United States
Environmental Protection
Agency
Municipal Environmental
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-065 Apr. 1984
Project Summary
/ i
Seattle Distribution System
Corrosion Control Study:
Volume II. Tolt River Water
Pilot Plant Study
Carlos E. Herrera and Brian P. Hoyt
The Seattle, Washington, Water
Department conducted a 6-month pilot
plant study of corrosion control of Tolt
River water through treatment with (1)
lime and sodium carbonate, (2) lime and
sodium bicarbonate, and (3) lime, so-
dium bicarbonate, and silicate. Con-
tinuous-flow pipe coupon tests were
conducted to determine corrosion rates,
penetration rates, and corrosion types
for copper, galvanized steel, and black
steel pipes. Metal leaching tests were
conducted using small-diameter pipes.
Results showed that using lime and
sodium carbonate, lime and sodium
bicarbonate, or lime, sodium bicar-
bonate, and silicate will significantly
reduce corrosion in home plumbing
systems. Copper and galvanized steel
showed the following respective corro-
sion rate reductions: 86% and 24% with
lime and sodium carbonate, 85% and
43% with lime and sodium bicarbonate,
and 83% and 49% with lime, sodium
bicarbonate, and silicate. Metal-
leaching tests showed that all the
treatments significantly reduced lead,
zinc, and copper levels in water. As a
result of this pilot study, lime and
sodium carbonate treatment is recom-
mended for both the Tolt and Cedar
River water supplies at an average
dosage of 1.7 mg/L CaO. This dose
should achieve an average distribution
system pH of 7.45 and 7.9 and an alka-
linity of 14 and 18 mg/L CaCO,, respec-
tively.
The Project Summary was developed
by EPA's Municipal Environmental Re-
search 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
back).
Introduction
Corrosion-related problems have plagued
water supply customers in the Seattle,
Washington, area for many years. Customer
complaints of rusty water, fixtures stained
red and blue, and metallic tastes are com-
mon. Furthermore, hot water galvanized and
copper pipes have an expected life of only
25 years in this system. Levels of lead, cop-
per, and iron in overnight-standing tap water
are also a concern, since they frequently ex-
ceed limits defined by the National Interim
Primary Drinking Water Regulations and the
National Secondary Drinking Water Regula-
tions. High levels of cadmium and zinc are
also found after overnight standing in home
plumbing, but they rarely exceed Federal
limits. Though the health impact of such
high metal levels poses no acute problem,
exposure should be reduced whenever
possible.
This corrosion of plumbing and the as-
sociated degradation in water quality has
been a major concern of the Seattle Water
Department (SWD) for many years. This
project is the result of a 1975 study commis-
sioned by the SWD to analyze the corrosion
problem and to recommend possible solu-
tions. The earlier study confirmed the ex-
istence, causes, and impacts of corrosive
water, and it identified alternative measures
for reducing the corrosiveness. The purpose
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of this project was to determine which of
three suggested treatments was most effec-
tive in controlling plumbing corrosion caused
by water from the Tolt River water supply
system.
Description of the Tolt
Supply System
Basic Characteristics
The SWD serves an averageof 161 million
gallons per day (MGD) of high quality water
to nearly 1 million people in the greater Seat-
tle area. The water originates in the Cascade
Mountains from two sources — the Cedar
and Tolt Rivers. The watersheds are well pro-
tected, and the water requires only disinfec-
tion with gaseous chlorine to meet Federal
standards.
The Tolt River water supply system was
developed in 1964 and serves about one-third
of the greater Seattle area. The remaining
two-thirds is supplied by the Cedar River
system. Because these mountain waters are
composed predominantly of rainfall and
snowmelt runoff, they are very soft and tend
to be highly corrosive to the unlined, metallic
pipes in home plumbing systems.
Causes of Corrosion
The corrosiveness of Tolt water results
from several related factors, including:
• Acidity, as indicated by low pH. (The
raw Tolt water pH is approximately 6.7;
after chlorination and fluoridation, pH is
reduced to 5.8 to 6.2).
