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HEALTH AND SANITATION
Rainwater harvesting:
dual systems and
backflow prevention
‘Dual-supply’ systems are an option worth exploring for
residences that cannot collect enough rainwater. In such
systems, potable harvested rainwater is supplemented with
water from a public water system with the appropriate
backflow prevention.
By Water Research Commission
In four of the five South African rainfall regions (winter,
early summer, mid-summer, late summer and very late
summer regions), rainfall occurs seasonally, requiring a
large storage capacity to hold enough water collected
during rain events to last through the dry spells.
Table 1: Advantages and
disadvantages associated
with top-up methods
Therefore, harvested rainwater can be a supplemental
water source in urban and peri-urban areas where
customers are already connected to the public water
supply infrastructure. There are two general dual systems
options available:
• Top-up: The rainwater storage tank can be partially
filled, either manually or automatically, with make-
up supplies of water from municipal (potable) or
private water sources.
• Bypass: The rainwater supply from the pressure
system can be shut off, either manually or
automatically, and water from municipal or private
sources can be directed through the rainwater
pressure piping. This option will not be further
explored, because a non-potable water system is
connected to a potable water system.
A rainwater harvesting (RWH) system can be topped up
manually or automatically. Each has advantages and
disadvantages as per Table 1.
APPLICABLE STANDARDS AND GUIDELINES FOR
DUAL SYSTEMS AND BACKFLOW PREVENTION
The applicable standards found in the South African
National Standards (SANS) are listed as per Table 2.
CONTROL EQUIPMENT
Control equipment used in make-up water systems consist
of water level sensors and valves (shut-off and solenoid).
Water level sensor
A device placed inside the tank to sense water level.
The basic systems only indicate the water level while
the more sophisticated systems can control (turn on
or off) warning lights, solenoid valves and/or pumps,
based on water level
Shut-off valve
A valve manually opened (or closed) to allow (or prevent)
the flow of water. Integrated into the RWH pressure
system to manage waterflow and isolate components of
the make-up system (for example, solenoid valves and
backflow preventers).
May 2019 Volume 25 I Number 3
www.plumbingafrica.co.za