Living creatures of the universe are made of five basic elements, viz., Earth, Water, Fire, Air and Sky, Obviously, water is one of the most important elements and no creature can survive without it. Despite having a great regard for water, we seem to have failed to address this sector seriously. Human being could not save and conserve water and it sources, probably because of its availability in abundance. But this irresponsible attitude resulted in deterioration of water bodies with respect to quantity and quality both. Now, situation has arrived when even a single drop of water matters. However. " better late than never", we have not realized the seriousness of this issue and initiated efforts to overcome those problems.
System of collection rainwater and conserving for future needs has traditionally been practiced in India. The traditional systems were time-tested wisdom of not only appropriate technology of Rainwater Harvesting, but also water management systems, where conservation of water was the prime concern. Traditional water harvesting systems were Bawaries, step wells, jhiries, lakes, tanks etc. These were the water storage bodies to domestic and irrigation demands. People were themselves responsible for maintenance to water sources and optimal use of water that could fulfill their needs.
What is Rainwater harvesting?
The term rainwater harvesting is being frequently used these days, however, the concept of water harvesting is not new for India. Water harvesting techniques had been evolved and developed centuries ago.
Why harvest rainwater ?
This is perhaps one of the most frequently asked question, as to why one should harvest rainwater. There are many reasons but following are some of the important ones.
To arrest ground water decline and augment ground water table To beneficent water quality in aquifers
To conserve surface water runoff during monsoon
To reduce soil erosion
To inculcate a culture of water conservation
Water management
If water management is not accorded the importance it deserves, the country can very much expect to find itself in troubled waters as the years roll by. Estimates of the Central Ground Water Board are that the reservoir of underground water will dry up entirely by 2025 in as many as fifteen States in India – if the present level of exploitation and misuse of underground water continues. By 2050, when more than 50 per cent of the Indian population is expected to shift to the cities, fresh drinking water is expected to get very scarce. A new category of refugees is expected to emerge around that time: the water migrants. Future wars, between or within nations will be fought on the issue of water.
India –which has 16 per cent of the world’s population, 2.45 per cent of the world’s land area and 4 per cent of the world’s water resources already has a grave drinking water crisis .
The reservoir of underground water ,estimated presently at 432 billion cubic meters (BCMs) has been declining at a rapid rate of 20 cms annually in as many as fifteen States with major metropolitan centers including Delhi, being estimated to go dry as early as 2015 on account of over-exploitation and misuse.
According to a study done by the New Delhi-based Central Ground Water Board, it will take just 2,600 additional tube wells running at an average of ten hours per day to exhaust the entire reserve of underground water in Delhi. Punjab, Haryana, Bihar, Andhra Pradesh, Gujarat, Karnataka, Madhya Pradesh, Maharashtra and Orissa have also been categorized by the Ground Water Board as the potentially ‘gray areas’.
Problem
The annual inter-State feuds over water are becoming more and more common in India . Per capita water availability in the country which was 5,000 cubic metres earlier , has dropped to 2,200 cubic meters. This is against the world figure of 8,500 cubic meters. As a result, India is fast approaching a phase of stressed water availability conditions.
Several perennial flows like the Ganga-Brahmaputra-Barak are becoming seasonal. Rivers are dying or declining and aquifers are getting over-pumped. Thus, people have to depend on limited municipal water supply. Consequently, they are forced to rely on their own resources. This scarcity has led to the birth of water markets with private entrepreneurs doing business in supplying water tankers. This, once again, is putting pressure on surface and groundwater sources which are fast depleting all over the country.
Eighty-five per cent of India’s urban population has access to drinking water but only 20 per cent of the available drinking water meets the health and safety standards.
Furthermore, there are serious inequities in the distribution of water. Consumption of water ranges from 16 liters per day to 3 liters per day depending on the city and the economic strata of the Indian consumer.
The water in rivers is wasted as it flows into the sea and is not properly harnessed. The debate on dams as a means of harnessing water continues to make this issue politically and environmentally sensitive. No clear ecologically stable and financially viable solution has emerged.The poor state of local and municipal authorities renders them unable to provide basic water to the cities. Strengthening of local bodies could lead to another means of addressing this issue.
