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In this article we will discuss about:- 1. Concept of Roof Water Harvesting 2. Amount of Roof Water Harvested 3. Conditions 4. Components 5. Process 6. Layout 7. Design 8. Maintenance 9. Advantages 10. Disadvantages.
Contents:
- Concept of Roof Water Harvesting
- Amount of Roof Water Harvested
- Conditions to RWH System
- Components of Roof Water Harvesting System
- Process of RWH System
- Layout of RWH System
- Design of Roof Water Harvesting System
- Maintenance of Roof Water Harvesting
- Advantages of Roof Water Harvesting
- Disadvantages of Roof Water Harvesting
1. Concept of Roof Water Harvesting:
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On occurrence of rainfall when rainwater falls onto the building’s roofs, then initially a part of that water gets lost as interception loss, and remaining part is converted into sheet flow, which flows over the roof surface and lastly falls over the ground surface. This water ultimately joins to the nearby stream/nala; and in this way, roof water is lost, if not properly stored at the safe point. However, if the roof water is channeled into a tank or any other water storage structure, then it can be utilized as per need.
Since, the roof surface is about to very clean in condition, so the roof water can be assumed to be in clean condition. Due to this property of roof water, the harvested roof water can be safely used for domestic purposes. The general requirements of harvested roof water are the convenience, quantity and quality, mainly. The convenience is normally available at every house on which rain falls. The quantity refers to the amount of obtainable water from the roof surface, which is normally very small as compared to the volume of rainfall on open ground surface.
At high demand level regarding domestic use, such as 50 litres per person per day, this technique is not able to fulfill the demand. However, for the demand of 15 to 20 litres per person per day, this may be quite good to fulfill the demands of households.
The quality of harvested rainwater varies with the seasons, the roof type and the complexity of the RWH system. Normally, the hard surfaces made of concrete materials or asbestos sheets produce the storable water of better physical quality.
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2. Amount of Roof Water Harvested
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The quantity of water that could be harvested from the roof surface depends on the rainfall depths and the roof area, mainly. For computing the quantity of harvested roof water, the following formula can be used –
Q = C x R x A … (16.8)
In which, R is the total annual rainfall (mm); A is the guttered roof area (sqm) and C is the runoff coefficient. The runoff coefficient considers the evaporation loss from the roof area and the losses between the roof and the storage point. Generally, its value varies from 0 to 1.0. For different roof surfaces the values of runoff coefficient are given in Table 16.17.
In assessing the amount of water to be expected from the RWH system, it is desirable to collect annual rainfall data; and also roughly to survey the size of roofs. The usable amount of harvested roof water is different than the harvested water. It is given by the following formula –
U = E x Q … (16.9)
in which, U is the usable quantity of harvested roof water; Q is the quantity of roof water and E is the storage efficiency, is less than 1.0.
The value of E depends on following factors:
i. Tank size (bigger is better but more expensive)
iii. Method, the water is drawn.
The total amount of water received from the rainfall over a specific area is called rainwater endowment of that specific area. And that portion of rainwater, which is effectively harvested is called water-harvesting potential. The water harvesting potential is given by the following relationship –
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Water harvesting potential = Rainfall (mm) x Collection efficiency
3. Conditions to RWH System
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In general, the roof water harvesting is not being appropriate, particularly where normal conditions for water harvesting are not met, water is cheap and sustainable, plentiful alternate supplies have already there or where very high water quantity and quality are required to supply.
The normal conditions for meeting the requirements of harvesting system are outlined as under:
i. The roof surface must be hard, rather vegetative. Such types of surfaces have very good potential of water yield for harvesting.
ii. There should be good amount of rainfall so that an adequate amount of water could reach to the guttered roof area.
iii. The harvested water should be equal to at least the planned rate of annual consumption per person.
iv. There should be available a suitable place/space within the housing plot to construct the water storage tank.
v. An extreme air-pollution should not be there in nearby area.
4. Components of Roof Water Harvesting System
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A roof water harvesting system mainly comprises following components:
1. Roof, and
2. Water storage tank
In addition, some RWH systems also have other components to make the system easier to manage or to improve the quality of the water. These components are described as under:
1. Roof:
A roof should be suitable to yield the water. The word “suitable” refers that feature of roof which could cause very little amount of rainwater absorption and does not pollute the water. The roofs made of tiles, metal sheets and asbestos sheets are most suitable for this purpose. The grass and palm-leaf roofs are generally not being suitable. The size of roof area also has very meaningful effect on roof water harvesting.
The larger the size of roof, the greater will be the run-off volume; and vice-versa. The rainwater reaching the roof in a year can be estimated as the annual rainfall times the roof plan area. In tropics about 85% of water runs off the roof; and remaining 15% is lost through evaporation and splashing. If the rainfall takes place in the form of light drizzle, then there is very little possibility of yield of runoff because of greater loss of water through evaporation and less availability of water for converting into runoff.
