The statements in the following discussion are based on research, field experience and the state of the art. It is necessary to point out, however, that when microbiocides are used in industrial water systems the possibility exists of side reactions which may occasionally alter their activity.

Microbiocides inhibit microorganisms in a variety of ways. Some alter the permeability of cell walls, thereby interfering with the vital life processes of the microbe. Heavy metals penetrate the cell wall and enter the cytoplasm, destroying protein groups essential for life. Surfactants damage the cell by reducing its permeability, disrupting the normal flow of nutrients into the cell and the discharge of its wastes; this denaturizes the protein, causing the organism to die.

Cationic surfactants, such as the quaternary ammonium compounds, adsorb to the cell membrane, chemically reacting with the negatively charged ions associated with the cell wall. Anionic surfactants reduce cell permeability and eventually dissolve the entire membrane. The isothiazolone molecule cleaves the disulfide linkage and denatures the proteins of a cell which are essential for energy production, causing death to the organism.

Other chemical agents such as the organosulfur compounds inhibit enzyme-substrate metabolite reactions. They competitively react with an enzyme in place of the normal metabolite reactions or noncompetitively attach to an enzyme at a point different from the normal metabolite and, as before, prevent the normal, life-sustaining enzyme reaction.

Oxidizing chemicals irreversibly oxidize protein groups, resulting in a loss of normal enzyme activity, and subsequently the rapid death of the cell.

A number of factors will determine the choice between an oxidizing and a nonoxidizing microbiocide. Wood deterioration has already been mentioned. The affect on corrosion rates on the system metallurgy is also a factor. The toxicant chosen must be capable of broad-range microbial control and the product of choice should be that which works best, regardless of its inhibitory mechanism.

The operating parameters of the industrial water system will also affect the choice of a microbiocide. Temperature, pH and system design are fundamental considerations in a decision involving oxidizing or nonoxidizing toxicants. In the following sections these factors will be discussed wherever their presence has been found to affect the efficacy of commonly used microbiocides.