RWH Technology

8/18/2019 RWH Technology

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Technology

A rainwater harvesting system comprises components of various stages -

transporting rainwater through pipes or drains, filtration,

and storage in tanks for reuse or recharge.

1) Catchment

2) Conveyance or conduit system

3) First flush

4) Filters5) Storage or recharge system

Effectiveness of the rainwater harvesting depends on appropriate

design of the systems. A few design tips to put the right water

harvesting system at the right place.

1) Design of storage tank

2) Design of groundwater recharge structures

Procedures and specifications for construction of storage and

recharge tanks are explained below. There are a lot of similarities

in the construction steps of both storage and recharge structures.

1) Masonry Tanks

2) Reinforced Cement Concrete Tank (RCC)

3) Ferro Cement Jars


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(1) Catchment

The catchment of a water harvesting system is the surface which directlyreceives 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 Gutters:

Channels all around the edge of a sloping roof to collect and transport rainwaterto 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.


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Conduits and channels

Conduits

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:

Diameter Of pipe (mm) Average rate of rainfall in mm/h

50 75 100 125 150 200

50 13.4 8.9 6.6 5.3 4.4 3.3

65 24.1 16.0 12.0 9.6 8.0 6.0

75 40.8 27.0 20.4 16.3 13.6 10.2


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100 85.4 57.0 42.7 34.2 28.5 21.3

125 - - 80.5 64.3 53.5 40.0

150 - - - - 83.6 62.7

mm/ h - millimeters per hour; m - meters

Source: National Building Code

First flushing

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.

(4) Filter


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Filters are used to remove suspended pollutants from rainwater collected over

roof. A filter unit is a chamber filled with filtering media such as fibre, coarse sand

and gravel layers to remove debris and dirt from water before it enters the storage

tank or recharge structure. Charcoal can be added for additional filtration.

Selection of a filter depends on followings: 1 Type of catchment1.Amount of silt load

2.Quality of runoff

3.Purpose of storage

4.Type of recharge structure

(i) Cloth filter:The simplest form of filter is a piece of fine cloth which is even

now used in areas like the north-east where they collect rainwater directly form

the roof into storage tanks. It is also known as saari filter in Gujarat where peopleuse a piece ofsaari filter(attire worn by Indian women) ordhoti filter (attire worn

by Indian men).

(ii) Charcoal water filterA simple charcoal filter can be made in a drum or an

earthen pot. The filter is made of gravel, sand and charcoal, all of which are

easily available.


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(iii) Sand filtersSand filters have commonly available sand as filter media. Sand

filters are easy and inexpensive to construct. These filters can be employed for

treatment of water to effectively remove turbidity (suspended particles like silt andclay), colour and microorganisms.

(iv) Inverted sand filter: It can filter medium to coarse sized sand & silt particles,

other floating debris along with bacterial contamination to limited extent.

Normally inverted sand filters are used in RWH.

Coarse sand1-2 mm

Gravel3-6 mm

Pebbles 15-25 mmBoulders 50-100 mm


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(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.

(vi) Varun


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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

http://www.rainwaterclub.org/http://www.rainwaterclub.org/


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(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 canbe 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


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quality of water stored in the container.

Recharge structures

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:

1. Filter mesh at entrance point of rooftop drains

2. Settlement chamber3. Filter bed

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 theexcess 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


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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


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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.

Design of storage tanks

The volume of the storage tank can be determined by the following factors:

1 Number of persons in the household: The greater the number of

persons, the greater the storage capacity required to achieve the same efficiency

of fewer people under the same roof area.

1. Per capita water requirement: This varies from household to household based on

habits and also from season to season. Consumption rate has an impact on the

storage systems design as well as the duration to which stored rainwater can last.

2. Average annual rainfall

3. Period of water scarcity: Apart from the total rainfall, the pattern of rainfall

-whether evenly distributed through the year or concentrated in certain periodswill determine the storage requirement. The more distributed the pattern, the

lesser the size.

4. Type and size of the catchment: Type of roofing material determines the

selection of the runoff coefficient for designs. Size could be assessed by

measuring the area covered by the catchment i.e., the length and horizontal

width. Larger the catchment, larger the size of the required cistern (tank).Dry

season demand versus supply approach

In this approach there are three options for determining the volume of storage:

1 Matching the capacity of the tank to the area of the roof1.Matching the capacity of the tank to the quantity of water required by its users

2.Choosing a tank size that is appropriate in terms of costs, resources and

construction methods 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.


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However a simplified calculation based on the following factors can give a rough

idea of the potential for rainwater colection.

Illustration

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. Theaverage 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 procedure:

Following details are available:

Area of the catchment (A) = 100 sq. m.

Average annual rainfall (R) = 611 mm (0.61 m)

Runoff coefficient (C) = 0.85 1. Calculate the maximum amount of rainfall that

can be harvested from the rooftop:Annual water harvesting potential = 100 x 0.6 x 0.85

;= 51 cu. m. (51,000 litres)

2. Determine the tank capacity: This is based on the dry period, i.e., the period

between the two consecutive rainy seasons. For example, with a monsoon

extending over four months, the dry season is of 245 days.

3. Calculate drinking water requirement for the family for the dry season

= 245 x 5 x 10

= 12,250 litres

As a safety factor, the tank should be built 20 per cent larger than required,i.e., 14,700 litres. This tank can meet the basic drinking water requirement

of a 5-member family for the dry period. A typical size of a rectangular tank

constructed in the basement will be about 4.0 m x 4.0 m x 1.0 m

Design of recharge structures and settlement tank

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


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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 usedmaterials 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


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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 acircle 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.

When constructing water tanks it is essential to adhere to a few basic yet criticalrules with respect to correct mixtures and applications of concrete and mortar.

These include: 1 Mixing cement, aggregate and water properly,

and not adding too much water

1.Applying the mortar or concrete within a maximum of half an hour of mixing

2.Curing cement work properly by keeping it moist and under shade for at least


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three weeks after its application.

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 Recharge well

a) Construction of a new recharge well

Step 1:Excavating the earth Step 2:Making a borehole to facilitate

groundwater recharging


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Step 3:Providing masonry or RCC walls in the excavated portion and thereafterproviding the filter materials. Step 4:Covering the tank made with a RCC or

stone slab provided with a manhole.


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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.


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