Corrosion control is an important aspect of safe drinking water supplies. The effects of corrosion which may not be evident without monitoring are an important issue concerning both public health and economical aspects. Chemical stability parameters of water quality in water treatment plants in Baghdad city can improve drinking water quality. The treated water quality from water treatment plants in Baghdad city was investigated along the water flow path in this study. The water quality parameters related to chemical stability included temperature, alkalinity as mg/L CaCO 3, calcium mg/L as Ca, pH and total dissolved solids (TDS) mg/L for different samples from WTPs within Baghdad city were investigated. The two water quality indices, Langelier saturation index (LSI) and the Ryznar stability index (RSI), were calculated in order to evaluate the chemical stability of the drinking water samples. The results of LSI and RSI of the effluents from Baghdad’s WTPs during 2000-2013 classified that corrosive water is produced and this indicates that the water is not safe for domestic use and will need the further treatment. The present study demonstrated the application of water stability indices in estimating/understanding the treated water chemical stability and appeared to be promising in the field of treated water quality management.
Stability of water is the tendency of water to either dissolve or deposit minerals varying with its chemical makeup. Water that tends to dissolve minerals is considered corrosive and that tends to deposit mineral is considered scaling. Corrosive water can dissolve minerals like calcium and magnesium, also can dissolve harmful metals such as lead and copper from plumbing utilities. Where scaling waters deposit a film of minerals on pipe walls and may prevents corrosion of metallic surfaces. If the scale deposition is too rapid, it also can be harmful and can damage appliances, such as water heaters, and increase pipe friction coefficients, in extreme cases, scale may clog pipes. Therefore, the most desirable water is one that is just slightly scaling [
Control of water quality in the distribution and plumbing systems seeks to preserve the basic characteristics of water during its conveyance from the point of production and treatment to the consumers tap. The finished water should be completely stable in its compositional and physical attributes. Also the conveyance system and accessory structures (pipelines, distributing reservoirs, mains and serves pipes) should be reactively inert to the water being conveyed. Most water quality parameters affect the corrosion process to some degree and that each pipe material is affected differently. Altering one parameter to subdue its effect on the corrosively of water may well change other water quality characteristics, perhaps rendering the water even more corrosive or less likely in some other way to meet drinking water standards [
Water quality changes in drinking water distribution systems occur as a result of complex and often interrelated chemical and biological processes [
Corrosive water is related to its pH, alkalinity, hardness, temperature, dissolved oxygen, carbon dioxide, total dissolved solids and other physical, chemical and biological factors. Water with high levels of sodium, chloride, or other ions will increase the conductivity of the water causing corrosion and also be accelerated by high: flow rates within the piping system, water temperature, dissolved O2 and CO2 and dissolved salts such as sulfates [
Although a number of indices have been developed, none has demonstrated the ability to accurately quantify and predict the corrosively or scaling of water. They can only give a probable indication. Experience has shown that if conditions encourage the formation of a protective calcium carbonate film, then corrosion will generally be minimized [
Pisigan and Singly [
This paper is to evaluate the stability of the water flowing in the distribution systems in Baghdad city from the existing water treatment plants. Two indices may be calculated to assess the corrosively of water due to problems associated with calcium carbonate scale formation. Probably the best way to qualitatively predict the formation of CaCO3 scale is through the use of the Langelier Saturation Index (LSI) and the Ryznar Stability index (RSI). The calculation of these indices is through the investigation of the chemical stability parameters including Temperature, Alkalinity as CaCO3, Calcium as Ca, pH and total dissolved solids (TDS) for the effluents from the WTPs within Baghdad city.
Tigris river water is considered the only source of potable water for the city of Baghdad, and the river divides the city into right (Karkh) and left (Risafa) sides with a flow direction from north to south. The study area within Baghdad City is located in the Mesopotamian alluvial plain between latitudes 33˚14'N - 33˚25'N and longitudes 44˚31'E - 44˚17'E, 30.5 to 34.85 m at sea level (a.s.l). The area is characterized by arid to semi-arid climate with dry hot summers and cold winters; the mean annual rainfall is about 151.8 mm [
All water treatment plants (WTPs) in Iraq are designed as conventional plants. This treatment process does not significantly affect the concentrations of the dissolved constituents, so the characteristics of the raw and treated water are quite the same. There are eight water treatment plants (WTPs) from the north to the south of Baghdad city, Al-Karkh, East Tigris, Al-Wathbah, Al-Karamah, Al-Qadisiya, Al-Dawrah, Al-Rashid, Al-Wahda WTPs (
Baghdad)-Water office which represents the quality of the treated water from the WTPs in Baghdad city according to WHO standards recorded for the period between 2000 until 2013. In order to evaluate the chemical stability of any type of water, many parameters are detected, such as Temperature, Alkalinity mg/l as CaCO3, pH, Total dissolved Solids (TDS), and concentration of Calcium (Ca). The variation in water stability with time and distance from the north to the south of Baghdad is to be indicated.
