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HEALTH AND SANITATION
Most rainwater system tanks are sized too small, resulting in a lot
of overflow, which is an inefficient method of collecting rainwater.
Catchment area
The size and nature of the catchment area
determine the amount of rainfall that can be
harvested. The run-off coefficient is defined
as a dimensionless value that estimates the
portion of rainfall that becomes run-off, taking
into account losses due to spillage, leakage,
catchment surface wetting, and evaporation.
According to the performance of a RWH
system, it is sensitive to the run-off coefficient
value only for small tank sizes.
Rainfall variability
The efficiency of a RWH system is largely
affected by the distribution patterns of rainfall.
The optimum size of RWH is likely to differ
in South Africa’s five rainfall regions; all year,
winter, early summer, mid-summer, late
summer, and very late summer regions
Water demand
It is the actual volume of water extracted from
the tank for various uses at a given time. When
optimising RWH system, scholars usually use
a single figure to express the potable water
demand. This is not a reflection of the actual
water demand, which varies throughout the
year. Moreover, no good correlation has been
established between the tank capacity of a RWH
system and the fixed daily potable water demand.
Therefore, a rainwater tank must never be sized
according to potable water demand only.
Reliability
Reliability is the probability that a system
can meet the expected demand. It can be
divided into two types: time-based reliability
and volumetric reliability. The former is the
probability that a reservoir will be able to meet
a certain demand on a specific time interval,
while the latter is the ratio of the amount of
water supplied to the total water demand
during the simulation period. The reliability
of the rainwater tank is very important for
domestic water conservation as it indicates the
ability of the tank to satisfy the demand of the
household on a given day.
Temporal resolution
Time intervals affect the quality of the simulation
results. The use of monthly rainfall data to
calculate the storage of a RWH tank results
in an underestimation of the required storage
capacity because it overlooks the temporal
distribution of rainfall. Hourly or daily time series
are said to provide a more accurate simulation of
system performance than monthly time intervals.
However, daily time steps have found more
preference in sizing RWH tanks because sub-daily
time series data are generally unavailable.
Tank sizes
The storage tank is the most expensive
component of a RWH system. The storage
capacity of the tank dictates the maximum
amount of water that can be stored. Most RWH
systems in the country are installed using a
rule of thumb law that does not involve proper
sizing; as a result, the RWH tanks installed are
either oversized or undersized tanks.
Storage tank optimisation is an important but
often overlooked design step of RWH systems.
Before the most cost-effective proportion of the
area of a roof to be used for harvesting the water
that will be stored in a tank can be determined,
the relationship between roof area and storage
capacity must be outlined, along with the major
parameters such as release rule and reliability,
interval used in simulation, record length of
rainfall data, and run-off coefficient. The release
rule can either be yield before spillage (YBS) or
yield after spillage (YAS). In the YBS rule, the water
is abstracted for use before the inflow. This leads
to an underestimation of the required storage
volume. The opposite applies with the YAS rule,
which is more conservative and therefore
usually preferred. PA
In the following article, we look at methods used to size RWH systems.
www.plumbingafrica.co.za
November 2018 Volume 24 I Number 9