Archive for the ‘Uncategorized’ Category


What Does “Viable But Not Culturable” Mean and Why Should I Care?

In microbiology, the term used to describe microbes that appear to be healthy and active by test methods other than culturing is viable but not culturable – VNBC. Since the term first came into vogue in the 1980s, it has always reminded me of the Monty Python skit in which the customer – played by John Cleese – and the shop owner – played by Michael Palin – debate whether the parrot that Mr. Cleese had just bought was dead or simply resting, check it out at The Parrot Sketch.

Michael Palin (left) and John Cleese (right) in Monty Python’s “Pet Shop Sketch” (1969).

The viability versus culturability debate

The issue is relevant for two reasons. First, if a fuel or other industrial process fluid system (think heat exchange fluids, metalworking fluids, lubricating oils and hydraulic fluids) is home to a population of microbes that are biodeteriogenic (i.e., causing damage to the fluid, the system, or both) but are not detected by culture testing, the risk of experiencing a failure event can high.

Second, the term VNBC has numerous meanings – depending on researchers’ focus. The varied definitions creates confusion among both microbiologists and others who rely on microbiological test results to drive maintenance decisions.

What does viable mean?

The Online Biology Dictionary defines viable as an adjective meaning (“1) Alive; capable of living, developing, or reproducing, as in a viable cell.” ASTM is a bit more helpful offering several similar definitions. From F2739 Guide for Quantifying Cell Viability within Biomaterial Scaffolds we get: “viable cell, n – a cell capable of metabolic activity that is structurally intact with a functioning cell membrane.” D7463 and E2694 offer: “viable microbial biomass, n – metabolically active (living) microorganisms.” These slight variations all agree that viability relates to a microbe’s ability to:

  • function under favorable physical and chemical conditions (more on this in a bit), or
  • to survive in an inactive (dormant) state under unfavorable conditions, and
  • to become active again once conditions improve.

What does culturable mean?

ASTM defines culturable as an adjective: “microorganisms that proliferate as indicated by the formation of colonies on solid growth media or the development of turbidity in liquid growth media under specific growth conditions.” This definition is used in several ASTM standard test methods, guides, and practices.
When microbes reproduce – i.e., proliferate – go through repeated cycles of division – on a solid or semi-solid medium, after approximately 30 generations (doubling cycles, or generations) they accumulate enough mass to form a visible colony. Thirty generations (230) yields approximately a billion cells. Liquid growth media become visibly turbid once the population density (cells mL-1) reaches approximately one million (106) cells – 20 generations. The duration of a single generation varies among microbial species and growth conditions. At present, known generation times range from 15 min for the fastest proliferating bacteria to >30 days (recent discoveries of deep earth microbes suggest that these microbes might have generation times measured in years or decades – the generation time for humans is approximately 30 years). The key point is that culturable, microbes reproduce in or on growth media under specific environmental conditions.

Before leaving our discussion of culturable lets consider time. Microbes with 15 min generation times will turn broth media turbid in 5h to 6h and form visible colonies on solid media within 8h to 10h. For microbes with a 1h generation time, detection as turbidity or colonies lengthens to 20h and 30h respectively. Many culture-based test protocols state that final observations are to be made after 3-days – sometimes 5-days. Any microbe with a generation time longer than 4h is unlikely to produce a visible colony within 5-days. They will not be detected unless observations are continued for a week or longer. For example, the culture test for sulfate reducing bacterial is not scored negative until after 30-days observation. If you end a culture test at 3-days, are all of the slower growing microbes non-culturable?

What are growth media?

Since the mid-1850s, microbiologists have developed thousands of different recipes designed to support microbial growth and proliferation (recall from an early post that growth refers to the increase in mass, and as noted above, proliferation refers to an increase in numbers). Some growth media are undefined. They are simple recipes made up of extracts from yeasts, soy, and animals. These are the components of media used for the most common culture test: the standard plate count. Other media are prepared from individual chemicals. Their recipes can include more than a dozen ingredients. Solid and semi-solid media include a gelling agent such as agar (extracted from seaweed), gelatin, or silica gel. One of the most frequently referenced microbiological media cookbooks – the Difco Manual – lists more than a thousand recipes. Each of these recipes was developed to detect one or more types of microbes. In addition to the diversity of recipe ingredients, growth media vary in pH, total nutrient concentration (some microbes cannot tolerate more than trace concentrations of nutrients), and salts concentration (ranging from deionized water to brine). The microbes targeted for recovery dictate post-inoculation incubation conditions. Some microbes require an oxygen-free (anoxic) environment. Others require special gas mixtures. Microbes also vary widely on the temperatures at which they will grow. Some only grow at temperatures close to freezing. Others require temperatures closer to boiling.

The growth medium defines the chemical environment and the incubation conditions define the physical environment in which microbes are cultured. No single growth medium is likely to support the proliferation of more than a tiny fraction of the different types of microbes present in an environmental sample. Many microbiologists estimate that <0.1 % of the microbes in a sample will be culturable in a given medium. Similarly, we suspect that for every microbe that has been cultured, there are at least a billion that haven’t.


Is my microbe really dead or simply resting? When conditions are unfavorable to either growth, proliferation or both, many different types of microbes have coping mechanisms. For nearly 200 years, we have recognized that some types of microbes can form endospores – their cell wall chemistry changes and metabolic activity ceases. Only in the past 20 years, we have come to recognize that non-spore-forming microbes can enter into a dormant state that enables them to survive unfavorable conditions for centuries or millennia – becoming metabolically active once conditions once again become favorable. Moreover, in some environments, although the microbes are metabolically active, the rate of their activity is so slow as to be nearly undetectable.

Within some fields of microbiology, VNBC refers to microbes that have been injured due to exposure to a microbicide. Pre-incubation in so-called recovery media – improves their ability to reproduce in or on growth media. In my opinion, this is a very myopic view of VNBC.

Microbial ecologists define VNBC as microbes that are metabolically active or dormant in their home environments but will not growth on the culture media and incubation conditions used to detect them.

I first experienced this phenomenon in the 1970s when I was testing water from oilfield production wells. Using radioactive carbon labelled nutrients to measure metabolic activity, my team routinely found that samples that yielded <1 CFU mL-1 (CFU – colony forming unit: “a viable microorganism or aggregate of viable microorganisms, which proliferate(s) in a culture medium to produce a viable colony.” ASTM E2896) held very active populations. Poisoned controls demonstrated that conversion from radiolabeled acetic acid or glucose to radiolabeled carbon dioxide was from metabolic activity – not from a non-biological (abiotic) process.

