Water Softeners and Legionnaires’ Disease


There are multiple well known and important benefits associated with use of water softeners. Properly designed and installed, when needed to control hardness in water, softeners can increase equipment life, reduce equipment maintenance, increase efficiency of heaters, reduce detergent usage and more. But as with many things there are also unintended consequences.

This webpage is being posted because equipment manufacturers have largely ignored the well documented issue of increased risk for Legionella and other OPPP’s associated with water softeners. Hopefully, the information on this webpage will provide those involved in specifying, designing, and operating plumbing systems, information necessary for addressing the Legionella risk associated with water softeners.

While Legionella is the most discussed waterborne pathogen there are other waterborne pathogens of concern including Mycobacterium and Pseudomonas. These deadly waterborne pathogens have been aptly described with the acronym OPPP’s which stands for Opportunistic Premise Plumbing Pathogens. For the remainder of this article only the term Legionella will be used because it is the most discussed and recognized OPPP.

This article Green Water Systems and Opportunistic Premise Plumbing Pathogens gives a detailed description of OPPP’s https://www.phcppros.com/articles/124-green-water-systems-and-opportunistic-premise-plumbing-pathogens

The Issue

A) Legionella Risk Associated with Softeners

Softeners can increase the risk for Legionella colonization in hot and cold-water systems by consuming disinfectant residual and providing a media for bacteria growth. There are several factors to consider with respect to softeners and risk potential.

Size: Softeners, even if properly sized for the application, will consume a small but significant percentage of free chlorine in the municipal supply. When oversized, softeners may consume most or all the free chlorine and if grossly oversized, softeners will even consume the more stable and persistent chloramine.

Based on my experience, softeners are frequently grossly oversized. Often the water softeners are two or three times larger than needed and on occasion more than five times larger than needed. When softeners are oversized there are multiple factors that increase Legionella risk because the water is retained in the softener longer;

  1. Increased time in the tank in contact with the resin will increase disinfectant decay.
  2. Increased time in the tank will increase water temperatures in the tank if the ambient room air temperature where the tank is located is higher than incoming water temperature. This higher water temperature will further increase disinfectant decay and accelerate bacteria growth.
  3. Reduced flow rate (velocity) through the tank increases potential for channeling. Channeling is the term used to describe when water flow through the softener is in limited channels rather than evenly flowing through the resin bed. Channeling can occur because of an equipment malfunction but can also occur when the flow rate through the softener is well below design because the softener is oversized which causes the water to follow the path of least resistance.
  4. Reduced backwashing frequency increases compaction of the resin bed and potential for increased buildup of particulate matter on the resin surface that aids in bacteria growth. Increased compaction also increases the potential for channeling.

Installation: Disinfectant decay occurs faster at higher temperatures. Thus softeners located in a warm mechanical room will likely experience more disinfectant loss and bacterial growth than softeners located in a cooler area.

Design: Dual resin tanks with more than 48 hours between switch over and regeneration can result in greater Legionella growth potential in the stagnant standby tank. Oversized softeners may need to regenerate only every 3 days, grossly oversized softeners may need to regenerate only every 7 days. Very few installations require softened water 100% of the time. If the hardness is not extremely high or there are periods of the day where there is little to no water usage such as 1 am to 3 am, then a single tank installation is frequently all that is needed.

Operation: Proper operation and maintenance is needed for softeners. Undetected or uncorrected equipment malfunction can cause channeling in the softener. Resin beads may become fouled with metals and require acid cleaning for effective ion exchange or may become fouled with biofilm and require disinfection for proper and safe operation.
Issues with water softener sizing, installation, design and / or operation can result in the softener becoming the equivalent of a petri dish, a culture broth, providing higher bacteria counts and lower disinfectant levels to the building water system. Lower disinfectant levels facilitate the growth and colonization of Legionella and other OPPP’s and often occur simultaneously where issues exist. In some cases, if the softener is 1.5 or 2 times larger than needed increasing regeneration from every three or four days to once every day or every other day can provide a cost effective solution where the increase in water and salt consumption is a lower cost than purchasing a new softener. But often the best course is to replace the oversized softener with a properly sized softener.

