The history of backflow prevention is a story of public health incidents over the years, in which health officials and members of the plumbing industry learned the dangers and health hazards associated with cross-connections after investigating backflow incidents that sickened and killed many people. 

From the late 1800s to the early 1900s, plumbing evolved from drawing water from wells, heating it on a wood stove, bathing in a washtub and using an outhouse, to piping in municipal water from treatment plants and routing waste to wastewater treatment plants. Miles and miles of utility water distribution systems served premise plumbing and process piping distribution systems that became more complex over time, creating many opportunities for cross-connections between potable and nonpotable piping systems. 

Before backflow preventers existed, cross-connections often occurred during back-siphonage or back-pressure events in plumbing systems. When water was supplied to fixtures or tanks with toxic or hazardous chemicals and a backflow condition existed, people were sickened or killed. The level of illness typically depended on the degree of hazard posed by the cross-connection. 

The plumbing industry studied and learned about the science of fluid hydraulics and the phenomenon associated with backflow at cross-connections — specifically back-siphonage and back-pressure — and how contaminated water can be drawn or forced into potable water systems. 

Health officials, engineers and the plumbing industry went to work to find ways to prevent backflow. Many stories are told about contaminants flowing to other parts of the water distribution system, causing a chemical or biological exposure incident. These backflow and back-siphonage incidents will be covered more in part 2 of this column. In part 1, we will cover the history and terminology.

The original backflow protection methods were rudimentary methods, such as single check valves, used to prevent backflow from contaminated sources. As they soon learned, check valves can foul and still allow toxic contaminants to siphon back into a water system when conditions are right. So, the air gap was another method used before testable backflow preventers were developed. 

A historic cross-connection event occurred at the 1933 Chicago World’s Fair, one of the earliest major cross-connection incidents reported. A major amoebic dysentery outbreak occurred during the 1933 event, causing 98 deaths and thousands of illnesses. Most victims had visited the Chicago fair and stayed in two hotels that reportedly routed their domestic water through sewage tanks to recover heat from the waste and pre-heat the water. 

According to reports, faulty plumbing, either a leaky connection or a corroded heat exchanger pipe wall, in the water tubes that ran through the cooling tanks in the evaporative cooling towers, occurred in these two major Chicago hotels that used a cooling tank system (the Congress Hotel and the Auditorium Hotel, located on opposite sides of LaSalle Street on the West side of South Michigan Avenue). 

The cross-connection allowed amoeba to be drawn into and contaminate the potable water supply through an evaporative cooling tank system. The pipe in the cooling tanks were eroded or corroded over time and allowed contaminants from the tanks to be drawn into the domestic water system. 

A Chicago Bureau of Sanitary Engineering employee, Joel Connolly, is credited with identifying the defective plumbing as the root cause of the Chicago World’s Fair outbreak. In today’s systems, a double-walled heat exchanger would be required, with the intermediate space vented to the atmosphere, so that if a breach occurred in either wall, it would drip onto the floor and be noticed before an actual crossover flow or contamination occurs.

The 1933 World’s Fair incident served as a catalyst for developing new types of backflow preventers and, later, for requiring backflow protection in the codes. It raised awareness of cross-connections in buildings. Water utility distribution system operators were made aware of these cross-connections and backflow issues. 

Soldiers returned home, started families and purchased new homes. This spurred new construction and urban sprawl, which led to the rapid expansion of water supply and wastewater treatment systems. During this time, the growing complexity of plumbing systems created many opportunities for cross-connections. 

Backflow history timeline

• 1933: The Chicago World’s Fair backflow incident, as previously mentioned, was a catalyst for raising health and safety awareness of cross-connections and the importance of identifying potential hazards and preventing backflow incidents. 

• 1940s: Cities began requiring backflow preventers on high-risk water lines to protect public water supplies as public health officials realized that untreated or contaminated water could backflow into clean drinking supplies. Backflow prevention has evolved from simple check valves to a science of mechanical assemblies, air gaps and backflow assemblies meeting industry standards.

• 1943: During WWII, a supply ship in the Los Angeles harbor was found to have highly contaminated harbor water in its potable water tanks. An investigation revealed this dangerous hazard was caused by a physical cross-connection between the city’s drinking water supply and the ship’s nonpotable harbor water. The cross-connection incident prompted local health officials and research engineers at the University of Southern California to do further research.

That incident in Los Angeles Harbor led to the 1944 creation of the Foundation for Cross-Connection Control and Hydraulic Research in response to its research on backflow-related cross-connections and contamination incidents. It continued to educate the industry and provide research, testing and certification in the field of backflow prevention. 