This means that culture testing invariably underestimates the microbial population density in tested samples. Conversely, because microbes that were dormant in the environment from which they were sampled can become active once transferred into or onto nutrient media, culture testing can overestimate the presence of metabolically cells. For example, most microbes suspended in fuels or lubricants are dormant, but can become active and form colonies on growth media. These issues do not make culture testing good or bad. Culture testing is still the only practical tool for obtaining microbial isolates that can be used for further testing. Moreover, culture testing is a useful condition monitoring tool if you are tracking changes over time. The more important limitation of culture testing as a condition monitoring tool is the delay between test initiation and the availability of test results. In the days or weeks it takes for microbes to form colonies on growth media, they are also continuing to proliferate in the system from which the sample was collected. This is where real-time (10 min) tests such as ASTM D4012, D7687, and E2694 have a major advantage over culture testing.

For more information on the most strategic use of culture or non-culture microbiology test methods, I invite you to contact me at


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.

The Science:

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.


Opportunity Cost at the Forecourt

In my first Fuel & Fuel System Microbiology post in November 2016, I wrote about the cost of repairing and remediating sites at which underground storage tanks had leaked. At the time, I have not fully considered that most of this cost burden was borne by insurance underwriters – not site owners.

Today, I want to return to a theme I’ve been writing about since long before my November 2016 blog post: retail site opportunity cost. As I write this blog, regular unleaded gasoline (RUL) in my part of New Jersey is retailing for $2.30/gal. For dispensers delivering 8 gpm instead of 10 gpm, this translates to approximately $144,000 per year per dispenser opportunity cost at urban sites that experience four hours per day of rush hour traffic – periods during which customers must wait in line to purchase fuel.

In this blog post I’ll share some simple computations on the relationship between dispenser flow rates and opportunity cost.

What is Opportunity Cost?

Opportunity cost is the difference between the economic value of the theoretically optimal use of an assist and the value realized by its actual use. At fuel retail sites that experience rush hour peaks, where the dispenser flowrate is the primary factor controlling fuel sales revenues, the opportunity cost is the difference between sales generated while dispensing at 10 gpm (U.S. EPA’s maximum permissible flowrate at retail dispensers) and sales generated while dispensing at slower flowrates.

Fuel Retail Opportunity Cost Model

As with all economic models, opportunity cost models are based on assumptions. For fuel retail assume:

  • 1. At state-of-the-art forecourts, submerged turbine pump (STP) capacity is sufficient to ensure that having multiple dispensers operating simultaneously does not reduce flowrate measurably.

  • 2. Absent flowrate restrictions, all dispensers can deliver 10 gpm.

  • 3. Customers can refuel their vehicles for 30 min per hour – the remaining 30 min are spent moving vehicles, making fuel-purchase selections, completing fuel sales transactions

  • 4. Although rush hour periods at urban fuel retail sites vary, four hours per day is the median – two hours each during the morning and afternoon commuting peaks.

  • 5. Rush hour periods occur five days per week.

  • 6. Current sales price (P) is $2.30/gal.

From these assumptions we can compute the annual opportunity cost (CO) per dispenser.

Where CO is the opportunity cost, P is the sales price in $s, Gmax is the flow rate @ 10 gpm, and Gobs is the observed (actual) flow rate. Note that Gobs can be for each dispenser or the average flow rate measured for two or more dispensers.

Computing CO for 8 gpm flowrate:

Field studies have shown that it is not uncommon for dispensers to operate at 5 gpm to 7 gpm. I’ll leave it to readers to do the math, based on their own flowrate, peak hour, and fuel price data. Don’t forget to multiply the opportunity cost per by the number of dispensers in your forecourt. If you own multiple urban retail sites, don’t forget to multiple your cost per site by the number of sites.

Data and Cognitive Dissonance

The two images under this blog’s title illustrate how our brains interpret what we see. Most viewers will alternate between seeing a man’s portrait and a woman reading a book in the left image. Similarly, when looking at the right image, most will alternate between seeing a goblet and two profiles. Some readers will only be able to see each photo as a single image – subject to only one interpretation.

In psychology, cognitive dissonance is the term used to describe a number of ways humans respond to new information – including how we behave when new infromation contradicts our previously held beliefs. I mention cognitive dissonance here because of the frequency with which I have encountered it whenever I have suggested that a fuel retailer test my model. The few who have go so far as to test their average dispenser flowrates have declined to compute the impact of their test results.

There’s a psychology term for willfully choosing to either refrain from collecting data that you don’t want to see, choosing not to do calculations that will lead to results you don’t want to believe, or both, but you’ll have to look that term up on your own.

The Big Questions

The model I’ve provided in this post addresses only fuel sales. It does not include convenience store (C-store) opportunity costs. For example, ask yourself how many prospective C-store customers choose not to visit the store when they believe that have either waited too long to pull up to a dispenser, that it has taken too long to fill their tank, or both. These delays are major factors contributing to customer fuel quality concerns. The fuel quality might be fine, but customer perception is that a refueling experience that take too long reflects uncertain fuel quality. One major petroleum company determined that they were losing 15 % of their customers due to fuel quality perceptions. In response, they invested heavily in a refinery to C-store cleaning program, followed by an upgraded condition monitoring and predictive maintenance program. They realized a tremendous return on their investment.

If you are a petroleum retailer who relies on a service company to perform the monthly checks required by the current underground storage tank regulations, what would your incremental costs be to include the additional condition monitoring and predictive maintenance actions needed to reduce your opportunity costs by 10 %? What would your return on investment be?

As always, please share your thoughts with me by writing to


I thank Michelle Hilger for inspiring me to write about today’s topic – the disconnect between the guidance NFPA provides about emergency generator fuel supplies and reality. Michelle chairs the Emergency Generating Systems Association’s (EGSA’s) Fuel: Fact or Fiction Working Group.

NFPA (National Fire Prevention Association) Standards Related to Fuel Storage

The NFPA provides fuel condition monitoring guidance in two documents:

  • NFPA 110, Standard for Emergency and Standby Power Systems; and
  • NFPA 25, Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems.

During her brief, presented to ASTM D02.14’s Fuel Microbiology Working Group in June 2018, Michelle reported that NFPA 110 prescribes the following:

  • Chapter 8.3.7 -A fuel quality test shall be performed at least annually using appropriate ASTM standards or manufacturer’s recommendation (Revision for 2019 Edition)

She also reported similar language in NFPA 25:

  • Chapter –Diesel fuel shall be tested for degradation no less than annually.
  • Chapter –Fuel degradation testing shall comply with ASTM D975, Standard Specification for Diesel Fuel Oils

Ensuring that fuel in emergency system storage tanks is fit for purpose is laudable, but insufficient.

