Problem 9.8: A cooling tower loses its biocide feed pump on

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

Question

A cooling tower loses its biocide feed pump on Friday. It will not be repaired until Monday. Using the biofilm lifecycle, the amplifier effect from the Suspended Solids chapter, and the condenser free body diagram from the Pillars chapter, describe the likely sequence of events over 72 hours across all four pillars.

Answer

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Friday: Biocide residual drops to zero within hours as remaining oxidant is consumed. Planktonic organisms begin reproducing unchecked in the warm recirculating water. Saturday: Planktonic populations multiply rapidly. Cells begin attaching to surfaces - particularly in low-flow zones, on fill media, and on heat-exchanger tubes. Initial EPS production begins. Existing biofilm communities, previously suppressed by biocide, resume active growth and begin expanding. Sunday–Monday: Biofilm thickens on heat-exchange surfaces, adding thermal resistance (fouling – Suspended Solids pillar). The insulating layer raises skin temperature on the condenser tubes, which increases CaCO₃ scaling pressure at the surface (Scale pillar) through inverse solubility. Under-deposit oxygen differential cells begin forming beneath biofilm, initiating localized corrosion (Corrosion pillar). The condenser’s ability to reject heat decreases – approach temperature rises, condensing pressure increases, and compressor efficiency drops (Pillars chapter free body diagram). A 72-hour biocide outage does not create a new problem. It accelerates the interaction cascade across all four pillars simultaneously.