Microbial Growth in Diesel Fuel:
Causes and Solutions with Fuel Polishing

Wes Cade, Service Manager, Felix Marine Industries

Fuel polishing is a filtration process that removes water, microbial contamination and particulate matter from petroleum-based fuels. The fuel polishing system of Felix Marine Industries (FMI) is the Clarus 250 model. This is a patented, four-stage filter and polishing machine that effectively removes water and solid contaminants down to ½ micron in size. The variable speed pump draws contaminated fuel through a sock filter, and then pumps the fuel through a one-micron pre-filter. After the pre-filter, the fuel is pumped through a water coalescing and separating two-part filter. The fuel then passes through the final two polishing filters for additional filtering down to ½ micron and where any residual water is trapped, before returning the fuel to the tank in nearly new condition.

Microbial contamination shows up in filters and tanks in what is commonly referred to as “sludge” or “algae”. These microbes feed off the hydrocarbons and water present in most diesel fuel. The microbes multiply with astonishing speed. These bugs and their waste products clog filters, fuel lines, and shut down engines. They have abrasive and caustic properties that corrode fuel tanks and cause premature wear of close tolerance machined surfaces, such as fuel pumps and injectors. In the industry, the microbes are referred to as Hydrocarbon Utilizing Microorganisms, or H.U.M. bugs.

The most common H.U.M. bugs found in petroleum products are Cladosporium Resinae and Pseudomonas Aeruginosa. H.U.M. bugs enter your fuel system from contaminated sources. Every stop in the commercial fuel distribution process has the potential for breeding these bugs, and it only takes a few microbes to start a problem in your boat fuel tank. H.U.M. bugs need water to sustain them and multiply. They don’t survive in pure fuel. Water shows up in fuel tanks and storage vessels from condensation on the inside of the tanks or by entering through breathers in moisture-laden air. In addition to creating a breeding ground for H.U.M. bugs, the water present in fuel combines with the sulfur in diesel fuel to form sulfuric acid. Since water is heavier than fuel, this sulfuric acid sits on the bottom of tanks, usually trapped in a “sludge” of H.U.M. bugs and their waste products.

At Felix Marine Industries, it is not unusual to find a boat that is only a few years old that has leaking fuel tanks. Most pleasure boaters don’t put nearly the number of hours on their engines as do commercial vessels, so therefore; the fuel sits in the tanks undisturbed for a long time. This creates the perfect breeding ground for H.U.M. bugs. Vessels that go through continual replenishment of their fuel have less chance of bug problems, as the fuel and the bugs are constantly being consumed by the engine. Pleasure boats often sit for months at a time without being run, especially during the winter season. Stagnant fuel sitting in slack tanks (less than full), where condensation is present, encourages growth of H.U.M. bugs. Keeping your tanks full and using a fuel treatment regularly, helps considerably. Unfortunately, one still can have the problem of the dead bugs, their waste products, and most importantly, - the sulfuric acid sludge sitting on the bottom of the tank.

At Felix Marine Industries - depending on the size and tank configuration, we use suction wands to remove the residue from tank bottoms. When tanks are baffled, it is often necessary to install removable hatches to access all compartments in the tank in order to fully remove all the contaminants. If the tank has been leaking, we steam clean the tank and repair the leaking seam or corroded and pitted bottom with different methods (according to the material of the tank). This process of fuel polishing, tank cleaning, and repair is an unwelcome expense to boaters, but the alternatives are many times more costly.

Replacement of fuel tanks in pleasure boats is rarely an easy chore due to the way most boats are constructed. The tanks are installed, and then everything else is built on top and around them. Contaminated fuel leads to poor engine performance, premature engine failure, shortened duty cycles, unplanned downtime, and expensive repairs. Early failure of injection pumps and injectors due to the abrasive and corrosive nature of these contaminants compounds the expense. (Hydrogen sulfide is a byproduct of some algae and causes pitting of injector surfaces).

In addition to the machinery costs, consider the price of safety of the boat owner’s family and his boat. Engines shut down in turbulent waters when the bottom “sludge bugs” are stirred up and plug filters, can be life-threatening if one is in jeopardy of going on the rocks or being swamped while dead in the water rolling around in the trough. The first sign of H.U.M. bugs should be addressed before it turns into a more serious problem. When you see accumulations of “sludge”, “algae”, or coffee ground-like deposits in your racor-type fuel filter bowl, or on your filter, don’t just change the filter and drop the hatch on the engine room. The problem will not go away. Take preventative measures right away to ensure a safe boating experience.

Felix Marine Industries steam cleans engine rooms and bilges. We do oil changes, as well as fuel polishing, mechanical and electrical installation, and repair. We are a full service yacht repair and detail business located within the Lake Union Drydock complex on South Lake Union. Please contact us for further information at (206) 286-6500.