Module 5.5 — Algae, biofilm, and bacteria in cooling towers: a fouling category with no real boiler-side equivalent, and how biocide programs manage it.
Click the biofilm or the oxygen-depleted zone to see its role. A biofilm creates an oxygen-depleted zone underneath it, setting up the same anode/cathode differential from Module 4.1.
Module 5.1 flagged biological growth as genuinely new territory in this course. The boiler/feedwater cycle (Tracks 1-4) is a closed, oxygen-controlled, chemically hostile environment to biological growth — recall Module 2.6's oxygen scavengers deliberately drive dissolved oxygen toward zero. Cooling towers are the opposite: warm, sunlit, oxygenated, and in continuous contact with atmosphere, which is close to an ideal environment for algae, bacteria, and fungi.
Biofilm forms when bacteria attach to a wetted surface and secrete a protective extracellular polymeric matrix, creating a structured community rather than free-floating organisms. Biofilm is more consequential than simple surface slime for two reasons: it's an effective insulator, degrading heat transfer at condenser tubes similarly to scale (Module 5.3), and it creates localized chemistry conditions at the metal surface beneath it — notably oxygen depletion — that can drive microbiologically influenced corrosion (MIC), a distinct corrosion category from anything in Track 4 but following the same anode/cathode logic from Module 4.1.
Algae growth, driven by sunlight exposure at open cooling tower basins and fill sections, can physically clog tower fill material (reducing cooling efficiency), contribute organic material that feeds bacterial growth, and create slippery surfaces that are a safety concern for personnel accessing tower structures. Algae control typically relies on limiting light penetration where practical and biocide treatment alongside bacterial control.
| Type | Mechanism | Consideration |
|---|---|---|
| Oxidizing biocides | Chlorine, bromine, or similar oxidizers that directly kill organisms through oxidative damage | Fast-acting, but can be consumed by other oxidizable material in the water, reducing effectiveness |
| Non-oxidizing biocides | Organic biocide compounds that disrupt specific biological processes (cell membrane, metabolism) | Longer-lasting residual, but organisms can develop resistance with prolonged single-product use |
Most cooling water biocide programs alternate between oxidizing and non-oxidizing biocides, or use both in combination, specifically to prevent microbial populations from developing resistance to any single treatment approach — a direct parallel to antibiotic resistance management in a medical context, applied here to industrial water systems.