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Modern Applied Science; Vol. 10, No. 3; 2016
ISSN 1913-1844 E-ISSN 1913-1852
Published by Canadian Center of Science and Education
166
Scale Formation and Corrosion of Drinking Water Pipes: A Case
Study of Drinking Water Distribution System of Shiraz City
Nima Tavanpour1, Masoud Noshadi1 & Navid Tavanpour2
1 Department of Water Engineering, College of Agriculture, Shiraz University, Shiraz, Iran
2 Department of Water Engineering, College of Agriculture, Shiraz Azad University, Shiraz, Iran
Correspondence: Masoud Noshadi, Department of Water Engineering, College of Agriculture, Shiraz University,
Shiraz, Iran. E-mail: esfehani.mohamad3@gmail.com
Received: January 18, 2016 Accepted: February 5, 2016 Online Published: February 29, 2016
doi:10.5539/mas.v10n3p166 URL: http://dx.doi.org/10.5539/mas.v10n3p166
Abstract
Due to increased consumption of drinking water in the world, there are attempts to apply new solutions for
accessing sufficient amounts of water with proper quality. In addition, efficient usage of energy and finding a
solution for reducing the scale-related problems in drinking water pipes are among major concerns of urban
water supply. Annually, significant budgets are assigned for removing the scale of pipes as well as home and
industrial devices. The present study endeavors to examine the scale-formation and corrosion potential of
drinking water in drinking water distribution network of Shiraz City (south of Iran). This study is of descriptive
type conducted through Langelier, Puckorius, Ryznar, Larson and aggressive indices and taking 144 samples of
different sources and points of distribution network. The results showed that the mean values of LI, RI, LS and
AI were 0.07 (slightly scale forming), 7.1 (non-scale forming), 1.2 (corrosive) and 14 (non-corrosive),
respectively. The mean of scale formation rate value in Shiraz drinking water pipes was 0.26 mm/y. Accordingly,
zones located in east, southeast and south of Shiraz have more problems of scale formation. Scale composition of
33 home pipe samples and 8 network pipe samples were analyzed by X-ray diffraction method. Results indicated
that main compositions in scale samples were calcium carbonate, calcium sulfate, magnesium carbonate,
magnesium sulfate, hematite, maghemite, magnetite, goethite, zinc oxide, gypsum, vivianite, dolomite,
hydroxyapatite and troilite. Main elements in scale samples were magnesium, silicon, phosphorus, sulfur, zinc,
copper and lead.
Keywords: scale-formation indices, corrosion, scale type, Shiraz
1. Introduction
In most of the cases, the mineral waters and springs used without typical treatment measures include mineral
materials but they should lack mineral materials of external origins (3). Because of increased consumption of
drinking water in all over the world, there are endeavors to find new solutions for accessing proper amounts of
water with good quality. Depending on source, different seasons of the year and the lands over which they flow,
these waters have highly varied compounds and diverse amounts of dissolved materials. Along with
hardness-causing ions, water may include other cations such as iron, sodium, potassium and different levels of
anions such as chloride, sulfate, nitrate, bicarbonate (5). Wherever natural water is used, settling of water scale is
one of the major problems. The drinking waters are usually saturated with calcium carbonate (19). Annually,
huge numbers are expended for removing the scale of pipes as well as home and industrial devices. Scale
formation occurs when water includes large quantities of calcium carbonate which may form lumps on the
surfaces. Excessive levels of scale-formation may be disruptive. Scale-formation may cause digestive problems
too. In addition, scale formation may reduce the internal diameter of channels of distribution and transfer
systems. As a result, the pipe may not have the capacity to transfer the expected amount of water. As a result, one
should repair the distribution system or replaces it; both of these options impose high economic expenses (12).
