Keeping a plumbing system free of Microorganisms

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Keeping Plumbing system free of microorganisms
by Abigail Cantor, P.E. (Chemical Engineer)

This is the third article in a series about the possibility of microorganisms growing in plumbing systems.  The first article warned that with high residence time, high surface area, and no disinfection, microorganisms can grow out-of-control in plumbing systems.  The second article provided a tour of a plumbing system, pointing out where and why microorganisms are likely to grow.

This article describes three actions that can prevent microorganisms from growing out-of-control in a plumbing system:

  1. Flushing
  2. Disinfecting
  3. Monitoring

Flushing Pipes


Unfortunately, while flushing of pipes with fresh water will keep the population of microorganisms down, it is not always practical for plumbing systems in buildings.  Flushing requires a large quantity of water, especially for buildings with a long, complicated piping system and a large volume of hot water storage.

In addition, a high velocity of water is required to remove biofilms from pipe walls once they are attached.  It may be impossible to reach these cleansing velocities if pipe diameter and tank volumes in the plumbing system are large or the water pressure is too low.  A high velocity, also, cannot be maintained in the multiple bends and branches of a building’s plumbing system.  Even if a scouring velocity can be achieved, some biofilms continue to adhere firmly to pipe walls.

Table 1 shows the flow rate of water required to achieve a flushing and scouring velocity of 6 feet per second.

Table 1.  Flow Rates of Water at a Scouring Velocity of 6 Feet per Second

Nominal Pipe Diameter (inches)

Internal Pipe Diameter (inches)

Flow at Scouring Velocity (gpm)


























  1. 1.     This table uses the internal diameter of Type L copper pipe
  2. 2.     Flow Rate = Velocity x Cross-Sectional Area of Pipe
  3. 3.     Unit conversion used in calculation: 12 inches = 1 foot
  4. 4.     Unit conversion in calculation: 448.8 cubic feet per second (cfs) = 1 gallon per minute (gpm)

Disinfecting Water


An alternative to flushing with a large volume or a high velocity of water is flushing with a low volume of disinfected water.   There are a number of types of disinfecting chemicals that can be used, but a common chemical for drinking water is sodium hypochlorite which provides chlorine as the active disinfecting ingredient.  Another name for sodium hypochlorite is bleach, which can be purchased at grocery and hardware stores.  It is important to purchase household bleach that has no additives along with the sodium hypochlorite.  In household bleach, about 5.7% of the product is active chlorine.  Even though this sounds like a small amount, the concentration of active chlorine in the bleach is very high —   about 62,700 mg/L chlorine.

Many municipal and community water systems use liquid sodium hypochlorite or gaseous chlorine to disinfect the drinking water before it enters the distribution system.  Enough chlorine is added to maintain a disinfecting concentration all the way to the farthest locations in the distribution system.  Disinfecting in a building that receives chlorinated drinking water would boost the existing chlorine concentration or replenish the chlorine concentration after on-site water treatment devices have removed the municipal disinfection.

Some municipal water systems use a combination of chlorine and ammonia to create monochloramine for disinfection.  Monochloramine is not as strong a disinfectant as chlorine and must be used at a higher concentration to achieve a similar disinfecting power as free chlorine.  In a building that receives chloraminated drinking water, household bleach can still be used for boosting or replenishing disinfection.  However, if monochloramine or ammonia is present in the water, it will take more bleach to reach the desired active chlorine concentration than if only chlorine was present.

Active chlorine is referred to as “free” chlorine because it is not combined with other chemicals and is available to react with microorganisms.  “Total” chlorine refers to free chlorine plus chemical compounds where chlorine is combined with other chemicals.  When disinfecting with sodium hypochlorite, the concentration of free chlorine is a measure of disinfecting power.  When disinfecting with monochloramine, total chlorine is used to estimate the disinfecting power because chlorine is combined with ammonia, but the measurement of total chlorine also includes any free chlorine and any other compounds of chlorine that have formed in the water.  A better measure of disinfection power, in this case, is to directly measure monochloramine concentration.  However, the Total Chlorine test kit is more widely used as a convenience.

