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Legionella Control in Roof Top Units

Roof top units

  1. Small water volume
  2. Humidification / dehumidification units
  3. Running in wet to dry in cycles of random lengths.

Answer first

Best Biocide Option and Recommendation

  • DBNPA (2,2-Dibromo-3-Nitrilopropionamide) – Hockey Pucks in a feeder in each tower
    1. Best for periodic treatment, as it acts fast and breaks down quickly.
    2. Best for systems that run wet/dry due to fast action.
    3. Ideal if the system experiences frequent drying, reducing residual buildup.
      1. Feed in perforated feeders that feed on contact with water
    4. Less effective for long-term biofilm control
    5. More costly per pound to buy
    6. Less costly to use – significant
    7. Less costly to set up feed system – 10:1

 

Second choice

  • Glutaraldehyde – Liquid applied to all make up water. Concern about getting biocide into unintended building waters
    1. Better for sustained biofilm control, as it remains effective longer.
    2. Slower acting but helps prevent Legionella regrowth in stagnant conditions.
    3. Works well if combined with periodic flushing or low-dose continuous application.

Discussion

Recommended Operating Strategy

  • Prevention & Routine Cleaning – Quarterly
    1. Regularly flush and clean basins to prevent biofilm buildup.
    2. Remove sediment or organic matter that can support bacterial growth.
  • Chemical Treatment Plan – Ongoing
    1. Use DBNPA for quick knockdowns (e.g., shock dosing when the system is restarted after a dry period).
    2. Apply low-dose Glutaraldehyde (if compatible with materials) to help prevent biofilm formation as indicated by testing – BART SLYM and Lab pseudomonas
    3. Consider alternating biocides to prevent bacterial resistance.
  • Water Management Best Practices
    1. Maintain water
      1. pH between 6.5 – 8.5 to ensure biocide effectiveness
      2. Conductivity to < 2000 to avoid excess hydraulic holding time
    2. Monitor temperature, as Legionella thrives in 77–113°F (25–45°C) – summer
    3. Use non-corrosive dispersants if needed to enhance biocide penetration in biofilms – in the event of required cleanings/ disinfections

Background

Both DBNPA (2,2-Dibromo-3-Nitrilopropionamide) and Glutaraldehyde are used as biocides against Legionella bacteria, but their effectiveness depends on various factors, including contact time, water conditions, and system application.

Effectiveness Against Legionella:

  • DBNPA
    1. A fast-acting, non-oxidizing biocide.
    2. Quickly penetrates bacterial cell walls and disrupts metabolism.
    3. Rapidly degrades in water, reducing long-term environmental impact.
    4. Works well in systems with low organic load.
    5. More effective for short-term, quick kill applications.
  • Glutaraldehyde
    1. A slow-acting, non-oxidizing biocide.
    2. Works by cross-linking bacterial proteins, disrupting cell function.
    3. Effective in biofilm penetration, where Legionella can thrive.
    4. More stable in water systems, providing longer-lasting control.
    5. Often used in combination with other biocides for enhanced effectiveness.

Which is More Effective?

  • For a rapid knockdown of Legionella, DBNPA is more effective due to its fast-acting properties.
  • For long-term control, Glutaraldehyde is preferred, especially in systems prone to biofilm formation.

Biocide Dosing Recommendations

  • DBNPA (Fast-Kill Biocide) – Shock Treatment
    1. Dose: 2–10 ppm (parts per million) active DBNPA in the water basin.
    2. Frequency: Slow dissolve feeders
    3. Contact Time: Allow at least 30–60 minutes before draining or cycling the unit.
    4. Notes: Decomposes rapidly; safe for intermittent wet/dry cycles.
  • DBNPA (2,2-Dibromo-3-Nitrilopropionamide) releases bromine as part of its antimicrobial action, but its primary mechanism of killing Legionella is different from traditional bromine-based disinfectants like sodium bromide.
    1. Rapid Cell Penetration – DBNPA quickly enters bacterial cells due to its small molecular size.
    2. Protein & Enzyme Disruption – It reacts with thiol (-SH) groups in bacterial enzymes, disrupting metabolism and leading to cell death.
    3. Limited Bromine Release – While DBNPA contains bromine atoms, it does not function like free bromine (Br₂ or HOBr). Instead, as DBNPA hydrolyzes in water, it slowly releases brominated compounds that can have some secondary antimicrobial effects.
    4. Rapid Decomposition – DBNPA degrades into less active byproducts (e.g., dibromoacetonitrile, dibromoacetamide), meaning its effectiveness diminishes quickly compared to stabilized bromine solutions.

 

  • Key Differences from Free Bromine Disinfection
    1. DBNPA is NOT a halogen donor like sodium bromide or chlorine-based treatments.
    2. It does not maintain a residual disinfectant effect after decomposition.
    3. It is best for short-term, rapid disinfection, whereas free bromine works better for continuous Legionella control.

Materials compatibility is similar

Glutaraldehyde          DBNPA                       

Aluminum                         N                         N                  Fins – minimal mist contact only

Brass                                 N                         N

Copper                              N                         N                  Some piping

Stainless Steel                   Y                          Y                  Basin

Zinc                                   N                         Y

Polycarbonate                   N                         N

PVC                                    Y                          N

Acrylic                               N                         N

Polystyrene                       N                         Y

Natural rubber                  N                        N

Chloroprene                      N                        Y

Neoprene                          Y                         N

 

Corrosion monitoring – coupons in the basin