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Sunday , 26th September 2021

Roof top Rain Water Harvesting (RRWH)

Section: Rainwater Harvesting pages are not under access control

Although rainwater harvesting has been implemented in all the Urban Local Bodies, it is now felt that there is a need to modify some of the structures. The recharge bore wells and open wells have now proved to be more successful to recharge deep fractures, fissures. Hence, the individual households may be advocated to practice the methodologies under “SuWaSeM”.

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 groundwater level of the area.

Rooftop Rain Water Harvesting can be done at any building, large or small, where each of the following form a part of the RWH structure. The RWH system mainly consist of catchment area, transportation, flushing and filter media (Sand gravel filter/Charcoal filter). The roof top rainwater is collected and stored for direct use or it can be recharged the groundwater level.

1. RRWH for direct use

  • Suitable roof
  • Gutter to collect water from the roof
  • Down pipe to divert water from gutter to storage tank
  • A pre-collection filter
  • Storage tank

2. RRWH for groundwater recharge

The rainwater collected from the rooftop can be used for recharging the groundwater aquifers by various kinds of structures to ensure percolation of rainwater in the ground instead of draining away from the surface.

  • Open well
  • Bore well
  • Recharge pit
  • Recharge trench
  • Recharge shaft
  • Percolation tanks

To sustain the benefits of RRWH systems in urban areas the concerned ULBs should take following initiatives:

  • The executive authority of the ULBs are at liberty to design and implement its own incentive systems for the successful adoption of RRWH system
  • To establish a mechanism to monitor 100% of RRWH provisions in all the buildings above 10002 m with annual physical verification while buildings less than 10002 m shall be monitored on the basis of 10% of random verification.

Activities to be taken up/Design/Methods

Rooftop Rain Water Harvesting systems can provide good quality of potable water, if the design features outlined below are taken into account:

  • The substances that go into the making of the roof should be non-toxic and chemically inert.
  • Roof surfaces should be smooth, hard and dense since they are easier to clean and are less likely to be damaged and release materials/fibres into the water.
  • Roof painting is not advisable since most paints contain toxic substances and may peel off.
  • No overhanging trees should be left near the roof.
  • Nesting of birds on the roof should be prevented.
  • All gutter ends should be fitted with a wire mesh screen to keep out leaves, etc.
  • Appropriate arrangement for discarding the first flow of rainfall should be made.
  • A hygienic soak away channel should be built at water outlets and a screened overflow pipe should be provided.
  • The storage tank should have a tight fitting roof that excludes light, a manhole cover and a flushing pipe at the base of the tank (for standing tanks).
  • There should be a reliable sanitary extraction device such as a gravity tap or a hand pump to avoid contamination of the water in the tank.
  • There should be no possibility of contaminated wastewater flowing into the tank (especially for tanks installed at ground level).
  • Water from other sources, unless it is a reliable source, should not be emptied into the tank through pipe connections or the manhole cover.

Maintenance

Roof top catchment tanks, like all water supplysystems, demand periodic management andmaintenance to ensure reliable and quality watersupply. If the various components of the system arenot regularly cleaned, water use is not properlymanaged, problems are not identified or necessaryrepairs not performed, the roof catchment system willcease to provide reliable and good quality water.Following is a time table of maintenance andmanagement requirements that can provide a basis formonitoring and checking:

  • During the rainy season, the whole system (roof catchment, gutters, pipes, screens, firstflush and overflow) 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 of all sediment and debris (the tank should be refilled afterwards with a few centimetres of clean water to prevent cracking). Ensure timely service (before the first rains are due) of all tank fixtures, including replacement of all worn screens and servicing of the outlet tap or hand pump.

The runoff water collected from roof tops can artificially recharge and augment the depleting ground water resources especially in the urban areas, where the natural recharge has diminished considerably. The areas having depth to water table greater than 8 m below ground level and underlain by permeable strata are suitable for artificial recharge.

Design

The design involves consideration of data onhydrological and hydrogeological aspects andhydro meteorological parameters. The backgroundinformation to be collected is as given below:

  • Layout plan of the area.
  • Demarcation of the roof, paved and open areas.
  • Delineation of storm water drains and flow of storm water.
  • Details of the existing ground water abstraction structures in and around the vicinity of the project site.
  • Computation of the runoff for recharge. Apart from the above mentioned parameters. Selection of appropriate recharge structure depends on the availability of space for construction of recharge structures and invert levels of storm water drains at inlets to recharge structures. While preparing the recharge scheme, depth and shape of the storage facility in recharge structure depends on the availability of runoff, depth of storm water drainage and space availability in an area. The recharge scheme as prepared may also be got vetted by appropriate authorities/TWAD to incorporate suggestions for improvement.

