Cross Connections and Backflow Prevention - NCCEH · Cross Connections and Backflow Prevention...

Cross Connections and

Backflow PreventionDeneen Spracklin, P.Eng.

Coordinator, Operator Education

Department of Environment and Conservation

What is a Cross Connection?A cross connection is defined as an actual or potential connection between a potable water supply and a non-potable water

Cross connections can exist anywhere within the distribution system. Some common examples include:

Wash basinsHose bibs (i.e. garden hose connected to a pesticide sprayer, filling a swimming pool, etc.)Auxiliary water suppliesIrrigation sprinkler systems

Why Do Cross Connections Exist?Plumbing is frequently installed by persons who are unaware of the inherent dangers of cross connections

Connections are made as a simple matter of convenience without regard to the dangerous situation that may be created

Inadequate protection on a known cross connection

Cross Connection ContaminationThe following conditions must be present for contamination to occur through a cross connection:

A cross connection exists between a potable and non-potable water source

The pressure in the distribution system either becomes negative (back siphonage) or the pressure of the non-potable source exceeds the pressure of the potable system (back pressure)

The cross connection is not protected, or it is protected and the equipment has failed

Cross Connections - Back Pressure

Back Pressure causes a reversal of normal flow direction in a system due to an increase in the downstream pressure above that of the potable supply pressure

Elevated plumbing/tanks

Pressure vessels

Back Siphonage causes a reversal of normal flow direction in a system because of a negative pressure (vacuum or partial vacuum) occurring in the system

Pressure in the water main is less than zero and foreign material is “sucked” into the system

Back Siphonage can occur under the following situationsLarge water main breaksWater main repairsWater system is shut down and drainedPeriods of high flow (i.e. fire fighting)

Cross Connections – Back Siphonage

Negative PressureNo system should be operated under vacuum under normal circumstances such as fire flows or flushing

During shutdown of the system however hydrants located on high points can be opened to allow air into the pipes while draining, which will prevent some if not all areas of vacuum in the system.

Negative Pressure in the System

There is a vacuum in the pipe where the hydraulic grade line drops below the pipe

Water Source

Ground Profile

Hydraulic Grade Line

Static Head

Waterline

Potential Cross Connection

Potential Cross Connection

Potential Cross Connection

Water service

Sewer Line

Water line connection to the sewer in order to run water to prevent freezing

Vacuum causes Backflow

Potential Cross Connection

Potential Cross Connection

Well

Town Water Supply

Pump

60 psi 40 psi

Backsiphonage Due to Large Leak

Cross Connections and Associated Health Effects

A potable water system can be cross connected with various types of non-potable sources

IndustrialCommercialResidential

These non-potable sources can introduce biological and chemical contaminants that can lead to illness and death

It is believed that the number of reported health impacts is a small percentage of the total experienced

Cross Connections and Reported Outbreaks

From 1981 to 1998, the Centre for Disease Control (US) documented 57 waterborne disease outbreaks related to cross connections resulting in 9,734 illnesses:

20 outbreaks (6333 illnesses) caused by microbiological contamination

15 outbreaks (679 illnesses) caused by chemical contamination

22 outbreaks (2722 illnesses) where the contamination was not reported

ChemicalPesticidesMetals (copper, chromium, etc.)Synthetic and Volatile Organic Compounds (i.e. ethylene glycol, propane, freon, etc.)Nitrates and nitrites (i.e. fertilizers)

BiologicalShigellaE.coliSalmonellaCyanobacteriaNorwalk virusGiardia

Cross Connection Contaminants

The following are some of the potential contaminants that can impact a cross connection

Reported Backflow Incidents

1981 – chlordane and heptachlor were back siphonedthrough a garden hose submerged in a termite exterminator’s tank truck

1982 – ethylene glycol back siphoned from an air conditioning system’s water holding tank into a group of dialysis machines, contributing to the death of several patients

1982 – during purging of a propane gas tank using a water hose connected to a hydrant, a backflow of propane gas occurred due to the pressure of the gas

1984 – during the shutdownof a water main to repair a valve, the backflow of water from a nursing home’s boiler caused burns to a employee of the water department

Reported Backflow Incidents

1990 – 1100 people at a country club became ill with an intestinal disorder after consuming contaminated water supplied from an unauthorized unprotected auxiliary well in close proximity to a malfunctioning sewage pumping station

1995 – pesticides were back siphoned into a distribution system when an accidental water main cut occurred while a farmer was diluting herbicides in a tank

1997 – a fire truck pump created backpressure on a fire hydrant before the valve was closed, forcing fire-fighting foam into the potable system

Reported Backflow Incidents

Other Impacts of Backflow IncidentsOther problems that can be associated with backflow incidents include:

Corrosion of distribution system materialsMicrobial and biofilm growthTaste, odour and colour problems

Contamination from cross connections does not only impact the area where the cross connection is located. The contamination can spread both upstream and downstream of the initial impact.