Especially, in the areas of low annual rainfall, the available water is sometimes not enough to capture in sufficient amount for satisfying the water needs of people living in the building. In this case, either the roof area must be extended, or roof water harvesting should only be for limited demand.
In order to have better performance, the roof should satisfy the following requirements:
i. More than 80% of the roof surface must be easy to connect with the gutter.
ii. The water that comes from the roof area must be free from poisonous contaminants, especially the dissolved contents.
iii. For the roof to yield good quality water, the roofing material must be impermeable.
If impermeable roof is not available, then roof area can be covered with polythene sheet or a tarpaulin coating. Generally, covering an organic roof with plastic sheet is not advisable.
Water-Storage:
Performance of RWH system depends very much on the size of water storage tank. A RWH system with large size water storage tank performs better than one with the small storage tank. A small storage tank say 500 litres capacity is likely to get overflow during intense storm. Or there is wastage of huge amount of run-off in case of small size storage area. Also, the stored water gets finished very soon.
However, a small size storage tank involves lesser cost than the bigger one; and gives cheaper water. The designer should therefore consider the cost and storage capacity of the tank to perform better. Water storage tank may be the plastic tank, drum or cistern. They can be installed above ground, underground or partly below the ground. A RWH system comprises only one water storage tank, but in the situations when they are cheaper to use then several tanks can also be used, around the roof area.
The basic requirements for water storage tanks in RWH system are mentioned as under:
i. In comparison to the quantum of water requirement, the water losses under seepage and evaporation should always be less than 5% of daily demand.
ii. There is no danger to the users.
iii. It should provide the quality water commensurate with its intended use.
For example – if it is to be used for drinking purposes, then tank should have following requirements:
(a) Fully covered to prevent entry of light and intrusion of mosquitoes and small creatures in tank water.
(b) Should be ventilated to prevent an aerobic decomposition of any matter.
iv. Ideally, it should also be affordable; durable; have a means by which the water can easily enter and withdrawn; have some arrangement to satisfactorily handle the tank overflow; and should be easy to clean and look attractive.
Gutter:
This a type of arrangement for allowing the runoff water from the roof surface to the water storage tank. Sometimes, it is also called down pipes. The gutters are open channels to carry water sideways, under the edge of the roof to a point just above the water storage tank.
Down pipes are the tubes, conveying the water down from the gutters to the entrance of the water storage tank. Amongst different ways of water transfer from the roof to the storage tank, the guttering is the most popular method because it keeps the run-off water clean. Gutter must be capable of catching the run-off from the roof and carrying the same to the storage.
Apart from above components of RWH system, there are few other components also, which are mainly the screens and filters, overflow arrangement, level gauge and pump to lift the water out of the underground tanks (if there).
A good gutter should include following features to work well:
i. Fully capable for catching the water as it runs off the roof.
ii. It should smoothly convey the water towards down pipe or outlet without overflow.
iii. It should be cheap.
iv. It should be durable and resistant.
v. It should be in optimum size, i.e. neither too small nor too large.
vi. It should be equipped with proper slope; and at the proper distance from the house wall/roof edge.
The size and shape of gutter significantly affect the ability to catch the runoff and to discharge the water in the storage tank. A gutter with very wide width and large cross-sectional area is found good for water flow through. The cost of gutter depends on its perimeter, therefore, the perimeter should not be very large. The factors such as ease of manufacture; ease of cleaning and ease of hanging affect the shape of gutter. In general, the trapezium shape with equal-size bottom and sides, and semi-circle shapes are very common.
The slope of gutter is towards downward the point where it is connected to the down pipe. The slope provides gradient to flow the water; and also causes an extra water carrying capacity to the gutter. For a given roof and rainfall, the gutter cost can be reduced by increasing the gutter slope. For making gutter’s capacity double, the slope should be increased four times. The slope may be uniform; or it may be kept more towards the outlet.
Usually, the roof edge is kept horizontal; and there should be a gap between sloping gutter and the horizontal roof-edge. The gap becomes big, when gutter slope is steep. In general, for most of the gutter sizes, the gap is kept lesser than the gutter width. In case of house buildings the gap should be less than 60 mm, whereas for a long school buildings it may be 150 mm. Fig. 16.20 illustrates the effects of gutter slope on the gap between the roof and the gutter.
Gutter Size:
The size of gutter mainly depends on the rainfall and wind conditions. During light rainfall and no wind, even a small gutter can intercept and carry all the roof runoff, efficiently. On the other hand, under intense rainfall, especially in case of corrugated roofing, there is risk of loss of runoff water, by overshooting the gutter or by causing the gutter overflow. For deciding the gutter size, the 2 mm/minute rainfall intensity is normally considered.
