United States
Environmental Protection
Agency
Industrial Environmental Research -
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-031 June 1983
Project Summary
Pilot Plant Treatment of
Acid Mine Drainage by
Reverse Osmosis
G. Lansing Blackshaw, Alfred W. Pappano, Garth E. Thomas, Jr., and
Shun-Yung Cheng
Studies were conducted at the EPA
Crown Mine Drainage Control Field
Site (a) to examine the performance of a
227.000 Ipd (60,000 gpd) reverse
osmosis (RO) unit at recovery levels of
50 percent through 90 percent for a
variety of dominantly ferrous iron acid
mine drainage (AMD) feed qualities, (b)
to evaluate the feasibility of using the
neutrolosis process to treat AMD, and
(c) to determine the AMD treatment
capability of a coupled 18.9 Ipm (5
gpm) sodium cycle cation exchange
(CIX)XRO system.
Neutrolosis studies were conducted
using both 227,000 Ipd (60,000 gpd)
and 15,100 Ipd (4,000 gpd) spiral-
wound membrane module RO units
utilizing lime, soda ash, and lime-soda
ash combinations as the neutralizing
agents. The neutrolosis process did not
increase system recovery beyond the
maximum recovery levels attainable by
once-through RO treatment of AMD.
Precipitate fouling and osmotic
pressure phenomena associated with
neutrolosis caused an increased
frequency of system operational and
shutdown problems.
Using the coupled CIX/RO system, a
greater than 90 percent recovery
operation was achieved by the RO unit
when the AMD feed was pretreated by
the CIX unit.
This report represents the second
phase of RO process research pertinent
to AMD treatment at the EPA Crown
Mine Drainage Control Field Site in
fulfillment of Contract Number 68-03-
0245 by West Virginia University under
the sponsorship of the U.S. Environ-
mental Protection Agency. This report
covers the period May 1973 to
November 1975.
This Project Summary was developed
by EPA's Industrial Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the
research project that is fully document-
ed in a separate report of the same title
(see Project Report ordering information
at back).
Introduction
The discharge of Acid Mine Drainage
(AMD) from both active and abandoned
coal mines throughout the coal-
producing areas in the United States has
resulted in a serious water pollution
problem since the first coal mining
operation in Pennsylvania began in 1761.
The problem is especially present in the
Appalachian region, where acid mine
discharges have contaminated many
freshwater streams, and has diminished
the available water supplies. As a result,
the control of this serious problem has
become a regional as well as a national
concern.
The formation of AMD is principally due
to oxidation of iron sulfide minerals
(pyrite or marcasite) that are found in coal
seams. After coal is mined, the FeS2 is
oxidized in the presence of air and
dissolved by water to form ferrous sulfate
and sulfuric acid. Further oxidation of the
ferrous sulfates produces ferric sulfate.
Then, through subsequent hydrolysis,
more sulfuric acid is produced along with
the formation of ferric hydroxide.
Therefore, a large quantity of iron sulfate
is present in the AMD, and the sulfuric
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acid formed decreases the pH value of the
acid mine water. This wastewater
infiltrates surface waters and other
groundwaters, thus lowering the pH and
increasing their dissolved solids content.
However, the resulting pH is generally
high enough to cause the precipitation of
ferric hydroxide, which ir\turn deposits in
stream beds and aquifiers and increases
the turbidity of the water. The acid mine
water also abounds in other metallic
pollutants, such as aluminum,
manganese, calcium and magnesium.
Moreover, it is found that the relative
pollutant concentrations and the
temperature of AMD vary considerably
with climatic conditions, and the extent to
which acid mine waters become a source
of pollution is also variable in different
coal mining areas.
Although various neutralization
treatment schemes can provide effluents
of sufficient quality to meet discharge
standards into natural waterways, they
normally cannot produce potable water.
The purpose of studies conducted by
West Virginia University has been the
pilot-plant investigation of both Reverse
Osmosis (RO) and coupled Ion
Exchange/Reverse Osmosis (CIX/RO)
processes that do have the potential of
yielding potable water from AMD.
This document summarizes the results
of both analytical and experimental work
conducted during the period May 1973
through November 1975 concerning
treatment of AMD by RO- and ClX-based
techniques to produce a high quality
product water. The basic studies have
involved pilot-plant scale operations at
the U.S. Environmental Protection
Agency (EPA) Crown Mine Drainage
Control Field Site, Rivesville, West
Virginia, and focused on:
• Evaluation of the operating
performance of a 227,000 Ipd
(60,000 gpd) rated produce output
RO unit, containing spiral-wound
cellulose acetate membrane
modules, in terms of its ability to
treat AMD at unit recoveries of 50
to 60 percent.
• Determination of the feasibility of
using lime, soda ash, and combined
lime-soda ash neutrolosis (a
combination of RO and neutrali-
zation) processes to treat AMD
focusing on enhanced overall
systems recovery and low-volume
waste yields.
