Environmental Protection Technology Series
LIMESTONE AND LIME NEUTRALIZATION OF
FERROUS IRON ACID MINE DRAINAGE
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special'1 Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-77-101
May 1977
LIMESTONE AND LIME NEUTRALIZATION OF
FERROUS IRON ACID MINE DRAINAGE
by
Roger C; Wilmoth
Resource Extraction and Handling Division
Crown Mine Drainage Control Field Site
Rivesville, West Virginia 26588
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Industrial Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
11
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FOREWORD
When energy and material resources are extracted, processed, converted,
and used, the related pollutional impacts on our environment and even on our
health often require that new and increasingly more efficient pollution con-
trol methods be used. The Industrial Environmental Research Laboratory -
Cincinnati (lERL-Ci) assists in developing and demonstrating new and improved
methodologies that will meet these needs both efficiently and economically.
This report evaluates the use of two commonly available neutralizing
agents—limestone and lime—for use in the treatment of acid mine drainage
(AMD), which is an industrial waste most closely associated with the mining
of coal. This study deals specifically with AMD where the iron is in the
ferrous state, which is the most difficult situation to treat. Because of
the recent emphasis on immediate expansion of the coal industry, the tech-
nology for the abatement of pollution from the mining operations needs re-
liable definition. The documentation of the cost, effectiveness, and
feasibility of the use of lime and of limestone is included in this report.
As such, this data will be of interest to regulatory agencies, individuals
involved in AMD treatment research, and—perhaps most importantly—to
industry as an aid to the design and/or modification of treatment facilities.
For further information contact the Resource Extraction and Handling Division.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
ill
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ABSTRACT
The U.S. Environmental Protection Agency (EPA) conducted a 2-yr study on
hydrated lime and rock-dust limestone neutralization of acid mine drainage
containing ferrous iron at the EPA Crown Mine Drainage Control Field Site near
Rivesville, West Virginia.
The study investigated optimization of the limestone process and its
feasibility in comparison with hydrated lime treatment. Operating parameters,
design factors, and reagent costs for both processes were determined. Effluent
quality was considered of prime importance in these investigations. Coagulants
were considered essential to successful thickener operation for both lime and
limestone treatment. Effluent total iron, suspended solids, and turbidity
values could be maintained below 3 mg/1, 10 mg/1, and 10 JTU, respectively, by
using coagulant addition.
Although the limestone process was demonstrated to be technically effec-
tive in ferrous iron treatment situations, the process was judged to be less
efficient overall in comparison with lime neutralization. The reaction and
aeration detention time requirements for the limestone process were two to
three times that for the lime process and overshadowed the reagent usage cost
advantage of the limestone process. The limestone process was thus judged
unfeasible for general application in ferrous iron acid mine drainage
situations.
This report covers a period from January I9Jk to January 1976.
IV
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CONTENTS
Foreword iii
Abstract iv
List of Figures vi
List of Tables viii
Acknowledgments x
1. Introduction 1
2. Conclusions 2
3. Recommendations 5
k. Procedures 6
5. Results 18
References ^92
Glossary =9^-
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FIGURES
Number
1 Schematic flow diagram for the EPA neutralization facility 7
2 Titration curves for lime and limestone 19
3 Theoretical ion solubility as a function of pH 21
k Effect of pH on ferrous iron solubility 23
5 Observed solubilities as a function of initial pH 2U
6 Observed solubility of ions during continuous-flow studies 25
7 Comparison of settling trends at pH 6.5 and pH 9-5 27
8 Settling trends for lime neutralization 30
9 Settling trends for limestone neutralization 31
10 Settling trends for combination limestone-lime neutralization ... 32
11 Comparison of settling trends at pH 7 33
12 Sludge percent solids @ 1 hr as a function of pH 35
13 Supernatant turbidity @ 1 hr as a function of pH 36
lit- Sludge percent solids % 2k hr as a function of pH 37
15 Supernatant turbidity § 2h hr as a function of pH 38
l6 Batch-scale combination limestone-lime feasibility test ko
17 Effect of the treatment pH on lime-usage costs 68
18 Sludge volume and effluent turbidity as functions of pH 69
19 Underflow solids concentration and dry solids production
for lime neutralization 70
20 Trend of the sludge solids during continuous sludge recycling
with no sludge discharge 86
vi
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FIGURES (COHT'D)
Kumber Page
21 Ferrous decline at various pH's for the EPA Crown discharge 88
22 Average oxidation rates as a function of pH 91
VI1
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TABLES
Number Fage
1 Typical Detention Times ...................... 1^
2 Crown Water Quality Data, 1 llh Thru 6/75 ............. 15
3 Manufacturers Chemical Analyses of Lime and Limestone ....... l6
k Speetroehemical Analysis of Germany Valley Limestone ....... IT
5 Sieve Analyses of As-Received Lime and Limestone ......... 17
6 Theoretical Ksp Constants for AMD Hydroxide Compounds ....... 20
7 Results Comparison as Related to Treatment pH ........... 28
8 Results Comparison as Related to Settling Time .......... 29
9 Combination Limestone-Lime Laboratory Study ............ 39
10 Research Plan for Limestone Investigations ............ k2
11 Neutralization Data Summaries for Limestone @ pH 6.5 and
1 Liter/Sec ........................... It 3
12 Chemical Analyses From the Limestone Neutralization Studies . . . .hk
13 Limestone Neutralization Investigating Increased Detention
Times .............................. 48
Limestone Neutralization With Sulfuric Acid Injection
15 Limestone Neutralization Comparing 0.5 Liter/Sec vs
1 Liter/Sec ........................... 50
16 Limestone Neutralization With Hydrogen Peroxide Addition ..... 52
17 Limestone Neutralization With Sludge Recycle ........... 53
18 Lime stone. Neutralization With Increased Recycle Rates ....... 55
19 Limestone Neutralization With Two Aerators in Series ....... 57
VI11
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TABLES (CONT'D)
Humber Page
20 Limestone Neutralization to pH 7 Using Two Aerators 59
21 Limestone Neutralization Using Two Aerators and Coagulants
@ 0.9 Liter/Sec 60
22 Limestone Neutralization Using Two Aerators and Coagulants
@ 0.9 Liter/Sec 6l
23 Research Plan for Lime Neutralization 6k
2h Characteristic Operational Data for Lime Neutralization
Studies 65
25 Characteristic Chemical Analyses for Lime Neutralization
Studies 66
26 Lime Neutralization Using Sludge Recycle 71
27 Chemical Analyses From the Lime Neutralization Studies 72
28 Lime Neutralization Investigating Recycling Rates 76
29 Lime Neutralization Using Coagulant Addition 77
30 Lime Neutralization Using Coagulant Addition @ pH 8 vs pH 7 • • • -79
31 Lime Neutralization With Coagulants Comparing Sludge Recycle
vs No Recycle 80
32 Lime Neutralization § pH 9 Using No Aeration (Ferrous
Precipitation) 82
33 Lime Neutralization Using No Aeration With Coagulant Addition ... 83
3^ Lime Neutralization; Aeration vs No Aeration 8U
ix
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ACKNOWLEDGMENTS
Thanks are extended to Ronald D. Hill, Eugene F. Harris, Robert B.
Scott, James L. Kennedy, Loretta J. Davis, Walter E. Grube, Jr., Paul H.
Moore, Robert M. Michael, Ralph S. Herron, Paula S. Arnett, Cathy J. Scott,
Joan R. Allender, and Frank J. Beafore for their helpful suggestions and
willing assistance during the course of these investigations.
A special thanks is extended to J. Randolph Lipscomb and Harry L.
Armentrout for their insight and invaluable contributions to the success of
this project.
The cooperation of Ray Henderson, Vince Ream, Ed Moore, Mike Ryan, and
Hershel Travis of Consolidation Coal Company is greatly appreciated.
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SECTION 1
INTRODUCTION
The Environmental Protection Agency's (EPA) Crown Mine Drainage Control
Field Site facility, located near Morgantown, West Virginia, "was established
in 1972 to investigate the treatment requirements of predominately ferrous
iron acid mine drainage. This was a partial follow-up of earlier studies
by the same staff at Norton, West Virginia, where the iron in the acid mine
drainage (AMD) was in the ferric state.
EPA employees designed the Crown facility and performed most of the
construction work. Contracts were awarded for the plant building shell,
drying beds, lagoon, storage bins, and miscellaneous earthwork, and EPA per-
sonnel completed the construction including wiring, plumbing, instrumentation,
and fabrication.
At present, lime neutralization is the commonly accepted method for treat-
ment of AMD discharges. Lime treatment has several disadvantages:
- It produces a low-density, high-volume sludge.
- Its highly reactive nature provides high potential for accidental
overtreatment.
- As a refined product, it is moderately expensive and is in
increasingly short supply.
Earlier studies by EPA and others have indicated that limestone treatment
(also combination limestone/lime) has advantages over lime in certain situa-
tions. Most important of these advantages are:
- Production of a low-volume, high-density sludge.
- Cheaper raw-materials costs.
- Cheaper usage costs in some instances.
These processes needed to be investigated under ferrous iron conditions
before they could be fully evaluated. This report documents the results of
research conducted to evaluate the lime and limestone processes (plus bench-
scale studies on combination limestone/lime) under ferrous iron conditions.
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SECTION 2
CONCLUSIONS
LIMESTONE TREATMENT OF FERROUS IRON AMD
Limestone utilization efficiencies were below 30 percent for ferrous
iron acid mine drainage neutralization to pH 6.5, which was about the highest
pH obtainable without sludge recycling. Effluent quality from the thickener
was terrible under the pH 6.5 conditions.
Recycling sludge at a rate of 20 percent of the influent flow rate in-
creased the utilization efficiency of limestone to the 50-percent range.
Effluent quality was still poor.
The use of two aerators in series provided adequate detention time for
ferrous iron oxidation. At least k-hr detention time was necessary to reduce
the ferrous iron level from 270 to 2.4 mg/1 at pH's near 7. The combination
of increased aeration time, better mixing, and sludge recycling enabled
limestone treatment to pH's as high as 7-4. Effluent suspended solids
(40 mg/l) and turbidity (60 JTU) were still high. Neutralization efficiencies
ranged from 50 to 80 percent.
Addition of coagulants reduced effluent iron levels (from 20 to 1 mg/l),
lowered turbidities (from 60 to 12 JTU), and improved suspended solids (from
40 to 9 mg/l). The coagulants greatly diminished pH probe fouling problems,
which were generally severe below pH 7*5»
The most satisfactory process operation involved incorporating a 20-
percent sludge recycle rate, maintaining pH's near 7-4, furnishing 30-min
detention time with vigorous mixing in the reactor, providing between 4 and
6 hr of detention time in the aeration system, and injecting coagulants at
about a 7-mg/l rate. Reagent costs for this process were approximately 1.3
cents/cu m (5 cents/1000 gal) for limestone plus approximately 2.6 cents/cu m
(10 cents/ 1000 gal) for coagulant. Coagulant addition was not optimized.
The thickener was most effective in clarification around 0.05 liter/sec/sq m
(0.07 gpm/sq. ft) of surface area.
2
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LIME TREATMENT OF FERROUS IRON AMD
Optimum overall operation of the EPA treatment system was at pH 8. At
this pH, probe fouling problems were almost nonexistent, effluent clarity was
improved, and iron oxidation was complete. Reagent cost was increased over
25 percent by treating to pH 8 rather than pH 7; an additional ^0-percent cost
increase was incurred by treating to pH 8.5 rather than to pH 8.
The thickener appeared to function most satisfactorily when the influent
suspended solids were between 5,000 and 10,000 mg/1. Sludge recycling was
employed to increase the influent solids to this range. Thickener effluents
contained between h and 10 mg/1 of suspended colloidal iron.
Coagulants were very effective in improving the thickener effluent qual-
ity. Iron values were consistently below 2 mg/1 using coagulants at pH 8.
Precipitating the iron without oxidation offered few advantages over the
oxidized version of the neutralization process. For ferrous precipitation,
pH's had to be elevated to around pH 9, resulting in an increase in lime cost
of 30 percent as compared to ferric treatment to pH 8. The ferrous hydroxide
sludge occupied more than 1.5 times the volume of the ferric sludge. Thickener
effluent quality was significantly better in the ferric situation.
Lime treatment to pH 8 required approximately J-m±n reaction time and less
than 2-hr oxidation time (utilizing a aludge recycling rate below 20 percent of
the influent rate) -and required coagulant addition at a rate under 5 mg/1.
Reagent costs for this process were approximately 2.9 cents/cu m (11 cents/1000
gal) for lime plus 1.9 cents/cu m (7 cents/1000 gal) for coagulant. Coagulant
addition was not optimized. Sludge settling rates with coagulants were greater
than k cm/sec (8 ft/hr) for unhindered settling.
COMPARISON OF LIMESTONE VS LIME
Limestone treatment of ferrous iron acid mine drainage involves reagent
usage costs somewhat less than is required for lime treatment. Overall costs
of the limestone process must include the very large reaction and oxidation
vessel size and increased power requirements. These and other operational
considerations render the limestone process to be considered unfeasible for
general application to AMD waters containing over 50 mg/1 of ferrous iron.
IRON OXIDATION
An average oxidation rate design factor of 2 mg/1 per min is suggested for
lime neutralization aeration facilities with influent ferrous levels below 300
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mg/1. On-site tests should be made to better determine the actual rate for
the specific conditions before proceeding to final design. It is imperative
that sufficient mixing capability be incorporated to provide enough top-to-
bottom turbulence to prevent solids accumulation in the aeration tank.
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SECTION 3
RECOMMENDATIONS
Accurate and reliable design criteria for oxidation time requirements
for acid mine waters are not available. The same applies to thickener design
criteria. A study should be undertaken of existing AMD neutralization facil-
ities to determine if an accurate model of these processes could be developed.
Sludge disposal from AMD treatment processes has received little research
effort to date. Since as much as 10 percent of the influent AMD can exit from
the plant as sludge, a significant effort to evaluate the disposal alterna-
tives, costs, and environmental effects is essential.
If the AMD should contain toxic pollutants, it is logical to assume that
at least some of these would be removed by neutralization. In this case, the
sludge would contain relatively high concentrations of toxic materials and
disposal considerations become even more critical.
The fact that the environment suffers from AMD pollution is unquestion-?
able; however, the alternative offered by lime and limestone treatment is the
introduction of high calcium and sulfate concentrations into the receiving
stream. An evaluation should be made of the overall long-term environmental
and economic effects of increased calcium and sulfate levels and of suggested
alternatives such as reverse osmosis, ion exchange, lime-soda softening, and
alumina-lime-soda treatment.
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SECTION k
PROCEDURES
DEFINITIONS
Definitions and an explanation of the calculations used in this report
are presented in the Glossary.
DESCRIPTION OF FACILITY
Acid mine water was obtained from the Stewarts Run borehole pump of
Christopher Coal Company that pumped water from the Pittsburgh Coal Seam
located approximately 85 m (280 ft) below the surface. This pump removed
water from an abandoned section of Mine 93 to prevent the drainage from
entering an active section of the mine. The borehole pump discharged approx-
imately 32 liters/sec (500 gpm) into a 3-8-cu m (lOOO-gal) concrete reservoir
constructed from a burial vault. The water used by the EPA facility was
pumped from the reservoir to the treatment plant—a distance of about 366 m
(1200 ft)—through a 102-mm (U-in) plastic pipe at a pressure of U80 kN/sq. m
(TO psi).
The AMD flowed through the neutralization facility as illustrated in
Figure 1. Processes A and B were identical in every respect. By having a
dual system such as this, it was possible to use one side as a control and
the other side as the variable and thus expedite the research efforts. The
dual system allowed immediate evaluation of the isolated variable under study.