The work at the foundation helped lead to the later development of testable backflow devices, such as the pressure vacuum breaker and the double-check valve backflow preventer assembly. During the post-WWII time, piping systems continued to grow in complexity and cross-connections occurred more frequently. With each incident and its resulting investigation, new methods or products were developed to prevent cross-connections.

• 1945: The International Association of Plumbing and Mechanical Officials (IAPMO) published the first edition of the Uniform Plumbing Code, helping standardize the installation of plumbing systems, including backflow prevention, through its basic principles.

• 1950s: The double-check valve assembly was invented and quickly became a primary testable backflow prevention assembly. It is called an assembly because it must have shut-off devices to work properly. 

The late John E. Matthews, PE, who served on many American Society of Sanitary Engineering (ASSE) committees, was a staunch advocate for calling them “assemblies” because an assembly of components works together and cannot be tested without all the components. Many early models were sold and installed without shut-off valves, which are essential for testing the device. Matthews fought hard to call them assemblies and have them tested and sold as assemblies. 

Today, we are experiencing a similar issue with mixing valves and other end-use faucets and fixture fittings. They need the inlet check valve to pass the cross-flow test, yet many are sold or supplied as an assembly. When they are sold without the inlet check valves, hot water can cross over into the cold water system. Many recent scald injuries and deaths have been attributed to manufacturers not selling or supplying check valves with their devices. 

• 1964: ASSE publishes the first backflow standard: ASSE 1001, Performance Requirements for Atmospheric-Type Vacuum Breakers. It has been working to make plumbing systems safe since a group of inspectors met in 1904 in the nation’s capital and formed the organization. Eventually, the reduced-pressure zone backflow preventer was developed for protection from high-hazard backflow applications. 

• 1973: The United States Environmental Protection Agency (EPA) is an independent federal agency of the United States government responsible for protecting human health and the environment by writing and enforcing regulations based on laws passed by Congress (such as the Clean Air Act and Clean Water Act). It released its first EPA Cross-Connection Control Manual, providing guidance to help states implement state-level backflow prevention safety measures.

• 1974: The Safe Drinking Water Act (SDWA) was enacted by Congress and signed into law by President Gerald Ford. It is the primary federal law ensuring the quality of Americans’ drinking water. It authorized the EPA to set national, health-based standards to protect against both naturally occurring and man-made contaminants in drinking water through enforcement at water treatment plants. 

• 1990s: The ASSE website, www.asse-plumbing.org, published the ASSE Series 5000, Professional Qualification Standards for Cross-Connection Control and Backflow Prevention. It was approved in 1991 and established as the first industry-consensus professional qualifications standard for plumbing and mechanical personnel, creating minimum requirements for testing and inspecting cross-connection control devices (backflow preventers). 

• 2023: ASSE published the fourth edition of IAPMO’s Backflow Prevention Reference Manual, which is now a joint publication with the United Association of Plumbers and Pipe Fitters (UA) Cross-Connection Control Manual. It is a collaboration between the UA and IAPMO through their Backflow Prevention Institute, headed by Sean Cleary. 

The reference manual covers every aspect of backflow prevention and cross-connection control, from identifying and testing the wide variety of backflow prevention assemblies to proper selection and installation of both testable assemblies and non-testable backflow prevention devices and methods. It is available through the IAPMO online bookstore at www.iapmostore.org.

Backflow prevention terminology 

1. Containment

Water utilities began to understand the hazards associated with cross-connections within buildings and started mandating backflow preventers at the building service entrances to protect public water mains from backflow. 

The concept of containment was to ensure that the public utility water distribution system was safe from contaminants within a customer’s premise plumbing system, where there could be unknown cross-connections. 

The American Water Works Association picked up on the containment concept to protect the public water supply. Requiring a backflow preventer at every building water service entrance eventually became a SWDA mandate for water utilities using containment to protect the public water supply from cross-connections, backflow or back-siphonage conditions within a customer’s property. 

2. Isolation

As new types of backflow prevention devices were developed for use with individual fixtures, and new industry standards were established for performance testing of these devices, plumbing codes began to require backflow preventers at each fixture or source of backflow. 

The location of each backflow preventer at each fixture or hazard is referred to as “isolation,” where each potential backflow or cross-connection hazard within a building is isolated to prevent a backflow or cross-connection incident within the building’s plumbing system. The intent is to isolate each hazard so that contaminants cannot flow back into the potable water system and emerge at another fixture. 

As different types of backflow preventers for specific applications were developed, they were added to the model plumbing codes in Chapter 6, Water Supply and Distribution. Each model plumbing code includes “Table 608.1 Application of Backflow Preventers.” The tables include columns that list the device, the degree of hazard, the application and the applicable standards. 

