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Microbiological control |
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The selection of a microbiocide involves several factors. First, it must be effective at inhibiting almost all microbial activity, as determined by the proper laboratory screening procedures. Second, it must be economical in a treatment program. This is often accomplished by combining a small amount of an expensive, but highly effective, microbiocide with another less expensive one, resulting in broad-spectrum control at reasonable costs. The use of oxidizing biocides is common in many industrial water applications. However, their success is contingent on proper frequency of application and dosage level. If a controlled feeding program is impractical, the use of these biocides may be limited by the potential for increased corrosion rates or industrial tower lumber deterioration. In this situation, the microbiocide choice would best be limited to nonoxidizers, which can provide effective control while leaving wood unaffected. Environmental discharge and disposal considerations constitute another factor which determines the choice of microbiocides. Disposal problems caused by toxicity have limited the use of certain microbiocides in many areas. In other situations, the microbiocide chosen must be easily detoxified before industrial system bleedoff reaches receiving streams. The frequency of treatment required also reflects cost considerations. Figure below shows suggested selection according to the pH.
As mentioned earlier, proper laboratory testing can establish a maximum tolerable bacterial population limit in the system. In Figure 5-18 those areas denoted as "acceptable programs" fulfill two basic requirements. They provide both a high percent reduction in bacterial count and an ideal period between dosings, which makes the program economically desirable. That portion of the graph labeled "high growth rate period" provides the proper percent of inhibition; the microorganisms quickly build up to the desired maximum level, however, making necessary the addition of more microbiocide to maintain control. In such a situation, the system may require daily dosage, which is often much too costly. An average of one to three normal "slug" doses per week can be considered ideal. There are cases, however, where the problem is so severe that more is needed. In that area of the graph labeled "percent kill low," the drawbacks of the program are obvious. The bacterial counts are close to the predetermined maximum level and may well lead to biofouling. The Mechanism of Microbiocidal Action 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
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