Processes for biofilm or microorganism growth control in an aqueous system such as a pulp, paper or board manufacturing system are described in which a halogenated hydantoin is added to the aqueous system in combination with haloamine, chlorine dioxide or a combination thereof. The halogenated hydantoin is preferably a fully or partially halogenated dialkyl hydantoin, and more preferably 5,5-dimethyl hydantoin or 5-methyl-5-ethyl hydantoin. The haloamine is preferably a monohaloamine, dihaloamine, trihaloamine, or a combination thereof, and more preferably monochloramine, monobromamine, bromochloroamine or a combination thereof. The halogenated hydantoin is preferably added to the aqueous system in portions of the system susceptible to gas phase corrosion, such as the short loop of the system, and the haloamine and chlorine dioxide are preferably added to other portions of the system. The halogenated hydantoin and haloamine and/or chlorine dioxide maintain good compatibility with each other in the absence of excess free chlorine.
