During the process of ,,enhanced coagulation" except for colloids and suspensions removal, the removal of associated organic compounds including DPD (Disinfection By-Products) precursors is crucial. It is often necessary to decrease color and turbidity of treated water to values which arc significantly lower than accepted for drinking water. On the basis of presented results of the research it was found out that under strict technological conditions coagulation of low mineralization waters ensures effective treatment, including significant THMs precursors removal, even when water is of low temperature. However, it is necessary to apply two different methods of coagulation (volumetric coagulation and direct filtration) dependently of water temperature with the usage of the same equipment (a flocculation chamber, a vertical sedimentation tank and a pressure rapid filter) including the same point of a coagulant dosing. During the periods of ,,high temperature" the treatment should be based on volumetric coagulation and during the periods of ,,low temperature" of water direct filtration should be applied.

The water of the Wisła-Czarne reservoir is of very low hardness and alkalinity. In spite ofhigh SUVA values it is not susceptible to enhanced coagulation. In order to achieve the assumed results, coagulation with ALS (aluminium sulphate) should be conducted in two optional technological systems - in a conventional system and in "in-bed" coagulation dependently ofwater quality and its temperature. Effective treatment with ALS is possible, even at low temperature of the water, but at strict technological parameters. However, because of significant variations of water quality, especially after rainstorms, it is very difficult to meet such requirements. Application of pre-hydrolyzed Flokor 1,2A instead ofALS enables to eliminate reagents to the pH adjustment and to apply "in-bed" coagulation when water supplied to the WTP is of low turbidity. To assure stable technological system operation, in aspect of raw water quality changes, some activities were also undertaken, i.e. modernization of rapid filters, which involved a drainage system and exchange of sand bed for anthracite-sand bed. Treatment based on direct filtration results in decrease of reagents usage and, what is especially important, effective DBPs precursors removal.