidation Rate.
After Stumm and Lee'^l)
25
-------
at pH 9.5, 10.0 and 10.8. Increasing the treatment pH from 9.5 to
10.0 resulted in a 20.5 percent greater sludge volume. When the pH
was increased from 9.5 to 10.8 a 50 percent increase in sludge
volume was observed. Treating the water below pH 9.5 did not
adequately precipitate the ferrous iron. This effect of pH on sludge
volume is shown in Figure 7.
This study was extended using two more mine waters. The second mine
water contained 2082 mg/1 acidity, 652 mg/1 total iron, 585 mg/1
ferrous iron. When treated to a pH of 10.0 and 10.5 and compared to
the initial treatment pH of 9.5, the sludge volume increased 25
and 30 percent respectively. The third water was the strongest of
the three waters examined (6500 mg/1 acidity, 2386 mg/1 total iron,
1500 mg/1 ferrous iron.) Treating this water at pH 9.5, 10.0 and
10.75 resulted in no significant differences in sludge volume.
explained that at higher treatment pH values the abundant
hydroxyl ions may attach themselves to the floes causing the floes
to be negatively charged and repellent to each other. This repellent
action may be the cause of the increased sludge volumes at higher
pH values.
Sludge Recirculation
Sludge recirculation is a concept that has been applied in the past
to waste pickle liquor treated with lime. Recently sludge recircu-
lation has referred to the treatment of coal mine drainage where a
portion of the precipitated sludge is returned to the beginning of the
treatment process for the purpose of reducing sludge volume and in-
creasing sedimentation rates.
The mechanism that controls the sludge characteristics during re-
circulation is not exactly understood. However, most authors agree
that the returned sludge acts as a nucleation site for further growth
and sedimentation.
Lovell(32) reported that sludge recirculation studies were initially con-
ducted at Pennsylvania State University on a lime treated mine water
containing 2060 mg/1 iron (mostly in the ferrous state) and 6800
mg/1 acidity. After approximately 85 recirculation cycles, the final
sludge volume was reduced from the 20 percent originally obtained
to 7.2 percent. From this and other pilot plant studies the following
recirculation process parameters were observed: iron oxidation
state, water composition, reaction time, reaction steps, neutrali-
zation levels, sludge recycle rates, and sludge blow down frequencies
and levels. Lovell later found that suspended solids level in the
reactor vessel is important and should be kept near 3000 mg/1.
26
-------
MLS. OF
SLUDGE
Ni
ANALYSIS OF WATER
Acidify 1041 ppm
Totol Iron 326 ppm
F«++ 291 ppm
Initial pH 3.15
PH ADJUSTED TO 10.8
pH ADJUSTED TO 10.0
pH ADJUSTED TO 9.5 8 9.0 (Sam* Curve)
10
TIME (HOURS)
Figure 7 - Influence of Treatment pH on Sludge Volume.
After Pudlo
(29)
-------
Initial sludge recirculation attempts reported by Lombardo^ were
very successful in increasing the solids content from a clarifier
underflow. Under normal conditions, the solids content from the
clarifier underflow was about 0.6 percent. After recirculation
was introduced into the system the solids content increased to
4-4.5 percent. Prior to sludge recirculation, sludge was being
discarded from the clarifier at the rate of 50 gallons per minute,
but after recirculation the sludge discharge rate was reduced to
seven gallons per minute. Lombardo also mentioned that another benefit
from sludge recirculation was the utilization of some of the alkalinity
in the sludge, resulting in a reduction of lime demands.
Sludge recirculation is being utilized in mine drainage treatment
on a limited scale probably due to a lack of understanding of
the process or because adequate land is available and a dense sludge
is not needed. An illustration of how sludge recirculation is
currently being used in mine drainage treatment is presented in Figure
8.
The research conducted on sludge recirculation has led to the discovery
of unique treatment methods that show promise in total treatment of
coal mine drainage. A discussion of these treatment methods follows.
w/>
MIME
ENTRY
LIME STORAGE
TANK
\
\
\
SLUDGE
RECIRCULA-n
r—1
\ !
SURFACE /
AERATOR
TWIN SETTLING TANKS
PRECIPITATE
RETURNED TO
MINED OUT AREA
PUMP
CLEAN WATER
OVERFLOW TO
^RECEIVING
STREAM
Figure 8 - Mine Drainage Treatment Plant Utilizing Sludge
Recirculation. After Goddard^33^
28
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High Density Sludge Process
The technology related to sludge recirculation and acid mine drain-
age has been recently enhanced by research reported bv Bethlehem
Steel Corporation and Bethlehem Mines Corporation.(34)
The "High Density Sludge Process" combines conventional acid mine
drainage treatment with sludge recirculation. However, the treat-
ment process is unique in that recirculated sludge is mixed with
the lime slurry prior to the neutralization process.
A conventional process and the high density sludge process are
compared in Figure 9.
The following advantages are reported to result from the high density
sludge process: a dense sludge containing 15 to 40 percent
solids; reduced sludge storage requirements; easier sludge dewatering
by filtering or centrifuging resulting from high sludge solids con-
centrations; applicability to high ferrous iron waters which are the
most difficult to treat; and inexpensive processing with lime and
air oxidation due to the simplicity of the process.(34)
Experimentation on the high density sludge process has shown that
the ferrous to ferric iron ratio has a limiting affect on the maximum
concentration of settled solids that can be produced. The current
findings on the relationship of ferrous iron content to the con-
centration of settled solids is listed in Table 3.
Table 3
Influence of Ferrous Iron Content on Sludge Settled Solids
Maximum
Ferrous iron, Concentration
Avg. % of of settled
Water Source total iron solids, %
Mines 32-33 AMD
(source discharge) 90 40
Synthetic AMD 95 50
Synthetic steel plant
waste 95 45
Steel plant waste 95 45
29
-------
Table 3 (Continued)
Influence of Ferrous Iron Content on Sludge Settled Solids
Maximum
Ferrous iron, Concentration
Avg. % of of settled
Water Source total iron solids, %
Mines 32-33 AMD 70
(shipped samples) (range 45-90) 22
Mine 32, supply-
shaft AMD 30 15
Mine 31 AMD
(shipped samples) 2 18
(34)
After Haines and Kostenbader
The ratio of solids recirculated to solids precipitated also had
a limiting affect on both sludge density and thickener size re-
quirements .
The chemistry involved in production of high density sludge by
this process is largely unknown. However, the sequence of unit
operations has been found to be the key to the effectiveness of the
operation. Mixing the lime slurry, recycle slurry and acid mine
drainage in the same tank was found to decrease settled solids con-
centration by 50 percent and increase thickener area requirements
by 100 percent. The only way the higher sludge solids content could be
realized was by mixing the lime slurry and recycle slurry prior to
the neutralization step.
Densator ^ Process
Sludge recirculation is also a key step in a proprietary apparatus
called the Infilco DensatorSP <35) This treatment device contains
a primary reaction zone, a secondary reaction zone, a sludge zone,
and a sludge storage zone all within a single tank. The major
advantage of this process, other than the single tank operation, is
that the sludge created is very dense and can be easily dewatered.
30
-------
CONVENTIONAL PROCESS
V
Lime Storage
WATER
1 ,
r — AMD
Neutralization
and Oxidation
Solids-Liquid
Separation
NEUTRAL
•EFFLUENT
WASTE SLUDGE
7 1% Solids
HIGH-DENSITY SLUDGE PROCESS
WATER
-AMD
Lime Storage
„
Sludge Reaction
i
RECYCLE SLUDGE
Neutralization
and Oxidation
AIR— *
So lids- Liquid
Stporotion
^
NEUTRAL
-^-EFFLUENl
WASTE SLUDGE
!5-40%Soli
-------
Laboratory studies were conducted applying this process to waste
pickle liquor with lime being used as the neutralizing agent.
Evans reported that sludge from the Densator ® contained 90 to 110
grams of dry solids per liter which was approximately 10 times the
concentration obtained in high rate solids contact units.(36)
The report further explained that the reaction involved in production
of dense ferrous hydroxide is one of cation exchange. In the primary
reaction zone, hydrogen and ferrous ions from the pickle liquor dis-
place calcium ions from recirculated sludge. In the secondary re-
action zone calcium ions provided from lime displace hydrogen ions
from the sludge to form water. The ferrous hydroxide acts as a
cation exchange material and precipitation of additional ferrous
hydroxide from the pickle liquor takes place as a result of ion
exchange, rather than by direct neutralization.
/«\
Filter leaf tests on Densator v*y sludge indicated that a vacuum
filter rate of 25 pounds of dry solids per hour per square foot of
filter area could be obtained with a cake containing approximately
37 percent solids.
/B\
A Densator ^ unit used in the treatment of coal mine drainage is
being studied by Pennsylvania State University at Hollywood,
Pennsylvania, but data is not available at this time (See Figure 10).
The Infilco Densator ® and Bethlehem's High Density Sludge Process
are similar due to the sludge recirculation step differing only at
the point where the recirculated sludge enters the treatment process.
Recirculated sludge from the Densator ® Plant is mixed with the
raw mine water prior to the addition of the lime feed. Recirculated
sludge from Bethlehem's High Density Sludge Process is mixed with
the lime slurry prior to the addition of the raw mine water.
