Working Paper No. 55

Very-Low-Cost Roofwater Harvesting In East Africa

7. TECHNOLOGY – DOMESTIC WATER STORAGE


CONTENTS

Definitions, abbreviations & costing units

1. Introduction

2. The Candidate technology (VLC-DRWH)

3. Water needs in the region and the possible contribution of VLC DRWH

4. Economics of DRWH

5. The management and social impact of DRWH — quotations and examples

6. Health aspects

7. Technology — domestic water storage

8. Technology - Other DRWH system components

9. Mechanisms of dissemination

10. Conclusions: Prospects for extensive take-up of DRWH in the Great Lakes area

11. Bibliography

APPENDICES

I. Participants at the RWH Seminar, 19th — 21st July 2000

II. Design drawings (VLC RWH systems)

III. Small tank costs

IV. Maps

V. Minutes of seminar held 19th & 20th July 2000 at Mbarara

VI. Partners in DRWH — organisational profiles

 

 

It is difficult to understand why certain technologies prosper over others. There are many examples of situations where inferior technologies thrive whilst the ideal (in the eyes of the technologist) is shelved or dropped in the dust bin. The reasons are often political or market driven, rather than technology driven and a good salesman can be a wonderful asset. In the case of developing countries, technologies which are well-suited to improving the lives of rural poor are also overlooked on occasions. Again, there are a variety of reasons, the main reasons usually being a poor access to knowledge and information, traditional cultural practises and a lack of political will. Small-scale RWH is one such technology that has been largely overlooked by the majority of poor rural households in LDC’s. In countries where the technology has been embraced (Thailand being the most prominent example), great benefits have been seen and large steps taken in alleviating the daily drudgery faced by householders in the task of meeting their water needs.

7.1 Requirements of a domestic water storage tank

Any vessel used for storage of potable water in a domestic context should have certain attributes. These are investigated below in some detail:

Strength

Any tank that is to store water must have sufficient strength. Water pressure inside the tank creates stresses, which, if not dealt with properly, can cause the tank to fail, which could in turn lead to serious damage of the tank and injury to persons and /or damage to surrounding buildings. Ideally a full engineering analysis should be carried out for any new tank design and tests carried out to confirm the findings. In practise, tanks are usually designed and built, based on previous experience with the material being used and/or previous experience with similar vessels. A good safety factor is usually incorporated in such cases. In Section 2 the shape of tanks was discussed. Existing tanks come in a number of common shapes. The relative merits of these shapes are discussed in Table 7.1

Table 7.1 Relative merits of some common tank shapes

Tank shape or type

Stresses

Material usage and construction

Cuboid

Stresses are unevenly distributed and difficult to calculate

The ratio between material usage and storage capacity is lower than for a cylindrical and doubly curved tank.

Construction is quite simple

Cylindrical

Stresses are more evenly distributed and are easier (though trivial) to calculate

There is an improvement in the material use to storage capacity ratio (a saving of 7.5% given a good height to diameter ratio)

Construction becomes more difficult with traditional materials e.g. bricks

Thai Jar Style (doubly curved tanks)

Stresses are ideally distributed if the proportions of the jar are correct

Material usage to capacity ratio is very good (savings of up to 20% over a cuboid) but construction can be very difficult, often relying on specialised moulds.

Impermeability

A water vessel should obviously be impermeable. This is achieved in one of a number of ways, depending on the material from which the tank is made. Some materials are inherently water proof e.g. corrugated steel sheets or fibre glass, and require no (or little) treatment to provide an impermeable barrier. Traditional materials, such as masonry and brick, are usually dealt with by applying an internal render of sand and cement, which can be treated with a water proofing agent or given a final coat of ‘nil’ (cement slurry). Ferrocement technology uses this concept by applying a cement slurry onto the wall of the tank when complete. Modern plastics may allow low-cost linings to be produced although little has been done in developing countries to develop a suitably sized off-the-shelf solution. Other modern materials, such as bituminous paints, suitable for use with potable water supplies, are slowly becoming available on the market in LDC’s.

