HEALTH AND SANITATION
43
Rainwater harvesting: design,
installation, maintenance
Rainwater harvesting (RWH) system
design and maintenance require
consideration of various aspects that
may affect efficiency and the calculations
required in proper system installations.
By Water Research Commission
DETERMINE RAINWATER STORAGE TANK CAPACITY
a. To determine the appropriate rainwater storage tank capacity, two
methods are available:
i. RWH models: Several RWH models have been developed to
calculate the optimal storage tank capacity given site-specific
details, including rainfall time series, catchment area and
material, and rainwater demands (often a constant demand).
ii. Rainwater storage tank sizing tables: Tables of optimal rainwater
tank capacities will be generated using RWH models for the
various rainfall zones of the country.
b. When selecting a tank size, consideration must be given to the
unused volume at the bottom and top of the tank (dead storage),
which reduces the effective storage volume.
c. Sizing the tank is often done without reference to the RWH model or
the rainwater storage tank sizing tables.
i. Consider the unused volume (typically referred to as the ‘dead
storage’) when selecting tank size;
ii. If unknown, assume 10 to 20% of tank capacity will be dead
storage; and
iii. The collection losses from pre-storage treatment devices (refer
to rainwater quality and treatment for details).
The type of material used for the rainwater tank
is a function of the:
a. Placement;
b. Storage volume requirements; and
c. Connected rainwater fixtures and desired quality.
DETERMINE THE LOCATION OF THE RAINWATER
STORAGE TANK
a. Ensure the location allows for:
i. proper drainage of rainwater through the conveyance network
(refer to rainwater catchment and conveyance);
ii. proper drainage of make-up water through top-up drainage
piping (refer to dual systems and backflow prevention for
details); and
iii. proper drainage of rainwater from the storage tank into an
appropriate stormwater discharge.
b. Identify the area(s) where the tank can be located.
www.plumbingafrica.co.za
i.
Ensure the location is permitted by applicable national/
provincial/ local codes and regulations and relevant by-laws.
Consult relevant authorities for details.
ii. Ensure the location has sufficient space for access above and
around the tanks for inspection and maintenance.
iii. Rainwater tanks are generally gravity fed with the rainwater
from the collection surface, thus they must be located below
the harvesting surface area. If, however, this is not possible,
a small intermediate storage tank with a pump can be used
to collect the water and then pump it at high volume to the
rainwater storage tanks.
iv. Minimise exposure of above ground tanks to direct sunlight
by covering them or installing them under a roof or trees. It
prevents excessive water temperature fluctuations and extends
the life of the tank.
TANK ACCESS AND OPENINGS
a. Tanks shall be provided with an access opening.
b. Access openings shall be a minimum of 450mm to facilitate
installation, inspection, and maintenance of components in the
rainwater storage tank.
c. Access openings shall have drip-proof, non-corrosive covers.
d. Openings that are larger than 100mm shall have lockable covers.
INSTALLATION OF THE RAINWATER STORAGE TANKS
Consult the tank manufacturer’s installation instructions regarding
recommended ground work or structure where the tank will be placed
INSTALLATION OF COMPONENTS IN THE RAINWATER STORAGE TANK
a. Components installed in the tank might include:
i. A pump or pump intake;
ii. Water level sensors and/or other types of control equipment; and
iii. Electrical wiring for the pump and control equipment.
b. Entry into the rainwater storage tank for the purpose of installing
components within the tank is not recommended.
c. If entry into the rainwater storage tank is required, it shall be performed
in accordance with the South African Safety, Health, Environment and
Continued on page 45 >>
April 2019 Volume 25 I Number 2