Nonoxidizing biocides

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. 

Antimicrobials and Antibiotics Specific metabolic inhibitors, electrophilic organics, cell signaling inhibitors Nonoxidizing Biocides... 

In general, the continuous, biostatic, slow-release bromination of smaller comfort cooling systems is also both practical and cost-effective. But it is the rationale for routinely recommending periodic applications of relatively expensive nonoxidizing biocides to cooling systems of any size, without benefit of proof of need, effectiveness, or program costs, that should be questioned. 

Biocide products, as sold by service companies, are generally available in both solid and liquid forms. Each form additionally offers various ready-to-use oxidizing and nonoxidizing chemistries. 

The concept of dispensing biocides or biostats in tablet form has recently been extended to the supply of certain nonoxidizers, such as 2,2-dibromo-3-nitrilopropionamide (DBNPA) (which has been widely available for many years as a liquid product and to a lesser extent as a powdered product). 

In general, the solid materials tend to act as biostats rather than biocides. Programs that use biostatic control in partnership with periodic applications of nonoxidizing biocides have proved to be extremely effective in many comfort cooling and general industrial cooling systems. 

Apart from sodium hypochlorite and sodium bromide solutions, almost all other liquid biocides (or biocide intermediates) are nonoxidizer types. Most nonoxidizers are manufactured by specialist chemical companies, who sell these branded products for scores of different sanitizing or disinfection applications, of which cooling water system treatment is only one. These chemicals may be merely renamed (according to subregistration permits) or they may be blended with other biocides or inhibitors to form new and different products with accordingly modified properties. Some biocides may only be available from the primary manufacturer, often as extremely concentrated and hazardous materials. These chemicals will then usually require some form of dilution, stabilization, and quality verification before being incorporated into a service company s product line. 

As with many patented deposit control agents (DCAs), the patents on certain proprietary nonoxidizing biocides, or their applications, are also beginning to expire, and, as a result, secondary manufacturers are entering the market and helping to bring down the prices of the more expensive formulatory materials. 

Since those days, various other bromine-based biocides and biocide intermediates have been introduced onto the global market or found renewed favor. The most notable are sodium bromide, isocyanurate/NaBr blends, and DBNPA (a nonoxidizer available as a powder or solution, and recently introduced in tablet form). Most bromine biocides employ chlorine in some form as an activator to produce the primary biocidal species hypobro-mous acid (HOBr). 

Fourth, the concept of a background bromine biostat, coupled with a flexible nonoxidizing biocide program, has provided the basis for clean cooling systems, with minimal biofilm formation and algal slimes. 

Even in many cooling systems that use only chlorine and appear to be clean and operating efficiently, it is often possible to provide housekeeping and efficiency improvements at little additional financial cost by modifying the existing programs to include a suitable nonoxidizing biocide. 

In Section 6.3.6, there are structures of many of the more common chemicals that act as nonoxidizing biocides. In addition, in Section 6.3.7, there are two tables that provide a guide to a suggested biocide selection in different types of cooling systems (Tables 6.3 and 6.4 on p. 229)... 

Basic Questions Before Selecting a Nonoxidizing Biocide... 

What type of nonoxidizing biocide(s) should be used ... 

The optimum selection and correct application of biocides (especially nonoxidizers) for any particular cooling system can be difficult to initially identify and, like the selection of polyelectrolytes for a wastewater plant, is often based on the experience of the water treatment company service engineer. 

Many of the nonoxidizing biocides employed in cooling systems today were originally developed for other applications, such as slimicides for pulp and paper, biostats for cosmetics, or sanitizers for food factories. The experience gained for these applications has proved to be useful for cooling water systems. Some products that have been available for several years have been, or are being, launched into new and different markets, especially where environmental concerns have a high profile. 

In order to provide a biostatic effect for any particular organism, nonoxidizers are required to be present at a certain minimum inhibitory concentration (MIC), and for a biocidal effect, at a minimum kill concentration... 

Notwithstanding the preceding limitations, a best effort starting-point guide for the selection of many of the more common nonoxidizing biocides is given in this section. 

From the preceding, it can be seen that there is a wide range of individual nonoxidizing biocides and blends available. None of the individual products are especially effective under all operating conditions or water qualities, although the newer individual biocides tend to perform better than older materials. However, several of these newer biocides are generally offered only by one or two primary manufacturers (examples are DBNPA, gluteraldehyde, isothiazolinones) and consequently are often the most expensive materials to use, on a cost-per-application basis. 

Combinations of newer nonoxidizing products (good biocidal potency but expensive) with some older materials (good penetrating ability and relatively low cost) may prove effective. Also, certain combinations of oxidizing biocides with nonoxidizers may prove suitable under many conditions. 

If the volume is large in relation to the total water consumption, the cost of microbiological control using standard nonoxidizing biocides may be prohibitively expensive and some innovative solution will probably need to be found. 

The biocide component will be tailored by a technical team, from a starting point of available products in a particular vendor s range. This biocide component is most usually a dual-alternating, nonoxidizer program, or an oxidizer/nonoxidizer combination, together with a suitable biodispersant. The selection process has been discussed in Chapter 6, and some starting-point selection charts provided (Tables 6.3 and 6.4). 

Fig. 9.2. Typical water treatment program dosing and control arrangements for industrial cooling system (schematic). Nonoxidizing biocides and biodispersant are typically periodically hand-dosed.

There are several biofilm cleaning and removal techniques in common use. They typically employ a program of several shock doses of various oxidizer/nonoxidizer biocide combinations, together with biodispersant, followed by heavy bleed. Particularly useful are the newer types of phosphonium salt biocides, which have an additional cleaning effect (e.g., THPS/TKHPS/TTPC). 

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