Durability

of storage tanks is a critical question. Engineering techniques for determining the durability (through accelerated ageing) are expensive and so the only way to properly ‘test’ a new technology is usually to apply the test of time. This is problematic when we are looking for a useful life of 20 — 30 years. Little information seems to be available on existing tanks and their useful life spans. The experience in Thailand (documented in Section ??) shows how some unsuitable technologies can be widely disseminated before major flaws appear. In the Thai case more than 50,000 bamboo reinforced mortar jars were manufactured, many of which failed due to termite and fungal attack on the bamboo.

Sufficient storage capacity

This topic is discussed in far more detail in other sections of this report. Many techniques are available in the RWH literature for determining the ideal size of a tank for full water coverage throughout the year, but none exists for determining the size with modified consumption (during the wet season for example), or for partial coverage.

Maintenance of water quality

A good storage vessel should maintain and improve the water quality. This is achieved in a number of ways:

  • a good fitting, light-proof cover will prevent debris, animals or humans from entering the tank and prevent light from causing algae growth 
  • water quality can enhanced by putting water into the tank and taking it out of the tank at the correct location — low-level tank entry and floating off-takes are devices designed to aid this approach 
  • good sanitary conditions around a tank will prevent disease being spread 
  • water extraction should be such that the water is not contaminated while being drawn 
  • filters improve water quality are discussed in a following section

No increase in health risk

Sometimes, with all good intentions, a water tank can become a serious health hazard. This is particularly the case when mosquitoes are allowed to breed in the tank. This can be avoided by sealing the tank well and preventing the mosquitoes entering and breeding by covering any openings with mosquito gauze.


 

 

7.2 Tank size — ideal tank size vs. affordability

Tank sizing techniques usually only consider the optimum size for a tank based on the rainfall available, the size of the catchment area, and the demand on the system. Little consideration is usually given to the affordability of the tank. It is assumed that the customer will be looking at capturing all the water from the roof or enough to meet all their demand. But in some cases, people will be happy with some water from their roof. In many cases, the customer may not be able to afford a tank suitable for catching the optimum amount of water. In such cases the tank size is determined by the tank cost and so, in this case, we need to maximise capacity for a given (low) cost.

Below, in Table 7.2 we have classified domestic tank sizes into three distinct groups – small, medium and large scale.

Table 7.2 Tank scale classification

Scale of domestic tanks

Description

Small-scale

Any tank or jar up to seven days storage or up to 1000 litres

Medium-scale

A tank up to several weeks storage or between 1000 and 20,000 litres storage

Large-scale

Any tank with several months of storage or above 20,000 litres storage capacity

Affordability is a strong function of tank size and tank design. The smaller the tank the cheaper it will be and the cheaper the construction materials and labour costs, the cheaper the tank will be. For increased affordability we are therefore looking at small-scale, locally produced RWH systems that use local materials. Local manufacture and use of local skills are design issues, and have been given great consideration during the design process described in Sections 7.4 and 7.8. Affordability is a function of a number of socio-economic factors and is decided at the household level.

As an indication of actual costs for a number of different tank types, a cost analysis of commonly available small and medium scale factory made tanks has been made, and compared with locally manufactured tanks. This is shown in Table 7.3 and shows the actual costs while Table 7.4 shows the cost per litre storage.

Table 7.3 Cost comparison between ‘imported’ and locally made tanks in East Africa
(all cost figures in Sterling)

Tank size (litres)

Plastic
Tanks (2)

GI
Tanks (3)

PBG
Tanks (4)

F/C jars and tanks

Brick jar (5)

Plastic tube jar (6)

Tarpaulin tank (7)

100

20

250

36

500 - 600

62

28(5)

21

750

88

33

1000

115

1500

158

2300 - 2500

219

72

3000

289

79(1)

4000

379

88

5000

463

100

6000

590

132

40

8000

747

147

10000 - 11000

976

159

155

264(1)

12000

207

Notes:

