ASTM D7978-14(2019)

5.1 This test method is intended to provide a tool for assessing whether fuel storage and distribution facilities or end user fuel tanks are subject to microbial growth and alert fuel suppliers or users to the potential for fuel quality or operational problems and/or the requirement for preventative or remedial measures.

5.2 This test method detects numbers of microbial colony forming units (CFU), the same detection parameter used in the laboratory standard procedures Practice D6974 and IP 385. However, whereas Practice D6974 and IP 385 provide separate assessment of numbers of viable aerobic bacteria CFU and numbers of viable fungal CFU, this test method provides a combined total count of viable aerobic bacteria and fungal CFU.

5.3 This test method is designed to detect a recognized group of microorganisms of significance in relation to contamination of distillate fuels, but it is recognized that microbiological culture techniques do not detect all microorganisms that can be present in a sample. Culturability is affected primarily by the ability of captured microbes to proliferate on the growth medium provided, under specific growth conditions. Consequently, a proportion of the active or inactive microbial population present in a sample can be viable but not detected by any one culture test.7 In this respect, the test is indicative of the extent of microbial contamination in a sample ,and it is assumed that when a fuel sample is significantly contaminated, some of the dominant microbial species present will be quantifiably detected, even if not all species present are culturable.

5.4 Many samples from fuel systems can be expected to contain a low level of “background” microbial contamination, which is not necessarily of operational significance. The minimum detection level of this test method is determined by the volume of specimen tested and is set such that microbial contamination will generally only be detected when it is at levels indicative of active proliferation.

5.5 The test will detect culturable bacteria and fungi that are metabolically active and dormant fungal spores. Presence of fungal spores in a fuel sample can be indicative of active microbial proliferation within a fuel tank or system, but at a point distant from the location sampled. Active microbial growth only occurs in free water, and this can be present only as isolated pockets at tank or system low points. Because fungal spores are more hydrophobic than active cells and fungal material (mycelium), they disperse more readily in fuel phase and are thus more readily detected when low points cannot be directly sampled and only fuel phase is present in samples.

5.6 This test method can determine whether microbial contamination in samples drawn from fuel tanks and systems is absent or present at light, moderate, and heavy levels.

5.7 The categorization of light, moderate, and heavy levels of contamination will depend on the fuel type, the sampling location, the facility sampled, and its specific operating circumstances.

5.8 Further guidance or interpretation of test results can be found in Guide D6469, in the Energy Institute Guidelines for the investigation of the microbial content of petroleum fuels, and for the implementation of avoidance and remedial strategies and in the IATA Guidance Material on Microbiological Contamination in Aircraft Fuel Tanks.

5.9 Testing can be conducted on a routine basis or to investigate incidents.

5.10 Microbiological tests are not intended to be used to determine compliance with absolute fuel specifications or limits. The implementation of specification limits for microbiological contamination in fuels is generally not appropriate, and microbial contamination levels cannot be used alone or directly to make inferences about fuel quality or fitness for use.

5.11 When interpreting results, it must be appreciated that the test result applies only to the specific sample and specimen tested and not necessarily to the bulk fuel. Microbiological contamination usually shows a highly heterogeneous distribution in fuel systems, and therefore, analysis of a single sample will rarely provide a complete assessment of the overall levels of contamination present.

5.12 Water phase will usually contain substantially higher numbers of microbial CFU than fuel phase and, consequently, a different interpretation of results is required.

1.1 This test method describes a procedure that can be used in the field or in a laboratory to quantify culturable, viable aerobic microorganisms present as contaminants in liquid fuels, including those blended with synthesized hydrocarbons or biofuels, with kinematic viscosities (at 40 °C) of ≤24 mm² s^-1 and heavy and residual fuels with kinematic viscosities (at 40 °C) of ≤700 mm² s^-1 and in fuel-associated water.