Policy
India’s national water policy gives overriding priority to drinking water. The policy requirements of urban development projects include a drinking water component. India is developing both its ground and surface water resources. Current policies prioritize the utilization of static reserves to prevent ground water mining but development of ground water mining is very intensive in Punjab, Haryana, Uttar Pradesh and some other parts of India.
India’s rainfall is temporal (with as much as 70 per cent rainfall occurring in four months) and the rain is also unevenly distributed. With the glacier or snow-fed surface or river water, there is the multiplying problem of pollution.
There are vast stretches of Indian rivers that are unable to support fish on account of the levels of pollution caused by the unwillingness or reluctance on the part of small and big industrialists to adhere to effluent treatment norms. While pollution is a problem, an equally important issue is to prevent the groundwater levels from declining any further. And it is here that the concept of rainwater harvesting comes into place.
How to harvest rainwater:Broadly there are two ways of harvesting rainwater:
(i) Surface runoff harvesting
(ii) Roof top rainwater harvesting
Surface runoff harvesting:
In urban area rainwater flows away as surface runoff. This runoff could be caught and used for recharging aquifers by adopting appropriate methods.
Roof top rainwater harvesting (RTRWH):
It is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the catchments, and the rainwater is collected from the roof of the house/building. It can either be stored in a tank or diverted to artificial recharge system. This method is less expensive and very effective and if implemented properly helps in augmenting the ground water level of the area.
Advantages of Rainwater Harvesting
1. Easy to Maintain: Utilizing the rainwater harvesting system provides certain advantages to the community. First of all, harvesting rainwater allows us to better utilize an energy resource. It is important to do so since drinking water is not easily renewable and it helps in reducing wastage. Systems for the collection of rainwater are based on simple technology.
The overall cost of their installation and operation is much lesser than that of water purifying or pumping systems. Maintenance requires little time and energy. The result is the collection of water that can be used in substantial ways even without purification.
2. Reducing Water Bills: Water collected in the rainwater harvesting system can be put to use for several non-drinking functions as well. For many families and small businesses, this leads to a large reduction in their utilities bill. On an industrial scale, harvesting rainwater can provide the needed amounts of water for many operations to take place smoothly without having to deplete the nearby water sources.
3. Suitable for Irrigation: As such, there is little requirement for building new infrastructure for the rainwater harvesting system. Most rooftops act as a workable catchment area, which can be linked to the harvesting system. This also lessens the impact on the environment by reducing use of fuel based machines.
Rainwater is free from many chemicals found in ground water, making it suitable for irrigation and watering gardens. In fact, storing large reservoirs of harvested water is a great idea for areas where forest fires and bush fires are common during summer months.
4. Reduces Demand on Ground Water: With increase in population, the demand for water is also continuously increasing. The end result is that many residential colonies and industries are extracting ground water to fulfill their daily demands. This has led to depletion of ground water which has gone to significant low level in some areas where there is huge water scarcity
5. Reduces Floods and Soil Erosion: During rainy season, rainwater is collected in large storage tanks which also helps in reducing floods in some low lying areas. Apart from this, it also helps in reducing soil erosion and contamination of surface water with pesticides and fertilizers from rainwater run-off which results in cleaner lakes and ponds.
6. Can be Used for Several Non-drinking Purposes: Rainwater when collected can be used for several non-drinking functions including flushing toilets, washing clothes, watering the garden, washing cars etc. It is unnecessary to use pure drinking water if all we need to use it for some other purpose rather than drinking.
Disadvantages of Rainwater Harvesting
1. Unpredictable Rainfall: Rainfall is hard to predict and sometimes little or no rainfall can limit the supply of rainwater. It is not advisable to depend on rainwater alone for all your water needs in areas where there is limited rainfall. Rainwater harvesting is suitable in those areas that receive plenty of rainfall.
2. Initial High Cost: Depending on the system’s size and technology level, a rainwater harvesting system may cost anywhere between $200 to $2000 and benefit from it cannot be derived until it is ready for use. Like solar panels, the cost can be recovered in 10-15 years which again depends on the amount of rainfall and sophistication of the system.
3. Regular Maintenance: Rainwater harvesting systems require regular maintenance as they may get prone to rodents, mosquitoes, algae growth, insects and lizards. They can become as breeding grounds for many animals if they are not properly maintained.
4. Certain Roof Types may Seep Chemicals or Animal Droppings: Certain types of roofs may seep chemicals, insects, dirt or animals droppings that can harm plants if it is used for watering the plants.
