FUEL & FUEL SYSTEM MICROBIOLOGY PART 28 – IS THE SULFUR IN HIGH SULFUR DIESEL TOXIC?
Diesel fuel biodeterioration is not affected by the fuel’s sulfur content.
There is a broadly embraced misperception about the relationship between diesel fuel’s sulfur content and its toxicity to microorganisms. This misperception is driven by two logical flaws.
Logical argument #1:
There has been an increase in the number of microbially contaminated fuel systems since the use of ultra-low-sulfur diesel (ULSD) became mandatory.
Therefore, microbial contamination in low sulfur diesel (LSD) and high sulfur diesel must have occurred less frequently than in currently does in ULSD.
Logical argument #2:
If argument #1 is valid, then the removed sulfur must have had a biostatic (ability to prevent microbes from growing) or biocidal effect.
“Get your facts first, and then you can distort them as much as you please.”
This quote was reportedly part of a session that Samuel Clemens (Mark Twain) has with young reporters sometime in the 1890s. More recently, in one of his many books on Zen, the philosopher Alan Watts, observed that humankind is unique in our uncanny ability to make precise and accurate observations only to use them to draw erroneous conclusions. Finally, in an earlier post I quoted Daniel Kahneman’s adage: “What you see is all there is.” (WYSIATI).
Logical argument #1 fallacies:
This argument assumes that the increased incidence of reports in a particular market sector (fuel retail) is equivalent to the increased incidence of microbial contamination in diesel fuels and fuel systems. But how do we know whether stakeholders are simply more aware of something that has been going on since diesel fuels were first used? The history of marine fuel oil biodeterioration that date back to the transitions from coal to oil and from burner oils to marine diesel fuel oil (more on this, in response to argument #2). Distillate aviation fuel biodeterioration has been recognized since the Korean War.
Additionally, the argument ignores various confounding factors (in statistics, a confounding factor is an unobserved variable that affects observed variables: in our case sulfur concentration and biodeterioration are observed variables. Before concluding that removing sulfur made diesel fuel more vulnerable to biodeterioration consider these four confounding factors (there are others, but these five illustrate the concept):
- Hydrotreatment to remove sulfur also removes aromatic compounds – especially high molecular weight, toxic, polynuclear aromatic compounds.
- During the past three decades, the fuel distribution infrastructure has evolved from vertically integrated control (the refiner controlled all stages from refinery to retail site) to fungible (common pipelines transport products from refinery tank farms to terminals from which independent and branded retailers draw product from tanks that can that can be mixtures of product from numerous refineries – >100 refineries produce product that is stored in in New Jersey terminal tanks). Fungible product comingling means that cradle to grave product stewardship is more complex than it was historically.
- Product transport from terminals to fleet operators and retailers is typically done my third-party transport companies. Switch-loading (a given tank compartment can carry gasoline on one trip and diesel on the next) is occurring more frequently. The probability of cross-contamination between two fuel-grades is a hotly debated issue at present.
- Although the trend is beginning to reverse itself, between 1990 and 2010, total diesel storage capacity shrunk annually as product demand grew. Consequently, residence time in terminal storage tanks has decreased. Although best practice is to give water and particulates time to settle before drawing product from a tank to the fueling rack, product demand can inspire terminal operators to begin drawing product early. Consequently, any water, particulates, or both that have not settled to below the suction zone will be transported with the fuel.
- Dispensing system technology has become more sophisticated. Systems that might not have be affected historically, are now failing – primarily due to corrosion damage. As a microbiologist, I’d like to think that all fuel system corrosion is microbiologically influenced corrosion (MIC). However, if ethanol enters diesel fuel systems (either because of switch loading or vapor recovery unit vapor comingling) it can be chemically oxidized to acetic acid. Therefore, unless other low molecular weight (4 to 6-carbon) organic acids are also present, high concentrations of acetic acid in fuel-associated water is likely to be a symptom of chemical – not microbial – activity.
Logical argument #2 fallacies:
This argument is built on argument #1’s house of cards. It falls apart if the statement: “There has been an increase in the number of microbially contaminated fuel systems since the use of ultra-low-sulfur diesel (ULSD) became mandatory.” is false. As noted above, increased incidence and increased reports are two very different concepts.
