NOVELTY - The reduction of impurities, which are formed by organic dyestuffs, biocatalytic substances, microorganisms and/or other biological material in a water system during production of paper from fibrous material in watery suspension, comprises generating gas discharge and/or plasma by application of a voltage to electrodes in water (23) or suspension. A voltage profile of the voltage has voltage pulses having pulse duration of 100-900 ns. A streamer discharge is generated in a voltage profile. The voltage produces an electric field of 1-10 Mv/cm.    USE - For reducing impurities that are formed by organic dyestuffs, biocatalytic substances, microorganisms and/or other biological material in a water system during production of paper (claimed).    ADVANTAGE - The procedure ensures fast and effective production of the paper without environmental pollution by the reduction of the impurities, which causes corrosion of paper making plant.    DETAILED DESCRIPTION - The reduction of impurities, which are formed by organic dyestuffs, biocatalytic substances, microorganisms and/or other biological material in a water system during production of paper from fibrous material in watery suspension, comprises generating gas discharge and/or plasma by application of a voltage to electrodes in water (23) or suspension. A voltage profile of the voltage has voltage pulses having pulse duration of 100-900 ns. A streamer discharge is generated in a voltage profile. The voltage produces an electric field of 1-10 Mv/cm. An ultraviolet radiation is produced through the voltage pulses. The concentration of the impurities in water or the suspension is measured by online. The concentration of protein and/or DNA-residue in water or suspension is measured by real-time or online for controlling and/or regulating of microbiological germination. A value of the concentration is used as control variable for voltage releasing the streamer discharge. Waste paper, wood matter, pulps and/or textile fiber are used as the fibrous material. The water system has a sieve water circulation as primary circulation, clear water circulation as secondary circulation and a circulation for purified wastewater as tertiary circulation. Chemically active substance and/or complementary colors are also used for the reduction of impurities.    DESCRIPTION OF DRAWING(S) - The figure shows a paper making plant.    Plasma reactor (11)    Material catcher (17)    Water (23)    Wet chamber area (24)    Power lines. (47)