Rainwater harvesting
Practical Action
The sand-charcoal-stone filter is often used for filtering rainwater entering a tank. This type
of filter is only suitable, however, where the inflow is slow to moderate, and will soon overflow
if the inflow exceeds the rate at which the water can percolate through the sand. Settling
tanks and partitions can be used to remove silt and other suspended solids from the water.
These are usually effective, but add significant additional cost if elaborate techniques are
used. Many systems found in the field rely simply on a piece of cloth or fine mosquito mesh
to act as the filter (and to prevent mosquitoes entering the tank).
Post storage filtration include such systems as the upflow sand filter or the twin compartment
candle filters commonly found in developing countries. Many other systems exist and can be
found in the appropriate water literature.
Sizing the system
Usually, the main calculation carried out by the designer when planning a domestic RWH
system will be to size the water tank correctly to give adequate storage capacity. The storage
requirement will be determined by a number of interrelated factors. They include:
local rainfall data and weather patterns
size of roof (or other) collection area
runoff coefficient (this varies between 0.5 and 0.9 depending on roof material and slope)
user numbers and consumption rates
The style of rainwater harvesting i.e. whether the system will provide total or partial supply
(see the next section) will also play a part in determining the system components and their
size.
There are a number of different methods used for sizing the tank. These methods vary in
complexity and sophistication. Some are readily carried out by relatively inexperienced, first-
time practitioners while others require computer software and trained engineers who
understand how to use the software. The choice of method used to design system
components will depend largely on the following factors:
the size and sophistication of the system and its components
the availability of the tools required for using a particular method (e.g. computers)
the skill and education levels of the practitioner / designer
Outlined below are two different methods for sizing RWH system components.
Method 1 : demand side approach
A very simple method is to calculate the largest storage requirement based on the
consumption rates and occupancy of the building.
As a simple example we can use the following typical data:
Consumption per capita per day, C = 20 litres
Number of people per household, n = 6
Longest average dry period = 25 days
Daily consumption = C x n = 120 litres
Storage requirement, T = 120 x 25 = 3,000 litres
This simple method assumes sufficient rainfall and catchment area, and is applicable in
areas where this is the situation. It is a method for acquiring rough estimates of tank size.
Method 2 : supply side approach
In low rainfall areas or areas where the rainfall is of uneven distribution, more care has to be
taken to size the storage properly. During some months of the year, there may be an excess
of water, while at other times there will be a deficit. If there is enough water throughout the
year to meet the demand, then sufficient storage will be required to bridge the periods of
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