What’s the Problem?

Some years ago, a maintenance engineer at a major resort hotel estimated that if there was a power outage and the hotel’s generators did not start and operate reliably, it would cost the hotel’s owners more than $2 million for each minute the facility was without power. Now ponder what the cost impact might be – in dollars and possibly lives – if a hospital’s emergency power system did not operate when needed (you can find some statistics if you research the impacts of recent hurricane disasters in Florida, Louisiana, Texas, and other southern states.

There is no debate that it is imperative to check the condition of stored fuel periodically. However, just determining that the bulk fuel meets ASTM D975 Table 1 specifications provides no information about the fuel system’s condition.

For several years – beginning with efforts to have proposed revisions incorporated into NFPA 110’s 2016 edition – the ESGA has been trying to educate NFPA stakeholders and broaden the scope of prescribed fuel system inspections.

For those of you who are new to this blog series, I invite you to start with my November 2016 post and read the entire series. This will help you to understand the rationale for ESGA’s efforts. In today’s post, I’ll just highlight two of the most important issues.

Fuel Quality Testing is a Snapshot

ASTM product fuel specifications were developed and are frequently revised to ensure that entities supplying and entities purchasing product have a common understanding of the criteria by which that product is defined as being fit for purpose. ASTM (and other consensus standard bodies – for example the International Standards Organization (ISO)) standards list parameters, test methods and pass/fail criteria that stakeholders can use to ensure that the product is fit-for-use at the time of sampling. Specification test results do not predict the product’s future condition. Under optimal storage conditions, emergency diesel generator fuel can be stored for prolonged periods (years). However, optimal storage isn’t always possible.

Fuel Versus Fuel System

Although my personal focus is microbial contamination and biodeterioration, in ASTM D6469 Guide for Microbial Contamination in Fuels and Fuel Systems I openly acknowledge that one major challenge to biodeterioration diagnosis is the number of symptoms that biodeterioration shares with non-biological (abiotic) fuel and fuel deterioration processes. Fuel can accumulate water and particulate matter. It can also become corrosive. Most engines can operate – at least for some time – using degraded fuel, but there’s a cost. CRC Report 667 Diesel Fuel Storage and Handling Guide (Coordinating Research Council, Alpharetta, GA, 2014) summarizes the most common fuel deterioration symptoms, their causes, and best practices for preventing problems. Two Energy Institute (EI) guidance documents are scheduled for publication in 2019:

  • Guidelines for the investigation of the microbial content of liquid fuels and for the implementation of avoidance and remedial strategies; and
  • Guidelines on detecting, controlling, and mitigating microbial growth in oils and fuels used at power generation facilities.

In the U.S., the Diesel Fuel Oil Group (DFOG) – a consortium of nuclear power industry professionals responsible for diesel fuel oil storage at power generation facilities – in collaboration with the Institute of Nuclear Power Operations (INPO) – has recently published:

Practice Guide: Management of diesel fuel quality for emergency diesel generators at nuclear power stations.

The publications coming from CRC, DFOG & INPO, and EI reflect the stakeholder community’s growing recognition of fuel system condition monitoring in addition to fuel quality testing.

What Needs to Happen

For many owner and operators, NFPA standards don’t just dictate minimum condition monitoring actions, they dictate all actions. If it isn’t prescribed in NFPA 110, then emergency generator system owners typically choose to avoid the incremental expense of fuel system condition monitoring. Historically, emergency standby diesel power generation system failures have cost tens of millions of dollars and countless lives.

EGSA has proposed new language for inclusion in NFPA 110 Chapter 8:

8.3.7 –Diesel fuel maintenance and testing shall begin the day of installation and first fill in order to establish a benchmark guideline for future comparison. Diesel fuel shall be tested for degradation no less than twice annually, with a minimum of six months between testing. All testing shall be performed using ASTM approved test methods and meet engine manufacturer requirements. Fuel testing shall be performed on all diesel fuel sources of EPSS. –Tests shall include at minimum, Microbial Contamination per guidelines referenced under ASTM D6469, Free Water and Sediment (ASTM D2709), and Biodiesel Concentration (ASTM D7371). Similar, modified, and proven methods recognized under ASTM shall be accepted. For acceptable values consult with the engine manufacturer and most current ASTM test documents -ASTM D975-18, and the Appendix X3.1.3 of ASTM D975-18, Standard Specification for Diesel Fuel Oils. –For diesel fuel stored consecutively for 12 months or longer, a diesel fuel stability test shall be performed annually. PetroOxy (ASTM D7545) is the accepted ASTM test method for S15 diesel fuels containing up to a biodiesel blend of 5% and less. Additional methods may be acceptable, refer to most current ASTM test documents -ASTM D975-18, and the Appendix X3.1.3 of ASTM D975-18, Standard Specification for Diesel Fuel Oils. –Any additional testing requirements shall be determined by equipment manufacturer, government regulations, recent test results, and geographical region. Refer to the most current NFPA 110 Annex A, ASTM D975 Appendix, and the CRC Report No. 667, Diesel Fuel Storage and Handling Guide for detailed testing and descriptions. –If diesel fuel is found to be outside of acceptable range in the testing listed in, the fuel shall be remediated to bring back to the required fuel quality for long-term storage specified under ASTM. Remediation may be in the form of fuel additives, polishing, tank cleaning, or diesel fuel replacement, and will be dependent of the test results received.

It’s time for NFPA to adopt these proposed changes and thereby substantially reduce the risk of emergency generators failing to operate when needed. Similar changes should also be made to NFPA 25.

As always, please share your thoughts with me by writing to For more information about EGSA’s activities, contact Michelle Hilger at


In my October What’s New blog post, I highlighted several of my primary takeaways from PEI’s 2018 convention. In this post, I’ll focus on one new commercial offering. If it works as promoted, this system can be a game changer for contamination control in underground storage tanks (UST). At the convention Veeder-Root showed a model of their new CleanDiesel In-Sump Fuel Conditioning System (ISFC). I haven’t seen any field data from retail or commercial sites using this system, so I’m not able to endorse the system (I asked Veeder-Root for permission to discuss their system but have no financial interest in their company or this filtration system). Still, I think it is an exciting innovation with great potential.

