Chlorination of drinking water is carried out to kill microbes that may exist in the water. It is known that this chlorination process could lead to the formation of "disinfectant by-products" (DBP's), some of which have been found to have carcinogenic properties. To date, trihalomethanes, haloacedic acids (HAAs), haloacetonitrjles (HANs), and haloketones (HKs) are some carcinogenic DBP's that are known to exist in the drinking water.

The chlorination of drinking water began in Mongolia in 1968, but to date the concentration of DBP's have not been surveyed. The aim of this study is to measure the concentration of THMs, HAAs, and HANs and to establish a relationship between precursors of some DBPs and the concentration of DBPs in the drinking water of Ulaanbaatar.

Three samples of water (raw, treated and tap water) was collected from stations A, B and V of the centralized system. Quality parameters of the drinking water were established using the universal EPA standards. The water concentrations of THMs, HAAs and HANs were measured using Gase chromatography, and calculated using Mathematical modeling.

If was found that the concentration of total organic compounds (TOC) in raw water samples were, 1.32 ±2.01mg/L compared to 0.68±1.23mg/L in treated water samples. It is thought thatTOC's at this concentration level provide a favorable environment for the absorption of UV-254 and THM's and the formation of the DBP's in Mongolia's drinking water. The concentrations of DBPs in Ulaanbaatar's drinking water were measured as follows; THMs = 5.247±3.14 mg/L, HAAs = 1.288 ±1.181 mg/L, HANs = 0.50 ±0.133 mg/L with the average content in tap water registering around 4.173±2.43 mg/L. The content of DBP's and inorganic compounds were found to be higher in the tap water at station V