الفهرس 
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Abstract
Chlorination is widely applied as a disinfectant during processes of drinking water treatment for destroy waterborne pathogenic organisms. Although its great useful for human beings, the chlorine reacts with natural organic matter (NOM) consequently, the undesirable disinfection byproducts (DBPs) forming. Among formed DBPs in chlorinated water, trihalomethanes (THMs) represent a significantly high fraction. THMs have adverse health effects on human beings and main three compounds of trihalomethane classified as probably carcinogenic to humans which are bromoform, bromodichloromethane and chloroform. Prediction of THM concentration is difficult due to the complex reactions and various hydrodynamic systems and chemicals used in drinking water treatment plants (DWTPs) and supply systems. However, high levels of THM precursors (e.g., Free Chlorine, Total Organic Carbon or Bromide Ion) indicator to possibility of THM formation. Purposes of current study are assessing THM and THM precursors levels in the main chlorinated treated drinking water sampled of Aseer region. Consequently, Determine the natural and operational factors that involved in THM formation. Over four consecutive seasons, conducted this study on main chlorinated treated drinking water sources in Aseer Region in the Kingdom of Saudi Arabia. Because of unique topography of Aseer and it’s depended on four drinking water sources (surface treated water, ground treated water, desalinated water and blended water of previous three types), that spatial variation helps to achieve purposes of this study. Study covered thirteen varied drinking water sources including five conventional filtration treatment plants of surface water and two non-conventional treatment plants of ground water in addition to six desalinated and blended drinking water reservoirs. Conducted analysis on 160 samples that were sampled during this study included: (4THM) compounds via Gas chromatography Mass Spectrometry (GC-MS), THM precursors (F.Cl, TOC, Br-, Temperature, Turbidity, Color) and water quality parameters tests which are the microbial, chemical and physical tests. Results have shown that THMs levels of ground treated water, desalinated water and blended water were within the permissible concentration, whereas average THM levels of surface treated water were exceeded the permissible concentration with maximum Total Trihalomethanes (TTHM) values ranged (111.16 ppb-208.76 ppb) with dominant of brominated THMs (i.e. bromoform (CHBr3) and dibromochloromethane (CHClBr2)). Results revealed that positive correlation of natural factors in determining the extent of THMs formation which are: Level of TOC, Color, free Chlorine concentration, Turbidity, Temperature and Bromide Ion and pH level respectively. In addition to operation factors which were: Post-chlorine dose, Pre-chlorine dose and Contact time respectively.