Martinson, Brett

Thomas, T.

English

Name not available

Better, faster, cheaper: research into roofwater harvesting for water supply in low income countries

Portsmouth University Research Portal (Pure)

C:\DOCUME~1\User\LOCALS~1\Temp\Better, Faster, Cheaper; Research into roofwater harvesting for water supply in low-income countries.doc 1 
ARCSA, Austin, Texas, August 2003 
Better, Faster, Cheaper; Research into roofwater 
harvesting for water supply in low-income countries 
D. Brett Martinson & Terry Thomas 
 
Abstract 
The water situation in many low-income countries is grim. More than one billion people have no access to clean 
drinking water and those that do often spend considerable time walking and queuing to collect it. Many water 
professionals are becoming worried about the increasingly difficult problems of finding and improving water 
sources while some existing water sources are now becoming depleted or polluted. Domestic roofwater 
harvesting (DRWH) provides an innovative solution to meeting water needs and can be implemented quickly 
and modularly. It is also very robust against risks of unexpected change. Renewed interest in the technology is 
reflected in the water policies of many developing countries, where it is increasingly being cited as a useful 
source of household water. 
This paper brings together a number of findings from two studies into DRWH by the Development Technology 
Unit of the University of Warwick and its partners the Lanka Rainwater Association, FAKT Germany, The 
Rural Development and Appropriate Technology Centre at the Indian Institute of Technology in Delhi, ACORD 
Uganda and Water Action in Ethiopia. The results presented here are a summary of a number of papers being 
presented at IRCSA 11, Mexico City. 
 
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1. Introduction 
The water situation in many low-income countries is 
grim. More than one billion people have no access 
to clean drinking water and those that do often 
spend considerable time walking and queuing to 
collect it. Many water professionals are becoming 
worried about the increasingly difficult problems of 
finding and improving water sources while some 
existing water sources are now becoming depleted 
or polluted. Domestic roofwater harvesting 
(DRWH) provides an innovative solution to meeting 
water needs and can be implemented quickly and 
modularly. It is also very robust against risks of 
unexpected change such as aquifers dropping or 
becoming polluted. Renewed interest in the 
technology is reflected in the water policies of many 
developing countries, where it is more and more 
being cited as a useful source of household water. 
Rainwater systems are decentralised and 
independent of topography and geology. They 
deliver water directly to the household, relieving the 
burden of water carrying, particularly from women 
and children. Implementation is similar to managing 
the installation of on-site sanitation and once 
systems are in-place they are owned by the 
householders who can then manage their own water 
supply. 
Roofwater harvesting does have a number of 
limitations, however. It is not suited to being used as 
a stand-alone water supply solution in any but the 
most water-stressed situations as the increase in tank 
capacity necessary to bridge a long dry season can 
be prohibitively expensive. The storage provided by 
a tank does, however give households good security 
against short-term failure of alternative sources. 
Niches where roofwater harvesting is particularly 
attractive include: 
• where groundwater is either difficult to 
secure or has been rendered unusable by 
fluoride, salinity or arsenic 
• where the main alternatives are surface 
water sources 
• where management of shared point sources 
has proved unsuitable 
• where the carriage of water is a particular 
burden on household members or where 
householders are prepared to invest in water 
convenience. 
Despite its advantages, domestic roofwater 
harvesting remains a niche technology and, when 
considered at all, is usually only considered when all 
other options have been eliminated. The problems 
come under three main categories, high cost, 
uncertain quality and difficulty in implementation. 
There is also a fourth category, lack of knowledge, 
which while important does not impinge upon 
DRWH’s viability and so will not be dealt with 
here. 
This paper brings together a number of findings 
from two studies by the Development Technology 
Unit of the University of Warwick and its partners 
the Lanka Rainwater Association, FAKT Germany, 
The Rural Development and Appropriate 
Technology Centre at the Indian Institute of 
Technology in Delhi, ACORD Uganda and Water 
Action in Ethiopia. The results presented here are a 
summary of a number of papers being presented at 
IRCSA 11, Mexico City. 
2. Cost 
The cost of domestic roofwater harvesting is usually 
seen as high by most water supply professionals. 
Table 1 shows the costs of a number of water supply 
options as reported by water professionals during 
interviews in 2001. 
Table 1: Per capita costs of water supply 
 Uganda Ethiopia 
Town 
water 
 $70 –90 pc 
Tube  
well 
$15–40pc (~500hh) 
< $150pc (~50hh) 
> $150pc (~500hh) 
Gravity 
scheme 
$15 $17-20 pc 
Rainwater 
Harvesting 
$30 pc  
Rainwater harvesting is about twice the cost of the 
cheapest competitor, but less expensive than deep 
groundwater in high-risk areas where wells could 
fail or sources are limited. 
Part of roofwater harvesting’s reputation as a high 
cost option is caused by the high expectations of 
water professionals themselves. Providers tend to 
think in terms of complete solutions, i.e. all water 
needs should be met by one source. In low-income 
C:\DOCUME~1\User\LOCALS~1\Temp\Better, Faster, Cheaper; Research into roofwater harvesting for water supply in low-income countries.doc 3 
countries this is rarely the case and householders 
tend to use three or four sources depending on need 
and availability. In this context relatively large 
storage tanks are unnecessary and costs can fall 
appreciably. Roofwater harvesting suffers from 
strong diseconomies of scale (see Figure 1) in terms 
of supplying water needs, a small (say 1,000 litre) 
tank may supply 70% of a households water needs 
over the year (mainly in the wet season) whereas a 
tank 5 times the size will supply 90%, only a 20% 
improvement. This is because water is drawn and 
replenished more often with a small system whereas 
a large one may only fill once or twice a year. 
Figure 1: Diminishing returns with increase in 
tank size 
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 10 20 30 40 50 60 70 80
Normalised tank size in days
V
a
lu
e
 r
a
ti
o
 
