Problem: Diesel bug

  • Mucous residues on filter

  • Mucous residues in fuel filter

  • Mucous residues in fuel filter

  • Mucous residues on filter

  • Mucous residues on filter

Diesel bug: Definition

The term diesel bug refers to the microbial contamination of diesel fuel. It can be considered a type of classic biofouling. This means the general damage to material caused by microorganisms growing on the material surface. More specifically, it is a mucous film or flakes of various organisms such as bacteria and fungi in a solution. Fundamentally, all surfaces and boundaries can develop such biofilms. Almost all industrial applications are affected, in particular the tank and engine system. One of the reasons for this is the increased use of methyl esters in place of petrodiesel and heavy fuel oils.

The topic is relevant for the preparation of fluids by means of fluid filters, in particular fuel filters, as the surface can become clogged by a biofilm. This results in the differential pressure increasing. In fuel filtration, this mainly occurs when water enters the fuel system and growth occurs at the boundary surface between water and fuel. A mucous film forms, varying in thickness. A biofilm of this type is caused by bacteria that populate the fluid boundary layer and feed from nutrients in the fuel. Forming a biofilm allows the organisms to survive in such extreme environmental conditions.

The bacteria living in a biofilm benefit from the film enormously. The film offers a degree of protection from mechanical stress. Apart from this, its irregular formation enables aerobic and anaerobic zones to develop, offering a habitat for a wide range of organisms. Within the biofilm, the individual bacteria engage in a vigorous exchange of genes. This makes it possible for the colonies to adapt to new conditions quickly and is one of the reasons for the increasing resistance to biocides. Generally, it can be said that bacteria in a biofilm are more capable of surviving and more resistant to external stress.

Background and Origin

Diesel bug arises when biofilm-producing bacteria adhere to a surface such as the surface of a filter element. This involves individual bacteria adhering to the surface. The forces driving this adhesion may include van der Waals forces, electrostatic attraction and the formation of hydrogen bonds and other physico-chemical processes. These forces are also major factors responsible for the formation and cohesion of the matrix structure. It is this matrix formation that gives a biofilm its characteristic properties and its typical appearance. This stable matrix can only form if the biofilm-producing bacteria populate a surface. They do this by initially spreading out across the entire surface side by side.
Once they have populated the entire area, the organisms begin to grow on top of each other in multiple layers.

To cohere effectively, this multi-layer matrix relies on “EPS” (= “extracellular polymeric substances”). These substances are biopolymers that the organisms surround themselves with. These polymers originate from the organisms themselves. They are metabolic products of the bacteria and the remains of dead organisms. More precisely, they are made up of polysaccharides, proteins, glycoproteins, glycolipids, extracellular DNA and other nucleic acids. The biopolymers surround the individual bacterial cells, holding them to together to form a firm, three-dimensional, heterogeneous compound structure. This matrix is what constitutes the actual biofilm. It has the appearance of slime. The matrix enables the microorganisms to be held together and also to fasten themselves firmly to a surface. Inside this structure, the organisms are extremely well protected from external influences. The only processes that occur inside the EPS structure are diffusion and convection processes, to supply the bacteria cells with substrates. The structure’s heterogeneous form enables convection flows to occur that allow substrate to be supplied to the organisms deep within the biofilm. The EPS also function as sources of nitrogen, carbon and phosphate. In spite of all this, there is a limit to how deep a biofilm can become. This is because a lack of oxygen or substrate can start to occur at certain points if the film becomes too thick. This causes organisms to die and thus entire portions of the film to break away. Furthermore, shear forces created by the fluid flows wear away the top layers. These processes combined bring about a balance between growth and erosion.

Typical diesel bug organisms include:


  • Bacteria: Bacillus subtilis, Bacillus licheniformis;
    Pseudomonas aeruginosa

  • Fungi: Hormoconis resinae

  • Yeasts: Yarrowia tropicalis

These are environmental germs that live in water, air and soil.

Because microorganisms have an exponential growth rate, diesel bug can be expected to advance rapidly from the initial infection to the point where effects can occur that damage machines and fuel filters.

