More than one half of drinking water demand in the Czech Republic is covered by surface water resources. The most frequent problems of the water treatment and supply are caused by the eutrophication of reservoirs. Biological parameters of drinking water quality based upon the microscopic analysis are included in local standards and commonly used in water works laboratories. Several new methods and criteria have been elaborated: microscopic evaluation of the separation efficiency of water treatment, categorization of microorganisms according to their separability, a screening bioassay for the assessment of biological stability of the water in course of the treatment and distribution, biological indicators of the eutrophication degree of reservoirs and their tributaries. A set of supplementary biological methods and criteria for special cases was published. Algal toxins have been detected not only in cyanobacterial water blooms in reservoirs but also in periphyton layers growing in different units of water works treatment trains. Two examples of the integrated system approach to the water supply from eutrophicated reservoirs inclusive of proposed preventive and remedial measures are selected from a recently finished extensive research project. Improvement actions in the resources protection, water works design and operation, laboratory control system and legislation are recommended.

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