Biofilm in cooling water systems

What biofilm actually is

Free-floating bacteria are called planktonic. They are the ones your water sample catches, and the ones your biocide dose was designed to kill. But bacteria overwhelmingly prefer not to float. Given a surface — tower fill, basin wall, pipe interior, the shell side of a heat exchanger — they attach, and then they secrete a matrix of polysaccharides and proteins around themselves. That matrix is the slime you can feel on a fouled tower fill. The bacteria plus the matrix is the biofilm.

This is not a passive lump. It is a structured community: channels for nutrients, layers with different oxygen levels, different species occupying different depths. It is closer to a small city than to a stain.

Why the same biocide stops working

The literature reports bacteria in a biofilm as 10 to 1,000 times more resistant to antimicrobial treatment than their planktonic form. Several mechanisms stack up:

• The matrix consumes the oxidiser. Chlorine or bromine reacts with the organic slime on the way in. The cells deep in the film may never meet an active dose at all.
• Diffusion is slow. The biocide has to travel through the matrix, not through water. The residual you measure in the bulk water is not the concentration at the pipe wall.
• The cells themselves change. Bacteria in a biofilm shift their metabolism and grow slower. Oxidising biocides work best on active, growing cells.
• There is a physical shelter. Extracellular material around the organisms protects them directly.

The Legionella connection: amoebae

Here is the part that surprises people. Legionella is not primarily a free-swimming organism in your tower. It is a parasite of amoebae.

Amoebae — single-celled protozoa — join the biofilm community and graze on the bacteria in it. When an amoeba ingests Legionella, the Legionella is not digested. It survives inside the amoeba and multiplies there, because the amoeba's interior turns out to be excellent growing conditions. The amoeba becomes a nursery, and a bunker.

So Legionella in a biofilm sits behind two barriers: the amoeba's membrane, and the biofilm matrix around the amoeba. The literature describes L. pneumophila in biofilms as extremely resistant to biocides for exactly this reason.

It also explains a pattern we see on site: a tower that tests clean, then produces a high count after a mechanical disturbance. Nothing grew overnight. Something knocked material off the wall and released what was already living there.

What biofilm costs in heat transfer

Biofilm is a very poor conductor of heat — it is mostly water held in an organic matrix, and it insulates far better per unit thickness than mineral scale does. A film thin enough that you would call the surface “a bit slimy” is already degrading your condenser approach temperature.

The mechanism is the same as scale, treated in the Langelier Saturation Index explained, but the penalty arrives at a fraction of the thickness. Worse, biofilm and scale help each other: the film traps suspended solids and gives minerals somewhere to nucleate, and the resulting deposit shelters more film.

Under-deposit corrosion: the bill you do not see

Biofilm does not just insulate. It creates a chemically distinct micro-environment against the metal — low oxygen, altered pH, and in the case of sulphate-reducing bacteria, actively corrosive metabolic products. This is microbiologically influenced corrosion, and it is aggressive and local. It does not thin the wall evenly. It puts a hole in it.

The failure mode is a pinhole leak in a pipe that, by average wall thickness, still had years of life in it. We cover the mechanism in metal corrosion in water systems.

Why you cannot dose your way out of it

The instinct when counts rise is to raise the oxidiser. It rarely works on established biofilm, and it has real costs: more corrosion, more chemical spend, more by-products. You are trying to push a reactive chemical through a layer specifically good at consuming it.

What actually works is a combination:

• Biodispersant first. A dispersant attacks the matrix rather than the cells — it loosens and lifts the film so the biocide can reach what is underneath. The Code of Practice's own cleaning procedure begins by circulating biodispersant through the system before disinfection, for exactly this reason.
• Physical cleaning. Mature film in a tower basin, on fill and on drift eliminators comes off mechanically, not chemically. This is why the Code requires cleaning and desludging at least every 6 months rather than just dosing.
• Rotate chemistries. Oxidiser alone rarely clears mature biofilm. Non-oxidising biocides work by different mechanisms — see effects of pH on various biocides.
• Then prevent, do not chase. Control suspended solids and nutrients, keep the residual continuous rather than intermittent, and keep flow up. Biofilm establishes fastest in dead legs and stagnant standby towers.

How to tell if you have it

• Heterotrophic colony count climbing while the residual looks normal. This is the classic signature — HCC has its own action ladder in the EMSD Code of Practice.
• Biocide demand rising — you need more chemical to hold the same residual. Something is consuming it.
• Condenser approach temperature drifting up with no scale to explain it.
• Erratic Legionella results — clean, clean, then a spike. Consistent with release from a reservoir rather than growth in the bulk water.
• Slime you can feel on the fill, the basin wall or under the drift eliminators. Look, do not just read the log sheet.

Frequently asked questions