68
TECHNICAL
<< Continued from page 67
Figure 1: Suggested
alternate water supply
application.
scenario, then devices falling into the A/A type, which
incorporate an unrestricted air gap design, are required,
but this is very seldom the case.
MISCONCEPTIONS
There are many misconceptions when the topic
of backflow prevention is brought to the table for
discussion. Frequently, the argument given is that a
simple non-return valve or a dual non-return valve
installation can be used to prevent back siphoning
or backflow of polluted water. This is not true, since
there is no way of monitoring the positive seal or
performance of simple non-return valves, be they of the
spring loaded or flap type.
Note: A non-return valve falls under type EB, fit only to
prevent backflow of category 1 fluids.
NATIONAL REGULATIONS
National regulation SANS 10252 -1 (water supply
installations for buildings). As an interesting and
informative exercise, refer to 7.4 of SANS 10252-1,
‘Preservation of water and water quality’, where it
states that reduced pressure zone backflow preventers
must be installed in any installation where there is a
risk of contaminated water or harmful substances being
back-siphoned or flowing back into a potable water
supply line.
The regulations state that backflow preventers shall
be installed in certain types of installations and
buildings, for example medical treatment of people and
animals, pharmaceutical or chemical research, and
manufacturing, dairies, nurseries, stadiums, abattoirs,
combined fire systems and so on.
Go and research this!
April 2018 Volume 24 I Number 2
RISK ASSESSMENT
Given the potential dangers of the phenomenon and
the requirements of current regulations, the risk of
pollution caused by backflow must be assessed for the
type of system and the characteristics of the fluid to be
conveyed.
A suitable backflow prevention device must be selected
based on the outcome of the assessment performed
by the system designer. In the case where it is not
possible to clearly verify the risk level, it is necessary
to hypothesise the greatest risk, and specify and install
accordingly.
ALTERNATE WATER SUPPLY SOURCES
With many areas suffering severe droughts coupled
to the lack of water supply infrastructure upgrades,
many organisations and homeowners are seeking
alternate water supply sources, such as the sinking of
boreholes, rainwater harvesting, and recycling or use
of grey water.
Municipalities are working with them and setting
guidelines for this and have clearly communicated
directives and procedures of what is deemed safe and
what is not. For example, because of the many varying
qualities of water extracted from boreholes, they have
limited the use of borehole water to flushing of toilets
and irrigation (see Figure 1).
Alternate water systems are subject to City approval,
while groundwater and surface water use are subject to
Department of Water & Sanitation (DWS) licensing.
Due to the health risks, no alternative water may be
used for drinking, cooking, or body washing, and
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