Biocides
The book groups biocides by how they attack organisms and, critically, by whether they can reach cells sheltered inside biofilm. The recurring lesson is that the limiting factor is access, not potency, so no single biocide controls established biology on its own.
| Biocide or class | Type | Mode of action | Strengths | Limits |
|---|---|---|---|---|
| Chlorine | Oxidizing | Steals electrons indiscriminately, damaging membranes, denaturing proteins, disrupting metabolism | Fast, suppresses planktonic populations, limits new colonization | pH sensitive (HOCl falls sharply above pH 8), consumed by EPS, attacks metals, elastomers, inhibitors |
| Bromine | Oxidizing | Oxidation, same blunt electron-stealing mechanism as chlorine | Fast, holds a higher fraction of active form at elevated pH, suited to alkaline cooling towers | Consumed by EPS before reaching inner cells, attacks unintended targets (metals, elastomers, inhibitors) |
| Chlorine dioxide (ClO₂) | Oxidizing | Oxidation | Fast-acting, pH stable | Oxidant, so consumed by EPS matrix like other oxidizers |
| Isothiazolone | Non-oxidizing | Targets specific biological functions (cell division, enzyme activity, membrane integrity) | Penetrates EPS better than oxidants, reaches sheltered zones, periodic slug dose | Dormant or slowed cells wait out exposure, selection pressure builds, rotate chemistries |
| Glutaraldehyde | Non-oxidizing | Targets specific biological functions | Penetrates EPS better than oxidants, reaches locations oxidizers cannot | Dormancy resistance, selection with repeated use, rotate chemistries |
| DBNPA | Non-oxidizing | Targets specific biological functions | Penetrates EPS better than oxidants, periodic slug dose for biofilm | Dormancy resistance, selection with repeated use, rotate chemistries |
| Quaternary amines | Non-oxidizing | Targets specific biological functions (membrane integrity) | Penetrates EPS better than oxidants, reaches sheltered surfaces | Dormancy resistance, selection with repeated use, rotate chemistries |
| Heat (thermal) | Thermal | Denatures proteins at sufficient temperature and exposure time, no reliance on chemistry or metabolism | Most honest biocide, no pH dependence, no selective chemistry, no uptake needed | Must reach every surface long enough, biofilms insulate, dead legs stay cool, partial heating leaves survivors |
Related concepts
Biofilm · Microbiologically influenced corrosion (MIC) · Sulfate-reducing bacteria (SRB) · Iron-related bacteria (IRB) · Acid-producing bacteria (APB) · Mechanical cleaning · Quorum sensing · Legionella
Part of The Blueprint of Water