B) Environmental Impact

Softeners have minimum and maximum exchange capacities. If you need maximum exchange capacity, you increase regeneration brining to maximum and for minimum exchange capacity you decrease regeneration brining to minimum. But the amount of water consumed in backwash will not be reduced. If a softener is 2, 3 or 5 times larger than needed then instead of regeneration every 24 or 48 hours it may need to be regenerated only once every two weeks. But this long of a period between regenerations results in more compaction of resin and more buildup of particulate in the bed. To reduce bacterial risk, softeners are recommended to be regenerated every day and at most every other day. Additionally, if there is a standby resin tank, and the softener is regenerated every 7 days then the standby tank is stagnant for 7 days, which will greatly increase bacterial growth and Legionella risk. Regenerating a softener every 24 or 48 hours that only needs regeneration every two or three weeks due to oversizing results in 7 or more times as much salt; purchased, loaded into brine tanks by maintenance personnel, and flushed to drain during regeneration.

If the municipal water has 140 ppm of hardness and the heater manufacturer recommends less than 50 ppm of hardness, then only 65% of the water needs to be softened. Getting a softener that is 35% or smaller in capacity will be a much lower cost, much smaller floor space plus environmental benefits of much less salt and water usage. So many factors related to softeners have an environmental impact because oversizing softeners not only increases the risk of Legionella growth, but increases water and salt usage as well.

Some studies have shown that zero hardness water can increase corrosion potential.

Conclusion – Sizing the softener to soften 100% of potable water has multiple negative impacts. Sizing the softener to deliver a desired percent blended hardness will reduce capital cost, water consumption, salt usage, Legionella risk and may reduce corrosion potential as well.

Examples of water softener blending valve:

GE – https://www.geappliances.com/ge/water-softeners.htm
Nobel – https://www.nobel.srl/en/products/softener-series-as-at-av/
Flotrol – https://www.flotrol.us.com/accessories-1/Bypass

C) Failure to Warn

While it is common to see manufacturers provide extensive WARNINGS on water heater owners’ manuals related to scald risk when temperatures are too high, partly in response to concerns for ‘FAILURE TO WARN’ product liability litigation, it is uncommon to see similar warnings on products that create known Legionella risk such as water heaters, carbon filters and water softeners. Industry is aware of the issue. There are papers documenting the correlation between Legionella and softeners going back decades.

Some water heater manufacturers are marketing certain heater designs as lowering Legionella risk but not mentioning that other designs are an increased Legionella risk. Likewise, some ice machine filter manufacturers are marketing low micron non-carbon filters as addressing Legionella risk associated with ice machine carbon filters but not mentioning on their carbon filter marketing that carbon filters are associated with increased Legionella risk.

More to Come

There will be more to come on this issue but for now the links below are documents from multiple sources related to this issue.

If you know of any good papers or articles that discuss the issue of pathogen risk associated with water softeners or carbon filters, please forward to me and I’ll review for including on this page.


Thank you to Dr. William Rhoads (william.rhoads@eawag.ch) and Dr. Rain Richard (rain.richard1@gmail.com) for volunteering their expertise and time in providing comments and research review in development of this document.

© 2021 Legionella Risk Management, Inc.
Cost Effective Engineering Solutions to Engineering Systems Pathogens

Published Papers investigating linkage of softeners to Legionella

Typically, the information collected in the papers below regarding softeners is limited often with the intent solely to do a statistical correlation between softeners and Legionella positives. None of the studies I have found investigate the key criteria that can be obtained quickly and easily. These are 1) disinfectant loss across the softener, 2) temperature gain across the softener, 3) ambient temperature of softener location vs ambient outside air temperature and 4) aerobic bacteria (HPC) gain across the softener. Another contributing factor that researchers should look at but that is more difficult to ascertain is the softener rated flow versus the system peak and average flow. The question most researchers have been looking to answer is if there is a higher risk with softener installation for Legionella growth. The question researchers should be looking to answer is the impact the 4 criteria listed above have on risk for bacteria growth in plumbing system softener installations.

Effect of Chlorine on Cation Exchange Resins D. G. Braithwaite, J. S. D’Amico and M.T. Thompson Ind. Eng. Chem. 1950, 42, 2, 312–315 Feb 1950
This first paper does not mention Legionella but shows that as early as 1950 there was an established and known correlation between softeners and bacteria growth and that industry was investigating methods to minimize that known risk.

Microbiological study of water-softener resins. Stamm, J.M., Engelhard, W.E., Parsons, J.E., 1969. Appl. Microbiol. 18, 376–386.