If it is desired to boost or replenish chlorine in a plumbing system, chemical injection equipment is needed.  Chemical injection equipment consists of a chemical feed pump with accessory valves for proper operation (Figure 1).  A multi-function valve, typically purchased with a chemical feed pump, provides for venting of air trapped in the suction line, pressure release, provision of required backpressure, and prevention of siphoning.  A foot valve at the end of the suction tubing in the chemical storage tank prevents backflow of chemical solution from the tubing back into the tank.  A chemical injector is a device with a ball check valve that connects into a threaded tee in the main pipeline and allows only forward flow of the chemical into the drinking water pipeline.  Additionally, a pulsation dampener and/or a static mixer (a pipe section with interior vanes) may be necessary for proper mixing of the chemical into the drinking water.  Automatic control of the chemical feed pump can be added by using a flow meter in the drinking water pipeline that sends an electric signal to the chemical feed pump to dose the chemical based on flow of the water.


Figure 1.  Typical Chemical Injection Equipment

To maintain disinfection throughout a plumbing system, sodium hypochlorite can be injected at critical locations where disinfection needs boosting or replenishing.  The article, “Critical Locations for Microbiological Growth in Plumbing Systems” explains that those locations are typically after certain water treatment devices.  For buildings fed by private wells, disinfection is also needed in the well water as it is discharged from the well pump.  Refer to the article for more detailed information as every plumbing system should be assessed individually for disinfection needs.

Care must be taken in selecting a proper dosage of chlorine for disinfecting plumbing systems.  Federal drinking water regulations demand that free chlorine concentration stay below 4 mg/L.  Many municipal and community water systems maintain a free chlorine concentration between 0.2 and 0.5 mg/L in the distribution system.  Some systems may go up to 1 mg/L.  Dosage of monochloramine in municipal water systems is typically between 1 and 3 mg/L total chlorine.  See Table 2 for a summary of typical disinfection concentrations.

Table 2.  Typical Disinfection Concentrations


Disinfection Concentration

Disinfectant Measured

Federal drinking water regulations Maximum of 4 mg/L allowable Free Chlorine
Chlorine disinfection of public water supplies Typically, 0.2 to 0.5 mg/L. Sometimes around 1 mg/L. Free Chlorine
Chloramine disinfection of public water supplies Typically, 1 to 3 mg/L Monochloramine;  Total Chlorine is used as an estimate
Chlorine disinfection of swimming pools 1 to 5 mg/L Free Chlorine
Conventional shock chlorination of wells 200 to 300 mg/L Free Chlorine


Chlorine can break down plastic components used in modern plumbing systems.  There is not well-documented information on the chemical compatibility of various plastics and chlorine, but when asked, many manufacturers of plastic components that come in contact with drinking water cite the drinking water regulation of 4 mg/L maximum.  However, the more exposure to higher chlorine concentrations over time, the shorter the life of the plastic components, so it is better to stay below 1 mg/L free chlorine.

Resin beads inside water softeners and ion exchange water treatment devices also break down over time in contact with higher chlorine concentrations.  Manufacturers will allow the resin to come in contact with 1 mg/L free chlorine for short time periods, but staying in the lower 0.2 to 0.5 mg/L range is better for the life of the beads.

The maximum allowable chlorine concentration in hot water systems is not known.  The higher temperatures push the chlorine to be more reactive and there is danger that the chlorine can corrode metals.  It is best to stay closer to 0.1 to 0.3 mg/L.

As was noted previously, the concentration of chlorine in household bleach is very high so only a small quantity of bleach is added to water to achieve free chlorine concentrations found in drinking water.  Table 3 lists the amount of bleach that should be added to water to reach drinkable concentrations.