Another reason for high cost is a narrow view of 
quality. While other products such as on-plot 
sanitation have for years been available in a range of 
qualities, DRWH systems have tended toward fairly 
high quality structures, partly for practical reasons 
of water soundness and safety and partly for more 
aesthetic reasons. 
Figure 2: unsustainable DRWH structure. 
 
Figure 2 shows such a structure. The house is wattle 
and daub, while the tank is a large ferrocement 
structure worth more than a year’s wages. It is 
unlikely, if not impossible for this kind of structure 
to be replicated by this family’s neighbours. 
To redress this situation the DTU has developed a 
tool called a “roofwater harvesting ladder” which 
can be used to present a range of designs to a 
community along with the trade-offs in terms of 
cost, any other commitment from the household 
such as labour, amount of water delivered and 
length of time the system is dry. Coupled with this a 
number of low-cost tanks have been developed 
which have reduced the cost of roofwater harvesting 
tanks by half, bringing DRWH into line with the 
cheapest water supply options available. Some of 
these designs are shown in Figure 3. After field 
testing the Dome and Enhanced Local Materials 
Roofed tanks are ready for use, while the Tube tank 
is most useful in a few niches (such as rapid water 
supply for refugees and a couple (such as the Wattle 
and Daub tank) need longer-term study before final 
recommendation. 
Figure 3: some of the tanks developed 
Dome tank 
 
 
Enhanced Local Materials 
Roofed tank 
  
Barrel Tank 
 
Polythene Tube tank 
  
Wattle and daub tank  
 
 
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3. Water quality 
The water quality of DRWH systems has often been 
in question, particularly when the water is used for 
drinking. Standards set by the WHO {WHO 1997 
#4130} suggest microbiological standards of zero 
E.Coli per 100ml, which is appropriate for treated 
water supply systems. Roofwater does occasionally 
meet these standards but the water quality is time 
dependent, strongly following the rainfall pattern 
(see Figure 4). 
Figure 4: E. Coli recorded from Rainwater 
Tanks in Alaba, Ethiopia (time averaged over 3 
days) 
0
1
10
100
26/Aug 26/Sep 26/Oct
E
.C
o
li
 (
C
o
u
n
t/
1
0
0
m
l)
0
10
20
30
40
50
60
R
a
in
fa
ll
 (
m
m
)
Rainfall E.Coli
 
