When cooling tower efficiency drops or a Legionella alert arises, biological contamination may be the culprit. The environment inside the cooling tower is prime for biological growth — warm, nutrient-rich water provides an ideal breeding ground where a single bacterial cell can multiply into millions in just hours. Understanding the issue, its causes and impacts, and how to address it effectively can help you design a risk-mitigation strategy for your facility.
Biological contamination is the growth of microorganisms in the water system. These contaminants, like fungi, algae and bacteria, form on surfaces that contact the water, creating a slime that’s challenging to eradicate. Each behaves slightly differently and poses different risks.
Knowing the contaminant type and activity helps you target your control efforts to the specific issue.
Bacteria are microscopic, single-celled organisms that reproduce independently, allowing them to colonize areas very quickly. There are two primary types you’ll encounter in cooling tower biological control — planktonic and sessile. Planktonic bacteria are free-swimming microorganisms that float in the water, while sessile bacteria permanently anchor themselves to a structure and form a biofilm.
This film encases and protects potentially dangerous pathogens like Legionella, which can also alternate between the two types. Once embedded in biofilm, Legionella thrives on the nutrients and protection it provides against disinfectants, making it especially difficult to eliminate. In favorable cooling tower environments, Legionella populations can double every three to four hours.
Algae pose challenges in cooling towers with infrastructure exposed to sunlight or the atmosphere, where light penetration supports photosynthesis. Unlike bacteria, they’re often visible as green or brown slime, typically accumulating within the tower fill and basin.
These microorganisms create a biomass that can quickly clog systems, blocking water flow and distribution and leading to reduced cooling efficiency.
While they also grow in moist environments, fungi are usually the least commonly identified biological contaminants in a water test. This lower frequency of occurrence tends to make them less problematic overall.
Cooling systems that feature wooden fill, structural supports or drift eliminators are more susceptible to fungi formation, which can cause rot and structural deterioration. Modern cooling towers often use nonwood materials, potentially reducing the risk of fungal growth.
Several conditions make cooling towers ideal environments for biological growth.
Water temperatures in cooling towers can range from 104° Fahrenheit (40° Celsius) at the inlet to 89.6° Fahrenheit (32° Celsius) at the outlet — a range at which many contaminants thrive or grow:
Nutrient-rich water substantially contributes to a favorable environment for microorganism growth. Dead microbial cells or biomass and airborne debris like pollen, dust, leaves and dirt entering the tower provide food for reproduction.
Process contamination is also a factor. Manufacturing byproducts — hydrocarbons, sugars, proteins, chemicals — entering the cooling water can cause nutrient spikes that accelerate biological growth.
Cooling tower design naturally relies on air contact for evaporative cooling, which means the system constantly circulates an abundant supply of oxygen for bacterial colonization and multiplication. Open tower designs also allow sunlight penetration, supporting algae growth.
Because cooling towers create such a favorable environment for biological contamination, treatment is never a one-time fix. High total organic carbon (TOC) and biochemical oxygen demand (BOD) often indicate that your protocols aren’t keeping pace with contamination and may result from:
Cooling tower water contamination negatively affects your equipment performance and operations in several key ways.
Bacterial colonization can lead to aerosol transmission of Legionnaires’ disease, which has more than 6,000 reported cases annually in the United States. About one in every 10 will die from complications, with the rate soaring to 25% of those who develop the disease in a healthcare facility.
The circulation of contaminated water can lead to Legionella outbreaks, creating legal liability or regulatory penalties. Improper water management protocols may also violate ASHRAE 188 and state laws in jurisdictions like New York and New Jersey, with costly penalties for noncompliance.
Biofilm formation on heat-exchange surfaces acts as insulation, preventing efficient heat transfer between the water and the air. A layer as thin as 0.1 millimeters can cause a corresponding 30% to 50% drop in thermal conductivity in some tower designs.
As the biofilm thickens, cooling capacity declines and energy consumption increases to compensate for reduced performance. The system must work harder to achieve the same cooling output, driving up operational costs while delivering diminished results.
Biological contamination accelerates equipment deterioration in multiple ways. Algae and bacterial biomass clog distribution nozzles, tower fill and basin screens, restricting water flow and reducing system effectiveness. Microbiologically influenced corrosion occurs when bacteria create localized conditions beneath biofilms that attack metal surfaces, causing pitting, leaks and premature equipment failure.
Addressing biological contamination requires a comprehensive approach rather than a single solution. Effective control programs combine three key elements:
Chardon Labs provides professional cooling tower water treatment services designed to manage biological contamination through customized programs, regular testing and expert technical support. Contact us to discuss a water management solution for your facility.
