United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/042 Nov. 1987
Project Summary
Corrosion in Water Distribution
Systems of the Pacific
Northwest
Steve H. Reiber, John F. Ferguson, Mark M. Benjamin, and
Dimitris Spyridakis
Continuous linear polarization mea-
surements (10-min increments) of
corrosion rates on copper surfaces
demonstrated an immediate response
to changing water quality conditions
found in Pacific Northwest distribution
systems. Significant relationships were
found between copper corrosion rates,
pH, and free chlorine residual in these
low-mineral waters. Regression analy-
sis of copper corrosion rates versus four
common chemical quality parameters
was used to develop a statistical model
of the relationship of copper corrosion
to changing water quality conditions.
This study evaluated the impact of
changing water quality conditions on
copper surfaces coated with varying
depths of oxide films. The presence of
a well aged film (greater than 50 /urn
depth) generally decreased the corro-
sion rate by more than 50 percent when
compared with corrosion on surfaces
coated with a relatively fresh film layer
(less than 20 /urn depth).
The Seattle Water Department's
program of corrosion control through
lime and soda ash addition was eval-
uated during its 12-month implemen-
tation period. A gradual pH increase of
approximately 2.0 units during this
period decreased the corrosion rates of
aged copper surfaces by approximately
9 /urn/year.
Selected chemical water quality
parameters were measured in the
distribution networks of Seattle,
Tacoma, and Anacortes. Significant
differences in quality variation were
found as a function of distribution level.
The higher the distribution level (i.e.,
the closer to the consumer tap), the
greater the degree of variation. More
elaborate treatment processes (rapid
sand filtration preceded by coagulation
and flocculation) induced a greater
degree of variation in the distribution
system than systems that did not filter
their water.
Particle counts and turbidity levels in
the distribution system were only
partially correlated. The strongest
turbidity correlation was found for
particles of less than 10-//m diameter,
which also constituted the great major-
ity of particles in all the waters
measured.
This Project Summary was devel-
oped by EPA's Water Engineering
Research Laboratory, Cincinnati, OH,
to announce key findings of the
research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
The Pacific Northwest has some excep-
tionally pristine surface water sources of
unusually low mineral content and high
clarity. These sources are extremely low
in alkalinity, pH, and buffer intensity, and
they are undersaturated with respect to
CaCOs- The potential for deposition of
protective carbonate films is minimal,
and excessive metal corrosion on plumb-
ing appurtenances is a common problem.
The general objective of this study was
to demonstrate the effectiveness and
capabilities of the U.S. Environmental
Protection Agency mobile water quality
monitoring laboratory (MWQML) in
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monitoring the variation of water quality
parameters and the impact of chemical
dosing on corrosion rates in water
systems of the Pacific Northwest. Other
activities included evalution of the
Seattle Water Department's corrosion
control program, determination of how
instantaneous copper corrosion rates are
affected by short-term variation in
selected water quality parameters, and
an assessment of the relationship
between distribution system water qual-
ity and distribution level, raw water
source, and treatment level.
The communities of Seattle, Tacoma,
and Anacortes participated in this Study.
Selection was based on raw water source
(which for all cases was surface water
originating on the western slopes of the
Cascade Mountains), the type of treat-
ment provided, and the history of
corrosion-related problems within the
system. Table 1 summarizes some basic
information about the sampling program
distribution systems
Procedures
The MWQML is a self-contained,
mobile, microprocessor-controlled labo-
ratory capable of continuously sampling,
analyzing, and recording 20 water quality
and corrosion parameters. Specific
capabilities of the MWQML are directed
at the automated monitoring of common
chemical quality parameters in the
distribution system, including pH,
temperature, conductivity, chlorine
residual, turbidity, alkalinity, hardness,
chloride, and others. Specialized instru-
mentation includes a three-channel
Petrolite* linear-polarization corrosion-
rate monitor, an HIAC/Royko particle
counter, and an loinics digital titrator.
Control and data acquisition functions
are provided by an HP 9845B micropro-
cessor. Data storage is on floppy disk.
Standard procedure was to locate the
MWQML at selected sampling sites
within the distribution systems for
periods of 2 to 4 weeks. This time was
necessary to develop a sufficient data
base of diurnal chemical variations.
Sampling sites included endline user
taps, raw water sources, reservoir
outlets, water blending points, and other
points of interest within the systems.
Data collected and stored by the MWQML
were periodically uploaded by means of
modem connection to the Cyber 1500
Table 1. Information Summary on Sampling Program Distribution Systems
Service
Community
Seattle
(North)
Tacoma
Anacortes
Raw Water
Pop.
500,000+
70,000+
20,000+
Source
So. Fork Toll River
Green River
Skagit River
Treatment
Screening and Cli ana
F-corrosion control
initiated 6/82
C/2 only
C/z, coagulation, floe-
culation, and rapid
sand filtration
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use
system at the University of Washington
Academic Computing Center. Basic
statistical analysis and data reduction
were performed using the 1984 edition
of "Minitab" from Pennsylvania State
University.
