Systems and methods for controlling microbial growth and/or activity in a gas field fluid or oil field fluid are provided, comprising: a) adding a first biocide component, such as glutaraldehyde, C12-16-alkyl dimethyl benzyl ammonium chloride (ADBAC quat), glutaraldehyde and ADBAC quat, tetrakis(hydroxymethyl) phosphonium sulfate, 2,2-dibromo- 3-nitrilopropionamide, [1,2-ethanediylbis(oxy)]bismethanol, 5-chloro-2- methyl-4-isothiazolin-3-one or chlorine dioxide and which is not dazomet to the gas field fluid or oil field fluid and b) after a delay of about 1 minute to about 30 minutes, adding a second biocide component , which is 3,5-dimethyl-1,3,5-thiadiazinane-2-thione (dazomet) to the gas field fluid or oil field fluid wherein the first biocide component and second biocide component are added in an amount effective to control microbial growth or activity.