All AMD transfer lines were 25-mm (l-in), Schedule 1*0 PVC. The sludge lines
were 13-mm Cs-in) plastic roll piping. PVC in-line strainers removed large
particulate matter from the AMD prior to entering the pressure regulators.
Plast-0-Matic Model PR075V pressure regulators were used to maintain relative-
ly constant downstream pressures to reduce flow fluctuations. C-E Invalco
Model W3/1000 turbine flow meters with Model ¥315 totalizers and rate indi-
cators totalized gallonage treated and displayed instantaneous rate of flow.
Accuracy of these meters was better than 1 percent of the full-scale flow of
10.3 liters/sec (6k gpm).
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PROCESS A
Reactor $™J«,!«i'£!«
Coagulant
Pressure
Regulator
_
Strainer Turbine
Flow
Meter
\pH [control
.1
rJt\****4**4*t*+nmtt ««
ACID MINE
—»—
DRAINAGE
Wastei
1 I V Aiv-w JLUUOt
.."—I \ Sludge Recycle W^r— m
Limestone -^ **—\ ?y
Magnetic I ill
Flow Meter/ „ ^
Composite
Sampler
Feeder
Valve
I * I Composite
'—'Sampler
PROCESS B
Reactor
Sludge Recycle Diverter
Coagulant
Strainer Turbine
Flow
Meter
Jiuugc nci,yuic ^V^ U)^
SLUDGE
HOLDING
TANK
Tap Water
I
TO DEWATERING
AND DISPOSAL
FACILITIES
Limestone
Feeder
Flow Meter
Composite
Sampler
Figure 1. Schematic flow diagram for the EPA neutralization facility.
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From the turbine meters, water could be channeled either:
l) Into the lime reactor, the aerator, and the thickener, or
2) Into the limestone reactor, the aerator, and the thickener, or
3) Into the limestone reactor, the lime reactor, the aerator and
the thickener.
The lime reactor tanks were type 30k stainless steel with effective capacities
of k$k liters (120 gal) and were equipped with Lightnin' Model NS-7, 3-hp,
1750-rpm mixers. High-density polyethylene tanks with 9-5-mm (3/8-in) thick
walls and effective capacities of 3785 liters (1000 gal) were used for the
limestone reactors. Lightnin' NS-7 mixers were installed on independent mounts
for use with the polyethylene tanks. In later studies, the NS-7 mixers on the
limestone reactors were replaced by 3-hp, 350-rpm, Lightnin' ND-UB mixers to
obtain increased turbulence. The NS-7 system provided a mixing (pumping) rate
of 93 liter/sec (lU70 gpm); the ND-to pumped 360 liter/sec (5710 gpm). The
limestone reactors were equipped with two baffles per tank. Each baffle con-
sisted of a metal outside support frame surrounding a l68-cm by 36-cm (66-in
by lij-in), ik x 1^-mesh, 30^-stainless steel screen. The screen baffles served
to improve mixing efficiency and to abrade the limestone particles in order to
remove reaction coatings.
BIF Model 25-06 helix-type volumetric dry feeders with variable-speed
drives were used to feed both hydrated lime and rock-dust limestone into the
respective reactors. The hoppers on top of the dry feeders held 0.2 cu m
(7-8 cu ft) of material. Syntron Model V-20 vibrators were bolted to the
side of each hopper to prevent bridging. The vibrators operated whenever the
dry feeders were operating. Typical observed dry feeder accuracies were
about +_ 5 percent on a weight basis. The operating time for each dry feeder
was accumulated on Eagle Model HK300A6 time totalizers reading directly in
minutes. These units were equipped with dynamic braking circuitry to prevent
the timers from coasting after the dry feeders stopped.
The lime and limestone dry feeders were refilled automatically from 36-
tonne (UO-ton) capacity Butler bolted-steel silos equipped with Vibra-screw
bin activators to prevent bridging. Jeffrey screw conveyors with 15-cm (6-
in) screws transported the neutralizer from the silos to the top of the dry
feeders. Monitor-brand bin level indicators were mounted in the dry feeder
hoppers to activate the conveyors when the level dropped.
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The storage silos were periodically pneumatically filled from l8-tonne
(20-ton) supply trucks. Butler Model CS9-90 bin-vent filters were mounted on
top of the silos to prevent dust from escaping while the tanks were being
loaded.
The neutralized water was pumped from the reactors into the aerators.
The aeration tanks were 3m (10 ft) in diameter by 1.8 m (6 ft) high with a
nominal water depth of 100 cm (3.25 ft); they contained 6800 liters (l800
gal). Four 25-cm (lO-in) solid baffles were installed at equal spacing
around the periphery of the tank. The tank itself was constructed of 6.^-mm
(%-in) hot-rolled steel coated with epoxy. Lightnin' Model LAR-10, 1 hp,
fixed-mount, surface aerators were installed on top of the aeration tanks.
These aerators were designed to transfer 1.8 kg/hr (h Ib/hr) of oxygen under
standard conditions.
Although the aerators were quite satisfactory in oxygen transfer, they
lacked sufficient top-to-bottom turbulence to keep heavier particles suspended.
This problem was most prevalent during limestone treatment studies where a
sizable accumulation of solids occurred in the bottom of the aeration tank
and thus decreased the effective detention time. A bottom-mixing prop was
installed on the aerator shaft later in the studies to successfully decrease
the solids accumulation problem.
After aeration, the treated water was pumped into the thickener for
clarification and sludge removal. FMC Link-Belt, Type-H thickeners were
used. These thickeners were 5-7 m (l8.T5 ft) long, 2.k m (8.0 ft) wide, and
2.7 m (9-0 ft) high (excluding sludge hoppers) and had an effective capacity
of 3^.8 cu m (9200 gal). Adjustable effluent weirs allowed the effective
depth to be varied from 2.7 m (9 ft) to 1.8 m (6 ft). Rakes moving at 0.005
m/sec (l fpm) pushed the sludge into a collection hopper on the influent end
of the thickener. The sludge hopper extended approximately 1.6 m (5-25 ft)
below the floor level .of the thickener and held 3.h cu m (900 gal) of sludge.
At 1 liter/sec (15.85 gpm), the surface overflow rate was 0.07 liter/sec/sq m
(0.11 gpm/sq ft).
Sludge was removed from the thickeners by timer-controlled centrifugal
pumps. Continuous removal was not possible because the lines became plugged
at low-flow rates. It was therefore necessary to pump the sludge for short
durations at high-flow rates. A Tork Model 12M8001 repeating-cycle timer
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•with a 12-min full-cycle time and 60 adjustable tabs to regulate on-off time
controlled the pumping duration and frequency. Typically, in non-recycling
situations, the sludge pumps operated 10 percent of the time (on 0.6 min and
off 5.1* min). Sludge pumping rates were about 0.3 liter/sec (5-5 gpm) through
a 13-mm (Jg-in) line corresponding to a linear flow velocity of 2.7 m/sec
(9 fps).
Sludge recycling was accomplished by utilizing Eagle Signal Series
HPTOO, adjustable-duration pulse-timers to control a pneumatically operated
diverter valve in the sludge line. When the Tork timer activated the sludge
pump, it simultaneously activated the Eagle pulse-timer. Sludge was recycled
into the appropriate reactor until the Eagle timer timed-out and switched the
sludge diverter valve in the waste direction.
Initially, Fischer X^Ot diverter valves were used for recycling sludge.
These valves had a 3l6-stainless steel body and ball with teflon seats.
During limestone sludge recycle studies, the tiny unreacted limestone particles
wedged between the teflon seats and the ball and scored both the seat and the
ball in just a couple of weeks of operation. Later, these valves were replaced
with Model 2600 NPT Red Valves with hypalon liners and PVC end caps.
Sludge flow was measured by Brooks Model 7185FB11KA 13-mm (%-in) magnetic
flow meters that totalized gallonage and displayed instantaneous rate-of-flow
(Model 7300B1A2C signal converters and Model 55UOBLA.UA1A1 totalizers).
Accuracy of these magnetic flow meters was 0.5 percent of the full-scale
indication of 0.63 liter/sec (10 gpm).
Since each magmeter was placed before the diverter valve, the output was
indicative of total sludge flow. To independently totalize the sludge quan-
tity being recycled and the quantity going to waste, the magmeter signal
converter output was channeled through a relay that switched (at the same
time the diverter valve switched) from one totalizer to another.
The waste sludge from both thickeners was pumped into a 1.5-cu m (UOO-
gal) reservoir located immediately adjacent to the thickeners. When the
sludge level reached the top of the reservoir, an automatic sequencer-
controlled pump transferred the sludge to disposal facilities and then flushed
the sludge lines with approximately 150 liters (Uo gal) of tap water. Sludge
flow to the disposal facilities was initially measured by volumetrically
determining the flow rate and multiplying by pumping time. Operating time of
10
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the pump was recorded on an Eagle elapsed-time meter. Later, this flow was
totalized electronically by remote slave counters on each magmeter waste
totalizer. Typical pumping rates to the disposal facilities were 1.1
liter/sec (IT gpm) through a 25-mm (l-in) roll plastic line corresponding to
a linear flow velocity of 2.1 m/sec (6.8 fps).
Both lime and limestone feed could be controlled either manually or
automatically. Automatic control was accomplished by Universal Interloc
(Uniloc) Model 1001 pH recorder-controllers that utilized on-off type control.
The pH controllers turned the dry feeders on when the pH dropped below the set
point and off when the pH increased beyond the set point. In order to reduce
probe fouling, the pH probes (Uniloc Model 321 immersion-type) were placed in
the influent area of the thickener. Probes that were placed in the reactors
fouled within a few hours. Even in the thickener, the probes fouled severely
at pH's below 7-5- It was necessary to clean the probes with HC1 as often as
three times per day at the lower-pH conditions. It took the probes approxi-
mately 1 hr to recover from each acid cleaning. Neutralizing the probes in a
pH 10 Na CO solution hastened the probe recovery. At pH levels above 7-55
probe fouling was not a problem and cleaning was routinely done once a day.
Effluent pH was monitored by similar Uniloc equipment with the pH
probes placed immediately in front of the thickener effluent weir.
Both influent (control) and effluent pH's for processes A and B were
recorded on Hewlett-Packard Model 7100B strip chart recorders. In the normal
mode of operation, the control set-point on the influent pH meter was ad-
justed until the desired effluent pH was achieved. Generally, the entire
system could be changed from one pH to another and stabilized within 36 hr.
The pH control system worked extremely well, as the effluent pH could be
maintained within +_ 0.1 pH for extended periods of time. The design flow
rate through each process was 1 liter/sec (15-9 gpm).
All process equipment was located indoors to isolate the variable of
weather from the data evaluation.
PHYSICAL MEASUREMENTS
Flow and Reagent Usage
Water and sludge flow rates and quantities were measured by turbine
meters and magnetic flow meters, respectively, as previously described.
Reagent usage was calculated by collecting and weighing a 3-min sample from
11
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the dry feeders at the beginning and end of each data set and averaging the
values. The average rate per minute was multiplied by the number of minutes
of dry feeder operation to determine the quantity of neutralizing agent used.
These reagent data were then related to the water quantity and influent and
effluent acidity and alkalinity values to calculate the various parameters of
interest such as usage, stoichiometric addition rate, and utilization effi-
ciency. Definitions and details of these calculations are presented in the
Glossary.
Water Sampling
The water quality from the borehole pump was quite variable. Fluctua-
tions in influent acidity were as large as Uo percent in a 2^-hr period.
Initially, the beginning and ending acidities for each data set were averaged
before being used in data calculations. Later, an automatic composite sampler
was built and installed to provide more reliable influent water quality data.
Sludge Settling Rates
Sludge samples were placed in 1000-ml graduates and the sludge/super-
natant interface was recorded as a function of time. In the case of limestone,
as well as lime at low pH's, a definite interface never clearly developed
during settling. In these cases, it was possible to observe the buildup of
solids in the bottom of the graduate.
Sludge Sampling
Representative sludge samples were quite difficult to obtain from the
process. Generally, the sludge was pumped in short-duration intervals on the
timed-basis previously described. Considerable variation in sludge density
was observed from the beginning to the end of the pumping cycle. All attempts
were made to sample near the midpoint of the cycle. Later, automatic composite
samplers were built and installed in the sludge lines at the discharges of
the thickener hoppers.
The clarifier rakes moved the sludge consistently to the edge of the
3^00-liter (900-gal) sludge hopper. Movement of the sludge from the top edge
of the hopper down the steep sides to the pump intake was often erratic and
thus created difficulties in obtaining representative samples. This was not
an operational problem but merely presented difficulties in data evaluation.
12
-------
DetentionTimes of the Various Vessels
Detention time determinations were made by instantaneously injecting
either salt or acid into the inlet of the vessel and monitoring the effluent
conductivity until it returned to a base level. The mean probable detention
time was calculated from a plot on probability paper (see Glossary for explan-
ation of calculation) and represents the point in time when the centroid of
the tracer mass passed through the vessel. Results of these studies are in
Table 1. The tracer appeared almost immediately in the effluents from all
reactors and aerators because of the mixing capability of these vessels. In
the thickeners, however, the first visible trace occurred at 120 min at both
l8.9-liter/m (5-gpm) and 56.8-liter/m (15-gpm) flow rates with the sludge
rakes operating. The sludge rakes significantly affected mixing in the
thickeners. In a static thickener with the rakes off, it took 50 hr after
tracer injection for the tracer to diffuse sufficiently to increase the con-
ductivity at the discharge end of the thickener to 500 micromhos/cm; with the
rakes on, the same conductivity was attained in Ik hr.
PROCEDURES FOR CHEMICAL ANALYSES
Conductivity and pH were measured potentiometrically. Total iron,
aluminum, magnesium, manganese, and calcium were determined by atomic absorp-
(2)
tion spectrophotometry. Sulfate was measured by adding barium to precipi-
tate the sulfate and analyzing for residual dissolved barium in the supernatant
(3)
by atomic absorption. Precipitation of the barium was accelerated by
(2)
centrifuging. EPA methods were used for total solids, suspended solids,
alkalinity, and turbidity determinations. Ferrous iron was determined by
colorimetrically titrating potassium dichromate against a p-Diphenylamine
sulfonic acid sodium salt indicator. A YSI Model 51 dissolved oxygen
(D.O.) meter was used for D.O. measurements. The Salotto acidity method
was used, in which hydrogen peroxide is added to oxidize the metals; a room-
temperature titration is then made to pH 7.3.
CHARACTERISTICS OF REACTANTS
Mine 93 Borehole Discharge
Table 2 presents the typical chemical quality of the AMD used for these
studies.
13
-------
Table 1. TYPICAL DETENTION TIMES
Test vessel
Effective volume at
time of test
Flow rate
liters
gal liter/sec
Theoretical Mean probable Efficiency
detention time, detention time, (T/To),
gpm min min percent
Limestone reactor A
Limestone reactor B
Lime reactor B
Lime reactor B
Aerator A
Aerator B
Thickener B
Thickener A
2U60
6810
6620
3^820
3^820
650
650
120
120
1800
1750
9200
9200
0.32
0.95
0.32
0.95
0.32
0.95
0.32
0.95
5
15
5
15
5
15
5
15
130 78.6 6l
U3 28.8 67
2k 16.6 69
8.0 6.1 76
360 273 76
117 92 79
18^0 (30.7 hr) lU35 (23.9 hr) 78
613 (10.2 hr) 515 (8.6 hr) 8U
-------
Table 2. CROWN WATER QUALITY DATA, 7/74 THRU 6/75
Parameter
pH
Specific conductance
Acidity as CaCO
Calc ium
Magnesium
Total iron
Ferrous iron
Sodium
Aluminum
Manganese
Sulfate
Alkalinity
Total dissolved solids
Temperature
Unit
PH
ymhos/cm
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
Vig/1
mg/1
mg/1
°C
Mean
5-04
3760
6140
370
110
300-
270
480
15
6
3040
17
1*320
13.8
Maximum
5-9
4000
1070
450
150
380
3ltO
670
36
8
3600
100
5170
17
Minimum
4.7
3400
155
300
55
250
160
280
6
4.3
2300
0
3250
9
Standard
deviation
—
260
120
40
20
39
34
95
9
1
300
25
480
2.3
-------
Lime and Limestone
The limestone and hydrated lime used in this study were obtained from
Germany Valley Limestone Company, Riverton, West Virginia. In order to
obtain the smallest particle size commercially available, the rock-dust form
of limestone was used. All tests in this study were made using the hydrated
form of lime.
