The structural formula of glutaraldehyde is shown in Figure 5-30. When dissolved in water, the active molecular forms which contain carboxyl groups react with amino groups in proteins and polyhydroxy compounds common in most microorganisms rendering control in water systems.

Chemically, the cell walls of bacteria contain a unique protein which, specifically, is a combination often different amino acids. The amino groups are typical organic bases and effective nucleophilic compounds.

Glutaraldehyde reacts with the proteins of the bacterial cell wall and the proteins within the internal portions of the cell via a nucleophilic substitution reaction. The reaction will be internal or external depending upon the pH of the water. Under alkaline water conditions, glutaraldehyde reacts with the free (unprotonated) amino groups on the outside of the cell wall. In doing so, glutaraldehyde cross-links and binds the amino groups on the cell wall preventing the cell from functioning by inhibiting nutrients from entering the cell and waste products from leaving the cell (Figure 5-31). Under acidic conditions, the amino 

groups on the outside of the cell wall are positively charged (or protonated). In this case, glutaraldehyde penetrates the cell, and once inside, functions in the same manner as it does at high alkalinity, cross-linking the free amino groups of the proteins (Figure 5-32). Since all microbial enzymes utilized for energy production are proteins or combinations of proteins, their structure becomes altered through cross-linking, thus rendering them nonspecific and unreactive. In this manner, glutaraldehyde functions as an enzyme poison, extremely reactive and broad spectrum in scope.

Glutaraldehyde shows particular strength in controlling sulfate-reducing bacteria. Published literature specifically points out the combination of glutaraldehyde and isothiazolone as being very effective in the control of this troublesome bacteria. Glutaraldehyde is also unaffected by oil contamination, making it a popular toxicant in refinery and oil production applications.

In reviewing the mode of microbiocidal action of glutaraldehyde, it is evident that it shows efficacy over a wide pH range. The ability of this material to work more rapidly at high pH makes it a popular choice in alkaline industrial water systems. A typical rate of kill curve at different pH's is shown in Figure. 

One area where this material shows reduced activity is in systems with high ammonia contamination. Since the toxic nature to microorganisms is derived from its reaction with proteins that are amine based, ammonia contamination ties up the active molecular form of glutaraldehyde, making it unavailable for biofouling control.