• Dissolved oxygen concentration at
saturated conditions.
• Insufficient calcium and bicarbonate
alkalinity in the water to form protective
calcium carbonate films on pipe
surfaces.
• A relatively high (halogen +
suHate) I alkalinity ratio; (halogen +
SO4= )/alk of 2.5 that results in condi-
tions favorable to pitting corrosion.
In 1970, three factors combined to inten-
sify the corrosiveness of this water supply.
First, the chlorine dosage at the open
distribution reservoir outlets was increased
to decrease the occurrence of positive
bacteriological samples within the distribu-
tion system. Second, at the request of the
U.S. Public Health Service, ammoniation of
the water supply was stopped to enable a
free chlorine residual to be maintained
throughout the distribution system. This
change from combined chlorination to free
chlorination was implemented to provide
quicker, more effective disinfection of the
unfiltered water supply. The third factor was
fluoridation with hydrofluorosilicic acid,
which began in 1970 based on a vote of the
Seattle citizens in 1968.
Internal Corrosion Study
In December 1975, the City of Seattle re-
tained a consulting engineering firm to per-
form a detailed analysis of the corrosion
problem and to recommend possible solu-
tions. The Internal Corrosion Study, which
included a 9-month pilot plant investigation,
confirmed the corrosiveness of Seattle
water, the causes of corrosion, and the im-
pacts associated with the corrosive water;
it also evaluated alternative measures to
reduce the corrosiveness of the water sup-
ply. Alternative methods to reduce corrosion
included changing the methods of disinfec-
tion and fluoridation, blending the water
supply with ground water supplies, and add-
ing corrosion-inhibiting chemicals.
Based on the findings of this study, an In-
ternal Corrosion Control Management Plan
was developed. Because the very low levels
of mineral solids, pH, and alkalinity con-
stitute the major causes of corrosiveness,
this plan was designed to correct the natural
deficiency of minerals in Seattle's water
through chemical addition.
The consultant recommended water qual-
ity goals using various chemical combina-
tions that included the addition of lime and
sodium bicarbonate. The actual selection of
chemical combinations and optimum
dosages became the task of the Seattle
Water Department.
Scope of Work
This research was performed to determine
which treatment best controls plumbing cor-
rosion caused by Tolt water — lime and
sodium carbonate, lime and sodium bicar-
bonate, or lime and sodium bicarbonate and
silicate. The scope of work included the
following:
1. Determining the corrosion rates,
penetration rates, and corrosion types
of copper, galvanized steel, and iron
pipe exposed to untreated water and to
water treated with lime and sodium car-
bonate, lime and sodium bicarbonate,
and lime, sodium bicarbonate, and
silicate;
2. Predicting increases or reductions in the
life of residential pipe;
3. Determining metal leaching levels from
galvanized pipe, copper pipe, lead/tin
and tin/antimony solder associated with
each treatment; and
4. Establishing the optimal full-scale
chemical dosages required for both
treatments.
From May 1980 to December 1980, four
continuous-flow corrosion test apparatus
were operated at Seattle's Tolt Pump Sta-
tion. Water used for the tests was either un-
treated (control) or treated with lime and
sodium carbonate, lime and sodium bicar-
bonate, or lime, sodium bicarbonate, and
silicate. This test site was chosen because
it has the same water quality that is delivered
to the SWD customers. The targeted water
quality characteristics for the tests are
presented in Table 1.
Corrosion coupon tests were used to
document average corrosion rates based on
weight loss, penetration rates based on pit
depths, and corrosion types based on visual
observations.
To evaluate the effects of treatment on the
quality of overnight-standing water, SWD
developed a new test method: Small-
diameter pipe sections attached to the main
test apparatus were used to determine cop-
per, lead, cadmium, and zinc leaching levels
from galvanized pipe, copper pipe, lead/tin
solder, and tin/antimony solder.
Procedures
The pilot test apparatus consisted of a
continuous-flow test unit and a metal pick-
up test unit (Figure 1). The continuous-flow
test unit contained four pipe segment
coupons (10.2 cm long and 2.54 cm in
diameter) of copper, galvanized iron, and
black steel pipe. A velocity of 0.30 m/s
through the pipe coupons was held stable
by the use of a constant head reservoir. The
coupons were removed from the test loop
periodically to be cleaned and weighed.
Weight loss for each coupon was calculated
and plotted over time for each metal and
treatment (Figure2). Average corrosion rates
were then calculated from these curves.
Results and Discussion
Treated water showed substantially lower
corrosion rates than the untreated control
(Table 2). Copper corrosion rates were ap-
proximately equal for the three treatments
and were measured in mils per year (mpy).
Treatment with lime, sodium bicarbonate,
and silicate produced the lowest zinc and
iron corrosion rates.
Penetration rates were determined for the
black steel coupons. The coupons were split
into quarters, and the pit depths were mea-
sured using either a pointed tip micrometer
or a binocular microscope with a graduated
fine focus adjustment. Penetration rates
were then calculated in mpy based on the
average of the 10 deepest pits on each
specimen.
The penetration rate results were not as
promising as those for corrosion rates.
Average iron penetration rates for the con-
trol and for water treated with lime and
sodium carbonate, lime and sodium bicar-
bonate, and lime, sodium bicarbonate, and
silicate were 44.6,49.1,43.7, and 53.7 mpy,
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Table 1. Targeted Water Quality for Pilot Tests
Characteristic*
pH
Alkalinity
Img/L CaC03
Chlorine
Residual
Lime dosage.
CaO
/Va2CO3
dosage
NaHCO3
dosage
NaO/SiO2
dosage
(as SiOJ
Loop #1
(Control,
Tolt
Distribution)
5.8-6.2
2.0
0.2-0.4
0
0
0
0
Loop #2
(Lime
+
Soda Ash)
7.85
14.5
0.2-0.4
1.5
10.0
0
0
Loop #3
(Lime
+
Bicarbonate)
7.95
21
0.2-0.4
4.5
0
20.0
0
Loop #4
(Lime +
Bicarbonate
+ Silicate)
7.95
21
0.2-0.4
3.5
0
20.0
4
' All characteristic measurements, except ph, are in mg/L.
Cot
1
istant Head
Reservoir
\
s c ^
/ Chemical
/ Pump
Static 1 -l l
Corrosion-
Inhibiting
Chemicals
/W/y»r "I 1 1 1 1
1 ' Galvanized Steel Pipe Coupons
~\ \
1 I
1
1
Black Steel Pipe Coupons
1 1 I
' 1 1
Copper Pipe Coupons
4 _ . — I
1,
rv.
Water Meter
-L
Metal Leaching Loop:Galvanized Steel Pipe. Iron Pipe.
Copper Pipe Plus SO/SO Lead/ Tin
Solder. Copper Pipe + 35/5 Tin/
Antimony Solder, and Copper Pipe
Figure 1. Pilot plant flow schematic.
respectively. These results demonstrated
that treatment produced increases or only
small reductions in penetration rates.
The metal leaching test unit consisted of
25- and 51-cm lengths of 0.635-cm-diameter
pipes attached to the continuous flow test
unit. The leaching pipe sections were made
of copper, copper plus 50/50 lead-tin (Pb-
Sn) solder, copper plus 95/5 tin-antimony
(Sn-Sb) solder, galvanized steel (new), and
black iron pipe. These sections were ana-
lyzed for copper, lead, zinc, cadmium, and
iron leaching. Two velocities were used (0.12
m/s and 0.18 m/s) in establishing corrosion
films in the metal leaching sections. The pipe
sections were then periodically removed, and
dissolved metals were measured in the
laboratory after approximately 24-hour con-
tract with test water.
The treatments resulted in substantial
reductions of lead and copper leaching from
copper pipe plus 50/50 lead-tin solder and
from copper pipe plus 95/5 tin-antimony
solder. Substantial reductions of zinc
leaching from galvanized pipe were also
realized.