Corrective actions may need to be taken following a backflow event to minimize the health impacts (i.e. flushing)

Funeral HomesWastewater Treatment PlantsHospital or Medical ClinicSeniors Home

Fish PlantsGreenhousesLaundriesDry CleaningAny many others

High Risk Establishments

Some high risk establishments you may find in your community:

Cross Connections - Degree of HazardThe degree of hazard is a function of both the probability that backflow may occur and the toxicity of the contaminant involved

High or severe hazard – a situation where the introduction of waterborne disease organisms, harmful chemicals, or other contaminant that presents an unreasonable risk to health

Low or minor hazard – the backflow situation would cause aesthetic problems with the potable water supply

Cross Connection Prevention Methods and Devices

There are a number of methods/devices to prevent backsiphonage and backpressure:

Air GapAtmospheric Vacuum BreakerPressure Vacuum BreakerDouble Check Valve AssemblyReduced Pressure Zone Devices

The degree of hazard, or potential risk determines the proper method or device to be used

Air GapAir gaps are non-mechanical, and provide the best means of backflow prevention where it is possible to use themAir gaps cannot be used without interrupting pressure flow conditions, so their primary use is at the end of the line

Air Gap Requirements

A proper air gap must provide a

separation of 2 pipe diameters, but can never be less than

25 mm (1 inch)

Atmospheric Vacuum BreakerAtmospheric Vacuum Breakers consist of a check valve that seals against an atmospheric vent when the water supply is turned onIf a negative pressure develops in the supply line, the loss of pressure permits the check valve to drop which seals the orifice, and at the same time the vent opens allowing air to enter the system to break the vacuum

Atmospheric Vacuum Breaker

Atmospheric Vacuum Breaker UseAtmospheric vacuum breakers may be used only where the device is:

never subjected to backpressureis not under continuous pressureis installed on the discharge side of the last control valveinstalled above the usage point

Atmospheric Vacuum Breakers are acceptable for low hazard situations

Once installed, AVBs can not be tested

Atmospheric Vacuum Breaker

Pressure Vacuum BreakerPressure Vacuum Breaker (PVB) evolved from the Atmospheric Vacuum Breaker in response to a need to have a device that could operate under continuous pressure and was testable

The PVB has a spring loaded check valve that will close tightly when water pressure drops to zero

A PVB must also have 2 shut off valves, and 2 test cocks for testing the unit

The PBV also has an air relief valve that opens when pressure drops to 1 psi to break siphon

Pressure Vacuum Breaker

Pressure Vacuum BreakerPressure Vacuum Breakers may be used only where the device is:

Not subjected to back pressure; prevents instances of back siphonage

Installed above the usage point (15 to 30 mm)

Installed in a location that is considered a low hazard

Pressure Vacuum Breaker

Double Check Valve AssemblyThe Double Check Valve Assembly consists of:

two spring-loaded, independently operating check valves

two shut-off valves (upstream and downstream of the check valves)

four test cocks for testing of the assembly

This assembly is suitable for protection against either backsiphonage or

backpressure, and is used in low to medium hazard situations

Double Check Valve Assembly

Normal Flow

Conditions

Reduced Pressure Zone DevicesA Reduced Pressure Zone device consists of:

two spring-loaded, independently operating check valves separated by a spring-loaded differential pressure relief valvetwo shut-off valves (upstream and downstream of the check valves)four test cocks for testing of the assembly

Reduced Pressure Zone Devices

Provides maximum protection against back siphonage and back pressure

They are used in situations where the hazard is considered high

As a “device”, the RP is the best choice where physical separation (air gaps) are impractical

During normal operation the pressure between the two check valves is maintained at a lower pressure than the supply pressure

Reduced Pressure Zone Devices

Reduced Pressure Zone Devices Operation

If downstream pressure increases, check valve 2 closes, preventing backflow

If an obstruction prevents valve 2 from closing completely, pressure in the central zone will increase, and the relief valve will open and flow will be discharged

If supply pressure drops below the minimum differential to open valve 1, pressure in the central chamber drops to atmospheric. If inlet pressure drops to less than atmospheric, the relief valve will stay open to discharge any water in the central chamber.

Reduced Pressure Zone Backflow Preventers

Reduced Pressure Zone Backflow Preventers

Reduced Pressure Zone Backflow Preventers

Reduced Pressure Zone Backflow Preventers

Reduced Pressure Zone Devices Relief Valve Discharge

Malfunction of either valve 1, 2 or 3 will result in the discharge of water through valve 3. No attempt should be made to plug this discharge, as this will disable the protection. Test, and make necessary repair ASAP.

A properly functioning device will periodically discharge small amounts of water due to fluctuation in line pressure

Because of the potential for discharge from the relief valve, this device should be installed in a location with adequate drainage….and an air gap

Backflow prevention devices need to be tested to ensure that they are operating correctly and providing protection

Generally, testing is required on an annual basis

Testing must be conducted by a certified tester

Any malfunctions should be corrected ASAP

Backflow Prevention Devices Testing and Repair

Backflow Prevention Devices Testing

Backflow prevention devices may have to be installed in parallel, especially for facilities where a water supply shutoff is unacceptable (i.e. hospitals, manufacturing processes)

Cross Connection Control ProgramsCross connections are difficult to control without a good cross connection programCross connection control programs may involves the following

Authority to implement and enforce a programPublic education programsTraining for operators/personnel that will be identifying cross connections, or installing and inspecting prevention equipmentRecord keeping and reportingInstalling and testing prevention devices

Questions

Contact Information:

Deneen Spracklin

(709)729-1158

dspracklin@gov.nl.ca

Check out the Operator Education, Training and Certification Section Webpagehttp://www.env.gov.nl.ca/env/Env/waterres/Template_OTEC.asp#mark