In case when rainfall intensity exceeds 2 mm/min, then there is spilling of excess runoff water. To avoid this happening the gutter slope is adjusted. The suitable gutter sizes for trapezoidal/ semi-circular gutters are given in Table 16.18. Note that if the gutter to be drained a small roof area, then a very small gutter size can be used. In such situations a PVC pipe as small as 50 mm outside diameter can be used as gutter.
2. Water Storage Tanks:
The function of storage tank is to collect the roof water via gutter. The size and types of tanks to be used for the purpose is an important task. The size should be such that it could easily store the rainwater of an individual rain event. In RWH system the cleaning of tank is also very important consideration, which should be easy to perform by a householder.
Various types of tanks, which are commonly used in different countries, for RWH are outlined as under:
i. Moulded Plastic Tank:
The plastic made tanks are very popular, worldwide. These tanks are usually made of High-Density Polyethylene (HDPE) or Glass Reinforced Plastic (GRP) materials. Special features of this type of tank is that they are handy, can be transported from one place to another, easily. Also, their weight is very less as compared to the metal-based tanks, as result quick to install. Nowadays, these tanks are available in different storage capacities.
ii. Drum Tank:
The drum tanks are mostly used in Sri Lanka, Ethiopia and Uganda in roof water harvesting system. In RWH system, 2 vertically stacked drums with welded seams are used to prevent the leakage. During rainfall, as the runoff flow takes place, the same is allowed to get store in the tank through gutter.
Particularly, for heavy rainfalls an option is there to add separate tank for receiving the excess flow. The drum tank is basically a portable type tank, which can be easily transported from one place to another, as per need. Also, if there is space problem regarding its installation, then wider base width tanks can be replaced with greater vertical height drums.
iii. Open-Frame Ferro-Cement Tank:
These tanks are used throughout Asian and African countries for RWH. As compared to the others, it is more expensive, but being more flexible in size. Its shape is in cylindrical form. And is made of BRC mesh or a network of reinforced bars. These bars provide a good strength to the tank.
iv. Pumpkin Tank:
The pumpkin tank was firstly introduced in Sri Lanka in the Community Water Supply and Sanitation Programme during 1995; and since then several thousand such tanks have been constructed in various parts of Sri Lanka. These tanks are made of Ferro cement materials. Shape of these tanks is spherical, formed by rounding the top and bottom, which casts the look of pumpkin.
v. Plate Tank:
This type of tank was developed in North-eastern Brazil. Nowadays, this has become most popular in Brazil. It is installed by keeping about 2/3 below the ground, and 1/3 above the ground. The construction is done by using plated mortar or sometimes concrete in 3-4 cm thickness and about 50 cm square. The plates are placed together and fixed by winding with the wire around construction. A layer of mortar is plastered inside and outside to finish the tank. The roof is also constructed from pre-cast parts, which are placed and plastered to produce the final tank.
vi. Interlocking Block Tank:
This tank was originally constructed in Thailand. It is constructed by means of several blocks which interlocked together. The blocks are constructed using bamboo-cement. Each block has interlocking provision, which efficiently transfers the load between the blocks and also reduces the mortar quantity for joining the blocks. For making the block with sufficient strength the thickness is kept little more, and a high level of compaction is used during manufacturing. Or sometimes reinforcement is also done for constructing the block.
vii. Brick-Lime Cistern:
This was developed in Brazil. It is constructed with the help of brick and lime. This tank is constructed totally underground with dome protruding. For construction, a pit of desired size is dug in the ground; and bricks are directly laid against the sides of excavation, and mortared together with the lime. Also, the inside surface is sealed with the help of lime-cement mix. And waterproofing is done by coating the cement slurry.
In addition to above mentioned tanks for RWH system, the mud tank, plastic tube tank, thatch tank etc., are also very common in few countries.
5. Process of RWH System
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The process diagram of RWH system (domestic) is presented in Fig. 16.21. In water harvesting process the roof area is the catchment for yielding the runoff. The runoff water is conveyed to the water storage tank by means of guttering system and down pipe. The stored runoff water in the tank is delivered by the tap, which is connected to the tank.
For making the roof water clean there is provided a filtering system or separation unit, before storage tank. Filtering system separates the foreign materials available in the roof water. The harvested water in the storage tank is required to treat for its safe use. The treatment includes chlorination, solar disinfection or use of a ceramic filler after the water is drawn from the tank.
6. Layout of RWH System
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The layout of system has a very important deal on gutter requirement. If the layout permits to locate the tanks anywhere, then it should always be taken into consideration that the size of gutter should be minimum. Various possible layouts of RWH systems with their details are presented in Table 16.19.