Corrosion rate measurements were
made with a three-channel Petrolite
model 1000 linear polarization instru-
ment. Two different sets of copper
electrodes were used to approximate
copper plumbing with different degrees
of oxide coating and age. The fresh
electrode set was relatively clean and
covered with a cuprous oxide layer of less
than 20 /urn depth. The aged electrode
set was coated with a heavy oxide layer
exceeding 50 /urn.
Results and Discussion
The Seattle Corrosion Control
Program
Seattle bega n its program of corrosion
control in June 1982. The program
called for the gradual increase of
distribution water pH and alkalinity
through the addition of lime and soda
ash. Target dosages for the two chem-
icals were 2.0 and 9.0 mg/L, respec-
tively, which resulted in an average
overall alkalinity increase of 5.0 to 17.0
mg/L (as CaCOs). The adjustment period
extended over 12 months. During this
period, copper corrosion rates were
measured on aged surfaces at selected
sites throughout the northern Seattle
distribution area. Figure 1 summarizes
weekly median pH values and aged
copper surface corrosion rates during
the adjustment period. Overall, the pH
was shifted from a median distribution
level of 5.8 at the beginning of the
program to a median level of 7.7 after
12 months of operation, producing a-
reciprocal decrease in corrosion rates
from a weekly average of 20 to 12/um/'
year, a decrease of approximately 40
percent.
Figure 2 presents a graph of weekly
median rates for aged copper corrosion
as a function of median pH. The inverse
relationship between pH and the distri-
bution copper corrosion rate is clear.
Water Quality Variation and
Copper Corrosion
Monitoring for short-term water
quality variations and their impact on
copper corrosion rates was conducted
at several points in each of the sampled
distribution systems. Figure 3 presents
a 5-day summary of water quality and
copper corrosion rate variations at one
end-line tap in the northern Seattle
distribution area. The strong positive
correlation of both the aged and fresh
electrode corrosion rates with the free
chlorine residual is apparent. Also clear
is a pronounced inverse relationship of
the corrosion rates with the pH changes,
including the larger pH spikes. Influence
of temperature and conductivity are not
readily apparent.
Multiple linear regression on lumped
data sets was performed to evaluate the
relative importance of the four water
quality parameters on both the fresh and
aged copper surfaces. A forward, step-
wise regression was used with four
independent parameters regressed in
the following order: (1) pH, (2) free
chlorine, (3) temperature, (4) conductiv-
ity. Table 2 presents the linear regres-
sion model developed for corrosion rates
on the aged and fresh surfaces. Table
3 presents the analysis of variance
(ANOVA) for the two regression models.
From this table, it is apparent that the
regression models account for 95 and
76 percent, respectively, of the overall
corrosion rate variance on the two
surfaces, and that in both cases, the free
chlorine residual is the most significant
predictor.
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w
9
8
' 7
6
5
,00
Ooooo
° °
°°00000
pH (Weekly Median)
oo0oo0oo
30
0 0000
n
°000000 00
Copper Corrosion Rates
°°0o°°o o 0°°°
40
SO
0 10 20 30
June 82 Weekly Progression
Figure 1. A verage weekly pH and copper corrosion rates in the Tolt distribution area during the first year of the control program.
60
10 T
9-
8-
7-
8°8g
8
Joo
0 0
Corrosion rates measured at
selected points throughout
the distribution system.
5 W 15
Copper Corrosion Rates um/yr
Figure 2. Copper corrosion rates in the Seattle System as a function of weekly median pH.
20
25
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30
25 <
20 <
Fresh Cu Electrode
Aged Cu Electrode
8 *
7 ,
18
17
16
15
Temperature
031
-J
^02
0.1'
0
Total Chlorine
24
48
0
9/6/83 Experimental Progression (hours)
Figure 3. Variation of water quality and corrosion rate with time (data set S906).
4
72
96
120
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Table 2.
Parameter
Linear Regression Models of Copper Corrosion on Aged and Fresh Surfaces
Coefficient
Units
AgedSurface
Fresh Surf ace
Corrosion rate
Y intercept
pH
Free chlorine residual
Temperature
Conductivity
lim/year
—
pH units
mg/L
°C
umhos/cm
0.553
-0.078
0.810
0.026
0.005
1.820
-0.091
1.013
0.013
0.0009
Table 3.
