Biofilm & Marine Heat Exchangers | Prevent At All Costs

Biofilm build up will restrict cooling waterflow and decrease marine heat exchanger efficiency if left untreated. This seemingly friendly biological slime accelerates corrosion and quickly wreaks havoc on marine engines causing costly repairs.

Marine heat exchangers that don’t receive regular preventative maintenance risk costly and damaging effects of biofilm build up. 

‍Part of AHI Marine heat exchanger article series

What is Biofilm

Algae, bacteria, and fungi found in soil, water, and air can quickly colonize cooling systems once introduced through water intake tubes if preventative measures aren’t taken. 

These microorganisms require water, organic nutrients (carbon, nitrogen, and phosphorus), and suitable environmental conditions—factors that are all present in heat exchanger systems. Ammonia, sulphur and hydrogen can be oxidised by some bacteria present.

Photosynthetic organisms like algae use sunlight as an energy source, while bacteria and fungi rely on organic or inorganic compounds in the water. 

Once established, these microbes grow rapidly and adapt to environmental changes, forming stable populations that can persist under varying oxygen levels, temperatures, and pH conditions.

This can cause severe, costly damage to marine heat exchanger metals, primarily through biofouling and microbially influenced corrosion (MIC).

Biofilm Build Up

Biofilm formation begins when sessile microorganisms adhere to wetted heat exchanger surfaces and secrete extracellular polymeric substances (EPS), primarily composed of polysaccharides and proteins. 

The sticky, gel-like matrix anchors the cells to the tube stack walls, preventing them from being washed away. The build up layer prevents oxidisers and toxins reaching the cells, as well as predatory organisms.

Biofilm Exponential Growth

Initially, biofilm development is slow as there is limited surface area of the sticky matrix for the microorganisms to adhere to.  

As colonising cells reproduce additional biofilm accumulates, creating a larger surface area to which new cells can attach. 

In addition to a greater surface area the structure becomes more complex and three dimensional, further trapping and adhering more and more sessile microorganisms. This eventually causes a runaway effect as the internal heat exchanger metals become overwhelmed with the thickening biofilm.

Factors Affecting Biofilm Growth

Marine heat exchangers present the perfect environment for the rapid growth of microorganism biofilm.

• Nutrient availability organic and inorganic matter steadily stream into the exchanger via the raw water intake fuelling growth
• Temperature marine engines provide optimal temperature conditions for rapid division of microorganism cells
• pH biofilm on heat exchanger metals gradually alter the local pH 
• Oxygenation dissolved oxygen promotes growth of aerobic bacteria. Anaerobic organisms thrive under biofilm surface as oxygen levels are depleted.
• Sunlight algae bloom in raw water in rivers and seawater under optimal conditions and present of ample sunlight

In high growth conditions, some bacteria can double their numbers every 20 to 30 minutes. Species of moulds can double their mass in as little as a few hours.

Effect of Biofilm On Marine Heat Exchangers

• Heat Transfer Efficiency: Biofilm water content acts as a thermal insulator. Performance begins to decrease once the biofilm thickness exceeds that of the laminar flow region surrounding it. 
• Microbiologically Influenced Corrosion (MIC): Microorganisms create local chemistry changes. Anaerobic bacteria, such as sulfate-reducing bacteria (SRB), thrive under the biofilm layer, producing hydrogen sulfide. This can lead to accelerated, localized pitting corrosion.
• Flow Restriction and Pressure Drop: Accumulated algae, slime, and trapped debris clog tubes, narrowing the flow diameter, which forces impeller pumps to work harder, increasing likelihood of breakages.‍
• Accelerated Galvanic Corrosion: During the day, photosynthetic algae produce oxygen, causing oxygen concentration differentials that accelerate galvanic corrosion on steel surfaces.

Biofilm vs Biofouling

• Biofilm is the established mesh of microorganisms inside the heat exhanger tubing.
• Biofouling refers to the aggregated mass of all organic matter including barnacles, muscles, plants and other microorganisms that adhere to marine hardware.
  

Next: Everything you Need to Know About Heat Exchanger Servicing