Problem 9.4: A tower runs at pH 84 a colleague says the

From Chapter 9 Problem Set. Level 2: Applied Thinking.

Question

A tower runs at pH 8.4; a colleague says the chlorine is useless and wants to switch to bromine. Under continuous feed, is he right? What does the biofilm’s internal pH do to his argument?

Answer

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He is not correct that “chlorine is useless.” At pH 8.4, the bulk water contains a much smaller fraction of chlorine as HOCl, so chlorine’s short-contact kill rate is reduced. That is where bromine has a real advantage: short contact time, high pH, and a need for rapid free-halogen activity. But a continuously fed cooling tower is not judged by short-contact speciation alone. If free chlorine residual is maintained for hours, contact time, residual persistence, oxidant demand, and surface access matter as much as the instantaneous HOCl percentage. The biofilm also weakens his argument because the pH inside and near the biofilm is often lower than the bulk water, which shifts more chlorine back toward HOCl where organisms are actually sheltered.