Analysis of Variance for Aged and Fresh Copper Surface Corrosion Regression
Models
Item
Total
Residual
Regression
pH
Free Chlorine
Temperature
Conductivity
Degrees of
Freedom
350
346
4
1
J
r
1
Sequential Sum
of Squares
Aged
2.357
0.118
2.238
0.354
1.491
0.385
0.008
Fresh
1.543
0.373
1.170
0.001
1.055
0.076
0.036
Coefficient of
Determination R2
Aged
0.950
0.15
0632
0.163
0.003
Fresh
0.758
0.001
0.684
O.050
0.023
Quality Variation in the
Distribution Network
In each of the sampled communities,
comparisons were made of specific
chemical parameters at different levels
of the distribution system to evaluate the
impact of residence time and system
contact on parameter variability. One
example appears in Figure 4 where free
chlorine residual at the point of dis-
charge from the Anacortes treatment
plant is compared with the residual
found at an end-line tap at the extreme
end of the distribution system. Water
discharged from the plant carries a
residual that varies erratically from 0.4
to 0.6 mg/L. The unevenness of the
residual is apparently due to minor flow
and dosage variations and incomplete
mixing in the clear well. Chlorine
residuals at the consumer tap are not
erratic, but they show a smooth and
pronounced diurnal variation. The high-
est concentration corresponds with the
period of peak demand, indicating that
retention time in the system affects
residual levels. Presumably, longer
contact times provide greater opportun-
ity for chlorine depletion through reac-
tion with plumbing materials and dis-
solved chlorine-consuming substances.
Similar variation and periodicity were
observed in other chemical and corro-
sion parameters in all of the distribution
systems examined.
Turbidity Versus Particle
Concentration
For the northern Seattle distribution
system, a comparison was made of
turbidity values measured by a
continuous-flow-ratio turbidimeter with
particle concentrations measured by an
HIAC/Royko counter. Figure 5 presents
4 days of concurrent turbidity and
particle data, with the latter broken
down into three different size ranges.
A moderately strong correlation exists
between the smallest particle size
concentration and the turbidity data (R
= .687). However, the correlation dimin-
ishes dramatically for the larger particle
concentrations. These and other results
indicate that the smaller particle sizes
(less than 2 fim diameter), which numer-
ically make up the great majority of
particles in all the distribution systems
examined, also contribute the most to
turbidity variation.
Regression analysis of turbidity data
using the three particle-size-range
concentrations as the independent
predictors showed that the particle
concentrations accounted for only 50
percent of the observed turbidity vari-
ation. Filtered particle samples were
examined by scanning electron micros-
copy, which revealed that the bulk of
the smaller particles consisted of sili-
ceous diatom skeletons and mineral
agglomerates. Evidence indicates that
the mineral agglomerates may be cor-
rosion products generated on exposed
steel surfaces in the distribution system.
Most of the observed diatom forms were
common to clean surface water sources
and may result from algae growth in
impoundments and on open reservoir
embankments.
Conclusions
This study has explored the relation-
ship between water quality variation
distribution system location and copper
corrosion rates. Some of the principal
conclusions drawn from the study are
as follows:
1. Seattle's program of corrosion
control through moderate pH
adjustment and alkalinity addition
has effectively reduced corrosion of
copper plumbing materials by
approximately 40 percent.
2. Free chlorine residual is the most
important of the common water
quality parameters for predicting
copper corrosion rates. In some
cases over half of the variation in
corrosion rate can be attributed to
variation in the free chlorine
residual.
3. Variable distribution system resi-
dence time as a result of demand
cycling can induce substantial
variation in pH, turbidity, and
chlorine residual of the delivered
water. Some treatment processes
(filtration, coagulation/floccula-
tion) induce further variation by
chemically destabilizing the water,
which is followed by reequilibra-
tion within the distribution system.
4. Turbidity is only partially useful as
a predictor of particle concentra-
tion. Small particles of less than 5-
/jm diameter correlate most closely
with turbidity levels, however, the
concentration variation of all size
ranges studied accounted for only
60 percent of the overall variation
in observed turbidity levels.
The full report was submitted in
fulfillment of Cooperative Agreement No.
810508 by the Department of Civil
Engineering, University of Washington,
under the sponsorship of the U.S.
Environmental Protection Agency.
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7 *
0.5 '
o
tb
0.5 i
•*•
24
Finished Water
Endline Tap
48 72
Experimental Progression (hours)
Figure 4. Comparison of free chlorine residuals in finished and delivered water in the Anacortes System.
96
120
20
10
0
200
700
2000
0.75'
0.5'
0.25
Particle Cone. (20-30 urn)
/?,„* = .304
Particle Cone. (10-20im)
flturb = .534
Particle Cone. (2-10 fjm)
= • 687
Turbidity
•#•
24
48 72
Experimental Progression (hours)
96
120
Figure 5. Comparison of turbidity and particle concentrations in the Seattle Water Distribution System (U. of Wa. Campus).
6
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John F. Ferguson, Mark M. Benjamin, Dimitris Spyridakis, and Steve H. Reiber
are with the University of Washington, Seattle, WA 98195.
Marvin Gardels is the EPA Project Officer (see below).
The complete report entitled "Corrosion in Water Distribution Systems of the
Pacific Northwest." (Order No. PB 87-197 521/AS; Cost: $13.95, subject
to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering 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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