When cooling tower water treatment programmes fail to control Legionella or other microbial threats, the instinct is usually to dose more biocide. Sometimes this works. Often it does not — because the real problem is not the concentration of bacteria in the water. It is the bacteria that are not in the water at all.
Biofilm changes the nature of microbial control in cooling towers so significantly that treating it as a simple extension of planktonic bacteria control will consistently produce inadequate results. Understanding why requires understanding what biofilm actually is and why it is so difficult to eliminate.
Planktonic Bacteria vs Biofilm: What the Difference Means
Planktonic bacteria are free-floating individual cells suspended in the bulk water. They are what standard water testing detects, what biocide dosing targets most directly, and what most water treatment programmes are designed around.
Biofilm is something structurally and functionally different. It is a community of microorganisms — bacteria, algae, fungi, and protozoa — that have attached to a surface and encased themselves in a self-produced matrix of extracellular polymeric substances (EPS). This matrix is primarily composed of polysaccharides, proteins, and nucleic acids. It acts as a scaffold, a nutrient reservoir, and — critically — a protective barrier.
Once established on heat exchanger surfaces, pipework, fill media, or the tower basin, biofilm does not behave like planktonic bacteria suspended in the same water. It behaves like a completely different biological entity with a dramatically higher tolerance for the conditions designed to kill it.
Why Biofilm Is Up to 1,000 Times Harder to Kill
Research has consistently demonstrated that bacteria within a biofilm require biocide concentrations up to 1,000 times higher than planktonic bacteria of the same species to achieve the same kill rate. Several distinct mechanisms contribute to this resistance, and they act simultaneously.
Physical barrier from the EPS matrix. The extracellular matrix through which biocide must diffuse is not a passive barrier. It actively reacts with and neutralises many oxidising biocides — chlorine, bromine, and chlorine dioxide — as they attempt to penetrate the biofilm. The biocide is consumed at the outer layers of the matrix before it can reach the bacteria deeper within. The deeper the biofilm, the more biocide is consumed in transit, and the less reaches the active microbial community.
Reduced metabolic activity and slow-growing phenotypes. Within a biofilm, bacteria in the deeper layers receive less oxygen and fewer nutrients than those at the surface. This forces them into a slow-growing or dormant metabolic state. Many biocides target actively metabolising bacteria — cells that are dividing and taking up nutrients. Slow-growing cells in the biofilm interior are simply less susceptible to these mechanisms, regardless of biocide concentration.
Persister cells. A subpopulation of bacteria within any biofilm adopts a phenotypic state of extreme dormancy called persister cells. These cells are not genetically resistant — they are phenotypically tolerant. They can survive extremely high biocide concentrations that kill every other cell in the community, then resume normal growth once the biocide concentration drops. Persister cells are what cause biofilm to recolonise after an apparently successful treatment — the bulk water may test clean, but the survivors in the biofilm remain.
Genetic exchange within the community. Biofilm creates physical proximity between different species and strains of bacteria, which accelerates the transfer of genetic resistance mechanisms. Bacteria that develop resistance to a biocide can transfer that resistance to neighbouring cells more rapidly in a biofilm environment than in planktonic suspension.
The Consequences for Cooling Tower Operation
These properties have direct and serious operational consequences.
Biofilm on heat exchanger surfaces is an insulator. Even a thin biofilm layer of 0.1 to 0.2 mm can reduce heat transfer efficiency by 10 to 25 percent, depending on surface area and flow conditions. In a chiller serving a large commercial or industrial building, this translates into measurable increases in energy consumption — the chiller must work harder to achieve the same cooling output.
Biofilm is the primary reservoir for Legionella in cooling towers. Legionella bacteria are not effectively controlled by targeting planktonic cells in the bulk water alone. They replicate preferentially within biofilm, particularly inside amoebae that colonise the biofilm community. The amoebae provide an intracellular environment that not only protects Legionella from biocide but actually enhances its virulence. A cooling tower with a Legionella problem and a well-maintained biocide programme almost always has a biofilm problem that the biocide programme is failing to address adequately.
Biofilm detachment introduces microbial spikes into the bulk water. Sections of mature biofilm periodically detach under hydraulic shear or nutrient fluctuation, releasing large numbers of bacteria directly into the bulk water in a short period. These spikes are the reason why planktonic bacteria counts can suddenly surge on a water test even when the dosing programme appears unchanged — the source is not the bulk water chemistry but the biofilm reservoir releasing cells episodically.
Why Standard Biocide Programmes Are Not Enough
A continuous or timer-based oxidising biocide programme controls planktonic bacteria effectively under normal operating conditions. It maintains residual biocide in the bulk water that kills free-floating cells quickly. But it does not penetrate established biofilm at the concentrations typically used.
Shock dosing — periodic high-concentration biocide addition — penetrates further into the biofilm matrix and kills a larger proportion of the microbial community. But it rarely achieves complete elimination, particularly the persister cell population. Without a mechanical or chemical treatment that physically removes or disperses the biofilm matrix itself, recolonisation typically occurs within days to weeks of a shock dose.
Effective biofilm control requires three things working together: an oxidising biocide to control planktonic bacteria and the outer layers of biofilm; a non-oxidising biocide with different mechanisms of action that penetrates the EPS matrix more effectively; and a dispersant chemistry that breaks down the extracellular matrix, exposing the embedded bacteria to biocide contact and allowing the biofilm to be flushed away in the bulk water flow.
Physical cleaning — particularly of heat exchangers, fill media, and the tower basin — is necessary on a regular basis to remove biofilm deposits that cannot be controlled chemically alone. No chemical programme eliminates the need for periodic physical cleaning in a system that has established biofilm.
Early Detection Is the Most Effective Strategy
Preventing biofilm establishment is substantially easier than eliminating it once established. Once a mature, deep biofilm community is present, it is extremely difficult to completely eradicate without mechanical intervention — and the 1,000-fold resistance differential means that attempting to do so with biocide alone typically leads to overdosing, chemical waste, and incomplete results.
Real-time monitoring of parameters that correlate with early biofilm development — ORP, turbidity, and in more advanced systems ATP (adenosine triphosphate) measurement — allows a water treatment programme to detect developing microbial activity before it transitions from planktonic growth to established biofilm. Responding to early signals with targeted dispersant treatment and shock biocide is far more effective than responding to a fully established biofilm that has been present for weeks or months.
The practical lesson is this: a cooling tower that tests clean on planktonic bacteria counts alone is not necessarily a clean cooling tower. The bacteria that matter most for both Legionella control and heat transfer efficiency are the ones attached to surfaces — and those require a fundamentally different approach than managing the bulk water chemistry.
Autoflo Technology supplies water treatment monitoring and control systems for cooling towers, including conductivity controllers, ORP monitoring, and biocide dosing systems. Contact us at info@autoflotechnology.com to discuss your cooling tower treatment programme.