The Infilco Densator ® and the Bethlehem's High Density Sludge Process
contradict each other particularly in the sequence of unit operations.
Further research will have to be conducted on both processes to find
their optimum usage in the treatment of coal mine drainage.
Elpo Treatment Process
The "Elpo I Treatment Process" is a proprietary method of treating
coal mine drainage involving the addition of a cationic polyelectrolyte
to aerated coal mine drainage followed by the addition of a neutra- .
lizing agent and a final step of adding an anionic polyelectrolyte.^
Investigators conducting pilot plant operations on the "Elpo" process
demonstrated that there was a marked increase in the rate of treated
32
-------
u>
u>
TOP OF BEAM
_EL_ 122142 _
OUTLET TO SETTLING
LAGOON
POLYELECTROLTTE 4. EL. I215.M
SAMPLE LIKE TO pH CONTROL
ji".\Jr;\* ii«-
*£.
LSLUDGE RETURN TO CONTROL BLOC. 11
t EL 120310
12" l'6" IV ' I 6" I IV I 1!"
•!• »|- ---i- 4"—4*—H
I.D OF CONC T_ANK 23'!V"_
Figure 10 - Densator^ Treatment Plant. Compliments of The Pennsylvania State University.
-------
water clarification and "the resulting sludge was more easily handled
than sludges generated by neutralization alone."(38) Data is
currently not available from the early pilot plant investigations.
The "Elpo II Treatment Process," a modified version of the "Elpo I"
process that utilizes sludge recirculation, is currently being studied
at L. R. Kimball, Consulting Engineers, Ebensburg, Pennsylvania.
Attempts are being made to define the reaction mechanisms and process
parameters involved in this treatment process.(38)
Magnetic Sludge
Bituminous Coal Research^?) studied synthetic and natural coal mine
water in an attempt to form magnetic sludge. A magnetic sludge could
then be treated using commercially available wet or dry magnetic
separators to remove the solids from the mine drainage treatment system.
The first tests were run with a synthetic mine water treated with sodium
hydroxide or lime. In both cases a black magnetic precipitate was
achieved at pH 10.0. Gentle aeration noticeably increased the rate of
conversion of the alkaline suspension to the magnetic product.
Later attempts to produce a magnetic sludge from natural coal mine
waters were unsuccessful. However, in further studies it was found
that under the proper conditions of pH, reaction temperature, and
low aluminum and magnesium concentrations a magnetic sludge could
be produced that exhibited a five-fold reduction in settled volume as
compared to conventional precipitates formed from lime neutralization.
Sludge thickening was attempted prior to the conversion to a magnetic
form by gravity settling with the addition of two coagulant aids.
The sludge thickening step was found to be advantageous in the
magnetic conversion process.
Additional sludge thickening was investigated with a centrifuge.
The solids content of gravity settled ferrous hydroxide sludges in-
creased from about two to nine percent by weight following centrifuging.
This increase in sludge concentration aided the subsequent magnetic
formation step. Centrifugation of the sludge prior to the magnetic
formation step also exhibited residual effects on the final settled
sludge volume and solids content. An example of this effect was
noted when a centrifuged sludge sample settled to 0.17 percent of
original treated volume and had a solids content of 17.7 percent by
weight. Similarly, a magnetic sludge sample prepared in the same
manner was allowed to gravity settle without the centrifugation
step. This sludge settled to a higher volume (0.54 percent of
the original volume) and had a solids content of 12.1 percent by
weight.
34
-------
The practical application of magnetic sludge treatment has not been
established, but further research is being conducted and an evaluation
as to its practicality will be made in the future.
Limestone Neutralization
Limestone (CaCO-) shows promise in the treatment of certain types
of mine drainages and has definite advantages, particularly in the
area of sludge control.
Conventional (Coarse Size Stone)
The feasibility of limestone treatment of coal mine drainage was
established by Tracy as early as 1913. (^0) A.S time passed limestone
was studied in hopes of replacing or being used in conjunction with
conventional lime neutralization of waste pickle liquor and other acid
wastes.(41.42,43,44) Later Braley(^) studied limestone neutralization
by passing the mine water through a flume containing five tons of one to
two inch limestone. This research presented a problem that has plagued
investigators from the beginning of limestone neutralization research,
the loss of limestone reactivity due to the coating of limestone by
the precipitation products iron and calcium sulfate.
Zurbuch'^"' solved the problem of limestone coating by placing the
limestone in a drum and placing the drum in a stream bed. The flow
of the stream caused the drum to turn like a water wheel and the sur-
face of the limestone was kept reactive due to the abrasive action
between the limestone particles.
Deul and Mihok^ ' conducted research into limestone neutralization
of coal mine drainage using a modified cement mixer as the reactor.
Exploratory tests were run on a batch and continuous basis using
high calcium, double screened (1-by 1/2-inch and 1/2-by 1/4-inch)
limestone. The raw mine water was fed into the reactor that con-
tained the coarse limestone and the treated water was monitored
for pH and ferrous iron concentrations. The researchers found
that mine waters containing low to moderate concentrations of iron
(less than 100 mg/1) could be treated with limestone to produce
a water that had a pH of 7 to 8 and an iron concentration of less
than 7 mg/1. Mine waters containing high ferrous iron con-
centrations required longer reaction times with limestone and
ultimately had to be supplemented with lime to reach acceptable
water quality in a short reaction time.
35
-------
Sludge settling rates and volumes of settled solids were also studied
for limestone, lime, and a combination limestone-lime treatments.
In every case where limestone was used for neutralization, either
in combination with lime or alone, the sludge settling rates and
volume of compacted solids were more favorable than where lime
alone was used. Figure 11 illustrates the favorable settling
characteristics of a limestone treated mine water.
Aeration
The use of mechanical aeration in limestone treatment has been
studied^ » » in an attempt to maximize the treatment
efficiency, but little work has been done on the effects of aeration
on the resulting sludge properties. As more research is conducted
into limestone neutralization further light may be shed on the
effects of aeration on limestone sludge properties.
Sludge Recirculation
(39)
Bituminous Coal Research briefly attempted sludge recirculation
with two natural mine waters and found that recirculated sludge
aided reducing the iron content in the raw water to a satisfactory
level. Optimum treatment results were obtained when sludge was in-
troduced with untreated mine water in the limestone reaction tank.
Other authors^ ' reported that sludge recirculation may aid the
treatment process by taking advantage of the alkalinity within the
sludge and further research is needed to confirm this possible aid
to treatment efficiency.
Biochemical Oxidation
The inability of limestone to effectively treat coal mine drainages
containing large quantities of ferrous iron has led to the discovery
of a promising treatment process. Glover^') reported on a biochemical
oxidation process that first converts the ferrous iron to the ferric
state. After the iron is oxidized the mine water is pumped upward
through a limestone reactor to neutralize the acid. The treated
water than flows into a clarifier where the ferric hydroxide and
calcium sulfate sludge precipitates. Upon completion of clarification,
the treated water is removed and the settled sludge is pumped to a
vacuum filtration unit for dewatering. Glover found that the sludge
created from this process exhibited poor initial settling character-
istics, but finally settled to a volume of about one percent of the
original volume treated and had a solids content of 9 - 12 percent by
weight. Sludge from lime treatment of the same mine drainage had a
36
-------
100
SETTLED
SOLIDS,
VOL. %
• 10 gal mixed with ISO-lb limestone in reactor for 4 min
D 10 gal mixed with 150-lb limestone in reactor for I min
5-g lime added and mixed for 2 min
A 20 gal mixed with 27-g lime in reactor for 2 min
40
20
20
40
SETTLING TIME, MIN.
60
Figure 11 - Settling Rate of Sludges From Limestone, Limestone and
Lime, and Lime Treated Mine Water. After Deul and
37
-------
relative volume of about 10 percent of the original treated mine
water and had a solids content of about 1.2 percent. A flow
diagram of the biological oxidation and limestone neutralization
process is presented in Figure 12.
LIMESTONE
GRIT
ACID
MINE —
DRAINAGE FLOW
*~ BALANC
\
BIOCHEMICAL
NG OXIDATION
SEDIMENT-
ATION
4 ACTIVE 1
\
LIMESTONE
" NEUTRALIZATION
SU
SLUDGE
SEQJMENT-
ATION
JDGE|
FILTRATION
TREATED
EFFLUENT
t
CAKE TO
WASTE.
Figure 12 - Flow Diagram of Complete Biochemical Oxidation and Limestone
Neutralization Process. After Glover(***'
38
-------
Rotating Drum
After early studies by Tracy(40)Braley^45), Zurbuch(46) and later
studies by Birch(5°) anj Calhoun^ ', the Bureau of Mines continued
research into rotating drum limestone treatment. Preliminary work
and pilot plant studies found that vigorous agitation of the
limestone in a drum present with the mine water provided substantial
abrasion to eliminate limestone coating and removed COo from solution.