  1. Costs take from 'Rainwater Catchment Systems for Domestic Supply' Gould and Peterson (1999). Costs are from 1998 and converted from Kenyan Shillings at a rate of 113.7 (15/8/2000) 
  2. Costs from price list, Poly Fibre (U) Ltd, P O Box 3626, Kampala, Uganda - cost of filter and tap not included. Factory made, spin moulded, plastic tanks 
  3. Costs from price list, Tank and tanks, PO Box 1219, Kampala, Uganda. Cost of filter and tap not included. These tanks are made from curved galvanised iron sheets which are riveted together and soldered to make them waterproof. Estimated useful life 15 years (by manufacturer) or 2 to 3 years (by local contact). These tanks are also available in Kampala or Masaka (2 hrs drive from Mbarara) 
  4. Partially below ground tank. Design by DTU. Approximately 10 have been built in SW Uganda of between 5,000 to 20,000 litres. Cost is for 10,800 litre tank not including handpump (approx. 10 extra), based on costing exercise carried out June 2000 
  5. Cost based on actual construction cost during study, July 2000. Cost includes tap and filter. See Section 7.8 for design detail and full cost breakdown 
  6. Cost based on actual construction cost during study, July 2000. Cost includes handpump and filter. See Section 7.8 for design detail and full cost breakdow 
  7. For detail see http:/../www.eng.warwick.ac.uk/DTU/cs/cs20.html. Costs based on actual construction costs, July 200 
  8. All costs (other than Note 1) were converted from Uganda Shilling prices converted at a rate of 2509 Shillings to the pound (15/8/200)

Table 7.4: Cost comparison — pence per litre storage capacity of tanks in East Africa

Tank size (litres)

Plastic
Tanks

GI
Tanks

PBG
Tanks

F/C jars and tanks

Brick jar

Plastic tube jar

Tarpaulin tank

100

19.6

250

14.5

500 - 600

12.5

5.6

3.4

750

11.8

4.4

1000

11.5

1500

10.5

2300 - 2500

9.5

2.9

3000

9.6

2.6

4000

9.5

2.2

5000

9.3

2.0

6000

9.8

2.2

0.7

8000

9.3

1.8

10000 - 11000

9.8

1.6

1.4

2.4

12000

1.7

As expected, economies of scale show the cost per litre dropping as tank size increases. Also, as expected, factory made tanks are generally more expensive than locally manufactured tanks. The general advantage of off-the-shelf, factory-made, plastic tanks is convenience, a good range of sizes and usually a guarantee of quality. The disadvantage is the high cost. The advantage of the GI sheet tanks is again off the shelf availability, but the quality is dubious with the manufacturer claiming a 15 year life and local contacts stating a more realistic figure to be 2 – 3 years. The usual mode of failure is that the base of the tank rots out and the usual method of repair is to surround the base with concrete. The cost is much lower than that of the plastic tanks. They are manufactured primarily on the outskirts of Kampala and some of the major Ugandan towns by micro-entrepreneurs, who sell small numbers of tanks. They also make gutters and downpipes from flat GI sheet. These tanks are found throughout Uganda, but not in very great numbers.

The figures given for the locally made tanks and jars are taken from the work carried out during the study (and documented in Section 7.5), as well as from the RWH literature for the region. It can be noted that the costs are generally lower than for the plastic tanks but in line with the GI tank costs. The expected useful life for the majority of the locally-made tanks is much higher than that of the GI tank. It is also noted that only one size is quoted for each of the small jars — this is because the costing exercise was only done for the work carried out under the study. Similar economies of scale would be expected for larger jar sizes using similar materials, but the design would need to be reconsidered. The aim of the small jars is to provide systems for poor rural households who don’t have sufficient money to purchase the larger tanks.







Figure 7.1
The Tarpailin Tank
Note the inlet into the side of the tank and the door at the front for scooping water

 

 

The tarpaulin tank, developed by the Rwandan refugees in Uganda uses a 5m x 4m polypropylene tarpaulin, which is fitted inside a lined pit with walls of poles and mud built up to about 1m around the pit. The outhouse-like building is roofed with corrugated iron sheet (see Figure 7.1). The simple design and use of predominantly local materials make this tank extremely cheap for the given, maximum 6000 litre, storage capacity. The cost per litre storage is only 7% that of the plastic tank of the equivalent size. Tarpaulins and corrugated iron sheet are available locally.

