Problem 9.9: A hospital cooling tower has excellent bulk

From Chapter 9 Problem Set. Level 3: System Integration.

Question

A hospital cooling tower has excellent bulk water chemistry: chlorine at 0.5 mg/L, conductivity in range, coupons acceptable, no visible scaling. Legionella culture comes back at 1,000 CFU/mL. Explain how this is possible in a “well-treated” system. Identify at least three potential harboring locations.

Answer

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Excellent bulk water chemistry does not guarantee clean surfaces. The 0.5 mg/L chlorine residual maintains the water column but may not penetrate established biofilms on pipe surfaces, fill media, or basin walls. The system’s pH determines how much of that chlorine is present as effective HOCl. Legionella replicates inside amoebae within biofilms, where it is doubly protected – by the host organism and by the EPS matrix. Three potential harboring locations: (1) Dead legs or low-use branches with stagnant, warm water and no biocide circulation. (2) Fill media surfaces where biofilm accumulates in low-flow zones between distribution nozzles. (3) The basin sump or areas beneath the tower where sediment and biological debris settle, providing shelter and nutrients. The system is “well-treated” by bulk water standards but has not been evaluated by surface standards. The Legionella result is a surface problem, not a water column problem.