Legionella Supplemental Disinfection

If short-term remediation does not have long-term effectiveness in eliminating legionella in your system, some facilities should consider continuous supplemental disinfection. However, the decision to add supplemental disinfection to your building water systems can be challenging. While a facility wants to reduce the risk of  Legionnaires’ Disease, there are capital and ongoing maintenance costs. It also requires modification to plumbing systems, knowledge of EPA permitting and disinfectant levels, routine monitoring, and preventive maintenance activities.

Luckily, ChemREADY has experts that can walk you through the decision-making process, to see if this could be a good fit for your facility.

Types of Secondary Disinfection

Several approaches can be considered for supplemental disinfection of potable water. There are many considerations for each solution to include required permitting, capital costs, availability of equipment, ongoing maintenance costs, EPA regulations, monitoring requirements, footprint of equipment, permitting timeline, water operator license requirements, and engineering constraints. Every state and municipality has different requirements and it’s important to understand those in order to make a decision.

Legionella supplemental disinfection should only be considered when shock disinfection and the water supply’s disinfection isn’t enough to continuously minimize the growth of Legionella bacteria along with other waterborne pathogens. There are a few common supplemental disinfection methods:

There are many continuous supplemental disinfection options available to end users. Each type of supplemental disinfection has advantages and disadvantages that are unique to every situation. The following list contains common supplemental disinfection methods:

  • Chlorination
  • Chlorine Dioxide 
  • Monochloramine 
  • Ozone 
  • Copper-silver ionization 


Chlorination is a common disinfectant used in public water supplies and in supplemental disinfection. If your incoming water supply has low levels of disinfectant (<0.5mg/L), your facility has the potential for on-going issues with Legionella bacteria and other waterborne pathogens. Chlorination can be used to supplement the existing disinfectant in your water supply. Supplemental chlorination systems inject additional chlorine into the potable water, either the cold water supply or the hot water system.

The EPA drinking water limit for Chlorine (as Cl2) is 4 mg/L while most manufacturer’s recommend a control range between 0.5 to 3.0 mg/L

Advantages to using supplemental chlorination are:

  1. Readily available and cost effective
  2. Effective in killing bacteria, viruses, and other organisms in water
  3. Easily monitored for disinfectant levels
  4. EPA-certified drinking water disinfectant.

Disadvantages of chlorination are:

  1. Will not fully penetrate biofilm. It oxidizes when in contact with bacteria quickly.
  2. Can react with organic materials in water to potentially make harmful disinfectant byproducts
  3. Can degrade piping materials and components over time
  4. Dissipates easily in hot water systems and becomes less effective

Chlorine Dioxide

Chlorine Dioxide (ClO2) is a versatile, broad-spectrum biocide, second only to ozone in biocidal efficacy. It selectively oxidizes biological pathogens while generating less disinfectant byproducts.

The EPA drinking water limit for Chlorine Dioxide (as ClO2) is 0.8 mg/L while most manufacturers recommend a control range between 0.1 to 0.7 mg/L.

Advantages of chlorine dioxide:

  1. More effective disinfectant than chlorine but not as strong of an oxidant as free chlorine
  2. Fewer disinfectant byproducts than chlorine
  3. Effective at low concentrations
  4. Does not cause an odor nuisance or affect the drinking water taste

Disadvantages of chlorine dioxide:

  1. Difficult to transport, most likely requiring generation on site
  2. Higher capital costs to install equipment for generation on site
  3. Decomposes rapidly, with a short shelf life
  4. 5-10 times more expensive than chlorine
  5. Vast safety measures in place due to explosive nature of chlorine dioxide gas


Monochloramine is one of the most effective disinfectants against Legionella, both in laboratory and field studies. Monochloramine is stable and has the ability to penetrate biofilm more effectively than chlorine, and has a wider pH working range than copper‑silver ionization and chlorine

The EPA drinking water limit for Monochloramine (as Cl2) is 4.0 mg/L while most manufacturers recommend a control range between 2.0 to 3.0 mg/L.

Advantages of monochloramine are:

  1. Readily penetrates biofilm and is less oxidizing, meaning longer stay times in water
  2. Does not dissipate in hot water, like chlorine and chlorine dioxide
  3. Less corrosive to plumbing and the least amount of disinfection byproducts compared to chlorine and chlorine dioxide

Disadvantages of monochloramine are:

  1. Requires generation on site with ammonia and chlorine, capital costs required for installing systems
  2. Requires the monitoring of free ammonia. High ammonia levels can increase corrosion in systems
  3. Cold water application can affect dialysis, fish tanks, and nitrification in water


Ozone is created when Oxygen (O2) molecules are broken apart by an energy source into oxygen atoms and collide with an oxygen molecule to form an unstable gas, ozone (O3). Ozone is a very strong oxidant and disinfectant.

Advantages of ozone:

  1. Ozone is more effective than chlorine in destroying bacteria and viruses
  2. Ozone only requires a short contact time to be effective
  3. Ozone decomposes rapidly and there are no harmful residuals that need removed.

Disadvantages of ozone:

  1. Highest capital and on-going maintenance costs make it hard to compete with less expensive methods for smaller facilities
  2. Ozone requires a maintenance team with high familiarity with the equipment and constant monitoring
  3. Ozone is very corrosive, requiring corrosion-resistant piping materials
  4. Very high energy requirements

Copper-Silver Ionization

Copper-Silver Ionization was the original technology used in combatting Legionella bacteria. Copper-Silver Ionization is an electrolysis process that introduces positively charged copper and silver ions into the water supply through an electrical current. The positive ions bind to the negatively charged cell walls of bacteria, eliminating them through the process.

The EPA drinking water limit for Copper is 1.3 mg/L while most manufacturers recommend a control range between 0.2 to 0.8 mg/L. The drinking water limit for Silver is 0.1 mg/L while most manufacturers recommend a control range between 0.01 to 0.08 mg/L.

Advantages of Copper-Silver Ionization:

  1. Effective in both cold and hot applications
  2. Not affected in pH ranges up to 9.00
  3. Long stay-times in both cold and hot, making a good long-term application

Disadvantages of Copper-Silver Ionization

  1. At pH levels above 9.00, only 10% effective
  2. If there is a high dissolved solid content, silver will precipitate and are not effective for disinfection
  3. High capital costs for installation of systems

Figuring out the best supplemental disinfection method can be a confusing and daunting task. If you feel your facility could benefit from the addition of a supplemental disinfection technology, contact our team of experts at ChemREADY today to discuss options.