Microorganisms: The effects of on-site water treatment
by Abigail Cantor, P.E.

This article is part of a series discussing the growth of microorganisms in plumbing systems.  As previously noted, many types of on-site water treatment equipment create conditions for microorganisms to grow and thrive by increasing the residence time of the water in the plumbing system with extra water storage volume, providing additional surface area for microorganisms to form biofilms on, and removing or using up any available disinfection to fight microorganisms.

The best practice in plumbing design is to provide on-site water treatment only when it is absolutely necessary to do so.  It is also important to select the appropriate treatment system, to determine a proper location for the equipment in the plumbing system, and to size the equipment so that volume and surface area are minimized.  Finally, automatic clean-in-place systems or manual cleaning protocols must be utilized to keep the equipment free of biofilms.

Determining Necessary Water Treatment

The first step in plumbing design is to determine specifically what contaminants, if any, are of concern in a building’s water source.  To do this, one must consider that there are numerous chemical compounds and types of microorganisms that can potentially contaminate drinking water.  Contaminants are identified and regulated in the United States with separate standards for municipal water systems, private water systems, and bottled water.  See Table 1.

If the building is connected to a public water system, the water has already been rigorously tested for the list of contaminants listed in Tables 2 and 3.  The results of those tests are public record, available at the regulatory agency that governs the state or territory where the building is located.  The results are also sent to each water utility customer annually in the Consumer Confidence Report.  There might be local issues to be concerned about, such as increasing concentrations of an industrial chemical in a public well.  In that case, the property owner should keep track of water utility plans to resolve the problem and attend water commission meetings, read the water utility website, or call the water quality manager.  If a building owner is not comfortable with the utility’s approach to removing the contaminant threat, then an on-site water treatment device should be used for removal.  It would be a rare and special case to need such a device.

It is possible for building plumbing systems to receive debris from municipal water distribution system piping.   Debris occurs in the distribution system when particles, like sand, settle out and when dissolved chemicals in the water, like manganese, iron, or aluminum, chemically precipitate out.  The possibility varies with the nature of the water and the water utility’s pipe cleaning and replacement program.  Debris can temporarily be entrained in the water during utility or road construction; it can happen seasonally due to water main flushing and other routine maintenance activities.  If a property owner experiences discolored water at an intolerable frequency, then on-site removal of the debris may be desired.

There is also the possibility that a building’s plumbing system leaches contaminants into the water.  Lead, copper, and iron are known to transfer from piping materials into water to varying degrees depending on characteristics of the individual water system.

If the property owner owns the water source, such as a private well, they must take the responsibilities of a water utility manager.  After complying with any state regulations on water quality for private water sources, the property owner must decide what other contaminants they might want to test for and, if significant, remove.  See Tables 2 and 3.  A common issue for private wells is high iron concentrations which can precipitate out and stain sinks and laundry.

For both private water sources and municipal water, the hardness of the water can be an issue.  Water hardness is mostly a measure of calcium and magnesium concentrations in the water.  Depending on other characteristics of the water, including temperature, the calcium and magnesium can precipitate out of the water as solid compounds.  The solids can cover heating surfaces in hot water heating systems, which in turn, will require more energy to heat the water; the buildup of solids will also reduce the life of the hot water heating tank.  For this reason, it is more economical to remove hardness from water where hardness is greater than about 120 mg/L as calcium carbonate (7 grains of hardness). Many people argue that hard water for cold domestic use should be softened; they state that hard water will clog pipes, create spots on glass shower doors, and react with soap so that it will not lather.  These are debatable arguments.  Even in locations with very high hardness (300 to 500 mg/L as calcium carbonate or 17.5 to 30 grains), cold un-softened water does not typically cause these severe problems.  Older residences in those locations only use softened water for the hot water system.  It is only recently, with modern plumbing practices, that cold water is also softened.

Determining Type of Water Treatment System

When specific contaminants have been identified in the water, then the proper contaminant removal technique can be selected.  The proper technique is the one that removes all contaminants of concern at the highest efficiency for the lowest financial and environmental costs.  Below are descriptions of common on-site treatment techniques.

