Method and composition for inhibiting growth of microorganisms including peracetic acid and a second organic biocide

a method for controlling the growth of microorganisms in pulp and paper mill process water including the step of administering a sufficient amount of a peracetic acid and a sufficient amount of a second organic biocide to the process water to reduce the growth of the microorganisms;

• said biocide being selected from the group consisting of isothizolin;
• methylene bisthiocyanate;
• DBNPA;
• carbamate;
• quaternary ammonium compounds;
• bronopol;
• 4,5-dichloro-1,2-dithio-3-one;
• and 4,5-dichloro-2-N-octyl-4-isothiazolin-3-one said amount of peracetic acid and biocide being selected to result in a synergy index of less than 1 wherein said synergy index is calculated by determining a first ratio of the amount of peracetic acid required to produce a level of microorganism growth control when added in combination with the amount of biocide to the amount of peracetic acid required to produce the level of growth control in the absence of the biocide, and adding the first ratio to a second ratio of the amount of biocide required to produce the level of growth control when added in combination with the amount of peracetic acid to the amount of oxidizing biocide required to produce the level of growth control in the absence of the peracetic acid.