The Issue

As I’ve discussed in previous articles – including my paraphrasing of Rebbeca Moore’s summary of contamination typically present in a 7,500-gal fuel delivery (i.e., 1 cup (∼0.25 L) of dirt + 1 to 2 gallons (3.8 L to 5.6 L) of water + up to 325 gallons of FAME (B5 ULSD is now included in ASTM Specification D975 Diesel Fuel Oils) + 1 gallon of glycerin + 5 to 40 gallons of additives). These concentrations of dirt and water are traces (<9 ppm by volume dirt and <200 ppm by volume water) on a per delivery basis, but they add up over time. For example, a UST receiving weekly ULSD deliveries will also received 52 gal to 104 gal (∼200 L to 400 L) of water per year. Even if only 10 % of the delivered water settles out, that’s plenty of water to provide an excellent niche in which microbes can facilitate microbiologically influenced corrosion – MIC (see
Fuel Microbiology Part 19 from April 2018
). Amongst fuel industry folks who specialize in product quality control, the consensus is to minimize water accumulation in UST. As I discussed in Part 19, keeping tanks water-free is easier said than done.

Veeder-Root’s CleanDiesel In-Sump Fuel Conditioning System

I’ve copied figure 1 from Veeder-Root’s ISFC flier and added a few component labels. According to Veeder-Root, the ISFC uses recirculated fuel to create turbulence in fluid at the bottom of the UST. This turbulence causes water, sludge and sediment to be suspended into the product. These resuspended contaminants are then pulled through a perforated inlet component and subsequently flow through a particulate filter and water separator before the polished fuel is recirculated back into the tank. In theory, the turbulence and contaminant capture zones are both sufficient to prevent contaminants from accumulating anywhere along the UST’s bottom.

Potential Impact

When I saw the ISFC demonstration at Veeder-Root’s PEI Convention booth, I thought that the design was simple, elegant, and promising. Its design includes modes that trigger polishing when water is detected by the automatic tank gauge or by a timer (system activated for some number of minutes per scheduled interval). My principal concerns were about the dimensions of the turbulent zone and the polishing system’s contaminant capture efficiency. If only a fraction of the UST bottom is resuspended, water, sludge, and sediment will still accumulate elsewhere along the UST bottom. Any particulate or water contamination that’s resuspended will begin to settle immediately after the recirculation cycle ends. This could create a mechanism for redistributing – rather than removing – contaminants. Another concern is system maintenance. Veeder-Root has estimates of expected filter life. Additionally, they have sensors to measure pressure drops across the filtration unit and water levels in the water separator. Still I can’t help but wonder if sites using the ISFC will quickly grow tired of either having in-house staff or contractors keep up with system maintenance (i.e., changing filters and removing water). My concern here is based on my experience with several petroleum companies who had installed filtration systems between their terminal tanks and racks, only to later remove them because of the filter element replacement costs.

Bottom Line

I think that the ISFC has great potential – even if it’s less than 100 % effective. It directly addresses many of the current challenges related to keeping water and particulates from accumulating on the bottom of UST. I hope that within the next year or two, Veeder-Root will be able to publish a case study or two reporting the impact of ISFC installation and operation at retail and commercial sites. I encourage anyone interested in learning more about the ISFC to contact Diane Sinosky, Veeder-Root’s Global Product Manager, ATG. Diane can be reached at Of course, I always welcome your fuel & fuel system comments and questions at


The Petroleum Equipment Institute (PEI) held its 2018 convention at the Las Vegas Convention Center from 07 to 10 October 2018. As usual, the PEI convention was held in conjunction with the much larger National Association of Convenience Store (NACS) convention. Today, I’ll focus on a few items that are particularly relevant to fuel and fuel system microbiology. I’m not going to attempt to provide anything approaching an overview of the entire convention. Instead I’ll report and discuss a few statements I heard from speakers during PEI’s Tuesday 09 October education sessions.

Regulatory issues

EPA Regulatory Update – Carolyn Hoskinson, Director of EPA’s Office of Underground Storage Tanks (OUST) and several members of her staff spoke to the current state of affairs regarding UST regulations. Tony Raia reported that with the 13 October 2018 compliance deadline looming, 32 states had updated their UST regulations to harmonize them with the 2015 updated US EPA regulations. Tony identified five state categories:
1. State Program Approval (SPA) States that have completed their updates and which are now in full compliance
2. Non-SPA States that have completed their updates to comply with the 2015 regulations
3. SPA States that have delayed revising their state regulations
4. SPA States that have updates in progress
5. Non-SPA States that have not yet updated their regulations per the US EPA 2015 regulations.
Bottom line is that we are entering a period during which there will be some confusion over compliance.

U.S. EPA UST Enforcement – Mark Barolo – the US EPA OUST official responsible for enforcement – noted that in nearly all cases, individual States were responsible for enforcement. Recognizing the confusion, Mark opined that inspectors were going to address violations on a case-by-case basis. Generally speaking, retailers who had been incompliance, had the required documentation, and demonstrated that they were making good-faith efforts to ensure that they remained in compliance, would experience less enforcement grief than those who have not. Mark’s colleague, Cho Yi Risher noted that the regulations do not prescribe the time permitted for site owners to repair or replace non-compliant equipment. Moreover, the inspections required by the 2015 regulations identify non-compliant equipment. There is no incentive for owners to institute predictive maintenance programs (see my 16 January 2017 post) that would detect failure trends before equipment became non-compliant.

Failure to detect uncontrolled microbial contamination and biodeterioration before they cause valves to seize or tanks and lines to leak is a false economy. A few pennies saved during regulation-mandated inspections can lead to remediation expenses in excess of $0.5 million.
Although for some of us, it’s hard to believe that the UST installed in 1987 – in compliance with the original UST regulations – are now beyond their 30-year warranty life. Discussion during the session’s question and answer period indicated that all stakeholders shared a common interest in ensuring that sites with tanks that were more than 30-years old would be able to continue to operate. I anticipate seeing articles to this issue in PEI Journal in the coming months.

Fuel Quality and Corrosion
Scott Boorse – PEI’s Technical Program Manager; recently retired from a major fuel retailer – made several observations that validated much of what I’ve been discussing in Fuel Microbiology What’s New posts. He suggested that in his experience, 100 % of all retail site fuel systems had some corrosion. He attributed much of this corrosion to the bacterial genus Acetobacter converting ethanol to acetic acid. I am convinced that most of the headspace and spill containment well acid production comes form chemical oxidation of ethanol to acetic acid. Microbes are involved in an estimated 50 % of all system corrosion issues, but – as I’ve written previously – microbes produce a variety of organic acids. These acids can react with chloride, sulfate, and nitrate in fuel-associated water to form organic bases (or salts) and strong, highly corrosive, inorganic acids – hydrochloric, sulfuric, and nitric acids, respectively. Still, Scott was on spot suggesting that UST system corrosion was much more wide-spread than most stakeholders realize.

