United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/099 Feb. 1987
4>EPA Project Summary
Limestone Bed Contactors for
Control of Corrosion at
Small Water Utilities
Raymond D. Letterman, Charles T. Driscoll, Jr., Marwan Haddad,
and H. Alan Hsu
A study was conducted to investigate
the use of limestone contactors for miti-
gating corrosion in small water supply
systems that use dilute acidic (corrosive)
water. As water is transported through a
packed bed of crushed limestone, calcium
carbonate dissolves and the pH, calcium
concentration, and alkalinity increase.
A mathematical model was derived for
use in contactor design. The model is
based on the interfacial transport of
calcium ion and relates the depth of lime-
stone required in the contactor to the
desired effluent water chemistry, influent
water chemistry, limestone particle size
and shape, limestone bed porosity, water
temperature, and superficial velocity. The
model was calibrated and tested using
laboratory column experiments.
In a field contactor monitored for 2.5
years, the water quality following treat-
ment (except for the initial few months)
was essentially constant. No gradual,
long-term degradation in performance was
noted. After several months of operation,
however, the rate of CaCO3 dissolution
was not as high as that observed in the
laboratory using fresh limestone. The rate
of dissolution was possibly reduced by an
alumino-silicate residue that remained
after the CaCO3 was dissolved from the
limestone matrix. A microbiological film
may also have been a limiting factor.
Field studies indicated that limestone
contactors can effectively reduce the
tendency of water to take up corrosion
byproducts (copper, lead, and zinc) from
surfaces in piping systems. First-flush
samples of cottage tap water receiving
untreated spring water showed copper and
lead concentrations that were significantly
higher than those in contactor-treated cot-
tage tap water (1.9 ± 0.31 mg Cu/L and
0.046 ± 0.004 mg Pb/L as opposed to
0.030 ± 0.037 mg Cu/L and 0.0084 ±
0.0084 mg Pb/L, respectively).
This Project Summary was developed
by EPA's Water Engineering Research Lab-
oratory, Cincinnati, OH, to announce key
findings of the research project that is futty
documented in a separate report of the
same title (see Project Report ordering in-
formation at back).
Introduction
In many areas of the United States,
homeowners and small public and private
water supply systems use water that is
potentially corrosive to metallic materials
used in the distribution system. Corrosion
is a concern to the owners and users of
small water supply systems because of
the potential health problems associated
with the ingestion of corrosion byprod-
ucts, the degradation of the aesthetic
quality of the water, and the significant
economic consequences of piping system
deterioration.
Corrosion and contamination of the
water by corrosion byproducts may be
caused by the use of dilute (low-ionic-
strength) acidic waters that generally have
low pH, alkalinity, and concentrations of
dissolved solids. Dilute acidic ground-
waters and surface waters are found in a
number of regions of the country, particu-
larly in regions underlain by siliceous bed-
rock. The waters naturally have low buffer-
ing capacity and are corrosive. They are
also prone to acidification by atmospheric
deposition of strong acids (acid precipita-
tion) or other factors such as changes in
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land use. In some areas (for example, the
Adirondack region of New York State); it is
possible that the corrosivity of water has
been increased by acid precipitation. In
any event, until recent concern developed
about acid precipitation and the resulting
deterioration of water quality, residents
and visitors tolerated or ignored the prob-
lems caused by corrosive water. Now this
indifference has changed to a significant
concern, and many home and resort own-
ers as well as those responsible for village
water supplies have begun to adopt tech-
niques designed to mitigate drinking water
corrosivity.
Low cost is a very important criterion in
establishing the feasibility of any corrosion
mitigation technique for a small water sup-
ply system. Also, the maintenance re-
quired should be minimal, and the tech-
nique should present a low potential for
public health hazard resulting from im-
proper construction, installation, or
maintenance. Limestone contactors are
water treatment devices that generally
meet these requirements.
In a limestone contactor, water is closed
to the atmosphere and flows through and
dissolves a packed bed of crushed lime-
stone. The chemistry of the water is
altered as the limestone dissolves. Sources
of CaCO3 other than high-calcium
limestone (e.g., seashells) are sometimes
used. Limestone contactors are simple but
effective devices with low capital cost and
minimum maintenance requirements. They
have been used to neutralize acid mine
drainage, acidic industrial wastes, and
dilute acidic surface waters.
The overall objective of this project was
to investigate the use of limestone contac-
tors for mitigating corrosion in small water
supply systems that use dilute acidic
water. The research plan included the
development and testing of a rational
method for contactor design and the eval-
uation of the field operation of a contactor
with respect to corrosion control and oper-
ation and maintenance problems. The
study had the following specific
objectives:
0)~to derive and test a mathematical
model for limestone contactor de-
sign, using laboratory, column-type
reactors,
(2) to develop design objectives by ex-
perimentally determining the rela-
tionship between contactor-treated
water quality and metal release from
pipes, and
(3) to evaluate the practical application
of the design equations and objec-
tives by monitoring the field per-
formance of full-scale contactors
and to determine the feasibility of
long-term operation and the type
and frequency of maintenance
required.
Results
As dilute acidic water is transported
through a packed bed of crushed lime-
stone, calcium carbonate in the limestone
dissolves, the pH, calcium ion concentra-
tion, and alkalinity increase, and these
characteristics generally tend to mitigate
the dissolution of corrosion byproducts
from surfaces in piping systems.
A laboratory study was conducted using
packed-column contactors (Figures 1 and
2). Results were used to develop and test
a numerical model for contactor design.
The design equations are based on the
interfacial transport of calcium ion. Limited
axial dispersion was also considered. Ac-
cording to the model, the depth of lime-
stone, L, required to achieve a given level
of treatment is given by
L =
1n[(Ceq - CbL)/(Ceq - 0^)]
Us
- 2 d
where a is the interfacial area of limestone
per unit volume of interstitial water, t is the
bed porosity, Us is the superficial velocity,
d is the effective diameter of the limestone
particles, and K0 is the overall CaC03
dissolution rate constant. Cbo is the in-
Figure 1. Laboratory columns with water supply and flow control system. Insert is a drawing
of a typical through-the-watl sampling tube.
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fluent calcium concentration. The quantity The results of this study indicate that
a is calculated using d and the particle K0 can be estimated using a correlation of
sphericity. dimensionless mass transfer parameters.
Plan
Limestone
Contactor
Chamber
Limestone
Contactor
Chamber
Figure 2. Diagram showing the installation of the baffled-box contactor in the spring at
Covewood.
For low values of a modified Reynolds
number (1< MRe < 30)
K0 = 5.70
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bon dioxide can have a significant effect
on the pH of the solution and hence on the
tendency of the water to dissolve corro-
sion byproducts. When the influent D1C is
high (e.g., greater than 10 mg C/U.
equilibration of the effluent with the at-
mosphere causes the pH to increase.
When the influent DIG concentration is
less than several mg C/L, the pH tends to
decrease.
Results of the study suggest that dilute
acidic waters facilitate the release of
elevated concentrations of trace metals
from metal piping systems. Passivation
films of most significance include
Cu2
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