5. Storage Limits: The collection and storage facilities may also impose some kind of restrictions as to how much rainwater you can use. During the heavy downpour, the collection systems may not be able to hold all rainwater which ends in going to to drains and rivers.
Role of Apartment Owners Association
Apartment owners can put pressure to builder while constructing apartment to implement rainwater harvesting either for direst use or for recharging groundwater. Apartment owners can form an association for implementing rainwater harvesting system in the apartment The cost can be divided equally by all owners. The cost of installing a rainwater harvesting is high but when comparing the amount saved on water bill for future, this cost is cheaper or nothing. There must be a separate pipeline for rainwater and for groundwater. Nowadays each apartment will have its own apartment owners association who play vital role in managing properties and working for welfare of residents of apartment. An active apartment owners association can easily implement rainwater harvesting and your association can become role model for apartments in your neighborhood or in your city
COMPUTATION OF ARTIFICIAL RECHARGE FROM ROOF TOP RAINWATER COLLECTION : Factors taken for computation :
Roof top area 100 sq.m. for individual house and 500 sq.m. for multi-storied building.
Average annual monsoon rainfall - 780 mm.
Effective annual rainfall contributing to recharge 70% - 550 mm.
Individual Houses Multi storied building
Roof top area
100 sq. m.
500 sq. m.
Total quantity available for recharge per annum
55 cu. m
275 cu. m.
Water available for 5 member Family
100 days
500 days
A few techniques of roof top rain water harvesting in urban areas are described as under:
(i) Roof Top Rainwater Harvesting through Recharge Pit:
a. In alluvial areas where permeable rocks are exposed on the land surface or at very shallow depth, roof top rain water harvesting can be done through recharge pits.
b. The technique is suitable for buildings having a roof area of 100 sq m and is constructed for recharging the shallow aquifers.
c. Recharge Pits may be of any shape and size and are generally constructed 1 to 2 m wide and 2 to 3 m deep which are back filled with boulders (5-20 cm), gravels (5-10 mm) and coarse sand (1.5-2 mm) in graded form— Boulders at the bottom, gravels in between the coarse sand at the top so that the silt content that will come with runoff will be deposited on FIG. 16.7. Roof top rain water harvesting through recharge pit the top of the coarse sand layer and can easily be removed. For smaller roof area, pit may be filled with broken bricks/cobbles.
d. A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the pit and a desalting/collection chamber may also be provided at the ground to arrest the flow of finer particles to the recharge pit.
e. The top layer of sand should be cleaned periodically to maintain the recharge rate.
f. By-pass arrangement is provided before the collection chamber to reject the first showers.
(ii) Roof Top Rain Water Harvesting through Recharge Trench:
a. Recharge trenches are suitable for buildings having roof area of 200-300 sq m and where permeable strata are available at shallow depths.
b. Trench may be 0.5 to 1 m wide, 1 to 1.5 m deep and 10 to 20 m long depending upon availability of water to be recharged.
c. These are back filled with boulders (5-20 cm), gravels (5-10 mm) and coarse sand (1.5-2 mm) in graded form—boulders at the bottom, gravel in between and coarse sand at the top so that the silt content that will come with runoff will be deposited on the top of the sand layer and can easily be removed.
d. A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the trench and a desilting/collection chamber may also be provided on ground to arrest the flow of finer particles to the trench.
e. By-pass arrangement is provided before the collection chamber to reject the first showers.
f. The top layer of sand should be cleaned periodically to maintain the recharge rate.
(iii) Roof Top Rain Water Harvesting through Existing Tubewells
a. In areas where the shallow aquifers have dried up and existing tubewells are tapping deeper aquifer, roof top rain water harvesting through existing tubewell can be adopted to recharge the deeper aquifers.
b. PVC pipes of 10 cm diameter are connected to roof drains to collect rain water. The first roof runoff is let off through the bottom of drain pipe. After closing the bottom pipe, the rain water of subsequent rain showers is taken through a T to an online PVC filter.
The filter may be provided before water enters the tube-well. The filter is 1-1.2 m in length and is made up of PVC pipe. Its diameter should vary depending on the area of roof, 15 cm if roof area is less than 150 sq m and 20 cm if the roof area is more.
