A water disinfection system using simultaneous multiple disinfectants includes a completely stirred tank reactor (CSTR) and a contact basin or clearwell acting as a plug flow reactor (PFR). The CSTR receives ammoniated water and free chlorine, and generates multiple disinfectants in primary disinfection water within the CSTR. The contact basin or clearwell receives primary disinfected water from the CSTR, the primary disinfected water containing the disinfectants. A method for water disinfection including mixing the non-disinfected treated water with ammonia to form ammoniated water, mixing the ammoniated water with free chlorine to form a primary disinfection water including multiple disinfectants, and maintaining the disinfectants in the primary disinfection water.