Problem 9.2: A mature biofilm on a condenser tube is

From Chapter 9 Problem Set. Level 1: Conceptual Mastery.

Question

A mature biofilm on a condenser tube is treated with a high dose of chlorine. Planktonic counts drop to near zero within 30 minutes. Two days later, counts are back to pre-treatment levels. Explain what happened using the concepts of EPS, diffusion limitation, and survivor repopulation.

Answer

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The high chlorine dose cleared the water column of planktonic organisms, which is what the bulk water test measures. However, the established biofilm on the condenser tube was protected by its EPS matrix, which consumed the oxidant through reaction before it could diffuse to the inner cells. Surviving organisms deep in the biofilm structure were never exposed to lethal concentrations. Once chlorine levels dropped, these survivors repopulated the EPS scaffold and shed new planktonic cells into the water column, returning counts to pre-treatment levels within 48 hours. The treatment killed the messengers but left the city intact.