Biocides vs. Biofilm – Understanding the "Persister" Threat

It is a common misconception in the industry: "If we dose biocide, the bacteria are dead." However, the reality is far more complex. In industrial water systems, up to 90% of the bacteria live not in the free-flowing water (planktonic), but attached to surfaces in a protective matrix known as biofilm. These sessile bacteria, or "persisters," are 1,000 to 1,500 times more resistant to biocides than their planktonic counterparts.

Biofilm is not just a slimy nuisance; it is a dynamic ecosystem that drives Microbiologically Influenced Corrosion (MIC) . MIC is responsible for up to 50% of all corrosion failures in the oil and gas industry and a significant percentage of pitting in cooling towers. Under the biofilm, anaerobic bacteria like Sulfate-Reducing Bacteria (SRB) thrive, producing hydrogen sulfide that aggressively attacks metal substrates.

Why is biofilm so resistant? The extracellular polymeric substance (EPS)—the "slime"—acts as a physical barrier, slowing the diffusion of biocide. Additionally, the nutrient gradients within the biofilm cause the bacteria to enter a slow-growing or starved state, rendering them insensitive to the metabolic disruption caused by many non-oxidizing biocides.

This reality has shifted the industry's approach from "shock and kill" to "penetrate and remove." The modern approach involves the use of biofilm dispersants—surfactants and polymers designed to break down the structural integrity of the EPS matrix. By using a dispersant alongside an oxidizing biocide, the biocide can effectively penetrate the biofilm and kill the underlying bacteria, which are then washed away by the system flow.

We are also seeing a resurgence in the use of Chlorine Dioxide (ClO2) . Unlike chlorine, ClO2 does not produce harmful THMs (trihalomethanes) and works effectively against biofilm because it is a "selective oxidant" that does not react extensively with ammonia or other organics, leaving more residual for the biofilm matrix. Furthermore, advanced ATP (Adenosine Triphosphate) testing has replaced the outdated "dip slide" method. ATP testing measures the total biological activity in the water—including the biofilm that has sloughed off—giving operators a real-time view of biofilm management.

The takeaway for operations: A successful microbiological program is not just about reducing the "colony count" in the bulk water; it is about actively managing the biofilm on the pipe walls. Dosing an oxidizing biocide without a robust dispersant is like spraying pesticide on a concrete wall—you kill the surface insects, but the nest inside remains untouched.