⟵ 09 Microbiology (Narrative) · Contents · 09 Microbiology (Problem Set) ⟶
Microbiology: Engineering Notes
“If you are reading this straight through, you can skip this section and lose nothing essential to the story. These notes are for the operators, engineers, and technicians who need to do the math.”
Measuring Biological Activity
No single test tells the whole story. Effective monitoring uses multiple methods and emphasizes trends.
| Method | What It Measures | Speed | Limitations |
|---|---|---|---|
| Heterotrophic Plate Count (HPC) | Culturable planktonic bacteria | 48–72 hr | Misses viable-but-nonculturable organisms; water column only |
| ATP (Adenosine Triphosphate) | Total biological energy | Minutes | Fast, field-ready; no species ID; water column unless surface swab |
| Dip Slides | Semi-quantitative planktonic count | 24–48 hr | Inexpensive; low precision; useful for trending |
| qPCR / Molecular | Specific organism DNA | Hours–days | Species-specific and sensitive; quantitative; expensive; requires lab |
| Coupons + Deposit Analysis | Indirect evidence of MIC | Weeks–months | Sulfide staining, black deposits, odor, and localized pitting can indicate MIC/SRB activity, but confirmation requires supporting evidence. |
Key principle: Bulk water tests (HPC, ATP, dip slides) measure planktonic populations. They do NOT reliably indicate biofilm activity on surfaces. A clean water column can coexist with a mature biofilm. Always supplement bulk testing with trend analysis and physical inspection.
Chlorine Chemistry and pH
Chlorine in water exists in two forms in equilibrium:
HOCl (hypochlorous acid) ↔ H⁺ + OCl⁻ (hypochlorite ion)
HOCl is a significantly more effective oxidizer than OCl⁻. The equilibrium is pH-dependent:
| pH | % HOCl | % OCl⁻ | Relative Effectiveness |
|---|---|---|---|
| 6.5 | ~90% | ~10% | Excellent |
| 7.0 | ~75% | ~25% | Very good |
| 7.5 | ~50% | ~50% | Good |
| 8.0 | ~22% | ~78% | Reduced |
| 8.5 | ~8% | ~92% | Poor |
| 9.0 | ~3% | ~97% | Very poor |
Operational implication: A cooling tower drifting from pH 7.5 to 8.5 loses roughly 85% of its chlorine effectiveness without any change in feed rate. pH control and biocide effectiveness are directly linked.
Biocide Program Design
Effective programs use rotation and coordination:
- Oxidizing biocides (chlorine, bromine, ClO₂): Continuous or intermittent feed for planktonic control. Fast-acting. pH-sensitive (chlorine) or pH-stable (bromine, ClO₂).
- Non-oxidizing biocides (isothiazolone, glutaraldehyde, DBNPA, quaternary amines): Periodic slug dose for biofilm penetration. Rotate chemistries to prevent selection.
- Dispersants / biodispersants: Used alongside biocides to lift deposits and expose sheltered surfaces. Critical for converting biocide access from the water column to the surface.
- Mechanical cleaning: Basin cleaning, hydroblasting of fill, side-stream filtration. Removes the shelter that chemical programs cannot reach.
Rule of thumb: If biocide demand is rising without a corresponding increase in system loading, biology is establishing faster than treatment is disrupting it. The problem is not potency. The problem is access.
Thermal Kill Reference
| Temperature | Approx. Kill Time | Notes |
|---|---|---|
| 140°F (60°C) | Hours | Many bacteria stressed; some survive extended exposure |
| 150°F (66°C) | Tens of minutes | Protein denaturation accelerates |
| 160°F (71°C) | Minutes | Most vegetative bacteria die rapidly |
| 170–180°F (77–82°C) | Seconds to <1 min | Very rapid kill |
| >212°F (100°C) | Effectively instant* | Boiler conditions; *spores may require higher pressure/time |
Legionella Risk Factors
Legionella risk is a function of system design and operation, not just water chemistry:
- Temperature: Growth range 77–113°F (25–45°C). Optimal ~95–105°F (35–40°C). Thermal control generally requires ≥140°F storage with ≥120°F distribution targets (varies by guidance), and periodic high-temp disinfection where feasible.
- Stagnation: Dead legs, low-use fixtures, seasonal systems, storage tanks without circulation.
- Biofilm: Legionella are often associated with biofilms and free-living amoebae/protozoa, which can protect them from disinfectants and support amplification.
- Aerosol generation: Cooling towers, decorative fountains, showerheads, and any system that generates fine water droplets.
- Susceptible population: Immunocompromised individuals, elderly, smokers, those with chronic lung disease.
ASHRAE 188 is widely used as the backbone for Legionella water management programs; some jurisdictions and facility types reference it directly or indirectly.1
⟵ 09 Microbiology (Narrative) · Contents · 09 Microbiology (Problem Set) ⟶
Footnotes
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ANSI/ASHRAE Standard 188-2021, Legionellosis: Risk Management for Building Water Systems. See also ASHRAE Guideline 12-2020, Managing the Risk of Legionellosis Associated with Building Water Systems, and the CDC Legionella water management program toolkit. Confirm the current edition at time of publication. ↩