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The dangers of backflow – solutions for public safety

If the proper precautionary measures are not taken, backflow can pose a serious threat to public health.  There are numerous documented cases of contaminants invading the public drinking water through cross connection/backflow problems, causing illness, diseases, and even death.

Mechanical backflow prevention devices and assemblies offer the best protection against cross connection hazards, and are now required by law.  Standards have been established regarding the function, manufacturing, installation, testing, and maintenance of backflow prevention devices and assemblies.

Part one: definitions

Backflow is the undesirable reversal of the flow of water or a mixture of water and other liquids, gases, or other substances into the distribution pipes of the potable water supply.  There are two ways for backflow to occur. The first way that backflow can occur is back siphonage.  Back siphonage is the reversal of water flow due to a drop in the water pressure on the supply side.  The most common cause of back siphonage is a water main break.  (See illustration and observe the garden hose cross connection.)  Other back siphonage conditions exist when there is a negative pressure in the supply piping, allowing downstream substances to be siphoned into the potable water supply.  Undersized pipes and high withdrawal rates can create vacuums, which contribute to the occurrence of back siphonage.

The second cause of backflow is backpressure.  What is backpressure?  It is when the pressure in the downstream piping rises above the supply pressure of the system, allowing downstream substances to be pushed into the potable water supply.  Some common causes of backpressure include thermal expansion, pump systems and elevated tanks. The type of occupancy of the premises, the design and construction of the system, and the manner in which it is used are major influences on the possible incidence of backflow.  Consequently, the degree of the hazard to which persons may be exposed varies from discomfort and minor illness to fatal, if the backflow of contaminants into the potable water system is not completely prevented.  Due to the many variables in systems, devices of different performance characteristics are needed, each tailored to the system and its protection needs

Understanding cross connections

Prior to listing which industries are most commonly affected by backflow or examining in detail specific legendary cases of backflow, we must first understand thoroughly the concept of what a cross connection is.

By-pass arrangements, jumper connections, removable sections, swivel or change-over devices and other temporary or permanent devices through which or because of which backflow can occur are considered to be cross connections.  There is a risk to public health if there is a cross connection between your water supply and a contaminated source.  Cross connections can occur in a number of situations.  A few examples of cross connections where a backflow incident could occur are:

  • Metal processing or chemical plants, where metals in solution or chemicals used for production can come into contact with the water supply.
  • Commercial or residential gardening, where chemical injectors, irrigation systems and garden hoses are connected to the water supply.
  • When a hose is left running in a planter or a container with mixed chemicals such as fertilizers or pesticides.
  • Car wash facilities, where a connection between the scrubber and rinse cycle pipe allows recycled water into the public drinking water supply.

By far, the most common cross connection arrangement involves the use of a garden hose.  Cross connections occur at home just as much as they do in industry.  Through increased public awareness programs and education, all of us need to be more careful on how we use our garden hoses.  Never submerge garden hoses in swimming pools, chemicals, and car radiators and do not connect them to aspirators.

A number of different industries pose a potential risk to public health through a cross connection.  Examples of some of these industries include:

  • Chemical Plants
  • Insect/Pest Control Businesses
  • Floral/Garden Nurseries
  • Golf Courses
  • RV Parks
  • Metal Processing Plants
  • Paint Manufacturers
  • Laundromats
  • Naval Bases and Shipyards
  • Car Washes
  • Mortuaries
  • Agricultural/Fertilizer/Pesticide Facilities
  • Petrochemical Plants
  • Dairy and Food Processing Plants

Part two: case histories

Definite proof of the potential menace to public health caused by backflow is borne out by case histories of actual occurrences.  Some of the more prevalent diseases, illnesses and outbreaks that have been caused over the years by the incidence of backflow include:

1800-1940 – Typhoid, dysentery and amoebiasis

1900 – Infectious hepatitis

1976 – Legionnaire’s disease (Philadelphia)

1985 – Salmonella outbreak affecting 100,000 people in the Chicago area.

Public health officials have long been aware of the impact that cross connections and accidental backflow play as a threat to the public health.  Because plumbing defects are so frequent and the opportunity for contaminants to invade the public drinking water through cross connections are so general, enteric infections caused by drinking water may occur at most any location and at any time.

The following documented cases of cross connection/backflow problems illustrate and emphasize how actual cross connections have compromised the water quality and the public health in the United States.

Case #1: burned in the shower

A resident of a small town in Alabama  jumped in the shower at 5 a.m. one morning in October 1986, and when he got out his body was covered with tiny blisters.  “The more I rubbed it, the worse it got”, the 60-year-old resident said. “It looked like someone took a blow torch and singed me.”

He and several other residents received medical treatment at the emergency room of the local hospital after the water system was contaminated with sodium hydroxide, a strong caustic solution.

Other residents claimed that, “It (the water) bubbled up and looked like Alka Seltzer.  I stuck my hand under the faucet and some blisters came up”. One neighbor’s head was covered with blisters after she washed her hair and others complained of burned throats or mouths after drinking the water.

