A process of producing a concentrated liquid biocide formulation is described. Mixed together are (a) bromine and (b) an aqueous solution of alkali metal salt of sulfamic acid having a pH of at least about 12, in amounts such that (i) the active bromine content of the solution is at least about 100,000 ppm (wt/wt), and (ii) the atom ratio of nitrogen to active bromine from (a) and (b) is greater than 1. A continuous process for producing the concentrated liquid biocide composition is provided. This process comprises continuously feeding into mixing apparatus (i) bromine and (ii) an aqueous solution of alkali metal salt of sulfamic acid; the desired product is withdrawn from the mixing apparatus at a rate sufficient to enable the continuous feeding to be maintained. Also described are methods for disinfecting surfaces and for sanitizing bodies of water using a single-feed, bromine-based biocide. These methods use concentrated liquid biocide compositions comprising biocidally active bromine as the biocide.