In general, one could point to the following problems or scale formation:
- Water leakage in sliding tapes, generation of stains on external surface of pipes (10)
- Pressure drop and reduced flow rate of water in pipes
- Bursting of pipes due to excessive pressure applied on them and reduced lifetime of water pipes
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- Clogging of filters and reduced efficiency of pumps
- Reduced operating efficiency of heating and cooling devices (5)
Ghezavati et al., (2009) analyzed the qualitative parameters of water and corrosion potential of water supply
sources for Bandar-Abbas City and Refinery. The results suggested that the parameters of chemical quality of
water were in normal ranges with exception of total dissolved solids which was higher than average (7). To
determine the corrosion and scale-forming potentials of Ilam City water distribution system, Avaz-pur et al.,
(2008) used significant scales such as Langelier saturation index, Ryznar stability index, and aggressive index. A
comparison of Langelier saturation index and Ryznar stability index for the three rivers supplying dam reservoir
water and the outpouring water of the dam to be used in Ilam City, suggests that the water supplied by dam
reservoir-which enters the storage tanks and distribution networks for drinking and industrial usages is corrosive
(2). Aghapur., et al (2009) examined the water corrosion and scale-forming levels in water transfer and
distribution facilities of Urmia. The results suggested that scale formation is related to no. 1 treatment center and
corrosiveness of water. It was also suggested that low levels of hardness and high levels of corrosion are
associated with increased risk of solution of heavy metals such as lead, cadmium, copper and zinc (1). Hosseini,
Shirmardi, Amini and Shafii (2009) examined the chemical and physical characteristics of waters in western and
mountainous regions of Iran based on Langelier and Ryznar indices. The results suggest the corrosiveness of
water. The Langelier index was in acceptable range and this signified water equilibrium and lack of need for
modification of final pH (9). Derosa (1993) studied scale formation in drinking water distribution network of 17
regions of England. The results suggested a significant effect on settlement and accumulation of particles. More
scale was found in main pipes with lower diameters that were located in downstream of distribution networks (4).
Tang et al (2006) analyzed the internal scale of pipes made of polyvinyl chloride, covered ductile iron, iron-cast
without cover and galvanized steel through XRD, RMS and XPS methods. The release of iron in coverless
iron-cast pipes reduced as percentage of magnetite increased. It manifested a reduction as percentage of hematite
declined. The transfer of dissolved iron and Siderite showed that as alkalinity and pH increased, the amount of
iron increased too (25). Peng et al (2010) used x-ray diffraction method to examine the structural and elemental
composition of solid materials and mobile scale that were separable from pipes of drinking water distribution
networks. The results suggest that among the main elements, iron was the major one. A comparison of relative
presence of these elements shows that hydraulic turbulences may impose relatively low effect on release of
magnesium, aluminum and zinc but it influences the release of organic carbon, non-organic carbon and
magnesium to a higher extent (17). Sarin (2002) analyzed corrosion scale of iron and steel pipes in their structure
and composition. As a result, goethite, magnetite and lepidocrocite were identified as the main elements of dried
samples. The corrosion scale included a broad shell-like layer that covered porous depots of iron oxide phases.
The results of this study suggested that there was a major difference between corrosion scale found in the two
different water supply networks (22). Kommineni et al (2008) set out to determine the reason of colored water
and to develop strategies for reducing it. This evaluation included tests on iron pipes cut out of distribution
network. They found out that alkalinity and acidity posed a significant effect on release on secondary products of
corrosion. In addition, certain tests were done on examination of advantages of adding corrosion inhibitors. By
addition of Dipotassium Orthophosphate and Tin (II) Chloride, the release of iron was reduced up to 40% (11).
Considering the problems caused by scale-formation in drinking water pipes and lack of relevant studies on
drinking water distribution network of Shiraz City, the significance of present study is evident. This research
Objectives are: 1- Measurement of scale-formation rate in water pipes of different zones of Shiraz Cityand 2-
Analysis of type of scale forming in old water pipes of different zones of Shiraz City.
Indices of Corrosion and Scale-forming Potential of Water
The commonly accepted indices for corrosion and scale-forming are: the Langelier saturation index (LSI),
Ryznar stability index (RSI), aggressive index, Puckorius scaling index (PSI), Larson-Skold index (LR), and
Aggressive index (AI). The existence of numerous indices show that there is no single and explicit solution for
determining the water corrosion and scale-forming. The corrosive tendency of water is a complicated
phenomenon the extent of which is not only a function of physical and chemical characteristics of water but also
of the water transfer system (14).
Langelier Index
LI = pH – pHs (1)
pHs = (9.3 + A + B) – (C + D) (2)
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A = (log[TDS]-1) /10 (3)
B = - 13.12 * (log(C+273)+34.55) (4)
C = log[Ca2+ as CaCO3] – 0.4 (5)
D =log[Alk as CaCO3] (6)
Where pHs is the pH of water saturated with calcium carbonate. A is total dissolved solids (mg/L), B refers to
temperature (oC), C represents the calcium hardness (mg/L as calcium carbonate), and D is total alkalinity (mg/L
as calcium carbonate).
Table 1. Interpretation of Langelier Index
Index Value Interpretation Index Value Interpretation
-5 Severe Corrosion 0 Balanced
-4 Mild Corrosion 0.5 Low Scale-forming
-3 Mild Corrosion 1 Moderate
Scale-forming
-2 Mild Corrosion 2 Moderate
Scale-forming
-1 Low Corrosion 3 Mean Scale-forming
-0.5 Low Corrosion 4 Severe Scale-forming
The Langelier index only shows the tendency of water to form scale and it cannot be used as a qualitative
indicator (14). This index is only conducive to control the calcium carbonate index in systems with low total
dissolved solids in not much high temperature ranges. The Langelier index could be more efficiently used in
systems with low current speed of water.
Ryznar Stability Index (RI)
Based on the report of scale-forming and corrosion result in urban water distribution network, the Ryznar
stability index (RI) could be defined in the following manner:
RI = 2 pHs – pH (7)
Table 2. Interpretation of Ryznar Stability Index
Index Value Interpretation
RI<5.5 Highly scale-forming
5.5<RI<6.2 Relatively scale-forming and corrosive
6.2<RI<6.8 Balanced
RI>8.5 Corrosive
The Langelier index is more reliable for application on still waters (e.g. water inside tanks and filters) while
Ryznar index is experimental and it could only be used for (water inside pipeline) with velocity of about 0.6
m/sec. The obtained results of Ryznar index show that this index is not suitable for the case of water saturation
(18).
Larson-Skold Index (LR)
This index was developed by Larson and Skold (1958) for examining the extent of corrosion by water which is
exposed to steel pipe lines with light carbonic structure and cast-iron pipes. The equation and interpretation of
the index is as follow:
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169
LS =
ـ+
4
2ـ
C
HCO3
ـ+C
CO3
2ـ
(8)
Where C refers to the concentration of each elements (in mg/L of calcium carbonate).
Table 3. Interpretation of Larson Index
Index Value Interpretation
LS<0.8 Formation of protective membrane without meditation of chloride and
phosphate ions
0.8<LS<1.2 Formation of protective membrane with meditation of chloride and
phosphate ions (in cases of high steel corrosion levels)
LS>1.2 Local corrosion
Aggressive Index (AI)
The aggressive index (AI) was developed to contribute to selection of proper pipeline for transfer of water in
transfer lines of distribution networks. It is based on water acidity and solubility of calcium carbonate (27). This
index is calculated as follow (20).
AI = pH + log[(A)(H)] (9)
Where A is total alkalinity (mg/L as calcium carbonate) and H refers to calcium hardness (mg/L as calcium
carbonate).
Table 4. Interpretation of Aggressive Index
Index Value Interpretation
AI<10 High water corrosion
AI=10-12 Moderate water corrosion
AI>12 Lack of water corrosion
2. Materials and Method
Shiraz City (capital of Fars province in south of Iran) with mean altitude of 1540 m above mean sea level and
total area of 10531 km2 is in the central part of the province. The mean altitude of the city is 1540 m above the
sea level. Shiraz lacks permanent road and only a number of dried-up flood channels exist which direct all
surficial waters towards Maharloo Lake during raining seasons. About 70 to 80% of the city’s demand for water
is satisfied through deep wells located in the city and its surrounding highlands and the rest are supplied by
DoroodZan Dam.
Sampling of Waters
To examine the condition of corrosion and scale-forming in water-distribution network of Shiraz, 25 samples of
water resources and 42 samples of houses in different points of Shiraz were taken. The sampling of waters was
done on three occasions during winter, spring and summer of 2010-2011. The sampling of waters was done on
three occasions during winter, spring and summer of 2010-2011. Shiraz was divided to 17 zones according to
areas covered by each resource (Figure 1).
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Figure 1. Zoning map of water reservoirs of Shiraz City and Sampling Points
Analysis of Waters
The parameters of acidity (pH meter), temperature (Thermometer; HM Digital, USA), total dissolved solids
(TDS meter), and electrical conductivity (EC Meter) were measured in sampling site. In addition, the parameters
of total alkalinity, total hardness and concentrations of anions including carbonate, bicarbonate, chloride and
sulfate as well as concentrations of cations such as calcium, magnesium, sodium, potassium and iron were
measured based on standard methods. It should be noted that the parameter of total dissolved solids was
measured in the intended site by portable machine and in the laboratory by determining the remaining dry weight
after evaporation). The mean values of these parameters, associated with sampling points in each area, were
defined as parameters of each zone. With the help of measured parameters, the Langelier, Ryznar, Larson,
Puckorius, and aggressive indices were determined for evaluation of corrosion or water scale-forming.
Scale-formation Rate and Type of Scale
From different regions of Shiraz city, 41 water pipes made of galvanized iron (from homes) and 8 pipes of urban
water distribution network were collected to evaluate the thickness and type of scales. To determine the
scale-formation rate (a), the following equation was used:
ε=ε
0
+α.t (10)
where ε is secondary diameter of pipe, ε
0
is primary diameter of pipe, a is scale-formation rate and t is time. The
lifetime of each sample of home pipe was determined by asking house owners. To analyze the type of scale, the
scale of each sample of pipe was removed by grindstone to be turned into powder by a pounder. Then, x-ray
diffraction method was used to determine the chemical composition of scale. The x-ray diffraction pattern was
analyzed by Xpert Software. The model of x-ray diffraction system used in the present study was D8 Advanced.
The x-ray diffraction source in this model was Cu. Furthremore, the intensity of wave was 40 map/40 kv and its
wavelength was 1.54 angstrom.
3. Results and Discussion
The values of water of chemical analysis for each zone and the spatial variation of parameters in different zones
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were determined by the analysis of variance (ANOVA) (table 5). The statistical analysis showed that the local
variations of parameters across different zones were significant at 5% probability level.
Table 5. Values of water chemical analysis of Waters in different Zone
Zone
numbe
r
Acidity
(pH)
To ta l
dissolved
solids
Calcium Magnesiu
m
Sulfate Chloride Hardnes
s (as
CaCO3)
Alkalinit
y (as
CaCO3)
………… ……..… mg/L …………………………………………………………………
1 7.4 375 77 22 49 19 282 220
2 7.7 460 97 26 128 24 355 194
3 7.4 472 95 30 120 25 356 204
4 7.4 520 103 34 146 28 389 221
5 7.3 600 114 39 186 32 440 222
6 7.5 527 108 31 156 28 400 212
7 7.1 706 131 43 223 55 500 223
8 7.2 491 94 33 115 25 370 225
9 7.5 626 107 49 203 39 471 222
10 6.9 719 120 58 216 65 537 260
11 7.1 681 135 40 221 46 501 228
12 7.2 713 134 48 242 65 442 223
13 7.6 437 82 29 96 23 327 206
14 7.05 843 150 51 290 82 586 228
15 7.2 834 145 53 281 88 580 227
16 7.7 440 81 32 107 23 333 198
17 7.3 756 136 48 264 65 538 196
Mean 7.3 600 112 39 179 43 436 218
Max 7.7 843 150 58 290 88 586 260
Min 6.9 375 77 22 49 19 283 194
Standard
deviation
0.26 167.1 26.4 11.6 79.5 28.6 107.7 18
P-value 0.004 0 0 0 0 0 0 0
Interpretatio
n
Significa
nt
Significa
nt
Significa
nt
Significant Significa
nt
Significa
nt
Significa
nt
Significa
nt
Table 6. National Standards of Drinking Water
Parameter Maximum of Desired Value
(mg/L)
Minimum of Desired Value
(mg/L)
Total Dissolved Solids 2000 500
Total Hardness (as CaCO3) 500 -
Calcium 200 75
Magnesium 150 -
Zinc 15 5
Chloride 400 200
Sulfate 400 250
Acidity (pH) 6.5-9.2 7-8.5
Chemical parameters
Total Dissolved Solids. The mean total solids dissolved in different zones was between 375 to 843 mg/L. The
total amount of dissolved solids in 65% of the zones (i.e. Zones no. 4, 5, 6, 7, 9, 10, 11, 12, 14, 15, 17) is higher
than maximum value of desirable level (but less than maximum desirable value). These zones are often located
in eastern, southeastern and southern regions of Shiraz.
Acidity (pH). The mean values of water acidity in different zones ranged between 6.9 and 7.1. Maximum and
minimum pH values were in zone no.2 and zone no.10 respectively. The values of acidity in all zones were in
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desirable and permitted range. Acidity values except zone no.10, were within the acidity range of 7-9.
Alkalinity. The mean value of alkalinity in different zones was between 194 to 228 mg/L. The least alkalinity
was associated with zone 2 and the highest was in zone of Central zone 11 and zone 14. Due to the fact that the
acidity values in the samples were less than 8, the alkalinity was in the form of bicarbonate. The alkalinity value
of less than 30 mg/L, in the form of calcium carbonate, causes water corrosion. If the alkalinity level of higher
than 65 mg/L is maintained as calcium carbonate, the extent of corrosion and release of iron decrease. In addition,
high alkalinity causes buffer property in water. In other words, acidity changes to a lesser extent. Therefore, the
values of alkalinity in drinking water of different parts of Shiraz are high which increases the corrosion inside of
water pipes. Indeed, the alkalinity values of higher than 100 mg/L in water samples leads to creation of calcium
carbonate which may negatively influence the water taste.
Calcium. The mean values of calcium in water of the zones were between 77 to 150 mg/L. The least and highest
values of calcium were associated with zone 1 and zone 15, respectively. The values of calcium in the zones
range between maximum extent of desirable value and minimum extent of permissible value. The main origin of
calcium in waters is limestone.
Magnesium. The mean values of magnesium in water of the zones were between 22 to 53mg/L. The minimum
and maximum values of magnesium were associated with zone 1 and zone 10, respectively. The values of
magnesium in the zones were within permitted range. The high concentration of magnesium was undesirable due
to scale-forming inside of pipelines. The main origin of magnesium in waters is Dolomite.
Hardness. The mean values of water hardness in the zones were between 282 to 586 mg/L. Therefore, the
waters of all zones are categorized in the group of highly hard waters. The least and highest values of hardness
were associated with zone 1 and zone 14, respectively. The values of hardness in 35% of the zones (i.e. zones no.
7, 10, 11, 14, 15, and 17) were higher than maximum permissible level. The main factor of water hardness in
Shiraz is the water originating from Karst regions. The most frequent type of hardness of water in Shiraz is the
temporary or carbonate hardness. It seems that the proper combination of groundwater resources with surface
water resources (DoroodZan Dam), with less hardness level, is influential on modification of hardness of water.
The high hardness contributes to formation of protective layer of calcium carbonate and inhibits the corrosion of
pipes. The excessive hardness increases the extent of scale-formation in pipes and frequency of blockages. As a
result, the current transfer potential decreases. The highest level of hardness of water in Shiraz city is temporary
or carbonate type (Tootoonchi, A., N. Naseri., and R. Sarkhoshiyan, 2003).
Sulfate. The mean values of sulfate in different zones were between 49 to 290 mg/L. The minimum and
maximum of sulfate concentration were associated with zone 1 and zone 14, respectively. The values of sulfate
in zones 14, 15, and 17 were higher than maximum desirable value. These zones are located in eastern,
southeastern and southern areas of Shiraz City.
Chloride. The mean values of chloride in different zones were between 19-88 mg/L. The least and highest
values of chloride were associated with zone 1 and zone 15, respectively. The values of chloride in all zones
were in permissible and desirable range. Chloride is the most active and corrosive ion.