Settling and compaction of precipitated solids was one of the main
investigations undertaken during this study. Mine water containing a
pH of 2.8, 36 mg/1 ferrous iron, 360 mg/1 total iron and 1700 rag/1
acidity was treated to the same end conditions (pH 6.9) using limestone
and lime as separate neutralizing agents. The treated slurry was
allowed to settle in separate cylinders each having a capacity of
4.9 liters. After 24 hours the limestone treated sludge settled
to approximately 2.5 percent of the original volume of treated
water while the lime sludge settled to only 12 percent. The settling
test was continued for 43 days with the limestone sludge settling to
less than two percent compared to the lime sludge that settled to
approximately 6.5 percent.(52) Results from this settling test are
shown in Figure 13.
In the same study, an attempt was also made to determine the
compaction of both lime and limestone sludges. Equal volumes of
lime and limestone sludges were prepared in separate glass cylinders
and the supernatant liquid was decanted. The authors reported that
absolute sludge compaction properties were not established; however,
results from the data indicated that there was no compaction for
the lime neutralized sludge and only slight compaction for the
limestone neutralized sludge.(52)
A full scale rotary drum limestone treatment plant has been built
by Rochester and Pittsburgh Coal Company(53) utilizing a drum three
feet in diameter and 30 feet long with four one inch rods welded
to the inside to provide lift for the limestone. The volume of sludge
from this installation was 25 percent of the volume of precipitates
produced from treatment with hydrated lime. Further sludge properties
such as percent solids were not reported.
Ground Limestone
Results of studies by Jacobs(44\ Hoak, et al.(42\ and Ford^
have shown that neutralization reaction rates are affected by the
39
-------
100
9 8
LIME
LIMESTONE
Figure 13 - Heights of Precipitated Sludge From Lime and
Limestone Neutralized Water. After Mihok, et
40
-------
fineness of the limestone used in the treatment of acid wastes and
coal mine drainage.
Wilmoth and Hill'' studied the treatment of high ferric iron (mean
value total iron 93 mg/1, 80 percent in ferric state) mine water using
lime, limestone "rock dust" and soda ash as neutralizing agents.
Both batch and continuous flow tests were conducted and it was
found that all three neutralizing agents were suitable for treating
the mine water under investigation.
Comparative tests were also conducted on the settling rates of the
sludges produced from the various neutralization agents. The lime
sludge exhibiting the fastest initial settling rate followed by the
soda ash and the limestone sludges. Limestone sludge settled to the
smallest volume while lime and soda ash sludges settled to higher
final volumes as illustrated in Figure 14.
Percent solids of the lime, limestone and soda ash sludges were
compared using the same water and treated to approximately the same
pH (pH 6.5 for lime and soda ash and pH 6.2 for limestone). After
24 hours settling the limestone sludge had a solids content of 9.5
percent as compared to 1.5 percent for lime and 0.7 percent for soda
ash. Table 4 presents the results from this test.(7'
Table 4
Percent Solids of Lime, Limestone and Soda Ash Sludges
Percent Solids
Neutralizing Agent Supernatant pH of Sludge
Lime
Limestone
Soda Ash
pH 6.5
pH 6.2
pH 6.5
1.5
9.5
0.7
* (7)
After 24 hours of undisturbed settling. After Wilmoth and Hill
41
-------
O LIME
A LIMESTONE
D SODA ASH
INITIAL HOT ACIDITY = 586 MG/L
30min. 60 min.
4hrs.
20hrs. 24hrs.
8hrs. I2hrs I6hrs.
TIME
Figure 14 - Comparison of Settling Rate for Lime, Limestone and Soda Ash Sludges,
After Wilmoth and /-7X
-------
Holland, et al. also conducted neutralization studies using lime,
limestone "rockdust" and limestone chips and observed the resulting
sludges. Sludge settling data was not presented. However, the authors
did indicate that considerably less sludge was produced from limestone
neutralizing agents as compared to similar treatments with lime. The
authors went on to point out that the apparent advantage of small sludge
volumes may not be meaningful, because under similar drying conditions
in sludge ponds both lime and limestone sludges could arrive at about
the same final volume.
Limestone-Lime Neutralization
The inefficient limestone treatment of mine drainage containing large
amounts of ferrous iron (above 100 mg/1) has, in part, induced studies
of a treatment method that combines limestone and lime into a split
treatment system.(47,48) Limestone is used to raise the pll of the
mine water to about 5.0 and then lime is added to raise the pH to the
required treatment level. The efficiency of operation is high when
the neutralizing agents are utilized in these pH ranges. It is hoped
the resultant combination treatment will produce superior treatment
results and reduced costs.
Results of preliminary studies on split treatment of coal mine drain-
age have shown that the proper combination of limestone and lime
produces a good effluent and superior sludge properties.(47,48)
Current research on split treatment is being conducted by Wilmoth at
EPA's Norton Mine Drainage Treatment Plant. Preliminary results indicate
that raw material cost reductions in excess of 20 percent can be achieved
with split treatment as compared to lime or limestone when operating
at pH 6.5.(55>
A study is being conducted by Peabody Coal Company at Carrier Mills,
Illinois under a grant from the Environmental Protection Agency (Grant
No. 14010 DAX) that provides for the construction of a full scale
limestone-lime demonstration plant. Results from this study may reveal
the most economic combination which will utilize the desirable char-
acteristics of lime (high pH) and limestone (low sludge volume).
43
-------
SLUDGE SETTLING
Treatment of coal mine drainage, like the treatment of many acidic
industrial wastes, is one of purification by chemical addition. The
acid is neutralized and specific pollutional material is removed by
the formation of insoluble products. Insoluble materials or precipi-
tated products are usually separated from their water environment by
sedimentation.
Sedimentation
Sedimentation of coal mine drainage sludge has been described by
Lovell(3) as having three phases: free settling and floe growth, a
transition zone, and compression. Different phases that can occur in
the general sedimentation process are illustrated in Figure 15. Early
floe formation in the liquid ready for settling is shown in Figure
15(a). Solids first appear in Zone D which consists of layers of
floes as in Figure 15(b). Immediately above Zone D is Zone C, commonly
called the transition layer, which has a solids composition that varies
between Zone D and the original slurry. Zone B is the original slurry
and Zone A is the clear liquid. As further settling occurs, all of
the solids settle to Zone D as can be seen in Figure 15(e). When
all the solids are settled into Zone D, compaction begins to occur.
The transition from settling to compaction is called the critical point.
In compaction, part of the liquid that was entrapped by the solids is
forced out when the weight of the deposit breaks down the structure of
the floes. Equilibrium finally occurs when the weight of the solids
equals the strength of the floes. At this point, settling stabilizes
since the height of the sludge is fixed.^56'
A plot of sludge height versus settling time is shown in Figure 16.
The curve is reasonably straight during early settling, but at some
point in time the critical point (Point C) is reached and compaction
occurs.
The amount of water soluble constituents affects, to a degree, the
settling rate and the density of the sludge.(3) This is due to the
quantity and floe size of minerals that precipitate out of solution.
Similarly, a sludge with a higher solids concentration exhibits a
definite interface between the clear supernatant liquid and the
settled solids, while in the case of a sludge with a lower solids
content, the interface is cloudy and not well defined.(47)
The type of neutralizing agent used also affects sludge settling.
In one settling study using synthetic coal mine water with limestone,
a well defined interface never formed between the clarified water and
the precipitate during the settling period. Rather, a continuous
45
-------
(a)
(b)
(d)
D-H
(e)
A-Clear liquid
B-Original slurry
C-Transition layer
D-Settled solids
Figure 15 - Batch Sedimentation. After McCabe
and Smith<56)
m
46
-------
cloudy gradient was apparent until complete settling had occurred.
Flocculation characteristics (agglomeration or sticking together
of solid particles) of lime neutralized coal mine sludge are not
well understood. One characteristic of flocculated particles
is the loose structure of the floes.(56) The bond between the
particles is weak and they retain water within their structure
while they settle. The second characteristic is that the floc-
culated particle settling mechanism is complicated and settling
takes place in phases as illustrated in Figure 15.
The precipitate from limestone neutralization exhibits a completely
different structure as compared to the lime precipitate. Lime-
stone neutralized solids are not hydrophilic (water holding) like
a lime sludge, but are actually crystalline gypsum with co-precipi-
tated iron oxide.(47) Sedimentation characteristics of solids
from limestone neutralized mine water are attributable to the attach-
ment of Fe(OH)-j to the gypsum formed on the surfaces of CaCCU
particles. This resulting structure cuts off the hydrophilic
structural formation normally exhibited by Fe(OH)-j. (57)
The importance of temperature on solid-fluid separation has been
recognized, '^8) but j.jas not been extensively studied with coal
mine drainage sludge. Stoke*s formula for settling velocity includes
a factor representing the coefficient of viscosity of the fluid.(59)
The coefficient of viscosity is temperature dependent, which under
normal conditions would allow settling to proceed faster as the
temperature increases. However, the temperature effect on settling
of coal mine drainage solids is not well understood.
Sedimentation Basin Design
The removal of solids from coal mine drainage by sedimentation requires
collection of the solids as they settle. Examples of sedimentation
basins used in coal mine drainage treatment are settling tanks,
lagoons and thickeners.
