Fuel-Performance Issues? Don’t Blame Biodiesel

Karen Potratz
Jun 25
6 min read

Carter Smith, a laboratory technician at Iowa Central Fuel Testing Laboratory, prepares to test a fuel sample for microbial growth. (Photo: Iowa Central Fuel Testing Laboratory)

Moisture and microbial contamination are common culprits.

Plugged filters. Sluggish acceleration. Gunky tanks. Too often, biodiesel unfairly takes the blame for these and other fuel-performance issues. In truth, high moisture content and microbial contamination in a fuel tank are the most common causes of fuel problems—whether in biodiesel or any other liquid fuel.

“Microbial contamination in diesel fuel has become more common with the adoption of ultra-low sulfur diesel,” points out Pete Probst, technical director of the Michigan Advanced Biofuels Coalition. “That’s because sulfur has antimicrobial properties and previously acted as a natural deterrent to microbial growth in fuel. But regardless of the type of fuel, it’s important for fuel suppliers and users to become educated on the hazards of contaminants and how to prevent and mitigate problems.”

Diagnosing fuel-quality issues can be like a crime-scene investigation, says Don Heck, director of the Iowa Central Fuel Testing Laboratory in Fort Dodge, Iowa. As an independent lab, ICFTL runs fuel-performance tests for most of the nation’s biodiesel producers, providing third-party verification under the BQ-9000 program. The lab also evaluates petroleum diesel, ethanol and other liquid fuels, as well as biofuel feedstocks.

“Whenever we get a sample for testing with a complaint of poor-quality fuel, our first recommendation is to test for moisture and microbial content,” Heck says. “These have the biggest impact on engine operations and fuel efficiency—much more than cetane, viscosity, cloud point or any other parameters we test fuel for.”

Microbes such as bacteria, molds and fungi are ever-present in the environment, making it virtually impossible to prevent them from entering fuel, Heck says. At high concentrations, these organisms can plug filters. Small microbial particles that pass through the filters can wear out fuel injectors. These issues are not unique to biodiesel blends.

Water, Microbes Go Hand in Hand

“Microbes are living organisms and can’t function without a source of water,” Heck says. “Therefore, microbial contamination in fuel is almost always associated with water in the storage tank.”

Water can contaminate fuel-storage tanks through leaks in tank walls, hoses or nozzles, or during fuel loading and dispensing. When temperatures fluctuate, moisture inside the tank can condense into water droplets that provide a medium for microbial growth. Water contamination by itself causes engine-performance issues and promotes corrosion in fuel systems. In winter, entrained water can freeze and form ice crystals that may clog fuel filters and lines.

When Water Causes Problems

Fuels commonly contain small amounts of entrained water—microscopic droplets that are suspended in the fuel. Even a freshly cleaned tank can have as much as 60 parts per million (ppm) of entrained water in the fuel. At low levels, entrained water remains in suspension, is dispensed with the fuel and disperses harmlessly during fuel combustion.

Typically, diesel fuel can hold entrained water up to 200 ppm to 250 ppm. Biodiesel has slightly higher water-holding capacity, especially at blends of B11 to B20 and higher.

“Trouble starts when entrained moisture reaches above 200 ppm to 250 ppm,” Heck says. “At higher levels of moisture content, water falls out of suspension in the fuel and creates a layer of free water at the bottom of the tank. That’s where microbes start to grow.”

ASTM specifically addresses water and particulate-matter contamination with the D2709 “Water and Sediment Test,” which allows a maximum of 500 ppm for water and debris combined. The Karl Fischer test is another, more sensitive method for measuring entrained-water content that allows action before major problems occur.

Fuel Sampling Best Practices

Proper sampling is key to accurate testing for water and microbial contamination. Because free water settles to the bottom of the tank, it’s important to collect fuel samples from the tank bottom. A Bacon Bomb tank-sampling device is specially designed for that purpose. Clean and disinfect your sampling equipment with alcohol to prevent cross contamination or false positives.

Visual observation can deliver a simple but not 100 percent accurate indication of fuel quality. Fill a clean jar or beaker with fuel and let it settle. The ASTM D4146 “Visual Inspection Procedures” method is the industry-standard test for visual clarity of diesel fuel at the point of delivery. High-quality fuel should be clear with good color. A cloudy fuel sample could be a sign of water and/or microbial contamination.