Corrosion and scale-forming indices. Considering the values of measured above parameters for different zones,
the indices of corrosion and scale-formation were calculated and spatial variations of indices values in different
zones were determined the analysis of variance (ANOVA) (Table 7). The statistical analysis showed that
variation of all indices except Ryznar index were not significant across different zones at 5% level of probability.
Table 7. Values of Langelier, Ryznar, Larson and Aggressive Indices of different Zones
Zone
number
Langelier index Ryznar index Larson index Aggressive index
value Interpretation value Interpretation value Interpretation value Interpretation
1 0.12 Low
Scale-forming
7.4 Lack of
Scale-forming
0.27 Scale-former 13.94 Non-corrosive
2 0.42 Low
Scale-forming
6.9 Lack of
Scale-forming
0.83 Probable
Corrosion
14.27 Non-corrosive
3 0.05 Low
Scale-forming
7.33 Lack of
Scale-forming
0.76 Scale-former 13.97 Non-corrosive
4 0.24 Low
Scale-forming
6.93 Lack of
Scale-forming
1.03 Probable
Corrosion
14.06 Non-corrosive
5 0.18 Low 6.95 Lack of 1.03 Probable 14.01 Non-corrosive
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Scale-forming Scale-forming Corrosion
6 0.17 Low
Scale-forming
7.2 Lack of
Scale-forming
0.96 Probable
Corrosion
14.05 Non-corrosive
7 -0.07 Low Corrosion 7.2 Lack of
Scale-forming
1.39 Corrosive 13.82 Non-corrosive
8 -0.08 Low Corrosion 7.4 Lack of
Scale-forming
0.7 Scale-former 13.84 Non-corrosive
9 0.24 Low
Scale-forming
6.97 Lack of
Scale-forming
1.20 Corrosive 14.18 Non-corrosive
10 -0.27 Low Corrosion 7.45 Lack of
Scale-forming
1.15 Corrosive 13.73 Non-corrosive
11 -0.1 Low Corrosion 7.3 Lack of
Scale-forming
1.28 Corrosive 13.87 Non-corrosive
12 0.04 Low
Scale-forming
7.08 Lack of
Scale-forming
1.54 Corrosive 13.97 Non-corrosive
13 0.27 Low
Scale-forming
7.02 Lack of
Scale-forming
0.68 Scale-forming 14.05 Non-corrosive
14 -0.06 Low Corrosion 7.16 Lack of
Scale-forming
1.82 Corrosive 13.88 Non-corrosive
15 0.06 Low
Scale-forming
7.15 Lack of
Scale-forming
1.77 Corrosive 13.95 Non-corrosive
16 0.24 Low
Scale-forming
7.18 Lack of
Scale-forming
0.67 Scale-former 14.17 Non-corrosive
17 0.2 Low
Scale-forming
6.92 Lack of
Scale-forming
1.66 Corrosive 14.1 Non-corrosive
Mean 0.08 - 7.15 - 1.1 - 14 -
Max 0.42 - 7.45 - 1.82 - 14.27 -
Min -0.27 - 6.9 - 0.27 - 13.73
Standard
deviation
0.2 - 0.26 - 0.49 - 0.17 -
P-value 0.504 Insignificant 0.447 Insignificant 0 significant 0.477 Insignificant
Role of Indices in Zones of Shiraz
Langelier and Ryznar Indices. The Langelier index values in all zones ranged between -0.27 to 0.42. In this
regard the least amount obtained (most corrosive) is related to Babakoohi wells (Zone 10) while the highest
value obtained (most scale-forming) is related to Maali-Abad reservoir (Zone 2). The Ryznar index values in all
zones were in the range of 6.9 to 7.45. The least (most scale-forming) and highest values of this index were
associated with Maali-Abad reservoir and Babakoohi wells (Zone 10). The Langelier index was close to
equilibrium in all zones. The Ryznar index for all zones showed lack of scale-forming. Considering the fact that
the Langelier index only represents the tendency of water to form scale or to corrosion, it cannot be used as an
index for quantitative measurement of corrosion or scale-forming. On the other hand, this index is preferably
used for still waters and for controlling formation of calcium carbonate scale in waters with low level of total
dissolved solids (TDS) and in this study, TDS is not in low level range. Therefore, it is recommended that
Langelier and Ryznar indices should be simultaneously used for more precise prediction and decision-making.
