Secondary Disinfection for Legionella and Other Waterborne Pathogens

Secondary disinfection (also called supplemental disinfection) refers to the implementation of additional water treatment methods to control waterborne pathogens beyond standard municipal treatment. This is especially critical in controlling Legionella pneumophila, the bacteria responsible for the majority of Legionnaires’ disease cases. Effective secondary disinfection strategies must address biofilms, complex plumbing systems, and specific water chemistry challenges. Below is an overview of secondary disinfection methods.

Oxidizing Chemical Secondary Disinfection Systems

Chlorination

Chlorination introduces chlorine (sodium hypochlorite) into the water to provide a residual disinfectant.

Advantages:

• Proven efficacy against a range of pathogens.
• Cost-effective and easy to procure.
• Works reasonably well for preventing biofilm growth.

Disadvantages:

• Is not effective at penetrating existing biofilm where over 90% of Legionella reside.
• Produces potentially harmful disinfection by-products like trihalomethanes (THMs).
• Can corrode pipes if improperly managed.

Monochloramines

Monochloramines, formed by mixing chlorine and ammonia, offer a more stable disinfectant than chlorine and chlorine dioxide.

Advantages:

• Long-lasting residual effect, well-suited for large, complex plumbing systems.
• Effective in penetrating and removing biofilms.
• Reduced formation of harmful by-products compared to free chlorine.

Disadvantages:

• Slower disinfection relative to other oxidants.
• May contribute to nitrification in the system.
• Some studies have shown a correlation between monochloramines and increased NTM levels.

Chlorine Dioxide

Chlorine Dioxide is introduced into the water as a dissolved gas and is a strong oxidizing disinfectant.

Advantages:

• Demonstrates strong biofilm control and Legionella eradication.
• Is fast acting against existing biofilm.
• Is effective in disinfecting a broad range of pathogens.

Disadvantages:

• Is limited in larger hot water systems due to its volatility in hot water and low EPA dosing limit.
• Requires one or more strong precursors, usually sodium chlorite and hypocaloric acid.
• Its disinfectant byproducts need to be tested daily or weekly.

Mixed Oxidant Solution (MOS)

MOS involves the on-site generation of a mixture of oxidants for enhanced disinfection.

Advantages:

• Demonstrates strong biofilm control and Legionella eradication.
• Is fast acting against existing biofilm.
• Is effective in disinfecting a broad range of pathogens.
• Is the safest of the oxidizing chemical disinfectants and requires only salt and water as precursors.

Disadvantages:

• Requires specialized generation equipment.
• Can corrode pipes if improperly managed.
• Produces potentially harmful disinfection by-products like trihalomethanes (THMs).

Ozonation

Ozone gas is dissolved in water to oxidize and eliminate Legionella and other pathogens.

Advantages:

• Powerful oxidizing agent effective against Legionella and biofilms.
• Produces no harmful residuals if managed properly.

Disadvantages:

• Especially short-lived residual effect.
• Due to short-lived residual, not effective against biofilm that is farther away.
• High capital and operational costs.
• May generate bromates as by-products in certain water chemistries.

Advanced Oxidation Process (AOP)

Combines oxidants like ozone, hydrogen peroxide, and UV light to produce hydroxyl radicals.

Advantages:

• Highly effective against Legionella and biofilms.
• Produces no harmful residuals.

Disadvantages:

• High capital, energy, and operational costs.
• Due to short-lived residual, not effective against biofilm that is farther away.
• Very short residual disinfection effect.

Non-Oxidizing Chemical Secondary Disinfection Systems

Copper-Silver Ionization

This method releases copper and silver ions into the water, which disrupt Legionella cell walls and inhibit and penetrate biofilms.

Advantages:

• Has longest longest-lasting residual effect for effective control of Legionella in biofilms.
• Does not use any oxidizing or hazardous precursors.
• Does not produce any disinfection by-products.
• Is completely independent of temperature.

Disadvantages:

• Can have high installation costs.
• Less is known about its ability to control other waterborne pathogens outside of Legionella.
• Slower disinfection relative to other oxidants.

Non-Chemical Secondary Disinfection Systems

Ultraviolet (UV) Light (Supplemental Disinfection Only)

UV light disrupts microbial DNA, rendering pathogens non-viable.

Advantages:

• Immediate disinfection with no chemical by-products.
• Effective against hard-to-kill organisms like NTM and protozoa, which are hard to kill with chemical disinfectants.

Disadvantages:

• No residual disinfection effect.
• Not effective against biofilm or water that does not pass through it.
• Requires water with lower suspended solids to be effective for optimal performance.

Nanobubbles

Nanobubbles (extremely small gas bubbles) enhance microbial disruption and water quality.

Advantages:

• Penetrate biofilms and destroy Legionella effectively.
• Eco-friendly with no harmful chemical by-products.

Disadvantages:

• Minimal residual disinfectant effect.
• Relatively new technology with limited large-scale data.

Dissolved Oxygen Removal

This method removes dissolved oxygen to create anaerobic conditions, inhibiting aerobic pathogens like Legionella to grow.

Advantages:

• Effective against Legionella and biofilms.
• Reduces corrosion and environmental risks due to oxygen removal
• Uses no precursors.

Disadvantages:

• Can have high installation costs.
• Less is known about its effectiveness against biofilms.
• Relatively new technology for Legionella control with limited data on effectiveness.

Legionella Control Considerations for Secondary Disinfection

The characteristics of Legionella make it important to utilize a truly independent Legionella expert like Legionella Control Systems because we are not tied to one particular disinfection method but have the freedom to recommend the best options for your unique facility and budget.

Selecting a supplemental disinfection strategy depends on system complexity, regulatory compliance, and specific pathogen risks. Often, a combination of methods is the most effective approach. For example, Point of Entry (POE) filtration, UV disinfection or AOP can serve as point-of-use treatments, while copper-silver ionization, mixed oxidants, or monochloramines offer broader residual control. Legionella Control Systems will consider factors such as:

• Biofilm Targeting: Methods like copper-silver ionization, monochloramines, chlorine dioxide, and mixed oxidants, are particularly effective.
• Residual Disinfection: Systems like monochloramines and copper-silver ionization provide lasting effects to prevent recolonization.
• Water Chemistry Optimization: Proper monitoring of pH, hardness, conductivity and other factors enhances effectiveness.

Contact Legionella Control Systems right now at 888-416-8626 or info@legionellacontrolsystems.com to discuss which secondary disinfection methods should be used in your facility.