ASTM D6469 standard guide for microbial contamination in fuels and fuel systems
11. Strategies for Controlling Microbial Growth
11.1 System Design - Strategies for controlling microbiological growth should begin with system design.
11.1.1 All tanks should be designed to facilitate water and bottom-solids removal, minimize contaminant entry, and facilitate sample collection.
11.1.2 Bulk tank bottom geometry should be cone-down (concave) with sufficient slope to permit sludge and sediment to migrate toward a central sump.
11.1.3 Floating of roof tanks should be fitted with a non-floating supraroof (false roof) to minimize precipitation and debris accumulation on the floating roof surface and to protect the above-floating-roof shell-surface from the environment. If this is not possible or economically feasible, tank bottoms should be checked daily for free water. Any free water found should be drained from the tank.
11.1.4 Horizontal, cylindrical, underground storage tanks should be fitted with a second access port at the opposite end from the fill-pipe to allow dipping the tank for water at both ends and pumping out bottom-water from the low-end.
11.1.5 During system design, biodeterioration control considerations are typically subordinate to other issues. Moreover, existing systems are not likely to be redesigned to accommodate biodeterioration prevention programs. Consequently, the other components of biodeterioration control strategies become more important.
11.2 Sampling and Analysis:
11.2.1 An adequate sampling and analysis program can be the most critical element of an effective biodeterioration control strategy. Although the details of the sampling program will depend on system operations and configuration, minimal sampling requirements will include bulk fuel and near-bottom fuel/bottom-water samples. Suitable sampling locations and techniques are paramount in determining both the presence and the extent of biological contamination. Microbiological contamination generally resides at a low point in a storage vessel or pipeline. Another potential location is filters. Biological contamination requires water to metabolize fuel, and filter media provide a perfect collection area for both water and the ability to hold matter.
11.2.2 In dealing with living organisms, it is important to sample with equipment that is free of contamination. Both samplers and containers should be sterile if possible. Sample containers should be new and kept closed prior to use.
11.2.3 Once contamination has been detected, further investigation and analysis can determine the extent of the problem.
11.3 Remediation:
11.3.1 Remediation really begins with prevention. Because microbes require water, an intensive water removal program is important. Many storage systems have been poorly designed in relationship to water removal, so extra effort may be necessary to determine the best method of removal. The best advice that can be given is that for each location, a sampling procedure may have to be written to accommodate the differences in tank and piping design.
11.3.2 After removing water from a system, a representative sample from that water may be tested for presence of biological activity. If the test is positive, it is likely that contamination is present in the facility. Additional steps, such as chemical treatment or tank cleaning, or both, may be required.
11.3.3 For systems that have high microbial loads, but no other gross evidence of contamination, water removal and biocide treatment usually suffice.
11.3.4 Water removal is never 100 % effective. Most tank configurations make it impossible to remove all water. Most bulk tanks with installed water removal systems still retain water after draining. Tank bottom configuration has a major impact on water removal capabilities. Flat and convex bottomed tanks retain the most water. Concave (optional) tank bottoms, with sumps drawing from the lowest point, retain the least amount of water.
11.3.5 Not all free-water accumulates in tank bottoms. The biofilm layer that accumulates on tank walls is typically greater than 90 % water. This creates a substantial, but difficult to sample, habitat for microorganisms. Data from bottom-water samples are used to estimate the likelihood of significant tank surface contamination. Generally, evidence of significant bottom-water contamination is predictive of significant tank surface contamination. In the absence of performance problems, acceptable bottom-water contamination levels also indicate acceptable tank-surface contamination levels. However, if performance problems indicate that there is significant microbial contamination, but bottom-water data are either negative or equivocal, the bottom-water data may not be a satisfactory indicator of overall microbial contamination within the tank.
11.4 Biocide Use:
11.4.1 By definition, biocides are toxic materials. They can, however, be used and handled safely. Users are advised to review and comply with the safety, handling, and disposal requirement information provided in each product's material safety data sheet (MSDS) and technical information literature.