Activated carbon filters

Activated carbon is carbon, typically from charcoal, that has been processed to make it very porous.  The more pores in the carbon, the higher the surface area.  The higher the surface area, the more specific contaminants can be pulled from the water to adhere to the carbon, a process called “adsorption”.

Different chemicals have different attractions to the carbon.  For example, heavier compounds have a greater attraction than lighter compounds.   This means carbon filters will not remove every type of contaminant.  In addition, the carbon can become saturated with contaminants and stop removing them.  Most importantly, just before the saturation point, the concentration of contaminants in the water flowing out of the filter begins increasing at a rapid rate.   Therefore, a carbon filter must be removed before the “breakpoint” of the least adsorptive contaminant or else the consumer will be drinking high levels of the contaminant that they intended to remove.  Filter manufacturers make assumptions as to what contaminants might typically be in water and set a common time when filters should be changed.   This may or may not be applicable in individual water systems.

Some carbon filters, such as certain ones that attach to sink faucets, are manufactured so that they combine treatment techniques within a small block of carbon.  Like other activated carbon filters, they have their limitations as to what contaminants can be treated and for how long.  In addition, the filters, themselves, can add contaminants to the water, based on compounds in the manufactured filter material.   There are research projects looking into this phenomenon.

Reverse osmosis and other membrane technologies

Reverse osmosis is a treatment technique that places a membrane barrier in the water.  The membrane is made of synthetic organic materials that do not have straight-through pores like a filter.  Instead, the pores are like a microscopic maze that can prevent many dissolved contaminants from passing through.  High pressure on the upstream side pushes the water, minus many of the contaminants, through the membrane.

There are other membrane technologies where the pores are straighter but very small.  Those technologies remove specific contaminants at lower pressures than reverse osmosis.

Membrane technologies prevent a percentage of the incoming water from going through the membrane, and instead, the water is sent down the drain to waste with the rejected contaminants.  The technique is not practical when it is too expensive to waste a percentage of the available water.  In addition, the synthetic organic membranes can dissolve in contact with some chemicals that might be in the water.  Chlorine used for disinfection is one of the chemicals and it is typically removed in a carbon filter upstream of the membrane.

Physical filters

Physical filters provide a physical barrier that can remove particles from water.  The filters can be made of sand or flossy material that will not allow particles of a certain size to pass through.

Ion exchange/water softeners

Ion exchange is a treatment process where one ion is taken out of the water and others are put into the water in its place.  An ion is an atom or molecule with either additional electrons or missing electrons; this gives the atom or molecule a negative or positive electric charge.   Water softeners are an example of an ion exchange process.  Here, calcium and magnesium, dissolved in the water as positively-charged ions, are “stuck to” negatively-charged ions.  When in contact with the ion exchange material, they are attracted and adhere to the material.  In exchange, the material releases two sodium ions for every calcium or magnesium ion; the sodium ions, then, form a union with the negatively-charged ions that were left behind in the water.  In the case of softening, the sodium concentration increases in the water.

At certain intervals, the ion exchange material must be cleaned to knock off the exchanged ions and replenish the original type of ions on the material.  In the case of water softeners, a solution of sodium chloride (brine) is used to flush out the ion exchange material.  This regeneration process creates a waste stream of chloride-laden water that is sent down the drain and out to the wastewater treatment plant.

Iron and manganese removal

Dissolved iron and manganese in the water eventually react with an oxidant like oxygen or chlorine in the water and precipitate out as a solid on pipe walls, sinks, and laundry.  To remove dissolved iron and manganese before it drops out elsewhere, an oxidant is pumped or bubbled into the water.  After a certain contact time, the iron and manganese are oxidized to a solid form and the particles are filtered out in a sand filter.

The filter must be backwashed periodically to clean the solids out and send them in a waste stream down the drain.

Sequestering

Sequestering is used to hold metals like iron and manganese in the water and prevent them from precipitating out as solids.  Traditionally, polyphosphate chemical products have been used in water systems to hold the metals in the water.  This is especially done when a small water utility or private property owner cannot afford a treatment process to remove iron and manganese.

