A previous article, “Microorganisms in Plumbing Systems”, warned that under certain conditions microorganisms can grow out-of-control in plumbing systems. When this happens, a “biofilm” of microorganisms and acidic enzymes that they secrete can become attached to pipe walls (Figure 1). This can lead to:
- Increased metals concentration (such as copper, iron, and lead) in the drinking water that comes in contact with the affected pipe
- Pitting and pinhole leaks in metal pipe
- Increased possibility of water-borne illnesses from microorganisms that are toxic to humans
Stagnating or slow moving water with inadequate disinfection allows microorganisms to grow rapidly. Microorganisms also need surface area to attach to – the more surface area, the more colonies of microorganisms can attach and grow.
This article identifies critical locations in a plumbing system where microbiological growth and biofilm formation tend to start. These locations are important to keep in mind as methods to prevent microbiological growth are discussed in future articles.
Biofilm in a Residential Copper Pipe
The light coating on the pipe in Figure 1 is a biofilm. Not all biofilms look like this. But, the one in Figure 1 is smooth, thick, and tightly adhered to the pipe wall.
The Water Source
A biofilm problem in a building can start with the water source. The water can carry microorganisms into a building’s plumbing system. If conditions are right in the plumbing system, the microorganism population will grow.
When private wells are a building’s water source, microorganisms can enter the well in the groundwater which has had contact with the surrounding soil and sometimes with surface water that has percolated down into the aquifer. Once in the well, microorganisms can form biofilms and attach to screens, casings, and piping. For this reason, wells should be cleaned periodically with high concentrations of chlorine solution. This is called shock chlorination and usually involves a solution of 200 to 300 mg/L free chlorine held in the well for twenty-four hours. Well owners may not realize the importance of this cleaning procedure and may neglect to have it done.
For buildings with water provided by a municipal or community water system, microorganisms are present in the water to varying degrees. In the drinking water industry, the emphasis is on microorganisms that are toxic to humans and can cause illness. However, the absence of illness-causing microorganisms in a water system does not mean that other microorganisms are not thriving. The lowest presence of microorganisms is found in municipal or community water systems where disinfection concentrations are maintained at effective levels and monitored throughout the distribution system. There are some water systems where disinfection is not maintained sufficiently and some where no disinfection is added at all. In those systems, the presence of microorganisms entering a building can be quite high.
The Water Service Line
A building is connected to a water source by means of a water service line. This can be piping between a building and a privately-owned well; or, it can be piping between a building and a water main in a municipal or community water distribution system. The longer that water stays in the water service line, the more likely it is that microorganisms will form biofilms on the pipe walls. This can lead to microbiologically influenced corrosion of the water service line with the effect of increasing metal concentrations in drinking water or creating leaks through holes in the pipe wall. Microorganisms from the biofilms can also be carried away by the passing water and taken into the building’s plumbing system to start new colonies.
Point of Entry Water Treatment
Water treatment is sometimes installed in buildings just as the water piping enters the building. This is called “point of entry” water treatment. Any water treatment device should be considered a critical location for microbiological growth in a water system for three reasons:
- Many water treatment devices include tanks that hold a large volume of water. Depending on water usage in the building, water might stay in a water treatment tank for an extended period of time. This long residence time in slow moving water allows for the growth of microorganisms.
- Along with the large volume of water in water treatment devices comes increased surface area. Many water treatment tanks are filled with particles that perform the water treatment, where the higher the surface area of the particle, the better the treatment. Examples of such particles are sand, ion-exchange resin beads, and granular activated carbon. The increased surface area gives great advantage to the attachment of microorganisms and the development of biofilms.
- Any disinfection in the water that enters most water treatment devices is removed in the device. The disinfection might be removed because that may be the purpose of the water treatment device, like granular activated carbon filters. It might also be removed because the disinfection is used up fighting microorganisms that have already taken up residence in the device. With the disinfection gone, downstream piping is no longer protected against the microorganisms that are carried out of the water treatment device in the flowing water.
Water softening is a type of water treatment device. It is typically installed farther downstream from the building’s point of entry, but all of the problems of microbiological growth discussed above affect water softeners.
In areas of the country where water high in calcium carbonate (also called “hard” water) is used, water softeners are needed to keep calcium carbonate out of hot water systems. This is because calcium carbonate can fall out of the water as solid particles that build up on heating surfaces. The energy required to transfer heat from the heating surface through the increasing layer of calcium carbonate and into the water becomes greater and greater. Therefore, it is more cost effective to remove the calcium carbonate before it enters the hot water system.
Unfortunately, if microbiological growth has occurred significantly in a water softener, the hot water system downstream receives microorganisms in the water and no disinfection to fight them.
Hot Water Systems
Hot water systems have some additional characteristics that encourage the growth of microorganisms. For example, if the storage tank is oversized for routine water usage, water is in the tank for an extended time, creating the conditions for microbiological growth. An oversized hot water storage tank is installed when there is an infrequent but large demand for hot water, such as for filling a large bathtub. When the tub is not in use, water has a long residence time in the tank making it prone to microbiological growth.
For large buildings, hot water recirculation systems are typically used so that hot water will be immediately received at each faucet, no matter how far away from the hot water storage tank. The recirculation system contributes to microbiological growth by adding more residence time and surface area to the plumbing system. It also helps to spread microorganisms from an infected site to other sites that have not yet been infected.
Point of Use Water Treatment
Water treatment devices at or near faucets are referred to as “point of use” water treatment. The same issues of microbiological growth can occur on those devices as discussed above. The good news is that there is little to no piping downstream of the device that can be affected by microbiological growth. The bad news is that the consumer is directly downstream of these devices. For this reason, the manufacturers’ instructions on equipment cleaning and replacement of filters must be followed closely.
Biofilms can form upstream and inside faucets as with any surface area in a plumbing system. Faucets and associated upstream plumbing that are reached a long time after the water has entered the building have a greater potential for microbiological growth. Faucets and associated upstream plumbing that are used infrequently also have a greater potential for microbiological growth.
In a plumbing system, the conditions of water stagnation, high surface area, and lack of disinfection contribute to the possibility that significant microbiological growth and biofilm formation can occur.
This article has discussed critical locations in a plumbing system where microbiological growth and biofilm formation tend to start, beginning at the water source and continuing through the water service line, point of entry water treatment, water softening, hot water system, point of use water treatment, and faucets. These locations are important to keep in mind as methods to prevent microbiological growth are discussed in future articles.
Abigail F. Cantor, P.E., Chemical Engineer
Process Research Solutions, LLC
PO Box 5593
Madison, WI 53705