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).

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   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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