EVALUATION OF TRIHALOMETHANES IN TAP WATER SAMPLES OF KARACHI CITY (PAKISTAN).
The chlorination of water containing natural organic matter leads to the formation of disinfection by-products. Trihalomethanes (THMs) are the major category of disinfection by-product in chlorinated drinking water. This study aimed to investigate the presence of THMs in tap water samples collected from different localities of Karachi city. Liquid-liquid extraction and gas chromatography with an electron capture detector was used for the determination of THMs. The concentration of THMs in tap water samples of Karachi was found below the WHO guideline values. The mean concentration of chloroform, bromodichloromethane and dibromochioromethane were found as 30.40, 1.04 and 0.09 p,g L' respectively. The relative standard deviations for these THM species were 1.15, 1.61 and 1.81 respectively. Bromoform (CHBr3) was not detected. On the basis of the results obtained by principal component analysis (PCA), CHC13 was found to be the most significant principal component.
The effect of free chlorine residual and pH on the concentration of THMs were also assessed.
KeyWords: Trihalomethanes, tap water, chlorination, PCA, WHO guideline value.
Disinfection with chlorine is a common water treatment process to kill microorganisms. The chlorination of water containing natural organic matter (NOM) also leads to the formation of trihalomethanes (THMs).
Chloroform (CHC13), Bromodichloromethane (CHC1213r), dibromochioromethane (CHC1Br2), and bromoform (CHBr3) are the four compounds belonging to the group of THMs. Several studies are reported about the carcinogenic and non-carcinogenic health effects of THMs. Due to the hazardous health effects, the concentration of THMs in water supplies are monitoring on regular basis in several countries. The maximum contaminant level (MCL) set by USEPA for the concentration of total THMs in drinking water is 80 jig L' (EPA, 1998). The concentration of total THMs regulated by the European Union in drinking water is 100 jig L' (EEC, 1998). WHO guideline values for CHC13, CHC1213r, CHC1Br2 and CHBr3 are 200, 60, 100 and 100 jig U' respectively (Nikolaou et al., 2002).
This study aimed to analyze THMs and its level in the tap water samples of Karachi city. It may be helpful in disseminating awareness about the presence of THMs in tap water samples and the risk associated with the consumption of water containing THMs to the inhabitants of Karachi city.
MATERIAL AND METHODS
Tap water samples were collected in 250 mL amber glass bottles containing 4 grams of ascorbic acid as a quenching reagent of residual chlorine. The bottles with Teflon coated rubber septa were then sealed and stored at 4degC before analysis. Samples were prepared for the analysis of THMs in tap water by using a modification of EPA Method 551.1, which includes Liquid-Liquid Extraction with methyl-tert-butyl ether (MTBE) (Nikolaou et al., 2002). 40 mL glass vial containing 35 mL water sample, 6 grams sodium sulphate anhydrous and 2 mL MTBE was sealed and shaken by hand for 1 minute and left undisturbed for 2 minutes. 1 jiL of the ether phase was then injected into a Perkin Elmer Clams 500 Gas Chromatograph equipped with an electron capture detector (ECD) for the quantitative determination of THM5. The gas chromatographic separation was achieved on a capillary column (30 m length x 0.53 mm I.D. and 1.5 jim film thickness).
The analytical conditions for LLE-GC-ECD method were: Nitrogen and Helium gas (99.9997% pure) were used with constant flow of l4psi. The oven temperature was kept at 80degC for 6 minutes. The temperature of the injector and detector were set at 200degC and 250degC respectively. For the calibration curve, standard solutions of THMs were prepared in concentrations ranging from 0.2 to 1 jig mU' by diluting the 100 ng jiL' THM standard (VOC-Mix 1, Dr. Ehrenstorfer (Germany)).
Concentration of individual trihalomethane species in tap water samples is depicted in Fig. 1. The total concentration of THMs was compared to the WHO guideline values (Fig. 2). Principal component analysis (PCA) was used as a multivariate statistical method to determine the significant principal components among the four THM species found in tap water samples. Table 1 lists the eigenvalue of each factor and the eigenvalue in terms of the percentage of variance explained. Scree-plot for the principal component model of the THM data is shown in Fig. 3. Fig. 4 represents the strong correlation between the concentration of chloroform and concentration of TTHMs. The concentration of free chlorine in tap water samples collected from different sampling sites of the Karachi city is depicted in Fig. 5. Effect of pH on TTHMs is presented in Fig. 6.