The incident began after an 8-inch water main that fed the town broke and was repaired.  While repairing the water main, one workman suffered leg burns from a chemical in the water and required medical treatment.  Measurements of the ph of the water were as high as 13 in some sections of the pipe.

Investigation into the cause of the problem led to a possible source of the contamination from a nearby chemical company that distributes chemicals such as sodium hydroxide.  The sodium hydroxide is brought to the plant in liquid form in bulk tanker trucks and is transferred to a holding tank and then pumped into 55-gallon drums.  When the water main broke, a truck driver was adding the water from the bottom of the tank truck instead of the top, and sodium hydroxide back-siphoned into the water main.

Case #2: human blood in the water system

Health Department officials cut off the water supply to a funeral home located in a large southern city after it was determined that human blood had contaminated the fresh water supply.  The chief plumbing inspector had received a telephone call advising that blood was coming from drinking fountains within the building.  Plumbing and county health department inspectors went to the scene and found evidence that the blood had been circulating in the water system within the building.  They immediately ordered the building cut off from the water system at the meter. City water and plumbing officials said that they did not think that the blood contamination had spread beyond the building. However, inspectors were sent into the neighborhood to check for possible contamination.

Investigation revealed that the funeral home had been using a hydraulic aspirator to drain fluids from the bodies of human remains as part of the embalming process.  The aspirator directly connected to the water supply system at a faucet outlet located on a sink in the preparation (embalming) room.  Water flow through the aspirator created suction that was utilized to draw body fluids through a hose and needle attached to the suction side of the aspirator.

The contamination of the funeral home potable water supply was caused by a combination of low water pressure in conjunction with the simultaneous use of the aspirator.  Instead of the body fluids flowing into the sanitary drain, they were drawn in the opposite direction into the potable water supply of the funeral home

Case #3: propane gas in the water mains

Hundreds of people were evacuated from their homes and businesses on an August afternoon in a town in Connecticut in 1982 as a result of propane entering the city water supply system.  Fires were reported in two homes and the town water supply was contaminated.  One five-room residence was gutted by a blaze resulting from propane gas bubbling and hissing from a bathroom toilet.  In another home, a washing machine explosion blew a woman against a wall.  Residents throughout the area reported hissing and bubbling noises coming from washing machines, sinks and toilets. Faucets sputtered out small streams of water mixed with gas and residents in the area were asked to evacuate their homes.

This near-disaster occurred when the gas company initiated immediate repair procedures in one, 30,000-gallon capacity liquid propane tank.  To start the repair, the tank was purged of residual propane by using water from one of two private fire hydrants located on the property.  Water purging is the preferred method of purging over the use of carbon dioxide since it is more positive and will float out any sludge as well as any gas vapors.  The purging consisted of hooking up a hose to one of the private fire hydrants located on the property and initiating flushing procedures.

Since the vapor pressure of the propane residual in the tank was 85 to 90 psi; and the water pressure was only 65 to 70 psi, propane gas backpressure backflowed into the water main.  It was estimated that the gas flowed into the water mains for about 20 minutes and that about 2,000 cubic feet of gas was involved.  This was approximately enough gas to fill one mile of an 8-inch water main.

Case #4: chlordane and heptachlor at the housing authority

The services to seventy-five apartments housing approximately three hundred people were contaminated with chlordane and heptachlor in a city in Pennsylvania in December 1980.  The insecticides entered the water supply system while an exterminating company was applying them as a preventative measure against termites.  While the pesticide contractor was mixing the chemicals in a tank truck with water from a garden hose coming from one of the apartments, a workman was cutting into a 6-inch main line to install a gate valve.  The end of the garden hose was submerged in the tank containing the pesticides, and at the same time, the water to the area was shut off and the lines were being drained prior to the installation of the gate valve.  When the workman cut the 6-inch line, water started to drain out of the cut, thereby setting up a back-siphonage condition. As a result, the chemicals were siphoned out of the truck, through the garden hose, and into the system, contaminating the seventy-five apartments.  Repeated efforts to clean and flush the lines were not satisfactory and it was finally decided to replace the water line and all the plumbing that was affected.  There were no reports of illness, but residents of the housing authority were told not to use any tap water for any purpose and they were given water that was trucked into the area by volunteer fire department personnel.  They were without their normal water supply for 27 days.

Case #5: heating system anti-freeze into potable water

Bangor Maine Water Department employees discovered poisonous anti-freeze in a homeowner’s heating system and water supply in November 1981.  The incident occurred when they shut off the service line to the home to make repairs.  With the flow of water to the house cut off, pressure in the lines in the house dropped and the anti-freeze, placed in the heating system to prevent freeze-up of an unused hot water heating system, drained out of the heating system into house water lines, and flowed out to the street.  If it had not been noticed, it would have entered the homeowner’s drinking water when the water pressure was restored.