• Examination of coupled sodium
cycle co-current CIX/RO systems
as a potentially high recovery AMD
treatment process.
Conclusions
The following major conclusions are
based on the results of the investigations
of RO-based AMD treatment techniques
conducted on ferrous mine discharge
waters at the EPA Crown Mine Drainage
Control Field Site, Rivesville, West
Virginia:
• The 227,000 Ipd (60,000 gpd) RO
unit, which represents a significant
scale-up compared to previous RO
studies of AMD treatment, is fully
capable of treating ferrous AMD
discharges to produce high quality
water with few operational diffi-
culties. Automated regulation of
product flow rates at preset levels
functioned reliably during more
than 4500 hours of operating time.
Module failures were minimal (4
out of 84) during the period, and
were the result of cracked module
casings. No deterioration of
membrane performance in terms of
flux decline or deterioration of ion
rejection was noticed during
normal operations at 50 through 90
percent unit recoveries. With
proper acidification of AMD (pH
2.8-3.0), and known recovery
limitations of the AMD feed based
on its cited characteristics, no iron
or gypsum fouling will occur.
Automatic low pressure shutdown
and acidified backflushing systems
worked well. The RO unit met all
design and operating specifications.
The only major problem encoun-
tered was erosion/corrosion of the
stub tubes connected at 90° angles
to the inner surface epoxy-coated
pressure vessels. This resulted in
pin-hole leaks in the stub tubes
themselves. Welding and epoxy
recoating of the defects was only a
short-term maintenance measure,
and the pressure vessels may
eventually have to be replaced.
• Lime neutrolosis proved to be both
technically and operationally
unfeasible, primarily due to exces-
sive CaS04 concentrations in the
neutralized supernatent recycled to
the RO unit. Several case studies
and experiments demonstrated
that when compared to once
through RO recovery operations on
an AMD with specified chemical
characteristics, lime neutrolosis is
not competitive when examined
from either an operational or
economic perspective.
Soda ash neutrolosis appeared to
be a feasible process, if care was
taken to raise pH levels to greater
than 9.5 to remove calcium
hardness. The economics of the
process are questionable for Crown
AMD, with chemical costs running
in excess of 53 cents per 1000 liters
($2.00 per 1000 gallons) of AMD
treated.
Lime-soda ash neutrolosis was
also an operationally viable
treatment method. Costs were
reduced since lime may be used to
elevate the RO brine to pH 8.0
before using soda ash softening to
raise pH levels to 9.5-10.0, which
removes better than 90 percent of
the calcium. Cost estimates of
chemicals show that it would
require approximately 40 cents per
1000 liters ($1.50 per 1000
gallons) to treat Crown AMD, plus
the expense of maintaining
accurate pH control in both the lime
and soda ash additive steps of the
process.
In both the soda ash and lime-soda
ash neutrolosis processes, a
problem of NaSO4 concentration
buildup in the supernatant
occurred during the run periods.
Even for the small amounts of
recycle conducted in the studies,
the increases in the RO brine
osmotic pressures due to this effect
were large. The net result to
maintain a pre-set product flow
under these conditions must be to
increase feed pressure, and hence
operating costs, at a possible
sacrifice of overall product quality.
The coupled sodium cycle CIX/RO
process showed an excellent
capability to function as a high
recovery AMD treatment system
without membrane fouling. The
Na2SO4 laden waste brine from the
process was utilized to provide up
to 40 percent of the regenerating
capacity of the resin bed, thus
reducing NaCI requirements in
regeneration. Again, with such a
high recovery (greater than 90
percent) mode of operation,
substantial osmotic pressures
existed within the RO system,
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tending to reduce product flux
which must be compensated for by
increased feed pressure on the RO
system. Product water quality from
the coupled system met potable
water standards except for low pH,
which could be easily buffered to
pH 7.0.
Recommendations
Based on the results and conclusions of
the investigations, the following
recommendations are made for addi-
tional studies related to RO-based and
coupled CIX/RO treatment of AMD:
• The economics of once-through RO
treatment of AMD should be fully
investigated to include capital,
operating, and maintenance costs
for a complete treatment system
including the RO unit and all
peripheral unit processes and
hardware, with special emphasis
on waste brine disposal costs.
• The cost-effectiveness of soda ash
and lime-soda ash neutrolosis
should be determined to establish
whether they are viable processes
for treatment of AMD, and to find
any advantages over a once-through
maximum recovery type RO opera-
tion.