It is now known that the use of polyphosphates carries negative side effects.  The polyphosphates not only can hold iron and manganese in the water but can also pull lead, copper, and iron from pipes and hold those metals in solution as well.  The consumer drinks any concentration of the metals being held in the water.  Polyphosphates also provide an essential nutrient, phosphorus, for the growth of microorganisms and in doing so, can aid in biofilm formation.  Finally, as the phosphorus from the polyphosphates eventually flows to waste, the wastewater treatment plants struggle with meeting stringent phosphorus discharge limits.

Disinfection

This series of articles on the growth of microorganisms and the formation of biofilms in piping systems has emphasized that disinfection of water is a main weapon against microorganisms.  With a clean piping system, it typically only takes a low dose of disinfection (0.3 to 0.5 mg/L free chlorine) to fight off intruding microorganisms and keep the piping system clean.

When on-site water treatment systems remove or use up disinfection in the incoming water, a dosing system should be added to replenish the disinfection in the water.  For owners of private water sources, continuous disinfection of the water before and after any treatment should be considered.

Some people complain about the taste of chlorine in their water.  If that is in issue, then drinking water can be left in a big pot open to the air or with cheesecloth covering it to allow the chlorine to transfer from the water into the air.  Alternatively, chlorine can be removed by a carbon filter at the drinking water faucet.  (Refer to the carbon filter discussion above.)

A more serious negative effect of disinfection is the possible formation of carcinogenic disinfection by-products.  This can occur when the water has high naturally-occurring organic carbon compounds that react with the chlorine.  If water is received from a municipal water system, disinfection by-products are tracked and minimized by regulation (Table 2); the property owner should re-chlorinate water within the concentration boundaries of the municipal utility.  For private water sources, the owner should become familiar with the disinfection by-product forming potential in the water and chlorinate accordingly.

Determining the Location of the Water Treatment System

Treatment systems located at the point in the plumbing where the water enters the building is called a point-of-entry water system.  Treatment systems located at the drinking water faucet are called point-of-use systems. The location of on-site water treatment in a building’s plumbing system is a critical design decision.

Point-of-entry systems treat the complete water flow to the building and are subsequently larger in size than point-of-use systems.  This creates a greater possibility of biofilm formation in the treatment equipment from increased surface area and retention of water.  It also increases the volume of water needed to clean and maintain the treatment system.  Many point-of-entry systems remove or use up incoming disinfection and so all of the piping downstream of the treatment system is not protected from the growth of microorganisms unless a disinfection dosing system is added.

Although smaller with little or no waste streams, point-of-use treatment systems must be installed for every drinking water faucet, while point-of-entry systems are installed at one location only.

Water treatment equipment for specific needs, such as water softeners for hot water, should be located as close to the specific need as possible.  Water softeners are typically located in a mechanical room adjacent to the water heating system.

Sizing Water Treatment Equipment

The goal of proper modern plumbing design should be to minimize volume of water retained and surface area of the treatment equipment.  The larger the size of the water treatment equipment, the more volume of water is retained on-site and the more surface area is available for biofilm formation.    As already discussed, one way to minimize volume is to eliminate treatment equipment unless absolutely necessary.  In addition, the volume of water to be treated should be carefully considered.  Divide the estimated total water use into: water for drinking/cooking, water for cleaning, and water for any significant purpose such as filling large bathtubs.  Design separate pipelines and treatment strategies for each purpose.

Cleaning Water Treatment Equipment

Various types of water treatment systems have cleaning cycles.  Sand filters must be backwashed to remove trapped solids.  Ion exchange material must be backwashed to remove solids and must be regenerated to replace ions.  Water treatment of this type has automatic clean-in-place cycles.  The cleaning water can be chlorinated to disinfect and fight developing biofilms routinely.  It is critical to work with the equipment manufacturer in setting up a cleaning water disinfection system; chlorine in too high a dose can destroy the treatment material.

Filters that require replacement of filter cartridges should be changed before or at the time recommended by the manufacturer to prevent breakthrough of contaminants and the development of biofilms.

