In the United States plumbing industry, focus on water quality and waterborne pathogens is increasing. For public water systems, the Environmental Protection Agency regulates 88 different contaminants. Of these, Legionella has received the most recent attention.
Legionella is the bacteria that can cause legionellosis. (Legionnaires’ disease is a form of legionellosis.) If not treated properly, legionellosis can be fatal. When legionellosis occurs, building owners and engineers may face legal action and damage from negative public relations surrounding the case. The best defense for a commercial plumbing system is to understand waterborne pathogens and how to minimize the risks they pose.
How Waterborne Pathogens Spread in Plumbing Systems
Legionella is a naturally occurring bacteria that affects the body’s respiratory system. Once aspirated, the bacteria establishes itself in the lungs, causing pneumonia-like symptoms. Legionella colonization in plumbing systems can pose a significant threat. Sources of Legionella in a building system may include piping systems, cooling towers, decorative fountains, standalone ice machines, and spas. Specific to piping systems, shower vapor or the mist from a lavatory aerator in which the bacteria exists can introduce Legionella into the respiratory system.
Temperature and flow conditions in a commercial plumbing system matter more than material type. Regardless of the piping, fitting, and sealing elements used, most modern plumbing systems are designed using a branchand- tee concept, which often results in dead legs. Dead legs result in stagnant water, and stagnant water poses a significant risk of harboring and fostering microbial growth.
Legionella bacteria that reside in stagnant water conditions will flourish within a broad temperature range, typically 95°F to 122°F. Because bacteria multiply exponentially every few minutes, undisturbed sections of piping can be the source of Legionella contamination for the entire system.
Legionella secretes a polymeric slime, forming a biofilm on the inside of the pipe wall. Once the biofilm forms, it serves as a protective blanket for bacteria living within it and can withstand extreme treatment methods such as thermal and chemical disinfection. If the colony is not rendered inert throughout the system, it will recolonize after treatment, regardless of the treatment method used.
Five Tips To Minimize Microbial Growth
One of the best approaches for minimizing the risk of Legionella forming in a building plumbing system is to prevent it from occurring in the first place. Design strategies can be coupled with good operational and maintenance practices and a secondary disinfection program to effectively prevent a problem before it begins.
The challenge with any type of disinfection, including thermal disinfection, is its reach. If the process is not complete, there is a risk of leaving bacteria and biofilm undisturbed throughout the system, allowing regrowth. To discourage microbial colonization, all parts of a system should experience flow and be maintained at the proper temperature.
Even with recirculation, stagnation can occur in dead legs because of the lack of water exchange, which represents a challenge to the plumbing system designer. Dead legs should be eliminated wherever possible.
One solution is to daisy-chain fixtures so their individual drops (dead legs) are avoided. Any time one fixture is used, the remaining fixtures will see flow up to the point of connection even if they are not used.
In the case of seldom-used remote fixtures, water can be forced through the fixture (up to the point of connection) with the use of a venturi. This concept manipulates the pressure across the fixture to induce flow, thereby eliminating the dead leg that serves that fixture.
With flow, temperatures can be better maintained, and disinfectant residual can reach much more of the system. These concepts require the designer to reconsider the branch-and-tee concept.
It is up to the system designer to determine when the risk of Legionella within the plumbing system requires specific attention as well as which disinfection methods and design concepts are best for minimizing that risk. Water quality and sound plumbing design involve far more than installing piping to connect fixtures. As plumbing requirements and risks evolve, so too must the plumbing industry.
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MCAA thanks Viega for being a benefactor of MCAA 2015.