Part three: solutions

Fire protection systems must be considered as non-potable systems due to the poor quality of water often found in them.  Listed below are several concerns the building owner and water purveyor should have with these systems.

  • The growth of offensive microorganisms, which can create taste and odor problems.
  • The leaching of metals such as zinc, cadmium, iron, copper, or lead into the water.
  • The addition of corrosion inhibitors, antifreeze, or other chemicals to protect systems.
  • Dry systems containing compressed air or nitrogen.
  • Systems which are constructed with unapproved or non-potable water piping or materials.
  • A loss of pressure on the potable water supply main, or an increase in pressure on the consumer’s system, which allows water from these systems to enter the potable supply.

If backflow from these systems should occur, the hazards will vary from a low health hazard or aesthetic concern to a high health hazard.

The purveyor may require a detector meter on the system to detect any unauthorized use or leakage within the system.  This is usually accomplished using a double check detector assembly, or reduced pressure detector assembly, depending on the degree of hazard determined by the purveyor.  A single detector check or single check valve is not considered an approved backflow preventer.

Most fire protection systems will have a fire department pumper connection.  In these cases, the pumper connection must be installed downstream of the backflow prevention assembly.

All fire protection systems that are connected to the water purveyor’s potable water system, either directly or indirectly on the property side of a potable water service, should be isolated with an approved backflow prevention assembly.  The level of backflow protection should be commensurate with the degree of hazard.

Degree of hazard

There are two factors to consider when determinating which method of controlling cross connections should be selected.  One factor is the type of cross connection involved: i.e., a direct type or an indirect type.  The other factor is the degree of hazard in other words, how serious a threat to public well-being is a particular material?  Degree of hazard may be that amount of threat to the health and well-being of the public.  It may be classified into three categories: lethal hazard, contaminant (high) hazard, pollutant (low) hazard.

  1. Lethal hazard is the highest degree we know.  Substances such as radioactive material and raw untreated sewage are in this class.
  2. Contaminant hazard is the next highest degree.  It involves an impairment of the quality of the water to an extent that creates a danger to public health through serious illness or death.  Materials such as chemicals and industrial fluids, which are poisonous, fall into this category.
  3. Pollutant hazard is a substance that would be objectionable in a water system, but not necessarily hazardous to health.  Examples of this would be edible food products, untreated steam and compressed air.

Protective methods and assemblies

Mechanical backflow prevention devices and assemblies offer the best protection against cross connection hazards, and are now required by law.  Per the Illinois plumbing code:

“All premises intended for human habitation or occupancy shall be provided with a potable water supply.  The potable water supply shall not be connected to non-potable water and shall be protected from backflow and back siphonage”.

Approval agencies, representing many diverse geographical areas and levels of government, have established performance criteria regarding the function, manufacturing, installation, testing, and maintenance of backflow prevention devices and assemblies.

The five recognized methods/assemblies for protecting against cross connection hazards are:

  1. Air gap may be used as protection for either type of cross connection, direct or indirect type, and for any degree of hazard.
  2. Atmospheric vacuum breaker may be used as protection for indirect type cross connections only, and for any degree of hazard.  This unit must be installed on the discharge side of the last control valve on a system so that it will not be subject to backpressures.
  3. Pressure vacuum breaker may be used as protection for indirect type cross connections only, and for any degree of hazard.  This unit must be installed so that it will not be subject to backpressure, but may be installed subject to continuous supply line pressures.
  4. Double check valve assembly may be used as protection for either type of cross connection, but it is limited for use in pollutant hazard situations.
  5. Reduced pressure principle backflow prevention assembly may be used as protection for either type of cross connection and it may be used in contaminant or pollutant hazard situations.

It is beyond the scope of this article to describe the operation of each of the five types of backflow prevention devices above.  Furthermore, the proper selection of the correct backflow prevention device is a complex process involving numerous variables. Of tremendous importance, however, is the need to select a qualified expert contractor, properly licensed, insured, and certified in cross connection control device inspections (CCCDI).  Scrutinizing your contractor’s safety performance record and checking their references are proven methods of ensuring a quality installation that will stand the test of time.

Furthermore, along with building ownership comes certain responsibilities and civic duties.  Not only is it the law to have backflow prevention devices installed in your building, the law requires that these devices be tested and certified at least once per year (Illinois Environmental Protection Agency) by an approved inspector.  Since most backflow prevention devices can be tested and certified for the nominal fee of just one or two hundred dollars, it makes no rational sense for a building owner to skip the annual test for economic reasons.  Building owners are advised to carefully consider the costs of defending a costly negligence lawsuit in the event of a backflow prevention device failure.  In this case, compliance with the law is a very small price to pay given the alternative.

Thomas Hartel is President of Valley Fire Protection Systems, located in Batavia, Illinois.  Valley is a full service, third generation fire protection contractor that also operates a backflow prevention division.  Mr. Hartel can be reached at (630) 761-3168.  The company maintains a website at

Credits:  Special thanks to Wilkins Backflow Preventers,  a Zurn Company.