The filter is provided with a reducer of 6.25 cm on both the sides. Filter is divided into three chambers by PVC screens so that filter material is not mixed up. The first chamber is filled up with gravel (6-10 mm), middle chamber with pebbles (12-20 mm) and last chamber with bigger pebbles (20-40 mm).
c. If the roof area is more, a filter pit may be provided. Rain water from roofs is taken to collection/desilting chambers located on ground. These collection chambers are interconnected as well as connected to the filter pit through pipes having a slope of 1: 15.
The filter pit may vary in shape and size depending upon available run off and are back-filled with graded material, boulder at the bottom, gravel in the middle and sand at the top with varying thickness (0.30-0.50 m) and may be separated by screen.
The pit is divided into two chambers, filter material is one chamber and other chamber is kept empty on accommodate excess filtered water and to monitor the quality of filtrated water. A connecting pipe with recharge well is provided at the bottom of the pit for recharging of filtered water through well.
(iv) Roof Top Rain Water Harvesting through Trench with Recharge Well:
a. In areas where the surface soil is impervious and large quantities of roof water or surface runoff is available within a very short period of heavy rainfall, the use of trench/pits is made to store the water in a filter media and subsequently recharge to ground water through specially constructed recharge wells.
b. This technique is ideally suited for area where permeable horizon is within 3 m below ground level.
c. Recharge well of 100-300 diameter is constructed to a depth of at least 3 to 5 m below the water level. Based on the lithology of the area well assembly is designed with slotted pipe against the shallow and deeper aquifer.
d. A later trench of 1.5 to 3 m width and 10 to 30 m length, depending upon the availability of water is constructed with the recharge well in the center.
e. The number of recharge wells in the trench can be decided on the basis of water availability and local vertical permeability of the rocks.
f. The trench is back-filled with boulders, gravels and coarse sand to act as a filter media for the recharge wells.
g. If the aquifer is available at greater depth say more than 20 m, a shallow shaft of 2 to 5 m diameter and 3-5 meters deep may be constructed depending upon availability of runoff. Inside the shaft a recharge well of 100-300 mm dia is constructed for recharging the available water to the deeper aquifers. At the bottom of the shaft a filter media is provided to avoid choking of recharge well.
A TYPICAL ROOF TOP
RAINWATER HARVESTING SYSTEM comprises of:
a. Roof catchment
b. Gutters
c. Down pipes
d. Rain water/ Storm water drains
e. Filter Chamber
f. Ground Water recharge structures like pit, trench, tube-well or combination of above structure
Rainwater Harvesting is a way to capture the rain water when it rains, store that water above ground or charge the underground and use it later. This happens naturally in open rural areas. But in congested, over-paved metropolitan cities, we need to create methods to capture the rain water.
Rainwater harvesting is a very ancient technique known to have existed for over 4000 years. This technique is being revived today to give back to nature what we take from it.
Rain water harvesting - a simple, economical and Eco-friendly method of water conservation is an ideal solution to recharge the ground water.
The rainwater that falls on the surface/ roof top is guided to bore wells or pits or new/ old/ abandoned wells through small diameter pipes to recharge the underground water which can be used later whenever required.
Rainwater can be harvested to the extent of 55,000 liters per 100 sq. meters area per year from rooftops
Roof top rainwater harvesting (RTRWH):
It is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the catchments, and the rainwater is collected from the roof of the house/building. It can either be stored in a tank or diverted to artificial recharge system. This method is less expensive and very effective and if implemented properly helps in augmenting the ground water level of the area.
Components of the roof top rainwater harvesting system
The illustrative design of the basic components of roof top rainwater harvesting system is given in the following typical schematic diagram/
The system mainly constitutes of following sub components:
Catchment
Transportation
First flush
Filter
The surface that receives rainfall directly is the catchment of rainwater harvesting system. It may be terrace, courtyard, or paved or unpaved open ground. The terrace may be flat RCC/stone roof or sloping roof. Therefore the catchment is the area, which actually contributes rainwater to the harvesting system.
Transportation
Rainwater from rooftop should be carried through down take water pipes or drains to storage/harvesting system. Water pipes should be UV resistant (ISI HDPE/PVC pipes) of required capacity. Water from sloping roofs could be caught through gutters and down take pipe. At terraces, mouth of the each drain should have wire mesh to restrict floating material.