Rebbeca Moore – GM and chair of the automotive industry’s Top Tier Detergent Gasoline and Diesel Fuel consortia – discussed the importance of fuel quality on engine performance. Top Tier is an auto industry sponsored compliance program intended to go beyond ASTM product specifications typically cited in state regulations. I’ll steer clear of the perennial debates between engine manufactures and petroleum producers that enliven our semi-annual ASTM D02 (Petroleum Products) subcommittee A (Gasoline and Oxygenated Fuels) and E (Burner, Diesel, Non-Aviation Gas Turbine, and Marine Fuels), but Rebbeca made an important point. ASTM specifications are often misused. They are meant to indicate whether a product (i.e., fuel) is fit for use at a single point and place in time (i.e., when and where the sample was collected). The petroleum industry’s infrastructure is vast and complex. Moreover, product ages (if it didn’t it wouldn’t combust so well in engines). Specification tests provide little information about how the product will age during storage.
Rebbeca illustrated the dilemma by listing the typical components of 7,500 gal of in-specification ULSD delivered to UST:
• 1 cup of dirt
• 1 to 2 gallons of water
• Up to 325 gallons of FAME (B5 ULSD is now included in ASTM Specification D975 Diesel Fuel Oils)
• 1 gallon of glycerin
• 5 to 40 gallons of additives
At sites that receive frequent deliveries, these trace amounts of dirt and water add up! Not surprisingly, along with measures that are outside the retail site or fleet owner’s control, Rebecca recommended more aggressive water removal and better dispenser filtration (there is an ongoing debate among stakeholders with some recommending that all dispensers have 5.0 µm, water absorbing filters and others arguing for 100 µm particulate filters). I share Rebecca’s view that all dispensers should have 5.0 µm, water absorbing filters. Marketers who are focused only on low rates and not product quality have argued for eliminating the filtration requirement completely.

Ryan Haerer – of US EPA’s OUST ( – wrapped up the session, sharing a few notable points. First, Ryan reminded attendees that UST regulations apply only to system components that are in contact with the soil. The US EPA does not regulate the condition of internal components that are not in direct contact with the soil (for example, submerged turbine pumps – STP – and their associated hardware). He also explained that under the UST regulations, there is a requirement that system components be compatible with the substance stored. This is likely to become interesting as new products (for example, E15 gasoline or substitution of ethanol with isobutyl alcohol) are introduced into the commercial fuel infrastructure – here I’m using interesting – in the same way it is used in of the phrase: “May you live in interesting times.” (Austen Chamberlain – British Foreign Secretary, 1924 to 1929 wrote that he had been told that this was an ancient Chinese curse, but his claim has never been verified).

Bringing it home

At this point we are enjoying an interesting paradox. Regulators, insurers, and an increasing number of retailers recognize that waiting until fuel systems fail is a problem. However, the system largely provides incentives for site owners to wait until failures have occurred. After failure, insurance covers component replacement costs. In many states, superfund monies cover remediation costs. When site owners invest in predictive maintenance, they only see the costs. Although there are benefits – not the least of which is customer satisfaction and a positive corporate image – they are intangible. How do we break the paradox?

Please share your thoughts on this issue with me at I’ll compile comments and post them anonymously as a future What’s New column.


Biocide treatment releases biomass – now what?

Disinfection using microbicides is only one element of the fuel system decontamination process. This month’s post covers what needs to be done after a fuel system has been treated with a microbicide.

When a moderately to heavily contaminated fuel system is treated with an effective biocide, masses of biofilm material – flocs – get suspended into the fuel. As illustrated in figure 1, some of this biomass quickly settles to the tank bottom and the rest is carried with the fuel to the submerged turbine pump. Dispenser filters are designed to capture fuel particulates and biomass flocs. When the fuel is clean, a 10 m fuel filter, mounted in a retail dispenser, can process 500,000 gal (1,900 m3) to a million gal (3,800 m3) of fuel before it needs to be replaced (Note: there are no consensus criteria for filter life. Some retailers replace dispenser filters when the flow is less than 8 gpm – 30 L min-1. Others wait until flow is less than 2 gpm – 7.6 L min-1. In my November 2016 post, I detailed the economics of dispenser flow rates. The opportunity costs caused by slow flow can be startlingly expensive! After a fuel system has been treated, the next step is to get the flocs out of the fuel.

Fig 1. Effect of biocide treatment on biofilm masses – a) photo of the bottom of an underground storage tank – submerged turbine pump is to left, just outside of view so that the left side of the photo is in line with tank’s bottom centerline; b) schematic representation of (a), showing biomass accumulation on either side of bottom-dead center; c) same as (B) but after biocide treatment – biomass flocs are not dispersed in the fuel.

Floc removal – fuel polishing

There are three options for floc removal. Option 1 is to stop using the dispensers that draw product from the treated tank, give the flocs a day or two to settle to the bottom, and then to vacuum out the bottom sludge, sediment, and water. I can’t think of any retailers or fleet operators who would choose to put a tank out of commission for a couple of days.

Option 2 is to let the dispenser filters do all the work. This can translate into multiple filter changes per day for several days (fig 2). When the fuel reaching the dispenser is loaded with biomass flocs, filters can plug after 2,000 gal (7.6 m-3) to 10,000 gal (38 m-3) have flowed through them. This option might be feasible at rural sites that sell fewer than 10,000 gal (38 m-3) per week but is not particularly practical at high volume facilities. Long fuel turnover periods (i.e., more than two days) give biomass flocs time to settle to the tank’s bottom. As is the case for option 1, settled sludge and sediment needs to be removed from the tank as soon as possible after biocide treatment. Optimally, this is done one or two days after the treatment.

Fig 2. Removing biomass flocs the hard way – flocs are pulled into the submerged turbine pump and carried to the dispenser filter. As shown in the inset, the dispenser filter quickly becomes covered with slime; preventing fuel from flowing to the dispenser’s nozzle.

Option 3 is to use a fuel polishing rig (fig 3). Filtration rigs come in numerous configurations. Filtration rigs come in nearly as many configurations as there are companies who offer fuel filtration services. The number of filter housings on a rig typically ranges from one to three – although there are rigs with more than three housings. Diverse filter media are available – each with advantages and disadvantages relative to the others. Filter housing designs differ by the types and number of filter elements they contain, and by fluid flow patterns they use to optimize filtration efficiency. I’ll leave it to the mechanics and engineers to provide details on filtration technology and rig design. In this post, I’ll describe a generic rig.

