The thesis entitled "Butoniga water conditioning plant in Istria and the process of infiltration of raw water" briefly describes the organization of the water supply system of the Istrian County with emphasis on the area of operation of Istarski Vodovod d.o.o. Buzet, and the process of water conditioning at the plant of the work unit RJ Butoniga is described in detail. The plant in the working unit of RJ Butoniga operates as an independent unit in the system of Istarski Vodovod d.o.o. Buzet and is one of the most modern water conditioning systems in the Republic of Croatia. The Butoniga plant in the process of water conditioning from the accumulation lake of the same name includes pre-treatment, main conditioning and auxiliary processes of wastewater treatment generated during cleaning and washing of filters and neutralization of wastewater from chemicals. The part of this thesis presents the collected data on the quantities of water produced in the Butoniga plant, and a comparison of the characteristic parameters of raw water and water after conditioning as well as a comparison of the characteristic parameters of water before and after the rapid filtration process. In the final part of this thesis, the filtration process, the characteristics of slow and fast filtration are described in more detail, and an example of the calculation of individual parameters in the filtration process is presented.