Summary

This article continues the series warning against the growth of microorganisms and the formation of biofilms in plumbing systems.  On-site water treatment systems can contribute to the growth of microorganisms by increasing  the retention time of water, increasing the surface area where biofilms can form, and by removing or using up disinfection in the water.

The first step in plumbing system design is to determine which contaminants in the water are essential to remove on-site.  In many buildings that receive municipal water, additional water treatment is not necessary.  There is a greater need for on-site water treatment when the building is served by a private water source where the property owner must manage their own personal water utility.  There are also special needs for water treatment such as the need to soften hard water before it enters a hot water heating system.

After it is determined what contaminants must be removed, the best removal system must be selected.  Every treatment system has advantages and disadvantages and has specific removal efficiencies for each individual contaminant.   The sizing of the water treatment equipment and its location in the plumbing system are also critical design choices affecting the growth of microorganisms.

Finally, all water treatment equipment must be cleaned and disinfected or filter cartridges replaced routinely to clean out and prevent the formation of biofilms.

Maintaining a high water quality, including the elimination of microbiological growth, is a very delicate balancing act that should be given the highest priority when designing a plumbing system.

Responsibilities in preventing microbiological growth in plumbing systems
by Abigail Cantor P.E. (Chemical Engineer)

This article describes the responsibilities in preventing microbiological growth from occurring in plumbing systems.  Those responsibilities are shared by the Plumbing Designer, the Building Contractor, the Municipal Water Utility Manager, and the Property Owner.

The Plumbing Designer

The prevention of microbiological growth in plumbing systems starts with the plumbing designer.  Two new design requirements must be met in order to prevent microbiological growth – the volume of water residing on-site must be minimized and the surface area that the water contacts must be minimized.  These design requirements must be balanced against the property owner’s list of desired plumbing fixtures and the plumbing code’s design criteria.

Considering the Number of Fixtures and Pipe Sizes

The property owner may have a long list of desired plumbing fixtures and showers with multiple sprays.  The plumbing code will base the size of the piping on the number of fixtures that can possibly operate at the same time.  The property owner needs to understand that there is a trade-off between having extra plumbing fixtures versus minimizing the volume of water stored in the piping.  The use of diverter valves as well as the property owner’s acceptance of fewer fixtures may assist in keeping pipe sizes smaller.

Considering Water Conservation Fixtures

Water conservation fixtures are becoming a necessity in some areas where drinking water resources are scarce.  With both new plumbing systems and the modification of existing plumbing systems, the lower water usage must be considered and the on-site storage of water be reduced accordingly.

Considering Tubs

The property owner’s selection of bath tubs and hot tubs will greatly influence the quantity of hot water that needs to be prepared and stored on-site.  The property owner needs to understand the trade-off between the number of tubs and the filling rate of each tub versus minimizing the volume of water stored on-site.

Tankless water heaters can be used to assist in lowering the volume of hot water that needs to be stored.  Acceptance of lower filling rates by the property owner will also help to lower the volume of hot water storage.

Considering Water Softening

With hard water, water softening is required before sending water into a hot water system.  Calcium carbonate from hard water will scale up the heating surfaces in the hot water system which will, in turn, increase the quantity of energy and the cost to heat the water.  The life of the hot water heating equipment will also be reduced with the build-up of scale.

However, water softeners increase both the volume of water stored and the surface area that the water is exposed to on-site.  They can become incubators for microorganisms.  If less hot water is required, then less water needs to be softened and the smaller the softener tank.  Again, the property owner’s acceptance of lower filling rates for tubs will lower the hot water volume requirement.

In addition, cold water does not need to be softened except in a few specific cases.  Water softeners can be smaller when only the water for the hot water system is softened.

Water softeners can also be outfitted with the dosing of chlorine and other biofilm-fighting chemicals to the brine tank so that disinfection of the media inside the softener tank can occur routinely and prevent the development of biofilms inside the tank.

Considering Other Water Treatment

A previous article pointed out that water should not be treated on-site unless absolutely necessary.  The article listed possible contaminants in water and steps to take in deciding whether or not removal of certain contaminants is necessary.

