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Report A1 - Current Technology for Storing Domestic Rainwater (Part 1)
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Report A1 - Current Technology for Storing Domestic Rainwater (Part 1)
The work in this report forms the basis for the current DTU roofwater harvesting Web Site. The site can be
found at:
http://www.eng.warwick.ac.uk/DTU/rainwaterharvesting/index.html
The work in this document is on-going and will be added to as the programme progresses. The aim is to collect
examples of DRWH practice from around the world and to provide a useful resource for practitioners of DRWH.
Many of the graphics shown in the Web Site are not shown in this document due to electronic storage
requirement limitations. The report is in two parts so that the document remains manageable. This is Part 1.
Contents of report (Part 1)
Current Technology for Storing Domestic Rainwater (Part 1) ............................................................................. 2
1. An Introduction to Domestic Roofwater Harvesting..................................................................................... 2
2. Styles of Roofwater Harvesting .................................................................................................................... 2
3. Components of a DRWH system.................................................................................................................. 4
Introduction .................................................................................................................................................. 4
Storage tanks and cisterns............................................................................................................................. 4
Collection surfaces........................................................................................................................................ 6
Guttering....................................................................................................................................................... 6
First flush systems......................................................................................................................................... 9
Filtration systems and settling tanks ........................................................................................................... 11
4. Sizing the DRWH system ........................................................................................................................... 12
Method 1 ˆ¢’Ǩ’Äú demand side approach ............................................................................................................. 13
Method 2 ˆ¢’Ǩ’Äú supply side approach ............................................................................................................... 14
Method 3 ˆ¢’Ǩ’Äú computer model....................................................................................................................... 16
Further comments ....................................................................................................................................... 17
Tank efficiency and the case for diminishing returns.................................................................................. 17
System features (that affect tank sizing) ..................................................................................................... 18
Rainfall data................................................................................................................................................ 18
Possible methods for estimating likely future RWH system performance with a tank of given size........... 18
5. Reasons for the current interest in DRWH ................................................................................................. 18
6.Criteria for the analysis of DRWH systems................................................................................................. 21
7.Health and DRWH....................................................................................................................................... 22
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Current Technology for Storing Domestic Rainwater
1. An Introduction to Domestic Roofwater Harvesting
A sufficient, clean drinking water supply is essential to life. Millions of people throughout the world still do not
have access to this basic necessity. After decades of work by governments and organisations to bring potable
water to the poorer people of the world, the situation is still dire. The reasons are many and varied. The poor of
the world cannot afford the capital intensive and technically complex traditional water supply systems which are
widely promoted by governments and agencies throughout the world.
Roof-water or rainwater harvesting (RWH) is an option which has been adopted in many areas of the world
where conventional water supply systems have failed to meet the needs of the people. It is a technique which has
been used since antiquity. Examples of RWH systems can be found in all the great civilisations throughout
history. The technology can be as simple or as complex as required. In many African countries this is often as
simple as placing a small container under the eaves of the roof to collect falling water during a storm. One 20
litre container of clean water captured from the roof can save a walk of many kilometres, in some cases, to the
nearest clean water source. In the industrialised countries of the world, sophisticated RWH systems have been
developed with the aim of reducing water bills or to meet the needs of remote communities or individual
households in arid regions. Traditionally, in Uganda rainwater is also collected from trees, using banana leaves
or stems as temporary gutters; up to 200 litres may be collected from a large tree in a single storm. Many
individuals and groups have taken the initiative and developed a wide variety of different RWH systems
throughout the world.
It is worth bearing in mind, however, that Domestic Rainwater Harvesting (DRWH) is not the definitive answer
to household water problems. There is a complex set of inter-related circumstances which have to be considered
when choosing the appropriate water source. Cost, climate, technology, hydrology, social and political elements
all play a role in the eventual choice of water supply scheme which is adopted for a given situation. RWH is only
one possible choice, but one which is often overlooked by planners, engineers and builders. The reason that
RWH is rarely considered is often due to lack of information ˆ¢’Ǩ’Äú both technical and otherwise. In many areas
where RWH has been introduced as part of a wider drinking water supply programme, it was at first unpopular,
simply because little was known about the technology by the eneficiaries. In most of these cases the technology
has quickly gained popularity as the user realises the benefits of a clean, reliable water source at the home. In
many cases RWH has been introduced as part of an integrated water supply system, where the town supply is
unreliable or where local water sources dry up for a part of the year, but is also often used as the sole water
source for a community or household. It is a technology which is flexible and adaptable to a very wide variety of
conditions, being used in the richest and the poorest societies on our planet, and in the wettest and the driest
regions of the world.