• A counter-current CIX system
coupled with RO should be studied,
because there is reason to believe
that even higher system recoveries
can be attained owing to reduced
hardness leakage from the ion
exhange side of the system. Also,
more detailed investigations
should be made to optimize
regenerant utilization when the
CaCI- Na2S04 tandem regener-
ation procedure is utilized. In this
respect, the goal would be to
minimize NaCI consumption, and
maximize Na2SO4 usage. The addi-
tion of a chemical antiprecipitant
agent to the Na2SO4 regenerant aid
should be investigated to inhibit the
onset of CaSO4 precipitation.
• A detailed study of the process
economics of a CIX/RO treatment
system should be performed, and a
comparative cost analysis made
with other types of RO-based
treatment techniques.
• A Manual of Practice pertinent to
treatment of AMD by RO
techniques should be prepared to
serve as a practical AMD treatment
guide for design and operating
personnel in the field. There
appears to be sufficient information
developed to date to produce such a
document.
• Membranes having high sodium
rejection should be used to raise
the sodium concentration in the
waste brine to determine the effect
of this increase upon the waste
brine as a regenerant aid and
simultaneously improve the
product quality by decreasing
sodium concentration .
• A determination of capacity as a
function of time should be done at
various NaCI dosages to find the
degree of the resin degradation
during long-term operations. This
experiment would permit the
quantification of fouling to be
expected when dealing with the
concentrations of iron encountered
in this study.
Description of the Pilot
Plant Facility
The Crown Mine Drainage Control
Field Site is located at the intersection of
Stewarts Run and Little Indian Creek on
10 acres of land at Crown, West Virginia,
which is approximately 20 km (12 miles)
southwest of Morgantown, West
Virginia. The mine drainage feed source,
which is capable of delivering more than
567,000 liters/day (150,000 gallons/day),
originates from a 156-mm (6-inch) bore
hole pump situated 366 m (1200 ft) from
the pilot plant location. The acid mine
waters are transported to the pilot plant
through a 78-mm (3-inch) polyvinylchlo-
ride (PVC) pipeline.
The 227,000 Ipd (60,000 gpd or 60 K)
and 15,000 Ipd (4,000 gpd or 4 K) RO
units, and the twin-bed sodium-cycle CIX
system are located in a 12.2 m x 30.5 m
(40 ft x 100 ft) light gauge steel building
which was erected by West Virginia
University at the Crown Mine Drainage
Control Field Site.
Reverse Osmosis
The RO process utilizes a semi-
permeable membrane through which
almost pure water (permeate) is
transported from a concentrated solution
(brine) to a dilute solution by applying a
pressure which is greater than the
natural osmotic pressure. Since the
membrane essentially rejects (is
impermeable to) the dissolved ions, they
are retained in the concentrated brine
solution. In this manner a feed such as
AMD can be demineralized to produce a
quantity of permeate separated from a
volume of concentrate that contains
almost all of the pollutants originally
present. The water flow rate (flux)
through the membrane is proportional to
the difference between the net applied
pressure and the natural osmotic
pressure of the concentrate, while salt
transport across the membrane surface
depends primarily on the salt
concentration gradient which is relatively
independent of the pressure.
The membrane is truly the key
component of an RO system. Many
variables can affect membrane
performance, and therefore, RO
performance.
Based on the success of previous
investigators to utilize RO to produce
nearly potable water from AMD and
simultaneously minimize iron fouling
problems, EPA decided to initiate pilot-
plant scale studies to further examine the
applicability of using RO-based processes
to treat AMD. In 1971, work was started
to install a 227,000-lpd (60,000-gpd)
spiral-wound RO membrane system
demonstration plant at the Crown Mine
Drainage Control Field Site, Rivesville,
West Virginia, to provide data on the RO
systems scale-up and its ability to sustain
long-term operations. Also to be studied
was the feasibility and reliability of
neutrolosis (a combination of RO and
neutralization) as a high recovery/low
waste treatment process for ferrous iron
type AMD. In 1973, a 240-hour
continuous shakedown run was
conducted at Crown using the 227,000-
lpd RO unit operating at a 50-percent
recovery on a once-through basis. It was
concluded that the RO unit did
demonstrate the capability of
successfully treating Crown AMD at this
recovery level without membrane fouling
or major operational difficulties.
60K Reverse Osmosis System
The 60K RO unit, manufactured by Gulf
Environmental Systems, Inc., has been
designed to nominally deliver a constant
product water output of 227,000 Ipd
(60,000 gpd). This is basically accomp-
lished by pneumatically operated auto-
matic flow control valves in the feed and
brine lines which, when preset to flow
rates to yield 2.65-lps (42 gpm) product
flow, will work in conjunction to adjust
system operating pressures to maintain
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constant product output and recovery. The
feed stream volume flow rate may vary
from 6.56 to 2.90 Ips (104 to 46 gpm),
which corresponds to designed operational
recoveries ranging from 40 percent to 90
percent. The system is composed of four
major assemblies: (1) pretreatment, and
product storage and blending, (2)
membrane-module pressure vessels and
high pressure pump assembly, (3) sam-
pling and pressure measurement assem-
bly, and (4) instrument console.