Work by professor W. F. Langelier, published in 1936 deals with the conditions at which water is in equilibrium with calcium carbonate. An equation developed by Langelier makes it possible to predict the tendency of calcium carbonate either to precipitate or to dissolve under varying conditions. Langelier Saturation Index (LSI) is an equilibrium model derived from the theoretical concept of saturation and provides an indicator of the degree of saturation of water with respect to calcium carbonate and can calculate from [
where:
pHa: the measured water pH.
pHs: the pH at which water with a given calcium content and alkalinity is in equilibrium with calcium carbonate.
The equation expresses the relationship of: pH, calcium, total alkalinity, dissolved solids, and temperature as they are related to the solubility of calcium carbonate in waters with pH of 6.5 to 9.5. This is known as the pHs:
where:
The Ryznar index is an empirical method for predicting scaling tendencies of water based on study of operating results with water at various saturation indices. The Stability Index developed by John Ryzner in 1944 used the Langelier Index (LSI) as a component in a new formula to improve the accuracy in predicting the scaling or corrosion tendencies of water [
The Ryznar index (RSI) takes the form:
The criteria used to give an indication of the stability indices can be summarized in
Water quality parameters from Baghdad’s WTPs compared with Iraqi and WHO drinking water standards are shown in
Alkalinity in water is due to the presence of weak acid systems that consume hydrogen ions produced by other reactions or produce hydrogen ions when they are needed by other reactions allowing chemical or biological
Index value | Water condition |
---|---|
LSI > 0 | Water is supersaturated with respect to calcium carbonate (CaCO3) and scale forming and CaCO3 precipitation may occur. |
LSI = 0 | Water is considered to be neutral. Neither scale-forming nor scale removing. Saturated, CaCO3 is in equilibrium. Borderline scale potential. |
LSI < 0 | Water is under saturated with respect to calcium carbonate. Under saturated water has a tendency to remove existing calcium carbonate protective coatings in pipelines and equipment. No potential to scale, the water will dissolve CaCO3. |
RSI ≤ 6 | Supersaturated, tend to precipitate CaCO3. The scale tendency increase as the index decrease. |
6 < RSI < 7 | Saturated, CaCO3 is in equilibrium. The calcium carbonate formation probably does not lead to a protective corrosion inhibitor film. |
LSI ≥ 7 | Under saturated, tend to dissolved CaCO3. Mild steel corrosion becomes an increasing problem. |
Parameter | Min. | Max. | Average | Drinking standards | |
---|---|---|---|---|---|
WHO | Iraqi | ||||
Temperature (˚C) | 21 | 22.81 | 21.89 | - | - |
Alkalinity as CaCO3 (mg/L) | 124.08 | 143 | 137.77 | 30 - 500 | 125 - 200 |
Calcium as Ca (mg/L) | 70 | 120.75 | 83.46 | 75 | 75 |
pH | 7.45 | 7.90 | 7.63 | 7 - 8.5 | 6.5 - 8.5 |
Total dissolved Solids TDS (mg/L) | 410 | 696.5 | 541.53 | 500 | 1000 |
activities to take place within water without changing the pH [
Calcium is the second most prevalent constituent in most surface waters and is generally among the most prevalent three or four ions in groundwater. Weathering and soil ion exchange reactions are the main sources of calcium in natural waters [
pH is important in water treatment as it directly influences the dosages of chemicals added to reduce hardness and coagulate particles [
Total dissolved solids (TDS) in water are due to inorganic salts. Principally these inorganic constituents are calcium, magnesium, sodium, and potassium salts of bicarbonate, chloride, sulfate, nitrate and phosphate. These compounds originate from weathering and leaching of rocks, soils and sediments. Also some of these compounds are added to the water in the treatment plants [
Temperature also affects the corrosion process. Higher water temperatures accelerate the rate of corrosion by increasing the rate of the cathodic reaction. The chemical reaction rate generally is doubled for every 8˚C increase in temperature [
The stability of the treated water from all WTPs in Baghdad was calculated according to the water quality of the effluents of these plants. The US Environmental Protection Agency (USEPA) has recommended the use of Langelier (LSI) and Ryznar (RSI) stability indexes by utilities to monitor the corrosion potential of water [