Table 3 presents the manufacturer's chemical analyses and cost for lime
and limestone.
Table 3. MANUFACTURER'S CHEMICAL ANALYSES OF LIME AND LIMESTONE
Parameter
CaO
MgO
CaCO equivalent
Si02
Q
Hydrated lime ,
minimum % composition
72.00
0.70
130
-
Rock-dust limestone,
minimum % composition
53.0
0.38
98.6
0.1*9
aLime cost $38.58/tonne ($35-00/ton) or 3.86 cents/kg (1.75 cents/lb)
delivered in bulk.
bLimestone cost $12.13/tonne ($11.00/ton) or 1.21 cents/kg (0.55 cents/lb)
delivered in bulk.
A spectrochemical analysis of the limestone was made by Bituminous Coal
Research and is reported in Table k. A dry sieve analysis of as-received lime
and limestone is presented in Table 5-
16
-------
Table U. SPECTROCHEMICAL ANALYSIS OF GERMANY VALLEY LIMESTONE
(percent)
Loss on
ignition Si02 A12°3 Fe2°3 Mg° Ca° Ti°2 Na2° K2° Mn°2
43.0
1.0 O.U3
0.15
1.16
97.0
O.OU
0.02 0.1 0.03
Table 5- SIEVE ANALYSES (DRY) OF AS-RECEIVED LIME AND LIMESTONE
Screen size
50 mesh
100 mesh
200 mesh
kOO mesh
(0.297 mm)
(O.lUp mm)
(O.OTlj- mm)
(0.037 mm)
Percent not passing sieve
Hydrated lime,
percent passing
92.9
62.5
35.0
13.3
U.I*
Rock-dust limestone ,
percent passing
97.7
83.6
56.6
15-7
2.1
17
-------
SECTION 5
RESULTS
While plant construction was underway, several bench-scale batch tests
were performed to provide basic data and guidance for full-scale, continuous-
flow tests to be performed later.
BENCH-SCALE BATCH TESTS
Titration Curves
A typical titration curve illustrating the comparative quantities of
limestone and lime required to increase pH is given in Figure 2. The flat
response in the limestone curve above pH 6 demonstrates two major problems in
the use of limestone—i.e., the inability to produce the high pH's necessary
for rapid iron oxidation and the difficulty of efficient control of the
reaction to prevent high reagent usage. The titrations were repeated on
samples in which hydrogen peroxide had been added to preoxidize the iron.
Above pH 7-5» the lime curves converge, indicating that iron oxidation had
occurred (in the ferrous sample) and the vast majority of potential acidity
had been released and was available for reaction with the lime titrant. In
the case of limestone, since the acidity present in the form of ferrous iron
is not released until the iron oxidizes and hydrolyzes, the ferric situation
should require a greater quantity of limestone as more acidity is available
for reaction with the titrant. This was not the case, however, as shown on
the graph where the ferric pH's were higher (above 2.0 g) than the ferrous
values for equal titrant additions. This same condition was duplicated in
later continuous-flow studies. For reasons not understood, limestone reacts
more completely in the ferric iron situation.
Solubilities of the Chemical Constituents of AMD
A review of current literature supplied solubility product (Ksp) values
for solubility calculations. Table 6 lists these values and Figure 3 shows
the concentration of the ions as a function of pH.
18
-------
\Q
12.0
10.5
9.0
7.5
6.0
4.5
3.0
SOLID LINES INDICATE IRON WAS PREOXIDIZED
I
1.5 3.0
4.5 6.0 7.5 9.0
NEUTRALIZE!* ADDED, grams/liter
Figure 2. Titration curves for lime and limestone.
10.5 12.0 13.5
-------
Table 6. THEORETICAL SOLUBILITY PRODUCT CONSTANTS FOR ACID MINE DRAINAGE HYDROXIDE COMPOUNDS
ro
o
Ion Solubility equation
Ferric iron
Ferrous iron
Aluminum
Manganese
Magnesium
Fe3
7e2"
A10~
Il3~
2
2
H3 = 6
H 2 = 8
~HT
OH"
L_OH_
_OH_
k
3 = 1
2
= 8
2
9
. Ksp
x 10-38
x ID'16
x ID'13
x ID'32
l U
x 10
-12
x 10
-------
ro
H
Al (as part of AI02" )._
PH
Figure 3. Theoretical ion solubility as a function of pH.
-------
The only Ksp value not in general agreement was that for ferrous iron.
(7)
The value quoted was determined "by Singer, et al., and a detailed compar-
ison with observed results is illustrated in Figure U.
AMD samples were neutralized to various pH levels using lime, and the
supernatants were analyzed to determine residual concentrations. These con-
centrations were plotted as a function of pH in Figure 5-
Comparing the theoretical solubilities with the observed values, ferric
iron precipitated approximately one-half pH unit later than predicted.
Aluminum precipitated as predicted but failed to return as A10_ , its ampho-
teric form. Ferrous iron and manganese precipitated about one and one-half
pH unit sooner than predicted. Magnesium dropped out about one-half pH unit
early.
Data from later continuous-flow studies in the pilot plant (Table 25)
were plotted in Figure 6 to illustrate the solubilities of aluminum, manga-
nese, magnesium, and ferric iron as a function of pH for further consideration
of this topic.
The continuous-flow results agreed quite well with both the theoretical
and lab studies for aluminum. As shown in the lab tests, aluminum failed to
redissolve at the higher pH levels. Magnesium began to precipitate around
pH 8 in the continuous-flow studies in comparison to pH 10 in the lab tests
and pH 9-6 in the theoretical curves. Manganese began to precipitate at a
lower pH during continuous-flow studies than in either lab studies or pre-
dicted by the theoretical curves (pH 5-5 vs pH 7.vs pH 9, respectively).
It is postulated that the deviations between the observed solubilities
and the theoretical values were caused by ionic strength effects of the high
TDS of the AMD and co-precipitation of these ions along with ferric hydroxide.
Ferric iron precipitation was strongly a function of the thickener
hydraulics during continuous-flow studies. The ferric hydroxide floe was
light and easily suspended. As a result, the ferric iron in the continuous-
flow studies precipitated considerably later than in the lab tests or pre-
dicted by the theoretical curves. Filtering the continuous-flow effluent
(Table 25) indicated almost complete removal of ferric hydroxide above pH
6.5. This was still much later than the lab value of pH k or the theoretical
value of pH 3.3.
-------
Theoretical Ferrous
Ksp = 8X10'16 (after Singer (7))
Fe2+ as function of pH
during neutralization
Fe2+ as function
of pH after settling
4.5
Figure 4. Effect of pH on ferrous iron solubility.
-------
8
9
"234567
PH
Figure 5. Observed solubilities as a function of initial pH (batch-scale studies).
10 11
12
-------
100
IX)
V/I
PH
Figure 6. Observed solubility of ions as a function of treatment pH as generated
from continuous-flow studies in the pilot plant.
-------
As the continuous-flow studies incorporated some degree of oxidation,
the ferrous iron parameter was of no value in this discussion. Virtually
complete ferrous precipitation did occur during pH 9 studies where the ferrous
iron values were reduced from 200 entering the thickener to a 1.0 mg/1 effluent,
as would have been expected by the theoretical curve.
BATCH-SCALE NEUTRALIZATION TESTS
A laboratory-scale study was performed to directly compare lime, limestone,
and combination limestone-lime neutralization to pH 6.5 and pH 9-5- Limestone
treatment was tested only at the 6.5 pH level. Since the tests were almost
simultaneously conducted, results can be directly compared.
Figure 7 shows the sludge settling trends. Neutralization to pH 6.5
resulted in very-low-volume sludges because of incomplete reactions at that
pH. Increasing the pH to 9-5 significantly increased sludge volumes as the
iron oxidized, hydrolyzed, and precipitated. Combination-treated sludge
settled more rapidly than lime sludge but the 2^-hr volumes were the same.
This result is not in agreement with later studies where combination sludge
settled to a lesser final volume than lime sludge.
Chemical and physical data for the tests are presented in Table 7 and 8.
The data in both tables are the same but are presented differently to make
comparisons easier.
Very little iron oxidation occurred at pH 6.5 in 2k- hr. At pH 9-5,
complete oxidation occurred in a matter of minutes. Unexpectedly, very
little loss in calcium and sulfate was detected by the chemical analyses.
SLUDGE CHARACTERISTICS
Bench-scale studies were made to investigate sludge characteristics as
related to the neutralizing agents used and the treatment pH.
Figures 8, 9, and 10 present settling trends as a function of pH for
lime, limestone, and limestone-lime treatment respectively. No clear sludge-
supernatant interface developed with any of the neutralizing agents below pH
6.5- Figure 11 is a composite drawing to compare the relative settling
trends of the three neutralizers at pH 7. Since the tests for each neutra-
lizing agent were made at separate times on separate AMD samples, the direct
comparison in Figure 11 is not strictly valid but may be useful as a guide.
At the end of 1 hr, the limestone sludge occupied a significantly smaller
volume than the other two sludges. The same situation was observed after 2k hr
of settling.
26
-------
IV)
—3
• Lime
A Limestone
• Limestone-Lime
15 min.
45 min.
30 min.
TIME
Figure 7. Comparison of settling trends at pH 6.5 and pH 9.5.
60 min. 24 hrs.
-------
Table 7. RESULTS COMPARISON AS RELATED TO TREATMENT pH
Parameter
Lime
Raw water pH 6.5 pH 9-5
Limestone-lime
pH 6.5 PH 9.5
Turbidity
Ferrous iron
Total iron
Aluminum
Manganese
Magnesium
Calcium
Sodium
Sulfate
Conductivity
Acidity
Alkalinity
Sludge % solids
1-hr Settling
7
530
550
50
9.2
180
1*50
620
U050
U800
1^50
62
350
360
3.1
11
200
790
6kO
3780
5200
25
0
0.5^
0.7
0.31
150
1020
610
3690
5050
0
80
390
390
3.2
12
200
7^0
620
3900
1*900
760
3.6
30
0
0.28
0.9
0.37
150
1010
610
3900
^950
0
9-3
2U-hr Settling
Turbidity
Ferrous iron
Total iron
Aluminum
Manganese
Magnesium
Calcium
Sodium
Sulfate
Conductivity
Acidity
Alkalinity
Sludge % solids
7
530
550
50
9.2
180
1*50
620
U050
U800
1^50
0
3^
3^0
3^0
1.1
11
180
720
600
39^0
U750
717
5
5.2
6
0
0.07
0.9
0.19
130
1010
590
3690
5000
0
79
6.5
1*2
380
380
1.2
12
180
730
590
39^0
^950
780
5
U.3
7
0
0.09
1.0
0.26
130
1050
590
3900
5100
0
81
6.1
Q
All units are mg/1 except for turbidity (JTU) and sludge percent solids.
28
-------
Table 8. RESULTS COMPARISON AS RELATED TO SETTLING TIME8
Parameter
Turbidity
Ferrous iron
Total iron
Aluminum
Manganese
Magnesium
Calcium
Sodium
Sulfate
Conductivity
Acidity
Alkalinity
Sludge % solids
Turbidity
Ferrous iron
Total iron
Aluminum
Manganese
Magnesium
Calcium
Sodium
Sulfate
Conductivity
Acidity
Alkalinity
Sludge % solids
Raw
water
7
530
550
50
9.2
180
1*50
620
1*050
1*800
ll*50
0
1
530
550
50
9.2
180
1*50
620
1*050
1*800
ll*50
0
Lime
1-hr
62
350
360
3.1
11
200
790
61*0
3780
5200
—
—
3.6
25
0
0.51*
0.7
0.31
150
1020
610
3690
5050
0
—
6.5
2i*-hr
31*
3l*0
3l*0
1.1
11
180
720
600
39^0
1*750
717
5
5.2
6
0
0.07
0.9
0.19
130
1010
590
3690
5000
0
79
6.5
Limestone
1-hr
At pH 6.5
120
1*70
1*90
3.7
12
200
710
630
391*0
5100
950
—
U.5
At pH 9.5
b
2l*-hr
U9
1*50
1*50
1.3
12
190
61*0
575
3780
1*950
9l*0
8
6.7
b
— — —
Combination
1-hr
80
390
390
3.2
12
200
71*0
620
3900
1*900
760
—
l*.l*
30
0
0.28
0.9
0.37
150
1010
610
3900
1+950
0
—
9.3
2l*-hr
1*2
380
380
1.2
12
180
730
590
39^0
1*950
780
5
1*.3
7
0
0.09
1.0
0.26
130
1050
590
3900
5100
0
81
6.1
aAll units are mg/1 except for turbidity (JTU) and sludge percent solids.
Not applicable.
29
-------
Ul
O
100
90
75
NOTE: INTERFACE NEVER CLEARLY FORMED BELOW pH 6.5
CO
°5b
~ 60
OS
0.
30
=3
CO
15
pH 9.5 and
pH 7.0
I
15 min.
30 min.
45 min.
TIME
Figure 8. Settling trends for lime neutralization.
I
60 min.
X -
24 hrs.
-------
U)
M
100
90
t± 60
oa
O
45
CS
30
15
NOTE: A CLEAR SETTLING INTERFACE NEVER DEVELOPED AT ANY pH LEVEL.
AT pH's BELOW pH 6.8, NO SETTLING INTERFACE OCCURRED.
I
15 min.
30 min.
TIME
t
45 min.
Figure 9. Settling trends for limestone neutralization.
60 min.
24 hrs.
-------
100
90
75
tu>
CD
(J
C9
NOTE: LIMESTONE WAS USED TO NEUTRALIZE (FIRST STAGE) TO pH 5.2;
LIME WAS USED TO ELEVATE pH TO LEVELS SHOWN ON CURVES.
NO SETTLING INTERFACE WAS OBSERVED BELOW pH 7.
15 min.
30 min.
TIME
45 min.
XI
V
60 min.
24
Figure 10. Settling trends for combination limestone-lime neutralization.
-------
U)
U)
Comb.
Limestone-Lime
pH 7.2
0
15 min.
30 min.
TIME
Figure 11. Comparison of settling trends at pH 7.
45 min.
60 min. 24 hrs.
-------
Effluent turbidity and sludge percent-solids determinations were made on
each sample after 1 hr and after 2k hr of settling. The results are shown in
Figures 12 through 15- Limestone effluent turbidity, after 1 hr was high in
comparison to lime and to combination treatment. At pH 6.8, limestone sludge
had a percent-solids content about four times as great as the other two
sludges.
Surprisingly, little change occurred in the sludge percent-solids
between the 1-hr and 2^-hr times in all the samples. Supernatant turbidity
values were significantly lower for all samples after 2k hr of settling as
compared to the 1-hr values.
In summary, limestone sludge settled fastest and compacted to the least
volume. Supernatant clarity from limestone treatment was poor, however, in
relation to lime treatment and combination treatment. Combination limestone-
lime sludge settled more rapidly than lime and settled to a final volume,
which was on the order of one-half that of lime-generated sludge at pH 7-
Supernatant clarity from combination treatment after 2k hr was superior to
that from lime treatment.
REAGENT COSTS FOR BATCH NEUTRALIZATION
Bench-scale tests were made to determine the feasibility of combination
limestone-lime treatment of the Crown AMD. One-liter samples were neutralized
to various predetermined pH's with limestone, allowing 20-min reaction time,
and then were treated with lime to pH 7.0 and further to pH 9-5 (for ferrous
oxidation or precipitation). The quantities of limestone and lime were
recorded. For comparison, samples were treated to pH 7.0 using limestone
alone. Lime was used alone to treat to pH 7.0 and pH 9.5.
The important consideration in this test was cost. Raw material costs
at Crown were: Limestone = $12.13 per tonne in bulk, and lime = $38.58 per
tonne. Table 9 and Figure 16 contain the data from the feasibility test.
Combination treatment using a first-stage limestone pH of 5.1 was the most
economical method of treating to pH 7.0. This combination (l.J cents/cu m)
was 57 percent cheaper than straight limestone treatment (3.9 cents/cu m)
and 9 percent cheaper than straight lime treatment (1.8 cents/cu m).
Treatment to pH 9-5 was most economical using combination treatment with
a first-stage limestone pH of 5.5. In this case, combination treatment (2.5
-------
Lime
Limestone
Limestone-Lime
5
PH
Figure 12. Sludge percent solids @ 1-hr, settling time as a function of pH.
-------
U)
ON
100
75
5 50
5
6
7
• Lime
A Limestone
• Limestone-Lime
10
11
8 9
PH
Figure 13. Supernatant turbidity @ 1-hr, settling time as a function of pH.
-------
—]
• Lime
ALimestone
• Limestone-Lime
8 9 10 11
PH
Figure 14. Sludge percent solids @ 24-hr, settling time as a function of pH.
-------
u>
OO
• Lime
A Limestone
• Limestone-Lime
E 20H
0
5
I
6
1
7
I
8
i
9
I
10
i
11
PH
Figure 15. Supernatant turbidity @ 24-hr, settling time as a function of pH.
-------
Table 9- COMBINATION LIMESTONE-LIME LABORATORY STUDY
U)
vo
Treatment
Limestone
Lime
Combination
n
it
"
"
"
11
First
stage
pH
6.8
5.1
5.3
5.5
5-7
5-9
6.1
6.3
Limestone
Amount ,
g/1
3.2300
0.021+0
0.0830
0.1180
0.1500
0.1730
0.1910
0.1+020
Lime
Cost,
0/cu m
3.917
0.029
0.101
0.11+3
0.182
0.210
0.232
0.1+87
Amt. to
PH 7,
g/1
0.1+820
0.1+310
o.i+i+io
0.1+380
0.1+800
0.1+780
0.1+130
0.1+110
Cost ,
(p I cu m
1.860
1.663
1.701
1.690
1.852
1.81+1+
1.593
1.586
Total
cost to
pH 7.0,
^/cu m
3.917
1.860
1.692
1.802
1.833
2.031+
2.051+
1.825
2.073
Lime
Amt. to
pH 9.5,
g/1
0.8280
0.6950
0.6660
0.6010
0.7380
0.6980
0.7130
0.7690
Cost ,
-------
16
4.23
_- 10
tst
ea
Straight lime
treatment
5.25
6.0
3.70
3.17
2.64
C9
2.11
1.592
1.06
5.5 5.75
FIRST-STAGE TREATMENT pH
Figure 16. Batch-scale combination limestone-lime feasibility test where influent water pH was 5.0.
6.25
-------
cents/cu m) was 23-percent cheaper than straight lime treatment to pH 9.5
(3.2 cents/cu m). Limestone was incapable of treating to pH 9.5.
The raw materials costs greatly influence the cost advantage of combi-
nation treatment. Any consideration of combination treatment would need to
include the delivered cost of bulk lime and rock dust for the plant site in
question.
FULL-SCALE, CONTINUOUS-FLOW STUDIES ON LIMESTONE NEUTRALIZATION
Introduction
The basis for these studies (i.e., pH, detention time, reagent addition
mode, reactor baffling system, etc.) was the earlier investigation by EPA
on the ferric iron AMD at Norton, West Virginia. In this earlier study, it
was determined that 20 to 30 min of detention time was required in the mixing
reactor, that wire-mesh reactor baffles increased efficiency, and that the
rock-dust form of limestone yielded utilization efficiencies in the range of
50 percent with treatment to pH 6.5-
The current EPA Facility provided kO min of reaction time in the lime-
stone reactor at 1 liter/sec (l6 gpm). Approximately 2 hr of detention time
was provided in the aerator (at 1 liter/sec) and approximately 10 hr of
settling time was available in the thickener.
Cost figures for limestone requirements are based upon $12.13/tonne
($11.00/ton).
The limestone investigations basically followed the research plan out-
lined in Table 10.
Test One - Basic Data
Basic data for the limestone neutralization process were generated
utilizing both Processes A and B operating at pH 6.5 and 1 liter/sec (l6 gpm).
Operating parameters are detailed in Table 11 and chemical data (from all the
limestone studies) are presented in Table 12. Utilization efficiencies were
very low (25 percent) and effluent qualities were poor (total iron, 120 mg/1;
ferrous iron, 110 mg/l). These efficiencies were well below observed values
from previous studies on ferric iron AMD.
Test Two - Additional Mixing and Longer Detention Times
To investigate the possibility that insufficient mixing was responsible
for the low utilization efficiencies, Process B utilized two reactors in
series. To increase detention times, the flow rates in both processes were
hi
-------
Table 10. RESEARCH PLAN FOR LIMESTONE INVESTIGATIONS
Order of study
Variable
Action investigated
1
2
3
k
5
6
7
8
10
11
None
Extra mixing
H SO, injection
Detention time
Preoxidation of iron
Sludge recycle
Recycle rate
Aeration detention
time
Aeration detention
time
Coagulant addition
Coagulant addition
and detention time
Basic data, pH 6.5, 1 liter/sec
Effect upon efficiency
Effect upon efficiency
Effect upon efficiency
Effect upon efficiency
Effect upon efficiency
Effect upon efficiency
Effect upon effluent
Iron levels
Effect upon effluent
iron levels
Effect upon effluent
iron levels
Effect upon effluent
iron levels
1*2
-------
Table 11. NEUTRALIZATION DATA SUMMARIES FOR
LIMESTONE % pH 6.5 AND 1 LITER/SEC
Effluent pH.
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometrie factor (influent acidity)
Sludge to waste, % of influent AMD
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
6.44
6.52
1.59
1.75
13.4
14.6
2.54
2.80
7-29
8.03
1.93
2.12
3.08
3.40
27
25
2.54
2.80
3.2
3.2
6.4
CK.4
0.77
1.13
2.2
3-7
52
52
Std. Dev.
0.05
0.04
0.2
0.2
1.4
1.2
0.2
0.2
0.2
0.7
0.2
0.2
0.2
0.2
1.8
3.4
0.2
0.2
0.3
0.2
8.7
16.5
1.1
2.0
2.7
6.5
12
11
Difference in reagent usage between Process
A and B, percent 10.2
-------
Table 12. CHEMICAL ANALYSES FROM THE LIMESTONE NEUTRALIZATION STUDIESC
Sample
Raw feed
Effluent A
Effluent B
•*=- Raw feed
Effluent A
Effluent B
Cond
1*000
1*000
1*000
Acid
530
230
210
TEST
710
120
120
pH
5.6
6.1*
6.5
TWO -
5.U
6.5
6-5
Ca Mg Total Fe2+
iron
TEST
1*10
510
520
ADDITIONAL
1*1*0
520
51*0
TEST THREE -
Raw feed (A)
Acid feed (B)
Effluent A
Effluent B
2000
1*000
3600
3800
560
720
190
21*0
5.2
3.9
6.5
6.5
1*10
1*10
1*90
500
ONE - BASIC DATA
iHo 280 270
ll*0 120 110
ll*0 120 105
Na
620
600
590
Al
8.8
1.1*
1.3
Mn
5-3
5.0
5.3
so,
31*00
3100
3100
Alk
35
61*
77
TDS
1*900
1*500
1*500
MIXING AND LONGER DETENTION TIMES
ll*0 360 310
130 60 1*8
130 55 38
560
550
570
17
1.6
1.5
7.3
5.1
5-2
3UOO
2900
3000
33
Ii5
39
1*970
1*210
1*31*0
SULFURIC ACID INJECTION
150 300 290
ll+O 300 290
ii*o 160 i^o
ii*o 180 170
570
560
1*60
500
6.3
10.1
1.2
1.3
6.5
6.5
6.2
6.2
3l*00
31*00
3000
3000
35
0
90
95
1*860
1*830
1*290
1*380
TEST FOUR - DETENTION TIME
Raw feed
Effluent A
Effluent B
3900
3900
3900
610
310
90
5.1
6.5
6.5
350
390
1*20
99 310 270
58 230 200
58 93 83
1*1*0
310
280
6.0
0.6
1.0
7
8
8
2900
2350
2050
12
7**
120
1*200
31*00
2900
(cont'd)
-------
Table 12. CHEMICAL ANALYSES FROM THE LIMESTONE NEUTRALIZATION STUDIES (cont'd)
•P-
v/i
Sample Cond
Acid
pH
Ca
Mg
Total Fe2+
iron
Na
Al
Mn
sok
Alk
TDS
TEST FIVE - PREOXIDATION OF IRON
Raw feed A 4 000
Feed B (HO) 4000
Effluent R 4000
Effluent B 4000
Raw feed
Effluent A
Effluent B
Raw feed
Effluent A
Effluent B
Raw feed 3400
Effluent 3400
Filtered eff.
6lO
590
230
13
630
270
120
640
150
300
TEST
745
5
5.2
3.1
6.5
6.5
5-1
6.47
6.54
4.8
6.4
6.2
EIGHT
4.8
7-1
360
34 0
420
380
TEST
350
370
440
TEST
360
590
440
- TWO
330
520
470
110
95
90
70
270 260
270 0
150 150
6.2 0
480
440
380
370
8.2
1.0
1.2
5.9
5-9
5.8
5.0
3000
2500
2400
2000
12
0
88
50
4200
3500
3500
2900
SIX - SLUDGE RECYCLE
92
86
92
SEVEN -
100
100
95
AERATORS
120
110
100
270 260
180 180
110 93
RECYCLE RATE
330 250
120 77
200 150
IN SERIES @ 0
290 280
23 0
1.8 0
4io
400
550
550
530
9.2
2.0
2.6
19
2.6
4.3
5.3
5-0
4.7
6.2
6.1
5.5
2790
2470
2600
3300
3300
3200
7
80
58
33
86
51
3920
3500
3670
4600
4700
4500
.6 LITER/SEC
390
380
370
24
2.7
1.3
5.1
3.3
3.0
2800
2500
13
46
4000
3500
(cont'd)
-------
Table 12. CHEMICAL ANALYSES FROM THE LIMESTONE NEUTRALIZATION STUDIES (cont'd)
cr\
Sample
Cond Acid pH
Ca
Mg
TEST NINE - TWO AERATORS
Raw feed
Effluent
Filtered eff
Raw feed
Effluent
Filtered eff
Raw feed
Effluent
Filtered eff
3950 680 1*.8
3950 11 6.7
TEST TEN - TWO AERATORS
3650 1*60 5-5
3650 o i.k
TEST ELEVEN - TWO AERATORS
3700 580 5-5
3700 2 l.k
350
670
620
IN
370
520
500
IN
350
1*80
110
110
110
SERIES §
110
110
110
SERIES H
95
87
87
Total Fe2+
iron
IN SERIES @ 0.
290 270
25 2.1*
6.7
Na
Al
Mn SO^
Alk
TDS
9 LITER/SEC
1*10
1*00
380
0.6 LITER /SEC WITH
230 220
0.9 0
0.2 0
§0.9 LITER/SEC
250 2l*0
3. ^ 0
0.1 0
1*70
kko
1*30
WITH
1*70
1*50
It50
27
2.5
0.8
5.
it!
COAGULANT
12
0.2
5-
3.
3.
COAGULANT
19
0.5
0.2
5.
3.
3.
6
7
2950
3000
8.8
37
1*100
ADDITION
9
0
2800
2700
8
It7
ItOOO
3700
ADDITION
1
8
8
2800
2ltOO
8
53
1*000
3ltOO
All units are mg/1 except for conductivity (micromhos/cm) and pH. Alkalinity and acidity are
expressed as CaCO .
-------
lowered from 1 liter/sec to 0.5 liter/sec (8 gpm). Results (Table 13) of the
study indicated that no difference in efficiency occurred from using two
reactors in series as compared to a single reactor; however, the increase in
detention time resulted in efficiencies of 52 percent as contrasted with the
25-percent values at Test One's 1-liter/sec flow rate. Effluent iron values
were significantly improved (50 mg/l), although far from satisfactory. This
increase in efficiency was later found to be in error and was postulated to
have been caused by the failure of the limestone supplier to adequately clean
the bulk truck of lime or quicklime prior to loading the rock dust. Conse-
quently, spikes of high efficiency occurred during the week of data collection.
This variance is illustrated by the standard deviation values for the effi-
ciency (Table 13).
Test Three - SulfuricAcid Injection
It was felt that the reaction of limestone would be more efficient if
the influent pH were lower than the pH 5 values of the Crown water. Since
more free acidity would be available at the lower pH, the probability of
acid-base collision and interaction would be much greater and the reaction
should be more efficient. To test this theory, sulfuric acid was injected
into Process B to lower the influent pH from 5-^- to 3-^. Flow rates were
returned to 1 liter/sec (15-9 gpm) for both processes. Contrary to expecta-
tions, results of the study (Table ik) indicated no significant difference in
efficiency because of influent pH. Even more disturbing was the fact that
efficiencies had fallen again to the 20-percent range. Effluent qualities
were still poor (Table 12).
Test Four - Detention Time
Since the 0 = 5 liter/sec test (Test Two) had indicated an apparently
higher efficiency, the effect of flow rate was directly compared by operating
Process A at 1 liter/sec and Process B at 0.5 liter/sec. The study indicated
no significant difference could be gained by operating at the lower flow rate
(Table 15). In fact, the lower flow-rate condition had a lower utilization
efficiency than the 1-liter/sec process. This was attributed to the slightly
higher effluent pH of Process B (6.U8 vs 6.58). Advantages were noted in
iron oxidation (Table 12) where the effluent ferrous iron values of Process B
were 83 mg/l as compared to 200 mg/l for Process A. Turbidities were about
-------
Table 13. NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE @ 0.5 LITER/SEC WHERE
PROCESS B UTILIZED AN ADDITIONAL REACTOR FOR MIXING
Process
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, Tbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids , percent
Effluent turbidity, JTU
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
6.1*8
6.50
1.16
1.18
9.7
9-9
1.80
1.81*
5.33
5.1*2
1,1*1
1.1*3
2.19
2.23
52
52
1.80
1.81*
6.1
5.1*
3.6
6.5
0.1*3
0.78
0.69
1.1+5
1*8
U7
Std. Dev.
0.03
0.03
0.5
0.5
U.I
3.8
0.8
0.7
2.3
2.1
0.6
0.6
0.9
0.9
16
19
0.8
0.7
0.1*
0.2
0.6
7-7
0.1
0.9
0.1
1.7
ll*
1-9
Difference in reagent usage between
Process A and B, percent 1.5
1*8
-------
Table lU. NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE @ 1 LITER /SEC WHERE
H SO, WAS IIJECTED INTO PROCESS B TO LOWER INFLUENT pH FROM 5.6
T
Process
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Weutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometrie factor (influent acidity)
Sludge to waste, % of influent AMD
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Effluent suspended solids, mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
6.50
6.1*9
2.87
3.88
23.9
32.1*
5-65
5-83
13.16
17-82
3.U8
U.71
6.85
7-07
16
17
5-7
5.8
3.1
2.6
22.6
22.6
2.71
2.71
8.7
10.7
59
61
70
80
Std. Dev.
0.07
0.06
0.8
0.7
6.1*
6.0
1.9
1.1+
3.5
3.3
0.9
0.9
2.3
1.7
U.9
k.O
1.9
1.1*
0.2
0.2
8.5
11
1.0
1.3
3.2
5-U
9
12
15
18
1*9
-------
Table 15- NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE @ 1 LITER/SEC
(PROCESS A) COMPARED TO 0.5 LITER/SEC (PROCESS B)
Process
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste , % of influent AMD
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Effluent suspended solids , mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
.A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
6.1*8
6.58
1.89
3. 72
15-7
31.0
2.88
5-75
8.66
17-06
2.29
U.51
3.1*9
6.97
23
19
2.9
5.8
3.2
U. 3
15.5
19-5
1.85
2.33
5.0
5.1*
68
60
70
56
Std. Dev.
0.05
0.08
0.8
0.9
6.9
7.2
1.2
1.5
3.8
U.O
1.0
1.1
1.5
1.8
7.8
3.9
1.2
1-5
0.3
1.0
12
6.2
i.U
0.7
k.k
1.0
17
10
30
12
Difference in reagent usage between
Process A and B, percent 97
50
-------
equal on both processes, but the effluent suspended solids were somewhat
lower on Process B. Overall effluent qualities were still poor.
Test Five - Freoxidation of Iron
Since the discrepancy in efficiency values between earlier^ results on
ferric iron AMD and the current studies had still not been resolved, it was
decided to directly compare ferrous vs ferric by oxidizing the iron in Process
B prior to entering the limestone reactor. This preoxidation was accomplished
by injecting hydrogen peroxide (HgOg) into the influent piping of Process B
and modifying the piping arrangement to facilitate in-line mixing. A Precision
Model 12321-11 diaphragm-type chemical pump was used to inject the H 0 in-
line. AMD flows in both processes were 1 liter/sec (l6 gpm).
An average addition rate of 35-percent HO during the study was 0.0007
ml H202/cu m AMD/ppm of ferrous iron (O.OOOT gal H 0/1000 gal AMD/ppm Fe2).
Efficiencies in the ferric situation were grossly better (Table l6) than
the ferrous situation. The 60-percent utilization efficiencies are in the
same ballpark as the observations at Norton. Virtually all aspects of
effluent quality (Table 12) were better for the ferric situation.
Unfortunately, no explanation is readily available for the reason that
limestone reacts better when the iron is in the ferric state. Discussion of
this observation with a variety of investigators has shed no additional light
on this situation.
Test Six - Sludge Recycling
To this point, the use of limestone in ferrous situations was not only
grossly inefficient but was also incapable of producing a satisfactory efflu-
ent. Because three-fourths of the limestone was going directly to the sludge
without reacting, sludge recycling was the logical step to increase utiliza-
tion efficiency.
Process B utilized sludge recycling at a rate of approximately 16-
percent of the AMD input rate of 1 liter/sec (l6 gpm).
The results (Table IT) were encouraging, as Process B efficiency was 51
percent as opposed to the 17-percent value in Process A. Ferrous iron oxida-
tion was better in Process B (93 vs 180 mg/1 remaining in the effluent).
Effluent suspended solids were equal for both processes.
51
-------
Table 16. NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE g 1 LITER/SEC WHERE
HYDROGEN PEROXIDE WAS INJECTED INTO PROCESS B TO OXIDIZE THE
FERROUS IRON
Process
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Effluent suspended solids, mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
6.1t9
6.58
2.72
1.06
22. 7
8.8
it. 52
l.?l*
12.U8
It. 87
3.30
1.29
5.1*8
2.11
19
62
U.5
1.8
3.1
2.6
21.U
11.2
2.56
1.35
7-9
U.9
60
1+6
82
3U
Std. Dev.
0.09
0.09
1.0
0.2
8. it
1.6
1.8
0.3
U. 6
0.9
1.2
0.2
2.2
O.lt
5.9
11
1.8
0.3
0.3
0.3
10.lt
It. 2
1.3
0.5
lt.0
1.9
19
5
16
3.6
Difference in reagent usage between
Process A and B, percent 156
-------
Table IT- NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE % pH 6 5 AND
1 LITER/SEC WHERE PROCESS B EMPLOYED SLUDGE RECYCLE
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Heutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids , percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids, mg/1
Process
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
6.1*7
6.50
2.1*3
0.87
20.3
7.2
3.95
1.1*1
11.15
3.98
2.95
1.06
1*.77
1.71
17
53
lt.0
1.1*
3.3
2.7
0
l6.lt
0
18.7
12.8
3.1
1.53
0.38
5.1
l.lt
57
52
lltOO
2800
57
57
Std. Dev.
0.1
0.1
0.3
0.7
2.2
0.7
0.1*
0.1
1.2
0.1*
0.3
0.1
0.5
0.1
3
11
0.1*
0..1
0.1
0.1
0
0.9
0
8.9
8.6
1.5
1.0
0.2
2.5
0.6
11
11
ll*0
900
26
15
Difference in reagent usage between
Process A and B, percent l8o
53
-------
Automatic pH control was not used for this study. The speeds of "both
dry feeders vere adjusted manually. Control-probe fouling was more severe in
recycling situations and was particularly bad in the pH range below T-5-
Sludge solids were reduced from 5 to 1.4 percent by recycling because
the increased utilization efficiency reduced the quantity of unreacted lime-
stone.
Although efficiency was improved by recycling, effluent quality remained
undesirable.
Test Seven - Recycle Rate
As the previous test had shown the efficacy of sludge recycling, it was
necessary to determine the rate of recycling that was most effective. The
hydraulic limitations of the equipment restricted flows to less than 1.1
liter/sec (l8 gpm). It was not possible, therefore, to increase the recycling
rate beyond the 16-percent rate of Test Six. To circumvent the hydraulic
limitations and increase the proportion of sludge recycled, the influent flow
rates on both processes were lowered to 0.5 liter/sec (8 gpm). The recycling
rate of Process A was adjusted to 16 percent (to be the same as Test Six) and
Process B recycled 6l percent. Actual flow rates through both processes
were:
Process A - 0.5 liter/sec AMD + 0.1 liter/sec sludge recycle = 0.6 liter/sec
(8 gpm AMD + 1.2 gpm sludge recycle = 9-2 gpm)
Process B - 0.5 liter/sec AMD +0.3 liter/sec sludge recycle = 0.8 liter/sec
(8 gpm AMD + 4.8 gpm sludge recycle =12.8 gpm)
There is significant time lag at 0.5 liter/sec between the inputs and
outlets of the system. This makes the reactions difficult to control auto-
matically (manual pH control was used) and also requires a long time to
establish equilibrium conditions. Since each entire system holds 44 cu m
(11,600 gal), a once-thru detention time is 2k hr at a 0.5-liter/sec (8-gpm)
flow rate.
The results of the study (Tables l8 and 12) were felt to be questionable
because of the variation and trends of the data throughout the duration (6
days) of the study. Efficiencies in both processes began near 200 percent
and generally decreased during the course of the test. It was impractical to
invest additional time in this particular investigation. The significant
findings from this study were:
54
-------
Table 18. NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE § pH 6.5 AND 0.5
LITER/SEC WHERE PROCESS A UTILIZED A 16-PERCENT RECYCLE RATE AND
PROCESS B UTILIZED A 60-PERCENT RATE
Process
Effluent pH
Neutralizer usage, kg/cu m
Heutralizer usage, lbs/1000 gal
Heutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, rbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor .suspended solids, mg/1
Effluent suspended solids, mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
6.57
6.33
0.7U
0.50
6.2
U.I
1.09
0.73
3-22
2.11
0.85
0.56
1.25
0.82
93
121
1.09
0.73
2.7
6.3
15.7
60.6
53
3U
9.0
It. 3
1.08
0.52
5-2
0.8
36
50
590
U20
39
U7
Std. Dev.
0.1
0.1
0.1
0.2
0.7
1-9
0.3
O.U
0.6
1.2
0.2
0.3
O.U
0.5
lU
56
0.3
O.U
0.2
3.0
0.8
U.9
52
19
9-0
2.7
1.1
0.3
U.2
0.3
8
19
1200
370
22
25
Difference in reagent usage between
Process A and B, percent 50
-------
1. The effluent quality of Process A was significantly better than
Process B.
2. The ferrous iron in effluent A was consistently and significantly
lower than B because the pH of A was slightly higher and the effec-
tive detention time in the B aerator was less. As both processes
were recycling sludge (predominantly ferric hydroxide), they weren't
recycling ferrous iron, per se. Since the throughput rate in the
B aerator was 0.8 liter/sec (12.8 gpm) as contrasted with 0.6
liter/sec (9.2 gpm) in Process B, the effective oxidation contact
time in Aerator B was 30 percent less than Process A. Any gains
in utilization efficiency were more than offset by losses in oxida-
tion capability.
Even at 0.5 liter/sec, the effluent ferrous iron values in Process B
were still near 80 mg/1.
Test Eight - Two Aerators In Series
At this point in time, it was apparent that although the limestone
process could probably be cost-competitive with lime, serious shortcomings
still existed in effluent quality (specifically iron oxidation).
To further investigate the iron oxidation problems, the aerators from
both Process A and B were connected in series to double the oxidation capa-
bility. A larger mixer (Lightnin1 Model KD-i4B) was installed on the limestone
reactor. A study was conducted utilizing this two-aerator mode with a 0,63-
liter/sec (lO-gpm) flow rate, manual pH control, and an 18-percent recycling
rate. Results of this study (Tables 19 and 12) indicated that sludge recy-
cling could increase the effluent pH to near 7 and could sustain the pH at
that point. All ferrous iron was oxidized, utilization efficiencies were
near 80 percent, and effluent quality was significantly improved over earlier
studies. The effluent total iron was high (23 mg/l), but practically all of
it was suspended iron, as filtered samples contained only 1.8 mg/1 of dis-
solved iron. It was felt that the addition of coagulant aids would improve
the effluent quality sufficiently to enable compliance with existing effluent
standards. At the 0.63-liter/sec flow rate, the detention time in the
aeration system (both aerators) was 6 hr.
56
-------
Table 19. NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE § pH 7 AM) 0.63
LITER/SEC UTILIZING TWO AERATORS IN SERIES
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
•3
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoiehiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids , mg/1
Effluent suspended solids, mg/1
Mean
7-19
0.93
7-7
1.26
U.25
1.12
1.53
83
1.26
1.9
18.0
U2
U.5
0.51*
o R
c. • O
57
7150
31
Std. Dev.
0.1
O.Olt
0.3
0.1
0.2
0.1
0.1
6.9
0.1
0.1
0.9
11
1.2
0.1
o 6
\J » \J
11
1450
8
57
-------
Test Nine - Two Aerators in Series & 0.9 Liter/Sec
Because the results of Test Eight were encouraging, the flow rate
through the system was increased from 0.63 liter/sec to 0.9 liter/sec to
determine if sufficient oxidation could be maintained with the shorter de-
tention time. Manual pH control was again utilized and sludge was recycled
at an 18-percent (of the influent AMD) rate.
Effluent pH's (Table 20) were sustainable near ?• Utilization effi-
ciency averaged 50 percent. Effluent ferrous iron averaged 2.U mg/1,
indicating incomplete oxidation at the higher flow rate. Total iron (Table
12) in the effluent was 25 mg/1, and the filtered samples contained 6.7
mg/1. Total system performance at this flow rate was inferior to that of
Test Eight. Oxidation detention time during this test was h hr at the
0-9-liter/sec flow rate.
Test Ten - Two Aerators in Series § 0.63 Liter/Sec With Coagulant Addition
The use of coagulants was studied on limestone treatment to attempt to
improve effluent clarity and quality. Dowell MlUU was injected at a T-mg/1
rate into the 2.5-cm (l-in) line from the aerator discharge pump to the
thickener influent. The limestone feed rate was manually controlled. Both
aerators were connected in series, and the water flow rate was 0.63 liter/sec
(10 gpm). The results indicated that coagulant addition (Table 21 and 12)
was very successful in improving effluent quality. Effluent turbidity was
reduced from 57 JTU (Table 19 - earlier test without coagulants) to 12 JTU;
effluent iron was reduced from 23 mg/1 to 1.8; and suspended solids were
lowered from 31 mg/1 to 9. Total chemical costs for this treatment were
1.3 cents/cu m (5 cents/1000 gal) for limestone plus 2.6 cents/cu m (10
cents/1000 gal) for coagulant. As the coagulant addition was not optimized,
some improvement could be made in this figure.
Test Eleven - Two Aerators in Series @ 0.9 Liter/Sec With Coagulant Addition
The AMD flow rate was increased in this test to 0=9 liter/sec (l^ gpm)
while maintaining approximately a 7-mg/l injection rate of MliiU coagulant.
Effluent turbidity was not adversely affected by the increased flow rate
(Table 22) but effluent iron values were somewhat higher than the previous
test (3.U vs 0.9 mg/1 - Table 12) at the lower flow rate.
58
-------
Table 20. NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE @ pH 7 AND 0.9 LITER/
SEC UTILIZING TWO AERATORS IN SERIES
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
•3
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometrie factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids, mg/1
Mean
6.9k
1.21+
10. k
2.17
5-70
1.51
2.63
51
2.09
2.2
17-9
50
6.1
0.73
3.3
6k
1+1+70
1+3
Std. Dev.
0.3
0.1
0.1+
0.3
0.2
0.1
0.1+
9-5
0.1+
0.1
0.7
28
3.5
0.1+
1.9
9
930
7
59
-------
Table 21. NEUTRALIZATION DATA SUMMARIES FOR LIMESTONE USING TWO AERATORS IN
SERIES @ 0.63 LITER/SEC (10 GPM) WITH COAGULANT ADDITION
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, l"bs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids, mg/1
Mean
7.3
1.06
8.9
2.38
It. 88
1.29
2.89
1*8
2.2k
2.0
26
1*6
3.8
O.U6
2.3
12
5600
8.8
Std. Dev.
0.2
0.3
2.2
0.7
1.2
0.3
0.9
Ik
0.9
0.2
3
2k
1.7
0.2
1.0
6
3500
1*.0
60
-------
Table 22. KEOTRALIZATIOI DATA SUMMARIES FOR LIMESTONE USING TWO AERATORS IN
SERIES § 0.9 LITER/SEC (l4 GPM) WITH COAGULANT ADDITION
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage , g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids, mg/1
Mean
7.3
.77
6.40
1.52
3.52
0.93
1.85
66
1.61*
1.5
18.6
28
2.8
0.3^
2.3
8.5
10200
' 9-3
Std. Dev.
0.1
0.2
1.6
0.4
0.9
0.2
0.5
11
0.3
0.1
0.8
21
1.2
0.1
1.0
3.7
4TOO
4.1
61
-------
Discussion of Limestone Studies
Several advantages and disadvantages to the use of limestone were noted
during the course of these investigations. Chief among the advantages was
the reagent usage cost that was, for the optimum case (Test Eight), on the
order of 1.3 cents/cu m (5 cents/1000 gal). Sludge solids content during
many of the tests was near 10 percent by weight. This solids advantage may
be more apparent than real, however, since much of the weight content is
unreacted limestone. The presence of unreacted limestone in the sludge is
obviously undesirable. Sludge recycling is a necessity because such a large
quantity of limestone ends up in the sludge.
On the disadvantage side, many comments can be made on the application
of limestone to ferrous iron situations. The low effluent pH (below 7.5)
requires excessive detention time in the oxidation system to enable adequate
ferrous to ferric conversion. This detention time requirement at Crown was
on the order of five to six hr. These long reaction and oxidation times would
require excessive energy input per unit volume of water for mixing and
oxidation processes and would similarly require large capital expenditures
and facility space for equipment.
The effluent turbidity for limestone-treated waters is high. Charles
/ o \
Ford of Bituminous Coal Research noted the high turbidity and felt that
it was caused by colloidal particles of limestone that remain in suspension.
Filtered effluent samples are quite clear and free from turbidity and color.
Coagulants are very effective in improving the turbidity problem associated
with the use of limestone, but their cost often exceeds the cost for the
limestone.
Operational difficulties associated with limestone treatment centered
around the accumulation of solids in the bottom of the aeration tanks where
violent mixing was not present. The solids and inerts in the sludge also
caused failure of the automatic valving by scoring the ball valves. This
erosion problem was overcome by using different valves, but it illustrated
the potential of the limestone for wear on moving parts. Because of the
high density of the limestone sludge, it was difficult to handle and move.
It demonstrated a tendency to bridge and rathole in the thickener. Periodic
cleaning the accumulation from the bottom of the aerators was a major task.
62
-------
The low effluent pH of limestone-neutralized AMD was in the range
(below pH 7.5) where maximum probe fouling problems occurred. Automatic pH
control in this situation was ineffective because of rapid fouling. Use of
coagulants, however, reduced the severity of the pH probe fouling problem.
In summary, although the limestone process was demonstrated to be
technically applicable for use on the majority of ferrous waters, it was
felt that the process simply wasn't feasible because of the aforementioned
reasons. In contrast, as demonstrated in Test Five, the limestone reagent
works quite well on ferric iron situations.
FULL-SCALE, CONTINUOUS-FLOW STUDIES ON HYDRATED-LIME NEUTRALIZATION
Introduction
Background data for lime—Ca(OH)_—neutralization is readily available
from a variety of sources. Approximately 300 AMD treatment plants using
lime are currently in operation in the Appalachian mining states alone.
Although a wealth of data was available on the subject, it was necessary to
develop background data on Crown AMD and EPA equipment to enable comparisons
with other treatment techniques at the Crown Field Site.
The EPA equipment provided 7-5 min of detention time in the lime
reactor at 1 liter/sec (l6 gpm). Approximately 2 hr of detention time was
provided in the aerator and 10 hr of settling time was available in the
thickener.
Cost figures for lime requirements are based upon $38.58/tonne ($35.00/
ton) in bulk.
The investigations on hydrated-lime treatment basically followed the
research plan outlined in Table 23.
Effect of pH
Tests to investigate the operational and effluent quality and to
generate basic data were conducted by varying the pH from 5.5 to 10.
Tables 2k and 25 illustrate typical data from the neutralization tests.
Optimum results in terms of effluent quality were obtained between pH 8 and
8.5. Lime utilization efficiency in this range was better than 60 percent.
No significant change in total dissolved solids (TDS) was observed throughout
the PH range of the study. Effluent iron values around k mg/1 (@ pH 8 to
8.5) were predominately caused by suspended ferric hydroxide as filtered
samples indicated less than 0.5 mg/1 of iron. It was felt that the use of
63
-------
Table 23. RESEARCH PLAN FOR LIME NEUTRALIZATION
Order of study
Variable
Action investigated
pH
Effect on effluent quality
Sludge recycling
Effect on quality and sludge
Rate of recycling
Effect on sludge density
Coagulant addition
Effect on effluent quality
Ferrous hydroxide
precipitation
Effect on effluent quality
Sludge density
Effect on sludge density
-------
Table 2U. CHARACTERISTIC OPERATIONAL DATA FOR LIME NEUTRALIZATION STUDIES
Effluent pH
Item
Heutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Weutralizer usage, g/cu m/ppm acidity
Cost, cents/1000 gala
Cost, cents/cu m
Cost, cents/10 cu m/ppm acidity
Utilization efficiency, percent
Stoichipmetric factor
Sludge to waste, percent of influent
Dry solids to waste, rbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids , percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids , mg/1
5.5
.082
.69
.177
1.20
.32
.U5
2l*0
.16
6.0
2.2
.27
.k
31
51*0
1*0
6.0
.359
3.0
.501*
5.25
1.39
1.95
128
.68
5.3
5.U
.65
1.2
1*8
580
75
6.5
.10*7
3-7
.661
6.51
1.72
2.55
115
.89
6.0
5.6
.67
1.1
5U
600
1*0
7-0
.506
U.2
.7^9
7.39
1.95
2.88
107
1.01
5.6
7.6
• 91
1.6
1*0
650
33
7.5
.521*
U.5
.795
7-92
2.10
3.07
102
1.07
7.1
6.6
.80
1.2
36
770
33
8.0
.621*
5-2
• 915
9.11
2.1*1
3.53
92
1.21*
6.1*
8.1*
1.00
1.5
30
880
21*
8.5
.873
8.0
1.256
12.71*
3.36
1*.81*
66
1.70
6.5
9.0
1.07
1.6
25
1300
28
9-0
.986
8.2
1.1*20
13.58
3.81
5.1*7
56
1.92
5.8
10.6
1.27
2.2
25
1500
35
9-5
1.108
9.2
1.1*96
16.18
U.27
5-77
52
2.03
6.1
13-5
1.62
2.6
20
151*0
22
10.0
1.228
10.2
1.659
17-93
U.73
6.1*0
^7
2.23
5-6
13.7
1.65
2.9
20
1550
28
Reagent cost based on bulk delivered prices of $38.58/metric ton ($35-00/ton) for hydrated lime.
To convert from cents/10 cu m/ppm acidity to cents/1000 gal/ppm acidity, multiply by 0.0037851*.
All values are means.
-------
Table 25. CHARACTERISTIC CHEMICAL ANALYSES FOR LIME NEUTRALIZATION STUDIES
o\
Raw water
pH
Acidity
Calcium
Magnesium
Total iron
Total iron, filtered sample
Ferrous iron
Sodium
Aluminum
Aluminum, filtered sample
Manganese
Sulfate
Alkalinity
Total dissolved solids
t».o
730
350
120
330
330
300
550
25
21
6
3200
0
1*1*00
5-5
1*80
380
120
260
250
250
560
3.5
3.0
5.7
3100
1*.6
1*300
6.0
110
1+90
120
62
50
1*6
560
2.1
1.7
U.5
2900
12
1*000
6.5
15
550
110
11
5
6.3
550
1.5
0.9
1*
3000
33
1*100
Effluent at specified
7.0
5
570
100
8.5
0.2
2.0
550
1.1*
1.0
3
3000
53
1*200
7-5
0
590
110
6.9
0.3
0
550
1.1*
0.6
l*
3000
63
1+000
8.0
0
630
110
5.9
0.2
0
550
0.9
0.6
3
3000
110
1*000
pH
8.5
0
600
70
3.9
0.1+
0
51+0
1.0
0.8
0.5
2900
83
1*300
9.0
0
630
53
3.0
0.3
0
530
0.9
0.3
0.2
2900
55
1*200
9-5
0
620
37
2.6
0.5
0
1*90
1.0
0.5
0.2
3000
1+2
1+300
10.0
0
61+0
21
1.8
0.5
0
1*70
0.8
0.3
0.1
2900
1+1+
1+200
All units are mg/1 except for pH. All values are means. Alkalinity and acidity are expressed as CaCO .
-------
coagulants would improve the effluent iron and turbidity levels. This was
confirmed in later studies.
Stoichiometric lime requirements ranged from 1.2 to 1.7 times the
amount of acidity (as determined by titration to pH 7-3) to treat the AMD
between PH 8 to 8.5- The cost for lime usage was increased by ^0 percent by
increasing the effluent PH from 8 to 8.5. Treatment to PH 8.5 was 70 percent
more expensive in terms of lime usage than treatment to pH 7.0.
Treatment above pH 8.5 did not increase effluent alkalinity. The
strong relationship between lime-usage cost and treatment pH is shown in
Figure 17. A definite inflection point was observed on the curve between pH
7-5 and 8.5.
Sludge volumes collected in 1000-ml graduates increased virtually
linearly with pH, as shown in Figure 18. An overlay of effluent turbidity
is also shown in this figure.
All the initial background studies were made using a sludge pumping
rate to waste of 6-percent of the influent AMD rate. A plot of underflow
solids levels and the dry solids production at the 6-percent sludge pumping
rate are contrasted with the same parameters at a 2.8-percent pumping rate
in Figure 19-
Effect of Sludge Recycling
A sludge recycling rate of 20 percent of the influent AMD flow was
studied in Process B as compared to no recycling in Process A. This study
was made using AMD flows of 0.95 liter/sec (15 gpm) treated to pH 8.0.
Results (Tables 26 and 27) indicated that a slight increase in lime
utilization efficiency was obtained by recycling (83 vs 7^ percent). Although
the reactor suspended solids were much,higher on B (because of the sludge
%
recycling), the effluent suspended solids levels were similar (3^ vs 28 mg/l)
as were the effluent turbidity values (29 vs 25 JTU). Effluent iron values
(Table 27) were lower in B than in A (12 vs 5-1 mg/l), apparently because
the higher influent solids level in the recycling process was more effective
in settling the suspended iron. Filtered samples of the effluents from both
processes contained less than 1 mg/l total iron.
The apparent increase in efficiency observed in Process B was probably
caused by the bulk lime absorbing carbon dioxide and partially recarbonating,
thus reducing the reactivity per unit volume. Recycling increased the
67
-------
24 -
20
CO
MO
CD
- 12
CJ>
LU
ca
0
5
9
6.0
5.0
CJ
4.0 42
3.0
LkJ
ca
I
LU
2.0
1.0
6 7 8
EFFLUENT pH
Figure 17. Effect of the treatment pH on lime-usage costs.
10
-------
0
5
6
9
7 8
TREATMENT pH
Figure 18. Sludge volume and effluent turbidity after 1-hour settling
time as functions of treatment pH for the lime neutralization
tests.
-------
CJ
0.
00
ae.
6
9
10
7 8
TREATMENT pH
Figure 19. Underflow solids concentration at approximately 6% and
2.8% pumping rates and the dry solids production rate
for the lime neutralization tests.
70
-------
Table 26. NEUTRALIZATION DATA SUMMARIES FOR LIME NEUTRALIZATION § pH 8.0
AND 0.95 LITER/SEC UTILIZING SLUDGE RECYCLE (20 PERCENT RATE) ON
PROCESS B
Process
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, rbs/1000 gal
Heutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, rbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids, mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
8.00
8.01
0.61*
.58
5.32
It. 85
1.18
1.08
9.33
8.1*9
2.1(6
2.2k
k.5k
It. 16
7lt
83
1-59
l.Ut
2.9
2.7
0
I9.lt
0
51
6.8
7.1
0.81
0.85
2.8
3.1
25
29
. 9^0
5300
3»*
28
Std. Dev.
0.09
0.06
0.07
0.08
0.6
0.7
0.2
0.1
1.1
1.2
0.3
0,3
0.6
0.5
8.5
8.1
0.2
0.2
0.2
0.1
0
1.1
0
8
0.6
1.2
0.1
0.1
0.2
0.6
5.3
lit
250
1375
17
22
Difference in reagent usage between
Process A and B, percent y'->
71
-------
Table 27. CHEMICAL ANALYSES FROM THE LIME NEUTRALIZATION STUDIES
a
Sample
Cond Acid
pH
Ca
Mg
Total
iron Fe
2+
Al
Mn
SO,
Alk
TDS
ro
SLUDGE RECYCLE STUDY
Raw feed
Effluent A
Effluent B
Filtered A
Filtered B
3100
2950
3000
560
0
0
l*.6
8.0
8.0
300
520
1*80
1*80
1*50
93
7!*
90
69
89
210
12
5-1
.12
.08
200
0
0
31*0
31*0
330
310
320
u
.62
.50
.35
.31
1*.5
1.1
1.6
.83
1.5
21*00
21*00
2300
0
88
95
3300
3300
3200
RATE-OF-RECYCLE STUDY
Raw feed
Effluent A
Effluent B
Filtered A
Filtered B
3900
1*000
1*000
520
0
0
5.1*
7-9
8.0
380
600
560
560
530
COAGULANT (A)
Raw feed
Effluent A
Effluent B
Filtered A
Filtered B
3800
1*000
1*000
1*90
0
0
5.1*
8.0
8.0
370
560
580
550
580
110
100
100
100
100
VS NO
100
90
90
87
90
250
10
12
0.07
0.06
COAGULANT
280
1.1*
5-3
0.10
0.12
230
0
0
(B)
270
0
0
500
1*90
1*90
1*70
1*80
STUDY
1*50
1*70
1*50
7.1
0.60
0.1*3
o.oi*
0.08
6.U
0.7
0.8
0.7
0.8
5-8
1.6
1.3
1.3
1.0
1*.8
0.8
1.0
0.9
0.9
3000
3000
2900
2800
2700
2700
37
ll*0
120
31
120
110
1*200
1*100
1*000
1*000
3800
3800
(cont'd)
-------
Table 27. CHEMICAL ANALYSES FROM THE LIME NEUTRALIZATION STUDIES8" (CONT'D)
Sample
Cond Acid
pH
Ca
Mg
Total
iron Fe
2+
Na
Al
Mn
Alk TDS
U)
COAGULANT g
Raw feed
Effluent A
Effluent B
Filtered A
Filtered B
1+000
1+100
1+100
520
0
0
COAGULANT 1
Raw feed
Effluent A
Effluent B
Filtered A
Filtered B
Raw feed
Effluent A
Filtered A
Raw feed
Effluent A
Filtered A
3600
3500
3500
3600
3800
3700
3700
550
0
0
1+80
0
580
0
5.7
7.9
7-0
\ pH 8
1+00
600
620
600
620
1 pH 8 COMPARING
H.7
7.8
7.9
FERROUS
5-3
9.0
FERROUS
5.5
8.9
1+30
620
560
620
560
(A) VS COAGULANT @ pH 7 (B)
130 280
120 2.1
120 10
110 0.10
120 0.30
NO RECYCLE (A)
130 270
85 2.3
110 2.0
80 1.7
100 1.7
260 510
0 500
0.7 1*90
VS SLUDGE
250 1+30
0 370
0 350
360
350
9.8
0.1+3
0.6l
0.10
0.21
RECYCLE
Ik
0.1
0.3
0.1
0.3
6.1+
1.6
k.9
1.5
b.7
(B)
6.1+
1.0
2.8
1.0
2.8
3200
3300
3300
3000
2700
2600
23
88
50
9
61+
88
1+500
1+700
1+1+00
1+300
3800
3600
HYDROXIDE PRECIPITATION STUDY
390
560
560
110 2l+0
100 1+.7
100 0.05
230 1+80
0.8 1+1+0
1+30
8.3
0.63
0.50
5.3
0.1+9
0.1+6
2900
2900
6.6
81
1+150
3850
HYDROXIDE ¥ITH COAGULANT STUDY
350
530
530
95 250
75 3.1
75 0.10
2l+0 1+70
0.25 1+60
0 1+60
19
0.8
0.5
5.1
0.37
0.32
2800
2500
8
73
3980
3500
(cont'd)
-------
-P-
Table 27. CHEMICAL ANALYSES FROM THE LIME NEUTRALIZATION STUDIESa (CONT'D)
Sample
Raw feed
Effluent A
Effluent B
Filtered A
Filtered B
Cond
3600
3600
3600
Acid pH
FERROUS
i+30 5-5
0 9.0
0 9.0
Ca
HYDROXIDE
^30
590
590
580
590
Mg
VS
110
83
85
83
85
Total
iron
FERRIC
250
1.7
6.7
0.06
0.0k
Fe2+
HYDROXIDE
200
0
1.3
0
0
Na Al
STUDY
530 12
k90 0.53
1*70 0.5lt
1*90 0.11
1*70 0.11
Mn SO^
5.8 3200
0.21 2700
0.56 2700
0.17
o.Uo
Alk TDS
19 Ul*80
32 3860
73 3850
All units are mg/1 except for conductivity (micromhos/cm) and pH. Alkalinity and acidity are
expressed as CaCO .
-------
effective reaction/mixing time of the calcium carbonate portion of the
neutralizing agent and allowed greater utilization. Tests of the purity of
the bulk lime itself indicated a CaO equivalence of 66 percent as compared
to the minimum composition of 72 percent for fresh lime (Table 3).
Effect of Sludge Recycle Rate
In the interim between this and the previous series of tests, several
changes were made in the facility during a 2-month downtime caused by pump
failure. New bottom mixers were installed on the aerator turbines in an
attempt to eliminate solids accumulation in the aeration tanks. A new load
of hydrated lime was obtained. Analyses of the lime indicated a Ca(OH)
equivalence of 93 percent, a CaCO equivalence of 125 percent, and a CaO
equivalence of 71 percent.
For this and future specialized studies, an additional pump was in-
stalled on each aerator to increase the flow capability from 1.25 to 2.5
liters/sec (20 to kO gpm). This study compared a 10-percent (of the influent
AMD) recycling rate (Process A) to a 30-percent (Process B) rate. Both
processes treated 1 liter/sec (l6 gpm) to pH 8.0. Results of the study
indicated that no increase in efficiency or improvement in effluent quality
was obtained from a higher recycling rate (Tables 27 and 28).
Effect of Coagulants on Effluent Quality
Several Dowell coagulants were tested in bench-scale studies to provide
guidance to plant studies. The anionic coagulants as a group were much more
effective than either non-ionic or cationic forms. In the EPA application,
Dowell MLhk appeared to be most effective. Although lab studies indicated
that a 1-mg/l addition rate was adequate, a 2-mg/l rate was necessary for
successful plant operation. The Mlkh was injected into the 2.5-cm (l-in)
line leading from the discharge of the single aerator pump to the thickener
inlet.
Two studies were made on coagulant usage. The first study compared
coagulant vs no-coagulant at pH 8; the second compared coagulant at pH 8 vs
coagulant at pH 7.
Coagulant vs No-Coagulant at pH 8—
The MlUU at 2 mg/1 was quite effective in improving clarification as
shown in Tables 29 and 27. Effluent turbidity was significantly better on
the coagulant process (A) as compared to no coagulant (B), i.e., 7 vs 25 JTU.
75
-------
Table 28. NEUTRALIZATION DATA SUMMARIES FOR LIME & 1 LITER/SEC WHERE
PROCESS A UTILIZED A 10-PERCENT SLUDGE RECYCLE RATE AND PROCESS B
UTILIZED A 30-PERCENT RATE
Process Mean
Effluent pH
Neutralizer usage, kg/cu m
Beutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste , kg/cu m
Underflow solids , percent
Effluent turbidity, JTU
Reactor suspended solids , mg/1
Effluent suspended solids, mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
7-9
7.9
0.6?
0.70
5-58
5.81*
1.1*0
1.1+6
9.8
10.2
2.58
2.70
5.39
5.63
68
63
1.89
1.97
1.26
1.20
10.1
29.0
62
150
7.6
6.2
0.93
0.7!*
7.3
6.1
39
21*
7000
13300
37
33
Std. Dev.
0.1
0.2
0.03
0.05
0.3
Q..k
0.2
0.2
0.1*
0.8
0.1
0.2
0.8
0.8
7-2
7.8
0.3
0.3
0.02
0.05
0.3
0.7
19
21
2.k
1.0
0.3
0.1
2.1
0.9
12
21
550
3900
10
13
Difference in reagent usage between
Process A and B, percent
k.6
-------
Table 29- NEUTRALIZATION DATA SUMMARIES FOR LIME @ 0.8 LITER/SEC (12 GPM)
AND pH 8 WHERE PROCESS A UTILIZED COAGULANT ADDITION
Process
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Heutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost , cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, percent of influent AMD
Sludge recycled, percent of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste , kg/cu m
Underflow solids , percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids, mg/1
Difference in reagent usage between
Process A and B, percent
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
8.0
8.0
0.76
0.71*
6.k
6.2
1.56
1.50
11.2
10.8
2.95
2.85
6.00
5.77
57
58
2.09
2.0h
2.k
2.1*
26.7
26.5
1*8
82
1+.3
7.1*
0.52
0.89
2.15
3.62
7
25
1*000
8600
9.0
18
3.k
Std. Dev.
0.0!*
0.06
0.1
0.1
0.5
0.5
0.2
0.2
1.0
0.9
0.3
0.2
0.9
0.6
10
6.9
0.3
0.2
o.oi*
0.08
0.1*
0.5
16
28
1.5
2.5
0.2
0.3
0.7
1.2
1-5
1*.2
2000
2700
5.7
10
11
-------
Effluent iron values were similarly improved (l.k vs 5«3 mg/l). These tests
vere conducted at AMD flow rates of 0.8 liter/sec (12 gpm) and recycling
rates of 23 percent of the influent AMD.
The Do-well fflM cost approximately $3.6U/kg ($1.65/lb) in bulk. The
add-on cost of a 2-mg/l addition rate amounts to 0.7 cents/cu m (2.7 cents/
1000 gal).
Coagulant at pH 8 vs Coagulant at pH 7—
All flow rates were the same as the previous test. Kikk was injected
into both processes at rates between 2 and 3 mg/l. The effluent pH on Pro-
cess B was lowered to pH 7; Process A was maintained at pH 8. Effluent
quality at pH 7 could not match the pH 8 conditions (Tables 27 and 30) in
terms of turbidity (27 vs 7 JTU), suspended solids (13 vs 9 mg/l), and iron
(10 vs 2.1 mg/l). During this particular comparison, twice as much lime was
required to treat to pH 8 than to pH 7-
Effluent quality at pH 7 was enhanced by the use of coagulants as
compared to earlier studies (Tables 2k and 25); e.g., suspended solids were
lower (13 vs 33 mg/l), iron was comparable (10 vs 8.5 mg/l), and turbidity
was improved (27 vs kO JTU).
Coagulant at pH 8 With Sludge Recycle vs Coagulant at pH 8 and Mo Recycle—
Coagulants were injected into both processes at a 3-mg/l rate. Flows
were maintained at 0.9 liter/sec (lU gpm) for each process. Process A re-
cycled no sludge; Process B recycled sludge at a 16-percent rate. In Table
31, the efficacy of sludge recycling is again demonstrated by the higher
utilization efficiencies observed in Process B as compared to A (79 vs 6l
percent). Very little difference was noted in effluent quality from the
processes.
Effect of Ferrous Hydroxide Precipitation
(n)
Current treatment techniques in Europe emphasize the possibility of
eliminating the oxidation step from the neutralization process and precip-
itating the iron as ferrous hydroxide at pH's around 9. Although the ferrous
hydroxide will, in most cases, eventually oxidize to ferric and release
additional acidity, sufficient buffering capacity is available at the higher
pH to compensate for the additional H ions.
Three studies were made on this concept at the Crown facility. In the
first, the AMD was mixed with lime and pumped directly to the thickener.
78
-------
Table 30. NEUTRALIZATION DATA SUMMARIES FOR LIME § 0.8 LITER/SEC (12 GPM)
USING COAGULANT AND COMPARING pH 8 (PROCESS A) VS pH 7 (PROCESS B)
Process
Effluent pH
Neutralizer usage, kg/cu m
Heutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids, mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
7.93
6.91
0.739
0.362
6.2
3.0
1.51
0.7k
10.8
5.3
2.85
l.itO
5.8U
2.83
55
108
2.03
0.99
2.6
2.5
29.lt
26.8
86
93
7.6
8.5
0.92
1..02
3.5
lt.1
7
27
9900
6600
9.3
13.3
Std. Dev.
0.1
0.1
0.1
0.1
0.6
0.5
0.2
0.1
1.0
0.8
0.3
0.2
0.7
0.2
7
9
0.2
0.1
0.2
0.1
2.7
2. It
t»9
38
It. 2
3.1*
0.5
O.U
2.1
1.6
2.3
11
1500
980
6. it
8.1
Difference in reagent usage between
Process A and B, percent
79
-------
Table 31 NEUTRALIZATION DATA SUMMARIES FOR LIME g 0.9 LITER/SEC AND pH 8
USING COAGULANTS WHERE PROCESS B UTILIZED SLUDGE RECYCLE
Effluent pH
Limestone usage, kg/cu m
Lime usage, kg/cu m
Cost, cents/1000 gal
Cost , cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, rbs/1000 gal
Dry solids to waste, Its/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Limestone reactor suspended solids, mg/1
Lime reactor suspended solids, mg/1
Effluent suspended solids, mg/1
Process
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
Mean
7.9
7.9
0
0
0.68
0.51*
9.95
7.91
2.63
2.09
5.^0
h.29
6l
79
1.90
1.50
2.k
2.2
0
15-9
0
k3
5.3
6.0
0.6U
0.73
2.8
3.3
10
7.9
-
920
8300
9.2
8.8
Std. Dev.
0.2
0.1
0
0
0.07
0.07
1.1
1.0
0.3
0.3
0.5
0.5
5-5
7.6
0.2
0.2
0.2
0.2
0.7
22
0.7
3.0
0.1
O.lt
O.U
1.7
5.0
2.9
-
2l*0
1760
2.0
3.7
Difference in reagent usage cost between
Process A and B, percent
26
80
-------
The mixer on the lime reactor was replaced with a small bench-scale mixer to
provide minimal agitation and minimal opportunity for oxidation to occur in
the reactor. A 1-liter/sec flow (l6 gpm) was treated to PH 9. No sludge
recycle or coagulant was added. The results of the test (Table 32 and 27)
indicated unexplained high-lime requirements as compared to earlier tests
using aeration at pH 9. Utilization efficiency averaged near M percent,
and the stoichiometric factor was 2.7 as compared to earlier ferric values
of 56 and 1.9, respectively. The effluent quality of the ferrous process
was comparable to earlier ferric conditions (e.g., turbidities were 26 vs 25
JTU, suspended solids were 23 vs 35 mg/1, and effluent iron levels were k vs
3 mg/1).
The ferrous hydroxide sludge exhibited very large floe size and slow
compaction characteristics. In comparison with ferric sludges, the ferrous
sludges occupied more than 1.5 times the volume.
In the second study of this series, Mlkk coagulant was injected at
approximately a 5-mg/l rate while maintaining the same basic operating
parameters as in the first study.
The coagulant addition greatly improved settling rates of the ferrous
sludge but did little for effluent clarity or quality (Tables 33 and 27).
Because of the unexplained high-lime requirement observed in the first
ferrous hydroxide study, an additional test was made directly comparing
ferrous (Process A) vs ferric hydroxide (Process B) conditions while keeping
all other parameters the same. The results (Tables 3^ and 27) indicated
virtually identical lime requirements for both processes. Both processes
utilized coagulant addition rates near 5 ppm. Effluent quality characteristics
of the ferric hydroxide situation were significantly superior to the ferrous;
e.g., turbidity was 5 vs 20 JTU, suspended solids averaged 6.0 vs 29 mg/1,
and total iron values were 1.7 vs 6.7 mg/1.
The continuing low utilization efficiencies of the lime remain unex-
plained.
Sludge Density Study
A limited attempt was made to maximize the sludge density obtainable
under the conditions at the Crown Field Site. For this purpose, a test was
conducted using lime neutralization to pH 8 and operating at a 1-liter/sec
(16-gpm) AMD flow rate. Sludge was continuously recycled into the reactor
81
-------
Table 32. NEUTRALIZATION DATA SUMMARIES FOR LIME @ 1 LITER/SEC AND pH 9
USING NO AERATION (FERROUS HYDROXIDE PRECIPITATION)
Effluent pH
Neutralizer usage, kg/cu m
Neutralizer usage, lbs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids , mg/1
Effluent suspended solids , mg/1
Mean
8.9
0.91
7-5
2.00
13.2
3.i+9
7.71
kk
2.70
2.1
0
0
7.U
0.89
5-1
26
1300
23
Std. Dev.
0.1
0.0^
0.3
0.3
0.5
0.1
1.1
5
o.u
1.1
0
0
0.6
0.1
0.3
h
200
9
82
-------
Table 33. NEUTRALIZATION DATA SUMMARIES FOR LIME @ 1 LITER/SEC (l6 GPM)
WITH NO AERATION USING COAGULANT ADDITION
Effluent pH
Neutralizer usage*, kg/cu m
Neutralizer usage, l"bs/1000 gal
Neutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Q
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids, mg/1
Effluent suspended solids , mg/1
Mean
8.9
0.91
7.56
1.79
13.2
3.50
6.92
U6
2.17
2.23
19-9
59
6.6
0.79
3.57
17
9500
37
Std. Dev.
0.1
0.1
0.2
0.2
O.U
0.1
0.9
6.2
0.7
0.1
0.7
22
2.U
0.3
1.2
h.Q
2500
12
83
-------
Table 3!*. NEUTRALIZATION DATA SUMMARIES FOR LIME § 0.95 LITER/SEC (15 GPM)
WHERE PROCESS B UTILIZED OXIDATION; PROCESS A HAD NO OXIDATION;
BOTH USED COAGULANTS
Process Mean
Effluent pH
Neutralizer usage, kg/cu m
Ueutralizer usage, l"bs/1000 gal
Heutralizer usage, g/cu m/ppm influent acidity
Cost, cents/1000 gal
Cost, cents/cu m
Cost, cents/10 cu m/ppm influent acidity
Utilization efficiency, percent
Stoichiometric factor (influent acidity)
Sludge to waste, % of influent AMD
Sludge recycled, % of influent AMD
Dry solids recycled, lbs/1000 gal
Dry solids to waste, lbs/1000 gal
Dry solids to waste, kg/cu m
Underflow solids, percent
Effluent turbidity, JTU
Reactor suspended solids , mg/1
Effluent suspended solids, mg/1
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
8.89
8.83
• 939
.936
7.81+
7.81
2.05
2.03
13.71
13.67
3.62
3.61
7-91
7.83
1+2
39
2.77
2.7!*
2.33
2.35
20.0
20.3
56
65
7.1
6.9
0.85
0.83
3.6
3.6
20
5
9250
7250
29
6.0
Std. Dev.
0.1
0.1
0.1
0.1
0.1+
0.8
0.3
0.2
0.7
1-5
0.2
O.ll
1.2
0.9
5.U
5.1
0.1+
0.3
0.3
0.1
1.1
0.8
30
12
3.3
2.2
0.1+
0.3
1.5
1.5
5.2
2.7
3200
2600
11
It. 5
Difference in reagent usage "between
Process A and B, percent 0.3
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at 0.35 liter/sec (5 = 5 gpm), vhich corresponds to a 3^-percent (of the
influent AMD) recycling rate. No sludge was discharged to waste. The
influent suspended solids and sludge percent solids were monitored in order
to observe the effects of the increasing solids concentration on the sludge
density. As shown in Figure 20, both the influent suspended solids and the
sludge percent solids increased uniformly until the sludge solids briefly
leveled off and then radically dropped. The influent suspended solids
lowered after the sludge solids dropped because less solids per unit volume
were being recycled. After dropping to approximately 5 percent, the sludge
percent solids began to increase again at approximately the same rate as was
observed in the early part of the test. Unfortunately, the test was quite
time-consuming and could not be continued.