Acid minefeedwater from the bore hole
pump enters the pilot plant facility and is
piped to a 9500-liter (2500-gallon)
holding tank. Then it is transferred by a
centrifugal pump to the pretreatment skid
where sulfuric acid is added to adjust the
pH to 2.9. The feed is subsequently
filtered by 20-micron cartridges to
remove suspended solids prior to
entering the main high pressure feed
pumps.
The volume discharged from these
pumps is automatically controlled by
means of a differential pressure flow
controller and control valve. The feed is
then distributed via manifolding to the
series-parallel array of 14 pressure
vessels, each containing six spiral-
wound ROGA®* modules. The concen-
trate flow is controlled similarly to the
feed flow, so that once the two controllers
are set, the unit will then operate at
constant recovery, and the unit pressure
will vary to maintain constant product
flow rates.
The product from each pressure vessel
is piped to a sampling panel where it can
be measured independently. Provisions
for measuring the osmotic pressure of
each tube, as well as the pressure and AP
of each bank of vessels are included. The
product streams are collected
downstream of the sampling valves and
piped to the 1330-liter (350-gallon)
elevated product water storage tank. The
product flow is measured by means of an
orifice-type rotameter and the water pH
is adjusted just prior to entering the tank.
This water can then be used to dilute the
feed stream at recovery levels above that
where calcium sulfate precipitation is
expected, or utilized for flushing the
pressure vessels during unattended or
scheduled shutdown.
A centrifugal pump, manual valve, and
orifice-type rotameter are used to deter-
mine the amount of product water
blended with the feedwater. The flushing
system includes a normally open valve
'Mention of trade names or commercial products
does not constitute endorsement or recommendation
for use
and electric-pneumatic controls. The
flushing sequence will begin at a preset
number of minutes after shutdown. A ball
valve in the line can be closed when
flushing is not desired.
Product water in excess of that needed
to maintain a proper inventory in the
storage tank has its pH raised to 6.5 before
discharging to a floor drain outlet to Little
Indian Creek. When not conducting
specific neutralization or neutrolosis
studies, waste brine is usually lime-
neutralized in a 2300-liter (600-gallon)
reactor vessel to pH levels of 5.5 to 6.5,
and then pumped to the Crown Mine
Drainage Control Field Site sludge settling
pond.
4K Reverse Osmosis System
The 4K RO unit, manufactured by Gulf
Environmental Systems, Inc., has been
designed to produce a nominal output of
15,000 Ipd (4000 gpd) of high quality
product water. The feed stream volume
can vary from 0.44 to 0.19 Ips (7 to 3
gpm), which corresponds to recoveries of
40 percent to 90 percent. This system is
also composed of four major assemblies:
(1) pretreatment assembly, (2) membrane-
module pressure vessels and high pressure
pump assembly, (3) sampling and pres-
sure measurement assembly, and (4)
instrument console.
AMD feedwater is withdrawn from the
same 9500-I (2500-gal) holding tank
utilized by the 60K RO system. This
feedwater is then transferred by a
centrifugal pump to the 4K RO unit.
Sulfuric acid is added to adjust the feed
pH to 2.9 in order to maintain iron constit-
uents in solution and control iron fouling
of the membranes. If a precipitation
inhibitor is to be used, it is also added at
this point. The feed is then filtered in the
10-micron cartridge filters to remove any
suspended solids.
The feed pressure to the unit is
controlled bypassing a portion of the feed
through a valved bypass line. The overall
unit recovery is adjusted by altering the
amount of the brine to be recycled with
the fresh feed to the unit, where
increasing the brine recycle also
increases the unit recovery of product
water. The resulting product water is
adjusted to pH 6.5 before it is discharged
to the drain. The waste brine is lime
neutralized, or it may be used as feed to
another process.
Reverse Osmosis Systems
Operations
One major aspect of the work
conducted at the Crown Mine Drainage
Control Field Site was the examination of
RO pilot plant performance under a
variety of AMD feed conditions. Studies
focused on the production of high quality
permeate from Crown AMD using the
60K RO unit at 50 to 90 percent recovery
levels, the investigation of chemical
pretreatment of AMD to alleviate or
retard precipitate fouling of RO
membranes, and the documentation of
operations maintenance problems
(including repair efforts) created by long-
term sustained usage of RO equipment in
the field.
As noted above, when a feed water
under applied pressure continuously
moves through an RO unit, water is
transported through the membrane. This
increases the salt concentration of the
water not passing through the membrane.
The percentage of the feedwater collected
as a permeate is defined as the recovery.
At 90-percent permeate recovery, the
brine concentration is 10timesthatof the
original feed. Thus, attempts to attain high
product recoveries can cause problems of
product quality deterioration, elevated
osmotic pressure, and ever-increasing
feed pressures.
