The main components of a rainwater harvesting system designed to directly reuse rainwater are:
Rainwater harvesting systems convey water from the rooftop through gutters and pipes to a storage (See videos for details on gutters & pipes, also see tips and precautions towards the end of this guide) .
To ensure good water quality management, the first rain separator (FRS) is designed so that the first 1-2 mm of rain is allowed to wash the rooftop and therefore ensures no accidental debris or dust is washed into the RWH storage from your clean catchment.
FRS is a critical part of the RWH system, as it ensures that dirty water gets filtered out. The FRS can be designed in many ways. One of the simplest ways to construct it is by putting in a cap or valve at the end of the downpipe that carries the rainwater. After every rain, you can simply open the cap/valve of the FRS to let out the first rain. You should also let water out of the FRS every time you clean the terrace. Watch water expert S. Vishwanath demonstrate this system here.
After the first rain accumulates at the FRS, the remaining quantity of rainwater passes through a filter, and then flows into the storage area in a barrel or sump.
At the filtration stage, only the physical impurities in the rainwater are removed. Remember, chemical or dissolved contaminants will still remain in the water.
Designing the filter can be a tricky job, as the rate of water flowing into it varies depending on the amount of rainfall. Water flowing into the filter may flow slow or fast; hence the filter design should ensure that water doesn’t overflow or be stagnant. The filter is usually designed to accommodate an inflow rate of 1 mm per minute rainfall intensity.
Filter media, or the material used for filtration, typically includes 40 mm or 20 mm jelly stones, Netlon mesh, and wood charcoal to absorb dissolved gases and avoid foul odour. The bigger the filter media, the faster the filtration. Cloth folded into eight (panche or the white dhoti) can also serve as filter media, but the cloth has to be kept clean and dry.
Filters are basically of two types, wall-mountable readymades, and DIY filters, and can be of made of different materials such as barrel, ferrocement or brick masonry. They can be installed either above the ground or below, depending on the points at which the pipes can be connected. Underground or subsurface filters are ideal for large catchments, typically for rooftops with area above 1000 sqm. These filters usually have two components – a silt trap or desilting chamber, and a twin-chamber filter.
This is the easiest RWH method. This video shows how a rain barrel can be installed. The rain barrel should be placed on a stand, and a tap installed at the bottom of the barrel, so that you can easily collect water from it. Ideally the barrel should be placed in areas where you use the most water, such as laundry area, car park, garden etc.
If you have an existing sump, it is easy to channel and store rainwater in it. But if the sump’s capacity is not large enough to hold the rooftop water runoff, you may need to build a new sump. The recommended size of the sump/barrel depends on the rooftop area and the number of members in the household. The sump can be made of RCC or brick masonry. This video explains how an RWH storage sump works.
In other cases, refer to this calculator. This tool can be used to design a sump of optimum volume knowing the rooftop that can be harvested from as well as the daily water demand.
If you do get high quality municipal water (as in the case of Cauvery supply in Bengaluru) and practice rainwater harvesting, you could use one sump to collect and store municipal water, and another for rain/borewell water. In apartments a separate overhead tanks would be useful to store pumped water from these two sumps and municipal water can be supplied to kitchens in each household for drinking and cooking purposes. And rain/borewell water can be supplied for all other purposes like bathing, washing clothes etc. Treated water from the apartment sewage treatment plant (STP) should be stored in a separate sump and tank, and used for toilet-flushing and gardening.
Costs vary widely depending on site conditions. But RWH system can be designed by giving consideration to budget, time and aesthetics.
Here are the average costs of retrofitting direct use RWH (these costs have been estimated based on pricing in 2017):
Here are the costs of the components: