United States
                    Environmental Protection
                    Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
                    Research and Development
EPA/600/SR-95/068    May 1995
&EFA         Project Summary
                    Calcium  Carbonate  Dissolution
                    Rate  in  Limestone  Contactors
                    Raymond D. Letterman
                     The project summarized here inves-
                    tigated some of the parameters and
                    relationships used to predict the per-
                    formance of limestone contactors. The
                    purpose of the project was to study the
                    effect of limestone composition, espe-
                    cially the dolomite [CaMg(CO,)2] and
                    impurity content of the stone, on the
                    kinetics of carbonate mineral dissolu-
                    tion and to determine the effect of tem-
                    perature on the rate of dissolution. The
                    rate of dissolution was determined by
                    using a rotating  disk apparatus and
                    samples of limestone of varied compo-
                    sition.
                     The  limestone samples included a
                    white marble and a selection of sedi-
                    mentary stones. The white marble con-
                    tained  a  significant amount  of silica
                    (approximately 35%). The major min-
                    eral constituents of the sedimentary
                    limestones ranged from approximately
                    100% calcite (CaCO,) to essentially pure
                    dolomite. The approximate  iron (Fe)
                    content of the stones ranged from 15
                    to 377 mg  Fe/IOOg and the  approxi-
                    mate aluminum content (Al) from 1 to
                    134mg Al/100g.
                     A heterogeneous reaction model for
                    mineral dissolution effectively explained
                    the results of the rotating disk experi-
                    ments  for all samples except the two
                    with the highest dolomite content. The
                    magnitude of the dissolution rate con-
                    stant for fresh stone decreased by ap-
                    proximately 60% as the calcite content
                    of the  stone decreased  from 0.92 to
                    0.09 g  CaCO3/g stone. The rate of dis-
solution of stones with a high dolomite
content may have been enhanced by
the presence of small amounts of cal-
cite. The rate of solubilization of mag-
nesium  (Mg)  was negligible in  all
samples except the two with the high-
est dolomite content (93 and 100 mass
percent dolomite).
  The overall dissolution rate constant
decreased as the amount of calcium
dissolved from the surface of the stone
increased. For a  given amount of cal-
cium dissolved per unit area of stone
surface, the magnitude of the percent-
age decrease  in the  dissolution rate
constant increased as the iron and alu-
minum content of the stone increased.
The effect of sample aging on the rate
of dissolution was lowest when the
weighted sum  of the  iron and alumi-
num content of the stone was less than
about 10 mg/g. The weighted sum is
equal to the aluminum content in mg
Al/g plus 0.30 times the iron content in
mg Fe/g. The presence of silica as the
principal impurity in the white marble
seemed to reduce the effective (cal-
cite)  surface area of the stone in pro-
portion  to the mass  of silica  in the
sample, but it did not appear to affect
the dissolution rate of the calcite sur-
face.
  The dissolution rate constant for cal-
cite increased with increasing tempera-
ture, from 0.38x10-9 cm/s  at 5C to
2.80x10-3 cm/s at 25C. The apparent
activation energy was 101 8 kJ/mol for
the surface reaction rate constant and
                                                                    Printed on Recycled Paper

-------
170.3 kJ/mol for the mass transfer rate
constant  in  the heterogeneous reac-
tion model.
  This Project Summary was developed
by  EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research  project
that is fully documented in a separate
report of the same title  (see  Project
Report ordering information  at the
back).

Background
  A limestone contactor is a treatment
device in which water flows through and
dissolves carbonate minerals (typically cal-
cium  carbonate) from a packed bed of
crushed limestone. Dissolution of calcium
carbonate  (under a closed-to-atmospheric-
carbon dioxide condition)  increases the
pH, alkalinity,  and dissolved inorganic car-
bon concentration of the water and de-
pletes the  amount of calcium carbonate in
the  bed. Limestone contactors are simple,
low-cost devices, which usually require
minimal maintenance and  are, therefore,
especially  suitable for small water  sup-
plies.
  In an earlier study (Letterman et al.,
1987), limestone contactors effectively re-
duced  the  dissolution  of  corrosion
byproducts, such as lead, copper,  and
zinc, from piping system surfaces. A math-
ematical model related the depth of lime-
stone needed to reach the desired effluent
water chemistry to the influent water chem-
istry, the limestone particle size and shape,
the limestone bed porosity, and the tem-
perature and superficial velocity of the
water. Limited field  experiments  showed
that contactor performance decreases as
the water temperature decreases.
  Another study (Haddad, 1986) monitored
the long-term  operation of a contactor con-
taining somewhat impure, high-calcium
limestone. Here, the author concluded that
as  the calcium carbonate  dissolved, the
rate of dissolution decreased because rela-
tively insoluble  impurities formed a  resi-
due layer. As the thickness of this layer
increased, the rate of transport of calcium
ion  from the calcium carbonate surface to
the bulk solution decreased, and, thus,
contactor  performance  decreased  with
time.
  Field   experiments  have  shown
(Letterman et al., 1987) that the tempera-
ture of the water flowing through a lime-
stone contactor can affect its performance.
For a given set  of design and operating
conditions, contactor performance tends
to decrease with decreasing temperature.
One of the objectives of this study was to
obtain a better understanding  of this rela-
tionship.
Experimental Materials,
Apparatus, and Methods

Limestones
  The  study was  conducted using  13
samples  of  limestone including  a white
marble (sample  WM) from a quarry in
Proctor,  VT, a  sedimentary limestone
(sample SL)  from a quarry near Boonville,
NY,  Black River limestone (sample BR)
from a quarry near Watertown, NY, and
10 samples  (samples A-J) from a dolo-
mite quarry near York, PA.
  A sample of each stone was powdered
and dissolved in concentrated hydrochlo-
ric acid.  Dilutions of this  solution were
used to determine the calcium, magne-
sium, iron, and  aluminum content of the
stone with a direct current plasma spec-
trometer and an  atomic absorption spec-
trophotometer.   For  a number  of  the
samples, some translucent material, prob-
ably quartz, remained after 2  days of dis-
solution. The measured calcium and
magnesium content of the samples was
used to estimate the calcite, dolomite, and
insoluble residue content of the samples.
In these calculations the magnesium was
assumed, based on x-ray diffraction and
thin-section photomicrography results, to
be associated only with dolomite.
  The  results of these calculations and
the measured iron and aluminum values
are listed in Table 1. In  several cases,
where the sum of the calcite and dolomite
fractions  was slightly greater than 100g/
100g of stone, the insoluble residue con-
tent was set  equal to zero.
  Samples WM,  SL and BR as well as a
number of the samples from the York do-
lomite quarry (samples A, B,  D, E and F)
are high calcium content limestones. Other
York samples (samples C, G, H, and I)
are predominately dolomite, and sample J
is essentially pure dolomite.
  The  WM  sample had  the highest  in-
soluble residue  content (36  g/100g) but
relatively low amounts of iron and alumi-
num (34 mg Al/100g and 71  mg Fe/100g
of stone). It is very likely that the insoluble
residue in this sample is quartz. Sample I,
from York, had the highest amount of iron
(377 mg Fe/100g) and sample H  had the
highest amount of aluminum (134 mg Al/
100g).
  The stone disks  used in  the  rotating
disk apparatus were prepared by cutting
3.10- or 2.45-cm-diameter, cylindrical cores
from pieces of rock collected at the quar-
ries. Each core was cut into  a number of
3-mm-thick disks using a rock saw. The
disk faces  were smoothed and polished
on a lapwheel with a silicon carbide abra-
sive. The back face and edge of the disks
were coated with plastic so that only the
polished face was available  for  dissolu-
tion. Each disk was  mounted in a Teflon-
coated1  brass holder (Figure 1). Between
dissolution  rate experiments, each stone
sample was "aged" by controlled  dissolu-
tion in dilute acid solution. The cumulative
amount  of  calcium  and  magnesium dis-
solved during aging was determined  by
measuring  their concentrations in the  di-
lute acid solution.

Rotating Disk Apparatus
  The reactor used in the  rotating disk
apparatus (Figure 1) was 14 cm in diam-
eter and the  clearance between the disk
and  the walls of the vessel  was greater
than 4 cm. The disk was centered about 3
cm above the bottom  of the vessel, and
its rotational  speed  was varied over the
range 200 to 1200 rpm.
  The  reactor was  constructed  with
double-glass walls. A water bath was used
to circulate water between  the walls to
maintain   the  reactor  contents  at
preselected temperatures in the range 4
  Mention of trade names or commercial products does
  not constitute endorsement or recommendation for
  use.
Table 1. Estimated Major and Minor Constituents of the Stone Samples (g/100g)

 Stone ID       Calcite       Dolomite       Insoluble          Fe
                                Al
WM
SL
BR
A
B
C
D
E
F
G
H
1
J
64
92
99
89
92
17
79
71
89
23
38
9
0
1
4
2
16
4
68
18
29
9
59
53
93
100
35
4
0
0
4
15
3
0
2
18
9
0
0
0.071
0.101
0.019
0.024
0.029
0.189
0.040
0.041
0.015
0.294
0.154
0.377
0.189
0.034
0.114
0.044
0.012
0.001
0.093
0.037
0.025
0.005
0.129
0.134
0.032
0.010

-------
                        Plexiglass
                       cover
                                           Drive shaft
                                                Drive shaft support
                                  L  Li,
                                       Limestone disk
                                                               Water
                       L
                           Reactor support
                                                                    iackete<
                                                                          reactor
Figure 1.  Schematic diagram of the rotating disk apparatus.
to 250.2C. The plexi-glass cover on the
reactor had holes for inserting the rotating
shaft, pH electrode, and wetted nitrogen
inlet tube. Additional holes were provided
for  measuring  the  temperature and
pipetting samples  for the calcium mea-
surement. The bulb of the pH  electrode
was located 1.5 cm from the rotating disk
and 3 cm above the bottom of the vessel.
  All solutions used in the  rotating disk
experiments were made with distilled and
deionized water that had been boiled for a
few minutes, several hours before use, to
remove carbon dioxide. Fisher analytical
grade (ACS Certified) chemicals were used
(KCI, N/10 HCI). The background electro-
lyte was 0.079 M KCI.

Methods
  A  free-drift  method, in which the  pH
was  allowed to increase as the carbonate
minerals dissolved  from  the stone, was
used in all experiments. Experimental  so-
lutions (600 ml) were prepared as needed
by adding potassium chloride and the re-
quired volume of acid to boiled water and
then transferring this to the reactor.
  Each experiment began  by raising  the
vessel and solution into place beneath  the
rotating disk and against the plexi-glass
cover.  Samples of solution (either 2- or 5-
ml volume) were withdrawn from the ves-
sel at 6 or 9 min intervals for a period of
1.5 hr with the use of an automatic pipette
(1  to 5 ml). The samples were stored in
polyurethane disposable test tubes at 4C
for no  longer than 2 days before the  ion
concentrations were measured by atomic
absorption spectrophotometry.

Experimental Results
  The  calcium  concentration  and  pH
change with time in a typical rotating disk
experimental run  are shown in Figures 2
and  3.  In this  example, the WM  stone
sample was used,  the rotational speed
was  600 rpm, and the initial acidity was
0.01  meq/L. At the end of the experiment
the pH was 9.04 and the calcium concen-
tration  was about 2 mg/L. For an initial
acidity of 0.01  meq/L and with no calcium
in  the  solution at t  = 0,  the  calculated
equilibrium calcium  concentration is 11.6
mg/L and  the calculated equilibrium pH is
10.02.
  The  overall  dissolution rate  constant,
k0, was determined for each experimental
run by using the  measured calcium con-
centrations and, in some cases, the mea-
sured magnesium concentrations. For the
stones that released  negligible amounts
of magnesium, the calcium concentrations
(Ct) were substituted in the  relationship,

  M  =  In {(C., - C,)/^ - CJKV/A)     (1)

  where C^ and Co are the equilibrium
and initial  calcium concentrations, respec-
tively, and A is  the  surface area of the
stone sample  disk exposed to  the solu-
tion.  Co was zero in all experiments. V, is
the volume of the solution in the rotating
disk  apparatus.  For sample WM and
samples A-J,  the limestone disk was 3.6
cm in diameter and, therefore, A was 10.17
cm2.  For the 3.1-cm diameter SL sample
and  the 2.5-cm-diameter BR sample, A
was 7.91 cm2 and 4.71 cm2, respectively.

-------
       0.0
                                       Time (hours)
Figure 2.  Calcium concentration in the rotating disk apparatus as a function of time: WM sample;
w = 600 rpm and initial acidity of 0.01 meq/L.
     10.0
      9.0 -
      60 -
      5.0
        0.0       02
                         0.4
                                  0.6       08       1.0

                                      Time (hours)
                                                           1.2       1.4
Figure 3. pH versus time for the rotating disk experiment of Figure 2.
                                                                                      The magnitude of C  was determined
                                                                                    for each experimental  run by  using  a
                                                                                    chemical equilibrium  model and  effec-
                                                                                    tive solubility products determined for  a
                                                                                    number  of the stone samples (Table 2).
                                                                                    For the  stone samples that were not in-
                                                                                    cluded in the solubility product experi-
                                                                                    ments, i.e., samples B, D,  E, G, H and J,
                                                                                    the average value of the effective solubil-
                                                                                    ity products (pK^ = 8.81) for the  samples
                                                                                    from  the same quarry was used. pKsp  =
                                                                                    8.35 was used for sample  BR because of
                                                                                    its similarity to sample SL.

                                                                                    Tables. Effective Solubility Products forCalcium
                                                                                    Carbonate and Calcium-Magnesium Carbonate
                                                                                    in Selected Limestone Samples.'

Stone sample ID
WM
SL
A
C
F
1
Negative log
of the effective
solubility product
8.20 + 0.07
8.35 0.06
8. 76 0.09
8.72 0.07
8.88 0.05
8.89 0.04
'Values are for 25C and infinite dilution.
                                                                                      As samples were withdrawn during an
                                                                                    experiment,  the  magnitude of  V,  de-
                                                                                    creased. A value of V, was calculated for
                                                                                    each value of C, using the relationship,
                                                                                      V = V  - nv
                                    (2)
  where Vo is the volume of the solution
in the  reactor at the start  of the experi-
ment,  v  is the  volume of each  sample
withdrawn  for the calcium  and magne-
sium measurements,  and  n is the  total
number of samples  withdrawn from the
reactor up  to that sample. In the dissolu-
tion rate experiments, Vo was 600 ml_ and
v was either 2 or 5 ml_.
  A straight line was fitted to the M ver-
sus time points using the method of  least
squares (M is given  by Equation 1). The
slope of this line  is  equal  to the overall
dissolution rate constant.
   Figure 4 is an M versus time plot  for a
fresh sample of  WM stone. In this experi-
ment, the disk rotational speed was 600
rpm, the  water temperature was 25C,
and the initial acidity was 0.01 meq/L. The
slope of the least squares line in Figure 4
yields an overall dissolution rate constant
of 3.3x1 O^cm/s.

Effect of Insoluble Residue on
the Dissolution Rate
   The  overall dissolution rate constant for
fresh calcitic stones (stones with low dolo-
mite content) tended to decrease as the

-------
      -2 -
      -4 -
      -8 -
     -10 -
     -12
       0.0       0.2      0.4       0.6       0.8       1.0

                                     Time (hours)
                                                           1.2
                                                                    1.4
Figure 4. Determination of the overall dissolution rate constant (ko) for the experiments of
Figures 2 and 3 (ko = o.oo33 cm/s).
estimated amount of insoluble residue in
the stone increased. We concluded that
the insoluble impurities reduced the area
of calcite exposed to the solution. To test
this hypothesis, we assumed that the area
of exposed  calcite is proportional to  the
mass percent of calcite in the stone. The
rate  constants  were "corrected"  for  the
residue content by dividing them by  the
mass percent of calcite in the stone. The
results of this  calculation,  listed  in  the
right-hand column of Table 3, show that
this correction  reduces  the effect of  the
residue content on the overall dissolution
rate constant.
  The corrected overall  dissolution  rate
constant  for the  coarse-grained WM
sample (3.12x10~3 cm/s) is somewhat less
               than the values of 3.51x10- and 3.75x10"3
               cm/s for the fine-grained SL and  BR
               samples.

               Effect of Aluminum and Iron
               Content on the Dissolution Rate
                 We observed that the extent to which
               the dissolution of calcium from the stone
               surface reduced the overall dissolution rate
               constant depended on the  aluminum and
               iron content of the stone.
                 Figure 5 shows the normalized overall
               dissolution rate  constant (i.e., the  mea-
               sured value  divided by the  initial,  fresh
               stone,  value, ko/kj  plotted versus  the
               amount of calcium dissolved from the sur-
               face of the stone,  Cad, for stones A through
               H. (The results for stones I  and J were
Table 3. Comparison of Experimental and Corrected Overall Dissolution Rate Constants (kjfor
Essentially Fresh Limestone Disks.'
          Stone
Mass %        Experimental kjdO3   Corrected k^xtO3
Calcite             (cm/s)             (cm/s)
Calcium dissolved in "aging" = 0.2 mg Ca/crrf
           WM                64
           SL                92
           BR                100

Calcium dissolved in "aging" = 0 mg Ca/crrf
            B                 92-
            F                 89
                    1.99
                    3.26
                    3.75
                    4.39
                    3.46
3.12
3.51
3.75
4.77
 3.89
'Small amounts of calcium had been dissolved from samples WM, SL, and BR before the first rate
constants were determined.
also plotted but are not shown in  Figure
5.) For stones C, I, G, and H, the overall
dissolution rate constant decreased by
more than 60% as the amount of calcium
dissolved  increased from 0 to 4 mg Ca/
cm2. For stones B, F, and J, the decrease
was less than 30%.
  Values  of k/k^ were interpolated from
Figure 5 at Cad = 2 mg Ca/cm2 and then
listed in Table 4 in rank order, from the
highest  (kyk^ = 0.90  for stone F)  to the
lowest (k^ =  0.23  for stone  G).  The
stones with the highest aluminum content
(> 25 mg Al/100g of stone) had the great-
est decrease in the overall dissolution rate
constant for this amount of calcium dis-
solved.  For  several  stones, especially
stone I with kyk^ = 0.36,  the  iron content
seemed to be an additional factor.
  Since both the iron  and aluminum con-
tent of the stone seem to determine how
sample  aging affects  the overall dissolu-
tion rate constant, a composite parameter
that includes a weighted combination  of
the  iron and aluminum  concentrations (aC^
+ bCFe) was derived, where C^  is the
aluminum  concentration  in mg AI/100 g
and  CFe is the iron concentration  in mg
Fe/100  g. The highest linear correlation
between k^ and the Fe+AI parameter
(r2 = 0.92) was obtained with  weighting
factors a=1  and b=0.3, i.e.,  (CAI + 0.30
CFe). The  quantity k/k^ and  correspond-
ing  values of (CA1 + 0.30 Cfe)  are listed in
Table 5.
  According to the results in Table 5, the
effect of iron and aluminum on the overall
dissolution rate constant will be minimized
if the quantity CAI + 0.30 CFe for the stone
is less than about 10 mg/100g.
  In a special  experiment, the brownish
residue  layer that formed on  the SL disk
was scraped into concentrated nitric acid,
ultrasonicated and the solution was ana-
lyzed for  total  soluble  aluminum.  The
soluble aluminum expressed as the
amount  per area of disk was 0.97 nmoles/
cm2  (26 ng/cm2). The scraped residue did
not dissolve completely in acid which sug-
gested the presence of alumino-silicates.
The overall dissolution rate constant for
the SL stone increased to 90% of its origi-
nal  value when  the  residue layer  was
scraped from the disk surface.

Conclusions and
Recommendations
  A heterogeneous reaction model for min-
eral dissolution, in which the rate of disso-
lution is controlled by a surface reaction
and a cation mass transfer resistance act-
ing  in series, effectively explained the re-
sults of the rotating disk experiments for
all samples except the two with the high-

-------
    1.2
                        sample A    samples   sample C   sample D
                          B     -&-     O--    X--
                        sample E    sample F   sample G   sample H
                                   468
                           Ca dissolved per unit area of disk (mg/sq cm)
                     10
Figure 5. Effect of amount of calcium dissolved from disk surface on the fractional decrease in the
overall dissolution rate constant.
Table 4. Effect of the Iron and Aluminum Content on the Fractional Decrease in the Overall Dissolution
Rate Constant (kj at Cad = 2mg Calcium Dissolved/sq cm of Limestone Surface.
Stone
ID
F
A
B
J
E
D
C
1
H
G
%
Calcite
88.9
88.9
92.1
0
70.7
78.9
16.5
8.8
38.3
22.9
kox10>'
(cm/s)
3.5
4.7
4.4
0.9
3.3
4.2
2.7
2.8
3.3
3.2
w
0.90
0.74
0.73
0.70
0.65
0.61
0.43
0.36
0.35
0.23
Fe
(mgFe/100g)
15
24
29
189
41
40
189
377
154
294
Al
(mgAI/100g)
5
12
1
10
25
37
93
32
134
129
' Interpolated from Figure 5 at Cad = 2mg Ca/crrf* kol = value when negligible Ca dissolved from the
stone.
Table 5. Effect of the Weighted Sum of Iron and Aluminum in the Limestone on the Fractional Decrease
in the Dissolution Rate Constant at 2 mg Calcium/sq cm of Limestone Surface.
      Stone
                               + 0.30C
ID
(mg/100g)
F
A
B
J
E
D
C
1
H
J
0.90
0.74
0.73
0.70
0.65
0.6J
0.43
0.36
0.35
0.23
10
19
10
67
37
49
149
145
180
217
'k = value when negligible Ca dissolved from the stone.
est dolomite content. For calcite and the
experimental conditions of this study, the
surface reaction rate was relatively large
and the rate of dissolution was essentially
mass transfer controlled. The results show
that a calcium ion diffusivity of 0.8 x 10~5
cnrWs (at 25C, can be used in predicting
the mass transfer resistance.
  The stone samples with the  highest cal-
cite content and lowest dolomite content
had the highest initial rates of  dissolution.
The magnitude of the overall dissolution
rate constant  for fresh stone decreased
by approximately 60% as the calcite con-
tent of the stone decreased from 0.92 to
0.09 g CaCO/g stone. The  rate of disso-
lution of stones with high dolomite content
may be enhanced by the presence of small
amounts of calcite. For example, the stone
that was essentially pure dolomite had a
dissolution rate constant that was  66%
less than  the  constant for  another dolo-
mitic stone with approximately 9% calcite.
When the high dolomite  content samples
were  fresh, it  appeared  that the calcium
carbonate component of  the dolomite dis-
solved faster  than the magnesium  car-
bonate component. The rate of dissolution
of magnesium was negligible in all samples
except the high dolomite content samples
(93 and 100 mass percent dolomite).
  The overall  dissolution rate constant
decreased as  the amount of calcium dis-
solved from the surface of  the stone in-
creased.   Analysis  of  several stone
surfaces, by scanning electron  microscopy
and x-ray energy spectroscopy, indicated
that the density of calcium  atoms on the
surface of the stone decreased and the
density of aluminum, silicon, and  iron in-
creased as calcium dissolved.  For a given
amount of calcium dissolved per unit area
of stone surface,  the magnitude of the
decrease  in the overall dissolution  rate
constant increased as the iron and alumi-
num content of the stone increased.  The
results suggest that the  effect of  sample
aging on the rate of dissolution is a  mini-
mum if the weighted sum of the iron and
aluminum content of the stone is less than
about 10 mg/g. The weighted sum is equal
to the aluminum content in mg Al/g  plus
0.30 times the iron content in mg Fe/g. To
minimize  the  negative effect  of  mineral
dissolution and  residue-layer  buildup on
the performance of a limestone contactor
during long-term operation, the iron  and
aluminum content should be less than this
weighted sum.
  The presence of silica as the principal
impurity in the white marble reduced the
effective surface  area of the  calcite  in
proportion to  the mass  of silica in the
sample but did not appear  to cause a