Conclusions
The following conclusions have been
reached for each chemical treatment alter-
native, based on the data in Table 3.
Lime and Sodium Carbonate
Treatment
Based on a matrix analysis of alternatives,
the lime and sodium carbonate treatment
produced the most cost effective protection
against corrosion and metal leaching. This
treatment produced better reductions of zinc
leaching from galvanized pipe and lead
leaching from lead/tin solder than did treat-
ment with lime and sodium bicarbonate or
with lime, sodium bicarbonate, and silicate.
Approximately equal reductions in copper
corrosion were achieved with all three
treatments (Figure 3).
Lime and sodium carbonate treatment is
by far the least expensive treatment; and
because both lime and sodium carbonate can
be used alone for pH control, a lime and
sodium carbonate system can maintain a
more consistent water quality during periods
of equipment breakdown.
Lime and Sodium Bicarbonate
Treatment
The lime and sodium bicarbonate treat-
ment did not produce the best results in any
category; but it did produce better reduc-
tions than the lime and sodium carbonate
treatment in three areas: galvanized steel
corrosion rates, copper leaching from cop-
per pipe, and lead leaching from tin /anti-
mony solder.
Lime, Sodium Bicarbonate,
and Silicate Treatment
The lime, sodium bicarbonate, and silicate
treatment demonstrated the best reductions
in four areas: copper leaching from copper
pipe, lead leaching from tin/antimony solder,
and galvanized steel and black iron corrosion
rates. Although the addition of silicate pro-
duced larger reductions in the corrosion of
these items, the added expense of feeding
silicate is not justified.
No Treatment
Untreated (control) water samples
demonstrated greater corrosion rates and
metal leachate levels than did treated water
except in three areas: black steel corrosion
rates, and lead and cadmium leaching from
galvanized pipe.
Recom mendations
Results of the pilot plant test program
demonstrate that lime and sodium carbonate
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9.0
8.0
7.0
6.0
5.0
§. 4.0
-------
3.0
2.5
"
8
fi 1.0
0.5
• = Loop ft 1 (Control)
ft = Loop #2 (Lime/Sodium Carbonate Treatment)
•Jfr = Loop #3 (Lime/Bicarbonate Treatment)
•k = Loop #4 (Lime/Sodium Bicarbonate/Silicate Treatment)
20
40
60
80
100 120 14Cij 160 180 200
0.12 m/s 0.18m/s
Time (Days)
Figure 3. Copper leaching from copper piping & 50/50 lead/tin solder during the Tolt pilot
plant test.
Table 4. Recommended Chemical Dosages, Target Water Quality, and Chemical Costs for the
Tolt Water Supply
Item
Treatment plant CO2
Treatment plant pH
Average total alkalinity ICaC03)
Atmospheric equilibrium pH @ 15°C
Minimum distribution pH
Estimated average distribution pH
Average distribution conductivity
(Halogen + SOf )/alk ratio
Average lime dosage fmg/L CaO)
Average sodium carbonate dosage (Na2CO3l
Chemical cost per year ($)
Present
Conditions
6.0*
6.0*
2.5*
6.95
5.6*
5.9*
27.0*
2.5-5.5t
—
—
-
With Lime Plus
Sodium Carbonate Treatment
0.0
8.3
14.0
7.75
7.45t
7.85
42.0
0.45-1. n
1.7
9.0
147,000t
* Based on SWD water quality monitoring data.
t By calculation.
t Based in cost of chemicals delivered to the Tolt regulating basin in June 1981.
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Carlos E. Herrera and Brian P. Hoyt are with Seattle Water Department, Seattle,
WA 98144.
Marvin C. Gardels is the EPA Project Officer (see below).
The complete report, entitled "Seattle Distribution System Corrosion Control
Study: Volume II. Tolt River Water Pilot Plant Study," (Order No. PB84-170 810;
Cost: $7.00, 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:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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