Biodeterioration of stored diesel oil: studies in Brazil

Abstract

The problems of hydrocarbon fuel storage in Brazil are particularly acute for diesel fuel. Visits to bus depots showed that many foremen did not understand the importance of draining water bottoms regularly and most systems were microbially contaminated. Common fungal isolates from refineries and distribution systems, Hormoconis resinae, Aspergillus niger, Aspergillus fumigatus, Paecilomyces variotii, and Candida silvicola, grew equally well in laboratory diesel/water systems with or without a chemical additive mixture, showing that this package of compounds neither promoted nor retarded fungal growth. Non-sterilised diesel was stored for 450 days over a water bottom, with or without an isothiazolone biocide, in the laboratory. The fungi most frequently detected in the non-biocide treated systems were H. resinae, A fumigatus, P. variotii, a Penicillium sp., and the yeasts, Rhodotorula glutinis and Candida silvicola. Bacterial isolates included oxidative Gram negative rods, sulphate-reducing bacteria and a Micrococcus sp. Biocide at 0.1 ppm maintained the systems clean for up to 30 days, and at 1 or 10 ppm for 400 days. After 400 days, the biomass (dry weight) from non-additive-containing diesel in control, 1 and 10 ppm biocide-containing systems was 24.6, 4.6 and 3.3 mg, respectively. The system treated with 0.1 ppm biocide yielded 38.2 mg biomass, indicating that sub-effective doses may lead to increased microbial growth. Within 24 h of addition of 10 ppm biocide to a highly contaminated control flask (145 days storage) there was a 2-log reduction in total aerobic bacterial and yeast population and the filamentous fungal count was >5 ml −1 .