Microbial growth on ion exchangers. Flemming, H.-C., 1987. Water Res. 21, 745–756

More than 10 Years of Unrecognized Nosocomial Transmission of Legionnaires’ Disease Among Transplant Patients. Jacob L. Kool, et al Infection Control & Hospital Epidemiology, 19(12), 898-904. doi:10.2307/30142014 Dec 1998

Presence of Legionella spp. in Hot Water Networks of Different Italian Residential Buildings: A Three-Year Survey Paola Borella et al Applied and Environmental Microbiology Volume 71, Issue 10, Pages 5805-5813 Oct 2005

Introduction of Monochloramine into a Municipal Water System: Impact on Colonization of Buildings by Legionella spp. Matthew R. Moore et al Applied and Environmental Microbiology p. 378–383 Jan 2006

Legionella pneumophila contamination of a dental unit water line system in a dental teaching centre. Ma’ayeh, S., Al-Hiyasat, A., Hindiyeh, M. and Khader, Y. (2008), International Journal of Dental Hygiene, 6: 48-55. Jan 2008

Legionella pneumophila occurrence in drinking water supplied by private wells K. Mapili, K. J. Pieper, D. Dai, A. Pruden, M. A. Edwards, M. Tang, W. J. Rhoads Lett Appl Microbiol, 70: 232-240 Jan 2020

Finding building water quality challenges in a 7 year old green school: implications for building design, sampling, and remediation – Environmental Science: Water Research & Technology (RSC Publishing) Kyungyeon Ra, Tolulope Odimayomi, Christian Ley, Tiong Gim Aw, Joan B. Rose, Andrew J. Whelton Environ. Sci.: Water Res. Technol. , 6, 2691-2703, Aug 2020

The impact of pipeline changes and temperature increase in a hospital historically colonised with Legionella. Quero, S., Párraga-Niño, N., Garcia-Núñez, M. et al. Sci Rep 11, 1916 Jan 2021.

Physical, Chemical, and Microbiological Water Quality Variation between City and Building and within Multistory Building Rain Richard, Kerry A. Hamilton, Paul Westerhoff, Treavor H. Boyer ACS EST Water 2021, 1, 6, 1369–1379 Apr 2021

Prevalence of opportunistic pathogens in a school building plumbing during periods of low water use and a transition to normal use by Tiong Gim Aw, Laura Scott, Kathryn Jordan, Kyungyeon Ra, Christian Ley, Andrew J. Whelton International Journal of Hygiene and Environmental Health Feb 2022

Presence of Legionella spp. in Hot Water Networks of Different Italian Residential Buildings: A Three-Year Survey Paola Borella et al Applied and Environmental Microbiology Volume 71, Issue 10, Pages 5805-5813 Oct

Water Softener content in selected Standards and Guidelines

Below is a listing of some standards and guidelines, along with associated excerpts discussing bacteria (including Legionella) growth potential and risk related to water softener installation, design, operation and maintenance.

ASHRAE (American Society of Heating Refrigerating and Air Conditioning Engineers) – Guideline 12 2020 Managing the Risk of Legionellosis Associated with Building Water Systems

“5.2.2 Plumbing Component Design, Selection, Installation, and Other Considerations. A number of physical, chemical, and operational Legionella control measures can be affected
by choices made when selecting plumbing components:” “k. Water processing equipment/potential effects on disinfectant residual. Some potable water system components, such as water softeners, water heaters, carbon filters, and ultraviolet (UV) devices, may reduce or eliminate disinfectant residual. The plumbing system design and component selection should consider the potential effects on downstream disinfectant residual levels.”

“5.3.6 Routine Cleaning and Maintenance. Routine cleaning and maintenance of building water system components reduces conditions that are supportive of Legionella growth, such as the accumulation of sediment in water storage tanks, and are important Legionella control measures. However, routine cleaning and maintenance alone do not ensure the control of Legionella. Building water systems components that should be considered for periodic cleaning and maintenance include faucets; showers; water storage tanks; filters; water softeners; electronic faucets; local and point-of-use mixing valves; water hammer arrestors; and non-flowthrough expansion tanks.”

ASHRAE (American Society of Heating Refrigerating and Air Conditioning Engineers) – Standard 188-2018 Legionellosis: Risk Management for Building Water Systems 2018

“7.1 Potable Water Systems. This section describes the preventive measures required for potable water systems. ….

