Investigation of Trihalomethanes formation in Greater Kandy Water Treatment Plant and its distribution
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Date
2015
Authors
Perera, M. G. N.
Weragoda, S. K.
Journal Title
Journal ISSN
Volume Title
Publisher
Uva Wellassa University of Sri Lanka
Abstract
No doubt that chlorination has been successfully used for the control of water borne infections
diseases for more than a century. Halogenated trihalomethanes (THMs) and haloacetic acids
(HAAs) are two major classes of disinfection byproducts (DBPs) commonly found in waters
disinfected with chlorine (Rook, 1974). The formation of the Trihalomethanes (THMs) was
investigated in Greater Kandy Water Treatment Plant (GKWTP) and distribution system which
serve drinking water to Kandy region, located in the middle province of Sri Lanka. Water samples
were taken from storage tank of GKWTP of the National Water Supply and Drainage Board (NWS
& DB), covering selected water quality and operational parameters that have direct influence on
THM formation. In addition THM formation at the distribution extremities were also studied.
Methodology
Water samples were taken from storage tanks of GKWTP after the chlorination of initial dosage
of 2 ppm chlorine. Water samples of 32 taken from selected distributed within six Divisional
Secretariat Divisions for the analysis. Primary Trihalomethanes were analyzed using Gas
Chromatography – ECD (Kuivinen, 1999). Formation of Trihalomethanes were analyzed in
storage tanks for 64 hours by doubling time and level of Trihalomethanes in distribution system.
Apart from THMs, pH, temperature, turbidity, were analyzed in raw water and treated water
collected at sampling points by pH meter, thermometer and turbidity meter respectively. Treated
water was also tested for free chlorine level and total chlorine level to observe impact on THM
formation by those parameters by colorimeter.
Result and Discussion
Measured THMs and other parameters for storage tanks in GKWTP summarized in Table 1 and
indicate in figure 1. This indicate that the trihalomethanes increase with the time in storage tanks.
Initially mean TTHMs was 17.09 µg/L when the initial dose of chlorine of 1.85 mg/L. When
doubling the reaction time formation of THMs were increased and finally it became 40.72 µg/L
when reaction time become 64 hours. Table 1 indicates that the free chlorine and total chlorine
decaying with time. Temperature is constant for the whole analysis as 25 C, as it highly depend
on temperature. Concentrations of CHCl3, CHCl2Br and TTHM of sampling locations indicate in
Table 2 in 32 sampling points. Table 2 summaries descriptive statistics for individual and total
THMs (TTHMs) with free chlorine in the water samples of distribution system. Highly variable
range of TTHMs concentrations were found (11.275 to 22.976 µg/L) in distribution system.
Chloroform (CHCl3) concentrations contribute a significant portion to the TTHMs (76%) while
Bromodichloromethane (CHCl2Br) contribute 24%.
Regression model for GKWTP. The regression equation is:
TTHM (µg/L) = 19.5 + 0.00580 Time (min) R-Square (adjusted) value = 95.9%; p < 0.05
Conclusions
When considering storage tanks conclude that the formation of trihalomethanes depend on
reaction time and free chlorine while temperature, pH, turbidity remains constant. CHCl3,
CHCl2Br and TTHMs levels at all locations were found lower than the guideline values regulated
in WHO and USEPA which is lower than 25 µg/L. Due to the free chlorine ranges to 0.2 to 0.7
mg/L in distribution system, conclude that residual chlorine exceeding its standard value of 0.2
mg/L for drinking water. Variation of trihalomethanes and free chlorine values of storage tanks
and distribution system, assume that chlorine reactions take place and evaporation of free chlorine
during analysis. Other two compounds of trihalomethanes such as Dibromochloromethane
(CHClBr2), and Bromoform (CHBr3) could not be detected in GKWTP.
Description
Keywords
Mineral Sciences, Materials Sciences, Water Treatment, Water Chemistry, Water Management