The aim of this web site is to enable readers to view a wide variety of these systems, with the aim of providing
interested parties with a selection of possible technical solutions to their water problems. We try to provide
guidelines for the sizing of RWH systems, a brief overview of the components of a RWHS, a critique for
examining the systems with a mind to their possible application, and look at the cost of the system (or at least the
material requirements). We also look at ways in which water quality can be improved and maintained before,
during and after storage.
2. Styles of Roofwater Harvesting
User regimes
Rainwater harvesting is used in many different ways. In some parts of the world it is used merely to capture
enough water during a storm to save a trip or two to the main water source. In this case only small storage
capacity is required, maybe just a few small pots to store enough water for a day or half a day. At the other end
of the spectrum we see, in arid areas of the world, systems which have sufficient collection surface area and
storage capacity to provide enough water to meet the full needs of the user. Between these two extremes exists a
wide variety of different user patterns or regimes. There are many variables that determine these patterns of
usage for RWH. Some of these are listed below:
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ˆ¢’Ñ¢¬¶ Rainfall quantity (mm/year) ˆ¢’Ǩ’Äú the total amount of water available to the consumer is a product of the total
available rainfall and the collection surface area. There is usually a loss coefficient included to allow for
evaporation and other losses. Mean annual rainfall data will tell us how much rain falls in an average year.
ˆ¢’Ñ¢¬¶ Rainfall pattern - climatic conditions vary widely throughout the world. The type of rainfall pattern, as well
as the total rainfall, which prevails will often determine the feasibility of a RWHS. A climate where rain
falls regularly throughout the year will mean that the storage requirement is low and hence the system cost
will be correspondingly low and vice versa. More detailed rainfall data is required to ascertain the rainfall
pattern. The more detailed the data available, the more accurately the system parameters can be defined.
ˆ¢’Ñ¢¬¶ Collection surface area (m2) - this, where rooftop catchment systems are used, is restricted by the size of the
roof of the dwelling. Sometimes other surfaces are used to supplement the rooftop catchment area.
ˆ¢’Ñ¢¬¶ Storage capacity (m3) - the storage tank is usually the most expensive component of the RWHS and so a
careful analysis of storage requirement against cost has to be carried out.
ˆ¢’Ñ¢¬¶ Daily consumption rate (litres/capita /day or lpcd) - this varies enormously ˆ¢’Ǩ’Äú from 10 ˆ¢’Ǩ’Äú 15 lpcd a day in
some parts of Africa to several hundred lpcd in some industrialised countries. This will have obvious
impacts on system specification.
ˆ¢’Ñ¢¬¶ Number of users - again this will greatly influence the requirements.
ˆ¢’Ñ¢¬¶ Cost ˆ¢’Ǩ’Äú a major factor in any scheme.
ˆ¢’Ñ¢¬¶ Alternative water sources ˆ¢’Ǩ’Äú where alternative water sources are available, this can make a significant
difference to the usage pattern. If there is a groundwater source within walking distance of the dwelling (say
within a kilometre or so), then a RWHS that can provide a reliable supply of water at the homestead for the
majority of the year, will have a significant impact to lifestyle of the user. Agreed, the user will still have to
cart water for the remainder of the year, but for the months when water is available at the dwelling there is a
great saving in time and energy. Another possible scenario is where rainwater is stored and used only for
drinking and cooking, the higher quality water demands, and a poorer quality water source, which may be
near the dwelling, is used for other activities.
ˆ¢’Ñ¢¬¶ Water management strategy ˆ¢’Ǩ’Äú whatever the conditions, a careful water management strategy is always a
prudent measure. In situations where there is a strong reliance on stored rainwater, there is a need to control
or manage the amount of water being used so that it does not dry up before expected.
Ideally, we would like to be able to classify the various common user regimes that are adopted. This can help us
to develop a nomenclature for dealing with the systems we will look at later. We can simply classify most
systems by the amount of ˆ¢’ǨÀúwater securityˆ¢’Ǩ’Ñ¢ or ˆ¢’ǨÀúreliabilityˆ¢’Ǩ’Ñ¢ afforded by the system. There are four types of user
regimes listed below:
Occasional (or opportunist) - water is collected occasionally with a small storage capacity, which allows the
user to store enough water for a maximum of, say, one or two days. During the wet season this means that the
user will benefit considerably from having such a system and most, if not all, of the user needs will be met during
this time. After a day or two of dry weather the user will have to return to using an alternative water source. This
type of system is ideally suited to a climate where there is a uniform, or bimodal, rainfall pattern with very few
dry days during the year and where an alternative water sources is close at hand.
Intermittent ˆ¢’Ǩ’Äú this type of pattern is one where the requirements of the user are met for a part of the year. A
typical scenario is where there is a single long rainy season and, during this time, most or all of the user needs
are met. During the dry season an alternative water source has to be used or, as we see in the Sri Lankan case,
water is carted/ bowsered in from a nearby river and stored in the RWH tank. Usually, a small or medium size
storage vessel is required to bridge the days when there is no rain.