First Flush
First flush is a device used to flush off the water received in first shower. The first shower of rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the probable contaminants of the atmosphere and the catchment roof. It will also help in cleaning of silt and other material deposited on roof during dry seasons Provisions of first rain separator should be made at outlet of each drainpipe.
Filter
There is always some skepticism regarding Roof Top Rainwater Harvesting since doubts are raised that rainwater may contaminate groundwater. There is remote possibility of this fear coming true if proper filter mechanism is not adopted. Secondly all care must be taken to see that underground sewer drains are not punctured and no leakage is taking place in close vicinity. Filters are used fro treatment of water to effectively remove turbidity, colour and microorganisms. After first flushing of rainfall, water should pass through filters. There are different types of filters in practice, but basic function is to purify water.
Sand Gravel Filter
These are commonly used filters, constructed by brick masonry and filleted by pebbles, gravel, and sand as shown in the figure. Each layer should be separated by wire mesh.
Charcoal Filter
Charcoal filter can be made in-situ or in a drum. Pebbles, gravel, sand and charcoal as shown in the figure should fill the drum or chamber. Each layer should be separated by wire mesh. Thin layer of charcoal is used to absorb odor if any.
PVC- Pipe filter
This filter can be made by PVC pipe of 1 to 1.20 m length; Diameter of pipe depends on the area of roof. Six inches dia. pipe is enough for a 1500 Sq. Ft. roof and 8 inches dia. pipe should be used for roofs more then 1500 Sq. Ft. Pipe is divided into three compartments by wire mesh. Each component should be filled with gravel and sand alternatively as shown in the figure. A layer of charcoal could also be inserted between two layers. Both ends of filter should have reduce of required size to connect inlet and outlet. This filter could be placed horizontally or vertically in the system.
Sponge Filter
It is a simple filter made from PVC drum having a layer of sponge in the middle of drum. It is the easiest and cheapest form filter, suitable for residential units.
Methods of Roof Top Rainwater Harvesting
Storage of Direct use
In this method rain water collected from the roof of the building is diverted to a storage tank. The storage tank has to be designed according to the water requirements, rainfall and catchment availability. Each drainpipe should have mesh filter at mouth and first flush device followed by filtration system before connecting to the storage tank. It is advisable that each tank should have excess water over flow system.
Excess water could be diverted to recharge system. Water from storage tank can be used for secondary purposes such as washing and gardening etc. This is the most cost effective way of rainwater harvesting. The main advantage of collecting and using the rainwater during rainy season is not only to save water from conventional sources, but also to save energy incurred on transportation and distribution of water at the doorstep. This also conserve groundwater, if it is being extracted to meet the demand when rains are on.
Recharging ground water aquifers
Ground water aquifers can be recharged by various kinds of structures to ensure percolation of rainwater in the ground instead of draining away from the surface. Commonly used recharging methods are:-
a) Recharging of bore wells
b) Recharging of dug wells.
c) Recharge pits
d) Recharge Trenches
e) Soak ways or Recharge Shafts
f) Percolation Tanks
Recharging of bore wells
Rainwater collected from rooftop of the building is diverted through drainpipes to settlement or filtration tank. After settlement filtered water is diverted to bore wells to recharge deep aquifers. Abandoned bore wells can also be used for recharge.
Optimum capacity of settlement tank/filtration tank can be designed on the basis of area of catchement, intensity of rainfall and recharge rate as discussed in design parameters. While recharging, entry of floating matter and silt should be restricted because it may clog the recharge structure. "first one or two shower should be flushed out through rain separator to avoid contamination. This is very important, and all care should be taken to ensure that this has been done."
Recharge Pits
Recharge pits are small pits of any shape rectangular, square or circular, contracted with brick or stone masonry wall with weep hole at regular intervals. to of pit can be covered with perforated covers. Bottom of pit should be filled with filter media.
The capacity of the pit can be designed on the basis of catchment area, rainfall intensity and recharge rate of soil. Usually the dimensions of the pit may be of 1 to 2 m width and 2 to 3 m deep depending on the depth of pervious strata. These pits are suitable for recharging of shallow aquifers, and small houses.
