Hydraulic ram pumps and Sling Pumps

2.2.2 The PVC foil tankSeveral industrial producers offer tanks of PVC foil. The foil is fixed inside a reinforcement meshframework or galvanized sheet cylinders, screwed together from sections. The tanks are availablefrom about 5.0 m³ up to 430 m³ Their considerable advantage lies in fast assembly and lowtransport costs. A reservoir of 9.25 m diameter (capacity 81.0 m³ can be transported on a small vanand be assembled within a couple of hours. No foundation is needed. Dismantling and reassemblyat another place can be carried out within a day or two. Apart from this advantage which is veryvaluable for cases requiring immediate action, for instance improvising a village water supply, thesystem has some weak points. Tanks of large capacity are uncovered, so evaporation is high andthere is a danger of pollution. More important for permanent use is the problem of ultraviolet rayinfluence on the PVC foil. Systems in use show signs of ultraviolet light effect on the material afterjust a few years. Otherwise the vulnerability to external force is great and tanks should always befenced in. For permanent rainwater catchment, although relatively cheap, this technique has itslimitations.2.2.3 The ferro-cement tank without mouldThis technique as explained by Laurie F. Childers of UNICEF Regional Office in Nairobi, Kenya, in1985 has been chosen by the author because of the unique advantages of this appropriatetechnology. There are many examples of such reservoirs in Kenya.This technique depends on the availability of welded reinforcement mesh. Since this is not to befound everywhere, other methods can be substituted.Firstly close attention has to be given to the cost of the material and the transport to the site. Anyother material used for this tank is more or less the same as for all ferro-cement tanks. The width ofthe roll of mesh or mats will be the height of the tank wall, about 1.80 m. This is certainly arestriction. Theoretically, it is possible to extend the height of the wall by using one and a half widthsof the mesh, overlapping it on a minimum of three fields and tying it together with the bottom circle,but this is not recommended. The entire structure becomes unstable and any vibration during theprocess of plastering will make the work very difficult. In addition a scaffold is needed which mightnot always be available. The fixing of the scaffold requires skilled workers.2.2.4 The ferro-cement tank with a factory-made mouldThe technique was described by N.J. Wilkinson, Botswana Technology Centre in his publication,and was chosen because of the considerable advantage it has for rainwater storage where all tanksare of the same size. Several examples of this are to be found in Botswana.This construction method can only be chosen if a factory or experienced workshop provides thefacilities for bending corrugated sheets and welding them neatly together. The technique is highlyappropriate in areas where a series of tanks are to be built. This is the case when new buildings likeschools are put up, and the design of the buildings already includes provision for rainwatercatchment. In such cases we can talk of a standardized tank.The mould can be used 10 - 15 times depending on the experience and careful handling of the staff.For larger projects it is advisable to have at least two moulds at the site. The advantage of thisconstruction method lies in the rationalization of the work. The masons become experienced andwork can be finished faster. With two moulds, the work can be organized with three crews. The firstcrew starts preparing the ground and then casts the foundation slab. The second erects the mouldand reinforces it, and the third crew does the plastering. The roof slab can be made by a fourth crewor by the first, depending on the amount of ground to be cleared. This technique should not beintroduced where only four or five reservoirs have to be constructed; in such a case the mould will13