Partial ˆ¢’Ǩ’Äú this type of pattern provides for partial coverage of the water requirements of the user, during the
whole of the year. An example of this type of system would be where a family gather rainwater to meet only the
high-quality needs, such as drinking or cooking, while other needs, such as bathing and clothes washing, are met
by a water source with a lower quality. This could be achieved either in an area with a uniform rainfall pattern
and with a small to medium storage capacity or in an area with a single (or two short) wet season(s) and a larger
storage capacity to cover the needs during the dry season.
Full ˆ¢’Ǩ’Äú with this type of system the total water demand of the user is met for the whole of the year by rainwater
only. This is sometimes the only option available in areas where other sources are unavailable. Sufficient a/
rainfall, b/ collection area, c/ storage capacity is required to meet the needs of the user and a careful feasibility
study must be carried out before hand to ensure that conditions are suitable. In areas where there is a bimodal
rainfall pattern (i.e. two rainy seasons) this type of system is far more attractive, as the tank will be recharged
during both wet seasons. Where there is a single (unimodal) wet season the storage capacity will normally be
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very large ˆ¢’Ǩ’Äú and therefore expensive. A strict water management strategy is required when such a system is used
to ensure that the water is used carefully and will last until the following wet season.
3. Components of a DRWH system
Introduction
Technically, DRWH systems vary in complexity. Some of the traditional Sri Lankan systems are no more that a
pot situated under a piece of cloth or plastic sheet tied at its corners to four poles. The cloth captures the water
and diverts it through a hole in its centre into the pot. On the other hand, some sophisticated systems used in the
industrialised nations, incorporate clever computer management systems, submersible pumps, and links into the
grey water and mains domestic plumbing systems.
Somewhere between these two extremes we find the typical DRWH system that is used in a typical developing
country scenario. Such a system will usually comprise a collection surface, a roof, a storage tank, and guttering
to transport the water from the roof to the storage tank. Other peripheral equipment is sometimes incorporated:
first flush systems to divert the dirty water which contains roof debris after prolonged dry periods; filtration
equipment and settling chambers to remove debris and contaminants before water enters the storage tank or
cistern.
In this section we will look at the various components commonly found in typical DRWH systems. In the Case
Studies section we will look at actual systems. In the Case Studies section, where possible, we have tried to look
at full systems, showing how the various components interact. In some cases, however, we have been able to
show only certain components of the system ˆ¢’Ǩ’Äú usually the tank or cistern as this is the most costly and critical
component of the DRWH system, and the area that has attracts most design attention.
Figure 1 ˆ¢’Ǩ’Äú Typical domestic roofwater harvesting system ˆ¢’Ǩ’Äú showing the main components of the system
Storage tanks and cisterns
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The water storage tank usually represents the biggest capital investment element of a DRWH system. It therefore
usually requires the most careful design ˆ¢’Ǩ’Äú to provide optimal storage capacity while keeping the cost as low as
possible. The catchment area is usually the existing rooftop, and guttering can often be obtained relatively
cheaply, or can be manufactured locally.
There are an almost unlimited number of options for storing water. Common vessels used for very small-scale
water storage in developing countries include such examples as plastic bowls and buckets, jerrycans, clay or
ceramic jars, cement jars, old oil drums, empty food containers, etc.
For storing larger quantities of water the system will usually require a tank or a cistern. For the purpose of this
document we will classify the tank as an above-ground storage vessel and the cistern as a below-ground or
under-ground storage vessel. These can vary in size from a cubic metre or so (1000 litres) up to hundreds of
cubic metres for large projects, but typically up to a maximum of 20 or 30 cubic metres for a domestic system.
There is a mind-boggling range of options open to the prospective rainwater harvester, with a wide variety of
shapes, materials, sizes and prices on offer. The choice will depend on a
number of technical and economic considerations. Some of these are listed below:
ˆ¢’Ñ¢¬¶ Space availability
ˆ¢’Ñ¢¬¶ Options available locally
ˆ¢’Ñ¢¬¶ Local traditions for water storage
ˆ¢’Ñ¢¬¶ Cost ˆ¢’Ǩ’Äú of purchasing new tank
ˆ¢’Ñ¢¬¶ Cost ˆ¢’Ǩ’Äú of materials and labour for construction
ˆ¢’Ñ¢¬¶ Materials and skills available locally
ˆ¢’Ñ¢¬¶ Ground conditions
ˆ¢’Ñ¢¬¶ Style of RWH (see link to this section)
One of the main choices will be whether to use a tank or a cistern. Both tanks and cisterns have their advantages
and disadvantages. Table 1 summarises the pros and cons of each.