To illustrate this point, consider the respiratory disease, legionellosis. The disease was given its name because the first recognized outbreak was among American Legion members attending a convention at the Bellevue-Strafford Hotel, in Philadelphia. It is beyond improbable that the bacterium that causes legionellosis – Legionella pneumophila – came into existence in 1976. However, in late July and early August 1976, after 221 American Legion convention attendees developed pneumonia-like symptoms, and 34 of the patients died, the medical establishment (physicians and epidemiologists) took note. It took a couple of years to figure out how to culture L. pneumophila, and there was wild speculation regarding the likely relationship between environmental conditions and the microorganism’s ability to grow. Forty years down the road, we know that L. pneumophila is ubiquitous – it can be found in many different environments where biofilms develop (relax – none yet recovered from fuel systems; but don’t relax too much – shower-head aerator screens tend teem with L. pneumophila). The good news is that only immunosuppressed individuals tend to develop the legionellosis.
What does this have to do with the relationship between sulfur concentration in fuel and biodeterioration risk? In both cases, the microbes causing the symptoms have been around for a long time. In the health sector, for centuries (if not millennia) L. pneumophila has caused an unknown percentage of all pneumonia cases, but it was never identified because there had never been (i.e., since the advent of modern medical microbiology, immunology, and epidemiology) such a large number of folks getting sick at the same time and place. Similarly, fuel biodeterioration was well known from the earliest days of gasoline and diesel production. However, there was no database documenting each biodeterioration event.
Prior to 2012 the upper limit for sulfur in marine diesel was 4.5 %. Before 1986, on-highway diesel typically had 0.1 % to 0.5 % (by volume) sulfur. If the sulfur in these historical fuels had been biostatic, fuel biodeterioration would have not occurred until ULSD came onto the market. Filter plugging on ships and aircraft had a more serious impact than filter plugging on dispensers, locomotives, and other land-based diesel fuel systems. However, efforts to control microbial contamination in the marine and aviation sectors were not general knowledge among fuel retailers and fleet operators. Ironically – because they ignored the biocidal effect of tetraethyl lead – folks were convinced that gasoline was too toxic to support microbial growth and that only diesel fuels and fuel systems were affected.
Despite all of this, isn’t it fair to say that ULSD biodeterioration is more pervasive than that of diesel grades with greater sulfur concentrations? My answer is: Not necessarily. There are no hard statistics on the average number of ULSD biodeterioration incidents per year since 1986 and there are certainly no reliable statistics for the decades before the switch to ULSD (or in off-highway systems using low or high sulfur diesel). The assessment that the incidence rate has increased since ULSD replaced other fuel grades for on-highway use is purely subjective. One more time: increased awareness (as in the case of legionellosis) is not the same as increased incidence. The switch to ULSD and biodiesel blends was highly visible to the industry. From the outset, stakeholders wanted to know what the change might do to their systems. Consequently, they now notice damage more quickly than they had in the past. Okay, this is an optimistic statement. In two recent fuel quality surveys, sites originally identified as control sites (no reported problems) had more microbial contamination and corrosion than he problem sites. In the more recent, US EPA-sponsored study, operators were unaware of any problems at 87 % of the moderately to heavily corroded sites.
There is no question that some organosulfur compounds are biocidal. For example, two of the few fuel-treatment biocides are mixtures of organosulfur compounds:
CIT/MIT (also referred to as CMIT): 5-Chloro-2-methyl-3(2H)-isothiazolone + 2-methyl-3(2H)-isothiazolone (isothiazolinones are ring structured molecules with the chemical formula: C₃H₃NOS).
MECT: 2-(Thiocyanomethylthio)benzothiazole + Methylene bis(thiocyanate) (the thio in each molecule’s name indicates that they are organosulfur compounds)
However, sulfur is one of the five primary elements (the other four are: carbon, hydrogen, nitrogen, and oxygen) on which all life depends.
Studies on fuel biodegradability have shown that the aromatic content, rather than the sulfur content is a primary factor affecting diesel biodegradability. Regardless of sulfur concentration, fuels with higher aromatic concentrations or more complex aromatic compounds biodegrade more slowly than more severely hydrotreated fuels from which aromatics have been substantially removed. The same hydrotreating process that removes sulfur also reduces fuel’s aromatic content. Note that although there are no aromatic biocides approved for fuel treatment, there are numerous aromatic biocides approved for other applications.
- Bottom line:
If ULSD fuels are more susceptible than higher sulfur content fuels are to biodeterioration, it is due to the reduced concentration of complex, toxic polynuclear aromatic compounds – not because of sulfur’s inherent toxicity.