The numbers of water stressed regions at various part of the world are increasing due to rapid growth of real estate. The urban areas are facing twin challenges of water scarcity and inadequate capacity of wastewater disposal systems. The rapid growth of urban population leads escalation of water demand. Meeting these increasing water demands with a geographically constrained water supply system is often a very difficult task. Large construction projects are also being taken up at areas where municipal water supply is yet to be made available. The developers are frequently exploring and abstracting groundwater to ensure the basic amenities at their housing projects. Conservation of ground water is important because it takes years to be replenished. In areas where ground water is used, care must be taken to minimize the quantity of water withdrawn and bring it on per with quantity of water being replenished.
According to Chatterjee (2008), the modern methods of rainwater harvesting can be broadly categorized under two:
1. Collection and storage of rainwater for direct use, and
2. Groundwater recharging.
The combination of these two are also practiced, where rainwater is collected and stored in containers for direct use while the collected rainwater in excess of the storage capacity is charged into ground (groundwater recharging).
Some of the modern systems that are being used for rain water harvesting are described below:
1. Artificial Recharging
1.1 Absorption pit method
A percolation / absorption pit is a hand bore made in the soil with the help of an augur and filled up with pebbles and river sand on top. The depth of these pits will be anywhere between 4 and 8 meters depending on the nature of the soil. If the soil is clayey, the pit has to be dug to a depth till a reasonably sandy stratum is reached. The diameter of these pits will be 25 cm (10 inches).
A square / circular collection chamber with silt arrester is provided at the top. An absorption pit should be constructed in the sloped area of the house veranda. The size of the absorption pit should be minimum of 5 feet in width and a maximum of 10 feet in depth. The pipe which is installed for collecting the rainwater from the rooftop and the veranda of the house should be linked to this absorption pit. Another pipe should be deployed for allowing the excess water to run off into the storm water drainage.
1.2 Absorption well method
These wells are constructed using cement rings readily available in the market. The diameter of these rings range from 2 ft to 6 ft. The depth to which these wells are dug depends on the nature of the soil and the diameter depends on the number of roof top pipes that are likely to be connected to each one of these wells. These wells are left unfilled and are covered with RCC slabs of suitable thickness to facilitate free pedestrian and vehicular movement on the ground.
Rainwater from the terrace is diverted to the existing open well using PVC pipes through a filter chamber. The minimum size of the filter chamber is 2" x 2" x 2" filled with broken bricks in the bottom and sand on the top. The chamber may be covered with RCC slab.
1.3 Well cum bore method
In areas where the soil is likely to be clayey upto say 15ft. and more, it is advisable to go in for a percolation well upto 10ft. or 15ft. and a hand bore pit within this well upto a depth of 10ft. to 15ft. from its bottom.
1.4 Recharge trench cum injection well
In this technique, 1 to 2 m. wide and 2 to 3 m. deep trench is excavated, the length of which depends on the site availability and volume of water to be handled. An injection well of 100 to 150 mm diameter is constructed, piercing through the layers of impermeable horizons to the potential aquifer reaching about 3 to 5 metres below water levels ( 1 to 10 m.) from the bottom of the trenches. Depending upon the volume of water to be injected, the number of injection wells can be increased to enhance the recharging rate.
1.5 Bore well Recharging
Recharging of bore wells should be done to prevent them from drying up and improve their water table. It is usually done by the following method. First, a pit should be dug in the region surrounding the casting pipe and cement rings should be installed in it. The size of the pit should be one meter in diameter and 10 feet in depth. At the bottom of the pit, filter holes should be made and a casting pipe with steel mesh should be fixed tightly to the bore well pipe. This casting pipe will function as a filter. A second layer of two feet height filled with jelly stones of 40 mm should follow this. The third layer of one foot height should be filled with jelly stones of 20 mm size. The fourth layer should be filled with charcoal. The exercise should be repeated till there in 3 feet of space left from the ground level. A nylon curtain should be spread on the layers and the remaining space of the pit should be filled with sand until one foot from the ground level. A pipe should be fixed to collect the rainwater from the roof and this pipe should be connected to the pit. A pipe should be fixed to prevent the excess water from running away to roadside drainage.
1.6 Ground Water Recharging
Ground water recharging in urban areas is done by collecting the rainwater from the rooftop and the portico of the house and by making it easily absorbed within the veranda space. The method used is absorption pit method.
Water falling on a flat rooftop should be made to run through a pipe connected to the roof and prevented from running off to drainage on the roadside. This water should be filtered and stored in tanks constructed for the purpose of storing rainwater. A good quality filter is needed for purifying the water. This is because, although the rain water is basically pure, the water flowing from the rooftop usually will contain waste material in it.
Harvesting methods can be implemented for the following:
2.1 For Individual Houses
Roof top rainwater can be diverted to the existing Open / bore well. Along with this, rainwater available in the open spaces around the building may be recharged into the ground through the following simple effective methods.
· Percolation pits (Small Houses).
· Recharge trench (Big houses / Apartments).
· Recharge wells (Large buildings / Industries).
Based on the size / area of the building and the underlying litho logical nature of the formation the said methods may be used either individually or in combination. To enhance nature recharge of rainwater avoids pavements since unpaved surfaces have more percolation rate.
2.2 Rainwater Harvesting in Group Houses
Utilize the open well if any, within the complex to divert the rainwater from the terrace into it. If not, construct a well for the purpose. The rainwater falling on the open space around the complex can be collected near the gate by providing a gutter with perforated lid. The collected water can be led through necessary piping arrangements into a recharge well of 1 meter diameter and 5 meter deep.
3. Groundwater Dams
Groundwater dams are structured that intercept or obstruct the natural flow of groundwater and provide storage for water underground. They have been used in several parts of the world, notably
The main principal of groundwater dam is that instead of storing the water in surface reservoirs, water is stored underground. The main advantages of water storage in groundwater dams are that evaporation losses are much less for water stored underground. Further, risk of contamination of the stored water from the surface is reduced because as parasites cannot breed in underground water.
There are two main types of groundwater dam:
3.1 Sub-surface Dam
A sub-surface dam intercepts or obstructs the flow of an aquifer and reduces the variation of the level of the groundwater table upstream of the dam. It is built entirely under the ground.
3.2 Sand storage dam
The sand storage dam is constructed above ground. Sand and soil particles transported during periods of high flow are allowed to deposit behind the dam, and water is stored in these soil deposits. The sand storage dam is constructed in layers to be deposited and finer material be washed downstream.
A groundwater dam can also be combination of these two types. When constructing a sub-surface dam in a river bed, one can increase the storage volume by letting the dam wall rise over the surface, thus causing additional accumulation of sediments. Similarity, when a sand-storage dam is constructed it is necessary to excavate a trench in the sand bed in order to reach bedrock, which can be used to create a sub-surface dam too.
4. Ferrocement Technology
One of the primary requirements of a water harvesting system is that of containers to store the harvested water in a hygienic condition. This need is more pronounced in high-rainfall areas, where it is more feasible to store water in containers for direct use, rather than for recharging the groundwater. Generally, in small domestic systems, the cost of constructing tanks with conventional materials like masonary or RCC is far more than that of the rainwater collection and piping component. Ferro cement can provide a low-cost and easy-to-build solution to the need for low-cost containers. This technology is particularly relevant for regions like Meghalaya, Arunachal Pradesh and Kerala, which have frequency of rainfall.