During a 150-hr, 50 to 90 percent
recovery checkout operation with the 60K
RO unit, operational difficulties were
minor in nature: a leak in the product
water recycle pump, sticking of the brine
flow controller, a malfunction of the
product recycle rotameter, and several
low suction pressure shutdowns due to
loss of raw AMD feed at the bore hole. In
all cases of automatic shutdown due to
low (<10 psig) feed pressure, the
automated backflushing operations with
pH 2.5 to 3.0 phosphoric acid solution
functioned well, as evidenced by the
return to pre-shutdown operating condi-
tions (flow rates, system pressures, water
quality) when the RO unit was restarted.
Repairs for the most part were
satisfactory; however, the incidence of
erosion-corrosion defects continued to
accelerate during the course of the two-
year investigations. The vendor has noted
that another of its RO units, having
identical pressure vessel design and
being utilized in brackish water
treatment, was experiencing erosion-
corrosion problems similar to those at
Crown. The vendor's recommendation
was that newly designed fiberglas
pressure vessels having end entry-exit
feed-brine ports be installed to replace
the current pressure vessel system.
Some comments should be made con-
cerning the performance and durability of
the spiral-wound cellulose acetate
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membrane-modules used in the 60K RO
unit. During more than 4,500 hours of
operation, the RO unit experienced only
four module failures due to cracks in the
module casing. No modules had to be
replaced because of inherent loss of
membrane rejection capability. Occasion-
ally, modules were removed and dissected
to examine iron or gypsum fouling
patterns, and other times, the central axis
product tubes were broken due to improper
loading procedures. When one considers
the types of investigations being conducted
that often resulted in potential CaS04
fouling situations and deliberate iron
fouling of the membranes, and the
numerous deliberate and unintentional
shutdown, membrane cleaning, and
start-up operations, the track record of
modular durability, performance and
lifetime must be rated as excellent.
Coupled Reverse Osmosis/
Neutralization Studies
The RO treatment of AMD often
produces a considerable volume of
concentrate which must undergo
treatment and disposal processes, even
though a significant amount of the feed
may be recovered as a high quality
permeate. In the majority of applications
of AMD treatment by RO, the RO unit
most likely would be utilized as an adjunct
to neutralization processes to produce a
given quantity of high quality water for a
particular purpose. The concentrate in
this case subsequently would be treated
in the neutralization/clarifier train
before a discharge to surface waters.
However, if it was desirable or necessary
to alleviate concentrate disposal
problems, a possible solution might be to
recycle some portion of the neutralized
brine to the RO unit for additional
treatment. Ideally, such a coupled
technique could yield a high permeate
recovery of the raw AMD and a small
amount of waste effluent. In some cases,
the only waste might be a sludge which
could be dried or disposed of in a landfill.
Previous work with RO/neutralization
processes was conducted by the EPA
staff at Norton, West Virginia. The name
'neutrolosis' was applied to the coupled
process in which the RO concentrate was
lime neutralized and recycled to the RO
unit. Although the Norton study showed a
an overall systems recovery of greater
than 98 percent, it was not clearly
determined how a long-term neutrolosis
operation would function with respect to
calcium sulfate fouling within the process
loop, nor was the overall feasibility of
operating in a neutrolosis mode explicitly
evaluated.
A major purpose of this study was to
carefully examine neutrolosis concepts.
Three variations of the ' coupled
RO/neutralization process were studied
using lime, soda ash, and combined lime-
soda ash as the respective neutralizing
agents. These processes were evaluated
from both theoretical considerations and
on the basis of experimental results.
Lime Neutrolosis
In the neutrolosis process, concentrate
from the RO unit is reacted with lime to
raise the pH of the concentrate to a
specified level. The reactor effluent is then
aerated to oxidize the iron to facilitate its
removal, and subsequently clarified to
produce a sludge and a low turbidity
supernatant. A large portion of the
supernatant is blended with the raw AMD
for further treatment in the RO unit,
which results in much of the concentrate
being continuously recycled in the
coupled system (Figure 1). The basis for
lime neutralization as a water treatment
process is that lime reacts with several
metals to yield fairly insoluble
hydroxides. The lime addition raises the
pH so that the solubility equilibrium is
shifted and the metals are precipitated
from the water.
All of the lime neutralization reactions
form calcium sulfate as one of the
products. In order to remove most of the
reactable components in AMD waters, it
would be necessary to add a considerable
amount of lime, thus greatly increasing
the calcium sulfate loading. For example,
in a typical sample of Crown water more
than 1.55 kg of hydrated lime must be
added to each 1000 liters of RO
concentrate neutralized. This is enough
to gypsum-saturate the water by itself,
but when added to the calcium sulfate
already present, a supersaturated
condition arises. Such a highly saturated
concentrate stream poses the great
problem with a lime neutrolosis type of
operation: namely, scaling of the
membranes with gypsum when the
recycle stream is introduced to the RO
unit.