  • Beginnender flächenmäßiger Bewuchs

  • Ausgebildete mehrschichtige Dieselpest-Struktur

Signs to look out for

  • Unpleasant, putrid odour

  • Cloudy fuel

  • Fuel discolouration (yellow, green, brown)

  • Slimy sedimentation in the fuel

  • Slimy residue on filters

  • Slimy residue in parts that convey fuel

Consequences and Effects

The consequences of diesel bug are particularly enormous where fuel consumption is concerned. For example, severe damage can be caused to metallic components. The reason for this is the corrosion of metal caused or accelerated by the microorganisms. One example of this is the corrosion of the metallic container walls of the fuel tank.

This results in deep indentations at certain points, caused by metal material degradation.
Alongside the consequences for parts that come into contact with the biofilm, there are many other negative effects that can occur. One of these problems is the metabolic products of the microorganisms causing the fuel to become more acidic. This involves various acids and free radicals that support the chemical processes of oxidation and the hydrolysis of the fuel. This results in the acidity of the diesel increasing to an unacceptable level in a matter of days.

The occurrence of biofouling in fuel systems is a major cause of clogged fuel lines and filters.

A particular problem is that when the filter becomes clogged this is not always a gradual process. An acute blockage of the filter can occur if a large amount of the biomass present in the fuel tank is taken in all at once. This results in the flow rate dropping severely and the differential pressure increasing. Consequently, the combustion engine is no longer supplied with sufficient fuel, preventing the engine from starting or causing severe damage to machine components during operation (e.g. cavitation damage to the low-pressure or high-pressure pump). Apart from the risk of free bacterial structures suddenly blocking the filter, classic biofouling can also occur on the filter surface. In this case, the consequences are largely identical.

Case Studies

  • Harvesting machines: Potatoe harvester starts with delay into harvesting season, because the fuel system has been massively infested by bacteria and filter as well as fuel-carrying components got blocked during harvesting-free season.

  • Power Gens Sets: Emergency power supply in hospitals or data centers is not guaranteed, because due to long fuel storage micro organisms spread in the system. Starting the engine is not possible anymore. Person or property damage is the consequence.

  • Motorboats: Damage of a maintenance vessel in a lockage of inland waterways because the diesel filter is blocked by bio mucus.

  • Leasing machines: Due to long downtimes the particle filters of excavator and wheel loader of a leasing organisation are highly infected and blocked by diesel bug. The machines do not start! The consequences are high financial losses of the organisation.

Solution: HYDAC Biomicron®

HYDAC has developed diesel filters for leading engine and machine manufacturers for many years. In the development of the innovative filter element technology Biomicron®, it has been possible to integrate an additional function never before available in this form in diesel filters. The development focused on the element’s ability to protect itself from microbial growth and the maintenance of the entire fuel system incl. tank.

Free water in the tank forms the basis for this phenomenon, as it promotes the microbial growth of bacteria, fungi and other microorganisms. This microbial contamination can affect all machines, particularly those subject to a certain amount of downtime (e.g. rental equipment, leasing fleets, seasonally used harvesting machines and diesel-powered emergency power generators).

Thanks to the innovative filter media, Biomicron® not only filters particles and water from the fluid but also prevents harmful organisms from spreading on the element surface.

Moreover, the micro-organisms that settle in the fuel get caught by separation mechanisms such as: inertia, diffusion, electrostatics as well as adhesive forces within the filter element. Through the interruption of the cell metabolism due to the Biomicron® technology the microbiological growth is approximately impeded.

Modern diesel engines are characterised by a very high fuel circulation volume, i.e. the fuel passes the installed filters and only a partial amount is combusted. The fuel surplus which the installed filters take care of, comes back into the fuel reservoir. The microbial pollution in this recirculating fuel is significantly reduced by the Biomicron® technology. This leads to a successive reduction of the pollution with micro-organisms in the fuel rank. In the end, it is the entire system that benefits from that.


