The components of rainwater harvesting systems include:
(2) conveyance or conduit system
(3) first flush
(5) storage or recharge system
The catchment of a water harvesting system is the surface which directly receives the rainfall and provides water to the system. It can be a paved area like a terrace or courtyard of a building, or an unpaved area like a lawn or open ground. A roof made of reinforced cement concrete (RCC), galvanised iron or corrugated sheets can also be used for water harvesting. Coarse mesh at the roof to prevent the passage of debris.
(2) Conveyance systems
Channels all around the edge of a sloping roof to collect and transport rainwater to the storage tank. Gutters can be semi-circular or rectangular and could be made using:
Locally available material such as plain galvanised iron sheet (20 to 22 gauge), folded to required shapes.
Semi-circular gutters of PVC material can be readily prepared by cutting those pipes into two equal semi-circular channels.
Bamboo or betel trunks cut vertically in half.
The size of the gutter should be according to the flow during the highest intensity rain. It is advisable to make them 10 to 15 per cent oversize.
Gutters need to be supported so they do not sag or fall off when loaded with water. The way in which gutters are fixed depends on the construction of the house; it is possible to fix iron or timber brackets into the walls, but for houses having wider eaves, some method of attachment to the rafters is necessary.
Conduits and channels
Conduits are pipelines or drains that carry rainwater from the catchment or rooftop area to the harvesting system. Conduits can be of any material like polyvinyl chloride (PVC) or galvanized iron (GI), materials that are commonly available.
The following table gives an idea about the diameter of pipe required for draining out rainwater based on rainfall intensity and roof area:
A first flush device is a valve that ensures that runoff from the first spell of rain is flushed out and does not enter the system. This needs to be done since the first spell of rain carries a relatively larger amount of pollutants from the air and catchment surface.
Selection of a filter depends on followings:
(iii) Sand filters
(v) Dewas filters
Most residents in Dewas, Madhya Pradesh, have wells in their houses. Formerly, all that those wells would do was extract groundwater. But then, the district administration of Dewas initiated a groundwater recharge scheme. The rooftop water was collected and allowed to pass through a filter system called the Dewas fillter, designed by Mohan Rao, district collecter of Dewas and engineers of the rural engineering services. The water thus filtered is put into the service tubewell.
S Vishwanath, a Bangalore water harvesting expert, has developed a rainwater filter "VARUN".
(vii) Desilting chambers-
Very effective & essential for runoff from unpaved and paved areas or from storm water drains carrying huge amount of silt, tree leaves and other debris
(viii) Weave wire filter
It is made up of stainless steel and also of rigid PVC. It can filter out suspended solids coming with runoff. This type of filter is incapable for filtering any bacteriological contaminants if presents. The degree of filtration is 100- 200 microns and capacity ranges between 5000- 45000 litres / hour. Therefore it can be used in the systems where rainwater is harvested for non potable purpose only.
(ix) Pop up filter
Mr Shiva Kumar of Bangalore developed this design. The filtration is the nylon sieve (60 mm dia) inserted inside rainwater pipe to arrest coarse particles. The advantage with this filter is that whenever the filter gets clogged, it comes out of the casing and easy to maintain.
(5) Storage facility
There are various options available for the construction of these tanks with respect to the shape, size and the material of construction.
Shape: Cylindrical, rectangular and square.
Material of construction: Reinforced cement concrete, (RCC), ferrocement, masonry, plastic (polyethylene) or metal (galvanised iron) sheets are commonly used.
Position of tank: Depending on space availability these tanks could be constructed above ground, partly underground or fully underground. Some maintenance measures like cleaning and disinfection are required to ensure the quality of water stored in the container.
Rainwater may be charged into the groundwater aquifers through any suitable structures like dugwells, borewells, recharge trenches and recharge pits.
1. Recharging of dugwells and abandoned tubewells.
In alluvial and hard rock areas, there are thousands of wells which have either gone dry or whose water levels have declined considerably. These can be recharged directly with rooftop run-off. Rainwater that is collected on the rooftop of the building is diverted by drainpipes to a settlement or filtration tank, from which it flows into the recharge well (borewell or dugwell).
Providing the following elements in the system can ensure the quality of water entering the recharge wells:
2. Settlement tank
Settlement tanks are used to remove silt and other floating impurities from rainwater. A settlement tank is like an ordinary storage container having provisions for inflow (bringing water from the catchment), outflow (carrying water to the recharge well) and overflow. A settlement tank can have an unpaved bottom surface to allow standing water to percolate into the soil.
In case of excess rainfall, the rate of recharge, especially of borewells, may not match the rate of rainfall. In such situations, the desilting chamber holds the excess amount of water till it is soaked up by the recharge structure. Thus, the settlement chamber acts like a buffer in the system.
Any container, (masonry or concrete underground tanks, old unused tanks, pre-fabricated PVC or ferrocement tanks) with adequate capacity of storage can be used as a settlement tank.
3. Recharging of service tubewells
In this case the rooftop runoff is not directly led into the service tubewells, to avoid chances of contamination of groundwater. Instead rainwater is collected in a recharge well, which is a temporary storage tank (located near the service tubewell), with a borehole, which is shallower than the water table. This borehole has to be provided with a casing pipe to prevent the caving in of soil, if the strata is loose. A filter chamber comprising of sand, gravel and boulders is provided to arrest the impurities.
4. Recharge pits
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. Design procedure is the same as that of a settlement tank.
5. Soakaways / Percolation pit
Percolation pits, one of the easiest and most effective means of harvesting rainwater, are generally not more than 60 x 60 x 60 cm pits, (designed on the basis of expected runoff as described for settlement tanks), filled with pebbles or brick jelly and river sand, covered with perforated concrete slabs wherever necessary.
