What is a Biofouling

Biofouling is the accumulation of biological material on surfaces that are regularly or continuously exposed to water. In pipes and drainage infrastructure, it can include bacteria, fungi, algae and other microorganisms, together with the extracellular material they produce and particles trapped within the resulting biological layer. The term is particularly associated with biofilms, which are structured microbial communities attached to a surface rather than organisms simply present in the flowing water.

Biofouling can develop on pipe walls, channels, tanks, filters, screens and other wet infrastructure. Its significance varies considerably between systems. A thin biofilm may have little measurable hydraulic effect, while substantial biological growth can increase surface roughness, retain sediment, contribute to odour or corrosion processes and interfere with equipment.

The presence of microorganisms alone does not mean that a drainage system has a biofouling problem. Microorganisms are common in water and wastewater environments. Biofouling becomes an operational issue when attached biological growth alters the condition or performance of the infrastructure.

How Biofilms Develop Inside Pipes

Biofouling usually begins at the interface between water and a solid surface. Pipe walls provide an area where microorganisms can attach, particularly when conditions allow nutrients and moisture to remain available.

After initial attachment, microorganisms can produce extracellular polymeric substances, often abbreviated to EPS. This material helps cells adhere to the surface and to one another, creating the matrix associated with a mature biofilm.

A biofilm is therefore not simply a layer of bacteria. Its matrix can contain water, microbial cells, organic substances and inorganic particles captured from the surrounding flow. The resulting structure can be irregular and can change as hydraulic and chemical conditions change.

Biofilm development can be considered as a sequence of interacting processes:

1. Organic and inorganic material conditions the wet surface.
2. Microorganisms attach to the surface.
3. Attached organisms grow and produce extracellular material.
4. The biofilm develops into a more complex structure.
5. Suspended particles can become trapped within the layer.
6. Portions of the biofilm detach when growth or hydraulic forces exceed its ability to remain attached.

Detachment is an important part of the process. Biofilms do not necessarily grow continuously until they fill a pipe. Flowing water applies shear forces to the surface, and sections can slough away before growth begins again.

The balance between growth and removal depends on local conditions. Nutrient availability, temperature, water chemistry, flow regime, surface characteristics and the microbial population all influence development.

Flow can have opposing effects. Very low movement may encourage settlement and create stable conditions for growth, while higher shear can limit the thickness of attached material or remove weakly attached sections. At the same time, moving water supplies nutrients to the biofilm, so increased flow does not automatically prevent biological growth.

Biofouling Is More Than Bacterial Growth

Different types of biological growth occur under different environmental conditions. Bacteria are particularly important because they can colonise surfaces without requiring light. Algae, by contrast, depend on light and are therefore more relevant to exposed channels, tanks and other locations where sufficient light reaches the wet surface.

Fungi can also occur in suitable environments, particularly where organic material is available. In wastewater infrastructure, biological deposits often consist of mixed communities rather than a single organism.

The distinction between biological growth and ordinary sediment is important during investigation.

In real drainage systems, these materials can occur together. A deposit removed from a pipe may contain biological matter, grease, mineral material and trapped sediment. Describing every soft deposit as biofilm would therefore be inaccurate.

Visual appearance alone may not establish its composition. Where the distinction matters for industrial operation, water treatment or corrosion investigation, sampling and laboratory analysis can provide information that CCTV inspection cannot.

Conditions That Influence Biofouling

Water availability is essential, but it is not enough on its own to predict how much biological material will develop. The environment must also support attachment and microbial activity.

Nutrients are one major factor. Wastewater contains organic material that can support microbial communities, although the composition and availability of nutrients vary greatly between systems. Industrial wastewater can create very different conditions from domestic foul drainage.

Temperature influences biological activity as well. Microbial communities have different preferred temperature ranges, and changes in temperature can alter growth rates and community composition. It would therefore be misleading to assign one universal temperature at which drainage biofouling begins or stops.

Surface properties can affect initial attachment. Roughness, material condition and existing deposits create different environments for colonisation. An old pipe with corrosion products and irregular surfaces does not present the same attachment conditions as a smooth new pipe.

Other relevant factors include:

• availability of biodegradable organic matter and other nutrients;
• water temperature and chemistry;
• dissolved oxygen and local oxygen gradients;
• flow velocity and hydraulic shear;
• frequency of wetting and drying;
• pipe surface condition and roughness;
• retention time;
• presence of existing sediment or deposits.

Conditions can vary within a single pipe. A biofilm surface exposed to flowing wastewater may have access to oxygen while deeper layers can experience very different conditions. Mature biofilms can therefore contain distinct microenvironments separated by very small distances.

Intermittently used drainage can behave differently from continuously flowing systems. Periods of low flow can leave wetted surfaces and retained material relatively undisturbed, while later high flows may remove part of the accumulated layer.

How Biofouling Can Affect Drainage Infrastructure

The hydraulic effect depends on the extent and character of the growth. A very thin biofilm does not automatically create a meaningful restriction, particularly in a large pipe. More substantial deposits can reduce the available flow area and increase effective surface roughness.

Biofilms can also trap particles that might otherwise continue downstream. This interaction between biological growth and sediment can create deposits that are more complex than either component alone.