A summary analysis of the tanks considered is given in Table 7.5

Table 7.5 Advantages and disadvantages of a variety of tank types

Tank type

Advantages

Disadvantages

Comments

Plastic tanks

  • Off the shelf convenience
  • Quality assured
  • Wide range of sizes
  • High cost
  • Central manufacture
  • High tooling costs
  • Local skills ignored
  • Transport costs extra

Factory made in large numbers

GI tanks

  • Off the shelf convenience
  • Reasonable initial cost
  • Moderate range of sizes
  • Low tooling costs for the manufacturer
  • Open to local manufacture
  • Doubtful quality
  • Transport costs extra

Made by micro-entrepreneurs in the major towns

PBG tank

  • Reasonable cost
  • Good range of sizes available
  • Low tooling costs
  • Suitable for local manufacture
  • Local skills enhanced
  • Use of many local materials
  • Transport costs embodied in material cost
  • Quality only assured through good workmanship
  • Water extraction device required to prevent contamination of water

DTU design. To date approximately 30 or 40 tanks have been built in SW Uganda by local artisans.

Locally manufactured small jars

  • Use of many local materials
  • Use of local skills
  • Low tooling costs — suitable to local artisans
  • Transport costs embodied in material cost
  • Suitable for poor rural households
  • Suitable for incremental adoption
  • Limited range of sizes for given design
  • Quality only assured through good workmanship

DTU designs dealt with in Section 7.8 These are new designs that have been prototyped and are currently under survey.

Tarpaulin tank

  • Very low cost
  • Uses skills available to most rural farmers
  • Uses only local resources (except tarpaulin and GI sheet)
  • Very few tools required
  • Significant storage capacity for small farms for irrigation or livestock
  • Suitable for poor rural households
  • Quality assured if new tarpaulin is used
  • Maximum size dependant on tarpaulin size
  • Some problems at present with termites eating poles and tarpaulin
  • Water extraction device required to prevent contamination of water

Tank developed by refugees in East Africa using UNHCR tarpaulin and now built in some number by ACORD and IVA / UNIFA in SW Uganda.

7.3 Choice of tank type

The type of tank that may be chosen will be dependent upon a number of factors:

  • space availability will determine the maximum dimensions and whether the tank will be above or below ground 
  • soil conditions determine whether a tank can be built below ground — rock causing excavation difficulties and sand being liable to subsidence during excavation 
  • the choice between factory made or locally made tanks is usually a function of wealth 
  • for low cost tanks (as defined above) the material and construction technique is usually dominated by what is available locally and what is affordable 
  • subsidies, often give as part of tank building programmes, can influence the type of tank that will be bought or built 
  • there are many other factors that influence the choice of tank

7.4 Materials for tank construction

The fundamentals of design for sustainability suggest that where possible, local skills and materials are used for manufacture. This should be carefully considered when designing RWH systems, particularly in rural areas of developing countries. A careful study of locally available skills and materials should be carried out before the design process begins. This can vary from dramatically from place to place, depending on natural resources, the range imported goods and tools and local building techniques (which are usually closely linked to availability of natural resources). Local knowledge is invaluable during such a survey. For the work described in Section 7.5, such a study was conducted and the findings are listed below in Table 7.6

Table 7.6 Resources and skills available close to the site at Mbarara town.

Local resources

Item

Availability

Comments

Sand

Good quality sand is difficult to find in the area. The sand used was transported 30kms from the Oruchinga Valley.

Sand of poor quality is available within one km.

Transport is needed and this costs up to six times the sand cost for the 30 km trip. Loading and offloading costs need to be considered — can be as much as the cost of the sand. Bulk purchase (4 tonne loads) is cheaper than buying small loads.

Aggregate

Available locally — about 5 kms from site

Again transport is needed and is costly. Loading and offloading to be considered. The stone is quarried and broken locally by hand.

Stone

Available locally at site.

