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An array of microcosms containing California Air Resources Board (CARB)-compliant, oxygenated S7-octane gasoline over nutrient-amended water was monitored over a 7-month period. The array included triplicate microcosms of each of four conditions: unchallenged control, challenged control and challenged with two different antimicrobial agent treatments. After 7 months, significant fuel chemistry and physical changes occurred in al1 the microcosms that were challenged with an uncharacterized microbial inoculum drawn from a contaminated fuel system. Most noteworthy was the average 67%loss of oxygenates and the marked shift from isoparaffins and normal paraffins to alkyl isoparaffins, coupled with a shift to higher carbon numbered compounds. Moreover, in the untreated, challenged control microcosms, mild-steel corrosion rates were approximately double, and filter-plugging rates were greater than four times those observed in the unchallenged control microcosms. Both antimicrobial agent treatments attenuated the physical and chemical changes. There were no significant physical or chemical changes in the unchallenged control microcosms, indicating that physical weathering during the test period played only a minor role in the changes. © 2001 Elsevier Science Ltd. All rights reserved

1. Introduction

Microbial growth at the expense of distillate petroleum fuels has been recognized for over a century (Atlas, 1984). At least three monographs address the topic of fuel microbiology (Beerstecher jr,7954; Davis, 1967; Atlas, 1884). Although there have been reports of gasoline biodeterioration (Hill and Koenig, 1995) most research has addressed middle distillate fuel biodeterioration (Littmann, 1980; Hill, 1984; Smith, 1988, 1991; Neihoff, 1988.

During the period 1992-1996, one of the authors (Passman) surveyed approximately 400 refinery, terminal and retail outlet storage tanks, ranging in size from 37.8 to 31,800 m³. Approximately 60% of all gasoline tanks surveyed contained significant levels of microbial contamination as evidenced by enzymatic activity, viable recovery methods and the presence of an intermediate zone (rag layer) between the fuel and bottom-water layers. However, the field surveys did not determine whether the observed levels of microbial contamination affected the commercial value of the fuels in contaminated systems. The present study was designed in order to determine whether microbial contaminants growing primarily in microcosm rag layers and bottom waters altered the chemistry of the overlying fuel significantly. Additionally, the experimental design to evaluated the effect of two antimicrobial pesticides presently approved for use in on-highway fuels in the United States (US EPA, 1994)

2. Materials and Methods

2.1. Chemicals

We used California Air Research Board (CARB) Phase 2 compliant, regular unleaded gasoline (RUL) for all microcosms. ASTM D 4814 (ASTM, 1999). defines properties of this fuel. This gasoline was augmented with 12% (w/w) of an oxygenate blend comprised primarily of methyl tertiary-butyl ether (MTBE) and tertiary amyl methyl ether (TAME). Sufficient product was obtained from a single production run to ensure that all testing was performed using the same fuel.

The two antimicrobial agents tested were methylenebis-thiocyanate (MBT, Buckman Laboratories, Memphis TN) and a blend of 4-(2-nitrobutyl)morpholine and 4,4t- (2-ethyl-2-nitrotrimethylene) dimorpholine (NMEND; AN- GUS Chemical, Buffalo Grove, IL). MBT was tested at 200, 100 and 50 ppm (as supplied; 40,20 and l0 ppm a.i.). NMEND was tested at 1000, 300 and 135 ppm (as supplied; 850, 255 and 115 ppm a.i). These doses represent the minimum, maximum and mid-range concentrations recommended by the respective manufacturers.

2.2 Organisms and Microcosms

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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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