Filtering and Separation of Harvested Roof Water:
If harvested roof water is sought to use for drinking purposes, then up to some degree the water is required to get filter for removing the foreign materials. This is carried out by using suitable filters/separators.
The selection of suitable filters for this purpose, the following criterion are commonly followed:
i. Capable to handle the flow produced by the individual rainfall.
ii. Should be easy to clean.
iii. They should not get block frequently, and the blockages should be easy to see.
iv. They should not become as a source of additional contamination, if left un-cleaned.
v. The cost should not be very high.
vi. The amount of water loss under washing the filter should be very less.
7. Design of Roof Water Harvesting System:
Various steps involved, are outlined as under:
i. Collection of Roof Water:
For this purpose clean the roof first. Also, avoid chemicals and other harmful materials on the roof surface. In surroundings if there are trees, then remove the fallen leaves from the roof carefully (everyday) especially during rainy season.
ii. Flow Down the Water:
Drain pipes are equipped for this purpose. Normally, total 3 drain-pipes are placed to convey the roof water down at the Rain Centre. The diameter of drainpipes is about 3-inch capable to sustain 6 Kg of water pressure.
iii. Flush the Rainwater:
Initially, the roof surface contains few foreign materials, such as leaves etc., which are carried by the flowing rainwater. This water should be flushed. For this purpose, initially few litres of collected water when starts raining, are flushed.
iv. Filter the Rainwater:
The filtering of collected rainwater is very essential to remove the foreign materials. For this purpose a suitable capacity filter chamber is provided in the system. The filter chamber is filled by 1/3 of its volume with the help of pebbles in 2 layers along with coarse sand between the two layers. Each drainpipe is equipped with individual filter chamber; however, a group of drain-pipes can also be equipped with single filter chamber, but the size of filter should be kept, accordingly.
v. Allow the Flow to the Sump:
Water from the filter chamber is allowed to get flow into the sump. The collected water in the sump can be used for the purpose.
vi. Allow the Excess Water to Ground Water Recharge:
This is done when ah excess rainwater is coming into the sump, which likely to get overflow. In this condition, the water that overflows the sump is directed to the well, from where water gets percolate into the underground.
The flow chart identifying the location of water harvesting in the entire flow system of rain water and snow melt water, is shown in Fig. 16.23.
8. Maintenance of Roof Water Harvesting:
The maintenance of RWH system is mainly required in following respects:
i. The obstructions around building roofs affecting the falling of rainfall over roof area should be carefully removed, otherwise, the net amount of rain likely to fall over the roof surface will get affected; and accordingly, the quantity of roof water, which is to be harvested, will also be affected.
ii. If there have been deposited few objects like vegetations or any other matters on the roof surface, then they should be removed, because they obstruct the flow of runoff. Sometimes, few vegetations are naturally grown at the roof surface, they must be removed.
iii. The gutter system should be properly inspected, if there is any leakage etc., then that should be repaired.
iv. The tank cleaning should be done properly. For this purpose, the tank should be inspected. If there is deposition of silts or any other types of materials, then they should be immediately cleaned.
9. Advantages of Roof Water Harvesting
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Various advantages of roof water harvesting are outlined as under:
i. Contributes to meet the increasing water demand.
ii. Recharges the groundwater.
iii. Enhances the availability of groundwater.
iv. Reduces the runoff, as result prevents the choking of drains and flood.
v. Reduces groundwater pollution.
vi. Provides self-sufficiency for water availability; and to supplement domestic water requirement during scare periods.
vii. It reduces the energy consumption for groundwater pumping. It is a thumb rule that, on rise of water table for 1 m height, there is saving of 0.4 KWH of electricity.
viii. It reduces the soil erosion.
ix. In saline or coastal areas, the roof water harvesting provides a means for storage of quality water.
x. It also provides a means for reducing the salinity level in saline area; and also maintains a balance between the fresh water and saline water interface.
xi. In islands, where there is limited extent of fresh water aquifers, the roof water harvesting generates a kind of fresh water resource to supply for domestic use.
xii. In desert area, where rainfall is very low, the roof water harvesting provides a kind of relief.
10. Disadvantages of Roof Water Harvesting
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i. Since, after getting rainfall over the ground surface, the rainwater moves over the land surfaces, where pollutants are there. The pollutants mix in the rainwater. In this way, the stored/harvested water is full of pollutants, leading to health problem. In India, the cities such as Ludhiana (Punjab), Kanpur (U.P.) etc., are facing such type of problems.
ii. Causes water quality deterioration by:
(a) Increasing the level of nitrates and fluorides in the water.
(b) High concentration of cadmium, lead, iron and chromium in the water.
(c) Increasing the bacteriological contamination levels in the water.