The general objective of this work was to examine the applicability of ozone for treating groundwater containing significant amounts of natural organic matter (NOM). The water, used in this work, is taken from the eastern Slavonijan region, and is originally used for preparation of drinking water for the city of Osijek. Ozonation is used in drinking water treatment to accomplish numerous treatment goals, including disinfection, removal of inorganic species, degradation of organic matter and as an aid to the coagulation-flocculation process. The emphasis of this work was to study the influence of ozonation on the degradation of natural organic matter, particle destabilization, and increase in the biodegradability of ozonation products as well as arsenic removal from water. For this purpose, an experimental set up was designed for laboratory experiments. It consisted of an ozone generator, contacting tank, thermostat and ozone measuring device. The results of the laboratory experiments have shown a small decrease in dissolved organic carbon and a substantial decrease in UV absorbance at 254 nm as well as an increase in biodegradability of ozonation products. BOD to COD ratio has been chosen as a biodegradability index. An increase of the BOD value due to the pretreatment with the ozone indicates greater tendency to biodegradation. Therefore, an increase in the BOD to COD ratio after ozonation indicates improved biodegradability due to an increase of lower molecular weight organic compounds such as aldehydes, carboxylic acids and keto acids that are more easily biodegradable. In addition, adsorption capacities of four different granular activated carbon (GAC) types were determined from isotherm experiments. The Freundlich model was selected to describe the adsorption of natural organic matter onto GAC and the prediction of their removal in larger-scale GAC applications. In order to confirm preliminary results obtained from laboratory experiments and to make a general conclusion, a pilot plant was designed. It consisted of an ozone generator, ozone contact column and two GAC filters. Through one of the GAC filters passed ozonated water and through another passed directly nonozonated water. The pilot study showed that ozonation of natural organic matter prior to adsorption onto GAC is beneficial because of the formation of biodegradable organic compounds that can be easily removed by biologically activated carbon filtration. Regarding the possibility that the maximum contaminant level (MCL) for total arsenic in drinking water will be substantially reduced in the near future, special attention has been devoted to arsenic removal from water. The results obtained from the pilot study indicate that arsenic removal efficiency depends on the amount of iron content in water. In order to encourage the adsorption and co-precipitation of arsenic with iron hydroxide precipitates, ferric chloride was added. For an arsenic concentration of approximately 50 μg/L, 1 mg/L Fe(III) was required to reduce arsenic concentration to less than 10 μg/L.