Rigs with multiple housings use filters in series. Fuel first passes through a coarse filter (for example, designed to remove particles and masses that are >100 m), and then through a polishing filter (nominal pore size between 1m and 5m). Figure 4 shows a 16-element filter housing that’s mounted on a skid. Some filtration rigs include a fuel-water separator. Others rely on coalescer filters to strip water out of the recirculating fuel.

Filtration rigs have one or more pumps to drive fuel recirculation. Most commonly, a pump pushes fuel into the tank to create turbulent flow. This turbulence helps to keep particles and biomass flocs in suspension. The pump’s discharge creates pressure and its intake creates a vacuum. The vacuum draws product through the return riser and line to the rig. In fig 3, the pump drives fuel into the tank via a stinger that is inserted into the fill tube fitting and draws fuel through a stinger that is dropped through the turbine riser fitting. The fuel discharge stinger can be rigid or flexible. Some stinger designs include high pressure nozzle meant to source biofilm residual material from tank walls.

Depending on how heavily contaminated the fuel is, three to seven fuel-volumes might need to recirculate through the filtration rig to complete the polishing process. Depending on rig design, filtration flow rates range from 75 gpm (0.28 m3 min-1) to 350 gpm (0.9 m3 min-1). For a tank containing 5,000 gal (19 m3) of fuel, this means that post-treatment fuel polishing can take from less than 30 min [(5,000 gal 350 gal min-1) x 3 cycles  23 min] to 8h [(5,000 gal  75 gal min-1) x 7 cycles  470 min  8h]. As I mentioned above, greater flow rate also helps to keep particles in suspension.

Fig 3. Removing biomass flocs the easy way – high-capacity filtration rig processes recirculating fuel at 75 gpm to 350 gpm. In this illustration pump pulls fuel through a 3-stage unit (fuel-water separator, coarse filter, and polishing filter) and discharges the filtered fuel – via stinger (in) back into the tank. The pump’s suction also creates sufficient vacuum to draw fuel, water, and particulates from the tank to the rig’s inlet. Using the filtration rig keeps dispenser filters (inset) in pristine condition.

Fig 4. High-capacity, two-stage filtration rig – inset shows 26 filter cartridges inside 1st-stage filter housing.

My fuel is now clean – is my system also clean?

The answer depends what you mean by clean. Periodic microbicide treatment and subsequent fuel polishing can be enough to prevent tank deterioration problems. However, if biodeterioration damage began before the tank was treated and fuel polished, more thorough cleaning might be needed.

Even the best fuel polishing equipment can only direct pressure at surfaces that are in direct contact with the fuel. High pressure systems can be used to remotely clean the surfaces of empty tanks. It might be necessary for workers to enter the tank (personnel performing this work must be properly trained and certified to operate in confined spaces) and clean its surfaces manually (fig 5).

Fig 1. Confined space entry to clean tank walls. Source:

The only way to know for certain that a tank is clean is by visual inspection. Available remote camera technology can only be used to see exposed surfaces (for example, see: They cannot be used to see surfaces that are below the fuel level. Consequently, for visual inspection by remote camera, tanks must first be emptied (product can be drawn into either a tank truck or frac tank). Confined space entry and direct inspection remains the most reliable means of evaluating tanks for cleanliness, coating condition, and corrosion.

Why do routine condition monitoring if visual inspection is the gold standard?

If direct observation is truly the only way to know how heavily contaminated a tank is, why bother with the various types of tests I’ve described in previous Fuel Microbiology blog posts? The answer lies in return on investment (ROI). Data from routine sample testing (see my December 2016 Blog post provides important infromation about the fuel system’s condition. Most frequently, easy tests that require little or no equipment, act as the canaries in the mine. If results indicate that something is changing for the worse, more advanced tests help to determine what is going on. One can run routine tests and complete preventive maintenance actions for years for the cost of a single visual inspection. Consequently, internal inspections are reserved for when data (or regulatory ordinances) indicate they are needed.

For more information about detecting and controlling microbial contamination in fuel systems, please contact me at


Take two gallons and call me in the morning – not!

In Part 21, I reviewed the three primary types of fuel treatment microbicides – classifying them by their respective solubilities in fuel and water. You’ll recall, that I recommend using products that are soluble in both fuel and water – what I call: universally soluble. If you don’t remember why I prefer universally soluble, fuel treatment biocide, please re-read Part 21.
In today’s post, I’ll discuss how to get the most effective results when you treat a fuel system with a microbicide. Spoiler alert, I do not recommend simply dumping the minimum dose (gal microbicide per gal fuel) into your system and hoping for the best.

Think strategically
Before treating a fuel system, ask yourself why you are adding microbicide. To many readers the answer will be obvious, but not necessarily correct. Yes, the objective is to kill microbes. However, have you first considered where the microbes are living, or how much biofilm has accumulated on fuel system surfaces? When you treat a heavily contaminated system, and biofilm sloughs off tank and pipe walls, where is it going to go? Is it reasonable to assume that a single dose will disinfect my system? How will you know if your treatment was effective? If you don’t think about these issues before you treat, you probably will afterwards.

Where are the microbes living?
Tank bottoms
Most often we base fuel treatment decisions on test results from bottoms samples. Growth at the fuel-water interface can be seen either as an invert (water in oil) emulsion (rag-layer), membrane-like layer (pellicle), or both (fig 1a). Profiles of bioburdens in fuel, interface, bottoms-water layers generally show that the greatest bioburden is in the interface layer (fig 1b).

Fig 1. Fuel-water interface. 1a: fuel over water, separated by rag-layer; 1b) schematic profile showing maximum bioburden within rag-layer, minimum in fuel, and intermediate bioburden in bottoms-water.

In Part 15 (November 2017), I wrote about biofilms. Biofilms are complex, slimy residues that can form at the fuel-water interface (as in fig 1a) and on fuel system surfaces, including bottom-sludge and sediment. Among their numerous fascinating properties, biofilms can act like a slime fortress – preventing microbicides from reaching biofilm microbes.

Treatment objective(s)
Based on the previous section, it should now be clear that most commonly, the objective is to disinfect fuel system surfaces. Treated surfaces can include any combination tank walls, pipe surfaces, valves, meters, and pumps. Treated surfaces will not include the tank ullage zone. If fuel is not in direct contact with a surface, neither will the microbicide. I’ll discuss disinfecting ullage surfaces in my next blog post (Part 23). If only tank surfaces need to be disinfected, then static soaking can be sufficient. However, if the objective includes disinfection of other fuel system component surfaces, the treated fuel will have to be recirculated to ensure that they are exposed to the microbicide.

