Microorganisms in Plumbing Systems

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Certain plumbing design choices and building contractor practices can promote the growth of microorganisms in new plumbing systems. This can lead to corrosion of metals, increased metals concentration in the drinking water, holes in pipe walls, and waterborne illnesses. Basic steps can prevent the problem from occurring in the first place.


The story is repeated over and over. Occupants of a new building notice either discolored water coming from the water faucets or a “rotten egg” odor coming from the hot water.

Typically, what will happen next is that the plumbing contractor, realizing that corrosion is occurring, will pull the sacrificial anode rod from the hot water storage tank. The rod in a hot water storage tank is there to slowly corrode over many years, with the rod’s metallic properties diverting the flow of electrons to sacrifice itself to corrosion and protect the storage tank. But, in these cases, the anode rod has greatly corroded over a few weeks or months. The plumbing contractor will replace the rod, only to see the new rod corrode quickly again.

Stray electrical currents or the connection of dissimilar metals are then blamed for the system-wide corrosion. Wires are added to connect various parts of the piping system for diverting electrons to a different path.

The problem persists.


What most people don’t know is that the problem is of microbiological origin. Microbiologically influenced corrosion (MIC) has not been appropriately understood or acknowledged in the drinking water industry—in municipal water systems or in plumbing systems.

Microorganisms are everywhere, and there are many different types of them. Some directly cause human illness and some do not. All waterborne microorganisms can grow into out-of-control populations when there are long periods of water stagnation or low flow and when disinfection chemicals are in inadequate concentrations. Under such conditions, microorganisms attach to plumbing system surfaces, both metal and non-metal. The microorganisms secrete an enzyme that forms a protective barrier around them, and they begin to multiply. This buildup of enzyme and microorganism colonies on surfaces is called a biofilm.

The biofilm is acidic and can create conditions at the pipe wall that allow metal to corrode. Metals that the plumbing sys- tem is made of, such as copper, iron, and even lead, have been found to corrode and their concentrations found to be increased in the drinking water when biofilms are present. Pinhole leaks in copper pipes have also been found. In addition, lower doses of chlorine and other disinfectants cannot reach the microorganisms protected in the biofilms. Instead, disinfectants get used up by reactions with the surface of the biofilms. This creates the environment for the growth of microorganisms that cause human illness if they are accidentally introduced into the water system.

Once a biofilm is firmly in place in a plumbing system, it is very difficult, if not impossible, to remove. It cannot be flushed with high-velocity water because the required water flows and pressures cannot be achieved in plumbing systems. The biofilm cannot be removed by disinfection because many modern plumbing materials, such as PEX® piping, cannot come in contact with the high concentrations of disinfection that are needed.


Proper plumbing design is the first line of defense against the growth of microorganisms in a plumbing system. In general, in water with low or no disinfection, whenever an excessive quantity of water is stored and excessive surface area is available in the plumbing system, microorganisms can get the upper hand and form biofilms.

Plumbing designers should carefully plan the capacity of the water system. In modern plumbing systems, the high hot waterflow demands of large bathtubs and Jacuzzis control the plumbing system design. This leads to installation of large water softeners and hot water storage tanks which are oversized for typical water usage in the building when the tubs are not in use. This creates a long residence time for water inside the plumbing system.

Another area where biofilm development is typically found is in hot water recirculation systems. In larger residences and buildings, hot water is re-circulated between the faucets throughout the building and the storage tank in order to provide water at the desired elevated temperature immediately when a faucet is opened. The recirculation piping adds extra storage of water and residence time in the plumbing system and helps spread microorganisms from a location of biofilm development to other parts of the hot water system.

Water conservation devices also increase the time that water spends in the plumbing system.

Water treatment devices cause issues in that many remove disinfection from the water, provide a large volume of water storage, and provide greatly increased surface area on the treatment media, such as on physical filters, granular activated carbon, and softener resin.

There is no room in this article to discuss plumbing design details. Just be aware in plumbing design that disinfected water should flow in the plumbing system with minimum residence time and minimum surface area contact.


Construction of a building takes months and sometimes over a year or more. During this time, any water that has been introduced into the water service line and the interior plumbing system is stagnating in the pipelines and forming biofilms. Building contractors need to be aware of this. Routine flushing and disinfection of all on-site pipelines should be performed. The disinfection concentration, and even the microbiological activity, can be measured and documented to show that no problems occurred under the building contractor’s watch.

Although there are many details that can help prevent microorganism growth, generally be aware that flushing, disinfection, and monitoring can prevent the growth of microorganisms and the development of biofilms in the piping system.


Everyone involved in the design and construction of buildings should be aware that microorganisms can and do significantly affect water quality in plumbing systems and can even weaken the pipe itself. Modern plumbing systems with large bathtubs, increased use of water treatment devices, materials of construction that cannot come in contact with high disinfection concentrations, and water conservation devices contribute to the likelihood that microorganisms will grow in the plumbing system and form biofilms on pipe and tank surfaces.

Plumbing designers can do their part in preventing the growth of microorganisms and the development of biofilms by minimizing the residence time of water in the plumbing system and the surface area that the water contacts. Building con- tractors can do their part by routinely flushing the pipelines, disinfecting the water, and documenting disinfection concentration and microbiological activity.



Abigail Cantor is the founder of Process Research Solutions, LLC, a chemical engineering consulting firm specializing in drinking water quality issues.


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