Working Paper No. 55
Very-Low-Cost Roofwater Harvesting In East Africa
7. TECHNOLOGY DOMESTIC WATER STORAGE
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 LDCs. 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:
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
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 LDCs.
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:
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
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.
7.3 Cost comparison between imported and locally made
tanks in East Africa
Table 7.4: Cost comparison pence per litre storage capacity of tanks in East Africa
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 dont have sufficient money to purchase the larger tanks.
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
7.3 Choice of tank type
The type of tank that may be chosen will be dependent upon a number of factors:
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.
7.5 Tank trials at Kyera Farm, Mbarara, as part of this Study
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:
(Technical drawings of each of the designs is given in the Appendix II. Sizes given are approximate)
The aim of this study was:
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
It is worth making a few general comments on the data presented in Table 7.7 and in the tables in Appendix III.
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.
6. Health aspects