Settling tanks have been constructed for wastewater sedimentation
in a great variety of designs depending partly upon the size, density
and flocculation properties of the solids. Some of the common settling
tank shapes are circular, square, or rectangular with either horizontal
or vertical flow patterns.
The earthen lagoon is the most commonly used sedimentation device
47
-------
in coal mine drainage treatment. The dimensions or design of
lagoons are not as critical as for settling tanks and generally lagoon
construction is dictated by the contour of the available land. Old
strip pits have often served as lagoons. When strip pits or land is
not available for lagoon construction, settling tanks or clarifier
thickeners with sludge withdrawal systems are employed.
Optimum dimensions for settling tanks at times seem to be ignored
especially when detention time is the major consideration rather
than performance needs. Of all tank shapes, the long narrow rectangular
model performs the best. Other tank designs perform inefficiently
for two reasons: (a) effective settling zone is reduced because the
inlet and outlet zones occupy too great a part of the total flow path
and (bi short circuiting and instability of flow prevail in short
tanks.*60>
Recently, wastewater gravity separation techniques have advanced
with the use of small diameter tubes as settling devices. The tubes
placed horizontally or at a steep inclination provide for efficient
clarification at substantially lower detention times as compared to
conventional clarifiers. In the horizontal designed tube system,
settled solids in the tubes can be removed by backwashing. In the
inclined tube design, the solids do not accumulate but continuously
slide down the tubes, countercurrent to the slurry flow.' '
48
-------
SLUDGE CONDITIONING
Conditioning of sludge traditionally has been used to enhance the
sludge dewatering rate. Sludge conditioning processes can be
broken down into two groups: (a) chemical conditioning and
(b) physical conditioning. Most of the sludge conditioning
operations for coal mine drainage sludge have evolved from the
treatment of sewage and industrial sludges.
Thickening
Sludge thickening has been defined as the process of removing water
from sludge after initial separation from the wastewater.(62; The
objective of thickening or concentrating sludge is to reduce the
volume of liquid sludge to be handled. The simplest, least
expensive and probably the oldest method of thickening is by gravity.
Gravity thickening and sedimentation are terms erroneously used
interchangeably by some authors. Although the mechanism of
settling is the same in both cases, the objectives are different.
Sedimentation implies water clarification, whereas thickening
requires a reduction in total sludge volume (increase in solids
content).
Lagooning is an example of gravity thickening which is currently
being extensively used in the treatment of coal mine drainage sludge.
In most cases sludge is perpetually stored in large lagoons where some
compaction occurs. However, settling tanks and mechanical thickeners
are also being used in continous operations where lagooning is not
feasible.
The mechanical thickener is a large, fairly shallow tank with slow
moving radial rakes driven from a central shaft. The treated slurry
usually flows down an inclined trough or is pumped from & centrally
located discharge pipe into the center of the thickener where the
slurry moves radially allowing the solids to settle to the bottom
of the tank and the clear water to overflow around the perimeter.
The rake arms rotate slowly and move the settled sludge to the center
of the tank, where the sludge flows through an opening to the inlet
of a sludge pump.(56)
The mechanical thickener allows for both settling and thickening.
The settling zones of a continuous thickener are not the same as
the settling zones established in batch settling. Basically two
zones are established, a free settling zone and a compression
zone. Movement of the rakes in the compression zone aids the
compaction of the settling particles by breaking up the floe and
49
-------
thus producing a more concentrated underflow than can be achieved
by simple settling.(56>
A new thickener system called the Hydraulic Rake ® Static Underflow
System is finding increased usage in coal mine drainage treatment.
This thickening system consists of a grid network on the floor of a
settling basin. The grid network is a series of parallel pipes
containing orifices through which the sludge is pumped. Each pipe
is brought independently outside of the thickener to a valve which
is operated pneumatically or electrically and connected to a timing
device. Each pipe is operated separately so that a limited area
of the bottom surface is being pumped. When the sludge has been
pumped from one area along the bottom, the first pipe is shut off
and the next one begins operation. This sequence produces what
is described as a "hydraulic rake". The manufacturer claims
the Hydraulic Rake ® can be applied to all settling tank designs
and, since settling is unimpeded, more efficient thickening results
Chemical
Chemical treatment or chemical coagulation has been defined as the
addition of chemicals (coagulants) for the purpose of de-stabilization
and aggregation of dispersed materials, followed by separation
of the aggregated material from the suspending liquid.\12) -^he
principal objective of the chemical coagulation process is to promote
the aggregation of the non-settleable or slow settling solids into
aggregates more amenable to sedimentation or filtration.
Recently the term "coagulant aid" has been applied to agents that
are used by themselves, or in addition to conventional coagulant
chemicals, to assist in the de-stabilization, flocculation, or sedi-
mentation phases of the coagulation process.(12) Organic poly-
electrolytes, also known as flocculants, fall into this classifi-
cation of chemical agents.
Polyelectrolyte coagulant aids are synthetic compounds that have a
very high molecular weight. These compounds can be characterized
according to their ionized form as cationic, anionic or nonionic.
Dorr-Oliver conducted bench-scale studies using polyelectrolytes on the
aerator overflow from different coal mine waters. Polyelectrolytes
improved the settling rate of the solids and at the same time reduced
the iron concentration of the treated water but did not increase
the solids concentration of the thickener underflow. Some flocculants
were pH sensitive making precise pH control necessary for effective
polyelectrolyte usage. Centrifugation and filtration tests were
50
-------
conducted on polyelectrolyte conditioned sludge with limited success.
Sludge pumping disturbed the conditioned sludge and the floe formation
never returned to the more desirable well flocculated form. Conclusions
drawn from this research were that even though improved settling
could be achieved no real benefits could be gained from flocculation
in further mechanical dewatering steps.(64)
(39)
Bituminous Coal Research studied sludge conditioning using anionic
polyelectrolytes, anionic electrolytes and inert solids. The effect
of the coagulant aids on the electrostatic charge of particles in
suspension was investigated using both ferrous and ferric hydroxide
sludges. Forces of mutual repulsion of suspended particles were
minimized at or near the isoelectric point (point of minimum zeta
potential) with the addition of proper coagulant aids. Reducing
the repulsive forces allowed the suspended particles to coagulate
and facilitated faster setting. The results from tests run using
the various coagulant aids on Fe(OH)2 are presented in Table 5.
In the same study the effect of adding both an inert solid (flyash)
and a polyelectrolyte (Calgon 240) to ferrous hydroxide particles
was observed. The addition of 500 mg/1 of flyash 30 seconds prior
to the polyelectrolyte addition reduced the amount of polyelectrolyte
required to achieve the isoelectric point by one mg/1. The flyash-
Calgon 240 combination appreciably increased the settling rate and
final settled sludge volume as illustrated in Figure 17.^3^)
Finally, studies were conducted on the effects of coagulant aids on
the zeta potential of freshly prepared ferric hydroxide particles
formed by limestone neutralization of synthetic coal mine water.
Generally, the amount of coagulant required to reach the isoelectric
point with ferric hydroxide sludges is much less than that required
with ferrous hydroxide. The reason for better coagulant performance
with ferric hydroxide sludges is not reported by the authors, probably
due to the complicated nature of the problem. The coagulant aids used
and their respective concentrations at the isoelectric point for both
feirous and ferric sludge are listed in Table 6.(39)
A flocculant feed system has been installed in a full scale mine drainage
treatment plant for the purpose of aiding settling.(•"' The flocculant
(.25 mg/1) was injected into the discharge from the aeration tank prior
to entry into the settling tanks. Reports on this sludge conditioning
attempt have yet to be published.
Freezing
Sludge freezing is an unusual method of sludge conditioning that
51
-------
Ui
Additive
Ludox HS-40
Ludox AS
Solution No. 24
Na4P207
Calgon C-55
Calgon 37
Lomar D
Priraafloc A-10
Purifloc A-21
Calgon 240
Poly-Floe 1130
M and D Clay
"Red Dog"
Fly Ash
Table 5
Effect of Coagulant Aids on Fe(OH).
4
Source DescrJ :ion
Cor.centra; ion at
Isoeiectric Point,
ppm
Added to i's(Oli)-,
E. 1. duPont de Nemours & Co.
E. I. duPont de Xemours & Co.
Philadelphia Quartz Co.
Fisher Scientific Co.
Calgon Corporation
Calgon Corporation
Diamond Shamrock Chemical Co.
Nopco Chemical Division
Rohm and Haas Co.
Dow Chemical Co.
Calgon Corporation
Betz Laboratories, Inc.
Kentucky-Tennessee Clay
Co., Inc.
Sewickley Township
Westmoreland County,
Pennsylvania
Colfax Power Station
Duquesne Light Co.