In extreme cases, the sample will settle into three distinct levels, with free water at the bottom, a microbial mat above the water and fuel settling to the top.

“If the sample looks bad, it probably is,” Heck says. “However, it’s important to realize that even a sample that appears to be clear and bright can have microbial growth. The only way to know for sure is to culture the fuel and see what grows.”

ASTM D6974 Test Process

ICFTL and other laboratories use the industry-standard ASTM D6974 method to measure microbial content in liquid fuel. The D6974 test involves running fuel samples through two separate filters that are then placed on two different culture plates to measure colony-forming units.

An incubated filter shows multiple contaminants in a submitted fuel sample, most likely molds, bacteria and yeasts. (Photo: Iowa Central Fuel Testing Laboratory)

One of these plates carries a culture medium optimized for growth of bacteria, such as E. coli. The medium on the other plate is optimized for growth of “mycologia”—a general term that encompasses species of yeasts, molds, fungi and other microbes. Based on the amount of microbial growth on the culture plate, lab technicians can determine the severity of contamination.

Biocides, Antiwater Agents

If microbes are detected, use a fuel-specific biocide treatment to kill the microbes. Most biocides are broad spectrum and are designed to kill all types of microbes. Consult with your fuel distributor for product recommendations. If possible, have the supplier apply the biocides during fuel delivery so the treatment can be evenly distributed throughout the tank. If applying biocides yourself, use caution as they are typically very toxic. Follow the manufacturers’ instructions and wear proper personal-protection equipment when handling these chemicals. After biocide treatment, expect to change fuel filters frequently as dead microbial material is caught in the filter.

Other additives are available to increase the fuel’s water-holding capacity—preventing the presence of free water that fosters microbial growth. Heck advises against using additives as a “band-aid” approach to fuel problems.

“I’ve seen a tank go from bad to worse by using too many additives that end up in a messy sludge,” he says. “In severe cases of contamination, it’s best to drain and clean the entire fuel system.”

Don’t Overlook Filters

Fuel-filter evaluation is an often-overlooked step in fuel-performance troubleshooting. If you experience ongoing filter plugging, Heck recommends sending in the filter for testing, not only the fuel.

Materials caught in a fuel filter might include microbial sludge, diesel waxes, oxidized fuel or polymer structures from accumulated additives. These are scraped off the filter and tested in the lab.

Photo 1: The fill-port lid seal on this underground storage tank is heavily degraded and needs replacement to help keep water and other contaminants out of the spill bucket. (Photo: Michigan Advanced Biofuels Coalition)

“The filter often represents the ‘smoking gun’ of fuel problems,” Heck says. “A fuel sample may only represent a gallon of fuel, but a fuel filter represents the residues and contaminants from thousands of gallons of fuel. This concentrating effect can make fuel analysis much more efficient and tell us a lot about what is happening in that fuel.”

Regular Fuel Maintenance

Once fuel problems are resolved, Probst recommends looking at tank bottoms every six months to ensure the fuel is free of water. It’s a good idea to test for microbial activity anytime free water is found.

Many suppliers offer fuel testing to customers at no charge. The Michigan Advanced Biofuels Coalition provides testing assistance to biofuels users in Michigan.

“Besides regular fuel testing, also keep an eye out for rust, cracks or any other tank imperfections that could become entry points for water and microbes,” Probst says.

Photo 2: This fill-port spill bucket had a loose plunger valve allowing dirt, water and microbes directly into the fuel below. This area should be kept clean and dry at all times. (Photo: Michigan Advanced Biofuels Coalition)

Probst encourages operators to inspect and clean the fill-port spill bucket and lid during monthly inspections of underground storage tanks (USTs). The fill-port lid has a seal that helps keep out water, but often it becomes degraded and allows contaminants to enter. (See Photo 1.)

In conclusion, realize that any type of fuel tank can become contaminated. Best practices for fuel handling and storage are the same with biodiesel as with other liquid fuels. Biodiesel remains a readily available, drop-in solution for diesel fleets and operators looking to reduce their carbon footprint and improve sustainability.

For more information about biobased diesel fuels, visit miadvancedbiofuelscoalition.com or email contact@miadvancedbiofuels.com.