In order to examine the performance of
an RO system when an actual lime
neutrolosis recycle operation occurs,
several experimental runs were
conducted. The calcium sulfate brine
saturation factor was selected as a signif-
icant variable, since its limiting value of
1.4 (based on ionic interference solubility
calculations) appeared widely applicable
to the RO treatment of a diversity of AMD
feeds. In an evaluation of RO systems
operation, it is the fouling potential of the
brine in the last bank of modules which is
of critical importance. For Crown AMD,
the 1.4 calcium sulfate saturation factor
represented CA++ concentrations of 750-
800 mg/l in the brine stream. It should
further be noted that based on non-ionic
Product
Water
Raw
AMD
Product T
Recycle \
I
T
60 K
RO Unit
Lime —
1
C
D
Filter U. V. I
(ft——
I I Filter
Neutralization
I
I Recycle
Air
Supernatant
Sludge
Figure 1. Process schematic for lime neutrolosis studies.
5
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interference calculations (comparison of
solubilities of CaS04 in brine with CaSO4
solubility in distilled H20), the 1.4 value
cited above corresponded to a non-ionic
interference CaS04 saturation factor of
2.45. These calculations are described in
detail in the main body of the report.
The "tap water neutrolosis" study
again provided evidence that the lime
neutrolosis mode of operation is not
comparable to a once-through recovery
process. Operational problems were also
encountered with respect to increased
acid consumption for feed pH adjustment
of the blended feed to the RO unit, and
more frequent replacement of cartridge
filters was due to the precipitation of
gypsum present in the supernatant.
These problems had been anticipated,
and, in fact, proved to be recurrent in all
lime neutrolosis schemes attempted.
In summary, the lime-based
neutrolosis process caused an intolerable
membrane fouling rate when attempting
to sustain a long-term, high overall RO
system recovery operation. Although
numerous attempts were made to run
lime neutrolosis experiments, all were
rather short-lived and unsuccessful due
to prefiltration system fouling and/or
deterioration of product water flux rates,
even when precipitation inhibitors were
introduced to the process.
Soda Ash Neutrolosis
Because of the gypsum fouling which
resulted from lime neutrolosis, attempts
were made to use soda ash (sodium
carbonate) as the neutralization agent in
place of the lime. It was expected that
calcium sulfate hardness could be
removed with the soda ash and thus
circumvent the principal difficulty which
was associated with lime neutrolosis.
Operating experience with soda ash
neutralization of AMD indicated that soda
ash was a very effective neutralizing
agent but that its cost was prohibitive
compared to lime and limestone.
Soda ash (Na2CO3) is used to raise the
pH of the water and thus acts to precipi-
tate the metals as insoluble hydroxides. As
discussed in the section on lime neutrol-
osis, the pH is controlled to remove most
of the various metals present. Addition of
soda ash to proper pH levels allows the
iron, manganese, aluminum, and magne-
sium to be removed to the same extent as
in the lime process. The great differences
between the two processes have to do
with the hardness removal and cost.
Bench-scale neutralization tests on
Crown AMD indicated that soda ash
neutralization of the RO brine to a pH
level of greater than 9.5 would be capable
of removing 90 percent of the calcium
hardness. This was the neutralization
bench mark pH value for the "closed loop"
experiment which followed, using the
60K RO unit.
Chemistry analyses for each day of the
four-day operation are shown in Table 1.
It can be seen that for a supernatant pH of
9.6, the calcium removal was better
than 95 percent; however, the Na2SO4
buildup in the supernatent increased ten-
fold during the operating period. This
verified the hypothesis that increased
sodium concentrations in the soda ash
neutrolosis mode of operation will occur.
An evaluation of soda ash consumption
during the "closed loop" operation indica-
ted that the neutralization chemical costs
themselves were greater than $0.53 per
m3 ($2.00 per 1,000 gal) of treated AMD,
which supported the earlier contention
that soda ash neutrolosis (at least from
Crown AMD) would be an expensive
process.
Combined Lime-Soda Ash
Neutrolosis
The lack of operational success of lime
neutrolosis and the high cost of soda ash
neutrolosis led to a brief preliminary
examination of a lime-soda ash
neutrolosis process, whereby waste
brine from the RO unit was neutralized to
pH 8.0 with lime, and then raised to pH
10.0 by adding more soda ash. These pH
values were established by bench-scale
experiments conducted with Crown AMD
RO waste brine, with the objective to
remove at least 90 percent of the calciu m
from the supernatant.