The presence of THMs in all water samples was confirmed in this study. However, the concentration of THMs in tap water samples was found below the WHO guideline values. The sum of the ratio of the concentration of each THM to its respective guideline value was found below 1. Chloroform was the only species that was found in a considerable amount in all water samples. PCA study of the THM species in the water samples were based on the eigenvalue-one criterion and the Scree-test. CHC13 was found to be the most significant principal component having the eigenvalue greater than 1. When the eigenvalues were plotted against the number of components in the Scree-plot, CHC12Br and CHC1Br2 were found less significant than the CHC13. The strong correlation between the concentration of chloroform and concentration of TTHMs is expected as chloroform makes up most of the TTHM level (Whitaker et al., 2003). The correlations between BDCM and DBCM with TTHM were also estimated.
The results of the correlation analysis indicated a weak correlation between TTHM and DBCM but no indication of correlation between TTHM and BDCM.
Concentration of chloroform is more likely to be influenced by the residence time and chlorine residual in the distribution system (Whitaker et al., 2003). More the residence time of water in the distribution system, the possibility of the formation of chloroform due to the reaction of residual chlorine with NOM also increases. Generally low concentration of THM in tap water samples may be due to the low level of chlorine residual. It is necessary to leave adequate chlorine residual in every part of the distribution system so as to maintain the chemical and microbial quality of the distributed water (Tamminen et al., 2008; Vasconcelos et al., 1997; Munavalli and Kumar, 2003). WHO set the free chlorine residual in a distribution network should be around 0.2 - 0.3 mg L' (Sarbatly and Krishnaiah, 2007; Tamminen et al., 2008). The concentration of free chlorine residual in almost all water samples is detected below the WHO range.
Concentration of chlorine in water distribution system decreases with time because of its consumption (AlJasser, 2007). Reaction of chlorine with water constituents including corrosion by-products (Al-Jasser, 2007; Zhang et al., 1992; Kiene et al., 1998; DiGiano and Zhang, 2005), microorganisms (Wable et al., 1991), organic impurities, ammonia compounds, iron and manganese ions lead to its consumption. Free chlorine can also be used up by reactions with biofilms formed on the pipe walls and reaction of chlorine with the pipe wall material itself (Al-Jasser, 2007; Wable et al., 1991; Zhang et al., 1992; Kiene et al., 1998; Tamminen et al., 2008; Frateur et al., 1999).
The main cause for the loss of free chlorine residual within distribution networks is a reaction of chlorine on the scales coating the inner pipe surfaces (DiGiano and Zhang, 2005). Maul et al. (1985) discussed that the concentration of both free and total chlorine residual decreases in the distribution system as the residence time increases while travelling from the water treatment plant. The free chlorine concentration of water also reduces when evaporation of chlorine increases at a temperature of around 25-35 degC (Sarbatly and Krishnaiah, 2007). The stability of hypochlorate also decreases by photo-degradation process in the presence of strong sunlight (Sarbatly and Krishnaiah, 2007; Nowell and Hoigne, 1992).
It is therefore justified that free chlorine content of the treated water when it leaves the distribution system might be within the WHO range, but during its distribution to the Karachi city through a network of primary distribution mains, pumping stations, reservoirs and secondary distribution pipeline, the concentration of free chlorine residual gradually decreases. This in turn leads to either very low or no chlorine residuals in the farthest parts of the distribution system as well as at the consumer's tap, which may results in high risk of bacterial contamination of drinking water.
pH is also a variable that influences the formation of THMs. It is reported that the concentration of THMs increases at higher pH (Whitaker et al., 2003; Chen and Weisel, 1998) but there is also some indication that the distribution of THM species is almost independent of pH (Nokes, 2003; Ichihashi et al., 1999). pH of all the water samples were recorded in the range of 6.7-8.2. No significant influence of pH on THM level was observed.
Table 1: Eigenvalues and explained variances for the THM concentration in tap water samples
Components###Eigenvalue###Explained variance %###Cumulative variance %
Presence of THMs in drinking water of Karachi was confirmed by this study. Therefore, it is a matter of utmost importance that the authorities responsible for providing drinking water to the city must design strategies
to improve the quality of raw water by reducing the NOMs and to remove the THMs after they are formed during the treatment and distribution processes.
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Corresponding author: E-mail: email@example.com
Zahida Karim, Majid Mumtaz and Tuba Kamal Department of Chemistry, University of Karachi, Karachi, Pakistan
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|Publication:||Journal of Basic & Applied Sciences|
|Date:||Jun 30, 2011|
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