The present invention provides methods and formulations for reducing or inhibiting increase in the concentration of microbes in a water-based fluid. The methods and formulations of the present invention use glutaraldehyde and a hydroxymethyl-substituted phosphorus compound selected from the group consisting of tetrakis(hydroxymethyl)phosphonium salts, C1-C3 alkyl- and alkenyltris(hydroxymethyl)phosphonium salts and tris(hydroxymethyl)phosphine, in a ratio of hydroxymethyl-substituted phosphorus compound to glutaraldehyde in the range of about 2:1 to about 7:1, or about 3.5:1 to about 7.5:1. The methods and formulations of the present invention can be useful in treating water contaminated with aerobic or anaerobic bacteria in oilfield and other industrial applications.
