Rainwater Harvesting – A Sustainable Tool [June 2012]

India conserves only around 1 per cent of rainwater and the remaining returns to the ocean, so there is a dire need to harvest rainwater, explains design and construction management professional – Vitthalrao Bhau Pise
Rainwater harvesting is the process of collecting and storing rainwater for reuse before it reaches the aquifer. It can be used to provide water for drinking, livestock, irrigation and industries.
Why to harvest rainwater? Demand for water is increased tremendously in the whole world with rising populations in cities, growth of industries and expansion in agriculture sector. We are tapping ground water reservoirs by using pumps in bore wells, open wells but we are not replenishing the resources. As a result groundwater levels are going down and water scarcity is created everywhere.
In cities, there is very little scope for water to soak into the ground due to massive constructions of houses, footpaths and roads. Whereas forests are getting reduced due to human habitation and cultivation, hills are losing its green cover, causing rainwater to flow down the slopes without getting soaked into ground. This is resulting into floodwater quickly flowing into rivers, however as soon as rain stops, the rivers dry up as there is no soaked water to make up in the dry period. If this water can be held back, it can seep into the ground to recharge groundwater.
We are withdrawing more water than what we replenish it, as a result water levels in the ground are getting depleted. So it is highly important to recharge the ground water through rainwater harvesting.
Water is the source of life. History shows that all our civilised society developed all along rivers. Water availability per capita is on the decline due to the increased demand for water and the increased population. Agriculture sector is the single largest consumer of water. Industrial demand for water is growing at fastest rate. Indian States like Rajasthan and Karnataka are drought prone regions. Groundwater from bore wells is increasingly getting depleted and polluted so it makes great sense for urban population to adopt roof rainwater harvesting to encourage self dependence and water conservation. It is being recognised that people from various sectors like agriculture, industries, and municipal corporations have to become partners in the process for water solution and not to remain simply as institutional beneficiaries.
India is blessed with adequate rainfall as a whole, still there are large regions having dry, drought prone area. In many other places the quality of groundwater is not good. In such places rainwater harvesting can provide lifetime water for survival and more potential for rainwater harvesting. The captured water can be used for household use or for recharging groundwater.
In the agriculture sector, crop productivity during scanty rainfall years can be increased by implementing suitable rain water harvesting methods on large scale by farmers, methods like filed bunds, waste weirs, compartment bunds, farm ponds, sunken ponds and planting tree species, contour cultivation. Farmers need to be trained through special training programs as participation of farmers is most important in such programs. This will help in long run in conserving soil and water.
Rainwater harvesting systems can be simple to construct from inexpensive local materials, and are potentially successful in most habitable locations. Roof rainwater may not be potable and may require filtration before consumption. The filtered water can then be used for all non‑potable uses like flushing, cleaning floors, gardening, washing cars etc.
Household rainfall catchment systems are appropriate in areas with an average rainfall greater than 200 mm (7.9 inch) per year; overflow from rainwater harvesting tank systems can be used to refill aquifers in a process called ground water recharge. The collected rainwater can be used to recharge the groundwater levels by using structures like dug wells, bore-wells, recharge trenches and recharge pits.
Water conservation in IndiaIndia receives largest amount of rainfall in the world. However, India conserves only around 1 per cent of rainwater and the remaining amount of rainwater returns to the ocean, whereas the developed countries conserve approximately 15-16 per cent of the available natural resources through rain/ice melting/river water etc. Therefore, we need to conserve water by adopting rain water harvesting.
Legislation on RWH in India and its importanceThe Central Ground Water Authority in India made it mandatory in the year 2001 for every household to have a rooftop rainwater harvesting system. The Government of India has made it compulsory to implement rain water harvesting (RWH) in the year 2003. Since then there is growing awareness on RWH. Now it is mandatory for new buildings in 18 out of 28 states and 4 out of 7 union territories. However, it is not strictly implemented.
The Government makes rules and regulations for people, without enforcement and penalisation system, so nobody complies and no one cares, this is the status of rainwater harvesting law. Everyone thinks that why to spend money on RWH when he gets his daily supply of water without doing it.