For every treatment device, water volume and surface area is increased in the plumbing system.  Each device must have a means of routine cleaning and disinfection, as was discussed with water softeners, in order to prevent them from becoming incubators for microorganisms and inoculating the downstream piping.

 

Considering Chemical Dosing into Piping

Water softening and other on-site water treatment devices can remove existing disinfection from the water.  Long residence times for water such as in hot water storage tanks also deplete the disinfection in the water.  Designers of commercial buildings where large volumes of water are required need to consider re-injecting chlorine or other biofilm-fighting chemicals approved for potable water after water treatment devices and before storage tanks.  A previous article described chemical dosing systems.

Some property owners, such as those with private wells, do not want to introduce disinfection because of its taste and smell in the water.  Disinfection chemicals can also form harmful by-products if dosed incorrectly.  But chemical disinfection, especially chlorine, is the most significant protection against the growth of microorganisms in modern water systems because of its disinfecting power at a reasonable cost and its ability to remain in the water to the far reaches of the plumbing system.

Considering Pipe Flushing Features

If high-volume, complicated plumbing systems are inevitable in a building, automatic pipe flushing should be considered.  Automatic valves can be placed at the ends of long pipe runs to move fresh water through the water line.  Such a valve needs to be slow-opening and closing so that water hammer does not occur as it does with a solenoid valve.  Also, this strategy wastes water which cannot be tolerated in areas where water resources are scarce.  Nevertheless, in some cases, automatic pipe flushing may be the only answer to keeping water fresh on-site.

Considering Sampling Taps

Small valves and faucets are inexpensive to install in pipelines.  Consider installing such taps before and after any water treatment or storage tank in the plumbing system so that water samples can be taken and the quality of water, as it changes throughout the plumbing system, can be monitored.  A previous article described critical locations in plumbing systems where microorganisms can grow.  This can be a guide for locating sampling taps.

The Building Contractor

 

The growth of microorganisms in plumbing systems begins during the building’s construction period when water pipes are filled with water.  The water usage, at that time, is not what the plumbing system was designed for and the water sits stagnant in the pipes for a long period of time.  Any disinfection in the water is quickly depleted.  Microorganisms can now thrive.  Building contractors should ensure and document that this does not occur on their watch.

Before a project begins, review the plumbing plans for the design features that have been discussed in these articles.  Every pipe run, water treatment device, and water storage tank has the potential for microbiological growth during the construction period.  Is there a way to monitor, flush, and disinfect each plumbing system feature during the construction period?  If not, work with the plumbing designer to provide the means to do so.

Before filling new pipes with water, make sure that they have been cleaned of debris and disinfected.

After filling new pipes with water, begin monitoring for and documenting chlorine concentration routinely and frequently.  A previous article describes how to do this with a relatively inexpensive field test kit.

Less frequently, monitor for microbiological activity.  A previous article described how to do this with water samples for Heterotrophic Plate Count (HPC) analysis.  Since that article was written, a better test has become commercially available.  It is a test for ATP and is described in a sidebar to this article.

Run treatment devices through their cleaning cycles on a routine basis during construction and refresh the water in piping and tanks.

The amount of plumbing system flushing can be reduced by dosing the water system with chlorine and biofilm-fighting chemicals used in potable water systems.  (See a second sidebar for a description of chemicals that can be used.)

Strategies to carry out these tasks in an economical fashion during construction need to be developed.  This is a new concept for the building industry, but it can and must be done based on the current knowledge about microbiological growth in water systems.

The Municipal Water Utility Manager

 

The role that microorganisms play in water systems is a new topic in the drinking water industry as it is in the plumbing industry.  Municipal water utility managers are just becoming aware that microorganisms and their biofilms can help to transfer piping metals like lead, copper, and iron to water.

Municipal water utility managers need to ensure and document that the water throughout the water distribution system stays properly disinfected and that microbiological activity is low.   Throughout the distribution system, managers need to:

• Monitoring for disinfection and microbiological activity
• Minimize water residence time (water age)
• Ensure a proper disinfection residual
• Perform uni-directional flushing on water mains to properly clean piping

Managers might want to refer to a Water Research Foundation report that demonstrates techniques to control water quality at the consumers’ taps (Project 4286: Distribution System Water Quality Control Demonstration).

