The growth of inline biofilm monitoring in cooling tower water treatment has been a genuine advance in how operators understand and manage biological fouling. But as adoption of biofilm sensors increases, a misconception has emerged: that monitoring for biofilm means you no longer need to monitor for algae. This is not correct. Biofilm and algae are different organisms with different biology, different growth requirements, and different consequences for cooling tower performance. A sensor designed to detect one does not detect the other.
What Biofilm Is and Where It Grows
Biofilm in a cooling tower is a community of bacteria — primarily heterotrophic bacteria including, in some cases, Legionella pneumophila — that attach to wetted surfaces and encase themselves in a self-produced matrix of extracellular polymeric substances. Biofilm grows wherever water contacts a surface: on fill media, in distribution headers, on basin floors, inside pipework, and on heat exchanger surfaces.
Biofilm is a subsurface and internal system problem. It is not dependent on light. It grows in the dark interior of pipework just as readily as it does in the open basin. It causes heat transfer reduction, corrosion under deposits, biocide demand increase, and, when Legionella is involved, a serious public health risk.
Inline biofilm sensors such as the Alvim Biofilm Sensor detect biofilm formation directly on a sensing surface in continuous contact with the circulating water. They measure the earliest stages of bacterial attachment before the biofilm has grown thick enough to cause operational problems. This early detection is precisely what makes them valuable.
What Algae Is and Where It Grows
Algae is a photosynthetic organism. It requires light to grow. In a cooling tower, algae establishes in areas where sunlight reaches the water: the open basin, the sides of the tower structure below the casing, distribution channels, and any open access points. Algae growth is a surface phenomenon and is fundamentally driven by light exposure, which is why shaded internal surfaces do not support algae but sun-exposed basin edges do.
Algae causes problems in cooling towers through several mechanisms. It physically blocks distribution nozzles and fill media, reducing water distribution uniformity and creating hot spots on the heat exchanger. It decomposes in the basin to create organic nutrient loading that feeds bacterial growth, including the heterotrophic bacteria that form biofilm. It increases turbidity in the circulating water. It can dislodge in clumps that travel into the system and foul downstream equipment.
Critically, algae growth is not detected by biofilm sensors. The Alvim sensor detects bacterial biofilm formation electrochemically. Algae is not bacteria. The sensor does not respond to algal growth, and a clean biofilm reading does not rule out a significant algae problem developing in the open basin or on sun-exposed surfaces.
Why the Two Problems Compound Each Other
The relationship between algae and biofilm is not simply that they are separate problems. They interact. Algae decomposition increases the dissolved organic carbon in the circulating water, which provides additional nutrients for heterotrophic bacteria and accelerates biofilm formation rates. A cooling tower with an uncontrolled algae problem will typically experience faster biofilm development than one where algae is properly managed. Treating biofilm without controlling algae means you are treating a symptom while leaving one of its drivers unaddressed.
Conversely, a biocide programme that is optimised for planktonic and biofilm bacteria may not be appropriately formulated for algae control. Algae requires specific algicidal chemistry, and a biocide effective against Legionella does not necessarily provide adequate algae control.
Monitoring Both Parameters
Effective cooling tower biological management requires visibility on both threats independently. The Alvim Biofilm Sensor provides continuous real-time data on bacterial biofilm accumulation on system surfaces — giving operators early warning of biofilm onset and immediate feedback on whether biocide treatments are working. The Pyxis range of water quality sensors and handheld analysers addresses the broader water quality parameters relevant to algae and biological control, including turbidity, free chlorine, and general water quality indices that indicate developing algae problems before they become visible fouling.
These two monitoring approaches are complementary by design. Neither substitutes for the other. Together they provide the complete biological monitoring picture that a cooling tower water management programme requires.
To discuss how Alvim and Pyxis monitoring can be integrated into your cooling tower treatment programme, contact us at info@autoflotechnology.com.