Sodiuia stabilized *
colloidal silica
Ammonia stabilized *
colloidal silica
Activated silica sol 100
Anionic electrolyte 100
Anionic mixture 75
Clay-anionic polyelectrolyte 35
mixture
Naphthalene sulfonate polymer 19
Anionic polyelectrolyte 15
Anionic polyelectrolyte 9
Anionic polyelectrolyte 5
Anionic polyelectrolyte 1
Ball clay, minus 400 mesh *
Similar to (completely burned)
coal ash, minus 200 mesh
Similar to coal ash but not 500
completely burned, minus 400
mesh
Isoeiectric point never achieved - remained electropositive. After Bituminous Coal Research
(39)
-------
200CH
^1500-
UJ
Q
hooo
500-
30 60 90
TIME- MINUTES
120
150
Figure 17 - Effect of Treatment with Flyash and Calgon 240
on Settling of Fe(OH)2 Sludge. After
Bituminous Coal Research *>39'
53
-------
Ul
*«
Name
Table 6
Effect of Coagulant Aids on Fe(OH) and Fe(OH).
Source
Concentration at Iso-
electric Point, ppm
Added to Fe(OH)2
Concentration at Iso-
electric Point, ppm
Added to Fe(OH)3
Poly-Floe 1130
Calgon 2AO
Purifloc A-21
Calgon C-55
Ludox AS
Primafloc A- 10
Ludox HS-40
Lomar D
Calgon 37
Solution 24
Na4P20?
Betz Laboratories, Inc.
Calgon Corp.
Dow Chemical Co.
Calgon Corp.
E. I. duPont de Nemours, Inc.
Rohm and Haas Co.
E. I. duPont de Nemours, Inc.
Diamond Shamrock Chemical Co.
Calgon Corp.
Philadelphia Quartz Co.
Fisher Scientific Co.
1
5
9
75
*
15
*
19
35
100
100
0.1
0.4
0.4
0.8
1.1
1.2
1.5
1.5
1.8
1.8
6.0
Isoelectric point never achieved - remained electropositive. After Bituminous Coal Research
(39)
-------
has found application in sewage and municipal water treatment. The
concept of sludge freezing has evolved from observations made on
sludge frozen by nature and later thawed resulting in improved sludge
dewatering characteristics. Early work on sewage sludge freezing
led observers to believe that freezing disrupted the cell walls re-
taining the internal moisture in sludge, thereby allowing water
release and drainage.'"*'
Early research on sludge freezing and dewatering was conducted in
Great Britain with mixed success. Clements, et al.(65) reported
initial success in sludge freezing prior to vacuum filtration
while Bruce, et al.'°"' reported negative results. Later Doe^°''
undertook preliminary freezing studies on washwater sludge and
found that after freezing and thawing the sludge had lost its
gelatinous consistency and sludge particle size was approximately
0.1 to 1.0 mm. The freeze conditioned sludge also settled quickly.
A 100 gram sample of freeze conditioned sludge was filtered in a
conventional filter funnel in a few minutes compared to 36 hours for
the same quantity of unfrozen sludge. This work led to the construc-
tion of a sludge freezing plant at Lancashire, England to condition
8500 gallons of washwater sludge per day.("°' However, high cost forced
the plant to be abandoned.
Further freezing studies by Katz and Mason^") were conducted in the
U.S.A. on activated sewage sludge. The investigators concluded that
freeze conditioned sludge can be dewatered by gravity draining using
wire screen cloth (40-80 mesh) and that the filtrate and filter cake
quality are equivalent or better than that produced by conventional
vacuum filtration.
Freeze conditioning of coal mine sludge has been attempted by the
Scientific Control Group of the National Coal Board, but data on
the effects on sludge solids and settled volume is not yet avail-
able. (70) Rumrael^ ' reported success in coal mine sludge con-
centration (.6 percent solids to 7.5 percent solids) by sludge
freezing, but did not follow up the research again due to the apparent
high cost.
Ultrasonic
Ultra high-frequency sound was proposed as an aid in the coagulation
of the hydrous iron (III) oxide formed by oxidation.(72) This
proposal was later investigated by Rozelle.^^) Results were dis-
appointing because a fixed frequency (80 kc/sec) was found to create
a dispersion effect rather than coagulation. The project personnel
concluded that further research would not be warranted unless a
55
-------
multiple-frequency generator was used. Rummel^ ' also investi-
gated sludge conditioning by ultrasonics (50 c/sec) without success.
Heating
Sludge heating is another less common sludge conditioning method
that has been applied to sewage treatment sludges. When sewage
sludge is exposed to heat and pressure, the cell structure is
destroyed, and the solids coagulate reducing the hydrophilic nature
of the solids.(62)
Rummel''1) attempted coal mine sludge heat conditioning, but did
not observe any apparent sludge dehydration which could have re-
sulted in improved sludge characteristics.
Artificial Seeding
Artificial seeding involves the introduction of a material which
acts as a nucleation site upon which the precipitate can grow.
As previously mentioned, considerable research has been conducted
on this subject using spent pickle liquor wastes. This research
eventually included sludge recirculation as discussed earlier in
this report.
Other
Solvent extraction, electrical treatment and bacteria treatment are
conditioning processes that have been attempted with sewage sludge
and found impracticable. Rummel(71) attempted to concentrate coal
mine sludge by electrophoretic and magnetic treatment. Both
attempts did not in any way affect the sludge structure. No recorded
attempts at coal mine sludge conditioning with either bacteria or
solvent extraction were found and due to the unsuccessful attempts
with sewage, the application of these conditioning processes to
coal mine sludge seems remote.
Summary of Sludge Conditioning
Several steps can be taken to precondition coal mine sludge for
further dewatering. Sludge thickening, either by stirring with the
conventional clarifier thickener or by the use of the Hydraulic Rake
56
-------
can be achieved.
Chemical treatment with coagulants such as organic polyelectrolytes
can dramatically improve the sludge settling rate, but does not
necessarily increase the solids content of the settled sludge. Un-
successful attempts have been made in the use of coagulants as sludge
conditioners prior to dewatering by centrifugation and vacuum filtration.
However, new flocculants are now available that may be applicable.
Sludge conditioning by artificial freezing has not been extensively
studied with coal mine sludge. However, the initial success with other
sludges indicates that application to coal mine sludges may be feasible.
57
-------
SLUDGE DEWATERING
The primary objective of any dewatering operation is to reduce the
moisture content (increase solids content) in the sludge to minimize
disposed volume. In terms of the operations that may be performed
on sludge, sludge dewatering follows the steps of liquid-solid
separation and sludge conditioning.
Figure 18 is a flow chart of unit operations (neutralization, clari-
fication, sludge conditioning, sludge dewatering) that could be used
in a future mine drainage treatment plant. Neutralization and
clarification are two operations currently employed in all coal mine
drainage treatment. Sludge conditioning operations such as polyelec-
trolyte addition and/or freezing may be conventional practices in the
future. However, they are not absolutely required in current sludge
dewatering technology.
MINE
TER
AMD
TREATMENT
>
1
*--
POLYELECTROLYTE
ADDITION
Cl ARIFSCATION
AND OR
THICKENING
SLUDGE
->-- TRtATED EFFLUENT
RECIRCULATION
SLUDGE
CONDITIONING
ij». Freezing,
Ftofyetectrolyte
Addition
U;LUDGE
WATERING
_ SOLIDS
DISPOSAL
Figure 18 - Flow Chart of Future AMD Treatment System.
59
-------
Vacuum Filtration
The revolving-drum is the most commonly used type of vacuum
filter and is characterized by a series of vacuum cells that run the
length of the drum. The drum turns at a slow peripheral speed
(one rpm or less) and passes through a reservoir containing the
sludge to be dewatered (See Figure 19). As the drum passes through
the sludge a vacuum of 12 to 26 inches of mercury is applied to
the submerged cells drawing a sludge cake to the surface of the
filter media. As the cell emerges from the sludge reservoir it is
partially dried by the vacuum and is then removed from the drum
by a scraper, a blast of air, or coiled springs. The sludge is then
carried away for further drying or disposal.>™'
Mechanical dewatering by vacuum filtration has been applied to
almost all types of sewage sludge and many industrial waste
water sludges. The ability to dewater a sludge is affected by
many variables, some of which are listed below:'°^)
Sludge Variables
1. Concentration of solids
2. Age and temperature
3. Viscosity
4. Compressibility
5. Chemical composition
6. Nature of solids
Operating Variables
1. Vacuum
2. Amount of drum submergence
3. Drum speed
4. Degree of agitation
5. Filter media
6. Conditioning of sludge prior to filtration
One of the first attempts to dewater lime treated coal mine drainage,
sludge by vacuum filtration was conducted by Rummel' ' on an
experimental basis in Germany. A method to dewater the sludge
was desired in the recovery of the iron (III) - oxyhdrate which was
used as purification material for gas, as a color pigment or as
smelting feed. The filtration tests were run on a unit that had a
3.2 square meter filter surface area. Optimum conditions were
obtained with a special nylon filter cloth and a drum speed that
corresponded to a 20 second drying time with a vacuum of .6 atraos-
60
-------
PNEUMATIC AND
HYDRAULIC SYSTEM
^SLUDGE SLUDGE
RESERVOIR CAKE
(a)
SLUDGE CAKE
DROPS OFF
STRINGS OR COILS
RETURN TO DRUM
Figure 19 - Sketches of Drum Vacuum Filters.
After Fair, et al
61
-------
pheres at immersion. The test apparatus had an output of .085 gpm/ft
and produced a filter cake that averaged approximately 23 percent
solids as compared to an original .6 percent solids.
