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Several of the major points that I made in 1995 need further consideration, based on both changes in the regulatory climate and field experience with microbial contamination control in surface transportation markets.

First, I'll comment to the term biocides that I used in the article's title. In the late 1990's the U.S. EPA, Office of Pesticides Programs (OPP) asked that that the terms microbicide or antimicrobial pesticide be used in reference to products intended for use to kill microbes. EPA officials were concerned that biocides inaccurately suggested that these chemicals were highly toxic to all organisms, including humans. Recognizing that many microbicides are used as food preservatives, OPP promoted the use of the kinder and gentler terms.

In the original article, I wrote: "Any product used for fuel treatment must be approved as a fuel additive." That comment was based on the original position communicated by U.S. EPA's Office of Transportation & Air Quality, Fuel & Fuel Additives Division. Subsequently, the Fuel & Fuel Additives Division clarified their position regarding microbicides. Their revised position is that for the most part, fuel-treatment microbicides are used to treat systems rather than fuels. Consequently, unless products are used as fuel additives (see my comments about microbicides as performance additives below), fueltreatment microbicides do not require approval as fuel additives and are not required to be substantially similar to the fuels in the systems being treated.

In my September 2005 discussion of water-soluble microbicides I focused on the wastefulness of treating bottom-water that was destined for waste treatment. While that argument remains valid, I'd like to offer a more compelling reason for avoiding the use of water-soluble microbicides in fuel-systems. Unless the bottom-water zone is continuous, the microbicide is only going to contact water at the point of treatment. In most bulk storage and underground storage tanks, the bottom is not flat. Consequently, unless the total water depth is greater than the height of the tank bottom surfaces' peaks and valleys, water will accumulate in pools that form in the various low points of the tank. Special measures may be taken to ensure uniform distribution of the water-soluble microbicide across the entire bottom area, but those same measures (aggressive mixing) are also likely to resuspend water and sediment into the fuel-phase. Under most conditions, treating a fuel tank with a water-soluble microbicide is unlikely to expose even the entire bottomwater associated population to the microbicide treatment. That's a poor investment.

The final point that I'll revisit is my suggestion of using fuel-treatment microbicides as performance additives.

I've reversed my 1995 position since writing Opening Your Biocides Toolbox. I've come to believe that the potential problems with this strategy far outweigh the potential benefits. To be effective, microbicides must be used at specified dosages. Under-dosing is likely to select for treatment-resistant microbes. As described in the original article, there are several processes that reduce the concentration of a microbicide once it has been added to a fuel system.

These same processes make it very difficult to determine the appropriate starting concentration for a microbicide used as a fuel additive. Moreover, only fuel-soluble or universally soluble microbicides that are approved as both antimicrobial pesticides and fuel additives may be used this way. This limits the choices to products that are not necessarily the most effective microbicides. Moreover, the economics of treatment are unfavorable. Systems holding fuels that contain microbicidal additives will still need periodic microbicide treatment to prevent system biodeterioration.

Bottom line, microbicides as fuel additives does not seem to be a good idea. As I've mentioned in several previous A Look Back articles, the best reference for microbicide use in fuels is to be found in Chapter 2 of ASTM Manual 47 - Fuel and Fuel System Microbiology: fundamentals, diagnostics, and contamination control - available from ASTM at .

. . .Continue to read rest of article (PDF).

Dr. Frederick Passman, PhD is a Certified Metalworking Fluids Specialist with over 35 years experience in Environmental & Industrial Microbiology. His company, Biodeterioration Control Associates, Inc. (BCA) provides clients with unparalleled expertise in Microbial Contamination Control.

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