Stones suitable for foundations and masonry work were available from previous work at the site.

Bricks

Good quality bricks manufactured about 40kms from the site. Poor quality bricks are manufactured locally.

There are the same concerns with transport and loading. The bricks are of reasonable quality but dimensionally irregular. Special (angle ended) bricks were needed and this had to be arranged in advance — a mould was supplied and the special bricks were made and burnt in the next available batch.

Wood / timber

Poles for building and for making ladders and scaffolding are available locally

We could harvest these from the site, as they were growing on the land.

Sawn timber is available in town or sometimes locally if trees are being felled and sawn by local farmers.

Materials available in the local market place (a selection)

Item

Unit

Comment

Cement

Bag 50kg

Used for most of our construction work

Chicken wire 1/2"

Roll (30 x 0.9m)

Used for ferrocement work

4mm mesh

Roll (30 x 0.9m)

Useful for sieves and for ferrocement work

Rebar 8mm

13m length

Reinforcing and cover manufacture

Rebar 6mm

13m length

Reinforcing and cover manufacture

Welded mesh

2 x 1 m sheet

For concrete reinforcement

Binding wire

kg

For tying rebar and other uses

Barbed wire (double strand)

Roll (600m)

Used for our rammed earth work

GI and plastic pipes and fittings

Wide variety of sizes and components available

Water extraction

Sisal rope

Roll

General purpose

Nails

kg

General purpose

Water proof cement

kg

For tank linings

Fencing staples

kg

General purpose

Plastic sheet (250 micron)

87cm wide roll — bought by the metre

For plastic tube tank — quality in local market is dubious as ends get easily scuffed

Tarpaulins

5m x 4m

For tarpaulin tank — available in some hardware shops or possibly from lcoal agencies dealing with refugees

Timber

Any size to requirements

General purpose (not accurately cut)

Common skills available in the area

Skill

Comments

Local pole and mud construction

Known to, and practised by, most rural farmers

Brick laying and rendering

Widely used and known to most masons

Stabilised earth technology

Not known locally

Stone masonry

Known to some masons but not widely practised

Ferrocement tank construction

There had been some previous training in the area, so a number of masons had been exposed to the technology. One local mason was very experienced and did good quality work.

Carpentry

Several carpentry workshops in town with a limited range of power tools available (thicknesser, planer, power saw, pillar drill, etc). Most carpentry shops specialise in furniture making. The quality of the work varied enormously.

Local village carpenters have no power tools and have limited skills. Accuracy of work is generally low.

Lathe work can be done but not very accurately.

Metal work

Welding equipment is available in town, but quality of work is not high at most workshops. No turning or toolmaking equipment available. Angle iron and flat bar available locally but few other profiled sections.

Other

A wide range of services are available in Kampala, 4 hrs drive from Mbarara

7.5 Tank trials at Kyera Farm, Mbarara, as part of this Study



Figure 7.2a
The Plastic Tube Tank

Figure 7.2b
The brick ja
r

Figure 7.2c
The Ferrocement Ja
r

 

 

 

A technical study was undertaken as part of the Feasibility Study to allow the study team to build and assess a number of small-scale RWH systems suitable for local manufacture in the region. The study was carried out at Kyera Farm, a training centre in organic farming techniques and rainwater harvesting techniques, based 8kms south of Mbarara, in SW Uganda. During the study 3 types of small storage vessels were investigated, namely:

  • a partially below ground plastic-lined tank of 600 litres 
  • a cylindrical brick jar of 750 litres 
  • a ferrocement jar of 500 litres 

(Technical drawings of each of the designs is given in the Appendix II. Sizes given are approximate)

The aim of this study was:

  • to test three designs of small storage vessel (one well established and two new designs) 
  • to build prototype / demonstration RWH systems at Kyera Farm to assess the skills and materials required for each of the designs, and their suitability for local manufacture 
  • to make improvements to the design based on early experiences with the prototypes 
  • to investigate the use of RWH on grass roofs 
  • to build a number of systems in the local community to allow a survey to be conducted — the survey will look at the technical suitability of the systems, as well as the use of the jars by householders and the benefits and savings brought about by the RWH system 
  • to carry out a full costing for each of the RWH system

7.6 The designs

The design of the jars was undertaken using the principles set out earlier in this chapter.