6. Recharge trenches
A recharge trench 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. In terms of recharge rates, recharge trenches are relatively less effective since the soil strata at depth of about 1.5 metres is generally less permeable. For recharging through recharge trenches, fewer precautions have to be taken to maintain the quality of the rainfall runoff. Runoff from both paved and unpaved catchments can be tapped.
7. Recharge troughs
To collect the runoff from paved or unpaved areas draining out of a compound, recharge troughs are commonly placed at the entrance of a residential/institutional complex.These structures are similar to recharge trenches except for the fact that the excavated portion is not filled with filter materials. In order to facilitate speedy recharge, boreholes are drilled at regular intervals in this trench. In design part, there is no need of incorporating the influence of filter materials. This structure is capable of harvesting only a limited amount of runoff because of the limitation with regard to size.
The volume of the storage tank can be determined by the following factors:
Dry season demand versus supply approach
In this approach there are three options for determining the volume of storage:
In practice the costs, resources and the construction methods tend to limit the tanks to smaller capacities than would otherwise be justified by roof areas or likely needs of consumers. For this reason elaborate calculations aimed at matching tank capacity to roof area is usually unnecessary. However a simplified calculation based on the following factors can give a rough idea of the potential for rainwater colection.
Suppose the system has to be designed for meeting drinking water requirement of a five-member family living in a building with a rooftop area of 100 sq. m. The average annual rainfall in the region is 600 mm (average annual rainfall in Delhi is 611 mm). Daily drinking water requirement per person (drinking and cooking) is 10 litres.
Design of groundwater recharge structures
Recharge of aquifers
For designing the optimum capacity of the tank, the following parameters need to be considered:
1.) Size of the catchment
2.) Intensity of rainfall
3.) Rate of recharge, which depends on the geology of the site
The capacity of the tank should be enough to retain the runoff occurring from conditions of peak rainfall intensity. The rate of recharge in comparison to runoff is a critical factor. However, since accurate recharge rates are not available without detailed geo-hydrological studies, the rates have to be assumed. The capacity of recharge tank is designed to retain runoff from at least 15 minutes rainfall of peak intensity. (For Delhi, peak hourly rainfall is 90 mm (based on 25 year frequency) and 15 minutes peak rainfall is 22.5 mm/hr, say, 25 mm, according to CGWB norms).
Design of a recharge trench
The methodology of design of a recharge trench is similar to that for a settlement tank. The difference is that the water-holding capacity of a recharge trench is less than its gross volume because it is filled with porous material. A factor of loose density of the media (void ratio) has to be applied to the equation. The void ratio of the filler material varies with the kind of material used, but for commonly used materials like brickbats, pebbles and gravel, a void ratio of 0.5 may be assumed.
Using the same method as used for designing a settlement tank:
Assuming a void ratio of 0.5, the required capacity of a recharge tank
= (100 x 0.025 x 0.85)/0.5
= 4.25 cu. m. (4,250 litres)
In designing a recharge trench, the length of the trench is an important factor. Once the required capacity is calculated, length can be calculated by considering a fixed depth and width.
There are a lot of similarities in the construction steps of both storage and recharge structures.
Construction of Storage Tanks
I. Masonry Tanks:
When building brick walls for water tanks, both horizontal and vertical joints are filled with mortar of a ration of 1:4. For obtaining maximum strength, lay out a circle of bricks or blocks on the foundation without mortar, with such spacing that no brick or block is cut to fit into the circle. A proper foundation of cement concrete will also have to be provided.
Covering the walls with polythene sheeting or plastic sacks, which must be properly secured against the walls using the sisal strings, does this. Water is poured between the wall and the sacks or polythene morning and evening for three weeks. The external wall can be made weather proof (if the tank is above the ground level) with two coats made of 1 part cement to 10 parts lime.
II. Reinforced Cement Concrete Tank (RCC)
Reinforced concrete tanks can be built above or below the ground. Concrete is durable and long-lasting, but is subject to cracking. An advantage of concrete cisterns is their ability to decrease the corrosiveness of rainwater by allowing the dissolution of calcium carbonate from the walls and floors. Each tank must have an overflow system for situations when excess water enters the tank. The overflow can be connected to the drainage system.
III. Ferro Cement Jars:
Ferrocement consists of a thin sheet of cement mortar which is reinforced with a cage made of wire mesh and steel bars. Because ferrocement is structurally more effectient than masonry, the thickness of the walls of the container are as low as 10 to 15 mm. Ferrocement components can be casted in any shape using suitable moulds. The technology is extremely simple to implement, and even semi-skilled workpersons can learn it with ease. Ferrocement requires only a few easily available materials - cement, sand, galvanized iron (GI) wire mesh, and mild steel (MS) bars - in small amounts compared to masonry and RCC.
Construction of roof for the tank
An assembly of mild steel elements is prepared as a skeletal frame for the roof. Chicken wire mesh is tied over it and plastered in cement mortar. The roof is provided with two openings. One is an opening of diameter 35 cm for accommodating the filter container. Another is a manhole with a 60 cm opening. The opening for the filter will be on one side of the roof. The manhole is provided at the centre of dome.
a) Construction of a new recharge well
Step 1: Excavating the earth
Step 2: Making a borehole to facilitate groundwater recharging
Step 3: Providing masonry or RCC walls in the excavated portion and thereafter providing the filter materials.
Step 4: Covering the tank made with a RCC or stone slab provided with a manhole.
b) Conversion of a dried up tube well into a recharge well
Step 1: Replace top few metres of the cast iron casing pipe of the dried tubewell with a perforated poly Vinyl chloride (PVC) pipe.
Step 2: Wrap the perforations with a screen-made of either coir screen or closely knit nylon mesh.