Under the recent project, a series of water quality 
measurements have been carried out on over 120 
systems at 6 sites at frequent intervals. This has 
provided a good picture of the water quality of 
roofwater harvesting systems and also highlighted 
several ways to improve it. 
Generally, roofwater appears as good or better than 
rural water sources but (as expected) not as good as 
chlorinated urban sources (Figure 5). As is apparent 
in Figure 4, the water also improves with time after 
the rain, mainly due to sedimentation and bacteria 
die-off. It takes an average of 3.5 to 4 days to 
achieve a 90% reduction in E.Coli numbers. 
Figure 5: Exceedance curves for stored 
roofwater and other water sources 
0
1
10
100
1,000
0%20%40%60%80%100%
Percentage of samples equal to or better 
than standard
E
. 
C
o
li
 (
c
o
u
n
t/
1
0
0
m
l)
Roofwater Control (rural)
Control (urban)
 
Figure 6 shows the paths of contaminants into a 
roofwater harvesting system. 
Figure 6: contamination paths for roofwater 
harvesting 
Microbiological 
contamination
Chemical 
contamination
Carried by 
vector
Blown dust
Rooftop
Direct Entry 
into tank
Residence in 
the tank
Carried by water 
entering the tank
Washed out 
of air
Tank outlet
Ingestion
tank environment
First flush
Filters
Overflow diversion
Inlet arrangements
Screens
Screens
Outlet arrangements
Post-tank processing
Roof environment 
Environmental reduction
Barriers
 
Research is currently underway to test the efficiency 
of a number of low-cost filters intended to stop 
contaminates from entering the tank with the water 
as well as a number of inlet and outlet arrangements 
for reducing mixing in the tank to allow die off to 
work more effectively. 
C:\DOCUME~1\User\LOCALS~1\Temp\Better, Faster, Cheaper; Research into roofwater harvesting for water supply in low-income countries.doc 5 
4. Implementation 
Roofwater harvesting is fundamentally different 
from most water supply options. These differences 
have profound effects on the management and 
implementation of any project involving roofwater 
harvesting: 
• It is based on a finite volume of water that can 
be depleted if not well managed, making it a 
poor candidate for community supply unless 
strong measures are taken to prevent overuse 
• It is strongly seasonal in nature meaning that 
there must also be another water source 
available. This source (or sources) must be able 
to cope with the demands of households using 
roofwater harvesting, especially as the largest 
demand will be in dry periods. It does not, 
however, have to be as high a quality 
• Domestic roofwater harvesting requires a large 
number of small civil works rather than the 
large-centralised works of most water projects, 
requiring different approaches to management 
• The cash flow of roofwater harvesting systems 
is that of a large up-front cost with extremely 
small maintenance charges. This is in contrast 
with most water supply where maintenance is a 
large part of the overall costs. Most projects are 
costed based on donor funded initial works with 
users paying for upkeep – this paradigm is 
unsuited to DRWH. 
These implementation issues remain inadequately 
researched and form the main barrier to further 
support of DRWH in formal water supply. 
5. Conclusions 
The state of the art of DRWH has advanced 
considerably in recent years. Costs can be reduced 
considerably and the water quality of stored 
rainwater is better understood and ways of 
improving it identified. While these efforts are in no 
way complete, the largest challenge remaining is the 
management and implementation of DRWH in 
water supply projects. 
 
This paper is an output from a project funded by the UK Department for International Development (DFID) for the benefit of developing 
countries. The views expressed are not necessarily those of the DFID 


Better, faster, cheaper: research into roofwater harvesting for water supply in low income countries

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