Roof Top RWH by Manohar Patil

7/29/2019 Roof Top RWH by Manohar Patil

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ROOF TOPRAIN WATER HARVESTING

Manohar Patil


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Why harvest rainwater

To arrest ground water

To beneficiate water quality in

aquifers To conserve surface water runoff

To reduce soil erosion


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What is RainwaterHarvesting

Collection

Storage

Recharge


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How to harvest rainwater

Broadly there are two ways of

harvesting rainwater Surface runoff harvesting

Roof top rainwater harvesting(RTRWH)


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Components of RTRWH system

The system mainly constitutes of

following sub components: Catchment

Transportation

First flush Filter


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Schematic diagram of a of typical roof toprainwater harvesting system

RECHARGE FACILITY

STORAGE FACILITY

FILTER

FIRST FLUSH ARRANGEMENT


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Commonly used Terms

Catchment

The surface that receives rainfall directly is thecatchment. Sloping roof or flat roof.

Transportation

Rainwater from rooftop should be carried throughdown take water pipes or drains to storage/harvestingsystem. Water pipes should be UV resistant (ISIHDPE/ PVC pipes) of required capacity.

First Flush

First flush is a device used to flush off the waterreceived in first shower. Provisions of first rainseparator should be made at outlet of each drainpipe.


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First Flush


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Filteration

Sand gravel filter

Charcoal filter PVC Pipe filter

Sponge filter


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Sand gravel filter

These arecommonlyused filters,constructedby brick

masonry andfilled bypebbles,gravels, andsand asshown in thefigure. Each

layer shouldbe separatedby wiremesh.

PEBBLE

GRAVEL

SAND


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Charcoal filter

Charcoal filtercan be madein-situ or in adrum. Thedrum orchambershould befilled bypebbles,gravels, sandand charcoalas shown in

the figure.Each layershould beseparated bywire mesh.

10 CM. GRAVEL LAYER

10 CM. CHACOAL LAYER

25 CM. SAND LAYER


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PVC Pipe filter

SAND

PVC PIPE FILTER

GRAVEL

MESH


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Sponge filter

It is a simplefilter madefrom PVCdrum havinga layer of

sponge inthe middle ofdrum. It isan easiest &cheapestform offilter,

suitable forresidentialunits.

OUTLET

SPONGE

PVC DRUM

COLLER CAP

PVC PIPE


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Methods of Rooftop Waterharvesting

Storage for Direct use

rain water collected from the roof is diverted tostorage tank.

storage tank has to be designed according to thewater requirements, rainfall and catchmentavailability.

Each drainpipe should have mesh filter at mouthand first flush device before connecting tostorage tank.

Each tank should have excess water over flow

system. Excess water could be diverted torecharge system. Water from storage tank can be used for

domestic and gardening purpose. It is most costeffective way of rainwater harvesting.


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Recharging ground wateraquifers

Ground water aquifers can be recharged byvarious kinds of structures to ensure percolationof rainwater in the ground instead of drainingaway from the surface. Commonly used

recharging methods are: -1) Recharging bore wells2) Recharge pits

3) Soakaways or Recharge Shafts

4) Recharging dug well

5) Recharge Trench

6) Percolation Tank


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Recharging bore wells


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Recharge pits

Perforated/honeycombed

30 cms filter media

From catchment

brickwork

RCC slab with

manhole


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Soak-aways or Recharge Shafts

60 cm x 60 cm. x 60 cmFilter bed sump

Perforated pipe

Perforated cover


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Recharging dug well

Filter bed

Runoff

Recharge dugwell

Perforated pipe

Runof

f


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Perforated cover

Filter media

Recharge trench


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Percolation tanksPercolation tanks are thesurface tanks, which canbe built, in big campuseswhere land is available andtopography is suitable.

Surface run-off and rooftop water can be divertedto this tank. Wateraccumulating in the tankpercolates in the soil to

augment the ground water.The stored water can beused directly for gardeningand raw use.


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Design parameters

To calculate maximum amount of rainfall that can beharvested from the rooftop, following details are required: -

Area of roof - (A)Average annual rainfall - (R)Runoff coefficient - (C)

Quantity of maximum annual

rainwater harvesting = A x R x C.


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Settlement tank

The capacity of the settlement tank should beenough to retain the runoff of

peak rainfall intensity period. Following data are

required to calculate capacity

Area of roof top - A

Peak rainfall in 15 min. - Pr

Runoff coefficient - CCapacity === A x Pr x C.


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Recharge tank/ filtrationchamber/ recharge trench

Water holding capacity of recharge tank/ filtrationchamber/ recharge trench is less then its gross volumebecause it is filled with porous material.factor for void ratio has to be applied in the calculationFollowing data required for the capacity: -

Area of roof top - APeak rainfall in 15 min - PrRunoff coefficient - CVoid ratio - DRequired capacity Q === A x Pr x C x DLength of recharge trench = Q / bxd

(Where b is width and d is depth of trench)


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Precautions

The roof or terraces used for harvesting should beclean

Do not store chemicals, rusting iron, manure ordetergent on the roof

Avoid using terraces for toilet by you and your pets

Provide gratings at mouth of each drainpipe onterraces to trap debris and floating materials

Prevent storage system from contamination

Provision of first rain separator should be made to

flush first rain Do not use polluted water to recharge ground water

Before recharging arrangements of filtering shouldbe provided


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Catch the waterwhere it falls

Thanks

Roof Top RWH by Manohar Patil

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