If the roof, the gutter, the first rain separator and the filter is kept clean, the collected rainwater will be crystal clear. This is an indication that good maintenance is being followed. If the water is however dirty in color or it smells bad, then it means that the system is not being kept clean. Even if the water is clear and does not smell still it must be checked for micro-biological contamination. The checking should daily for the first one month and then weekly if the water is clear and not foul smelling. For this one must use a H2S strip test bottle. A schematic representation of H2S test is shown in the Fig 13 given below.
Fig: H2S Strip test bottle. Fill it with water. If the water turns black it needs to be treated
Though rainwater as it falls from the clouds is very pure, it does pick up dirt, dust and bacteria once it falls on the roof. It is very necessary to therefore check the quality of the water before using it for consumption. Once it is established that the rainwater is not micro-biologically contaminated it can then be consumed directly. However if the H2S strip test suggests that water has bacteria in it, it must then be treated before it can be used for drinking.
The method suggested for treating for bacteria is chlorination. Liquid chlorine or chlorine tablets are available for treatment of water. Depending on the volume of the rainwater in the tank, chlorine needs to be added to disinfect the water. Chlorination should be carried out every time there is rain and a fresh infusion of water into the tank. Using a chloroscope, residual chlorine of 0.20 mg/litre should be established before the water is used for drinking
Another form of deactivating bacteria and making water fit for consumption is called SODIS – Solar disinfection of water. In this method, rainwater is kept in a PET bottle or a glass bottle in the sun for 6 hours. One side of the bottle is painted black. The black surface is kept on the ground. With a combination of UV disinfection and infra red heat sterilization the water becomes fit for consumption. In cloudy weather the bottles need to be kept in the sun longer. A typical way of disinfecting rainwater is shown in Fig 14.
Fig : Solar disinfection or SODIS using a bottle painted half black
Cost of Rainwater Harvesting
The cost of each recharge structure varies from place to place. The approximate cost of the following structures is given in the table shown below.
Table1 : Cost of rainwater harvesting structures