All cities in the world are growing at an alarming rate due to industrialisation, increase in population, and high migration from rural to urban area. Imagine if there is scarcity of water, what will we drink? How will we bath, wash, cook, build houses, run industries, cultivate crops, generate electricity? Rain water harvesting is an ancient technique but has lost its importance and now it is re-emerged as a great tool to overcome drought situation faced in each country.
Components of rainwater harvestingAny rainwater harvesting system has three components: Catchment area, conveyance of rainwater runoff and storage for later use. Catchment: Any surface such as building roof, paved/unpaved ground, natural streams can act as the catchment for rainwater harvesting. Based on the type of catchment rainwater harvesting may be classified into three categories. They are: • Roof top rainwater harvesting • Storm water harvesting from paved, unpaved footpaths, roads as catchment • Flood water harvesting from natural streams, channels as catchment. Rooftops have higher run-off coefficients and less contamination as compared to paved areas, footpaths and roads. Large paved/unpaved areas with adequate catchment management can provide good quantity and adequate quality of runoff water for use. Unpaved areas during heavy storms can act as rainwater catchment.
Conveyance: Rainwater gutters and rainwater down take pipes are conveyance systems for rooftop catchment. They are designed properly to take care of high intensity of rainfall. Storm water drains pipes and French drains with pebbles are also conveyance systems for storm water catchment. Debris and dirt removal is required to be done through filtration before it is either stored in storage tank or before it is let out to recharge structure.
Storage: Ground level tank, underground sump, surface lined ponds and large lakes serve as storage units in RWH system. Even soil itself serves as storage in the form of artificial recharge of rainwater for wells and bore-wells.
Roof water harvesting is in general 20 per cent of the total rainfall and water can be collected in tanks for potable water through filters. Ferro cement tanks or tanks made up of plastered brick walls are more suitable. It is most important to harvest runoff from open area instead of roof area. Life of domestic RWH system is 10-15 years. The flooding of an area can be avoided by injecting the rainwater into the soil. Urban runoff is polluted with sewage so rainwater is taken through separate gutters called storm water drains therefore it can be easily harvested. It is actually safe to inject all storm water into the soil. Estimate shows that only 20 – 25 per cent of the water is used for drinking, cooking etc. While 75 to 80 per cent is used for cleaning, bathing, washing and flushing the toilet. This indicates that almost 80 per cent of potable water goes down the drain as wastewater. This is not cost effective and we need to conserve and practice rainwater harvesting.
How to harvest rainwater?We have to collect water from the rooftop using gutters, and down take pipes. The water needs to be filtered before storing it into a sump or tank for later use. It can also be recharged into the ground through a soak pit or we can lead the water into a well to increase groundwater content. Artificial recharge systemsRecharge systems are structures similar to bore/tube wells but constructed for augmenting the groundwater storage in deeper aquifers through supply of water either through gravity or under pressure. Since they are manmade they are also sometimes known as artificial recharge systems.
Recharge structure designWater collected from terrace areas or open areas is first diverted to a desalting chamber and then into a recharge structure which is a bore dug up to the aquifer level. PVC casing pipe is inserted into the bore well till rocky formation is found. This is to prevent the loose soil from entering the bore and clogging it. A 200 micron “V” wire filter is attached at the bottom of the bore to further filter the water and also prevent sand & pebbles from entering the bore hole.
Groundwater recharge or deep percolation is a hydrologic process where water moves downward from surface water to groundwater. Groundwater is recharged naturally by rain and snow melt and to a smaller extent by surface water (rivers and lakes). Recharge, paving, manmade development activities like building projects can result in enhanced surface runoff and reduction in recharge. Groundwater recharge is an important process for sustainable ground water management; artificial groundwater recharge is becoming increasingly important in India, where over-pumping of groundwater by farmers has led to underground resources becoming depleted. Recharge can help move excess salts that accumulate in the root zone to deeper soil layers
Practical implementation of rain water harvesting In Tamil Nadu, rainwater harvesting is made compulsory for every building to avoid groundwater depletion. It has given excellent results. In Rajasthan, rainwater harvesting is practiced by the people of Thar desert. There are many ancient water harvesting systems in Rajasthan, which have now been revived.
FiltrationFiltration is most important process in the purification of water. It involves allowing water to pass through a filter media. Filtration involves removal of suspended and bacterial impurities present in water.
