Components of Rainwater Harvesting
Rainwater Harvesting (RWH) is the act of capturing rainwater and either storing it for use or recharging it into the ground. In Apartments, water from clean catchments (roof) can be routed properly through suitable conduits into a filter. Filtered water can be stored in storage structures for subsequent use.
Surface run off can be used to recharge the ground water. This is achieved by recharge wells and/or direct tube-well recharge. Following are important Components of Rainwater Harvesting system.
- Conduit / Pipes
- Filtration
- Storage Structures
- Recharge Structures
Conduits/Pipes
Rainwater pipes/conduits carry water from the roof top to the ground. These pipes have to be connected and drawn to the required location by providing sufficient slope so that the water flows with gravity and no water is stored in the pipes after rain stops. The pipes can also be laid underground but sufficient care has to be taken in order to avoid damage to the pipe because of the vehicular movement. The cost of piping depends on various factors like quality of pipe and length of piping works to be done and many other site conditions.
Filtration
Rainwater has to be filtered for silt and suspended impurities before allowing into any of the storage structures in order to store it for a longer time.
Below are some of the filtration methods which can be implemented.
- First Rain Separator: First rain separator is a mechanism where in first few minutes of rain which carries most of the impurities from the roof are not allowed to pass into the filter or storage structures. First rain separators play a major role in rainwater harvesting structures by reducing the amount of silt / debris flowing into the filter or storage structures. This in turn reduces maintenance of filter and cleaning of the storage structure. First rain separators can also act as bypass arrangement if the rain water is not to be collected.
Design: Provide a capacity of 0.2 to 0.3 Litres per square meter of the roof area in case the roof is maintained clean.
- First Rain separator by using Y Joints in the Conduit: This can be designed by providing a Y joint for the rainwater pipes at about 8 – 10 feet height and closing the end of the down flow pipe using and threaded end cap. The Other end of the Y joint can be connected to the Filter or storage Structure. First few minutes of rain collects in the pipe till the water level rises to the Y joint level once the water starts flowing to the filter or storage structure. The silt and dirt in the water collects at the bottom of the pipe which has to be cleaned by opening the end cap at least once in a week. A drain hole has to be provided on the cap which ensures that the pipe gets emptied after every rain.
- Filter: Rainwater from the roof carries dust and suspended particles from the roof top. These impurities have to be filtered before allowing the water for storage. This can be achieved by passing the rain water through a chamber having layers of graded aggregates and a layer of charcoal. The suspended particles get trapped in the aggregate bed and charcoal adsorbs gases and odour if any. Filter also reduces the velocity of water and hence helps in sedimentation of fine particles in the filter.
Design: The size of the filter depends on the roof area and the cleanliness of the roof. The filter can be designed to a capacity of 0.5 to 0.8 Liters per square meter of the roof area.
Typical filter constructed at site
- Underground / Subsurface Filters: In case it is not possible to provide filters over the ground, underground filters can be constructed. The principle of filtration remains the same but the filter will have 2 chambers. The water flows through the filter media and moves to the second chamber through the underground pipe.
Typical Subsurface filter installed at site
Storage Structures
There are various options available for the construction of these tanks with respect to the shape, size and the material of construction.
Design of Underground Sump: If a new underground sump has to be constructed for the above capacity the size of the sump would be 1.5 M x 1.5 M x 1.2 M or 4 feet x 5 feet x 5 feet (Every Cubic feet holds approximately 28 Ltrs of water). Underground sumps up to 20,000 Litres can be constructed of brick or solid concrete masonry. If the required volume is higher the sump must be constructed of RCC with proper design.
Note: In most cases additional infrastructure like Sumps or recharge wells may not be required. Existing tanks can easily be used to store rainwater.
- Cost of Construction of a New Sump: A new underground sump constructed out of brick masonry will cost approximately Rs. 11 per liter and RCC sump costs approximately Rs. 14 per liter. For the given tank capacity, the cost of construction in Brick Masonry will be 2700 x 11 = Rs. 29,700.
The cost of Construction in RCC would be 2700 x 14 = Rs. 37,800
- Storage in Rain Barrels: Rainwater can also be stored in HDPE tanks for secondary uses. This procedure involves construction of a small pedestal and placing the HDPE Tank on the Pedestal and connecting the rainwater pipe to the Tank after First rain Separation unit. The water stored in the rain barrel can only be used for non potable purposes like gardening, car washing etc… Proper overflow pipe should be provided in order to avoid water logging near the rain barrel.
Typical Rain barrel installed at site
Approximate Cost of Rain barrel Installation
Recharge Structures
Ground water can be recharged using recharge wells. Ground water recharge helps in reviving the underground water level and can act as an additional source of water if recharged consistently.
- Recharge well: Recharge wells are constructed by digging the earth, placing concrete rings and packing the outer space of the ring using boulders.
Recharge Wells constructed at site
Below table can guide you to decide on the size and depth of the recharge well for the required capacity.
Note: The Cost mentioned is approximate and varies on many factors like the soil condition, ground water level, presence of rocks etc.