The annual LSI and RSI for the period 2000-2013; are shown in
LSI | Temp | Alkalinity | Ca | pH | TDS | |
---|---|---|---|---|---|---|
LSI | 1 | |||||
Temp | 0.424(**) | 1 | ||||
Alkalinity | −0.221(*) | −0.163 | 1 | |||
Ca | 0.286(**) | −0.119 | −0.553(**) | 1 | ||
pH | 0.507(**) | 0.381(**) | 0.227(*) | 0.198(*) | 1 | |
TDS | 0.120 | −0.235(**) | −0.447(**) | 0.925(**) | 0.129 | 1 |
**Correlation is significant at the 0.01 level (2-tailed). *Correlation is significant at the 0.05 level (2-tailed).
RSI | Temp | Alkalinity | Ca | pH | TDS | |
---|---|---|---|---|---|---|
RSI | 1 | |||||
Temp | −0.468(**) | 1 | ||||
Alkalinity | 0.306(**) | −0.163 | 1 | |||
Ca | −0.280(**) | −0.119 | −0.553(**) | 1 | ||
pH | −0.311(**) | 0.381(**) | 0.227(*) | 0.198(*) | 1 | |
TDS | −0.127 | −0.235(**) | −0.447(**) | 0.925(**) | 0.129 | 1 |
**Correlation is significant at the 0.01 level (2-tailed). *Correlation is significant at the 0.05 level (2-tailed).
Year | Al-Karkh | East Tigris | Al-Wathbah | Al-Karamah | Al-Qadisiya | Al-Dawrah | Al-Rashid | Al-Wahda |
---|---|---|---|---|---|---|---|---|
LSI | −0.122 | 0.032 | 0.042 | 0.195 | 0.168 | −0.006 | 0.045 | −0.012 |
RSI | 7.785 | 7.595 | 7.505 | 7.344 | 7.338 | 7.540 | 7.473 | 7.551 |
Year | 2000 | 2001 | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 | 2012 | 2013 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
LSI | 0.156 | 0.074 | 0.067 | 0.081 | 0.002 | 0.147 | 0.071 | 0.370 | 0.120 | −0.082 | −0.136 | −0.065 | 0.021 | 0.002 |
RSI | 7.38 | 7.54 | 7.5 | 7.53 | 7.59 | 7.40 | 7.52 | 7.17 | 7.34 | 7.66 | 7.68 | 7.63 | 7.54 | 7.58 |
from year 2000-2008 and 2012-2013 are positive values indicating scale forming water while from 2009-2011 are negative values or corrosive water. While RSI results from period 2000-2013 show more than 7 as under saturated, tend to dissolved CaCO3 or corrosive water.
The differences between LSI and RSI shown in
As the choice of any index may propose different description water stability, so selecting the most appropriate index conforming to the actual situation of any system is the most sophisticated stage in making predictions. According to the present study:
1) The treated water from the WTPs in Baghdad is considered corrosive. So these plants need to pay attention to achieve national water quality standards and reduce corrosion and corrosion by-products, with corrective action that should be made to prevent corrosion in water supply networks.
2) Water quality affects the corrositivity of the treated water flowing from the WTPs into the distribution system in Baghdad city.
3) The calculated stability indices LSI and RSI indicated that the treated water from the plants was corrosive.
4) Using LSI together with the RSI contributes to more accurate prediction of the scaling or corrosive tendencies of water.
Control of water quality in the distribution system seeks to preserve the basic characteristics of water during its conveyance from the point of production and treatment to the consumers tap. The finished water from the treatment plants should be completely stable in its compositional and physical attributes. Several methods could be applied in the treatment plants to produce stable water like pH adjustment or adding corrosion inhibitors.
Our special gratitude and many thanks are forwarded to “Baghdad Mayoralty-Water Office” for supplying the data and without their support and encouragement the work would have not been done.