The recharge wells use gravel as the base material, which is placed at the bottom of the well. The thickness of the layer of base material varies from 45 to 60 cm. It is graded and laid in layers of 15 cm. The size of the gravel increases from top to bottom.
On this a layer of coarse sand, ranging in thickness from 60 to 90 cm is placed above the gravel. The effective size of sand used varies from 0.35 to 0.60 mm and its uniformity coefficient is between 1.20 and 1.70, ensuring increased rate of filtration on account of the increase in the void space between particles when compared to slow sand filters.
Charcoal Sand filter: A simple charcoal filter can be made in a drum or an earthen pot. The filter is made of gravel, sand and charcoal, all of which are easily available.
Sand filter: Sand filters have commonly available sand as filter media. Sand filters are easy and inexpensive to construct. These filters can be employed for treatment of water to effectively remove turbidity (suspended particles like silt and clay), colour and microorganisms.
Filter for large rooftops: Filter system is designed with three concentric circular chambers in which the outer chamber is filled with sand, the middle one with coarse aggregate and the inner-most layer with pebbles.
This way the area of filtration is increased for sand, in relation to coarse aggregate and pebbles. Rainwater reaches the centre core and is collected in the sump where it is treated with few tablets of chlorine and is made ready for consumption.
VARUN filters: Bangalore-based water harvesting expert S Viswanath has developed a rainwater filter called as ‘VARUN’. It can handle a 50 mm per hour intensity rainfall from a 50 square metre roof area, this means the product is relatively standardised. For new house builders we can therefore recommend the number of downpipes they have to optimise on and the number of filters they will need.     'VARUN' is made from a 90 litre high density poly ethylene (HDPE) drum. The lid is turned over and holes are punched in it. This is the first sieve which keeps out large leaves, twigs etc. Rainwater coming out of the lid sieve then passes through three layers of sponge and a 150 mm thick layer of coarse sand. Presence of sponge makes the cleaning process very easy. Remove the first layer of sponge and soak/clean it in a bucket of water which can be used for vegetation. The sand needs no cleaning at all.
Sump: A storage provision to collect filtered water is called sump. It can be cylindrical, rectangular and square in shape made up of RCC, ferro-cement concrete, brick masonry, or of PVC cleaning and disinfection of the sump has to be ensured to maintain the quality of water stored in the sump for direct use.
Recharge StructuresRecharging well: Rainwater is collected in a recharge well which acts as a temporary storage tank near the service tube well and it is shallower than the water table. This borehole has to be provided with a casing pipe to prevent the caving in of soil, a filter chamber comprising of sand, gravel and boulders is provided to arrest the impurities. Recharge pit: A recharge pit is 1.5m to 3m wide and 2m to 3m deep. The excavated pit is lined with a brick/stone wall with openings (weep-holes) at regular intervals. The top area of the pit can be covered with a perforated cover.
Soak ways/Percolation pit: Percolation pit is one of the easiest and most effective means of harvesting rainwater. It is not more than 60 x 60 x 60 cm pit, filled with pebbles or brick jelly and river sand and covered with perforated concrete slabs wherever necessary.   Recharge trenches: It is a continuous trench excavated in the ground and refilled with porous media like pebbles, boulders or broken bricks. A recharge trench can be 0.5 m to 1 m wide and 1 m to 1.5 m deep. The length of the recharge trench is decided as per the amount of runoff expected. The recharge trench should be periodically cleaned of accumulated debris to maintain the intake capacity.
Modified injection well: A modified injection well is 500 mm diameter bore hole drilled to the desired depth preferably 2 to 3 m below the water table in the area. Inside this hole a slotted casing pipe of 200 mm diameter is inserted. The annular space between the borehole and the pipe is filled with gravel and developed with a compressor till it gives clear water. To stop the suspended solids from entering the recharge tube well, a filter mechanism is provided at the top.  Vitthalrao Bhau Pise, BE (Civil), MIE, Design & Construction Management Professional. Mr. Pise has more than four decades of comprehensive construction experience including 16 years of consultancy in architectural and RCC designs of residential, institutional, public utility structures. He has successfully completed many residential, industrial, and public utility projects as a Project Manager in private sector. He has special interest in water supply, rainwater harvesting, and sewerage projects.

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