Methods for disinfecting surfaces and for sanitizing bodies of water using a single-feed, bromine-based biocide are described. These methods use concentrated liquid biocide compositions comprising biocidally active bromine as the biocide. Also described is a process of producing the concentrated liquid biocide composition: mixed together are (a) bromine chloride and (b) an aqueous solution of alkali metal salt of sulfamic acid having a pH of at least about 7, in amounts such that (i) the active bromine content of the composition is at least about 100,000 ppm (wt/wt), and (ii) the atom ratio of nitrogen to active bromine in the composition is greater than 0.93. Use of bromine chloride as the source of the active bromine in the process is advantageous because in the resulting aqueous compositions, all of the bromine of the bromine chloride is made available as active bromine in solution. In other words, the chlorine of the bromine chloride is converted in the process to dissolved alkali metal chloride salt, thereby liberating all of the bromine as the active bromine content of the biocidal composition.