-------
reduction  in the dissolution rate of  the
calcite surface.
  The dissolution rate of calcite increased
with  increasing  temperature,  from
0.38x10"3 cm/s at 5C to 2.80x10'3 cm/s at
25C. The apparent activation energy de-
termined for the surface reaction rate con-
stant in the heterogeneous reaction model
was 101 8 kJ/mol, a value that is signifi-
cantly larger than literature values (46 to
63  kJ/mol). The apparent activation  en-
ergy for the mass  transfer rate constant
was 170.3 kJ/mol,  which is  consistent
with values in the literature for mass trans-
fer controlled kinetics.

References
  Haddad, M.,  1986.  Modeling  of Lime-
stone   Dissolution   in  Packed  Bed
Contactors Treating Dilute Acidic Water.
Ph.D. Dissertation,  Department of Civil
Engineering, Syracuse University.
  Letterman, R. D., C. T. Driscoll,  Jr., M.
Haddad and H. A. Hsu,  1987. Limestone
Bed  Contactors for Control of Corrosion
at Small Water Utilities.  A  Report for the
Water Engineering Research  Laboratory,
Office of Research and Development, U.S.
Environmental Protection Agency, Cincin-
nati, OH (EPA/600/S2-86/099).
  Letterman,  R D., M. Haddad and C. T.
Driscoll,  1991.  Limestone  Contactors:
Steady-State  Design Relationships, Jour-
nal  of  Environ. Eng.. Am.  Soc. of Civil
Engineers, 117:339-358.
  The full report was submitted in fulfill-
ment of CR 814926 by Syracuse Univer-
sity under the  sponsorship  of  the  U.S.
Environmental Protection Agency.

-------
Raymond D. Letterman is with the Department of Civil and Environmental
  Engineering, Syracuse University, Syracuse, NY 13244-1190.
Jeffrey Q. Adams is the EPA Project Officer (see below).
The complete report, entitled "Calcium Carbonate Dissolution Rate in Lime-
    stone Contactors," (Order No. PB9S-222733; Cost: $27.00, 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 Officer can be contacted at:
        Risk Reduction Engineering Laboratory
        U.S. Environmental Protection Agency
        Cincinnati, OH 45268
   United States
   Environmental Protection Agency
   Center for Environmental Research Information
   Cincinnati, OH 45268

   Official Business
   Penalty for Private Use
   $300
      BULK RATE
POSTAGE & FEES PAID
         EPA
   PERMIT No. G-35
   EPA/600/SR-95/068
                                                                         .
                                                                      ffif-REBIWV
                                                                       CHICflBOD. 68684

-------