Choosing the correct dose
Dosage is the volume of microbicide added per gallon of fuel (recall from Part 21 that I recommend against doing before removing bottoms-water). All microbicides list minimum and maximum dosages on their container labels. The minimum dose is based on laboratory tests that can provide optimistic results. I always recommend using the maximum permissible dose. Maximum dosage is based on the regulatory agency’s toxicological risk assessment of the microbicide’s active ingredient(s).
I recommend using the maximum permissible dose because the concentration of microbicide available to kill microbes begins to decrease once the product has been added to the fuel. Collectively, the factors contributing to the disappearance of microbicide active ingredient are called demand. There are chemical, physical, and microbiological demands. Figure 2 illustrates how microbicide concentration can decrease over time. The time axis in fig. 2 can range from hours to months.

Fig 2. Microbicide demand curve.

The most common physical demand is dilution. Each time untreated fuel is added to a tank containing treated fuel, it dilutes the microbicide concentration. Figure 3 illustrates what can happen once the microbicide concentration decreases to below its critical concentration – the minimum concentration at which the active ingredient is effective. Note how active ingredients that are quite effective when used as directed can actually stimulate growth once their concentration is less than the critical concentration. In fig. 2, microbial populations exposed to sub-critical microbicide concentrations are more than three orders greater than those in untreated systems. Conversely, at concentrations ≤50 % of the maximum permissible dose, the microbicide is fully effective.

Fig 3. Microbial population response to different microbicide doses (hormesis).

Dosing plan
Although a single dose is often sufficient when treating a lightly contaminated system, it can be insufficient for disinfecting moderately to heavily contaminated systems. The reason is microbicide demand. An effective treatment exposes microbes to adequate concentration of active ingredient for a sufficient period of time (the soak period). The optimal soak period is between 24h and 48h. Except for long-term storage tanks, operators rarely have the luxury of allowing their fuel tanks to stand idle for this long. Depending on the fuel turnover rate, it might be necessary to add microbicide in order to maintain the active ingredient’s effective concentration for at least 24h (this is most commonly an issue at sites that receive more then one fuel delivery per day).

Additionally, active ingredients are used up as they react with microbes. The more heavily contaminated a tank is, the more quickly the available microbicide concentration will decrease. Figure 4 illustrates a point I made above, regarding biofilms. The initial treatment is unlikely to remove the entire biofilm or to kill microbes deep within the biofilm matrix. One or more follow-up treatments might be needed to fully eradicate the biofilm community (fig. 4c, d, and e). Each treatment will cause masses (flocs) of biofilm material to slough away from the surface to which it was originally attached. Some of these flocs will settle to the tank’s bottom. Those that don’t will remain suspended in the fuel and be transported to filters. Rapid filter plugging is a common result of effective biofilm destruction.

Fig 4. Biocide interacting with biofilm – a) biofilm accumulation on a surface; b) first biocide dose penetrates into the biofilm partially, causing some biofilm material to slough off; c) second biocide dose treats most of the remaining biofilm; d) third does disinfects surface; e) after effective treatment, surface is biofilm-free.

When tanks or sufficiently contaminated to cause filter plugging after biocide treatment, polishing, fuel tank cleaning or both should be part of the remedial effort. In part 23, I’ll write about fuel polishing and tank cleaning. In the meantime, if you have questions or comments about today’s post, please contact me at

Microbes are ubiquitous. There are extraordinarily few habitats on earth where thriving, microbial communities have not been detected. In practical terms, this means that it is unlikely that operators will ever have a completely sterile fuel system or that they will reduce their fuel system biodeterioration risk to zero. Biodeterioration can still occur in the best maintained fuel systems. However, the risk of it occurring in an inadequately maintained system is much more likely


Fig 1. From Rime of the Ancient Mariner, Samuel Taylor Coleridge, 1798

Water, water everywhere…

Samuel Coleridge’s infamous mariner paid dearly for having killed an albatross (figure 1). Do fuel quality managers and personnel responsible for fuel system integrity pay dearly for underestimating the ability of small (<1 oz; 30 mL) pools of fuel-associated-water left behind after water has been nominally purged from a fuel tank? A water bit is any small volume of water that remains in a fuel tank after dewatering (my personal, technical definition).
In Part 20, I wrote: “No water means no bugs. Is it as easy as all that?” I also explained why the short answer to the question was: “No.” For emphasis, I’ll again share figure 3 from Part 20 (figure 2, here):

Fig 2. Scale: how 2 mm of water appears to a bacterial cell.

A – a 6’6” tall man standing at the base of Mt. Kilimanjaro; B- a bacterial cell “standing” in a pool of water that is >2 mm deep; the ratios between the height of Mt. Kilimanjaro and the man in A, and between the depth of the pool of water and the bacterial cell in B are the approximately the same.

What can we do about these traces of water?
I confess that I am not a big fan of dispersants. When water dispersants are used routinely as fuel additives, the dispersed water can act as a corrosive agent; damaging engine components. However, when used to complete the job started by draining or vacuuming most of the free-water out of a tank, dispersants can be quite effective.
Figure 3 illustrates how dispersants work. Most dispersants are organic molecules that have a polar (charged; water-soluble) head and a non-polar (non-charged; fuel-soluble) tail. When added to fuel over water (figure 3b), they move towards anywhere where fuel contacts water (figure 3c) and trap tiny (typically <1 µm; 0.0004 in dia) fuel droplets. The fuel “sees” only the dispersant’s non-polar tails, so the droplets disperse uniformly throughout the fuel (figure 3d). The dispersed droplets (micelles) get transported with the fuel and evaporate during combustion in the engine cylinder.

Fig 3. Dispersant action: a) fuel over bottoms-water; b) dispersant added to fuel – inset shows dispersant molecule with polar head and non-polar tail; c) dispersant heads and tails align in water an fuel phases, respectively; d) dispersants form micelles with water droplet trapped in center; typical droplet size is < 1 μm dia.
The use of dispersants is controversial. Dispersant manufacturers and marketers focus on dispersant effectiveness in keeping free-water from accumulating in fuel systems. Moreover, under most circumstances, microbes are unlikely to make use of water trapped within dispersant micelles. Conversely, engine manufacturers focus on the potential for dispersed water to corrode and erode injector nozzles; particularly on modern, high-pressure, common-rail diesel engines. Interestingly, there is at least one additive manufacturer that has tried to promote water-emulsion diesel fuels – diesel with a dispersant that enables the fuel to hold as much as 25 % water. As a microbiologist, I was looking forward to investigating microbial contamination problems in systems that handled 25-75 water-in-diesel blends. But that’s another discussion.
In my next blog, I’ll focus on fuel treatment biocides. In the meantime, if you have questions or comments about today’s post, please contact me at

Microbes are ubiquitous. There are extraordinarily few habitats on earth where thriving, microbial communities have not been detected. In practical terms, this means that it is unlikely that operators will ever have a completely sterile fuel system or that they will reduce their fuel system biodeterioration risk to zero. Biodeterioration can still occur in the best maintained fuel systems. However, the risk of it occurring in an inadequately maintained system is much more likely.