The most suitable recharge structures for roof top rain water harvesting are:

  • Recharge pits
  • Recharge trenches
  • Recharge through dry or operational dugwells
  • Recharge through abandoned/existing tube wells
  • Recharge wells, etc.

Recharge Pits

  • In alluvial areas where permeable rocks are exposed on the land surface or at very shallow depth, recharge pits are suitable for artificial recharge of water collected from the roof tops.
  • The technique is suitable for buildings having a roof area of 100 m2 the recharge pits are constructed for recharging the shallow aquifers.
  • Recharge pits may be of any shape and size and are generally constructed I to 2 m wide and 2 to 3 m deep which are backfilled 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 and coarse sand at the top so that the silt content that will come with runoff will be deposited on the top of the coarse sand layer and can easily be removed. For smaller roof area, pit may be filled with broken bricks/cobbles.
  • A mesh should be provided at the roof so that leaves or any other solid waste/debris are prevented from entering the pit and a desilting/ collection chamber may also be provided at the ground to arrest the flow of finer particles to the recharge pit.
  • The top layer of sand should be cleaned periodically to maintain the recharge rate.

Recharge Trenches

  • Recharge trenches are suitable for buildings having roof area of 200-300 m2 and where permeable strata is available at shallow depths.
  • 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.
  • These are backfilled 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.
  • 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.
  • The top layer of sand should be cleaned periodically to maintain the recharge rate.

Recharge through Dry or Operational Dug Wells

  • Dry/operational dug wells if exist in the area may be utilized as recharge structures after cleaning and desilting the same.
  • Recharge water is guided through a pipe from desilting chamber to the bottom of the well or below the water level to avoid scouring of bottom and entrapment of air bubbles in the aquifer.
  • Recharge water should be silt-free. For removing the silt content, the runoff water should pass either through a desilting chamber or filter chamber.
  • Periodic chlorination should be done for controlling the bacteriological contamination in operational dug well.
  • Wire mesh filter should be provided just before the inlet to avoid entry of any foreign material, tree leaves, etc., in to the dug well.

Recharge through Abandoned/Existing Tube Wells

  • Abandoned/existing tube wells may be used as recharge structures.
  • The abandoned tube well should be properly developed before use as recharge structure.
  • PVC pipes of 10 cm diameter are connected to roof drains to collect rainwater.
  • The first roof runoff is drained through the bottom of drain pipe if existing tube well is used as recharge structure. After closing the bottom pipe, the rainwater of subsequent rain showers is taken through a 'Tee' to an online PVC filter in case of small roofs. If the roof area is larger, a filter pit may be provided. Rainwater 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.
  • A connecting pipe with recharge well is provided at the bottom of the pit for recharging of filtered water through well.
  • Wire mesh filter should be provided just before the inlet to avoid entry of any foreign material, tree leaves. etc., in to the system.

Recharge Wells

  • In areas where the aquifers are overlain by a considerable thickness of impervious formation, a new recharge tube well can be constructed for recharging the harvested rainwater.
  • It is used for recharging single/multiple aquifers.
  • A settlement-cum-storage tank is constructed near the tube well for settlement of silt particles and storage of excess water.
  • Roof top water is diverted to the settlement tank through pipes.
  • Clear water of storage tank is diverted to the recharge tube well for recharge.
  • It is suitable for recharging roof top rainwater of big buildings/blocks.
  • If runoff availability is less then online filter may be used in the pipe line connecting roof water with recharge well.

Construction of Recharge Well

These are drilled by deploying the appropriate rig unitor by hand boring as per the site conditions and depthof the tube wells.

A well assembly of pipes with diameters varying from100 to 250 mm may be lowered throughout the depth.Both M.S. and PVC pipes can be used. PVC pipes arerigid, light pipes in 6 or 9 m lengths available in alldiameters. The main advantage of PVC pipes is theirresistance to corrosion and slots of the pipes will notclose with time. As the slotted pipes in recharge wellsare in fluctuation zones of water levels, slots of M.S.pipes may become closed due to rusting. The maindrawback of PVC pipes is that, these pipes cannot beused in large diameter recharge wells. M.S. Pipes maybe coated with bituminous coating to avoid rusting.