Table 3.  Free Chlorine Concentrations and Volumes of Household Bleach (5.7% active chlorine) in 100 Gallons of Water

Free Chlorine Concentration (mg/L)

mL of Household Bleach in 100 gallons of Water

Ounces of Household Bleach in 100 gallons of Water





































Note:   To measure milliliters (mL) of bleach, purchase a container marked in milliliters, called a “graduated cylinder”, from a laboratory supply company.  Graduated cylinders come in various sizes with various precision of measurement, such as a 10 mL cylinder marked in gradations of 0.2 mL or a 25 mL cylinder marked in gradations of 0.5 mL.

Monitoring Water Quality


To prevent the growth of microorganisms in pipes, the goal is to provide just enough fresh water and just enough disinfection to continuously expose the pipes to a minimum of about 0.3 mg/L free chlorine in the water. Routine monitoring of chlorine concentrations throughout the plumbing system is necessary to determine if the pipes are getting the proper exposure to disinfected water.

Field test kits are available for measuring chlorine concentrations in water.  An example of a field test kit is the Pocket Colorimeter™ II from the Hach Company.  The test kit can measure both free and total chlorine concentration.  With sodium hypochlorite disinfection, free chlorine concentration should be measured.   With chloramine disinfection, total chlorine concentration should be measured as an estimate of monochloramine concentration.  The instructions for measuring the chlorine concentration come with the field test kit and are simple to follow.  These are relatively inexpensive, simple, and convenient tests that can routinely guide flushing and disinfection dosing in piping systems.

Unfortunately, the control of microorganisms is a little more complicated than merely maintaining a specific disinfection concentration.  The conditions in piping systems and water environments vary and microorganisms can still grow at disinfection concentrations that are typically effective elsewhere.  Monitoring of “microbiological activity” should be performed in addition to monitoring disinfection concentration.

The best method of monitoring for microbiological activity has not been determined at this time.  However, one test is typically used to estimate microbiological activity.  It is called Heterotrophic Plate Count (HPC).  The test should be performed by a commercial laboratory that uses a special nutrient in the incubation dish called R2A.  There are many laboratories that will perform an HPC test, but not all will use the R2A. Table 4 lists the criteria that the laboratory should use to run the test.   The tests are about $30 a sample but overnight transport to the laboratory should be considered in the overall cost. After finding a laboratory that performs these tests according to the criteria listed in Table 4, have the laboratory send sample bottles for HPC_R2A sampling in disinfected water.  To take HPC_R2A water samples from a plumbing system, the water in the plumbing system must sit stagnant for a minimum of six hours.  It is good to use a similar stagnation time each time.  When it is time to take samples, wipe the faucet or sample tap opening inside and out with an alcohol wipe, wiping off any excess alcohol.  Open the sample bottle carefully because it has been sterilized to prevent contamination of the water sample.  Do not touch the inside of the bottle, the rim of the bottle, or the inside of the cap.  Capture the first-draw stagnation water in the sample bottle and fill the bottle to the indicated line.  Cap and label the sample bottle.  Put the bottle on ice in a cooler and get the sample to the laboratory within twenty-four hours.

Table 4.  Laboratory Requirements for the HPC_R2A Water Analysis

HPC_R2A Criteria
HPC=Heterotrophic Plate Count
Send sampler sealed sterile bottles containing sodium thiosulfate to deactivate disinfection
For analysis, use R2A growth media in the incubation dishes
Incubation temperature range: 25 to 28 degrees Centigrade
Incubation period: 5 to 7 days
Run two dilutions: 1 mL sample per dish and 0.1 mL sample per dish
Run each dilution in duplicate


There is another test that has been found to be a better measure of microbiological activity than HPC.  It is a test that measures chemical compounds of metabolism (ATP) for organisms in the water.  A second generation method has been developed but is not readily available at this time in water testing laboratories.



Flushing of pipelines and disinfection of water are the available tools for preventing out-of-control microbiological growth in plumbing systems.  In order to determine the right amount of flushing balanced with the right amount of disinfection, water quality throughout the plumbing system must be monitored routinely.

This article describes the details of flushing and disinfecting plumbing systems and testing for chlorine and microbiological activity.  Future articles will use this information to suggest a practical approach for building and plumbing contractors for preventing out-of-control microbiological growth in modern plumbing systems.

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