When water treatment professionals talk about bacteria in an industrial water system, they are often talking about two very different things without clearly distinguishing between them. The distinction matters enormously, because the same species of bacterium behaves in fundamentally different ways depending on whether it is floating freely in the bulk water or attached to a surface. The difference between these two states — planktonic and sessile — determines how difficult the bacteria are to detect, how resistant they are to treatment, and what kind of damage they cause.
Planktonic Bacteria: The Free-Floating State
Planktonic bacteria are individual cells suspended in the bulk water of a system. They move with the flow, are exposed directly to whatever chemicals are present in the water, and exist as discrete organisms rather than in organised communities. When a water sample is taken from a system and sent to a laboratory for bacterial count analysis, the result reflects the planktonic bacteria in that sample. Colony-forming unit counts, total viable counts, and most rapid microbiological tests measure planktonic organisms.
Planktonic bacteria are relatively vulnerable to biocidal treatment. In direct contact with a correctly dosed biocide at an adequate concentration, most planktonic bacteria are killed effectively. This is why biocide programmes, when correctly formulated and dosed, can maintain acceptable planktonic counts in bulk water testing.
The problem is that a low planktonic count does not tell you that the system is biologically clean. It tells you that the bulk water has an acceptable level of free-floating bacteria at the moment the sample was taken. It says nothing about what is happening on the surfaces that the water contacts.
Sessile Bacteria: The Biofilm State
When bacteria attach to a surface and remain there, they transition from the planktonic to the sessile state. This transition is not passive. It is an active biological response to environmental conditions, and it changes the bacteria profoundly.
In the sessile state, bacteria alter their gene expression, their metabolic activity, and critically, they begin producing extracellular polymeric substances — a complex matrix of polysaccharides, proteins, nucleic acids, and lipids that surrounds and protects the community. This matrix is what gives biofilm its characteristic slimy texture and, more importantly, its resistance properties.
Sessile bacteria within a mature biofilm are the same species as their planktonic counterparts. But their behaviour, their vulnerability to treatment, and their relationship to the surface they inhabit are entirely different.
How Biofilm Forms: The Four Stages
Biofilm formation is a sequential process that proceeds through recognisable stages once conditions allow initial bacterial attachment.
The first stage is surface conditioning. Before bacteria arrive, the surface is conditioned by organic and inorganic molecules from the water — proteins, polysaccharides, and mineral ions — that adsorb onto the surface and change its surface energy. This conditioning layer makes the surface more receptive to bacterial attachment.
The second stage is initial attachment. Individual planktonic bacteria come into contact with the conditioned surface and attach reversibly. At this point, attachment is weak and the bacteria can still detach and return to the planktonic state. This is the stage at which intervention is most effective — if the conditions that allowed initial attachment are disrupted, biofilm development stops here.
The third stage is irreversible attachment and early biofilm development. Bacteria that remain attached begin producing extracellular polymeric substances, anchoring themselves to the surface and to each other. They recruit additional planktonic bacteria and begin forming microcolonies. At this stage, the biofilm is still thin and relatively permeable to biocides, but it is no longer easily removed by flow or simple chemical treatment.
The fourth stage is biofilm maturation. The microcolonies develop into a complex three-dimensional structure with water channels that distribute nutrients and remove waste products. The extracellular matrix thickens. The community may include multiple species, some of which provide protection to others. The resistance of the biofilm to biocides at this stage is orders of magnitude greater than the resistance of the same bacteria in planktonic form. Studies have demonstrated resistance factors of 100 to 1,000 times compared to planktonic cells of the same species.
The fifth stage is dispersal. Portions of the mature biofilm detach and release planktonic bacteria back into the bulk water. These dispersed cells may colonise new surfaces downstream, propagating biofilm through the system, and they briefly show up in bulk water counts — often long after the biofilm that produced them has been established for some time.
Why This Matters for Water Treatment Monitoring
The implications for how industrial water systems are monitored and treated are significant. A biocide programme calibrated to maintain acceptable planktonic counts is not necessarily controlling the biofilm on system surfaces. A bulk water sample with a clean result does not confirm that heat exchanger surfaces, pipe interiors, or cooling tower fill media are free of biofilm. The two states require different detection methods and, in some cases, different treatment approaches.
Direct biofilm monitoring — measuring biofilm accumulation on a representative surface in the water circuit rather than sampling the bulk water — is the only way to obtain meaningful information about the sessile bacterial population that causes the most significant operational problems: heat transfer reduction, corrosion under deposits, and Legionella harbourage in mature biofilm communities.
The Alvim Biofilm Sensor provides this direct surface measurement continuously and in real time, giving operators the visibility into the sessile bacterial state that bulk water testing cannot provide. To discuss how direct biofilm monitoring can complement your existing water treatment programme, contact us at info@autoflotechnology.com.