Tank (above ground)
Cistern (below ground)
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Pros
Above ground structure allows for
easy inspection for cracks or
leakage
Many existing designs to choose
from
Can be easily purchased ˆ¢’ǨÀúoff-the-
shelfˆ¢’Ǩ’Ñ¢ in most market centres
Can be manufactured from a wide
variety of materials
Easy to construct for traditional
materials
Water extraction can be by gravity
in many cases
Can be raised above ground level
to increase water pressure
generally cheaper
more difficult to empty by leaving tap on
require little or no space above ground
unobtrusive
surrounding ground gives support
allowing lower wall thickness.
Cons Require space
Generally more expensive
More easily damaged
Prone to attack from weather
Failure can be dangerous
water extraction is more problematic ˆ¢’Ǩ’Äú
often requiring a pump
leaks or failures are more difficult to
detect
contamination of the tank from
groundwater is more common
tree roots can damage the structure
there is danger to children and small
animals if tank cover is left off
flotation of the cistern may occur if
groundwater level is high and cistern is
empty
heavy vehicles driving over a cistern can
cause damage
Table 1. Pros and Cons of Tanks and Cisterns
Much work has been carried out on the development of the ideal tank for DRWH. The Case Studies section of
this Web Site show a wide variety of tanks that have been built in many countries throughout the world.
Collection surfaces
For domestic rainwater harvesting the most common surface for collection of water is the roof of the dwelling.
Many other surfaces can and are used: courtyards, threshing areas, paved walking areas, plastic sheeting, trees,
etc. Most dwellings, however, have a roof. The style, construction and material of the roof affect its suitability as
a collection surface for water. Typical materials for roofing include corrugated iron sheet, asbestos sheet; tiles (a
wide variety is found), slate, and thatch (from a variety of organic materials).
Guttering
Guttering is used to transport rainwater from the roof to the storage vessel. Guttering comes in a wide variety of
shapes and forms, ranging from the factory made PVC type to home made guttering using bamboo or folded
metal sheet. Guttering is usually fixed to the building just below the roof and catches the water as it falls from
the roof. For a detailed analysis of the performance of various types of guttering see the DTU working paper
titled ˆ¢’ǨÀúGuttering Design for Rainwater Harvesting ˆ¢’Ǩ’Äú with special reference to conditions in Ugandaˆ¢’Ǩ’Ñ¢.
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Below are shown some of the common types of guttering and fixings.
2a - rectangular section guttering fixed to rafter
2b - semi circular trough gutter fixed to facia board
2c - timber V-shaped trough fixed with wire to rafter
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2d - one configuration for guttering with thatch roof
2e - V-shaped gutter with wall mount
Figure 2 ˆ¢’Ǩ’Äú a variety of guttering types showing possible fixings
Manufacturing low-cost gutters. Factory made gutters are usually expensive and beyond the reach of the poor of
developing countries. They are seldom used for very low-cost systems. The alternative is usually to manufacture
gutters from materials that can be found cheaply in the locality. There are a number of techniques that have been
developed to help meet this demand; one such technique is described below.
V- shaped gutters from galvanised steel sheet can be simply made by cutting and folding flat galvanised steel
sheet. Such sheet is readily available in most market centres and can be worked with tools that are commonly
found in a modestly equipped workshop. One simple technique is to clamp the cut sheet between two lengths of
straight timber and then to fold the sheet along the edge of the wood. A strengthening edge can be made be
folding the sheet through 90o and then completing the edge
with a hammer on a hard flat surface. The better the grade of steel sheet that is used, the more durable and hard-
wearing the product. Fitting a downpipe to V-shaped guttering can be problematic and the V-shaped guttering
will often be continued to the tank rather than changing to the customary circular pipe section downpipe.
Methods for fixing gutters are shown in figure 4.
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Figure 3 ˆ¢’Ǩ’Äú folding galvanised steel sheet to make V-shaped guttering
Rectangular section gutters can be made in a similar way. It is somewhat easier to fit downpipes to rectangular
section guttering.
First flush systems
Variations in Rainwater Quality from Roof Catchment
The quality of rainwater from a tile and a galvanised-iron type roof catchment were analysed over a period of 5
months. Examination of staggered 1 litre samples collected during a rainfall event showed that the
concentrations of various pollutants were high in the first litre but decreased in subsequent samples with few
exceptions. Faecal coliform and total coliform counts ranged from 8-13 (tile roof) and 4-8 (iron roof) to 41-75
(tile roof) and 25-63 (iron roof) colonies per 100 ml, respectively. However, no faecal coliforms were detected
in the fourth and fifth litre samples from both roofs. The pH of rainwater collected from the open was acidic but
increased slightly after falling on the roofs. The average zinc concentrations in the run-off from the galvanised-
iron roof was about 5-fold higher compared to the tile roof, indicating leaching action but was well below the
WHO limits for drinking water quality. Lead concentrations remained consistently high in all samples collected
and exceeded the WHO guidelines by a factor of 3.5. For the roof area studied, a ' foul flush ' volume of 5l.
would be the minimum to safeguard against microbiological contamination but the high metals content in the
water indicate the need for some form of treatment. Rainfall intensity and the number of dry days preceding a
rainfall event significantly affect the quality of run-off water from the catchment systems.