"Operation Yellowboy" studies included filtration tests on coal
mine sludge thickener underflow from numerous hydrated lime treated
mine waters. A 0.1 square foot filter leaf to simulate the
conditions of drum and disc filter operations was employed using a
relatively tight nylon cloth as the filter media. Filter cakes were
formed with solids contents as high as 29.8 percent. Little advantage
was gained from the filtration of a thickener underflow that had
been conditioned with a flocculating agent.(^4) ^ rotary vacuum
filter was incorporated as the dewatering unit in the scale-up design.
Sludge from biochemical oxidation and limestone neutralization
treatment has very good vacuum filtration possibilities. This
sludge was dewatered on a one square foot model rotary vacuum
drum filter. The filter cake had a thickness of 3/32 inch with a
45 percent solids content and exhibited good discharge and handling
characteristics. Glover'^^' calculated that vacuum filters with 250
square feet of filtration area operating 10 hours/day, 7 days/
week could dewater the sludge created from the treatment of 1,000,000
gallons/day of coal mine drainage containing approximately 300 mg/1
of dissolved iron.
Vacuum Filtration and Filter Aids
At times certain sludges which are slimy or contain very fine solids
form a dense, impermeable cake that "blinds" the filter medium and
hinders the flow of liquid through the filter. To combat this
situation filter aids are used to increase the porosity of the cake
so the filtrate can flow at a reasonable rate.'->°'
Filter aids have two different applications, as an addition (admix)
to the sludge slurry or as a precoating to the filter surface.^6)
Depending upon the use of the filter cake, the filter aid may be
separated from the cake or if the solids have no value, the total
mixture may be discarded.
Bituminous coal used as an admix filter aid to coal mine sludge has
been studied with mixed success. Test results indicated that the
addition of coal aided the filtration slightly, but at the same
time filter area requirements increased.^"^'
62
-------
Successful laboratory and pilot plant studies by Brown, et al. have
been conducted using diatomaceous earth as the precoat for rotary
vacuum filtration of coal mine drainage sludge produced from various
mine waters and treatment methods. Filter cake from hydrated lime,
limestone-lime and limestone treated mine waters averaged 22, 45
and 63 percent solids respectively. A Water Quality Office,
Environmental Protection Agency, Report 14010 DII presents these re-
sults in detail.
Another investigation into precoat vacuum filtration of coal mine
drainage sludge is currently being conducted using a full scale
vacuum filtration unit at the EPA - Penn State Experimental Mine
Drainage Treatment Plant located at Hollywood, Pennsylvania. This
unit has 113 square feet of filtering area which is precoated with
either diatomaceous earth or fine coal.(76) Results from this investi-
gations will be published when available.
Porous Bed Drying
The use of a porous bed for sludge dewatering is common and the
literature is abundant with theory and applications.(62,74,77)
Various types of granular filtering materials have been used for
porous bed drying some of which are natural silica sand, crushed
anthracite, crushed magnetite and garnet sands. The most common
application of porous beds is in municipal and industrial treat-
ment where the sludge or slurry is passed through sized sand and
gravel to remove suspended solids.
Dewatering occurs in porous beds by drainage and evaporation.
Some of the parameters that affect the design and usage of porous
beds are listed below: (*>2)
1. Weather conditions
2. Land values and proximity of residences
3. Sludge characteristics
4. Use of sludge conditioning aids
5. Subsoil permeability
Weather is the most important and to a certain degree uncontrollable
parameter to be considered in porous bed drying. Some of the weather
factors are rate and amount of precipitation, percentage of sunshine,
air temperature, relative humidity and wind velocity.
Various design modifications have been attempted in porous bed
drying, most of which relate to the use of the sand bed. Some of
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the design modifications are as follows.
(62)
1. Covering the drying bed in some manner to protect the sludge
from adverse weather conditions.
2. Covering the bottom of the bed with asphalt or concrete to
facilitate removal of the dried sludge.
3. Heating the drying beds.
4. Conditioning the sludges with materials such as organic
flocculants, polymeric flocculants, sawdust, anthracite or
activated carbon.
The sludge drying process is affected by the nature and moisture
content of the discharged sludge. Vogler and Rudolfs(?8) studied
paper mill white-water sludge in the laboratory. They found that
as the initial sludge solids content decreased the final cake
moisture decreased and the cake was easier to handle.
The solids content at which various sludges are "liftable" (condition
of sludge when removal is possible) differs considerably. Burd(62)
in a review of sand bed drying stated that some sludges are con-
sidered liftable at 55 percent solids while other sludges can be
lifted at 16 percent solids.
Rummel^ ' considered drying beds as a method of coal mine sludge
enrichment, but decided that the idea was not feasible due to the
large amount of sludge to be treated and the unfavorable weather
conditions in the area.
Yeh and Jenkins' ' conducted laboratory sand bed drying tests on
a coal mine sludge (one percent solids) that was being discharged
into a lagoon at the rate of 800 gpm. Results from calculations
showed that the filter area requirements for one day of sludge
production (sludge depth 1 foot) would be 1.8 acres. Drying time
was approximated to be 10 days assuming similar laboratory weather
conditions.
A coal mine sludge drying basin is currently being studied at
Hollywood, Pennsylvania which was designed to achieve dewatering
by combined percolation, evaporation and freezing. A drainage
tile system for discharge of the percolate is constructed on a
base of compacted clay. The tile is covered with 24 inches of well-
burnt red dog and topped with two inches of sand. The drying basin
is divided into three sections separated by concrete walls. One
section is open to the sun, the next is covered with plastic supported
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by an aluminum frame and open at the ends and the third section has
a close-fitting plastic cover, which simulates a solar still.^'
Data has yet to be published from this research.
Lovell found that during sludge removal attempts, a rubber wheeled
vehicle could not negotiate well in the basin due to the presence
of the red dog. The vehicle also compacted the red dog which
impeded the flow of the filtrate to the drain tiles. Lovell
plans to replace the red dog with graded limestone.
Pressure Filtration
Pressure filtration, like vacuum filtration, operates on the principle
of a pressure differential across the filter media. In pressure
filtration the material to be filtered is forced under pressure
against a filter media that catches the solids while the liquid passes
through. Among the common enclosed pressure filtration systems are
shell-and-leaf filters and plate and frame presses. The shell-and-leaf
filters are characterized by filter leaves suspended inside a shell
into which the material to be filtered is charged under pressure. The
plate and frame press utilizes filter media such as cloth, screens or
paper held between a metal, wood, or rubber plate and a frame assembly
to form a frame unit. The material to be filtered is placed within the
frame under pressure.' '
Pressure filtration of waste sludge has not been widely accepted
in the United States due to the inherent batch operations involving
high labor and maintenance costs.(°^) However, pressure filtration
is finding increased usage in England and other European countries.
Lime, aluminum chloride, aluminum chlorohydrate and ferric salts
have been used overseas as sludge conditioning agents prior to
pressure filtration.(62) Polyelectrolytes and flyash were among
the conditioning agents studied by Gerlich'®^' during sewage sludge
dewatering investigations. The addition of flyash, used both as an
admix and precoat during pressure filtration, helped produced a filter
cake which contained 50 percent solids.
Pressure filtration of coal mine drainage sludge was investigated
by Rummel.(71) The sludge was aged 14 days in a large basin where
it thickened to 1.2 percent solids prior to filtering. The solids
content of the filter cake ranged between 20 to 30 percent after
filtering. The output varied from operation to operation with the
Kelly Filter being the most successful unit. Filtration rate was 50
liters of sludge per square meter of filter surface per hour
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9
(.061 gpm/ft ). It was concluded that filter output was not sufficient
to handle large quantities of sludge.
Cycloning
The likelihood of sludge dewatering with a cyclone is doubtful due to
the similar specific gravities of coal mine drainage sludge and water,
Rummel(71) discussed the possibility of sludge dewatering with a
hydrocyclone and concluded that the density of the sludge (1.003)
was too close to water for practical liquid-solid separation.
Centrifugation
Centrifuges separate solids from liquid by sedimentation and centri-
fugal force. Sludge enters the centrifuge through a feed tube
located in the center of the machine. The sludge is accelerated
and distributed to the periphery of the bowl or basket where the
solids are compacted by centrifugal force against the walls of the
bowl. The separated liquid is discharged at one end while the solids
are pushed out the other end by a screw conveyor or are cut away
with a knife mechanism. Various centrifuge models are available
for dewatering waste sludge, the most effective model being the
horizontal, cylindrical-conical, solid bowl machine.(*>2)
Ruramel^ ' considered centrifuging but concluded that high speed
centrifuges would fail to separate the low density coal mine sludge.
Centrifuge tests were also conducted during "Operation Yellowboy"
studies using a Merco Z-l-L solid bowl model on coal mine drainage
sludge thickener underflow. The centrifuge cake averaged 30 percent
solids or greater and the suspended solids recovery was virtually
100 percent. However, the desired feed rate could not be achieved
and power costs were high. The centrifuge was therefore eliminated
in the scale-up design.(64)
Centrifugation as a method of dewatering coal mine sludge has
not received a thorough investigation. The conclusions drawn from
the early attempts at Centrifugation seem to have been premature.