Soak away or Recharge Shafts
Soak away or recharge shafts are provided where upper layer of soil is alluvial or less pervious. These are bored hole of 30 cm dia. up to 10 to 15 m deep, depending on depth of pervious layer. Bore should be lined with slotted/perforated PVC/MS pipe to prevent collapse of the vertical sides. At the top of soak away required size sump is constructed to retain runoff before the filters through soak away. Sump should be filled with filter media.
Recharging of dug wells
Dug well can be used as recharge structure. Rainwater from the rooftop is diverted to dug wells after passing it through filtration bed. Cleaning and desalting of dug well should be done regularly to enhance the recharge rate. The filtration method suggested for bore well recharging could be used.
Recharge Trenches
Recharge trench in provided where upper impervious layer of soil is shallow. It is a trench excavated on the ground and refilled with porous media like pebbles, boulder or brickbats. it is usually made for harvesting the surface runoff. Bore wells can also be provided inside the trench as recharge shafts to enhance percolation. The length of the trench is decided as per the amount of runoff expected. This method is suitable for small houses, playgrounds, parks and roadside drains. The recharge trench can be of size 0.50 to 1.0 m wide and 1.0 to 1.5 m deep.
Percolation tanks
Percolation tanks are artificially created surface water bodies, submerging a land area with adequate permeability to facilitate sufficient percolation to recharge the ground water. These can be built in big campuses where land is available and topography is suitable.
Surface run-off and roof top water can be diverted to this tank. Water accumulating in the tank percolates in the solid to augment the ground water. The stored water can be used directly for gardening and raw use. Percolation tanks should be built in gardens, open spaces and roadside green belts of urban area.
Do's and Don’ts
Harvested rainwater is used for direct usage or for recharging aquifers. It is most important to ensure that the rainwater caught is free from pollutants. Following precautionary measures should be taken while harvesting rainwater:-Roof or terraces uses for harvesting should be clean, free from dust, algal plants etc.
Roof should not be painted since most paints contain toxic substances and may peel off.
Do not store chemicals, rusting iron, manure or detergent on the roof.
Nesting of birds on the roof should be prevented.
Terraces should not be used for toilets either by human beings or by pets.
Provide gratings at mouth of each drainpipe on terraces to trap leaves debris and floating materials.
Provision of first rain separator should be made to flush off first rains.
Do not use polluted water to recharge ground water.
Ground water should only be recharged by rainwater.
Before recharging, suitable arrangements of filtering should be provided.
Filter media should be cleaned before every monsoon season.
During rainy season, the whole system (roof catchment, pipes, screens, first flush, filters, tanks) should be checked before and after each rain and preferably cleaned after every dry period exceeding a month.
At the end of the dry season and just before the first shower of rain is anticipated, the storage tank should be scrubbed and flushed off all sediments and debris.
ATTRIBUTES OF GROUNDWATER :
There is more ground water than surface water
Ground water is less expensive and economic resource.
Ground water is sustainable and reliable source of water supply.
Ground water is relatively less vulnerable to pollution
Ground water is usually of high bacteriological purity.
Ground water is free of pathogenic organisms.
Ground water needs little treatment before use.
Ground water has no turbidity and colour.
Ground water has distinct health advantage as art alternative for lower sanitary quality surface water.
Ground water is usually universally available.
Ground water resource can be instantly developed and used.
There is no conveyance losses in ground water based supplies.
Ground water has low vulnerability to drought.
Ground water is key to life in arid and semi-arid regions.
Ground water is source of dry weather flow in rivers and streams.
Working of Rainwater Harvesting
When collecting rainwater it is very important to keep the water clean and safe. The first spell of rain carries a relatively larger amount of pollutants from the air and catchments surface. Without a first- flush system this will be washed off the roof and directly into your rainwater tanks causing the water to become dirty and possibly dangerous. The First- Flush Diverter is installed anywhere before the inlet of the water tank and the runoff from the first spell of rain is flushed out by using a first-flush before rainwater entering to the system or storage tank. There are several possible choices to selectively collect clean water for the storage tanks. The most common is the down-pipe flap. With this flap it is possible to direct the first flush of water flow through the down-pipe, while later rainfall is diverted into a storage tank. The filter is used to remove suspended pollutants from rainwater collected over roof. A filter unit is a chamber filled with filtering media such as fiber, coarse sand and gravel layers to remove debris and dirt from water before it enters the storage tank or recharge pit.
