Introduction The principle oxidants in the drinking system are dissolved oxygen and chlorine, and the corrosion of water pipe is going alone because of oxidants reaction. Since 1930, pH control chemicals such as NaOH, Ca(OH)2, NaHCO3, and phosphate corrosion inhibitors have been used to prevent the corrosion of water pipes. However, to directly add Ca(OH)2at inlet of water well in production process of drinking water may cause a problem to make clouding phenomenon in tap water. The objective of this study removes such problems as the turbidity, strengthen more safety of water. Experimental Materials – The iron(carbon steel) was in the form of sheet 0.04 cm in thick and the following analysis: Fe 99.62%, Mn 0.26%, Cu 0.08%, Ni 0.05%, Mo 0.01%. specimens cut from this sheet measured 15cm×1.2cm. The chemicals used as corrosion inhibitors were NaOH, Ca(OH)2, and H3PO4 and tap water used as blank. NaOCl was used to disinfect the drinking water made by the demo plant. In order to examine the effect of disinfectant residual concentration according to adding lime water and corrosion inhibitor such as ortho phosphate(H3PO4), the residual concentration of disinfectant was measured in carbon steel water pipe 50 m in length at the corrosion pilot plant and the temperature of solution was in room temperature between 5°C in the winter to 35°C in the summer. Chlorine residual concentration in the water pipes 50 m in length was measured for 5, 10, 20, 40, 60 min, respectively. The electrochemical tests was measured using Solartron Potentiostat 1480 and the test was performed in a typical three electrode cell which consist of carbon steel as working electrode, saturated calomel electrode as reference electrode, platinum counter electrode. Result s HOCl and OCl- in drinking water which chlorine was added as disinfectant reach in equilibrium state at nearly pH 7.5. As pH increases, more of OCl- exists than the strong disinfectant, HOCl-, Increasing HOCl below pH 7.5 is more benefit in terms disinfectant efficiency. Figure 2 above shows the concentration of free residual chlorine with time. Running drinking water in the pipe, adding PO4 decreased concentration of chlorine from 0.48 to 0.28 ppm for 40 min, NaOH decreased concentration of chlorine from 0.45 to 0.10ppm, Ca(OH)2 decreased concentration of chlorine from 0.24 to 0.10 ppm, but tap water as blank degreased concentration of chlorine from 0.40 to 0.07 ppm for only 10 minutes. Of three corrosion inhibitors, PO4 showed the highest reduced efficiency for the concentration of residual chlorine. To inhibit the reaction of chlorine with metal water pipe is similar to inhibit the corrosion reaction. Reference US EPA, Disinfection Profiling and Benchmarking, EPA Guidance Manual, August 1999. American Society for Testing and Materials, G 4-95(1996). Figure 1
1. Introduction Water pipe corrosion corrodes the iron by the oxidation reduction reaction, and the corrosion rate is accelerated by the oxidizing agent used as disinfectant in the tap water. The major oxidizing agent in tap water are ozone and chlorine. pH, alkalinity and residual chlorine concentration are factors of internal corrosion. We are replacing the ductile cast iron pipe with inner coating and stainless steel pipe. However, it is still used in many cases, such as gray cast iron pipes and galvanized steel pipes without internal coatings in Korea. Chlorine is a very strong oxidant for all metals and organics, and the reaction between chlorine and iron is as follows. HOCl + 2Fe2+ + 5H2O → 2Fe(OH)3↓ + Cl- + 5H+ Recently, as the problem with sinkholes has arisen on the a buried road as water pipes, sewage pipes, etc, interest in corrosion has been growing. The purpose of this study is to evaluate the effect of residual chlorine on the corrosion factor by analyzing the water quality and to evaluate the necessity of the proper residual chlorine on in the water purification plant. 2. Experimental Method In order to evaluate the corrosion of residual chlorine concentration of tap water, we were carried out Tafel experiment and water quality analysis. The corrosion rate was measured by a Tafel experiment using an electrochemical corrosion measurement system (PAR VMP-3). For tap water in Seoul, corrosion evaluation was performed according to alkalinity and hardness, residual chlorine concentraion. 