Thermal Drying
Thermal drying of sludges is accomplished by the introduction of
hot gases to rapidly remove moisture from the solids. Thermal
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drying has been applied to sewage sludges. In most sewage sludge
thermal drying applications the dried product is sold as fertilizer
to offset a portion of the operating costs. The basic advantages
of thermal drying are the reduction of harmful pathogenic micro-
organisms, odor destruction, and massive reduction in sludge
volume.(62)
The following types of thermal dryers are applicable to sludge
drying: (1) flash, (2) multiple hearth, (3) rotary drum and (4)
atomizing spray dryers. All of these units use hot gases for
drying and possess the capability to dry wastewater sludges to
less than 10 percent moisture.("^)
Due to high fuel requirements, thermal drying of coal mine sludge
is economically unattractive. If waste heat were available or if the
dried product had some economic value, thermal drying might be
practical. No attempts to dewater coal mine sludge by thermal
drying techniques were found in the literature.
Screening
Screening is used to dewater sludge by applying the basic principles of
gravity filtration. The material to be dewatered is applied to
a fine mesh screen that catches the solids and allows the liquid
phase to pass through.
Dewatering of sludge by screening, particularly with vibrating
screens, has been successfully applied on a pilot plant basis to sewage
sludge. This operation consists of a series of three screens. First,
a coarse stationary screen (8 x 24 mm) removes the large solids. Next,
a sonic screen (1.2 - 2 mm) and three sonic filters (varying between
0.1 - 0.5 mm) remove the small solids. The last screen and filters are
vibrated by electromagnetic vibrators. With the introduction of a roll
press in the system, sewage sludge was dewatered to 35-40 percent
solids/82'
The application of screening to coal mine sludge dewatering seems
remote due to the inability of the screens to catch fine solids.
However, if large, firm floes could be produced from a sludge conditioning
process, screening may become feasible.
Flotation
The use of sludge flotation has been applied to thickening or
clarification operations. Solids are separated from the water by the
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attachment of minute air bubbles which drag the solids along as they
move to the top of the flotation cell.
Flotation experiments have been attempted on coal mine drainage
sludge using various flotation agents. Insignificant solids removal
was reported with the conclusion that, under the conditions examined,
the flotation separation of solids from the sludge slurry was un-
satisfactory.
Lagooning
The lagoon serves three purposes in the treatment of coal mine drain-
age: as a clarification basin, as a sludge dewatering area, and finally
as a sludge storage area.
When a single lagoon performs all three functions the sludge dewatering
function is served the least effectively. In a single lagoon, only
slight compaction occurs. The presence of surface water undergoing
clarification does not allow the sludge to be exposed to the beneficial
effects of evaporation and natural freezing. Thus, the single lagoon
cannot be considered an effective sludge dewatering device.
Another type of lagooning system that is receiving increased attention
is the series system. Kosowski and Henderson'"^) designed a series
system in which the first lagoon catches most of the sludge as it preci-
pitates and settles. The second lagoon "polishes" the treated water
allowing for complete sedimentation. The designers report that the
system was built to minimize sludge "plugging", to allow visual obser-
vation of the flow and to reduce sludge short circuiting. The series
lagoon system has the same disadvantage as the single lagoon due to
the presence of water.
Lagooning operations at times take advantage of evaporation and natural
freezing brought about through atmospheric conditions.'8^' Where land
is available the construction of two lagoons each having sufficient
retention time to handle the entire flow appears advantageous. This
dual or parallel system allows for the alternate use of lagoons so
that sludge in the inactive lagoon can dewater, compact and dry. How-
ever, to take full advantage of the dual lagoon system the inactive,
drying lagoon must be decanted of surface water. A covering of water
will not allow the sludge to be fully exposed to the drying effects of
the sun, wind, and freezing conditions.
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A variation of the dual lagoon system has been reported by Holland,
et al.'l' on an experimental mine drainage treatment plant. Sludge
was pumped from the inactive lagoon to a separate sludge drying
lagoon. While in the drying lagoon, sludge was allowed to undergo
evaporation, freezing and compaction. Holland found that if the
sludge was left undisturbed the percent solids may increase by
as much as 12 to 20 percent over a three week period.
Pudlo ^ ' extended this work by quantifying the effect of evaporation
on sludge volume. He first simulated a normal single lagoon by
determining the percent solids and the cubic feet per pound of dry
solids of sludge samples settled in a 1000 ml graduated cylinder.
Samples were then taken from Holland's sludge drying lagoon and
analyzed in the same manner. By comparing the samples taken from
the drying lagoon to the samples from the graduated cylinder,
Pudlo found that evaporation reduced the volume of the sludge 90
percent as compared to normal single lagoon conditions. Data from
this study is presented in Table 7.
Table 7
Effect of Evaporation on Sludge Volume
Cubic feet per pound of Cubic feet per pound
dry solids from sludge of dry solids from
in 1000 ml graduate sludge in pond
Sample No. Sample No.
1 0.44 1 0.047
2 0.40 2 0.047
3 0.35 3 0.052
4 0.36 4 0.049
5 0.042
6 0.035
(29)
After Pudlo
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In summary lagoon drying can substantially reduce sludge volume, but
requires large land areas. If land is unavailable, other dewatering
techniques may be required to handle the sludge.
Summary of Sludge Dewatering
The most promising method of mechanically dewatering coal mine
sludge is by vacuum filtration. Lime and limestone coal mine
sludges have been successfully dewatered using both conventional rotary
vacuum filtration and precoat rotary vacuum filtration.
Sand bed filtration appears to be a feasible dewatering technique.
However, data is not available from the research currently being
conducted on this process and a critical analysis cannot be made.
Pressure filtration of coal mine sludge has been studied on a limited
scale. Initial results were reasonably successful, but the inherent
disadvantages of the batch dewatering process plus the lack of high
filter output favored vacuum filtration. The initial study on pressure
filtration was conducted almost 15 years ago suggesting that pressure
filtration research should be updated.
Centrifugation studies conducted on coal mine sludge used the solid
bowl model throughout the investigation. Other models such as the
basket type centrifuge may be applicable to coal mine sludge. There-
fore, centrifugation should receive further study.
A summary of mine drainage sludge dewatering methods and results
is presented in Table 8.
Table 8
Coal Mine Sludge Dewatering Attempts
Dewatering Method Percent Solids of Cake
Vacuum Filtration 23-45
Precoat Vacuum Filtration 22-63
Sand Bed Drying* 12-33
Pressure Filtration 20-30
Centrifugation 30 average
Single Lagoon .5-4.5
Drying Lagoon 12-20
No data available, results projected by author.
Percent solids of input sludge (.5 - 9.5 percent).
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COST COMPARISON BETWEEN METHODS FOR DEWATERING
Sludge dewatering costs as related specifically to coal nine drainage
have not been reported in any depth. This is due to the almost
universal use of lagooning which frequently serves both dewatering
and disposal functions.
Since data is generally not available for various coal mine drainage
dewatering schemes, a study into other sludges and their dewatering
economics is in order. It must be understood that average cost
figures from one type of sludge cannot be directly compared to
coal mine drainage sludge.
A general review has been made on economic data related to the
average sludge handling and disposal costs of sewage sludge.' '
Table 9 presents the capital and operating costs of various sludge
handling systems as they relate to sewage sludge. The costs in this
table do not include any conditioning costs or costs for ultimate
disposal.
As can be expected lagooning is the least expensive treatment
method. Heat drying is the most expensive and the costs of other
dewatering techniques falls between the two extremes.
Table 9
Costs of Sludge Handling System For Sewage Sludge
Capital and Operating
Sludge Handling System Costs ($/ Dry Ton)
Average
Lagooning 2
Sand Bed Drying
Centrifugation 12
Vacuum Filtration 15
Heat Drying 35
(62)
After Burd
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METHODS OF SLUDGE DISPOSAL
The quantity of sludge created by the lime or limestone neutralization
process is large. When suitable land is available, lagooning is
the normal disposal method. Holland, et al.W aptly described the
land requirements for sludge disposal in a report related to an experi-
mental treatment plant. This plant treated water at the rate of 200
gpm, 16 hours per day, 5 days per week (approximately 52,000,000
gallons per year) and if kept in continuous operation would have pro-
duced 4 acre feet/year of settled sludge.
The indiscriminate disposal of sludge created from the treatment of
coal mine drainage could result in a pollution problem as serious
in some cases as the original problem caused by mine drainage.
Besides the adverse environmental effects of sludge dumpings, there
is the possible danger of slippage or subsidence caused by the
storage of sludges above grade in residential areas.
Lagooning for the purpose of water clarification and sludge thickening
is almost universally practiced. The natural extension of this
practice is to leave the sludge in the lagoon for long periods of
time. This is in fact the most common disposal method currently in use.
Problems can evolve from this practice, especially when the sludge
starts to occupy a large portion of the lagoon volume. If the
occupied volume becomes large enough to reduce required retention
time, short circuiting may introduce solids into local streams.
It is possible to alleviate this problem by pumping the settled
sludge into a separate lagoon for further dewatering and drying.
In this case the sludge can be exposed to evaporation and freezing
conditions which can substantially increase the solids content.