The plastic lined tank was developed as a new innovation, specifically aimed at reducing costs. It is an adaptation of a larger partially below ground tank developed by the DTU in Uganda. The tank was designed in such a way that plastic tubular sheet, available in the local market, could be used to line a hole dug to a suitable diameter. The above ground section of the tank is made of brick. The handpump used with this tank was designed during the project and generated considerable interest, enough to warrant a short training course for local NGO technical staff.

In the case of the ferrocement jar, the design was taken from the RWH literature (Watt, 1978) and adapted slightly to suit local conditions. The size of the jar was increased from 250 litres, as suggested by Watt, to 500 litres. A tap was incorporated, and the jar set on a plinth, to allow water to be extracted without contamination. Chicken wire was added to the cement jar described by Watt, to give added strength and a combined cover and filter was incorporated to help improve and maintain water quality.

The cylindrical brick tank was developed as it was seen to be a tank, which very closely matches local skills, materials and known building techniques. Brick manufacture is common in the area and brick building techniques well known. The jar is cylindrical, which, as described earlier in this section, reduces stresses and gives a good material:capacity ratio.

It was decided that three designs should be developed, in order that a choice would be available to local artisans and to their ‘customers’.

Further information and design drawings are given in Appendix II

7.7 Small tank costs

A detailed costing of the small RWH storage vessels was undertaken and a breakdown of the costs are given in Appendix III. A brief summary of the costs is given in Table 7.7 to allow for easy comparison.

Table 7.7 Cost comparison between the jars constructed at Kyera

Type of jar

Size (litres)

Cost (USh)

Cost per litre storage capacity (USh)

Brick jar

750

83,000

110

Ferro-cement jar

500

70,000

140

Plastic tube jar

600

51,500

86

It is worth making a few general comments on the data presented in Table 7.7 and in the tables in Appendix III.

  • Cement is a major expense. A bag of cement costs 3 times the daily wage of a mason. For the jars constructed the cost of cement is dominant — 42% of the material cost for the f/c jar, 45% for the brick jar and 24% for the plastic tube jar. Reducing cement content can significantly reduce cost 
  • Irregular brick size increase cement content as extra mortar is used to fill the gaps. It is worth carrying out a quality control exercise at the brick manufacturing plant 
  • Water extraction can be made cheaper in most cases, but then there is the increased risk of contamination 
  • Further cost reduction exercises should be carried out e.g. reducing f/c wall thickness through proper experimentation, possibly omitting chicken wire from f/c tank, using more locally available materials such as wood poles and mud. It is worth bearing in mind that the jars constructed at Kyera were demonstration/ prototype jars and were constructed to a high standard

7.8 Training





Figure 7.3
Classroom sessions for the masons gave an opportunity to reinforce the techniques being taught as well as allowing the masons to discuss concerns and make suggestion
s

 

 

 

As part of the study, training was given to eight masons, 4 taken from the local community and 4 taken from a pool of masons who work closely with a local farmers organisation (IVA, Mbarara) who are already building RWH systems. The training was for a period of 6 weeks and was primarily ‘on-the-job’ training, with instruction being given by the project technician and with a classroom component included at the end of the period to re-cap on the work undertaken during the training. Feedback from the masons on the practical implications of the designs was absorbed and often changes implemented directly as a result of suggestions.

A series of Photo Manuals for the construction of these small RWH systems have been developed based on the work carried out at Kyera Farm. They can be found elseware on the DTU Web Site or obtained directly in hard copy from the DTU.

A pump training course was arranged as a result of high levels of interest shown by people attending the programme seminar. This course in Low-cost Handpump Manufacture was run over a two-day period on the 22nd and 23rd August 2000 by Vince Whitehead, a Warwick mature student (and experienced machinist) who is working at Kyera Farm voluntarily during his summer break.

 

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6. Health aspects
 
 

8. Technology - Other DRWH system components

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