The basic chemical reactions which
would be involved in lime-soda ash
neutralization have already been
discussed. In this type of process the
concentrate is first lime-treated to
remove many of the metals with the
cheaper lime. The more expensive soda
ash is then added to remove the residual
calcium as calcium carbonate; the goal is
to take advantage of the best features of
the two neutralization agents. Lime-soda
ash treatment has been utilized
extensively for the treatment of both
industrial water and drinking water.
However, application to AMD treatment
is very recent, and there have been no
other reported studies of a neutrolosis
Table 1. Chemistry Analysis for "Closed Loop" Soda Ash Neutrolosis (All Concentrations in mg/l)
Day 1
Brine
Supernatant
Product
Day 2
Brine
Supernatant
Product
Day 3
Brine
Supernatant
Product
Day 4
Brine
Supernatant
Product
Ca
470
17
0.35
420
16
0.65
450
18
0.66
470
17
0.60
Mg
140
95
0.2
230
140
0.3
230
150
0.37
210
160
0.34
Total
fe
230
0.7
0.3
230
6
0.5
240
2.20
0.32
350
2.2
0.6
Fe+2
220
0
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type process which utilizes both lime and
soda, ash.
Lime and soda ash consumption during
the study was calculated to cost approxi-
mately $0.40 per m3 ($ 1.50 per 1,000 gal)
of Crown AMD waters treated. Although
less than soda ash neutrolosis itself, it is
still quite costly. Again, the economics of
the situation need to be evaluated before
any decision is made to promote lime-
soda ash neutrolosis as a cost-effective
technology. In any neutrolosis process,
there will be added equipment costs and
involved operational and control proce-
dures which must be carefully monitored,
e.g., accurate pH control in neutralization
procedures and maintenance of low
turbidity conditions in supernatant
effluents from clarifiers.
Acid Mine Drainage Treatment
by Ion Exchange
Ion exchange (IX) has been widely used
for many years to produce limited
quantities of potable water from brackish
water, but its application to AMD
treatment is relatively recent. Based on
the promising results of previous coupled
IX/RO systems studies, investigations
were conducted at the Crown Mine
Drainage Control Field Site to determine
the applicability of such coupling
processes for AMD treatment. A sodium
form strong acid ion exchanger was used
as a softening pretreatment step prior to
purification in a spiral-wound membrane
15,000-lpd (4,000-gpd) RO unit. The
exhausted resin was regenerated by both
NaCI solution and by a combined
NaCI/Na2SO4-loaded waste brine from
the RO unit. The specific goal of the
studies was to determine the recovery
potential of the coupled system, and the
effect of utilizing Na2SO4 waste brine as a
regenerant aid to NaCI for capacity
restoration of the exhausted IX resins.
Twin-Bed Ion Exchange System
The heart of this system is a twin-bed
sodium-form IX unit, manufactured by
Culligan International Company, that
consists of two pressure tanks, 0.61 m
(24 inch) in diameter and 2.43 m (8 ft)
high, each containing 0.41 m3 (14.5 ft3) of
resin. The tanks are hot rolled steel rated
at 690 kN/m2 (100 psi) with a 0.1-mm
vinyl-ester lining for corrosion
resistanee. AMD feed is transferred to the
IX unit from the indoor holding tank by
means of a centrifugal pump. Before the
AMD is fed to the IX column in service,
the pH is adjusted to 2.8 by addition of
sulfuric acid in order to prevent iron
precipitation in the resin bed. The AMD
passes through the servicing exchanger
at a flow rate of 0.22 to 0.25 Ips (3.5 to 3.9
gpm).
During the downflow exhaustion
process, samples of softened water from
the ion exchanger are taken at intervals to
obtain leakage and free mineral acidity
(FMA) data. In addition, a representative
sample (AMD composite) of the average
influent water to the resin is continually
collected at hourly intervals throughout
the entire exhaustion run. This composite
sample is used to calculate the average
cationic loading to the coupled Ion
Exchange (CIX), and with the total service
time, is used to compute the IX capacity.
After the exhaustion process of the in-
service IX column is completed, the
alternate tank on standby simultaneously
goes into service, thus maintaining a
continuous AMD treatment process. The
exhausted ion exchanger is then
backwashed by sending a strong flow of
AMD upward at the rate of 1.0 - 1.1 Ips
(4.3 gpm) in the downflowdirection. After
the regenerant has passed through the
resin, a slow rinse, followed by a fast
rinse, is applied to complete the
regeneration cycle and return the
exchange resin bed to a standby service
mode. The operation is classified as co-
current, since it utilizes both
regeneration and exhaustion in a
downflow mode.
The IX process is def i ned as a reversible
replacement of the exchangeable cations
between a resin and the feed solution,
where the exchange resin contains ionic
groups which are chemically linked to the
polymer structure of the resin in which
the reactions occur. The polymeric
portion of the resin must be highly cross-
linked so that the solubility of the resin is
negligible. Moreover, the resin structure
must be chemically stable so that essenti-
ally no degradation occurs during use.
