Problem 9.5: A 4inch branch off a cooling tower
From Chapter 9 Problem Set. Level 2: Applied Thinking.
Question
A 4-inch branch off a cooling tower recirculation loop has been valved off for 6 months. The water inside is at 85°F. No biocide reaches it. Describe the biological progression you would expect from month 1 through month 6, and explain why this dead leg is a Legionella risk.
Answer
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Month 1: Planktonic organisms already present in the stagnant water begin attaching to pipe surfaces. Initial EPS production begins. Month 2–3: Without biocide access or flow to disrupt attachment, biofilm matures. Oxygen is consumed at the metal surface, creating anaerobic zones. SRB may begin colonizing the base of the biofilm. Month 4–6: A mature, multi-species community develops with established gradients. The 85°F water temperature is within Legionella’s growth range, though below its optimal growth range(~95–105°F). Legionella replicates inside protozoa (amoebae) that thrive within the biofilm. No biocide reaches the branch. No flow disrupts the community. No thermal treatment exceeds growth temperature. This dead leg is a Legionella reservoir that, if reconnected to the system, could release organisms into the circulating water and potentially into aerosol-generating components.