The presented report deals with the description of raw water quality focusing mainly on the nature of the contained organic substances, then a comprehensive evaluation of the operational efficiency of existing aggregation and laboratory optimization of coagulation operating parameters - especially the dose of coagulant and reaction pH. Coagulation in combination with sedimentation and sand filtration must be understood not only as the first stage of water treatment before ultrafiltration, but also as protection of the membranes themselves from their clogging, especially by macromolecular organic substances. From this point of view, it is necessary to operate coagulation with subsequent separation of aggregates with the maximum possible efficiency.