Ever increasing demands for reduced fresh water use in the paper and pulp industry lead into more closed circuits. The result of this is less clean cooling water in circuits than before. Accumulating impurities provide nutrients for microbes. In addition to increase in impurities also temperatures often rise to levels that are more optimal to microbes than before. All this results in problems when accumulating biomass in conjunction with dirt and impurities start to clog passages in heat exchangers and cooling tower cores.