Problem 9.6: A facility has used the same nonoxidizing

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

Question

A facility has used the same non-oxidizing biocide (isothiazolone) on a weekly slug-dose schedule for 18 months. Biological counts have been rising steadily for the last 6 months despite consistent dosing. Explain what is likely happening and propose a modified program.

Answer

Show answer

Eighteen months of the same non-oxidizing biocide has created sustained selection pressure. Organisms tolerant to isothiazolone – through thicker EPS, dormancy responses, or altered uptake pathways – have been preferentially selected. The community has adapted, not disappeared. A modified program should: (a) introduce a different non-oxidizing chemistry (e.g., glutaraldehyde or DBNPA) to disrupt the selection pressure, (b) alternate between at least two non-oxidizing chemistries on a rotating schedule, (c) add a biodispersant to expose biofilm-sheltered organisms to chemical treatment, and (d) consider mechanical cleaning of accessible surfaces to physically remove the adapted biofilm community before restarting chemical treatment.