HEALTH AND SANITATION
suitable backflow prevention device or a visible air gap.
To ensure that backflow does not take place, backflow
prevention devices (also called backflow preventers) are
required to be installed in conjunction with any domestic
rainwater harvesting system.
Cross-connections
Traditionally, a cross-connection is any actual or physical
connection between a potable water supply and any
source of non-potable liquid. When talking about rainwater
harvesting, a cross-connection is defined as any actual or
potential connection between municipal water supply and
harvested rainwater.
Risks of cross-connection are traditionally minimised
through:
• Sealed and pressurised pipe systems to prevent non-
drinking water from entering the water supply.
• Backflow prevention devices fitted to high-risk newly
developed or redeveloped properties. These devices
prevent the reverse flow of water from a potentially
contaminated source into the drinking water supply.
• Monitoring compliance with plumbing regulations,
designed to prevent and minimise backflow
contamination.
BACKFLOW PREVENTION
A backflow event occurs when there is a physical connection
between the potable water system and the rainwater
distribution system. When such a connection is present,
rainwater may unintentionally be drawn into the potable
water supply of the house, or drawn into the entire water
supply of a municipality causing contamination of the potable
water system. Backflow prevention measures should be
applied on two distinct levels:
• Zone protection — Backflow prevention device is
installed at the point of an actual cross-connection to
protect residents of the building from backflow.
• Premise isolation — Backflow prevention device is
installed on the potable water piping entering a building,
in case zone protection fails or in case of a future
unintentional or clandestine cross-connection. Serves to
protect users of the municipal system from backflow.
In addition to above listed backflow prevention measures, codes
and regulations require the separation of potable and non-
potable pipes and the labelling of non-potable plumbing pipes.
Air gap
An air gap is one of the simplest methods of preventing
backflow and involves a physical separation between two
sections of pipe that is open to the atmosphere. This physical
break prevents the backflow of water since, even if rainwater
backed up from the tank to the gap, it would spill from the
gap and not come into contact with the potable water supply.
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The air gap must be located higher than the overflow
drainage piping from the tank and the overflow drainage
piping must remain free of blockage so that excess
rainwater flows to the overflow system and does not back
up and overflow at the air gap. It also acts as a visual
inspection point to detect trickle top-up system failure.
Check valve
Check valves (clack valve, non-return valve, or one-way
valve) are mechanical valves that permit the flow in only
one direction, preventing process flow from reversing.
Water flows in the desired direction and opens the valve,
while backflow forces the valve closed. The mechanics of
check valve operation are not complicated.
Atmospheric vacuum breaker
In an atmospheric vacuum breaker, a poppet is pushed to
the upper seat when water flows through the assembly: the
air inlet valve is closed. The poppet is pulled to the lower
seat when the flow stops or when there is a back-siphonage
condition: the air inlet valve opens.
Spill-resistant vacuum breaker
Spill-resistant vacuum breaker is designed to prevent
contamination of the potable water supply due to back-
siphonage. It consists of one loaded check valve and one
loaded air inlet valve.
Double check valve
A double check valve or double check assembly is a
backflow prevention device designed to protect water
supplies from contamination. It consists of two check
valves assembled in series. This employs two operating
principles: firstly, one check valve will still act, even if
the other is jammed wide open. Secondly, the closure of
one valve reduces the pressure differential across the
other, allowing a more reliable seal and avoiding even
minor leakage.
Reduced pressure zone device
A reduced pressure zone device is a type of backflow
prevention device used to protect water supplies from
contamination. The device is similar to a double check
valve, with an additional relief valve between the two
loaded check valves. If one or both check valves are
fouled and cannot prevent backflow, the contaminant
or pollutant exits through the vent of the relief valve. A
reduced pressure zone assembly provides protection
against back-siphonage and backpressure and can be
used under continuous operation. PA
In the next issue, we look at the design and installation of
dual systems and backflow prevention.
May 2019 Volume 25 I Number 3