Potable water that is chemically and bacteriogically safe for human consumption
should be provided for any community. Internal corrosion of piping is a serious
problem in drinking water industry. According to ISO 8044 standard, corrosion
is a physico-chemical reaction between metal and its surrounding environment.
Corrosion changes metal properties. In addition to not being economic, corrosive
water has the potential of degrading water quality and therefore affects the
health of consumers (Mahvi and Eslami, 2005). Corrosion
tends to increase heavy metals concentration. In general, different water resources
have different water characteristics and therefore its tendency to corrode the
water pipe is also different. The potential to form a protective layer containing
a mixed precipitate of calcium carbonate and iron oxides depends on many different
chemical and biological parameters such as pH, residual chlorine, hardness,
temperature, total dissolved solids, alkalinity, acidity, dissolved salts, dissolved
gases and microorganism (Viessman and Hammer, 2004).
Corrosion products can shield microorganisms from disinfectants. These microorganisms
can cause many problems such as slimes, bad odor and taste (Bina
and Porzamani, 2005). Atasoy and Yesilnacar (2009)
also used Langelier Index and Ryznar Index to determine the degree of scale
formation tendency/corrosivity of groundwater samples. The groundwater of Harran
plain in Turkey had intense corrosion tendency. The amount of CO2
from the soil zone respiration and high sulfate concentration contributed the
corrosivity of groundwater (Atasoy and Yesilnacar, 2009).
A current study indicated that the loss of chlorine dioxide in corroded iron
pipes was significant and its effect was more than the total organic carbon
(Zhang et al., 2008). According to Edwards, in
many countries like Japan, England and Australia the cost of corrosion was almost
3 to 4% of their total income (Edwards, 2002).Therefore,
corrosion control is an effective way to maintain water quality and providing
the health of consumer. According to Environmental Protection Agency (EPA) standard,
drinking water should not be corrosive.
The Langelier Saturation Index (LSI) and the Ryzner Stability Index (RSI) are
commonly used as the indicators of water scale potential. The quality of tap
water resources regarding the potential of scale formation and corrosivity in
Tafila, Jordan was determined by various indices, LSI, RSI and the calcium carbonate
precipitation potential (Al-Rawajfeh and Al-Shamaileh, 2007).
Melidits also used the marble test and LSI value to determine the corrosion
or deposition effect of Paradises well water, in Hrysoypolis in Greece (Melidis
et al., 2007). The tendency of water to precipitate
CaCO3 or to dissolve it was determined by these indices. However,
using the LSI and RSI as corrosion index should be done very conservatively.
Calcium carbonate precipitation is inhibited by the presence of certain compounds.
Conversely, the formation of CaCO3 precipitate under-saturated water
condition was observed (Lisitsin et al., 2005).
Measuring metal concentration in the water distribution system is considered
the best way to determine population exposure and it is the most appropriate
method to assess the corrosion rate (Eaton et al., 1995).
Considering the fact that pipes are used for a long period of time, corrosion
control is very important to maintain water quality and pipe integrity. Since,
there were limited time and facility and lack of such study in Shiraz water
resources, LSI was used to predict the potential of scale formation and corrosivity.
This study aimed at determining the corrosivity or scale formation potential
of the groundwater and surface water in Shiraz, South of Iran using LSI. For
this purpose, 78 wells and one dam resource and 39 points through the network
distribution system in a time interval of three months in Shiraz were analyzed
for the parameters of temperature, pH, total alkalinity, calcium hardness, Total
Dissolved Solids (TDS) and Electrical Conductivity (EC).
MATERIALS AND METHODS
Experiments: Samples were collected from selected sites for chemical analysis.
Study site description: Shiraz is situated between the 29th and 38th
degrees of longitude, and between the 40 and 52 degrees of latitude, and is
1500 feet above the sea. Shiraz has a moderate climate. It is located in the
South of Iran (Fig. 1). Precipitation is mainly in late fall,
Winter and early Spring. Shiraz drinking water is supplied by 122 deep wells
(78 of them are in use) of which 81 wells are alluvial and 41 wells are calcareous,
which are located within or outside Shiraz. In addition, Dorodzan dam, which
is located 80 km East of Shiraz, is the only surface water resource which is
used to supply Shiraz drinking water. This study was conducted in three months
from July through late October in 2007. City map was provided and then was divided
into 32 regions. Randomized sampling was done inside the regions including 39
points in the network distribution system and 79 water resources including wells
and Dorodzan dam.
||The location of sampling points from 79 water resources (•)
and 39 (*) stations in network distribution system in Shiraz
Analytical methods: The samples were collected in 300 mL polyethylene
bottles. According to the standard method, the samples were analyzed for total
alkalinity and calcium hardness (Eaton et al., 1995).
The temperature, pH, electrical conductivity and Total Dissolved Solids (TDS)
were determined at the sampling point in the field by Aqua-conductivity TDS
and temperature meter.
