Page 338 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch09_297-332.qxd 8/2/10 9:54 PM Page 298

298 Chapter 9 Cross-Connection Control
Cross-connections are the links through which it is possible for contaminating materi-
als to enter a potable water supply. The contaminant enters the potable water system when
the pressure of the polluted source exceeds the pressure of the potable source. The action
may be called backsiphonage or backflow. Essentially it is reversal of a hydraulic gradient,
and it can be produced by a variety of circumstances.
One might assume that the steps for detecting and eliminating cross-connections
would be elementary and obvious. The reality, however, is that cross-connections can
appear in many subtle forms and in unsuspected places. Reversal of pressure in the water
may be freakish and unpredictable. The probability of contamination of drinking water
through a cross-connection occurring within a single plumbing system may seem remote,
but considering the multitude of similar systems, the probability is great. Cross-
connections exist for these reasons:
1. Plumbing is frequently installed by persons who are unaware of the inherent
dangers of cross-connections.
2. Oftentimes, connections are made as a simple matter of convenience without re-
gard to dangerous situations that might be created.
3. Connections are made with reliance on inadequate protection such as a single valve
or other mechanical device.
To combat the dangers of cross-connections and backflow connections, education in
their recognition and prevention is needed. First, plumbing installers must know that
hydraulic and pollutional factors may combine to produce a sanitary hazard if a cross-
connection is present. Second, they must realize that reliable and simple standard backflow
prevention devices and methods are available that can be substituted for convenient but
dangerous direct connections. Third, plumbing installers must understand that the hazards
resulting from direct connections greatly outweigh the convenience gained from a quick
and direct connection.

9.2 PUBLIC HEALTH SIGNIFICANCE OF CROSS-CONNECTIONS

Public health officials have long been aware of the threat to public health that cross-
connections represent. Because plumbing defects are so frequent and the opportunities
for contaminants to invade the public drinking water through cross-connections so gen-
eral, enteric illnesses caused by drinking water may occur at most any location and at
any time.
The following documented cases of cross-connection problems illustrate and empha-
size how cross-connections have compromised water quality and public health.

9.2.1 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 freshwater
supply. The chief plumbing inspector had received a telephone call advising that blood was
coming from drinking fountains within the funeral home building. Plumbing and county
health department inspectors went to the scene and found evidence that blood had been cir-
culating 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, in-
spectors were sent into the neighborhood to check for possible contamination.