Environmental Engineering Reference
In-Depth Information
Backflow Prevention Devices
Backflow-prevention devices are designed to prevent backflow, which is the
reversal of the normal and intended direction of water flow in a water sys-
tem. Backflow is a potential problem in a water system because it can spread
contaminated water back through a distribution system. For example, pollu-
tion or backflow at uncontrolled cross-connections (any actual or potential
connection between the public water supply and a source of contamina-
tion) can allow pollutants or contaminants to enter the potable water sys-
tem. More specifically, backflow from private plumbing systems, industrial
areas, hospitals, and other hazardous contaminant-containing systems into
public water mains and wells poses serious public health risks and security
problems. Cross-contamination from private plumbing systems can contain
biological hazards (such as bacteria or viruses) or toxic substances that can
contaminate and sicken an entire population in the event of backflow. The
majority of historical incidences of backflow have been accidental, but grow-
ing concern that contaminants could be intentionally backfed into a system
is prompting increased awareness among private homeowners, businesses,
industries, and areas most vulnerable to intentional strikes; therefore, back-
flow prevention is a major component of water system protection.
Backflow may occur under two types of conditions: backpressure and back-
siphonage.
Backpressure
is a reversal of normal flow direction within a piping
system resulting from the downstream pressure being higher than the supply
pressure. These reductions in the supply pressure occur whenever the amount
of water being used exceeds the amount of water supplied, such as during
water-main flushing, fire fighting, or breaks in water mains.
Backsiphonage
is a
reversal of normal flow direction within a piping system caused by negative
pressure in the supply piping (the reversal of normal flow in a system caused by
a vacuum or partial vacuum within the water supply piping). Backsiphonage
can occur due to high velocity in a pipeline, a line repair or break that is lower
than a service point, or a lowered main pressure caused by a high water with-
drawal rate, such as during fire fighting or water-main flushing.
To prevent backflow, various types of backflow preventers are appropriate
for use. The primary types of backflow preventers are
• Air gap drains
• Double check valves
• Reduced pressure principle assemblies
• Pressure vacuum breakers
Biometric Security Systems
Biometrics involves measuring the unique physical characteristics or traits of
the human body. Any aspect of the body that is measurably different from
person to person—for example, fingerprints or eye characteristics—can serve