After excavation of the recharge trench/shaft orfiltration chamber is over, pipes should be rechecked and cleaned with wire brush. Depth sounding ofrecharge wells should be taken with tape to make surethat no silt or soil has gone into the recharge wellsduring the excavation of trench/shaft. Width of slots inrecharge well should be in accordance with the aquifersystem encountered. Slotted pipes should be placedagainst the aquifer or dried-up aquifers encounteredin the recharge wells. A slotted pipe at the top of therecharge well will need to be placed to permit the entryof clean/clear water into the recharge well.

The annular space around the well assembly may beshrouded with appropriate size of gravel. The gravelshould be washed so that it is silt-free. The rechargetube well should be developed by low capacity aircompressor or by bailing method as required.The wellmay also be cleaned and developed by pouring thewater from outside if required. The water levels of thetube well should be recorded and the well covered withcap with a provision to monitor the well in future. A vent pipe of about one inch diameter is alsorecommended which can act as escape for gases and for measuring the water levels. Once the rechargetrench or shaft is constructed around the recharge tube well, recharge wells may be developed with hand bailers to avoid the disturbance of filter media.

Filters: Generally, the following two types of filters are used:

1.Online Filter

  • This filter is used when availability of runoff as well as recharge rate of recharge well is less.
  • Manufactured from reinforced engineering plastic material.
  • Available in various sizes and flow rates ranging from 3 to 25 m3/h.
  • Easy to open and clean.

2.Purpose Built Filter

  • The filter material recommended is coarse sand of 1.5 to 2 mm size at the top, followed by gravel of 5 to 10 mm size and boulders of 5 to 20 cm at bottom. The thickness of each layer should be about 0.5 m. Coarse sand should be placed at the top so that the silt content that comes with runoff will be deposited on the top of the coarse sand/pea gravel and can easily be removed. For smaller roof area the pit may be filled with over burnt broken bricks/cobbles.
  • After excavation of filter chamber, boulders and gravel should be filled up first to the foundation of wall of the structure.
  • After filling of boulder and gravel, filter material should be covered with polythene/jute bags to avoid spilling of construction material, which may damage the filter bed. After the construction of walls, the polythene/jute bags should be removed and the sand/pea gravels filled up to the recommended depth as per the design.
  • Filter media should be free from silt and any other foreign material. Before putting the filter material into the chamber, filter material should be sieved and washed to remove all the finer material. During operation the scouring effect of flow of water into the structure should be checked upon and if flow is disturbing the filter media, the water can be released near the filter media. This can be done by providing an 'I' shape joint in the inlet pipe in trench.
  • Regular inspection of filter material is essential in recharge structures. Silt deposited on the filter media should be cleaned regularly. Once in a year the top 5-10 cm sand/pea gravel layer should also be scraped to maintain a constant recharge rate through filter material.
  • Growth of grass or bushes hampers the filtration rate of the chamber. The grass and bushes should be cleared regularly.

Maintenance of Catchment Area, Water Drains and Recharge Structures

  • The catchments should be neat and clean. The roof top/terrace of the building spaces around the buildings should not be used for dumping of unwanted items and scrap material.
  • The washing machine water having heavy dose of detergents should not be allowed to enter into the water drains which are connected with recharge structures.
  • Open water drains covered with perforated detachable RCC slabs are best as the maintenance of these drains is easy and pollution, especially bacteriological pollution, can be avoided. If the storm water drainage is through pipe system, provide manholes andchambers at regular intervals as well as close to the suspected silt and waste accumulation places within the channel.
  • Protect the drainage system from tree leaves, polythene bags, plastic bottles and pouches of eatables.
  • Put up sign boards mentioning that the campus of building is equipped with rainwater harvesting system which is being recharged to the ground water system.Mention the ill effects and health impacts if the storm water drains are not properly maintained. Educate the 'Staff’ maintaining the storm water drains to keep the drains neat and clean.
  • Provide wire mesh filter just before the inlet. Provide silt check wall with in the drain bed at a convenient place. If more silt is expected provide check wall at regular intervals in the storm water drains.
  • The periodic removal of the material deposited on the surface be done by scraping the silt accumulated on top of the filter bed regularly.
  • Precaution should be taken to avoid domestic waste water entering into the recharge structures.
  • Recharge tube wells should be developed periodically by hand bailers to avoid clogging of the slots.
  • Before the arrival of monsoon, the roof top as well as drains should be properly cleaned.
  • Length and placement of the slotted pipe should be finalized after drilling of pilot hole for tube well.
  • Recharge water should be introduced into the structure at its lowest point to prevent erosion and disturbance of filter material.
  • A wire mesh should be placed at the entrance of recharge structures.
  • Periodic cleaning of collection chambers should be carried out to remove the plastic bags, leaves, etc. which may choke the entry of water recharge structures.