Source: Yaziz, M.I. Gunting, H. Sapari, N.,Ghazali, A.W.
Pertanian Malaysia Univ. Serdang, Dept. of Environmental Sciences.
Citation: Water Research WATRAG Vol. 23, No. 6, p 761-765, June 1989. 1 fig, 5 tab, 3 ref.
Debris, dirt and dust will collect on the roof of a building or other collection area. When the first rains arrive,
this unwanted matter will be washed into the tank. This will cause contamination of the water and the quality will
be reduced. Many RWH systems therefore incorporate a system for diverting this ˆ¢’ǨÀúfirst flushˆ¢’Ǩ’Ñ¢ water so that it
doesnˆ¢’Ǩ’Ñ¢t enter the tank.
There are a number of simple systems which are commonly used and also a number other, slightly more
complex, arrangements. The simpler ideas are based on a simple manually operated arrangement whereby the
inlet pipe is moved away from the tank inlet and then replaced again once the initial first flush has been diverted.
Clamp
Length of
straight-edged
timber
Fold
Galvanised
sheet to be
folded
Sheet folded with
and finished
hammer to give 90
0
V-shaped guttering
Finished guttering
section showing
strengthening
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This method has obvious drawbacks in that there has to be a person present who will remember to move the
pipe. Slightly more sophisticated methods include arrangements such as those shown in Figure 4 below, where
the stopper in the inlet chamber can be removed to allow the first flush to be diverted.
Figure 4 ˆ¢’Ǩ’Äú First flush device using removable stopper in bottom of inlet chamber (above) and using
diversion pipe (below)
Other systems use tipping gutters to achieve the same purpose. The most common system (as shown in Figure 5
below) uses a bucket which accepts the first flush and the weight of this water off-balances a tipping gutter which
then diverts the water back into the tank. The bucket then empties slowly through a small-bore pipe and
automatically resets. The process will repeat itself from time to time if the rain continues to fall, which can be a
problem where water is really at a premium. In this case a tap can be fitted to the bucket and will be operated
manually. The quantity of water that is flushed is dependent on the force required to lift the guttering. This can
be adjusted to suit the needs of the user.
Figure 5 ˆ¢’Ǩ’Äú the tipping gutter first flush system
Another system that is used relies on a floating ball that forms a seal once sufficient water has been diverted (see
Figure 6 below). The seal is usually made as the balls rises into the apex of an inverted cone. The ball seals the
top of the ˆ¢’ǨÀúwasteˆ¢’Ǩ’Ñ¢ water chamber and the diverted water is slowly released, as with the bucket system above,
through a small bore pipe. Again the alternative is to use a tap. In some systems (notably one factory
manufactured system from Australia) the top receiving chamber is designed such that a vortex is formed and any
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particles in the water are held in suspension in the vortex while only clean water passes into the storage tank. The
ˆ¢’ǨÀúwasteˆ¢’Ǩ’Ñ¢ water can be used for irrigating garden plants or other suitable application. The debris has to be removed
from the lower chamber occasionally.
Figure 6 ˆ¢’Ǩ’Äú the floating ball first flush system
Although the more sophisticated methods provide a much more elegant means of rejecting the first flush water,
practitioners often recommend that very simple, easily maintained systems be used, as these are more likely to be
repaired if failure occurs.
Filtration systems and settling tanks
Again, there are a wide variety of systems available for treating water before, during and after storage. The level
of sophistication also varies, from extremely high-tech to very rudimentary. A German company, WISY, have
developed an ingenious filter which fits into a vertical downpipe and acts as both filter and first-flush system.
The filter cleverly takes in water through a very fine (0.17mm) mesh while allowing silt and debris to continue
down the pipe. The efficiency of the filter is over 90%. This filter is commonly used in European systems.
The simple trash rack has been used in some systems but this type of filter has a number of problems attached:
firstly it only removes large debris; and secondly the rack can become clogged easily and requires regular
cleaning.
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, 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
effective where used but add significant additional cost if elaborate techniques are used.
Post storage filtration include such systems as the upflow sand filter shown in Figure 7. Many other systems exist
and can be found in the appropriate water literature.
