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HEALTH AND SANITATION
Rainwater harvesting
storage tanks
The storage container (cistern, tank) is often the most visible or
recognisable component of a rainwater harvesting (RWH) system
where the captured rainwater is diverted to and stored for later use
By Water Research Commission
The main purpose of the storage tank is to store water
that is safe to use, while preventing its access to
children or animals. There are several topics related
to storage containers and you should go through each
before making a decision on purchasing one.
There are a number of different RWH systems available
with a range of features, depending on the manufacturer.
These systems can be grouped into three basic types of
RWH systems:
• water collected in storage tank(s) and pumped
directly to points of use;
• water collected in storage tank(s) and fed by gravity
to points of use; and
• water collected in storage tank(s), pumped to an
elevated cistern and fed by gravity to the points of use.
All the systems listed above have a reservoir that is
used to store rainwater harvested from roof catchments
referred to as a rainwater storage tank.
Table 5.1: Standards
and codes applicable to
rainwater storage tanks
APPLICABLE STANDARDS AND GUIDELINES FOR
RAINWATER STORAGE TANKS
The applicable standards found in the South African
National Standards are listed in Table 5.1.
APPLICABLE CODES, STANDARDS
AND GUIDELINES SELECTED PROVISIONS AND DESIGN
AND INSTALLATION IMPLICATIONS
SANS 10252-1:2012
Water supply and drainage for buildings
Part 1: Water supply installations for
buildings. 5.4.6 Storage tanks
6.5 Storage tanks (6.5.1 and 6.5.2)
9.3.2 Storage tanks
SANS 10100-1:2000
Structural use of concrete
Part 1: Design Concrete reservoir
SANS 1200 G:1982
Concrete (structural) Testing of concrete reservoirs
November 2018 Volume 24 I Number 9
TANK CAPACITY
As a rule of thumb, the larger the tank, the greater the
volume of rainwater that can be collected and stored
during rainfall events (collection efficiency). However, this
is true only up to a certain point — after which other
factors, such as local rainfall patterns, roof catchment
area, and rainwater demand, will limit the amount of
rainfall that can be collected and utilised by the system.
Thus, for any RWH system with a given roof catchment
area, rainwater demands, and local rainfall patterns, the
storage capacity of the tank can be described as either:
• Too small — Much of the collected rainwater
overflows during rainfall events. Significant
improvements in collection efficiency can be
achieved with minor incremental increases in
storage volume.
• Optimum — Rainwater tanks in this range provide
the best balance between collection efficiency of
the RWH system and minimising its size and cost.
• Too large — Rainwater tanks in this range
rarely fill to capacity. A smaller tank can be
used without a significant drop in the collection
efficiency of the RWH system. An oversized
rainwater storage tank, however, may be
desirable if stormwater management is a strong
driver for installing an RWH system.
TANK SIZING
The correct sizing of a RWH tank is important in order
to avoid extra costs incurred when the tank is oversized
and for avoiding low efficiency when it is undersized. The
effectiveness of a RWH system depends on factors such
as the catchment size, rainfall variability, temporal and
spatial variation, the water demand, and the tank size.
Tank sizes have an important role since they dictate the
maximum amount of water that gets stored. However,
there must be other attributes that must be considered:
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