Because the value of Ryzner index for all water resources was not more than 6, the waters tend towards
corrosion and the thin layer does not form or cannot protect the internal wall of the tubes. On the
Larson Index. The Larson index values in all zones ranged from 0.27 to 1.82. The minimum value of the index
(i.e. most scale-forming) was related to Dokoohak wells (Zone 1) and the maximum value (i.e. most corrosive)
belonged to Sorsore reservoir (Zone 14). This index is associated with corrosion of steel and iron-cast pipes.
Based on the index, there is a tendency toward corrosion in most of the zones.
Aggressive Index. The mean values of aggressive index in all zones ranged from 13.73 to 14.27. The minimum
and maximum values of the index were related to Babakoohi wells (Zone 10) and Maali Abad Reservoir (Zone
2), respectively. The aggressive index is specific to asbestos-cement pipes which are used in some parts of
distribution network pipes. Based on this index, the waters were not corrosive to asbestos-cement pipes in all
zones.
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Table 8. Rates of Scale-formation of Zones
Number
of Zone Source Mean Rate of
Scale-forming (mm/year)
1 Do-Kohak wells 0.18
2 Maali-Abad reservoir 0.17
3 Derak reservoir 0.13
4 Chamran wells 0.19
5 Chamran and Chogia wells 0.3
6 Abivardi reservoir 0.19
7 Namazi treatment plant 0.28
8 Dare-Chenari reservoir 0.28
9 Sheikh Agha reservoir 0.34
10 Baba-Koohi wells -
11 Central storage (Anbar Markazi) reservoir 0.29
12 Haft-Tanan wells 0.3
13 Kharame reservoir 0.25
14 Sorsore reservoir 0.38
15 Golha reservoir and wells of Nasr Boulevard 0.41
16 Mohammadieh reservoir 0.26
17 Sabz-Poshan reservoir 0.34
Mean --- 0.26
Max --- 0.41
Min --- 0.13
Analysis of Scale-forming Rate in Zones
The most significant factors affecting the scale-forming rate are hardness, acidity, alkalinity, pressure and values
of chloride and sulfate. The increase in the extent of hardness, acidity, alkalinity and pressure leads to increase in
the rate of scale-forming. The anions of chloride and sulfate are corrosive and their increased amounts in water
leads to decreased rate of scale-forming. The least rate of scale-forming is associated with Derak reservoir (Zone
3) and the highest value of scale-forming rate was observed in Golha reservoir and wells of Nasr boulevard
(Zone 15). It should be noted that the zone of Dokoohak wells (Zone 1) had mean scale-forming rate despite of
lowest level of hardness. This is probably due to high water pressure as well as high corrosion rate. As a result,
more corrosion products were found in the zone. Probably, the main factor underlying the highest scale-forming
rate of Golha reservoir (Zone 3) is significantly high level of hardness in this zone. In general, the rate of
scale-formation in drinking water pipes of the houses in Shiraz is 0.26 mm per year
Figure 2. Atlas of water quality in different zones of Shieaz city
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Interpretation of Scale Type Analyses
The presence of goethite and magnetite in most of the formed scale represent a shell-like layer in them. The
phases of iron-oxide, calcium carbonate and magnesium carbonate were found in most of the formed scale and
lepidocrocite was found in some of them. In addition, the presence of goethite, magnetite and magnemite in most
of the formed scale imply the presence of oxygen in waters. Alongside, Siderite was found in a limited number
of samples.
Iron
As a result of scale analysis, iron was rarely found in the samples in its elemental form. It was often present in
oxide, hydroxide and complex forms such as goethite, magnetite, and magnemite among many others.
Calcium.The presence of calcium in scale samples was due to its natural inclusion in regional water. It was the
result of deposition and decomposition of calcite, dolomite, hydroxyapatite and gypsum (CaSO4.2H2O). These
mineral materials were present in most of the samples.
