32
HEALTH AND SANITATION
Rainwater harvesting: design,
installation and maintenance for dual
systems and back flow prevention
The option of “dual-supply” systems is an option worth
exploring for residences which 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
DESIGN AND INSTALLATION
Select the type of dual system
• Automatic top-up system
• Manual top-up system
• No make-up system
Plan the layout of the top-up system
a. A top-up system is generally composed of the
following:
i. water level sensors located in the rainwater storage tank;
ii. s olenoid valve located on the potable water supply pipe;
iii. an air gap;
iv. top-up drainage piping conveying make-up water to
the rainwater storage tank; and
v. electrical conduits containing wiring from water level
sensors and pumps.
A commercial rainwater
harvesting and UV
filtration system.
b. Determine the location of the solenoid valve and air gap in
accordance with the guidelines provided below.
c. Plan route of top-up drainage piping from the air gap
to the tank.
d. Plan route of electrical conduits from the location of the
solenoid valve and power supply to the tank.
e. Contact the municipality and service providers to ensure
that there are no buried service lines (gas, electricity,
water, stormwater, wastewater, phone or cable lines) in the
area where digging will take place to accommodate the
buried top-up drainage piping and/or the electrical conduit.
June 2019 Volume 25 I Number 4
Water level sensors
a. Select the appropriate water level sensors for the RWH
system (float switch, ultrasonic level sensor or other).
b. Float switches
o Select the type of float switch.
• Solenoid valve actuation is typically provided by
an N/C float switch, for top-up systems.
• Pump dry run protection is typically provided by
an N/O float switch.
o Electrical requirements
• The voltage rating of the float switch must
match that of the device it controls (120V or 240V).
• The power rating (watts [W] or horsepower [HP])
of the float switch must be sufficient to carry the
total load of the device it controls or, alternatively,
float switches may be low voltage and used to
activate the pump through relays in a control panel.
• Spliced electrical wiring must be watertight and its
electrical rating is determined by the loads handled
by the float switch and the total length of wiring.
• All electrical connections for float switches
must be made by a licensed electrician in
accordance with the manufacturer’s instructions.
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