11.4.2 In the United States two groups within the Environmental Protection Agency regulate fuel biocide use. The pesticides group issues restrictions for all industrial biocides (see 40 CFR 152), while the air quality group issues restrictions for all fuel additives (see 40 CFR 79). Only biocides meeting both pesticide and fuel additive restrictions should be used to treat fuel systems. In addition, end-user groups (for example, the aviation industry and the U.S. Department of Defense) may place further restrictions on biocide selection options. Finally, biocides are required by law to be be registered in the state(s) in which they are used. Outside the United States different countries have unique biocide regulatory requirements. Always check with manufacturers or the appropriate local authorities.
11.4.3 There are three major groups of fuel biocides: fuel soluble, water soluble, and universally soluble.
11.4.3.1 Fuel soluble biocides are unstable or insoluble in water. Their principal advantage is that they reside in the fuel phase and can be transported throughout the fuel system. Their primary disadvantage is that they are typically inactivated by water, where the microbes tend to grow.
11.4.3.2 Water-soluble biocides are insoluble in fuel. They tend to be inexpensive and are best used to shock-treat bottom-water contamination in tanks that are not drained routinely. The microbes found in bottom-water can contribute to the wastewater treatment process. Consequently, there is little value in using a biocide to kill microbes in water that is destined for waste treatment. Water-soluble biocides do not persist in the fuel phase enough to diffuse into system surface biofilms. Consequently, they tend to be effective only against bottom-water populations.
11.4.3.3 Universally soluble biocides are stable in both fuel and water. Typically, these products are primarily fuel-soluble, with sufficient water solubility to perform in both phases. Like fuel soluble biocides, universally soluble products can be transported throughout the fuel system. Their water solubility makes them equally effective against biofilm and bottom-water microbes. Their principal disadvantage is their high cost relative to the other two fuel biocide groups.
11.4.3.4 Biocide treatment frequency and dose levels are both system and biocide product specific. Test Method E 1259 addresses fuel biocide performance testing. Fuel system operators should consult with manufacturers or others with biocide-use expertise before using fuel treatment biocides.
11.5 Tank Cleaning:
11.5.1 The option of tank cleaning is expensive, potentially disruptive, and risky. A plan of action needs to be carefully considered before this step is utilized. Care should be exercised in the selection of a contractor because of the liability of a system's owner is exposed to should a spill occur or waste be disposed of improperly.
11.5.2 Heavily contaminated systems generally require tank and pipe cleaning in conjunction with biocide treatment. The most effective programs include a three-step process.
11.5.2.1 Systems are first shock-treated with biocide. Biocide-treated surface biofilms will slough-off system walls and accumulate in tank bottoms.
11.5.2.2 Next, systems are cleaned. There are a variety of tank-cleaning strategies offered by service companies.
NOTE 3 - Certain commonly used materials, such as strong detergents, are not suitable for cleaning aviation fuel systems. Any cleaning material used should first be checked for compatibility with both the fuel and fuel system.
11.5.2.3 Fuel system operators should evaluate alternative recommendations to ensure that the proposed methods meet their needs.
11.5.2.4 After cleaning, the freshly charged fuel system should be retreated with a second biocide dose. This treatment decontaminates surfaces that may not have been reached either by the initial dose (biocide is consumed as it kills microbes) or subsequent cleaning (microbes protected by surface irregularities, called aspersions, may escape mechanical cleaning). If stored fuel had been temporarily removed to accommodate tank cleaning, filtering it as it is pumped back into the cleaned tank may reduce the risk of recontaminating the tank with microbes that may be present in the original fuel.
12. Keywords
12.1 biocides; biodegradation; biodeterioration; biological contamination; contamination; fuel quality; microbial contamination; microbially induced corrosion; microbiological testing; sampling