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Figure 7 ˆ¢’Ǩ’Äú Upflow sand filter for post treatment of stored water.
4. Sizing the DRWH system
Usually, the main calculation when designing a DRWH 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:
ˆ¢’Ǩ¬¢ local rainfall data and weather patterns
ˆ¢’Ǩ¬¢ roof (or other) collection area
ˆ¢’Ǩ¬¢ runoff coefficient (this varies between 0.5 and 0.9 depending on roof material and slope)
ˆ¢’Ǩ¬¢ user numbers and consumption rates
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The style of rainwater harvesting (see Rainwater Harvesting Styles) will also play a part in determining the
system components.
There are a number of different methods for sizing system components. These methods vary in complexity and
sophistication. Some are readily carried out by relatively inexperienced first-time practitioners; others require
computer software and trained engineers who understand how to use this software. The choice of method used to
design system components will depend largely on the following factors:
ˆ¢’Ǩ¬¢ the size and sophistication of the system and its components
ˆ¢’Ǩ¬¢ the availability of the tools required for using a particular method (e.g. computers)
ˆ¢’Ǩ¬¢ the skill and education levels of the practitioner / designer
Below we will outline 3 different methods for sizing RWH system components.
Method 1 ˆ¢’Ǩ’Äú 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 ˆ¢’Ǩ’Äú 20 litres
Number of people per household, n ˆ¢’Ǩ’Äú 6
Longest average dry period ˆ¢’Ǩ’Äú 25 days
Annual consumption = C x n x 365 = 43,800 litres
Storage requirement, T = 43,800 x 25 = 3,000 litres
365
Roof area = l x b
Maximum harvestable water =
Roof area x total annual rainfall
x runoff coefficient
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This simple method assumes sufficient catchment area and rainfall and catchment area which is adequate, and is
therefore only applicable in areas where this is the situation. It is a method for acquiring rough estimates of tank
size.
Method 2 ˆ¢’Ǩ’Äú 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 (see figure 1 below). If there is sufficient water throughout the year to meet the demand, then
sufficient storage will be required to bridge the periods of scarcity. As storage is expensive, this should be done
carefully to avoid unnecessary expense.
The example given here is a simple spreadsheet calculation for a site in North Western Tanzania. The rainfall
statistics were gleaned from a nurse at the local hospital who had been keeping records for the previous 12 years.
Average figures for the rainfall data were used to simplify the calculation, and no reliability calculation is done.
This is a typical field approach to RWH storage sizing.
Example
Site: Medical dispensary, Ruganzu, Biharamulo District, Kagera, Tanzania (1997)
Demand:
Number of staff: 7
Staff consumption: 45 litres per day x 7 = 315 litres per day
Patients: 40
Patient consumption : 10 litres per day x 40 = 400 litres per day
Total demand: 715 litres per day or 260.97m
3
per month
Roof area: 190m
2
Runoff coefficient (for new corrugated GI roof): 0.9
Average annual rainfall: 1056mm per year
Annual available water (assuming all is collected) = 190 x 1.056 x 0.9 = 180.58m
3
Daily available water = 180.58 / 365 = 0.4947 m
3
/ day or 494.7 litres per day or 150.48m
3
per month
Figure 8 ˆ¢’Ǩ’Äú average annual rainfall for the District of Biharamulo.
So, if we want to supply water all the year to meet the needs of the dispensary, the
demand cannot exceed 494.7 litres per day. The expected demand cannot be met by
the available harvested water. Careful water management will therefore be required.
Figure 9 below shows the comparison of water harvested and the amount that can be supplied to the dispensary
using all the water which is harvested. It can be noted that there is a single rainy season. The first month that the
Average Rainfall for Biharamulo District
114
101
136
214
75
3
5
15
47
88
124
134
0
50
100
150
200
250
1
2
3
4
5
6
7
8
9
10
11
12
Month
mm
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rainfall on the roof meets the demand is October. If we therefore assume that the tank is empty at the end of
September we can form a graph of cumulative harvested water and cumulative demand and from this we can
calculate the maximum storage requirement for the dispensary.
Figure 9 ˆ¢’Ǩ’Äú comparison of the harvestable water and the demand for each month.
Figure 10 ˆ¢’Ǩ’Äú showing the predicted cumulative inflow and outflow from the tank. The maximum storage
requirement occurs in April.