It is felt that the drop in the sludge solids was caused by rat-holing
in the sludge hopper of the thickener. It is conjectured that the sludge
reached a density and viscosity sufficient to impede effective movement into
the pump intake and as a result, the pump withdrew a column of water extending
from the intake to the top of the sludge bed and found the less dense sludge
easier to draw than the sludge immediately surrounding the intake.
Approximately 1900 liters (500 gal) of sludge resulted from the neutra-
lization of approximately 1100 cu m (300,000 gal) of AMD. On a mass balance
basis, approximately 725 kg (l600 Ib) of dry solids should have resulted
from the neutralization; approximately 360 kg (800 Ibs) remained in the
thickener hopper when the supernatant water was decanted.
IRON OXIDATION
AMD containing ferrous iron levels greater than 50 mg/1 usually requires
aeration during the neutralization process to oxidize the iron to the ferric
form for precipitation. To design an aeration system,*'it is necessary to
have a good estimate of the ferrous iron oxidation rate. The complexity of
the iron oxidation process is well known. ~ Almost all the process
parameters (e.g., pH, dissolved oxygen level, air bubble size, amount of
light, types and concentrations of micro-organisms, temperature, as well as
the concentrations of such ions as ferrous iron, ferric iron, aluminum,
manganese, copper, aluminum, and silica) affect the oxidation rate in one
manner or another. Of these parameters, pH, ferrous iron content, and
dissolved oxygen concentration have the greatest influence. Only pH can be
85
-------
CUMULATIVE WATER NEUTRALIZED, cu m
21 42 63
Co
ON
Suspended solids (influent)
30,000
- 25,000
20,000
toJO
E
15,000
CO
- 10,000
- 5,000 z
80,000 160,000 240,000 320,000
CUMULATIVE WATER NEUTRALIZED, gallons
Figure 20. Trends of the sludge solids and thickener influent suspended solids during continuous sludge
recirculation (zero sludge discharge) while maintaining a 1 liter/sec (16 gpm) AMD influent rate
and lime neutralizing to pH8.
-------
controlled, since the aerator should saturate the water with oxygen and the
influent ferrous iron level is predetermined.
Only limited data on rate constants for AMD oxidation by aeration at
neutral or basic pH's have been presented in the literature. Most of these
rate constants were developed from either synthetic AMD or non-AMD solutions
evaluated in laboratory studies. Unfortunately, valid extrapolation of
laboratory data from synthetic solutions to real-world situations is often a
difficult task.
Initial designs for the Crown facility were based on field oxidation
studies at the test site using the AMD to be treated. The results of the
field study were then compared with values from the literature and wide
discrepancies were noted. These discrepancies prompted the design of a
flexible oxidation system including considerable over-capacity to cover most
contingencies. Even so, the detention capacity was insufficient for several
of the tests made during the course of these investigations.
Typical ferrous decline curves were generated from laboratory oxidation
tests (Figure 21) and illustrate the relative effects of treatment pH on the
oxidation.
In 197^-5 EPA conducted a limited investigation on the oxidation process
and included field observations on nine treatment plants. The major
conclusions from these investigations were as follows:
—Few, if any, of the aeration systems achieved complete
oxidation of the ferrous iron prior to entering the sedimenta-
tion basin. Complete oxidation in the aeration tank would seem
desirable but in reality was not necessary in the cases observed
because the water went to sizable lagoons where the oxidation was
eventually completed. Where thickeners are used, complete oxida-
tion in the aerator (to 1 mg/1 or so) is a necessity because de-
tention times in the thickeners are short.
Time-cyclic feeding of the neutralizing reagent was com-
mon but was detrimental to aerator performance unless the fre-
quency was closely spaced. Aeration systems performing unsatis-
factorily could be improved by constant neutralizer addition.
--All the facilities visited suffered from poor mixing in
the oxidation systems as evidenced by massive accumulations of
87
-------
0 10 20 30 40 50 60 70
TIME, minutes
Figure 21. Ferrous decline of various pH's for the EPA Crown
discharge as determined in the laboratory.
-------
solids in the bottoms and around the periphery of the aera-
tion tanks or ponds. These accumulations grossly reduced
the detention times and contributed to short-circuiting and
inefficient oxygen transfer. Use of intermittent lime addi-
tion further complicated the inadequacy of the processes.
Oxidation ponds over half-full of solids were common. In-
creased mixing capability in terms of either bottom turbines
and/or peripheral mixers must be incorporated into existing
and future plants in order to eliminate this condition.
—A review was made of the current literature relative
to oxidation rates in AMD treatment situations. No satis-
factory guidelines were available to reliably predict the
required oxidation time for an aeration system. EPA empiri-
cally developed a generalized equation for the oxidation time
requirement as a function of treatment pH and ferrous iron
content:
t = 1 In C where C = initial ferrous cone, in me/1
- _o_ o &l
C C = final ferrous cone, in mg/1
t = time, in minutes
k » 3.17883 x 10-V)6'11075 ± ^
This equation was developed from laboratory batch studies
using nine different AMD waters. Even though the batch-type
test data from each site conformed to first-order rate reaction
procedures, the resulting rate constants differed significantly
for each site visited. Also, the batch-test data did not cor-
relate well with observed conditions at operating treatment
plants in the field. Considering all inequities, however, the
equation could be useful for preliminary estimates and trouble-
shooting in relation to oxidation problems. It must be empha-
sized that individual on-site tests should be made on the AMD
to be treated before designing an aeration system and that some
difficulty may be encountered in accurately extrapolating these
data to full-scale conditions.
89
-------
Oxidation data from the current pilot-plant studies involving lime and
limestone treatment under the wide range of conditions investigated at Crown
were tested for conformance to first-order rate reaction characteristics.
Too much variance was present to reliably apply first-order reaction pro-
cedures. These data were then grouped together and the average oxidation
rates were calculated for each of lUo different data sets. Figure 22 shows
the compilation of the data where the average rate is expressed as a function
of treatment pH. Linear regression analyses of the data generated the
equation:
Average oxidation rate, mg/1 per min = 2.03(pH)-8.26
Again, considerable scatter is evident in the data in Figure 22.
Assuming normal neutralization techniques would treat to a minimum of pH 1,
a rough estimate of the average oxidation rate for design purposes on in-
fluent ferrous irons below 300 mg/1 would be 2 mg/1 per minute. Use of this
figure should provide considerable overcapacity in the majority of situa-
tions; however, overcapacity in the pollution abatement field should be the
rule rather than the exception in order that most contingencies can be
accommodated. As was observed in the earlier EPA oxidation report, the
nine sites visited probably had adequate oxidation capability when the
plants started; but increases in flow because of constantly expanding mine
area, decreases in water quality because of increases in pyritic contact,
and deterioration of the effectiveness of the aeration system because of
solids accumulation all led to a general decline in the overall effective-
ness of the process. It is felt that it would be cheaper to overdesign in
the first place than to retrofit expanded facilities in an existing plant.
90
-------
tan
2
I 2
5
X
o
UJ
ta
r -»*
r-w 4*5
-,*-
^
Linear Regression Equation:
Average oxidation rate, mg/l/min = 2.03|pH)-8.26
Conditions of test: _
Ferrous iron below 300 mg/l
H Limestone Data
• Lime Data
I
I
6789
TREATMENT pH
Figure 22. Average oxidation rates from 140 data sets of limestone and
of lime neutralization shown as a function of the treatment pH.
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REFERENCES
1. Wilmoth, Roger C., Lime and Limestone-Lime Neutralization of Acid Mine
Drainage, U.S. Environmental Protection Agency Report 670/2-7^-051,
Cincinnati, Ohio. 197^.
2. U.S. Environmental Protection Agency, Methods for Chemical Analysis of
Water and Wastes. 1971.
3. Dunk, R., M. A. Mostyn, and H. C. Hoare, General Procedure for Indirect
Determination of Sulfate, Analytical Methods for Atomic Absorption
Spectrophotometry, Perkin-Elmer Corporation, Worwalk, Connecticut.
March 1971.
U. U.S. Steel Corporation, Sampling and Analyses of Coal and Coke and By-
products, Third Edition, 1929.
5- Salotto, B. V., et al., Procedure for Determination of Mine Waste
Acidity, paper given at the 15^-th National Meeting of the American
Chemical Society, Chicago, Illinois. 1966.
6. Bituminous Coal Research, Inc., Studies on Limestone Treatment of Acid
Mine Drainage, Federal Water Quality Administration, Research Series
1U010 EIZ 01/70, Washington, D. C. 1970.
7- Singer, Philip C. and Werner Stumm, Kinetics of the Oxidation of
Ferrous Iron, Second Symposium on Coal Mine Drainage Research,
May 1968, Pittsburgh, Pennsylvania.
8. Ford, Charles, personal communication. June 1975.
9- Hill, Ronald D., personal communication. September 1975.
10. WiJmoth, Roger C., James L. Kennedy, and Ronald D. Hill, Observations
on Iron Oxidation Rates in Acid Mine Drainage Treatment Plants, Fifth
Symposium on Coal Mine Drainage Treatment Research, October 1975,
Louisville, Kentucky.
11. Stumm, Werner, Oxygenation of Ferrous Iron, First Symposium on Acid
Mine Drainage Research, Pittsburgh, Pennsylvania. May 1965.
12. Kim, Ann G., An Experimental Study of Ferrous Iron Oxidation in Acid
Mine Water, Second Symposium on Coal Mine Drainage Research, Pittsburgh,
Pennsylvania. May 1968.
92
-------
13. Mihok, E. A., and C. E. Chamberlain, Factors in Neutralizing Acid Mine
Waters with Limestone, Second Symposium on Coal Mine Drainage Research,
Pittsburgh, Pennsylvania. May 1968.
Ik. Holland, Charles T., James L. Corsaro, and Douglas J. Ladish, Factors
in the Design of an Acid Drainage Treatment Plant, Second Symposium
on Coal Mine Drainage Research, Pittsburgh, Pennsylvania. May 1968.
15. Holland, Charles T., Robert C. Berkshire, and Daniel F. Golden,
An Experimental Investigation of the Treatment of Acid Mine Water
Containing High Concentrations of Ferrous Iron with Limestone, Third
Symposium on Coal Mine Drainage Resea- , Pittsburgh, Pennsylvania.
May 1970.
l6. Selmeezi, Joseph G., The Design of Oxidation Systems for Mine Water
Discharges, Fourth Symposium on Coal Mine Drainage Research, Pittsburgh,
Pennsylvania. April 1972.
17. Lovell, Harold L., An Appraisal of Neutralization Processes to Treat
Coal Mine Drainage, U.S. Environmental Protection Agency Technology
Series Report, EPA-670/2-73-093, Washington, D- C. November 1973.
18. Singer, Philip C., and Werner Stumm, Oxygenation of Ferrous Iron:
The Rate-Determining Step in the Formulation of Acidic Mine Drainage,
U.S. Environmental Protection Agency Water Pollution Control Research
Series Report, DAST-28, lUOlO 06/69, Washington,. D. C. June 1969-
19. Mihok, Edward A., Plant Design and Cost Estimates for Limestone Treat-
ment, U.S. Dept. of Interior Mine Water Research Series, U.S. Bureau
of Mines RI 7368, Washington, D. C. April 1970.
93
-------
GLOSSARY
CALCULATIONS AND DEFINITIONS
Mean Probable Detention Time: A mathematical approximation of the average
actual time a flowing liquid is detained in a vessel. The value is deter-
mined by tracer studies in which the tracer concentration exiting from the
tank is monitored in respect to time. Two values are plotted on probability
paper for each measurement point; i.e., cumulative tracer quantities to each
point divided by the total cumulative tracer quantity measured (percent
passing) and the ratio of elapsed time of each measurement divided by the
theoretical detention time of the vessel. The theoretical time equals the
tank volume divided by the flow rate. The resulting probability plot is
interpolated to derive the time for 50-percent passing.
Detention Efficiency: The ratio of mean probable detention time to theo-
retical detention time expressed as a percentage.
Utilization Efficiency: A measure of the proportion of a neutralizer that
reacts with the acid water as compared to the amount originally added.
Since alkalinity imparted to the water is considered a benefit, the formula
for utilization efficiency is:
Alkalinity Used
Utilization Efficiency = Alkalinity Added
therefore, Influent Acidity - Effluent Acidity +
Utilization Efficiency = Effluent Alkalinity (all as CaCO )
Alkalinity Added (as CaCOj 5
Stoichiometric Factor: The ratio of amount of neutralizer required to treat
the original amount of acid present:
Stoichiometric Factor = Alkalinity Added (as CaCO )
Influent Acidity (as CaCO )
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-77-101
3. RECIPIENT'S ACCESSION-NO.
4, TITLE AND SUBTITLE
LIMESTONE AND LIME NEUTRALIZATION OF FERROUS IRON
ACID MINE DRAINAGE
5. REPORT DATE
May 1977 issuing date
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Roger C. Wilmoth
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Resource Extraction and Handling Division
Crown Mine Drainage Control Field Site
Box 555, Rivesville, W. Va. 26588
10. PROGRAM ELEMENT NO.
1BB610
11. CONTRACT/GRANT NO.
05-03-01A-07-01D
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Industrial Environmental Research Laboratory-Gin., OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio *i5268
Final 1/7U - 1/76
14. SPONSORING AGENCY CODE
EPA/600/12
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The U.S. Environmental Protection Agency conducted a 2-yr study on hydrated
lime and rock-dust limestone neutralization of acid mine drainage containing ferrous
iron at the EPA Crown Mine Drainage Control Field Site near Rivesville, West Virginia.
The study investigated optimization of the limestone process and its feasibility
in comparison with hydrated lime treatment. Operating parameters, design factors, and
reagent costs for both processes were determined. Effluent quality was considered of
prime importance in these investigations. Coagulants were considered essential to
successful thickener operation for both lime and limestone treatment. Effluent iron,
suspended solids, and turbidity values could be maintained below 3 mg/1, 10 mg/1, and
10 JTU, respectively, using coagulant addition.
Although the limestone process was demonstrated to be technically effective in
ferrous iron treatment situations, the process was judged to be less efficient overall
in comparison with lime neutralization. The reaction and aeration detention time re-
quirements for the limestone process were two to three times that for the lime process
and overshadowed the reagent usage cost advantage of the limestone process. The lime-
stone process was thus judged unfeasible for general application in ferrous iron acid
mine drainage situations.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Limestone
Calcium hydroxides
Neutralizing
Drainage-mine (excavations)
Cost comparison
Surface drainage
13. DISTRIBUTION STATEMENT
Release to public
HHHHM||B||M^M>BHWB^B^M^HMIMBaMH>MB
EPA Form 2220-1 (9-73)
b.lDENTIFIERS/OPEN ENDED TERMS
Acid mine drainage
Coal mine drainage
Ferric iron
West Virginia
19. SECURITY CLASS (ThisReport}
JJnclassified
20. SECURITY CLASS (Thispage)
Unclassified...
COSATl Field/Group
13B
21. NO. OF PAGES
105
22. PRICE
95 ,v U.S. GOVERNMENT PRINTING OFFICE, 1977-757-056/6^35 Region No. 5-11
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