Potential operational effects include:

• increased roughness of internal surfaces;
• retention of suspended solids;
• local reduction in available flow area;
• fouling of screens, filters, sensors or other equipment;
• increased cleaning requirements;
• odour-related problems under suitable conditions;
• contribution to some forms of microbiologically influenced corrosion.

The corrosion issue requires careful distinction. Biofilm formation does not mean that a pipe will necessarily corrode. However, microbial activity can alter local chemical conditions at a material surface. Certain microorganisms and microbial communities can contribute to processes collectively described as microbiologically influenced corrosion, or MIC.

In sewer environments, concrete deterioration can involve biological processes associated with sulphur compounds. Hydrogen sulphide generated within wastewater can enter the sewer atmosphere and undergo further transformations on moist surfaces. Under appropriate conditions, sulphur-oxidising microorganisms can contribute to the production of sulphuric acid, which attacks cementitious materials.

That process is more specific than general biofouling and should not be treated as synonymous with it. Biofouling describes biological accumulation on a surface, whereas MIC describes corrosion in which microorganisms influence the corrosion process.

Odour has a similar distinction. A visible biofilm does not by itself prove the source of an odour. Drainage odours can be associated with stagnant wastewater, trapped organic matter, sewer gases and several biological or chemical processes. Investigation should establish the actual cause rather than assuming that biological growth is responsible.

Biofouling in Foul Drains, Surface Water Systems and Other Infrastructure

The importance of biofouling varies with the type of drainage infrastructure. Domestic foul drains receive wastewater containing organic matter and microorganisms, but they also experience regular flushing from toilets, sinks, showers and appliances. In many ordinary blockage investigations, fats, wipes, foreign objects, scale, structural defects or root intrusion may be more immediately significant than biofouling alone.

Larger wastewater infrastructure presents different conditions. Sewers, pumping stations, wet wells and treatment equipment can provide extensive permanently wet surfaces where biological growth interacts with wastewater over long periods.

Surface water infrastructure is different again. Rainwater drains can experience long dry periods followed by high flows. Open channels, ponds and exposed drainage structures can support algae where light and nutrients are available, whereas algae cannot develop normally in completely dark buried pipework because photosynthesis requires light.

Biofouling is also important outside conventional gravity drainage. Water treatment equipment, cooling systems, membranes and some industrial pipelines can be particularly sensitive because even relatively small amounts of biological growth may affect heat transfer, filtration or process efficiency.

The operational significance must therefore be judged against the function of the system. A biological layer that is insignificant in a large gravity sewer may be unacceptable on a fine filter or membrane.

Detecting and Assessing Biological Deposits

In accessible drainage systems, biological growth may first be identified during routine inspection or cleaning. CCTV can reveal deposits, changes in pipe surface appearance and areas where material has accumulated, but it has limitations.

A camera image does not reliably identify microorganisms or determine the chemical composition of a deposit. Soft material visible on the pipe wall may contain grease, sediment and biological matter in different proportions.

Physical sampling becomes more relevant when determining the composition is important to the investigation. Laboratory methods can be used to examine microbial populations, organic content or other characteristics depending on the purpose of the assessment.

The investigation should also consider where deposits occur. Repeated fouling at one location can indicate local conditions that differ from the rest of the system, such as reduced flow, a poorly draining section or accumulation around an obstruction.

Useful observations include:

• extent and thickness of visible deposits;
• whether material is attached to the wall or lying loose on the invert;
• location relative to changes in gradient or pipe geometry;
• presence of sediment, grease or scale;
• evidence of structural deterioration;
• whether fouling returns rapidly after cleaning.

These observations help distinguish the presence of biological material from the underlying conditions allowing it to become operationally significant.

Removing Biofouling and Limiting Regrowth

Mechanical and hydraulic cleaning can remove biological deposits from many drainage surfaces. Water jetting can detach soft material, while other cleaning techniques may be required where deposits have combined with scale, grease or more resistant substances.

Removal does not make the surface permanently free from microorganisms. Where water, nutrients and suitable environmental conditions remain, recolonisation can begin after cleaning.

Effective management therefore depends partly on why the fouling developed. If a section of pipe retains wastewater because of poor gradient, simply cleaning the surface may provide only temporary improvement. Likewise, persistent accumulation around a damaged joint or obstruction may require the physical defect to be addressed.

Measures relevant to controlling repeated fouling can include restoring effective flow, removing accumulated sediment and organic material, correcting drainage defects and maintaining equipment where biological growth affects its operation.

Chemical control is used in some water and industrial systems, but chemicals should not be introduced into drainage infrastructure simply because biological material is present. Compatibility with pipe materials, downstream treatment processes, discharge requirements and safety all need to be considered.

Biofouling is best understood as an interaction between microorganisms, the water they inhabit and the surface on which they grow. In drainage infrastructure, its importance ranges from a thin biological layer with little practical effect to substantial fouling that traps solids, alters hydraulic conditions or contributes to material deterioration. Identifying the deposit correctly and understanding why it developed are therefore more useful than treating all biological growth as an isolated blockage that only needs to be removed.