Month
Rainfall (mm) Rainfall
harvested
(cubic metres)
Cumulative
rainfall
harvested
(cubic metres)
Demand
(based on
total
utilisation)
Cumulative
demand (cubic
metres)
Difference
between
column 4 and
6
Oct
88
15.05
15.05
15.05
15.05
0.00
Nov
124
21.20
36.25
15.05
30.10
6.16
Dec
134
22.91
59.17
15.05
45.14
14.02
0.00
20.00
40.00
60.00
80.00
100.00
120.00
140.00
160.00
180.00
200.00
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Month
Cubic metres (cumulative)
Line:
Cumulative
demand
Bars:
Cumulative
harvested
rainwater
Maximum storage
requirement occurs
in April:
50.54 cubic metres
Comparison of harvestable water and demand
0.00
5.00
10.00
15.00
20.00
25.00
30.00
35.00
40.00
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Cubic metres
Demand line
Monthly
harvestable
water
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Jan
114
19.49
78.66
15.05
60.19
18.47
Feb
101
17.27
95.93
15.05
75.24
20.69
Mar
136
23.26
119.19
15.05
90.29
28.90
Apr
214
36.59
155.78
15.05
105.34
50.45
May
75
12.83
168.61
15.05
120.38
48.22
Jun
3
0.51
169.12
15.05
135.43
33.69
Jul
5
0.86
169.97
15.05
150.48
19.49
Aug
15
2.57
172.54
15.05
165.53
7.01
Sep
47
8.04
180.58
15.05
180.58
0.00
Totals
180.58
180.58
Table 1 - shows the spreadsheet calculation for sizing the storage tank. It takes into consideration the
accumulated inflow and outflow from the tank and the capacity of the tank is calculated as the greatest
excess of water over and above consumption. This occurs in April with a storage requirement of 50.45
cubic metres. All this water will have to be stored to cover the shortfall during the dry period.
Method 3 ˆ¢’Ǩ’Äú computer model
There are several computer-based programmes for calculating tank size quite accurately. One such programme,
known as SimTanka, has been written by an Indian organisation and is available free of charge on the World
Wide Web. The Ajit Foundation is a registered non-profit voluntary organisation with its main office in Jaipur,
India and its community resource centre in Bikaner, India.
SimTanka is a software programme for simulating performance of rainwater harvesting systems with covered
water storage tank. Such systems are called Tanka in western parts of the state of Rajasthan in India.
The idea of a computer simulation is to predict the performance of a rainwater harvesting system based on the
mathematical model of the actual system. In particular SimTanka simulates the fluctuating rainfall on which the
rainwater harvesting system is dependent.
Rainwater harvesting systems are often designed using some statistical indicator of the rainfall for a given place,
like the average rainfall. When the rainfall is meagre and shows large fluctuations then a design based on any
single statistical indicator can be
misleading. SimTanka takes into account the fluctuations in the rainfall, giving each fluctuation its right
importance for determining the size of the rainwater harvesting system. The result of the simulation allows you
to design a rainwater harvesting
system that will meet demands reliably, that is, it allows you to find the minimum catchment area and the
smallest possible storage tank that will meet your demand with probability of up to 95% in spite of the
fluctuations in the rainfall. Or you can use SimTanka to find out what fraction of your total demand can be met
reliably.
SimTanka requires at least 15 years of monthly rainfall records for the place at which the rainwater harvesting
system is located. If you do not have the rainfall record for the place then the rainfall record from the nearest
place which has the same PATTERN of rainfall can be used.
The included utility, RainRecorder, is used for entering the rainfall data. Daily consumption per person is also
entered and then the software will calculate optimum storage size or catchment size depending on the
requirements of the user. SimTanka also calculates the reliability of the system based on the rainfall data of the
previous 15 years.
SimTanka is free and is and was developed by the Ajit Foundation in the spirit that it might be useful for meeting
the water needs of small communities in a sustainable and reliable manner. But no guaranties of any kind are
implied.
For more information or to download the software see their website at
http://www.geocities.com/Rainforest/Canopy/4805/
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(Source: the information given here is taken from this website).
Further comments
These methods outlined above can be further refined where necessary to use daily rainfall data. This is
particularly important in areas where rainfall is more evenly distributed and more sensitive calculations are
necessary.
Rainfall data can be obtained from a variety of sources. The first point of call should be the national
meteorological organisation for the country in question. In some developing countries, however, statistics are
limited due to lack of resources and other sources are often worth seeking. Local Water Departments or
organisations, local hospitals or schools are all possible sources of information.
In reality the cost of the tank materials will often govern the choice of tank size. In other cases, such as large
RWH programmes, standard sizes of tank are used regardless of consumption patterns, roof size or number of
individual users.
Tank efficiency and the case for diminishing returns
On days when rainfall is heavy, the flow into a tank is higher than the outflow drawn by water users. A small
tank will soon become full and then start to overflow. An inefficient system is one where, taken over say a year,
that overflow constitutes a significant fraction of the water flowing into the tank. Insufficient storage volume is
however not the only cause of inefficiency: poor guttering will fail to catch water during intense rain, leaking
tanks will lose water, and an ˆ¢’ǨÀúoversizeˆ¢’Ǩ’Ñ¢ roof will intercept more rainfall than is needed.