A complete IX operation cycle contains
the following four steps:
(1) Exhaustion Process - During
exhaustion, the AMD to be treated
is continuously fed through the
bed of IX resin, and the softening
capability of the ion exchanger is
gradually depleted.
(2) Backwash Process - Effective
backwash is required to maintain
a clean resin bed. The purpose of
the backwash process is to expand
the IX bed and to break up any
channels that may have formed in
the column.
(3) Regeneration Process - The
regeneration process is applied to
restore the capacity of the ion
exchanger by passing a
regenerant solution through the
exhausted resin bed.
(4) Rinse Process - After the
regenerant solution has been
applied to the ion exchanger,
the water or liquid to be treated is
used to rinse the IX bed before it is
returned to the exhaustion
process. The purpose of the rinse
process is to complete the
contacting and remove spent
regenerant from the ion exchanger.
Coupled Cation Exchange/
Reverse Osmosis and
Treatment Systems
Due to an inability to use RO as a single
unit process to treat Crown AMD (or mine
drainages of similar quality) at recoveries
greater than 50 percent, the failure of
lime neutrolosis as a feasible AMD
treatment alternative and the apparent
high cost of soda ash and lime-soda ash
neutrolosis processes, an AMD treat-
ment scheme which offered a potential
for high recovery performance was
investigated. One promising process
appeared to be a coupled CIX/RO system
in which an ion exchanger operating on a
sodium cycle removes calcium and other
+2 valence and +3 valence cations from
the AMD, replaces them with sodium
ions yielding essentially an Na2SO4
solution, and transfers this calcium-free
effluent to an RO unit to be processed at
high recovery.
Because of the high product water
recovery that can now be obtained from
the coupled IX/RO process, the volume of
the waste brine obtained is also
decreased. Moreover, the concentration
of Na2SO4 in this waste brine is high
enough to assist with the regeneration of
the exhausted IX resin. In this mode, the
brine is stored and later recycled back to
the IX unit to make up a portion of the
regenerant stream.
Observations of the experimental work
indicated that high permeate recoveries
(90 to 93 percent) could be maintained in
the RO unit throughout the period of
operation without any evidence of
membrane fouling. The performance of
the coupled system can be analyzed using
Table 2, which presents a representative
chemical analysis of AMD feed, softened
feed, blended feed, brine, and product at
salt dosage of 240 kg/m3 (15 Ib NaCI/ft3).
-------
Table 2. Representative Chemistry Analysis at a Salt Dosage of 240 kg/m3 (15 Ib/ft3)
Component
Ca
Mg
At
Ferrous Fe
Mn
Na
Total Fe
SO4
Conductance
pH
AMD
Water
380
110
4.4
250
4.5
480
270
2,900
3.800
4.5
Softened
Feed (SF)
6.0
0.6
0.2
2.5
0.02
1.200
2.6
2,900
5,500
2.8
Blended
Feed (BF)
45
4.6
0.9
12
0.2
8,200
12
28,000
27,000
2.7
Brine
71
7.0
1.2
18
0.4
12,000
19
48.000
40.000
2.5
Salt Rejection * Salt Rejection* Salt Rejection*
Product in SF (%) in BF (%) in AMD (%)
0.2
0.01
trace
trace
trace
58
0.01
260
650
3.4
96.7
98.3
>99.9
>99.9
>99.9
95.2
99.6
91.0
88.2
99.6
99.8
>99.9
>99.9
>99.9
99.3
99.9
99.1
97.6
>99.9
>99.9
>99.9
>99.9
>99.9
87.9
>99.9
91.0
82.9
All units are mg/l except pH and conductance (Mmhos/cm)
*Salt Rejection (%) *eed ^f.BF. AMD) - Product
Feed (SF, BF, AMD)
Table 2 also presents the salt rejections
for each specific ion.
The results obtained in these studies
demonstrated that the coupled sodium-
form exchanger and 4K RO unit were
capable of high-recovery treatment of
AMD at Crown, WV. When compared to
the use of each treatment system sepa-
rately, it was found that not only was the
RO water recovery effectively enhanced
by using IX pretreatment, but also NaCI
regenerant savings were realized with
the aid of Na2S04 waste brine discharged
from the RO unit.
-X 100%
G. L Blackshaw, A. W. Pappano, G. E. Thomas, Jr., and S-Y Cheng are with
Department of Chemical Engineering, West Virginia University, Morgantown,
West Virginia 26506
Robert B. Scott and Roger C. Wilmouth are the EPA Project Officers (see below).
The complete report, entitled "Pilot Plant Treatment of Acid Mine Drainage by
Reverse Osmosis," (Order No. PB 83-191437; Cost: $14.50, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officers can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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