Definition: Biofilm is a gelatinous layer that forms in cooling water systems on heat transfer surfaces, fill material, and basins. Biofilm consists of microbial cells, the polysaccharide biopolymer they produce, and debris extracted from the recirculating cooling water.
Problems Caused by Biofilm:
- Physical plugging of cooling tower fill, tubes, and water passages
- Accelerated corrosion
- Reduced heat exchanger efficiency
- Increased risk of Legionnaires’ Disease
The gelatinous mass of biofilm can obstruct water flow through the cooling tower fill and tubing. Build-up of biofilm reduces the normal ΔT heat rejection efficiency of the fill. There have even been reported incidences of fill collapsing due to the weight of biofilm build-up.
Areas underneath biofilm deposits are more prone to corrosion due to biofilm excretion products. This phenomenon is known as Microbiologically Influenced Corrosion (MIC).
Reduction in Efficiency
Biofilm has a thermal conductivity of just 0.6 compared to calcium carbonate at 2.6, meaning that it is over 4X as resistant to heat transfer. Calculations show that a biofilm only 0.045” thick on the condenser tubes of a centrifugal chiller results in a 35% reduction in heat transfer. For a 200-ton chiller operating at 50% annual average load, at $0.05/kWh, this would increase annual power costs of $26,280 by an additional $9,198.
Increased Risk of Disease
The wet environment in cooling towers is a prime location for Legionella-based biofilm formation, which is continuously eroded and dispersed through the aerosols produced during normal cooling tower operation. Legionnaires’ disease results when a sufficient number of the airborne bacteria are inhaled. Legionella outbreaks are a real threat with a recent example in Toronto, Canada causing 21 deaths at a nursing home, and ultimately resulting in a S600-million class action lawsuit. Other recent examples include a Florida hotel, a Pennsylvania nursing home, a N.Y. correctional facility, and a South Dakota restaurant.