Manganese.The presence of manganese in scale samples is probably due to their natural inclusion in waters and
settlement of oxide and magnesium hydroxide in water.
Magnesium.It seems that presence of magnesium in scale samples is due to formation and decomposition of
mineral materials such as dolomite (CaMg(CO3)2). This compound was found in most of the samples.
Calcium Carbonate.In all of the samples, calcium carbonate was present because these waters are excessively
hard and rich with calcium and bicarbonate. Among these phases, calcium is thermodynamically the most
suitable phase in temperature and pressure of the environment. During settlement from pure calcium bicarbonate,
it is the dominant phase. Aragonite and vaterite are kinesthetically similar and they may appear as the first forms.
They also may turn into calcite as time passes. Calcite usually deposits in room temperature and in most of the
samples, one could find calcite phase. In majority of samples, vaterite phase also existed. Because aragonite
often deposits when temperature is higher than 65 °C, this type of phase was not identified in the sample.
Type of Hardness.In most of the water samples, the concentration of bicarbonate ion was higher than sulfate
and chloride ions. In addition, the analysis of scale samples through x-ray diffraction method showed that
carbonate compounds of calcium and magnesium ions are more than their non-carbonate compounds. Therefore,
the most frequent type of hardness in waters of Shiraz city is temporary one.
Phosphorus.The presence of phosphorus in scale samples may be associated with formation of hydroxyapatite,
accumulation of phosphate in biofilm and cellular materials. The accumulation of phosphorus in scale-forms
demands the presence of phosphorus in treated water.
Sulfur.The presence of sulfur in scale samples is probably due to existence of troilite which is a typical
component in iron corrosion deposit sub-layer. The results of x-ray diffraction analysis refer to presence of this
compound in the samples.
Silica.Silica was present in almost all of the scale samples.
Aluminum.The detection of aluminum in scale samples could be attributed to its natural presence in water,
deposition of alumina and aluminum hydroxide along with formation of aluminosilicates. In addition, aluminum
may have entered the cement coatings through separation.
Zinc.The underlying reason for presence of zinc in samples may be their natural presence in source water and
corrosion of galvanized iron pipes.
Lead.Lead was present in most of the scale samples. The detection of lead in scale samples might be due to its
natural presence in source water or corrosion of brass fittings of water piping systems.
Cadmium and Arsenic.The source of cadmium and arsenic is their natural presence in water sources or
corrosion of galvanized pipes.
4. Conclusion
The showed that the mean value of Langelier, Ryznar, Puckorius, Larson, and aggressive indices in water
distribution network of Shiraz city were respectively 0.07 (low scale-formation), 7.12 (lack of scale-formation),
6.67 (corrosive), 1.2 (corrosive) and 13.96 (non-corrosive). Based on Langelier index, 71% of waters in studied
zones were scale-forming. Based on Ryzner index, all of the zones lacked the characteristic of scale-formation.
Based on Larson index for iron-cast and steel pipes, 29% of zones are scale-forming, 23% are probably
corrosive and 48% of them are corrosive. Based on aggressive index, none of the zones are corrosive for
asbestos-cement pipes. The highest rate of scale-formation was associated with Zone 15 (Golha reservoir) and
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the least value of the index belonged to Zone 3 (Derak reservoir). In general, the rate of scale-formation in
drinking water pipes of the houses in Shiraz is 0.26 mm per year. The zones located in eastern, south-eastern and
southern zones of water distribution network have more problems in regard to scale-formation while northern
and northwestern zones had the best conditions in regard to scale-formation. In general, certain arrangements
should be made to put the water resources in relatively balanced conditions and to create a proper membrane of
calcium carbonate on the interior layer of pipes for protecting them against corrosion. On the other hand, the
conditions should be modified in a way that excessive scale-formation of waters and consequent reduction of
transportation capacity are prevented. Results indicated that main compositions in scale samples were calcium
carbonate, calcium sulfate, magnesium carbonate, magnesium sulfate, hematite, maghemite, magnetite, goethite,
zinc oxide, gypsum, vivianite, dolomite, hydroxyapatite and troilite. Main elements in scale samples were
magnesium, silicon, phosphorus, sulfur, zinc, copper and lead.
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