One of the more popular proprietary toxicants utilized in industrial water applications is 5-chloro-2-methyl-4-isothiazolin-3-one which makes use of a blend of two specific isomers shown in Figure 5-27. This material has gained widespread acceptance because of its effectiveness on a broad spectrum of microbes in a wide variety of water types.

Bacteria obtain their energy from a complex series of oxidation-reduction reactions which are catalyzed by proteins called enzymes. There is a specific enzyme for each reaction in this complex series. Any disruption of this sequence will result in cell death. In addition, other cellular proteins serve as a source for the essential amino acids necessary for life. Almost all proteins possess covalent disulfide bonds as part of their structure. The active isothiazolone molecule cleaves this disulfide linkage and denatures the protein. Denaturation inactivates the enzymes and cellular proteins essential for metabolism causing death to the organism (Figure 5-28).

Isothiazolone is effective against bacteria (including sulfate-reducers), fungi and algae. This is not always the case with many products because of the manner in which they function. For instance, a product that kills an organism by tying up an essential metabolite will function only on specific organisms. Such a product will be ineffective on an organism that does not require that metabolite.

Isothiazolone has the added benefit of retaining its effectiveness across the spectrum of pH values normally encountered in industrial water systems and is not hindered in any way by waters high in dissolved or suspended solids, oil contamination or nitrogen-bearing compounds.

The one situation that should be avoided when using this toxicant is waters high in sulfide concentration. Since the primary mode of microbiocidal action is the tying up of disulfide linkages present in the cell, isothiazolone will be consumed by high levels of sulfide and not be available for microbe control.

Environmental characteristics of isothiazolone are also quite attractive with degradation of the toxicant from the discharge stream occurring via a number of pathways, including hydrolysis, biodegradability and photolytic breakdown.