|Rainwater Reservoirs above Ground Structures for Roof Catchment (GTZ, 1989, 102 p.)|
When you order a corrugated galvanized iron tank, ask for the measurements or find one from which you can take the measurement. The height between the gutter outlet (where the downpipe is connected) and the ground is your clear height. If the tank is 2.20 m high and you have 3.00 m clear height, prepare to build a plinth about 0.5 m above ground. If you can make a higher plinth, remember the advantage in case you want to connect a hose to distribute the water on a vegetable field. Pressure will be higher if the tank is elevated as high as possible.
If the tank is supplied before you could build the plinth, make sure the tank is stored safely. The larger the tank, the more it is affected by storms. More than once, a tank not properly stored has been blown away by storms and as a result badly damaged. The supplier only has to guarantee safe transport with loading and unloading. Check the tank for damage.
The plinth can easily be built of cement blocks. If they are hollow blocks use them upside down and fill the chambers of every course with concrete. Remove the topsoil on an area of slightly larger diameter than the tank and build a circular wall as high as needed (see Fig. 4.1). Place four steel anchors into the joint of the first or second course of the blockwork in such a way that they are opposite each other in pairs. This means one anchor at each quarter point of the circumference. They are used to fasten the tank with a steel rope. The space inside the circular wall should be filled with soil or gravel and be well compacted. Waste material is usually not suitable since it is very difficult to compact. Compaction should be done mechanically in layers not exceeding 300 mm, see Fig. 4.2. If possible the filled-up plinth should be compacted by water as well. Only after the filling is very well compacted should a concrete layer not less than 50 mm be applied. The top of the concrete must be flat, level and smooth.
It is common but not recommended to put the tank on top of the plinth. The problem is that after the tank is filled with water, occasionally depending on the difference in temperature between the water and the concrete plinth, condensation water will develop between the bottom of the tank and the concrete slab. This condensation water cannot evaporate easily because of the tied joint between tank and slab. Over the years, this water will cause corrosion to the tank's bottom. This can be avoided if a layer of 20-mm timber boards is placed on top of the plinth (see Fig. 4.3), before the tank is fixed in position. The advantage of this is double. Firstly, the timber will hinder the development of condensation and therefore this will occur less often. Secondly, if condensation develops because of high humidity and air temperature but low temperature of the water tank, this condensation can easily evaporate through gaps between the timber boards which allow ventilation. Another advantage of this layer of timber is that it is a relatively soft material and will act as a buffer between the hard concrete and the hard but very thin tank bottom. The timber must be treated.
The interior of corrugated iron tanks should always be painted with special water tank paint. This paint, usually black, is produced on a bitumen basis. The manufacturer indicates that the paint does not affect drinking water. Since most of these types of paints develop toxic fumes, painting inside the tank can only be done with breathing masks, but these are often not available. Another method is to pour paint into the horizontal tank through the inspection hole and to spread it by rolling the tank carefully. This protection paint reduces the corrosion of the metal from the inside and, by doing so, extends the life of the tank. The paint should also be used for the outside bottom of the tank and especially the soldered joints between bottom and corrugated tank wall covering, as well as two corrugations, before the tank is put up.
Every corrugated iron tank must be fixed in position with steel ropes or heavy 16-gauge fence wire. The larger the tank the greater the risk of storm damage if the tank is empty and not tied down to the plinth (see Fig. 4.4). A tank supplied by two gutters is shown in Fig. 4.5.
If a corrugated iron tank is used not only for rainwater but also for raw water from boreholes or wells, and if this water is considered to be contaminated, try to find out whether the contamination is caused by minerals. Water with a high content of salt or manganese accelerates the process of corrosion. Tanks used for corrosive water should be repainted at least every five years.