LSI was determined by the following relationship:
where, pH is measured pH of the water and pHS is pH at CaCO3 saturation and is determined by the following equation:
||Constants based on ionic strength and temperature
||Negative logarithm of the calcium ion concentration, mole L-1
||Negative logarithm of the total alkalinity, equivalents L-1
||Negative ionic strength coefficients at water temperature
A negative number of LSI indicates corrosive water and there is no potential to scale. A positive number of LSI indicates over saturated and it can precipitate calcium carbonate. If LSI is zero, water is at equilibrium.
Equation 1 was used to measure LSI and then the potential of scale formation and corrosion of water sample were determined.
Pearson coefficient in SPSS 11.5 software was used for data analysis.
RESULTS AND DISCUSSION
To determine the corrosion potential, different chemical water quality parameters such as temperature, pH, total alkalinity, calcium hardness, Total Dissolved Solids (TDS) and Electrical Conductivity (EC) were measured in the 39 points in the network distribution of water supply municipality of Shiraz and 79 different water resources including wells and dam resources in a time interval of three months. The pH of the samples ranged from 6.92 to 8.3. Calcium hardness concentration ranged from 79 to 490 mg L-1 it as CaCO3. The lowest concentration of calcium hardness (79 mg L-1 it as CaCO3) was found in the North part of Shiraz, Derak Mountain. The highest TDS concentration (1398 mg L-1) it was found near the cement factory. The majority of water samples have a high concentration of TDS. The range for EC is 297 to 2020 μmho cm-1.
According to LSI value, water scale formation and corrosion potential for water samples from different network distribution system and water resources were presented, respectively in Table 1 and 2. The value of LSI for water samples in Shiraz network distribution system and drinking water resources in respect to the equilibrium line were presented, respectively in Fig. 2 and 3.
Shiraz water resources had medium to very hard water (calcium hardness of 118 to 419 mg L-1 it as CaCO3) except three water resources with calcium hardness of less than 100 mg L-1 it as CaCO3 located in the Northern Shiraz (Derak mountain). According to LSI value, water had a low to medium potential for scale formation (92.4%) and only 1.3% of the water resource had a trace potential of corrosion. The result of chemical water quality parameters in Dorodzan dam indicated that water had a mild hardness (130 mg L-1 it as CaCO3) and therefore had a moderate scaling potential at total alkalinity of 200 mg L-1 it as CaCO3 and pH equal to 7.9.
Results also indicated that water samples from the network distribution system
were ranged from mild to very hard (118 to 490 mg L-1 it as CaCO3).
The LSI value was ranged from (0.23 to 0.96). In general, water did not have
a corrosion potential and even a protective CaCO3 lining on pipes
could be formed.
||The potential of scale formation and corrosivity of water
according to LSI value in Shiraz network distribution water samples
||The potential of scale formation and corrosivity of water
according to LSI value in Shiraz water resources samples
According to our study, the mean value of LSI for 118 water samples was 0.417. Samples exhibited a slight scaling potential, according to LSI value.
Since, Shiraz network distribution water system is fed by many different water resources with different water quality parameters, the mixing of water resources in pipes caused a trace to moderate scale formation potential making it more suitable for drinking, considering physico-chemical properties.
Pearson coefficient with a confidence level of 95% (α = 95%) was used for correlation between the LSI value and the quality parameters (temperature and pH). Data analysis showed significant difference between LSI and temperature and pH (p<0.001) and a positive correlation was observed.
value of LSI for water samples in Shiraz network distribution system in
respect to the equilibrium line
value of LSI for water samples in Shiraz drinking water resources in respect
to the equilibrium line
Mahvi and Eslami indicated that Zanjan (Central Iran) water resources were
corrosive, based on LSI value. They concluded a high possibility of damaging
in the pipes (Mahvi and Eslami, 2005). However, the
quality of tap water in Bagbedarn, Esfahan was not corrosive and at the same
time, water had a tendency to form a protective layer in the pipes (Bina
and Porzamani, 2005). Although, the quality of tap water resources regarding
the potential of scale formation and corrosivity in Tafila, Jordan indicated
corrosion conditions, this image changes upon heating and evaporation of water
with the release of CO2 (Al-Rawajfeh and Al-Shamaileh,
2007). Atasoy and Yesilnacar (2009) also showed that
the groundwater of Harran plain in Turkey had intense corrosion tendency. Their
results showed that precipitation, excessive irrigation, and change in groundwater
level caused seasonal variation in corrosive characteristics (Atasoy
and Yesilnacar, 2009).
One hundred and eighteen tap water samples, 79 water resources and 39 stations
in Shiraz network distribution system were used to determine the potential of
scale formation and corrosion. Langelier Saturation Index (LSI) confirmed that
the potable water of municipality Shiraz was not corrosive and did not cause
any damage in the network distribution system pipes. Furthermore, its water
had a high tendency to form a protective layer in pipes. However, water authorities
should monitor corrosion rates on a regular basis to determine changes of corrosiveness
during seasons with high or low precipitation. Measuring metal concentration
especially heavy metals contents in Shiraz water distribution system is also
recommended. Because of the severe health effects of heavy metals, it is recommended
to control the level of the heavy metals contents in Shiraz drinking water.
The authors would like to thank Shiraz University of Medical Sciences for the financial support (contract No.3855). Also Shiraz Organization of Water and Wastewater is highly appreciated for their assistance in sampling from water resources.