Selected biocides

For information on the general activities and mechanisms of action of biocides, the reader is referred to recent reviews on this subject (McDonnell and Russell, 1999 Donlan and Costerlon, 2002) and to other ehapters of this book. We have not attempted to present an exhaustive overview on the entire range of bioeides and their efTieaey 

In water at pH values between 4 and 7, chlorine predominantly exists as hypochlorous acid (HOCl) it is in equilibrium with the hypochlorite ion (OCf), which predominates above pH 9. Hypochlorous acid is the rapidly acting component, and is mainly responsible for the microbicidal activity of chlorine. Thus it is suggested that low pH values enhance the biocidal activity, whereas high pH values ( 7) favour hypochlorite ion-mediated disruption and detachment of mature biofilms (Characklis, 1990). In this respect, data from the literature for chlorine efficacy on biofilms are difficult to compare because either different pH values were used or pH values are not mentioned. Moreover, most studies focus on the killing of biofilm organisms, while a limited number of studies also include the aspect of biofilm detachment from surfaces (Table 1). 

System Material Organisms Age of biofilm Product Contact time Concentration Reduction in viable cell counts Detachment of biofilms References 

Pipe system PVC pseudomonads, mycobacteria 8 w sodium hypochlorite 7d 10-15 ppm free chlorine incomplete incomplete Vess et ah, 1993 

Tube system silicone tap water biofilm 50 d chlorine 24 h 0.3mg/L CI2, lOmg/L CI2 none, 4 log units none Exner et ah, 1987 

---

Although there are many biocide alternatives available on the market, for example enzyme technology or bio-dispersants, there appears to be a continued requirement for the use of biocides in order to reduce the levels of microbiological contamination entering the paper making process. The increased awareness of environmental and safety aspects will continue to play an important role on the selection of biocides for paper making processes. The use of legislation to select biocides must be done in parallel with each plants internal risk assessment. No one biocide active will meet all the criteria set out by different European countries and hence the use of these actives must be carefully assessed on a plant by plant basis. 

Customer preference is also an increasingly important consideration when selecting biocides for use in products. Large manufacturers such as Ford, Volvo, BMW and SKF, to name but a few, are producing lists of materials they do not want to see in their factories. Many biocides are on these lists. Currently, they are not prohibited, but this may only be a matter of time. 

Colinas C, Ingham E, Molina R (1994) Population responses of target and non-target forest soil-organisms to selected biocides. Soil Biol Biochem 26 41 17... 

Bartolome, M.C. and Sanchez-Fortun, S. Effects of selected biocides used in the disinfection of cooling towers on toxicity of bioaccumulation in Arfe/ni a larvae. Environ. Toxicol. Chem., 24(12) 3137-3142, 2005. 

Figure 16.8 Structures of selected biocides. See Table 16.4 for persistence data.

Selective toxicity. If toxic compounds are to be used for controlling diseases, pests, and parasites, it is important to develop selective biocides, toxic to the target organism but less toxic to other organisms, particularly humans. 

Industrial consumption of organotins, for example, rose from about 5000 tons in 1965 to about 35,000 tons in 1985. The uses of nontoxic organotin compounds (R2SnX2 and RSnXs types) account for about 67% of the total world production, although use of RsSnX types as selective biocides has increased disproportionately. Tin now has more of its organometallic derivatives in commercial use than any other element. 

The impact of each biocide on microbial populations, together with the compatibility studies, can be used to select the most effective biocidal treatment. The biocides are first evaluated for their ability to inhibit metabolic activity at the time of maximum growth activity this maximum time is determined from growth curves of the microbial populations of the formation and river wastes. Growth curves are then measured for the selected biocide(s) at different concentrations to determine the optimum level of biocide required and the period necessary for alternate slugging of the injection waters. The biocidal treatment is then determined from the growth-concentration curves and the compatibility studies. 

Introduction. To operate a successful microbial control program, it is essential to use standard field methods for evaluating microbial populations, screening and selecting biocides that will be effective in controlling microbial populations in the paper system. This section outlines, in detail, field microbiological methods used to monitor bacterial populations and to measure their response to biological control measures. 

Finally, criteria for selecting biocides suitable for paint formulation and its intended end use are considered. 

Follow the manufacturer s recommendation for level of selected biocide and guidelines on point and method of introduction. 

Sanchez-Fortun, S. Llorente, M. T. Castano, A. Genotoxic effects of selected biocides on RTG-2 fish cells by means of a modified Fasf Micromefhod Assay. Aquat. Toxicol. 2005, 73, 55-64. 

A classification by chemical type is given ia Table 1. It does not attempt to be either rigorous or complete. Clearly, some materials could appear ia more than one of these classifications, eg, polyethylene waxes [9002-88 ] can be classified ia both synthetic waxes and polyolefins, and fiuorosihcones ia sihcones and fiuoropolymers. The broad classes of release materials available are given ia the chemical class column, the principal types ia the chemical subdivision column, and one or two important selections ia the specific examples column. Many commercial products are difficult to place ia any classification scheme. Some are of proprietary composition and many are mixtures. For example, metallic soaps are often used ia combination with hydrocarbon waxes to produce finely dispersed suspensions. Many products also contain formulating aids such as solvents, emulsifiers, and biocides. 

Water quality is important, not only from an environmental point of view but also in relation to the type of packing to be specified. Analysis of the circulating water is simple to obtain, but it is very seldom offered to the cooling tower designer. The quality, or lack of it, will determine the type of pack to be used, the selection of structural materials and whether the tower should be induced or forced draft, counterflow or crossflow. Water treatment, in the shape of chemicals to control pH and to act as counter-corrosion agents or as biocides, all has a bearing on tower selection. 

It is only very recently that organic componnds synthesized by humans have begun to exert a selection pressure upon natural populations, with the consequent emergence of resistant strains. Pesticides are a prime example and will be the principal subject of the present section. It should be mentioned, however, that other types of biocides (e.g., antibiotics and disinfectants) can produce a similar response in microbial populations that are exposed to them. 

The clinical relevance of biocide resistance of antibiotic-resistant staphylococci is, however, unclear. It has been claimed that the resistance of these organisms to cationic-type biocides confers a selective advantage, i.e. survival, when such disinfectants are employed clinically. However, the in-use concentrations are several times higher than those to which the organisms are resistant. 

Chlorine dioxide has been evaluated as a replacement for chlorine [1630]. Gaseous chlorine as a biocide for industrial applications is declining because of safety and environmental and community impact considerations. Various alternatives have been explored, for example, bromo-chorodimethyl hydan-toin (BCDMH), nonoxidizing biocides, ozone, and chlorine dioxide. Chlorine dioxide offers some unique advantages because of its selectivity, effectiveness over a wide pH range, and speed of kill. Safety and cost considerations have restricted its use as a viable replacement. 

---