Ironically, because of modern plumbing design practices, the growth of microorganisms can occur whether or not there is a high microbiological activity in the water entering a building.  However, water utility personnel should document that the municipal system did not contribute significantly to any building’s plumbing problem.

The Property Owner

 

Property owners need to understand the dilemmas in designing and installing modern plumbing systems as have been discussed in these articles.  That way, they can make better decisions in planning their buildings’ plumbing systems.

If a property owner uses a private well instead of municipal system water, that person must take on the responsibilities of a water utility manager.  They need to know what contaminants should be removed from the water and the best method of removal.  They also need to consider how to keep the water and the system disinfected.  They must monitor for disinfection and microbiological activity and ensure that their water system is operating safely.

Conclusions

 

Preventing the growth of microorganisms in plumbing systems is a new reality for modern plumbing systems.  There has not been a lot of experience in designing systems with minimal volume and surface area while still utilizing the new plumbing fixtures that are available today.  But, the plumbing industry can start by being aware of the microbiological growth issues and design dilemmas.  Eventually, people in the plumbing and construction industry will figure out how to accomplish this task economically and efficiently.

This pushes people away from techniques that they have always used and what they have always known to be true.  But, the industry must move in this direction or else the future will be filled with ruined plumbing systems in new buildings, angry property owners, and liability lawsuits for plumbing and building contractors.

This series of articles does not provide all the answers needed to tackle this problem; they were written to raise awareness of the problem and to encourage the plumbing profession to consider the repercussions of the volume of water stored on-site and the surface area that the water is exposed to.  The growth of microorganisms in plumbing systems can be prevented with everyone’s participation.

 

A New Method of Testing for Microbiological Activity

 

In a previous article, a method of measuring microbiological activity in water was presented.  It was a method to measure the number of heterotrophic bacteria colonies that can grow from a water sample (HPC analysis).  But, there are other types of bacteria in water besides heterotrophs.  Nevertheless, the HPC analysis was used because it was the only economical and commercially available test.

A new type of test now meets the economics of the HPC analysis.  It is a test for adenosine triphosphate (ATP), the energy molecule found in living organisms.  This test will measure the presence of any type of living organism in the water sample, not just heterotrophic bacteria.

Unfortunately, there are only a few laboratories and professors that have the equipment to run this test.  For example, only one small laboratory in Wisconsin can run the ATP test.  Nevertheless, it is worthwhile to pursue the use of this test by inquiring at commercial laboratories and by contacting major manufacturers of the equipment (http://www.luminultra.com/Industry-Applications/water-treatment.html).

New Chemicals to Clean Out Biofilms

Chlorine is the most common chemical used for disinfection of drinking water.  The chemical is effective in preventing biofilms from forming.  However, once the biofilms have formed, it is difficult for the chlorine to penetrate the biofilm and remove it from surfaces.  In addition, many modern plumbing materials cannot be exposed to high concentrations of chlorine, so dosages that can be used are limited.

There are new chemicals on the market that can penetrate and destroy biofilms.  For use in drinking water, the chemical must be approved by NSF International (http://nsf.org) as an acceptable additive.  When determining new chemicals to use for biofilm removal, look for the NSF approval and follow-up on references of organizations that have used the product.

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)
3/8	0.430	3
1/2	0.545	4
5/8	0.668	7
3/4	0.785	9
1	1.025	15
1-1/4	1.265	24
1-1/2	1.505	33
2	1.985	58

Notes:

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

Circumstance	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
0.10	0.6	0.02
0.20	1.2	0.04
0.30	1.8	0.06
0.40	2.4	0.08
0.50	3.0	0.10
1.00	6.0	0.20
1.50	9.1	0.31
2.00	12	0.41
2.50	15	0.51
3.00	18	0.61
3.50	21	0.71
4.00	24	0.82

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.

Summary

 

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.