  • Purely synthetic filter material
  • Two-stage element structure: coalescent effect and hydrophobic barrier for water separation
  • Filtration ratings: 5, 7, 10, 30 µm
  • Special filter element self-protection from microbial contamination

Fuel system incl. pre-filter with Biomicron®

Your Benefits


  • Triple function: reliable particle filtration, water separation and filter self-protection from microbial contamination thanks to two-stage element structure and innovative filter media

  • Prolonged filter life expectancy in microbially contaminated systems thanks to element self-protection

  • Reliable suppression of bacterial growth in the filter (e.g. diesel bug) across the element’s entire service life

  • Increased machine availability, even for seasonal machines, thanks to reliable particle filtration and water separation



  • The outer sleeve of the element must not be removed!
  • Avoid direct contact of the filter mesh pack with the skin or eyes!
  • When in contact with the skin wash effected areas well with soap and water!
  • When in contact with the eyes: Rinse cautiously with water for several minutes. Eventually remove contact lenses if possible. Countinue rinsing. 
  • After ingestionImmediately give call to the POISON CONTROL CENTRE or medical doctor if feeling unwell. 
  • Should not come into the hands of children.
  • Store dry and air-tight.
  • Protect from moisture.
  • It is only suitable for use in HYDAC fuel filters.



As usual for used fuel filters the product itself (13 05 01*solid waste from grit chambers and oil/water separators) and also the water discharged by this product (13 05 07* oily water from oil/water separators) must be disposed as hazardous waste.

The relevant regulations in the country concerned with regard to ground water pollution, used oil and waste must be complied with.


  • Pre-filter HDP BC 240

  • Pre-filter HDP BC 600 with integrated, electrical pump

  • Double module pre-filter HDP BC 1200

  • Pre-filter HDP HT 1800 - Fully automated system for water discharge

  • Pre-filter HDP HT 1200 - Fully automated system for water discharge

  • Pre-filter HDP HT 600 - Fully automated system for water discharge

  • Pre-filter HDP BC 340

  • Pre-filter HDP BC 600 with small and big hand priming pump

  • Triple module pre-filter HDP BC 1800


  • Filter element technology Dieselmicron®

  • Filter element technology Biomicron®


Mobile machines and commercial vehicles are subject to the toughest working conditions all over the world. To ensure smooth running of vehicles and to protect both the engine and the whole drive system from damage, optimum diesel fuel conditioning is particularly important.

With HYDAC Diesel PreCare (HDP), HYDAC offers a modern system for diesel pre-filtration which protects vehicle manufacturers and operators from failures, breakdowns and expensive service interventions. The HDP achieves outstanding performance data with its two-stage water removal, superb filtration characteristics and the use of synthetic media.
In addition to the fully automatic drainage plug&play solution for user-independent, fully automatic dewatering, even during intake operation (HDP HighTech), the HDP is available as a user-dependent solution (HDP BestCost). Both designs are also available in double and triple versions and can thus be used even for larger flow rates.

Both systems are designed for use as pre-filters on the suction side and as such protect all the pumps and components in the fuel system from water and contamination.


Particularly in the main flow on the pressure side, diesel filtration is becoming increasingly important, as the filter elements and modules used for main filtration today must already be able to meet the cleanliness requirements of tomorrow. Efficient fuel filtration should thus achieve a cleanliness class of 12/9/6, even when subjected to engine vibration and load cycle changes. Machine users and OEMs also demand application-specific elements with the highest possible contamination retention capacity coupled with compact dimensions. Compatibility with biofuels and environmentally friendly disposal are also common requirements and they are set to become increasingly important, along with filter module application customised to suit the engine.

To address these trends, HYDAC Diesel MainCare was developed – a cup filter system which ensures efficient and reliable filtration of diesel fuel in the main flow.

In conjunction with our customers, we develop main filter modules whose technical data meets the customer’s requirements and specifications.

HYDAC Dieselmicron®

New element technology: Dieselmicron filter element with two-stage system

An innovative filter element in two-stage design which combines outstanding dewatering irrespective of the contamination level with excellent particle separation at the same time.
Particle filtration and droplet coalescence (stage 1)
By using purely synthetic filter media, a high contamination retention capacity and steady coalescence are guaranteed.
Water separation (stage 2)

A hydrophobic barrier on the strainer tube guarantees reliable separation of the water droplets formed during stage 1.