To determine the required device type, a hazard classification is performed to verify whether the potential cross-connection is high or low hazard. 

3. Back-siphonage backflow

Back-siphonage is the reversal of normal flow in a system caused by a negative pressure (vacuum or partial vacuum) in the supply piping. Vacuum breakers were developed to open under negative pressure, allowing air into the piping system to break the siphon. For this reason, the vacuum breaker must be located above the outlet by the prescribed distance required for the type of vacuum breaker. 

It can occur when a water main break or fire causes the pressure in the water main to drop below the pressure in a building’s water supply. Water will backflow or siphon back into the water main when negative pressure pulls contaminants upstream. Back-siphonage can be created when a low water supply pressure due to nearby firefighting operations causes a negative pressure in the water main, or when there are repairs or breaks in the city water main. 

The low upstream pressure is similar to sipping through a straw, causing a flow in the opposite direction. Water always flows from high pressure to low pressure. When the supply pressure drops significantly, downstream pressures can be higher than the upstream pressures, leading to a reversal of flow while the upstream pressure is low. 

4. Backpressure backflow

This is the reversal of normal flow in a system due to downstream pressure exceeding the supply pressure. This is common in high-rise buildings when a pump fails or a check valve is fouled. 

Back-pressure backflow conditions are created whenever the downstream pressure exceeds the upstream supply pressure, which is possible in installations such as boilers or water heating systems when they heat up and the water expands. This is a result of thermal expansion. It can also occur in elevated tanks and pipe risers in high-rise buildings or in large buildings with pressure-boosting pump systems. 

If a single check valve leaks or a bypass valve is opened, or if the downstream pressure exceeds the upstream pressure at pressure-reducing valves with internal reliefs, backflow can occur. Water always flows from high pressure to low pressure.

5. Cross-connections

A cross-connection is a direct piping arrangement that allows the potable water supply to be connected to a line containing a contaminant. Some examples are: 

• A garden hose attached to a hose outlet with the end of the hose lying in a pool of contaminated liquid or other liquid contaminants. 

• A garden hose attached to hose valve connections on two different piping systems. 

• A garden hose connected to the faucet outlet of the service sink, with the end of the hose submerged in a sink full of detergent or cleaning chemicals.

• A hose or piping connection from bottom-fed tanks connecting directly to a pumper truck or other piping system. 

• Water supply lines connected to boilers without proper backflow prevention.

Ironically, the ordinary garden hose is the most common offender in cross-connections and backflow incidents because it can be easily connected to the potable water supply and used for a variety of potentially dangerous applications.

6. Continuous pressure

Continuous pressure occurs when a backflow preventer is used where the pressure is supplied for more than 12 hours at a time. 

Intermittent pressure, such as that supplied to a lab faucet vacuum breaker, is acceptable for a nonpressure, atmospheric vacuum breaker because the supply is periodically turned on and shut off. An atmospheric vacuum breaker on a hose valve should never be subjected to continuous pressure unless the backflow device is rated for the continuous pressure type and clearly identified for this service. 

Pressure-type vacuum breakers have a small spring that holds them open with a small amount of force, which is easily overcome by city water pressure, keeping them closed. If a downstream valve and the vacuum breaker are subjected to continuous pressure, the spring in the pressure vacuum breaker should open when city water pressure is lost.

7. Pollution or pollutant

This is a specific type of contaminant that degrades water quality to the point where it becomes hazardous to human health or the environment.

• Origin. Almost always linked to human activities, such as industrial discharge, agricultural runoff or poor sewage management.

• Hazards. Pollutants pose a proven biological, chemical or physical risk to users.

• Examples. Heavy metals (such as lead), pesticides, pharmaceuticals and toxic chemicals.

8. Contaminant or contamination

The EPA’s SDWA broadly defines contamination or a contaminant as any physical, chemical, biological or radiological substance in water. 

• Origin. It can be naturally occurring (such as arsenic leaching from bedrock) or introduced by humans.

• Safety. Many contaminants are harmless at low levels and do not affect potability.

• Examples. Trace minerals, harmless sediment or naturally occurring calcium and magnesium. A nonhazardous-to-human-health contaminant could be backflow from a food contaminant, such as lemon-lime flavor from a beverage processing facility.

Basic methods or devices used for backflow protection

The five basic methods/devices:

Air gap (method);

Atmospheric vacuum breakers (includes hose-type vacuum breakers);

Pressure vacuum breakers; 

Double-check valve assembly;

Reduced-pressure zone assembly.

Next month, we will continue with the types of backflow prevention devices, some examples of backflow incidents and what is typically included in a backflow prevention ordinance.