This being the "Power Generating" forum, I assume you're using Sodium Hypochlorite as a biocide in an open cooling water system. If this is the case, I would offer some more thoughts :
There are basically two types of industrial biocides, oxidising and non-oxidising. Halogens (chlorine, bromine compounds) are oxidising. Whilst they can be effective at killinf free bacteria, the halogen levels tend to fall fairly quickly in the cooling water, particularly where the system is exposed to sunlight. Virtually continuous dosing may be required to maintain halogen concentration at a level that kills bacteria.
Non-oxidising compounds are more like "industrial poisons", and vary from manufacturer to manufacturer. Their effectiveness at maintaining bacteria levels depends upon their constitution.
In Britain, the Health and Safety Executive issue guidelines for cooling water systems, in respect of legionella. The document is known as "L8" and can be obtained from the HSE website (at a cost). This recommends alternate dosing of two types of biocide, since bacteria has been known to develop immunity over time to a single biocide.