Problem 9.8: On Friday afternoon a cooling towers biocide
From Chapter 9 Problem Set. Level 3: System Integration.
Question
On Friday afternoon, a cooling tower’s biocide feed pump fails. It won’t be repaired until Monday morning. The water is warm. The system keeps recirculating. For 72 hours, nothing is fed. Using the biofilm lifecycle from this chapter, and what you already know about scale and corrosion, trace the likely sequence of events across the weekend. Where does biology go first? What does its growth do to the heat-transfer surfaces, and how does that pull scale and corrosion in behind it?
Answer
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The order is reliable; the timing is not. A biocide residual suppresses the population, but it never empties the system. Pull it, and the first thing that happens is chemical, not biological: the last oxidant is consumed and nothing replaces it. Only then does biology move, and it moves in a set order. Freed planktonic cells multiply in the warm water, find surfaces, and attach; EPS production ramps up and any suppressed biofilm resumes growth, thickening where it matters most, on the heat-transfer surfaces. That film is an insulator. The surface beneath it runs hotter than the bulk water, and you’ve met both consequences: calcium carbonate has inverse solubility, so a hotter surface scales more eagerly, while respiration beneath the film consumes the oxygen the metal can’t replenish, setting up the oxygen-differential cell from the Corrosion chapter. Under-deposit corrosion begins. How far a system travels in three days depends on temperature, nutrient load, and how much biofilm was already there when the pump failed. A warm, fouled system moves far in a weekend; a cold, clean one barely stirs. The lesson is the direction, not the calendar: biology reaches the surface first, and scale and corrosion follow it in. Killing the organisms Monday won’t undo that. The shelter is already built.