The sludge can then be periodically removed to landfill operations,
however no record of this practice has been found.
Disposal of coal mine sludge to abandoned deep mines is being used
when conditions warrant.^2'»33' Like other disposal methods,
underground disposal can lead to other problems. When factors
such as the geological and legal environment permit and underground
disposal is feasible, the cost savings for this method can be sub-
stantial.
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REGULATIONS CONCERNING SLUDGE DISPOSAL
Disposal problems of coal mine drainage sludge have been recognized
by state agencies, particularly in the Commonwealth of Pennsylvania.
The Sanitary Water Board, Pennsylvania Department of Health, has
established guidelines for the underground disposal of sludge from
coal mine drainage treatment. '") The main criteria for underground
sludge disposal is that the sludge must have a pH of 7.0 or above
and that all the iron present must be in the ferric state. Other
factors that have to be considered are the location of disposal,
mine hydrology, quality of the water in the mine where the sludge is
to be disposed, sludge characteristics, and geology.
A major concern with underground disposal is the possibility of
the sludge redissolving. Lovell'^' reported that work has been
done on sludge dissolution but that further study is needed. To
alleviate this problem the Commonwealth of Pennsylvania requires
the mine operator to supply data to show that the disposed sludges
will not affect present or future discharges from the mine pool.
In addition to underground disposal data, Pennsylvania requires
detailed information on sludge lagooning such as lagoon depth,
capacity, expected life and geologic data. A description must also
be submitted on proposed methods of removal and disposal of the con-
tents of the lagoon during and after its expected life.
When sludge is to be dispersed on land, indepth data must be prepared
on the type of waste, location, bedrock, solids and ground water of
the disposal site. If the site is also used for sanitary landfill
operations, the landfill must be approved by the Pennsylvania
Department of Health.
The recent public concern over pollution will probably lead to
further regulations of sludge disposal on both the state and Federal
level.
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USES FOR SLUDGE
One of the first commercial applications of coal mine drainage
sludge was found when coal mine drainage treatment was still in
its infancy. Sludge underflow from a thickener was dried on a
steam heated rotary drum, and the dried material was sold as a
component for a gas purification sponge used to remove hydrogen
sulfide from gas generated during the combustion of bituminous
coal. Studies were also made using the same sludge as a soil
conditioning agent. It was found that fifteen to twenty pounds
per acre of dried precipitate could increase crop yield by up
to 75 percent. However, quantities in excess of that amount
were toxic. Plans were also made for construction of a calcining
plant to convert the sludge precipitate into paint pigment, but
were never implemented.(•")
(79)
Jeh and Jenkins attempted greenhouse experiments using mine
drainage sludge. They found that a mixture of mine drainage sludge,
sewage sludge and mine spoil (approximately 25% - 30% - 45%
respectively) substantially increased plant growth. The authors
suggest that this mixture could be used in revegetation of areas
disturbed by surface mining.
Other investigations have been made into the utilization of coal
mine drainage sludge in four areas: additives used in the building
materials industry; recovery of iron; the application of gypsum
technology to the sulfate portion of the sludge; and separation
of the major chemical components.(86) These investigations were
generally technical feasibility studies and did not include
economic consideration.
Osman, et al.(86) concluded that small amounts of sludge (1-2%)
could be added in some cases to clay to induce color or texture
changes in structural brick. The use of sludge in cement manu-
facture and in concrete was not attractive.
Sludge was also successfully pelletized and used as a component of
blast furnace feed.(86'
Due to the fact that the calcium sulfate in the sludge is not
cementitious, the application of elementary gypsum technology
to form plaster of paris was unsuccessful. Other processes such
as the Merseburg reaction and the OSW process appear attractive
but will require considerable process development before they
can be applied to coal mine sludge.^ '
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Wet sieving was reported to be a feasible method for separating
sludge into its iron and sulfate constituents. Even more
effective separation could occur if prior nucleation (crystal
growth) was introduced in the mine drainage treatment system.* '
The authors also felt that raw sludge from thickeners could be
used in soil engineering as pond wall sealant or drilling mud.'8")
To summarize, there is no known major practical use for coal mine
sludge. If a use could be found, an economic payback could be
realized which might offset, at least in part, the substantial
costs of treating coal mine drainage. Therefore, it is recommended
that further research be conducted into the use of, or by-product
recovery from coal mine sludge.
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ACKNOWLEDGEMENTS
Mr. Edward A. Moss, the principal investigator for E.P.A. Grant
14010 FJX, authored this report.
Mr. Edwin B. Wilson, now associated with Bethlehem Steel Corporation,
submitted the proposal for this project and his efforts on this
project are gratefully acknowledged.
Thanks are also due to Messrs. David J. Akers, Jr., Gary Myland,
Larry G. Shaffer and James Pappajohn for their assistance in the
collection of the literature for this report. A special thanks
are due to Mr. Richard B. Miter for his aid in the preparation of
the section on chemical analysis.
The financial support of this project by the Environmental Protection
Agency and the State of West Virginia, Coal Research Bureau,
Joseph W. Leonard, Director, is acknowledged with sincere thanks.
Mr. Charles F. Cockrell, Acting Director of Research at the Coal
Research Bureau is acknowledged for his guidance and editing of
this report.
Mr. Charles McFadden prepared the illustrations for this report. The
cooperation of the secretarial staff of the Coal Research Bureau is
gratefully acknowledaed.
A significant objective of this project was to investigate practical
means of abatinq mine drainane pollution. Such research projects,
intended to assist in the prevention of pollution of water by
industry, are required by Section 6 b of the Water Pollution Control
Act, as amended. This project of EPA was conducted under the
direction of the Pollution Control Analysis Section, Ernst P. Hall,
Chief, Dr. James M. Shackelford, Project Manager, and Roger C. Wilmoth,
Project Officer.
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81
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88
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GLOSSARY OF TERMS
Admix - Particulate materials that are added to sludge which tend to
form a porous, permeable and rigid lattice structure. This
structure helps retain solids but allows passage of liquid
during filtration.
Clarifier Underflow - Settled sludge that is pumped or otherwise
removed from the bottom of a clarifier.
Decantation - Removal of clarified water from sedimentation basin.
Diatomaceous Earth - Skeletal remains of single cell plant life (fossil
silica) that can be used as a sludge admix or
precoat during filtration.
FPM - Feet per minute.
Flocculation - Agglomeration or tying up of finely suspended material
in large masses that settle quickly.
Floes - Suspended particles that are joined together to form a larger
mass.
Flyash - Unburned fine inorganic residue resulting from the combustion
of coal used in power generation.
GPP - Gallons per day.
Mg/1 - Milligrams per liter or parts per million.
Mine Spoil - The overburden material removed in gaining access to the
mineral material in surface mining.
Organic Polyelectrolytes - Synthetic compounds that aid the agglomeration
of finely suspended material during water
clarification processes.
j>H - Measurement of the negative log of the immediate hydrogen ion
concentration in moles per liter.
Rock Dust - Limestone that is crushed so that 65-80 percent will
pass through a 200 mesh screen.
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Roll Press - Dewatering apparatus that removes water from sewage
sludge by passing the sludge through a series of rolls
rotating in opposite directions.
Short Circuiting - Inflow to settling basin reaches outlet in less
than the theoretical detention period.
Slaking - Quicklime (CaO) is converted to a hydrate form (putty,
slurry or milk-of-lime).
Synthetic Coal Mine Drainage - Man-made mixture of the major con-
stituents of coal mine drainage.
Zeta Potential - A measure of electrostatic charge on particles in
suspension.
90
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Accession Number
Subject
Field & Group
05D
SELECTED WATER RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Organization
Coal Research Bureau, West Virginia University
Title
Dewatering of Mine Drainage Sludge
10
Authors)
Boss, Edward A0
22
11
16
Date
12
Pages
Project Number
EPA Project No«
14010 FJX
1 c Contract Number
^i | Note
Citation
23
Descriptors (Starred First)
* *
Sludge Treatment, Acid Mine Water, Dewatering, Neutralization, Lime, Limestones,
Sludge Disposal
25 Identifiers (Starred First)
Sludge Conditioning, Sludge Thickening
27
Abstract
This report is a literature review on thickening and dewatering of sludge resulting from
lime or limestone neutralization of coal mine drainage.
The effects of mine water constituents and methods of treatment on the physical and
chemical characteristics of the resulting sludge are described0 Such current practices
as aeration, recirculation and neutralization are discussed. Additional techniques at
various stages of development, such as thickening, conditioning, and dewatering are
evaluated for use in coal mine drainage treatment.
The most promising coal mine sludge dewatering technique appears to be vacuum filtration.
Other methods such as sand bed filtration, pressure filtration and centrifugation may
also be applicable,.
Recommendations are made as to the areas in coal mine drainage treatment and sludge
densification that need further research.
Abstractor
Edward A0 Moss
institution Coal Research Bureau, Wefit Virginia
NR-,102 (REV. OCT. 1868)
WRSIC
ER RESOURCES SCIENTI FIC INFORMATION CENTER
U S. DEPARTMENT OF THE INTERIOR
WASHINGTON. D.C. 20240
* GPO: 1 96 9— 324-444
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