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.

Results

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