Storage efficiency (%) =100 x (1 - overflow / inflow) provided that inflow<demand
System efficiency (%) = 100 x water used / water falling on the roof
In the dry season, a small tank may run dry, forcing users to seek water from alternative sources. Unreliability
might be expressed as either the fraction of time (e.g. of days) when the tank is dry or the fraction of annual
water use that has to be drawn from elsewhere. A RWH system may show unreliability not only because storage
is small, but because the roof area is insufficient. The figure below shows how reliability, expressed as a fraction
of year, varies with storage volume (expressed as a multiple of daily consumption) for two locations close to the
Equator and therefore both with double rainy seasons.
AVAILABILITY OF RAINWATER SUPPLY (as fraction of year)
Ratio of storage volume to daily water consumption
D/R = Ratio of daily consumption to mean collectable rainwater
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1
2
4
8
16
32
64
128
256
Kyenjojo D/R=.8
Mbarara D/R=.8
Mbarara D/R=1.5
From this graph one can see that increasing storage size, and therefore cost, gives diminishing returns. For
example look at the left hand column of each triplet (Kyenjojo with roof sized such that average annual water
demand is only 80% of average annual roof runoff). Assuming a say 100 litres per day demand, shows that
increasing storage from 1 day (100 l) to 16 days (1600 l) raises the reliability from 31% to 78%, but storage has
to be increased as high as 128 days (12,800 l) to achieve 99% reliability. Such high reliability is so expensive
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that it is an unrealistic design objective for a DRW system in a poor country. In any case, as we shall see below,
users may change behaviour so as to reduce the effective unreliability of their systems.
System features (that affect tank sizing)
ˆ¢’Ñ¢¬¶ An oversize roof slightly extent compensates for an undersize tank.
ˆ¢’Ñ¢¬¶ If users are able and willing to adjust their consumption downwards during dry seasons, or when they find
water levels in their tank lower than average, tanks can be sized smaller.
ˆ¢’Ñ¢¬¶ ˆ¢’ǨÀúPartialˆ¢’Ǩ’Ñ¢ RWH systems, either where it is accepted that RW will not meet needs throughout the year or
where rainwater is only used to meet specific water needs like cooking/drinking, can be built with
surprisingly small tanks.
ˆ¢’Ñ¢¬¶ The reliability level appropriate to the design of a RWH system rises with the cost (in money, effort or even
ill-health) of the alternative source that is used when the tank runs dry.
Rainfall data
Rainfall is very variable, especially where annual precipitation is less than 500mm. It also varies with location,
so that data from a rain gauging station 20km away may be misleading when applied to the site of the RWH
system. From the lowest to the highest quality of rainfall data we can think of at least 6 categories:
1. No numerical data available, but of course local people know quite well the seasonality of precipitation and
which crops will grow (with what sort of water-stress failure rate).
2. There is no numerical data, but RWH has been practised for long enough locally for people to have a feel
for what is an adequate tank size.
3. Only annual average rainfall is available, probably at a somewhat distant recording point, plus local
knowledge of seasonality.
4. Monthly rainfall, averaged over at least 4 years, can be obtained.
5. Actual monthly rainfall records for at least 4 years, and preferably 7 years, are available for the site or for a
location sufficiently nearby to give confidence or allow some systematic correction to be applied.
6. Daily rainfall data for a relevant location and lasting at least 4 years is available.
Daily data is adequate for all design methods except perhaps the optimisation of gutters for which rainfall
intensity data is useful (e.g. the fraction of annual precipitation falling at a rate faster than say 1mm per minute).
Rainfall data can be expensive to purchase and is often hard to locate even where it exists. Obviously methods of
sizing tanks that require as an input rainfall data of a sort that is not locally available should not be used.
Possible methods for estimating likely future RWH system performance with a tank
of given size
1. Ask local people with existing RWH systems and a comparable roof size what tank size they used and what
has been their experience with it.
2. Mean dry season duration x daily dry season consumption. This may define an ˆ¢’ǨÀúidealˆ¢’Ǩ’Ñ¢ below which the
designer chooses the ˆ¢’ǨÀúbiggest affordableˆ¢’Ǩ’Ñ¢.
3. Model reliability or efficiency using mean monthly rainfall data. (One variant of this is to compare the
cumulative supply and usage loci to determine the maximum deviation of the former below the latter)
4. Test different tank sizes against actual monthly rainfall data over several recent years (or for tanks
equivalent to less than 2 months water demand actual daily data) and thereby deciding the best reliability v
cost trade-off.
5. Reasons for the current interest in DRWH
In the last decade there has been a large increase in the amount