I’ll open this post with a disclaimer. Microbes are ubiquitous. There are extraordinarily few habitats on earth where thriving, microbial communities have not been detected. In practical terms, this means that it is unlikely that operators will ever have a completely sterile fuel system or that they will reduce their fuel system biodeterioration risk to zero. Biodeterioration can still occur in the best maintained fuel systems. However, the risk of it occurring in an inadequately maintained system is much more likely.


I’ll also take this opportunity to remind readers that biodeterioration (damage caused by organisms) and bioremediation (using microbes or other organisms to degrade or remove toxic or noxious chemicals) are the flip-sides of the biodegradation coin (figure 1).

Figure 1. Just as the two sides of a coin are its obverse (front) and reverse (back) sides, the two sides of biodegradation are bioremediation and biodeterioration.

Microbes degrade fuel quality and fuel system components. In high-turnover retail systems, product deterioration is unlikely. I consider any tank that is refilled at least weekly to be a high-turnover (or high-throughput) system. The time that product spends in the storage tank is too short for degradation to occur. Studies that investigate the rate at which microbes change fuel chemistry, typically show substantial changes after a month or longer. Consequently, fuel in tanks used for emergency generators, or seasonally operated equipment is at greater biodeterioration risk than fuel in retail underground storage tanks (UST), or frequently operated vehicles. That’s all I want to say about fuel biodeterioration for now. I’ll return to the topic in a future blog post.

For now, I’ll focus on fuel system biodeterioration. Most of the damage caused to fuel system components is caused by biofilm communities (see post #16 Microbes cause damage either directly or indirectly (more on this in a future post). The most obvious indications of biodeterioration are filter plugging and corrosion. Although it’s typically the first indication of a biodeterioration problem, filter plugging is a late symptom. I often compare it to a heart attack; a late – but often first recognized – symptom of coronary disease.

So how do we substantially reduce fuel system biodeterioration risk? Step one is cost-effective condition monitoring (CM). Step two is cost-effective predictive maintenance (PdM; see The former drives the latter. Why do I emphasize cost-effectiveness? As I see it, there is as little justification for investing $10,000 per year to detect problems that might cause $1,000 per year of damage, as there is in refraining from spending $10,000 per year to detect problems that could cost $100,000 per year. I’m not suggesting that any fuel system CM program should cost $10,000 per year. An effective program can cost less than $2,000 per year. My point here is that before setting up a CM/PdM program, stakeholders should invest a bit of time and effort to determine their actual annual biodeterioration-related costs.

Opportunity Cost:

Nearly two decades ago, I first argued that a 10% flow-rate loss at high-traffic, retail sites, can easily translate to more than $100,000 per dispenser per year opportunity cost (Passman, F.J., 1999. “Microbes and Fuel Retailing: The Hidden Costs of Quality.” Nat. Petrol. News 91 [7]: pp: 20-23). My model did not include lost C-store revenues related to customer discontent with their fueling experience. Retailers who have tested my model have invariably been shocked by the huge impact of seemingly minor flow-rate reductions on fuel sales volumes. I’m still trying to understand the psychology behind retailers’ general reluctance to even test my model (for model details, contact me at Bottom line: the return on investment (ROI) for well-designed and executed, CM can easily be >$1,000 return on each $1 invested.

Condition Monitoring:

In Parts 2 through 18 of this series, I’ve written about the details of condition monitoring. I won’t repeat that information here. Instead, I’ll offer a few basic guidelines:

1. Testing hierarchy – CM plans should include two or more tiers. Tests that are easiest and least expensive to perform should be done most frequently (checking UST for bottoms-water accumulation and dispenser flow-rate checks are great examples of Tier 1 tests). Tier 2 tests include bottom-sample visual inspection (optimally, samples should be collected from the fill, automatic tank gauge, and submerged turbine ports). A simple microbiological test (for example ASTM D7687) is indicated whenever the bottom-sample is turbid or when it includes water. When Tier 2 tests indicate that the biodeterioration risk is moderate to high, Tier 3 tests (generally performed by a qualified laboratory) are used to confirm the risk.

2. Test method selection – notwithstanding the examples I mentioned under Testing hierarchy each site owner should develop a CM plan that best meets their needs. You can read my test specific blog posts (or ASTM D6469) for discussions of the benefits and limitations of each test method.

3. Testing frequency – my rule of thumb, after you have determined how often a test parameter is likely to indicate an increased biodeterioration risk, divide that period in three. That gives you the optimal test interval. Testing more often typically translates into greater costs without any real ROI. Testing less frequently increases the risk of having to perform corrective – rather than preventive – maintenance actions.

4. Understanding trends – each of the three previous guidelines depends on a basic understanding of trends. For example, even fuel with an ISO 4406 cleanliness rating of 18/16/13 (see, will eventually plug dispenser filters. Consequently, dispenser flow-rates will invariably decrease. How many operators know what normal looks like? How many have a control limit? Typically, dispenser filters can process >250,000 gal of fuel before the flow rate will fall below 7 gpm. Replacing fuel filters when the flow-rate is <7 gpm strikes a balance between the opportunity maintenance costs. Knowing whether the flow rate has fallen to <7 gpm after 50,000 gal or 500,000 gal of fuel have been filtered serves to trigger additional sampling and testing. If the amount of fuel filtered before substantial flow reduction occurs is much less than expected, then additional testing is indicated.


In summary, microbial contamination control depends on a good CM program that is linked to a good PdM program. A reasonable investment in CM and PdM should be a fraction of the likely cost impact of not having those programs in place. A cost-effective CM program is driven by an understanding of system trends, definition of the methods that will provide the most useful, actionable information; selection of which tests to run; and determination of sampling and testing frequency. In my next blog, I’ll focus on preventive measures. In the meantime, if you have questions or comments about today’s post, please contact me at



  • Consulting Services
  • Condition Monitoring
  • Microbial Audits
  • Training and Education
  • Biocide Market Opportunity Analysis
  • Antimicrobial Pesticide Selection