Method and composition for inhibiting growth of microorganisms including peracetic acid and a non-oxidizing biocide

A method for controlling the growth of microorganisms in industrial process water including the step of administering a sufficient amount of a peracetic acid and a sufficient amount of a non-oxidizing biocide to the industrial process water to inhibit the growth of the microorganisms, said non-oxidizing biocide selected from the group consisting of decylthioethylamine, orthophthaldehyde, dodecylguanidine hydrochloride, 1-(3-chloroallyl)-3,5,7-triaza-1-azoniaadamantane chloride, dibromo dicyanobutane and bis(trichloromethyl)sulfone, said non-oxidizing 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 inhibition when added in combination with the amount of non-oxidizing biocide to the amount of peracetic acid required to produce the level of growth inhibition in the absence of the non-oxidizing biocide, and adding the first ratio to a second ratio of the amount of non-oxidizing biocide required to produce the level of growth inhibition when added in combination with the amount of peracetic acid to the amount of non-oxidizing biocide required to produce the level of growth inhibition in the absence of the peracetic acid.