7.1.2 System Maintenance. The Program documents shall include procedures for….
c. maintenance and monitoring procedures based on equipment manufacturers’ instructions for cleaning, disinfection, replacement of system components, and other treatments the Program

Team decides are necessary for the following:

  1. Hot-water and cold-water storage tanks
  2. Ice machines
  3. Water-hammer arrestors
  4. Expansion tanks
  5. Water filters”

CDC (Centers for Disease Control and Prevention) – Environmental Infection Control Guidelines 2003

Pg 63 “Both softeners and deionizers are significant reservoirs of bacteria and do not remove endotoxin. … Oversized diameter and length decrease fluid flow and increase bacterial reservoir for both treated water and centrally-prepared dialysate.”

Pg 224 “Box C.1. Potential sampling sites for Legionella spp. in health-care facilities*
• Potable water systems incoming water main, water softener unit, holding tanks, cisterns, water heater tanks (at the inflows and outflows)”

CIBSE (Chartered Institution of Building Service Engineers) – Minimising the risk of Legionnaires’ disease TM13 2013

“5.2 Cold water services systems …Where water softeners or filters are used, these should be checked and cleaned on a regular, routine basis, and manufacturers’ recommendations should be followed. They can become colonised with micro-organisms and therefore should also be capable of being disinfected as required.”

EPA (US Environmental Protection Agency) Technologies for Legionella Control in Premise Plumbing Systems: Scientific Literature Review Sep 2016

“An environmental assessment of the various components of a facility’s premise plumbing system can help determine vulnerabilities. These elements commonly include consideration of hot and cold water temperatures, proper service of heating components, water softeners, water fixtures (e.g., showers), spas, water features, humidifiers and cooling towers.”

ESGLI (European Study Group for Legionella Infections) – European Technical Guidelines for Prevention, Control and Investigation of Infections Caused by Legionella species Jun 2017

“3.163 Maintaining the cleanliness of water softeners and filters is important and best achieved by following the manufacturers’ recommendations. Coarse filters and strainers should be checked and cleaned regularly to prevent the build-up of organic contaminants.”

“3.178 A simple description and plan of the system and its location within and around the building. This should identify piping routes, storage and header tanks, hot water storage heaters and relevant items of plant, especially water softeners, filters, strainers, pumps and all water outlets.”

EWGLI (European Working Group for Legionella Infections) – Technical Guidelines for the Investigation, Control and Prevention of Travel Associated Legionnaires’ Disease Sept 2011

“Water softening In hard water areas the water supply to the hot water system should be softened to reduce scale formation. Water softeners can occasionally become colonized with microorganisms including legionellae. For this reason it is advisable to have a sample point installed just downstream of the softener to enable water samples to be collected at least once a year and to be tested for the aerobic colony count and Legionellae. The aerobic colony count should be compared with that of the incoming mains supply water.”

VDI (Association of German Engineers) – Guideline DVGW 6023 Hygiene in drinking-water installations – Requirements for planning, execution, operation and maintenance Apr 2013

“Ion-exchanger systems (such as water softeners) should be dimensioned as small as possible. The total capacity of the system, expressed in mol × m3, must not exceed the 72-hour demand during specified normal operation.”

VA (Department of Veterans Affairs) – Plumbing Design Manual May 2021

10.11 WATER SOFTENING SYSTEM “10.11.2 When Required (a) Entire medical center: Provide softening equipment when total hardness exceeds 170 mg/L [170 ppm] as CaCO3. Blend equipment effluent to a hardness of approximately 50 mg/L [50 ppm]. Design triplex softeners, each furnishing 50% of the maximum flow rate and exchange capacity. Provide a hard water bypass. … (c) Hot-water supply: Provide softening equipment when total hardness exceeds 50 mg/L [50 ppm]. Design triplex softeners, each furnishing 50% of the maximum flow rate at an exchange capacity required for peak boiler feed-water make-up. Locate regeneration alarm in office of boiler plant operator.”

WHO (World Health Organization) – Guidelines for Drinking-water Quality Volume 1 Recommendations 2008

“Elevated HPC occur especially in stagnant parts of piped distribution systems, in domestic plumbing, in some bottled water and in plumbed-in devices such as softeners, carbon filters and vending machines.”