cross connections and backflow prevention - ncceh · cross connections and backflow prevention...
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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
Check out the Operator Education, Training and Certification Section Webpagehttp://www.env.gov.nl.ca/env/Env/waterres/Template_OTEC.asp#mark