Abstract
Microbial contamination of fuel has been the cause of several problems in transportation and storage of these products. Due to the lack of previous studies related to these problems in Costa Rica, bacterial quality was evaluated biannually in automotive fuels stored in the four oil distribution facilities of the Costa Rican Petroleum Refinery (RECOPE). In 12 oil storage tanks, for a total of 96 samples, mesophilic, heterotrophic aerobic/facultative counts (ASTM D6974-04) and identification of bacteria presented in regular gas, premium gas and diesel from the bottom and superior part of the tanks were done; in the samples containing an aqueous phase, sulfate reducing bacteria (SRB) were also quantified by the most probable number technique (MPN), according to the ASTM D4412-84 standard. The higher contamination was shown at the bottom of the tanks (populations up to 10(4) UFC/l), especially if there was accumulated water, in which case populations reached 10(8) UFC/l. The most contaminated fuel was diesel (counts up to 10(4) UFC/l), whereas the less contaminated was premium gas. The less contaminated fuels were from the facilities of La Garita and Barranca, whereas the most contaminated were from Ochomogo. Nevertheless, the quantified populations did not cause significant alteration in quality physicochemical parameters in the samples analyzed. A total of 149 bacterial strains were isolated, 136 (91.3%) Gram positive and 13 (8.7%) Gram negative. The most frequent genera were Staphylococcus (24.0%), Micrococcus (21.9%), Bacillus (18.8%) and Kocuria (11.5%) among Gram positive bacteria and Pseudomonas (7.3%) among Gram negative bacteria. The majority of these genera have been found as fuel contaminants or even as degraders of this kind of products; nevertheless, some species for which their appearance or growth in hydrocarbons have not been described were found with low frequencies. SRB were present in counts up to 10(5) MPN/l in 42.9% of water containing samples (including all from diesel tanks), indicating biocorrosion processes risk in fuel transport and storage systems. From the findings in this study it is recommended to give a frequent maintenance to fuel containers, based on continuous drainage and removal of accumulated water, antimicrobial agent addition and microbial quality monitoring in country's fuels.
Highlights
Microorganisms may be introduced in fuel containers through different ways: during condensation processes in refineries or transported along with dust and water through tank vents
The numbers of viable bacteria and fungi recovered from fuel-phase samples are several orders of magnitude smaller than those found in waterphase samples, fuel-phase microorganisms are often the most readily available indicators to evaluate microbial contamination in this kind of products (Yemashova et al 2007)
The presence of microorganisms in fuel storage systems causes an increase in water content due to potential microbial degradation of hydrocarbons, and their metabolic activity leads to peroxide and acid formation, increase in viscosity, decrease in thermal stability and volatility, as well as increase in suspended solids in the form of sludge and corrosion residues, among others, that may lead to filtration problems, equipment deterioration and a general loss in fuel quality (Kartavtseva et al 1989, Gaylarde et al 1999, Yemashova et al 2007)
Summary
Microorganisms may be introduced in fuel containers through different ways: during condensation processes in refineries or transported along with dust and water through tank vents. Among bacteria able to grow in fuel, genera such as Acinetobacter, Alcaligenes, Bacillus, Pseudomonas, Flavobacterium, Aeromonas, Achromobacter, Arthrobacter, Nocardia, Rhodococcus and Micrococcus have been reported (Gaylarde et al 1999, Emtiazi et al 2005, Yemashova et al 2007, Neilson & Allard 2008) To grow in this environment, microorganisms require the capacity to metabolically transform hydrocarbons in order to employ them as carbon and energy source. Several studies have described biodegradation mechanisms of different hydrocarbons (Yemashova et al 2007, Neilson & Allard 2008); in the case of alkanes in aerobic conditions, the mechanism involves a terminal hydroxylation followed by dehydrogenations, until forming the respective alcohols, aldehydes and carboxylic acids, or alternatively, a subterminal hydroxylation followed by oxidation to ketones (Neilson & Allard 2008)
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