3. Summary of results The tap water quality of Seoul was as follows that residual chlorine is 0.4 mg/L, and alkalinity is 50 mg/L as CaCO3, with a pH of 7.3. The results of experiments on the rate of corrosion of Carbon Steel specimens to residual chlorine in seoul tap water, can be seen in Fig. 1. As a result of the Tafel experiment according to the initial residual chlorine concentration under the same conditions (same LI index), The corrosion rate was 0.01 mmpy at 0 mg/L of residual chlorine and 0.074mmpy at 0.8mg/L of residual chlorine, corrosion rate for residual chlorine in carbon steel specimens was more than 7 times. It was confirmed that the higher the concentration of residual chlorine, the faster the corrosion rate was increased. The corrosivity of tap water in Korea is evaluated by the LI index, but residual chlorine has a large effect on corrosion, so it is required to add residual chlorine for the tap water corrosion evaluation. It is reducing the concentration of residual chlorine for reduce the chlorine odor of tap water in the distribution water system in Korea. But, residual chlorine guideline for corrosion reduction is necessary in Korea. Figure 1
Introduction Corrosion control of treated water is an important aspect of safe drinking water supplies. Highly corrosive water can cause system failures or result in health problems because of dissolved lead and other heavy metals from plumbing utilities. This paper is to evaluate corrosivity of water flowing in the distribution systems from water treatment plants in Korea Experimental The langelier Saturation index(LI), a measure of solution’s ability to dissolve or deposit calcium carbonate, is often used as an indicator of the corrosivity of water. LI is an equilibrium model derived from the theoretical concept of saturation. It indicates the degree of saturation of water with respect to calcium carbonate and can be calculated from LSI = pHa -pHs Where: pHa : the measured water pH pHs : the pH at which water with a given calcium and alkalinity as CaCO3 is an equilibrium with calcium carbonate. The LI was calculated from the chemical parameters including Temperature, Alkalinity, Calcium, pH and Conductivity at major 70 WTPs in Korea from July 2014 to October 2015. Result s Water quality parameters of treated water in Korean WTPs are shown in Fig. 1. From the recorded data , alkalinity ranged between 4.7 and 108.0 mg/L on average 31.6 mg/L as CaCO3. The concentration of Ca ranged between 1.86 and 44.8 on average 19.1mg/L in all WTPs. The result of pH varied from 6.33 to 8.40 on average 7.12, indicating sub-acid to sub-akaline in nature. The calculated LI of raw and treated water were low (Fig.2). Mean LI values in raw water of WTPs were -0.93, -1.41, -1.68, and -1.80 for the Han, Nakdong, Keum, and Sumjin river basin, and values in treated water were -1.37, -1.78, -1.86, and -2.50 respectively. LI of raw water from tributaries of four major rivers were relatively low and the geological characteristics of granite in Korea was considered as a main cause of it. Increased corrosivity of treated water resulted from the decrease of pH and alkalinity, which occurred during the purification process especially coagulant treatment. If the corrosion management guidelines were set to the Japan’s guideline(LI≥ -1.0), 61 water treatment plants among 70 were classified as which were required corrosion control(87%). Based on the correlation results between LI and water quality parameters, pH and calcium concentration were confirmed as the major components for LI(Table 1). Therefore, pH and Calcium concentration control is considered as an effective method for the management of corrosivity of tap water According to investigated LI, the treated water in Korean WTPs during 2014~2015 can be classified as corrosive water. This result indicates that the further treatment will be needed for the safe domestic use of water. Reference A. B. Richard, E. M. Nancy, A. C. David, Strategies for assessing optimized corrosion control treatment of lead and copper, American Water Works Association, 2013, 105(5), pp. 62-75 Figure 1
The tap water used in Seoul was found to be corrosive. Its corrosivity was effectively reduced by that the additions of alkali agent such as NaOH, Ca(OH) 2 and corrosion inhibitor such as H 3 PO 4 . For the corrosion test, carbon steel pipe 50 m long was exposed to the drinking water produced by a pilot plant at 36.5 ℃, similar to the existing process where it takes about 20 minutes to reduce the initial chlorine content of 0.5 ㎎/L to 0.05 ㎎/L. CO 2 and Ca(OH) 2 was added not only to control the Langelier index (LI) above -1.0 and but also, to increase the duration time of residual chlorine by about 6 times. The persistence effect of residual chlorine was in the order of H 3 PO 4 > Ca(OH) 2 > NaOH. Measurements of weight loss showed that corrosion inhibition was effective in order of Ca (OH) 2 > H 3 PO 4 > NaOH > no addition, where the concentrations of Ca(OH) 2 and phosphate were 5 ~ 10 ㎎/L (as Ca 2+) and 1 ㎎/L (as PO 4 3-), respectively.
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