The present invention provides a method of treating an aqueous system for biocide control, by adding an acidified solution that includes monoalkyldithiocarbamate salt and alkyl isothiocyanate to an aqueous system. A biocide feed system is also provided that is configured to generate a reproducible level of alkyl isothiocyanate. The method and system provide effective levels of alkyl isothiocyanate for rapid biocidal activity, along with levels of monoalkyldithiocarbamate salt which can decompose to form additional alkyl isothiocyanate and provide longer-term biocidal activity.